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Managing Wastewater in Coastal Urban Areas (1993)

Chapter: D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...

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Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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D Engineering and Management Options for Controlling Coastal Environmental Water Quality

INTRODUCTION

As discussed in the main body of this report, approaches to wastewater management and engineering in coastal areas should be designed to suit the characteristics of the surrounding environment. This appendix discusses five engineering and management options for wastewater and storm water in coastal urban areas: source control, wastewater treatment systems, disinfection, combined sewer overflows, and nonpoint source controls. Wastewater outfalls are not included here, but are discussed in Appendix C in connection with transport and fates in the coastal environment.

Whereas traditional wastewater management with its single medium focus revolves around treatment and disposal, a fully integrated approach addresses a broader range of considerations including source control; potential impacts on all environmental media (water, air, and land); water, energy, and other natural resource conservation; recycle and reuse; and nonpoint source control.

SOURCE CONTROL

The three basic source control alternatives, which may be practiced independently or concurrently in any municipality, are pollution prevention, pretreatment, and recycle and reuse.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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Pollution Prevention

Pollution prevention is the common sense notion of trying to prevent or reduce pollution at the source before it is created. It may include a wide range of activities, programs, and techniques. Elimination or minimization of water pollutants at the source is becoming more important as wastewater treatment plant effluent criteria become more strict. Once a pollutant is discharged into a sewer system, it is diluted by several orders of magnitude and usually much more difficult to remove. Analysis and treatment of these diluted pollutants can be difficult and expensive. Depending on the pollutant's dominant characteristics, it may volatilize into the atmosphere, biodegrade, settle out with the sludge, or pass through into the final effluent.

Pretreatment

Pretreatment refers to the treatment of wastewater at industries or commercial establishments before it is discharged to a sewer system. Pretreatment of wastewater reduces the release of conventional and toxic pollutants into the system. Pretreatment processes include physical and chemical treatment and biological treatment. These processes typically result in some type of cross-media transfer of pollutants from wastewater to land or air. For example, chemical or biological processes produce residuals that contain concentrated levels of pollutants removed in treatment. Thermal and biological processes, however, can destroy all or most of some compounds, but others will concentrate in residuals or escape to the atmosphere.

To date, pretreatment has been the main approach used by the federal government to control the discharge of industrial or commercial waste to publicly owned treatment works (POTWs). Environmental Protection Agency (EPA) effluent guidelines are based on the best available control technology. The enforcement of federal pretreatment standards by POTWs has helped reduce the amounts of contaminants, especially metals and some toxic organics, from being discharged to the nation's waterways.

Recycling and Reuse

Recycling and reuse involve transformation of potential waste materials into products. Internal recycling and reuse occurs when a material that has served its original purpose and could become a waste is recovered and reused at the site of waste generation; the material is controlled by the waste generator. Internal recycling by industry can involve the installation of closed-loop or in-process recycling systems. Internal recycling takes place in the home when, for example, vegetable wastes are composted rather than disposed as garbage. External recycling and reuse occurs away from the site of waste generation. External recycling is a multiple-step process

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

involving separation and collection of the material, transport to a recycling center and/or a reprocessing facility, and resale to a new user.

Pollution Prevention in Municipal Wastewater Management—Background and Definitions

In the recently enacted 1990 Federal Pollution Prevention Act, source reduction (or source control) has been interpreted by the EPA as ''any practice that reduces the amount of any hazardous substance, pollutant, or contaminant entering any wastestream prior to recycling, treatment and disposal" (42 U.S.C. 13101 et seq.).1 Pollution prevention in the context of coastal municipal wastewater management refers to the use of materials, processes, or practices that eliminate or reduce the creation of pollutants, either toxic or conventional, or wastes (e.g., plastics, paper) at the source. Sources can be domestic, commercial, institutional, or industrial. Pollution prevention includes any on-site source reduction or substitution undertaken prior to discharge to a municipal sewer system to reduce the total volume or quantity of pollutants generated or hazardous materials used in order to minimize the impact of the waste itself. It also includes practices that reduce the use of water, energy, or other natural resources at the source. Actions taken away from the source of the waste-generating activity, including off-site treatment of wastes or off-site recycling, are not considered pollution prevention activities by the EPA. These activities may still serve to improve wastewater influent quality.

Pollution prevention options include product changes, technology modifications, raw materials and process changes, and operational changes. In the context of municipal wastewater management, some of the chief pollution prevention activities include source reduction, water conservation, energy conservation, and some approaches to nonpoint source control.

In applying pollution prevention concepts to the field of municipal wastewater management, all sources—domestic, commercial, institutional, and industrial—are potentially significant. Similarly, all chemicals and materials are assessed, including conventional pollutants such as total suspended solids (TSS) and biochemical oxygen demand (BOD), paper and plastics, as well as toxics. For example, at one municipality, the elimination of plastics may be the central pollution prevention project; at another, the minimization of silver and mercury amalgams from dentists' offices may be more effective. The banning of phosphate detergents is another example of source reduction or product substitution that can change wastewater characteristics significantly.

1  

References to United States Code are cited with the title followed by "U.S.C." and the section.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×
Energy Conservation and Energy Recovery

Energy conservation and recovery can be important components of an integrated coastal management program for POTWs. Municipal POTWs generally consume less than one percent of the electrical energy demand of the communities they serve. Nonetheless, optimizing existing treatment processes, limiting sludge production, and using less energy intensive treatment methods have the potential to make a significant reduction in energy demands. Energy recovery using methane gas—a natural by-product of anaerobic sludge digestion—can meet over 50 percent of the electricity needs of a POTW (CSDOC 1989).

Nonpoint Source Control

Nonpoint source control options include a range of activities to limit urban and agricultural runoff and atmospheric deposition into waterways that in turn degrade the coastal environment. Some of these approaches can be considered pollution prevention activities, others are structural and fall under the treatment category. The subject of nonpoint source control options is addressed later in this appendix.

Pollution Prevention Programs
Implementation

At present there is no federal mandate to implement pollution prevention programs in municipalities and/or municipal wastewater management districts. Key ingredients in the implementation of pollution prevention programs in municipalities include 1) setting definitions and goals; 2) conducting an inventory of all resources and pollutants, including, especially, those subject to cross-media transfer; 3) systematically examining each problem pollutant to determine how it can be prevented or minimized; 4) establishing a prioritization, reporting, and tracking system for pollutants; 5) undertaking preventive routine operation and maintenance inspection programs to help eliminate unwanted plant shut-downs and avoidable discharges; and 6) implementing specific projects or actions. Examples of regulatory options that encourage the implementation of pollution prevention practices are shown in Table D.1.

Examples of Pollution Prevention Programs

Orange County, California. A wide range of pollution prevention activities have been instituted and are being incorporated into the existing

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.1 Local Pollution Prevention Regulatory Options

Indirect Inducements

Direct Requirements

Regulatory flexibility

Mandatory hazardous waste management plans

Aggressive enforcement of local pollution discharge limits

Mandatory pollution prevention requirements incorporated into discharge permits, including the requirement of standard reduction technologies

Development of more comprehensive and stringent limits

 

 

Incentives tied to reduced regulatory requirements

Nonprescriptive effluent guidelines

 

Development of control mechanisms for commercial and small industrial dischargers

Mandatory recycling requirements for industrial dischargers

 

Investigation of pollution prevention opportunities required by statute prior to planning of new treatment facilities

Option to use mass-based wastewater discharge limits

 

Development of waste exchanges and other technical assistance for regional industry to encourage waste reduction

 

Low interest loans

 

source reduction program at the Orange County Sanitation Districts. Principally, pollution prevention and waste minimization have been used as a tool to assist dischargers in attaining compliance. A program is being developed to train field inspectors in pollution prevention inspections, to coordinate multi-agency pollution prevention work, to hold workshops and other public education events, to mandate wastewater reductions of pretreatment program permittees, to apply mass emission limits instead of concentration limits for permittees, to require the implementation of pollution prevention techniques, and to provide technical assistance to permittees in violation of their permits.

Springfield, Massachusetts. While the pretreatment program at the 67 million gallons per day (MGD) Springfield Regional Wastewater Treatment Plant has effectively controlled the industrial discharge of cadmium, nonindustrial sources and/or nonpoint sources interfere with a Type I classification for the composting and marketing of its sludge. The Springfield pollution prevention pilot project is designed to monitor and quantify the nonpoint source pollutant load. It will be coupled with a pollution prevention educational outreach project targeted at reducing illegal discharges to storm sewers.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

Winston-Salem, North Carolina. The city of Winston-Salem is establishing a pilot project at a large POTW by developing a model pollution program designed to meet the needs of both large and small POTWs and integrating pollution prevention evaluation techniques into existing pretreatment program elements.

Cincinnati, Ohio. Cincinnati's Metropolitan Sewer District is developing a pilot program similar to that of Winston-Salem with assistance from the Ohio Environmental Protection Agency. The program will incorporate pollution prevention techniques into the ongoing pretreatment program to reduce loadings to the POTWs.

Economic Advantages of Pollution Prevention

Added regulations, higher industrial treatment and landfilling expenses, and increased liability costs have caused industrial and governmental leaders to reevaluate end-of-pipe pollution control measures in favor of front-end actions. Some of the economic advantages of pollution prevention include reduced on-site capital and operational waste-treatment costs; reduced transportation and disposal costs for wastes sent to an off-site location; reduced compliance costs for permits, monitoring, and enforcement; lower risk of spills, accidents, and emergencies; lower long-term environmental liability and insurance costs; reduced production costs through better management and efficient use of raw materials, transportation, and energy; improvements in process, product quality, and product yield resulting from a reexamination of current practice and the institution of better controls; income derived from the sale or reuse of waste; reduced sewer-use fees; better employee morale; and better public relations.

Economic Advantages of Recycling and Reuse

Because hazardous waste disposal fees can be a major component of an industry's annual operation and maintenance costs, zero sludge production may be attractive on economic as well as environmental grounds. A study sponsored by the California Department of Health Services compared the annual waste management operating costs for two similar circuit board plants for two different treatment plants assuming 10-year lifetimes. Cost data for the first plant were based on an installed conventional treatment system with sludge handling equipment. Cost data for the second plant were based on an installed recovery system with zero sludge production. The results are shown in Table D.2.

In this study, the total annual cost of the zero sludge production alternative is 9 percent higher than the conventional sludge system. However,

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.2 Comparison of Conventional Sludge Treatment Versus Zero Sludge Production System (Source: Cal-Tech Management Associates 1987. Reprinted, by permission, from Cal-Tech Management Associates, 1987.)

 

Conventional Sludge System

Zero Sludge Production System

Total Capital Cost

$450,000

$1,250,000

Capital Recovery1

$67,000

$186,000

Operation and Maintenance2

$43,000

$75,000

Labor

$75,000

$50,000

Chemicals and Power

$75,000

$54,000

Water- In/Out

$22,000

$2,000

Sludge Disposal & Fees

$48,000

0

Miscellaneous

$10,000

$5,000

Total Annual Cost

$340,000

$370,000

1 Assuming an 8 percent opportunity cost.

2 Assuming 12 percent of the original capital cost per annum.

only quantifiable costs have been used in defining the cost parameters. No attempt was made to examine less tangible future costs such as the value of recovered metals, increasing land disposal costs, legal liabilities, or insurance costs. Had these nonquantifiable costs been factored in, it is possible that the zero sludge production system would become the preferred option on economic as well as environmental grounds.

Pollution Prevention or Pretreatment?

An integrated coastal management plan for a given area would compare the environmental and cost benefits of pollution prevention with pretreatment to determine which is most advantageous. The following exercise shows the advantages and trade-offs in each set of activities.

Environmental Benefits-Pretreatment

In the 1991 National Pretreatment Program Report to Congress, the EPA concluded that categorical standards and local industrial discharge limits implemented by POTWs had brought about significant reductions in toxic pollutant loadings from regulated industries (EPA 1991a). The EPA estimated that metals loadings have been reduced by 95 percent to annual loadings of 14 million pounds, and organic loadings have been reduced by

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

between 40 and 75 percent to annual loadings of 65 million pounds. The EPA also concluded that the planned development of additional categorical standards would further reduce loadings of toxic pollutants to POTWs.

In 1988 and 1989, the Association of Metropolitan Sewerage Agencies conducted a survey of its members to determine the effectiveness of their pretreatment programs. It found that the mass discharge of 10 heavy metals had decreased 69 percent and that the mass discharge of cyanide and selected organics had decreased 66 percent (AMSA 1990). These findings confirmed the EPA's Domestic Sewage Study conclusion that "The pretreatment program has been an effective means in reducing the mass discharge of many hazardous constituents to POTWs" (EPA 1986).

This success in the reduction of heavy metals through pretreatment is illustrated by the example of the County Sanitation Districts of Orange County, California. In 1976, the Sanitation Districts adopted a new industrial source reduction ordinance which included numerical limitations on all industrial discharges. The influent heavy metals reductions at the two regional wastewater treatment plants in the 15 years of record are shown in Figure D.1.

FIGURE D.1 Annual mass inflows of various metals to the County Sanitation Districts of Orange County wastewater treatment plants. (Reprinted, by permission, from County Sanitation Districts of Orange County, California.)

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×
Environmental Benefits—Pollution Prevention

Information on the effectiveness of practices for preventing or reducing discharges to urban wastewater collection systems and receiving waters is available. However, much of it is case-specific and thus complicated to compare with the more extensive information available on pretreatment.

In 1985, the Robbins Company, a metal finishing and plating operation in Attleboro, Massachusetts, was in violation of its water discharge permit and was named a major polluter of the Ten Mile River that empties into Rhode Island's Narragansett Bay. When federal and state officials announced plans to tighten discharge limits further, the company was faced with four options as shown in Table D.3 (Berube and Nash 1991).

The Robbins Company management realized that a pollution prevention approach, through the use of a closed-loop system, although risky, was its best choice. Figures D.2a, D.2b, and D.2c show the Robbins Company's success for the years 1985 through 1990 in water conservation, chemical use, and sludge production after implementing a pollution prevention program. These figures show a 97 percent, 98 percent and 100 percent reduction, respectively, in the use of caustic soda, acid, and chlorine and nearly 100 percent reductions in water use and sludge production. The benefits of such a program are that the reductions can occur in every aspect of the process leading to multiple environmental improvements.

Cost-Benefit Ratios

Although some studies have examined the economics of waste management alternatives for selected industries, such as metal finishers and printed

TABLE D.3 Four Pollution Management Options at the Robbins Company (Source: Berube and Nash 1991. Reprinted, by permission, from the Robbins Company.)

Options

Effect on Compliance

Capital and Operation and Maintenance Cost

Do nothing

Completely out of compliance

In compliance now but probably not in the future

Full compliance

Fines up to $10,000 per day

$250,000 capital

$120,000/yr O&M

Upgrade present system

 

 

Build a full wastewater treatment plant

Modify the process and build a closed-loop system

 

$500,000 capital,

$120,000/yr O&M

$250,000 capital

$21,000/yr O&M

 

Full compliance

 

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

FIGURE D.2a Pollution prevention at the Robbins Company, Attleboro, Massachusetts-Water Usage. (Source: Chatel 1992. Reprinted, by permission, from the Robbins Company.)

circuit board facilities, to date there have been few comparisons of the cost benefits of pretreatment and pollution prevention.

One comparison performed by the EPA looked at the costs of end-of-pipe treatment for an electroplating facility with and without pollution prevention versus a waste recovery system for an electroplating facility (EPA 1979). The three options evaluated were: 1) a system with standard single-stage running rinses and no pollution prevention, 2) a system with pollution prevention in the form of counter-current rinses instead of single-stage rinses, and 3) a system with pollution prevention in the form of recovery units installed after each plating operation in addition to counter-current rinses.

As shown in Table D.4, the cost of the system with counter-current rinses and recovery units provided an annual cost savings of over 50 percent in comparison to the other systems evaluated.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

FIGURE D.2b Pollution prevention at the Robbins Company, Attleboro, Massachusetts—Chemical Usage. (Source: Chatel 1992. Reprinted, by permission, from the Robbins Company.)

Grants for Small Business. While many basic pollution prevention practices (e.g., good housekeeping and operational practices, systematic maintenance, and training of personnel) require marginal or no capital investment, other more fundamental practices such as pretreatment systems, production equipment modifications, or raw material substitutions require up-front investment of funds for development, research, engineering, and equipment. Although pollution prevention may represent financial benefits, these benefits may take several years to amortize the original capital investment. Large businesses and corporations are usually able to support the burden of long term capital returns. Small businesses may not be. Consequently, small businesses are often reluctant, and frequently find it impossible, to engage in advantageous pollution prevention or pretreatment programs without federal, state or local financial assistance, such as grants.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

FIGURE D.2c Pollution prevention at the Robbins Company, Attleboro, Massachusetts—Sludge production. (Source: Chatel 1992. Reprinted, by permission, from the Robbins Company.)

TABLE D.4 EPA Comparison of Pretreatment Versus Pollution Prevention Cost (Source: EPA 1979)

 

No Pollution Prevention

Pollution Prevention

Pollution Prevention

 

Standard

Single Stage

Rinses

Counter-Current

Rinses

Counter-Current

Rinses + Plating

Recovery Systems

Captial Costs1

Annual Costs2

$192,000

$90,000

$186,000

$87,000

$162,000

$40,000

1 The capital costs include equipment for treatment of wastewater and sludge.

2 The annual costs include annualized capital (10 year life), depreciation, operation and maintenance, energy and power, and sludge disposal costs.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

Grants may be used as seed money for small businesses to get pollution prevention and pretreatment programs started.

Quality Certification of Technology. Pollution prevention and pretreatment often involve innovation and new processes and technologies that are unfamiliar to the average business. Many businesses do not have the resources, personnel, or expertise to pursue and obtain good technical information. Unscrupulous vendors and lack of knowledge of technological realities may result in the improper application of a control technology or practice. One problem faced by industry is when a vendor who provides the equipment goes out of business or reneges on guarantees. Then businesses are faced with the problem of replacing the entire pollution control system or paying heavy fines with no redress. In order to provide small business with the necessary resources and confidence to acquire new technologies, a strict quality certification program of technology and vendors could certify the vendors and technology.

Integrated Multi-Media Permitting. The current permitting system responds to single media statues and regulations such as the Clean Air Act, Clean Water Act, and Resource Conservation and Recovery Act. Because of the single media interests addressed by these statutes, the requirements of one ignores impacts on other media. Single media permitting imposes standards and technology that may result in an increase of emissions to another media, and conflict with another set of permitting requirements. An integrated multi-media permitting program would address the balance between the three media—air, water, and land—providing an integrated system for environmental protection. As an incentive for business in general, and small business in particular, it would eliminate some of the duplication of reporting, audits, inspection, monitoring and permitting of the current system. Most importantly, multi-media permitting would allow for an integrated environmental protection program that addresses all media equally and results in the greatest and most cost-effective benefit to the environment.

Conclusions

Pollution prevention programs are generating a different attitude in industries and communities, where pretreatment regulations are based on the command-and-control philosophy of environmental protection. Pollution prevention offers a new type of incentive to better business practice and community relations. Pollution prevention, like labor productivity and energy consumption, can become a measure of productivity and efficiency of industrial operations. For businesses, pollution prevention may be a way to improve profitability and competitiveness.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

While enforcement and compliance often provide the impetus for pollution prevention programs, the existing framework of regulations is not sufficient to implement successful pollution prevention programs. There are several key elements necessary for a workable system:

Regulatory Flexibility—e.g., use of alternative requirements such as mass-emission limits as the compliance basis for dischargers and latitude in imposing compliance and enforcement schedules on industry to permit innovative approaches and make allowances for failed technology and development of incentives;

POTWs as Co-Regulators—POTWs must have the flexibility to develop a prevention program tailored to local conditions and the tools to implement the program;

Institutional Structure that Supports a Multi-Media Perspective; and

Parallel Programs—Federal, state, and local governments should initiate grants for small businesses, quality certification of technology, integrated multi-media permitting.

Several factors make the economic determination of pollution prevention benefits a difficult problem. First, because there are innumerable ways to reduce waste, oftentimes each pollution prevention opportunity must be considered on a case-by-case basis. Second, there are a number of nonquantifiable economic benefits to pollution prevention programs, including decreased liability and improved public relations. Nevertheless, some U.S. companies subject to regulation under the Clean Water Act have verified that pollution prevention pays for itself relatively quickly, especially compared with the time needed to comply with regulations (e.g., obtain regulatory permits, and site and construct waste management facilities) (NHSRC 1991, EPA 1992a).

The information presented in this report indicates that the total operating costs of facilities with pollution prevention equipment can be economically competitive with conventional pretreatment systems, even when nonquantifiable costs are not factored into the final result. Recognizing that more research and data collection are essential to a sound analysis of environmental and economic costs and benefits, the following preliminary conclusion can be reached:

  • A break-even cost situation between the pollution prevention approach and the pretreatment approach to source control has nearly been reached across a broad spectrum of industries. If higher discharge limitations and/or higher off-site waste disposal costs are applied in the future, businesses with pollution prevention programs in place will have an economic advantage.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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MUNICIPAL WASTEWATER TREATMENT

Introduction

Ultimately, after appropriate treatment, wastewater collected from cities and towns must be returned to the land or water. The complex question of which contaminants in wastewater should be removed to protect the environment, to what extent, and where they should be placed must be answered in light of an analysis of local conditions, environmental risks, scientific knowledge, engineering judgment, and economic feasibility.

The total, 20-year capital cost to upgrade U.S. municipal sewerage systems is $110 billion (for the design year of 2010 in 1990 dollars) according to the 1990 Needs Survey Report to Congress (EPA 1991a). The cost for construction of conventional secondary ($37.3 billion) and advanced systems ($11.79 billion) totals $49.0 billion. A decade ago, the United States government paid about 75 percent of the costs to construct new treatment plants. In 1990, the Federal Construction Grants Program was terminated and replaced with a revolving loan program administered by the states for the EPA. Today, more than 80 percent of the cost burden falls on local ratepayers.

Advances in Municipal Wastewater Treatment Systems

A selection of some important advances in municipal wastewater treatment can be organized into five areas: 1) optimization of primary stages of treatment, 2) innovations in biological treatment processes, 3) natural wastewater treatment systems, 4) water reclamation and reuse as an alternative to discharge to receiving waters, and 5) innovations that offer flexibility and/ or special capabilities.

Optimization of Primary Stage(s) of Treatment

Economic and space constraints at existing and new sites have provided the impetus to optimize the primary stages of treatment through high-rate settlers, primary filtration, fine screens, and chemically-enhanced primary treatment. Improved primary treatment results in a reduction of larger organic and inorganic particles. Because small (less than 1.0 micrometer) organic particles in wastewater can be biologically degraded about 4 times faster than larger particles (Force 1991), optimization of primary treatment can increase the biodegradation rate in subsequent treatment, which can result in improved performance or cost savings. The first three of these technologies are discussed here; chemically-enhanced primary treatment is covered in the section on Municipal Wastewater Treatment Systems.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

High-Rate Settlers. High-rate settlers consist of parallel plates or tubes in sedimentation tanks inclined at an angle of between 30 to 60 degrees from the horizontal. Beginning with an idea first proposed in the early 1900s on the value of maximizing the surface area of a public water supply settling tank (Hazen 1904), the concept was adapted to wastewater applications in the 1970s. High-rate settlers have significantly smaller land-area and basin-size requirements than conventional settlers. They have been operated at increased overflow rates compared with conventional primary treatment facilities, often with chemicals for increased removal efficiency. However, increased maintenance problems caused by the accumulation of grease and other debris must be balanced against the performance advantages. High-rate settlers are used in a number of locations in France, Monaco, the province of Quebec, Canada, and the United States (Forsell and Hedstrom 1975, Leblanc 1987).

Primary Effluent Filtration. Primary effluent filtration makes use of a shallow bed of single-size, fine-grain sand with an underdrain and air pulsing system to filter primary effluent. By reducing the amount of larger-sized organics by filtration, the biodegradation rate of the remaining organic material can be increased in subsequent biological secondary treatment processes. Alternatively, primary effluent filtration can serve as a final treatment step if the prevailing water-quality objectives can be met with the effluent quality achieved by this treatment method. Primary effluent filtration performance efficiencies vary widely for TSS and 5-day biochemical oxygen demand (BOD5) (Matsumoto 1991). In pilot-scale studies conducted at the County Sanitation Districts of Orange County, California in 1982, average concentrations of 25 mg/l TSS and 104 mg/l BOD5 were achieved for municipal wastewater (Hydroclear Corporation, unpublished data, January 1983). Full-scale capital and operations and maintenance estimates for this technology on which to determine its relative cost-effectiveness have not been developed.

Fine Screens. Where coarse screens have been used in the preliminary stage of treatment, the development of better screening materials and systems over the last 20 years has led to the use of fine screens as a substitute for or as a means of upgrading conventional primary sedimentation. Fine screen designs include inclined, continuous self-cleaning types, and rotary drum disk types, which are cleaned by spraywater. Screens are made of stainless steel with mesh sizes ranging from 0.001 to 0.25 inches. Suspended solids removals are between 15 percent to 30 percent when the units are used as substitutes for conventional primary treatment. When used as an effluent screen to upgrade primary performance, 15 percent additional removal is typically achieved. Screens have the advantages of being inex-

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

pensive, more compact, and having low maintenance requirements (Marshall 1987, WEF/ASCE 1992).

Advances in Biological Treatment Processes

Advances in biological treatment processes have emphasized the importance of space, energy, and cost savings, as well as the need to incorporate further flexibility into these systems. Some of the significant advances include the development of biological aerated filters, sequencing batch reactors, high biomass systems, and biological nutrient removal systems. Further developments in the area of biotechnology are anticipated in the near future. These technologies are discussed here. Nutrient removal systems are addressed in the section on Municipal Wastewater Treatment Systems.

Biological Aerated Filters. Biological aerated filters were developed in France during the early 1980s and are in use today on a full-scale basis at about 100 facilities in Europe, Japan, and Canada. These systems employ shale, aluminum silicate, or expanded polystyrene to foster the growth of high concentrations of bacteria. Depending on the design, wastewater and air are introduced from the top or bottom of the media in a counter-current or co-current manner. The reactors can be used for BOD and/or ammonia removal. Biological aerated filters eliminate the need for final clarifiers and may be cost-competitive relative to other biological systems at low influent concentrations and loadings. This process is compact in areal dimensions, but has significant energy requirements and high operation and maintenance costs.

Sequencing Batch Reactors. Sequencing batch reactors are an elementary form of biological treatment in which the aeration, settling, and decant phases of each treatment cycle occur in a single reactor. The steps of operation include:

  • fill - the reactor is filled to a predetermined level (with or without aeration),

  • react - the introduction of air allows the aerobic degradation of carbon, ammonia, and other degradable compounds,

  • settle - solids are allowed to settle to the bottom of the reactor without mixing or aeration,

  • decant - clarified effluent is withdrawn from the reactor, and

  • idle - waste sludge is removed (as necessary).

Prior to 1984, there were four sequencing batch reactor facilities operating in the United States. Since then, over 150 new sequencing batch reactor

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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facilities have been built in this country, 80 percent of which have flows less than I MGD. The largest U.S. plant has a flow of 9.2 MGD flow and is located in Cleveland, Tennessee. The advantages of a sequencing batch system compared with a continuous flow system include the ability to absorb both hydraulic and organic shock loads, to hold wastewater until regulatory limits are met, and to remove phosphorus during the anoxic fill period. N-removal is possible through long react and idle periods. Sequencing batch reactor systems contain no secondary clarifiers or associated return sludge facilities (Heidman 1990). However, post-reactor treatment facilities either must be sized excessively or use flow equalization because of the intermittent, high-flow discharges.

High Biomass Systems. High biomass systems include various forms of inert media within aeration basins, which support the growth of fixed film organisms as a supplement to the suspended biological growth. This technology can lower the solids loading rate on subsequent clarifiers, thereby improving the treatment capacity within existing basins. The media can increase the solids retention time and lower the food-to-microorganism (F/ M) ratio, thus permitting or improving nitrification, reducing aeration tank volumes for new facilities, and reducing secondary clarifier surface area requirements. It can be incorporated in a new treatment facility and can also increase the capacity of existing activated sludge systems, thereby forestalling or avoiding new tank construction. Significant long-term, full-scale data on these systems are currently limited to facilities in Europe and Japan and are scant.

Natural Wastewater Treatment Systems

Natural wastewater treatment systems include various methods of wastewater application to land (e.g., crop irrigation, landscape irrigation, and groundwater recharge). They also include the use of constructed wetlands and floating aquatic plant systems. The majority of natural land-based wastewater treatment systems are preceded by a minimum of primary treatment. Slow-rate, rapid infiltration, and overland flow processes are the predominant municipal natural treatment systems in use today. Table D.5 summarizes effluent quality of these three types.

Constructed wetlands can be free water surface or subsurface flow systems, in which the treatment takes place as the water flows through the stems and roots of the wetlands vegetation. The former type may be designed with the goal of establishing new wildlife habitats or enhancing adjacent natural areas. The use of floating aquatic plants is a variant on the wetlands theme. Typical species used include water hyacinth and duckweed.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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TABLE D.5 Comparison of Average Effluent Quality of Three Natural Treatment Systems (Source: WPCF 1990, EPA 1992b)

in mg/l:

Slow-Rate

Rapid Infiltration

Overland Flow

TSS

< 2

2

10 - 30

BOD5

< 1

2- 10

15 - 30

Total Phosphorus

< 1

< 1- 5

4 - 6

Total Nitrogen

1 -8

10 - 20

5 - 15

Ammonia Nitrogen

< 0.5

< 0.5

> 4

Land-area requirements for natural wastewater treatment systems can be quite large relative to more conventional systems, and thus these systems are generally not feasible for use in urban areas. Under an integrated coastal management plan, natural wastewater treatment systems could be used as a complement to conventional in regions where the required land area is available. Table D.6 presents a range of the land-area requirements for all of the systems described here.

Solar Aquatic Systems. Aquaculture is the growth of fish and other aquatic organisms for food production. Wastewater has been applied to such systems around the world where the production of biomass remains the chief objective. In the United States, a new aquaculture/wastewater concept modification has been developed. Solar aquatic systems combine alternative energy use with the selective use of aquatic plants and animals for the purpose of obtaining a clean wastewater effluent. To date, five demonstration facilities have been built in New England, and several other facilities have been designed or built in other parts of the United States and in Swe

TABLE D.6 Land-Area Requirements for Land-Based Wastewater Systems (After Primary Treatment) (Source: Metcalf and Eddy 1991. Reprinted, by permission, from McGraw-Hill, Inc., 1991.)

 

Area Required (Acres/MGD)

Slow-Rate

50 - 550

Rapid Infiltration

5 - 60

Overland Flow

6 - 50

Natural or Constructed Wetlands

20 - 60

Floating Aquatic Plants

20 - 60

Conventional Activated Sludge

0.4

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

den. The technology is currently applied only to small flow facilities of less than 1 MGD because of cost and land requirement considerations.

The solar aquatics design makes use of a series of aerated tanks to accept the pretreated and flow-equalized effluent. After the first bank of tanks, the flow is passed to a constructed wetland, then on to another series of aerated tanks. As wastewater flow progresses through the system, phytoplankton, bacteria, and stress-tolerant plants give way to zooplankton and finally fish and other higher life forms. The animals, including the fish, snails, clams, and crayfish feed on sludge after microbial treatment. Water polishing and final nutrient removal is completed by flowers, shrubs, and trees. Retention times vary from 4 days for sewage up to 11 days for septage lagoon supernatant, each at constant flow. These times do not include required pretreatment systems.

Most of the treatment achieved in these systems is attributed to bacteria attached to floating aquatic plants, with little evidence that fish contribute directly to treatment. The health risks associated with the use of aquatic organisms requires further investigation. Controlled pilot-scale studies have reported removal efficiencies for TSS and BOD5 of over 98 percent and nitrogen from 85 to 95 percent, with 14 of 15 EPA volatile organic compounds found in the influent but none in the effluent and effluent fecal coliforms of less than 1,000 per 100 milliliters (Guterstam and Todd 1990, Teal and Peterson 1990).

Water Reclamation and Reuse as an Alternative to Wastewater Discharge

Wastewater reclamation and reuse has become increasingly important in water resources planning as the demand for water increases due to increased population, drought conditions, and decreasing supplies of high quality surface water and ground water. The use of reclaimed water allows municipalities to meet specific water needs while increasing their long-term water supply reliability. Coastal states, as shown in Figures D.3a and D.3b, are among the largest wastewater producers and also have high water demands. Therefore, significant water reuse projects have been implemented in water-short areas such as California, Florida, and Texas. Currently California recycles over 400,000 acre feet of water per year. This volume is expected to increase by 50 percent by the year 2010 (see Table D.7) (Arora 1992).

Wastewater reuse provides the option of putting constraints on discharge to surface waters and the marine environment. Recognition of environmentally sensitive conditions in coastal and estuarine waters, such as those of western Florida and northern California, has resulted in strict discharge limits or prohibition of wastewater discharge. For example, waste-

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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FIGURE D.3a Total treated wastewater design flows by state. (Source: EPA 1992b)

FIGURE D.3b Total fresh water demands by state. (Source: EPA 1992b)

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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TABLE D.7 Present and Projected Annual Use of Reclaimed Wastewater in California in 1,000s of Acre-Feet (Source: Arora 1992. Reprinted, by permission, from Water Environment Federation, 1992.)

 

Year

Increase 1980-2010

Hydrologic Area

1980

1990

2000

2010

 

North Coast

9

10

10

10

1

San Francisco Bay

10

11

13

15

5

Central Coast

9

25

27

27

18

Los Angeles

59

101

196

267

208

Santa Ana

29

47

73

78

49

San Diego

9

43

55

55

46

Sacramento Basin

21

22

23

25

4

San Joaquin Basin1

23

25

29

33

10

Tulare Lake Basin2

67

78

86

99

32

North Lahontan

6

6

7

8

2

South Lahontan

4

13

15

15

11

Colorado River Basin

4

202

332

452

412

Total

250

401

567

677

427

1 Does not include planned reclamation of agricultural drainage water.

2 Includes reclaimed agricultural return flows (normally lost to the Salton Sea) for power plant cooling.

water discharge into the Russian River in northern California is prohibited during the summer months.

Increasingly stricter discharge limits have made many communities, such as Vero Beach, Florida, implement water reuse. In anticipation of these stricter discharge limits, there is an increasing need for coastal communities to include water reclamation and conservation in local and regional planning. An integrated coastal management approach could facilitate environmental and economic priority-setting to address discharge and reuse issues.

Economic Feasibility of Water Reuse. Wastewater reclamation costs, including treatment and distribution range widely from $200 per acre foot to almost $1,500 per acre foot in total cost. Thus, to understand the costs of wastewater recycling correctly, it is important to be aware of the various assumptions and factors used in developing cost estimates. Costs associated with secondary treatment of municipal wastewater are normally considered as water pollution control costs and are not included in wastewater reclamation and reuse costs (Asano 1991).

To aid in determining cost-effectiveness of water reclamation and reuse programs, the EPA has formulated ''Cost Effectiveness Analysis Guidelines"

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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(40 CFR, Part 35). Care must be taken in accounting for factors such as environmental impacts and quality of life, which are not traditionally accounted for in cost-benefit analyses. In November 1991, the Florida Department of Environmental Regulation produced "Guidelines for Preparation of Reuse Feasibility Studies for Applicants Having Responsibility for Wastewater Management" in order to standardize economic evaluations (FDER 1991). These guidelines consider two alternatives 1) no action, and 2) the implementation of a water reuse system. The criteria set forth in the guidelines gives recycled water equal value to the potable water conserved through its use (EPA 1992b).

Cost effectiveness and economical feasibility of water recycling programs are difficult to evaluate due to the complex nature of the water supply issue. Unrealistically low potable water rates are a major obstacle in establishing the economic feasibility of water recycling projects. Increasing potable water prices will aid in making water recycling economically feasible.

Water Conservation

In the United States, water historically has been a cheap commodity, demand for it has generally been met, and there has been little incentive to curb its use. With the mounting environmental costs of developing new supplies and with shortages occurring in drought-prone and/or densely populated areas, water conservation is gaining increasing credibility. In Massachusetts, obstacles to the development of new supplies has led to rigorous leak detection programs, public education campaigns to encourage voluntary conservation, domestic device retrofitting efforts, and cooperative programs with industry. These combined efforts have led to the reduction of upstream water use at the Massachusetts Water Resources Authority by 17 percent over 5 years (M. Conner, Harbor Studies, Massachusetts Water Resources Authority, personal communication, 1992). In California, the drought of 1986-1992, the second longest of the century, has taxed an already burdened water-delivery system and led to drought survival efforts that include mandatory rationing, conservation programs for domestic and industrial water use, water waste restrictions, and excess water-use penalties. These efforts have resulted in decreases in wastewater flows of between 10 percent and 40 percent at various municipal wastewater treatment plants (Bruskin and Lindstrom 1992). Reduced wastewater flows can lower operating costs at existing facilities and lower costs, postpone, and reduce the capacity of future wastewater treatment facilities where inflow and filtration are minimized. In coastal cities where infiltration and inflow are not controlled, water conservation alone will not reduce flows enough to cause a reduction in facility needs.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×
Advances that Offer Flexibility and/or Special Capabilities

Membrane Filtration. Membranes made from cellulose acetate, polyamides, or combinations of polymers separate suspended, colloidal or dissolved particles from wastewater when the liquid flows through the membrane. The smaller the membrane pore, the smaller the rejected species and the more costly the membrane technology. Membrane processes can be used for removing fine particles, turbidity, trihalomethane (THM) precursors, specific organics or for disinfection. The five membrane processes with the greatest potential for wastewater applications are reverse osmosis, nanofiltration, ultrafiltration, microfiltration, and electrodialysis.

Magnetite Process. The magnetite process begins with a rapid-mixing step in which metal salts and/or a polymer are added to the wastewater to flocculate the solids. The floc is then seeded with magnetite (Fe304), a highly magnetic material. The mixture is passed over a magnet, on which the particles collect. The magnetite particles can be reused repeatedly by regeneration with alkali, which strips off the adsorbed contaminants and reactivates the surface. This concept has been studied on a limited scale in the United States under EPA support, and, without chemical addition, is currently being tested in Australia (D. Dallis, Sala, Inc., Wellesley, Massachusetts, personal communication 1990; WBNSWG 1991).

Natural Chemical Coagulants. Research is being conducted on the use of a variety of new natural substances such as seeds, carrageenan (a seaweed product from algae), vegetable gums, chitosan (shell extract), ashes,. starches, bark resins, and other biodegradable, renewable, and/or nonpetrochemical-based substances capable of removing constituents from wastewater, either selectively or comprehensively. Seeds from two moringa species native to the sub-Himalayan region of India have out-performed metal salts in municipal wastewater and water treatment tests (Folkard 1986). Chitosan has comparable performance characteristics to metal salts at dosages 5 to 10 times less than those typically used in chemically enhanced treatment (Murcott and Harleman 1992b). Insoluble starch xanthate, developed by the U.S. Department of Agriculture, adsorbs heavy metals from wastewater and has been found to be especially effective in removing aluminum, zinc, chromium, cadmium, mercury, nickel, copper, tin, silver, and gold (Hauck and Masoomian 1990). Recycled sludge from drinking water treatment processes has also been used as a wastewater coagulant.

Institutional Barriers to Innovation

The EPA Innovative and Alternative Technology Program, phased out in 1990, has been the primary federal effort to promote innovation in waste-

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

water treatment technology. The program was most successful in small communities but had little impact on large municipalities in coastal urban areas. Any future federal program to promote innovation should address existing barriers to steady progress in improving wastewater treatment technologies. Some of these barriers are

  1. Reluctance of design engineers to try new technologies when there is no incentive to do so;

  2. Reluctance of decisionmakers in local municipalities to spend public funds for technologies involving a substantial degree of technical risk;

  3. Reluctance of state agencies to approve the first installation of a new technology in their state;

  4. Delayed implementation: There is an unreasonable delay between the demonstration of new technology and its recognition in the regulatory process. This may be due to inertia in the process or an inability to adopt new technology or science in an incremental manner;

  5. Potential performance problems of new technologies experienced after implementation;

  6. Reputations of engineers and of state and local water officials may be damaged if a project fails;

  7. Conservative state design standards: Most states continue to be governed by the conservative "Ten State Standards," which may preclude optimal design;

  8. Lack of performance specifications which would permit more competition in design choices; and

  9. Many engineering firms are simply not familiar with new technologies.

Municipal Wastewater Treatment Systems

A large number of technically feasible wastewater treatment technologies are currently available. Ten representative systems, arranged roughly from the simplest to the most complex, have been selected to demonstrate the wide range of treatment capabilities and costs. The ten systems are all proven technologies in full-scale operation in the United States. The data presented below for these ten systems can be used to make comparative judgments regarding performance, to estimate the approximate costs of meeting various effluent discharge standards, and to compare the costs of point and nonpoint source treatment options.

The ten wastewater treatment systems are as follows:

  1. Primary (PRI)

  2. Chemically enhanced primary (CEPT)

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×
  1. low-dose chemically-enhanced primary (CEPT)

  2. high-dose chemically-enhanced primary (HD-CEPT)

  1. Conventional primary + biological treatment (BIO)

  2. Chemically-enhanced primary + biological treatment (CEPT-BIO)

  3. Primary or chemically enhanced primary + nutrient removal (NUTR)

  4. System 5 + gravity filtration (NUTR-FILT)

  5. System 5 + high lime + filtration (NUTR-HI-FILT)

  6. System 5 + granular activated carbon + filtration (NUTR-FILT-GAC)

  7. System 5 + high lime + filtration + granular activated carbon (NUTRHI-FILT-GAC)

  8. System 9 + reverse osmosis (NUTR-HI-FILT-GAC-RO).

Common elements for all treatment systems presented in this report include influent pumping, preliminary treatment (bar screens and grit removal), effluent disinfection (chlorination and dechlorination), effluent pumping, and a sludge processing system consisting of dissolved air flotation of biological sludges, and anaerobic digestion of combined primary and thickened biological sludges, followed by belt press dewatering.

Description of Ten Wastewater Treatment Systems

1. Primary Treatment

Primary treatment is a physical process that involves gravity separation of settleable and floatable solids from the influent wastewater stream. Removal of settleable solids takes out some associated pollutants, including organic matter, nutrients, heavy metals, toxic organics, and pathogens. Other physical separation processes, such as fine screens and filters can be included in this treatment step.

2. Chemically-Enhanced Primary Treatment

Chemically-enhanced primary treatment is a modification of the primary clarification process through the use of chemical coagulants, typically metal salts and/or organic polyelectrolytes. By varying the chemical dose, the performance of chemically-enhanced clarification systems can be adjusted to increase the removal of suspended solids, BOD, and/or total phosphorus. Chemically-enhanced primary treatment facilities can be divided into two categories:

a. Low-dose chemically-enhanced primary treatment is used mainly for increasing the removal of suspended solids, BOD, metals, and toxic organics. A low-dose chemically-enhanced primary treatment plant is defined as the addition of a metal salt or other primary coagulant in concentrations between 5 mg/l and 100 mg/l, with or without the application of a polymer, prior to primary clarification.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

b. High-dose chemically-enhanced primary treatment is used mainly to increase the removal of suspended solids, BOD, metals, and toxic organics in addition to the removal of phosphorus. The added metal salts react with soluble ortho-phosphate in the influent wastewater, producing a precipitate that is removed in the waste sludge. High-dose chemically-enhanced treatment is defined as the addition of a metal salt or other primary coagulant in concentrations greater than 100 mg/l, with or without the application of a polymer, prior to primary clarification.

Chemically-enhanced primary treatment provides opportunities for size reduction of follow-on treatment; if iron salts are used, the control of hydrogen sulfide, a major cause of odor problems; and in some cases, the potential for production of extra methane as a fuel source.

3. Conventional Primary + Biological Treatment

Conventional biological treatment systems, often classified as either suspended (e.g., activated sludge) or attached growth systems (e.g., trickling filters), use a diverse culture of microorganisms to break down organic matter in the wastewater, oxidizing a portion and converting the remainder into biological solids. Organic contaminants are removed by biodegradation and volatilization. Nondegradable suspended contaminants are removed by physical entrapment and subsequent removal with the generated biomass. Some soluble constituents (i.e., heavy metals) are removed by adsorption on the biomass. Some nutrient removal occurs through incorporation into the generated biomass. Biological treatment systems convert some influent organic nitrogen and urea to ammonia, thereby increasing the ammonia concentration making it more biologically available upon effluent discharge. Some biological systems are operated to convert ammonia to nitrate. Gas-liquid mass transfer is required to supply oxygen to the biological process, and this often results in stripping and gaseous discharge of volatile compounds. With disinfection, this effluent is sometimes used for irrigation of agricultural lands. In fact, effluent from all of the following treatment trains can be reclaimed for certain uses.

4. Chemically-Enhanced Primary + Biological Treatment

Chemically-enhanced primary + biological treatment involves the use of metal salts and polymers with either a conventional or innovative biological treatment system. Chemically-enhanced primary treatment has three major effects on a biological system-enhanced removal of phosphorus (HDCEPT), potential improved BOD removal by the biological system due to enhanced efficiency of the primary treatment stage, and increased sludge quantity. Raw sludge production is increased in the chemically-enhanced

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

primary stage and decreased in the biological stage such that the overall amount of raw sludge is greater by 10 percent to 20 percent compared with a conventional primary + biological treatment system. However, there is less than a 10 percent difference in the overall amount of digested sludge produced by the two systems due to the destruction of the volatile content in the chemically-enhanced primary stage (Chaudhary et al. 1991).

5. Nutrient Removal

Wastewater treatment systems can be configured to remove the nutrients nitrogen and/or phosphorus. Nitrogen removal is accomplished by an extension of the conventional biological system to incorporate the biochemical processes of nitrification and denitrification. Nitrification is the oxidation of ammonia and organic nitrogen to nitrate nitrogen. The process is mediated by the activity of a specialized class of autotrophic bacteria that can be grown in conventional activated sludge biological systems by extending the biological solids residence time resulting in more complete biodegradation of organic matter. Nitrogen removal is subsequently obtained by denitrification whereby the nitrate nitrogen is reduced to nitrogen gas and then released into the atmosphere.

Phosphorus removal can be accomplished by chemical or biological means. High-dose metal salts addition, as described in the section on chemically enhanced treatment, results in phosphorus removal. Alternatively, biological phosphorus removal can be accomplished through the selection of high phosphorus content microorganisms, resulting in a greater mass of phosphorus in the excess biological solids removed. Biological phosphorus removal systems are more capital cost-intensive and less operations and maintenance cost-intensive than chemical phosphorus removal systems and their efficiency can vary depending on a number of factors. Consequently, biological phosphorus removal systems typically incorporate some degree of chemical addition (usually for polishing) to ensure reliability and low phosphorus concentrations in the effluent.

6. Nutrient Removal with Gravity Filtration

This alternative includes a filtration system in addition to the nutrient removal system. This combination will remove additional quantities of TSS, along with other contaminants associated with the TSS (such as BOD5, nitrogen, phosphorus, and heavy metals). The capability to add chemicals to the effluent filters is also provided, allowing further removal of phosphorus and other pollutants. Sludge production is increased slightly with this alternative relative to the nutrient removal system alone due to the increased removal of pollutants. Effluent filtration will increase the removal of pathogenic organisms, metals, and toxics from the treated effluent and enhance the

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

performance of downstream disinfection processes. This system is used in some areas to produce water for use in urban irrigation.

7. Nutrient Removal with High Lime

This alternative follows a nutrient removal system with a high lime treatment system. High lime treatment involves the addition of lime (CaO) to elevate the pH of the treated effluent to over 11.0. This process provides considerable removal of phosphorus and heavy metals, removal of high molecular weight residual organics, and disinfection of the treated effluent. Solids produced as a result of lime addition are separated in a downstream clarifier and the pH of the clarifier effluent is adjusted back to neutral using either carbon dioxide or acid. Alternatively, two pH adjustment steps (referred to as two-stage recarbonation) can be used to remove excess calcium and magnesium from the treated effluent. In the first step, the pH of the clarified effluent is reduced to 9.3 to allow precipitation of calcium carbonate, which is removed in a downstream clarifier. The pH of the effluent from this second clarifier is subsequently reduced to 7.0. Because a high level of phosphorus removal will be obtained independent of influent phosphorus concentration, upstream removal of phosphorus by either biological or chemical means is not necessary.

Large quantities of chemical sludge are produced in the high lime treatment process. The chemical sludge can either be dewatered and land-filled or it can be processed to thermally regenerate lime, suitable for reuse in the same process. In either case, processing of the lime sludge is relatively difficult and expensive. For the purposes of this analysis, thermal regeneration of the lime is assumed.

8. Nutrient Removal and Granular Activated Carbon

This alternative adds the granular activated carbon (GAC) system to the NUTR-FILT system. GAC is a physical process for the removal of residual organic materials, including toxic organics, from a treated wastewater effluent. Some heavy metals may be removed as well. Treated effluent is applied to downflow packed beds containing granular activated carbon. Residual organic materials are removed as the treated effluent passes through the packed beds. GAC has a fixed capacity for removal of organics and, when this capacity is fully utilized, the beds must be removed from service and the exhausted GAC regenerated. Thermal regeneration is typically used and is assumed for this case.

9. Nutrient Removal with High Lime and Granular Activated Carbon

This alternative, the first of two potable reuse alternatives described, adds high lime and GAC to the nutrient removal system. This process with

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

disinfection has typically been applied to wastewater that is reclaimed for indirect potable reuse.

10. Nutrient Removal with High Lime, Granular Activated Carbon, and Reverse Osmosis

This alternative, the second of two potable reuse alternatives described, involves an additional treatment step, a reverse osmosis (RO) system, after System 9. The RO process is applied typically when dissolved solids removal is required to prevent salt build-up within a recycling system. RO involves application of highly treated effluent under high pressure to a membrane. It allows the water to flow through but is not permeable to (i.e., rejects) dissolved solids. Dissolved solids are concentrated as a brine in a reject stream. RO's best application is as a complementary process with GAC to accomplish complete removals of a broad spectrum of pollutants. Operating costs for the RO process are high due to the energy costs of maintaining high pressure, limited membrane life due to fouling, and the high cost of brine disposal. Owing to its high cost, RO has been applied primarily for water reuse applications in areas where water is scarce and expensive. In many instances, only a portion of the treated effluent is processed through RO.

Matrix of Performance and Cost Summary Tables

Data for the ten systems are summarized in the Matrix of Performance and Cost Summary Tables D.8a, D.8b, D.8c, and D.8d.2 Performance comparisons are made on the basis of conventional parameters.

Performance and Costs

The performance of each of the 10 wastewater treatment systems was assessed based on two surveys of over 100 U.S. POTWs undertaken in 1990

2  

These tables subdivide the ten systems into two categories. Systems 1-4 are those for which significant performance data exist, based on the two above-mentioned surveys. Average effluent concentration and average percentage removal values are based on these data. Systems 5-10 are ones for which fewer data exist. Average effluent concentration and average percentage removal values for these systems are based on the technical literature and professional judgment.

The assumptions used to standardize results for Systems 5-10 are as follows:

Plant size = 20 MGD

TSS influent concentration = 250 mg/l

BOD5 influent concentration = 250 mg/l

Total phosphorus influent concentration = 8 mg/l

Total nitrogen influent concentration = 35 mg/l

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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TABLE D.8a Average Influent/Effluent Concentrations and Percentage Removals for Systems 1-4

 

Primary

(1)

Low-Dose Chemical Primary

(2a)

High-Dose Chemical Primary

(2b)

Biological

(3)

Chemical Primary + Biological

(4)

TSS (mg/l)

214/93

182/52

177/13

234/14

186/10

BOD5 (mg/l)

202/139

168/80

146/33

203/16

174/9

TP (mg/l)

6/4

6/2

5/0.3

6/3

5/1

TN (mg/l)

30/23

30/19

30/—

28/19

16/—

NH4 (mg/l)

14/—

15/13

Oil & grease (mg/l)

41/20

42/12

36/6

50/<

TSS (%)

55

71

92

93

93

BOD5 (%)

30

55

78

92

95

TP (%)

38

63

93

38

87

TN (%)

15

37

40

31

31

Oil & grease (%)

51

71

82

98

NOTE: TSS = total suspended solids, BOD5 = 5-day biochemical oxygen demand, TP = total phosphorus, TN = total nitrogen, NH4 = ammonia nitrogen,—= data insufficient or unavailable.

and 19913 and also on the technical literature and the professional experience of members of the Committee on Wastewater Management in Coastal Urban Areas and its Panel on Source Control and Treatment Technologies.

Costs for the ten wastewater treatment systems were estimated by a professional engineering firm with confirmation provided by the two surveys. Many factors can affect the cost of a wastewater treatment system and, as a consequence, the cost of a specific wastewater treatment facility may vary significantly from the general costs presented in this section.

Costs are expressed as capital cost, operation and maintenance cost, and total cost. Capital cost is expressed in two sets of units: dollars per gallon per day of installed capacity, and dollars per mission gallons.4 Operation and maintenance costs, and total costs are expressed in dollars per million

3  

Information on the two nationwide surveys conducted to obtain data on these ten candidate systems and the screening criteria used to select them has been published as a separate document: Performance and Innovation in Wastewater Treatment—Technical Note #36, January, 1992, by Murcott, S., and Harleman, D., Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts.

4  

Total capital cost is calculated as follows:

Total capital cost = (annual cost in $/MG) x (20 MGD) x (365 days/year) x (uniform series present worth factor)

Uniform series present worth factor = [(1 + i)n - 1]/[i(1 + i)n], where i = 8% and n = 20 years.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.8b Costs for Systems 1-4

 

Primary

(1)

Low-Dose Chemical Primary

(2a)

High-Dose Chemical Primary

(2b)

Biological

(3)

Low-Dose Chemical Primary + Biological

(4)

Capital Cost ($/gpd)

0.9-1.1

1.1-1.4

1.2-1.8

2.4-2.6

2.6-2.9

Capital Cost ($/MG)

245-310

320-400

400

610-720

750-870

O & M Cost ($/MG)

205-240

230-280

250-350

320-410

350-450

Total Cost ($/MG)

450-550

550-680

650-750

930-1,130

1,050-1,150

gallons. All costs are annualized costs. Assumptions used in computing these costs include an 8 percent interest rate for a 20 MGD facility with a design period of 20 years. Land costs are not included. For systems 5 to 10, lime recalcination and other increased sludge production were internalized into the cost so that the additional sludge shows up as an increased cost rather than as increased sludge.

TABLE D.8c Average Influent/Effluent Concentrations and Percentage Removals for Systems 5-10

 

Nutrient Removal

(5)

Nutrient Removal + Filtration

(6)

Nutrient Removal + High Lime + Filtration

(7)

Nutrient Removal + Filtration + GAC

(8)

Nutrient Removal + High Lime + Filtration + GAC

(9)

Nutrient Removal + High Lime + Filtration + GAC + Reverse Osmosis

(10)

TSS (mg/l)

250/15

250/5

250/3

250/2

250/2

250/0

BOD5 (mg/l)

250/15

250/5

250/3

250/3

250/2

250/1

TP (mg/l)

8/1.5

8/1

8/0.1

8/0.5

8/0.1

8/<0.1

TN (mg/l)

35/3

35/2

35/2

35/1.5

35/1.5

35/<1

NH4 (mg/l)

—/0.5

—/0.5

—/0.4

—/0.5

—/0.4

—/<0.1

TSS (%)

94

98

99

99

99

100

BOD5 (%)

94

98

99

99

99

100

TP (%)

81

88

99

94

99

100

TN (%)

91

94

94

96

96

97

NOTE: TSS = total suspended solids, BOD5 = 5-day biochemical oxygen demand, TP = total phosphorus, TN = total nitrogen, NH4 = ammonia nitrogen, - = data insufficient or unavailable.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.8d Costs for Systems 5-10

 

Nutrient Removal (5)

Nutrient Removal + Filtration (6)

Nutrient Removal + High Lime + Filtration (7)

Nutrient Removal + Filtration + GAC (8)

Nutrient Removal + High Lime + Filtration + GAC (9)

Nutrient Removal + High Lime + Filtration + GAC + Reverse Osmosis (10)

Capital Cost ($/gpd)

2.9-3.3

3.5-3.9

5.2-5.6

4.5-4. 9

6.1- 6.7

8.5-9.5

Capital Cost ($/MG)

750-870

890-1,140

1,300-1,700

1,150-1,450

1,500-1,800

7,000-2,500

O & M Cost ($/MG)

500-580

560-660

1,100-1,300

850-950

1,350-1,650

2,500-3,000

Total Cost ($/MG)

1,250-1,450

1,450-1,800

2,400-3,000

2,000-2,400

2,900-3,500

4,500-5,500

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×
Qualitative Comparisons

Ten Systems. The data presented in the Matrix of Performance and Cost Summary Tables have been used to develop cost-performance relationships for the removal of various pollutants. Figures D.4 and D.5 plot the data, providing TSS, BOD, P, and N performance-cost relationships for all the systems. The figures show ranges both of performance and cost. The numbering of the ten systems on these figures follows the same order as given above.

Several observations are evident from an examination of these four figures:

  • Unit total treatment costs increase exponentially as a higher level of treatment is provided. In general, the point at which treatment costs begin to increase quite rapidly and produce only small improvements in improved effluent concentrations occurs at a unit total treatment cost on the order of $800 to $1,200 per million gallons treated.

  • Smooth curves are indicated for TSS and BOD5 performance-cost plots (Figures D.4a and D.4b). However, smooth curves are not obtained when nutrient control (total nitrogen and/or total phosphorus) is taken into consideration (Figures D.5a and D.5b). Here, the cost for conventional primary + biological treatment (System 3) is significantly higher than the cost for either primary (System 1) or low-dose chemically-enhanced pri-

FIGURE D.4a Total suspended solids performance and cost relationship.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

mary (Systems 2a and 2b), even though there is no advantage in terms of total phosphorus effluent concentration. In other words, conventional primary + biological treatment is not a particularly cost-effective strategy for phosphorus removal if phosphorus removal is the only treatment objective. The same can be said for conventional primary + biological treatment with regard to total nitrogen removal.

  • The more advanced technologies such as HI-LIME, GAC, and RO significantly increase costs while producing little additional removal of TSS, BOD5, or nutrients. These technologies generally lie on the other side of the cost and performance curve for these parameters. Their major purpose is for the removal of specific toxic organic compounds, heavy metals, and other unconventional pollutants, which must be controlled for specific reuse or discharge applications.

Comparison of Low-Dose Chemically-Enhanced Primary with Conventional Primary + Biological Treatment. Data were collected on 34 primary and 19 low-dose chemically-enhanced primary treatment plants through the two above-mentioned national surveys. These data are used to compare low-dose chemically-enhanced primary treatment performance with conventional primary and conventional primary plus biological treatment. In most

FIGURE D.4b Five-day biochemical oxygen demand (BOD5) performance and cost relationship.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

FIGURE D.5a Total phosphorus performance and cost relationship.

TABLE D.9 Comparison of Low-Dose Chemically-Enhanced Primary Treatment with Conventional Primary Treatment Plus Activated Sludge

Constituent

Low-Dose Chemically Enhanced Primary

Conventional Primary + Biological

TSS Effluent (mg/l)

52

14

BOD5 Effluent (mg/l)

80

16

Total Phosphorus Effluent (mg/l)

2

3

Total Nitrogen Effluent (mg/l)

19

19

Oil & Grease Effluent (mg/l)

6 - 12

0 - 1

Raw Sludge (lb solids/lb TSS removed)

1.3 - 1.5

1.8

Cadmium Effluent (glg/l)

6 - 20

6 - 20

Copper Effluent (lag/l)

130 - 220

90 - 150

Chromium Effluent (ag/l)

150 - 300

140 - 280

PAH1 Effluent (gg/l)

1 - 6

1 - 6

Pathogens (% removed)

< 99%

< 99%

Total Energy Use (kWh/yr x 10-3)

290

700

Capital Cost ($/gpd)

1.1 - 1.4

2.4 - 2.6

O & M Cost ($/MG)

230 - 280

320 - 410

Total Cost ($)

550 - 680

930 - 1130

1 PAH = polycyclic aromatic hydrocarbons.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

FIGURE D.5b Total nitrogen performance and cost relationship.5

cases, significant removals of organic materials and metals are achieved only by the most complex treatment trains.

Table D.9 summarizes performance characteristics for two systems, low-dose chemically-enhanced primary (System 2a) and conventional primary treatment + biological treatment (System 3), for the purpose of showing a side-by-side comparison of their respective efficiencies. This comparison shows that, on average, chemically-enhanced primary treatment is comparable to conventional primary + biological treatment in phosphorus, nitrogen, cadmium, and PAH removal, and there is little difference in their removals of copper and chromium. Conventional primary + biological treatment removes TSS, BODS, and oil and grease to lower levels. Low-dose chemically-enhanced treatment produces roughly 20 percent less sludge. Neither treatment method without add-on disinfection processes adequately removes pathogenic organisms. Conventional primary + biological treatment uses approximately 2.5 times as much total energy. Total cost of chemically-enhanced primary treatment is approximately half that of conventional primary + biological treatment. This is the average capability of these two systems based on full-scale yearly operations data in the United States in 1989 and 1990.

5  

Data were not available for Systems 2b and 4.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

Comparison of Conventional Primary Treatment and Low-Dose Chemically-Enhanced Primary Treatment. Figures D.6a and D.6b show graphs of average TSS and BOD5 effluent concentration versus overflow rate at full-scale POTWs in the United States. Overflow rate, a measure of treatment process efficiency, is the wastewater flow divided by the surface area of the treatment tank. Figure D.6a compares TSS effluent concentration versus overflow rate for primary with low-dose chemically-enhanced primary. The average primary TSS effluent concentration is 93 mg/l with a standard deviation of 34. The chemically-enhanced primary TSS effluent concentration averages 52 mg/l with a standard deviation of 21 mg/l.

Figures D.7a and D.7b are similar plots comparing BOD5 effluent concentration versus overflow rate for primary and low-dose chemically-enhanced primary treatment. The average primary BOD5 effluent concentration is 139 mg/l with a standard deviation of 50. The average low-dose chemically-enhanced primary BOD5 effluent concentration is 80 mg/l with a standard deviation of 36 mg/l.

These figures show the advantages of low-dose chemically-enhanced primary over conventional primary treatment. They also show no obvious decrease in removal efficiency with increasing overflow rates for plants with overflow rates up to 2,400 gpd/sf (except for primary effluent BOD5). These data suggest that design overflow rates may be unnecessarily conservative for primary and low-dose chemically-enhanced primary treatment, but more controlled studies are necessary to verify this suggestion.

Seven-day average data on the performance of low-dose chemically-enhanced primary treatment come from two plants: Point Loma Treatment Plant, San Diego, California and Hyperion Wastewater Treatment Plant, Los Angeles, California. These two southern California low-dose chemically-enhanced primary facilities remove approximately 80 percent TSS and 55 percent BOD5 at overflow rates between 1,600 and 2,100 gpd/sf. Comparing the performance at these well-operated plants with the average performance treatment as shown in the Matrix of Performance and Cost Summary Tables, a 48 percent increase in TSS performance and an 77 percent increase in BOD5 performance over conventional primary treatment is observed.

Toxic Organics and Metals

The ten representative wastewater treatment systems provide different capabilities in terms of their ability to remove toxic organics and heavy metals. Table D.10 lists a number of toxic organics and heavy metals and

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

FIGURE D.6a TSS removal efficiency for average primary and chemically-enhanced primary treatment.

FIGURE D.6b BOD5 removal efficiency for average primary and chemically-enhanced primary treatment.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

FIGURE D.7a State-of-the-art chemically-enhanced primary TSS removal efficiency at San Diego (Point Loma) and Los Angeles (Hyperion) POTWs.

FIGURE D.7b State-of-the-art chemically-enhanced primary BOD5 removal efficiency at San Diego (Point Loma) and Los Angeles (Hyperion) POTWs.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

shows a range of expected effluent concentrations attainable by each of the ten treatment systems.6

Fats, Oil, and Grease

The wastewater parameter known as fats, oil, and grease (FOG) refers to a number of compounds. The term grease generally includes fats, oils, and waxes, while fats and oils are compounds (esters) of alcohol or glycerol (glycerin) with fatty acids. In domestic wastewater, fats and oils typically come from butter, margarine, lard, and cooking oil. However, oils are not only of animal or vegetable origin but also come from petroleum products. FOG can cause problems in wastewater plant operations, both in the sewers and in the plant itself. It can contribute to the loss of hydraulic capacity, clogging of screens, poor grit separation, poor settling in sedimentation tanks, and interference with biological processes. It also causes aesthetic and toxicity problems if discharged into receiving waters (Metcalf and Eddy 1991).

The parameter measured in U.S. wastewater treatment plants is termed oil and grease. Data on oil and grease removal efficiency for Systems 1-3, based on the two above-mentioned POTW surveys is presented in Table D.1. The table shows an increasing improvement in removal of FOG with the increasing level of treatment. This trend is consistent with most literature reports.

6  

Procedure for determining the treatment performance levels:

  1. Begin with extensive data base for activated sludge (ASs effluent to determine the range for typical AS effluents (column 3).

  2. Review metals data base (Esmund et al. 1980) and toxic organics data base (McCarty et al. 1980) to determine typical influent concentration levels and AS treatment efficiencies. Using a consistent AS treatment efficiency applied to column 3, determine range for typical influent concentration.

  3. For column 1 and 2, apply treatment efficiency factors from Hannah et al. (1986). Infer from filtration data from Esmund et al. (1980) when not available in Hannah et al. (1986).

  4. For columns 4 and 5, apply a chemical treatment efficiency factor: Esmund et al. (1980) for metals, McCarty et al. (1980) for toxic organics, to column 3 and 4.

  5. For column 6, apply a filtration efficiency factor: Esmund et al. (1980) for metals, McCarty et al. (1980) for toxic organics, to column 5.

  6. For column 7, apply a high lime and filtration factor: Esmund et al. (1980) for metals, McCarty et al. (1980) for toxic organics, to column 4.

  7. For column 8, apply a GAC efficiency factor: Esmund et al. (1980) and McCarty et al. (1980), to column 7.

  8. For column 9, apply a GAC factor to column 8.

  9. Column 10 was obtained from McCarty et al. (1980). Inferences were made when needed.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.10 Typical Effluent Concentrations of Organics and Metals for Selected Treatment Trains

 

Treatment Train Effluent Concentration (micrograms/liter)

Constituent

Influent

1

2

3

4

Chloroform

7-60

7-60

5.6-48

1.0-9.0

1.0-9.0

Bromodichloromethane

0.3-1.7

0.3-1.7

0.3-1.7

0.1-.05

0.1-0.5

Dibromochloromethane

1.0-6.0

1.0-6.0

1.0-6.0

0.1-0.7

0.1-0.7

Bromoform

0.3-1.2

0.2-1.0

0.2-1.0

0.1-0.4

0.1-0.4

Carbon Tetrachloride

1.0-8.0

1.0-8.0

1.0-8.0

0.2-2.0

0.2-2.0

1,2-Dichloroethane

5.0-15.0

5.0-15.0

3.9-11.7

0.8-2.4

0.8-2.4

1, , I -Trichloroethane

7.5-12.5

7.5-12.5

7.5-12.5

3.0-5.0

3.0-5.0

Tetrachloroethylene

1.0-4.0

1.0-4.0

1.0-4.0

0.5-2.0

0.5-2.0

Trichlorothylene

1.0-2.0

1.0-2.0

1.0-2.0

0.5-1.0

0.5-1.0

Xylene

0.06-0.2

0.06-0.2

0.06-0.2

0.03-0.1

0.03-0.1

Chlorobenzene

1.0-25.0

0.8-20.0

0.7-18.0

0.1-2.5

0.1-2.5

1,2-Dichlorobenzene

1.0-8.0

0.8-6.4

0.7-5.6

0.1-0.8

0.1-0.8

1,3-Dichlorobenzene

1.0-8.0

0.8-6.4

0.7-5.6

0.1-0.8

0.1-0.8

1,4-Dichlorobenzene

15.0-25.0

12.0-20.0

10.0-17.5

1.5-2.5

1.5-2.5

1,2,4-Trichlorobenzene

1.0-5.0

0.8-4.0

0.7-3.5

0.1-05

0.1-0.5

Ethylbenzene

0.4-15.0

0.3-13.0

0.3-9.0

0.04-1.5

0.04-1.5

Naphthalene

1.0-20.0

0.2-17.4

0.2-15.4

0.03-0.6

0.03-0.6

1-Methylnaphthalene

0.33-30.0

0.29-26.1

0.25-23.1

0.01-0.9

0.01-0.9

2-Methylnaphthalene

033-30.0

0.29-26.1

0.25-23.1

0.01-0.9

0.01-0.9

Dimethylphthalate

33-106

21-67

5.0-16.0

5.0-16.0

3.2-10.4

Diisobutylphthalate

20-33

12-21

3.0-5.0

3.0-5.0

1.9-3.2

Bis-[2-ethylhexyl] phthalate

66-200

41-126

10.0-30.0

10.0-30.0

6.5-19.5

PCBs

5.0-33

3.1-20.7

0.55-3.6

0.5-3.3

0.3-2.6

Arsenic

9-22

9-22

9-22

8-20

5.6-14.0

Barium

120-160

120-160

120-160

60-80

60-80

Boron

300-500

300-500

300-500

300-500

300-500

Cadmium

6.6-22.2

5.8-19.5

5.8-19.5

3.0-10.0

2.2-7.3

Chromium

160-320

149-297

137-275

40-80

12-24

Copper

167-267

134-214

94-150

50-30

31-50

Iron

600-1600

600-1600

300-800

300-800

150-400

Lead

100-150

70-105

50-80

40-60

32-48

Manganese

41-81

37-73

33-65

30-60

21-42

Mercury

0.25-2.5

0.2-2.0

0.2-2.0

0.1-1.0

0.08-0.8

Nickel

93-147

88-140

79-126

70-110

60-95

Selenium

4.2-15.0

3.8-13.5

3.8-13.5

1.0-3.5

0.9-3.1

Silver

0.4-6.7

0.4-6.7

0.4-6.7

0.2-3.0

0.2-3.0

Zinc

250-400

225-360

225-360

100-160

70-112

NOTE: Influent values attempt to be representative of concentrations entering POTWs. However, values can be quite variable depending on the nature of the service area. MDL = minimum detection level

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

5

6

7

8

9

10

1.0-9.0

1.0-9.0

1.0-9.0

1.0-9.0

1.0-9.0

0.1-1.0

0.1-0.5

0.1-0.5

0.1-0.5

0.04-0.2

0.04-0.2

0.02-0.1

0.1-0.7

0.1-0.7

0.1-0.7

0.03-0.2

0.03-0.2

0.01-0.08

0.1-0.4

0.1-0.4

0.1-0.4

0.02-0.08

0.02-0.08

0.01-0.03

0.2-2.0

0.2-2.0

0.2-2.0

0.1-1.6

0.1-1.6

0.01-0.16

0.8-2.4

0.8-2.4

0.8-2.4

0.2-0.6

0.2-0.6

0.02-0.06

3.0-5.0

3.0-5.0

3.0-5.0

0.1-1.2

0.1-1.2

0.01-0.1

0.5-20

0.5-2.0

0.5-2.0

0.05-0.2

0.05-0.2

0.05-0.2

0.5-1.0

0.5-1.0

0.5-1.0

0.35-0.7

0.35-0.7

0.35-0.7

0.03-0.1

0.03-0.1

0.03-0.1

0.01-0.03

0.01-0.03

0.01-0.03

0.1-2.5

0.1-2.5

0.1-2.5

0.01-0.02

0.01-0.02

0.01-0.02

0.1-0.8

0.1-0.8

0.07-0.6

0.03-0.3

0.03-03

0.02-0.2

0.1-0.8

0.1-0.8

0.05-0.4

0.05-0.4

0.02-0.2

0.01-0.1

1.5-2.5

1.5-2.5

0.9-1.5

0.4-0.7

0.4-0.7

0.3-0.6

0.1-0.5

0.1-0.5

0.03-0.15

0.01-0.05

0.01-0.05

0.01-0.05

0.04-1.5

0.04-1.5

0.04-1.5

0.03-1.1

0.03-1.1

0.03-1.1

0.03-0.6

0.03-0.6

0.02-0.5

0.01-0.02

0.01-0.02

0.01-0.02

0.01-0.9

0.01-0.9

0.01-0.9

0.01-0.9

0.01-0.9

0.004-0.36

0.01-0.9

0.01-0.9

0.01-0.9

0.01-0.9

0.01-0.9

0.004-0.36

3.2-10.4

3.2-10.4

3.2-10.4

1.1-3.7

1.1-3.7

0.46-1.5

1.9-3.2

1.9-3.2

1.9-3.2

0.24-0.41

0.24-0.41

0.17-0.29

6.5-19.5

6.5-19.5

6.5-19.5

5.9-17.7

5.9-17.7

2.2-6.5

0.3-2.6

0.3-2.6

0.3-2.6

0.1-0.3

0.1-0.3

0.1-03

5.6-14.0

5.0-12.6

1.4-3.6

5.0-12.6

1.4-3.6

<MDL

60-80

60-80

60-80

60-80

60-80

2.0-5.0

300-500

300-500

300-500

300-500

300-500

100-300

2.2-73

2.2-7.3

1.4-4.7

2.1-6.9

1.3-4.5

0.7-2.0

12-24

9-18

8-16

5.4-10.8

4.8-9.6

0.2-2.0

31-50

31-50

15-24

15-25

7.0-12.0

1.0-10.0

150-400

120-320

30-80

84-224

21-56

20-30

32-48

27-41

18-27

16-25

11-16

1.0-3.0

21-42

17-34

5.6-11.2

13.6-27.2

5.0-10.0

1.0-4.0

0.08-0.8

0.08-0.8

0.07-0.7

0.06-0.6

0.05-0.5

<MDL

60-95

60-95

49-77

50-79

41-64

4.0-10.0

0.9-3.1

0.7-2.6

0.6-2.1

0.35-1.3

0.3-1.1

<MDL

0.2-3.0

0.2-3.0

0.12-1.8

0.2-3.0

0.12-1.8

0.1-1.2

70-112

70-112

40-64

45-73

34-54

5.0-30.0

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.11 Oil and Grease Influent and Effluent Concentrations and Percent Removal for Systems 1-3 (Source: Murcott and Harleman 1992a)

 

Primary (I)

Low-Dose Chemical Primary (2a)

High-Dose Chemical Primary (2b)

Biological (3)

Oil & grease inf/eff (mg/l)

41/20

42/12

36/6

50/1

Oil & grease (% removal)

51

71

82

98

Sludge
Sludge Quantity

The quantity of sludge produced depends mainly on the amount of TSS and BOD5 removed. As performance efficiency in terms of their removal increases, sludge quantity generally increases. This relationship is reflected in Table D.12 which gives sludge production values for Systems 1 to 10.

Sludge Treatment Costs

The cost of sludge treatment is calculated as the cost following sludge stabilization and dewatering.7 In this analysis, four alternative sludge management options were considered: 1) land disposal in a dedicated landfill or a refuse landfill, 2) composting with the give-away of the finished product, 3) incineration with the disposal of ash to landfill, and 4) direct land application. In all cases, it is assumed that sludges of 20 percent and 30 percent solids are produced by dewatering the sludge from the treatment plant, thus giving a band of costs. The incineration and composting operations are assumed to be at the plant site, while the ultimate land-disposal facility is assumed to be 100 miles from the treatment plant. This distance is not excessive for large cities. Transport costs are considered for all alternatives. In addition, anaerobic digestion costs were not factored into the

7  

The costs are mainly derived from the Handbook for Estimating Sludge Management Costs (EPA 1985a), which contains cost curves for the various sludge treatment and disposal processes, all in 1983 dollars. Conversion from 1983 dollars to 1991 dollars is by using the Engineering News Record Construction Costs Index. This index was 4,006 in 1983 and is presently at 4,818. Therefore all of the 1983 costs are converted to 1991 dollars by multiplying by 1.2. Where the cost curves did not cover the full range of sludge production for larger plants, the curves were extrapolated. The costs thus derived were checked against the results of the National Sewage Sludge Survey (EPA 1989a) and found to be in agreement.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.12 Sludge Production in Systems 1 to 101 (Source: Murcott and Harleman 1992a)

Sludge Production for Systems 1-4

Primary (1)

Low-Dose Chemical Primary1 (2a)

High-Dose Chemical Primary1 (2b)

Biological (3)

Chemical Primary + Biological (4)

1 lb solids/lb TSS removed

1.3 - 1.5 lb solids/lb TSS removed

1.3 - 2 lb solids/lb TSS removed

1 lb solids/lb BOD5 removed (bio sldg only) or 1.8 lb solids/lb TSS rem (conv pri+bio sldg)

1.2 lb solids/lb BOD removed (bio sldg only) or 2.2 lb solids/lb TSS rem (chem pri+bio sldg)

Sludge for Systems 5-10 (expressed as lb/million gallons)

Nutrient Removal (5)

Nutrient Removal + Filtration (6)

Nutrient Removal + High Lime + Filtration (7)

Nutrient Removal + Filtration + GAC (8)

Nutrient Removal + High Lime + Filtration + GAC (9)

Nutrient Removal + High Lime + Filtration + GAC + Reverse Os. (10)

2500

2750

2750

2750

2750

2750

1 All chemical primary sludge data in this table has been computed according to the formulas described in Murcott and Harleman (1992a).

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

incineration alternative. Further assumptions are listed under each alternative.

All of the sludge management options listed include final disposal. In the case of landfilling, the sludge is deposited in a dedicated landfill and covered with soil or deposited in a municipal solid waste landfill. In the latter case, the sludge assists in the anaerobic decomposition of the solid waste and may be considered a beneficial addition to the landfill. Composting is an aerobic decomposition process, usually carried out in either open piles or windrows or within closed vessels. In the case of sludge composting, a bulking agent is required to make it possible for oxygen to penetrate the compost piles. Typical bulking agents are sawdust, leaves, bark, or shredded paper. The product of sludge composting is typically an excellent soil additive or conditioner, and a useful fertilizer. Incineration may be considered the ultimate oxidation process, resulting in a mostly inorganic ash, which also must be disposed of, usually in a dedicated landfill. Finally, direct land application of sludge involves the addition of sludge to either agricultural or nonagricultural land such that the sludge is assimilated into the soil.

Land Disposal

If sludge at either 20 percent or 30 percent solids is transported to a dedicated landfill, the cost includes the land cost. This is assumed to be not very different from the disposal of sludge in a refuse landfill, where the cost of sludge disposal would be prorated. Liquid sludge disposal is not considered because of the high cost of truck haul for large treatment plants and the inability of most to pass the paint filter test required for such sludges to be permitted at a landfill.

The transport and land disposal calculated for this alternative would be the minimum sludge-handling cost for any treatment plant. The cost is calculated as the sum of the land disposal and transportation costs, as shown in Table D.13.

Composting

It is assumed that composting is by the static aerated pile method with wood chips or similar materials used as the bulking agent. No credit is given for the sale of the compost, which is often the case. Exceptions usually yield less than $10 per ton. The published curves did not extend to the required plant size and were extrapolated. Costs are calculated for both 20 percent and 30 percent feed sludge. Transportation figures are for dewatered sludge, assuming a 60 percent solids concentration of the finished compost. The cost results are shown in Table D.14.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.13 Cost of Land Disposal

Dry Tons of Sludge Disposed Per Day

Annual Cost of Land Disposal ($ million)

Annual Cost of Transport ($ million)

Total Annual Cost ($ million)

 

Dewatered sludge of 20 percent solids

 

20

0.7

1.8

2.5

60

1.5

4.0

6.4

120

3.0

9.1

12.1

180

4.2

13.3

17.5

 

Dewatered sludge of 30 percent solids

 

30

0.7

1.8

2.5

90

1.5

4.0

6.4

180

3.0

9.1

12.1

270

4.2

13.3

17.5

TABLE D.14 Cost of Composting

Dry Tons of Sludge Composted & Disposed of Per Day

Annual Cost of Composting ($ million)

Annual Cost of Transport ($ million)

Total Annual Cost ($ million)

 

Dewatered sludge feed of 20 percent solids

 

3

1.2

0.1

1.3

9

2.7

0.3

3.0

18

4.3

0.6

4.9

30

6.5

0.9

7.4

60

20.1

1.8

21.9

 

Dewatered sludge feed of 30 percent solids

 

3

1.1

0.1

1.2

9

2.3

0.3

2.6

18

3.7

0.6

4.3

30

5.4

0.9

6.3

60

14.6

1.8

16.4

NOTE: If the compost cannot be given away or sold, it must be disposed of on land. Compost may be used in landfill disposal as the daily cover requirement.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×
Incineration

A fluidized bed incinerator is assumed. Feed solids are assumed as both 20 percent and 30 percent suspended solids with 70 percent volatile. The process operates for 24 hours per day, 360 days per year. A reduction of 70 percent by weight is assumed and the ash is destined for a land disposal facility 100 miles from the treatment plant. The land disposal costs and transport costs are assumed to be the same as for the land disposal, reduced by 70 percent.

The incineration cost curves in the Handbook for Estimating Sludge Management Costs (EPA 1985a) are limited to sludge quantities less than 30 dry tons per day, and the higher values are therefore extrapolated.

If chemically-enhanced primary treatment is used in a wastewater treatment facility, the quantity of sludge is increased relative to primary treatment and the unit heating value of the sludge incinerated is decreased. It is assumed here that the heating value for sludge produced in chemically-enhanced treatment plants is 25 percent less than in conventional primary + biological plants and that the amount of auxiliary fuel needed to run the incinerator is increased by 25 percent. The cost of fuel oil is assumed at $1.50 per gallon.

Since incineration of sludge precludes the necessity for digestion, credit is given for the savings in digestion costs. The results of the calculations are shown in Table D.15.

TABLE D.15 Cost of Incineration

Dry Tons of Sludge Incinerated Per Day

Annual Cost of Incineration (incl. Transport & Disposal) ($ million)

Total Annual Cost of Incineration + Credit Given for Digestion ($ million)

 

Dewatered sludge of 20 percent solids

 

9

3.7

2.1

18

5.8

3.6

30

7.8

5.3

90

22.7

19.0

270

67.2

59.9

 

Dewatered sludge of 30 percent solids

 

9

2.8

1.2

18

4.7

2.5

30

6.1

3.7

90

21.9

18.2

270

62.2

54.9

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.16 Cost of Incineration of Chemically-Enhanced Primary Sludge

Dry Tons of Sludge Incinerated Per Day

Total Annual Cost1 of Incineration for a Dewatered Sludge Feed of 20% Solids ($ million)

Total Annual Cost1 of Incineration for a Dewatered Sludge Feed of 30% Solids ($ million)

9

2.3

1.4

18

3.9

2.8

30

5.8

4.1

90

20.3

19.6

270

6.4

59

1 Total annual cost includes transport and disposal, with credit given for digestion and added cost of fuel for low BTU sludge.

If chemically-enhanced wastewater treatment is employed, the costs are as shown in Table D.16. Note that the quantity of sludge to be incinerated is greater for chemically-enhanced primary treatment plants than for conventional primary treatment.

Direct Land Application

In direct land application, treated sludge is hauled to a site in a liquid or dewatered state and injected into, or spread and incorporated into the soil. The nutrient-rich organic matter in the sludge provides a food source for microbiological organisms and earthworms, which provide nutrients for crop uptake and benefit soil structure. The beneficial reuse of treated sewage sludge increases crop production and reduces the potential pollution from the use of chemical fertilizers. The organic material in the sludge also increases the soil's ability to store water.

The costs of managing sludge by direct land application typically range from $100 to $150 per dry ton. In some urban areas on the east coast, direct land application costs can be as high as $750 per dry ton. The costs include either the purchase or contracting of dedicated farm land and the cost of transportation to the sites. Costs of transportation for land application are a function of the distance between the farm land and the treatment facility. Cost estimates for transportation range from a low of approximately $.50 per dry ton per mile to $2.50 per dry ton per mile. Application costs vary greatly, but generally are less than $50 per dry ton. These transportation and land application rates do not include the potential for crop and backhaul rebates.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×
Energy Use in Municipal Wastewater Treatment

Although the efficiency of energy recycle and recovery in municipal sewage treatment and disposal has improved since the passage of the Clean Water Act of 1972, U.S. POTWs remain net consumers of energy. U.S. POTWs consumed an estimated 257 x 1012 Btu/yr of energy in 1990, which represents 0.32 percent of total national energy use or approximately 4 percent of total electricity use (Jones 1991). This increase represents a doubling of energy used by POTWs since 1972.

Wastewater facilities typically account for 15 percent or more of a municipal energy budget (Jones 1991). The amount of energy consumed at POTWs and the inefficient use of that energy means that there are significant opportunities for energy conservation and demand-side management. Minimizing pumping and improving the delivery of process air are examples of where significant energy savings can be obtained.

Energy in POTWs can be described by the terms primary energy and secondary energy. Primary energy is the energy employed in the operation of a facility, such as electricity used in various processes and in space heating. Secondary energy can be defined as the energy needed in the manufacture of materials to construct a POTW facility; the construction of the facility itself; and energy associated with chemical use, labor, and transportation. The main energy sources for POTWs are electric power, natural gas or propane, and diesel fuel or gasoline.

Approximately 25 percent to 40 percent of the annual costs of running U.S. POTWs are primary energy costs, with the operation of the facilities accounting for the major share of the energy consumption. The energy use associated with operating treatment plants is a function of level of treatment, plant size, location, and pumping needs. Pumping can be a substantial energy-consuming process, especially in the headworks and outfall. The costs associated with energy use are financed by user charges. Table D.17 gives the energy requirements for Systems 1-4.

Energy consumption can be reduced by any of several means, including reduction of energy demand by process optimization and energy efficient installations, innovation in design and operation of treatment systems, and improvements in energy recovery from digester gas or incineration. Recovery and use of digester gas as a fuel source for space heating, steam generation, electricity, incineration, and other internal needs is the major form of energy reclamation in sewage treatment. Sludge volatile solids serve as an energy source in incineration and in anaerobic digestion.

DISINFECTION

Sources of pathogens include discharges from sewage treatment plants, combined sewer overflows (CSOs), and stormwater runoff. Water fowl and

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.17 Energy Requirements for Treatment Systems 1-4 for a 1 MGD Plant, Energy Usage (kWh/yr x 10-3) (After Tchobanoglous and Schroeder 1985. Tchobanoglous/Schroeder, Water Quality © 1985 by Addison Wesley Publishing Company, Inc. Reprinted with permission of the publisher.)

 

Primary Energy

Secondary Energy

 

 

Electricity

Fuel

Plant Construction

Chemicals

Parts & Supplies

Total

Primary

50

90

44

8

192

Low-Dose Chemical Primary1

58

95

54

70

15

292

Biological

 

 

 

 

 

 

—Activated Sludge

237

130

209

180

156

165

60

60

34

30

696

565

—Trickling Filter

 

 

 

 

 

 

Chemical Primary + Biological2

177

200

132

100

32

641

1 Low-Dose Chemical Primary includes increased energy requirements due to mixing, flocculation, additional sludge collection, and transport.

2 Chemical Primary + Biological assumes a 50 percent reduction in the size of the biological system due to the enhanced removal from the Chemical Primary stage.

animal wastes, failing septic tank leaching systems, discharge from boats, and storm drain systems are other potential sources.

Disinfection Methods

Methods of disinfection include 1) the addition of chemicals; 2) the use of physical agents such as light or heat; 3) mechanical methods; and 4) exposure to electromagnetic, acoustic, or particle radiation. Disinfection technologies are considered as add-ons to any of the ten treatment trains already discussed and to other treatment systems. They can also be used for treating combined sewer overflow discharges. A variety of the present and future disinfection methods, the major advantages and disadvantages of each method, and selected costs are given below.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×
Chlorination

Chlorine (Cl2), calcium hypochlorite (Ca(OCl)2), or sodium hypochlorite (NaOCl) are the most widely used disinfectants in the United States and the rest of the world. Chlorination is inexpensive, widely available, and has a long history of proven effectiveness. However, organic compounds typically present in treatment wastewater can combine with chlorine to form toxic chloro-organic compounds and excess free-chlorine is toxic to many aquatic species. Chloroform, the best-known trihalomethane, is a documented animal carcinogen. Chloramines, especially monochloramine, form in wastewater in the presence of ammonia. These compounds are stable and do not dissipate. Because the high chlorine dose required for break point chlorination are not used, chloramine disinfection is less expensive than free chlorine disinfection. However, chloramines are also less reactive than free-chlorine. Some chloro-organics are potential carcinogens, mutagens, or toxins (WPCF 1986). Recent Occupational Safety and Health Administration standards for safety require intensive training and other expensive measures for systems employing chlorine gas. Sodium hypochlorite does not have the same safety issues associated with it as does gaseous chlorine.

Chlorine dioxide (C102) has been used in the past for wastewater treatment disinfection where phenolic wastes are present. It is used as an alternative disinfectant of raw water supplies. Its advantages are that it does not produce THMs and is a very effective bactericide and viricide over a broad pH range. Because it does not react with ammonia, it can provide effective disinfection at relatively low applied concentrations. Its disadvantages include high capital cost for the C102-generating equipment, and a lesser understanding of its toxicity. Chlorine dioxide produces chlorate and chlorite which may have subsequent impacts on the environment.

Capital costs of chlorination systems can range from minor for cases where adequate contact time can be attained in the outfall to substantial, where a contact tank and mechanical mixing must be provided. Operation and maintenance costs for chlorine systems are about $15 per million gallons treated (at a dose rate of 5 mg/l).

Dechlorination

Dechlorination is the process of removing chlorine residuals after chlorination. Dechlorination typically is done by either chemical means or with granular activated carbon (GAC). Chemical methods involve the injection of sulfur dioxide(SO2), sodium sulfite (NaSO3), sodium bisulfite (NaHSO3), or sodium metabisulfite (Na2S2O5) following the chlorine contact tanks. Like sulfur dioxide, the salts produce the same active ion, sulfite (SO) upon dissolution in water. Due to cost, GAC for dechlorination generally is

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

limited to those instances where a GAC process is already in place at a wastewater treatment plant to remove toxic organics. The GAC can be a gravity or pressure bed system.

Dechlorination by chemical means is a simple, relatively inexpensive, and effective method. However, while reduced sulfur ions react rapidly with free ions and combined chlorine residuals, they do not always completely remove organic chloramines or chloro-organics. Additional treatment may be needed to further reduce total organic halogens (TOX, where X is chlorine, bromine, or iodine), which are potential carcinogens, mutagens, or toxins. It is estimated that municipal and industrial treatment plants using some 100,000 to 200,000 tons of chlorine per year produce several thousand tons of chloro-organics per year (WPCF 1986).

GAC is effective in removing many residuals. However, it is also expensive and therefore used only in those instances where high levels of organic removal are required. Operations and maintenance costs total about $20 per million gallons treated.

Ozone Disinfection

Ozonation of drinking water has a long history, especially in Europe. However, its application to wastewater has not been widely accepted. Ozonation produces fewer toxic by-products than chlorination and, because it reduces the need to store large quantities of chlorine in urban areas, it is safer to operate than a chlorine system. Because ozone is the strongest oxidizing agent and disinfectant used in wastewater treatment, only small doses and short contact times are needed. The efficiency of ozone disinfection is also independent of pH in a range between pH 6-10 and of temperature in a range between 36-86 degrees Fahrenheit.

Ozone use has been limited in the United States, however, because of its relatively high cost. Ozonation has higher capital and operating costs than chlorination. It is energy intensive, requiring 16-24 kilowatt hours of electricity per kilogram of ozone. Ozone disinfection of wastewater for a city of 500,000 could require 10,000 kWh/day of electricity. Also, ozone does not maintain a residual concentration in the treated water which allows for the possible regrowth of microorganisms after disinfection.

Ultraviolet Irradiation

Ultraviolet (UV) irradiation disinfection is a developing technology and is receiving increasing attention and application in the United States. UV disinfection has been demonstrated to be effective on a variety of organisms, particularly on viruses. It is as effective or more effective than chlorination or ozonation (WPCF 1986) and leaves no toxic residue. UV irradia-

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

tion disinfection equipment is inexpensive and occupies little space, and is relatively easy to maintain and operate.

As with ozonation, however, UV disinfection leaves no residual inactivation agents. Thus, the regrowth of microorganisms after disinfection is possible. Also, some microorganisms may be able to repair damage done by UV disinfection if exposure is not fatal. Investigators have found that suspended and dissolved matter and water itself absorb UV radiation; thus the efficacy of this process is compromised when particulate matter is elevated (Qualls et al. 1985). Because UV disinfection is a relatively new technology and is generally more widely applicable to smaller POTWs, it requires further development for larger installations.

Electron Beam

This technology uses high speed electrons to kill microorganisms and toxic organic compounds. The technology, used for many years to preserve food and disinfect medical supplies, is now being tested as to its feasibility in treating wastewater (Jones 1991) but has been successful in treating sludges in limited studies (EPA 1989c).

Efficacy of Disinfection Methods in Pathogen Inactivation

Disinfection is the term used for the selective destruction of pathogenic organisms in order to protect the health of people and other animal life. The efficacy of different disinfection methods is a function of the concentration of the chemical agent or intensity and nature of the physical agent, the contact time, the temperature, and the number or types of pathogenic organisms present. It also depends on water quality factors (e.g., the amount of solids, dissolved organic material, inorganic compounds, and pH), treatment plant design, and level of treatment. Generally, the relative resistance of microorganisms can be listed in the following order, from the most resistant to least: parasitic cysts and acid fast bacteria, bacterial spores, viruses, vegetative bacteria.

Viruses are less easily inactivated than bacterial indicator organisms. These differences in sensitivity are one reason indicator bacteria do not adequately specify the effectiveness of disinfection measures in the inactivation of parasites and viruses (see Appendix B). Table D.18 shows the relative resistance of waterborne microorganisms for chloramines, chlorine, chlorine dioxide, and ozone (Sobsey 1989). The ranges of CTs, the product of the disinfection concentration and the contact time, for 99 percent inactivation reflect the variety of water disinfection conditions during experimentation as well as the variety of viruses, cysts, and bacterial cultures used.

The inactivation of microorganisms in chlorinated activated sludge ef-

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

fluents is highly variable. Disinfection reduces pathogen levels, especially when the effluent is free of suspended solids, but effectiveness is reduced in the presence of higher concentrations of suspended and colloidal solids. Depending on the level of nitrification, chlorination results in 0.8 to 1.3 log10 reduction of fecal streptococci and 0.1 and 0.5 log10 reduction of F-specific coliphages, used as surrogates for human enteric viruses. Table D.19 summarizes these data.

Table D.20 summarizes several surveys that have evaluated the occurrence of enteric viruses in chlorinated and unchlorinated primary + biologi

TABLE D.18 Ranges of CT1 for 99 Percent Inactivation (Source: Sobsey 1989)

 

E. coli

Enteric Viruses

Giardia Cysts

Chloramines

113

345 - 2,100

430 - 1,400

Chlorine

0.6 - 2.7

0.3 - 12.0

12 - 1,012

Chlorine Dioxide

0.5

0.2 - 6.7

2.7 - 15.5

Ozone

0.006 - 0.02

0.006 - 0.72

0.53 - 4.23

1 CT is the product of the disinfectant concentration (mg/l) and the contact time (minutes).

TABLE D.19 Log10 Reduction of Microorganisms in Chlorinated Activated Sludge Effluents (Source: Nieuwstad et al. 1988)

 

Fecal Streptococci

F-specific Phage

Activated Sludge

0.8

0.1

Moderately Nitrified

1.4

0.2

Nitrified

1.3

0.5

TABLE D.20 Enteric Virus Levels in Treated Wastewater (Source: Rose and Gerba 1991, Asano et al. 1992)

 

% of Positive Samples

Average PFU/100 liters

BIO + Chlorination

31

70

13

130

BIO + Unchlorinated

67

79

BIO + Filtration + Chlorination

8

0.8

7

1.25

not detected

0.13

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

cal treatment effluents (System 3) and in chlorinated activated sludge effluents with filtration (Rose and Gerba 1991, Asano et al. 1992). Viruses were recovered from 31 percent to 70 percent of the samples at concentrations averaging between 13 and 130 plaque forming units per 100 liter in chlorinated and unchlorinated primary + biological treatment effluents. Filtration with chlorination reduced the prevalence and concentrations by 10- to 100-fold. This reduction was the result both of the improved physical removal of viruses due to the filtration and the enhanced inactivation due to the higher quality effluent.

COMBINED SEWER OVERFLOW CONTROLS

History and Problems

In many older cities across the United States—particularly the northeast seaboard cities and Seattle, Portland, and San Francisco on the west coast—combined sewer systems were constructed to provide drainage and wastewater disposal services. As shown in Figure D.8, combined sewers are designed to convey both wastewater and surface drainage from residential and business areas to a discharge location. During dry weather, an interceptor sewer accepts wastewater from the combined sewer and conveys it to a treatment plant. During rain events, the limited capacity of the interceptor sewer allows only a portion of the wastewater/stormwater mixture to be carried to a treatment plant. The remainder discharges from the combined sewer into nearby creeks, open channels, and rivers. The discharged mixture of wastewater and stormwater is called a combined sewer overflow (CSO). The construction of combined sewer systems in the United States ceased for the most part after 1945.

Technical uncertainties and difficulties in developing cost-effective measures to mitigate CSOs has plagued regulators since the beginning of the century. The first federal legislation to address this problem, the River and Harbor Act of 1899, exempted CSOs from regulation. The Clean Water Act of 1972 removed this exemption, but even after the July 1, 1988 deadline for municipal compliance with the Clean Water Act, there had been only partial elimination of some dry weather overflows and limited elimination of wet weather overflows.

The uncertainty on how to address CSOs emanates from concerns regarding the adequacy of technical and financial resources available to address them compared with their priority as a water quality problem. Federal uncertainty and lack of a consistent national program to fund and administer CSO control programs has resulted in uneven application of the law in the United States. Even so, several cities (Chicago, San Francisco, and Milwaukee) have undertaken large-scale programs to mitigate overflows from

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

FIGURE D.8 Combined sewer operation. (Source: Camp Dresser & McKee, Inc. 1991. Reprinted, by permission, from Camp Dresser & McKee, Inc., 1991.)

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

their combined sewer systems. But many of the more recent advances in CSO control technology come from western Europe. In Europe, research and development for promoting new CSO controls and creating ways and methods to design system-wide holistic controls flourishes. For example, in a 1990 CSO research and development program (Project Rainfall), the West German government spent more in one year than the U.S. EPA did in the past eight years. Similar programs of this scale are under way in the United Kingdom, France, The Netherlands, and Canada.

The U.S. government, however, has now initiated a formal CSO regulatory program in response to the Water Quality Act Amendments of 1987. On September 8, 1989, the U.S. EPA published its National Combined Sewer Overflow Strategy (Federal Register 1989). This strategy required six technology-based limitations as a minimum best control technology and best available technology, which is established on a best professional judgement basis:

  1. Proper operation and regular maintenance programs for the sewer system and combined sewer overflow points,

  2. Maximum use of the collection system for storage,

  3. Review and modification of pretreatment programs to assure CSO impacts are minimized,

  4. Maximization of flow to the POTW for treatment,

  5. Prohibition of dry weather overflows, and

  6. Control of solid and floatable materials in CSO discharges.

The following section describes 5 categories of CSO controls that represent the state-of-the-art of this technology.

  1. Source Controls,

  2. Flow System Optimization,

  3. Sewer Separation,

  4. Satellite Treatment, and

  5. Off-Line Storage.

The first 3 categories apply, essentially, to the EPA's minimum requirements 1 to 5. The fourth and fifth categories: satellite treatment and off-line storage, relate to the EPA's item 6.

CSO Technologies
Source Controls

The source controls cited here deal with reducing the amount of pollutants that accumulate during dry weather on the land surface, streets, and

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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within sewer systems. Minimizing these accumulations means that during rainstorms there will be a smaller pollutant mass discharged from the urban land areas to the receiving waters. While it is straightforward to postulate that this is the cleanest and most obvious class of controls, in real practice, the opposite is generally true. Long-term dependency on a labor force to perform these types of controls simply has not worked.

Street Sweeping. Although the major objective of street sweeping is to enhance roadway appearance, periodic removal of surface accumulations of litter, debris, dust, and dirt also reduces the transport of such materials into the sewer system. Common methods of street sweeping include manual sweeping and the use of mechanical sweepers utilizing brooms or vacuums. Most communities now practicing street sweeping rely mainly on one of the mechanical methods, which loosen debris from the street surface, pick it up, and store it for later disposal. Street sweeping cannot be performed during wet weather or during periods of ice and snow accumulation.

The technology of street sweeping and its pollutant removal effectiveness has been assessed extensively in the last decade. Almost all manuals for urban stormwater water-quality management cover the topic (FHA 1985). The extensive statistical reviews of this practice (11 sites and 5 pollutants) during the EPA's National Urban Runoff Project indicated that no significant reductions in pollutant concentrations are realized, although they could occur in certain site-specific cases (EPA 1983).

The EPA concludes that street sweeping in most of the United States is appropriate for aesthetic purposes but has limited water-quality benefits. Street flushing, which is only practiced in a few cities in the United States but is common in Europe, provides better removal of fine particles than street sweeping but is only feasible in combined sewer areas.

Catch-Basin Cleaning. Catch-basins are commonly cleaned with mechanical bucket devices or vacuums. Catch-basin cleaning removes heavy solids, eliminating possible sources for silt deposition problems in downstream sewers. When properly designed and maintained, catch-basins are effective in removing both coarse solids and floatables. However, the finer solids are not effectively removed. Catch-basins accumulate solids and liquids which, when flushed by runoff, can contribute significant pollutant loads. Frequent vacuum or suction-type (eductor) cleaning of catch-basins can remove accumulated pollutant material and maintain catch-basins' removal efficiency. However, catch-basins are not frequently cleaned because of high labor requirements (CH2M Hill 1989, FHA 1985).

Sewer Flushing. First flush includes the wet weather scouring of wastewater solids deposited during dry weather in combined sewer systems. First flush

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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TABLE D.21 Effectiveness of Single 300-Gallon Manual Flush (Tanker) to Scour, Entrain, and Transport Materials within 12'' to 18" Laterals

Pollutant removed in a length of:

250 feet

700 feet

1,000 feet

Organic & Nutrient Deposits (BOD5, TP, TN)

75% - 90%

65% - 75%

35% - 45%

TSS Deposits

75%

55% - 65%

18% - 25%

also includes the transport of loose solid particles from the urban ground surface to the sewerage system. These particles settle out in the combined system and are flushed during periods of larger flows. The magnitude of these combined loadings during runoff periods has been estimated to be as much as 30 percent of the total daily dry weather sewage loadings (CH2M Hill 1989).

Sewer flushing involves scouring and transport of deposited pollutants to the wastewater treatment plant during dry weather when there is sufficient interceptor capacity to convey these flows. In 1979, a three-year research and development program sponsored by the EPA was conducted in the Dorchester area of Boston to determine the pollution reduction potential of flushing combined sewer laterals (Pisano 1978, 1979). It was concluded that small volume flushing would transport organics, nutrients, and heavy metals sufficient distances to make the option feasible and attractive. Relevant conclusions are listed in Table D.21.

Sewer flushing of large diameter combined sewers was investigated in the Elizabeth, New Jersey, CSO Facility Plan (Clinton Bogert Associates 1991, Kaufman and Lai 1978). The plan concluded that daily flushing of troublesome deposition sections within seven subareas using 12 automatic flushing systems reduced the first flush overflow pollutant loadings by about 28 percent.

Flow System Optimization

This method involves adjusting the flow controls within existing pipe systems to maximize the carrying capacity of interceptors or to take advantage of unused large pipe storage during wet weather. This control is very efficient in the capture of small storms (less than 0.1 inch), costing under $200 per acre.

Combined sewer systems have flow regulation structures that divert wet weather flow to the POTW through the interceptor at rates normally from 1.5 to 3 times average dry weather flow. Any greater flow escapes and discharges to the environment. When the wet weather flow exceeds this

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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rate, the POTW sees only a small fraction of the potential first flush. Many North American communities have increased their POTW capacity in order to maintain higher ratios of wet to dry flow to better capture the first flush. New types of flow controllers are needed to increase the first flush capture without overloading the POTW during the remainder of the storm in cases where interceptors can accept the increased flows.

Enhanced Flow Regulation and Static In-Line Control. Static control regulators have the operational advantage of no moving parts. Interception is limited to preset levels. Typically, static control regulators cannot maximize intercepted flow before a spill. The standard type of controllers include: fixed orifices, drop inlets, leaping and side-spill weirs, siphons, and manually operated gates.

In the last ten years a new type of static flow controller called a vortex throttle has improved in-line storage of combined sewage at regulator chambers for later drainback to the POTW. Vortex devices work using no moving parts or external energy supply. Besides providing a more positive degree of flow control than orifices or spill weirs, they require less maintenance than mechanical float-operated controllers.

The largest U.S. system-wide configuration of vortex flow controllers for combined sewer system control is in Saginaw, Michigan. In the mid-1980s, the city converted 21 flow regulators to static vortex types at a cost of $200 per acre served. In the spring of 1988, Saginaw reported that the amount of wet sludge processed during rainfall events had increased by about 12 tons per day during runoff events preceded by several days of antecedent dry weather (J. Anderson, Supt. of Wastewater Treatment, Department of Public Utilities, City Saginaw, Michigan, personal communication, 1988).

The Marigot project in Laval, Quebec includes 13 new regulators controlling combined sewer flow to a new tunnel more than three miles long with pumpage to a new POTW located on the southerly side of the island of Laval, adjacent to Montreal, Canada. This project is the largest in North America in terms of number of units and the scale of the technology. Construction was completed in 1987.

Dynamic In-Line Storage and Real Time Control. Variable control is provided by regulators that control a gate or similar structure opened in response to an external signal. This controller can adjust the amount of interception and is connected to a central control that optimizes selection of overflow time and location in response to actual system flow and rainfall conditions. Most variable controls require constant maintenance to ensure proper operation.

Seattle, Washington, maintains the largest computerized in-system con-

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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trol program in the United States. It monitors pumping stations, regulator stations, POTWs, and rain gages and uses programmed information to provide control.

The Northeast Ohio Region Sewer District has operated and maintained in-line storage systems in Metropolitan Cleveland for nearly 20 years. Over 140 in-line systems are in operation using motorized sluice gates and inflatable plastic fabric dams (fabridams) controlled by computers. Cleveland's system fabridam failure rate initially approached 30 percent because of design and installation problems. As a result, the Northeast Ohio Region Sewer District continues to upgrade design and installation quality control. It is estimated that the automated program eliminates 98 percent overflows for a 0.12 inch rain. Ohio's experience demonstrates that a slow and progressive correction approach is preferable to one of immediate full-scale implementation (Hudson 1990).

Sewer Separation

Sewer separation is a method of minimizing the amount of street runoff that mixes with sanitary sewage. However, complete separation is difficult and prohibitively expensive to achieve and street runoff can be quite polluted. Thus, in contrast to 20 or 30 years ago, the practice of sewer separation within combined-sewered areas is not practiced on a large scale today. Separation is still practiced to solve pollution problems within small portions of combined sewer areas connected to separated systems or to solve flooding problems within combined systems where there is inadequate flow capacity.

Conventional Full and Partial Separation. Sewer separation has several different meanings. Complete separation means strict separation of all sanitary, commercial, and industrial sewage flows into a system that is separated from a storm sewer system serving the same area. Partial separation generally means construction of a new storm sewer system to handle street runoff load, i.e., pipes connecting to all the catch-basins. The effectiveness of stormwater inflow reduction for complete separation systems is about 95 percent. Partial separation effectiveness ranges from 50 percent to 85 percent.

Separation is viewed as a viable CSO control in some limited circumstances, where substantial separation already exists. However, the federal stormwater permit program promulgated in 1991 has lead most CSO-impacted communities to reject prior separation as a CSO control approach and to express serious concern about separating large, new areas. Areas impacted by basement flooding will continue to separate in order to reduce health hazards and property damage.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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The cost of a separation project is dependent on the degree of prior separation; the configuration of the existing system; the size, population density, and geography of the area; and the design objective. A complete separation cost was developed for the CSO catchment area in Boston (CH2M Hill 1988). Total construction costs varied from $60,000 per acre for partially separated residential neighborhoods to $190,000 per acre for entirely combined downtown areas.

Flow Slipping. Flow slipping involves the use of inlet control in urbanized areas to manage the stormwater entering existing combined sewer systems. It has its roots in Scandinavia and the United Kingdom where the concept has been used on undersized combined sewers to relieve basement flooding and to mitigate the volume and frequency of overflows. This practice is widely used for basement flooding and drainage control in Ohio, Maine, Illinois, and Quebec and is used as a CSO control throughout Ontario Province in Canada.

Flow restrictors placed within stormwater catch-basins are widely used to induce overland flow away from sensitive areas to more attractive capture/storage locations (Smisson 1981; Pisano 1982a, b; Wisner 1984; Walesh 1985; Havens & Emerson Engineers 1987; TWA and HRD 1987). In the last several years, Boston has conducted a long term field evaluation of this concept and has noted no adverse flooding or deposition problems. It intends to utilize this concept to optimize on-going separation projects (S. Shea, Head of Sewer Construction, Boston Water & Sewer Commission, personal communication, 1991).

In Hartford, Connecticut, preliminary CSO plans for the Franklin Avenue District (a 1,000 acre CSO catchment) included extensive use of flow slipping within combined sewer areas, and new or existing storm drain outlets. Average costs for conventional street load sewer separation within the district are about $56,000 per acre. Flow slipping separation costs ranged from $4,000 to about $21,000 per acre.

High-Rate Satellite Treatment

Often it is impossible to capture and treat all combined sewage even from low production rainstorms. Most POTWs can treat no more than about 0.003 inches per hour (on the average) of rainfall. Federal and some state CSO control agencies are considering requiring capture and secondary treatment of one-year storm events, equalling approximately one inch per hour, which could represent more than 300 times the capacity of a POTW. The costs of near-surface retention, deep storage facilities, and treatment facilities to process these returned flows from storage can be enormous.

For example, the current CSO policy for metropolitan Toronto and area

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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municipalities restricts the system to one overflow per recreational season for critical receiving water areas and 90 percent volumetric control (annual) for all other areas. Up-system near-surface tanks and waterfront tunnels will cost about $350 million, while the balance of the estimated $1.3 billion program is associated with additional new conveyance pipes and treatment capacity at existing POTWs to handle new growth and additional wet weather flows. These flows are gradually sent to the POTW over an average of three days. Toronto is considering satellite treatment to decrease the amount of retained storage and to decrease new POTW capital investment. Satellite treatment is usually distributed throughout a collection system at the outfalls of large combined sewer trunks.

There are only a few practical high-rate treatment processes in modern CSO control design practice. These include screening, vortex separation, and vortex separation with storage. Treatment units such as dissolved air floatation, dual-media high-rate filtration, high-gradient magnetic separation, and powdered activated carbon-alum coagulation (all preceded by screening) are operations that have been used with some success in experimental demonstration projects. For a variety of technical reasons, these unit operations are not practical in full-scale operation where intermittent, heavy shock CSO flows, debris, and pollutant loadings occur.

Combinations of vortex separation with conventional near-surface storage are being used in several projects. Several recently constructed facilities in Decatur, Illinois, using new German vortex separators coupled with relatively small volumes of storage, provide higher degrees of pollutant removal than conventional high-rate treatment schemes. At the same time, they reduce maintenance requirements and reduce the amount of required storage relative to conventional near surface storage. For an investment of roughly $2,000 to $4,000 per acre, the storage and vortex treatment concept consumes very little land space. During peak design flow conditions, this system can achieve solids removals at levels between those obtained in preliminary and primary treatment.

Screening Facilities. Screening technology has an inconsistent classification system, using such terms as bar, coarse, fine, or micro, depending on the screen construction and spacing. Bar and coarse screens are used to remove gross floatable and settleable materials. Coarse screens are used as a pre-treatment protective measure for vortex separation or off-line storage facilities, especially where pumping is required. Fine screens and micro screens are usually used at POTWs and centralized CSO treatment facilities. Types include static screens, hydraulic sieves, drum screens, and vertical rotating screens.

In practical terms, because it operates reliably, mechanical screening of the type used in POTW headworks is the only screening method that is

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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viable for remote satellite operations. Micro strainers require constant maintenance and frequently are blinded with solids. They are recommended only at continuously staffed operations.

Five major screening and disinfection facilities (200-300-MGD range) were recently designed in Atlanta, Georgia. Each satellite plant includes a series of parallel screening channels with preset overflow diversion weirs for bypass. Flows within the design capacity of the facility pass through coarse and then fine screens, followed by chlorination and discharge. The facilities are intended to reduce fecal coliform and remove floatable and settleable solids larger than 3/8 inch in diameter. Presently, the $90 million construction program is under way. Average cost per million gallons treated per day for the 5 facilities is estimated at $10,000, which is approximately one-half of one percent of the cost of conventional wastewater treatment.

Vortex Solids Separators. Vortex solids separators coupled with storage facilities can provide a significant degree of coarse solids removal that is cost-effective and requires low maintenance. A vortex solids separator is a small, compact, solids separation device with no moving parts. If the unit is used as a combined sewer regulator, dry weather sewage passes through unimpeded. If the device is intended to operate only as an off-line treatment unit, then storm flows are deflected by gravity or by pumpage into the unit. During wet weather the unit's outflow is restricted, causing the unit to fill and inducing a swirling vortex-like operation. Settleable grit and floatable matter are rapidly removed. Concentrated foul matter is sent to the POTW, while the cleaner, treated flow discharges to the receiving waters or into temporary storage for later treatment at the POTW.

A type of vortex separator developed in the early 1970s, the swirl concentrator has been extensively tested in the United States (Drehwing 1979, Sullivan et al. 1982, Wordelman 1984, Heinking and Wilcoxon 1985, Hunsinger 1987), and the performance results are extremely mixed. In many early applications, the device was intended to remove substantial amounts of suspended solids, while its original design was only intended to remove coarse grit and floatables at regulation chambers. Other deficiencies relate to excessive vessel turbulence at design flow. Also, combined sewage contains finer materials that do not readily separate.

There are 19 swirl concentrators in the United States, which have a total design flow capacity of 888 MGD, including two under construction in Euclid, Ohio (EPA 1989b). With the exception of Decatur, Illinois, all installations are stand-alone, off-line devices. The largest swirl concentrator complex in the United States is the Robert F. Kennedy facility in Washington D.C. The site contains three 57-foot diameter units with a total design flow of 400 MGD. In the mid-1980s, a vortex separator was developed which appears to achieve better removal efficiencies than the swirl

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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concentrator (Brombach 1987, Pisano et al. 1990). Presently there are 13 such units in seven U.S. projects, which have a total design capacity of 1.2 billion gallons per day.

In general terms, properly designed U.S. vortex separators remove 15 percent to 35 percent of settleable solids, with higher removals associated with first flush. Construction costs of vortex vessels range from $5,000 to about $8,500 per acre treated. A recent full-scale vortex separator investigation in Tengen, Germany, has concluded that this technology achieves settleable solids removals of at most 60 percent (Brombach et al. 1992).

Off-Line Storage

Storage facilities, basins, or tunnels have been extensively used to capture excess runoff during storm events. Storage allows the maximum use of existing dry weather treatment facilities and is often the best low-cost solution to CSO problems. Combined sewage flow is stored until the treatment facilities can treat the excess flows. Off-line storage requires detention facilities, basins or tunnels, and the facilities for either draining by gravity or pumping flow to and from storage.

Utilization of relatively small volumes of retention storage can be effective in retaining pollutant loadings from small, frequent, storm events. Loadings from the larger storms usually represents only a small fraction of the total annual CSO. This is sometimes overlooked. Small amounts of system storage (in-line or off-line), on the order of 200 to 400 cubic feet per acre, can effectively address much of the CSO problem.

There are a number of different storage strategies in use. The first is called upstream stormwater hybrid. The concept is a British and Scandinavian inlet control practice, where catch-basins are restricted to force street loads to move over land to new catch-basin intakes. These intakes discharge into shallow off-line storage tanks which have throttled outlets leading back into the sewer system. This practice is widely used to solve basement flooding problems and is often viewed as a CSO control. The value of this approach is limited because it typically is not possible to provide large volumes of surface storage that can be gravity-drained back into sewer systems. This approach benefits congested residential areas because it can be used as a last resort in situations where there is no other good, cost-effective way of creating near-surface storage. This approach is used in the Cleveland and Chicago areas and has recently been proposed in Hartford, Connecticut.

The second type is near-surface contaminated upstream storage and is popular in Europe. This technique includes small volume storage that captures first flush flows in excess of several times the average dry weather flow. The captured flow is returned to the sewer system for treatment after

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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the storm passes. Over 12,000 such tanks in Germany, Switzerland, and Holland presently exist with a median size of about 150,000 gallons. Because the trend in the United States has been toward much larger downstream facilities, very few of these types of tanks are used.

The third type is near-surface contaminated downstream storage. There are only about 20 in the United States, and they are usually of 1 to 4 million gallons. The largest facility, in Sacramento, California, has a storage volume in excess of 25 million gallons. The greatest concentration of large near-surface retention and detention facilities is in Michigan, where very stringent performance standards of the one-year, six-hour storm with secondary treatment have been adopted. Cincinnati has included in its CSO Master Plan a number of such tanks. New York City is currently designing several near-surface storage tanks with storage volumes on the order of 10 to 40 million gallons.

One negative feature of large near-surface storage tanks is the large amount of land required. However, if properly designed, the covered surface of the tanks can be used for tennis courts, basketball courts, or parking. Some designs include soil covering and a planted park area. Another negative feature is the problem of cleaning the large tanks of accumulated sediments and organic material. Because the solids loadings on downstream treatment can be significant, cleanout timing is important. Capture performance of such tanks is high if total retention is provided.

The fourth and fifth methods of storage are deep tunnels and reservoirs in bedrock, either decentralized or consolidated. Large caverns have been excavated in Chicago, Illinois, Milwaukee, Wisconsin, and Rochester, New York for storage and subsequent treatment. Such systems are also proposed for Cincinnati, Ohio and Boston, Massachusetts. These approaches are becoming more attractive because the costs of tunneling have dramatically decreased in the last decade.

The last method described is the moat storage as employed by the city of San Francisco. This system will eliminate 85 percent of the city's CSOs by adding storage in large underground conveyance boxes that effectively ring the city along the shoreline. The construction cost for a service population of 727,000 was $1.35 billion. Flexible bags located in the receiving water have been demonstrated in small scale applications represent a low-cost variation of this approach if it proves to be successful on the large scale.

The costs of near-surface storage typically ranges between $5,000 to $15,000 per acre. There are a number of factors that influence overall cost. In general, the most expensive facilities are underground, rectangular tanks resting on poor soils and requiring odor treatment in urban settings. Maintenance cost is also significant since the tanks must be visited by a maintenance crew to flush settled solids from the tanks after every storm. Such

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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tanks in New York City presently cost about $4 to $5 per gallon of storage generated. Storage systems can be very effective in CSO control. Annual overflow reductions on the order of 80 percent to 90 percent are possible, but maintenance is costly because of the need to remove heavy solids regularly from the storage facilities.

Integrating CSO Control Techniques

A $4 million full-scale demonstration project in Metro Toronto, begun in May 1991 is exploring alternative CSO control strategies including a vortex solids separator, detention tanks, chemical addition, and alternative disinfection schemes (i.e., conventional chlorination and dechlorination versus ultraviolet treatment). The central idea of this demonstration project is to ascertain what portion of the CSO flows can be "safely treated and acceptably discharged" in a satellite context such that POTW costs can be reduced. The project is funded by Metro Toronto Works, Environment Canada, and the Ontario Ministry of Environment.

This new idea of storage and high-rate treatment can be appreciated by a review of the Decatur, Illinois, CSO control projects. Projects at 3 sites: McKinley Ave (40 MGD), 7th Ward (113 MGD), and Lincoln Park (416 MGD) are discussed here.

The concept is to direct all flow through coarse screens. A portion of the first flush is captured in retention storage tanks and the rest passes through vortex separators. The underflow from the vortex separators is pumped into the retention storage tank. When the retention storage tank is full, the vortex separator continues to operate, but underflow is set to zero. Some separation occurs, but efficiency is greatly diminished. The retention storage tanks are provided with aerators and mixers to prevent odors and facilitate cleanout. Washdown is performed using water cannons. Following a storm event, the vortex separator pump station dewaters the tank with pumpout sent to the nearby interceptor. Typical ratios of wet weather flow volume to dry weather flow volume interception ranges from 3 to 5.

Storage provided by these projects ranges from 113 to 217 cubic feet per acre. The 416 MGD Lincoln Park facility covers 3.4 acres. A conventional near-surface storage facility would occupy at least three times that area and, under certain storm conditions, still have overflow. The facility consists of two screening buildings, two flow deflection chambers, one vortex flow divider, four vortex separators, one pump building, and the first flush storage tank. The 113 MGD 7th Ward facility, which provides preliminary treatment (15 percent settleable solids removal at a design flow of 174 cubic feet per second), covers 2.2 acres. A number of unit costs are presented in Table D.22.

The cost of vortex separators per MGD design flow are $4,125/MGD

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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TABLE D.22 CSO Project—Decatur, Illinois, Construction Costs Only (1991)1 (Source: Pisano and Wolf 1991. Reprinted, by permission, from Water Environment Federation, 1991.)

Parameter

McKinley

7th Ward

Lincoln Park

Tributary Area (acre)

661

860

2,491

Design Area (MGD)

40

113

416

Site Area (acre)

1.5

2.2

3.4

Cost ($ in million)

1.755

3.947

7.84

Cost ($/acre)

2,655

4,134

3,147

First Flush Storage (cf/acre)

101

117

73

Storage Underflow (cf/acre)

27

100

40

Total Storage (cf/acre)

138

217

1771

Cost Storage ($/gal)

1.09

0.76

N/A

Diameter Vortex Sep (ft)

25

44

44

Cost Vortex Sep ($1,000)

171

492

N/A

N/A = not available.

1 Additional in-line storage = 60 cf/acre.

and $3,125/MGD for similar projects in Burlington, Vermont and Saginaw, Michigan. The unit volumetric cost of storage within the vortex separators is $1.66 and $2.43 per gallon respectively.

It is believed that providing vortex separators with conventional facilities would provide the greatest operational flexibility. The future of satellite treatment will likely couple near-surface storage with high-rate vortex solids separation treatment devices. This approach provides the flexibility to expand facilities to meet more stringent regulatory control requirements. There is a limit to the removal of solids by physical means. Chemical addition to detention storage is an option to increase removal. If the requirement is to obtain fewer overflows per year, then adding more retention storage is about the only option available.

Summary of Comparative Performance of CSO Control Technologies

The efficacy and cost of the various CSO control technologies is summarized in Tables D.23, D.24, and D.25. Table D.23 compares the relative advantages, Table D.24 presents the pollutant removal capability, and Table D.25 shows the comparative costs of each CSO technology.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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TABLE D.23 Comparative Advantages of Each Technology

 

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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F: Previous effectiveness

Unproven effectiveness

Moderate effectiveness

Proven effectiveness

G: Wide use applicability

Low potential for wide use

Medium potential for wide use

High potential for wide use

H: Removal effectiveness

< 25% control of overflow volume and solids removal

25% - 60% control of overflow volume and solids control

> 60% control of overflow volume and solids control

TABLE D.24 Comparative Pollutant Removals

 

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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TABLE D.25 Comparative Capital Costs for CSO Control Options

CSO Control Options

Capital Costs ($ per acre)

Source Controls

 

Catch-basin Cleaning

Not Applicable

Street Sweeping

Not Applicable

Sewer Flushing

$100 - $500

Flow System Optimization

 

Enhanced Flow Regulation and Static

 

In-Line Control

$100 - $500

Dynamic In-Line Storage

$500 - $1,000

Real Time Control

$500 - $1,000

System Flow Reduction

 

Conventional Full Separation

$50,000 - $100,000

Conventional Partial Separation

$10,000- $50,000

Flow Slipping

$5,000 - $10,000

High Rate Satellite Treatment

 

Screening

$2,000 - $5,000

Vortex Solids Separators

$2,000 - $5,000

Vortex Separators and Storage

$5,000 - $10,000

Combination

 

Off-Line Storage

 

Upstream Stormwater Hybrid

$10,000 - $50,000

Near-Surface Contaminated Upstream Storage

$5,000 - $10,000

Near-Surface Contaminated Downstream Storage

$5,000 - $10,000

Decentralized Deep Storage

$10,000 - $50,000

Consolidated Deep Storage

$10,000 - $50,000

NONPOINT SOURCE MANAGEMENT OPTIONS

Introduction

According the joint EPA/National Oceanic and Atmospheric Administration document Proposed Development and Approved Guidance-State Coastal Nonpoint Pollution Control Programs, nonpoint source pollution has become the largest single factor preventing the attainment of water quality standards nationwide (EPA 1991b). For estuarine waters, current best estimates show that of the approximately 75 percent of waters assessed, 10 percent are threatened and 35 percent are impaired. Nonpoint

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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source pollution is an important component of these threats and impairments. The leading sources of nonpoint pollution in estuarine waters are urban and agricultural runoff. More specifically, these include erosion from construction sites; runoff from urban areas; erosion from agricultural lands, streambeds, and roadways; runoff from livestock production areas; and runoff from farmland contaminated by fertilizers, pesticides, and herbicides. Pollutants from nonpoint sources are carried to the coast primarily by surface water through the action of rainfall runoff, snow melt, and ground water seepage. Principal pollutants of concern include sediment, nutrients, bacteria, metals, organics, and oil and grease.

Nonpoint sources include atmospheric deposition. Atmospheric pollutants deposit directly onto coastal water bodies in the form of precipitation or dry deposition. In the Great Lakes, the principal source of PCBs is through atmospheric deposition. In the Chesapeake Bay, atmospheric deposition is a major source of nitrogen to the bay.

Direct atmospheric deposition on coastal waters is probably a small source in the mass balance of most pollutants in most cases. However, deposition on land areas and subsequent delivery to the shore by storm channels and rivers can be significant, as, for example, in southern California, where urban storm runoff was the main pathway for lead transport before the use of unleaded gasoline was mandated.

Nonurban sources, especially agriculture and forestry, can generate large quantities of nonpoint source pollution. Indeed, in bays fronted by farms or harvested forests, nonurban activities may be the predominant source of pollutants such as pesticides; and naturally occurring substances, such as nitrogen and minerals. The largest source of nitrogen in the Chesapeake Bay comes from upstream agriculture in the fertile river valleys. The largest source of nitrogen and copper in Puget Sound is forestland runoff, the result of heavy logging and subsequent erosion. The high acidity of urban precipitation likely contributes to the elevated levels of metals found in urban runoff.

The Problem of Characterizing and Controlling Nonpoint Sources

Unlike sewage treatment plants and other point sources which discharge at relatively constant rates, nonpoint sources deliver pollutants in pulses linked to storm events. The quantity and type of pollutant contained in nonpoint sources depends on the human activity, the intensity and duration of precipitation, and the time between storms. The combination of the randomness of rainfall with the varying level of human activity makes controlling nonpoint sources relatively difficult.

In part because of the complex nature of nonpoint sources and in part because nonpoint sources were not historically recognized as a significant

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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pollution source, nonpoint source pollution control efforts have taken a back seat to point source control efforts. From the Clean Water Act's enactment in 1972 until its reauthorization in 1987, total national spending on nonpoint source pollution controls totaled only 6 percent of the amount spent on point source pollution (EPA 1990). In the 1987 Clean Water Act reauthorization, Congress required that the EPA establish a national nonpoint source pollution control program. However, funding for nonpoint source pollution control efforts has been limited. In 1990, the EPA still allocated less than 6 percent of its water pollution control budget to nonpoint sources (GAO 1990).

Because of the relative lack of attention devoted to nonpoint sources, information about their character and the effectiveness of control measures is wanting. This increases the difficulty of implementing control measures.

The Composition of Urban Runoff

Tables D.26 and D.27 show ranges of contaminant levels found in urban runoff for several contaminants of concern, plus lead and suspended solids, as reported in the technical literature. Table D.26 presents average pollutant concentrations. Table D.27 shows annual pollutant loadings from runoff per hectare of land area for overall urban land and four specific land uses: residential, commercial, industrial, and highway. These loadings vary greatly from location to location. Tables D.26 and D.27 show that there is considerable range in the reported data.

Where do these pollutants come from? How does land use affect the quantity of pollution generated? Which pollutants represent the greatest risk?

Urbanization creates nonpoint source pollution problems by increasing the volume of runoff and by broadening the spectrum of substances that accumulate on land. Pavement covers large percentages of urban surface areas and reduces the opportunity for stormwater to filter into the ground. In rural areas or places with large pervious surfaces, surface runoff events are generated only during large storms and snow melt. In urban areas, even relatively small storms (a few tenths of an inch) can create significant runoff.

Urbanization also creates nonpoint source pollution problems by increasing the number of contaminant sources and expanding the variety of contaminants. Any by-product of human activity deposited on an impervious surface and not removed by street cleaning, wind, or decay eventually ends up in surface runoff. The sources of contaminants encompass all human urban activities and include the following:

Traffic—Traffic is directly responsible for the deposition of substantial amounts of toxic hydrocarbons, metals, asbestos, and oils from exhaust

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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TABLE D.26 Pollutant Concentrations in Urban Runoff Ranges of Values Reported in Technical Literature

Contaminant Class

Contaminant

Reported Concentrations

Reference

Nutrients

Nitrogen (mg/l)

5.6 - 7.1

EPA 1983

 

Phosphorus (mg/l)

0.4 - 0.5

EPA 1983

Aesthetics

Oil and grease (mg/l)

4.1 - 15.3

10

13

Stenstrom et al. 1984

Wakeham 1977

Eganhouse and Kaplan 1981

Suspended Solids

TSS (mg/l)

71 - 1,194

141 - 224

SCCWRP 1990

EPA 1983

Metals

Cadmium (lag/l)

Chromium (pg/l)

Copper (ag/l)

3.3 - 4.2

11 - 43

17 - 138

19

38 - 48

 

SCCWRP 1990

SCCWRP 1990

SCCWRP 1990

Marsalek 1986

EPA 1983

 

Lead (lg/l)

23 - 242

90

161 - 204

SCCWRP 1990

Marsalek 1986

EPA 1983

Pathogens

Fecal coliforms

(MPN/ 100 ml)

1,000 - 21,000

> 2,000

EPA 1983

Olivieri et al. 1977

pipes, tire wear, solids carried on tires and vehicle bodies, and lubrication fluid loss.

Litter-Litter—deposits contain items such as cans, broken glass, bottles, pull tabs, papers, building materials, plastic, vegetation, dead animals and insects, and animal waste that eventually wash into storm sewers.

Atmospheric deposition—Cadmium, strontium, zinc, nickel, lead, nutri ents and many organic hazardous chemicals are transported with atmospheric fallout from local or distant combustion sources and city dust. Atmospheric pollutants find their way into runoff by either settling directly to the ground or becoming entrained in falling rain drops. In larger cities, the deposition rate of atmospheric particulates in wet and dry fallout ranges from 63 kg/ hectare/month to more than 270 kg/hectare/month (Novotny and Chesters 1981).

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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TABLE D.27 Annual Pollutant Loadings from Urban Runoff (pollutant loading units = kg/ha-year)

Class

Contaminant

Residential

Commercial

Industrial

Highway

Urban Land

Reference

Metals

Cadmium

0.013 - 0.016

0.016

0.024

N/A

N/A

Marsalek 1978

 

 

0.026 - 0.028

0.028

0.044

N/A

N/A

Marsalek 1978

 

 

0.09 - 0.11

N/A

N/A

N/A

N/A

Whipple et al. 1978

 

Copper

0.045 - 0.049

0.049

0.077

N/A

N/A

Marsalek 1978

 

 

0.03

0.07 -0.13

0.29 - 1.3

N/A

0.02 - 0.21

Sonzogni 1980

 

Lead

0.157 - 0.174

0.174

0.269

N/A

N/A

Marsalek 1978

 

 

0.01- 0.90

2.70

2.7

4.96

N/A

Bannerman et al. 1984

 

 

0.06

0.17 - 1.1

2.2 - 7

N/A

0.14 -0.5

Sonzogni 1980

 

 

1.0 - 2.3

N/A

N/A

N/A

N/A

Whipple et al. 1978

 

PAHs

2.7 x 10-6

5.9 x 10-6

8.4 x 10-5

1.8 x 10-4

2.2 x 10-5

Hoffman et al. 1984, Hoffman 1985

Nutrient

Nitrogen

5.0- 7.3

1.9- 11

1.9- 14

N/A

0.2- 18

Sonzogni 1980

 

 

9-11.2

11.2

7.8

N/A

N/A

Marsalek 1978

 

 

N/A

N/A

N/A

N/A

5

Beaulac and Reckhow 1982

 

 

N/A

N/A

N/A

N/A

2 - 9

Uttormark et al. 1974

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

 

Phosphorus

0.04 - 1.12

1.49

1.49

1.04

N/A

Bannerman et al. 1984

 

 

1.6- 3.4

3.4

2.2

N/A

N/A

Marsalek 1978

 

 

1.2 - 8.0

N/A

N/A

N/A

N/A

Whipple et al. 1978

 

 

N/A

N/A

N/A

N/A

0.3 - 4.8

Sonzogni 1980

 

 

N/A

N/A

N/A

N/A

1

Beaulac and Reckhow 1982

 

 

N/A

N/A

N/A

N/A

1.1

Uttormark et al. 1974

Aesthetics

Oil & Grease

3.67

65.4

26.9

31.6

12.4

Stenstrom et al. 1984

 

 

1.8

5.8

140

78

21

Hoffman et al. 1983

 

 

N/A

N/A

N/A

N/A

12.8

Eganhouse and Kaplan 1981

Sus. Solids

TSS

11 - 487

957

957

979

N/A

Bannerman et al. 1984

 

 

360 - 390

360

672

N/A

N/A

Marsalek 1978

 

 

620 - 2,300

50 -830

450 - 1,700

N/A

200 - 4,800

Sonzogni 1980

 

 

N/A

N/A

N/A

N/A

141 -224

EPA 1983

N/A = Not available

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

Plant debris—In residential areas, fallen leaves and grass clippings dominate street refuse composition. In the fall, a mature tree can produce from 15 to 25 kilograms (dry weight) of organic leaf residue containing significant amounts of nutrients (Heaney and Huber 1973). The fallen leaves are about 90 percent organic and contain from 0.04 percent to 0.28 percent phosphorus, and thus if washed into receiving waters, may produce excess nutrients and oxygen-demand as the leaves decay.

Lawn chemicals—Lawn chemicals can potentially contaminate runoff with pesticides, herbicides, and excess nutrients.

Deicing chemicals—The road salt used in most of the northern United States is composed mainly of sodium chloride with added calcium chloride and calcium sulfate, but sand-salt mixtures applied to snow-covered roads may also contain significant amounts of phosphorus, lead, and zinc (Oberts 1986).

Erosion—Stream channel erosion in an urban watershed is two to three times that under predevelopment conditions. Except at construction sites, urban pervious surfaces in humid areas are usually well protected from sheet and rill erosion by vegetation. However, in arid areas where droughts have forced a transition from lawns to less water-demanding landscapes using native arid plants and wood or stone mulches, erosion of pervious lands can be quite extensive.

Septic systems—Septic systems, though not a problem in sewered urban areas, are a significant source of suburban pollution in the coastal zone. Pollution from septic systems enters water via two potential pathways: subsurface transport of mobile pollutants (primarily nitrate) and effluent surfacing from failing systems.

Cross-connections and illicit discharges into storm sewers—Nonstormwater discharges in storm sewers originate from vehicle maintenance activities and from sewage and industrial wastewater leaking from sanitary sewers and failing septic systems into storm-sewered areas. However, almost any type of pollution may find its way into urban storm sewers by illicit discharges and accidental spills. Deliberate dumping of used oil or waste paint into storm sewers and catch basins is especially common and troublesome. Leaking underground storage tanks and leachate from landfills and hazardous waste disposal sites might also infiltrate sewers. The detection, identification, and elimination of such discharges is a major focus of the EPA's new stormwater permit program.

Combined sewerlsanitary systems—Without special control measures, overflows from combined systems occur on average between 40 to 80 times per year. CSOs harbor all the pollutants found in municipal wastewater, including pathogenic microorganisms, trash, and unpleasant odors, and may carry objectionable debris such as the medical waste found on east coast beaches in recent years. Today, between 15,000 and 20,000 CSO discharge

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

points remain in operation (AMSA 1988), serving a total population of about 37 million. They are contained in 1,100 to 1,300 distinct combined collection systems (EPA 1978).

The Effect of Different Land Uses on Urban Pollutant Loading

Micro-level studies have reported that pollutant concentrations in runoff vary significantly with land use. That does seems logical—in industrial areas and roadways, more combustion occurs and more anthropogenic chemicals are used, so one should expect higher concentrations of combustion byproducts and industrial chemicals in that runoff. However, this logic has not been borne out in the most comprehensive national study of urban runoff carried out to date.

The study, the EPA's National Urban Runoff Project (NURP), evaluated contaminant concentrations in runoff in 28 cities between 1978 and 1983. When NURP researchers compiled the national data and analyzed it statistically, the result was surprising: there appeared to be no statistical correlation between pollutant concentrations and three typical urban land uses—residential, mixed, and commercial (EPA 1983). Only pollutant concentrations from open/nonurban lands were significantly different from the three land-use types.

Having determined that land-use category appears to be of little utility in explaining overall site-to-site, storm-to-storm variability of urban runoff, NURP researchers concluded that for estimating pollutant concentrations at unmonitored sites, the best general characterization may be obtained by pooling data for all land uses (other than the open, nonurban ones).

As shown in Table D.27, the NURP studies do not disprove that land use influences unit loadings of pollutants (expressed as mass per unit land area). Pollutant loadings could vary with land use even if one assumes a constant pollutant concentration for all land uses. The smallest loadings are typical for suburban areas with natural surface drainage, which allows much runoff to percolate through the soils. The highest pollutant loadings are emitted from highly paved, urban, industrial centers in which very little runoff is absorbed by the ground.

And so, the question of how urban land use influences pollutant levels in runoff is unresolved.

The Most Significant Contaminants in Urban Runoff

Tables D.26 and D.27 show that urban runoff may transport measurable quantities of all of the pollutants of concern. Which categories of contaminants are most significant in urban runoff?

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×
Metals and Organics

In addition to analyzing the effect of land use on urban runoff, the NURP studies evaluated nationwide data to determine which pollutants occur most commonly. They found that toxic metals are by far the most prevalent priority pollutants in urban runoff (EPA 1983). Copper, lead, and zinc were present in 91 percent of the samples. Other frequently detected inorganic pollutants included arsenic, chromium, cadmium, nickel, and cyanide. Hazardous organic pollutants were detected less frequently and at lower concentrations than metals. The most commonly found organic waste was the plasticizer bis (2-ethylhexyl) phthalate (found in 22 percent of runoff samples), followed by the pesticide a -hexachloro-cyclohexane (a-BHC) (found in 20 percent of the samples). An additional 11 organic pollutants were reported with detection frequencies between 10 percent and 20 percent. Nutrients and oxygen-depleting organics were also detected but in less significant quantities than both toxic metals and toxic organic chemicals.

Pathogens

The NURP and other studies found levels of fecal coliform bacteria in urban runoff high enough to signal a potential health risk, as shown in Table D.26. However, the degree of health risk posed by these coliform levels is unknown. Since fecal coliforms in urban runoff come mainly from animal waste, fecal coliform counts may not be useful in identifying human health risks. Some researchers have reported the presence of human pathogens in runoff. For example, Olivieri et al. (1977) found that Baltimore storm runoff samples with fecal coliform densities greater than 2,000/100 milliliters were 95 percent positive for Salmonella. Ranges of Salmonella densities in urban runoff from Baltimore were less than 1 to more than 11,000 per 10 liters. These researchers also analyzed six stormwater flows for viruses; all tested positive (Olivieri et al. 1977).

Suspended Solids: Pollutant Transporters

The NURP study found that urban runoff carries high quantities of sediment. This is significant from a water quality standpoint because other studies have shown that a large portion of pollutants carried in runoff are bound to sediment. For example, Marsalek (1986) measured much higher concentrations of metals and PAHs in runoff-borne sediment than in the runoff water itself. Table D.28 shows pollutant concentrations in runoff solids as measured by Ellis (1986) for different land uses.

That a large quantity of pollutants is bound to solids is important for

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.28 Pollutant Concentrations in Solids from Urban Runoff (After Ellis 1986. Reprinted, by permission, from Springer-Verlag New York, Inc., 1986.)

Class

Pollutant

Residential

Commercial

Industrial

Highways

Metals

Cadmium (jig/gram)

2.7 - 3.2

2.9

3.6

2.1 - 10.2

Metals

Lead (jlg/gram)

1,570 - 1,980

2,330

1,390

450 - 2,346

Pathogens

Fecal coli (MPN/gram)

25,621 - 82,500

36,900

30,700

18,768 - 38,000

Nutrients

Nitrogen (lig/gram)

460 - 610

410 - 420

430

223 - 1,600

reducing pollutant loads delivered by runoff. It means that control measures designed to remove solids will also remove other contaminants. However, a complicating factor is that most pollutants are associated with smaller particles. Studies by Sartor et al. (1974) indicated that more than 50 percent of the sediment-borne phosphorus is contained in the 6 percent of particles smaller than 43 microns. Consequently, systems designed to remove only larger solids will be ineffective for removing most solids-borne pollutants.

Management Options
Introduction

In comparison to wastewater treatment technology, urban runoff treatment technology is in its infancy. By 1991, only the states of Florida and Maryland required urban runoff treatment. There are only a few cities with such requirements, notably Seattle and Bellevue, Washington. Thus, it is from these areas that the field has gained most of the knowledge about the design and performance of urban runoff quality controls. There are few data on the performance of these controls because monitoring of facility performance is not required. For information on removal efficiencies, it is necessary to rely on data gathered in the National Urban Runoff Program conducted in the late 1970s and early 1980s (EPA 1983) and miscellaneous research studies conducted since. For the most part, these data were collected on existing prototype facilities, most of which had not been designed with stormwater treatment in mind but rather simply as drainage facilities. Some had to be retrofitted with other controls to increase detention time. This circumstance limits the usefulness of the data for developing design criteria.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

Still, a lot has been learned. Sufficient information is available to design structures into the drainage systems that will definitely improve the quality of urban runoff. Exactly how much improvement will result from a given system is difficult to predict, but estimates are possible. The following sections present an overview of current knowledge of urban runoff quality controls. They address structural and nonstructural types of control, design hydrology and pollutant removal efficiencies for structural controls, cost for implementation, and next steps required in applications and research.

Types of Controls and Control Philosophy

There are two basic controls of pollution in urban runoff: source reduction and structural control. Source reduction prevents the pollutants from ever coming in contact with rainwater or runoff. When it is cost-effective, source reduction is the better approach because the pollutant never gets into the runoff and therefore never enters receiving waters. Source reduction practices include street sweeping, and mitigation of illicit connections and illegal dumping. Many also consider land-use regulations and restrictions as a form of source reduction. Most source reductions can be used in existing as well as in new developments.

Structural controls are those that remove pollution from urban runoff by either reducing the amount of runoff or by providing some type of treatment. Controls that reduce the amount of runoff include reducing impervious areas and increasing infiltration. Treatment controls include sedimentation and biological removal. Typical structural controls include grassy swales, buffer strips, infiltration devices, detention basins, and wetlands.

Source Reduction of Pollution in Urban Runoff

Source reduction of pollution in urban runoff prevents or minimizes the potential of pollutants into contact with rainfall or runoff. Source reduction controls are viewed by most nonpoint source experts as the most cost-effective runoff quality control for developed urban areas, especially for developments more than 20 or 25 years old. The most common source reduction measures are

  • elimination of illicit connections,

  • mitigation of illegal dumping,

  • coverage of chemical storage areas,

  • prevention and containment of spills,

  • minimization of chemical applications,

  • street sweeping and catch basin cleaning, and

  • erosion control.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

Illicit connections are defined by the EPA as connections to storm drains that contain discharges other than surface runoff. They may or may not be illegal, but they simply do not belong there. Illicit connections might be direct or indirect. A direct connection would be a sanitary sewer connected directly to a storm drain or the connection of a floor drain to the storm drain. A study in Michigan (Murray 1989) revealed that over 60 percent of service stations and auto repair stores had connections directly to storm drains. These connections accounted for 35 percent of all illicit connections to stormsewers in the study area.

In this report, illegal dumping means the intentional disposal of chemicals and waste materials into a storm drainage system. Some illegal dumping is done in known violation of existing laws; other illegal dumping is done simply in ignorance. Examples of the former are septage haulers emptying their trucks into storm sewers and chemical users washing their excess or waste supplies into storm drains. Ignorance is generally the case when an individual changes oil over a catch-basin, dumps waste household chemicals into the gutter, or sweeps yard trash into a catch-basin.

Covering chemical storage areas, loading docks, and other areas at industrial or commercial facilities where contact with rainfall or runoff occurs is a cost-effective control. Spill prevention and containment is a housekeeping measure. Storing chemicals away from storm drains and diking chemical storage areas can prevent contaminants from entering storm sewers.

Control of chemical application rates in areas exposed to rainfall and runoff is another source reduction measure. This control applies to fertilizer, pesticide, and herbicide application in private and public properties for gardening and pest control. There has been significant progress over the past 10 years in developing technology to minimize highway deicing application rates without compromising safety (Lord 1989).

Street sweeping is a debated control practice because it is difficult to determine its effectiveness, while more benefits are accruable to catch-basin cleaning.

Surface erosion from developed watersheds is generally less than that under natural conditions. However, during construction, it can be a major problem. The principal impact on receiving water is due to sedimentation since erosion from construction sites usually consists of natural soil. Although the technology for erosion control on construction sites is well-developed (VSCC 1984), erosion control requirements for construction sites are rarely enforced.

Land-use controls include zoning to minimize pollutant loads to receiving waters. Runoff pollution is directly related to the intensity of development. Low-density residential development is characterized by having a smaller impervious area and smaller amount of human activity than higher

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

density residential and/or commercial areas. Thus, where runoff controls are difficult to implement due to terrain or topography such as steep slopes or shorefront development, zoning for low density development becomes a source reduction practice. While density limitations have not been used much in the past due to pressure from developers, land-use controls are likely to be used more extensively in shoreline areas, especially estuarine waters, for runoff quality management, in the future.

Source reduction is implemented by two methods: education and ordinances. Education is an important component of source reduction strategies because the public generally has little idea about where storm drains go. Many communities are embarking on public education programs that include workshops, videotapes, and flyers alerting the public to runoff pollution and what they can do to reduce it. The city of Ann Arbor, Michigan, has undertaken a program to paint a fish on storm drain inlets accompanied by the text: ''Do not dump, drains to stream." Appendix E provides further information on the role of education.

Ordinances are most effective in controlling illicit connections, illegal dumping, chemical storage location and covering, erosion from new construction sites, and land use. However, ordinances are ineffective unless they are backed up with adequate enforcement.

Structural Controls

Structural controls are those devices that are designed into a drainage system to remove pollutants from runoff. These devices include swales, filter strips, infiltration basins and trenches, detention facilities, and artificial wetlands. Treatment facilities used for municipal wastewater and industrial wastes are not generally considered viable for stormwater treatment because 1) they are extremely expensive, and 2) they are designed to perform under continuous and fairly uniform loading. Stormwater runoff is highly variable in the frequency and magnitude of both runoff and pollutant concentrations; therefore it does not lend itself to treatment by those practices.

It is commonly but erroneously thought that design storms for sizing water quality controls should be the same as those used for the design of drainage facilities. Design storms for urban drainage systems are large, infrequent storm events ranging from the 5-year storm to the 25-year storm. But the damage done to a receiving water ecosystem by uncontrolled pollutant wash-off in the 25-year storm is inconsequential compared with the hydraulic damage that results naturally to aquatic habitats from such a storm. The same can be said for the five-year storm. Design storms for runoff quality control are small, frequent events smaller than the 1-year storm.

Figure D.9a shows the percentage of annual runoff that will be captured

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

by detention basins of various sizes for six U.S. cities. This information was obtained by analyzing hourly rainfall data over more than 10 years at each city. A 24-hour drawdown time for each basin was used because that is the detention time required for effective pollution removal (Grizzard et al. 1986). For all six cities, less than 1 inch (0.03 million gallons per acre) of detention storage is required to capture 90 percent of the runoff and detain it for 24 hours; for Cincinnati, Detroit, Tucson, and Butte, less than 0.5 inches (0.014 MG/acre) is required. Further study of these curves indicates that the most cost-effective basin size (generally taken at the knee of the curve) varies from 0.7 inches in San Francisco to 0.18 inches in Butte and captures 80 percent and 90 percent of the annual runoff respectively. Thus 80 percent or 90 percent capture of annual runoff would appear to be a good technology-based standard for urban runoff quality control.

Figure D.9b shows how many times per year each basin is expected to fill and overflow. The black dot on Figure D.9b indicates the basin volume required for each city to capture 90 percent of the annual runoff. This figure shows that the overflow frequency of basins sized to capture 90 percent of the runoff volume overflow three to ten times per year, suggesting that the design storm is on the order of about a one-month to a four-month storm.

Structural controls vary from site controls. Site controls attempt to reduce runoff rate and volume at or near the point where the rainfall hits the ground surface. Regional controls (usually detention devices) serve an area of 50 to 100 acres. The following types of structural controls are common:

  • minimization of directly-connected impervious areas,

  • swales and filter strips,

  • porous pavement and parking blocks,

  • infiltration devices such as trenches and basins, and

  • detention devices.

Minimization of Directly Connected Impervious Areas. Impervious areas are those that do not absorb any rainfall—rooftops, sidewalks, driveways, streets, and parking lots. The minimization of directly connected impervious areas is by far the most effective method of runoff quality control because it delays the concentration of flows into the improved drainage system and maximizes the opportunity for rainfall to infiltrate at or near the point at which it falls.

Swales and Filter Strips. Swales, or grassed waterways, and filter strips are among the oldest stormwater control measures, having been used alongside streets and highways for many years. These devices slow the runoff, giving solids an opportunity to settle or be filtered out by vegetation

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

FIGURE D.9a Runoff capture efficiency versus unit storage volume. (Source: Roesner et al. 1991. Reprinted, by permission, from American Society of Civil Engineers, 1991.)

FIGURE D.9b Basin overflow frequency versus unit storage volume. (Source: Roesner et al. 1991. Reprinted, by permission, from American Society of Civil Engineers, 1991.)

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

and provide an opportunity for the runoff to infiltrate into the ground. To be useful as a treatment device, a swale should have a flat bottom or very flat side slopes so that the water-quality design storm can be conveyed with less than 3 inches of water in the channel. The channel should be planted with vegetation suitable for soil stabilization and nutrient uptake.

A filter strip is simply a mildly sloped strip of land across which stormwater from a street, parking lot, rooftop, or other impervious surface sheet-flows before entering adjacent receiving waters. Minimum length of a filter strip should be 20 feet, and its slope should be such that erosion does not occur, except infrequently during large storms.

Swales and filter strips are widely used for urban runoff treatment in Florida and metropolitan Seattle. They have been used to a limited extent in Virginia and Maryland. They are best suited to non-arid areas.

Porous Pavement and Parking Blocks. Porous pavement has excellent potential for use in parking areas. When properly designed and carefully installed and maintained, porous pavement can have load-bearing strength and longevity similar to conventional pavement. In addition, porous pavement can help to reduce the amount of land needed for stormwater management, preserving the natural water balance at a given site. However, porous pavement is only feasible on sites with permeable soils, fairly flat slopes, and relatively deep water table and bedrock levels. The risk of clogging is high, and once clogging occurs it is difficult and costly to correct.

Another effective site-control device is parking blocks or modular pavement. These are hollow concrete blocks similar to, but smaller than, those used in construction. In parking lots for retail stores, sports arenas, civic theaters, etc., where more than half the parking area is used less than 20 percent of the time, the use of parking blocks in the less-used portions gives them a more attractive appearance and reduces runoff quantity, flow rates, and pollution from these areas.

Infiltration Devices. Infiltration devices are those stormwater quality control measures that completely capture runoff from the water-quality design storm and allow it to infiltrate the ground. They are the most effective stormwater quality control devices that can be implemented because pollutants in the infiltrated flow are removed from the runoff. Advantages of infiltration devices are that they help to maintain the natural water balance of a site and can be integrated into a site's landscaped and open areas. Disadvantages can include a fairly high rate of failure due to unsuitable soils; the need for frequent maintenance; and possible nuisance factors, such as odors, mosquitos, or soggy ground.

An infiltration basin is made by constructing an embankment or by excavating in or down to relatively permeable soils. The basin will tempo-

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

rarily store stormwater until it infiltrates through the bottom and sides of the system. However, the infiltration basin can actually be a landscape depression within open or recreational areas. Infiltration basins generally serve areas ranging from a front yard to areas of five or ten acres in size.

Infiltration trenches, which can be located on the surface of the ground or buried beneath the surface, are usually designed to serve areas of from five to ten acres and are especially appropriate in urban areas where land costs are very high. Stored runoff infiltrates into the surrounding soil.

Detention Devices. Detention basins are widely used throughout the United States to reduce runoff peaks for drainage control (i.e., peak-flow attenuation of the 2-, 10-, 25-, and 100-year storms). They are designed with fixed flow rate outfall pipes. Because these basins are designed for peak shaving of runoff from large storms, the small storms of interest for urban runoff quality control pass through them with little or no detention or pollutant removal. (Peak runoff rates for small storms are much less than the outlet capacity of these detention basins.) However, studies in Virginia and Maryland (Grizzard et al. 1986) and at several other locations (Zariello 1989) indicate that these basins can be effectively retrofitted with adjustable outflow controls to increase detention times and pollutant removal efficiency for small storms. Care must be taken with these retrofit devices not to reduce the flood control effectiveness for which these facilities were designed.

For treatment of urban runoff, detention devices fall into three basic categories: extended detention, detention with filtration, and wet detention. The removal mechanism of extended detention is sedimentation; detention with filtration combines sedimentation with filtration; wet detention incorporates sedimentation and biochemical removal.

Extended detention basins are the most common type of detention basin used around the country. These basins are dry between storms and capture small storms and the first flush from big storms. They then release it slowly. Since stormwater pollutants tend to be associated primarily with very small particles (very fine sands, silts, and clays), relatively long detention times (20 to 40 hours) are required to achieve appreciable removal of suspended pollutants. Even with such extended detention times, however, removal of dissolved stormwater pollutants does not occur. Typical removal efficiencies for extended detention basins are (Hartigan 1989):

TSS: 80% - 90%

TN: 20% - 30%

Pb: 70% - 80%

BOD: 20% - 40%

TP: 20% - 30%

Zn: 40% - 50%

One of the stormwater treatment practices commonly used in Florida is detention coupled with filtration in which stored stormwater is discharged

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

through a filter. Typical filtration systems include bottom or side-bank sand or natural soil filters. Experience in Florida and Texas indicates significant difficulties associated with the design, construction, and especially the maintenance of stormwater filters. It is not a question of if a filter will clog, but when, and who will maintain the filter when clogging becomes a problem. Thus filters should be used when timely maintenance is assured. Livingston et al. (1988) describes design details.

Wet detention basins are characterized by a permanent pool of water and a shallow littoral zone around the perimeter that occupies 30 percent to 50 percent of the pond area. The volume of the pond itself is equal to the runoff from the wettest two weeks of an average year. This volume of water is significant in most locales, and is what distinguishes a wet pond from an extended detention basin that may have a small permanent pool associated with it. The removal of pollutants in a wet detention system is accomplished by gravity settling and biological uptake of nutrients by aquatic plants and phytoplankton metabolism. A wet pond is the best detention facility for use in locations where nutrients are of concern because they remove two to three times as much phosphorus as extended detention ponds and 1.3 to two times as much total nitrogen, if the plants are harvested.

Artificial Wetlands for Stormwater Quality Enhancement. Wetlands provide water quality enhancement through sedimentation, filtration, absorption, and biological processes. They also provide flood protection through water storage and conveyance. The incorporation of wetlands into a comprehensive stormwater management system achieves wetland preservation and revitalization (Hartigan 1989). While much research has been completed on the ability of wetlands to remove wastewater pollutants (EPA 1985b, Martin 1988), many questions remain. For example, how long can a wetland continue to remove stormwater pollutants effectively? What type of maintenance is required and at what frequency? What is the ultimate fate of pollutants in wetland habitats and how do they affect the wetland ecosystem? Furthermore, treatment wetlands fall under the jurisdiction of the federal wetlands protection law (Clean Water Act, Section 401[k]), which limits the way in which routine maintenance can be done. Design guidance relative to constructed wetlands can be found in Maryland Water Resources Administration (1987) and Livingston (1989).

Retrofitting Structural Controls to Existing Developments. The idea of retrofitting structural controls into an existing setting appears a formidable task at first blush. However, there are a number of ways to retrofit at reasonable cost.

Two devices that are fairly simple to retrofit are oil-water separators and water quality inlets. These can be installed at stormwater inlets in park-

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

ing lots, service stations, and other areas where oils and greases may be in the runoff. Infiltration trenches have been retrofitted at a number of roadways in Maryland. Also, the city of Orlando, Florida, replaced its entire downtown storm drainage system with infiltration trenches in order to protect its urban lakes from runoff pollution.

Other devices such as street replacement or storm sewer system improvements can be retrofitted as part of infrastructure repairs. For example, porous pavement, modular pavement, or geotextile fabric should be considered as a replacement in parking areas that are in need of upgrading.

There are many ways to retrofit runoff treatment devices to existing development. To do so requires ingenuity and knowledge of how specific treatment devices work to remove pollutants from runoff.

Pollutant Removal Efficiencies of Various Treatment Practices. Table D.29 shows the relative efficiency of various urban runoff quality controls in removing pollutants. This table is based on data collected in the late 1970s and early 1980s. More recent data (Roesner et al. 1989) indicate that wet ponds, filter strips, and swales perform much better than shown, if properly designed. Notice that where suspended sediment removal is good, removal of other pollutants is good. This is because many of the noxious pollutants in urban runoff are attached to particulate matter. As a rule of thumb, if the solids can be removed from the runoff, most of the noxious pollutants will also be removed. This rule does not hold true for nitrogen or bacteria.

Table D.30 shows removal efficiencies for constituents of concern. The table was developed under the assumptions that 1) copper, cadmium, and chromium will behave in the same way as trace metals and lead, 2) PAHs will be associated primarily with suspended solids and behave in the same way as heavy metals, 3) coliforms are an adequate predictor of enterovirus, and 4) oil and grease are 80 percent to 100 percent removed in any properly designed stormwater treatment device.

Rating of Runoff Treatment Practices

The matrix in Table D.31 provides a relative rating of structural controls. This table summarizes the characteristics of the practices described above. It is noteworthy that, in contrast to wastewater treatment processes, most of the practices are not operationally difficult.

Costs for Stormwater Quality Controls

Cost data for source controls are not available. Studies of source control practices (Murray 1989) do not contain information on costs for illicit connection detection or removal. Little information is available on costs of

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.29 Comparative Pollutant Removal of Urban Runoff Quality Controls (From Schueler 1987. Reprinted, by permission, from Metropolitan Washington Council of Governments, 1987.)

 

structural controls other than that collected by the Metropolitan Washington Council of Governments (MWCOG). A MWCOG study (Wiegand et al. 1986) drew together cost data from a survey of engineering estimates and bids for 65 infiltration and detention facilities built since 1982 in the Metropolitan Washington area. Based on these data, regression equations for cost versus volume were developed.

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

 

TABLE D.30 Comparative Removal of Pollutants of Concern by Runoff Treatment Practices

 

Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
×

TABLE D.31 Rating of Runoff Treatment Practices

 

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Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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Page 392
Suggested Citation:"D ENGINEERING AND MANAGEMENT OPTIONS FOR CONTROLLING COASTAL ENVIRONMENTAL...." National Research Council. 1993. Managing Wastewater in Coastal Urban Areas. Washington, DC: The National Academies Press. doi: 10.17226/2049.
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Page 393
Next: E POLICY OPTIONS AND TOOLS FOR CONTROLLING COASTAL ENVIRONMENTAL WATER QUALITY »
Managing Wastewater in Coastal Urban Areas Get This Book
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Close to one-half of all Americans live in coastal counties. The resulting flood of wastewater, stormwater, and pollutants discharged into coastal waters is a major concern. This book offers a well-delineated approach to integrated coastal management beginning with wastewater and stormwater control.

The committee presents an overview of current management practices and problems. The core of the volume is a detailed model for integrated coastal management, offering basic principles and methods, a direction for moving from general concerns to day-to-day activities, specific steps from goal setting through monitoring performance, and a base of scientific and technical information. Success stories from the Chesapeake and Santa Monica bays are included.

The volume discusses potential barriers to integrated coastal management and how they may be overcome and suggests steps for introducing this concept into current programs and legislation.

This practical volume will be important to anyone concerned about management of coastal waters: policymakers, resource and municipal managers, environmental professionals, concerned community groups, and researchers, as well as faculty and students in environmental studies.

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