4

Wastewater Reclamation Technology

Treatment processes in wastewater reclamation are employed either singly or in combination to achieve reclaimed water quality goals. Considering the key unit processes and operations commonly used in water reclamation (see Figure 4-1), an almost endless number of treatment process flow diagrams can be developed to meet the water quality requirements of a certain reuse application.

Many factors may affect the choice of water reclamation technology. Key factors include the type of water reuse application, reclaimed water quality objectives, the wastewater characteristics of the source water, compatibility with existing conditions, process flexibility, operating and maintenance requirements, energy and chemical requirements, personnel and staffing requirements, residual disposal options, and environmental constraints (Asano et al., 2007). Decisions on treatment design are also influenced by water rights, economics, institutional issues, and public confidence. The relative importance of some of these factors is likely going to change in the future. With the current desire to limit greenhouse gas emissions and introduction of carbon taxes, energy-intense processes likely will be viewed much less favorable than today. This chapter focuses on treatment processes—characterized as preliminary, primary, secondary, and advanced and including both natural and engineered processes—that can be used to meet water quality objectives of a reuse project and their treatment effectiveness. The efficiency in removing certain constituent classes, energy requirements, residual generation, and costs of these treatment processes are qualitatively summarized in Table 4-1. Economic, social, and institutional considerations that also influence the choice of reclamation technologies are addressed in Chapters 9 and 10.

PRELIMINARY, PRIMARY, AND SECONDARY TREATMENT

Wastewater treatment in the United States typically includes preliminary treatment steps in addition to primary and secondary treatment. Preliminary steps include measuring the flow coming into the plant, screening out large solid materials, and grit removal to protect equipment against unnecessary wear. Primary treatment targets settleable matter and scum that floats to the surface. As shown in Table 2-1, only 1.3 percent of wastewater treatment plant effluents in the United States are discharged after receiving less than secondary treatment because of site-specific waivers (EPA, 2008b).

Secondary treatment processes are employed to remove total suspended solids, dissolved organic matter (measured as biochemical oxygen demand), and, with increasing frequency, nutrients. Secondary treatment processes usually consist of aerated activated sludge basins with return activated sludge or fixed-media filters with recycle flow (e.g., trickling filters; rotating biocontactors), followed by final solids separation via settling or membrane filtration and disinfection (Figure 4-1) (Tchobanoglous et al., 2002).

Advances over the past 20 years in membrane bioreactor (MBR) technologies have resulted in an alternative to conventional activated sludge processes



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 67
4 Wastewater Reclamation Technology Treatment processes in wastewater reclamation are cial, and institutional considerations that also influence employed either singly or in combination to achieve the choice of reclamation technologies are addressed in reclaimed water quality goals. Considering the key Chapters 9 and 10. unit processes and operations commonly used in water reclamation (see Figure 4-1), an almost endless number PRELIMINARY, PRIMARY, AND of treatment process flow diagrams can be developed to SECONDARY TREATMENT meet the water quality requirements of a certain reuse application. Wastewater treatment in the United States typi- Many factors may affect the choice of water recla- cally includes preliminary treatment steps in addition mation technology. Key factors include the type of wa- to primary and secondary treatment. Preliminary steps ter reuse application, reclaimed water quality objectives, include measuring the flow coming into the plant, the wastewater characteristics of the source water, com- screening out large solid materials, and grit removal to patibility with existing conditions, process flexibility, protect equipment against unnecessary wear. Primary operating and maintenance requirements, energy and treatment targets settleable matter and scum that floats chemical requirements, personnel and staffing require- to the surface. As shown in Table 2-1, only 1.3 percent ments, residual disposal options, and environmental of wastewater treatment plant effluents in the United constraints (Asano et al., 2007). Decisions on treatment States are discharged after receiving less than second- design are also influenced by water rights, economics, ary treatment because of site-specific waivers (EPA, institutional issues, and public confidence. The relative 2008b). importance of some of these factors is likely going to Secondary treatment processes are employed to change in the future. With the current desire to limit remove total suspended solids, dissolved organic matter greenhouse gas emissions and introduction of carbon (measured as biochemical oxygen demand), and, with taxes, energy-intense processes likely will be viewed increasing frequency, nutrients. Secondary treatment much less favorable than today. This chapter focuses processes usually consist of aerated activated sludge on treatment processes—characterized as preliminary, basins with return activated sludge or fixed-media primary, secondary, and advanced and including both filters with recycle flow (e.g., trickling filters; rotating natural and engineered processes—that can be used to biocontactors), followed by final solids separation via meet water quality objectives of a reuse project and their settling or membrane filtration and disinfection (Fig- treatment effectiveness. The efficiency in removing cer- ure 4-1) (Tchobanoglous et al., 2002). tain constituent classes, energy requirements, residual Advances over the past 20 years in membrane generation, and costs of these treatment processes are bioreactor (MBR) technologies have resulted in an qualitatively summarized in Table 4-1. Economic, so- alternative to conventional activated sludge processes 67

OCR for page 67
68 WATER REUSE Engineered Unit Processes and Operation Engineered Natural Processes Preliminary Primary Secondary Advanced Product Water duct W r Product Water roduct Product Water roduct Product W duct Water Low rate processes S Stabiliza on ponds D Disinfec on Disinfec on Disinfec on Aerated lagoons High rate processes Ac vated sludge Trickling filters Organics O i Managed Aquifer dA Nitrogen Removal Rota ng bioreactors R Removal Recharge Nitrifica on/ Carbon Soil-aquifer denitrifica on adsorp on treatment Gas stripping Oxida on High rate processes Riverbank Break point filtra on (ozone, AOP) Membrane chlorina on bioreactors Ar ficial recharge Nanofiltra on and recovery R Reverse Osmosis Phosphorus Removal Wetlands Dissolved Solids Biologically Removal Natural Chemically So ening Constructed Screening Electrodialysis Se Sedimenta on Sedimenta on Grit removal Nanofiltra on Suspended Solids S d d S lid Reverse Osmosis Removal Reservoirs Chem. Coagula on Filtra on Sludge P essing Sludge Processing R Residuals: -B -Brines -S -Spent carbon Disposa Disposal -S -Spent resins Disposal al -Sludges FIGURE 4-1 Treatment processes commonly used in water reclamation. Note that some or all of the numerous steps represented under advanced processes may be employed, depending on the end-product water quality desired and whether engineered natural processes are also used. All possible combinations are not displayed here. that does not require primary treatment and second- ever, membrane fouling and its consequences regarding ary sedimentation (LeClech et al., 2006). Instead, plant maintenance and operating costs limit the wide- raw wastewater can be directly applied to a bioreac- spread application of MBRs (LeClech et al., 2006; van tor with submerged microfiltration or ultrafiltration Nieuwenhuijzen et al., 2008). Challenges that require membranes. These applications may only employ a fine research relate to maintaining productivity (or flux, i.e., screen as a preliminary treatment step. MBR processes the amount of water produced per membrane area) and combine the advantage of complete solids removal, a minimizing the effects of membrane fouling. Other significant disinfection capability, high-rate and high- MBR research needs include the effluent quality that efficiency organic and nutrient removal, and a small can be achieved and improvements in oxygen transfer footprint (Stephenson et al., 2000). In the past 10 years, and membrane aeration to lower operational costs of reductions in the cost of membrane modules, extended MBRs (van Nieuwenhuijzen et al., 2008). life expectancy of the membranes, and advances in In the United States, 45 percent of wastewater process design and operation have resulted in many treatment plant effluent as of 2004 received only pri- domestic and industrial applications using MBRs. Its mary and secondary treatment (see Table 2-1). EPA integrated design, which can be scaled down more eas- (2008b) reported that 49 percent of all wastewater ily than conventional secondary treatment processes, treatment plant effluent received “greater than second- can facilitate decentralized water reclamation. How- ary” treatment. This could include MBR treatment or

OCR for page 67
TABLE 4-1 Treatment Processes and Efficiencies to Remove Constituents of Concern during Water Reclamation Constituents of Concern Pathogens Bromate Trace Organics and Energy Residual Process Protozoa Bacteria Viruses Nitrate TDS Boron Chorate Metals DBPs Nonpolar Polar Requirements Generationa Cost Engineered Systems: Physical Filtration Moderate Moderate Low None None None None Low None None None Low Low Low PAC/GAC Low Low Low None None None Low Low Moderate High Low Low Low Moderate MF/UF High Moderate Low None None None Low Low Low Low None Moderate Low Moderate NF/RO High High High High High Moderate High High Moderate High High High High High Engineered Systems: Chemical Chloramine Low Moderate Low None None None None None None None None Low None Low Chlorine Moderate High High None None None None None None Low to Low to Low None Low moderate moderate Ozone Moderate High High None None None None None Low High High High None High UV High High Moderate None None None None None None None None Moderate None Low High High High None None None None None Low High High High None High UV/H2O2 Engineered Systems: Biological BAC Low Low Low None to None None Low Low Low to Moderate Moderate Low None to low Low low moderate Natural Systems SAT High High Moderate High None None Low to High High High Moderate Low None Low moderate to High Riverbank High High Moderate High None None Low to High High High Moderate Low None Low Filtration moderate to High Direct inj. Moderate Low Low Low None None Low to High Moderate Low None Moderate None Low to moderate moderateb ASR Moderate Moderate Moderate Moderate None None Low High Moderate Moderate Low to Low None Low moderate Wetlands Low to Low to Low Moderate None None Low Moderate Low Low to Low Low None Low moderate moderate to high moderate Reservoirs Low to Low Low Low to None None Low Moderate Low Low Low Low None Low moderate moderate to high NOTE: The qualitative values in the table represent the consensus best professional judgment of the committee. aLow represents little generation of residuals, high represents significant amounts of residual generation; bHigh when required pretreatment is considered. 69

OCR for page 67
70 WATER REUSE any combination of the treatment processes described methods for disinfection, as discussed later in this in the following sections. chapter. The effectiveness of each of the disinfectants against pathogens is a function of the amount of dis- DISINFECTION infectant added, the contact time provided, and water Disinfection processes are those that are deliber- quality variables that may compete for the disinfectant ately designed for the reduction of pathogens. Patho- or modulate its effectiveness. Once decay (or in the case gens generally targeted for reduction are bacteria (e.g., of UV, absorbance of energy) is taken into account, a Salmonella, Shigella), viruses (e.g., norovirus, adenovi- first approximation to effectiveness is the product of rus), and protozoa (e.g., Giardia, Cryptosporidium) (see residual concentration (C) (or in the case of UV deliv- also Chapter 3). ered power intensity [I]) and contact time (t). There Common agents used for disinfection in waste- is a relationship between the C∙t “product” (actually water reclamation are chlorine (applied as gaseous integrated over the contact time of a disinfection reac- chlorine or liquid hypochlorite) and ultraviolet (UV) tor, taking into account hydraulic imperfections) and irradiation. Only chlorine is purchased as a chemi- the degree of microbial inactivation. This concept is cal in commerce. Chlorine dioxide, ozone, and UV schematically illustrated in Figure 4-2. are generated on-site. In drinking water applications, The relationships between C ∙t a nd microbial chlorine and hypochlorite remain the most common inactivation may be affected by water quality (e.g., disinfectants, although they are decreasing in preva- temperature, turbidity, pH). For chlorine in particular, lence (Table 4-2). Chloramines are formed from either there is a strong effect of pH, with disinfection being chlorine or hypochlorite if appropriate amounts of am- more effective below pH 7.6 (when hypochlorous acid monia are present (as in wastewater) or if ammonia is [HOCl] predominates) than above pH 7.6 (where hypochlorite [OCl–] predominates) (Fair et al., 1948). deliberately added. Although chlorine or hypochlorites are still the most prevalent disinfection processes used The impact of turbidity on disinfection has been known in wastewater applications, UV is much more common for a long time and is particularly problematic in dis- and chlorine dioxide and ozone are less common than infection of wastewater effluents (Hejkal et al., 1979). However, in drinking water, when the turbidity is <1 in drinking water applications (Asano et al., 2007). Membrane processes can also remove many pathogens, turbidity unit (TU), the effect of turbidity on disinfec- although they are not considered reliable stand-alone tion is minimal (LeChevallier et al., 1981). This has also been confirmed on experiments with actual waters, demonstrating that 0.45-µm filtration had minimal effect on disinfection of water (by chlorine or chlorine TABLE 4-2 Drinking Water Disinfection Practices dioxide) in waters with initial turbidity <2 TU (Barbeau According to 1998 and 2007 AWWA Surveys et al., 2005). Other water quality factors, the nature of which remains unknown, may modulate disinfection Percent of Drinking Water Utilities Using effectiveness for both chlorine (Haas et al., 1996) and Disinfectant 1998 2007 ozone (Finch et al., 2001). It should also be noted that disinfection and the competing decay and demand pro- Chlorine gas 70 61 Chloramines 11 30 cesses are nonlinear. Therefore, a more detailed consid- Sodium hypochlorite 22 31 eration of these nonlinearities as coupled to hydraulics Onsite generation of hypochlorite 2 8 Calcium hypochlorite 4 8 is needed for a full engineering design (Bellamy et al., Chlorine dioxide 4 8 1998; Bartrand et al., 2009). Ozone 2 9 In general, in most disinfection approaches ex- UV 0 2 cept UV, bacteria are more easily disinfected (lower NOTE: Percentages sum to more than 100 because some utilities use multiple disinfectants. required C∙t) than viruses, which are in turn more SOURCE: AWWA Disinfection Systems Committee (2008); AWWA easily disinfected than protozoa. With UV, protozoa, Water Quality Division Disinfection Systems Committee (2000). are somewhat more sensitive than viruses (particularly

OCR for page 67
71 WASTEWATER RECLAMATION TECHNOLOGY FIGURE 4-2 Required “C∙t” or “I∙t” for 99 percent inactivation of different organisms by different disinfectants at pH 7, 20-25 °C. SOURCE: Crittenden et al. (2005). adenovirus, the most UV-resistant class of viruses) R02129 produce disinfection byproducts analogous to those can Figure roduced by chlorination (Tibbets, 1995; Richardson p4-2 ( Jacangelo et al., 2002). bitmapped 1994; van Nieuwenhuijsen et al., 2000; Hua and Chemical disinfectants (i.e., chlorine, ozone, chlo- et al., rine dioxide) are known to produce characteristic dis- Reckhow, 2007). infection byproducts (Minear and Amy, 1996; see also Chapter 3). The spectrum of these will not be reviewed ADVANCED ENGINEERED TREATMENT in this report, but in general, chlorine and ozone can re- act with organic materials to produce stable disinfection Advanced engineered unit processes and operations byproducts (which may or may not be halogenated). can be grouped into engineering systems targeting the For chlorine, these include trihalomethanes, trihalo- removal of nutrients and organic constituents, reduc- acetic acids, haloaldehydes, and haloamines. Ozone tion of total dissolved solids (TDS) or salinity, and can react with bromide that may be present to produce provision of additional treatment barriers to pathogens bromine and, in turn, brominated byproducts, includ- (Figure 4-1). Nutrients can be reduced by biological ing bromate. Chlorine dioxide can produce chlorite and nitrification/denitrification processes, gas stripping, chlorate, and depending on the mode of production of breakpoint chlorination, and chemical precipitation. chlorine dioxide, chlorine may also be present, which Organic constituents can be further removed by various

OCR for page 67
72 WATER REUSE advanced processes, including activated carbon, chemi- upon a more consistent feedwater quality and steady cal oxidation (ozone, advanced oxidation processes operational conditions. Biological and chemical phos- [AOPs]), nanofiltration (NF), and reverse osmosis phorus removal can result in effluent concentrations of (RO). Dissolved solids are retained during softening, less than 0.5 mg P/L (see Table 3-2). electrodialysis, NF, and RO. Various processes can be combined to produce the desired effluent water qual- Suspended Solids Removal ity depending on the reuse requirements, source water quality, waste disposal considerations, treatment cost, Filtration is a key unit operation in water reclama- and energy needs. tion, providing a separation of suspended and colloidal particles, including microorganisms, from water. The three main purposes of filtration are to (1) allow a Nutrient Removal more effective disinfection; (2) provide pretreatment for Nutrient removal is often required in reuse applica- subsequent advanced treatment steps, such as carbon tions where streamflow augmentation or groundwater adsorption, membrane filtration, or chemical oxidation; recharge is practiced to prevent eutrophication or ni- and (3) remove chemically precipitated phosphorus trate contamination of shallow groundwater. Nutrient (Asano et al., 2007). Filtration operations most com- removal can be either an integral part of the secondary monly used in water reclamation are depth, surface, and biological treatment system or an add-on process to an membrane filtration. existing conventional treatment scheme. Depth filtration is the most common method used All of the biological processes for nitrogen removal for the filtration of wastewater effluents in water recla- include an aerobic zone in which biological nitrification mation. In addition to providing supplemental removal occurs. An anoxic zone and proper retention time is of suspended solids including any sorbed contaminants, then provided to allow biological denitrification (con- depth filtration is especially important as a conditioning version to nitrogen gas) to reduce the concentrations of step for effective disinfection. At larger reuse facilities (>1,000 m3/d or >4 MGD), mono- and dual-media nitrate to less than 8 mg N/L as illustrated in Table 3-2 (Tchobanoglous et al., 2002). Gas stripping for removal filters are most commonly used for wastewater filtra- of ammonia or breakpoint chlorination as the primary tion with gravity or pressure as the driving force. Both means for nitrogen removal is not commonly employed mono- and dual-media filters using sand and anthracite in wastewater reclamation applications in the United have typical filtration rates between 2,900 and 8,600 gal/ft2 per day (4,900–14,600 L/m2 per hour) while States. To accomplish biological phosphorus removal achieving effluent turbidities between 0.3 and 4 neph- via phosphorus-storing bacteria, a sequence of an an- elometric turbidity units (NTU). Because large plants aerobic zone followed by an aerobic zone is required with many filters usually do not practice wasting of the (for more detailed information see Tchobanoglous et initial filtrate after backwash (filter-to-waste), effluent al., 2002). Phosphorus removal can also be achieved qualities with elevated initial turbidity are commonly by chemical precipitation by adding metal salts (e.g., observed, and as a consequence, the overall effluent Ca(II), Al(III), Fe(III)) with a subsequent filtration fol- quality can be less consistent in granular media filtra- lowing the activated sludge system. Although chemical tion plants compared with reclaimed water provided by precipitation for phosphorus removal is practiced in a membrane filtration plant. many water reclamation facilities, biological phos- As an alternative to depth filtration, surface filtra- phorus removal requires no chemical input. Biological tion can be used as pretreatment for membrane filtra- phosphorus removal, however, requires a dedicated tion or UV disinfection. In surface filtration, particulate anaerobic zone and modifications to the activated matter is removed by mechanical sieving by passing sludge process, which usually is more costly during a water through thin filter material that is composed of plant retrofit than an upgrade to chemical precipitation. cloth fabrics of different weaves, woven metal fabrics, A biological phosphorus removal process is also more and a variety of synthetic materials with openings challenging to control and maintain because it depends between 10 to 30 m or larger. Surface filters can be

OCR for page 67
73 WASTEWATER RECLAMATION TECHNOLOGY operated at higher filtration rates (3,600–30,000 gal/ft2 surize the feedwater and circulate it through the mem- per day; 6,100–51,000 L/m2 per hour) while achieving brane. Pressurized MF and UF units can be operated lower effluent turbidities than conventional sand filters. in two hydraulic flow regimes, either in cross-flow or Membrane filters, such as microfiltration (MF) and dead-in filtration mode. In a submerged system, mem- ultrafiltration (UF), are also surface filtration devices, brane elements are immersed in the feedwater tank but they exhibit pore sizes in the range from 0.08 to 2 and permeate is withdrawn through the membrane m for MF and 0.005 to 0.2 m for UF. In addition to by applying a vacuum. The key operational parameter removing suspended matter, MF and UF can remove that determines the efficiency of MF and UF mem- large organic molecules, large colloidal particles, and branes and operating costs is flux, which is the rate of many microorganisms. The advantages of membrane water flow volume per membrane area. Factors affect- filtration as compared with conventional filtration are ing the flux rate include the applied pressure, fouling the smaller space requirements, reduced labor require- potential, and reclaimed water characteristics (Zhang ments, ease of process automation, more effective et al., 2006). Flux can be maintained by appropriate pathogen removal (in particular with respect to pro- cross-flow velocities, backflushing, air scouring, and tozoa and bacteria), and potentially reduced chemical chemical cleaning of membranes. Typically, MF and demand. An additional advantage is the generation of UF processes operate at flux rates ranging from 28 to 110 gal/ft2 per day (48 to 190 L/m2 per hour) (Asano a consistent effluent quality with respect to suspended matter and pathogens. This treatment usually results et al., 2007). in effluent turbidities well below 1 NTU (Asano et al., MF and UF membranes are effective in removing 2007). The drawbacks of this technology are potentially microorganisms (Figure 4-3). It is generally believed higher capital costs, the limited life span of membranes that MF can remove 90 to 99.999 percent (1 to 5 logs) requiring replacement, the complexity of the operation, of bacteria and protozoa, and 0 to 99 percent (0 to 2 and the potential for irreversible membrane fouling logs) of viruses (EPA, 2001; Crittenden et al., 2005). that reduces productivity. Unlike robust conventional However, filtration efficiencies vary with the type of media filters, membrane systems require a higher de- membrane and the physical and chemical character- gree of maintenance and strategies directed to achieve istics of the wastewater, resulting in a wide range of optimal performance. More detail about MF and UF removal efficiencies for pathogens (NRC, 1998). MF membranes and their operation in reuse applications is and certain UF membranes should not be relied upon provided in the following sections as well as in Asano et al. (2007). Removal of Organic Matter and Trace Organic Chemicals The following sections describe processes that are designed to remove organic matter and trace organic chemicals from reclaimed water. These processes in- clude membrane filtration (MF, UF, NF, and RO), adsorption onto activated carbon, biological filtration, and chemical oxidation (chlorine, chloramines, ozone, and UV irradiation). Microfiltration and Ultrafiltration MF and UF membrane processes can be configured FIGURE 4-3 P ore size distribution of a microfiltration using pressurized or submerged membrane modules. In membrane. the pressurized configuration, a pump is used to pres- SOURCE: Pera-Titus and Llorens (2007). R02129 Figure 4-3 bitmapped

OCR for page 67
74 WATER REUSE Nanofiltration or Reverse Osmosis for complete removal of viruses for several reasons (Asano et al., 2007). First, whereas the terms micro- For reuse projects that require removal of dis- and ultrafiltration nominally refer to pore sizes that solved solids and trace organic chemicals and where have cutoff characteristics as shown in Figure 4-3, the a consistent water quality is desired, the use of inte- actual pore sizes in today’s commercial membranes grated membrane systems incorporating MF or UF often vary over a wide range. Second, experience has followed by NF or RO may be required. RO and NF shown that today’s membrane systems sometimes are pressure-driven membrane processes that separate experience problems with integrity during use for a dissolved constituents from a feedstream into a con- variety of reasons. Although membrane integrity tests centrate and permeate stream (Figure 4-4). Treating have been developed and these tests are widely used, reclaimed water with RO and NF membranes usually they are not suitable for detecting imperfections small results in product water recoveries of 70 to 85 percent. enough to allow viruses to pass. Thus, the use of NF or RO results in a net loss of water Nevertheless, it is generally believed that the resources through disposal of the brine concentrate. RO new generation of filtration systems has significantly applications in water reuse have been favored in coastal improved performance for microbial removal. For settings where the RO concentrate can be conveniently example, Orange County Water District (OCWD) discharged to the ocean, but inland applications using compared the MF filtration result of their current RO are restricted because of limited options for brine groundwater replenishment system (GWRS) operation disposal (see NRC [2008b] for an in-depth discussion initiated in 2008 (see Table 2-3) with data collected of alternatives for concentrate disposal and associated during Interim Water Factor 21 (IWF21), the precur- issues). Thus, existing inland water reuse installations sor to the GWRS project, started in 2004. Although employing RO membranes are limited in capacity and the influent water quality was similar for both projects, commonly discharge brine to the sewer or a receiving IWF21 MF filtrate showed breakthrough of total co- stream provided that there is enough dilution capacity. liform in 58 percent of the samples and Giardia cysts Most commonly used RO and NF membranes in 23 percent of samples, whereas both were absent in provide apparent molecular weight cutoffs of less than the GWRS MF filtrates (OCWD, 2009). However, 150 and 300 Daltons, respectively, and are therefore MF did not eliminate viruses. Coliphages were present highly efficient in the removal of organic matter and in GWRS after MF treatment. The geometric mean selective for trace organic chemicals. Some of the of male-specific coliphage was 134 plaque-forming organic constituents that are only partially removed units (pfu)/100 mL in MF-treated water (OCWD, by NF and RO membranes while still achieving total 2009). Combining MF with chlorination is likely to organic carbon (TOC) concentrations of less than 0.5 improve the rate of virus removal. The OCWD reports mg/L are low-molecular-weight organic acids and significant reduction of coliphage in the MF feed in neutrals (e.g., N-nitrosodimethylamine [NDMA], the presence the chloramine residual. Male-specific 1,4-dioxane) as well as certain disinfection byprod- coliphage dropped from a geometric mean of 1,800 ucts (e.g., chloroform) (Bellona et al., 2008). Recent pfu/100 mL in the previous year to 28 pfu/100 mL in advances in membrane development have resulted in the MF feed and they were absent in the MF filtrate low-pressure RO membranes and NF membranes that (OCWD, 2010). can be operated at significantly lower feed pressure MF and UF membranes sometimes in combina- while providing approximately the same product water tion with coagulation can also physically retain large quality. However, certain monovalent ions (e.g., Cl–, dissolved organic molecules and colloidal particles. Na+, NO3–) are only partially rejected by NF, and NF Effluent organic matter and hydrophobic trace organic membranes result in product water with higher TDS chemicals can also adsorb to virgin MF and UF mem- than RO (Bellona et al., 2008). branes, but this initial adsorption capacity is quickly Today, most integrated membrane systems applied exhausted. Thus, adsorption of trace organic chemicals in reuse employ RO rather than NF. However, certain is not an effective mechanism in steady-state operation low-pressure NF membranes offer opportunities for of low-pressure membrane filters.

OCR for page 67
75 WASTEWATER RECLAMATION TECHNOLOGY FIGURE 4-4 Substances and contaminants nominally removed by pressure-driven membrane processes. SOURCE: Cath (2010). wider applications in water reclamation projects be- R02129s well. Leaks around the seals and connectors were a cause they have lower energy requirements and can suspected as the cause of reduced microbial removal Figure 4-4 achieve selective rejection of salts and organic constitu- efficiency, but once faulty connectors and an obviously bitmapped ents that results in less concentrated brine streams. For flawed membrane element were identified and replaced, wastewater applications, RO and ultra-low-pressure rejection of bacteria and protozoa seemed absolute, but RO membrane facilities typically operate at feed pres- the removal of the surrogate coliphage MS2 remained sures between 1,000 and 2,100 kPa (approximately slightly above 2.5 logs (99.7 percent). Expansion of 150–300 psi) in order to produce between 8.5 and 12.5 both bench- and pilot-scale testing to include a variety 2 per day (13.5 and 20 L/m2 per hour) of permeate gal/ft of manufacturers revealed that the quality of brackish (Lopez-Ramirez et al., 2006). NF membranes, while water RO membranes ranged widely, with one manu- achieving a similar product water quality with respect facturer consistently demonstrating complete rejection to TOC and trace organic chemicals, can be operated in both types of tests. Though systematic tests are not at 2 to 4 times lower feed pressures, resulting in sig- available, newer RO systems may have significantly im- nificantly greater energy savings than conventional RO proved performance for microbial removal. Recent tests membranes (Bellona and Drewes, 2007). have shown promising results (Lozier et al., 2006) and RO and NF membranes, in theory, should remove data collected in 2008 at OCWD’s GWRS revealed all pathogens from the feedwater because they are de- the absence of native coliphage in 1-L samples of RO effluent,1 which indicated an improvement from the signed to remove relatively small molecules. However, some earlier testing results have shown that the removal earlier pilot study (27 percent RO breakthrough rates) of virus surrogates (coliphage) seeded in front of RO is using an older generation of membranes (OCWD, sometimes incomplete. For example, studies conducted 2010). by the City of San Diego noted coliphage breakthrough in the permeate of the RO system at concentrations up 1 Randomly selected RO permeate samples taken from each of to 103 pfu/100 mL (Adham et al., 1998). Early tests 15 RO trains, each sampled three times (M. Wehner, OCWD, showed inadequate removal of protozoa and bacteria personal communication, 2011)

OCR for page 67
76 WATER REUSE Activated Carbon to reduce BDOC with the aid of indigenous bacteria present in the feedwater. Additionally, the use of bio- In water reclamation, adsorption processes are filtration after ozone also has been shown to reduce sometimes used to remove dissolved constituents by the formation of some byproducts formed during accumulation on a solid phase. Activated carbon is a secondary disinfection with chlorine (Wert et al., common adsorbent, which is employed as powdered 2007). Some studies have also demonstrated that the activated carbon (PAC) with a grain diameter of less byproducts from ozonation of trace organic chemicals, than 0.074 mm or granular activated carbon (GAC), such as steroid hormones and pharmaceuticals, also which has a particle diameter greater than 0.1 mm. are largely biodegradable (Stalter et al., 2010); there- D uring water reclamation, PAC can be added di- fore, there is growing support for the use of biofiltra- rectly to the activated sludge process or solids contact tion after ozone or AOP. Although biofiltration alone processes, upstream of a tertiary filtration step. GAC may provide some direct benefit in terms of removing is used in pressure and gravity filtration. Activated trace organic chemicals, it has generally been shown to carbon is efficient for the removal of many regulated be only marginally effective without a prior oxidation s ynthetic organic compounds as well as unregu- step ( Juhna and Melin, 2006). lated trace organic chemicals exhibiting properties Biological filtration can be accomplished using of high and moderate hydrophobicity (e.g., steroid traditional media (i.e., sand/anthracite) or using ac- h ormones, triclosan, bisphenol A) (Snyder et al., tivated carbon (biologically activated carbon [BAC]). 2006a). Although PAC needs to be disposed of after Although some studies have suggested that activated its adsorption capacity is reached, GAC can be re- carbon is superior for supporting biological growth, generated either on- or offsite, providing this practice mainly because of superior adherence of the biofilm to is more cost-effective than disposing it via landfills. the GAC, there are some conflicting reports that show Onsite GAC regeneration is only cost-effective for approximately equal performance using anthracite large installations and is currently not practiced by ( Wert et al., 2008). Some studies have demonstrated any water reclamation facility in the United States. that BAC is capable of adsorption as well as biological GAC adsorbents are characterized by short empty- degradation; however, the adsorptive capacity of the bed contact times (i.e., 5-30 min) and preferably a BAC will eventually be reduced as the micropores in large throughput volume (i.e., bed volumes of 2,000 the carbon structure become blocked and the adsorp- to 20,000 m3/m3) (Asano et al., 2007). tive capacity subsequently becomes exhausted. At this point, fresh GAC will be required to restore the Biological Filtration adsorptive capacity, but effective biological activity as measured by reduction of AOC or BDOC will take As mentioned previously in this chapter, the use time to establish. The amount of time needed to de- of strong oxidants, such as ozone or ozone/peroxide velop a biologically active filter will depend on water and UV/peroxide, results in the formation of various quality, water temperature, and operational parameters. biodegradable byproducts (Wert et al., 2007). For An important factor in establishing and maintaining instance, simple aldehydes, ketones, and carboxylic an active biofilm is the backwash frequency with chlo- acids are produced as ozone oxidizes organic matter rinated water. in water. The aggregate measurements commonly One major disadvantage of using biological filtra- employed to assess the biodegradability of transfor- tion is the detachment of biofilm and likely detection mation products is assimilable organic carbon (AOC) of bacteria in filtered water. Although these bacteria (Hammes and Egli, 2005) and biodegradable dis - are not harmful, the detection of heterotrophic bacteria solved organic carbon (BDOC) (Servais et al., 1987). could in some cases lead to regulatory violations. In This readily biodegradable carbon has been implicated those cases, biofiltration would generally be followed in the acceleration and promotion of biofilm growth in by a disinfection step, such as chlorination or UV distribution systems. Thus, drinking water treatment irradiation. facilities usually employ biofiltration after ozonation

OCR for page 67
77 WASTEWATER RECLAMATION TECHNOLOGY Chemical Oxidation not as effectively oxidized (Westerhoff et al., 2005). However, the ability of chlorine to effectively oxidize Chemical oxidation is commonly employed in trace organic chemicals, including steroid hormones, is water treatment to achieve disinfection, as described a function of contact time and dose. More importantly, previously in this chapter; however, oxidants are also chlorine is not expected to mineralize trace organic used to remove tastes, odors, and color and to improve chemicals, but rather to transform them into new the removal of metals (Singer and Reckhow, 2010). products (Vanderford et al., 2008), which may in fact O xidants used for water treatment include chlorine, be more toxic than the original molecule. chloramine, ozone, permanganate, chlorine dioxide, and ferrate. Advanced oxidation relies upon formation Chloramines. Chloramines are not nearly as effective of powerful radical species, primarily hydroxyl radicals as oxidants and thus play a much smaller role in trace (OH∙) and is rapidly gaining in use for the oxidation organic chemical oxidation. Snyder (2007) demon- of more resistant chemicals, such as many trace or- strated that a dose of 3 mg/L chloramines and a contact ganic chemicals and industrial solvents (Esplugas et time of 24 hours was able to effectively oxidize phenolic al., 2007). The most commonly employed advanced steroid hormones (e.g., estrone, estradiol, estriol, ethi- oxidation techniques in water reclamation use hydrogen nyl estradiol) as well as triclosan and acetaminophen; peroxide coupled with UV light or ozone gas. The UV however, the vast majority of trace organic chemicals light itself is not strictly an oxidant but it does selec- studied were not significant oxidized. Therefore, al- tively transform a small group of compounds sensitive though chloramines play an important role in reduction to direct photolysis (e.g., NDMA, iohexol, triclosan, of membrane fouling and disinfection, only minimal acetaminophen, diclofenac, sulfamethoxazole) (Pereira expected benefit in oxidation of trace organic chemicals et al., 2007; Snyder et al., 2007; Yuan et al., 2009; and will result. Moreover, careful evaluation of nitrosamine Sanches et al., 2010). formation should be undertaken when using chlora- Very few oxidative technologies are employed at mines, considering the carcinogenic potency of these operational conditions capable of mineralizing organic byproducts (see Choi et al., 2002; Mitch et al., 2003; materials in water. Even the most promising advanced Haas, 2010). oxidation techniques using ozone and UV irradiation combined with peroxide will result in only a minor Ozone. Ozone (O3) is a powerful oxidant and disin- (if any) measurable reduction of dissolved organic fectant that decays rapidly and leaves no appreciable carbon (DOC). Regardless of the oxidation technique residual in reclaimed water during storage and distribu- deployed and superior performance of trace organic tion. Ozone-enriched oxygen is generally added to wa- chemical removal, some transformation products will ter through diffusers producing fine bubbles, and once result that are often uncharacterized (see Chapter 3 for dissolved in water, ozone quickly undergoes a cascade additional discussions of transformation byproducts). of reactions, including decomposition into hydroxyl The most commonly used oxidation methods for the radicals (OH∙), hydroperoxyl radical (HO2), and su- removal of trace organic contaminants are described peroxide ion (O2–). These radicals along with molecular below. ozone will rapidly react with organic matter, carbonate, bicarbonate, reduced metals, and other constituents in Chlorine. Chlorine, defined here as the combination of water. The reactions mediated by the hydroxyl radical chlorine gas, HOCl, and OCl–, reacts selectively with are relatively nonselective, whereas molecular ozone is electron-rich bonds of organic chemicals (e.g., double more selective (Elovitz et al., 2000). bonds in aromatic hydrocarbons) (Minear and Amy, Because of ozone’s ability to oxidize organic chemi- 1996). Recently, several reports have shown that many cals, it has been widely applied in water treatment for trace organic chemicals containing reactive functional taste and odor control, color removal, and to reduce groups can be oxidized by free chlorine (Adams et al., concentrations of trace organic chemicals. At dosages 2002; Deborde et al., 2004; Lee et al., 2004, Pinkston commonly employed for disinfection, the vast majority and Sedlak, 2004; Westerhoff et al., 2005), while ke- of contaminants can be effectively converted into trans- tone steroids (e.g., testosterone and progesterone) are formation products (Snyder et al., 2006c). Although

OCR for page 67
78 WATER REUSE several studies have shown that ozone effectively re- to remove excess peroxide from the UV-AOP effluent. duces estrogenic potency in reclaimed water (Snyder Although UV-AOP does form transformation prod- et al., 2006c), recent publications have suggested that ucts (i.e., it does not result in mineralization of organic biologically active filters be included after the ozone compounds), it does not form bromate. Additionally, process in order to remove biodegradable byproducts UV alone at elevated dosages or in combination with formed during ozonation (Stalter et al., 2010). For po- hydrogen peroxide (UV-AOP) effectively removes table reuse applications, ozonation could also be applied NDMA. after soil aquifer treatment (SAT), which combines UV-AOP efficacy, however, is quite susceptible the benefits of a more selective oxidation of remaining to water quality and requires proper pretreatment. In chemicals persistent to biodegradation and a lower many potable reuse applications, UV-AOP is applied ozone demand due to reduced DOC concentrations after RO treatment to negate the detrimental impacts in the recovered water. of water quality, such as suspended and particulate It is well known that in the presence of bromide, matter and DOC. UV-AOP applications generally will ozone can form bromate, a toxic byproduct. There are require extensive pretreatment to increase UV trans- steps that can be employed to mitigate the formation of mittance; however, recent studies have demonstrated bromate, such as the use of chlorine and ammonia be- that UV-AOP can be also effective in advanced-treated fore ozone addition (von Gunten, 2003). Some reports effluents (Rosario-Ortiz et al., 2010). have shown that ozone applied before chloramination also results in the oxidation of nitrosamine precursors Removal of Dissolved Solids (Lee et al., 2007). However, ozone also has been shown to form some nitrosamines directly (von Gunten et al., Domestic and commercial uses of public water 2010). supplies result in an increase in the mineral content of Ozone can play an important role in water recla- municipal wastewater. This increase can be problematic mation, but the process is more energy intensive and where drinking water supplies are already elevated in operationally complex than chlorination. In cases where TDS and regional water reuse is already occurring, trace organic chemical removal (e.g., pharmaceuticals, resulting in partially closed water and salt cycles. Hard steroid hormones) is important, ozone is a viable option water can also be a problem because it results in the pro- and does not result in a residuals stream like NF or RO liferation of self-regenerating water softeners, which membrane processes or in spent media as with activated discharge their regenerant into the wastewater collec- carbon. However, ozone does not provide a complete tion system. To mitigate salinity problems associated barrier to trace organic chemicals, and there are certain with local water reuse activities, especially in inland chemicals that are not amendable to oxidation (e.g., applications, partial desalination of reclaimed water chlorinated flame retardants; artificial sweeteners) especially for potable reuse projects may be required. (Snyder et al., 2006c). In addition to pressure-driven membrane-based separation processes, such as NF and RO, as discussed UV irradiation. UV light at doses commonly employed above, current-driven membrane processes, such as for disinfection (40–80 mJ/cm2) is largely ineffective electrodialysis (ED) or electrodialysis reversal (EDR), for trace organic chemical removal. In a recent study can be used to separate salts. Nevertheless, ED and that investigated the removal of trace organic chemicals EDR are not commonly employed in water reclamation from water, none of the target compounds investigated and currently only one facility in Southern California were well removed (>80 percent oxidized) using UV at is using EDR to remove TDS at a demonstration-scale disinfection doses (Snyder, 2007). However, when UV facility. Precipitative softening can also be used for doses are significantly increased (generally by 10-fold) partial demineralization (mainly to remove hardness) and high doses of hydrogen peroxide (5 mg/L and and is currently employed for this purpose in the City higher) are added, most trace organic chemicals were of Aurora’s Prairie Waters Project, Colorado (see also effectively oxidized (Snyder et al., 2006c). Activated Box 4-1). carbon is sometimes employed to catalytically remove hydrogen peroxide, and other chemicals can be used

OCR for page 67
79 WASTEWATER RECLAMATION TECHNOLOGY use systems and the general role of environmental buf- BOX 4-1 fers in potable reuse projects are described in Chapter Prairie Waters Project, Aurora, Colorado 2, but water quality improvements provided by these surface and subsurface natural systems are described in The Prairie Waters Project, established by the City of the subsequent sections. Aurora, Colorado, in 2010, is a potable water reuse aug- mentation project that will increase Aurora’s water supply by 20 percent; delivering up to 9 MGD (34,000 m3/d). The Subsurface Managed Natural Systems project is using return flows discharged to the South Platte River downstream of Denver. This water is recovered through Subsurface managed natural systems can be used to a series of 17 vertical riverbank filtration wells, followed by enhance water quality and/or to provide natural stor- artificial recharge and recovery (ARR), providing a retention age for reclaimed water. These systems include surface time of approximately 30 days in the subsurface. The water is spreading basins, vadose zone wells, and riverbank subsequently pumped to an advanced water treatment plant. filtration wells, which take advantage of attenuation The water treatment plant consists of precipitative softening, processes that occur in the vadose zone and saturated UV-AOP, biologically active carbon filtration, and granular activated carbon (GAC) filtration. At a ratio of 2:1, the final aquifer. Other processes, such as aquifer storage and product water is blended with Aurora’s current supply using recovery (ASR) and direct injection wells, introduce mountain runoff water prior to disinfection and final distribu- highly treated reclaimed water directly into a potable tion. Precipitative softening is employed to maintain a hard- aquifer. ness level that is similar to Aurora’s current supply. Riverbank In general, subsurface treatment applications offer filtration and ARR are very efficient in removing pathogens, numerous advantages. These systems typically require organic carbon, trace organic chemicals, and nitrate (Hoppe- Jones et al., 2010). UV-AOP and GAC serve as an additional a low degree of maintenance, and the energy require- barrier for trace organic chemicals that might survive after the ments are low. The input of chemicals usually is not natural treatment process. The treatment scheme was selected required, and the operation is residual free. Tempera- because alternatives such as reverse osmosis with zero liquid ture equilibration of water is achieved during subsurface discharge of brine or wetland treatment instead of riverbank storage and excursions in water quality are buffered due filtration were cost-prohibitive or not viable. to dispersion in the subsurface and dilution with native SOURCE: http://www.prairiewaters.org. groundwater. However, subsurface applications require that a substantial aquifer be available and that it be characterized by an extensive site assessment. Although the advantages seem to outweigh the disadvantages ENGINEERED NATURAL PROCESSES from an operational standpoint, the lack of clear and standardized guidance for design and operation of these Natural processes in water reclamation are usu- system limits wider establishment of managed subsur- ally employed in combination with aboveground face treatment systems. Lack of process understanding engineered processes and consist of managed aquifer can result in less-than-optimal performance or physical recharge systems and natural or constructed wetlands footprints or retention times that are larger than needed (Figure 4-1). Natural systems can be considered as mul- for the desired water quality improvements. Some tiobjective treatment processes targeting the removal of installations might also exhibit deterioration of water pathogens, particulate and suspended matter, DOC, quality in the recovered water due to biogeochemical trace organic chemicals, and nutrients, either as the key reactions in the subsurface that were not anticipated. treatment process or as an add-on polishing step. All natural treatment processes combine the advantage of Surface Spreading or Soil Aquifer Treatment a low carbon footprint (i.e., little to no chemical input, low energy needs) with little to no residual generation. Surface spreading basins allow reclaimed water The drawbacks of these processes are the required to infiltrate slowly through the vadose zone, where footprint and a suitable geology, which might not be sorption, filtration, and biodegradation can enhance available where the use of natural treatment systems is the water quality (also called soil aquifer treatment). desired. Examples of managed natural processes in re- Recharge basins for surface spreading operations are

OCR for page 67
80 WATER REUSE often located in, or adjacent to, floodplains, character- bination of filtration and biotransformation processes ized by soils with high permeability. In some instances, during subsurface treatment is very efficient for the excavation is necessary to remove surface soils of low inactivation of pathogens, especially viruses (Schijven et permeability. For mosquito control and to maintain al., 2000, 2002; Quanrud et al., 2003; Azadpour-Keeley permeability during operation with reclaimed water, and Ward, 2005; Gupta et al., 2009). Attenuation of recharge basins are usually operated in alternate wet pathogens depends primarily on three mechanisms— and dry cycles. As the recharge basin dries out, dis- straining, inactivation, and attachment to aquifer grains solved oxygen penetrates into the subsurface, facilitat- (McDowell-Boyer et al., 1986). Findings from field ing biochemical transformation processes, and organic studies demonstrated that infiltration into a relatively material accumulated on the soil surface will desiccate, homogeneous sandy aquifer can achieve up to 8 log allowing for the recovery of infiltration rates (Fox et virus removal over a distance of 30 m in about 25 days al., 2001). (Dizer et al., 1984; Yates et al., 1985; Powelson et al., The removal of organic matter during SAT is 1990; Schijven et al., 1999, 2000). During SAT in the highly efficient and largely independent of the level of Dan Region Project, Israel, Icekson-Tal et al. (2003) aboveground treatment. Biodegradable organic carbon measured 5.3 log removal of total coliform and 4.5 log that is not attenuated during wastewater treatment removal of fecal coliform bacteria. The efficient removal represents an electron donor for microorganisms in the of fecal and total coliform bacteria during subsurface subsurface and is readily removed during groundwater treatment and essentially their absence in groundwater recharge (Drewes and Fox, 2000; Rauch-Williams and abstraction wells after SAT or riverbank filtration was Drewes, 2006). Monitoring efforts revealed consistent confirmed by various other studies (Fox et al., 2001; removal of TOC between 70 and 90 percent at full- Hijnen et al., 2005b; Levantesi et al., 2010). Other field scale SAT facilities that were in operation for several studies have focused on attenuation of protozoa, and decades (Quanrud et al., 2003; Drewes et al., 2006; findings suggest that efficient removal occurs during Amy and Drewes, 2007; Lin et al., 2008; Laws et al., passage across the surface water—groundwater inter- 2011). The removal of easily biodegradable organic face and lesser removal is observed during groundwater carbon in the infiltration zone usually results in deple- transport away from this interface (Schijven et al., tion of oxygen and the creation of anoxic conditions. 1998). Further details on pathogen attenuation during Although this transition is advantageous to achieve SAT are provided in Chapter 7. An example of the denitrification, it might also lead to the solubilization degree of attenuation for various microbial and chemi- of reduced manganese, iron, and arsenic from native cal constituents that can be achieved in SAT systems is aquifer materials. If these interactions occur, appro- illustrated in Tables A-7 and A-9 (Appendix A). priate post-treatment is required after recovery of the Nitrogen removal needs to be carefully managed recharged groundwater. when reclaimed water is applied with total nitrogen Previous studies have characterized the transfor- concentrations in excess of 20 mg N/L. At such high mation and removal of select trace organic chemicals concentrations, the wetting and drying cycles of the during SAT for travel times ranging from ~1 day to 8 spreading basins cannot meet the nitrogenous oxygen years (Drewes et al., 2003a. Montgomery-Brown et demand (in excess of 100 mg/L), resulting in incom- al., 2003; Snyder et al., 2004; Grünheid et al., 2005; plete nitrification. Ammonium is usually removed by Massmann et al., 2006; Amy and Drewes, 2007). Sev- cation exchange onto soil particles during wetting eral studies also report efficient removal of NDMA and cycles, followed by nitrification of the adsorbed ammo- other nitrosamines under both oxic and anoxic subsur- nium during drying cycles. Nitrate is not adsorbed to face conditions (Sharp et al., 2005; Drewes et al., 2006; soils, but if sufficient carbon is present to create anoxic Nalinakumari et al., 2010). A case study conducted at conditions, nitrate can be removed via denitrification a facility in Southern California (Box 4-2) illustrates during subsequent passage in the subsurface (Fox et the efficiency of short-term SAT for the attenuation al., 2001). Reclaimed water with nitrate concentra- of trace organic chemicals in reclaimed water (Laws tions in excess of 10 mg N/L can result in incomplete et al., 2011). denitrification when applied to groundwater recharge Previous studies have demonstrated that the com- basins because the biodegradable organic carbon usu-

OCR for page 67
81 WASTEWATER RECLAMATION TECHNOLOGY than 10-4 m/s. In riverbank filtration, constant scour ally present in a secondary or advanced-treated effluent will be insufficient to achieve complete denitrification. forces due to streamflow prevent the accumulation of For potable reuse projects, different regulatory particulate and colloidal organic matter in the infiltra- requirements exist regarding the minimum retention tion layer. time of reclaimed water in the subsurface prior to Biodegradation of organic matter represents a key extraction. The primary intent of these regulations is attenuation mechanism of riverbank filtration processes to provide additional protection against pathogens in (Kühn and Müller, 2000; Hoppe-Jones et al., 2010). A groundwater recharge projects and to provide time for bioactive filtration layer forms near the water/sediment corrective action in the event that substandard water interface where dissolved oxygen concentrations are is inadvertently recharged. Regulations in the state highest, which can cause significant removal of DOC of Washington require a minimum of 6 months of during the initial phase of infiltration (first meter). hydraulic retention time in the subsurface for surface Conditions can quickly transition from oxic to anoxic spreading operations and a minimum of 12 months as the water travels with increasing distance from the for direct injection projects before the water can be river through the subsurface, although the oxidation- recovered as a potable water source (Washington De- reduction gradient depends on site specific conditions, partment of Health and Washington Department of such as DOC and ammonia concentrations in the river Ecology, 1997), while California’s draft groundwater (Hiscock and Grischek, 2002; Ray et al., 2008). recharge regulations require a minimum of 2 months More than 5-log removal of pathogen surrogate in the subsurface for both surface spreading and injec- microorganisms (e.g., bacteria, viruses, and parasites) tion projects to provide time for corrective action if has been reported in riverbank filtration under steady- substandard water is inadvertently recharged (CDPH, state conditions, with variations of ±1-log removal 2011). Others have defined minimum setbacks (i.e., efficiency associated with individual microorganism horizontal separation) between reclaimed water surface characteristics (Medema et al., 2000). Havelaar et al. spreading operations and potable wells (e.g., 500 ft [150 (1995) reported removal in excess of 5 logs for total m] in Florida; 2,000 ft [610 m] in Washington) (FDEP, coliform during transport of river water over a 30-m 2006; Washington Department of Health and Wash- distance from the Rhine River and over a 25-m distance ington Department of Ecology, 1997). However, these from the Meuse River to a well. Total coliforms were setbacks or minimal retention times are frequently not rarely detected in riverbank-filtered waters, with 5.5- based on scientific findings but represent a conserva- and 6.1-log reductions in average concentrations in tive estimate to provide additional removal credits wells relative to river water (Weiss et al., 2005). Have- for pathogens in case of a failure in the aboveground laar et al. (1995) reported 3.1-log removal of protozoa treatment train. Reuse regulations are discussed in more surrogates during transport over a 30-m distance from detail in Chapter 10. the Rhine River to a well and 3.6-log removal over a 25-m distance from the Meuse River to a well. Schijven et al. (1998) measured 1.9-log removal for protozoa Riverbank Filtration surrogates over a 2-m distance from a canal. This find- Riverbank filtration has been practiced in the ing is consistent with field monitoring results from a United States for more than 50 years for domestic riverbank filtration site in Wyoming, where Gollnitz drinking water supplies utilizing streams that might et al. (2005) observed a 2-log removal of Cryptospo- have been compromised in their quality due to the dis- ridium surrogates in groundwater wells characterized charge of wastewater effluents or other waste streams by flowpaths between 20 and 984 ft (6 and 300 m). (Ray et al., 2008). Recently, water reuse projects have At a riverbank filtration site at the Great Miami River, integrated riverbank filtration into their treatment Gollnitz et al. (2003) reported a 5-log removal of pro- process train to take advantage of the benefits of this tozoa surrogates in a production well located 98 ft (30 natural treatment system (see Box 4-1). Aquifers used m) off the river. for riverbank filtration usually consist of alluvial sand Numerous research projects have documented the and gravel deposits, with thickness ranging from 15- removal of trace organic compounds during riverbank 200 feet (5–60 m) and a hydraulic conductivity higher filtration. For example, Ray et al. (1998) and Vers-

OCR for page 67
82 WATER REUSE BOX 4-2 Montebello Forebay Groundwater Recharge Operation, California In the United States, drinking water augmentation with reclaimed water was pioneered by the County Sanitation Districts of Los Angeles County (CSDLAC) and the Water Replenishment District of Southern California (WRD) by establishing groundwater recharge spreading operations with reclaimed water in Pico Rivera, California in the early 1960s. Laws et al. (2011) studied the fate and transport of bulk organic matter and a suite of 22 trace organic chemicals during the surface-spreading recharge operation using a smaller but well-instrumented test basin at this facility. Two monitor- ing wells were located at the side of the recharge basin and lysimeters were installed beneath the basin (see figure below). Based on ion signatures it appeared that all of the samples collected originated from reclaimed water that was applied to the basin; however, the samples from the deeper wells (PR 8 and 10) appeared to have been diluted by native groundwater. Instrumentation of groundwater recharge test basin associated with monitoring data provided in table below. SOURCE: Laws et al. (2011). R02129 traeten et al. (1999) reported 50 to 75 percent removal Figure 4-5 river water and bank-filtered water (Kühn and in both of the herbicide atrazine during riverbank filtration,bitmapped 2000; Schmidt et al., 2004; Hoppe-Jones et al., Müller, although the underlying removal mechanisms were 2010; Maeng et al., 2010). A partial reduction in con- not clear. Despite the success of riverbank filtration in centration was only achieved under certain redox con- removing numerous compounds, certain trace organic ditions and through dilution with local groundwater. chemicals have been regularly found in the product wa- ter of riverbank filtration systems, including urotropin Direct Injection (an aliphatic amine) and 1,5-naphthalindisulfonate (an aromatic sulfonate) (Brauch et al., 2000), antiepileptic Direct injection of reclaimed water may occur in drugs (e.g., carbamazepine, primidone), a blood-lipid both saturated and unsaturated aquifers using wells regulator (e.g., clofibric acid), antibiotics (e.g., sulfa- that are constructed like regular pumping wells. In the methoxazole), and x-ray contrast media were present United States, OCWD pioneered direct injection of

OCR for page 67
83 WASTEWATER RECLAMATION TECHNOLOGY Over a travel time of less than three days in the upper aquifer, approximately 55 percent of the total organic carbon was removed (from 7.8 mg/L to 3.5±0.3 mg/L), and overall removal increased to 79 percent with increased travel time (60 days). Most of the observed removal occurred in the vadose zone (<2.4 m) because of its aerobic conditions. Attentuation of trace organic chemicals also occurred in the vadose zone, where concentra - tions decreased within the first 2.4 m (~10 hours). After 60 days travel time, the concentrations of monitored trace organic chemicals decreased further (see table below). Concentrations of primidone, carbamazepine, trimethoprim, N,N-diethyl-meta-toluamide (DEET), meprobamate, tris (2-chloroethyl) phosphate (TCEP), tris (2-chloroisopropyl) phosphate (TCPP), and triclosan were reduced less than 10 percent in the upper aquifer but contaminant attenuation increased with travel time (Laws et al., 2011). Concentration of Select Trace Organic Chemicals in Reclaimed Water After Surface Spreading Avg. Avg. (MLS 8-PR 11) (PR8, PR10) Compound Basin (10-70 hrs) (60 days) <5 Atrazine 5±0.2 4.0±0.1 TCEP 400 402±15 128±39 6,483±87 TCPP 7,200 5 797±188 <1000 <50 Benzophenone 68±27 DEET 320 238±60 50±12 <25 OCR for page 67
84 WATER REUSE for example, redox change can result in dissolution of must frequently satisfy competing demands and mul- certain constituents from the soil matrix, including tiple objectives. For example, in addition to providing iron, manganese, or arsenic. water quality benefits, engineered treatment wetlands Infiltration rates of direct injection wells are much frequently serve as habitat for birds and provide recre- higher than infiltration rates in spreading basins, al- ational and educational benefits for the community. In though direct injection wells can become clogged at addition, they have the potential to serve as breeding the interface of the gravel envelope of a well and the grounds for mosquitoes and other vectors. aquifer. Considerable research has been conducted to Another important difference between surface understand factors that contribute to clogging and to and subsurface systems is the way in which the water develop approaches to evaluate the clogging potential flows. With the exception of fractured bedrock, the soil due to biological activity and suspended solids (Asano and groundwater systems used in managed subsurface et al., 2007). These approaches can assess the relative treatment processes lead to predictable flow patterns clogging potential of different waters, but they cannot and residence times in the subsurface. In addition, the provide an absolute prediction of clogging in injection high surface area provided by soil and geological mate- wells. Therefore, more reliable design and operational rials provides ample surface area for microbial growth, criteria are needed for a sustainable operation. The facilitating biological attenuation processes. In contrast, costs of direct injection wells can also be significant managed surface systems often exhibit preferential flow where deep aquifers are used for storage, which in- and lower biological activity. As a result, a poorly man- creases the well construction costs as well as the energy aged natural system has a higher potential for providing costs for injecting water to maintain proper infiltration less-effective treatment than expected, with hydraulic rates (Asano et al., 2007). short-circuiting and low biological activity leading to A high level of pretreatment is usually employed little contaminant attenuation. to minimize the risk of clogging and to avoid costs for redeveloping clogged injection wells. In several potable Treatment Wetlands reuse applications in the United States, RO treatment is employed prior to direct injection (Table 2-3, Chap- Treatment wetlands have been used to treat re- ter 2). This degree of treatment, however, reduces the claimed water for nonpotable and potable reuse (see biodegradable organic carbon, thereby limiting the Box 2-10). Treatment wetlands are built as either sub- biological activities in the subsurface environments surface-flow or surface-flow systems. Subsurface-flow and reducing the effectiveness of the natural subsur- wetlands consist of plants growing within a gravel bed face treatment with respect to achieving attenuation through which reclaimed water flows whereas surface- of contaminants. At OCWD in the early 2000s, low- flow systems consist of wetland plants growing in molecular-weight compounds, such as NDMA, were anywhere from 0.5 to 2 feet (0.15 to 0.6 m) of flowing present in RO permeate and persisted after direct surface water with occasional deeper areas to enhance i njection during subsurface transport, presumably mixing and provide habitat (Kadlec and Knight, 1996). because co-metabolic reactions that can remove these Subsurface wetlands are more common in colder cli- compounds were not adequately stimulated in the aqui- mates and in locations where there are concerns about fer (Drewes et al., 2006; Sharp et al., 2007). contact with contaminants in the reclaimed water (e.g., when wetlands are used for treatment of primary efflu- ent). With respect to water reclamation, subsurface- Surface Managed Natural Systems flow wetlands may be better suited for decentralized In addition to providing aesthetic benefits and treatment of primary or secondary effluent (e.g., septic providing habitat and recreational opportunities, man- tank effluent) than wastewater from full-scale treat- aged natural surface water systems can provide benefits ment plants. with respect to water quality. One of the main differ- Surface-flow wetlands are less expensive to build ences between surface and subsurface managed natural and maintain and provide better habitat and aes - systems is that managers of surface water systems thetic benefits and are therefore more common in

OCR for page 67
85 WASTEWATER RECLAMATION TECHNOLOGY warmer climates. Ammonia is usually removed from dominant within wetlands, meaning that a large frac- reclaimed water through nitrification prior to discharge tion of the flow receives little treatment (Lightbody et to surface-flow wetlands because ammonia toxicity af- al., 2008). Therefore, active management of surface- fects the growth of plants and can be detrimental to flow treatment wetlands is crucial to achieving effec- resident fish that control mosquitoes. tive treatment. Surface-flow wetlands frequently provide good re- moval of contaminants present in wastewater effluent. Reservoirs In particular, ample data indicate that surface-flow wet- lands remove nitrate through denitrification in anoxic As mentioned previously, surface-water reservoirs zones, and phosphorus through settling of particulate frequently are managed to preserve or enhance water phosphate and uptake by growing plants (Kadlec and quality. Procedures for proper management of reser- Knight, 1996). Wetlands also are effective in the re- voirs that receive reclaimed water are not well estab- moval of particles that settle out at the low flow veloci- lished because there are a limited number of reservoirs ties encountered in the wetland. As a result, wetlands that receive reclaimed water and the contribution of re- provide removal of particle-associated pathogens and claimed water to the overall volume of the reservoirs is metals. Aerobic microorganisms living near the air– typically small. Concentrations of trace organic chemi- water interface and nitrate-reducing microbes below cals usually are quite low and it is difficult to assess the the surface also can transform organic contaminants potential for removal from reservoirs. There is a clear as they metabolize decaying plants and organic mat- research need to better understand the contribution of ter present in the reclaimed water. Concentrations of various attenuation processes (i.e., biotransformation, certain trace organic chemicals, such as trihalomethane photolysis, sorption to particulate matter, and dilution) disinfection byproducts, also can decrease in treatment for trace organic chemicals and pathogens in surface wetlands through volatilization (Rostad et al., 2000). reservoirs receiving reclaimed water. Laboratory microcosm studies demonstrate the ability Despite these limitations, insight into the potential of microorganisms and organic compounds in wetlands importance for attenuation of contaminants in reser- to transform numerous trace organic chemicals (Gross voirs can be made from data on reservoirs and lakes et al., 2004; Matamoros et al., 2005; Matamoros and that receive discharges of wastewater effluent. For Bayona, 2006; Waltman et al., 2006). example, Poiger et al. (2001) demonstrated that the Comparison of results from laboratory- or pilot- pharmaceutical diclofenac underwent photolysis in the scale wetland studies with full-scale systems often surface of Lake Griefensee in Switzerland that receives indicates that hydraulic short-circuiting can result a significant fraction of its overall flow from wastewater in significant decreases in treatment efficacy. For treatment plants. Monitoring data and models of the example, between 30 and 40 percent of the steroid stratified lake demonstrated that diclofenac concen- hormones entering a pilot-scale surface treatment trations were significantly lower in the epilimnion of wetland were removed over a hydraulic residence time the lake because photolysis rapidly transformed the of approximately 2 days (Gray and Sedlak, 2005). The compound. Thus, for those compounds that undergo associated full-scale wetland, which had nearly identi- photolysis (e.g., diclofenac, sulfamethoxazole) as well as cal plant species and a nominal hydraulic residence waterborne pathogens that are inactivated by sunlight, time of over a week should have achieved removals ex- the surface-to-volume ratio of the reservoir and the ceeding 90 percent, but monitoring of the inflow and depth of the drinking water plant intake both could be outflow of the full-scale system failed to show signifi- important to the concentration of contaminants in the cant removals. Presumably, this apparent discrepancy water entering the treatment plant. is due to hydraulic short-circuiting, which has been In recent years, there has been increasing federal observed in tracer tests of the full-scale system (Lin et attention to the impacts of nutrients on surface water al., 2003). These types of findings are consistent with ecosystems. EPA has encouraged states to develop and tracer studies of full-scale treatment wetlands that fre- adopt numeric nutrient criteria for nitrogen and phos- quently show that preferential flow paths can become

OCR for page 67
86 WATER REUSE options for brine disposal are limited in inland ar- phorus, which could affect the viability of surface dis- eas. Many of the earliest potable water reuse projects charge of reclaimed water without nutrient removal.2 were established in coastal communities where brine concentrate from RO systems could be disposed of CONCLUSIONS by ocean discharge. As a result, many coastal utilities A portfolio of treatment options, including en- still favor RO to mitigate salinity and risks from trace gineered and managed natural treatment processes, organic chemicals to produce high-quality water for exists to mitigate microbial and chemical contami- potable reuse. However, limited cost-effective con- nants in reclaimed water, facilitating a multitude of centrate disposal alternatives hinder the application of process combinations that can be tailored to meet membrane technologies for water reuse in inland com- specific water quality objectives. Advanced treatment munities. Instead, inland potable water reuse projects processes are capable of also addressing contemporary are increasingly relying on treatment trains that do not water quality issues related to potable reuse involving include RO, such as process combinations that involve emerging pathogens or trace organic chemicals. Ways managed natural treatment systems, activated carbon, to integrate these technologies through alternative ozonation, or AOP. The lack of clear and standardized guidance for system designs that ensure water quality are discussed design and operation of engineered natural systems in Chapter 5. Advances in membrane filtration have made is the biggest deterrent to their expanded use, in membrane-based processes particularly attractive for particular for potable reuse applications. Engineered reuse applications. Membrane advances have resulted natural systems that replace certain advanced treat- in treatment approaches for nonpotable and potable ment unit processes are compelling from an opera- reuse applications that are associated with a smaller tional standpoint, but little is known how operating space requirement, reduced labor requirement, ease of conditions could be modified and retention times process automation, more effective pathogen removal shortened to achieve a predictable water quality while (in particular with respect to protozoa and bacteria), using a smaller footprint. Additional research is needed consistent effluent quality, and potentially reduced to elucidate key attenuation processes in engineered chemical demand. The drawbacks of this technology natural systems and quantify their effects on microbial are potentially higher capital costs, the limited life span and chemical constituents of concern so that guidance of membranes, the complexity of the operation, and for design and operation can ultimately be developed. the potential for irreversible membrane fouling that Although each application will still require a thorough reduces productivity. Unlike robust conventional media site-specific assessment, general design standards and filters, membrane systems require a higher degree of operating procedures as well as appropriate monitoring maintenance and strategies directed to achieve optimal approaches can foster a wider application of natural performance. systems as part of reuse schemes. Environmentally sustainable and cost-effective 2 See http://water.epa.gov/scitech/swguidance/standards/ criteria/nutrients/progress.cfm.