Safe Water From Every Tap Improving Water Service to Small Communities (1997) / Chapter Skim
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3: Technologies for the Small System
3: Technologies for the Small System
Pages 48-131
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From page 48... ...
has ruled that all surface waters must be filtered and disinfected before consumption unless the purveyor can justify avoidance of filtration; some surface waters also need to be treated with additional processes to remove chemical contaminants before they are suitable for use as drinking water. Many ground water sources are disinfected, and many are treated to remove nuisance chemicals (such as iron and manganese)
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From page 49... ...
For very small systems, treatment may not be a feasible alternative because of the high cost of having a treatment system designed and installed and the complexity of maintaining it. Historically, the design of drinking water treatment systems has been driven by the need to remove microbial contaminants and turbidity.
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From page 50... ...
50 TABLE 3-2 Treatment Technologies by Contaminant Type Disinfectants/Oxidants Air Stripping Systems Free Ultraviolet Membrane Cl2 NH 2Cl ClO2 O3 Radiation KMnO4 Aeration Aeration General water quality parameters Turbidity Color X X Disinfection byproduct precursors Taste and odor X X X X X Biological contaminants Algae Protozoa X X Bacteria X X X X X Viruses X X X X X Organic chemicals Volatile organic compounds X X (VOCs) Semivolatile compounds X Pesticides Biodegradable organic matter SAFE WATER FROM EVERY TAP
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From page 51... ...
Inorganic chemicals Hardness (calcium and magnesium) Iron X X X X X Manganese X X X X X Arsenic Selenium Thallium Fluoride Radon X X Radium Uranium Cations Anions TECHNOLOGIES FOR THE SMALL SYSTEM Total dissolved solids Nitrate Ammonia continued on next page 51
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From page 52... ...
52 TABLE 3-2 Continued Adsorption Systems Powdered Granular Activated Activated Ion Activated Carbon Carbon Exchange Alumina General water quality parameters Turbidity Color X X Disinfection byproduct precursors X X Taste and odor X X Biological contaminants Algae X Protozoa X Bacteria X Viruses X Organic chemicals VOCs X X Semivolatile compounds X X Pesticides X X Biodegradable organic matter X X SAFE WATER FROM EVERY TAP
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From page 53... ...
Inorganic chemicals Hardness X Iron Manganese Arsenic X Selenium X Thallium X Fluoride X X Radon Radium X Uranium Cations X Anions X Total dissolved solids X Nitrate X TECHNOLOGIES FOR THE SMALL SYSTEM Ammonia X continued on next page 53
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From page 54... ...
54 TABLE 3-2 Continued Membrane Processes Electrodialysis/ Reverse Electrodialysis Microfiltration Ultrafiltration Nanofiltration Osmosis Reversal General water quality parameters Turbidity X X X Color X X X Disinfection byproduct precursor X X X Taste and odor Biological contaminants Algae X X X Protozoa X X X X Bacteria X X X Viruses X X Organic chemicals VOCs Semivolatile compounds X Pesticides X X Biodegradable organic matter SAFE WATER FROM EVERY TAP
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From page 55... ...
Inorganic chemicals Hardness X X X Iron X Manganese X Arsenic X X Selenium X X Thallium X X Fluoride X X Radon Radium X X Uranium X X Cations X X Anions X X Total dissolved solids X X TECHNOLOGIES FOR THE SMALL SYSTEM Nitrate X X Ammonia continued on next page 55
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From page 56... ...
56 TABLE 3-2 Continued Filtration Systems Dissolved Diatomaceous Slow Bag/ Direct Conventional Air Earth Sand Cartridge Lime Filtration Filtration Flotation Filtration Filtration Filters Softening General water quality parameters Turbidity X X X X X X Color X X X Disinfection byproduct X X X precursors Taste and odor X Biological contaminants Algae X X X Protozoa X X X X X X X Bacteria X X X X X X Viruses X X X X X X Organic chemicals VOCs Semivolatile compounds Pesticides Biodegradable organic Xa Xa Xa Xa SAFE WATER FROM EVERY TAP
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From page 57... ...
Inorganic chemicals Hardness X Iron X X X X X Manganese X X X X X Arsenic X X Selenium X Thallium Fluoride Radon Radium X Uranium Cations X Anions Total dissolved solids TECHNOLOGIES FOR THE SMALL SYSTEM Nitrate Ammonia Xa Xa Xa Xa aOperated in biologically active mode.
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From page 58... ...
A number of the treatment processes listed in Table 3-2 and described in more detail below are available to small communities as package plants. The term "package plant" is not intended to convey the concept of a complete water treatment plant in a package.
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From page 59... ...
Table 3-4 shows operating considerations -- raw water quality, operator skills, monitoring requirements, and costs -- for common treatment processes. As shown in the table, different treatment processes have different requirements for source water quality.
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From page 60... ...
TABLE 3-3 Capital Considerations for Treatment Technologies 60 State of Relative Technology Contaminants Technology Capital Cost All water sources Disinfection Microbiological contaminants Free Cl2 Conventional Low NH 2Cl Conventional Low ClO2 Accepted Low O3 Accepted Medium Ultraviolet radiation Accepted Medium Corrosion control Prevention of system corrosion, lead, copper Chemical feeders Conventional Low Limestone contactor Accepted Medium Membrane filtration systems Turbidity, protozoa (Giardia and Cryptosporidium) , algae, and the following: Microfiltration Some bacteria Accepted Medium Ultrafiltration Bacteria, some viruses, some color Accepted Medium Nanofiltration Bacteria, viruses, color, some organic Emerging Medium chemicals, hardness Reverse osmosis Bacteria, viruses, humic acids, some organic Conventional Medium chemicals, inorganic chemicals, hardness, radium, salts Electrodialysis/electrodialysis Inorganic chemicals (charged)
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From page 61... ...
Organic chemicals, tastes and odors, radon, Accepted Medium/high microorganisms Lime softening Hardness, iron, manganese, turbidity Accepted High Ground water sources Aeration Diffused air Volatile organic chemicals, radon, tastes and odors Accepted Low Mechanical aeration Volatile organic chemicals, radon, tastes and odors Accepted Low Tray aerators Volatile organic chemicals, radon, tastes and odors Conventional Low/medium TECHNOLOGIES FOR THE SMALL SYSTEM Packed tower aeration Volatile organic chemicals, radon, tastes and odors Conventional Medium Membrane aeration Volatile and semivolatile organic chemicals Emerging Medium radon, tastes and odors Oxidation/filtration Permanganate Reduced iron and manganese, organic chemicals, Conventional Low radon, tastes and odors O3 Reduced iron and manganese, organic chemicals, Accepted High tastes and odors continued on next page 61
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From page 62... ...
TABLE 3-3 Continued 62 State of Relative Technology Contaminants Technology Capital Cost Ion exchange Inorganic chemicals, radium, nitrate Accepted Medium Activated alumina Arsenic, thallium, selenium, fluoride, Accepted High other inorganic chemicals Surface water sources Coagulation-filtration Turbidity, color, disinfection byproduct precursors, microorganisms, algae, iron, manganese, biodegradable organic matter,a ammoniaa Direct filtration Accepted High Conventional, with sedimentation Conventional High Dissolved air flotation Accepted High Diatomaceous earth filtration Turbidity, algae, Giardia, Cryptosporidium, Accepted Medium/high biodegradable organic matter,a ammonia a Slow sand filtration Turbidity, microorganisms, biodegradable organic matter,a ammonia,a tastes and odors Uncovered filters Conventional in some states Medium Covered filters Conventional in some states Medium/high Bag filters and cartridge filters Giardia cysts and Cryptosporidium oocysts Accepted in some states Low SAFE WATER FROM EVERY TAP aIf operated in biologically active mode.
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From page 63... ...
However, because it does a poor job of killing Giardia and Cryptosporidium, UV radiation is not an accepted means for disinfecting surface waters, unless they have already been treated in a way that would physically remove the cysts and oocysts of the Giardia and Cryptosporidium. The chemical disinfectants used in drinking water treatment are free chlorine, chloramine, ozone, and chlorine dioxide.
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From page 64... ...
All waters Granular activated carbon (GAC) Surface water may require prefiltration Lime softening All waters Ground water sources Air stripping Diffused air All ground waters Mechanical aeration All ground waters Tray aeration All ground waters Packed tower aeration All ground waters Membrane aeration All ground waters Oxidation/filtration Tray aerators All ground waters Permanganate All ground waters O3 All ground waters Cl2 All ground waters Ion exchange All ground waters Activated alumina All ground waters Surface water sources Coagulation-filtration Direct filtration Needs high raw water quality Conventional, with sedimentation Can treat wide range of water quality Dissolved air flotation Very high algae OK, high color OK, moderate turbidity Diatomaceous earth filtration Needs very high water quality Slow sand filtration Needs very high water quality Bag and cartridge filters Need very high quality water a Refer to text for detailed description of water quality needs.
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From page 65... ...
TECHNOLOGIES FOR THE SMALL SYSTEM 65 Operator Skill Monitoring Relative Operating Level Required Requirements Cost Basic Low Low Basic Low Low Intermediate High Low Intermediate Low Medium Basic Low Low Basic Low Medium Basic Low Low Basic Low Low Basic Low Medium Basic Low Medium/high Advanced Medium High Advanced Medium High Intermediate Low Medium/high Basic Low/medium Medium/high Advanced High High Basic Low Low Basic Low Low Basic Low Low Intermediate Low Medium Intermediate Low Medium Basic Low Low Intermediate Medium Medium Intermediate Low Low Basic Low Low Intermediate Medium Medium/high Advanced Medium High Advanced High Medium/high Advanced High Medium/high Advanced High Medium/high Intermediate Medium Medium/high Basic Low Low, with good raw water Basic Low Low/high; depends on cycle length
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From page 66... ...
Since 1990, a considerable number of ozone facilities have come on line, although the percentage of utilities using ozone remains small compared to the percentages using chloramine or free chlorine. If ozone is used for disinfection of surface waters, the ozone can break down complex organic molecules into smaller organic molecules or molecular fragments that are more readily used by bacteria as a food source.
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From page 67... ...
The intensity or rigor of chemical disinfection provided in the treatment plant, before water is delivered to customers, is assessed in terms of CT, in which C is the residual concentration of the disinfectant in milligrams per liter and T is the time in minutes for which the water and disinfectant chemical were in contact. The product of these parameters is a measure of the effectiveness of disinfection and is used to determine compliance with drinking water standards.
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From page 68... ...
Disinfection byproduct compliance is more likely to be a problem for small water systems treating surface waters than for those treating ground waters because surface water sources tend to contain more natural organic matter that forms byproducts when mixed with disinfectants. Formation of byproducts depends on the quality of the source water and on the disinfectant used.
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From page 69... ...
A shortcoming of many small systems, particularly those with package plants, is the small amount of disinfection contact time (T) available.
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From page 70... ...
This type of feeder is most appropriate for disinfection of ground water, but a clever operator probably could adapt it to treatment of surface waters. Another type of chlorine feeder works on the erosion feed principle.
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From page 71... ...
They also revert to orthophosphates, and this is thought to be a major reason for their effectiveness in controlling lead and copper concentrations at the tap. Some ground waters have high concentrations of carbon dioxide (CO2)
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From page 72... ...
For orthophosphates and polyphosphates, pH control is important, because the orthophosphates work best at a pH in the range of 7.2 to 7.8 for lead and copper control. Aeration to strip CO2 from ground water could result in oxidation of dissolved iron and thus might be inappropriate for some waters or might require use of additional treatment processes for removal of precipitated iron.
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From page 73... ...
Use of aeration for stripping CO2 from ground water also is a manageable process for small systems, although it must be carefully controlled to prevent excessive calcium carbonate precipitation in the distribution system. Membrane Filtration Systems How the Process Works Once considered a viable technology only for desalination, membranes are increasingly employed for removal of bacteria and other microorganisms, particulate material, and natural organic matter, which can impart color, tastes, and odors to the water and react with disinfectants to form disinfection byproducts.
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From page 74... ...
, by permission. ©1995 by the American Water Works Association.
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From page 75... ...
diameter each, with the selective skin layer either on the interior or exterior surface of the tube. If the skin is on the interior surface of the tubes, pressurized source water is fed through the inside of the tubes, permeate water passes through the pores in the membrane, and the concentrate water with its impurities remains inside the fibers.
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From page 76... ...
To avoid this, water must be relatively free of particulate material prior to entering a membrane module. Surface waters may require pretreatment by a more conventional treatment process prior to polishing by membrane filtration, although membrane systems are capable of tolerating a lower quality surface water than direct filtration systems (discussed later in this chapter)
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From page 77... ...
Unlike conventional treatment processes, in which approximately 5 to 10 percent of the influent water is discharged as waste, membrane processes produce waste streams amounting to as much as 15 percent of the total treated water volume. Because little or no chemical treatment is used in a membrane system, the concentrate stream usually contains only the contaminants found in the source water (although at much higher concentrations)
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From page 78... ...
In addition, for ground water sources not needing pretreatment, membrane technologies are relatively simple to install, and the systems require little more than a feed pump, a cleaning pump, the membrane modules, and some holding tanks. Most experts expect that membrane filtration will be used with greater frequency in small systems as the complexity of conventional treatment processes for small systems increases.
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From page 79... ...
Reverse osmosis produces a larger volume of reject concentrate solution than membrane filtration. The concentrate volume can be as much as 25 to 50 percent of the raw water volume.
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From page 80... ...
Humic materials, particulate matter, microorganisms, and viruses are generally removed in the process, but the bypass water will add microbiological contaminants to the treated water when the two are mixed to reduce corrosiveness. Also, leaks of concentrate water containing bacteria and viruses can occur around o-ring seals under the high operating pressures of a reverse osmosis system.
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From page 81... ...
Prior to installation of a reverse osmosis system, the town's approximately 1,200 residents experienced problems with the deep-well ground water they use. The source water has high levels of dissolved solids, which imparted a salty taste to the water and damaged equipment such as water heaters and washing machines.
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From page 82... ...
. Hydrogen sulfide is highly unlikely to be present in surface water and would generally be a concern only for ground water sources.
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From page 83... ...
but are trapped by the cation selective membranes (C) ; sodium ions move toward the cathode and pass through the cation selective membranes but are trapped by the anion selective membranes.
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From page 84... ...
. Like water treated by reverse osmosis and nanofiltration, ED/EDR water is corrosive, so some bypassed water may be needed to stabilize the product water.
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From page 85... ...
Because of its small particle size, adsorption to the surface occurs quickly. The normal contact time of mixing basins used for other elements of water treatment is sometimes sufficient for contaminant adsorption onto PAC.
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From page 86... ...
Even a small amount of PAC passing through the system can cause the water to turn gray. In addition, if PAC enters a sample vial used for determining whether the treated water meets drinking water standards, the apparent aqueous concentration of the target contaminants can exceed regulatory standards because these contaminants will be concentrated on the PAC.
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From page 87... ...
However, the combination of variable source water quality and the complexity of the chemistry of lime softening may make lime softening too complex for small systems that use surface water sources. Lime softening may be more appropriate for small systems that use ground water because of the relatively uniform quality of ground water.
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From page 88... ...
Prefabricated lime softening equipment is available for use by small systems. TECHNOLOGIES FOR SYSTEMS WITH GROUND WATER SOURCES Ground water sources generally require less treatment than surface water supplies or ground water supplies that are under the influence of surface water.
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From page 89... ...
As mentioned in Chapter 2, the most common chemical contaminants in ground water are nitrate, fluoride, and volatile organic chemicals. In addition to the technologies mentioned above, which are suitable to both surface and ground water systems, some technologies are best suited to the types of contamination found in ground water.
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From page 90... ...
Pretreatment for removal of microorganisms, iron, manganese, and excessive particulate matter is important for this design. Packed tower aerators have been used for decades in water and wastewater treatment.
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From page 91... ...
TECHNOLOGIES FOR THE SMALL SYSTEM 91 FIGURE 3-6 Cutaway diagram of a tray aerator in a package treatment system. SOURCE: Courtesy of General Filter Company, Ames, Iowa.
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From page 92... ...
, particulate matter, calcium precipitates, or iron precipitates. Reduced iron or manganese in ground water will oxidize when exposed to air and will precipitate.
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From page 93... ...
However, because of the difficulty of controlling the chemistry of such reactions when water quality varies, as in surface water, it is unlikely that a small surface water system would use oxidation/filtration. The primary use of this technology by small systems is for removal of iron and manganese from ground water sources.
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From page 94... ...
The expense of operation derives from the chemical use in most cases and is therefore directly related to the source water quality. Monitoring and Operating Requirements Oxidation followed by filtration is a relatively simple process.
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From page 95... ...
Suitability for Small Systems Oxidation using chlorine or potassium permanganate is frequently applied in small ground water systems. The dosing is relatively easy, requires simple equipment, and is fairly inexpensive.
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From page 96... ...
In particular, reduced iron species in ground water may become oxidized when the water is exposed to oxygen in the atmosphere and form precipitates that can damage the resin.
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From page 97... ...
Ion exchange is a common water treatment technology, available in point-of-use and point-of-entry devices as well as fullscale treatment plants. It is readily adaptable to small treatment plants.
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From page 98... ...
Generally, the cost of an activated alumina system, including capital and operating costs, is quite high compared to other water treatment processes. In addition, operation of these systems requires advanced knowledge of water treatment principles and practice.
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From page 99... ...
TECHNOLOGIES FOR SYSTEMS WITH SURFACE WATER SOURCES Historically, the primary concerns when treating surface water have been inactivation of microbial contaminants to prevent the spread of waterborne disease and removal of turbidity to make the water more palatable and to ensure that particles that may harbor microorganisms are not conveyed to the consumer's tap. As a consequence, the EPA's Surface Water Treatment Rule (SWTR)
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From page 100... ...
Source water quality should be relatively stable. If raw water turbidity can increase by a factor of 10 in one day's time, the direct filtration process may not be appropriate.
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From page 101... ...
Another difficult condition for water treatment is the combination of high color and moderate to high turbidity. The pH that is best for color removal may be different from the pH that is best for turbidity removal.
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From page 102... ...
In addition, sample taps should be provided so the operator can obtain samples of raw water, pretreated water, and filtered water for analysis. When equipment is entrusted to part-time operators, the foremost operating requirement for any small system treatment process is simplicity and ease of operation.
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From page 103... ...
Because of the relatively short detention times in package plants, many small systems treating surface water with package plants may need to provide for separate treated water storage facilities at the plant site to attain adequate CT values. The availability of package plants has encouraged application of coagulation/filtration technology to small systems.
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From page 104... ...
In the DAF process, raw water is coagulated and flocculated. Flocculated water flows to a basin where the floc is floated to the water surface by a cloud of microscopic bubbles, in contrast to conventional treatment employing a sedimentation process in which the solids settle to the bottom.
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From page 105... ...
This reinforces the concept that a water treatment plant employing DAF for solids separation in pretreatment should be considered the equivalent of a conventional treatment plant for regulatory compliance purposes. The very short detention times in flocculation and flotation, however, mean that storage may be needed after filtration to increase the disinfectant contact time.
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From page 106... ...
Industries have long used the process for filtration of liquids. The technology was developed for potable water treatment during World War II.
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From page 107... ...
is fed through the filter. During a filter run, removal of particulate matter in the raw water, plus the accumulation of body feed diatomaceous earth material, causes the head loss to build up in the filter.
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From page 108... ...
If the nature of the turbidity-causing particulate matter remains stable, it may be possible to establish a ratio between the raw water turbidity and the appropriate dose of diatomaceous earth for use in the body feed. This situation might apply to treatment of lake water, for example.
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From page 109... ...
Slow Sand Filtration How the Process Works In slow sand filtration, biological action breaks down some organic matter, and some inert suspended particles are physically removed from the water. Slow sand filtration was the original form of water treatment used by municipalities in the nineteenth century and is now considered a low-technology approach to water treatment.
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From page 110... ...
Slow sand filters are not very effective at removing disinfection byproduct precursors or color. If the biological action within a filter bed were effective for removal of organic matter of this type, biological action in lakes and rivers would have already removed the organic matter from the source water.
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From page 111... ...
The package slow sand filter produced filtered water turbidity averaging less than 1 formazin turbidity unit (FTU) through the study, and filtered water turbidity remained at less than 2 FTU even when raw water turbidity was as high as 94
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From page 112... ...
are suitable for application to slow sand filters without pretreatment or process modifications such as the use of a GAC layer in the filter. When used with source water of appropriate quality, however, this process may be the most suitable choice for small systems that must filter surface water.
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From page 113... ...
Appropriate Water Quality and Performance Capabilities Because filtration of some types of particles can blind bag and cartridge filters, these filters are appropriate only for high-quality waters. In fact, source water quality for bag filters should be higher than the quality for slow sand filters.
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From page 114... ...
In addition, these filters will not remove chemical contaminants present in solution. Bag filters and cartridge filters are most appropriate for treatment of very-high-quality source waters for removal of protozoan cysts and are best suited for very small systems, such as those serving fewer than 500 people.
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From page 115... ...
The types of water treatment problems addressed at a given site will determine to a large extent the level of remote monitoring and control desired. If a short-term disfunction in a system could result in a high risk of an acute health effect such as breakthrough of Giardia or Cryptosporidium, or nitrate levels high enough to cause methemoglobinemia in infants, a high level of remote monitoring and control is advisable.
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From page 116... ...
water treatment devices in their customers' homes or distributing bottled water. These alternatives generally are appropriate for system-wide use only for very small systems, particularly those serving 500 or fewer people.
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From page 117... ...
The discharge line should be installed with an air gap so a cross-connection between wastewater and drinking water will not occur. Reverse osmosis and other membrane technologies are among the fastest-developing types of technology with possible applications for POE systems.
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From page 118... ...
118 SAFE WATER FROM EVERY TAP FIGURE 3-10 Examples of under-the-sink POU units. The top unit treats all the water flowing to the kitchen faucet.
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From page 119... ...
Appropriate Water Quality Although POE and POU systems may in some instances be used to treat surface waters, in a regulatory setting they would be appropriate only for ground water because of the frequency of monitoring (daily) necessary with surface water treatment and because of the necessity of ensuring thorough disinfection of surface water.
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From page 120... ...
However, replacement based on time may result in equipment being replaced too early or too late. The former case would waste resources, while replacing equipment too late could result in the consumption of drinking water that exceeds one or more of the drinking water standards.
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From page 121... ...
For a community consisting of 50 homes and served by a central treatment facility, regulatory compliance monitoring for most of the regulated contaminants could be done at the discharge point from the treatment plant or at the point of entry to the distribution system. If POE or POU devices were used instead of central treatment, the community of 50 homes would have 50 water treatment devices, any one of which might possibly malfunction or reach its capacity for effective treatment at some time.
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From page 122... ...
• Effective monitoring: A monitoring program must be developed and approved by the state regulatory agency before POE or POU systems are installed. Such a monitoring program must ensure that the systems provide health protection equivalent to that which would be provided by central water treatment meeting all primary and secondary standards.
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From page 123... ...
NSF International has issued seven standards related to the testing of POE and POU devices: 1. standard 42, which covers the ability of GAC and mechanical filtration to improve the aesthetic qualities of drinking water; 2.
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From page 124... ...
Table 3-5 shows a cost comparison for using POE versus adding a GAC treatment system to the water treatment plant for a community with between 10 and 50 households (Goodrich et al., 1992)
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From page 125... ...
However, the objections to using POE and POU treatment devices are substantial and have merit, particularly as the system size increases and the complexity of monitoring and servicing the devices increases. Using centralized water treatment should be the preferred option for very small systems, and POE or POU treatment should be considered only if centralized treatment is not possible.
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From page 126... ...
. The majority of bottled water is purchased for aesthetic reasons rather than for quality reasons related to drinking water regulations.
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From page 127... ...
Army Corps of Engineers' Emergency Water Plan, and the EPA's National Contingency Plan under the Superfund act. In addition, the EPA rules specify that bottled water, like POU devices, may be used on a temporary basis to avoid anunreasonable risk to health or as a condition of a variance or exemption to drinking water regulations.
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From page 128... ...
However, other strategies, such as use of ozone prior to treatment followed by use of chloramine in the water distribution system, may be needed to minimize the formation of disinfection byproducts that are already or will soon be regulated. • For small systems using ground water sources, the most commonly reported chemical contaminants influencing the selection of water treatment systems are nitrate, fluoride, and volatile organic compounds.
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From page 129... ...
1991. Source water quality and pretreatment options for slow sand filters.
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From page 130... ...
1992. Point-of-Use/Point-of-Entry for Drinking Water Treatment.
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From page 131... ...
drinking water by reverse osmosis. Journal of the American Water Works Asso ciation 72(4)
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