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11 Elements of Public Water Supplies Fundamentally, a water supply system may be described as consisting of three basic components: the source of supply, the processing or treatment of the water, and the distribution of water to the users. Water from the source is conveyed to the treatment plant by conduits or aqueducts, either by pressure or open-channel flow. Following treatment, the water enters the distribution system directly or is transported to it via supply conduits. SOURCE For a public water supply, the raw water source must provide a quantity sufficient to meet all municipal, institutional, and industrial uses as well as the fire-fighting demand. Either surface water or groundwater may be used. Although most water systems are supplied from only one source, there are instances when both surface water and groundwater sources are utilized. Surface water is drawn from large rivers or lakes. Even a small stream may be suitable if it is impounded by a dam. Groundwater is normally ob- tained by sinking wells into the saturated zone located beneath the water table. RAW WATER QUALITY AND TREATMENT The quality of surface waters varies. Characteristically, such waters con- tain microorganisms as well as inorganic and organic particulate matter 9

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10 DRINKING WATER AND HEALTH and dissolved solids. They may also have an undesirable color' taste, and odor. Surface waters are subject to contamination by sewage from cities, industrial wastes, agricultural runoff, and waste from animals and birds. The temperature of surface waters fluctuates with climatic variations. Although groundwaters are also subject to contamination as a result of human activities, they are often clear, colorless, and possess lower concen- trations of organic matter and microorganisms than does surface water because of the natural filtration effected by the percolation of water through soil, sand, or gravel. Conversely, the mineral content, including calcium and magnesium ions the main contributors to "water hardness" may be higher in groundwater than in the nearby surface waters. In general, the mineral content of groundwaters reflects the mineral characteristics of the soil in the area. Over time, the quality of groundwaters is usually more constant than that of surface waters. The temperatures of groundwaters are also more constant, normally ap- proaching the average annual temperature of the region instead of the constant fluctuations reflected in the temperatures of surface waters. To be made acceptable for a public water supply, groundwater may re- quire only disinfection to ensure adequate health protection. On the other hand, it may be necessary to remove certain objectionable constituents in the water and/or to reduce others to tolerable limits, depending upon the type of contamination, applicable criteria or standards, and/or the desire of the users. Surface waters normally require more extensive treatment than do groundwaters. Treatment of raw water might include coagula- tion, sedimentation, filtration, softening, and iron removal in addition to disinfection. The corrosiveness of surface waters and groundwaters varies widely, depending on their pH, hardness, and other characteristics. Some waters may also contain dissolved minerals, which deposit on the inside of pipelines, resulting in scale formation. Highly corrosive raw waters may be treated to reduce this property in conjunction with other types of treat- ment required. The temperature of treated water is generally the same as that of raw water. Slight changes may be produced by ambient air temperature during the detention time in the treatment plant. High water temperatures accelerate corrosive action and decrease the viscosity of water. DISTRIBUTION OF WATER That portion of a public water system transporting water from the treat- ment plant to the users is called the distribution system. Physical aspects

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Elements of Public Water Supplies 11 such as the design, construction. and operation of such systems can have important impacts on water quality. The complexity of and demands on these systems make them the most costly single element in the water sup- ply system. To avoid possible contamination and because it is delivered to the con- sumers under pressure. treated or finished Hater is transported in con- duits or pipes rather than by open channels. In addition to a network of interconnecting mains or pipes, water distribution systems normally in clude storage facilities, valves, fire hydrants, service connections to user facilities, and perhaps pumping facilities. The ability of a distribution system to deliver an adequate quantity of water to meet the present and projected demands of the domestic, commercial, and industrial users and to provide the necessary flow for fire protection depends upon the carrying capacity of the system's network of pipes. In all but the largest systems, the flow that is necessary to combat a major fire is usually the major factor determining requirements for the amount of water to be stored, the size of mains in the system, and the pressure to be maintained. Fire flow stan- dards require a minimum residual water pressure of 20 pounds per square inch gauge (psi") during flow. It is common practice to maintain pressures of 60 to 75 psig in industrial and commercial areas and 30 to 50 psig in residential areas. Distribution system mains and pipes must be designed to withstand these pressures. The flow of water distribution systems may be controlled either by gravity or by pressure (pumping). Often, public water supply systems use some combination of both. In gravity systems, water is impounded at strategic locations sufficiently elevated to create the working pressure re- quired to move the water to the points of demand. When elevated im- poundment or storage is impractical, the required working pressure is provided by pumps within the system. In these pressure systems, the pumps are normally located at the treatment plant and perhaps within the distribution system. In combined systems, facilities for water storage are often provided along with provision for pumping. This type of system pro- vides for the storage of water during times of least demand while assuring that a sufficient quantity is available to meet the peak demand. Typically, water is pumped directly into the distribution system. The quantity of water exceeding the demand automatically feeds into a storage facility or reservoir. A system may also be designed so that pumps supply the water storage facility directly; the water, in turn, might flow into the distribution system by gravity. Reservoirs may be located at the beginning of a distribution system, i.e., immediately following water treatment, or at an intermediate point in the system. The stored water may be used to meet fluctuating demands or

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12 DRINKING WATER AND HEALTH to equalize rates of flow or operating pressures on the system. The reser- voirs may be classified as underground, ground level, elevated, or stand- pipe. An underground reservoir or basin, either open or covered, may be at or below grade level and formed either by excavation or embankment. It is customary to line such reservoirs with concrete, Gunite, asphalt or an asphalt membrane, or butyl rubber. A standpipe consists of a cylindrical shell with a flat bottom resting on a foundation at ground level. An elevated reservoir is a tank supported above ground by a structural framework. Steel and wood have been used in the construction of stand- pipes and elevated tanks, which are normally covered. It is preferable to use covered reservoirs for treated water because water in open reservoirs is subject to falling dust, dust-borne microorganisms, and soot; to con- tamination by animals, including birds and human beings; and to algal growth. It may be necessary to control algae and microbial slime growths in open distribution reservoirs by adding copper sulfate and/or chlorine to the water. Furthermore, to ensure adequate disinfection, it is generally believed that there should be a sufficient chlorine residual throughout the distribution system. In a large distribution system, dechlorination of the water may be required. This is often accomplished at the distribution reservoirs. The detailed layout of a distribution system and its flow characteristics depend upon the area to be served and its topography, the street plan, the location of the source of supply, and other variables. Regardless of the type of system, there is usually at least one primal feeder line or transmission main that transports a large quantity of finished water from the treatment plant and/or pumping facility to a specific location within the system. If the distribution system is large, there may be more than one transmission main, each serving a specific geographical area within the overall system. This flow is then distributed locally to the users through a series of progressively smaller pipes or mains. The buildings being served are connected to the mains by small pipes called service lines or connec- tions. This network of various sizes of interconnecting pipes is normally designed as a grid with a series of loops to avoid dead ends. The result is a circulating system capable of supplying water to all points within the system, sustaining service even if a section must be removed for maintenance and repair or if a portion of the system must be taken out of service because of contamination. To accomplish this, all distribution systems should have sufficient numbers, types, and sizes of valves so that different sections can be isolated. Mains are usually made of cast iron, ductile iron, steel, reinforced con- crete, plastic, or asbestos-cement. The type of pipe used is dictated by cost

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Elements of Public Water Supplies 13 considerations, local conditions, and the size pipe required. The piping material for service lines, i.e., household connections, may be galvanized wrought iron, lead, galvanized steel, copper, plastic, cast iron, or ductile iron. Of these, copper appears to be the most widely used. Lead, copper, zinc, aluminum, and such alloys as brass, bronze, and stainless steel may also be used in addition to ferrous metals in pumps, small pipes, valves, and other appurtenances. Linings may be used to prevent corrosion and/or to reduce the roughness of pipes. For example, iron and steel pipes and fittings are often lined with cement mortar and/or with bituminous material. Plastic piping may also be used in water distribution systems, especially in household connections. Thermoplastic material used in plastic pipe include polyvinyl chloride (PVC), polyethylene (PE), ac~ylonitrile-butadiene-styrene (ABS), polybutylene (PB) plastic, and fiberglass-reinforced plastic (FRP). Piping used in the distribution system is manufactured in different lengths, depending upon the material and size, and must be joined. Several types of joints are used with pipes of a given material. The joints for cast-iron or ductile-iron pipes may be either bell-and-spigot, mechanical, flanged, threaded, or push-on (rubber gasket). With many joints, such as the bell-and-spigot, it is necessary to fill the space created in the joining of two ends of pipe. With cast-iron pipe, for example, the space may be packed or caulked with hemp or jute, and then lead poured into the joint to complete the seal. Thus, materials used for joints include lead and lead substitutes, sulfur compounds, cement mortar mixtures, and rubber together with asbestos, hemp, jute, and other substances ap- plied as packing. Sections of steel pipe may be joined by welding, rubber gasket seals, threading, or mechanical coupling. Sections of asbestos- cement pipe are usually coupled with a push-on joint and a rubber ring. Plastic mains usually have push-on or roll-on joints, while flared, com- pression, clamp, or solvent joints are used with service lines. The carrying capacity of mains and smaller pipes is a function of their size and length, the pressure, and the resistance to flow, i.e., internal fric- tion, bends, or turns in the pipe, joints, control valves, and other devices. The internal surface of pipe, regardless of the material from which it is made, offers resistance to water flow. For example, new steel and unlined cast-iron or ductile-iron pipe have approximately equal resistance, while that associated with cement-lined cast-iron or ductile-iron, asbestos- cement, and plastic pipe is somewhat less. Encrustation caused by tuber- culation, rust, and sedimentary deposits of various salts, such as iron and manganese precipitates, will also increase the resistance of water flow. Tuberculation is believed to result from the corrosive action of water on metal pipes. The tubercles formed by the accumulation of corrosion prod

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14 DRINKING WATER AND HEALTH ucts often resemble barnacles. Microorganisms, through their bio- chemical reactions, are also involved in corrosion and the formation of tubercles. Sulfate-reducing bacteria may be involved in the latter process. The growth of other microorganisms, including the iron bacteria. causes the build-up of biological slimes, which also contributes to frictional resistance. In a distribution system, these events may lead to a deteriora- tion in the quality of water delivered to the users. It is beyond the scope of this report to review established engineering practice as it relates to the proper design, construction, and operation of distribution systems. It should be recognized, however, that random or accidental events, such as pipe breakage, or situations leading to cross- connections or back-siphonage, may severely affect the chemical or bacteriological quality of water in distribution systems and, thus, the water delivered to the users. Table II-1 lists several recent incidents of this type and their consequences. The quality of water in distribution systems may also be affected by cross-connections, which can be any direct or indirect physical connection TABLE II-1 Some Recent Incidents Caused by Hydraulic Problems Within Distribution Systems and Resulting Effectsa Incident Result Extermination contractor caused 3 gallons (11.4 liters) of chlordane to be back- siphoned into the distribution system. Wastewater from a meat processing plant contaminated plant's potable water. Water from fire-protection system in a steam electricity-generating plant contaminated plant's potable water. Ethylene glycol from fire-protection system contaminated potable water sup- ply at two Air Force bases. Boiler in a school was receiving a heavy dose of chemical at the same time the water supply to the school was shut off. Loss of pressure resulted in back- siphonage of treated boiler water into drinking water. Contamination of drinking water. No im- mediate effects reported. U.S. Department of Agriculture destroyed approximately 2.9 million pounds (1.3 million kg) of pork. Gastroenteritis suffered by 31 of 160 em- ployees. No hospitalizations. "Temporary" illnesses reported. Two students were hospitalized, and 11 were treated and released. a From Springer. 1980.

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Elements of Public Water Supplies 15 or structural arrangement that permits nonpotable water or water of ques- tionable or known poor quality to enter or back-flow into a potable water supply (Angele, 19741. An arrangement whereby a safe water system- is physically joined to a system containing unsafe water or wastewater is con- sidered a direct connection. If the arrangement is such that unsafe water may be blown, sucked, or otherwise diverted into a safe system, the con- nection is considered indirect. Contaminated water may enter a potable supply either through the distribution system or through a defect in the user's plumbing system. Cross-connections, together with back-flow or back-siphonage, are the most critical factors in protecting a distribution system from contamination. Back-siphonage occurs when a potable or safe system is at a pressure less than atmospheric; in this situation, the at- mospheric pressure on the unsafe system will force the flow toward the partial vacuum associated with the safe system. Prevention of back-flow may be accomplished by using vacuum breakers designed to admit air to break any vacuum in a water main or pipe, swing connections that permit a connection either to a potable water supply or to another source of water but not to both simultaneously, air gaps, and reduced pressure back-flow preventers, i.e., a device with at least two independently acting check valves separated by an automatic pressure differential relief valve. APPROACH TO THE STUDY Although the quality of water in a public water supply may be acceptable immediately after treatment, it may deteriorate before it reaches the users. This may result from either chemical or biological transformations. Public water supplies are disinfected to inactivate infectious agents, to protect the users against possible recontamination, and to control subse- quent microbial growths that might alter the quality of the water. For these reasons, it is normal practice to add chlorine to a water supply to provide a residual concentration that will persist until the water reaches the user. However, small quantities of chlorine or the loss of chlorine residual in a distribution system may lead to microbial regrowth s and/or the development of slime growth, which may in turn affect the turbidity of the water or cause taste and odor problems. For example, the depletion of dissolved oxygen resulting from microbial activity may promote the pro- duction of hydrogen sulfide by sulfate-reducing bacteria. Furthermore, the microbial production or release of metabolic products, e.g., endotox- ins or extracellular products of algae, may affect the health of the users directly. There is evidence that the corrosion of unlined steel, cast-iron,

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16 DRINKING WATER "D HEALTH and ductile-iron mains may be significantly influenced by microbial activ- ity. Thus, it is possible for microorganisms to alter the quality of water in distribution systems before it reaches the users. The corrosion of metals may not only change the surface properties of pipe but also produce soluble corrosion products, which in turn may affect the quality of the water. There is also the possibility that certain consti- tuents in cast-iron, asbestos-cement, concrete, plastic, and other pipe materials may be leached into the water. The formation of scale and deposits on the wall of pipes during periods of low-velocity flow may lead to the release or resuspension of associated materials when the velocity of the water is increased. This report reviews the factors or potential conditions associated with water distribution systems and their effects on water quality, with par- ticular attention to their possible impact on the health of users of public water supplies. The discussions focus on finished water, i.e., quality changes occurring between the time the water leaves a treatment plant and the time it reaches the users. Chemical control at the treatment plant is considered only as it affects changes of quality in the finished water in the distribution system. Having reviewed and evaluated those conditions or factors influencing the deterioration of water quality in distribution systems and, in a sense, determined what is known and unknown, the committee was able to make recommendations regarding control pro- cedures and to identify existing research needs. It is beyond the scope of this report to consider in depth the physical reliability or integrity of a public water supply system. However, * is im- portant to recognize that the quality of water in distribution systems can be adversely affected if the system is not designed, constructed, and main- tained according to accepted engineering practice. For example, a cross- connection in the system, together with back-flow or back-siphoning, which permits an unsafe water or even wastewater to enter the system, represents a most serious source of potential contamination. This altera- tion in water quality can impose a direct health risk upon the users or make the water aesthetically unacceptable. Leaks or mechanical breaks in the piping of the distribution system can have the same effect. In this sense, attention must be given to proper repair procedures or, for that matter, to the installation of piping, e.g., the need to adequately disinfect new or repaired distributional pipe before placing it in service. Further, pumping equipment must be adequate to meet the demands of the system, and reserve units, ready to be placed in service immediately when needed, must be available. Low-velocity flow, whereby the water has extended con- tact with pipes, can also cause a deterioration in water quality. Dead ends in the distribution system or the oversizing of pipes or mains can con

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Elements of Public Water Supplies 17 tribute to this stagnation. Finally, it should be pointed out that open distribution or service reservoirs in the system can also lead to contamina- tion of the water before it reaches the users. Again, the importance of hav- ing a properly engineered and operated distribution system as it relates to reliability cannot be overemphasized. Water supply personnel must be constantly vigilant of defects and problems associated with distribution systems as they might impact water quality. For example, each public water system needs a continuous cross-connection control program. In considering the reliability of a distribution system, it would seem ap- propr~ate to point out that water purveyors are legally responsible if any illness or death results from a system defect. REFERENCES Angele, G. J., Sr. 1974. Cross Connections and Backflow Prevention, 2nd Ed. American Water Works Association, Denver, Colorado. Springer, E. R. 1980. A sip could be fatal. Pp. 81-88 in Proceedings, American Water Works Association Distribution System Symposium. American Water Works Association, Denver, Colorado.