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CHAPTER 18 MICROBIOLOGICAL QUALITY CONTROL IN DISTRIBUTION SYSTEMS Edwin E Geldreich, M.S Consulting Microbiologist Cincinnati, Ohio Mark LeChevallier, Ph.D Research Director American Water Works Service Co Voorhees, New Jersey The purpose of a water supply distribution system is to deliver to each consumer safe drinking water that is also adequate in quantity and acceptable in terms of taste, odor, and appearance Historically, the initial network of pipes was a response to present community needs that eventually created a legacy of problems of inadequate supply and low pressure as the population density increased (Frontinus, 1973; Baker, 1981) To resolve the problems caused by increasing water demand along the distribution route, reservoir storage was created Pressure pumping to move water to far reaches of the supply lines and standpipes was incorporated to afford relief from surges of pressure In some areas, population growth exceeded the capacity of a water resource, so other sources of water were incorporated and additional treatment plants were built to feed into the distribution network Another response was to consolidate neighboring water systems and interconnect the associated distribution pipe networks GENERAL CONSIDERATIONS FOR CONTAMINATION PREVENTION Today (1999), community expansion plans are more fully developed and include the engineering of utility service so that careful consideration is given to meeting future projected water supply needs Advanced planning provides the opportunity to 18.1 18.2 CHAPTER EIGHTEEN design the pipe network as a grid with a series of loops to avoid dead ends The objective is to produce a circulating system capable of supplying high quality water to all areas while at the same time permitting any section to be isolated for maintenance, repair, or decontamination without interrupting service to all other areas To ensure delivery of a high-quality municipal potable water supply to each consumer, managers of public water supply systems must be continually vigilant for any intrusions of contamination or occurrences of microbial degradation in the distribution network This job is complicated by the very nature of the distribution system— a complex network of mains, fire hydrants, valves, auxiliary pumping, chlorination substations, storage reservoirs, standpipes, and service lines Following the intrusion of microbial contamination, any of these component parts may serve as a habitat suitable for colonization by certain microorganisms in the surviving flora The persistence and possible growth of organisms in the pipe network are influenced by a variety of environmental conditions that include physical and chemical characteristics of the water, system age, type of pipe materials, and the availability of sites suitable for colonization (often located in slow-flow sections, dead ends, and areas of pipe corrosion activity) ENGINEERING CONSIDERATIONS FOR CONTAMINATION PREVENTION Many public water utilities make substantial efforts to expand their distribution networks to keep up with continuing suburban growth Urban renewal and highway construction projects may at times require the relocation or enlargement of portions of the distribution network Corrosion, unstable soil, faulting, land subsidence, extreme low temperatures, and other physical stresses often cause line breaks and necessitate repair or replacement of pipe sections To avoid possible bacteriological contamination of the water supply during these construction projects, a rigorous protective protocol must be followed Distribution System Construction Practices The American Water Works Association has developed standard procedures that are used, with variations, by most of the water supply industry for disinfecting water mains (AWWA, 1986) In essence, these recommendations recognize six areas of concern: (1) protection of new pipe sections at the construction site; (2) restriction on the use of joint-packing materials; (3) preliminary flushing of pipe sections; (4) pipe disinfection; (5) final flushing; and (6) bacteriological testing for pipe disinfections Pipe sections, fittings, and valves stockpiled in yard areas or at the construction site should be protected from soil, seepages from water or sewer line leaks, stormwater runoff, and habitation by pets and wildlife (Becker, 1969; Russelman, 1969) Each of these contamination sources may deposit significant fecal material in the interior of pipe sections awaiting installation Septic tank drain fields, subsurface water in areas of poor drainage or high water table, and seasonal or flash flooding may also introduce significant contamination into unprotected pipe sections Fecal material introduced by contaminating sources may become lodged in pipe fittings and valves Thus, such sites become protected habitats from which coliforms and any associated pathogens in the contaminated material may be released into the bulk flow of water MICROBIOLOGICAL QUALITY CONTROL IN DISTRIBUTION SYSTEMS 18.3 supply Common-sense protective measures include providing end covers for these pipe materials, drainage of standing water from trenches before pipeline assembly, and flushing of new construction or line repairs to remove all visible signs of debris and soil (Suckling, 1943; Davis, 1951) Pipe-Joining Materials Gasket seals of pipe joints can be a source of bacterial contamination in new pipes (Hutchinson, 1971) Annular spaces in joints provide a protected habitat for continued survival and possible multiplication of a variety of bacteria in the distribution network In these instances, although the heterotrophic plate count (HPC) and any coliform occurrences may be temporarily reduced by main disinfection, bacteria soon become reestablished from the residual population harbored in some jointpacking materials In this regrowth process, the variety of organisms and dominance of strains change, often restructuring the bacterial flora to a predominant population of Pseudomonas aeruginosa, Chromobacter strains, Enterobacter aerogenes, or Klebsiella pneumoniae Thus, where the pattern of organisms present is predominantly one bacterial strain, a search for a protective habitat in joint-packing materials or impacted material in pipe sections should be made (Calvert, 1939; Adam and Kingsbury, 1937; Taylor, 1950, 1967–1968; Schoenen, 1986; Schubert, 1967) Nonporous materials such as molded or tubular plastic, rubber, and treated paper products are preferable Lubricants used in seals must be nonnutritive to avoid bacterial growth in protected joint spaces Efforts to develop bacteriostatic lubricants have resulted in the inclusion of various quaternary ammonium compounds that minimize contamination from pipe joint spaces (Hutchinson, 1974) The National Sanitation Foundation (NSF International, Ann Arbor, Michigan) has proposed a test method for evaluating the biological growth potential of materials that contact drinking water (Bellen et al., 1993) Although the method has not yet received final approval, some manufactures have submitted their products for testing, and in many cases the results are available from NSF upon request Water Supply Storage Reservoirs Water use in a community varies continuously as a reflection of the activities of the general public and local industries While industrial uses of potable water may be more predictable, expecting water treatment operations to gear production to those frequent and sudden changes in water demand from all consumers is impractical For this reason storage reservoirs are an essential element of the distribution network These water supply reserves supplement water flows in distribution during periods of fluctuating demand on the system, providing storage of water during offpeak periods; equalize operational water pressures; and augment water supply from production wells that must be pumped at a uniform rate Storage reservoirs also provide a protective reserve of drinking water to guard against discontinuance of water treatment during chemical spills in the source water, flooding of well fields, transmission line breaks, and power failures An important secondary consideration is providing adequate storage capacity for fire emergencies Finished water reservoirs may be located near the beginning of a distribution system, but most often they are situated near the extremities of the system Local topography plays an important part in determining the use of low-level or high-level reservoirs Underground storage basins are usually formed by excavation, while 18.4 CHAPTER EIGHTEEN ground-level reservoirs are constructed by earth embankment.The sides and bottom of such reservoirs are lined with concrete, Gunite, asphalt, or with a plastic sheet to prevent or reduce water loss in storage (Harem, Bielman, and Worth, 1976) In earthquake zones, reinforced concrete or a series of flat-bed steel compartments is mandatory Reinforced concrete is often selected because of its minimal rate of deterioration from water contact Elevated storage tanks and standpipes are constructed of steel, with an interior coating applied to retard corrosion (Wade, 1974) Care must be taken to prevent potential contamination of the high-quality water entering storage reservoirs and standpipes One area of concern is in the application of coating compounds over the inner walls of tanks to maintain tank integrity Organic polymer solvents in bituminous coating materials may not entirely evaporate even after several weeks of ventilation As a consequence, the water supply in storage may become contaminated from the solvent-charged air and from contact at the sidewall Some of these compounds are assimilable (biodegradable) organics that support growth of heterotrophic bacteria during warm water periods (Schoenen, 1986; Thofern, Schoenen, and Tuschewitzki, 1987; Mackle, 1988; Bernhardt and Liesen, 1988) Liner materials, also used to prevent water loss, may contain bitumen, chlorinated rubber, epoxy resin, or a tar-epoxy resin combination that will eventually be colonized by microbial growth and slime development (Schoenen, 1986) PVC film and PVC coating materials are other sources of microbial activity Nonhardening sealants (containing polyamide and silicone) used in expansion joints should not be overlooked as a possible source of microbial habitation Water volumes in large reservoirs mix and interchange slowly with water that is actually distributed to service lines In some instances, water storage may become stratified and experience a complete mixing only after a sudden change in ambient air temperature or as a consequence of a significant water loss in the system caused by a major main break or intensive system flushing (Geldreich et al., 1992; Clark et al., 1996) Standpipes, in contrast, provide a fluctuating storage of water during a downsurge, thereby providing surge relief in the system Abrupt changes in water flow that sometimes occur during surge relief can disturb sediment deposits, moving viable bacteria from biofilm sites into the main flow of water Reservoirs of treated water should be covered whenever possible to avoid contamination of the supply from bird excrements (Alter, 1954; Fennel, James, and Morris, 1974), air contaminants, and surface water runoff The health concern with bird excrement is that this wildlife may be infected with Salmonella and protozoans pathogenic to man Within the wildlife population in every area (as is true for any community of people), a persistent pool of infected individuals exists that sheds pathogenic organisms through fecal excretions Seagulls are scavengers and often are found at landfill locations and waste discharge sites, searching for food which is often contaminated by a variety of pathogens At night, birds often return inland to roost in aquatic areas, such as source water impoundments and open finished water reservoirs, thereby introducing pathogens through their fecal excrements (Fennel, James, and Morris, 1974) Pigeons roosting in elevated storage tanks were believed to be the source of Salmonella typhimurium that contaminated the water supply of a small community in Missouri (Geldreich et al., 1992; Clark et al., 1996) Air pollution contaminants and surface water runoff can contribute dirt, decaying leaves, lawn fertilizers, and accidental spills to a water supply that is not covered Such materials increase productivity of the water by providing support to food chain organisms and nitrogen-phosphate requirements for algal blooms This degrades the treated water quality Covered distribution system storage structures also are subject to occasional contamination because of air movement in or out of the vents as a result of water move- MICROBIOLOGICAL QUALITY CONTROL IN DISTRIBUTION SYSTEMS 18.5 ment in the structure During air transfer, the covered reservoir is exposed to fallout of dust and air pollution contaminants from the inflowing air Vent ports or conduits from the service reservoir to the open air should be equipped with suitable air filters to safeguard the water quality from airborne contaminants Such air filters must be replaced periodically to prevent a serious loss of air transfer or create an undesirable vacuum Birds and rodents may also gain access through air vents that have defective screen protection Bird or rodent excrement around the vents may enter the water supply and become transported into the distribution system before dilution and residual disinfection are able to dissipate and inactivate the associated organisms FACTORS CONTRIBUTING TO MICROBIAL QUALITY DETERIORATION Factors contributing to deterioration of microbial quality may be associated with source water quality, treatment processes, or distribution network operation and maintenance The following sections review each of these areas Source Water Quality Bacteria in distributed water may originate from the source water High-quality groundwater can be characterized as containing