Water Quality Deterioration in Distribution Systems: Part 2
Microbial Processes in Distribution Mains
Tastes and Odors
Commonly, consumer dissatisfaction with water quality is expressed in terms of unpleasant taste and odor. The results of a comprehensive survey of taste and odor problems related to algal growths were reported by Lendall (1946).1 He proposed activated carbon treatment, breakpoint chlorination and chlorine dioxide as remedial measures.
The significance of algae and other interference organisms in water supplies was further detailed and illustrated in a series of reviews by Palmer and Tarzwell (1955)2 and Palmer (1960, 1961).3, 4 These reviews also depicted iron bacteria and protozoans associated with water quality deterioration in distribution systems.
Reviews and studies suggesting possible taste and odor problems involving actinomycetes were reported by Waksman (1959),5 Silvey and Roach (1959, 1964)6, 7 and Silvey (1963, 1966).8,9 Silvey and Roach (1959)6 concluded that both algae and actinomycetes were responsible for taste and odor problems in water supplies. Subsequently, Silvey and Roach (1964)7 reported increases in Gram negative organisms, confirming Baylis’s earlier observations regarding the Baltimore water system (1930).10 While Alcaligenes, Aerobacter and Pseudomonas groups were found in water with little pollution, field data suggested that microbiotic cycles involving blue-green algae, actinomycetes and heterotrophic bacteria were responsible for the development of taste and odor-producing compounds in source waters.
Tastes-and-odors originating from sulfate reduction or organic metabolites were also reported.11, 12 A review of the literature on the sources of taste and odors in waters, including anthropogenic sources, was published by Lin (1976).12
An extensive update of more recent studies, many of which relate to the identification and classification of the chemical and sensory nature of the taste and odor-producing compounds found in water, was published by AWWARF (1987).13
Associated with Corrosion
As early as 1891, Garrett14 suggested the rate of lead corrosion might be influenced by the ammonium ion, nitrite and nitrate formed by bacterial action.
In 1910, Gaines14, 15 postulated that the corrosion of iron in both soil and aqueous environments might be caused by sulfate-reducing, sulfur-oxidizing and iron bacteria since Gallionella ferruginea was isolated from corrosion products on a buried steel conduit. High concentrations of sulfur and organic matter also were noted. Gaines proposal of biologically-mediated corrosion initially was not accepted.
In 1923, von Wolzengen Kuhr14, 16 reported that sulfate-reducing bacteria corroded iron in the absence of oxygen. Von Wolzengen Kuhr and Van der Vlugt (1938)17 presented further evidence of anaerobic corrosion. They attributed the ‘graphitization’ of cast iron pipes to the activities of sulfate-reducing bacteria.
In the United States, confirmation of the role of anaerobic bacteria in the corrosion of iron pipes resulted from the studies of Hadley (1939),18 Pomeroy (1941),19 Starkey (1945)20 and Starkey and Wright (1945).21 In addition to sulfate-reducing bacteria, the researchers found large masses and encrustations of iron sulfides clogging cast iron pipe. Pomeroy (1945)22 also reported more rapid corrosion when iron pipe was exposed to oxygen-bearing water intermittently as opposed to constantly. This result implied that additional periods of anoxia accelerated corrosion of cast iron pipe.
Bunker (1939),23 Butline (1955)24 and Lewis (1965)25 isolated sulfate-reducing bacteria from tubercles. The latter two investigators enumerated 1,000 and 280,000 organisms per gram of tubercle, respectively. Lewis reported no detectable sulfate-reducing organisms in the water.
Sulfur bacteria range from complex filamentous Beggiatoa and Thiothrix species to the rod-shaped Thiobacillus species. Their involvement in sulfuric acid formation and corrosion of iron piping was reported by Booth (1966). 26
Sources of Microorganisms Found in Water Distribution Systems
Aftergrowth: Contamination of Finished Water by Distribution System Piping
Early bacteriologists noted that organisms found in the distribution system reflected those present in the source waters.27 Over the past half-century, confidence has grown in the ability of current water treatment processes to remove or inactivate organisms initially present in the source water. Increases in organism populations during distribution, as indicated by turbidity, coliform and heterotrophic plate count measurements, are now generally believed to result from the recruitment of organisms from colonized distribution system piping. The distribution system is generally believed to be contaminating the virtually organism-free, treated water.
Serious distribution system problems were detailed by Larson et al. (1960)28 in a paper describing remedial actions taken in the Hammond, Illinois, water system. The authors expressed confidence that "treatment practices commonly employed can produce any quality of water that may be desired" but lamented that "a perfectly clear, soft and safe water entering the distribution system may be unrecognizable as such at the household tap."
Geldreich et al. (1972)29 concluded, "once microorganisms enter the distribution system, they may be harbored in protective slime and sediments that develop in portions of the system." They reported that maintaining a residual chlorine level in the distribution system could control this population. A subsequent report by Geldreich et al. (1977)30 noted the "potential for bacterial regrowth during warm water periods" and recommended a systematic flushing program in addition to the maintenance of a chlorine residual. A third report reaffirmed earlier observations that "whereas the water quality is excellent as it leaves the treatment plant, the consumer may be using tap water of considerably lower quality."31
Despite finding relatively low surface populations of bacteria on distribution piping, Nagy and Olson (1985)32 concluded that "a likely explanation for the similarity between microorganisms on pipe surfaces and in drinking waters is that detachment of microorganisms from the pipe surface and their re-entrainment into the passing water may account for the majority of bacteria in potable water."
Overall, biological growths (biofilms) embedded in sediments or entrained in accumulations on the surface of distribution system piping have long been viewed as a major source of subsequent microbial contamination of treated waters.10, 33—36 As a result, remedial programs, such as flushing and chlorination, have primarily been directed at controlling attached growths on the internal surfaces of the distribution system.
Microbiologists have hypothesized that a significant portion of the bacterial community in most aquatic environments is in exogenous dormancy–a state of neither activity nor death.38—40 Exogenous dormancy is defined as a condition in which development is delayed because of unfavorable chemical or physical conditions of the environment. It is an adaptation in which enzyme activity is minimal, thereby contributing to the survival of the organism. For example, starvation may cause bacteria to decrease in both size and activity. Release of the stress would permit normal development. Conceivably, disinfectants would have significantly less effect on dormant cells than on metabolically active cells whose enzyme systems are more susceptible to inactivation. Since bacteria attached to larger particles occupy a microenvironment higher in nutrient concentrations than the surrounding water, epibacteria are both more active40—43 as well as more susceptible to removal by the physical processes of a filtration plant.
If the exogenous dormancy hypothesis is correct, the numerous source water planktonic cells that penetrate filtration plants may be the very cells that have survived disinfection because of their low level of metabolic activity. Their slow recovery and growth rates on culture media also may cause them to tend to escape enumeration as heterotrophic plate count colonies.
Previous studies on both Legionella pneumophila44 and Pseudomonas aeruginosa45 have demonstrated that slowly-growing cells cultured in tap water were far more resistant to inactivation by a disinfectant than rapidly-growing active cells cultured in a rich medium.
Recovery and Regrowth of Organisms Penetrating Treatment Facilities
Aftergrowth was the term used to describe microbial contamination of distributed water by organisms detached from the surface of distribution system piping or by other organisms external to the source water.10 Where aftergrowth is the main source of cells reaching the consumer, treated water that may be virtually organism-free would exhibit increases in cell numbers during distribution as cells were recruited from pipe surfaces or entered from cross-connections. Aftergrowth may operationally be defined as microbial contamination of distributed water during distribution by recruitment of organisms from internal pipe surfaces or cross—connections.
In order to describe the concept and develop the rationale for more scientifically delineating the origin of organisms found in distributed drinking water, the term regrowth was defined46 to distinguish organisms whose growth had been arrested during treatment, but were capable of growing again in distributed waters after chlorine dissipation and time for metabolic repair of the cells passing through the plant.
The distinction between regrowth and aftergrowth should be of particular importance to water utilities seeking to monitor and mitigate microbial problems in distribution systems. Since regrowth involves the passage of source water organisms through the treatment plant, control of regrowth is achieved by consistently maintaining effective physical removal. This is primarily accomplished by ensuring the entrainment of source water organisms during floc formation and providing adequate time for floc growth and sedimentation.47
Alternately, since aftergrowth occurs within the distribution system, control measures must be directed toward arresting the microbial activity of the organisms present in the distribution system. Conventional methods for the control of aftergrowth, such as flushing, swabbing (pigging), main disinfection and periodic breakpoint chlorination, would be expected to have a limited effect on regrowth where significant numbers of planktonic organisms are constantly being passed into the system through the treatment plant.
Part 3 of this four-part series will summarize studies directed at controlling microbial growths in distribution systems supplied by ground waters containing ferrous ion and naturally-occurring microbial nutrients, such as methane and ammonium ion and will appear in the January issue.
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