Bacterial Regrowth in Drinking Water Distribution Systems: A Comparison of Durham and Raleigh

Studies have shown that "adequate1' residual disinfectant concentration leaving the water treatment plant does not guarantee control of bacterial regrowth. From a regulatory point of view utilities are concerned with the presence of bacteria in the bulk water of their distribution systems. However, these bacteria very likely originate from attached growth on the pipe walls. The relationship between attached and unattached (bulk-water) bacteria is complex and not well understood. This research was directed toward investigating the factors influencing both attached and unattached bacteria in two distribution systems that use different disinfectant strategies. The City of Durham uses chlorine while the City of Raleigh uses chloramines by addition of ammonia to chlorine. This comparison is especially important in North Carolina because many water utilities are interested in switching from chlorine to chloramines in order to meet new U.S. Environmental Protection Agency (EPA) regulations related to reduction of chlorine disinfection byproducts. Regrowth was measured by heterotrophic plate count (HPC) using E A agar. An experimental apparatus and analytical procedures were developed to measure the attachment of bacteria. A comparison was made of attachment to cast iron, ductile iron, and glass during exposure to low and high concentrations of chlorine and chloramine. Monthly sampling of bulk water over 17 months at 10 stations provided about 160 measurements of each chemical and microbial parameter considered important in assessing the bacterial regrowth problem. The average assimilable organic carbon (AOC) was found to be high enough to support bacterial regrowth if the disinfectant residual concentration was low but not necessarily as low as the detection limit of about 0.2 mg/L. Maintenance of sufficient disinfectant residual depends on control of oxidant demand, but this could strongly depend on minimizing corrosion within the system. Chloramine seems to offer greater promise in controlling this growth because it persists at higher concentrations for a longer time in the distribution system. Notwithstanding its stability, given enough time and the presence of oxidant demand, chloramine will decrease as was evident at one of the 10 stations included in Raleigh. In addition, excess ammonia led to nitrification which hrther decreased chloramine residual. The effectiveness of controlling regrowth by either flushing (Durham), or switching from chloramine to chlorine for one month (Raleigh) is doubtfd. This research showed that the HPC returned in a short time to values found before either control measure. Measurement of attached growth showed that high concentrations existed on the surface of cast iron and ductile iron pipe material in the absence of disinfectant residual. More important, attached growth existed even when the bulk-water disinfectant concentration was high; this was true for both chloramine and chlorine residuals. A simple regression model revealed that the most important factor affecting HPC was disinfectant residual although to a much lesser extent, temperature, AOC, and pH were shown to be important. However, a more powerfbl statistical approach combined with more frequent data collection at more sampling sites would be needed to yield better results from regression modeling but at considerable expense. The alternative is a mechanistic model of the regrowth process or better yet, a mechanistic model that accounts for uncertainty in parameter estimation.