Nitrification in chloraminated drinking water can have a number of adverse effects on water quality, including a loss of total chlorine and ammonia-N and an increase in the concentration of heterotrophic plate count bacteria and nitrite. To understand how nitrification develops, a study was conducted to examine the factors that influence the occurrence of ammonia-oxidizing bacteria (AOB) in a chloraminated distribution system. Samples were collected over an 18-month period from a raw-water source, a conventional treatment plant effluent, and two covered, finished-water reservoirs that previously experienced nitrification episodes. Sediment and biofilm samples were collected from the interior wall surfaces of two finished-water pipelines and one of the covered reservoirs. The AOB were enumerated by a most-probable-number technique, and isolates were isolated and identified. The resistance of naturally occurring AOB to chloramines and free chlorine was also examined. The results of the monitoring program indicated that the levels of AOB, identified as members of the genus Nitrosomonas, were seasonally dependent in both source and finished waters, with the highest levels observed in the warm summer months. The concentrations of AOB in the two reservoirs, both of which have floating covers made of synthetic rubber (Hypalon; E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.), had most probable numbers that ranged from <0.2 to >300/ml and correlated significantly with temperature and levels of heterotrophic plate count bacteria. No AOB were detected in the chloraminated reservoirs when the water temperature was below 16 to 18°C. The study indicated that nitrifiers occur throughout the chloraminated distribution system. Higher concentrations of AOB were found in the reservoir and pipe sediment materials than in the pipe biofilm samples. The AOB were approximately 13 times more resistant to monochloramine than to free chlorine. After 33 min of exposure to 1.0 mg of monochloramine per liter (pH 8.2, 23°C), 99% of an AOB culture was inactivated. The amounts of this disinfectant that are currently used (1.5 mg/liter at a 3:1 ratio of chlorine to ammonia-N) may be inadequate to control the growth of these organisms in the distribution system.

Previous studies have shown that nitrification of chloraminated drinking water can have deleterious effects on water quality. These studies also showed that the cause of nitrification is the oxidation of ammonia (used to form chloramines) to nitrite by autochthonous nitrifying bacteria. In this study, bench‐scale experiments were conducted with fully treated drinking water to determine the optimum chloramine application conditions necessary to prevent nitrification in the distribution system. These experiments examined the survival and regrowth of nitrifying bacteria after exposure to three chloramine dosages, three weight ratios of chlorine to ammonia‐nitrogen, three temperatures, and two contact times. The results indicate that the control of ammonia‐oxidizing bacteria is highly dependent on temperature.

Laboratory and field studies of five Texas distribution systems suggest that chlorite ion has significant potential for controlling nitrification in chloraminated water. Controlling nitrification is essential if chloramines are to be a viable alternative disinfectant scheme for distribution systems in all types of environments. This article reviews problems associated with nitrification and presents laboratory and field evidence for using the chlorite ion (ClO2–) to control nitrification in distribution systems. Laboratory experiments in this study showed that even low dosages of ClO2– (0.05 mg/L) can inactivate 3–4 logs of ammonia‐oxidizing bacteria (AOB) over several hours. Higher concentrations of ClO2– inactivate all of the AOB in as little as 30 minutes. Field investigations at five Texas water utilities showed that the presence of ClO2– in the distribution systems resulted in less loss of chloramines and ammonia–nitrogen and thus less nitrification than in those systems in which ClO2– was not present. The ease of use of ClO2– is compared with other, more traditional nitrification control measures.

Biology and Physiology Ecology and Occurrence in Potable Water Prevention and Control