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designed to carefully control the chlorine to ammonia ratio and avoid excess ammonia in the downstream system. The effects of the booster station on nitrification have not yet been fully assessed (Potts et al. 2001). Free chlorine is more effective at inactivating ammonia -oxidizing bacteria colonies than chloramines (Wolfe et al. 1990). As a result, breakpoint chlorination is also used by utilities to treat nitrifying bacteria. According to Schrempp et al. (1994), mechanically cleaning pipelines; draining and cleaning reservoirs; and dead-end, unidirectional and continuous flushing; were not sufficient to control nitrification at one mid -western utility. When these strategies were replaced with breakpoint chlorination, nitrification was controlled and target residuals could be maintained. Some systems using breakpoint chlorination have reported an initial increase in HPC bacteria and total coliform levels immediately following treatment that is probably attributable to biofilm sloughing (Odell et al. 1996, Wilczak et al. 1996). 3.2 Absence of Sunlight Although monochloramine will degrade when exposed to the atmosphere at varying rates depending on the amount of sunlight, wind, and temperature (Wilczak, 2001), nitrifiers are very sensitive to near UV, visual, and fluorescent light; consequently, nitrification episodes in distribution systems occur in the dark (in covered reservoirs, pipelines, taps, etc.) (Wolfe et al, 2001). Wolfe et al (2001) also report that nitrifiers do have an excision repair mechanism for DNA repair; therefore low levels of nitrifiers may be recovered from partially shaded reservoirs or channels. The Long -Term 2 Enhanced Surface Water Treatment Rule will require utilities to either cover uncovered finish water reservoirs or treat reservoir discharge to the distribution system to achieve a 4 -log virus inactivation, unless the State/Primacy Agency determines that existing risk mitigation is adequate. The potential increase in nitrification episodes associated with covering uncovered storage facilities within chloraminated systems was not addressed within the literature. 3.3 Operations Activities Nitrifying bacteria are slow growing organisms, and nitrification problems usually occur in large reservoirs or low -flow sections of the distribution system. According to Kirmeyer et al (1995), operational practices that ensure short residence time and circulation within the distribution system can minimize nitrification problems. Low circulation areas of the distribution system such as dead -ends and reservoirs are prime areas for nitrification occurrence since detention time and sediment buildup can be much greater than in other parts of the system. Water temperature has a strong effect on the growth rate of nitrifying bacteria. Numerous researchers have documented that nitrification episodes are more common during the warmer months. Most strains of nitrifiers grow optimally at temperatures between 25 and 30°C (Watson, Valos, and Waterbury, 198 1) but nitrification has occurred over a wide range of temperatures (8- 26°C) (Kirmeyer et al. 1995). Operations activities that lead to decreased water age may also result in decreased bulk water temperatures. Prepared by AWWA with assistance from Economic and Engineering Services, Inc. 11