Symptoms indicative of nitrification occurred in the Ann Arbor, Mich., system during the late summer of 1990. The episode was characterized by decreases in monochloramine residual and increases in heterotrophic plate count (HPC) bacteria and nitrite. The cause of the nitrification was most probably a high dosage of ammonia combined with granular activated carbon filtration and discontinuation of the hydrant flushing program. The incident was ultimately halted by free chlorination of the distribution system. During free chlorination, an unexpected increase in HPC bacteria was observed in areas not affected by nitrification. Recommendations for future control of nitrification include more stringent control of ammonia, increased monitoring of nitrite and nitrate, development and implementation of a hydrant flushing program, and experimentation with the effects of pH.

Concerns about water quality and possible intentional contamination of water distribution systems are making on-line monitoring an increasingly important priority for many water utilities. The city of Ann Arbor (Mich.) evaluated different water quality monitoring parameters, tested multiple manufacturers' monitoring equipment, and evaluated how to effectively locate monitoring equipment within the distribution system to address these two concerns. A suite of modeling tools was used in this case study. Total chlorine, ultraviolet absorbance at 254 nm, conductivity, and dissolved oxygen were selected for monitoring based on pilot testing. When balancing costs and benefits, four stations for monitoring water quality and four stations for monitoring contamination events were found to be sufficient for the city. Only one location was common between water quality and security sites, and the number of security monitors needed was not affected by system demands. It was confirmed that it is critical to minimize response time in order to mitigate the effects of a contamination event.

Nitrification affects many utilities that use chloramine as the finished water disinfectant, degrading water quality in the distribution system. Control of nitrification is a challenge for these utilities, and traditional nitrification‐control strategies are not always effective. In many cases, the utility must free‐chlorinate the distribution system to handle nitrification water quality problems, defeating the purpose of chloramination—to avoid regulatory problems with disinfection by‐products (DBPs). Utilities need additional nitrification control strategies. This study evaluated historical water quality data and nitrification occurrence at the Ann Arbor (Mich.) Water Plant (AAWP) and identified a correlation between high drinking water pH and the absence of nitrification. AAWP then performed experiments to evaluate the effects on nitrification of maintaining the finished water at pH 8.5 versus 9.4. The higher pH appeared to inhibit nitrification. AAWP then undertook an eight‐year assessment using high pH as an operational parameter. Significant nitrification was not observed in seven of the eight years, whereas in the four years preceding the assessment, when the finished water was pH 8.7–8.9, nitrification occurred every summer. Elevating the pH of the finished water reduced the frequency of nitrification in Ann Arbor—reducing the need to free‐chlorinate the distribution system (and the production of DBPs) and maintaining the chloramine residual in the distribution system.

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For any drinking water utility using chloramine disinfection, nitrification is a serious concern. The most important steps utilities can take to manage nitrification are to thoroughly understand their systems' chloramine chemistry, establish an effective monitoring strategy, and deploy multiple preventive strategies holistically throughout their distribution systems.