1990年夏末，密歇根州安娜堡系统出现了硝化作用的症状。该事件的特点是一氯胺残留量减少，异养平板计数（HPC）细菌和亚硝酸盐增加。硝化作用的原因很可能是高剂量的氨与颗粒活性炭过滤结合，以及消防栓冲洗程序的中断。该事件最终因配电系统的免费氯化而停止。在自由氯化过程中，在未受硝化影响的区域观察到HPC细菌意外增加。未来控制硝化作用的建议包括更严格地控制氨，加强对亚硝酸盐和硝酸盐的监测，制定和实施消防栓冲洗程序，以及对pH值影响的实验。包括21篇参考文献、表格和图表。

Symptoms indicative of nitrification occurred in the Ann Arbor, Michigan, 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. Includes 21 references, tables, figures.