Effectiveness of High pH in Controlling Nitrification

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.