Significance of trihalomethanes in preventing distribution system nitrification

Chloramination is popular in drinking water treatment because it can provide microbial control, but unlike chlorination it results in much less formation of disinfection by-products (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs). Unfortunately, nitrification in drinking water distribution systems is a widespread issue when chloramination is employed as a residual disinfection process. Nitrification is undesirable because the disinfectant residual can be lost and re-growth of bacteria may occur. Nitrification is a well understood process where ammonia-oxidizing bacteria (AOB) oxidize ammonia into nitrite (NO2-), which is then converted to nitrate (NO3-) by nitrite-oxidizing bacteria (NOB). Recent research has shown that AOB are able to biodegrade THMs through an enzymatic process known as cometabolism. The cometabolism by-products are highly reactive substances thought to be capable of either damaging or killing AOB. These observations led to a working hypothesis that under certain conditions and THM concentrations, THMs play a significant role in preventing distribution system nitrification. This research was composed of two distinct tasks aimed at determining how background concentrations of THMs, in the absence of residual disinfectant, impact nitrifying biofilms in a mock distribution system under ideal conditions for microbial growth. For the first task, nitrifying biofilms were developed in annular reactors and were challenged by increasing concentrations of THMs until nitrification was halted. In the next task, THMs were continuously fed to the reactors to determine the concentration of THMs necessary to prevent the initial onset of nitrification. The results from these experiments will be used to design future experiments to investigate the coupled effects of monochloramine and THMs in preventing the onset of nitrification. The goal of this research is to advance our understanding of distribution system nitrification by examining the role that THMs play. This improved understanding will allow utilities to more accurately assess the potential for nitrification in their distribution systems and to anticipate nitrification problems that may arise as a result of treatment process modifications.