Chloramination: Its effect on distribution system water quality

Postdisinfection using chloramines can limit coliform regrowth in pipeline biofilms and formation of disinfection by-products (DBPs) in distribution systems. Two utilities that have had occurrences of total coliform converted from free chlorine to chloramines. Water in both distribution systems was monitored for biological and chemical quality for a year after the conversion, Coliform occurrences were eliminated in one system and reduced fourfold in the other. However, excessive concentrations of assimilable organic carbon (up to 375 mu g/L) were characteristic of the latter system, as was pitting corrosion. Both factors may play a critical role in limiting coliform occurrence. Postdisinfection using chloramines rather than free chlorine has advantages including decreased coliform concentrations, decreased heterotrophic plate count and DBPs, and improved maintenance of a disinfectant residual. However chloramination alone may not be sufficient to control coliform concentrations in all distribution systems.