The Los Angeles Department of Water and Power (LADWP) purchases some of its water from a wholesale supplier in the Southern California area, the Metropolitan Water District (MWD). After their change of disinfectant from chlorine to chloramine, in 1985, unexplained disinfectant residual decreases in the distribution system occurred. Seminars conducted by MWD helped point out that biological nitrification, or oxidation of ammonia to nitrite and nitrate, could be responsible. Acting on that assumption, a sampling program was initiated to identify if and where nitrification was occurring. After nitrification was identified, several options were discussed to eliminate it from the distribution system. Chlorination to maintain a free residual in the distribution system was considered the best method for dealing with the problem. Customer awareness, operational constraints, among other impacts were, and continue to be, major water quality concerns.