AWWA JAW18196

The Metropolitan Water District of Southern California (Metropolitan) has investigated the impact of various points of application of chlorine and ammonia on the coliform content and standard plate count of water moving through a pilot water treatment plant. Conventional pour-plate techniques and a new membrane-filter standard plate count (m-SPC) were used to compare free residual chlorine disinfection with chloramine disinfection. Results indicate that concurrent addition of ammonia and chlorine with more than 3 h of contact time produces trihalomethane (THM) concentrations equivalent to those produced by preammoniation. Concurrent addition of chlorine and ammonia produces lower counts of m-SPC bacteria than does preammoniation. Concurrent addition produces water of acceptable bacteriological quality. The m-SPC total count procedure was found to be a more sensitive indicator of comparative disinfection than the pourplate count method or coliform bacteria count. Chloramination of Metropolitan's waters effectively reduces trihalomethanes and, with longer contact times, provides disinfection that is statistically equivalent to that observed with free chlorine. Includes 51 references, tables, figures.