The effect of GAC filtration on bacterial regrowth and nitrification in a simulated water main

A 16-month pilot study in two similar 1200 m water mains was conducted to d etermine the effects of granular activated carbon (GAC) filtration on drink ing water quality in a distribution system. The results demonstrated that d espite the higher initial disinfectant residue, the increase in bacteria in the conventionally treated and postozonated water was higher than in the w ater additionally treated with GAC filtration and u.v.-disinfection. Accord ingly, a significant decline in assimilable organic carbon in the postozona ted water was observed throughout the main, whereas in the GAG-filtered wat er this decline was shifted to the GAC filters. In the GAC filtered water t he conversion of ammonia to nitrite and nitrite to nitrate was more intense than in postozonated water. The findings confirm that GAC filtration incre ases the biological stability of drinking water even when treating cold hum ic waters in which biodegradation is generally limited by phosphorus. Howev er, it appears that biological treatment favours the slow kinetics of nitri fying bacteria, thus allowing nitrification to occur even under cold water conditions in a distribution system.