A method of stimulating nitrification at low SRT by elevating pCO.sub.2 during aeration is disclosed. The improvement on solids settling performance when elevated pCO.sub.2 was supplied after 2 hours within the React cycle is consistent with the previous results that identified inorganic carbon as a potential remedy to poor settling and bulking sludge problems in activated sludge systems. Elevated pCO.sub.2 increases the concentration of carbon dioxide and lowers the pH, which improve nitrification. The specific growth rate of nitrifying bacteria is sensitive to pCO.sub.2, pH, and dissolved oxygen (DO). The DO is a function of the aeration rate. Elevating the pCO.sub.2 and lowering the aeration rate provides conditions for nitrification rates that are comparable to conventional systems. However, the lower aeration rate yields significant energy cost savings.