An activated sludge treatment plant for integrated removal of carbon, nitrogen and phosphorus

During the last decade more stringent effluent requirements concerning the nutrients effluent values have been imposed by legislation and social concern. This paper presents a pilot-scale activated sludge prototype system which combines the UCT concept and the step denitrification cascade for integrated removal of carbon, nitrogen and phosphorus. The experimental unit includes an anaerobic selector and stepwise feeding in subsequent anoxic and oxic vessels for removal of BOD5/COD, suspended solids, nitrogen, and phosphorus. Raw municipal wastewater with influent flow rates ranging between 48-120 L/d was fed to the unit at residence times of 7-18 h and specific BOD5 loading rates of 0.23-1.08 kg BOD5/(kg MLVSS-d). The results of a full year operation show high removal efficiencies of organic matter of about 88% as total COD and 92% removal for BOD5 complete nitrification (93-99% removal of ammonia-nitrogen), high removal of Total Kjeldahl Nitrogen ranging between 87 and 97% and total nitrogen removal through denitrification of 70%. Phosphorus removal is fair because of the intense fluctuations in the incoming phosphate concentration. Moreover, plant configuration provides operational conditions that suppress filamentous bacteria and favour growth of floc-formers, leads to high sludge settleability characteristics (SVI < 130 ml/g).