LAUSR

A full-scale wastewater treatment plant with a 5500 population equivalent was retrofitted with a pre-denitrification submerged anoxic filter (SAnoF) in order to achieve new total nitrogen (TN) consent of 35 mg/L. A 36 m 3 SAnoF was installed downstream the primary settling tanks. The optimal operation of the anoxic SAnoF was investigated by varying the recirculation ratio, the carbon to nitrate ratio and the hydraulic retention time. After stable operation was achieved, nitrate was removed by 80% at a loading of 0.5 kg NO3/m 3 .d and retention time of 60 minutes. The SAnoF presented a number of advantages, including use of internal carbon for denitrification, decrease of carbon load to the trickling filter by 30% and production of alkalinity required for nitrification in the trickling filter (11 mg CaCO3/mg NH4 removed). Overall the SAnoF was satisfactory and the effluent TN concentration reached 20-25 mg TN/L.