Experimental Studies on a New Process for Nitrogen Removal with Immobilized Microbes in Oxic Tanks Using Sewage Treatment Plant of Commercial Scale

In order to upgrade quality of the effluent from Otsu Water Reclamation Plant, faced to Lake Biwa, experimental studies mainly on nitrogen treatment of sewage were conducted for more than 3 years, using a modified reactor of former conventional activated sludge process without any HRT change on the deep aeration tank with water depth of 9.5m. Influent flow rates were around 11800 m3/day for a series of experiments.The new nitrogen removal process studied was a modification of the anoxic-oxic process, where the biological reactor had a poorly aerated tank installed between the anoxic tank and two oxic tanks with carriers for immobilization of nitrifiers.Nitrogen removal efficiency obtained in this process were 10 to 18% higher than that of theoretical values, which were expected to be obtained in the single stage anoxic-oxic process without the poorly aerated tank. It was revealed that the poorly aerated tank positively contributed to the high nitrogen removal efficiency obtained in this process. This new process with higher nitrogen removal efficiency seemed to bring a merit for operational cost, because a relatively smaller total recycle flow rate was required and energy for pumping was saved.In this system with relatively short ASRT, sludge detached from the carriers seemed to stay in the suspended sludge for a while and to contribute for nitrification by the suspended sludge.