COD and nitrogen removal by biofilms growing on gas permeable membranes

A bioreactor was constructed and used to treat a synthetic wastewater containing ammonium acetate and trace nutrients for about 190 days. The reactor was aerated by means of bundles of gas-permeable hollow-fiber membranes that were installed in the reactor. The membranes provided a specific surface area of 422 m2/m3 and the external surface of the membranes rapidly became covered in an active biofilm. The membrane bundles were agitated by an internal gas recycle. The gas bubbles in the water encouraged fiber-fiber contact and were intended to control biofilm growth. Chemical oxygen demand (COD) removals in excess of 95% were achieved in a 6 h nominal detention time. Nitrification developed rapidly and complete oxidation of the influent ammonium was evident within 20 days. Even though the reactor was equipped with a large membrane surface area, the oxygen was consumed within the biofilm growing on the membrane surface. As a result, the external dissolved oxygen (DO) dropped to zero and the reactor was able to support essentially complete denitrification. After about 3 months of operation the reactor showed excellent removals of both COD and inorganic nitrogen but the performance could not be sustained. Excess biofilm accumulation eventually contributed to a deterioration in process performance. This study demonstrates that while membrane aeration can provide simultaneous BOD and N removal in the same reactor, the membrane modules/bioreactor must be designed to allow for the development of thick biofilms. In addition, options for controlling the biofilm thickness need to be investigated.