Abstract
An Under-Over Stream (UOS) -type biofilm reactor equipped with a filtering material in the form of long threads for treating domestic wastewater from a district mainly using a primitive-type wastewater-treating tank was developed. The proposed biofilm reactor in which flow was directed under and over baffles facilitated securing of high levels of dissolved oxygen and gave good contact between the filtering material and wastewater. In the UOS-type reactor, the sludge adhered to the filtering material was removed by shifting the material from the inlet side to outlet side at a point when the cleaning performance at the inlet side is lowered. An year-long test for domestic wastewater treatment was performed with a reactor that had an effective volume of 1.6m3and with an automatic switching between the wastewater inlet and outlet points at given time intervals. As a result, when the water temperature is 12°C or higher, the sludge was removed to a level such that COD was lowered from about 25-30 mg/L to about 10-15 mg/L and BOD to about 5 mg/L at a residence time of 6.7h. In few case, the suspended solids were found at a concentration of about 10 mg/L. The filtering material did not require cleaning during the one-year test period. Nitrogen was present mostly in the form of ammonium in the original domestic wastewater. However, when the water temperature is 10°C or higher, more than 90% of nitrogen in the form of ammonium was oxidized to nitrate in the biofilm reactor.
