Feasibility study of the pilot-scale sewage treatment system combining of UASB and aerated fixed bed reactor under ambient temperature

Abstract

A feasibility test of a 17m³ pilot-scale sewage treatment system was carried out by continuous feeding of raw municipal sewage under ambient temperature conditions. The system consisted of an UASB and an aerated fixed bed reactor as a first unit and as a second unit, respectively. A part of the effluentfrom the fixed bed reactor was returned to the influent of UASB in order to supply sulfate sources. The total BOD of 148-162mg/L at influent was reduced desirably to 11-25mg/L at final effluents with a condition of the overall HRT of 24hr and the recirculation ratio of 2-0.3 throughout the experimental period. The final effluent also showed superior quality with respect to low SS (10-32mg/L) and high phosphorus removal efficiency. Vial activity tests of retained sludge in the UASB was conducted to evaluate an impact of temperature on the activity of microorganism. The methane-producing activities from acetate and H₂/CO₂ gas at 10°C were only 2% and 0% of those at 35°C, respectively. On the other hand, sulfate-reducing activities of the UASB sludge showed relatively high at 10°C such as 18% for acetate and 9% for H₂/CO₂ gas compared to the activities at 35°C. Therefore, BOD oxidization by sulfate reduction in the UASB could be dominant rather than methane production under low temperature condition. This sulfate-reducing activity tended to be proportional to the copy number of adenosine-5'-phosphosulfate (APS) reductase genes in DNA extracted from the sludge.