Treatment of low-strength wastewater at mesophilic and psychrophilic conditions using immobilized anaerobic biomass

An anaerobic immobilized bio-plates reactor (AnIBPR) of effective volume of 14.5 L and bio-plate packing ratio of 27.5% was used to treat low-strength synthetic wastewater (COD of 400 mg/L) at a hydraulic retention time (HRT) of 16 h. Treatment performance was investigated at different temperatures. The results showed that the reactor AnIBPR achieved COD removal of 86-92% at mesophilic (35 °C and 25 °C) and psychrophilic (15 °C) conditions. Dissolved methane (CH4) was found to be oversaturated in the reactor (saturation factor of 1.03-1.21), increasing with decreasing temperature. Increased dissolved CH4 content at 15 °C, albeit at decreased production, was attributed to significant mass transfer limitation due to lower KLa at low temperature, resulting in oversaturation of dissolved CH4. Increasing treatment temperature to mesophilic conditions led to increased reaction rates and yields. At steady state, gas-phase CH4 content was 75-83% and the overall yield was 0.22-0.26 L/g COD removed. Steady-state volatile fatty acids (VFAs) were measured at 3.3-12 mg COD/L, small amounts that were unlikely to hamper treatment performance.