Energy saving system with high effluent quality for municipal sewage treatment by UASB–DHS

An up-flow anaerobic sludge blanket (UASB) – down-flow hanging sponge (DHS) was applied to Japanese municipal sewage treatment, and its treatability, energy consumption, and sludge production were evaluated. The designed sewage load was 50 m³/d. The sewage typically had a chemical oxygen demand (COD) of 402 mg/L, a suspended solids (SS) content of 167 mg/L, and a temperature of 17–29 °C. The UASB and DHS exhibited theoretical hydraulic retention times of 9.7 and 2.5 h, respectively. The entire system was operated without temperature control. Operation was started with mesophilic anaerobic digested sludge for the UASB and various sponge media for the DHS. Continuous operational data suggest that although the cellulose decomposition and methanogenic process in the UASB are temperature sensitive, stable operation can be obtained by maintaining a satisfactory sludge volume index and sludge concentration. For the DHS, the cube-type medium G3-2 offers superior filling rates, biological preservation and operational execution. The SS derived from the DHS contaminated the effluent but could be removed by optional sand filtration. A comparison with conventional activated sludge (CAS) treatment confirmed that this system is adequate for municipal sewage treatment, with an estimated energy requirement and excess sludge production approximately 75 and 85% less than those of CAS, respectively.