A down-flow hanging sponge (DHS) reactor is a novel, unaerated, aerobic, biofilm reactor that is used to polish effluent received from an up-flow anaerobic sludge blanket (UASB) reactor for treating municipal sewage. A full-scale DHS reactor was constructed for post-treatment of a full-scale UASB reactor at a municipal sewage treatment plant in India. Performance of the DHS reactor was evaluated with respect to organic removal over 1800 days of continuous operation. The UASB + DHS system consistently produced effluent with chemical oxygen demand (COD), biochemical oxygen demand (BOD), and suspended solids (SS) values of 37, 6.0 and 19 mg L (1), on average, respectively. The sludge yield of the DHS reactor was estimated to be 0.04 kg SS kg (1) COD removed or 0.12 kg SS kg (1) BOD removed, which is considerably lower than other aerobic treatment methods that have been employed for polishing UASB effluent. (C) 2015 Elsevier Ltd. All rights reserved.