Characterization of the retained sludge in a down-flow hanging sponge (DHS) reactor with emphasis on its low excess sludge production

► This study characterized retained sludge in a DHS fed with UASB treated sewage. ► The average excess sludge production rate was 0.09 g SS g−1 COD removed. ► The DHS reactor maintained a high sludge concentration of 26.9 gVSS L−1 sponge. ► The endogenous respiration rate of DHS sludge was relatively high. ► The numbers of microfauna were one order of magnitude greater in activated sludge.