Treatment of municipal wastewater with aerobic granular sludge

Treatment of municipal wastewaters with aerobic granular sludge (AGS) has been extensively researched in the past decade and has now become a mature option for implementation. Aerobic granules are distinguished from activated sludge flocs through their larger size and more compact and spherical structure. Due to these properties, granules settle rapidly and can therefore contribute to compact treatment processes through high sludge concentrations and short settling times. In this review, the factors that promote granulation in treatment processes are identified and discussed and the experience of municipal wastewater treatment with AGS at laboratory-, pilot-, and full-scale are critically evaluated. The most important factors to promote granulation include exposing the biomass to relatively high concentrations of contaminants in sequencing batch reactors, promoting slow-growing microorganisms and applying a relatively short settling time. Enhanced biological phosphorus removal is preferably integrated with AGS and the large size of the granules makes simultaneous nitrification (at the surface of the granules) and denitrification (at the inner, anoxic parts) feasible. We propose directions for future research including further optimization of AGS to obtain stable and low effluent nutrient concentrations in line with increasingly stringent upcoming effluent demands.