Formation of aerobic granules for the treatment of real and low-strength municipal wastewater using a sequencing batch reactor operated at constant volume.

This study aimed at evaluating the formation of aerobic granular sludge (AGS) for the treatment of real and low-strength municipal wastewater using a column sequencing batch reactor (SBR) operated in fill-draw mode (constant volume). The focus was on understanding how the wastewater upflow velocity (VWW) applied during the anaerobic feeding influenced the sludge properties and in turn the substrate conversion. Two different strategies were tested: (1) washing-out the flocs by imposing high wastewater upflow velocities (between 5.9 and 16 m h-1) during the anaerobic feeding (Approach #1) and (2) selective utilization of organic carbon during the anaerobic feeding (1 m h-1) combined with a selective sludge withdrawal (Approach #2). A column SBR of 190 L was operated in constant volume during 1500 days and fed with real and low-strength municipal wastewater. The formation of AGS with SVI30 of around 80 mL gTSS-1 was observed either at very low (1 m h-1) or at high VWW (16 m h-1). At 16 m h-1 the AGS was mainly composed of large and round granules (d > 0.63 mm) with a fluffy surface, while at 1 m h-1 the sludge was dominated by small granules (0.25 < d < 0.63 mm). The AGS contained a significant fraction of flocs during the whole operational period. A considerable and continuous washout of biomass occurred at VWW higher than 5.9 m h-1 (Approach #1) due to the lower settling velocity of the AGS fed with municipal wastewater. The low sludge retention observed at VWW higher than 5.9 m h-1 deteriorated the substrate conversion and in turn the effluent quality. High solid concentrations (and thus solid retention time) were maintained during Approach #2 (VWW of 1 m h-1), which resulted in an excellent effluent quality. The study demonstrated that the formation of AGS is possible during the treatment of real and low-strength municipal wastewater in a SBR operated at constant volume. Low wastewater upflow velocities should be applied during the anaerobic feeding phase in order to ensure enough biomass retention and efficient substrate removal.