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

Abstract

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 (V_WW) 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 g_TSS^-1 was observed either at very low (1 m h^-1) or at high V_WW (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 V_WW 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 V_WW 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 (V_WW 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.

Keywords: Aerobic granules; Anaerobic feeding; Low-strength municipal wastewater; Simultaneous fill-draw; Wastewater upflow velocity.