Efficient aerobic granular sludge production in simultaneous feeding and drawing sequencing batch reactors fed with low-strength municipal wastewater under high organic loading rate conditions

To establish rapid and efficient aerobic granular sludge (AGS) production for augmentation strategy to mainstream activated sludge process, AGS production was evaluated under high organic loading rate (OLR) conditions in a side-stream simultaneous feeding and drawing sequencing batch reactor (SFD-SBR) fed with low-strength municipal wastewater. Specifically, three SFD-SBRs were operated under different OLRs (> 1.3 kg-tCOD m- 3 d(-1)) with a 100% volume exchange ratio (VER). Rapid granulation (SVI30 < 80 mL g(-1) and granule fraction of -70%) was achieved within 40 days in all reactors. However, an excessively high OLR (2.0 kg-tCOD m- 3 d(-1)) tended to labilize the AGS settleability (SVI5 of -120 mL g-1) despite the larger AGS size due to the outgrowth of fluffy flocs and filamentous bacteria. Furthermore, after granulation, application of a long settling time (10 min) to maintain a low sludge blanket level during the feeding period was key for stable and efficient AGS production (0.17 g-SS g-tCOD-1) under a high OLR of 1.6 kg-tCOD m- 3 d(-1)compared to AGS production with a short settling time (0.10 g-SS g-tCOD-1). Taken together, this study demonstrated that application of low selection pressure and moderate up-flow linear velocity enables stable and efficient AGS production in SFD-SBRs fed with low-strength municipal wastewater under high VER and OLR conditions.