Aerobic sludge granulation in a Reverse Flow Baffled Reactor (RFBR) operated in continuous-flow mode for wastewater treatment

• It is first time to cultivate aerobic granules in continuous-flow RFBR.
• Several cells used, flow direction reversed, settling time controlled in RFBR.
• Periodic feast-famine, high H/D, and without sludge pump for granulation.
• Raw wastewater constituent aided in granulation was analyzed.
• Bacterial community variations occurred in RFBR and SBR.

A novel Reverse Flow Baffled Reactor (RFBR) was constructed to achieve aerobic granulation in continuous-flow process in order to treat the high quantity of influent wastewater. Municipal wastewater was fed to a 120 L reactor and successfully produced aerobic granules revealing a sludge volume index (SVI) of 33 mL/g. Aerobic granules possessed higher extracellular polymeric substances (EPS) content than seed sludge. The protein/polysaccharide (PN/PS) ratio in the EPS was determined to be about 10:1. Bacterial community analysis revealed that most of the species (Bacteroidetes, Nitrospira and Proteobacteria) found in the seed sludge were preserved in the reactor, except Pedobacter species that was washed out from the reactor. Moreover, different bacterial species were identified in RFBR and SBR (sequencing batch reactor) granules due to different process and operational parameters. Presence of Fe, Ca, Al, Si and P in wastewater were aggregated in granules (acted as a core) and enhanced the granulation. However, the RFBR was operated in periodic feast-famine condition, short settling time, high height/diameter (H/D) ratio, and without sludge return pump, which led to successful granulation in continuous-flow mode.