Aerobic granulation in a modified oxidation ditch with an adjustable volume intraclarifier

•Aerobic granulation was achieved in modified oxidation ditch (MOD, continuous flow).•Adjustable volume intraclarifier was to control settling time and delete sludge pump.•Bacterial community of sludge in MOD was slight different compared to granule SBR.•MOD mode played a key role in granulation though raw wastewater had positive effect.

A modified oxidation ditch (MOD) with an adjustable volume intraclarifier was proposed and used to achieve aerobic sludge granulation in continuous flow process. This MOD with working volume of 60 L treated onsite wastewater from a town. Excellent aerobic granules with mean diameter of 600 μm and sludge volume index (SVI) of 44 mL/g were obtained in 120 day. Bacterial community analysis revealed that most species from seed sludge were preserved in both MOD and granule SBR (G-SBR) except bacteria (Bacteroidetes) might be easily washed out during granulation. Some different bacterial communities were found in sludges from sequencing batch and continuous flow reactors. Presence of metal ions and inorganics in raw wastewater had positive effect on granule formation, but an adjustable volume intraclarifier for controlling selection pressure and deleting return sludge pump played a key role in aerobic sludge granulation.