Diameter control and stability maintenance of aerobic granular sludge in an A/O/A SBR

• Small (<800 μm) and stable aerobic granules were obtained with low energy input.
• Simultaneous nitrification, denitrification and phosphate removal was realized.
• LB PS and TB PS fascinate the formation and stability maintenance of granules.
• Combined selective pressures was critical to control the size of aerobic granules.

Diameter is the key factor that influencing the stability of granules. In this study, combined selective pressure generated by low strength wastewater (chemical oxygen demand, COD, 200 ± 40 mg/L), low hydrodynamic shear force (superficial gas velocity of 0.55 cm/s and agitation of 180 rpm), different sludge discharge mode and a sequencing batch bioreactor (SBR) with an alternating anaerobic/aerobic/anoxic (A/O/A) operational process were employed to limit the diameter of aerobic granules. Energy consumption was reduced as low superficial gas velocity and agitation were employed. Granulation process was completed after 40 days. The matured granules with small diameter (d (0.9) < 800 μm) were stable without disaggregation during the experiment period (220 days). The extracellular polysaccharide/extracellular protein (PS/PN) ratio increased along with granulation process, and maintained at a stable level in the following time. After granulation, the removal efficiencies of COD, NH4+–N, total nitrogen (TN) and phosphate (P) reached 99%, 98%, 90% and 99%, respectively. The majority of TN was consumed during aerobic stage after complete granulation. The effective TN removal was mainly due to the activities of denitrifying phosphate-accumulating organisms (DNPAOs) and denitrifying glycogen-accumulating organisms (DNGAOs).