Cultivation of aerobic granules under different pre-anaerobic reaction times in sequencing batch reactors

•Investigate the effect of pre-anaerobic reaction time on aerobic granulation.•Fluffy granules with poor settlement formed under long pre-anaerobic period.•Pre-anaerobic time of 85 min positively affect the stability of granules.•Higher tightly bound-EPS did not guarantee the stability of granular structure.•Big granules consumed higher TB-EPS mainly in the form of polysaccharides.

This work investigated the influence of pre-anaerobic reaction time on aerobic granulation in sequencing batch reactors (SBRs). For this purpose, a purely aerobic SBR (R1) was operated as a reference, while two other reactors, namely R2 and R3, were running with anaerobic/aerobic reaction times of 85 min/265 min and 175 min/175 min, respectively. Results showed that the compact granules cultivated in R1 disappeared in the early days of operation and gradually transited to large-sized ones with filamentous microorganisms as the backbone mostly ranging in size between 2 and 5 mm. The aerobic granules in R2 appeared to be more compact in structure, while the cultivated granules in R3 showed a fluffy, loose and dispersed structure morphology with poorer settling ability (SVI30 = 673 mL/g at the end of experiment). Investigation of tightly bound extracellular polymeric substances (TB-EPS) showed that the aerobic granules in R2 produced lower TB-EPS in comparison with two other reactors, which indicates that higher TB-EPS quantity in R1 and R3 did not guarantee the stability of granular structure. As a result, pre-anaerobic reaction time of 85 min in R2 positively affected the compactness and structural stability of granules, while more than 91% of influent COD could remove all the time.