Effect of sludge discharge positions on steady-state aerobic granules in sequencing batch reactor (SBR)

We have investigated the effect of sludge discharge location on the steady-state aerobic granules in sequencing batch reactors (SBRs). Two SBRs were operated concurrently with the same sludge retention time using sludge discharge ports at: (a) the reactor bottom in R1; and (b) the reactor middle-lower level in R2. Results indicate that both reactors could maintain sludge granulation and stable operation, but the two different sludge discharge methods resulted in significantly different aerobic granule characteristics. Over 30 days, the chemical oxygen demand (COD) removal of the two reactors was maintained at similar levels (above 96%), and typical bioflocs were not observed. The average aerobic granule size in R2 was twice that in R1, as settling velocity increased in proportion to size increment. Meanwhile, the production yields of polysaccharide and protein content in R2 were always higher than those in R1. However, due to mass transfer limitations and the presence of anaerobes in the aerobic granule cores, larger granules had a tendency to disintegrate in R2. Thus, we conclude that a sludge discharge port situated at the reactor bottom is beneficial for aerobic granule stability, and enhances the potential for long-term aerobic granule SBR operation.