Improving aerobic sludge granulation in sequential batch reactor by natural drying: Effluent sludge recovery and feeding back into reactor.

One of the main problems in treating high volumes of wastewater is the long startup time required aerobic granular sludge (AGS), and this issue significantly limits the broad application of advanced AGS technology. To promote rapid AGS formation in the startup phase, a method was developed involving the recovery and natural drying of effluent sludge prior to feeding it back into the sequencing batch reactor (SBR). An analysis of the process shows that supplemented naturally dried sludge swiftly promoted sludge aggregation and granular sludge formation in the reactor, and feeding the SBR with naturally dried sludge aggregates (1.75 ± 0.05 g/L seven times) significantly shortened the granulation time in the startup phase by 14 days. In addition, MLSS, SVI30, SVI30/SVI5, and the average granule size of AGS in the reactor were maintained at 4.66 g/L, 47.4 mL/g, 0.93, and 2.8 mm, respectively. When fed back into the bioreactor, the aggregates acted as nuclei/carriers in the rapid granulation and played a significant role in rendering the SBR operation stable. This approach could be used to eliminate the random granules aggregation-disintegration mechanism that occurs in the initial stage of AGS formation. The study results reveal that the removal rate of COD and NH4+-N were above 95% and 96%, respectively. Furthermore, this approach requires less energy and significantly reduces the amount of sludge produced (as the effluent sludge is reused).