Disintegration and re-granulation of aged algal-bacterial aerobic granular sludge in wastewater treatment

Long-term stability in practical applications is one of the crucial issues challenging the sustainable operation of aerobic granular sludge (AGS)-based wastewater treatment systems. Numerous attempts have been made to improve its long-term stability, among which introducing microalgae to bacterial AGS systems seems promising. This study investigated and recorded the whole process of disintegration and re-granulation of the aged algal-bacterial AGS in a 42-day operation. The disintegration of granules was signaled by broken structure and more effluent biomass discharge, resulting in deterioration of phosphorus removal from 86 % on day 1 to 7 % on day 20 and some slight decrease in chlorophyll a content. The broken granules were found to be filamentous flocs or granule fragments, and the latter with coexisting microalgae possibly accounted for the re-granulation and system recovery. Large proportions of fragments could be developed into new granules on day 30 under no additional strategy adoption. And the effluent biomass, chlorophyll a content and phosphorus removal can recover to their initial levels within 34 days' operation. Interestingly, organics and ammonia removals were not affected, which kept around 99 % during the whole test period. Results from this study provide a more in-depth understanding of the stability of algal-bacterial AGS and its long-term operation management.