Aerobic granular sludge process: a fast growing biological treatment for sustainable wastewater treatment

Aerobic granular sludge (AGS) was developed as a better replacement for activated sludge (AS) and overcoming sludge-water separation issues in the biological treatment of municipal and industrial wastewaters. AGS consists of dense self-immobilized microbial granules that have compact microbial structure, high biopolymer content and higher settling velocities. Granular microbial growth allows effective biomasswater separation, higher biomass concentrations in the bioreactor and simultaneous nitrification, denitrification and phosphate removal from wastewater in a single treatment tank. In this way, AGS process minimizes the land footprint (50%-75% lower) and energy (30%-48% lower) of wastewater treatment plants (WWTPs) as compared to conventional AS process. In the last 20 years, AGS process has grown into a mature biotechnological solution from laboratory-scale to full-scale WWTPs. The other benefits of AGS process are lower sludge production and resource utility of excess sludge. Prevailing microbial groups and their unique metabolism contributes to lower sludge production in AGS process. Extraction of alginate-like exopolymer increases the resource recovery and aid in the sludge management. However, the long start-up periods for AGS formation and disintegration of AGS are seen as constraints for widespread implementation of AGS technology. Extensive research in this area already identified key operational parameters and strategies for improved start-up of granulation. The bioreactor operating conditions such as anaerobic feeding, feast-famine regime and short settling periods impose selection of slow-growing bacteria with unique metabolic traits and favour granulation. Anaerobic feeding and sludge removal strategies are important in maintaining the stability of granules. Further studies on molecular aspects of granulation would be crucial for widespread implementation of this emerging technology in WWTPs.