Achieving activated sludge granulation and evaluating the performance of an innovative anaerobic-anoxic sequencing batch reactor system

Granular sludge (GS) technology is a cutting-edge advancement in wastewater (WW) treatment, enabling the simultaneous removal of carbon (C), nitrogen (N), and phosphorus (P) in a single reactor. The process relies on granular microbial aggregates that support the growth of different communities, including nitrifiers, heterotrophic bacteria, polyphosphate-accumulating organisms (PAOs), and glycogen-accumulating organisms (GAOs). The granules enhance pollutant removal through spatially distinct metabolic activities, and PAOs play a key role in P removal by storing it intracellularly as polyphosphate, allowing P to be removed from WW and recovered. This study investigates the operation of an anaerobic-anoxic sequencing batch reactor (SBR) system fed on acetate as C source using nitrate as the sole electron acceptor. Operating in 4-hour cycles, divided into 90-min anaerobic and 150-min anoxic phases, the system promoted biomass granulation, developing dense granules from conventional flocculent sludge. Experimental results demonstrated the effective removal of organics, nitrate, and P, with low P release-to-C uptake ratio and high specific P uptake. The system also achieved excellent sludge settleability (SVI equal to 85.4 mL/g TSS after 5 minutes), minimal solids discharge (12 ± 8 mg TSS/L), and high-quality effluent, demonstrating that anoxic granular sludge technology could be a sustainable solution for advanced WW treatment and nutrient recovery in case of appropriate WW composition.