Successful granulation and microbial differentiation of activated sludge in anaerobic/anoxic/aerobic (A2O) reactor with two-zone sedimentation tank treating municipal sewage.

A continuous pilot-scale A2O reactor with a two-zone sedimentation tank (A2O-TST) was constructed for the formation of aerobic granular sludge (AGS) to treat real municipal sewage. The characteristics of sludge, nutrient removal performance and the corresponding microbial ecology dynamics were studied during granulation process. Experimental results indicated that AGS with a mean particle size of 210 μm and sludge volume index after 30 min of 47.5 mL/g was successfully formed with effluent COD, total nitrogen and total phosphorus concentrations in the reactor reaching 22.8, 3.5 and 0.2 mg/L, respectively. Furthermore, high throughput data indicated that granules in settling tank-1 (ST-1) harbored slow-growing autotrophic organisms like Nitrosomonas and Nitrospira, while the flocs in settling tank-2 (ST-2) were dominated by fast-growing heterotrophic organisms including Ca. Accumulibacter, Dechloromonas, Flavobacterium, Arcobacter and Halomonas. Simulation results using computational fluid dynamics and discrete element method (CFD-DEM) modeling verified that the selection pressure created by the TST separator contributed to the retention of heavy granules (>1.011 kg/m3 density) in ST-1 zone and the withdrawal of light flocs (<1.011 kg/m3 density) from ST-2 zone. Therefore, the segregation of biomass using the TST system provides an opportunity to select for desired microbial populations and to optimize the nitrogen and phosphorus removal performance of the A2O-TST reactor. This study could add a guiding sight into the application of two-sludge system based on AGS technology for upgrading traditional A2O process.