A novel two-stage aerobic granular sludge system for simultaneous nutrient removal from municipal wastewater with low C/N ratios

In this study, a novel two-stage aerobic granular sludge (AGS) system was developed to treat municipal wastewater. High removal efficiencies of phosphorus (91%) and nitrogen (81%) were obtained under a low influent carbon/nitrogen ratio of 5.4. The high nutrient removal was attributed to microbial segregation in the two sequencing batch reactors (SBRs). In the first reactor, the high abundance of polyphosphate-accumulating organisms (Candidatus Accumulibacter 15.3%) promoted a high rate of enhanced biological phosphorus removal (EBPR). In the second reactor, residual ammonium was autotrophically removed via partial nitritation/ anammox (PN/A). Moreover, granular sludge was maintained in both SBRs with a sludge volume index of 40-80 mL/g, which reduced the settling time and improved the operational stability. Furthermore, batch tests and mass balance analysis revealed that most influent phosphorus (68%) and some organics (33%) were recovered from the waste sludge as struvite and methane, respectively. Overall, this study illustrates that the novel two-stage AGS system integrating EBPR and PN/A is promising for enhancing nutrient removal, as well as resource and bioenergy recovery from municipal wastewater.