Continuous flow aerobic granular sludge: recent developments and applications

Abstract

Abstract Aerobic granular sludge (AGS) in continuous-flow reactors (CFRs) has attracted significant interest, with notable progress in research and application over the past two decades. Cumulative studies have shown that AGS-CFRs exhibit comparable morphology, settleability, and pollutant removal efficiency to AGS cultivated in sequencing batch reactors, despite their smaller particle sizes. Shear force and selection pressure are the primary drivers of granulation. While not mandatory for granulation, feast/famine conditions play a crucial role in ensuring long-term stability and nutrient removal. Additionally, bioaugmentation can facilitate the granulation process. Furthermore, this paper comprehensively assesses the application of AGS-CFRs in full-scale wastewater treatment plants (WWTPs). Currently, AGS-CFRs have been implemented in nine WWTPs, encompassing two distinct processes. Hydrocyclone-based densified activated sludge significantly enhances sludge density, settleability, and biological phosphorus removal efficiency, thus increasing treatment capacity. The microaerobic–aerobic configuration with internal separators can induce granulation, ensuring long-term stability, eliminating the need for external clarifiers, and reducing land and energy requirements. This review demonstrates the high potential of AGS-CFRs for intensifying existing WWTPs with minimal retrofitting needs. However, further research is required in granulation mechanisms, long-term stability, and nutrient removal to promote the widespread adoption of AGS.