Impact of additive application on the establishment of fast and stable aerobic granulation

Aerobic granular sludge (AGS) is a microbial biofilm self-aggregation, which is effective for nutrient and pollutant removal, through the development of dense microbial layers bound together with extracellular polymeric substances (EPSs). However, long start-up times and granule disintegration are still challenges ahead. An array of external additives, including ion chelating agents, sludge-based enhancers, and magnetic influence have been tested to overcome these barriers. The application of such additives may promote enhanced EPS production, neutralization of charges on the bacterial surface, acts as a core-induced agent, or as a bridge to connect EPSs and cell surfaces. Although additives may improve the granule formation without reducing treatment efficiencies, there are still environmental concerns due to the fate and toxicity of discharged excess sludge. This mini-review identifies an array of external additives and their mechanisms to improve granulation properties, and proposes discussion about the technical and economic viability of these additives. KEY POINTS: • Additives reduce granulation time and repair granule disintegration. • Biopolymer-based additives fulfill technical and environmental requirements. • Sludge-based additives are cheap and in line with the resource recovery concept. • The need for environmental-friendly additives for aerobic granular sludge process. • External additives affect granular biomass size distribution.