A unified theory for upscaling aerobic granular sludge sequencing batch reactors

Aerobic granular sludge sequencing batch reactors (SBR) are a promising technology for treating wastewater. Increasing evidence suggests that aerobic granulation in SBRs is driven by selection pressures exerted on microorganisms. Three major selection pressures have been identified as follows: settling time, volume exchange ratio and discharge time. This review demonstrates that these three major selection pressures can all be unified to one, the minimal settling velocity of bioparticles, that determines aerobic granulation in SBRs. The unified selection pressure theory is a useful guide for manipulating and optimizing the formation and characteristics of aerobic granules in SBRs. Furthermore, the unified theory provides a single engineering basis for scale up of aerobic granular sludge SBRs.