Selection pressure theory for aerobic granulation - an overview

Aerobic granulation technology is an improvement over conventional activated sludge process, where cell-to-cell interactions lead to the formation of stable granules with excellent settleability. Selection pressures are the factors that can strongly influence the formation and growth of aerobic granules. Settling time and the volume exchange ratio (VER) will decide how long the bioparticles would remain in the system, and thereby the hydraulic retention time. The minimum settling velocity needed for the particles to be retained in the reactor can be achieved by proper control of settling time and VER. Hydrodynamic shear force can affect granule formation and shaping, density and structure of the granules, and production of polysaccharides. Thus settling time, volume exchange ratio and hydrodynamic shear force can be considered as selection pressures in aerobic granulation process. The effects of these three parameters in aerobic granulation process in sequencing batch reactors (SBRs) are reviewed in this paper.