ABSTRACT
This chapter provides an overview of the major selection pressures for aerobic granulation in sequencing batch reactors (SBRs), and describes a unified selection pressure theory. In the study of aerobic granulation by two coaggregating bacterial strains, it was found that the coaggregating bacterial strains could produce autoinducer-like signals during aerobic granulation. The outer shape and size of microbial aggregates are determined by the interactive strength/pattern between aggregates and of hydrodynamic shear force, microbial species, and substrate loading rate. It is believed that the structure of mature aerobic granules is determined by hydrodynamic shear force, but there is no concrete evidence to show that shear force is a primary inducer of aerobic granulation in SBRs. An SBR is operated in a sequencing cycle of feeding, aeration, settling, and discharge of supernatant. The chapter shows that aerobic granulation in SBRs is driven by selection pressures exerted on microorganisms.
