Aerobic granules were considered to be a special case of biofilm composing of self-immobilized cells. During the last 20 years, intensive research in the field of biological wastewater treatment and other applications demonstrated that biofilms were often more efficient for water purification than suspended activated sludge. To date, the application of aerobic granular sludgewas regarded as one of the promising biotechnologies in wastewater treatment. The first patent was granted by Heijnen and van Loosdrecht (1998). de Kreuk et al. (2007a,b) provided comments on the state of the art for the aerobic granulation process. Liu and Tay (2004) and Maximova and Dahl (2006) provided an up to date summary of the current understanding towards the bioaggregation processes.
Granular sludge was first described for strictly anaerobic systems in 1980 (Lettinga et al., 1980) and only by the late 1990s had the formation and application of aerobic granules been reported (Morgenroth et al., 1997, Beun et al., 1999, Dangcong et al., 1999). The anaerobic granulation technology exhibited several drawbacks
that included a long start-up period, a relatively high operating temperature, unsuitability for low strength organic wastewater, and low efficiency in the removal of nutrients (N and P) from wastewater.This resulted in the development of aerobic granular technology which became a popular topic of discussion for environmental engineers.
Compact structured, biologically efficient aerobic sludge granules with wide diverse microbial species and excellent settling capabilities have been developed in sequencing batch reactors (SBR) (Morgenroth et al., 1997; Beun et al., 1999; Tay et al., 2001a; Yang et al., 2003; Liu and Tay, 2004, Adav et al., 2007a). Formation by self immobilization of bacteria as hypothesized by several researchers (Kim et al., 2004; McSwain et al., 2004a; Qin et al., 2004a,b;Wang et al., 2004; Hu et al., 2005; Liu et al., 2005), the aerobic granules were densely packed microbial aggregates and their densities were much higher than that of conventional activated sludge. In addition, the aerobic granules were known to exhibit attributes of:
1) Regular, smooth and nearly round in shape
2) Excellent settleability
3) Dense and strong microbial structure
4) High biomass retention
5) Ability to withstand at high organic loading
6) Tolerance to toxicity
