Microbial granulation is a process exploited in biological wastewater treatment whereby bacteria are organized into highly structured suspended granules that are capable of removing biodegradable organic matter, nitrogen, and phosphorus. Parts of microbial granule, probably, have coordinated physiological functions, i.e. cell growth, metabolism, interactions, biosynthesis, transport, consumption, and storage of nutrients.
Microbial granulation differentiated from flocculation and formation of microbial flocs by the following definition: granules making up granular activated sludge are aggregates of microbial origin (no carrier material is intentionally involved or added), which do not coagulate under reduced hydrodynamic shear, and which settle significantly faster than activated sludge flocs (de Kreuk et al., 2005). Microbial granules are usually spheres with diameter from 0.5 to 4 mm. Microbial flocs formed in conventional wastewater biological treatment due to the recycling from secondary settling tank are loose aggregates with undefined shape and size from 0.05 to 0.2 mm. Microbial granules are formed under aerobic conditions with such selection factors as settling time from 2 to 10 min (Tay et al., 2001c; Qin et al., 2004) and high aeration rate ensuring superficial upflow air velocity above 1.2cm/s in a column sequencing batch reactor (SBR) (Tay et al., 2001b; Liu and Tay, 2002, 2004). The primary aim of the formation of strong microbial granules and their application in industrial and municipal wastewater treatment is to avoid the construction of secondary settling tanks or to diminish their size. Formed granules have not only settling velocity higher than 10 m h-1 (Beun et al., 1999) but showed several other advantages over microbial flocs of conventional activated sludge, including reduced biomass yield (Tay et al., 2003b) and higher resistance to toxic compounds due to its compact structure (Glancer et al., 1994; Jiang et al., 2002; Bergsma-Vlami et al., 2005; Tay et al., 2005). Therefore, activated granular sludge systems are developing for the treatment of industrial wastewater and for application in the places where land is a premium. Aerobic granular sludge has been successfully used to treat real industrial wastewater like dairy effluents (Arrojo et al., 2004; Schwarzenbeck et al., 2005).
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