The essential role of hydrodynamic shear force in the formation of biofilm and granular sludge.

Biofilm and granular sludge processes are promising biotechnology for wastewater treatment. The formation, structure and metabolism of immobilized microbial community are associated very closely with hydrodynamic shear force in reactors. Therefore, this paper attempts to review the essential role of shear force in the formation and performance of biofilm and granular sludge. More compact, stable and denser biofilms, aerobic and anaerobic granules form at relatively higher hydrodynamic shear force. It is clearly shown that shear force has significant influences on the structure, mass transfer, production of exopolysaccharides, metabolic/genetic behaviours of biofilms, aerobic and anaerobic granules. In an engineering sense, hydrodynamic shear force can be manipulated, as a control parameter, to enhance microbial granulation process. It can be concluded that the knowledge regarding the effects of hydrodynamic shear force on biofilms and granules is far from complete and much research is still needed to fully understand the relevant mechanisms. Some of these future research niches are therefore outlined.