The time scale of shear and its importance to fouling potential in MBRs

Shear is used to control particulate deposition on the membrane surface in membrane bioreactors (MBRs). However, shear also affects the physical and physiological properties of MBR biomass. The current study examines this relationship. The effect of short-term shear was studied by shearing biomass from three activated sludge wastewater treatment plants. The concentration of soluble EPS and fouling potential were measured for sheared and unsheared sludge. Long-term shear experiments were accomplished with two identical MBRs which were operated in parallel. The biomass in one reactor was exposed to seven times greater shear forces than the other. The concentrations of floc-associated and soluble EPS as well as the fouling potential were characterized for both the high and low shear biomass. The fouling potential was quantified using dead-end filtration flux decline experiments. Floc-associated EPS was extracted and quantified. Soluble EPS was characterized using size exclusion chromatography. Short-term increases in shear increase the fouling potential because EPS is released into solution from the floc-matrix. However, biomass grown under long-term high shear conditions produced less floc-associated and soluble EPS, and therefore had a lower fouling potential compared to biomass grown under low shear conditions. Thus, although short-term increases in shear would be detrimental to MBR process performance, high shear can be beneficial, not only to control particulate fouling, but also to reduce the biological production of soluble foulants in MBR systems.