The influence of shear on extracellular polymeric substance production in membrane bioreactors

Shear is used to control particulate fouling in membrane bioreactors (MBRs) and is therefore a key process parameter. However, shear also affects the physical and physiological properties of MBR biomass. This research examines the relationship between the level of shear and microbial extracellular polymeric substance (EPS) production in MBRs. Different mathematical model structures are evaluated to describe shear-induced changes in floc-associated and soluble EPS production.;The relationship between EPS production and the level of shear was first examined with MBRs that decoupled soluble EPS production from retention and degradation by using a large membrane pore size of about 20 mum. Differences in the level of shear were achieved through mechanical mixing. It was demonstrated that biomass grown in high shear conditions had lower floc-associated and soluble EPS production compared to biomass grown in a lower shear environment. The decreased soluble EPS production under high shear conditions was caused by two factors: (1) the lower concentration of floc-associated EPS and (2) the production of stickier floc-associated EPS that is more erosion resistant in the high shear reactor.;The production and degradation of soluble EPS compounds was further investigated using traditional microfiltration MBRs operated with differing aeration intensity. Increased aeration had no direct effect on soluble or floc-associated EPS production in the microfiltration MBR. However, increased aeration intensity had a significant effect on predatory organisms. Large aquatic earthworms, A. hemprichi, proliferated under lower shear conditions but A. hemprichi were never observed in the high shear reactor. Predation by A. hemprichi resulted in increased floc-associated and soluble EPS production in the low shear MBR. Thus, differences in the mixing conditions indirectly resulted in increased soluble EPS concentrations and larger fouling potential in the low shear MBR but not in the high shear MBR.;The experimental results of the current study show two dominant mechanisms of soluble EPS production in MBRs: (1) erosion of floc-associated EPS and (2) biomass decay caused by predation. Soluble EPS produced by erosion was readily biodegradable while soluble EPS produced by biomass decay which was slowly degradable. It was further demonstrated that mathematical models of soluble EPS production in MBRs should include both erosion of floc-associated EPS and biomass decay. EPS production is tightly linked with fouling in MBRs. The current work demonstrates the link between MBR operation (i.e., level of shear) and EPS production. Ultimately, an understanding of the complex mechanisms of EPS production and their inter-relationship with process parameters will help facilitate the evolution of MBR operational philosophy toward strategies that both minimize the production of biological foulants in addition to reducing the deposition of foulants on the membrane surface.