Fouling in membrane bioreactors with various concentrations of dead cells

The effects of sludge viability on process performance and membrane fouling in a membrane bioreactor were investigated. A laboratory-scale submerged membrane bioreactor (SMBR) containing a flat-sheet microfiltration membrane was used at various percentages of living cellular flocs and dead cells. The effect of live to dead cells ratio on the extracellular polymeric substances (EPS) concentration was also studied. The proportion of dead cells affected the type of EPS in the mixed liquor as well as hydrophobicity of activated sludge. Tightly bound EPS (TB-EPS) mitigated membrane fouling, whereas loosely bound EPS (LB-EPS) caused severe membrane fouling. The results show that the content of TB-EPS and LB-EPS increases with dead cells portion. Furthermore, the ratio of TB-EPS to LB-EPS was presented as an important parameter in MBR performance. It was showed that this ratio is a good representative of flocculability of activated sludge and fouling intensity. Compared to the live cells, the dead cells had lower hydrophobicity and induced membrane fouling so that membrane permeability of the viable activated sludge was twice of the dead cell culture. The potential of zone settling velocity (ZSV) to be considered as a key parameter in MBR operation was also reported.

Research highlights
► The proportion of loosely and tightly bound EPS in activated sludge depends to cell viability and affect membrane fouling.
► High percent of dead cells in activated sludge results in more irreversible fouling in MBR systems.
► Hydrophobicity of activated sludge as one of its surface properties decreased by cellular death.
► Zone Settling Velocity (ZSV) was introduced as a potential parameter for usual evaluation in MBR plants instead of SVI.