Influence of sludge discharge on sludge settleability and membrane flux in a membrane bioreactor

In this study, the correlations between sludge characteristics, amount of extracellular polymeric substances (EPS) and membrane fouling were investigated and analysed, using continuous sludge discharges from a membrane bioreactor (MBR). The results showed that continuous sludge discharges improved sludge settleability, but not always weakened membrane fouling. The changes in the EPS amount, caused by continuous sludge discharge from MBRs, could resolve this situation well. Soluble EPS concentration could be regulated, to some extent, by sludge discharges. But it was embarrassed to artificially adjust bound EPS amount because the MBRs included living microorganisms and their metabolites. After sludge had been discharged four times, the sludge volume index (SVI) was in the range of 50-60 mL.gSS(-1) with a VSS:SS ratio of about 0.8. Sludge settleability mainly depended on the bound EPS content. The bound EPS content and its components presented a closer relationship to sludge hydrophobicity than to the membrane flux. Soluble EPS dominated both the EPS content on the membrane surface and membrane fouling.