Analysis of filterability of mixed liquor in membrane bioreactor with complete sludge retention

A laboratory-scale membrane bioreactor(MBR)was operated for 100 days with complete sludge retention in order to investigate the filterability of mixed liquor.Soluble microbial products(SMP),molecular weight(Mw)distributions of SMP,proteins and carbohydrates in the extracellular polymeric substances(EPS)were periodically monitored.A modified fouling index(MFI)was used to investigate the filterability of mixed sludge.It is closely related to the mixed liquor suspended solids(MLSS),SMP and EPS.The results showed that the MFI increased rapidly after 30 days of operation,which indicated that the long term operation of a MBR with no sludge discharge had a negative influence on the filterability of mixed sludge.The concentration of SMP,especially the SMP with Mw10 kDa,was positively correlated to the filterability of mixed sludge.MLSS concentration had a complex interaction with the filterability of mixed sludge in MBR,and the existence of a threshold above which the MLSS concentration had a influence on the filterability of mixed sludge were also found.The protein fractions in the EPS were biodegradable and available for microorganism metabolism after about 60 days.Test results demonstrated that carbohydrates in the EPS had a negative effect on the filterability of mixed sludge.