The effect of organic loading on process performance and membrane fouling in a submerged membrane bioreactor treating municipal wastewater

Pilot-scale and bench-scale SMBRs were operated on municipal wastewater primary effluent at mean cell residence times (MCRTs) between 2 and 10 d to investigate the effect of the organic loading rate (F/M) on the SMBR process and membrane performance. A control bench-scale CMAS reactor with gravity sedimentation was operated in parallel with, and at the same MCRTs as, the pilot-scale SMBR. In general, SMBRs produced a higher effluent quality with lower turbidity, COD and suspended solids (TSS) (non-detect) concentrations than the control reactor. The SMBR processes completely nitrified (effluent NH3-N < 1 mg/L) at all MCRTs > 2 d. Mixed liquor properties (EPS, SMP, soluble COD and colloidal material), believed to effect membrane filtration of activated sludge, were quantified for each steady-state condition. Because a membrane provided solid-liquid separation in the SMBR, there was no selective pressure for sludge to settle and the sludge EPS content was lower than in the CMAS reactor. Because the membrane retains materials larger than the membrane pores, the total mixed liquor SMP content increased with F/M in the SMBR processes while the SMP content was relatively constant with F/M in the CMAS reactor. The pilot-scale and bench-scale SMBR processes were operated at constant membrane fluxes (30 and 17 LMH), constant MLSS concentrations (8 and 1.5 g/L), and constant coarse bubble aeration flow rates for all conditions tested. The steady-state membrane fouling rate increased significantly for both SMBR processes as the F/M increased and correlated well with the total SMP concentration. The importance of the cake layer and adsorbed foulants to the total filtration resistance was estimated at the end of each condition. Filtration resistance was always mostly attributable to the adsorbed foulants but the significance of cake resistance increased with F/M. Batch filtration experiments on mixed liquor from low and high F/M conditions (0.54 and 2.34 gCOD/gVSS·d) revealed an increased total resistance to filtration. In addition, the batch filtration experiments demonstrated that the resistance attributable to the MLSS themselves (no soluble materials) increased 6-fold. Molecular weight (MW) fractionation of SMP and EPS revealed that protein and carbohydrate MW distributions were lower at MCRT of 10 d compared to 2 d.