Membrane fouling study and identifying fouling microorganisms in membrane bioreactors treating municipal wastewater

Abstract

Two bench membrane bioreactors (MBRs), Bench Enviroquip and Bench Ionics, and four pilot MBRs were operated at Honouliui wastewater treatment plant (WWTP) treating municipal wastewater at 10-d, 20-d, 30-d, 40-d mean cell residence time (MCRT), and no sludge wasting conditions for membrane fouling study and identification of fouling microorganisms. Results indicated that bench and pilot MBRs produced high quality effluent water. Higher critical flux was obtained for bench Ionics MBR (39.3 LMH) than for bench Enviroquip MBR (37.4 LMH). No sludge wasting condition showed higher conditioning and steady-state fouling rates. It was found that SMP concentration had no or little effect on membrane fouling process, while carbohydrate EPS concentration appeared to have a significant impact on steady-state fouling rate. Shock loading condition by glucose addition and no anoxic zone condition following the normal 20-d MCRT operation were also evaluated and results did not show any increase of membrane fouling, indicating bench MBRs are capable of handling shock loading and variance in operating schemes without compromising membrane performance. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) fingerprinting analysis and 16S rRNA clone library analysis were performed for the bench-scale MBRs. Betaproteobacteria was found to be the dominant bacterial group in MBR mixed liquors (47%). Pseudomonas sp. might involve in membrane fouling process since it was detected extensively in biofilm. The presence of Rhodobacteraceae sp. and Brevundionas sp. might increase membrane fouling too. Laboratory bench MBR with polyvinylidene fluoride (PVDF) and polyvinyl chloride (PVC) ultrafiltration membrane modules were operated to study optimum ultrafiltation membrane cleaning methods. Majority of the membrane resistance at fouled condition was attributed to cake resistance. Vacuum permeation with Clorox® disinfectant + industrial alcohol or propylene glycol solution was found to be the best cleaning method with the highest membrane flux recovery and lowest membrane re-fouling rates. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and Energy-dispersive X-ray spectroscopy (EDX) analysis indicated less contaminants were observed or detected in ultrafiltration membranes with the optimum cleaning method. DGGE fingerprinting and clone library analyses showed that bacteria responsible for fouling belonged to Betaproteobacteria, Deltaporteobacteria and Bacteroidetes.