Occurrence and removal of microbial indicators from municipal wastewaters by nine different MBR systems.

Nine different membrane bioreactor (MBR) systems with different process configurations (submerged and external), membrane geometries (hollow-fiber, flat-sheet, and tubular), membrane materials (polyethersulfone (PES), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE)) and membrane nominal pore sizes (0.03-0.2 µm) were evaluated to assess the impact of influent microbial concentration, membrane pore size and membrane material and geometries on removal of microbial indicators by MBR technology. The log removal values (LRVs) for microbial indicators increased as the influent concentrations increased. Among the wide range of MBR systems evaluated, the total and fecal coliform bacteria and indigenous MS-2 coliphage were detected in 32, 9 and 15% of the samples, respectively; the 50th percentile LRVs were measured at 6.6, 5.9 and 4.5 logs, respectively. The nominal pore sizes of the membranes, membrane materials and geometries did not show a strong correlation with the LRVs.

Peak flux performance and microbial removal by selected membrane bioreactor systems.

A pilot study was conducted over a period of 18 months at the Point Loma Wastewater Treatment Plant (PLWWTP) in San Diego, CA to evaluate the operational and water quality performance of six selected membrane bioreactor (MBR) systems at average and peak flux operation. Each of these systems was operated at peak flux for 4 h a day for six consecutive days to assess peak flux performance. Virus seeding studies were also conducted during peak flux operation to assess the capability of these systems to reject MS-2 coliphage. When operating at steady state, these MBR systems achieved an effluent BOD concentration of <2 mg/L and a turbidity of <0.1 NTU. Peak flux for the MBR systems ranged from 56 to 76 L/m2/h (liters per square meter per hour) with peaking factors in the range of 1.5-3.2. When switching from average to peak flux operation, a reversible drop of 22-32% in temperature-corrected permeability was observed for all submerged MBR systems. The percent drop in permeability increased as MLSS concentration in the membrane tank increased from 11,100 mg/L to 15,300 mg/L and was observed to be highest for the system operating at highest MLSS concentration. Such trends were not observed with an external MBR system. Each MBR system was able to sustain a 4-h-a-day peak flow for six consecutive days with only moderate membrane fouling. The membrane fouling was quantified by measuring the drop in temperature-corrected permeability. This drop ranged from 13 to 33% over six days for different submerged MBR systems. The MBR systems achieved microbial removal in the range of 5.8-6.9 logs for total coliform bacteria, >5.5 to >6.0 logs for fecal coliform bacteria and 2.6 to >3.4 logs for indigenous MS-2 coliphages. When operating at peak flux, seeded MS-2 coliphage removal ranged from 1.0 to 4.4 logs, respectively. The higher log removal values (LRVs) for indigenous MS-2 coliphage among different MBR systems were probably the result of particle association of indigenous coliphage. Differences in membrane pore size (0.04-0.2 microm) amongst the MBR systems evaluated did not have a substantial impact on indigenous MS-2 coliphage removal, but seeded MS-2 coliphage removal varied among the different MBR systems.

Performance investigation of membrane bioreactor systems during municipal wastewater reclamation.

Four commercially available membrane bioreactor (MBR) systems were operated at the pilot scale, to investigate performance during the reclamation of municipal wastewater. The MBR performance was evaluated under a variety of operating conditions, including two types of feed wastewater (raw and advanced primary effluent), hydraulic retention times (HRTs) ranging from 2 to 6 hours, and permeate fluxes between 20 and 41 lmh. Test results showed that MBR systems were capable of operating on advanced primary effluent, despite the possible presence of coagulant and/or polymer residual, with minimal membrane fouling. Membrane performance data generated during this study was also used to quantify the relationship between permeate flux and membrane fouling. Cleaning intervals at various flux conditions were estimated as follows: 69 days at 20 lmh, 58 days at 25 lmh, and 30 days for operation between 31 and 41 lmh. It was also demonstrated that the MBR process could be optimized to operate with minimal fouling under high hydraulic (flux = 37 lmh) and organic loading (HRT = 2 hours and food-to-microorganism ratio = 0.33 g COD/g VSS x d) conditions. Water quality monitoring conducted throughout the study showed that each MBR system consistently produced an oxidized (5-day biochemical oxygen demand < 2 mg/L) and nitrified (ammonia < 1 mg-N/L) effluent low in particulate matter (turbidity < 0.1 NTU), under all conditions tested.