Impact of MBR cleaning and breaching on passage of selected microorganisms and subsequent inactivation by free chlorine.

Membrane bioreactors (MBRs) produce better quality effluent compared to conventional treatment processes but they are still subjected to the same disinfection requirements as conventional processes by many regulatory agencies. A research study consisting of bench-, pilot- and full-scale studies was conducted to characterize effluents produced from an MBR system operating under routine and challenged conditions and to assess the disinfection requirements for these effluents. Membrane cleaning did not seem to pose a substantial risk with respect to passage of target microorganisms; however, the membrane under breached conditions (turbidity > 0.5 NTU) resulted in an increase in a total coliform bacterial concentration up to 8500 CFU/100 mL. Adenoviruses were always detected in MBR filtrate samples by PCR (method detection limit of 10(3) genome copies per 25 µL reaction) irrespective of the membrane cleaning or breaching status. Passage of MS-2 bacteriophage through a breached membrane was lower compared to total coliform bacteria potentially due to their lower densities in the mixed liquor. Despite an increase in microbial concentration, a free chlorine CT of 30 mg-min/L was sufficient to achieve greater than 5-log removal of seeded MS-2 bacteriophage and removal of total coliform bacteria at or below the method detection limit (2 CFU/100 mL) for samples with a filtrate turbidity of 1.0 NTU. If such lower CT were to be employed, a significant decrease in plant footprint and operational costs could be realized.

Characterization of effluent water qualities from satellite membrane bioreactor facilities.

Membrane bioreactors (MBRs) are often a preferred treatment technology for satellite water recycling facilities since they produce consistent effluent water quality with a small footprint and require little or no supervision. While the water quality produced from centralized MBRs has been widely reported, there is no study in the literature addressing the effluent quality from a broad range of satellite facilities. Thus, a study was conducted to characterize effluent water qualities produced by satellite MBRs with respect to organic, inorganic, physical and microbial parameters. Results from sampling 38 satellite MBR facilities across the U.S. demonstrated that 90% of these facilities produced nitrified (NH4-N <0.4 mg/L-N) effluents that have low organic carbon (TOC <8.1 mg/L), turbidities of <0.7 NTU, total coliform bacterial concentrations <100 CFU/100 mL and indigenous MS-2 bacteriophage concentrations <21 PFU/100 mL. Multiple sampling events from selected satellite facilities demonstrated process capability to consistently produce effluent with low concentrations of ammonia, TOC and turbidity. UV-254 transmittance values varied substantially during multiple sampling events indicating a need for attention in designing downstream UV disinfection systems. Although enteroviruses, rotaviruses and hepatitis A viruses (HAV) were absent in all samples, adenoviruses were detected in effluents of all nine MBR facilities sampled. The presence of Giardia cysts in filtrate samples of two of nine MBR facilities sampled demonstrated the need for an appropriate disinfection process at these facilities.

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.