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samples were tested for presence of total coliforms, fecal coliforms, and/or E. coli bacteria in the MBR permeate. The results of the sampling clearly showed that the effluent from the MBR systems without an additional disinfection step met pathogen-based surface water discharge standards for treatment plants and were below limits set for water bodies used for recreation. A summary of the results of this study are shown in Table 1 below. Table 1: Study Data Measuring Microbial Contaminants in MBR Permeate Total Samples Taken Samples Less Than Detection Limit Range of Plant Specific Geometric Means* Geometric Mean of All Samples* 95th Percentile of All Samples * E. Coli 772 80% <1 to 1.4 1.1 3.4 Fecal Coliform 924 65% <1 to 2.9 1.6 10 Total Coliform 346 52% <1 to 10.9 2.6 34 * Units CFU/100mL As the industry accepts the MBR process as a viable means of disinfection, it will be imperative to understand how plant operations will ensure that the MBR system is capable of meeting these microbial limitations. Studies have shown that in MBR operation, with a supported membrane, substantial membrane damage was required to cause a significant increase in contaminant levels. Minor breaches in membrane integrity had negligible impact because of contaminant sorption onto much larger floc in the activated sludge, which are largely rejected even by damaged membranes, and the impact of the dynamic filtration layer. Online membrane permeate turbidity monitoring — which allows for an online means of displaying solids concentrations in the effluent and ensuring membrane integrity — was sufficiently responsive to detect membrane damage that resulted in a significant increase in contaminant levels in the MBR permeate; a turbidity threshold of 0.2 NTU (nephelometric turbidity units) was appropriate for verification of contaminant removal below required limits. (For MBR applications, the measurement of online turbidity is considered the operational standard for monitoring performance of the system.) It is important to note that all of the current guidelines are based on absolute values in the effluent, and therefore the treatment efficacy needs to be independent of the influent concentration of the contaminants. The Cost Of Disinfection The addition of a post-membrane disinfection step — such as chlorination/dechlorination, UV treatment, or ozonation — to the MBR flowsheet comes with significant cost, from both a capital expenditure and operating standpoint. The total costs associated with the incorporation of a UV disinfection system into a treatment scheme are significantly more than just the purchase costs of the equipment. It was estimated that the capital cost of including this equipment represents 3.3 percent of the overall capital cost of the project, and the operations and maintenance (O&M;) cost represents 2.9 percent of the anticipated total O&M; of the entire facility. Regulatory Cooperation Recent cases demonstrate that in certain jurisdictions regulators have granted municipalities the allowance to bypass disinfection due to the permeate quality achieved by the MBR systems. The following two cases provide examples of local regulatory bodies allowing municipalities to shut down their UV disinfection systems. Hutchinson Wastewater Treatment Facility — City of Hutchinson, MN In 2008, the City of Hutchinson Wastewater Treatment Facility completed a major expansion that included a GE LEAPmbr* filtration system running in parallel to the existing oxidation ditch activated sludge process. The MBR system added 1.27 MGD of capacity to the plant for a total treatment capacity of 3.67 MGD with the ability to treat a peak wet-weather flow of 9.62 MGD. The design included a UV disinfection facility for bacteria removal to reach the city's permit requirements for fecal coliforms of a monthly mean value of <200 MPN (most probable number) per 100 mL. Soon after startup, testing was conducted to review the MBR system's ability to independently meet the discharge requirements without postdisinfection. During the testing, sampling was done as specified by the permit requirements from the MBR effluent and was tested for fecal coliform and E. coli. The results of the testing demonstrated that the amount of fecal coliform and E. coli in the MBR effluent was negligible for all tests. Having validated the MBR's ability to meet the permit requirements without disinfection, the Minnesota Pollution Control Agency (MPCA) granted the city of Hutchinson wateronline.com ■ Water Innovations ACTIVATEDSLUDGE 15 GE's LEAPmbr technology installed at the city of Hutchinson, MN