MEMBRANE FILTRATION ACTIVATED SLUDGE: The Next Era of Treatment Case Study in the Town of Cohasset, MA

The Town of Cohasset, MA desired to upgrade and expand their 72,000 gpd extended aeration wastewater treatment plant (WWTP) to satisfy State mandates. The WWTP was almost entirely surrounded by wetlands, leaving minimal room for expansion. In addition, the Town needed to satisfy water quality issues with its discharge to Class SA waters with any upgrade measure. Tutela Engineering Associates, Inc. (TEA) of Wilmington, MA was retained to evaluate the feasibility of upgrading and expanding the plant within the current design limitations. Numerous technologies were evaluated, but only Biological Aerated Filters (BAFs) and Membrane Filtration Activated Sludge (MFAS) methods were deemed feasible alternatives. After a detailed evaluation of both technologies, (MFAS) was chosen for use to upgrade the Cohasset, MA municipal (WWTP). The use of MFAS would satisfy the tight site conditions, maximize the use of existing tankage and provide tertiary grade effluent quality to satisfy stringent permit limitations relative to its discharge to Class SA waters.Based upon an evaluation of existing facilities in Canada and in light of manufacturer recommendation, the MFAS system was designed, bid and constructed in the course of about two years. The modular membrane system was retrofit into existing surface aeration tankage in a sequenced construction process to maintain WWTP operation. Influent screening, an anoxic tank and ultraviolet (UV) disinfection were also provided with the upgraded facility. The MFAS process expanded the WWTP capacity over four fold, to 300,000 gpd, and simultaneously reduced effluent pollutant concentrations by two-thirds. The MFAS process has been on line for approximately a year performing exceptionally. More recent innovations have increased membrane system capacities and significantly reduced operational costs. The process has proved to be an affordable, environmentally beneficial measure demonstrating it as a viable, highly reliable solution for the upgrade of municipal treatment facilities.