Comparison of Three Filtration Echnologies to Meet Tertiary Level Discharge and Unrestricted Reuse Requirements

Metropolitan Wastewater Management Commission (MWMC) was formed in 1977 through an intergovernmental agreement between Lane County and the cities of Eugene and Springfield, Oregon. MWMC was created to construct and operate regional wastewater facilities, including the Water Pollution Control Facility (WPCF), the Biosolids Management Facility, the Biocycle Farm, and four collection system pump stations. MWMC provides wastewater service to more than 250,000 residents. In 2004, the MWMC decided to add tertiary filtration to the 14,000-m3/d (30- mgd) WPCF.The benefits of adding filtration would be twofold. First, discharging filtered effluent to the river would reduce the concentration and mass of biochemical oxygen demand (BOD) and total suspended solids (TSS) to help meet discharge permit limits. Second, by producing a Class A recycled water for use in irrigation and industrial processes, MWMC would reduce the thermal loads to the Willamette River by diverting a portion of the relatively warm effluent from the river.To select the most appropriate filtration technology, in 2008 MWMC conducted parallel pilot testing of cloth disk filtration, compressible medium filtration, and granular medium filtration. These three different technologies have not previously been tested side by side for secondary effluent filtration.The goals of the testing were to:evaluate the performance of the filters with respect to Class A recycled water criteria,gain hands-on experience with operation and maintenance of the filters,evaluate the filters' reliability and performance as affected by variations in filter influent quality and flow rate,determine and/or confirm design criteria for the filter hydraulic loading rate,determine backwash water requirements including the backwash water ratio, andevaluate the head loss developmentThe results from this testing were used to design the filtration facilities, which have a total treatment capacity of 53,000 m3/d (14 mgd).