Hui Lin, PE
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Fox Metro Water Reclamation District (WRD) in Oswego, Ill., started up in the 1920s with a trickling filter system designed to treat an average daily flow of 8 MGD. Growth and new stringent effluent regulations during the 1960s and 1970s resulted in the installation of traveling bridge sand filters for tertiary filtration in the 1970s and conversion of the trickling filter system to an activated sludge treatment process in 1982.
Fox Metro currently serves a population of almost 250,000 and has a treatment capacity of 42 MGD average daily flow and 85 MGD peak flow. Its treatment scheme includes a bar screen, grit removal, primary clarifiers, aeration tanks, secondary clarifiers, traveling bridge sand filters, AquaDiamond cloth media filters and chlorine disinfection tanks. The plant discharges its treated effluent directly into the Fox River.
Problem
The sand filters could not handle high flows in excess of 100 MGD (almost 20 percent more than the plant's rating) caused by rainfall, melting snow and high solids concentrations of 100 mg/L TSS or more produced by solids carryover from the secondary clarifiers. To solve its maintenance and performance issues, Fox Metro began researching other tertiary filtration technologies for a more effective alternative.
Solution
Fox Metro retrofitted three of its nine existing 16- by 110-foot traveling bridge sand filters with 80-foot-long AquaDiamond cloth media filters in 2005. The three retrofitted AquaDiamond filters provide an average design flow capacity of 36 MGD and peak flow capacity of 72 MGD. The maximum hydraulic flow is almost 2.5 times that of three equivalent size sand filters. The plant currently uses the remaining sand filters to aid in peak flow conditions until more AquaDiamond cloth media filters are installed.
The AquaDiamond cloth media filter has a low profile and handles high solids-loading excursions better than the sand filters. Each cloth media filter tank provides 3.25 pounds TSS/ft2 per day of solids loading capacity. The cloth media filter costs less to install, operate and maintain over the life of the equipment as compared to sand filters. The lower costs are primarily due to a much smaller footprint, reduced building size and fewer components to install and maintain.
