DIRECT FILTRATION: LOW COST ALTERNATIVE TO TERTIARY CLARIFICATION EXPANSION

Direct filtration for total phosphorous removal, which is filtering secondary effluent conditioned with alum but not taken to a tertiary clarifier, has been successfully implemented at the Rock Creek Advanced Wastewater Treatment Facility (AWTF). When compared to expansion with high-rate clarification, implementation of direct filtration saved significant capital costs and will reduce chemical use and power consumption. The direct filtration facilities were constructed in 2004 for approximately 400,000—approximately 7 percent of the estimated costs of adding high-rate clarification. Since start-up, effluent from direct filtration blended with filtered tertiary clarifier effluent has had an average total phosphorous concentration of 0.08 milligrams per liter (mg/L) and the direct filtration process has used 20 percent of the alum used in tertiary clarification.