USING MEMBRANES TO MEET A STRICT PHOSPHORUS DISCHARGE LIMIT

A membrane filtration pilot plant was operated at the City of Ashland, Oregon Wastewater Treatment Plant from August to November 2000. The purpose of the pilot plant evaluation was to determine the suitability of the membrane process to cost-effectively treat the secondary wastewater from the Ashland WWTP to meet required water quality criteria, specifically phosphorus concentrations.Phosphorus concentrations in the secondary effluent ranged from 1.5 to 4.5 mg/L. Coagulated filter effluent phosphorus concentrations ranged from 0.05 to 0.5 mg/L in the submerged system and with higher concentrations in the pressure system. The key to good removal was determined to be coagulant dose and flocculation time. Minimal phosphorus was removed during the first run without the addition of coagulant, but with 60 mg/L of alum added, phosphorus was easily removed from the effluent.As more water bodies are designated water quality limited, treatment plant discharge limits are reduced to levels that can no longer be met by conventional secondary treatment. The Oregon DEQ has begun setting total phosphorus limits in discharges to many streams at 0.07 mg/L. Results from the pilot study at the City of Ashland WWTP demonstrate that membrane filtration is a viable option for meeting strict BOD, TSS and total phosphorus permit limits. This paper will present the results from the pilot study for both membrane systems and discuss how results were used to develop full-scale design criteria.