Pilot Study Application of Tertiary Clarification and Filtration to Meet Proposed Ultra Low Phosphorous Discharge Limits on the Spokane River

The Spokane River flows through northern Idaho and eastern Washington. The Washington State Department of Ecology is proposing to implement an interim phosphorous TMDL (total maximum daily load) of 50 ppb, and a final TMDL of 10 ppb on all point source discharges to the river. Biological treatment alone is not sufficient to meet these levels, requiring the addition of tertiary physical/chemical treatment to existing systems. One treatment method available is co-precipitation by coagulation, clarification and filtration.Pilot studies were conducted on a municipal and industrial wastewater source that currently exceed the proposed TMDL. At each site two treatment processes were investigated. Process Train 1 utilized up-flow buoyant media clarification followed by mixed media filtration. Train 2 was comprised of a three stage process combining high rate sedimentation ahead of Train 1. Chemical treatment focused on co-precipitation with aluminum sulfate or ferric chloride and polymer addition.At the municipal site Train 1 averaged 97% reduction in total phosphorous at an alum dose of 75 ppm. All effluent phosphorous samples met the 50 ppb proposed TMDL and 20% met the 10 ppb proposed TMDL. At the industrial site the complex nature of the wastewater inhibited co-precipitation, requiring alum dosages in excess of 300 ppm to achieve an average 70% reduction in total phosphorous. Effluent phosphorous measured ≤ 100 ppb in 74% of samples, and ≤ 50 ppb in 21% of samples. Train 2 achieved similar phosphorous removals, with the added benefit of extending buoyant clarifier flush intervals 800% and filter backwash intervals 200 – 400%, corresponding to a 25 – 75% reduction in reject waste production.