Pilot Scale Evaluation of Coagulation and Ultrafiltration for Phosphorus Removal

The City of Boise, Idaho (City) is evaluating approaches for improved phosphorus removal at its West Boise WRF (WBWRF) in response to changes in the Lower Boise River TMDL which established new, more stringent, phosphorus discharge limits. The City is evaluating various strategies to meet the combined mass load limit, which may result in limits at West Boise WRF from 0.1 mg/L to 0.35 mg/L TP. After completion of ongoing improvement projects and optimization activities it is anticipated that secondary effluent TP concentration will be 1 mg/L. Preliminary desktop evaluations recommended a split flow, tertiary Coagulation/Flocculation/Filtration (direct filtration) process using ultrafiltration membranes. A pilot study was conducted to verify required coagulant doses and associated impacts on membrane performance and sizing. The pilot study included 4 coagulants and 3 hollow fiber membrane systems. Membrane system testing included 2 submerged, vacuum driven ultrafiltration systems and 1 positive pressure, cartridge type ultrafiltration system. Coagulant testing showed that RE300, a coagulant mixture consisting primarily of cerium chloride required the lowest dose to reduce phosphorus from 1 mg/L to <0.1 mg/L. Required coagulant doses in ascending order were RE300 <Ferric Chloride<Alum<Poly Aluminum Chloride (PACl) Results of the membrane pilot test indicated that membrane fouling increased with coagulant dose, regardless of the coagulant type. At the range of doses needed to reduce phosphorus from 1 mg/L to 0.1 mg/L in the secondary effluent (12 mg/L – 35 mg/L) reliable flux rates for the two best performing membrane systems was 15 gallons/ft2/day (gfd). This flux rate is significantly less than was assumed in the desktop evaluation that recommended membranes, making membranes less attractive from the cost perspective. This has prompted the City to re-evaluate its options, including the use of a clarification step in front of the membrane process to reduce fouling, and increase flux and reliability. That work is ongoing.