PHOSPHORUS REMOVAL IN A MEMBRANE REACTOR SYSTEM: A FULL-SCALE WASTEWATER DEMONSTRATION STUDY

The Arapahoe County Water and Wastewater Authority (ACWWA) is located in the Cherry Creek Reservoir watershed in the Denver metropolitan area. Stringent phosphorus limits in wastewater discharges are required to protect the downstream reservoir. Annual mass phosphorus loadings for treatment facilities are based on meeting a total phosphorus concentration of 0.05 milligrams per liter (mg/L), or less, in the treated effluent discharged in the watershed.The ACWWA owns and operates a 2.4-million-gallon-per-day (mgd) membrane bioreactor (MBR) wastewater treatment facility (WWTF). The MBR consists of vacuum operated membranes (Zenon) placed in the aerobic reactor. The treatment process also includes an anoxic basin for denitrification. Phosphorus removal in the MBR is accomplished by the addition of precipitant chemical to the anoxic basin. Precipitated solids are removed by the membranes and wasted with the waste activated sludge pumped from the aerobic reactor.A special testing program was developed for the ACWWA with the goal of demonstrating that the MBR could meet the 0.05 mg/L phosphorus concentration on a consistent, reliable basis. The testing was planned for the wintertime to experience lower wastewater temperatures.The testing was also performed to compare ferric chloride and sodium aluminate for phosphorus removal and impact to operations. Ferric chloride was used for a 15-week period and sodium aluminate was applied for a separate 12-week period.Results of the demonstration using ferric chloride showed that a dosage from between 80 and 100 mg/L reduced the effluent total phosphorus concentration to 0.05 mg/L, or less. The average effluent total phosphorus concentration was 0.035 mg/L during the period of ferric chloride addition.Results of the demonstration using sodium aluminate showed that a dosage from between 50 and 90 mg/L reduced the effluent total phosphorus concentration to 0.05 mg/L, or less. The average effluent total phosphorus concentration was 0.026 mg/L during the period of sodium aluminate addition. Phosphorus concentrations as low as 0.011 mg/L were experienced.This MBR demonstration study was successful in removing phosphorus in wastewater effluent to concentration well below 0.05 mg/L using either ferric chloride or sodium aluminate. This concentration was attained without the addition of a second chemical for adjusting pH. Effluent total nitrogen concentrations were consistently less than 7 mg/L during this testing. The operation staff preferred sodium aluminate to ferric chloride since the pH conditions were comparatively more stable and overall effluent phosphorus concentrations were lower.