Plant-Scale Phosphorus Removal by Secondary Addition of Alum

Many municipal wastewater treatment plants discharge their effluents into the Great Lakes. Several of the Great Lakes are experiencing excessive eutrophication which results in the accelerated aging of these lakes. The Lake Michigan Enforcement Conference required the removal of 85 percent phosphorus removal from any major wastewater effluents being discharged into the Lake Michigan watershed by December, 1972. One of the first plants in this watershed designed and operated with phosphorus removal facilities is the Muskego, Wisconsin Wastewater Treatment Plant. The Muskego, Wisconsin Northeast Sanitary District Wastewater Treatment Plant is designed to treat an average flow of 0.5 MGD. Approximately 85 percent of the wastewater is derived from domestic sources, with the remaining 15 percent derived from commercial and light industrial sources. The treatment process consists of a contact stabilization unit followed by an effluent polishing lagoon. Lagoon effluent is chlorinated before discharge to the Root River, a tributary to Lake Michigan. Waste activated sludge is aerobically digested prior to dewatering on sand drying beds. A research program was carried out during 1972-73 to (1) evaluate the effectiveness of alum for phosphorus removal on a plant scale basis, (2) determine the optimum point of alum addition, (3) determine the optimum Al to P ratio required for phosphorus removal, and (4) evaluate the effect of the polishing lagoon on phosphorus removal. The research program entailed 24-hour composite sampling of the raw wastewater and treatment plant effluent. Composite samples were collected at these points during a period when aluminum sulfate was added to the head of the aeration tank (Condition 1), the settling tank still well (Condition 2) and the end of the aeration tank (Condition 3). In addition, composite samples were collected at these points prior to any alum addition. A separate program was established to test the effect of aluminum sulfate addition on the lagoon effluent phosphorus concentration. This program included the collection of daily grab samples combined with periodic composite sampling of the lagoon effluent. All composite samples collected were analyzed for pH, chlorides turbidity, alkalinity, Biochemical Oxygen Demand (BOD5), Chemical Oxygen Demand (COD), ammonia nitrogen, organic nitrogen, total volatile and fixed residue, volatile and fixed suspended solids and various forms of phosphorus. Lagoon grab samples were analyzed for pH, turbidity and various forms of phosphorus. Aluminum sulfate addition to the Muskego Wastewater Treatment Plant didn\u27t significantly enhance the removals of BOD5, COD, suspended solids and total Kjeldahl nitrogen. However, overall removal of phosphorus was significantly increased. Phosphorus removal did not appear to be sensitive to the exact point of addition, with comparable results obtained from alum addition to the beginning and end of the aeration tank and the stilling well of the settling zone. An Al:P molar ratio of approximately 3:1 was required to achieve the desired 85 percent phosphorus removal. Improved phosphorus removals at a lower Al:P molar ratio of approximately 2:1 appeared possible with improved solids separation. The lagoon effluent phosphorus concentrations averaged about 1.32 mg/1 as THP. The polishing lagoon improved overall phosphorus removal from an average of 77.1 percent to 83.5 percent. Therefore, total system effluent phosphorus concentration of 1.0 mg/1 or less appeared possible with proper management of alum addition. The chemical cost of phosphorus removal at the Muskego, Wisconsin Wastewater Treatment Plant averaged $0.56 per 1000 gallons of wastewater treatment during this study