One of many hurdles that had to be overcome to bring this agreement together was obtaining approval from MPCA. MPCA’s primary concern that needed to be addressed in the agreement between the parties and the design of the new facilities was the discharge of phosphorus. The City was one of the first municipalities in Minnesota to agree to an effluent phosphorus limitation in the mid 1990’s, which strengthened the City’s negotiating position with MPCA for this project. The City’s original phosphorus discharge limit was established as a maximum annual mass discharge of 15,000 kg, which allowed the City to discharge an average phosphorus concentration of 1.8 mg/L at their current annual average flow of approximately 6.0 mgd.
The concept for returning the cooling water from the power plant was to blend it with effluent from the plant in the existing chlorine contact basin. At times the plant effluent could be predominantly cooling water return with phosphorus and TDS level fours times higher than the water originally supplied. With concerns that the concentration based effluent limits would be applied to the “dehydrated” effluent, an intermediate point for monitoring total phosphorus prior to use by the power plant was proposed and accepted by MPCA. A monthly average phosphorus limit of 0.9 mg/L at the intermediate point was agreed to by the City and MPCA. This concentration limit was targeted to prevent an increase in the in-stream phosphorus concentration accounting for the reduction of dilution water resulting from the evaporation of up to 4 mgd of water in the Calpine cooling towers.
To allow this project to move forward, an innovative treatment strategy was developed along with the creative discharge compliance solution. The water quality requirements for the reuse water were consistent with California’s Title 22 Reuse Regulations. The reuse water also required low phosphorus concentrations to reduce scaling in the cooling towers. The treatment strategy developed to provide Title 22 reuse water to Calpine is shown schematically on Figure 1. An Actiflo ballasted flocculation process was provided to coagulate secondary effluent and to precipitate phosphorus. The Actiflo process was designed to treat an average daily flow of 12 mgd and peak daily flow of 18 mgd.
Initially MPCA was requiring a 1.0 mg/L limit at the discharge point to the Minnesota River. To comply with this discharge requirement, the water sent to Calpine had to have a phosphorus concentration of less than 0.25 mg/L. After further negotiation with MPCA they relaxed their position allowing the discharge compliance point for phosphorus to be set downstream of the Actiflo process. Cloth media tertiary filters were provided after the Actiflo process to ensure the turbidity requirements of the Title 22 regulation could be maintained. A chlorine contact basin was constructed to provide advanced disinfection using sodium hypochlorite prior to pumping the reuse water to Calpine. Approximately 2 mgd of flow will be returned from Calpine back to the Mankato WWTP for discharge to the Minnesota River under the City’s NPDES permit.