Tertiary Phosphorus Removal: An Effective Treatment Strategy for Reuse Applications

In late 2004, the City of Mankato, MN and the Calpine Corporation, an independent power production company, entered an agreement to treat and reuse the City's wastewater treatment plant effluent to supply cooling water for a proposed 640-megawatt combined-cycle power plant. The Mankato Wastewater Treatment Plant has an average day design capacity of 11.25 mgd, and prior to the tertiary treatment upgrade the liquid treatment facilities consisted of preliminary treatment, primary clarification, conventional activated sludge secondary treatment designed to meet seasonal ammonia limits, and standard disinfection for surface water discharge to the Minnesota River. Phosphorus removal was being accomplished by chemically precipitating phosphorus with ferric chloride in the primary and/or secondary clarifiers.To allow this project to move forward, a tertiary treatment system was designed and constructed to provide the necessary reuse water quality for the power plant cooling water, and to position the City of Mankato to be able to meet the forthcoming more stringent phosphorus discharge limits. 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 that had been designed for blowdown based on 4:1 cycling of the cooling water (75% of the supply water is evaporated). The Calpine Energy Center requires as much as 6.0 mgd of reuse water and returns a maximum of 2 mgd to the Mankato WWTP for discharge to the Minnesota River under the City's NPDES permit.The tertiary treatment strategy developed to provide the low phosphorus Title 22 reuse water to Calpine includes ballasted flocculation for phosphorus and turbidity removal, cloth media filtration, and advanced disinfection using sodium hypochlorite. The tertiary treatment systems are designed to treat an average daily flow of 12 mgd and peak daily flow of 18 mgd. There are a number of factors that contributed to the selection of the tertiary treatment strategy. As much as 70 percent of the phosphorus load on the Mankato WWTP is from a soy bean processing facility that discharges a highly variable load, making it difficult to reliably achieve low effluent phosphorus by chemically precipitating phosphorus in the primary and secondary clarifiers. Online phosphorus analyzers can now be used more effectively to monitor and control chemical feed to efficiently and reliably achieve low effluent phosphorus from the tertiary treatment process. Tertiary treatment also positions the City to meet more stringent future phosphorus limits as the plant service area grows. The City's phosphorus discharge limit prior to the upgrade 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 Minnesota Pollution control Agency (MPCA) had indicated that Mankato's phosphorus discharge limit would become more stringent in the next permit cycle, and that the limit would be based on a monthly average discharge concentration of 1 mg/L rather than the annual average mass limit. The new 1.0 mg/L limit will apply up to the current annual average permitted flow of 11.25 mgd, but would be reduced to maintain the same mass discharge at future flows above 11.25 mgd.The plant effluent total phosphorus concentration has been significantly lower after the start-up of tertiary treatment process, averaging less than 0.5 mg/L total phosphorus. With the implementation of this project, phosphorus precipitation can be accomplished in the primary clarifiers, secondary clarifiers, or the new tertiary treatment process. The operational staff is still working to find the right balance of chemical dose for each application point to optimize plant performance and to reduce overall use of chemical. The ability of the process to reduce effluent total phosphorus well below the City's current limit has also provided opportunities for nutrient trading along the Minnesota River. The City has already made arrangements with one industry, and continues to look for other opportunities.Development of this project from the conceptual stages through design and implementation will be discussed, along with a summary of the treatment technology evaluation and reasons for final selection. Start-up issues and "lessons-learned" during commissioning of the tertiary treatment facilities will be identified, along with discussion of how these issues were resolved. A few of these issues include:• Ferric dose must be carefully controlled to optimize performance and avoid iron precipitation downstream of Actiflo process.• Coagulant dose location must be selected to maximize chemical reaction time• Ferrous oxidation to ferric can result in iron precipitate downstream of Actiflo and/or cloth media filters• Phosphate analyzer location, installation, and calibration is critical for optimum process performance• Coagulant dose must be paced continuously with plant influent to provide optimum dose during diurnal low flows and peak flows• Lead/lag operation of Actiflo units is necessary to optimize performance during low flow conditions• Optimum ferric dose to primary clarifiers and Actiflo must be identified to enhance performance and reduce overall use of chemicals.Operational data will be reviewed to identify correlations for point of chemical application (primary or tertiary treatment), chemical dose, overall sludge production, and plant performance. Actual chemical dosages (on a molar basis, Fe/P) for the Mankato facility will be compared to theoretical values and other published values for chemical phosphorus precipitation.