Faribault Energy Park, a 300-MW gas-fired combined-cycle facility located in southern Minnesota, represents a significant investment in sustainable energy infrastructure. Owned by the Minnesota Municipal Power Agency (MMPA) and operated by NAES Corporation, the plant was commissioned in 2007.
Faribault Energy Park initially used mobile ion exchange trailers for its steam makeup water. This approach was a workaround due to restrictions on discharging wastewater from a permanent Reverse Osmosis (RO) system. The mobile trailers posed significant operational challenges, especially during Minnesota's harsh winters, creating icy conditions and safety hazards for plant personnel. Furthermore, the plant's water source—a pond system fed by a well and rainwater—was subject to high turbidity peaks and variable temperatures, creating a difficult environment for a standard membrane-based water treatment system. The existing multi-media filter (MMF) was also deemed inadequate to pretreat the water to the quality required by an RO system, particularly regarding turbidity and silt density index (SDI).
To address these issues, plant management developed a comprehensive plan to install a permanent, in-house water treatment system. The first and most critical step was to overcome the regulatory barrier. The Minnesota Municipal Power Agency (MMPA), in collaboration with Veolia, presented a detailed plan to municipal regulators. The plan demonstrated that the RO concentrate (reject water) was of similar quality to the existing system and would account for less than 0.01% of the cooling system volume, resulting in a minimal impact on the final blowdown water composition. This data-driven approach secured the necessary approval to discharge the RO reject into the cooling tower.
With regulatory approval granted, plant personnel drafted a detailed Request for Quotation (RFQ) and evaluated proposals from various water industry OEMs.
To meet the high-purity boiler feedwater specifications, the core design selected was a two-pass RO system, with the final product water polished by service exchange mixed bed deionization (MBDI) vessels. To handle the difficult raw water, Ultrafiltration (UF) was chosen as the pretreatment method over alternatives like coagulation and a second MMF. UF was selected for its ability to provide superior and consistent water quality (filtering particles down to 0.1 micron), its fully automated operation including clean-in-place (CIP) cycles, and its resilience to the variable water quality and temperature. This avoided the high operator involvement required by conventional systems. Veolia was selected as the OEM, providing a compact, skid-mounted UF and two-pass RO system. The mechanical design and installation were adeptly managed in- house by the plant's team, who collaborated with Veolia on the process design and with a local electrical contractor for the electrical work.
The permanent water treatment system was successfully installed in 2010, delivering immediate and significant benefits. The new system enhanced operational efficiency, improved on-site safety by eliminating the winter hazards associated with the mobile trailers, and began generating cost savings compared to the previous rental setup. The project's innovative and effective design was formally recognized when it received a best practice award from the Combined Cycle Journal in 2012.
A long-term analysis conducted fourteen years after commissioning confirmed the project's enduring success. The review validated that the original business case for the permanent system had been fully realized, with its performance and cost savings meeting or exceeding the initial projections. The integrated system—comprising Ultrafiltration, two-pass Reverse Osmosis, and MBDI polishing—has proven to be a robust and reliable long-term solution, effectively managing a challenging water source while consistently delivering the high-purity water essential for the plant's efficient and safe operation.
The UF/RO system is located in the Clean Water Building. An overall site water layout is as shown in Figure 1.
结果
The Ultrafiltration (UF) System is cleaned with the standard automatic maintenance clean 3-4 times per week, with an offline recovery clean once per quarter. The UF membranes were replaced after 7 years of service versus the original projection of 5 years. Savings versus budget for UF membranes totalled ~$58,000 USD.
The feed turbidity to the UF unit varies significantly as the existing MMF upstream is providing coarse filtration of pond water while UF filtrate is exceeding feed specifications for the RO.
The RO maintenance has included quarterly membrane cleanings and replacement of a few instruments, rebuilding of a couple valve actuators and standard pump maintenance over the 14 years in service. It is estimated that an average of 15 minutes per day is spent on data collection and general maintenance.
Based on water sampling completed in March 2022 with original membranes operating for 12 years, water quality shown in Table 2. RO membranes were replaced in year 14 (February 2024) versus the original projection of 5 years. The membrane reloads took plant personnel 4 hours working in conjunction with OEM support. Savings based on membrane life versus budget is ~$10,000 USD.
The business case for the permanent water treatment system was overwhelmingly successful, exceeding its initial projections by delivering a simple payback in just 1.52 years, faster than the anticipated 1.8 years. Over a 14-year period, the system generated $4.9 million in operating cost savings for MMPA stakeholders, primarily by eliminating the recurring fees for mobile ion exchange trailers. Beyond the financial gains, the project had a significant positive environmental and safety impact; it is estimated to have avoided 25,000 miles of semi-truck transport, equating to a reduction of 37 metric tons of CO2, while also mitigating critical safety hazards associated with transportation, ice buildup, and weather, resulting in no recordable accidents related to the water system.
Review of all available data demonstrates the project met or exceeded the initial investment case as well as contributed towards stated objectives of the stakeholders including the Minnesota Municipal Power Authority, NAES Corporation, and the MMPA member cities, Board of Directors, and community. The project lived up to its 2012 Award for best-in-class design and has exceeded expectations delivering critical financial and environmental benefits for the MMPA stakeholders.