Best Practices – Design – Faribault Energy Park

Permanent RO system with UF a win-win

Faribault Energy Park/Minnesota River Station
Owned by Minnesota Municipal Power Agency
Operated by NAES Corp 

Challenge. Soon after accepting care, custody, and control of a newly constructed GE 7FA combined-cycle facility in fall 2007, plant staff faced freeze-related challenges, including the initial water treatment set-up shown in Fig 1. The facility was using rental trailers equipped with demineralizers for water treatment and they were exposed to ambient conditions.

   During winter months, these trailers and ancillary equipment were susceptible to freezing and had the potential to impact plant availability. Processes also associated with these trailers led to the formation of a potential slip hazard from ice around the trailer. Plant staff, together with the owner, wanted to develop an economic solution to this demineralizer set-up.

Solution. Plant O&M staff, led by operations manager Doug Klar and a third-party service provider (on behalf of owner), decided a first priority was to plan and construct a building over the plant’s mixed-bed rental trailer to prevent freezing, eliminate the slip hazards associated with ice around the trailer, and increase plant reliability.                   

   Even though the plant’s discharge contract prohibited both reverse osmosis (RO) and softener waste streams, the operator and owner decided to optimistically design a building with enough room to accommodate a future permanent water treatment system.

 In parallel with building construction in 2009, the site successfully negotiated a discharge contract language change that allowed discharge of RO reject water, providing the waste stream impact was first approved by the city.

 
   A year and a half later, when funds were approved for a permanent water treatment train, the impact study prepared by the operator was presented to the city and the proposed equipment was subsequently approved.      

   In the development phase that followed, plant personnel prepared a detailed RFP, reviewed multiple bids and negotiated with at least six prospective water treatment OEMs. This extensive research and bidding process provided plant staff the opportunity to learn much about the advantages and disadvantages of available water treatment technologies.          

   Throughout the design review phase, we frequently updated the owner with the pros, cons and costs associated with each technology. The operator prepared and presented a return-on-investment (ROI) analysis to demonstrate the benefits of an integrated RO system as a cost-effective way to replace the trailer rentals while providing other inherent water quality improvements.

   After extensive review, the owner and operator agreed to pre-treat the RO with an ultra-filtration (UF) unit rather than adding a second multimedia filter (MMF). While ultra-filtration is more expensive than MMF technology, it provided superior pretreatment protection for the RO. UF can filter particulate matter down to 0.1 micron as compared to MMF’s 20-micron capability. The UF was installed downstream of an existing MMF that continues to provide “whole plant” filtration for both the cooling tower and demineralizer plant.          

   An appropriately sized rental mixed-bed bottle was chosen to polish the RO product water, eliminating the need for the customary acid and caustic tanks required for regenerating a permanently installed mixed-bed vessel. An electrodeionization (EDI) unit was ruled out because of its higher capital and maintenance costs and the limited electric service available to the RO building.

   Plant personnel voluntarily took on the scope of work to design, procure, and install all interconnecting piping, valves, instrumentation, plant air, and DCS screens, resulting in additional savings. New electrical feeder breakers and wiring were subcontracted to a qualified third party to ensure relay coordination was not adversely impacted by the additional load.

Results. The dry, climate-controlled building with an ultrafiltration/RO system was commissioned (Fig 2). Slip hazards are reduced and the plant has a higher quality demineralized water supply than before. The RO reject stream goes to the cooling tower and is cycled through until blowdown or evaporation.

   Most people gain a great deal of satisfaction when we create and build. Not only did we save monetarily by taking on those portions of the project we were qualified to perform, the team built a sense of ownership that manifested into O&M excellence and owner satisfaction.