ROs Retain & Produce Power
In 1992, a reverse osmosis (RO) system utilizing 168 DOW Filmtec BW30-8040 elements was installed to pretreat the feedwater to the ion-exchange system for the Antelope Valley Station (AVS) just outside of Beulah, N.D. The RO ion-exchange system and other pretreatment equipment were installed to provide boiler feedwater to the generating station.
After 17 years, the original RO elements are still in service, well exceeding the industry-expected life span. The durability and cleanability of RO elements, combined with proper pretreatment and good plant maintenance, makes such an achievement possible.
Power-generation facilities rely on pure water to make steam, turn turbines and generate electricity. Located seven miles northwest of the town of Beulah, the Antelope Valley Station (AVS)—a lignite-based electric generating station with a capacity of 900,000 kW—relies on pure water to provide electricity to member electric distribution systems throughout the upper Midwest.
The station is operated by Basin Electric Power Cooperative and was established in 1984, making it the newest coal-based power plant in the state. An integral part of Basin Electric’s generating and transmission network, most energy produced at AVS is sent to a substation near Huron, S.D., where it is then delivered to Basin Electric’s member systems.
The feedwater source for AVS is Lake Sakakawea, which is a reservoir of Missouri River water. Filmtec elements were selected to be installed in the AVS because of their high productivity, long membrane life and durability. After 17 years, these elements are still efficiently producing clean process water from this surface water source.
The system is made up of four trains of a 5-2 array of vessels, each containing six elements. The feedwater arrives from the lake via a 9-mile pipeline, where it goes through a prechlorination step before the clarifiers. From the clarifiers, it is cold-lime-softened, and liquid ferric sulfate and a polymer are added as coagulation aids.
The water is then pumped to a clear well that holds 1 million gal and onto the main plant, where it goes through a sand/anthracite filter and then a separate carbon filtration step. Additional pretreatment steps include 5-um cartridge filters, addition of sulfuric acid to reduce the pH to 7.0 and a temperature increase to about 75°F before the water is finally pumped to the RO system. A heat exchanger that uses waste heat from the plant increases the temperature of the feedwater.
The pretreatment system, coupled with the well-maintained RO system, is what ultimately gives the RO membranes long life and durability, enabling them to produce pure process water for power generation. The RO system receives a chemical cleaning once each quarter, and O-rings on the interconnectors and adaptors are changed out as needed. While preheating the feedwater is not a requirement for the operation of RO systems, in this case the higher feedwater temperature lowers the feed pressure requirement and has contributed to extending the life of the membranes beyond that which is normal in an industrial environment.
After 17 years of operation, the RO elements at AVS show close to the same performance as when they were first installed, reducing conductivity from 450 umhos to 12 umhos while running at 75% recovery.
The following is a summary of current RO plant data:
- Temperature: 75°F
- Stage 1 pressure: 225 psi
- Stage 2 pressure: 185 psi
- Concentrate pressure: 160 psi
- Feed conductivity: 450 umhos
- Concentrate flow: 173 gal per minute (gpm)
- Concentrate conductivity: 1,390 umhos
- Product flow: 520 gpm
- Product conductivity: 12 umhos
The long life achieved at AVS is to the credit of the operators and maintenance personnel. Their commitment to keeping the elements clean through a regular cleaning program and replacing worn O-rings has kept the system running more than four times the normal life span of RO elements that are operated on surface water feed.
The Filmtec BW30-8040 element originally installed at this site, with a nominal active surface area of 330 sq ft, was a predecessor to today’s BW30-365 element, which has a nominal active surface area of 365 sq ft. The BW30-365, and now the new interlocking BW30-400/34i, are traditional element options for system designers, original equipment manufacturers and system operators who require consistently high performance and maximum element life when treating difficult feedwaters.
The new BW30-400/34i elements are essential to the next generation of technology, enabling the power generation industry to reduce system operating and maintenance costs through improved water quality and extended membrane life. Furthermore, these elements offer improved RO system productivity by setting a new standard for biological and organic fouling resistance. They feature the industry’s thickest feed spacer (34 mil) to lessen the impact of fouling.
In addition, the elements can be cleaned over a wide pH range of 1.0 to 13.0 for effective cleaning, and they are best suited for systems operating on challenging feed streams. When complex pretreatment is not an option, or where maximum cleanability delivers value, these new elements are a practical solution.