Given a power generation facility, the end product is the electricity that is produced from the energy that steam supplies to the system. From now on we can say that uninterrupted steam production is essential to achieve this. This directly conveys the importance of feed water quality for the boilers. During the last fifteen years, the use of reverse osmosis in power generation facilities has become increasingly popular with new construction facilities. However, reverse osmosis is very easily modified for boiler water pretreatment systems in large and old power generation facilities, regardless of the fuel source. This article provides 5 operational parameters to consider before purchasing an RO system for your power generation facility.
Reverse Osmosis Industrial Parameter No. # 1: cost of wastewater treatment
Wastewater treatment or disposal costs are constantly increasing. For those plants where cost has become punitive, it may make more economic sense to design a wastewater reverse osmosis system with additional stages to reduce wastewater to the minimum amount possible. In some, “no vacuum” power plants, specialized reverse osmosis systems and other equipment such as crystallizers may be required.
To provide an example of how progressive multiplexing can significantly reduce wastewater volume, keep in mind that 400 GPM, 400-micrometre streams can be reduced to just 7.5 GPM with a three-stage system. Conductivity, of course, increases exponentially from 400 μm to 21,320 μm!
Reverse Osmosis Industrial Parameter No. # 2 – Choose Cellulose Acetate (CA) or Polyamide Composite (PA) Films
Cellulose acetate and polyamide films differ greatly in their method of physical and chemical resistance. Liquid temperature, pH and chlorine resistance are just 3 examples of the properties of liquids that make you prefer one over another. Operating pressure is another important factor. CA membranes can operate at much higher pressures (over 450 psi) compared to PA films (300 psi max).
Residual chlorine content, common in most municipal water system streams, can be well tolerated by CA membranes, but if PA membranes are used, they must be chemically neutralized or removed by activated carbon filtration. A complete detailed comparison of the CA and PA membrane types is provided in the technical document referenced below.
Anticipate your RO system maintenance requirements Contamination of reverse osmosis membranes will occur. Prevention is the best way to tackle this problem. Pretreatment of common dust reverse osmosis feedwater will reduce maintenance burden and ensure longer RO operation between membrane cleaning or replacement.
Reverse Osmosis Industrial Parameter No. 3: Prevent scale formation on reverse osmosis membranes by hardness, strontium or barium.
The three most common means of preventing volume build-up in power generation RO systems are;
- Feed acid to control pH,
- Installing a laxative before the reverse osmosis system
- Anti-scale feeding.
Reverse Osmosis Industrial Parameter No. 4: avoid microbiological contamination of the membrane.
Microbiological contamination generally does not cause as much damage as a metallic scale, but it can drastically reduce the efficiency of a reverse osmosis system. Again, preventing contamination is a much more effective strategy than treatment. In general, microbiological contamination of reverse osmosis systems is avoided by controlling the methyl bromide content in the reverse osmosis feed water to the maximum with a biocide.
Reverse Osmosis Industrial Parameter No. 5: Avoid organic non-microbiological contamination of the reverse osmosis membrane.
This is most successful in controlling the COD of the reverse osmosis feed water. If the reverse osmosis feedwater is non-polluting plant service water (or does not contain waste or recycled water), then the COD is always a decaying colour or plant. Both can be removed by flushing the appropriate source or anion exchanger.
If the reverse osmosis feed water contains a recycled or waste component, then the COD can be almost anything. In this case, appropriate primary treatment using secondary (biological) treatment of the waste or a recirculated stream will be required to reduce COD to levels such that non-biological organic contamination of the RO membranes does not occur.