RO Brine Wastewater Recycling

The electricity generation process requires and consumes large volumes of high-purity water for boiler make-up and for cooling. Sources of water supply can vary and each water source has unique characteristics, including organic matters, dissolved minerals, microorganism growth and chemical contaminants. Each of these inherent characteristics can cause difficulties in a power plant. Therefore, the water has to be treated to minimize potential problems, since these problems could often result in either reduction of plant efficiency or large capital costs. Reverse Osmosis (RO) is widely adopted by power producers for purifying boiler feed water, makeup water and in zero-liquid discharge (ZLD) applications. . RO produces a clean stream of high-purity water, and a smaller stream of concentrated waste, referred to as reject or brine. The RO brine contains a wide variety of inorganic and organic dissolved solids with TDS concentration up to several thousand mg/L. For some plant locations, RO brine can either bedisposed of without any additional treatment or minimized prior to disposal. For those plant locations where water supply is limited and disposal is not a viable option, recycle of RO brine will have to be implemented.

Based on the types and quantities of fouling substances identified in the brine solution, a chemical treatment process is developed to counteract each of the fouling factors. The chemical treatment may take the form of precipitation, adsorption, pH adjustment and microbial control. The chemistries are evaluated for their compatibility and combined effect. The treatment process is carried out in a two- or three- stage chemical reaction. The chemical treatment will typically include one or more of the following processes: (1) Chemical softening for scaling control, (2) Silica removal for fouling prevention, (3) Organic reduction & oxidant destruction, and (4) pH optimization for the integrated chemistries After chemical reaction, the pre-treated brine solution from the process water RO system is processed through the tubular microfiltration membrane filters designed for separation of the incompatible precipitates from water. During operation, clear filtrate permeates through the membrane, while the suspended solids retained in the re-circulation loop are periodically purged for further de-watering. The filtrate is directed to a brine recycle RO system designed for processing of high TDS solution.

A case study is presented in the Article to review the design, operation and performance of a 250 GPM RO brine recycling system installed in a combined-cycle power plant. After 5 years of operation, the RO brine wastewater recycling plant has demonstrated that a ZLD system with RO desalination coupled with a high-flux tubular membrane filtration process as pre-treatment is a technically viable and economically affordable solution to address today's water scarcity and regulatory requirements in the power industry.