What Are Useful Membrane Technology Applications for Power Plant Water Treatment and Reuse
By Joseph Wong, PE, BCEE
Membrane processes are relatively new when compared to conventional treatment processes such as granular media filtration and ion-exchange (IX) deionization (DI) in power plant water treatment. Membrane processes generally include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), hyperfiltration (HF) or reverse osmosis (RO), electrodialysis (ED)/electrodialysis reversal (EDR), and electrodeionization (EDI). Newer developments include forward osmosis (FO), membrane distillation, and electrodialysis metathesis (EDM). These membrane processes are used to remove solids (suspended, colloidal, and dissolved) from water. However, the membrane contactor (MC) is another membrane process used to separate gases from water.
The power industry started using membrane technologies in water treatment approximately 30 years ago with the advent of RO technology and the applications have evolved over the years. To the author’s knowledge there are five categories of membrane technology used in power plants. These include
- Boiler feed and NOx injection water treatment
- Removal of dissolved gases in water for boiler feed
- Recycled water treatment (municipal and industrial)
- Cooling tower blowdown recovery
- Concentration of solids for zero liquid discharge
Boiler Feed and NOx Injection
Power plants typically require highly purified (deionized) water for use as makeup water to steam generators (boilers) and/or for injection into the turbine combustors to control the formation of environmentally undesirable nitrogen oxides (NOx) in the turbine exhaustion gas. The higher the boiler pressure, the higher the water quality requirement, especially for silica and total dissolved solids (TDS) concentrations. Conventional power plant water treatment processes using surface water as source include coagulation, flocculation, sedimentation, granular media filtration (GMF), granular activated carbon (GAC), cation exchange, degasification, anion exchange, and mixed-bed ion exchange (MBIX). The product water is basically high-purity water. A modern power plant may use coagulation, MF/UF, RO, and EDI to achieve similar water quality objective as the conventional treatment processes.
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