Case study: Integrating membrane processes with evaporation to achieve economical zero liquid discharge at the Doswell Combined Cycle Facility

Abstract

New restrictive permitting, operating and environmental requirements placed on independent power producers, utility power plants, chemical processing plants, refineries and other industrial installations are mandating zero liquid discharge of wastewater to sewers, rivers, deep wells and the like. Economic reclamation and reuse of wastewater is becoming essential. Since the mid-70's, vapor compression evaporation has been the preferred means to achieve zero liquid discharge. Evaporation recovers about 95% of a wastewater as distillate for reuse. Waste brine can be reduced to solids in a crystallizer/dewatering device. However, evaporation alone can be an expensive option when flow rates are large or when wastewater is not extremely high in total dissolved solids (TDS). As a result, grassroots power plants and other industrial facilities often find it difficult to economically justify a zero liquid discharge system. One way to solve this problem is to integrate membrane processes with evaporation. By combining membrane processes such as electrodialysis reversal (EDR) and/or reverse osmosis (RO) with evaporation, the synergism makes zero liquid discharge a much easier goal to meet. An example of this hybrid approach for economical zero liquid discharge is the installation of EDR plus RO along with evaporation and crystallization at the Doswell Combined Cycle Power Plant in Hanover County, Virginia. Use of membrane processes to preconcentrate and reclaim plant wastewaters has significantly reduced the evaporator feed volume, resulting in a 62% downsizing of the evaporator system. Without the EDR and RO system, the evaporator/crystallizer system would have treated 250 gallons per minute (gpm) of wastewater at a total dissolved solids concentration of 2900 parts per million (ppm). Using the combined EDR and RO systems to pretreat wastewater helped reduce the evaporator feed volume to about 90 gpm with a higher TDS concentration of 9700 ppm. Compared to straight evaporation/crystallization, the hybrid approach reduced overall capital costs by $900,000 and saves energy and operating costs by $680 per operating day.