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<br />o <br />L'" <br />'!l,;:j"1 <br />N <br />c' <br />Cj <br /> <br />with the final wastewater disposal In evaporation ponds. The cooling tower <br />concentration factor Is the same as the evaporation pond option. <br /> <br />Vapor compression evaporators combined with evaporation ponds Is a standard <br />method for achieving zero discharge In many western power plants. This com- <br />bination Is straight-forward in operating philosophy and has generally been <br />more cost-effective than evaporation ponds alone for wastewater treatment and <br />d I sposa I. <br /> <br />When appl led to the Lower Virgin River Unit, the estimated vapor compression <br />evaporator capacity required to evaporate the high volume of cooling tower <br />blowdown was found to be economically prohibitive. This eliminated the option <br />from further consideration. <br /> <br />Lime/Soda Raw Water Softenlng/VCt/Evaporation Ponds <br /> <br />This option uses lime/soda softening to reduce the calcium, magnesium, silica <br />and phosphate In the water supplied to the cooling towers, ash and scrubber <br />systems. Recyclable wastewaters are concentrated In vapor compression <br />evaporators with the non-recyclable wastewaters and sludges discharged to <br />evaporation ponds. High quality water from the wastewater evaporators Is used <br />as makeup to the condensate system. <br /> <br />Makeup lime/soda softening combined with vapor compression evaporators Is used <br />In numerous electric utility plants based zero or limited discharge design. <br />This process option met the screening criteria and Is Included for further <br />evaluation. <br /> <br />Lime/Soda Raw Water Softening/Recycle Softenlng/YCE/Evaporatlon Ponds <br /> <br />Lime/soda softening Is used to reduce the calcium, magnesium, silica and phos- <br />phate In the water suppl led to the cooling towers. The cooling tower cycles <br />of concentration are controlled to supply the FGD and ash systems water <br />requirements with cooling tower blowdown. The blowdown Is softened to reduce <br />the calcium, magnesium and silica levels to prevent scaling In the scrubber <br />and ash systems. Recyclable wastewaters are processed In vapor compression <br />evaporators with the non-recyclable wastewaters and sludges discharged to <br />evaporation ponds. HIgh quality water from the evaporators is used as makeup <br />for the condensate system with excess production available for blending with <br />softened water to supply the ash and scrubber systems. <br /> <br />Although the Individual components meet the screening requirements, the water <br />suppl led to the scrubber from the process option can contain very high <br />chlorides. The estimated chloride concentration In the scrubber Is 20,000 <br />mg/I If the Lower Ylrgln River unit Is the makeup source to the cooling <br />towers. The secondary effluent, lower in chlorides, results In a chloride <br />concentration of 12,000 mg/I In the scrubber. <br /> <br />This configuration using Lower VirgIn River unit water was not considered fur- <br />ther because of the high chloride concentrations In the blowdown. Using <br />secondary effluent, It Is evaluated as Option V-SE which is the base case op- <br /> <br />4-12 <br />