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I ('1 ~ "'l" N C' c. process, however, conservative design and operating measures were assumed In Its application. Combined makeup and sldestream lime/soda softening has shown promise In a num- ber of power plant water management studies and In field tests [4J[5J[6J. EPRI pilot plant tests conducted at Comanche Station In Colorado and at North Valmy Station In Nevada confirmed the technical potential of the process. Tests conducted by Stone and Webster for San Diego Gas and Electric Company using the Palo Verde Irrigation Outfal I Drain water also confirmed the techni- cal and economic potential of the process. The combined makeup and sldestream lime/soda softening application remains to be demonstrated In a full scale power plant system. Southwest Public Service Company (SWPSCO) conducted pilot tests for using secondary municipal sewage as cooling tower makeup where the calcium phosphate solubility product limited the cycles of concentration. They considered makeup softening, sldestream softening, and combined makeup and sldestream softening. They also evaluated lime, caustic, and lime/caustic mixes. They found the op- timum choice to be combined makeup and sldestream softening In separate ves- sels. Lime was used In the makeup unit with a lime/caustic mixture In the sldestream unit. Plans are to Incorporate this system Into the final plant des Ign. " ,> The use of lime/soda softening for pretreatment of wastewater has been demonstrated at a sufficient number of power plants to consider It a commer- cially proven application. As with sldestream lime/soda softening, a conser- vative design approach Is necessary to ensure accurate cost estimates and operating success. Lime/soda softening for raw water makeup treatment, sldestream treatment and combined makeup and sldestream treatment was selected for process screening In the study based on demonstrated performance and economic potential. Reverse O~mo~l~ Reverse osmosis Is a treatment process for the production of high purity water by passing water through a semipermeable membrane which allows the passage of water but not the dissolved solids. The driving force Is provided by a high pressure pumping allowing the membranes reject both cations and anions and unionized species such as silica. Reverse osmosis (RO) Is applicable to raw water treatment, sldestream treat- ment of the cooling tower circulating water and to the concentration of was- tewaters. Present RO technology Includes spiral wound elements, hollow fine fiber elements and tubular elements. Each Is applicable to the treatment of raw water. Spiral wound and tubular elements are generally considered more applicable to wastewater applications because of their higher resistance to fouling. For either raw water source, RO would qualify as a commercially proven tech- nology for boiler makeup water treatment, with numerous power plant 4-4 I I, I