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regeneration efficiency. Problems with high suspended solids in the regenerant have been avoided by conducting the regeneration upflow rather than downflow. These streams are low in NaCI concentration (1% to 3%) as compared with a saturated brine solution (10%). However, the "free" nature of the regenerant provides favorable economics. Ion exchange softening removes hardness only. Silica, another major scale forming constituent in most waters, Is not removed. The ideal application for this approach is on water supplies with high hardness, low sil ica, and low suspended sol ids. If, however, the si Ilca content of the water is a limiting factor for cycles of concentration, silica-specific removal techniques can be appl ied to the circulating water system at a favorable overall cost. Lime/Soda Softening Lime softening Is one of the most common make-up treatment techniques. It in- volves removal of Impurities by chemical reaction with combinations of lime, soda ash, and caustic with the fol lowing effects: . Calcium hardness precipitates as hardness as magnesium hydroxide; phate as calcium phosphate calcium carbonate and magnesium calcium precipitates orthophos- ;.,: " . Bicarbonate alkal inlty Is first converted to carbonate form and then removed as calcium carbonate ;, . Silica comes out as an Insoluble silicon-magnesium complex when it reacts with, or is adsorbed on, sol id magnesium hydroxide. . Turbidity from suspended solids and magnesium hydroxide and calcium carbonate sludge are removed by settling and/or filtration. e , 3 " A more recent appl icatlon of I ime softening Is for sidestream treatment. The sidestream is taken from the recirculating line downstream from the condenser to take advantage of the Increased efficiency of silica removal by magnesium hydroxide and the lower calcium carbonate solubility at the higher temperature prevail ing at this point In the system. Filters following the treating unit assist in the removal of suspended calcium carbonate and magnesium hydroxide. Filter wash water is recovered by returning it to the sidestream treating unit. Generally a sldestream softener with associated chemical feed equipment is several times smaller than a makeup softener for the same generating unit. This Is accompl ished by the concentrating effect of the circulating water sys- tem which provides the softener unit with a higher concentration of dissolved sol ids than would be encountered in the makeup position. -,: ;;.' Sidestream I ime/soda softening has emerged through recent power plant ex- periences at Martin Drake Station in Colorado, Coolwater Station In California and North Valmy Station In Nevada as viable appl ications. Additional ex- periences in domestic Industrial plants and In foreign power plants further supports the viabll ity of sidestream softening. In the technology evaluations for Task 3 sidestream lime/soda softening was considered a commercially proven ;~' " ".',1 4-3 .' '.', ~.~-