The Piñon Pine Plant of Sierra Pacific Power Company near Reno, Nevada is a zero discharge facility. To maintain zero discharge operation a side-stream softener is used to control circulating cooling water (circ-water) and discharge pond water quality. During plant startup, this softener was not performing to its design objectives, which specified maximums of 30 ppm calcium as Ca, 10 ppm magnesium as Mg, and 20 ppm silica as SiO2 in the softened water. Jar tests were conducted to determine the best chemicals and optimum dosages for the side-stream softener. The optimum pH range for softening was in the range of 11.3–11.5. Replacing the Polyaluminum Hydroxychlorosulfate (PAC) coagulant with 25–50 mg/L of Mg(OH)2 greatly improved flocculation. In addition to Mg(OH)2 solution, MgSO4 was required at a dosage of 50 mg/L as Mg to reduce the silica concentration to 20 mg/L as SiO2. The optimum dosage of Na2CO3 (soda ash) was 750 mg/L to reduce the calcium concentration to 30 mg/L as Ca. The optimum dosage of flocculent (a Polyacrylamide based anionic polymer) was 10–15 mg/L. Dispersant addition was adversely effecting the softening process.

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