A drinking water treatment plant provides 100 million gallons per day (MGD) of treated water to its city. It is considering adding fluoride as sodium fluorosilicate (Na2SiF6) to the water to promote dental hygiene. The utility needs to stay within the US EPA guidelines of 0.7 mg/L (for dental health) and 2 mg/L (secondary standard) of F-. (Note: the maximum level of F− allowed is 4 mg/L, but such levels are not desirable.) The source water to the treatment plant has an F− concentration of 0.5 mg/L and a dissolved silica concentration of 8 mg/L (as SiO2). a. How much sodium fluorosilicate does the treatment plant need to add (per liter) to be at the minimum and maximum levels of the US EPA guidelines? b. How much sodium fluorosilicate would need to be added each day (kg/day) at the water treatment plant to reach the two F− levels?
