Using carbon dioxide to cope with fluctuations in raw water pH and maintain effective conventional treatment

The Truckee River is the primary source of drinking water for the metropolitan areas of Reno and Sparks in Nevada. During the warmer months of the year, the river experiences dramatic diurnal fluctuations in pH from a low of about pH 7.5 to as high as pH 9.2. These fluctuations in pH can significantly impact conventional surface water treatment, especially coagulation and filtration. The use of carbon dioxide (CO₂) to adjust and maintain the pH of the raw water was evaluated during bench-scale, pilot-scale, and full-scale testing. The objectives of the testing were to minimize the required coagulant dose, minimize the concentration of residual aluminum, and maintain low particle counts in the filter effluent. Polyaluminum chloride (PACl), aluminum sulfate, and ferric chloride were used as primary coagulants during testing.; Full-scale tests using PACl demonstrated that using CO₂ to adjust the raw water pH to a target between pH 7.5 and pH 7.8 minimized overall chemical costs and stabilized treatment by reducing the need to frequently adjust the PACl dose to compensate for fluctuations in raw water pH. As a result of this testing, a permanent full-scale CO₂ feed system will be installed at the Chalk Bluff Water Treatment Plant. (Abstract shortened by UMI.)...