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

During the warmer months of the year, the Truckee River experiences dramatic diurnal fluctuations in pH from a low of about pH 7.5 to as high as pH 9.2. These pH fluctuations can significantly impact conventional surface water treatment, especially coagulation and filtration. The use of carbon dioxide (CO₂) to maintain the raw water pH was evaluated during bench-scale, pilot-scale, and full-scale testing using polyaluminium chloride (PACl) as the primary coagulant. The objectives were to minimize the required PACl dose and the concentration of residual aluminium as well as maintain low filter effluent particle counts. Results indicated that particle removal could be improved by either increasing the PACl dose or by reducing the raw water pH. For a fixed PACl dose, filter performance consistently improved as pH decreased. An increase in raw water pH required a corresponding increase in PACl dose to maintain filter performance. To minimize the PACl dose, the raw water pH must be maintained to ensure consistent filter performance. Full-scale tests demonstrated that using CO₂ to adjust the raw water pH to a target of 7.5 to 7.8 minimized overall chemical costs and eliminated the need to frequently adjust PACl dose to compensate for pH fluctuations.