AWWA WQTC60730

According to the Stage 1 Disinfection Byproduct Rule (DBP), utilities must comply with a trihalomethane (THM) maximum contaminant level (MCL) of 0.080 mg/L and a haloacetic acids (HAA) MCL of 0.060 mg/L. The formation of THMs and HAAs are impacted by total organic carbon, bromide, pH, temperature, chlorine dose, and contact time. As the elevation of Lake Mead continues to decrease due to drought, these water quality parameters will result in changes more conducive to forming THMs and HAAs potentially near the MCL. The current regulations coupled with water quality changes have prompted the Southern Nevada Water Authority (SNWA) to investigate various strategies to reduce THM and HAA concentrations. Bench scale experiments were performed to identify ways to optimize the current treatment plant performance without a major capital improvement. Results showed ozonation reduced THM concentrations 4-7% at a 1.0-log Cryptosporidium inactivation, and 7-17% at a 2.0-log Cryptosporidium inactivation. Biological filtration did not provide any substantial THM (< 5%) or HAA (<10%) reduction. Rechlorination results lowered THM concentration by 10-15% when a 1.8 mg/L free chlorine residual was rechlorinated with 0.7 mg/L Cl2 after 48 hours, compared to an initial free chlorine residual of 2.5 mg/L. A temperature increase from 16.7°C to 23.7°C increased the chlorine decay rate 30-40% and increased THM formation by 30%. These bench scale results provide SNWA alternatives to continue compliance with the Stage 1 DBP Rule and proposed Stage 2 DBP rule without requiring a major capital improvement. Includes table, figures.