Innovating Chlorination Approaches to Tackle Stringent THM Limits

Presentation Description: The City of Vacaville Easterly WWTP is facing an unintended consequence of an upgrade to the treatment process from BOD removal to Biological Nutrient Removal. Ammonia removal in the secondary process has led to significant generation of THMs due to free chlorine being the dominant disinfectant instead of chloramines. The WWTP is on a time schedule order (TSO) to meet more stringent THM limits in its final effluent set forth by CRWQCB. To address the challenge, a study was conducted to explore alternative methods of chlorination that could effectively and reliably reduce THMs while also ensuring compliance with the microbiological requirement of Total coliform 2.2 MPN/100mL. The study included both bench-scale dose-response tests and a full-scale demonstration. Various chlorination methods were examined at the bench-scale, ultimately identifying that the "step-dosing" approach effectively reduced THMs. The subsequent field demonstration validated the feasibility and reliability of "step-dosing" as an alternative approach in full-scale operations. The significance of this study can be summarized as follows: 1) Reliable regulatory compliance: The full-scale demonstration has provided evidence of the step dosing strategy's reliability in achieving regulatory compliance for THMs in chlorinated effluent. The THM concentrations at the end of the basin were significantly lower than the stringent regulatory limits, ensuring adherence to future compliance. 2) Easy implementation: Implementing the step-dosing strategy requires minimal modifications to add a second dosing point in the existing contact basin. 3) Better process control: The use of two dosing points in step dosing enables precise control of chlorine doses and end-of-basin residual concentrations, which can be further enhanced through residual-pacing due to the short contact time and stable residuals. 4) O&M cost savings: The reduced contact time and CT values can lead to a decrease in required contact volume, resulting in O&M cost savings. Additionally, the controllable chlorine doses lead to potential savings on chlorine usage, contributing to overall cost reductions. Based on the outcomes of this study, the adoption of step-dosing as an operational improvement is recommended at the Easterly WWTP. The step-dosing chlorination strategy offers an enhanced approach that can be beneficial for other facilities facing similar challenges.