Optimizing Disinfection Operations for Discharge and Recycling: Las Vegas Experience

Introduction: The City of Las Vegas Water Pollution Control Facility's disinfection system includes chlorination and dechlorination for two key purposes: ensuring compliance with NPDES permit requirements for surface water discharge and meeting water reclamation standards for year-round reuse. Sodium hypochlorite is used to disinfect the final effluent in six chlorine contact basins, divided into two groups. Sodium bisulfite is then used to neutralize residual chlorine before discharge (Figure 1).

Challenges: The dual focus on discharge compliance and water recycling highlights the importance of maintaining reliable disinfection operations. To achieve this, the facility has faced several operational challenges in ensuring consistent and effective disinfection performance.
- Challenge of flow-pacing dosing control — A reliable chlorine dose is crucial for producing pathogen-free recycled water. Flow from the filters is split between two chlorine contact basin groups, with one group dedicated to recycled water disinfection. This splitting causes significant flowrate fluctuations, leading to variable signals sent to hypochlorite metering pumps, causing inconsistent dosing. Conventional flow-pacing cannot adapt to these fluctuations, resulting in frequent dosing malfunctions.
- Challenge of residual-pacing dosing control — Chlorine is applied to each contact basin group (Figure 2)., while residual is monitored at the combined effluent. As the flow proportions from each group fluctuate, the combined residual reading changes. Conventional residual-pacing cannot adjust to these variations, leading to frequent over- and under-dosing, which affects optimal disinfection performance.
- Challenge of dechlorination dosing control — Surface water discharge must meet a chlorine residual limit of less than 0.1 mg/L. Residual monitoring is affected by filter backwash cycles and high dissolved oxygen levels, causing variable readings that disrupt the dechlorination dosing system and often lead to overdosing of the dechlorination chemical.

Improvement and Optimization Approach: Recent system enhancements have focused on addressing these challenges by improving the operational efficiency and reliability of chemical dosing operations.
-Dedicated Dosing Control Loop — Chlorine residual analyzers are installed at each contact basin group and the effluent location. They monitor residuals for dosing control and cross-check each other for accuracy and reliability.
-Enhanced Flexibility of Chemical Dosing - A swing pump arrangement was implemented to improve chlorine dosing flexibility. Dedicated metering pumps serve each contact basin group without increasing the total pump count. Together with the analyzers, this setup allows for more precise and responsive chemical dosing control.
-Out-of-the-Box Dechlorination Control Strategy - After field trials, water quality monitoring, and data analysis, the facility developed a new dechlorination dosing strategy. Despite variable flow rates and fluctuating residuals, water quality, especially ammonia levels, remained stable. The new system uses hypochlorite metering, verified by residual analyzers (Figure 3), with a lag time to allow dechlorination pumps to respond to changes in chlorine concentration (Figure 4).
Performance Results: A series of disinfection process improvements, control optimizations, and operational adjustments have resulted in noticeable improvements in system performance, as demonstrated by the following:
- Consistent Regulatory Compliance - Fecal coliform levels consistently remained below the NPDES limit over the three-year period (Figure 5). Chlorine residuals before dechlorination ranged from 3-5 mg/L, consistently approaching zero after dechlorination, in line with regulatory limits (Figure 6).
- Efficient Chemical Use - Bisulfite dosages ranged from 8-10 mg/L. Residual sulfite levels remained stable at 2.5 mg/L or lower (Figure 7). The calculated dechlorination ratio averaged at 2.7, which is in line with typical ratio for dechlorination practice, ensuring effective chlorine neutralization without excessive chemical use (Figure 8).
- Operational Cost Savings - By optimizing sodium bisulfite usage, significant cost savings are realized. A reduction of 2 mg/L in bisulfite dosage results in an annual savings of nearly 300,000 pounds of sodium bisulfite, translating to over $85k in cost savings at current prices.
- Enhanced Water Quality- Dissolved oxygen levels in the effluent ranged from 5.5 to 8.5 mg/L, which is higher than typical dechlorinated streams due to the consumption of dissolved oxygen by sodium bisulfite (Figure 9). By minimizing bisulfite usage, DO levels are better preserved, benefiting aquatic life.

Significance: This paper outlines the outcomes of disinfection improvements, focusing on chemical feed control optimizations, which enhance compliance, chemical efficiency, and overall disinfection operations for both discharge and recycling. These adjustments not only boost treatment efficiency but also offer long-term operational benefits. The City of Las Vegas Water Pollution Control Facility demonstrates a balance between compliance, cost-efficiency, and environmental stewardship, providing a scalable model for other wastewater treatment facilities disinfection practices. This approach proves that sustainability and regulatory compliance can be achieved together and replicated elsewhere.