Optimizing De-chlorination by Altering Sodium Bisulfite Dosing Strategy: A Computational Fluid Dynamics Study

De-chlorination is the process of removing excess residual chlorine when discharging to the environment to reduce risk of aquatic toxicity and disinfection by-product (DBP) formation. For de-chlorination, chemical agents such as sodium bisulfite (SBS) or sodium sulfite are typically added to the chlorinated water to reduce residual chlorine levels. Excessive quantities of de-chlorination chemicals can have negative pH impacts on downstream receiving waters. Effective utilization of de-chlorination agents without overdosing has challenges related to chemical injection, mixing, and reaction time prior to compliance monitoring. Designing de-chlorination agent dosing and mixing strategies for full-scale treatment facilities using rules of thumb or simple models has not always led to optimal system performance. Fortunately, computational fluid dynamics (CFD) modeling coupling three-dimensional flow fields with chemical mixing and reactions allows us to simulate the de-chlorination process in detail.

This study collaboratively developed by Carollo Engineers and the City of Sunnyvale, CA, (City) a CFD model consisting of a mechanistic model for the reactions between chlorine and SBS with the reaction rate constant fitted by physical experimental data from Basu and De Souza (2011), shown in Figure 1. Ideally it takes 1.46 parts SBS to reduce 1 part chorine by weight. As the SBS is mixed into the fluid, the reaction proceeds in time while flow travels downstream to the monitor location, with SBS dose adjusted based the monitor readings. The model is suitable for use in evaluating de-chlorination and easily identifying areas of incomplete reaction due to both insufficient chemical dose mixing and reaction time.

The de-chlorination model was used to study the de-chlorination process in the chlorine contact tank (CCT) effluent channel of a wastewater facility operated by the City. The City is considering improvements to their SBS dosing including chemical injection approach (diffuser or nozzle), downstream mixing approach including vertical mechanical mixers or a static mixer system, dosing locations and dosing rates. Figure 2 shows the alternative configurations used for model testing. De-chlorination performance was evaluated for each SBS dosing and mixing strategy using the CFD model. For testing it was assumed that there was a 2 mg/l chlorine residual to be removed.

Figure 3 compares the calculated velocity field for both mixing approaches. The left image shows the velocity is not uniform in the channel downstream from the first mechanical mixer, with higher velocity along the floor and lower velocity in the upper water column, whereas the static mixer shown in the right image promotes a more inform velocity field. Figure 4 shows typical results for de-chlorination simulation in this system. The model shows slight variability in chlorine residual across the sample plain with a slightly higher concentration on the inside wall. In this facility a sample diffuser is used providing a composite reading, but if a point sampler was used, it would likely read high leading to the feed system supplying more SBS that required. The static mixer performs slightly better due to the more uniform velocity field allowing for a more consistent reaction time in the channel.

The model was also used to test impacts of influent flow rates and the operation of CCT channel combinations on de-chlorination, with the complete range of results summarized in Table 1. Scenarios 1 and 2 showed the highest reduction in chlorine, as expected with the longest reaction time between SBS addition and the monitoring location, Scenario 4 had lower chlorine reduction with less time for the de-chlorination reaction between CCT2 and the sample location.

For an annual average plant flow of 10 mgd with static mixers and operation primarily of CCT3 and CCT4, the efficiency improvement is estimated at 2.5%. This level of performance improvement reduces the SBS need by approximately 2,200 pounds for a similar level of de-chlorination per year as compared to the mechanical mixers. Excess SBS has both operational costs for the facility and risk of excess SBS discharge to the environment.

The results show the model is accurately able to capture the time varying reaction rate and spatial variability. This study provides useful information and insights in de-chlorination operations for wastewater facility operators, managers, and designers.

This presentation is relevant to utilities, consultants, and consultants that have de-chlorination requirements. The synergistic approach of CFD modeling with real-world data allows facilities to make educated decisions on what is required to improve their de-chlorination compliance. This presentation will also outline how this approach can be expanded for use at other facilities, considering mixing, the reaction time, and monitor locations.