CHLORINE CONTACT OPTIMIZATION UTILIZING CFD MODELING

MAIN AUTHOR:

Melissa L. A. Tafilaku, P.E. Black & Veatch International Company

9000 Regency Parkway, Suite 200 Cary, North Carolina 27518

Phone: (919) 462-7513 Fax: (919) 462-8356

Email: tafilakuml@bv.com

CO-AUTHORS: Michael B. Shafer, P.E., Black & Veatch International Company

Marllon Laboto, Black & Veatch International Company

ABSTRACT Chlorine contact time (CT) is an area of treatment plant design that often receives little focus when compared to other treatment processes. Typically, the clearwells have significantly more volume than required to achieve the CT, so plants have not optimized that process. However, that excess volume provides more time for disinfection byproduct (DBP) formation. The requirements of the pending Stage 2 DBP regulations have demanded that municipalities become increasingly focused on the control of DBP formation. Optimizing chlorine contact basin volumes, layouts, and efficiencies can have a significant impact on controlling DBP formations. Bench scale modeling for the Durham, NC project provided results indicating that reducing free chlorine contact time could reduce DBP formation between 20 % and 40 %. To optimize chlorine contact time, smaller basins with more efficient baffling are required to reduce the actual contact time while maintaining the disinfection effectiveness. During design of the Brown Treatment Plant upgrades, computational fluid dynamic modeling was utilized to optimize the chlorine contact basin efficiency by evaluating basin layout and alternative baffle configurations. Rectangular and circular chlorine contact basin layouts were evaluated on a cost benefit basis. Initial analyses showed a rectangular configuration would provide approximately 10% higher baffling factor, the cost savings of a circular tank was approximately 27%. Therefore, the circular tank was selected. During detailed design, computational fluid dynamics was again utilized to evaluate alternative baffle configurations to optimize the efficiency of the chlorine contact tank. Additionally, the design included flexibility in the chlorine contact basin to allow the plant staff to either operate the chlorine contact basin at a fixed water level with a constant volume or to allow the water level in the basin to float based on downstream water levels thus decreasing the contact time in the chlorine contact basin.

KEYWORDS: Water Treatment, Disinfection Byproducts, Computational Fluid Dynamics Modeling, Chlorine Contact Basin

INTRODUCTION Compliance with the Stage 2 Disinfectants/Disinfection Byproducts Rule is an important issue for water utility planning over the next few years and has required municipalities to become more focused on the control of Disinfection Byproducts (DBP) formation. The City of Durham has conducted bench-scale testing to identify treatment strategies at both their Wade G. Brown WTP (BWTP) and their Williams WTP (WWTP) to reduce disinfection byproduct formation. The results of the testing indicated that the formation of trihalomethane (THM) and haleoacetic acids (HAA) could be reduced by 25 to 30 percent by utilizing ferric sulfate for coagulation instead of alum. Additionally, the testing indicated that reducing the total chlorine contact time from 10 hours to 2 hours would yield a reduction in THM and HAA5 of approximately 20 to 40 percent. Combining

the reductions in formation available with decreased chlorine contact time with those from coagulant conversion to ferric sulfate would yield a combined percent reduction of approximately 50 percent and 43 percent, respectively, for THMs and HAAs. These reductions in DBP formations would allow the City to be in compliance with the pending Stage 2 Disinfectants/Disinfection Byproducts Rule. Based on the results and recommendations from the bench-scale model testing, the City completed the conversion to ferric sulfate in December of 2008. The ferric sulfate has been effective at reducing the DBP formation for the last seven quarterly samples by approximately 40 percent for THMs and 34 percent for HAAs. The implementation of the recommendation for reduced chlorine contact time was decided to be incorporated into the Upgrade and Expansion Project for the BWTP. Currently, the BWTP utilizes two 5 million gallon (MG) clearwells for contacting. The clearwells are operated in series with free chlorine being fed upstream of the first clearwell and ammonia being added upstream of the second clearwell to convert the free chlorine to chloramines in an effort to stop the formation of DBPs. The existing conditions in the clearwells are not well controlled due to the fact that these facilities were originally designed for storage purposes and are not equipped with any baffles to prevent short circuiting. Also, the clearwells have significantly more volume than required to achieve the CT which has a significant impact on the formation of THMs and HAA5. For the Upgrade and Expansion Project for the BWTP, a dedicated chlorine contact basin was designed which included the optimization of the chlorine contact time, volume, and baffle efficiency. During design of the Brown Treatment Plant upgrades, computational fluid dynamic modeling was utilized to optimize the chlorine contact basin efficiency by evaluating basin layout and alternative baffle configurations. The goal of this evaluation was to identify the ideal basin volume for varying plant flow conditions, determine the most cost effective basin layout, and determine the most efficient baffle layout utilizing

METHODOLOGY Due to issues with excessive contact times and the formation of disinfection by-products when utilizing the existing 5MG clearwells, optimizing the volume of the new chlorine contact basin to provide the required contact time while minimizing excess contact was evaluated over the flow range for the BWTP. As a result of these evaluations, a new 1.4 MG chlorine contact basin was designed in accordance with the parameters denoted in Table 1 which will provide sufficient contact volume for a plant influent flow of 54MGD which provides some future expansion capacity above the design flow of 42 MGD for the Upgrade and Expansion Project. Table 1 - Brown WTP Chlorine Contact Basin Design Parameters

Constituents Design Point Range

Min Temperature, C 5 5 -30

Chlorine Residual, mg/L 1 0.4 2.8

Assumed Log Removal Credit 1 0-2.5

Log Removal Required 0.5 0.5 - 3

pH 6.5 6 7.5

Baffle Factor 0.60 0.5 0.7

To minimize construction costs, rectangular and circular chlorine contact basin layouts were evaluated on a cost benefit basis. It was determined that a 1.4 MG rectangular concrete basin would cost approximately 27% more than a pre-stressed concrete circular basin of the same volume and would have an approximately 10% higher baffling efficiency than a circular basin based on our previous project experience. The slightly reduced volume required due to the increased baffling efficiency for the rectangular basin was not sufficient to offset the 27% cost increase. Therefore, pre-stressed concrete circular basins were selected for the BWTP chlorine contact basin.

Inlet

Outlet

To match the existing on-site 5 MG clearwells, a 20 foot side water depth was selected for the design of the chlorine contact basin which resulted in a tank diameter of 115 feet. For additional flexibility in contact volume, two effluent pipes were included in the design. One of the effluent pipes was designed as a standpipe to constantly maintain the 1.4 MG contact volume in the tank and the other effluent pipe was designed as an outlet at the floor of the tank to allow the water level in the basin to be hydraulically controlled by the water level in the downstream clearwells thus reducing the available contact volume. Since the required contact time is dependent of flow and temperature among other variables, this effluent arrangement allows the City to decrease the contact volume and contact time for lower flows and warmer temperatures. In addition to providing the required contact time, the chlorine contact basin also needed to be sufficiently baffled to achieve plug flow conditions and good hydraulic efficiency to minimize recirculating regions that could increase the risk of DBP formation. The layout of the baffles in the basin was considered to be a critical design element and a minimum baffling efficiency of 60% or a baffle factor of 0.6 was targeted. Since very little published information exists regarding baffle efficiency optimization for circular basins, it was determined that CFD modeling would be needed to predict the performance of various baffle layouts. Three different baffle configurations were developed for the circular basin and each of the configurations were evaluated using CFD modeling. Two models were run for each baffle layout including a Steady State Simulation and a Tracer Simulation. All modeling was performed using the Ansys CFX 12 software with a constant flow rate equal to the design flow for the BWTP Upgrade and Expansion Project of 42 MGD.

RESULTS The results showed that two of the three baffle layouts exceeded the minimum required baffling efficiency of 60%. Baffle Layout. Figures 1 thru Figure 3 illustrate the three different baffle layouts. These layouts were developed based on previous project experience and recommendations from pre-stressed circular concrete tank vendors.

Figure 1 Curved Baffles Layout

Figure 2 Straight Baffles 1 Layout

Figure 3 Straight Baffles 2 Layout Steady State Simulation Results. The Steady State CFD Simulation predicts the three dimensional flow path through the tank. The results of the simulation indicate the extent to which plug flow occurs. Figure 4 shows the steady state results obtained for the three layouts. Flow separation is clearly visible at the end of each baffle, resulting in areas of low velocity. It is also clearly visible that the Straight Baffles 2 layout has significantly less flow separation and velocity differentials than the other two layouts. This suggests that the full area of the tank is being utilized for plug flow rather than only the flow area adjacent to the baffle walls as in the Curved Baffle layout and the Straight Baffle 1 layout. The streamline flow illustrations in Figure 5 also clearly indicate the areas of the tank not being utilized. For the Curved Baffle and Straight Baffle 1 layouts, Figure 5 suggests that a significantly larger area of the tank is not being utilized with these baffle configurations than with the Straight Baffles 2 layout. Areas of zero velocity or no streamline flow are areas where recirculating regions could exist leading to longer contact times and the potential for increased DBP formation.

Inlet

Outlet

Inlet

Outlet

Curved Baffles

Straight Baffles 1

Straight Baffles 2

Figure 4 Steady state results at mid depth

Curved Baffles

Straight Baffles 1

Straight Baffles 2

Figure 5 Steady state streamline results

Tracer Simulation Results. The Tracer CFD Simulation imitates the passage of a tracer through the tank. The T10, the time for 10% of the tracer to pass through the tank, can be evaluated by plotting the concentration versus time of the tracer at the outlet of the tank. The hydraulic efficiency/baffling efficiency of the tank can be calculated by dividing the T10 time by the theoretical retention time for the tank. Figure 6 to Figure 9 show the tracer results obtained for the three layouts. The T10 for the Curved Baffles layout is 31 minutes. A comparison of Figure 6 with Figure 8 shows that the Straight Baffles 2 layout provides an improved performance over the Curved Baffles layout with a lower tracer concentration at the outlet and a T10 time of 33 minutes. Conversely, Figure 7 indicates that the Straight Baffles 1 layout yields slightly impaired performance with a T10 time of 27 minutes.

Time 1 min

Time 25 min

Time 31 min

T10 = 31 min

Figure 6 Curved Baffles - Tracer concentration after one, twenty five and thirty one minutes

Time 1 min

Time 25 min

Time 31 min

T10 = 27 min

Figure 7 Straight Baffles 1 - Tracer concentration after one, twenty five and thirty one minutes

Time 1 min

Time 25 min

Time 31 min

T10 = 33 min

Figure 8 Straight Baffles 2 - Tracer concentration after one, twenty five and thirty one minutes

Table 2 shows the T10 time found for all three layouts and also the tank baffle efficiency, related to the theoretical retention time. The theoretical retention time is calculated by dividing the tank volume by the design flow rate.

Table 2 - Tank Baffle Efficiency

Baffle Layout Theoretical retention time

(minutes) T10

(minutes) Efficiency (%)

Curved Baffles 50.6 30.8 61.7

Straight Baffles 1 50.6 26.8 53.1

Straight Baffles 2 50.6 33.2 65.7

RECOMMENDATIONS The efficiency for the Curved Baffles and the Straight Baffles 2 layout were both higher than the minimum design target of 60%. The Curved Baffles layout was determined to be slightly less efficient than the Straight Baffles 2 layout. In addition to the decreased efficiency, the Curved Baffles configuration may result in higher construction costs than the straight baffle layouts due to the curvature of the baffles requiring additional supports or possibly more complicated form work. The most inefficient baffle layout modeled was the Straight Baffles 1 layout. The calculated tank efficiency for this layout does not meet the design target efficiency and suggests that the baffle configuration may not perform well hydraulically for this installation. For the BWTP Upgrades and Expansion Project, the Straight Baffles 2 layout was incorporated into the design. The plug flow characteristics, greater tank utilization, and resulting higher baffling efficiency made it the obvious choice over the other two layout modeled.