Maintain Disinfection Residuals and Reduce Flushing With Chlorine Dioxide—Shelby Hughes, Rhea

FWRJ Maintain Disinfection Residuals and Reduce Flushing With Chlorine Dioxide

Shelby Hughes, Rhea Dorris, and Madison Rice

Consecutive drinking water systems have limited control over their influent water quality, yet they must maintain disinfection residuals throughout the extent of their distribution systems, creating unique water quality challenges. The Summertree Water Distribution System (Summertree) is owned and operated by Utilities Inc. of Florida (UIF), located in New Port Richey in Pasco County.

The existing Summertree system has approximately 11.5 mi of water main, varying from 2 in. to 12 in. in diameter. The water main material is variable, but generally consists of polyvinyl chloride (PVC), ductile iron, and high-density polyethylene (HDPE) pipe. In December 2016, UIF interconnected with the Pasco County Utilities (Pasco) distribution system and began purchasing potable water for delivery to UIF’s Summertree customers. Thereafter, UIF decommissioned the existing wells and water treatment plant. Pasco County receives water from Tampa Bay Water, which uses chloramination for primary disinfection.

The Florida Department of Environmental Protection (FDEP) requires, by Florida Administrative Code (FAC) Rule 62-555, that chloramine residuals are maintained above 0.6 mg/L. Following the interconnection with Pasco’s distribution system, the Summertree system required frequent flushing to maintain adequate chloramine residuals at the perimeter of the service area. Chloramine residuals observed during testing of Pasco’s water at the point of connection (POC) were inconsistent, contributing to the difficulty of meeting minimum chloramine residual at remote points in the system. Additionally, the system’s susceptibility to high water age in outlying areas increased the degradation of chloramine residuals. Seasonal population changes and low water use further exacerbated the high water age issue. Frequent flushing was successful at reducing the water age and maintaining adequate chloramine residuals, but consistently wasted large volumes of purchased potable water.

Utilizing chlorine dioxide as an oxidant was identified as a potential solution to help maintain disinfectant residuals throughout the Summertree system and reduce the need for flushing. In the Summertree Water Distribution System Analysis Report, completed by KimleyHorn in 2017 (Summertree Analysis[1] , 2017), pressure and constituent modeling, along with the analysis of various field and laboratory data to assess water quality, resulted in the recommendation that UIF implement a chlorine dioxide storage and injection system at the POC to maintain residuals throughout the system. Based on this recommendation, other utilities’ successes with similar systems, and historical knowledge of chlorine dioxide use, UIF completed a chlorine dioxide pilot program to promote residual retention throughout the Summertree system.

The pilot program was necessary to confirm the optimal chlorine dioxide dosage and to demonstrate the ability of chlorine dioxide to reliably maintain the system residual. The results of this pilot study confirm that utilizing chlorine dioxide as an oxidant successfully maintained the chloramine residual throughout the Summertree system and reduced the need for flushing.

During the pilot, data were collected and analyzed, leading to the development of this

Shelby Hughes, P.E., Rhea Dorris, P.E., and Madison Rice, E.I., are with Kimley-Horn and Associates Inc. in St. Petersburg.

article that identifies the value in continuing the addition of chlorine dioxide at Summertree for the benefit of the utility, its customers, and the environment.

Preliminary Analysis of Need

The water quality analysis completed as part of the 2017 Summertree analysis was performed to identify recommended improvements to address observed deficiencies within the distribution system. A hydraulic model was developed to analyze system pressure, water age, and water quality.

System pressure was determined to be sufficient to meet FDEP’s regulatory requirements for pressure during peak demand and fire flow scenarios. The hydraulic model was also used to determine the water age in the distribution system from the POC. Water age was assessed because it’s a parameter used to indicate water quality. The higher water ages, the more time it has to undergo chemical and biological reactions, resulting in reduced chloramine residuals. It should be noted that the water age was not considered within the Pasco system; the water age at the POC was assumed to be zero hours for this analysis.

Under average day demand conditions without flushing, the water age of the system from the POC was 48 hours or less, except for streets at the extent of the system, and several dead ends. Water age along the extent of the system ranged from 49 to 254 hours (two to 10.5 days). The water age at the dead ends was approximately 60 hours (two and a half days).

The water quality analysis for the 2017 Summertree analysis was performed at 17 locations throughout the distribution system and the POC to characterize the water coming from Pasco’s distribution system. The POC samples were all within compliance of U.S. Environmental Protection Agency (EPA) regulations. Chloramine residual ranged from 0 to 1.5 mg/L throughout the Summertree system. The lowest chloramine residual was found at the extent of the system and dead ends. The total chloramine residual at the POC ranged from 0.8 to 1.5 mg/L.

The inconsistent chloramine residual coming from Pasco contributed to the difficulty of maintaining sufficient residual in the Summertree system. The samples were tested for evidence of nitrification, which is an issue that can plague distribution systems using chloramine disinfection. Nitrate and nitrite were found in all samples taken in the distribution system, with the average nitrite and nitrate concentrations being 0.44 parts per mil (ppm) and 0.70 ppm, respectively. Nitrate was 0.44 mg/L on average at the POC. This indicates that nitrification, an ammonia oxidation process performed by bacteria, was occurring in the Summertree and Pasco systems. The presence of biological growth within the pipelines contributes to the degradation of disinfectant residual, increasing the difficulty of maintaining the minimum required residual at the system extent.

It should be noted that since the completion of the 2017 Summertree analysis, Pasco implemented changes to the distribution system to reduce water age in remote areas, increasing the chloramine residual at the POC. While the chloramine residual is now consistently higher, it’s still variable.

Figure 4. Chlorine Dioxide Injection Point Figure 5. Point of Connection Sampling Point

Figure 6. Sampling Point Exhibit

Use of Chlorine Dioxide as an Oxidant

Chlorine dioxide is a strong and selective oxidizer and offers several advantages in the treatment and distribution of drinking water. Chlorine dioxide forms fewer disinfection byproducts (DBPs) than traditional chlorine and chloramine treatments. It also can be used at lower concentrations and shorter contact times to achieve equivalent disinfection than the contact times and concentrations required by chlorine and chloramine disinfection. Chlorine dioxide is also less reactive to changes in pH and has been proven more effective over a broader range of pH than free chlorine[2] . Continued on page 56 Florida Water Resources Journal • August 2022 55

Continued from page 55

Chlorine dioxide is as effective as chlorine disinfection against viruses, bacteria, and fungi, and more effective at the inactivation of Giardia and Cryptosporidium parvum[3]. The reduction of biological growth in the system through chlorine dioxide addition allows the chloramine residuals to persist throughout the extent of the system by reducing potential reactants that contribute to residual degradation; therefore, introducing chlorine dioxide to reduce or eliminate biological growth can maintain the chloramine residual within the system for a longer period and consequently reduce or even eliminate the need for flushing.

Pilot Study

Utilizing the results of the Summertree analysis to set up dosing parameters, and coordinating with Applied Oxidation LLC for equipment and chemicals, the full-scale pilot study was designed. A pilot testing approval package was submitted to FDEP. After approval and communication with customers, the 90-day pilot study was implemented at the Summertree POC. The 90-day time frame was selected as the minimum duration expected to allow the chlorine dioxide to react with and break down any existing biological growth within the system, even to the extremities of the pipe network.

Pilot Setup and Equipment

The pilot program included the physical components to mix, store, and inject the powder-generated chlorine dioxide into the distribution system at the POC. The physical equipment required to complete the full-scale pilot test includes the following components: S Chlorine dioxide mixing tank – A 125-gal

HDPE tank for mixing the two-component chlorine dioxide powder and solution water. S 15-gal-per-minute (gpm) magnetic drive transfer pump – A pump to transfer the fully mixed chlorine dioxide solution from the mixing tank to the storage tank. S Chlorine dioxide storage tank – A double-

walled 275-gal HDPE tank for the storage of the 0.3 percent chlorine dioxide solution S FlexPro feed pump – A pump to transfer the chlorine dioxide solution from the storage tank to the chlorine dioxide injection point after the POC with Pasco. S Grab sample analyzer – One handheld analyzer (Palin Test Unit) for routine daily monitoring of the chlorine dioxide residual and chlorite at each of the sampling stations identified. S Sampling points – Sampling taps located within the distribution system are used to pull representative grab samples of the treated water: • POC • Sample Point 1 (first customer) • Sample Point 2 (location of average water age) • Sample Point 3 (perimeter of the distribution system reflecting maximum water age) S Shade tent – To prevent excessive exposure to ultraviolet (UV) radiation, which would

Figure 8. Total Chlorine in Summertree System

otherwise cause degradation of chlorine dioxide.

The dosing rate was initiated at 0.5 mg/L and adjusted manually as needed. Chlorine dioxide and chlorite were monitored daily with a handheld grab sample analyzer. All normal water quality measurements continued during the pilot study. These physical components were inspected once per day as the operations staff was completing its sampling efforts, as well as during the routine operation and maintenance protocol.

Photographs of the pilot equipment are shown in Figures 1 through 5, sampling locations within the Summertree system are shown in Figure 6, and a process flow diagram of the pilot is shown in Figure 7.

Results and Observations

Regulatory Compliance Sampling

Throughout the pilot study, total chlorine residual (as Cl2), chlorine dioxide, and chlorite were monitored daily using the handheld analyzer at the POC at Sample Point 1, Sample Point 2, and Sample Point 3. The Cl2 was monitored as a substitute for chloramine due to its ease of measurement. The total chlorine had an average of 3.98 ppm and a maximum single sample reading of 5.0 ppm across the Summertree system. The total chlorine at the POC was variable, but consistently remained near the FDEP limit of 4 mg/L. Figure 8 shows the total chlorine throughout the entire system during the 90-day pilot study.

The residual chlorine dioxide had an average of 0.05 ppm and a maximum of 0.39 ppm across the Summertree system. Many chlorine dioxide readings were measured as 0.02 ppm, which is the lower detection limit of the handheld analyzer. The chlorine dioxide residual at the POC consistently remained well below the EPA maximum residual disinfectant level (MRDL) of 0.80 ppm, averaging at 0.08 ppm, with a maximum residual of 0.39 ppm. Figure 9 shows the chlorine dioxide throughout the entire system during the 90-day pilot study.

Chlorite is a byproduct formed by the aqueous dissolution of chlorine dioxide and therefore increases as chlorine dioxide is consumed; it’s also an EPA-regulated primary contaminant. The chlorite residual was an average of 0.27 ppm and a maximum of 0.91 ppm across the Summertree system. The chlorite residual at the POC consistently remained below the EPA maximum contaminant level (MCL) of 1 ppm, averaging at 0.21 ppm, with a maximum residual of 0.70 ppm. The chlorite levels throughout the system are shown in Figure 10.

Systemwide Disinfection Residual Results

Total chlorine is monitored daily by UIF operators. Results from January 2020 through January 2021 were analyzed to evaluate the effect of chlorine dioxide on overall residual permanence. Total chlorine is measured at the POC and two random points within the system. Before the chlorine dioxide pilot study, there was, on average, a 43 percent reduction in total chlorine from the POC to the random sampling points throughout the distribution system. During the 90-day pilot study, the reduction in total chlorine throughout the system averaged 18 percent. The addition of chlorine dioxide significantly enhanced the system’s ability to maintain disinfectant residuals.

Figure 11 shows the total chlorine residuals at the POC, two random sampling points within the Summertree system, and the system average residual. The volume of water flushed throughout the Summertree system is also displayed to show the effect the chlorine dioxide had on flushing activity. During the 90-day pilot study, a 96 percent reduction in monthly flushing was observed, resulting in substantial cost savings by a reduction of 96 percent of purchased water, as well as reduced waste of potable water. As evidenced in the figure, the increased clustering of the data during the piloting period indicated greater stability in the system in maintaining the chlorine residual over time.

Disinfection Byproduct Results

The DBP samples were collected every 30 days (days 30, 60, and 90) during the pilot study. The DBP samples demonstrate compliance with FDEP requirements for the formation of total trihalomethanes (TTHMs) and haloacetic acids (HAA5).

Consistent sampling locations were utilized for the TTHMs and HAA5 distribution system analysis. The POC represents the point that the Summertree system connects to the Pasco County distribution system, with an assumed water age of 0 to 16 hours. This assumption was made in the Summertree analysis as a baseline for water age and constituent modeling. Sample Point 3 represents the maximum residence time location, with an approximate water age of 49 to 96 hours.

Figures 12 and 13 show the DBP results at the POC and Sample Point 3, indicating that the TTHMs and HAA5 remained at approximately one-third of their respective FDEP limits of

Figure 10. Chlorite Residual in Summertree System

Figure 11. Total Chlorine Residuals and Flushing Volumes

Continued from page 57 80 parts per bil (ppb) and 60 ppb throughout the 90-day study, which is expected for a chloramine disinfection system. These results show that using chlorine dioxide as an oxidant for the Summertree system will not contribute to the formation of DBPs.

Operations Concerns

Throughout the 90-day pilot study, the chlorine dioxide dosing pump was not operational on days 18, 32, 36, and 46 due to the clogging of a ball valve. It was determined that the pump was oversized, and it was replaced with a smaller pump on Day 50. This solved the problem and no further clogging occurred throughout the pilot study.

Figure 12. Total Trihalomethanes Results

Figure 13. Haloacetic Acids Results

Figure 14. Sampling Extension Total Chlorine Residuals and Flushing Volumes

Conclusion

The overall goal of this pilot study was to reduce the need for flushing and maintaining disinfectant residual at the extent of the Summertree system. The results of the study confirm that utilizing chlorine dioxide as an oxidant maintained the chloramine residual throughout the Summertree system. The addition of chlorine dioxide reduced the degradation in total chlorine residuals throughout the system from 43 to 18 percent, displaying its ability to stabilize disinfectant residuals. The concentrations of chlorine dioxide, chlorite, and DBPs all remained compliant with FDEP’s maximum allowed values throughout the 90-day testing period.

Monthly flushing was reduced by 96 percent with the introduction of chlorine dioxide dosing. The UIF purchases water from Pasco County at a rate of $3.69 per 1,000 gals. From January 2018 to January 2019, the estimated volume of potable water lost due to flushing was 17,134,294 gal, costing UIF approximately $63,218. The anticipated annual flushing volume based on the 90-day pilot study results is 251,600 gal, with an anticipated cost of $929. Implementing the utilization of chlorine dioxide permanently will pay for itself after 19 months of operation and result in an annual savings of $45,490.

The chlorine dioxide dosing proved to be highly effective at maintaining residual, while saving potable water and reducing costs due to flushing. Based on these favorable results, permanent installation of the chlorine dioxide dosing system was recommended at Summertree. To further optimize the system, the sampling was extended for an additional 90 days and was completed on April 16, 2021. This additional testing showed the effect of chlorine dioxide with a wider variation of residuals at the POC.

Figure 14 shows the results of the additional testing. Following the successful testing, a preliminary design report and permit application were submitted to FDEP for the permanent installation of the chlorine dioxide dosing system, and the system was certified in July 2021. The sizing and installation of the necessary equipment were completed as part of the pilot study and the same equipment and site plan were used for permanent installation, with the addition of a HydroAct Chlorine Dioxide Analyzer to automatically monitor chlorine dioxide and chlorite at the POC. The online analyzer reduced the time spent by operators

collecting grab samples and allows for safe and continuous monitoring at the POC.

Recommendations

To continue improving the UIF system, it’s recommended that chlorine dioxide be dosed intermittently to optimize dosage and operational costs. Dosing may also be optimized based on the time of year to accommodate the seasonal population.

Before installing a permanent chlorine dioxide dosing system for other utilities, it’s imperative to completely understand the process before investigating its use.

The following recommendations are based on lessons learned from the Summertree pilot study: S Chlorine dioxide is proven to be an effective tool to maintain residuals in distribution systems suffering from high water age and inconsistent influent residuals, but it’s still recommended to perform field and laboratory testing to verify the compatibility with the system’s water. A full pilotscale study is recommended prior to the installation of a permanent chlorine dioxide dosing system to understand how chlorine dioxide interacts with the system’s specific water quality. The pilot study can also serve to optimize the dosing of chlorine dioxide to decrease operational costs. Chlorite, chlorine dioxide’s nonorganic byproduct, should be considered and kept below EPA’s

MCL of 1 mg/L. S As chlorine dioxide has not been used extensively in potable water applications, it’s important to gain understanding and consensus from state and local regulators.

Effective communication during all steps of the pilot study process is crucial and will help further the science of chlorine dioxide treatment. S The Summertree pilot utilized a twocomponent powder chlorine dioxide generation system, but other systems, such as generators, can produce chlorine dioxide based on demand. Powder generation was feasible for Summertree, which is a small distribution system, because a large amount of powder was not required, and batches did not have to be mixed often. Larger systems may desire a chlorine dioxide generation method that is more robust or automated.

Other considerations, such as chemical safety, goal usage of chlorine dioxide, operator training, and redundancy needs, should also be understood when choosing a chlorine dioxide generation system. S Prompt and direct public communication is recommended before implementing chlorine dioxide in a treatment process.

It’s important to emphasize the benefits of chlorine dioxide to customers.

Resources

[1] Kimley-Horn and Associates Inc., 2017.

“Summertree Water Distribution System

Analysis.” [2] Gates, Don, et al., 2011. “State of the Science of Chlorine Dioxide in Drinking Water.”

Water Research Foundation. [3] Holden, Glenn W., 2017. “Chlorine Dioxide

Preoxidation for DBP Reduction.” Journal -

American Water Works Association, vol. 109, pp. 36–43.doi:10.5942/jawwa.2017.109.0089. S

Continued from page 35 advocacy, and the exchange of knowledge. Headquartered in Kansas City, Mo., it has an office in Washington, D.C., and 63 chapters and 97 branches throughout North America.

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The City of Delray Beach was awarded the 2021 Sustainable Practices Recognition Award from the American Concrete Pavement Association (ACPA) in recognition of its use of pervious pavements in the Osceola Park Neighborhood Project. The award is presented to the organization that demonstrates leadership by implementing sustainable and resilient design and construction practices that consider societal, environmental, and economic factors.

The award was formally presented to Mayor Shelly Petrolia by Amy Wedel, director of concrete pavements of the Florida Chapter of ACPA.

Terrence Moore, city manager, said, “We recognized the need for vision and long-term consideration, while addressing the needs of our community. By employing the latest technological innovations and proven best practices, we will reduce stormwater runoff and flooding for decades to come.”

Pervious pavement provides multiple benefits to the city and its residents by reducing localized flooding events, protecting water quality, and recharging the local aquifer. The use of pervious concrete allows for the intake of water into the concrete, which acts as a retention area that helps reduce runoff. As the water enters through the open cells of the pavement, aerobic bacteria help to break down harmful pollutants and chemicals.

The Osceola Park Neighborhood Improvement project provides multiple benefits to the community, including the permeable pavement for alleys, bioswales in parkways, and other ways to treat stormwater runoff.

To learn more about the project, visit OsceolaParkProject.com.

NEWS BEAT

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The Environmental Integrity Project (EIP) has released a national report, “The Clean Water Act at 50: Promises Half Kept at the Half-Century Mark,” which examines water pollution data in all 50 states on the 50th anniversary of the federal Clean Water Act.

The report concludes that 50 percent of 1.4 million river and stream miles nationally, which have been studied in recent years, are so polluted they are classified as “impaired” (despite the Clean Water Act’s promise of making all waterways “fishable and swimmable” by 1983), using the most recent state water quality reports from the U.S. Environmental Protection Agency (EPA).

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The report includes detailed maps and charts, with the most recent available water pollution impairment data for the U.S.

For copies of the report and accompanying spreadsheet, contact Ari Phillips, senior writer and editor at EIP, at aphillips@environmentalintegrity.org. Continued on page 63