F W R J
Maintain Disinfection Residuals and Reduce Flushing With Chlorine Dioxide Shelby Hughes, Rhea Dorris, and Madison Rice
C
onsecutive 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
Figure 1. Chlorine Dioxide Mixing (left) and Storage (right) Tanks
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,
Figure 2. Chlorine Dioxide Transfer Pump
54 August 2022 • Florida Water Resources Journal
Shelby Hughes, P.E., Rhea Dorris, P.E., and Madison Rice, E.I., are with Kimley-Horn and Associates Inc. in St. Petersburg.
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
Figure 3. Chlorine Dioxide Storage Tank (left), Dosing Pump (center), and Flow Meter (right)
