The most common disinfectants for potable water, chlorine and chloramine, are increasingly criticized for their disadvantages and hazards.

Chlorine poses safety and health risks, specifically disinfection byproducts (DBPs). Chlorine dioxide (ClO2) mitigates these risks and still performs effectively as a strong oxidizer for drinking water. It is used in Europe and the U.S. mainly for disinfection and preoxidation for color removal and DBP control. Its use can be tailored to specific needs and treatment goals. Approved ClO2 generation systems include powder mixes combined in water, gas generation and injection, and ion exchange.

Testing of ClO2 at a Florida water utility documented its effectiveness as a disinfectant and its ability to reduce total trihalomethanes (TTHMs) and haloacetic acids (HAA5) in the distribution system.

Needing an alternative

Pluris Holdings owns and operates the Wedgefield Potable Water and Wastewater Utility in central Florida. With the onset of the Stage 2 Disinfectant/Disinfectant Byproducts Rule (D/DBPR), the utility tried to comply with DBP limits through removal of organics before disinfection. Using chlorine, the utility began approaching the Stage 2 D/DBPR limits and sought alternative means of compliance. Chloramines were not considered because of distribution system challenges.

After field-testing and laboratory evaluation of ClO2 products, the utility launched a full-scale pilot test in its distribution system that included demand testing and simulated distribution system testing of ClO2 as a primary disinfectant over a five-day water age analysis. The results supported a full transition to ClO2, showing significant potential to reduce TTHMs in the distribution system. A full-scale pilot testing approval package was submitted and approved by Florida Department of Environmental Protection.

The study goals included a gradual transition from chlorine disinfection to ClO2, vigorous field and laboratory testing of the treatment process to ensure public safety, and compliance with the regulations. The utility and on-site staff completed extensive sampling, which helped significantly in determining the effect of each process adjustment.

Careful monitoring

The pilot study was designed to inject a premixed 0.3 percent ClO2 solution into the ground storage tank to take advantage of organics and sulfide removal through the existing treatment processes. Online analyzers monitored the residual ClO2 along with chlorite at the point of entry; the hand-held Palintest was used at various process points and in the distribution system.

Utility staff closely monitored the residuals as the transition to ClO2 extended through the distribution system. The dosage was initiated at 1.3 ppm based on the demand testing and laboratory analysis. The dose was increased or decreased to maintain the desired 0.2 mg/L ClO2 distribution system residual. Once that residual was obtained, the chlorine dosage was reduced gradually to zero. Careful attention was paid to the residuals during the transition. Online monitoring of chlorite and ClO2 included high and low alarms to ensure compliance.

The results confirmed the success of ClO2 as a primary disinfectant in reducing TTHMs and, to a lesser extent, HAA5. TTHMs declined from more than 110 ppb in November 2016 to 20 ppb after about two months and were nondetectable by June 2017, near the end of the 180-day pilot (Figure 1). TTHMs were also undetectable in September 2017 compliance samples. HAA5 concentrations also declined (Figure 2). The level increased early in the study but then steadily decreased to a reduction of slightly below 40 percent to 65 percent.

Compliance with regulations was imperative throughout the pilot period, especially for ClO2 and chlorite. ClO2 was maintained below the maximum residual disinfectant level (MRDL) of 0.8 ppm and chlorite below the maximum contaminant level (MCL) of 1.0 ppm. Chlorine presence had a noticeable effect on the chlorite analysis: Chlorite concentrations fluctuated at the beginning of the study and ultimately leveled off at 0.6 to 0.8 ppm. The daily ClO2 residual dropped below the 0.2 ppm residual requirement early in pilot sampling and peaked shortly after.

Pilot data revealed 99 percent reduction in TTHMs and about 50 percent reduction in HAA5s in the first 180-day pilot test, proving the effectiveness of ClO2 in potable water disinfection. The utility has seen no adverse issues in the distribution system since the transition to ClO2.

ClO2 proved highly effective at minimizing DBPs while saving capital costs versus treatment upgrades. It is imperative to fully understand the process before investigating its use. These recommendations are based on lessons learned:

ClO2 is proven effective in maintaining compliance with Stage 2 D/DBPR. Focus pilot studies on the effects of ClO2 on a system’s specific water quality; identify the optimal ClO2 dosing for cost savings. ClO2 demand will stabilize as existing biofilm is cleaned.

Maintain chlorite ion concentrations under the MCL limit of 1.0 mg/L.

Gain understanding and consensus from regulatory agencies; compliance must be maintained. Accurate assessment from laboratory and hand-held analyzers is imperative for preventing unnecessary public concern.

Understand and review available options for ClO2 generation. Important factors include operator training and availability, redundancy, and safety. 

Undertake proactive and direct public communication emphasizing the benefits of ClO2 and comparing its safety to typical disinfectants. Although ClO2 is not new, the public may be concerned about the use of an unfamiliar chemical.  

Utilities should carefully consider the implementation of ClO2 in a water production or distribution facility. While ClO2 is effective in maintaining disinfectant residuals and improving water quality aesthetics, the appropriate process addition may be as a preoxidant rather than as the primary disinfectant.

ClO2 has shown promise as a strong disinfectant for utilities aspiring to reduce DBPs without the capital cost of high-end treatment or the routine maintenance challenges of chloramines. It should be considered as an alternative disinfectant when DBP or distribution system challenges are present.

About the authors

Lance Littrell, P.E., (lance.littrell@kimley-horn.com) is a registered professional engineer with Kimley-Horn and Associates. Joe Kuhns (jkuhns@plurisusa.com) is a regional manager with Pluris Wedgefield.