Water Modeling

Modeling Disinfection with Chlorine Dioxide

Advantages of Chlorine Dioxide
The substitution of chlorine by chlorine dioxide as a disinfectant in drinking water treatment is of growing interest. While chlorine has been effective for reducing most microbial pathogens to safe levels, it reacts with naturally-occurring matter in the water to form trihalomethanes (THMs) and haloacetic acids (HAAs) as disinfection by-products (DBPs). In December, 1998, the U.S. EPA finalized the Stage1 Disinfectants/Disinfection By-product (D/DBP) Rule setting a maximum allowable concentration for total THMs of 0.080 mg/L, and for the sum of five HAAs at 0.06 mg/L.  The maximum allowable concentration for chlorine in the treated water is 4.0 mg/L.

Among the reported advantages of chlorine dioxide over chlorine are a better ability to inactivate Cryptosporidium cysts, and elimination of the formation of THMs and HAAs.

Concerns
Although chlorine dioxide does not produce THMs and HAAs, it does produce a different set of disinfection by-products, chiefly the chlorite ion, as well as lower concentrations of the chlorate ion. Typically, the ClO2 reacts either with dissolved organic matter, or with dissolved metals such as manganese (II), to accept an electron and become a chlorite ion. Chlorate ion may form due to reaction of the chlorite ion with hypochlorous acid present as an impurity in chlorine dioxide generators. As a rule of thumb, the concentration of chlorite formed is in the range of 50 to 70 % of the applied ClO2 dose.

The maximum residual disinfectant level for chlorine dioxide set by the U.S. EPA in the Stage 1 D/DBP Rule is 0.8 mg/L. Under the same rule, the maximum allowable contaminant level (MACL) for chlorite is 1.0 mg/L.

Chlorine Dioxide Modeling in WatPro
In WatPro, the models for consumption of ClO2, and formation of chlorite and chlorate are taken from newly published university research. The expressions are expressed as functions of water quality and chlorine dioxide related parameters, including:

As well, significant 2-factor interaction terms are included in the relationships.

The model equations were developed from testing with 8 different waters sources, and then validated with an external data set from waters throughout North America (Ann Arbor, MI, Vancouver, BC, Calgary, AB and Brantford, ON). The inactivation of Giardia and viruses by ClO2 in WatPro are calculated by a polynomial interpolation routine. The procedure estimates the log reduction of the microorganisms by fitting the calculated Ct parameter to a polynomial expression derived from inactivation tables in the EPA’s Surface Water Treatment Rule (EPA, 1989).

WatPro is very useful in the evaluation of the substitution of chlorine by chlorine dioxide. Through modeling, one can determine microbial reductions and levels of DBPs formed at comparable disinfectant doses. Once the doses of disinfectants are known, cost comparisons may be conducted to see which is the most cost-effective.


References

EPA (1989). Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources. Report No. EPA-570/4-89-018. Office of Drinking Water, U.S. Environmental Protection Agency, Washington, D.C. October.