(205a) Optimizing Drinking Water Disinfection: Balancing Corrosion, Byproduct Formation, and Pathogen Removal

Currently, treatment plants most commonly disinfect drinking water with free chlorine to remove pathogens. Free chlorine, however, is prone to forming carcinogenic disinfection byproducts from reactions with organic matter. Ill-considered application of disinfectants and complications from cleanup of their byproducts, recently resulted in catastrophic corrosion and discharge of lead into the water supplies of Washington D.C. and Flint, Michigan. To avoid a recurrence of such mistakes, the strategy for drinking water disinfection in vulnerable, lead-containing supply networks was reconsidered. Specifically, we evaluated the merits and drawbacks of replacing free chlorine with chlorine dioxide in systems with lead infrastructure.

Free chlorine, the most commonly used disinfectant in drinking water treatment, is a powerful oxidant that readily reacts with viral and bacterial pathogens, but also with other water constituents, including dissolved organic matter and metallic infrastructure surfaces. Reactions with organic matter produce hazardous byproducts, while reactions with infrastructure surfaces can increase the concentration of metals, including lead, in drinking water. Some water treatment facilities have substituted free chlorine with chloramines, an alternative disinfectant, to meet disinfection byproduct regulations set by the Environmental Protection Agency. However, chloramines produce their own set of toxic byproducts and have lower disinfection efficiency than free chlorine. Removal of disinfection byproducts and the precursors to said byproducts can exacerbate corrosion in the metal pipes of water distribution systems.

Unlike free chlorine, chlorine dioxide has a significantly lower propensity to form organic byproducts, and it is as potent, and in some cases, more potent than free chlorine for the inactivation of bacteria and viruses. In a properly managed dose, chlorine dioxide can deliver effective disinfection of drinking water¹. Chlorine dioxide is more expensive to use for drinking water treatment than free chlorine and it can decay in water to form chlorite and chlorate, two potentially hazardous inorganic byproducts. It has been assumed that chlorine dioxide will behave similarly to free chlorine with regards to lead corrosion mechanisms because of their similar oxidation reduction potentials, but there have been few studies to confirm.

Using three different lead minerals, a series of experiments were performed to measure the kinetics of chlorine dioxide decay and byproduct formation, considering chlorine dioxide concentration, pH, presence of relevant anions, and presence of natural organic matter. Bicarbonate/carbonate, sulfate, silicate, bromide and phosphate are common anions in drinking water systems and were all considered in this study for the following reasons. The chloride to sulfate mass ratio has been shown to influence lead leaching². The bromide to bromate pathway is a concern for drinking water standards. Phosphate is commonly added to drinking water to control corrosion³. Finally, natural organic matter has also been shown to be an important variable when analyzing corrosion dynamics and chlorine dioxide reactions⁴.

Spectrophotometry was used to standardized chlorine dioxide concentrations at 359nm. Ion chromatography, by a Dionex 5000 Reagent-Free ion chromatograph, was used to measure byproducts (chlorite and chlorate) and specified anions. All experiments were batch reactions, conducted in the dark under constant agitation of a stir plate. The influencing factors on the kinetics of chlorine dioxide decay and byproduct formation in the presence of lead minerals will be presented and discussed. Our data will be contrasted with the results of published results on other metals including nickel, copper, and iron.

Aging lead infrastructure will continue to be an issue in the United States. The replacement of distribution systems could take decades and at high environmental, economic, and social costs due to our poor understanding of corrosion and byproduct formation mechanisms. There is a demand for alternative disinfectants that have the capability to protect society from waterborne diseases without polluting our environment with lead and toxic byproducts. This work is intended to provide guidance, on the viability of chlorine dioxide as a disinfectant, to avoid costs and dangers associated with lead corrosion.

References

(1) Gagnon, G. A.; O’Leary, K. C.; Volk, C. J.; Chauret, C.; Stover, L.; Andrews, R. C. J. Environ. Eng. 2004, 130(11), 1269–1279.

(2) Edwards, M.; Triantafyllidou, S. J. / Am. Water Work. Assoc. 2007, 99(7).

(3) Dodrill, D.; Edwards, M. Am. Water Work. Assoc. 1995, No. C, 74–85.

(4) Liu, C.; Von Gunten, U.; Croue, J. P. Environ. Sci. Technol. 2013, 47 (15), 8365–8372.