Optimizing Drinking Water Disinfection: Balancing Corrosion, Byproduct Formation, and Pathogen Removal

Abstract

The oxidants used in water treatment to inactivate pathogens are powerful and, consequently, react with other constituents they encounter, notably organic matter and pipe corrosion scale. Moreover, the complex relationships between said reactions remains poorly understood. Reactions with organic matter produce disinfection byproducts, many of which are regulated by the United States Environmental Protection Agency (EPA) due to their toxicity. To remove these byproducts and meet EPA standards, water treatment facilities add chemicals that can exacerbate corrosion and increase the concentration of dissolved metals in drinking water.

Chlorine dioxide, the focus of this dissertation, has been used as an alternative to free chlorine, the most commonly used disinfectant, because it does not produce organic disinfection byproducts. Additionally, chlorine dioxide has a disinfecting power equal to or higher than that of free chlorine, its disinfection capabilities are independent of pH, and it can be used as either a primary or secondary disinfectant. From a corrosion standpoint, chlorine dioxide has a high oxidation-reduction potential, which promotes the formation of passivating scale layers on metal pipe surfaces, thereby preventing dissolution of heavy metals into drinking water. Chlorine dioxide does, however, produce two toxic inorganic byproducts, chlorite and chlorate.

Despite the drawbacks associated with inorganic byproduct formation, chlorine dioxide is a disinfectant worthy of investigation with regards to three reactions: pathogen disinfection mechanisms; drinking water pipe corrosion; and formation of inorganic byproducts. The first part of this dissertation addresses the inactivation of the H1N1 influenza virus using computational models. Both computational and experimental methods identified tryptophan 153, an amino acid residue key in the binding of H1N1 to its human host cell, as the primary target of chlorine dioxide oxidation.

Part two of this work shows results from batch reactor experiments of chlorine dioxide with lead and copper minerals commonly found in corrosion scale layers. Decay of chlorine dioxide in the presence of lead oxide and lead carbonate was significantly faster and produced different byproducts than decay in the presence of cupric oxide. It was further revealed that the relationship between pH and reaction rate is likely dependent upon surface charge for lead oxide but not for cupric oxide.

These findings were the impetus for the third and final part of this dissertation which employed computational methods to model the subtle differences between surface adsorption on cupric oxide and lead oxide, of either the chlorine dioxide monomer or dimer, in the presence or absence of hydroxide. The results of the calculations suggest that the chlorine dioxide degradation pathway on the cupric oxide surface favors dimerization of chlorine dioxide and its ensuing disproportionation into chlorite and chlorate, whereas the lead oxide surface favors direct electron transfer and formation of chlorite.

These findings add to the body of knowledge on the alternative disinfectant, chlorine dioxide, and its chemical interactions with pathogens and corrosion scale. The results suggest that chlorine dioxide may have highly specific mechanisms of virus inactivation and computational methods could be valuable tools for elucidating these mechanisms. Further conclusions suggest that chlorine dioxide decay caused by mineral scales in lead-containing water supply networks may be more pronounced than in those assembled from copper pipes.