1. Significance and Chemistry
Oxidants are added to water supplies and wastewater primarily for disinfection. Other beneficial uses include slime removal, oxidation of undesirable inorganic species (e.g., ferrous ion, reduced manganese, sulfide, and ammonia) and oxidation of organic constituents (e.g., taste- and odor-producing compounds). Oxidant demand is the difference between the added oxidant dose and the residual oxidant concentration measured after a prescribed contact time at a given pH and temperature. Oxidant requirement is the oxidant dose required to achieve a given oxidant residual at a prescribed contact time, pH, and temperature.
The fate of oxidants in water and wastewater is complex. For example, chlorine reacts with sample constituents by three general pathways: oxidation, addition, and substitution. First, chlorine can oxidize reduced species, such as Fe2+, Mn2+, and sulfide. In these reactions, chlorine is reduced to inorganic chloride (Cl−). Second, chlorine can add to olefins and other double-bond-containing organic compounds to produce chlorinated organic compounds. Third, chlorine can substitute onto chemical substrates. The addition and substitution reactions produce organochlorine species (e.g., chlorination of phenol to chlorophenols) or active chlorine species (e.g., chlorination of ammonia to produce monochloramine). Chlorine reacts with naturally occurring organic compounds by a combination of these mechanisms to generate such products as trihalomethanes. For more information, see Sections 4500-Cl (chlorine), 4500-ClO2 (chlorine dioxide), and 4500-O3 (ozone).
Oxidant demand and oxidant requirement are significantly affected by the sample’s chemical and physical characteristics and the manner in which oxidant consumption is measured. In particular, oxidant reactivity is influenced by temperature, pH, contact time, and oxidant dose. Oxidant demand and oxidant requirement are defined operationally by the analytical method used to determine the residual oxidant concentration. Report sample temperature, pH, contact time, oxidant dose, and analytical method with oxidant demand or oxidant requirement. Sample temperature strongly affects reaction kinetics and thus the demand exerted in a given contact time. Sample pH affects the form of the oxidant and the nature and extent of the demand. For example, ozone is unstable at high pH values, and ozone demand is especially sensitive to sample pH. Oxidant demand increases with time; the demand must be defined for a given contact time. Oxidant demand also depends on oxidant dose. Increasing oxidant dose usually increases demand, but it is incorrect to assume that doubling the oxidant dose doubles the oxidant demand. For these reasons, it is difficult to extrapolate oxidant demand data from one set of conditions to another. Always study oxidant consumption under the range of conditions expected in the field.
Oxidant consumption is used to evaluate oxidant demand and oxidant requirement. Report consumption values according to the study’s objective. For example, report chlorine demand as follows: “The sample dosed at 5.0 mg/L consumed 3.9 mg/L after 24 h at 20 °C and pH 7.1, as measured by amperometric titration.” By contrast, report ozone requirement as follows: “The sample required a dose of 2.1 mg/L to achieve an ozone residual of 0.5 mg/L after 20 min at 15 °C and pH 6.5, as measured by the indigo method.”