Drinking Water Treatability Database

Local Navigation


Figure 1: Chlorine application with conventional treatment.
Figure 1: Chlorine application with conventional treatment.

Chlorine is a strong oxidant commonly used in water treatment for oxidation and disinfection. As an oxidant, chlorine is applied to control biological growth and to remove color, taste and odor compounds, iron and manganese, and other dissolved inorganic contaminants such as arsenic. As a primary disinfectant, chlorine is applied to disinfect and to control microbial activity in the distribution system. It is also used as a secondary disinfectant after chlorine, ozone, UV irradiation, or chlorine dioxide. Chlorine is commonly applied at one or two points during treatment. Downstream residual chlorine concentrations make chlorination concurrent with other treatment processes. Figure 1 shows multiple possible chlorine (Cl2) application points during conventional treatment. Chlorine residuals are common during filtration to inhibit microbial (biofilm) growth on filter media that could increase filter head loss (pressure) build up.

Chlorine is available as compressed elemental gas, sodium hypochlorite solution (NaOCl) or solid calcium hypochlorite (Ca(OCl)2). All forms of chlorine, when applied to water, form hypochlorous acid (HOCl). Gaseous chlorine acidifies the water and reduces the alkalinity, whereas the liquid and solid forms of chlorine increase the pH and the alkalinity at the application point. The pH of the water will affect the dominating chlorine species such that HOCl dominates at lower pH, while the hypochlorite ion (OCl-) dominates at higher pH. Of the two species, HOCl is the stronger oxidant. Therefore, chlorine is more effective as an oxidant and a disinfectant at lower pH. Both forms, HOCl and OCl-, are referred to as free chlorine.

The concentration (C), contact time (T), pH and temperature affect the effectiveness of chlorine application. The product of concentration and time (CT) is the most important operational parameter in disinfection and inactivation. Although increasing the dose increases the ability of chlorine to oxidize and disinfect, it may also lead to taste and odor issues and to the formation of disinfection byproducts (DBPs) by chlorine's reaction with natural organic matter (NOM). The dose is also affected by the application point, chlorine demand of the water, and desired residual concentration. Total organic carbon (TOC) and ultraviolet absorbance (UV) are two measures of DBP-reactive NOM and of chlorine demand.