Chlorine Disinfection for Drinking Water

10/09/25

“How can we use chlorine to make water safe to drink and swim?”

All sorts of pathogens harmful to human health reside in water. We need to disinfect water before we drink it. But what chemical should we use? Well, it turns out that chlorine is an incredibly useful tool. We can add chlorine in the form of chlorine gas (Cl2), sodium hypochlorite (NaOCl), and calcium hypochlorite (Ca(OCL)2) to water. The chlorine will then react with the water to form hypochlorous acid (HOCL) and hypochlorous acid (HCL). Hypochlorous acid may then dissociate into hydrogen ions (H+) and Hypochlorite ion (OCl-). The dissociation process is pH level and temperature dependent, and the HOCL and OCL- amounts are equal at pH levels of 7.5 and 20 degrees Celsius. Below a pH of 7.5, HOCL is dominant, and OCL- is dominant above 7.5. HOCL and OCL disinfect water by breaking down the cell walls and internal components of pathogens. HOCL is 80 times more effective at destroying E.coli than OCL- is. In Chlorine Disinfection for Drinking Water, there are at least some chlorine residuals left after the process ends. The amount of time and residual concentration the water is in contact with the chlorine is incredibly important for the disinfection process. Chlorine disinfection is used at a wide range of scales, from swimming pools to gargantuan treatment plants.