What is the desirable chlorine compound in water treatment?

The desirable chlorine compound in water treatment is hypochlorous acid, represented as HOCl. This compound is a strong disinfectant and is highly effective in killing bacteria, viruses, and other pathogens in water.

HOCl is formed when chlorine gas (Cl2) is dissolved in water, making it the most active form of chlorine in disinfection processes. Its efficacy comes from its ability to penetrate microbial cell walls, leading to the destruction of harmful organisms.

Other chlorine compounds listed, like sodium chloride (NaCl), while they do have a role in water treatment, do not function directly as disinfectants. Chlorine gas (Cl2), while effective, is a hazardous material that requires careful handling and is less desirable for direct use in many treatment scenarios due to safety concerns. Calcium hypochlorite (Ca(OCl)2) is another form of chlorine used in water treatment, and while it generates HOCl when dissolved in water, it is not as effective as having the HOCl directly present, particularly in achieving rapid disinfection.

Thus, hypochlorous acid is the preferred compound for its high disinfection capability and safety in practice when used in appropriate concentrations in treated water.