Which of the following is not a chlorine compound used in water treatment?

Hydrogen peroxide is the correct choice as it is not a chlorine compound used in water treatment. Unlike sodium hypochlorite, calcium hypochlorite, and gas chlorine, which all contain chlorine and are employed for their powerful disinfectant properties in water treatment processes, hydrogen peroxide operates differently. It serves as an oxidizer and can be used for disinfection; however, it does not release chlorine in the water and therefore does not fit the definition of a chlorine compound.

In contrast, sodium hypochlorite and calcium hypochlorite are both chlorine sources that contribute to the chlorination process to eliminate bacteria and other pathogens. Gas chlorine is pure chlorine gas that is also utilized for disinfection purposes in various water treatment systems.