Generally, when groundwater is used as drinking water, sodium hypochlorite is often used as a disinfectant. It may also be used as an oxidant to remove iron　and manganese contained in water. In addition to sodium hypochlorite, bactericides that can be used in tap water include chlorine-based oxidants such as highly bleached powder　and chlorinated isocyanuric acid, potassium permanganate, ozone, ultraviolet rays,　and chlorine dioxide, but they are inexpensive. Sodium hypochlorite is used in general water treatment because it is easy to handle.

Sodium hypochlorite reacts as shown below when dissolved in water to generate effective chlorine.

NaClO　＋　H₂O　↔　HClO　+　NaOH

Effective chlorine refers to chlorine that has bactericidal　and oxidizing powers. Most of the inorganic chlorides such as sodium chloride, which become chloride ions when dissolved in water,　and the organic chlorine compounds combined with organic compounds do not have bactericidal　or oxidizing powers, so they are not called effective chlorine.

Residual chlorine is the active chlorine that remains even after the active chlorine causes bactericidal　and oxidative effects in water. Residual chlorine includes free residual chlorine　and combined residual chlorine. Of these, hypochlorous acid　and hypochlorite ions are called free residual chlorine,　and chloramines produced by reacting with ammonia are called combined residual chlorine.

There are no restrictions on residual chlorine in tap water quality standards, but the Waterworks Law Enforcement Regulations require that free residual chlorine: 0.1 mg / L　or more be detected at the end of the faucet.
A typical example of using it as an oxidizing agent is when it is insolubilized　and removed by oxidizing metal ions such as iron ions　and manganese ions contained in groundwater to generate hydroxides.