Chlorine, the most widely used disinfectant for municipal wastewater, destroys target organisms by oxidation of cellular material. It may be applied as chlorine gas, hypochlorite solutions, and other chlorine compounds in solid or liquid form.

Chlorination is a well-established technology. Presently, chlorine is more cost-effective than either UV or ozone disinfection (except when dechlorination is required and fire code requirements must be met). The chlorine residual that remains in the wastewater effluent can prolong disinfection even after initial treatment and can be measured to evaluate the effectiveness. Chlorine disinfection is also reliable and effective against a wide spectrum of pathogenic organisms and is effective in oxidizing certain organic and inorganic compounds including certain noxious odors.

But is has to be considered that the chlorine residual, even at low concentrations, is toxic to aquatic life and may require dechlorination. All forms of chlorine are highly corrosive and toxic. Thus, storage, shipping, and handling pose a risk, requiring increased safety regulations. In some cases Chlorine oxidizes organic matter in wastewater, creating more hazardous compounds. In all cases the level of total dissolved solids is increased in the treated effluent. Another problem in developing counries can be that some parasitic species have shown resistance to low doses of chlorine ans the long-term effects of discharging dechlorinated compounds into the Environment are unknown.

Table 4: Chlorine dosage ranges according to wastewater type (Rowe & Abdel-Magib, 1995)