In which treatment stage is chlorination commonly applied?

Chlorination is commonly applied during the tertiary treatment stage of sewage treatment. This stage is designed to further improve the quality of wastewater after primary and secondary treatments have been completed. Tertiary treatment aims to remove remaining contaminants, nutrients, and pathogens that may be present in the effluent. Chlorination serves as a disinfection process, effectively killing or inactivating harmful microorganisms, thereby ensuring that the treated water is safe for discharge into the environment or for potential reuse.

In contrast, primary treatment focuses mainly on physical processes to remove large solids and settleable organic matter, while secondary treatment involves biological processes to reduce suspended solids and organic pollutants. Because the goal of primary and secondary treatments is not specifically to disinfect or remove pathogens to the extent that tertiary treatment does, chlorination is not typically applied during these stages. Therefore, it is the tertiary treatment stage where chlorination plays a crucial role in enhancing water quality and ensuring public health and environmental safety.