Disinfection is an important step in ensuring that either water or wastewater are safe. Systems add disinfectants to destroy microorganisms that can cause disease in humans. Public water treatment systems are required to disinfect water obtained from different sources, surface or groundwater.

While Primary disinfection is designed to achieve the desired level of microorganism kill and inactivation; Secondary disinfection maintains a disinfectant residual in the finished water, preventing the regrowth of microorganisms.

Excellent results are obtained when disinfection methods are combined with conventional treatment, such as coagulation, flocculation, sedimentation and filtration.   Sometimes, insoluble oxides form when chlorine, chlorine dioxide or ozone are added. Both ozonization and chlorination may cause flocculation of dissolved organics, thus increasing turbidity.