In treating drinking water, utilities typically disinfect water twice. During primary disinfection, their goal is to kill or inactivate microorganisms present in water arriving from a source, such as a lake or river. At this stage, utilities can use chlorine-based disinfectants as well as techniques such as ozonation or UV disinfection.
Ozonation uses ozone—a molecule containing three oxygen atoms—to kill microbes.
UV disinfection relies on the power of ultra violet light, the kind that is responsible for sunburns! UV rays can also inactivate or kill microorganisms.
However, these forms of disinfection do not remain in the water long enough to offer protection as the water travels through the distribution system and into consumer’s homes. Secondary treatment is designed to prevent organisms from re-growing as the water travels from the treatment plant, through the distribution system pipes, all the way to consumers' homes. Only chlorine-based disinfectants are approved for this purpose.
Chlorine has been widely used as a disinfectant since the early 1900s. Chloramine and chlorine dioxide are two similar alternatives to chlorine.
Keep in mind that all chemical disinfectants cause DBPs. Utilities must balance the need to protect the public from water-borne illnesses while keeping DBPs at safe concentrations. Efforts began in the late 1970s to manage potential health risks associated with DBPs without compromising disinfection.
Utilities can control certain factors that influence the production of DBPs such as the amount of disinfectant used and the amount of organic material or minerals present during disinfection. Other factors such as temperature, pH and reaction time also affect DBP production.