The Chlorine Residual: A Public Health Safeguard

Chlorine and chlorine-based disinfectants are used worldwide to destroy germs in drinking water and swimming pools. One of the reasons for the widespread use of chlorine disinfectants is that they provide a “residual” level of protection against waterborne pathogens. A chlorine residual is a low level of chlorine remaining in water after its initial application. It constitutes an important safeguard against the risk of subsequent microbial contamination after treatment—a unique and significant benefit for public healthⁱ.

What happens to chlorine when it is added to drinking water or swimming pools?  According to the US Centers for Disease Control & Prevention’s Chlorine Residual Testing Fact Sheet, chlorine proceeds through a series of reactions:

1. The Chlorine Demand is Satisfied:  Upon initial dosing, chlorine reacts with any organic matter in water.  The amount of chlorine used in these reactions is known as the “chlorine demand” of the water.  Raw water taken from lakes and streams for drinking water treatment is likely to have a high chlorine demand based on the presence of natural organic material, e.g., decaying plant and animal matter. Groundwater, which normally contains far lower levels of organic matter, has a low chlorine demand.
2. Combined Chlorine Forms:  When the chlorine demand of the water is satisfied, some portion of the remaining chlorine reacts with nitrogen in the water to form compounds known as chloramines.  Nitrogen-containing compounds may result from decaying organic matter in raw water secured for drinking water treatment.  In swimming pools, swimmers add nitrogen-containing compounds to the water in the form of substances such as urine and perspiration. Chloramines may impart a chemical odor to water, which is sometimes inaccurately described as a “chlorine” odor.  The chlorine that combines chemically with nitrogen and nitrogen-containing compounds is known as “combined chlorine.”
3. Free Chlorine Destroys Germs:  Chlorine remaining in water after the chlorine demand is satisfied and combined chlorine is formed is known as “free chlorine.”  This is the chlorine portion available for disinfection.  Many waterborne germs are either killed or rendered incapable of reproducing, helping to prevent waterborne disease outbreaks. The time required to destroy viruses, bacteria and parasites present in raw water at a given chlorine concentration varies with the organism and is known as the “contact time.”
4. A Chlorine Residual Remains:  Following a given contact time during which chlorine destroys germs, some chlorine remains in the water.  This remaining, or residual level, acts as a safeguard against additional microbial contamination that, in the case of swimming pools, for example, could be introduced as more swimmers enter the pool. Chlorine and bromine are unique in their ability to impart this kind of protection. EPA requires all US facilities that treat water to maintain a chlorine residual of no more than 4 parts per million, whether chlorine is used as a primary disinfectant or not.  Swimming pool operators generally maintain a chlorine residual of 1 to 3 parts per million.  Swimming pools that are treated primarily with metal ions, such as copper, require a low level of chlorine to provide residual protection.

A Safety Marker Too

By monitoring the chlorine residual throughout a drinking water distribution system, water treatment operators can quickly identify points at which the residual declines or disappears. A sudden decline in the chlorine residual could indicate a leak in the drinking water distribution system. Swimming pool operators monitor the chlorine residual regularly. As the number of swimmers and conditions in the pool varies, the disinfectant level can be adjusted to maintain the chlorine residual.

In both drinking water and swimming pools, the chlorine residual represents a smart use of chemistry and provides a remarkable public health safeguard!

Chris Wiant, M.P.H., Ph.D., is president and CEO of the Caring for Colorado Foundation. He is also chair of the Water Quality & Health Council.

ⁱOne other disinfectant—bromine–provides a residual level of protection in water. Bromine is used in swimming pools, but more frequently in spas. It is not used in drinking water disinfection.