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Biological Contaminant Removal

The purpose of treating water is to provide drinking water safe for consumption. Disinfection is a process used to reduce biological contamination in water. The Surface Water Treatment Rule requires that public water systems (PWSs) filter and disinfects surface water sources to reduce the occurrence of waterborne diseases. In addition, the Ground Water Rule requires PWSs to disinfect their well water supplies.

PWSs must maintain a residual level of disinfectant in the distribution system. A disinfectant residual will limit the growth of organisms within the system and may afford some protection against contamination from without. The disappearance of the residual provides an indication of a problem in the treatment process.


Chlorination is the most common disinfection method. It is a popular choice because of its residual disinfection capabilities. Plus its effectiveness is very simple to test. There are a number of methods of delivery and chemical reactions used for chlorination, including:

  • chlorine gas,
  • chloramines,
  • chlorine dioxide, and
  • sodium hypochlorite.

The goal of all these methods is to release free chlorine in the form of hypochlorite or in the case of chloramines combined available chlorine. Selection of the type of chlorination system should include consideration of:

  • capital and operating costs,
  • operation and maintenance requirements,
  • code restrictions,
  • containment requirements, and
  • safety concerns.

Note: The use of chlorine gas for chlorination is more hazardous, frequently more expensive, and less used by small systems.

Chlorine reduces bacteria levels, but it also reacts with other organic impurities present in water. It produces various Disinfection By-Products (DBPs) that are listed as probable carcinogens. Other disadvantages of chlorination are:

  • undesirable tastes and odors,
  • requirement of additional equipment (such as tanks) to guarantee proper contact time,
  • extra time to monitor and ensure proper residual concentration level, and
  • performs poorly in reducing viruses (such as hepatitis A) and protozoa (such as Cryptosporidia and Giardia).

Small system operators continue to disinfect water using common household liquid bleach or swimming pool chlorine. Sometimes small drinking water systems use solid calcium hypochlorite, which is sold as a dry solid or in the form of tablets for use in proprietary dispensers. This method of disinfection is expensive and suitable mainly for low-flow applications. Plus, the use of calcium can lead to scale formation.

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Ultraviolet Light

Contaminated water is exposed to UV light, which penetrates the cell walls of an organism. UV disrupts the organism's genetic material, which impairs the organism. A special lamp generates the radiation that creates UV light by striking an electric arc through low-pressure mercury vapor. Research shows that the optimum UV wavelength range to destroy bacteria is between 250 nm and 270 nm.

The use of UV light for water disinfection has been a proven process for many years. The advantages of this process are that it does not use any chemicals. It is effective for impairing Cryptosporidium, a microscopic parasite that causes intestinal infection characterized by diarrhea. However, UV does not provide residual disinfection.

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Ozone is a colorless, very unstable gas with a very short life. It is effective in removing bacteria relatively quickly. Due to the gas’ instability and short life, ozone generators are used on site. The generating equipment is costly. Therefore, ozonation is costly compared to other disinfection technologies. Ozone reacts with bromide resulting in the formation of highly carcinogenic DBPs. In PWSs, UV equipment or biological filters are typically installed to remove ozone residuals prior to filtration.

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Ultraviolet Light and Ozonation (UV/O3) – An Advanced Oxidation Process for Disinfection and Destruction

EPA has focused on the smallest systems (population 25–500) and on technologies that are easy to operate and maintain. Package plants are alternative treatment systems and technologies packaged together. They are perceived as "high tech" and are sometimes more expensive. However, alternative technologies are easier to operate, monitor, service, and less expensive to maintain and service in the long term.

EPA evaluated a packaged UV/O3 system for removal of microorganisms. It was an advanced oxidation process (AOP), which destroys organic and microbial contaminants in drinking water. The UV/O3 system achieved the highest removal rates for bacterial contamination. The UV/O3 disinfection technology is also useful in removing chemical organic contaminants, such as synthetic fuels.

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