5.2.The disinfection mode of action

The inactivation of microbiological contamination in natural or untreated water through chemical inactivation, is one of the final stages for reduction of pathogenic microflora in drinking water. Combinations of water purification steps (oxidation, coagulation, settling, disinfection, and filtration) make (drinking) water safe after production. In order to protect the water from microbiological contamination, at the end of the purification process, many countries apply an extra measure. This is a second disinfection step in order to protect the water from microbiological contamination in the water distribution system. During this secondary disinfection process, the kind of disinfectant that is usually used is different than the one used at the early stage of the process. This type of disinfection ensures that microorganisms will not multiply in the water during the distribution in the system. They can remain in the water after the first disinfection step or can end up in the water during back flushing of contaminated water (which can contain groundwater microorganisms due to cracks in the plumbing).

Disinfection commonly takes place because of disruption effect on microbial cell wall, or caused changes in cell permeability, protoplasm or enzyme activity (because of a structural change in enzymes). All these disturbances in cell activity cause reduction or termination of propagation of microorganisms and their elimination from the system. Oxidizing disinfectants also demolish organic matter in the water, causing lack of nutrients.

There are several different disinfectants, which either kill or deactivate pathogenic microorganisms. Examples of disinfectants are chlorine containing substances, peroxide, bromine, silver-copper, ozone, and UV. All disinfectants have benefits and drawbacks and can be used for water disinfection depending on the circumstances.

Besides drinking water disinfection, it may also be applied in swimming pools and cooling towers. Water disinfection is a very important factor for these applications.

Swimming pools contain a large variety of contamination, originating mainly from swimmers. The contamination contains microorganisms, among other things.

To prevent swimmers from getting infected by pathogenic microorganisms, swimming water must be disinfected. Swimming pool water is often circulated. Before the water is returned to the swimming pool, it is purified. The purification includes disinfection.

Cooling towers are used to cool down process water. After that the water can be reused. Within cooling towers circumstances are ideal for growth and multiplication of microorganisms. Biofilm development is a major problem in cooling towers, because this promotes corrosion and blocks the system.

Another problem in cooling towers, as well as in ventilation systems, is the development of Legionella bacteria. These bacteria spread through aerosols and can cause Legionnaires' disease – a very serious disease that resembles pneumonia. Many countries now have legal standards, determining that the development of Legionella bacteria in cooling towers should be prevented by disinfection of cooling water.

In the early 1970s, it was found that chemical disinfectants can form specific by-products. When this was discovered, research started on health effects of these by-products. Today, there are legal standards indicating maximum levels of disinfection by-products in drinking water. Methods to lower the concentration of disinfection byproducts in drinking water have also been researched.