Water disinfection means the removal, deactivation or killing of pathogenic microorganisms. Microorganisms are destroyed or deactivated, resulting in termination of growth and reproduction. When microorganisms are not removed from drinking water, drinking water usage will cause people to fall sick.

The table below shows some common, water-transmitted diseases as well as the organisms (pathogens) which cause each disease.

The primary goal of disinfection is to ensure that the water is safe to drink and does not contain any disease-causing microorganisms.  The best way to ensure pathogen-free drinking water is to make sure that the pathogens never enter the water in the first place.  However, this may be a difficult matter in a surface water supply which is fed by a large watershed.  Most treatments plants choose to remove or kill pathogens in water rather than to ensure that the entire watershed is free of pathogens.
Pathogens can be removed from water through physical or chemical processes.  Sedimentation and filtration, may remove a large percentage of bacteria and other microorganisms from the water by physical means.

Disinfection is different from sterilization, which is the complete destruction of all organisms found in water and which is usually expensive and unnecessary.  Disinfection is a required part of the water treatment process while sterilization is not.

The goal of disinfection is to remove or inactivate all disease-causing organisms in water.  However, testing for each type of pathogen individually would be costly and inefficient. So total coliform is the most frequently used indicator of disinfection efficiency.

Coliform bacteria are often found in the guts of warm-blooded animals such as humans, but can also be found in plants, soil, water, or air.  It is relatively simple to test for the number of Coliform bacteria found in water, and their presence indicates that other pathogenic bacteria are also likely to be present.  If disinfection removes all of the coliforms from the water, then it can be safely assumed that the other disease-causing microorganisms have also been removed.

For disinfection of water the following disinfectants/methods are normally used:

– Chlorine (Cl₂)

– Hypo

– Ozone (O₃)

– Ultra Violet (UV) Light.   

Chlorination (Gas)

Chlorine is very effective for removing almost all microbial pathogens and is appropriate as both a primary and secondary disinfectant.

A basic system consists of a chlorine cylinder, cylinder-mounted chlorine gas vacuum regulator, a chlorine gas injector, and a contact tank or pipe.

Chlorination (Sodium hypochlorite solution)

Sodium hypochlorite is available as a solution in concentrations of 5 to 15 percent chlorine, but is more expensive than chlorine gas (as available chlorine).

Sodium hypochlorite is easier to handle than gaseous chlorine or calcium hypochlorite.

A basic liquid chlorination system, or hypo chlorinator, includes two metering pumps (one serving as a standby), a solution tank, a diffuser (to inject the solution into the water), and tubing.

Ozonation

Ozone, an allotrope of oxygen having 3 atoms to each molecule, is a powerful oxidizing and disinfecting agent. It is formed by passing dry air through a system of high voltage electrodes.

Requiring shorter contact time and dosage than chlorine, ozone is widely used as a primary disinfectant in many parts of the world. Ozone does not directly produce halogenated organic materials unless a bromide ion is present.

Ozone gas is unstable and must be generated onsite. A secondary disinfectant, usually chlorine, is required because ozone does not maintain an adequate residual in water.

Ozonation equipment includes air preparation equipment; an ozone generator, contactor, destruction unit; and instrumentation and controls. The capital costs of ozonation systems are relatively high.

Ultraviolet Light (UV)

Ultraviolet (UV) radiation is generated by a special lamp. When it penetrates the cell wall of an organism, the cell’s genetic material is disrupted and the cell is unable to reproduce.

UV radiation effectively destroys bacteria and viruses. As with ozone, a secondary disinfectant must be used to prevent re growth of microorganisms.

UV radiation is be attractive as a primary disinfectant for small systems

UV radiation may not inactivate Giardia lamblia or Cryptosporidium cysts, and should be used only by groundwater systems not directly influenced by surface water—where there is virtually no risk of protozoan cyst contamination. UV radiation is unsuitable for water with high levels of suspended solids, turbidity, color, or soluble organic matter. These materials can react with or absorb the UV radiation, reducing the disinfection performance.

The basic equipment consists of UV lamps and a reactor.