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Chlorine is an inexpensive treatment option used to improve water’s taste and clarity while knocking out many microorganisms like bacteria and viruses. However, the process does have limitations. Giardia and Cryptosporidium are generally resistant to chlorine unless it is used in higher doses than those generally preferred for treatment. The presence of these parasites may necessitate source water pretreatment.

Chlorine also removes substances like manganese, iron, and hydrogen sulfide, which can taint water taste.

Chlorination can be scaled to fit any system size. Chlorine use is also relatively simple, and treatment systems do not require extensive technical expertise.

Chlorination can be accomplished with several different products. Chlorine is stored as liquid in pressurized containers and injected as a gas directly into source water. This process must be carefully regulated and orchestrated, because chlorine gas is a dangerous—even lethal—toxin.

Another, more expensive chlorination option is treatment with a sodium hypochlorite solution. This solution is corrosive but far less dangerous and easier to handle than chlorine gas. The liquid is simply diluted and then mixed with source water to effect disinfection.

Chlorination can also be achieved with a solid disinfectant—calcium hypochlorite. This material is corrosive and can react explosively when it comes into contact with organic materials. However, these powders, granules, and tablets may all be stored in bulk and used effectively for up to a year. In all of its forms, calcium hypochlorite dissolves easily in water.

These chlorination methods all require some time to work—disinfection does not happen instantly. Required doses also change with variations in water quality so that source water monitoring, particularly of surface waters, is an important part of the treatment process.

Chlorine treatment has some residual effects. Among the most noticeable is an unpleasant taste in treated water. But other after-effects may be more significant. Residual amounts of chlorine remain in treated water supplies. This chemical content continues to protect treated water from reinfection, and can be beneficial for water subjected to long periods of storage or time-consuming distribution over large areas.

Unfortunately, too much residual chlorine may also produce chemical byproducts, some of which may be carcinogenic. These health hazards are generally considered minor, however, compared to the effects of pathogens in water left untreated.

It is relatively simple and cheap to manufacture chlorine, and to transport it as sodium—or calcium hypochlorite. It also requires little training to use. These qualities have made it popular as a point-of-use treatment even in impoverished areas despite its limitations in killing parasites. In conjunction with safe storage and water—and food-handling practices, use of chlorination has produced significant drops in diarrheal disease in many locations.


Chloramines are an inexpensive treatment option, but are they not typically suitable as a “primary” disinfectant system. This process effectively treats many bacteria but does less well against other contaminants. Because of their limitations, chloramines are often employed as a secondary disinfectant step to be used with source water that has been previously treated by another method.

Chloramines are valuable as a secondary treatment because they provide long-lasting residual protection. These additives are more stable and longer lasting than those produced by chlorination, and therefore provide outstanding extended protection against bacterial reinfection. This is an important consideration for waters that will be stored for long periods of time or distributed over great distances.

Chloramines are formed when chlorine and ammonia are mixed in water. The process requires skilled operation and significant mixing infrastructure. The two additive substances must be combined in the proper proportions or the process is not as effective.

Typically, however, chloramine treatment is an effective bacteria-killing option which produces less of a residual aftertaste than chlorination.


Chlorine dioxide is effective against Giardia, bacteria, viruses, and, to some extent, Cryptosporidium. It is often combined with other treatment methods, such as chlorination or ozonation, because unlike these other treatments chlorine dioxide is not known to produce carcinogens.

However, the process of creating chlorine dioxide is complicated. It requires skilled technicians and careful monitoring. These technical requirements limit its usefulness for many small systems.

Like chlorine and chloramines, chlorine dioxide is used in distribution systems, but it breaks down over time faster than chlorine.


Ozone (O3) is a powerful oxidizing agent and an effective primary disinfectant.

This oxygen-rich molecule is pumped into water systems to eliminate biological contaminants like bacteria, viruses, Giardia, Cryptosporidium and organic chemicals. It’s also effective for oxidizing and removing iron, sulfur, manganese, and other inorganic substances.

Ozone gas is unstable and quickly reverts back to a normal oxygen molecule (O2) with two atoms instead of three. Because of this condition, it cannot easily be stored or transported. Instead, treatment facilities create ozone on site by forcing dry air through an arrangement of electrodes.

Once ozone has been created, it is forced into contact with the source water and mixed for an appropriate contact time. Because ozone is pure oxygen it produces no residual tastes or odors in water.

Unfortunately, it also provides no lasting residual protection. If water is to be stored for long periods of time, or distributed over long distances, it may be necessary to supplement ozonation with a long-lasting residual treatment like chlorine or chloramines.

Ozonation has been known to produce unwanted byproducts, such as bromate, which may be harmful to human health.

Ozone systems are uncommon in much of the world; but they are infrastructure intensive, and they can be expensive to implement. In addition, the operation and upkeep of these of these systems requires skilled personnel who may not be available in all areas.

Ultraviolet radiation

For many water systems, treatment may be as simple as shining a light on the problem.

Ultraviolet (UV) light, an invisible part of the electromagnetic spectrum, is used to cleanse drinking water of harmful microorganisms. Mercury lamps can replicate the sun’s rays and mimic their natural purification processes.

The UV process is an attractive option in many cases because it is chemical-free and because it requires only a simple and affordable infrastructure investment.

In small-scale systems UV light is typically used where the power supply is dependable, and is not often used to treat surface water sources. Turbid, particle-rich water can create problems for UV rays, which may not be able to achieve the penetration necessary for complete disinfection. This problem is sometimes solved by preceding UV irradiation with filtration, sedimentation or other processes designed to remove waterborne particles before UV light is introduced.

Those considering UV disinfection should also consider its limited protection time. Exposure to UV rays is a one-time process that kills microorganisms—but does not prevent them from returning again. UV irradiation is sometimes supplemented by chemical additives such as chlorine or chloramines to protect the newly disinfected water from becoming contaminated once again.

Alternatively, UV irradiation may simply be used in situations where treated water will be consumed quickly rather than stored for future use. In this respect, UV systems have become popular in-home accessories in regions with reliable electric power supply.

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