14.6 Chlorination
Chlorination, used at both household and large-scale levels, is one of the most effective and widely used methods for disinfecting water and making it safe to drink. Whatever the level, it is important that the correct quantity of chlorine is added to remove all impurities.
Which of the water treatment methods described earlier for household use involve chlorination?
Three of the four chemical disinfection methods described are chlorination methods. Chlorine solution, PUR and Wuha Agar all use chlorine as the disinfecting agent.
At municipal level, various terms are used to describe aspects of the chlorination process. Chlorine dosage is the amount of chlorine added to the water system in milligrams per litre (mg/l). Chlorine demand is the amount of chlorine that combines with the impurities and therefore is no longer available as a disinfecting agent. The chlorine that remains in the water after the chlorine demand has been satisfied is called free chlorine residual. A certain amount of residual chlorine is a good idea because it protects against future recontamination.
The orthotolidine-arsenite test (OTA) is used to determine the amount of free chlorine residual. When orthotolidine reagent is added to water containing chlorine, a greenish-yellow colour will appear. The intensity of the colour is measured against a chart to determine the amount of free available residual chlorine in the water. The amount of residual chlorine needs to be in the range of 0.2–0.5 mg/l if it is to prevent recontamination with bacteria. The OTA test requires a special test kit. If required, this should be available from your district environmental health office.
The benefits of point-of-use chlorination include:
- Chlorine is proven to be effective in the reduction of bacteria and most viruses.
- The residual chlorine is effective in protection against recontamination.
- It is easy to use.
- Chlorine is easily available at low cost.
The drawbacks of chlorine treatment include:
- It provides relatively low protection against some viruses and parasites.
- Lower effectiveness in water contaminated with organic and certain inorganic compounds.
- Potential objections to taste and odour.
- Some people have concerns about the potential long-term carcinogenic effects of chlorination byproducts.
14.5 Large-scale water treatment