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Chlorination is a chemical disinfection process. Chlorine is added to the water to be treated as either gaseous chlorine, or more commonly in emergency/development programmes as a solution made from some form of hypochlorite, either sodium or calcium. When added to the water the chlorine forms hydochlorous acid and chloride ions. In fact a succession of equilibrium reactions occur. In addition to forming acid and ions the chlorine will react with any metal ions present in the water and any nitogen based compounds in the water such as ammonia, to form various chloramine compounds. The chloramines are known as combined chlorine, whereas the acid and chloride ions are known as free chlorine.

As the chlorine is added to the water, the residual chlorine concentration will decrease with dose as the reactions that form chloramines occur. Once these reactions have occurred the concentration of free chlorine will increase with the dose. The point at which the chlorine residual concentration begins to increase in proportion to the dose added is the break point.

Due to the complexity of the chemical equilibria it is not possible to calculate the dose of chlorine required for the break point to be reached from first principles. This must be determined empirically through the use of jar tests. In jar tests fixed masses of chlorine are added sequentially to known volumes of raw water, and the chlorine demand is determined by constructing a dose-residual curve.

Chlorine residual is measured by clorimetry, ion sensitive electrodes, and colour comparators. In most field settings simple colour comparators are used. DPD 1 tablets are used to measure the free chlorine. While DPD 2 & 3 tablets are used to measure combined chlorine as chloramines.

For chlorination to be an effective disinfectant the raw water must have a maximum turbidity of <5NTU. The water must also be free from organics, as the chlorine will react with these to form trihalomethanes (THM’s). The formation of THM’s imparts a strong chlorine taste and odour to the water, which results in it being unpalatable. THM’s are thought to be carcinogens, and chronic exposure has been linked with bowel cancer. The EU has set maximum acceptable concentration limits for THM’s. Hence unless the raw water source is a protected groundwater source, some form of pre-treatment will generally be required for effective chlorination, generally involving filtration, coagulation and flocculation or both.

Contact time required for complete disinfection depends on temperature and the pH of the water. It is normally assumed that a 0.5mg/l free residual chlorine after 30 minute contact time indicates complete disinfection. It is important to ensure full mixing of the hypochlorite solution with the raw water, so the solution should be added under turbulent mixing conditions, for example at the inlet to a tank.

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