Breakpoint Chlorination in Water Treatment Process

Breakpoint Chlorination in Water Treatment Process

In this article, we are going to explain about Breakpoint Chlorination. Where water already contains ammonia the production of chloramine is unavoidable when chlorine is added. To ensure the production of free chlorine to enhance bacterial kill, substantially more chlorine may have to be added because the additional chlorine at first only causes a reduction of the chloramines by oxidation.

Only when this reaction is completed does the addition of further chlorine produce free chlorine. Stoichiometrically the breakdown of ammonia to nitrogen commences at a chlorine:ammonia (as N) ratio of 5:1 and completes at a ratio of 7.6:1.* In practice, the ratio for complete breakdown could be as much as 10:1 and is pH-dependent. The point at which the free chlorine begins to form is called the ‘breakpoint’ for the water, and adding enough chlorine to exceed this is called ‘breakpoint chlorination’.

In laboratory experiments, observed that for neutral to slightly alkaline pH when the ratio of chlorine to ammonia (as N) is less than 5:1 (by weight) the residual was mainly NH2C1; breakpoint occurred at 9.5:1 for pH 6; between 8.2:1 and 8.4:1 for pH 7 to 8; and 8.5:1 for pH 9. As the ratio increased to 10:1 and above there was a decrease in combined chlorine accompanied by increases in NC13 and free C12.

Apart from the advantage of producing free chlorine, breakpoint chlorination can sometimes reduce taste and odour problems. The foregoing reactions are complex, being dependent on numerous factors such as temperature, pH and contact time.

The breakpoint reaction could take about 20 minutes to complete and depends on the water quality. In some waters the ammonia content may be so high (0.5-1.0 mg/1) that the amount of chlorine to be applied to achieve breakpoint is uneconomic and other means to reduce the ammonia first should be adopted. This is called Breakpoint Chlorination in water treatment process.