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Disinfection with chlorine

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Harmful bacteria can be killed by

• the addition of chlorine gas or alternatively
• by bubbling ozonated air throught the water or
• by shining ultra violet light on the water.

Chlorine gas reacts with water to form hydrochloric acid (HCl) and hypochlorous acid (HClO) also known as chloric(I) acid. Both these acids are strong and therefore fully dissociated in solution.

Cl₂(aq) + H₂O(l)HCl(aq) + HClO(aq)   or

Cl₂(aq) + H₂O(l)2H⁺(aq) + Cl^-(aq) + ClO^-(aq)

The hypochlorite ion or chlorate(I) ion (ClO^-) is an oxidising agent and is able to oxidise bacteria thus killing them.

ClO^-(aq) + bacteriaCl^-(aq) + CO₂ and H₂O etc.

Ozonated air functions in a similar way to chlorine resulting in the oxidation and destruction of bacteria, however, it has the additional advantage that the disinfected water is better tasting than that produced by chlorination. A dose as low a 1 part per million can kill all the bacteria present in the water in as little as 10 minutes.

The ozone is usually generated on site by an electric-arc discharge in air.