Chlorination

Chlorine is mainly used in the disinfection process in the following forms :

  • chlorine gas Cl2 stored under pressure in liquid phase (bottles and tanks),
  • (liquid) sodium hypochlorite NaOCI,
  • (solid) calcium hypochlorite Ca (CIO) 2, 2H20.

The first two forms - chlorine gas and sodium hypochlorite - are used extensively in the water treatment industry.

Using calcium hypochlorite can only be justified in certain countries where it is difficult to obtain chlorine gas or sodium hypochlorite for practical or economic reasons (availability of reagents, transport).

In all three cases it is the same disinfectant agent that results from the reagents coming into contact with the water that is to be treated. It is HClO - hypochlorous acid. Chlorine can also be used in the form of chlorine dioxide(ClO2) and chloramine.

Chlorine gas

At atmospheric pressure chlorine is a yellow greenish gas 2.5 times denser than air and has a characteristic unpleasant odour.

It is stored in liquid form in 50 kg bottles or in 1000 kg tanks under a pressure of around 5 bars.

Chlorine gas is dissolved in the service water using a hydro ejector with an average concentration of 1 to 2 g/l.

The resulting solution is chlorinated water or "chlorine water" which will be used as a disinfectant.

Sodium hypochlorite


Similar to chlorine, sodium hypochlorite is added in the correct proportions following a preliminary dissolving process in the service water, resulting in the creation of hypochlorous acid.

This reaction is accompanied by an increase in pH levels. This may lead to scaling if the dilution water is hard. "47/50" industrial sodium hypochlorite titrates 47-50 chlorometric degrees, i.e. around 150 g of active chlorine per liter (1 chlorometric degree being equivalent to 3.17 g of active chlorine).

Whatever the reagent used, the resulting hypochlorous acid is a weak acid that partially dissociates in water in equilibrium with the hypochlorite ion.

Although hypochlorite possesses germicidal properties, the real disinfecting agent is the hypochlorous acid. One of the reasons for this is that the bacterial membranes are negatively charged lipid environments in which ions are not soluble (CIO-), whilst the chemical structure of the hypochlorous acid enables it to penetrate the cytoplasmic membranes of the micro-organisms.

Hypochlorous acid is often referred to by the term "free available chlorine" as opposed to a reserve of free chlorine in the form of hypochlorite CIO-, and combined chlorine consisting of chloramines (mono, dichlor and trichloramines which are a result of the effect of chlorine on ammonia and organic chloramines formed from the effect of chlorine on the amine functions of certain organic molecules).

  • total chlorine = CI2 + HCIO + CIO-. + chlorine from chloramines
  • free chlorine = CI2 + HCIO + CIO-.
  • combined chlorine = chlorine from chloramines