Method of treating water with chlorine dioxide

1. Field of the Invention

The invention relates to a method of treating water with chlorine dioxide (ClO₂).

2. Description of the Related Art

Chlorine dioxide is used in water treatment because of its high bactericidal, virucidal and algicidal activity. Due to the explosive tendency of gaseous chlorine dioxide (c>300 g/m³) and chlorine dioxide solutions (c>26 g/l), chlorine dioxide cannot be stored in compressed form or in solutions of relatively high concentration. Owing to these chemical properties, chlorine dioxide must be produced at the point of use. This is conventionally achieved by mixing basic chemicals in special reactors of chlorine dioxide generation systems. The chemical storage vessels, the metering appliances and also the reactor of the chlorine dioxide systems form a locally linked unit of apparatus which is generally erected in rooms accessed by people.

There are a plurality of methods for synthesizing ClO₂ which are used commercially for water treatment, but principally three underlying methods are used. These methods use sodium chlorite (NaClO₂) as one of the starting materials. The underlying chemistry of the three methods is explained below. The substances used in these methods are termed starting chemicals, or else reactants.

1. Method Using Sodium Chlorite and Strong Acid

In the first method, a strong acid is used together with sodium chlorite. The strong acid is usually hydrochloric acid or sulphuric acid. When hydrochloric acid is used the reaction stoichiometry is as follows:

5NaClO₂+4HCl→4ClO₂+5NaCl+2H₂O

In addition, chlorine dioxide can be formed with the use of sulphuric acid in accordance with the following reaction:

10NaClO₂+5H₂SO₄→8ClO₂+5Na₂SO₄+2HCI+4H₂O

2. Method Starting from Sodium Chlorite and Chlorine

This method uses gaseous chlorine together with sodium chlorite. The reaction proceeds in two stages, first with the formation of hydrochloric acid.

Cl₂+H₂O→HOCl+HCl