Break-Point Chlorination and Ozonization

In the case of strongly polluted surface water, chlorination is the first purification step and is carried out after removal of any coarse foreign matter. Sufficient chlorine is added to ensure a free chlorine concentration of ca. 0.2 to 0.5 mg/L in the water after treatment (break-point chlorination). Chlorine reacts with water forming hydrochloric acid and the hypochlorite anion, depending upon the pH. 

The last two processes are undesirable chlorophenols have a very strong taste and some of the aliphatic chlorohydrocarbons (e.g. chloroform) are also suspected of being carcinogenic. It is therefore usual to perform the chlorination only up to the chloramine stage and to carry out the further elimination of impurities, e.g. microbiological degradation processes, on activated charcoal. 

The most important alternative to chlorination of water is ozonization in which the above-mentioned disadvantages occur to a much lesser extent. However, the higher cost of ozonization is a problem. Ozonization helps subsequent flocculation and biological degradation on activated charcoal. About 0.2 to 1.0 g of ozone is required per of water, in exceptional cases up to 3 g/m. A further alternative is treatment with chlorine dioxide (from sodium chlorite and chlorine), in which there is less formation of organochloro-compounds than in the case of chlorination (see Section 1.7.4.2.5). 

In the Federal Republic of Germany ozonization, as pre-ozonization - a post ozonization step being inserted before flocculation (see Section 1.1.2.2) - has largely supplanted break-point chlorination. 

Aeration is sufficient to oxidize and thereby flocculate out iron and manganese ions in the treatment of groundwater, as well as serving to increase the pH by expelling the unbound carbon dioxide. 

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