The removal of some Pesticides

The pesticides which can be removed by means of ClO2 are DMDT (dimethoxydichlorine) and aldrin [42]. Herbicides, such as paraquat and diquat, are eliminated in a few minutes at pH above 8. For the pre-oxidation and reduction of organic pollutants, required dosages are between 0.5 and 2 mg/l, with contact times usually as low as 15 to 30 minutes, depending on the water characteristics; in the case of post-disinfection, 0.2 - 0.4 mg/l of ClO2 are generally used. At these dosages, the potential chlorite residual is such that it does not constitute any health hazard. Such values are strictly linked to the conditions and structure of the distribution network of drinking water which, in some cases, can even require further ClO2 feeding along the line to maintain a minimum residual concentration of disinfectant.

The action of chlorine dioxide in pre-oxidation is related, above all, to the organic matter, present both in a dissolved form (DOC – Dissolved Organic Carbon) and in colloidal form. The pH and temperature, which can strongly influence other pre-oxidation treatments (a classic case is hypochlorite), are irrelevant in the case of chlorine dioxide. The eventual chlorite residual can be easily removed through successive passage via Granular Activated Carbon (GAC) filters, usually installed in a drinking water plant to improve both the chemical and microbiological quality of the treated water.