Influence of drinking water treatments on chlorine dioxide consumption and chlorite/chlorate formation

Disinfection is the last treatment stage of a Drinking Water Treatment Plant (DWTP) and is
carried out to maintain a residual concentration of disinfectant in the water distribution
system. Chlorine dioxide (ClO2) is a widely used chemical employed for this purpose. The
aim of this work was to evaluate the influence of several treatments on chlorine dioxide
consumption and on chlorite and chlorate formation in the final oxidation/disinfection
stage. A number of tests was performed at laboratory scale employing water samples
collected from the DWTP of Cremona (Italy). The following processes were studied:
oxidation with potassium permanganate, chlorine dioxide and sodium hypochlorite,
coagulation/flocculation with ferric chloride and aluminum sulfate, filtration and adsorption
onto activated carbon. The results showed that the chlorine dioxide demand is high if
sodium hypochlorite or potassium permanganate are employed in pre-oxidation. On the
other hand, chlorine dioxide leads to the highest production of chlorite and chlorate. The
coagulation/flocculation process after pre-oxidation shows that chlorine dioxide demand
decreases if potassium permanganate is employed as an oxidant, both with ferric chloride
and aluminum sulfate. Therefore, the combination of these processes leads to a lower
production of chlorite and chlorate. Aluminum sulfate is preferable in terms of the chlorine
dioxide demand reduction and minimization of the chlorite and chlorate formation.
Activated carbon is the most effective solution as it reduced the chlorine dioxide consumption
by about 50% and the DBP formation by about 20e40%.