Chlorine dioxide is a highly potent biocide that has a higher efficacy than hypochlorite in destroying protozoa, bacteria and viruses, without allowing them to build a tolerance.  It is effective over a wide pH range as both a biocide and metal precipitant and has gained popularity because it creates significantly lower concentrations of disinfection byproducts such as trihalomethanes (THM’s) and haloacetic acids (HAA’s) when compared to chlorine. However, at least one study has shown that THM formation is still significant when naturally occuring organic matter (NOM) such as humic acids are present (A.A.Stevens, “Reaction Products of Chlorine Dioxide”,1982).

Chlorine dioxide is a powerful oxidizer and can damage polyamide reverse osmosis (RO) membranes, so a reducing agent such as sodium bisulfite is usually dosed as a precautionary measure.

The use of chlorine dioxide at a low residual that would deplete prior to contact with the RO membrane surface  has been a hot topic.  But as with any other biocide, when the residual depletes to sub-lethal doses prior to membrane contact, biofouling will be enhanced.  This was confirmed in a 2010 publication by Shemesh et al, “The biocide chlorine dioxide stimulates biofilm formation in Bacillus subtilis by activation of the histidine kinase KinC.”

Chlorine dioxide can be used in the permeate but only at low dosages because it is reduced to chlorite and chlorate ions which can cause anemia and affect the nervous system.  In the United States, the EPA limits the allowable residual of chlorine dioxide in drinking water to 0.8 ppm.