Chlorite removal with granular activated carbon

! Chlorite (ClO2 ) is the dominant degradation by-product coming from chlorine dioxide reactions. Compliance to the maximum concentration of 200 µg/l, established by new Italian regulations for drinking water (D. Lgs. 31/2001), can be accomplished by reduction of chlorine dioxide dosage during water treatment or by chlorite removal. In this work granular activated carbon (GAC) as a technology for chlorite removal was investigated; particularly, the interference between chlorite and organic matter removal on GAC active sites and the applicability of a “chemical regeneration” process in order to improve GAC efficiency for organic matter and chlorite removal were studied. Different batch and column tests were performed, both on distilled and surface water spiked with chlorite; both on virgin, exhausted and thermally regenerated carbon; and on exhausted GAC after chemical regeneration by acid, basic and basic–acid solutions. The results demonstrate that GAC can be an efficient method for chlorite removal.

Regenerated GAC obviously gives good removal both for organic matter (about 70–80%) and for chlorite (100%); for exhausted GAC organic matter removal is reduced from 40-50% (without chlorite) to 5–7% (when water is spiked with chlorite), while signifiant removal (about 50%) is observed for chlorite. Among the experimented regenerations, base– acid washing gives the best results: chlorite and organic matter removal is improved 15–20% and 10–15%, respectively (with respect to exhausted GAC), until 1200 h of operation. It is possible to conclude that GAC can be adopted as an efficient system for chlorite removal in potable water production plants where chlorine dioxide is applied in a pre-oxidation step. Acknowledgments The authors acknowledge Engineer Silvia Vanotti; all the personnel of Caffaro S.p.a. of Brescia involved in this research (for assistance in performing the experimental tests, sample collection and analytical determinations); and SMAT of Torino for the supply of water and GAC.