CHAPTER 20: Evaluation of Chlorite and Chlorate Removal by Activated Carbon in a Pilot Scale Drinking Water Treatment Plant

Chlorine dioxide (ClO₂) applied in the disinfection of drinking water can produce chlorite (ClO₂⁻) and chlorate (ClO₃⁻). The World Health Organization recommends a guideline value (GV) of 700 µg L⁻¹ for both of these compounds in drinking water, since they can cause oxidative damage to human red blood cells. This work considers a drinking water treatment plant with high chlorine dioxide consumption in disinfection resulting in chlorite concentrations in treated water exceeding the WHO GV. The plant, treating a maximum water flow of 38.9 ML d⁻¹, produces drinking water for the city of Cremona (76,000 inhabitants) in the North of Italy. The treatment train comprises aeration, biofiltration, chemical oxidation with potassium permanganate and ferric chloride, sand filtration, and final disinfection with chlorine dioxide. Regular monitoring was carried out for chlorine dioxide demand, UV absorbance at 254 nm (UV₂₅₄) and permanganate index (PI) at the inlet and at the outlet of each treatment stage. A pilot scale study was carried out to evaluate chlorite removal with activated carbon. Two columns filled with two different mineral activated carbons were installed at the outlet of the sand filter of the plant. Each column was tested under two conditions: 249 L h⁻¹ flow with a 10 minute empty bed contact time (EBCT) and 124.5 L h⁻¹ flow with a 20 minute EBCT. The chlorine dioxide demand, UV₂₅₄, PI, and chlorite and chlorate concentration were analyzed at the inlet and at the outlet of each column. The results of this study showed a significant reduction in chlorine dioxide demand, UV₂₅₄ and PI, close to 70%. With a 10 minute EBCT with both carbons chlorite is reduced by 20–40% during the first two weeks of operation of the columns; from the 3^rd to the 6^th week the reduction decreases with values of 11–17%, and then below 10%. With a 10 minute EBCT, chlorate concentration at the outlet of both columns was quite variable during the operating period.