Disinfection with Chlorine and Chlorine Dioxide

Abstract

Disinfection with chlorine dioxide was compared to disinfection with chlorine in wastewater from three treatment sequences: (1) Conventional activated‐sludge treatment; (2) activated‐sludge/nitrification; and (3) activated‐sludge/nitrification/filtration treatment. The comparisons were made based on correlations of fractional survival of coliform bacteria with the product of residual concentration times contact time for each disinfectant. Laboratory experiments were conducted in a complete randomized block design, with mass dose of disinfectant (3 levels) and contact time (3 levels) as independent variables. The laboratory experiments were conducted in a four‐liter batch reactor. In general, both chlorine and chlorine dioxide give decreased survival ratios for increases in dose or contact time. In all cases, chlorine dioxide achieved a given fractional kill with an equal or lower residual time product than chlorine. Chlorine dioxide demand is greater in conventional secondary effluent than the chlorine demand, but less than that for chlorine in nitrified effluents. The chloramines found in non‐nitrified wastewaters are much less effective in disinfection than “free” chlorine. The disinfection effectiveness of chlorine dioxide, and to a lesser extent that of chlorine, is enhanced by the filtration process in both conventional secondary effluent and in nitrified effluent.