High-rate Disinfection of Combined Sewer Overflow Using Chlorine Dioxide

This presentation is a state-of-the-art review of chlorine dioxide (ClO2) used for high-rate disinfection of combined sewer overflow (CSO). The review includes bench-, pilot-, and fullscale studies on the use of ClO2 as a disinfecting agent for a variety of wastewaters. Specific topics discussed include the need for high-rate disinfection processes for CSO, the evaluation of ClO2 as a high-rate disinfecting agent, the effectiveness of ClO2 as a single disinfectant and in combination with chlorine (Cl2), the factors affecting ClO2 disinfection performance, the advantages and disadvantages of ClO2 disinfection, and the methods of ClO2 generation. The effective disinfection of CSO requires use of high-rate disinfection processes with powerful microbe-killing capabilities. Due to the high flowrates, volumes, and Cl2 demand of CSO, effective treatment requires relatively high Cl2 concentration, resulting in a high level of toxic byproducts and Cl2 residuals in receiving waters, which have a negative impact on aquatic life. As a result, alternative disinfection processes, which are efficient and cost effective, provide a measurable residual, are safe to handle, and produce a minimum amount of hazardous byproducts are being investigated.Chlorine dioxide appears to have a potential to be successfully used in high-rate disinfection. The full-scale verification of ClO2 disinfection efficiency however, is necessary to determine its effectiveness for intermittent use. When compared to other alternatives to chlorination technologies, ClO2 appears to be an effective disinfectant having the ability to kill a wide spectrum of microorganisms (Cl2-resistant viruses and protozoa.), with its activity being independent of pH and providing excellent disinfection at a fraction of the Cl2 dosage and contact time, making it cost effective. Because ClO2 is a more powerful oxidant than Cl2, lower levels of ClO2 can be used to get the same level of inactivation with a reduced formation of halogenated organic compound byproducts, if any, and at a lower cost. Because ClO2 is generated onsite, the method of its generation must be carefully considered. Due to the safety considerations involved with the transport of Cl2, generation methods without the use of gaseous Cl2 are more attractive. In general, ClO2 appears to be effective for high-rate disinfection and a suitable Cl2 replacement.