Use of chlorine dioxide for the disinfection of treated waste-water (secondary effluent)

Present contribution reports on the leading references underlying the application of chlorine dioxide in the treatment of secondary effluents of urban wastewater treatment plants. Also data of a case study on a full scale application in Deauville (Normandy-France) are reported. Chlorine dioxide has at least four major advantages for disinfection of secondary effluents: The disinfecting power is not affected by changes in pH in the range of 5 to 9. Ammonia and Kjeldahl nitrogen (TKN) do not affect the germicidal efficiency. Nitrite, if present is oxidized into nitrate. Bromide ion is not significantly oxidized Chlorine dioxide is a powerful inactivating agent of viruses, which is also able to inactivate spores and cysts like Clostridium perfringens and Cryptosporidium parvum. Virucidal action is increasing at alkaline pH. There is no significant format ion of trihalomethanes (THM's); formation of AOX remains marginal. The necessary dose depends on the water quality to be treated. It is generally in the range of 2-5 mg/L; (exceptional maximal doses reported range 12 mg/L). With adequate mixing, two to three minutes contact time are sufficient, but 10-15 min. are recommendable. For the case of Deauville, the avg. dose was 4.8 mg/L (1994); the residual at discharge point avg. 0.2 mg/L. An abatement of 2.5 to 4 Logs decay of enterobacteria were achieved by the disinfection of the treatment and 1.5 to 2 Logs for Clostidium perfringens spores. These removal rates are "additional" to the ones achieved by other previous treatment steps. The cost-efficiency of chlorine dioxide for disinfection of such effluents compares favourably with other disinfectants.