Chlorine dioxide (ClO2) is a useful secondary disinfectant for shock treatment to control for Cryptosporidium and to control biofilm. It is not a primary disinfectant and does not produce free chlorine. When combined with hypochlorite it reduces the contact time required for control of Cryptosporidium, reducing the time required to close a pool for shock treatment. Table 1 compares the contact time (CT) for 3 log reduction in Cryptosporidium when combining ClO2 with hypochlorite.
Table 1: Comparison of contact time for 3 log reduction of Cryptosporidium using hypochlorite to form free chlorine and ClO2, independently and in combination (Murphy et al. 2014).
Murphy, J.L., Haas, C.N., Arrowood, M.J., Hlavsa, M.C., Beach, M.J. & Hill, V.R. (2014). Efficacy of chlorine dioxide tablets on inactivation of Cryptosporidium oocysts. Environmental Science and Technology, 48, 5849-5856. doi: 10.1021/es500644d
NHMRC, NRMMC (2011). Australian Drinking Water Guidelines Paper 6 National Water Quality Management Strategy. National Health and Medical Research Council, National Resource Management Ministerial Council, Commonwealth of Australia: Canberra, Australia.
World Health Organization (WHO) (2006). Guidelines for safe recreational water environments: volume 2 swimming pools and similar environments. World Health Organization: Geneva, Switzerland.