Chlorine is very effective in many situations in disinfecting effluent from wastewater treatment plants. Chlorine’s mode of action is well understood. It works well on numerous organisms, can reduce odors, is cost-effective, and leaves a residual that continues to kill the organisms. The problem is that chlorine residual can be toxic to aquatic life, highly corrosive and toxic to people, oxidizes organic chemicals at times into more dangerous substances, and does not work well in high concentrations of chlorine-demanding substances such as combined sewer overflows. Some parasites are resistant to low doses of chlorine including Cryptosporidium parvum, Entamoeba histolytica, and Giardia lamblia.
Chlorine dioxide is an effective disinfectant of wastewater. It is used as a gas which is generated from excess chlorine on-site since it is unstable, reactive, and difficult to transport. It oxidizes phenols but does not produce trihalomethanes unless there is an excess of chlorine remaining after the generation of the chlorine dioxide, which means that the individuals generating the chlorine dioxide must be very careful with the amount of chlorine being used. The chlorine dioxide will not form chloramines but it can produce toxic byproducts including chlorite and chlorate.