Chlorine
Chlorine, a strong oxidant, has a long history of use as a water disinfectant, but is found to be quite toxic to all aquatic organisms. Chlorine combines with organic compounds in the water to form trihalomethanes (THM). Because of this, it is highly regulated. Since chlorine is already used in many areas, both Canada and the United States encourage and/or require only limited use. It is normally added to water systems as a gas or as the hypochlorite salt, NaOCl. The common aquatic test organism, >Daphnia magna, is one of the most sensitive freshwater animals in laboratory testing with chlorine according to USEPA reports cited in EPRI (1993).
Chlorine dioxide
Chlorine dioxide (ClO2) has to be prepared on site. It may not be as toxic to the zebra mussel as chlorine, but it does not form THM. Exxon Chemical (year of publication unavailable) and Rusznak et al. (1994) discuss this biocide in terms of its effects on the zebra mussel and environmental safety. The Exxon information sheet highlights the differences between chlorine dioxide and chlorine. Chlorine dioxide reacts with organic matter in water by an oxidation reaction, forming chlorite ions that undergo a secondary reaction to form chloride ions or salt and water. The information sheet emphasizes that ClO2 is not similar to chlorine and does not result in the formation of trihalomethanes (THM), dioxins, chlorinated benzene, toluene, or the xylene (BTX) compounds that have been associated with the use of chlorine.
Management and Control Contents