Toxicity of Ozone to Fish Larvae andDaphnia magna

Ozone can be used as an alternative to chlorination to control biofouling in cooling water systems. The possible negative environmental impact of a discharge of ozone-containing cooling water was investigated. The acute toxicity of dissolved ozone was determined for fish larvae of three species [Cyprinus carpio(at 27°C),Leuciscus idus(at 27°C) andClarias gariepinus(at 32°C)] and toDaphnia magna(at 21 and 27°C). The results indicate that ozone is very harmful to aquatic life. Daphnids are more sensitive to ozone than fish larvae. The mean 48-h LC50value for the larvae amounts to about 35 μg/liter, while the 48-h NOEC forD. magnawas 11 μg/liter (at 21°C). It was concluded that, to protect aquatic life, discharged cooling water should not contain any dissolved ozone. This can be achieved in practice by mixing the treated cooling water with a source of organic substances before discharge, as free ozone will react immediately with organic matter and thus disappear.