Among the disinfection processes of urban waste water, ozonation and UV irradiation are fast gathering interest. To avoid what happened with chlorine it is necessary to have a look at the formation of by-products and the risks of toxicity when such processes are used. We were already working on this type of problem in 1984, although we did not make a point of communicating in extenso the results obtained during a 2-year period of pilot plant and laboratory studies at La Roche sur Yon (Vendée, France). The present paper reminds of the changes in chemical nature of the effluent after an ozonation from 6 to 12 g O3/m3 resulting in an abatement of fecal coliforms and fecal streptococci to the extent of 3 to 4 logarithmic units. No evidence of compounds known for their toxicity was brought to light.

The toxicity tests performed on fish (Brachydanio rerio) and crustacean (Artemia nauplii) showed that the secondary effluent upstream or dowstream of the ozonation did not have toxic effects on the used biological material except for the fish when strong ammonia nitrogen concentrations occur. The toxicity tests performed on green algae (Scenedesmus subspicatus) showed that ozonated effluent seemed less conductive to algal growth than non ozonated effluent.

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