The pulp and paper industry is facing more stringent regulations on the quality of effluent discharges to receiving waters, especially for bleached pulp mills. In most cases, these regulations concern mainly AOX concentrations, biotoxicity and COD level in the final discharges. An effective way to improve pollution balance would be to use ozone as an external treatment to meet environmental regulations. The first part of this work has consisted in the identification of the polluting steps in pulp and paper mill processes in terms of toxic effects. After identification, effluents streams that contribute the most to the global pollution of the mill were selected for ozone treatment. For bleaching effluents, the goal was to eliminate acute toxicity and chlorophenolics to increase biodegradability. Depending on the nature of the effluent, complete detoxification and an increase of biodegradability can be obtained with low ozone doses (0.5-1 kg/ADt of pulp). Ozone has also been applied on the final effluent discharges before and after biological treatment. In hardwood and softwood pulp and paper mill final discharges, the residual COD can be eliminated with a catalyzed ozone treatment. This treatment may allow a water quality sufficient for reutilisation which could lower water consumption of the pulp processes.

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