A practical scheme for assessment of the functional biodegradability of industrial effluents in terms ofdegradable and persistent toxicity is suggested, and the general problems of biodegradability characterization of complex mixtures is discussed. Persistent toxicity is assessed after a long-term stabilization (aging) of the effluent brought about by incubating an effluent sample with at least an equal volume of test medium (nutrient enriched surface water or mineral medium) containing a natural consortium of microorganisms. Degradable toxicity is the toxicity of the fresh effluent less its persistent toxicity. The two categories of toxicity should be managed differently. While for example dilution in the receiving water can be an effective means of mitigating degradable toxicity, dilution is in principle an insufficient control instrument for persistent toxicity. A degradability study with an unbleached sulphite pulp mill effluent, highly toxic to algae, is presented as a case history to illustrate the suggested biodegradability assessment scheme and the results' interpretation.

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