A method and apparatus are disclosed for dehalogenating and further biodegrading organic compounds, including halogenated organic compounds, present in an aqueous mixture, the mixture comprising the waste effluent produced in a continuous high flow rate by an industrial plant such as a bleach pulp or paper mill using chlorine and chlorine compounds. The aqueous mixture is passed through at least one combination of a first oxygen-enriched liquid zone and a second zone containing a mixed population of methylotrophic and heterotrophic microorganisms supported on a substrate bed. A first gas including oxygen is flowed through the first zone and second gas consisting substantially of a low- molecular- weight alkane is flowed through the second zone as the aqueous mixture passes through the first and second zones. The microorganisms supported by the bed dehalogenated and further biodegrade the organic compounds in the aqueous liquid flowing through the bed as they aerobically metabolize the low-molecular-weight alkane. The first zone may be hydraulically coupled to an upstream aerobic biopond for decreasing the total organic carbon and biochemical oxygen demand of the aqueous mixture before the mixture is passed through the first and second zones. A plurality of paired first and second zones may be hydraulically interconnected to achieve a higher degree of dehalogenation and further biodegradation of organic compounds in the aqueous mixture flowing therethrough and/or to accommodate higher flow rates.
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