Operationally exhausted metal working fluids are chemically mixed, produced in largequantities (400 000 tonnes year in the U.K.), and potentially environmentally toxic.It is essential to develop more reliable and economical approaches for their disposal.We investigated the effectiveness of a defined bacterial consortium, constructedspecifically for treating metal-working fluid (MWF), and contrasted its performanceto that of undefined inocula from activated sludge. Construction of the consortiumwas based on knowledge of the diversity of bacterial communities that naturallycolonize MWF and determination of their catabolic abilities and tolerance to thechemical constituents. Chemical analysis of the inoculated MWF bioreactor revealedthat, after 100 h at 28
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C, the defined inoculum reduced the pollution load by over80% from an initial chemical oxygen demand of approximately 48 000 mg L
-1. Theinocula performance was approximately 50% more effective than that of the undefinedmicrobial community from the activated sludge. Furthermore, the performance of theconstructed consortium was more reproducible than that of an undefined community,an essential feature for bioaugmentation treatment of industrial wastes.