Traditional treatment methods of waste metalworking fluids produce an aqueous phase containing toxic components and with a high Chemical Oxygen Demand (COD). In this paper, a biological solution is proposed to further reduce both the effluent COD and its toxicity. A method to develop a bioconsortium from microbes found in the waste metalworking fluid is proposed. The development took place in three phases: feasibility, bioconsortium. development and optimisation. Flask tests have been used to show the feasibility of using the metalworking fluid indigenous microbial community for the degradation of the nanofiltration permeate of the metalworking fluid. A suspended bioreactor allowed the development of a better-adapted consortium. Finally, a fixed bed bioreactor inoculated with the developed bioconsortium. was set up and run for 8 months to test the bioconsortium's robustness and to optimise the biological process. A bioconsortium was successfully developed using a simple method and a 90% reduction in the original nanofiltration COD level was achieved by the fixed bed bioreactor. (c) 2005 Society of Chemical Industry