The biological treatment of spent synthetic metalworking fluid (MWF) has been investigated in many studies, with most reporting the presence of undegraded components in the residual waste. In this study a hybrid approach to waste treatment of spent MWF, involving both physical treatment and microbial degradation, was investigated. The effect of the removal of a synthetic polymer on biological degradation was investigated in a bioreactor system. Polymer removal was found to have little effect on the degradation efficiency of the remaining components but resulted in a waste product with 14% lower chemical oxygen demand. An additional benefit of this approach is that it enables the possibility of recycling of the recovered components, with the aim of economising on future product formulation and reducing waste discharge. (c) 2006 Society of Chemical Industry.