The dynamic behaviour of membrane bioreactors (MBRs) with negligible biomass growth due to the presence of toxic substances was modelled to simulate a pilot plant test in an industrial degreasing unit. Modelling was done by introducing a substrate biodegradation rate into a material balance equation of an MBR. This was followed by a discrimination procedure between the simulated and measured values, from which best-fit specific substrate uptake rates (q(sp)) were determined at different air supply rates. The results showed clear improvement of q(sp) with increased air supply, indicating a possible increase in the active cell population in the reactor due to reduced toxicity. The highest specific substrate uptake rate (q(sp) = 0.08 g(substrate) g(biomass)(-1) d(-1)) determined was assumed equal to the maintenance coefficient (q(m)) for the biodegradation of the substrate under the test conditions. The understanding of the dynamics provided a reference in subsequent regeneration system improvement. (c) 2006 Society of Chemical Industry.