A novel technique has been used to determine the effective diffusion coefficients for 1,1,2‐trichloroethane (TCE), a nonreacting tracer, in biofilms growing on the external surface of a silicone rubber membrane tube during degradation of 1,2‐dichloroethane (DCE) by Xanthobacter autotrophicus GJ10 and monochlorobenzene (MCB) by Pseudomonas JS150. Experiments were carried out in a single tube extractive membrane bioreactor (STEMB), whose configuration makes it possible to measure the transmembrane flux of substrates. A video imaging technique (VIT) was employed for in situ biofilm thickness measurement and recording. Diffusion coefficients of TCE in the biofilms and TCE mass transfer coefficients in the liquid films adjacent to the biofilms were determined simultaneously using a resistances‐in‐series diffusion model. It was found that the flux and overall mass transfer coefficient of TCE decrease with increasing biofilm thickness, showing the importance of biofilm diffusion on the mass transfer process. Similar fluxes were observed for the nonreacting tracer (TCE) and the reactive substrates (MCB or DCE), suggesting that membrane‐attached biofilm systems can be rate controlled primarily by substrate diffusion. The TCE diffusion coefficient in the JS150 biofilm appeared to be dependent on biofilm thickness, decreasing markedly for biofilm thicknesses of > 1 mm. The values of the TCE diffusion coefficients in the JS150 biofilms <1‐mm thick are approximately twice those in water and fall to around 30% of the water value for biofilms > 1‐mm thick. The TCE diffusion coefficients in the GJ10 biofilms were apparently constant at about the water value. The change in the diffusion coefficient for the JS150 biofilms is attributed to the influence of eddy diffusion and convective flow on transport in the thinner (<1‐mm thick) biofilms. © 1998 John Wiley & Sons, Inc. Biotechnol Bioeng 59:80–89, 1998.

Control of Membrane Attached Biofilm (MAB) formation and accumulation is a key aspect in the operation of Extractive Membrane Bioreactors (EMBs). In this work, MAB control was attempted in a novel EMB configuration which presents two innovative aspects: the presence of a biphasic biomedium and a contained liquid membrane module. This reactor, where the benefits of high shear forces and the use of a biphasic biomedium are effectively combined, was operated without any biofilm formation or reduction in organic substrate flux over time.