DETERMINATION OF POLLUTANT DIFFUSION-COEFFICIENTS IN NATURALLY FORMEDBIOFILMS USING A SINGLE TUBE EXTRACTIVE MEMBRANE BIOREACTOR

A novel technique has been used to determine the effective diffusion c oefficients 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 a utotrophicus GJ10 and monochlorobenzene (MCB) by Pseudomonas JS150. Ex periments were carried out in a single tube extractive membrane biorea ctor (STEMB), whose configuration makes it possible to measure the tra nsmembrane flux of substrates. A video imaging technique (VIT) was emp loyed for in situ biofilm thickness measurement and recording. Diffusi on coefficients of TCE in the biofilms and TCE mass transfer coefficie nts in the liquid films adjacent to the biofilms were determined simul taneously using a resistances-in-series diffusion model, it was found that the flux and overall mass transfer coefficient of TCE decrease wi th increasing biofilm thickness, showing the importance of biofilm dif fusion 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 control led primarily by substrate diffusion. The TCE diffusion coefficient in the JS150 biofilm appeared to be dependent on biofilm thickness, decr easing markedly for biofilm thicknesses of >1 mm. The values of the TC E diffusion coefficients in the JS150 biofilms <1-mm thick are approxi mately 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 b iofilms were apparently constant at about the water value. The change in the diffusion coefficient for the JS150 biofilms is attributed to t he influence of eddy diffusion a nd convective flow on transport in th e thinner (<1-mm thick) biofilms. (C) 1998 John Wiley & Sons, Inc.