The biological removal of volatile o-, m-, p-xylene and ethylbenzene from the bulk fluid was investigated in a stirred laboratory reactor. The biofilm was grown on a gas-permeable silicone membrane tubing through which oxygen was supplied. A mathematical simulation could adequately reproduce the experimental values for the biofilm thickness and the concentrations for several species for both sides of the biofilm: in the bulk liquid and in the gas phase. A high xylene conversion plateau of more than 90% was determined by the chronologically occurrence of two sub-maxima: conversion by suspended biomass and by the biofilm. The two maxima can be explained by the spatial transport processes of oxygen and xylene in the biofilm. The first sub-maximum of the conversion by suspended biomass is determined by a maximum oxygen flux through the membrane-bound biofilm. Later oxygen becomes limiting in the bulk and less xylene is degraded by suspended biomass and a maximum transport of xylene into the biofilm occurred, which led to a maximum conversion of xylene within the biofilm. For biodegradable organic compounds, the biofilm can totally reduce the transfer of these compounds into the gas phase. If compounds are non-degradable a biofilm with a thickness of 2 mm reduces the transfer into the gas phase only by 35%. Therefore, the application of pure oxygen and low volumetric gas rates are necessary to reduce the transfer of compounds that are not biodegradable.

The transfer of CO2 into the gas phase of the membrane-bound biofilm is very important for the stabilization of the pH in the biofilm and the bulk. The transfer of CO2 from the biofilm into the gas phase is up to 6 times higher than the transfer of CO2 from the biofilm into the bulk. The pH-minimum in the biofilm is not more than 0.15 pH-units below the pH in the bulk.