MODELING THE SPATIAL-DISTRIBUTION AND DYNAMICS OF A XYLENE-DEGRADING MICROBIAL-POPULATION IN A MEMBRANE-BOUND BIOFILM

In a stirred laboratory reactor a biofilm was grown on a gas-permeable silicone membrane. Xylene was added to the bulk fluid as the only car bon source and oxygen was supplied through the membrane. For 66 days t he reactor was operated under varying experimental conditions. The con centrations of five components in the reactor effluent and of two in t he gas outlet were measured almost daily, the biofilm thickness was de termined several times. The measured time series were used to calibrat e a mathematical model which calculates pH, aerobic degradation of xyl ene, and microbial growth, decay and conversion to inert particulate m aterial. The calibrated model was then used to analyze the spatial dis tribution and dynamics of the xylene-degrading heterotrophic populatio n in the biofilm. The analysis revealed that as long as oxygen was sup plied as air heterotrophic cells only grew within a thin zone of about two hundred micrometers near the membrane. Upon the change from air t o pure oxygen this zone shifted instantaneously by half of the biofilm thickness of about two millimeters towards the biofilm surface. The i mmediate ability of the biofilm to convert xylene in a zone which so f ar had been strictly anaerobic, indicates the presence of a heterotrop hic population in this zone. The conclusion is that this population ha d been transported from the aerobic to the anaerobic zone and that tra nsport and growth of microbial cells are processes which are equally i mportant in biofilms.