BIOFILM MORPHOLOGY IN POROUS-MEDIA, A STUDY WITH MICROSCOPIC AND IMAGE TECHNIQUES

Biofilm activity, behaviour and our ability to control biofilms depend s to a large extent on mass transfer phenomena in the biofilm, at the biofilm-liquid interface and in the bulk liquid. Biofilms respond to c hanging mass transfer conditions by adjusting morphology, thereby opti mising the exchange of matter with their surroundings. Observing biofi lm morphology and mass transfer in relevant fluid dynamic conditions c an therefore yield essential information to understand and model biofi lm behaviour. Lack of such knowledge, as the case is with regards to b iofilm behaviour in various porous media, such as sandstone reservoirs , limits our ability to predict biofilm effects. A transparent porous media replica of a sandstone reservoir with cybernetic image processin g has been designed to study biofilm related transport phenomena in po rous media. The porous medium was inoculated with a mixed bacterial cu lture and fed a sterile nutrient solution in a once through flow mode. The biofilm was observed by microscopy with automated image analysis. This novel integrated software/hardware cybernetic design allows near real-time, essentially simultaneous, surveillance of several critical sites in the porous network and facilitates selective recording and c ompilation of observations as a function of the biological activity at each particular site. Biofilm biomass distribution in space and time (morphology and morphological changes) are thereby recorded at a repre sentative selection of sites in the porous structure. Local in-pore fl ow velocity measurements were carried out by measuring the velocity of suspended particulate matter such as detached cells or clusters of ce lls. The influence of biofilm morphology on convective mass transport could thereby be observed and recorded. This effect, on a meso scale, was also monitored by sensitive, automated pressure drop measurements across the porous medium cell. Important observations so far include: Bioweb; the biofilm morphology in porous media is very different from the ''classical film'', as it appears more like a spider web where eac h strand varies in size and shape. The biofilm influences the convecti ve flow through pores both locally within pores and effecting the flow distribution between pores. Pores with high initial permeability ther eby become less permeable, diverting more now to less permeable zones in the porous matrix. Large variations in this picture were observed, demonstrating the need for a sophisticated experimental apparatus with high sampling capacity to investigate such an intricate system. The o bserved biofilm behaviour in porous media has important theoretical an d practical implications. Flow diversion and permeability effects are of immediate practical importance, improving the prospects for biologi cal treatment of reservoirs. The information obtained in this study wi ll be applied in mathematical simulations of ground water reservoirs, bioremediation and biological enhanced oil recovery. (C) 1997 IAWQ. Pu blished by Elsevier Science Ltd.