DYNAMIC MODELING OF THE POPULATION-DISTRIBUTION IN THE ANAEROBIC GRANULAR BIOFILM

Significant downward gradient of glucotrophic (acidogenic) activity wa s experimentally observed as anaerobic granules were stripped off thei r surface bacteria. This indicated a clear predominance of fermentativ e bacteria in the external layer of the granules. This is consistent w ith the multilayer microstructural model evidenced by scanning electro n microscopy (SEM) of cleaved granules. In contrast, acetoclastic acti vities were evenly distributed along the granule depth even though SEM had shown the core of granules of various sources to be almost exclus ively composed of Methanosaeta-like bacteria. A detailed distributed d iffusion-reaction mathematical model was developed to predict local su bstance and biomass concentrations at any depth within the granular bi ofilm. This intragranular model combines the kinetics of ten substance s and eight trophic groups of microorganisms as well as physico-chemic al constraints such as ionic dissociation and gas-liquid equilibrium, internal and bulk-surface interfacial diffusion mechanisms. Predicted results of this intragranular kinetic model have shown a drastic downw ard glucose gradient towards the centre, as a concentration of 10 mu M in the bulk liquid boundary layer dropped to almost 0 at a distance l ess than 100 mu m below the surface. In contrast, the model predicted very low to no gradients of acetate. However, the model predicted pH v alues 1 mm inside the granule to be over one pH unit more alkaline tha n in the bulk liquid. Hence this significant pH gradient could be the major factor which gives a competitive advantage for Methanosaeta-like population to proliferate in the granule core.