Evolutional changes in interior structures of mixed population biofilms grown on domestic wastewater were quantitatively analyzed using a cryosectioning technique and an image analysis. Meanwhile, transport of particulates into the biofilms was also experimentally investigated using fluorescent microbeads as tracers to relate the biofilm structure and particulate transport into the biofilm. Microscopic observation of the cryomicrotomy biofilm sections indicated the biofilms were very porous and consisted of interwinded filamentous biomass acting as a framework of the biofilm. A honeycomb structure was often found, which would make the biofilm more resistant to water flow. There were micropores with the diameter of about 10 μm microcolony aggregates attached to filamentous biomass and macropores with the diameter of 20–200 μm in the biomass matrix. These pores did not clog during two months of cultivation. Areal porosity was about 30% in the bottom biofilm and more than 90% in the surface. Significant difference in transport efficiency was not observed for various sizes of microbeads due to the presence of macropores. Therefore, even 10 μm tracer beads could quickly traverse throughout a biofilm 640 μm thick via water channels or macropores and then penetrated into the micropores. Convective transport from the bulk to the bottom biofilm, rather than molecular diffusion, was responsible for this rapid transport. Based on experimental results, it can be concluded that the biofilm structure seems to be well designed to maximize the transport efficiency of substrates and products and the strength of biofilm structure.

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