Croissance bacterienne sur charbon actif granule. Investigation au microscope electronique a balayage

Abstract

Recent work characterising biological development in granular activated carbon filters (GAC) has contributed to a better understanding of the role of bacterial growth in the efficiency of adsorption processes in water treatment. Among the techniques involved in biological GAC studies, scanning electron microscopy (SEM) can be used to describe the physical relations between bacterial populations and activated carbon surface. In this study, an effective technique is presented for two-phased fixation (2% paraformaldehyde-2.5% glutaraldehyde and 1% osmium tetroxyde), dehydration and critical point drying of “biological” activated carbon particles. Microscopic examination of GAC after 5 days of filtration (drinking water artificially contaminated with 1,0 mg l −1 of an anionic surfactant) showed the nature of the fixation sites of organic matter and the initially scattered bacterial growth sites. The microbiological colonization appears then along surface crevices and holes (Fig. 1) where substrate concentration occurs and where organisms are shielded from fluid shear forces. Bacteria attach to the carbon surface by secreting a network-like polysaccharide matrix (Fig. 2). After 15 days of filtration (Fig. 3), biological maturation of the carbon induces the progressive formation of a membranous organic film (surfactant precipitation and bioflocculation) making it difficult to observe bacterial development on the carbon surface (Fig. 4). However, there is no evidence that a continuous biofilm (uniform layer of bacteria in a slimy shell) develops around the carbon granules. The observations support the assumption that structural relationships between microorganisms and carbon surface are modified considerably during the accumulation of organic floc and cellular debris. This organic support medium enhances the biodegradation of substrates that would obtained from the carbon surface alone. Therefore, it appears that the effective metabolization of organic adsorbates in GAC filtration units is only slightly connected with the efficiency of physico-chemical adsorption processes.