CHANNEL STRUCTURES IN AEROBIC BIOFILMS OF FIXED-FILM REACTORS TREATING CONTAMINATED GROUNDWATER

Scanning electron microscopy, confocal scanning laser microscopy, and fatty acid methyl ester profiles were used to study the development, o rganization, and structure of aerobic multispecies biofilm communities in granular activated-carbon (GAC) fluidized-bed reactors treating pe troleum-contaminated groundwaters. The sequential development of biofi lm structure was studied in a laboratory reactor fed toluene-amended g round-water and colonized by the indigenous aquifer populations. Durin g the early stages of colonization, microcolonies were observed primar ily in crevices and other regions sheltered from hydraulic shear force s. Eventually, these microcolonies grew over the entire surface of the GAC. This growth led to the development of discrete discontinuous mul tilayer biofilm structures. Cell-free channel-like structures of varia ble sizes were observed to interconnect the surface film with the deep inner layers. These interconnections appeared to increase the biologi cal surface area per unit volume ratio, which may facilitate transport of substrates into and waste products out of deep regions of the biof ilm at rates greater than possible by diffusion alone. These architect ural features were also observed in biofilms from four field-scale GAC reactors that were in commercial operation treating petroleum-contami nated groundwaters. These shared features suggest that formation of ce ll-free channel structures and their maintenance may be a general micr obial strategy to deal with the problem of limiting diffusive transpor t in thick biofilms typical of fluidized-bed reactors.