SUPPORT MEDIA FOR MICROBIAL ADHESION IN AN ANAEROBIC FLUIDIZED-BED REACTOR

The most significant variable in anaerobic digestion in an anaerobic fluidized-bed reactor (AFBR) is the selection of the support medium for microbial adhesion. Using eight kinds of media, cristobalite, zeolite, vermiculite, granular active carbon, granulated clay, pottery stone, volcanic ash, and slag, we examined the physical properties of each medium, microbial adhesion, loading rates of organic matter and removal efficiencies in an AFBR. It appeared that good performance as a support medium was associated with rougher surfaces rather than with larger surface areas, because, although cristobalite had a much smaller surface area (50 m2/g) than that of the granular active carbon (1,125 m2/g), it had a very rough surface with many tubercular processes, by which a maximum loading rate of TOC of 8 g/l·d could be achieved in a synthetic wastewater. Moreover, it appeared to be important that the surface of the medium has a positive charge judging from the difference in performance between cristobalite and zeolite. That is, the two media were charged positively and negatively at pH 7, respectively. As a result, microorganisms, charged negatively in general, could adhere more easily to cristobalite than to zeolite, which was confirmed under a scanning electron microscope (SEM) and by amounts of microbial cells adhering (85 mg cells/g). The upflow linear velocity to allow twice the expanded volume (defined as the ratio of the expanded height to the static height) was decreased to half (0.13 cm/s) by microbial adhesion. In conclusion, a suitable medium for adherence of microorganisms in AFBR should have a rough and positively charged surface rather than a large surface area. © 1990.