Microbial distribution in UASB granules and its resulting effects

Microscopic (SEM and TEM) examinations of biogranules sampled from various UASB (Upflow Anaerobic Sludge Blanket) reactors indicated that microbes are densely packed. The microbial distribution is strongly dependent upon the degradation thermodynamics and kinetics of individual substrates, Biogranul es degrading carbohydrates exhibited typically a layered distribution with a surface layer populated with hydrolytic/fermentative acidogens, a mid-lay er comprising syntrophic colonies and an interior comprising acetotrophic m ethanogens. On the other hand, those substrates having a rate-limiting hydr olytic/fermentative step did not exhibit any layered pattern; instead, bact eria were interwined and distributed evenly. These observations have two im plications. Biogranules are developed through evolution instead of random a ggregation of suspended microbes. Furthermore, biogranules should be less v ulnerable to the changes of mixed liquor condition, because the large major ity of microbes inside the biogranules are shielded from the hostile mixed liquor environment. The latter is supported by experimental evidence that b iogranules are more resistant than suspended sludge to the toxicity of hydr ogen sulfide, heavy metals and aromatic pollutants in wastewater.