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. Biogranules degrading carbohydrates exhibited typically a layered distribution with a surface layer populated with hydrolytic/fermentative acidogens, a mid-layer comprising syntrophic colonies and an interior comprising acetotrophic methanogens. On the other hand, those substrates having a rate-limiting hydrolytic/fermentative step did not exhibit any layered pattern; instead, bacteria were interwined and distributed evenly. These observations have two implications. Biogranules are developed through evolution instead of random aggregation of suspended microbes. Furthermore, biogranules should be less vulnerable to the changes of mixed liquor condition, because the large majority of microbes inside the biogranules are shielded from the hostile mixed liquor environment. The latter is supported by experimental evidence that biogranules are more resistant than suspended sludge to the toxicity of hydrogen sulfide, heavy metals and aromatic pollutants in wastewater.