The typical UASB granule has 3 distinct layers: The following is an example when sucrose substrate is used
The outer layer is a mixture of various types of bacteria and includes some scattered colonies of Methanosaeta spp. and Methanosarcina spp. Acidogens are concentrated on the outer layer. The middle layer is dominated by syntrophic microcolonies with some scattered Methanosaeta spp. and Methanosarcina spp. The interior layer, which is also known as the centre core, is densely packed with short rod-shaped Methanosaeta spp. and Methanosarcina spp [3].
Usually, the product of the external layer will be the substrate of the next inner layer. An example would be in the situation when biopolymers are degraded by the acidogens into short chained volatile fatty acids (SCFA) or alcohols. The fatty acids or alcohols then diffuse down a concentration gradient to the middle layer. The middle layer bacteria, known as acetogenic bacteria, use the SCFA and produce acetate. Acetate is the substrate for the methanogens, Methanosaeta spp. and Methanosarcina spp. [2][3].
One or more of the layers could be missing or undistinguishable when certain wastewaters are used. The layers could be limited to the degradation of carbohydrate substrates. For example, granules, which degrade glutamate, do not have any layers. The rationale behind these seems to be related to the rate of uptake of the substrates for acidogenesis. The initial uptake via the layers for acidogenesis will be low if glutamate was the substrate; thus, it is reasonable that no layers are observed [2].
Methanosaeta spp. and Methanosarcina spp. are found in all layers of the sludge. This structure supports that these bacteria are responsible for the initial granulation [1][2][3