Multi-layered granules with Methanothrix aggregates as nucleation centres (1990): McLeod et al, working with a UASB-filter hybrid reactor, suggested a hypoth-esis in which the Methanothrix aggregates function as nucleation centres that initiate granule development of sucrose degrading granules. Acetate produ-cers, including H2-producing acetogens would then attach to this framework, providing the substrate to the Methanothrix and, together with H2-consuming organisms, form a second layer around the Methano-thrix   core. Consecutively, fermentative bacteria adhere to this small aggregate forming the exterior layer of the granule, where they are in contact with their substrates, present in the bulk solution. The products of the fermentative bacteria would then serve as substrates to the underlying acetogens. Moreover, the fact that also methanogen-like organisms were found in the exterior layer lead to the idea that these H2-consuming organ-isms could consume any free H2, avoiding its diffusion into the second layer, where other H2-consuming organisms would then be able to remove the remaining H2produced by the acetogens. Thus, such a spatial arrangement of the different trophic groups guarantees a high level of acetogenic activity.