At the low loading rates (low selection pressure)applied during the initial phase of the start-up of a UASB reactor, Methanothrix filaments will grow in and on small flocs present in the seed sludge leading to the formation of a ‘bulking’ anaerobic sludge. When a high selection pressure is applied, Methano-thrix, that has a high affinity to attach to all kind of surfaces, attach to carrier materials originating from the seed sludge or from the wastewater thus forming filamentous granules (type B).

More compact Methanothrix granules (rod granules, type A) are thought to be formed by the colonisation of the central cavities of Methanosarcina clumps by Methanothrix bacteria, which have a higher acetate affinity, eventually leading to a loss of the outer layer of Methanothrix.Another explanation for these rod-type granules can be the filling of the filamentous granules with more bacteria, leading to a more compact Methanothrix granule.