2.2.2. Growth of ‘microbial nuclei’

The ‘‘Spaghetti theory’’ (1987): Wiegant proposed a ‘Spaghetti theory’ on sludge granulation in UASB reactors treating acidified wastewaters, solutions of acetate or mixtures of VFA with predominant Metha-nothrix   bacteria.Although reactors with predominant Methanosarcina species can perform ‘spontaneous gran-ulation’, this type of granules has less practical importance in the UASB reactors as they bring

operational problems Therefore, when the relative concentration of Methanothrix     bacteria is not high enough, a strong selection towards these bacteria has to be imposed.This can be done by using low acetate concentrations during the start-up phase, as Methano-thrix has a higher substrate affinity for acetate compared to Methanosarcina. Wiegant divides the granule formation in two phases:

 

1.formation of precursors

2.actual growth of the granules from these precursors.

 

The first step is considered the most crucial part of the granule formation.Initially, Methanothrix bacteria form very small aggregates, due to the turbulence generated

by the gas production, or attach to finely dispersed matter.The concentration of suspended solids should not be too high, otherwise the increase in size of the

aggregates will be too slow.Selection for aggregates is done by imposing an increasing upflow velocity.Once the precursors are formed and a proper step-up routine is followed, granulation is inevitable.The growth of the individual bacteria and the entrapment of non-attached bacteria lead to the growth of the precursor particles to form granules, which due to the hydraulic shear forces of the upflowing biogas acquire a spherical shape.The granules in this phase still present a filamentous appearance, like a ball of spaghetti formed of very long Methanothrix filaments, of which part is loose and part in bundles.With time, rod-type granules are formed from these filamentous granules at a high biomass retention time, due to the increase in the density of the bacterial growth. Similarly to Wiegant, Chen and Lun formulated a hypothesis for the anaerobic sludge granulation in a UASB fed with fermented alcohol stillage divided in two steps:

 

1.nucleus formation

2.nucleus growing into a granule.