upflow velocities, suspended solids in the effluent or seed sludge, attrition and removal of excess sludge from the reactor are considered as the factors responsible for granulation.

Selection pressure theory(1983): The essence of the granulation process in a UASB reactor, in this theory, is believed to be the continuous selection of sludge particles that occurs in the reactor.The selection pressure can be regarded as the sum of the hydraulic loading rate and the gas loading rate (dependent on the sludge loading rate).Both factors are important in the selection between sludge components with different settling characteristics. Under conditions of high selection pressure, light and dispersed sludge will be washed out while heavier components can be retained in the reactor. Thus, growth of finely dispersed sludge is minimised and the bacterial growth is delegated to a limited number of growth-nuclei, that can consist of inert organic and inorganic carrier materials or small bacterial aggregates present in

the seed sludge. These growth nuclei increase in size until a certain maximum size, after which parts of the granules detach, producing a new generation of growth

nuclei, and so on. The first generation consists of relatively voluminous aggregates, but gradually they become denser due to bacterial growth on the outside and inside of the aggregates.Moreover, bacterial growth is stimulated in the more voluminous aggregates as the substrate can penetrate deeper in the aggregates due to less diffusion limitation and lower volumetric bacterial activity inside these aggregates as compared to denser aggregates.The filamentous granules that exist in the initial stages of the granulation process become denser due to this ageing

process.

Under conditions of low selection pressure, growth will take place mainly as dispersed biomass, which gives rise to the formation of a bulking type of sludge.In

anaerobic reactors, the predominant organism is Metha-nothrix, which can form very long filaments (200–300 mm).When these organisms grow without attach-ment to a solid support particle, a loosely intertwined structure of filaments, with very poor settling character-istics will be obtained.Moreover, through the attach-ment of gas bubbles to these loosely intertwined filaments, the sludge even has a tendency to float.

Growth of colonised suspended solids (1994): Pereboom states that granules originate from fines formed by attrition and from colonisation of suspended solids from the influent.Moreover, according to this author, granule size increase is only due to growth and therefore, the concentric layers observed on sliced granules are

related to small fluctuations in growth conditions. Pereboom reported that the most significant process limiting the maximum granule size in normal operation is the regular discharge of surplus biomass. Reactor turbulence and internal gas production appear to have no influence on the size distribution.These shear