Attrition model proposed that granules originate from fines formed by attrition and from colonization of suspended solids from the influent
(Pereboom, 1994). According to Pereboom (1994), increase in granule size is only due to microbial growth and therefore the concentric layers observed on sliced granules are related to small fluctuations in growth conditions.
It was 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 appeared to have no influence on the size distribution. The shear forces due to liquid and gas turbulence are not responsible for breaking or disintegrating of granules, and only cause attrition of small particles from the granules. The attrition is not expected to be significant to the removal of large granules.
According to Pereboom (1994), the granular size distribution in UASB reactors seems to be the result of growth from small particles (being washed into the reactor or developed in the reactor by attrition) into larger granules and the removal of representative amounts of granules from all size classes by sludge discharge. Besides, wastewaters of high concentrations of suspended solids would result in narrow granule size distributions, while influent of little or no suspended solids would lead to good distribution of size.
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