Internal Circulation (IC) reactors can be operated at higher reactor volume loading rates than Upflow Anaerobic Sludge Blanket (UASB) reactors. This results in increased gas production rates and subsequently higher average shear rates in IC-reactors. Furthermore, the liquid upflow velocity is 8-20 times higher, still granules develop successfully in IC-reactors. To investigate the granule development in IC-reactors and elucidate the process limitations with respect to granule development and biomass retention, granule samples from three full scale IC-reactors are characterized. Characterization included size distribution, strength, settling velocity, density, ash content and methanogenic activity. Granules were compared with samples from UASB reactors treating similar types of wastewaters. A hydrodynamic model was developed to describe the liquid circulation in IC reactors. The average shear rate in IC reactors is approximately twice as high compared to UASB-reactors. The two stage design of the IC-reactor allows 3-6 times higher loading rate. The experimental results showed that IC-granules are larger than UASB-granules grown on similar wastewater, while the strength of IC-granules is lower as a result of the higher sludge loading rate. Although wash-out is slightly enhanced in IC-reactors, the conditions in the second stage are tranquil enough to ensure adequate biomass retention in IC-reactors. The development of characteristic IC-granules after seeding proceeds within a few months. Physical characteristics of granules are determined mainly by biological factors.

The strength of microbial granules is still poorly understood. The granule strength is defined as the resistance to attrition and/or breaking by a mechanical force or the liquid shear stress. The strength of various types of microbial granules were studied. The objective was to develop an in vitro strength characterisation test and to be able to predict granule behaviour in full scale systems. Abrasion experiments were conducted in stirred tanks and bubble columns, while the influence of viscosity and medium composition, shear rate and particle size were studied. The medium viscosity did not influence the relation between shear rate and abrasion. Measurements in demineralized water or media containing complexing agents resulted in lower granule strengths presumably the result of the removal of calcium from the granule matrix. In bubble columns attrition is the predominant abrasion process. In full scale reactors, e.g. UASB, attrition is apparently insignificant. In stirred tanks also granule breakage was observed. The abrasion rate is not directly related to the stirrer tip speed. Larger methanogenic granules showed more abrasion compared to small ones. In contrast larger nitrifying granules were stronger than small ones. A standardized strength characterization test was developed to be able to compare strengths of different microbial granules. The strength of slow growing systems appeared to be higher as compared to fast growing systems. From an ecological point of view, cells growing slowly in continuous operated systems gain advantage from attachment and shear resistance or strength.

The size distribution of methanogenic granules from ten full scale UASB and Internal Circulation (IC) reactors were studied at regular time intervals. A size distribution model was developed based on these data and additional lab-scale experiments. Full scale data were used, since the hydrodynamic conditions in full scale reactors influencing the size distribution cannot be simulated properly in lab scale reactors. It was concluded that breaking and/or disintegration did not significantly contribute to the size distribution. Washout of granules is under normal conditions negligible, since the settling velocity is much higher than the liquid upflow velocity. Gradient measurements showed that the sludge bed is well mixed. Thus, sluicing will not systematically remove large granules. The most significant process limiting the maximum granule size in normal operation is the regular sluicing of surplus biomass. Shear forces appear to have no influence on the size distribution. High numbers of precursors in the influent result in short size distributions, while in wastewaters with little or no suspended solids wide size distributions can be found. The developed size distribution model accurately described the wider size distributions found in IC-reactors. IC-granules are larger and less strong as compared to UASB granules. Furthermore, it was concluded that operating the systems at high loadings results in granules with a lower strength. The density of methanogenic granules is strongly correlated with the ash content.

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