Granular sludge formation and it`s activity change are the most important factors in achieving successful start-up and operation of UASB reactor. Nevertheless, the detailed mechanism is still unknown. On the basic of the experiments in laboratory-scale UASB reactor, the effect of hydrogen partial pressure on sludge granulation was evaluated. Size distribution method and specific metabolic activity of the sludge with the operation time were used as a means for estimating the degree of the sludge granulation. At the constant hydrogen loading, the granulation increased as starvation periods in hydrogen supply increased, resulting in high organic removal efficiency. It was evidient that hydrogen play very important role in granulation and sludge granulation was achieved through mutual symbiosis between hydrogen utilizing bacteria and hydrogen producing bacteria under the hydrogen deficient conditions.

Effluent recycling effect on UASB reactor performances is known as an important operational factor. In the present study, the possibility of intermittent recycle in UASB process for saving the power consumption was examined at different organic loading and various operational modes in recycle time period. The organic removal efficiencies of the reactors operated with the intermittent effluent recycle were considerably higher compared to those without the effluent recycle. In the intermittent recycle mode, the organic removal efficiencies slightly decreased as the non-recycle time period in the operational mode increased. Proper ratio of recycle and non-recycle time period in the mode seemed to be required to prevent the produced biogas from accumulation in the sludge bed, which caused dead zone in the reactor and sludge loss when the gas was escaped from the bed at the certain pressure.

This study was aimed to evaluate the effects of effluent recycling on the UASB reactor performances at the various organic loading rates and influent substrate concentrations. The organic removal efficiency of the reactors operated with effluent recycle were above 85%. However, the efficiencies of the reactors operated without the recycle were below 40% even though the effort to increase the efficiencies was made by changing the influent substrate concentrations and the organic loading rates, and introducing the effluent recycle at the final stage of the experiment. It was realized that the certain amount of effluent recycling from the start-up stage in UASB reactors seemed to be necessary to provide the effective contact chances between the substrate and granular sludge for better performances of the UASB