This research was carried out in order to study the effect of the upflow liquid velocity (ULV), the COD concentration, the volumetric loading rate (VLR) and the hydraulic loading rate (HLR) on the start-up and steady-state performance of UASB reactors treating soluble substrate with 3,000 mgCOD/l at mesophyllic temperature (35°C).

Three lab-reactors (UASB) were constructed with different liquid volumes, and also with different cross-sectional areas. The three UASB reactors were started up with the same biological loading rate (BLR), but the start-up procedures were based on different criteria.

After the start-up had been carried out successfully with the formation of the granules, all the three UASB reactors were operated under steady-state conditions for up to eight different HRT(s), namely: 16, 14, 12, 10, 8, 6, 4 and 2 hours.

For each of the three reactors the kinetic parameters were determined using the Monod model. Analysis of the influence of the operational factors and reactor design on the kinetic coefficients is described. The results indicate that the liquid upflow velocity has considerable effect on the sludge in this type of system, thereby acting as a selective process in the cultivation of the biomass.

Analysis of sensitivity on the kinetic parameters indicated which was the most sensitive kinetic parameter for these reactors.

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