Caracterização hidrodinâmica de biorreator combinado anóxico-aeróbio de leito fixo

Abstract

The present study evaluated the hydrodynamic characteristics of a pilot scale fixed-bed anoxic-aerobic combined bioreactor that operates continuously, with upward flow and recirculation. It was used salt as tracer and fed by a step-input injection. The bioreactor consisted of five compartments, called modules, the first three of which were anaerobic and after the third module there was the insertion of an aeration flow (2.5 m³.h-1), where the latter two were aerobically operated. Five different stages were evaluated, aiming to verify the influence on the flow regime in the bioreactor of the variations in the flows, insertion of aeration and variation in the recirculation ratios. In the first two stages, the hydrodynamic characteristics of the three anaerobic modules that compose the bioreactor were evaluated with flow rates of 0.06 l.min-1 and 0.12 l.min-1 respectively. In the third stage the aeration flow was inserted. In the last two stages the effluent was submitted to recirculation in ratios of 1:1 and 2:1, respectively. The classical models of Levenspiel, Small Dispersion (PD), Large Dispersion – Open Vessel (GD-TA), Tanks in series (N-CSTR) and Danckwerts model were tested for mathematical modeling. The evaluated bioreactor presented three distinct flow profiles. During the first two steps the flow profile remained close to the piston flow (PF) which proved by obtained value of dispersion number (D/µ STAGE I = 0,01) and (D/µ STAGE II = 0,008); with the insertion of the aeration, there was a change in the hydrodynamic behavior of the bioreactor, and it started to have a flow regime between: PF and perfect mixture (CSTR) (D/µ STAGE III = 0,12). In evaluating the influence of the recirculation, it was recorded that the insertion of a recirculation flow had a direct effect on the flow, which became a CSTR regime (D/µ STAGE IV and V = 0,21). No influence was observed on the dispersion number as the recirculation ratios varied under the conditions proposed by the study. Through the flow profiles of the curves of all stages and through the calculated hydraulic residence time, there were strong indications of the presence of dead zones inside the reactor, which allows the fluid to stagnate and take preferential paths.