Remoção de cromo (vi) e carbono orgânico total em biorreatores híbridos sequenciais

Abstract

This work was intended to evaluate the performance of sequential hybrid bioreactor systems, without aeration and with intermittent aeration, when removing chrome (VI), Total chrome, Total organic carbon (COT) from an effluent containing Cr (VI). Chemical Oxygen Demand (DQO) and the biomass concentration also were monitored. As inoculum in the bioreactors, mixed cultures were used from microorganisms originated in the industrial sludge, previously adapted, and with Cr source (VI), the potassium dichromate (K2Cr2O7). It was initially performed a blank test, before adding chrome to the system, with the purpose to verify the influence of the metal presence in the COT removal. Were performed 3 kinetic assays, in chrome hexavalent initial concentrations of 80, 120 and 150 mg/L. After each kinetic, it was evaluated the removal during the bioreactor regeneration, with the same effluent, but with no chrome. The Cr (VI) concentration was defined by the colorimetric method using diphenylcarbazide 3500-Cr D, according to APHA (2005). The total chrome concentration was measured in the atomic absorption Spectrophotometer AA-7000 Shimadzu. The characterization regarding COT was performed by the using the technique catalytic combustion at high temperature, using the TOC-VCPH-ASI+TNM-1 analyzer from Shimadzu. For kinetic with concentration of 80 mg/L de Cr (VI), the results showed that after 600 hours of operation the bioreactors did not saturated, obtaining 87% of global removal of Cr (VI) and 62% of total chrome. For kinetics of 120 and 150 mg/L, inside the operation time studied, only the nor aerated bioreactor had saturation, after 228 and 216 operation hours, respectively. The chrome removal, in the TDH adopted, were satisfactory in all concentrations studied. In the regeneration, it was verified, in all assays performed, that the chrome concentration in the bioreactor output became practically nil in a maximum of 72 hours of operation. In the blank test, the COT removal reached was 64% while in the 3 assays using chrome, it was around 55 % (80 mg/L), 41 % (120 mg/L) and 31 % (150 mg/L). In the bed regeneration, the COT removal increased, indicating that the presence of chrome affects the COT removal, due to the possible metal accumulation. The DQO concentration had a similar profile to the COT concentration kinetic. After addition of a new inoculum to the system, the kinetic assay was repeated in concentration of 150 mg/L of Cr (VI), being that in 600 operation hours, the bioreactors did not saturated, obtaining removal of 67% for Cr (VI) and 46% for total Cr. The COT removal was not significantly changed. The biomass concentration remained practically constant in the experiment, except in the beginning of each kinetic, where it was verified the occurrence of death and cell lysis, due to the increase of the Cr (VI) concentration added. However, during the process the mixed culture readapted to the new conditions. It was performed the cell opening by ultra sound where its noticed a low quantity of chrome retained inside it, by bioaccumulation. This indicates that most of the chrome is bio-absorbed in the cell surface (deposited), of living and dead cells, or bio accumulated in living cells. Given the above, the hybrid bioreactor system, operating in series, showed promising to the chrome removal. However, some changes in the system and in the process itself, as the periodical reposition of biomass, are needed to a better performance.