Biodegradación anaerobia de tricloroetileno (TCE) en un reactor UASB

Abstract

Current different reports indicate that sulfate reduction (SR) is an alternative to be used in wastewater treatment from many types of industrial effluents containing high sulfate concentrations as well as various organic pollutants such a trichloroethylene (TCE). Therefore, the general aim of this work was to study the biological process of sulfate reduction in order to use it for the biodegradation of TCE to less toxic compounds such as ethene using an upflow anaerobic sludge blanket (UASB) reactor inoculated with hydrothermal vents sediments. First at all, SR was carried out in long term batch-fed microcosms experiments incubated at mesophilic temperature using an initial sulfate concentration of 4000 mg/L and two different electron donors, lactate (500 mg DQO/L) and a mixture of volatile fatty acids (VFAs) (350 mg DQO/L). For lactate, SR reached up to 95% in 403 days and for VFAs SR was 95% in 209 days. Once SR was established in the microcosms, TCE was added to a final concentration of 100 and 260 μM for experiments with lactate and VFAs respectively. TCE biodegradation was 89% in 8 days for lactate experiments and 70% in 5 days for incubations using VFAs as substrate. At a second stage, the UASB reactor was inoculated with the sediments (4.7 g VSS/L) along with a sulfate concentration of 4000 mg/L and operated in a semi-continuous regime for a little longer than one year in order to enrich the sediments in sulfate reducing bacteria (SRB) within a robust microbial community. The operation of the reactor was divided in eight periods. Acclimation stage includes from period one to five (309 days), a span of time in which the bioreactor was maintained under appropriate conditions in order to enrich it in adequate biomass. During these periods, SR reached 98% with a H2S concentration of 2900 mg/L corresponding to 70% of sulfate conversion. The values obtained for the sulfate reducing activity (SRA) (188 mg DQO-H2S/g VSS*d) demonstrate the appropriate development of SRB in the inoculum of the bioreactor. During period VI (days 310-319) SR kinetics were carried out at hydraulic retention time (HRT) of 12 h at an initial sulfate concentration of 4000 mg/L, yielding 98% of sulfate reduction in the first 4 h. In this kinetics, the total H2S concentration was 1200 mg/L (which is equivalent to 30% of sulfate converted to H2S) along with a COD removal of 70% at the end of the kinetics. The TCE biodegradation under sulfate reducing conditions (initial sulfate concentration 4000 mg/L) in the UASB was conducted in period VII (days 320-348), for which two concentrations of TCE were tested (250 and 300 μM) along with three different HRTs (24, 12 and 8 h) for each TCE concentration. The best biodegradation efficiencies were 77% of TCE removal (TCE initial concentration 260 μM) at HRT of 8 h and 74% of TCE removal (TCE initial concentration 300 μM) at HRT of 12 h respectively. Under the established conditions for TCE biodegradation in the UASB reactor, SR was of 98% within the first 5 h at each TCE concentration and HRT. Finally, the objective of period VIII (days 349-365) was to determine SR, H2S production and COD consumption in the UASB after the period in which TCE was biodegraded in order to estimate the damage (if any) that TCE had caused to the microbial community in the reactor. The overall results of this work demonstrate that the reactor can function efficiently for both, sulfate and TCE removal in short periods of time. The present work will be used as background for precipitation of metals using the same reactor. Ongoing experiments to describe the microbial ecology of the UASB reactor are being conducted.