Avaliação de diferentes estratégias de remoção de BTEX em condições anaeróbias

Abstract

The compounds benzene, toluene, ethylbenzene and xylenes (BTEX) are monoaromatic hydrocarbons compounds which represent a high risk for the environment, mainly due to their toxic and carcinogenic characteristics. Among the methods usually applied for the removal of BTEX from contaminated waters, anaerobic treatment has drawn attention especially because of its low cost. Accordingly, anaerobic biodegradation of BTEX was assessed under methanogenic, denitrifying and sulfidogenic conditions. Assays were performed in three bitstream anaerobic bioreactors that were fed with a synthetic solution of BTEX (~ 3 mg.L-1 of each compound) solubilized in ethanol and operated at 27°C. The concentrations of BTEX were determined by chromatography. In methanogenic bioreactor were analyzed the following influences: 1) the The hydraulic retention time (HRT) (48 h, 36 h and 24 h), 2) the shock loading, 3) the effluent recycling , 4) and microaerobic system. The sulfidogenic reactor was operated with a HRT of 48 h with different DQO/SO4-2 ratios of approximately 12, 5, 2.5, and stoichiometric with different organic fillers. The denitrifying reactor was also operated with a HRT of 48 h in the rations DQO/NO3-1 of approximately 12 and 5. The reactors evaluated were quite stable during all phases of the experiment. About BTEX removal, in general, benzene showed the lower removal efficiencies, regardless of the electron acceptor, indicating the difficulty of biodegradation of this compound under anaerobic conditions. The highest removal efficiencies were observed for toluene and xylenes, reaching up to 90% removal. These values take into account possible interferences of adsorption and volatilization. It was also noted that there should be a synergy between the different compounds that could have a strong effect on the removal efficiencies of BTEX. Comparing the three reactors, it was noted that there was significant improvement in the removal efficiency of the compounds in the presence of nitrate or sulfate. The methanogenic reactor showed high COD removal efficiencies higher than 80% on average for the phases with a high substrate concentration, producing biogas (mainly in the form of methane) not accumulating volatile fatty acids, and with enough alkalinity to buffer the medium at neutral pH. The phase that achieved the best results BTEX removal (about 94%) was the one that had a microaerobic system added to methanogenic reactor, with COD removal efficiencies exceeding 85%.