Application of ultrasound and air stripping for the removal of aromatic hydrocarbons from spent sulfidic caustic for use in autotrophic denitrification as an electron donor

Spent sulfidic caustic (SSC) produced from petroleum industry can be reused to denitrify nitrate-nitrogen via a biological nitrogen removal process as an electron donor for sulfur-based autotrophic denitrification, because it has a large amount of dissolved sulfur. However, SSC has to be refined because it also contains some aromatic hydrocarbons, typically benzene, toluene, ethylbenzene, xylene (BTEX) and phenol that are recalcitrant organic compounds. In this study, laboratory-scale ultrasound irradiation and air stripping treatment were applied in order to remove these aromatic hydrocarbons. In the ultrasound system, both BTEX and phenol were exponentially removed by ultrasound irradiation during 60 min of reaction time to give the greatest removal efficiency of about 80%. Whereas, about 95% removal efficiency of BTEX was achieved, but not any significant phenol removal, within 30 min in the air stripping system, indicating that air stripping was a more efficient method than ultrasound irradiation. However, since air stripping did not remove any significant phenol, an additional process for degrading phenol was required. Accordingly, we applied a combined ultrasound and air stripping process. In these experiments, the removal efficiencies of BTEX and phenol were improved compared to the application of ultrasound and air stripping alone. Thus, the combined ultrasound and air stripping treatment is appropriate for refining SSC.