ENHANCED BIODEGRADATION OF VOLATILE ORGANIC COMPOUNDS (VOCS) BY COMBINING A BIOREACTOR WITH NON-THERMAL PLASMAS TECHNOLOGY

Xiang-Qian Wang, Chao Wu, Su-Jing Li and Wei Li

College of Chemical and Biological Engineering, Zhejiang University, Zhejiang, China

Abstract

Volatile organic compounds (VOCs) and odorous compounds discharged into the environment create ecological and health hazards. Among VOCs, a considerable number of BETXs (benzene, ethylbenzene, toluene and xylenes) into the environment originates from industrial facilities. Organic odorous pollutants are usually related to mercaptans (R-SH), organic sulfides (R-S-R), and disulfides (R-S-S-R). Due to the high volatility, potential toxicity and the extensive use of these organic pollutants, most environmental releases partition to the atmosphere. As a consequence, it is inevitable to adapt effective technologies for the treatment of industrial VOC emissions, along with the more strict emission regulations in China.

Waste gas treatment processes using bioreactors have gained popularity in control of VOCs and odour, since they offer a cost effective and environment friendly alternative to conventional air pollution control technologies. However, industrial VOC emissions usually contained several recalcitrant pollutants (e.g., xylenes, toluene and mercaptans realased from the pharmaceutical industry), making it difficult for a conventional bioreactor to efficiently remove all of the target pollutants and strictly meet discharge limits in force. Therefore, one additional pretreatment step is required for removing these recalcitrant pollutants emitted from the industrial processes. Due to the high oxidation potential and reactivity of ozone and other reactive species, non-thermal plasmas technology has been widely used for removing recalcitrant pollutants from industrial VOC emissions. Nowadays, combination of a bioreactor with some physicalchemical approaches has attracted increasing attention in order to overcome the weaknesses of biodegradation-alone systems.

In this study, an innovative design of biotreatment system for the removal of the recalcitrant compounds by integrating pretreatment steps of non-thermal plasmas oxidation and water absorption with a biotrickling filter was studied. In this integrated process, a dielectric barrier discharge (DBD) reactor was adopted as a pretreatment step in order to transfer these recalcitrant compounds to easily biodegradable compounds. Furthermore, water absorption followed with the DBD reactor was designed to keep the stability of a biotrickling filter from the side-effect of overdose ozone, and simultaneously enhance the absorption of target pollutants into liquid medium. As the key process for the removal of VOCs and odorous compounds, a biotrickling filter inoculated with two new isolates (Pandoraea sp. WL1 and Pseudomonas sp. WL2) for removing a mixture of two model pollutants (p-xylene and ethyl mercaptan), was acclimated to steady-state operation. In this process, the integrated BTF system with a dielectric barrier discharge (DBD) reactor as the main pretreatment step showed a better performance (removal efficiency of 70%~100%) than that of the BTF-alone system (removal efficiency of 58%~77%) with total VOC concentrations of 300~1500 mg/m3 (1:1 ratio for ethyl mercaptan paired with p-xylene). Key interactions identified include inhibition of p-xylene degradation and enhancement of ethyl mercaptan degradation for a paired mixture relative to single substrate degradation. These indicate that this novel bioreactor could effectively overcome these operational limitations for poorly soluble and recalcitrant pollutants in the industrial application.

Keywords: Enhanced biodegradation, biotrickling filter, dielectric barrier discharge, volatile organic compounds.