Removal of Gaseous O-Xylene in the Two-Liquid Phase Biotrickling Filters and Airlift Bioreactors

O-xylene is the most recalcitrant among the three xylene isomers which are released into the atmosphere as a kind of fugitive emission from industrial production with healthy risk and environmental pollutions. Biotechnologies show off their advantages of completely degrading low-concentration contaminants into end-products like carbon dioxide, water, biomass, salts even biofuels since the 1920s. However, the practically insolubility of o-xylene may cause obvious deterioration in the mass transfer, thus reducing the performance of bioreactors.

Recently, an emerging biotechnology for removal of hydrophobic volatile organic compounds (VOCs) is the two-liquid phase bioreactor (TLPB). In a TLPB, an organic liquid-phase, which is always immiscible, non-biodegradable and biocompatible, is added to for an improvement of the VOC dissolution. A large variety of bioreactor configurations have been utilized in the research of TLPB so far, of which the continuous stirred tank bioreactor (CSTB) is the most popular system. Nevertheless, as a traditional bioreactor, the biotrickling filter (BTF) also showed an elimination capacity improvement of more than 100% relative to the one without silicone oil as non-aqueous phase. Although the airlift bioreactors (ALBR) didn’t have a significant enhancement after adding silicone oil, it would be different for the abatement of gas-phase o-xylene. Actually, the literatures about the removal of o-xylene from off-gas in the BTF and ALBR using silicone oil as non-aqueous phase are sparse.

The main aim of this research was to make a comparison observation on the performance of a two-liquid phase (TLP) BTF and ALBR for the biodegradation of o-xylene existing in contaminated air under different empty bed residence times (30-60 s), inlet loading rates (0-100 g m^-3 h^-1) as well as the response to shock-loads. The pilot plant consisted of two identical bioreactors with or without silicone oil to water phase ratio of 5% operating in parallel. Removal efficiency increased as the EBRTs were improved and inlet concentrations of o-xylene were decreased, exhibiting higher elimination capacities at higher inlet loading rate. Under shock-load condition, adding 5% silicone oil, removal efficiency of the biotrickling filter was dropped 23%, while that of the airlift bioreactors was essentially maintained at approximately 90%  when the inlet concentration was increased from 500 mg m^-3 to 1500 mg m^-3. When inlet concentration was recovered, the two-liquid phase airlift bioreactor could restore its removal efficiency to 100%, but the two-liquid phase biotrickling filter could not, which reveals that the former was more stable than the latter because of fully stirring and protection from silicone oil.