The application of biological processes for waste-gas treatment has broadened due to the wide variety of cross-level research that has been carried out in recent years. Researchers have made successful attempts to study the mechanism of pollutant removal, fundamental microbial ecology in bioprocesses, the waste-gas flow pattern and other specific properties in detail, in different bioreactor configurations. This review shows that biological treatment systems can be more effective, reaching >90% removal efficiencies, compared to physico-chemical techniques for handling moderately high loading rates of volatile organic/inorganic compounds present in waste-gases. The novel bioreactors described in this review has shown to handle high loading rates of volatile organic compounds (example: gas-phase toluene, styrene) under both steady and transient-state operations, and can easily overcome operating problems like clogging, channeling, pH control and oxygen limitations experienced during biofilter and biotrickling filter operation. For waste-gas stream containing mixtures of organic and inorganic pollutants, two-stage bioreactors have shown to perform better, as they are able to operate independently, using specific microorganisms that target these pollutants individually. The performance of suspended-growth bioreactors for handling hydrophobic volatile organic compounds has improved tremendously in the last few years, thanks to the ability of the non-aqueous organic phase that has almost completely eliminated the commonly reported mass transfer limitations. Further research should be directed towards developing bioreactors that can provide high interfacial area with high mass transfer rates, robustness for scale-up, tolerance to high temperatures, and can perform at very low gas-residence times (<15 s).

Acknowledgements The authors thank the Spanish Ministry of Science and Innovation (Project: CTM2010-15796/TECNO), and European FEDER funds for financial support. Eldon R. Rene thanks the Spanish Ministry of Science and Innovation (MICINN), for his "Juan de La Cierva"-post doctoral contract. The PhD research contract of María Montes was funded through project CTM2007-62700/TECNO and from a fellowship offered by the University of La Coruña for doctoral students. We also thank M. Estefanía López, PhD student, for assisting us in depicting some of the high-quality figures provided in this chapter.

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