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Two-phase partitioning bioreactors (TPPBs) in environmental biotechnology are based on the addition of a non-aqueous phase (NAP) into a biological process in order to overcome both mass-transfer limitations from the gas to aqueous phase and pollutant-mediated inhibitions. Despite constituting a robust and reliable technology in terms of pollutant biodegradation rates and process stability in wastewater, soil, and gas treatment applications, this superior performance only applies for a restricted number of pollutants or contamination events. Severe limitations such as high energy requirements, high costs of some NAPs, foaming, or pollutant sequestration challenge the full-scale application of this technology. The introduction of solid NAPs into this research field has opened a promising pathway for the future development of TPPBs. Finally, this work reviews fundamental aspects of NAP selection and mass transfer and identifies the niches for future research: low energy-demand bioreactor designs, experimental determination of partial mass transfers, and solid NAP tailoring.

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Silicone oil 20 and 200 cSt, a perfluorocarbon (FC40TM), heptamethylnonane, Kraton, Elvax, and Desmopan were evaluated for their ability to enhance oxygen transfer in stirred tank and airlift reactors (STR and ALR, respectively). None of the vectors tested was either toxic or biodegradable and they exhibited a moderate affinity for oxygen (gas/vector partitioning coefficients K(g)/(v) = C(g) times C(v)(-1) ranging from 3 to 5.1). FC40 was highly volatile, while KratonTM and ElvaxTM exhibited a low thermal stability, which constitutes a serious handicap for their implementation in fermentations. Silicone oil 20 cSt and Desmopan supported the highest oxygen transfer rates under abiotic conditions in both STR and ALR designs, with enhancement factors of up to 90% and 250%, respectively, compared to control tests (deprived of vector). The fact that these vectors showed the highest K (g/v) proved that, besides the classical selection criteria, the in situ hydrodynamic behavior (which affects K ( L ) a) must be considered for vector selection. The use of silicone oil 20 cSt and Desmopan in glucose-supplemented Saccharomyces cerevisiae fermentations resulted in a two- and threefold increase in biomass productions, respectively. The better performance of Desmopan in terms of biomass growth enhancement, together with the absence of the operational problems inherent to the use of liquid vectors (such as intensive foaming, high cost, and difficult solvent recovery), make solid vectors a promising and cost-effective alternative in the future developments of two-phase partitioning bioreactors.

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