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Foam-bed reactors are known for the physical-chemical treatment of gases [15]. Although foam-beds are not yet applied in industry, continuous research is being conducted in this field. These reactor types are advantageous for contaminant removal of gas streams because of their increased interfacial area. Due to the large interfacial area, high mass transfer rates can be achieved, facilitating the absorption of compounds from the gas phase to the liquid phase. In the liquid phase, these compounds are absorbed and can be modified via chemical reactions. Next to large interfacial area, foam-beds are advantageous because they offer a low pressure drop and increased contact time between the gas and liquid phases [16-18]. Foam-beds can also be operated as bioreactors. Similarly to foam-beds for chemical treatments, bioactive foam reactors are also used for contaminant removal of gas streams. However, instead of chemical reaction of the absorbed pollutants, the microorganisms dispersed in the foam phase are responsible for the degradation [19]. The degrading microorganisms are actively growing at high cell density in the liquid phase of the foams. When microorganisms are applied, a biocompatible surfactant is required. Also for these biological foam-beds the advantage relies on the increased surface area of the foam leading to high pollutant removal rates. The absorption of organic pollutants can be further facilitated by the introduction of an organic-phase dispersed in the liquid phase of the foam as an emulsion [20, 21]. Additionally, our goal was to characterize the potential of this novel cultivation concept in a theoretical model, which can serve as a basis for further optimization and scale-up of foam-bed photobioreactors. In Chapter 2 the proof-of-principle of the foam-bed photobioreactor concept is described. In that study, we determined whether microalgae cultivation in liquid foam-beds is possible. For the development of a practical and operative reactor design, foam formation and foam break up were studied in detail. In order to break the foam, different foam breaking methods were compared and evaluated. As a foam stabilizing agent, bovine serum albumin (BSA) was applied because it is a natural, non-toxic protein with good foaming capacity. Foam formation was This chapter

A novel liquid foam-bed photobioreactor has been shown to hold potential as an innovative technology for microalgae production. In this study, a foam stabilizing agent has been selected which fits the requirements of use in a liquid foam-bed photobioreactor. Four criteria were used for an optimal surfactant: the surfactant should have good foaming properties, should not be rapidly biodegradable, should drag up microalgae in the foam formed, and it should not be toxic for microalgae. Ten different surfactants (nonionic, cationic, and anionic) and two microalgae genera (Chlorella and Scenedesmus) were compared on the above-mentioned criteria. The comparison showed the following facts. Firstly, poloxameric surfactants (Pluronic F68 and Pluronic P84) have acceptable foaming properties described by intermediate foam stability and liquid holdup and small bubble size. Secondly, the natural surfactants (BSA and Saponin) and Tween 20 were easily biodegraded by bacteria within 3 days. Thirdly, for all surfactants tested the microalgae concentration is reduced in the foam phase compared to the liquid phase with exception of the cationic surfactant CTAB. Lastly, only BSA, Saponin, Tween 20, and the two Pluronics were not toxic at concentrations of 10 CMC or higher. The findings of this study indicate that the Pluronics (F68 and P84) are the best surfactants regarding the above-mentioned criteria. Since Pluronic F68 performed slightly better, this surfactant is recommended for application in a liquid foam-bed photobioreactor. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:711-720, 2018.

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