Potential of a liquid foam-bed photobioreactor for microalgae cultivation

Liquid foams: Properties, structures, prevailing phenomena and their applications in chemical/biochemical processes

Liquid foams are gas-liquid dispersions with flexible structures that provide high gas-liquid interfaces. This property nominates liquid foams as excellent gas-liquid contactors, systems that are widely used in the chemical and biochemical industries. However, challenges such as a lack of comprehensive understanding and foam instability have historically hindered their widespread industrial use in most applications. It was not until the recent development of nanofluidics, nanotechnology, surface science, and other related fields that the understanding, analysis, and control of foam phenomena improved. This led to the development of innovative stabilization techniques and foam-based unit operations in chemical and biochemical processes, each of which requires in-depth and exclusive reviews to fully comprehend their potential and limitations and to identify areas for further improvement and innovation. This paper reviews the foams, the common phenomena in them, the characteristics that make them suitable for chemical/biochemical engineering, reports on their current applications and recent developments in this field.

Modeling and Simulation of Photobioreactors with Computational Fluid Dynamics—A Comprehensive Review

Computational Fluid Dynamics (CFD) have been frequently applied to model the growth conditions in photobioreactors, which are affected in a complex way by multiple, interacting physical processes. We review common photobioreactor types and discuss the processes occurring therein as well as how these processes have been considered in previous CFD models. The analysis reveals that CFD models of photobioreactors do often not consider state-of-the-art modeling approaches. As a comprehensive photobioreactor model consists of several sub-models, we review the most relevant models for the simulation of fluid flows, light propagation, heat and mass transfer and growth kinetics as well as state-of-the-art models for turbulence and interphase forces, revealing their strength and deficiencies. In addition, we review the population balance equation, breakage and coalescence models and discretization methods since the predicted bubble size distribution critically depends on them. This comprehensive overview of the available models provides a unique toolbox for generating CFD models of photobioreactors. Directions future research should take are also discussed, mainly consisting of an extensive experimental validation of the single models for specific photobioreactor geometries, as well as more complete and sophisticated integrated models by virtue of the constant increase of the computational capacity.

Potential Impact of Biodegradable Surfactants on Foam-Based Microalgal Cultures

Microalgae cultivation in liquid foams is a promising concept which requires the use of a surfactant as a foam stabilizing agent. The biodegradable character of a surfactant is a key aspect regarding its applicability in a liquid foam-bed photobioreactor (LF-PBR), since it might influence microalgal growth and the stability of the foam-based cultivation. In this work, the effects of the biodegradable surfactants bovine serum albumin (BSA), Saponin and Tween 20 on the whole microbial community of microalgal cultures (i.e., microalgal and bacterial populations) were studied. The three surfactants enhanced bacterial and microalgal growth in non-axenic microalgal cultures, but they differed in their efficiency to sustain bacterial growth. In this sense, Saponin was proven to enhance the growth of S. obliquus-associated bacteria in microalgae-free cultures, and to sustain it even when other nutrients were lacking, suggesting that Saponin can be used as an energy and nutrients source by these bacteria. The degradation and consumption of Saponin by S. obliquus-associated bacteria was also confirmed by the foaming capacity decrease in Saponin-added bacterial cultures. The biodegradable character of BSA, Saponin and Tween 20 reduces their suitability to be used in a LF-PBR since they would not be able to maintain stable foaming.

Potential utilization of bioproducts from microalgae for the quality enhancement of natural products

Microalgae are autotroph organisms that utilise light energy to synthesize various high-value bioactive compounds such as polysaccharides, proteins and lipids. Due to its fast growth rate and capability to survive in harsh environment, microalgae nowadays are applied in various industrial areas. The process of obtaining microalgae-based biomolecules starts with the selection of suitable microalgae strain, cultivation, followed by downstream processing of the biomass (i.e., pre-treatment, harvesting, extraction and purification). The end products of the processes are biofuels and other valuable bioproducts. Nevertheless, low production yield and high-cost downstream processes are the emerging bottlenecks which need to be addressed in the upscaling of extracted compounds from microalgae biomass. To conclude, tremendous efforts are required to overcome these challenges to revolutionize microalgae into a novel and green factory of different bioactive compounds for industrial necessities to satisfy and fulfil global demands.