Design of closed photobioreactors for algal cultivation

Abstract

Apart from their indispensable role as solar-driven oxygen factories, microalgae act as powerful microbial cell factories for production of various intra- or extracellular bio-products like proteins, lipids, pigments, well-known and exotic carbohydrates, biopolyesters, antibiotics or bio-hydrogen. These products can serve the demandsof various markets such as the fuel- and energy sector, cosmetic industry, pharmaceutical industry, convenience- and functional food, and agriculture, or even constitute novel raw-materials for manufacturing of biodegradable plastic materials. Efficient output of these products by using selected microalgal species requires the adaptation of the cultivation system to the special requirements of different microalgae. Factors like protection against microbial contamination, optimized nutrient supply, tailored illumination, sufficient outgassing of the produced oxygen, and maintaining pH-value and temperature in the optimum range have to be taken into account when designing an algae-based production platform for bio-products. Simple, well-known open cultivation systems are operating at typical natural environmental conditions which are far below the real biosynthetic potential of these microbial cell factories. As a common consequence, such systems only produce modest cell densities at low volumetric productivity. Closedsystems allow for the adaptation of process conditions to the optimum values inherent in the different species, provide the possibility to implement more effective illumination systems, prevent water loss by evaporation, avoid the entrance of competing microbes into the system, and circumvent the release of the algal cells into the environment. Hence, high output for desired algal bio-products requires the development of sophisticated closed photobioreactor (PBR) systems; they are designed based both on deep understanding for microbial processes and on process engineering know-how. Such optimized design, mimicking natures strategies for light harvest, constitutes the pre-requisite for economic success of phototrophic biotechnology that now is already announced since decades. The chapter at hands offers a detailed overview of different used typesof photobioreactors for cultivation of microalgae, highlighting their opportunities, advantages and constraints, devotes special attention to the scalability of different PBR systems, and provides examples for successful (semi)industrial implementations.