Over the last decade climate change and a strategy of natural resources preservation led to a dramatic increase in the interest for aquatic biomass. Along with mitigation of the environmental pressure and a reduction of waste streams such as CO2 and heat emissions, microalgae can supply food, feed, pharmaceutical and power industries with a number of value added products. Microalgae are microscopic cells growing in aqueous media. They are able to use sunlight energy via photosynthesis and to transform inorganic compounds into valuable organic materials. Compared to terrestrial crops, they possess enormous growth rates without a dependency on fertile soil.
Microalgal growth and composition is affected by many different factors, namely light and nutrient availability, pH and temperature. In general, cultivation processes can be divided into open and closed systems. The design depends significantly on the strain, cost of land, the potential application of the final product, the source of CO2 and the nutrients required. Open systems, mainly represented by open ponds, consist of shallow channels where the algal culture is circulated by pumps or paddle wheels. They offer relatively low construction and operating cost and are simple to maintain. However, open systems are susceptive to contaminations and weathering and have a reduced availability of sunlight for the single cells. The disadvantages of open systems can be avoided by closed and transparent photobioreactors. Their design parameters are leading to a basic classification into tubular, flat panel or column systems. Next to a better control of parameters like temperature, pH and nutrient concentration, these systems ensure a lower contamination risk, better mixing and a better availability of sunlight. However, they come with a higher cost, difficult scale up, material wear, and with a sophisticated cooling system.