Photobioreactor

A photobioreactor is a bioreactor that utilizes a light source to cultivate phototrophic microorganisms. These organisms use photosynthesis to generate biomass from light and carbon dioxide and include plants, mosses, macroalgae, microalgae, cyanobacteria and purple bacteria. Within the artificial environment of a photobioreactor, specific conditions are carefully controlled for respective species. Thus, a photobioreactor allows much higher growth rates and purity levels than anywhere in nature or habitats similar to nature. Hypothetically, phototropic biomass could be derived from nutrient-rich wastewater and flue gas carbon dioxide in a photobioreactor.

The first approach for the controlled production of phototrophic organisms was and still is a natural open pond or artificial raceway pond. Therein, the culture suspension, which contains all necessary nutrients and carbon dioxide, is pumped around in a cycle, being directly illuminated from sunlight via the liquid’s surface. This construction principle is the simplest way of production for phototrophic organisms. But due to their depth (up to 0.3 m) and the related reduced average light supply, open systems only reach limited areal productivity rates. In addition, the consumption of energy is relatively high, as high amounts of water containing low product concentration have to be processed. Open space is expensive in areas with a dense population, while water is rare in others. Using open technologies causes high losses of water due to evaporation into the atmosphere.

Since the 1950s several approaches have been conducted to develop closed systems, which theoretically provide higher cell densities of phototrophic organisms and therefore a lower demand of water to be pumped than open systems. In addition, closed construction avoids system-related water losses and the risk of contamination through landing water birds or dust is minimized. All modern photobioreactors have tried to balance between a thin layer of culture suspension, optimized light application, low pumping energy consumption, capital expenditure and microbial purity. Many different systems have been tested, but only a few approaches were able to perform at an industrial scale.

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