Process development in photobioreactors

Flat panel airlift photobioreactor

© Fraunhofer IGB

Outdoor facility at the Fraunhofer CBP

The most important process parameter in the mass cultivation of microalgae in photobioreactors is the light intensity, which has an impact on every algal cell in the photobioreactor volume. This determines the biomass productivity and thus the growth rate and cell concentration of the algae in the reactor. To achieve high cell concentrations, the light availability for every individual cell in the photobioreactor has to be increased.

The photobioreactor system developed and patented (Patent number WO 00926833.5; EP 1326959) at the Fraunhofer IGB and scaled-up by the Fraunhofer spin-off Subitec GmbH takes these parameters into account. Airlift-driven intermixing combined with static mixers offers efficient distribution of light with a low energy input for intermixing and low shear forces taking effect on the algal cells. Due to the static mixers, uprising gas bubbles induce definite vortices in the interconnected reactor compartments. In these definite vortices algal cells are transported at short intervals to the reactor surface to intercept high light intensities and then transported back to the dark. Sufficient CO2 and O2 mass transfer for unlimited growth is ensured by the combination of the airlift-driven principle and static mixers. The flat panel airlift (FPA) reactor is well-suited for small-scale and large-scale production of microalgae. The reactor itself is inexpensively made from two deep-drawn plastic sheets including static mixers, manufactured by twin-sheet technology.

In a scale-up process the volume of the FPA reactor was increased from 5 liters lab scale to 30 liters and finally to 180 liters by Subitec GmbH. The scale-up step to a pilot plant consists of linking several reactor modules (each 180 liters).

Automation of photobioreactors

To design an outdoor process which is independent of light and temperature, the Fraunhofer IGB developed an automation concept with an easily accessible measuring technique. The automation concept was achieved – in line with the current industry standard – with the aid of a programmable logic controller (SIMATIC S7-1200, Siemens).

Both reactor temperature and pH are controlled. Control of pH is achieved by control of CO2 concentration in the supply air: the higher the CO2 concentration in the supply air, the more becomes dissolved as carbon dioxide in the culture medium. This lowers the pH value. This is counteracted by the ammonium dissolved in the medium: the higher the ammonium concentration, the higher the pH value in the culture medium. If in such a system the pH value is constantly regulated by means of the carbon dioxide concentration in the supply air, this allows conclusions to be drawn about the ammonium concentration in the reactor. This correlation was used to determine the consumption of nutrients in the reactor. On the basis of these calculations, we were able to successfully control feeding cycles and exclude nutrient and carbon dioxide limitation.

When setting up the control software, it was ensured that it was very user and operator-friendly. The overall process is visualized on a display screen and all online data continuously recorded. The control software is constructed in a modular way and can therefore be implemented easily in new production facilities.

Advantages of automation system

© Fraunhofer IGB

Process visualization on the display screen of the SIMATIC S7-1200 controller.

  • Continuous process monitoring
  • Automated feeding and harvesting cycles possible


By estimating the amount of ammonium in the culture via CO2 concentration in the supplied air:
Allows constant nutrient supply

  • Allows consistent nutrient concentration in the culture due to low feeding amounts
  • Feeding of nutrients depends on nutrient consumption and is independent of weather conditions and therefore suitable for outdoor production
  • Growth limitations by culture medium components are detectable (via decreasing ammonium consumption rates)
  • Monitoring of growth is possible if correlation factor of nutrient demand per gram biomass is known

Reference projects

Robust automation concept for the outdoor production of algal biomass

 

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