Process development in 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

• 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
  
  

For the production of tailor-made microalgae biomass of P. tricornutum, a diatom containing significant amounts of laminarin as the major storage product, a two-stage process was developed at Fraunhofer IGB and established at the Fraunhofer CBP pilot plant facilities in Leuna, Germany. Laminarin is an energy and carbon storage molecule in P. tricornutum and is of interest for the application in the food, feed and agricultural sector as it has immunomodulatory properties.

Novel two-stage production process

In the first stage, biomass was produced under optimum growth conditions. In the second stage laminarin accumulation was induced by N-limitation starting with 5 g dry weight per liter (DW/L) and the final harvest concentration was achieved at 10 g DW/L. Under these nutrient deprived conditions, not only biomass concentration doubled but laminarin accumulated up to 25 percent (w/w) of dry weight. The biomass contained also > 1 percent fucoxanthin and > 3 percent EPA. After harvesting, fresh culture media was provided to a small amount of culture and the cycle started again. The culture recovered after every subsequent cycle even in spite of 10 days in disadvantageous conditions.

Scale-up with modularized reactor composite

For this two-stage process, an accelerated scale-up strategy was established starting in a 30 L flat panel airlift reactor (FPA) with artificial illumination. This pre-culture was transferred to a novel 900 L (5 x 180 L FPAs) modularized greenhouse reactor composite and then transferred to a final production volume of 2700 L in additional 10 x 180 L FPA reactors outdoors for a long-term production experiment. From April to September over 80 kg biomass were produced with half the production volume compared to previous experiments with an average productivity of 0.4 g/L*d.

Literature

[1] Brinitzer, G.; Kuhnhardt, C.; Frick, K.; Derwenskus, F.; Schmid-Staiger, U.  (2019) Tailor-made production of chrysolaminarin-rich P. tricornutum biomass in a two-stage process with flat panel airlift reactors in pilot scale, Talk AlgalEurope 2019

Although the basic mechanism of microalgae growth has been well studied, there are only a few mathematical models that can be used to model microalgae growth. Such models are particularly important for the large-scale cultivation of microalgae and serve as a basis for a robust, predictive control system. An essential component of this system are algorithms that enable automated optimization of microalgae growth. So-called machine learning has been widely used for prediction and optimization in different areas. To predict the growth behavior of the microalgae Phaeodactylum tricornutum in outdoor cultivation, so-called Support Vector Machines (SVM) were used. The results show that the SVM-based model can predict the growth rate of Phaeodactylum tricornutum with a correlation coefficient of 88%. At the same time, a model with Monod kinetics yields a correlation coefficient of 82%. These two models will be further validated on both laboratory and pilot scale in order to establish a model-predictive control for microalgae production.