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.

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

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

Scale-up of microalgae-based processes and customized production of functional ingredients

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.

Comparison of the biomass accumulation in 2019 with previous experiments.

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.

Comparison of the scaling strategy based on the volume used in scale-up and final production with previous experiments.

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

Machine learning for algae cultivation

Simulation of light distribution in a FPA reactor.

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.

Reference projects

FuTuReS – Evaluation of a biorefinery approach for the production of fucoxanthin and EPA on an industrial scale

The aim of the project is the economic and ecological characterization of a process for co-production of the carotenoid fucoxanthin and the omega 3 fatty acid eicosapentaenoic acid (EPA) with the diatom Phaeodactylum tricornutum on an industrial scale. For this purpose, real process data is used, and the use of residual material flows as well as surplus electricity for artificial lighting, is taken into account.

Automation concept for outdoor production of algal biomass

Oil from microalgae is a potential alternative to plant biofuels and is considered to be a "third generation" biofuel. Compared to the cultivation of higher plants, there are numerous advantages: a higher yield per area, reduced water requirements and the possibility of cultivating microalgae on land that cannot be used for agriculture.