Microalgae production in liquid foam-bed photobioreactors

Large-scale and low-cost production systems for microalgae are a necessity in order to exploit their huge potential. The goal of this study is the development of a novel microalgae cultivation design that could enable economically feasible cultivation by reducing biomass production costs.

In a foam-bed reactor small gas bubbles are generated via a gas distributor on the bottom of the reactor that are passed through a thin liquid layer of microalgae culture, containing foam stabilising agents. This results in foam formation in the reactor. The foam formed drags microalgal cells upward, generating a microalgae-enriched foam. Due to the continuous gas supply, the generated foam bubbles rise. Since growing microalgae continuously take up carbon dioxide and produce oxygen, the gas in the foam bubbles has to be refreshed in order to avoid CO2 limitation and O2 accumulation.

The cultivation of algae in liquid-foams might have several advantages over other cultivation systems. These advantages are the increased mass transfer and CO2 removal efficiency due to the big gas/liquid interfacial area offered by the foam and also due to the increased gas residence time in the photobioreactor. Besides, the pressure drop in the reactor is severely reduced and all will lead to a large reduction in energy costs for aeration. Moreover, the biomass density might be increased due to the shortened light path and consequently harvesting and processing costs can be severely reduced. Finally, the reduced pressure drop in the system leads to reduced material requirements which is beneficial for large scale systems. These benefits might give rise to an economically feasible algae cultivation prospect.

Aim

The objective of the project is to develop a liquid foam-bed photobioreactor as an innovative microalgae cultivation concept. The productivity of the reactor will be maximized by the development of efficient methods for foam formation and break-up, and by optimizing the operational parameters of the photobioreactor. In order to obtain the optimal reactor design, modelling of the system is inevitable. The final goal of the project is to maximize the productivity of the bench scale photobioreactor and design a conceptual large foam-bed photobioreactor.

Approach

Firstly, the project aims to reveal suitable foam stabilising agents and foam break-up methods in order to get controllable foam formation in a functional foam-bed reactor. To form foam, different foam stabilising agents will be assessed and different microalgae strains will be evaluated. In order to separate the gas phase from the liquid phase containing algae, different methods for foam break-up will be tested, which are either based on self-drainage of the foam or based on the utilisation of foam breaking devices.

Next, the performance of the foam-bed photobioreactor will be investigated, described and possibly further improved. The performance is described by productivity, biomass density, gas flow rates and pressure drop in the reactor. The reactor performance will be improved by optimizing the different operational parameters applied (gas flow rates, surfactant concentration, recirculation rates, batch vs. continuous operation), and also the reactor design will be improved by building a second generation lab scale photobioreactor. We also aim to develop a mathematical model describing microalgae growth in foam-bed photobioreactors, including light penetration, mass transfer and microalgal growth.

There are different thesis topics available for BSc or MSc thesis within the project! If you are interested, please feel free to contact me!

Acknowledgements

The work is performed in the framework of the MIRACLES project, which is supported by the European Commission through the 7th Framework Program under Grant Agreement No. 613588.