Spaceship EAC: Development of an interface between PBR and C.R.O.P.® for nutrient supply of microalgae

Abstract

The present thesis deals with the development of an interface between a photobioreactor (PBR) and a biofilter system for nutrient supply of microalgae. First, the working principle of the Combined Regenerative Organic food Production® or C.R.O.P.® system, developed by the German Aerospace Center (DLR), is described. This includes the underlying nutrient cycles and chemical processes, the set-up of the filter unit, as well as advantages and applications. The chosen type of microalgae, Chlorella vulgaris, is explained and its cultivation in photobioreactors is demonstrated. Four different cultivation approaches are compared, with one being selected for use in this work; two reference cultivation cycles are conducted at the European Astronaut Centre (EAC) with an airlift photobioreactor. The results are explained and compared to previous studies. A separate bottle cultivation with different nitrate concentrations is conducted in order to investigate the influence of nitrate concentrations on the growth rate of microalgae. Then, the interaction between a biofilter and PBR is explained; its importance and meaning are outlined and present challenges are outlined. Before the nutrient solution, produced by the C.R.O.P.® filter, can be added to the PBR, bacteria has to be removed. Therefore, different sterilisation methods are explained and compared. Crossflow filtration and membrane diffusion are chosen as the most promising options. Two different types of dialysis membranes are tested in standalone experiments to prove the concept of nutrient diffusion. The experimental set-up and the results are explained. Selectivity of certain substances was not observed with the given membranes. One of the membranes is tested in algae solution with C.R.O.P.® as fertiliser; growth of the algae culture proves the possibility to supply microalgae with nutrients by the means of membrane diffusion. Possible connection methods of PBR and C.R.O.P.® are investigated to minimise human interaction and to automate the cultivation process as much as possible. A nutrient exchange chamber is designed using rapid prototyping, into which membranes can be inserted. The chamber can be implemented into a bypass of the PBR. Lastly, automated feeding of the algae culture using a computer controlled peristaltic pump and a crossflow filter is investigated. The planned set-up and its advantages are explained, encountered problems are shown, and experimental methods for a cultivation with this new set-ups is explained. This cultivation shows promising results, as the highest optical density ever achieved at EAC was observed.