Process optimisation for the reuse of growth medium in the cultivation of marine microalgae in a closed photobioreactor

Microalgae have application in aquaculture, as feed, functional food, chemicals, pharmaceuticals, cosmetics and biofuels. Despite their high potential, widespread large-scale cultivation of microalgae especially for low-value applications struggles to get off the ground. To increase the economic viability of bulk cultivation, production costs need to be reduced. Recycling growth medium not only allows salvaging the unused nutrients remaining in the water after harvesting, but also permits to reduce water demand and to avoid or reduce waste water treatment costs. To be able to implement a recycling method in a fully automated production system of microalgae, several challenges such as cleaning of spent medium, replenishment of nutrients and comprehending positive or negative effects on microalgal growth needed to be addressed. This study investigated a method to rejuvenate centrifuge supernatant in a robust and inexpensive way based on high pH induced flocculation and sandfiltration. Next, nutrient consumptions, with and without medium reuse, were mapped and dosing strategies for an automated replenishment were formulated. Besides, understanding was gained in the effect of medium recirculation on the cultivation of Nannochloropsis sp. and Tisochrysis lutea, separately and across species borders. The results of this thesis led to the implementation of a medium recycling method in a fully automated production system of microalgae, which in turn contributes to the economic viability of a large-scale cultivation of microalgae