Galdieria sulphuraria (G. sulphuraria ) is a eukaryotic, extremophilic, spherical and unicellular species of red algae. G. sulphuraria can grow at very low pH-values (pH 0.05 – 5.0) and high temperatures (35 – 56 °C). The growth conditions of G. sulphuraria make it suitable for axenic cultivation because the low pH and high temperature minimalize the risk of microbial contamination. Next to its ability to remove nutrients in wastewater treatment, G. sulphuraria is a prospective producer of a valuable product, Phycocyanin (PC), a thermostable blue pigment-protein complex, which is used as, among others food additive and food colorant.

These characteristics of G. sulphuraria lead its selection by Evides Industriewater for the uptake of the ammonium present in the reverse osmosis (RO) concentrate of New Energy and REsources from Urban Sanitation (NEREUS). NEREUS focuses on the re-use of nutrients present in wastewater, among others ammonium. One of the goals of NEREUS is to re-use the ammonium present in the RO concentrate with the use of algae. In order to recover ammonium from the RO concentrate of NEREUS, it is necessary to test whether G. sulphuraria is capable of growing on such medium. The possibility of cultivating of G. sulphuraria on the RO concentrate from water and resource recovery pilot plant of NEREUS was investigated in this thesis.

The objectives of this thesis were to find the optimal growing conditions and assess the biomass growth and nutrients consumption. Screening experiments with synthetic Allen medium, which is usually used for the cultivation of the G. sulphuraria, were conducted to obtain the best growing conditions of G. sulphuraria. In order to understand the best growing conditions for the cultivation of G. sulphuraria, the effects of several factors were investigated, which are: 1) different metabolism, 2) different nitrogen sources and concentrations (ammonium: 100 – 1000 mgNH4+-N/L and nitrate: 247 mgNO3--N/L), 3) different carbon sources (glucose, bicarbonate and CO2) and different glucose concentrations (C:N = 5:1 and 10:1), 4) different phosphate concentrations (N:P = 37:1 and 7.2:1), 5) culture densities. Ammonium with mixotrophic metabolism turned out to be the best nitrogen source. Biomass concentration on ammonium was four times higher than on nitrate. Increasing the ammonium concentration from 200 mgNH4+-N/L to 1000 mgNH4+-N/L resulted in around 25% more biomass and no firm conclusions could be drawn from the experiment performed with different phosphate concentrations. No significant increase in the growth of G. sulphuraria was observed between Carbon:Nitrogen (C:N) ratio = 5:1 and 10:1. Furthermore, culture densities higher than 0.7 g/L of biomass resulted to a slower growth of G. sulphuraria.

Experiment with synthetic RO concentrate shows that there was light limitation involved during the cultivation. Highest and fastest growth (µmax = 0.78 day-1) was observed in the mix of 40% real RO concentrate and 60% synthetic RO concentrate medium culture. Growth inhibition was observed in cultures containing RO concentrate of NEREUS. Still, G. sulphuraria did grow on RO concentrate of NEREUS. This work is contributing to the scientific and engineering community in the field of microalgae.