Schädler T, Neumann-Cip A-C, Wieland K, Glöckler D, Haisch C, Brück T, Weuster-Botz D Applied Sciences (3 June 2020)
(1) Background: Recycling of water and non-converted nutrients is considered to be a necessity for an economically viable production of microalgal biomass as a renewable feedstock. However, medium recycling might also have a negative impact on algal growth and productivity due to the accumulation of growth-inhibiting substances. (2) Methods: Consecutive batch processes with repeated water recycling after harvesting of algal biomass were performed with the saline microalga Microchloropsis salina in open thin-layer cascade photobioreactors operated at a physically simulated Mediterranean summer climate. The impact of water recycling on culture performance was studied and the composition of the recycled water was analyzed. (3) Results: Water recycling had no adverse effect on microalgal growth and biomass productivity (14.9−21.3 g m−2 d−1) if all necessary nutrients were regularly replenished and KNO3 was replaced by urea as the nitrogen source to prevent the accumulation of K+ ions. Dissolved organic carbon accumulated in recycled water, probably promoting mixotrophic growth. (4) Conclusion: This study shows that repeated recycling of water is feasible even in high-density cultivation processes with M. salina of more than 30 g L−1 cell dry weight, increasing culture performance while reducing nutrient consumption and circumventing wastewater production.