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A liquid digestate rich in ammonium nitrogen (8.3 g L−1) was collected from an agricultural biogas plant and supplied to microalgae cultures as their only nitrogen source. Chlorella vulgaris was cultivated for up to 21 days, both under controlled conditions in laboratory-scale glass-column photobioreactors as well as outdoors in an open pilot-scale thin-layer photobioreactor. By systematically addressing issues associated with the use of liquid digestate (i.e., turbidity, nutrient imbalance, ammonium toxicity, and acidification), microalgae were robustly cultivated at a high density and cultures achieved a net biomass dry weight of between 10 and 14 g L−1, and a productivity of up to 0.93 g L−1 d−1 (93% of maximum expectation). Cultivation in the thin-layer photobioreactor achieved areal productivities between 7 and 10 g m−2 d−1. Water acidification due to the uptake of ammonium by microalgae was prevented by a controlled addition of NaOH. A detailed mass balance showed that, despite high removal efficiencies (approximately 3% of the supplied nitrogen remained in the medium), microalgae assimilated only 40–60% of the supplied nitrogen and, consequently, a large amount of nitrogen was lost to the atmosphere. [Display omitted] • Liquid digestate was used as nitrogen source to cultivate microalgae. • Chlorella vulgaris was cultivated successfully at laboratory and pilot scales. • Algal biomass reached net dry weights of up to 13.7 g L−1. • Acidification of the growth medium and ammonium toxicity were controlled. • 40–60% of the nitrogen was lost to the atmosphere during the cultivation. [ABSTRACT FROM AUTHOR]