Enhanced microalgal biomass cultivation on anaerobic digestate depends on species selection, media characteristics and ammonium concentration

Microalgae cultivation on anaerobic digestate can contribute to the recovery and reuse of nutrients, but special attention is needed to achieve optimum digestate pre-treatment to support algal growth. Assessing culture performance based on feedstock dilution factors as the sole pre-treatment overlooks temporal variability in digestate composition, leading to poor performance reproducibility and potential operational failure at large scale. To address this, we implemented a one-variable approach to systematically isolate and evaluate the effects of key growth-limiting factors across digestates from pig waste, food waste and sewage sludge. We screened six microalgal species, identifying Chlorella sorokiniana and Chlorella vulgaris as the most resilient species, although their performance varied according to digestate source and dilution factor. Using the one-variable framework, we assessed the impact of increasing ammonium (NH 4 –N) concentrations and their interactions with turbidity and organic matter in autoclaved digestate. This allowed for a systematic assessment of the effect of increasing ammonium concentrations on microalgal growth and interactions with other digestate characteristics, while removing bacterial effects. C. sorokinana was more affected by organic matter and NH 4 –N concentrations than NH 4 –N alone. C. vulgaris was more affected by turbidity but displayed improved growth in high organic-content with elevated NH 4 –N levels (up to 300 mg-N L −1 ). These findings are interpreted within the context of photosynthesis, respiration and NH 4 –N assimilation and they were used to demonstrate that microalgal growth is highly dependent on species selection and media characteristics, that cannot be reproduced by simply choosing a set dilution factor. Tailoring cultivation conditions based on this framework is therefore essential to improve reproducibility and scalability in digestate-based microalgal systems.