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 digestate dilution factors as the sole pre-treatment overlooks temporal variability in effluent composition, leading to poor performance reproducibility and potential operational failure at large scale. We evaluated the growth of microalgae on digestate liquors from anaerobic digesters processing pig waste, food waste and sewage sludge, by screening six microalgal species. Chlorella sorokiniana and Chlorella vulgaris were identified as the most resilient species, but their performance varied according to digestate source and dilution factor. A one-variable approach was adopted to pinpoint individual factors affecting growth in all digestate sources. This allowed for a systematic assessment of the effect of increasing ammonium (NH ₄ –N) concentrations on microalgal growth and interactions with other digestate characteristics. 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 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 strategies to media properties is essential for optimising microalgal growth and ensuring scalability in microalgal based systems using anaerobic digestates.