Upcycling food waste digestate into single-cell microalgae protein through a mixotrophic cultivation approach

The effective use of digestate remains a significant challenge in adopting anaerobic digestion technology for sustainable urban food waste management. This study investigates the potential of converting food waste-derived digestate (FWD) into single-cell protein through mixotrophic cultivation of Chlorella sorokiniana UTEX 1230. The liquid fraction of the digestate (LD) was diluted to 10 % to maintain ammonium-nitrogen (NH₄⁺-N) levels within tolerable ranges (up to 250 mg/L) and enriched with macro- and micronutrients such as magnesium, calcium, sulfur, phosphorus, and trace elements to enhance microalgae growth. Glucose and acetate were tested at different organic carbon-to-nitrogen (Organic-C/N) ratios to assess their effects on biomass production, protein content, and NH₄⁺-N removal. Results indicated that increasing the Organic-C/N ratios to 5 (glucose) or 10 (acetate) enhanced biomass concentration and NH₄⁺-N removal rates. Although protein content declined with higher Organic-C/N ratios, total protein production still increased. Glucose at an Organic-C/N ratio of 5 achieved the highest biomass yield (up to 2.5 g/L), while acetate required an Organic-C/N ratio of 10 to reach comparable levels, pointing to its lower metabolic efficiency. Additionally, biomass protein content under glucose supplementation (40-41 %) surpassed that obtained with acetate (34-36 %). Combining acetate with 60-80 % glucose at a constant Organic-C/N ratio of 5 improved biomass production, but did not match the protein levels observed with glucose alone. Overall, the mixotrophic cultivation approach achieved higher biomass, superior NH₄⁺-N removal rates, and over 40 % protein content, confirming its effectiveness in converting FWD into single-cell protein.