Nitrogen upcycling and carbon capture from food waste through dark fermentation coupled with microalgae and hydrogen oxidising bacteria cultivation

The production of alternative protein sources such as single-cell protein (SCP) is expected to help mitigate the adverse environmental impacts of current feed and food production methods. This mitigation is especially relevant when SCP production is integrated with a carbon capture and nitrogen recovery system from nitrogen-rich wastes. The present study investigated a novel integrated approach to upcycle both the biogas containing hydrogen and carbon dioxide produced from the dark fermentation (DF) of food waste, and the nitrogen recovered through stripping from the liquid fermentate into protein-rich biomass. SCP was produced using a co-culture of the microalga Chlorella vulgaris and the hydrogen-oxidising bacterium Cupriavidus necator. The fermentate, characterised by an ammonium concentration of around 170.0 ± 36.2 mg N-NH4+/L, was transferred from a continuous 1 L fermenter to a 2 L stripping column, where an NH4+-N removal efficiency of 24 % was achieved. The NH3-laden air flow was supplied to the algal-bacterial culture growing in a 4 L column photobioreactor (PBR). The highest biomass concentration achieved in the PBR was 0.78 ± 0.02 g VSS/L under test conditions, while it reached up to 1.51 ± 0.04 g VSS/L under supply of chemical nitrogen salts and synthetic biogas. The algal-bacterial culture contained 31.6 ± 0.2 % protein/VSS under test conditions and eight of the nine essential amino acids were detected. Overall, the findings of this study highlight the potential of the proposed DF-microbial conversion system for carbon capture and nitrogen upcycling, while identifying ammonia stripping as the main bottleneck for future optimisation.