Enabling protein-rich microalgae production from anaerobic digestate through direct aeration stripping for gaseous nitrogen and carbon supply

Efficient removal and recovery of nitrogen (N) from N-rich waste streams are essential to mitigate their environmental impact and to enhance the sustainability and circularity of N-demanding processes, such as the production of protein-rich microalgal biomass. This study investigated the integration of a direct aeration stripping process, recovering gaseous ammonia and CO₂ from anaerobic digestate, with the growth of a mixed microalgal culture in a photobioreactor, targeting the conversion of the recovered gases into protein-rich microalgae while preventing cross-contamination from the waste digestate matrix. Various air flow rates and system configurations were tested to optimise the stripping efficiency of ammonia nitrogen while simultaneously evaluating microalgal growth and protein accumulation. Results were compared to control conditions having nitrogen supplied either as mineral ammonium or as anaerobic digestate added directly in the growth medium. The highest microalgal biomass concentration (1239.6 ± 294.3 mg VSS/L) was achieved using a closed system with an aeration flow rate of 2 L/h. Under the same conditions, ammonia stripping efficiency reached 69.9 %, alongside with the highest assimilation of nitrogen in the form of protein (61.6 ± 2.6 %). The essential amino acid (EAA) profile of the microalgal biomass produced through direct aeration stripping aligned with literature values and WHO/FAO nutritional guidelines for adult requirements, with particularly high levels of histidine, leucine, and lysine (0.75, 3.14, and 2.85 g EAA/100 g biomass, respectively). These findings highlight the potential of direct aeration N-stripping as an effective strategy for nitrogen recovery from waste streams, enabling its upcycling into valuable microalgal biomass.