From waste to high-value nitrogen: Optimizing and validating direct nitrogen stripping from anaerobic digestate for sustainable microbial protein production

In the quest for sustainable and circular alternative protein sources, low-cost and contaminant-safe resource recovery techniques are urged. This study investigated and optimized a zero-chemical, low-temperature direct air stripping process from anaerobic digestate for clean nitrogen (N) recovery and its upcycling into microbial protein (MP) produced from cheese whey permeate (CWP). Key operational parameters, including air-to-digestate (A:D) ratio, temperature, initial ammonium nitrogen concentration, and batch vs. continuous operation, were assessed. Stripping efficiencies of up to 99.9 % were achieved within 24 h under batch conditions with A:D ratios of 2:1 and 4:1 vvm at mesophilic (25, 35, 45 °C) and thermophilic (55 °C) temperatures. Under continuous operation, up to 61 % stripping efficiency was obtained at 45 °C with an A:D ratio of 4:1 vvm. The latter operating condition was selected to integrate the direct air stripping process with the aerobic MP production step, leveraging the aeration flow of the latter. This N-recovery strategy was compared to other N-supply routes such as the direct addition of N-rich digestate or of NH4Cl to CWP. The impact of heavy metals in digestate on biomass growth and MP quality was also investigated. Biomass concentration reached approximately 17 g TSS·L−1 when nitrogen was directly supplied through digestate, whereas using re-solubilised stripped ammonia as nitrogen source resulted in a lower biomass growth, reaching 4.78 g TSS·L−1. Although heavy metals did not inhibit biomass growth, they compromised the quality of the final MP product when digestate was supplied directly as nitrogen source.