Upcycling anaerobic digestion streams into feed-grade protein for increased environmental sustainability

Biogas plants have long been recognized as well-established systems for converting waste into energy and organic fertilizers. However, biogas plants are expected to go beyond their primary functions to enhance the sustainability gains from the circular bioeconomy. Carbon (CO 2 and CH 4 ) and nitrogen streams from anaerobic digestion (AD) facilities can be converted into higher-value products, such as microbial proteins (MPs). While two dominant pathways are known for MP production from AD streams, namely methane-oxidizing bacteria (MOB) and hydrogen-oxidizing bacteria (HOB), there is a lack of comprehensive research comparing these two pathways from an environmental sustainability viewpoint. Furthermore, the extent to which the environmental sustainability of MP production platforms depends on feedstock characteristics is yet to be scrutinized. To address the above-mentioned research questions, four biogas plants treating different types of biowaste were selected as case studies. Specifically, the effects of the C/N ratio in the input streams, biogas upgrading technologies (water scrubbing and biological biogas upgrading), and microbial platforms (HOB and MOB) were investigated. Environmental impacts were assessed using consequential life cycle assessment (CLCA), with Denmark as the spatial boundary and 2030 as the temporal boundary. Across all scenarios, the MOB pathway demonstrated superior environmental performance compared with the other pathways. Furthermore, the composition of the feedstock and quantity of biogas play key roles in the total environmental gains. Finally, the sources of uncertainty and opportunities for future improvements are identified to pave the way for land-independent feed-grade protein production using locally available biowastes. • Combining microbial protein production with anaerobic digestion and biogas plants. • MOB pathway promotes more environmental sustainability than HOB pathway. • The influence of feedstock characteristics on the environmental impacts was assessed. • The impacts of using external carbon to balance feedstock C/N ratio were scrutinized. • The role of external carbon sources with and without the CCS technology was detected.