Anaerobic digestion as a platform for nitrogen upcycling and carbon capture by means of hydrogen-oxidizing bacteria: a case study

The security of the water, energy and food (WEF) nexus is being increasingly challenged by global climate change and population growth. Among the various anthropogenic processes that are central in the WEF nexus, wastewater treatment must also be revised in order to move away from the conventional dissipative technology and progress toward a more sustainable and regenerative approach. In wastewater treatment plants (WWTPs), useful resources such as reactive nitrogen species (ammonium, nitrite and nitrate) are usually dissipated by means of energy intensive processes (nitrification/denitrification) into their non-reactive form: dinitrogen gas. Reactive nitrogen can instead be recovered as ammonia after anaerobic digestion of sewage sludge and be subsequently upcycled as main constituent of new useful products such as edible microbial protein (MP). Microbial re-assimilation of recovered nitrogen can be carried out at efficiencies close to 100%, and can be driven by renewable electron donors such as biogas and hydrogen that are present or producible on-site of the WWTPs. In the ideal scenario here proposed, hydrogen-oxidizing bacteria, able to capture CO2 while re-assimilating nitrogen into MP, can bring about novel circular economy perspectives. In our case study, the potential of on-site nitrogen upcycling and carbon capture by hydrogen-oxidizing bacteria has been analyzed in the context of an ideal 100,000 inhabitant equivalents WWTP. Our calculations show that about 23% of the nitrogen stripped from digestate could be re-assimilated and up to almost 1 ton of CO2 could be captured into MP by exclusively making use of the renewable electron donors that are producible on-site.