Sustainable additives for the regulation of NH3 concentration and emissions during the production of biomethane and biohydrogen: A review

: This study reviews the recent advances and innovations in the application of additives to improve biomethane and biohydrogen production. Biochar, nanostructured materials, novel biopolymers, zeolites, and clays are described in terms of chemical composition, properties and impact on anaerobic digestion, dark fermentation, and photofermentation. These additives can have both a simple physical effect of microbial adhesion and growth, and a more complex biochemical impact on the regulation of key parameters for CH4 and H2 production: in this study, these effects in different experimental conditions are reviewed and described. The considered parameters include pH, volatile fatty acids (VFA), C:N ratio, and NH3; additionally, the global impact on the total production yield of biogas and bioH2 is reviewed. A special focus is given to NH3, due to its strong inhibition effect towards methanogens, and its contribution to digestate quality, leaching, and emissions into the atmosphere.