Biological methanation: Strategies for in-situ and ex-situ upgrading in anaerobic digestion

• Biological methanation was assessed in-situ and ex-situ. • A 24-hour batch ex-situ system produced 3.7 L CH 4 L VR −1 d −1 at 96% methane content. • High hydrogen loadings boost performance while adversely affecting efficiency. • Elevated hydrogen concentrations hamper in-situ acetogenesis process. • Concepts for full-scale methanation strategies are proposed to upgrade biogas. This study investigated in-situ and ex-situ biological methanation strategies for biogas upgrading potential. The addition and circulation of hydrogen with a ceramic gas diffuser unit revealed positive effects on the methanogenic process. A short-term maximum methane productivity of 2.5 L CH 4 per L reactor volume per day (L VR −1 d −1 ) was obtained in-situ. Adverse effects of elevated dissolved hydrogen concentrations on acetogenesis became evident. Ex-situ methanation in a reactor subjected to gas recirculation for recurrent 24 h periods achieved methane formation rates of 3.7 L CH 4 L VR −1 d −1 . A biomethane with methane concentrations in excess of 96% successfully demonstrated the potential for gas grid injection. A theoretic model supplying gases continuously into a sequential ex-situ reactor system and steadily displacing the upgraded biogas confirmed similar methane formation performance and was advanced to a full-scale concept. Gas conversion efficiency of 95% producing biomethane at 85% methane content was attained. A hybrid model, where an in-situ grass digester is followed by an ex-situ reactor, is proposed as a novel upgrading strategy.