Production of biogas with high calorific value from wastewater via hydrogen assisted anaerobic digestion

Afrooz Bayat (University of Canterbury), Ricardo Bello-Mendoza  (University of Canterbuy), Aaron Marshal (University of Canterbury)

Anaerobic digestion is a well-known technology for wastewater treatment and biogas production. This process has been used all over the world for treating a wide range of waste and wastewater streams with biogas being one of its most valuable products. However, there are still a few challenges with this technology such as low methane content of the biogas. In general, the biogas primarily contains about 60% of methane and 40% of CO2. The CO2 content of the biogas lowers the calorific value of this valuable and renewable gas and limits its industrial applications. Therefore, biogas upgrading mechanisms have been studied to remove CO2 and other unwanted gases from the biogas. Biogas upgrading has gained attention particularly because many countries are moving towards renewable fuel production. Upgraded biogas can be an appealing substitute for natural gas.

Hydrogen-assisted biological biogas upgrading has been explored to convert CO2 to methane gas with the help of CO2-consuming hydrogenotrophic methanogens. These microorganisms use hydrogen and CO2 and produce methane gas as the results of their biological activities. However, hydrogen gas can take other pathways in mixed culture systems such as anaerobic digesters where volatile fatty acids are produced as a result of biological reaction of CO2 and H2. In this context, the aim of this study was to identify the H2 injection rate required to increase the methane content in the biogas.

The results demonstrated a positive effect of hydrogen addition on biogas upgrading. The methane content of the biogas in H2-assisted anaerobic digestion (70%) was higher than that of conventional anaerobic digestion (45%). The results showed that after acclimation of anaerobic microorganisms to the added hydrogen, hydrogen gas uptake and sequestration of CO2 occurred which resulted in the production of high quality biogas.