Effects of high-pressure anaerobic digestion up to 30 bar on pH-value, production kinetics and specific methane yield

The production of biogas for combined heat and power generation represents a common method in Germany. An alternative is the purification of biogas to biomethane and injection into the natural gas grid to decouple gas production from usage in terms of space and time. The concept of pressurized two-stage anaerobic digestion integrates biogas production, upgrading and pressure boosting within one process. The increasing solubility of CO2 in process liquid at high pressures results in high methane contents in gaseous phase and in drop of pH. To investigate the effects of high initial pressures (10, 20, 30 bar) and dissolved CO2 on pH-value, production kinetics and specific methane yields, pressurized batch methane reactors were built up. Additionally, a method for indirect measuring of pH-value was determined. The results of the experiment showed a decrease in pH from 7 to 6.31 (10 bar) and 6.25 (30 bar). Furthermore, neither a significant influence of initial pressures on the pressure increase nor on the degradation of organics and the specific methane yields was observed. However, the results show that anaerobic digestion at high pressures up to 30 bar might be a promising alternative to post purification and pressure boosting applications. •The influence of initial pressure on anaerobic digestion up to 30 bar was examined experimentally.•During digestion process the pH-value drops beyond the optimal range for anaerobic digestion.•The initial pressure has no influence on the pressure increase rates.•The initial pressure has no effect on the degradation of the organics.•The initial pressure doesn't affect the specific methane yield.