Predicting pH rise as a control measure for integration of CO2 biomethanisation with anaerobic digestion

In-situ CO2 biomethanisation offers a means to combine biogas upgrading with increased methane productivity, but its potential contribution to power-to-gas is often ignored due to concerns over process stability and control. The research presents an equation derived from fundamental chemical equilibria which predicts the relationship between partial CO2 pressure and digester pH, and allows estimation of the maximum achievable biogas methane content compatible with stable operation. A rapid experimental determination was also developed to support these predictions. The results were validated by long-term experiments using synthetic feedstock with different ammonia concentrations (2 and 3 g N L-1). Further trials carried out using food waste and sewage sludge as substrates showed stable operation at biogas methane contents of 92 and 90% CH4 and pH 8.5 and 7.9, respectively. CO2 biomethanisation was successfully demonstrated in a food waste digester with a total ammoniacal nitrogen of 4.8 g N L-1 with volumetric methane production enhanced by more than 2 times, from 2.29 to 5.01 L CH4 per L digester per day. The predictive approach used is applicable to digesters fed on different feedstocks and to hybrid systems with biomethanisation of both endogenous and exogenous CO2; and offers a basis for both process design guidelines and operational control. The output from the work thus provides engineers, operators and plant designers with a valuable tool for the successful implementation of in-situ biomethanisation in anaerobic digesters.