Assessing the syngas biomethanation in anaerobic sludge digestion under different syngas loading rates and homogenisation

Abstract

Gasification offers digestate conversion into syngas, which can be metabolised to CH4 by a mixed anaerobic consortium. However, low gas-liquid mass transfer and CO presence can limit further scale-up. The present study assessed syngas biomethanation (55% H2, 45% CO) in a thermophilic lab-scale continuous anaerobic digester processing sewage sludge. Different syngas loading rates and mechanical stirring compared to pneumatic mixing were investigated. Results showed that anaerobic culture acclimation for CO utilisation is unnecessary. Moreover, the mechanical stirring replacement for the pneumatic system enhanced the gas-liquid mass transfer rate. Hence, the biological conversion of CO and H2 was improved by 30%. Nevertheless, increased pH to 8.6 resulted in acetic acid accumulation (>7.5 g/L). These findings improve syngas biomethanation efficiency and suggest including a proper pH control before further scale-up.