Membrane-free dissolved hydrogen monitoring in anaerobic digestion

Dissolved hydrogen (dH2) is an important parameter in anaerobic digestion (AD) processes, in particular in a two-stage operation mode encompassing separate hydrolytic/acidogenic and methanogenic stages. Then, monitoring of dH2 as a substrate of hydrogenotrophic methanogenesis is essential to avoid feast and famine conditions. Despite its significance, there is currently a lack of established monitoring systems which are capable of reliably measuring dH2 concentrations in culture broth. To address this challenge, we propose a novel measurement system, which is based on the membrane-free extraction of hydrogen in an extraction chamber and the subsequent analysis by a metal-oxide (MOX) gas sensor. The response time of the MOX sensor lies in the range of seconds, while the entire measurement process completes within a total processing time of about 70 min). This study explores the measurement performance of the dH2 sensor in the hydrolytic/acidogenic and methanogenic stage during lab-scale anaerobic digestion. The measurement principle was consistently applied for over three months. During this period, the methanogenic stage was partly sparged with gaseous hydrogen to monitor the dH2 response afterwards. The dH2 sensor responded reliably to these and other dynamic changes. Depending on the process conditions, concentrations between < 10 and > 4,000 Pa were detected, corresponding to dH2 concentrations of < 0.074 to > 30 µmol L−1. The findings demonstrated the importance of dH2 monitoring and show that it facilitates the control of H2 addition, thereby preventing both under- and oversupply during methanogenesis.