H 2 addition through a submerged membrane for in-situ biogas upgrading in the anaerobic digestion of sewage sludge.

•Gas recirculation and H2 supply through a submerged membrane for in-situ upgrading.•94% H2 utilization efficiency was found at H2 flow rate of 0.87 L Lreactor−1 d−1.•CH4 production rate increased from 0.38 to 0.54 L Lreactor−1 d−1.•An increase of 42% in CH4 production was observed in the in-situ upgrading reactor.•Organic matter removal was not compromised despite the high pH level (8.1)