Biomethanation potential of macroalgae Ulva spp. and Gracilaria spp. and in co-digestion with waste activated sludge

► BMP assays were performed at different TS concentrations of three macroalgae. ► The maximum specific methane production was 196 ± 9 L CH4 kg−1 VS with 2.5% TSUlvasp. ► Co-digestion of Ulva sp. with mixed sludge increased the methane production rate. ► Use anoxic marine sediment as inoculum had no positive effect on the CH4 production. ► The limiting step of the overall anaerobic digestion process was the hydrolysis.