Continuous anaerobic co-digestion of Ulva biomass and cheese whey at varying substrate mixing ratios: Different responses in two reactors with different operating regimes

•The feasibility of anaerobic co-digestion of Ulva with cheese whey was examined.•Co-digestion was beneficial to the methanation of Ulva compared to mono-digestion.•The optimal fraction of Ulva in the substrate mixture is suggested to be 50-75%.•Start-up conditions had a crucial effect on the evolution of microbial community.•Ulva had a greater influence than whey in determining the community structure.

The feasibility of co-digestion of Ulva with whey was investigated at varying substrate mixing ratios in two continuous reactors run with increasing and decreasing proportions of Ulva, respectively. Co-digestion with whey proved beneficial to the biomethanation of Ulva, with the methane yield being greater by up to 1.6-fold in co-digestion phases than in the Ulva mono-digestion phases. The experimental reactors responded differently, in terms of process performance and community structure, to the changes in the substrate mixing ratio. This can be attributed to the different operating regimes between two reactors, which may have caused the microbial communities to develop in different ways to acclimate. Methanosaeta-related populations were the predominant methanogens responsible for the production of methane regardless of different substrate mixing ratios in both reactors. Considering the methane recovery and the Ulva treatment capacity, the optimal fraction of Ulva in the substrate mixture is suggested to be 50-75%.