Reaching the operating limit of the continuous multiple tube reactor: Still a promising technology for maximizing fermentative biohydrogen production?

This study simulated the application of the continuous multiple tube reactor (CMTR), a promising technology capable to replace fixed-film reactors in biohydrogen (bioH2) production via dark fermentation, in the processing of sucrose-rich wastewater (COD of up to 16gL−1). The long-term impacts on both the bioH2 evolution and the dynamics of biomass growth and retention were assessed, aiming to identify the operating limits of the reactor. The bioH2 production was maximized (1445 NmL H2 L−1 d−1) when using moderately concentrated wastewaters (COD = 8gL−1) at an OLR of 48.0g COD L−1 d−1, with much lower hydrogenogenic activity (< 200 NmL H2 L−1 d−1) observed at OLR levels of 72.0 and 96.0g COD L−1 d−1. Inhibition of the fermentative activity by excess substrate and limitations in the retention capacity of suspended biomass in the reactor still limit the application of higher OLR at the current stage of the technology. Strategies stimulating the cell growth (nitrogen dosing) and biofilm formation (calcium dosing) have potential to expand the operating limit of the CMTR without requiring constructive modifications. [Display omitted] •High COD (up to 16gL−1) was applied in the continuous multiple tube reactor (CMTR).•BioH2 production is maximized at COD = 8.0gL−1, regardless of the OLR.•Inhibition by substrate and poor suspended biomass retention limit using higher COD.•The CMTR seems suitable for processing nitrogen- and calcium-rich wastewaters.•The biomass retention rate peaked (196.6mg VSS L−1 d−1) when OLR = 48kg COD m−3 d−1.