Impacts of micro- to nano-sized carbon supplements on mixed and archaea-free halophilic cultures when used for bioenergy recovery from saline wastewater

Due to the growing interest in enhancing dark fermentation and anaerobic digestion via carbon-based additives to exploit different lucrative synergies, process-based optimizations and mechanistic analyses are crucial. Herein, we studied the impacts of carbon supplements on halophilic microbial cultures as a process intensification way towards bioenergy recovery from saline wastewater. Three additives, i.e., powdered activated carbon, multiwall carbon nanotubes and graphene, were examined. Two sets of parallel batch experiments were conducted using archaea-free (for dark fermentation) and mixed (for anaerobic digestion) halophilic cultures at different supplement doses. Adding 30 mg/L of graphene and 100 mg/L of powdered activated carbon exhibited the maximum improvements in dark fermentation (H2 -production) and anaerobic digestion (biogas production), i.e., -28% and -27% over the control, respectively. This underscored the dose-dependent impact of different-sized carbon amendments. Both the electron transport system activity and extracellular polymeric substances were improved with all amendments, indicating a stimulated electron transfer flux. However, distinct bacterial compositions were identified when using mixed and archaea-free cultures along with indicators for direct interspecies electron transfer establishment in the presence of archaea. Beyond this, such findings also supported the expanded role of conductive additives on H2-producing bacteria, including Clostridium. According to a techno-economic analysis, powdered activated carbon emerged as the best for optimizing bioenergy recovery (in terms of H2 or biogas) from saline wastewater, providing a solid basis for relevant applications.