Hydrochar-Facilitated Anaerobic Digestion: Evidence for Direct Interspecies Electron Transfer Mediated through Surface Oxygen-Containing Functional Groups

Acceleration of the anaerobic digestion
(AD) process is crucial
to achieving energy-efficient recycling of organic wastes. Hydrochar
is produced by hydrothermal liquefaction of biomass, yet its application
in the AD process is rarely reported. The present study showed that
sewage sludge-derived hydrochar (SH) enhanced the methane production
rate of glucose by 37%. SH increased the methane production rate from
acetate but did not affect acidification and the methane production
rate from H2/CO2. SH enhanced hydrogenotrophic
methanogenesis, which could be due to direct interspecies electron
transfer (DIET) by converting H+, e–,
and CO2 to methane. Trichococcus and Methanosaeta were dominant in the AD process with SH. Label-free
proteomic analysis showed Methanosaeta was involved
in DIET as reflected by the up-regulation of proteins involved in
hydrogenotrophic methanogenesis. Hydrochars derived from corn straw
(CH), Enteromorpha algae (EH), and poplar wood (PH),
as well as activated carbon (AC), were also tested in the AD process.
SH, CH, and EH obviously increased the methane production rates, which
were 39%, 15%, and 20% higher than the control experiment, respectively.
It was neither electrical conductivity nor the total redox property
of hydrochars and AC but the abundances of surface oxygen-containing
functional groups that correlated to the methane production rates.