Magnetite
Triggering Enhanced Direct Interspecies
Electron Transfer: A Scavenger for the Blockage of Electron Transfer
in Anaerobic Digestion of High-Solids Sewage Sludge
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Abstract
At present, high-solids anaerobic
digestion of sewage sludge has
drawn great attention due to the superiority of its small land area
footprint and low energy consumption. However, a high organic loading
rate may cause acids accumulation and ammonia inhibition, thus leading
to an inhibited pseudo-steady state in which electron transfer through
interspecies hydrogen transfer (IHT) between acetogens and methanogens
is blocked. In this study, adding 50 mg/g TS (total solid) magnetite
clearly reduced the accumulation of short-chain fatty acids and accelerated
methane production by 26.6%. As demonstrated, the individual processes
of anaerobic digestion could not be improved by magnetite when methanogenesis
was interrupted. Analyzing stable carbon isotopes and investigating
the methanogenesis pathways using acetate and H<sub>2</sub>/CO<sub>2</sub> as substrates together proved that direct interspecies electron
transfer (DIET) was enhanced by magnetite. Metatranscriptomic analysis
and determination of key enzymes showed that IHT could be partially
substituted by enhanced DIET, and acetate-dependent methanogenesis
was improved after the blockage of electron transfer was scavenged.
Additionally, the expression of both pili and c-type cytochromes was
found to decrease, indicating that magnetite could replace their roles
for efficient electron transfer between acetogens and methanogens;
thus, a robust chain of electron transfer was established