Distinct Mechanisms between Free Iron Species and Magnetite Addition in Anaerobic Digestion on Alleviating Ammonia Inhibition

Ammonia inhibition often occurs during anaerobic digestion
(AD)
of the protein-rich substrate. Iron-containing substances were proved
to be efficient in alleviating the ammonia stress. However, the mechanisms
behind, especially the distinct impacts of different forms of iron
materials, are not fully revealed. Here, the alleviating performances
of FeCl3 and Fe3O4 on AD systems
under ammonia stress were investigated. Moreover, the mechanisms behind
these were revealed and compared at the transcriptional level. Results
showed that FeCl3 and Fe3O4 additions
with an equal amount of iron element content (1.29 mM) led to the
increased cumulative biogas and methane yields under an ammonia concentration
of 3 g/L. Furthermore, the addition of iron-containing substances
alleviated the accumulation of volatile fatty acids (VFAs) and extracellular
polymeric substances (soluble carbohydrates and protein) caused by
ammonia stress, which also had an obvious positive effect on the electron
transfer capability. Microbial analysis demonstrated that the microbes
(e.g., orders Methanosarcinales, Clostridiales, and Syntrophobacterales) associated with direct
interspecies electron transfer (DIET), syntrophic acetate oxidization,
and degradation of organic compounds were enriched. Metatranscriptomic
analysis showed that ammonia inhibited the AD process by disrupting
cellular redox homeostasis, infecting the ATPase activity, affecting
cellular energy supply, inhibiting methane-producing enzyme activity,
and suppressing the expression of cell conductive structure genes.
Meanwhile, the addition of FeCl3 and Fe3O4 enhanced the cellular basal metabolism and energy supply,
as well as microbial electron transfer and enzymic activities on methanogenesis.
Metatranscriptomic analysis indicated that the addition of free iron
species (FeCl3) can relieve the ammonia stress on syntrophic
propionate and acetate oxidizing bacteria, enhance DIET by stimulating
the synthesis of c-type cytochrome, and thus promote methane production.
Meanwhile, Fe3O4 may promote methane production
by stimulating the expression of related genes and facilitating electron
transfer in the AD system as a capacitor. Overall, the results demonstrated
that ferric chloride and magnetite can alleviate the ammonia inhibition
in the AD process of high-nitrogen waste through different mechanisms.