Further
reducing total nitrogen (TN) and total phosphorus (TP)
in the secondary effluent needs to be realized effectively and in
an eco-friendly manner. Herein, four pyrite/sawdust composite-based
biofilters were established to treat simulated secondary effluent
for 304 days. The results demonstrated that effluent TN and TP concentrations
from biofilters under the optimal hydraulic retention time (HRT) of
3.5 h were stable at <2.0 and 0.1 mg/L, respectively, and no significant
differences were observed between inoculated sludge sources. The pyrite/sawdust
composite-based biofilters had low N2O, CH4,
and CO2 emissions, and the effluent’s DOM was mainly
composed of five fluorescence components. Moreover, mixotrophic denitrifiers
(Thiothrix) and sulfate-reducing bacteria (Desulfosporosinus) contributing to microbial nitrogen and
sulfur cycles were enriched in the biofilm. Co-occurrence network
analysis deciphered that Chlorobaculum and Desulfobacterales were key genera, which formed an obvious
sulfur cycle process that strengthened the denitrification capacity.
The higher abundances of genes encoding extracellular electron transport
(EET) chains/mediators revealed that pyrite not only functioned as
an electron conduit to stimulate direct interspecies electron transfer
by flagella but also facilitated EET-associated enzymes for denitrification.
This study comprehensively evaluates the water–gas–biofilm
phases of pyrite/sawdust composite-based biofilters during a long-term
study, providing an in-depth understanding of boosted electron transfer
in pyrite-based mixotrophic denitrification systems.