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.

There is a lack of information on denitrification of saline wastewaters, such as those from marine recirculating aquaculture systems (RAS), ion exchange brines and wastewater in areas where sea water is used for toilet flushing. In this study, side-by-side microcosms were used to compare methanol, fish waste (FW), wood chips, elemental sulfur (S0) and a combination of wood chips and sulfur for saline wastewater denitrification. The highest denitrification rate was obtained with methanol (23.4 g N/(m3·d)), followed by FW (4.5 g N/(m3·d)), S0 (3.5 g N/(m3·d)), eucalyptus mulch (2.6 g N/(m3·d)), and eucalyptus mulch with sulfur (2.2 g N/(m3·d)). Significant differences were observed in denitrification rate for different wood species (pine > oak ≫ eucalyptus) due to differences in readily biodegradable organic carbon released. A pine wood-sulfur heterotrophic-autotrophic denitrification (P-WSHAD) process provided a high denitrification rate (7.2-11.9 g N/(m3·d)), with lower alkalinity consumption and sulfate generation than sulfur alone.