This study developed a two-stage biofilter utilizing pyrite/sawdust composites to treat actual secondary effluent with high dissolved oxygen (DO) concentrations (3-8.5 mg/L) over a period of 169 days. The findings demonstrated that the two-stage pyrite-based biofilters achieved advanced purification of the real secondary effluent, maintaining effluent concentrations of total nitrogen (TN) and total phosphorus (TP) below 2.0 mg/L and 0.5 mg/L, respectively, with an influent TN of 10 mg/L and a temperature of >21 degrees C. The Rhodocyclaceae family is the predominant mixotrophic denitrifying bacteria in both the first-class (FC) and second-class (SC) systems. Sulfate-reducing bacteria (i.e., Desulfrispora and Desulfatirhabdium) might be keystone species in the SC system, underscoring that the sulfate reduction process enhanced denitrification under low DO conditions. Differential functional gene analysis exposed that high DO might suppress the activity of Complex III, NAR, NIR, NOR, and NOS, leading to slow-unstable electron transport and consumption in the denitrification process. Moreover, the diminished expression of S and Fe cycling genes (soxA/B/Z/X, aprA/B, dsrA/B, ABC.FEV<middle dot>S, and korA/B/C) in the FC system indicated that high DO predominantly might inhibit the Sox pathway, dissimilatory sulfate reduction process, Fe2+/Fe3+ transfer, and biological Fe2+ oxidation system. The dormancy of the S and Fe cycles induced by high DO levels primarily accounted for the diminished performance of the pyrite-based biofilters. This study offers novel insights into the extensive application of pyrite-based composites and enhances the understanding of DO effects on S and Fe cycles in the nitrogen removal process of municipal tailwater.

The complexity and variability of actual secondary effluent from wastewater treatment plants (WWTPs) pose significant treatment challenges. In this study, a two-stage biofilter packed with ZVI/Poly-3-hydroxybutyrate-cohyroxyvelate/sawdust (ZPS) composites was innovatively constructed to treat actual secondary effluent with high influent dissolved oxygen (DO) concentrations (3-8.5 mg/L) for 143 days in a WWTP. Results showed that advanced purification of real secondary effluent was achieved, and the effluent concentrations of TN and TP were stable below 2.0 mg/L and 0.1 mg/L, respectively, at influent TN < 10 mg/L. Microbial community analysis identified unclassified\_f\_\_Rhodocyclaceae as the dominant denitrifiers in both first-class (FC) and second-class (SC) systems. The shift in dominant Fe-related bacteria from Ferritrophicum to Clostridium sensu stricto\_7 from the FC to SC system with DO decreased suggested that ZVI's triple role in oxygen-capturing reagent, denitrification and organic matter decomposition. Co-occurrence network analysis deciphered that Thermomonas and Clostridium\_sensu\_stricto\_10 were key genera in SC system, which formed an obvious Fe redox cycle process that bolsters denitrification under low DO levels. Differential functional gene analysis revealed that high DO could inhibit the activity of Cyt c, NOR and NOS, resulting in a slow and unstable electron transport and consumption in denitrification process. Furthermore, the down-regulation iron cycling genes (feoA and ABC.FEV.S) in FC system suggested that high DO mainly inhibited the Fe2+/Fe3+ transfer system. An inactive Fe cycle at high DO levels highlights the important role of Fe cycle in iron-based denitrification process. These findings advanced the understanding of effected mechanism of DO on nitrogen removal mediated by ZPS composites in actual tailwater treatment. Additionally, the novel ZPS composites can be combined with the removal of antibiotics, and other toxic or harmful substances within wastewater to expand their application.

The reactive substance consisting manganese oxides (MnOx) and solid carbon have been reported to be effective in polishing secondary wastewater; however, the treatment characteristics and mechanism remains limited. In this study, MnOx/carbon (Mn-C) composites were applied in biofilters to evaluate simultaneous removal of nitrate and sulfamethoxazole (SMX), with the single carbon composites as control. Results showed that the effluent concentrations of NO3 –N and SMX were below 2.87 mg L- 1 and 7.97 mu g L- 1 under hydraulic retention time (HRT) of 6 h. The intermittent aeration optimized Mn cycle with treatment performance improved under lower HRT and Mn(II) accumulation decreased. Mn-C composites could reduce the emission of N2O, CO2 and CH4. The dominant genera gradually evolved from fermentation to glycogen aggregation, and from heterotrophic/sulfur autotrophic to heterotrophic denitrifiers by intracellular substance and manganese autotrophic/heterotrophic bacteria. Microbial network analysis indicated higher antagonism, lower modularity and shorter average path among microbes in Mn-C biofilters, which highlighted microbial differentiation and faster electron transfer. Improved functions of denitrification and Mn respiration, and the increasing genes encoding electron transfer chain, including NADH dehydrogenase, Cytc and ubiquinone, further elucidated the superiority of Mn-C com- posites. These results improved our understanding of Mn-C composites application in low-carbon wastewater treatment.