Application of pilot-scale two-stage ZVI-based biofilter for advanced nitrogen and phosphorus removal from the actual secondary effluent under high DO conditions: Focusing on the effect of DO on electron transfer and Fe cycle

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.