Waterworks sludge coupled with biochar and pyrite as wetland substrates for enhanced nitrogen removal and greenhouse gases mitigation: First study on synergistic mechanisms

This study demonstrates the promising potential of combining waterworks sludge (WS) with a pyrite novel lightweight substrate (PNLS) at a volumetric ratio of 2:1 to serve as a supplementary carbon source as substrates in constructed wetlands (CW) systems. In addition to enhancing wastewater purification, such substrates can reduce greenhouse gas (GHG) emissions, simultaneously. The incorporation of the sulfur–iron (S-Fe) composite derived from pyrite significantly enhanced nutrient removal and effectively mitigated GHG emissions. The PNLS amendment notably improved nitrate removal, achieving optimal total nitrogen (TN) removal efficiency (75.9 ± 3.7 %) through synergistic autotrophic denitrification and Feammox pathways. Concurrently, total phosphorus (TP) removal efficiency reached 94.3 ± 1.0 %. Compared with the WS-only substrate system, the WS-PNLS configuration achieved the lowest integrated global warming potentials (GWP) at 27.33 mg/m²/h CO₂-eq., representing a 46.4 % reduction in GHG emissions. Furthermore, the abundances of functional genes related to nitrification (AOA and AOB), denitrification (nirS, nirK, and nosZ), autotrophic denitrifiers, and methanotrophs were significantly enriched supporting a mixotrophic microbial environment conducive to GHG mitigation. Overall, this study proposed a novel substrate strategy for overcoming the limitations of low-carbon wastewater treatment in CWs while simultaneously achieving effective GHG emission reduction.