Sewage sludge biochar addition improves nitrogen and sulphate removal in vertical flow constructed wetlands: A resource circularity approach

The sewage sludge biochar (SSB)-linked shifts in nitrogen and sulphur metabolism and microbial functioning in vertical flow constructed wetlands (VFCWs) are unclear. This study investigated SSB as a functional amendment to enhance nitrogen and sulphate (SO 4 2− ) removal in VFCWs planted with Typha macrophyte. The results showed that VFCW (with SSB) achieved a significantly higher chemical oxygen demand (COD) removal efficiency (84.41%) compared to control VFCW (without SSB). Meanwhile, VFCW (SSB) exhibited a 16.4% increase in nitrification efficacy, with 86.75% and 40.57% removals of ammonium nitrogen (NH 4 + -N) and total inorganic nitrogen (TIN), respectively, compared with the control VFCW. VFCW (SSB) exhibited a higher reduction (41.49%) in SO 4 2− than that of the control (45.01%), suggesting stable redox conditions maintained by plant–microbe interactions. Metagenomic analysis revealed a 34.2–37.77% increase in nitrification genes (e.g., amoA, nxrB ), and 7.67–20.2% in DNRA genes (e.g., narG, napA, norB, nirB ) in VFCW amended with SSB. Meanwhile, VFCW with SSB showed enrichment of genes involved in the sulphur cycle (e.g., soxX, soxA ). Apart from microbial-mediated sulphur metabolism, sulphur-driven denitrification and metal-SO 4 −2 precipitation were also found to be prominent mechanisms for sulphur reduction in VFCW (SSB), supported by X-ray photoelectron spectroscopy (XPS) analysis of SSB. These findings suggest that SSB can be utilized as a sustainable carbon source in VFCW to support effective microbial functioning, aligning with a circular engineering approach in wastewater treatment processes.