Enhancing simultaneous nitrogen and phosphorus removal in secondary effluent by a novel coal gangue-sulfur autotrophic biofilter: performance and microbial mechanisms

Abstract

Conventional sulfur autotrophic denitrification (SAD) is constrained by effluent acidification and inability to remove phosphorus, while the resource utilization of coal gangue (CG), a massive solid waste, remains challenging. To address both issues, this study developed a novel composite filler by integrating elemental sulfur (S⁰) with CG and established a coal gangue-based sulfur autotrophic denitrification (CGSAD) biofilter. The system was comprehensively evaluated across various hydraulic retention times (HRT: 0.5–4 h) and temperatures (10–30 °C). During 132 d of operation, CGSAD maintained an average NO₃⁻-N removal efficiency of 82.86% even at a short HRT of 1 h. It demonstrated superior low-temperature adaptability, removing 77.27% of NO₃⁻-N and 53.87% of PO₄³⁻-P at 10 °C, significantly outperforming conventional SAD. Phosphorus was mainly removed via metal-phosphate precipitation, driven by H⁺ released from autotrophic denitrification that promoted metal dissolution from CG and subsequent precipitation, as verified by SEM-EDS. Sulfur-oxidizing bacteria (e.g., Thiobacillus, Sulfurimonas) were highly enriched as core functional genera. Metagenomic and microbial community analyses confirmed the enrichment of key functional genes and the formation of a resilient biofilm with abundant extracellular polymeric substances, supporting system stability under low-temperature stress. The CGSAD system also exhibited effective pH self-buffering without external alkalinity supplementation. Overall, this process provides a sustainable and efficient solution for simultaneous N and P removal from low C/N wastewater, enabling resource utilization of coal gangue while overcoming key bottlenecks of traditional SAD.

Graphical abstract

Keywords

Denitrification / Phosphorous removal / Biofilter / Coal gangue / Secondary effluent

Highlight

	● An innovative CGSAD system with coal gangue-S0 ceramic biocarriers was constructed.
	● CGSAD system delivers enhanced performance even at low temperatures and short HRT.
	● Thiobacillus was enriched and predominant genera serving for denitrification.
	● The mechanisms of simultaneous N and P removal in CGSAD system were elucidated.
	● A new insight was provided for the resource utilization of coal gangue.

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