Elemental sulfur enhances autotrophic denitrifying phosphorus removal from carbon-deficient wastewater through microbial synergy and electron transfer optimization

The deficient organic carbon sources frequently constrain biological phosphorus removal in urban wastewater treatment. While elemental sulfur (S⁰) serves as an economical electron donor for autotrophic denitrification, its capacity to offer supplementary electron donor to enhance denitrifying phosphorus removal (DPR) under carbon-deficient conditions (< 200 mg COD/L) remains unexplored. To address this gap, a long-term reactor with and without S⁰ supplementation was operated for 178 days. Results demonstrated that S⁰ effectively replaced partial carbon demand, elevating phosphorus removal efficiency from 93.3% (200 mg COD/L) to 95.1% under carbon-deficient conditions (150 mg COD/L + 67.5 mg/L S⁰). Sulfur conversion analysis revealed heightened S⁰ utilization during carbon limitation, corroborated by typical cycle tests. Microbial analyses indicated S⁰ enrichment of community richness (Ace: +1.14%) and diversity (Shannon: +2.0%), while molecular ecological networks exhibited enhanced complexity (connectance: +100%) and stability (robustness: +100∼206.3%). Crucially, S⁰ amplified synergistic interactions between polyphosphate-accumulating organisms (PAOs; Rhodobacteraceae, Dechloromonas, Sediminibacterium) and sulfur-driven denitrifiers (Terrimonas, Arenimonas). Random forest analysis confirmed S⁰-mediated upregulation of key functional genes: phosphorus metabolism (ppk, ppx), sulfur oxidation (soxB, dsrA), and electron transfer (nuoF, coxA, cytc), thereby optimizing electron flux and ATP synthesis for metabolic demands. This work establishes an S⁰-assisted DPR strategy that leverages synergistic microbial partnerships and enhanced electron transport to overcome carbon deficiency in wastewater treatment.