Anthraquinone-2-sulfonate enhances endogenous denitrification and phosphorus removal: Electron shuttle-mediated...

Endogenous denitrification (EnD) and denitrifying phosphorus removal (DPR) offer distinct advantages for low-carbon wastewater treatment, yet the nutrient removal performance is often constrained by inefficient electron transfer and nitrite/free nitrous acid (FNA) inhibition. Here, we demonstrate that anthraquinone-2-sulfonate (AQS) acts as an effective redox mediator to overcome these bottlenecks. With nitrate (NO₃^--N) as the electron acceptor, the addition of 0.05 mmol/L AQS significantly amplified the electron transfer system activity (ETSA) by 3.66-fold. Consequently, this enhancement promoted the NO₃^--N removal rate to 25.90 mg/(g VSS·h) (12.65-fold increase) and increased the phosphorus uptake rate to 3.69 mg/(g VSS·h) (1.95-fold improvement), achieving removal efficiencies of 96.22±1.00 % and 96.03±2.98 % for phosphorus and nitrogen, respectively. Moreover, when nitrite (NO₂^--N) served as the electron acceptor, AQS enhanced the phosphorus uptake rate and nitrogen removal rate by 1.69-fold and 1.54-fold, respectively. Microbial analysis revealed a robust syntrophic partnership wherein Thauera, Candidatus Competibacter and Defluviicoccus (functioning as denitrifying glycogen-accumulating organisms) efficiently reduced NO₃^--N to NO₂^--N, which was subsequently scavenged by Dechloromonas and Candidatus Accumulibacter clade Ⅱ (functioning as denitrifying polyphosphate-accumulating organisms) for coupled phosphorus uptake. Metagenomic analysis further indicated that AQS facilitated electron transfer from Complexes I/Ⅱ to nitrate reductase and Complex Ⅲ, accelerating NO₂^--N generation while alleviating FNA toxicity via coupled electron transfer from Cyt c to nitrite reductase. Crucially, this accelerated electron flux potentially intensified the proton motive force, suggesting an enhanced capacity for ATP generation to fuel the upregulation of phosphate transport (pit/pst) and polyphosphate synthesis (ppk) genes. These findings highlight AQS as a promising strategy to regulate electron transfer kinetics and metabolic coupling for advanced nutrient removal.