Effects of C/N ratio and sulfate concentration on simultaneous partial nitrification and denitrifying phosphorus removal (SPNDPR) process

Abstract

Simultaneous partial nitrification and denitrifying phosphorus removal (SPNDPR) is a promising strategy for treating low carbon-to-nitrogen (C/N) ratio wastewater. This study explored the establishment, performance, and microbial mechanisms of the SPNDPR process in a sequencing batch reactor (SBR) under varying C/N ratios (6.67–3.33) and sulfate concentrations (50–300 mg/L). Operated under low dissolved oxygen (DO) and intermittent aeration, the system achieved high removal efficiencies for ammonium nitrogen (NH₄⁺-N, 99.88 ± 0.55 %), total inorganic nitrogen (TIN, 79.90 ± 7.17 %), PO₄³ ⁻-P (91.67 ± 3.72 %), and chemical oxygen demand (COD, 92.60 ± 0.27 %). The ratio of ammonia-oxidizing bacteria (AOB, Nitrosomonas) to nitrite-oxidizing bacteria (NOB, Nitrospira) increased from 0.34 to 2.09, indicating successful partial nitrification. Polyphosphate-accumulating organisms (PAOs, Candidatus Accumulibacter) and glycogen-accumulating organisms (GAOs, Candidatus Competibacter) were effectively enriched, facilitating phosphorus removal and denitrifying activity. At a C/N ratio of 5, nutrient removal remained stable; however, further reduction to 3.33 led to decreased TIN (83.99 ± 6.34 %) and PO₄^3--P (88.08 ± 4.37 %) removal. Increasing influent sulfate concentrations adversely affected performance, with removal efficiencies of TIN and PO₄^3--P declining to 87.22 ± 3.57 % and 60.22 ± 9.47 %, respectively, at 300 mg/L. High sulfate levels also altered the microbial community, with sulfate-reducing bacteria (SRB), Desulfomicrobium (1.81 %) and Desulfosporosinus (5.01 %), becoming prevalent.