Composition characterization and transformation mechanism of dissolved organic phosphorus in wastewater treatment using 31P NMR spectroscopy

The migration and transformation of phosphorus components in wastewater treatment plants (WWTPs) play a crucial role in the convergence and circulation of phosphorus. However, the composition and variation of dissolved organic phosphorus (DOP) in WWTPs were unclear because of its complex nature, hindering its efficient detection. In this study, the DOP species and their transformation during the treatment process in WWTP were comprehensively analyzed. First, two enrichment methods were assessed for their effectiveness at facilitating wastewater analysis: lyophilization and aluminum salt precipitation. Aluminum salt precipitation was found to be better because its application allowed ³¹P nuclear magnetic resonance (³¹P NMR) spectroscopy to identify more species in the secondary effluent: orthophosphate (Ortho-P) (81.1%–89.3% of the dissolved total phosphorus), pyrophosphates (Pyro-P) (0%–2.3%), orthophosphate monoesters (Mono-P) (7.0%–10.77%), orthophosphate diesters (Di-P) (1.0%–2.96%), and phosphonate (Phos-P) (1.7%–5.16%). Furthermore, the variation and transformation mechanism of phosphorus, particularly those of DOP, during the entire sewage-treatment process were elucidated. Among the treatment steps, biological treatment combined tertiary treatment achieved better DOP removal efficiencies. Therein, biological treatment mainly removed Mono-P and Di-P with removal efficiencies of 33.3% and 41.7% compared with the effluent of the grit chamber. Di-P has higher bioavailability and is more easily converted and utilized by microorganisms than Mono-P. However, Phos-P, with low bioavailability, was hardly utilized by microorganisms, which showed only 18.4% removal efficiency in biological treatment. In tertiary treatment, coagulation process exhibited higher removal ability of Ortho-P (69.1%) and partial removal efficiencies of DOP, resulting in an increase in the DOP proportion in TP. In addition, Phos-P could not be effectively removed through the biological treatment and was only partially reduced via the adsorption process by large particles, zoogloea or multinuclear hydroxyl complexes. The results of this study can provide a theoretical basis for efficient phosphorus removal in WWTPs.