Objective With the large-scale application of organic phosphorus tanning agents, represented by tetra hydroxymethyl phosphonium salt (THPs), in the fur processing industry, the accompanying tanning wastewater contains high concentrations of unreacted organic phosphorus, additives, and dissolved organic matter from skin proteins, significantly increasing the difficulty of wastewater treatment. This study systematically investigated the removal characteristics of total phosphorus (TP) and the residual patterns of organic phosphorus (OP) in fur processing wastewater, addressing the industry-wide challenge of unstable TP compliance.

Methods A typical fur processing enterprise was selected, and key pollution indicators were monitored throughout the phosphorus tanning and wastewater treatment processes. Microfiltration-ultrafiltration membrane fractionation was employed to analyze the distribution of TP, OP, and total organic carbon (TOC) across different relative molecular mass ranges. Pearson correlation analysis was applied to evaluate the co-removal efficiency of multiple pollutants. The structural composition of dissolved organic matter (DOM) was characterized using three-dimensional excitation-emission matrix (3D-EEM) spectroscopy, while X-ray photoelectron spectroscopy (XPS) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) were utilized to identify DOM composition at the molecular level.

Results Approximately 30% of OP was not effectively utilized during the tanning process and entered the wastewater along with additives, fats, and hide scraps, resulting in a high pollution load. Discrepancies were observed between operational data from the coagulation-sedimentation and two-stage biological (O-A-O) process and Pearson correlation analysis. Although the system generally conformed to the basic principles of removing nitrogen and phosphorus from high-load wastewater and the stable removal rates of over 85% were achieved for inorganic phosphorus (IP), TOC, and total nitrogen (TN), the removal efficiency of OP was only 67%. The analysis of relative molecular weight distribution revealed that both OP and TOC tended to shift toward smaller molecules during the treatment process. Among them, OP was consistently enriched in the ＜0.002 μm fraction and became the dominant existing form of phosphorus pollution. During the biological treatment stage, OP was not removed synchronously with the reduction of TOC and the content of OP in the ＜0.002 μm range remained stable, indicating that the main form of phosphorus in the system had shifted to the difficult-to-degrade OP. Further analysis of DOM was conducted using 3D-EEM, XPS, and FT-ICR-MS. It was found that the residual organics in the biologically treated effluent were mainly composed of lipids (CHO), proteins, and amino acids (CHON), with a weighted average double bond equivalent (DBE_wa) of 4.5 and a weighted average nominal oxidation state of carbon (NOSC_wa) of -1.0, collectively reflecting the difficult-to-degrade characteristics. The coexistence of these refractory organics with stable OP further constrained the treatment effect of the wastewater.

Conclusions The low utilization efficiency of phosphorus tanning agents (＜70%) led to the enrichment of OP in the tanning wastewater, and approximately 84.4% of OP was distributed in the small molecule range of ＜0.002 μm. Although the O-A-O process of this enterprise achieved relatively high removal rates for carbon and nitrogen (TOC 86.1%, TN 85.9%), the TP removal rate was only 76.73% and 33.5% of OP remained. The main reasons can be explained by the difficulty in removing low-molecular-weight OP, the absence of polyphosphate-accumulating organisms, and the lack of sludge discharge. Improvements should involve the introduction of a polyphosphate-accumulating organism enrichment unit and the establishment of a sludge discharge system. The persistent presence of low-molecular-weight OP is the key factor restricting the attainment of TP standards, and its coexistence with recalcitrant organics further increases the difficulty of treatment.