Characteristics of dissolved organic matters and their relationship with nitrogen in wastewater from sewage treatment plants in dry season

To address the need for studying the variations in dissolved organic matters (DOMs) in wastewater from sewage treatment plants (STPs) and their correlation with nitrogen transformation, three-dimensional fluorescence spectroscopy, parallel factor analysis, and correlation analysis were employed, and STPs in a city in southwestern China was chosen as the research object to investigate the changes in DOM fluorescence components across different process units and their correlation with nitrogen transformation during the dry season. The results showed that: (1) DOMs in the wastewater of the STPs consisted primarily of four fluorescence components, that was protein-like components C1 (tyrosine-like), C2 (tryptophan-like), and humic-like components C3 and C4. The influent of the STPs was primarily dominated by protein-like components, which accounted for an average of 66.5% of the total fluorescence intensity. Among the protein-like components, C1 had a relatively high content, representing an average of 54.6% of the fluorescence intensity. The effluent was primarily dominated by humic-like components, which accounted for an average of 71.7% of the total fluorescence intensity. Among the protein-like components, C2 had a higher content, representing an average of 99.8% of the fluorescence intensity. (2) The fluorescence intensity of DOMs generally decreased as the treatment process progressed. In particular, the fluorescence intensity of component C1 gradually approached zero. On the other hand, the humic-like components remained relatively stable and did not change throughout the treatment process. (3) The fluorescence index (FI) values of DOMs after passing through the biological treatment units of the STPs were all above 1.9, suggesting that DOMs were predominantly converted into endogenous sources. (4) A strong correlation was observed among \mathrmNH_4^+\text-\mathrmN , dissolved total nitrogen (DTN), component C1, and the humification index (HIX) in the STPs. Multiple regression could effectively predict the concentrations of \mathrmNH_4^+\text-\mathrmN and DTN in water. A recommendation was made for the STPs to develop a more comprehensive model using the extensive data on DOM spectral properties and nitrogen (\mathrmNH_4^+\text-\mathrmN and DTN). This model would be able to predict the nitrogen trends in both the effluent and receiving water bodies.