Quantitative study on the structure-bioavailability relationship of dissolved organic nitrogen in wastewater treatment plant effluent

This study systematically investigated the relationship between the structure properties and biological characteristics of dissolved organic nitrogen (DON) in the effluents from municipal wastewater treatment plants. Ultrafiltration, Fourier transform infrared (FTIR) spectroscopy, ultraviolet (UV) spectroscopy, and excitation-emission matrix (EEM) fluorescence spectroscopy were used to characterize the structure of organic matters in the effluent samples, and the bioavailability of DON was determined by algal/bacterial-based bioassay. The quantitative analysis of EEM spectra conducted by fluorescence regional integration method showed that the organic portion of all samples was mainly consistent with fulvic acid and protein. Combined with the bioassay results, a positive correlation between the DON bioavailability and the protein content (sum of region I and region II) (r = 0.80, P < 0.02) and soluble microbial byproduct-like materials (region IV) (r = 0.76, P < 0.03) were observed. Nevertheless, the humic substances content represented by the region III and V would negatively affect the DON bioavailability. High humification degree (high HIX value) (r =  − 0.77, P < 0.03) was related to low bioavailability. Furthermore, according to UV spectroscopy results, strong aromaticity (high UV254 values) (r =  − 0.78, P < 0.03) suggested low DON bioavailability. One protein-like component (C3) and two humic-like components (C1 and C2) were identified via fluorescence excitation-emission matrices-parallel factor analysis (EEM-PARAFAC), and component C3 values were positively correlated to the BAN/DON ratio (r = 0.74, P < 0.03). The ultrafiltration showed that the low molecular weight DON (< 3 kDa) accounted for 30–73% of the total DON, and no notable relationship was observed for DON molecular weight and its bioavailability.