Separation and characterization of refractory colored dissolved effluent organic matter in a full-scale industrial park wastewater treatment plant

Colored dissolved organic matter (DOM) is a significant indicator of refractory DOM in wastewaters, and fluorescent DOM is an essential part indicating colorants. However, little is known about the composition and contribution of colored DOM to wastewater. This study provided some insights on the persistent yellowish color in biological effluent through use of a multi-characterization approach, and evaluated the effect of two advanced treatments (O₃ and granular active carbon (GAC)) in a full-scale wastewater treatment plant. The multi-characterization technique incorporated resin fractionation, excitation-emission matrix spectroscopy (EEM) combined with fluorescence regional integration (FRI), size-exclusion chromatography (SEC), and X-ray photoelectron spectroscopy (XPS) analysis. The fractionation results showed that hydrophobic acid (HPOA) and hydrophilic (HPI) substances are abundant in colorants, and HPI-type colorants are comparatively resistant or unable to be removed through GAC and O₃ individually. FRI-based EEMs showed that F₃ (fulvic acid–like organics) and F₅ (humic acid–like organics) mainly account for the yellowish color, and their combined fractions of total colorants are 50%, 31%, and 48% in biological, biological + O₃, and biological + GAC effluents, respectively. SEC for measurement of the apparent molecular weight revealed that these colorants may have molecular weights in the range 2–5 kDa. The XPS analysis indicated that these colorants possess ether or hydroxyl and nitro (C-O/C-N) chromophoric groups with conjugated aromatic structures. For C-O/C-N, O₃ showed good removal efficiency overall. GAC showed exceptionally high efficiency for HPOA but very low efficacy toward HPI-type colorants in terms of C-O/C-N chromophoric functional group removal.