Refractory DOM in Industrial Wastewater: Formation and Selective Oxidation of AOPs

Abstract The performances of advanced oxidation processes (AOPs) for the removal of refractory dissolved organic matter (DOM) are significantly different due to the various degradation mechanisms and competitive interactions between DOM and AOPs. This research traced the variation and removal of DOM components in tannery wastewater (TW) during conventional treatment, which includes coagulation, hydrolytic acidification/anoxic (H/A), and anaerobic/aerobic (A/O) stages. Electrochemical oxidation (EO) and ozone oxidation (OO) AOPs were conducted to treat a conventional treatment effluent based on selective oxidation and performance. The results demonstrated that the refractory DOM of the secondary effluent of the TW was mainly the hydrophilic neutrals (HIN) fraction (44.1%), which consisted of fatty acids and aliphatic compounds, and the hydrophobic acids (HOA) fraction (28.5%), which included the carboxylic-rich alicyclic molecules (CRAMs) and complex polycyclic aromatic structures. During the conventional treatment, lower molecular weight (MW) aliphatic and protein substances were preferentially removed by the H/A stage, while the A/O stage more completely eliminated hydrophobic aromatic substances of a lower MW. However, humus-like substances increased in both scenarios. With respect to the two AOPs, OO preferentially removed substances with a lower MW, and the HOA fraction converting to hydrophilic humus-like substances followed with more derived by-products. Comparing to OO, EO performed better on the removal of aromatic compounds, especially the humus-like substances (polycyclic aromatic and CRAM-like substances) in HOA fraction, therefore significantly improved the biodegradability of the effluent. The results indicate that EO has a better selectivity for humus-like substances and is suitable for the treatment of refractory DOM containing chloride.