From 2022, industrial wastewater discharge regulations in South Korea will replace chemical oxygen demand (COD_Mn) with total organic carbon (TOC). A shift from COD_Mn to TOC is a pioneering change in protecting water bodies from organic contaminants. However, several industries are struggling to meet these TOC requirements even though their effluents met the COD_Mn limits. Effluent COD_Mn/TOC ratios (1.28 ± 0.64) found in our study were lower than the COD_Mn/TOC coefficients (1.33–1.80) suggested by the Ministry of Environment in South Korea. Aliphatic and particulate organic matter contents in effluents likely influenced the COD_Mn/TOC ratio. Regardless of the industrial category, dissolved organic carbon often consists of low molecular weight neutrals, hydrophobic organic carbon, and protein-like substances in raw and treated industrial wastewaters. The present study also revealed that TOC and COD_Mn represented different organic matter fractions in the paper mill and oil refinery wastewater, whereas the industrial park wastewater showed similar dissolved organic matter characteristics. Specifically, COD_Mn was effective in the determination of humic content in paper mill wastewater but was underestimated in oil refinery wastewater. Additionally, only paper mill effluents exceeded the TOC requirements (4 of 6 samples) and required an additional post-treatment process owing to higher organic loads.

Since 2021, national standards of South Korea for industrial wastewater discharge to surface water have changed from chemical oxygen demand to total organic carbon for the organic matter. Conventional organic matter parameters (e.g., biochemical oxygen demand, chemical oxygen demand and total organic carbon) are limited means of understanding the behavior of dissolved organic matter in industrial wastewater treatment processes. Thus, the current study used advanced dissolved organic matter characterization tools (e.g., fluorescence excitation emission matrix and size exclusion chromatography-organic carbon detection) to scrutinize industrial wastewater characteristics from three full-scale industrial wastewater treatment plants (IWTPs). The tools were conducive to tracking industrial wastewater sources of total organic carbon, influencing the overall performance of IWTPs, and proposing alternative processes to lower total organic carbon concentration in the effluent. The results of this study suggest that the diagnosis of IWTPs based on dissolved organic matter characteristics could be a useful tool for providing more insight into total organic carbon management.