Why COD Mn Is Not Suitable for Assessing NOM Concentration in Source Waters

ABSTRACT Total organic carbon (TOC) and permanganate chemical oxygen demand (COD Mn ) are the most commonly employed methods to determine the amount of natural organic matter (NOM) in raw and drinking water. However, the usage of COD Mn is questionable due to its variable degrees of oxidation for different types of organics. Our study investigates the ratio of COD Mn to TOC in source waters with diverse NOM compositions characterized by specific UV absorbance (SUVA) and total organic nitrogen (TON). Herein, mountainous water sources with high amounts of humic substances (high SUVA, low TON) exhibit approximately 1.8–2.5 times higher COD Mn values than lowland algae‐laden sources (low SUVA, high TON) despite having the same TOC values. The determination of COD Mn , therefore, can give misleading information about the amount of organics in waters with different NOM character and underestimate the NOM concentration in lowland algae‐laden water sources, which could have implications for drinking water quality.