Characterization of ultraviolet-quenching dissolved organic matter (DOM) in mature and young leachates before and after biological pre-treatment

The discharge of municipal landfill leachate into publicly owned treatment works (POTWs) is a common leachate management practice in the United States. However, the benefits of this option have diminished in many POTWs because leachate can significantly interfere with UV disinfection due to the introduction of UV-quenching substances (UVQS). This study aims to characterize UV-quenching dissolved organic matter (DOM) in different raw and sequencing batch reactor (SBR) pre-treated landfill leachates. Nine leachate samples, including four raw mature leachates, four SBR pre-treated mature leachates, and one young raw leachate, were investigated. The mature leachates, regardless of raw or SBR pre-treated ones, had lower biodegradability (low BOD₅/COD), higher specific UV absorbance (SUVA), greater hydrophobic fraction, and more high molecular weight (MW) organic compounds than the young leachate. SUVA served as a useful indicator for the characterization of UV-quenching DOM, which exponentially declined with increasing BOD₅/COD. The high SUVA of the mature leachates suggests a highly aromatic and hydrophobic character with a high fraction of aquatic humic matter and high MW compounds. In contrast, the low SUVA of the young leachate implies the presence of a high fraction of non-humic matter, a highly aliphatic character, and more low MW molecules. After the mature leachates were biologically pre-treated, the SUVA increased due to the partial degradation of high MW compounds. Results from fluorescence excitation-emission matrix (EEM) analysis could effectively reflect the variation of hydrophobic DOM, but poorly indicated the presence of hydrophilic DOM.