Characterization of dissolved organic matter in effluents from wastewater treatment plants

Abstract

Dissolved organic matter (DOM) in effluents from sewage and human-wastes treatment plants (STPEs and HWTPEs) was fractionated using resin adsorbents into six classes: aquatic humic substances (AHS), hydrophobic bases (HoB), hydrophobic neutrals (HoN), hydrophilic acids (HiA), hydrophilic bases (HiB), and hydrophilic neutrals (HiN). DOM-fraction distribution varied substantially depending on the kind of wastewater and the type of treatment process. AHS and HiA dominated in all effluents, collectively accounting for more than 55% of the DOM measured as dissolved organic carbon (DOC). In particular, HiA were the most abundant fraction in the effluents, constituting 32–74% of the DOM. AHS appeared to be the second most dominant fraction and varied considerably, accounting for 3–28% of the DOM. The HoN fraction also varied from 0–21%. AHS, HoN, and HiA were the fractions that changed substantially and differed characteristically among the samples studied. The ratios of ultraviolet absorbance to DOC (UV : DOC ratio) in all effluents exhibited a common relationship: AHS>total DOM>HiA. Nevertheless, the ratios were substantially different between STPEs and HWTPEs and among HWTPEs. For HWTPEs, the effluent from the chemical coagulation process had the highest UV : DOC ratios. On the other hand, the effluent from the ultrafiltration and activated carbon adsorption processes had the lowest ratios. Molecular size distribution of the effluents was determined by size exclusion chromatography and found to exhibit a relatively narrow size range and low weight-averaged molecular weights ranging from 380 to 830 g mol −1. The weight-averaged molecular weight of DOM increased as the UV : DOC ratio of total DOM increased.