Disproportionate Changes in Composition and Molecular Size Spectra of Dissolved Organic Matter between Influent and Effluent from a Major Metropolitan Wastewater Treatment Plant

Abstract

Dynamic changes in the abundance, composition, and size spectra of dissolved organic matter (DOM) in different size-fractions between influent and effluent from Milwaukee metropolitan wastewater treatment plants (WWTPs) were evaluated using size-fractionation, excitation–emission matrix (EEM) coupled with parallel factor analysis (PARAFAC), and ΔEEM approaches. Up to 82.7% of DOC, 43.1% of total dissolved nitrogen (TDN), and 52.4% of chromophoric-DOM (CDOM) were removed from the influent, indicating different reactivity/degrability and disproportionate removal between DOM species. Within the CDOM pool, both the aromaticity and humification-index increased after treatments while the biological-index remained similar, showing a preferential removal of autochthonous DOM and the protein-like components, resulting in a higher abundance of humic-like components in the effluent but little change in the DOM size spectra. Up to 3%–5% differences were measured in the bulk DOM concentrations between the 0.22 and 0.7 μm filtrates. In addition, DOM in the 0.22–0.7 μm size-fraction contained almost only protein-like components, showing a high DOM heterogeneity. High levels of photochemically and biologically labile DOM in the effluent, with a higher molecular weight and lower DOC/TDN ratio, may alter the microbial community and biogeochemical processes in coastal Lake Michigan. Long-term and concurrent characterization of DOM in both effluent and receiving waters is needed to better understand the environmental/ecological roles of DOM as it moves through WWTPs and into the natural environment.