Correlating fluorescence spectral properties with DOM molecular weight and size distribution in wastewater treatment systems

This study explores the potential of a fluorescence excitation-emission matrix (EEM) for indication of DOM molecular weight/size (MW) levels in wastewater treatment systems by systematically investigating the correlations between various fluorescence spectral properties and MW levels. Three types of DOMs from a membrane bioreactor and an oxidation ditch were fractionated into nine grades with apparent MW cut-offs (MWCOs) of 0.7, 0.22 and 0.05 μm and 100, 30, 10, 3, 1 and 0.5 kDa. The EEM overall fluorescence intensity per unit total organic carbon (FI/TOC) and the overall FI per unit UV absorbance (FI/UVA), which represent the gross fluorescence efficiency and apparent quantum yield, respectively, were found to increase significantly as the MWCO decreased from 0.7 μm to 1 kDa. Further elaboration of the excitation (Ex) and emission (Em) spectra revealed that the smaller- and larger-MW fractions tend to occur in the lower-wavelength (Ex < 300 nm, Em < 280 nm) and higher-wavelength (Em > 400 nm) regions, respectively. The EEM spectra in these particular regions produced highly significant relationships between the relative intensity and log-MWCO (from 0.7 μm to 1 kDa) with R² > 0.83 at specific Ex/Em positions. The optimal Ex/Em positions for the relationships differed according to the source of the DOM that varies with process types (e.g. membrane bioreactor vs. oxidation ditch) and process stages (e.g. sludge mixed liquor vs. membrane effluent). The results of this study provide new insights with regard to the possible use of fluorescence spectroscopy for qualitative or quasi-quantitative indication of DOM MW in wastewater treatment systems.