Novel insights into destruction mechanisms in a hybrid membrane process for simultaneous sludge thickening and digestion by characterization of dissolved organic matter

Three-dimensional excitation–emission matrix (EEM) fluorescence spectroscopy and gel filtration chromatography (GFC) were employed to characterize dissolved organic matter (DOM) in a hybrid membrane process for simultaneous sludge thickening and digestion (MSTD). The MSTD process was operated under a sequencing batch mode, and the operating time of each cycle was 15 d from the initial fresh sludge to the final thickened and digested sludge. Results showed that the molecular weight (MW) distribution of DOM in sludge supernatant became broader after the MSTD process, and three main peaks including Peak A and Peak B related to protein-like substances and Peak C associated with humic acid-like substances could be identified from the EEM fluorescence spectra of all DOM samples in the MSTD process. The quantitative analysis of the fluorescence spectra of DOM in sludge supernatant showed that the fluorescence intensity (FI) of Peak A and Peak B in sludge supernatant correlated well with the concentration of soluble protein, MLSS destruction efficiency and mean particle size, suggesting that the variations of EEM fluorescence spectra of DOM in sludge supernatant could be used to evaluate the floc destruction mechanisms of the MSTD process. It was also found that the FI of Peak A slightly increased before 8 d but rapidly went up after 8 d, and the FI of Peak B changed slightly before 8 d but increased after 8 d, which implied that the floc destruction mechanisms were varied in one cycle of the MSTD process.

► In the MSTD process, three main peaks related to protein-like and humic acid-like substances could be identified from the fluorescence spectra of all DOM samples.
► The FI of Peak A and B in sludge supernatant correlated well with the concentration of soluble protein, destruction efficiency and mean particle size.
► The FI of Peak A slightly increased before 8 d but rapidly went up after 8 d, and the FI of Peak B changed slightly before 8 d but increased after 8 d.