Effect of recycling filter backwash water on characteristic variability of dissolved organic matter in coagulation sedimentation process

Abstract

In this study, the physicochemical characteristics of the dissolved organic matter (DOM) in simulated raw water, corresponding filter backwash water (FBWW) and treated water at three recycling ratios of 2, 5, and 8%, including control without FBWW were evaluated using bulk properties, in addition to advanced characterization by molecular weight distribution (MWD), hydrophobicity, and fluorescence. The DOM characteristics of raw water and FBWW were primarily identified and compared, and it was found that DOM in FBWW was roughly hydrophilic as compared with raw water, and the FBWW was more refractory due to a substantial increment of low-MW (< 3 KDa). Additionally, humic-like fluorophore (Peak A) and protein-like fluorophore (Peak T1), with maxima at the Ex/Em wavelengths of 245.0–265.0 nm/400.0–435.0 nm, 280.0–305.0 nm/305.0–345.0 nm, respectively, were the two main components in raw water and FBWW. Recycling trial results indicated that DOM removal efficiencies were improved with comparison to control. Specifically, all chemical fractions except hydrophobic neutral could enhance removal, but MWD was highly depended on the physical nature of raw water and FBWW. Fluorescence spectra of treated water indicated recycling FBWW could effectively improve humic-like substance (Peak A) removal, but the protein-like matter (Peak T1) was resistant to elimination or was enriched.