Characteristics of trihalomethane and haloacetic acid precursors in filter backwash and sedimentation sludge waters during drinking water treatment

Abstract

Trihalomethanes (THMs) and haloacetic acids (HAAs) are the most common disinfection byproducts posing human health risks. This study investigated the precursors of THMs and HAAs in filter backwash water (FBW) and sedimentation sludge water (SSW) by the methods of ultrafiltration membranes, synthetic resins and excitation-emission matrix (EEM) spectra. The concentrations of dissolved organic carbon (DOC) in FBW and SSW ranged from 1.7 to 2.7 mg/L and 2.7 to 5.8 mg/L, respectively. After chlorination, THMs and HAAs ranging from 223 to 608 μg/L and 26 to 110 μg/L were generated in FBW and SSW, respectively. There were more THM precursors than HAAs in FBW and SSW, but the formation of HAAs was often affected by water quality in terms of DOC, specific ultraviolet absorbance (SUVA), etc. High concentrations of bromo-trihalomethanes (Br-THMs) were generated in FBW and SSW, while monochloroacetic acid (MCAA) was the dominant species. The effect of bromide concentration on the formation of total THMs was greater than that of HAAs. Fluorescence analysis revealed that aromatic protein-like compounds were the dominant dissolved organic matter (DOM) in FBW and SSW. The concentrations of THMs and HAAs in the MW < 3 kDa fraction accounted for 58–80% and 62–74% of the whole sample, respectively. Hydrophilic (HPI) organic matter with molecular weight (MW) < 3 kDa was the main precursor of THMs and HAAs in FBW and SSW. Both HPI and hydrophobic (HPO) fractions were proven to play a considerable role in the formation of HAAs. Therefore, the corresponding treatment strategies should be considered to remove low-MW, HPI and HPO organics according to the water quality of FBW and SSW before recycling in order to avoid the formation of THMs and HAAs.