Natural organic matter (NOM) in surface water with low specific UV absorbance in Tianjin, China, was fractionated using XAD resins and ultra-filtration membranes into different groups based on hydrophobicity and molecular weight (MW), respectively. The effects of the pre-chlorination process on each fraction were also studied. The hydrophilic acid (HPIA) fraction was the main organic component of NOM and the most significant precursor of trihalomethanes (THMs) and haloacetic acids (HAAs) formation among the other two fractions (hydrophobic acid, HPOA and transphilic acid, TPHA). HPIA was the most reactive precursor to the formation of THMs, while the HPOA was the most reactive precursor to the formation of HAAs. Pre-chlorination tests were carried out at different contact times. Some of the HPOA and TPHA fractions transformed into HPIA fraction after oxidation of 180 min as deduced from the increase of HPIA and the decreases of HPOA and TPHA fractions. On the other hand, the MW <2 kDa fraction was predominant in source water (approximately 60%) and played the most important role in the formation of THMs and HAAs. Pre-chlorination caused the cleavage of large molecules and altered the MW distribution towards smaller molecules.

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