Insight into changes during coagulation in NOM reactivity for trihalomethanes and haloacetic acids formation

Natural organic matter (NOM) in raw water can contribute in many ways to the poor quality of drinking water, including the formation of disinfection byproducts such as trihalomethanes (THM) and haloacetic acids (HAA) during disinfection. This paper investigates the role of individual NOM fractions on changes in THM and HAA formation during coagulation with iron chloride (FeCl3) and a combination of polyaluminium chloride and iron chloride (FeCl3/PACl). The dissolved organic carbon (DOC) in the raw water and after coagulation was fractionated into four fractions, based on their hydrophobicity. Fractionation showed that most of the DOC (68%) in the raw water comes from the fulvic acid fraction, yielding 41% of the total THM precursors and 21% of the total HAA precursors. Both coagulants remove the humic acid fraction, but result in different changes to the reactivity of the remaining NOM fractions towards THM and HAA formation, indicating that coagulation occurs by different pathways, depending upon the type of coagulant used. In particular, significant changes in the reactivities of the hydrophilic acidic and non-acidic fractions were observed.

► The effect of coagulation by FeCl3 and FeCl3/PACl on NOM structure was investigated.
► Coagulation with FeCl3/PACl was more efficient for NOM removal than FeCl3.
► Coagulation changes the reactivities of NOM fractions towards THM and HAA formation.
► THMFP and HAAFP increased significantly in the HPI-A after FeCl3 coagulation.
► FeCl3/PACl coagulation decreased HAAFP in the HPI-NA fraction and increased THMFP.