Changes in optical properties and molecular composition of dissolved organic matter and formation of disinfection by-products during conventional water treatment processes

Abstract

Dissolved organic matter (DOM) is present ubiquitously in natural water environments. It is inevitable that DOM enters water treatment plants (WTPs) via raw water. When in a WTP, DOM introduces some optional issues and reacts with disinfectants forming harmful disinfection by-products (DBPs). Due to the complexity of DOM, specific portions of DOM are removed and/or transformed differently in treatment processes. In this study, DOM in raw waters and processed waters of two main WTPs, in Bangkok, Thailand, was investigated, namely, Bangkhen Water Treatment Plant (BK-WTP) and Maha Sawat Water Treatment Plant (MH-WTP). Treatment processes of both WTPs consisted of coagulation/flocculation followed by sedimentation, sand filtration, and disinfection. Changes in organic concentration, fluorometric properties, and molecular composition of DOM were illustrated. Our results showed that the treatment processes of both WTPs removed different portions of DOM, although the WTPs possessed identical treatment processes. Dissolved organic carbon (DOC) concentrations of BK-WTP decreased drastically from 6.3 mg C per L in raw water to 3.5 mg C per L after sedimentation and remained relatively stable throughout the treatment chain. In MH-WTP, DOC concentrations gradually decreased from 1.9 mg C per L in raw water to 1.2 mg C per L after disinfection. Raw water of BK-WTP showed exclusively terrestrial DOM features, while that of MH-WTP showed terrestrial DOM features with an addition of microbial DOM features. Sedimentation and filtration of BK-WTP removed those terrestrial DOM features characterized by humic acid- and fulvic acid-like FDOM and unsaturated molecular DOM features. The processes of MH-WTP indicated otherwise, wherein microbial FDOM features were selectively removed by sedimentation and filtration. As a result, FDOM and DOM components left to the disinfection process were different between the WTPs resulting in different DBP formations. Comparing before and after disinfection samples, we could extract a list of unknown DBPs and the fate of their putative precursors in the prior samples showing their treatability by the treatment processes.