Quantifying and characterizing major DOC fractions in water treatment processes: A simplified SPE method without recovering sorbed compounds.

Abstract

Practitioners often use specific UV₂₅₄ absorption (SUVA) as an indicator of hydrophobic (HPO) dissolved organic carbon (DOC) to evaluate removal efficiency and estimate trihalomethane formation potential (THMFP). However, current fractionation methods, such as solid-phase extraction (SPE), primarily focus on recovering sorbed compounds, and assume that treatment impacts only the quantity, not the characteristics, of DOC fractions. Additionally, varying recovery rates and inconsistent fractionation pH definitions complicate cross-study comparisons of hydrophilic and HPO DOC composition. To address these issues, we tested three pH fractionation approaches (pH 3, pH 7, and sequential adjustment) and observed significant differences in DOC content, SUVA, and specific THMFP (STHMFP) between SPE filtrates at pH 3 and pH 7, which were most likely because of ionizable acidic DOC compounds becoming HPO at lower pH levels. Based on these findings, we developed a new fractionation method to estimate the quantity and characteristics of major DOC fractions-hydrophilic neutral (HPIN), total acidic (TA = HPOA + HPIA), and hydrophobic neutral (HPON)-without the need to recover sorbed fractions. Applying this method in a conventional coagulation/softening plant revealed HPON decreased while the relative amounts of HPI and TA increased after the treatment. However, the treated water HPI exhibited significantly higher STHMFP and contained approximately twice the proportion of low-molecular-weight compounds than raw water HPI, highlighting significant changes in both the content and properties of DOC fractions throughout the treatment process. Our study indicates that the contribution of HPI DOC fraction to SUVA and STHMFP in treated water is greater than that of HPO DOC. PRACTITIONER POINTS: A two-stage ENV to estimate major DOC fractions without recovering sorbed compounds. One ENV cartridge at pH 3 can effectively isolate HPI DOC, replacing sequential ENV. Coagulation and lime/soda softening altered characteristics of DOC fractions. HPI DOC in treated water contributes to SUVA and STHMFP more than HPO fraction.