Submitted to: Water Quality Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/17/2004
Publication Date: 11/14/2004
Citation: Chow, A.T., Guo, F., Gao, S., Brewer, R. 2004. Filter Pore Size Selection for Characterizing Trihalomethane Precursors From California Delta Soils. Water Quality Symposium Proceedings. Interpretive Summary: High concentrations of dissolved organic carbon (DOC) in water lead to formation of health hazardous compounds including trihalomethanes (THMs) during drinking water treatment processes (e.g., chlorination). The physical sizes of DOC that serve as THM precursors are not well understood. This research investigated the propensity of organic carbon fractions to form THMs from natural soil extracts that passed through various pore-size filters. This study identified that the major contributions of soil organic carbon to water and to form THMs are the fractions of DOC passing through 0.1 'm. Further identification and characterization of THM precursors should be focused on the finer sized DOC instead of the traditionally used size of 0.45 'm. In addition to research in this field, the information is also valuable for water utilities to adopt effective water treatment technologies when removal of organic carbon in drinking water source is needed.
Technical Abstract: Filters with a pore size of 0.45 'm have been arbitrarily used for isolating dissolved organic carbon (DOC) in water. Recent studies have shown that organic carbon fractions passing filters of smaller pores differ from DOC in chemical properties, and subsequently in their propensity to form disinfection by-products. Further physical fractionation enables better characterization of organic matter by separating the heterogeneous organic matter into fractions with more homogeneous properties. In this study, we examined the trihalomethane formation potential (THMFP) from water extracts of organic fractions of a mineral soil (Scriber Clay Loam) and an organic soil (Rindge Muck) in the Sacramento-San Joaquin Delta. Organic carbon from the Delta soils, especially peat soils, is of concern because they have been identified as important sources of trihalomethane (THM) precursors in the Delta drinking water sources. In addition, the effects of Na+ and Ca2+ on the partitioning of organic carbon and THM precursors in these soils were evaluated. Water extracts (1:100, soil : water) were sequentially filtered through a series of pore size filters: 1.2, 0.45, 0.1 and 0.025 'm. Each filtrate was analyzed for total organic carbon, UV absorbance at 254 nm (UV254), and THMFP. The authors defined organic matter with size fractions of 1.2-0.45 'm, 0.45-0.1 'm, 0.1-0.025 'm and smaller than 0.025 'm as particulate (POC), colloidal (COC), fine colloidal (FCOC) and dissolved organic carbons (DOC), respectively. Results showed that the dominant fraction in all extracts was DOC, followed in a decreasing order by COC, POC and FCOC. The DOC from the organic soil was 85%, whereas it was 57% from the mineral soil. Filtrates of water extracts through 0.45 'm from the mineral soil contained 36% COC. Such a high content of colloidal materials in water caused high turbidity and resulted in abnormally high UV254 values. Monovalent sodium had little impact on the distribution of each fraction, but divalent calcium significantly reduced COC. The difference in THMFP among size fractionations was relatively small. Thus, the authors believe that DOC contains the majority of THM precursors. Ideally, DOC, organic fractions that passed through 0.025 'm, should be used in characterizing THM precursors in water; however, slow filtration makes it impractical for large volumes. The difference in total organic carbon of 0.1 'm and 0.025 'm filtrates was only about 2%. Thus, a 0.1 'm pore-size filter is recommended for THM characterization.