| Summary: | 博士 === 國立成功大學 === 環境工程學系 === 105 === Natural organic matter (NOM) is a complex mixture of organic materials present in natural waters, and can cause problems with regard to undesirable color, taste and odor. Disinfection by-products (DBPs), of which trihalomethanes (THMs) and haloacetic acids (HAAs) are the major groups, were formed when NOM reacts with a disinfectant such as chlorine, in water treatment process. The characteristics of NOM have been shown to affect the formation of DBPs. However, there is conflicting information regarding which NOM within the hydrophobic and hydrophilic fractions are dominant precursor for THMs and HAAs formation. Therefore, more attention should be given to NOM characterization, which is complemented by DBPs formation analysis, and NOM removal processes, to obtain a better understanding of the types of NOM present in source water, and their subsequent removal or transformation through the water treatment process train for controlling DBPs formation.
The objective of this research was to characterize NOM and its removal related to DBPs formation potential (DBPFP) in source water and treated water through high performance size exclution chromatography (HPSEC) and fluorescence excitation emission matrices (F-EEMs) analysis. The scopes of this research include (1) the evaluation of removal of NOM, which was contained in the effluent of slow sand filters, by alum coagulation under various dosages. In addition to non-purgeable dissolved organic matter (NPDOC) and trihalomethanes formation potential (THMFP) measurement, HPSEC with organic carbon and ultraviolet detectors (HPSEC-OCD-UVD) was used to characterize the various organic fractions contained in the water before and after coagulation, (2) ultrafiltration (UF) membrane and alum coagulation was compared for their capacity to remove different fractions of NOM from water by using HPSEC-OCD-UVD. At the same time, the removal of DBPs precursors was measured by THMFP and haloacetic acid formation potential (HAAFP). (3) NOM in water samples from a source water, as well as the treated water after coagulation with or without potassium permanganate (KMnO4) preoxidation, was characterized by using HPSEC-OCD and F–EEMs with parallel factor (PARAFAC) analysis, besides bulk parameters, such as dissolved organic carbon (DOC) and ultraviolet light absorbance at 254 nm (UV254).
The results show that source water from effluent of slow sand filter contain more aromatic humic substances as detected by HPSEC-OCD and HPSEC-UVD, in addition to aliphatic biopolymers. It reveals that among all NOM fractions, alum coagulation mainly removed hydrophobic aromatic, which is mainly humic substances. The reduction in THMFP was found to be higher than that of NPDOC under specific alum dosage, and the former was also found to be proportional to the corresponding reduction in the area of hydrophobic aromatic fraction, mostly humic substances, as obtained from HPSEC chromatogram with peak-fitting.
Characterization and removal of NOM from water using UF membrane and alum coagulation show that the UF membrane mainly removed the aliphatic biopolymer fraction, while alum coagulation mainly removed the aromatic humic substances fraction. Combining these results, it is conjectured that the aliphatic biopolymer fraction is the major precursor for THMs, while the humic substances fraction is major precursor for HAAs. This study highlighted that UF membrane and coagulation have different capacity in efficiently removing specific fraction of NOM, which eventually could affect DBP formation in the finished water.
The study of NOM in source water and treated water after coagulation with or without KMnO4 preoxidation show that KMnO4 preoxidation caused the breakdown of high molecular weight (MW) organics into low MW ones. All organics, whether those that existed in the source water or those generated by KMnO4 preoxidation, could be partly removed by coagulation. Combining the derived organic fractions obtained from HPSEC-OCD with peak-fitting and from F-EEMs with PARAFAC on the same sample, humic substances have been specified as the main organic composition. Further, the predictive models for THMFP and HAAFP based on organic fractions from HPSEC-OCD have higher accuracy than those based on the components from PARAFAC modeling. These models provide useful tools to specify the organic fractions from HPSEC-OCD and F-EEMs that constitute active precursors towards THMs or HAAs formation in water. Further, the comprehension of the major organic precursors could be beneficial for choosing the appropriate water treatment process for DBPs control.
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