The relationship between NOM removal and DBPs formation

• Main Authors: Yung-ChenChou, 周泳辰
• Other Authors: Hsuan-Hsien Yeh
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/uj3x7v

碩士 === 國立成功大學 === 環境工程學系 === 102 === The relationship between NOM removal and DBPs formation Yung-Chen Chou Hsuan-Hsien Yeh Department of Environmental Engineering, NCKU. SUMMARY Due to its adverse effect on human health, disinfection by-products (DBPs) problem, especially trihalomethane (THM) and haloacetic acid (HAA), has received considerable attention recently. The effective way to control the DBPs formation is avoiding the contact between natural organic matter (NOM) and chlorine. The main purpose of this study is to understand the relationship between the removal of NOM species and DBPs formation. Two water samples, Luju raw water (LJ) and Taihu slow sand filtration effluent (TH), were chosen as the targets. Coagulation tests combined with permanganate (KMnO4) preoxidation were conducted for LJ, while coagulation and UF filtration were conducted for TH. Water quality analysis included pH, turbidity, UV254, NPDOC, THMFP (FP, formation potential) and HAA9FP. Moreover, high performance size exclusion chromatography (HPSEC) with UV/Vis and OC detectors was used for characterize NOM, based on molecular size distribution. The coagulation results of both LJ and TH waters indicated that organics in the Peak a (Biopolymers) and Peak b (Humic substances) of HPSEC-OCD chromatograms were important DBPs precursors. The results from coagulation combined with KMnO4 preoxidation with LJ water indicated that the treated water had lower THMFP than that from sole coagulation. The reason might be that the LMW organics, produced by KMnO4 preoxidation, were still DBPs precursors, and which could be removed by coagulation. The result from UF filtration of TH water shows that Peak a was the major organic fraction removed by UF membrane. The corresponding reduction in THMFP and HAA9FP also indicates that Peak a was important DBPs precursors. Key words: Coagulation, DBPs, KMnO4, NOM, HPSEC INTRODUCTION In Taiwan, about 70 % of the source water for public water supply come from reservoirs. However, based on the water quality monitoring program conducted by the Environmental Protection Administration, many major reservoirs have eutrophication problem, which may cause the increase in NOM concentration and further DBPs problem. Due to its adverse effect on human health, DBPs problem has drawn much attention recently. The effective way to control DBPs formation is avoiding the contact between NOM and chlorine. Therefore, if we could decrease the NOM concentration before chlorination by pretreatment processes, such as coagulation, membrane filtration; and understand which NOM species could be important DBPs precursors, it would be a benefit for controlling DBPs formation. In this study, two water samples, Luju raw water (LJ) and Taihu slow sand filtration effluent (TH), were chosen as the targets. Coagulation test combined with permanganate (KMnO4) preoxidation were conducted for LJ, while coagulation followed by UF filtration were for TH. Water quality analysis included pH, turbidity, UV254, NPDOC, THMFP and HAA9FP. Moreover, HPSEC connected with OC and UV detector was used to characterize the NOM based on molecular size distribution. Also the peak-fit method was applied to model the NOM species area in HPSEC chromatogram. The main purpose of this study is to understand the relationship between NOM removal and DBPs formation in different processes. MATERIALS AND METHODS Coagulation test: The samples were coagulated by alum (Al2(SO4)3．18H2O, Merck, Germany) and FeCl3 (Katayama, Japan) with the conventional jar test apparatus (Jar tester, Phillips ＆ Bird, Richmond, Virginia). Six 1.0 Liter jars were used and each was filled with the pre-filtered source water. KMnO4 oxidation test: KMnO4 stock solution was prepared by dissolving 10.0 g KMnO4 (Merck, Germany) in 1 liter deionized water (MilliQ water, Millipore). Then KMnO4 oxidation test was carried out with the conventional jar test apparatus (same procedure as coagulation test). The KMnO4 residual concentration measurement was based on Standard Methods for the Examination of Water and Wastewater (20th ed., APHA, AWWA, ＆ WEF. 1998). UF membrane filtration: Membrane filtration was conducted with bench-scale dead-end membrane testing system under room temperature (23 ± 2 oC). The membrane materials included PVC (Polyvinyl Chloride), CA (Cellulose Acetate), PVDF (Polyvinylidene Fluorides) and PS (Polysulfone). Water quality analysis: The analysis items included nonpurgeable dissolved organic carbon (NPDOC), ultraviolet absorbance at 254 nm (UV254) and turbidity. Also the samples were analyzed for DBPFP (THMFP and HAA9FP). All analysis procedures were based on Standard Methods. Turbidity measurement was carried out by a turbidity meter (Model 2100N, Hach, USA). NPDOC was measured by using a total organic carbon analyzer (Model TOC-500, Shimadzu, Kyoto, Japan). UV254 was measured by a UV/Vis spectrophotometer (Model U-2001, Hitachi, Japan). THMFP and HAA9FP were analyzed by gas chromatograph with electron capture detector (GC-ECD, Agilent - 6890 series, USA) Samples for NPDOC, UV254 and DBPFP measurement were filtered through 0.45 μm membrane (Advantec, Japan) before analysis. The composition of dissolved organic matter: High performance liquid chromatography (HPLC, LC-20 ATV, Shimadzu, Japan)- size exclusion chromatography (SEC) was conducted with sequential on-line detectors consisting of UVD (254 nm, SPD-20A, UV-VIS detector, Shimadzu) and OCD (modified Sievers Total Organic Carbon Analyzer 900 Turbo, GE Water ＆ Process Technologies). Chromatograms were analyzed using PeakFit software (Version 4.12, Systat Software Inc., USA, CA) to resolve the overlapped peaks and to determine the area under each peak. RESULTS AND DISCUSSION The coagulation result of LJ indicates that the Peak a (Biopolymers) and Peak b (Humic substances) in HPSEC-OCD were the main organic fractions removed, and alum had much better removal on these two NOM fractions than FeCl3. The coagulation result of TH also confirms that Peak a and Peak b could be removed more effectively than other peaks. Furthermore, the decrease of DBPs is similar to the trend of NOM removal in coagulation. Therefore, Peak a and Peak b were assumed to be the important precursors for THMFP and HAA9FP. In permanganate preoxidation test for LJ, the result shows that KMnO4 could oxidize high molecular NOM into lower molecular ones, but no removal was observed. KMnO4 preoxidation could slightly lower DBPs formation, and it was also noticed that the low molecular NOM, generated by oxidizing high molecular NOM, could still form DBPs. In the part of coagulation combined with KMnO4 preoxidation (1 mg/L), the results shows that preoxidation caused the decrease in the area of Peak a, and the increase in the Peak c (Building blocks) and Peak d (LMW acid and humics), probably through the oxidation and breakdown of large MW organics into lower ones. Further, the results also show that these lower MW organics could be removed in the following coagulation step under higher coagulant dosages. In DBPs test, the preoxidation dosage used did not change the DBPs formation, but the following coagulation could lower THMFP better than sole coagulation. It is conjectured that low MW organic species, which were formed by preoxidation of high MW species (Peak a), still could be the precursors of THM. However, these also could be removed by coagulation. Further, the result from UF filtration of TH water shows the Peak a was the major organic fraction removed by UF membrane. The corresponding reduction in THMFP and HAA9FP also supports the previous statement that Peak a was important DBPs precursors. CONCLUSION In this study, there are several important findings: (1) Peak a (Biopolymers) and Peak b (Humic substances) in HPSEC-OCD could be important DBPs precursors, (2) Coagulation with KMnO4 preoxidation could lower THMFP than sole coagulation, (3) In UF filtration, it confirmed that Peak a (Biopolymers) could be important DBPs precursors.