Characterization of Natural Organic Matter in Advanced Water Treatment Processes for DBPs Control

Transcription

1 Characterization of Natural Organic Matter in Advanced Water Treatment Processes for DBPs Control H.C. Kim*, M.J. Yu*, G.N. Myung*, J.Y. Koo* and Y.H. Kim** *Department of Environmental Engineering, University of Seoul, 90 Junnondong, Dongdaemungu, Seoul, , Korea ( **Department of Environmental Management, Shinheung College, Howondong, Euijungbusi, Kyungido, , Korea ( Abstract A demonstration plant consisted of granular activated carbon (GAC) and ozone/gac process was operated using filtered water of a conventional water treatment plant as influent to study feasibility of introducing advanced water treatment. Natural organic matter (NOM) from process waters at the demonstration plant was isolated into hydrophobic and hydrophilic fractions by physicochemical fractionation methods to investigate its characteristics. In the case of hydrophobic fraction (i.e. humic substances; HS), the structural and chemical characteristics were investigated using various spectroscopic methods. Through ozonation and carbon adsorption, the hydrophobic fraction and hydrophilic fraction gradually decreased from 22.0 % to 14.7 % and from 38.6 % to 20.8 % based on the content in raw water, respectively. Formation of trihalomethanes (THMs) was highly influenced by hydrophobic fraction, whereas haloacetic acids formation potential (HAAFP) depended more on the hydrophilic fraction. Concentration of phenolic group in the HS gradually decreased from 60.5 % to 15.8 % through the water treatment and a higher yield of THMs resulted from chlorination of NOM with a higher phenolic content. The structural changes of HS identified from FT-IR and 1 H-NMR were consistent with the results from the isolation of functional groups in the HS fractionated using A-21 resin. Decreases of ratio of UV absorbance at 253nm and 203nm, respectively (A 253 /A 203 ) and DBPFPs/DOC showed consistent trends, therefore, the A 253 /A 203 ratio may be a good indicator of tendency for the DBPs formation. Keywords DBPs/DBPFPs; FT-IR; 1 H-NMR; humic substances (HS); hydrophobicity; natural organic matter (NOM) Introduction Humic substances (HS), which are described as heterogeneous polyfunctional polymers formed through the breakdown of plant and animal tissues and/or synthesis of products by chemical and biological processes (Thurman, 1985). Because of their complex polymeric properties, HS are still among the least understood and characterized components in the environment. To understand the role of HS in water chemistry, it is often necessary to fractionate natural organic matter (NOM). Various methods have been used to isolate NOM from natural water and the XAD resin method has been reported in many applications for fractionation of NOM (Aiken et al., 1979; Leenheer, 1981; Yu et al., 2002). One of the major problems for NOM including HS is the production of disinfection byproducts (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs), and NOM is 2004 IWA Publishing. 2nd IWA Leading-Edge Conference on Water and Wastewater Treatment Technologies edited by Mark Van Loosdrecht & Jonathan Clement. ISBN:

2 98 2nd IWA Leading-Edge Conference on Water and Wastewater Treatment Technologies widely known as precursors of DBPs (Bellar et al., 1974; Rook, 1974). Ozonation followed by biological activated carbon adsorption is one of the promising processes to remove NOM from raw water. Some of NOM is removed in the conventional drinking water treatment process system such as coagulation/sedimentation and sand filtration. The rest should be removed before releasing the water to the distribution system by oxidation and activated carbon adsorption. It is generally known that formation of DBPs highly depends on the organic matter content, but there are many other factors such as organic matter composition and water treatment methods. Although many studies on the DBPs formation have been carried out, the reduction of the DBPs concentration in distribution is still an important issue. The alterations in structural and chemical characteristics of NOM by physicochemical water treatment processes have been shown to cause changes in reactivity with disinfectants (Schnoor et al., 1979; Vartiainen et al., 1988). The main purposes of this paper were to compare the structural and chemical characteristics of NOM fractionated from process waters in the advanced water treatment, and to investigate the relationships between the formation of DBPs and various characteristics of NOM i.e. hydrophobicity, functionality and different chromatography. Knowledge about the interactions of chlorine with NOM considering characteristics of the NOM is essential to establish optimal treatment strategy for DBPs control. Therefore, we attempted to estimate the characteristics of DBPs generated by the different properties of the NOM in aqueous environments, and to characterize behavior of NOM as DBPs precursors through advanced water treatment. Materials and Methods The first phase of this study was to fractionate NOM from process waters in 10 months operation into hydrophobic and hydrophilic fractions with XAD resin at the demonstration plant. On the other hand, using a weak-base secondary amine resin, hydrophobic fraction (i.e. humic substances; HS) in NOM was isolated into phenolic and carboxylic functional groups. In the second phase study, these fractions obtained in the first phase were analyzed with following analytical techniques: dissolved organic carbon (DOC), UV-abs, specific ultra-violet absorbance (SUVA) and disinfection by-product formation potentials (DBPFPs). And then, HS samples were selectively extracted and analyzed by Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance ( 1 H-NMR) spectroscopy. Demonstration plant The demonstration plant is located at the Gueui Water Treatment Plant (GWTP) in Seoul, Korea. GWTP adopts conventional water treatment consisting of pre-chlorination, coagulation/sedimentation, sand filtration and chlorination, and uses Han River water as a water source. The demonstration plant that consisted of two trains, granular activated carbon (GAC) and Ozone/GAC respectively, was operated using filtered water of GWTP as influent to study feasibility for introduction of advanced water treatment. Table 1 outlines the operational parameters for the demonstration plant. Each train was optimized based on total organic carbon (TOC) removal efficiency and economical aspect, and designed to treat 40m 3 /day of sand filtered water.

3 NOM in Advanced Water Treatment Processes for DBPs Control 99 Table 1 Operating parameters of the advanced water treatment demonstration plant Process Parameters Design values Ozonation Output Contactor Contact time Ozone dose Residual ozone 40 g/hr L W H: 1.7 m 0.3 m 5.5 m 10 min 1~1.5 mg/l 0.05~0.1 mg/l GAC adsorption Raw material Particle size Effective size Uniformity coefficient Apparent density EBCT a LV b Bed volume Bituminous coal type 0.9~1.1 mm (12 40 mesh) 0.55~0.75 mm 1.9 (Max.) 0.44 g/ml (Min.) 15 min 11 m/hr ID H: 450 mm 6 m a Empty bed contact time, b Line velocity Fractionation and extraction of NOM NOM from various water samples was fractionated and extracted using the methods conducted by Thurman and Malcolm (Thurman and Malcolm, 1981). A glass column (ID: 3 cm) was filled with XAD resin (Amberite XAD-7HP, Rohm & Haas Co., France) up to 40 cm. 50 L of the water filtered with 0.45 µm membrane filter was passed through the glass column with flow rate of 10~15 ml/min, and the effluent collected is termed the hydrophilic fraction. 0.1N NaOH was used to extract HS (i.e. hydrophobic fraction) adsorbed in the resin bed. And then, Na + ions in the solution were substituted with H + using cationic exchange resin (Amberite IRC-50, Rohm & Haas Co., France). Each fraction collected was analyzed for various physicochemical analyses. Isolation of phenolic and carboxylic groups A secondary amine weak base [~N(CH 3 ) 2 ] resin (Amberlyst A-21, Rohm & Haas Co., France) was used for separating phenolic and carboxylic groups present in the HS (Lin et al., 2001). The HS solution after passing through the resin column contains little phenolic functional group by strong interaction between neutral nitrogen of the resin and the phenolic functional group present in the HS solution at neutral ph. The effluent collected is termed the carboxylic fraction. FT-IR and 1 H-NMR HS powder obtained through freeze-drying HS solution substituted with H + was analyzed for its structural and chemical characteristics. IR spectra were obtained by scanning HS extracted from process waters. KBr (FT-IR Grade, Aldrich Co., USA) was mixed with HS in the ratio of 100 to 1 and the IR spectra of the mixture were scanned by IR spectrophotometer (Infinity Gold 60AR, Thermo Mattson, USA). A NMR analyzer (Avance 400, Bruker, Germany) was used to obtain the 1 H-NMR spectra of HS. Approximately 50 mg of HS powder was added to 0.5 ml D 2 O in a 10 mm NMR tube. The signal for D 2 O was used as reference and set to 4.8 ppm chemical shift.

4 100 2nd IWA Leading-Edge Conference on Water and Wastewater Treatment Technologies Determination of physicochemical characteristics DOC was determined with TOC analyzer (DC-180, Dohrmann, USA) after filtration of the samples with 0.45 µm membrane filters. A UV-spectrophotometer (UV-2101PC, Shimadzu, Japan) was used to determine the absorbance at various wavelengths. For DBPFPs test, solutions were chlorinated at ph 8 and 20 C for 48 hr. In case of THMs, using the headspace method, volatile compounds in samples were concentrated and injected into GC (HP-5890, Hewlett Packard, USA). GC/MSD (3800CP, Varian, USA) was used for determining HAAs by slightly modifying EPA Methods (USEPA, 1995). Results and Discussion Removal of NOM and DBPs in water treatment processes The removal trends of TOC, SUVA and DBPs in process waters at the demonstration plant are shown in Figure 1. TOC decreased from 2.67 to 1.23 mg/l during conventional water treatment and decreased to 0.72 mg/l through ozonation and carbon adsorption. Also, SUVA values gradually lowered with decrease of UV absorbance. In case of carbon adsorption process alone, TOC decreased from 1.23 to 0.70 mg/l. However, DBPs formed during prechlorination were not effectively removed by ozonation, and THMs even increased in the range of 29.6~31.1 µg/l after carbon adsorption. On the other hand, HAAs were effectively removed by carbon adsorption with/without pre-ozonation, indicating that the adsorption capacity of HAAs by activated carbon was relatively higher than those of THMs. (a) Ozone/GAC (b) Only GAC Figure 1. Variation in treated water quality at the demonstration plant The variation in distribution of DOC concentration of hydrophobic and hydrophilic fractions in NOM during water treatment is shown in Figure 2. Hydrophobic fraction and hydrophilic fraction decreased from 46.1 % to 22.0 % and 53.9 % to 38.6 % through conventional water treatment based on the content of raw water, respectively. Through ozonation and carbon adsorption, the hydrophobic fraction and hydrophilic fraction gradually decreased from 22.0 % to 14.7 % (from 0.45 to 0.30 mgdoc/l) and from 38.6 % to 20.8 % (from 0.78 to 0.42 mgdoc/l), respectively. In case of carbon adsorption process alone without pre-ozonation, both fractions decreased from 22.0 % to 15.1 % (from 0.45 to 0.31 mgdoc/l) and from 38.6 % to 19.6 % (from 0.78 to 0.42 mgdoc/l), respectively as shown in Figure 2.

5 NOM in Advanced Water Treatment Processes for DBPs Control 101 (a) Ozone/GAC (b) Only GAC Figure 2. Distribution of NOM fractionated from process waters at the demonstration plant These results are indicating that hydrophobicity of the carbon surface decreased in 10 months operation of the demonstration plant. That is, the hydrophobicity of activated carbon surface is reduced by hydrophilic oxygenated functional groups increased by interaction with oxidants such as dissolved oxygen, free chlorine and residual ozone in water. Although not all of the surface functional groups are hydrophilic, the adsorption capacity of hydrophilic compounds was enhanced by increase in hydrophilic sites. As a result, the activated carbon adsorption resulted in an early breakthrough of hydrophobic compounds including THMs. Therefore the adsorption capacity of hydrophilic compounds by activated carbon was relatively higher than those of hydrophobic compounds. And, ozonation and carbon adsorption processes mainly contributed to reduction of HAAs precursor rather than decrease of THMs precursor. DBPFPs characteristics of NOM fractionated into hydrophobic and hydrophilic fractions from process waters are shown in Table 2. THMFP/DOC values of hydrophobic and hydrophilic fractions of raw water were 85.6 µg/mg and 43.3 µg/mg, and HAAFP/DOC values of each fraction were 20.2 µg/mg and 33.7 µg/mg. Through ozonation and carbon adsorption, THMFP/DOC value of hydrophobic fraction was gradually decreased from 38.4 µg/mg to 24.8 µg/mg and HAAFP/DOC value of hydrophilic fraction was decreased from 19.9 µg/mg to 16.7 µg/mg. The yield of THMs formed by hydrophobic fraction as compared to the HAAs yield by the same fraction was significantly high. That is, the formation of THMs was mainly influenced by hydrophobic fraction, while HAAFP depends more on the hydrophilic fraction.

6 102 2nd IWA Leading-Edge Conference on Water and Wastewater Treatment Technologies Table 2. DBPs formation characteristic of NOM fractionated from process waters at the demonstration plant Item Raw water (TP a /HP b ) Filtration (TP a /HP b ) Ozone (TP a /HP b ) Ozone/GAC (TP a /HP b ) Only GAC (TP a /HP b ) Unfractionated water 55.6/ / / / /15.3 Hydrophobic fraction (A 253 /A 203 ) c 85.6/20.2 (0.160) 38.4/13.8 (0.102) 30.3/10.3 (0.073) 24.8/9.0 (0.055) 25.6/9.7 (0.062) Hydrophilic fraction 43.3/ / / / /14.1 a THMFP/DOC (µg/mg) b HAAFP/DOC (µg/mg) c The ratio of UV absorbance at 253 nm and 203 nm, respectively A 253 /A 203 ratio of the hydrophobic fraction (i.e. humic substances) was analyzed and the results are shown in Table 2. A 253 /A 203 ratio decreased from to by conventional water treatment system. As the A 253 /A 203 ratio gradually decreased from to through ozonation and carbon adsorption, DBPs formation potential decreased. The changes in A 253 /A 203 ratio of HS suggest that aromatic rings substituted with various functional groups in the HS molecules are structurally altered by physicochemical water treatment. The A 253 /A 203 ratio is low for unsubstituted aromatic ring structures and increases for HS in which the aromatic rings are highly substituted with OH, carbonyl, ester and carboxylic groups. These groups are also considered to participate preferentially in reactions generating DBPs. Decreases of A 253 /A 203 ratio and DBPFPs/DOC showed consistent trends, therefore, the A 253 /A 203 ratio may be a good indicator of the tendency for the DBPs formation. Structural and chemical characteristics of HS Determination of functional groups in HS HS was isolated from process waters in the water treatment and then fractionated into the phenolic and carboxylic groups using A-21 resin. The variation in distribution of molar concentration of each group through treatment process is shown in Figure 3. (a) Ozone/GAC (b) Only GAC Figure 3. Functional group distribution of HS in NOM fractionated from process waters at the demonstration plant

7 NOM in Advanced Water Treatment Processes for DBPs Control 103 The concentration of phenolic group and carboxylic group decreased from 60.5 % to 24.3 % and 39.5 % to 39.1 % through conventional water treatment, respectively. The phenolic group decreased by 36.2 % through conventional treatment while the carboxylic group only changed by 0.4 %. This suggests the reduction in A253/A203 ratio from 0.16 to after conventional treatment was solely due to the removal of phenolic group. That is, A253/A203 ratio depended more on the phenolic group content than carboxylic group content in the HS. In addition, it was recognized that the content and substitution type of phenolic ring structure in the HS molecule highly influenced DBP formation. Through ozonation and carbon adsorption, the concentration of phenolic group and carboxylic group decreased from 24.3 % to 15.8 % (from 4.9 to 3.2 µm/l as phenolic-oh) and from 39.1 % to 29.1 % (from 7.8 to 5.8 µm/l as COOH), respectively. The content of phenolic group decreased from 24.3 % to 18.4 % by pre-ozonation, however adsorption capacity of phenolic group was decreased by structural alteration through pre-ozonation. In the case of carbon adsorption process alone, both groups decreased from 24.3 % to 16.2 % (from 4.9 to 3.3 µm/l as phenolic-oh) and from 39.1 % to 30.3 % (from 7.8 to 6.1 µm/l as COOH), respectively as shown in Figure 3. In advanced processes such as ozone/gac and GAC alone, A 253 /A 203 ratio decreased as phenolic group decreased. FT-IR spectra of HS The IR spectra of HS extracted from ozonated and carbon adsorbed waters in advanced water treatment system are shown in Figure 4. The band at around 3400cm -1 is generally attributed to OH groups and bands at 2995~2965 cm -1 are assigned to C-H, C-H 2 and C-H 3 stretching of the aliphatic groups. The bands at 1646~1640 cm -1 and 1560~1551 cm -1 are attributed to C=O stretching vibration of carboxylic acids and ketones/quinones, respectively. The bands at around 1450 and 1410 cm -1 are attributed to C-H deformation of aliphatic and CH 3 groups, respectively. Also, bands in the 1280~1137 cm -1 region are attributed to C-O stretching of esters, ethers and phenols, and band at around 830 cm -1 can be assigned to OH stretching vibration of carboxylic groups. (a) Filtered water (b) Ozonated water (c) Ozonated and GAC adsorbed water (d) GAC adsorbed water Figure 4 FT-IR spectra of HS extracted from process waters at the demonstration plant

8 104 2nd IWA Leading-Edge Conference on Water and Wastewater Treatment Technologies From this result, the peak area of 1646~1640 cm -1 was not changed much by ozonation, while this peak area decreased by carbon adsorption after pre-ozonation. In case of carbon adsorption process alone without pre-ozonation, the peak area also decreased. It suggested that hydrophobicity of carboxylic acids in the HS was decreased by ozonation, the hydrophilic compounds were selectively adsorbed by activated carbon. The structural changes of HS through water treatment process from FT-IR were consistent with the result from the isolation of phenolic group and carboxylic group using A-21 resin. 1 H-NMR spectra of HS 1 H-NMR spectra were obtained for basic structural information of HS extracted from process waters at the demonstration plant. For a more effective comparison, in accordance with literature results of assignments for the chemical shift in 1 H-NMR spectra, each spectrum was quantitatively analyzed and the results are shown in Table 3 (Ma et al., 2001). Table 3. 1 H-NMR chemical shift regions and their relative contributions Chemical shift region Relative contributions (%) (ppm) Raw water Filtration Ozone Ozone/GAC Only GAC I (0.0~1.6) a II (1.6~3.2) b III (3.2~4.3) c IV (6.0~8.5) d P Al /P Ar ratio e a Aliphatic methyl and methylene b Protons of the methyl and methylene groups α to aromatic rings, protons on carbons in α position to carbonyl, carboxylic acid, ester, or amino acid c Protons on carbon of hydroxyl, ester and ether, and protons on methyl, methylene, and methyne carbons directly bonded to oxygen and nitrogen d Aromatic protons including quinones, phenols, oxygen containing hetero-aromatics e The ratio of aliphatic protons to aromatic protons (Region I and II/Region IV) Percentage of the region III (3.2~4.3ppm) including olefin and acetylene compounds decreased more than region IV (6.0~8.5ppm) including aromatic compounds by ozonation. It was caused by the reduction of specific reaction sites. That is, since specific reaction sites such as aromatic ring structures in the HS molecule were already attacked by chlorine through pre-chlorination in conventional water treatment process, specific reaction sites to react with ozone or OH radical through ozonation were reduced. Therefore, other unsaturated sites such as olefin and acetylene bonds were directly or indirectly oxidized by ozonation. On the other hand, the ratio of aliphatic protons to aromatic protons (P Al /P Ar ) was increased from 5.8 to 9.1 by conventional water treatment system as shown in Table 3. This ratio increased by ozonation, while it was relatively decreased by carbon adsorption. And percentage of the region IV increased by carbon adsorption with/without pre-ozonation. It was because that aliphatic group in the HS was selectively adsorbed to activated carbon, and portion of aromatic protons relatively increased. These results were mostly consistent with the result from FT-IR spectra of the HS.

9 NOM in Advanced Water Treatment Processes for DBPs Control 105 Summary and Conclusions The formation of THMs was highly influenced by hydrophobic fraction, while HAAFP depended more on the hydrophilic fraction. Also, a higher yield of THMs resulted from chlorination of NOM with a higher phenolic content. Through conventional water treatment, the hydrophobic fraction was reduced more than the hydrophilic fraction and the phenolic group was mainly removed compared to the carboxylic group. It was found carbon adsorption with/without pre-ozonation decreased the hydrophilic fraction than the hydrophobic fraction. Therefore residual hydrophilic NOM after conventional treatment could to be removed by advanced treatment such as ozone/gac and GAC alone to reduce HAAs formation. GAC alone appeared virtually as effective as ozone/gac in the results. However one of these can be suggested considering the necessity of selective oxidation and adsorption of each NOM constituent, and reactivation frequency of GAC. Amount and species of DBPs produced were more influenced by chemical and structural characteristics such as hydrophobicity and functionality rather than merely molecular size of organic matter. The structural alteration of the HS caused the change in A 253 /A 203 ratio, therefore the A 253 /A 203 ratio may be a good indicator for changes in reactivity of the NOM with chlorine. The formation of DBPs may be predicted by monitoring the A 253 /A 203 ratio, a relatively simple measurement. References Thurman E.M. (1985). Organic Geochemical of Natural Waters, Kluwer Academic, Boston, MA, USA, 497. Aiken G.R., Thurman E.M., Malcolm R.L. and Walton H.F. (1979). Comparison of XAD macroporous resins for the concentration of fulvic acid from aqueous solution, Anal. Chem. 51, Leenheer J.A. (1981). Comprehensive approach to preparative isolation and fractionation of dissolved organic carbon from natural waters and wastewaters, Environ. Sci. Technol. 15, Yu M.J., Kim Y.H., Han I. and Kim H.C. (2002). Ozonation of Han River humic substances, Water Sci. Technol. 46(11-12), Bellar T.A., Lichtenberg J.J. and Korner R.C. (1974). The occurrence of organohalide in chlorinated drinking water, J. AWWA 66, Rook J.J. (1974). Formation of haloform during chlorination of natural waters, Water Treat. Exam. 23, Schnoor J.L., Nitzschke J.L., Lucas R.D. and Veenstra J.N. (1979). Trihalomethanes yields as a function of precursor molecular weight, Environ. Sci. Technol. 13, Vartiainen T., Liimatainen A., Kauranen P. and Hiisvirta L. (1988). Relations between drinking water mutagenicity and water quality parameters, Chemosphere 17, Thurman E.M. and Malcolm R.L. (1981). Preparative isolation of aquatic humic substances, Environ. Sci. Technol. 15, Lin C-F., Liu S-H. and Hao O.J. (2001). Effect of functional groups of humic substances on UF performance, Water Res. 35(10), USEPA. (1995). Methods for the determination of organic compounds in drinking water: supplement 3. EPA600R Ma H., Allen H.E. and Yin Y. (2001). Characterization of isolated fractions of dissolved organic matter from natural waters and a wastewater effluent, Water Res. 35(4),

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