Natural organic matter and formation of trihalomethanes in two water treatment processes

Transcription

1 Desalination 210 (2007) Natural organic matter and formation of trihalomethanes in two water treatment processes H. Wong a, K.M. Mok b *, X.J. Fan a a The Macau Water Supply Co. Ltd., 718, Avenida do Conselheiro Borja, Macau SAR, China b University of Macau, Faculty of Science and Technology, Department of Civil and Environmental Engineering, Av. Padre Tomás Pereira S.J., Taipa, Macau SAR, China Tel ; Fax ; kmmok@umac.mo Received 4 November 2005; revised 15 March 2006; accepted 11 May 2006 Abstract The investigation involved the study of performance of two local plants slightly different treatment processes for the removal of natural organic matter (NOM) which comprises trihalomethane (THM) precursors. Ultrafiltration separated the contained NOM into various apparent molecular weight (AMW) fractions to reveal the two processes NOM removal efficiencies, in terms of dissolved organic carbon (DOC), UV absorbance (UV254) and THM formation potential (THMFP). DOC and UV254 levels of source water at both plants tested low and they concentrated in the portion with AMW less than 3,000 Daltons. The combined expression of UV254 with DOC as specific ultraviolet light absorbance (SUVA) indicated that the source water of Macau had a value less than 3 L/mg-m. Therefore the main DOC content was fulvic in character, which is difficult to remove and favors the formation of brominated THMs. Low DOC removal percentages recorded in both processes confirm this observation. However, both processes showed similar removal efficiency of around 50% on humic substances and THM precursors quantified by measurements of UV254 and THMFP, respectively. Keywords: Natural organic matter; Humic substance; Trihalomethanes; Water treatment process; Chlorination 1. Introduction Organic matter suspended in natural water is a key precursor to chlorination byproducts such *Corresponding author. as trihalomethanes (THMs), and thus NOM is the focus of this study. Macau, a small coastal city in South China, has three water treatment plants, two on the Macau peninsula and one on the Coloane Island [1]. The present study was carried out in Presented at the 9th Environmental Science and Technology Symposium, September 1 3, 2005, Rhodes, Greece. Organized by the Global NEST organization and prepared with the editorial help of the University of Aegean, Mytilene, Greece and the University of Salerno, Fisciano (SA), Italy /07/$ See front matter 2007 Elsevier B.V. All rights reserved. doi: /j.desal

2 H. Wong et al. / Desalination 210 (2007) the two Macau peninsula plants. The first site is the main Ilha Verde Water Treatment Plant (IV Plant). It uses the conventional water treatment process that includes pre-chlorination, coagulation, flocculation, sedimentation, filtration and final disinfection. The raw water processed in this plant is imported directly from the West River, a tributary of the Pearl River. Due to the intake location is near the estuary outlet, the water taken may be affected by seawater intrusion during the dry season. The second plant is the Main Storage Reservoir Water Treatment Plant (MSR Plant). The MSR Plant processes water by pre-chlorination, coagulation, filtration and final disinfection. This process uses a dual-layer direct filtration process with filtering material including anthracite and sand. Water treated in the MSR Plant flows from the main storage reservoir which contains a mixture of water from the West River and local rainwater. The main storage reservoir retains water for some time before treatment. Water from this reservoir also supports the IV Plant and contributes to salinity reduction by dilution during the dry season. Since these two plants use slightly different treatment processes, their performance in removing natural organic matter (NOM) and THM precursors was studied and compared. 2. Experimental methods Water samples of untreated and treated product water at different steps in both treatment processes were collected on the same day in December of Analyses of the samples were carried out in the Laboratory and Research Center of the Macau Water Supply Co. Ltd. Samples were characterized by dissolved organic carbon (DOC), UV absorbance (254 nm at ph 7.5), and trihalomethane formation potential (THMFP) under the conditions of 25 C, ph 7.5, chlorine dosage of 20 mg/l with a 72-h reaction time. A TOC analyzer (Shimadzu TOC-5000A) measured DOC with an auto-sampler (Shimadzu ASI-5000A), while a UV-visible spectrophotometer (Shimadzu UV-2401) detected UV254. Measurements of ph were done by a TOA ph meter (Scientific HM- 30S). Concentrations of THMs were measured by a gas chromatography system (Pekin Elmer Auto System XL) through uploading the water sample to its automatic headspace sampler (Pekin Elmer HS 40XL). The GC column used in the system was an SPB-5 (30 m 0.25 mm, with id 2.5 µm). Molecular weight fractionation of dissolved organic matter was achieved by ultrafiltration. Note that filtration by a 0.45 µm membrane (Millipore, Φ 47 mm) removed suspended solids from the raw water before ultrafiltration. Apparent molecular weight (AMW) distributions were determined using a stirred cell (Amicon Inc., model 8400, effective volume 350 ml) with membranes characterized by nominal AMW cutoffs of 1,000, 3,000, 10,000, 100,000 Daltons (YM1, YM3, YM10, YM100; Amicon Inc.). Sample aliquots were processed through the membranes in parallel under gas pressure, yielding a series of permeates, each of which contain all molecular weight fractions below the corresponding AMW cut-offs. This parallel processing arrangement would minimize mass loss during ultrafiltration. 3. Results and analysis Three parameters (DOC, UV254 and THMFP) were used to assess organic content characteristics of the water samples and NOM removal performance of the two treatment processes, hence subsequently the effects of NOM on formation of THMs. The value of DOC indicates the amount of dissolved organic carbon while the value of UV254 gives representation in concentration of humic substances. The combination of these measures yields the specific ultraviolet light absorbance, SUVA = UV254/DOC, which is a good surrogate for representation of humic content [2] as well as a good predictor for the aromatic-carbon content of NOM in water [3]. Meanwhile THMFP is designed to give the THM formation potential of water, and can be used to give the

3 46 H. Wong et al. / Desalination 210 (2007) corresponding THM reactivity or yield, defined as the generated THMFP per unit of DOC. In the following sections, characteristics of the source and treated water in terms of these parameters at the two plants are reported first, and then comparisons between them are presented. Table 1 summarizes the main water quality parameters at the two plants The conventional treatment process the IV Plant Fig. 1 presents the variations in the values of DOC, UV254, and THMFP with respect to AMW distributions before and at different treatment steps. Note that the term raw water in the figure stands for the source or untreated water, coagulated water stands for water after the pre-chlorination and coagulation step, filtrated water Table 1 Main parameters of the source water at the two water treatment plants Water quality parameter IV Plant MSR Plant Color, mg/l Pt/Co 4 4 Turbidity, NTU Temperature, C ph Hardness, mg/l Ca Alkalinity, mg/l CaCO DOC, mg/l 1.7 UV254, m THMFP, µg/l stands for water after the sedimentation and filtration step, and treated water stands for water after the final chlorination step. Commentary is DOC (mg/l) (a) UV254 (m-1) (b) <1000 <3000 <10000 < TOTAL TTHMFP (µmol/l)* (c) THM Reactivity (µgthm/mgc) (THM Reactivity) (THM Reactivity) (SUVA) (SUVA) (d) SUVA (L/mg-m) 0 10 <1000 <3000 <10000 < TOTAL 0.5 Fig. 1. Characteristics of raw and treated water in terms of AMW fractions at the IV Plant; (a) DOC; (b) UV254; (c) THMFP; (d) SUVA and THM reactivity.

4 H. Wong et al. / Desalination 210 (2007) mainly on characteristics of the raw and treated water as quality of the final product is the main concern. Some interesting features observed in between treatment steps are also discussed. Fig. 1a indicates that the DOC concentration in raw water of this plant was low ( mg/l). The result also shows that only 16% of DOC was removed when water was completely treated, and none of the removed was from the fraction with AMW less than 1,000 Daltons. This low total DOC removal percentage could be due to the organic matter in the source water being concentrated in the fraction with AMW less than 3,000 Daltons, which occupied about 72% of the total DOC concentration, and 63% (about 46% of the total) was in the portion with AMW less than 1,000 Daltons. In the meantime, the nature of this organic matter at the low AMW fractions is also important and it could be revealed by the values of the UV254 and SUVA discussed below. Looking at the UV254 values in Fig. 1b, the raw water had a total value of 2.9 m 1. A relatively large contribution to it was from the fraction with AMW less than 1,000 Daltons which UV254 reading was 1.7 m 1 ; this is 59% of the total value. This percentage is over two times larger than the 25% from the second largest contributor lying in the AMW fraction between 3,000 to 10,000 Daltons. This finding together with the DOC distribution discussed earlier suggests that organic matter and humic substances, which are precursors for THM formation during chlorination, were concentrated in the small molecular weight fractions of the raw water. Different from the DOC removal, the UV254 values (hence humic substances) in Fig. 1b show that 45% of humic materials were removed after treatment. The removal percentage was also relatively high (63.5%) even at the fraction with AMW less than 1,000 Daltons. With DOC and UV254 values, the SUVA variation with respect to AMW distribution is shown in Fig. 1d. The SUVA values of the source water samples were about 2.1 L/mg-m or lower in all AMW inclusion ranges except in the smallest one (less than 1,000 Daltons) which had a value of 2.5 L/mg-m. Nonetheless, all these SUVA readings are still considered low. It is known that an SUVA value less than 3 L/mg-m indicates that the main content of DOC is more fulvic, and therefore difficult to remove in treatment [2,4]. This explains the low to null DOC removal percentages discussed earlier. Meanwhile, the higher SUVA value observed in the lowest AMW fraction of raw water may imply that there were more hydrophobic materials which could react with chlorine to form THMs at that AMW range. Overall, the SUVA values of the treated water were lowered in all AMW inclusion ranges after treatment, as shown in Fig. 1d. The corresponding effects of these on THM formation are shown by the THMFP results in Fig. 1c. Total removal efficiency of THM precursors after complete treatment was 51%, and the major portions of removed precursors occurred in the larger AMW inclusion ranges. The efficiency of removal was limited at the fraction with AMW less than 1,000 Daltons: hence, the dominant AMW fraction of THM precursors in the treated water was in that range. Its value was 46% of the total in raw water and 95% of that in treated water. In other words, the precursors with AMW less than 1,000 Daltons were the most difficult to be removed by the conventional treatment process. Therefore these low-amw fractions had major effects on THM formation. The THM reactivities or yields of the water examined are shown in Fig. 1d. The total THM reactivity values for raw and treated water were 91.4 µg THM /mg C and 53.2 µg THM /mg C respectively, showing a reduction after treatment. Meanwhile, variations of the reactivity value with different AMW inclusion range for both the raw and treated water showed similar trends: i.e. reactivity decreased with increase of AMW inclusion range. The highest reactivity was observed to occur at the fraction with AMW less than 1,000 Daltons, which is consistent with the THM precursor removal efficiency mentioned earlier. In

5 48 H. Wong et al. / Desalination 210 (2007) addition, the relations of the SUVA and THM reactivity of both raw and treated water shown together in Fig. 1d indicate that their variation trends were similar, which suggests SUVA is a good indicator for revealing reactivity of THMs in water. It is noted that there was slight DOC and UV254 mass shifting in coagulated and treated water occurring in the AMW-including fractions of less than 1,000 Daltons (Fig. 1 a), less than 10,000 Daltons (Fig. 1a) and less than 3,000 Daltons (Fig. 1b). They were suspected to have been caused by the oxidizing effect of chlorine, similar to that by ozonation, which could breakdown higher molecular weight humic materials into matter of lower molecular weight with little effect on the total amount of organic carbon [5,6]. Such oxidation would also increase the amount of biodegradable substances in water [5,7]. In addition, chlorination was actually shown to be able to lower humic material concentration at larger AMW fractions [8]. Therefore, the slight increases of concentration of DOC and UV254 observed in coagulated and treated water in smaller AMW fractions may be caused by the actions of pre-chlorination and final chlorination used in treatment, respectively The direct filtration process the MSR Plant Variations on the values of DOC, UV254, and THMFP with respect to AMW distributions before and at different treatment step in this plant are presented in Fig. 2. Note that the term filtrated water here stands for water after the filtration step without any sedimentation process as used in the conventional treatment process. Again, discussions are mainly on results of the raw and treated water. DOC (mg/l) (a) UV254 (m -1 ) (b) Apparent Molecular Weight (daltons) TTHMFP (µmol/l) * (Dismissed) (c) THM Reactivity (µgthm/mgc) (THM Reactivity) (THM Reactivity) (SUVA) (SUVA) (d) SUVA (L/mg-m) 0 30 Fig. 2. Characteristics of raw and treated water in terms of AMW fractions at the MSR Plant; (a) DOC; (b) UV254; (c) THMFP, (d) SUVA and THM reactivity.

6 H. Wong et al. / Desalination 210 (2007) Fig. 2a shows that the DOC concentration in raw water was also low (1.7 mg/l). The amount of DOC removed by the treatment process was none or almost none in all AMW inclusion ranges. This was due to the organic matter with AMW less than 1,000 Daltons in the source water occupying about 62% of the total DOC concentration. This value was much higher than the 46% found in the IV Plant and this portion is known to be difficult to remove. Checking the UV254 values in Fig. 2b, the raw water had a total value of 3.4 m 1 and the biggest contribution was from the fraction with AMW less than 1,000 Daltons which UV254 value was 1.38 m 1 and it was 41% of the total value. However contributions from other fractions were significant too, e.g. there were 32% from the part with AMW between 3,000 10,000 Daltons and 21% from that with 10, ,000 Daltons. Comparing the UV254 values of the raw and treated water in Fig. 2b shows that a total of 51% of humic materials were removed by treatment, and the removal efficiency was also relatively high at various AMW inclusion ranges. Variation of the SUVA value with respect to AMW distribution is shown in Fig. 2d. All SUVA values of source water were found very low: 2.0 L/mg-m and lower. Therefore, the main content in DOC was more fulvic and would be difficult to remove. This again explains the almostnone DOC removal percentage observed earlier. Even without the removal of DOC, the SUVA values for water at all AMW ranges dropped after treatment, showing a good removal efficiency of humic substances by the direct filtration process. The THMFP and THM reactivity results are shown in Figs. 2c and 2d, respectively. It is noted that the water samples taken from the filtration step for THMFP tests were mishandled, so that information is dismissed from Fig. 2d. Nonetheless, it can be seen that the total removal efficiency of THM precursors after complete treatment was 44%. In general, the trend is similar to that found at IV Plant. THM reactivity increased with the AMW inclusion range. The value of THM reactivity for raw water was 84.9 µg THM/mg C and that for treated water was 47.6 µg THM/mg C, showing a reduction after treatment. Looking at the changes of the THM reactivity value with respect to AMW distribution for both raw and treated water, it was found that reactivity generally increased with the increase in the AMW inclusion range. This relationship differs from that observed at IV Plant and that difference is suspected to be caused by the larger DOC concentration observed in the lower AMW fraction at MSR Plant. Hence the THM reactivity, defined as the generated THMFP per unit of DOC, would have a relatively larger denominator at the MSR Plant when reactivity of the small AMW inclusion fraction was calculated. Furthermore, checking the variation trends of THM reactivity and of SUVA with respect to AMW distribution of both raw and treated water in Fig. 2d shows that they were similar, SUVA is again being shown to be able to act as a good indicator for revealing the behavior of THM reactivity in water. Similar to observations in the IV Plant, there was also slight DOC and UV254 mass shifting in coagulated and treated water in the AMW-including fractions of less than 1,000, 3000 and 10,000 Daltons (Figs. 2 a and 2b). This again is believed to be caused by the oxidizing effect of chlorine during processes of pre-chlorination and final chlorination, respectively, as discussed earlier Performance comparison of the two treatment processes Table 2 summarizes the parameters in percentages used for comparing performance in removing natural organic matter and THM precursors in the two treatment processes. The removal percentages in Table 2 were calculated by taking the ratio of the difference between the values before and after treatment of a parameter to its value before treatment observed in the specified AMW inclusion range; e.g. the UV254 measurements for

7 50 H. Wong et al. / Desalination 210 (2007) Table 2 Removal percentages of DOC, UV254 and THMFP after treatment at the two water treatment plants AMW range (Daltons) IV Plant DOC UV254 THMFP MSR Plant DOC UV254 < 1, < 3, < 10, < 100, Total THMFP water with AMW less than 3,000 Daltons at the IV Plant were 2.02 m 1 and 1.30 m 1 before and after treatment: the removal percentage was then ( ) / % = 35.6%. It can be seen that both plants had similar removal percentages in UV254 and THMFP, but the DOC removal percentages were quite different with the MSR plant being almost zero compared to about 16% at the IV Plant. This is quite surprising as both plants received their raw water from the same origin, except that the water used by the MSR plant would experience some retention time in a reservoir before being treated. This retention period may be the reason for the difference, since during reservoir storage micro-biodegradation would take place, decomposing larger molecular weight organic matter into smaller one, especially that with AMW less than 1,000 Daltons. This was seen in the raw water of the MSR plant which had about 62% of the total DOC concentrated in the organic matter with AMW less than 1,000 Daltons, while that from the IV Plant had only 46%. This small molecular weight organic matter is known to be hard to remove in treatment. In addition, the lack of a sedimentation step in the direct filtration process, compared with the total 45 min of coagulation and sedimentation step used in the conventional process, could also contribute to this DOC removal deviation between the two sites. At last, it could be stated that the two water treatment processes studied preferentially removed humic substances over other constituents of aquatic organic matter. A similar observation was given in [9]. 4. Conclusions The performance of the conventional chlorination water treatment process and the direct filtration water treatment process on organic matter removal hence on reduction of THM formation potential was studied. Both plants processes exhibited comparable removal percentages for humic substance (UV254) and for THM precursors (THMFP), but larger differences in DOC removal. The difference in DOC removal was considered to be a result of a change of characteristics of the raw water treated at the MSR Plant due to that water s extra retention time in the reservoir prior to treatment. Meanwhile the absence of the sedimentation step used in the direct filtration process of that plant may have also contributed to the difference. Nonetheless, it is noted that the aquatic organic content (DOC) of the source water at both sites was actually quite low and it concentrated in the portion of AMW less than 3,000 Daltons. More importantly, the SUVA values of the water were found always below 3 L/mg-m before and after treatment. This means that the main organic content was more fulvic in character, which is known to be difficult to remove. Although the current study looked at THM formation in chlorination as a whole, the speciation char-

8 H. Wong et al. / Desalination 210 (2007) acteristic of THMs is very important since trihalomethanes have four species (chloroform, bromodichloromethane, di-bromochloromethane and bromoform) each with different carcinogenicity. It is known that fulvic acid generally contains more of the aliphatic ketone groups than aqueous humic acid, and aliphatic ketones are more reactive with bromine than chlorine, hence favoring the formation of brominated THMs [10]. Therefore, the source water treated at the two plants favors the formation of brominated THMs if ambient conditions are met; i.e. when seawater intrudes into the source water, increasing the concentration of bromide ion. This is the case during the dry season in Macau [11]. Acknowledgements This study was supported by the Research Committee of University of Macau (RG023/01-02S/MKM/FST, RG047/02-03S/MKM/FST) and the Macau Water Supply Co. Ltd. Dr. William Bruce Guthrie of the University of Macau proofread this article. Comments and suggestions from referees are highly appreciated. References [1] H. Wong, Effects of bromide and natural organic matter on the formation of trihalomethanes in chlorination of drinking water, MSc thesis, University of Macau, Macau, [2] J.K. Edzwald and J. Van Benschoten, Aluminum coagulation of natural organic matter. Chemical Water and Wastewater Treatment, H.H. Hahn, and R. Klute, eds., Springer-Verlag, Berlin, [3] Y.P. Chin, G. Aiken and E. O Loughlin, Molecular weight, polydispersity, and spectroscopic properties of aquatic humic substances. Environ. Sci. Technol., 28(11) (1994) [4] J.K. Edzwald, Coagulation in drinking water treatment: particles, organics, and coagulants. Wat. Sci. Tech., 27(11) (1993) [5] B. Langlais, D.A. Reckhow and D.R. Brink, Ozone in Water Treatment: Application and Engineering: Cooperative Research Report. Chelsea, Mich., Lewis Publishers, [6] L. Tan and G.L. Amy, Comparing ozonation and membrane separation for color removal and disinfection by-product control. J. AWWA, 83(5) (1991) [7] P. Servais, G. Billen and M.C. Hascoët, Determination of the biodegradable fraction of dissolved organic matter in waters. Wat. Res., 21(4) (1987) [8] G. Becher, G.E. Carlberg, E.T. Gjessing, J.K. Hongslo and S. Monarca, High-performance size exclusion chromatography of chlorinated natural humic water and mutagenicity studies using the microscale fluctuation assay. Environ. Sci. Technol., 19(5) (1985) [9] M.R. Collins, G.L. Amy and C. Steelink, Molecular weight distribution, carboxylic acidity, and humic substances content of aquatic organic matter: implications for removal during water treatment. Environ. Sci. Technol., 20(10) (1986) [10] J.A. Leenheer, M.A. Wilson, and R.L. Malcolm, Presence and potential significance of aromatic ketone groups in aquatic humic substances. Organic Geochem., 11(4) (1987) [11] K.M. Mok, H. Wong, and X.J. Fan, Modeling bromide effects on the speciation of trihalomethanes formation in chlorinated drinking water. Global NEST J., 7(1) (2005) 1 16.