Seasonal variations of disinfection by-product precursors profile and their removal through surface water treatment plants

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a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m

w w w. e l s e v i e r. c o m / l o c a t e / s c i t o t e n v

Seasonal variations of disinfection by-product precursors profile and their removal through surface water treatment plants Vedat Uyak a,⁎, Kadir Ozdemir b , Ismail Toroz b a b

Department of Environmental Engineering, Faculty of Engineering, Pamukkale University, 20020, Kinikli, Denizli, Turkey Department of Environmental Engineering, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey

A R T I C LE I N FO

AB S T R A C T

Article history:

A sampling program has been undertaken to investigate the variations of disinfection by-

Received 27 May 2007

products (DBPs) formation and nature and fate of natural organic matter (NOM) through

Received in revised form

water treatment plants in Istanbul. Specific focus has been given to the effect seasonal

21 September 2007

changes on the formation of DBPs and organic precursors levels. Water samples were

Accepted 28 September 2007

collected from the three reservoirs inlet and within three major water treatment plants of

Available online 13 November 2007

Istanbul, Turkey. Changes in the dissolved organic carbon (DOC), ultraviolet absorbance at 254 nm (UV254), specific ultraviolet absorbance (SUVA), trihalomethane formation potential

Keywords:

(THMFP), and haloacetic acids formation potential (HAAFP) were measured for both the

Precursors removal

treated and raw water samples. The variations of THM and HAA concentrations within

Trihalomethanes (THMs)

treatment processes were monitored and also successfully assessed. The reactivity of the

Haloacetic acids (HAAs)

organic matter changed throughout the year with the lowest reactivity (THMFP and HAAFP)

Istanbul

in winter, increasing in spring and reaching a maximum in fall season. This corresponded to the water being easier to treat in fall and an increase in the proportion of hydrophobic content. Understanding the seasonal changes in organic matter character and their reactivity with treatment chemicals should lead to a better optimization of the treatment processes and a more consistent water quality. © 2007 Elsevier B.V. All rights reserved.

1.

Introduction

Chlorination is widely used in several countries to ensure a safe drinking water. This technique is sometimes the only step, and commonly it is added after the conventional treatment processes. The chlorine reacts with natural organic matter (NOM) that has not been completely removed in the treatment and it forms trihalomethanes (THMs) and haloacetic acids (HAAs). Several studies reported that these compounds have been related to the occurrence of cancer, growth retardation, spontaneous abortion, and congenital cardiac defects (Ivancev et al., 2002; Dodds et al., 1999; Yang et al., 2000; Cedergren et al., 2002). Recently, special attention has

been given to the water treatment processes due to the increasing concern with the protection of people life. New strict regulations for water quality and water sources have been imposed in different countries. These regulations should ensure the safety of drinking water through the elimination, or reduction to a minimum concentration, of the hazardous substances in water. The maximum contaminant level (MCL) of THMs and HAAs was set to 80 and 60 μg/l by EPA, respectively (Pontius, 1993; USEPA, 2003a,b). On the other hand, the European Union (EU) regulation limits for total THM concentration in drinking water are 100 μg/l (EC, 1998). Thus, a new strict European legislation for drinking water quality was created, Directive No: 98/83/CE, and subsequently transposed

⁎ Corresponding author. Tel.: +90 2582953316; fax: +90 2582953262. E-mail addresses: [email protected], [email protected] (V. Uyak). 0048-9697/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2007.09.046

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to the Turkish legislation, which entered into force in February 17, 2005. The current THM limit in Turkey is 150 μg/l until 2012, after this date, the limit is going to be reduced to 100 μg/l (TMH, 2005). Up to now, there is no any HAA limit in Turkish drinking water regulations, but in the near future, there will be a HAA standard in the regulations because of adverse health effects of these hazardous substances. The total concentration of disinfection by products (DBPs) and the distribution of the individual species in the chlorinated water depend on the raw water characteristics and operational parameters during the treatment. Parameters that should be taken into consideration are the pH, total applied chlorine dose, temperature, bromide concentration, contact time, NOM levels and structure. It was noted that the NOM is difficult to measure because there is no single parameter that can provide a complete characterization of NOM. Thus, several surrogate parameters are used to describe the NOM properties. Dissolved Organic Carbon (DOC), Ultraviolet Absorbance at 254 nm (UV254), Specific Ultraviolet Absorbance (SUVA) and DBP Formation Potential (DBPFP) are commonly used as surrogate parameters of DBP precursors. Due to erosion and the deforestation of the watershed surrounding the water sources used for the drinking water treatment plants in Istanbul, high values of colour and DOC can be expected in the rainy season. In Buyukcekmece drinking water plant of Istanbul, a high chlorine concentration up to 6 mg/l is used to overcome DOC problems. As a consequence of the use of this higher level of chlorine doses, the presence of DBPs can be expected. Three major drinking water plants in Istanbul were sampled, at Kagithane (KTP), Omerli (OTP), Buyukcekmece (BTP). These three plants were chosen because they use different raw water and one of them (OTP) uses ozone as a preoxidant agent instead of chlorine. The impact of raw water quality and operational parameters on the DBPs precursors removal and formation of DBPs of THMs and HAAs from drinking water was studied in three treatment plants in Istanbul, Turkey. This metropolitan city is located in the west of Turkey, and it has approximately 16 million inhabitants. During the last decades, the demand

for drinking water in Istanbul has strongly increased due to the rapid growth of the population and the number of organized industrial plants zones. In Istanbul, there are three drinking water treatment plant complexes using conventional treatment as the main treatment step. Three surface water supplies of Terkos, Omerli, and Buyukcekmece lakes are used as sources of raw water to these plants (Table 1). In the treatment plants, source water are treated to comply with EU water quality limits (EC, 1998). Oxidation of natural organic matter (NOM) in raw water performed with chlorine in Kagithane and Buyukcekmece treatment plants, while Omerli treatment plant uses ozone as a preoxidizing agent (Uyak and Toroz, 2007a). The purpose of this work is to investigate the seasonal changes on DBPs precursors removal efficiencies of three treatment plants and associated THM and HAA formation from raw water through finished water of Istanbul water treatment plants.

2.

Materials and methods

2.1.

Sampling procedure

In this study, three major water treatment plants were considered: Kagithane Treatment Plant (KTP), Omerli Treatment Plant (OTP), and Buyukcekmece Treatment Plant (BTP). Together, they supply water to approximately, 16 million people in Istanbul. In addition these plants take their water from different types of surface sources. Tables 1 and 2 show average raw water quality parameters and operational parameters of these treatment plants, respectively. All plants use conventional treatment processes except Omerli uses preozonation instead of prechlorination. Further, they have similar post-disinfection strategies, with chlorine being a principal disinfectant chemical. Raw water samples were collected from three lakes of Terkos, Omerli, and Buyukcekmece in Istanbul. These three lakes provide more than two million cubic meters water to water treatment plants daily.

Table 1 – Characterization of surface water supplies quality parameters in Istanbul Parameters

pH Turbidity Color Conductivity Alkalinity Hardness Temperature Br− DOC TOC UV254 SUVA THMFP HAAFP

Unit

– NTU mg/l Pt–Co μS/cm mg/l CaCO3 mg/l CaCO3 °C μg/l mg/l mg/l 1/cm l/mg×m μg/l μg/l

Terkos Lake Water (TLW)

Omerli Lake Water (OLW)

Buyucekmece Lake Water (BLW)

Range

Average

Range

Average

Range

Average

7.50–8.40 1.6–5.5 14–25 160–350 85–140 95–154 7.6–25.7 85–340 3.02–7.45 3.81–8.47 0.062–0.194 2.15–3.58 217–388 178–406

8.12 2.8 18 310 123 133 15.9 145 4.76 4.43 0.118 2.66 258 317

6.52–7.74 1.5–4.6 5–14 210–405 58–74 53–79 8.5–24.4 55–96 3.21–5.20 3.56–7.84 0.045–0.142 1.62–2.70 177–253 185–365

7.05 2.2 9 265 64 69 17.2 65 3.90 3.85 0.075 1.95 213 243

7.47–8.65 1.4–6.3 12–36 420–680 108–167 112–170 6.8–23.7 68–420 3.88–5.84 3.48–8.15 0.052–0.166 2.06–3.61 186–322 165–359

8.1 2.4 21 510 145 150 16.5 240 4.55 4.28 0.100 2.33 237 247

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Table 2 – Istanbul water treatment plants average operational parameters Plants

KTP OTP BTP

Flow rate (m3/day)

600,000 1200,000 400,000

Preoxidation

Coagulation

Chlorination (mg/l)

Ozonation (Mg/l)

Alum (mg/l)

1.3 – 1.6

– 1.8 –

45.0

Disinfection

Ferric (mg/l)

Chlorination (mg/l)

35.0

1.4 1.2 1.8

50.0

KTP: Kagithane Treatment Plant, OTP: Omerli Treatment Plant, BTP: Buyukcekmece Treatment Plant.

Raw water samples were stored at 5 L glass containers and, kept at + 4 °C, they were transported to the laboratory. Besides, treatment plants samples were collected over a period of 10 months at each sampling points. Four sampling points were chosen in each plant (Table 2). Sample 1 (S1) was used to determine the physical, and chemical characteristics of the raw water used in treatment plants. This sample was also used to analyze the DBP precursor parameters (DOC, UV254, SUVA, THM formation potential (THMFP), and HAA formation potential (HAAFP)). Sample 2 (S2) and sample 3 (S3) were taken to determine the removal of DBP precursors after conventional coagulation processes, and after sand filtration, respectively. Sample 4 (S4) was taken for analysis to determine the presence of THMs and HAAs in the treatment plants effluents. In addition to this, several samples were taken each of respective sampling points to determine the variation of THMs and HAAs concentrations within water treatment plants.

2.2.

Glassware and reagents

The glassware used during study included washing with detergent, rinsing with tap water, ultrapure water, and placing in an oven at +150 °C for 1 h. n-Pentane, methanol purge and trap grade, methyl–tert–butyl ether (MTBE) for organic trace analysis, potassium iodide, sodium sulfite, sodium sulfate anhydrous, and sulfuric acid for analysis were obtained from Merck. Ultrapure water was from Sartorious water purification system.

2.3.

Analytical procedure

All water samples were analyzed based on procedures described in Standard Methods (APHA, 1998). All standard solutions were prepared in ultra pure water (Sartorious Co., Germany). Further, raw water samples were filtered using 0.45 μm cellulose acetate filters before analyses and chlorination. DOC analysis was conducted by the high temperature combustion method according to 3510B using a Shimadzu5000A TOC analyzer equipped with an auto-sampler. The minimum quantification limit of the analyzer was 0.1 mg/l. UV254 absorbance readings were carried out by a Shimadzu 1601 UV Visible spectrophotometer at a wavelength of 254 nm (APHA, 1998). THMFP and HAAFP test was conducted according to the procedure described in Standard Methods of 5710B (APHA, 1998). The method involved buffering samples with phosphate buffer at pH value of 7.0, chlorinating samples with excess free

chlorine and storing the sample at +25 °C for seven days to allow the reaction to approach completion. The chlorinated samples were placed into 125-ml amber glass bottles with polypropylene screw caps and TFE-faced septa. The vials were carefully filled for preventing the trapping of air bubbles inside. Then they were incubated at +25 °C until seven days. Then, residual chlorine was determined according to the DPD colorimetric method (APHA, 1998) and the quenching agent of sodium sulfite was added for depletion of residual chlorine. THM measurement was conducted using EPA Method 551.1 of liquid–liquid extraction (LLE) with pentane (USEPA, 1990). For HAA analysis, EPA Method 552.3 acidic methanol esterification was performed (USEPA, 2003a,b). THM calibration standards were prepared using certified commercial mix solutions (AccuStandard, Inc., purity N 99%). The four THM species were chloroform (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM), and bromoform (TBM). Besides, five HAA commercial mix solutions in MTBE were obtained from AccuStandard and were also accompanied with certificates of analysis (purity N 99%). HAA calibration standards were prepared using certified commercial mix solutions (AccuStandard, Inc., purity N 99%). The five HAA compounds were monochloroacetic acid (MCAA), monobromoacetic acid (MBAA), dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), and dibromoacetic acid (DBAA). All stock solutions were stored at − 4 °C and used for up to 3 months for calibration studies. Both THM and HAA analysis was performed with the HP 6890 Series II Gas Chromatograph equipped with a micro Electron Capture Detector (GC-μECD). A capillary column of DB-1 (30 m × 0.32 mm I.D. × 1.0 μm, J&W Science) was used. Injections of samples were made in split/ splitless mode, with helium as carrier gas and nitrogen as makeup gas.

3.

Results and discussion

3.1.

Characterization of the raw water quality

Table 1 summarizes the average measured raw water quality parameters in three surface water supplies of Istanbul. In general, the results showed that the highest UV254 absorbance was observed in Terkos Lake Water (TLW) (0.118 cm− 1), followed by Buyukcekmece Lake Water (BLW) (0.100 cm− 1), while Omerli Lake Water (OLW) showed the lowest level of UV254 absorbance value of 0.075 cm− 1. On the other hand, the highest SUVA value the ratio UV254/DOC of water was observed in TLW, BLW, and OLW, respectively. SUVA has

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Fig. 3 – Seasonal variations of DOC levels for Istanbul water supplies. Fig. 1 – Seasonal variations of THM formation for Istanbul water supplies.

3.2. Seasonal variations of DBP formation potentials in water reservoirs been found to be a good predictor of the carbon aromaticity content of the NOM and DBP formation in water (Liang and Singer, 2003). Reckhow et al. (1990) have found that DBP formation increases with the increased activated aromatic content of the NOM. Similarly, the highest THMFP concentration of 258 μg/l was detected in TLW, due to the highest DOC and UV254 content of water. Besides, the THMFP values of OLW, and BLW were 213, and 237, μg/l, respectively. The ranges of average HAAFP levels were found to be 243–317, μg/l, respectively. While, the highest HAAFP concentration (317 μg/l) was found in TLW, the lowest (243 μg/l) was measured in OLW, which has low level of organic matter (3.85 mg/l) and SUVA (1.95 L/mg⁎m) values. As summarized in Table 1, the relatively high level of bromide (240 μg/l) was measured in BLW. Previous studies in this region indicated that the BLW source is located near Marmara Sea coastlines, and therefore, usually there is a sea water intrusion to this water source (Toroz and Uyak, 2005). Bromide concentration in BLW was followed by TLW, and OLW, in decreasing order, respectively.

A seasonal variation of THM and HAA formation potential in chlorinated water supplies of Istanbul is presented in Figs. 1 and 2, respectively. For all water studied, the average highest THM concentrations were observed in spring and fall season, while the lowest THM concentrations were obtained in summer season. Similar to THM trends, the highest average HAA concentrations were found in spring and fall. It was concluded that seasonal variations of DBP were related to changes in NOM quantity and characteristics of water sources. THMs and HAAs are produced as a result of reactions between chlorine and organic matters. A higher DOC level is thus likely to produce more THMs and HAAs. As shown in Fig. 3, increase of DOC concentrations was observed at fall and spring season in general. These observations indicate a shift not only in the quantity but also in the composition of NOM following precipitation and suggest that runoff leached humic substances from the upper soil layer (Volk et al., 2005). Soil and hydrology affect NOM, as hydrologic conditions define the flow paths that water takes in transporting DOC to surface

Fig. 2 – Seasonal variations of HAA formation for Istanbul water supplies.

Fig. 4 – Formation of THM species in chlorinated Istanbul water supplies.

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connected to the formation of brominated species of disinfection by products. The current Turkish drinking water regulation includes 150 μg/l THM limit (Uyak and Toroz, 2006), however, the future Turkish water regulation should be focused on the not only occurrence of THM but also HAA. The HAAs have not been regulated in the Turkey and EU, but during this investigation they often occurred in significant levels, sometimes exceeding the THMs levels, which highlight the importance of their monitoring in drinking water.

3.3. DBP precursors removal performances of water treatment plants

Fig. 5 – Formation of HAA species in chlorinated Istanbul water supplies.

water supplies like rivers and lakes, and interact with soil horizons of differing mineral and inorganic character (Aiken and Cotsaris, 1995). On the other hand, UV254 is a reasonable surrogate for DOC concentrations, not only in natural waters (Brandstetter et al., 1996), but also in other environments such as sediment pore waters (Deflandre and Gagne, 2001). However, Volk and colleagues reported that the strength of the relationship between UV254 and DOC diminishes under changing hydrologic regimes, suggesting that sources of DOC and their quality change with hydrology (Volk et al., 2005). For current study, the water samples of TLW and BLW collected during the spring snowmelt revealed a nearly 2 fold increase of DOC concentrations (Fig. 3), while UV254 values of 0.320 and 0.314 cm− 1 for TLW and BLW was tripled, respectively, demonstrating the limitation of using UV254 to predict DOC concentrations (Aiken and Cotsaris, 1995; Edzwald et al., 1985). On the other hand, the formation of THM and HAA species in chlorinated water of TLW, OLW, and BLW is presented in Figs. 4 and 5. In samples from TLW and OLW, the dominant species of THM was TCM, followed by BDCM, while for BLW, the major THM species was TCM, BDCM, and DBCM. A variety of HAAs were also detected in chlorinated samples from TLW, OLW, and BLW. The major HAA species was TCAA and DCAA. On the other hand, due to moderate level of bromide ion (240 μg/l) in BLW, 31.5 μg/l of DBAA (N13% of total HAA) was formed (Fig. 5). The maximum concentrations of THM detected in chlorinated raw samples during study were 185.9 μg/l for TCM, 74.8 μg/l for BDCM, 48.5 μg/l for DBCM, and 7.4 μg/l for TBM (Fig. 4). For HAA, the maximum concentrations were 12.0 μg/l for MCAA, 7.1 μg/l for BCAA, 149.6 μg/l for DCAA, 149.0 μg/l for TCAA, and 31.5 μg/l DBAA (Fig. 5). As observed previously (Uyak et al., 2007), in chlorinated water from BLW, the speciation of DBP is a little different from TLW and OLW. As shown in Figs. 4, and 5, the brominated species of DBP observed was DBCM and DBAA in BLW. Since previous study by Uyak (2006) indicated that, DBCM was found to contribute a higher cancer risk to Istanbul residents than did the other three THM compounds, thus, the preventation of Marmara Sea water intrusion to BLW may decrease the risk

Fig. 6 shows the variations of DBP precursors removal performance of each plant studied. The aim of this study was also to evaluate the how operations of different treatment plants with different raw water quality impact DBP precursors levels in treated water. Organic precursors removal by three treatment plants were assessed by monitoring treated water DOC, UV254, SUVA, THMFP, and HAAFP value. As mentioned by Uyak and Toroz (2007a), NOM content and alkalinity of raw water affect the coagulation performance of plant. The error bars on the bar graph show the standard deviations during DBP precursors removal (Fig. 6). The removal of DOC ranged between 9 and 28%. Further, the removal of aromatic materials of UV254 was in the range of 10 and 37%. In three plants, the UV254 material was always removed to a greater extent than DOC (Crozes et al., 1995; Rizzo et al., 2004). In addition, the average percent removal of SUVA of KTP, OTP, and BTP was about 6, 12, and 16, respectively. The treatment performance of THMFP was ranged between 17 and 40%. While, treatment plants removal efficiencies of HAAFP were in the range of 5 and 27. The highest removal amount of DOC, UV254, and THMFP was obtained for OTP among in three plants. It was reported that the relatively high percentage removals achieved are a result of the relatively low pH of coagulation (pH: 6–7) (Singer et al., 1995). Further, BTP has the highest treatment efficiency SUVA and HAAFP with 16 and 21%, respectively. It was reported that the chemical structure and compositions of NOM could be changed by physicochemical

Fig. 6 – Removal percent of DBPs precursors profile for water treatment plants.

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water treatment processes and the difference in NOM has been shown to cause changes in its reactivity with disinfectants (Kim and Yu, 2007; Kanokkantapong et al., 2006a). Several people reported that THMFP reduction is attributed to removal of THM precursors (Rizzo et al., 2005; Uyak and Toroz, 2005). DBP precursors are composed of a mixture of compounds varying in size, structure and composition (Goslan et al., 2006). It was determined that the chemical properties of the DOC, as well as other physicochemical characteristics of the raw water will determine the degree of DBP precursors removal by treatment processes, especially coagulation process (Goslan et al., 2002, 2006). Analysis of raw water alkalinity and DOC levels showed that quantity of DOC and alkalinity influenced treatment efficiency. Higher NOM removal was observed for OTP with average DOC value of 3.85 mg/l with an alkalinity of 64 mg/l CaCO3 (Fig. 6). It is likely that the improved reductions in DBPFP were related to the lower pH of coagulation as well as the increased DOC removal. Several people have reported that DOC removal appeared to be a conservative indicator for removal of DBPFP with precursors preferentially removed by coagulation (Bell et al., 1998). Uyak and Toroz (2007a) determined that the correlation between DOC removal and THMFP removal was moderate with a R2 value of 0.77. On the other hand, they found that the relationship between THMFP removal and UV254 absorbing compounds removal was slightly higher, with a correlation coefficient of 0.80. Their experimental results show that waters higher in UV254 tend to exert a higher chlorine demand and the higher concentrations of chlorine applied to satisfy that demand may have resulted in greater THM formation (Uyak and Toroz, 2007a; Crozes et al., 1995). Besides, some scientists reported that DBP formation potentials decreased by breakdown of large molecular weight of NOM as well as reduction in aromaticity of NOM molecule through the water treatment processes (Kanokkantapong et al., 2006b). However, amount and species of DBPs produced were more influenced by chemical and structural characteristics such as hydrophobicity and functionality rather than mere breakdown of NOM (Kim and Yu, 2005). As shown in Fig. 7, the reactivity of the organic matter changed throughout the year with the lowest reactivity (THMFP and HAAFP) in winter, increasing in spring and reaching a maximum in fall season. The highest specific THM level was observed in BLW (66 μg/mg), followed by TLW (65 μg/mg) and OLW (55 μg/mg), respectively, in fall season. For specific HAA level, similarly, the highest specific HAA concentration of 70 μg/mg was detected in BLW, due to the highest HAAFP and DOC content of water (Fig. 7). On the other hand, the characteristics of NOM moieties and their chlorine reactivity vary by season in all three Istanbul waters studied, significantly affecting DBP formations. Fig. 7 describes that the formation of THMs and HAAs have similar levels of dependence on NOM. Thus, the chlorine reactivity of NOM present in these source waters is similar significantly for THMs and HAAs. Further, the water is becoming easier to treat in fall and an increase in the proportion of hydrophobic content. Several researchers have observed that DOC removal during treatment is usually higher for waters with elevated DOC concentrations that contain high molecular weight organic molecules and a high hydrophobic content of humic substances (high

Fig. 7 – Seasonal variations of DBP reactivity for Istanbul water supplies.

SUVA) (Owen et al., 1995; Jacangelo et al., 1995). NOM input and output from water reservoirs, reservoir stratification, and NOM decay by various means are also affected by seasonal changes (Volk et al., 2005). All these complex interactions appear to affect DBP formations seasonally. On the other hand, the formation of HAAs is known to be influenced by the hydrophilic fraction of NOM while the formation of THMs is influenced by the hydrophobic fraction (Kim and Yu, 2005).

3.4. Variations of DBP occurrences within water treatment plants The chlorine dosage and reaction time influence the detected concentration and formation rate of DBPs within water treatment plants. In the first treatment process of preoxidation step, THMs and HAAs are formed as soon as chlorine is dosed to water. During the forthcoming processes of sedimentation, filtration and disinfection, DBPs were formed by reacting chlorine with residual precursors in water. In Istanbul, the level of residual chlorine is maintained in the range of 1.0–2.0 mg/l to prevent microbial contamination in the distribution system. Fig. 8 shows the results of THMs analysis within three water treatment plants. THMs and HAAs

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measurements were performed at four sampling points for each plant. The THM concentrations were increased with increased retention time in treatment plants. However, oxidation of organic matter at preoxidation process of OTP has been performed with ozone, and chlorine addition to water performed before filtration. Thus, formation of chlorination by products of THMs and HAAs was occurred after sedimentation unit (Figs. 8 and 9). The averages highest THMs concentrations at finished water was obtained as 65 μg/l for KTP, while OTP THMs value at finished water was found to be 32 μg/l as an average lowest THMs level. Further, chlorine addition to water in treatment plants is occurred at preoxidation and before filtration steps. Therefore, as long as chlorine is dosed, the THMs and HAAs are occurred in water. Similar to THMs, HAAs concentrations were found to be risen during treatment processes. As depicted in Fig. 9, The HAA determined for three treatment plants of KTP, OTP, and BTP were 32, 54, and 45 μg/l, respectively. In contrast to THMs observations, the average highest HAAs concentrations were detected in OTP. This result is attributed to hydrophilic character of ozonated raw water of OTP. As stated in many studies, ozonation of NOM is not resulted in complete oxidation of organic matter in water (Kanokkantapong et al., 2006b). Besides, NOM oxidation with ozone is resulted in low molecular weight organic matter fractions in water. After this preozonation process, chlorination of low molecular organic structures results in formation of HAA in water. Recently, several researchers determined that hydrophilic fraction of water has been found to be the precursor for HAAs (Kanokkantapong et al., 2006b; Kim et al., 2006). Therefore, the average HAAs level was found to be the highest concentration for OTP among Istanbul treatment plants. After postdisinfection process, THMs concentrations increase was expected, as the contact between the free chlorine residual and natural organic matter continued throughout the distribution system (Uyak and Toroz, 2007b). On the other hand, it was observed that distribution networks of THMs concentrations of KTP and BTP were expected to be higher than EPA and EU THM limit because of the higher level of treated water THM values of 62, 58 μg/l (Fig. 8), for KTP, and BTP, respectively. Such expected high levels of distribution system THMs may be attributed to the poor organic matter removal efficiency of coagulation process and high level of

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Fig. 9 – Formation of HAAs within water treatment plants. applied chlorine dosages at prechlorination and postdisinfectison steps in BTP and KTP.

4.

Conclusions

Results from this investigation show that seasonal variations of DBP were related to changes in NOM quantity and characteristics of water sources. The reactivity of the organic matter has shown some variation throughout the year with the lowest reactivity (THMFP and HAAFP) in winter, increasing in spring and usually reaching a maximum in fall season. On the other hand, increase of DOC concentrations was observed at fall and spring season in general. These observations indicate a shift not only in the quantity but also in the composition of NOM following precipitation and suggest that runoff leached humic substances from the upper soil layer Besides, results from this investigation indicate that water was becoming more easier to treat in fall and an increase in the proportion of hydrophobic content was observed. It was determined that the chemical properties of the DOC, as well as other physicochemical characteristics of the raw water will determine the degree of DBP precursors removal by treatment processes, especially coagulation process. Understanding the seasonal changes of organic matter character and their reactivity with treatment chemicals should lead to a better optimization of the treatment processes and a more consistent water quality. Moreover, research on alternative DBP precursors removal methods (e.g. Granular Activated Carbon (GAC) Adsorption, Membrane Filtration, and Magnetic Ion Exchange (MIEX)) should be conducted to evaluate their applicability in the case of the drinking water of Istanbul. Finally, the HAAs have not been regulated in the Turkey and EU, but during this investigation they often occurred in significant levels, sometimes exceeding the THMs levels, which highlight the importance of their monitoring in drinking water.

REFERENCES

Fig. 8 – Formation of THMs within water treatment plants.

Aiken G, Cotsaris E. Soil and Hydrology: their effects on NOM. J Am Water Works Assoc 1995;87:36–45. APHA, AWWA, WEF. Standard Methods for the Examination of Water and Wastewater. 20th ed. Washington: DC; 1998.

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