DSpace at VNU: Trihalomethane formation by chlorination of ammonium- and bromide-containing groundwater in water supplies of Hanoi, Vietnam

Water Research 37 (2003) 3242–3252 Trihalomethane formation by chlorination of ammonium- and bromide-containing groundwater in water supplies of Hanoi, Vietnam Hong Anh Duonga, Michael Bergb,*, Minh Hang Hoanga, Hung Viet Phama, Herve! Gallardb, Walter Gigerb, Urs von Guntenb a Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, Nguyen Trai Street 334, Hanoi, Viet Nam b EAWAG, Swiss Federal Institute for Environmental Science and Technology, Ueberlandstrasse 133, CH-8600 Dubendorf, Switzerland Received 16 October 2002; received in revised form 10 February 2003; accepted 12 February 2003 Abstract The occurrence and the fate of trihalomethanes (THMs) in the water supply system of Hanoi City, Vietnam was investigated from 1998 to 2001 The chlorination efficiency, THM speciation, and, THM formation potential (THMFP) was determined in the water works and in tap water With regard to THM formation, three types of groundwater resources were identified: (I) high bromide, (II) low bromide, and (III) high bromide combined with high ammonia and high dissolved organic carbon (DOC) concentrations Under typical treatment conditions (total chlorine residual 0.5– 0.8 mg/L), the total THM formation was always below WHO, EU, and USEPA drinking water standards and decreased in the order type I>type II>type III, although the THMFP was >400 mg/L for type III water The speciation showed >80% of bromo-THMs in type I water due to the noticeable high bromide level (p140 mg/L) In type II water, the bromo-THMs still accounted for some 40% although the bromide concentration is significantly lower (p30 mg/L) In contrast, only traces of bromo-THMs were formed (B5%) in type III water, despite bromide levels were high (p240 mg/L) This observation could be explained by competition kinetics of chlorine reacting with ammonia and bromide Based on chlorine exposure (CT) estimations, it was concluded that the current chlorination practice for type I and II waters is sufficient for X2-log inactivation of Giardia lamblia cysts However, in type III water the applied chlorine is masked as chloramine with a much lower disinfection efficiency In addition to high levels of ammonia, type III groundwater is also contaminated by arsenic that is not satisfactory removed during treatment Nnitrosodimethylamine, a potential carcinogen suspected to be formed during chloramination processes, was below the detection limit of 0.02 mg/L in type III water r 2003 Elsevier Science Ltd All rights reserved Keywords: Disinfection by-products; Water distribution system; Trihalomethane formation potential; Competition kinetics; Chlorine exposure; N-nitrosodimethylamine (NDMA); Hanoi; Vietnam; Bromide; Ammonium Introduction Hanoi has a strongly increasing water demand due to the rapid growth of the urban population (3.5 Mio *Corresponding author Tel.: +41-1-823-5078; fax: +41-1823-5058 E-mail address: michael.berg@eawag.ch (M Berg) inhabitants in 2001, urban area 84 km2) The main water resources for Hanoi are reduced groundwaters containing variable levels of dissolved iron(II) and manganese(II) ranging from 1–25 mg/L and 0.2–3 mg/L, respectively Moreover, Hanoi’s groundwater contains excessive concentrations of arsenic that are partly removed in the WTPs to lower but not fully acceptable levels (25–91 mg/L) [1] The groundwaters pumped from 0043-1354/03/$ - see front matter r 2003 Elsevier Science Ltd All rights reserved doi:10.1016/S0043-1354(03)00138-6 H.A Duong et al / Water Research 37 (2003) 3242–3252 several locations have significantly differing chemical compositions that are influenced by varying proportions of bank infiltration from the Red River [1] Since the beginning of the 20th century chlorination has been a key-treatment for improving the microbiological safety in drinking waters However, an undesired formation of disinfection by-products (DBPs) results from the reaction of chlorine with natural organic matter (NOM) and includes products such as trihalomethanes (CHCl3, CHCl2Br, CHClBr2, CHBr3) [2] which might have adverse health effects [3–5] The total concentration of trihalomethanes (THMs) and the formation of individual THM species in chlorinated water strongly depend on the composition of the raw water, on operational parameters during water treatment and on the residual chlorine in the distribution systems [6] Aiming at minimizing the cancer risk, the United States Environmental Protection Agency (USEPA), the World Health Organization (WHO), and the European Union (EU) introduced regulations for THMs in drinking water Whereas the USEPA and the EU regulate total THM concentrations as 80 or 100 mg/L, respectively, the WHO provides guidelines for individual THM compounds [4] In Vietnam, chlorination is generally applied for drinking water disinfection The requirement for the minimum residual chlorine concentration in water at the outlet of water treatment plants (WTPs) and in the distribution system is 0.5 and 0.3 mg/L, respectively [7] 3243 So far, no information on THM concentrations in drinking water was available To determine THM levels in the urban area of Hanoi, the eight major WTPs of this city were investigated (see Fig 1) Based on the differences in dissolved organic carbon (DOC), UV254 absorption, bromide concentration and chlorine demand (mainly ammonium and ferrous iron content), it was inferred that substantial variations in THM formation can be expected in the eight Hanoi WTPs A direct comparison of the eight WTPs is favored by the fact that very similar treatment trains are applied In addition to full-scale data, chlorination experiments with natural groundwater were carried out in the laboratory to evaluate the maximum THM formation potential (THMFP) and THM yields for various chlorine doses Based on these data, a relation between chlorine exposure and THM formation was established These results may be used as a tool to adjust the chlorine doses, which is needed to achieve efficient disinfection and acceptable levels of THMs in the drinking waters of Hanoi Materials and methods 2.1 Reagents If not specified, chemicals were reagent grade from Fluka (Buchs, Switzerland) or Merck (Darmstadt, Fig Map illustrating northern Vietnam and the locations of the eight studied Hanoi WTPs The areas of differing groundwater quality (types I, II and III) are indicated The numbers 1–8 refer to the following WTPs: 1, Mai Dich; 2, Ngoc Ha; 3, Ngo Si Lien; 4, Yen Phu; 5, Luong Yen; 6, Ha Dinh; 7, Tuong Mai; and 8, Phap Van H.A Duong et al / Water Research 37 (2003) 3242–3252 3244 2.3.2 Distribution system Two sampling campaigns (May and September 2001) were performed in three water distribution systems to investigate THMs in household tap waters These water distribution systems primarily provide drinking water from WTPs 3, and representing the three different groundwater types of Hanoi Finished water of the WTPs and tap water samples of the corresponding distribution system were collected at the same day Germany) A standard stock solution containing the four THMs (1 mg/mL of each in methanol) and the internal standard p-bromofluorobenzene (1 mg/mL in methanol) was supplied by Tokyo Kasei Kogyo (ChuoKu, Tokyo, Japan) Working standard solutions of THMs (1, 10, and 100 mg/L) were prepared from the stock solution Calibrations were conducted with five concentration levels in de-ionized and boiled water Chlorine solutions of 0.4% and 0.04% were prepared from NaOCl 4% (Aldrich, Steinheim, Germany) by dilution with water 2.4 Analytical methods Water temperature, pH, dissolved oxygen (detection limit 0.1 mg/L) (Paqualab instrument ELLE, Leighton Buzzard, UK) and residual chlorine (detection limit 0.01 mg/L) were measured at the sample collection sites Samples for DOC, cations, anions and coliforms were collected according to Standard Methods [8] The DOC concentrations were measured by a Shimadzu TOC– 5000A analyzer Ammonium and bromide were determined by ion chromatography with conductivity detection (HPLC 10A, Shimadzu, Japan) UV absorbance was measured with a UV/Vis spectrophotometer (Shimadzu 1201, Japan) at 254 nm in 10 mm quartz cells Total coliforms in water were determined by the membrane filtration method [8] The chlorine and chloramine concentrations were determined by the ABTS method [9] 2.2 Water treatment plants in Hanoi The eight major WTPs (see Fig and Table 1) produce roughly 450,000 m3 drinking water per day Groundwater from 30 to 70 m depth is used as raw water All WTPs operate with similar treatment trains including aeration, settling, sand filtration, chlorine disinfection, and storage in a reservoir Chlorine is applied as gas and the applied disinfectant dose is adjusted manually to maintain a total chlorine residual of 0.5–0.8 mg/L after the reservoir 2.3 Sampling campaigns 2.3.1 Water treatment plants Water samples of raw water from the aeration jet, treated water before disinfection (after Fe removal), and finished water at the outlet of the reservoir were collected Nine sampling campaigns were conducted between April 1998 and May 2001 2.5 Analysis of THMs Duplicate samples for THM analysis were collected in 44-mL glass vials and were capped with PTFE-faced Table Individual THM concentrations and chemical parameters in treated and finished waters of Hanoi water treatment plants Groundwatera Type I b Water treatment plants 1, and Type II Type III and 6, and c DOC UV254 NH+ BrÀ CHCl3 CHCl2Br CHClBr2 CHBr3 a mg/L mÀ1 mg N/L mg/L treated water , concentration range (classification) 0.6–1.3 (low) 0.9–1.1 (low) 0.1–1.4 (low) 0.7–1.2 (low) o2 (low) o2 (low) 50–140 (high) o20–30 (low) 2.0–6.4 4.0–16.0 5–25 70–240 mg/L mg/L mg/L mg/L finished waterd, concentration range (average)e o0.3–11.1 (2.2) 0.9–7.7 (4.3) 0.5–7.3 (2.4) o0.2–5.6 (2.4) 0.3–22.3 (6.3) o0.2–3.8 (1.8) 1.2–18.5 (6.6) o0.2–3.7 (0.5) 0.3–21.5 (4.2) o0.2–3.6 (0.3) o0.2–3.7 (0.3) o0.2 (high) (high) (high) (high) The areas of differing groundwater quality are indicated in Figure The numbers 1–8 of the water treatment plants refer to Figure c Treated waters were collected before chlorination Number of samples: 9–15 d Finished waters collected at the outlets of the WTPs Number of samples: 10–17, chlorine dose o1.5 mg/L, contact time 30 e Average concentration from finished water samples collected between April 1998 and May 2001 b H.A Duong et al / Water Research 37 (2003) 3242–3252 silica septa Household taps were flushed for 15 prior to sampling The 44-mL sample vials contained 0.15 mg sodium sulfite (50 mL of a g/L Na2SO3 solution) to quench residual chlorine The samples were stored at 4 C Head-space gas chromatography with mass spectrometry detection was used to analyze THMs in water Sample volumes of 10 mL were filled in 20 mL glass vials and spiked with 50 mL of the internal standard (10 mg/L p-bromofluorobenzene) The vials were immediately sealed with Teflon coated septa and aluminum crimpcaps After equilibration (30 at 30 C) a volume of mL of the head-space was injected splitless into a gas chromatograph (DB 624 column, 60 m  0.32 mm, 0.32 mm film, J&W Scientific, CA) coupled to a mass spectrometer (GC/MS QP5000 Shimadzu, Japan) The temperature program was: 50 C (1 min), 7 C/min to 120 C (10 min), 12 C/min to 200 C (5 min) The eluting analytes were recorded in the selective ion monitoring (SIM) mode and quantified by internal standard calibration Recoveries of the four THMs, determined in spiked samples at levels of 1, 10 and 20 mg/L, were in the range of 94–114% (n ¼ 7) The method detection limits (MDLs) were determined from the standard deviation of 1-mg/L spiked samples (n ¼ 7; sigma) following the procedure described by [10] The corresponding MDLs for CHCl3, CHCl2Br, CHClBr2 and CHBr3 were 0.3, 0.2, 0.2 and 0.2 mg/L, respectively Nnitrosodimethylamine (NDMA) was analyzed with gas chromatography and thermal energy analysis detection (GC-TEA, detection limit 0.02 mg/L), following the official method 982.12 of the Association of Analytical Communities (AOAC, Gaithersburg, MD) 2.6 Laboratory chlorination experiments Laboratory chlorination experiments were carried out with treated waters collected before disinfection from the WTPs 1–4 and 6–8 Samples were collected in 5-L glass bottles and stored at 4 C until the chlorination experiments The water samples were buffered with mM phosphate and adjusted to the desired pH with NaOH For chlorination experiments, the required chlorine dose was added to a laboratory batch system and the solution was well mixed for 30 s Then the water was immediately portioned into 44-mL glass vials that were sealed with Teflon-lined screw caps After filling and sealing the head-space-free samples, the vials were kept in a thermostated water bath at 25 C For each desired reaction time, a sample was removed, and, residual total chlorine and chloramine were determined [11] THM concentrations were quantified as described above The THMFP of the treated waters was determined as total THM formed during a reaction time of days at 25 C (pH 8.0 or 7.0) and by maintaining a free chlorine 3245 residual throughout the experiment A chlorine dose of mg/L was used for treated water from WTPs and 4, and, of 160 mg/L for treated water from WTP 8, respectively Results and discussions 3.1 Water quality parameters and THM formation (water types I, II, and III) Table summarizes the water quality parameters of the three water types I, II and III (see Fig 1) that affect the THM formation during chlorination No THMs were detectable before chlorination 3.1.1 DOC and UV254 Water of types I and II had low levels of DOC (B1 mg/L) and low UV absorbances (o1.5 mÀ1) indicating a low content of NOM and aromaticity (Table 1) Therefore, the concentration of THM precursors in these waters can be expected to be low In water type III, DOC and UV254 were relatively high The highest values were measured in treated water of WTP (DOC 6.4 mg/L, UV254 16 mÀ1) reflecting a very high content of NOM and a high potential for THM formation 3.1.2 Bromide Relatively high levels of bromide in the range of 50– 140 and 70–240 mg/L were observed in water types I and III, respectively (Table 1) In water type II, bromide was near the detection limit of 20 mg/L Bromide is important for THM formation because it is oxidized by chlorine to hypobromous acid, which contributes to the formation of bromo-THMs [12] 3.1.3 Ammonium The fast reaction of chlorine with ammonia leads to masking of chlorine in excess of ammonia by formation of chloramine If chloramine is the dominant species, the THM yields in chlorinated water is reduced because of the lower reactivity of chloramine with NOM [13] Such a situation was observed for water type III (WTPs 6, and 8) where extremely high ammonium concentrations of 5–25 mg/L are present Ammonium is believed to originate from mineralization of peat which is abundant in the subsurface of the type III area [1] 3.2 Occurrence of THM in finished waters at the WTP outlets Fig shows the concentrations of THMs in finished waters at the outlet (reservoir, h contact time) of WTPs using groundwater types I, II and III They are in the range of 5–56, 2–18 and 0.3–22 mg/L, respectively The high variability of the total THM concentrations can be H.A Duong et al / Water Research 37 (2003) 3242–3252 3246 100 50 average minimum 40 (n = to 8) 30 20 10 WTP CHBr3 maximum I II groundwater type III molar distribution (%) total THM conc (µg/L) 60 CHClBr2 80 CHCl2Br 60 CHCl3 40 20 I II III groundwater type Fig Average concentration and range of total THM concentrations measured in finished waters at the outlets of the Hanoi WTPs Fig Molar distribution of THM species in finished waters of the Hanoi water types I, II and III attributed to unsteady chlorine doses, variations in ammonium, DOC and Fe(II) concentrations The groundwater composition is not constant because the 10–20 wells belonging to each Hanoi WTP are operated intermittently The THM formation in finished water was generally in the following order of water sources: type I>type II>type III Due to the relatively low DOC levels in water types I and II, the absolute levels of THMs are quite low Higher levels of THMs would be expected in water type III (high DOC) However, ammonium is dominating this system (see below) The distribution of the four THMs in finished waters is illustrated in Fig In the WTPs 1, and (water type I), brominated THMs (>85%) were the dominant and most abundant species In finished water from WTP and (water type II), chloroform was formed in similar concentrations as the brominated THMs In comparison to water type II, the higher ratio of brominated THMs in water type I can be explained by the 3–5 times higher bromide levels (see Table 1) The highest bromide levels were present in groundwater type III, yet only small amounts of brominated THMs were detected in the treated waters (chloroform 90% of the total THM) This is due to masking of chlorine by ammonia which prevents bromide oxidation (see below) Table Trihalomethane formation potential (THMFP) for treated Hanoi groundwaters 3.3 Influence of ammonium on THM formation and speciation The THMFP determined for the three waters type, namely WTP (type I), WTP (type II) and WTP (type III), were 103, 59 and 406 mg/L, respectively (see Table 2) The high THMFP found for water type III corresponds to its high content of NOM represented by the DOC values (see Tables and 2) However, as shown in Fig 2, the total THM concentrations in the finished WTP WTP WTP Type Ia Type IIa Type IIIa Treated waterb DOC mg/L 0.9 UV254 mÀ1 0.5 BrÀ mg/L 86 NH+ mg N/L 0.2 Laboratory conditions Chlorine dose mg/L pH buffered to THMFP mg/L as CHCl3 103 0.8 0.9 24 0.13 6.4 15.0 160 18.5 59 160 406 a The areas of differing groundwater quality are indicated in Fig b Treated waters were collected before chlorination water type III are 20–200 times lower than the THMFP This can be explained by the formation of THM along the breakpoint chlorination curve Fig shows that only traces of THMs are formed for chlorine doses in the far pre-peakpoint region, whereas a noticeable increase of THMs is observed near the peakpoint Between the peakpoint and the breakpoint, chloroform formation sharply increases with increasing chlorine dose A comparison of breakpoint chlorination curves is shown in Fig for five of the eight WTPs Peakpoint to breakpoint chlorination is applied in WTPs of types I and II (Fig 5a) whereas far pre-peakpoint chlorination is obvious for the WTPs type III (Fig 5b) Fig 5b shows that the high ammonium concentration (5–25 mg/L) present in the investigated waters of WTP 6–8 consume 40–130 mg/L chlorine For water type III it can therefore H.A Duong et al / Water Research 37 (2003) 3242–3252 15 0.3 0.2 CHCl3 0.1 CHBr3 0.0 (a) 0.5 0.4 10 CHCl2Br 1.0 2.0 3.0 chlorine dose (mg(L) 4.0 total THM as CHCl3 WTP CHCl3 0.3 25 20 15 0.2 CHCl2Br 0.1 CHClBr2 CHBr3 0.0 0.0 (b) 1.0 2.0 3.0 chlorine dose (mg(L) 10 4.0 HOCl ỵ NOM- products-THM k1 ẳ ? 1ị HOCl ỵ NH3 - NH2 Cl ỵ H2 O k2 ẳ 4:2 106 M1 s1 2ị (see Ref [14]), HOCl ỵ NH2 Cl- NHCl2 ỵ H2 O k3 ẳ 1:1 103 M1 s1 3ị (see Ref [15]), HOCl ỵ Br - HOBr ỵ Cl À1 s breakpoint WTP 0.6 breakpoint WTP WTP 0.13 mg/L NH4 0.4 0.2 0.5 ð4Þ 1.0 1.5 2.0 chlorine dose (mg/L) 70 applied chlorine dose ≤1.5 mg/L 60 type III 50 WTP 18 mg/L NH4 40 30 WTP 7 mg/L NH4 20 10 be inferred that the far pre-breakpoint chlorination leads to the formation of monochloramine, and consequently, low THM concentrations are present in the finished waters of WTP 6, and (see Table and Fig 2) In terms of disinfection, Fig shows that the chlorine dose (0.8–1.5 mg/L) applied is critical for WTP (only slightly above breakpoint) and insufficient for water type III The THM formation in the investigated waters is controlled by the following competition kinetics: k4 ¼ 1:55  10 M 0.8 (a) Fig Breakpoint curves and formation of THMs (contact time 24 h, pH 8.1) Laboratory experiments with treated waters collected before chlorination in the Hanoi WTPs Mai Dich (WTP 1, type I) and Yen Phu (WTP 4, type II) WTP 0.03 mg/L NH4 1.0 0.0 30 residual chlorine range of applied chlorine dose type I and II 1.2 residual chlorine (mg/L) CHClBr2 residual chlorine (mg/L) WTP 0.0 residual chlorine (mg/L) total THM as CHCl3 THM conc (µg/L) 0.4 1.4 20 residual chlorine THM conc (µg/L) residual chlorine (mg/L) 0.5 3247 WTP 6 mg/L NH4 (b) 50 100 150 200 chlorine dose (mg/L) Fig Breakpoint curves derived from contact times of 24 h at pH 8.0 for: (a) WTP and and (b) WTP 6–8 The range of the chlorine dose applied in the WTPs is indicated (see Ref [16]), HOBr ỵ NOM- products-THM k5 ẳ ? ð5Þ The kinetics of reactions (1) and (5) are not known in absolute terms, however, reaction (5) is much faster than reaction (1) Therefore, it is important to know to which extent bromide is oxidized to assess the potential for the formation of bromo-THMs Because ammonia is the À main sink for chlorine (HOCl/OCl ) in the pre-breakpoint region, the extent of bromide oxidation can be estimated from a competition kinetics calculation involving reactions (2)–(4) The fraction of bromide being oxidized during the phase where ammonia is in excess is in the range of a few percent However, near the peakpoint or from the peakpoint to the breakpoint, the fraction of bromide being oxidized becomes larger This is due to the slower kinetics of reaction (3) as compared to reaction (2) (B4 orders of magnitude) The fraction f(HOCl, BrÀ) of HOCl reacting with BrÀ can be calculated as follows: pre-peakpoint: f HOCl; Br ị ẳ k4 ẵBr k2 ẵNH3 ỵ k4 ẵBr 6ị H.A Duong et al / Water Research 37 (2003) 3242–3252 3248 peak- to breakpoint: f HOCl; Br ị ẳ k4 ẵBr : k3 ẵNH2 Cl ỵ k4 ẵBr 7ị If the residual concentration of HOCl is known ([HOCl]res) the absolute amount of oxidized bromide can be estimated as ẵHOBr ẳ ẵHOClres f ðHOCl; BrÀ Þ: ð8Þ Because reaction (3) is a relatively slow process, HOCl at the peakpoint can be determined as ẵHOClres ẳ ẵHOClo ẵNHỵ o ; 9ị [NH+ ]o where [HOCl]o is the applied chlorine dose and is the initial ammonium concentration Based on these considerations, estimates of the bromide oxidation to HOBr were made for water types I and III, and summarized in Table The calculations show that in the case of water type I up to 80% of the bromide can be oxidized to HOBr which then further reacts with NOM according to reaction (5) This explains the high fraction of bromo-THMs found in this water (Fig 3) Water type II has a similar water quality, however, the bromide levels are considerably lower which explains the lower formation of bromoTHMs (Fig 3) For the chlorine doses applied in water type III, bromide oxidation will be very minor (Table 3) Therefore, low levels of bromo-THMs are expected (Fig 3), even though the bromide levels in the raw water are very high (up to 240 mg/L) 3.4 Estimation of the disinfection efficiency via the THM formation Fig shows the relationship between THM formation (expressed as CHCl3) and chlorine exposure for various chlorine doses derived from laboratory experiments with water types I and II The chlorine exposure (CT) is calculated as the integral under a chlorine concentration time curve [17] The CT values can be used to estimate the inactivation efficiency of micro- organisms The data plotted in Figs 6a and b for water types I and II show that the CT requirements [18,19] for a 2-log inactivation of E coli, a 3-log inactivation of viruses and a 2-log inactivation of Giardia lamblia cysts can be achieved while the total THM concentration remains below 10 mg/L which is significantly lower than the typical drinking water standard To reach the same germ inactivation in waters of type III where active chlorine is mainly present as chloramines, CT values of 360 and 500 mg/L are required for virus and G lamblia cysts, respectively [18] Yet, for this purpose, significantly higher chlorine doses of 10–30 mg/L would be necessary for the WTPs 6, and However, such high chlorine doses might lead to substantially higher THM concentrations (see Table 2) and possibly result in the formation of NDMA (see below) 3.5 Chlorine residual concentrations in tap water samples of the distribution systems Results from a sampling campaign in three distribution systems are presented in Table Samples were taken at increasing distance from the WTP Using the distance from the WTP to the sampling point and the guideline limits for the flow rate in the distribution system provided in the Water Master Plan of Hanoi [20], the average residence times in the distribution system were estimated to be less than h for all tap water sampling points Active residual chlorine concentrations generally decreased with increasing distance from the WTPs The active chlorine residual in the distribution systems of WTPs and was free chlorine It was maintained at concentrations of 0.5–1.4 mg/L However, in the distribution system of WTP (groundwater type III), the disinfectant is chloramine and its concentration did not meet the Vietnamese requirement for residual disinfectant concentrations (X0.3 mg/L) The concentration of chloramine residual decreased quickly after a distance of 500 m from WTP The fast consumption of Table Estimated formation of HOBr during chlorination of pretreated raw water from groundwater resources in Hanoi Water type I Parameter À Br NH+ NH3 HOCl dose HOClres f(HOCl, BrÀ) HOBr Conversion of BrÀ Water type III Conc Peak- to breakpoint Conc Peak- to breakpoint p100 mg/L 0.2 mg/L p1.25  10À6 M 1.4  10À5 M  10À8 M 1.7  10À5 M 0.3  10À5 M 33%  10À6 M 80% 200 mg/L 20 mg/L 2.5  10À6 M 1.4  10À3 M  10À6 M 1.4  10À5 M 51.4  10À5 M 1.3  10À4 51  10À9 M 50.04% 1.2 mg/L mg/L 51 mg/L H.A Duong et al / Water Research 37 (2003) 3242–3252 THM concentrations decreased in the distribution system of WTP after 540 m distance There are several possible hypothesis for this observation: (i) leaking of pipelines and dilution, (ii) volatilization of THMs, and (iii) reductive degradation of THMs Hypothesis (i) is not very likely because the decrease of THMs and oxygen are different A massive volatilization of THMs would probably need a longer residence time and smaller pipeline diameters Therefore, it is most likely that a biotic or abiotic degradation of THMs occurs under low oxygen conditions Virus (CT = 1), E.coli (CT < 0.05) Giardia lamblia cysts (CT = 25) total THM as CHCl3 (µg/L) 12 type I 10 0 50 100 150 200 3.7 N-nitrosodimethylamine formation 200 Samples from WTP and its distribution system were also checked for a possible formation of NDMA which is a probable human carcinogen It has been shown that NDMA is formed during chloramination processes due to the reaction of monochloramine with dimethylamine [21] Yet, NDMA concentrations were below the detection limit of 0.02 mg/L in the finished water as well as in the five tap water samples analyzed For the currently applied chlorine dose (p1.5 mg/L) in type III WTPs, the NDMA formation potential can therefore be considered negligible Should chlorine doses be increased, the risk of NDMA formation has to be considered again chlorine exposure (CT), (mg/L.min) (a) total THM as CHCl3 (µg/L) 30 type II 25 20 15 10 0 (b) 25 50 100 150 3249 chlorine exposure (CT), (mg/L.min) Fig THM formation during chlorination of treated waters at 25 C and pH 7.5 for various chlorine doses (a) Water type I, chlorine dose 1.1 mg/L (diamonds), 1.9 mg/L (squares); (b) water type II, chlorine dose 0.5 mg/L (diamonds), 1.0 mg/L (squares), 1.9 mg/L (triangles) Required CT values for E coli, virus, and Giardia lamlia cysts are indicated chloramine near WTP may be due to the oxidation of remaining Fe(II) and Mn(II) or a biological reduction of NH2Cl 3.6 Trihalomethane occurrence in tap water As shown in Table the total THM concentration analyzed in the tap water samples were below the drinking water standards of the EU and the USEPA The speciation of THMs in the finished waters and in the tap waters was the same In the distribution systems of WTPs and having relatively high free chlorine residuals and dissolved oxygen of more than mg/L, the total THM concentrations increased with increasing distance from the plant No coliforms were present in the tap waters investigated Based on the estimates on chlorine CT (Fig 6), the measured THM concentrations indicate CT values which possibly guarantee a good inactivation, even for G lamblia cysts Interestingly, the Conclusions and recommendations This study shows that the THM concentrations in all Hanoi WTPs were below WHO, EU and USEPA drinking water standards For water type III with high DOC and bromide levels and a THMFP of >400 mg/L, higher total THM concentrations as well as a higher proportion of bromo-THMs were expected The low THM formation could be explained by competition kinetics of chlorine for the oxidation of ammonia and bromide, as well as by the short residence times (o1 h) in the Hanoi distribution system The following conclusions and recommendations can be drawn from the findings obtained through this study Based on estimations of chlorine exposure (CT), it can be hypothesized that for the inactivation of coliforms, virus, and Giardia lamblia cysts, the chlorine dose of 0.8–1.5 mg/L applied for disinfection by the Hanoi water works is inefficient for water type III, and critical for water types I and II if ammonium concentrations exceed 0.1 mg/L With regard to the residual chlorine of 0.3 mg/L required in the distribution system and tap water, it is recommended to maintain a steady chlorine dose of 1.5 mg/L for water types I and II if ammonium concentrations are below 0.15 mg/L a Tap water from distribution system Treated water Finished water Tap water from distribution system Treated water Finished water 11 18 28 35 40 540 900 1400 1750 2000 14 16 270 720 800 360 2 15 32 50 Estimated residence time (min) 90 90 180 360 750 1600 2500 Distance from WTP (m) Below detection limit (see Materials and Methods) WTP (type III) WTP (type II) Treated water Finished water WTP (type I) Tap water from distribution system Sample Water treatment plant 27.7 27.4 27.6 27.4 27.4 6.8 6.8 6.9 6.7 6.8 6.8 6.8 30.2 27.7 6.8 7.3 7.2 7.3 28.6 27.8 27.5 29.5 7.2 7.3 27.0 27.6 7.2 27.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 27.1 27.3 27.0 27.8 28.1 27.3 29.1 6.7 pH 27.1 T ( C) 1.5 0.9 0.9 0.7 0.4 1.8 2.3 3.9 4.3 3.3 4.0 4.3 2.1 2.7 2.3 2.0 2.0 2.1 2.5 DO (mg O2/L) 0.11 0.06 0.04 0.03 0.03 0.12 0.15 0.75 0.47 0.45 0.86 0.87 1.39 1.34 1.33 1.25 0.98 0.90 1.44 Total residual chlorine (mg/L) 2.9 2.1 1.8 0.7 0.5 1.6 0.8 19.2 23.4 25.4 18.3 17.1 4.9 6.3 5.4 6.4 6.0 6.4 3.0 CHCl3 (mg/L) a a a a a a a a a a a a a 6.2 8.0 7.9 6.1 5.2 18.4 20.7 22.7 26.6 30.0 25.5 16.6 CHClBr2 (mg/L) a 17.1 22.2 22.7 16.7 15.1 13.2 14.4 15.7 18.0 20.0 17.8 9.6 CHCl2Br (mg/L) Table Individual THM concentrations and water quality parameters in three Hanoi water distribution systems (September 2001) a a a a a a a 3.2 2.6 3.7 0.0 1.6 20.5 16.6 20.9 26.5 30.4 25.9 18.8 CHBr3 (mg/L) 2.9 2.1 1.8 0.7 0.5 1.6 0.8 36.7 45.4 48.2 34.0 31.8 34.6 36.5 39.7 47.3 52.1 45.2 28.4 Total THM as CHCl3 (mg/L) 0 0 0 0 0 0 0 0 0 0 Coliform (MPN/ 100 mL) 3250 H.A Duong et al / Water Research 37 (2003) 3242–3252 H.A Duong et al / Water Research 37 (2003) 3242–3252 Lower chlorine doses and/or ammonium concentration >0.15 mg/L result in residual chlorine concentrations o0.3 mg/L Due to the high ammonium levels of X15 mg/L in water type III, chlorine is mainly transformed to chloramine which is a considerably less efficient disinfectant than chlorine (B100 times) The chlorine doses of 10–30 mg/L necessary to reach a 2-log inactivation of Giardia lamblia cysts might cause substantially higher THM concentrations and possibly form NDMA A satisfactory quality of drinking water derived from water type III therefore requires multistage treatment for removal of Fe, Mn, NH4, and DOC In addition, a primary disinfection stage should be implemented Chlorine, THM and oxygen concentrations decreased in the distribution system of water type III (WTP 6) In this water, oxygen is possibly consumed by nitrification of ammonia in the reservoir and the distribution system THMs are then possibly degraded in the resulting anaerobic milieu Even though this process is desired, low oxygen conditions are unfavorable with regard to several water quality issues such as corrosion, taste and odor Acknowledgements This study was funded by the Swiss Agency for Development and Cooperation (SDC) in the framework of the Swiss–Vietnamese Cooperation Project ESTNV (Environmental Science and Technology in Northern Vietnam) We are indebted to the Hanoi Water Business Company for their cooperation and sampling assistance [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] References [1] Berg M, Tran HC, Nguyen TC, Pham HV, Schertenleib R, Giger W Arsenic contamination of groundwater and drinking water in Vietnam: a human health threat Environ Sci Technol 2001;35:2621–6 [2] Singer PC, Reckhow DA In: Letterman RD, editor Water quality and treatment, a handbook for community water supplies, 5th ed New York: 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Water Res 2001;35: 2095–9 USEPA Disinfection profiling and benchmarking guidance manual, Appendix C: CT values for inactivation achieved by various disinfectants, EPA-815-R-99-013, August 1999, Washington, DC, 1999 Langlais B, Reckhow DA, Brink DR, editors Ozone in water treatment: application and engineering, 2nd ed Chelsea, MI: Lewis Publishers, 1991 p 221 3252 H.A Duong et al / Water Research 37 (2003) 3242–3252 [20] Water Master Plan of Hanoi City for the period of 1993–2001 The Social Republic of Vietnam, Hanoi People’s Committee and The Republic of Finland, Finnish International Development Agency FINNIDA, Hanoi, Vietnam, vol 1, 1993 [21] Mitch WA, Sedlak DL Formation of N-nitrosodimethylamine (NDMA) from dimethylamine during chlorination Environ Sci Technol 2002;36:588–95  ... hydroxyatrazine, desethylatrazine, and deisopropylatrazine in natural waters Anal Chem 1995;67:1860–5 Gallard H, von Gunten U Chlorination of natural organic matter: kinetics of chlorination and of. .. occurrence of trihalomethanes in the drinking water in Greece Chemosphere 2000;41: 1761–7 Vietnamese Ministry of Health Standards for drinking water, Hanoi, Vietnam, 1988 Standard Methods In: Eaton... below the drinking water standards of the EU and the USEPA The speciation of THMs in the finished waters and in the tap waters was the same In the distribution systems of WTPs and having relatively