Speciation and distribution of trihalomethanes in the drinking water of Hong Kong

Environment International, Vol. 25, No. 5, pp. 605-611. 1999 Copyright 01999 Elsevier Science Ltd Printed in the USA. All rights reserved 0160-4120/99/$-see front matter

PI1SO160-4120(99)00033-l

SPECIATION AND DISTRIBUTION OF TRIHALOMETHANES IN THE DRINKING WATER OF HONG KONG Jimmy C. Yu* and Lai-nor Cheng Department of Chemistry and Centre for Environmental Studies, The Chinese University of Hong Kong, Shatin. New Territories, Hong Kong

EI 9807-10 M (Received ISduty 1998;accepted 7 March 1999)

Chlorine is commonly used as a disinfectant in the drinking water treatment process. However, chlorine may react with organic precursors such as humic substances in water to form trihalomethanes (THMs). Since THMs are known carcinogens, their concentrations in water should be monitored. The aim of this research was to measure the concentrations of THMs in drinking water in the 19 districts of Hong Kong. This study also reports, for the first time, trihalomethanesformation potential (THM-FP) of the drinking water in Hong Kong. While the THM-FP values measured are all below the WHO guideline values, the total concentrations of THMs in tap water samples from 4 out ofthe 19 districts exceed the U.S. and U.K. standards. The measurements show total THM levels of 104 ug/L, 106 pg/L, 115 pg/L, and 131 ug/L in Yuen Long, Tsuen Wan, North, and Tai PO, respectively. Such high levels of organic precursors could be an indication of the deteriorating state of the Dongjiang River in Guangdong, where over 70% of Hong Kong’s drinking water originates. 831999 Elsevier Science Ltd

INTRODUCTION In Hong Kong, almost all fresh water comes either directly from the Dongjiang River in Guangdong or from one of the reservoirs in the territory. The treatment works of the Water Supplies Department purify the water before it is provided for public use. The final step of water purification is disinfection, a procedure for protecting drinking water during distribution against external contamination and re-growth of bacteria (Minear and Amy 1996). Since the early 19OOs, chlorine, being easy and ready to use, especially in its most common form of hypochlorite, has been the major disinfectant introduced into drinking water for preventing waterborne diseases (Houston 19 13).

Bellar et al. (1974) reported that chloro and bromo trihalomethanes (THMs) were present in treated drinking water and implied that chlorination was the cause. Although chlorine can effectivey kill the bacteria, it also reacts with organic precursors (e.g., humic substances) existing in water and produces side-products such as THMs. The most common THMs found in tap water are chloroform, bromodichloromethane, chlorodibromomethane, and bromoform (Twort and Law 1994). In order to minimize THM formation, alternative disinfectants (e.g., ozone, chloramines, and chlorine dioxide) have been tested. Although many of them help to minimize the THMs, they also create potential new problems. Ozone cannot give a residual effect during

*E-mail: [email protected] 605

606

J.C. Yu and L.-N. Cheng

distribution and is of higher cost. Chloramines are not as effective as chlorine and have toxicological properties. Chlorine dioxide may generate inorganic pollutants such as chlorite and chlorate (Bryant et al. 1992; White 1992). Hence, chlorine is still widely used over the world as a water disinfectant. Scientific research on THMs in drinking water is mainly focused on: 1) the treatment of THM precursors prior to chlorination; and 2) the removal and destruction of THMs formed during chlorination. Many techniques have been developed for the treatment of THM precursors. These include physical separation methods such as adsorption (Chen 1997), coagulation (Crozes et al. 1995), and membrane filtration (Jacangelo et al. 1995). Advanced oxidation processes such as H,O, in the presence of UV light (Symons and Worley 1995), O3 or UV/O, (Ferguson et al. 1991; Backlund 1994), and TiO, photocatalysis (Hand et al. 1995) have also been used to treat THM precursors. For the removal of THMs from drinking water, packed-tower air stripping/aeration methods (Adams and Clark 1991; Roberts and Levy 1985) were found to be cost-effective. Several advanced THM destruction methods have also been proposed, including TiO, photocatalysis (Kormann et al. 1991), and high-energy electron-beam irradiation (Cooper et al. 1993). In this study, THM concentrations in tap water across Hong Kong were measured and compared with the World Health Organization (WHO) guideline values. A WHO guideline value is the concentration in drinking water associated with an excess lifetime cancer risk of 10” (one additional cancer per 100 000 of the population ingesting drinking water containing the substances at the guideline value for 70 y), assuming a daily per capita consumption of 2 L by a person weighing 60 kg. The respective guideline values for CHCl,, CHCl,Br, CHClBr,, and CI-IBr, are 200, 60, 100, and 100 ug/L (WHO 1993). In addition, the sum of ratio of concentration of each to its respective guideline value should not exceed 1. c

C CHCI zBr

CHCI,

GVCHCI

3

+

GvCHC,2Br

C CHCIBr,

+ G ‘CHCIBr,

c +

CHBr,

“CHB,

I

1

3

Twort and Law (1994) reported that many countries set a maximum total THM level of 100 l&L. Note that

total THM levels (TTHMs) = [CHCl,] + [CHCl,Br] + [CHClBr,] + [CHBr,]. The THM-formation potential (THM-FP) for each of the samples collected was also assessed in this study. In a THM-FP measurement, a sample is first dosed with enough chlorine to maintain a free chlorine residual throughout the test and then stored under selected conditions of temperature and pH for a selected time (Symons et al. 1996). The incubation time is usually 7 d to ensure complete reaction between chlorine and organic compounds in water (White 1992). Since there is a strong correlation between THM-FP and the concentration of organic carbon, THM-FP may be considered as an indirect measurement of organic precursors in water (Miller et al. 1990; Dojlido and Best 1993; Canale et al. 1997). This is a significant water quality parameter that should be evaluated annually (White 1992).

EXPERIMENTAL Sample collection

Triplicate samples were collected on 3 different dates from 57 locations representing the 19 districts of Hong Kong during the period 18 March 1997 to 26 May 1997. Before sample collection, the tap was opened and run for approximately 5 s, until a steady stream was achieved. Two bottles were collected each time, one for THM measurement and the other for THM-FP determination. Accordingly, each 68-mL sample bottle was fully tilled with water and tightly stoppered. It was postulated that, if headspace existed in sample bottles, some volatile THMs might escape from liquid phase to gas phase, leading to its loss. Sample pretreatment

and storage

For measurement of THM level in tap water, the sample bottle was pre-added with approximately 0.3 g Na,S,O, in order to remove the residual Cl, in the tap water during sample collection. The sample was stored in a refrigerator at 4°C (Greenberg et al. 1992). For measurement of THM-FP in tap water, the pH was maintained at 7:O by adding 0.5 mL of a phosphate buffer into each sample bottle. A NaOCl solution (100 g/L, 2.8 uL) was added to provide excess free chlorine with chlorine residue of l-5 mg/L. The sample was then maintained at 25°C in a water bath for 7 d (Aoki and Kawakami 1992).

607

THMs in Hong Kong drinking water

Reagents All chemicals used were of reagent grade or higher. A Milli-Q system provided the deionized water for this study. Procedure

Pentane extraction. A 20-mL water sample was pipetted into a 25mL conical flask fitted with a ground glass stopper. Pentane (2 mL) was then added. The flask was tightly stoppered and shaken for 10 min. The pentane extract was transferred with a dropper to a 1.5-mL vial with minimum headspace. Pentane extract (1 FL) was then injected into the capillary GC column (USEPA 1979).

Instrumentation. The models of gas chromatograph, detector, and GC injector used were HP 5890 series II, HP 5972 MSD, and HP 7673, respectively.

GCparameters. The injector port was a split/splitless liner, where the temperature was set at 160°C. The column used was Supelco PTE-5 (30 m x 0.25 mm x 0.25 pm; bonded; poly (5% diphenyl/95% dimethylsiloxane). The flow rate was adjusted to 1.5 mL/min with helium as the carrier gas. The split ratio was 10: 1.

GCoven temperatureprogram. The initial temperature was set at 32°C and held for 2 min. Then, the temperature was gradually increased to 37°C at a rate of S”C/min for 1 min. Finally, the temperature was increased at a rate of 30”C/min for 3 min to 127°C. The total running time for the temperature program was 6 min. MS (SIM parameters. For each of the THMs, two prominent ion fragments were chosen for detection. For CHCl, (retention time 2.40 min) and CHCl,Br (retention time 3.58 min), the selected ions were 83 and 85 m/z and the detection period was at the 2”d to 4* minute. For CHClBr, (retention time 4.72 min), the selected ions were 127 and 129 m/z and the detection period was at the 4* to 5* minute. For CHBr, (retention time 5.65 min), the selected ions were 171 and 173 m/z and the detection period was at the 5” to 6* minute. The dwell time for each ion in a group and the solvent delay time were set to be 100 msec and 2 min, respectively. The EM voltage was optimized by the autotune process. The MS transfer line temperature was set at 200°C.

Stockstandardsolution. Methanol (9.8 mL) was placed in a IO-mL ground-glass stoppered volumetric flask. It was weighed to the nearest 0.1 mg. Using a micropipet, appropriate amounts (-10 mg) of the four THMs pure standards were added and reweighed. Precaution was taken to ensure that pure standards fell directly into alcohol without contacting the flask neck. The solution was diluted to volume, stoppered, and then mixed by inverting the flask several times. The standard solution was transferred to 1.5-mL vials and stored with minimum headspace at 4°C. Concentration (in mg/mL) was calculated from the net gain in weight multiplied by the corresponding purity of pure standard.

Calibration standards. Solutions

containing 0.07 to 160 ug/L of THMs were prepared by spiking appropriate amounts of the diluted stock standard into 20 mL of deionized water and extracting with 2 mL of pentane. The extracted solvent was transferred to a 1.5-mL vial. Measurement was performed in an identical manner as the samples to compensate for possible extraction losses. RESULTS AND DISCUSSION

Figure 1 shows a typical GC chromatogram of THMs in a tap water sample. The detection limits for this method were estimated to be 0.03 ug/L for CHCl,, 0.05 ug/L for CHBrCl,, 0.04 ug/L for CHBr,Cl, and 0.03 ug/L for CHBr,. The results of THM measurements on 17 1 samples from 57 locations in Hong Kong are summarized in Table 1. Table 2 shows the THM levels, THM ratio, and total THMs in the 19 districts of Hong Kong. Figure 2 is a map of Hong Kong showing the distribution of THM levels. The results of THM-FP measurements in the 19 districts are summarized in Table 3. The THM and THM-FP levels in the Islands (including Lantau Island and Peng Chau) are significantly lower than the other districts. This reflects the low concentrations of organic precursors in the water of Lantau Island and Peng Chau. The Shek Pik Reservoir supplies the southern coast of Lantau Island (e.g., Mui Wo and Cheung Sha), Peng Chau, and Cheung Chau, where all raw water is collected from the island itself. The rest of Hong Kong gets most of its water from the Dongjiang (East River) in Guangdong. This river provides over 70% of Hong Kong’s annual water demand of 720 million cubic metres. Water extracted from the river is pumped over a series of dams and pipelines across the border at Muk Wu to Hong Kong. The water received at Muk Wu is delivered

608

J.C. Yu and L.-N. Cheng

Abrmclrncc

6500

CHCl3

I

6000 -: 5500: 5000: 4500 4000 -. 3500.. 3000

:

2500 2000

-.

1500

:

CHC&Br

n

1000-: 500 -: ‘~ime(min)__> 0 ~-‘-..‘-

J,\

+----9.. 2.50

8 I, 3.00

CHCIBrz CHBr,

I * .‘-

0’.

3.50

1.1”

4.00

4.50

5.00

5.50

Fig. 1. Typical GC chromatogram of THMS.

Table 1. THMs levels in 17 1 tap water samples from 57 locations in Hong Kong. THMs levels &g/L) Average

Min

Max

WHO Guideline values @g/L)

CHCll

47.08

2.25

104.68

CHBrCl,

11.04

1.90

21.42

200 60

CHBr,Cl

2.40

0.58

8.96

100

CHBr,

0.06

ND

2.56

100

Sum of THMs ratio

0.44

0.06

0.77


-

-

TIMh4S

60.58

along three aqueduct systems to the Tai Lam Chung Reservoir, Shatin Treatment Works, the Plover Cove Reservoir, and the High Island Reservoir or Pak Kong Treatment Works. While the THM-FP values measured are all below the WHO guideline values, the total concentrations of THMs in tap water samples from 4 out of the 19 districts are at or above the U.S. and U.K. standards. The four districts (Tsuen Wan, Yuen Long, North, and Tai PO) are all located in the New Territories, a region of Hong Kong just across the border from Guangdong. Such high levels of THM may be attributed to the higher mixing ratio of Dongjiang water with local reservoir water. The relatively high level of organic precursors could also be an indication of the deteriorating state of the Dongjiang River. It is known that the

water quality of the Dongjiang has deteriorated significantly over the years. The pollution is mainly caused by the massive increase in discharge of untreated wastewater from nearby farms (fertilisers, pesticides), factories (chemicals, heavy metals), and inhabitants (domestic sewage). To avoid further deterioration and to improve the present situation, the Guangdong Authority has recently decided to build a biological nitrification plant to treat the micro-organic pollutants. Moreover, industrial sewage must be treated to meet the required standard before discharge. In addition, the Hong Kong Government and the Guangdong Authority are considering the feasibility of constructing a new closed water supply aqueduct to keep pollution away. These measures would ensure the supply of high quality drinking water for the residents of Hong Kong.

609

THMs in Hong Kong drinking water

Table 2. Summaty of THMs levels in tap water of the 19 districts in Hong Kong. THMs levels (pg/L) District

CHCl,

CHC1,Br

CHClBr,

CI-IBr,

THMs ratio

TTHMs

1. Central & Western

43.5

11.3

2.36

ND

0.43

57.2

2. Wan Chai

33.7

12.1

3.53

ND

0.41

49.3

3. Eastern

37.9

13.6

4.15

ND

0.46

55.6

4. Southern

39.4

10.7

2.79

0.05

0.40

52.9

5. Yau Tsim

50.9

11.4

2.16

ND

0.47

64.4

6. Mong Kok

51.2

13.3

2.46

ND

0.50

66.9

7. Sham Shui PO

46.5

10.1

1.62

ND

0.42

58.2

8. Kowloon City

50.9

12.1

2.11

ND

0.48

65.0

9. Wong Tai Sin

51.2

12.7

2.25

ND

0.49

66.1

12.5

3.45

0.05

0.44

56.4

1.52

ND

0.40

57.3 71.9

10. Kwun Tong

40.4

11. Kwai Tsing

46.5

12. Tsuen Wan

60.6

9.98

1.31

ND

0.48

13. Tuen Mun

53.6

9.38

1.37

ND

0.44

64.4

14. Yuen Long

60.8

9.73

1.22

ND

0.48

71.8

8.33

66.0

9.25

15. North

56.8

0.83

ND

0.43

16. Tai PO

75.1

10.9

1.24

ND

0.57

87.2

17. Shatin

48.8

11.1

1.93

ND

0.45

61.8

18. Sai Kung

42.8

17.2

5.56

0.04

0.56

65.6

4.15

0.92

0.16

15.8

19. Islands

5.71

5.04

Fig. 2. Distribution of TI-IMs in tap water of the 19 districts of Hong Kong.

610

J.C. Yu and L.-N. Cheng

Table 3. Summary of THM-FP levels in tap water of the 19 districts in Hong Kong. THM-FP (ug/L) District

CHCl,

CHCI,Br

CHClBr,

CHBr,

THMs ratio

TTHMs

67.1

14.3

2.69

ND

0.60

84.1

2. Wan Chai

55.0

14.8

3.77

0.08

0.56

73.6

3. Eastern

62.3

17.2

4.55

0.12

0.64

84.2

4. Southern

55.0

14.0

3.24

0.07

0.54

72.3

5. Yau Tsim

77.5

14.7

2.65

ND

0.66

94.9

6. Mong Kok

79.1

15.2

2.47

ND

0.67

96.8

7. Sham Shui PO

66.4

12.1

1.88

ND

0.55

80.4

8. Kowloon City

78.2

15.2

2.37

ND

0.67

95.8

9. Wong Tai Sin

73.6

15.4

2.58

ND

0.65

91.6

10. Kwun Tong

56.5

15.4

4.02

0.09

0.58

76.0

11. Kwai Tsing

81.4

11.9

1.76

ND

0.62

95.0

12. Tsuen Wan

92.0

12.4

1.60

ND

0.68

13. Tuen Mun

72.6

10.2

1.44

ND

0.55

14. Yuen Long

90.3

11.9

1.44

ND

0.66

104

15. North

101.6

12.6

1.20

ND

0.73

115

16. Tai PO

115.7

13.9

1.55

ND

0.82

131

17. Shatin

75.1

14.3

2.42

ND

0.64

91.8

18. Sai Kung

60.9

18.9

5.74

0.06

0.68

85.6

19. Islands

13.2

7.2

5.36

1.29

0.25

27.0

1. Central & Western

CONCLUSIONS On the basis of the results of the study, it may be concluded that the tap water in all districts of Hong Kong is safe for drinking in terms of THM levels. The water on the Islands is of the best quality, while the tap water in the New Territories could be a cause for concern judging from the relatively high THM-FP levels.

Acknowledgmen+-This research is supported by the Lee Hysan Foundation Research Grant Scheme of the United College, the Chinese University of Hong Kong. The authors would like to thank T.L. Cheung at the Water Supplies Department for technical assistance.

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THMs in Hong Kong drinking water

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