Disinfection by-products and their precursors in a water treatment plant in North China: Seasonal changes and fraction analysis

SC IE N CE OF T H E TOT AL E N V I RO N ME N T 3 9 7 ( 2 00 8 ) 1 4 0–1 47

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

Disinfection by-products and their precursors in a water treatment plant in North China: Seasonal changes and fraction analysis Chao Chena,⁎, Xiao-jian Zhanga , Ling-xia Zhua , Jing Liua , Wen-jie Heb , Hong-da Hanb a

Department of Environmental Science and Engineering, Tsinghua University, Beijing, 100084, China Tianjin Waterworks Group Co. Ltd, Tianjin, 300040, China

b

AR TIC LE I N FO

ABS TR ACT

Article history:

A one-year-long monitoring project was conducted to assay the concentrations of THMs,

Received 5 October 2007

HAAs and their formation potential along the conventional process in a water treatment

Received in revised form

plant in North China. Subsequent investigations of organic matter fractionation and the

15 February 2008

contribution of the algae to the precursor were also conducted to trace the source of the

Accepted 19 February 2008

DBPs. The results showed that the concentration of DBPs and their formation potential

Available online 8 April 2008

varied with the seasons. The highest concentrations of THMs and the highest HAAs formation potential, each almost 500 µg/L, were detected in autumn and the lowest were in

Keywords:

spring, no more than 100 µg/L. Both organic matter and algae were found to be important

Disinfection by-product

DBP precursors. The hydrophobic acid fraction in dissolved organic matter has the highest

Precursor

formation potential for both THM and HAA. Algae contribute about 20% to 50% of the total

Organic fraction

formation potential during an algal bloom. The efficiency of each unit process for DBPs and

Algae

precursors was also assayed. Unfortunately, the conventional drinking water treatment

Trihalomethane

process is limited in its efficiency for precursor removal. The pre-chlorination and filtration

Haloacetic acid

process had a negative effect on DBP or precursor removal.

Conventional treatment process

1.

Introduction

Since trihalomethanes (THMs) were discovered in chlorinated water in the 1970s by Rook (1974), disinfection by-products (DBPs) have become a focus of attention in water treatment. More than 700 species of DBPs have been confirmed, among which trihalomethanes and haloacetic acids (HAAs) were the two DBP groups found in the highest concentrations and most commonly detected in chlorinated water all over the world. The main DBP precursor was generally considered to be natural organic matter (NOM), which was defined as the complex matrix of naturally occurring organic materials present in natural waters. The NOM can significantly affect many aspects of water treatment, including the performance of the unit processes (i.e.,

© 2008 Elsevier B.V. All rights reserved.

oxidation, coagulation and adsorption), application of disinfectants, and biological stability. As a result, NOM affects potable water quality by contributing to disinfection by-products, biological regrowth in the distribution system, color, taste, and odor (Owen et al., 1995; Singer, 1999). The concentrations of DBPs and their precursors in water treatment processes are basic information for their hazard analysis, regulation and process reconstruction. DBP surveys have been conducted in many countries since the 1980s, first in the U.S.A. and then in Europe, Australia and East Asia. A survey by Krasner et al. (1989) of 35 water treatment plants in America found that the mean THM value was 34 μg/L in spring, 44 μg/L in summer, 40 μg/L in autumn and 30 μg/L in winter. A survey by Arora et al. (1997) of THM and HAA in 100 water

⁎ Corresponding author. Tel.: +86 10 62781779; fax: +86 10 62785687. E-mail address: [email protected] (C. Chen) 0048-9697/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2008.02.032

141

S CIE N CE OF T H E TOT AL E N V I RO N ME N T 3 9 7 ( 2 00 8 ) 1 4 0–1 47

treatment plants in the U.S.A. found that 20% and 60% of water treatment plants could not meet the requirements for THM in DBP rule I and DBP rule II respectively. 16% and 52% of water treatment plants exceeded the requirement for HAA in DBP rule I and DBP rule II respectively. The surveys conducted by Williams et al. (1997) in Canada and Simpson and Hayes (1998) in Australia also found much higher DBP concentrations in certain plants. The higher chlorine dosage and pre-chlorination were responsible for the higher DBP concentrations. The situations in Europe and East Asia seem less serious. A survey of 35 water treatment plants in Finland by Nissinen et al. (2002) and a survey in four provinces in Spain by Villanueva et al. (2003) found fairly low DBP formation, which may be attributed to the usage of alternative disinfectants, such as chloramines or ozone. The reason for the modest DBP concentrations in East Asia must be the fairly low precursor concentrations. Yoon et al. (2003) found that the precursor concentrations in South Korea were only 1/3 of those in America. Duong et al. (2003) found that the maximum THM concentration was about 50 μg/L in Hanoi, Viet Nam. There have been some studies on DBPs and their precursors in China. Zhou et al. (2004) conducted a HAA survey in several cities in China. They reported that the concentrations of HAAs were usually not more than 25 μg/L in drinking water. In a previous assay of HAA yield in a water treatment plant in Beijing, Li et al. (2001) reported that the highest HAA concentration detected was about 30 μg/L, owing to prechlorination in Autumn. Since China covers a vast territory with different climatic and geographical conditions, the surface water quality varies greatly in different areas. Generally, the organic matter concentration in lakes or reservoirs is much higher than in rivers, according to monitoring data. The organic matter concentration in North China is higher than in South China. The TOC concentration of the source water varies from 3 mg/L to 9 mg/L in different cities around China (Chen et al., 2007), which means that the precursor concentration was quite different in different surface water source in this country and the DBP concentration should also be different. Compared with the former reports in China, the DBP concentration detected in one plant in Tianjin city was much higher. Therefore, it was considered to be a good site for investigating the characteristics of HAA and THM formation. A monthly assay lasting for one year during 2003 was

conducted to investigate the DBPs and precursors in a water treatment plant in Tianjin.

2.

Materials and methods

2.1.

Water treatment plant and source water

Tianjin is the 3rd largest metropolis in China. It is located in North China, 150 km to the east of Beijing. For many years, Tianjin city has had a water shortage. The source water of Tianjin's water treatment plants is transferred for more than 100 km through a drain, the Luan River or the Yellow River. The source water quality of the tested water treatment plant is shown in Table 1. The investigated water treatment plant has a capacity of 300,000 m3/d. The water purification process is a conventional coagulation–sedimentation–filtration–disinfection process. 17 to 25 mg/L of coagulant, FeCl3 and 4–5 mg/L of free chlorine were added together in the mixing well. The pH adjustment for enhanced coagulation was not applied because the alkalinity was as high as 180 mg/L. The addition point for pre-chlorination was shifted to the source water pump station to inactivate algae during the algal bloom in autumn. The following horizontal flow tank has a hydraulic retention time (HRT) of about 30 min. The filtration tank contains dual media of charcoal and sand. The upper charcoal medium has a particle size of 1.0–1.2 mm and a depth of 300 mm. The lower sand medium has a particle size of 0.8–1.2 mm and a depth of 400 mm. Free chlorine and ammonia were added to the connecting pipe between the filtration tank and the clear well. The dosage of post-chloramination was adjusted to maintain a residual chlorine level of 1.0–1.5 mg/L in the effluent of the clear well, which would guarantee the residual chlorine level required by the national water standard (over 0.05 mg/L) at the end of the large distribution system.

2.2.

Monitoring procedure

A total of ten assays were conducted on about the 25th of each month in 2003, except in May and June when sampling had to be cancelled because of the severe SARS epidemic at that time in North China.

Table 1 – Source water quality of the B water plant during one year Parameters

Spring (Mar–May) Mar

pH T (degree centigrade) TOC (mg/l) CODMn (mg/l) NH3–N (mg/l) Chlorophyll (μg/l) Algae (million/l)

Apr

May

Summer (Jul–Aug) Jun

Jul

Aug

Autumn (Sep–Nov) Sep

Oct

Nov

Winter (Dec–Feb) Dec

Jan

Feb

8.22–8.46 8.42–8.51 8.35–8.56 7.66–7.88 7.75–7.84 7.68–7.94 7.74–7.91 8.04–8.31 8.19–8.51 8.32–8.46 8.17–8.33 8.10–8.21 6.6–9.0 7.8–14.2 13.2–17.0 15.8–19.9 19.5–27.2 25.1–29.2 14.5–25.3 8.5–13.3 4.5–9.3 3.3–7.4 0.4–2.2 0.7–1.4 2.2–2.8 2.4–3.2 2.8–3.4 3.2–3.8 3.7–4.2 4.1–4.6 4.9–5.6 4.1–4.6 3.5–4.3 1.9–2.1 2.0–2.1 2.0–2.2 2.7–3.3 3.2–4.0 3.7–4.2 2.8–3.8 3.3–4.2 3.7–4.8 4.5–5.0 4.3–4.6 4.0–4.3 3.1–3.2 3.1–3.4 3.1–3.3 0.15–0.19 0.18–0.22 0.19–0.24 0.14–0.18 0.16–0.20 0.18–0.21 0.17–0.21 0.13–0.18 0.11–0.14 0.11–0.12 0.11–0.13 0.12–0.13 0 2.7–4.7 3.9–6.3 5.8–15.3 14.6–19.3 20.5–27.3 5.82–18.5 4.86–6.02 3.88–6.23 0–5.59 0 0 0

3.3–6.5

7.4–12.1 14.8–18.8 17.5–19.8 20.8–25.8 19.9–34.2 17.7–21.6 15.9–18.2

0–4.1

0

0

142

SC IE N CE OF T H E TOT AL E N V I RO N ME N T 3 9 7 ( 2 00 8 ) 1 4 0–1 47

Fig. 1 – Flowchart of water treatment process and sampling sites.

In each assay, five samples were collected at the influent point of the water treatment plant, the mixing well, the outlet of the settling tank and at the effluents of the filtration tank and the water treatment plant, as shown in Fig. 1. 250 ml of each water sample was used to completely fill an ambercolored glass bottle with a TFE-lined screw cap for DBP analysis. 5 ml of a 0.05 mol/L sodium hyposulfite solution was previously placed in the bottle to quench the residual chlorine in the water. Another 250 ml of each water sample was placed in another amber-colored glass bottle for DBP formation potential (DBPFP) analysis. A phosphate buffer solution and excess sodium hypochlorite solution were added in advance to allow the complete formation of DBPs. The water samples were transported to our laboratory in Beijing within 4 h in portable ice boxes.

2.3.

Analytical methods

THMs were measured by headspace-gas chromatography (Ministry of Health, China, 2001; Wylie, 1988). The five HAAs proposed for regulation by China and the U.S.A. were assayed by derivation-gas chromatography. We could not determine the other four HAAs until 2004 because of the lack of standards. The sample water was pre-treated by microextraction at a pH of 0.5 with methyl tertiary butyl ether and methylation with acidic methanol (Xie et al., 2002). A Shimadzu GC-17A gas chromatographer with an electron capture detector and an HP-5 capillary column was used. The assays for indicating DBP precursors include the formation potential, UV absorbance, the SDS (Simulated Distribution System) test and the UFC (Uniform Formation Condition) test. The first two assays were chosen to describe the total DBPs formed in the distribution system because part of the distribution system in the Tianjin water treatment plant has a HRT of as long as 3 days. The THM formation potential (THMFP) and the HAA formation potential (HAAFP) were determined according to methods 5710 B and D (APHA, 1995). Under standard conditions, the samples were buffered at a pH of 7.0, chlorinated with an excess of free chlorine, and stored at 25 °C for 7 d to allow the reaction to approach completion.

2.4.

When isolating the DOC, 60 L of raw water was first filtered with a 0.45 μm membrane to remove the suspended solids and particles. The concentration of the initial DOC was determined by a Shimadzu TOC-5000 analyzer. Then, the raw water was pumped through the filtration columns sequentially. Finally, different fractions of organic matter absorbed on different kinds of resin were eluted by sodium hydroxide or hydrochloric acid or methanol separately. The analytical procedure is shown in Fig. 2. The concentration of each organic fraction was determined as the DOC and their contributions to the DBP precursors were determined as the THMFP and HAAFP.

2.5.

Plankton contribution to the precursors

Plankton was suspected to contribute much of the precursors after one year of assays. In the next high-algae-laden period in autumn 2004, experiments were conducted to verify its contribution. The plankton concentration and the species in raw water were monitored by microscopy firstly. The slides were viewed under an Axioskop 2 microscope (Carl Zeiss, Jena, Germany). Direct counts were carried out at magnifications of ×100–×400, according to the sizes of the species, and expressed as the number of individuals per ml of mixed liquor. The

Isolation and fractionation of DOC

The analytical procedure for preparative dissolved organic carbon (DOC) fractionation followed that developed by Leeheer (1981). The Amberlite XAD-8 resin, Bio-Rad AG-MP-50 cationexchange resin and Duolite A-7 anion-exchange resin were purified by Soxhlet extraction before isolation. After cleanup, each kind of resin was packed separately into glass columns for further isolation.

Fig. 2 – Analytical procedure for preparative dissolved organic carbon fractionation(solid arrow: filtration, broken arrow: elution).

S CIE N CE OF T H E TOT AL E N V I RO N ME N T 3 9 7 ( 2 00 8 ) 1 4 0–1 47

dominant plankton species were blue and green cyanobacteria and diatoms. 5 L of raw water was filtrated with a 0.45 µm membrane and the plankton was retained on the membranes. About 40 pieces of membrane were used for filtration and all were rinsed with pure water to release the attached algae. All the rinse water was collected and diluted to 1 L. The organic matter in this algal solution was composed mainly of algal bodies and secretions and free of other dissolved organic matter (DOM), a well-known DBP precursor. The DBPFP of this solution was assayed according to methods 5710 B and D (APHA). The DBPFP of four THMs and nine HAAs was assayed in this experiment. The algal concentration in raw water was measured as about 11 million/L and the concentration in the rinse water was condensed to about 55 million/L for DBPFP determination. The algal contribution to the precursor was quantified by the THMFP and HAAFP as per million algae rather than per mg/L DOC because the DOC concentration could not be determined since the algae were larger than 0.45 µm.

3.

Results and discussion

3.1. Changes in DBP concentration along the water treatment process in different seasons The THM and HAA concentrations varied in the different seasons during the year, as shown in Fig. 3. To make the discussion brief, the authors chose March as the representative month of spring, July as representative of summer, September as representative of autumn and December as representative of winter. These months had characteristics typical of the different seasons. For example, the highest algal concentration occurred in September, which is representative of the algal blooms in autumn. In contrast, the water quality in March is almost at its best, as there is still very limited planktonic production with the cool spring weather in North China. The concentrations were highest in autumn, almost 50 µg/l, and also very high in summer but lowest in spring, less than 10 µg/L. The high DBP concentrations in autumn and summer could be attributed to a higher organic matter concentration, a higher chlorine dosage and the higher temperatures. The reason for the higher DBP concentrations in winter compared

143

with spring could be the biomass, which was produced in summer and autumn, was not degraded completely in winter. Concerning the DBP dynamics along the process, THM increased almost continuously while HAA increased before filtration and decreased appreciably in the filtration tank. Prechlorination contributed a large amount of the DBP yield due to the higher concentration of organic matter and the higher dosage of free chlorine. In autumn, the application point was moved forward in the pump station in the reservoir to inactivate the algae, which contributed 45% of the total THM (TTHM) and 60% of the total five HAAs (THAA5). The application point of pre-chlorination in the other seasons was during coagulation, yielding 40%–70% of TTHM and 40%–60% of THAA5. This showed that DBP formation was much faster in the first 5 min in the mixing well. Sedimentation had no effect on DBP removal. Both THM and HAA concentration decreased slightly after filtration. The decrease in THM may be explained by volatilization during water falling as proved by subsequent experiments in our laboratory. The decrease in HAA could be explained by microbiological biodegradation in the filtration tank. (Xie et al., 2002). There was a moderate increase of THM and a moderate decrease of HAA in the effluent because post-chloramination did not result in a great increase in DBP formation (Carlson and Hardy, 1998). The DBP concentrations detected at the end of distribution system, which are not shown in this paper, did not increase significantly as compared to the in-plant effluent, since chloramination resulted in only moderate THM and HAA formation (Diehl et al., 2000). The differences in the dynamics of these two DBPs along the process can be ascribed to their physical and chemical properties. THM is easily volatilized and HAA is easily biodegraded (Xie and Zhou, 2002). Thus, different measures can be designed for the removal of these two DBPs. The concentration of TTHM was almost equal to that of THAA5. Also, the concentration of THAA5 was almost the same as that of THAA9 because there was a moderate bromide concentration in the source water and limited fractions of brominated trihaloacetic acids in all HAAs. Roberts et al. (2002) reported that THAA9 concentrations were appreciably higher than THAA6 concentrations and demonstrated a central tendency for THAA9 concentrations in treated drinking waters to be approximately equal to TTHM concentrations with the first 12 months of ICR (Information Collection Rule) data in

Fig. 3 – Changes in DBP concentration along the water treatment process in each season.

144

SC IE N CE OF T H E TOT AL E N V I RO N ME N T 3 9 7 ( 2 00 8 ) 1 4 0–1 47

Fig. 4 – Change in DBPFP concentration along water treatment process in each season.

the U.S.A. Our investigation was in accordance with their research. The lower yields of THM and HAA, as compared with the much higher precursor concentration in Fig. 4, suggests that those precursors might react with chlorine slowly to produce DBPs.

3.2. DBP precursor dynamics along the process in different seasons The concentrations of the precursors THM and HAA also varied in different seasons, as shown in Fig. 4. The concentrations during the year were highest in autumn and lowest in spring. This phenomenon seemed be closely linked with different organic matter concentrations, the dynamics of plankton production and the varying temperatures in the different seasons. The precursor concentrations in winter were still high. The reason could be the biomass, which was produced in summer and autumn, was not degraded completely in winter. The precursor concentrations fluctuated along the treatment process. They decreased greatly after coagulation and sedimentation, increased after filtration and decreased again after post-chloramination. Pre-chlorination with a high chlorine dosage oxidized the organic matter and resulted in a notable decrease of precursors during coagulation. Sedimentation resulted in moderate DBP precursor removal during most of the year except in autumn, when it remarkably removed 70 µg/L of the THM precursor and 150 µg/L of the HAA precursor. This suggested that the DBP precursor partly originated from particulate matters such as algae. Filtration resulted in a distinct increase of the THM and HAA precursors, due to the accumulation of organic matter and algae in the filter and these materials were probably strong precursors of DBP (Chen et al., 2007). The chloramine added to the filtration effluent could react with the THM and HAA precursors at a moderate rate in the clear well and distribution system (Carlson and Hardy, 1998). Although there was still a high concentration of precursors, the DBP concentrations in the distribution system met the drinking water standard, owing to the moderate DBP yield of monochloramine. Few measurements over the standard of 80 µg/L of THM and 60 µg/L of THAA5 were detected, mostly in summer and autumn.

The precursor accumulation in the filtration tank should be regarded seriously. The increase of organic concentration in the filtration effluent has also been found in other water treatment plants in China recently (Chen et al., 2007). Particles, organic material and algae were retained or adsorbed among the filter media and accumulated more and more during the period of filtration. The organic matter would be desorbed from the media into the effluent and algae would release secretions into the effluent, which greatly increases the precursors in the filtration effluent. This phenomenon is a reminder that organic accumulation should be regarded as important an operational parameter as turbidity and water head loss in the filtration process. It is interesting that the THMFP often increased after filtration, while the HAAFP hardly increased, which implies that the organic matter accumulated in filtration was more like the THM precursor than the HAA precursor, or that the HAA was more biodegradable. This suggestion could be partly confirmed by the report of Graham et al. (1998) that the typical blooms in a reservoir could produce a substantial fraction of the THM precursor. Although the relationship between algae or filtration accumulation with the HAA precursor was not

Fig. 5 – Change in DBPFP concentration in source water in each month (Note: A total of ten assays were conducted in each month in 2003, except in May and June when sampling had to be cancelled because of the severe SARS epidemic at that time in North China.).

S CIE N CE OF T H E TOT AL E N V I RO N ME N T 3 9 7 ( 2 00 8 ) 1 4 0–1 47

Table 2 – DBP contribution of different fractions of dissolved organic matter Fraction

TOTAL HPOA HPOB HPON HPIA HPIB HPIN

RATE (%) THMFP (µg/mg C) THMs contribution (%) HAAFP (µg/mg C) HAA contribution (%)

100 38.66

35.6 66.6

3.6 4.35

12.3 34.97

26.5 34.35

4.1 17.9 9.16 5.71

100

61.4

0.4

11.1

23.5

1

118.23

20.26

47.42

45.81 40.42 42.67

60.1

1.0

8.3

70.37 100

17.3

2.4

2.6

10.9

Note: This experiment was conducted in November 2003. Basic raw water quality: TOC = 3.78 mg/l, UV254 = 8.1/m, SUVA = 2.14 l/mg m.

reported as far as we know, this phenomenon attracted us to investigate the contribution of the algae to the precursor.

3.3.

DBP precursor concentrations in the source water

The DBPFP concentration in raw water in each month in 2003 is shown in Fig. 5. It seems that DBPFP has a positive relationship with the CODMn and temperature, but they do not have a good linear relationship since there could be lags between the temperature peak and the algal count peak, as well as between the algal peak and the TTHMFP and THAA5FP peaks. Some researchers used a multi-regression equation to predict the precursor concentration with many indexes such as TOC, UV254, temperature and bromide. However, such approaches need much longer monitoring and much more data, which was beyond the scope of the study. The DBPFP concentration probably has a good relationship with the plankton productivity and their accumulation in water. The concentration of precursors increased rapidly with increasing temperatures and decreased much slower when temperatures dropped. The DBPFP concentration was higher in early winter than in spring at the same temperature. Its change profile was similar to that of the algal concentration in water during a year. The biomass of algae also reached its maximum in autumn and decreased slowly.

145

3.4. Fractions of organic matter and their contribution to precursors Organic matter in water is very complex. Isolation and fractionation was fairly simple and effective methods are well accepted to investigate their characteristics, although the resin absorption and desorption could change the properties of organic matter to some degree (Hua and Reckhow, 2007). The dissolved organic matter in the source water was isolated and fractionated into 6 fractions in November 2003 and the THMFP and HAAFP of each fraction was determined, as shown in Table 2. Generally, this source water has a relatively low SUVA value and DBPFP concentration compared with the Colorado River. The SUVA value shows the degree of humus formation and has a close relationship with the DBP precursors (Owen et al., 1995). This difference could partly explain why the actual DBP formation in the water treatment plant in Tianjin was not as high as in Colorado. The concentration of DBPFP in this filtered water was lower than the data in Fig. 5, and the difference could be due to the contribution of the algae or particulate organic matter in that water sample. The HPOAs (Hydrophobic Acids), including humic acid and fulvic acid, were the strongest precursor with a THMFP contribution of 66.6 µg/mg C and a HAAFP contribution of 118.23 µg/mg C. This finding was consistent with previous studies (Krasner et al., 1996; Goslan et al., 2002). It was also the largest fraction (35.6%) in all the dissolved organic matter. Therefore, HPOAs contributed more than 60% of each of the THM and HAA precursors. HPIA and HPON were also important precursors, with DBPFP concentrations of about 35 µg/mg C and 45 µg/mg C, respectively. Other fractions contributed moderately to DBP formation. The characteristics of the organic matter varied with different water sources. Wang (2001) found 62% of HPOAs and just 24% of total hydrophilic matter in the organic matter in the source water in Chengdu city, Southwest China. Compared with these results, there was a larger amount of hydrophilic organic matter (48.5%) in the Tianjin water. Even in the same fraction, the formation potential also varied with the different water sources. In Chengdu, the HPON had the highest HAAFP of 51.95 µg/mg C. HPOAs had a much lower HAAFP of 25.25 µg/mg C but the largest contribution to the HAA concentration, owing to the highest fraction rate.

Fig. 6 – The contribution of algae to the DBPFP (The HAAFP concentration, left and THMFP concentration, right, are in units of μg/L. The algae concentration for the DBPFP test is 55 million/L).

146

SC IE N CE OF T H E TOT AL E N V I RO N ME N T 3 9 7 ( 2 00 8 ) 1 4 0–1 47

To simplify the procedure of fractionation, we divided the dissolved organic matter into just two fractions, the hydrophobic and the hydrophilic, by one step of isolation with XAD-8 resin. According to Table 2, the hydrophobic fraction contributed 51.5% of the DOC but about 70% of both DBPFPs. The hydrophobic NOM fractions also exert the greater dominance on coagulation control (Sharp et al., 2006). From this result, we can conclude that the hydrophobic matter should first be removed before it can react with chlorine.

further increases in the concentration of DBPs in the plant effluent. Generally, the conventional drinking water treatment process employed by the water treatment plant showed a limited efficiency of precursor removal. Also, the pre-chlorination and filtration process had a negative effect on DBP or precursor removal.

Acknowledgements 3.5.

Algal contribution to the precursors

The contribution of algae to the DBP precursors was determined by THMFP and HAAFP assays for the isolated algal solution, as shown in Fig. 6. The algal concentration in the isolated solution was 55 million/L and the concentrations of THMFP and HAAFP were 91.93 µg/L and 217.84 µg/L, especially. Dividing the DBPFP concentration by the algal concentration, the contribution of the alga was quantified as 3.96 µg THAA9/million algae and 1.67 µg TTHM/million algae. During algal blooms in summer or autumn, when the algal concentration would be 20 to 80 million per liter, the DBP precursors originating from algae would be dozens to hundreds of µg/L, which would account for about 20% to 50% of the total formation potential, according to Figs. 4 and 6. Moreover, the algae contributed much more to the HAAFP than the THMFP. This phenomenon leaves us with a question for further investigation, to find which component in algae favors HAA formation rather than THM. It was noted in the previous paragraph that a higher concentration of THMFP than HAAFP was measured in the filtration effluent, as shown in Fig. 4. The explanations of this seeming contradiction could include that most of the algae was removed before filtration. The bromine-containing DBPs were not dominant in the total DBPs in this determination since the initial bromide concentration was limited in the source water.

4.

Conclusion

It was found in this study that both dissolved organic matter and algae were important DBP precursors. The hydrophobic acid fraction in the dissolved organic matter has the highest formation potentials for both THM and HAA. High concentrations of algae also resulted in large proportions of the DBP precursors and their actual formation in summer and autumn. The DBP formation potential varied with changes in seasons or temperature. The highest DBPFP concentration was determined to be as high as almost 500 µg/L for both THM and HAA in autumn, which could be attributed to the higher concentrations of dissolved organic matter and algae at that time. The concentration of DBPs also varied with the seasons. The highest DBPs yield in the effluent of the water treatment plant was determined as less than 50 µg/L, which was only 10% of the total formation potential. The lower yield of THM and HAA suggests that these precursors would react with chlorine slowly to produce DBPs. Also, the transformation of free chlorine to chloramines before the clear well helped to limit

This research was one part of the project (2002AA601140) funded by the Ministry of Science and Technology and the project (50238020) funded by the National Science Foundation Committee, PR China. The authors thank Enfu Wang, Xiuli Wang and Wenhua Li in Tianjin Waterworks Co. Ltd. for their participation in this one-year monitoring study.

REFERENCES APHA, AWWA, WEF. Standard Methods for the Examination of Water and Wastewater, 19th edition; Washington DC, USA: American Public Health Association; 1995. Arora H, LeChevallier MW, Dixon KL. DBP occurrence survey. J AWWA 1997;89(6):60–8. Carlson M, Hardy D. Controlling DBPs with monochloramine. J AWWA 1998;90(2):95–102. Chen Chao, Zhang Xiaojian, He Wenjie, Lu Wei, Han Hongda. Comparison of seven kinds of drinking water treatment processes to enhance organic material removal: a pilot test. Sci. Total Environ. 2007;382:93–102. Diehl AC, Speitel Jr GE, Symons JM, Krasner SW, Hwang CJ, Barrett SE. DBP formation during chloramination. J AWWA 2000;92(6):76–92. Duong Hong Anh, Berg M, Hoang Minh Hang, Pham Hung Viet, Gallard H, Giger W, et al. Trihalomethane formation by chlorination of ammonium- and bromide-containing groundwater in water supplies of Hanoi, Vietnam. Water Res 2003;37:3242–52. Graham NJD, Wardlaw VE, Perry R, Jiang Jia-qian. The significance of algae as trihalomethane precursors. Water Sci Technol 1998;37(2):83–9. Goslan EH, Fearing DA, Banks J, Wilson D, Hills P, Campbell AT, et al. Seasonal variations in the disinfection by-product precursor profile of a reservoir water. J Water Supply 2002:475–82. Hua Guanghui, Reckhow DA. Characterization of disinfection byproduct precursors based on hydrophobicity and molecular size. Environ Sci Technol 2007;41:3309–15. Krasner S, Croue JP, Buffle J, Perdue EM. Three approaches for characterizing NOM. J AWWA 1996;88(6):66–79. Krasner S, McGuire MJ, Jacangelo JG, Patania NL, Reagan KM, Aieta EM. The occurrence of disinfection by-products in US drinking water. J AWWA 1989;81(8):41–53. Leeheer JA. Comprehensive approach to reparative isolation and fractionation of dissolved organic carbon from natural waters and wastewaters. Environ Sci Technol 1981;15(5):578–87. Li Shuang, Zhang Xiaojian, Liu Wenjun, Cao Lili, Wang Zhansheng. Formation and evolution of haloacetic acids in drinking water of Beijing City. J Environ Sci Health Part A 2001;36A(4):475–81. Ministry of Health, PRC. China. Sanitary standard for drinking water quality; 2001. Nissinen TK, Miettinen IT, Martikanen PL, Vartiainen T. Disinfection by-products in Finnish drinking water. Chemosphere 2002;48:9–20.

S CIE N CE OF T H E TOT AL E N V I RO N ME N T 3 9 7 ( 2 00 8 ) 1 4 0–1 47

Owen DM, Amy GL, Chowdhury ZK, Paode R, McCoy G, Viscosil K. NOM characterization and treatability. J AWWA 1995;87(1):46–63. Roberts MG, Singer PC, Obolensky A. Comparing total HAA and total THM concentrations using ICR data. J AWWA 2002;94(1):103–15. Rook JJ. Formation of haloforms during chlorination of natural water. Water Treat Exam 1974;23:234–6. Sharp EL, Parsons SA, Jefferson B. Seasonal variations in natural organic matter and its impact on coagulation in water treatment. Sci Total Environ 2006;363:183–94. Simpson K, Hayes KP. Drinking water disinfection by-products: an Australian perspective. Water Res 1998;32(5):1522–8. Singer PC. Humic substances as precursors for potentially harmful disinfection by-products. Water Sci Technol 1999;40(9):25–30. Villanueva CM, Kogevinas M, Grimalt JO. Haloacetic acids and trihalomethanes in finished drinking waters from heterogeneous sources. Water Res 2003;37(4):953–8. Wang Lihua (2001). Study on haloacetic acids formation and characteristics of their precursors. PhD thesis. Department of

147

Environmental Science and Engineering, Tsinghua University, Beijing, China. (in Chinese). Williams DT, Lebel GL, Benoit FM. Disinfection by-products in Canadian drinking water. Chemosphere 1997;34(2):299–316. Wylie PL. Comparing headspace with purge and trap for analysis of volatile priority pollutants. J AWWA 1988;80(10):65–72. Xie Yuefeng, Zhou Haojiang. Use of BAC for HAA removal-part 2, column study. J AWWA 2002;94(5):126–32. Xie Yuefeng, Rashid I, Zhou Haojiang. Acidic methanol methylation for HAA analysis: limitations and possible solutions. J AWWA 2002;94(11):115–23. Yoon Jeyong, Choi Youshik, Cho Soonhang, Lee Dongsoo. Low trihalomethane formation in Korean drinking water. Sci. Total Environ. 2003;302:157–66. Zhou Hong, Zhang Xiaojian, Wang Zhansheng. Occurrence of haloacetic acids in drinking water in certain cities of China. Biomed Environ Sci 2004;17(3):299–308.