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

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Abstract

A one-year-long monitoring project was conducted to assay the concentrations of THMs, HAAs and their formation potential along the conventional process in a water treatment plant in North China. Subsequent investigations of organic matter fractionation and the contribution of the algae to the precursor were also conducted to trace the source of the DBPs. The results showed that the concentration of DBPs and their formation potential 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 spring, no more than 100 µg/L. Both organic matter and algae were found to be important DBP precursors. The hydrophobic acid fraction in dissolved organic matter has the highest formation potential for both THM and HAA. Algae contribute about 20% to 50% of the total formation potential during an algal bloom. The efficiency of each unit process for DBPs and precursors was also assayed. Unfortunately, the conventional drinking water treatment process is limited in its efficiency for precursor removal. The pre-chlorination and filtration process had a negative effect on DBP or precursor removal.

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., 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 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 pre-chlorination 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.

Section snippets

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

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

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

Acknowledgements

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.

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