The determination and fate of disinfection by-products from ozonation of polluted raw water
Introduction
Currently, ozone applications for water treatment are numerous and widespread all over the world. Ozone is becoming a popular disinfectant due to its effectiveness for killing harmful microorganisms and also because it does not produce significant concentrations of trihalomethanes (THMs) or other chlorinated disinfection by-products (DBPs). However, the increased use of ozonation for drinking water treatment has led to a reevaluation of the chemistry involved in the ozonation of waters that contain natural organic matter (NOM) and bromide (Patel, 1992, Krasner, 1993, Amy, 1994, von Gunten, 2003a).
In the presence of bromide, ozonation of natural waters leads to the formation of hypobromous acid (HOBr), hypobromide ion (OBr−), bromate and brominated organic by-products. Haag and Hoigné (1983) have shown that ozone oxidizes bromide to form HOBr/OBr− under drinking water treatment conditions. Hypobromite was found to be further oxidized to bromate or to a species that regenerates bromide, while HOBr reacted with NOM to form brominated organic by-products. The HOBr/OBr− equilibrium distribution is pH dependent.
The major organic DBPs resulting from ozone treatment of surface water or groundwater have been identified as low molecular weight aliphatic organics, in particular aldehydes, carboxylic acids and ketones (Richardson et al., 1999). On the other hand, ozonation of bromide-containing water can produce brominated DBPs including bromate (BrO3−), bromoform (CHBr3), bromoacetic acids (BAA), dibromoacetone (DBA) and dibromoacetonitrile (DBAN), and some of these inorganic and organic by-products are potential carcinogens (Weinberg et al., 1993, Siddiqui et al., 1995, von Gunten, 2003b). Furthermore, ozonation of drinking water transforms NOM into a more biodegradable form. This can cause significant bacterial regrowth in the distribution system, if biodegradable organic matter is not removed by subsequent treatment steps (Van der Kooij et al., 1989).
Most of the ozonation DBPs that have been reported previously were found in laboratory reactions of ozone with isolated humic substances or natural water (with added bromide) (Xiong et al., 1992, Andrews and Huck, 1994). Other than several studies monitoring for ozonation by-products, there have been very few studies, which follow the formation of ozonation by-products from actual sources. As a result, there is still much uncertainty over what chemical by-products are formed when ozone is used to treat organics- and ammonia-rich source waters. This article summarizes the results of research conducted to analyze ozonation DBPs and using high-resolution electron-impact mass spectrometry to identify the unknown compounds in ozonated waters derived from polluted surface waters.
Section snippets
Water source
Water samples were collected from the Feng-San Reservoir (abbreviated by FSR), which is the major source of public water supply for Greater Kaohsiung area, the second largest metropolis in Taiwan with a population over 2 million and the location of major heavy industries. Owing to upstream discharge of farming, industrial and domestic wastes, the reservoir has become eutrophic. As a result, the chemical dosage for prechlorination can be higher than 70 mg/l, and the trihalomethane (THM)
Carbonyl compound formation
Carbonyl moieties are expected to be prevalent among the disinfection by-products of natural waters. These by-products generally serve as carbon source for bacteria, potentially causing regrowth problems in distribution systems. An upcoming occurrence study of in ozonated water DBPs will determine quantitatively many of the carbonyl compounds reported here.
The major carbonyl compounds products resulting from ozonation of polluted waters in this study were carboxylic acids and aldehydes. Over
Conclusions
This work examined several major classes of ozonation by-products. As utilities consider switching from free chlorine to ozone as the primary disinfectant in order to minimize the formation of chlorinated DBPs during pre-disinfection process; meanwhile, the potential for producing other DBPs must be addressed. In particular, ozonation of waters containing rich organics and bromide may produce organic by-products. Based on an experimental design of ozonation of polluted raw water affected by
Acknowledgements
The authors thank the National Science Council, R.O.C. and Research Committee of Hung Kuang University for financial support of this research under contract no. NSC 91-2211-E-241-003 and HKC-92-A-60, respectively.
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