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Research Progress of Typical Taste and Odor Compounds Produced By Applied Mechanics and Materials Vols. 316-317 (2013) pp 698-702 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.316-317.698 Research Progress of Typical Taste and Odor Compounds Produced by Chlorination in Drinking Water MAO Minmin, ZHANG Kejia *, ZHANG Tuqiao, YU Hongliang College of Civil Engineering and Architecture, Zhejiang University, Hangzhou, 310058, China *Corresponding Author, PhD., E-mail: [email protected] Key words: chlorination; odor and taste compounds; disinfection by-products; precursor; electronic nose Abstract: As disinfection by-products, the taste and odor (T&O) compounds were produced by chlorination at the end of water treatment. The existence of T&O compounds causes the pipe water with unpleasant odor and reducing water quality. Based on the latest research achievements at home and abroad, the characteristics and formation mechanisms of typical T&O compounds produced by chlorination are introduced. The new analytical method of electronic nose is also expounded. In addition, the common control technologies, such as pre-oxidation, activated carbon adsorption and bio-membrane, are reviewed. Since the precursors of T&O compounds shows small molecular, stable structure and low concentration, some coupling technologies including ozone-GAC, permanganate- PAC, and bio-membrane and GAC treatments, are employed effectively. 1. Introduction With the improvement of people's living standards, people as well put forward to the quality requirements of the drinking water. The quality of drinking water is a direct factor on human health. Taste and Odor (T&O) is one of the important indicators of water quality evaluation.Chlorine disinfection commonly used in drinking water treatment plants at home and abroad has advantages of economy ，efficiency and maintenance of residual chlorine in the pipe network to prevent the regeneration of pathogenic microorganisms. Therefore, chlorine disinfection is an important and primary treatment to prevent water-borne diseases and protect the biological safety of drinking water. However, since the 1970s, the chlorine in the disinfection process can react with compounds in the water was discovered to generate toxic T&O compounds such as chlorophenol, halogenated aldehyde and trichloroanisole[1]. In 1977, U.S. EPA promulgated “The Law of Water Treatment ” amendments which clearly defined that among 129 priority pollutants in 65 class, there are about 70 kinds of chlorinated organics and some compounds, like 2-chlorophenol, 2,4,6-trichlorophenol, 2,4-dichlorophenol, et al. could produce unpleasant taste and odor. Therefore, how to effectively control and remove the toxic T&O compounds in drinking water has become a new direction in the international field. 2. T&O Compounds Produced by Chlorine Disinfection 2.1 Chlorophenol It usually contains chlorophenol, di-, tri- and penta-chlorophenol et al. Generally the chlorophenol produce pungent, pesticides, phenol flavor. It is poisonous to living creature and human reproductive system. Olfactory threshold number(TON) were 10, 40 and 300 µg/L. China's drinking water standard (GB5749-2006) regulates the volatile phenol does not exceed 0.002 mg/L. 2.2 Chloroanisole When anisole appears in the drinking water, anisole will react with chlorine generating chloroanisole, di-, tri-chloroanisole (TCA) in which TCA is the most common. TCA’s odor is described as earthy, musty. TON of TCA is 0.05 ~ 10 ng / L. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 123.157.213.111-05/03/13,07:47:05) Applied Mechanics and Materials Vols. 316-317 699 2.3 Halogenated aldehyde It is primarily consisted of chloroacetaldehyde, dichloroacetaldehyde and chloral in which chloral is more contented in drinking water. Chloral formed from humic acid with chlorine is a colorless, volatile with pungent odor oily liquid. At present, WHO stipulates the ceiling concentration of chloral is 10 µg/L. China's drinking water quality standards specified chloral limits is10 µg/L. 2.4 Haloketone It includes tri-, tetra-, five-and hexa-chloroacetone, and 1, 1, 1 - trichloroacetone has been detected in a number of countries. Trichloroacetone an important intermediate to the conformation of chloroform is a yellowish, highly toxic damaging chromosomes and DNA, strongly stimulate smell liquid. Currently, enough information for the recommended health guideline values has not been get internationally. 2.5 Halogenated nitromethane It mainly refers to chloropicrin. Chloropicrin is a colorless, oily, volatile with spicy pungent taste, toxic, liquid which can damage the small bronchi and alveoli, leading to toxic pneumonitis and pulmonary edema.TON is 1.1 mg/L. Enough information recommending the health guidance values in water has not been lawed up to now. 2.6 Halogen acetonitrile Chloroacetonitrile, di-, tri-chloroacetonitrile are included. Chloroacetonitrile, highly toxic chemical, has a pungent odor usually used for the preparation of pesticides, intermediates of organic synthesis. WHO restricted total chlorineacetonitrile in drinking water should be lower than 70 µg/L. 2.7 Trihalomethanes and Haloacetic acids Chloroform is a colorless, transparent, highly volatile with special smell sweet, low toxicity liquid. Long-term contacting will have narcotic even carcinogenic effect to central nervous system. Haloacetic acids (dichloroacetic acid and trichloroacetic acid) both are colorless, low toxicity, strong corrosive liquid with a pungent odor. WHO lawed the chloroform’s limit is 30 µg/L in drinking water while China is 60 µg/L. 3. Generation Mechanism of T&O Compounds during Disinfection Researching the formation mechanisms and sources of T&O during disinfection is a significant step to control toxic odor compounds generated effectively. Formation mechanisms of T&O compounds are different between precursors. 3.1 Chlorophenol It is caused by chlorine and phenol in a substitution reaction[2]. First, a hydrogen atom on the benzene ring was replaced by chlorine sequentially generating chlorophenol, di-, tri-chlorophenol. Then, enolisable, hydrolysis and the decarburization reaction were proceeded. At last, trihalomethyl methane (THMs) and haloacetic acids (HAAs) are generated. Thus, chlorophenol is an important intermediate product to form THMs and HAAs. 3.2 Haloketone It chiefly refers to compounds that hydrogen on the methyl group of acetone is replaced by halogen[3]. The mechanism of 1, 1,1-trichloroacetone formed by chlorine oxidation with trichloroacetone is shown in figure.1 700 Energy Engineering and Environmental Engineering Fig.1 The mechanism of formation pathway of chloroacetone during chlorination In alkaline environment, at first, carbon(C) linked with hydroxyl in acetone is reacted with the adjacent C to form double bond .In the meaning time, the double bond in hydroxy group becomes a single bond, and then reacts with the hypochlorite generating chloroacetone. Finally, di- and tri-chloroacetone are produced in the same way successively. Xu et al. showed that the yield of l, l, 1-trichloroacetone decreased as the pH increased, and unchanged with the increase in contact time and chlorine. 3.3 Halogenated nitromethane It is resulted from reaction of chlorine and organic nitrogen. Hydrophilic amino acids, amino sugars and other organic nitrogen compounds are thought to be precursors. Formation mechanisms are different between different precursors. Clear pathway of the chloropicrin has not been detected currently. 3.4 Trihalomethanes and haloacetic Acids(THMs and HAAs) Humic acid and fulvic acid (humus as representative ingredient) are the major precursors to generate THMs and HAAs due to the basic unit of humus is similar to resorcinol. Boyoe et al. studied the formation of chloroform through chlorinating resorcinol. Fig.2 The formation mechanism pathway of chloroform with the chlorination of second inter-phenol 3.5 Products formed by chlorine with nitrogenous compounds Except halogenated nitromethane ，other kinds of T&O compounds would be formed in the reaction of chlorine and nitrogenous compounds. Ingrid [4] took the phenylalanine as a precursor for test. At different Cl/N ratio, reaction mechanism is different. Fig.3 The mechanism of formation pathway of phenylacetaldehyde and phenylacetonitrile during chlorination Applied Mechanics and Materials Vols. 316-317 701 4. Detection Technology of T&O Compounds 4.1 Conventional detection technologies Sensory detection, instrument detection and other detection methods are the conventional qualitative and quantitative techniques. 4.2 Electronic nose detection technology With the development of sensor technology, application of electronic nose technology has been widely used in the field of detecting T&O compounds. Consisted of sampling operation, gas sensor array and signal processing system, electronic nose is a novel equipment developed at the 1990s which accord to the different response pattern from electronic systems to recognize the compounds. Short response time, fast detection , avoiding human error, good repeatability are the biggest advantages. 5. Controlling Technology Based on the mechanism of chlorine disinfection, two ideas to remove T&O compounds are proposed. First, before chlorine disinfection, remove the precursors. Second, eliminate the T&O compounds after the formation. Water after the chlorine disinfection is generally directly into the pipe network system, so the main
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