Study On Formation Mechanism Of Taste And Odor Compounds-Chloroanisoles In Drinking Water

In recent years, with the improvement of people’s living standards, people as well put forward to the quality requirements of the drinking water. Taste and odor is one of the important indicators of water quality evaluation. But, currently, drinking water is frequently reported to have taste and odor. Earthy, septic, fishy, aromatic and grassy odors are always occurred in drinking water among which earthy smell is the most unbearable and common smell.2,4,6-trichloroanisole (TCA), as one of the source of earthy smell, was originally found in wine and later found in the end of pipe water. Because the olfactory threshold of TCA is very low (0.05-10ng/L), So it is the target of this research.In this study, the method of using large volume concentration, solid-phase micro-extraction and gas chromatography-mass spectrometry was applied to study the targets. First the TCA and other taste and odor compounds which are often detected in eutrophic water were researched as the targets. They are called:2-methylisoborneol (2-MIB), geosmin (GSM),β-Cyclocitral, β-Ionone and dimethyl trisulfide (DMTS). In this way, it can not only detect the distribution of TCA in the water, but also can simultaneously detect the distribution of the other five kinds of smell taste substances. As a result, the method can be applied to the detection of taste and odor compounds in actual water body more widely. The test of the distribution of six smelling compounds showed that:the concentrations of DMTS,β-Ionone and β-Cyclocitral were relatively higher in the lake and reservoir water, generally in μg～level. The concentration of β-Cyclocitral in the lake has reached358μg/L.2-MIB and GSM exist in each of these water bodies and in the lake,2-MIB is2.4μg/L. TCA was primary detected in distribution system and lake water. The concentration of TCA in the end of distribution system is5.49ng/L. It is beyond the olfactory threshold of TCA and is higher than the lake’s.In the process of reaction between anisole with chlorine during the chlorine disinfection, the results indicated that:pH value is a determining factor which directly decides whether the reaction will occur or not. Only under the condition of acid and weak acid, the anisole will be chlorinated. It means H+is playing acid catalysis role while Fe3+.Al3+also has catalysis to the generation of chloroanisoles because of their special electronic track. The lower the pH, the reaction rate is greater. Because of the influence of-O-functional group of anisole, it is easier for the para-position to be replaced than the ortho-position. In addition to, chlorine dosage, initial concentration, temperature all have influences on the generation and rate of chloroanisoles. They respectively have a good relationship between factor and generation of chloroanisoles. The impact strength is:initial concentration of chlorine> chlorine dosage> temperature. The formation models for CA and DCA are B=0.4757e-00112t-0.2178e-0.476t and C=0.9702e-0.00711t-0.0443e-0.476t-0.924e-0.0112t, respectively.In the process of transformation from2,4,6-trichlorophenol (TCP) to TCA with the microbial methylation, the results indicated that:the transformation from TCP to TCA must be under the condition of microbial effects. The ability of transformation from TCP to TCA between Escherichia coli, Cyclotella, Chlorella, Anabaena are different. The organics which as the source of methyl must be have independent methyl. Temperature has a huge impact on the generation of TCA. Under the35℃, the transforming ability of microbial methylation is the largest. Because Mg2+, Mn2+are a necessary trace element of microbial, so they can improve the transforming ability of microbial methylation while Zn2+has inhibiting effect on the formation due to it is heavy metal which is toxic to microorganism. Anaerobic and supersaturated aerobic environment are not conducive to the reaction. The test in actual water body showed that:the important factors are TOC and the aquatic environment. The greater the TOC, the generation of TCA is larger. Under the same TOC, the quantity of microorganism is more in seal water body, so the generation of TCA is larger.Experimental results show that:Chloroanisoles generally are not formed during chlorine disinfection process, but from methylation in the end of the pipe under the action of microorganism Therefore the best way to control the chloroanisoles and earthy odor in the water is to control the amount of microorganism and organic matter.