| Chlorine is currently the most widely used oxidizer and disinfectant in water treatment plant, but the Trihalomethanes (THMs) is one of the main chlorination by-products and has been recognized as a cause of Carcinogenicity-Mutagenicity-Teratogenicity. THMs contains four kinds of halogenated alkanes (CHCl3, CHCl2Br, CHClBr2 and CHBr3), their precursors are mainly of natural organic matter. Raw water contains more trihalomethanes precursors, with larger Trihalomethanes Formation Potential (THMFP). Studies have shown that prechlorination and chlorine disinfection is the main source of THMs, so removing the generated THMs and reducing the THMFP in prechloridized water are effective method to reduce the THMs of the tap water.Coagulation-Dissolved air flotation(D AF) process with the advantage of treating low temperature, low turbidity and high algae-laden water will be gradually applied to the water treatment process, instead of the traditional coagulation-sedimentation process. Counter current dissolved air flotation (CCDAF) is a new type of flotation technique. Compared with the traditional co-current DAF, the advantages of this process are that the potential for floc damage by recycle water is eliminated, the counter-flow can exert capturing of bubble/floc aggregate suspension bed and bubble blanket so that the collision and adhesion rate of floc and bubble is enhanced, and the efficiency of treatment is improved. This dissertation studied the effect of generated THMs and the THMFP in prechloridized water by coagulation-DAF process. The influencing factors on flotation effects were studied by the coagulation-DAF jar test and the coagulation-CCDAF dynamic state test, and the efficiency of treatment was enhanced by optimizing the operational parameters and improving operating mode.The experimental results are hoped to provide references and supports in improving the treatment process and the quality of drinking water safety.The raw water was treated by jar test of flocculation-DAF, and the results were as follows:FeCl3 was chosen as the coagulant of subsequent tests by contrasting treatment effects of iron salt and aluminum salt coagulants. Flocculation conditions, flocculation time and dosage have a significant effect in the floc form and strength, thereby affecting the removal effect of the THMs, THMFP and other indicators. Different flocculation modes had marked influences on floc strength, and When floc form formed by constant and tapered flocculation was similar, the floc strength formed by the tapered flocculation was greater. floc form and strength influenced jointly the flotation treating effect, and by the tapered flocculation during which the dosage, stirring strength and time of flocculation reaction were proper, the flocs with looser structure, more branches and greater strength can effectively adsorb organic matter and were more easily removed by flotation. structure of flotation tank and performance of releaser should be improved in order to distribute the microbubbles evenly throughout the space, reduce partial turbulent intensity, try to extend the contact time of bubbles and flocs and increase the collision probability, so the treatment efficiency can be enhanced in practice.Coagulation-DAF process is conducive to the removal of volatile components of THMs and Coagulation-DAF process on the removal of THMs, THMFP and organic matter were better than the coagulation-sedimentation process. Removal effect of THMFP and organic matter in each molecular weight distribution range by Coagulation-DAF process were better than by coagulation-sedimentation process. But the two processes are mainly removing the THMFP and the organic matter of large molecular weight. And the removal effects of small molecular weight THMFP and organic matter were worse. Enhanced coagulation using powder activated carbon-DAF process could significantly enhance the removal of THMs and small molecular weight THMFP and organic matter. The removal rate of THMs, THMFP, TOC and UV254 were respectively 47.3%,64.8%,64.6%and 69.8%.In the dynamic test, the raw water was treated respectively by conventional single-recycle and novel step-recycle coagulation-CCDAF on the basis of jar test. The results were as follows:step-recycle mode of dissolved air water can strengthen capturing of bubbles and flocs association suspension bed, and extend the collision time of smaller flocs and bubbles because of reducing the thickness of suspension bed and increasing that of transition bed; compared with the biggest hydraulic load (9.8m/h), that of step-recycle could reach 14.8m/h which was almost increased by 51%, and the treatment efficiency was largely improved; the flow ratio of fixed releaser F and mobile one M and the distance between M and the inlet of raw water influenced removal effect remarkably, and the removal effect was better when the flow of M is 2 to 3 times as much as that of F and the distance was kept between 60 to 90 centimeters; when the FeCl3 adding amount was 35mg/L, the fast stirring strength was 648s-1, the two-grade tapered flocculation values were 107s-1 and 21s-1 respectively, the recycle rate was 15%, the rate of sludge discharge was about 10%, and the hydraulic load was 13.8m/h, the pollutant removals by the process with step-recycle were that the removal rates of turbidity, THMs, THMFP, TOC and UV254 were respectively 77.5-87.9%,37.7-47.7%,55.3-64.1%,47.5-62.1%, 52.1-65.6%. |
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