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Comparing of Flotation and Sedimentation Process for Treating Trihalomethanes

Xiaoming Sun 1,2, Qishan Wang 1,2, Yanfang Liu1, Wenshu Zhu 1,2

1 College of Environment Science and Engineering, Nankai University, Tianjin 300071, China. 2 The Guangxi Key Laboratory of Environmental Engineering, Protection and Assessment, Guilin 541004, China.

E-mail: sunxm52@126.com

Abstract-The treatment of prechloridized water by the coagulation-dissolved air flotation process and the coagulation-sedimentation process was examined, the removal of trihalomethanes (THMs) and trihalomethanes formation potential (THMFP) were studied. The results show that the removal rate of THM and THMFP treated with flotation process is 37% and 55% respectively, which is respectively 18% and 21% higher than with sedimentation process (19% and 34%); After flotation process, the value of UV254/TOC is decreased significantly, which indicates that the THMFP is decreased significantly, and the formation of THMs during the follow-up chlorination process can be reduced obviously; the removal of THMFP in each molecular weight range treated with flotation is better than with sedimentation, those two processes mainly remove the THMFP of larger molecular weight range and the removal efficiency of THMFP in smaller molecular weight range is poor.

Keywords-flotation; sedimentation; trihalomethanes; molecular weight distribution

1. Introduction

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 [1, 2]. 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

Supported by the Research funds of The Guangxi Key Laboratory of Environmental Engineering, Protection and Assessment (0804K024)

prechlorination and chlorine disinfection is the main source of THMs, so removing the generated THMs and reducing the THMFP in raw water are effective method to reduce the THMs of the tap water [3]. Coagulation-flotation 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 [4]. The removal effect of THMs and THMFP by flotation process and sedimentation process will be compared in this work. The main objectives are to provide technological guidance and parameters for further research and the operation of water treatment plant.

2. Experimental Materials and methods

2.1. Experimental Materials

The coagulant is FeCl3, and the water quality is summarized in Table 1.

Table 1. Water quality

pH Turbidity

(NTU)

UV254

(cm-1)

DOC

(mg/L)

THMFP

(μg/L)

8.10 8.75 0.103 4.165 224.7

2.2. Experimental equipment

The coagulation-sedimentation experimental equipment is the TS-6 programmable mixing instrument and the reactor volume is 1 L.

The coagulation-flotation experimental equipment is self-made apparatus including the reactor and the dissolved air system. The coagulation and flotation process is carried out in the reactor. The reactor is made of plexiglass, and the size is 100mm×100mm ×300mm. Its effective volume is 2L (Figure. 1). Dissolved air system consists of air compressors, high pressure pump and dissolved gas tank.

978-1-4244-4713-8/10/$25.00 ©2010 IEEE

Figure 1. Schematic diagram of coagulation-flotation reactor

2.3. Experimental Methods

Coagulation-sedimentation experiments: ① Infusing 1L prechloridized water into the reactor; ② Adding coagulant (FeCl3 dosage of 55 mg/L) and starting the mixing program. Coagulation conditions: mixing 500 rpm, duration of 2 min; the first reaction 150 rpm, duration of 10 min; the second reaction 50 rpm, duration of 10 min; ③ Putting it aside for 30 minutes, then sampling and testing the water quality.

Coagulation-flotation experiments: ① Infusing 2L prechloridized water into the reactor; ② Adding coagulant (FeCl3 dosage of 47mg/L) and starting the mixing program. Coagulation conditions: mixing 500 rpm, duration of 1 min; The first response 150 rpm, duration of 5min; the second reaction 50 rpm, duration of 5 min; ③ Opening the valve and infusing 200 mL dissolved-gas water into the reactor (recycling ratio 10%), stirring slowly (speed of 20 rpm) and the micro-bubbles adhere to the flocs; ④ Putting it aside for 7 min, then sampling and testing the water quality.

2.4. Analytical Instruments and Methods

THM: Angilent 6890 gas chromatograph; TOC: Shimadzu TOC-VCPH analyzer; UV254: DR/4000U UV-visible spectrophotometer; Turbidity: HACH 2100N Turbidimeter; pH value: Mettler Toledo 320 pH meter.

THMFP determination: Firstly the sample was put into a incubator (at 25 士 2 ℃) with excessive dosage chlorine (as C12) at PH 7.0 士 0.2 (adjust by phosphate buffer solution), after reacting 5d, ascorbic acid was added to stop the react and the production of THMs is the THMFP of the water.

Molecular weight distribution [5-8]: the relative molecular weight distribution of organic matter was measured by ultrafiltration membrane method. After filtration of 0.45 μm

filter membrane, the water samples are filtered respectively through MWCO (Molecular weight cut off) for 30k, 10k, 6k and 2k ultrafiltration membrane, and then the quality of filtered liquid was measured.

3. Results

3.1. Removal of THMs

Figure 2. Removal of THMs by flotation and sedimentation process

It is shown in Figure 2 that the removal rate of THMs treated with flotation and sedimentation process is respectively 37% and 19%. The removal rate of THMs monomer (CHCl3, CHCl2Br, CHClBr2 and CHBr3) by those tow processes is 29%~44% and 13%~25%. The removal rate of the four monomers treated with flotation process were increased by 24%、19%、14% and 5% than with sedimentation process. Thus, flotation process has a greater advantage than the sedimentation for the removal of THMs. But with the increase of monomer molecular weight the removal rate of monomer is decreasing. In the flotation process, the micro-bubbles can blow off the volatile THMs, thereby increasing the removal rate of THMs. However, with the increase of the molecular weight, the volatility of THMs monomer gradually decreases, so the monomers are more and more difficult to be blown off, resulting in lower removal rate of the weightier monomers. The removal of THMs treated with traditional coagulation-sedimentation process is limited, how to make full use of the flotation process to increase the removal rate of THMs remains to be further studied.

3.2. Removal of THMFP

Removal of THMFP by flotation and sedimentation process shows in Figure 3.

0%

10%

20%

30%

40%

50%

CHCl3 CHCl2Br CHClBr2 CHBr3 THMs

FlotationSedimentation

Rem

oval

rate

Figure 3. Removal of THMFP by flotation and sedimentation process

The removal rate of THMFP by flotation is 55% which is 21% higher than by sedimentation, and the removal rate of the four monomers treated with flotation were increased by 6%~37% than with sedimentation process. Flotation can be seen more conducive to reduce trihalomethanes precursors, which can effectively reduce the generation of THMs.

3.3. Removal of organic matter

Figure 4 shows that the removal rate of turbidity, UV254 and TOC by flotation is 16%、24% and 12% higher than by sedimentation. As the UV254 is positively related to the precursors of THMs, the value of UV254/TOC is often used to examine the formation characteristics of THMs[9]. After flotation and sedimentation process, the UV254/TOC is 0.0175 and 0.024 respectively. The value of UV254/TOC treated with flotation process is decreased significantly, which show that the THMFP is decreased significantly.

Figure 4. Removal of Turbidity, UV254 and TOC by flotation and

sedimentation process

3.4. Removal of THMFP in different molecular weight range

As shown in Figure 5, the THMFP of raw water mainly distributed in the molecular weight of <2 × 103 and >30 × 103. THMFP of these two intervals are 38% and 25% of the total

content. THMFP removal of different molecular weight ranges treated with flotation and sedimentation is shown in Figure 6.

0

50

100

150

200

250

>30 30～10 10～6 6～2 <2 total

Molecular Weight

THM

FP (μ

g/L)

Figure 5. THMFP of different molecular weight ranges in raw water

The THMFP removal rate is over 75% in the molecular weight range of > 6×103 treated with flotation process and the removal rate is 68% in the range of > 30×103 treated with sedimentation. The THMFP removal rate is gradually reduced as the decreases of molecular weight and the removal effect is worst in the range of < 2×103 by the two processes. It can be seen that the two processes are mainly to remove the macromolecular organic. But the removal of THMFP by flotation is better than by sedimentation in any molecular weight range.

0%

20%

40%

60%

80%

100%

>30 30～10 10～6 6～2 <2 total

Molecular Weight

rem

oval

rate

Sedimentation Flotation

Figure 6. THMFP removal of different molecular weight ranges treated with

flotation and sedimentation

4. Conclusions

The removals rate of THM and THMFP treated with flotation process is 37% and 55%, which is respectively 18% and 21% higher than with sedimentation process (19% and 34%).

After flotation process, the value of UV254/TOC is decreased significantly, which show that the THMFP is decreased significantly, and the formation of THMs during the follow-up chlorination process can be reduced obviously.

0%

20%

40%

60%

80%

100%

Turbidity UV254 TOC

Rem

oval

rate

FlotationSedimentation

0%

10%

20%

30%

40%

50%

60%

70%

CHCl3 CHCl2Br CHClBr2 CHBr3 THMs

Sedimentation Flotation

Rem

oval

rate

The removal efficiency of THMFP in each molecular weight range treated with flotation is better than with sedimentation, the two processes mainly remove the THMFP of larger molecular weight range and the removal of THMFP in smaller molecular weight range is poor.

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