응집·침전·여과를 이용한 중금속 폐수 처리

강 창덕1,2, 심 상준1, 김 우식2

한국과학기술연구원 청정기술연구센터1, 경희대학교 화학공학과2

Treatment of heavy metal wastewater

by coagulation, flocculation, sedimentation, and filtration

C. D. Kang1,2, S. J. Sim1, and W. S. Kim2

Clean Technology Research Center, Korea Institute of Science and Technology1

Department of Chemical Engineering, Kyung Hee University2

Introduction

Chemical precipitation is based on the fact that many of the metal ions form relatively insoluble hydroxides and consists of hydroxide precipitation of the metals at an alkaline pH, followed by removal of the resulting solids by simple sedimentation and sometimes filtration. However, this method presents several limitations from a technical point of view, which requires a relatively large amount of space for the clarifier due to long settling times and results in a wet, bulky sludge that is not easily handle [1,2].

In order to make up for these weak points in the chemical precipitation, addition of coagulant is proposed. In this study, two continuous processes using coagulation by ferric chloride (FeCl3) [3] and limestone were developed for removal of copper in the wastewater on the basis of results from jar test experiments. One is the process using coagulation and filtration and the other is the process using coagulation, flocculation and settling.

In a former process, the removal of copper in the wastewater was achieved by coagulation and coprecipitation with ferric chloride as a coagulant. The process consisted of limestone aeration bed for the formation of amorphous ferric hydroxide flocs and sedimentation tank coupled with stainless steel screen membrane for the separation of formed flocs and treated water.

In a second process, the removal of copper in the wastewater was achieved by coagulation using the ferric chloride as a coagulant, and flocculation using the polymer with polyacrylamide group followed for bridging of the formed flocs [4]. The continuous process consisted of the limestone aeration bed for the formation of amorphous ferric hydroxide flocs including copper ions, rapid-mixing tank for the mixing of the flocs and flocculant, slow-mixing tank for the growing of the flocs and sedimentation tank with lamellae for the separation of the flocs and treated water.

Continuous removal of copper was carried out under various conditions by the processes.

Experimental

Raw water containing 10 mg/l of copper was prepared by dilution of 1,000 mg/l of copper stock solution that was made by dissolving 97% CuCl2 (Kanto, Japan) in deionized water. 3,000 mg/l of stock FeCl3 solution prepared by dissolving 97% FeCl3 (Aldrich, USA) in deionized water was used as a coagulant varying concentration of Fe3+. 0.1% SA 407 solution was prepared for a flocculant by dissolving SA 407 powder (Songwon, Korea) in deionized water, which was anionic or nonionic flocculant as polymer with polyacrylamide group, and was used varying dosage of that.

Raw limestone was obtained from Dae-Gog mining company at Danyang, Chungbuk, Korea. The raw limestone was crushed, screened and graded by 7∼10 mesh size with sieve.

(1) The continuous process using coagulation and filtration

FeCl3 solution was added to the 10 mg/l of copper contaminated water. After sufficient mixing, wastewater was supplied to the bottom of the limestone aeration bed at the rate of 50 ml/min concurrently with up-flow aeration using 50 l/min of air. The limestone aeration bed was packed with 1,200 g of limestone granules having 7∼10 mesh size. The working volume of the limestone bed was 1.7 l. The treated water eluted from the limestone aeration bed was settled in the sedimentation tank coupled with membrane cartridge, and non-sediments were filtered by the stainless steel screen membrane. When the vacuum pressure generated inside the membrane by a fouling on the membrane surface during the filtration was increased up to 10 cmHg, backwash was automatically performed with 10 l/min of air in order to prevent the damage of the membrane surface.

(2) The continuous process using coagulation, flocculation, and settling

FeCl3 solution was added to the 10 mg/l of copper contaminated water. After sufficient mixing, wastewater was supplied to the bottom of the limestone aeration bed concurrently with up-flow aeration using 50 l/min of air. The limestone aeration bed was packed with 1,200 g of limestone granules having 7∼10 mesh size and working volume of that was 1.7 l. Treatment water through the limestone aeration bed flowed to the rapid-mixing tank which the flocculant (0.1% SA 407 solution) was added continuously and then stirred at 150 rpm. After rapid mixing, the wastewater flowed to the slow-mixing tank and stirred slowly at 40 rpm. The wastewater which coagulation and flocculation were achieved through the former units induced to the sedimentation tank with lamellae, and separation of formed flocs and final treatment water was performed continuously.

Both of the processes were operated at room temperature (about 15℃).

Results and discussion

(1) The continuous process using coagulation and filtration

As the pH condition in the limestone aeration bed was maintained over 7.5, the formation of the ferric hydroxide flocs and coprecipitation of the copper with the formed flocs were achieved in the continuous operation. As the result, it was found that both copper in the raw water and iron added as coagulant were removed over 99.5% (Fig. 1).  In order to investigate effect of pore size of the stainless steel screen membrane on the filtration for treatment water, continuous process experiments were carried out varying the pore size of the membrane from 1000 to 2000 mesh size (Fig. 2). In cases of 1450 and 2000 mesh size of the stainless steel screen membrane, fine particles as well as amorphous flocs and non-sediments were filtered efficiently and residual copper concentration of the treatment water was below 0.05 mg/l. When the stainless steel screen membrane of 1000 mesh size was used, the residual copper concentration of the treatment water was over 0.65 mg/l higher than the former case.

(2) The continuous process using coagulation, flocculation, and settling

When the wastewater was supplied at rate of 42.5 ml/min, the pH condition in the limestone aeration bed was maintained around 7.6 and formation of the ferric hydroxide flocs including copper ions was achieved. Flocculation of the flocs was performed efficiently with 1.0 ml/l of 0.1% SA 407 solution in the rapid-mixing tank and slow-mixing tank subsequently. Continuous separation of the flocs and treated water in the sedimentation tank equipped with lamellae was employed too. As the result, it was found that both copper in the raw water and ferric ions added as coagulant were removed over 99% (Fig. 3).  In order to investigate effect of polymer flocculant (0.1% SA 407 solution) dosage on copper removal through the continuous process, the continuous process experiments were carried out varying dosage of polymer flocculant provided continuously in the rapid-mixing tank (Fig. 4). When 10 mg/l of copper contaminated water was supplied at rate of 42.5 ml/min, residual copper concentration of the treatment water was very low (0.08 mg/l) with small dosage of flocculant, but the removal efficiency of copper was not significant to the dosage of flocculant. When inflow rate of the raw water was 85.0 ml/min (a half of hydraulic retention time), although removal efficiency of copper was lower than the former case to some degree, the residual concentration of copper was nearly constant on the variation of the flocculant dosage like the former case. It seems that removal efficiency of copper depends on the hydraulic retention time in the continuous process rather than the dosage of polymer flocculant.

References

1. L. Charerntanyarak, Wat. Sci. Tech., 39(10), 135-138(1999)

2. T. E. Higgins and V. E. Sater, Environmental Progress, 3(1), 12-25(1984)

3. C. E. Martinez and M. B. Mcbride, Environ. Sci. Technol., 32, 743-748(1998)

4. J. J. Chack, V. Rubino, S. Mcfarland, P. J. Kransnoff and J. Liubicich,  Wat. Environ. and Technol., 6, 49-53(1994)

Fig. . Residual metal concentration and pH profiles in effluent from the limestone aeration bed, filtrated with filter paper having 5 □ of pore size.

Fig. . Effect of pore size of stainless steel screen membrane. (Flux of the membrane : 306 l/m2·hr, Results after 8-hr operation)

Fig. 4. Effect of polymer flocculant dosage on copper removal in the continuous process. (Results after 10-hr operation)

Fig. 3. Profiles of residual metal concentration of the treatment water and pH of the effluent from the limestone aeration bed.