3-Rahmani

JRHS 2008; 8(1): 18-24

Copyright © Journal of Research in Health Sciences

Removal of Water Turbidity by the Electrocoagulation Method

Rahmani AR (PhD)

Department of Environmental Health Engineering, Center for Health Research, Hamadan University of Medical Sciences, Iran

*Corresponding author: P.O. Box: 4171, Hamadan, Iran. E-mail: rahmani@umsha.ac.ir.

Received: 5 November 2007; Accepted: 22 March 2008

Abstract

Background: Electrocoagulation is a technique involving the electrolytic addition of coagulating metal ions directly from sacrificial electrodes. These ions coagulate with turbidity agents in the water, in a similar manner to the addition of coagulating chemicals such as alum and ferric chloride, and al­low the easier removal of the pollutants. Purpose of this study was to conduct experimental investiga­tion of water turbidity removal using the electrocoagulation method.

Methods: Removal of turbidity from raw water in batch system was investigated by different voltage (10, 15, 20, 25, 30V), electrodes (Al, Fe and St) and electrolyzes time (0 to 40 min.), electrodes dis­tance 2 cm and pH=7.5.

Results: The experimental results showed that the removal efficiency depends on the electrolyze time, types of electrodes and the applied current. From the experiments carried out at 20V, it was found that in 20 minutes the removal efficiency for Al, Fe and St electrodes was 93, 91 and 51 percent respec­tively. Based on turbidity removal efficiency, Al is prior to Fe and St as sacrificial electrode material.

Conclusion: In an era when environmental phenomena attract a great attention, electrocoagulation meth­ods can be said to be a promising cleaning and purifying method for water treatment.

Keywords: Water, Treatment, Turbidity, Electrocoagulation

Introduction

The presence of particulate materials such as algae, clays, silts, organic particles and solu­ble substances in water often causes it to get turbid or colored. The settle ability of the par­ticulate depends on the density of the mate­rial and the size of particles. The parti­cles with density more than water should eventually settle due to gravitational force. Small particles, especially those with density close to water such as bacteria and colloidal particles may never settle and remain sus­pended in the water. Therefore, agglomera­tion of particles into a larger floc is a neces­sary step for their removal by sedimentation (1).

The conventional treatment method consists of adding metal salts (aluminum, iron etc.), de­stabilization of colloidal particles (which is called coagulation), followed by floccula­tion and sedimentation. In this way the applica­tion of chemical reagents like alum, lime, soda ash etc, which are widely used in good quantity, becomes imperative for clarifi­cation. This method of treatment has cer­tain drawbacks like handling large quanti­ties of chemicals, proper assessment of re­quirements, feeding of chemicals and produc­tion of large volume of sludge caus­ing disposal problem and loss of water (2).

During recent decades research on electricity applied directly in water treatment has pro­gressed well, making it an attractive method for coagulation or clarification of water, usu­ally known as the electro-coagula­tion/elec­tro­chemical method (3). In this method di­rect current is passed through alumi­num/iron plates suspended in water (2-5).

This system causes sacrificial electrode ions to move into an electrolyte. Undesirable con­taminants are removed either by chemical reac­tion and precipitation or by causing colloi­dal materials to coalesce. They are then removed by electrolytic flotation, or sedimen­tation and filtration. Disinfection is also accomplished by anodic oxidation (6). Donini at el. pointed out that the mecha­nisms of coagulation were similar for elec­tro-coagulation and aluminum salts treat­ment (7). The difference is mainly in the way aluminum ions are delivered. Compared to water treatment with aluminum sulphate or ferrichloride, electrochemical alumi­num/iron generation has several distinct ad­vantages. Aluminum or iron is introduced with­out corresponding sulphate ions. Also there is no need for an alkalinity supply to give a reaction. By eliminating competing ani­ons using a highly pure Al or Fe source, lower metal residuals are obtained and less sludge is produced (50-70 percent). The ad­justment of Al or Fe ion dose in the water can be done easily by manipulating the dial for control of current (2).

The following electrode reactions for Al oc­cur in this process (2):

Anode: Al0   A13+ + 3e-                      [1]

Cathode: 2H20 + 2e-   20H + H2        [2]

AI0 + 3H2   Al (OH)3 + 1.5H2            [3]

Pzhegorlinski et al. determined the contribu­tion of the individual reactions of equations 1 to 3. Each of these reactions was evaluated by the weight loss of the corresponding elec­trode and the volume and composition of the collected hydrogen (8).

The nascent oxygen produced is a very power­ful oxidizer and oxidizes metals pre­sent in water. The nascent Al reacts with the water to form insoluble hydroxides and the colloid destabilization process is therefore analogous to that obtained with traditional metal salts. For completing the treatment, elec­trocoagulation is followed by the usual separation processes, i.e., sedimentation, flota­tion, filtration, etc (2).

The main objective of this research was to con­duct experimental investigation of water turbidity removal by using the electrochemi­cal method. Since iron, aluminum and Stainless steel electrodes have not been com­pared in detail for the treatment of turbid wa­ter, it is the purpose of this study to compare the turbidity removal by electrocoagulation us­ing Stainless steel, aluminum and iron elec­trode materials. In addition, the effects of current density and treatment time on the process performance are explored.

Methods

Turbid water preparation

In this study the clay was supplied from Hama­dan mines in the west of Iran during 2006.  The collected samples were sieved and the fractions below 230 mesh were used for the study. The turbid sample was pre­pared by mixing 0.15 to 0.25-wt% of the 230 mesh clay fraction in 1000 ml water. Turbid­ity in samples was measured as NTU before and after electrolysis by using the turbidime­tre (Hach 2100 N). All analyses were made done according to the standard methods (9).

Experimental Set-up and Measurements

In these experiments, the electrochemical cell consisted of a 500-ml beaker and two se­ries electrodes. Aluminum, iron and stainless steel plates (15.0 x 4.0 cm) were used as elec­trodes. They were treated with the solu­tion of HCl (15%Wt.) for cleaning prior to use. The beaker was filled with 250ml of sam­ple turbid water, (pH=7.5), and the elec­trode plates were held suspended 2 cm apart in the water. The electrodes arrangement con­sisted of three cathodes interspersed with three anodes connected by brass rods to each other arranged as a parallel electrode plates.

Experiments were done similarly via the same electrolyzes time, electrodes distance and voltage intensity for all types of elec­trodes. To evaluate the direct current effect on turbidity removal, the samples were ex­posed to different voltage (10, 15, 20, 25, 30V) for 40 minutes respectively. In this study current was changed from 50 to 400 mA according to voltage. Primary turbidity measurement was done and then samples were taken periodically each 10 minutes for measurement of turbidity. In each run pH and EC were measured. Power was supplied to the electrodes with a DC Power Supply. A magnetic stirrer was used for stirring. Cell cur­rent and voltage were measured using Am­meter and Voltmeter. All experiments were conducted at ambient temperature (nomi­nally 20°C). The experimental appara­tus as shown in figure 1 was set up in Hamadan University of medical sciences in 2006.


Figure 1: Experimental apparatus

Results

The effects of direct current on water turbidity removal are shown in Figure 2 to 4 for all combina­tions of electrodes. The results show that turbidity removal is increased with increasing voltage. It is found that the Al electrodes had the highest efficiency. The variation of pH and EC was low and neglect able.

Figure 2: Effect of current intensity on water turbidity removal by Al electrodes

Figure 3: Effect of current intensity on water turbidity removal by Fe electrodes

Figure 4: Effect of current intensity on water turbidity removal by St electrodes

Discussion

Electro-coagulation has been accepted as an ideal technology to upgrade water quality for a long time and it has been successfully ap­plied to a wide range of pollutants in even wider range of reactor designs (10). Turbid­ity removal occurs as the result of destabiliza­tion of colloids due to the effect of the electric field generated between the elec­trodes and the reactions with coagulating compounds formed in situ during anode oxida­tion, followed by a subsequent flotation of agglomerates of the particles (11). 

Turbidity removal rate by Al electrodes ap­pears to be higher than that by Fe and St elec­trodes. Turbidity removal as a function of electrodes and voltage in 2 cm distance be­tween electrodes and 40 min contact time are compared in Figure 5.

When Al electrodes are used, it was found that the reaction required lower current. When Fe or St electrodes are used, it is neces­sary to increase voltage. As seen in figure 2 the Al electrode requires at most 10-15 min. for good removal efficiencies, while for iron electrode the time is increased to 20-25 min according to fig. 3. According to fig 4 the removal efficiency in case of St electrode is less than Al and Fe electrodes and the time required for reaching to same efficiency in­creased significantly. On the other hand, by comparing Figure 2-4, it is easily seen that the cur­rent density and operating time have simi­lar effects on process performance.


Figure 5: Comparison of turbidity removal as a function of electrodes type (Potential: 10-30V, contact time: 40 min and electrodes distance: 2 cm)

It is thought that increasing electrolyze time or current intensity improves the efficiency of turbidity removal by faster producing hy­drolyze products. During electrochemical treatment, when a potential is applied be­tween electrodes, hydroxyl ions and Al3+ or Fe3+ are generated at the cathode and anode re­spectively. It is known that these products are responsible for flocculation. The possible combination of various hydrolysis products is endless and one or more of them may be re­sponsible for the observed action of floccula­tion (5, 11).

It was shown that the efficiency of water tur­bidity removal was depended significantly on the applied current intensity and elec­trodes material. These enhancing effects are at­tributed to the increase in the driving force of the electrode reaction and the increase in current voltage. This is because potential is the major driving force for the respective phe­nomena of interest in electrochemical reac­tors (12).

The results show that turbidity removal in this study is comparable to similar experi­ments in the literature as discussed below:

M. Han et al. compared effectiveness of the electrocoagulation with conventional chemi­cal coagulation through a set of batch experi­ments. He concluded that the electrocoagula­tion is more efficient than chemical coagula­tion for turbidity removal (4).  Lai CL et al. in­dicated that electrocoagulation with Al/Fe pair electrode was very efficient and able to achieve 96.5% turbidity removal in less than 30 min (13). Kobya et al. studied treatment of textile wastewater by electrocoagulation us­ing iron and aluminum electrodes. The re­sults showed that turbidity removal for the Al electrode was as high as 98% in pH<6. So for both materials, it is clear that turbidity removal shows the same trend (3). Bayramo­glu et al. show that pH is an important operat­ing factor influencing the performance of electrochemical process. The turbidity re­moval in acidic medium for the Al electrode is as high as 98% and for iron in the pH range 3 and 7 is 98 and 75 percent respec­tively. They concluded that in acidic me­dium, higher removal efficiencies are ob­tained with Al, while in natural and weak alka­line medium iron is more efficient (14). 

In conclusion, the efficiency of electrochemi­cal methods for turbidity re­moval was examined in this study. By the ex­periments carried out at 10 V. and 135 mA current intensity, it was seen that a 10-15 min­ute period is sufficient for Al electrodes. By this way at the case of Fe a little bit more time or voltage was required. When the ef­fects of voltage, electrode material and combi­nation of them on turbidity removal were examined, as it was expected, increas­ing current intensity increased the efficiency. It was found that 100-300 mA is sufficient in a large scale for turbidity removal of water.

Acknowledgement

I gratefully acknowledge financial support for this project from the Environmental Health Engineering Dept., Faculty of Health, Hamadan University of Medical Sciences. I also appreciate Ms Gordan S. and, Alborzi M. for their kindly helps and efforts.

References

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