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Environmental Sciences ELSEVIER Procedia Environmental Sciences 10 (2011) 1159 - 1164
2011 3rd International Conference on Environmental Science and Information Application Technology (ESIAT 2011)
Landfill leachate treatment using electrocoagulation
Xiangdong Li, Junke Song, Jiandong Guo, Zhithao Wang, Qiyan Feng
School of Environment Science and Spatial Informatics, China University of Mining and Technology, Xuzhou, 221116,China
Abstract
This study investigated the efficiency of electrocoagulation in removing ammonia nitrogen (NH3-N) and chemical oxygen demand (COD) from leachate. The paper studies the factors affecting the efficiency of removing NH3-N and COD of leachate, such as electrode material ,current density, electrolysis time , Cl- concentration and pH of solution. Treatment was carried out in a discontinuous system,and the results show that the electrocoagulation can be applied to leachate pre-treatment. The operating conditions were current density of 4.96 mA/cm2, the raw pH, Cl-concentration of 2319 mg/L, operating time of 90 min with Fe electrode, the highest COD and NH3-N removal efficiencies were 49.8% and 38.6%, respectively.
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Conference ESIAT2011 Organiz ation Committee.
Keywords:Electrocoagulation; Landfill leachate; Iron electrode
1. Introduction
Landfilling is the most widely used method for the waste treatment. Leachate is commonly generated from precipitation, surface run-off, and infiltration or intrusion of groundwater percolating through the landfill[1]. Leachate is difficult to be treated to satisfy the discharge standards for its variable composition and high proportion of refractory materials[2].Many treatment methods have been used to treat the leachate, such as advanced oxidation techniques, membrane processes, biological processes, coagulation-flocculation methods and so on[3].For the characteristics of leachate change with advancing years of the landfill, these methods have some shortages such as decreasing treatment efficiencies and increasing cost[1]. Therefore, some effective and economical treatment methods need to be developed to solve these problem.
Electrocoagulation(EC) is a simple and efficient electrochemical methods for the purification of water and wastewaters[4]. Electrochemical methods is characterized by its less coagulantion required, decreased amount of sludge, simple equipment, easy operation[5]. Electrocoagulation is an efficient method for the treatment of metalladen wastewater, potable water, textile dyeing wastewaters, tannery wastewater pre-treatment, restaurant wastewater and so on[6]. The technology lies at the intersection of three more fundamental technologies: electrochemistry, coagulation and flotation[7]. Upon the application of direct current, electrocoagulation involves the generation of coagulants in situ by dissolving sacrificial anodes. When aluminum is the anode material, the
* Corresponding author. Tel.: +86-516-13813460754 E-mail address: xdli123@126.com.
1878-0296 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Conference ESIAT2011 Organization Committee. doi: 10.1016/j.proenv.2011.09.185
electrochemical reaction is the formation of Al3+ which combine with water and hydroxyl ions to form
corresponding hydroxydes and/or polyhydroxides [8]
Al—3 e-—>Al3+ (1)
Al3+ +nH2O ^ Al(OH)n3'n+nH+ (2)
The electrochemical dissolution of iron anode is much more complex for there are two oxidation state of iron species: Fe2+ and Fe3+ . According to solution pH and the dissolved oxygen, Fe2+ species can be potentially oxidized to the Fe3+, and finally it hydrolyses to form the hydroxide[9]:
Fe^Fe2+ + 2e- (3)
4Fe2+ +O2 +4H+^4Fe3++ 2H2O (4)
Fe3++ 3H2O^ Fe(OH)3 + 3H+ (5)
The main reaction at the cathode is hydrogen evolution:
2H2O +2e-^H2 + 2OH- (6)
The purpose of this paper is to study the removal efficiency of COD and NH3-N from landfill leathate by electrocoagulation.
2. Experimental
2.1. landfill leachate
In the experimental study, leachate from the adjusting tank of Yanqun Landfill in Xuzhou was used. The Yanqun Landfill was put into commission in 2005. The properties of the leachate were analyzed and values were given in Table 1.
Table 1 The properties of leachate from Yanqun Landfill
p H COD NH3-N BOD5 TP SO42- Cl- Turbidity
meter pH (mg/L) (mg/L) ((mg/L) (mg/L)) (mg/L) (mg/L)) (NTU)
Value 6.4~7.3 2566 386 960 6.29 650 819 314
2.2. Experimental device
The schematic experimental setup is shown in Fig. 1. The batch electrocoagulation cell was constructed of plexiglas with the dimensions of 150mmX 120mmX 110mm.A digital dc power supply (WYK-503, 0-55V, 0-3.5A) was used to give a regulated electricity current. The monopolar electrocoagulation unit consisted of ten electrodes, each electrode was 120mm X 70mm, the electrodes were dipped in the electrochemical reactor to a depth of 80 mm, and the distance between electrodes was 10 mm. In this discontinuous system, 1 liter leachate was injected into the water cell and the solution was agitated with a magnetic stirrer by the speed of 200 rpm. During the experiments, we changed the electrode material, current density, electrolysis time, pH of solution and concentration of Cl-. After 90 minutes settling, we measured the COD and NH3-N of the effluent.
Figure 1. The schematic experimental setup in this study
2.3. Analysis
The COD was measured by the dichromate method, and NH3-N was tested by the Nessler's reagent spectrophotometry. The determination of TP was Ammonium molybdate spectrophotometric method. The pH and turbidity are measured by a pH monitorr (WTW-526) and a turbidity monitor (WZS-185), respectively. All the runs were performed at room temperature of 25 centigrade.
3. Results and discussion
3.1. Effect of electrode materials
Different electrode materials affect the performance of the electrocoagulation process[10].The most widely used electrode materials are aluminum and iron for their cheap, readily available, and effective[11].In this study , Fe and Al electrodes were compared on the condition that: 2.98 mA/cm2 current density, 50 min electrolysis time, 6.5 pH. As can be seen in Fig. 2, the iron anodes give better COD and NH3-N removal performance than the aluminum anodes. When the time is 30 min, the results for COD(32.7%) and NH3-N (24.8%) with iron electrode are more effective than that for aluminium electrode, the COD and NH3-N removal efficiencies with aluminium electrode are 21.3 and 20.8% , respectively. The settleability of particle formed by Fe(OH)3 is better than that formed by Al(OH)3. In this study, the iron electrode is regarded as the optimum choice.
Figure 2. Effect of electrode materials on COD and NH3-N removal (left:COD;right: NH3-N) 3.2. Effect of electrolysis time
The effect of electrolysis time was investigated in the range 5 to 110 minutes by the following conditions: 6.4 initial pH, 2.98 mA/cm2 current density values. As can be seen in Fig. 3, an increase in the time from 5 to 90 minutes yield an increase in the efficiency of COD removal from 18.8 to 45.0% and the NH3-N removal from 9.4 to 44.1%, it does not change significantly after 90 min. When the electrolysis time increases, the concentration of iron ions and their hydroxide flocs increase, also the rate of bubble-generation increases. The pollutants in leachate were removed by the effect of coagulation and flotation. High electrical energy consumption with the increasing time, the optimum time of electrolysis is 90 min.
5 10 15 20 30 40 50 70 90 110 tiine(min)
Figure 3. Effect of electrolysis time on COD and NH3-N removal
3.3. Effect of current density
The current density is an important operating factor which determines the coagulant dosage[12]. The investigated current densities were 1.98, 2.98, 3.97,4.96 and 5.95 mA/cm2. Fig. 4 depicts the effect of current density on the removal efficiencies by the following conditions: 6.8 pH, 30 min electrolysis time. As shows in Fig. 4, the removal efficiency of COD at current density 1.98 to 4.96 mA/cm2 increase from 23.6 to 93% and from 19.8 to 32% for NH3-N removal, it does not change significantly at higher current density. By the increasing of current density, the extent of anodic dissolution of iron increases, resulting in a greater amount of hydroxide flocs for the removal of pollutants. Moreover, the rate of bubble-generation increases and the bubble size decreases with the increasing of current density, resulting in a faster removal of pollutants by H2 flotation [13].
-■—COD —A—NH3 -N
current density(mA/cm2)
Figure 4. Effect of current density on COD and NH3-N removal
3.4. Effect of initial pH
In the electrocoagulation process, pH of the solution is known to play an important role [14].To examine its effect, the initial pH is adjusted to 3.9, 5.8, 6.7, 7.5 and 10.1. Removal efficiencies of COD and NH3-N are presented in Fig.5 with constant operating time of 30 min and current density of 2.98 mA/cm2. The maximum removal of COD and NH3-N were observed at initial pH 6.7 and 7.5, and the efficiencies of COD and NH3-N were 35.3% and 25.3%.The drop of removal efficiency occurred when pH tends towards acidic or basic values. When the initial pH is increased from 3.9 to 7.1, the effluent pH increased from 5.9 to 7.8, and the influent pH is lower than the initial pH when the initial pH is greater than 7.1. The pH could be increased by the hydrogen generation at the electrocoagulation cathodes and the liberation of hydroxide ions from iron hydroxide[15]. To take into account the raw water pH, it is not necessary to regulate raw water pH.
Figure 5. Effect of initial pH on COD and NH3-N removal
3.5. Effect of CF addition
The electrical conductivity of the solution is an important parameter for saving electric energy [16].To increases the electric conductivity of the solution, NaCl was used as the supporting electrolyte.The time is 30 min and the current density is 2.98 mA/cm2. The effects of Cl- on the removal efficiency were studied at 819, 1500, 2000, 2500 and 3000 mg/L levels. As is seen in Fig. 6, An increase in the concentration of Cl- from 819 mg/L to 2500 mg/L
yield an increase in the efficiency of COD removal from 29.8 to 38.9% and the NH3-N removal from 23.5 to 32.6%, it does not change significantly after 2500mg/l. With the Cl" concentration increases, the passivation of the electrode can be relief and the ability of electric conduction can be improve[16]. During the coagulation process, the Cl" will be discharged at the anode to generate Cl2, then the Cl2 can be chemically convert to ClO" which can oxidize the pollutants effectively[17]. To take into account the cost, the optimum concentration of Cl" is 2000 to 2500 mg/L.
-■— COD —±-NH 3 -N
819 1500 2000 2500 3000
Cl- (mg/1)
Figure 6. Effect of Cl on COD and NH3-N removal
4. Conclusions
Based on the experiment of treatment of leachate by electrocoagulation, this paper studies the factors affecting the removal efficiencies. The results indicate that electrocoagulation can be used to the leachate preprocessing. Under conditions of iron electrode, 4.96 mA/cm2 current density, 2319 mg/L Cl" concentration, 90 min electrolysis time and unchanged the raw pH, the removal efficiencies of COD, NH3-N, TP ,BOD5 and turbidity are 49.8, 38.6, 82.2, 84.4 and 69.7%, respectively.
Acknowledgements
The authors would like to acknowledge the project of environmental protection commonweal industry research of China (No.201109011) and the major scientific and technological special project in developing great oil & gas field and coal bed gas(No.2011ZX05060-005) supporting this research
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