Degradation and biodegradability improvement of the Landfill Leachate using Electrocoagulation with Iron and Aluminum Electrodes: A Comparative Study
Bharath M1*, B M Krishna 2, Manoj Kumar 3
1 Research Scholar, 2Associate Professor, 3Professor
Department of Environmental Engineering, Sri Jayachamarajendra College of Engineering, Mysuru 570006, India
*Corresponding author. Email: 1bharath571989@gmail.com
Abstract
This present study investigates the comparative study of iron and aluminum electrode for the treatment of landfill leachate by the Batch Electrocoagulation (EC) technique. The electrocoagulation performance used to determine the removal efficiency of COD and Color. The effects of an operating condition such as electrode material, stirring speed, inter-electrode distance, electrolysis time, initial pH, and applied voltage were studied to evaluate the performance of the electrode. The electrodes were arranged in a monopolar mode by using different cell voltages of 4, 6, 8, 10, and 12V for 180 min of electrolysis time (ET) with a varying inter-electrode distance between 1 to 4cm. The iron and aluminum electrodes can be successfully used as anodes and cathodes for the treatment process, which makes the process more efficient and easier to maintain. Based on the obtained results, it was observed that there was an increase in BOD/COD ratio from 0.11 to 0.79. The maximum removal of COD and Color was found to be 76.5% and 67.2%, respectively, and was accomplished with 105 min optimum electrolysis time with a pH of 9.25 using an Iron electrode. In the case of the aluminum electrode, the BOD/COD ratio was increased from 0.11 to 0.66. Over 78.4% of COD and 77.0% of Color removal was obtained with 90 min optimum electrolysis duration and pH 9.3 with an optimum 10 V and an optimum inter-electrode distance of 1 cm. However, the aluminum electrode is superior to iron as a sacrificial electrode material in terms of Color and COD removal efficiency. The aluminum electrode significantly treated landfill leachate by electrocoagulation method under optimum experimental conditions.
Keywords – Landfill Leachate; Electrocoagulation; Iron Electrode; Aluminum Electrode.
- Introduction
Leachate can be defined as water (rainwater or groundwater) that has percolated through solid waste. Rainfall is the main contributor to the generation of leachate(Abbas et al. 2009). Many factors affect the quality of leachates such as age, seasonal weather variation, precipitate, waste type, and waste composition, landfill leachate composition mainly depending on the age of the landfill (Silva et al. 2004). The main characteristic of leachate is BOD, COD, BOD/COD ratio, suspended solids, pH, ammonia-nitrogen, and heavy metals. Leachate may contain a large quantity of organic matter, biodegradable, humic-type constituents, chlorinated organic, and inorganic salts (Renou et al. 2008). Pre-treatment is required for the landfill leachate to meet the standards for its discharge as direct disposal into surface water or sewer. Many wastewater treatment technologies have been used to treat landfill leachate such as membrane process(Alizadeh et al. 2015) (Amokrane et al. 1997), Sequencing Batch Reactor (SBR) (Bashir et al. 2010; Khosravi et al. 2017; Laitinen et al. 2006; Neczaj et al. 2005), Coagulation-Flocculation (Amokrane et al. 1997), Constructed Wetland (Ogata et al. 2015) and Thermophilic Membrane Bioreactor (Visvanathan et al. 2007) have been used in the literature.
The electrocoagulation process has proven to be more economical, highly efficient in the removal of pollutants, and has been considered as a promising treatment technology. Hence EC process was applied for a variety of wastewater treatments such as Dairy wastewater (Kushwaha et al. 2010). Potato Chips manufacturing wastewater (Kobya et al. 2006). Distillery wastewater (Farshi et al. 2013) (Krishna et al. 2010), Dye wastewater (Riadi et al. 2017), Restaurant wastewater (Chen et al. 2000), Health care wastewater (Singh et al. 2018). The mechanism of EC reactions is as follows from equations 1 through 12.
Anode:
Fe(s) à Fe2+ (aq) + 2e– 1
Fe2+ (aq) + 2OH– (aq) àFe (OH)2(s) 2
Cathode:
2H2O +2e–àH2(g) +2OH– 3
Overall:
Fe(s) + 2H2O àFe(OH)2(s) + H2(g) 4
Oxidation:
2Cl–à Cl2 + 2e– 5
Cl2(g) + H2O àHOCl + H+ + Cl‑ 6
Fe(OH)2 + HOClà Fe(OH)3(s) + Cl– 7
Fe2+à Fe3+ + e– 8
Fe3+ + 3H2O à Fe(OH)3 + 3H+ 9
Depending on the pH range, the ferric ions generated from the electrocoagulation process may result in the formation of monomeric ions, ferric hydroxo complexes with hydroxide ions and polymeric species such as, Fe(OH)2+, Fe(OH)2+, Fe2(OH)24+, Fe(OH)4–, Fe(H2O)2+, Fe(H2O)5(OH)2+, Fe(H2O)4(OH)2+, Fe(H2O)8(OH)24+, Fe2(H2O)6(OH)42+, which finally converts into Fe(OH)3. The larger surface area resulted from freshly formed Fe(OH)3 are advantageous for the adsorption of soluble organic compounds and trapping of colloidal particles.( Kobya et al. 2003; Feng et al. 2007). The reaction when the iron is used as an electrode is given below:
nFe(OH)3à Fen(OH)3n 10
The amount and variety of hydrolysis products formed by anodic dissolution significantly depend on electrolysis time, when the iron is used as an electrode. The formed Fe(OH)n(s) complexes are in the form of gelatinous suspension. These gelatinous complexes may play a perfect role in the effective removal of pollutants. The various processes may be involved in the pollutant removal process such as neutralization of charge, adsorption, electrostatic attraction, and complexation.
The variety of monomeric and polymeric species formed due to Al3+and OH– ions generated by electrode reactions include Al(OH)2+, Al(OH)+2, Al(OH)4+2, Al(OH)–4, and Al6(OH)3+15, Al7(OH)4+17, Al8(OH)4+20, Al13O4(OH)7+24, Al13(OH)5+34 respectively. All these monomeric and polymeric species finally lead to the formation of Al(OH)3. The reaction when aluminum is used as an electrode is given below:
At anode: Al → Al3+ + 3e– 11
At cathode: 3 H2O + 3e → 3/2 H2 + 3OH– 12
Sparse research in open literature focusing on the BOD/COD ratio during and after electrocoagulation treatment of landfill leachate promotes this research work. The main significance of this research work is to study the degradation and biodegradability of landfill leachate aiming BOD/COD ratio and to optimize the process parameters such as inter-electrode distance, electrolysis time and voltage (current density) with main focus on COD and Color removal, and compare the investigations between iron and aluminum electrode.
- Materials and Methodology
- Study Area
The area selected for the study is in Mysuru city, Karnataka. It is located at 12.30°N 76.65°E, with an average altitude of 770 meters. The dumpsite was situated at Vidyaranyapuram, Mysuru, Karnataka. The dumping of waste in this area is being used from the past 6-7 years. The area consists of accumulated waste of about 2, 50,000 cum, and the area used for dumping of waste is approximately 41.47 acres. The present study attempts to treat landfill leachate using the electrocoagulation process. The sample landfill leachate was collected in the tank wherein; the leachate is coming from the pipes, which have shown in Figure. 2. The various physical and chemical parameters were analyzed in this study. The physical and chemical parameters in the initial characterization of the sample are shown in Table 1.
TABLE 1 Characterization of the landfill leachate
| No. | Parameters | Concentration |
| 1 | pH | 8.67 |
| 2 | Conductivity | 38.5 mS/cm |
| 3 | Turbidity | 140NTU |
| 4 | Total solids | 16760(mgL-1) |
| 5 | Total Dissolved Solids | 14580(mgL-1) |
| 6 | COD | 13760(mgL-1) |
| 7 | Phosphate | 208.5(mgL-1) |
| 8 | Total suspended solids | 1648(mgL-1) |
| 9 | Nitrates | 97.3(mgL-1) |
| 10 | BOD | 1519(mgL-1) |
| 11 | Chloride | 7034(mgL-1) |
| 12 | BOD/COD | 0.11 (mgL-1) |
| 13 | Color | 8750 PCU |
- Experimental Setup for Electrocoagulation
Electrochemical experiments were conducted on a plexi-glass laboratory scale. Batch electrochemical reactor 11cm × 14cm × 13cm of 2 L capacity with the working volume of 1.75L at room temperature was used in the setup. The reactor was kept under the process of continuous agitation using magnetic stirrer with 250 rpm to avoid the formation of concentration gradients. The T-shaped electrodes with the material of Iron and Aluminum plates with a size of 5 cm × 7 cm were used as both anode and cathode electrode (35 cm2 effective surface area). At the bottom of the electrodes, the gap of 2 cm was maintained to facilitate continuous and easy stirring. Before each treatment process, the electrodes were cleaned and degreased. The power supply used to run all experimental conditions was DC power. The distance between anode and cathode electrode was varied from 1cm to 4cm, wherein the voltage used in the electrolysis process was 4V. The duration of the electrolytic process was done for 180 mins with 15 mins time interval. Every 15 mins, the sample was collected for further processing. The collected samples after electrolysis were used to analyze the parameters such as voltage (current density), electrolysis duration, COD, Color, and pH. Among these analyzed parameters, pH, Electrolysis duration, as well as the distance between electrode and voltage (current density) was optimized in this study. The experimental setup for electrocoagulation to the lab-scale process is shown in Figure 1.
Figure 1 Experimental set up of electrocoagulation treatment in a lab-scale
- Result and Discussion
- Influence of inter-electrode distance:
The distance between the inter-electrodes has been studied as one of the parameters to minimize the consumption of electricity in the treatment of landfill leachate. The distance between the electrodes was varied at 1, 2, 3, and 4 cm. The increased percentage removal of COD and Color was observed with the decreased inter-electrode distance from 4 to 1 cm for both Fe and Al electrodes. The obtained results show the insignificant effect of inter-electrode distance on COD and Color removal percentage. The maximum removal efficiency was observed at 1 cm for the shortest distance between the electrodes with an electrode area of 35 cm2. Suppose the spacing was less than 1cm, it would disallow the flow of liquid absorbate in the intermediate space of the electrode and hence impede the removal efficiency.
Figure 2 (a-b) Effect of inter-electrode distance on percentage removal of COD in the leachate treatment by EC with Al and Fe electrode
The results show that the electrochemical method has significant efficiency in the removal of COD and Color. Greater efficiency was observed; 62.1% & 60.5% of COD and 52.0% & 47.5% of Color was removed by iron and aluminum electrode respectively, as shown in Figure 2(a-b). Figure 3(a-b) shows the efficiency of COD and Color, respectively, using different inter-electrode distance at the same experimental conditions. The ohmic potential drop is proportional to the distance between the electrodes. (Alizadeh et al. 2015).
As the inter-electrode distance is increased, the energy consumption also increases due to the electrostatic effect of the distance between the electrodes. The electric field can be controlled by changing the applied current, but once the distance between the electrodes changes, the electric current also varies. (Bouhezila et al. 2011). Further experiments were carried out by keeping 1cm spacing as optimum inter-electrode distance.
Figure 3 (a-b) Effect of inter-electrode distance on percentage removal of Color in the leachate treatment by EC with Al and Fe electrode
- Influence of applied voltage on the process efficiency
In electrocoagulation, voltage and electrolysis time are the important operational parameters to be set effectively, for the crucial removal of leachate under defined electrical energy and consumption of power. The electrocoagulation experiment was carried out for different voltages such as 4V, 6V, 8V, 10V, and 12V.
The removal of COD and Color was found to be extreme at 10V for both Fe and Al electrodes ascribed to the reaction between organic compounds with that of Fe and Al ions and formation of insoluble products (Alimohammadi et al. 2017). The removal efficiencies for iron and aluminum was found to be 76.5% & 78.4% of COD respectively as shown in Figure 4(a-b) and 67.21% & 77.0% of Color respectively as shown in Figure 5(a-b), at an optimum 10V and optimum time of 105 min for Fe and 90 min for Al electrodes. The results suggested that, if the voltage in the electrocoagulation increases, the treatment efficiency also increases. The aluminum electrode was more efficient as compared to the iron electrodes. Another research group had shown that an increased voltage resulted in an increased treatment efficiency; this might lead to increased coagulant dose and bubble generation rate (Golder et al. 2007). Further increases in voltage would result in the faster dissolution of anode material. Removal of COD by an electrochemical method is by oxidizing organic matter, and producing oxidant agents such as hydroxyl radicals (•OH) or hypochlorite (HOCl) (if Cl− is present) (Pirsaheb et al. 2016). It was observed that the COD removal decreased after 100 min electrolysis time because the chloride ions in the wastewater go exhausted in the influence of high voltage (Singh et al. 2019). It was noticed that the rate of COD and Color removal was relatively high at 10 V compared to 12V.
Figure 4(a-b) Effect of applied voltage on leachate treatment by EC (COD Removal) for Al and Fe electrode.
Hence, iron and aluminum electrodes can be successfully used as anode and cathodes for electrocoagulation processes due to its increased efficiency and easier maintenance. Faraday’s law explains that the applied current was directly proportional to the amount of ionized metal. Hence the COD removal was high, and an increased current density was noticed.
Figure 5(a-b) Effect of applied voltage on leachate treatment by EC (Color Removal) for Al and Fe electrode.
When the current density increases, it would result in the generation of more magnesium ions, which was favorable for co-precipitation and electrocoagulation methods. (Asselin et al. 2008), this group has shown that a decrease in COD level was due to the destabilization of colloidal organic compounds and combined effects of cathodic reduction. They have also observed a thin brownish layer deposited on the surface of the cathodic electrode after the process of electrocoagulation, which is an indication of the cathodic reduction phenomenon.
- Influence of pH changes during electrochemical treatment:
It was observed that the pH of the landfill leachate in the electrocoagulation process had been raised from 8.67-9.25 for iron electrode and 8.67- 9.31 for aluminum electrode at 10V as shown in Figure 6(a-b). The highest COD and Color removal were obtained at pH 9.25 for the iron electrode and pH 9.31 for the aluminum electrode. At the end of the processing time, the results specified that based on the activeness of the anode and cathode, pH in the process will increase. This is due to the foremost activities at the cathode (Ilhan et al. 2008), which resulted from the generation of hydroxide ions at the cathode through the electrochemical reduction of water (Oumar et al. 2016). De-colorization of effluent is very low at acidic pH of the medium, whereas it is very high at neutral or alkaline conditions (Huda et al. 2017). The formed iron hydroxides remain as suspension, which induces the removal of pollutants through adsorption, coagulation, and co-precipitation under alkaline conditions (Gengec et al. 2012). This leads to the increased removal of Color under neutral and alkaline pH. In the electrocoagulation process, the pH of the water was found to be high due to an ammonia stripping process (Ilhan et al. 2008). Some of the research groups have found that the variation in the pH of the medium did not significantly alter the removal of COD in the treatment process (Deng and Englehardt 2007). Another research group also reported that the COD removal in the treatment process at alkaline condition, i.e. pH 8.9 and 10 was achieved, 4% higher compared to the neutral condition, i.e. 7.5 (Wang et al. 2001). Hence, it is proved that the alkaline condition is more favorable for the treatment of landfill leachate wastewater.
Figure 6(a-b) Effect of applied voltage on leachate treatment by EC (pH) for Al and Fe electrode.
- Effect of BOD/COD ratio in the electrocoagulation process
In Figure 7(a-b), it is observed that there was an improvement in the biodegradability of landfill leachate evaluated through the evolution of the BOD/COD ratio. When iron was used as an electrode, it was observed that the ratio of BOD/COD was increased from 0.11 to 0.79. It was found to have the maximum removal percentage of COD and Color at the optimum experimental condition at 10V with an inter-electrode distance of 1cm for 180min. Similarly, when aluminum was used as electrode BOD/COD ratio was found to be increased from 0.11 to 0.66 at an optimum experimental condition. As time passes, COD degraded with the time for the iron electrode. There was an improvement in the BOD/COD ratio. When the voltage increases, the degradation of COD also increases, and that increases the effluent BOD/COD ratio. In case of the aluminum electrode, gradually BOD/COD value increases along with the time and COD degradation is high when compared to the iron electrode, and it is therefore confirmed that BOD/COD ratio increases with the increase in voltage, and that can be observed with both the iron and aluminum electrodes. This is due to an increasing voltage which increases the overall potential essential for the generation of chlorine/hypochlorite. The low BOD/COD ratio in the effluent indicates that it contains recalcitrant substances which are not easily biodegradable or non- biodegradable material present in leachate (Visvanathan et al. 2007).
Figure 7(a-b) Effect of BOD/COD ratio on leachate treatment by EC for Al and Fe electrode.
- Electrode dissolution pattern
The electrode dissolution (ED) has a vital role in the electrocoagulation process, whereas it offers information on the amount of consumption of electrode per kilogram of removed COD per cubic meter of wastewater to be treated. This helps to estimate the operational treatment cost. The removal of contaminants/pollutants from the wastewater will be assisted by electrode dissolution by the formation of electro-floc, which is an essential part of any of the treatment methods. Figure 8(a-b) represents ED for different cell voltages such as 4,6,8,10, and 12V with 1cm of inter-electrode distance and Fe and Al electrodes arranged in a monopolar mode in a batch reactor, 24.3g and 5.86g of electrode dissolution of Fe anode and Al anode electrode for 105 min and 90 min electrolysis duration respectively, as shown in the Figure. Aluminum electrodes are more effective than the iron electrodes. In monopolar mode, ED will be strongly influenced by the position of the electrodes and the applied voltage with the corresponding current across two electrodes.
The higher electrode dissolution rate is obtained at the positive terminal (anode) connected electrode as compared to the negative terminal (cathode) connected electrode. In cathode, there was low electrical resistance and hence ED is least. When the anode and cathode electrodes are close to each other, it shows the increased oxidation rate producing more coagulant generation because of the higher current between the electrodes. If the inter-electrode distance is too close, it can cause short-circuiting during the treatment process. (Singh et al. 2018).
Figure 8(a-b) Effect of electrode dissolution pattern for Fe and Al electrode
- Conclusions
The present paper shows the performance of electrocoagulation using the Al and Fe electrode for the treatment of landfill leachate. The effect of initial pH, inter-electrode distance, electrolysis time and applied voltage were studied on COD and Color removal. Experiments were carried out for different applied voltages 4, 6, 8, 10, and 12V using Al and Fe electrodes for an electrolysis time of 180 min. The electrodes were placed at an inter-electrode distance varying from 1 to 4 cm and connected in a monopolar mode. The maximum removal of COD and Color using the Fe electrode was found to be 76.5% and 67.2% respectively for an applied voltage of 10 V, pH 9.25, and 1 cm distance for 105 min electrolysis time. At that time, the BOD/COD ratio increased from 0.11 to 0.79. Removal of COD and Color using Al electrodes were found to be 78.4% and 77.0% respectively for an applied voltage of 10 V, pH 9.3, and 1 cm distance for 90 min electrolysis time. At that time, the BOD/COD ratio increased from 0.11 to 0.66. The overall data thus showed that aluminum electrode was more efficient than iron electrode material in treating landfill leachate, in terms of COD and Color removal.
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