Bio-Electro-Fenton: A novel method for treating leachate in Da Phuoc Landfill, Viet Nam

Science & Technology Development Journal, 23(1):461-469 Open Access Full Text Article Research Article Faculty of Environment, University of Science, Vietnam National University - Ho Chi Minh City Correspondence Ho Truong NamHai, Faculty of Environment, University of Science, Vietnam National University - Ho Chi Minh City Email: htnhai@hcmus.edu.vn History  Received: 2019-12-14  Accepted: 2020-02-17  Published: 2020-03-19 DOI : 10.32508/stdj.v23i1.1736 Copyright © VNU-HCM Press. This is an open- access article distributed under the terms of the Creative Commons Attribution 4.0 International license. Bio-Electro-Fenton: a novel method for treating leachate in Da Phuoc Landfill, Vietnam Ho Nhut Linh, Ho Truong NamHai* Use your smartphone to scan this QR code and download this article ABSTRACT Introduction: Leachate is a noticeable pollution problem because it contains a considerable amount of persistent organic pollutants (POPs). If leachate isn't treated thoroughly, its leak will negatively affect the environment. Therefore, appropriate treatment technologies are required to remove them. Bio-Electro-Fenton (BEF) is a newmethod usingmicroorganisms such as electrolytes to convert chemical energy into electricity to help createH2O2 support advancedoxidation process (AOPs). Realizing the potentials that BEF brings, this study applies BEF to assess the effectiveness of leachate treatment at Da Phuoc landfill (operation period > 12 years), Ho Chi Minh City, which to save costs and energy for Fenton process. Methods: The BEF pilot scale model (30 x 10 x 10 cm) is divided by a proton exchange membrane (PEM) (Nafion®112) into two chambers (anode and cathode). Cathode chamber used a graphite electrode, the anode chamber used a carbon fabric electrode. The experiments aimed to determine the optimal conditions of parameters affecting the BEF system by determining the efficiency of COD removal and BOD5/COD ratio in leachate. Results: At optimal conditions of the model including pH 3, [Fe2+ ] = 4g/L, current intensity = 1A, reaction time 60 minutes and airflow = 12 L/min, as a result COD was reduced by 68.2  1.04 % from 4950 14 mgO2/L to 1574.1  51.4 mgO2/L, the ratio of BOD5/COD = 0.1 Conclusion: The study result showed thatBio-electro-Fenton process is effective for wastewater with high concen- trations of pollutant and difficult to treat as leachate suggesting that the appropriate method for pre-treatment processes support the thorough elimination of pollutants. Key words: Leachate, Bio-Electro-Fenton, AOPs, treatment. INTRODUCE The population explosion and industrialization in re- cent years have increased the demand for consump- tion of goods, materials, and energy which leads to a rapidly increasing amount of domestic waste gen- erated. According to the estimation of The National Environment Statistics report in 2017 of The Ministry of Natural Resources and Environment, the amount of domestic solid waste in urban areas increases by an average of 10 - 16% per year. The majority of do- mestic waste in Vietnam has been treated by the land- fill method. When using this method, a consider- able amount of leachate will be generated, which does harm to the environment because leachate contains heavy metals, ammonium, and POPs. The composi- tion and characteristics of leachate are complicated by seasonal changes and burial time hence leachate treat- ment is extremely sophisticated. There are many different methods for treating leachate such as flocculation, adsorption, oxidation, etc. AOPs are themost outstanding treatmentmethod which form hydroxyl (OH) free radicals to decom- pose organic pollutants base on characteristics such as non-selective pollutants oxidation, easily react at room temperature. Typical AOPs such as tradition Fenton and Electro-Fenton have been proven to effi- ciently treat leachate. However, the traditional Fen- ton process requires the supply of a large amount of H2O2 and Fe2+. Besides the amount of reagent added is unstable and requires partial treatment of the chem- icals remain after the reaction. For Electro-Fenton, the process requires large energy for the generation of H2O2 . To deal with these disadvantages, it is nec- essary to progress a low-cost Fenton process which ensures treatment efficiency, therefore BEF method was invented. The BEF method uses microorganisms to decompose organic matter creating energy to form H2O2 for Fenton processes. As a result of the BEF method is a multipurpose method which save costs by reducing electrical energy consumption and using chemicals. The BEF is a completely new technology. In Viet- nam, there has not been any previous article about BEF application in wastewater treatment. Therefore, in this study, we optimize the parameters affecting the BEFmethod in a complexmatrix, with the purpose of Cite this article : Nhut Linh H, Truong Nam Hai H. Bio-Electro-Fenton: a novel method for treating leachate in Da Phuoc Landfill, Vietnam. Sci. Tech. Dev. J.; 23(1):461-469. 461 Science & Technology Development Journal, 23(1):461-469 evaluating the applicability of the method in leachate treatment. The experiments had been conducted to find optimal conditions through COD treatment effi- ciency and BOD 5/COD ratio of leachate. MATERIALS ANDMETHODS Sampling Samples had collected at the Da Phuoc Solid Waste Treatment Complex in Ho Chi Minh City in March 2019. Sampling, transportation, and preservation techniques complied with TCVN 5999: 1995. Sam- ples had precipitated and stored in 2 plastic containers 30L. According to Table 1, leachate has neutral pH, COD is 4950 mgO2/L, BOD5 is 1500  59.7 mgO2/L, and BOD5/COD ratio of 0.3 is quite low. Da Phuoc land- fill came into operating in 2007, after over 10 years, themain component of leachate is organic substances, which are difficult or non-biodegradable. Leachate is gradually shifted to the stable phase. BEF pilot system and operation BEF pilot scale model (30 x 10 x 10 cm) divided by a PEM (Nafion r112) into two chambers. The vol- ume of each chamber (anode and cathode chambers) was 1.5 L with a working volume of 1.134 L. Cathode chamber used a graphite electrode, the anode cham- ber used a carbon fabric electrode with size (7.5 x 5 x 0.4 cm). The electrodes connected with copper wire of 2 mm diameter and 40 cm length through an exter- nal transistor of 100W and DC supply with voltage 0 - 30V, current 0 - 5A to adjust to each test requirement (Figure 1a, c). At the first stage of the survey, anode chamber was loaded with anaerobic sludge and 500 mL of artifi- cial wastewater (glucose 30 g/L, KH2 PO4 4.33 g/L, Na2HPO4 2 g/L, NH 4Cl 0.2 g/L, KCl 0.13 g/L) 1 to help anaerobicmicroorganisms grow and develop sta- bly. The microorganisms decompose glucose to H+ and produce electrons (Equation (1)): C6H12O6+6H2O! 6CO2+24H++24e (1) Then ion H+ passed through the PEM to the cath- ode chamber. Due to the potential difference in volt- age, electrons formed at the anode chamberwill trans- fer the external resistor and to the cathode electrode. In the cathode chamber, the pump supplies oxygen for the reaction (Equation (2)) to form H2O2. The amount of H2O2 produced will immediately react with the amount of Fe (II) added to form hydroxyl radicals (OH) (Equation (3)) which oxidize persis- tent organic compounds in leachate (Equation (4)). In addition, the energy generated helps restore Fe2+ (Equation (5)). O2+2H++2e! H2O2 (2) H2O2+Fe2+! OH+OH+Fe3+ (3) OH+RP! oxidationproducts (CO2;H2O) (4) Fe3++ e! Fe2+ (5) All chemicals and reagents used for the experiments were of analytical grade and supplied by Merck (Ger- many). In each experiment, 500 mL of leachate was added at the cathode chamber, pH was adjusted us- ing NaOH 1N, H2SO4 1N. The agents used in the Fenton process includes FeSO4.7H2O 5%. All exper- iments were performed at 30oC temperature and at- mospheric pressure in a batch mode manner. Data are representative with three replicates and their av- erage are reported. Chemical analysis pH value of wastewater wasmeasured by Schott - LAB 850 - Germany. The parameters COD, BOD5, Total Phosphor, Total Nitrogen, and TSS were determined according to the Standard Methods for the examina- tion of water and wastewater2. COD, BOD5 were observed throughout the exper- iment. For the Bicromate method, the amount of residual H2O2 and Fe2+ in sample after the reaction affect the results of COD determination according to the reaction (Equations (6) and (7))3. Cr2O27 +3H2O2+8H +! 2Cr3++ 3O2+7H2O (6) Cr2O27 +Fe 2++H+! Cr3++Fe3++H2O (7) In this study, reaction in sample after treatmentwill be stopped immediately by adding NaOH 2.5 N to pH 10 - 11 to precipitate iron, then heat at 70-800Cwithin 30 minutes to completely remove residual H2O2 before conducting COD analysis4. Data processing - Processing efficiency (H%) is calculated according to the formula: H = C0 C C0 100 Where: C0 is the initial concentration (mg/L) C is the final concentration (mg/L) 462 Science & Technology Development Journal, 23(1):461-469 Table 1: Composition, characteristics of Da Phuoc leachate Parameter Unit Result Viet Nam National Technical Regula- tion – 25:2009/MONRE Column B1 pH - 7.8 - BOD5 mgO2/L 1500 59.7 100 COD mgO2/L 4950 14 400 BOD5/COD - 0.3 - Total Phosphor mg/L 12.4 0.43 - [Fe] total mg/L 44.1 2.24 - Total Kjeldahl Nitrogen mg/L 1477 62.6 60 Ammonium mg/L 589.67 22.11 25 Total Suspended Solid mg/L 15.3 - Figure 1: (a) Photograph (b) Electro transfer mechanism in the BEF system (c)Schematic drawing of the experi- mental setup of the BEF system 463 Science & Technology Development Journal, 23(1):461-469 Figure 2: COD removal efficiency (%) (SD) and BOD5/COD ratio of treated leachate following various pH in cathode chamber onthe Bio-Electro-Fenton process. Reaction conditions: [Fe2+]= 1.4 g/L, reaction time 60 minutes, current intensity = 1 A, airflow = 4 Lair/minute RESULTS To determine the maximum treatment efficiency of the method for leachate as well as evaluate the influ- ence of important parameters in the BEF model, ex- periments are performed to optimize each parameter in a complex matrix of effects. The result show sim- ilarity to other Fenton processes, the optimal pH of BEF is at 3 (Figure 2). A larger concentration of Fe2+ catalyst will increase COD treatment efficiency significantly, up to 4 g/L. However, when concentration is too high, it will reduce processing efficiency. Treatment efficiency tends to decrease at increasing iron concentration (Figure 3). The COD removal efficiency decreased from 54.82  2.04 % to 51.22 1.53 %, while the BOD5/COD ratio increased when concentration [Fe2+] increased from 1 g/L to 1.4 g/L, respectively. When the concentration [Fe2+] is from 1.4 g/L to 4 g/L, the treatment efficiency increases linearly with [Fe2+]. The BOD5/COD ratio reached the highest values of 0.15 at 3 g/L and the lowest value of 0.04 at 6 g/L. For current intensity, the addition of an external cur- rent to the system helps to accelerate the Fenton pro- cess due to electrons are increased with electrons made from anaerobic organisms decompose glucose in the anode chamber. The optimum current for the system is recorded at 1 A (Figure 4). The reaction time and airflow rate provided to the sys- tem were also surveyed. (Figures 5 and 6) The reac- tion time of 60 minutes, the airflow rate of 12 L/min yiel ded the highest COD removal efficiency, reaching 62.42 0.99 % and 68.20 1.04 %, respectively. DISCUSSION Effect of pH on the Bio-Electro-Fenton pro- cess Both of case pH is too low and too high are effect to the efficiency of the Fenton process4. The Fenton pro- cess can be inhibited because of very low pH values (pH < 3). Fe2+ , which exists as Fe[H2O6]2+ , has a slower reaction rate with H2O2 than Fe[H2O5]2+ at pH = 3 which leads to less hydroxyl (OH) genera- tion5. Besides, when the pH is too low H+ will react with H2O2 to form peroxone (H3 O2+) according to the reaction H2O2 + H+ ! H3 O2+. The peroxone do not react with Fe2+ therefore it will decrease OH formation efficiency which leads to reduces the pro- cessing efficiency of the Fenton process. At high pH (pH  4), the formation of ferrous/ferric hydroxide complexes leads to the catalyst deactiva- tion, which decreases the quantity of OH6. In ad- dition, the decomposition of H2O2 into H2O and O2 also increases with increasing pH7,8. The accumula- tion of protons due to the slow and insufficient pro- tons diffusion through membrane would cause a de- crease of pH in anode chamber6. Therefore, the cath- ode electrode material as a source to self-regulate the supply of Fe2+ under neutral conditions is necessary to reduce the cost of pH adjusting chemicals. 464 Science & Technology Development Journal, 23(1):461-469 Figure 3: COD removal efficiency (%) (SD) and BOD5/CODratio of treated leachate following various Fe2+concentration in cathode chamber on the Bio-Electro-Fenton process. Reaction conditions: pH = 3, re- action time 60 minutes, current intensity = 1 A, airflow = 4 Lair/minute Figure 4: COD removal efficiency (%) (SD) and BOD5/CODratio of treated leachate following various cur- rent intensity incathode chamber on the Bio-Electro-Fenton process. Reaction conditions: pH = 3, reaction time 60 minutes, [Fe2+] = 4 g/L, airflow = 4 Lair/minute 465 Science & Technology Development Journal, 23(1):461-469 Figure 5: COD removal efficiency (%) (SD) and BOD5/COD ratio of treated leachate following various re- action times on the Bio-Electro-Fenton process. Reaction conditions: pH = 3, [Fe2+] = 4 g/L, current intensity = 1 A, airflow = 4Lair/minute Figure 6: COD removal efficiency (%) (SD) and BOD5/COD ratio of treated leachate following various air- flow into cathode chamber on the Bio-Electro-Fenton process. Reaction conditions: pH = 3, [Fe2+] = 4 g/L, reaction time 60 minutes, current intensity = 1A 466 Science & Technology Development Journal, 23(1):461-469 Effect of [Fe2+ ] on the Bio-Electro-Fenton process Fe2+ is an extremely important factor that directly af- fects the Fenton process. Considering that the H2O2 production rate and yield could be constant at de- fined conditions in bio-electro-chemical system, Fe2+ as catalyst could be a key factor for the final treatment performance9. As a result, a certain amount of Fe2+ saves chemicals and makes the process more efficient. At low [Fe2+ ] concentration, hydroxyl (OH) pro- duced just enough to oxidize the biodegradable or- ganic compounds. When the dose of Fe2+ increased, amount of hydroxyl is more produced. The oxida- tion of organics withOHoccurs throughwell-known pathways, principally H atom abstraction (mainly from aliphatics) and addition to C = C bonds (mainly with aromatics leading to the formation of hydrox- ylated aromatic derivatives)10. The persistent or- ganics changes into a biodegradable form, increasing the BOD value, increasing the BOD5/COD ratio and leading to a reduction in processing efficiency. At higher concentration [Fe2+], from 4 g/L to 6 g/L, treatment efficiency decreases due to re- duction of radicalsOH according to the reaction (Equation (8))11: OH+Fe2+! Fe3++OH (8) In addition, the Fe3+ ions formed can react with H2O2 to reduce the mineralization of organic sub- stances (Equation (9))12 : H2O2+Fe3+! Fe2++OOH+H+ (9) Excess iron salts increase the amount of dissolved salt (TDS) and conductivity. Besides, after stopping the reaction, treated wastewater must be adjusted to neu- tral pH. pH raising create a large amount of iron de- posits in the sludge12. Effect of current intensity on the Bio- Electro-Fenton process The current intensity produced by themicroorganism system in the cathode chamber to create H2O, which is an extremely important catalyst in the BEF. Higher current will increase the amount of H2O2 , thus in- creasing the number of (OH) hydroxyl radicals in the electrolyte environment. However, in the experi- ments, the efficiency of the current generation by the microorganism system was quite low, therefore to in- crease the processing efficiency, the experiment used an external power (DC) to connect the BEF system to supplement the process. The technology converts from microbial electrolysis cells (MEC) to the micro- bial fuel cell (MFC). Besides, increasing the current too highwill affect processing efficiencywhile wasting a significant amount of energy. In this case, optimized current intensity is usually chosen to attain the max- imum H2O2 production rate and yield, and its value quite depends on the cathode material used 6. The current increased (0,5 to 1A) with the increas- ing rate of pollutant degradation (49.60  2.17 % to 62.42 1.81 %) since more HO are formed at a given time. However, the COD concentration decreased as the current increased above 1A, the efficiency de- creased from 62.42  1.81 % to 50.75  1.72 % at 2.5A. The current cannot be increased indefinitely since the cathode potential would be changed by the applied voltage, resulting in the side reactions, and thereby decreasing current efficiency and pollutants removal efficiency. The side reactions can involve: (i) high current density by adding high external volt- age would enhance the H2O2 electrochemical reduc- tion through (Equation (10)), (ii) the H2O2 reaction with Fe3+ via (Equation (11)) and (Equation (12)) (iii) the destruction of OH with H2O2 and Fe2+ via (Equation (13)) and (Equation (8))6 , (iv) The reac- tions forming H2 at the cathode are more dominant via (Equation (14))4. H2O2+2H++2e! 2H2O (10) Fe3++H2O2 ! Fe2++HO2+H+ (11) Fe3++HO2 ! Fe2++O2+H+ (12) H2O2+ OH! H2O+HO2 (13) 2H++2e! H2 (14) Effect of reaction time on the Bio-Electro- Fenton process The time needed to complete a Fenton reaction will depend on the many variables such as catalyst dose and wastewater strength. For more complex or more concentrated wastes as leachate, the reaction in var- ious studies fluctuated between 30 minutes and 3 hours13. COD removal efficiency increased gradually and reached the highest of 62.42  0.99 % at 60 min- utes, BOD in leachate also increased from 50mgO2/L to 191 mgO2/L. The complex organic is decom- posed into simpler organic substances, thus reduced COD concentration, increased BOD concentration in wastewater, contributed to an increase BOD5/COD 467 Science & Technology Development Journal, 23(1):461-469 Figure 7: Samples of current intensity after processing. ratio. Increasing the reaction time will create more OH radicals to form H2O2, while also fostering the Fenton reactions occur more to thoroughly oxidize the pollution. After the equilibrium time, COD concentration de- creased, the BOD5/COD ratio did not change signifi- cantly. The BOD concentration raised, this can be ex- plained by the persistent organic pollutant degrada- tion which still continues to occur but tends to slow down. In addition, intermediates created are more difficult to oxidize, which inhibits the Fenton reac- tion. The amount of Fe2+ when being regenerated will be oxidized to Fe2O3, thus reducing treatment ef- ficiency 4. Effect of airflow on the Bio-Electro-Fenton process Oxy is one of the key parameters in the BEF system, which is the electron acceptor in the cathode cham- ber to produce H2O2. High airflow rate could en- hance the dissolved oxygen in solution and promote the oxygen mass transfer rate, and thus, is beneficial forH2O2 production and accumulation in bioelectro- chemical systems6. It is important to set the optimal airflow rate because if the speed is too low, it will not maintain enough dissolved oxygen and if the speed is too high, it will cost a lot of operating energy for the system. COD treatment efficiency increased with increasing airflow rate and reached 68.20  1.04 % (Figure 6) at the maximum airflow rate of 12 L air/minute. The reason is that the excessive high airflow rate could also disturb the mass transfer between catholyte and elec- trode and lead to a low catalytic efficiency for pol- lutants degradation by the Fenton process6. The ra- tio of BOD5/COD also increased significantly due to (OH) hydroxyl free radicals non-selectively re- acts with POPs compounds to create more easily biodegradable compounds. However, when continuing to increase the airflow rate, from 12 to 16 L air/minute, COD treatment effi- ciency decreased from 68.20 1.04 % to 62.22 1.12 %, respectively. This is explained by the extremely high airflow rate which leads to a chemical imbalance of reaction (2) O2 + 2H+ + 2e!H2O2 and reduces the accumulation of H2O2 14 . CONCLUSIONS The results obtained in the study show that the use of the Bio-electro-Fenton process is effective for wastew- ater with large concentrations and difficult to treat as leachate. At optimal conditions of the model includ- ing pH 3, [Fe2+] = 4g/L, current intensity = 1A, re- action time 60 minutes and airflow = 12 LO2/min, as a result COD from 4950  14 mgO2/L to 1574.1  51.4 mgO2/L (decreased 68.2  1.04 %). The ratio of BOD5/COD decreased from 0.3 to 0.1 due to Fen- ton reactions which reduced a large amount of easily biodegradable organic matter, suggesting that the ap- propriatemethod for pre-treatment processes support to thoroughly eliminate pollutants. LIST OF ABBREVIATIONS AOPs: Advanced oxidation process BEF: Bio-electro-Fenton PEM: Proton exchange membrane POPs: Persistent organic pollutants AUTHORS’ CONTRIBUTIONS The author Ho Nhut Linh did the experiment. The author Ho Truong NamHai discussed the results and 468 Science & Technology Development Journal, 23(1):461-469 wrote the final manuscript. All authors approved the final manuscript. COMPETING INTERESTS The authors declare that they have no competing in- terests. 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