Bio-Electro-Fenton: a novel method for treating leachate in Da Phuoc Landfill, Vietnam

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 concentrations of pollutant and difficult to treat as leachate suggesting that the appropriate method for pre-treatment processes support the thorough elimination of pollutants.


INTRODUCE
The population explosion and industrialization in recent years have increased the demand for consumption of goods, materials, and energy which leads to a rapidly increasing amount of domestic waste generated. 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 domestic waste in Vietnam has been treated by the landfill method. When using this method, a considerable amount of leachate will be generated, which does harm to the environment because leachate contains heavy metals, ammonium, and POPs. The composition and characteristics of leachate are complicated by seasonal changes and burial time hence leachate treatment is extremely sophisticated. There are many different methods for treating leachate such as flocculation, adsorption, oxidation, etc. AOPs are the most outstanding treatment method which form hydroxyl ( • OH) free radicals to decompose 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 efficiently treat leachate. However, the traditional Fenton process requires the supply of a large amount of H 2 O 2 and Fe 2+ . Besides the amount of reagent added is unstable and requires partial treatment of the chemicals remain after the reaction. For Electro-Fenton, the process requires large energy for the generation of H 2 O 2 . To deal with these disadvantages, it is necessary 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 H 2 O 2 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 Vietnam, there has not been any previous article about BEF application in wastewater treatment. Therefore, in this study, we optimize the parameters affecting the BEF method in a complex matrix, with the purpose of evaluating the applicability of the method in leachate treatment. The experiments had been conducted to find optimal conditions through COD treatment efficiency and BOD 5 /COD ratio of leachate.

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. Samples had precipitated and stored in 2 plastic containers 30L. According to Table 1, leachate has neutral pH, COD is 4950 mgO 2 /L, BOD 5 is 1500 ± 59.7 mgO 2 /L, and BOD 5 /COD ratio of 0.3 is quite low. Da Phuoc landfill came into operating in 2007, after over 10 years, the main 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 ® 112) into two chambers. The volume 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 chamber 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 external transistor of 100 Ω 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 artificial wastewater (glucose 30 g/L, KH 2 PO 4 4.33 g/L, Na 2 HPO 4 2 g/L, NH 4 Cl 0.2 g/L, KCl 0.13 g/L) 1 to help anaerobic microorganisms grow and develop stably. The microorganisms decompose glucose to H + and produce electrons (Equation (1)): Then ion H + passed through the PEM to the cathode chamber. Due to the potential difference in voltage, electrons formed at the anode chamber will transfer the external resistor and to the cathode electrode.
In the cathode chamber, the pump supplies oxygen for the reaction (Equation (2)) to form H 2 O 2 . The amount of H 2 O 2 produced will immediately react with the amount of Fe (II) added to form hydroxyl radicals ( • OH) (Equation (3)) which oxidize persistent organic compounds in leachate (Equation (4)).
In this study, reaction in sample after treatment will be stopped immediately by adding NaOH 2.5 N to pH 10 -11 to precipitate iron, then heat at 70-80 0 C within 30 minutes to completely remove residual H 2 O 2 before conducting COD analysis 4 .

Data processing
-Processing efficiency (H%) is calculated according to the formula: Where: C 0 is the initial concentration (mg/L) C is the final concentration (mg/L)

RESULTS
To determine the maximum treatment efficiency of the method for leachate as well as evaluate the influence of important parameters in the BEF model, experiments are performed to optimize each parameter in a complex matrix of effects. The result show similarity to other Fenton processes, the optimal pH of BEF is at 3 (Figure 2). A larger concentration of Fe 2+ 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 BOD 5 /COD ratio increased when concentration [Fe 2+ ] increased from 1 g/L to 1.4 g/L, respectively. When the concentration [Fe 2+ ] is from 1.4 g/L to 4 g/L, the treatment efficiency increases linearly with [Fe 2+ ]. The BOD 5 /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 current to the system helps to accelerate the Fenton process 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 system 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.

Effect of pH on the Bio-Electro-Fenton process
Both of case pH is too low and too high are effect to the efficiency of the Fenton process 4 7,8 . The accumulation of protons due to the slow and insufficient protons diffusion through membrane would cause a decrease of pH in anode chamber 6 . Therefore, the cathode electrode material as a source to self-regulate the supply of Fe 2+ under neutral conditions is necessary to reduce the cost of pH adjusting chemicals.

Effect of [Fe 2+ ] on the Bio-Electro-Fenton process
Fe 2+ is an extremely important factor that directly affects the Fenton process. Considering that the H 2 O 2 production rate and yield could be constant at defined conditions in bio-electro-chemical system, Fe 2+ as catalyst could be a key factor for the final treatment performance 9 . As a result, a certain amount of Fe 2+ saves chemicals and makes the process more efficient. At low [Fe 2+ ] concentration, hydroxyl ( • OH) produced just enough to oxidize the biodegradable organic compounds. When the dose of Fe 2+ increased, amount of hydroxyl is more produced. The oxidation of organics with • OH occurs through well-known pathways, principally H atom abstraction (mainly from aliphatics) and addition to C = C bonds (mainly with aromatics leading to the formation of hydroxylated aromatic derivatives) 10 . The persistent organics changes into a biodegradable form, increasing the BOD value, increasing the BOD 5 /COD ratio and leading to a reduction in processing efficiency. At higher concentration [Fe 2+ ], from 4 g/L to 6 g/L, treatment efficiency decreases due to reduction of radicals • OH according to the reaction (Equation (8)) 11 : In addition, the Fe 3+ ions formed can react with H 2 O 2 to reduce the mineralization of organic substances (Equation (9)) 12 : Excess iron salts increase the amount of dissolved salt (TDS) and conductivity. Besides, after stopping the reaction, treated wastewater must be adjusted to neutral pH. pH raising create a large amount of iron deposits in the sludge 12 .

Effect of current intensity on the Bio-Electro-Fenton process
The current intensity produced by the microorganism system in the cathode chamber to create H 2 O, which is an extremely important catalyst in the BEF. Higher current will increase the amount of H 2 O 2 , thus increasing the number of ( • OH) hydroxyl radicals in the electrolyte environment. However, in the experiments, the efficiency of the current generation by the microorganism system was quite low, therefore to increase 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 microbial fuel cell (MFC). Besides, increasing the current too high will affect processing efficiency while wasting a significant amount of energy. In this case, optimized current intensity is usually chosen to attain the maximum H 2 O 2 production rate and yield, and its value quite depends on the cathode material used 6

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 various studies fluctuated between 30 minutes and 3 hours 13 . COD removal efficiency increased gradually and reached the highest of 62.42 ± 0.99 % at 60 minutes, BOD in leachate also increased from 50 mgO 2 /L to 191 mgO 2 /L. The complex organic is decomposed into simpler organic substances, thus reduced COD concentration, increased BOD concentration in wastewater, contributed to an increase BOD 5 /COD wrote the final manuscript. All authors approved the final manuscript.