Treatment of landfill leachate in a membraneless bio-cathode microbal fuel cell and production of electrical energy

The aim of this study is to treat the leachate that is originated from solid waste landfill via a new and promising technology that is named as Microbial Fuel Cell (MFC). It is also an alternative method for wastewater treatment instead of conventional treatment systems such as anaerobic treatment and activated sludge process. MFC systems have also the potential of electrical energy production beside with the wastewater treatment. So it is aimed to determine the treatment efficiency of a membraneless bio-cathode MFC and the power production rate when treating leachate. In order to select parameters to observe when the process is on, it is best to know the characteistics of the leachate. The leachate is taken from a landfill in Konya region, South of Turkey. It is well known that landfill leachate has a complex content and can be highly organic polluted. Leachate may include heavy metal concentrations, non-biodegradable organics and excess amount of ammonia which has inhibitory effect to microorganisms. As a result, treatment of leachate becomes very difficult and expensive. On the other hand microbial fuel cell technology is a new method to produce bioenergy while targetting to overcome the disadvantages such as high expenditures for aeration process and high amount of waste sludge that are faced during the activated sludge process. In an MFC, the basic principle is to recover the electrical energy equivalent that soluble organic molecules have. Carbohydrates, proteins and volatile fatty acids are used by bacteria for metabolic activities and energy. The remaining part of the organic materials can be directly recovered as electrical energy. At the anode chamber of an MFC organic molecules are degraded lastly into electrons (e-), protons (H+) and CO2. The microorganisms are responsible of this process are called "anodophilic bacteria". They are mostly grown on a conductible surface, anode electrode. After their metabolic activities, anodophilics transfer the electrons to the anode surface, protons to the environment. Electrons are delivered to the cathode chamber by an external electrical circuit that contains a resistor connected to the both electrodes in the anode and cathode chambers. While electrons are transferred to the cathode, protons are selectively flow from anode through a membrane to the cathode. At the cathode chamber, electrons, protons and an electron acceptors such as, oxygen (O2), potassium hexacyanoferrate (also known as ferricyanide, K3Fe(CN)6, nitrate (NO3-) and sulfate (SO4-) come together and form water and the other final elements. In all bioelectrochemical systems (BES) physical structures as well as environmental conditions such as pH, conductivity, temperature etc. are very important for the effective power production and wastewater treatment. The anode and cathode chambers` shape, feeding type, substrate type, electrodes (granular carbon, carbon sheet, grafit plate, carbon rod, carbon brush), electrode surface area, catalyzer metals ( platinum, titanium, manganese, etc.), membrane type, membrane surface area are the most important physical structures of the MFC systems. Sometimes these structural specifications make the MFC`s cost not affordable. So it is crucial to select cost effective materials and maintain the MFC process sustainable. According to the general knowledge given above, two similar MFCs were constructed for this study. One of them was aerated membraneless bio-cathode MFC and the other one was the anaerobic membraneless bio-cathode MFC. MFCs were composed of two identical chambers, 0,85 L volume of anode and 0,85 L volume of cathode. Leachate was feed to the anode chamber and it flowed from the bottom/top of the anode to the bottom of the cathode. After leachate left he cathode chamber it was collected and analyzed. Since the study was separated into two periods, granular carbon was used as electrodes in both chambers at the first period, between day 1 and day 51. At the second period, between day 52 and day 103, carbon cloth was used as the anode electrode and granular carbon was used as the cathode electrode. The main operational difference between the two periods was the cross sectional area that water and ions could penetrate from anode chamber to the cathode chamber. At the first period the cross sectional area between the chambers was only 0.2 cm2 that leachate flowed from anode to cathode. At the second period the cross sectional area between the anode and the cathode was 9 cm2. And j-cloth was used to seperate the chambers and prevent short circuit between the chambers at the secon period. The reason that cross sectional area and the electrodes were changed at the second period was the oxidation and corrosion of the copper wire at the aerated bio-cathode. At the second period titanium wire was used to connect the electrodes to the circuit instead of copper wire. Both periods were also seperated into two parts according to the organic loading rates (OLR) of the feeding to determine the effect of OLR on the MFCs. There was no additives used to adjust the pH of the MFCs and no expensive materials, catalyzers and membranes were used in the construction of the MFCs. Finally the effects of organic loading rate, the amount of area that ions and water flowed through from anode to cathode and the feeding type on MFC`s performance were observed. It was seen that cross sectional flow area between chambers is very important for the power and current production. When the cross sectional area between anode and cathode increased from 0.2 cm2 to 9 cm2, power production increased from 5-6 mW/m3 to 15-20 mW/m3. And after increasing the OLR from 0.88 g COD/L.d to 1.75 g COD/L.d at the second part of second period, power production increased from 15-20 mW/m3 to 35-40 mW/m3. Thus a current density of 149 mA/m3 and a power density of 37,7 mW/m3 were obtained with the organic loading rate of 1,75 g COD/L.day and hydraulic retention time of 2 days in a membraneless bio-cathode microbial fuel cell. With this simple and low cost constructed MFC, depending on feeding type and organic loading rate, chemical oxygen removal ratio between 30-50 % and ammonium removal ratio between 10-30 % were achieved. At the end of the study, it is thought that MFC performance was inhibited due to the high ammonium concentration (1350~1400 mg NH4+/L) and high pH value (pH≈8,2) in the reactor environment that was originated from the leachate. When the power and current production of this membraneless bio-cathode MFC compared to other MFC studies conducted with leachate, our results might seem lower than the others. It can be seen from other studies, power productions as high as 500 mW/m3 and treatment efficiencies as high as % 70-90 are achieved. It is because, generally high cost materials, catalyzers and reactors and operational interference are applied to these studies. On the other hand the results obtained in our study are appropriate with the results that are obtained from membraneless MFC studies. So it can be concluded that although membrane is expensive to be used in MFC studies, it is very effective to increase the power outputs. To increase the power production and treatment efficiencies of the MFCs in studies such as ours, low cost metals that can fasten the reaction can be used as catalyzers on the electrode material. Electrodes can be covered with these metals. Also, since leachate include heavy metals that can interrupt the microorganism activity, they should be settled prior to feeding. Lastly, because leachate has high ammonium concentration, it can be a good alternative to mix the leachate with a wastewater that is known to be rich in carbohydrate. So the leachate`s C/N ratio will be suitable for the microorganisms. If not anything, adjusting pH with a proper solution is a good way to surpass the ammonium inhibition.