Ethanolamine Degradation and Energy Recovery using Single Air-Cathode Microbial Fuel Cell

Abstract

Monoenthanolamine solution is used to remove sour gases such as H2S and CO2 in power plants. Ethanolamine is also used for alkalinization of water in steam cycles of power plants, including nuclear power plants with pressurized water reactors. This alkalinization is performed to control corrosion of metal components and pH. However, ETA remains in high concentration in discharged water system. It is difficult to degrade naturally, besides, the degradation product can cause deterioration of water, increasing the concentration of loading of organic matter or nutrient salts. Therefore, the appropriate treatment of wastewater containing ethanolamine is required. In this study, in order to degrade ETA from wastewater of power plant, microbial fuel cell was used. The microbial fuel cell is recent technology which works on principle of bio-electrochemical technology. It is new process which directly produces electricity from organics of wastewater, attaching electrochemically active bacteria (EAB) to electrode. Microbial fuel cell can be directly used to produce electricity without additional purification or energy conversion process. Thus, this is very cost-effective than other energy reproduction technology. Especially, anaerobic precesses do not require additional high efficiency aeration equipment. Also, it is expected effect of sludge reduction because electron is retrieved during anaerobic respiration metabolic process by electrochemically active bacteria. Laboratory scale single air-cathode microbial fuel cell has a single chamber and a separator which is placed between anode and cathode electrodes to prevent short circuit. Additionally it has simple configuration than existing two-chamber system and dose not need additional facility for aeration into the system because it uses oxygen from air not the water. According to many reports, it is known that ETA is decomposed into acetate by anaerobic microorganism and most MFCs have used acetate as substrate for microorganism. Therefore, the experiment was conducted using ETA as substrate for anaerobic microorganism. The system was expected degrade COD and T-N simultaneously, and generate power. The single air-cathode MFC for degradation of ETA appeared high COD reduction, besides, treatment time was also shorter of 8 days compared to existing anaerobic treatment. The nitrogen from ETA was expected to be removed by simultaneous nitrification and denitrification (SNdN). However, it is believed that ammonia loss occurred due to ammonium volatilization around the cathode by physicochemical mechanisms. The ETA-MFC was expected to show better electrochemical efficiency than the Acetate-MFC because the hydrogen formed by ETA is slightly more than acetate. However the overall Coulombic efficiencies of ETA-MFC and Acetate-MFC were 23.66% and 27.94% respectively. This is the reason why some ETA might contribute to fermentation or methanogenesis instead of power production. In the experiment to investigate appropriate separator, the maximum voltage of PP felt-MFC was obtained the highest, showing 0.614 V. The power densities of MFCs were 297 mW/m2 (PEM-MFC), 583.7 mW/m2 (CEM-MFC) and 268.8 mW/m2 (PP felt-MFC) and the Coulombic efficiencies were obtained 25.13% (PEM-MFC), 23.66% (CEM-MFC) and 10.47% (PP felt-MFC) respectively. At 1000 Ω, the internal resistances of MFCs obtained were 38 (PEM-MFC), 27 (CEM-MFC) and 7 Ω (PP felt-MFC). Overall the internal resistances of MFC were obtained by slope in polarization curve, showing 208 (PEM-MFC), 132 (CEM-MFC) and 304 Ω (PP felt-MFC). The most important reason for this is oxygen diffusion by use of the porous fabrics (PP felt) instead of membrane. Oxygen diffusion through air-cathode can lead to substrate loss due to aerobic oxidation by bacteria in the anode chamber. Therefore, the power production was obstructed as much as substrate used by aerobic oxidation. Although the selection of PP felt as separator for MFC makes Coulombic efficiency lower than membrane, it can lead to power generation improvement due to ease in transfer of protons. Additionally, the PP felt-MFC has several advantages for wastewater treatment. First, the organic matter as substrate of MFC is free because it is wastewater. Therefore, its substrate loss by oxygen diffusion is not a problem. Second, the PP felt is very cheaper than other separators. This is very important because high prices of membrane was one of the major disadvantage in MFC research. Third, it is good in terms of ETA treatment because it requires less time for each treatment cycle, compared to anaerobic treatment. Finally the power can also be obtained. Therefore, the single air-cathode MFC used PP felt as separator is a promising system for ETA degradation in terms of treatment of ETA.