Enhanced processing of exhaust gas and power generation by connecting mini-tubular microbial fuel cells in series with a biotrickling filter
Graphical abstract
Introduction
Ethyl acetate (EA) that is used in the coatings industry is often soluble in water (solubility is 0.083 g/mL at 20 °C, KH is 1.34 × 10−4 atm m3/mol and vapor pressure is 93 mmHg at 25 °C [1]), so EA is commonly used as a solvent for coatings such as inks and paints. Long-term exposure to EA can cause adverse health effects. The biological treatment of low-concentration volatile organic compounds (VOCs) that are contained in waste gas involves mild reaction conditions, simplicity of implementation, low operating cost, and the generation of small amounts of secondary pollutants. Biotrickling filters (BTFs) are commonly used to treat highly water-soluble contaminants such as ketones, alcohols and hydrogen sulfide [2]. The BTF is one of the best air pollution control devices for treating gaseous VOCs or odor. The treatment mechanism involves the introduction of VOCs-containing exhaust gas into a packed bed in the BTF; the VOCs reach the biofilm reaction zone through the gas-liquid interface and so diffuse into the biofilm under the influence of the concentration gradient, before being metabolized by the microbes in the biofilm. Accordingly, the VOCs are converted into less harmful substances such as carbon dioxide and water [3].
Microbial fuel cells (MFCs) have been successfully investigated on generating electricity while treating wastewater, such as refinery wastewater [4], dairy wastewater [5] and azo dye wastewater [6]. Among them, the air-cathode microbial fuel cell (AC-MFC) uses oxygen as an electron acceptor, not only reducing the system construction cost but also providing low internal resistance [7], thus improving its generating of power [8,9]. In recent years, the MFC has been combined with the BTF to form a biotrickling filter-type microbial fuel cell (BTF-MFC) [10]. For example, Wu and Lin [11] used a polyvinyl alcohol-membrane electrode assembly (PVA-MEA) as a cathode and conductive coke as an anode, which were packed in a BTF-MFC, which was then successfully used to treat gaseous pollutants. According to Wu et al. [12], BTF-MFC can effectively promote the flow of electrons, and its pollutant removal efficiency is superior to that of a conventional BTF. However, the proton exchange membrane (PEM) in their BTF-MFC had low water retention, resulting in poor proton transfer, reducing the power output efficiency [13,14]. Lin et al. [15] improved the PEM using PVA-hydrogel (PVA-H), effectively improving the uneven distribution of the entering waste gas and the poor water retention of the PEM. Liu et al. [16] evaluated the removal of isopropanol by a cylindrical BTF-MFC under shock-load and shut-down events by simulating the concentration fluctuations under field conditions.
The performance of the BTF-MFC has been somewhat improved, but its disadvantages remain; these include an oversized anode chamber and uneven distribution of microorganisms on the packing material, which result in channeling of the incoming exhaust gas stream. Therefore, in this work, a porous ceramic ring with high conductivity is used as the anode and the cathode, favoring a uniform distribution of microorganisms on the packing material. Winfield et al. [17] indicated that the performance of an MFC can be improved by optimizing the electrode materials or connecting multiple micro MFCs. Walter et al. [18] established that multiple MFCs in series can provide higher power densities with small anode electrode volumes. Accordingly, the design of series reactor has been attracting attention.
The tubular microbial fuel cell (Tube-MFC) herein has a conductive porous ceramic ring (CPCR) as the anode and cathode material [19], and a polyvinyl alcohol hydrogel (PVA-H) as a proton exchange membrane [20,21]. The Tube-MFC was integrated into the upper layer of a BTF-MFC to form a test tube-biotrickling filter-microbial fuel cell (Tube-BTF-MFC) system. The commercially available CPCR that is used in this work is coated with a carbonaceous organic material and is calcined at a high temperature. The CPCR exhibits high porosity, low electrical resistance, high surface area, high mechanical strength, high corrosion resistance and a 3D structure [22,23]. Therefore, when it is used as an anode material, it is suitable for microbial attachment and the adsorption of organic pollutants [24]; when it is used as a cathode material, its porosity increases the contact area of the cathode with oxygen in the atmosphere. For a given efficiency of exhaust gas removal and power generation, the required CPCR volume is therefore lower when the CPCR is packed into the anode chamber of the Tube-BTF-MFC.
The main goal of this study is to improve the removal efficiency of pollutants in exhaust gas with concomitant MFC power generation. The novelty of this study is the combination of BTF-MFC with tube-MFC using PVA-H as PEM and CPCR as electrode. After integration, a new tube-BTF-MFC can be constructed for processing EA vapor. The optimal empty bed residence time (EBRT) of the Tube-BTF-MFC is obtained, and then the relationships among the removal efficiency (RE), the elimination capacity (EC) and the power outputs of the MFC under various organic loading (OL) were established for the optimal EBRT. When the operation of the MFC was stable under each set of parameter settings, the power density of the MFCs was monitored, and the power generation performances of the types of MFC were compared.
Section snippets
Fabrication of Tube-MFC
Molasses waste was coated on a ceramic ring as a carrier and calcined at 1100 °C in a vacuum. The resulting CPCR was used as a cathode and anode material. A 10% PVA colloidal solution was prepared by mixing DI water (90 mL) with PVA beads (10 g) (BF-26, Chang Chun Petrochemical, Taiwan), and kept in an autoclave (121 °C, 1.2 kg/m2 and 30 min). The Tube-MFC was a porous plastic centrifuge tube (10 cm3) that consisted of the CPCR as the electrodes and 10% PVA-H as the PEM.
The Tube-MFC is
Effects of OL on output powers and removal efficiency of BTF-MFC
Fig. 3 (a) reveals that the output power increases slowly from 0.43 mW to 1.61 mW when the BTF-MFC is in the acclimation period (OL at 16.2 g/m3·h). The increase in power is presumably attributable to the fact that when the microorganism gradually forms a biofilm on the coke surface, the oxygen in the pores is consumed and when the oxygen has been consumed so none remains to receive electrons, electrons are transferred to the anode, gradually increasing the output power.
Fig. 3 (a) shows that
Conclusion
A conductive porous ceramic ring (CPCR), prepared by carbonizing waste molasses at 1100 °C, can be used as an anode in a tubular microbial fuel cell (Tube-MFC) and embedded in a biotrickling filter (BTF)-MFC tank. Since the CPCR, which served as an anode of the Tube-MFC, is a good biological carrier and favors microbial attachment, it can effectively improve the efficiency of an MFC in treating exhaust gas. In addition, each Tube-MFC is a separate MFC in the chamber of the Tube-BTF-MFC, so the
CRediT authorship contribution statement
Shu-Hui Liu: Conceptualization, Investigation, Writing - original draft, Funding acquisition. Sih-Hua Fu: Data curation, Validation. Chia-Ying Chen: Visualization, Methodology. Chi-Wen Lin: Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan, for financially supporting this research under Contract No. MOST 107-2637-E-412-001 and MOST 108-2637-E-224-005.
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2022, Renewable EnergyCitation Excerpt :MFCs are often used in wastewater treatment studies as they have a high removal efficiency [4,5]. There are only a few studies on the treatment of gaseous pollutants, such as mixed exhaust gas toluene/ethylbenzene/xylene [6], toluene [7–9], benzene [10], o-xylene [11], isopropanol [12,13], dimethyl sulfide [14], ethyl acetate [15–17], acetone [18] and styrene [19]. In addition, microbial electrolysis cells that are similar to MFC can also process gaseous pollutants like chlorobenzene [20,21] and nitrogen oxides [22,23].