JP2020092071A - Microbial fuel cell - Google Patents

Abstract

To provide a novel microbial fuel cell technology with a high organic matter removal rate making it possible to use an existing facility with a slight modification at low cost for wastewater treatment with low organic matter concentration and obtain electric energy with high efficiency.SOLUTION: The microbial fuel cell has at least a pair of electrodes, a wastewater containing organic matter, and a wastewater treatment tank. A biofilm containing a microorganism having an extracellular electron transfer ability of decomposing an organic substance contained in wastewater to release an electron is formed on an anode of the electrodes. The anode has a layered structure or a structure for surrounding wastewater and includes a region in which a surface area of the anode is 2.5 to 18 cm/cmwith respect to a volume of wastewater surrounded by the anode. A surface area of the cathode paired with the anode is 5 to 35% of the surface area of the anode.SELECTED DRAWING: Figure 1

Description

The present invention relates to a microbial fuel cell. The present invention also relates to a method for efficiently extracting electric energy from wastewater having a low organic matter concentration using a microbial fuel cell.

The potential energy of wastewater is about 9.3 times the energy used in the treatment plant to treat the wastewater by the activated sludge method (Shizas, I. (2004) Journal of Energy Engineering, 130(2)45- It is highly estimated that it is 53), and it is important to collect and use this in order to form a recycling-based society.

As an energy recovery technology for wastewater treatment, research on anaerobic treatment such as methane fermentation and hydrogen fermentation is under way. However, methane fermentation requires a generator to use the recovered biogas as electric power, and requires pretreatment of the gas to remove hydrogen sulfide contained in the biogas. There are also safety issues such as gas explosion and poisoning with hydrogen sulfide.

On the other hand, there is a so-called microbial fuel cell that takes out electric energy from organic substances by utilizing the metabolic function of microorganisms. The present inventors have previously taken out electric energy from wastewater and at the same time recover phosphorus-containing precipitates. A method (Patent Document 1) is proposed. According to this method, it is possible to purify wastewater having a high organic matter concentration, particularly including livestock excrement, and not only obtain electric energy but also recover reusable resources contained in the wastewater at the same time. ..

The basic structure of the microbial fuel cell is that special microorganisms are grown on the anode and brought into contact with wastewater containing organic substances, and the cathode is brought into contact with an oxidant such as oxygen in the air. It is to connect. As a result, an oxidation reaction by a microorganism that uses an organic substance as an electron donor proceeds at the anode, and a reduction reaction that uses oxygen as an electron acceptor proceeds at the cathode to obtain electric power.

For example, regarding a specific structure of a power generation device, a technique relating to a cathode using a porous material or the like (Patent Document 2), a working electrode formed of carbon fiber, a counter electrode in contact with an oxidizing substance, and the working electrode An electrolytic device (Patent Document 3) in which an ion-permeable film is formed between the counter electrode and the like has been proposed. In addition, there is also a cassette (patent document 4) in which a negative electrode and a positive electrode are combined into one cassette, and a plurality of these cassettes are connected to form a microbial fuel cell.

Microbial fuel cells have high theoretical energy efficiency and are expected to be put to practical use as wastewater treatment technology for the next generation, but when treating wastewater with low organic matter concentration, the amount of power generation is extremely small and energy must be recovered. There was a problem that was virtually meaningless. Further, such wastewater cannot be said to have a sufficiently high organic matter removal rate, and there is a problem that a long residence time, that is, a facility required for treating the same amount of wastewater becomes large.

By the way, most of the wastewater has a low organic matter concentration as represented by sewage (COD _Cr : 150 to 300 mg/L). Therefore, the wastewater from which a significant amount of energy can be recovered by the microbial fuel cell is It was quite limited in quantity and type.

In the case of wastewater having a low organic matter concentration, the amount of power generation itself increases by increasing both the anode area and the cathode area per treatment tank volume, so it seems that the above problems can be solved. However, in practice, since a cathode such as platinum is often loaded with a catalyst such as platinum, another problem such as an increase in cost, an increase in the size of the entire processing apparatus and a complicated structure of the apparatus is also caused. Will occur. In some cases, no catalyst is used for the cathode, but in that case the power generation performance will be significantly reduced.

In addition, in a paper and the like, there is a report that the power generation capacity did not increase even if the anode area was increased in the wastewater treatment by the microbial fuel cell (Non-Patent Document 1). Furthermore, in an air-cathode single-tank microbial fuel cell with the same anode area and cathode area, the anode hardly contributes as a limiting factor for power generation (about 1/10 of the cathode), and the capacity of the cathode is increased by increasing the area. There is a report that increasing the power consumption was effective in improving power generation (Non-patent Document 2). As described above, it has been thought that increasing the area of the cathode is effective in increasing the power generation amount of the microbial fuel cell. However, if the area of the cathode is increased, the above-mentioned problem occurs, so that it is difficult to increase the amount of power generation from sewage or the like having a low organic matter concentration using a microbial fuel cell by a practical means.

JP, 2013-84597, A JP, 2004-342412, A JP, 2006-159112, A JP, 2009-93861, A

Lanas, Vanessa, and Bruce E. Logan. "Evaluation of multi-brush anode systems in microbial fuel cells." Bioresource technology 148 (2013): 379-385. Fan, Yanzhen, Evan Sharbrough, and Hong Liu. "Quantification of the internal resistance distribution of microbial fuel cells." Environmental science & technology 42.21 (2008): 8101-8107. Kayako Hirooka*, Osamu Ichihashi*, Phosphorus recovery from artificial wastewater by microbial fuel cell and its effect on power generation, Bioresource Technology, 137 (2013): 368?375 (*co-first authors). Kazuki Iida, Ai Umeda, Naoko Yoshida, Basic Research for Application to Sewage Treatment Plant of Microbial Fuel Cell, Koei Forum No. 24/2016 .3 (https://www.n-koei.co.jp /rd/thesis/pdf/201603/forum24_009.pdf)

For wastewater treatment with low organic matter concentration, the above problems can be solved, existing equipment can be used at low cost with slight modification, electric energy can be obtained with high efficiency, and organic matter removal speed is fast. To provide a novel microbial fuel cell technology.

In the present invention, it has been found that the above-mentioned problems can be solved by intentionally executing a method of increasing the anode area while keeping the cathode area constant, which has only been negatively reported.

That is, the microbial fuel cell of the present invention has at least a pair of electrodes, a wastewater containing an organic substance and the like, and a wastewater treatment tank, and a microorganism having an extracellular electron transfer ability to decompose the organic substance and release an electron. A biofilm containing is formed on the anode of the electrode, and the anode has a layered structure, a structure surrounding the wastewater, or a structure in which strips are bundled and hung, and the volume of the wastewater surrounded by the anode is , The surface area (wetted area) of the anode is 2.5 to 18 cm ² /cm ³ , and the surface area (wetted area) of the cathode paired with the anode is equal to the surface area of the anode. It is characterized by being 5 to 35%.

In the present invention, the surface area of the anode in contact with a unit volume of waste water in the treatment tank is made as large as possible, while the surface area of the cathode in contact with the waste water is set as small as 5% to 35% or less of the surface area of the anode. And This is because it is possible to use a material such as a non-woven fabric formed of carbon fiber capable of retaining microorganisms on the anode side, and it is not necessary to carry a catalyst such as platinum on the cathode side, so that it can be handled at low cost. .. Also, by increasing the surface area on the anode side, it is possible to obtain electric energy with high efficiency even in wastewater having a low organic matter concentration, and at the same time, it is possible to remove organic matter in the wastewater faster.

The “surface area” of the anode and the cathode refers to the area of the part where the anode and the cathode are in contact with the wastewater (wetted area). Only one surface of the air cathode is in contact with wastewater, and the other surface is in contact with air. On the other hand, since the anode is basically immersed in the waste water for use, both surfaces come into contact with the waste water, but the part that comes out on the water surface and is not in contact with the waste water is not counted as the surface area.

The “layered structure” of the anode means, for example, a state in which a plurality of flat plate-shaped anodes are arranged in parallel at an appropriate interval. The surface of each anode does not have to be parallel or evenly spaced apart, but since biofilms are formed on the front and back surfaces of each anode, if the spacing is too small, biofilms formed on each other will be formed. As a result, the anode gap disappears and wastewater cannot flow in, which is not preferable from the viewpoint of treatment efficiency.

The term “structure surrounding the wastewater” means that the anode is formed into a tubular shape, a window is provided in a part of the tubular shape, or a cross section taken along a plane perpendicular to the anode surface is It refers to each structure such as a letter shape, a U shape, and an S shape.

In addition, "a structure in which strips are bundled and hung" refers to a structure in which multiple strip-shaped anodes are installed with one end bundled and the other end not bundled, with the bundled side facing up. Say.

This is because these structures can increase the contact area with the anode per unit volume of waste water.

The “volume of wastewater surrounded by the anode” means a three-dimensional volume formed when the anode as a pair of electrodes is wrapped with a wrapping paper or furoshiki. For example, when the outermost surface of the anode having a layered or surrounding structure is one surface of the solid, or the outermost end portion in the circumferential direction of the anode forms a ridge line of the solid to form a closed region. The volume of a virtual solid. In short, it means that the highest density of the anodes arranged in the wastewater treatment tank is used as a reference. The above-mentioned "anode as a pair of electrodes" is determined by whether or not it is paired with the corresponding cathode. That is, even if they are not connected by a connecting member or the like, they are regarded as the same anode if they are electrically connected, and conversely if they are connected only by a non-conductive connecting member, they are regarded as another anode. Strictly speaking, the volume includes the volume of the anode and the connecting member, but in the present invention, these are collectively defined as “volume of waste water surrounded by the anode”.

Electrons (e ^- ) produced by oxidizing microorganisms of the anode in waste water move to the cathode side and reduce oxygen by the action of a catalyst. In the process, the electrons work by the load on the wiring to obtain electric power. Therefore, since the wastewater near the anode surface is directly related to the generation of electricity, its volume becomes important.

The biofilm-forming microorganisms in the microbial fuel cell are characterized by containing one or more bacteria that do not require addition of a mediator that transfers electrons to the anode. In general, in a microbial fuel cell, electrons produced by microorganisms are received by electrodes and supplied to an external circuit as electric energy. At that time, a microorganism that directly exchanges electrons with the electrode and a substance called a mediator (for example, neutral red, methylene blue, thionine, etc.) from the microorganism absorbs the electron, and this is a so-called intermediary that transfers the electron to the electrode. There are microorganisms that need a role. In the present invention, a microorganism that does not require the addition of this mediator is particularly suitable.

This is because many mediators are rarely contained in wastewater, so it is necessary to add them when using them, but this requires labor and cost and results in an increase in the number of mediator recovery processes. .. However, for microorganisms that transfer electrons to the electrode by utilizing mediators produced by themselves or other microorganisms present in the system, this kind of microorganism is also preferable because the step of adding or recovering the mediator is not necessary. ..

As a microorganism having an extracellular electron transfer ability to decompose the organic matter and release an electron, it is generally known that it belongs to any of the phylum Acidobacteria, Proteobacteria, Firmicutes, Cyanobacteria, and Bacteroidetes. More specifically, iron-reducing bacteria belonging to Gammaproteobanteria genus Shewanella and Deltaproteobacteria genus Geobacter are well known. Although various kinds of bacteria may exist in the biofilm formed on the anode of the microbial fuel cell, the main bacteria are those having extracellular electron transfer ability because the selective pressure that is preferable for the electron-producing microorganisms is applied on the anode side. Will increase in proportion.

The method of forming a biofilm on the anode is, for example, when using carbon felt as the anode, an environment containing a large amount of microorganisms such as activated sludge collected in an actual wastewater treatment plant, paddy soil, and lake bottom mud on the surface of the felt. The sample may be applied. As a result, various microorganisms in sludge are inoculated on the felt surface, but the selective pressure at the anode increases the proportion of microorganisms mainly having extracellular electron transfer ability. Of course, a method may be used in which various microorganisms are separated or accumulated from the above environmental sample, only microorganisms having extracellular electron transfer ability are selected in advance, and then applied to the anode surface. In any case, those suitable for the growth environment are selected to form the biofilm.

The term "biofilm" means "a structure formed by microorganisms", in which "microorganisms such as molds and bacteria adhere to the surface of an object and proliferate, and a film covering the surface with secretions and deposits. Say.

The anode used in the present invention preferably has a structure in which layers are laminated with an appropriate gap. If the volume of the entire anode is large to increase the area, it may be difficult to apply it to the conventional wastewater treatment tank, and if the design of the treatment tank needs to be changed, the cost will increase accordingly. Is.

Further, the cathode used in the present invention may be a system utilizing dissolved oxygen in wastewater, but a system utilizing oxygen in air (referred to as an air cathode) is preferable. One side of the air cathode is in contact with wastewater, and the other side is in contact with air. Therefore, only air needs to be circulated, and there is an advantage that aeration is not required.

Furthermore, the microbial fuel cell of the present invention is suitable when the organic matter concentration of waste water is low. When the organic matter concentration is high, the conventional type (so-called anode and cathode have the same projected area, that is, the surface area (wetted area) is 50% of the anode type) This is because it is difficult to recognize an increase in power generation performance commensurate with. More specifically, it is desirable that the organic matter concentration is applied to wastewater having a chemical oxygen demand (COD _Cr ) value of 30 to 300 mg/L (more preferably 40 to 180 mg/L). When used in the treatment of wastewater having a higher organic matter concentration, the location where the treatment proceeds and the organic matter concentration is reduced to 30 to 300 mg/L (more preferably 40 to 180 mg/L) (such as downstream of the treatment tank). It is desirable to apply to.

The microbial fuel cell of the present invention can increase the amount of power generation and the ability to remove organic matter even with wastewater having a low organic matter concentration, which has been conventionally difficult to apply. Therefore, it can be used for treating a large amount of wastewater such as city sewage, and can also be used by putting it into a conventional treatment tank, so that the scope of application of the technology can be greatly expanded.

In addition, although the recovery and effective use of energy have not been considered at all in the wastewater treatment so far, the use of the microbial fuel cell of the present invention helps to build a recycling-based society.

FIG. 1 is a diagram showing an example of the structure (layered structure) of the anode according to the present embodiment. FIG. 2 is a diagram showing the “enclosed volume” in the anode illustrated in FIG. 1. FIG. 3 is a diagram showing an example of the structure of the anode according to the present embodiment (a structure in which strips are bundled and hung). FIG. 4 is a diagram showing an example of a structure in which a cylindrical anode is adopted and the “enclosed volume”. FIG. 5 is a schematic diagram showing an example of the configuration of the microbial fuel cell according to this embodiment. FIG. 6 is a schematic diagram of a circulation type operation system of the microbial fuel cell in the example. FIG. 7 is a graph showing the change in the relationship between the acetic acid concentration of waste water and the maximum anode current when the surface area of the anode was increased in Example 1. FIG. 8 is a graph showing a change in the relationship between the organic matter concentration in wastewater and electric power when the anode surface area is increased in Example 2. FIG. 9 is a graph showing the average power when the organic matter concentration in the wastewater is high and the average power when the organic matter concentration is low in Example 2. FIG. 10 is a graph showing the power generation status of the microbial fuel cell in Example 3.

Since microbial fuel cells place importance on the recovery of electric energy, until now, wastewater with a high organic matter concentration (such as wastewater related to livestock), which is a favorable condition for power generation, has been the main target. However, in reality, the amount of wastewater with low organic matter concentration such as city sewage is overwhelmingly large. Therefore, an object of the present invention is to provide a microbial fuel cell applicable to such wastewater.

According to the present invention, by setting the anode area larger than the cathode area, it is possible to sufficiently recover electricity from wastewater having a low organic matter concentration, and it is possible to provide a technique with a high organic matter removal rate. More specifically, a biofilm having at least a pair of electrodes, a wastewater containing an organic substance and the like, and a wastewater treatment tank, a biofilm containing a microorganism having an extracellular electron transfer ability to decompose the organic substance and release an electron. It is formed on the anode of the electrode, and the anode has a layered structure, a structure surrounding the wastewater, or a structure in which strips are bundled and hung, and the volume of the wastewater surrounded by the anode is larger than that of the anode. The surface area of the cathode, which includes a region having a surface area of 2.5 to 18 cm ² /cm ³ , more preferably 3.1 to ¹² cm ² /cm ³ , and which is paired with the anode is The present invention relates to a microbial fuel cell having a surface area of 5 to 35%, more preferably 7.5 to 30%.

Furthermore, the surface area of the anode that has the best balance between cost and capacity depends on the organic matter concentration in the wastewater. For example, in an actual wastewater treatment facility, the organic matter concentration is high upstream of the treatment tank (close to the wastewater inlet), and the organic matter concentration becomes lower toward the downstream of the treatment tank. Therefore, it is considered preferable to set the anode surface area small in the upstream of the treatment tank and set the anode surface area to increase in the downstream.

Both or one of the electrodes may be divided into a plurality of electrodes. Carbon fiber, carbon felt, carbon paper, carbon rod, etc. are suitable as the material on the anode side that easily forms a growth environment for microorganisms and easily conducts electricity, but is not limited to the above as long as it has good electrical conductivity. Absent. Particularly, in order to increase the area of the anode, for example, it is preferable that the carbon papers are arranged at appropriate intervals to form a layer.

FIG. 1 shows an example in which the anode has a layered structure. The upper side is a schematic diagram and the lower side is a photograph of the prototype. In this example, a carbon paper [large] (1) of the same size is prepared, and a carbon paper [small] (2) having the same thickness and a small size as the carbon paper [large] is overlaid on the carbon paper [1]. [Large] is placed on both ends, and carbon paper [Large] is repeatedly laid on it to connect them to form a five-layer structure. With such a structure, it is possible to secure a void for forming a biofilm and a wastewater flow path between the layers of the carbon paper [large]. However, if the gap is less than 1 mm, the biofilm is formed to reduce the gap, and it becomes difficult for waste water to flow in, resulting in a decrease in treatment efficiency. If it is greater than 8 mm, the volume of the entire anode becomes bulky. Therefore, it tends to be difficult to apply it to an installation site or an existing treatment facility.

The five-layer structure in FIG. 1 is merely an example, and may be four layers or less or six layers or more. Further, the both ends may be formed so as to be connected to each other, or may be connected at four corners of the carbon paper or at appropriate places, and their arrangement may be the same or different for each interval of each carbon paper. However, it is desirable to set each layer as parallel as possible to the flow of wastewater. In the example of FIG. 1, by flowing the waste water from the front side of the paper surface to the back surface direction or the reverse direction, the entire surface of the anode can be effectively utilized without obstructing the flow.

Since a biofilm is not formed in the part where the carbon papers contact each other, it is basically not included in the calculation of the surface area of the anode. Strictly speaking, the surface of the material such as carbon paper has irregularities due to the fine structure, but the calculation of the surface area does not include these fine structures, and the general surface area calculation method assuming that the surface is flat is used. Seek based on. However, if there is unevenness of 0.1 mm or more, which is expected to increase the effective area of the anode, the surface area is calculated without treating it as being flat.

It is not always necessary to use a conductive material to connect each layer of the layered structure anode, but when carbon paper is used as the anode material, for example, the carbon paper of each layer is electrically connected through a connecting member or an external circuit. If not, some carbon paper will not function as an anode and will be wasted.

Similarly, an example of the layered structure is a structure folded in an accordion shape. Since this structure does not require a connecting member as described above, it is easy to process and can be reduced or expanded depending on the thickness of the biofilm formed on the surface, so it can be adjusted according to the situation of wastewater. Is excellent at. However, there is another aspect that the structure is not always stable because it can be expanded and contracted.

An example of the structure of the anode that surrounds the wastewater is a cylindrical shape represented by a cylindrical tube. Considering that waste water is allowed to pass through the cylinders in this shape, a structure in which several cylinders are bundled at appropriate intervals, or by connecting or contacting the side surface of one cylinder with the side surface of another cylinder, It may have a structure forming a raft (FIG. 4 shows an example of forming a raft). In any case, it is preferable that the biofilm is formed not only on the inner side of the cylinder but also on the outer surface thereof, and that part is brought into contact with the wastewater to effectively utilize the anode surface. It should be noted that the inflow and discharge of wastewater may be further promoted by providing windows at any one or a plurality of locations on the cylinder.

As another example of the structure for surrounding the wastewater, it is preferable to imagine a groove-shaped structure formed by removing one or more adjacent side surfaces of a prismatic structure having a polygonal bottom surface, for example. When cut along a plane perpendicular to the side surface, the cut surface may be U-shaped, U-shaped, S-shaped or the like.

The definition of the “volume of wastewater surrounded by the anode” is as described above, but it will be described using the example of the anode (3) shown in FIG. 1, for example. As shown in FIG. 2, the surface of the uppermost quadrangle of the anode and the surface of the lowermost quadrangle are the top and bottom surfaces, and the four corners of each quadrangle are connected to form the volume of a quadrangular prism. The same applies to the case of the accordion shape.

On the other hand, in the case of a tubular shape as shown in FIG. 4, for example, the same volume shows the volume. Four cylindrical tubes (4) are arranged at intervals and are connected by a connecting member (5) to form an anode (6). When the entire anode is wrapped with wrapping paper, a solid body having the shape shown by the line (7) is formed. This volume is the “volume of waste water surrounded by the anode”. The four arrows shown in the figure indicate the direction in which the wastewater flows.

The present invention requires that the surface area of the anode is in the range of ^2.5 to 18 cm ² /cm ³ with respect to the volume of waste water surrounded by the anode.

As mentioned in the prior art, increasing the area of the cathode has been the first choice so far in order to increase the power generation of the microbial fuel cell. Certainly, if the area of the cathode is increased, the amount of power generation itself is increased, but the cost of the cathode is higher than that of the anode because it requires the use of a catalyst and the like, and there is a problem that the total cost is significantly increased. In the present invention, by setting the area of the cathode smaller than the area of the anode as described above, it is possible to generate power with high efficiency from wastewater having a low organic matter concentration while suppressing the cost, and also to improve the purification effect of wastewater. it can.

The surface area of the cathode is preferably 5 to 35% of the surface area of the anode. If it is less than 5%, the reaction of the cathode becomes rate-determining and the power generation efficiency is reduced. If it is more than 35%, the power generation amount itself increases, but this is not preferable in terms of cost increase.

The microbial fuel cell of the present invention is particularly suitable when the organic matter concentration of waste water is low. When the organic matter concentration is high, the conventional type may be applied, and it can be said that it is difficult to demand the increase in power generation performance commensurate with increasing the anode area in terms of cost. More specifically, it is applied to wastewater having an organic matter concentration of 30 to 300 mg/L (more preferably 40 to 180 mg/L) in terms of chemical oxygen demand (COD _Cr ) when contacting the anode. Is desirable. INDUSTRIAL APPLICABILITY Since the present invention can efficiently generate electricity and remove organic matters from general municipal wastewater, it is expected to expand the application range of microbial fuel cells.

Hereinafter, some examples will be shown in order to specifically clarify the present invention.

(Example 1)
A one-tank type air cathode microbial fuel cell (8) as shown in FIG. 5 was prepared. The volume of the anode tank (wastewater treatment tank) (9) was 100 ml. As the anode (10), the layered structure shown in FIG. 1 was prepared in three layers of two layers, five layers, and ten layers, which were respectively incorporated into different microbial fuel cells. The details of the method for producing the anode are as follows. First, a carbon paper having a thickness of 0.37 mm (Toray Carbon Paper TGP-H-120) was used as an anode material, and this was cut into large and small pieces of carbon paper. Next, as a paste for sticking the carbon papers together, a conductive mixed liquid was prepared in which carbon black and 40% PTFE dispersion liquid were mixed at a ratio of 1 mg:120 μL. Then, large and small carbon papers were alternately laminated at a constant ratio to form a layered structure. For the two-layer anode, two large pieces of carbon paper were used, and a connecting member formed by stacking 35 small pieces on each other was sandwiched between the two large pieces for connection. Similarly, five large pieces were used for the five-layer anode, and eight small pieces were stacked as the connecting member sandwiched between the layers of the large piece. For the 10-layer anode, 10 large pieces were used, and as the connecting member sandwiched between the layers of the large piece, 3 small pieces were stacked. As a result, three types of anodes with the same “volume of wastewater surrounded by anodes” but different surface areas of about 160 cm ² (2 layers), 340 cm ² (5 layers), and 630 cm ² (10 layers) were obtained. Was created. The surface areas for these “volumes of wastewater surrounded by the anode” are about 3.1 cm ² /cm ³ (2 layers), 6.5 cm ² /cm ³ (5 layers), and 12 cm ² /cm ³ (10 layers), respectively. ) Became. Finally, these were fired to complete the anode.

Further, as a conventional anode, an anode having only one piece of carbon paper (surface area: 94 cm ² ) cut into a circle with a diameter of 7.7 cm was also prepared.
For the connection between the anode and the external circuit, in the case of a 10-layer anode, a carbon felt (11) cut into 1 cm square was brought into contact with the anode by pressing it with a carbon rod (12) and pulled out to the outside of the anode tank. The carbon rod was sandwiched between clips of the external circuit. In the case of a 5-layer or 2-layer anode, a platinum wire was passed between the anode layers and brought into contact with each other, and the platinum wire drawn out of the anode tank was sandwiched by clips of an external circuit.

The air cathode (13) is a carbon paper (Toray Carbon Paper TGP-H-120, Teflon treated: 5% wt. wet proofing manufactured by Toray Industries, Inc.) by the method of Cheng et al. (Cheng, S., Liu, H. and Logan). , BE (2006) Increased performance of single-chamber microbial fuel cells using an improved cathode structure, Electrochemistry Communications, 8(3), 489-494.). /Cm ² ), and PTFE was applied to the side in contact with the outside air to form an air diffusion layer. The liquid contact area of the air cathode was 47 cm ² (circular shape having a diameter of 7.7 cm).

The adaptation of these microbial fuel cells was performed using artificial wastewater. As the planting source, the anodic biofilm of the microbial fuel cell, which already produces good power, was used. The waste water was circulated using a pump (20) in such a direction that it was introduced from the waste water inlet hole (14) on the bottom surface of the microbial fuel cell and was discharged from the outflow hole (15) on the upper surface. For artificial wastewater, several types of phosphate-based buffers (NaH ₂ PO ₄ : 36 mM, Na ₂ HPO ₄ : 64 mM, NH ₄ Cl: 12 mM, KCl: 3.5 mM) with a pH of 6.8 were used. Vitamin (about 55 mg/L), mineral (about 60 mg/L), and substrate (sodium acetate) were added. The detailed composition of vitamins and minerals is described in Non-Patent Document 3. Phosphate buffer, vitamins and minerals were mixed in advance and circulated inside the microbial fuel cell, and the substrate was continuously injected using a syringe pump (19) (load is about 3 mmol/L/day to about 8 mmol/L/day. Increase gradually until day). The external resistance (18) on the external circuit was replaced with 100Ω in the initial stage of the operation, and then with 10Ω after the voltage reached 0.25V. A schematic diagram of this circulation type operation system is shown in FIG.

The voltage of the microbial fuel cell during acclimatization was measured with a digital multimeter (17) (midi LOGGER GL200A, Graphtec Co., Ltd.) having an automatic data collection capability. The electric current and the electric power were calculated according to Equation 1. Here, I: current (A), E: voltage (V), P: power (W), R: resistance (Ω).

As a result of the above-mentioned acclimatization operation, the microbial fuel cell that has generated good power generation was evaluated for the ability of the anode for power generation. For the evaluation, an electrochemical method called liania sweep voltammetry (LSV) was used. In this method, a device called a potentiostat is used to measure the current while sweeping (changing) the potential of the anode. In this evaluation, the sweep range was from the open circuit potential to -0.1 V vs. Ag/AgCl, and the sweep speed was 1 mV/s. This measurement was carried out with three kinds of artificial wastewater to which acetic acid having an organic matter concentration (COD _Cr ) of 50 mg/L, 100 mg/L, and 1000 mg/L was added.

FIG. 7 shows the relationship between the organic matter concentration of waste water and the maximum anode current obtained from the LSV measurement results when the anode surface area was changed. It can be seen that the maximum current increases as the anode surface area increases. This effect is particularly remarkable when the organic substance concentration is low. That is, it can be seen that by increasing the surface area of the anode, a relatively high power generation capacity can be exhibited even with a low organic matter concentration.

(Example 2)
Using the same microbial fuel cell as in Example 1, without using a potentiostat, instead of attaching an external resistance of 10Ω to treat the artificial wastewater, the organic matter concentration (COD _Cr ) and electric power in the wastewater were treated. A test for examining the change with time was performed. In Example 1, the power generation capacity of the anode at a specific organic matter concentration (COD _Cr ) was evaluated, whereas in this evaluation, the change in power generation accompanying a decrease in the organic matter concentration (COD _Cr ) at a constant (10Ω) external resistance. Can be revealed. Further, in this test, the artificial wastewater was prepared by adding only sodium acetate (initial concentration: about 180 mg/L) to a phosphate buffer (NaH ₂ PO ₄ : 36 mM, Na ₂ HPO ₄ : 64 mM) having a pH of 6.8. I used one.

The voltage of the microbial fuel cell was measured with a digital multimeter (midi LOGGER GL200A, Graphtec Co., Ltd.) having an automatic data collection capability. The electric power was obtained according to the above-mentioned formula 1.

The COD _Cr concentration of wastewater is measured according to the Standard Method (method 5220D) (Reference: APHA (1995) Standard Methods for the Examination of Water and Wastewater (19th ed.), American Public Health Association, Washington DC). Company).

FIG. 8 shows the relationship between the organic matter concentration in the wastewater and the electric power when the artificial wastewater is treated. In the conventional anode, the decrease in the electric power due to the decrease in the organic matter concentration has already started at 180 mg/L, but in the anode with a surface area of 3.1 cm ² /cm ³ (two layers) or more, the power generation capacity is up to about 90 mg/L. The anode having a surface area of 6.5 cm ² /cm ³ (5 layers) and 12 cm ² /cm ³ (10 layers) did not decrease and exhibited high power generation ability even at a lower organic substance concentration. That is, it can be seen that by increasing the surface area of the anode, it is possible to maintain a high electric power even when the organic substance concentration is low.

Further, FIG. 9 shows that when the organic matter concentration in the artificial wastewater is high (COD _Cr : about 180 mg/L→about 130 mg/L) and when the treatment progresses to a low concentration (COD _Cr : about 130 mg/L→about 40 mg/ It is a comparison of the average power of L). It can be seen that the effect of improving the electric power by increasing the anode area is greater when the organic substance concentration is low than when the organic substance concentration is high.

(Example 3)
In the same microbial fuel cell as in Example 1, using a microbial fuel cell using an anode of 12 cm ² /cm ³ (10 layers), an operation of treating actual sewage (soluble COD _Cr : about 200 mg/L) was performed. went. As the planting source, the anode biofilm of the microbial fuel cell, which has already generated good power, was used, and sewage was used from the stage of acclimatization operation. The external resistance was set to 100Ω at the start of operation, and was replaced with 50Ω after the voltage reached 0.25V. As the operation system, the circulation system of FIG. 6 from which the inflow line of sodium acetate was removed was used, and the waste water bottle (21) was replaced with a fresh sewage every 1 to several days.

The voltage of the microbial fuel cell was measured with a digital multimeter (midi LOGGER GL200A, Graphtec Co., Ltd.) having an automatic data collection capability. The electric power was obtained according to the above-mentioned formula 1. Furthermore, the power generation capacity of the microbial fuel cell was evaluated by LSV. Specifically, the current was measured with a potentiostat while sweeping the voltage of the microbial fuel cell. In this evaluation, the sweep range was from open circuit voltage to 0 V vs. Ag/AgCl, and the sweep speed was 1 mV/s.

The microbial fuel cell increased power generation about one month after the start of operation, and started power generation in a stable cycle. FIG. 10 shows the power generation status from the 46th day to the 54th day of the operation period. The results are shown in terms of the amount of power generation per volume of the anode tank for ease of comparison with other literature. The maximum electric power per anode tank volume reaches 8.7 W/m ^3, which is the value reported so far in the microbial fuel cell for treating sewage (Non-patent Document 4, 0.085 W/m ³ per anode volume). It was 100 times or more compared to the amount of power generation and calculated from the anode volume in the tank volume). Each time the waste water bottle filled with sewage was replaced, the power density per anode tank volume increased to about 8 W/m ³ , and after being maintained for several hours to 1 day, it gradually decreased. It is considered that this is because the organic matter in the sewage was consumed as the power was generated, and the concentration of organic matter decreased. Further, as a result of LSV, the maximum power density per tank volume of the sewage reactor was even higher at about 13 W/m ³ .

The microbial fuel cell of the present invention is particularly effective in treating wastewater having a low organic matter concentration such as city sewage, and is expected to be specifically used for municipal wastewater treatment, industrial wastewater treatment having a low organic matter concentration, and the like. To be done. Further, it is also effective in the place where the organic matter concentration becomes low as the treatment progresses in the wastewater treatment with high organic matter concentration. Further, even in a microbial fuel cell of the type that purifies sediments in lakes and marine areas, the low concentration of organic matter is the cause of the low power generation capacity, so it is considered that it is highly applicable to this field.

1 carbon paper [large]
2 Carbon paper [small]
3 6 10 Anode 4 4 Cylindrical tubes 5 Connection member 7 Solid showing "volume of wastewater surrounded by anode" 8 16 Single-chamber air cathode microbial fuel cell 9 Anode tank (wastewater treatment tank)
11 Cut carbon felt 12 Carbon rod 13 Air cathode 14 Waste water inflow hole 15 Waste water outflow hole 17 Digital multimeter 18 External resistance 19 Syringe pump 20 Circulation pump 21 Waste water bottle

Claims (2)

At least a pair of electrodes, a wastewater containing organic matter, and a wastewater treatment tank,
A biofilm is formed on the anode of the electrode, which contains a microorganism having an extracellular electron transfer ability of decomposing the organic matter and releasing an electron.
The anode has a layered structure or a structure for surrounding wastewater,
With respect to the volume of waste water surrounded by the anode, the surface area of the anode includes a region in which the surface area of the anode is 2.5 to 18 cm ² /cm ³ , and the surface area of the cathode paired with the anode is the same as that of the anode. A microbial fuel cell characterized by having a surface area of 5 to 35%. The microbial fuel cell according to claim 1, wherein the organic matter concentration when the wastewater comes into contact with the anode is in the range of 30 to 300 mg/L in terms of chemical oxygen demand (COD _Cr ).