EP2160786A2

EP2160786A2 - Bioelectrochemical reactor

Info

Publication number: EP2160786A2
Application number: EP07866557A
Authority: EP
Inventors: Knowledge Limited Power
Original assignee: Power Knowledge Ltd
Priority date: 2006-10-03
Filing date: 2007-10-01
Publication date: 2010-03-10
Also published as: WO2008059331A2; RU2006134751A; WO2008059331A3

Abstract

Disclosed is a bioelectrochemical reactor (1) for the generation of electricity from the oxidation by microbes of chemical species in a liquid, the reactor comprising an inlet (2), an outlet (3), at least one cathodic zone (5) and at least one anode (4), the interior of the reactor being configured to define a flow path for liquid from the inlet to the outlet which passes between the cathodic zone and the anode, the flow path comprising a first region and a second region wherein the first region is an acidogenic region where microbes generate more carbon dioxide in use than the second region, the reactor including a gas retaining volume which extends from the first region to the second region and which is operable to retain a gas so that carbon dioxide generated at the first region in use is transported to the second region in the gas phase, where it suppresses methane generation by microbes at the second region.

Description

Bioelectrochemical Reactor

Field of the invention

The present invention relates to a bioelectrochemical reactor for the generation of electricity from the oxidation by microbes of chemical species in a liquid.

Background to the invention

Anaerobic microorganisms generate reducing equivalents such as quinones, NADH etc. during oxidative metabolic processes, for example glycolysis. These reducing equivalents can transfer electrons to conductive materials, such as graphite, and thereby change the redox state of the conductive material. This phenomenon allows the accumulation of negative electric charge from microbial processes at a collector. Several processes based on oxygen reduction are known which lead to the accumulation of positive electric charge on another conductive material. By closing a loop between these conductive materials, an electrical current can be generated. This basic idea is realised, more or less successfully, in various types of devices called microbial fuel cells.

Microbial fuel cells may be used in a variety of circumstances where it is desirable to generate electricity from the microbial oxidation of chemical species. The oxidisable chemical species function as fuel for the microbial fuel cell. The invention will now be illustrated with reference to the application of generating electricity while treating waste water, such as industrial and domestic effluent, by the oxidation of organic and inorganic molecules found in such waste water. Nevertheless, the microbial fuel cell is useful for other applications. Microbial fuel cells may also be used to remove pollutants from contaminated water.

Research to date on the use of microbial fuel cells for the generation of electricity from organic materials in waste water has focused on the optimisation of electrode materials, and reactor design, as well as the development of catalysts for the effective transfer of electrons from reducing equivalents to the anode.

EP 0 827 229 A2 (Korea Institute of Science and Technology) discloses a mediator- less microbial fuel cell, including an anaerobic anodic zone which comprises anodic electrodes manufactured from graphite felt or from graphite in the form of tubes. This fuel cell also contains a cathodic zone with cathodic electrodes. Some embodiments of the fuel cell contain porous glass or a polymeric ion-exchange membrane to separate the anodic and cathodic zones. Microorganisms, which may reduce metal ions such as iron, cobalt and some other ions, were used as a biocatalyst. This publication disclosed the ability of microorganisms to transfer electrical charge directly through their cell walls to an anodic electrode without requiring the transfer of reducing equivalents through the surrounding medium.

A disadvantage of this arrangement arises from the requirement for direct contact between the microorganism and the electrode surface for mediator-less electron transfer to occur. Direct contact can be achieved by forming a biofilm on the electrode surface or by short-term contacts between microorganisms which are free in solution and the anode surface. However, the presence of a biofilm on an anode surface clogs fuel transport to the microbes which are in contact with the electrode surface, decreasing charge generation by those microbes. In embodiments where free microbes make short-term contacts with the electrode surface, charge transfer is limited by the rate of transport of microorganisms to the electrode, resulting in the generation of only very low current levels, even where the fuel cell contains several anodic electrodes and is connected as a battery.

US 2005/0208343 (Korea Institute of Science and Technology) discloses a membrane-less and mediator-less fuel cell, including a vertical cylindrical reactor. Waste water, which contains organic species, is fed in through an inlet at the bottom from where it passes upwards. Cleaned effluent exits the reactor from the top of the uppermost chamber. The reactor contains an anode in the form of porous graphite adjacent to the inlet. The surface of the graphite may be covered by a metal catalyst such as platinum. The aerated cathode is located near the top of the reactor, which is adapted such that the distance between the anode and the cathode may be varied.

This fuel cell has several disadvantages, which reduce the amount of current which is generated. The absence of a membrane between the anodic and cathodic zones leads to the loss of electricity due to the nonelectrochemical oxidation in the cathodic zone of reducing equivalents which were produced by microbes in the anodic zone. The use of a single anodic electrode inside the reactor gives rise to the generation of a potential difference between different regions of the anode, which lie in different parts of the liquid phase, with different redox potentials, leading to a loss of electricity due to the generation of intrinsic electric current within the electrode. Moreover, where the waste water is not sufficiently buffered at the end of the anaerobic zone, one would expect an increase of the pH of the medium. This leads to methane generation which reduces the efficiency of fuel transformation into electricity.

WO 2006/072112 (Washington University) discloses a mediator-less fuel cell, including a vertical cylindrical reactor. Waste water, which contains organic species, is fed into the inlet, which is located at the bottom of reactor. After passing upwards through biomass, the cleaned effluent passes out of the reactor through an outlet at the top. Granules of activated carbon are used as anodic electrodes and the proportion of the volume of the anodic zone which is occupied by the anodic granules is approximately 50%. The surface of the granules may be covered with a metal catalyst, such as platinum. The electrical charge which is generated in the anodic granules is collected on the electrical collectors, which are in the form of graphite tubes located in the anodic zone. The collectors stretch from the bottom of reactor up to the top of the reactor. The cathodes take the form of U-shape tubes. The outside surface of this tube includes an ion-exchange membrane. Cathodic potential is generated by feeding potassium ferricyanide through this tube. The liquid phase may be recycled between the inlet and outlet of the anodic zone.

Such a reactor has several disadvantages. Different regions of the electricity collectors, which stretch from the bottom to the top of the reactor, lie in different parts of the liquid phase which have different redox potentials. The redox potential varies along the height of the reactor leading to the generation of potential differences on the collectors and the consequent loss of power output due to the generation of intrinsic electric current inside the collectors. The existence of different types of microorganisms inside the reactor sharply increases the risk of microorganisms adhering to the surface of the granules. During long-term working periods, this may increase the resistance of the charge transfer to the conductive collector. Moreover, in the case of low buffered streams, one would again expect an increase of pH of the medium at the end of anaerobic zone, which leads to methane generation, and a consequential decrease in the efficiency of fuel transformation into electricity.

The invention aims to provide an improved bioelectrochemical reactor for the generation of electricity from the oxidation of chemical species by microbes in a liquid.

Summary of the invention

According to a first aspect of the present invention there is provided a bioelectrochemical reactor for the generation of electricity from the oxidation by microbes of chemical species in a liquid, the reactor comprising an inlet, an outlet, at least one cathodic zone and at least one anode, the interior of the reactor being configured to define a flow path for liquid from the inlet to the outlet which passes between the cathodic zone and the anode.

Preferably, the flow path comprises a first region and a second region wherein the first region is an acidogenic region where microbes generate more carbon dioxide in use than the second region, the reactor including a gas retaining volume which extends from the first region to the second region and which is operable to retain a gas so that carbon dioxide generated at the first region in use is transported to the second region in the gas phase, where it suppresses methane generation by microbes at the second region.

By a "gas retaining volume for retaining a gas phase" we mean a volume through which a gas may pass whilst remaining as a gas without dissolving in or passing through a liquid, for example a volume defined by the internal walls of the reactor and the surface of the liquid. Gas generated at the first region can pass through the gas retaining volume and dissolve at the second region. The gas retaining volume is typically enclosed to restrict the egress of carbon dioxide from the gas phase. However, the gas retaining volume may not be gas tight and some carbon dioxide may be lost from the reactor in use. Typically, the reactor will be configured such that carbon dioxide from the first region is transported to the second region through the gas retaining volume by passive methods, for example diffusion. The first region typically comprises a portion of the flow path which is close to the inlet. Although less carbon dioxide is generated in the second region, typically at least some carbon dioxide will be generated in the second region. The first region may comprise or consist of the region where the rate of carbon dioxide production is greatest. The second region may comprise or consist of the region where the rate of methane production would be greatest were it not for transport of carbon dioxide from the first region by virtue of the gas retaining volume. In use, it may be that the second region produces a relatively low amount of methane, or no methane, due to the transport of carbon dioxide from the first region. The second region may anyway comprise or consist of the region where the rate of methane production is greatest.

Preferably, the reactor comprises a plurality of cathodic zones. Preferably, the reactor comprises a plurality of anodes. A plurality of anodes (for example, two) may be associated with each cathodic zone. A cathodic zone and one or more anodes associated with the cathodic zone may be connected through an external electrical circuit in use to generate electricity. Typically, the flow path extends between each cathodic zone and one or more anodes associated with the respective cathodic zone. The term "anodic zone" is used herein to refer to the liquid in the flow path and the one or more anodes.

Typically, the first region includes the region of the interior of the reactor where the flow path first passes close to an anode. Preferably, the gas retaining volume extends from before the first location where the liquid passes between a cathodic zone and an associated anode, to after the final location where the liquid passes between the said cathodic zone and an associated anode. More preferably, the gas retaining volume extends from before the first location where the liquid passes between a cathodic zone and an associated anode, to after the final location where the liquid passes between a cathodic zone and an associated anode. Typically, the gas retaining volume extends from the inlet of the reactor to the outlet.

Each cathodic zone typically has an external face, in contact with the liquid in the flow path, which preferably comprises an ion-exchange membrane. Each anode has at least one external face, in contact with the liquid in the flow path in use. Preferably, the external face of the at least one cathodic zone and an opposed external face of a corresponding anode are parallel, with the liquid flow path therebetween. Preferably, the external face of the at least one cathodic zone and an opposed external face of a corresponding anode are planar.

The cross-sectional area of the flow path between the external face of the at least one cathodic zone and a corresponding external face of a corresponding anode may be less than the cross-sectional area of the flow path at at least one other location within the flow path. Preferably, the rate of flow of the liquid between the external face of the at least one cathodic zone and a corresponding external face of a corresponding anode is greater than the mean rate of flow of liquid within the flow path. We have found that this arrangement improves the current which is generated by increasing the convective component of the mass transfer of reducing equivalents.

Where a cathodic zone is associated with more than one anode, for example, two anodes, the cross-sectional area of the flow path between the external face of the said cathodic zone and the corresponding external face of each anode may be less than the cross-sectional area of the flow path between the regions where the liquid flows between the external face of the said cathodic zone and the corresponding external faces of corresponding anodes.

Preferably, the flow path is such that the liquid flows in a generally horizontal orientation between each cathodic zone and corresponding anode. Preferably, the flow path is such that the liquid flows generally horizontally through the reactor as a whole. This facilitates the creation of a gas phase extending above the liquid in the flow path. Typically, the inlet is located towards or at the base of the interior of the reactor. Typically, the outlet is located towards the top of the interior of the reactor. The height of the outlet typically defines the height of the liquid within the reactor. The anodes may extend above the liquid within the reactor in use. The cathodic zones typically extend above the liquid within the reactor in use.

The interior walls of the reactor define at least one chamber having a base and side walls within which at least one cathodic zone and at least one anode are located. The reactor may comprise a single chamber, however the reactor could comprise a plurality of chambers that comprise at least one cathodic zone and at least one anode. At least one of the said cathodic zones may contact a side wall such that liquid flows only around the side of the cathodic zone which is not in contact with a side wall. The reactor may be configured such that the direction of flow of the liquid along the flow path reverses at least once, and preferably a plurality of times. This enables a longer flow path to be provided within the reaction than would otherwise be the case and improves mass transfer.

The one or more cathodic zones and the one or more anodes may each extend across the majority of the width of a chamber, and each successive cathodic zone or anode may be in contact with alternating side walls of the chamber, such that the flow path extends around each successive cathodic zone or anode within the chamber and alternates in direction, typically along at least the majority of the length of the chamber. Two anodes may be provided between each successive cathodic zone.

The reactor may be configured such that liquid flows in one direction past a first face of the cathodic zone, between the cathodic zone and a corresponding anode, and then passes around an end of the cathodic zone and flows in the opposite direction past an opposed second face of the cathodic zone, between the cathodic zone and a second corresponding anode.

Where a chamber includes two or more cathodic zones, a cathodic zone may contact each of two opposed side walls of the chamber. The liquid may flow around a cathodic zone which is in contact with a first side wall and then flow around a cathodic zone which is in contact with the opposite side wall. A cathodic zone may be formed by a container having opposite ion-permeable faces bounded by ion-exchange membranes. A cathodic zone may comprise a cathode coated with a metal-organic catalyst.

The one or more cathodic zones may be in the form of parallelepipeds, typically cuboids. The one or more cathodic zones may extend through a lid of the reactor.

The one or more anodes may be in the form of a parallepiped, typically a cuboid. The one or more anodes may be rectangular planes. The flow path may extend past one planar face of an anode, around one edge of the anode and then past the opposite planar face of the anode. The one or more anodes may comprise graphite. The surface of the one or more anodes may comprise a plurality of granules of activated carbon. The granules of activated carbon are preferably attached to the anodes with long-term low resistance contacts. The surface of one or more of the anodes may comprise a metal catalyst. For example, granules of activated carbon on the surface of one or more anodes may comprise a metal catalyst. The surface of one or more anodes may comprise a mediator. For example, a mediator may be attached to the granules of activated carbon on the surface of one or more anodes, for example by physical absorption. Accordingly, the surface of the anodes may have a different chemical composition. Typically, the anodes will be made from the same material, e.g. graphite, with the surface of at least two of the anodes having different materials attached thereto, for example granules of activated carbon with different materials (such as catalysts and/or mediators) included therein.

The liquid is typically waste water, such as domestic or industrial effluent. The liquid typically comprises organic species which are oxidised by the microbes in the reactor. The liquid may comprise organic species which are oxidised by the microbes in the reactor, such as metals, sulphides and so forth. The reactor may be used not only to generate electricity but to simultaneously treat liquids, such as waste water, by oxidising chemical species within the liquid.

Microbes may be present in the liquid which is received through the inlet. Microbes may be introduced into the liquid within the reactor. Typically, a mediator is present within the reactor to facilitate the transfer of reducing equivalents from the microbes to the external face of the anodic zone. The mediator may be in solution in the liquid within the reactor.

The reactor may comprise a conduit for directing liquid from the outlet, or a region of the flow path which is near the outlet, to the inlet, or a region of the flow path which is near the inlet. The conduit may be attached to the outlet, or to a pipe which is downstream of the outlet. The conduit may be attached to the inlet, or to a pipe which is upstream of the inlet. Accordingly, a proportion of the liquid within the reactor may be recirculated. This increases the amount of power which is generated from the fuel in a given volume of liquid.

According to a second aspect of the present invention there is provided a bioelectrochemical reactor for the generation of electricity from the oxidation by microbes of chemical species in a liquid, the reactor comprising an inlet, an outlet, at least one cathodic zone and at least one anode, the interior of the reactor being configured to define a flow path for liquid from the inlet to the outlet, between the anode and the cathodic zone, wherein the anode and the cathodic zone comprise opposed parallel surfaces, and the interior of the reactor is configured such that liquid flows in a generally horizontal orientation between the opposed parallel surfaces of the anode and the cathodic zone.

Typically, the opposed surface of the anode and the cathodic zone are planar. Preferably, the opposed surfaces of the anode and the cathodic zone are vertical.

Preferably, the cross-sectional area of the flow path between the external face of at least one cathodic zone and at least one external face of a corresponding anode is less that the cross-sectional area of the flow path at at least one other location within the flow path.

Optional features correspond to the features discussed in relation to the first aspect of the present invention above.

According to a third aspect of the present invention there is provided a bioelectrochemical reactor for the generation of electricity from the oxidation by microbes of chemical species in a liquid, the reactor comprising an inlet, an outlet, at least one cathodic zone and at least one anode, the interior of the reactor being configured to define a flow path for liquid from the inlet to the outlet, between the anode and the cathodic zone, wherein the anode and the cathodic zone comprise opposed parallel surfaces, and cross-sectional area of the flow path between the external face of at least one cathodic zone and at least one external face of a corresponding anode is less that the cross-sectional area of the flow path at at least one other location within the flow path.

Optional features correspond to the features discussed in relation to the first and second aspects of the present invention above.

According to a fourth aspect of the present invention there is provided a reactor, which uses organics of waste water as a fuel for the generation of electricity, which reactor comprises anodic and cathodic zones located in one vessel and separated by ion-exchange membrane, wherein granules of activated carbon are used as anodic electrodes and graphite electrodes are used as collector of electricity zone.

The reactor may be a horizontally located sectionalised vessel. The sectioning of the reactor may be performed by anodic electrodes and cathodic containers. Preferably, the reactor gas phase which is located under the upper lid of the reactor has a contact with the liquid phase along the whole path of liquid flow.

The anodic electrodes may be in the form of rectangular graphite planes, comprising electricity collectors and granules of activated carbon fixedly attached to the surface of the collectors.

The granules of activated carbon preferably have a time stable low resistance contact with the anodic collectors. The granules of activated carbon which are attached to the anodic electrodes may be coated with different catalysts depending on the location of the particular anode in the reactor.

Typically, the plane of the anodic electrodes is perpendicular to the bottom plane and side planes of the reactor. Preferably, each plane has no gap with one side surface of the chamber within which it is located, but the opposite surface of the plane has a gap with the opposite side wall of the chamber within which it is located for liquid flow.

The cathodic electrodes may be located in several rectangular cathodic containers with two holes in its opposite side planes, which are hermetically covered by ion- exchange membrane. Cathodic containers may be placed inside the anodic zone through rectangular holes in the upper lid of the reactor in such a way that each cathodic container stands inside the anodic zone between two anodic electrodes.

Preferably, each cathodic container has no gap with one side wall of the reactor, but the opposite surface of the cathodic container has a gap with the opposite side surface of the reactor for liquid flow. The cathodic electrodes may be coated with metal-organic catalysts.

Description of the Drawings

An example embodiment of the present invention will now be illustrated with reference to the following Figures in which: Figure 1 is a plan view of the components of a bioelectrochemical reactor, in which the path of liquid flow is illustrated by arrows;

Figure 2 is a cross-section through the bioelectrochemical reactor of Figure 1 ;

Figure 3 is an electrical circuit diagram illustrating the connections between anodic and cathodic electrodes;

Figure 4 is a graph of current output and voltage versus time during a charging- discharging process. The discharging process corresponds to a short-circuit operating mode and the charging process corresponds to an open circuit operating mode; and

Figure 5 is a graph of the output current in milliAmperes versus liquid retention time with and without recycling.

Detailed Description of an Example Embodiment

Figure 1 is a plan view of the components of a bioelectrochemical reactor which functions as a microbial fuel cell that is adapted to generate electricity from the oxidation by microbes of chemical species within a liquid. Figure 2 is a cross-section through the bioelectrochemical reactor. The bioelectrochemical reactor comprises a single chamber sectionalized vessel 1 , the interior and exterior of which are in the shape of a rectangular parallelepiped, having an inlet 2 for receiving a liquid which is to be treated, such as waste water, and an outlet 3 through which treated liquid leaves the reactor. The inlet is located near the bottom of the interior of the reactor. The outlet is located some distance above the bottom of the interior of the reactor and, in use, the interior of the reactor will be filled with liquid up to the level of the outlet. The interior walls of the rector, and the anodes 4 and cathodic containers 5 discussed below, define a flow path for liquid which is illustrated by the arrows in Figure 1.

The reactor is sectioned by rectangular vertically disposed planar anodes which are perpendicular to both the flat base and planar side walls of the reactor. The width of each anode is slightly less then the width of the interior of the reactor. The height of each anode is slightly less then the height of the interior of the reactor but higher than the outlet so that the anodes extend above the surface of the liquid within the reactor. The reactor is covered by a lid 6. There is a gap between the surface of the liquid and the reactor lid and so there is a common gas phase which extends from the inlet to the outlet, above the liquid.

Each anode stands on the reactor base and has a gap on only one side around which liquid can flow. The other side of each anode is in gapless contact with a side of the reactor. This arrangement of separate anodes defines the flow path of liquid inside the anodic zone. Internal electric currents within the anodes are less than would be the case if the anodes were not separate, as different parts of each anode lie in different parts of the liquid phase which will have different redox potentials in use.

Each anode is a composite comprising graphite planes, which serve as electricity collectors, to the surface of which granules of activated carbon have been attached using low electrical resistance contacts. This kind of electrical contact between the activated carbon granules and the graphite collector leads to a constant low value of electrical resistance between the granules and the graphite planes during long-term working periods. This anode design sharply reduces the volume of activated carbon which is required and increases the amount of biomass which can be included in the anodic zone.

The granules of activated carbon attached to the anodes which are closest to the reactor inlet contain a metal catalyst, such as platinum or palladium, which is selected dependent on the chemical species which are to be oxidised. This is where one would expect more intensive hydrogen emission than at other parts of the flow path as the more intensive acidogenic stage of biological degradation of organics would be expected to occur at this location in use.

The other anodes contain mediators, which are attached to the surfaces of the activated carbon granules only by physical adsorption. This increases the electrical capacity of the composite anodes. The energy threshold for electron transfer from the reducing equivalents to the anodic granules is less than the energy threshold would be for electron transfer from the reducing equivalents directly to the graphite planes.

Cathodic zones are formed by cathodic containers in the shape of rectangular parallelepipeds with open tops. The cathodic containers are placed inside the anodic zone via rectangular holes in the upper lid of the reactor. Each cathodic zone is placed between two anodic electrodes.

Each cathodic zone contains cathodic electrodes 7 and cathodic electrolyte. However, the cathodic zones could alternatively contain cathodes which could work without cathodic electrolyte (so-called dry cathodes). Two opposite surfaces of each container include holes 8, which are covered by rolled ion-exchange membrane 9. The cathodic electrodes are joined to the same holes from the inside of each container. Each container has an aerator 10 for the aeration of cathodic electrolyte, or for the air blasting of dry cathodes, as appropriate.

As with the anodes, the cathodic containers stand on the bottom of the anodic zone in such a way that there is a gap through which liquid can flow between the cathodic container and the side walls of the chamber on only one side of the cathodic container. The arrangement defines vertical channels for liquid flow between anodes and cathodic zones and minimizes anode-cathode distance, which minimises the internal electrical resistance of the biological source of energy. Moreover minimizing the anode-cathode distance makes it possible to increase the linear liquid flow rate between each cathodic zone and corresponding anodes, when the volume liquid flow rate is constant. This intensifies the transport of reducing equivalents to the surface of the anode surface by means of the convective component of mass transfer.

The external surfaces of the cathodic electrodes are covered with a catalyst such as phthalocyanine which increases the cathodic potential due to the reaction O₂ + 4H⁺ + 4e^" =≠ 2H₂O. In contrast to ferricyanides, the presence of phthalocyanines on the cathodic surface decreases the pH value of the cathodic electrolyte and increases the equilibrium potential of the cathode. The intensive transport of hydrogen ions into the cathodic zone leads to a decrease in the pH value of the cathodic electrolyte.

The volume under the reactor lid, above the liquid, constitutes a gas retaining volume which reactor retains a gas phase which extends from the inlet to the outlet of the reactor. The vertical height of the gas retaining volume and thus the volume of the gas phase depends on the height of the outlet.

In use, the liquid flow path within the anodic zone is filled with anaerobic sludge, which contains different groups of microorganisms. Waste water is pumped into the anodic zone through the inlet and liquid flows around the flow path as illustrated by the arrows shown in Figure 1. The flow direction reverses each time the liquid reaches a side wall of the reactor because to the gaps between successive anodes or cathodic containers and a side wall of the reactor. The cleaned effluent passes out through the outlet.

Reducing equivalents are generated during microbial metabolic processes, and electrons from these reducing equivalents are transferred to the anodes. A cathodic potential is formed by the reaction O₂ + 4H⁺ + 4e^" ≠= 2H₂O and enhanced by the cathodic catalyst. As the anodes and cathodes are connected in a circuit, electricity is generated.

The portion of the flow path which is closest to the inlet forms an acidogenic zone and microbes in this zone generate significant amounts of carbon dioxide which diffuse through the gas retaining volume above the liquid and dissolves in the liquid phase further downstream in the flow path. This has the effect of reducing the pH at other locations in the reactor, suppressing methanogenic processes and transforming chemical species within the liquid, which function as fuel, into electricity more effectively.

Experiment 1

An experiment was carried out to determine the maximal values of the open circuit voltage and closed circuit current in the example bioelectrochemical reactor described above. The anodes and cathodes were each electrically connected in parallel according to the circuit illustrated in Figure 3. In the open circuit mode, the anodes and cathodes were disconnected. In the closed circuit mode, the resistance was approximately one Ohm.

Prior to making electrical measurements the reactor was operated for five days in open circuit mode. The flow rate was low such that the residency time of liquid within the reactor was 2.2 days. A 3g/L solution of sucrose in water was introduced into the inlet, as fuel. Once the voltage reached its maximum value, the current determining resistor of an external electric circuit was set to zero resistance and the circuit was closed. After a short period, the reactor was again switched into open circuit mode. The change in current and voltage during this test is shown in Figure 4. It was found that, in short circuit mode, the current value was 75 mA while the maximal power per reactor volume was 45 mW/L. This is approximately three times the current achieved by the upflow microbial fuel cell disclosed in WO2006/072112.

Experiment 2

A second experiment measured the electrical parameters of the bioelectrochemical reactor under steady stable conditions with and without recycling of the liquid phase in the anodic zone. As before, the anodes and cathodes were connected as illustrated in Figure 3. The current determining resistors were 20 Ohm. Tests were carried out with retention times of the liquid phase in the anodic zones of 2.2, 0.73, 0.24 (days). For each retention time, an experiment was carried out without recycling and also with recycling of the liquid phase from the reactor outlet to the inlet. The coefficient of recycling, (i.e. the ratio of recycled liquid flow to the influx of new liquid) was 40, 13.3, 4.4 respectively. In the experiments where the retention times were 0.73 and 0.24 days respectively, the current measurements with or without recycling were carried out after one day, once the reactor reached steady-state conditions. In the experiments where the retention time was 2.2 days, the current measurements were carried out after two days, once the reactor had reached steady-state conditions. Figure 5 shows the steady-state current values corresponding to each of the different working modes of the reactor. The results show that a higher current value is associated with a reduced retention time of the liquid phase (with or without recycling). This results from the increased contribution of the convective component of mass transfer of reducing equivalents to anode surface when the retention time is lower.

The reactor used in the experiments described above had a liquid capacity of 1.1 litres in the anodic zones and this volume was been used to calculate specific parameters. The volume of liquid in the cathodic zones was 0.5 litres. The reactor can readily be scaled up 100 times by volume or more. The common gas phase in the reactor facilitates the suppression of methanogenic processes, increasing the efficiency of the generation of electricity from the oxidation of fuel by the microbes. This is especially useful when treating low-buffered waste waters. The chemical composition of the anodes may be customised for use with liquids including specific chemical species, by selecting appropriate metal catalysts and mediators.

Further modification and variations may be made within the scope of the invention herein disclosed.

Claims

1. A bioelectrochemical reactor for the generation of electricity from the oxidation by microbes of chemical species in a liquid, the reactor comprising an inlet, an outlet, at least one cathodic zone and at least one anode, the interior of the reactor being configured to define a flow path for liquid from the inlet to the outlet which passes between the cathodic zone and the anode, the flow path comprising a first region and a second region wherein the first region is an acidogenic region where microbes generate more carbon dioxide in use than the second region, the reactor including a gas retaining volume which extends from the first region to the second region and which is operable to retain a gas so that carbon dioxide generated at the first region in use is transported to the second region in the gas phase, where it suppresses methane generation by microbes at the second region.

2. A bioelectrochemical reactor according to claim 1 , wherein carbon dioxide from the first region is transported to the second region through the gas retaining volume by passive methods.

3. A bioelectrochemical reactor according to claim 1 or claim 2, wherein the first region includes the region of the flow path where the rate of carbon dioxide production is greatest and the second region includes the region of the flow path where the rate of methane production would be greatest were it not for transport of carbon dioxide from the first region by virtue of the gas retaining volume.

4. A bioelectrochemical reactor according to any one preceding claim, wherein a plurality of anodes are associated with each cathodic zone.

5. A bioelectrochemical reactor according to any one preceding claim, wherein the gas retaining volume extends from before the first location where the liquid passes between a cathodic zone and an associated anode, to after the final location where the liquid passes between the said cathodic zone and an associated anode.

6. A bioelectrochemical reactor according to any one preceding claim, wherein each cathodic zone has an external face, in contact with the liquid in the flow path, each anode has at least one external face, in contact with the liquid in the flow path in use, the external face of the at least one cathodic zone and an opposed external face of a corresponding anode are parallel, with the liquid flow path therebetween.

7. A bioelectrochemical reactor according to claim 6, wherein the external face of the at least one cathodic zone and an opposed external face of a corresponding anode are parallel.

8. A bioelectrochemical reactor according to any one preceding claim, wherein the cross-sectional area of the flow path between the external face of the at least one cathodic zone and a corresponding external face of a corresponding anode is less than the cross-sectional area of the flow path at at least one other location within the flow path.

9. A bioelectrochemical reactor according to claim 8, wherein the rate of flow of the liquid between the external face of the at least one cathodic zone and a corresponding external face of a corresponding anode is greater than the mean rate of flow of liquid within the flow path.

10. A bioelectrochemical reactor according to any one preceding claim, wherein the flow path is such that the liquid flows in a generally horizontal orientation between each cathodic zone and corresponding anode.

11. A bioelectrochemical reactor according to any on preceding claim which is configured such that the direction of flow of the liquid along the flow path reverses at least once, and preferably a plurality of times.

12. A bioelectrochemical reactor according to claim 11 , wherein the reactor is configured such that liquid flows in one direction past a first face of the cathodic zone, between the cathodic zone and a corresponding anode, and then passes around an end of the cathodic zone and flows in the opposite direction past an opposed second face of the cathodic zone, between the cathodic zone and a second corresponding anode.

13. A bioelectrochemical reactor according to claim 11 or claim 12, wherein the flow path extends past one planar face of an anode, around one edge of the anode and then past the opposite planar face of the anode.

14. A bioelectrochemical reactor according to any one of claims 11 to 13, wherein the reactor comprises one or more chambers having side walls, wherein at least one chamber comprises two or more cathodic zones and a cathodic zone contacts each of two opposed side walls of the chamber and the liquid flows around a cathodic zone which is in contact with a first side wall and then around a cathodic zone which is in contact with the opposite side wall.

15. A bioelectrochemical reactor according to any one preceding claim, wherein the reactor comprises one or more chambers having side walls and comprise one or more cathodic zones and one or more andoes, wherein the one or more cathodic zones and the one or more anodes within at least one chamber each extend across the majority of the width of the chamber and each successive cathodic zone or anode is in contact with an alternating side of the chamber, such that the flow path extends around each successive cathodic zone or anode and alternates in direction.

16. A bioelectrochemical reactor according to any one preceding claim, wherein the one or more anodes comprise graphite, and the surface of the one or more anodes comprises a plurality of granules of activated carbon.

17. A bioelectrochemical reactor according to any one preceding claim, comprising a plurality of anodes which are made from the same material with the surface of at least two of the anodes having different catalysts and/or mediators attached thereto.

18. A bioelectrochemical reactor according to any one preceding claim, wherein a proportion of the liquid within the reactor is recirculated.

19. A bioelectrochemical reactor for the generation of electricity from the oxidation by microbes of chemical species in a liquid, the reactor comprising an inlet, an outlet, at least one cathodic zone and at least one anode, the interior of the reactor being configured to define a flow path for liquid from the inlet to the outlet, between the anode and the cathodic zone, wherein the anode and the cathodic zone comprise opposed parallel planar surfaces, and the interior of the reactor is configured such that liquid flows in a generally horizontal orientation between the opposed parallel planar surfaces of the anode and the cathodic zone.

20. A bioelectrochemical reactor wherein the opposed surfaces of the anode and the cathodic zone are vertical.

21. A bioelectrochemical reactor according to claim 19 or claim 20, which is configured such that the direction of flow of the liquid along the flow path reverses at least once.

22. A bioelectrochemical reactor according to any one of claims 19 to 21 , wherein the cross-sectional area of the flow path between the external face of at least one cathodic zone and at least one opposed parallel planar face of a corresponding anode is less that the cross-sectional area of the flow path at least one other location within the flow path.

23. A bioelectrochemical reactor for the generation of electricity from the oxidation by microbes of chemical species in a liquid, the reactor comprising an inlet, an outlet, at least one cathodic zone and at least one anode, the interior of the reactor being configured to define a flow path for liquid from the inlet to the outlet, between the anode and the cathodic zone, wherein the anode and the cathodic zone comprise opposed parallel surfaces, and cross-sectional area of the flow path between the external face of at least one cathodic zone and at least one external face of a corresponding anode is less that the cross- sectional area of the flow path at at least one other location within the flow path.

24. A reactor, which uses organics of waste water as a fuel for the generation of electricity, which reactor comprises anodic and cathodic zones located in one vessel and separated by ion-exchange membrane, wherein granules of activated carbon are used as anodic electrodes and graphite electrodes are used as collector of electricity zone.