WO2011038453A1

WO2011038453A1 - Bioelectrochemical system

Info

Publication number: WO2011038453A1
Application number: PCT/AU2010/001277
Authority: WO
Inventors: Shelley Therese Brown; Rene Rozendal; Korneel Rabaey
Original assignee: The University Of Queensland
Priority date: 2009-09-29
Filing date: 2010-09-29
Publication date: 2011-04-07

Abstract

A bioelectrochemical system comprises at least one compartment selected from an anode compartment or a cathode compartment and two or more other compartments selected from the other of an anode compartment or a cathode compartment. Each anode compartment is separated from a cathode compartment by an ion permeable membrane.

Description

BIOELECTROCHEMICAL SYSTEM FIELD OF THE INVENTION The present invention relates to a bioelectrochemical system. BACKGROUND TO THE INVENTION

Bioelectrochemical systems typically comprise a housing or a vessel containing an anode compartment and a cathode compartment. The anode compartment contains an anode and the cathode compartment contains a cathode. One of the compartments, typically the anode compartment, contains electrochemically active microorganisms that oxidise material (such as organic material and/or inorganic material) present in the anode compartment. This results in the transfer of electrons to the anode. The anode is electrically connected or coupled to a counter electrode (cathode) at which a cathodic reaction takes place. The cathodic reaction is a reduction reaction. The cathode compartment may contain electrochemically active microorganisms. As a result of the electrical connection between the anode and the cathode, the electrode reactions can occur and electrons can flow from the anode to the cathode. The bioelectrochemical system may operate as a fuel cell (in which case electrical energy is produced) or as an electrolysis cell (in which case, electrical energy is fed to the bioelectrochemical system) (Rozendal, R. A., H. V. M. Hamelers, K. Rabaey, J. Keller, and C. J. N. Buisman. 2008. Towards practical implementation of bioelectrochemical wastewater treatment. Trends in Biotechnology 26:450-459). The skilled person will understand that one or both of the anode compartment and the cathode compartment may contain electrochemically active microorganisms.

In order to maintain an electrical circuit and electrical charge balance (i.e., electroneutrality) in the bioelectrochemical apparatus, ions are typically allowed to flow between the anode and the cathode. For example, cations may pass from the anode compartment to the cathode compartment or anions may pass from the cathode compartment to the anode compartment (or both anions and cations may pass between the respective compartments). An ion permeable membrane is typically used to ensure that the flow of ions takes place. A liquid or solution is provided to the anode compartment and a liquid or solution is also provided to the cathode compartment. Appropriate feeding arrangements and remove arrangements may be provided for feeding and removing the liquid or solutions to and from the anode compartments and the cathode compartments. The solution being removed from the anode compartment (which has effectively being "treated" by passing through the anode compartment) may be subsequently fed to the cathode compartment. Alternatively, different liquids and solutions may be fed to the anode compartment and the cathode compartment, respectively.

BRIEF DESCRIPTION OF THE INVENTION

In a first aspect, the present invention provides a bioelectrochemical system comprising:

- at least one compartment selected from an anode compartment or a cathode compartment; and

- two or more other compartments selected from the other of an anode compartment or a cathode compartment;

- wherein each anode compartment is separated from a cathode compartment by an ion permeable membrane.

In some embodiments of the first aspect of the present invention, the bioelectrochemical system comprises a plurality of anode compartments and a plurality of cathode compartments, each anode compartment being adjacent to at least one cathode compartment.

In some embodiments of the bioelectrochemical system of the first aspect of the present invention, the system comprises a plurality of anode compartments and at least one cathode compartment. The at least one cathode compartment may include a plurality of cathodes.

In other embodiments of the first aspect of the present invention, the bioelectrochemical system may comprise a plurality of cathode compartments and at least one anode compartment. The at least one cathode compartment may include a plurality of anodes.

In a second aspect, the present invention provides a bioelectrochemical system comprising a plurality of anodes, and a plurality of cathodes, each anode being adjacent to at least one cathode, each anode being separated from an adjacent cathode by an ion permeable membrane, wherein the anodes are electrically connected to each other in parallel and the cathodes are electrically connected to each other in parallel. In the second aspect of the present invention, it will be appreciated that the anodes will also be electrically connected to the cathodes in order to maintain an electrical circuit in the system.

In some embodiments, the electrical circuit may comprise a conductor having very low resistance such that the conductor acts as an electrical short circuit between the anode and the cathode. In other embodiments, a power supply may be included in the electrical circuit.

In some embodiments, a single ion permeable membrane is used to separate each anode electrode from an adjacent cathode electrode. In this embodiment, the single ion permeable membrane may be interwoven between the anode electrodes and the cathode electrodes. Ion permeable membranes suitable for use in the present invention include any ion permeable membranes that may be used in bioelectrochemical systems (Kim et al., Environ. Sci. Technol., 2007, 41 , 1004-1009; Rozendal et al., Water Sci. Technol., 2008, 57, 1757-1762). Such ion permeable membranes may include ion exchange membranes, such as cation exchange membranes and anion exchange membranes. Porous membranes, such as microfiltration membranes, ultrafiltration membranes, and nanofiltration membranes, may also be used in the bioelectrochemical system used in the present invention. The ion permeable membrane facilitates the transport of positively and/or negatively charged ions through the membrane, which compensates for the flow of the negatively charged electrons from anode to cathode and thus maintains electroneutrality in the system. In other embodiments, a plurality of ion permeable membranes, such as a plurality of separate ion permeable membranes, are used to separate the anodes from the cathodes. In one embodiment, each of the anodes or each of the cathodes are surrounded by an ion permeable membrane. A plurality of ion permeable membranes may be provided in this regard. The ion permeable membranes surrounding each of the anodes or each of the cathodes may comprise an ion permeable membrane in the form of a lamellae or an envelope. In some embodiments, the ion permeable membrane envelops an anode or a cathode. In some embodiments, a plurality of lamellae or envelopes of ion permeable membranes are formed and a plurality of electrode chambers are created by connecting the plurality of lamellae or envelopes at opposite ends.

In all aspects of the present invention, it is possible to feed the anode compartments with liquid or solution from a sump or a manifold. Similarly, it is possible to feed all of the cathode compartments with liquid or solution from a sump or a manifold.

In some bioelectrochemical systems, it has been found that the surface area of either the anode or the cathode places a limitation on the maximum rate of reaction or conversion taking place in that compartment. In most bioelectrochemical systems, it is the anode compartment that is so limited. As the functioning of the anode compartment is linked to the functioning of the cathode compartment, the rate limitations occurring in the anode compartment also result in the same rate limitations occurring in the cathode compartment. Therefore, in some embodiments of the present invention, the anode has a larger surface area then the cathode. In this manner, the adverse outcomes arising from the surface area limiting effects taking place on the anode electrode can be minimised by virtue of the increased surface area of the anodes, when compared to the surface area of the cathodes . In some embodiments, the surface area of the anodes may be up to twice as large as the surface area of the cathodes. In some embodiments, one or more (or even each) anode compartment may be provided with two anodes and the cathode compartments may be provided with one cathode. In other embodiments, one or more anode compartments may be provided with an anode that has a larger surface area than the surface area of a cathode. In systems where the bioelectrochemical system is limited at the cathode, similar arrangements as described above may be used, but with the cathodes having greater surface area than the anodes. In a third aspect, the present invention provides a bioelectrochemical system comprising one or more anodes and two or more cathodes or two or more anodes and one or more cathodes, wherein each anode is positioned adjacent to a cathode and each anode is separated from an adjacent cathode by an ion permeable membrane. In some embodiments of this aspect of the invention, the number of anodes equals the number of cathodes.

In a further embodiment, a combination of multiple anodes and multiple cathodes is contained within one reactor unit, which can be connected to one or more similar reactor units in a serial electrical connection. As a result, multiple reactor units can be assembled into a stack type configuration, as known to a person skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic side view of an arrangement of anode compartments and cathodes compartments in accordance with one embodiment of the present invention;

Figure 2 is a schematic plan view of an arrangement of anode compartments and cathode compartments in accordance with another embodiment of the present invention;

Figure 3 is a schematic plan view of an arrangement of anode compartments and cathode compartments in accordance with another embodiment of the present invention; Figure 4 is a schematic side view of an arrangement of anode compartments and cathode compartments in accordance with a further embodiment of the present invention; Figure 5 is a schematic side view of an arrangement of anode compartments and cathode compartments in accordance with another embodiment of the present invention; Figure 6 is a schematic side view of an arrangement of a compartments and cathode compartments in accordance with another embodiment of the present invention. In the embodiment shown in figure 6, the cathodes are connected to each other in parallel and the anodes are connected to each other in parallel; Figure 7 shows a schematic top view of an apparatus in accordance with an embodiment of the present invention;

Figure 8 shows a schematic side view of the apparatus shown in Figure 7; Figure 9 shows a schematic overview of overall reactor connections and feed streams in accordance with an embodiment of the present invention;

Figure 10A and 10B show a graph of evolution of the current over time during the first (A) and second (B) test run on synthetic feed. The current gradually exceeded 1 A in both cases, at which point the potentiostatic control became unstable. To restrict the current, the anode potential was lowered to -350 mV vs Ag/AgCl on day 39 for run 2; and

Figure 1 1 shows a graph of evolution of current for a set anode potential of -200 mV versus Ag/AgCl using real wastewater. A weekly pattern including a peak (on Mondays) can be observed. This Monday peak likely corresponds to higher pH and lower organics load at start up of the existing treatment plant. Overall, the baseline current gradually increased over the test period. DETAILED DESCRIPTION OF THE DRAWINGS

It will be appreciated that the drawings have been provided for the purposes of illustrating preferred embodiments of the present invention. Therefore, it will be understood that the present invention should not be considered to be limited to the features as shown in the present invention.

Figure 1 shows a schematic side view of an arrangement of anode compartments and cathode compartments suitable for use in a bioelectrochemical system in accordance with the one embodiment of the present invention. The embodiment shown in figure 1 includes a first anode 10 positioned in a first anode compartment 12, a first cathode 14 positioned in a first cathode compartment 16, a second anode 18 positioned in a second anode compartment 20 and a second cathode 22 positioned in a second cathode compartment 24. The electrodes 10, 14, 18, 22 may be positioned in a larger vessel (not shown). Each of the electrodes 10, 14, 18, 22 have electrical connections 1 1, 15, 19, 21 extending therefrom. This allows the electrodes to be connected to the appropriate electrical circuits required to maintain electrical charge balance in each of the compartments.

As can be seen from figure 1, each anode compartment is positioned such that it is adjacent to a cathode compartment. Desirably, the compartments are arranged such that a sequence of anode compartment, cathode compartment, anode compartment, cathode compartment, etc, is maintained. Therefore, each anode compartment is adjacent to a cathode compartment and each cathode compartment is adjacent to an anode compartment. Each anode compartment is separated from an adjacent cathode compartment and by use of an appropriate ion permeable membrane. For example, anode compartment 12 is separated from cathode compartment 16 by membrane 17. Similarly, cathode compartment 16 is separated from anode compartment 20 by membrane 21. Similarly, anode compartment 20 is separated from cathode compartment 24 by membrane 25. A further membrane 13 is shown positioned to the left of anode compartment 12. It will be appreciated that membrane 13 may be used to separate anode compartment 12 from an adjacent cathode compartment located to the left of anode compartment 12. Alternatively, the sequence of anode compartments and cathode compartments may end at the left-hand edge of anode compartment 12. In this case, membrane 13 will not be required and it can be replaced by the wall of the vessel that contains the compartments. In a similar fashion, if the sequence of anode compartments and cathode compartments continues to the right of cathode compartment 24, a further membrane will be provided to the right of cathode compartment 24. Alternatively, if the sequence ends at the right-hand edge of cathode compartment 24, a wall of the vessel that contains the compartments will be positioned to the right-hand side of cathode compartment 24.

In figure 1 , a plurality of spaced, essentially parallel membranes are provided in order to separate the respective anode compartments from the adjacent cathode compartments and to separate the cathode compartments from the adjacent anode compartments. Suitably, the lower ends of the membranes are joined or sealed to the bottom of the vessel in order to prevent liquid transfer between the anode compartments and the cathode compartments.

Figure 2 shows an alternative arrangement of anodes and cathodes suitable for use in a bioelectrochemical system in accordance with another embodiment of the present invention. In figure 2, a plurality of anode electrodes 30, 32 are provided and a plurality of cathode electrodes 34, 36 are also provided. Each anode is surrounded by an envelope or lamella formed from an ion permerable membrane. For example, anode 30 is surrounded by an envelope or lamella 31 formed from an ion selective membrane. Similarly, anode 32 is surrounded by an envelope or lamellar 35 formed from an ion permeable membrane. In this manner, anode 30 is positioned inside a anode compartment 33 that is defined in part by the envelope or lamella 31 of the ion permeable membrane. Similarly, anode 32 is positioned inside anode compartment 37 that is formed at least in part by envelope or lamella 35 of the ion permeable membrane. The cathodes 34, 36 may be also positioned within separate cathode compartments or the cathodes 34, 36 may be positioned within a single larger cathode compartment. Figure 3 shows an alternative arrangement of the ion permeable membrane for separating the anodes from the cathodes. In figure 3, instead of using a plurality of separate ion permeable membranes, a single ion permeable membrane 40 is used to separate anode 41 from cathode 42, and to separate cathode 42 from anode 43, and to separate anode 43 from cathode 44. In this regard, the ion exchange membrane 40 is effectively interwoven between the adjacent anodes and cathodes to thereby separate the anodes from the adjacent cathodes.

Figure 4 shows a schematic side view of a further embodiment of the present invention. In the embodiment shown in figure 4, the apparatus 50 includes an anode compartment 51, an adjacent cathode compartment 52, an anode compartment 53 and a cathode compartment 54. This sequence of anode compartment/cathode compartment may continue to the left of anode compartment 51 or to the right of cathode compartment 54.

The cathode compartment 52 is defined by an envelope or lamella of an ion permeable membrane 55. The envelope or lamella of ion permeable membrane 55 may be formed by taking two separate ion permeable membranes, placing them on either side of the cathode and welding or otherwise joining their ends 56, 57 together. Similarly, cathode compartment 54 is formed from an envelope or lamella 58 of an ion permeable membrane.

Each cathode compartment contains a cathode. For example, cathode compartment 52 contains cathode 59. Cathode compartment 54 contains cathode 60.

As can be seen from figure 4, each anode compartment contains 2 anodes. For example, anode compartment 51 contains anodes 61 and 62. Similarly, anode compartment 53 contains anodes 63 and 64. Therefore, the apparatus 50 shown in figure 4 provides a bioelectrochemical system in which the surface area of the anodes is approximately double the surface area of the cathodes (by virtue of their being twice as many anodes as cathodes). Consequently, if the anode reactions are rate-limiting in the overall bioelectrochemical process, the overall throughput of the apparatus 50 can be increased by virtue of the increased surface area of anodes.

Although not shown in figure 4, the person skilled in the art will appreciate that the anodes will be in electrical connection with the cathodes in order to complete an electrical circuit. Similarly, appropriate feed and effluent arrangements can be made to ensure that appropriate fluids or solutions are fed to and removed from the anode compartments and the cathode compartments.

The person skilled in the art would also readily understand that if a bioelectrochemical system is cathode limited, the anodes and cathodes shown in figure 4 may be transposed (such that the anodes of figure 4 become cathodes and the cathodes of figure 4 become anodes).

Figure 5 shows a schematic side view of an apparatus in accordance with another embodiment of the present invention. The arrangement of anode compartments and cathode compartments shown in figure 5 is generally similar to that shown in figure 4 and, for convenience, like features are denoted by like reference numerals. As shown in figure 5, anode compartment 51 is provided with two anodes 61, 62. Adjacent cathode compartment 52 is provided with cathode 59. Anode compartment 53 is provided with two anodes 63, 64. Cathode compartment 54 is provided with cathode 60. Additional anode compartments and cathode compartments are also included in the embodiment shown in figure 5. The cathode compartments are defined, at least in part, by a lamella or envelope of ion permeable membrane, such as shown by reference numerals 55 and 58. The anode compartments and cathode compartments are contained within a larger vessel 70.

The vessel 70 includes an inlet 71 through which a feed solution is fed to the vessel. As can be seen from figure 5, the anode compartments are open to the feed solution and therefore the feed solution flows into a bottom sump region (denoted by reference numeral 72) and then flows upwardly (as shown by arrows 73) through the anode compartments. The solution leaving the anode compartments flows upwardly and out of the vessel 70 (as shown by arrows 74). Therefore, a simple feed arrangement for feeding solution to the anode compartments is provided, which feed arrangement has only a single inlet 71 to the vessel 70. As there is only a single inlet 71 to the vessel 70, the risk of clogging of the inlet is minimised. As a person skilled in the art will appreciate, the position and number of inlets can be varied according to the requirements and situation in which the bioelectrochemical system is deployed. Further, effluent solution leaving the anode compartments may simply overflow from the vessel 70 for subsequent collection. Alternatively, the vessel 70 may be provided with a liquid outlet through which the effluent solution passes. In certain embodiments, the bottom sump may contain structures to enable solids separation prior to entry of the aqueous solution in the bioelectrochemical system.

Appropriate electrochemical solutions may be fed to the cathode compartments by use of a manifold arrangement that passes the solution to the cathode compartments. This arrangement is not shown in figure 5. However, as the person skilled in the art will appreciate, the envelopes or lamellae of the ion permeable membranes may have open regions or ends that are placed in fluid communication with fluid inlets and fluid outlets to respectively supply and withdraw electrochemical solutions from the cathode compartments.

Figure 6 shows another arrangement of anodes and cathodes suitable for use in the present invention. The arrangement shown in figure 6 is generally similar to that shown in figure 1 and, for convenience, like reference numerals are used to denote like parts. These features need not be described further. As shown in figure 6, the respective anodes 10, 18, etc are electrically connected to each other in parallel, using an anode current collection line 80 with appropriate respective anode electrical lines 11, 19 extending to the respective anodes 10, 18 or current collectors connecting to the respective anodes 10,18. Similarly, the respective cathodes 14, 22 are electrically connected to each other in parallel, for example, using a cathode current supply line 82 that has respective cathode electrical lines 15, 21 extending to the respective cathodes 14, 22. The person skilled in the art will appreciate that the anodes and cathodes will also be electrically connected to each other in order to complete an electrical circuit. One such possible connection is shown with dashed connection 84. The connection 84 may be provided with an additional power supply 85, if required or desired. Figures 7 and 8 provides a schematic overview of a reactor 100 in accordance with another embodiment of the present invention. Figure 7 shows a top view whilst figure 8 shows a side view of the reactor 100. A lamellar type reactor was constructed by creating 2 welded cation exchange membrane (CMI-7000, Membranes International Inc.) envelopes 101 of 1 cm thickness. Two anodic chambers were located inside the envelopes 101 while an additional single sheet membrane 1 1 1 was used as a third anode chamber as depicted in the left panel of Figure 7. The membranes were clamped and glued in a bottom and top groove (see Figure 8), surrounding an opening. Inside the membrane envelopes 101, on both sides a graphite felt anode 102 was inserted and clamped to the sides by inserting a corrugated stainless steel mesh 103. As a cathode, either only a corrugated stainless steel mesh 104 or this mesh plus two finely woven stainless steel meshes 1 14 were inserted in the cathode chamber. All corrugated meshes were welded on the side of the compartment to perpendicular stainless steel rods(e.g. cathode collector rod 105 anode collector rod 106), which electrically connected them to either an anode collector plate 107 or a cathode collector plate 1 16. The reactor was then connected to recirculation and feed circuits as shown in Figure 9.

As shown in figure 9, the reactor matrix 100 has a bottom sump 120 and a top sump 122 that are in fluid communication with the anode chambers. Anode feed solution is fed via feed line 124. Anode effluent is removed via effluent line 126. A pressure vent 128 is provided in the anode and forward line 126 to facilitate venting of any excessive gases that may be generated by the anode reactions. Anode feed line 124 is fed with various feed materials, which may include the bulk feed 130 and concentrated feed 132. As can be seen, the anode effluent is partly sent to disposal via line 134 and partly recycled to the anode feed line 124 via recycled line 136.

Similarly, the cathode compartments are fed with cathode feed solution via cathode feed line 140. Fluid from the cathode chambers are removed by cathode effluent line 142. The cathode effluent may be fed to a buffer vessel 144 that can be used to store cathode effluent. The cathode effluent may be partly sent to disposal via disposal 146 and partly recycled to the cathode feed line 140 via recycle line 148. Appropriate valves 150, 152, 154 and 156 are provided to control flow of the various streams. Example

A bioelectrochemical system was constructed in accordance with the embodiment shown in figures 7 to 9. In this system, the total anode liquid volume was 1.02L, the total cathode volume was 0.61L. These volumes do not include the sumps and a small space next to the cathodes, including wall thickness the total reactor volume was ultimately 3.313L.

Initial tests were conducted in a laboratory. The inoculum for the initial start up of the reactor was obtained from a lab scale microbial fuel cell, fed with wastewater from the mixing tank of a brewery wastewater treatment plant as well as from a pilot scale microbial fuel cell fed with brewery wastewater. During the lab based runs the anode was fed with a mixture of two media. The basic medium (initially 6.9 L d^"1, increased up to 30 L d^"1) contained per liter: 0.1 g NH₄CI, 0.1 g KH₂P0₄, 0.1 g MgS0₄.7H₂0, 0.02 g CaCl₂.2H₂0 and 1 ml of nutrient solution as described in Rabaey, K.; Ossieur, W.; Verhaege, M; Verstraete, W., Continuous microbial fuel cells convert carbohydrates to electricity. Wat Sci. Technol. 2005, 52, (1-2), 515-523.

To this medium, a concentrate containing sodium acetate (as appropriate for increasing current, starting at 3.93 g acetate L^"1) and NaHC0₃ (variable quantity to ensure pH neutrality of the incoming concentrate) was added as required to achieve a target current density depending on the status of the reactor. The flow of this concentrate was varied to achieve increasing loading rates (starting rate was 0.7 L d^"1),

1 3 1

to a maximum loading of 9.89 g acetate d^" (10.27 kg COD m^" anode d^" ). The anode was recirculated at 7 L d^"1, which roughly represents a 1/1 recirculation. The increasing concentrate addition caused increasing conductivities of the anode medium over time, i.e. for run 2 the conductivity increased from 4.29 to 9.02 mS cm^"1. The cathode was continuously fed with a salt solution (1 g NaCl L^"1), at a rate of 0.7 L d^"1, and recirculated at a rate of 7 L d^"1.

The operational period can be divided in three runs: (i) first lab based run (ii) second lab based run and (iii) brewery based run. During the first run, the cathode only contained the corrugated mesh as cathode and current collector. The system was operated for 64 days, during which the anode feed was progressively increased by increasing both concentrate concentration and flow. The experiment was terminated shortly after a failure due to gas production. Imperfect sealing between anode and cathode was observed, therefore the reactor was dismantled and rebuilt. At this stage (second lab based run) the finer meshes were inserted into the cathodes to serve as electrode, next to the corrugated mesh as current collector. The system was then operated for 46 days. After this period, the reactor was moved to a brewery where "mixing tank" wastewater was fed to the reactor. The composition of the incoming wastewater can be seen in Table 1. The influent was mixed in (1/1) with anaerobic digester effluent to achieve a higher influent pH and gain more alkalinity (composition also in Table 1). The cathode flow was 0.71 L d^"1, the anode influent flow was varied between 51 and 702 L d^"1.

After start-up (E_AN - -0.12 V vs Ag/AgCl fixed), the reactor had a considerable lag phase of 15 days (Figure 10A). It is important to note that when setting this anode potential versus a reference with a potentiostat (three-electrode setting), the anode process is still spontaneous - microorganisms will provide a current depending on the set potential. The potentiostat will if required provide the additional power to convey the anodic current towards the cathode, making this approach different than the conventional "two-electrode" approach where a cell voltage is applied, rather than an electrode potential. After this period, the current rapidly increased, on day 19 the anode potential was lowered to E_AN ⁼ -0.30 V vs Ag/AgCl. During this period, the feed supply was progressively increased to supply up to 9.89 g acetate per day, which was equivalent to a maximum theoretical current of 1.5 A. The pH of the cathode did not reach high values, i.e. the highest value achieved was 10.57. This could either be caused by back diffusion of hydroxyl ions or cross over of some anode fluid to the cathode. Upon inspecting the reactor, a small leak was discovered in the membrane sealing between the anode and the cathode chamber. The reactor was further operated using a pump both for the cathode influent and effluent at equal flow rates to prevent crossover of anode fluid to the cathode. Over time, the current increased to 1.015 A on day 62. At that point, the applied voltage over the BES was 1.77 V, which gives a calculated cathode potential of -2.07 V vs Ag/AgCl. As the cathode was not provided with a separate reference electrode, this value is off by the ohmic resistance of the system and the pH related potential difference. Impedance spectroscopy was performed, giving an estimated ohmic resistance of 0.14 Ω. Expressed as a volumetric resistance (considering 1.63 L volume in total) this implies that the resistance was 0.23 πιΩ m³.

During the current generation, increasing amounts of gas bubbles were formed. Likely, these gas bubbles were C0₂, produced during the oxidation of acetate and released through consumption of bicarbonate alkalinity at decreasing pH. Upon reaching the maximal current, the potentiostatic control became unstable on day 62. Possibly, this was due to the increased gas production, pushing down the headspace in the anode, leading to a temporary disconnection of the reference electrode from the electrolyte. After this disruption, the system was restarted for a short period, delivering current at about 0.8 A. Polarization curves could not be recorded, as the current during such an analysis was well beyond the range of the used potentiostat.

Second lab run. Cathode stainless steel mesh electrodes were inserted in the cathode compartments to increase the cathode surface area and thus improve overall cathode catalysis. After reassembly full hydraulic separation between the anode and the cathode was observed. The system was similar to the first test run, progressively supplied with more feed as required for the current generation, over a period of 46 days. Surprisingly, although the anodes used for the first test run were re-used for this second test run, a lag phase of about 18 days was observed before current started to increase (Figure 10B). The reasons for this long lag phase are as yet unknown. In this second run, a consistent increase of the current was obtained, reaching on average 0.977 ± 0.039 A on day 36 (maximum 1.054 A). After reaching these values, the potentiostatic control became unstable, and the anode potential was decreased to EA = -0.350 V vs Ag/AgCl to restrict current.

The pH of the anode effluent remained quite constant at 7.00 ± 0.35. Based on the influent and effluent concentrations, the acetate removal was 61 ± 20% over the experimental period. The pH of the cathode liquid gradually increased (average 12.5 ± 1.6 after the lag phase) reaching a value of 13.93 on day 42. This corresponds to a 3.4 %WT concentration of hydroxyl expressed as NaOH. On that day, the average current generated was 0.710 ± 0.100 A, which leads to an efficiency of current to caustic conversion of 96%. At the anode, the coulombic efficiency for acetate oxidation was 63%) (removal 75%), leading to overall acetate to caustic coulombic efficiency of 61%. On average for the stable period from day 39 till 46, the pH in the cathode was 13.76 ± 0.12 for an averaged current of 0.607 ± 0.019 A, corresponding to a efficiency of current to caustic conversion of 76%. The acetic acid removal in that period was 60 ± 27%, which theoretically allows 0.898 ± 0.404 A. Thus, for the 7 day, stable period, the average acetic acid to current conversion was 68 ± 2%, while the acetic acid to caustic conversion was 52%. The conductivity of the catholyte significantly increased over time, and exceeded 50 mS cm^"1 by day 33 (Influent conductivity 2.44 ± 0.59 cm^"'). The applied voltage over the BES was 1.202 ± 0.006 V for the 7 day period.

After 46 days of operation on acetate in the second lab based run, the reactor was transferred to a brewery, and initially connected to mixing tank effluent. Mixing tank effluent is a mix of anaerobic digester effluent and fermented liquor - plant operators perform this mixing to improve pH of the digester feed. Over a week cycle, the pH and the fatty acid content changed considerably, due to different levels of activity and cleaning in place at the brewery site. This variation in pH and organics content led to the cyclical behaviour of the current (Figure 1 1). To allow for a higher base-line current during the remainder of the week, anaerobic digester effluent (pH ~ 6.8) was mixed in with the existing feed at a 1/1 ratio. Table 1 shows typical ananysis for the mixing tank effluent and the anaerobic digester effluent. During anaerobic digestion, alkalinity is generated and thus a mix-in of digester effluent will improve the buffering capacity of the BES influent. This modification did not completely alleviate the weekly fluctuations, however it was maintained throughout the experimental period. The slow increase of the current indicates that the microbial community had to adapt considerably to the change in feed conditions. Over time, the baseline current consistently increased, with the weekly peak reaching a value of 0.38 A. Table 2 summarizes the analytical data over time for the influent and effluent of the anode and the cathode. The caustic strength was low for this experimental period: while theoretically the pH could go up to 13.4 for an assumed average current density of 0.2A on a daily basis, 12.54 was the maximal measured value. The increased conductivity over time confirms the transport of cations from anode to cathode.

Table 1. Representative composition of the mixing tank wastewater and anaerobic digester effluent obtained at the brewery. All concentration values are given in mg L^"1.

Mixing tank Anaerobic digester pH 6.1 6.8

Alkalinity (as HC03-) 242 856

Volatile fatty acids

Acetic acid 226 29

Propionic acid 307 26

i-butyric acid 3 1

n-butyric acid 125 2

i-valeric acid 3 1

n-valeric acid 120 2

hexanoic acid 6 <1

Ammonia-N 101 187

Cations

Calcium 16 15

Sodium 191 187

Potassium 17 15

Anions

Chloride 53.2 1 17

Sulfide-S <1 <1

Sulfite-S <1 <1

Sulfate-S 5.2 <1

Thiosulfate-S <1 <1

Soluble COD 2258 221

Total COD 2906 n.a.

Table 2. Overall in- and output parameters for the operation on site, using real wastewater.

The voltage over the BESs increased more than proportionally with the current over time. To investigate whether this was due to scaling at the cathode, the system was stopped at day 33, and a 1M hydrochloric acid solution was recirculated through the cathode for 5 minutes. Upon restarting the BES, the voltage over the system was considerably lowered for a comparable current, indicating that calcium scaling may have occurred at this high pH. As the incoming flow through the anode is considerable and the calcium content low, the difference between in- and effluent of the anode cannot be measured accurately. Therefore, a calcium balance could not be made for this experiment. Possible scaling in the cathode would furthermore prevent assessing the crossover from the cathode perspective.

The approach here allows to scale up BESs without compromising on the ohmic resistance of the system, which was considered a key impediment to scale up. Indeed, the ohmic resistance measured was only about 0.14 Ω. The key reasons for this is the continued close spacing of anode and cathode, as well as the use of current collectors (stainless steel) to compensate for the low conductivity of the anode. The latter are typically graphite based, and have a conductivity about two orders of magnitude lower than steel. It was noted that upon achieving higher currents, the gas production in the system considerably increased. This gas subsequently ended up in the recirculation circuit (1/1 recirculation). Upon entry in the reactor, the gas bubbles cause more turbulence than liquid would. On a short timescale, the effect of these gas bubbles could be verified by observing the fluctuation in current when either gas (increase) or liquid (decrease) was passing through the reactor.

Embodiments of the present invention provide a number of benefits when compared with previous bioelectrochemical systems. For example, the present invention allows for the provision of bioelectrochemical system units that can be used with other bioelectrochemical system units in a modular manner. This allows for easy scale up by simply adding further modules to the overall system. Some embodiments of the present invention also allow for parallel current collection/connection between multiple anodes and parallel current collection/connection between multiple cathodes. This simplifies both manufacture and operation of the system. In some embodiments of the present invention, sumps or manifolds may be used to feed and/or remove liquid or solution from the respective anodes and cathodes. Again, this simplifies construction and operation. Furthermore, the risk of clogging of the liquid inlet and outlet can be decreased, for example, by having one common inlet for a multitude of anodes or cathodes, as described previously.

In some embodiments of the present invention, improved conversion or rate of reaction can be obtained by increasing the surface area of the electrodes in the limiting electrode reaction compartments. Typically, this will be the anode reaction and in these embodiments the surface area of the anodes can be increased relative to the surface area of the cathodes, for example, up to twice the anode surface area compared to the cathode surface area.

Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. It will be understood that the present invention encompasses all such variations and modifications that fall within its spirit and scope.

Claims

1. A bioelectrochemical system comprising:

2. A bioelectrochemical system as claimed in claim 1 wherein the bioelectrochemical system comprises a plurality of anode compartments and a plurality of cathode compartments, each anode compartment being adjacent to at least one cathode compartment.

3. A bioelectrochemical system as claimed in claim 1 or claim 2 wherein the bioelectrochemical system comprises a plurality of anode compartments and at least one cathode compartment.

4. A bioelectrochemical system as claimed in claim 3 wherein the at least one cathode compartment includes a plurality of cathodes.

5. A bioelectrochemical system as claimed in claim 1 wherein the bioelectrochemical system comprises a plurality of cathode compartments and at least one anode compartment.

6. A bioelectrochemical system as claimed in claim 5 wherein the at least one anode compartment includes a plurality of anodes.

7. A bioelectrochemical system comprising a plurality of anodes, and a plurality of cathodes, each anode being adjacent to at least one cathode, each anode being separated from an adjacent cathode by an ion permeable membrane, wherein the anodes are electrically connected to each other in parallel and the cathodes are electrically connected to each other in parallel.

8. A bioelectrochemical system as claimed in claim 7 wherein the anodes are electrically connected to the cathodes in order to maintain an electrical circuit in the system.

9. A bioelectrochemical system as claimed in claim 7 or claim 8 wherein a single ion permeable membrane is used to separate each anode from an adjacent cathode.

10. A bioelectrochemical system as claimed in claim 9 wherein the single ion permeable membrane is interwoven between the anodes and the cathodes.

11. A bioelectrochemical system as claimed in claim 7 or claim 8 wherein a plurality of separate ion permeable membranes, are used to separate the anodes from the cathodes.

12. A bioelectrochemical system as claimed in claim 1 1 wherein each of the anodes or each of the cathodes is surrounded by an ion permeable membrane.

13. A bioelectrochemical system as claimed in claim 12 wherein the ion permeable membrane surrounding each of the anodes or each of the cathodes comprises an ion permeable membrane in the form of a lamella or an envelope.

14. A bioelectrochemical system as claimed in claim 13 wherein a plurality of lamellae or envelopes of ion permeable membranes are formed and a plurality of electrode chambers are created by connecting the plurality of lamellae or envelopes at opposite ends.

15. A bioelectrochemical system as claimed in any one of the preceding claims wherein the bioelectrical system comprises an anode compartment feed arrangement comprising a sump or a manifold for feeding a liquid or a solution to anode compartments.

16. A bioelectrochemical system as claimed in any one of the preceding claims wherein the bioelectrical system comprises a cathode compartment feed arrangement comprising a sump or a manifold for feeding a liquid or a solution to cathode compartments.

17. A bioelectrochemical system as claimed in any one of the preceding claims wherein the bioelectrical system comprises an anode compartment fluid removal arrangement comprising a sump or a manifold for removing a liquid or a solution from anode compartments.

18. A bioelectrochemical system as claimed in any one of the preceding claims wherein the bioelectrical system comprises a cathode compartment fluid removal arrangement comprising a sump or a manifold for removing a liquid or a solution from cathode compartments.

19. A bioelectrochemical system as claimed in any one of the preceding claims wherein the anode electrode has a larger surface area then the cathode electrodes.

20. A bioelectrochemical system as claimed in claim 19 wherein the surface area of the anode electrodes is up to twice as large as the surface area of the cathode electrodes.

21. A bioelectrochemical system comprising one or more anodes and two or more cathodes or two or more anodes and one or more cathodes, wherein each anode is positioned adjacent to a cathode and each anode is separated from an adjacent cathode by an ion permeable membrane.

22. A bioelectrochemical system as claimed in claim 21 wherein the number of anodes equals the number of cathodes.

23. A bioelectrochemical system as claimed in any one of the preceding claims wherein each anode compartment is adjacent to a cathode compartment and each cathode compartment is adjacent to an anode compartment, and each anode compartment is separated from an adjacent cathode compartment by an ion permeable membrane.