Method for controlling the operation of a microbial fuel cell arrangement and microbial fuel cell arrangement

Abstract

The invention relates to a method for controlling operation of a microbial fuel cell arrangement having at least one microbial fuel cell unit. The unit comprises an anode and a cathode, which are connected with each other via an external electrical circuit. In the method an influent flow of liquid medium, which comprises organic substance(s), is fed to the microbial fuel cell unit, and at least a part of the organic substance(s) are converted into electrical energy in the microbial fuel cell unit by using microorganisms. Potential of the anode is measured against a reference electrode and obtaining measurement value(s), and the measured value(s) are used for controlling the feed of the influent flow to the microbial fuel cell unit. An effluent flow of treated liquid medium is removed from the microbial fuel cell unit. The invention relates also to microbial fuel cell, which comprises a controller, which is arranged in functional contact with the means for measuring the potential of the anode and with the means for adjusting the influent flow to the inlet.

Claims

1. A method for controlling operation of a microbial fuel cell arrangement having at least one microbial fuel cell unit, which unit comprises an anode and a cathode, which are connected with each other via an external electrical circuit, the method comprising feeding an influent flow of liquid medium, which comprises organic substance(s), to the microbial fuel cell unit, converting at least a part of the organic substance(s) into electrical energy in the microbial fuel cell unit by using microorganisms, measuring potential of the anode against a reference electrode and obtaining measurement value(s), and using the measured value(s) for controlling the feed of the influent flow to the microbial fuel cell unit, and removing an effluent flow of treated liquid medium from the microbial fuel cell unit.

2. The method according to claim 1, wherein feed of the influent flow is adjusted on basis of the obtained measurement value(s) for the anode potential.

3. The method according to claim 1, wherein feed rate of the influent flow is adjusted on basis of the obtained measurement value(s) for the anode potential.

4. The method according to claim 1, wherein the arrangement comprises a plurality of microbial fuel cell units arranged in series, whereby the anode potential of at least one microbial fuel cell unit is measured and the feed of the influent flow to the arrangement is controlled and/or adjusted.

5. The method according to claim 1, further comprising predetermining a minimum and/or maximum limit value for the potential of the anode, and adjusting the feed of influent flow when the potential of the anode deviates outside the minimum and/or maximum limit value.

6. The method according to claim 1, wherein value for an additional parameter, which is selected from pH, concentration of organic substance(s) and/or redox value of the influent flow and/or the effluent flow, is measured and obtained measurement value for the additional parameter is used for control and/or adjustment of the feed of the influent flow.

7. The method according to claim 6, further comprising predetermining an upper and/or a lower limit value for the additional parameter, and adjusting the feed of the influent flow when the value of the additional parameter deviates outside the upper and/or lower limit value.

8. The method according to claim 6, further comprising predetermining the minimum and maximum limit values for the potential of the anode; measuring the potential of the anode against the reference electrode, and observing that the potential of the anode is within an accepted range defined by the minimum and the maximum limit values; and measuring the value of at least one additional parameter and using the obtained measurement value for at least one additional parameter for adjusting the feed of the influent flow.

9. The method according to claim 6, further comprising predetermining the minimum and maximum limit values for the potential of the anode; measuring the potential of the anode against the reference electrode, and observing that the anode potential outside the minimum and the maximum limit values; and measuring the value of at least one additional parameter and using the obtained measurement value for the additional parameter for adjustment of the feed of the influent flow.

10. The method according to claim 1, wherein a wait time is applied between two successive adjustments of influent feed.

11. The method according to claim 1, wherein at least one minimum and maximum feed level is determined for the feed of the influent flow.

12. The method according to claim 1, wherein the liquid medium is municipal or agricultural wastewater, an effluent from pulp and paper industry process; an effluent from oil and gas industry process; an effluent from mining process; or the liquid medium originates from food or beverage industry.

13. A microbial fuel cell arrangement, comprising at least one microbial fuel cell unit comprising a cell reactor having an inlet and outlet for liquid medium, as well as an anode and a cathode arranged in the cell reactor and connected with each other through an external electrical circuit, means for measuring potential of the anode against a reference electrode, means for adjusting influent flow to the inlet, a controller, which is arranged in functional contact with the means for measuring the potential of the anode and with the means for adjusting the influent flow to the inlet.

14. The arrangement according to claim 13, wherein the arrangement comprises at least one additional sensor selected from a group of pH sensors, sensors for measuring concentration of organic substance(s) and/or redox sensor.

15. The arrangement according to claim 13, wherein the arrangement comprises memory means for storing the measured anode potential values and/or calculation means for calculating an average anode potential value from the measured anode potential values.

Description

EXPERIMENTAL

[0054] Some embodiments of the invention are described in the following non-limiting examples.

[0055] Arrangement Construction

[0056] A microbial fuel cell arrangement comprising of a MFC reactor, various sensors, pumps and a feed controller was designed and evaluated in laboratory scale.

[0057] Three similar microbial fuel cell reactors were used in the examples. Each reactor comprised an anode, which was a carbon cloth (FuelCellsEtc GDL-CT) and a cathode, which was a carbon cloth with catalyst and waterproof layer (FuelCellsEtc GDL-CT with 2 mg/cm.sup.2 platinum and PTFE). Anode chamber volume was 25 ml in all examples. Active electrode areas were 50 cm.sup.2. The reactor configuration was flat sheet. A separator, which was tissue, was placed between the anode and cathode

[0058] A pH electrode and a PT100 thermometer were arranged in a pre-fermentation vessel, from which the feed of liquid medium was pumped to the reactor inlet of the microbial fuel cell. The pre-fermentation vessel and microbial fuel cell were arranged close to each other in order to avoid unnecessary degradation of organic substances in the influent feeding line. The reactor inlet was located in the lower part of the anode chamber of the reactor and the effluent outlet was located on the upper part of the anode chamber of the reactor. A reference electrode was placed in the effluent flow close to the reactor outlet and pH electrode was placed in the effluent flow close to the reference electrode. The effluent flow was led to an effluent vessel.

[0059] Used pH electrodes were obtained from Van London-Phoenix, type PHO-5533501. Reference electrodes were from BASi, type RE-5B Ag/AgCl. Pumps were Masterflex L/S 07522-30. Each pH electrode was connected to a signal converter and the signal converters were connected to Siemens IO cards. The controller was S7-300 314 Programmable Logic Controller (PLC) using 4 Siemens S7-300 IO cards: 2 analog inputs and 2 analog outputs. The setup included a Siemens TP177 touch screen as user interface. Thermometer, anode electrode, reference electrode and pumps were connected to the IO cards. Temperature compensation for pH was done within the control logic based on the individual pH signals and temperature of pre-fermented wastewater as measured by the PT100.

[0060] Controller sent also 4-20 mA signals to datalogger (Agilent 34972A). The signals included the pH measurements, temperature measurement and flow orders.

[0061] Operation of the Arrangement

[0062] The three identical microbial fuel cell arrangements were inoculated at the same time. The inoculation lasted 3 days. The arrangements were operated for 66 days in total. The arrangements were operated at 27.60.6 C. temperature.

[0063] The arrangements were fed with pre-fermented brewery wastewater. The hydraulic retention time of the pre-fermentation vessel varied between 16-24 hours. The soluble COD of the pre-fermented wastewater varied between 3300700 mg/I during the testing period.

[0064] Each arrangement had its own pump for feeding pre-fermented brewery wastewater as liquid medium. After inoculation all three arrangements started receiving liquid medium at a flow rate 2 ml/h. From day 11 onwards liquid medium was fed to two arrangements at a rate determined by the measurement results obtained from the arrangements. Liquid medium was fed through the reactor at a flow rate, which varied between 2-20 ml/h. Adjustments to the feed were done manually for one arrangement based on the anode potential and COD reduction. For this arrangement the feed rate was 2 ml/h during operation days 3-59, and 3 ml/h during operation days 59-66.

[0065] When deemed necessary, an analysis of soluble COD was performed two to three times a week. Analysis of the soluble COD was performed on a sample from pre-fermentation vessel and on samples at the end of effluent line. Weights of effluent vessels were measured at the same time.

[0066] A variable external resistor was connected between anode and cathode. Its value was changed manually from initial 500 Ohm to 60 Ohm at the end of the test period in four intermediate steps. Potentials of the anode, cathode and reference electrode were measured at 3-10 minute intervals and recorded with the datalogger. The cell voltage and external resistor value were used to calculate power and current. All power production (W/m.sup.3) results and COD removal (kg/m.sup.3/d) results are expressed in relation to anode chamber volume.

[0067] Control Logic for the Arrangements

[0068] Automatic control logic was programmed by using the anode potential, pH of the influent flow and pH of the effluent flow as parameters. If the anode potential exceeded its predetermined level and/or the effluent pH fell below its predetermined level, following criteria was used to determine whether flow rate needed to be increased or decreased:

[0069] 1) In case the anode potential had exceeded the predetermined level, but the effluent pH had not fallen below the predetermined level, and there had not been a feed rate change within a defined wait time, the feed rate was increased by a predetermined amount of 10%.

[0070] 2) In case anode potential had exceeded the predetermined level and the effluent pH had fallen below the predetermined level, and there had not been a feed rate change within a defined wait time, the feed rate was decreased by a predetermined amount of 25%.

[0071] 3) In case the anode potential was still within the accepted range, but the effluent pH had fallen below the predetermined level and there had not been a feed rate change within a defined wait time, the feed rate was decreased by a predetermined amount. If the influent pH was above its predetermined level, the feed rate was decreased by of 10%, otherwise the decrease was 25%.

[0072] A rule for maximising feed rate was included: when the anode potential was below a predetermined level and the effluent pH was above a predetermined level, after every n.sup.th wait time period a 10% increase was applied to feed rate.

[0073] Feed was adjusted after a wait time. Wait time was calculated as follows: wait time (in hours)=(Hydraulic Retention Time)/3. If the feed rate was changed based on changes in the anode potential or effluent pH, an additional delay of 0.5 hour was added in wait time calculation.

[0074] The control logic defined for each MFC arrangement the flow order, i.e. the actual amount of liquid medium that was fed to the MFC arrangement under one hour. Operator could manually change the flow orders defined by the control logic to start a new experiment. The influent pumps feeding the liquid medium were operated in 1 minute periods. The control logic adjusted the time when pumping was on during the period to achieve the defined flow order. The pumping speed during pumping intervals was defined by the maximum flow limit.

[0075] The limit values for anode potential, influent flow pH and effluent flow pH, which were used in control logic, could be changed during operation. Other operator definable parameters were wait time multiplier n and maximum feed flow. Following values or value ranges were used for limits and variables during the test: [0076] Anode potential: 380-420 mV, vs. Ag/AgCl 3 M NaCl reference electrode for pre-fermented brewery wastewater, 28 C. operation temperature, mixed bacterial culture; [0077] Effluent flow pH: 6.5-6.8, for pre-fermented brewery wastewater and air cathodes; [0078] Influent flow pH: 6.3-6.7, for pre-fermented brewery wastewater and HRT 1-13 hours; [0079] Wait time multiplier n: 4-8; [0080] Minimum flow: 2 ml/h; [0081] Maximum flow: 5-20 ml/h

Example 1

[0082] There was a sudden increase in anode potential. Effluent flow pH was above the defined upper limit value of 6.6. Controller increased feed rate twice until anode potential leveled below the defined limit 420 mV.

[0083] FIG. 1 shows the anode potential, flow order and effluent flow pH for controlled MFC arrangement 1 used in Example 1.

Example 2

[0084] Anode potential rose above the defined limit of 420 mV. Six feed rate increases were applied until effluent flow pH decreased below the defined limit value of 6.6. The low pH value was considered to indicate overfeeding. After that the controller decreased the feed rate, four feed rate decreases were applied. Example 2 shows that the defined anode potential limit was possibly too low and resulted in unnecessary feed rate changes. However, the controller was able to adjust and readjust feed rate accordingly.

[0085] FIG. 2 shows the anode potential, flow order and effluent flow pH for controlled MFC arrangement 1 in Example 2.

Example 3

[0086] External resistor was changed from 125 Ohm to 90 Ohm during the experiment at time 15:20. Anode potential rose momentarily. Effluent feed pH was above the defined limit value of 6.8. Controller increased feed rate twice until anode potential leveled below the defined limit value of 400 mV. As the change in external circuit was substantial, it affected also the anode potential and controller adjusted feed rate of the liquid medium accordingly.

[0087] FIG. 3 shows the anode potential, flow order and effluent flow pH for controlled MFC arrangement 1 used in Example 3.

Example 4

[0088] External resistor was changed from 75 Ohm to 60 Ohm during the experiment at time 13:00 and anode potential rose above the defined limit value of 395 mV. Effluent flow pH was above the defined limit value of 6.5. Controller increased feed rate twice until anode potential leveled below the defined maximum limit value. As the effluent flow pH remained above the limit, feed rate was increased as an additional adjustment using wait time multiplier value n=8.

[0089] FIG. 4 shows the anode potential, flow order and effluent flow pH for controlled MFC arrangement 2 used in Example 4.

Example 5: Comparison of Performance of MFC Arrangements

[0090] Coulombic efficiency is compared for the controlled MFC arrangements 1 and 2 and the reference MFC.

[0091] The coulombic efficiency (CE %) is calculated using the ratio of total coulombs obtained, C.sub.out to the theoretical amount, C.sub.in, available from complete influent oxidation:

CE %=C.sub.out/C.sub.in100%=(It)/((FnCODV)/M)

[0092] where

[0093] I is the average daily average current (A), calculated from cell voltage and resistor value;

[0094] t is the time interval;

[0095] M is the molecular weight of oxygen;

[0096] F is the Faraday constant;

[0097] n is the number of electrons exchanged per mole of oxygen;

[0098] COD is the weight of removed amount in soluble COD;

[0099] V is the volume of reactor anode chamber.

[0100] FIG. 5 shows the coulombic efficiency for the MFC arrangements in Example 5.

[0101] It can be observed from FIG. 5 that the reference MFC arrangement had after day 30 of the test period usually a higher coulombic efficiency value than the controlled MFC arrangements 1 and 2. This outcome is expected as the controller was programmed to increase feed at intervals if the anode potential and effluent flow pH values were in the defined range. The target for the control of the controlled MFC arrangements was to maximize COD removal without impairing exoelectrogenic activity. The result was a decrease in the coulombic efficiency, as the current could not be increased at the same time.

[0102] FIG. 8 shows the coulombic efficiency for the MFC arrangements in Example 5.

[0103] Maximum power point (MPP) data for MFC arrangements are shown in Table 1. MPP data was obtained from linear sweep voltammetry (LSV) scans using two electrodes and a potentiostat (Ivium nSTAT). First scans were performed at day 9 and last scans were performed at day 62.

[0104] Even though the controlled MFC arrangements were run to maximize COD reduction, power production results of LSV scans for all three MFCs are similar. Thus the feed rate has not been too high to degrade the electroactive biofilm. The target of the example was achieved: higher COD removal without power decrease. For comparison purposes the average COD removal rate during the test period is included in table 1.

TABLE-US-00001 TABLE 1 Maximum power point and COD removal results for Example 5 Number Average of Maximum of Average of of LSV MPP results MPP results COD removal Reactor scans (W/m.sup.3) (W/m.sup.3) (kg/m.sup.3/d) Reference MFC 3 137 155 3.5 Controlled 4 129 162 4.6 MFC 1 Controlled 4 140 154 4.5 MFC 2

[0105] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.