Method for nitrogen recovery from an ammonium comprising fluid and bio-electrochemical system

Abstract

A method is disclosed for nitrogen recovery from an ammonium including fluid and a bio-electrochemical system for the same. In an embodiment, the method includes providing an anode compartment including an anode; providing a cathode compartment including a cathode, wherein the compartments are separated by at least one ion exchange membrane; providing the ammonium comprising fluid in the anode compartment and a second fluid in the cathode compartment; applying a voltage between the anode and the cathode; and extracting nitrogen from the cathode compartment.

Claims

1. Method for nitrogen recovery from an ammonium comprising fluid, comprising: providing an anode compartment comprising an anode; providing a cathode compartment comprising a cathode, wherein the compartments are separated by at least one ion exchange membrane; providing the ammonium comprising fluid in the anode compartment and a second fluid in the cathode compartment; applying a voltage between the anode and the cathode; and extracting nitrogen from the cathode compartment.

2. Method according to claim 1, comprising the step of providing a bio-electrode as the anode and/or the cathode.

3. Method according to claim 1, comprising the step of providing the fluid in the cathode compartment with a pH above 7.

4. Method according to claim 3, comprising the step of providing the fluid in the cathode compartment with a pH above 8.

5. Method according to claim 4, comprising the step of providing the fluid in the cathode compartment with a pH above 10.

6. Method according to claim 1, wherein extracting the nitrogen comprises extracting ammonia, the method further comprising the step of feeding the ammonia to a fuel cell.

7. Method according to claim 1, wherein extracting the nitrogen comprises extracting molecular nitrogen, the method further comprising the step of providing an additional anode.

8. Method according to claim 7, wherein providing an additional anode comprises providing the additional anode in the cathode compartment.

9. Method according to claim 7, further comprising the steps of: providing an additional compartment between the anode compartment and the cathode compartment and arranging the additional anode in the additional compartment; separating the anode compartment from the additional compartment by a first ion exchange membrane; and separating the additional compartment from the cathode compartment by a second ion exchange membrane.

10. Method according to claim 7, comprising the step of feeding fluid from the anode compartment to the compartment comprising the additional anode.

11. Method according to claim 1, comprising the step of supplying oxygen to the cathode compartment.

12. Method according to claim 1, comprising providing a urine comprising fluid as ammonium comprising fluid.

13. Method according to claim 1, comprising providing an ammonium comprising fluid having an ammonium nitrogen concentration ≅0.5 g/l.

14. Method according to claim 13, comprising providing an ammonium comprising fluid having an ammonium nitrogen concentration >1 g/l.

15. Method according to claim 14, comprising providing an ammonium comprising fluid having an ammonium nitrogen concentration >10 g/l.

16. Bio-electrochemical system for nitrogen recovery from an ammonium comprising fluid, comprising: an anode compartment comprising an anode; a cathode compartment comprising a cathode; wherein the compartments are separated by at least one ion exchange membrane; a power supply connected to the anode and the cathode; and an additional anode, wherein a fluid outlet of the anode compartment is connected to a fluid inlet of the compartment that comprises the additional anode.

17. Bio-electrochemical system according to claim 16, wherein the additional anode is arranged in the cathode compartment.

18. Bio-electrochemical system according to claim 16, wherein an additional compartment is arranged between the anode compartment and the cathode compartment, the additional compartment comprising the additional anode, wherein the anode compartment is separated from the additional compartment by a first ion exchange membrane and the additional compartment is separated from the cathode compartment by a second ion exchange membrane.

19. Bio-electrochemical system according to claim 16, comprising a gas outlet that is connected to a fuel cell.

Description

(1) Further advantages and details of the invention are elucidated using the accompanying drawings, wherein:

(2) FIG. 1 illustrates a first embodiment of the method according to the invention, wherein hydrogen and ammonia are produced;

(3) FIG. 2 illustrates a second embodiment of the method according to the invention wherein ammonia is produced;

(4) FIG. 3 illustrates a third embodiment of the method according to the invention wherein an additional anode in the cathode compartment is provided and molecular nitrogen and hydrogen are produced;

(5) FIG. 4 illustrates a fourth embodiment of the method according to the invention, wherein an additional anode in the cathode compartment is provided and molecular nitrogen and hydrogen is produced;

(6) FIG. 5 illustrates a fifth embodiment of the method according to the invention wherein an additional compartment is provided and an additional anode is provided within this additional compartment;

(7) FIG. 6 shows experimental results in the form of a graph of pH, current and cell voltage over time; and

(8) FIG. 7 shows a block diagram of a combination of an MAP process and the method according to the invention.

(9) In FIGS. 1-5, similar components have been given the same reference numeral increased with a hundreds, e.g. element 14 in FIG. 1 is similar to element 114, 214, 314, 414 in FIGS. 2, 3, 4 and 5 respectively.

(10) A first bio-electrochemical system 2 (FIG. 1) comprises an anode compartment 4 with a bio-anode 6 and a cathode compartment 8 with a cathode 10. The anode compartment 4 and the cathode compartment 8 are separated by a cation exchange membrane 12. The bio-anode 6 and the cathode 10 are connected with a power supply 14.

(11) The anode compartment 4 comprises an inlet 16 for supplying urine to the anode compartment 4, as indicated with the arrow. The cathode compartment 8 comprises an outlet 18 for extraction of a hydrogen ammonia mixture (H.sub.2/NH.sub.3). The anode compartment comprises an outlet 20 for releasing effluent. Preferably, the effluent is released periodically, for example by means of a valve controlled by a control mechanism. The effluent is optionally fed to a system for removal of phosphorus. This is in particular efficient due to the low carbonate/bicarbonate concentration of the effluent. Furthermore, the low pH of the anode is beneficial for phosphorus recovery, since carbonate (CO.sub.3.sup.2−) can be easily stripped as CO.sub.2 at a low pH:
H.sub.2CO.sub.3 H.sub.2O+CO.sub.2
H.sub.2CO.sub.3 HCO.sub.3.sup.−+H.sup.+(pKa=6.3)
HCO.sub.3.sup.− H.sup.++CO.sub.3.sup.2− (pKa=10.05)

(12) Less carbonate in the solution means that less calcium carbonate (CaCO.sub.3) will be formed during the recovery of phosphate as a calcium phosphate (e.g. hydroxyapatite, Ca.sub.5 (PO.sub.4).sub.3(OH)). Therefore, phosphate recovery becomes more efficient.

(13) The outlet 18 of the cathode compartment 8 is connected to a fuel cell 22 for the production of electricity. The amount of power produced by fuel cell 22 exceeds the power consumption of power supply 14 and a part of the produced power is directed to the power supply 14 to power the process.

(14) The urine supplied to the anode compartment 4 via inlet 16 comprises ammonium ions (NH.sub.4.sup.+) and organic compounds (COD), such as acetate ions (CH.sub.3COO.sup.−). The bacteria of the bio-anode 6 will oxidize the acetate according to the following reaction:
CH.sub.3COO.sup.−+4H.sub.2O.fwdarw.2HCO.sub.3.sup.−+9H.sup.++8e.sup.−

(15) For every mole of acetate eight moles of electrons are produced at the bio-anode 6.

(16) The power supply 14 connected to these electrodes provides an additional driving force for the ammonium transport from anode compartment 4 to the cathode compartment 8 (arrow A). Due to this added energy, higher current densities are achieved, which lead to a high ammonium transport. At the cathode 10 protons (H.sup.+) are reduced to form hydrogen (H.sub.2):
3H.sub.2O.fwdarw.H.sub.2+2OH+H.sub.2O

(17) Due to the proton consumption of the cathode 10, the pH of the cathode compartment 8 increases. This enables the reaction of ammonium ions in the cathode compartment according to the reaction:
NH.sub.4.sup.++OH.sup.−.fwdarw.NH.sub.3+H.sub.2O

(18) Therefore, the produced hydrogen and ammonia can be extracted from outlet 18 and fed to a modified fuel cell 22. The modified fuel cell can produce electricity from both hydrogen and ammonia. Alternatively, the modified fuel cell is replaced by a solid oxide fuel cell (SOFC).

(19) Bio-electrochemical system 102 (FIG. 2) comprises an anode compartment 104 with a bio-anode 106 and a cathode compartment 108 with a cathode 110. Again, the anode compartment 104 and cathode compartment 108 are separated by a cation exchange membrane 112. The anode 106 and cathode 110 are connected to a power supply 114. Urine is provided to the anode compartment 104 through inlet 116. Ammonia (NH.sub.3) which is produced in the cathode compartment 108 is extracted through outlet 118. Effluent from the anode compartment 104 is released periodically via outlet 120.

(20) The ammonia extracted via outlet 118 is fed to an ammonia fuel cell 122 to produce electricity. Again, the energy produced exceeds the energy requirement of the power supply 114 and a part of the electricity can be used to power the power supply 114. The cathode compartment 108 further comprises an inlet 121 for providing oxygen (O.sub.2) to the fluid in the cathode compartment 108.

(21) In the anode compartment 104 the same reactions take place as described with respect to FIG. 1. In the cathode compartment the availability of oxygen (O.sub.2) enables a reaction at the cathode:
4H.sup.++O.sub.2+4e.sup.−.fwdarw.2H.sub.2O

(22) Therefore, substantially no hydrogen is produced and the outlet 118 of the cathode compartment 108 releases predominantly ammonia.

(23) The bio-electrochemical system 202 (FIG. 3) comprises an anode compartment 204 with a bio-anode 206, a second compartment 208 comprising a cathode 210 and an additional anode 211. The cathode 210 and the additional anode 211 are separated from the anode 206 by a cation exchange membrane 212. The anode 206, the cathode 210 and the additional anode 211 are connected to a power supply 214.

(24) In this example, the voltage of the anode 206 and the additional anode 211 is substantially the same, since the anode 206 and additional anode 211 are connected in parallel. It is noted that this need not be the case and the voltage of the electrodes may be controlled individually according to the invention.

(25) The anode compartment 204 comprises an inlet 216 for providing urine and the cathode compartment 208 comprises a first outlet 217 for releasing molecular nitrogen (N.sub.2) and a second outlet 218 for releasing hydrogen (H.sub.2). Furthermore the anode compartment 204 comprises an outlet 220 for periodically releasing effluent fluid.

(26) The reactions in the anode compartment 204 are the same as described above. In the second compartment 208 an additional reaction occurs due to the additional anode 211. At the additional anode 211 the ammonia (NH.sub.3) in the compartment 208 is decomposed according to the following reaction:
2NH.sub.3.fwdarw.N.sub.2+6H.sup.++6e.sup.−.

(27) At the cathode protons are oxidized to form hydrogen. Therefore in compartment 208, molecular nitrogen is formed at the additional anode 211 and hydrogen is formed at the cathode 210, such that outlet 217 substantially comprises N.sub.2 (g) and outlet 218 substantially comprises H.sub.2 (g).

(28) Bio-electrical system 302 (FIG. 4) shows as a similar arrangement as in FIG. 3. A difference between the system of FIG. 3 and FIG. 4 is that in system 302 the second compartment 308 comprises an inlet 322 which is connected to outlet 320 of the anode compartment 304 such that effluent from the anode compartment 304 is directed into the second compartment 308. Preferably the fluid is fed periodically to the second compartment 308.

(29) The effluent exiting the anode compartment 304 may still comprise some ammonia which can be further decomposed by exposing it to the additional anode 311 in the second compartment 308.

(30) In system 402 (FIG. 5), three compartments are provided: an anode compartment 404 comprising a bio-anode 406, a cathode compartment 408 comprising a cathode 410 and an additional compartment 409 comprising an additional anode 411. The anode compartment 404 is separated from the additional compartment 409 by a cation exchange membrane 412 and the additional anode compartment 409 is separated from the cathode compartment 408 by an anion exchange membrane 413. Anode 406, anode 411 and cathode 410 are connected to power supply 414. Urine is provided via inlet 416. Nitrogen (N.sub.2) is released via outlet 418.

(31) The effluent from the anode compartment 404 is fed from the outlet 420 of the anode compartment 404 to the inlet 422 of the additional compartment 409. Fluid can exit the additional compartment 409 via outlet 424. Furthermore, the cathode compartment 408 comprises an inlet 421 for O.sub.2 supply.

(32) In the cathode compartment oxygen (O.sub.2) and protons (H.sup.+) form water (H.sub.2O). Hydroxide ions (OH.sup.−) pass through the anion exchange membrane 413 (arrow B). Therefore, no hydrogen is produced and all the ammonia in the urine is decomposed into water and molecular nitrogen N.sub.2 (g), which exits via outlet 418.

(33) Optionally, cathode compartment 408 comprises an outlet for surplus gas.

(34) An experiment has been performed using system 202 of FIG. 3. The system was operated for three days on real urine and produced 678.08 ml of gas, composed of 86% H.sub.2, 0.05% O.sub.2 and 6% N.sub.2. The graph of FIG. 6 shows further results of the experiment. The potential of the anode and the cathode was measured relative to a Ag/AgCl reference electrode. The graph shows a constant voltage level during the 3 days for both the anode (E.sub.anode), graph 508, as the system (E.sub.cell), graph 500. The current, graph 502, dropped slightly during three days operation. The pH levels of the anode, graph 506, and cathode, graph 504, are constant. Furthermore, ammonia was recovered in the experiment. In a further experiment, the system was operated stably for more than 270 days.

(35) In a further experiment, H.sub.2 and NH.sub.3 was produced using bio-electrochemical system 2 of FIG. 1. The experiments were conducted using different types of membranes. The results are shown in the table below.

(36) TABLE-US-00001 Membrane COD in i η CE N removal η H.sub.2 type (mg/L) (A/m.sup.2) (%) (%) (%) AEM (s) 475.7 28.27 95.78 1.88 94.06 CEM (s) 480.2 8.07 87.13 18.21 86.9 CEM (r) 700.8 15.74 99.5 32.6 86 CEM (r) 658 22.61 99.1 34.3 90.9

(37) The first column of the table shows the membrane type: cation exchange membrane (CEM) (membrane 12) or anion exchange membrane (AEM) (instead of membrane 12). The column further denotes whether synthetic wastewater (s) or real wastewater (r) was used. In this case, the real wastewater was urine.

(38) The second column shows the chemical oxygen demand (COD) of the fluid in mg/L. COD is a sum parameter of the oxidizable compounds in the fluid.

(39) The third column shows the current density (i) in A/m.sup.2.

(40) The fourth column shows the Coulombic efficiency (η CE), which is the efficiency of conversion of COD to electrons.

(41) The fifth column shows the nitrogen removal from the influent as a percentage.

(42) The last column shows the hydrogen efficiency (η H.sub.2), which is the efficiency of current to hydrogen gas.

(43) It is noted that the use of a CEM is particularly advantageous for removing nitrogen from a fluid, as a high removal is achieved at relatively low current densities, while at the same time providing a high hydrogen efficiency.

(44) The method according the invention is advantageously combined with further treatment steps. For example, the method according to the invention is combined with a Magnesium Ammonium Phopsphate (MAP) precipitation process (FIG. 7).

(45) Urine is supplied to a short term storage α, which enables hydrolysis of urea to form NH.sub.3. Furthermore, storage α serves as a buffer to regulate the inflow to the system. From storage α the urine flows to a struvite reactor β, wherein magnesium phosphate ammonium (MAP), MgNH.sub.4PO.sub.4.6H.sub.2O, is formed. For example, NH.sub.4.sup.+, Mg.sup.2+ and PO.sub.4.sup.3− react at elevated pH to form MAP. The precipitated MAP is extracted, as indicated by the arrow P, and can for example be used as a fertilizer. Therefore, in this first treatment step, both ammonium and phosphorus are removed from the fluid. However, not all ammonium will be precipitated.

(46) The supernatant effluent from reactor β is fed to reactor γ, in which the method according the invention is applied. In this example, reactor γ corresponds to the system described in relation to FIG. 1, i.e. a bio-electrochemical system for NH.sub.3/H.sub.2 production. However, instead of this system, any other system and method according to the invention can be used for reactor γ.

(47) The treated urine leaving reactor γ will be depleted from nitrogen, COD and phosphorus. The hydrogen/ammonia mixture is fed to a solid oxide fuel cell (SOFC) δ. Also oxygen is supplied to fuel cell δ. Other types of fuel cells may be used as well, in particular when a different method according to the invention is used in reactor Y.

(48) Energy will be produced in system δ and the end products are N.sub.2 and H.sub.2O, which are harmless to the environment.

(49) The present invention is by no means limited to the above described embodiments thereof. The rights sought are defined by the following claims within the scope of which many modifications can be envisaged. For example, the method according to the invention may be operated batchwise or continuously on an ammonium comprising fluid.