WO2020109494A1

WO2020109494A1 - A process to convert total ammonia nitrogen

Info

Publication number: WO2020109494A1
Application number: PCT/EP2019/082953
Authority: WO
Inventors: ter HEIJNE ANNEMIEKE; Johannes Bernardus Maria Klok; Rieks DE RINK; Cees Jan Nico Buisman; Dandan Liu
Original assignee: Paqell B.V.
Priority date: 2018-11-30
Filing date: 2019-11-28
Publication date: 2020-06-04
Also published as: EP3887319A1; CN113365952A

Abstract

The invention is directed to a process to oxidise total ammonia nitrogen, also referred to as TAN, as dissolved in an aqueous solution (1) by (i) contacting the aqueous solution with oxidised ammonia- oxidizing bacteria (AOB) under anaerobic conditions wherein TAN is converted and reduced ammonia-oxidizing bacteria is obtained and (ii) wherein in a separate step the reduced ammonia-oxidizing bacteria are oxidised in a separate step by transfer of electrons to an anode (4) of an electrochemical cell (3) to obtain the oxidised ammonia-oxidizing bacteria.

Description

A PROCESS TO CONVERT TOTAL AMMONIA NITROGEN

The invention is directed to oxidise total ammonia nitrogen (TAN) as dissolved in an aqueous solution by contacting the aqueous solution with ammonia-oxidizing bacteria.

Such processes are known for waste water treatment. TAN dissolved in waste water is converted in an aerobic process to nitrite in the presence of ammonia oxidizing bacteria (AOB). TAN is defined as the total amount of dissolved nitrogen in the forms of NH3 and NH4⁺. The nitrite is subsequently converted to nitrate in the presence of nitrite oxidizing bacteria (NOB). Nitrate can subsequently be converted to nitrogen by denitrification in the presence of denitrifying bacteria, using organic matter as electron donor, in a subsequent anaerobic process.

One of the disadvantages of the above process is that in the aerobic first step nitrous oxide (N2O) may be formed, which is a strong greenhouse gas. In typical waste water treatment processes this gaseous by-product can easily escape to the environment. Furthermore, a large amount of electrical energy is required for aeration, in order to remove the TAN from the wastewater.

US2011/0183159 describes a process for the oxidation of ammonia in a microbial fuel cell (MFC). The microbial fuel cell is provided with an anode and a cathode. A first fluid is in contact with the anode and contains microorganisms capable of catalysing the oxidation of ammonium. A second fluid is in contact with the cathode and contains microorganisms capable of catalysing the reduction of nitrite. The conversion of ammonia takes place in a biofilm as present at the surface of the electrode. This process is an improvement with respect to the above described aerobic waste water treatment processes in that significantly less nitrous oxide will be formed and that less energy is required.

A problem of the process described in US2011/0183159 is that the conversion of ammonia is low. The object of the present invention is to provide a process which has the advantages of US2011/0183159 with respect to the low formation of nitrous oxide and at the same time provide a higher conversion of ammonia.

The object of the present invention is to provide a process for converting ammonia which does not have the above disadvantages.

These objectives can be achieved by the following process. A process to oxidise total ammonia nitrogen, also referred to as TAN, as dissolved in an aqueous solution by (i) contacting the aqueous solution with oxidised ammonia-oxidizing bacteria under anaerobic conditions wherein TAN is converted and reduced ammonia-oxidizing bacteria are obtained and (ii) wherein in a separate step the reduced ammonia-oxidizing bacteria are oxidised by transfer of electrons to an anode of an electrochemical cell to obtain the oxidised ammonia-oxidizing bacteria.

Applicants found that TAN can be converted in an anaerobic process by the process according to the invention thereby significantly lowering the formation of nitrous oxides. By performing step (i) and step (ii) as separate steps a higher ammonia conversion is observed. It is not entirely understood why such a process would have a higher ammonia conversion. It may be because the ammonia conversion does not take place in a biofilm as in the prior art process. It may also be that the driving force of electron transfer in the electrochemical cell is enlarged as a result of performing step (i) and step (ii) separately. A further advantage is that energy can be produced instead of consumed as in the prior art processes. Further advantages will become clear when discussing the invention in more detail.

Although the exact mechanism is not yet fully understood applicants believe that the following reactions may take place. When an aqueous solution comprising TAN is contacted with oxidised ammonia-oxidizing bacteria under anaerobic conditions in step (i) according to the invention oxidation of TAN to nitrate takes place which may be described by:

NH₃ + bac⁴⁺ + 3 H₂0 -> NO3- + bac⁴ + 9 H⁺ (1 a) Here, bac⁴⁺ is an oxidized ammonia-oxidizing bacteria and TAN is represented as ammonia (NH3). While the ammonia is oxidized, the bacteria is reduced or at least has taken up reduced components in a 8-electron reaction thereby obtaining bac^4-.

Oxidation of ammonia to nitrite may also take place. This reaction may be described by:

NH₃ + bac³⁺ + 2 H₂0 -> N0₂ + bac³ + 7 H⁺ (1 b)

Here, bac³⁺ is an oxidized ammonia-oxidizing bacteria. While the ammonia is oxidized, the bacteria is reduced or at least has taken up reduced components in a 6-electron reaction thereby obtaining bac^3-.

In the process according to the invention the reduced bacteria (bac) is regenerated in step (ii) to obtain oxidised ammonia-oxidizing bacteria. This electrochemical anode reaction is given by:

and

Bac^3- -> bac³⁺ + 6e^_ (2a)

In the process according to the invention the electrons as supplied by the reduced ammonia-oxidizing bacteria are not directly transferred to oxygen. Instead the electrons are transferred to an anode of an electrochemical cell. The material of the anode may be any conductive material, for example stainless steel or titanium optionally provided with a metal coating. A preferred anode are graphite or carbon- based electrodes. Such an electrochemical cell used in step (ii) will also comprise of a cathode wherein the electrons as released at the anode are used to reduce another counter compound. If such a counter compound has a more positive electrode potential than the electrode potential of the reduced ammonia-oxidizing bacteria an electric current between the anode and the cathode results and electric power may be produced. If the counter compound has a more negative electrode potential than the electrode potential of the reduced ammonia-oxidizing bacteria the required transfer of electrons and resulting electric current between said electrodes is achieved by applying an electric potential difference between the anode and cathode. In a so- called three electrode cell, an anode potential of between -0.4 and +1.0V versus a Ag/AgCI reference electrode may be applied. The choice for the anode potential will depend on the desired rate and selectivity of the process.

The redox potential of the aqueous solution in step (i) is suitably different than the redox potential of the aqueous solution in step (ii) as measured versus a Ag/AgCI reference electrode. Preferably the redox potential of the aqueous solution in step (i) is more negative than the redox potential of the aqueous solution in step (ii) as measured versus a Ag/AgCI reference electrode. The redox potential may thus be chosen for step (i) to optimise the conditions for ammonia conversion and for step (ii) to optimise the oxidation of the reduced ammonia-oxidizing bacteria.

The anode and cathode may be present in the same space, more specifically in the same vessel, wherein the ammonia-oxidizing bacteria may also contact the cathode. Preferably the space in which the anode is present is separated from the space in which the cathode is present by a semi-permeable membrane. The ammonia-oxidizing bacteria cannot pass such a semi-permeable membrane.

Suitably the electrochemical cell is comprised of a space for the anode and a space for the cathode as separated by a semi-permeable membrane and wherein no ammonia-oxidizing bacteria are present in the space for the cathode. Such a membrane may be an ion-selective membrane for transport of cations from anode to cathode. Such cations may be any cation which is present in higher concentrations. Examples of cations are H⁺, K⁺ and Na⁺. The membrane may also be an ion- selective membrane for transport of anions from cathode to anode. Examples of anions are OH- and SO42- .

In a first preferred embodiment the electrochemical cell comprises a cathode which transfers electrons to an electron acceptor having a more positive potential than the electrode potential of the anode and wherein as a result of this difference in potential between anode and cathode an electric current between said electrodes results. In order to increase the reaction rate an additional potential may be applied from an external source. The compound at the cathode, i.e. the counter compound, may for example be oxygen which electrochemical reaction at the cathode may be given by:

2 0₂ + 8 H⁺ + 8 e- -> 4 H₂0 (3)

Another possible counter compound is ferric iron (Fe3⁺). The reaction at the surface of the cathode may be uncatalyzed or catalyzed. The material of the cathode may be graphite-based or carbon-based (uncatalyzed) or metal-based. Examples of catalyzed cathodes are mixed metal oxide coatings containing Pt, Ir, or other noble metals, on a conductive support like titanium. Possible catalysts are Pt, Ir, Cu and microorganisms as present as a biofilm on the cathode.

In a second preferred embodiment the electrochemical cell comprises a cathode which transfers electrons to a compound having a more negative potential than the electrode potential of the anode and wherein a current is provided between anode and cathode such that the transfer of electrons can take place. In this embodiment an electric current is generated between said electrodes, for which power input is required. The advantage of this second option is that higher rates can be achieved by increasing the applied electric potential.

Possible counter compounds having a more negative electrode potential than the electrode potential of the reduced ammonia-oxidizing bacteria are for example, Cu2⁺ and hydronium ions. A preferred counter compound is the hydronium ion which allows the production of hydrogen. This electrochemical cathode reaction may be given by:

8 e- + 16 H+ ~> 8 H₂ (3)

Other possible reactions which may be performed at the cathode is the reduction of hydronium ions together with CO2 to methane or other chemicals like acetate, ethanol, or medium chain fatty acids. The thus produced hydrogen or methane or the hydrocarbons are useful by-products of the process according to the invention.

In the presence of ammonia oxidizing bacteria (AOB) in step (i) nitrite and nitrate may be formed according to reactions (1a) and (1 b) described above. To further convert the nitrite to nitrate it is preferred that also oxidised nitrite oxidizing bacteria (NOB) are present in step (i). This allows the formed nitrite to be converted to nitrate. The reduced nitrite oxidizing bacteria which are obtained in step (i) may be beneficially oxidised by transfer of electrons to the anode of the electrochemical cell to obtain the oxidised nitrite-oxidizing bacteria in step (ii) of the invention. In this way both the ammonia oxidizing bacteria (AOB) and the nitrite oxidizing bacteria (NOB) can simultaneously and in the absence of oxygen, i.e. the anaerobic conditions, convert ammonia into nitrate according to the following general formula:

NH₃ + bac⁴⁺ + 3 H₂0 -> NO3- + bac⁴ + 9 H⁺ (3)

Preferably no nitrite-oxidizing bacteria are present in the space for the cathode of the above described electrochemical cell comprising of a space for the anode and a space for the cathode as separated by a semi-permeable membrane.

In a more preferred embodiment the potential in the anode potential is so chosen that the selectivity of the process to prepare nitrite is increased, and nitrate formation is suppressed. The formed nitrite can subsequently be converted to nitrogen by the anaerobic reaction of the so-called Annamox process which may take place in step (i): NH₄ ⁺ + N0₂ N₂ + 2 H₂0

The contacting in (i) and the oxidation of the reduced ammonia-oxidizing bacteria and optional reduced nitrite oxidizing bacteria in (ii) takes place in separate steps. By separate steps is here meant that the contacting in step (i) and the oxidation of the reduced bacteria takes place in two separate steps and

consequently in at least two separate vessels or reactors, for example step (i) is performed in a conventional anaerobic bioreactor vessel where the bacteria and ammonia may be intimately contacted. Step (ii) is performed in a electrochemical cell. Although in such a process a large part of the conversion of ammonia by the oxidised bacteria will take place in a first step it cannot be excluded that part of the reaction will also take place when the reduced bacteria are oxidised in (ii).

Preferably step (i) is performed in an anaerobic reactor vessel which is fluidly connected to the space for the anode of the electrochemical cell. More preferably part of the content of the anaerobic reactor vessel is continuously supplied to the space of for the anode of the electrochemical cell of step (ii) and part of the content of the space for the anode is continuously recycled to the anaerobic reactor vessel. To the anaerobic reactor vessel the aqueous solution comprising ammonia may be continuously fed to be treated. A next part of the content of the space for the anode may be discharged as a treated aqueous solution comprising a reduced total ammonia nitrogen. This treated aqueous solution may be supplied to a denitrification bioreactor to convert any nitrate still present in the treated aqueous solution to dinitrogen. Such a reactor may be a state of the art anaerobic denitrification reactor, for example of the so-called Annamox process.

The reduced ammonia-oxidizing bacteria and optional reduced nitrite oxidizing bacteria are oxidised by transfer of electrons to an anode of an electrochemical cell to obtain the oxidised bacteria. The electrochemical cell may be any cell which comprises an anode and cathode as described above. The anode and cathode may be present as flat plates or as co-axial tubular parts as present in a vessel. The anode and cathode may also be comprised in a single tubular part wherein preferably the anode is present at its exterior and the cathode is present at its interior. One or more of such tubes may be placed such that the anode at its exterior may contact the reduced bacteria. Through the tube another solution may flow comprising the compound which will perform the earlier referred to electrochemical reaction at the cathode.

If a desired degree of regeneration of the ammonia-oxidizing bacteria is not obtained in (ii) it may be advantageous to oxidize the remaining reduced ammonia- oxidizing bacteria by direct contacting with, for example, oxygen. Although some of the advantages of the invention will then be less it still results in a process which overall does not have the high nitrous oxide production as in the prior art process. A further advantage is that hydrogen may be produced which can be used as a source of energy, for example as a transportation fuel. Thus suitably part of the reduced bacteria are oxidised by transfer of electrons to an anode of an electrochemical cell to obtain the oxidised bacteria and another part of the reduced bacteria are oxidised directly by contacting the reduced ammonia oxidizing bacteria with oxygen such to achieve a desired degree of regeneration of the oxidizing bacteria.

The contacting (i) of the aqueous solution comprising ammonia with oxidised ammonia-oxidizing bacteria is performed under anaerobic conditions. With anaerobic conditions is meant in the absence of molecular oxygen. No molecular oxygen is supplied and/or present during such contacting. Preferably such contacting is performed in the absence of other oxidants. Anaerobic conditions is here meant‘in the absence of molecular oxygen’ wherein the concentration of molecular oxygen in the aqueous solution is at most 1 mM, more preferably at most 0.1 mM.

The pH of the aqueous solution is suitably between 5 and 9. The temperature and pressure conditions may be such that the aqueous solution remains liquid.

Practical conditions are ambient conditions.

The ammonia-oxidizing bacteria (AOB) may be any ammonia-oxidizing bacteria, such as heterotropic gram-negative bacteria, autotrophic bacteria and anaerobic ammonia-oxidizing bacteria. A preferred ammonia-oxidizing bacteria is of one of the heterotropic gram-negative family Nitrobacteraceae. Autotrophic AOB are, on genus-level, Nitrosospira and Nitrosomonas. Anaerobic ammonia-oxidizing bacteria may be of the order Planktomycetales. The nitrite oxidizing bacteria (NOB) may be any nitrite-oxidizing bacteria, preferably the ammonia-oxidizing bacteria belonging to the phylum Nitrospira. The bacteria may be used as such, i.e. may be present as planktonic cells the aqueous solution or may be supported on a dispersed carrier.

The bacteria may be cultivated from bacteria found in simultaneous nitrification and denitrification process of a typical municipal wastewater treatment plant, for example an Oxygen Ditch plant. A sample of bacteria as recovered from such a facility may be added to a process according to the invention and in time the desired ammonia-oxidizing bacteria will be selected. Alternatively the desired ammonia- oxidizing bacteria may also be cultivated in a process set-up wherein instead of step (ii) of the present invention an aerobic bioreactor is used to which for example air is provided. In time the desired ammonia-oxidizing bacteria will accumulate. It has been found that thus cultivated bacteria can be advantageously be used in the process according to the present invention.

The process may advantageously be used to treat waste water containing TAN, digestate, manure, sour water stripper off-gas and nitrogen containing streams obtained in refinery hydrotreaters. The ammonia as present in gaseous streams may be absorbed directly or indirectly into the aqueous solution in which the process according to the invention is performed.

The invention will be illustrated by Figure 1. Figure 1 shows an aqueous solution in which ammonia is dissolved (1 ) as a feed to an anaerobic reactor (2). In anaerobic reactor (2) ammonia is converted in the presence of oxidised ammonia oxidizing bacteria and oxidised nitrite oxidizing bacteria. Most of the oxidised bacteria will be converted to reduced bacteria. Any nitrogen gas formed in anaerobic reactor (2) is discharged via (7a). In electrochemical cell (3) the reduced bacteria are oxidised by transfer of electrons to an anode (4). Oxidised bacteria are obtained. Liquid effluent as obtained at the anode side of the electrochemical cell is in part (via stream (7)) recycled to anaerobic reactor (2) and in part discharged to a

denitrification bioreactor (10) via (8). Between anode (4) and cathode (5) an electric potential is created to drive the reaction at anode and cathode. Between the space in which the anode (4) is present and the space in which the cathode (5) is present an ion-selective membrane (6) is positioned. At cathode (5) hydrogen is prepared and discharged as stream (9). Hydrogen may in a subsequent step be reacted with carbon dioxide to prepare methane or used as such. In the denitrification bioreactor

(10), which may be a state of the art anaerobic denitrification reactor, the nitrate still present in the aqueous solution (8) as fed to this reactor is converted to dinitrogen

(11 ). An aqueous solution poor in dissolved ammonia (12) is discharged from reactor (10). Bioreactor (10) may also be an anaerobic reactor of the so-called Annamox process in which nitrite is converted to dinitrogen.

The invention will be illustrated by the following non-limiting examples.

Example 1

Flocculant sludge in reactor solution was taken from an activated sludge plant with bio-P removal, treating domestic wastewater, in Bennekom, The Netherlands. This flocculant sludge was composed of a mixture of ammonium oxidizing, nitrite oxidizing and phosphate accumulating bacteria. The flocculant sludge with reactor solution was aerated overnight, to achieve full oxidation. Then, a 3 mM ammonium chloride solution was fed to the sludge with reactor solution. After 3 hours, the flocculant sludge with reactor solution was transferred to an electrochemical cell.

This electrochemical cell consisted of a graphite rod anode, a reference electrode (Ag/AgCI, 3 M KCI), and a graphite felt cathode (counter electrode). The

electrochemical cell containing flocculant sludge was flushed with N2 gas for 15 min to remove oxygen. The anode potential was controlled at +0.3 V vs. Ag/AgCI, 3M KCI in a so-called three electrode set-up and the resulting current was measured during 150 seconds. Control experiments were performed on the reactor solution without bacteria but containing ammonia. To obtain this reactor solution without bacteria, the solution was centrifuged at 10,000 rpm for 10 minutes, and the solution was transferred to the electrochemical cell. Current was measured at the same anode potential of 0.3 V vs. Ag/AgCI. The total charge produced in presence of bacteria was 20.6 mC, whereas the reactor solution with ammonia but without bacteria produced 0.8 mC as also shown in Figure 2, showing that indeed bacteria can transfer charge from ammonia to the electrode.

Example 2

The sludge, after overnight aeration, was fed with 3 mM ammonium. The

concentration of ammonia was measured at different time points. After 12 hours, the ammonia concentration decreased to below detection limit, showing that the bacteria removed ammonia from solution in absence of an electron acceptor. This is believed to result in the charging of bacteria, after which current can be harvested from the bacteria in the electrochemical cell as in example 1.

Example 3

In a system consisting of an aerobic reactor of 2 L and an anaerobic reactor of 0.5 L fluidly connected to the aerobic reactor an aqueous solution was added consisting of bacteria as harvested as a sludge at the from an activated sludge plant with bio-P removal, treating domestic wastewater, in Bennekom, The Netherlands of Example 1. Two different aqueous streams were supplied to the anaerobic reactor:1 ) ammonium chloride with a concentration of 82 mg/I at an inflow rate of 30 ml/h, 2) trace element for bacteria growth and a phosphate buffer (100 mM) at the same inflow rate of 30 ml/h. Part of the content of the anaerobic reactor was supplied at a constant rate of 1.26 l/h to the aerobic reactor and at the same rate part of the content of the aerobic reactor was supplied to the anaerobic reactor. The liquid circulation between anaerobic and aerobic chamber resulted in the hydraulic retention time (HRT) in the anaerobic chamber was approximately 20 min. To the aerobic reactor air was supplied via an air stone to oxidise the reduced ammonia- oxidizing bacteria.

The Cyclic voltammetry (CV) scans of the bacteria in the system were measured at the start, after 14 days and after 30 days and are shown in Figure 3. On the 30 days of operating the system of example 3, it was observed that the overall biological content decreased significantly due to biomass wash-out. Figure 3 shows that the bacteria which were growing showed a pair of oxidation and reduction peaks at 0.6 V and 0.4 V vs. Ag/AgCI, respectively. The peak currents for both oxidation and reduction reaction increased during the cultivation of desired ammonia oxidizing bacteria. These results suggests the invention has an effective selection of large charge - discharge capacity (i.e. selection for ammonia conversion). This shows that an aerobic-anaerobic system may be used to provide for the selection of desired ammonia-oxidizing bacteria when starting from a sludge as obtained in a

simultaneous nitrification and denitrification process.

Example 4

50 ml_ of solution containing ammonia-oxidizing bacteria as obtained after 30 days in Example 3 was taken from the anaerobic chamber and added to an electrochemical cell. This set-up is similar as in Example 1 , except for the anode electrode, which was a graphite felt with a size of 5 x 2 x 1 cm (length x width x height). To remove trace amounts of dissolved oxygen in this 50 ml_ of solution, Argon (Ar) was used to flush the solution for 30 min. The NH4CI stock solution (21 mg-N/l) was then added to the electrochemical cell leading to a ammonium

concentration of 6 mg-N/l. During 20 min of open cell period (i.e. no applied discharge anode potential), the ammonium concentration decreased by 35%. After 20 minutes, the anode potential was controlled at +0.3 V vs. Ag/AgCI in the electrochemical cell and the resulting current was measured during 30 min. Control (with ammonia-oxidizing bacteria but without ammonium) and blank (supernatant of the solution (without bacterium) but with ammonium) experiments were also performed in the same way. However, no detectable decrease of ammonium concentration was measured in the blank experiment and the currents generated in both control and blank experiment were significantly lower than that obtained in the experiment with both bacteria and ammonium. This shows that the function of ammonia-oxidizing bacteria cultured in this invention can be considered as a living- capacitor, which can store electrons from ammonium oxidation process in absence of any additional electron acceptor (e.g. oxygen).

Claims

1. A process to oxidise total ammonia nitrogen, also referred to as TAN, as dissolved in an aqueous solution by (i) contacting the aqueous solution with oxidised ammonia-oxidizing bacteria (AOB) under anaerobic conditions wherein TAN is converted and reduced ammonia-oxidizing bacteria is obtained and (ii) wherein in a separate step the reduced ammonia-oxidizing bacteria are oxidised by transfer of electrons to an anode of an

electrochemical cell to obtain the oxidised ammonia-oxidizing bacteria.

2. A process according to claim 1 , wherein the redox potential of the aqueous solution in step (i) is more negative than the redox potential of the aqueous solution in step (ii) as measured versus a Ag/AgCI reference electrode.

3. A process according to any one of claims 1 -2, wherein the electrochemical cell comprises a cathode which transfers electrons to an electron acceptor having a more positive potential than the electrode potential of the anode and wherein as a result of this difference in potential between anode and cathode an electric current between said electrodes results.

4. A process according to any one of claims 1 -2, wherein the electrochemical cell comprises a cathode which transfers electrons to a compound having a more negative potential than the electrode potential of the anode and wherein a potential is provided between anode and cathode such that the transfer of electrons can take place.

5. A process according to any one of claims 1 -4, wherein the electrochemical cell is comprised of a space for the anode and a space for the cathode as separated by a semi-permeable membrane and wherein no ammonia- oxidizing bacteria are present in the space for the cathode.

6. A process according to any one of claims 1 -5, wherein step (i) is performed in a reactor vessel which is separate from the electrochemical cell of step (ii).

7. A process according to any one of claims 1 -6, wherein in step (i) also oxidised nitrite oxidizing bacteria (NOB) are present, wherein nitrite is converted to nitrate and reduced nitrite oxidizing bacteria are obtained and wherein in separate step (ii) reduced nitrite-oxidizing bacteria are oxidised by transfer of electrons to the anode of the electrochemical cell to obtain the oxidised nitrite- oxidizing bacteria.

8. A process according to claim 7, wherein the electrochemical cell is comprised of a space for the anode and a space for the cathode as separated by a semi- permeable membrane and wherein no nitrite-oxidizing bacteria are present in the space for the cathode.

9. A process according to any one of claims 1 -8, wherein the anaerobic

conditions are defined as a concentration of molecular oxygen in the aqueous solution of at most 0.1 mM.

10. A process according to any one of claims 1 -9, wherein the pH of the aqueous solution is between 5 and 9.

11. A process according to any one of claims 3-10, wherein in (ii) a potential of between -0.4 to +0.8 V is applied to the anode versus a Ag/AgCI reference electrode (resulting in a potential difference between the anode and the cathode).

12. A process according to any one of claims 1 -11 , wherein the aqueous solution is waste water containing TAN, reject water, digestate, manure, sour water stripper offgas or nitrogen containing streams obtained in refinery

hydrotreaters.

13. A process according to any one of claims 1 -12, wherein the anode potential is chosen such that the selectivity of the process to prepare nitrite is increased, and nitrate formation is suppressed and wherein the nitrite as present in the resulting aqueous solution is converted in a separate step to dinitrogen (N2) by the anaerobic reaction of the so-called Annamox process.

14. A process according to any one of claims 1 -13, wherein step (i) is performed in an anaerobic reactor vessel which is fluidly connected to the space for the anode of the electrochemical cell and wherein part of the content of the anaerobic reactor vessel is continuously supplied to the space of for the anode of the electrochemical cell of step (ii) and part of the content of the space for the anode is continuously recycled to the anaerobic reactor vessel and wherein to the anaerobic reactor vessel the aqueous solution comprising total ammonia nitrogen is continuously fed and wherein a next part of the content of the space for the anode is continuously discharged as a treated aqueous solution comprising a reduced total ammonia nitrogen.

15. A process according to claim 14, wherein the treated aqueous solution is supplied to a denitrification bioreactor to convert any nitrate still present in the treated aqueous solution to dinitrogen.