WO2019109139A1

WO2019109139A1 - Ammonia recovery apparatus and method

Info

Publication number: WO2019109139A1
Application number: PCT/AU2018/051298
Authority: WO
Inventors: Trevor David Waite; Changyong ZHANG; Jinxing Ma; Di HE
Original assignee: Newsouth Innovations Pty Limited
Priority date: 2017-12-04
Filing date: 2018-12-04
Publication date: 2019-06-13
Also published as: CN111655629A

Abstract

Use of flow-electrode capacitive deionization for ammonia reclamation. Embodiments extend to the use of flat-sheet membranes are used; use of a common replenishment tank for the anode and cathode flow materials. Specific current densities, hydraulic retention times, carbon content for the flow materials and operating pH is specified.

Description

AMMONIA RECOVERY APPARATUS AND METHOD

Technical Field

A process and corresponding apparatus are disclosed for recovery of ammonia. Background Art

A rapidly increasing world population and improvement in living standards have resulted in the demand for greater food production, with this production strongly dependent on the utilization of fertilizers rich in nitrogen (N) and phosphorus (P) to maintain soil fertility and increase crop yields. Ammonia, as the key component in the synthesis of most popular Nitrogen fertilizers (including urea, ammonium nitrate and ammonium sulfate), is responsible for 15-2.5% of the annual global energy consumption. The principal means by which ammonia is synthesized is the Haber-Bosch process which is energy intensive, and releases significant carbon dioxide into the atmosphere. While expenditure on ammonia production for fertilizer use is increasing, water contamination has become increasingly severe, especially in relatively recently industrialized countries such as China and India, with ammonia nitrogen recognized to be one of the principal contaminants. Ammonia nitrogen can result in severe eutrophication, acute and chronic toxicity to aquatic creatures, and by- products produced during drinking water disinfection. As a consequence, the removal of ammonia nitrogen from wastewaters is desirable.

A variety of methods are available for removal of ammonia from wastewaters including nitrification/denitrification, air stripping, break-point chlorination and electrochemical oxidation. However, each of these technologies exhibits its own limitations ranging from high energy consumption, low efficiency or generation of secondary pollution.

Adsorption by zeolites via ion exchange is one way of removing ammonium ions from an aqueous stream with brine solutions subsequently used to regenerate the zeolite and obtain a highly concentrated ammonia solution. Another approach to ammonia preconcentration involves the application of membrane separation technologies such as reverse osmosis (RO) or nanofiltration (NF). However, these technologies either require large quantities of chemicals for regeneration purposes or suffer from high energy consumption.

Flow-electrode capacitive deionization (FCDI) has known applications in seawater desalination and energy storage.

Summary of the Disclosure An embodiment extends to a process for recovery of ammonia in aqueous solution, the process comprising the steps of:

providing a capacitive deionizer having a cathode electrode, an anode electrode and a channel through which the solution flows situated between the cathode electrode and the anode electrode, the cathode electrode comprising cathode material, the anode comprising anode material, the deionizer comprising a cation exchange membrane between the channel and the cathode electrode and an anion exchange membrane between the channel and the anode electrode;

allowing the aqueous solution to flow through the channel whilst providing a potential difference between the anode electrode and the cathode electrode, thereby removing dissolved ammonia from solution through the cation exchange membrane and capturing the dissolved ammonia in the cathode electrode material; and

removing the dissolved ammonia from the cathode electrode material. The dissolved ammonia may be removed from the cathode electrode material by diffusion of the gaseous ammonia through a gas permeable membrane. The dissolved ammonia may be in equilibrium with gaseous ammonia.

The capacitive deioniser may be a flow-electrode capacitive deioniser and the anode electrode and the cathode electrode may then comprise

corresponding flow channels through which the cathode material and the anode material flow.

The process may comprise the further step of removing the dissolved ammonia with a membrane contactor. Alternatively, or in addition, the dissolved ammonia may be removed by degassing.

The membrane contactor may comprise a first conduit and a second conduit separated by a contactor membrane, the process comprising allowing the cathode material to flow in the first conduit and an acid to flow in the second conduit so that ammonia is removed from the cathode material through the contactor membrane to interact with the acid to form an ammonia salt.

The acid may be sulphuric acid. Alternatively, the acid may be phosphoric acid or nitric acid.

A flow rate through the channel of the capacitive deionizer may be set so that a hydraulic retention time of the channel is longer than 1 minute. The flow rate may be set so that the hydraulic retention time is longer than 1.4 minutes.

The hydraulic retention time of the channel may be less than 5 minutes. Preferably, the hydraulic retention time of the channel is less than 3 minutes.

A pH of the cathode material whilst in the cathode may be at least 9.25.

The pH of the cathode material whilst in the cathode may be less than 12. The pH of the cathode may be the pH of the cathode material whilst in the flow channel of the cathode electrode. A current density between the anode and the cathode may be between:

2.8 A rrr² and 17.2 A m ².

The current density may be less than 50 A nr². The current density may be between 11 A nr² and 12 A nr². The current density may be about 11.5 A nr².

In a further embodiment, the current density may be between 6 A nr² and 7 A nr². The current density may be 6.8 A nr².

The cathode material and the anode material may comprise a common material and the cathode material and the anode material may be replenished in corresponding chambers.

The cathode material and the anode material may comprise a common material and the cathode material and the anode material may be mixed in a common chamber. In this embodiment, there may be a current density of 6.8 A nr² and a hydraulic retention time of 1.48 min.

The common material may comprise carbon in a weight percentage of, or more than, 2%. The common material may comprise carbon in a weight percentage of, or less than 10%. The common material may comprise carbon in a weight percentage of about 5%.

A further embodiment of the invention extends to apparatus for recovery of ammonia in aqueous solution, the apparatus comprising:

a capacitive deionizer having a cathode electrode, an anode electrode and a channel through which the solution flows situated between the cathode electrode and the anode electrode, the cathode electrode comprising cathode material, the anode comprising anode material, the deionizer comprising a cation exchange membrane between the channel and the cathode electrode and an anion exchange membrane between the channel and the anode electrode; and

an ammonia collector for removing the dissolved ammonia from the cathode electrode material.

The capacitive deioniser may be a flow-electrode capacitive deioniser and the anode electrode and the cathode electrode may comprise corresponding flow channels through which the cathode material and the anode material flow.

The ammonia collector may comprise a membrane contactor. Alternatively, the ammonia collector may be a degasser.

The membrane contactor may be a hollow fibre membrane contactor. The membrane contactor may comprise a flat-sheet membrane

arrangement.

The apparatus may comprise a gas permeable membrane and a contactor channel wherein the gas permeable membrane is arranged between the cathode flow channel and the contactor channel, the contactor channel being arranged between the cathode flow channel and a cathode conductor.

The contactor channel may be connected to an acid reservoir by a conduit so that an acid solution may flow from the acid reservoir to the contactor channel.

The cathode material and the anode material may comprise a common material and wherein the cathode material and the anode material are replenished in corresponding chambers. In this embodiment, there may be a current density of 6.8 A nr² and a hydraulic retention time of 1.48 min.

The cathode material and the anode material comprise a common material and wherein the cathode material and the anode material are mixed in a common chamber. The common material may comprise carbon in a weight percentage of, or more than, 2%. The common material may comprise carbon in a weight percentage of, or less than 10%. The common material may comprise carbon in a weight percentage of about 5%.

Brief Description of the Drawings Notwithstanding any other forms which may fall within the scope of the system and method as set forth in the Summary, specific embodiments will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of the apparatus according to an

embodiment of the invention;

Figure 2 is a schematic diagram of a membrane contactor as used in an embodiment of the invention;

Figure 3 is a schematic diagram illustrating the movement of ions through components of the apparatus of Figure 1 ; Figure 4A is a schematic diagram illustrating a hollow fibre membrane contactor for use with the embodiment of Figure 1 ;

Figure 4B is a schematic diagram of a tube plate of the hollow fibre membrane contactor of Figure 4A;

Figure 5 is a schematic diagram of apparatus according to a further embodiment of the invention; and

Figures 6 to 26 are graphs illustrating operation of the apparatus of Figures 1 or 5 under different operating parameters. Detailed Description of Specific Embodiment

Figure 1 is a schematic diagram of an apparatus 10 according to an embodiment of the invention. The apparatus 10 includes a gasket 12 in which a channel 14 is formed. In use, water having ammonia in solution is passed through the channel 14, as described in further detail below. Although the channel 14 is illustrated as a rectangular channel, it is to be realised that other shapes and arrangements for the channel are possible.

Ion-exchange membranes are arranged to either side of the gasket 12. An anion exchange membrane 16 is arranged on the first side of the gasket 12 and a cation exchange membrane 18 is arranged of the other side of the gasket 12. In this embodiment the cation exchange membrane is a CEM-Type I membrane and the anion exchange membrane 16 is an AEM-Type I membrane. The distance between the ion exchange membranes is about 500 pm.

A flow gasket 20 is arranged on the opposite side of anion exchange membrane 16. The flow gasket 20 is arranged around an acrylic sheet 24 into which a flow channel 36 has been machined. An acrylic end plate 28 has a graphite film 32 adhered to one side thereof and is arranged so that the graphite sheet 32 faces the flow gasket 20. In this embodiment, silicone with a nylon sheet (100-mesh) is used to form the channels. Similarly, a second flow gasket 22 contacts anion exchange membrane 18.

Flow gasket 22 includes an acrylic sheet 26 into which a flow channel 38 has been machined. A second acrylic end plate 30 has a graphite film 34 adhered to one side thereof and is arranged so that the graphite film 34 faces the flow gasket 22.

An anode material reservoir 52 is connected by conduit 46 to the flow channel 36. Similarly, a cathode material reservoir 54 is connected by conduit 48 to the flow channel 38. In this embodiment, the cathode material and the anode material each comprise graphite slurries. During use, the graphite slurries are pumped from the reservoir through the corresponding conduit to the flow channel (pumps not shown in Figure 1). The graphite film 32 mounted to acrylic end plate 28 is connected to the positive terminal of a cell 40 and graphite film 34 connected to end plate 30 is connected to the negative terminal of the cell 40. Therefore, during use, graphite film 32 mounted on an plate 28 and flow channel 36 in acrylic sheet 24 months in gasket 20 through which a graphite slurry flows, forms anion 44. Graphite film 34 mounted on end plate 30, together with flow channel 38 formed in acrylic sheet 26 mounted in gasket 22 through which a graphite slurry flows, forms cation 50. Therefore, the device 10 forms a flow-electrode capacitive deioniser. In the embodiment of Figure 1 , the flow channels 36 and 38 are formed as serpentine flow channels. The flow channels have a square cross-section with dimensions of 3 mm x 3 mm. The effective contact area between the iron-exchange membranes and the flow electrodes is 34.9 cm². It is to be realised that

embodiments of the process and apparatus are not limited to the shape, size or arrangement of these flow channels, and other arrangements may be suitable.

A membrane contactor 60 is placed in the cation exchange reservoir 38. Figure 2 is a schematic representation of the membrane contactor 60. The membrane contactor 60 includes a first fluid reservoir 62 having an inlet 68 and an outlet 70 and a second fluid reservoir 64 having an inlet 72 and an outlet 74. A gas permeable membrane 66 separates the first fluid reservoir 62 from the second fluid reservoir 64. During use, the first fluid (the graphite slurry used in the cation 50 of Figure 1) enters the first fluid reservoir 62 via the inlet 68 annexes fire the outlet 70.

A second fluid (in this embodiment, sulphuric acid, as described in greater detail below) enters the second fluid reservoir 64 and exits by outlet 74. In this

embodiment, both the cathode and anode slurries comprise 18.2 MW cm Milli-Q water (Millipore) with 100-mesh DARCO^® activated charcoal from Sigma Aldrich.

The use of the apparatus 10 will now be described with reference to Figure 3. Figure 3 illustrates a portion of the apparatus 10 illustrated in Figure 1. It is to be understood that the illustration is not to scale. Figure 3 illustrates the flow channel 36 situated in between graphite film 32 forming a current collector and the anion exchange membrane 16 at the anion 44 illustrated in Figure 1. Similarly, cation exchange membrane 14, flow channel 38 and graphite film 34 of the cathode 50 are illustrated. Cation exchange membrane 18 and anion exchange membrane 16 are separated by channel 14. As previously described, a graphite slurry is transported through the flow channels 36 and 38. Relatively large carbon molecules 90 are present in the graphite slurry. Wastewater containing sodium chloride and ammonia in solution is pumped through channel 14 and a potential difference is applied between graphite films 32 and 34. Under the influence of the potential difference, and through action of the anion and cation exchange membranes, chloride ions will tend to migrate to the flow channel 36 of the anode and sodium and ammonium ions will tend to migrate to the flow channel 38 of the cathode. The graphite slurry of the cation is then transported via conduit 48 to the first chamber 62 of the membrane contactor 60 contained within the cathode material reservoir 54. The second chamber 64 of the membrane contactor 60 contains sulphuric acid 92. As the ammonia degases from the solution in the first chamber 62 of the membrane contactor 60, it combines with the sulphuric acid 92 to form an ammonium sulphate solution 96. The ammonium sulphate 96 can then be removed from solution and used commercially.

The charged flow electrodes were continuously cycled between the flow channels and their respective circulation tanks, with the polypropylene (PP) hollow- fibre membrane contactor (Figures 2 and 3) placed in the negatively charged flow electrode circulation tank. The total length of the membranes used in this work was 30 cm, with outer diameter of 2 mm, wall thickness of 0.1 mm, pore size of 0.45 mM, and total effective surface area of 18.8 cm².

Figure 4A illustrates a hollow fibre membrane contactor 100 which may be used as the membrane contactor 60 of Figure 1. The hollow fibre membrane contactor 100 includes a shell 102 and two base plates 104 mounted with the shell 102. A number of hollow fibre members 108 are mounted between the two base plates 104. The cathode slurry enters through the inlet 118 in the direction of arrow 110 and exits the through an outlet 120 in the direction of arrow 112. Similarly, a solution of sulphuric acid enters through inlet 122 in the direction of arrow 114 and the solution, now containing ammonium sulphate, exits through outlet 124 in the direction of arrow 116.

Figure 5 is a schematic diagram of an apparatus 150 according to a further embodiment. The embodiment of Figure 5 is similar to that of Figure 1 and the same reference numerals have been used to describe common components. The apparatus 150 of Figure 5 differs from the apparatus 10 of Figure 1 in that the hollow fibre membrane contactor 60 is replaced by a flat-sheet membrane contactor arrangement. The apparatus 150 includes a contactor gasket 152 within which a contactor channel 156 is formed. A gas permeable contactor membrane 154 is provided between the second flow gasket 22 and the contactor gasket 152. The contactor gasket 152 abuts the second end plate 30. The contactor channel 156 is connected via conduit 158 to acid reservoir 160. During use, sulphuric acid in solution contained within the acid reservoir 160 is transported by the conduit 158 in the direction of arrow 162 through the contactor channel 156. In a similar manner to the hollow fibre membrane contactor 60 of the embodiment of Figure 1 , ammonia ions in solution in the cathode slurry contained within flow channel 38 will degas over the gas permeable membrane 154 and combine with the sulphuric acid contained within contactor channel 156 to then form ammonium sulphate. The ammonium sulphate can then be used as a fertilizer in liquid form or removed from solution by precipitation.

The inventors have found that certain parameters influence the efficiency of apparatus according to embodiments. It is to be realised however that there is a trade-off between cost, benefit and speed and that the most desirable arrangement may be dependent upon economic factors such as the market value of the power needed or the price at which end products can be sold. In particular, the inventors have found that the hydraulic retention time (HRT), the current density of the capacitive deioniser and the pH of the cathode slurry influence the rate or efficiency at which ammonia may be extracted.

Results of the ion removal in the apparatus 10 of Figure 1 at different charging current densities and HRTs are summarized in Figures 6A and 6B.

Relatively stable effluent conductivity was observed. To compare the influence of current densities on ions removal, the flow rate of wastewater was fixed at 1.70 mL min^-1 with a corresponding HRT of 1.48 min while the applied current densities were varied from 2.8 to 17.2 A rrr². As can be seen from Figure 6A, the reduction of the feed water conductivity is positively related to the charging current density; for instance, at a charging current density of 2.8 A nr², the steady-state conductivity of the effluent was 2250 pS cm^-1 with this concentration decreasing significantly to 120 pS cm^-1 at 17.2 A nr². The desalination performance of apparatus 10 is also dependent on the HRT. At an HRT of 0.98 min, the extent of reduction in

conductivity of the feed solution was -50%. In contrast, when using a longer HRT over 1.96 min, the feed water conductivity decreased to less than 100 pS cm^-1 (Figure 6B).

NH4⁺-N and Na⁺ removal efficiencies under different experimental conditions are shown in Figure 7 A and 7B. The applied current densities and HRTs may have positive effects on NH4⁺ and Na⁺ removal. At a low current density of 2.8 A nr², the steady-state effluent NH4⁺-N and Na⁺ concentration were 18.7 mg L ¹ and 282.8 mg L^_1 respectively with corresponding removal efficiencies of 57.0% and 22.6%. When a higher current density of 17.2 A nr² was applied, the effluent NhV-N and Na⁺ concentrations decreased to lower levels, with the removal efficiencies of both ions reaching nearly 100% (Figure 7A). Variation in HRT exhibited similar impacts on NhV-N and Na⁺ removal. At a short HRT of 0.98 min, 79.2% and 48.7% NH₄ ⁺-N and Na⁺ were successfully removed from the feed water, while these values increased to 96.6% and 94.5% at a longer HRT of 2.94 min (Figure 7B).

These findings also indicated that the apparatus 10 had a higher selectivity of NH4⁺-N compared to Na⁺, particularly at lower current density or HRT. For instance, the ratio of removal efficiency between NH4⁺-N and Na⁺ (i.e., RE (NH4⁺-N )/RE (Na⁺)) was 2.5 at a current density of 2.8 A rrr², but gradually decreased to 1.8, 1.34 and 1.0 at higher current densities of 5.8, 1 1.2 and 17.2 A rrr². A similar trend was observed for the effect of change in HRT with RE (NH4⁺-N )/RE (Na⁺) declining from 1.6 to 1.0 on increase in HRT from 0.98 min to 2.94 min. Without being tied to theory, this

phenomenon might be ascribed to the fact that ammonia ion has a smaller size and higher mobility compared to the sodium ion and, as a result exhibiting a higher rate of transport (higher selectivity) from the feed water into the cathodic flow electrode during the desalination process.

Current efficiency of the apparatus 10 was nearly 100% at low current densities (< 1.5 A rrr²) and/or short HRTs (< 1.96 min). However, a decrease in current efficiency was observed if a high current density or long HRT was applied, with values of 83.0% for a current density of 17.2 A rrr² and 61.4% for an HRT of 2.94 min.

Higher current density and longer HRT resulted in better effluent water quality with lower salt concentration, thus, increasing the internal resistance. In this case, the voltage of FCDI cell may increase accordingly and induce the occurrence of Faradaic reactions (i.e., oxygen reduction, water splitting), therefore, decreasing the current efficiency at higher current density.

The pH variation in the cathodic flow electrode during charging of the apparatus 10 is shown in Figures 8A and 8B. In all cases, the pH of the cathode gradually changed from neutral to alkaline. At a low current density of 2.8 A rrr², the final pH reached 10.5, but rose to above 12 when higher current densities were applied. The shaded areas in Figures 8A and 8B represent the region where the pH was higher than the pK_a of NH4⁺ (9.3). In this region, almost all of the NH4⁺ will be deprotonated and transformed into dissolved NH3, thereby providing excellent prerequisites for ammonia separation and recovery from the bulk solution via membrane stripping. Only 30 min was required at current densities of 11.5 and 17.2 A nr² for the cathode pH to increase to values greater than 9.3 whereas more than 100 min was required at 2.8 A rrr². In contrast, variation in HRT (with current density fixed at 11.5 A rrr²) showed negligible effects on pH. In all cases, the cathode pH rose to values higher than 9.3 in 30 min and reached a steady-state value of 12.

Without being tied to theory, the inventors believe that Faradaic processes in the cathode chamber (such as oxygen reduction) lead to the cathodic pH fluctuation. Higher current densities (and higher charging voltages) facilitate the onset of Faradaic reactions leading to a more rapid rise in pH in the cathode. In contrast, changing HRTs has little influence on the occurrence of Faradaic processes and, as such, will not significantly affect the pH.

The voltage ( U) across the electrical circuit was recorded every five seconds using a Vernier voltage probe connected to the Sensor DAQ. The variation of pH in the flow electrode was monitored by an F-51 pH meter (Horiba, Japan). Samples from the influent, effluent, cathodic flow electrode, and acid stripping solution in the membrane contactor 60 were collected every 30 min. All samples were filtered through 0.45 pm filters (Millipore) prior to further analysis. Ion concentrations of ammonia and sodium were measured by ICS-3000 ion chromatograph (Dionex,

U.S.) and ICP-OES (Agilent Varian vista pro 710) respectively.

Current efficiency and energy consumption are two key parameters used to evaluate the performance of the system. Current efficiency (%) can be expressed as:

where Cij_nt is the initial influent concentration of ion species, Ci,_en the effluent concentration of ion species, vthe flow rate of the wastewater stream, / the applied current density, Fthe Faraday constant (96485.3 C mol^-1) and n, the charge of certain ion species (i.e., 1 for NH4⁺ and Cl ). The electrical energy consumption for wastewater treatment (and deionization) (kWh rrr³) was calculated according to: 1000 (2)

where Ut is the voltage of the FCDI. Specific energy consumption for NhV-N removal (kWh kg 1ST¹) can be calculated as:

where Ut is the voltage of the FCDI, / is the current density, CNH4_+-N,inf and C_NH4_+-N,eff are influent and effluent NH4⁺-N concentrations in the feed water. Specific energy consumption for NH4⁺-N recovery (kWh kg 1ST¹) can be calculated as: f U_t x Idt

EC NH ₊ -N =— - v c ΐoqq . (4)

NH ⁺ -N,acid ^ac^ where Ut is the voltage across circuit of the apparatus 10, / is the current density, C_NH4_{+-N,a d} is the ammonia concentration in the acidic receiving solution at time t, V/_acid is the volume of the receiving solution (65 ml_). The removal efficiency of certain ion (%) can be calculated according to:

C „

RE i = (1 -— ) x 100 (5)

C inf

The ion selectivity can be expressed by the ratio of the ion removal efficiency, i.e., the NhV-N selectivity towards Na⁺ is calculated as F/E_Nm_{+ N} / /Sj_a+.The ammonia recovery efficiency ( P , %) can be calculated according to:

Finally, the distribution of the ions (ammonia and sodium) in the effluent, acid solution, cathode slurry was obtained by dividing the total amount of a particular ion contained in the effluent, acid solution, cathode slurry with the total amount of that ion in the influent. Through the mass balance principle, the remaining ions were considered to be the amount of ions absorbed on the carbon particles or stripped into the air (these parameters cannot be measured directly).

Dissolved ammonium-nitrogen concentrations (including NH₃-N and NHLN) measured in the acidic receiving solution and cathode slurries for different applied current densities and HRTs during a four hour operation are shown in Figures 9A to 9D. From these results, it is can be concluded that ammonium ions removed from the feed water stream were effectively transferred to the acid recovery solution with final NH4⁺-N concentrations of 51.1 ± 1.2, 100.1 ± 9.6, 147.2 ± 0.6, 174.9 ± 11.4 mg L^_1 at 2.8, 5.8, 1 1.5 and 17.2 A rrr² respectively (Figure 9A). In the first one hour, the NH₄ ⁺-N concentration in the acid solution increased relatively slowly, mainly because the initial low pH in the cathode chamber hindered transformation of NH₄ ⁺ into NH₃ and its subsequent ammonia transport across the gas membrane, thereby leading to a low rate of ammonia uptake by the acid solution. On analysing the NH₄ ⁺-N concentration in the cathode solution, a sharp increase was observed in the first hour, indicating that the NH₄ ⁺-N was removed from the feed solution and was accumulated in the cathode chamber during this period (Figure 9C). In the following three hours, dissolved NH4⁺-N in the cathode slurry reached a relatively stable concentration of 35-40 mg L ¹ , demonstrating that a steady-state was attained. For the apparatus 10 operated at different HRTs, shorter HRTs favoured the recovery of the ammonia in the acid solution, but reached a limit when the HRT was lower than 1.48 min (Figure 9B).

There are two processes of ammonia-nitrogen migration in the apparatus 10, that is: (i) NH4⁺-N transfer from the influent to cathode chamber across the cation exchange membrane 18 of Figure 1 and (ii) IMH3-N transfer from the cathode chamber to the acidic receiving solution through the gas-permeable hydrophobic membrane 66, Figure 2. These two fluxes should be the same when steady state operation is reached.

From Figure 10A, we can clearly see that the applied current density played an important role in ammonia recovery. An increase in the current density resulted in better ammonia removal efficiency as well as higher ammonia recovery rate. 55-65% of the ammonia present can be recovered by the membrane contactor as (NH4)2SC>4 (an effective fertilizer) while about 15% of the ammonia remains in the cathode solution and 20% was not detected (possibly being electro-absorbed on the carbon particles or stripped into the air). A means of recovering this relatively stable -35% remaining ammonia from the cathode chamber represents a significant challenge for the future. In contrast to the effect of current density, HRT has a minor influence on the ammonia recovery efficiency especially when the HRT is longer than 1.48 min, with a similar recovery rate of 50-60% (Figure 10B).

As shown in Table 1 , the energy demand for the water desalination and ammonia recovery processes ranged from 0.05-1.95 kWh rrr³, depending on the applied current density and HRT. The lowest energy demand for per Kg NhV-N recovery was 6.1 KWh. When a current density of 11.5 A nr² and HRT of 1.48 min were used, an acceptable effluent water quality (NH4⁺-N: 4 mg L ¹, NaCI: 281.8 mg L^_1) was obtained at a low energy consumption of 0.51 kWh nr³. In this case, the energy cost was 0.037 US$ nr³, while the generated NH4⁺-N fertilizer can be used to earn 0.064 US$ nr³, i.e. , almost twice the cost of the energy consumed.

Figures 11 to 14 applied to the apparatus 150 illustrated in Figure 5. Figure 11 illustrates the conductivity over time at a current density corresponding to a current of 20 mA, a HRT of 3.92 minutes and a voltage between 2.5 and 3.0 V. Figure 12 shows the pH of the cathode slurry over time. As shown, the pH stabilised at 11 indicating that the ammonia stripping process occurs just as efficiently, if not more so than with the apparatus 10 of Figure 1. The grey shaded area represents the region where the pH was higher than the pK_a of NH4⁺ (9.3).

Figure 13 compares the concentration of ammonia at cathode flow chamber for the apparatus 10 (labelled as“1st CapAmm”) to that of the apparatus 150

(labelled as“2nd CapAmm”). Shown, the ammonia concentration for the apparatus 150 of Figure 5 reached a steady-state faster than that of the apparatus 10.

Furthermore, the steady state concentration is significantly lower.

Figure 14 compares the concentration of ammonia in the acid reservoir 160 of Figure 5. It would appear that the acid stripping for the apparatus 150 of Figure 5 is significantly faster than that of the apparatus 10.

Figures 15(a) and (b) illustrates two modes of operation of the embodiment of Figure 5. It is to be realised that the illustrations of Figure 15 are schematic in nature and only the relevant features of the apparatus are shown. The configuration of Figure 15(a) corresponds to the configuration in Figure 5 where the electrode slurry from the cathode (contained in flow gasket 22) is replenished at chamber 54 and the electrode slurry from the anion (contained in flow gasket 20) is replenished at separate chamber 52. This configuration and corresponding mode of operation are referred to as the isolated closed-cycle (ICC) configuration. Figure 15 (b) illustrates a different configuration where both the anion and cathode electrode slurries are replenished from the same chamber 22. This configuration and corresponding mode of operation are referred to as the short- circuited closed-cycle (SCC) configuration. Figures 16 to 20 reflect ion removal and ammonia recovery performance of the embodiment of Figure 5 running in ICC mode using flow electrodes of different carbon contents. Under a constant charging current density of 6.8 A nr² and HRT of 1.48 min, an increase in the carbon content may have positive effects on ion removal, with much lower steady-state effluent conductivity (<1000 ps cm^-1) and higher NH4+ and Na+ removal efficiencies observed at higher carbon mass loadings of 5 wt% and 10 wt% (Figure 16 and Figure 15(a)). The current efficiencies of the system were relatively low (63.7% and 71.9%) at carbon contents of 0 wt% and 2 wt% but rose to higher values (-90%) at 5 wt% and 10 wt% (Figure 16(b)).

Meanwhile, cathode pH can increase to higher than the pKa of NH4+ (9.3) in all cases (Figure 17), leading to the successful recovery of ammonia in the acid chamber (Figures 19 and 20). In this embodiment, the majority of ammonia was recovered as ammonium sulfate in the acid chamber for the operating parameters considered.

An inverse correlation for these parameters between carbon contents and the recovery efficiency was noted. Without being constrained by theory it has been hypothesised that this may be due to the adsorption/trapping of ammonia on the carbon particles (Figure 20). In addition, it was noted for certain operating parameters that an increase in the carbon content significantly reduced the cell voltage and electric energy consumption (Figure 21). Specifically, the control experiment at 0 wt% carbon (similar to the electrodialysis process) required a high electric energy consumption of 45.2 kWh kg-1 N but decreased to 35.8, 24.8 and 21.2 kWh kg^-1 N at carbon contents of 2 wt%, 5 wt% and 10 wt%, respectively (Figure 16(b)).

It may be that increasing the carbon content enhances the ion removal and ammonia recovery efficiency in the system due to the increased extent of direct contact between carbon particles that is achieved at the higher carbon loadings. Indeed, it has been hypothesised that the formation of a conductive 3D

interconnected particle network at high carbon densities in flow electrodes can facilitate electron transfer and decrease the internal resistance of the whole cell.

Note that using 5 wt% carbon loading achieved very similar performance to that of 10 wt% carbon (Figure 16b)). Therefore, in view of the low residual NH4+-N retained on the carbon particles, superior current efficiency and lower risk of pipe and flow channel blocking, a flow-electrode with a 5 wt% carbon content may be preferred for certain embodiments. Figures 22(a) and 22(b) clearly show that both current density and HRT have a positive effect on ion removal, with higher NH4+ removal efficiency achieved than Na+ due to the fact that NH4+ has a slightly smaller hydrated radius (3.31 A) than Na+ (3.58 A).41 , 42 More importantly, Faradaic reactions such as 02 reduction occurring in cathode flow electrode resulted in the cathodic pH increasing to values higher than the pKa of NH4+ thus providing ideal conditions for ammonia transformation and extraction via membrane stripping (Figures 22(c) and 22(d)).

It can be seen from Figures 23 that negligible accumulation of ammonia (~20 mg L^_1) in cathode was observed for this mode of operation. In contrast, continuous increase in ammonia concentrations was found in the acid chamber. For the ICC mode of operation, at a current density of 6.8 A nr² and HRT of 1.48 min, nearly 150 mg L ¹ ammonia nitrogen was recovered in the acid chamber, which represents a 3.5 fold increase in concentration to that of the feed wastewater. A summary of the ammonia distribution in the system is provided in Figures 24(a) and 24(b), which shows that the majority of ammonium ions removed from the feed wastewater may be finally transferred to the acid chamber.

For example, 65.8% of the influent ammonia was transferred and fixed as ammonium sulfate in the acid chamber at an applied current density of 6.8 A nr² and HRT of 1.48 min, while 6.2% remained in the liquid phase of the cathode with the missing portion (16.8%) electrosorbed on the carbon particles and/or escaped into the air. Meanwhile, higher current density and/or longer HRT relates to more efficient desalination that lowers the conductivity of the solution in the desalination chamber (i.e. , < 200 pS cm^-1 at a current density of 10.4 A nr² and HRT of 2.94 min) which, in turn, may increase the cell internal resistance and voltage across the cell (Figure 25). Electric energy consumption for ammonia recovery can be calculated with values of 9.6, 13.3, 24.8 and 48.9 kWh kg^-1 N corresponding to 1.8, 3.4, 6.8 and 10.4 A nr² respectively (Figure 24(c)). On extending the HRT from 0.98 to 2.94 min, the energy consumption increased by 3.2 times (from 18.7 to 79.1 kWh kg ¹ N) (Figure 24(d)).

In certain embodiments, in particular, but not limited to those where the ICC mode is used, taking the ion removal efficiency, ammonia recovery performance and energy consumption into consideration, a current density of 6.8 A nr² and HRT of 1.48 min was applied. While it is reported that SCC operation of the flow electrode is conducive to maintaining the“infinite” capacitance of the flow electrode in the charging step as a result of the continuous charge neutralization and electrode regeneration in the ex-situ apparatus, there is a critical concern that the

circumneutral pH value of the SCC flow electrode is unfavourable for the ammonia transformation and recovery in this application. Therefore, comparison of the ICC and SCC operation of the flow electrode was conducted under the preferred conditions determined above (i.e., a charging current density of 6.8 A nr² and HRT of 1.48) (Figure 15). A control experiment involving adjustment of the pH of SCC flow electrode to ~11 (similar to the average cathode pH of ICC mode) was carried out in parallel.

Figure 25 depicts the results of a 2-h charging stage followed by a 20-min reverse-current discharging step to recover energy. In ICC mode, 77.8% of the influent ammonia could be recovered in the acid chamber, potentially due to the higher cathode pH. However, the recovery ratio in SCC mode dropped to 12.6% without pH adjustment though the use of pH adjustment led to 68.9% recovery of ammonia (Figure 25(a) and 25(b)). The application of a short-term discharging step process (20 min) may result in further improvement in ammonia recovery efficiency to -80% in ICC mode (Figure 25(b)). This finding may be ascribed to the desorption and migration of the trapped ammonia on the carbon surface on increasing the cathode potential to zero or even positive values. The change in cell voltage in ICC mode from the charging stage to discharging stage may suggest that only a small amount of energy could be recovered (2.1%, as shown in Figure 25(c)) with this low percentage possibly a result of (i) the self-discharging of the flow electrode during recirculation and (ii) loss of charge at the electrode/electrolyte interface as a result of the occurrence of Faradaic reactions. Overall, the net electric energy consumption was 20.4, 11.0 and 70.4 kWh kg^-1 N respectively for ICC mode and SCC operation with and without pH adjustment of the flow electrode (Figure 26(d)). Although the SCC mode with pH adjustment consumed the lowest electrical energy, it should be noted that extra chemicals (-5 kg NaOH kg^-1 N, equivalent to -60 kWh kg^-1 N) were required to drive the ammonia recovery process. In certain embodiments, the ICC mode is more favourable for recirculation of the flow electrode in the system.

In a further embodiment, the ammonia dissolved in the cathode slurry may be removed by degassing. In this embodiment, the apparatus includes a degasser instead of a membrane contactor. The degasser may be a collector arranged over a surface of a reservoir of the cathode material which captures ammonia degassing from the cathode material. It is to be realised that degassing may be combined with a contactor membrane.

Table 1

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word“comprise” or variations such as“comprises” or“comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the process and apparatus.

It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the process and apparatus.

Claims

Claims:

1. A process for ammonia recovery in aqueous solution comprising the steps of:

allowing the water to flow through the channel whilst providing a potential difference between the anode electrode and the cathode electrode, thereby removing dissolved ammonia from solution through the cation exchange membrane and capturing the dissolved ammonia in the cathode electrode material; and

removing the dissolved ammonia from the cathode electrode material.

2. The process according to claim 1 wherein the dissolved ammonia is removed from the cathode electrode material by diffusion of gaseous ammonia through a gas permeable membrane.

3. The process according to claim 1 or claim 2 wherein the capacitive deioniser is a flow-electrode capacitive deioniser and the anode electrode and the cathode electrode comprise corresponding flow channels through which the cathode material and the anode material flow.

4. The process according to claim 3 comprising the further step of removing the dissolved ammonia with a membrane contactor.

5. The process according to claim 4 wherein the membrane contactor comprises a first conduit and a second conduit separated by a contactor membrane, the process comprising allowing the cathode material to flow in the first conduit and an acid to flow in the second conduit so that ammonia is removed from the cathode material through the contactor membrane to interact with the acid to form an ammonium salt.

6. The process according to claim 5 wherein the acid is sulphuric acid.

7. The process according to any of claims 3 to 6 wherein a flow rate through the channel of the capacitive deionizer is set so that a hydraulic retention time of the channel is longer than 1 minute, preferably longer than 1.4 minutes.

8. The process according to claim 7 wherein the hydraulic retention time of the channel is less than 5 minutes.

9. The process according to any of claims 3 to 8 wherein a pH of the cathode material whilst in the cathode is at least 9.25.

10. The process according to claim 9 wherein the pH of the cathode material whilst in the cathode is less than 12.

11. The process according to any preceding claim wherein a current density between the anode and the cathode is between:

2.8 A rm² and 17.2 A m ².

12. The process according to claim 11 wherein the current density is between 11 A nr² and 12 A nr².

13. The process according to claim 11 wherein the current density is between 6 A nr² and 7 A nr².

14. The process according to any of claims 3 to 13 wherein the cathode material and the anode material comprise a common material and wherein the cathode material and the anode material are replenished in corresponding chambers.

15. The process according to any of claims 3 to 13 wherein the cathode material and the anode material comprise a common material and wherein the cathode material and the anode material are mixed in a common chamber.

16. The process according to claim 14 or claim 15 wherein the common material comprises carbon in a weight percentage of about 5%.

17. An apparatus for recovery of ammonia in aqueous solution, the apparatus comprising: a capacitive deionizer having a cathode electrode, an anode electrode and a channel through which the solution flows situated between the cathode electrode and the anode electrode, the cathode electrode comprising cathode material, the anode comprising anode material, the deionizer comprising a cation exchange membrane between the channel and the cathode electrode and an anion exchange membrane between the channel and the anode electrode; and

an ammonia collector from the cathode electrode material.

18. The apparatus according to claim 17 wherein the capacitive deioniser is a flow-electrode capacitive deioniser and the anode electrode and the cathode electrode comprise corresponding flow channels through which the cathode material and the anode material flow.

19. The apparatus according to claim 18 wherein the ammonia collector comprises a membrane contactor.

20. The apparatus according to claim 19 wherein the membrane contactor is a hollow fibre membrane contactor.

21. The apparatus according to claim 19 wherein the membrane contactor comprises a flat-sheet membrane arrangement.

22. The apparatus according to claim 21 comprising a gas permeable membrane and a contactor channel wherein the gas permeable membrane is arranged between the cathode flow channel and the contactor channel, the contactor channel being arranged between the cathode flow channel and a cathode conductor.

23. The apparatus according to claim 22 wherein the contactor channel is connected to an acid reservoir by a conduit so that an acid solution may flow from the acid reservoir to the contactor channel.

24. The apparatus according to any of claims 18 to 23 wherein the cathode material and the anode material comprise a common material and wherein the cathode material and the anode material are replenished in corresponding chambers.

25. The apparatus according to any of claims 18 to 23 wherein the cathode material and the anode material comprise a common material and wherein the cathode material and the anode material are mixed in a common chamber.

26. The apparatus according to claim 24 or claim 25 wherein the common material comprises carbon in a weight percentage of about 5%.