Technical Field

[0001] The present disclosure generally relates to a system and process for removing organic contaminants and recovering caustic soda from an alkaline liquid, such as but not limited to a process stream or effluent, in particular Bayer liquors. In particular, the system and process employ electrochemically active and other microorganisms to oxidise organic contaminants and facilitate caustic soda recovery. The system and process may additionally be employed to produce hydrogen from an alkaline liquid with organic contaminants.

Background

[0002] Aluminium is typically extracted from bauxite ores in a refinery process known as the Bayer process, wherein the ores are reacted with caustic liquor (Bayer liquor) under elevated temperature. Most of the bauxite deposits in Australia contain a high content of silica, which requires the use of higher amounts of caustic soda in the Bayer process, raising the overall refining cost. Moreover, the refinery process is severely hampered by the accumulation of organics, particularly sodium oxalate (Na ₂ C ₂ 0 ₄ ) in the Bayer liquor. This is particularly relevant to Australian alumina refineries because of the high organic content of the bauxite ore. Efficient removal of these organics is therefore vital to maintain process productivity and to lower the overall refinery cost.

[0003] Aerobic biodegradation of oxalate is a proven treatment solution for Bayer liquors, but it does not allow the recovery of caustic soda.

[0004] Bio-electrochemical systems (BES) is an emerging wastewater treatment and resource recovery technology. A unique feature of this technology lies in its effective use of electrodes to stimulate and control microbial degradation of organic matter.

[0005] Practical application of BES for large-scale treatment of wastewater, however, has been largely hindered by inherent wastewater properties which prevent the wastewater from behaving as an electrolyte for the purposes of electrochemical treatment, namely (i) low ionic conductivity (typically in a range of 1 to 2 mS/cm); and (ii) low pH buffering capacity (typically in a range of 5 to 10 mM) for the neutralisation of the acidity produced from the bio-electrochemical anodic oxidation of organic matter.

[0006] The present disclosure seeks to overcome at least some of the above- mentioned disadvantages.

Summary

[0007] The present disclosure provides a system and process for removing organic contaminants and recovering caustic soda from an alkaline liquid such as, but not limited to a process stream or effluent, in particular Bayer liquors.

[0008] In a first aspect, the disclosure provides a bio-electrochemical system for removing organic contaminants and recovering caustic soda from an alkaline liquid, said bio-electrochemical system comprising:

an anodic chamber having an anode, wherein the anodic chamber is configured to receive the alkaline liquid;

at least one electrochemically active microorganism disposed in proximity to the anode, wherein the at least one electrochemically active microorganism is capable of catalysing oxidation of organic contaminants to carbon dioxide, thereby removing at least some of the organic contaminants from the alkaline liquid to produce a partially treated alkaline liquid;

a cathodic chamber having a cathode arranged to produce hydroxide ions; a cation selective membrane configured to separate the anodic and cathodic chambers and allow cations to migrate from the anodic chamber to the cathodic chamber, the hydroxide ions and cations in the cathodic chamber producing caustic soda;

a bioreactor in fluid communication with the anodic chamber in an arrangement whereby the bioreactor receives the partially treated alkaline liquid and facilitates biodegradation of remaining organic contaminants in the partially treated alkaline liquid to produce a treated alkaline liquid, the bioreactor being, optionally, in fluid

communication with the cathodic chamber in an arrangement whereby the bioreactor discharges at least a portion of the treated alkaline liquid to the cathodic chamber; and at least one power source configured to apply a voltage between the anode and the cathode.

[0009] In some embodiments, a catholyte of the cathodic chamber may comprise at least the portion of treated alkaline liquid discharged from the bioreactor to the cathodic chamber.

[0010] In some embodiments, the cathodic chamber may be configured to discharge a stream comprising the treated alkaline liquid and the recovered caustic soda into an alkaline liquid circuit, a pond or another location.

[0011] In other embodiments, the cathodic chamber may be configured to discharge the recovered caustic soda to an alkaline liquid circuit, pond or other location.

[0012] In some embodiments, the bioreactor may be configured to discharge the treated alkaline liquid to an alkaline liquid circuit, a pond or another location.

[0013] In some embodiments, the bioreactor may be configured to receive a diluent and/or one or more nutrients. Alternatively, or additionally, the anodic chamber may be configured to receive a diluent and/or one or more nutrients. The one or more nutrients may include, but are not limited to, nitrogen (N), phosphorus (P) and/or organic carbon (C).

[0014] In a second aspect, the disclosure provides a process for removing organic contaminants and recovering caustic soda from an alkaline liquid, said process comprising the steps of:

providing an electrolytic cell having an anodic chamber and a cathodic chamber separated by a cation selective membrane, wherein the anodic chamber has an anode and at least one electrochemically active microorganism in proximity to the anode, and the cathodic chamber has a cathode;

treating the alkaline liquid in the anodic chamber by contacting the alkaline liquid with the anode and the at least one electrochemically active microorganism in the anodic chamber; and

applying a voltage to the electrolytic cell sufficient to cause:

(i) oxidation of the organic contaminants in the anodic chamber thereby removing at least some of the organic contaminants from the alkaline solution to produce a partially treated alkaline liquid,

(ii) production of hydroxide ions in the cathodic chamber, and

(iii) migration of cations from the anodic chamber to the cathodic chamber to produce caustic soda in the cathodic chamber;

passing the partially treated alkaline liquid to a bioreactor to facilitate

biodegradation of remaining organic contaminants in the partially treated alkaline liquid to produce a treated alkaline liquid, and, optionally, mixing at least a portion of the treated alkaline liquid with the caustic soda solution to produce an alkaline liquid having recovered caustic soda..

[0015] In one embodiment, prior to passing the partially treated alkaline liquid to the bioreactor, the process may comprise mixing the partially treated alkaline liquid with a diluent and/or one or more nutrients. Alternatively, the treated alkaline liquid may be mixed with a diluent and/or one or more nutrients in the bioreactor.

[0016] In an alternative embodiment, prior to treating the alkaline liquid in the anodic chamber, the process may comprise mixing the alkaline liquid with a diluent and/or one or more nutrients. Alternatively, the alkaline liquid may be mixed with a diluent and/or one or more nutrients in the anodic chamber.

[0017] In one embodiment the anode and the cathode may comprise the same or different materials. For example, the anode and the cathode may comprise graphite granules.

[0018] In one embodiment the at least one electrochemically active microorganism may be capable of catalysing one or more organic compounds selected from a group comprising an acetate, an oxalate, a succinate, a malonate, a formate compound and salts thereof. In one embodiment, the at least one electrochemically active

microorganism comprises an electrochemically active microorganism originating from a culture collection, an activated sludge, the bioreactor or a natural or engineered environment. In one embodiment, the at least one electrochemically active microorganism may comprise one or more members of the phyla Proteobacteria; Firmicutes; Bacterioidetes; Tenencutes; Actinobacteria. [0019] In one embodiment , the cation selective membrane may be a sodium ion selective membrane.

[0020] In one embodiment, the voltage applied to the electrolytic cell may be in a range of 0-5000 mV and, more particularly; 0-3000 mV. In some embodiments, the voltage applied to the electrolytic cell may be in a range of 0-800 mV.

[0021] In one embodiment, the potential applied to the anode is in a range of -600 to 600 mV vs Ag/AgCI, in particular in a range of 600 to -300 mV vs Ag/AgCI.

Brief Description of Drawings

[0022] Preferred embodiments of the present disclosure will now be further described and illustrated, by way of example only, with reference to the accompanying drawings in which:

[0023] Figure 1 shows a schematic representation of a dual chamber electrolytic cell for removing organic contaminants and recovering caustic soda from highly alkaline solutions as described herein;

[0024] Figure 2 shows a schematic representation of one embodiment of a system for removing organic contaminants and recovering caustic soda from highly alkaline solutions as described herein;

[0025] Figure 3 shows a schematic representation of a further embodiment of the system and process as described herein with reference to the Examples;

[0026] Figure 4 is a graphical representation of current production from reactor R1 and R2 over a period of 300 days;

[0027] Figure 5A is a graphical representation of current production over the first 54 days of operation of an embodiment of the bio-electrochemical system with sodium oxalate and Figure 5B is a graphical representation of oxalate concentration of the outflow and inflow streams of reactor R1 and R2; [0028] Figure 6 is a graphical representation of current production from anodic oxidation of simple organic substances commonly present in Bayer liquors (A: sodium acetate; B: sodium formate; C: sodium succinate and sodium malonate);

[0029] Figure 7 is a graphical representation of reactor R1 performance at different anode potentials from -600 to 300 mV vs Ag/AgCI at pH 9.0 with sodium oxalate and sodium acetate as electron donors; Figure 7A shows current production and anode potential over time; Figure 7B shows variation of average current produced at different posed anode potentials; Figure 7C shows oxalate, acetate and COD removal percentages within the range of anode potentials;

[0030] Figure 8 is a graphical representation of the production of caustic soda solution in the cathodic chamber of Reactor R1 at anode potential -300mV vs Ag/AgCI, anolyte pH 9.0 and COD loading rate 5.1 kgCOD/m3.day (HRT 3h) with sodium oxalate and sodium acetate as electrone donors. (Figure 8A: Steady current production and anode and cathode electrode potentials over the experimental time period. Figure 8B: Increase of catholyte Na ⁺ and NH ₄ ⁺ ion concentration during the experimental time. Figure 8C: Oxalate and acetate removal percentages during the experiment. Figure 8D: Anolyte and catholyte pH varying with the time. Figure 8E. Increase of catholyte total alkalinity and catholyte electrical conductivity over the experimental time. Figure 8F: Cumulative energy input into the system;

[0031] Figures 9A and 9B are respective graphical representations of current production at different anolyte pH in the anodic chamber of Reactor R1 at anode potential -300 mV vs Ag/AgCI, 2.55 kg COD/m ³ . d loading rate with sodium oxalate and sodium acetate as electron donors. Figure 9A shows the variation of current production with anolyte pH over the experimental time period. Figure 9B shows the variation of anolyte pH vs average current production;

[0032] Figure 10 is a graphical representation of oxalate removal rate within the bioreactor over time;

[0033] Figure 11 is a graphical representation of the effect of pH in the bioreactor on the oxalate removal rate over time; and [0034] Figure 12 is a graphical representation of the current production profile during start-up of the bioelectrochemical system with the anode coated with an aerobic oxaalte degrading microbial biofilm established from the bioreactor.

Description of Embodiments

[0035] The present invention is described in the following various non-limiting embodiments, which relate to a method and a system for removing organic

contaminants and recovering caustic soda from highly alkaline solutions.

GENERAL TERMS

[0036] Throughout this specification, unless specifically stated otherwise or the context requires otherwise, reference to a single step, composition of matter, group of steps or group of compositions of matter shall be taken to encompass one and a plurality (i.e. one or more) of those steps, compositions of matter, groups of steps or groups of compositions of matter. Thus, as used herein, the singular forms "a", "an" and "the" include plural aspects unless the context clearly dictates otherwise. For example, reference to "a" includes a single as well as two or more; reference to "an" includes a single as well as two or more; reference to "the" includes a single as well as two or more and so forth.

[0037] Each example of the present disclosure described herein is to be applied mutatis mutandis to each and every other example unless specifically stated otherwise. The present disclosure is not to be limited in scope by the specific examples described herein, which are intended for the purpose of exemplification only. Functionally-equivalent products, compositions and methods are clearly within the scope of the disclosure as described herein.

[0038] The term "and/or", e.g., "X and/or Y" shall be understood to mean either "X and Y" or "X or Y" and shall be taken to provide explicit support for both meanings or for either meaning.

[0039] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[0040] It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

SPECIFIC TERMS

[0042] The term Organic contaminants' as used herein refers to organic material, such as but not limited to leaves, wood and humus, which have dissolved under the highly alkaline conditions (pH>12) of the alkaline liquid as soluble organic compounds. Generally, the organic contaminants are expressed as COD ('chemical oxygen demand') and may be present as alkali metal and alkaline earth metal salts (e.g.

sodium, potassium, magnesium and so forth) of a range of molecular weight organic compounds including, but not limited to, acetate, oxalate, malonates, humates and so forth. Humates comprise high molecular weight organic compounds having at least twenty carbon atoms. Humates are commonly present in bauxites.

[0043] The expression 'treated alkaline liquid' as used herein refers to an alkaline liquid having a reduced organic contaminants content. The expression 'reduced organic contaminants content' as used herein refers to a liquid which has undergone a treatment process to remove organic contaminants therefrom and consequently the liquid's organic contaminant content is less than its organic contaminant content prior to undergoing the treatment process. A reference to 'partially treated' may mean a reduction in the content of at least one type of organic contaminant (e.g. acetate) in the absence of, or with minimal, reduction in the content of other organic contaminants in the liquid.

[0044] The term 'caustic soda' as used herein predominantly refers to aqueous sodium hydroxide solutions but may also comprise aqueous hydroxide solutions having one or more types of counter-cations including, but not limited to, potassium, magnesium, calcium or other alkali metal or alkaline earth metal cations.

[0045] The term 'alkaline liquid' as used herein refers to an aqueous solution having a pH >8, typically pH >13, derived from an industrial, agricultural, food and beverage production, or chemical process, wastewaters and/or effluents. Exemplary alkaline liquids include, but are not limited to, diluted or undiluted Bayer liquors (i.e. caustic aluminate liquors containing sodium hydroxide and sodium aluminate utilised in the processing of bauxite to alumina trihydrate), liquids produced by dissolving oxalate or other organics cake generated by alumina or other industries, pulp and paper processing liquors (i.e. mixtures of wood pulp, sodium hydroxide and sodium sulphide), and process solutions from manufacture of alkaline batteries, bio-diesel fuels, soaps, medications, detergents and cleaning products.

[0046] The term 'cation selective membrane' as used herein refers to a semipermeable membrane which relies on an electrical potential or a concentration gradient to selectively transport cationic species through pores in the semi-permeable membrane. The cation selective membrane may be comprised of glass, solid salt precipitates or polymers. The polymeric membranes generally comprise a polymeric binder or support as the supporting matrix and a cation-selective carrier . The cation- selective carrier is a compound which is capable of sequentially complexing the desired cation and transporting the cation across the membrane-solution interface and de-complexing the cation.

[0047] The terms 'catholyte' and 'anolyte', respectively, as used herein refer to the aqueous electrolyte solution contained in the cathodic and anodic chambers of an electrolytic cell. An electrolyte is a substance, such as an acid or a salt, that when dissolved in an aqueous solution conducts an electric current. [0048] The term 'electrochemically active microorganism' refers to a microorganism capable of transferring electrons to electrodes or accepting electrons from electrodes while consuming organic matter, typically by oxidising organic matter to carbon dioxide.

[0049] The term 'bioreactor' as used herein refers to a container configured to facilitate a chemical process therein involving microorganisms, such as a bacterial, archaeal, yeast, fungal or viral species or biochemically active substances derived from such organisms. For example, the biochemically active substance may be a chemical such as an enzyme or hormone. This process can either be aerobic (i.e. in the presence of oxygen) or anaerobic (i.e. in the absence of oxygen).

[0050] The term 'biofilm' as used herein refers to a community of bacteria and/or other microorganisms embedded in a layer of a self-produced matrix of extracellular polymeric substance which adheres to a surface.

[0051] The term 'activated sludge' as used herein refers to a mass of aerobic or facultatively anaerobic microorganisms capable of breaking down organic matter into carbon dioxide, water, and other inorganic compounds. Activated sludge may be sourced from culture collections and/or natural or engineered environments including, but not limited to, soil, surface water, ground water, marine sediment, freshwater sediment, brackish water sediment, solid or liquid waste streams from mineral processing plants, red mud, aerated sewage, sewage treatment plants, or wastewater treatment plants.

BIO-ELECTROCHEMICAL SYSTEM

[0052] The present disclosure relates to a bio-electrochemical system for removing organic contaminants and recovering caustic soda from an alkaline liquid, such as but not limited to an alkaline process stream or an effluent in particular Bayer liquors.

[0053] With reference to Figure 1 a dual chamber electrolytic cell 12 for use in a bio- electrochemical system 10 for removing contaminants and recovering caustic soda from an alkaline liquid is shown and described in detail below. [0054] The dual chamber electrolytic cell 12 comprises an anodic chamber 14 having an anode 16, a cathodic chamber 18 having a cathode 20, and a power source 22 configured to apply a voltage between the anode 16 and the cathode 20.

[0055] The anode 16 and the cathode 20 may be made of the same or different materials. In this particular embodiment, the anode 16 and the cathode 20 comprise graphite granules (3-5 mm). Other suitable materials include, but are not limited to, activated carbon, biochar, stainless steel, titanium, titanium coated with mixed metal oxides, wire mesh, carbon mesh, carbon cloth, carbon fibre, carbon felt, carbon granules, graphite in a form other than granules or a combination thereof.

[0056] The working volume of the anodic and cathodic chambers 14, 18 may determine the hydraulic retention time (HRT) of the liquid in the respective chamber. In general, the smaller the working volume the shorter the HRT with a particular flow rate.

[0057] The anodic and cathodic chambers 14, 18, may be separated by a cation selective membrane 24. In use, the cation selective membrane is arranged to allow migration therethrough of cations from the anodic chamber to the cathodic chamber. In one particular embodiment, the cation selective membrane may be a sodium selective membrane.

[0058] The dual chamber electrolytic cell 12 further includes at least one

electrochemically active microorganism disposed in proximity to the anode 16. For example, the anode 16 may support a biofilm 26 thereon, wherein the biofilm 26 comprises the at least one electrochemically active microorganism.

[0059] The at least one electrochemically active microorganism comprises an organic compound-oxidising microorganism. The at least one electrochemically active microorganism may be capable of catalysing oxidation of one or more organic compounds including, but not limited to, an acetate, an oxalate, a succinate, a malonate, a formate compound, or salts thereof and so forth.

[0060] The at least one electrochemically active microorganism may comprise an activated sludge. However, any organic compound-oxidising microorganism may be suitable for use in the bio-electrochemical system as disclosed herein. Other suitable electrochemically active microorganisms include, but are not limited to, members of the phyla Proteobacteria; Firmicutes; Bacterioidetes; Tenencutes; Actinobacteria.

[0061] The dual chamber electrolytic cell 12 may be configured to operate in batch mode, continuous mode or sequencing batch mode. In batch mode operation, the anodic chamber 14 is charged with a fixed volume of alkaline liquid containing organic contaminants and is treated until a desired endpoint (i.e. concentration of organic contaminants therein) is achieved before the next fixed volume of alkaline liquid is treated. In a continuous mode operation, a continuous flow of alkaline solution containing organic contaminants is provided to the anodic chamber 14 with a concurrent flow of treated alkaline liquid out of the anodic chamber 14 to maintain a constant volume in the anodic chamber 14. In sequencing batch mode a portion of the treated alkaline liquid volume is replaced with untreated alkaline liquid after each treatment cycle. Accordingly, the dual chamber electrolytic cell 12 may be provided with a pump (not shown) and a flow control means to control the respective flows of alkaline liquid containing organic contaminants and treated alkaline liquid in and out of the anodic chamber 14.

[0062] With reference to Figure 1 , the anodic chamber 14 is provided with an inlet 28 in fluid communication with an alkaline liquor process circuit 100 to receive an influent comprising an alkaline liquor containing organic contaminants via line 1 1. The alkaline liquor containing organic contaminants may optionally comprise a diluent delivered through line 13 and/or one or more nutrients such as but not limited to N, P and organic C delivered through line 15. The diluent and/or the one or more nutrients may be mixed with the alkaline liquor to provide an environment in which the at least one electrochemically active microorganism may thrive. When mixed with the alkaline liquor, the diluent and/or the one or more nutrients may reduce the pH and/or ionic strength of the alkaline liquor and/or provide nutrients to sustain growth and viability of the at least one electrochemically active microorganism in the anodic chamber 14.

[0063] The inventors have found that although the at least one electrochemically active microorganism is capable of catalysing oxidation of organic contaminants to carbon dioxide, in use, it is not possible to completely remove all organic contaminants from the alkaline liquid in the anodic chamber 14. Accordingly, in use, the at least one electrochemically active microorganism disposed in proximity to the anode 16 in the anodic chamber 14 removes at least some of the organic contaminants from the alkaline liquid to produce a partially treated alkaline liquid.

[0064] The anodic chamber 14 is also provided with an outlet 30 to discharge the partially treated alkaline liquid.

[0065] The cathodic chamber 18 may be provided with an inlet 32 to receive a liquid, such as a catholyte, and an outlet 34 to discharge a caustic soda solution recovered therein. As shown in Figure 1 , the outlet 34 may be configured in fluid communication with the alkaline liquor process circuit 100 via line 23 to discharge the recovered caustic soda solution directly to the alkaline liquor process circuit 100. Optionally, a portion of the recovered caustic soda solution may be re-directed from the alkaline liquor process circuit 100.

[0066] The cathodic chamber 18 may optionally be configured to receive a portion of the partially treated alkaline liquid from the anodic chamber 14 via line 19. It will be appreciated that the dual chamber electrolytic cell 12 may be further provided with a pump (not shown) and a flow control means to control respective flows of catholyte and recovered caustic soda solution in and out of the cathodic chamber 18.

[0067] The inventors have found that the partially treated alkaline liquid from the anodic chamber 14 may be further treated in a bioreactor to remove remaining organic contaminants and produce a treated alkaline liquid.

[0068] With reference to Figure 2, where like reference numerals are used to refer to like parts, one embodiment of a bio-electrochemical system 10 for removing organic contaminants and recovering caustic soda from an alkaline liquid is shown.

[0069] In this particular embodiment, the bio-electrochemical system 10 comprises the dual chamber electrolytic cell 12 as described above and a bioreactor 36. This embodiment further treats the partially treated alkaline liquid discharged from the anodic chamber 14 containing remaining organic contaminants. Said partially treated alkaline liquid may undergo a bio-oxidation process to produce a treated alkaline liquid comprising an alkaline liquid substantially reduced in organic contaminants.

[0070] The bioreactor 36 comprises a vessel 38 for hosting microorganisms capable of oxidising organic contaminants. The microorganisms may be suspended in the vessel 38. Alternatively, the microorganisms may be provided as a biofilm on a supporting substrate. In this embodiment, the supporting substrate may be the same material as the anode 16 in the anodic chamber 14 of the dual chamber electrolytic cell 12. For example, the supporting substrate for the biofilm may be graphite granules. Advantageously, for embodiments wherein the supporting substrate for the biofilm in the bioreactor 36 is the same material as the anode 16 in the anodic chamber 14, the supporting substrate and the biofilm from the bioreactor may be employed to assist start-up (or alternatively a back-up) of the anodic chamber 14 of the dual chamber electrolytic cell 12.

[0071] It will be appreciated that in some embodiments, the supporting substrate may be a different material from the anode 16 in the anodic chamber 14 of the dual chamber electrolytic cell 12.

[0072] The vessel 38 may be provided with an agitator (not shown) for mixing the contents thereof and maintaining a homogenous mixture, thereby providing better mass transfer of nutrients and oxygen. The bioreactor 36 may be provided with one or more baffles within the vessel 38 to interrupt the formation of a vortex therein. The formation of a vortex is usually highly undesirable because the centre of gravity of the system changes and increases the consumption of power. The vessel 38 may also be provided with a sparger to supply an oxygen-containing gas to the vessel's contents and a temperature control system to maintain the vessel's contents at a constant desired temperature.

[0073] The anodic and cathodic chambers 14, 18 may be configured to be in fluid communication with the bioreactor 36.

[0074] In contrast to the embodiment described with reference to Figure 1 , the outlet 30 of the anodic chamber 14 shown in Figure 2 may be configured to be in fluid communication with an inlet 40 in the bioreactor 36. In this way, the partially treated alkaline liquid may be discharged directly to the bioreactor 36 for further treatment therein to produce a treated alkaline liquid which is substantially reduced in organic contaminant content.

[0075] In turn, the inlet 32 of the cathodic chamber 18 may be configured in fluid communication with an outlet 42 in the bioreactor 36 to receive all or a portion of the treated alkaline liquid. In this way, the bioreactor 36 may conveniently provide a catholyte comprising the treated alkaline liquid for the cathodic chamber 18, eliminating or reducing the need to source a catholyte from an external source.

[0076] In the embodiment shown in Figure 2, the caustic soda solution recovered in the cathodic chamber 18 may be mixed with the received treated alkaline liquid to produce a stream of treated alkaline liquid and recovered caustic soda. Said stream may subsequently be discharged from the cathodic chamber 18 via outlet 34 and line 23 into the alkaline liquor process circuit 100, pond or other location.

[0077] Integration of the bioreactor 36 with the dual chamber electrolytic cell 12 as described above advantageously facilitates return of the stream of treated alkaline liquid and recovered caustic soda to the alkaline liquor process circuit 100, thereby reducing the need to source alternative liquor elsewhere for the same purpose and associated operating costs by lowering consumption of reagents, in particular caustic soda, for the alkaline liquor process circuit 100.

[0078] It will be appreciated that in an alternative embodiment, the treated alkaline liquid or a portion thereof from the bioreactor 36 may not be discharged into the cathodic chamber 18 but may be discharged into an alternative process stream, pond or other location via line 44. In this way, the caustic soda solution produced in the cathodic chamber 18 is not diluted by mixing with the treated alkaline liquid from the bioreactor 36.

[0079] The bio-electrochemical system 10 may be configured to operate in batch mode, continuous mode or sequencing batch mode as described above.

PROCESS FOR REMOVING ORGANIC CONTAMINANTS AND RECOVERING CAUSTIC SODA [0080] The disclosure also relates to a process for removing organic contaminants and recovering caustic soda from an alkaline liquid, such as but not limited to an alkaline process stream or an effluent.

[0081] In some embodiments, the process may be performed by employing the bio- electrochemical systems 10 described above.

[0082] The process may comprise treating the alkaline liquid by contacting the alkaline liquid with the anode 16 and the biofilm 26 encompassing the at least one electrochemically active microorganism in the anodic chamber 14. It will be appreciated that the cathodic chamber 18 may be fed with a catholyte to complete the electrolytic cell.

[0083] The catholyte may be an aqueous solution of any one or more of a suitable electrolyte, including but not limited to NaCI, KN0 ₃ , KCI, NH ₄ N0 ₃ , NaHC0 ₃ , KHC0 ₃ and so forth. In one particular embodiment, the catholyte may be sodium chloride solution having a concentration in a range of 0.01 M to 3.5M NaCI. In some

applications of the process, it may be convenient to use as the catholyte other plant process streams, groundwater having a suitable dissolved solids content, saline water, seawater, hypersaline water, waste streams from desalination plants and so forth.

[0084] The process also comprises applying a voltage between the anode 16 and the cathode 20 of the electrolytic cell 12. In one embodiment, the voltage applied to the electrolytic cell 12 is in a range of 0 to 5000 mV and, more particularly, in a range of 0 to 3000 mV. In some embodiments, the voltage applied to the electrolytic cell 12 is in a range of 0 to 800 mV. Oxidation of the organic compounds in the alkaline liquid is catalysed by the electrochemically active microorganism at a suitable anode potential in the range of -600 to +600 mV vs Ag/AgCI, in particular -600 to +300 mV vs Ag/AgCI.

[0085] At least some of the organic contaminants are oxidised at the anode. For example, acetate and oxalate oxidation may be represented by reactions (1) and (2), respectively, as follows:

(1) CH ₃ COO ^" + 4H ₂ 0→ 2HC0 ₃ ^" + 9H ⁺ + 8e ^" (2) C ₂ 0 ₄ ^2" → 2C0 ₂ + 2e

[0086] The voltage applied to the electrolytic cell 12 is sufficient to cause reduction of water to hydroxide and hydrogen (H ₂ ) in the cathodic chamber 18 according to reaction (3):

(3) 2H ₂ 0 + 2e ^" → H ₂ +20H ^"

[0087] and migration of cations, such as sodium, from the anodic chamber 14 to the cathodic chamber 18 through the cation selective membrane 24. The migrated cations combine with the hydroxide ions produced in the cathodic chamber to produce a caustic soda solution.

[0088] In the embodiment as described with reference to Figure 2, the partially treated alkaline liquid may be discharged into the bioreactor 36 and subjected to a bio- oxidation process therein to biodegrade the remaining organic contaminants in the partially treated alkaline liquid to produce a treated alkaline liquid comprising an alkaline liquid substantially reduced in organic contaminants.

[0089] The treated alkaline liquid or a portion thereof from the bioreactor 36 may then be passed to the cathodic chamber 18 whereupon it is mixed with caustic soda solution produced in the cathodic chamber 18 to produce an alkaline liquid having recovered caustic soda. Said treated alkaline liquid may then be discharged into an alkaline liquid circuit, such as but not limited to an alkaline liquor process circuit 100, pond or other location.

[0090] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. Examples

[0091] The invention is further illustrated by the following examples. The examples are provided for illustrative purposes only. They are not to be construed as limiting the scope or content of the invention in any way.

Example 1. Bio-electrochemical Systems, Process Set-up and General Operation

[0092] Two dual-chamber bio-electrochemical reactors 10 were constructed according to the schematic shown in Figure 3: Reactor 1 (R1) was fed with an ammonium-N supplemented synthetic alkaline liquor, whereas Reactor 2 (R2) was fed with an ammonium-N deficient synthetic alkaline liquor. Each reactor consisted of an anodic chamber 14 and a cathodic chamber 18 of identical size(14 cm x 12 cm x 2 cm), which were separated by a cation exchange membrane 24 (surface area 168 cm ² ) (Ultrex CMI-7000, Membrane International Inc.) (Figure 3). Both chambers 14, 18 were loaded with identical conductive graphite granules (3-5 mm diameter, KAIYU Industrial (HK) Ltd.; both half cells were loaded with 285 g of the graphite granules each) to behave as the anode 16 and the cathode 20, respectively. This reduced the void volume of each chamber 14, 18 from 336 to 250 ml_. The specific surface area of the graphite granules was 1.308 ± 0.003 m ² /g as determined by using BET (Brunauer- Emmett-Teller) method (CSIRO Process Science and Engineering, Waterford, Western Australia). Four graphite rods (5 mm diameter, length 12 cm) were used as current collector to enable electric connection between the graphite granules and the external circuit. The reactors were operated as a three-electrode system coupled to a potentiostat (VMP3, BioLogic). The anodic chamber 14 was inoculated with bacteria and the anode 16 therein is termed as the working electrode (here anode). The cathode 20 in the cathodic chamber 18 is termed as the counter electrode (which functioned mostly as a cathode). The working electrode was polarized against a silver- silver chloride (Ag/AgCI) reference electrode 50 (MF-2079 Bioanalytical Systems, USA) at a defined potential using the potentiostat. The reference electrode 50 was mounted in the anodic chamber 14 and was embedded within the granular graphite working electrode (anode 16). A total liquid volume of 0.5 and 2.0 L was continuously recirculated through the anodic and the cathodic chambers 14, 18 via two separate external recirculation bottles 52, 54 (0.25 and 2.0 L), respectively, at a recirculation rate of approximately 14 L/h. The recirculation bottle 52 was intermittently sparged (every 20 min for 30 second) with nitrogen gas to create an anaerobic environment in the anodic chamber 16 and to facilitate nitrogen fixing mechanism in reactor R2. The process was operated at 22±2°C.

[0093] The anodic chambers 16 of both reactors 10 were operated in either batch or continuous modes. When the reactors 10 were operated in continuous mode, fresh anolyte (maintained at 4°C in a refrigerator) was continuously introduced at a specified flow rate into the external recirculation bottle 52 and an equal volume of the old anolyte was extracted (and discarded) from the recirculation line 56 using a peristaltic pump (Masterflex® Cole-Parmer L/S pump drive fitted with a Model 77202-60

Masterflex® pump head; Norprene® tubing 06404-14). The cathodic chambers 18 of the reactors were exclusively operated in batch mode. The catholyte was occasionally renewed via recirculation line 58. The BES process was continuously monitored and controlled using a computer program (LabVI EW). The working electrode potential and current of the BES were monitored via the potentiostat. All electrode potentials (mV) refer to values against Ag/AgCI reference electrode (ca. +197 mV vs. standard hydrogen electrode (Bard and Faulkner 2001 )). The pH of the working electrolyte was continuously monitored using in-line pH sensors (TPS Ltd. Co. , Australia). All signals were regularly recorded to an Excel spreadsheet via the computer programme interfaced with a National Instrument™ data acquisition card (DAQ) 60 and a computer 62. The system also included pH probes 64 and conductivity probes 66

Example 2. Synthetic alkaline process liquor and reactor electrolyte

[0094] A synthetic medium that mimics highly saline and alkaline process streams was used as the influent of the bio-electrochemical reactors. Unless stated otherwise, sodium oxalate 3.35 g/L (25 mM) and/ or sodium acetate (5 mM) were used as the carbon source, and NaCI 25 g/L was added to increase the solution salinity. The pH value of the feed solution was maintained at above 10 by adding 2M NaOH solution. The nutrients medium used for R1 consisted of (mg/L): NH ₄ CI, 535; NaHC0 ₃ , 125; MgS0 ₄ -7H ₂ 0, 51 ; CaCI ₂ -2H ₂ 0, 15; and Κ ₂ ΗΡ0 ₄ ·3Η ₂ 0, 20.52 and 1 .25 mL/L of trace element which had the composition of (g/L): ZnS0 ₄ -7H ₂ 0, 0.43; FeS0 ₄ - 7H ₂ 0, 5;

CoCI ₂ -6H ₂ 0, 0.24; MnCI ₂ -4H ₂ 0, 0.99; CuS0 ₄ -5H ₂ 0, 0.25; NaMo0 ₄ -2H ₂ 0, 0.22; NiCI ₂ -6H ₂ 0, 0.19; NaSeO ₄ - 10H ₂ O, 0.21 ; ethylenediaminetetraacetic acid (EDTA) 15, H ₃ BO ₃ , 0.014; and NaWCy2H ₂ 0, 0.05. The same nutrient medium without NH ₄ CI was used as the influent of the anodic chamber for R2. Unless stated otherwise, this medium was used as the electrolyte in the anodic chamber throughout the entire study. The catholyte of both reactors (R1 and R2) also contained a similar NaCI concentration of (25 g/L) as the anolyte. This was chosen with the anticipation that after the organic content is removed, the anodic effluent would form the influent stream of the cathodic chamber. In some trials, the effect of other common organics, namely sodium formate, sodium succinate and sodium malonate was also tested via spiking experiments.

Example 3. Reactor start-up and acclimatisation of electrochemically active anodic biofilm

[0095] Establishment of electrochemically active biofilm in the anodic chambers of both reactors (R1 and R2) was initiated by adding a returned activated sludge (RAS) collected from a municipal wastewater treatment plant (ca. 2 g mixed liquor suspended solids (MLSS)/L) in Perth, Western Australia. The RAS was filtered through a metal screen to remove large particles (> ca. 1 mm). The inoculation was done by injecting a predefined volume of the filtered sludge into the anodic recirculation line using a 60- ml_ plastic syringe. During the initial start-up period, yeast extract was added to the anolyte (50 mg/L final concentration) as bacterial growth supplement. Both the anolyte and catholyte were renewed regularly (and prior to each specific experiments as described below) to avoid accumulation of unwanted chemical species. After inoculation with the activated sludge, the working electrode (anode) was maintained at a constant potential of +200 mV vs Ag/AgCI and the synthetic alkaline liquor was continuously fed into the anodic chamber to obtain a hydraulic retention time (HRT) of one day. Over this period, sodium oxalate was used as the sole carbon source and influent pH was maintained at 10. The use of sodium oxalate alone was preferable as it is considered as the key contaminant in Bayer liquors. Anodic current production and oxalate removal in the anolyte were used as the parameters to indicate the

establishment of biofilm activity over the start up period (first 26 days). Since no obvious current was recorded during this period, on day 27 both R1 and R2 were switched into batch mode operation, and fixed amounts of sodium acetate solution (2.5 mmoles) were injected into the anodic recirculation line of each reactor on day 35 and day 51 to test if the biofilm could readily convert acetate (another key organic contaminant in Bayer liquor and a more readily degradable substrate and electron donor) into electrical current. Since current was produced in response to the acetate spikes, from day 54 onwards the synthetic alkaline liquor was supplemented with sodium acetate (5 rtiM). The in-reactor pH was maintained at 9 by dosing NaOH (0.5M) to overcome anodic acidification.

[0096] The reactors R1 and R2 were maintained continuously for more than 300 days under different experimental conditions to analyse the potential and performance of the BES configuration on organics destruction in synthetic alkaline liquor. Figure 4 shows the current production of the two reactors over the total experimental time period.

[0097] Over the start-up period with sodium oxalate as sole electron and carbon source the current production of the both reactors were very low approximately 1 mA for 1 day HRT and the oxalate removal was negligible, which indicates that the bacteria were not able to utilise oxalate for current generation, regardless of whether the synthetic media was supplied with nitrogen source or not. Figure 5 shows the current production and oxalate concentration of inflow and outflow streams over the first 54 days of operation with sodium oxalate as only carbon source.

[0098] Figure 5A shows a residual current of ~1 mA and immediate response of biofilm to yeast extract and sodium acetate addition. Oxalate concentration in in-flow and out-flow streams were unchanged over this period for both reactors (Figure 5B). This indicates that bacteria were inactive in oxidizing oxalate under tested conditions and showed preference towards using yeast extract or sodium acetate as carbon and energy source.

Example 4. Ability of established biofilm to oxidise organic contaminants in Alkaline liquors

[0099] Spike tests were carried out at a steady current production to study the capability of the anodic biofilm to oxidise simple organic compounds such as sodium formate, sodium succinate and sodium malonate. The reactors were operated in batch mode and steady low background-current production to observe a clear response in current production after substrate injection. On day 86, sodium formate (2.5 mmoles) was injected to the both reactors to observe a sudden increase in current production and after current had decreased back to steady level another sodium formate (5 mmoles) was added to the system to confirm the observation. The same procedure was carried out for the other spike tests as well. On day 233, sodium succinate (2.5 mmoles) was added to the reactor R1 followed by sodium malonate (2.5 mmoles) addition. To confirm the effective utilization of above mentioned organics, the response of current production and columbic efficiency were tested.

[0100] The ability of the established biofilm in the reactors to also degrade acetate was tested.. On day 35, 2.5 mmoles of sodium acetate was added to both reactors to check the response in current production. A clear increase on current production was observed immediately after the addition of sodium acetate (Figure 6). Although, the biofilm was not previously exposed to acetate, the biofilm could immediately oxidise acetate using the graphite as an electron acceptor. In order to study the acetate oxidizing capability of the biofilm several sodium acetate spike tests were conducted during the first 54 days.

[0101] A separate study was conducted to determine how much the produced current was derived from the amount of coulombs added as acetate only (i.e.

coulombic efficiency, CE). Reactors were set up at batch mode and 2.5 mmoles of sodium acetate was injected to each reactor. As shown in Figure 6, current production immediately increased. The CE in R1 was approximately 77%, which is similar to most other BES studies that used acetate as the sole energy donor.

[0102] A spike test was carried out to find out the response of anodic biofilm to sodium formate. Initially, 2.5mmoles of sodium formate was injected to both reactors to check if the anodic biofilm could oxidise formate for current generation. Similar to the sodium acetate addition, sodium formate addition triggered the current production (Figure 6). In this case, both reactor R1 and R2 reached CE of more than 95%. The reproducibility of the result was confirmed by injecting sodium formate (5 mmoles) again after the current reached the background level. This study proved that the established biofilm could anodically oxidise the sodium formate at high pH and high salinity conditions.

[0103] Reactor R1 was further used to test the ability of the established biofilm to oxidise sodium succinate and sodium malonate. Figure 6 C shows the results of sodium succinate and sodium malonate spike tests for reactor R1. Sodium succinate gave a positive outcome in current production with CE of 23%. However, sodium malonate addition did not increase the current production. The anodic biofilm in reactor R1 showed difficulty to anodically oxidise malonate.

[0104] Based on above results from day 54 onwards, 5 mM of sodium acetate was added as a supplementary electron donor for both reactors. The reactor R1 and R2 produced an averaged current of 7 mA with COD removal of 60% and 6 mA with COD removal of 40% respectively at anolyte pH 9 and 2 days HRT (loading rate 0.34 kg COD/m ³ .d) with sodium oxalate and sodium acetate as electron donors.

Example 5. Effect of anode potential

[0105] Figures 7A and 7B show the variation of current production over time at different anode potentials for reactor R1. A sudden increase on current production from ~4 mA to -100 mA was observed at anode potential of -400 mV vs Ag/AgCI. Then further increase in current up to approximately 125 mA at -300 mV vs Ag/AgCI and fairly stable current of 120 mA at poised anode potentials from -200 to +300 mV vs Ag/AgCI were recorded.

[0106] Importantly, Figure 7C shows nearly 100% acetate removal in active phase of anodic biofilm at anode potentials from -300 to +300mV vs Ag/AgCI. The average oxalate removal during this period was low as 4% and COD removal percentage was approximately 40% at 3 h HRT with loading rate of 5.4 kg COD/m ³ .d.

Example 6. Alkalinity in catholyte and caustic recovery

[0107] Figure 8B shows the increase of catholyte Na ⁺ and NH ₄ ⁺ ion concentration with time in reactor R1. There was a continuous increase in both ion concentrations in the catholyte due to migration through the cation exchange membrane other than protons. Figure 8E shows further evidence of cation migration in the linear increase in catholyte electrical conductivity during the experiment. There was negligible difference in anion concentrations in catholyte and no pH control, so this catholyte conductivity increase was caused by mainly Na ⁺ and NH ₄ ⁺ ion migration.

[0108] In Figure 8E, the anolyte pH was maintained at pH 9.0 throughout the experimental run and initial catholyte (NaCI solution 25 g/L) pH was 6.8 and with the current production the catholyte pH increased gradually to 13.7 within 12 days. The catholyte total alkalinity reached maximum value of 580 mmoles OH7L at the end of 12 days of operation. In fact, the Na ⁺ concentration was very high in catholyte (Figure 8B) compared to other cations, this alkaline solution was considered as caustic soda (NaOH). These results confirmed the feasibility of producing caustic solution from synthetic alkaline liquor.

[0109] Energy input to produce 0.58 M caustic solution within 12 days was 35.7 Wh. Example 7: Effect of anolyte pH on current production

[01 10] Figures 9A and 9B show the variation of current production in reactor R1 over time at different anolyte pH, at an anode potential of -300 mV vs Ag/AgCI and 2.55 kg COD/m ³ .d loading rate with sodium oxalate and sodium acetate as electron donors. The current production at different anolyte pH in the anodic chamber confirmed the alkaphilic nature of the biofilm in the anodic chamber of the bio-electrochemical system.

Example 8: Oxalate removal rate

[01 11] Figures 10 and 1 1 show the oxalate removal rate over time, and the oxalate concentration at different controlled pH values, respectively, in an bioreactor coupled with a dual chamber electrolytic cell as described herein.

[01 12] With respect to Figure 10, the bioreactor was operated at a batch cycle length of from 7 days to 2 hours with a sodium oxalate loading from 100 mg/L to 1 g/L. The pH in the bioreactor was 9.5-10.0 during operation. There was good oxalate removal efficacy in the bioreactor, suggesting that coupling the bioreactor with the dual chamber electrolytic cell may further reduce the amount of organic contaminants in the alkaline liquid. Figure 10 also shows that there was successful establishment of an aerobic oxalate degrading microbial biofilm on the conductive graphite granules which, in turn, may be used to readily start up the anode of the anodic chamber in the bio- electrochemical system.

[01 13] With respect to Figure 11 , the bioreactor was operated at a 4 h batch cycle at controlled pH levels of pH 8, 9, 10 and 1 1. Sodium oxalate loading was 2g/L and dissolved oxygen (DO) input was 8 mg/L during each experimental run. Under these operating conditions, the oxalate concentration steadily decreased at these alkaline pH conditions, thereby demonstrating the suitability of the bioreactor to accept a treated alkaline liquid from the anodic chamber to further remove organic contaminants therefrom.

Example 9: Bio-electrochemical system coupled with bioreactor

[01 14] A summary of process parameters for operation of the bio-electrochemical system coupled with a bioreactor configured for removing organic contaminants and caustic soda recovery from an alkaline stream is provided in Table 1.

[01 15] Figure 12 shows the current production profile during start-up of the bio- electrochemical system with an anode comprising graphite granules coated with the aerobic oxalate degrading microbial biofilm established from the bioreactor.

[01 16] The energy consumption of this process is compared with similar studies for caustic soda recovery from alkaline solutions with organic contaminants in Table 2.

Table 1

Table 2

Reactor type Feed type Caustic CE of Energy Reference strength caustic Input

(wt%) production (kWh/kg

(%) caustic)

Lamellar BES Sodium 3.4 76 1.06 (Rabaey et acetate al. 2010) solution

Two chamber Sewage 0.61 51 5.25 (Pikaar et electrochemical al. 2011) cell

Two chamber Synthetic 0.63 81 0.64 CSIRO BES alkaline study

liquor