Membrane cleaning process

Abstract

A method of removing biofouling from a membrane is disclosed. The method comprises contacting the membrane with a solution comprising free ammonia. The solution may be obtained by anaerobic digestion of sludge produced in wastewater treatment.

Description

Membrane cleaning processField of the disclosure

[0001] The present disclosure broadly relates to a process for cleaning biofouling from membranes.Background of the disclosure

[0002] Any discussion of the prior art throughout this specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0003] The treatment and recycling of water, such as wastewater, often involves membrane filtration. During membrane filtration, contaminants in the feedwater such as microorganisms, colloidal particles, and / or solute macromolecules can be deposited or adsorbed onto the membrane pores and surface, resulting in the pores of the membrane becoming blocked, a process known as membrane fouling. This increases the transmembrane pressure and reduces the permeate flux, recovery rate, and the produced water quality of membrane processes. Membrane fouling requires frequent cleaning or replacement of membranes, thereby increasing the operating costs of a wastewater treatment plant. Based on the foulants, membrane fouling can be divided into biofouling (caused by microorganisms), organic fouling (caused by oils, proteins, polyelectrolytes, and humic substances), inorganic scaling (caused by mineral precipitates), and colloidal fouling (caused by clays and flocs).

[0004] Biofouling is a type of membrane fouling that is caused by the formation of biofilm on the membrane surface due to the attachment, growth, and multiplication of microorganisms found in the feedwater. There are two key components of the biofilm, the microorganisms themselves and extracellular polymeric substances (EPS). EPS are excreted by microorganisms during their metabolic processes, and contain mainly proteins and polysaccharides. The formation of biofilms leads to biofouling and eventually the biodegradation of membrane polymers and other components, shortening their lifespan. Biofouling often occurs in wastewater recycling plants and seawater desalination plants. Costs attributed to biofouling are estimated to be at least around 20% of operating expenditure for a typical wastewater recycling plant, and can be as high as 30%. As physical methods are difficult to remove biofouling from membranes, periodic chemical cleaning has been required to remove biofouling.

[0005] Conventional chemical cleaning of biofouling uses specific chemical solutions to remove the biofouling (i.e. microorganisms and EPS) from the membrane using forward flushing or backward flushing. To date, common chemical cleaning solutions for biofouling have included sodium hydroxide (NaOH), ethylenediamine tetraacetic acid (EDTA), and formaldehyde. Such conventional chemical cleaning agents must be sourced externally to the plant where the membranes are used, can be costly, and require appropriate treatment before discharge to prevent environmental contamination and toxicity.Summary of the disclosure

[0006] In a first aspect of the disclosure, there is provided a method of removing biofouling from a membrane, the method comprising contacting the membrane with a solution comprising free ammonia.

[0007] The following options may be used in conjunction with the first aspect of the disclosure, either individually or in any combination.

[0008] The method of the first aspect of the disclosure may comprise (i) preparing a solution comprising free ammonia by anaerobic digestion of sludge produced in wastewater treatment, and (ii) contacting the membrane with the solution comprising free ammonia.

[0009] The membrane may be selected from the group consisting of a reverse osmosis membrane, a nanofiltration membrane, an ultrafiltration membrane, and a microfiltration membrane.

[0010] The membrane may be comprised of a polymer. The membrane may be comprised of one or more materials selected from the group consisting of cellulose acetate, polysulfone coated with polyamides, polyethersulfone (PES), polytetrafluoroethylene, polyamide, cellulose nitrate, and polycarbonate. The membrane may be comprised of a first layer comprising polyester, a second layer comprising polysulfone, and a third layer comprising polyamide. The membrane may be a symmetric membrane or an asymmetric membrane.

[0011] The biofouling may comprise microorganisms and / or extracellular polymeric substances.

[0012] The solution comprising free ammonia may be an aqueous solution. The solution comprising free ammonia may comprise at least about 200 mg NH3-N / L. The solution comprising free ammonia may comprise between about 200 mg NH3-N / L and about 15,000 mg NH3-N / L. The solution comprising free ammonia may have a pH of between about 8 and about 14.

[0013] The contacting may comprise soaking the membrane in the solution comprising free ammonia. The contacting may comprise passing the solution comprising free ammonia through the membrane. The contacting may comprise passing the solution comprising free ammonia tangentially across a surface of the membrane. It may comprise any two or all of these.

[0014] The contacting may be carried out for a duration of at least about 5 min.

[0015] The method of the first aspect of the disclosure may further comprise rinsing the membrane with water, prior to contacting the membrane with a solution comprising free ammonia. The method may further comprise rinsing the membrane with water, after contacting the membrane with a solution comprising free ammonia.

[0016] Following the method of the first aspect of the disclosure, the amount of biofouling present on the membrane may be reduced. The number of viable microorganism cells on the membrane may be reduced by at least about 10%. The amount of extracellular polymeric substances (EPS) on the membrane may be reduced by at least about 10%. After contacting the membrane with a solution comprising free ammonia, the water permeability of the membrane may be increased by at least about 1%.

[0017] In a second aspect of the disclosure, there is provided a method of wastewater treatment, the method comprising:(i) subjecting an influent comprising wastewater to biological treatment to provide a treated influent;(ii) clarifying the treated influent to provide a clarified influent and a sludge;(iii) passing the clarified influent through a membrane to provide an effluent comprising treated wastewater;(iv) subjecting the sludge to anaerobic digestion to provide a solution comprising free ammonia; and(v) contacting the membrane with the solution comprising free ammonia obtained in step (iv) to remove biofouling from the membrane

[0018] The following options may be used in conjunction with the second aspect of the disclosure, either individually or in any combination.

[0019] In the method of the second aspect of the disclosure, contacting the membrane with the solution comprising free ammonia obtained in step (iv) to remove biofouling from the membrane may result in a residual solution comprising free ammonia, and step (i) may comprise subjecting an influent and the residual solution comprising free ammonia to biological treatment to provide a treated influent.

[0020] The membrane may be selected from the group consisting of a reverse osmosis membrane, a nanofiltration membrane, an ultrafiltration membrane, and a microfiltration membrane.

[0021] The membrane may be comprised of a polymer. The membrane may be comprised of one or more materials selected from the group consisting of cellulose acetate, polysulfone coated with polyamides, polyethersulfone (PES), polytetrafluoroethylene, polyamide, cellulose nitrate, and polycarbonate. The membrane may be comprised of first layer comprising polyester, second layer comprising polysulfone, and a third layer comprising polyamide.

[0022] The biofouling may comprise microorganisms and / or extracellular polymeric substances.

[0023] The solution comprising free ammonia may be an aqueous solution. The solution comprising free ammonia may comprise at least about 200 mg NH3-N / L. The solution comprising free ammonia may comprise between about 200 mg NH3-N / L and about 15,000 mg NH3-N / L. The solution comprising free ammonia may have a pH of between about 8 and about 14.

[0024] The contacting may comprise soaking the membrane in the solution comprising free ammonia. The contacting may comprise passing the solution comprising free ammonia through the membrane. The contacting may comprise passing the solution comprising free ammonia tangentially across a surface of the membrane. It may comprise any two or all of these.

[0025] The contacting may be carried out for a duration of at least about 5 min.

[0026] The method of the second aspect of the disclosure may further comprise rinsing the membrane with water, prior to step (v) contacting the membrane with the solution comprising free ammonia obtained in step (iv). The method may further comprise rinsing the membrane with water after to step (v).

[0027] Following the method of the second aspect of the disclosure, the amount of biofouling present on the membrane may be reduced. The number of viable microorganism cells on themembrane may be reduced by at least 10%. The amount of extracellular polymeric substances on the membrane may be reduced by at least 10%. After contacting the membrane with the solution comprising free ammonia, water permeability of the membrane may be increased. The water permeability of the membrane is increased by at least about 1%.

[0028] In a third aspect of the disclosure, there is provided a wastewater treatment plant (1 ) comprising: a biological treatment unit (2), which is adapted to receive an influent (3) comprising wastewater and expel a treated influent (4); a clarifier (5) in fluid communication with the biological treatment unit (2), which is adapted to receive the treated influent (4) and separately expel a clarified influent (6) and a sludge (7); a membrane unit (8) in fluid communication with the clarifier (5), which is adapted to receive the clarified influent (6), filter the clarified influent (6), and expel an effluent (9) comprising treated wastewater; an anaerobic digestion unit (10), which is adapted to receive the sludge (7) and generate a solution comprising free ammonia; a free ammonia recycling unit (12) in fluid communication with the anaerobic digestion unit (10), which is adapted to receive the solution comprising free ammonia; wherein the free ammonia recycling unit (12) is in fluid communication with the membrane unit (8), so as to deliver free ammonia to the membrane unit to remove biofouling from the membrane unit

[0029] The following options may be used in conjunction with the third aspect of the disclosure, either individually or in any combination.

[0030] The membrane unit (8) may be in fluid communication with the biological treatment unit (2).

[0031] The wastewater treatment plant may comprise a plurality of membrane units (8), wherein the plurality of membrane units (8) are joined in series and in fluid communication with one another and with the clarifier (5).

[0032] The wastewater treatment plant may further comprise an additional clarifier (1 1 ), in fluid communication with the clarifier (5) and with the anaerobic digestion unit (10), which is adapted to receive and thicken the sludge (7).

[0033] The biological treatment unit (2) may comprise an aeration tank.

[0034] The clarifier (5) may comprise a settling tank.

[0035] The membrane unit (8) may comprise a membrane as described in respect of the first aspect of the disclosure.Definitions

[0036] The following are some definitions that may be helpful in understanding the description of the present disclosure. These are intended as general definitions and should in no way limit the scope of the present disclosure to those terms alone, but are put forth for a better understanding of the following description.

[0037] Throughout this specification, unless the context requires otherwise, 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.

[0038] The terms "a" and "an" are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.

[0039] In the context of this specification the term "about" is understood to refer to ±10% of the recited value.

[0040] The term "may" encompasses both positive and negative statements, and may therefore be construed as "may or may not", unless the context implies otherwise. Thus for example, the statement "A may be B" encompasses both of the statements "A is B" and "A is not B".

[0041] Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of 1 .0 to 5.0 is intended to include all sub-ranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 5.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 5.0, such as 2.1 to 4.5. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited herein is intended to include all higher numerical limitations subsumed therein.

[0042] Any description of prior art documents herein, or statements herein derived from or based on those documents, is not an admission that the documents or derived statements are part of the common general knowledge of the relevant art.

[0043] For the purposes of description, all documents referred to herein are hereby incorporated by reference in their entirety unless otherwise statedBrief Description of the Drawings

[0044] Figure 1. A conventional wastewater treatment plant.

[0045] Figure 2. A wastewater treatment plant according to an embodiment of the present disclosure.Detailed Description

[0046] The inventors have surprisingly found that a solution comprising free ammonia can effectively remove biofouling from membranes, such as reverse osmosis membranes. Conveniently, as free ammonia is generated in the normal course of operation of a wastewater treatment plant, it can be recycled for use in cleaning biofouling from membrane units forming part of the plant.

[0047] In a first aspect of the disclosure, there is provided a method of removing biofouling from a membrane, the method comprising contacting the membrane with a solution comprising free ammonia.

[0048] In a second aspect of the disclosure, there is provided a method of wastewater treatment, the method comprising:(i) subjecting an influent comprising wastewater to biological treatment to provide a treated influent;(ii) clarifying the treated influent to provide a clarified influent and a sludge;(iii) passing the clarified influent through a membrane to provide an effluent comprising treated wastewater; and(iv) subjecting the sludge to anaerobic digestion to provide a solution comprising free ammonia; and(v) contacting the membrane with the solution comprising free ammonia obtained in step (iv) to remove biofouling from the membrane.

[0049] In the first and second aspects of the disclosure, the biofouling may comprise microorganisms and / or extracellular polymeric substances (EPS). The microorganisms may comprise living and / or dead bacteria, metazoans, protozoa and / or fungi. The microorganisms may comprise species selected from the group consisting of Achromobacter, Acinetobacter, Alcaligenes, Chironomus, Crustacea, Desulfovibrio, Flavobacterium, Geotrichum, Gordon ia, Micrococcus, Microtrix, Nitrobacter, Nitrosomonas, Pseudomonas, Rotifera, Sphaerotilus, Tubifex, Vorticella, Zoogloea, and ramigera, including mixtures of any two or more of these. EPS are polymeric substances which are excreted by microorganisms, and may comprise polysaccharides, lipids, nucleic acids, proteins, and / or lipopolysaccharides as well as mixtures of any two or more of these.

[0050] In the first and second aspects of the disclosure, the membrane may be a reverse osmosis membrane, a nanofiltration membrane, an ultrafiltration membrane, or a microfiltration membrane. The membrane may be a reverse osmosis membrane. The membrane may be comprised of a material selected from cellulose acetate, polysulfone coated with aromatic polyamides, polyethersulfone (PES), polytetrafluoroethylene, polyamide, cellulose nitrate, and polycarbonate. The membrane may be a composite polyamide membrane. The membrane may comprise a layer of polyethersulfone or polysulfone coated with a layer of polyamide, optionally aromatic polyamide. The membrane may be comprised of a first layer comprising polyester, a second layer comprising polysulfone, and a third layer comprising polyamide. In the present specification, a microfiltration membrane has a mean pore size of about 0.1 to about 10 microns, an ultrafiltration membrane has a mean pore size of about 0.01 to about 0.1 micron and a molecular weight cut-off about 10 to about 500kDa, and a nanofiltration membrane has a mean pore size of about 0.001 to about 0.01 microns and a molecular weight cut-off about 0.2 to about 10kDa. A reverse osmosis membrane typically has a molecular weight cut-off below about 200Da. It will be recognised that the molecular weight cut-offs described above are not absolute and will depend on multiple factors including the shape of the molecule, the hydrodynamic radius of the molecule etc.

[0051] In the first and second aspects of the disclosure, the membrane is contacted with a solution comprising free ammonia. Free ammonia is understood to be NH3(as opposed to NH4+). When ammonia is dissolved in water, it may, depending on the pH, be in the form of NH3and / or NH4+. Free ammonia may also be referred to as ammoniacal nitrogen, which is designated NH3-N. In wastewater treatment plants, free ammonia is a by-product of the anaerobic digestion of sludge, and can be obtained directly onsite. In conventional wastewaterrecycling, the free ammonia from the anaerobic digestion liquor is returned as a waste product to the biological wastewater recycling process (e.g. in the conventional wastewater treatment plant of Figure 1 ). Thus in one embodiment, the free ammonia solution may be obtained from the anaerobic digestion of sludge in wastewater treatment. In another embodiment, the free ammonia solution may be obtained from a synthetic source, such as by the reaction of ammonium chloride with an base, such as sodium hydroxide, in water.

[0052] In the first and second aspects of the disclosure, the membrane may be contacted with a solution comprising free ammonia in a concentration of at least about 200 mg NH3-N / L, or at least about 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 1000, 2500, 5000, 7500, 10,000, 12,500 or at least about 15,000 mg NH3-N / L. The membrane may be contacted with a solution comprising free ammonia in a concentration of between about 200 mg NH3-N / L and about 15,000 mg NH3-N / L, or between about 200-300, 200-400, 200-500, 200-600, 200-700, 200-800, 200-1000, 300-400, 300-500, 300-600, 300-700, 300-800, 300-900, 400-500, 400- 600, 400-700, 400-800, 400-1000, 500-600, 500-700, 500-800, 500-900, 500-1000, 600-700, 600-800, 600-900, 600-1000, 700-800, 700-900, 700-1000, 800-900, 800-1000, 900-1000, 1000-2500, 1000-5000, 1000-7500, 1000-10,000, 1000-12,500, 2500-5000, 2500-7500, 2500-10,000, 2500-12,500, 2500-15,000, 5000-7500, 5000-10,000, 5000-12,500, 5000- 15,000, 7500-10,000, 7500-12,500, 7500-15,000, 10,000-12,500, 10,000-15,000, or between about 12,500 NH3-N / L and about 15,000 NH3-N / L. The membrane may be contacted with a solution comprising free ammonia in a concentration of between about 200 mg NH3-N / L and about 600 mg NH3-N / L. The membrane may be contacted with a solution comprising free ammonia in a concentration of between about 5000 mg NH3-N / L and about 15,000 mg NH3- N / L. The membrane may be contacted with a solution comprising free ammonia in a concentration of about 200 mg NH3-N / L, or about 300, 400, 500, 600, 700, 800, 900, 1000, 2500, 5000, 7500, 10,000, 12,500 or about 15,000 mg NH3-N / L. The membrane may be contacted with a solution comprising free ammonia in a concentration of about 10,000 mg NH3-N / L.

[0053] The solution comprising free ammonia may be an aqueous solution, that is, may comprise ammonia dissolved in water. The solution comprising free ammonia may have a pH of greater than about 7, or greater than about 8, 9, 10, 11 , 12, or greater than about 13. The solution comprising free ammonia may have a pH of between about 8 and about 14, or between about 8-9, 8-10, 9-1 1 , 8-12, 8-13, 9-10, 9-1 1 , 9-12, 9-13, 9-14, 10-1 1 , 10-12, 10-13, 10-14, 11 -12, 11 -13, 1 1 -14, 12-13, 12-14, or between about 13 and about 14. The solution comprising free ammonia may have a pH of about 8, or a pH of about 9, 10, 11 , 12, 13, or about 14. The solution comprising free ammonia may have a pH of about 9. The solutioncomprising free ammonia may have a pH of about 8.9. The solution comprising free ammonia may have a pH of about 11 .

[0054] In the first and second aspects of the disclosure, the membrane is contacted with a solution comprising free ammonia. The contacting may be carried out in any suitable manner. For example, the membrane may be soaked in the solution comprising free ammonia, the free ammonia solution may be passed through the membrane, or the free ammonia may be passed tangentially across a surface of the membrane (referred to as cross-flow). The contacting may be carried out for a suitable duration of time, until the biofouling is sufficiently removed. The contacting may be carried out for a duration of at least about 5 min, or at least about 1 hour, 1 day, or at least about 1 week. The contacting may be carried out for a duration of between about 5 min and about 1 week, or between about 5 min and about 1 hour, or between about 5 min and about 1 day, or between about 1 hour and about 1 day, or between about 1 hour and about 1 week, or between about 1 day and about 1 week. The contacting may be carried out for a duration of at least about 15 hours, or at least about 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, or at least about 30 hours. The contacting may be carried out for between about 15 and about 30 hours, or between about 15-20, 15-25, 20-25, 20-30, or between about 25 and about 30 hours. The contacting may be carried out for between about 20 and about 25 hours. The contacting may be carried out for a duration of about 15 hours, or about 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, or about 30 hours. The contacting may be carried out for a duration of about 22 hours. The contacting may be carried out for a duration of about 24 hours. The membrane may be contacted with the solution comprising free ammonia at ambient temperature or at some other suitable temperature. It may be at a temperature between about 5 and about 30°C, or at a temperature between about 5 and 25, 5 and 20, 5 and 10, 10 and 30, 20 and 30, 15 and 25, 15 and 20 or 20 and 25°C, for example at a temperature of about 5, 10, 15, 20, 25 or 30°C.

[0055] The first and second aspects of the disclosure may further comprise rinsing the membrane with water, optionally deionised water, prior to contacting the membrane with a solution comprising free ammonia. In the second aspect of the invention, the solution comprising free ammonia is obtained in step (iv). In some embodiments, the step of contacting the membrane with a solution comprising free ammonia may be followed by a step of rinsing the membrane with water, optionally deionised water.

[0056] In the first aspect of the disclosure, the solution comprising free ammonia may be produced in the course of wastewater treatment. In some embodiments, the solution comprising free ammonia may be produced by anaerobic digestion of sludge produced in wastewater treatment. That is, the solution comprising free ammonia may be obtained fromthe anaerobic digestion of sludge in wastewater treatment. It may be obtained by the action of anaerobic bacteria on the sludge. The anaerobic bacteria may be, or may comprise, acidogenic bacteria. Thus, in some embodiments, there is provided a method of removing biofouling from a membrane, the method comprising (i) preparing a solution comprising free ammonia by anaerobic digestion of sludge produced in wastewater treatment, and (ii) contacting the membrane with the solution comprising free ammonia. The production of sludge during wastewater treatment and generation of ammonia during anaerobic digestion is described in relation to the second and third aspects of the disclosure. Alternatively, in the first aspect of the disclosure, the free ammonia solution may be obtained from a synthetic source, such as by the reaction of ammonium chloride with an base, such as sodium hydroxide, in water.

[0057] Following the method of the first or second aspects of the disclosure, the amount of biofouling present on the membrane may be reduced. The amount by which the biofouling is reduced may be determined by comparing the number of viable microorganisms on the membrane before and after carrying out the method. The number of viable microorganisms can be determined by conventional, commercially available live / dead staining kits, using a viability dye. The amount of viable microorganisms on the membrane may also be determined by comparing the amount of adenosine triphosphate on the membrane before and after carrying out the method. After carrying out the method of the first or second aspects of the disclosure, the number of viable microorganism cells, or the amount of adenosine triphosphate, on the membrane may be reduced, compared to the number of viable microorganism cells, or the amount of adenosine triphosphate, on the membrane prior to carrying out the method. The number of viable cells, or the amount of adenosine triphosphate, may be reduced by at least about 10%, or by at least about 20, 30, 40, 50 60, 70, 80, 90 or at least about 95%. The number of viable cells may be reduced by at least about 50%. The number of viable cells may be reduced by at least about 70%. The reduction in the number of viable microorganism cells, or in the amount of adenosine triphosphate, may depend on the length of exposure of the membrane to the ammonia solution.

[0058] The amount by which the biofouling is reduced may be determined by comparing the amount of EPS on the membrane before and after carrying out the method of the first or second aspects of the disclosure. The amount by which the biofouling is reduced may be determined by comparing amounts of proteins and polysaccharides on the membrane before and after carrying out the method of the first or second aspects of the disclosure. The amount of proteins and polysaccharides on the membrane may be determined using conventional, commercially available assay kits. After carrying out the method of the first or second aspectsof the disclosure, the amount of EPS on the membrane may be reduced, compared to the amount of EPS on the membrane prior to carrying out the method. The amount of EPS may be reduced by at least about 10%, or by at least about 20, 30, 40, 50, 60, 70, 80, 90 or at least about 95%. The amount of EPS may be reduced by at least about 50%. The amount of EPS may be reduced by at least about 60%. The reduction in the biofouling may depend on the length of exposure of the membrane to the ammonia solution.

[0059] The amount by which the biofouling is reduced may be determined by measuring the water permeability of the membrane before and after carrying out the method of the first or second aspects of the disclosure. Permeability may be determined by measuring the amount of deionised water that passes through the membrane over a set period, at a fixed pressure and water flux. After carrying out the method of the first or second aspects of the disclosure, the water permeability of the membrane may be increased, compared to the water permeability of the membrane prior to carrying out the method. The water permeability of the membrane may be increased by at least about 1 %, or by at least about 2, 5, 10, 15, or at least about 20%. The water permeability of the membrane may be increased by between about 1% and about 20%, or by between about 1 -2, 1 -5, 1 -10, 1 -15, 2-5, 2-10, 2-15, 2-20, 5-10, 5-15, 10-15, 10-20, or by between about 15% and about 20%. The water permeability of the membrane may be increased by between about 1 % and about 15%. The water permeability of the membrane may be increased by between about 10% and about 15%. Permeability may be measured by determining the flux of deionised water at a specified pressure, or by measuring the pressure required to achieve a specified flux of deionised water, each at a specified temperature. The skilled person will recognise that a suitable specified flux or specified pressure will depend on the nature of the membrane.

[0060] The method of the second aspect of the disclosure is a method of wastewater treatment. In step (i), an influent comprising wastewater is subjected to biological treatment to provide a treated influent. The wastewater may be from any source, such as for example, domestic wastewater, industrial wastewater, and / or agricultural wastewater. Biological treatment involves utilising microorganisms to degrade and decompose organic matter present in the influent. For example, the biological treatment may comprise holding the influent in an aeration tank for a period of time while aerobic microorganisms degrade and decompose organic matter present in the influent. During the biological treatment, microorganisms commonly clump together forming flocs.

[0061] Biological treatment is followed by step (ii) of clarifying the treated influent in a clarifier, or settling tank. In this step, the flocs formed in step (ii) settle to the bottom of the clarifier forming a sludge, separating it from the clarified influent.

[0062] In step (iii) the clarified influent may be further treated by membrane filtration to provide effluent comprising treated wastewater which may be suitable to be released from the treatment plant (optionally following further disinfection steps such as chlorine and / or UV treatment), while in step (iv) the sludge is subjected to anaerobic digestion. Anaerobic digestion involves the degradation and decomposition of the sludge by anaerobic microorganisms in low oxygen environments, or in the absence of oxygen. Anaerobic digestion generates ammonia as microorganisms digest organic nitrogen compounds, and thus provides a solution comprising free ammonia.

[0063] The free ammonia solution generated in step (iv) may then be contacted with the membrane in order to remove biofouling in step (v).

[0064] In one embodiment, contacting the membrane with the solution comprising free ammonia obtained in step (iv) to remove biofouling from the membrane results in a residual solution comprising free ammonia. A residual solution comprising free ammonia is a solution comprising free ammonia which has been previously contacted with the membrane to remove biofouling. This solution may be subject to biological treatment together with the influent, such that step (i) comprises subjecting an influent and the residual solution comprising free ammonia to biological treatment to provide a treated influent. Thus the residual solution may be recycled into the process.

[0065] In a third aspect of the disclosure, there is provided a wastewater treatment plant comprising: a biological treatment unit, which is adapted to receive an influent comprising wastewater and expel a treated influent; a clarifier in fluid communication with the biological treatment unit, which is adapted to receive the treated influent and separately expel a clarified influent and a sludge; a membrane unit in fluid communication with the clarifier, which is adapted to receive the clarified influent, filter the clarified influent, and expel an effluent comprising treated wastewater; an anaerobic digestion unit, which is adapted to receive the sludge and generate a solution comprising free ammonia; a free ammonia recycling unit in fluid communication with the anaerobic digestion unit, which is adapted to receive the solution comprising free ammonia;wherein the free ammonia recycling unit is in fluid communication with the membrane unit, so as to deliver free ammonia to the membrane unit to remove biofouling from the membrane unit.

[0066] The wastewater treatment plant (1 ), for example as depicted in Figure 2, may be adapted to carry out the method of the second aspect of the disclosure. The biological treatment unit (2) is adapted to receive an influent (3). The influent (3) may be held for a period of time in the biological treatment unit (2) so as to allow aerobic microorganisms to degrade and decompose organic matter present in the influent (3). In some embodiments the biological treatment unit (2) is, or comprises, an aeration tank. The aeration tank may be any conventional aeration tank suitable for use in wastewater treatment which provides an environment promoting the growth of aerobic bacteria. The aeration tank may be a tank comprising an inlet to receive influent, an outlet to expel treated influent, and a means for producing air bubbles. Aeration may be provided by, for example, a fountain, a floating surface aerator, a paddlewheel aerator, a sprayer, a jet aerator, a coarse or fine bubble aerator or some other type of aerator, or by a combination of these. The influent (3) may comprise wastewater as described in respect of the second aspect of the disclosure. Treated influent(4) is expelled from the biological treatment unit (2) and introduced into the clarifier (5), which is in fluid communication with biological treatment unit (2).

[0067] The clarifier (5) may be, or comprise, a settling tank. The clarifier (5) is adapted to receive the treated influent (4), which separates by settling in the clarifier (5) into clarified influent (6) and sludge (7), and is filtered. The clarified influent (6) is expelled from the clarifier(5) and caused to flow to the membrane unit (8), which is in fluid communication with the clarifier (5). The sludge (7) is expelled from the clarifier (5) and caused to flow to the anaerobic digestion unit (10), which may be in fluid communication with the clarifier (5).

[0068] The membrane unit (8) may comprise a membrane as described in respect of the first and second aspects of the disclosure. The clarified influent (6) is filtered through the membrane unit (8) to provide an effluent (9) which is expelled from the membrane unit (8) and may be suitable for release from the wastewater treatment plant (1 ) (optionally following further disinfection steps such as chlorine and / or UV treatment). In some embodiments, such as that depicted in Figure 2, the wastewater treatment plant may comprise a plurality of membrane units, wherein the plurality of membrane units are joined in series and are in fluid communication with one another and with the clarifier (5).

[0069] In some embodiments, such as that depicted in Figure 2, the waste water treatment plant (1 ) further comprises an additional clarifier (11 ), which is in fluid communication with theclarifier (5) and with the anaerobic digestion unit (10). In this embodiment, the sludge (7) is caused to flow from the clarifier (5) to the additional clarifier (1 1 ), which serves to thicken the sludge (7) prior to anaerobic digestion.

[0070] The anaerobic digestion unit (10) may be any conventional anaerobic digestion unit suitable for use in wastewater treatment which provides an environment promoting the growth of anaerobic bacteria. The anaerobic digestion unit may be a sealed vessel, such as a tank or covered pit, having an inlet for receiving sludge (7), and an outlet for expelling treated sludge. In the anaerobic digestion unit (10) the sludge (7) is subject to degradation and decomposition by anaerobic microorganisms in the absence of oxygen. This process generates ammonia as microorganisms convert organic nitrogen compounds to ammonia. The ammonia may be recovered from the sludge in the anaerobic digestion unit, and ammonia is caused to flow to the free ammonia recycling unit (12), which is in fluid communication with the anaerobic digestion unit (10). Specifically, as the free ammonia solution may be found in the supernatant liquid of the sludge in the anaerobic digestion unit, it can be recovered by centrifugation. Thus in one embodiment, the free ammonia recycling unit (12) comprises a centrifuge.

[0071] The free ammonia recycling unit (12) is also in fluid communication with the membrane unit (8), such that ammonia generated in the anaerobic digestion unit (10) may be utilised to remove biofouling on membranes comprised in the membrane unit (8). In some embodiments, removing biofouling from membranes in the membrane unit (8) results in a residual solution comprising free ammonia. A residual solution comprising free ammonia is a solution comprising free ammonia which has been previously contacted with a membrane. Thus, the membrane unit (8) may also be in fluid communication with the biological treatment unit (2), such that the residual solution comprising free ammonia may be caused to flow back in to the biological treatment unit (2) as part of the influent (3).Examples

[0072] The present disclosure is further described below by reference to the following nonlimiting examples.Preparation of free ammonia solution

[0073] Free ammonia (FA) solutions were prepared using sodium nitrite (>99%, Chem- supply), NaOH (>98%, Sigma-Aldrich), and deionised water. The FA concentration was calculated based on the total nitrite concentration, the pH, and the temperature as follows: FA concentration (as mg NH3 / L) = S(NH4+-N+NH3-N) x 10pH / (Kb / Kw-i-10pH) (according to Anthonisen, A.C., Loehr, R.C., Prakasam, T.B.S. and Srinath, E.G., 1976. Inhibition ofnitrification by ammonia and nitrous acid. Journal (Water Pollution Control Federation), pp. 835-852). S(NH4+-N+NH3-N) corresponds to the total ammoniacal nitrogen concentration. Kb / Kw is equal to e6’344 / (273+T).Membrane cleaning tests

[0074] Two sets of cleaning were conducted using FA solutions, namely bench-scale soak cleaning tests, and cross-flow cleaning tests, on thin-film composite polyamide reverse osmosis (RO) membranes collected from a municipal wastewater recycling plant in Sydney, Australia.Bench scale soak test

[0075] A set of bench-scale soak cleaning tests was carried out as a pre-experiment to determine and select the concentration of FA for reducing biofouling on the RO membrane. The FA concentration ranged from 85-560 NH3-N / L, pH 8.9. Small pieces of RO membranes were completely submerged in a beaker containing a specific concentration of FA solution. The beaker was then placed on an orbital shaker and agitated at 120 rpm for 24 hours. The proportions of viable cells on the membrane pieces before and after 24 h cleaning were tested by cutting small pieces of the membrane and staining them with fluorescent viability dye. Live and dead cells were visualised and counted using fluorescence microscopy.Cross-flow cleaning test

[0076] Based on the results of the bench-scale soak cleaning, FA solutions of 310-560 mg NH3-N / L, pH 8.9 were further tested through cross-flow cleaning to remove biofouling on RO membranes. Membrane coupons (150 cm2of membrane active surface) along with respective feed spacers were obtained from the fouled RO membranes.

[0077] Cross-flow cleaning tests were carried out to simulate a real membrane cleaning process in the following steps: i. Rinse the membrane with deionised water for 2 hours to remove biomass at the external layer of biofilm. ii. Recirculation of cleaning solution (i.e. FA solution) for 22 hours through the membrane. iii. Rinse the membrane with deionised water for 15 min to remove any residual FA solution.

[0078] The proportion of viable cells was measured, as well as adenosine triphosphate levels (ATP), proteins, polysaccharides, and water permeability, for the fouled membranes beforeand after the cross-flow cleaning. ATP level is an indicator for living microorganisms on the membrane. Protein and polysaccharides were used to reflect the EPS levels on the membrane. Water permeability is an indicator of membrane performance.ResultsBench scale soak test

[0079] After soak cleaning, the proportion of live cells on the membrane was reduced by 75% when the concentration of FA exceeded 310 NH3-N / L. As NaOH was used to adjust the pH of FA solutions, the effect of NaOH solutions under the same pH (8.9) was also tested. The NaOH solution only removed 20% of the live cells on the membrane. This implies that the FA solutions, rather than the NaOH added for adjusting the pH, is the effective agent for the reduction in live cell number. Thus, the FA solutions with a concentration range of 310-560 mg NH3-N / L, pH 8.9 were further selected for the cross-flow cleaning tests.Cross-flow cleaning test

[0080] After cleaning with FA solutions (310-560 mg NH3-N / L, pH 8.9), the percentage of viable cells on the membrane was reduced by over 90%. The ATP, proteins, and polysaccharides in the biofouling layers were reduced by 88-98%, 64-87%, and 68-91 % respectively, after FA cleaning. The removal of ATP, proteins, and polysaccharides was positively correlated with the FA concentration in the cleaning solution. The water permeability of the membrane also increased by 10-13% after cleaning. This indicates that FA solutions effectively killed or inhibited the microorganisms in the fouling layer (up to 98% reduction), facilitated EPS destruction (up to 90% reduction), and partially restored the membrane performance.

[0081] Those skilled in the art will appreciate that the disclosure described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the disclosure includes all such variations and modifications. The disclosure also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of an two or more of said steps, features, compositions and compounds.

Claims

AMENDED CLAIMS received by the International Bureau on 10 October 2024 (10.10.2024)

1. A method of removing biofouling from a membrane, the method comprising:(i) preparing a solution comprising free ammonia by anaerobic digestion of sludge produced in wastewater treatment; and(ii) contacting the membrane with the solution comprising free ammonia.

2. The method of claim 1 , wherein the membrane is selected from the group consisting of a reverse osmosis membrane, a nanofiltration membrane, an ultrafiltration membrane and a microfiltration membrane.

3. The method of claim 1 or claim 2, wherein the membrane is comprised of a polymer.

4. The method of any one of claims 1 to 3, wherein the membrane is comprised of one or more materials selected from the group consisting of cellulose acetate, polysulfone coated with polyamides, polyethersulfone (PES), polytetrafluoroethylene, polyamide, cellulose nitrate and polycarbonate.

5. The method of any one of claims 1 to 4, wherein the membrane is comprised of a first layer comprising polyester, a second layer comprising polysulfone and a third layer comprising polyamide.

6. The method of any one of claims 1 to 5, wherein the biofouling comprises microorganisms and / or extracellular polymeric substances (EPS).

7. The method of any one of claims 1 to 6, wherein the solution comprising free ammonia is an aqueous solution.

8. The method of any one of claims 1 to 7, wherein the solution comprising free ammonia comprises at least about 200 mg NH3-N / L.

9. The method of any one of claims 1 to 8, wherein the solution comprising free ammonia comprises between about 200 mg NH3-N / L and about 15,000 mg NH3-N / L.

10. The method of any one of claims 1 to 9, wherein the solution comprising free ammonia has a pH of between about 8 and about 14.AMENDED SHEET (ARTICLE 19)

11. The method of any one of claims 1 to 10, wherein the contacting comprises soaking the membrane in the solution comprising free ammonia.

12. The method of any one of claims 1 to 10, wherein the contacting comprises passing the solution comprising free ammonia through the membrane.

13. The method of any one of claims 1 to 10, wherein the contacting comprises passing the solution comprising free ammonia tangentially across a surface of the membrane.

14. The method of any one of claims 1 to 13, wherein the contacting is carried out for a duration of at least about 5 min.

15. The method of any one of claims 1 to 14, wherein the method further comprises rinsing the membrane with water, prior to contacting the membrane with a solution comprising free ammonia.

16. The method of claim 15, wherein the method further comprises rinsing the membrane with water, after contacting the membrane with a solution comprising free ammonia.

17. The method of any one of claims 1 to 16, wherein the amount of biofouling present on the membrane is reduced.

18. The method of any one of claims 1 to 17, wherein the number of viable microorganism cells on the membrane is reduced by at least about 10%.

19. The method of any one of claims 1 to 18, wherein the amount of EPS on the membrane is reduced by at least about 10%.

20. The method of any one of claims 1 to 19, wherein, after contacting the membrane with a solution comprising free ammonia, the water permeability of the membrane is increased by at least about 1 %.

21. A method of wastewater treatment, the method comprising:(i) subjecting an influent comprising wastewater to biological treatment to provide a treated influent;(ii) clarifying the treated influent to provide a clarified influent and a sludge;(iii) passing the clarified influent through a membrane to provide an effluent comprising treated wastewater;(iv) subjecting the sludge to anaerobic digestion to provide a solution comprising free ammonia; andAMENDED SHEET (ARTICLE 19)(v) contacting the membrane with the solution comprising free ammonia obtained in step (iv) to remove biofouling from the membrane.

22. The method of claim 21 , wherein contacting the membrane with the solution comprising free ammonia obtained in step (iv) to remove biofouling from the membrane results in a residual solution comprising free ammonia, and step (i) comprises subjecting an influent and the residual solution comprising free ammonia to biological treatment to provide a treated influent.

23. The method of claim 21 or claim 22, wherein the membrane is selected from the group consisting of a reverse osmosis membrane, a nanofiltration membrane, an ultrafiltration membrane and a microfiltration membrane.

24. The method of any one of claims 21 to 23, wherein the membrane is comprised of a polymer.

25. The method of any one of claims 21 to 24, wherein the membrane is comprised of one or more materials selected from the group consisting of cellulose acetate, polysulfone coated with polyamides, polyethersulfone (PES), polytetrafluoroethylene, polyamide, cellulose nitrate and polycarbonate.

26. The method of any one of claims 21 to 25, wherein the membrane is comprised of first layer comprising polyester, second layer comprising polysulfone and a third layer comprising polyamide.

27. The method of any one of claims 21 to 26, wherein the biofouling comprises microorganisms and / or EPS.

28. The method of any one of claims 21 to 27, wherein the solution comprising free ammonia is an aqueous solution.

29. The method of any one of claims 21 to 28, wherein the solution comprising free ammonia comprises at least about 200 mg NH3-N / L.

30. The method of any one of claims 21 to 29, wherein the solution comprising free ammonia comprises between about 200 mg NH3-N / L and about 15,000 mg NH3-N / L.

31. The method of any one of claims 21 to 30, wherein the solution comprising free ammonia has a pH of between about 8 and about 14.AMENDED SHEET (ARTICLE 19)

32. The method of any one of claims 21 to 31 , wherein the contacting comprises soaking the membrane in the solution comprising free ammonia.

33. The method of any one of claims 21 to 31 , wherein the contacting comprises passing the solution comprising free ammonia through the membrane.

34. The method of any one of claims 21 to 31 , wherein the contacting comprises passing the solution comprising free ammonia tangentially across a surface of the membrane.

35. The method of any one of claims 21 to 34, wherein the contacting is carried out for a duration of at least about 5 min.

36. The method of any one of claims 21 to 35, wherein the method further comprises rinsing the membrane with water, prior to step (v) contacting the membrane with the solution comprising free ammonia obtained in step (iv).

37. The method of claim 36, wherein the method further comprises rinsing the membrane with water after step (v).

38. The method of any one of claims 21 to 37, wherein the amount of biofouling present on the membrane is reduced.

39. The method of any one of claims 21 to 38, wherein the number of viable microorganism cells on the membrane is reduced by at least 10%.

40. The method of any one of claims 21 to 39, wherein the amount of EPS on the membrane is reduced by at least 10%.

41. The method of any one of claims 21 to 40, wherein, after contacting the membrane with the solution comprising free ammonia, water permeability of the membrane is increased.

42. The method of claim 41 , wherein water permeability of the membrane is increased by at least about 1%.

43. A wastewater treatment plant (1 ) comprising: a biological treatment unit (2), which is adapted to receive an influent (3) comprising wastewater and expel a treated influent (4); a clarifier (5) in fluid communication with the biological treatment unit (2), which is adapted to receive the treated influent (4) and separately expel a clarified influent (6) and a sludge (7);AMENDED SHEET (ARTICLE 19)a membrane unit (8) in fluid communication with the clarifier (5), which is adapted to receive the clarified influent (6), filter the clarified influent (6), and expel an effluent (9) comprising treated wastewater; an anaerobic digestion unit (10), which is adapted to receive the sludge (7) and generate a solution comprising free ammonia; a free ammonia recycling unit (12) in fluid communication with the anaerobic digestion unit (10), which is adapted to receive the solution comprising free ammonia; wherein the free ammonia recycling unit (12) is in fluid communication with the membrane unit (8), so as to deliver free ammonia to the membrane unit to remove biofouling from the membrane unit.

44. The wastewater treatment plant of claim 43, wherein the membrane unit (8) is in fluid communication with the biological treatment unit (2).

45. The wastewater treatment plant of claim 43 or claim 44, wherein the wastewater treatment plant comprises a plurality of membrane units (8), wherein the plurality of membrane units (8) are joined in series and in fluid communication with one another and with the clarifier (5).

46. The wastewater treatment plant of any one of claims 43 to 45, further comprising an additional clarifier (11), in fluid communication with the clarifier (5) and with the anaerobic digestion unit (10), which is adapted to receive and thicken the sludge (7).

47. The wastewater treatment plant of any one of claims 43 to 46, wherein the biological treatment unit (2) comprises an aeration tank.

48. The wastewater treatment plant of any one of claims 43 to 47, wherein the clarifier (5) comprises a settling tank.

49. The wastewater treatment plant of any one of claims 43 to 48, wherein the membrane unit (8) comprises a membrane according to any one of claims 2 to 5.AMENDED SHEET (ARTICLE 19)

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