Background

[0001] The present invention relates to a process for separating a solvent, for example, water from a feed solution.

[0002] In water purification methods, water is separated from an impure solution, for example, a saline solution. Various methods of water purification are known. An example of such a method is reverse osmosis. In reverse osmosis, water is forced from a region of high solute concentration through a semipermeable membrane to a region of low solute concentration by applying a hydraulic pressure in excess of the osmotic pressure of the high solute concentration solution. Reverse osmosis is commonly used, for example, to obtain drinking water from seawater. Reverse osmosis is also used to separate water from, for example, industrial waste streams. By using reverse osmosis to treat industrial waste streams, it is possible to generate relatively clean water from industrial waste, while reducing the volume of undesirable waste requiring disposal or further treatment.

[0003] Reverse osmosis requires relatively high pressures to be exerted on the high solute concentration side of the membrane. For instance, to desalinate seawater by conventional reverse osmosis techniques, pressures as high as 82 barg are commonly used to increase the recovery of product water. This places a significant energy burden on desalination methods that rely on conventional reverse osmosis. Moreover, streams having higher solute concentrations than seawater may require even higher hydraulic pressures to be applied. Many commercially available reverse osmosis membranes are unsuitable for withstanding hydraulic pressures of greater than 82 barg. Accordingly, this can impose a limitation on the concentration of feed solutions that can be treated using commercially available reverse osmosis membrane, which effectively limits the maximum concentration of the concentrated feed stream to an osmotic pressure equivalent to the maximum hydraulic pressure rating of the reverse osmosis membrane and pressure vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1 is a schematic illustration of a system for carrying out a process according to a first example of the present disclosure; Figure 2 is a schematic illustration of a system for carrying out a process according to a second example of the present disclosure;

Figure 3 is a schematic illustration of a system for carrying out a process according to a third example of the present disclosure;

Figure 4 is a schematic illustration of a system for carrying out a process according to a fourth example of the present disclosure;

Figure 5 is a schematic illustration of a system for carrying out a process according to a fifth example of the present disclosure;

Figure 6 is a schematic illustration of a system for carrying out a process according to a sixth example of the present disclosure;

Figure 7 is a schematic illustration of a system for carrying out a process according to a seventh example of the present disclosure;

Figure 8 is a schematic illustration of a system for carrying out a process according to an eighth example of the present disclosure;

Figure 9 is a schematic illustration of a system for carrying out a process according to a ninth example of the present disclosure;

Figure 10 is a schematic illustration of a system for carrying out a process according to a tenth example of the present disclosure; and

Figure 11 is a schematic illustration of a system for carrying out a process according to an eleventh example of the present disclosure

DETAILED DESCRIPTION

[0005] Throughout the description and claims of this specification, the words“comprise” and“contain” and variations of them mean“including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0006] Features, integers, characteristics, and compounds in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0007] According to the present invention, there is provided a process for separating solvent from a feed solution, said process comprising:

[0008] contacting the feed solution with one side of a first semi-permeable membrane;

[0009] applying hydraulic pressure to the feed solution, such that solvent from the feed solution flows through the first semi-permeable membrane by reverse osmosis to provide a permeate solution on the permeate-side of the first semi-permeable membrane and a residual solution on the feed-side of the first semi-permeable membrane; and

[0010] feeding a portion of the feed solution or a portion of the residual solution to the permeate-side of the first semi-permeable membrane.

[0011] Preferably, the solvent is water. The feed solution may be a salt solution, for example, a saline solution. The feed solution may be an impure water stream, for example, saline ground water or surface water, brine and seawater. Other examples include waste water streams, lake water, river water and pond water. Examples of waste water streams include industrial or agricultural waste water streams. Alternatively, the feed solution may be a salt solution that is prepared by dissolving an osmotic agent in water.

[0012] The present inventors have found that, by feeding a portion of the feed solution or a portion of the residual solution to the permeate-side of the first semi-permeable membrane, the osmotic pressure difference across the first semi-permeable membrane may be reduced. As a result, the hydraulic pressure required to induce solvent flow from the feed solution by reverse osmosis may be reduced. The reverse osmosis step, therefore, can become osmotically assisted (i.e. osmotically assisted reverse osmosis or “OARO”). Accordingly, the flux across the semi-permeable membrane is higher compared to that achievable using reverse osmosis alone operating under the same hydraulic pressure limitations. In other words, to achieve the same level of flux across the semi- permeable membrane, lower hydraulic pressures may be employed. An important advantage of the present invention is that it allows highly concentrated feed solutions to be treated at hydraulic pressures that are within the hydraulic pressure ratings of conventional reverse osmosis membranes (e.g.82 barg or less). With conventional reverse osmosis techniques, such highly concentrated feed solutions would require hydraulic pressures in excess of the maximum hydraulic pressure rating of most conventional reverse osmosis membranes (e.g. above 82 barg).

[0013] In one embodiment, it is a portion of the feed solution that is fed to the permeate- side of the first semi-permeable membrane.

[0014] Alternatively or additionally, it is a portion of the residual solution that is fed to the permeate-side of the first semi-permeable membrane. Making use of a portion of the residual solution to feed to the permeate-side of the first semi-permeable membrane may be advantageous because of the concentrated nature of the residual solution. A lower flow of residual solution to the permeate-side of the first semi-permeable membrane may be required in comparison to a flow of feed solution, for example, in order to provide the same brine concentration. This may result from the higher osmotic pressure of the residual solution. In addition, a more concentrated residual solution may be obtained for a given hydraulic pressure. In one embodiment, where the first semi-permeable membrane is a hollow fibre membrane and wherein the draw solution is on the fibre bore, feeding a portion of the residual solution may be more energy efficient than feeding, for example, a portion of the feed solution. The lower pressure drop down the fibre bore may mean than less energy is required.

[0015] Preferably, the feed solution to the first semi-permeable membrane may be produced by contacting an initial solution with one side of an initial semi-permeable membrane. Hydraulic pressure may be applied to the initial solution, such that solvent from the initial solution can flow through the initial semi-permeable membrane by reverse osmosis to provide an initial permeate solution on the permeate-side of the initial semi- permeable membrane and an initial residual solution on the feed-side of the initial semi- permeable membrane. The initial residual solution may be employed as the feed solution to the first semi-permeable membrane. The initial permeate solution may be withdrawn, for example, for use or further purification. Withdrawing the initial permeate solution for use after the initial reverse osmosis stage may advantageously produce a higher quality permeate, in comparison to feeding the initial permeate solution to further reverse osmosis steps. This may result from the lower concentration in the feed/concentrate side of the initial semi-permeable membrane. Additionally, the inclusion of an initial reverse osmosis step may provide economic benefits, as conventional, widely used reverse osmosis may be employed. In one embodiment, a portion of the permeate solution on the permeate side of the first semi-permeable membrane may be recycled as a feed to the initial semi- permeable membrane. [0016] In one embodiment, the hydraulic pressure applied to the initial solution may be used at least in part to apply hydraulic pressure to the feed solution that is contacted with the first semi-permeable membrane. For example, a pump may be used to apply hydraulic pressure to drive the initial solution through the initial semi-permeable membrane. This hydraulic pressure may also be used fully or in part to drive downstream membrane separation steps.

[0017] In one embodiment, the process further comprises withdrawing at least a portion of the residual solution on the feed-side of the first semi-permeable membrane and contacting the withdrawn solution with one side of a further semi-permeable membrane as a further feed solution. Hydraulic pressure may be applied to the further feed solution in contact with the further semi-permeable membrane to cause solvent from the further feed solution to flow through the further semi-permeable membrane by reverse osmosis to provide a further permeate solution on the permeate-side of the further semi-permeable membrane and a further residual solution on the feed-side of the further semi-permeable membrane. A portion of the further feed solution or a portion of the further residual solution may be fed to the permeate-side of the further semi-permeable membrane. As explained above, this can reduce the osmotic pressure difference across the further semi- permeable membrane. The hydraulic pressure required to induce solvent flow from the further feed solution by reverse osmosis may hence be reduced.

[0018] While hydraulic pressure may be applied using a dedicated pump, it is possible to use a pump employed to apply hydraulic pressure to the initial solution in an initial reverse osmosis step to apply hydraulic pressure for one or more of any subsequent osmotically assisted reverse osmosis steps.

[0019] In some embodiments, a series of further semi-permeable membranes may be provided downstream of the first semi-permeable membrane. The residual solution from any one of these membranes may be withdrawn and contacted with a downstream membrane as a feed solution for the downstream membrane. A portion of the feed solution to the downstream membrane or a portion of the residual solution on the feed-side of the downstream membrane may be fed to the permeate-side of the downstream membrane.

[0020] Where the process includes the use of an initial semi-permeable membrane as described above, a portion of the further permeate solution on the permeate side of the further semi-permeable membrane may be recycled as or as part of a feed to the initial semi-permeable membrane.

[0021] In another embodiment, the feed solution to the first semi-permeable membrane is produced by contacting an initial solution with one side of an initial semi-permeable membrane. Hydraulic pressure may be applied to the initial solution, such that solvent from the initial solution flows through the initial semi-permeable membrane by reverse osmosis to provide an initial permeate solution on the permeate-side of the initial semi- permeable membrane and an initial residual solution on the feed-side of the initial semi- permeable membrane. The initial residual solution may then be contacted with one side of a further semi-permeable membrane. Hydraulic pressure may be applied to the initial residual solution, such that solvent from the initial residual solution flows through the further semi-permeable membrane by reverse osmosis to provide a further permeate solution on the permeate-side of the further semi-permeable membrane and a further residual solution on the feed-side of the further semi-permeable membrane. The further residual solution may then be used as the feed to the first semi-permeable membrane.

[0022] In some embodiments, the initial residual solution may flow through a series of further semi-permeable membranes, each producing its respective permeate and residual solutions. At least one of these residual solutions may be used as the feed to the first semi-permeable membrane.

[0023] In some embodiments, at least a portion of the further residual solution from the (or one of the) further semi-permeable membrane is withdrawn and fed to the permeate- side of the first semi-permeable membrane.

[0024] In some embodiments, at least a portion of the first permeate solution from the first semi-permeable membrane is recycled as a feed to the initial semi-permeable membrane.

[0025] In some embodiments, the hydraulic pressure applied to the initial solution is used at least in part to apply hydraulic pressure to the feed solution that is contacted with the first semi-permeable membrane and/or the further feed solution that is contacted with the or at least one of the further semi-permeable membranes.

[0026] The feed solution and/or the initial solution may be any solution, such as an aqueous solution. The feed solution and/or the initial solution may be a salt solution, for example, an aqueous salt solution. In some embodiments, the feed solution and/or the initial solution may contain a plurality of dissolved salts. In some embodiments, the feed solution and/or the initial solution is an aqueous solution of sodium chloride. Examples of suitable feed solutions and/or the initial solutions include saline ground water or surface water, brine and seawater. Other examples include waste water streams, lake water, river water and pond water. Examples of waste water streams include industrial or agricultural waste water streams.

[0027] The feed solution and/or the initial solution may be a solution of one or more osmotic agents. Suitable osmotic agents include salts, such as inorganic salts. Suitable salts include salts of ammonium and metals, such as alkali metals (e.g. Li, Na, K) and alkali earth metals (e.g. Mg and Ca). The salts may be fluorides, chlorides, bromides, iodides, sulphates, sulphites, sulphides, carbonates, hydrogencarbonates, nitrates, nitrites, nitrides, phosphates, aluminates, borates, bromates, carbides, chlorides, perchlorates, hypochlorates, chromates, fluorosilicates, fluorosulphates, silicates, cyanides and cyanates. One or more salts may be present. An example may be sodium chloride.

[0028] The total dissolved salt concentration of the feed solution and/or the initial solution may be at least 5,000 mg/I, for example, 5,000 to 250,000 mg/I. In one example, the total dissolved salt concentration of the feed solution and/or the initial solution to the first semi- permeable membrane is at least 30,000 mg/I. The osmotic pressure of the feed may be at least 4 barg, for example, 4 to 320 barg.

[0029] In some examples, the initial solution may be produced by contacting an impure solution with one side of a forward osmosis membrane, and contacting the opposite side of the forward osmosis membrane with an initial solution precursor. The osmotic pressure (solute concentration) of the initial solution precursor may be higher than the osmotic pressure (solute concentration) of the impure solution such that solvent from the impure solution flows across the forward osmosis membrane by forward osmosis to dilute the initial solution precursor to produce the initial solution.

[0030] The impure solution may be saline ground water or surface water, brine and seawater. Other examples include waste water streams, lake water, river water and pond water. Examples of waste water streams include industrial or agricultural waste water streams.

[0031] The initial solution and/or initial solution precursor may be formed by dissolving an osmotic agent in a solvent, for example, water.

[0032] Suitable osmotic agents include salts, such as sodium chloride. Other examples of salts include salts of ammonium and metals, such as alkali metals (e.g. Li, Na, K) and alkali earth metals (e.g. Mg and Ca). The salts may be fluorides, chlorides, bromides, iodides, sulphates, sulphites, sulphides, carbonates, hydrogencarbonates, nitrates, nitrites, nitrides, phosphates, aluminates, borates, bromates, carbides, chlorides, perchlorates, hypochlorates, chromates, fluorosilicates, fluorosulphates, silicates, cyanides and cyanates. One or more salts may be employed.

[0033] The semi-permeable membrane(s) employed in the present invention may be nanofiltration or reverse osmosis membranes. Preferably, the semi-permeable membrane is a reverse osmosis membrane. Where more than two membranes are employed, the membranes may be the same or different. In one embodiment, the semi-permeable membrane(s) are all reverse osmosis membranes. In another embodiment, the semi- permeable membrane(s) are all nanofiltration membranes. In yet another embodiment, both nanofiltration and reverse osmosis membranes are employed as the semi-permeable membrane(s).

[0034] Any suitable reverse osmosis membrane may be used in the present invention. For example, the reverse osmosis membrane may have an average (e.g. mean) pore size of 0.5 to 80 Angstroms, preferably, 2 to 50 Angstroms. In a preferred embodiment, the membrane has an average (e.g. mean) pore size of from 3 to 30 Angstroms. Pore size (e.g. mean pore size) may be measured using any suitable technique. For example, a differential flow method may be employed (Japan Membrane Journal, vol. 29; no. 4; pp. 227 -235 (2004)) or the use of salts, uncharged solutes and atomic force microscopy (Journal of Membrane Science 126 (1997) 91-105).

[0035] Suitable reverse osmosis membranes include integral membranes and composite membranes. Specific examples of suitable membranes include membranes formed of cellulose acetate (CA) and/or cellulose triacetate (CTA) , such as or similar to those used in the study of McCutcheon et al . Desalination 174 (2005) 1-11 and membranes formed of polyamide (PA). An array of membranes may be employed.

[0036] The reverse osmosis membrane may be planar or take the form of a tube or hollow fibre. For example, a tubular configuration of hollow fine fibre membranes may be used. If desired, the membrane may be supported on a supporting structure, such as a mesh support. When a planar membrane is employed, the sheet may be rolled such that it defines a spiral in cross-section. When a tubular membrane is employed, one or more tubular membranes may be contained within a housing or shell.

[0037] The reverse osmosis membrane may be operated at an elevated pressure to drive the (liquid) solution through the membrane. For example, the reverse osmosis step may be carried out at a pressure of 25 to 120 bar, preferably 50 to 100 bar, more preferably 60 to 80 bar.

[0038] Optionally, a scale inhibitor, anti-scaling or anti-fouling additive may be added to any one of the solutions in contact with any of the membranes. Preferably, the scale inhibitor, anti-scaling or anti-fouling additive may be re-circulated between the retentate- side of one membrane and a permeate-side of another or vice-versa.

[0039] These and other aspects of the present invention will now be described with reference to the accompanying figures. Referring to Figure 1 , this represents a schematic illustration of a system for carrying out a process according to a first example of the present disclosure. The system 10 comprises a reverse osmosis unit 12 comprising a first semi-permeable membrane 14. A feed solution 16 is contacted with one side of the membrane 14 and hydraulic pressure is applied so that solvent (e.g. water) from the feed solution 16 flows through the membrane 14 by reverse osmosis to provide a permeate solution 18 on the permeate-side 14b of the first semi-permeable membrane and a residual solution 20 on the feed-side 14a of the first semi-permeable membrane. A portion 16a of the feed solution 16 is fed to the permeate-side 14b of the membrane 14. This can reduce osmotic pressure difference across the membrane 14. As a result, the hydraulic pressure required to induce solvent flow from the feed solution by reverse osmosis may be reduced.

[0040] The residual solution 20 may be removed, for example, as a concentrated waste product, for example, for disposal or further processing or use.

[0041] Figure 2 depicts a schematic illustration of a system for carrying out a process according to a second example of the present disclosure. The system 10 of Figure 2 is similar to the system of Figure 1 and like parts have been labelled with like numerals. Unlike the system 10 of Figure 1 , however, it is a portion 20a of the residual solution 20 that is recycled to the permeate-side 14b of the membrane 14 to reduce the osmotic pressure difference across the membrane 14.

[0042] Figure 3 depicts a schematic illustration of a system for carrying out a process according to a third example of the present disclosure. The system 10 of Figure 3 is similar to the system 10 of Figure 1 and like parts have been labelled with like numerals. However, the apparatus of Figure 3 further includes an initial reverse osmosis unit 22 comprising an initial semi-permeable membrane 24. A pump 26 is also provided.

[0043] When operating the process using the system 10 of Figure 3, the feed solution 16 is produced by contacting an initial solution 28 with one side of an initial semi-permeable membrane 24. Using pump 26, hydraulic pressure is applied to the initial solution 28, such that solvent from the initial solution 28 flows through the initial semi-permeable membrane 24 by reverse osmosis to provide an initial permeate solution 30 on the permeate-side of the initial semi-permeable membrane and an initial residual solution 32 on the feed-side of the initial semi-permeable membrane. The initial residual solution 32 is employed as the feed solution 16 to the first semi-permeable membrane 14. The initial permeate solution 30 may be withdrawn and used or further purified. The pump 26 may be used to deliver the hydraulic pressure required for the downstream reverse osmosis units.

[0044] The permeate solution 18 on the permeate-side of the first semi-permeable membrane 14 may be recycled for use as part of the initial solution 28. [0045] Figure 4 depicts a schematic illustration of a system for carrying out a process according to a fourth example of the present disclosure. The system 10 of Figure 4 is similar to the system of Figure 3 and like parts have been labelled with like numerals. Rather than feeding a portion 16a of the feed solution 16 to the permeate-side 14b of the first semi-permeable membrane 14, however, it is a portion 20a of the residual solution 20 on the feed-side 14a of the first semi-permeable membrane 14 that fed to the permeate- side 14b of the first semi-permeable membrane 14.

[0046] Figure 5 depicts a schematic illustration of a system for carrying out a process according to a fifth example of the present disclosure. The system 10 of Figure 5 is similar to the system 10 of Figure 3 and like parts have been labelled with like numerals.

However, in the system 10 of Figure 5, the residual solution 20 from the feed-side 14a of the first semi-permeable membrane is withdrawn and contacted with one side of a further semi-permeable membrane 34 as a further feed solution.

[0047] Hydraulic pressure delivered by pump 26 or other means is applied to the further feed solution in contact with the further semi-permeable membrane 34, such that solvent from the further feed solution flows through the further semi-permeable membrane 34 by reverse osmosis. This provides a further permeate solution 36 on the permeate-side of the further semi-permeable membrane and a further residual solution 38 on the feed-side of the further semi-permeable membrane 34. A portion of the further feed solution 20b is fed to the permeate-side of the second semi-permeable membrane. The further residual solution 38 may be disposed or concentrated further for disposal. At least a portion of the further permeate solution 36 may be recycled as at least part of the initial solution 28.

[0048] Figure 6 depicts a schematic illustration of a system for carrying out a process according to a sixth example of the present disclosure. The system 10 of Figure 6 is similar to the system 10 of Figure 5 and like parts have been labelled with like numerals. However, in Figure 6, it is a portion 20a of the residual solution 20 that is recycled to the permeate-side 14b of the first semi-permeable membrane 14 to reduce the osmotic difference across the membrane 14. It is also a portion 38a of the further residual solution 38 on the feed-side of the further semi-permeable membrane 34 that is recycled to the permeate-side of the further semi-permeable membrane.

[0049] Figure 7 depicts a schematic illustration of a system for carrying out a process according to a seventh example of the present disclosure. The system 10 of Figure 7 is similar to the system 10 of Figure 1 and like parts have been labelled with like numerals. However, in Figure 7, the feed solution 16 is produced by contacting an initial solution 100 with one side of an initial semi-permeable membrane 110. Hydraulic pressure may be applied to the initial solution, for example, via pump 112, such that solvent from the initial solution 100 flows through the initial semi-permeable membrane 110 by reverse osmosis to provide an initial residual solution 114 on the feed-side of the initial semi-permeable membrane and an initial permeate solution 116 on the permeate-side of the initial semi- permeable membrane. The initial permeate solution 116 may be withdrawn for use or further purification. The initial residual solution 114 is contacted with one side of a further semi-permeable membrane 118. Hydraulic pressure may be applied to the initial residual solution 114, for example, using pump 112, such that solvent from the initial residual solution 114 flows through the further semi-permeable membrane 118 by reverse osmosis. This flow of solvent provides a further permeate solution 120 on the permeate-side of the further semi-permeable membrane 118 and a further residual solution 122 on the feed-side of the further semi-permeable membrane 118. The further residual solution 122 may be used as a feed to the first semi-permeable membrane 14.

[0050] The permeate solution 18 from the first semi-permeable membrane 14 may be fed to the permeate-side of the further semi-permeable membrane 118. The further permeate solution 120 on the permeate-side of the further semi-permeable membrane 118 may be recycled for use as or as part of the initial solution 100.

[0051] Figure 8 depicts a schematic illustration of a system for carrying out a process according to an eighth example of the present disclosure. The system 10 of Figure 8 is similar to the system 10 of Figure 7 and like parts have been labelled with like numerals. However, in Figure 6, it is a portion 20a of the residual solution 20 that is recycled to the permeate-side 14b of the first semi-permeable membrane 14 to reduce the osmotic difference across the membrane 14.

[0052] Figure 9 depicts a schematic illustration of a system for carrying out a process according to a ninth example of the present disclosure. The system 10 of Figure 9 is similar to the system 10 of Figure 8 and like parts have been labelled with like numerals. However, instead of the permeate solution 18 from the first semi-permeable membrane 14 being fed to the permeate side of the further semi-permeable membrane 118, it is a portion 20c of the residual solution 20 from the feed-side 14a of the first semi-permeable membrane 14 that is fed to the permeate side of the further semi-permeable membrane 118. The permeate solution 18 from the first semi-permeable membrane 14 is recycled for use as part of the initial solution 100.

[0053] Figure 10 depicts a schematic illustration of a system for carrying out a process according to a tenth example of the present disclosure. The system 10 of Figure 10 is similar to the system 10 of Figure 9 and like parts have been labelled with like numerals. However, in Figure 10, the initial solution 100 is produced by contacting an impure solution 200 with one side of a forward osmosis membrane 210. The opposite side of the forward osmosis membrane 210 is contacted with an initial solution precursor 212, wherein the solute concentration of the initial solution precursor 212 is higher than the solute concentration of the impure solution such that solvent from the impure solution flows across the forward osmosis membrane by forward osmosis to dilute the initial solution precursor to produce the initial solution 100. The initial solution 100 may be stored, for example, in storage vessel 216 prior to contact with the initial semi-permeable membrane 110. The concentrated impure solution 214 on the feed-side of the forward osmosis membrane 210 may be withdrawn and optionally discarded or further concentrated.

[0054] The initial solution precursor 212 may be formed by dissolving osmotic agent 218 in water. The initial solution precursor 212 may be formed from by recycling at least a portion of the residual solution 20 from the first semi-permeable membrane 14 to the permeate-side of the forward osmosis membrane 210. If desired, a bleed of the recycled residual solution 20 may be removed and discarded via line 220.

[0055] Figure 11 depicts a schematic illustration of a system for carrying out a process according to an eleventh example of the present disclosure. In Figure 11 , hydraulic pressure is applied using a pump to drive an initial feed 300 through a reverse osmosis membrane 310 under reverse osmosis conditions. The permeate 312 is withdrawn as product, while the residual solution 314 is contacted with a semi-permeable membrane 316. Residual pressure from the pump is used to drive the residual solution through the membrane. However, a draw solution 318 containing added osmotic agent 328 is contacted with the opposite side of the semi-permeable membrane 316 so as to reduce the osmotic pressure differential across the membrane 316. This osmotically assists the reverse osmosis step across the membrane 316. The permeate through the membrane 316 is withdrawn as dilute draw solution and stored in tank 322, while the residual solution 324 from membrane 316 is withdrawn as concentrate.

[0056] To regenerate the diluted draw solution in tank 322, a portion of the draw solution is withdrawn and pumped 325 through a reverse osmosis membrane 326. This generates a permeate 328 that can be withdrawn as product, and a residual solution 330. The residual solution 330 is contacted with a semi-permeable membrane 332. Pressure applied using the pump 325 for the preceding reverse osmosis step may be used to drive the residual solution 330 through membrane 332 to produce a permeate, which is recycled to tank 322. A portion 336 of the residual solution 334, however, is fed to the permeate side of the membrane 332 to osmotically assist reverse osmosis across membrane 332. The remainder of the residual solution 334 is recycled as regenerated draw solution to the membrane 316. Osmotic agent 328 may be added to the regenerated draw solution. A bleed 338 may also may be used to withdraw some of the regenerated draw solution, for example, as a bleed for disposal or treatment e.g. to reduce the risk of build-up of unwanted impurities within the circulating draw solution.

[0057] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.