Field of the Invention The invention relates to the field of osmotic systems and particularly to reverse osmosis systems for desalinating water or pressure retarded osmosis systems for generating energy.

Background of the Invention Desalination (RO)

The primary use of osmotic membrane systems at present is for desalinating water by means of reverse osmosis, (F. Carpenter US Patent 3156645 Sea-well conversion of salt water); the most important application being to provide drinking water from the ocean for water security. Reverse osmosis plants consume energy, usually in electrical form, take in seawater and eject concentrated brine as a byproduct of the freshwater production. Most seawater desalination plants are built to have sufficient capacity to meet the highest foreseeable demand for water. Hence, it is common to have desalination plants run at variable capacity, depending on demand for water, considering alternative supplies from other water sources (rivers, aquifers etc.). In general, a percentage of the desalination plant's machinery and semipermeable mem- branes are not utilized unless the plant is being operated at peak capacity.

For example:

- Adelaide Desalination Plant, Australia. This plant was built to provide 100 GL/yr of fresh water, however, because water is available also in metropolitan reservoirs and from the Murray River, the desalination plant operates at only 8 GL/yr, as of January

2016; a utilization of 8% of the peak capacity of the plant.

- swcd ² , Santa Cruz Desalination Project, USA. This project is proposed to provide 2.5 MG/D and is expected to run at half its capacity in non-drought years and at full ca- pacity during drought. Having components in the plant sit idle may not, on its own, be a problem. However, if there is a potential to utilize these idle components in a way which reduces energy consumption, then there is a clear opportunity to reduce operational costs. A sufficient reduction in operational cost can allow the construction of a plant that provides water to meet seasonal demand, which otherwise could not be built because the normalized cost of the plant would not justify the low utilization in day-to-day operation.

Osmotic Energy Generation (OEG) and Storage (OES) Where solutions of differing concentration are available, energy can be recovered from the difference in osmotic pressure between them. There is a large potential for osmotic energy generation (OEG) from the salinity difference between fresh and salt water at river estuaries (A.T. Jones and W. Finley, Recent development in salinity gradient power, Oceans 2003, Proc, 4 (2003) 2284-2287). Osmotic membrane systems have been studied extensively for energy generation by combining fresh water with seawater or other solutions via Pressure Retarded Osmosis (PRO), a process closely related to RO. Energy is recovered by recombining the two fluids and a diluted mixture of the feed and draw solution is ejected. (S. Loeb USPatent 3906250 Method and apparatus for generating power utilizing pressure- retarded-osmosis)

RO and PRO can be used in tandem for energy storage: a saline solution such as seawater is separated into freshwater and concentrated brine using RO, which are stored. PRO can also be useful in recovering a portion of the energy used in desalination, thus reducing the cost of freshwater.

OEG systems are generally conceived under the assumption of constant feed flows of low- (feed) and high- concentration (draw) solution since this allows for efficient operational parameters to be established for the semipermeable membrane used in the osmotic process. However in many situations the supply of draw and feed solutions could vary, for example in recovering energy by mixing brine effluent from a desalination plant with treated waste water effluent from a wastewater treatment plant. A practical OEG system for desalination energy recovery will have to adapt to variation in the supply flows while still providing high efficien- efficiency at any flow state.

It has been recognised that a flexibly designed treatment process would be required to accommodate variations in the quality of feed water, such as total dissolved solids or salinity, that can not be dealt with by adjustments to a fixed RO system (Michelle Chapman Frank Leitz Andrew Tiffenbach , Variable Salinity Desalination , S&T Research Program X9316 Bureau of Reclamation, Department of the Interior, August 2013). The utility of single stage and multistage configurations was demonstrated as a method of dealing with variations in the quality of available feed water in that work. It was also noted that variations in energy supply occur and might be compensated by system reconfiguration but no method has been proposed for doing so.

The Adaptive Membrane Systems (AMS) of the present invention can provide improved efficiency either as to energy use in desalination by RO or as to energy production in PRO when conditions vary in the supply of input resources (whether energy, low salinity or high salinity solution) or in the demand for output products (whether freshwater or energy). The inventive adaptive RO and PRO membrane systems will allow full utilization of system components which otherwise might be unutilized if standard methods are used for responding to changes in demand or supply. With AMS, unused components are redeployed to increase the energy efficiency of the system. AMS does this by adapting the system configuration to permit multistage operation when that is advantageous.

It is well recognised that the brine ejected from a RO desalination system still contains water that can be extracted in a second stage of RO, and that under proper operational conditions the overall extraction can be more energetically efficient with multiple stages. (US Patent Application US2017/0320016 Al "Multistage Reverse Osmosis Systems and Methods" Quantum J. Wei, Konan Killan McGovern, John H Lienhard; Bharadwaj D, Struchtrup H. Large scale energy storage using multistage osmotic processes: approaching high efficiency and energy density. Sustainable Energy Fuels. 2017 May 3; 1(3):599— 614; and Bharadwaj D, Fyles TM, Struchtrup H. Multistage Pressure-Retarded Osmo-sis. Journal of Non-Equilibrium Thermodynamics. 2016;41(4):327-347). Similarly it is known that the mixture of freshwater feed and brine draw solutions that is ejected from a PRO system can provide additional energy through further dilution in a second stage. (Bharadwaj D, Struchtrup H. Large scale energy storage using multistage osmotic processes: approaching high efficiency and energy density. Sustainable Energy Fuels. 2017 May 3;1(3):599-614; and Bharadwaj D, Fyles TM, Struchtrup H. Multistage Pressure-Retarded Osmo-sis. Journal of Non-Equilibrium Thermodynamics. 2016;41(4):327-347). The inventive adaptive membrane systems take advantage of that opportunity by reassigning equipment already present in RO or PRO systems to permit multistage operation through reconfiguring the overall system to adapt to changes in demand or supply.

Brief Description of the Drawings

Fig. 1 is a representation of a single train of a pressure retarded osmosis system typical of the prior art.

Fig. 2a illustrates the regulation of freshwater production in a reverse osmosis system of the prior art. Fig 2b illustrates the regulation of freshwater production in reverse osmosis system according to the invention

Fig 3a shows three trains of a reverse osmosis desalination system of the prior art Fig 3b shows three trains of a reverse osmosis desalination system reconfigured for greater efficiency and reduced freshwater production, according to the invention.

Fig 4a shows a reverse osmosis desalination system using the pressure centre design, according to the prior art. Fig 4b shows a reconfiguration of the pressure centre system of Fig 4a for greater efficiency and reduced freshwater production, according to the invention.

Fig 5a shows three PRO trains acccording to the prior art.

Fig 5b shows a reconfiguration of the PRO trains of Fig 5a for greater efficiency, according to the invention.

Description of the Drawings

Single Stage and Multistage Trains

In the following a "train" is a fully functional unit for reverse or pressure retarded osmosis, and is comprised of at least one pressure vessel having an osmotic membrane (a "membrane module") and the pressure exchangers, turbines, pumps and other support structures required to carry out reverse or pressure retarded osmosis. In the present disclosure the term "multistage" refers to the interconnection of membrane modules so that one receives a fluid from another and in combination they operate in a more thermodynamically efficient manner than do individual modules. Simple connection of the outputs of some membrane modules to the inputs of others does not alone provide for multistage operation according to the sense of this disclosure. Interconnection of membrane modules to provide improved thermodynamic efficiency normally involves an adjustment of the pressure of the solution flowing through the connection and therefore the inclusion of appropriate pressure modifying apparatus between the modules. In some types of trains, notably those employing "batch" or "semi-batch" operation, the inputs and outputs of modules are at ambient pressure. Multistage operation in the sense of improving thermodynamic efficiency can not employ pressure modification in such a situation. With trains of this type other parameters than pressure will be modified to achieve multistage operation. It should be noted that in osmotic systems of the prior art, a single stage train may use more than one membrane module connected in series. Various such arrangements of components are possible which are not multistage according to this criterion. To illustrate the point, Fig 1 shows a single train 90 of a PRO system which employs two membrane modules connected in sequence, but constitutes a single stage train. Input freshwater feed flow 100 is divided to two (or possibly more) membrane modules 102,103. Draw solution entering port 101 passes through PEX 104 and then through both membrane modules in sequence before going to turbine 106. Such a single stage train configuration is useful for use with hollow fibre membrane modules, which are subject to restricted flow of freshwater.

PRO train 90 does not contain two stages in the sense used here to discuss the inventive Adaptive Osmotic Membrane Systems. The pressures at both the feed and draw sides of the membranes of modules 102, 103 are the same (apart from unavoidable pressure losses due to friction), and the two membrane modules are really acting in parallel as a single extended membrane. Adaptive Reverse Osmosis Systems

It is not generally possible to regulate the production of an individual RO train because its components achieve adequate operational efficiency over only a small range of pressures and flows. Regulation of the production of freshwater in the prior art is instead achieved by adding or idling individual units of a bank of parallel trains. The concept is shown in Fig 2a. A bank of four reverse osmosis trains is shown operating in parallel on the left. When production must be reduced, some trains are simply taken out of service, as shown by the crosses at the right. Regulation in this way increases or reduces the amount of power and seawater used and the amount of freshwater produced, but it does not alter the energetic efficiency of that production. Because many RO plants operate at reduced capacity for considerable periods, the prior art approach results in much idle capital investment.

Fig 2b shows the concept for adapting production according to the present invention of Adaptive Osmosis Systems. In response to reduced demand some trains are again removed from the bank of parallel trains at the left, but in this case the trains that are freed are reconfigured as subsequent stages for the trains that remain in use. In this way the intake and production are reduced as before, but in the new configuration the multistage RO systems can operate more efficiently than the previous single stage systems, thus further reducing the demand for power at the reduced production. In general the capital cost of multistage designs is usually not justified by the resulting increase in efficiency but in this case multistage operation is attained using equipment already present which can be returned to its original function when high production operation is again required.

While some configurations for multiple stage RO trains are known (e.g. US 2017/0320016A1) systems that adapt to operational demand by reconfiguring multiple single stage trains into a different number of multistage trains has not previously been known. The present invention does not envisage a particular of multistage system; many possibilities are available for the detailed arrangement of components in the multistage configuration.

Figure 3a (PRIOR ART), shows a typical reverse osmosis plant layout. The apparatus within each dashed box represents a "train". Many trains of identical or similar makeup may be used in parallel as building blocks in larger osmotic membrane systems to produce large quantities of freshwater. Three such trains 7,8, and 9 are shown. The seawater inputs 1, and freshwater 2 and brine outputs 3, may be combined into manifolds (not shown) for ingress into and egress from the RO system installation.

For clarity in describing the reconfiguration of single stage systems to multistage it is useful to review the operation of a single reverse osmosis desalination train 7. First, seawater is introduced at port 1. (While the production of water from a concentrated solution by reverse osmosis has many applications, it is commonly employed for the purpose of extracting freshwater from seawater. In the following, for convenience, "seawater" is used to denote the fluid from which freshwater is extracted.)

The incoming sea water is and carried to input port 501 of pressure exchanger (PEX) 5. PEX such as 5 have the property that when separate fluid flows having different pressures are introduced at ports 501 and 502 they exit, respectively, from ports 503 and 504. However, the fluid exiting port 504 acquires the pressure of the fluid introduced at port 501, and vice versa, i.e. the pressures are exchanged between the two flows. Pressure exchange may be accom- accomplished with turbines that drive pumps (turbopumps) or devices in which hydrostatic pressure is directly exchanged between fluids in a rotating drum. Modern PEX devices can be very efficient, introducing only a small constant pressure drop to each flow. (Compensation pumps (not shown) would be included to compensate for this.) Further, they can operate over a relatively large range of flow without losing efficiency. However, the most efficient types require that both inputs have the same volumetric flow.

PEX 5 raises the pressure of the incoming seawater to that required for reverse osmosis. However, the high-pressure flow entering PEX 5 at port 502 is missing a portion that has previously been expressed through the membrane as product freshwater. To ensure PEX 5 receives equal input flows an equivalent portion of the incoming seawater bypasses it and goes through pump 6 which raises this portion to the same pressure as at output port 503. Ideally, this portion of seawater flow equals the flow of freshwater produced, and the power put into pump 6 is the input power that is thermodynamically required to produce the freshwater flow.

RO module 4 consists of two chambers 405, 406 separated by an osmotic membrane 407. Pressurized seawater is carried into chamber 406 through port 401. The fluid in chamber 406 is at a pressure exceeding the osmotic pressure of the seawater, under which conditions some water permeates through the membrane into chamber 405 and is removed through port 404 as freshwater product.Freshwater may also be introduced at port 402 (not shown) in which case the permeation adds to this flow and the aggregate flow exits out port 404. The remaining fluid in chamber 406 thereby becomes more concentrated and will be termed "brine". The brine exits chamber 406 via port 403 and is returned to PEX port 502. In the PEX the brine transfers its pressure to the incoming seawater and is then exhausted out of PEX port 503, and leaves the train at port 3. An adaptive RO system according to the present invention is shown in Fig 3b. Consider the three trains 7,8 and 9 of the prior art system of Fig 3a each generating 1 m ³ /hour of fresh drinking water and 1 m ³ /hour of brine, using lkW to operate the pumps. An aggregate pro- production of 3 mVhr of fresh water is produced for a consumption of 3 kW. Suppose now that the water demand is only 2 m ³ /h. Reducing pump power by 33% in each train would either drop the pressure below the osmotic pressure, and reverse osmosis would stop, or reduce the flowrate, which could take the membranes away from their optimum operational conditions. Hence, the conventional way of responding would be to shut down one of the trains, say train 9, thus reducing power consumption to 2 kW and producing 2 m ³ /h by operating the remaining two trains, say train 7 and 8, at the optimum operational conditions, retaining the desalination process's previous energy efficiency. (The amount of energy consumed to produce unit amount of fresh water corresponds to the desalination process's energy efficiency.)

An Adaptive Membrane System according to the present invention, as shown in Fig 3b, provides for component trains to rearranged into a multistage configuration. Multistage operation allows water production with higher efficiency than single stage operation. Such a rearrangement of components can allow the system to consume less power to generate the desired amount of freshwater, as compared with shutting down one train. Most of the extra components required for multistage operation are in this instance cost-free because otherwise they would be idled.

The rearrangement shown in Fig 3b interconnects trains 7 and 8 in Fig 3a to provide a single, two stage RO system. In the new configuration there are two RO systems in parallel, thus reducing the production of the overall system of three parallel trains by about 33%, as before, but one of them is a two-stage system and is more efficient than a single stage system therefore providing more energy efficient operation for the whole RO desalination plant in its reduced production mode.

Connections altered to effect the reconfiguration are shown by dashed lines in Fig 3b. The connection between the high-pressure output port 403 of membrane module 4a and the high- pressure input port 502 of PEX 5a is removed and the high-pressure input to PEX 5b is reconnected to receive the brine output from membrane module 4a (brine consists of seawater that was input at port 401 that has been concentrated by the removal of some freshwater). The brine output of the first module is thus connected to the original seawater input of the other, placing them in series. The connection is made through a pressure modulating device, PEX 5b, that adjusts the pressures in the two modules to be suitable for the concentrations in each. Pump 3b boosts the pressure of the bypassed brine in the second stage to conform also with the higher osmotic pressure of the higher concentration there. The required pressure change is less than is needed in pump 3a, and therefore this pump does not require the same power. By this reconfiguration the RO system is able to produce more than half the previous aggregate freshwater output with the available power, or alternatively produce the same amount of water with lower power consumption.

Such rearrangement can allow the system to consume less power to generate equal amount of freshwater, by comparison with the alternative of shutting down one train. Most of the extra components required for multistage operation are in this instance cost-free because otherwise they would be idled. However, in some cases certain of the existing components may not be suitable for the multistage operation. It is within the scope of the invention to provide alternative components to be used in the reconfiguration from single to multiple stage operation. An alternative method of producing more freshwater than can be obtained from a single train, termed the "Pressure Centre" design, aggregates multiple components in parallel, and then assembles them into the equivalent of a single large train. A Pressure Centre equivalent of the three-train system of Figure 3a is shown in Figure 4a (PRIOR ART). Groups of components that perform the various functions of the large train are labelled to correspond to Figure 3, but with an "a" appended to the label. The groupings of pumps 6a, PEX 5a, and membrane modules 4a perform the same functions as in a single train but are collectively capable of handling larger total flows, which are aggregated into manifolds. The parallel seawater inputs to osmotic modules 4b,4c and 4d enter ports 401b, 401c and 401d, and the concentrated brine exits the modules at ports 403b,403c,403d in parallel, while the freshwater product is collected in parallel out of ports 404b,404c,404d into the manifold that exits at output 2a. Given the same component capabilities, the parallel train design and the pressure centre designs have equivalent performance. However, by separating the pumping, pressure exchange and osmosis functions, some extra flexibility may be obtained in the pressure centre approach. For example the capacity of the five pumps comprising the pumping centre 6a might be temporarily met by four pumps,so that for maintenance or other purpose a pump might be removed without seriously impacting system performance.

According to the prior art the output of the pressure centre design would be regulated in the same manner as the parallel train design, by shutting down individual membrane units and adjusting the PEX and pumping capacity that is then required. In prior art this entails shutting down one of the membrane modules 4a in Fig 4a, and idling some of the pumps 6a and PEX elements 5a to match the reduced requirement.

According to the present invention the output of the system of Fig 4a can be regulated more efficiently as an adaptive membrane system multistage reconfiguration. It will be readily appreciated that by altering the parallel fluid connections shown within module grouping 4a of Fig 4a, and including pumps 11 and 12, a series arrangement of these modules, as is shown in Fig 4b, can be obtained, in which all three of the membrane modules 4a, 4b, and 4c remain in use. The fluid connections between the membrane modules have been rearranged in Fig 4b so that after being pressurised and passing through the first membrane module, losing some freshwater by reverse osmosis, the input seawater passes to pump 1 1. Although the seawater has been concentrated to become brine in the first membrane module, there will still be the possibility of extracting more water from the brine by reverse osmosis, using a higher pressure to take account of the increased osmotic pressure of the brine. Pump 11 raises the pressure in the fluid connection between modules 4b and 4c to provide the required pressure. The brine concentration is again increased by reverse osmosis in module 4c and the further concentrated brine passes out through another fluid connection to membrane module 4d. Pump 12 in this fluid connection again increases the pressure of the brine so that in module 4d a third reverse osmosis step occurs. The freshwater outputs from the exit ports 404b,404c and 404d of the modules are collected into the output manifold 2b. After leaving module 4d via port 403d the thrice-concentrated brine is returned via a fluid connection to the PEX to pressurise the incoming seawater. However, the returned brine is at a higher pressure than would be required at first stage membrane module 4b input port 401b. Turbine 13 in the fluid connection between port 403d and the PEX drops the pressure of the brine by the total pressure that has been added by pumps 11 and 12, thus returning the brine to the first stage pressure. The returned brine enters the PEX stack and pressurises the incoming seawater before being exhausted at system port 3b.

In the process of reducing the brine pressure the turbine recovers some of the power used in pumps 11 and 12. Ignoring losses, the power recovered is ideally the product of the total pressure increment provided in pumps 11 and 12 and the component of the brine flow that does not pass through the membranes. The ideal recovered power equals the power used in the pumps in excess of what is thermodynamically required to desalinate the water produced. This power is used to offset the input requirement of those pumps (the power transfer is via electrical path 14). It should be noted that pumps 11 and 12 provide a relatively small increment of pressure to a relatively large flow, as compared with bypass pumps 6, and for this reason it may be necessary to use different pumps for the two functions.

The examples above are illustrative. Many different multistage configurations may be used to control the production and efficiency of RO systems according to the present invention. While certain multistage configurations may be more suitable than others for the adaptation of particular types of single stage system, there is no requirement for a specific multistage design to be used in reconfiguring a particular type of single stage reverse osmosis system. Fluid connections can be altered to reconfigure arrangement of modules by means of valves controlling the route taken by fluids through a pre-built system of pipes, or by demounting and rearranging pipes to provide the required connections. Some connections required in one configuration may not be used in another.

Adaptive Pressure Retarded Osmosis Systems Figure 5a (PRIOR ART), shows typical PRO topology. There are three parallel PRO trains with separate inputs and outputs. High concentration (draw) solution is introduced to train 17 at port 12a and carried to input port 501a of pressure exchanger (PEX) 15a. Freshwater "feed" solution introduced at port 13a is carried to port 412a. A high concentration "draw" solution flows through port 12a into port 513 of PEX 15a and then to the other side of the membrane through port 411. The draw solution is held at a higher pressure than the feed solution. By osmosis, some water from the feed solution permeates through the membrane into the draw chamber. This permeation flow is retarded by the draw solution pressure and the extra flow acquires that pressure. (This feature underlies the term "pressure-retarded osmosis".) The extra pressurised flow is then separated to drive turbine 16a for the generation of electricity, while the remainder goes to port 512 of pressure exchanger 15a to raise the pressure of the incoming draw solution. While three trains are shown, more can be added, using more freshwater and draw solution to generate more electricity. Alternatively, a pressure centre design is possible, in which the membranes, PEX and turbines are aggregated into separate units.

Consider the three single stage PRO trains 17, 18 and 19 shown in Fig 5a operating in parallel, each generating power from turbine 16a,b,c respectively, and having optimum draw flow rates into ports 12a,b,c, and feed flow into 13a,b,c to accommodate the performance of the modules, PEX and turbines. Suppose now that the supply of feed solution is reduced by one third. The conventional way of responding would be to shut down one of the trains, e.g. stop the flow into ports 12c, 13c, and continue to supply optimal flow to ports 12a,b and 13a,b thus using all the available feed flow and producing power from the two trains 17a and 17b , i.e. two thirds of the previous aggregate power output.

An adaptive PRO system according to the invention would continue to use the components of all three trains by rearranging them into a multistage configuration. Since multistage operation allows higher power generation efficiency than does a single stage, such rearrangement can allow the system to generate a greater overall power level from the reduced input flow than could be obtained simply by shutting down one train. The majority of the extra components required for multistage operation are in this instance cost-free because otherwise they would be idled. Some additional components would be required to effect the rearrangement, for ex- example valves to control the flow of fluids and alternative pipe routings.

One possible multistage configuration is shown in Figure 5b. The system shown is a three- stage PRO system reconfigured from the parallel PRO system shown in Fig 5a. Feed solution (i.e. fresh water) enters at port 2b and is carried to all three membrane units 4b, 4c and 4d. Draw (i.e.brine) solution enters at port lb and is carried to a pressure exchanger 5b array where it acquires the pressure needed in the first stage by exchange of pressure from previously used draw fluid. A compensation pump (not shown) makes up a small loss of pressure that occurs in this exchange.

In the first stage membrane unit 4b a portion of the feed fluid permeates through a membrane and dilutes draw fluid on the other side, acquiring the pressure on the draw side that has been established by the pressure exchanger 5b. Draw fluid that exits from membrane unit 4b is diluted by the portion of feed fluid that has permeated. This draw fluid passes to membrane unit 4c where it is separated from feed fluid by a membrane. On its passage from membrane unit 4b to membrane unit 4c the draw solution passes through an interstage turbine 20 which converts some of the power in the fluid flow to mechanical energy, lowering the pressure. The pressure of the diluted draw solution entering membrane unit 4c is thus reduced to a lower pressure than in membrane unit 4b, appropriate for pressure retarded osmosis at its reduced concentration as it enters membrane unit 4c.

The draw solution then passes on to the third membrane unit 4d through pressure reducing turbine 22 in similar fashion, and so on through as many other membrane units stages as may be provided. After exiting the final one, the flow of diluted draw solution is divided by a flow separator 18 into two streams. One stream has flow rate equal to the aggregate of the permeation through all the membranes, and the same pressure as that of the draw solution of the final stage. This enters a turbine 22 where it generates the useful output power from the system, and exits the system at ambient pressure. The other flow of diluted draw solution passes through a pump unit 6b which raises its pressure to the pressure required on the draw solution side of the first membrane unit 4b. The diluted draw solution then enters the pressure exchanger unit 5b where it pressurises the incoming brine, after which the diluted draw solu- solution is exhausted via port 3b.

Turbines 20 and 21 provide power to drive pump unit 6b thus recovering energy that would be lost in adjusting the full flow of draw solution to the pressures required in the multistage system.

As in the case of Adaptive RO Systems, various multistage PRO configurations may be used according to the present invention. While certain multistage configurations may be more suitable than others for the adaptation of particular types of single stage PRO system, there is no requirement for a specific multistage design to be used in reconfiguring a particular type of single stage reverse osmosis system, since the reconfiguration is done by reconnecting or rearranging fluid paths in a pipe network by means of valves or altered pipe connections. Operation of Adaptive Membrane Systems

Adaptive membrane systems provide for the regulation of efficiency in response to varing conditions. Such regulation may be a response to production requirements, whether fresh water from an RO system, or energy from a PRO system, or to the availability of inputs - saltwater and energy with RO, saltwater and freshwater in the case of PRO. Regulation of production may occur on a wide variety of time scales and in response to a wide variety of determinations and measurements.

For example, a seawater desalination plant may require to run in the efficient, low production multistage mode during a season when freshwater from rivers contributes significantly to reservoir contents, and then to alter to the less efficient high production, single stage parallel mode for an emergency in a drought season in which there is no other input to freshwater reservoirs except desalination. In such a case the determination of when to convert between configurations is a matter of policy as much as of measurements. The conversion might take relatively long time and be accomplished by the manual setting of valves or even the physical reconnection of pipes. On the other hand, a PRO energy storage system may have to react on quite short time scales, perhaps hourly, to the availability of environmental or grid power to be stored. In such a case the reconfiguration might be made by means of remotely activated valves set under the control of an automated system.

It will be clear that adaptive membrane systems may take many detailed forms and may be reconfigured between single and multistage operation by many methods, and controlled in response to many different criteria, to provide a desired production capability and efficiency of operation. Regulating the production of osmotic membrane systems by reconfiguration of trains between single and multistage dispositions according to any criterion for so doing is within the scope of the invention.

Configuration of Adaptive Membrane Systems

It will be apparent from the above that configurations of RO and PRO systems in their high production and high efficiency forms may vary widely. Adaptive membrane systems according to the present invention are characterised by the capability to reconfigure between a configuration having least one single stage train, and a system having at least one thermodynamically multistage train, where a thermodynamically multistage train is one that operates more efficiently than multiple single stage trains. Particular implementations of such a reconfigurable system are within the scope of the invention.