the block circuit diagram in fig1 shows a delivery pump 1 for introducing salt water 10 into a pressure compensating device 2 under a first pressure p 1 . the same salt water 11 which however is now subjected to a high working pressure p 2 is passed from the pressure compensating device 2 to the membrane module 3 . there a part of the salt water 11 passes through the membrane 6 which is preferably in the form of a so - called crossflow membrane , for example 25 % of the salt water 11 , it is desalinated in doing so and it is discharged in the form of desalinated water 12 . the remaining part of the salt water 11 , for example 75 %, cannot pass through the membrane 6 but flows along the surface of the membrane 6 into the connecting conduit 5 , by way of which it is discharged from the membrane module 3 as concentrated salt water 13 . the concentrated salt water 13 which in that case is still at a high pressure which approximately corresponds to the pressure p 2 but is somewhat lower is then passed to the pressure compensating device 2 again . there , that high pressure p 2 is used in a manner that is still to be described in detail hereinafter for the purposes of acting with pressure on the salt water introduced into the pressure compensating device 2 , and feeding it to the membrane module 3 at the inlet thereof . at the same time that pressure is used in the pressure compensating device to definitively discharge concentrated salt water 14 therein , by way of the discharge conduit 4 , and to feed unconcentrated salt water 10 to the pressure compensating device 2 . all the described procedures take place in that case simultaneously and continuously so that there is no need for a high - pressure pump for subsequently delivering the high working pressure and desalinated water 12 is continuously available . as was described hereinbefore , particularly when using a crossflow membrane 6 it is necessary for the flow of the salt water over the surface of the membrane to be maintained continuously and under a uniformly high pressure as otherwise salt molecules can be deposited at the surface of the membrane , and such molecules can result in damage to the membrane or an interruption in operation of the system . by virtue of various circumstances however it can happen that the pressure p 2 of the salt water discharged from the pressure compensating device 2 briefly falls so greatly that the flow over the surface of the membrane would be reduced or even interrupted . desalination would then admittedly still take place ; it will be noted however that the membrane could be damaged as the concentrated salt water 13 cannot flow away out of the membrane module 3 . in order in such a situation to maintain the pressure p 2 and the flow , there is therefore provided in accordance with the invention a reservoir 15 which in such a situation passes additional water into the membrane module 3 and thus ensures that the high working pressure p 2 remains maintained and the flow over the surface of the membrane is not reduced . fig2 shows a specific configuration of an apparatus according to the invention . it has two identical piston / cylinder devices 401 , 402 with two aligned mutually opposite cylinders which each have a respective inlet chamber 201 , 202 for receiving the salt water and a respective outlet chamber 101 , 102 for receiving the concentrated salt water 13 . arranged within each of the piston / cylinder devices 401 , 402 is a respective special piston 301 , 302 which subdivides the piston interior into the above - mentioned chambers and which in the figure is displaceable in the horizontal direction within the piston / cylinder device . from the delivery pump 1 , a respective feed conduit with a passive non - return valve 7 leads to the inlet chambers 201 , 202 . the non - return valves 7 in that case are of such a configuration that they open and permit a through flow when the pressure in the feed conduit is greater than in the inlet chambers 201 , 202 . comparable non - return valves 8 which however involve a different through - flow direction are disposed in the feed conduits from the inlet chambers 201 , 202 to the membrane module 3 . in contrast , actively switchable main valves v 3 , v 6 and v 1 , v 4 respectively are arranged in the feed conduits 5 from the membrane module 3 to the outlet chambers 101 , 102 and in the discharge conduits 4 from the outlet chambers 101 , 102 ; the feed flow of the concentrated salt water 13 from the membrane module 3 and the discharge flow of the concentrated salt water 14 out of the pressure compensating device 2 respectively can be controlled by way of the above - mentioned main valves . the pistons 301 , 302 are fixedly connected together by means of a piston rod 30 . pinions 40 which for example can be driven by electric gear motors and which engage into a tooth arrangement provided on the piston rod 30 can drive the piston rod 30 and by way thereof the pistons 301 , 302 in order to compensate for pressure losses . the pistons are arranged in such a way that they operate in opposite phase relationship . when therefore one piston is disposed in a position in which the volume of the inlet chamber 202 is at a maximum and the volume of the outlet chamber 102 is at a minimum , then the other piston which is connected by way of the piston rod 30 is in a position in which the volume of the inlet chamber 201 is at a minimum and the volume of the outlet chamber 101 is at a maximum ( see fig2 ). in that situation the inlet chamber 202 is filled with water and the outlet chamber 101 is filled with concentrated salt water . the valves v 1 , v 3 , v 4 and v 6 which are illustrated here as switches are controlled in such a way that v 3 and v 4 are now closed while v 1 and v 6 are opened . in this connection opening a valve signifies producing a flow communication in order to allow a through - flow , for which purpose the valve is purely mechanically opened . similarly closing a valve signifies interrupting a flow communication in order to prevent a through - flow , for which purpose the valve is purely mechanically closed . by virtue of the main valve v 1 being open , firstly the pressure of the concentrated salt water in the outlet chamber 101 escapes . by virtue of opening of the main valve v 6 the outlet chamber 102 is subjected to the effect of pressure ( for example about 65 bars ) and the concentrated salt water flows into that chamber . at the same time the salt water disposed in the inlet chamber 202 is pressed to the membrane module 3 by the piston subjected to pressure . as the pistons are arranged in such a way that they operate in opposite phase relationship , introduction of the concentrate which is subjected to pressure ( for example 65 bars ) into the outlet chamber 102 by the piston rod 30 causes movement of the other piston 301 which as a result empties the pressure - less outlet chamber 101 . at the same time a reduced pressure is produced in the inlet chamber 201 and sucks in the salt water and fills that chamber . when the outlet chamber 102 is filled the main valves are suitably controlled and the opposite procedure takes place . as the membrane module is preferably operated at about 70 bars in order to provide for a sufficiently high level of fresh water , and at a maximum about 5 - 10 bars occur as a pressure loss at the membrane , at least the above - mentioned pressure of about 65 bars of the concentrated salt water is still available at the concentrate discharge 5 of the membrane module 3 . in order to maintain the flow of the water along the surface of the membrane 6 during the operation of switching over the direction of movement of the pistons 301 , 302 , in particular at the moment when the pistons 301 , 302 are stopped , in accordance with the invention there is provided an additional piston / cylinder device 403 , referred to hereinafter as the piston reservoir . it has three chambers , namely a feed water chamber , inlet chamber 203 , which is connected to the feed conduit for the salt water 11 which is fed in , a concentrate chamber , outlet chamber 103 , connected to the concentrate conduit 5 and a pressure chamber 503 . in that arrangement the pressure chamber 503 is connected on the one hand by way of an active valve v 7 to the feed conduit 11 and on the other hand directly to a pressure reservoir 20 , preferably a bladder reservoir . during operation the valve v 7 is always closed , it only serves to be able to fill up the circuit comprising the pressure chamber 503 and the pressure reservoir 20 again with the pressure fluid , for example a hydraulic fluid , after an interruption in operation , and to restore the required high pressure in the pressure reservoir 20 . if the effective piston area of the piston 303 in the concentrate chamber 103 is about three quarters of the piston area in the feed water chamber 203 and the piston area in the pressure chamber 503 is about a quarter of that area , the pressure distributions are as follows . the feed water chamber 203 is subjected to a pressure of about 70 bars in operation . that results in up to 280 bars in the circuit comprising the pressure chamber 104 and the pressure reservoir 20 . they are however not attained in operation . the operating pressure in that region is between about 200 and 210 bars . at the time of switching over the direction of movement of the pistons 301 , 302 , a pressure of about 70 bars acts on the piston 303 , from the feed water chamber 203 . the pressure in the reservoir 20 is only 160 bars . then , from there , because of the smaller piston area in the pressure chamber 503 , the pressure acting is about 160 / 4 , that is to say about 40 bars . the pressure in the concentrate circuit , that is to say the pressure of the concentrated salt water 13 discharged from the membrane module 3 , is about 68 bars . that pressure acts on an area which includes three quarters of the piston area . consequently a pressure of about 51 bars acts here . those two pressures act in the same direction and are thus added to give a total of about 91 bars . only the approximately 70 bars in the feed water chamber 203 acts against that resulting pressure . accordingly there is a sufficiently high pressure to press the piston 203 downwardly in the illustrated position and thus to maintain the flow over the membrane 6 . even if it is only a pressure of about 60 bars that is taken as the basis for the concentrate circuit , that still affords a proportion of 45 bars in the concentrate chamber 103 . even if the pressure in the pressure reservoir 20 is only 120 bars , that results in a further 30 bars , so that there is still an overall pressure of 75 bars , which allows the flow over the membrane 6 to be maintained . the piston reservoir 403 can be controlled in such a way that it is only in the case of a pressure drop in the connecting conduit between the inlet chambers 201 , 202 and the membrane module 3 or a reduction in the flow over the membrane 6 , that an additional pressure is exerted on that connecting conduit . for that purpose it is possible to provide for example suitable sensors which detect such a pressure drop or a reduction in flow and which trigger the appropriate pressure control procedure . in addition it is also possible to provide valves which are suitably controlled for that purpose in the concentrate conduit 5 between the membrane module 3 and the piston / cylinder device 403 , which are opened if necessary , in order to produce the described movement of the piston 303 downwardly by the introduction of a pressure into the concentrate chamber 103 . if in contrast such pressure assistance is not required , such a valve can also be closed again so that , because of the higher pressure in the feed water chamber 203 in relation to the pressure chamber 503 , the piston 303 is moved upwardly again and thus remains virtually in the readiness position . in the case of the piston reservoir 403 according to the invention however such a control can be omitted as it can automatically set the specified pressure conditions in operation and the desired effect is achieved by separate control . on the one hand then feed water can flow out of the chamber 203 and on the other hand concentrate can flow out of the membrane module 3 , into the chamber 103 , so that the flow over the membrane 6 is maintained . in addition it is also possible to additionally provide secondary or bypass valves parallel to the described main valves v 1 , v 3 , v 4 , v 6 in order to reduce the loading on the main valves and thus to increase the service life thereof . in addition it is also possible to provide one or more quantitative flow limiting devices which are intended to prevent an abrupt pressure compensation effect insofar as they limit the maximum quantitative through - flow and thus contribute to a gradual compensation of pressure and slow changes in pressure , instead of abrupt pressure fluctuations . elements of that kind and further elements are described and illustrated in above - mentioned wo 02 / 41979 a1 to which reference is hereby expressly directed and the description of which is to be deemed to be included herein . the basic mode of operation of such an apparatus with two piston / cylinder devices is also discussed in detail therein , and reference is also made thereto . the invention can also be used in relation to apparatuses of a different configuration for the desalination of water by reverse osmosis , which for example , instead of the illustrated two piston / cylinder devices , have another number of such devices , for example one or three piston / cylinder devices . they can in principle also be of a different configuration . the configuration of the reservoir in the form of the piston / cylinder device with three chambers , as is shown in fig2 , is also not absolutely necessary but in principle can also be of a different nature . all of the above u . s . patents , u . s . patent application publications , u . s . patent applications , foreign patents , foreign patent applications and non - patent publications referred to in this specification and / or listed in the application data sheet , are incorporated herein by reference , in their entirety . from the foregoing it will be appreciated that , although specific embodiments of the invention have been described herein for purposes of illustration , various modifications may be made without deviating from the spirit and scope of the invention . accordingly , the invention is not limited except as by the appended claims .