Abstract:
A method of treatment of liquid including supplying liquid to be treated to at least one liquid treatment module having a liquid inlet, a permeate outlet and a brine outlet, monitoring liquid pressure within the at least one liquid treatment module and upon exceedance of a liquid pressure threshold in the at least one liquid treatment module, reducing the liquid pressure in the at least one liquid treatment module by performing at least one of the following functions: opening a liquid pressure reducing valve at the brine outlet, increasing a liquid volume output of a circulation pump which removes brine from the at least one liquid treatment module, equilibrating liquid pressures between a liquid pressure inside the at least one liquid treatment module and inside a liquid feed tank and opening a liquid pressure reducing valve at the liquid inlet.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is hereby made to the following patent applications of the present inventor, the disclosures of which are hereby incorporated by reference: 
         [0002]    U.S. patent application Ser. No. 13/603,028, filed Sep. 4, 2012, entitled “SYSTEM AND METHOD FOR DESALINATION OF WATER”, published as U.S. Patent Publication No. 2014/0061129; 
         [0003]    U.S. patent application Ser. No. 14/145,068, filed Dec. 31, 2013, entitled “SYSTEM AND METHOD FOR DESALINATION OF WATER”, published as U.S. Patent Publication No. 2014/0110337; and 
         [0004]    PCT Patent Application No. PCT/IL2013/050744, filed Sep. 2, 2013, entitled SYSTEM AND METHOD FOR TREATMENT OF WATER, published as International Publication No. WO 2014/037940. 
     
    
     FIELD OF THE INVENTION 
       [0005]    The present invention relates to water treatment systems and methods. 
       BACKGROUND OF THE INVENTION 
       [0006]    Various types of water treatment systems and methods are known. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention seeks to provide improved water treatment systems and methods. There is thus provided in accordance with a preferred embodiment of the present invention a method of treatment of liquid including supplying liquid to be treated to at least one liquid treatment module employing at least one of reverse osmosis and nanofiltration membranes, the at least one module having a liquid inlet, a permeate outlet and a brine outlet, monitoring liquid pressure within the at least one liquid treatment module and upon exceedance of a liquid pressure threshold in the at least one liquid treatment module, representing exceedence of a salinity threshold in the liquid therein, reducing the liquid pressure in the at least one liquid treatment module by performing at least one of the following functions: opening a liquid pressure reducing valve at the brine outlet, thereby reducing the liquid pressure at the brine outlet to a level above atmospheric pressure which exceeds osmotic pressure of the liquid in the at least one liquid treatment module, increasing a liquid volume output of a circulation pump which removes brine from the at least one liquid treatment module and supplies liquid to the water inlet from a liquid volume output at times other than upon and immediately following the exceedance, equilibrating liquid pressures between a liquid pressure inside the at least one liquid treatment module and inside a liquid feed tank and opening a liquid pressure reducing valve at the liquid inlet, thereby providing a pressure reducing backflow liquid path from the liquid inlet of the at least one liquid treatment module which bypasses a liquid pressure increasing pump upstream of the liquid inlet. 
         [0008]    In accordance with a preferred embodiment of the present invention upon exceedance of a liquid pressure threshold in the at least one liquid treatment module, representing exceedence of a salinity threshold in the liquid therein, the liquid pressure in the at least one liquid treatment module is reduced by performing at least two of the following functions: opening a liquid pressure reducing valve at the brine outlet, thereby reducing the liquid pressure at the brine outlet to a level above atmospheric pressure which exceeds osmotic pressure of the liquid in the at least one liquid treatment module, increasing a liquid volume output of a circulation pump which removes brine from the at least one liquid treatment module and supplies liquid to the water inlet from a liquid volume output at times other than upon and immediately following the exceedance, equilibrating liquid pressures between a liquid pressure inside the at least one liquid treatment module and inside a liquid feed tank and opening a liquid pressure reducing valve at the liquid inlet, thereby providing a pressure reducing backflow liquid path from the liquid inlet of the at least one liquid treatment module which bypasses a liquid pressure increasing pump upstream of the liquid inlet. 
         [0009]    In accordance with a preferred embodiment of the present invention upon exceedance of a liquid pressure threshold in the at least one liquid treatment module, representing exceedence of a salinity threshold in the liquid therein, the liquid pressure in the at least one liquid treatment module is reduced by performing at least three of the following functions: opening a liquid pressure reducing valve at the brine outlet, thereby reducing the liquid pressure at the brine outlet to a level above atmospheric pressure which exceeds osmotic pressure of the liquid in the at least one liquid treatment module, increasing a liquid volume output of a circulation pump which removes brine from the at least one liquid treatment module and supplies liquid to the water inlet from a liquid volume output at times other than upon and immediately following the exceedance, equilibrating liquid pressures between a liquid pressure inside the at least one liquid treatment module and inside a liquid feed tank and opening a liquid pressure reducing valve at the liquid inlet, thereby providing a pressure reducing backflow liquid path from the liquid inlet of the at least one liquid treatment module which bypasses a liquid pressure increasing pump upstream of the liquid inlet. 
         [0010]    In accordance with a preferred embodiment of the present invention upon exceedance of a liquid pressure threshold in the at least one liquid treatment module, representing exceedence of a salinity threshold in the liquid therein, the liquid pressure in the at least one liquid treatment module is reduced by performing all of the following functions: opening a liquid pressure reducing valve at the brine outlet, thereby reducing the liquid pressure at the brine outlet to a level above atmospheric pressure which exceeds osmotic pressure of the liquid in the at least one liquid treatment module, increasing a liquid volume output of a circulation pump which removes brine from the at least one liquid treatment module and supplies liquid to the water inlet from a liquid volume output at times other than upon and immediately following the exceedance, equilibrating liquid pressures between a liquid pressure inside the at least one liquid treatment module and inside a liquid feed tank and opening a liquid pressure reducing valve at the liquid inlet, thereby providing a pressure reducing backflow liquid path from the liquid inlet of the at least one liquid treatment module which bypasses a liquid pressure increasing pump upstream of the liquid inlet. 
         [0011]    There is also provided in accordance with another preferred embodiment of the present invention a method of treatment of liquid including: supplying at least one water treatment module including at least one membrane and having a feed water inlet at a feed side of the at least one membrane, a permeate outlet at a permeate side of the at least one membrane and a brine outlet at a brine side of at least one membrane, pressurizing feed water supplied to the feed water inlet by employing a pump which normally maintains a fixed output feed water volume notwithstanding variations in water pressure at an outlet thereof, the energy consumption of the pump being a function of the variations in water pressure at the outlet, monitoring the water pressure at the outlet of the pump and when a predetermined high pressure threshold is reached at the outlet of the pump, immediately making changes in the water supply to the module, thereby to cause immediate lowering of the water pressure at the outlet of the pump, to a pressure below the osmotic pressure at the feed side of part but not all of the module, thereby immediately reducing the energy consumption of the pump, thereby providing an overall energy cost savings per unit of water treated. 
         [0012]    There is further provided in accordance with yet another preferred embodiment of the present invention a method for treatment of liquid in at least one liquid treatment module including at least a high pressure pump and a circulation pump, the method including upon occurrence of an operational threshold representing exceedence of a salinity threshold in the at least one liquid treatment module, effecting the following: removing brine from the at least one liquid treatment module and reducing, at an enhanced speed, pressure of the liquid in the at least one liquid treatment module by at least one of the following: opening a pressure reducing valve downstream of the high pressure pump, increasing the volume output of the circulation pump from its volume output when functioning as a concentrate circulation pump to a higher volume output when functioning as a feed water pump and passing liquid from downstream of the high pressure feed pump to upstream of the high pressure feed pump. 
         [0013]    Preferably, the passing liquid from downstream of the high pressure feed pump to upstream of the high pressure feed pump includes passing the liquid via a flow restrictor arranged in parallel to the high pressure feed pump. 
         [0014]    In accordance with a preferred embodiment of the present invention the pressure of the liquid in the at least one liquid treatment module is reduced at an enhanced speed by at least two of the following: opening a pressure reducing valve downstream of the high pressure pump, increasing the volume output of the circulation pump from its volume output when functioning as a concentrate circulation pump to a higher volume output when functioning as a feed water pump and passing liquid from downstream of the high pressure feed pump to upstream of the high pressure feed pump. Alternatively, the pressure of the liquid in the at least one liquid treatment module is reduced at an enhanced speed by all of the following: opening a pressure reducing valve downstream of the high pressure pump, increasing the volume output of the circulation pump from its volume output when functioning as a concentrate circulation pump to a higher volume output when functioning as a feed water pump and passing liquid from downstream of the high pressure feed pump to upstream of the high pressure feed pump. 
         [0015]    Preferably, the liquid treatment module is a water treatment module including at least one of at least one reverse osmosis membrane and at least one nanofiltration membrane and is operative for treatment of at least one of sea water, brackish water and waste water. 
         [0016]    There is even further provided in accordance with another preferred embodiment of the present invention a water treatment system including: at least one liquid treatment module operative to receive feed water at a water inlet thereof and to separate the feed water into permeate and concentrate, the permeate constituting treated water, the at least one liquid treatment module having a brine outlet for release of concentrate whose salinity is such that an operational threshold of the system is exceeded, a liquid pressure reducing valve at the brine outlet, a high pressure pump, operative to pressurize liquid to be treated received at a liquid feed inlet and to provide a pressurized feed water output to the at least one liquid treatment module, a feed water flow rate sensor located upstream of the high pressure pump and providing a feed water flow rate output, a pump controller receiving the feed water flow rate output and controlling the operation of the high pressure pump, a liquid pressure sensor for providing an output indication of liquid pressure at at least one of an inlet to the at least one liquid treatment module, an outlet from the at least one liquid treatment module and within the at least one liquid treatment module, a circulation pump which removes brine from the at least one liquid treatment module and supplies liquid to the water inlet, a liquid feed tank, a system controller receiving at least the output indication of liquid pressure within the at least one liquid treatment module, the system controller being operative, upon exceedance of a liquid pressure threshold in the at least one liquid treatment module, representing exceedence of a salinity threshold in the liquid therein, reducing the liquid pressure in the at least one liquid treatment module by performing at least one of the following functions: opening the liquid pressure reducing valve at the brine outlet, thereby reducing the liquid pressure at the brine outlet to a level above atmospheric pressure which exceeds osmotic pressure of the liquid in the at least one liquid treatment module, increasing a liquid volume output of the circulation pump, which removes brine from the at least one liquid treatment module and supplies liquid to the water inlet from a liquid volume output at times other than upon and immediately following the exceedance, equilibrating liquid pressures between a liquid pressure inside the at least one liquid treatment module and inside the liquid feed tank and opening a liquid pressure reducing valve at the liquid inlet, thereby providing a pressure reducing backflow liquid path from the liquid inlet of the at least one liquid treatment module which bypasses the high pressure pump upstream of the liquid inlet. 
         [0017]    In accordance with a preferred embodiment of the present invention the system controller is operative, upon exceedance of a liquid pressure threshold in the at least one liquid treatment module, representing exceedence of a salinity threshold in the liquid therein, reducing the liquid pressure in the at least one liquid treatment module by performing at least two of the following functions: opening the liquid pressure reducing valve at the brine outlet, thereby reducing the liquid pressure at the brine outlet to a level above atmospheric pressure which exceeds osmotic pressure of the liquid in the at least one liquid treatment module, increasing a liquid volume output of the circulation pump, which removes brine from the at least one liquid treatment module and supplies liquid to the water inlet from a liquid volume output at times other than upon and immediately following the exceedance, equilibrating liquid pressures between a liquid pressure inside the at least one liquid treatment module and inside the liquid feed tank and opening a liquid pressure reducing valve at the liquid inlet, thereby providing a pressure reducing backflow liquid path from the liquid inlet of the at least one liquid treatment module which bypasses the high pressure pump upstream of the liquid inlet. 
         [0018]    Alternatively, the system controller is operative, upon exceedance of a liquid pressure threshold in the at least one liquid treatment module, representing exceedence of a salinity threshold in the liquid therein, reducing the liquid pressure in the at least one liquid treatment module by performing at least three of the following functions: opening the liquid pressure reducing valve at the brine outlet, thereby reducing the liquid pressure at the brine outlet to a level above atmospheric pressure which exceeds osmotic pressure of the liquid in the at least one liquid treatment module, increasing a liquid volume output of the circulation pump, which removes brine from the at least one liquid treatment module and supplies liquid to the water inlet from a liquid volume output at times other than upon and immediately following the exceedance, equilibrating liquid pressures between a liquid pressure inside the at least one liquid treatment module and inside the liquid feed tank and opening a liquid pressure reducing valve at the liquid inlet, thereby providing a pressure reducing backflow liquid path from the liquid inlet of the at least one liquid treatment module which bypasses the high pressure pump upstream of the liquid inlet. 
         [0019]    In another alternative embodiment, the system controller is operative, upon exceedance of a liquid pressure threshold in the at least one liquid treatment module, representing exceedence of a salinity threshold in the liquid therein, to reduce the liquid pressure in the at least one liquid treatment module by performing all of the following functions: opening the liquid pressure reducing valve at the brine outlet, thereby reducing the liquid pressure at the brine outlet to a level above atmospheric pressure which exceeds osmotic pressure of the liquid in the at least one liquid treatment module, increasing a liquid volume output of the circulation pump, which removes brine from the at least one liquid treatment module and supplies liquid to the water inlet from a liquid volume output at times other than upon and immediately following the exceedance, equilibrating liquid pressures between a liquid pressure inside the at least one liquid treatment module and inside the liquid feed tank and opening a liquid pressure reducing valve at the liquid inlet, thereby providing a pressure reducing backflow liquid path from the liquid inlet of the at least one liquid treatment module which bypasses the high pressure pump upstream of the liquid inlet. 
         [0020]    There is yet further provided in accordance with still another preferred embodiment of the present invention a water treatment system including at least one water treatment module including at least one membrane and having a feed water inlet at a feed side of the at least one membrane, a permeate outlet at a permeate side of the at least one membrane and a brine outlet at a brine side of at least one membrane, a high pressure pump, which normally maintains a fixed output feed water volume notwithstanding variations in water pressure at an outlet thereof, a system controller receiving at least the output indication of liquid pressure within the at least one liquid treatment module, the system controller being operative, upon exceedance of a liquid pressure threshold in the at least one liquid treatment module, representing exceedence of a salinity threshold in the liquid therein, to reduce the liquid pressure in the at least one liquid treatment module by performing at least one of the following functions: pressurizing feed water supplied to the feed water inlet by employing the high pressure pump, the energy consumption of the high pressure pump being a function of the variations in water pressure at the outlet, monitoring the water pressure at the outlet of the high pressure pump and when a predetermined high pressure threshold is reached at the outlet of the high pressure pump, immediately making changes in the water supply to the at least one liquid treatment module, thereby to cause immediate lowering of the water pressure at the outlet of the high pressure pump, to a pressure below the osmotic pressure at the feed side of part but not all of the at least one liquid treatment module, thereby immediately reducing the energy consumption of the high pressure pump, thereby provide an overall energy cost savings per unit of water treated. 
         [0021]    There is still further provided in accordance with yet another preferred embodiment of the present invention a water treatment system including at least one water treatment module including at least one membrane and having a feed water inlet at a feed side of the at least one membrane, a permeate outlet at a permeate side of the at least one membrane and a brine outlet at a brine side of at least one membrane, a high pressure pump, which normally maintains a fixed output feed water volume notwithstanding variations in water pressure at an outlet thereof, a pressure reducing valve downstream of the high pressure feed pump, a circulation pump, a system controller receiving at least the output indication of liquid pressure within the at least one liquid treatment module, the system controller being operative, upon exceedance of a liquid pressure threshold in the at least one liquid treatment module, representing exceedence of a salinity threshold in the liquid therein, to reduce the liquid pressure in the at least one liquid treatment module by performing at least one of the following functions: upon occurrence of an operational threshold representing exceedence of a salinity threshold in the at least one liquid treatment module, effecting the following: removing brine from the at least one liquid treatment module and reducing, at an enhanced speed, pressure of the liquid in the at least one liquid treatment module by at least one of the following: opening the pressure reducing valve downstream of the high pressure feed pump, increasing the volume output of the circulation pump from its volume output when functioning as a concentrate circulation pump to a higher volume output when functioning as a feed water pump and passing liquid from downstream of the high pressure feed pump to upstream of the high pressure feed pump. 
         [0022]    Preferably, the water treatment system also includes a flow restrictor arranged in parallel to the high pressure feed pump and the passing liquid from downstream of the high pressure feed pump to upstream of the high pressure feed pump includes passing the liquid via the flow restrictor. 
         [0023]    In accordance with a preferred embodiment of the present invention the pressure of the liquid in the at least one liquid treatment module is reduced at an enhanced speed by at least two of the following: opening the pressure reducing valve downstream of the high pressure feed pump, increasing the volume output of the circulation pump from its volume output when functioning as a concentrate circulation pump to a higher volume output when functioning as a feed water pump and passing liquid from downstream of the high pressure feed pump to upstream of the high pressure feed pump. Alternatively, the pressure of the liquid in the at least one liquid treatment module is reduced at an enhanced speed by at least three of the following: opening the pressure reducing valve downstream of the high pressure feed pump, increasing the volume output of the circulation pump from its volume output when functioning as a concentrate circulation pump to a higher volume output when functioning as a feed water pump and passing liquid from downstream of the high pressure feed pump to upstream of the high pressure feed pump. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
           [0025]      FIG. 1  is a simplified illustration of a system for water treatment constructed and operative in accordance with a preferred embodiment of the present invention; 
           [0026]      FIGS. 2A and 2B  are simplified illustrations of examples of periodic variations in feed water pressure and osmotic pressure in embodiments of the system of  FIG. 1 , each showing a distinction from the prior art; 
           [0027]      FIGS. 3A, 3B, 3C and 3D  are simplified illustrations of liquid flows in the system of  FIG. 1  at various stages in the periodic variation in feed water pressure and osmotic pressure shown in  FIG. 2A  in accordance with one embodiment of the present invention; 
           [0028]      FIGS. 4A, 4B, 4C and 4D  are simplified illustrations of liquid flows in the system of  FIG. 1  at various stages in the periodic variation in feed water pressure and osmotic pressure shown in  FIG. 2A  in accordance with another embodiment of the present invention; 
           [0029]      FIGS. 5A, 5B and 5C  are simplified illustrations of liquid flows in the system of  FIG. 1  at various stages in the periodic variation in feed water pressure and osmotic pressure shown in  FIG. 2A  in accordance with still another embodiment of the present invention; 
           [0030]      FIGS. 6A, 6B, 6C and 6D  are simplified illustrations of liquid flows in the system of  FIG. 1  at various stages in the periodic variation in feed water pressure and osmotic pressure shown in  FIG. 2B  in accordance with yet another embodiment of the present invention; 
           [0031]      FIGS. 7A, 7B, 7C and 7D  are simplified illustrations of liquid flows in the system of  FIG. 1  at various stages in the periodic variation in feed water pressure and osmotic pressure shown in  FIG. 2B  in accordance with a further embodiment of the present invention; 
           [0032]      FIGS. 8A, 8B, 8C and 8D  are simplified illustrations of liquid flows in the system of  FIG. 1  at various stages in the periodic variation in feed water pressure and osmotic pressure shown in  FIG. 2B  in accordance with still a further embodiment of the present invention; 
           [0033]      FIGS. 9A, 9B, 9C and 9D  are simplified illustrations of liquid flows in the system of  FIG. 1  at various stages in the periodic variation in feed water pressure and osmotic pressure shown in  FIG. 2A  in accordance with yet a further embodiment of the present invention; 
           [0034]      FIGS. 10A, 10B, 10C and 10D  are simplified illustrations of liquid flows in the system of  FIG. 1  at various stages in the periodic variation in feed water pressure and osmotic pressure shown in  FIG. 2A  in accordance with an alternative embodiment of the present invention; and 
           [0035]      FIGS. 11A, 11B, 11C and 11D  are simplified illustrations of liquid flows in the system of  FIG. 1  at various stages in the periodic variation in feed water pressure and osmotic pressure shown in  FIG. 2B  in accordance with a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0036]    Reference is now made to  FIG. 1 , which is a simplified illustration of a liquid treatment system constructed and operative in accordance with a preferred embodiment of the present invention, and to  FIGS. 2A and 2B , which are simplified time-line illustrations of examples of the operation of the system of  FIG. 1 , wherein seawater is being desalinated. 
         [0037]    The liquid treatment system of  FIG. 1  comprises at least one liquid treatment module, preferably water treatment module  100 , comprising reverse osmosis membranes and/or nanofiltration membranes. The system of  FIG. 1  is operative for treatment of liquid to be treated, such as feed water, which may be, for example, sea water, brackish water or waste water. 
         [0038]    Water treatment module  100  is described in applicant&#39;s U.S. patent application Ser. No. 13/603,028, filed Sep. 4, 2012 and entitled SYSTEM AND METHOD FOR DESALINATION OF WATER, and published as U.S. Published Patent Application No. 2014/0061129 on Mar. 6, 2014, the disclosure of which is hereby incorporated by reference. FIG. 2A in U.S. patent application Ser. No. 13/603,028 illustrates a water treatment module, here designated by reference numeral  100 . 
         [0039]    For the purposes of the description which follows, the following definitions will be employed: 
         [0040]    feed water—water to be treated by the system, such as saline solution, sea water, brackish water or waste water; 
         [0041]    mixed feed water—water supplied to water treatment module  100 , which may include feed water and water that was previously treated in the water treatment module  100  and is being resupplied to the water treatment module for further treatment; 
         [0042]    module feed side water—water at a feed side, as distinguished from a permeate side, of module  100 . The salinity of the module feed side water increases as the module feed side water passes through the module from an initial salinity, which represents the salinity of the mixed feed water, to a concentrate salinity, which represents the salinity of the concentrate output at an output of the feed side, as distinguished from the permeate side, of module  100 . 
         [0043]    It is an inherent feature of water treatment modules  100  that the osmotic pressure at the feed side thereof increases over time as the salinity at the feed side increases until such time as the salinity at the feed side is reduced, thereby reducing the osmotic pressure. As described hereinbelow, the salinity of the mixed feed water at the feed side is preferably reduced by supplying only feed water instead of a mixture of feed water and recirculated concentrate. 
         [0044]    An increase in osmotic pressure requires a corresponding increase in water pressure at the feed side of the module  100  in order to maintain permeate production. This increase is provided automatically by a high pressure pump  102 , preferably operative to pressurize the liquid to be treated to typical pressures of approximately 15 bar for brackish water and up to approximately 70 bar for sea water. 
         [0045]    High pressure pump  102  may be any suitable type of pump, such as a positive displacement pump. An example of a preferred positive displacement pump is a Danfoss APP 21-43 high pressure pump, commercially available from Danfoss A/S Nordborgvej 81, 6430 Nordborg, Denmark. High pressure pump  102  is preferably controlled by the output of a feed water flow rate sensor  104  upstream of pump  102 , which is received by a pump controller  106 , preferably an ABB ACS800-U1 controller, commercially available from ABB Inc. MS 3L7 29801 Euclid Ave, Wickliffe, Ohio 44092-2530, USA. 
         [0046]    Preferably, a mixed feed water pressure sensor  108  is provided at the feed side of module  100 . A typical graph of mixed feed water pressure as measured by pressure sensor  108  over time and verses the osmotic pressure at the feed side of the module  100  appears in an enlargement forming part of  FIG. 1  and in  FIGS. 2A and 2B . Alternatively or additionally, a water pressure sensor may be provided at a suitable location within module  100  or at a module outlet for measuring the pressure of liquid flowing therethrough. 
         [0047]    The variation of feed water pressure over time typically has a periodicity of a few minutes, typically between 3-30 minutes in seawater desalination and possibly longer in brackish water desalination. 
         [0048]    As seen in  FIG. 1 , water treatment module includes a plurality of pressure vessels  110  arranged in parallel. Each pressure vessel  110  preferably includes a plurality of membrane elements  112 , typically eight in number, only four being shown in the drawing for the sake of conciseness. Pressure vessels  110  are commercially available from various vendors, for example BEL Composite Industries Ltd, Industrial Zone, Kiryat Yehudit, P.O.B. 4, 84100 Beer Sheva, Israel, and membrane elements  112  are commercially available from various vendors, for example LG NanoH2O, 750 Lairport Street, El Segundo, Calif. 90245. 
         [0049]    Liquid to be treated is supplied at a liquid feed inlet and is pressurized by high pressure pump  102 , preferably operative to pressurize the liquid to be treated to typical pressures of approximately 15 bar for brackish water and up to approximately 70 bar for sea water. 
         [0050]    The liquid to be treated, hereinafter referred to as feed water, wherein the definition of “feed water” encompasses, inter alia, saline solution, brackish water, sea water and waste water, is supplied via a feed manifold  114  to the parallel pressure vessels  110 . Treated water, hereinafter referred to as permeate, wherein the definition of “permeate” encompasses, inter alia, “product water”, from each of pressure vessels  110 , is preferably supplied via a permeate manifold  116  to a permeate outlet  118 . 
         [0051]    Concentrate from each of pressure vessels  110  is preferably supplied via a concentrate manifold  120  to a recirculation conduit  122 , which directs the concentrate back to feed manifold  114 , downstream of pump  102 , via a recirculation control valve  124  by employing a circulation pump  126 . A concentrate pressure sensor  128 , a concentrate conductivity sensor  129  and a concentrate flow rate sensor  130  are preferably provided downstream of concentrate manifold  120 . 
         [0052]    Concentrate from module  100  may also be provided from pressure vessels  110  via concentrate manifold  120  to a brine outlet  132  via a brine outlet control valve  134  for flushing. Concentrate that exits module  100  and is not recirculated is referred to herein as brine. Preferably, the salinity of the brine that is flushed is greater than the salinity of the concentrate that is recirculated via conduit  122 . 
         [0053]    In some embodiments of the present invention, brine from each of pressure vessels  110  may be supplied from concentrate manifold  120  via an auxiliary brine replacement conduit  136 , an auxiliary tank feed conduit  138  and an auxiliary brine replacement control valve  140  to an auxiliary feed water tank  142 . Preferably, the salinity of the brine supplied to auxiliary feed water tank  142  exceeds the salinity of the concentrate that is recirculated via conduit  122 . 
         [0054]    It is appreciated that, as described further hereinbelow, the decision to recirculate the liquid as concentrate or to flush the liquid as brine may be a function of the salinity of the liquid, a function of the pressure of the liquid, a function of a rate of accumulation of foulants on membrane elements, a system energy efficiency rating, or may be based on a predetermined time schedule or any other suitable method. Additionally or alternatively, the selection of the threshold may be predetermined to be a suitable threshold which will avoid or minimize precipitation of foulants on the membrane elements  112  in module  100 . 
         [0055]    During the flushing of brine, the recirculation control valve  124  is closed. The auxiliary feed water tank  142  is preferably filled, prior to the opening of auxiliary brine replacement control valve  140 , with feed water by an auxiliary feed water pump  144 . Brine driven by circulation pump  126  drives the feed water from auxiliary feed water tank  142  to feed manifold  114  via an auxiliary feed water conduit  146  and an auxiliary feed water control valve  148 . An auxiliary water flow sensor  150  is provided upstream of auxiliary feed water tank  142 . After full replacement of brine by feed water in module  100 , the recirculation control valve  124  is opened and auxiliary brine replacement control valve  140  and auxiliary feed water control valve  148  are closed. Then auxiliary feed water pump  144  fills auxiliary feed water tank  142  with feed water, which drives the brine to an auxiliary brine outlet  152  via an auxiliary brine outlet tank control valve  154 . 
         [0056]    It is appreciated that, alternatively, elements  136 - 154  may be obviated. 
         [0057]    In some embodiments of the present invention, water pressure at the feed side of the module  100  may be quickly reduced at desired points in time by operation of a recycle conduit control valve  156  to redirect feed water from downstream of high pressure feed pump  102  to upstream of pump  102  through a conduit  158  and preferably through a flow restrictor  160 , which limits the pressure reduction to a pressure above atmospheric pressure, which pressure exceeds the osmotic pressure of the feed water at the feed side of module  100 . 
         [0058]    In accordance with a preferred embodiment of the present invention there is provided a Feed Pressure Management (FPM) Controller  162 , which controls the operation of the high pressure feed pump  102 , circulation pump  126 , auxiliary feed water pump  144 , recirculation control valve  124 , brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148 , auxiliary brine outlet tank control valve  154 , and recycle conduit control valve  156  for adjusting the pressure at which desalination takes place at the at least one water treatment module. 
         [0059]    As seen in  FIGS. 2A and 2B , which will be described in detail hereinbelow, and refer to examples wherein seawater is being desalinated, periodic variations in the feed water pressure during water treatment correspond to periodic variations in the osmotic pressure, which corresponds to the salinity of the feed water supplied to module  100 , as measurable by a conductivity sensor (not shown) typically downstream of pressure sensor  108 . 
         [0060]    Control over the variation of the feed water pressure may be achieved in various ways, such as according to the flow rate measured by flow sensor  104  and, alternatively or additionally, according to the salinity of the water being supplied to the feed side of the module  100 , which may include feed water received from pump  102 , recirculated water received from recirculation conduit  122 , auxiliary feed water received via auxiliary conduit  146  and combinations thereof. 
         [0061]    Alternatively, the feed pressure may be varied in accordance with a predetermined time schedule. As a further alternative, the desired feed pressure may be reached by employing recycle conduit  158  with or without flow restrictor  160 . Other alternative methodologies for control over the variation of the feed pressure may be employed. 
         [0062]    FPM controller  162  is operative to periodically open and close recirculation control valve  124 , brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148 , auxiliary brine outlet tank control valve  154  and recycle conduit control valve  156  in accordance with a predetermined time schedule or alternatively, for example, in response to either sensed salinity of the concentrate, for example as per an output of sensor  129 , or exceedance of a predetermined maximum feed pressure threshold, for example as per an output of sensor  108  or sensor  128 . 
         [0063]    FPM controller  162  is also preferably operative to periodically activate auxiliary feed water pump  144  and may also be operative to change the flow rate of circulation pump  126 . Other alternative algorithms for control of opening and closing recirculation control valve  124 , brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148 , auxiliary brine outlet tank control valve  154  and recycle conduit control valve  156 , and for control of the operation of the high pressure feed pump  102 , circulation pump  126  and auxiliary feed water pump  144  may be employed. 
         [0064]    In some embodiments of the invention, once the concentration of the concentrate increases to a predetermined level at which continued water treatment is deemed not to be practicable, the FPM controller  162  opens auxiliary brine replacement control valve  140  and allows brine to flow from concentrate manifold  120 , via auxiliary brine replacement conduit  136  and auxiliary tank feed water conduit  138 , to auxiliary feed water tank  142 . In some embodiments of the invention, FPM controller  162  also closes recirculation control valve  124  at about the same time. The volume of the brine flowing out of the system may be measured by concentrate flow rate sensor  130 . Feed water that was in auxiliary feed water tank  142  is driven by circulation pump  126  to flow via auxiliary feed water conduit  146  and auxiliary feed water control valve  148  to feed manifold  114 . Feed water, having salinity which is significantly lower than that of the brine, thus enters module  100 . 
         [0065]    Various methodologies for ensuring that the feed water pressure is above atmospheric pressure and above the osmotic pressure of the feed water at the feed side of module  100  at all times are described hereinbelow with reference to  FIGS. 2A &amp; 2B  and to  FIGS. 3A-11D . Ensuring that the feed water pressure remains above atmospheric pressure and above the osmotic pressure of the feed water at the feed side of module  100  prevents overshooting of the feed pressure when feed water from high pressure feed pump  102  enters the module  100 , which overshooting commonly occurs in prior art systems. 
         [0066]    It is appreciated that the term ‘overshooting’ as used herein refers to operating the high pressure feed pump  102  at an excessively high pressure relative to the osmotic pressure of the feed water, which is typically caused when the controller causes the system to supply feed water, as from auxiliary feed water tank  142 , instead of recirculated concentrate to modules  100 , without modifying the pressure of feed water supplied via high pressure feed pump  102 . The operation of the high pressure feed pump  102  at an excessively high pressure relative to the osmotic pressure of the feed water, increases the energy required to operate the system. 
         [0067]    In the description which follows, it is to be appreciated that the pressure values given for the various embodiments described hereinbelow and shown in  FIGS. 2A &amp; 2B  are values associated with membrane sea water desalination. Different pressure values will apply to desalination of brackish water and other types of feed water. 
         [0068]    In steady state normal operation of the system, prior to initiation of a periodic process of replacing the concentrate in the water treatment module  100  with feed water, brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148 , auxiliary brine outlet tank control valve  154  and recycle conduit control valve  156  are all closed and recirculation control valve  124  is open. 
         [0069]    In this steady state normal operation, concentrate from concentrate manifold  120  is directed back to the input of feed manifold  114  via recirculation conduit  122  and recirculation control valve  124 , as shown in  FIG. 1  by an arrow labeled CONCENTRATE representing the recirculation flow in recirculation conduit  122 . In feed manifold  114 , the concentrate is mixed with feed water, as shown in  FIG. 1  by an arrow labeled MIXED, representing the mixed flow in the feed manifold  114 , and the mixed flow enters pressure vessels  110  of module  100  for treatment. The above-described flow in steady state normal operation is represented in solid black lines in  FIGS. 3A, 4A, 5A, 6A, 7A, 8A, 9A, 10A and 11A . 
         [0070]    The feed pressure gradually increases as the salinity of the mixed water being supplied to the membrane elements  112  increases. Once the concentration of the concentrate increases to a predetermined level at which continued water treatment is deemed not to be practicable, the periodic process of replacing the concentrate in the water treatment module  100  with feed water is initiated.  FIGS. 3A-11D  illustrate various techniques for replacing the concentrate in the water treatment module  100  with feed water at a pressure above atmospheric pressure, which exceeds the osmotic pressure of the feed water at the feed side of module  100  without causing overshooting. 
         [0071]    Typically, the predetermined level of concentrate concentration deemed not to be practicable to continue treating, is based on one of a number of operational considerations, such as rate of accumulation of foulants and energy efficiency. 
         [0072]    Reference in now made to  FIG. 2A , which illustrates the periodic variations in mixed feed water pressure as measured by pressure sensor  108 , and in the mixed feed water osmotic pressure estimated to exist at pressure sensor  108  when circulation pump  126  is operating at a normal pumping rate. The estimated mixed feed water osmotic pressure at pressure sensor  108  is generally a function of the salinity of the water at the feed side of the membranes  112  in module  100 . 
         [0073]    Periodic variations in the mixed feed water pressure during water treatment correspond to periodic variations in the salinity of the feed water entering module  100 , as measurable by the conductivity sensor, typically located downstream of pressure sensor  108 . The y-axis represents pressure in seawater desalination, and the x-axis represent time. Dashed vertical lines ‘A’ represent points in time where a threshold, such as a maximum feed water pressure threshold, is reached. 
         [0074]    When the threshold is reached, pressure vessels  110  are still filled with concentrate, whose salt concentration continues to increase as it moves through the pressure vessel. Therefore, both mixed feed water osmotic pressure and mixed feed water pressure continue to increase, until feed water enters the pressure vessels  110  replacing the concentrate therein. Typically, reaching the threshold causes the system to begin the brine flushing process. 
         [0075]    In  FIGS. 2A and 2B , the mixed feed water osmotic pressure line  170  represents the estimated osmotic pressure of the mixed feed water. Thus, when feed water from pump  102  is mixed with concentrate from recirculation conduit  122 , the mixed osmotic pressure gradually increases as seen in the gradual slope of line  170 . Once a threshold, such as a salinity threshold or a pressure threshold, is reached the controller  162  preferably begins the flushing process. During the flushing process, concentrate is not recirculated back to feed manifold  114 , thus only feed water enters feed manifold  114  and the mixed osmotic pressure decreases sharply, as shown in the sharp decline in line  170 . 
         [0076]    Line  175  in  FIG. 2A  illustrates the behavior of the mixed feed water pressure in the prior art, represented by the teaching of U.S. Pat. No. 8,025,804. Line  180  illustrates the mixed feed water pressure being controlled by the controller such that the mixed feed water pressure needed for reverse osmosis desalination process is maintained not only during the gradual increase of mixed feed water pressure, when concentrate is being recirculated back to feed manifold  114 , but also at times of flushing the brine in pressure vessels  110  by feeding only feed water without recirculated concentrate. The difference between line  175  and line  180  illustrates the energy benefit of operation at lower pressures for desalination of sea water, thus saving energy. 
         [0077]    In both  FIGS. 2A and 2B , line  190  illustrates the osmotic pressure of the module feed side water. 
         [0078]    As described further hereinbelow,  FIG. 2B  is similar to  FIG. 2A , except that the values are provided for the embodiment where circulation pump  126  is operating at a higher than normal pumping rate, generating a higher flow. 
         [0079]    Reference is now made to  FIGS. 3A-3D , which are simplified illustrations of water flows in a first embodiment of a water treatment system of the type shown in  FIG. 1 . 
         [0080]      FIG. 3A  shows the flow during normal steady state operation of the system in solid black. 
         [0081]      FIG. 3B  shows, in solid black lines, the flow that takes place, once the concentration of the concentrate increases to a predetermined level. At this stage, the FPM controller  162  opens auxiliary brine replacement control valve  140 , closes recirculation control valve  124  and opens auxiliary feed water control valve  148 . Brine from concentrate manifold  120  flows through auxiliary brine replacement conduit  136  and auxiliary brine replacement control valve  140  via auxiliary tank feed conduit  138  to auxiliary feed water tank  142 . The auxiliary feed water tank  142  is filled with feed water prior to the opening of auxiliary brine replacement control valve  140 , as described hereinbelow. The brine enters the auxiliary feed water tank  142  and drives feed water from tank  142  to feed manifold  114  via auxiliary feed water conduit  146  and auxiliary feed water control valve  148 . 
         [0082]    It is noted that water in auxiliary feed water tank  142  may be maintained at the same pressure as that of the brine, such as by maintaining auxiliary brine replacement control valve  140  in an open state as the water pressure in the system gradually increases. Alternatively, the water in the auxiliary feed water tank  142  may be maintained at a pressure which is much lower than the pressure of the brine but above atmospheric pressure by operation of auxiliary feed water pump  144 , as described hereinbelow. 
         [0083]    During flushing, FPM controller  162  preferably opens recycle conduit control valve  156 , resulting in a water flow from a location downstream of pump  102  to a location upstream of pump  102 , optionally thorough a restrictor  160 , thus lowering the feed water pressure at manifold  114  to a pressure above atmospheric pressure, which pressure exceeds the osmotic pressure of the feed water at the feed side of module  100 . 
         [0084]    Concentrate flow rate sensor  130  measures the cumulative volume of brine flowing from concentrate manifold  120  and thus measures the cumulative volume of feed water entering feed manifold  114  via auxiliary feed water conduit  146  and auxiliary feed water control valve  148 , which replaces the brine in module  100 . 
         [0085]    After complete replacement of brine with feed water in module  100 , FPM controller  162  reopens recirculation control valve  124  and closes auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156 , providing a liquid flow as shown in  FIG. 3C , which may be identical to the liquid flow illustrated in  FIG. 3A , in which the operation of circulation pump  126  and of high pressure pump  102  supplies mixed feed water to module  100 . 
         [0086]    Thereafter, FPM controller  162  periodically activates auxiliary feed water pump  144  and opens auxiliary brine outlet tank control valve  154  to flush all brine from auxiliary feed water tank  142  through an auxiliary brine outlet  152  to a location outside of the at least one water treatment system, and to fill the auxiliary feed tank  142  with feed water for future replacement of the brine in module  100 . This flow is shown in solid black lines in  FIG. 3D . 
         [0087]    Following full replacement of brine with feed water in auxiliary feed water tank  142 , as measured by auxiliary flow sensor  150 , FPM controller  162  closes auxiliary brine outlet tank control valve  154  and terminates operation of auxiliary feed water pump  144 . 
         [0088]    Reference is now made to  FIGS. 4A-4D , which are simplified illustrations of water flows in a second embodiment of a water treatment system of the type shown in  FIG. 1 . 
         [0089]    Prior to initiation of removal of the concentrate from the module  100 , brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156  are closed and recirculation control valve  124  is open. The concentrate from concentrate manifold  120  is directed back to input of feed manifold  114  via recirculation conduit  122  and recirculation control valve  124 , as shown by an arrow labeled CONCENTRATE ( FIG. 1 ), representing the recirculation flow in the recirculation conduit  122 . In feed manifold  114 , the concentrate is mixed with feed water, as shown by an arrow labeled MIXED ( FIG. 1 ), representing the mixed flow in the feed manifold  114 . Thus, a mixed flow enters pressure vessels  110  for further treatment. The water flow for this stage is shown in a solid black line in  FIG. 4A . 
         [0090]    The feed pressure thereafter gradually increases, as the salinity of the mixed water being supplied to membrane elements  112  increases, and the above-described recirculation process continues. 
         [0091]    Once the concentration of the concentrate reaches a threshold, such as a predetermined salinity level at which continued water treatment is deemed not to be practicable, FPM controller  162  opens auxiliary brine replacement control valve  140 , which is approximately at atmospheric pressure, thus reducing the water pressure within module  100  to a pressure between the pressure of the concentrate at module  100  and the pressure of the feed water in the auxiliary feed tank  142 . 
         [0092]    Immediately thereafter, FPM controller  162  closes recirculation control valve  124  and opens auxiliary feed water control valve  148 . Feed water from auxiliary feed tank  142  flows via auxiliary feed water conduit  146  and feed water control valve  148  to feed manifold  114 , as shown in  FIG. 4B , thus supplying feed water to module  100  at a pressure above the osmotic pressure of the feed water and slightly higher than the pressure required for reverse osmosis to occur. 
         [0093]    Concentrate flow rate sensor  130  measures the cumulative volume of brine flowing from concentrate manifold  120 , and thus measures the cumulative volume of feed water entering feed manifold  114  via the auxiliary feed water conduit  146  and the auxiliary feed water control valve  148 , which replaces the brine in module  100 . 
         [0094]    After full replacement of the brine with feed water in module  100 , FPM controller  162  reopens recirculation control valve  124 , and closes auxiliary brine replacement control valve  140  and auxiliary feed water control valve  148 , providing a liquid flow as shown in  FIG. 4C , which may be identical to the liquid flow illustrated in  FIG. 4A , in which the operation of high pressure pump  102  and the circulation pump  126  supplies mixed feed water to module  100 . 
         [0095]    Thereafter, the FPM controller  162  periodically activates the auxiliary feed water pump  144  and opens auxiliary brine outlet tank control valve  154  to flush all brine from auxiliary feed water tank  142  through an auxiliary brine outlet  152  to a location outside of the at least one water treatment system and fill the auxiliary feed tank  142  with feed water for further replacement of the brine in module  100  with feed water as described hereinabove, as seen in  FIG. 4D . Following full replacement of brine with feed water in auxiliary feed water tank  142 , as measured by auxiliary flow sensor  150 , FPM controller  162  closes auxiliary brine outlet tank control valve  154  and terminates operation of auxiliary feed water pump  144 . 
         [0096]    Reference is now made to  FIGS. 5A-5C , which are simplified illustrations of water flows in another embodiment of a water treatment system of the type shown in  FIG. 1 . 
         [0097]    Prior to initiation of removal of the concentrate from module  100 , brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156  are closed and recirculation control valve  124  is opened. The concentrate from concentrate manifold  120  is directed back to the input of feed manifold  114  via recirculation conduit  122  and recirculation control valve  124 , as shown by an arrow labeled CONCENTRATE ( FIG. 1 ), representing the recirculation flow in the recirculation conduit  122 . In feed manifold  114 , the concentrate is mixed with feed water, as shown by an arrow labeled MIXED ( FIG. 1 ) representing the mixed flow in the feed manifold  114 . Thus, a mixed flow enters pressure vessels  110  for further treatment. The water flow for this stage is shown in a solid black line in  FIG. 5A . 
         [0098]    The feed pressure thereafter gradually increases as the salinity of the mixed water being supplied to the membrane elements  112  increases, and the above-described recirculation process continues. 
         [0099]    Once the concentration of the concentrate reaches a threshold, such as a predetermined salinity level at which continued water treatment is deemed not to be practicable, FPM controller  162  preferably closes concentrate recirculation control valve  124  and at least partially opens brine outlet control valve  134 , thus allowing brine to flow via brine outlet  132  to a location outside of the at least one water treatment system, thus reducing the system pressure to a pressure above the atmospheric pressure, which pressure exceeds the osmotic pressure of the feed water at the feed side of module  100 . The brine is all flushed through brine outlet  132  from the at least one water treatment system, while high pressure feed pump  102  continues to pump feed water to feed side of module  100 , as shown in  FIG. 5B . It is possible to increase the flow rate of pump  102  at this stage in order to speed the flushing of the brine. 
         [0100]    Concentrate flow rate sensor  130  measures the cumulative volume of brine flowing from concentrate manifold  120 , and thus measures when the full flushing of the brine out of module  100  has been completed. 
         [0101]    After fully flushing the brine from module  100 , FPM controller  162  reopens recirculation control valve  124  and closes brine outlet control valve  134 , providing a liquid flow as shown in  FIG. 5C , which may be identical to the liquid flow illustrated in  FIG. 5A , in which the operation of high pressure pump  102  and the circulation pump  126  supplies mixed feed water to module  100 . 
         [0102]    Reference is now made to  FIGS. 6A-6D , which are simplified illustrations of water flows in yet another embodiment of a water treatment system of the type shown in  FIG. 1 . 
         [0103]    Prior to initiation of removal of the concentrate from module  100 , brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156  are closed and recirculation control valve  124  is open. The concentrate from concentrate manifold  120  is directed back to the input of feed manifold  114  via recirculation conduit  122  and recirculation control valve  124 , as shown by an arrow labeled CONCENTRATE ( FIG. 1 ), representing the recirculation flow in the recirculation conduit  122 . In feed manifold  114 , the concentrate is mixed with feed water, as shown by an arrow labeled MIXED ( FIG. 1 ), representing the mixed flow in the feed manifold  114 . Thus, a mixed flow enters pressure vessels  110  for further treatment. The water flow for this stage is shown in a solid black line in  FIG. 6A . 
         [0104]    The feed pressure thereafter gradually increases, as the salinity of the mixed liquid being supplied to the membrane elements  112  increases, and the above-described recirculation process continues. 
         [0105]    Once the concentration of the concentrate reaches a threshold, such as a predetermined pressure level, FPM controller  162  opens auxiliary brine replacement control valve  140 , closes recirculation control valve  124  and opens auxiliary feed water control valve  148 . Brine from concentrate manifold  120  flows through auxiliary brine replacement conduit  136 , auxiliary brine replacement control valve  140  and auxiliary tank feed conduit  138  to auxiliary feed water tank  142 . 
         [0106]    The auxiliary feed water tank  142  is filled with feed water prior to the opening of brine replacement control valve  140 , as described hereinbelow. Brine entering the auxiliary feed water tank  142  drives feed water in it to feed manifold  114  via an auxiliary feed water conduit  146  and an auxiliary feed water control valve  148 . 
         [0107]    The water flow for this stage is shown in a solid black line in  FIG. 6B . 
         [0108]    It is appreciated that the water in auxiliary feed water tank  142  may be maintained at a pressure generally the same as the pressure of the brine, such as by maintaining auxiliary opening brine replacement control valve  140  in an open state as the pressure in the system gradually increases. Alternatively, the water in the auxiliary feed water tank  142  may be maintained at a pressure which is much lower than the pressure of the brine but above the atmospheric pressure by operation of the auxiliary feed water pump  144  as described hereinbelow. 
         [0109]    In the embodiment of  FIGS. 6A-6D , FPM controller  162  preferably increases the flow rate of circulation pump  126  during flushing to achieve faster replacement of the brine from module  100  with feed water from auxiliary feed tank  142 , hence, reducing the time required for the brine to be flushed from module  100  and replaced by feed water. A typical graph of flow rate over time, as measured by concentrate flow rate sensor  130 , located downstream of circulation pump  126 , appears in an enlargement forming part of  FIG. 6B , in which line  200  represents the flow rate over time. 
         [0110]    Concentrate flow rate sensor  130  measures the cumulative volume of brine flowing from concentrate manifold  120  and thus measures the cumulative volume of feed water entering feed manifold  114  via the auxiliary feed water conduit  146  and the auxiliary feed water control valve  148 , which replaces the brine in module  100 . 
         [0111]    After complete replacement of the brine with feed water in module  100 , FPM controller  162  reopens recirculation control valve  124 , closes auxiliary brine replacement control valve  140  and auxiliary feed water control valve  148 , and reduces the flow rate of circulation pump  126  to the flow rate prior to opening concentrate recirculation control valve  124 , providing a liquid flow as shown in  FIG. 6C , which may be identical to the liquid flow illustrated in  FIG. 6A , in which the operation of high pressure pump  102  and the circulation pump  126  supplies mixed feed water to module  100 . 
         [0112]    Thereafter, FPM controller  162  periodically activates auxiliary feed water pump  144  and opens auxiliary brine outlet tank control valve  154  to flush all brine from auxiliary feed water tank  142  through an auxiliary brine outlet  152  to a location outside of the at least one water treatment system, and fill the auxiliary feed tank  142  with feed water for further replacement of the brine in module  100  with feed water as described hereinabove, as seen in  FIG. 6D . 
         [0113]    Following full replacement of brine with feed water in auxiliary feed water tank  142 , as measured by auxiliary flow rate sensor  150 , FPM controller  162  closes auxiliary brine outlet tank control valve  154  and terminates operation of auxiliary feed water pump  144 . 
         [0114]    Reference in now additionally made to  FIG. 2B , wherein periodic variations in the feed water pressure during water treatment correspond to periodic variations in the salinity of the feed water entering module  100 , as measurable by a conductivity sensor (not shown), typically located downstream of pressure sensor  108 . As in  FIG. 2A  described hereinabove, the y-axis represents the pressure variations in seawater desalination, and the x-axis represent the time. Dashed vertical line ‘A’ represents the time where a threshold, such as a maximum feed water pressure threshold, is reached 
         [0115]    When the threshold is reached, pressure vessels  110  are still filled with concentrate and both mixed osmotic pressure and mixed feed water pressure continue to increase, until feed water enters the pressure vessels  110  replacing the concentrate therein. Typically, reaching the threshold causes the system to begin the brine flushing process. 
         [0116]    As described hereinabove, the mixed feed water osmotic pressure line  170  represents the estimated osmotic pressure of mixed feed water at module  100  inlet. Thus, when feed water from pump  102  is mixed with concentrate from recirculation conduit  122 , the mixed osmotic pressure gradually increases as seen in the gradual slope of line  170 . Once a threshold, such as a salinity threshold or a pressure threshold, is reached the controller  162  preferably begins the flushing process. During the flushing process, concentrate is not recirculated back to feed manifold  114 , thus only feed water enters feed manifold  114  and the mixed osmotic pressure decreases sharply, as shown in the sharp decline in line  170 . 
         [0117]    Line  175  in  FIG. 2B  illustrates the behavior of the mixed feed water pressure in the prior art, represented by the teaching of U.S. Pat. No. 8,025,804. Line  180  illustrates the pressure being controlled by the controller such that the delta pressure needed for reverse osmosis desalination process is maintained not only during the gradual increase of mixed feed water pressure, when concentrate is being recirculated back to feed manifold  114 , but also at times of flushing the brine in pressure vessels  110  by feeding only feed water without recirculated concentrate. As seen in  FIG. 2B , increasing of flow rate of circulation pump  126 , vis-à-vis the flow rate of circulation pump  126  used in the example of  FIG. 2A , causes the mixed osmotic pressure line  170 , as well as the mixed feed water pressure line  180 , to decrease even more rapidly than in the example shown in  FIG. 2A . In  FIG. 2B , the increased difference between line  175  and line  180 , vis-à-vis the difference between the two lines in  FIG. 2A , illustrates the increased benefit of operation at lower pressures when the flow rate is increased, thus saving additional energy beyond the additional energy required to operate the circulation pump  126  at a higher flow rate. 
         [0118]    Reference is now made to  FIGS. 7A-7D , which are simplified illustrations of water flows in a further embodiment of a water treatment system of the type shown in  FIG. 1 . 
         [0119]    Prior to initiation of removal of the concentrate from module  100 , brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156  are closed and recirculation control valve  124  is open. The concentrate from concentrate manifold  120  is directed back to the input of feed manifold  114  via recirculation conduit  122  and recirculation control valve  124 , as shown by an arrow labeled CONCENTRATE ( FIG. 1 ), representing the recirculation flow in the recirculation conduit  122 . In feed manifold  114 , the concentrate is mixed with feed water, as shown by an arrow labeled MIXED ( FIG. 1 ), representing the mixed flow in the feed conduit  114 . Thus, a mixed flow enters pressure vessels  110  for further treatment. The water flow for this stage is shown in a solid black line in  FIG. 7A . 
         [0120]    The feed pressure thereafter gradually increases as the salinity of the mixed water being supplied to the membrane elements  112  increases, and the above-described recirculation process continues. 
         [0121]    Once the concentration of the concentrate reaches a threshold, such as a predetermined salinity level at which continued water treatment is deemed not to be practicable, FPM controller  162  opens auxiliary brine replacement control valve  140 , closes recirculation control valve  124  and opens auxiliary feed water control valve  148 . The FPM controller also increases flow rate produced by circulation pump  126 . Brine from concentrate manifold  120  flows through auxiliary brine replacement conduit  136 , auxiliary tank feed conduit  138  and auxiliary brine replacement control valve  140  to auxiliary feed water tank  142 . The auxiliary feed water tank  142  is filled with feed water prior to the opening of auxiliary brine replacement control valve  140 , as described hereinbelow. The brine enters the auxiliary feed water tank  142  and drives feed water from tank  142  to feed manifold  114  via auxiliary feed water conduit  146  and auxiliary feed water control valve  148 . It is appreciated that water in auxiliary feed water tank  142  may be at the same pressure as the pressure of the brine, such as by maintaining auxiliary brine replacement control valve  140  in an open state as the water pressure in the system gradually increases. Alternatively, the water in the auxiliary feed water tank  142  may be maintained at a pressure which is much lower than the pressure of the brine but above the atmospheric pressure by operation of the auxiliary feed water pump  144  as described hereinbelow. 
         [0122]    In this embodiment, during flushing the FPM controller  162  preferably opens recycle conduit control valve  156 , resulting in a water flow from a location downstream of pump  102  to a location upstream of pump  102 , preferably thorough a restrictor  160 , thus lowering the feed water pressure at manifold  114  to a pressure above atmospheric pressure, which pressure exceeds the osmotic pressure of the feed water at the feed side of module  100 . The water flow for this stage is shown in a solid black line in  FIG. 7B . 
         [0123]    A typical graph of flow rate over time, as measured by concentrate flow rate sensor  130 , located downstream of circulation pump  126 , appears in an enlargement forming part of  FIG. 7B , in which line  200  represents the flow rate over time. 
         [0124]    Concentrate flow rate sensor  130  measures the cumulative volume of brine flowing from concentrate manifold  120 , and thus measures the cumulative volume of feed water entering feed manifold  114  via the auxiliary feed water conduit  146  and the auxiliary feed water control valve  148 , which replaces the brine in module  100 . 
         [0125]    After complete replacement of brine with feed water in module  100 , FPM controller  162  reopens recirculation control valve  124 , and closes auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156 , providing a liquid flow as shown in  FIG. 7C , which may be identical to the liquid flow illustrated in  FIG. 7A , in which the operation of high pressure pump  102  and the circulation pump  126  supplies mixed feed water to module  100 . 
         [0126]    Thereafter, the FPM controller  162  periodically activates the auxiliary feed water pump  144  and opens auxiliary brine outlet tank control valve  154  to flush all brine from auxiliary feed water tank  142  through an auxiliary brine outlet  152  to a location outside of the at least one water treatment system, and fill the auxiliary feed tank  142  with feed water for further replacement of brine in module  100 . This flow is shown in solid black lines in  FIG. 7D . 
         [0127]    Following full replacement of brine with feed water in auxiliary feed water tank  142 , as measured by auxiliary flow sensor  150 , the FPM controller  162  closes auxiliary brine outlet tank control valve  154  and terminates operation of the auxiliary feed water pump  144 . 
         [0128]    Reference is now made to  FIGS. 8A-8D , which are simplified illustrations of water flows in yet another embodiment of a water treatment system of the type shown in  FIG. 1 . 
         [0129]    Prior to initiation of removal of the concentrate from module  100 , brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156  are closed and recirculation control valve  124  is open. The concentrate from concentrate manifold  120  is directed back to the input of feed manifold  114  via recirculation conduit  122  and recirculation control valve  124 , as shown by an arrow labeled CONCENTRATE (FIG.  1 ), representing the recirculation flow in the recirculation conduit  122 . In feed manifold  114 , the concentrate is mixed with feed water, as shown by an arrow labeled MIXED ( FIG. 1 ), representing the mixed flow in the feed manifold  114 . Thus, a mixed flow enters pressure vessels  110  for further treatment. The water flow for this stage is shown in a solid black line in  FIG. 8A . 
         [0130]    The feed pressure thereafter gradually increases as the salinity of the mixed water being supplied to the membrane elements  112  increases, and the above-described recirculation process continues. 
         [0131]    Once the concentration of the concentrate reaches a threshold, such as a predetermined salinity level at which continued water treatment is deemed not to be practicable, FPM controller  162  opens auxiliary brine replacement control valve  140 , which is approximately at atmospheric pressure, thus reducing the water pressure within module  100  to a pressure between the pressure of the concentrate at module  100  and the pressure of the feed water in the auxiliary feed tank  142 . 
         [0132]    Immediately thereafter, FPM controller  162  closes recirculation control valve  124  and opens auxiliary feed water control valve  148 . Feed water from auxiliary feed tank  142  flows via auxiliary feed water conduit  146  and feed water control valve  144  to feed manifold  114 , as shown in  FIG. 8B , thus supplying feed water to module  100  at a pressure above the osmotic pressure of the feed water and slightly higher than the pressure required for reverse osmosis to occur. 
         [0133]    In the embodiment illustrated in  FIGS. 8A-8D , FPM controller  162  also increases the flow rate of circulation pump  126  during flushing to achieve faster replacement of the brine from module  100  with feed water from auxiliary feed tank  142 , hence, reducing the time required for the brine to be flushed from module  100  and replaced by feed water. A typical graph of flow rate over time, as measured by concentrate flow rate sensor  130 , located downstream of circulation pump  126 , appears in an enlargement forming part of  FIG. 8B , in which line  200  represents the flow rate over time. 
         [0134]    The water flow for this stage is shown in a solid black line in  FIG. 8B . 
         [0135]    Concentrate flow rate sensor  130  measures the cumulative volume of brine flowing from concentrate manifold  120  and thus measures the cumulative volume of feed water entering feed manifold  114  via the auxiliary feed water conduit  146  and the auxiliary feed water control valve  148 , which replaces the brine in module  100 . 
         [0136]    After complete replacement of the brine with feed water in module  100 , FPM controller  162  reopens recirculation control valve  124 , closes auxiliary brine replacement control valve  140  and auxiliary feed water control valve  148  and reduces the flow rate of circulation pump  126  to the flow rate prior to opening concentrate recirculation control valve  124 , providing a liquid flow as shown in solid black lines in  FIG. 8C , which may be identical to the liquid flow illustrated in  FIG. 8A , in which the operation of high pressure pump  102  and circulation pump  126  supplies mixed feed water to module  100 . 
         [0137]    Thereafter, the FPM controller  162  periodically activates the auxiliary feed water pump  144  and opens auxiliary brine outlet tank control valve  154  to flush all brine from auxiliary feed water tank  142  through an auxiliary brine outlet  152  to a location outside of the at least one water treatment system, and fill the auxiliary feed tank  142  with feed water for further replacement of the brine in module  100 . This flow is shown in solid black lines in  FIG. 8D . 
         [0138]    Following full replacement of brine with feed water in auxiliary feed water tank  142 , as measured by auxiliary flow rate sensor  150 , the FPM controller  162  closes auxiliary brine outlet tank control valve  154  and terminates operation of the auxiliary feed water pump  144 . 
         [0139]    Reference is now made to  FIGS. 9A-9D , which are simplified illustrations of water flows in yet another embodiment of a water treatment system of the type shown in  FIG. 1 . 
         [0140]    Prior to initiation of removal of the concentrate from module  100 , brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156  are closed and recirculation control valve  124  is open. The concentrate from concentrate manifold  120  is directed back to the input of feed manifold  114  via recirculation conduit  122  and recirculation control valve  124 , as shown by an arrow labeled CONCENTRATE ( FIG. 1 ), representing the recirculation flow in the recirculation conduit  122 . In feed manifold  114 , the concentrate is mixed with feed water, as shown by an arrow labeled MIXED ( FIG. 1 ), representing the mixed flow in the feed conduit  114 . Thus, a mixed flow enters pressure vessels  110  for further treatment. The water flow for this stage is shown in a solid black line in  FIG. 9A . 
         [0141]    The feed pressure thereafter gradually increases as the salinity of the mixed water being supplied to the membrane elements  112  increases, and the above-described recirculation process continues. 
         [0142]    Once the concentration of the concentrate reaches a threshold, such as a predetermined salinity level at which continued water treatment is deemed not to be practicable, FPM controller  162  at least partially opens brine outlet control valve  134 , thus allowing brine to flow to via brine outlet  132  to a location outside of the at least one water treatment system, thus reducing the system pressure in module  100 . 
         [0143]    Additionally, in the embodiment shown in  FIGS. 9A-9D , during flushing the FPM controller  162  preferably opens recycle conduit control valve  156 , resulting in a water flow from a location downstream of pump  102  to a location upstream of pump  102 , preferably thorough a restrictor  160 , thus maintaining the feed water pressure at manifold  114  at a pressure above atmospheric pressure, which pressure exceeds the osmotic pressure of the feed water at the feed side of module  100 , as seen in  FIG. 9B . 
         [0144]    Immediately thereafter, FPM controller also opens auxiliary brine replacement control valve  140 , closes recirculation control valve  124  and opens auxiliary feed water control valve  148 . Brine from concentrate manifold  120  flows through auxiliary brine replacement conduit  136 , auxiliary tank feed conduit  138  and auxiliary brine replacement control valve  140  to auxiliary feed water tank  142 . The auxiliary feed water tank  142  is filled with feed water prior to the opening of auxiliary brine replacement control valve  140 , as described hereinbelow. The brine enters the auxiliary feed water tank  142  and drives feed water from tank  142  to feed manifold  114  via auxiliary feed water conduit  146  and auxiliary feed water control valve  148 . It is appreciated that water in auxiliary feed water tank  142  may be at the same pressure as the pressure of the brine, such as by maintaining auxiliary brine replacement control valve  140  in an open state as the water pressure in the system gradually increases. Alternatively, the water in the auxiliary feed water tank  142  may be maintained at a pressure which is much lower than the pressure of the brine but above the atmospheric pressure by operation of the auxiliary feed water pump  144  work as described hereinbelow. The water flow for this stage is shown in a solid black line in  FIG. 9B . 
         [0145]    Concentrate flow rate sensor  130  measures the cumulative volume of brine flowing from concentrate manifold  120  and thus measures when the full flushing of the brine from module  100  has been completed. After fully flushing the brine from module  100 , FPM controller  162  reopens recirculation control valve  124 , and closes brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156 , providing a liquid flow as shown in  FIG. 9C , which may be identical to the liquid flow illustrated in  FIG. 9A , in which the operation of high pressure pump  102  and circulation pump  126  supplies mixed feed water to module  100 . 
         [0146]    Thereafter, the FPM controller  162  periodically activates the auxiliary feed water pump  144  and opens auxiliary brine outlet tank control valve  154  to flush all brine from auxiliary feed water tank  142  through an auxiliary brine outlet  152  to a location outside of the at least one water treatment system, and fill the auxiliary feed tank  142  with feed water for further replacement of the brine in module  100 . This flow is shown in solid black lines in  FIG. 9D . 
         [0147]    Following full replacement of brine with feed water in auxiliary feed water tank  142 , as measured by auxiliary flow sensor  150 , the FPM controller  162  closes auxiliary brine outlet tank control valve  154  and terminates operation of the auxiliary feed water pump  144 . 
         [0148]    It is appreciated that in the embodiment shown in  FIG. 9B , as described hereinabove, a portion of the brine exits via brine outlet  132  and thus is not replaced with water from auxiliary feed water tank  142 . The volume of water in module  100  may be replenished by increasing the flow rate of high pressure pump  102  or by any other suitable method. 
         [0149]    Reference is now made to  FIGS. 10A-10D , which are simplified illustrations of water flows in still another embodiment of a water treatment system of the type shown in  FIG. 1 . 
         [0150]    Prior to initiation of removal of the concentrate from module  100 , brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156  are closed and recirculation control valve  124  is open. The concentrate from concentrate manifold  120  is directed back to the input of feed manifold  114  via recirculation conduit  122  and recirculation control valve  124 , as shown by an arrow labeled CONCENTRATE ( FIG. 1 ), representing the recirculation flow in the recirculation conduit  122 . In feed manifold  114 , the concentrate is mixed with feed water, as shown by an arrow labeled MIXED ( FIG. 1 ), representing the mixed flow in the feed conduit  114 . Thus, the mixed flow enters pressure vessels  110  for further treatment. The water flow for this stage is shown in a solid black line in  FIG. 10A . 
         [0151]    The feed pressure thereafter gradually increases as the salinity of the mixed water being supplied to the membrane elements  112  increases, and the above-described recirculation process continues. 
         [0152]    Once the concentration of the concentrate reaches a threshold, such as a predetermined salinity level at which continued water treatment is deemed not to be practicable, FPM controller  162  opens auxiliary brine replacement control valve  140 , which is approximately at atmospheric pressure, thus reducing the water pressure within module  100  to a pressure between the pressure of the concentrate at module  100  and the pressure of the feed water in the auxiliary feed tank  142 . 
         [0153]    Immediately thereafter, FPM controller  162  closes recirculation control valve  124  and opens auxiliary feed water control valve  148 . Brine from concentrate manifold  120  flows through auxiliary brine replacement conduit  136 , auxiliary tank feed conduit  138  and auxiliary brine replacement control valve  140  to auxiliary feed water tank  142 . The auxiliary feed water tank  142  is filled with feed water prior to the opening of auxiliary brine replacement control valve  140 , as described hereinbelow. The brine entering the auxiliary feed water tank  142  drives feed water in it to feed manifold  114  via auxiliary feed water conduit  146  and auxiliary feed water control valve  148 . It is appreciated that water in auxiliary feed water tank  142  may be maintained at a pressure generally the same as the pressure of the brine, such as by maintaining auxiliary brine replacement control valve  140  in an open state as the pressure in the system gradually increases. Alternatively, the water in the auxiliary feed water tank  142  may be maintained at a pressure which is much lower than the pressure of the brine but above the atmospheric pressure by operation of the auxiliary feed water pump  144  as described hereinbelow. 
         [0154]    Additionally, in the embodiment of  FIGS. 10A-10D , during flushing the FPM controller  162  preferably opens recycle conduit control valve  156 , resulting in a water flow from a location downstream of pump  102  to a location upstream of pump  102 , preferably thorough a restrictor  160 , thus lowering the feed water pressure at manifold  114  to a pressure above atmospheric pressure, which pressure exceeds the osmotic pressure of the feed water at the feed side of module  100 . 
         [0155]    Concentrate flow rate sensor  130  measures the cumulative volume of brine flowing from concentrate manifold  120  and thus measures the cumulative volume of feed water entering feed manifold  114  via the auxiliary feed water conduit  146  and the auxiliary feed water control valve  148 , which replaces the brine in module  100 . 
         [0156]    After complete replacement of the brine with feed water in module  100 , FPM controller  162  reopens recirculation control valve  124 , and closes auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156 , providing a liquid flow as shown in  FIG. 10C , which may be identical to the liquid flow illustrated in  FIG. 10A  hereinabove. 
         [0157]    Thereafter, the FPM controller  162  periodically activates the auxiliary feed water pump  144  and opens auxiliary brine outlet tank control valve  154  to flush all brine from auxiliary feed water tank  142  through an auxiliary brine outlet  152  to a location outside of the at least one water treatment system, and fill the auxiliary feed tank  142  with feed water for further replacement of the brine in module  100 . This flow is shown in solid black lines in  FIG. 10D . 
         [0158]    Following full replacement of brine with feed water in auxiliary feed water tank  142 , as measured by auxiliary flow sensor  150 , the FPM controller  162  closes auxiliary brine outlet tank control valve  154  and terminates operation of the auxiliary feed water pump  144 . 
         [0159]    Reference is now made to  FIGS. 11A-11D , which are simplified illustrations of water flows in another water treatment system of the type shown in  FIG. 1 . 
         [0160]    Prior to initiation of removal of the concentrate from module  100 , brine outlet control valve  134 , auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156  are closed and recirculation control valve  124  is open. The concentrate from concentrate manifold  120  is directed back to the input of feed manifold  114  via recirculation conduit  122  and recirculation control valve  124 , as shown by an arrow labeled CONCENTRATE (FIG.  1 ), representing the recirculation flow in the recirculation conduit  122 . In feed manifold  114 , the concentrate is mixed with feed water, as shown by an arrow labeled MIXED ( FIG. 1 ), representing the mixed flow in the feed conduit  114 . Thus, a mixed flow enters pressure vessels  110  for further treatment. The water flow for this stage is shown in a solid black line in  FIG. 11A . 
         [0161]    The feed pressure thereafter gradually increases as the salinity of the mixed water being supplied to the membrane elements  112  increases, and the above-described recirculation process continues. 
         [0162]    Once the concentration of the concentrate reaches a threshold, such as a predetermined salinity level at which continued water treatment is deemed not to be practicable, FPM controller  162  opens auxiliary brine replacement control valve  140  that is connected to auxiliary feed water tank  142  through auxiliary brine replacement conduit  136  and auxiliary tank feed conduit  138 . In this embodiment, the water in auxiliary feed water tank  142  is approximately at atmospheric pressure, thus opening auxiliary brine replacement control valve  140  reduces the water pressure within module  100  to a pressure between the pressure of the concentrate at module  100  and the pressure of the feed water in the auxiliary feed tank  142 . 
         [0163]    Immediately thereafter, FPM controller  162  closes recirculation control valve  124  and opens auxiliary feed water control valve  148 . Brine from concentrate manifold  120  flows through auxiliary brine replacement conduit  136 , auxiliary tank feed conduit  138  and auxiliary brine replacement control valve  140  to auxiliary feed water tank  142 . The auxiliary feed water tank  142  is filled with feed water prior to the opening of auxiliary brine replacement control valve  140 , as described hereinbelow. The brine entering the auxiliary feed water tank  142  drives feed water from auxiliary feed tank  142  via auxiliary feed water conduit  146  and auxiliary feed water control valve  148  to feed manifold  114 , thus supplying feed water to module  100  at a pressure above the osmotic pressure of the feed water and not much higher than the pressure required for a reverse osmosis to occur. 
         [0164]    In this embodiment, during flushing, the FPM controller  162  preferably also opens recycle conduit control valve  156 , resulting in a water flow from a location downstream of pump  102  to a location upstream of pump  102 , preferably through a restrictor  160 , thus lowering the feed water pressure at manifold  114  to a pressure above atmospheric pressure, which pressure exceeds the osmotic pressure of the feed water at the feed side of module  100 . In this embodiment illustrated in  FIGS. 11A-11D , FPM controller also increases the flow rate produced by circulation pump  126  during flushing to achieve a faster replacement of the brine from module  100  with feed water from auxiliary feed tank  142 , hence, reducing the time required for the brine to be flushed from module  100  and replaced by feed water. A typical graph of flow rate over time, as measured by concentrate flow rate sensor  130 , located downstream of circulation pump  126 , appears in an enlargement forming part of  FIG. 11B , in which line  200  represents the flow rate over time. 
         [0165]    The water flow for this stage is shown in a solid black line in  FIG. 11B . 
         [0166]    Concentrate flow rate sensor  130  measures the cumulative volume of brine flowing from concentrate manifold  120  and thus measures the cumulative volume of feed water entering feed manifold  114  via the auxiliary feed water conduit  146  and the auxiliary feed water control valve  148 , which replaces the brine in module  100 . 
         [0167]    After complete replacement of brine with feed water in module  100 , FPM controller  162  reopens recirculation control valve  124 , closes auxiliary brine replacement control valve  140 , auxiliary feed water control valve  148  and recycle conduit control valve  156 , and reduces the flow rate of circulation pump  126  to the flow rate prior to opening recirculation control valve  124 , providing a liquid flow as shown in solid black lines in  FIG. 11C , which may be identical to the liquid flow illustrates in  FIG. 11A , in which the operation of high pressure pump  102  and circulation pump  126  supplies water to module  100 . 
         [0168]    Thereafter, the FPM controller  162  periodically activates the auxiliary feed water pump  144  and opens auxiliary brine outlet tank control valve  154  to flush all brine from auxiliary feed water tank  142  through an auxiliary brine outlet  152  to a location outside of the at least one water treatment system, and fill the auxiliary feed tank  142  with feed water for further replacement of brine in module  100 . This flow is shown in a solid black line in  FIG. 11D . 
         [0169]    Following full replacement of brine with feed water in auxiliary feed water tank  142 , as measured by auxiliary flow sensor  150 , the FPM controller  162  closes auxiliary brine outlet tank control valve  154  and terminates operation of the auxiliary feed water pump  144 . 
         [0170]    It will be appreciated by persons skilled in the art that the present invention is not limited by what has been specifically shown and described hereinabove. Rather the scope of the invention includes both combinations and sub-combinations of features described and shown hereinabove as well as modifications thereof which would occur to persons reading the foregoing description and which are not in the prior art.