Abstract:
A liquid pumping system comprises a plurality of liquid pumps and a hydraulic drive unit. Each liquid pump is driven by a separate hydraulic cylinder. The hydraulic cylinders are powered by a shared hydraulic pump through a valve set. A valve set controller is configured to operate the valve set. A liquid pumping process comprises distributing an initial flow of pressurized hydraulic fluid between the hydraulic cylinders. The hydraulic cylinders move through a cycle in a phased relationship to provide a constant sum of flow rates from the liquid pumps. A membrane filtration system combines the liquid pumping system with a membrane unit. In a water treating process, feed water is pumped through the membrane unit. Brine from the membrane unit is returned to each liquid pump while that liquid pump is feeding water to the membrane unit.

Description:
FIELD 
       [0001]    This invention relates to devices and processes for pumping liquids with energy recovery; to membrane filtration, for example by reverse osmosis; and to desalination. 
       BACKGROUND 
       [0002]    Many areas of the world do not have adequate fresh water supplies but they are near seawater. Seawater can be desalinated using reverse osmosis (RO). During RO, the feed water must be pressurized above the osmotic pressure of the feed water. The feed water becomes concentrated during this process and its osmotic pressure increases. Feed water pressures for seawater reverse osmosis (SWRO) are typically in a range of 50-70 bar (approximately 725 psi to 1015 psi). 
         [0003]    Pressurizing the seawater in an RO system consumes energy. One approach to reduce energy consumption is to recover energy from the residual pressure of the brine after it leaves an RO module. An energy recovery pumping system is described by Childs et al. in U.S. Pat. No. 6,017,200 entitled “Integrated Pumping and/or Energy Recovery System.” This approach uses multiple water cylinders moving in a phased relationship to provide pressurized feed water to a RO membrane unit. One side of a piston in the water cylinder drives the feed water to the RO membrane unit while the other side of the piston receives brine from the RO membrane unit. The pressure of the brine reduces the power required to move the piston. Each water cylinder is connected to a separate hydraulic pump and hydraulic cylinder combination to move the piston in the water cylinder according to a desired velocity profile and to provide the additional energy required to pressurize the feed water. 
         [0004]    U.S. patent application Ser. No. 13/250,463, entitled “Energy Recovery Desalination”, by D&#39;Artenay et al. describes an energy recovery pumping system that makes various improvements to the Childs et al. system. For example, each of the hydraulic pumps has an adjustable swash plate to change the rate and direction of hydraulic fluid flow to its associated hydraulic cylinder. Inner and outer control loops are used to modify the position of the swash plate so that the water cylinder connected to the hydraulic cylinder follows an intended velocity profile more closely. 
       SUMMARY 
       [0005]    A liquid pumping system is described in this specification that comprises a plurality of liquid pumps and a hydraulic drive unit. Each liquid pump is driven by a separate hydraulic cylinder. The hydraulic cylinders are powered by a shared hydraulic pump through a valve set. The valve set is operated by a valve set controller. The valve set controller is configured to distribute a flow of hydraulic fluid from the hydraulic pump between the hydraulic cylinders such that the liquid pumps operate in a phased relationship to each other. Preferably, the total liquid flow produced from the liquid pumps is generally constant over a period of time in which the hydraulic pump produces a generally constant output. Optionally, the valve set may comprise a set of valves, for example a proportional directional control valve for each hydraulic cylinder, connected in parallel to the hydraulic pump. 
         [0006]    A membrane filtration system is described in this specification that uses the liquid pumping system to provide feed water to a membrane unit. A water circuit is configured such that each liquid pump receives pressurized brine from the membrane unit while pumping water. The membrane unit may be a reverse osmosis unit. 
         [0007]    Processes are described in this specification for pumping a liquid and for treating water. The liquid pumping process comprises a step of providing an initial flow of pressurized hydraulic fluid. The initial flow of pressurized hydraulic fluid is distributed between a plurality of hydraulic cylinders such that, over a period of time in which the initial flow is essentially constant, the sum of the distributed flows is also essentially constant but the hydraulic cylinders move in a phased relationship to each other. Each hydraulic cylinder drives a liquid pump. In the water treating process, water is pumped to a membrane unit. Brine from the membrane unit is provided to each liquid pump while that liquid pump is feeding water to the membrane unit. Preferably, the liquid pumps produce a generally constant flow of feed water to the membrane unit. Optionally, the membrane unit may be a reverse osmosis unit. 
         [0008]    The processes and systems provide useful alternative ways and means for pumping liquids or treating water. In at least some cases, the processes and systems may provide one or more benefits relative to the systems described by Childs et al. and D&#39;Artenay et al., or other high pressure pumping systems with energy recovery, such as reduced energy consumption, reduced parts count or cost, or reduced maintenance. Without limitation, the processes and systems may be used in the desalination industry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram of a water treatment system having a pumping system combined with a membrane unit. 
           [0010]      FIG. 1A  is a schematic diagram of a water cylinder for use with the system of  FIG. 1 . 
           [0011]      FIG. 1B  is a schematic diagram of a hydraulic delivery unit for use with the system of  FIG. 1 . 
           [0012]      FIG. 1C  is a schematic diagram of a control valve for use with the hydraulic delivery unit of  FIG. 1B . 
           [0013]      FIG. 1D  is a cross section of the control valve in  FIG. 1C . 
           [0014]      FIG. 2A  is an intended water pump velocity profile for a single water pump. 
           [0015]      FIG. 2B  is an intended water pump velocity profile for three water pumps. 
           [0016]      FIG. 3  depicts simulation results from computer modeling of the system of  FIG. 1  in operation. 
           [0017]      FIG. 4  is a schematic of a process for controlling a water treatment system as in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  shows a system  10  for treating water. The system  10  comprises a feed water source  12 , a pumping system  11  and a membrane unit  16 , for example a reverse osmosis unit. The pumping system  11  provides feed water from the source  12  to the membrane unit  16 , preferably at a high pressure and generally constant flow rate. The flow rate may be varied by an operator from time to time. However, the flow rate is constant in the sense that it is generally the same as a fixed reference value, for example within about 10% of the reference value, for a period of time. During the period of time, which may be an hour or more, the components of the pumping system  11  may move through many, for example 10 or more or 100 or more, cycles. 
         [0019]    The pumping system  11  has two or more water cylinders  14  and a hydraulic drive unit  18 . The water cylinders  14 , and the valves and conduits of a water circuit connecting them to the membrane unit  16 , may be similar to those described in U.S. Pat. No. 6,017,200, entitled “Integrated Pumping and/or Energy Recovery System”, U.S. patent application Ser. No. 13/250,463 entitled “Energy Recovery Desalination” and U.S. patent application Ser. No. 13/250,674 entitled “Valve System for Pressure Recovery in IPER”, which are incorporated herein by reference. The pumping system  11  shown has three water cylinders  14  but alternatively there may be two, three, four or other numbers of water cylinders  14 . Alternatively, other types of water pumps may be used in place of the water cylinders  14 . The pumping system  11  may also be used to pump other liquids. 
         [0020]    Feed water, for example seawater, brackish water, groundwater, boiler feed water or wastewater, flows from the feed water source  12  to the water cylinders  14  via low pressure feed pipes  20 . The feed water is pressurized within the water cylinders  14  and directed to the membrane unit  16  via high pressure feed pipes  22 . Each water cylinder  14  goes through approximately the same cycle but the cycles have a phased relationship to each other such that at any given point in time each water cylinder  14  is in a different part of its cycle. 
         [0021]    The membrane unit  16  separates the feed water into a low pressure stream of low-solute permeate and a high pressure stream of high-solute brine, alternatively called concentrate or retentate. The permeate is withdrawn from the membrane unit  16  for various uses, for example drinking water, through permeate pipe  23 . The brine is directed back to the water cylinders  14 , via high pressure brine pipes  24 . Each water cylinder  14  receives brine while providing feed water such that the pressure of the brine can be used to help pressurize the feed water. Low-pressure brine, after being used to help generate feed water pressure, is directed from the water cylinder  14  for waste, recycling or reuse via low pressure brine pipes  25 . The water cylinders  14  are dual acting pumps that pump feed water on both a forward and a reverse stroke. 
         [0022]    Variations of the system  10  may have two, three or more single acting or dual acting water cylinders  14 . The description immediately below will focus on one water cylinder  14  and the movement of a single reciprocating assembly  26  that is part of the water cylinder  14 . However, other parts of the description and figures may refer to a particular water cylinder  14 ,  14   1 ,  14   11  or to a set of the water cylinders  14 ,  14   1 ,  14   11 . 
         [0023]    Referring to  FIG. 1A , each water cylinder  14  has a first and a second water piston chamber  28 ,  28   A . In the example of  FIG. 1A , the water piston chambers  28 ,  28   A  are located in a single housing, but alternatively they may be located in separate housings. Each water piston chamber  28 ,  28   A  has a water piston  32 . The water pistons  32  separate the water piston chambers  28 ,  28   A  into feed water working chambers  34  and concentrate working chambers  36 . Each water cylinder  14 , therefore, has first and second feed water working chambers  34 ,  34   A  and a first and a second concentrate working chambers  36 ,  36   A . Preferably, the feed water working chambers  34 ,  34   A  are at the ends of the water cylinder  14  and the concentrate working chambers  36 ,  36   A  are at the middle of the water cylinder  14 . Optionally, other configurations of water cylinder  14  may be used. 
         [0024]    The water pistons  32  are mechanically coupled to each other by a connecting rod  38 . The connecting rod  38  extends through a dividing wall between the concentrate working chambers  36 ,  36   A  and out of the water cylinder  14  through bearing and seal assemblies (not shown), which minimize or prevent pressure or fluid leaks. The connecting rod  38  and the dual-acting pistons  32  are collectively referred to as the reciprocating assembly  26 . 
         [0025]    The reciprocating assembly  26  is connected to a piston rod  40  of a hydraulic piston  52  (see  FIG. 1B ). In the example of  FIG. 1 , the piston rod  40  and the reciprocating assembly  26  move in unison and have the same acceleration, the same velocity and the same direction of travel during the same period of time. Alternatively, other connections may be provided between the piston rod  40  and the reciprocating assembly  26  such that there is a transformation between the movement of the piston rod  40  and the reciprocating assembly  26 . For example, the piston rod  40  and the reciprocating assembly  26  may be connected by a gear set, lever or hydraulic transducer such that the reciprocating assembly  26  moves through a shorter or longer stroke or in a reverse direction relative to the piston rod  40 . 
         [0026]    Each water cylinder  14  comprises water cylinder valves  70  that control the flow of liquid into and out of the water cylinders  14 . Opening and closing of the water cylinder valves  70  is controlled by a controller  90  in association with the movement of the reciprocating assembly  26 . Optionally, the water cylinder valves  70  may be similar to those described in U.S. Pat. No. 6,017,200, entitled “Integrated Pumping and/or Energy Recovery System”, U.S. patent application Ser. No. 13/250,463 entitled “Energy Recovery Desalination” and U.S. patent application Ser. No. 13/250,674 entitled “Valve System for Pressure Recovery in IPER”. 
         [0027]    While the reciprocating assembly  26  moves forwards, or upwards as it is oriented in  FIG. 1A , the water cylinder valves  70  are configured such that: feed water in the upper working chamber  34  flows out to a high pressure feed pipe  22 ; brine flows into the upper concentrate working chamber  36  from a high pressure brine pipe  24 ; water flows out of the lower concentrate working chamber  36 A to a low pressure brine pipe  25 ; and, feed water flows into the lower feed water working chamber  34   A  from a low pressure feed pipe  20 . While the reciprocating assembly  26  moves in reverse, or downwards as it is oriented in  FIG. 1A , the water cylinder valves  70  are configured such that: feed water flows into the upper working chamber  34  from a low pressure feed pipe  20 ; brine flows out of the upper concentrate working chamber  36  to a low pressure brine pipe  25 ; water flows into the lower concentrate working chamber  36 A from a high pressure brine pipe  24 ; and, feed water flows out of the lower feed water working chamber  34   A  to a high pressure feed pipe  22 . The water cylinder valves  70  are re-configured near or during dwell periods between forward and reverse movements of the reciprocating assembly  26 . In this way, energy is recovered from the pressurized brine to help provide pressurized feed water to the membrane unit  16 . 
         [0028]      FIG. 1B  shows the hydraulic drive unit  18 . The hydraulic drive unit  18  has a hydraulic pump  42 , two or more hydraulic cylinders  44 , a valve set  45  and a controller  90 . Optionally, the valve set  45  may have a control valve  46  for each hydraulic cylinder  44 . Each hydraulic cylinder  44  has a hydraulic piston  52  connected to a piston rod  40 . Referring to  FIG. 1A , each piston rod  40  is connected to the reciprocating assembly  26  of a water cylinder  14 . 
         [0029]    The hydraulic piston  52  optionally includes an extension  41  that extends from the first side  56  of the hydraulic piston  52 . The extension  41  preferably has a different cross-sectional area than the piston rod  40 . In particular, the extension  41  may have a smaller cross-sectional area than the piston rod  40 . The first side  56  and the second side  60  of the hydraulic piston  52  preferably have different surface areas. In particular, the first side  56  of the hydraulic piston  52  preferably has a larger surface area than the second side  60 . For example, the ratio of the surface areas of the first and second side  56 ,  60  may be within about 10% of the ratio of the forces acting on the water pistons  32  as they move in the forward and reverse directions within the water cylinder  14 . The ratio of forces is calculated by equation (1) below and it is equal to the piston surface area within the first feed water working chamber  34  (PSA  34 ) subtracted by the piston surface area within the first concentrate working chamber  36  (PSA  36 ) relative to the piston surface area within the second feed water working chamber  34   A  (PSA  34   A ) subtracted by the piston surface area within the second concentrate working chamber  36   A  (PSA  36   A ): 
         [0000]      Ratio of forces=(PSA 34−PSA 36):(PSA 34 A −PSA 36 A )  (1).
 
         [0030]    The ratio of forces can be within a range of about 1:1 to about 1.25:1. The ratio of hydraulic piston  52  surface areas is selected to help balance a pressure differential that arises between the hydraulic cylinders  44 ,  44 ′,  44 ″ resulting from the connecting rod  38  extending through the second feed water working chamber  34   A  but not the first feed water working chamber  34 . Alternatively, but not preferably, the connecting rod  38  may be extended through the first feed water working chamber  34 . 
         [0031]    If the extension  41  is not included, optionally the piston rod  40  may be re-sized to ensure that the ratio of hydraulic piston  52  surface areas is still within 10% of the ratio of forces acting on the water pistons  32 . If this results in piston rod  40  being too small to withstand the hydraulic forces, the surface areas of the water pistons  32  can be modified to ensure that a sufficiently large piston rod  40  diameter is used while simultaneously providing the desired ratio of the hydraulic piston  52  surface areas. 
         [0032]    Over a period of time, for example an hour or more, when a generally constant flow of feed water to the membrane unit  16  is desired, the hydraulic pump  42  is operated at a generally constant output. The hydraulic pump  42  provides a generally constant flow of hydraulic fluid at a generally constant pressure through supply pipes  50  to the valve set  45 . The hydraulic pump  42  may be one of a number of variable displacement pumps, including but not limited to: axial piston pumps, bent axis pumps and pressure compensated variable displacement pumps. Alternatively, the hydraulic pump  42  may be one of a number of fixed displacement pumps, including but not limited to: rotary vane pumps, piston pumps and diaphragm pumps, with a motor that may be controlled by a variable frequency drive unit. Return pipes  51  conduct hydraulic fluid returning from the valve set  45  to a hydraulic fluid reservoir  49 . Optionally, a filter may be provided in the return pipes  51 . 
         [0033]    Optionally, the hydraulic pump  42  may supply hydraulic fluid to the valve set  45  through an accumulator  48  to accommodate temporary pressure increases or decreases in the supply pipes  50 . Optionally, the hydraulic drive unit  18  may further comprise a pressure relief loop  96  with a pressure relief valve  97 . The pressure relief loop  96  connects the supply line  50  to the hydraulic fluid reservoir  49 . The pressure relief valve  97  opens if pressure in the supply line  50  exceeds a pre-set pressure indicating a failure in the hydraulic drive unit  18  or the pumping system  11 . 
         [0034]    For each hydraulic cylinder  44 , a forward feed pipe  54  connects the valve set  45  to a chamber of the hydraulic cylinder  44  in communication with the first side  56  of the hydraulic piston  52 . A reverse feed pipe  58  connects the valves set  45  to another chamber of the hydraulic cylinder  44  in communication with and a second side  60  of the hydraulic piston  52 . 
         [0035]    The valve set  45  receives a generally constant flow of hydraulic fluid from the hydraulic pump  42  and distributes the hydraulic fluid between the hydraulic cylinders  44 . For example, in relation to each hydraulic cylinder  44 , the valve set  45  may direct pressurized hydraulic fluid to the first side  56  of the hydraulic piston  52  or to the second side  60  of the hydraulic piston  52 , or the valves set  45  may stop the flow of hydraulic fluid to the hydraulic cylinder  44 . The valve set  45  may also return hydraulic fluid from the hydraulic cylinder  44  to the hydraulic fluid reservoir  49 . The valve set  45  may be configured such that low pressure returning hydraulic fluid flows through the same, or a different, valve body that the pressurized hydraulic fluid flows through. 
         [0036]    Optionally, the valve set  45  may comprise a separate control valve  46  for each hydraulic cylinder  44 . The control valve  46  may be, for example, one or more servo valves, preferably with actuator feedback. Alternatively, the control valve  46  may be one or more four-way, proportional directional control valves. Table 1 below provides a summary of some of the available positions of a four-way, proportional directional control valve  46 . Each control valve  46  is able to transition between position 1 and position 2 and between position 2 and position 3. The individual control valves  46  in the valve set  45  are operated in a phased relationship to each other. However, operation of the control valves  46  is coordinated such that the sum of the flow rates of pressurized hydraulic fluid to forward feed pipes  54  and reverse feed pipes  58 , which is essentially the same as the sum of the flow rates in the individual supply pipes  50 ′,  50 ″ and  50 ′″, is essentially constant over a period of time in which the flow rate in the supply pipe  50  is essentially constant. The operation of the control valves  46  can also be coordinated to minimize pressure losses across each control valve  46 . This is achieved by directing a position profile of each control valve  46  to closely follow the shape of a velocity profile  138  of the associated water piston  32 , as described further below. In Table 1, positions 1 and 3 represent nominal fully open positions. However, these positions may be partially open, for example 80% to 98% open, positions in the physical valves to allow for a controller to correct errors by temporarily more fully opening a valve, as will be described further below. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Position 1 
                 Position 2 
                 Position 3 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Supply pipe 50 
                 OPEN to forward 
                 CLOSED 
                 OPEN to reverse 
               
               
                   
                 feed pipe 54 
                   
                 feed pipe 58 
               
               
                 Return pipe 51 
                 OPEN to reverse feed 
                 CLOSED 
                 OPEN to forward 
               
               
                   
                 pipe 58 
                   
                 feed pipe 54 
               
               
                   
               
             
          
         
       
     
         [0037]    Preferably, the control valve  46  has a controllable transition speed between the three positions. Preferably, the rate of flow through the valve while transitioning or at a certain time is a known or determinable function of the location of control valve  46  between positions. Optionally, a throttle valve may be integrated with the control valve  46  to vary the flow rate through the control valve in position 1 or position 3. However, it is typically more energy efficient to control flow rate in position 1 or position 3 by varying the output of the hydraulic pump  42 . 
         [0038]      FIG. 1C  is a schematic of a four way, proportional directional control valve that may be used for each control valve  46 . The control valve  46  has a valve body  62 , a spool  64  and an actuator  66 . The spool  64  has a series of lands and ports configured such that when the spool  64  is moved to the left or right different connections are made.  FIG. 1D  is an example of a possible internal configuration of a spool valve. In particular, the spool  64  is shown in  FIG. 1C  in position 2 of Table 1. Moving the spool  64  to the right puts the control valve  46  in position 1 of Table 1. Moving the spool  64  to the left puts the control valve  46  in position 3 of Table 1. While moving between position 2 and position 1, partially restricted flow paths are provided according to position 2. While moving between position 2 and positions 3, partially restricted flow paths are provided according to position 3. 
         [0039]    The spool  64  is moved by the actuator  66 . The spool  64  may move by sliding or rotating. The actuator  66  may be a mechanical actuator, a pilot-valve system, an electronic servo system or a combination of devices. The actuator  66  is connected to the controller  90  and moves the control valve  46  when instructed by the controller  90 . Preferably, the actuator  66  includes an internal controller  67  that receives the instructions from the controller  90  and instructs the actuator  66  to move the control valve  46 . The actuator  66  can move the spool  64  at a predetermined rate of speed. However, it is preferable for the controller  90  to control both the timing and rate of moving the spool  64 . Varying the position of the spool  64  alters the velocity of the hydraulic cylinder  44 . Varying the rate of movement of the spool  64  alters the acceleration or deceleration of the hydraulic cylinder  44 . The control valve  46  preferably includes a spool position transducer  65 , for example a linear variable differential transformer (LVDT), which feeds into a control loop within the internal controller  67  so that the position of the spool  64  can be adjusted if required to better match the position instructed by the controller  90  at a particular time. The rate of movement of the spool  64  may be implemented as a series of changes of position over time rather than as a rate directly. 
         [0040]    The controller  90  preferably includes one or more programmable devices such as a processor or microprocessor, computer, Field Programmable Gate Array, or programmable logic controller (PLC). Alternatively or additionally, the controller  90  may comprise one or more non-programmable control elements, such as a timer or pneumatic or electric circuit, capable of implementing a sequence of operations. The controller  90  is preferably the same controller that is used to control the water cylinder valves  70  and the hydraulic pump  42 . Optionally, multiple controllers may be used, preferably connected to a master controller. 
         [0041]      FIG. 2A  shows the velocity profile  138  of a single reciprocating assembly  26 . The hydraulic piston  52  and piston rod  40  attached to this reciprocating assembly  26  follow the same velocity profile  138 . In general, the reciprocating assembly  26  moves through a repeated cycle of movements. In each cycle, the reciprocating assembly  26  first moves in a forward direction, then stops for a dwell period, then moves in the reverse direction, then stops for a dwell period. The movement in the forward direction has an acceleration phase, a constant velocity phase and a deceleration phase. Similarly, the movement in the reverse direction has an acceleration phase, a constant velocity phase and a deceleration phase. For example, Table 2 shows the motions and positions (as defined in Table 1) of a control valve  46  during the cycle. With other valve sets  45 , one or more valves are moved by the controller  90  as required to provide similar connections between the supply pipe  50  and the return pipe  51 , and the forward feed pipe  54  and the reverse feed pipe  58 , of a hydraulic cylinder  44 . 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Reference 
                   
                   
               
               
                 numeral 
                   
                 Control valve 46 movement 
               
               
                 (FIG. 2A) 
                 Cycle phase 
                 or position 
               
               
                   
               
             
             
               
                 202 
                 Accelerating forward 
                 Moving from position 2 to 
               
               
                   
                   
                 position 1 
               
               
                 204 
                 Constant velocity forward 
                 Position 1 
               
               
                 206 
                 Decelerating forward 
                 Moving from position 1 to 
               
               
                   
                   
                 position 2 
               
               
                 208 
                 Dwell 
                 Position 2 
               
               
                 210 
                 Accelerating reverse 
                 Moving from position 2 to 
               
               
                   
                   
                 position 3 
               
               
                 212 
                 Constant velocity reverse 
                 Position 3 
               
               
                 214 
                 Decelerating reverse 
                 Moving from position 3 to 
               
               
                   
                   
                 position 2 
               
               
                 216 
                 Dwell 
                 Position 2 
               
               
                   
               
             
          
         
       
     
         [0042]    The cycle is implemented by the controller  90  moving the one or more valves of the valve set  45 . For example, the controller  90  may have a velocity reference chart that represents the velocity profile  138 , or a related position reference chart giving the desired position of the reciprocating assemblies  26  over time, or both. The velocity reference chart is pre-calculated and stored in the memory of the controller  90 . The controller  90  is programmed to poll the velocity reference chart, for example at regular time intervals, to determine the required velocity at that time. At each time interval, the controller  90  instructs the valve set  45  to move one or more control valves  46 , or hold one or more control valves  46  in position, as required. Accelerations and decelerations are caused by moving a control valve  46  between positions from one time interval to another. The required spool positions and changes in positions over time are obtained by the controller  90  referencing the velocity reference chart and sending instructions to the control valves  46  to move the spools  64  to positions of the spools  64  predicted, according to a chart or formula in the memory of the controller  90  relating reciprocating assembly  26  velocity to spool  64  position, to give the velocity of the reciprocating assemblies  26  specified for that time interval. 
         [0043]    In greater detail, and as depicted in  FIG. 4 , at each time interval the controller  90  instructs the actuator  66  to implement the required spool  64  positions by sending a master command to the internal controller  67 , which in turn commands the actuator  66  of the control valve  46  ( 300 ) to move the spool  64  to the specified position. The controller  90  locates the required velocity by looking up the velocity value in the velocity reference chart that corresponds to current time ( 302 ). Current time may be indicated by a clock or timer in the controller  90 . The controller  90  then determines what control valve  46  spool position should provide the required water cylinder velocity and generates an initial master command  304 . The controller  90  can go through the process in  FIG. 4  and send a master command to the internal controller  67  at a pre-determined frequency, for example once every 1 ms. Preferably, the master command is an electronic signal within a range of about −10 V and about 10 V. Each end of this signal range represents an instruction to move the control valve  46  to either the first or third position and a 0 V signal represents an instruction to move the control valve  46  to the second position. 
         [0044]    The controller  90  receives information regarding the position of each reciprocating assembly  26  ( 308 ). For example, the controller  90  receives the positional information from an assembly position transducer  63  (shown in  FIG. 1A ) located on, or within, each reciprocating assembly  26 , its associated piston rod  40  or the associated hydraulic piston  52 . Optionally, the assembly position transducer  63  may be a LVDT sensor. The assembly position transducer  63  feeds into a control loop within the controller  90  so that the position of each reciprocating assembly  26  can be adjusted, if required, to better match a desired position profile of each reciprocating assembly  26 , which is an integration of its velocity profile. 
         [0045]      FIG. 2B  shows a desired assembly sequence  136 . The assembly sequence  136  includes the velocity profiles  138 ,  138 ′ and  138 ″ of three reciprocating assemblies  26 ,  26   1 ,  26   11  over a period of time. The three velocity profiles  138 ,  138 ′ and  138 ″ are the same, but positioned out of phase, or with a relative time delay, such that the reciprocating assemblies  26 ,  26   1 ,  26   11  are not moving in the same direction at the same speed at the same time. Due to the operation of the water valves  70  described above, movement of a reciprocating assembly  26  in either direction produces a flow of feed water to the membrane unit  16 . The sum of the absolute values of the velocities of the reciprocating assemblies  26 ,  26   1 ,  26   11  is generally constant. The feed flow rate to the membrane unit  16  is also generally constant. The sum of the flow rates of hydraulic fluid in supply pipes  50 ′,  50 ″ and  50 ′″ is also generally constant. Similarly, the sum of the flow rates in return pipe  51 ′,  51 ″ and  51 ′″; the sum of the flow rates in forward feed pipes  54 ,  54 ′ and  54 ′; and, the sum of the flow rates in reverse feed pipes  58 ′,  58 ″ and  58 ′″ are also generally constant. 
         [0046]    The controller  90  instructs the valve set  45  to implement the three velocity profiles  138 ,  138 ′ and  138 ″ in the phased relationship. Where the valve set  45  comprises three control valves  46 , each control valve  46  moves through the same cycle but at different times. For example, at time A in  FIG. 2B , control valve  46  is in, or close to, position 3; control valve  46 ′ is moving from position 1 to position 2; and, control valve  46 ″ is moving from position 2 to position 1. At time B in  FIG. 2B , control valve  46  is in or close to position 2; control valves  46 ′ is in or close to position 3 and control valve  46 ″ is in or close to position 1. 
         [0047]    During the velocity profile  138 , there are four generally distinct pressures that occur within the system  10 . The first pressure P1 is the pressure that supplies the feed water from the source  12  to the water cylinder  14 . P1 can be provided by a variety of known pumps. The second pressure P2, which is higher than P1, is the pressure exerted on the feed water from the water cylinder  14  to the membrane unit  16 . P2 is provided by the movement of the reciprocating assembly  26 . The third pressure P3 is the pressure of the concentrate as it leaves the membrane unit  16  to return to the water cylinder  14 . P3 is less than P2 since some of the energy is used to drive a filtration process of the membrane unit  16 . The fourth pressure P4 is the pressure of the concentrate as it leaves the water cylinder  14  to the waste or recycling stream. P4 is less than P3. For example, P1 may be in the range of 5 to 100 p.s.i.; P2 may be in the range of 600 to 1000 p.s.i.; P3 may be in the range of 500 to 950 p.s.i.; and P4 may be in the range of 1 to 50 p.s.i. 
         [0048]    Preferably, the controller  90  includes an independent proportional, integral and derivative (PID) loop for each control valve  46  (see  FIG. 4 ). Each PID loop receives the positional information input from the assembly position transducer  63  and generates a PID output command that modifies the master command that the controller  90  sends to the associated control valve  46 . Within each PID loop, the assembly positional information is compared against a stored position reference chart of an ideal position of the reciprocating assembly  26  during the assembly sequence  136  ( 310 ). The position reference chart is pre-calculated and stored in the memory of the controller  90 . The comparison step identifies a positional error value  312  that is used to generate at least part of the PID output command by multiplying the positional error by a proportional gain term  314 . Part of the PID output command can also be generated by multiplying an integral of the positional error, over time, with an integral gain term  316 . The PID output command can also include a derivative of the positional error, over time, multiplied by a derivative gain term  318 . The gain terms can be pre-calculated, based upon testing of the system  10 , and saved in the memory of the controller  90 . The three corrected signals of the multiplication steps are then summed to produce a final PID output command  320 . The final PID output command is added to the master command, which may modify the master command  322 . The final PID output command can increase or decrease the amplitude of the master command. The modified master command signal is sent from the controller  90  to the internal controller  67  to change the position of the spool  64 . Optionally the PID loop may be based on positional information from the last time period rather than the current time period. 
         [0049]    To allow for a PID output command indicating a more fully open valve position at any time, the position of each spool  64  may be between 80 and 98% open when the associated reciprocated assembly  26  is at maximum velocity specified in the velocity profile  138 . This may be achieved by multiplying the master command by a further correction factor  306 . This permits the spool  64  to move to a more open position and increase the flow of hydraulic fluid to the hydraulic cylinder  44  to correct the position or velocity of the reciprocating assembly  26  even if the reciprocating assembly  26  is already moving at the maximum velocity specified in the velocity profile  138 . 
         [0050]    Optionally, the positional information from the assembly position transducer  63  may be used to modify the output of the hydraulic pump  42 . The positional information is received by the controller  90  and the positional information is mathematically transformed into a calculated change in hydraulic fluid flow rate through the control valve  46  that will be required to correct an error in position. The calculated change in hydraulic fluid flow rate is then multiplied by a proportional gain to provide a hydraulic command signal. The hydraulic command signal is sent to the hydraulic pump  42  to cause the pump to vary its output. For example, when the hydraulic pump  42  is a fixed displacement pump that is regulated by a variable frequency drive, the hydraulic command signal is sent to the variable frequency drive to change the hydraulic output. As another example, the hydraulic pump  42  can be an open circuit, pressure-compensated variable frequency pump with an internal control loop. The internal control loop includes a pump controller and a pressure sensor. The hydraulic command signal modifies a pressure threshold set-value within the pump controller so that when the pressure sensor senses an error between the actual pressure and the pressure threshold, the controller can change the hydraulic output to better match the pressure within the pump to the pressure threshold value. This altered hydraulic output also contributes to having the reciprocating assemblies  26 ,  26 ′,  26 ″ in the correct position and at the correct velocity during the cycle. 
         [0051]    Preferably, the internal controller  67  receives spool positional information from the spool position transducer  65 , which is compared with the instructed position provided by the last master command received. Any error between the spool positional information and the instructed position provides an error signal that is multiplied by a proportional gain to provide a new command signal. The new command signal is sent to the actuator  66  to move the spool  64  to, or closer to, the instructed position. The derivative and integral of the error signal can also be multiplied by individual gains and added to the new command signal to the actuator  66 . 
         [0052]    The system  10 , with three water cylinders  14 ,  14 ′,  14 ″ and three, 4-way, 3-position proportional directional dual pilot control valves  46 , was simulated with MATLAB/Simulink software.  FIG. 3  depicts the software modeling results. Panel (i) depicts the velocity (inches/second) of each reciprocating assembly  26 ,  26   1 ,  26   11  over time (seconds). Velocities in the range of 0 to 10 represent movement in the forward direction and the range of 0 to −10 represents movement in the reverse direction. The results indicate that the simulated reciprocating assembly  26   11  closely follows the assembly sequence  136 . Panel (ii) depicts the simulated position (inches) over time (seconds) of one reciprocating assembly  26 . The simulated position follows the assembly sequence  136  so closely that the lines are indiscernible. Panel (iii) depicts the total flow rate (inches 3 /second) over time (seconds) of hydraulic fluid from hydraulic pump to the simulated three hydraulic cylinders  14 ,  14   1 ,  14   11  (collectively “simulated”) and the ideal hydraulic flow rates. The results indicate that the simulated values closely follow the ideal values, which are scalable and reflect a constant flow of feed water to the membrane unit  16 . Panel (iv) depicts the position of the spools  64 ,  64   1 ,  64   11  over time (seconds) with “0” representing the second position. The first position and the third position are represented by “100” and “−100”, respectively. 
         [0053]    Based upon computer simulations of the water treatment system, the system  10  has approximately 7% less specific power consumption than a system using three hydraulic pumps (one for each water cylinder) with swash plates. In the simulation, a significant portion of this difference was attributed to not idling hydraulic pumps during the dwell periods and the lack of an auxiliary parasitic charge pump that is used with swash-plate piston pumps. 
         [0054]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art.