Abstract:
A water treatment system combines a microfiltration or ultrafiltration membrane system with a downstream reverse osmosis membrane system. The MF or UF system has multiple trains of immersed membrane modules. The trains are connected to a common permeate pump. The permeate pump discharges directly into the inlet of an RO feed pump. The membrane trains are each subjected to the same suction. The permeate pumps are operated to provide the required flow to the RO feed pump at or above the minimum inlet pressure of the RO feed pump.

Description:
FIELD 
       [0001]    The invention is in the field of membrane processes for water treatment such as microfiltration, ultrafiltration and reverse osmosis. 
       BACKGROUND 
       [0002]    The following discussion is not an admission that anything discussed below is common general knowledge or citable as prior art. 
         [0003]    Some water treatment processes use multiple membrane treatment steps in series. In particular, microfiltration (MF) or ultrafiltration (UF) membranes may be used to pre-treat water prior to a nanofiltration (NF) or reverse osmosis (RO) step. Such combined processes are used for example in sea water desalination, wastewater recovery and in some industrial water treatment plants. 
         [0004]    For example, a presentation entitled “Desalination Technology Overview” presented by James C. Lozier at the April 2011 Water Resources Research Center Conference in Yuma, Ariz., USA, describes a seawater RO plant. In this plant, there are three trains of UF or MF immersed membranes each having a suction pump delivering permeate to a break tank. From the break tank, water is pumped through a set of parallel RO trains. The RO permeate is stored in tanks and then transferred to a distribution system. 
       INTRODUCTION 
       [0005]    The following section is intended to introduce the reader to the detailed description to follow and not to limit or define any claims. One or more inventions may be comprised of a combination or sub-combination of elements or steps described in this introduction or in other parts of this specification. 
         [0006]    A water treatment system combines a microfiltration or ultrafiltration membrane system with a downstream reverse osmosis membrane system. The UF or MF membrane system has multiple trains of immersed MF or UF membrane modules. The trains are connected to a common set of one or more permeate pumps. The permeate pumps discharge directly into the inlet of an RO feed pump. In a water treatment process, the membrane trains are each subjected to the same suction. The permeate pumps are operated to provide the required flow to the RO feed pump at or above the minimum inlet pressure of the feed pumps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic process flow diagram of a water treatment system. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]      FIG. 1  shows a water treatment system  10 . The water treatment system  10  combines a UF or MF system  12  with an RO system  14 , which may be replaced by a nanofiltration system or a combined nanofiltration and RO system for some applications. The water treatment system  10  may be used, for example, for sea water desalination, for wastewater recovery and reuse, or for various industrial water treatment applications. 
         [0009]    The UF or MF system  12  has many membrane modules that are hydraulically connected to form a train  16 . In particular, all of the membrane modules in a train  16  discharge permeate, alternatively called filtrate, into a common filtrate header  18 . The modules in a train  16  may also be physically connected, or they may be separated into smaller physical groups such as cassettes. The membrane modules may be, for example, ZeeWeed™ modules sold by GE Water and Process Technologies. 
         [0010]    Each train  16  is located in a separate tank  20 . The tanks  20  may be distinct structures or regions separated by partitions in a larger tank. Optionally, two or more trains  16  may be located in a common tank  20  but in that case the two or more trains  16  are treated as one larger train  16  unless they can be separately monitored and individually shut down. Although two trains  16  are shown in  FIG. 1 , more trains  16  may be provided. A larger number of trains  16  reduces the effect of one train  16  being temporarily out of service, for example for a deconcentration or cleaning procedure. Further, in the water treatment system  10  there are no pumps dedicated to a individual train  16  and only two valves, and very little if any instrumentation, dedicated to an individual train  16 . Accordingly, the incremental cost of subdividing the total number of membrane modules into a greater number of trains  16  is not large. For example, five or more trains  16  may be used. 
         [0011]    Feed water is provided to the tanks  20  from a feed inlet  22  connected to a feed distribution manifold  24 . The feed distribution manifold  24  is in turn connected to a tank inlet  26  associated with each tank  20 . The tank inlet  26  is located near the bottom of a tank  20  or at least in part below ordinary water levels in the tank  20 . In this way, the division of the total feed flow between the tanks  20  is affected by the relative water level in the tanks  20 . If one train  16  fouls and begins to produces less permeate, the water level in that tank  20  will rise. The rising water level will both reduce the rate of feed flow into thank tank  20  and increase the transmembrane pressure (TMP) across that train  16  until a new equilibrium is reached. Since the train  16  has fouled, its flux will be lower at the new equilibrium but the increase in TMP will moderate the decrease in flux. Conversely, if a first train  16  is more permeable than a second train  16 , the water level in the first tank  20  will be lower than in the second tank  20 . The TMP of the first train  16  will be lower than the TMP of the second train  16 . Although the first train  16  will still have a higher flux, its reduced TMP will reduce the difference in flux between the first and second trains. At an equilibrium condition, more feed water will flow to the first tank  20 . The head loss to the first tank  20  minus the head loss to flow to the second tank  20  will be equal to the difference in water level equal. In this way, feed water is automatically divided between the tanks  20  as required to accommodate different fluxes in each tank  20 , and TMP is automatically adjusted in a way that tends to dampen differences in permeability between the trains  16 . 
         [0012]    Each tank inlet  26  has a feed valve  28 . The feed valve  28  is fully open while the associated train  16  is operating in a permeation phase of its cycle. However, the feed valve  28  is closed when the train  16  enters a deconcentration or cleaning cycle, or during an optional flux test to be described further below. In a deconcentration cycle or cleaning cycle, the feed valve  28  is closed to isolate the associated tank  20  from the feed inlet  22 . A permeate flow control valve (FCV)  30  in the filtrate header  18  is also closed. Optionally, the train  16  may continue to produce permeate for a period of time after the feed valve  28  is closed to reduce the volume of water in the tank  20 . Optionally, a backwash pump  34  may be operated to flow permeate from a backwash tank  36 , with or without cleaning chemicals, through an opened backwash valve  38 , and to the train  16 . Other optional cleaning processes involve filling the tank  20  with a cleaning solution and soaking the membranes or permeating cleaning solution through the membranes. At some point in a cleaning or deconcentration cycle, a drain valve  32  may be opened to drain the tank  20  of accumulated solids or cleaning solutions. To refill the tank, the feed valve  28  is opened after the drain valve  32  is closed. During the refilling, the feed valve  28  may be opened to a predetermined partially open position to avoid filling the tank  20  too rapidly, which may damage the membranes. Once the tank is full, and the permeate FCV  30  is open, the feed valve  28  is left fully open. 
         [0013]    The filtrate headers  18  of the trains  16  are connected to a common plant permeate pipe  40 . The plant permeate pipe  40  is in turn connected to the inlet of a permeate pump  42 . The permeate pump  42  discharges to a connecting pipe  44 . Although a single permeate pump  42  is shown in  FIG. 1 , there would typically be a set of pumps connected in parallel to the plant permeate pipe  40  and all discharging to the connecting pipe  44 . 
         [0014]    The permeate FCVs  30  are typically left fully open while a train  16  is producing permeate. The permeate FCVs  30  are not operated to produce the same flux from each train  16 . However, if a train  16  is exceeding its maximum permissible flux (which may be greater than its typical design flux) then the permeate FCV  30  is partially closed to prevent over-fluxing of that train  16 . 
         [0015]    The permeate pump  42  creates a partial vacuum, which is shared across all of the trains  16 . The vacuum applied to the trains  16  will be generally equal between the trains  16  but for some variation due to different flows, and head losses, in the filtrate headers  18 . Since the vacuum is generally equal between trains  16 , the flow drawn from each train  16  will vary according to the permeability of the membranes in each train  16 , subject to dampening cause by variations in water level discussed earlier, and variations in head loss in the filtrate headers  18 . 
         [0016]    The resulting variances in permeate flow between trains  16  is tolerated as long as none of the trains  16  exceed their maximum flux. This is more energy efficient than partially closing the permeate FCVs ( 30 ) to balance the trains  16 . If the flow variances result in a train  16  exceeding its maximum flux, the permeate FCV  30  can be closed by a pre-determined amount, or by an amount calculated or predicted to bring the train  16  back under its maximum flux. If a permeate FCV  30  is closed by a pre-determined amount, this adds resistance to the train  16  being over-fluxed, thus reducing its flux for the next permeate cycle. If the train  16  is still above the maximum permitted flux in the next permeate cycle, then the permeate FCV  30  is closed by another pre-determined amount. 
         [0017]    To determine whether a train  16  is exceeding its maximum flux, a flow indicator-transmitter (FIT) can be placed in the filtrate header  18 . Alternatively, the flux can be estimated by measuring the rate of water level decrease in a tank  20  when the tank inlet  26  is closed and permeate continues. This can occur at the start of a deconcentration or cleaning sequence, or in a separate flux test. Alternatively, since the water level in the tanks  20  is related to flux through the trains  16 , a low water level in a tank  20 , or a large difference in water level between two or more tanks  20 , indicates a high flux in the train  16  in the tank  20  with a low water level. The water level of a tank  20 , or the difference in water level between a tank  20  and the average water level or another tank  20 , can be correlated to flux and used to indicate whether a train  16  is exceeding its maximum flux. In another alternative, flow in a train  16  is estimated by measuring the pressure in its associated tank inlet  26  or in all tank inlets  26 . The difference in pressure in a tank inlet  26  relative to the static head in the tank  20  can be correlated to the flow velocity, and flow rate, into the tank inlet  26 . Similarly, pressure in the tank inlet  26  less static head in the tank  20  can be compared between tanks  20  to determine if one train  16  is above its maximum permissible flux. 
         [0018]    The permeate pump  42  discharges directly into the inlet of an RO feed pump  46 . Although only one RO feed pump  46  is shown, a set of RO feed pumps  46  connected in parallel is likely to be used. The permeate pump  42  acts as a booster pump delivering water to the RO feed pump  46  at a pressure at or above the specified minimum inlet pressure of the RO feed pump  46 , or at least sufficient to prevent cavitation in the RO feed pump  46 . For example, the permeate pump  42  may discharge water at a pressure of 125 kPa or more. 
         [0019]    The RO feed pump  46  in turn delivers water at high pressure to a set of one or more RO units  48 . Each RO unit  48  may be, for example, a set of spiral would membrane elements arranged end to end in a pressure vessel. Optionally, the RO units  48  may be replaced with nanofiltration units or a combination of RO and nanofiltration units. 
         [0020]    Permeate from the RO units  48  is collected in appropriate pipes and exits through a product water outlet  50 . On its way to the product water outlet  50 , the RO permeate passes through a permeate FIT  52 , typically a magnetic flowmeter, that measures the flow of RO permeate. The permeate FIT  52  is connected to a variable frequency drive (VFD)  60  that controls the speed of rotation of permeate pump  42 . Brine from the RO units  48 , also called rententate or reject water, is collected in appropriate pipes and exits through a brine FIT  56 . The brine FIT  56  is connected to a brine valve  58 . For both of the FITs  52 ,  56 , their connection as shown in  FIG. 1  and discussed above is intended to indicate the primary part of the system  10  controlled by each FIT  52 ,  56 . The FITs  52 ,  56  are also connected to appropriate controllers, for example a programmable logic controller (PLC) or computer, electrical circuitry and servos as required to operate the VFD  60  and brine valve  58 . A controller (not shown) may receives signals from both FITSs  52 ,  56  and, optionally, other sensors in the system  10  and coordinates the control of multiple components in the system  10 . 
         [0021]    In one example of a control scheme, a desired product water (RO permeate) flow rate and recovery rate are selected. A reject flow rate is calculated that will provide the selected recovery rate at the RO permeate flow rate. In a feedback control loop, the brine valve  58  is modulated based on signals from the brine FIT  56  as required to maintain the calculated flow through the brine outlet  54 . In another feedback control loop, the VFD  60  modulates the speed of permeate pump  42  as required to produce the desired RO permeate flow, as sensed by RO permeate FIT  52 . RO feed pump  46  generally runs at a constant velocity over extended periods of time, for example a day or more. However, RO feed pump  46  may be driven by a VFD and operate at various speeds. For example, the controller may be able to direct the RO feed pump  46  to operate at one of a set of available speeds. If the permeate pump  42  is unable to deliver the required flow, then the RO feed pump  46  may be instructed to operate at a higher speed. Conversely, if pressure at the inlet to the RO feed pump  46  falls below a minimum allowed pressure, the RO feed pump  46  may be instructed to operate at a lower speed. Dynamic adjustments to the RO permeate flow rate, for example over periods of time of one hour or less, are made by adjusting the speed of the permeate pump  42 . The permeate pump  42  preferably outputs water at a generally constant flow rate, although possibly with variations of up to 5% above or below a preselected flow rate. 
         [0022]    Variations in the speed of the permeate pump  42  cause corresponding variations in the TMP applied to the UF trains  16 . As discussed above, permeate FCVs  30  are provided in the filtrate headers  18 . However, the permeate FCVs are not used to balance flows between trains or to ensure that any particular flow rate is produced from each train  16 . Instead, the permeate FCVs  30  remain fully open as long as a train  16  does not exceed its maximum flux. While a train  16  is isolated for a deconcentration or cleaning procedures, the TMP and flux of the remaining trains  16  increases. The total membrane surface area of the trains  16  is designed to allow at least one train  16  to be isolated without exceeding the maximum flux of all of the remaining trains  16  and to accommodate at least some irreversible fouling over the service life of the membranes. A train  16  typically exceeds its maximum flux only while another train  16  is isolated. The maximum flux may be larger than the typical design maximum flux of the same membranes, where the typical design maximum flux assumes continuous operation at that flux. However, if a train  16  chronically exceeds its maximum flux, that may indicate that one or more other trains  16  with lower flux may require replacement of some or all of its membrane modules or an intense recovery cleaning. 
         [0023]    This written description uses examples to disclose the invention 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.