Abstract:
System and method for purifying and recycling spent brine in a water softener are provided. The system may be made up of a cation exchange resin tank fluidly coupled for passing spent brine comprising monovalent and divalent ions. A fluid mixer valve is coupled to the resin tank and to a water tank to dilute the spent brine to a desired concentration of a regenerant salt, e.g., NaCl. An ion-separation device is fluidly coupled to the fluid mixer valve to receive the diluted spent brine and separate the diluted spent brine into first and second streams. The first of the streams comprises monovalent ions and the second of the streams comprises divalent ions.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is generally related to water softeners, and, more particularly, to system and techniques for the filtration and reuse of spent brine, such as may be produced during the regeneration of salt-based ion-exchange water softening systems. 
     Residential water softeners typically use cation exchange resins, which remove ions, such as calcium and magnesium ions, which commonly contribute to the hardness of water. During the ion exchange process, the resin releases a cation, e.g., a divalent cation, into the softened water. Periodically, preferably when the resin bed becomes saturated with the divalent cations, the resin is regenerated by flushing it with a concentrated regenerant aqueous solution, such as salt (e.g., sodium chloride) brine. In the process, the cations producing hardness (calcium and magnesium, for instance) are released into the regeneration stream that is disposed of into the municipal sewer system. 
     Unfortunately, excess sodium chloride from the regenerant brine solution is also discarded to the sewer system. Because many municipalities nowadays treat sewer water for agricultural irrigation or other purposes, discharge of brine is often no longer acceptable because the discharged brine would introduce excessive salinity to the agricultural fields and present environmental pollution of the ponds and nearby lands being irrigated. As a consequence, many municipalities may enact regulations effectively banning the discharge of brine from the regeneration of residential water softeners. 
     In addition, residential users must periodically buy pelletized salt in heavy bags and carry them to the location of the ion exchange softener, which is often not easily accessible. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In view of the foregoing considerations, the inventors of the present invention have innovatively recognized a brine regeneration system by which the mixture of monovalent ions (e.g., brine salt) and divalent ions (e.g., hardness-causing ions) in the spent regeneration stream are separated. This allows for the brine to be recycled and reused for additional cycles of resin regeneration while the hardness ions may be discharged in an environmentally friendly fashion to the sewer system. 
     Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof, a water softener comprising a cation exchange resin tank fluidly coupled for passing spent brine comprising monovalent and divalent ions. A fluid mixer valve is coupled to the resin tank and to a water tank to dilute the spent brine to a desired concentration of a regenerant salt (sodium chloride or NaCl). An ion-separation device is fluidly coupled to the fluid mixer valve to receive the diluted spent brine and separate the diluted spent brine into first and second streams. The first of the streams substantially comprises monovalent ions and the second of the streams substantially comprises divalent ions. 
     In another aspect thereof, the present invention further fulfills the foregoing needs by providing a method for purifying and recycling spent brine in a water softener. The method allows passing from a cation exchange resin tank spent brine comprising monovalent and divalent ions. The method further allows for diluting the spent brine to a desired concentration of a regenerant salt. The diluted spent brine is separated into first and second streams. The first of the streams substantially comprises monovalent ions and the second of the streams substantially comprises divalent ions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention will become apparent from the following detailed description of the invention when read with the accompanying drawings in which: 
         FIG. 1  illustrates a schematic representation of an exemplary system for purifying spent brine in a water softener; and 
         FIG. 2  illustrates a schematic representation of an exemplary system for purifying and recycling spent brine in a water softener. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Below is generic background information solely for the purpose of providing a cursory description of one exemplary operation of a typical water softener. This background information in no way should be construed as limiting the scope of the present invention described further below. As shown in  FIG. 1 , an exemplary water conditioner system  10  may comprise at least two tanks: a resin tank  12  and a brine (e.g., salt) tank  14 . The resin tank is filled with a resin that, for example, comprises relatively small beads of a suitable material, e.g., cross-linked polystyrene sulfonic acid. This resin may be referred to in the art as a cation resin. The beads may be constructed to exhibit a permanent electrical charge. The charge may cause the beads to attract positively charged ions. For example, the resin may be initially placed into service with Na +  ions on the beads. When the hardness ions (e.g., calcium or Ca 2+  and magnesium or Mg 2+ ) come in contact with the sodium ions (Na + ), such ions would displace the Na +  on the beads. The Na +  is eventually dissolved into the water. This sodium generally leaves the resin tank and may be delivered to the tap with the treated water. When most of the Na +  is removed from the resin beads, regeneration equipment should start the regeneration process. The resin may then be regenerated by drawing in a relatively high concentration of brine (e.g., NaCl or KCl) solution from the brine tank. This salt solution is washed over the depleted resin. The salt solution may contain Na +  and chloride ions (Cl − ). The Na +  is placed back onto the resin beads and the Ca 2+ , Mg 2+  and Cl −  are washed down the drain. The resin may then be rinsed with fresh water to remove any remaining residual salt. Additional water may be added to the brine tank to dissolve salt for the next regeneration cycle. The equipment may then command a service mode and there would be treated water available from the softener. 
     In accordance with aspects of the present invention, the exemplary embodiment illustrated in  FIG. 1  uses a nanofiltration membrane  20  to separate the spent brine into two streams: stream  22  comprises regenerated or purified brine (essentially monovalent ions, such as sodium and chloride) and stream  24  comprises waste hardness (essentially divalent ions, such as calcium, magnesium and carbonates) in order to deal with the environmental requirements for discharge to sewer. It will be appreciated by those skilled in the art that a nanofiltration membrane is just one example of a device that can separate the monovalent ions from the divalent ions that may be present in the spent brine. For some applications, a “loose” reverse osmosis (RO) device, electrodialysis device, or deionization device may be used in lieu of or in combination with the nanofiltration membrane. 
     The foregoing embodiment may be useful for cases where the nanofiltration membrane  20  is capable of separating the divalent ions in the presence of a high concentration of monovalent ions. In practice, many nanofiltration membranes (as presently available in the market) may not be able to achieve complete separation of hardness at high monovalent ion concentration. As an example, presently commercially available nanofiltration membranes will separate divalent ions from water at efficiencies approaching 99% or better whenever the concentration of monovalent ions is substantially close to zero (e.g., approximately below 0.1% wt NaCl). However, the efficiency to separate divalent ions that produce hardness may decrease to approximately 50% when the monovalent concentration is at approximately 1% wt NaCl or higher. 
     In one exemplary embodiment, the concentrated brine from a regeneration cycle of the ion exchange softener, may be typically saturated at approximately near 20–26% NaCl. It would be desirable to reuse the brine multiple times without discharging it to the sewer system. As described in greater detail below in the context of  FIG. 2 , another embodiment is contemplated to effectively separate hardness when the nanofiltration membranes alone may not be able to achieve the separation to a satisfactorily high degree in the presence of high brine (e.g., NaCl) concentrations. 
     Referring Now to  FIG. 2 : 
     A concentrated brine (stream  50 ) for regenerating an ion exchange softening system resin may be obtained in a brine storage tank  52  from pelletized salt (sodium chloride), for example. Stream  50  leaving the brine storage tank is passed to an ion-exchange resin bed in a resin tank  54  where the concentration of sodium may be reduced from close to saturation (approximately 26% wt NaCl or so) to approximately from about ⅕ th  to about 1/10 th  of its saturation point (stream  56 ). This stream is commonly called spent brine because it comprises a residual amount of brine plus the divalent cations removed from the resin bed during resin regeneration. Stream  56  may be collected in a spent brine storage tank  58 . 
     Stream  56  may be diluted with a stream  60 , which may comprise either fresh water or predominantly softened water, from a ratio of approximately about 1:1 to about 10:1 or higher, passing through a suitable mixing valve  62  or equivalent device. The purpose of this diluting step is to reduce the concentration of sodium chloride in stream  56  so as to produce a diluted stream  64  equal to or below the concentration of monovalent ions that a nanofiltration membrane  66  can tolerate to separate the divalent (hardness) ions at high efficiencies (typically 70% or higher, and up to about 99% hardness removal in one exemplary embodiment). 
     A pumping device  68  can be used to draw fluid from the spent brine holding tank  58  and/or a recycled water tank  70  to the nanofiltration membrane  66 . In one exemplary embodiment, typical pressures for a stream  72  entering the nanofiltration membrane  66  need not be much higher than pressures found in residential city water supplies. If necessary for a given application, the pressure can be increased slightly with the pumping device  68  (or an auxiliary pump) to approximately about 100–150 psi. 
     Stream  72  comprises a diluted hard water stream that enters the nanofiltration membrane  66  and is separated into two streams: a concentrated stream of divalent ions, such as calcium, magnesium and carbonates (stream  74 ) and a diluted softened stream (stream  75 ) that comprises just the monovalent ions (e.g., sodium and chloride) and is essentially free of hardness. Stream  74  is discharged to the sewer as a safe discharge essentially free of the sodium chloride monovalent ions. In one exemplary embodiment, multiple passes through nanofiltration membrane  66  may be performed in an optional loop arrangement (not shown in  FIG. 2 ) to achieve higher separation efficiencies. 
     The diluted soft stream (stream  75 ) may be pumped through a pumping device  76  to a sufficiently high level of pressure, e.g., approximately 70 psi or higher, and passes through a reverse osmosis (RO)-type membrane device  78 , where the demineralized water (stream  80 ) is available for further recirculation and eventually the dilution of stream  56 . The purpose of RO device  78  may be two-fold: 1) to reclaim the water present in stream  75  (i.e, the dilute spent brine) for further utilization, and 2) to reconstitute the brine for further reuse. The reclaimed water stream  80  is sent back to the recycled water tank  70  for further use as diluent. The reconstituted brine stream  82  is sent to brine storage  52  to make more brine and/or increase its salt concentration. 
     In one exemplary embodiment, a bleed stream  75 A, a bleed stream  82 A, and a bleed stream  60 A could be added off stream  75 , stream  82 , and stream  60 , respectively, to allow for draining or discharging of fluids used in the cleaning or maintenance of membrane device  66 , membrane device  78 , and storage device  70 , respectively. 
     Unlike sea water desalination, the pressure in the RO membrane device does not have to be very high because the osmotic pressure of a diluted water stream is relatively low. In some applications one may use a loose reverse osmosis (RO) device, an electrodialysis device, or a deionization device in lieu of RO device  78 . 
     As will be appreciate by those skilled in the art, a general rule typically employed in water treatment is that for every 100 mg/L of ions in the treated water, the osmotic pressure that needs to be overcome is about 1 psi. For instance, assuming that stream  75  comprises a total dissolved solids (TDS) concentration of 1% wt (10,000 mg/L or ppm), this would require approximately 100 psi of osmotic pressure, which is a pressure level readily achievable with a small pump in a residential system. The monovalent ions present in stream  75  are separated and concentrated in stream  82 , the reconstituted brine. 
     In one exemplary embodiment, a design goal would be to provide a relatively high water separation efficiency in the RO membrane, for example at least 50% or higher, in order to bring the concentration of monovalent ions to at least 3% wt NaCl, so that most of the water is recovered as stream  80  and sent to the recycled water tank  70 . The reconstituted brine (stream  82 ) will then comprise a small volume of liquid with high a concentration of NaCl and can be used for further regeneration of the water softener cation exchange resin as regenerated brine. 
     It is further desirable to achieve good separation efficiency of the divalent ions at the nanofiltration membrane  66  so that the working capacity for water hardness removal of the cation exchange resin does not decrease with time. It is also desirable to have a good separation (permeation) efficiency of water from the RO-type membrane to avoid the possibility that the brine storage tank  52  overflows with low salt concentration liquid. 
     In operation, performing separations in a diluted stream loop as illustrated in  FIG. 2  allows the nanofiltration membrane  66  to operate more efficiently (i.e., achieve a higher level of separation between monovalent and divalent ions) while avoiding a reduction in the softening capacity of the ion exchange resin bed. 
     While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.