Patent Publication Number: US-11027989-B2

Title: Membrane filtration system with concentrate staging and concentrate recirculation, switchable stages, or both

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 14/775,821, filed Sep. 14, 2015, which is a National Stage Entry of International Application No. PCT/CN2013/072588, filed Mar. 14, 2013. 
    
    
     FIELD 
     This specification relates to membrane filtration, for example reverse osmosis or nanofiltration. 
     BACKGROUND 
     Reverse osmosis (RO) and nanofiltration (NF) membranes are typically used in the form of elements, also called modules, such as spiral wound elements, hollow fiber elements or tubular elements. A number of elements, typically between 1 and 8, are mounted in series in a pressure vessel, alternatively called a housing, with a feed inlet, concentrate outlet, and permeate outlet. Multiple pressure vessels may be connected together in parallel to form a bank, alternatively called a stage, in a filtration system. The stages of a filtration system are collectively referred to as a membrane block. 
     A filtration system may have multiple stages connected together in various configurations. In concentrate staging, alternatively called a multi-stage array, feed water is first pumped into a first stage of elements. Concentrate from each upstream stage is fed to each downstream stage. The concentrate port of the last stage is fitted with a concentrate valve. The flow and pressure through the membrane block are controlled by the feed pump and concentrate valve. Permeate flows from each stage to a common permeate header. The concentrate staging increases permeate recovery. Filtration systems with high recovery rates, for example 80% or more, typically have at least two stages. 
     BRIEF SUMMARY OF THE INVENTION 
     Although concentrate staging increases the recovery rate of a system, the flow rate of concentrate declines in each stage. With some types of wastewater, the flow rate in a high recovery filtration system may be insufficient to prevent fouling in the last stage. 
     In a filtration system described in this specification, two or more stages are connected together so as to provide concentrate staging. The last stage has a recirculation pump and conduits configured to provide concentrate recirculation. In another filtration system, valves and conduits of the system are arranged to allow the order in which feed water flows through a portion of a first and a last stage to be switched. The portion may be the entire first stage or less than the entire first stage. In an embodiment, with or without the order of flow switched, the system provides concentrate staging and concentrate recirculation in the stage that receives concentrate last. 
     In a filtration process described in this specification, feed water is separated into permeate and a first concentrate. The first concentrate is separated into permeate and a second concentrate. Part of the second concentrate portion is recycled and mixed with the first concentrate. In another process, the order of flow is switched between a portion of a first stage and a last stage of a filtration system at some times. The portion may be the entire first stage or a portion that is less than the entire first stage. In an embodiment, in each order, the system implements a filtration process having both concentrate staging and concentrate recirculation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic process flow diagram of a filtration system. 
         FIG. 2  is a schematic process flow diagram of a second filtration system. 
         FIG. 3  is a schematic process flow diagram of a third filtration system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a filtration system  10  treats feed water  12  to produce effluent  14 . The filtration system  10  has a feed pump  22 , a first stage  16 , a last stage  20 , a recirculation pump  24  and brine  26 . In an embodiment, the system  10  also has one or more intermediate stages  18 . In the system  10 , the last stage  20  may be called the third stage although in other systems the last stage  20  could be the second stage, fourth stage or another stage. Also in the system  10 , the intermediate stage  18  may be called the second stage although in other systems intermediate stages could include second, third, fourth or more stages. 
     Each stage comprises a set of one or more membrane filtration elements, for example nanofiltration or reverse osmosis elements. In an embodiment multiple elements in a stage are provided in series in a pressure vessel. Larger stages may comprise multiple pressure vessels plumbed in parallel, for example with a feed manifold connected to an inlet of each of the 8 pressure vessels, a concentrate manifold connected to a concentrate outlet of each of the 8 pressure vessels, and a permeate manifold connected to a permeate outlet of each of the 8 pressure vessels. In an embodiment first stage  16  is larger (i.e. has more of the same size elements) than the intermediate stage  18 . In an embodiment last stage  20  is larger than the intermediate stage  18 . The first stage  16  is at least as large as the last stage  20 . 
     The filtration system also has valves V 1  to V 9  and various conduits configured to provide the flow paths described below. Valves V 1  to V 9  may move between fully opened and fully closed positions. However, in an embodiment, valve V 9  is a throttle valve that may be set to various intermediate positions. The flow and pressure through the membrane block are controlled by the feed pump  22  or valve V 9  or both. 
     In a first configuration as shown, valves V 1 , V 3 , V 5  and V 7  are open. Valves V 2 , V 4 , V 6  and V 8  are closed. Valve V 9  is at least partially open to provide a bleed of brine  26  at a selected flow rate. The system  10  can also be operated in a second configuration with valves V 1 , V 3 , V 5  and V 7  closed; valves V 2 , V 4 , V 6  and V 8  open; and, valve V 9  at least partially open. In both configurations, the flow rate of the brine  26  is more particularly 20% or less of the flow rate of the feed water  22 . 80% or more of the feed water  22  is recovered as permeate  14 . 
     In the first configuration, feed pump  22  pumps feed water  12  to the first stage  16 . First stage permeate  28  is produced and becomes part of the effluent  14 . First stage concentrate  30  is also produced and flows to the intermediate stage  18 . First stage concentrate  30  flowing through a feed side of the intermediate stage  18  is separated into second stage permeate  32  and second stage concentrate  34 . Second stage permeate  32  becomes part of the effluent  14 . Second stage concentrate  34  flows to the recirculation pump  24 . The recirculation pump  24  pumps the second stage concentrate  34  to the last stage  20 . Third stage permeate  36  is produced and becomes part of the effluent  14 . Third stage concentrate  38  is partially discharged as brine  26  and partially returned to the feed side of the recirculation pump  24 . A portion of third stage concentrate  38  is thereby recirculated to the third stage  20 . In summary, the system  10  operates with concentrate staging between the stages  16 ,  18 ,  20  and with concentrate recirculation in the last stage  20 . 
     In the second configuration, the system  10  again operates with concentrate staging between the stages  16 ,  18 ,  20  and with concentrate recirculation in the stage receiving concentrate last. However, in this case the third stage  20  receives the feed water  12  first; the direction of concentrate staging is from the third stage  20  to the second stage  18  to the first stage  16 ; and first stage  16  operates with concentrate recycle, alternatively called feed and bleed. 
     The recirculation pump  24  and the order of flow are switched between through the first stage  16  and last stage  20  when the valves are moved from the first configuration to the second configuration. More particularly, however, the direction of flow through the feed sides of the stages  16 ,  18 ,  20  does not change when the valves are moved from the first configuration to the second configuration. Although some elements and pressure vessels may be configured for reversible flow, others have components such as brine seals or permeate collectors that only operate with flow in one direction, or are optimized for flow in one direction. In systems with multiple intermediate stages, the order of flow in an embodiment as between the intermediate stages also does not change when the valves are moved from the first configuration to the second configuration. The relative number of elements, or the piping or pumping system, may be optimized for flow in one direction between multiple intermediate stages. 
     The use of concentrate staging allows for a high recovery rate, for example 80% or more or between 85% and 95%. Concentrate recirculation in the last stage increases the flow rate through the last stage to help inhibit fouling. Overall permeate quality remains high since the first and second stages operate at reasonable feed side concentrations. The cost and energy consumption of the recirculation pump is limited to what is required by the last stage. However, in some cases, the last stage may still foul. Switching the order of flow at least at some times allows the last stage to be flushed with feed water to help further inhibit, or in some cases remove, fouling. In particular, some soluble organic compounds in difficult to treat wastewater can cause fouling in the last stage despite the concentrate recirculation. However, exposing the last stage to un-concentrated feed water at some times flushes the organic fouling layer from the last stage. In the system  10 , the last stage  20  may be switched with the first stage  16  and receive un-concentrated feed water  12  for up to half of the operating time of the system  10 . 
     In order to facilitate switching in an embodiment, the first stage  16  and last stage  20  are the same size, or at least about the same size. If the first stage  16  is materially larger than the last stage  20 , then the last stage may be switched with a portion of the first stage  16  that is less than the entire first stage  16 . In this case, the portion is more particularly the same size, or at least about the same size, as the last stage  20 . For example, a portion of the first stage  16 , which may be the entire first stage  16  or less than the entire first stage, that is switched with the last stage  20  may have a number of elements or pressure vessels, or both, that is within 25% of the corresponding number or numbers in the last stage  20 . 
     The system  10  can be used to treat a variety of feed water  12 . However, the system  10  is particularly adapted to providing high (80% or more) recovery from difficult to treat wastewater. The feed water  12  may have 200 mg/L or more of chemical oxygen demand (COD). Difficult waste waters include, for example, landfill leachate, coking plant wastewater, reverse osmosis brine and cooling tower blowdown. Optionally, recovery may be increased further by treating the brine, for example with a thermal evaporator, crystallizer, zero liquid discharger (ZLD) or physical-chemical treatment system. 
     EXAMPLES 
     In a first example, a three stage nanofiltration (NF) system  10  was designed for treating an industrial effluent with greater than 200 mg/of chemical oxygen demand (COD), 90 m3/h of permeate flow, and 90% recovery. The system  10  was arranged as shown in  FIG. 1 . The feed pump  22  is a high pressure pump rated for 100 m3/h of output and 110 m of head. The recirculation pump  24  is rated for 110 m3/h of output and 40 m of head. 
     The system has 126 nominal 8 inch (20 cm) NF spiral wound elements inserted into 21 pressure vessels. Each pressure vessel holds 6 elements in series. The system has three stages  16 ,  18 ,  20 . The first  16  and last 20 stages each have 8 pressure vessels plumbed in parallel. The intermediate stage  18  has 5 pressure vessels plumbed in parallel. 
     The first and last stages  16 ,  20  are identical and the recirculation pump  24 , and the order of flow, can be switched between them. In one valve configuration, as shown in  FIG. 1 , valves V 1 , V 3 , V 5  and V 7  are open and valves V 2 , V 4 , V 6  and V 8  are closed. The feed water  12  is pumped to the first stage  16  by the high pressure feed pump  22 , the concentrate from the first stage  16  is fed to the intermediate stage  18 , and the concentrate from the intermediate stage  18  is fed to the last stage  20  through the recirculation pump  24 . In a second valve configuration in which the first stage  16  and the last stage  20  are switched, valves V 2 , V 4 , V 6  and V 8  open and valves V 1 , V 3 , V 5  and V 7  are closed. The feed water  12  was pumped to the last stage  20  by the high pressure pump, the concentrate from the third stage  20  is fed to the intermediate stage  18  and the concentrate from the intermediate stage  18  is fed to the first stage  16  through the recirculation pump  24 . In both configurations, valve V 9  is a control valve operated to control the concentrate flow. A high cross-flow rate of 6-8 m3/h of concentrate per NF element was achieved in all stages. 
     In a second example, shown in  FIG. 2 , a second system  40  was designed for treating an industrial effluent with over 200 mg/l of COD, 90 m3/h of permeate flow, and 95% of recovery. The second system  40  has four stages  16 ,  18 ,  19  and  20 . The first stage is divided into two portions, a first portion  16 A and a second portion  16 B. The recirculation pump  24  and the order of flow can be switched between the last stage  20  and the first portion  16 A of the first stage  16 . When switched, the first portion  16 A of the first stage  16  receives concentrate last and the last stage  20  receives feed water first in parallel with the second portion  16 B of the first stage. 
     The feed pump  22  is a high pressure pump rated for 100 m3/h of output and 110 m of head. The second system  40  also has a booster pump  46  rated for 40 m3/h of output and 30 m of head, and a recirculation pump  24  rated for 56 m3/h of output and 40 m of head. 
     The NF system has 138 nominal 8 inch (20 cm) NF elements inserted in 23 pressure vessels. Each pressure vessel holds 6 elements in series. The first stage  16  has eight pressure vessels. Five of these pressure vessels are plumbed in parallel and make up the first portion  16 A. The remaining three pressure vessels are plumbed in parallel and make up the second portion  16 B. A first intermediate stage  18  has six pressure vessels plumbed in parallel. A second intermediate stage  19  has three pressure vessels plumbed in parallel. The last stage  20  has five pressure vessels plumed in parallel.  5  respectively. First portion  16 A and third stage  20  are identical to facilitate switching between them. Second portion  16 B always receives feed water  12  directly from the feed pump  22 . 
     When the valves are configured in a first configuration as shown in  FIG. 2 , valves V 1 , V 3 , V 5  and V 7  are open and valves V 2 , V 4 , V 6  and V 8  are closed. The feed water  12  is pumped to the first portion  16 A and the second portion  16 B of the first stage  16  by the feed pump  12 . Concentrate from the first stage  16  is fed to the first intermediate stage  18  and then to the second intermediate stage through the booster pump  46 . Concentrate from the second intermediate stage  19  is fed to the last stage  20  through the recirculation pump  24 . When the valves are configured in a second configuration such that the first portion  16 A and the last stage  20  are switched, valves V 2 , V 4 , V 6  and V 8  are open and valves V 1 , V 3 , V 5  and V 7  are closed. The feed water  12  is pumped to the last stage  20  and the second portion  16 B by the high pressure pump  22 . The concentrate from the last stage  20  and the first portion  16 B is fed to the first intermediate stage  18  and then to the second intermediate stage through the booster pump  46 . Concentrate from the second intermediate stage  19  is fed to the first portion  16 A through the recirculation pump  24 . Valve V 9  is used as a control valve to control the concentrate flow in both configurations. 
     Optionally, though not shown in  FIG. 2 , the valves and conduits of the second system  40  could be changed such that at some times another selected portion of the first stage  16  receives the concentrate last and the last stage  20  receives feed water first in parallel with the remainder of the first stage. However, it is desirable for the portion of the first stage  16  that is switched with the last stage  20  to have the same number of elements and pressure vessels as the last stage  20  such that the recirculation pump  24  and second system  40  as a whole works well in both configurations. In the second system  20 , the first stage  16  does not have twice as many pressure vessels as the last stage  20  as a result of optimizing the system design. While it would be possible to rotate which five of the eight pressure vessels of the first stage  16  are switched with the last stage  20 , this complication is typically not justified since the primary purpose of switching the order of flow is to allow the last stage  20  a period of being exposed to un-concentrated feed water  12 . If the first stage  16  happened to have twice as many elements and pressure vessels  20 , it would be easier to rotate which portion of the first stage  16  is switched with the last stage  20 . However, in most cases, the ability to switch the last stage  20  with either portion  16 A,  16 B in the first stage  16  would not justify altering an optimized choice of the number of elements and pressure vessels in each stage. These comments assume that fouling in the last stage  20  can be adequately controlled by exposing the last stage  20  to un-concentrated feed water  12  for one half of the system operating time or less. If not, then the system could be modified to allow for a method of operation such that the pressure vessels of the first stage  16  receive concentrate last more than half of the operating time, but a system of rotation between them results in each individual pressure vessel receiving concentrate last for less than half of the operating time. 
     In a third example, shown in  FIG. 3 , a third system  42  was designed for treating an industrial effluent with over 200 mg/l of COD, 90 m3/h of permeate flow, and 90% of recovery. A recirculation pump  24  was used for the last stage  20 . The third system  42  is similar to system  10  of  FIG. 1  but without valves and conduits allowing the order of the first stage  16  and last stage  20  to be reversed. 
     The third system  42  has 126 nominal 8 inch (20 cm) NF elements inserted in 21 pressure vessels. Each pressure vessel holds 6 elements in series. A high pressure feed pump  22  is rated for 100 m3/h of output and 110 m of head. A recirculation pump  24  is rated for 110 m3/h of output and 40 m of head. The thirds system  42  has three stages  16 ,  18 ,  20 . The first stage  16  and last stage  20  each have 8 pressure vessels plumbed in parallel. The intermediate stage  18  has 5 pressure vessels plumbed in parallel. The feed water  12  is pumped to a feed inlet of the first stage  16  by the high pressure pump  22 . The concentrate from the first stage  16  is fed to a feed inlet of the intermediate stage  18 . Concentrate from the intermediate stage  18  is to a feed inlet of the third stage  20  through the recirculation pump  24 . 
     This written description uses examples to disclose the invention and 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.