WO2020244999A1

WO2020244999A1 - Membrane filtration assembly

Info

Publication number: WO2020244999A1
Application number: PCT/EP2020/064708
Authority: WO
Inventors: Tinto Johnichan ALENCHERRY; Karthick MANICKAM; Rajeesh Kumar RAMACHANDRAN; Himadri SEKHAR KUMAR
Original assignee: Unilever N.V.; Unilever Plc; Conopco, Inc., D/B/A Unilever
Priority date: 2019-06-03
Filing date: 2020-05-27
Publication date: 2020-12-10

Abstract

The present invention relates generally to a water treatment assembly and in particular to a reverse osmosis assembly. It is an object of the present invention to provide a membrane assembly for treating water which increases the permeate volume without altering the existing membrane module. It is yet another object of the present invention to provide a membrane assembly for enhancing the membrane life. At least some of the above objects is achieved by providing a membrane assembly for treating water with a flow resistance element disposed on the permeate line for increasing the pressure in at least part of the permeate line upstream the flow resistance element and a valve means located between the pump and the membrane module; wherein the valve means is configured to intermittently change the direction of flow of the feed water discharged from the pump to enter the membrane module through the concentrate line and exit the membrane module through the feed line.

Description

Membrane filtration assembly

Field of the invention

The present invention relates generally to a water treatment assembly and in particular to a reverse osmosis assembly.

Background of the invention

Water scarcity is increasing as fresh water sources are deteriorating in terms of both quality and quantity. This is increasing the reliance on groundwater, seawater, agricultural water recovery and potable water reuse. The concentration of minerals in various water sources at different geographies is also on the rise and there is a demand for removing minerals and salt from brackish water.

The most frequently used technology to meet the demand is reverse osmosis (RO) which is efficient in removing salt from most water sources. However, continuous increase in the salt content in water has led to poor life of in-home use reverse osmosis devices.

Some of the constraints to the successful use of membrane separation processes includes phenomena known as concentration polarization and fouling. Concentration polarization occurs when a concentration gradient of the retained components is formed at or near the membrane surface. Fouling is the deposition of material, referred to as foulant, on the membrane surface or in its pores, leading to a change in membrane behavior or even complete plugging of the membrane. These phenomena manifest over time and increase operating pressure whereby beyond a certain point any further increase in operating time and pressure does not increase the permeate flux. The severity of the effects of these phenomena varies with the membrane type, operation conditions and the composition of the feed water. In recent times, increase in concentration of minerals and salts in water sources at various geographies poses a challenge to produce potable water without affecting the membrane life. It is desired to increase the volume of purified water produced from feed water having higher salt concentration without altering the existing membrane structure and membrane life.

One of the ways of improving permeate flux in existing membrane module is by increasing the transmembrane pressure which enhances the driving force essential for increasing the permeate flux. However, it is believed that an increase in

transmembrane pressure also corresponds with an increase in the energy utilized for producing the purified water.

EP1457460 A2 (Millipore Corporation, 2004) discloses a system and method of purifying water which provides excellent operating conditions for the selective permeability membrane. The water purification system includes a selective permeability membrane and a flowrate regulator disposed in the permeate flow path from the selective permeability membrane to maintain a substantially constant predetermined permeate flowrate. US6168714 B1 (Mias et al., 2001) discloses a filtration system for improving the average transmembrane flux by periodic reversal of the direction of flow of the feed stream in the membrane module, while maintaining the cross-flow. The filtration system includes a feed supply for providing a feed solution; a feed pump connected to the feed supply, a cross-flow membrane filter connected downstream of the feed pump for separating the feed into a permeate and a retentate; and a valve manifold assembly located between the feed pump and the cross-flow membrane filter for selectively reversing the flow of the feed through the cross-flow membrane filter. The periodic reversal of the direction of flow of the feed stream keeps the system in a

hydrodynamically transient state and prevents the formation of an undesirable stable boundary layer at the membrane system. Filtration system of US6168714 B1 is disclosed to treat feed solution with a solute concentration ranging from 0.01 wt.% to 5 wt.%. The present inventors have found that the filtration system employed in US6168714 B1 when used for treating feed water with higher solute concentrations as is commonly found in water sources in many regions, the filtration system may require reversal of flow in close succession making it inefficient to treat feed water for household use.

Thus, there remains a need for an improved membrane filtration assembly for treating water which provides increased permeate flux and membrane life without

compromising on the quality of the treated permeate water and without increasing the energy consumption.

It is thus an object of the present invention to provide a membrane assembly for treating feed water which increases the permeate volume without significantly altering the existing membrane module.

It is yet another object of the present invention to provide a membrane assembly for enhancing the membrane life.

It is a further object of the present invention to provide a simple, efficient and inexpensive membrane assembly for increasing the permeate volume.

It is also an object of the present invention to provide a membrane assembly which increases permeate volume without increasing the transmembrane pressure as the membrane life increases.

Summary of the invention

At least some of the above objects is achieved by providing a membrane filtration assembly for treating feed water which comprises a flow resistance element (4) disposed on the permeate line (5) for increasing the pressure in at least part of the permeate line (5) upstream the flow resistance element (4) and a valve means (8) located between the pump (1) and the membrane module (3); wherein the valve means (8) is configured to intermittently change the direction of flow such that the feed water discharged from the pump (1) enters the membrane module (3) through the

concentrate line (7) and exit the membrane module (3) through the feed line (2).

It was found that the membrane filtration assembly (100) for treating feed water according to the present invention having the flow resistance element (4) and the valve means (8) configured to intermittently change the direction of flow of feed water as already described herein increases the volume of permeate water produced by the membrane assembly using existing membrane module along with providing enhanced membrane life. It is further found that an increase in the volume of the permeate water is achieved without increasing the energy consumption.

Throughout the description of the invention, the term“change in direction” means that the flow of feed water discharged from the pump changes direction from a first direction to a second direction which is opposite to that of the first direction. In the first direction feed water discharged from the pump enters the membrane module through the feed line and the concentrate water exits the membrane module through the concentrate line. In the second direction the feed water discharged from the pump enter the membrane module through the concentrate line and the water along with the contaminants back-flushed from the membrane exits the membrane module through the feed line. The feed water preferably flows in the second direction for the same duration as that in the first direction before changing the direction.

Throughout the description of the invention, the term“concentrate line” means the line or conduit for the flow of concentrate water or wastewater from the membrane module.

Throughout the description of the invention, the term“permeate line” means the line or conduit for the flow of purified water recovered from the membrane module.

Feed or feed stream refers to the liquid that is to be filtered by the membrane module.

Permeate water is the liquid that has passed through (permeated) the filtration membrane module. It can also be referred to as filtrate. Retentate water is the liquid that is retained on the feed side of the filtration membrane module. It can also be referred to as concentrate water.

Concentration polarization should be understood to mean the accumulation of the retained molecules (gel layer) on the surface of a membrane module and can be caused by a combination of factors: transmembrane pressure, crossflow velocity, feed water viscosity, and solute concentration.

Transmembrane flux is measured as litres of permeate water flowing per square meter of membrane surface per hour (L/m².hr).

Detailed description of the invention

According to a first aspect of the present invention disclosed is a membrane filtration assembly (100) comprising a pump (1), a membrane module (3), a concentrate flow restricting means (6), a flow resistance element (4) and a valve means (8).

Pump

The membrane assembly according to the present invention includes a pump (1) adapted to discharge incoming feed water under a pump pressure.

Although various types of pumps may be employed, the pump is typically a positive displacement pump, preferably a piston operated pump. Preferably the pump is adapted to generate a pump pressure adequate to provide desirable backpressure in the concentrate line (7) to ensure proper filtration. Preferably the pressure of the feed water discharged from the pump (1) is higher than the osmotic pressure of the feed water.

A preferred pump is a positive-displacement pump preferably equipped with a variable- speed pump head and preferably a controller to vary the flow rates.

Preferably the membrane assembly includes a feed valve upstream the pump and operably connected to the pump for controlling the flow rate of the incoming feed water entering the membrane module. The feed valve is connected to the pump such that the feed valve is open when the pump is powered and closed when the pump is switched off. Membrane module

The membrane assembly according to the present invention includes a membrane module (3) connected to (a) a feed line (2) in fluid communication with the pump (1), (b) a permeate line (5) adapted for connection to a dispenser of permeate water and, (c) a concentrate line (7) adapted for connection with a drain of concentrate water.

Preferably the membrane module comprises a membrane selected from ultrafiltration membrane, nanofiltration membrane, microfiltration membrane, reverse osmosis membrane. Preferably the membrane module is a reverse osmosis membrane. Preferably the membrane is spiral wound or hollow-fibre membrane, most preferably a spiral wound membrane. In the membrane module according to the present invention the membrane is preferably a cross-flow type membrane. In a preferred embodiment the membrane is a cross-flow reverse osmosis membrane. The material of construction of the reverse osmosis membrane preferably includes any commercially available standard material for construction, preferably selected from polysulfone, polyvinylidene fluoride (PDVF) and cellulose acetate. Preferably the incoming feed water entering the feed port of the membrane module, in a reverse osmosis membrane has a feed pressure of at least 40psig.

Preferably the reverse osmosis membrane according to the present invention is packed into a pressure vessel. The pressure vessel used in the present invention are not particularly limited but preferably include a solid structure capable of withstanding pressure associated with operating conditions. The vessel structure preferably includes a chamber having an inner periphery corresponding to that of the outer periphery of the membrane module to be housed therein. The length of the chamber preferably corresponds to the length of the membrane module or the combined length of more than one membrane module to be sequentially (axially) loaded, example 2 to 8 module. The pressure vessel may also include one or more end plates that seal the chamber once loaded with one or more membrane module. The vessel further includes at least one fluid inlet (feed) and two fluid outlets (concentrate and permeate), preferably located at opposite ends of the chamber. The orientation of the pressure vessel is not particularly limited, example either horizontal or vertical orientations may be used.

Concentrate flow restricting means

The membrane assembly according to the present invention includes a concentrate flow restricting means (6) disposed on the concentrate line (7). The concentrate line is adapted for connection with a drain for concentrate water. The concentrate flow restricting means is adapted for draining concentrate water at a predetermined rate through the concentrate line.

Preferably the concentrate flow restricting means (6) is an on-off valve, solenoid value or a throttle valve, still preferably a throttle valve.

Preferably the predetermined rate for draining the concentrate water through the concentrate line is from 200 mL/minute (3.34x1 O ⁶ cubic metre/second) to 1000 mL/minute (1.67x1 O ⁵ cubic metre/second) per membrane module most preferably from 400 mL/minute (6.67x10 ^e cubic metre/second) to 600 mL/minute (1.0x10⁵ cubic metre/second) per membrane module.

Flow resistance element

The membrane assembly according to the present invention includes a flow resistance element (4) disposed on the permeate line (5) for increasing the pressure in at least part of the permeate line (5) upstream the flow resistance element (4). Increase in permeate pressure is believed to increase the membrane permeability by providing a pressure gradient across membrane, preferably at constant transmembrane pressure. Preferably the flow resistance element (4) is adapted to maintain a constant transmembrane pressure between 10 psi (68.95 kPa) to 20 psi (13.79 kPa).

The flow resistance element (4) is preferably a variable valve. Preferably the flow resistance element is a control valve, more preferably a deformable member for reducing the effective flow section through the permeate line (5). For example, the deformable member can be an elastic eyelet forming a variable orifice and preferably made of elastomer. The deformable member exerts greater resistance to flow at higher permeate flow rates or greater pressure drops across the flow resistance element. The flow resistance element can include an orifice that becomes partially obstructed or changes shape, i.e. narrowing as permeate flow increases and opening as permeate flow decreases. The flow resistance element can comprise a plate in which a fixed orifice is formed but is preferably a variable orifice adjustable manually or by an automatic adjusting means.

For reasons of economy, the flow resistance element preferably comprises a needle valve or a capillary tube. The flow resistance element is preferably a capillary tube with internal diameter ranging between 0.2 mm to 0.8 mm, with length varying from 5 mm to 50 mm. The capillary tube can be made with any material which could withstand a pressure of 200 psi (1378.95 kPa).

Preferably the flow resistance element is either manually adjustable or operated by an automatic adjusting means. Alternately, the flow resistance element may include a motorized flow rate regulation valve operated by an automatic adjusting means.

The flow resistance element preferably exerts resistance to the flow of the permeate water in the permeate line that varies as a function of permeate flow rate, i.e.

increasing the resistance as permeate flow increases. Preferably the flow resistance element is adjustable to either increase the resistance with increase in the permeate flow rate or reduce the resistance with decrease in the permeate flow rate.

"Resistance" (R) is defined as the ratio of pressure drop (Dr) to flow (F), i.e. R=Ap/F. The flow resistance element increases resistance as flow (or pressure drop) across the flow resistance element increases. In this way, flow across the flow resistance element can be maintained relatively constant in operation over a desired pressure range.

The degree of pressure drop created by the flow resistance element may be optimized based upon the characteristics of the membrane assembly, for example; number of modules, quality of feed liquid, feed operating pressure, etc. The assembly may preferably include a plurality flow resistance element spaced along the permeate line, each providing a successive pressure drop. Valve means

The membrane filtration assembly according to the present invention includes a valve means (8) located between the pump (1) and the membrane module (3); wherein the valve means (8) is configured to intermittently change the direction of flow of the feed water discharged from the pump (1) to enter the membrane module (3) through the concentrate line (7) and exit the membrane module (3) through the feed line (2). The water exiting the feed line (2) including the contaminants drains out through the concentrate flow restricting means (6) disposed on the concentrate line and is drained out of the membrane filtration assembly. Preferably the valve means may be any conventional diverting valve means. The diverting valve means is adapted to permit feed water from the pump to flow in the feed line and pass through the membrane module into the concentrate line in one position (production cycle), where the feed water is filtered via conventional filtering medium in the membrane module. In another position (cleaning cycle), the diverting valve means directs feed water in a reverse direction to the production cycle. The diverting valve means permits feed water from the pump to flow in the concentrate line and pass through the membrane module into the feed line. The feed water in the cleaning cycle backwashes the membrane and the diverting valve further directs the backwashed water containing the contaminants to a drain.

The valve means according to the present invention is preferably configured to change the direction of the flow of the feed water in response to reaching a predetermined process set point. Preferably the process set point is a predetermined time, preferably from 5 minutes to 90 minutes, of feed water flow across the membrane module in a selected direction. In another embodiment, the process set point is a predetermined permeate flux level preferably from 20% to 80% of the permeate flux without flow resistance, from the membrane module. In yet another embodiment the process set point is a function of the frequency of operation of the pump. The valve means may be selected from but is not limited to multiport valve or a slide valve. The valve means may be selected from a plurality of manual valve, a plurality of electrical valves, or a single 5/2 directional control valve (DCV). Most preferably the valve means is a 5/2 directional control valve (DCV).

The 5/2 directional control valve (DCV) includes an input port (S) connectable to the pump and has 4 output ports (A, B, C and D) which are connectable to the feed line of the membrane module, concentrate line of the membrane module and 2 output ports are connectable to the concentrate flow restricting means. Preferably the 5/2 directional control valve (DCV) periodically reverse the direction of cross-flow across the RO membrane at specified time.

Preferably the membrane filtration assembly according to the present invention includes a control means for receiving signal and controlling the operation of the valve means in response to reaching a predetermined process set point.

The combination of the flow resistance element along with the valve means according to the present invention provides for a membrane filtration assembly having enhanced membrane permeability and flux and where the membrane life is enhanced without undesirable scaling due to concentration polarization. The input port (S) of the valve means is connected to the pump and the 4 output ports (A, B, C and D) are connected to feed port of RO module, concentrate port of RO module and to concentrate flow restricting means as shown in figure 2.

According to a second aspect of the present invention provided is a method for treating feed water using the membrane filtration assembly according to the first aspect, the method comprising the steps of:

i supplying feed water to be treated to a pump (1) for discharging the feed water under pressure;

ii feeding the pressurized feed water to the membrane module (3) through a feed line (2) in fluid communication with the pump (1); iii causing the feed water to flow through the membrane module (3) to separate the feed water into a purified permeate water stream which flows out through the permeate line (5) and a concentrate water stream which flows out through the concentrate line (7);

iv resisting the flow of the permeate flowing through the permeate line (5) by a flow resistance element (4) for increasing the pressure in the permeate line (5) upstream the flow resistance element (4);

v intermittently changing the direction of flow of the feed water from the pump (1) by a valve means (8) which changes the direction and diverts the feed water to enter the membrane module (3) through the concentrate line (7) and exit the membrane module (3) through the feed line (2).

According to a third aspect of the present invention, provided is an use of a flow resistance element (4) disposed on the permeate line (5) wherein the flow resistance element (4) is adapted to increase the pressure in at least part of the permeate line upstream the flow resistance element and a valve means (8) located between the pump (1) and the membrane module(3); wherein the valve means (8) is configured to intermittently change the direction of flow of the feed water discharged from the pump (1) to enter the membrane module (3) through the concentrate line (7) and exit the membrane module (3) through the feed line (2) in a membrane filtration assembly according to the first aspect for treating feed water with LSI 0.5 to 3 to improve the permeate flux while enhancing membrane life.

These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment when considered with the drawings.

Brief description of the figures

Figure 1 is a schematic representation of the membrane assembly (100) in accordance with the present invention showing the flow resistance element (4). Figure 2 is a schematic depiction of the membrane assembly (100) showing the valve means (8) in accordance to the present invention.

Detailed description of the figures

Figure 1 is a schematic representation of a membrane assembly 100 according to the invention showing the flow resistance element 4. The membrane assembly 100 comprises a pressure vessel having a filter space holding a spiral wound cross flow membrane module 3. The pressure vessel of the membrane module 3 comprises a feed port connected to the feed line 2 at one end of the filter space and in fluid communication with the pump 1 , and a concentrate port connected to the concentrate line 7 at the other end of the filter space. The feed port connected to the feed line 2 feeds a liquid through the membrane module 3 in a feed flow direction from the feed line 2 towards the concentrate line 7 during the production cycle. A concentrate flow restricting means 6 is disposed on the concentrate line 7 for controlling the concentrate flow rate. A permeate outlet is provided in the pressure vessel for pure water permeating from the membrane module 3 to flow into the permeate line 5. A flow resistance element 4 is positioned on the permeate line 5 to increase the pressure in a section of the permeate line upstream of the flow resistance element 4. One or more standardized control valves can be applied as flow resistance element 4. Preferably the flow resistance element 4 is a control valve, preferably connected with a control unit. In this way, the flow resistance element settings can be adjusted during the process, for instance to correct for pressure changes during the process. Alternatively, a flow resistance element 4 can be a local reduction of the diameter of the permeate line 5. In the filter assembly of Figure 1 , a system with one flow resistance element 4 is shown as an example. If desired, multiple flow restriction element may be applied. The flow resistance element 4 creates a pressure in at least a part of the membrane module 3 upstream the flow resistance element 4, this reduces the maximum pressure difference along the membrane module 3. The pump 1 provides a pressure difference over the feed side of the membrane module 3 for feeding the liquid through the feed line 2 along a feed flow direction in the module. This creates on the membrane module, a transmembrane pressure such that the pressure is higher at the feed side than at the permeate side. The transmembrane pressure induces transport of liquid through the membrane module to the permeate side.

As shown in Figure 2, the membrane assembly according to the present invention includes a valve means 8 located between the pump 1 and the membrane module 3. In a preferred embodiment the valve means 8 is a single 5/2 directional control valve connected between the pump 1 and the membrane module 3. In the production cycle, the pressurized water from the pump 1 enters the 5/2 directional control valve 8 through the input port S and exists through the output port A in a direction towards the feed line 2 connected to the membrane module 3. The feed water thereafter travels through the membrane module 3 and exists the membrane module and into the concentrate line 7, thereafter it re-enters the valve means 8 through the output port B and is then pushed out through the output port C into the concentrate line 7 and is drained out of the membrane assembly through the concentrate flow restricting means 6.

In the cleaning cycle, the valve means 8 reverses the direction of flow of feed water discharged from the pump 1. In the cleaning cycle, the pressurized water from the pump 1 enters the 5/2 directional control valve 8 through the input port S and flows out of the valve means 8 through the output port B in a direction towards the concentrate line 7 connected to the concentrate outlet at the concentrate side of the membrane module. The feed water travels through the membrane module 3 cleaning the membrane and carrying the deposits on the membrane along with it towards a feed side of the membrane module connected to the feed line 2 e and thereafter exists the membrane module to flows into the feed line 2, thereafter the feed water containing the contaminants enters the valve means 8 through the output port A and thereafter flows out of the valve means 8 through the output port D and into the concentrate line 7 and is drained out of the membrane assembly through the concentrate flow restricting means 6. In a membrane assembly preferably including a reverse osmosis membrane module, the cleaning of the membrane is by way of periodic reversal of the flow direction of the feed water through the membrane module from the feed line to the concentrate line. Precipitation fouling or the scaling of the membrane due to the deposition of the sparingly soluble salts and minerals on the surface of the membrane reduces the membrane performance. Periodic reversal reduces precipitation fouling, when the reversal of the flow direction of the feed water is conducted before the induction time for precipitation of sparingly soluble salts like CaCCh. Induction time refers to the time between the moment solution reaches supersaturation and the onset of precipitation. The induction time (T) is calculated based on the super saturation and any increment in the value of the super-saturation results in shorter induction time (T). Induction time refers to the time in between the solution reaching supersaturation and the onset of precipitation.

A water which is super-saturated in CaCCh is alternately indicated by Langelier Saturation Index (LSI). Water with LSI greater than 0, is supersaturated with respect to calcium carbonate (CaCCh) and scale forming, while that with LSI lesser than 0 is undersaturated with respect to calcium carbonate. Further feed water with LSI between 0 and 0.5 is slightly scale forming while LSI above 0.5 indicate strong scale forming

(Desalination 136 (2001) 243-254). For high LSI system induction time is zero as the feed water itself is supersaturated. Thus, the periodic reversal will not reduce the precipitation potential as induction time is closer to 0.

Examples

Preparation of test water: Test water with a solute concentration of 2000 ppm was prepared by adding sodium chloride and sodium bicarbonate salts along with calcium chloride and magnesium sulphate into distilled water. The test water had an LSI of 1.1.

The test water was passed through different comparative membrane assembly and the membrane assembly according to the present invention. The details of the various membrane assembly are as given hereinafter. Control membrane assembly (C): In the control (C) set up the feed test water was pressurized using a pump and was fed to the pressure vessel enclosing the membrane module. The feed water was allowed to flow through the membrane module in a feed flow direction from the feed line towards the concentrate line. The pure water permeated from the membrane and was collected from the permeate line. The concentrated water moved towards the concentrate outlet of the pressure vessel connected to the concentrate line and is drained out through the concentrate flow restricting means disposed on the concentrate line. In the control set up a flow resistance element 4 was not provided on the permeate line. In the control membrane assembly, a valve means 8 was also not provided.

Comparative membrane assembly (Ex A): In a first comparative example (Ex A) the set-up of the membrane assembly was similar to the control (C) except that the assembly is modified by the addition of a capillary tube placed in the permeate line between the permeate port of the pressure vessel and the dispenser of permeate water. The capillary tube serves as the flow resistance element 4 disposed on the permeate line 5 for increasing the pressure in at least part of the permeate line upstream the flow resistance element 4. In the first comparative example (Ex A) the membrane assembly was not provided with a valve means 8.

Comparative membrane assembly (Ex B): In a second comparative example (Ex B) the set-up of the membrane assembly was similar to the control (C) except that the assembly was modified by the addition of a 5/2 directional control valve located in between the pump and the reverse osmosis module. The 5/2 directional control valve was operationally connected to the reverse osmosis module placed in the pressure vessel. The solenoid of the 5/2 direction valve is charged alternately every 15 minutes to reverse the direction of flow of the feed test water pumped by the pump through the reverse osmosis membrane in the pressure vessel.

Inventive membrane assembly (Ex 1): In an example according to the present invention (Ex 1) the set-up of the membrane assembly was similar to Ex B except that the assembly is modified by the addition of a capillary tube placed in the permeate line between the permeate port of the pressure vessel and the dispenser of permeate water. The capillary tube serves as the flow resistance element disposed on the permeate line for increasing the pressure in at least part of the permeate line upstream the flow resistance element.

In each of the above membrane assembly the TDS and flow rate of the permeate water is constantly monitored along with monitoring the TDS of feed water. The life of the membrane was defined as the point when the flow rate of permeate water drops below 100 mL/minute (1.67x10⁶ cubic metre/second). The salt removal was calculated as the amount of salt removed in permeate water compared to the input TDS in terms of percentage. Recovery was calculated as the total permeate water obtained from the membrane assembly compared to the total input feed water passed through the membrane assembly. The calculated values were recorded and is provided in table 1 below.

Table 1

The data shows that when treating hard water with a higher salt concentration (LSI of 1.1), as compared to the control the comparative membrane assembly of Ex A which uses a low feed pressure and has a flow resistance element for increasing the permeate pressure shows an improvement in the life of the membrane but there is a reduction in the salt removal.

In the comparative membrane assembly of Ex B which incorporates a flow reversal of the feed water at an interval of 15 minutes a reduction in the life of the membrane and reduction in performance of the membrane was observed. As compared to the above two comparative examples (Ex A and Ex B), the membrane assembly of Ex 1 according to the present invention shows an improvement in the membrane life without compromising on the salt removal from the water. The permeate water of Ex 1 is comparable with the water from the control in terms of the salt removal% but has a higher membrane life.

Claims

1 A membrane filtration assembly (100) for treating feed water, the assembly (100) comprising:

i a pump (1) adapted to discharge incoming feed water under a pressure; ii a membrane module (3) connected to (a) a feed line (2) in fluid

communication with the pump (1), (b) a permeate line (5) adapted for connection to a dispenser of permeate water and, (c) a concentrate line (7) adapted for connection with a drain of concentrate water;

iii a concentrate flow restricting means (6) disposed on the concentrate line

(7);

iv a flow resistance element (4) disposed on the permeate line (5) for

increasing the pressure in at least part of the permeate line upstream the flow resistance element (4);

v a valve means (8) located between the pump (1) and the membrane

module (3);

wherein the valve means (8) is configured to intermittently change the direction of flow of the feed water discharged from the pump (1) to enter the membrane module (3) through the concentrate line (7) and exit the membrane module (3) through the feed line (2).

2 An assembly according to claim 1 wherein the valve means (8) is configured to change the direction of flow of the feed water in response to reaching a predetermined process set point.

3 An assembly according to claim 2 wherein said process set point is a

predetermined time of feed flow across said membrane module (3) in a selected direction.

4 An assembly according to claim 2 wherein said process set point is a

predetermined permeate flux level. A membrane system according to claim 2 wherein said process set point is a function of frequency of operation of the pump (3). An assembly according to claim 1 or 2 wherein the assembly comprises a control means for receiving signal and controlling the valve means (8) in response to reaching a predetermined process set point. An assembly according to any one of the preceding claims wherein the valve means (8) is a 5/2 directional valve. An assembly according to any one of the preceding claims wherein the flow resistance element (4) provides a resistance to the flow through the permeate line (5) that varies as a function of permeate flow rate. An assembly according to claim 8 wherein the flow resistance element is adjustable to either increase the resistance with increase in the permeate flow rate or reduce the resistance with decrease in the permeate flow rate. An assembly according to claim 8 wherein the flow resistance element (4) is a variable valve, preferably a control valve. An assembly according to any one of the preceding claims wherein the assembly comprises a control means for receiving signal and controlling the flow resistant element (4) in response to changing permeate flow rate. An assembly according to any one of the preceding claims wherein the membrane module (3) is a cross-flow reverse osmosis membrane, preferably spirally wound. An assembly according to any one of the preceding claims wherein the assembly comprises a feed valve upstream the pump and operably connected to the pump (1) for controlling the flow rate of incoming feed water entering the membrane module (3). A method for treating water using the membrane filtration assembly according to any one of the preceding claims 1 to 12, said method comprising the steps of: i supplying feed water to be treated to a pump (1) for discharging the feed water under pressure;

ii feeding the pressurized feed water to the membrane module (3) through a feed line (2) in fluid communication with the pump (1);

iii causing the feed water to flow through the membrane module (3) to separate the feed water into a purified permeate water stream which flows out through the permeate line (5) and a concentrate water stream which flows out through the concentrate line (7);

iv resisting the flow of the permeate water stream flowing through the permeate line (5) by a flow resistance element (4) for increasing the pressure in at least part of the permeate line (5) upstream the flow resistance element (4); wherein direction of flow of the feed water discharged from the pump (1) is intermittently reversed by a valve means (8) which diverts the feed water to enter the membrane module (3) through the concentrate line (7) and exit the membrane module (3) through the feed line (2). Use of a flow resistance element (4) disposed on the permeate line (5) wherein the flow resistance element (4) is adapted to increase the pressure in at least part of the permeate line upstream the flow resistance element and a valve means (8) located between the pump (1) and the membrane module(3); wherein the valve means (8) is configured to intermittently change the direction of flow of the feed water discharged from the pump (1) to enter the membrane module (3) through the concentrate line (7) and exit the membrane module (3) through the feed line (2) in a membrane filtration assembly according to the first aspect for treating feed water with LSI 0.5 to 3 for improving the permeate flux while enhancing membrane life.