WO1997038786A1

WO1997038786A1 - Filter membranes with controlled product flow and systems using same

Info

Publication number: WO1997038786A1
Application number: PCT/US1997/006418
Authority: WO
Inventors: Robert L. Bradley; David R. Anderson
Original assignee: United States Filter Corporation
Priority date: 1996-04-12
Filing date: 1997-04-11
Publication date: 1997-10-23

Abstract

Filter membranes with controlled product flow as well as filter vessels (100-112) and filtration systems (80) which use the same. In one embodiment, the present invention provides a filter for purifying a fluid. The filter comprises a filter membrane and a flow control device (96). The flow control device (96) is in fluid communication with the product outlet (128) of the filter membrane such that the flow control device (96) supplies a backpressure to the product outlet (128) to control the flow rate of a fluid through the product outlet. Filter vessels (100-112) having a plurality of such filter membranes arranged in series are also described. Furthermore, the present invention provides filtration systems which include a plurality of such filter vessels arranged in series and/or parallel. The present invention also provides methods of using such filter membranes, filter vessels and filtration systems.

Description

FILTER MEMBRANES WITH CONTROLLED PRODUCT FLOW AND SYSTEMS

USING SAME

BACKGROUND OF THE INVENTION

JL Field of the Invention

The present invention relates generally to filter membranes with controlled product

flow and filtration systems using the same.

2. Description of the Related Art

Filter membranes are commonly used to purify water and other fluids. Various

membrane configurations have been proposed, including spiral wound, hollow fiber and plate

and frame. Fluid flowing in a feed stream enters the filter membrane and is separated into a

filtered product stream and an unfiltered or brine waste stream. To increase efficiency and

reduce cost associated with fluid purification, it is desirable to dispose filter membranes in a

series configuration such that the waste stream from one filter membrane becomes the feed

stream of the next filter membrane. Therefore, filter membranes are often arranged in series

within a filter vessel, and the filter vessels, themselves, are also disposed in series such that

the waste stream from a first filter vessel serves as the feed stream into the next filter vessel.

This series arrangement of filter vessels is commonly referred to as staging.

While an ideal filtration system would include a large number of staged filter vessels,

such a system is not normally practical because, as a feed stream flows through a filter vessel,

friction between the stream and each filter membrane reduces the pressure of the stream. As

a result, the pressure of the waste stream leaving the filter vessel is less than the pressure of

the feed stream entering the same filter vessel. Ultimately, after passing through a number of filter vessels, the pressure of the waste stream is reduced to a pressure too low to serve as the

feed stream ofthe next filter vessel.

To alleviate this problem, the pressure ofthe feed stream into the first filter vessel

may be increased, causing the waste stream pressure to increase as well. However, this can

cause overfluxing ofthe upstream filter membranes, premature fouling of these membranes,

increased maintenance and time associated with the filter vessel, and a reduced duty cycle for

the filter vessel. Alternatively, the number of filter vessels used in a filtration system may be

decreased, but this decreases the level of filtration achieved by the system. Another option is

to provide a pump between stages of filter vessels, but this increases the power necessary to

utilize the filtration system, resulting in relatively high costs.

Filtration systems that include ultrafilter membranes are particularly susceptible to the

problem of decreased waste stream pressure because the pressure drop between the feed

stream and waste stream of an ultrafilter membrane is comparatively high. As a result, an

unpurified fluid can pass through only a relatively small number of ultrafilter membranes

before the waste stream pressure becomes too low to act as the feed stream for another filter

membrane. Therefore, filter vessels do not typically contain more than about four ultrafilter

membranes, and filter vessels having ultrafilters are not normally staged.

SUMMARY OF THE INVENTION

Therefore, it is an object ofthe present invention to provide a filtration system having

a number of filter membranes arranged in series such that the pressure of the waste stream of

any filter membrane is not too low to serve as the feed stream for the next filter membrane.

It is another object ofthe present invention to provide a filter vessel having a plurality of filter membranes arranged in series such that the product flow rate from each filter

membrane is approximately the same such that overfluxing of upstream membranes is

substantially reduced.

It is yet another object ofthe present invention to provide such a filter vessel such that

the transmembrane pressure across each filter membrane is approximately the same such that

the product flow rate is approximately the same for each membrane such that overfluxing of

upstream membranes is substantially reduced.

It is a further object ofthe present invention to provide a filtration system having

staged filter vessels with a comparatively high number of filter membranes are arranged in

series such that cost and increase the efficiency associated with purifying fluids.

To achieve these and other objects, the present invention provides an improved filter

membrane designed such that a plurality ofthe filter membranes can be arranged in series

without having the waste stream pressure drop too low to serve as the feed stream for the next

filter membrane. This is attained by controlling the pressure differential between the feed

stream and the product stream of each membrane such that a predetermined value for this

pressure differential may be obtained independent ofthe feed stream pressure. Since the

product flow rate of each membrane depends on this pressure differential, the product flow

rate of each membrane is also be controlled independent ofthe feed stream pressure. By

controlling the product flow rate of each membrane, the feed stream pressure ofthe most

upstream membrane can be increased to a relatively high value without overfluxing and

premature fouling of this membrane. At the same time, the increased feed stream pressure

allows a comparatively large number of filter membranes to be arranged in series without the

waste stream pressure becoming too low for the waste stream to act as the feed stream for another filter vessel.

In one illustrative embodiment, the present invention provides a filter for purifying a

fluid. The filter comprises a filter membrane and a flow control device. The filter membrane

has an inlet, a product outlet and a waste outlet. The flow control device is in fluid

communication with the product outlet ofthe filter membrane and supplies a backpressure to

the product outlet to control flow rate of fluid through the product outlet.

In another illustrative embodiment, the present invention provides a filter vessel for

purifying a fluid. At least two filter membranes are arranged in series within the filter vessel.

The inlet ofthe first filter membrane is in fluid communication with the inlet ofthe filter

vessel to allow a fluid to flow therebetween, and the waste outlet ofthe first filter membrane

is in fluid communication with the inlet ofthe second filter membrane so that the waste

stream ofthe first filter membrane serves as the feed stream ofthe second filter membrane. A

flow control device is in fluid communication with the product outlet ofthe filter vessel, first

filter membrane or the second filter membrane such that the flow control device supplies a

back pressure to the product outlet with which it is in fluid communication to control the flow

rate ofthe fluid through the product outlet.

In yet another illustrative embodiment, the present invention provides a filter vessel

that has the at least two filter membranes arranged in series such that the product flow rate is

approximately the same for both filter membranes.

In a further illustrative embodiment, the present invention provides a filter vessel that

has the at least two filter membranes arranged in series such that the transmembrane pressure

is approximately the same for both filter membranes.

In still a further illustrative embodiment, the present invention provides a method for filtering a fluid with a filtration system. The method comprises the steps of: providing a first

filter vessel having a plurality of filter membranes arranged in series; introducing a feed

stream at a first pressure into the first filter vessel; and maintaining a relatively constant

product flow rate through each ofthe filter membranes.

In yet a further illustrative embodiment, the present invention provides a filtration

system which includes first and second filter vessels arranged such that the waste outlet of the

first filter vessel is in fluid communication with the inlet ofthe second filter vessel. Each

filter vessel includes a plurality of filter membranes.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, advantages and features ofthe present invention will be more clearly

appreciated from the following detailed description when taken in conjunction with the

accompanying drawings, in which:

Fig. 1 is a schematic view of one embodiment of a filter according to the present

invention;

Fig. 2 is a schematic view of one embodiment of a filter vessel according to the

present invention;

Fig. 3 is a schematic view of one embodiment of a filtration system according to the

present invention; and

Fig. 4 is a schematic view of one embodiment of a filtration system according to the related art. DETAILED DESCRIPTION

The present invention relates to an improved filter 10 (Fig. 1) which is designed so

that a plurality of such filters can be arranged in series while maintaining a relatively high

waste stream pressure. Filter 10 includes a filter membrane 12, an inlet 14, a feed path that

terminates at a waste outlet 16, a product path that terminates at a product outlet 18 and a

flow control device 20. A portion of a feed fluid may pass through membrane 12 along the

waste path without being purified and exit through waste outlet 16. Alternatively, the fluid

may be purified by filter membrane 12 and leave through product outlet 18. After flowing

through product outlet 18, the purified fluid passes through flow control device 20.

The flow rate of purified fluid that exits filter 10 through product outlet 18 depends

approximately linearly on the transmembrane pressure of filter 10 which is the pressure

differential between the fluid entering filter 10 at inlet 14 and the purified fluid leaving filter

10 at outlet 18. Therefore, as the transmembrane pressure of membrane 10 increases, the

flow rate of fluid through product outlet 18 increases. Flow control device 20 is designed to

supply a back pressure to the purified fluid flowing through product outlet 18 such that the

transmembrane pressure can be controlled while the pressure of the feed fluid at inlet 14 is

varied. Since the flow rate of fluid through product outlet 18 depends on the transmembrane

pressure of membrane 10, device 20 also controls the product flow rate while the pressure of

the feed fluid is varied. For example, by varying the back pressure supplied by flow control

device 20 to maintain a constant transmembrane pressure across filter 10, the pressure of the

feed fluid at inlet 14 may be increased without increasing the product flow rate and without

causing overfluxing of filter membrane 12.

A "flow control" device may be any device that can be placed in fluid communication with the product outlet of a filter membrane so that the device is capable of supplying a back

pressure to the product outlet ofthe filter membrane. Preferably, a flow control device is a

static flow control device or a dynamically responsive flow control device. Static flow

control devices supply a back pressure to the product outlet independent ofthe flow rate of

purified fluid through the product outlet. Static flow control devices include fixed orifice

devices, capillary devices and valves. An example of a fixed orifice device is a flat plate

having an orifice disposed therethrough such that the fluid flowing through the product outlet

passes through the orifice and is partially blocked by the flat plate. The size ofthe orifice

should be selected to achieve the desired flow of purified fluid using the equation:

where h_f is the pressure ofthe product stream in feet of water, Q is the flow rate ofthe

product stream, d is the diameter ofthe orifice in a direction normal to the path ofthe product

stream through the orifice, and k is a constant value of 0.0044. A capillary device may be

formed from a coiled tube. The length and diameter of the coiled tube should be chosen such

that a predetermined amount of purified fluid passes through the coiled tube. A valve

appropriate for use as a flow control device may be formed from a plurality of ball valves.

The ball valves should be selected manually to obtain the desired amount of purified fluid.

Dynamically responsive flow control devices supply a back pressure to the product

outlet that depends upon the flow rate of purified fluid through the product outlet.

Dynamically responsive flow control devices may be variable orifice flow control devices.

Such devices may include a flat plate having an orifice disposed therein and a rubber grommet located within the fixed orifice. As the pressure ofthe fluid leaving product outlet

18 increases, the diameter ofthe rubber grommet decreases. The reduced diameter ofthe

grommet increases the back pressure to outlet 18, reducing the flow rate of purified fluid

through outlet 18. One example of such a flow control device is a Dole valve.

Filter membranes appropriate for use in the present invention include, but are not

limited to, ultrafiltration membranes, microfiltration membranes and reverse osmosis

filtration membranes. Preferably, a filter membrane is an ultrafiltration membrane. The filter

membrane may be configured as a spiral wound membrane, a hollow fiber membrane or a

plate and frame membrane. Preferably, a filter membrane is a spiral wound membrane. In a

particularly preferred embodiment, a filter membrane is a spiral wound ultrafiltration

membrane.

Fig. 2 depicts an embodiment of a filter vessel 30 which includes a plurality of filter

membranes arranged in series such that the transmembrane pressure and product flow rate of

each filter membrane can be controlled to allow the feed pressure of vessel 30 to be increased

without resulting in overfluxing ofthe upstream filter membranes. Filter vessel 30 includes

inlet 32, waste outlet 34, product outlet 36, flow control device 38 and filters 40, 50, 60 and

70. Filters 40, 50, 60 and 70 have membranes 42, 52, 62 and 72 as well as inlets 44, 54, 64

and 74, respectively. In addition, filters 40, 50, 60 and 70 have waste outlets 46, 56, 66 and

76 and product outlets 48, 58, 68 and 78, respectively. Waste outlets 46, 56 and 66 are in

fluid communication with inlets 52, 62 and 72, respectively. Filters 40, 50 and 60 further

include flow control device 49, 59 and 69, respectively. Flow control devices 49, 59 and 69

supply a back pressure to outlets 48, 58 and 68, respectively.

Filters 40, 50, 60 and 70 are arranged within filter vessel 30 such that a purified fluid flowing through outlet 48, 58, 68 or 78 exits vessel 30 through outlet 36. Therefore, the flow

rate of purified fluid through outlet 36 is the sum ofthe flow rates through outlets 48. 58, 68

and 78. After passing through outlet 36, the purified fluid then passes through flow control

device 38 which is designed to supply a back pressure through outlets 48, 58, 68 and 78 to

achieve a predetermined flow rate of purified fluid leaving filter vessel 30 through outlet 36.

It is to be noted that with this arrangement of filter vessel 30, since the flow rates of purified

fluid flowing through outlets 36, 48, 58 and 68 are controlled, the flow rate of purified fluid

passing through outlet 78 may be controlled without the use of an additional flow control

device. Furthermore, the total flow rate of purified fluid exiting filter vessel 30 through

product outlet 36 can be controlled independent ofthe pressure ofthe fluid at inlet 32 of

vessel 30.

As discussed above, the pressure of an unpurified stream decreases as it flows through

each filter membrane in a filter vessel, so the pressure of a feed fluid at a filter vessel inlet is

higher than the pressure of the waste stream leaving the same filter vessel. This leads to

overfluxing ofthe upstream membranes and premature fouling of these membranes.

However, with the configuration of filter membranes and flow control devices depicted in

Fig. 2, the pressure ofthe feed stream at inlet 32 may be increased to increase the pressure of

the stream flowing through waste outlet 34 without causing overfluxing of filter membranes

40, 50 or 60.

For embodiments in which it is desirable to reduce the amount of overfluxing of

upstream filter membranes in a filter vessel to avoid premature fouling of these membranes,

the flow rates of purified fluid through the product outlets of each filter should be

approximately equal. Preferably, these flow rates vary by less than approximately 20%, more preferably less than approximately 10% and most preferably less than approximately 5%. To

achieve an approximately uniform flow rate of purified fluid through each product outlet,

back pressure supplied by each flow control device is predetermined such that the

transmembrane pressure across each filter should be approximately equal notwithstanding the

decrease in the pressure ofthe feed stream for each successive filter membrane. For static

flow control devices, the decrease in feed stream pressure of a filter membrane may be

compensated for by increasing the orifice ofthe flow control device in fluid communication

with that filter membrane. For dynamic flow control devices, the size ofthe orifice of a flow

control device in fluid communication with a filter membrane changes in response to the

decrease in feed stream pressure for that filter membrane to maintain a substantially constant

product flow rate for each filter membrane. Preferably, the variation between the

transmembrane pressure of each membrane is less than approximately 20%, more preferably

less than approximately 10% and most preferably less than approximately 5%.

Although an arrangement of flow control devices for a filter vessel is shown in Fig. 2,

it is to be appreciated that the present invention is not limited by this arrangement. According

to the present invention, the arrangement of flow control devices within a filter vessel is

limited only in that the filter vessel should have at least one flow control device. Therefore, a

filter vessel may have a flow control device in fluid communication with the product outlet of

each filter as well as the product outlet ofthe filter vessel. Alternatively, a filter vessel may

only have a flow control device in fluid communication with the filter vessel, without having

a flow control device for any ofthe filters. Other arrangements of flow control devices are

contemplated to be within the scope of the present invention. Preferably, however, a filter

vessel is arranged such that a dynamically responsive flow control device is in fluid communication with the product outlet ofthe filter vessel and all but the most downstream

filter membrane have a flow control device in fluid communication with their respective

product outlets.

Although depicted in Fig. 2 as having four filters, a filter vessel many any number of

filters. Typically, however, a filter vessel has from two to four filters. The filter membranes

disposed within the filters of a filter vessel may be the same or different types. Preferably,

however, the filter membranes disposed are all the same type and have approximately the

same pore size and active membrane area. In a particularly preferred embodiment, a filter

vessel has from two to four spiral wound ultrafiltration membranes, each membrane having

approximately the same pore size and active membrane area.

As noted above, control devices may be incorporated into filter vessels such that the

amount of purified fluid passing therethrough may be controlled independently ofthe

pressure ofthe fluid at the inlet ofthe filter vessel. Thus, the pressure ofthe fluid at the

vessel inlet may be increased without changing the amount of purified fluid flowing through

the filter vessel and without causing overfluxing ofthe upstream filter membranes. As a

result, the pressure ofthe unfiltered fluid exiting the filter vessel through the waste outlet can

be increased to allow this unfiltered fluid to serve as the feed stream of another filter vessel,

allowing filter vessels to be staged within a filtration system.

Fig. 3 depicts an embodiment of a staged filtration system 80 including filter vessels

arranged in series such that a high level purification of a fluid can be achieved without

excessive overfluxing or premature fouling ofthe upstream filter membranes housed within

the filter vessels. Filtration system 80 includes a first filter vessel bank 82 and a second filter

vessel bank 84 as well as a pump 86, a recirculation loop 88, a drain 92, a system inlet 94 and flow control device 96. First filter vessel bank 82 includes filter vessels 100, 102, 104 and

106, and second filter vessel bank 84 includes filter vessels 108, 110 and 1 12.

Feed stream 81 and recirculation stream 83 (described below) are combined and flow

through pump 86 to form inlet stream 85 which is input to system inlet 94. As a fluid in

stream 85 passes through system 80, it may be filtered by a filter within filter vessel 100, 102,

104 or 106 and enter first product stream 120. Alternatively, the fluid may flow through first

filter vessel bank 82 without being filtered and enter first waste stream 122. A fluid in stream

122 flows into second filter vessel bank 84. After entering vessel bank 84, the fluid may be

filtered by filter vessels 108, 1 10 or 112 and enter second product stream 124, or the fluid

may flow through vessel bank 84 without being filtered and enter system waste stream 126.

A fluid that is filtered by system 80 enters stream 120 or 124 which combine to form

system product stream 128. A fluid flowing in stream 128 passes through flow control device

96 which is designed to supply a back pressure such that the flow rate of stream 128 is

controlled. Although not shown in Fig. 3, a flow control device may be disposed along

stream 120 to control the flow rate ofthe purified fluid flowing from first vessel bank 82,

and/or a flow control device may be disposed along stream 124 to control the flow rate ofthe

purified fluid flowing from second filter vessel bank 84. In addition, any or all ofthe filter

vessels and filter membranes may have flow control devices as described herein.

A fluid that passes through system 80 without being filtered enters system waste

stream 126. The flow of stream 126 is controlled by valves 130 and 132 such that a fluid in

stream 126 may flow to drain 92 or enter recirculation loop 88. A fluid flowing through loop

88 is combined with the feed stream 81, passes through pump 86 and re-enters filtration

system 80. The filter vessels within a filter vessel bank are in a parallel arrangement such that the

inlets of the filter vessels are each in fluid communication with a common feed stream, and

the outlets ofthe filter vessels are each in fluid communication with a common product

and/or waste stream for the filter vessels. Although shown in Fig. 2 as including three or four

filter vessel arranged in parallel, a filter vessel bank is not limited by this number of filter

vessels. Instead, a filter vessel bank may have any number of filter vessels equal to or greater

than one. In addition, the filter vessels in a filter vessel bank need not all be arranged in

parallel. For example, a filter vessel bank may include portions in which filter vessels are

arranged in series and portions in which filter vessels are arranged in parallel. Alternatively,

a filter vessel bank may include more than one portion in which filter vessels are arranged in

parallel. In addition, a filter vessel bank may have multiple parallel filter vessel portions as

well as portions in which filter vessels are arranged in series.

The following examples are intended to demonstrate certain aspects ofthe present

invention and are in no way intended to be construed as limiting.

EXAMPLE 1

A typical ultrafilter membrane that can be used in the present invention is an OSMO^®

ultrafiltration membrane (Model 815PT3PS, available from Osmonics, located in

Minnetonka, MN). Table 1 demonstrates various parameters for an ultrafilter vessel having

three such ultrafiltration membranes arranged in series. The ultrafilter vessel has an operating

temperature of 25 °C. The pressure ofthe feed stream at the inlet ofthe filter vessel is 30

pounds per square inch (PSI), and the flow rate ofthe fluid at the filter vessel inlet is 51

gallons per minute (GPM). Relative to feed stream ofthe filter vessel, membrane 1 is the

most upstream ultrafiltration membrane, and membrane 3 is the most downstream ultrafiltration membrane. Reject corresponds to the flow rate of the waste stream from each ultrafiltration membrane, and avg. press is the average pressure within an ultrafiltration membrane. In addition, product is the flow rate of the filtered product that flows from each ultrafiltration membrane, and total represents the total flow rate of filtered product from the ultrafilter vessel. Furthermore, dP is the pressure drop across each membrane. As can be seen from Table 1 , without the use of a flow control device to provide product side throttling, the product flow rate from each ultrafiltration membrane in the ultrafilter vessel is different. As a result, membrane 1 is exposed to a higher flux of fluid than membranes 2 or 3.

Table 1

EXAMPLE 2 Fig. 4 shows a non-staged ultrafiltration system 200 designed to provide 160 GPM of filtered fluid. The system includes a feed stream 201, a pump 202, a filter vessel bank 204 having a valved inlet 206. a product stream outlet 208 and a waste stream outlet 210. The waste stream may flow to a waste drain 212 along flow path 21 1 or pass through a recirculation loop 214 to feed stream 200 along flow path 213. The flow rate along paths 21 1 and 213 is regulated by valves 215 and 217. With this arrangement, the total flow rate into system 200 is the sum of the flow rates of stream 201 and 213.

Filter vessel bank 204 includes filter vessels 216-234 arranged in parallel such that the inlet each filter vessel is in fluid communication with the inlet 206, the waste outlet of each

filter vessel is in fluid communication with waste outlet 210, and the product outlet of each

vessel is in fluid communication with product outlet 208. Each filter vessel includes three

Model 815PT3PS OSMO^® ultrafiltration membranes arranged in series.

To provide the desired product flow rate of 160 GPM, the total flow rate into filter

vessel bank 204 is 510 GPM, and the flow rate of streams 201 and 213 are 176 GPM and 334

GPM, respectively. Pump 202 operates at 50 horsepower (HP) to supply filter vessel bank

204 with the 510 GPM at a backing pressure of 50 PSI. Since only 16 GPM of the fluid

flows to drain 212, filtration system 200 has a 90% recovery rate.

EXAMPLE 3

An ultrafiltration system designed to provide a product flow rate of 160 GPM was

arranged similar to the system depicted in Fig. 3. Fixed orifice flow control devices were

used for the filters and Dole valves as the flow control device for the filter vessels. Each filter

vessel included four OSMO^® 815PT3PS ultrafiltration membranes arranged in series. The

feed stream for the system had a flow rate of 176 GPM and the flow rate ofthe fluid in the

recirculation loop was 90 GPM. The pump operated at 25 HP such that the fluid at the inlet

ofthe filtration system had a flow rate of 266 GPM and a backing pressure of 90 PSI. The

waste stream flowing to the drain had a flow rate of 16 GPM.

Table 2 demonstrates various parameters for this filtration system. Stages 1 and 2

relate to the filters in the first filter and second vessel banks, respectively, from the most

upstream filter to the most downstream filter. Feed corresponds to the flow rate ofthe feed

stream for each membrane, and brine corresponds to the flow rate of the waste stream from each membrane. In addition, product is the flow rate of fluid from each membrane, and delta

p is the difference between the inlet pressure and the outlet pressure of each membrane.

Pressure feed, net driv, trans P, pressure brine, orifice and drill represent the pressure ofthe

fluid at the filter inlet, the back pressure against the outlet ofthe filter, the transmembrane

pressure, the pressure ofthe waste stream, the diameter ofthe orifice in the flow control

device and the size ofthe drill bit used to form the orifice, respectively.

Table 2 indicates that utilizing flow control devices in accordance with the present

invention can result in a constant product flow rate for each membrane in a filtration system.

Because the flow rate of purified fluid from each product outlet is approximately the same,

overfluxing ofthe upstream filters ofthe filtration is reduced.

Table 2

Having thus described certain embodiments ofthe present invention, various

alterations, modifications and improvements will readily occur to those skilled in the art.

Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention.

What is claimed is:

Claims

1. A filter vessel for purifying a fluid, the filter vessel having an inlet, a waste outlet and

a product outlet, the filter vessel comprising:

a housing;

a first filter disposed within the housing, the first filter comprising:

a filter membrane having an inlet in fluid communication with the inlet ofthe

filter vessel, a waste outlet and a product outlet;

a second filter disposed within the housing, the second filter comprising:

a filter membrane having an inlet in fluid communication with the waste outlet

ofthe filter membrane ofthe first filter, a waste outlet in fluid communication with the waste

outlet ofthe filter vessel and a product outlet in fluid communication with the product outlet

ofthe filter vessel; and

a first flow control device in fluid communication with an outlet selected from the

group consisting of the product outlet ofthe filter vessel, the product outlet ofthe first filter

and the product outlet ofthe second filter, the first flow control device being capable of

supplying a backpressure to the outlet to control a flow rate of a fluid through the outlet.

2. The filter vessel according to claim 1, wherein the first flow control device is in fluid

communication with the product outlet ofthe filter vessel for supplying a backpressure to the product outlet ofthe filter vessel to control a flow rate of a fluid through the product outlet of

the filter vessel.

3. The filter vessel according to claim 2, further comprising a second flow control device in fluid communication with the product outlet ofthe first filter membrane for supplying a

backpressure to the product outlet ofthe first filter membrane to control a flow rate of a fluid

through the product outlet ofthe first filter membrane.

4. The filter vessel according to claim 3, further comprising a third flow control device is

in fluid communication with the product outlet ofthe second filter membrane for supplying a

backpressure to the product outlet ofthe second filter membrane to control a flow rate of a

fluid through the product outlet ofthe second filter membrane.

5. The filter vessel according to claim 2, further comprising a second flow control device

in fluid communication with the product outlet of the second filter membrane for supplying a

back pressure to the product outlet ofthe second membrane to control a flow rate of a fluid

through the product outlet of the second membrane.

6. The filter vessel according to claim 1 , wherein the first flow control device is in fluid

communication with the product outlet ofthe first filter membrane for supplying a

through the product outlet ofthe first filter membrane.

7. The filter vessel according to claim 6, further comprising a second flow control device

fluid through the product outlet ofthe second filter membrane.

8. The filter vessel according to claim 7, wherein the first flow control device supplies a

back pressure to the product outlet ofthe first filter membrane such that the first filter

membrane has a first transmembrane pressure, and wherein the second flow control device

supplies a back pressure to the product outlet ofthe second filter membrane such that the

second filter membrane has a second transmembrane pressure approximately equal to the first

transmembrane pressure.

9. The filter vessel according to claim 7, wherein the first flow control device supplies a

back pressure to the product outlet ofthe first filter membrane such that the filter membrane

has a first product flow rate, and wherein the second flow control device supplies a back

pressure to the product outlet ofthe second filter membrane such that the second filter

membrane has a second product flow rate approximately equal to the first product flow rate.

10. The filter vessel according to claim 1 , wherein the first flow control device is in fluid

communication with the product outlet ofthe second filter membrane for supplying a

fluid through the product outlet of the second filter membrane.

11. The filter vessel according to claim 1 , wherein the flow control device is a stationary

flow control device.

12. The filter vessel according to claim 1, wherein the flow control device is a fixed orifice device.

13. The filter vessel according to claim 1, wherein the flow control device is a capillary

device.

14. The filter vessel according to claim 1 , wherein the flow control device is valve device.

15. The filter vessel according to claim 1 , wherein the flow control device is a

dynamically responsive flow control device.

16. The filter vessel according to claim 1 , wherein the flow control device is a Dole valve.

17. The filter vessel according to claim 1 , wherein the first filter membrane is an

ultrafilter membrane.

18. The filter vessel according to claim 17, wherein the second filter membrane is an

ultrafilter membrane.

19. The filter vessel according to claim 1 , wherein the second filter membrane is an

ultrafilter membrane.

20. A filter vessel for purifying a fluid, the filter vessel having an inlet, a waste outlet and

a product outlet, the filter vessel comprising: a first filter disposed within the filter vessel, the first filter comprising:

a filter membrane having an inlet in fluid communication with the inlet of the

filter vessel, a waste outlet, a product outlet and a first product flow rate; and

a second filter disposed within the filter vessel, the second filter comprising:

a filter membrane having an inlet in fluid communication with the waste outlet

outlet ofthe filter vessel, a product outlet in fluid communication with the product outlet of

the filter vessel and a second product flow rate approximately equal to the first product flow

rate.

21. The filter vessel according to claim 20, wherein the first filter membrane is an

ultrafilter membrane.

22. The filter vessel according to claim 21, wherein the second filter membrane is an

ultrafilter membrane .

23. The filter vessel according to claim 20, wherein the second filter membrane is an

ultrafilter membrane.

24. A filter vessel for purifying a fluid, the filter vessel having an inlet, a waste outlet and

a product outlet, the filter vessel comprising:

a first filter disposed within the filter vessel, the first filter comprising:

a filter membrane having a an inlet in fluid communication with the inlet of the filter vessel, a waste outlet, a product outlet and a first transmembrane pressure; and

a filter membrane having an inlet fluid communication with the waste outlet of

the filter membrane ofthe first filter, a waste outlet in fluid communication with the waste

outlet ofthe filter vessel, a product outlet in fluid commumcation with the product outlet of

the filter vessel and a second transmembrane pressure approximately equal to the first

transmembrane pressure.

25. The filter vessel according to claim 24, wherein the first filter membrane is an

ultrafilter membrane.

26. The filter vessel according to claim 25, wherein the second filter membrane is an

ultrafilter membrane.

27. The filter vessel according to claim 24, wherein the second filter membrane is an

ultrafilter membrane.

28. A method of purifying a fluid with a filtration system, the method comprising the steps of:

providing a first filter vessel having a plurality of filter membranes arranged in series;

introducing a feed stream at a first pressure into the first filter vessel; and

maintaining a relatively constant product flow rate through each of the filter

membranes.

29. The method according to claim 28, wherein the step of maintaining a relatively

constant product flow rate includes the step of:

filtering a portion ofthe feed stream with at least one ofthe plurality of filter

membranes to form a purified stream; and

passing the purified stream through a flow control device in fluid communication with

a product outlet of the at least one of the plurality of filter membranes.

30. The method according to claim 29, wherein the step of filtering a portion ofthe feed

stream with at least one ofthe plurality of filter membranes to form a purified stream includes

the step of: filtering a portion ofthe feed stream with each ofthe plurality of filter membranes to

form a purified stream.

31. The method according to claim 30, wherein the step of passing the purified stream

through a flow control device in fluid communication with a product outlet ofthe at least one

ofthe plurality of filter membranes includes the step of:

passing the purified stream through a plurality of flow control devices, each ofthe

plurality of flow control devices being in fluid communication with one of the plurality of

filter membranes.

32. The method according to claim 28, further comprising the steps of:

providing a second filter vessel having a plurality of filter membranes arranged in series; transferring a waste stream from the first filter vessel to the second filter vessel;

introducing the waste stream into the second filter vessel; and

maintaining a relatively constant product flow rate through each of the filter

membranes in the second filter vessel.

33. The method according to claim 32, wherein the step of maintaining a relatively

constant product flow rate includes the step of:

filtering a portion of the feed stream with at least one ofthe plurality of filter

membranes to form a purified stream; and

a product outlet of the at least one ofthe plurality of filter membranes.

34. The method according to claim 33, wherein the step of passing the purified stream

ofthe plurality of filter membranes includes the step of:

plurality of flow control devices being in fluid communication with one ofthe plurality of

filter membranes.

35. A filtration system for purifying a fluid, the filtration system having an inlet, a waste

outlet and a product outlet, the filtration system comprising:

a first filter vessel bank having an inlet in fluid communication with the inlet ofthe filtration system, a product outlet, a waste outlet and at least one filter vessel, the at least one

filter vessel having a product outlet in fluid communication with the product outlet ofthe first

filter vessel bank, a waste outlet in fluid communication with the waste outlet of the first filter

vessel bank, an inlet in fluid communication with the inlet ofthe first filter vessel bank, and

at least one filter, the at least one filter including a filter membrane having an inlet in fluid

communication with the inlet ofthe at least one filter vessel, a product outlet in fluid

communication with the product outlet ofthe at least one filter vessel and a waste outlet in

fluid communication with the waste outlet ofthe at least one filter vessel; and

a second filter vessel bank having an inlet in fluid communication with the waste

outlet ofthe first filter vessel bank, a waste outlet in fluid communication with the waste

outlet ofthe filtration system, a product outlet in fluid communication with the product outlet

ofthe filtration system, and at least one filter vessel, the at least one filter vessel having a

product outlet in fluid communication with the product outlet of the second filter vessel bank,

a waste outlet in fluid communication with the waste outlet of the second filter vessel bank,

an inlet in fluid communication with the inlet ofthe second filter vessel bank, and a filter, the

filter having an inlet in fluid communication with the inlet ofthe at least one filter vessel of

the second filter vessel bank, a product outlet in fluid communication with the product outlet

ofthe at least one filter vessel ofthe second filter vessel bank, and a waste outlet in fluid

communication with the waste outlet of the at least one filter vessel ofthe second filter vessel

bank,

wherein the at least one first filter vessel ofthe first filter vessel bank further includes

a flow control device in fluid communication with the product outlet ofthe at least one filter

ofthe at least one filter vessel ofthe first filter vessel bank.

36. The filtration system according to claim 35, wherein the at least one filter vessel ofthe

first filter vessel bank includes a first plurality of filter vessels, each of the first plurality of

filter vessels having an inlet in fluid communication with the inlet ofthe first filter vessel

bank, a product outlet in fluid communication with the product outlet of the first filter vessel

bank, and a waste outlet in fluid communication with the waste outlet ofthe first filter vessel

bank, wherein the at least one first filter vessel ofthe first filter vessel bank further includes a

flow control device in fluid communication with the product outlet ofthe at least one filter of

the at least one filter vessel of the first filter vessel bank.

37. The filtration system according to claim 36, wherein the at least one filter vessel ofthe

second filter vessel bank includes a second plurality of filter vessels, each ofthe first plurality

of filter vessels having an inlet in fluid communication with the inlet ofthe second filter

vessel bank, a product outlet in fluid communication with the product outlet ofthe second

filter vessel bank, and a waste outlet in fluid communication with the waste outlet ofthe

second filter vessel bank.

38. The filtration system according to claim 35, wherein the at least one filter vessel ofthe

second filter vessel bank includes a second plurality of filter vessels, each of the first plurality

vessel bank, a product outlet in fluid communication with the product outlet of the second

filter vessel bank, and a waste outlet in fluid communication with the waste outlet of the

second filter vessel bank.

39. The filtration system according to claim 35, wherein the at least one first filter vessel

ofthe second filter vessel bank further includes a flow control device in fluid communication

with the product outlet of the at least one filter ofthe at least one filter vessel ofthe second

filter vessel bank.

40. The filtration system according to claim 35, wherein the at least one first filter vessel

of the second filter vessel bank further includes a flow control device in fluid communication

filter vessel bank.

41. The filtration system according to claim 35, wherein the filter membrane ofthe at

least one filter ofthe at least one filter vessel ofthe first filter vessel bank is an ultrafilter

membrane.

42. The filtration system according to claim 41 , wherein the filter membrane of the at

least one filter ofthe at least one filter vessel ofthe second filter vessel bank is an ultrafilter

membrane.

43. The filtration system according to claim 35, wherein the filter membrane of the at

least one filter of the at least one filter vessel ofthe second filter vessel bank is an ultrafilter

membrane.