CA2313740A1

CA2313740A1 - Multi-stage filtration and softening module and reduced scaling operation

Info

Publication number: CA2313740A1
Application number: CA002313740A
Authority: CA
Inventors: Hidayat Husain; Henry Behmann; Pierre Cote
Original assignee: Zenon Environmental Inc
Current assignee: GE Zenon ULC
Priority date: 2000-07-07
Filing date: 2000-07-07
Publication date: 2002-01-07
Also published as: TW508265B

Abstract

A filtration module comprising has a plurality of preceding or succeeding stages of hollow fibre membranes. The lumens of the membranes are the retentate or feed side. The lumens of the membranes of the stages are connected in series between a module inlet and retentate outlet. A permeate collection plenum is in direct fluid communication with each stage. The stages are connected by caps typically having dividers located at the ends of the module. The caps and dividers collect the membranes into the stages such that the surface area of the membranes of each preceding stage is between 1 and 2.5 times the surface area of the membranes of a directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage.
Liquid, typically water, being filtered flows through the stages in series with interstage flows through the caps and dividers being generally parallel to the periphery of the module. Optionally, the caps and dividers re-collect the membranes into stages such that the same surface area relationship applies when feed flows through the module in a reverse direction. The module is used to filter water and when optionally fitted with hollow fibre membranes adapted to selectively reject hardness causing salts is used to remove hardness. Carbon dioxide or other suitable acids may be added to the feed water to control scaling.

Description

T~'~ Ig: Multi-Stage Filtration and Softening Module and Reduced Scaling Operation Field ofthe Invention This invention relates to a multi-stage filtration module and to processes for using such a module to filter water and to remove harness, particularly in small scale systems.
Background ofthe Invention Hollow fibre semi-permeable membranes are useful for filtering solids rich fluids. Membranes in the ultrafiltration, nanofiltration and reverse osmosis range are also useful for separating salts. For example, U.S. Patent No. 5,152,901 describes a nanofiltration membrane material capable of filtering out suspended solids and large organic molecules and generally rejecting calcium salts while generally permeating sodium salts. Such a membrane, and others with similar characteristics, are useful for filtering and softening a potable or domestic water supply.
Membranes as described above may be used in the form of hollow fibres operated in an inside-out flow mode. The hollow fibres are suspended between a pair of opposed tube sheets or headers. The headers fluidly separate the lumens of the membranes from their outer surfaces. Thus, pressurized feed water can be supplied to the lumens of one end of the membranes, permeate can be collected as it leaves the outer surface of the membranes, and a concentrate or retentate can be extracted from the lumens at the other end of the membranes.
Various characteristics of hollow fibre membranes, however, make them difficult to use in such an inside-out flow mode. For example, the inner diameter of the hollow fibre is small which results in significant pressure and flux reductions towards the outlet end of long hollow fibres.
This often results in significantly reduced flux through the fibres near their outlet ends. The problem is most significant when the feed pressure is low, for example when treating water obtained from an ordinary municipal water distribution network without using pumps to pressurize the feed water.
U.S. Patent No. 5,013,437 describes one method of attempting to correct the problem of pressure and flux loss in long fibres. In an embodiment of that patent, an inside-out hollow fibre filtration module is split into two stages. The retentate from the first stage becomes the feed for the second stage. The ratio of the surface areas of the first to the second stages is preferably about 1.5 : 1 to 2.25 : 1. This helps to increase the pressure and velocity of the retentate from the first stage as it becomes the feed to the second stage such that both stages have more nearly equal pressure drops. The stages are arranged concentrically, however, and permeate, particularly from the second stage, must flow along the outside of the fibres to reach an outlet port. With a reasonable packing density of hollow fibre membranes, the head loss in the permeate flow is substantial. Thus the transmembrane pressure differential across the membranes of the second stage is significantly reduced resulting in decreased production.
A similar principle has also been used in large scale systems using spiral wound membranes. A large number of membrane modules are arranged in stages. Each successive stage has fewer modules than the preceding stage and the retentate from preceding stages becomes the feed of the succeeding stages. The number of modules in succeeding stages is chosen to maintain an approximately constant feed or retentate velocity through the system. Such a system is both large and complex and not suited to residential or small commercial systems.
Another characteristic of hollow fibre membranes, is that their pores become fouled over time, for example, because of carbonate scaling. In large scale systems, carbonate scaling may be addressed by partially softening the feed water using resin exchange beds or by adding an acid or an anti-scalant to the feed water. Such techniques are generally not practicable in small scale systems.
Makers of small scale membrane filtration systems typically try to address the problems above by using a single stage filtration module and recirculating the retentate to the feed inlet. This technique requires a high rejection, high permeability membrane which must be operated at a very low per pass recovery. This leads to rapid fouling and either frequent cleaning or replacement of the membranes. Energy costs are also high because of the high rate of recirculation.
Summary of the Invention It is an object of the invention to improve on the prior art. It is another object of the invention to provide a filtration module, particularly one that is useful for small scale filtration of water. It is another object of the invention to provide a process to reduce scaling of such a module used to soften water and to provide a module that may be used with that process.
In various aspects, the invention provides a filtration module having a plurality of preceding or succeeding stages (some stages being both preceding and succeeding) of hollow fibre membranes suspended between opposed headers. The lumens of the hollow fibre membranes are open at first and second ends of the stages. A module feed inlet is connected in fluid communication with the first end of a first stage. The remaining stages are connected in series behind the first stage with fluid connections between the second end of each preceding stages and the first end of each directly succeeding stage. A module outlet is connected in fluid communication with the second end of a last stage. A permeate collection plenum surrounds the stages and is in fluid communication with each stage. Permeate from each stage may flow to the permeate plenum without flowing through other stages. The surface area of the membranes of each preceding stage is between 1 and 2.5 times the surface area of the membranes of a directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage.
To construct the connections between the stages, the outer surfaces of the membranes are sealed to the headers while their lumens are made open at the distal faces of the headers. A first cap covers the distal face of one header and a second cap covers the distal face of the other header. The permeate plenum includes the space between the proximal faces of the headers and an outer shell. Dividers within one or both of the caps collect groups of the membranes into the stages while leaving open fluid connections between the second end of each preceding stage and the first end of each directly succeeding stage. The module inlet and module retentate outlet are provided in the caps so as to be in fluid communication with the first end of the first stage and the second end of the last stage respectively. Thus feed water enters the first end of the first stage and the portion not permeated exits the second end of the first stage. From there, the second end cap directs it to the first end of the second stage. The water not permeated in the second stage arrives at the first cap. In a two stage device, the water not permeated then leaves the module. In a module with more stages, the first cap redirects the water to the first end of another stage and the water not permeated flows to the second cap and so on until the second end of the last stage is reached.
The stages are arranged so that each is adjacent the perimeter of the module and interstage flows are generally parallel to the periphery of the module. Further, all permeate can flow directly through the path of minimum head loss to the permeate plenum which includes space within the group of membranes and, optionally, around the perimeter of the module. For example, the stages may be configured as sectors of a cylinder.

The dividers are fitted with valves and arranged such that when feed water flows into the module in a reverse direction, entering through the module retentate outlet, the dividers re-collect the groups of membranes into second preceding and second succeeding stages having first and 5 second ends. The dividers leave open fluid connections generally parallel to the periphery of the module between the second end of each second preceding stage and the first end of each second succeeding stage. In the re-collection of the membranes, the surface area of the membranes of each second preceding stage is between 1 and 2.5 times the surface area of the membranes of a second directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage. This is achieved by using one way valves opening in a direction such that the grouping and re-grouping of membranes is performed by the action of liquid flowing through the module, ie. opening valves where the pressure differential is in the direction that the valve opens and closing valves where the pressure differential is opposite the direction that the valves open.
Such a module is used to filter water and can be used to remove hardness when optionally fitted with hollow fibre membranes adapted to selectively reject hardness causing salts. Water to be filtered flows through the stages in series while a filtered and optionally softened permeate is collected from the outer surfaces of the membranes. When the module is used to provide a softened permeate, carbonate scale may form in the membranes. To control scaling, carbon dioxide or other suitable acids may be added to the feed water before the feed water enters the lumens of the hollow fibre membranes. The carbon dioxide may be added continuously to the feed water in amounts such that the Langelier Index is zero or slightly negative. Alternatively, the carbon dioxide may added to the feed water from time to time, preferably when the need for permeate is low and either no permeate is produced or the permeate produced during these times is discarded. Further, the direction of flow through the module can be reversed while carbon dioxide is being added to apply the maximum concentration of acid to the most heavily scaled stage.
Brief Description of Drawings Embodiments of the invention will be described below with reference to the following figures.
Figure 1 shows a partially cut away elevation of a membrane module which may be used as a four stage module.
Figure 2 shows a plan view of the module of Figure 1 with top cap removed.
Figures 3 and 4 show the forward and reverse flow respectively through a three stage module.
Figures 5 and 6 show the forward and reverse flow respectively through a four stage module.
Description of Embodiments Figures 1 and 2 show a filtration module 10. The module 10 has a plurality of filtering hollow fibre membranes 12 suspended between opposed headers 14. The membranes are typically in the reverse osmosis, nanofiltration or ultrafiltration range, preferably in the nanofiltration range and more preferably able to selectively retain hardness causing salts and permeate softened water. The ends 16 of the membranes 12 are potted in a closely spaced relationship in the headers 14 such that their outer surfaces are sealed to the headers 14 and the lumens of the membranes 12 are open at the distal faces of the headers 14. A first cap 20 and a second cap 22 cover the distal faces of the headers 14 and are sealed to the headers 14. The membranes 12 are arranged into groups 24 each group separated by an area of the headers 14 having no membranes 12 potted in it. The membranes 12 may be maintained in groups 24 during potting by wrapping ends of groups in an expandable plastic mesh. Dividers 26 within one or both of the caps 20, 22 (and optionally formed as part of the caps 20, 22) extend from the distal surface of the caps 20, 22 to sealingly contact some or all of the areas of the headers 14 having no membranes 12. Optionally, the dividers 26 may be inserted into the headers 14 during potting in which case the dividers 26 help separate groups 14 of membranes 12 and the dividers 26 become bonded to the headers 14. Some or all of the dividers 26 may have openings which may include one or more one way valves 28, typically flap valves, located within them.
The perimeter of the module 10 is surrounded by a casing 30. The volume inside of the casing 30 between the proximal faces of the headers 14 and not occupied by membranes 12 forms a permeate plenum 32.
The permeate plenum 32 includes the space around the membranes 12 and may also include an open space adjacent the periphery of the module 10 in direct fluid communication with each of the groups of membranes 12. A permeate outlet 34 in fluid communication with the permeate plenum 32 allows permeate to be removed from the module 10. Thus water permeated through the membranes 12 in a group of membranes 12 can flow directly to the permeate outlet 34 through the path of least resistance and is not required to flow through the groups 24 in a selected path. This can be achieved by arranging the groups of membranes 12 as sectors of a cylinder as shown. Other configurations are also possible.
For example, in a square or rectangular module groups of membranes 12 of various sizes can be located on either side of a centre line of the module. A module feed inlet 36 admits feed water into one of the caps 20, 22. Retentate leaving the module 10 flows out of a module feed outlet 38 also located in one of the caps 20, 22.

Referring to Figures 3 and 4, a three stage module 110 is divided into four groups 124a,b,c,d of membranes (not shown). The arrangement and potting of membranes in groups 124 allows the dividers 26 to isolate groups 124 with no membranes improperly crossing over divisions between stages. The size of the groups are 1/6, 1/6, 1/3 and 1/3 respectively of the size of the entire amount of membranes. Dividers 26 comprise solid dividers 40 and one way dividers 42 in the locations shown. The one way dividers 42 open to allow flow in the direction shown. The dividers 26, 40, 42 divide the groups 124 into stages I, II and III depending on the direction of feed flow.
In Figure 3, feed flows first into group 124a through the module feed inlet 36 in the first cap 20. The feed may flow into group 124d and groups 124a and 124d form stage I. Feed/retentate flows in stage I to the second cap 22 where it flows over to group 124c which forms stage II.
Feed/retentate in stage II flows back to the first cap 20 where it flows over to group 124b which forms stage III. Feed/retentate is prevented from flowing back into stage I through the one way divider 42 by the greater pressure in stage I, a pre-requisite for having flow from stage I to stage II.
Feed/retentate flowing in stage III flows to the second cap 22 where it leaves the module 110 through the module retentate outlet 38. Through all stages, permeate flows from each stage directly to the permeate plenum (not shown) and out through the permeate outlet (not shown).
Stages I, II and III thus involve 1/2, 1/3 and 1/6 of the total amount of membranes respectively.
In Figure 4, the feed and retentate flows are reversed. Feed flows first into group 124b through the module retentate outlet 38 in second cap 22. The feed may flow into group 124c and groups 124b and 124c form stage I. Feed/retentate flows in stage I to the first cap 20 where it flows over to group 124d which forms stage II. Feed/retentate in stage II flows back to the second cap 22 where it flows over to group 124a which forms stage III. Feed/retentate flowing in stage III flows to the first cap 20 where it leaves the module 110 through the module feed inlet 36. Stages I, II and III thus still involve 1/2, 1/3 and 1/6 of the total amount of membranes respectively. As above, in some places undesired flow through the one way dividers 42 is prevented by the pressure gradient between phases I, II
and III.
Figures 5 and 6 show a four stage module 210 of similar operation.
Other modules with up to six stages can also be created. Modules with even more stages might also be possible, but the complexity of such a module would be a concern. The four stage module has five groups 224a,b,c,d,e of membranes (not shown). The size of the groups are 1/8, 1/4, 1/4, 1/4 and 1/8 respectively of the size of the entire amount of membranes. Dividers 26, 40, 42 as discussed above divide the groups 224 into stages I, II and III depending on the direction of feed flow.
In Figure 5, feed flows first into group 224a through the module feed inlet 36 in the first cap 20. The feed may flow into group 224b and groups 224a and 224b form stage I. Feed/retentate flows in stage I to the second cap 22 where it flows over to group 124c which forms stage II.
Feed/retentate in stage II flows back to the first cap 20 where it flows over to group 224d which forms stage III. Feed/retentate flowing in stage III
flows to the second cap 22 where it flows over to group 224e which forms stage IV. Feed/retentate flowing in stage IV flows to the first cap 20 where it leaves the module 210 through the module retentate outlet 38. Through all stages, permeate flows from each stage directly to the permeate plenum (not shown) and out through the permeate outlet (not shown).
Stages I, II, III and IV thus involve 3/8, 1/4, 1/4 and 1/8 of the total amount of membranes respectively.
In Figure 6, the feed and retentate flows are reversed. Feed flows first into group 224e through the module retentate outlet 38 in first cap 20.
The feed may flow into group 224d and groups 124d and 124e form stage I. Feed/retentate flows in stage I to the second cap 22 where it flows over to group 124c which forms stage II. Feed/retentate in stage II flows back to the first cap 20 where it flows over to group 124b which forms stage III.
Feed/retentate flowing in stage III flows to the second cap 22 where flows over to group 124a which forms stage IV. Feed/retentate in stage IV flows 5 back to the first cap 20 where it leaves the module 210 through the module feed inlet 36. Stages I, II and III thus still involve 3/8, 1/4, 1/4 and 1/8 of the total amount of membranes respectively. As above, in some places in both Figures 5 and 6, undesired flow through the one way dividers 42 is prevented by the pressure gradient between phases I, II, III and IV.
In summary, the modules 10, 110, 210 are provided with a plurality of preceding or succeeding stages (I, II, II etc.), some stages being both preceding and succeeding stages. The module feed inlet 36 into one of the caps 20, 22 is connected in fluid communication with the first end of a first stage. The dividers 26, 40, 42 collect the groups 24 of membranes 12 into preceding and succeeding stages having first and second ends, the lumens of the membranes open to the ends. The dividers 26, 40, 42 also leave open fluid connections created by the caps 20, 22 between the second end of each preceding stage and the first end of each directly succeeding stage. The fluid connections between stages permit an interstage flow of retentate/feed that is generally parallel to the periphery of the module 10, 110, 210. For example, with the pie shaped stages illustrated, interstage flows flow around the centre of the module although it is not necessary that the interstage flows be geometrically perfect circles. The module retentate outlet 38 from one of the caps 20, 22 is in fluid communication with the second end of a last stage. The surface area of the membranes of each preceding stage is between 1 and 2.5 times the surface area of the membranes of a directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage.
This exact sizes of the stages can be selected to provide a nearly uniform velocity through the module 10 despite permeation with limited variation in velocity between the stages.

The dividers 26, 40, 42 are fitted with valves and arranged such that when feed water flows into the module in a reverse direction, entering through the module retentate outlet 38, the dividers 26, 40, 42 re-collect groups 24 of membranes 12 into second preceding and second succeeding stages having first and second ends, the lumens of the membranes open to the ends. Under reverse flow the dividers 26, 40, 42 leave open fluid connections created by the caps 20, 22 between the second end of each second preceding stages and the first end of each second succeeding stage. The surface area of the membranes of each second preceding stage is between 1 and 2.5 times the surface area of the membranes of a second directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage. The valves are one way valves 28 and, to the extent that they are required to move, they open in a direction such that the grouping of and re-grouping of the stages in fonrvard and reverse flow is performed by the action of liquid flowing through the module 10.
To permit flow through the module 10 to change directions, feed and retentate lines to and from the module 10 are provided with valves, typically solenoid valves, that allow each line to be connected to either the module feed inlet 36 or the module retentate outlet 38. The valves are operated simultaneously by a PLC or timer such that both the feed and retentate lines are not both connected to the same point on the module 10 at the same time.
Modules can also be constructed to used with flow in one direction only. Such modules are simpler to construct but may have a shorter service life than a module with reversing flow. Nevertheless, for small systems, a module designed for flow in one direction only may be more cost efficient. Referring to Figure 3, a three stage module for flow in one direction only is made by providing a solid divider in the first cap 20 between groups 124c and 124d and between groups 124b and 124a and in the second cap 22 between groups 124c and 124b and between groups 124b and 124a. All other dividers 26, 40, 42 shown in Figure 3 are omitted. Referring to Figure 5, a four stage module for flow in one direction only is made by providing a solid divider in the first cap 20 between groups 224b and 224c, groups 224d and 224e and groups 224e and 224a and in the second cap 22 between groups 224c and 224d and groups 224e and 224a. All other dividers 26, 40, 42 shown in Figure 3 are omitted.
Where the module 10 is used to soften water, the water to be filtered flows into a first end of the lumens of the membranes 12 which are chosen to selectively reject, ie. retain, hardness causing salts. A softened permeate is collected from the outer surfaces of the membranes 12 and a retentate is collected from the second end of the lumens of the membranes 12 and either exits the module 10 or flows to the next stage.
Hardness causing salts thus build up in the lumens of the membranes 12, particularly in the last stage. Periodically reversing the direction of feed flow through the hollow fibre membranes, such that water to be filtered flows into the second end of the lumens and retentate flows out of the first end of the lumens, helps distribute this scaling more evenly and extend the life of the module 10.
To further extend the life of the module, carbon dioxide is added to the feed water before the feed water enters the lumens of the hollow fibre membranes. Other acids may be used for the scale control, but carbon dioxide is preferred. Carbon dioxide is non-hazardous and suitable for human ingestion. Carbon dioxide is also self limiting for very hard waters with buffering capability, that is excessive dosages do not result in very low pH and potentially unsafe water quality.
In one method, a small amount of carbon dioxide is added continuously to the feed water. The flux of carbon dioxide is selected so that the Langelier Index is zero or slightly negative at which point the feed water is non-scaling but only minimally corrosive. At such doses, however, scaling is still controlled for most feed waters.
In another method, carbon dioxide is added to the feed water only from time to time, for example once a day, and preferably when the need for permeate is low. During such descaling events, the permeate outlet 34 may be closed with a valve to prevent permeation or permeate produced during the descaling discarded. In either case, higher fluxes of carbon dioxide can be used for rapid or intensive cleaning. The two methods above may also be combined, providing continuous carbon dioxide addition to the feed and a once a day intensive descaling.
The two methods above may also be advantageously combined with flow reversal as described further above. With carbon dioxide continuously added to the feed, the supply of carbon dioxide switches between the module feed inlet 36 and the module retentate outlet 38 along with the feed water. Thus, the first and last stages of the module 10 alternate between relatively low hardness water with high carbon dioxide concentration and relatively high hardness water with low carbon dioxide concentration, the carbon dioxide concentration decreasing with travel through the module 10. Thus the carbon dioxide is added to the feed flow while the feed flows first into the most heavily scaled part of the module at least during a period right after the flow is reversed.
With carbon dioxide added periodically, the flow reversal is also done only periodically and timed to coincide with the addition of carbon dioxide to the feed. Thus, for most of the day feed flows in the forward direction and scale builds up in the last stage. During an off-peak period, flow is reversed and carbon dioxide is added to the feed. Thus the carbon dioxide is added to the feed flow while the feed flows first into the most heavily scaled part of the module.

Examples A. 0.5 mm internal diameter coated nanofiltration membranes which selectively reject (ie. retains) hardness causing salts were used in a series of four tests. In the tests, the membranes were used to filter and soften a very hard and scaling feed water with total hardness exceeding 3000 mg/L. After six hours of operation, flux through the membranes had dropped noticeably to varying degrees. A carbon dioxide solution with a pH of 6.3 was circulated through the membranes. Flux through the membranes recovered completely in three of the tests.
B. Two Desal DL1812 spiral wound nanofiltration modules were operated at 50% recovery and approximately 99 psi TMP. The feed was scale forming in nature with a positive Ryznar index. Carbon dioxide was injected continuously into the feed of one of the modules to reduce its pH
from 8.0 to 6.5. The flux of the module without carbon dioxide added to the feed stabilized at 0.20 gfd/psi. The flux of the module with carbon dioxide added to the feed stabilized at .26 gfd/psi, a 30% improvement.
The embodiments described above are subject to various modifications within the scope of the invention which is defined by the following claims.

Claims

1. A filtration module comprising:
(a) a plurality of preceding or succeeding stages of hollow fibre membranes suspended between opposed headers and having first and second ends, the lumens of the fibres open to the first and second ends;
(b) a module feed inlet in fluid communication with the first end of a first stage;
(c) fluid connections between the second end of each preceding stages and the first end of each directly succeeding stage, the fluid connections adapted to facilitate interstage flows generally parallel to the periphery of the module;
(d) a module outlet in fluid communication with the second end of a last stage; and, (e) a permeate collection plenum having a permeate outlet, the permeate collection plenum being in fluid communication with each stage.

2. The module of claim 1 wherein the surface area of the membranes of each preceding stage is between 1 and 2.5 times the surface area of the membranes of a directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage.

3. A filtration module comprising:
(a) a plurality of groups of hollow fibre membranes suspended between a pair of opposed headers such that their outer surfaces are sealed to the headers and their lumens are open at the distal faces of the headers;
(b) a first cap covering the distal face of one header;
(c) a second cap covering a distal face of the other header;
(d) a permeate plenum having a permeate outlet and being in fluid communication with the groups of hollow fibre membranes such that permeate from each stage may flow to the permeate plenum;

(e) dividers within one or more of the caps, the dividers (i) collecting the groups of membranes into preceding and succeeding stages having first and second ends, the lumens of the membranes open to the ends, and (ii) leaving open fluid connections between the second end of each preceding stages and the first end of each directly succeeding stage, the fluid connections adapted to facilitate interstage flows generally parallel to the periphery of the module;
(f) a module retentate outlet from one of the caps in fluid communication with the second end of a last stage; and, (g) a module inlet to one of the caps in fluid communication with a first end of a first stage.

4. The module of claim 3 wherein the surface area of the membranes of each preceding stage is between 1 and 2.5 times the surface area of the membranes of a directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage.

5. The invention of claim 4 wherein, (a) the dividers are fitted with valves and arranged such that when feed water flows into the module in a reverse direction, entering through the module retentate outlet, the dividers (i) re-collect groups of membranes second preceding and second succeeding stages having first and second ends, the lumens of the membranes open to the ends, and (ii) leave open fluid connections between the second end of each second preceding stages and the first end of each second succeeding stage, the fluid connections adapted to facilitate interstage flows generally parallel to the periphery of the module; and, (b) the surface area of the membranes of each second preceding stage is between 1 and 2.5 times the surface area of the membranes of a second directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage.

6. The invention of claim 5 wherein the valves are one way valves opening in a direction such that the grouping of claim 3 and re-grouping of claim 5 are performed by the action of liquid flowing through the module.

7. A process for filtering water to remove hardness comprising the steps of:
(a) flowing water to be filtered into a first end of lumens of hollow fibre membranes adapted to selectively reject hardness causing salts;
(b) collecting a softened permeate from the outer surfaces of the membranes;
(c) collecting a retentate from the second end of the lumens of the membranes; and, (d) periodically reversing the direction of feed flow through the membranes such that water to be filtered flows into the second end of the lumens and retentate flows out of the first end of the lumens.

8. A process for filtering water to remove hardness comprising the steps of:
(a) flowing water to be filtered against a feed or retentate side of hollow fibre membranes adapted to selectively reject hardness causing salts;
(b) collecting a softened permeate from a permeate side of the membranes;
(c) collecting a retentate from the membranes; and, (d) adding carbon dioxide to the feed water before the feed water enters the lumens of the hollow fibre membranes.

9. The process of claim 8 wherein the carbon dioxide is added continuously to the feed water in amounts such that the Langelier Index is zero or slightly negative.

10. The process of claim 8 wherein the carbon dioxide is added to the feed water from time to time at times when the need for permeate is low and permeate is either not produced while carbon dioxide is added to the feed or permeate produced while carbon dioxide is added to the feed is discarded.

11. The process of any of claims 8 through 10 wherein the membranes are hollow fibre membranes and the feed/retentate side is the lumens of the membranes and further comprising periodically reversing the direction of feed flow through the hollow fibres membranes such that water to be filtered flows into the second end of the lumens and retentate flows out of the first end of the lumens wherein the carbon dioxide is added to the feed flow while the feed flows first into the most heavily scaled part of the module.

12. A filtration process comprising the steps of:
(a) providing a plurality of preceding or succeeding stages of hollow fibre membranes having first and second ends;
(b) feeding feed water into the first end of a first stage;
(c) flowing retentate generally parallel to the periphery of the module from the second end of each preceding stage into the first end of its directly succeeding stage;
(d) withdrawing a retentate from the module from the second end of a last stage; and, (e) collecting permeate from each stage.

13. The process of claim 12 wherein the surface area of the membranes of each preceding stage is between 1 and 2.5 times the surface area of the membranes of a directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage.

14. The process of claim 12 further comprising periodically reversing the direction of feed flow through the hollow fibres membranes such that water to be filtered flows into the second end of the lumens and retentate flows out of the first end of the lumens wherein the carbon dioxide is added to the feed flow while the feed flows first into the most heavily scaled part of the module.

15. A filtration process comprising the steps of:
(a) providing a plurality of groups of hollow fibre membranes;
(b) collecting the groups of membranes into preceding and succeeding stages having first and second ends, the lumens of the membranes open to the ends, while leaving open fluid connections generally parallel to the periphery of the module between the second ends of the membranes of each preceding stages and the first ends of the membranes of each directly succeeding stage;
(c) feeding feed water into the first end of a first stage;
(d) flowing retentate from the second end of each preceding stage into the first end of its directly succeeding stage;
(e) withdrawing a retentate from the module from the second end of a last stage; and, (f) collecting permeate from each stage without such permeate first flowing through other stages.

16. The method of claim 15 wherein the surface area of the membranes of each preceding stage is between 1 and 2.5 times the surface area of the membranes of a directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage.

17. The invention of claim 16 further comprising the steps of, (a) periodically flowing feed water flows into the module in a reverse direction, the feed water entering through the module retentate outlet; and, (b) while the flow of feed is so reversed, (i) re-collecting groups of membranes into second preceding and second succeeding stages having first and second ends, the lumens of the membranes open to the ends, and (ii) providing open fluid connections generally parallel to the periphery of the module between the second end of each second preceding stages and the first end of each second succeeding stage,

18. The process of claim 17 wherein the surface area of the membranes of each second preceding stage is between 1 and 2.5 times the surface area of the membranes of a second directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage.

19. The process of claim 17 further comprising periodically reversing the direction of feed flow through the hollow fibres membranes such that water to be filtered flows into the second end of the lumens and retentate flows out of the first end of the lumens and adding carbon dioxide to the feed flow while the direction of feed flow is reversed.

20. The process of claim 18 further comprising periodically reversing the direction of feed flow through the hollow fibres membranes such that water to be filtered flows into the second end of the lumens and retentate flows out of the first end of the lumens and adding carbon dioxide to the feed flow while the direction of feed flow is reversed.