WO2007043879A1

WO2007043879A1 - Apparatus for the purification of water and a method for its use

Info

Publication number: WO2007043879A1
Application number: PCT/NL2006/050231
Authority: WO
Inventors: Leo Peter Wessels; Sebastiaan Gerard Jozef Heijman; Emile Robin Cornelissen
Original assignee: Kiwa Water Research B.V.
Priority date: 2005-10-07
Filing date: 2006-09-20
Publication date: 2007-04-19
Also published as: NL1030142C2

Abstract

The present invention relates to an apparatus for the purification, in particular the desalination of a liquid, more specifically water, using a pressure pipe and spiral- wound membrane module. The pressure pipe comprises several parallel disposed spiral-wound membrane modules and its orientation is preferably vertical. The material of the pressure pipe is able to withstand a pressure of 3-80 bars and is glass-fibre reinforced plastic, stainless steel or Rilsan- coated steel and the diameters of the pressure pipe is 600- 2000 mm. The invention further relates to a method for the desalination of a liquid, preferably water, using the apparatus of the invention. The membranes can be cleaned by means of flushing with water and air.

Description

Apparatus for the purification of water and a method for its use

FIELD

The invention relates to an apparatus for the purification, in particular the desalination of a liquid, more specifically water, using a pressure pipe and spiral-wound membrane module. The invention further relates to a method for the purification, in particular the desalination of a liquid in an apparatus, wherein the liquid to be filtered is fed to a pressure pipe.

PRIOR ART

Such an apparatus and method are described in the Dutch patent No. 1019130 relating to the purification of surface water or effluent from waste-water purification.

The Dutch patent NL 1019130 describes an apparatus comprising one or more vertically disposed pressure pipes in witch one single spiral-wound nanofiltration or hyperfiltra- tion membrane is provided per pressure pipe so that the pre- purification necessary is only minimal. This pre-purification may consist of microsieving and quick-run filtration. An essential aspect is that the pressure pipes containing the spiral-wound membranes are disposed vertically in the direction of the longitudinal axes, and always the following operational steps are alternated:

1. The production of permeate with a continuous dis- charge of the concentrate

2. Hydraulic flushing over the membrane with water or a combination of water and gas, for example, air.

According to the method described in the Dutch pat- ent No. 1019130, the floating substances, solid particles, bacteria, viruses and solutes, such as salts, dissolved organic matter, pesticides and the like, are removed simultaneously by using at least one single vertically disposed pressure pipe containing one spiral-wound nanofiltration or hy- perfiltration membrane (1 or 1,5 m length) per pressure pipe. To this end the following steps are repeated:

1. Production of permeate with simultaneous and continuous discharge of concentrate and 2. Flushing over the membrane without simultaneous production of permeate.

DSAUBACKS OF THE PRIOR ART

Spiral-wound membranes are known to be very suscep- tible to fouling. This is mainly fouling as a result of scaling (deposit of salt on the membrane) , biofouling {growth of biomass, which may clog the feed channels) and organic fouling (organic material being adsorbed to the membrane) . The deposited particles cause clogging of the feed spaces between the membranes, while particular nutrients in the feed water may induce the growth of biomass. This biomass grows between the spacer and is able to adhere very strongly to everything. The result of one thing and another is that the biomass is very difficult to remove. Scaling is prevented by adjusting to a lower output

(with respect to the product/feed ratio) and to realise a particular longitudinal flow by means of 'staging' or by using an anti-sealant. 'Staging' is understood to mean that the passage through the purification process occurs in suc- cessive steps, wherein one step entails the passage through one or several parallel disposed pressure pipes. 'Staging' serves to realise a proper flow over the membranes at the feed side.

Biofouling is prevented by (periodically or con- tinuously) supplying bacterocides or by periodically cleaning with chemicals, or by extensive pre-purification or a combination of these.

Organic fouling can only be prevented by choosing a suitable membrane or by chemical control . Chemical cleaning involves a permeate stream (usually heated to 30 - 50⁰C) flowing over the membrane. To this the cleaning chemicals are added. The liquid is flushed over the membrane at the feed side and after flowing through the membranes (in practice for example 6 or 7, connected in series), it returns to a tank. It is therefor a case of recirculation, with the fouling from the first membrane module flowing through the second, third, fourth, fifth and sixth module before returning into the purification tank.

Although the Dutch patent NL 1019130 has improved the hydraulic cleanability of membranes, there are some persisting problems. For example, the parallel connection of the membranes in the existing pressure pipes (8 inch 0) means that the pressure pipes take up much space in the building, and much piping is needed to connect the pressure pipes. However, due to the fact that only one membrane module can be placed per pressure pipe, the installation costs are high.

It is an object of the present invention to provide an improved apparatus. It is a further object of the invention to provide an apparatus, wherein the above mentioned drawbacks are at least partly removed.

DESCRIPTION OF THE INVENTION At least one of the above mentioned objects is achieved with an apparatus for the purification, in particular the desalination of a liquid, more specifically water, by using a pressure pipe and a spiral-wound membrane module, characterised in that in the pressure pipe a plurality of standard spiral-wound membrane modules are provided, arranged in parallel.

The invention further relates to a method for the purification, in particular the desalination of a liquid in an apparatus, wherein the liquid to be filtered is fed to a pressure pipe, characterised in that the liquid to be filtered is fed through at least two membrane elements placed in parallel in the pressure pipe so as to form a concentrate stream and a filtrate stream, and that the membrane modules can be cleaned hydraulically by reversing the flow with re- spect to the flow during operation.

It is especially preferable for such a method to be characterised in that said apparatus corresponds to one or several of the claims 1 to 7. The apparatus according to the invention is preferably embodied as shown in the appended drawing. The invention will now be elucidated with reference to the Figures 1 and 2. Figure 1 shows a schematic cross-sectional view of a pressure pipe 1 according to the invention, comprising membranes 2 that are disposed parallel in relation to each other. Figure 1 further shows the feed pipes 11 the central permeate tubes 6 in the membrane modules, and the concentrate outlet pipe 15. Figure 2 shows a schematic longitudinal side view of an embodiment of an apparatus according to the invention, comprising a pressure pipe 1 provided at both sides with a flange 20, 21. Only one spiral-wound membrane module 2 is shown. The pressure pipe possesses a top lid 3 and a bottom lid 4, attached to the respective sides of the pressure pipe by means of fixing means 17. The lids are fixed in relation to each other by means of the concentrate outlet pipe 15 and/or by way of the tie rod (not shown) extending through the lids, so as to distribute the pressure to be absorbed by the lids. The top lid 3 and bottom lid 4 are connected with the concentrate outlet pipe 15 via fixing means 16. The permeate tubes 6 provided centrally in each membrane module are closed off in the bottom plate by means of a closure 7, and fixed to the top lid 3 by means of interconnector 8. Via connecting space 9 which is defined by the top lid 3 and a fixing plate 10 that serves to hold the modules in position when the pressure pipe is being opened, the membrane modules (of which only one is shown) are at the topside in open communication with feed pipes 11, through which the water to be purified is fed into the pressure pipe. At the bottom side, the membrane modules are via concentrate collecting space 12, defined by bottom lid 4 and sealing plate 13 and via concentrate outlet openings 14 in open communication with concentrate outlet pipe 15, which is sealed at its bottom side 19 and via which the concentrate leaves the pressure pipe. Between the membrane modules and the closing plate rubber 0- rings 18 are provided to prevent leaking from the collecting space 9 to the concentrate collecting space 12. Via the con- centrate outlet pipe 15 an inlet pipe 5 for air is provided extending into the concentrate collecting space 12 to facilitate hydraulic flushing of the membrane modules with water and air. In a favourable embodiment, a plurality of such pressure pipes is then serially arranged in succession to realise the necessary 'staging'. As the approach flow is divided over various membranes in a pressure pipe instead of over various pressure pipes, considerably fewer high-pressure connections and a shorter network of high-pressure pipes are needed. This saves costs.

Because the apparatus according to the invention does not use, for example, 2 'stages' in series with pressure pipes of 6 or 7 meter length, in which several membranes are placed in series one behind the other, but rather membranes connected in parallel within a pressure pipe, more advantages are gained. By increasing the 'staging' possibilities, the hydraulic design reaches maximum flexibility. Depending on the salinity and the recovery etc, the new pressure pipe makes it possible to realise a more optimal 'staging¹. This keeps the hydraulic losses over the feed channels to a minimum and realises the maximally possible flux from the membranes.

Per stage different sorts of membranes may be used (NF or RO, low-fouling membranes, different makes etc) .

Expanding the 'staging' also makes it possible to improve monitoring: per pressure pipe with parallel connected membranes, the MTC (Mass Transport Coefficient, a measure for the membrane's permeability) can be determined from flow and pressure measurements for each individual membrane. In this way scaling in the last 'stage' can be detected sooner and organic fouling can also be detected more accurately. Per pressure vessel, monitoring of the increase of pressure over the feed channels is also improved. In this way the occur- rence of biofouling can be detected sooner and the change of the pressure as a result of biofouling can also be monitored more easily. In addition, parallel connection of the membranes in the new pressure vessels with one permeate discharge per individual membrane module also provides the possibility of online determining for each membrane the integrity and age- ing. This is possible because per membrane module the electric conductivity (EC) or the sulphate content or another indicator parameter can be measured. Comparing these parameters provides a good control with respect to the density and retention of the membrane, whether there are any leaks. When problems with respect to integrity are discovered, the respective membrane module can be found more quickly with the aid of electric conductivity (EC) measurements, but may also simply be taken out of operation by disconnecting and closing off the permeate discharge. Once dis- connection and closing off have taken place, the pressure pipe with the remaining modules may resume operation. However, these manoeuvres do require high-pressure piping and high-pressure valves in the permeate discharges.

Per pressure pipe all the modules can be tested for leakage/density by means of a vacuum test without having to dismantle the membranes. The pressure pipes according to the prior art require for this purpose that all the modules are removed from the pressure pipes and that each individual membrane is subjected to the vacuum test in a test unit built especially for the purpose.

The parallel connection of the membrane modules in accordance with the present invention affords the possibility of selectively cleaning per stage. The first stage may, for example, be cleaned more frequently in order to prevent the biofouling from penetrating further into the installation. Also, the dead biomass can be flushed out more easily, without simultaneously rinsing through other membranes, which in this way might become fouled with organic rest material, which at a later stage could induce more serious biofouling. Selective cleaning is possible over the last modules in the 'staging' (susceptible to scaling) and for the removal of organic fouling. Selective cleaning limits the consumption of water and chemicals, which also provides an economic advan- tage. Furthermore, the suitable chemicals and cleaning methods can be applied at the appropriate place and time.

In contrast with the prior art apparatus where several modules are permeate-coupled (in practice often per 6, but other numbers are also possible) , the membranes here are not coupled to each other, so that there is less chance of leakage from feed side to permeate side.

The distribution of water over the membrane modules is better due to less complex piping (feed pipes distribute the water more evenly over, for example, three large streams than over 12 smaller ones) .

Because the set-up (monitoring, cleaning, etc) of the apparatus and method according to the invention is easier to control (more flexible) , the production of water with such an apparatus is also more energy efficient.

The pressure pipe needs to be made of sufficiently strong material because it has to withstand pressures of 3 bars, with nanofiltration, to 80 bars, with seawater filtration. Suitable materials are glass-fibre reinforced plastic (GRP) , or steel (stainless steel or Rilsan-coated steel) . However, other materials may also be used. GRP is generally available in a 30-bars pressure category up to 1400 mm 0. Larger diameters and/or higher pressure categories are available as special products. The pressure pipe preferably has a diameter of approximately 600 mm to 2000 mm so as to allow room for the membranes with a diameter of 8 - 20 inches (20.3 - 50.8 cm), but larger diameters also fall within the scope of the invention. In the present-day prior art, a much-used diameter for membrane modules is 8 inches. A trend has recently started wherein membrane modules with a larger diameter are being developed.

In the apparatus according to the invention the pressure pipes may be disposed horizontally and vertically. The vertical orientation is preferred because in that case it is possible to clean hydraulically with an air-water mixture. This embodiment also allows the apparatus to be vented and drained and it is easier to build in and remove membrane modules.

The apparatus according to the invention comprises much less 'air' in the stacks than the apparatus according to the prior art and there is much less (small-scale) connecting work needed. The footprint of the apparatus is comparable to the prior art, the amount of piping (and thus man-hours during construction) is much less, so that the total space required is less. In contrast with the prior art (applying ultrafiltration) , the present invention only requires a simple pre-purification because particles and biofouling can be removed more easily and therefor cause fewer problems. This makes the apparatus according to the invention significantly cheaper. A further advantage is the excellent ability to flow over the exterior of the membrane elements, so that their exterior can be cleaned very conveniently.

The method according to the invention is suitably realised in the apparatus as shown in the figures. It is, for example, possible to filter saltwater. When the embodiment shown in the Figures 1 and 2 is used, the water to be purified is fed into a pressure pipe 1 provided at both sides with a flange and with top lid 3 and bottom lids 4 fastened to the flanges 20, 21. Said lids are in relation to each other fixed by means of concentrate outlet pipe 15 and/or by means of tie-rods extending through the pressure pipe. Via feed pipes 11 the water to be purified enters the collecting space 9 defined by the top lid 3 and a fastening plate 10. Via collecting space 9 the feed pipes 11 are in open communi- cation with several spiral wound parallel disposed membrane modules 2 through which the water is forced. The purified water subsequently leaves the pressure pipe at the top side via the central permeate tubes 6, which are closed off in the bottom plate by means of a closure 7 and fixed to the top lid 3 by means of interconnector 8. The concentrate leaves the membrane modules at the bottom sides, and arrives in the concentrate collecting space 12 defined by bottom lid 4 and closing plate 13. Via concentrate outlet openings 14 the concentrate is subsequently discharged to concentrate outlet pipe 15, which is closed off at its bottom side 19. The concentrate then leaves the pressure pipe at the top side of the concentrate outlet pipe 15. It appears that in the course of using the method, the membrane modules become more and more fouled. This clogs the spacer of the membrane and purification is more inefficient. In order to solve this problem the fouling deposit is removed, thereby cleaning the membrane modules. This is achieved by flushing over the membrane modules, the direction of flow being reversed with respect to the flow during operation, in accordance with Dutch patent application NL 1019130. However, in accordance with the invention a mixture of air and water may be used. After reversing the direction of flow, air is supplied together with the water, wherein gaseous air may be admixed, or wherein air dissolved in water may be admixed. It is preferred to admix the gaseous form, but if it is necessary for the correct ratio between air and water, the air may also be supplied in solution (saturated water) . Mem- brane cleaning preferably takes place in the following steps. The water stream is contacted with the air only when it has an elevated pressure. This pressure is preferably 1-6 bars. At a pressure below 1 bar not enough air is dissolved, while using a pressure above 6 bars is very expensive. Contacting air and water preferably occurs in a saturation vessel where the water stream is sprayed over a contact bed in the presence of air. The use of a contact bed provides optimal renewal of the contact surface between air and water. Contacting under high pressure will cause air to go into solution. Subsequently the water stream (saturated with air at elevated pressure) is supplied to the membrane modules.

Finally, the pressure over the membrane is reduced, causing a spontaneous return of the air present from the solute to the gaseous form. The air may then be used for the hydraulic cleaning.

After that the cleaning will be started anew. The advantage of such a cleaning method is that the air is properly distributed over all the membrane modules, so that a maximum uniformity of cleaning is obtained.

In a particular embodiment of the above method, the membranes are cleaned by flushing with the ordinary feed water for the installation. This affords the advantage that no extra, already purified water needs to be used, which saves costs and is more convenient in use.

In a particular embodiment of the method it is fur- ther possible to combine (before, simultaneously, after, or at a completely different moment) with the hydraulic cleaning an optional combination with chemical cleaning agents. Hydraulic cleaning on its own is not always enough to remove biomass. After a chemical shock it is easier to hydraulically flush out biomass. The chemicals to be used (for example copper sulphate, a saline solution, or other biocide) serve to kill and/or loosen the biomass that has formed from the spacer and the membrane.

The chemical solution preferably comprises copper sulphate or other salts. Copper sulphate works very well for killing micro-organisms and this effectively inhibits the increase of biomass.

In an advantageous embodiment of the method, the chemical solution for cleaning the membranes can be recovered and purified, and subsequently be reused.

This recovery is preferably realised by using an ultrafiltration step. The recovered chemicals may be concentrated by means of a nanofiltration step or a reverse osmosis step. A further apparatus for the purification of water is known from the Japanese patent specification JP 2001259381, wherein spiral wound membrane modules are arranged in a multi-stage fashion and wherein concentrated water from the membrane module on an front-stage side is used as raw water of the membrane module on a rear-stage side.

The Japanese patent JP 2001259381 is in conflict with NL 1019130. However, neither of the two patents describes a system that comprises a pressure pipe with a plu- rality of elements being disposed parallel to each other. Both patents do, however, described the vertical positioning of modules (i.e. 1 per pressure pipe) and hydraulic cleaning using water and air going with the flow as well as in counter flow. The present patent application is an extension of both said patents with respect to the apparatus and method for the purification of water, as well with respect to the cleaning of the membranes used.

Claims

CIAIMS

1. An apparatus for the purification, in particular the desalination of a liquid, more specifically water, using a pressure pipe and a spiral-wound membrane module, characterised in that in the pressure pipe a plurality of standard spiral-wound membrane modules are provided, arranged in parallel.

2. An apparatus according to claim 1, characterised in that

- the pressure pipe (1) is provided at both sides with a flange (20, 21) , in which pressure pipe a plurality of spiral-wound membrane modules are provided,

- and the pressure pipe is provided with a top lid (3) and bottom lid (4) attached to the respective flanges of the pressure pipe by means of fixing means (17), - and the lids are in relation to each other fixed by- means of the pressure pipe and the concentrate outlet pipe (15), and/or by means of tie-rods extending through the pressure pipe,

- and with the aid of fixing means (16) top (3) and bottom lid (4) are connected with the concentrate outlet pipe, and the permeate tubes (6) are closed off in the bottom plate by means of a closure (7) and fixed to the top lid

(3) by means of interconnector (8),

- and at their top side, the membrane modules are via the collecting space (9) , defined by the top lid (3} and a fastening plate (10) in open communication with feed pipes (11), via which the water to be purified is fed into the pressure pipe,

- and at the bottom side, the membrane modules are via concentrate collecting space (12) , defined by bottom lid

(4) and sealing plate (13) and via concentrate outlet openings (14) in open communication with concentrate outlet pipe (15), which is sealed at its bottom side (19) , and via which the concentrate leaves the pressure pipe, and between the membrane modules and the closing plate rubber 0-rings (18) are provided, and via the concentrate outlet pipe {15} an inlet pipe (5) for air is provided extending into the concentrate collecting space (12) to facilitate hydraulic flushing of the membrane modules .

3. An apparatus according to one of the claims 1 to 2, characterised in that several pressure pipes are placed in series.

4. An apparatus according to claim l_f characterised in that the pressure pipe is made of a material that is able to withstand a pressure of from 3 to 80 bars.

5. An apparatus according to claim 1, characterised in that the material is glass-fibre reinforced plastic.

6. An apparatus according to one of the claims 1 to

5, characterised in that the diameter of the pressure pipe is 600-2000 mm.

7. An apparatus according to claim 6, characterised in that the pressure pipes are disposed vertically.

8. A method for the purification, in particular the desalination of a liquid in an apparatus, wherein the liquid to be filtered is fed to a pressure pipe, characterised in that the liquid to be filtered is fed through at least two membrane elements placed in parallel arrangement in the pressure pipe so as to form a concentrate stream and a filtrate, and that after fouling, the membrane modules can be cleaned hydraulically using water and/or gas by reversing the flow with respect to the flow during operation, and after which the method for the purification is resumed.

9. A method according to claim 8, characterised in that said apparatus corresponds to one or several of the claims 1 to 7.

10. A method for the purification, in particular the desalination of a liquid, making use of the apparatus in accordance to one or several of the claims 1 to 7, wherein the water to be purified is fed into a pressure pipe (1) provided at both sides with a flange and with top (3) and bottom lids (4) , which are in relation to each other fixed by means of the pressure pipe and the concentrate outlet pipe (15), and/or by means of tie-rods extending through the pressure pipe, and via feed pipes (11) enters the collecting space (9) defined by the top lid (3) and a fastening plate (10), wherein via collecting space (9) the feed pipes (11) are in open communication with several spiral wound parallel disposed membrane modules (2) through which the water is then forced, after which the purified water subsequently leaves the pressure pipe at the top side via the central permeate tubes (6), which are closed off in the bottom plate by means of a closure (7) and fixed to the top lid (3) by means of interconnector (8), and the concentrate leaves the membrane modules at the bottom sides, and arrives in the concentrate collecting space (12) defined by bottom lid (4) and closing plate (13), and after which via concentrate outlet openings (14) the concentrate is discharged to concentrate outlet pipe (15), which is closed off at its bottom side (19), the concentrate then leaves the pressure pipe at the top side of the concentrate outlet pipe (15), after fouling followed by hydraulic cleaning of the membrane modules, wherein the flow is reversed with respect to the flow during operation, flushing being performed using a mixture of air and water and going through the steps of

- contacting the water stream with the air at an elevated pressure,

- supplying the water stream saturated with air to the membrane modules, reducing the pressure over the membrane, causing the air present to return from the solute to the gaseous form. in order to realise an even distribution of water and air in the membrane modules, subsequent to which the method of purification is continued.

11. A method according to claim 10, characterised in that with hydraulically cleaning the membrane modules, said pressure at which the water stream is contacted with the air is 1-6 bars.

12. A method according to claim 10 or 11, characterised in that during said hydraulically cleaning of the membrane modules, the water stream is contacted with the air in a saturation vessel and the water stream is sprayed over a contact bed in the presence of air.

13. A method according to claims 10 to 12, characterised in that during said hydraulically cleaning of the membrane modules the ordinary feed water for the installation is used for flushing.

14. A method according to claims 10 to 13, characterised in that during said hydraulically cleaning of the membrane modules, (before, simultaneously, after, or at a completely different moment) the hydraulic cleaning is combined with an optional combination of chemical cleaning agents to kill the biomass.

15. A method according to claim 14, characterised in that the chemical solution comprises copper sulphate.

16. A method according to claims 10 to 15, characterised in that furthermore the chemicals are recovered and purified, and subsequently be reused.

17. A method according to claim 16, characterised in that recovery is realised by using an ultrafiltration step.

18. A method according to claims 16 to 17, characterised in that the recovered chemicals are concentrated by means of step in accordance with one of the techniques of nanofiltration or reverse osmosis.