WO2004037396A1

WO2004037396A1 - Method of operating a filter unit, an apparatus to be used therewith and a capillary filtration module

Info

Publication number: WO2004037396A1
Application number: PCT/NL2003/000722
Authority: WO
Inventors: Leo Peter Wessels
Original assignee: Dhv Water B.V.
Priority date: 2002-10-24
Filing date: 2003-10-23
Publication date: 2004-05-06
Also published as: AU2003274826A1; NL1021744C2

Abstract

The invention relates to a method of operating a filter unit (1) with capillary membrane filters (2), wherein at least one end (3) of the capillary membrane filters (2) is potted in a sealing material (4), wherein a movement of the capillary membrane filters (2) near the sealing material (4) during operation is reduced by at least one of the following steps: I) removing gas present at the permeate side (6) in the filter unit (1), which gas was initially contained in the water and which is released therefrom, and II) providing a resilient material (7) around the capillary membrane filters (2), abutting against the sealing material.(4) Thanks to the method according to the invention, rupturing of the capillary membrane filters (2) is greatly reduced. This means a significant extension of the life of the filter unit (1), resulting in considerable savings.

Description

Method of operating a filter unit, an apparatus to be used therewith and a capillary membrane filtration module

The invention relates to a method of operating a filter unit with capillary membrane filters, wherein at least one end of the capillary membrane filters is potted in a sealing material. The term "membrane" as used in this de- scription refers in particular to a capillary membrane filter.

Such a method and such a filter unit are generally known. These filter units are used in particular when purifying surface water and domestic effluent. In this process, floating particles that have adsorbed multifarious substances as well as bacteria, viruses and protozoa, are removed. Generally speaking, these filter units consist of capillary membrane filters formed as elongated thin tubes of filter material of a diameter usually ranging between 0.2 and 10 mm. The two ends of these capillary membrane filters are usually potted into a sealing material, so that they are completely enclosed near their two ends. It is therefore only possible to transport water to the other side of the sealing material through the membrane filter, in the process of which it is filtered. However, another possibility is that one end of the capillary membrane filter is potted in a sealing material and the other end of the capillary membrane filter is closed.

In general the ultrafiltration membranes are operated at overpressure. The feed is put under pressure so that the water to be filtered passes the membrane, and the contaminants stay behind at the feed side. As usually no discharge of concentrate takes place during the production of filtered water, the contaminants accumulate at the feed side of the membrane. For this reason the membranes need to be backwashed after some time, allowing the resulting concentrate to be discharged together with the flushed fouling. During backwashing, the water's direction of flow through the membrane is reversed. The problem with these known membrane filtration units is that during use, the membranes rupture or tear. This causes them to leak, resulting in unfiltered water being fed to the permeate side. These leaking membrane capillaries must be repaired. In general this is done by sealing the end of the membrane capillary by putting a prop into the end potted in the sealing material. The membrane module can then be replaced into the filtration unit. The drawback is that these repair operations are time-consuming and expensive and that the membrane becomes contaminated at the permeate side. It is the object of the invention to provide a solution for the problem of leaking filter units with capillary membrane filters. In order to achieve this object, the invention provides a method as mentioned in the preamble, characterised in that a movement of the capillary membrane filters near the sealing material during operation is reduced by at least one of the following steps:

1) removing gas present at the permeate side in the filter unit, which gas was initially contained in the water and which is released therefrom, and

2) providing a resilient material around the capillary membrane filters, abutting against the sealing material.

The measures in accordance with the invention have been shown to considerably reduce leakage through capillary membrane filters. The reason for this is not altogether obvious. Possibly the gas produces a great mechanical load on the capillary membrane filters during backwashing, due to the fact that during backwashing the pressure at the permeate side increases, causing a sudden compression of the gas present. Gas may be introduced into the filter unit due to temperature differences in the water or due to feeding gas- supersaturated water, or during the production of membranes, and may be retained there. The presence of gas, for example at the permeate side, seems to have disastrous consequences for the membranes. During filtration, the pressure at the permeate side of these membranes will be relatively low, whereas backwash- ing causes an instantaneous rise of the pressure at the permeate side. Such a rise in pressure at the permeate side effects a sudden compression of the locally present gaseous gas. This compression causes an undesirable abrupt movement of the membrane capillaries. Especially if the gas is located near the sealing material in which the capillary membrane filters are potted, the load on the capillary membrane filters is such that it cannot be absorbed by their resilience. The regular performance of the backwash step will cause fatigue and ultimately rupturing or tearing of the membrane capillaries.

An advantageous embodiment of the invention is characterised in that the gas is contacted with gas-unsaturated water, for the purpose of dissolving the gas therein. According to a further embodiment, the degassed water is selected from: a) degassed water, preferably vacuum-degassed water, b) cooled water, having a temperature lower than the temperature of the water to be filtered, c) compressed water, having a pressure higher than a filtration pressure. The use of vacuum-degassed water is especially preferred, because such' water can be thoroughly degassed relatively easily. The use of compressed water has the advan- tage that such a facility can simply be incorporated in already existing installations. The use of cooled water also provides a suitable method, but requires relatively much energy to enhance its ability to absorb an increased amount of gas as desired. Of course, it is also possible to combine the above- mentioned possibilities a, b, and c. This may even result in a further improvement of degassing.

The method according to the invention of removing gas from the filter unit will always require a certain amount of time. Since backwashing a filter unit using cleaning chemicals generally requires the liquid containing chemicals that is used for backwashing to remain in the unit for some time (standing time or soaking time, during which no liquid is let in or out) , and since degassing takes up some time, said removal of gasses can conveniently be carried out during backwashing of the filter unit. Degassing also needs to take place periodically and can therefore be best combined with the periodical chemical cleaning.

According to a further preferred embodiment the removal of the gas takes place at the permeate side of the filtration unit, by introducing the previously degassed liquid via the backwash passage. The invention can be applied especially successfully with filtration units comprising capillary ultrafiltration membranes. Under the usual filtration conditions such membranes are not permeable to air. Only at higher pressures, generally in the order of 2.5 bars and higher, will wet ultrafiltration membranes become permeable to air.

It is therefore not possible for gaseous air to escape through a wet ultrafiltration membrane (whose pores have a diameter of less than 0.1 micrometers) unless a pressure above 2 to 2.5 bars is used to press the air through the membrane pores. Microfiltration membranes have larger pores than ultrafiltration membranes (the size of the pores of a microfiltration membrane ranges from 0.1 to 1 micrometer). Wet microfiltration membranes are permeable to gaseous air at a lower pressure than ultrafiltration membranes. Similarly with ultrafiltration, there is at least one and preferably several microfiltration membranes incorporated in the modules, which in accordance with a further preferred embodiment are potted at the peripheral side of the module, so as to allow easy degassing of the modules during backwashing with water. Indeed, the applied backwash pressure will allow the gas present at the permeate side to escape via the microfiltration membranes. In a standard module of 9000 membranes, only a few microfiltration membranes need to be incorporated in order to maintain a satisfactory level of virus removal. To be able to differentiate between them, the microfiltration membranes preferably have a different, preferably a larger diameter than the ultrafiltration membranes. This allows the microfiltration membranes in the module to be sealed quickly in an integrity test. Such integrity tests are carried out by applying overpressure (using air) to the module while immersing the module in a tank with water. Air will then escape via the broken ultrafiltration membranes. Since microfiltration membranes allow air to pass through even when they are intact, these membranes must be sealed prior to the integrity test.

Another solution, albeit less elegant, is not to provide the ultrafiltration modules with any microfiltration membranes, but to drill a few small holes (having a maximum diameter of, for example, 0.2 mm, preferably maximally 0.1 mm, still more preferably maximally 0.05 mm) in the outer wall of the membrane module, for the de-aeration of the modules. This violates the integrity of the membrane module by intentionally making leaks along which the air that is present at the permeate side can escape. Nevertheless, such a solution is conceivable, since in present-day practice the membranes regularly rupture of themselves. In view of the fact that the membranes often have a diameter between 0.4 and 2.5 mm, a de-aeration hole of less than 0.4 mm, preferably maximally 0.1 to 0.2 mm is favoured if this helps to avoid the frequent occurrence of the 0.4-2.5 mm membranes rupturing.

According to another aspect of the invention, the method is characterised in that the filter unit is disposed vertically, and at a highest position comprises an opening for discharging gas so as to lead the gas outside the range of the capillary membrane filters. Such an embodiment will effectively keep gas outside the range of the capillary membrane filters. When elevating the pressure at the permeate side, there is no movement of the capillary membrane filters since there is no gas near the capillaries and consequently no compression of gas takes place near the capillaries. According to a particular preferred embodiment, the opening is in communication with an outlet for the removal of gas from the filtration unit accumulated via the opening.

According to still an other aspect of the invention, the method is characterised in that the resilient material is a gel. Gel refers to any resilient material allowing the capillary membrane filters to make a slight movement, with the degree of movement in the gel at a particular pressure equalling a value between the degree of movement in the liquid and the possible degree of movement in the sealing material. By making the gel sufficiently thick, the degree of movement will gradually decrease from the free surface of the gel up to the sealing material. This prevents fatigue of the capillary membrane filters on the rim of the sealing mate- rial, and the movement takes place over a greater length of the capillary. There is no abrupt transition from free movement in the liquid to a stationary position in the sealing material.

It is particularly preferred for the resilient material, for example the gel, to be a material that is insoluble in the liquid to be filtered, or that does not comprise any component that is soluble in the liquid to be filtered.

In accordance with a further aspect, the invention relates to an apparatus for filtering a liquid with the aid of ultrafiltration membranes, which apparatus is characterised in that an inlet for backwashing the apparatus is in communication with a supply of degassed water. This provides a very convenient way of carrying out the method according to the invention.

Finally, according to a last aspect, the invention relates to a capillary membrane filtration module, wherein at least one end of tubular filtration elements is potted in a sealing material, and wherein the filtration elements are surrounded near the sealing material by a resilient material or casing abutting against the sealing material. Such a filtration module may be conveniently used in known filtration units.

As mentioned, the method according to the invention is implemented in a backwash step of the filter unit. In general, a filter unit is backwashed several times with clean water and subsequently once with chemical additives for a chemical cleaning. The cleaning chemicals are metered into the backwash water and the membranes are subjected to the cleaning chemicals for a pre-determined length of time (the so-called "standing time" or "soaking time"). In accordance with the invention, such cleaning can be carried out accord- ing to a preferred embodiment using gas-unsaturated water, in order to dissolve the gas present in the filter unit. This is a particularly effective manner of removing gas at the permeate side of the filter. After the said standing time, the backwash water is flushed out of the apparatus and the re- suiting concentrate thus located at the feed side of the filter unit, is discharged. When flushing, it must be ensured that the added chemicals are completely removed.

Apart from the possibility of carrying out the method for the removal of gas simultaneously with a chemical cleaning, this method may also be performed in addition to a normal hydraulic backwash.

The fact that both chemical cleaning and the dissolution of gas require a certain standing time, makes a combined operation especially advantageous. If, instead of using degassed water as mentioned above the pressure is elevated so as to bring the gas into solution, the method is preferably as follows. During backwashing backwash water is fed into the filter unit, after which the pressure is elevated by putting the backwash pipe under high pressure and simultaneously keeping the feed and the concentrate outlet closed. This high pressure is maintained for a pre-determined period of time without discharging any liquid. Finally, after degassing has taken place, the backwash water is discharged either via the con- centrate outlet, or via the product discharge from the permeate side. This latter possibility is only applicable if no chemicals are present at the permeate side. After that the filter unit may be used again for filtering liquid.

With pressure elevation also, the method may be carried out either prior to, simultaneously with or after completion of a chemical cleaning or during backwashing.

The figures show two embodiments in accordance with the invention. Fig. 1 shows a part of a membrane filtration apparatus 1, with capillary membrane filters 2. Their ends 3 are potted in a sealing material 4. This sealing material 4 forms a barrier between the feed side 5 and the permeate side 6. As can be seen, a gel 7 is applied to abut against the sealing material 4, with the gel 7 surrounding the capillaries 2.

Fig. 2 shows a part of a membrane filtration apparatus 1 according to another aspect of the invention. Identical reference numbers have the same meaning as in fig. 1. At a highest point 8 at the permeate side 6 an outlet opening 9 is formed, through which the gas 12 present can escape to a gas discharge side 10. The gas can be discharged via a valve 11. It will be obvious that combinations of the above- mentioned methods are possible. The invention can be used with practically all existing filtration units employing capillary membrane filters. These filtration units may be comprised either of horizontal pressure tubes with serially arrayed membranes, or of units wherein the membranes are arrayed in a vertical and/or parallel orientation. In comparison with the known apparatuses, only a vacuum degasser or a pressure elevation unit on the backwash feed pipe is required. This may be arranged as by-pass or in line. The control programmes may be supplemented by suitable control programmes wherein, for example, the frequency of the secondary cycle with vacuum degassing or pressure elevation, and the settings of the primary (backwash) cycle can easily be modified by a person skilled in the art.

It will be obvious, that the invention is not lim- ited to the specific embodiments mentioned heretofore.

Moreover, the invention may be implemented by means of combinations of measures, which in the foregoing have been mentioned only separately.

Claims

1. A method of operating a filter unit with capillary membrane filters in a membrane module, wherein at least one end of the capillary membrane filters is potted in a sealing material, characterised in that a movement of the capillary membrane filters near the sealing material during operation is reduced by at least one of the following steps: I) removing gas present at the permeate side in the filter unit, which gas was initially contained in the water and which is released therefrom, and II) providing a resilient material around the capillary membrane filters, abutting against the sealing material.

2. A method according to claim 1, characterised in that the gas is contacted with gas-unsaturated water, for the purpose of dissolving the gas therein.

3. A method according to claim 2, characterised in that the degassed water is selected from: a) degassed water, preferably vacuum-degassed water, b) cooled water, having a temperature lower than the temperature of the water to be filtered, c) compressed water, having a pressure higher than a filtration pressure.

4. A method according to one or more of the preceding claims 2 and 3, characterised in that degassing takes place during backwashing or chemical cleaning of the filter unit.

5. A method according to one or more of the preceding claims 2 to 4, characterised in that the steps are carried out at the permeate side of the filter unit.

6. A method according to one or more of the preced- ing claims, characterised in that the membrane module comprises ultrafiltration membranes.

7. A method according to claim 6, characterised in that the membrane module comprises 1 to 500, preferably 1 to 250, and more preferably 1 to 25 microfiltration membranes, at least some of which microfiltration membranes are posi- tioned at least partly at a highest position in the membrane module.

8. A method according to claim 7, characterised in that the capillaries of the microfiltration membrane in the membrane module have a visibly different, preferably a larger, diameter than the capillaries of the ultrafiltration membranes.

9. A method according to one or more of the claims 7 and 8, characterised in that the microfiltration membranes are potted at the peripheral side of the membrane module.

10. A method according to one or more of the preceding claims, characterised in that the wall of the membrane module comprises at least one hole having a diameter of less than 0.4 mm, preferably maximally 0.2 mm, more preferably maximally 0.1 mm, and still more preferably maximally 0.05 mm, wherein at least one hole is provided in the apparatus, and bringing the module under pressure in order to degas the module via the at least one hole located at the substantially highest position.

11. A method according to one or more of the preceding claims, characterised in that the filter unit is disposed vertically, and at a highest position comprises an opening for discharging gas so as to lead the gas outside the range of the capillary membrane filters.

12. A method according to one or more of the preceding claims, characterised in that the resilient material is a gel.

13. A method according to one or more of the preceding claims, characterised in that the resilient material is a material that is insoluble in the liquid to be filtered.

14. An apparatus for filtering a liquid with the aid of ultrafiltration membranes, characterised in that an inlet for backwashing the apparatus is in communication with a supply of degassed water.

15. A capillary membrane filter module, wherein at least one end of tubular filtration elements is potted in a sealing material, characterised in that the filtration ele- ments are near the sealing material surrounded by a resilient material abutting against the sealing material.