AS A SWIMMING POOL

TECHNICAL FIELD

The present invention is related to the filtration of water and, in particular, to the filtration of water in water recirculation systems, such as swimming pools.

STATE OF THE ART

In water recirculation systems such as swimming pools, water is generally repetitively circulated through a filter in order to remove particles suspended in the water and thereby keep the water in adequate conditions in terms of, for example, the transparency of the water and its salubrity. Many different kinds of filters are known in the context of water recirculation systems. For example, sand filters are often considered to be efficient but involve certain drawbacks related to, for example, their maintenance. Another kind of filter frequently used in the context of swimming pools is the so-called cartridge filter. A swimming pool cartridge filter generally comprises a container or housing with an inlet for water to be filtrated and an outlet for the filtrated water, and a filter cartridge arranged in the container so that the fluid passes through a filtration medium forming part of the filter cartridge. Generally, the filtration medium comprises a pleated sheet of a porous material, such as paper. Many filter cartridges used in swimming pool filters have a substantially cylindrical shape, with the pleats arranged in parallel with the longitudinal axis of the cylinder, and with caps or similar arranged at both of the longitudinal ends of the pleated filtration medium. The filtration medium is pleated in order to provide for a relatively large surface of filter material within a relatively limited space. The filter cartridge is thus a cylinder shaped member with pleated and porous walls. Generally, at least one of the ends of the filter cartridge features an opening for letting out fluid. The fluid enters the housing under pressure, passes through the filtration medium (such as the pleated filter material) and exits through the end opening of the cartridge. Some of the particles carried by the water are retained in the filtration medium.

A problem is that while the water is being recirculated through the filter, particles filtered out from the water accumulate in the filtration medium. This accumulation of particles increases resistance to flow and thereby tends to reduce the flow. The increased resistance to flow can to a certain extent be compensated by an increase in the pressure (such as by using more powerful pumps), but sooner or later the filtration medium has to be replaced or cleaned. Cleaning a cartridge filter can be rather laborious and is often perceived as a "dirty" task. In many prior art cartridge filters, the filter cartridge has to be removed manually, and the operator then cleans the filtration medium by directing a stream or jet of water against it, for example, using a garden hose or similar. In many prior art cartridge filters using a layer of porous material such as paper, the particles tend to get stuck within the filter material, away from the surface of the material, so that the water stream has to be applied under relatively high pressure and for a considerable amount of time in order to bring the particles back out of the material, that is, in order to extract them from the filtration medium. The fact that particles tend to get stuck within the filter material makes the task of cleaning the filtration medium even more laborious.

It has also been found that in spite of the efforts, the cleaning is generally not 100% effective and some particles tend to remain within the filtration medium. With time, such remaining particles tend to accumulate in the filtration medium, whereby its efficiency deteriorates.

Traditionally, swimming pool cartridge filters have often featured rather selective filtration media, that is, cartridge filters have often been provided with filtration media designed to retain, as far as possible, all particles having a size exceeding a specified limit. That is, many filtration media seem to have been designed to have pores of a specific size, that is, featuring a pore size distribution curve with a substantial and relatively narrow peak around a certain pore size. It is understood that this may appear attractive to users, as these filters to a certain extent seem to "guarantee" that particles having a size above a certain threshold will be retained by the filtration medium when the water passes through the filter. That is, this kind of selective filter performance may at a first look appear attractive to swimming pool operators, and traditionally such filtration media have frequently been used. However, when designed to retain also relatively small particles, this kind of filtration media tend to get clogged relatively rapidly, requiring frequent repetition of the cleaning operations.

Attempts have been made to provide for so-called self-cleaning cartridge filters. US- 5989419-A and US-6156213-A both teach cartridge filters in which a filter cartridge is arranged to be cleaned within the housing by subjecting the cartridge to water jets from jet nozzles, causing the cartridge to spin around its longitudinal axis. The water jets in combination with the centrifugal forces generated by the spinning of the cartridge are supposed to provide for the desired cleaning of the filter material, by removing debris and particles retained on the surface and possibly also within the filter material. However, to create the centrifugal forces, the cartridge has to be spun at a rather high speed. This requires a sophisticated and potentially costly arrangement of the cartridge in relation to the housing, involving low-friction bearings, which can be problematic in view of the relatively "dirty" environment. There is a risk that the bearings will jam with time, unless carefully maintained and cleaned. Also, this requires a water outlet arrangement suitable for emptying the filter housing prior to initiating the cleaning operation, as with water present inside the housing, it appears to be difficult to achieve a sufficiently high rotational speed of the cartridge to produce the desired centrifugal forces.

The discussion regarding the state of the art has so far focused on swimming pools, but the same or similar problems arise also in other kinds of water recirculation systems, wherein the water in one or more tanks or deposits is treated by repetitively circulating the water through one or more filters. Examples of such water recirculation systems are water tanks for fish farming, certain processes and installations for treatment of waste water, etc.

As indicated above, prior art swimming pool cartridge filters generally use a filtration medium of porous material, such as paper. However, in the general art of filter technology, there are many other materials. Some of these filter materials use so-called nanofibers.

US-8303693-B2 discloses a filtration medium for facemasks and cabin filters, including a fine filter layer having a plurality of nanofibers, and a coarse filter layer having a plurality of microfibers attached to the fine filter layer. The coarse filter layer is positioned upstream of the fine filter layer.

US-8308834-B2 discloses a composite filter media structure including a base substrate and a nanofiber layer deposited on one side of the base substrate, aimed at overcoming drawbacks involved with prior art filter media in relation to the filtering of inlet air in the field of power generation gas turbines.

The use of nanofiber filter layers is also known or at least suggested in the field of water treatment.

For example, Christian Burger, et al., "Nanofibrous Materials and Their Applications", Annu. Rev. Mater. Res. 2006, 36, pages 333-368, suggest the use of a three-tier approach and mention possible applications in the field of treatment of produced water, ballast water, and desalination.

Also Renuga Gopal, et al., "Electrospun nanofibrous filtration membrane", Journal of Membrane Science 281 (2006), pages 581 -586, suggest the use of a "sandwich" structure and applications in the field of pre-treatment of water. DESCRIPTION OF THE INVENTION

The present invention aims at overcoming at least some of the drawbacks known from prior art arrangements, by improving both the filtration medium and the filter assembly and the way in which they co-operate.

A first aspect of the invention relates to a filtration medium for a filter for a water recirculation system, comprising a first layer arranged between and adhered to a second layer and a third layer, the first layer being a non-woven nanofiber layer.

In the present document, the term "water recirculation system" refers to a system comprising one or more tanks or deposits for water, arranged for treating the water by repetitively circulating the water through one or more filters. The system can comprise, in addition to the filter and the deposit, conduits suitable for displacing water from the deposit to the filter and back to the deposit, as well as means such as pumps for displacing the water. Examples of water recirculation systems are swimming pools, some deposits for fish farming, etc.

In the present document, the term "nanofiber layer" refers to a layer comprising nanofibers, that is, fibers having a diameter of less than 1 micron (μιη) and preferably a diameter of not less than 5 or 1 nanometer (nm). Most of the fibers of the nanofiber layers are nanofibers. For example, more than 90%, such as more than 95% or more than 99%, such as 100%, of the fibers making up the nanofiber layer are nanofibers. The nanofiber layer can be obtained by techniques such as electrospinning, force spinning or melt spinning, techniques that are well known in the art.

The use of a nanofiber layer has been found to provide for special advantages in the context of a filter for a water recirculation system, such as a swimming pool filter. Due to the way in which a nanofiber layer is prepared, the fibers can be arranged in an aleatory or pseudo- aleatory manner, thereby providing for a broad pore size distribution, with a substantial amount of both large and small pores. This means that when water is passing through the nanofiber layer, particles of a given size may get stuck in the nanofiber layer or may traverse the nanofiber layer, passing through the filtration medium. Some larger particles may pass through the filtration medium while some substantially smaller particles may be retained by the filtration medium.

This approach differs from the one underlying the conventionally preferred selective filtration media discussed above, designed to remove all particles having a size above a certain specific threshold present in the water passing through the filtration medium. Although, as explained above, that kind of selective filtration media may seem attractive to many users, it has been found that they involve certain drawbacks when it comes to finding the right balance between the size of particles to be retained -it is generally preferred that even fairly small particles be retained, such as particles having a size in the order of 1 μιη- and the desire of low resistance to flow (so as not to require the use of unnecessary powerful pumps) and long time between successive cleaning cycles. It is generally desired to avoid the need for frequent cleaning. A problem with a filtration medium arranged for retaining all particles above a certain size when the water passes through the filter once is that if it is designed to retain even small particles, it will get clogged rapidly when in use. This means that cleaning of the filter will have to be carried out relatively frequently. The frequency with which cleaning has to be carried out can be reduced by using more powerful pumps, but this increases the costs -due to the additional cost of more powerful pumps and the larger amount of energy consumed by the pumps- and cleaning may still be necessary at a fairly high frequency.

It has been found that this problem can be mitigated by the use of a filter with a nanofiber layer. Due to the aleatory or pseudo-aleatory distribution of fibers, when water passes through such a nanofiber layer, some very small particles may become retained by the nanofiber layer, whereas some larger particles may pass through it, depending on the kind of pore or pores faced by the respective particle. Whereas this might intuitively be perceived as a non-optimal behavior of the filter, in the context of a filter for a water recirculation system, such as a swimming pool filter, this behavior may actually be preferred. Contrary to many other applications where a fluid passes through a filter only once, in water recirculation systems such as swimming pools there is a constant re-circulation of water. The water is constantly circulated through the filter or filters of the installation. Thus, with time, all particles -or at least all particles above a minimum size basically corresponding to a minimum size of the pores of the filter- will be retained. However, the filter will not be rapidly blocked in the first "sweep". Thus, the buildup of particulate clogging the filter will take longertime when this kind of nanofiber filter is used, and this will allow also smaller particles to become retained, thereby providing for an enhanced over-all removal of particles from the water, while allowing for a relatively long time between successive filter cleaning cycles.

Another advantage of the use of a nanofiber layer is related to the way in which particles are retained by the nanofiber layer, namely, substantially at the surface of the nanofiber layer. This is a difference compared to what happens in prior art paper filters, where particles tend to get retained within the filter material. This difference implies that the nanofiber layer can be easier to clean by applying a water stream to the filtration medium.

The fact that the particles are substantially retained at the surface of the nanofiber layer also implies that less particles will remain in the filtration medium after a cleaning operation, thereby preventing or at least delaying the deterioration of the quality of the filter due to accumulation of particles remaining in the filtration medium after cleaning.

Cleaning can also be carried out at a relatively low pressure, preferably within a filter housing and preferably without any need for emptying the filter housing by letting the water out of the housing, and preferably operating with the water flow at the normal pressure, without need for operating pumps at higher power during the cleaning cycle. This provides for important advantages in terms of costs and environmental-friendliness.

The filtration medium can for example be obtained by applying the nanofiber layer onto the second layer by electrospinning, force spinning or melt spinning, and thereafter applying the third layer onto the nanofiber layer, providing for adherence by applying heat and pressure, for example, using a calender roll. It has been found that in this way it is possible to obtain a filtration medium in which the three layers stick together but which is not unduly compressed. A too high compression may give rise to an undesirably reduced pore size.

Thus, it can be said that filtration medium according to the invention may tend to be less selective than many traditional filtration media, in that the pore size distribution, which can be determined by the nanofiber layer, does not show the same substantial peak around a specific threshold value. However, the filtration medium may indeed be more efficient when considered in the context of a water recirculation system such as a swimming pool, where water is constantly or repetitively being recirculated. Here, the filtration medium will end up capturing both large and small particles as the water is being made to repetitively flow through the filtration medium.

With other words, the use of the nanofiber layer facilitates the creation of a filter with a substantial amount of both small and large pores. The average pore size may be larger than the average pore size of many prior art systems, which facilitates water circulation and prevents the filter from becoming clogged rapidly. However, the presence of small pores implies that also small particles will become trapped, sooner or later. At the same time, the nanofiber layer retains particles at its surface, facilitating the cleaning operations.

In some embodiments of the invention, the second layer and the third layer are coarse filter layers and the first layer is a fine filter layer. Generally, in these embodiments of the invention, the nanofiber layer is a relatively fine filter layer arranged to retain a substantial part of the particles passing through the second layer, and the third layer is selected to be relatively coarse so as not to provide for unnecessary resistance to flow and so as not to retain particles that have not been retained by the nanofiber layer, so that cleaning of the filtration medium can be efficiently achieved by directing a water jet onto the second layer. The second layer can be coarse to allow particles to reach the surface of the nanofiber layer and to allow the water stream or streams used for cleaning to reach the nanofiber layer. The terms "coarse" and "fine" are relative terms and are used to denote that the first layer is a finer filter layer than the second and third layers, so that the main filter function is performed by the first layer. The main purpose of the second and third layers is to provide for support for the nanofiber layer and to protect it, whereas the purpose of the nanofiber layer is to retain particles. In some embodiments of the invention, the second and the third layers feature a higher permeability to airthan the first layer. In some embodiments of the invention, the first layerfeatures on average finer pores than the second layer and the third layer.

Whereas some particles and debris may be retained in the coarse layer arranged upstream of the nanofiber layer when the filtration medium is in use, it is desired that most particles be retained by the nanofiber layer. Especially, except for relatively large particles, the intention is that particles be retained by the nanofiber layer.

In some embodiments of the invention, the second layer and the third layer each features a higher air permeability than the first layer. That is, in these embodiments of the invention, both the second layer and the third layer are generally more permeable to air than the first layer, which is consistent with their function of offering support and protection rather serving to filter particles out of the water, a task that is substantially entrusted to the first layer. The high air permeability of the second layer and the third layer contributes to improve the efficiency of the cleaning operation, when cleaning is carried out by applying a stream of a fluid such as water to the surface of the second or third layer. The relatively high permeability allows the stream to reach the first layer so as to remove the particles trapped at the surface of the first layer. Also, the relatively high air permeability favors a low resistance to flow.

In some embodiments of the invention, the second layer and the third layer substantially consist of fibers having a diameter larger than 1 micron. As the second and third layers are relatively coarse layers, in some embodiments of the invention they substantially consist of fibers featuring a relatively large diameter, such as a diameter of more than 1 micron on average, such as more than 10 microns on average, for example, between 10 microns and 40 microns. Most of the fibers of the first and second layers, such as more than 90%, such as more than 95% or more than 99%, such as 100% of the fibers making up these layers, can have a diameter larger than 1 micron, for example, between 10 and 40 microns.

In some embodiments of the invention, the second layer and the third layer are substantially identical. In some embodiments of the invention, the second layer and the third layer are identical or similar, so that the filtration medium is substantially symmetric in cross section, so that any of the second and the third layer can be arranged to be upstream when the filtration medium is in use, without affecting the way in which the filtration medium will perform.

In some embodiments of the invention, the second layer and the third layer are non- woven layers. One advantage provided by embodying the second and the third layers as non- woven layers is that it easy to provide non-woven materials that do not fray easily, thereby further enhancing the durability of the filtration medium.

In some embodiments of the invention, the first layer is substantially made of polyethylene terephthalate. This material, also known as PET, has been found to prevent the growth of fungi and may be especially useful in the context of swimming pool filters. However, in other embodiments of the invention other materials can be used, such as other polyester materials, polyacrylonitrile, polyvinylidene fluoride, poly(amide 6), polyurethane, or a combination thereof. In some embodiments of the invention, the second layer and the third layer are made of the same material as the first layer.

In some embodiments of the invention, the first layer has a grammage of more than 0.1 g/m ² and less than 20 g/m ² , preferably more than 0.5 g/m ² and less than 0.7 g/m ² .

In some embodiments of the invention, each of the second layer and the third layer has a grammage of more than 45 g/m ² and less than 55 g/m ² . These ranges of grammages are considered appropriate for, for example, swimming pool filters.

In some embodiments of the invention, the nanofibers of the first layer have a diameter of more than 400 nm and less than 600 nm. This range has been found to be appropriate for, for example, swimming pool filters.

In some embodiments of the invention, in a cross section of the filtration medium, each layer has a thickness, the thickness of the first layer being smaller than the thickness of the second layer and smaller than the thickness of the third layer. That is, the thickness of the first layer -that is, its width when seen in a cross section of the filtration medium- is smaller than the thickness of the other layers. The first layer does not need to have a substantial thickness, as it is primarily intended that particles be retained at the surface of the first layer, and as the support and consistency of the ensemble to a substantial extent can be provided by the coarser second and third layers.

In some embodiments of the invention, the filtration medium is pleated in accordance with a plurality of parallel lines. This pleated filter material can be incorporated into a cartridge, forming a filtration medium cylinder with a wall thickness substantially corresponding to the pleat depth.

In some embodiments of the invention, the distance between said parallel lines is not larger than 6 cm, preferably not larger than 4 cm. Traditionally, in swimming pool filter cartridges using paper as the material for the filtration medium, the distance between the parallel pleat lines has been larger than 4 cm and even larger than 6 cm. However, it has been found that when using a filtration medium involving a nanofiber layer as described above, smaller pleats can be advantageous to make the cleaning even more efficient. This enhanced cleaning efficiency is considered to compensate the fact that the reduced pleat depth may involve a reduced amount of effective surface of the filtration medium and thus, to a certain extent, increase the frequency with which the filter has to be cleaned.

In some embodiments of the invention, at least 10% of the pores have a diameter of less than or equal to 20 μιη, preferably less than or equal to 18 μιη, more preferably less than or equal to 16 μιη, and at least 10% of the pores have a diameter of more than 40 μιη, preferably more than 50 μιη, more preferably more than 54 μιη, as determined in accordance with ASTM F-316-03 Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test (201 1 ),Test Method B. A combination of a substantial amount of relatively small pores and a substantial amount of relatively large pores has been found to be advantageous for use in water recirculation systems, as explained above.

Another aspect of the invention relates to a filter cartridge for a filter assembly for a water recirculation system, comprising a pleated filtration medium, the filtration medium being a filtration medium as described above. The filtration medium is arranged to form a wall of the filter cartridge, extending between two end portions of the filter cartridge. This filter cartridge can be used in a filter assembly for a water recirculation system, such as a swimming pool, and has proven to facilitate the cleaning operations, for example, by allowing for the use of relatively simple internal cleaning means such as a diffuser, that is, an element with a plurality of outlet openings or nozzles for directing water streams or jets against the filtration medium.

Another aspect of the invention relates to a filter assembly for a water recirculation system, comprising: a container with an inlet and an outlet;

a cartridge with a filtration medium, the cartridge being rotatably arranged inside the container;

at least one cleaning diffuser with a plurality of openings directed towards the filtration medium, the diffuser being placed within the container, between a wall of the container and the filtration medium.

Thereby, cleaning of the filtration medium from outside, that is, from the upstream end, can be carried out without any need to remove the cartridge from the container. This "clean hands" approach is advantageous for users. Especially, it has been found that when using a filtration medium as described above, cleaning can be carried out with the described assembly without any need to use high pressure water jets, and even without emptying the filter assembly. It has been found that the normal operating pressures of for example a swimming pool system can often be sufficient to generate the water jets or streams necessary to achieve sufficient cleaning, that is, without any need to use different pumps than the ones normally used for generating the circulation of the water.

In some embodiments of the invention, the filter assembly comprises a connecting pipe between the outlet and a first end of the cartridge, the connecting pipe being equipped with a pipe section which is inserted into the cartridge through a first end of the cartridge, the cartridge being equipped with a continuous perimeter brush facing and pressing against the pipe section, or the pipe section being equipped with a continuous perimeter brush facing and pressing against the cartridge, so that a rotatable support arrangement is configured in which the brush is a barrier for the passage of impurities from one side of the filtration medium to the other side of the filtration medium. The inventors have found that this support arrangement very effectively prevents the passage of impurities, allows the rotation of the cartridge about its axis -for example, when actuated manually-, and is relatively inexpensive to manufacture, as it does not require the use of bearings, nor space for them. Also, the absence of bearings implies a reduced risk for jamming of the device, for example, due to accumulation of dirt.

In some embodiments of the invention, the rotatable support arrangement comprises an integral circular support surface of the container, on which a complementary circular support surface of the cartridge is supported, these surfaces surrounding the pipe section.

In some embodiments of the invention, the inlet is provided for letting in unfiltered water, the outlet is provided for letting out filtered water, the rotatable support arrangement is a rotatable support arrangement for rotation of the cartridge around a vertical axis, and the first end of the cartridge is a bottom end of the cartridge.

In some embodiments of the invention, the diffuser extends substantially in parallel with the cartridge, and the diffuser comprises at least two columns of openings, a first one of said columns being longitudinally displaced in relation to a second one of said columns, so that areas between subsequent openings in the first one of said columns overlap with respective openings in the second one of said columns, when viewed perpendicularly to a longitudinal axis of the cartridge. It has been found that this arrangement, with two columns or rows of openings displaced in relation to each other so that the openings of one row overlap with respective spaces between the openings in the other row, favors efficient cleaning. One reason for this seems to be that the pressure inside the diffuser is kept sufficiently high along the entire axial extension of the columns of openings, so that the water jets or streams exiting the diffuser provide the same or at least a similar cleaning effect in correspondence with the entire axial extension of the diffuser. If instead of a plurality of openings one single slot is used, the pressure has been found to decrease substantially along the slot, so that the cleaning is much more efficient in correspondence with the upstream portion of the diffuser than in correspondence with the downstream portion thereof. The use of overlapping openings makes sure that jets are applied to the cartridge in a continuous manner, along the axial extension thereof, in spite of the use of a plurality of openings instead of one continuous slot.

In some embodiments of the invention, the filter assembly comprises drive means for rotating the cartridge, said drive means comprising at least one of a handle for manual actuation arranged outside the housing, an electric motor, and a drive means actuated by water flow. Whereas a handle for manual actuation, for example, a handle operatively connected to the filter cartridge such as to a shaft thereof so that the turning of the handle by a user causes the cartridge to rotate around its longitudinal axis, may at first appear to be inappropriate for "automatic" cleaning, it has been found to imply a safe, reliable and cost- effective means for carrying out the cleaning operation. The bias against manual cleaning has been found to be due to the "dirty" aspects thereof, rather than due to the need for some manual work. Actually, users may often prefer to manually turn the filter, so as to have control over the duration and velocity of the cleaning operation. Also, and even if electric motors or even water driven drive means (including for example some kind of turbine) may be appropriate, the manual drive means may often be preferred due to their reliability. Also, even if the assembly includes electrically or hydraulically powered drive means for rotating the cartridge, the presence of means for manually rotating the filter may be appreciated as back- up drive means, thus making sure that the filtration medium can be appropriately cleaned also in the case of a failure in the electrically or hydraulically actuated drive means.

In some embodiments of the invention, the integral circular support surface of the container or the complementary circular support surface of the cartridge comprises two side frames that are extended on both sides of the complementary circular support of the cartridge, or the integral circular support surface of the container, respectively, whereby a mutual fitting is configured between the cartridge and the container.

In some embodiments of the invention, the brush is attached to the cartridge and has the bristles arranged tilted upwards or downwards according to the radial direction inwards.

In some embodiments of the invention, the brush is attached to the pipe section and has the bristles arranged tilted upwards or downwards according to the radial direction outwards.

In some embodiments of the invention, the connecting pipe has an intermediate bypass that is connected to a lower end of the diffuser. This allows water to be channelled in order to clean the filter.

In some embodiments of the invention, the pipe section comprises at its upper end a non-return valve arranged to allow water to flow in the direction going from inside the filter to the outlet and to prevent flow in the opposite direction.

In some embodiments of the invention, the assembly comprises in the intermediate bypass a non-return valve arranged to allow water to flow in the direction going towards the diffuser.

In some embodiments of the invention, the non-return valves are swing flap valves. This kind of valves is simple and reliable.

In some embodiments of the invention, the cartridge comprises a top drive extension that crosses the container, so that it can be rotated from outside. In some embodiments of the invention, the top drive extension is attached to a motor or to a handle.

In some embodiments of the invention, the openings of the diffuser are elongated openings distributed in two or more columns and displaced vertically such that they overlap horizontally to cover the entire height of the filtration medium. These columns can extend along the entire axial length of the filtration medium.

In some embodiments of the invention, the filter assembly comprises a Teflon or polyamide joint arranged between the two support surfaces, to provide for a minimum friction coefficient and absorb irregularities.

In some embodiments of the invention, the assembly comprises a deflector element arranged opposite the inlet, designed to divert the water inflow to contribute to the distribution of the flow around the filtration medium. In some embodiments of the invention, the deflector element comprises a cylindrical section fitted to the inlet, which extends into the container by a surface and a support of the deflector surface.

In some embodiments of the invention, the cartridge comprises a base arranged at one end of the cartridge, and the brush is provided on a member, such as an annular member, configured to fit into an opening in the base. This can serve to simplify the design and manufacture of the cartridge as such, and also makes it possible to replace the cartridge without replacing the brush, and vice-versa. This kind of modular arrangement is often preferred to minimize the waste of material. In some embodiments of the invention, the base and the member can both be plastic, and the brush can for example be integrated with the member by injection moulding the member so that it incorporates the bristles of the brush. The bristles can be arranged in correspondence with an opening in the member.

In some embodiments of the invention, the filtration medium is a filtration medium as the one described above.

A further aspect of the invention relates to the use of a filtration medium as described above, or of a filter assembly as described above, for removing particles from the water of a water recirculation system, such as a swimming pool.

A further aspect of the invention relates to a method for removing particles from the water of a water recirculation system, such as a swimming pool, comprising circulating the water through a filtration medium as described above, or through a filter assembly as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate embodiments of the invention, which should not be interpreted as restricting the scope of the invention, but just as examples of how the invention can be carried out. The drawings comprise the following figures:

Figure 1 is a cross sectional view in accordance with the central axis of a swimming pool cartridge filter assembly, in accordance with an embodiment of the invention.

Figure 2 is a perspective view of the filter cartridge of this embodiment.

Figure 3 is a perspective view of the assembly in accordance with this embodiment of the invention, with a portion cut away.

Figure 4 is a detail perspective view of a portion of the assembly in accordance with this embodiment, showing the flow deflector arranged at the inlet of the container.

Figure 5 is a perspective view of the lower part of the assembly in accordance with this embodiment, showing the rotatable support arrangement.

Figure 6 is an enlarged view of a portion of the part shown in figure 5, in which the details that make up the rotatable support arrangement can be seen.

Figures 7 and 8 are perspective views of the deflector used in this embodiment of the invention.

Figures 9 to 12 show various options for the arrangement of the brush between the pipe section that is inserted through the bottom of the cartridge and the cartridge, in accordance with different variants of this embodiment of the invention.

Figures 13A and 13B are schematic front and perspective views, respectively, of the diffuser of the filter assembly in accordance with this embodiment of the invention.

Figures 14 and 15 schematically illustrate a cross section of a filtration medium in accordance with an embodiment of the invention.

Figure 16A is a scanning electron microscope (SEM) image (showing an area corresponding to 505x475 microns) of the second layer of the filtration medium, in accordance with an embodiment of the invention.

Figure 16B is an SEM image (showing an area corresponding to 505x475 microns) of the second layer and the first layer in accordance with an embodiment of the invention.

Figure 17 is a diagram illustrating differences in pore size distribution between different filtration media. DESCRIPTION OF WAYS OF CARRYING OUT THE INVENTION

As shown in figures 1 , 5 and 6, the filter set or assembly, in accordance with an embodiment of the invention, comprises:

- a container 2 with an inlet 3 for letting in unfiltered water and an outlet 4 for letting out filtered water;

- a cartridge 5 equipped with a filtration medium 6, arranged inside the container 2;

- a rotatable support arrangement 7 of the cartridge 5, so that the cartridge 5 can rotate inside the container 2, around a vertical axis Γ;

- at least one cleaning diffuser 8 arranged between the container 2 and the filtration medium 6, with the diffuser 8 equipped with a plurality of openings 81 directed towards the filtration medium 6;

- a connecting pipe 9 between the outlet 4 and the bottom of the cartridge 5, with the pipe 9 equipped with a pipe section 91 which is inserted into the cartridge 5 through the bottom thereof.

In this document, the expression "rotatable support arrangement" refers to those elements of the container and of the cartridge that allow the latter to be supported on the former and be able to rotate with respect to it.

In particular, according to this embodiment of the invention and as shown in figure 6, the rotatable support arrangement 7 comprises an integral circular support surface 21 of the container 2 on which a complementary circular support surface 51 of the cartridge 5 is supported, and these surfaces surround the pipe section 91. The cartridge 5 orthe pipe section 91 is equipped with a continuous perimeter brush 10 facing and pressing the pipe section 91 or the cartridge 5, respectively, such that a rotatable support arrangement is configured on the vertical axis Γ wherein the brush is a barrier to the passage of impurities from one side of the filtration medium 6 to the other. The brush 10 is arranged at the inner perimeter of an annular member 53, configured to fit into an opening in a base 52 of the cartridge 5, as schematically shown in figures 1 , 3, 5 and 6.

As can be seen in figures 1 and 6, the complementary circular support surface 51 of the cartridge 5 comprises two side frames C1 , C2 that extend along both sides of the circular support surface 21 of the container 2, so that a mutual fitting is configured between the cartridge 5 and the container 2. In a variant of this embodiment, this arrangement is reversed, that is, the frames are joined to surface 21 .

It is preferred that the brush 10 have bristles arranged tilted upwards in the radial direction inwards, while a downward tilted arrangement is not ruled out. Another possibility is a symmetrical arrangement with respect to a horizontal plane. The possibility of the brush being attached to the pipe section 91 instead of to the cartridge is also contemplated. Four different options are shown in figures 9 to 12.

The connecting pipe 9 has an intermediate bypass 1 1 which is connected to the lower end of the diffuser 8. The pipe section 91 comprises, at its upper end, a non-return valve 92 arranged to allow water to flow in the direction going from inside the cartridge towards the outlet 4 and to prevent flow in the opposite direction. A non-return valve 12 is foreseen in the intermediate bypass 1 1 arranged to allow water to flow in the direction that goes from the connection 4 to the diffuser 8. With this arrangement, the reverse flow allows the water to be channelled through the diffuser 8, so that it impacts against the filtration medium 6, preferably along all of its height. In this embodiment, the non-return valves 12, 92 are swing flap valves.

As can be seen in figure 1 , the cartridge 5 comprises an upper actuation extension 61 that crosses the container 2, so that it can be rotated from the outside, either by a handle 62 as shown in figure 3, or by a motor (not shown).

The openings of the diffuser 81 are elongated openings distributed in two or more columns as shown in figures 13A and 13B, and are displaced vertically in relation to each other such that they overlap horizontally to cover the entire height of filter 6. This arrangement is particularly advantageous for providing effective cleaning, and may constitute on its own a separate invention.

In this embodiment, a Teflon ^® or polyamide joint J1 is arranged between the two support surfaces 21 , 51 , to provide for a minimum friction coefficient and absorb irregularities.

In this embodiment, the assembly further comprises a deflector element 31 arranged opposite the inlet 3, designed to divert the water inflow to contribute to the distribution of the flow around the filtration medium 6. Specifically, as shown in figures 7 and 8, the deflector element 31 comprises a cylindrical section 32 adjustable to the inlet 3, which extends into the container 2 by a surface 33 and a support 34 of the deflector surface 33. This form of deflector allows the water flow to be diverted tangentially to the filter material, thereby distributing the inlet flow to this filter material. This contributes to homogenising the way in which the filtered impurities are deposited onto the filtration medium.

As indicated above, figures 13A and 13B schematically illustrate the arrangement of the openings 81 in the diffuser 8, through which water is directed onto the filtration medium 6. The inventors found that using only one elongated opening did not produce totally satisfactory results, possibly due to a pressure drop in the axial direction, from the upstream to the downstream end of the elongated opening. On the other hand, using one row of openings likewise failed to produce the desired results in terms of efficiency and efficacy. The inventors found, however, that especially good results could be obtained by using at least two rows of openings, extending along the longitudinal axis of the diffuser, and with the openings displaced in relation to each other so that at every cross section of the filter cartridge, there is at least one portion of an opening 81 facing the filtration medium 6. However, also other diffuser layouts are obviously within the scope of the invention.

Figure 14 schematically illustrates a filtration medium 6 in accordance with an embodiment of the invention and which can be used in, for example, an assembly as described above. The filtration medium comprises a relatively fine nanofiber layer 101 sandwiched between two coarser non-woven layers 102 and 103 of thicker fibers, the non-woven layers 102 and 103 generally featuring larger pores than the nanofiber layer 101. The purpose of this arrangement is to provide support for the nanofiber layer 101 and to protect it, while at the same time allowing most of the particles present in the water to reach the nanofiber layer; the flow direction is indicated with an arrow in figure 14. It is schematically illustrated how some larger debris 201 is retained at the surface of the second layer 102, whereas some particles, generally some relatively large particles 202, are retained within the second layer. On the other hand, most of the particles 203 are retained by the first layer 101 , that is, the nanofiber layer. As explained above, particles that pass through the nanofiber layer 101 will most likely also pass through the third layer 103, as this layer is a relatively coarse layer, similar or identical to the second layer 102. These particles may then be retained by the nanofiber layer 101 the next time they pass through the filter, due to the re-circulation of the water.

The nanofiber layer 101 generally retains the particles 203 at the surface of the nanofiber layer 101 , thereby making it relatively easy to remove them. Figure 15 schematically illustrates how water jets or streams 300, generated by the diffuser 8, can be directed onto the filtration medium 6. Due to the coarseness and permeability of the second layer 102, the water jets reach the first layer 101 , that is, the nanofiber layer, and can then wash away the particles 203 that have accumulated on the surface of the first layer, as schematically illustrated in figure 15.

Figure 16A is an image showing an area corresponding to 505x475 microns (μιη) of the second layer in accordance with an embodiment of the invention. The image has been obtained using an SEM. Here, the non-woven structure comprising a plurality of PET fibers with a diameter in the order of 15 microns is shown.

Figure 16B is an SEM image showing an area corresponding to 505x475 microns of the second layer and the first layer in accordance with an embodiment of the invention. Here, the first layer has been applied onto the second layer by electrospinning. The first layer comprises fibers having a diameter that is much smaller than the diameter of the fibers of the second layer. Generally, the fibers of the first layer have diameters smaller than one micron, and are therefore referred to as nanofibers. It is clear to the eye that the first layer represents a finer mesh structure than the second, coarser, layer. Thus, it is clear that the second layer, which when the filtration medium is in use is intended to be arranged upstream of the first layer, will generally only retain some relatively large particles, so that most particles -and especially most of the smaller particles corresponding to the vast majority of the particles typically suspended in swimming pool water- will reach the finer mesh formed by the nanofibers of the first layer.

After application of the first layer, the third layer is applied onto the first layer. The third layer has a structure similar or identical to the structure of the second layer, for symmetry; thereby, the filtration medium can be used with any of the second and third layers facing the upstream direction, thereby reducing the risk for mistakes in the manufacture of the cartridges. Thus, the third layer is not likely to retain particles that slip through the first layer. This avoids or at least reduces the risk of clogging of the third layer.

Figure 17 is a diagram illustrating differences in pore size distribution between a series of different filtration media, where pore sizes have been determined in accordance with ASTM F-316-03 Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test (201 1 ), Test Method B. Curve 400 corresponds to an embodiment of the present invention, curve 401 to a first product available in the marketplace (namely, a filtration medium commercialized by Sacopa SAU under the name "TERRA™ 75"; the product reference was 19786, and the tested sample had barcode number 8420382521993), curve 402 to a second product available in the marketplace (namely, a filtration medium commercialized by Pentair Water Pool and Spa, Inc under the name "STA- Rite Posi Clear™ PCX75"; the product reference was 25230-0075S, and the tested sample had the number 4968026150006L), and curve 403 to a third product available in the marketplace (namely, a filtration medium commercialized by Hayward Industries under the name "Star-Clear ^® Plus", with product reference CX0500-RE).

The embodiment of the present invention was a filter comprising three layers, as follows:

an upstream layer consisting of a non-woven fabric of white Texpun™, 50 gr/m ² , 100% polyester;

a middle layer consisting of PET nanofibers having a diameter of approximately 500 nm, and with a grammage of 0.6 g/m ² ;

a downstream layer consisting of a non-woven fabric of white Texpun™, 50 gr/m ² , 100% polyester. Both the embodiment of the present invention and the three products available in the Marketplace were designed for the treatment of water at a flow rate of 10-12 m ³ /h.

Tests were performed on these art products in accordance with ASTM F-316-03 Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test (201 1 ), Test Method B, to determine the cumulative pore size distribution, which was found to be as indicated in the table below:

* Pore size for which the indicated percentage of the pores has a diameter that is smaller or equal than the indicated one. For example, the indication of 16 microns in the 10% column means that 10% of the pores have a diameter equal to or smaller than 16 microns.

The table shows how, for example, the TERRA™ 75 sample generally features larger pores than the embodiment of the invention, whereas STA-Rite Posi Clear™ PCX75 has 80% of the pores within the range from 20 to 32 microns, and Star-Clear ^® Plus has 80% of its pores within the range from 22.5 to 38 microns, with 40% of the pores in the range from 22.5 to 26 microns (the lower end point of the range not being included, the upper end point being included). The embodiment of the present invention appears to feature a larger percentage of pores in the range below or equal to 16 microns than any of the other samples, and a larger percentage of pores in the range above 32 and 38 microns, respectively, than the samples of STA-Rite Posi Clear™ PCX75 and Star-Clear ^® Plus, respectively. That is, the embodiment of the present invention features a combination of a substantial percentage of relatively small pores and a substantial percentage of relatively large pores.

It is clear that the embodiment of the present invention features a pore size distribution conceptually different from the ones represented by curves 402 and 403, respectively, which reflect the traditional approach aiming at retaining all particles larger than a predetermined threshold size. As explained above, this prima facie attractive approach has been found to involve the drawback that it is difficult to find a balance between the need to retain relatively small particles, and the desire for a long time of operation between successive cleaning cycles.

Contrarily, both the embodiment of the present invention and the TERRA™ 75 product feature a broader pore distribution, thereby reducing the risk for rapid clogging of the filter. However, as evidenced by for example curve 400, also the filtration medium in accordance with the present invention can have a substantial percentage of relatively small pores, even below the pore size were curves 402 and 403 peak. Thus, and whereas with the arrangement as per this embodiment of the present invention many relatively large particles will slip through the filtration medium when the water passes through it for the first time, the repetitive recirculation of water will make sure that with time, both small and large particles will get trapped. This is considered to be due to the aleatory or pseudo-aleatory distribution of the nanofibers, inherent to processes such as electrospinning or force spinning. The nanofiber approach facilitates the implementation of a broad pore size spread with a relatively large proportion of small pores in combination with a substantial proportion of substantially larger pores. That is, the first layer can feature a relatively low flow resistance and some relatively large pores, while simultaneously featuring a substantial proportion of pores sufficiently small for retaining also relatively small particles.

In this text, the term "comprises" and its derivations (such as "comprising", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

When ranges are indicated, these ranges include the end points, unless something else is specified.

The invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.