This invention is related to a method and an apparatus for filtering liquids preferably aqtreous liquids such as raw water intended for drinking or similar, water in aquariums, swimming pools and similar, sewage or eøpecially final-filtering of sewage water after puri¬ fication, industrial water of various types such as cooling water and also other hydrophilic or hydro- phobic liquids such as salt solutions, acids, such as sulf ric acid, oils, gasoline and other hydro¬ carbon containing products and also gases; for remov¬ ing liquid or solid impurities such as sulfuric acid or fog.

The filter material used according to the invention consists essentially of fibrous materials especially of inorganic type, such as mineral wool, preferably stone or rock wool. A suitable type is basalt fibres which in addition to basalt may also comprise e.g. lime e.g. 10-30 per cent by weight lime (calcium oxide) and 90-70 per cent basalt or similar minerals or starting materials which give a similar final com¬ position. Suitable main consituents are 45-50 per cent Si0 ₂ , 10-15 per cent A1 ₂ 0 ₃ , 10-18 per cent CaO, 6-12 per cent MgO. 6-12 per cent FeO, optionally 1-5 per cent K.,0 + Na ₂ 0 and up to 1-3 per cent MnO and up to 1-3 per cent i0 ₂ . An example is 47 per cent Si0 ₂ , 14 per cent A1 ₂ 0 ₃ , 1 per cent Ti0 ₂ , 8 per cent FeO, 16 per cent CaO, 10 per cent MgO, 1 per cent MnO. 2 per cent Na ₂ 0, 1 per cent K _n 0. The fibre material is preferably bound with a resinous binder and non- hydrophobic or is made hydrophilic.

The filter material used according to the invention comprises preferably the fibres oriented in the direction of a plane, preferably randomly oriented in said plane.

Preferably about 50-90 per cent and especially 60-80 per cent e.g. about 70 per cent of the fibres are directed in the direction of said plane which means that the fibres form an angle of less than 45 in re¬ lation to said plane and especially an angle of less than 30°, especially an angle of less than 20°. The rest -of the fibres are suitably randomly oriented in other directions.

The fibre thickness of the filter material may vary within broad limits but is preferably mainly within th range of about 1-20 m and especially 1-10 m, espec¬ ially a thickness of about 2-7 pm, with which is meant that the fibres along a main part of the fibre length have a thickness within said range.

The density of the filter material in non-compressed condition or in the compressed condition used for filtering, is suitably within the range up to 200 kg/m

₃ and preferably up to 150 kg/m , e.g. up to 120 or 100 kg/m 3 and can exceed 50, e.g. exceed 70 or 90 kg/m3.

The filter material should also exhibit a certain stiff ness which may be expressed as compressibility in per cent under a stated load per surface area, calculated normal to the main direction plane of the fibres accord ing to definition above: a total compressibility under

2 a load of 0.05 kp/cm (= 0.5 m water head) on the upper surface of a cylinder with a height of 100 mm and a diameter of about 100 mm of at most 50 per cent, pre¬ ferably at most 40 per cent especially at most 30 per cent and normally at most 10-20 per cent in dry or wet condition.

According to the invention filtering through the fibre material is performed in such a way that the filtered medium is withdrawn from the filter material in a

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direction which is essentially in the main, direction of the plane of the fibres according to the definition above or so. that the liquid flows essentially in said direction through the filter. The fibre material is suitably prepared as a web with the main direction plane of the fibres coinciding with the main direction of the web. Such a web is cut into suitable pieces, usually in a right angle to the direction of the web and the main direction plane of the fibre. A piece of such a filter material or filter web is suitably arranged in a filter holder provided with an outlet means which permits the withdrawal qf the filtered media through a surface of the filter arranged transverse to and preferably per¬ pendicular to the direction plane of the fibres.

Figure 1 shows a piece of filter material 1 with a bottom surface S_. with the sides A and B, and a vertical side surface S ₂ with sides B and C and a second vertical side surface S, with the sides A and C. The side S ₃ is supposed to be parallel with the main direction plane P of the fibres and consists e.g. of the side surface of a web material prepared e.g. by a spinning or spraying of a melt to fibres against a support belt (lay-down belt) . The direction perpendicular to said plane N is shown on the figure.

According to the invention the filtered medium is with¬ drawn through one or more of the side surfaces S., , S~ or optionally S _3> On figure 1 ₍ ' hypothetical outlets U with sides a and b are shown with broken lines.

Usually the filter material is used in the form of pieces with rectangular or square side surfaces which are mutually perpendicular, but of course it is a ^* lso possible to use filter pieces of other shapes arranged so that the filtered medium flows out from the filter material in the main direction plane of the fibres.

It is preferable that the outflow opening U is restrict ed by lines at a certain distance d to the peripheral line of surface S. , through which one achieves that the filtered medium which is supposed to have free access to the entire surface S ₃ , is forced to flow through the filter material for a certain distance alon the path to the outlet ϋ. The distance d is suitably 1-5 cm, preferably about 2-4 cm. Thus a shortest flow path of 1-5 cm, preferably about 2-4 cm through the filter material is usually desired in order to give a sufficient filtering effect. Said distance comprises preferably a direction perpendicular to the plane P. If the flow is in the direction of plane P a larger minimum flow path extension is usually desired e.g. up to 10 cm and preferably at least 1-3 cm.

Figure 2 shows examples of suitable filter holders. Figure 2A shows the filter holder from the side and figure 2B from above. The filter holder comprises a ^: bottom plate 21 of extended shape with a lengthwise extending opening 22 formed by flange 23 extending up¬ ward from the bottom plate 21. Below the bottom plate 21 with the opening 22 is a box 24 with an outlet 25 in the shape of a short tube. A framework 26 protrudes from the bottom plate 21. When using the filter holder a filter slab F is arranged in the frame 26 so that an outlet surface of the filter slab F is in contact with the outlet flange 23. The filter slab is preferably urged into the frame so that the flange 23 penetrates into a surface which is perpendicular to the main direction plane of the fibrese.g. the side surface S_. on figure 1. Hereby the seal between the filter mat¬ erial and the filter holder is improved and the filtere medium is forced to pass through a sufficient thickness of the filter material before it flows out through the opening 22.

The -extension of the filter material perpendicular to the main direction plane of the fibres, i.e. the direction N may vary within broad limits but is usually within the range 2-30 cm and preferably 5-15 cm, e.g. about 10 cm. The extension of the filter material in the direction of the fibre direction plan may also vary within broad limits, partly depending upon the thickness in the direction N. The extension h in a direction perpendicular to the flow-out opening or flow-out sur- face of the filter may e.g. be 1-10 times the thickness t and preferably 2-4 times said thickness, e.g. 3 times that thickness. The extension in the direction h should be selected so that the filtering effect of the entire filter can be used, i.e. not larger than an extension which permits deposition of an essential quantity of filtered material in a part of the filter material remote from the outflow opening before parts of the filter material closer to the flowout opening are clogged to such an extent that the flow of the filtered medium through the filter material from the remote parts to the flowout opening reaches an unacceptably low level. Common values of h are up to 1 m, preferably up to 0.8 , in some cases up to 0.6 or 0.4 m. h should usually be at least 0.05-0.1 m, preferably at least 0.2 m, in many cases 0.4-0.6 m, but at least large enough to give a sufficient filtering effect.

In order to improve the flow through the filter from the outer part of the filter, which is the most active for the filtration, to the outflow opening through the inner parts of the filter material, the inner parts of said material may consist of a more pervious material than the outer parts. Furthermore channels which im¬ prove the flow of the filtrate to the outflow area of the filter material may be formed in the inner parts of the filter material. Said channels may e.g. be achieved with elongated means which are pressed into

the filter material in the direction of the main direct ion plane of the fibres essentially in the direction towards the outflow opening or outflow area of the fibr material. Said means can optionally remain in the filte material in which case they should preferably have such a shape that paths or channels with reduced flow resis¬ tance are formed in the filter material, preferably paths*•extending from the outflow area of the filter and into the filter material. As examples of such channel shaped means may be mentioned: perforated tubes preferably provided with a point , rods of star shaped e.g. cross shape, cross section, etc. Said means may be arranged fastened in the filter holder or inserted into the filter material before said material is arrang ed in the filter holder.

One way to improve the permeability of the filter in the desired flow direction, preferably the main direct¬ ion plane of the fibres, is to form or shape elongated channels in said direction in the inner parts of the filter layer especially in the centre of said layer. Said channels may e.g. be bored or drilled into the filter layer from a side surface which is perpendicular to the main direction plane of the fibres, e.g. the side surface S-. on figure 1 , e.g. in the form of a number of essentially parallel holes such as is indi¬ cated on figure 3 which shows a side view of a paral¬ lel epipedical piece of filter material. The holes or channels may e.g. have a diameter of up to 50 mm, pre- ferably up to 30 mm or up to 20 mm. The lower limit depends on the desired flow rate and may e.g. be 5 mm or 10 mm. The distance between the holes may vary but may e.g. amount to 0.1-10 times the thickness of the filter layer, preferably 0.2-2 times said thickness, e.g. 0.5-1 times said thickness (B on figure 1). The filter material should preferably have such a stiffness or the diameter of the holes should be selected suffic¬ iently large so that the filter material remains self-

supporting and retains it shape under the influence of the flow-through forces, i.e. so that said holes re- ^" .main open and with a sufficient cross section under the influence of the flow speed which is forseen for the filtration and are not collapsed by the pressure differ¬ ence. Said holes may also be supported with means which are inserted into the holes as mentioned above.

The filter holder shown on figure 2 may be modified in many ways , e.g. with some other shape of the support of the outflow area of the filter in which case the opening 22 on figure 2 can be substituted with two or more openings with support for the filter material in between, e.g. a mesh or an expanded metal sheet, or a surface or sheet provided with round holes or holes of other shape. Figure 4 a and b show a bottom surface 30 provided with a number of round holes 31 with up¬ wardly extending flanges 32, which are intended to penetrate into the side surface S- according to figure 1 of a filter slab and act as number of outlets from the filter arranged in a row. Said outlet openings 31 may also cooperate with and preferably be arranged coinciding with channels or holes in the filter slab which are perpendicular against the bottom surface 30 and improve the penetration.

Furthermore the filter material may be provided with an extended surface, preferably of the type mentioned in the Swedish patent application 7804690-1. When using a filter prepared from a web material, the area extend¬ ing recesses, such as slots, holes, etc, are preferably formed extending from both side surfaces which are parallel with the main direction plane of the fibres to a suitable depth e.g. to 1/3 or 1/4 of the thickness from each side surface. The area increase can be com¬ bined with a penetration improving way of forming the

core of filter material e.g. with a more pervious material in said core or by forming flow-through chan¬ nels in the core of the filter material as mentioned above. A filter material of sufficient thickness should of course be left to remain between the area increasing recesses and the flow-through channels or similar means

The area increase may also be achieved by inserting means from the side surfaces of the filter which open up recesses in the side surfaces of the filter material Said means may consist of parts of a holder. The in¬ serted means consist of a number of spikes or teeth or similar means, preferably pointed spikes or teeth of sheet metal material which may be punched and bent up from the surface of a sheet in right angle to it.

Preferably said surface increasing or surface opening means are inserted to a depth of up to half the thick¬ ness or more, e.g. up to 10 cm, up to 5 cm or up to 3 or 2 cm. The minimum depth may vary ,e.g. at least 0,5 cm, at least 1 cm. at least 3 cm, at least 5 cm or may also amount to a certain part of the thickness of the filter layer e.g. at least 0.1. at least 0.2, at least 0.4 times that thickness. The recesses or surface in¬ creasing shapes are also arranged so that a sufficient minimum filter thickness remains to optional internal flow improving channels etc in the inner part of the filter slab e.g. at least 1 cm and preferably at least 2 or 3 cm remain to said means so that a sufficient filtering effect is ensured.

In order to improve the flow through the core of the filter material one may also in the middle of the filte material in or after the production step by joining to pieces of filter material arrange a central flow improving means, e.g. perforated corrugated cardboard or a similar means. Said means is thus arranged in the middle of the filter layer and essentially parallel wit

the main direction plane of the fibres. Said layer may extend to all side surfaces of the filter material which are perpendicular to the main direction plane or may be embedded so that said layer extends to only three, two or one or optionally not to any of said side surfaces.

A filter according to the invention can be operated with high faltering flow speed of the liquid. A flow speed, calculated on the outer surface of the filter, of above 10, usually above 20 and in many cases above 50 or even above 100 metres/hour or higher can be used, especially with the rockwool material mentioned above, especially a material of basalt-lime type. A corresponding pressure drop through the filter may amount to up to 500 cms hydrostatic head, preferably up to 200 or up to 100 cms hydrostatic head. In many cases a hydrostatic head of up to 50 cms or up to 10 or ^' 20 cms is sufficient. Frequently a hydrostatic head of 1 - 5 cms is sufficient.

It is also possible to arrange outside the filter mate¬ rial according to the invention a somewhat coarser filter material which removes coarser impurities, e.g. impurities with a size above 10 microns, 100 microns or 500 microns or even coarser impurities, e.g. with a size above 1 or 10 mms.

In practical use, especially of the materials mentioned above, the surface area load calculated on the outlet area ϋ in most cases is from about 2500 to normally 1250 - 1000 metres/hour.

The height of the filter sheet from the outflow area is not restricted to the values stated above but is determined on the first hand by the manageability and may be e.g. up to 10 times the thickness or above, e.g. up to 15 or 20 times the thickness. The minimum height can also vary and is determined by practical decisions,

provided that the filtering effect is sufficient, e.g. 0.5, 1, 2 , 3 or 5 times the filter thickness.

A suitable embodiment of the apparatus and method according to the invention is disclosed in detail with reference to Figure 5. Figure 5 discloses a vertically arranged cylindrical filter holder 1 with a cover 2, a liqui inlet 3a and a liquid outlet 3b. The apparatus shown with part of the container wall 1 removed in orde to improve the clarity of the figure. Filter material rods 4 are arranged in the container. The figure shows two such filter rods of elongated shape, e.g. 10 x 10 c in cross section and a length extension which is severa times the size of the side, e.g. 50 - 100 cms, e.g. 90 One of the smaller side ends of the filter rod contacts an outlet tube 5 arranged in a wall 6 in the bottom par of the container.

In order to achieve a more even distribution of the inflowing liquid over the surface of the filter rods 4 baffel sheet 7 is arranged immediately above the bottom wall 6 and extending close to the wall of the container but with a pass-by gap remaining between the wall of th container 1 and the baffel sheet 7. Hereby liquid whic is introduced between the bottom wall 6 and the baffel sheet 7 is distributed around the periphery of the shee 7 and flows through the gap between said sheet and the mantle surface of the container 1 and upwards along sai mantle surface. Optionally the baffel sheet 7 may also be provided with a centrally arranged opening 8 which permits the flow of liquid between the filter rods 4 centrally in the container 1. The figure shows how liquid from the inlet 3 flows downwardly to the bottom wall 6 through a tube 9.

Many other arrangements are, of course, usable but it i suitable to achieve a flow of the introduced liquid alo

the mantle surface of the container 1 so that the liquid hereby is distributed as even as possible along the filter rods 4.

Figure 4 is a view from above of a cross section through the container 1 and shows the sheet 6 and outlet tubes 5 on which the filter rods shall be arranged. Support means .11 for the filter rods 4 are indicated. Said means consist of vertically arranged pieces of 90° V-section and are intended to guide three vertical edges of a filter rod with square cross section.

Figure 7 discloses another embodiment of the bottom according to which the baffel sheet 7 is provided with short tube pieces 12 extending through the bottom sheet 7 and surrounding the outlet tubes 5 which extend through the bottom sheet 6. . Between said short tube sockets 12 and the outlet tubes 5 are arranged suitable sealing means, e.g. sealing rings 12 or sealing sleeves, e.g. rubber tubes 13 which may also be arranged outside the short tube sockets 12 as disclosed to the left on Figure 7. The filter rods 4 can then be arranged to seal against the upper end of the short tube sockets 12. The filter rods can be lifted out from the container 1 either separately or as a unit together with the baffel sheet 7.

Suitable dimensions of the device are e.g. a diameter of 25 - 40 cms of the container 1, e.g. 30 cms, a diameter of 40 - 60 mms of the short tube sockets 12 , e.g. an outer diameter of 54 mms and a height of the pipe sockets 12 above and below the sheet 7 of about 15 - 30 mms on each side.

In order to improve the sealing of the outlet from the filter rod to the outlet tube 5 or tube socket 12 it is also suitable to arrange on said end of the filter rod a liquid impermeable envelope extending a few cms up to

about 15 - 20 cms and preferably about 10 cms from the end of the filter rod along the longitudinal side surfa ^■ of the filter rod and enclosing the filter rod. Preferably said envelope also encloses the outlet end surface of the filter rod and comprises an opening in t envelope which permits the outflow of liquid from the filter rod. Said envelope may e.g. consist of a thin plastic film or plastic bag or plastic shrink film, arranged on the end of the filter rod and retained on i e.g. with an elastic string or in another way. Said impermeable envelope prevents by-pass flow of liquid between the filter rod and the end of the outlet tube, especially if said envelope is arranged sealing also against the outlet tube 5 or the tube socket 12, e.g. with an elastic string or similar means. Furthermore, in this way clogging of the outlet end of the filter ro is prevented so that an unhindered flow-through the out let end is secured.

Figure 8 shows schematically "-a part of a filter rod 4 with an envelope 13 retained with elastic rubber strings 14 at the filter rod and the outlet tube 5 or tube sock 12 resp.

According to the invention it has also been revealed that the shape of the outflow and the flow path of the liquid from the filter are critical for the functioning of the filter, especially for preventing a compression the filter close to the outflow area. Figure 9 shows a section through the outlet of a filter device with a piece of filter material 91 resting on an outlet 92 consisting of a tube socket arranged on a bottom plate 9 The filter element 91 is maintained in a desired positio on the bottom plate with a retainer 94 extending upwards from the bottom plate 93. According to the invention i is essential to influence on the distribution of the fl speed within a cross section of the outlet socket 92,

preferably so that the flow speed is as even as possible within a cross section through the outlet stud 92 or so that the flow speed increases from the center and towards the walls at least within a part of the radial extension in the cross section through the outlet stud 92 from a central line 95 towards the wall 92. The figure shows with arrows the radial extension r to a point p in the cross -section, and the radial extension r to the wall of socket 92 from the central axis 95. Figure 10 shows a graph of the flow speed profile within the cross section, i.e. the distribution of the flow speed v as a function of the radial extension r from the central axis 95. Figure 10 shows a first curve a with a comparatively even flow profile which exhibits a moderate maximum at the center and decreases only slowly with increasing value of r. The figure furthermore shows a curve d with a maximum within the area between the. center and the wall of the stud. Flow speed profiles of said two types can be used.

According to the invention an improved flow condition at the outlet 92 can be achieved by arranging an axial flow restricting means or baffel means close to the outlet 92 which exerts a deflecting and baffling effect on the liquid flow through the outlet 92 and deflects at least parts of the liquid flow in the transverse direction from the axis 95 counted. This can be achieved e.g. by arranging a disc-shaped means in the flow direction close to the outlet 92, e.g. a disc 96 on Figure 9. Figure 9 b shows a section along the line B - B on Figure 9 a. The figure shows the arrangement of the openings 97 in the damper disc 96. After passing through the openings 97 the liquid flows to an outlet 98 consisting of a pipe socket. Said pipe socket comprises a choke disc 99 which is intended to restrict the flow through the filter 91. The damper disc 96 is arranged at a suitable distance h-. from the outlet which gives the desired flow profile

through the outlet 92. A suitable distance h, can eas be determined with the aid of simple tests in each particular case. It is preferable that the radial extension of the damper disc 96 from the axis 95 is larger than the extension of the outlet 92.

With a bredth size S 1 of the filter material 91 of ab 50 - 2.00 and preferably 100 mms a diameter of the outl 92 of about 20 - 70, preferably 30 - 60 and especially about 40 - 60 and particularly about 50 mms and a distance h, of 10 - 50, especially about 15 - 40 and preferably about 25 mms is suitable. With the shape of the damper disc disclosed on Figure 9 a side size S ₂ of the flow-through openings in the damper disc of about 5 - 25 and preferably about 10 - 20, particularly about 15 mms can be suitable. The choking opening in the choke disc 99 can under said conditions have a diameter of about 5 - 25 mms and e.g. together with a filter rod with a side size S of 100 x 100 mms and a vertical extension of 900 mms be adapted to give a flow-through of about 1 - 5 m /h and preferably about 2.5 m /h.

The average flow speed in the outlet stud 92 may amount to e.g. 0.1 - 5 metres/second, preferably about 0.15 - 2 metres/second and especially 0.2 - 0.5 metres/second and particularly about 0.35 metres/second.

Figure 11 discloses a further embodiment of outlet tube of a filter container of the type shown on Figure 4. A outlet manifold 101 comprises a number of internally threaded tube sockets 102 in which are inserted externa threaded outlet tube pieces 103 of which one is shown o the figure. On the preferably bevelled upper edge of t outlet tube piece- 103 a filter element is arranged as disclosed above. By turning the tube piece 103 the distance s between the lower edge 105 of the tube piece and a bottom wall 106 of the manifold means 101 can be

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adjusted so that the damping effect mentioned in connection with Figures 9 and 10 can be achieved.

Figure 12 shows a section through a filter container seen from above with the manifold means 101 according to Figure 11 arranged in the filter container 121. The manifold means is provided with 4 sleeves 102 and outlet tubes 103 arranged therein for engaging 4 filter elements as disclosed above.

When using rod-shaped filter elements of the size 100 x 100 x 900 mms of the type mentioned above an inner diameter d of the outlet tube of 20 - 50 and preferably 30 - 40 mms _. is suitable. In this case as in connection with other embodiments of devices according to the invention it is suitable to arrange a supporting means, such as mesh, gauze, perforated disc over the outlet opening if said opening is in contact with the filter element in order to support said element and prevent that the filter material is pressed out through the outlet opening. A suitable damping effect can be achieved with d = 38 mms and a distance s = 5 - 30 mms, preferably

3 about 20 mms when the flow is above about 1 - 5 m pe Er hour and outlet opening. A flow of up to about 7.5 m~ per hour is preferred under the conditions stated above, but also higher flow values, e.g. up to 10 or 20 per hour can- be used, especially if the vertical extension of the filter is restricted to up to 60 cms.

The distance s is usually selected to between 5 and 30 mms, e.g. 10 - 20 mms.

The outlet speed in the outlet opening is selected with regard to the characteristics of the filter material and suitable values are stated above. Suitable values are e.g. 0.1 - 5, e.g. 0.3 - 2 metres/second. Under the conditions stated above for Figures 11 and 12 0.5 - 1.5

A- -κ c

and especially 0.8 - 1.3 metres/second can be suitable.

The filter material, the method of filtering and the filtering device according to the invention are accordin to an embodiment characterized by maintaining a high value of the ratio of the size of the filter element inlet area to the outlet area. The value F of said rati is usually above 10, preferably above 25, especially above 50 and frequently above 100 or above 250. For a filter rod of a square cross section and the size

100 x 100 x 900 mms said ratio F may amount to about 30 or above, for a filter disc of the size 900 x 100 x 450 to about 50 and for a circular filter rod of 65 mms diameter, vertical extension 100 mms to about 340. The area range* referred to also corresponds to a large average distance between the outlet surface and inlet surface of the filter element and a correspondingly lon flow path through the filter element, e.g. an average flow path extension or distance of at least 10 cms, preferably at least 20 cms or at least 40 cms. Frequent the flow field within at least 20%, preferably at least 40%, e.g. at least 50% or at least 75% exhibits a main composant direction which coincides with the direction o a fibre orientation plane according to the definition above.

A filter element can according to the invention be used so that the entire element acts as a filter medium. Outlets from the filter element can be arranged by apply an outlet opening of the filtering device against an outer surface of the filter element, which preferably exhibits the area ratio stated above to the inlet openin area which preferably comprises the rest of or at least 90 or 75% of the surface of the filter element. The outlet surface of the element can also be submerged into a depression in the material, e.g. in the shape of an internal channel, as disclosed above.

Suitable values of the distribution of fibre diameter and fibre length of the filter material according to the invention are disclosed on Figures 13 and 14. For a material according to the invention the values of the 5 or 6 highest of the interval columns disclosed on said figures are preferably between 0.2 and 5 and especially between 0.5 and 2 times the values of each column disclosed on the figures.

Figure 15 shows examples of suitable compressibility values of filter materials of various volume weights. The compressibility is preferably at most 10 and especially at most 5 or at most 2 times said values.

In order to improve the permeability and other characteristics the filter material may be entirely or partly coated with or treated so that a hydrophilic surface layer is obtained on the fibres, e.g. by coating with a per se known material of said type. :

Fibre main direction plane of the fibre material is also intended to comprise the lay-down plane of a fibre material which is laid-down on a support surface of flat shape or other shape to form a fibre material layer, also if the fibres in the material do not exhibit the major directionality mentioned above in a particular plane but are rather randomly distributed in space, in which case the filter material layer frequently exhibits a sub-layer structure with a plurality of stacked parallel sub-layers in which the fibres may be oriented in a preferred direction, as mentioned above, or may be more or less randomly oriented in space.