fig1 shows a schematic drawing of a preferred embodiment of a device according to the present invention . the device comprises a water supply 1 connected to a water pump 2 . the output of the pump is connected to a heater 3 which preferably comprises a coiled stainless - steel pipe in a die cast aluminium thermoblock ( not shown ). power is supplied to the thermoblock sufficient to heat the water to the desired temperature as it passes though the pipe . the output of the heater is connected to a filter 5 which is connected to the discharge or injection head 4 of the device . said discharge head 4 comprises a chamber 9 for housing a cartridge 7 . the term “ cartridge ” refers , in the general concept of the invention , to a single - use package such as pod , capsule or sachet of various packaging materials containing nutritional ingredients . the term “ pump ” means any technical means that can transport a liquid at a sufficient positive pressure from the water supply to the injection head . this means can be a pressure pump such as a piston pump , a diaphragm pump , a peristaltic pump , a gear pump , a centrifugal pump , a progressive cavity pump , a hand pump and the like . furthermore , flow control means 2 a are provided preferably between the pump 2 and the heater 3 of the device . moreover , temperature control means 3 a are provided preferably between the heater 3 and the discharge head 4 . hence , the temperature and the flow of a liquid within the device can be controlled and adjusted . in addition , an air pump 6 is connected to discharge head 4 in order to enable the flushing of any residua within a cartridge 7 into a receptacle disposed beneath the cartridge 7 . the cartridge 7 which is shown only schematically contains approximately 10 ml of ingredients . the ingredients are preferably provided in a powdered or in a liquid concentrated form within the cartridge 7 . the preparation of a 100 - ml single serving of a nutritional beverage such as infant formula with the device according to the present invention will now be described . the cartridge 7 is located in the cartridge receiving chamber 9 of the discharge head 4 such that an injection element ( not shown ) protruding into the chamber 9 punctures a membrane sealing the cartridge 7 . flow control means 2 a and temperature control means 3 a are set such that a desired quantity of water at the desired temperature is provided to the cartridge . thereby , water is preferably pumped at a pressure of about 0 . 2 bar from a water tank 1 ( or a water supply line from outside ) to the heater 3 where it is heated to a preferred temperature between 25 and 40 ° c . the heated water is then passed through the filter 5 to the chamber 9 of the discharge head 4 at a preferred pressure between 2 and 7 bar and dispensed from there into a cartridge 7 which contains a unit dose of a nutritional composition such as an infant formula . when the pressure in cartridge 7 reaches a predetermined value which is preferably 2 . 5 bars , the cartridge opens and the reconstituted infant formula is dispensed directly from the cartridge into a suitable receptacle such as a baby bottle ( not shown ). the air pump 6 is then operated to flush any remaining liquid out of the cartridge and into the receptacle . in addition , a steam generator 8 is connected to the discharge head 4 and to the filter 5 . in operation , the chamber 9 of the discharge head 4 and the filter 5 may be sterilised as often as desired by passing steam from the steam generator through the discharge head 4 and the filter 5 . fig2 shows a preferred embodiment of a filter card 20 according to the present invention . the filter card 20 comprises a frame 21 designed for housing a filter membrane ( shown in fig3 ) and at least one rigid perforated backing member 24 . the frame 21 are preferably of rigid plastic or metal material equipped with a semicircular ending 21 a which is the ending to be first introduced into an aperture such as a slot provided at a device and designed to receive the filter card 20 . due to the semicircular ending 21 a , the filter card 20 can easily be inserted into a provided slot of the device , even if a user inserts the filter card 20 in a slanted way or offset the central axis of the aperture . furthermore , the filter card 20 comprises guiding means 22 in order to prevent a user from inserting the filter card 20 into a provided slot in an incorrect side . preferably , the guiding means 22 is at least one protruding ridge provided at one of the edges of the filter card 20 and designed to be received by a corresponding groove 22 a ( shown in fig3 ) provided at the aperture of the device , the filter card 20 is designed to be inserted to . preferably , the guiding means 22 are an integral part of the frame 21 . as shown in fig3 , the frame 21 comprises a preferably circular aperture 23 for housing the filter membrane 30 and at least one perforated backing member 24 . more preferably , at least one filter membrane 30 is placed or sandwiched between at least two backing members 24 which are situated in parallel and in close contact to the filter membrane 30 . furthermore , the backing members 24 are perforated in order to enable a flow of fluid through the filter membrane 30 and thus through the filter card 20 . the diameter of the perforations of the backing members 24 is preferably wider than the nominal pore size of the provided filter membrane 30 . thus , the flow resistance of the backing members 24 does not influence the overall flow resistance of the filter card 20 in a negative way . moreover , in the case when two or more perforated backing members 24 are provided to the filter card 20 , the arrangement of the backing members 24 is adjusted such that the perforations of the backing members are aligned . hence , water which is passed through the filter card 20 is not deflected . the backing members 24 are preferably designed to be supported by support members 25 a ( shown in fig3 ) of the device to which the filter card 20 is intended to be connected . said support members 25 a may for instance be ridges or the like designed to abut on the backing members when the filter card 20 is connected to the device . therefore , the surface of the backing members 24 comprises support areas 25 onto which the support members 25 a may abut . the support areas 25 are preferably uniformly distributed over the surface of the backing member 24 . the support areas 25 do not comprise any perforations such that the support members 25 a abutting on the support areas 25 do not deflect any water passed through the perforations of the backing members 24 . accordingly , the support areas 25 of the backing members 24 abutting on given support members 25 a enable a stable position of the filter card 20 when being connected to a device for preparing a nutritional composition . the risk of deflection or breaking of the filter card 20 or particularly of the filter membrane 30 when water at high pressure is passed through the perforations of the backing members 24 can thus effectively be minimized . furthermore , the protection of the filter membrane 30 against deflection or breaking when water is passed through the filter card 20 may be enhanced by providing one perforated backing member 24 of sufficient rigidity to each side of the filter membrane 30 . thereby , the outer ones of said backing members 24 on each side of the filter membrane 30 have a wider diameter than the inner ones of the backing members 24 which are disposed in close contact to the filter membrane 30 . hence , liquid which is passed through the backing members 24 and through the filter membrane 30 is not deflected by the outer ones of the backing members 24 . furthermore , to enable a flow of water through the filter membrane 30 and the backing members 24 , the perforations of all backing members 24 disposed on either side of the filter membrane 30 are arranged such that they are aligned . moreover , the filter card 20 comprises sealing means 26 for sealing the filter card 20 when being connected to a water flowpath 50 ( shown in fig3 ). the sealing means 26 are disposed at the aperture 23 of the frame 21 . preferably , the sealing means 26 is an o - ring or the like suitable for sealing the aperture 23 and thus the filter card 20 when water is passed through it . the sealing means 26 are preferably provided at either side of the filter card 20 . the sealing means 26 may as well be provided at the water flowpath 50 which the filter card 20 is designed to be connected to . as shown in the drawing , the sealing means is preferably a lip - seal . the lip - seal is especially advantageous since pressure exerted onto the lip - seal enhances the sealing effect by pressing the lip against the counter part . thus , the higher the pressure in the filter chamber , the higher the pressure which presses the lip against the counterpart . hence , the preferred embodiment according to the figure is able to withstand pressure as high as 16 bars , for instance , without leaking . moreover , the sealing means 26 , the backing members 24 and the filter membrane 30 are designed to be easily removed from the aperture 23 of the filter card 20 . therefore , fastening means 27 ( see fig2 ) are provided which can be connected to the aperture 23 of the filter card 20 . thereby , said fastening means 27 preferably enclose the filter membrane 30 , the backing members 24 and the sealing means 26 . the fastening means 27 is preferably a clamping piece or the like , designed to be connected to the aperture 23 . of course , the fastening means may as well be provided as an integral part of the aperture 23 . hence , when the filter card 20 is not connected to the designated water flowpath 50 , a user may easily change the filter membrane 30 , the backing members 24 or the sealing means 26 . accordingly , a repeatedly exchange of the filter membrane 30 may be performed in a convenient way in order to enhance the protection provided by the filter card 20 against pathogens . fig3 shows an exploded assembly drawing of a preferred embodiment of the filter card 20 being connected to the water flowpath 50 . thereby , the two enclosing members 40 a , 40 b are connected between the water flowpath 50 in order to enable a connection between the water flowpath 50 and the filter card 20 . the two enclosing members 40 a , 40 b enclose an aperture 42 formed by their surfaces 41 a , 41 b into which the filter card 20 can be inserted . thereby , the semicircular ending 21 a of the filter card 20 abuts on a semicircular recess 21 b at the bottom of the aperture 42 enclosed by the two enclosing members 40 a , 40 b . furthermore , a linear groove 22 a is preferably provided at one side of the aperture 42 and , preferably , one of the two enclosing members 40 a , 40 b . the linear groove 22 a is designed to receive the ridge 22 formed at one side of the filter card 20 . thus , the filter card 20 can only be inserted into the aperture formed by the two enclosing members 40 a , 40 b in the correct side . each of the two enclosing members 40 a , 40 b comprises a circular recess 51 at its inside which is concentrically arranged and which is connected to the water flowpath 50 . hence , the diameter of the water flowpath 50 can be adapted to the diameter of the filter membrane 30 provided at the aperture 23 of the filter card 20 . accordingly , the whole surface of the filter membrane 30 provided at the filter card 20 can be used for filtering water . when water is passed through the water flowpath 50 and thus trough the recesses 51 , the recesses 51 are sealed by the sealing means provided at the frame 21 of the filter card 20 . of course , the sealing means 26 may as well be provided on the enclosing member around the recesses 51 . as can be seen from fig3 , support members 25 a are provided within the recesses 51 of the enclosing members 40 a , 40 b . these support members 25 a are preferably cylindrical protruding ridges which are abutting on the support areas 25 provided at the perforated backing members 24 , when the filter card 20 is inserted into the aperture 42 enclosed by the two enclosing members 40 a , 40 b . thus , deflection and / or breaking of the perforated backing members 24 and thus of the filter membrane 30 can effectively be prevented . in order to fully enclose the filter card 20 and fix its position within the aperture 42 , the enclosing members 40 a , 40 b can be moved relatively to each other . hence , after the filter card 20 is inserted into the aperture 42 , the two enclosing members 40 a , 40 b can be approached and the position of the filter card 20 is fixed at a stable position and in a water - tight manner within the aperture 42 . in the same manner , releasing the filter card 20 can be achieved by distancing the two enclosing members 40 a , 40 b from each other . in order to enable a relative movement of the enclosing members 40 a , 40 b , at least one of the enclosing members is connected to a closing mechanism ( not shown ) such as a lever or a motor which can be operated by a user . in case the filter card 20 is disconnected from the device by distancing the enclosing members 40 a , 40 b from each other , the remaining water within the water path 50 and the circular recess 51 upstream of the filter is released in a controlled manner through an opening 51 a provided at the bottom of the aperture 42 . a collector below the opening 51 a ( not shown ) is connected to a drip tray ( not shown ) so that the fluid can be discharged from the device . furthermore , the aperture 23 of the filter card 20 comprises a circular support 28 for mounting the filter membrane 30 and the perforated backing members 24 thereon . thereby , the support 28 comprises locating means 29 a , 29 b for determining the correct position of the filter membrane 30 and the backing members 24 when being mounted at the support 28 of the aperture 23 . the locating means 29 a provided at the support 28 interlock with locating means 30 a provided at the filter membrane 30 and thus prevent relative rotation of the backing member and filter membrane and enable a user to correctly mount the filter membrane 30 at the support 28 of the filter card 20 . preferably , the locating means 29 a is at least one protruding ridge which fits in at least one correct recess 30 a provided at the circumference of the filter membrane 30 . the backing members 24 are preferably provided with recesses 24 a which are equally distributed over the circumference of the backing members 24 . hence , these recesses 24 a interlock with corresponding protruding ridges 29 b which are disposed at the circumference of the support 28 of the filter card 20 . thereby , the locating means 29 b , 24 a for the backing members 24 are designed such that the perforations of the backing members 24 on either side of the filter membrane 30 are aligned . accordingly , a user can effectively be guided to mount the filter membrane 30 and the perforated backing members 24 at the filter card 20 in a correct way . in the shown embodiment , two filter membranes 30 are mounted at the support 28 of the filter card 20 which are sandwiched between two perforated holders 24 . it may as well be possible to mount three or more filter membranes 30 to influence the filtration properties of the filter card 20 . a proximity sensor 54 may be provided at the bottom of the aperture 42 for inserting the filter card 20 . the sensor enables to detect a presence of the filter card 20 within the aperture 42 . the proximity sensor 54 is preferably connected to at least the pump 2 ( see fig1 ) of the device . thereby , the sensor 54 preferably transmits a signal in response to the presence of the filter card 20 within the aperture 42 . it can thus be ensured that the operation of the pump 2 is only possible when the filter card 20 is fully inserted into the aperture 42 . therefore , it can be prevented that a user unintendedly operates the device without having the filter card 20 fully inserted into the aperture 42 . fig4 shows a preferred embodiment of the enclosing members 40 a , 40 b enclosing a filter card 20 when being inserted into the aperture 42 and thus being connected to the water flowpath 50 . as can be seen from the figure , the filter membrane 30 is sandwiched between two perforated backing members 24 . the perforations thereof are aligned such that water passed through the filter card 20 is not deflected . the water flowpath 50 is connected to a circular recess 51 formed in each of the enclosing members 40 a , 40 b . within the circular recess , protruding support members 25 a are disposed , which are designed to abut on the support areas 25 of the perforated backing members 24 when the filter card 20 is inserted into the aperture 42 . thereby , the support areas 25 of the backing members 24 do not comprise any perforations . as can be seen in fig4 , a circular recess 51 b for housing sealing means is provided at both enclosing members 40 a , 40 b . the circular recess 51 b is preferably concentric to the circular recess 51 housing the protruding support members 25 a . the circular recess 51 b for housing the sealing means 26 is disposed radially outside of the circular recess 51 . thus , the sealing means 26 housed by the circular recess 51 b enable an efficient sealing of the aperture 42 when the filter card 20 is provided within the aperture 42 and when water is passed through the filter card 20 . the sealing means 26 is preferably an o - ring or the like . moreover , the sealing means 26 are preferably exchangeable . hence , when the sealing means 26 are subjected to deterioration , a user may easily exchange the sealing means 26 . as previously described , the sealing means 26 may as well be provided at the frame 21 of the filter card 20 . for ensuring a safe and stable defined position of the filter card 20 when the filter card 20 is fully inserted into the aperture 42 , the two enclosing members 40 a , 40 b can be brought in tight contact to the filter card 20 by enable a relative movement of the two enclosing members , as already described . alternatively , locking means 51 a , 51 b may be provided at the frame 21 of the filter card 20 . as can be seen from fig4 , the locking means are e . g . a snap fit . thereby , a protruding ridge 52 a , preferably provided at the frame 21 of the filter card 20 , interacts with at least one corresponding recess provided at the snap fit member 52 b . by pressing the member 52 b in direction of the shown arrow 53 c , the user may loosen the connection between the filter card 20 and the snap fit member 52 b and thus , the filter card 20 may be withdrawn from the aperture 42 . the snap fit member 52 b is preferably an integral part of one of the enclosing members 40 a , 40 b . in another preferred embodiment of the filter card 20 , the defined safe and stable position of the filter card 20 within the aperture 42 is ensured by frictional forces ( press fitting ) between the frame 21 of the filter card 20 and the adjacent faces 42 a , 42 b . hence , the fit of the frame 21 within the aperture 42 is chosen such that a user has to apply a certain force onto the filter card 20 in order to fully insert the filter card 20 into the aperture 42 . the same applies for removing the filter card 20 from the aperture 42 . fig5 a and 5 b show a preferred embodiment of a backing member 24 . thereby , the backing member 24 has individual arcuated recesses 53 a and ridges 53 b which are formed to enable a perfect fit of two equal backing members 24 in a predefined angular position . accordingly , the recesses 53 a and ridges 53 b of the first backing member 24 interlock with provided ridges 53 b and recesses 53 a of the second backing member 24 . hence , a relative rotation of the backing members 24 can be prevented . moreover , as the relative angular position of the two backing members 24 is defined by the recesses 53 a and ridges 53 b , the perforation of the two backing members 24 can be effectively aligned . as shown in the figure , support areas 25 are provided at the backing member 24 in order to provide a surface onto which the support members 25 a ( see fig3 ) of the device can abut . as can be seen from fig5 a , the recesses and ridges are preferably arranged symmetrically about the perpendicular axes a 1 and a 2 . moreover , a semicircular recess 53 c and a protruding semicircular part 53 d are formed at opposing endings of the backing member 24 . hence , when two equal backing members 24 are connected to each other , the semicircular recess 53 c and protrusion 53 d of the two backing members 24 interlock with each other and the correct predefined assembly position of the two backing members 24 is obtained . between the two backing members 24 , a filter membrane 30 ( not shown ) is placed which preferably has an outer diameter which is bigger than the diameter of the portion of the backing member 24 which is perforated . as shown in fig5 b , the protruding recesses 53 a and ridges 53 b may vary in their dimensions . for example , the height of the recesses 53 a and ridges 53 b may be adjusted to enable a correct fitting of the two backing members 24 when assembled . moreover , the recesses and ridges may overlap as indicated by reference b in fig5 a . thus , a tight and correct fitting of the two backing members can be obtained . in manufacturing process of the filter card 20 , the filter membrane 30 is placed on one of the backing members 24 . then , the second backing member 24 is placed onto the first backing member 24 , wherein their relative position to each other is defined by the arcuated recesses 53 a and ridges 53 b provided on their joint face . to secure the assembly , the frame 21 is injected . thereby , injection ports are preferably provided at the backing members 24 in order to enable plastic material to pass through during the injection process and hence further secure the membrane between the backing members 24 . moreover , sealing means 26 which may be provided at outer surfaces of the backing members 24 can be placed in provided recesses at the backing members 24 before the injection process . accordingly , the sealing means 26 can be secured to the backing members 24 by the injection process as well . fig6 shows a sectional drawing of a preferred embodiment of a filter card 20 . as can be seen from the figure , the filter membrane 30 is sandwiched between two perforated backing members 24 . the perforation of the backing members 24 is designed such that the diameter of the perforations widens towards the filter membrane 30 , i . e . the diameter of the perforations is bigger at the surface adjacent to the filter membrane 30 than at the outer surface of the backing member 24 being directed towards the water flowpath 50 ( see fig4 ). as can be seen from fig6 , the two backing members 24 are interlocking due to provided circular recesses 53 a and ridges 53 b on the backing members 24 . the position of the ridges 53 b is chosen such that the filter membrane 30 is as close as possible to the protruding inner edge 53 c of the protruding ridges 53 b and / or recesses 53 a . hence , the filter membrane 30 can efficiently be held in place and a tight enclosing of the filter membrane is obtained . moreover , a lip - seal 26 is provided to the outer surfaces of the backing members 24 in order to seal the filter assembly when the filter card 20 is in connection with the water flowpath 50 . as already described , the sealing means 26 and the backing members 24 can be effectively connected to the frame 21 of the filter card 20 by means of the injection process during manufacturing of the frame 21 . fig7 is a sectional side view of another preferred embodiment of a filter card 20 . in this embodiment , an o - ring 56 is placed in a provided circular recess 61 at one of the backing members 24 . accordingly , the outer joint face between the filter membrane 30 and the backing members 24 is sealed and water cannot by - pass the filter membrane 30 . preferably , the recess 61 and hence the o - ring 56 is provided at the backing member 24 which is situated on the upstream side of the filter card 20 . instead of the shown o - ring 56 , plastic material may be provided to the outer edge of the filter membrane 30 such that the plastic material is compressed by the backing members 24 when the filter card 20 is assembled . such plastic material can e . g . be applied to the filter membrane ( s ) by means of rubber overmoulding . accordingly , effective sealing of the filter membrane 30 can be provided such that water passed through the filter card 20 is prevented from by - passing the filter membrane 30 through the joint face of the two backing members 24 . fig8 shows an exploded assembly drawing of another preferred embodiment of the filter card . as shown in the figure , the backing means 24 are an integral part of the frame 21 of the filter card 20 . the backing means 24 are relatively large compared to the other embodiments . the backing means 24 may be grids formed in the frame 21 . thus , dedicated support members 25 a ( see fig4 ) provided at the device can abut on the grid 24 in order to support a stable position of the filter card 20 when it is inserted into the aperture 42 ( see fig4 ) of the device . the frame 21 comprises two parts which can be assembled in order to enclose a filter membrane 30 there between . to enable a correct assembly of the frame 21 , locating means 55 a are provided . preferably , the locating means 55 a are protruding and / or recessed bolts arranged circularly around the grids 24 . thus , the two parts of the frame 21 can be effectively assembled and a relative motion of the frame parts 21 after the assembly is prevented . as shown in fig8 , at least one filter membrane 30 is sandwiched between the grids 24 and thus can be effectively supported when water is passed trough the filter membrane 30 . the direction of the preferred water flow through the assembly is indicated by arrow 60 . preferably , two or more filter membranes 30 are situated between the grids 24 . thereby , the filter membranes 30 are of different pore sizes . more preferably , the filter membrane situated relatively upstream has a wider pore size than the filter membrane ( s ) situated relatively downstream . for instance , the upstream filter membrane has a nominal pore size of 0 . 65 μm and the downstream filter membrane has a nominal pore size of 0 . 22 μm . furthermore , an additional membrane support 24 b may be provided between the grids 24 in order to further support the filter membrane ( s ) 30 . preferably , the support member 24 b is a circular aluminium grid wherein the openings of the grid have a smaller size than the backing members 24 of the frame 21 . more preferably , the additional support 24 b is positioned downstream of the filter membrane ( s ) 30 . in addition , locating means 55 b are preferably provided at the membrane support 24 b which interact with the locating means 55 a provided at the frame 21 in order to hold the support 24 b at a fixed position relative to the frame 21 . preferably , the locating means are apertures formed close to the outer edge of the circular support member 24 b . in assembly process , the two frame parts 21 can be manufactured e . g . injected as separate pieces . after that , the additional backing member 24 b may be fixed , e . g . glued , onto one of the backing members 24 . of course , two backing members 24 b may be provided at each side of the filter membrane ( s ) 30 . fig9 shows a preferred embodiment of the closing mechanism 70 of the device which is provided in order to enclose the filter card 20 between the two enclosing members 40 a , 40 b . the closing mechanism is preferably a lever connected to at least one of the enclosing members 40 a , 40 b which are moveable relative to each other . alternatively , a dedicated motor which may be operated by a push button can be provided in order to enable a relative movement of the two enclosing members 40 a , 40 b . as can be seen from the figure , the two enclosing members 40 a , 40 b are arranged such that a dedicated aperture 42 for inserting the filter card 20 is formed between them . when the filter card 20 is fully inserted by a user , as shown by the figure , the user may operate the closing mechanism 70 in order to enclose the filter card 20 . thereby , the closing mechanism 70 transmits mechanical forces in order to move the enclosing member 40 b towards the other enclosing member 40 a , as indicated by arrows 80 . hence , the two enclosing members 40 a , 40 b are approached and the filter card 20 situated between them is fixed in its position . accordingly , the filter card 20 can be secured at a stable position in the aperture 42 . moreover , when the enclosing members 40 a , 40 b are approached , the sealing means 26 provided at the enclosing members 40 a , 40 b are pressed against the surface of the filter card 20 , thereby enabling a water - tight sealing of the assembly . when a user intends to remove the filter card 20 , the closing mechanism is to be operated in advance in order to separate the two enclosing members 40 a , 40 b . although the present invention has been described with reference to preferred embodiments thereof , many modifications and alternations may be made by a person having ordinary skill in the art without departing from the scope of this invention which is defined by the appended claims . for example the filter membrane may be provided in another embodiment of a filter frame means such that the filter membrane can be exchanged relatively easy by any operator of the device . it may as well be possible to mount more than two filter membranes at the filter card in order to enhance the filtration properties of the filter card . moreover , other filtration media could be incorporated by the removable filter card which enables a sufficient filtration of water . it may as well be possible to design a filter card having two or more apertures , each of them housing at least one filter membrane and at least one backing member , in order to filter water passed through two or more water flowpaths of a device to which the filter card is connected . moreover , different embodiments of locating devices for the filter membrane and the backing members may be provided at the frame means of the filter card in order to enable the correct mounting of the filter membrane and the backing members .