1 A CONTACT FILTER BLOCK, A METHOD OF PRODUCING STRUCTURAL ELEMENTS OF A CONTACT FILTER BLOCK AND AN APPARATUS FOR PRODUCING STRUCTURAL ELEMENTS OF A CONTACT FILTER BLOCK 5 The present invention relates to a method of producing structural elements of a contact filter block, a contact filter block obtained by the method and an apparatus for producing structural elements of a contact filter block using the method. Related techniques have been described in publications such as US 10 2004/0022973, US 3,758,359, US 4,089,667, US 5,015,123, US 5,772,870, US 5,882,510, US 6,217,800, US 6,333,088, US 6,342,158, US 6,551 ,818, EP 1 142 837 A and KR 9611746. Reference is made to the above mentioned patent publications and the above US patents are hereby incorporated in the present specification by reference in their entirety for all purposes. 15 A first aspect of the present invention relates to a method of producing tubular elements of a contact filter block using an apparatus including: a reservoir for storing granulate material having a first melting point, said reservoir having a first inlet and a first outlet, said granulate material including a foaming additive having a second melting point, 20 a heating and pressurising unit including a chamber having a second inlet and a second outlet, said second inlet in communication with said first outlet, a path of travel defined from said second inlet to said second outlet, said heating unit including heating and pressurising elements distributed along said path of travel, 25 an extruder unit mounted downstream relative to said heating and pressurising unit at said second outlet, said extruder unit further including an extruder heating unit, said extruder unit including two oppositely rotating extruder heads defining an extruder outlet, said extruder unit generating a mesh-type tubular structure, 30 a cooling unit mounted at said extruder unit for rapidly cooling said granulate material, and a measuring and cutting device for measuring and cutting elements having a predetermined length within a specific interval, 2 the method including at least the steps of: supplying said granulate material to said heating and pressurising unit from said reservoir via said second inlet, conveying said granulate material along said path of travel, 5 heating said granulate material to an elevated temperature and pressurising said granulate material to an elevated pressure while conveying said granulate material along said path in said heating and pressurising unit, said heating and pressurising performed according to a specific heating and pressurising profile, 10 transferring said heated and pressurised granulate material to said extruder unit, heating said granulate material within said extruder heating unit to a temperature above said first melting point and/or at or above said second melting point, 15 extruding said mesh-type tubular structure from said granulate material, said heating of said granulate material causing an expansion of said foaming additive causing said mesh-type tubular structure to obtain a porous structure, cooling said extruded mesh-type tubular structure by said cooling unit according to a specific cooling profile thereby stopping or halting expansion of 20 said granulate material and/or said foaming additive and locking or fixating said porous structure of said mesh-type tubular structure, and cutting said mesh-type tubular structure into discrete tubular elements using said measuring and cutting unit. The basic idea according to the present invention is to provide 25 improvements relating to contact filter blocks, in particular to the tubular elements of the contact filter blocks for improving the properties of the contact filter blocks. According to a first embodiment of the method according to the first aspect of the present invention, the cutting of the mesh-type tubular structure involves cutting the mesh-type tubular structure into individual or discrete tubular elements 30 which are in a further step joined together side by side for producing a contact filter block. The individual or discrete tubular elements may be joined or sealed together by means of an adhesive or by the application of heat. The step of joining or assembling the individual and discrete tubular elements together may 3 provide a contact filter block of the conventional cubic configuration or a substantially box-like configuration or may alternatively provide a contact filter block having a base surface from which the individual or discrete tubular elements extend, preferably perpendicularly from the base surface. The base 5 surface may have any relevant geometrical configuration such as a square, a rectangle, a circle, a triangle, a polygonal or a rhombic configuration. According to an alternative or second embodiment of the method according to the first aspect of the present invention, the cutting of the mesh-type tubular structure involves transforming the mesh-type tubular structure into a 10 plane structure by cutting lengthwise the mesh-type tubular structure and forming the plane web produced from the mesh-type tubular structure into a corrugated structure and further cutting transversally the corrugated structure into individual corrugated plate elements. In the process of producing the corrugated plate elements from the mesh-type tubular structure, the cutting lengthwise and the 15 cutting transversally may be performed in one action before the individual corrugated plate elements be deformed into the corrugated configuration by the application of heat and/or pressure to the element. Alternatively, the mesh-type tubular structure may be cut lengthwise in a separate step, whereupon for providing a web which is forced through a deforming dye by the application of 20 heat and/or pressure to the web for producing a corrugated web, which web is then cut into individual corrugated plate elements. By the provision of corrugated structural plate elements, a further advantage in relation to the volume or space used for transportation is reduced as the individual corrugated plate elements may be positioned on top of one another 25 occupying a fairly small space as no or substantially no space is left between the corrugated plate elements. At the site of the application, the corrugated plate elements are joined together back to back as the corrugations of the one corrugated plate element are joined to the corrugations of the adjacent corrugated plate elements, thereby providing in the inter space between the individual 30 adjacent corrugations through-going apertures similar to the apertures produced by the tubular elements produced in accordance with the embodiment of the method according to the first aspect of the present invention.
4 In the present context, the term 'a foaming additive' is to be construed as a generic term including any material present in solid, liquid or gaseous phase which allows the granulate material to be expanded for providing the porous structure of the mesh-type tubular structure produced in accordance with the 5 method according to the first aspect of the present invention. Furthermore, granulate material is also to be construed including any material conventionally used in the polymer or plastics industry in the extrusion technical field and may include solid granulate material or polymer material included in a semi-pasteous or liquid state dependent on the actual melting point of the polymer composition. 10 It is further construed that any two-component polymer system, the one component constituting a component similar to the granulate material and the second component constituting a component similar to the foaming additive, which system provides that the two components are brought together, a physical and chemical reaction similar to the reaction caused by the straightforward 15 addition of the foaming additive to the granulate material within the heating and pressurising unit is also to be contemplated as part of the components used or usable in accordance with the teachings of the present invention. The primary use of the structural elements produced according to the first aspect of the present invention is as elements in a filter block. The method is 20 primarily directed towards the advantageous properties of the structural elements, namely the enlarged surface originating from the release of the foaming additive. The increase in surface area is contemplated to be 10-50%, 50% to 100% or even more compared to structural elements produced according to traditional production methods. 25 In the present specification, the term a tubular element is to be construed as a geometrical element having a generator such as a circle, an ellipse, a square, a triangle or a rectangle and being generated by moving the geometrical generator along a generator line. Furthermore, in the present context, the terms heating and pressurising 30 unit and extruder unit are defining two sections of an apparatus conventionally known as an extruder. In the present context, the above terms heating and pressurising unit and extruder units are used solely for the purpose of defining 5 specific properties and functions of the respective parts or sections of the extruder. Furthermore, it is to be understood that the term extruder unit may be considered equivalent to or identical to the part of the conventional extruder 5 commonly known as the dye head. Additionally, the method is contemplated to reduce the amount of material used for producing the structural elements by 20% to 50%, or even more, as other production methods for producing elements having a large surface require larger amounts of material. 10 The large surface of the structural elements is contemplated to provide space for one, two or more colonies of bacteria, of aerobic and/or anaerobic types. The combination of aerobic and anaerobic bacteria is contemplated to ensure an advantageous rate of decomposition of waste or biological material in water. Also, the structural elements are contemplated to ensure that a large 15 amount of bacteria will grow on the structural elements within a short period of time, such as within two weeks. It is contemplated that anaerobic bacteria will grow at, around or near the joints of the mesh-type structure and aerobic bacteria will grow at or on the parts between the joints. Dependent on the heating of the granulate material and also dependent on 20 the cooling of the extruded mesh-type tubular structure after extrusion the individual strands of the mesh-type structure may be pressed together provided the strands are allowed to flow prior to cooling or alternatively, the individual strands of the mesh-type structure may be positioned on top of one another providing a more irregularly configurate type structure improving or increasing the 25 joints of the mesh- type structure as compared to a structure in which the individual strands of the mesh-type structure are joint or pressed together into a shallow structure. It is, furthermore, contemplated that the more irregularly configurated structure may preserve the bacteria from being killed if the contact filter block is exposed to toxic material, which is harmful or mortal to the bacteria. 30 The reservoir may be constituted by a simple funnel into which a granulate material is poured automatically, semi-automatically or manually. Alternatively, the reservoir may be a tank or the like, for continuously supplying granulate material to the apparatus. The reservoir may have at least two inlets and/or 6 outlets for refilling the reservoir and for delivering the granulate material to the apparatus. The heating and pressurising unit receives the granulate material from the reservoir and heats and pressurises the granulate material. The granulate 5 material is heated up to but preferably not beyond the melting point of the granulate material. In alternative embodiments, the granulate material may be heated above or beyond the melting point. The pressure to which the granulate material is pressurised is preferably within the interval 5 to 5000 bar, such as 5 to 50 bar, such as 50 to 1000 bar, 10 such as 60 to 900 bar, such as 65 to 850 bar, such as 75 to 750 bar, such as 100 to 650 bar, such as 150 to 600 bar, such as 200 to 500 bar, such as 275 to 450 bar, such as 300 to 400 bar, such as 325 to 375 bar, such as 5 to 25 bar, such as 25 to 45 bar, such as 45 to 75 bar, such as 75 to 150 bar, such as 150 to 200 bar, such as 200 to 275 bar, such as 275 to 340 bar, such as 340 to 375 bar, such as 15 375 to 425 bar, such as 425 to 500 bar, such as 500 to 550 bar, such as 550 to 625 bar, such as 625 to 700 bar, such as 700 to 775 bar, such as 775 to 850 bar, such as 850 to 1000 bar, preferably around 350 bar. The extruder heating unit provides an additional heating of the material. When the extruder heating unit heats the material, the foaming additive is 20 contemplated to reach its foaming state and cause an expansion or creation of bubbles in the material, thereby creating a porous structure in the material. In alternative embodiments, the extruder heating unit may be omitted, and the heating and pressurising unit may provide the needed heat for the foaming additive to expand. 25 For ensuring that the porous structure in the material is fixed or not changed, a cooling unit is provided. The cooling unit cools the material after it exits the extruder unit which is contemplated to stop the expansion of the foaming additive. The cooling may be obtained by using water, air, any fluid or liquid. The extruder unit extrudes the mesh-type tubular structure from the heated 30 material. In the presently preferred embodiment of the present invention, the extruder unit includes two oppositely rotating extruder heads or two parallel moving extruder heads, and the extruder heads define the extruder outlet or dye head. The oppositely rotating extruder heads provide a relative motion between 7 the rotating extruder heads as the one head may be stationary or be moving at a lower speed in the one direction as compared to the other head, or alternatively, the other head may be stationary or rotating oppositely to the first head or at a lower speed as compared to the first head. 5 Furthermore, for providing specifically configurated mesh-type tubular structure, the rotation may be oscillating or have varying speeds of one or both of the rotating extruder heads for providing a specific configuration of the mesh-type structure of the mesh-type tubular structure. Further, the extruder unit may include a multitude of nozzles each defining a cross-section having a substantially 10 square, round, semi-round, rectangular, oblong, triangular, semi-hyperbolic, trapeze, inverse trapeze shaped or any combinations thereof, preferably round. In the present context, the term nozzle is used for describing the outlets of the extruder. The measuring and cutting device is used for measuring and cutting the 15 mesh-type tubular structure into discrete elements having a predetermined length within a specific interval, i.e. the resulting discrete elements may have an overall dimension with a measurement uncertainty. The measurements may be performed by machine visual inspection, mechanical determination of the length of the elements or any other automatic or semi-automatic methods or techniques. 20 In performing the method according to the first aspect of the present invention, the granulate material is supplied from the reservoir into the heating and pressurising unit. The heating and pressurising unit has an inlet through which the granulate material is supplied. The inlet may be in fluid communication with the reservoir. 25 The granulate material is conveyed along the path of travel defined in the heating and pressurising unit while the heating unit heats the granulate material. The heating is performed according to a specific heating profile. In a first instance the heating elements may emit the same amount of heat at all positions along the path of travel, i.e. the same number of watts is emitted 30 from all elements. In a second instance the heating element or elements may emit an increasing or decreasing amount of heat along the path of travel to ensure that the temperature of the granulate material is near the melting point at the outlet of the heating and pressurising unit. In a third instance, the specific 8 heating profile may include one or more temperature peaks along the path of travel, such as short and/or longer bursts of heat emitted from the heating elements or element. The temperature peaks may be substantially equal or in the alternative different, both in terms of amount and width/duration, further 5 alternatively, a combination of two or more substantially equal peaks and one or more peaks different from the two. Further, the speed at which the granulate material is conveyed along the path of travel may be adapted to ensure that the temperature of the granulate material is near or at the melting point of the granulate material at the outlet of the 10 heating and pressurising unit. The heating and pressurising unit may also increase or decrease the pressure of the granulate material as it is conveyed from the inlet to the outlet of the heating and pressurising unit. The change in pressure may be performed according to a specific pressurising profile. 15 After the material has been extruded, the extruder heating unit may provide extra heat for heating and thereby causing the foaming additive to expand and thereby create a porous structure in the material. The cooling unit cools the material to a temperature below the temperature at which the foaming additive expands. 20 The granulate material used in the method according to the first aspect of the present invention is preferably HDPE. Further, the granulate material may further include an amount of PP or LDPE or a combination thereof, and/or may include PVS, NYLON, ABS or any other polymer materials. According to the teachings of the present invention, the weight of the 25 foaming additive may constitute 0,1% to 50%, such as 1 % to 40%, such as 5% to 35%, such as 8% to 20%, such as 9% to 12%, such as 0,1 % to 5%, such as 5% to 15%, such as 15% to 25%, such as 25% to 35%, such as 35% to 45%, such as 45% to 50%, preferably 3-5% or 5-10% of the weight of the granulate material. According to the basic realisation of the method according to the present 30 invention, the granulate material may be heated to 140 to 300 degrees centigrade, dependent on the polymer material or materials in question. Additionally, the granulate material may further include a colouring additive, such as green, black or any other colour.
9 Particularly, the method according to the first aspect of the present invention may further comprise providing a feed screw in the path of travel, the feed screw defining a cylindrical or conical geometry. The feed screw may then constitute a conveyor for conveying the material from the inlet of the heating and 5 pressurising unit to the outlet thereof. The geometrical structure of the feed screw may perform the pressurising of the material as it is conveyed along the path of travel. A second aspect of the present invention relates to a contact filter block for the construction of filters for biological treatment of waste water comprising: 10 a plurality of individual tubular elements which are joined together each having an mesh-type outside surface, the individual tubular elements being produced using the method according to the first aspect of the invention, said individual tubular elements having a porous structure. Said plurality may comprise e.g. 4, 6, 8, 10, 12, 16 or even more individual 15 tubular elements, such as e.g. 25, 36, 64 or 100. The porous structure of the contact filter block is contemplated to provide a large surface on which a multitude of bacteria may grow. The bacteria are then contemplated to break-down or decompose waste in water flowing through the contact filter block. The contact filter block may be positioned in the ground or in a 20 waste water treatment facility. In a basic realisation of the contact filter block according to the second aspect of the present invention, it is contemplated that the porous structure may have a surface of no less than 25-800 m 2 /m 3 , such as 90-100 m 2 /m 3 , 100-150 m 2 /m 3 , 150-200 m 2 /m 3 , or 200-300 m 2 /m 3 . 25 As discussed earlier, the large surface area is contemplated to provide space for large amounts of bacteria for decomposing waste and/or biological material in water. The surface roughness may be further described by the parameters Ra, Rz and Rmax. The surface roughness is measured in pm. 30 In embodiments of the present invention, the parameter Ra has a value in the interval 5 to 1000, such as 7 to 700, such as 10 to 600, such as 25 to 500, such as 50 to 450, such as 75 to 350, such as 100 to 300, such as 150 to 250, such as 7 to 15, such as 15 to 30, such as 30 to 50, such as 50 to 75, such as 75 10 to 150, such as 150 to 225, such as 225 to 300, such as 300 to 350, such as 350 to 400, such as 400 to 500, all measures being Ra, Rz and Rmax. In embodiments of the present invention, the parameter Rz has a value in the interval 5 to 4000, such as 20 to 2000, such as 25 to 1800, such as 40 to 5 1500, such as 75 to 1300, such as 125 to 1200, such as 200 to 1100, such as 275 to 1000, such as 350 to 850, such as 400 to 700, such as 450 to 600, such as 500 to 550, such as 20 to 50, such as 50 to 100, such as 100 to 150, such as 150 to 225, such as 225 to 300, such as 300 to 375, such as 375 to 450, such as 450 to 550, such as 550 to 675, such as 675 to 775, such as 775 to 850, such as 10 850 to 950, such as 950 to 1100, such as 1100 to 1250, such as 1250 to 1400, such as 1400 to 1600, such as 1600 to 1800, such as 1800 to 2000, all measures being Ra, Rz and Rmax. In embodiments of the present invention, the parameter Rmax has a value in the interval 5 to 5000, such as 30 to 3000, such as 40 to 2800, such as 50 to 15 2600, such as 75 to 2400, such as 150 to 2200, such as 200 to 2000, such as 275 to 1800, such as 375 to 1700, such as 450 to 1550, such as 550 to 1400, such as 650 to 1200, such as 775 to 1000, such as 850 to 950, such as 30 to 40, such as 40 to 50, such as 50 to 75, such as 75 to 100, such as 100 to 175, such as 175 to 275, such as 275 to 350, such as 350 to 475, such as 475 to 575, such 20 as 575 to 700, such as 700 to 900, such as 900 to 1200, such as 1200 to 1500, such as 1500 to 1800, such as 1800 to 2000, such as 2000 to 2300, such as 2300 to 2600, such as 2600 to 2800, such as 2800 to 3000, all measures being Ra, Rz and Rmax. A third aspect of the present invention relates to an apparatus for 25 producing tubular elements of a contact filter block, the apparatus including: a reservoir for storing granulate material having a first melting point, said reservoir having a first inlet and a first outlet, said granulate material including a foaming additive having a second melting point, a heating and pressurising unit including a chamber having a second inlet 30 and a second outlet, said second inlet in communication with said first outlet, a path of travel defined from said second inlet to said second outlet, said heating unit comprising heating and pressurising elements distributed along said path of travel, 11 an extruder unit mounted downstream relative to said heating and pressurising unit at said second outlet, said extruder unit further including an extruder heating unit, said extruder unit including two oppositely rotating extruder heads defining an extruder outlet, said extruder unit generating a mesh-type 5 tubular structure, a cooling unit mounted at said extruder unit for rapidly cooling said mesh type tubular structure, and a measuring and cutting device for measuring and cutting tubular elements having a predetermined length from said tubular structure. 10 The present invention will be further described with reference to the attached drawings in which: Fig. 1a is a schematic illustration of an apparatus for producing a mesh type tubular structure, Fig. 1 b is a schematic illustration of a first embodiment of an apparatus for 15 processing the mesh-type tubular structure into tubular elements, Fig. 1c is a schematic illustration of a second embodiment of processing the mesh- type tubular structure into corrugated plate elements, Fig. 2 is a schematic illustration of a part of an extruder unit, Fig. 3 is a schematic illustration of the extruder unit of fig. 2, 20 Fig. 4 is schematic illustrations of alternative embodiments of extruder unit outlets, Fig. 5 is a schematic illustration of a plurality of tubular elements connected in a filter block, Fig. 6 is a zoomed schematic illustration of a part of a tubular element, 25 Fig. 7a is a schematic illustration of a pile of corrugated plate elements, and Fig. 7b is a schematic illustration of a plurality of corrugated plate elements connected in a filter block. Fig. 1 a is a schematic illustration of an apparatus 10 for producing a mesh 30 type tubular structure. The apparatus 10 comprises a first section 12 and a second section 14. The first section 12 receives granulate material 16 via an inlet 18 constituted by a funnel. Alternatively, the inlet 18 may be in contact with a larger reservoir for continuously supplying granulate material 16 to the apparatus 12 10. The granulate material comprises a foaming additive that, when heated above a certain temperature, expands and thereby creates bobbles in the melted or soft granulate material. When the foaming additive expands the granulate material is probably not granulate any more, but a soft mass. 5 The first section 12 receives the granulate material which is then fed to a screw conveyor 20 defining an axis or path of travel along which the granulate material is transported. The screw conveyor 20 is positioned in the first section 12. Along the path of travel defined by the screw conveyor 20, a number of 10 heating elements, alternatively a single heating element, is distributed. The heating element or elements, not illustrated in Fig. 1a, heat the granulate material 16 while it is conveyed by the screw conveyor from the inlet of the first section 12 to an outlet 22 of the first section. The granulate material 16 is heated to a temperature just below the 15 melting point of the granulate material 16. In the presently preferred embodiment of the present invention, the screw conveyor 20 compresses the granulate material as it is conveyed along the path of travel. The heating of the granulate material 16 is performed according to a specific heating profile. In one instance the heating elements may emit the same 20 amount of heat at all positions along the path of travel and the speed of which the screw conveyor 20 conveys the granulate material 16 may be adapted to ensure that the temperature of the granulate material is above the melting point of the granulate material at the outlet 22. In a second instance the heating element or elements may emit an 25 increasing or decreasing amount of heat along the path of travel to ensure that the temperature of the granulate material 16 is near the melting point of the granulate material at the outlet 22. Thirdly, the specific heating profile may include one or more temperature peaks along the path of travel, such as short or long bursts of heat emitted from 30 the heating elements or element. The temperature peaks may be substantially equal, both in terms of amount and width/duration. The screw conveyor 20 may be formed as a simple screw having a substantially constant diameter or height. In the alternative the screw conveyor 20 13 may be formed as a screw conveyor with gradually increasing or decreasing height so that the granulate material 16 is further compressed due to the gradually reduced size or space. In the embodiment where the screw conveyor 20 has a substantially constant height along the path of travel, the granulate 5 material 16 may still be compressed, e.g. by the "new" or cold granulate material pushing the "old" or hot granulate material 16 located nearer the outlet 22 of the first section. The operation of the heating elements and/or the screw conveyor is controlled by an external unit 24, e.g. a computer or specialised hardware and/or 10 software monitoring the operation of the screw conveyor and/or the heating elements by receiving information regarding the number of revolutions per second of the screw conveyor and pressure and/or temperature information e.g. via sensors located inside the first section 12. The unit 24 may ensure that the temperature of the heating elements and/or the material is within predetermined 15 ranges. In Fig. 1a, the unit 24 is connected to the first section 12 via a cable 26 for transferring information and control signals back and forth between the unit 24 and the first section 12. Alternatively, the cable or wire 26 may be constituted by other links such as RF, IR, HF or other wireless transfer techniques. 20 From the outlet 22 of the first section 12, the heated and near-melted granulate material is transferred into the second section via an inlet 28. The material is fed to an extruder unit 30 including an additional heater unit, not illustrated, for heating the material above its melting point. As the material includes a foaming additive that is released above a certain temperature, the 25 foaming material will enter a gaseous state thereby creating a porous structure in the material. The expansion or transfer of the foaming additive into gas is preferably done after the material has passed though the extruder unit 30. To ensure that the porous structure is fixed, a cooling unit cools the material shortly after the porous structure has been established. 30 In Fig. 1a, the mesh-type tubular structure 34 exits the second section 14 via the outlet 36. The tubular mesh-structure 34 will be further processed as illustrated in Fig. 1 b, or alternatively as illustrated in Fig. 1 c.
14 In Fig. 1b, the structure 34 is cut into individual tubular elements using a cutting saw 38 located in a tower 40, the cutting wheel being mounted on a reciprocating arm moving in the direction of the arrow 42. The structure 34 is cut into smaller pieces that may be assembled in larger structures such as 44 which 5 is a structure comprising 25 short pieces of the tubular mesh-structure 34. The 25 individual pieces of the tubular mesh-structure are then transferred into a heater unit 46 melting the ends of the pieces of the tubular mesh-structure so that the individual pieces are assembled and/or locked in a fixed structure which may then be used for purposes such as cleaning water, such as waste water. 10 In Fig. 1c, the mesh-type tubular structure 34 is cut lengthwise by means of a cutting knife 43 causing the mesh-type tubular structure to be converted into a substantially planar structure as is illustrated by 35. The substantially planar structure is introduced into a heating and pressing dye 47, in which the substantially planar structure 35 is converted into a corrugated structure, as is 15 indicated by the reference numeral 37, whereupon the corrugated structure is introduced into a cutting station, in which the cutting saw 38 cuts the corrugated structure into individual corrugated plate elements, one of which is designated the reference numeral 45. The corrugated plate elements are to be described in greater details below with reference to Figs. 7a and 7b. In an alternative method 20 of providing the corrugated plate element 45, the planar structure shown in Fig. 1c is cut into individual rectangular plane elements, which are then introduced into a set of forming and pressing tools providing the corrugated configuration. In the presently preferred embodiment of the present invention, the extruder unit 30 is constituted by two oppositely rotating parts illustrated in greater 25 details in Figs. 2, 3 and 4. As seen from Fig. 2, the extruded material exits the extruder unit at extruder outlets 32 distributed in a circle. The distribution of the extruder outlets 32 ensures along with the rotation of the extruder heads 52 and 54 that the resulting structure is a tubular mesh-type structure. 30 Fig. 3 schematically illustrates an extruder comprising two extruder heads 52 and 54. The extruder heads 52 and 54 are two oppositely rotating circular heads. In other embodiments, the extruder unit may comprise two or more 15 parallel moving parts, two parts moving or rotating in the same direction, or parts not moving or rotating themselves. In Fig. 4, three alternative geometrical configurations of the extruder outlets are illustrated, referenced 32', 32", 32"', respectively. In the presently 5 preferred embodiment of the present invention, the extruder outlets 32 define a substantially circular geometrical cross section, however, as illustrated in Fig. 4, the extruder outlets may define a substantially square 32', star-shaped 32", triangular 32"' or any polygon configurations such as elliptical, rectangular, truncated pyramidal, inverse truncated pyramidal, hexagonal, pentagonal or any 10 combinations thereof. Fig. 5 is a schematic illustration of an assembly of pieces of the tubular mesh-type structure 34, assembled into a simple structure 44. The individual mesh-type tubular structure elements are joined together at the ends of the structures, however embodiments wherein the elements are joined together at 15 other locations along the longitudinal axis of the elements may be envisioned as well as embodiments where the elements are joined along the entire lengths of the elements. Fig. 6 is a schematic closer view of an element 48 illustrating the expanded surface of the threads 50 constituting the mesh-structure of the tubular element 20 48. The threads 50 have an expanded surface due to the manufacturing process as described in relation to Fig. 1 wherein the foaming additives release a gas creating bubbles in the material. The enlarged surface is contemplated to allow a great or large amount of bacteria to grow on the structure or thread, e.g. the bacteria located nearest the thread 50 will be covered by other bacteria thereby 25 allowing substantially only anaerobic bacteria to grow on the thread 50 itself. In Fig. 7a, a pile of individual corrugated plate elements 45 is shown, which pile constitutes a highly compact structure as compared to the structure 44, i.e. the contact filter block 44 shown in Fig. 5. The saving of space by the provision of the corrugated plate element 45 is contemplated to provide a further advantage of 30 the contact filter block as compared to the conventional contact filter blocks known in the art. In Fig. 7b, a contact filter block 44' is shown, which contact filter block is assembled from the corrugated plate elements 45, also shown in Fig. 7a. The corrugated plate elements 45 are joined together in Fig. 7b in a structure 16 "back-to-back", as the corrugations of the one plate are joined to the opposite corrugations of the adjacent plate which has been turned upside down as compared to the pile shown in Fig. 7a. The structures 44' shown in Fig. 5 may be fixed by means of an adhesive or alternatively by the application of heat to the 5 joined corrugations or at the end parts of the structure. The combination of anaerobic and aerobic bacteria is contemplated to provide an improved cleaning effect on wastewater, as they each digest or decompose different types of waste.