Filtration apparatus

Abstract

A method for filtering a liquid (2), such as wastewater, is described. The liquid has a total suspended solids content of between 1 and 50 g / L, and the method uses a liquid-permeable filter element (9) with a first and a second face (10, 11) to produce a filtered liquid (4) with a total suspended solids content not exceeding 10 mg / L. The method consists of circulating the liquid-permeable filter element through itself, such that, in a first position, an area of ​​the first face of the filter element is under pressure, and the pressure across the filter element is greater than 0 and less than or equal to 5.9 kPa (60 cmH2O), and, in a second position, the area is either not under pressure or under pressure, and the solids accumulated on the first face of the filter element can be removed by directing at least one jet from the second face of the filter element through the filter element to the first face.The filter element has a pore size and is operated at a speed such that the permeation flow is between 200 L / (m2h) and 5000 L / (m2h), and the thickness of the layer of accumulated solids when removed is between 0 and 6 cm.

Description

Filtration apparatus Field The present invention relates to the filtration of a liquid, such as wastewater, to obtain a filtered fluid and / or thicken the liquid or recover the solid content. The present invention also relates to the apparatus for filtering a liquid, such as wastewater, to obtain a filtered fluid and / or thicken the liquid or recover the solid content. Background Filtration of a liquid, particularly a suspension containing suspended solids, can be used in several different applications, including municipal, industrial, and agricultural wastewater treatment processes. In some cases, such as municipal wastewater treatment, filtration can be used to obtain a filtered liquid with a lower suspended solids content. However, in other cases, filtration can be used to thicken the liquid or to recover solid content (such as fibers or debris). One type of filter is a rotary disc filter. Examples of rotary disc filters can be found in US patents 5,759,397 A, 4,639,315 A, and WO 91 / 12064 A1 (also published as US 5,296,143 A). Disc filters are also described in WO 2014 / 170533 A1 and EP 2514500 A1. Rotary disc filters can achieve high flow rates, operate continuously for extended periods, and are generally resistant to clogging. However, rotary disc filters tend to use filter cloths with larger pore sizes and therefore sacrifice a lower degree of filtration. Another type of filter is a membrane bioreactor. Membrane bioreactors tend to have a much smaller effective pore size. A smaller pore size helps increase the degree of filtration, but this comes at the expense of flow rate (several magnitude resolutions lower than rotating disk filters) and a tendency for the membrane to clog. Therefore, membrane bioreactors tend to be used in specialized, low-flow applications. Document JP 2016159241 A describes a membrane filtration system used in water treatment that includes flocculant injection means for injecting an excessive amount of a flocculant to flocculate organic matter contained in raw water and cause membrane clogging, microbubble injection means for injecting microbubbles required to enlarge an additional portion of the flocculant that does not act in flocculating the organic matter that causes membrane clogging, and membrane filtration means for filtering the target water to be treated into which the microbubbles are injected. Summary According to a first aspect of the present invention, an apparatus for filtering a liquid is provided as specified in claim 1. According to a second aspect of the present invention, a method for filtering a liquid is provided as specified in claim 11. Optional features are specified in the dependent claims. Brief description of the drawings The following will describe certain embodiments of the present invention, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an apparatus for filtering a liquid; Figure 2 is a schematic perspective view of a filtering element arrangement; Figure 3 is a schematic perspective view of an alternative arrangement of the filtering element; Figure 4 schematically illustrates a disk; Figure 5 schematically illustrates different stages during filtering; Figure 6 schematically illustrates a suspension, a substrate, and a filtrate when the substrate is introduced into the suspension; Figure 7 schematically illustrates a suspension, a substrate, and a filtrate when the substrate is removed from the suspension; Figure 8 is a schematic block diagram of a computer system for controlling the filtering apparatus; Figure 9 is a perspective view of a structure for a rotary disk filter; Figure 10 is a perspective view of a longitudinal member of the structure that combines with internal cylindrical portions of the same structure; Figure 11 is an enlarged detail shown in Figure 10; Figure 12 is a perspective view of a modular component that is assembled with an inner cylindrical portion of the structure shown in Figure 9; Figure 13 is an exploded perspective view of a modular component of the structure according to the present invention, together with an inner cylindrical portion; Figure 14 is a partially exploded perspective view of the structure shown in Figure 12; Figure 15 is an assembled perspective view of the structure shown in Figure 14; Figure 16 is a perspective view of a rotary disc thickening vacuum filter in a purification vessel; Figure 17 is a schematic side view, in partial section, of the rotary disc filter structure and its backwash apparatus; Figure 18 is an enlarged detail circled as shown in Figure 17; Figure 19 is a perspective view of the rotary disk filter shown in Figure 17 with parts removed for clarity of illustration; Figure 20 is a perspective view of the backwash apparatus shown in Figure 19 with parts removed for clarity of illustration; Figure 21 is a far left view of the rotary disc filter shown in Figure 19, which shows the backwash apparatus in a first position; Figure 22 is a far left view of the rotary disc filter shown in Figure 19, which shows the backwash apparatus in a second position; Figure 23 is a far left view of the rotary disk filter; Figure 24 is a far left view of the rotary disc filter shown in Figure 19, which shows a disc washing apparatus; Figure 25 is a perspective view showing the spray tubes of the disc washing apparatus shown in Figure 24; Figure 26 is a perspective view of a thickening filter machine; Figure 27 is a partial perspective view of the disk structure of the machine in Figure 26; Figure 28 is a longitudinal section of the machine in Figure 26 submerged in a container (not shown); Figure 29 is an enlarged schematic detail of a portion of the disk shown in Figure 28; Figure 30 is an enlarged schematic detail of a support; Figure 31 is an exploded perspective view of the support shown in Figure 30 with some parts removed for clarity of the illustration; Figure 32 is a perspective view of the assembled support shown in Figure 31 with some parts removed for clarity of illustration; Figure 33 is a perspective view of an internal labyrinth-shaped sealing element in the support shown in Figure 30; Figure 34 is an end view of the external labyrinth-shaped sealing element in Figure 33; Figure 35 is a section of the element shown in Figure 34 taken along line AA; Figure 36 is an enlarged detail of the element shown in Figure 35; Figure 37 is a perspective view of an external, maze-shaped sealing element shown in Figure 30; Figure 38 is an end view of the external labyrinth-shaped sealing element shown in Figure 37; Figure 39 is a section of the element shown in Figure 38 taken along line BB; Figure 40 is an enlarged detail of the element shown in Figure 39; Figure 41 is a perspective view of the backwash apparatus shown in Figure 19 with the addition of a pressure compensation tube; Figure 42 is an end view showing the pressure compensation tube shown in Figure 41; Figure 43 illustrates a first outlet arrangement of a filtering machine comprising a spillway and a valve; Figure 44 illustrates a second outlet arrangement of a filtering machine comprising a valve without a spillway; Figure 45 illustrates a third outlet arrangement of a filtering machine comprising a suction pump; Figure 46 illustrates a fourth outlet arrangement of a filtration machine comprising a submersible pump; and Figure 47 illustrates a fifth outlet arrangement of a filtering machine comprising a suction pump and a dependent pipe. Detailed description of certain achievements Similar items are then indicated with similar reference numbers. Introduction With reference to Figure 1, the apparatus 1 is shown for filtering a suspension 2 (referred to herein as "liquid" or "feed"), such as wastewater, by removing the solid content 3 from the liquid 2 to produce the filtered liquid 4. The apparatus 1 includes a container 5, such as a tank, which is filled with liquid 2 to a level L. Level L can be maintained by feeding liquid 2 into container 5 on the influent side as filtered liquid 4 (also referred to herein as "filtrate") is withdrawn from the effluent side. As will be explained in more detail later, the level L of liquid 2 and the level Lf of filtered liquid 4 can be varied to control the filtration process. The apparatus 1 includes the gas feeder(s) 6 (or "aerators") for introducing gas bubbles 7, preferably air, into the liquid 2 and a filter machine 8 (only the (p / o) part shown in Figure 1) is at least partially immersed in the liquid 2 above the gas feeder 6. In the present description, the filter machine 8 may be referred to simply as the "filter". The filtering machine 8 includes one or more liquid-permeable filtering elements 9 (in the present description also referred to as "filtering elements", "filter sectors", "substrates" or "meshes"), each filtering element 9 having a first and a second opposing face 10, 11 (in the present description also referred to as "outer and inner faces" or "outer and inner faces"), which can circulate (or "cycle") in and out of the liquid 2. While a filter element 9 is immersed in liquid 2, liquid 2 can pass through it under pressure. As liquid 2 passes through filter element 9, the solid content 3 is removed from liquid 2 and deposited (or "accumulated") as a layer 14 on the first face 10 of filter element 9. In this description, layer 14 is also referred to as the "deposited layer," "accumulated layer," "sludge layer," "cake," and "dynamic membrane." One or more internal nozzles 15 (also referred to as a "spray bar" in this description) may be used to direct the spray(s) or jet(s) 16 of a liquid (such as the filtered fluid 4) towards the inner face 11 of the filter element(s) 9 to control the thickness of the layer 14 deposited on the filter elements 9. The spray 16 may be used to maintain a thin boundary layer 17 of the fluid between the outer face 10 of the filter element 9 and the layer 14. One or more external nozzles 18 may be used to direct the external spray or jet(s) 19 of a liquid (such as the filtered fluid 4) towards the outer face 10 of the filter elements 9 to help dislodge the layer 14. Apparatus 1 is arranged to control the rate of solid deposition, the degree of compaction of the solids, and the rate of reintroduction of solids back into the liquid 2. Whereas in a membrane bioreactor the deposition of solids on a membrane is discouraged or avoided, in the apparatus 1 described herein, the deposited layer 14, which is permeable to liquids, is used to help provide further finer filtration. For a liquid-permeable filter element 9 having a mesh free passage of 5 to 40 µm, measurements have shown that the deposited layer 14 has an equivalent nominal pore size of between 0.04 and 0.4 µm. With reference also to Figure 2, the filter elements 9 take the form of thin, sectorial, and annular filter elements 9 (hereafter also referred to as "filter sectors") that are assembled to form an annular filter 20. As will be described in more detail below, each filter sector 9 can be attached to a respective 'U' shaped element ("support" or "frame") that allows the filter sectors to be replaced. As will also be described in more detail later, a pair of axially offset annular filters 20 form part of a disk 31 (Figure 4) having an annular space 34 (Figure 4) in which liquid flows from opposite sides through submerged filter elements 9. The filter machine 8 comprises a series (or "battery") of such disks 31 (Figure 26) arranged in a row. The annular filter 20 is partially submerged, oriented perpendicularly or inclined to the liquid level L of the liquid 2, and rotates around a rotation axis 21. Therefore, the filter elements 9 pass into the liquid 2, move through the liquid 2, and then leave the liquid 2 to travel along a circular path. While a filter element 9 is in the liquid 2, the liquid 2 is forced under pressure through the filter element 9. Although the present description describes embodiments of filter machines that take the form of rotating disc filter machines having filter elements 9 that have an annular sector shape, the filter machine and the filter element(s) 9 can take other forms that allow them to circulate in and out of the liquid 2. With reference to Figure 3, a filter element 9 can take the form of a cylindrical filter element 9 that forms a cylindrical filter 20'. Alternatively, the filter elements 9 can take the form of curved parts that can be assembled to form the cylindrical filter 20'. The cylindrical filter 20' has a central axis that also serves as its axis 21' of rotation. The cylindrical filter 20' is partially submerged, with its central axis 21' oriented parallel to the liquid level L 2. Other forms and arrangements of filter elements may be used, such as a tape or band of liquid-permeable filter material. With reference to Figure 4, the filtering machine 8 includes a plurality (although only one is shown for clarity) of vertically oriented discs 31 that are horizontally (or at an angle) separated, each disc 31 comprising first and second annular filters 20, each annular filter 20 comprising filtering elements 9, preferably made of plastic. The disks 31 are partially submerged in fluid 2, which has an immersion (i.e., proportion of the disk area that is submerged) between 40 and 75%. Therefore, a first portion 32 of each disk 31 (shown unshaded) is above the level of fluid 2 (i.e., not submerged), and a second portion 33 of disk 31 is submerged (shown shaded). An annular space 34 is defined between the first and second annular filters 20 in which the filtered liquid 4 accumulates. The filtered liquid 4 is extracted through an inner space 35. The filtering machine 8 is arranged so that the discs 31 rotate at between 0 and 10 revolutions per minute (0 and 10 (2 / 60) rad / s). With reference to Figure 5, the electrostatic attraction and repulsion between liquid 2, filter elements 9, and sludge 14 can affect the filtration process. For example, water is hydrophilic and carries a negative charge. Filter elements 9, particularly if made of a plastic such as polyethylene, tend to be highly hydrophobic and carry a positive charge. Sludge 14 tends to be hydrophobic, carrying a small positive charge. During an initial period between t = 0 and t = t1 (0 ≤ t ≤ t1), when a substrate 9 first enters liquid 2 and the height difference is between 0 and 1 (0 ≤ 1), the substrate 9 provides substantially no resistance to the flow of liquid 2 through it. Because the height difference is small, the flux F is low, between 0 and F1 (0 ≤ F ≤ F1). The value of t1 is approximately 1 s and 1 is approximately 0.5 cm. The substrate 9 is generally free of mud 14. Solid particles 3 larger than the substrate openings (i.e., the pores), which have a pore size P, tend to repel each other. However, as the substrate 9 is submerged further and the height difference increases, the fluid flow 2 through the substrate 9 forces the solid particles 3 towards the substrate 9, thus forming a thin cake 14. During a second period between t = t1 and t = t2, the height difference increases and is between 1 and 2 (1 2). Substrate 9 begins to provide some resistance to the flow of liquid 2 through substrate 9. Particles 3 begin to accumulate loosely on substrate 9 and a cake 14 begins to form. The value of t2 is approximately 2 s and 2 = 1 cm. With reference also to Figure 6, the thin cake 14 restricts the flow of liquid 2, such as water and smaller particles, and in this way the cake layer 14 grows, but water 2 still flows through the cake layer 14 and the substrate. As the cake 14 develops, the positive charge of the substrate 9 and the sludge 14 and the negative charge of the water 2 help to form a thin water boundary layer 22 between the substrate and the cake. During a third period between t = t2 and t = t3, the height difference increases even further. Therefore, the flux F increases. The value of t3 is approximately 5 s and 3 = 5 cm. As the cake rotates, the pressure drops across the cake 14 and the substrate decreases, the water speed decreases, which facilitates the filtration process. The highest flow occurs when cake 14 and substrate 9 enter filtrate 4. However, as cake 14 becomes thicker, the flow F will begin to decrease. With reference also to Figure 7, when cake 14 is extracted from the solid suspension 2, gravity acts on the water boundary layer 22, which is aided by the subsequent wetting of the substrate. Once the force on cake 14 due to gravity becomes greater than the boundary friction, cake 14 slides into the solid suspension, leaving a clean substrate 9. The process is repeated when the substrate is reintroduced into suspension 2. As mentioned previously, the filtration processes described herein can benefit from the use of a plastic mesh, such as polyethylene, with a mesh size of, for example, 20 µm. These processes can further benefit from capillary action, which is a suction effect, in conjunction with surface tension. Capillary force can be very strong and depends on the material used to form the passages or channels (in other words, the pores in the substrate), the dimensions of the passages or channels ("capillary"), as well as the location of the water source and destination. Furthermore, a film of water on the inner surface of the substrate (i.e., the filtrate side) is exposed to the air, and therefore surface tension also pulls the water into the capillary and accelerates its movement.At the same time, this force creates an equal but opposite force that repels solids and creates a larger positive charge on the mesh. Therefore, ensuring that the filter material 9 is fully wetted when exposed to air can help promote filtration and can result in surprisingly high flow rates and high-quality filtration. With reference to Figures 1 and 4, this process is carried out by circulating the substrate 9 through the liquid 2. When a portion of the disk 31 enters the liquid 2, the solid content 3 adheres to and bonds with the disk 31. As the disk 31 continues to rotate, there is further accumulation of the solid content 3. At the end of a quarter rotation, the accumulated layer 14 can have a thickness, t, of between 0.5 and 10 mm. The thickness, t, depends on the solid content 3 and the flow rate, F. The driving force used to drive the liquid 2 through the substrate 9 and cause the deposition of the solid content 3 is generated by a differential level, , between the outside and inside of the filter, i.e., between the first and second faces 10, 11. The driving force is typically between 3 cm and 5 cm (i.e., 3 cm 5 cm), but can be as high as 0.4 m (0.4 m). The filtration process is characterized by a fluid flow through a specific substrate surface and can be defined in terms of a flow rate, F (measured in l / (m2h) or "LMH"). The filtration cycle allows flow rates equal to or greater than 250 l / (m2h) and equal to or less than 5,000 l / (m2h) at the maximum level difference, i.e., for = 40 cm. The rotation (i.e., circulation) of the disc 31 is used not only to deploy the substrate 9 in the liquid 2 and thereby allow the dynamic membrane 14 to form, but also to remove the substrate 9 from the liquid 2 and control the formation of the dynamic membrane 14. The deposition of solid content 3, if not controlled, can continue until substrate 9 becomes clogged and partially dehydrated. To prevent dehydration, a filtrate spray 16 (i.e., filtered fluid 4) or fluid from another source may be applied to the inner surface 11 of the substrate 9 above or below the fluid level, L. Internal spraying 16, in combination with biological aeration or additional machine aeration, can help prevent excessive solids buildup. The combination of rotation, aeration, an external filtrate spray 19 (or other clean fluid), height differential, substrate pore size (or "aperture size"), and internal spraying helps maintain the desired thickness, t, of the dynamic membrane 14 for a given application. Table A below sets out different programs (adjustment of rotation speed and time, use of backwashing and aeration) for filtration. Table A A thicker dynamic membrane 14 can be used to increase the degree of filtration (i.e., to improve separation). The maximum level differential and dynamic membrane thickness can be selected to achieve the required degree of separation and filtrate quality. As disc 31 rotates further, the dynamic membrane 14 is pulled out of fluid 2 and exposed to air. The film of fluid 17 between substrate 9 and dynamic membrane 14 allows membrane 14 to detach from substrate 9 by gravity and fall back into fluid 2. Spraying can help create a membrane-free substrate area. A film of water may be present on the membrane-free substrate. A combination of gravity and internal and external spraying can create a membrane-free substrate area that may have a water film. With reference also to Figure 8, the filtering apparatus 1 includes a computer system 41 comprising at least one processor 42, memory 43, non-volatile storage 44, and a network interface 45. The computer system 41 executes the control program 46, which is loaded into memory 43 from storage 44 and executed by the processor(s) 42. The computer system 41 controls one or more motors 47 that drive the rotation of the series of filters and pump(s) and / or valves 48 for the spray bars 15, 18. The computer system 41 can receive signals from sensors 49, for example, level sensors. Control program 46 can be used to switch the machine between different operating states according to the level differential. For example, for a relatively large level differential, program 46 can make discs 31 rotate faster and use more intense spraying and aeration. For a relatively small level differential, program 46 can make discs 31 rotate more slowly (or not at all) and use intense spraying (or none at all) and / or less aeration (or none at all). Excessive solids deposition can lead to dehydration of the dynamic membrane 14 and result in cake filtration. Cake filtration can cause localized cake dehydration, resulting in a dense, fixed, immobile layer. The external spray bar 18 can be used to remove the cake. The spray bar 19 is angled to cut the cake away from the substrate 9 rather than breaking it up and forcing it through the substrate 9. This form of recovery cleaning can be used, for example, if the level differential cannot be sufficiently reduced otherwise. The deposited layer 14, once removed, falls back into fluid 2 where it mixes with the fluid (it is called "resuspended") and is redistributed within fluid 2. The filtration machine 8 can process fluid 2, which has a suspended solids content of 2 g / l, 20 g / l, or 40 g / l, and can even rise to a localized suspended solids content of 50 g / l in the vicinity of the filters. The dynamic membrane 14 has been measured above 3-5% dry matter (DM) and falls into fluid 2 (5% DM = 50 g / l). Structure for a rotary disk filter With reference to Figure 9, a perspective view of a rotary disk filter structure 50 is shown. The disks 31 are coaxially separated from each other. Each disk 31 carries a multiplicity of filter sector face pairs (not shown) and communicates with a central interior space. Each disk 31 is enclosed by a cylindrical outer sleeve 52. The rotary filter comprises two circular end plates 53, each of which is provided in the center of an opening 55 for a rotating support. A cylindrical portion 54 separates two adjacent disks 31 and, in turn, delimits the central interior space of the disk filter. The end plates 53 can contain filters to increase the filter area. This can be used when only a few disks 31 are used. For example, by providing filters on the end plates in a four-disc drive, the number of filter faces increases from six to eight. With reference now to Figure 10, a perspective view is shown of a longitudinal member of the structure that combines with internal cylindrical portions of the same structure. The longitudinal members 56 (referred to herein as "vertical members") extend radially from the cylindrical portions 54. The set of longitudinal members 56 shown in Figure 10 is one of twelve sets of longitudinal members 56 provided in the structure. As shown in Figure 14, the longitudinal members 56 are spaced at 30-degree intervals around a central axis. Each longitudinal member 56 is fabricated from successive square-wave modular components, as shown in more detail in Figure 11, which is an enlarged detail of Figure 10. Figures 12 and 13 are a perspective view of an assembled and disassembled modular component, respectively, of the structure, which is combined with an interior cylindrical portion of the structure. With reference to Figures 12 and 13, the modular component, which is shaped like a square wave, is combined with an inner cylindrical portion. Specifically, each square wave is manufactured with a U-shaped element 58. The U-shaped element 58 has a wave channel 59 that is fixed to the cylindrical portion 54, and an ascending wave edge 60 and a descending wave edge 61 to support the consecutive filter sectors. Each U-shaped element 58 also has a wave crest 63 that interacts with the other wave crests, which are equidistant from one or the other circular end plate 53, to support a portion of the outer cylindrical sleeve 52 (Figure 9). Each 'U' shaped element 58 preferably has an upward wave edge 60 and a downward wave edge 61 to which support guides 64 are welded, which are channel-shaped with a 'U' shaped cross-section to support the face filter sectors 9. A pair of rings, specifically a proximal ring 65 and a distal ring 66, separated from each other, are mounted on each side of the cylindrical portion 54 to support the lower ends of the support guides 64. Each proximal ring 65, which is in contact with the inner cylindrical portion 54, is provided with angularly equidistant grooves 67, the number of which is equal to the number of longitudinal members of the structure. Accordingly, the wave ring 59 has a pair of grooves 68 coaxial with the rising wave edge 60 and the falling wave edge 61. The grooves 68 are designed to fit with a pair of angularly equidistant grooves 67 in the ring in contact with the cylindrical portion 54. The U-shaped element 58 also has the wave crest 63, which is adapted for connection to a subsequent U-shaped element 58. Fittingly, the wave crest 63 supports face plates 69, which are adapted to support the outer cylindrical sleeve portion 52. Figures 14 and 15 are a partially exploded perspective view of the structure according to Figure 12 and an assembled perspective view of the structure according to Figure 14. With reference first to Figure 14, modular components 57 are added for each inner cylindrical portion 54, of which there are twelve in the present embodiment. The modular components 57 are welded together sequentially to form the assembled structure 50 of Figure 16. It should be evident that the modular component at one end has a ridge 63 for joining to the circular end plate that is added to the ridge 63 of its 'U'-shaped element. The stable and robust 50 structure allows for quick replacement of the filter sectors. Filtration apparatus Figure 16 is a perspective view of a first example of the rotary disc filter machine 8 for filtering a liquid 2. In the following description, the liquid 2 to be filtered is referred to as wastewater for simplicity. The rotary disc filter machine 8 can be used to thicken the liquid 2. With reference to Figure 16, the rotary disc filter machine 8 (hereafter referred to as the "rotary disc filter" or simply the "filter") is placed inside a container 5 (or "vessel") resting on a platform 70. Air feeders 6 are placed at the bottom of the vessel 5. The level L of the wastewater 2 to be treated covers a large part of the filter 8. The filter 8 comprises a structure 50, the nature of which will become clearer from the rest of the description, which is rotatably mounted within a frame 73. More precisely, it is this frame 73 that rests on the platform 70. A platform is not required. For example, the frame 73 may be fitted with legs (not shown) that rest on the floor of the vessel 5, or the frame may be fixed to the wall(s) of the vessel 5. The frame 73 supports a cover 74 with trapdoors constructed of metal. The filtration machine 8 is equipped with a filtered water supply system 75. A first arrow F indicates the direction of rotation of structure 50 within the filter machine 8. A set of second arrows U schematically indicates the flow outlet of filtered water 4 from one side (or "end") of the filter machine 8. Figure 17 is a schematic side view, in partial section at the top, of the structure 50 of the filtering machine 8 and its backwashing apparatus. With reference to Figure 17, there is a similar flow outlet for filtered water 4 on the opposite side of the filter machine 8. The flow outlet U from both sides of the filter machine 8 is transferred in a tube 76 that connects to a flange 77. Alternatively, the filter machine 8 can be implemented with a single outlet U. Figure 18 shows part of structure 50 in greater detail. With reference to Figures 17 and 18, the structure 50 is formed by two circular end plates 53 mounted on external supports 79, 80 that are rotatable about an x-axis by means of a geared motor arrangement 47 and transmission 81. The external supports 79, 80 are located in openings 87 that are centrally positioned in the circular end plates 53 to create the two opposing flow outlets U for the filtered water. Clearly, in the case of a single outlet U, a single opening 87 in a single end plate is sufficient. For simplicity, this embodiment is not illustrated. A plurality of disks 31 are fixed coaxially to the two circular end plates 53. The disks 31, in plan view, take the form of a circular ring with an inner radius ry and an outer radius R. Each disk 12 carries, between the inner radius r and the outer radius R, a multiplicity of filter sectors 9, oriented face to face with each other and delimiting an internal disk compartment 84 that is peripherally closed at the outer radius R by means of a cylindrical sleeve 52 and opens at the inner radius r towards a shared inner space 35 that is formed by the cylindrical portions 54 that separate one disk from another. Each filter sector 9 comprises, in a support frame, a fabric, mesh or substrate or filter material having holes or openings in the range of 2 to 40 µm. Including manufacturing tolerances, the pores have a size of 2 ± 1 µm, 4 ± 1 µm, 6 ± 1 µm, 8 ± 1 µm, 10 ± 1 µm, 12 ± 1 µm, 14 ± 1 µm, 16 ± 1 µm, 18 ± 1 µm, 20 ± 1 µm, 22 ± 1 µm, 24 ± 1 µm, 26 ± 1 µm, 28 ± 1 µm, 30 ± 1 µm, 32 ± 1 µm, 34 ± 1 µm, 36 ± 1 µm, 38 ± 1 µm or 40 ± 1 µm. Tests were performed on the filtration machine using nine filter sectors with different pore sizes. The resulting flow rate and filtrate quality (expressed in terms of TSS) are set out in Table B below. Table B The filter cloth or material is selected from the group that includes polyester, polyethylene, PTFE, stainless steel, and nylon. In Figure 17, the arrows ar indicate the liquid 2 to be filtered, the level of which in the vessel is identified as L. Air bubbles 7 are supplied from the air feeders 6 (Figure 16). The air bubbles 7 can be macrobubbles, microbubbles, or nanobubbles. It should be noted that the liquid 2 to be filtered could be different from the wastewater. The supply of air bubbles 7 creates turbulent movement outside the filter 8. This movement can help stabilize the thickness of the sludge layer 14 (Figure 1) that forms on the outer surface of the filter sections 9 and the flow rate that the filter 8 can achieve. The filtered water 4 has a level Lf that can rise in the tube 76. The supply system 75 for providing filtered water includes a reservoir 78 that communicates with the inside of the filter 8 through the opening 87. It should be understood that the level Lf of the filtered water in the reservoir 78 is the same as the level of the filtered water inside the filter disc 31. This is because the flow of filtered water, which flows by gravity from the opening 87, is due to the fact that the lower edges of the external filter outlet tubes, which are oversized relative to the flow rate and therefore practically empty, determine the level of the filtered water Lf and simultaneously determine the internal level, which is the same. When the filter is submerged in the container 5, the level of filtered water Lf is below the water level L by an amount that depends on the dimensions of the filter. This difference in level causes the water to move from the inside to the outside of the filter.Essentially, at the moment of immersion of the machine in the container, a hydraulic differential pressure is generated inside the container, which causes the water to pass through the filter membranes. Normally, the liquid 2 to be filtered contains suspended solids with a concentration of 1 g / l, 40 g / l, or even 50 g / l of TSS. The distance between the level L of the liquid 2 to be filtered in vessel 5 and the level Lf of the filtered liquid 4 within the rotary filter is between 0 and 0.4 m; therefore, a reduced pressure is generated within the same range inside the rotary filter. A deposited layer with a thickness between 0 and 0.06 m forms on the filter membranes. The flow of the filtered liquid 4 is between 0 l / (m2h) and 5000 l / (m2h) with a content of suspended solids in the filtered liquid 4 between 0 and 10 mg / l. The rotational speed of the discs is between 0 and 1,047 rad / s (10 rpm). The number of discs (31) is chosen within the range of 1 to 40, and their outer diameter within the range of 0.5 m to 4 m. Figures 19 and 20 show the supply system (75) for providing filtered water for washing operations. Figure 16 is a perspective view of the rotary disc filter with parts removed for clarity. With reference to Figures 19 and 20, the supply system 75 comprises a pump 48 that is submerged in the tank 78 with its lower end below the Lf level of the filtered water 4. The supply system 75 for supplying filtered water 4 includes, in addition to the pump 48, a series of instruments for its operation, specifically pressure gauges 89, pressure switches 90, a cartridge filter 91, which is provided in a transverse section of the tube 92, which extends towards the backwash apparatus 93 and towards a longitudinal supply tube 97 of the washing apparatus described below. The backwash apparatus comprises a first L-shaped connector 95, which leads to a second L-shaped connector 96, which in turn leads to a backwash connecting tube 97 that passes through the shared interior space 35 and is oscillatingly mounted around a bracket 98 on the second L-shaped connector 96. The bracket 98 is located at opposite ends of the filter machine 8, near its circular end plates 53. The connecting tube 97 is coaxial with the x-axis of the filter machine 8 structure 50 (Figure 17). The circular end plate 53 is shown without any filter sector. A plurality of spray tubes 99 extend orthogonally from the backwash connecting tube 97 and into the inner compartment of the disc 84. An optional oscillation device may be provided on each of the circular end plates 53 (only one of which is shown in Figure 16) at the second 'L'-shaped connector 96 near the connecting tube 97, to cause rotation of the backwash connecting tube 97 and thus of the spray tubes 99 about supports 98 that are coaxial with the x-axis. The arrangement of the backwash apparatus 93 described above is shown more clearly in the perspective view of Figure 17. In this figure, it can be seen more clearly that each spray tube 99 (Figure 19) has a plurality of opposing nozzles 15 (Figure 1) directed towards the filter sectors 9 (not shown in Figure 20). Figures 21 and 22 are a left-hand view of the rotary disc filter showing the backwash apparatus 93 in the first and second positions, respectively. A device 101 for oscillating the backwash connection tube 97 is optionally provided in the opening 87 to communicate with the shared interior space 35. The oscillating device 101 comprises a toothed ring 102 fixedly mounted on the outer surface of the opening 87. The toothed ring 102 is engaged with an eccentric pinion 103 rotatably mounted on the backwash apparatus near the backwash connection tube 97. It should be understood that counterclockwise rotation of the filter structure 50 8 results in rotation of the eccentric pinion 103 in the opposite direction, causing the backwash connection tube 97 to oscillate about the support 98 in the two positions shown in Figures 21 and 22. This movement is produced by a toothed cam and an eccentric cam follower.As a result of this oscillating motion, the filtered water jets from nozzles 15 of spray tubes 99 do not always strike the filter sectors 9 at the same point, but rather provide a substantial backwashing effect over the entire surface of these sectors. To achieve this, the number of teeth on the pinion and ring gear is chosen such that there are prime numbers among them (i.e., they have no common factors). Therefore, the nozzles do not pass through the same point, thus helping to avoid or prevent the formation of striations on the membranes of the filter sectors. With reference to Figure 23, a variant of the backwash apparatus is shown. The backwash apparatus variant differs from the backwash apparatus described above by virtue of a cam and a cam follower oscillation device 101'. In this variant, the cam is a lobed cam 102', and the cam follower is a roller 103' which, by following the lobed cam 102', causes the same oscillation of the backwash connection tube 97. In this variant, the lobed cam 102' is fixedly mounted in the opening 87, and the roller cam follower 103' is rotatably mounted on the backwash apparatus 93 near the backwash connection tube 97. Figure 24 is a far-left view of the rotary disc filter showing an external disc washing unit. Figure 25 is a perspective view of the spray tubes of the disc washing unit. With reference to Figures 19, 21, and 22, the supply system 75 for supplying filtered water 4 has an extension 94 downstream of the branch to the first L-shaped connector 95. The extension 94 is used to supply a backwashing apparatus 105 (Figure 24) comprising a backwashing connection tube 106, mounted outside the filter structure 50 parallel to the x-axis, and a plurality of spray tubes 18 that connect to the backwashing connection tube 106 and extend at least individually between one disc 31 and another of the structure 50, to act externally on the filter sectors 9 with downward-directed, tangential water jets. Backwashing precedes backwashing.The resulting effect is that the layer of sludge adhering to the outer surfaces of the filter sectors is removed as soon as they emerge from the water to be filtered, due to the rotation of the discs, while very low pressure is exerted perpendicularly on the filter sectors. This can considerably increase the effectiveness of the subsequent backwash, which is intended to clear the pores of the filter sectors of any impurities that may have clogged them. Tangential washing, as well as backwashing, uses or can use the filtered water drawn from one of the outlet tanks by a pump. The air bubble feed creates turbulent movement on and around the filter sectors. This movement helps stabilize the thickness of the sludge layer that forms on the outer surface of the filter sectors and the flow rate achievable with this type of thickener filter. It also ensures the quality of the filtered water through hydrodynamic filtration. The rotating motion of the discs creates a hydraulic / centrifugal thrust of the thickened sludge into the vessel. Filtration apparatus Figure 26 is a perspective view of a second example of the rotary disc filter machine 8 for filtering a liquid 2. The second rotary disc filter 8 is similar to the first example of rotary disc filter 8 except that there are four discs 31. Figure 27 is a partial perspective view of the disc structure 50. Figure 28 is a longitudinal section of the machine being immersed in a vessel. With reference to Figures 26 to 28, the filter structure 50 comprises a plurality of discs 31 including first and second end discs 310, 311. The discs 31 are in the form of a circular ring with an inner radius r, and an outer radius R. The number of discs is in the range of 1 to 40, and their diameter is 0.5 to 4 m. When the number of discs is one, the machine could be called a drum machine. The end discs 310, 311 comprise a circular end plate 51, which is reinforced with supports 108 arranged radially confluent (i.e., "fused") in a flange opening 109. Each disc 31 carries, between the inner radius r, and the outer radius R, a multiplicity of filter sectors 9, oriented face to face with each other and delimiting an inner disc compartment 84. The inner disc compartment 84 is peripherally closed at the outer radius R by a cylindrical sleeve 52 and opens at the inner radius r into a shared inner space 35 formed by cylindrical portions 54 (Figure 19) separating one disc from another. Figure 29 is an enlarged schematic detail of a portion of an end disk 31. With reference to Figure 29, the filter material 114, made of mesh, substrate, or other filter material, is thicker at its periphery at an edge 115, which is received and held in a trapezoidal frame 116. On the side of the filter material 114 facing outwards from the disc 31, and also on the same side of the other discs, a cake or layer 14 forms, deposited onto the filter cloth or material with a thickness between 0 and 0.06 m. The trapezoidal frame 116, which holds the filter sectors 9 on the disc and is generally 0.007 m wide, allows the deposition of the thick layer or cake, which contributes to the filtration efficiency. Additionally, the thickness of the cake is one of the factors that affect the outflow of machine 8. A peculiar converging shape of the edges allows the cake to remain in position to counteract the effect of the disk rotation and the attraction of gravity.The cake thickness is controlled by a control unit that measures the outlet flow and the level of water to be filtered into vessel 5 and determines the intensity of aeration with macrobubbles, microbubbles and nanobubbles in vessel 5. The circular end plates 51 are mounted on an external support 79, 80 (Figure 28) and can be rotated about an x-axis, preferably by means of two gearboxes 119 and the power transmission shaft assemblies 120, which are shown schematically at opposite ends of the machine along the x-axis. It is intended that two assemblies share the rotational load of the filter structure 50 and achieve more balanced rotation. Figure 30 is an enlarged view of the external support 79. With reference to Figure 30, the external support 79 (and, likewise, the other external support 80) supports a flow outlet opening 121 as a tubular joint connected by a first flange 122 to the flange opening 159 of the circular end plate 51. The external support 79 preferably has a sliding friction or bushing 123 in a bushing bearing sleeve 124 welded to a second flange 125 attached to the machine frame. The bushing 123 is internally close to the flow outlet opening 121 and has at each end a proximal labyrinth seal 126 and a distal labyrinth seal 127. The labyrinth seals are coaxial to the flow outlet opening 121 and to the bushing bearing sleeve 124. Figures 31 and 32 are an exploded and assembled perspective view of the external support 79, with some parts omitted for clarity. The flow outlet opening 121 is divided into a proximal segment 128, which is intended to house the proximal labyrinth seal 126 by means of tabs 129, an intermediate segment 130 adjacent externally to the bushing 123, and a connecting segment 131, which is intended to be inserted into a separate segment 132, on which a gear 133 is mounted, forming part of the power transmission shaft 120 (Figure 30). The connecting segment 131 and the separate segment 132 are securely connected by means of threaded couplings 138 in the corresponding holes 134, 135. In the separate segment 132 there is a distal segment 136, which is intended to house the distal labyrinth seal 127 by means of tabs 137.On the outer sleeve 124, lubrication nozzles 139 are provided, which are connected by means of channels 140 to the internal space of the distal and proximal labyrinth seals 126, 127 and to the inner part of the bushing that is delimited by the flow outlet opening 121. In the flow outlet opening 121, rigid stops, which are not described in detail, are provided to stop the bushing 123 and the labyrinth seals 126, 127. Figure 33 is a perspective view of an internal element of the labyrinth seal. Figure 34 is an end view of the internal element of the labyrinth seal shown in Figure 33. Figure 35 is a section of the internal element of the labyrinth seal, taken along line AA, and Figure 36 is an enlarged view of the section. Figure 37 is a perspective view of an external element of the labyrinth seal. Figure 38 is an end view of the external element of the labyrinth seal shown in Figure 37. Figure 39 is a section of the external element of the labyrinth seal, taken along line BB, and Figure 40 is an enlarged view of the section. With reference to Figures 33 to 40, the labyrinth seals 126, 127 are composed of grooved coaxial elements. Each labyrinth seal comprises the inner ring 141 with external grooves and the outer ring 142 with internal grooves. The inner ring 141 and the outer ring 142 are mutually coaxial and couple to define an internal space between the rings. The inner ring 141 is rigidly connected to the flow outlet opening 121 and to the separating segment 132 by tabs 129, 138 and is secured by grooves 143. The outer ring 142 is rigidly connected to the bushing bearing sleeve 124 by coupling means inserted into corresponding holes 144 machined into the outer ring 142 and the bushing bearing sleeve 124, respectively. Figures 36 and 40 show the preferred sizes for slotted profiles to help ensure optimal sealing. The inner and outer rings 141 and 142 are preferably made of Teflon (RTM) due to its low friction and long service life characteristics. Because of the material's elasticity, the outer ring 142 is cut to fit onto the inner ring 141, and the resulting labyrinth seal assembly is then mounted on both bracket 79 and bracket 80. Referring again to Figure 28, the shared internal space 35 of the filter structure 50 is divided by a wall 145, which hydraulically separates it into two distinct regions. Because of this division of the internal space, in the event of a mechanical failure of one filter sector, the machine can continue operating by isolating the portion containing the filter disc from the broken sector. To this end, each of the two parts is equipped with a respective turbidity sensor 146 (Figure 26) to monitor the turbidity of the filtered liquid 4, and motorized valves 147 to shut off the flow of the filtered liquid 4. The motorized valves 147 are located in the filtered liquid outlet pipe 148. Each outlet pipe 148 is connected upstream to a filtered liquid tank 149. It should be understood that the shared interior space 35 of the filter structure 50 can be divided into more than two parts to obtain better performance in the event of a mechanical failure of a filter sector. The liquid 2 that is to be filtered contains suspended solids with a concentration from 1 g / l to 50 g / l. The distance between the level L of the liquid 2 to be filtered in the vessel and the level Lf of the filtered liquid 4 inside the rotary filter is between 0 and 3 m; therefore, a reduced pressure is generated within the same range inside the rotary filter. A deposited layer with a thickness between 0 and 0.06 m forms on the filter membranes, as previously mentioned. The flow of the filtered liquid 4 is between 0 l / (m2h) and 5000 l / (m2h). The suspended solids content in the filtered liquid 4 is between 0 and 50 mg / l. The rotational speed of the discs is between 0 and 1,047 rad / s (10 rpm). As explained above, feeding air in the form of macrobubbles, microbubbles, or nanobubbles by means of air supply 6 helps to create turbulent motion outside the filter structure 50. This motion can help to stabilize the thickness of the sludge layer 14 that forms on the outer surface of the filter sectors 9 and the flow regime that can be achieved by the filter structure 50. At both ends of the filtration machine 8, a supply system 75 is provided to supply filtered water 4, which is contained in the reservoir 149 that communicates with the interior of the filter structure 50 through the flow outlet opening 121 (Figure 30). It should be understood that the level Lf of the filtered water in the reservoir 149 is the same as the level of the filtered water inside the filter structure 50. When the filter is immersed in the container 5, the level of filtered water Lf is below the level L of the water to be filtered by an amount that depends on the dimensions of the filter structure 50. Essentially, at the moment of immersion of the machine in the container, a hydraulic differential pressure is generated inside the container, causing the water to pass through the filter sectors 9. Figures 26 and 28 also show the supply system 75 for providing filtered water for washing operations. The supply system 75 comprises a pump 150 submerged in the reservoir 149 with its lower end below the level Lf of the filtered water 4. It can be seen that the supply system 75 for providing filtered water 4 includes the pump 150 as well as a series of instruments for its operation, including a cartridge filter. These instruments are provided in a cross-section of the pipe 151, which leads to a backwash apparatus 152 (Figure 41). A backwash apparatus 105 is also provided with nozzles 153 (Figure 28) that are positioned externally to the discs above the level L of the liquid to be filtered into the vessel V. With reference to Figure 41, the backwash apparatus 152 includes separate tubes 154 for each part of the shared interior space 35 of the filter structure 50. A plurality of spray tubes 155 are derived orthogonally from the backwash connecting tube 154 and extend into the inner compartment of the disc 84. Each spray tube 155 has a plurality of opposing nozzles 156 that are positioned above the level L of the water or liquid to be filtered 4 and directed towards the filter sectors 9. If necessary, external disc washing can be activated. The resulting effect is that the layer of sludge adhering to the outer faces of the filter sectors is removed as soon as they emerge from the water to be filtered, due to the rotation of the discs, while very low pressure is applied perpendicularly to the filter sectors. This considerably increases the effectiveness of the subsequent backwash, whose sole purpose is to clear the pores or openings of the filter sectors 9 of any impurities that may have clogged them. Tangential washing, as well as backwashing, uses filtered water drawn from one of the outlet tanks by a pump. Depending on the liquid to be filtered, it may be advantageous to maintain a deposited layer on the filter material 114 of the filter sector 9. It is important that the filter material 114 is not forced to become concave into the inner space of the disc in the frame 116 of the filter sectors 9. To address this problem, which is caused by the filtrate flow outlet but exacerbated by fabric washing and the reduced pressure created in the internal space 83 of the discs, a pressure equalization tube 157 is provided, connecting the internal space 83 of the discs to the external atmosphere of the machine. The pressure equalization tube 157, which has a bent inner end 158, has a slightly inclined length 159 around the flow outlet opening 21, preferably inclined at 3° to the horizontal, which can aid with drainage.This connection with the external environment allows for an increase in internal pressure and a reduction in the concavity of the filter material 114 in the filter sector, which could lead to reduced performance and possibly mechanical failure of the filter material 114 itself. If atmospheric pressure is insufficient to counteract the concavity determined by the flow of filtrate through the filter sectors, the pressure compensation tube 157 is connected to a pump to obtain, locally, for example, in the vicinity of an internal surface of the filter material 114 in the filter sector, a pressure higher than atmospheric pressure. Although the above description refers to a disc filter machine, it can be implemented as a drum when the number of discs is one. While intended for wastewater treatment, it should be understood that other liquids can also be treated, and obviously, washing and backwashing must be carried out with the filtered liquid. The filter machine's components can also act as thickeners. By extracting the filtered liquid from a container holding a mixture of liquid and solid or semi-solid substances, the effect of increasing the concentration of these substances in the container is also achieved. Monitor fluid levels As explained above, the flow rate and filtration quality can be adjusted by varying the levels L and Lf of the liquid to be filtered and the filtered liquid 2 and 4. These levels can be varied independently of each other. With reference to Figures 9, 17, and 18, and also to Figures 43 to 47, the level, L, of the liquid to be filtered must not rise above a maximum level, Lmax, which corresponds to the inner radial portion of the filter element 9 (Figure 9) or the top of the cylindrical portion 54 (Figure 17), as this may impede the ability of the backwashing apparatus 93 (Figure 20) to properly clean the filter element 9. The filtering level, Lf, can be controlled so that it is between a minimum level, Lfmin, and a maximum level, Lfmax, by using a number of different arrangements. With reference to Figure 18, the portion below pipe 76 can serve as a spillway that can be used to control the Lf level. With reference particularly to Figure 43, in a first modified arrangement, the filtrate level Lf can be controlled by using an internal spillway 161 (or "wall") in the tank 78. The filtrate level Lf passing through the spillway increases marginally at higher flow rates. With reference particularly to Figure 44, in a second modified arrangement, the filtration level Lf can be controlled by using valve 147 which can be in the form of a proportional flow valve. The filtration level can then be controlled by using a pump that pumps the filtrate out of tank 78 to a desired level. With reference particularly to Figure 45, in a third modified arrangement, the filtration level Lf can be controlled by using a suction tube 162 that depends on a suction pump 164. The filtration level Lf can be detected by using a level sensor 163. With reference particularly to Figure 46, in a fourth modified arrangement, the filtrate level Lf can be controlled by using a submersible pump 164 that pumps the filtrate to a higher level through the tube 165. A level sensor (not shown) can be used. With reference particularly to Figure 47, in a fifth modified arrangement, the filtrate level Lf can be controlled by using a stepped suction tube arrangement 166, 167, 168 that relies on a suction pump 164. The stepped suction tube arrangement 166, 167, 168 extends into the inner compartment 84, below the lower part of the cylindrical portion 54. A level sensor (not shown) can be used. Therefore, the height Lf of the filtration can be variably adjusted to improve the performance of the filtration machine 8 by controlling the height difference, , through the filter element 9, with or without controlling the level to be filtered, L. Experimental results Tables I, II, III and IV show the results of using the filtering machine under different operating conditions. In the tables, columns A through K are as follows: A is the pressure difference between the inlet and outlet of the machine, expressed in cmH2O B is the flow rate at the machine outlet, expressed in m3 / h C is the flow rate, expressed in l / m2. It is the flow rate (column B) that is divided by the active surface area of ​​the machine (column H) and converted from cubic meters to liters. D is the controller frequency, expressed as %, where 100% = 0.66 rpm at 50 Hz. E is the air flow rate in the container, in m3 / h under normal conditions. F is the water pressure in the backwash pipes, in bar (= 1 kPa) . G is the air flow rate in the membrane bioreactor, in m3 / h under normal conditions. H is the active area of ​​the filter material, that is, the surface area of ​​the filter material that is submerged in water, in m2. It corresponds to the surface area of ​​a machine with four discs of 1700 mm in diameter each, which are submerged up to their axis of rotation. I are suspended solids, in mg / l. J is the turbidity at the machine outlet, in mg / l. K is the electrical energy absorbed by the machine, in kW. Table I Table II Table III Table IV continuation Modifications It will be noted that several modifications can be made to the embodiments described above. Such modifications may involve equivalent features and other features already known in the design, manufacture, and use of filtering machines and their component parts, which may be used instead of or in addition to the features already described herein. The features of one embodiment may be substituted for or supplemented by features of another embodiment. Although the above description has referred to the rotary disc filter intended for the treatment of wastewater, it should be understood that other liquids can also be treated, and obviously the washing and backwashing must be carried out with the filtered liquid. The filter can act as a thickener. Extracting the filtered liquid from a container holding a mixture of liquid and solid or semi-solid substances has the effect of increasing the concentration of these substances in the container.

Claims

1. Apparatus comprising: a container (7) to be filled with the liquid to be filtered, a filtering machine (8), which in turn includes a liquid-permeable filter element (9) having first and second faces (10, 11), the liquid-permeable filter element being at least partially immersed in a liquid, the liquid-permeable filter element being arranged to be circulated through the liquid such that: in a first position, an area of ​​the first face of the filter element is subjected to pressurized liquid, and the pressure across the filter element is greater than 0 and less than or equal to 5.9 kPa; and in a second position, the area is not subjected to pressurized liquid or is subjected to liquid at a lower pressure,at least one nozzle (15) for directing at least one jet (16) from the second face of the filter element through the filter element to the first face of the filter element to remove and / or assist in removing solids accumulated on the first face of the filter element; characterized in that: the apparatus further comprises: a gas feeder(s) (6) for introducing gas bubbles (7) of macrobubble and nanobubble size into the liquid to create turbulent motion externally and in the immediate vicinity of the filter sections.

2. The apparatus of claim 1, wherein the pore size is between 2 and 40 µm.

3. The apparatus of claim 1 or 2, wherein the liquid-permeable filter element (9) includes pores having a range of different pore sizes, the pore sizes being between 2 and 40 µm.

4. The apparatus of any one of claims 1 to 3,wherein the liquid-permeable filtering element (9) comprises a mesh.

5. The apparatus of any one of claims 1 to 4, configured such that, in the second position, the area is above the liquid.

6. The apparatus of any one of claims 1 to 5, further comprising: a container (5) for a liquid (2) up to a level (L) for its operation.

7. The apparatus of any one of claims 1 to 6, further comprising: a control system for providing a height difference between the liquid and the filtered liquid (4) across the filtering element (9) to control the pressure across the filtering element (9).

8. The apparatus of claim 7,wherein the control system comprises a computer system (47); at least one valve (147) and / or at least one pump (164) controllable by the computer system (47) for controlling the outlet of filtered liquid from a reservoir (78).

9. The apparatus of any one of claims 1 to 8, comprising: a reservoir (78) for receiving the filtered liquid (4), the reservoir comprising a spillway (161) having a lower portion and an upper portion and a first height, h, between the lower and upper portions, the spillway dividing the reservoir into first and second portions; an inlet for supplying filtered water to the first portion of the reservoir and an outlet for receiving filtered liquid, the outlet being provided at a second height between the lower and upper portions of the spillway.

10. The apparatus of any one of claims 1 to 9,where the liquid is wastewater.

11. A method for filtering a liquid having total suspended solids of between 1 and 50 g / l by using a liquid-permeable filter element (9) having first and second faces (10, 11) to produce a filtered liquid (4) having total suspended solids of no more than 10 mg / l, wherein the liquid-permeable filter element is at least partially immersed in a liquid and is arranged to be circulated through the liquid, the method comprising: circulating the liquid-permeable filter element through the liquid such that: in a first position, an area of ​​the first face of the filter element is subjected to pressurized liquid, and the pressure across the filter element is greater than 0 and less than or equal to 5.9 kPa; and in a second position, the area is not subjected to pressurized liquid or is subjected to liquid at a lower pressure,and the solids accumulated on the first face of the filter element can be removed by directing at least one jet to the second face of the filter element through the filter element to the first face of the filter element; wherein the filter element has a pore size and the filter element is circulated at a speed such that the permeation flux is between 200 L / (m²h) and 5,000 L / (m²h) and the thickness of a layer of accumulated solids when removed is between 0 and 6 cm, characterized in that the method further comprises: introducing gas bubbles (7) having the size of macrobubbles and nanobubbles into the liquid (2), to create turbulent motion externally and in the immediate vicinity of the filter sectors.

12. The method of claim 11,wherein the filter element has a pore size and the filter element is circulated at a speed such that the permeation flow rate is between 200 L / (m²h) and 5,000 L / (m²h) and the thickness of a layer of accumulated solids when removed is between 0 and 6 cm.

13. A computer program that, when executed by a computer, causes the computer to perform the method of claim 11 or 12.

14. A system comprising: the apparatus of any one of claims 1 to 10; an assembly of one or more sensors arranged to monitor the apparatus; an assembly of one or more actuators and / or an assembly of one or more motors and / or pumps arranged to control the apparatus; and a computer system arranged to receive signal(s) from the assembly of one or more sensors and to provide control signals to control the assembly of one or more actuators and / or the assembly of one or more motors and / or pumps.

15. The system of claim 14,where the computer system is configured to control the rate of solid deposition onto the filter element, the degree of solid compaction, and / or the rate of solid reintroduction back into the liquid.

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