Sewage distribution apparatus

Abstract

Rotational sewage distribution apparatus comprising: a rotatable drive mechanism, flow control apparatus which directs the flow of sewage from the inlet (202 Fig 2) onto the drive mechanism causing th

Description

FIELD The present disclosure relates to a sewage distribution system. Aspects of the disclosure relate to a rotational sewage distribution apparatus; and to an associated sewage filtration system. BACKGROUND It is well-known to treat sewage liquid by passing said sewage liquid through a filter bed formed of filter media. Such treatment systems are commonly termed “trickling filter” systems. One system which has been used for distributing sewage liquid to such a filter bed includes a sewage liquid reservoir which is connected via an inlet pipe to a rotational sewage distribution apparatus. The rotational sewage distribution apparatus includes longitudinally extending distribution channels which are situated overthe filter bed. Sewage liquid flows from the sewage liquid reservoir along the inlet pipe, into the rotational sewage distribution apparatus, along the longitudinally extending distribution channels and onto the filter bed. When a flow of sewage liquid is passed through the rotational sewage distribution apparatus, the rotational sewage distribution apparatus causes rotation of the longitudinally extending distribution channels about a vertical axis. In this way, the longitudinally extending distribution channels rotate over the filter bed and thereby distribute the sewage liquid overthe entire surface of the filter bed overtime. Such a system is described in UK patent number GB2181187B. It shall be appreciated that the operation of the rotational sewage distribution apparatus is often tailored to achieve a desired flushing intensity to optimise performance of the sewage filtration system. However, in some circumstances, the longitudinally extending distribution channels may become imbalanced thereby causing the rotational sewage distribution apparatus to “see-saw” which can lead to an inconsistent distribution of sewage liquid across the filter bed. This issue is particularly prevalent in systems which feature large filter beds (and hence longer distribution channels) and / or during dry weather periods in which sewage treatment systems tend to experience intermittent flow. It is therefore an aim of the present invention to provide a rotational sewage distribution apparatus which exhibits improved stability during periods of intermittent flow. SUMMARY The present teachings provide a rotational sewage distribution apparatus, a sewage distribution system, and a sewage filtration system according to the appended claims. A first aspect of the disclosure provides a rotational sewage distribution apparatus. The rotational sewage distribution apparatus comprises: a drive mechanism which is rotatable about a rotation axis; a flow control apparatus for receiving a flow of sewage liquid from an inlet pipe and directing the flow of sewage liquid onto the drive mechanism so as to cause the drive mechanism to rotate about the rotation axis; one or more longitudinally extending distribution arms for receiving sewage liquid from the flow control apparatus and distributing said sewage liquid onto a filter bed; and a transmission arrangement operable to convert rotation of the drive mechanism about the rotation axis to rotation of the one or more longitudinally extending distribution arms about a vertical axis, wherein the transmission arrangement comprises: an upper bearing; a lower bearing; and a gear positioned between the upper and lower bearings, wherein the gear defines a radial axis which extends in a direction perpendicular to the vertical axis; wherein the upper bearing is vertically offset from the radial axis of the gear by a first distance (Di), and wherein the lower bearing is vertically offset from the radial axis of the gear by a second distance (D2) which is greater than the first distance (Di). In exemplary embodiments, the second distance (D2) may be at least 50% greaterthan the first distance (Di). In exemplary embodiments, the second distance (D2) may be at least 100% greater than the first distance (Di). In exemplary embodiments, the second distance (D2) may be at least 200% greater than the first distance (Di). In exemplary embodiments, the second distance (D2) may be at least 400% greater than the first distance (Di). In exemplary embodiments, the upper bearing and the lower bearing may be spaced apart in the vertical direction by a distance (Hi) of at least 150mm. In exemplary embodiments, the upper bearing and the lower bearing may be spaced apart in the vertical direction by a distance (Hi) of at least 200mm. In exemplary embodiments, the upper bearing and the lower bearing may be spaced apart in the vertical direction by a distance (Hi) of at least 250mm. In exemplary embodiments, the upper bearing and the lower bearing may be spaced apart in the vertical direction by a distance (Hi) of at least 300mm. In exemplary embodiments, the lower bearing may be vertically offset from the radial axis of the gear by a distance (D2) of at least 75mm. In exemplary embodiments, the lower bearing may be vertically offset from the radial axis of the gear by a distance (D2) of at least 100mm. In exemplary embodiments, the lower bearing may be vertically offset from the radial axis of the gear by a distance (D2) of at least 150mm. In exemplary embodiments, the lower bearing may be vertically offset from the radial axis of the gear by a distance (D2) of at least 200mm. In exemplary embodiments, the lower bearing may be vertically offset from the radial axis of the gear by a distance (D2) of at least 250mm. In exemplary embodiments, the upper bearing may be vertically offset from the radial axis of the gear by a distance (Di) of no more than 70mm. In exemplary embodiments, the upper bearing may be vertically offset from the radial axis of the gear by a distance (Di) of no more than 55mm. In exemplary embodiments, the upper bearing may be vertically offset from the radial axis of the gear by a distance (Di) of no less than 40mm. In exemplary embodiments, the radial axis of the gear may be vertically aligned with the rotation axis of the drive mechanism. In exemplary embodiments, the rotation axis may be a horizontal axis. In exemplary embodiments, the transmission arrangement may further comprise a transmission housing to which the one or more longitudinally extending distribution arms are coupled. In exemplary embodiment, the transmission arrangement may be configured such that the transmission housing and the one or more longitudinally extending distribution arms rotate together about the vertical axis. In exemplary embodiments, the drive mechanism may comprise a paddle wheel. In exemplary embodiments, the transmission arrangement may further comprise an input shaft coupled to the drive mechanism, and a worm screw operable to transfer rotational motion of the input shaft onto the gear. In exemplary embodiments, the drive mechanism may comprise a first paddle wheel coupled to a first end of the input shaft and a second paddle wheel coupled to a second end of the input shaft. In exemplary embodiments, the flow control apparatus may comprise: a first outlet proximal a first side of the flow control apparatus configured to direct the flow of sewage liquid onto the first paddle wheel; and a second outlet proximal a second side of the flow control apparatus configured to direct the flow of sewage liquid onto the second paddle wheel. A second aspect of the disclosure provides a sewage filtration system. The sewage filtration system comprises: the rotational sewage distribution apparatus according to the first aspect of the disclosure; a sewage liquid source; an inlet pipe for providing a flow of sewage liquid from the sewage liquid source to the rotational sewage distribution apparatus; and a filter media tank comprising a filter bed and a vertical shaft fixed to the filter media tank in a centre of the filter bed, wherein said vertical shaft defines the vertical axis about which the one or more longitudinally extending distribution arms rotate. In exemplary embodiments, the filter bed may have a diameter of at least 10m. In exemplary embodiments, the filter bed may have a diameter of at least 15m. In exemplary embodiments, the filter bed may have a diameter of at least 20m. It shall be appreciated that optional features of the first aspect of the disclosure may be combined with the second aspect of the disclosure. BRIEF DESCRIPTION OF DRAWINGS Embodiments will now be described by way of example only with reference to the accompanying figures, in which: Figure 1 is a plan view of a sewage filtration system including a sewage distribution system according to an embodiment; Figure 2 is a cross-sectional view of the sewage filtration system of Figure 1, taken along plane A-A; Figure 3 is a cross-sectional view of a sewage distribution system and a rotational sewage distribution apparatus for the sewage filtration system of Figures 1 and 2; and Figure 4 is a cross-sectional view of a transmission arrangement of the rotational sewage distribution apparatus illustrated in Figure 3. DETAILED DESCRIPTION In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments and the teachings. However, those skilled in the art will understand that: the present teachings may be practiced without these specific details or with known equivalents of these specific details; that the present teachings are not limited to the described embodiments; and, that the present teachings may be practiced in a variety of alternative embodiments. It will also be appreciated that well known methods, procedures, components, and systems may not have been described in detail. References to vertical and horizontal in the present disclosure should be understood to be in relation to the respective apparatus / system when installed and in use. Referring firstly to Figures 1 and 2, a sewage filtration system is indicated at 300. The sewage filtration system 300 includes a sewage liquid source 302, a filter media tank 304, a filter bed 306 provided within the filter media tank 304, and a sewage distribution system 200. The sewage distribution system 200 has an inlet pipe 202 for providing a flow of sewage liquid from the sewage liquid source 302 to a rotational sewage distribution apparatus 100. In this embodiment, the sewage liquid source 302 is a sewage liquid reservoir, and the inlet pipe 202 is coupled to an opening in a side of the sewage liquid reservoir 302, so that sewage liquid can flow passively along the inlet pipe 202 under the force of gravity. In alternative embodiments, a pump or other device may be used to actively drive sewage liquid along the inlet pipe 202. The inlet pipe 202 is arranged above the rotational sewage distribution apparatus 100 so that sewage liquid can enter an apparatus inlet of the rotational sewage distribution apparatus 100 from above (as will be described in more detail below). In the illustrated embodiment, the inlet pipe 202 is supported at one or more different positions along its length by a support structure 204. The illustrated support structure 204 includes a vertical post 206, and one or more support arms 208 coupled between the inlet pipe 202 and the vertical post 206. In particular, the support structure 204 includes support arms 208 which are coupled to the inlet pipe 202 at different positions along a length of the inlet pipe 202. In the illustrated embodiment, there are three support arms 208, but other suitable numbers of support arms 208 may be provided. In other embodiments, any other suitable support structure 204 may be used. The rotational sewage distribution apparatus 100 includes one or more longitudinally extending distribution arms 114. In particular, the illustrated rotational sewage distribution apparatus 100 includes a pair of longitudinally extending distribution arms 114, although it will be appreciated that in other embodiments there may be more or less than two longitudinally extending distribution arms 114 (e.g., one, three, four, five or more). The longitudinally extending distribution arms 114 are arranged above the filter bed 306 and each define a respective channel 115 extending along a length thereof. In this way, sewage liquid which is input from the inlet pipe 202 to the rotational sewage distribution apparatus 100 can be distributed along the longitudinally extending distribution channels 115 for distribution onto the filter bed 306. It shall be appreciated that the filter bed 306 will typically feature a diameter of at least 10m, optionally at least 15m, and further optionally at least 20m and, as such, the one or more longitudinally extending distribution arms 114 may each have a length of at least 4m, optionally at least 6m, and further optionally at least 9m. As will be described in more detail below, the rotational sewage distribution apparatus 100 includes a rotation mechanism for rotating the rotational sewage distribution apparatus 100 about a vertical axis 104. In this embodiment, the vertical axis 104 is defined by a vertical shaft 102 which is fixed to the filter media tank 304 in a centre of the filter bed 306. In this way, as the rotational sewage distribution apparatus 100 is caused to rotate by the rotation mechanism, the longitudinally extending distribution arms 114 rotate above the filter bed 306 (e.g., as illustrated by the block arrows on Figure 1, and the dashed lines on Figure 1 illustrating a position at which the rotational sewage distribution apparatus 100 will have rotated after a period of time). This causes sewage liquid to be distributed evenly throughout the filter bed 306. The sewage filtration system 300 described above may be referred to as a “trickling filter” system. Referring now to Figure 3, the rotational sewage distribution apparatus 100 will be described in more detail. Notably, the rotation mechanism which causes the rotational sewage distribution apparatus 100 to rotate includes a drive mechanism which is rotatable about a rotation axis and a transmission arrangement 130 operable to convert rotation of the drive mechanism about the rotation axis to rotation of the longitudinally extending distribution arms 114 about the vertical axis 104 defined by the vertical shaft 102. In the illustrated embodiment, drive mechanism includes one or more paddle wheels 106A, 106B which are rotatable about a paddle wheel axis 108. In particular, the paddle wheels 106A, 106B are rotated by a flow of sewage liquid supplied to the rotational sewage distribution apparatus 100 by the inlet pipe 202, as will be described in more detail below. In the illustrated embodiment, the paddle wheel axis 108 is a horizontal axis, but the paddle wheel axis 108 may be a vertical axis or a transverse axis arranged at an angle to both horizontal and vertical directions. In the illustrated embodiment, there is a left paddle wheel 106A and a right paddle wheel 106B, although it will be understood that more or less than two paddle wheels 106A, 106B may be provided in other configurations. In the illustrated embodiment, the left and right paddle wheels 106A, 106B are coaxially aligned (i.e., they share the same paddle wheel axis 108). In other embodiments, the left and right paddle wheels 106A, 106B have different paddle wheel axes 108 (e.g., parallel but spaced-apart axes). The transmission 130 is configured to translate rotation of the paddle wheels 106A, 106B about the paddle wheel axis 108 to rotation of the rotational sewage distribution apparatus 100 about the vertical axis 104. For example, the transmission 130 may include one or more gears or other mechanical components for converting rotation in one direction to rotation in another direction. The transmission 130 allows work done by sewage liquid rotating the paddle wheels 106A, 106B to be harnessed to rotate the longitudinally extending distribution arms 114 about the vertical shaft 102. In other words, this removes the need for an external energy supply (e.g., electricity for powering an electric motor). In the illustrated embodiment, the rotational sewage distribution apparatus 100 includes an outlet tray arrangement 112 extending beneath the paddle wheels 106A, 106B to receive sewage liquid (e.g., from the paddle wheels 106A, 106B). The outlet tray arrangement 112 is configured to direct the sewage liquid to the longitudinally extending distribution channels 115. In the illustrated embodiment, the outlet tray arrangement 112 includes: a left outlet tray 112A extending beneath the left paddle wheel 106A to receive sewage liquid from the left paddle wheel 106A; and a right outlet tray 112B extending beneath the right paddle wheel 106B to receive sewage liquid from the right paddle wheel 106B. In other embodiments, the outlet tray arrangement 112 may be defined by a single outlet tray. For example, in embodiments where there is a single paddle wheel, a single outlet tray may be used. Alternatively, a single outlet tray may be provided to extend under both the left and right paddle wheels 106A, 106B with a sealing arrangement provided in the centre proximal to the vertical shaft 102, to permit rotation of the single outlet tray around the shaft. Each outlet tray 112A, 112B includes a base surface 122 and one or more sides 124. A lower surface of the longitudinally extending distribution channels 115 is provided at or below the level of the base surface 122 of the respective outlet tray 112A, 112B. In particular, the illustrated configuration has the lower surface of the longitudinally extending distribution channels 115 provided below the level of the base surface 122 of the respective outlet tray 112A, 112B. Further, in the illustrated embodiment the lower surface of the longitudinally extending distribution channels 115 tapers upwards along a length of the longitudinally extending distribution arms 114. Openings 126 are provided in the one or more sides 124 of each outlet tray 112A, 112B. In this way, sewage liquid flows from each outlet tray 112A, 112B to the longitudinally extending distribution channels 115. The longitudinally extending distribution channels 115 may be open channels (i.e., having an open upper end). In this way, once a level of sewage liquid in the longitudinally extending channels 115 reaches the open upper end, the sewage liquid passes over the open upper end and falls onto the filter bed 306 below. In some embodiments, the longitudinally extending distribution channels 115 may include sides with a series of recesses. In this way, sewage liquid is able to pass through the recesses and fall onto the filter bed 306 before the sewage liquid level reaches the open upper end. Alternatively, the longitudinally extending channels 115 may be closed channels (e.g., pipes) with a series of apertures along a length of the channel, such that sewage liquid can pass through the apertures and fall onto the filter bed 306 below. In the illustrated embodiment, a cross-sectional area of each longitudinally extending distribution channel 115 decreases along its length in a radially outboard direction (with respect to the vertical axis 104). In this way, the cross-sectional area reduces as the amount of sewage liquid in the longitudinally extending channels 115 decreases (due to falling out onto the filter bed 306). This allows a velocity of sewage liquid to be maintained along the length of the longitudinally extending channels 115. In the illustrated embodiment, the decreasing cross-sectional area is achieved by having a lower surface of the longitudinally extending distribution channels 115 taper upwards from the respective outlet tray 112A, 112B towards a tip of the longitudinally extending distribution channel 115, as illustrated in Figures 2 and 3. A further decrease in the cross-sectional area of the longitudinally extending distribution channels 115 is achieved by having a narrowing width along the length of the longitudinally extending distribution channels 115, as illustrated in Figure 1. In the illustrated embodiment, the rotational sewage distribution apparatus 100 also includes a flow control apparatus 10 for directing the flow of sewage liquid onto the paddle wheels 106A, 106B. The flow control apparatus 10 includes a housing 12 defining an internal volume 14 for receiving a flow of sewage liquid. The housing 12 also has a cover portion 24 and left and right sides 26A, 26B extending between a base 22 and the cover portion 24. The flow control apparatus 10 includes a housing inlet 28 for receiving the flow of sewage liquid from the inlet pipe 202 into the internal volume 14. In the illustrated embodiment, the housing inlet 28 is an opening in the cover portion 24. The vertical axis 104 passes through the housing inlet 28, so that sewage liquid can fall from the inlet pipe 202 through the housing inlet 28 and into the internal volume 14 of the flow control apparatus 10 regardless of the rotational orientation of the flow control apparatus 10. The flow control apparatus 10 also includes a housing outlet 30A, 30B through which a flow of sewage liquid exits the internal volume 14. In particular, there is a left housing outlet 30A proximal the left side 26A of the housing 12 and a right housing outlet 30B proximal the right side 26B of the housing 12. Each housing outlet 30A, 30B, is configured to direct the flow of sewage liquid from the internal volume 14 onto the respective paddle wheel 106A, 106B to cause rotation of the paddle wheel 106A, 106B about the paddle wheel axis 108. It will therefore be understood that the number of housing outlets 30A, 30B may differ in accordance with variations in the number of paddle wheels 106A, 106B. Alternatively, there may be multiple housing outlets 30A, 30B configured to direct sewage liquid onto the same paddle wheel 106A, 106B (i.e., at different axial positions along a length of the paddle wheel 106A, 106Bwith respect to the paddle wheel axis 108). Referring now to Figures 3 and 4, the transmission 130 of the rotational sewage distribution apparatus 100, indicated at 130, shall be described in further detail. The transmission 130 includes an input shaft 132, a toothed gear (or worm wheel) 134 and an output hub 136 for rotatably coupling the transmission arrangement 130 to the vertical shaft 102 to facilitate rotation of the apparatus 100 about the vertical axis 104. As shown in Figure 3, the input shaft 132 is coupled to the drive mechanism and is configured for transferring rotational motion of the drive mechanism onto the toothed gear 134. In the illustrated embodiment, the input shaft 132 comprises a first end which is coupled to the left paddle wheel 106A and a second end, opposite said first end, which is coupled to the right paddle wheel 106B. As such, in the illustrated embodiment, the paddle wheel axis 108 is defined by the input shaft 132. However, in other embodiments (e.g., in which only one paddle wheel 106 is provided) the input shaft 132 may be coupled to the drive mechanism at only one end thereof. The input shaft 132 is configured to rotate along with the left 106A and right 106B paddle wheels such that, when the paddle wheels 106A, 106B are rotated by a flow of sewage liquid, the input shaft 132 is also caused to rotate about the paddle wheel axis 108. The input shaft 132 is further coupled to the toothed gear 134. In the illustrated embodiment, the input shaft 132 is coupled to the toothed gear 134 via a worm screw 138 which is provided between the first and second ends of the input shaft 134 such that rotation of the input shaft 132 about the paddle wheel axis 108 causes subsequent rotation of the worm screw 138 about the same paddle wheel axis 108. The worm screw 138 comprise a helical thread which is configured to mesh with the toothed gear 134 such that rotational motion of the worm screw 138 about the paddle wheel axis 108 causes subsequent rotation of the toothed gear 134. As shown in Figure 4, the toothed gear 134 is arranged to rotate about the vertical axis 104 and hence the worm screw 138 and toothed gear 134 together allow rotation of the left 106A and right 106B paddle wheels about the horizontal paddle wheel axis 108 to be translated into rotation of the toothed gear 134 about the vertical axis 104. The rotational motion of the toothed gear 134 about the vertical axis 104 is subsequently conveyed onto the output hub 136 thereby causing the rotational sewage distribution apparatus 100 to rotate about the vertical shaft 102. It shall be appreciated that the transmission arrangement 130 will typically feature a gear ratio of at least 20:1 leading to a significant reduction in speed (and a corresponding increase in torque) of the toothed gear 134 compared to the worm screw 138 to help ensure that the longitudinally extending distribution arms 114 pass over the filter bed 306 at speeds which are suitable for achieving a desired flushing intensity. Referring still to Figure 4, the transmission arrangement 130 further comprises a pair of upper 140 and lower 142 bearings positioned along the vertical axis 104 for supporting rotation of the rotational sewage distribution apparatus 100 about the vertical shaft 102. In the illustrated embodiment, the transmission arrangement 130 also features a transmission housing 150 having a top surface 152, a bottom surface 154 and one or more sides 156 which together define an internal volume 158 in which the toothed gear 134, and the pair of upper 140 and lower 142 bearings, are housed. Openings are also provided in the one or more sides 156 of the transmission housing 132 for receiving the input shaft 132. In the illustrated embodiment, the transmission housing 150 comprises a pair of openings provided in the left and right sides thereof, which are coaxially aligned with the paddle wheel axis 108, to enable the input shaft 132 to extend laterally across the housing 150 between the left 106A and right 106B paddle wheels respectively. The transmission housing 150 also features a further opening provided in its bottom surface 154 which is coaxially aligned with the vertical axis 104 to enable the vertical shaft 102 to be received in the output hub 136. However, it shall be appreciated that in other embodiments, the transmission housing 150 may comprise a different number of openings and / or may comprise openings provided in different locations about the transmission housing 150. For example, in some embodiments, the transmission housing 150 may comprise openings in the front and rear sides 156 of the housing 150. Alternatively, in further embodiments, the input shaft 132 may only extend partially across the transmission housing 150 and hence an opening may be provided in only one side 156 of the housing 150. As shown in Figure 3, the transmission housing 150 is coupled to the pair of longitudinally extending distribution arms 114 and, as such, the transmission arrangement 130 (and the associated drive mechanism coupled thereto) are each configured to rotate about the vertical shaft 102 during use, along with the longitudinally extending distributions arms 114. However, it shall be appreciated that in other embodiments, only the longitudinally extending distribution arms 114 may rotate about the vertical shaft 102 whilst the remaining components of the rotational sewage distribution apparatus 100 may remain relatively static. Referring back to Figure 4, the toothed gear 134 is positioned between the upper 140 and lower 142 bearings and defines a radial axis 144 which extends in a direction perpendicular to the vertical axis 104. As shown in Figure 4, in the illustrated embodiment, the radial axis 144 of the toothed gear 134 is vertically aligned (or co-planar) with the paddle wheel axis 108. The upper bearing 140 is positioned above the toothed gear 134 and is vertically offset from the radial axis 144 by a first distance (Di). It shall be appreciated that the first distance (Di) may be no more than 70mm, optionally no more than 55mm, and further optionally no more than 40mm. The lower bearing is positioned below the toothed gear 134 and is vertically offset from the radial axis 144 by a second distance (D2) which is greater than the first distance (Di). In other words, the upper 140 and lower 142 bearings are spaced asymmetrically about the radial axis 144. It shall be appreciated that the second distance (D2) may be at least 75mm, at least 100mm, at least 150mm, at least 200mm, or optionally at least 250mm. As such, the second distance (D2) may be at least 50% greater, at least 100% greater, at least 200% greater, or optionally at least 400% greater than the first distance (Di). Importantly, it has been found that by spacing the upper 140 and lower 142 bearings asymmetrically about the toothed gear 134, the respective bearing gap (Hi) defined between the upper 140 and lower 142 bearings can be increased. For example, in the illustrated embodiment, the upper 140 and lower 142 bearings may be spaced apart in the vertical direction by a distance (Hi) of at least 150mm, at least 200mm, at least 250mm, or optionally at least 300mm. It shall be appreciated that increasing the size of the aforementioned bearing gap (Hi) helps to improve the overall stability of the apparatus 100 and hence helps to prevent the longitudinally extending distribution arms 114 from “see-sawing” during use. Furthermore, since the rotational sewage distribution apparatus 100 of the present disclosure increases the bearing gap size (Hi) by lowering the height of the lower bearing 142, the overall height of the apparatus 100, and hence the drop distance between the apparatus 100 and the sewage liquid source 302, remains un-impacted. This enables a greater amount of liquid flow to be passed through filtration systems 300 which employ the aforementioned apparatus 100, which, in turn, helps to reduce the likelihood of system overflows during periods of high demand. The one or more embodiments are described above by way of example only and it will be appreciated that the variations are possible without departing from the scope of protection afforded by the appended claims. It should also be noted that whilst the appended claims set out particular combinations of features described above, the scope of the present disclosure is not limited to the particular combinations hereafter claimed, but instead extends to encompass any combination of features herein disclosed.

Claims

1. A rotational sewage distribution apparatus comprising:a drive mechanism which is rotatable about a rotation axis;a flow control apparatus for receiving a flow of sewage liquid from an inlet pipe and directing the flow of sewage liquid onto the drive mechanism so as to cause the drive mechanism to rotate about the rotation axis;one or more longitudinally extending distribution arms for receiving sewage liquid from the flow control apparatus and distributing said sewage liquid onto a filter bed; anda transmission arrangement operable to convert rotation of the drive mechanism about the rotation axis to rotation of the one or more longitudinally extending distribution arms about a vertical axis, wherein the transmission arrangement comprises:an upper bearing;a lower bearing; anda gear positioned between the upper and lower bearings,wherein the gear defines a radial axis which extends in a direction perpendicular to the vertical axis;wherein the upper bearing is vertically offset from the radial axis of the gear by a first distance (Di), andwherein the lower bearing is vertically offset from the radial axis of the gear by a second distance (D2) which is greater than the first distance (Di).

2. The rotational sewage distribution apparatus according to claim 1, wherein the second distance (D2) is at least 50% greater than the first distance (Di).

3. The rotational sewage distribution apparatus according to claim 2, wherein the second distance (D2) is at least 100% greater than the first distance (Di).

4. The rotational sewage distribution apparatus according to claim 3, wherein the second distance (D2) is at least 200% greater than the first distance (Di).

5. The rotational sewage distribution apparatus according to claim 4, wherein the second distance (D2) is at least 400% greater than the first distance (Di).

6. The rotational sewage distribution apparatus according to any preceding claim, wherein the upper bearing and the lower bearing are spaced apart in the vertical direction by a distance (Hi) of at least 150mm.

7. The rotational sewage distribution apparatus according to claim 6, wherein the upper bearing and the lower bearing are spaced apart in the vertical direction by a distance (Hi) of at least 200mm.

8. The rotational sewage distribution apparatus according to claim 7, wherein the upper bearing and the lower bearing are spaced apart in the vertical direction by a distance (Hi) of at least 250mm.

9. The rotational sewage distribution apparatus according to claim 8, wherein the upper bearing and the lower bearing are spaced apart in the vertical direction by a distance (Hi) of at least 300mm.

10. The rotational sewage distribution apparatus according to any preceding claim, wherein the lower bearing is vertically offset from the radial axis of the gear by a distance (D2) of at least 75mm.

11. The rotational sewage distribution apparatus according to claim 10, wherein the lower bearing is vertically offset from the radial axis of the gear by a distance (D2) of at least 100mm.

12. The rotational sewage distribution apparatus according to claim 11, wherein the lower bearing is vertically offset from the radial axis of the gear by a distance (D2) of at least 150mm.

13. The rotational sewage distribution apparatus according to claim 12, wherein the lower bearing is vertically offset from the radial axis of the gear by a distance (D2) of at least 200mm.

14. The rotational sewage distribution apparatus according to claim 13, wherein the lower bearing is vertically offset from the radial axis of the gear by a distance (D2) of at least 250mm.

15. The rotational sewage distribution apparatus according to any preceding claim, wherein the upper bearing is vertically offset from the radial axis of the gear by a distance (Di) of no more than 70mm.

16. The rotational sewage distribution apparatus according to any preceding claim, wherein the upper bearing is vertically offset from the radial axis of the gear by a distance (Di) of no less than 55mm.

17. The rotational sewage distribution apparatus according to any preceding claim, wherein the radial axis of the gear is vertically aligned with the rotation axis of the drive mechanism.

18. The rotational sewage distribution apparatus according to any preceding claim, wherein the rotation axis is a horizontal axis.

19. The rotational sewage distribution apparatus according to any preceding claim, wherein the transmission arrangement further comprises a transmission housing to which the one or more longitudinally extending distribution arms are coupled, and wherein the transmission arrangement is configured such that the transmission housing and the one or more longitudinally extending distribution arms rotate together about the vertical axis.

20. The rotational sewage distribution apparatus according to any preceding claim, wherein the drive mechanism comprises a paddle wheel.

21. The rotational sewage distribution apparatus according to any preceding claim, wherein the transmission arrangement further comprises:an input shaft coupled to the drive mechanism; anda worm screw operable to transfer rotational motion of the input shaft onto the gear.

22. The rotational sewage distribution apparatus according to claim 21, wherein the drive mechanism comprises a first paddle wheel coupled to a first end of the input shaft and a second paddle wheel coupled to a second end of the input shaft.

23. The rotational sewage distribution apparatus according to claim 22, wherein the flow control apparatus comprises:a first outlet proximal a first side of the flow control apparatus configured to direct the flow of sewage liquid onto the first paddle wheel; anda second outlet proximal a second side of the flow control apparatus configured to direct the flow of sewage liquid onto the second paddle wheel.

24. A sewage filtration system comprising:the rotational sewage distribution apparatus according to any preceding claim, a sewage liquid source;an inlet pipe for providing a flow of sewage liquid from the sewage liquid source to the rotational sewage distribution apparatus; anda filter media tank comprising a filter bed and a vertical shaft fixed to the filter media tank in a centre of the filter bed, wherein said vertical shaft defines the vertical axis about which the one or more longitudinally extending distribution arms rotate.

25. The sewage filtration system according to claim 24, wherein the filter bed has a diameter of at least 10m, optionally at least 15m, and further optionally at least 20m.A

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