WO2010128283A2

WO2010128283A2 - Rainwater harvesting system

Info

Publication number: WO2010128283A2
Application number: PCT/GB2010/000887
Authority: WO
Inventors: David George; Yewande Akinola
Original assignee: Ove Arup & Partners International Limited
Priority date: 2009-05-05
Filing date: 2010-05-05
Publication date: 2010-11-11
Also published as: WO2010128283A3; GB2470017B; GB2470017A; GB0907705D0

Abstract

A rainwater harvesting system comprises a roof (20) of a building, and a roof rainwater outlet (10) in fluid communication with a water supply system of said building. The rainwater outlet (10) comprises a filter (3) to attenuate the flow of rainwater from said roof (20) through said outlet (10) to said water supply system.

Description

Rainwater Harvesting System

This present invention relates to a rainwater harvesting system and in particular a rainwater filter outlet for use in such a system.

It is well known to collect water from the rooftops or other surfaces of buildings. Traditionally rainwater falling on a roof is diverted into a system of gutters and drainpipes which carry the rainwater into the sewers. It is usually desired to remove water from the roof as quickly as possible to reduce the chances of water leaking into the building.

Heavy rainfall can be problematic as it can cause 'instantaneous runoff or 'flash flooding'. These terms refer to flooding that occurs when sewers overflow due to a sudden, large flow of water. This problem is especially acute in urban areas, as there is much less permeable ground into which the rain can soak naturally, hi urban areas, the majority of the ground is covered by buildings, roads, pavements etc., into which water cannot soak.

More recently, due to environmental and economic considerations, rainwater has been collected and used, rather than being directed straight into the sewers. The collected rainwater can be used for outdoor purposes such as the irrigation of gardens or can be used domestically within buildings for non-drinking purposes, such as for flushing toilets. Use of rainwater in this way, reduces the amount of (potable) mains water required. This has economic and environmental benefits.

Traditionally, rainwater collected for use within a building is stored in a tank somewhere within or nearby the building. When the level of rainwater in this tank is not sufficient, mains water can be added to the tank to increase the level. The water from this tank is then supplied to, for example, the toilet to be flushed. Often the water will need to be pumped from this tank to its destination.

The tank, associated piping and pumps may be rather bulky and inconvenient and expensive to install within a building. Due to the considerable weight of the laden tank, the floor below the tank may also have to be reinforced. For this reason, the tank is usually located in a basement or cellar which necessitates the use of a pump as mentioned above.

The Applicant has realised that in addition to saving mains water, rainwater harvesting systems can also be used to attenuate the flow of storm rainwater into the sewers. Instead of directing rainwater directly into the sewers, the Applicant has realised that large amounts of rainfall can be stored, at least temporarily, in or on the roof. This rainwater can then be used domestically, rather than being wasted. In other words, the roof can be used as a reservoir to store the rainwater.

Such a system is at least partially made possible by the advances in the waterproofing of modern roofing_systems. It is now not so necessary to divert rainfall off the roof as soon as possible.

Such a system has the added advantage that less rainwater has to be stored within the building, so less extensive tanks and plumbing are required.

According to a first aspect of the present invention, there is provided a rainwater harvesting system, comprising a roof of a building and a roof rainwater outlet in fluid communication with a water supply system of the building. The rainwater outlet comprises a filter to attenuate the flow of rainwater from the roof through the outlet to the internal water supply system.

It should be understood that the rainwater harvesting system of the present invention can be used on any building having a roof or other surface which may receive and potentially accumulate rainwater. As used herein the term roof is intended to encompass all such surfaces, for example verandas, terraces and so on.

For example, the invention could be used on a residential property such a house or a block of apartments, or on a commercial building such as an office block or a municipal building. Preferably, the roof is a substantially flat or low pitch roof.

It should be understood that the water supply system can be any system for supplying water, i.e. rainwater, to the interior of the building or to a supply associated with the building but external thereto (such as an outside tap for gardening or car washing use). An internal water supply system could be used, for example, to flush toilets or for washing machines.

The term 'rainwater' should be understood to mean any type of atmospheric precipitation, such as rainfall, snow, sleet, hail, dew etc. By attenuating the flow or rainwater from the roof to the water supply system, heavy rainfall can be prevented from flooding the supply system, which may result in damage thereto or in excess rainwater being discharged into the sewers. The rain harvesting system of the present invention potentially provides a more steady flow of rainwater into the water supply system.

Using a filter to attenuate the flow also provides an added advantage of filtering the rainwater prior to it flowing into the water supply system. The water supplied from this system may thus be substantially cleaner than unfiltered rainwater. Preferably, the rainwater harvesting system is configured to retain rainwater on the roof and/or in said outlet. Retaining rainwater on the roof or in the outlet, i.e. in or adjacent the roof, is beneficial as discussed above. The rainwater can be released from the roof in a more controlled manner.

Preferably, the rainwater outlet comprises a conduit, the filter being disposed within the conduit. In a preferred embodiment, the conduit is removably mounted within a housing mounted to the roof.

Preferably, the filter comprises a filtration medium. Preferably, the filtration medium is sand, grit or gravel. Preferably, the filtration medium is contained within a water permeable casing such a permeable membrane or a cartridge. Preferably, the water permeable membrane is a non-woven fabric.

Preferably, the filtration medium provides microbiological filtration. In the preferred embodiment, a 'slow-sand' filter is utilised to provide such microbiological filtration. Slow-sand filters are often used in water purification devices. They are simple devices containing a column of fine particles such as sand or fine gravel. They usually contain no mechanical parts or added chemicals and are environmentally friendly. Slow sand filters work through the formation of an active biological layer at the top of the column of sand. The gelatinous biological layer formed, known as a 'Schmutzdecke¹, serves to filter the water passing therethrough. Solid particles in the water are trapped in the biological layer. The Schmutzdecke also serves to reduce the amount of bacterial activity in the water, i.e. the water undergoes microbiological filtration. Slow sand filters gradually become less efficient as the Schmutzdecke layer becomes too thick and reduces the water flow through the sand filter too much. At this time, the sand filter must be replaced, washed or replenished.

In order to sustain the Schmutzdecke, it must be kept moist at all times. Preferably, therefore, the conduit comprises a main body for receiving rainwater from the roof and a sump for receiving rainwater from the main body and retaining a portion of rainwater received therein, wherein the retained portion of water keeps at least a portion of the filter wet. This may be sufficient to keep the entire filter moist at all times. In the preferred embodiment, this is necessary to maintain the Schmutzdecke of the slow sand filter, as discussed above.

Preferably, the filter is disposed at least partially within the main body. Preferably, the retained water thus has to pass through at least a portion of the filter to reach the sump.

Preferably, at least a portion of the filter, the main body and the sump are arranged substantially coaxially. This provides a more compact arrangement.

In the preferred embodiment, the main body has upper and lower cylindrical sections, with the upper section being of a larger diameter than the lower section, such that there is a step formed between the upper and lower sections. The sump may be provided separately from the main body, with the sump preferably supported by the main body. In this embodiment, the filter may be cylindrical in shape and be received at least partially in the lower section of the main body. The filter may protrude from the lower end of the lower section of the main body into the sump.

In another embodiment, the conduit may comprise an integrally formed main body and sump. In either embodiment, this means that the main body and sump may be removed together from any supporting housing for maintenance of the filter.

The conduit maybe cylindrical with a substantially uniform cross-section. The filter may be located in the conduit between the main body and the sump. In this embodiment, the filter may be annular in shape. Preferably, the system comprises means for adjusting the fluid flow path between the main body and the sump. This may provide further attenuation of the flow of rainwater from the roof to the water supply system and a level of control of the amount of attenuation. The means for adjusting the fluid flow path may be a valve.

Preferably, the valve may be operated by varying the relative positions of the main body and the sump. Preferably, the main body and sump may comprise complementary threads such that their relative positions can be varied. Alternatively, other inter-engaging formations could be provided on the main body and the sump, such as ribs and notches.

Preferably, the system further comprises a cap covering the entrance of said outlet. This 'frost' cap is preferably configured to seal the outlet to reduce evaporation of water in the outlet and/or to insulate the outlet to prevent water in the outlet freezing. The 'entrance' of the outlet should be understood to refer to the inlet of the outlet, i.e. the point through which rainwater from the roof enters the outlet.

In the preferred embodiment, the frost cap serves to protect the slow sand filter. Firstly, it prevents the slow sand filter from drying out in dry conditions. Secondly, it prevents the slow sand filter from freezing, which would also kill the biological layer.

Preferably, the frost cap is buoyant, such that in use when there is a sufficient level of rainwater on the roof, the frost cap floats upon said rainwater and the entrance of said outlet is opened and when there is an insufficient level of rainwater on the roof, the entrance is closed by the frost cap. There may be a minimum nonzero level of rainwater of the roof that is required to lift the frost cap and open the entrance of the outlet.

Preferably, the system comprises a cover disposed on the roof over the outlet, the cover having a plurality of apertures through which rainwater can flow into the outlet. The cover serves to protect the rainwater outlet and preferably the frost cap. It also provides a coarse level of filtration as the apertures may be sized to prevent debris, such as leaves, twigs etc., entering the outlet.

Preferably, the cover is substantially cylindrical or dome shaped.

Preferably, the system further comprises a filtration membrane disposed on the outer surface of the cover, the membrane covering the apertures. The filtration membrane provides primary filtration for the rainwater flowing into the outlet through the apertures. Preferably, the filtration membrane comprises a non-woven fabric.

Preferably, the outlet has a longitudinal axis, which is disposed substantially vertically. Preferably, the system comprises a plurality of the rainwater outlets. The plurality of outlets may be spaced over the roof surface.

According to a second aspect of the present invention, there is provided, a filter unit for use in a rainwater harvesting system, comprising a body, a filter disposed in the body, and rainwater retaining means configured to retain a portion of the rainwater flowing through the body to keep at least a portion of the filter moist.

The second aspect of the present invention provides a filter unit for use in a rainwater harvesting system, in particular in a rainwater outlet of a rainwater harvesting system installed on a roof of a building. The filter unit serves to filter the rainwater prior to it flowing into a water supply system of a building and may also serve to attenuate the flow of rainwater from the roof into the water supply system. The rainwater has to pass through at least a portion of the filter to reach the sump. The retained water is thus filtered. The rainwater retaining means serves to keep the filter moist which may aid filtration.

This further aspect of the present invention may include any of the features described with respect to the first aspect.

Preferably, the filter comprises a filtration medium. Preferably, the filtration medium is sand, grit or gravel. Preferably, the filtration medium is contained within a water permeable casing such a permeable membrane or a cartridge. Preferably, the water permeable membrane is a non-woven fabric. Preferably, the filter provides microbiological filtration. As described above with respect to the first aspect of the present invention, in the preferred embodiment, the microbiological filtration is provided by a slow sand filter. The slow sand filter can be kept moist by the rainwater retaining means.

The filter is disposed at least partially within the rainwater retaining means. Preferably, the retaining means is a sump. Preferably, at least a portion of the filter, the body and the rainwater retaining means are arranged substantially coaxially. In the preferred embodiment, the body has upper and lower cylindrical sections, with the upper section being of a larger diameter than the lower section, such that there is a step formed between the upper and lower sections. The rainwater retaining means may be provided separately from the body, with the sump preferably supported by the main body. In this embodiment, the filter may be cylindrical in shape and be received at least partially in the lower section of the body. The filter may protrude from the lower end of the lower section of the body into the rainwater retaining means.

In another embodiment, the conduit may comprise an integrally formed body and rainwater retaining means. The conduit may be cylindrical with a substantially uniform cross-section. The filter may be located in the conduit between the body and the rainwater retaining means. In this embodiment, the filter may be annular in shape.

Preferably, the filter unit comprises means for adjusting the fluid flow path between the body and the rainwater retaining means. Reducing the cross-sectional area of the fluid flow path provides further attenuation of the flow of rainwater from the roof to the water supply system. An adjustable fluid flow path provides a method of controlling the amount of attenuation. The means for adjusting the fluid flow path may be a valve. Preferably, the valve may be operated by varying the relative positions of the body and the rainwater retaining means. Preferably, the body and the rainwater retaining means may comprise complementary threads such that their relative positions can be varied. Alternatively, other engaging formations could be provided on the main body and the sump, such as ribs and notches. The present invention extends to a rainwater filter outlet for use in a rainwater harvesting system, comprising the filter unit as described above and a cover having a plurality of apertures.

Preferably, the cover is substantially cylindrical or dome shaped. The cover may be stainless steel. Preferably, the rainwater filter outlet further comprises a filtration membrane disposed on the outer surface of the cover, the membrane covering the apertures. Preferably, the filtration membrane comprises a non- woven fabric. Preferably, the rainwater filter outlet further comprises a buoyant frost cap, which is preferably configured to reduce evaporation of rainwater in the body and/or to prevent rainwater in the body freezing.

The present invention extends to a rainwater harvesting system comprising a roof and a rainwater filter outlet as described above.

Some preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows an expanded view of a rainwater filter outlet according to a preferred embodiment of the present invention;

Figure 2 shows a sectional view of a rainwater harvesting system comprising the rainwater filter outlet of Figure 1 ;

Figure 3 shows a sectional view of the rainwater harvesting system of Figure 2 during rainfall; Figure 4 shows a sectional view of the rainwater harvesting system of Figure

2 in a post-rainfall condition;

Figure 5 shows a sectional view of the rainwater harvesting system of Figure 2 dismantled for maintenance;

Figure 6a to 6c show sectional views of the outlet trap of the rainwater filter outlet of Figure 1;

Figures 7a and 7b show sectional views of a rainwater harvesting system comprising a rainwater filter outlet according to another embodiment of the present invention; and

Figure 8 shows a building comprising a rainwater harvesting system according to the present invention.

Figures 1 and 2 show a rainwater harvesting system comprising a rainwater filter outlet 10 according to a preferred embodiment of the present invention. The outlet 10 has a stepped cylindrical housing 1, a conduit 30 which is removably mounted in the housing 1, a cylindrical filter cartridge 3, a frost cap 4, a cover 7 and a primary filtration membrane 6. The conduit 30 comprises a main body 2 having an upper cylindrical section 2a and a lower cylindrical section 2b. The upper section 2a is of a larger diameter than the lower section 2b. The lower section 2b holds the filter cartridge 3. An annular seal 9 is disposed in an annular seat 22 (shown only in Figure 2) around the outer surface of the upper section 2a.

In use, the housing 1 is mounted on a roof 20. The lower end Ib of the housing 1 defines an entrance 25 into a water supply system. An annular flange Ia of the housing 1 sits flush with the upper surface 21 of the roof 20 and is secured thereto with, for example, a screw fixing. The main body 2 is mounted to be located wholly within the housing 1. The seal 9 engages the inner surface Ib of the housing 1 to form a friction fit, such that all water flowing into the outlet 10 flows into the main body 2 and through filter cartridge 3 into sump 8 and then through entrance 25 into the downstream water supply system. An annular rib or lip 24 (shown only in Figure 2) is provided on the housing 1 to engage with the annular seat 22 to locate the main body 2 in the correct position within the housing 1.

Frost cap 4 sits at the upper end of the housing 1 to close an entrance 26 thereto. The frost cap 4 is made of an insulating material, for example a plastics material such as expanded polystyrene, such that it protects the filter cartridge 3 from extremes of temperature, i.e. cold and hot temperatures. When in place in the entrance 26, the frost cap 4 also reduces evaporation of water from the conduit 30 (i.e. from main body 2 and filter cartridge 3).

Above the frost cap 4, is located a cover 5. The cover 5 sits on the upper flange 1 a of the main housing 1. The flange 1 a may be provided with locating means, such as an upstanding annular rib or ring Id, to assist in locating the cover 5. Alternatively, the cover 5 may be fastened to the flange 1 a, e.g. by screws etc. The cover 5 comprises apertures 7, shown as vertical slots. These apertures 7 allow the flow of rainwater from the roof 20 into the outlet 10. The primary filtration membrane 6 is a non-woven material which is wrapped around the cover 5 and covers apertures 7. It is preferably annular in shape. It provides primary filtration, to prevent debris such as leaves, twigs, insects etc., entering the rainwater filter outlet.

In use, the filter cartridge 3 is disposed within the conduit 30. The filter cartridge 3 is contained partially within the lower part 2b of the main body 2 but protrudes therefrom into the coaxially arranged sump 8. The filter cartridge 3 comprises a particulate filtration medium, such as sand, grit or gravel, contained within a flexible water permeable membrane 3 a (Fig. 2). Alternatively, the filtration medium may be contained in a rigid cartridge having apertures, for example made of a plastics material. Over time, the medium forms a slow-sand filter having an upper bacterial 'Schmutzdecke' layer.

Fig. 3 shows the rainwater harvesting system in use in during rainfall. Rainfall forms a level 11 of rainwater on roof 20. When the level 11 is higher than the lower level of the apertures 7 in cover 5, rainwater flows through the apertures 7 and causes the frost cap 4 to float, i.e. move upwards such that entrance 26 is opened. Water can then flow into main body 2 and flow through filter cartridge 3. As water flows through the slow sand filter 3, it is filtered. Solid particles are trapped by the filtration medium and the Schmutzdecke, while the latter also serves to kill bacteria in the rainwater.

Water flows from the filter cartridge 3 into the sump 8. When the level of water in the sump 8 is higher than the upper lip 8a of the sump 8, water will flow over the lip 8a and down into the entrance 25 of the water supply system. This will clearly happen if the water level in the main body 2 is higher than the level 8a. The lip 8a thus acts as a weir.

The filter cartridge 3 serves to attenuate the flow of rainwater from the roof 20 to the water supply system. In heavy rainfall conditions, rainwater will build up in the upper section 2a of the main body 2 and "back-up' onto the roof surface 21. The roof 20 is thus acting as a reservoir. Water will flow through the filter cartridge 3 into the sump 8 at an attenuated rate and then into the water supply system. The rainwater can thus be released from the roof 20 at a controlled rate. This avoids a deluge into the water supply system, which results in less storage capacity within the building being required and/or less overflow from the water supply system into the sewers.

Fig. 4 shows the state of the rainwater filter outlet after rainfall has stopped. Water has drained out of the upper part of the main body 2 through filter cartridge 3. A residual level of (filtered) water 12 (at level 8a) remains in the sump 8. This residual water keeps the filter 3 moist. While the upper part of the filter cartridge 3 is above the residual water level 12, capillary action will ensure that the upper part of the filter 3, i.e. the Schmutzdecke, remains moist. This keeps the Schmutzdecke alive. Fig. 5 shows the rainwater filter outlet 10 dismantled so that it can be serviced. For example, the slow sand filter cartridge 3 will need to be refurbished or regenerated. Other parts of the rainwater filter outlet, such as main body 2 and sump 8 may also need to be cleaned. Figs 6a to 6c show views of the conduit 30 with the main body 2 and sump 8 being at different relative positions. The distance 16 between the lower edge 2c of the lower section 2a of the main body 2 and the inner lower surface 8a of the sump 8 defines a fluid flow path between the main body 2 and the sump 8. This distance can be varied (prior to installation) to vary the depth of the fluid flow path between the main body 2 and the sump 8. The lower part 2a of the main body 2 has an external thread 13. The sump 8 has a complementary thread 14 on the end of spoked or apertured web 15. The sump 8 can then be rotated relative to the main body 2 to adjust the separation 16. Fig 6c shows a small separation 16, which will result in the most restricted fluid flow path through which flow will be most attenuated. Fig 7a shows a rainwater harvester system comprising a rainwater filter outlet 110 according to an alternative embodiment of the present invention. Fig 7b shows a view of the outlet of Fig 7a being dismantled for maintenance. The outlet 110 comprises a conduit 130 mounted in a complementary shaped housing 101 in a roof 120. The conduit 130 is substantially cylindrical in shape except for a downwardly extending pipe 131 which, in use, is connected to an inlet of a water supply system of a building (not shown). Conduit 130 comprises a main body 102 having a sump 108 at its lower end. The sump retains a permanent level 150 of water in the conduit and thus extends downwardly from level 150. Main body 102 extends upwardly from this level up to horizontal flange 102a. The cap 104, cover 105 and filtration membrane 106 substantially correspond to those described in relation to the outlet shown in Figs 1 to 6. The system of Figs 7a and 7b further comprises a waterproofing membrane 125 position along the horizontal flange 102a of main body 102 and along the roof surface 121. hi order that water is retained in the sump, an upwardly extending pipe 140 is provided. This pipe is disposed within the downpipe 131 and may be secured thereto. The upper edge 141 of the up-pipe 140 defines the permanent water level 150 in the sump, as can be seen in Fig 7a. A filtration system 150 is provided. The filtration system 150 comprises a filter cartridge 103 and a sleeve 151. The filter cartridge 103 is annular in shape. It comprises a filtration medium which forms a slow sand filter as previously discussed in relation to the previous embodiment. The filtration medium is contained in casing 103 a having apertures 103b on its lower surface. The filter cartridge 103 is mounted to the sleeve 150 which extends perpendicularly to the filter cartridge. The annular filter cartridge extends around the sleeve 150.

The sleeve 150 has a handle 152 to ease removal and a lower flange 153. The sleeve also has apertures 154 through which filtered water can flow. The sleeve is made of a rigid material such as metal or plastic. In use, as shown in Fig 7a, the filtration system 150 is positioned within the conduit 102. The lower flange 153 sits on lower inner edge 108b of sump 108. The seal 109 engages an inner surface of the sump 108. Rainfall flowing from the roof can flow through filtration medium 106 and cover 105. The cap 104 will be lifted and the rainfall will flow into conduit 130. When the level of water in the conduit 130 is over weir level 142 (which corresponds to the level of upper edge 141 of pipe 140), water will flow to the water supply system through the filter and water will flow over the edge 141 of pipe 140 and into pipe 140. The upper edge 141 acts as a weir. The filtration cartridge is submerged beneath the permanent water level 142 so that the slow said filter remains wet, thus maintaining the Schmutzdecke.

Fig 8 shows a building 200 having a rainwater harvesting system according to the present invention. The building has a roof 220, in which a rainwater filter outlet 210 is mounted. A level of rainwater 230 is formed on roof 220. The rainwater filter outlet 210 feeds into a water supply system 240 of the building 220. The water supply system 240 comprises a tank 241 into which the filtered rainwater flows and supply line 242 which supplied water to flush toilets 243. When necessary the reservoir 241 may be topped by the mains water from mains supply line 244. It will be appreciated that the present invention has been described by way of example only and that various modification may be made to the apparatus described without departing from the scope of the present invention as defined by the claims.

It should also be understood that the term roof as used herein encompasses any building surface which is capable of receiving and potentially accumulating rainwater and encompasses terraces, verandas and so on.

Claims

1. A rainwater harvesting system, comprising: a roof of a building; and a roof rainwater outlet in fluid communication with a water supply system of said building, said rainwater outlet comprising a filter to attenuate the flow of rainwater from said roof through said outlet to said water supply system.

2. The system of claim 1 , wherein said rainwater harvesting system is configured to retain rainwater on said roof and/or in said outlet.

3. The system of claim 1 or 2, wherein said rainwater outlet comprises a conduit, said filter being disposed at least partially within said conduit,

4. The system of claim 3 wherein said conduit is removably mounted in a housing mounted to said roof.

5. The system of any preceding claim, wherein said filter comprises a filtration medium.

6. The system of claim 5, wherein said filtration medium is sand, grit or gravel.

7. The system of claim 5 or 6, wherein said filtration medium is contained within a water permeable casing.

8. The system of claim 5, wherein said water permeable casing is a water permeable membrane or a cartridge.

9. The system of any of claims 5 to 8, wherein said filter provides microbiological filtration.

10. The system of any preceding claim, wherein said conduit, comprises a main body for receiving rainwater from said roof and a sump for receiving rainwater from said main body and retaining a portion of rainwater received therein, wherein said retained portion of water keeps at least said portion of said filter moist.

11. The system of claim 10, wherein said sump is provided separately from and supported by said main body.

12. The system of claim 10, wherein said sump is provided integrally with said main body.

13. The system of any of claims 10 to 12, wherein said filter is disposed at least partially within said main body.

14. The system of claim 13, wherein at least a portion of said filter, said main body and said sump are arranged substantially coaxially.

15. The system of any of claims 10 to 14, wherein said system comprises means for adjusting a fluid flow path between said main body and said sump.

16. The system of claim 15, wherein said adjusting means comprises means for varying the relative positions of said main body and said sump.

17. The system of claim 16, wherein said means for varying the relative positions comprises complementary formations on said main body and said sump.

18. The system of any preceding claim, further comprising a cap covering an entrance of said outlet.

19. The system of claim 18, wherein said cap is buoyant.

20. The system of any preceding claim, further comprising a cover disposed on said roof over said outlet, said cover having a plurality of apertures through which rainwater can flow into said outlet.

21. The system of claim 20, wherein said cover is substantially cylindrical or dome shaped.

22. The system of claim 20 or 21 , further comprising a filtration membrane disposed on the outer surface of said cover, said membrane covering said apertures.

23. The system of claim 22, wherein said filtration membrane comprises a non- woven fabric.

24. The system of any preceding claim, wherein said outlet has a longitudinal axis, said axis being disposed substantially vertically.

25. The system of any preceding claim, comprising a plurality of said rainwater outlets.

26. A filter unit for use in a rainwater harvesting system, comprising: a body; a filter disposed in said body; and rainwater retaining means configured to retain a portion of the rainwater flowing through said body to keep at least a portion of said filter moist.

27. The filter unit of claim 26, wherein said filter comprises a filtration medium.

28. The filter unit of claim 27, wherein said filtration medium is sand, grit or gravel.

29. The filter unit of claim 27 or 28, wherein said filtration medium is contained within a water permeable casing.

30. The filter unit of claim 29, wherein said water permeable casing is a water permeable membrane or a cartridge.

31. The filter unit of any of claims 27 to 30, wherein said filter provides microbiological filtration.

32. The filter of claim 31 , wherein at least a portion of said filter, said body and said retaining means are arranged substantially coaxially.

33. The filter unit of any of claims 26 to 32, further comprising means for adjusting a fluid flow path between said body and said retaining means.

34. The filter unit of claim 33, wherein said adjusting means comprises means for varying the relative positions of said body said retaining means.

35. The filter unit of claim 34, wherein said means for varying the relative positions comprises complementary formations on said body and said retaining means.

36. A rainwater filter outlet for use in a rainwater harvesting system, comprising: the filter unit of any of claims 26 to 35; and a cover having a plurality of apertures.

37. The rainwater filter outlet of claim 36, wherein said cover is substantially cylindrical or dome shaped.

38. The rainwater filter outlet of claim 37, further comprising a filtration membrane disposed on the outer surface of said cover, said membrane covering said apertures.

39. The rainwater filter outlet of claim 38, wherein said filtration membrane comprises a non-woven fabric.

40. The rainwater filter outlet of claim 39, further comprising a buoyant cap.

41. A rainwater harvesting system comprising: a roof; and a rainwater filter outlet as claimed in any of claims 36 to 40.

42. A rainwater harvesting system, comprising: a roof of a building; and a water supply system of said building; and a filter arranged between said roof and said water supply system to attenuate the flow of rainwater from said roof to said water supply system.

43. The system of claim 42 wherein the filter is arranged within a rainwater outlet.