WO1996010676A1

WO1996010676A1 - Separation device

Info

Publication number: WO1996010676A1
Application number: PCT/GB1995/002314
Authority: WO
Inventors: John Bell Mckeown
Original assignee: Motherwell Bridge Fabricators Limited
Priority date: 1994-09-30
Filing date: 1995-09-29
Publication date: 1996-04-11
Also published as: AU692605B2; CA2199023A1; HK1005988A1; EP0783614A1; NO971398D0; JPH10509634A; NZ293315A; NO971398L; AU3572595A

Abstract

A low maintenance separation device intended for removing liquid-borne solids from a carrier liquid includes a hydrocyclone separator (105) comprising a liquid vortex-forming chamber (130) with an inlet (150) for receiving a solids-contaminated liquid feed, and an outlet (145) for gravity-assisted outflow of liquids and solids, wherein the chamber is provided with a screen (125) positioned in an upper portion of the chamber, and arranged to interfere with raised liquid levels so as to inhibit solids passage whilst permitting liquid through-flow, the said screen having a surface which is at least partially foraminated and of a concave shape.

Description

SEPARATION DEVICE

This invention relates to a low maintenance separation device intended for removing liquid-borne solids from the carrier liquid by causing the liquid to enter a curved wall chamber and form a vortex prior to exiting therefrom normally under the influence of gravity. The rotational flow characteristics created in the chamber result in a flow velocity differential between liquid at the chamber wall and that in the centre of chamber whereby the contaminant solids are accumulated for collection and separation from the liquid at the outlet of the chamber. Such devices are generally known in the art and are commonly referred to as hydrocyclones. In particular, though not exclusively, this invention relates to hydrocyclones used in combined sewer overflows.

A combined sewer is one where domestic sewage is combined with other forms of waste water from roads, roofs, land drains, etc. During periods of heavy rainfall the flow through such a sewer increases dramatically and can exceed the working capacity of any sewerage system or sewage treatment plant situated downstream. As a consequence the excess must be discharged without full treatment.

The problem of how to cleanse excess storm water has been addressed in a number of ways. An early system involved the use of a simple overflow weir. This was not however particularly effective as it allowed a considerable amount of solid matter to overflow and be discharged. The addition of a mechanically raked screen was tried as a means of reducing this emission.

In recent years, a method of separation using the properties of vortex flow has been used with reasonable success. In this method a high velocity stream of liquid, containing solid matter, is introduced tangentially into a cylindrical chamber with an outlet at its base. The resultant bending of the inlet stream into a circular cyclonic path produces a vortex which results in dynamic separation of the components of the feed stream. Thus polluting solids are discharged through the outlet before being passed to a sewage treatment facility, while the remaining liquid can be collected from the chamber and discharged without treatment.

The advantages of a system that operates on this principle are that it requires little maintenance and it can be situated below ground. Tests however have shown that while the system is efficient at removing settleable and floating solids, it has difficulty in removing some types of buoyant material. This material, termed neutral density solids (NDS) , is composed of such items as condoms, the backing of sanitary towels, hypodermic syringes, etc. These solids are unfortunately the ones which create visible and unsightly pollution, are in the most part non-biodegradable and, if discharged in the vicinity of a recreational body of water, can constitute a significant public health risk. Also new legislation coming into force in the near future will require that NDS above a certain two dimensional size will have to be removed before overflow water can be discharged.

It has been suggested that screens may be incorporated into vortex separation equipment but it has been found that these rapidly become blocked and ineffective. In addition, with the separator unit commonly buried below ground, it is extremely difficult to gain access in order to clear the blockage. Therefore it is an object of the present invention to obviate or mitigate at least some of the aforementioned problems.

According to the present invention there is provided a hydrocyclone separator comprising a liquid vortex-forming chamber provided with an inlet for supply of a solids- contaminated liquid feed, and an outlet for gravity-assisted outflow of liquids and solids, wherein the chamber is provided with a screen positioned in an upper portion of the chamber, and arranged to interfere with raised liquid levels so as to inhibit solids passage whilst permitting liquid through-flow, the said screen having a surface which is at least partially foraminated and of a concave shape.

A significant advantage is derived from the concave shape of the screen in that as liquid level rises in the chamber, e.g. during periods of high flow or overflow conditions, the rotational velocity is increased by contact with the tapered surface, thereby providing an additional scouring action which assists in clearing the foraminated portion(s) of the screen. Furthermore whenever liquid levels recede, the sloping surface of the screen will encourage any solids trapped on the screen to drop off under the influence of gravity. Likewise depending materials, i.e. entrapped lengths of solids hanging from the screen will tend to be caught in the receding swirling liquid flow to be dragged free of the screen.

In one configuration, the screen has the form of an inverted dish or shallow cone and the surface thereof has a major portion which is foraminated whilst the minor portion is continuous to direct or deflect flow. Baffles for controlling or enhancing liquid flow across the screen may also be provided on the screen surface or in close proximity thereto. In one arrangement the foraminated surfaces are set back from non-foraminated surfaces and the overall surface configuration is arranged to optimise scouring flow currents across the foraminated portions.

The concave screen surface may be manufactured from, or coated with a non-, or low-stick material. A suitable material of appropriate durability and non-stick characteristics would be a fluorocarbon polymer resin such as polytetrafluoroethylene (ptfe) such as that type known generally by the Trade Mark TEFLON. It is also possible to design the screen surface to discourage solids retention or adherence after high liquid levels subside. This may be done by providing a plurality of surface projections juxtaposed with respective apertures in the screen surface which projections present a sloping surface against the direction of flow to divert solids past the adjacent aperture. Such projections may be discrete deflector formations adjacent an aperture or may be a ridge deflector aligned with a series of such apertures. Conveniently the surface projection may be generally wedge- shaped having a first surface forming a shallow incline upwards from the screen surface and a second surface forming a steep return thereafter. The aperture for permitting liquid passage may be either immediately adjacent to the steep return surface or may optionally be formed therein. In either event, the momentum of solids carried in the liquid passing rapidly over the projections should cause these solids to by-pass the apertures and thereby obviate or mitigate clogging of the foraminated portions of the screen.

Optionally, the screen may be provided with means for clearing or dislodging solids, such means may be either driven brushes, or water jet applicators, or a combination thereof.

The brushes may be driven by means of a hydraulic or electric motor, or by energy generated by the swirling motion of fluid within the vortex chamber.

The separator may be provided with means to lower the level of fluid contained within the vortex chamber during sustained periods of high inlet flow-rate e.g. during and after storm conditions. Said means may comprise at least one siphon tube communicating between the vortex chamber and the chamber outlet, or may take the form of a vortex breaker intended to collapse the air core of the vortex and hence increase the flow-rate of fluid through the chamber outlet. It is the intention of such means to cause the sudden lowering of the fluid level within the vortex chamber and hence to cause solids trapped against the screen to fall into the vortex chamber.

The screen may be provided with a chimney extending vertically upwards, the purpose of which is, when elevated fluid levels are present within the vortex chamber, to cause the fluid level to rise yet further and subsequently prime and then actuate the at least one siphon tube.

The screen may have a surface which is at least partially foraminated. In a preferred embodiment there is provided a frusto-conical screen where the non-foraminated portions comprise a quadrant of said screen and a circumferential band extending inwardly from said screens outermost edge.

The interior of the vortex chamber may be provided with baffle means and/or flow direction means to enhance the vortex forming properties of the chamber and/or to direct solid matter to the vicinity of the chamber outlet.

Preferably the vortex chamber is provided with baffles around the tangential inlet which serve to direct inlet fluid stream both downwards and onto the wall of the chamber.

The chamber may also be provided with at least one baffle which protrudes inwards from the wall of the chamber and extends from the base of the chamber to screen atop the chamber. In a preferred embodiment there is provided a single baffle adjacent to the inlet.

The separator may be configured such that, in use, the fluid level within the separator is maintained above the base of the screen. If the separator is furnished with an annular collection chamber surrounding the vortex chamber, the fluid level may be maintained above the base of the screen by the provision of a weir surrounding the outlet from said annular collection chamber. Alternatively, the separator may be provided with a collection chamber, with communication between said vortex chamber and said collection chamber being permitted by virtue of an opening in the wall of the vortex chamber, said opening being positioned above the base of the screen.

In use, the separator may either be positioned so as to interrupt the flow in a combined sewer or be provided in an "off-line" position with provisions to enable it to be brought "online" as and when it is needed.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which is shown:

Figure 1. a side view of a conventional vortex separator;

Figure 2. an overhead view of the separator referred to in Figure 1;

Figure 3. a side view of a vortex separator in accordance with the present invention,*

Figure 4. an overhead view of the separator referred to in Figure 3;

Figure 5. a side view of a separator in accordance with the present invention and incorporating a water jet system,* Figure 6 a representation on an enlarged scale of a possible configuration for at least a portion of the screen;

Figure 7. a detailed view of the screen perforations;

Figure 8 an alternative embodiment of the construction of the screen perforations;

Figure 9. a side view of a hydro-mechanical cleaning mechanism for use with a separator in accordance with the present invention.

Figure 10 a side view of a vortex separator in accordance with an aspect of the present invention;

Figure 11 a partial plan view of the vortex separator of Figure 10;

Figure 12 a schematic side view of a vortex separator equipped with water jet apparatus and mechanical brushing apparatus;

Figure 13 a side view of a vortex separator equipped with a siphon tube;

Figure 14 a partial plan view of the vortex separator of Figure 13,*

Figs. 15A-15H. side and plan views of alternative screen configurations;

Figs. 16A-16D. plan views of alternative vortex chamber baffle configurations; Figure 17 sectional view indicated by line A-A on Figure 16B;

Figure 18. sectional view indicated by line B-B on Figure 16B;

Figure 19 side view of a vortex separator in accordance with an alternative embodiment of the present invention;

Figure 20. plan view of the vortex separator of Figure 19;

Figure 21 side view of a vortex separator in accordance with an alternative embodiment of the present invention; and

Figure 22. partial plan view of the vortex separator of Figure 21.

Referring firstly to figures 1 and 2 there is shown a separator 1 comprising a cylindrical outer casing 2 incorporating a sloping base 13, and a cylindrical inner vessel 3 with a conical base 9. A circumferential lip 4 provided with an overflow portion (not shown) as part of the circumference, extends from the outer surface of the inner vessel 3 to the inner surface of the outer casing 2. This has the effect of dividing the interior of the separator 1 into three distinct spaces: a vortex chamber 5, an annular channel 6 formed between the inner vessel 3, the outer casing 2 and the lip 4 , and an outlet chamber 7. The latter is defined by the conical base 9, the lip 4, the outer casing 2 and its sloping base 13. Communication between the vortex chamber 5 and the outlet chamber 7 is achieved by an aperture 8 situated at the centre of the inner vessels conical base 9. The upper edge of the inner vessel 3 defines a peripheral, circumferential weir 10. Outlet chamber 7 is provided with orifice 11 to allow any matter present to be removed via channel 12. Tangential inlet 14 is provided for introducing liquid into vortex chamber 5. Outlet duct 17 permits the removal of liquid present in the annular chamber 6.

In use, a liquid mixture of water and sewage is introduced as a high velocity stream 15 into the vortex chamber 5 through tangential inlet channel 14 located close to the bottom of the chamber. The stream 15 impinges upon the cylindrical inner wall of the inner vessel 3 with the result that a circular cyclonic stream 16, or vortex, is created. Under normal operating conditions all the liquid mixture entering the vortex chamber 5 through inlet 14 is passed through aperture 8 and into the outlet chamber 7. From there it is removed via outlet channel 12 for appropriate treatment. However, when increased flow conditions are experienced, i.e. during or immediately after a storm, the volume of liquid entering the vortex chamber 5 exceeds that which can be removed for subsequent specialist treatment. Accordingly liquid will begin to accumulate in the vortex chamber 5 , its upper limit being defined by the circumferential weir 10. As a result of the elevated flow rate through tangential inlet 14 due to storm conditions, the forces generated by the swirling motion of stream 16 are sufficient to transport most types of solid matter through aperture 8 into outlet chamber 7. It will be understood that the mechanics of vortex flow produced by a high velocity stream tangentially entering a cylindrical vessel will be apparent to any appropriately skilled person with a knowledge of hydrodynamics. Excess liquid not used to transport solid matter through aperture 8 spills over weir 10 and into the annular channel 6 where it can be removed via outlet duct 17. Arrows 18 and 18' indicate the movement of liquid from the vortex chamber 5 to the annular channel 6.

Whilst the device shown in Figures 1 and 2 is successful at removing most types of solid matter contained in domestic sewage it is ineffective at separating NDS. These solids do not become trapped in the vortex 16 and are transported over weir 10 and into channel 6. Referring now to figures 3 and 4 there is shown a separator 19 operating on the same principles described hereinbefore and adapted to prevent NDS from passing between the vortex chamber 5 and the annular channel 6. This is achieved by means of a circular baffle 20 and a screen 21. The screen 21 is concave with respect to the vortex chamber 5, abuts onto ^' the circumferential weir 10 and serves to enclose the vortex chamber 5. The screen is provided with a plurality of perforations 22 so as to allow water ejected from the vortex chamber 5 to pass to the annular channel 6. The size of the perforations 22 is such that neutral density material is prevented from reaching the annular channel 6 and is confined within the vortex chamber 5. The screen 21 is provided with an aperture 23 at its crown which acts as an emergency overflow passage should the perforations 22 become obstructed.

The circular baffle 20 is positioned within the vortex chamber 5 and is provided with a central baffle 24. The baffle 24 is aligned with the vortex chamber outlet aperture 8. The surface of the baffle 20 is contoured to promote the movement of liquid from the inner surface of the screen 20 to the central baffle 24 and to increase the velocity of the liquid coming into contact with the inner surface of screen 21. Arrows 25 and 25' indicate the direction of these currents. This action is intended to scavenge any neutral density material which may have become trapped against the screen perforations 22, thus reducing the possibility of a blockage being formed. The currents 25 and 25' are also intended to transport the material removed from the screen 21 through the baffle 24 and into the vortex 16. Thus the neutral density material is introduced into the centre of the vortex 16, as opposed to its periphery when introduced tangentially, and is conveyed through aperture 8 into the outlet chamber 7. Once the elevated flow conditions have passed, i.e. the extra storm water in the combined sewer has been disposed of, the liquid level in the vortex chamber 5 recedes back below that of the circumferential weir 10. As before all the sewage/water mixture entering the vortex chamber 5 via tangential inlet 14 is passed through aperture 8 and into the outlet chamber 7. The screen 21 is shaped such that any solid matter left in contact with it by the receding liquid level will fall, under the influence of gravity, into the vortex chamber 5. The manufacture of the screen 21 from a non-stick material or the presence of a non-stick coating on its surface further enhances this self cleaning ability.

To aid the action of the scouring currents 25 and 25 ' , figure 5. illustrates an embodiment of the present invention incorporating the possible use of a pressurised water jet system. Nozzle units 26 and 26" are positioned outwith the vortex chamber 5 and above the perforated sections of screen 21. Water jets 27 and 27' are directed by nozzles 26 and

26' through the screen perforations 22. The action of these jets is intended to dislodge any solid material adhering to the inner surface of screen 21 which is resistant to the scouring currents 25 and 25'.

Figure 6. shows a possible means of positioning the perforations 22 upon the screen 21. The portion of the screen 21 containing the perforations 22 is stepped back from the main body 28. The action of scouring currents 25 across the stepped portion 28 produces eddy currents 29.

The flow disturbance produced by eddy currents 29 enhances the ability of the scouring currents 25 to remove neutral density matter in the vicinity of the perforations 22.

Figures 7. and 8. illustrate two possible embodiments for the arrangement of screen perforations 22. Firstly figure 7. shows a reciprocating saw-tooth surface profile 31 for the surface of the screen 21, with individual perforations 22 present in the shorter side of each tooth. The action of scouring current 25 produces an eddy current 30 adjacent to each perforation 22. The recirculatory motion imparted to the liquid adjacent to the perforations 22 discourages solid material from adhering to the screen

21. Figure 8. shows an alternative method of achieving the aforementioned effect. The screen 21 is covered alternately with perforations 22 and fin shaped projections 32. The projections 32 protrude into the scouring currents 23 and disturb the flow to discourage solid material from adhering to the screen 21.

It is possible that the screen 21 will need to be physically cleaned on occasion. Figure 9 illustrates a possible configuration for such a system adapted for use in an underground separator. One or more high pressure water jetting units 36 are positioned within the outer casing 2 and above the screen 21. The jetting units 36 may be built into the separator 19 or inserted through holes in the casing 39 when required. To aid the plurality of water jets 37 provided by units 36, a mechanical cleaning device may also be incorporated. In alternative embodiments (not shown) , such mechanical devices may be used alone without water jets. Drive unit 35 is connected via drive shaft 40 and articulated coupling 34 to cleaning brush 33. When drive unit 35 is activated, drive shaft 40 rotates causing brush 33 to move around the entirety of the outer surface of the screen 21. The disturbance supplied by the brush 33 and the water jets 37 is sufficient to remove stubborn material from the screen 21. The drive unit 35, shaft 36 and brush 33 assembly may be removed through down shaft 38 for the purposes of maintenance. The mechanical cleaning mechanism may be powered by any suitable means including electricity or solar or wind power. Smaller separators may incorporate a manual system powered by hand. The action of the cleaning mechanism may also be powered by hydraulic energy generated by the rotating liquid within the device. The intervals at which the screen 21 is physically cleaned may be predetermined according to a set pattern or in response to external stimuli. For example the drive unit 35 may incorporate a control system which activates the cleaning mechanism after a period of heavy rainfall. This could be achieved by means of a liquid level detector within the separator. Optionally the cleaning brush 33 may be positioned within the vortex chamber 5 and hence facilitate cleaning of the inner surface of the screen 21.

Referring now to Figures 10 and 11 there is shown a separator 105 comprising a cylindrical outer casing 110, a cylindrical inner vessel 115 with a conical base 120 and a frusto-conical screen 125 mounted atop the cylindrical inner vessel 115. The cylindrical inner vessel 115 and screen 125 serve to form a vortex chamber 130 while the cylindrical outer casing 110 and the cylindrical inner vessel 115 define an annular collection chamber 135. A collection chamber 140 is provided below the vortex chamber 130 with communication between said chambers 130, 140 being permitted by means of an aperture 145 in the conical base 120. The vortex chamber 130 is further provided with a tangentially disposed inlet duct 150 , while the annular chamber 135 and the collection chamber 140 are provided with outlet ducts 155 and 160 respectively.

In use, a liquid mixture of water and sewage is introduced through the tangential inlet duct 150. The inlet stream impinges upon the cylindrical wall of the vortex chamber 130 with the result that a circular cyclonic stream or vortex 165 is created. Under normal operating conditions, i.e. except those experienced during and after sustained periods of heavy rainfall, storms and the like, all the liquid mixture entering the vortex chamber 130 is passed through the aperture 145 to the collection chamber 140, from where it is removed for appropriate cleansing treatment. However, when increased inlet flow conditions are experienced, i.e. during or immediately after a storm, the volume of liquid entering the vortex chamber 30 exceeds that which can be removed for specialist treatment. Hence the liquid level 168 within the vortex chamber 130 rises above the edge 170 of the cylindrical inner vessel 115 and excess fluid passes through perforations present in the screen 125 and into the annular chamber 135. The screen 125 prevents matter such as neutral density solids (NDS) reaching the annular chamber 135 and subsequently being discharged without treatment.

The separator 105 may be provided with apparatus to ensure the screen does not become blocked with solid and semi-solid matter. The scouring action of the vortex 165 against the screen 125 discourages such matter from adhering to the screen 125, however additional means may be required to keep the screen 125 clear. Figure 12 shows schematic representations of mechanical brushing 175 and water jet 180 apparatus adapted to prevent solid matter from adhering to the screen 125. The brushing apparatus 175 may be powered by either an electric or hydraulic motor 185, or by energy generated by the swirling motion of fluid within the vortex chamber.

During sustained periods of high inlet flow, i.e. during and after prolonged storm conditions when the flow velocity typically peaks in the region of 1 to 4 m/s, solid matter may become trapped against the screen 125 despite measures taken to avoid such an eventuality. It has been found that a sudden drop in the liquid level 168 within the vortex chamber 130 is advantageous in removing said solid matter. The hydrodynamic pressure holding the solid matter against the screen 125 is thus removed and said solid matter falls from the screen 125 into the vortex chamber 130.

The sudden drop may be caused by a vortex breaker device (not shown) which can collapse the air core 190 of the vortex and increase the flow of liquid through the vortex chamber aperture 145. Alternatively siphon tubes 195 may be provided between the vortex chamber 130 and the collection chamber 140 as shown in Figures 13 and 14. In use, the surface level 200 of the liquid within the siphon tube 195 is equal to that within the vortex chamber 130. Should the liquid level 168 within the vortex chamber 130 rise high enough to prime the rising leg 205 of the siphon tube 195, said siphon tube 195 will then proceed to vent liquid from the vortex chamber 130 to the collection chamber 140. The separator 105 may be provided with as many tubes as is necessary to achieve the required level drop within the vortex chamber 130. The amount by which the level 168 will fall is governed by the inlet 210 to the siphon tube 195 which, in the example shown in Figure 13, is just below the base 215 of the screen 125. The screen 125 may also be provided with a chimney (not shown) extending centrally from the top of the screen 125. The provision of the chimney causes, during certain storm operating conditions, the rapid rise of the liquid level 168 within the vortex chamber 130 and hence aid in the priming of the siphon tube 195.

The foraminated and non-foraminated portions of the screen 125 may be arranged in a number of configurations to aid the scouring action of the vortex 165. Figures 15A to 15H show various possible configurations for f usto-conical screens 125 where the non-foraminated portion of the screen 125 is a circumferential band 220 or portion thereof 225, a quadrant 230 of the screen 125 or a combination of any of the above. A preferred embodiment is shown in Figures 15E and 15F where the non-foraminated portion of screen comprises a quadrant 230 of said screen 125 and a circumferential band 220 extending inwards from said screens outermost edge.

Baffle and flow direction means may be provided within the vortex chamber 130 to aid in the formation the vortex 165 and to direct solid matter towards the centre of said vortex. Said baffle means and flow direction means may be positioned around the inlet duct 150 in order to concentrate and/or direct the flow-stream entering the vortex chamber 130. Baffles may also be provided around the inner surface 235 of the vortex chamber. Figures 16A to 16D show various possible positions for baffles and flow directors within the vortex chamber 130. Tests have shown that the configuration shown in Figure 16B comprising an inlet extension 240 and a wall baffle 245 to be successful in achieving the desired results. Figures 17 and 18 show the sectional views indicated by arrows A-A and B-B.

Referring to Figures 16B, 17 and 18 there is shown an extension 240 of the inlet duct 150 which projects into the vortex chamber 130 and which comprises a side wall 250 and an upper wall 255. Said walls 250, 255 extend both from the inner surface 125 and conical base 120 of the vortex chamber 130 and serve to reduce the cross sectional area of the inlet duct 150. In an alternative embodiment, the extension 240 may serve to direct the inlet flow without reducing the cross sectional area of the in let duct 150. The walls 250, 255 in reducing the cross sectional area of the inlet duct 150 direct the inlet flow both toward the base 120 and the inner surface 235 of the vortex chamber 130. The wall baffle 245 extends from the conical base 120 of the vortex chamber 130 to the screen 125 atop said vortex chamber 130 and comprises a concave leading edge 260 and a concave trailing edge 265, the leading edge 260 having a radius of curvature greater than that of the trailing edge 265. Baffles and flow direction means provided within the separator may be adjustable allowing their positions to be altered depending, for example, on the inlet flow characteristics.

The separator devices 105 described hereinbefore operate on the principal that under normal inlet flow conditions all the liquid, solid and semi-solid matter entering the vortex chamber 130 is conveyed through the vortex chamber aperture 145, while the screen 125 is only employed under operating conditions experienced, for example, during and after a storm. Figures 19 to 22 show two possible embodiments in which the liquid level is maintained above the base of the screen.

Figures 19 and 20 show a separator 300 substantially as described hereinbefore comprising a vortex chamber 330 with a frusto conical screen 325, an annular chamber 335, a collection chamber 340, a tangential inlet duct 350 and outlet ducts 355, 360 from the annular and collection chambers 335, 340. The annular chamber outlet 355 is shrouded by a weir 365, the upper lip 370 of which is higher than the base 315 of the screen 325, and hence the liquid level 368 with the vortex chamber 330 must exceed the level of the lip 370 before fluid can pass to the annular chamber outlet 355.

Figures 21 and 22 represent an alternative embodiment wherein there is shown a separator 400 without an annular collection chamber. The wall 410 of the vortex chamber 430 extend above the base 415 of the screen 425 and there is provided an overflow box 470 which abuts the vortex chamber 430. An opening 475 is provided in the wall 410 of the vortex chamber 430 which permits liquid to flow from the vortex chamber 430 into the overflow box 470. Once again the lower lip 465 of the opening 475 is above the base 415 of the screen 425 and thus the liquid within the vortex chamber 430 is maintained at an elevated level. The advantages of maintaining the fluid level above the base of the screen include a greater screen area in contact with liquid within the vortex chamber 430 at lower flow rates, and automatic backwash when storm flow abates.

It should be understood that the embodiments of the invention described hereinbefore are given by way of example only, and these are intended to illustrate not limit the scope of the invention. Those appropriately skilled in this art will appreciate that the invention herein disclosed provides a simple and relatively easily constructed solution to long-standing problems experienced in this art.

Claims

1. A hydrocyclone separator (105) comprising a liquid vortex-forming chamber (130) provided with an inlet (150) for supply of a solids-contaminated liquid feed, and an outlet (145) for gravity-assisted outflow of liquids and solids, characterised in that the chamber is provided with a screen (125) positioned in an upper portion of the chamber, and arranged to interfere with raised liquid levels so as to inhibit solids passage whilst permitting liquid through- flow, the said screen having a surface which is at least partially foraminated and of a concave shape.

2. A hydrocyclone separator according to claim 1 characterised in that the screen has the form of an inverted dish or shallow cone and the surface thereof has a major portion which is foraminated whilst the minor portion is continuous to direct or deflect flow within the chamber.

3. A hydrocyclone separator according to claim 2 characterised in that the minor portion is an area of the screen surface which is approximately sector-shaped.

4. A hydrocyclone separator according to claim 3 characterised in that the minor portion is a quadrant of the screen surface.

5. A hydrocyclone separator according to any one of claims 1 to 4 characterised in that at least one baffle (240) is provided adjacent to a chamber wall in close proximity to the inlet for controlling or enhancing liquid flow as a vortex within the chamber.

6. A hydrocyclone separator according to any one of claims 1 to 5 characterised in that at least one baffle (245) is provided at the chamber wall on, or in close proximity to the screen for controlling or enhancing liquid flow across the screen surface.

7. A hydrocyclone separator according to any one of the preceding claims characterised by the provision of screen clearing aids such as driven brushes (175) or wiper blades, or water jetting nozzles (180) .

8. Use of a hydrocyclone according to any one of the preceding claims in a combined sewer overflow system.

9. A method of improving the performance of a combined sewer overflow system comprising a hydrocyclone for separation of solids from liquids, characterised by the provision of an inverted dish-shaped screen arranged in the hydrocyclone to interfere with raised liquid levels exceeding normal hydrocyclone capacity e.g. arising from storm conditions, so as to inhibit solids passage whilst permitting liquid through-flow, and the selection of the design parameters of the hydrocyclone so as to provide, during substantially the whole operational performance of the hydrocyclone under conditions exceeding normal capacity, in addition to a cyclonic liquid flow, a central air cone above the liquid vortex extending from the screen down to an outlet in the base of the hydrocyclone.

10. In a combined sewer overflow system comprising a hydrocyclone for separation of solids from liquids, the improvement consisting of the provision of an inverted dish- shaped screen arranged in the hydrocyclone to interfere with raised liquid levels exceeding normal hydrocyclone capacity e.g. arising from storm conditions, so as to inhibit solids passage whilst permitting liquid through-flow, the solids inhibited from passing the screen being generally removable from the screen surface by liquid scouring created by vortex flow within the hydrocyclone during the presence of raised liquid levels in the hydrocyclone or by gravity upon decline of liquid levels therein.