WO2006012687A1

WO2006012687A1 - A low shear centrifugal separator

Info

Publication number: WO2006012687A1
Application number: PCT/AU2005/001151
Authority: WO
Inventors: Edwin Edmunds; David Edward John Garman; Allan Meyer; Lincoln Black; Hayel Smair
Original assignee: Filtra Limited
Priority date: 2004-08-05
Filing date: 2005-08-03
Publication date: 2006-02-09
Also published as: AR050107A1; EP1796844A1; TW200607567A

Abstract

A centrifugal separator including: a spiralling conduit (17) rotatable around a central axis (y); an inlet (15) delivering a fluid flow including entrained solids or particles into one end of the spiralling conduit; a first outlet for receiving a portion of the fluid flow nearest a radially outward section of the spiralling conduit; and a second outlet for receiving the remainder of the fluid flow; wherein, the rotating spiralling conduit is effective in imparting a radially outward force to the entrained solids or particles in the fluid flow, whereby the entrained particles or solids are concentrated on the radially outward section of the spiralling conduit. Preferably, the outlets) are located at a point remote from the inlet along the spiralling conduit. Preferably, the spiralling conduit is housed around the inside of a cylindrical housing (10). Preferably, the second outlet is positioned for receiving a portion of the fluid flow nearest a radially inward section of the spiralling conduit. Preferably, there is a series of first or second outlets.

Description

A Low Shear Centrifugal Separator

The present invention relates to a centrifugal separator and in particular to a centrifugal separator that removes or reduces the quantity of entrained particles or solids contained within a fluid stream.

Background of the Invention

The separation of entrained solids and particles from liquid process streams remains a fundamental issue in many processes in a variety of industries.

An example is the separation of entrained particles or biomass from liquid process streams during a sewage treatment process. In this process stream, the entrained particles and biomass are characterised by having a very low density, are fragile in terms of physical composition and as such do not lend themselves to conventional separation processes.

Furthermore, it is desirable the biological materials present within a sewage stream are not subject to high shear forces nor the addition of further chemical agents during a separation process, so that the biomass materials maybe recycled back into the sewage treatment process to be reused.

The standard method of physical separation in sewage treatment process streams is sedimentation, and it usually takes the form of a clarifier. However, clarifiers are costly to construct, taking up 40% of the budget of a conventional sewage treatment plant and in addition they take up approximately 60% of the land area of the plant. Furthermore, in order to increase the capacity of the sewage treatment plant to take into account a growth in population or increased rainfall, another clarifier would need to be built making this a financial and sometimes logistic impossibility. A number of separation processes have been developed in an attempt to overcome the limitations of a clarifier. These include dissolved air floatation in a variety of forms from simple aeration systems to rapid capture with synthetic or natural materials. However in all cases, the addition of further chemicals is required and/or the equipment involves a large cost and sizeable footprint.

Another possibility when considering a replacement process for a clarifier in a sewage treatment plant, is the use of traditional centrifugal separators. However, these devices typically subject the process stream to high shear forces to produce a highly compacted solids output that can destroy the structure of the biomass such that it cannot be recycled back into the sewage treatment process.

The separation of sewage treatment process stream is typical of many problems experienced in many industries. Particularly, where the entrained solids or particles are not readily settled, or it is necessary to have their physical characteristics preserved without extreme compaction or chemical modification, or where there is limited space.

An alternative form of separation is filtration, which involves passing a fluid to be separated through a porous medium. Solid material is captured either due to the surface active properties of the filter medium or the size of the solid material relative to the filter apertures. Screening is a filtration process that relies solely on the relative size of the apertures and incoming solid material. A primary disadvantage of the filtration process is that the medium becomes fouled with time and must be cleaned. To achieve high removal rates for fine particles, back-flushing with complex process monitoring and control is required to ensure that filtration continues to be effective.

Taking into consideration the points highlighted above, the present invention seeks to provide a centrifugal separator which utilises centrifugal force to aid in the separation of entrained particles or solids whilst maintaining a high continuous processing rate. Furthermore, the present invention seeks to provide a centrifugal separator that does not subject the process stream to high shear forces, and that doesn't require the addition of any further chemical agents to the process stream.

Summary of the Invention

According to one aspect the present invention provides a centrifugal separator including: a spiralling conduit rotatable around a central axis; an inlet delivering a fluid flow including entrained solids or particles into one end of the spiralling conduit; a first outlet at a point remote from the inlet along the spiralling conduit for receiving a portion of the fluid flow nearest a radially outward section of the spiralling conduit; and a second outlet for receiving the remainder of the fluid flow; wherein, the rotating spiralling conduit is effective in imparting a radially outward force to the entrained solids or particles in the fluid flow, whereby the entrained particles or solids are concentrated on the radially outward section of the spiralling conduit.

The second outlet according to this aspect is preferably located at a point after the first outlet along the spiralling conduit. More preferably the second outlet is located at a sufficient distance after the first outlet whereby in use, the pressure differential between the first outlet and the second outlet is substantially equal.

Preferably, the spiralling conduit is housed around the inside of a cylindrical housing. The spiralling conduit may be rotatable in both the clockwise and counter-clockwise directions.

The cross-section of the spiralling conduit may have a circular or orthogonal appearance. Preferably, the cross-section is substantially orthogonal in shape, such as for example a rectangular shape where the length is longer than its height.

Alternatively, the cross-section of the spiralling conduit is substantially trapezoidal wherein the two parallel portions of the trapezoidal cross section are the radially inside surface and the radially outside surface of the spiralling conduit. Preferably, the length of - A -

the radially inside surface of the cross section of the spiralling conduit is longer than the radially outside surface wherein the bottom and top surfaces of the cross section of the spiralling conduit taper from the radially inside surface to the radially outside surface.

Preferably, the inside edges of the spiralling conduit are curved to avoid instances of fouling within the spiralling conduit. Preferably, the inside surface of the spiralling conduit has a low friction co-efficient and is more preferably coated with a non-stick material such as for example TEFLON, in order to reduce/avoid the instances of fouling.

Preferably, the length of the spiralling conduit circles around its axis greater than three times and more preferably the length of the spiralling conduit circles around its axis greater than 15 times and even more preferably greater than 25 times.

Preferably, the radially outward portion of the flow is separated from the remainder of the flow by a dividing portion situated within the spiralling conduit. Preferably, the dividing portion is substantially parallel to the radially inside and radially outside surfaces of the spiralling conduit. More preferably the radially outward portion of the flow is separated by the dividing portion prior to the first outlet, and even more preferably a distance of 5 cm to 75 cm along the spiralling conduit prior to the first outlet.

Preferably, the centrifugal separator further includes an offtake assembly for receiving the fluid flow from the first and second outlets wherein the offtake assembly includes: a first receiving portion for receiving the fluid flow from the first outlet; and, a second receiving portion for receiving the flow from the second outlet.

Preferably, the second receiving portion includes a sufficiently large cavity to receive the flow from the second outlet, whereby there is substantially no reverse pressure effect acting on the second outlet.

Preferably, the first receiving portion includes a sufficiently large cavity to receive the flow from the first outlet, whereby there is substantially no reverse pressure effect acting on the first outlet. Preferably, the second receiving portion is larger than the first receiving portion.

The offtake assembly may further include a first outlet in communication with the first receiving portion and a second outlet in communication with the second receiving portion.

The centrifugal separator may be arranged whereby the inlet is located at a height greater than the outlets such that gravitational force also aids the fluid moving through the centrifugal separator. More preferably the centrifugal separator is arranged in a vertical alignment. Alternatively, the centrifugal separator may be arranged such that the fluid flows in a horizontal direction from inlet to outlets.

The inlet may further include a pumping portion that aids in imparting a radially outward force to the fluid being delivered into the spiralling conduit. Preferably, the inlet is located on the central axis of the spiralling conduit and the pumping portion delivers the fluid to the beginning of the spiralling conduit.

Preferably, the radially inward wall of the spiralling conduit is at a constant radius from the central axis.

Alternatively, the distance from the inward wall of the spiralling conduit to the central axis increases along the spiralling conduit towards the first and second outlets.

According to another alternative, the distance from the inward wall of the spiralling conduit to the central axis decreases along the spiralling conduit towards the first and second outlets.

The spiralling conduit is preferably rotated at speeds of between 150 RPM to 4000 RPM and more preferably between 300 and 1000 RPM.

The pitch of the spiralling conduit may also be varied within the centrifugal separator. By increasing the steepness of the pitch, you increase the flow rate of the fluid passing through the spiralling conduit and thereby decrease the residence time of the fluid moving through the centrifugal separator. The pitch of the spiralling conduit may be calculated by measuring the vertical distance up the spiral for one revolution.

According to another aspect the present invention provides a centrifugal separator including: a spiralling conduit rotatable around a central axis; an inlet delivering a fluid flow including entrained solids or particles into one end of the spiralling conduit; a first outlet at a point remote from the inlet along the spiralling conduit for receiving a portion of the fluid flow nearest a radially outward section of the spiralling conduit; and a second outlet at a point remote from the inlet along the spiralling conduit for receiving a portion of the fluid nearest a radially inward section of the spiralling conduit; wherein, the rotating spiralling conduit is effective in imparting a radially outward force to the entrained solids or particles in the fluid flow, whereby the entrained particles or solids are concentrated on the radially outward section of the spiralling conduit.

The second outlet according to this aspect maybe located at a point before or after the first outlet along the spiralling conduit. Preferably, if the second outlet is located before the first outlet, the portion of the flow leaving through the second outlet is taken out from the inside of the spiralling conduit. Furthermore, there may be more than one second outlet sequentially positioned along the spiralling conduit each taking a portion of the fluid nearest a radially inward section of the spiralling conduit.

According to another aspect the present invention provides a centrifugal separator including: a spiralling conduit rotatable around a central axis; an inlet delivering a fluid flow including entrained solids or particles into one end of the spiralling conduit; a plurality of first outlets at sequential points remote from the inlet along the spiralling conduit for receiving a portion of the fluid flow nearest a radially outward section of the spiralling conduit; and a second outlet for receiving the remainder of the fluid flow; wherein, the rotating spiralling conduit is effective in imparting a radially outward force to the entrained solids or particles in the fluid flow, whereby the entrained particles or solids are concentrated on the radially outward section of the spiralling conduit.

By varying the various parameters of the centrifugal separator, such as for example: the pitch of the spiralling conduit; the length of the spiralling conduit; the radial distance from the central axis to the radially inward wall of the spiralling conduit; the speed at which the spiralling conduit is rotated; and/or the orientation of the centrifugal separator, the centrifugal separator may be tailored for a variety of different applications. Specifically, the revolutions per minute (RPM) and the radial distance from the central axis to the radially inward wall of the spiralling conduit directly impact on the centrifugal loading on the fluid moving through the centrifugal separator.

The centrifugal separator may be used in various applications where removal of entrained or suspended particles or solids is desirable from a fluid stream. Entrained or suspended particles may be in the form of a fluid of different density, such as for example oil in water or alternatively gas bubbles in a liquid stream. Solids can be any solid or immiscible liquid found in a fluid stream, and a fluid may be any liquid or gas.

The centrifugal separator may be external to the medium being separated by it, or alternatively, the separator may be submerged within it similar to a submersible pump, such as for example inside a biological reactor in a waste water treatment plant. Furthermore, the several centrifugal separators may be linked in series to provide an even greater separation result. In such an arrangement, each centrifugal separator in series may be individually designed to separate out solids with particular characteristics.

According to one aspect, the present invention may be used to remove entrained particles or solids from flow streams in wastewater treatment plants. In this particular application, the entrained particles or solids may be as small as microorganisms as well as various solids from biological sources. Furthermore, the centrifugal separator may be used for water purification applications.

According to another aspect, the centrifugal separator may be used in accordance with other applications where the separation of particles or solids from a fluid stream is desirable, such as for example applications found in the manufacture of paper products; oil refining, mining, and the purification of industrial waste water.

Brief Description of the Drawings

The present invention will become better understood from the following detailed description of a preferred but non-limiting embodiment thereof, described in connection with the accompanying drawings, wherein:

Figure 1 illustrates a cross-sectional view of a centrifugal separator in accordance with one aspect of the present invention;

Figure 2 illustrates a perspective view detailing the spiralling conduit of the centrifugal separator; Figure 3 illustrates the cross-sectional view of the inlet to the centrifugal separator;

Figure 4 illustrates an alternative cross-sectional view of the inlet of the centrifugal separator;

Figure 5 illustrates the centrifugal separator highlighting the first and second outlet points; Figure 6 illustrates a schematic view of the first and second outlets of the centrifugal separator;

Figure 7 illustrates an alternative schematic view looking down vertically onto the first and second outlets of the centrifugal separator;

Figure 8 illustrates a schematic view of a first outlet of the centrifugal separator; and,

Figure 9 illustrates a schematic view of a second outlet of the centrifugal separator; Figure 10 illustrates a schematic cross-sectional view of a centrifugal separator in accordance with, another aspect of the present invention;

Figure 11 illustrates a detailed cross-sectional view of one part of the spiralling conduit including a first outlet; Figure 12 illustrates a detailed cross-sectional view from above of a section of the spiralling conduit including the first and second outlets;

Figure 13 illustrates a cross-sectional view from above and one side of the centrifugal separator; and,

Figure 14 illustrates a side elevation of the centrifugal separator with the offtake assembly shown as transparent for clarity.

Detailed description of the drawings

Referring to figure 1, there is shown a cut away cross section of a centrifugal separator 10 with a fluid inlet 15 at the top of the centrifugal separator 10. The centrifugal separator 10 comprises a cylindrical body that includes a spiralling conduit 17 around the inside. The centrifugal separator 10 is able to rotate around a central axis y driven by a drive motor (not shown).

Fluid including entrained or suspended solids enters the centrifugal separator 10 via the inlet 15 where it then proceeds through the spiralling conduit 17. As the centrifugal separator 10 rotates, the rotational force combined with the spiralling pathway of the conduit 17, acts upon the entrained or suspended particles in the fluid. As a result, the entrained or suspended particles move towards the radially outward section of the conduit 17 closest to the outside body portion of the centrifugal separator 10.

Once the entrained or suspended solids are concentrated at the radially outward section of the conduit 17, this section of the fluid flow is directed off through the first outlet, thereby concentrating the entrained or suspended solids into this flow stream.

The remainder of the fluid flow continues on through the centrifugal separator 10 and out of the second output. The flow from this stream is much lower in concentration of the entrained or suspended particles. It is also envisaged that further first outlets may be present along further points along the length of the spiralling conduit, where more of the flow from the radially outward section of the conduit is separated out from the remainder of the flow passing out via the second output. This results in less flow exiting the second output, but results in a lower concentration of entrained or suspended solids.

At figure 2, detail B shows a magnified view of a section of the spiralling conduit 17. As can be seen, the cross-section of the conduit is substantially rectangular in shape with the width of the cross-section greater than the height. Furthermore, the corners of the rectangular cross-section 22 at the radially outward section are curved so as to prevent build up of material and subsequent fouling of the centrifugal separator 10. The inside of the radially outward section of the conduit may also be coated with a non stick surface coating such as TEFLON, or alternatively, the entire inside surface of the conduit may be coated as such.

Referring now to figures 3 and 4, there is shown the inlet portion of the centrifugal separator in more detail. As can be seen, as the fluid including the entrained or suspended particles enters the inlet, it proceeds along pumping portion 25 which leads the fluid into the spiralling conduit and aids in imparting a radially outward force into the fluid.

Referring now to figures 5 and 6, there is shown the first outlet 30 and the second outlet 35 which are located towards the end of the spiralling pathway (not shown). Referring now to figure 7 there is depicted an alternative perspective looking down onto the first outlet 30 and second outlet 35. Here the portion of the flow 40 closest to the radially outward wall of the spiralling conduit 17 is directed out of the first outlet 30 by a protrusion or division 45 into the path of the flow within the spiralling conduit 17. The protrusion or division may be part way extended 46 or fully extended 47 into the path of the flow of the spiralling conduit where in effect the protrusion or division cuts the flow into the portion containing the majority of the entrained solids, that being the portion of the flow 40 closest the radially outward wall of the spiralling conduit 17, and into the remaining portion of the flow 50 which is substantially free of the entrained solids.

As is shown, the second outlet 35 is immediately after the first outlet 30. This provides a significant advantage to the pressure differential between the two outlets providing more solids and entrained particles are present in the portion of the flow 40 captured by the first outlet 30.

Referring to figure 8 and 9 there is shown an alternative view of the first outlet 30 along the section B-B and section C-C shown in figures 6 and 7. Here it is shown that the first outlet 30 directs the portion of the flow nearest the radially outward wall of the spiralling conduit downward into a collection point 60 of an offtake assembly. Conversely the second outlet 35 directs the remainder of the flow upward into a collection point 65 of the offtake assembly. The collection points 60 and 65 of the offtake assembly do not rotate with the spiralling conduit 17 but remain static and direct the respective flows away from the centrifugal separator.

The present invention will become better understood from the following example of preferred but non-limiting embodiments thereof.

Example 1

A centrifugal separator, in accordance with that depicted in figures 1 to 9, and with the dimensions indicated in Table 1 below, was operated for a period of 60 minutes with an input flow rate of 30 L/min. Water was fed into the centrifugal separator which included entrained finely ground PLIOLITE with a specific gravity of 1.03 at a concentration of 150 -200 mg/L. On passing through the centrifugal separator the process stream was divided into the concentrate exiting from the first outlet, and the filtrate exiting from the second outlet. Table 1

The fluid received as concentrate and filtrate was analysed at 10 minute intervals and the Total Suspended Solids (TSS) of the fluid captured in each output stream was measured. These results are tabulated in Table 2.

Table 2

As can be seen from the results, the centrifugal separator has increased the Total Suspended Solids in the concentrate stream exiting from the first outlet by more than 500%.

Referring now to figures 10 to 14, there is shown a centrifugal separator in an alternative embodiment of the present invention. Referring to figure 10, there is shown a cross sectional perspective view of a centrifugal separator 100 which includes an inlet 105 for fluid flow including entrained particles, a first outlet 110 for receiving the flow with the majority of the entrained particles after separation, and a second outlet 115 for receiving the remainder of the flow. Once the fluid including the entrained particles enters the inlet 105 it moves onto a spiralling conduit 120 that winds around the inside of the cylindrical housing 125. The spiralling conduit 120 circles around itself 32 times in this particular embodiment at a constant distance from its axis. During operation, the spiralling conduit 120 is rotated at speeds of between 100 to 2000 RPM, whereby the centrifugal force created in conjunction with the flow rate of the fluid passing through the spiralling conduit imparts a radially outward force on to the entrained particles. Under this force, the entrained particles present in the fluid move to the radially outside surface of the spiralling conduit 120.

Referring now to figure 11, the cross sectional shape of the spiralling conduit 120 is clearly displayed. The cross-section is in the shape of a trapezoid whereby the radially inside surface 140 and the radially outside surface 130 are parallel, with the bottom surface 145 and the top surface 150 tapering together towards the radially outside surface 130. When the spiralling conduit 120 is used which possesses such a shaped cross-section, it has been found to facilitate the movement of entrained solids to the radially outside surface 130 as the process fluid passes through the separator 100.

A flow divider or protrusion 155 is shown within the top most segment of the spiralling conduit 120. This divider 155 effectively cuts the flow of the fluid passing through the spiralling conduit 120. The fluid flow closest the radially outside surface 130 of the spiralling conduit 120 including the majority of the entrained solids is separated from the remainder of the flow by the divider 155 and passes to the first inlet 110.

Referring now to figure 12, the relationship between the first and second outlets is more clearly shown with a detailed cross-sectional view from above. The fluid including the majority of the entrained solids and closest to the radially outside surface 130 of the spiralling conduit 120 is divided from the remainder of the flow by the divider and proceeds to the first outlet 110. The remainder of the flow proceeds further down the spiralling conduit 120 until it exits from the second outlet 115.

The divider 155 is positioned a substantial distance before the first outlet 110 of approximately 20 cm in this particular embodiment. By dividing the fluid flow well before the first outlet 110, any turbulence or pressure differential provided by the fluid exiting from the outlet 110 does not effect the point along the spiralling conduit where the fluid is separated by the divider 155.

Referring now to figure 13 there is shown the centrifugal separator 100 in conjunction with an offtake housing 160. In operation the centrifugal separator 100 rotates around the axis •point y, whilst the offtake housing 160 remains stationary. The first and second inlets 110, 115 rotate with the centrifugal separator 100 and release fluid into a first and second receiving cavities 165, 170 respectively. The fluid once caught in the first receiving cavity 165 and the second receiving cavity 170 is then released from the arrangement via a first outlet 175 and a second outlet 180 respectively.

Referring now the figure 14, it can be seen that the receiving cavity 170 for receiving the majority of the portion of the fluid flow via the second outlet 115 is quite large with a substantial drop from the height of the second outlet 115. This provides that the fluid being received by this cavity 160 does not have any reverse pressure effect on fluid being separated further down the spiralling conduit 120 from the outlets.

Example 2

A centrifugal separator, in accordance with that depicted in figures 10 to 14, and with the dimensions indicated in Table 2 below, was operated continuously in a stable manner for three hours at a time. The unit operated continuously with the volumetric mass balance 100% accurate, without the required addition of dilution water. Table 2

The unit consistently achieved a 'solids' stream with at least 4-6 times the input streams solids loading, in 10 - 20% of the water volume over a range of operating conditions including RPM' s and barrel flow rates. Using a Nylon suspension (to represent entrained particles) these results are achieved between 400 and 800 rpm's. These results have been achieved with flows ranging between 1 - 1.5 L/sec.

The unit consistently achieved a 'liquid' stream with less than 10 - 15% the input streams solids loading, in 80 - 90% of the water volume over a range of operating conditions including RPM's and barrel flow rates. Using a Nylon suspension these results are achieved between 600 and 1000 rpm's. These results have been achieved with flows ranging between 1 - 1.5 L/sec.

Finally, it can be understood that the inventive concept in any of its aspects can be incorporated in many different constructions so that generality of the preceding description is not superseded by the particularity of the attached drawings. Various alterations, modifications and/or additions may be incorporated into the various constructions and arrangements of parts without departing from the spirit or ambit of the present invention.

Claims

The claims

1. A centrifugal separator including: a spiralling conduit rotatable around a central axis; an inlet delivering a fluid flow including entrained solids or particles into one end of the spiralling conduit; a first outlet at a point remote from the inlet along the spiralling conduit for receiving a portion of the fluid flow nearest a radially outward section of the spiralling conduit; and a second outlet for receiving the remainder of the fluid flow; wherein, the rotating spiralling conduit is effective in imparting a radially outward force to the entrained solids or particles in the fluid flow, whereby the entrained particles or solids are concentrated on the radially outward section of the spiralling conduit.

2. A centrifugal separator according to claim 1 wherein the second outlet is located at a point after the first outlet along the spiralling conduit.

3. A centrifugal separator according to claim 2 wherein the second outlet is located at a sufficient distance after the first outlet whereby in use, the pressure differential between the first outlet and the second outlet is substantially equal.

4. A centrifugal separator according to any one of claims 1 to 3 wherein the spiralling conduit is housed around the inside of a cylindrical housing.

5. A centrifugal separator according to any one of claims 1 to 4 wherein the spiralling conduit is rotatable in a counter-clockwise or clockwise direction.

6. A centrifugal separator according to any one of claims 1 to 5 wherein the cross-section of the spiralling conduit is substantially orthogonal.

7. A centrifugal separator according to any one of claims 1 to 5 wherein the cross-section of the spiralling conduit is substantially trapezoidal.

8. A centrifugal separator according to claim 7 wherein the two parallel portions of the trapezoidal cross section are the radially inside surface and the radially outside surface of the spiralling conduit.

9. A centrifugal separator according to claims 7 and 8 wherein the length of the radially inside surface of the cross section of the spiralling conduit is longer than the radially outside surface.

10. A centrifugal separator according to any one of claims 7 to 9 wherein the bottom and top surfaces of the cross section of the spiralling conduit taper from the radially inside surface to the radially outside surface.

11. A centrifugal separator according to any one of claims 1 to 10 wherein the comers of the cross section of the spiralling conduit are substantially curved.

12. A centrifugal separator according to any one of claims 1 to 11 wherein the inside surface of the spiralling conduit has a low-friction coefficient and is preferably coated with a non-stick material such as for example TEFLON.

13. A centrifugal separator according to any one of claims 1 to 12 wherein the length of the spiralling conduit circles around its axis greater than three times.

14. A centrifugal separator according to any one of claims 1 to 12 wherein the length of the spiralling conduit circles around its axis greater than 15 times and more preferably greater than 25 times.

15. A centrifugal separator according to any one of the proceeding claims wherein the radially outward portion of the flow is separated from the remainder of the flow by a dividing portion within the spiralling conduit.

16. A centrifugal separator according to claim 15 wherein the dividing portion is parallel to the radially inside and radially outside surfaces of the spiralling conduit.

17. A centrifugal separator according to claim 15 or claim 16 wherein the radially outward portion of the flow is separated by the dividing portion prior to the first outlet, and preferably 5 cm to 75 cm along the spiralling conduit prior to the first outlet.

18. A centrifugal separator according to any one of claims 1 to 17 further including an offtake assembly for receiving the fluid flow from the first and second outlets wherein the offtake assembly includes: a first receiving portion for receiving the fluid flow from the first outlet; and, a second receiving portion for receiving the flow from the second outlet.

19. A centrifugal separator according to claim 18 wherein the second receiving portion includes a sufficiently large cavity to receive the flow from the second outlet, whereby there is substantially no reverse pressure effect acting on the second outlet.

20. A centrifugal separator according to claim 18 or 19 wherein the first receiving portion includes a sufficiently large cavity to receive the flow from the first outlet, whereby there is substantially no reverse pressure effect acting on the first outlet.

21. A centrifugal separator according to any one of claims 18 to 20 wherein the second receiving portion is larger than the first receiving portion.

22. A centrifugal separator according to any one of claims 18 to 22 wherein the offtake assembly further includes a first outlet in communication with the first receiving portion and a second outlet in communication with the second receiving portion.

23. A centrifugal separator according to any one of claims 1 to 22 wherein the centrifugal separator is arranged whereby the inlet is located at a height greater than the outlets such that gravitational force also aids the fluid moving through the centrifugal separator and preferably in a substantially vertical alignment.

24. A centrifugal separator according to any one of claims 1 to 23 wherein the inlet includes a pumping portion that aids in imparting a radially outward force to the fluid being delivered into the spiralling conduit.

25. A centrifugal separator according to claim 24 wherein the inlet is located on the central axis of the spiralling conduit and the pumping portion delivers the fluid to the beginning of the spiralling conduit.

26. A centrifugal separator according to any one of claims 1 to 25 wherein the spiralling conduit is at a constant radius from the central axis.

27. A centrifugal separator according to any one of claims 1 to 25 wherein the distance from the inward wall of the spiralling conduit to the central axis increases along the spiralling conduit towards the first and second outlets.

28. A centrifugal separator according to any one of claims 1 to 25 wherein the distance from the inward wall of the spiralling conduit to the central axis decreases along the spiralling conduit towards the first and second outlets.

29. A centrifugal separator according to any one of claims 1 to 28 wherein the spiralling conduit is rotatable at speeds of between 150 RPM to 4000 RPM and more preferably between 300 and 1000 RPM.

30. Use of the centrifugal separator according to any one of claims 1 to 29 to remove entrained particles or solids from flow streams in wastewater treatment plants.

31. Use of the centrifugal separator according to any one of claims 1 to 29 for water purification.

32. Use of the centrifugal separator according to any one of claims 1 to 29 for the purification of industrial wastewater.

33. A centrifugal separator including: a spiralling conduit rotatable around a central axis; an inlet delivering a fluid flow including entrained solids or particles into one end of the spiralling conduit; a first outlet at a point remote from the inlet along the spiralling conduit for receiving a portion of the fluid flow nearest a radially outward section of the spiralling conduit; and a second outlet at a point remote from the inlet along the spiralling conduit for receiving a portion of the fluid nearest a radially inward section of the spiralling conduit; wherein, the rotating spiralling conduit is effective in imparting a radially outward force to the entrained solids or particles in the fluid flow, whereby the entrained particles or solids are concentrated on the radially outward section of the spiralling conduit.

34. A centrifugal separator according to claim 33 wherein the second outlet is located before the first outlet, and the portion of the flow leaving through the second outlet is taken out from the inside of the spiralling conduit.

35. A centrifugal separator according to claim 33 or claim 34 wherein there is a plurality of second outlets sequentially positioned along the spiralling conduit, each second outlet taking a portion of the fluid nearest a radially inward section of the spiralling conduit.

36. A centrifugal separator including: a spiralling conduit rotatable around a central axis; an inlet delivering a fluid flow including entrained solids or particles into one end of the spiralling conduit; a plurality of first outlets at sequential points remote from the inlet along the spiralling conduit for receiving a portion of the fluid flow nearest a radially outward section of the spiralling conduit; and a second outlet for receiving the remainder of the fluid flow; wherein, the rotating spiralling conduit is effective in imparting a radially outward force to the entrained solids or particles in the fluid flow, whereby the entrained particles or solids are concentrated on the radially outward section of the spiralling conduit.