EP1299600A1

EP1299600A1 - Undershot overflow filter

Info

Publication number: EP1299600A1
Application number: EP01949111A
Authority: EP
Inventors: Scott Wade Roy; Allan K. Wallace
Original assignee: Ecosol Pty Ltd
Current assignee: Ecosol Pty Ltd
Priority date: 2000-07-12
Filing date: 2001-07-12
Publication date: 2003-04-09
Also published as: CA2415216A1; KR20030040343A; NZ523794A; EP1299600A4; WO2002004755A1; JP2004502063A

Abstract

A filter (10) for concentrating a fluid stream (72-74) containing particulate matter (74-76) or for filtering from a fluid stream certain material is described. The filter consists of a plurality of solid elements (a, b, c, d) in an array having gaps (g) between adjacent solid elements through which spill flow of the fluid passes. The fluid stream (72-74) and filter (10) are orientated with respect to each other such that the spill flow of fluid through adjacent gaps is bounded by a dividing streamline that terminates on a solid element intermediate adjacent gaps (g). The stream and filter elements are also arranged so that the incidence of the dividing stream line to the plane of the array is such that particulate matter larger than the gaps are deflected out of the spill flow between the gaps. To optimally maintain the incidence at least two chambers are located on the spill flow side of the filter. To further optimise the operation of the filter the one or ore chambers each have spill flow controls sized and positioned to allow spill flow to exit each chamber so as to maintain the incidence. An example of the use for the filter is given in concentrating sewage at locations where sewage overflow would otherwise occur.

Description

UNDERSHOT OVERFLOW FILTER

This invention relates to a fluid filter apparatus for use in fluid/ particulate matter separation/ concentration, and in one particular example of the separation of fluid from fluid entrained with waste material, particularly sewage waste.

BACKGROUND

The process of filtering fluids and their fluid borne elements can be viewed in two ways. In one way it is the act of removing an unwanted part of what is flowing in the fluid much like the action of sieving. In another way it is the act of separating a portion of the fluid out of the fluid and thus concentrating the remaining fluid.

Clearly, a filter element can be used in either operation but what is clear from the known prior art is that regardless of the desired action there will be blocking of the filter to some degree. Once a portion of the filter is blocked its efficiency reduces and over time the filter will completely block. Maintenance of such filters is an unwelcome and typically expensive necessity.

In this specification, a filter element used in a suitable filter arrangement will be described in relation to its application to storm water and sewage overflow conditions and in particular to a concentration application in overflow conditions. It should however, be understood that the filter element and the filter arrangement concepts discussed herein, are used as an example only and that the arrangements described can with appropriate adjustment be used for separation and concentration applications.

It is a practice in some countries or with some water authorities in certain countries to run both storm water and sewage in the same pipe system. The pipe system carrying sewage typically terminates at either a treatment plant or a water outflow sometimes into rivers and other times into the sea. Regardless of whether sewage runs in pipes separated from storm water or combined overflow conditions can arise.

Engineers responsible for designing such systems can identify the maximum volume of combined storm water and sewage that can be accommodated into the water treatment plant and hence the capacities of respective pipe systems flowing into that treatment plant.

When these maximums are exceeded, at one or more of the potential bottlenecks in the pipe systems, alternative flow paths including pipes and open-air conduits communicate the overflow to other parts of the system or directly to an acceptable outflow point.

The simplest way of providing an overflow mechanism is to include a weir over which flow volumes greater than the maximum calculated, can be directed away from the main flow of storm water and sewage.

A simple weir has the benefit of being easy to implement and can be installed either at the time of the creation of the pipe system or with difficulty, retrofitted into an existing pipe work system.

The great disadvantage of simple weirs is that the storm water and sewage is diverted untreated into the alternative route. In some instances, this untreated overflow is exposed to the atmosphere as it travels along open culverts and the like thus creating a potential health hazard. Furthermore, weirs create a head-loss in the pipe system, which is often unacceptable and requires expensive changes to the existing pipe work.

In some systems when flow volumes are greater than the maximum that can be accommodated, the pipe systems fill and the contents backup and at certain upstream inspection points, untreated overflow escapes the pipe system onto roads, parks and footpaths. This occurrence is liable to create health problems and becomes an embarrassment to the relevant water authority.

Not unsurprisingly, a common alternative to a simple weir is the use of a filter element which is intended to remove solid wastes from the overflow and hence lessen the health related risks. This concentrating function is designed to leave the unwanted sewage in the pipe and only overflow relatively clean fluid. Such a filter is designed typically to come into operation only when the overflow condition occurs. Unfortunately, the filters and filter arrangements tried thus far, will eventually block and become inoperative. Surprisingly, it is not the solid sewage waste that causes the majority of blockages but in fact cellulose fibres are the cause. These fibres are the result of the breakdown of toilet paper. Cellulose fibres are long and easily bridge between the elements of the various filters used thus far. As time progresses, the fibres build up until a portion of the available flow path through the filter is blocked. This portion grows, eventually ceasing the flow of fluid through the filter.

Worse still, even though a first period of overflow ceases, accumulated fibres do not simply fall from the filter elements but harden and form a massive paper mache-like obstruction over portions of the filter. This built up mass is limpid-like and does not dislodge under normal environmental conditions and requires manual, mechanical or high pressure back washing to remove it from the filter element.

If maintenance is not undertaken on a regular basis, expensive as it is, then it is to be expected that the mass will increase and the filter will eventually become completely blocked.

When a filter does become blocked, it merely means that filtered storm water/ sewage will not be able to overflow as required through the filter and that alternative overflow mechanisms must also be put in place. These alternative mechanisms allow unf iltered storm water and sewage to be diverted from the main flow pipe system and ultimately are no better than a simple weir. Alternatively, incoming sewage backups the existing pipe system and overflows at some other point further away from the treatment plant.

Thus, it is an object of the invention to reduce, minimize or eliminate some at least of the above-mentioned problems or at least provide an alternative, not only for storm water and sewage pipe works systems but also for any filter element used for separation or concentration.

Specific embodiments of the invention will now be described in some further detail with reference to and as illustrated in the accompanying figures. These embodiments are illustrative, and not meant to be restrictive of the scope of the invention.

BRIEF DESCRIPTION OF THE INVENTION

In a broad aspect, the invention is a filter for concentrating or separating in a fluid stream containing particulate matter, the filter consists of: a plurality of solid elements in an array having gaps between adjacent solid elements through which spill flow of the fluid passes, and the fluid stream and filter orientated with respect to each other such that the spill flow of fluid through adjacent gaps is bounded by a dividing stream line that terminates on a solid element intermediate adjacent gaps, and further arranged so that the incidence of the dividing stream line to the plane of the array is such that particulate matter larger than said gaps are deflected out of the spill flow between said gaps.

In a further aspect of the invention the filter consists of at least two chambers on the spill flow side of the filter arranged to receive spill flow so as to maintain substantially the same incidence along the fluid flow length of the filter adjacent each said chamber. In yet a further aspect of the invention of the filter described above, the one or more chambers have spill flow control means sized and positioned to allow spill flow to exit each chamber so as to maintain the incidence.

Suggestions and descriptions of other embodiments may be included within the scope of the invention, but they may not be illustrated in the accompanying figures or alternatively features of the invention may be shown in the figures but not described in the specification.

BRIEF DESCRIPTION OF THE FIGURES

Fig 1 depicts a top perspective view of a filter arrangement according to the invention;

Fig 2 depicts a cutaway version of the filter arrangement of Fig 1;

Fig 3 depicts a top view of the filter arrangement of Fig 1;

Fig 4 depicts a side view of the filter arrangement of Fig 1;

Fig 5 depicts a further embodiment of the filter arrangement of Fig 1;

Fig 6 depicts a close up perspective view of the filter element;

Fig 7 depicts a side view of the principle of dividing streamline separation,

Fig 7a depicts a variant of the filter element depicted in Fig 7;

Fig 8 depicts a top view of a further embodiment of a filter assembly showing individual partitions located above a filter element;

Fig 9 depicts a side view of the filter assembly of Fig. 8;

Fig 10 depicts a perspective cut away view of the filter element used in the embodiments for Figs. 8 and 9; and

Fig 11 depicts a top persecutive view of the filter element used in the embodiments for Figs. 8 and 9.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

A number of embodiments of inline filter arrangements will be described. The embodiments described first can be considered larger scale variants of a specific portion of the later embodiments. The described embodiments described may have advantages in applications not explicitly referred to herein but the later embodiments have been found to be particularly useful in the storm water/ sewage overflow situation, but of course may have use in other applications.

In the embodiment depicted in Figs. 1 to 5, an inline filter arrangement 10 is installed along an existing storm water/ sewage pipe 12 in which storm water and sewage flow from pipe 12 to pipe 12' through the filter arrangement 10. An overflow pipe 14 runs parallel to pipe 12' for a distance after the filter arrangement 10.

The inline filter arrangement depicted in this embodiment is preferably arranged so that it has the same fall as the pipe 12 between the filter arrangement inlet 16 to the filter arrangement outlet 18.

The inline filter arrangement can be used at any location along or at the termination point of a storm water pipe system. The filter arrangement may also be used other than in-ground situations and may also be used in storm water only systems as well as industrial fluid filtering applications.

Fluid and entrained pollution/ sewage enters the filter arrangement 10 via pipe 12 and inlet 16 so as to pass below a filter element 20 which in this embodiment comprises a "cheese grate" screen as will be described in detail later in the specification.

As best depicted in Figs 2, 3 and 4 the filter element 20 in this embodiment is shown running parallel to the lower surface 22 of the filter arrangement 10. Arrows marked 'M' show the continuation flow of incoming storm water and sewage while arrows marked 'F' indicate the flow of filtered water (spill flow). In other embodiments not shown, the filter element 20 may be sloped so that it is closer to the lower surface 22 at the outlet end than the inlet end of the filter arrangement 10. In another embodiment not shown, the filter may be angled so that the side of the filter is higher on the side of the filter arrangement at which wall 24 is located and lower on the opposite side which is nearer the middle of the filter arrangement 10.

The height of the filter element 20 above the lower surface 22 is generally determined with knowledge about the range of the rate of incoming storm water and sewage entering the filter arrangement and the particular rate is known, about 50% of the pipe work inner diameter, that should be reached before the incoming fluid needs to be concentrated so as to produce a spill flow. In this embodiment, as the particular incoming flow rate range is known, the height of the filter element is set above the lower surface 22 at approximately 50% of the diameter of the incoming and pipe 12. This setting is such that at that a predetermined incoming flow rate, storm water and sewage will rise to that level and the filter/ concentration action will commence.

The final height of the filter is also dependent upon other factors, such as filter element 20 efficiency, the dimensions of the incoming pipe 12, the filter assembly dimensions in length and breadth and the outgoing pipe 12' dimensions (which are typically the same as the incoming pipe 12). Thus even though in Figs 1 to 5 the length of the filter arrangement 10 seems long compared with the diameters of the pipe work (12, 12' and 14) that length may be shorter or longer depending on the volumes of incoming and spill flow desired and the type of fluid or entrained particulate matter.

Once the filter arrangement is in operation and a spill flow is produced, filtered fluid flowing through the filter element 20 falls off the side of the filter element nearest the middle of the filter assembly 10 into a overflow chamber 25 which is preferably of equal volumetric dimensions to that of the chamber on the filter side of the filter arrangement 10. The overflow chamber 25 communicates filtered fluid to the filtered fluid outlet 26 of the filter arrangement, which is in fluid communication with overflow pipe 14. The reason the overflow chamber 25 is the same dimensions as the filter size dimensions is so that if necessary incoming fluid can be diverted without restriction.

The term "undershot overflow filter" best describes the use of the filter element in this embodiment as the bulk of fluid containing particulate matter (contaminates) flows under the filter element (continuing flow) and the filtering (concentration) function of the filter arrangement is used to cope with an overflow (spill flow) condition.

The flow of incoming fluid is arranged to submerge the filter element 20 to a depth where the vertical components of velocity of the fluid are small relative to the flow passing below the filter so that fluid passes up through the filter element but not the particles contained in the fluid passing below except those smaller than the gap. This characteristic of the filter arrangement is described in greater detail later in the specification and is referred to as dividing stream separation. The conditions to achieve dividing stream separation can be achieved in a number of ways. When using the embodiment disclosed in Fig 1, an arrangement of weirs and/ or header tanks can be positioned downstream of the filter assembly to create some of the conditions required.

The filter arrangement 10 into which the filter element 20 is positioned is used in conjunction with weirs and/ or header tanks to control the relative velocities of the water flowing parallel and normal to the filter mesh so that the storm water sewage approaches the plane of the filter element 20, in this example a mesh, at an angle of less than approximately 3% which is an expression representative of the velocity components normal and parallel to the plane of the filter mesh. This is referred to as "the incidence" in this specification. There is a face velocity (normal component) defined as the spill flow volume rate divided by the face area of the filter element from whence it came. There is a continuation flow velocity (parallel component) defined as the mean velocity component parallel to the filter element of that continuation flow at a point outside the boundary layer.

The particular incidence of 3% in this embodiment is merely preferable and is used by way of example only and is not meant to be limiting in any way.

The incidence can also be varied by adjusting the depth of the spill flow fluid above the filter element relative to the depth of the continuation flow that eventually flows downstream of the filter.

Methods and apparatus for adjusting both of these depths include the placement of valves, restrictions and weirs external and internal of the filter arrangement. As the incoming flow rate changes, the depths can be optimised and controlled by, for example, using notched weirs where the depth of water upstream of the weir has a desired relationship to the flow over the weir or by using apertures in multiple isolating chambers above portions of the filter element as will be described in more detail later in this specification.

In some applications it may be beneficial to use a non-planar filter element so that the incidence of the fluid to the filter element at various regions along the filter element can be made uniform over the entire area of the filter element for the widest range of input flow rates. The incidence in this case is then referred to by way of reference to the velocity ratio below the local plane of the filter element ie. that average plane in this embodiment below a portion of the filter element. Thus although not shown in any of the figures the filter element may be curved or comprise planar sections set at different angles relative to adjacent sections so as to provide a curved (non-planar) shape but will have an average plane for any particular region of the filter element for the purposes of referring to the incidence.

A further embodiment of the filter arrangement is depicted in Fig 5 that shows a wall 28 positioned at one side and rising above the filter element 20 along the longitudinal center line of the filter arrangement 10. The wall 28 causes fluid and small amounts of entrained pollution, that has flowed up through the filter element 20, to flow in the direction of the continuation flow 'M' until it reaches aperture 30 in wall 28. The filtered fluid flow 'F' (spill flow) passes through the aperture and falls into the overflow chamber 25 and then flows into the overflow pipe 14. Again the length of the filter arrangement 10 is shown to be long with respect to the diameter of the pipe work but this is merely an example and it could be longer or shorter dependant on the sewage pipe work.

In other applications the filter arrangement is likely to be different but the spatial arrangement of the filter element to the flows will be the same.

At the inlet region of the filter arrangement 10, the inlet 16 opens to a continuation flow chamber 32. This chamber has a lower surface common to the lower surface 22 of the filter arrangement such that incoming fluid flows towards the outlet 18 of the filter arrangement along the same gradient. The incoming fluid will continue to flow as continuation flow 'M', through the filter arrangement 10 until the incoming flow of water is such that the level of fluid in the filter arrangement rises above the filter element 20. Even though the fluid will continue to flow through the inlet 16 and outlet 18 of the filter arrangement 10, it will also pass a portion of the fluid (spill flow) flowing into the arrangement through the filter element 20 thus allowing more fluid to pass into the filter arrangement 10. It has been found preferable to shape the region at the entrance of the filter 34 so as to streamline the flow of, in this example, storm water and sewage under the dividing wall 36 which precedes the filter element 20. This streamlining comes into effect when the flow of fluid into and out of the filter arrangement is such that the level of the incoming storm water and sewage rises above the filter so as to smooth the passage of the continuation flow under the filter element.

In this embodiment, as depicted in Fig 4, a rounded half circle shape 35 is located across the bottom of wall 36 so as to streamline the flow of water entering the filter arrangement 10 from the incoming pipe 12. Reduction of turbulence at the location above the bottom level of the wall 36 eliminates an undesirable re-circulation region below wall 36.

Likewise, at the end of the filter element 20 of the filter apparatus, it is preferable to locate a sloped ramp 37 at the bottom of the wall 36 above the filter element that is closest to the outlet 18. The sloped ramp is shown in Fig. 4 extending into the outlet pipe 12' but the ramp could be located completely within the filter arrangement 10. The reduction of turbulence at the outlet reduces turbulence at the end of the filter element that would otherwise adversely affect the effectiveness of the filter arrangement.

Also at the inlet end of the filter arrangement 10, there exists a wall 38 (weir) separating the continuation flow chamber 32 from the overflow chamber 25. Once the spill flow rate is exceeded and ever increases there will be a point, at which the incoming flow is large enough so as to require a bypass mechanism in addition to that, which is provided by the filter element. In this embodiment, storm water and sewage in the pass forward flow chamber 32 rises above the wall 38 and drops into the overflow chamber 25 and out of the filter arrangement via overflow pipe 14. An overflow mechanism of this type could also be provided further upstream as well as, or as an alternative to, that provided in the embodiment described.

The critical overflow rate will likely be defined by the water authority engineers based on calculated needs and the capacity of the existing pipe work system to provide an alternative route for the overflow fluid that unfortunately will be unfiltered.

In an example of an overflow mechanism being provided external of the filter arrangement, the wall 38 will be extended to the roof of the filter arrangement or be otherwise configured so as to ensure that no overflow of unfiltered fluid enters the overflow chamber 25 or overflow pipe 14. Unfiltered overflow will then need to be directed by an upstream weir to another filter-like arrangement or to an acceptable outfall or even a different treatment plant to that which it would otherwise have been expected to be directed to.

The filter element 20 comprises in this embodiment, a "cheese grate" mesh, a detailed illustration of which is provided in Fig 6.

Referring to Fig 4, arrows indicate how fluid containing sewage flows under the filter element (mesh) 20. While doing so, a certain portion of the fluid flows backward through the mesh (relative to the continuation flow) and then flows above the mesh 20. It has been found that the flow pattern above the mesh 20 is a good indicator of how evenly the mesh is operating. One filtered fluid flow arrangement is to have the filtered flow (spill flow) move sideways off the mesh 20 and into the overflow chamber 25. When the mesh 20 is working evenly, the water has little or no flow in the same direction as the main flow, i.e. no stream-wise flow and it only moves sideways. A stream-wise component above the mesh 20 seems to indicate that there is some re-circulation. The sideways movement appears to result from the momentum of spill flow falling into the overflow chamber 25 and water surface tension forces.

In this embodiment the flow above the mesh is shown moving in the same direction as the continuation flow, both flows being in the direction of respective outlets 26 and 18. The water flowing above the mesh of course, is filtered and first falls into the overflow chamber 25 and then eventually exits the filter arrangement 10 through outlet 26 into overflow pipe 14.

Fig. 6 depicts a bottom view of a portion of one mesh type (referred to herein as a cheese grate mesh) with the arrow 'M' showing the direction of the continuation flow of the storm/ sewer fluid.

Fig 7 pictorially depicts the mechanism that produces the results described above and that is evident in other embodiments that are yet to be described in this specification. The mechanism is referred to herein as "dividing streamline separation".

A portion of a cross section of the "cheese grate" mesh and or a slat-like grate array as will be used in another embodiment as depicted in Fig 7.

Solid lines show the mesh/ grate elements and the dotted lines in the water are virtual in the sense that they do not exist but will in this particular example, assist the depiction of the method of operation of the filter. Line 70 represents the top of the continuation flow chamber 32 in particular, the lowest portion of partition wall 36. Unfiltered fluid is shown flowing into the region below the filter element 20 and dotted line 72 defines a height XI of fluid which will flow upwards into the gap between grate element (a) and grate element (b). Upwards flow of water (spill flow) and not entrained particulate matter, occurs as a result of a number of influences. Control of the previously mentioned incidence angle of the continuation flow onto the mesh along its length is an important influence on the successful use of the filter arrangement. Dividing streamline separation contributes to non-blocking of the mesh because the fluid borne particles passing through the mesh are much smaller than the separation between the mesh apertures, viz. the dimensions of XI and larger particles are drawn further along the filter arrangement. This arrangement appears counter-intuitive and indirect contrast to the way in which existing screens and filters work that eventually trap particles between their elements because the particles are being drawn towards and into the gaps between solid elements of the mesh/ grate and are larger than the provided gap.

Particles are larger than the gap bridge across the gap between mesh elements whatever their spacing or orientation.

Therefore in this filter arrangement, any entrained particle or cellulose fibre flowing in the stream having less than height XI will pass through the filter element while particles and other fibres, the bulk of which primarily fit below dotted line 72, will flow past that particular portion of the filter element.

However, fluid and particles which flow fully within dotted lines 72 and 74 that defines height X2, will flow through the gap between grate elements (b) and (c). Likewise, the fluid and particles flowing fully within the dotted lines 74 and 76 that defines height X3 will flow through grate elements (c) and (d) respectively. Generally, particles having a centre of gravity below a respective line will be drawn past the streams heading towards gaps in the filter element.

Thus it can be seen from the pictorial representation in Fig 7 that there exists a dividing streamline 100 that terminates on each solid element of the filter element and that flows that gaps between the solid elements is bounded by those stream lines. Particulate matter that fits between the solid elements and the bounds of the dividing streamline ie. one dimension being XI will flow between the gap between adjacent solid elements with the spill flow. Particulate matter that does not fit will not be drawn into the gaps and thus not bridge the gap and cause a partial blockage of the filter element.

A filter configuration of this type has been found to be non-blocking in most conditions including low and high flow rates and regardless of the nature of the entrained contaminates, for example and in particular, long cellulose fibres such as those described previously. The dotted lines depicted are merely schematic and will vary in shape and spacing dependent on flow rates (continuation and spill) so as to reflect the principle as understood and described herein.

The "cheese grate" mesh has been found useful in certain circumstances in that there is little or no collection of debris, however other mesh or grating types could be used and have a similar or better results. For example, the mesh or grate may be formed of a parallel array of rectangular, square or round bars which are located laterally with respect to the flow path.

A yet further embodiment of the filter arrangement is depicted in Figs 8 and 9 that show ten (10) independent compartments above an elongate filter element. The filter element comprises an array of parallel louvres located laterally with respect to the continuation flow and with each generally planar filter element angled with respect to the flow so that the gaps between louvres are directed in an opposite direction to the continuation flow M as is depicted in Fig 10. The arrangement of the louvres in cross-section is not unlike that depicted in Fig 7 so that a condition for dividing streamline separation can occur.

However, there are a number of characteristics that need to be controlled if the benefit of a non-blocking undershot overflow filtering arrangement is to be achieved and optimised. These characteristics have been identified at this time by way of modelling and experimentation.

Referring to the filter element depicted in Fig 7 and in particular an element in the form of an array of louvres, the length L of the louvre, the thickness T, the gap G and the pitch P of the individual louvres of the grill are all variables relevant to some of the characteristics of such an a filter arrangement. It is preferable to flatten the lower edge of the louvres, which lie at the interface of the filter element to pass forward flow as is depicted in Fig 7a. This modification is intended to lessen the possibility of turbulence at this region.

Pass forward flow is tangential and spill flow is normal to the plane of the louvre panel. The face velocity is the mean spill flow velocity component that is normal to the plane of the filter element 20. It is numerically equal to the spill flow volume rate divided by the face area of the panel of the filter element as described previously.

It is a preferable feature to maintain the face velocity substantially constant over the length (along the direction of flow) of the panel of the filter element, in this example the array of louvres.

Spill flow is that flow of filtered water filtered fluid leaving the filter apparatus and the spill ratio is a ratio of the volume of filtered outflow (spill flow) over the total fluid flowing into the filter assembly. Thus a filter assembly accepting 21 litres per second having a spill flow of 10 litres per second has a spill ratio of approximately 48 per cent. The continuation flow would be 11 litres per second.

The higher the spill ratio, the better the performance of the filter as long as the filter elements does not collect debris and become blocked. It may be that it is possible to optimise the filter arrangement for non-blocking at a relatively high spill ratio but that at a lower spill ratio blocking or fouling could occur.

It has been recognised that there are a number of variables that need to be optimised to achieve the best possible combination of spill ratio and filter non-fouling across the broadest possible range of input flow rates wherein the spill ratio is greater than 1.

One of those characteristics is the incidence of the continuation flow to the plane of the array of louvres. This incidence has been found to be preferably in the range 1 to 3% as described previously and if maintained along the length of the filter element then fouling is very unlikely or non-existent. Maintenance of a constant incident would appear to enhance the principle of dividing stream separation discussed previously and lead to a maximum of spill flow from a filter of any given size.

However, to achieve uniformity of this characteristic it is firstly necessary to keep the filter length relatively short as wall friction and stream wise gradients of velocity cause variations in the pressure differential across the louvre array, leading to non- uniform incidence. The pressure differential across the louvre array is the driving force producing the face velocity. As the length of the filter element increases, the ratio of energy loss to the total energy of the fluid increases and a non-linear gradient of total energy occurs. The angle of incidence becomes non-uniform along the length of the filter and if too high, causes fouling, and if too low, reduces the average face velocity.

Modifying the tilt of the filter element can initially and at certain continuation flow rates be beneficial, but beyond a certain angle (which is different for different flow rates) the positive effect can transform into a negative effect such that face velocity not only reduces but reverses, ie, the filtered fluid above the filter element is sucked back into the continuation flow. Unfortunately, to have a high spill ratio and an appropriate face velocity, the angle of inclination of the filter element becomes too great and too much friction loss occurs with the consequences described above.

Compartmentalising the filter, that is, shortening the filter length and keeping the parameter of spill ratio, uniform face velocity and incidence constant within the region below the filter and respective partition helps to achieve a desirable outcome. The face velocity can be difficult to control.

It is possible, to at least in an active system, control the spill flow from each compartment. However, an active system is not an ideal arrangement as it adds complexity and cost to a filter arrangement that in the stormwater pipe system is mostly remote of maintenance personnel. Such a system using moveable weirs or valve actuations is possible in appropriate application such as industrial filtering where specific spill ratios may be required and maintenance of the mechanical/ hydraulic elements is less of an issue.

Of particular benefit in some application and particularly in the sewage application used as an example herein is the provision of a passive means that at least within acceptable boundaries, controls the spill flow from each chamber thereby maintaining the desired incidence without the need for mechanical/ hydraulic elements.

The challenge therefore is to passively account for different spill flows where there exists a wide range of input flow rates.

Some of the devices that may be used as an outlet control for the spill flow control feature include a broad crested weir, an orifice, a combination of the two, a v-notch control and a slit. A simple circularly shaped orifice is preferred. The outlet control device is preferably located above the filter element on a sidewall of a respective chamber. The circular orifice should be positioned so as to provide a head of water (acting like a weir) above the level of the filter element at a height such that not only is the spill flow from each chamber known (because of the known aperture area), but which also maintains a head in the chamber that is beneficial to the maintenance of the incidence and the inter-related face velocity. Additional orifices above an^" existing one can be used to regulate spill flow when higher flow rates into the filter arrangement are encountered. Those orifices may be the same or different to those previously mentioned.

Fig 11 depicts a filter arrangement of the above configuration in operation and the spill flow can be seen emitting from three circularly shaped orifices located one above the other in each of the chambers used in that embodiment.

In the arrangement depicted in Figs 8, 9 and 10 the spill flow aperture is shown as circular holes arranged substantially one above the other however a slot of appropriate dimension could also be used for the purposes described.

Consideration can also be given to the aspect ratio of the filter elements such that the chamber used to encompass a filter element is wider than it is long. It would also be possible to use a cylindrical filter element such that the fluid to be concentrated is swirled into the inside of the cylinder and chambers are arranged about the outside of the chamber to create ideal conditions for dividing streamline separation to occur. A conically shaped filter with a truncated apex may also work.

Such arrangements of course may well have more difficulty in being retrofitted to an existing pipe system, but may provide flexibility in certain circumstances.

Indeed, any configuration of filter element 20 which exhibits the non-blocking characteristics desired, using a dividing streamline separation principle is likely to be useful in certain circumstances including as mentioned previously in industrial applications requiring non-blocking filters to separate fluid from fluids entrained with particulate matter (particulate that may include long thin strands of material).

It is a matter of experimentation with filter shapes and slopes, spill flow rate, spill flow aperture shape and location, face velocity, incidence as well as filter element sizes and aperture sizes to identify the most ideal arrangement for various installation conditions as well as configuring the most suitable continuation and spill flow rates for a range of input flow conditions.

It will be appreciated by those skilled in the art that the invention is not restricted in its use to a particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/ or features described or depicted herein. It will be appreciated that various modifications can be made without departing from the principle of the invention. Therefore, the invention should be understood to include all such modifications within its scope.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A filter for concentrating or separating in a fluid stream containing particulate matter, said filter consists of: a) a plurality of solid elements in an array having gaps between adjacent solid elements through which spill flow of said fluid passes, and said fluid stream and filter orientated with respect to each other such that the spill flow of fluid through adjacent gaps is bounded by a dividing stream line that terminates on a solid element intermediate adjacent gaps, and further arranged so that the incidence of the dividing stream line to the plane of said array is such that particulate matter larger than said gaps are deflected out of the spill flow between said gaps.

2. A filter according to claim 1 further consisting of at least two chambers on the spill flow side of said filter arranged to receive spill flow so as to maintain substantially the same incidence along the fluid flow length of said filter adjacent each said chamber.

3. A filter according to claim 2 wherein one or more of said chambers have spill flow control means sized and positioned to allow said spill flow to exit a said chamber so as to maintain said incidence.

4. A filter according to claim 3 wherein said fluid flow past said filter or a portion thereof is a continuation flow and said spill flow control is arranged to maintain a ratio of continuation flow to spill flow for each of said plurality of chambers.

5. A filter according to claim 1 wherein said solid elements are planar and their width is greater than their depth which is greater than their thickness and said gaps are uniform in size.

6. A filter according to claim 5 wherein said planar elements of said filter array are set so that said gaps allow spill flow to flow substantially in the opposite direction to said fluid stream flow direction.

7. A filter according to claim 5 wherein said solid elements are arranged in a planar array.

8. A filter according to claim 1 wherein said solid elements are arranged in a circular array.

9. A filter according to claim 1 wherein said fluid stream flows below said filter array and said spill flow flows up through said filter array.

10. A filter according to claim 3 wherein said spill flow control means consists of one or more circular apertures in a wall of said chamber located above said filter element located so as to allow spill flow to exit said chamber in a controlled manner.