GB2542837A

GB2542837A - Media filter

Info

Publication number: GB2542837A
Application number: GB1517358.6A
Authority: GB
Inventors: Cupples Stephen
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-10-01
Filing date: 2015-10-01
Publication date: 2017-04-05
Also published as: GB201517358D0; WO2017055794A1

Abstract

A water filter 100 comprises a vessel 102 which in use receives multiple layers of particulate filter media 118a-d. A first inlet 128 is provided through which water to be filtered is introduced to the vessel. The first inlet is arranged above the level of the filter media and is oriented to cause a body of water 124 above the filter media to rotate. An outlet 122 for exhaust of filtered water from the vessel is positioned such that water being filtered flows down through the vessel passing through the filter media to reach the outlet. According to one aspect of the invention the first inlet comprises a velocity adjuster (144, Fig. 8b) which defines a cross-sectional area for flow of water, wherein the velocity adjuster is replaceable or adjustable to enable the cross-sectional area to be varied. In another aspect, the inlet has at least one opening which is upwardly inclined to project water into a region above the inlet. In a further aspect, the vessel has a cylindrical side wall and the inlet is a tube that enters the vessel through the cylindrical side wall. Other inventions are also claimed.

Description

MEDIA FILTER

The present invention relates to water filtration and more specifically to an improved performance media filter.

Media filters have a wide range of applications in relation to industrial, commercial and municipal water filtration. They can be divided into gravity filters and pressure filters. A typical pressure-type media filter has a cylindrical pressure vessel fabricated in metal and containing various layers of particulate filter media which are graded according to size, with a fine layer at the top and the coarsest layer (the layer with the largest particle size) at the bottom. During normal operation the water to be filtered flows downwardly through the pressure vessel from an inlet at the top to an outlet at the bottom. Water flow can be reversed periodically to fluidise the filter media and thereby clean and regenerate it, a process known as "backwashing". A known pressure-type media filter 10 is represented in simplified form in Figure 1. Within its pressure vessel 12 are multiple graded layers of particulate filter media comprising, from the bottom layer up and in order: pea gravel 14, coarse sand or grit 16, finer grit 18, and fine sand 20.

In this example the total media depth is of the order of 1 m. to 1.2 m. A range of filter media can also be used. Sand and gravel are common, but alternatively or additionally use may be made of chemically active media (for example granular activated carbon), of catalytic media (for example Birm, a granular catalytic agent which is commercially available and well known to the skilled person), of modified natural media such as very fine sand, and of heat treated modified recycled glass. Some specific high performance filter media exploit static electrical charge acquired by their constituent particles ("Zeta potential") to improve filter efficiency.

The coarsest media 14 located at the bottom of the filter 10 helps to equalise water and air distribution in both normal filtration and during backwashing. The water fills the vessel and its pipework during filtration and backwash, any entrapped air being exhausted by automatic vent valves.

Water to be filtered is in this example introduced through an inlet pipe 22 which enters the pressure vessel 12 through its cylindrical side wall but is typically elbowed upwardly to provide an inlet opening 24 above the level of the topmost layer of filter media. From there the water passes down through the layers of filter media to an outlet 26 at the base of the pressure vessel 12. The outlet 26 in this example has multiple radial arms 27. The topmost media layer 20 forms an interface with the incoming water to be filtered, and in a standard filter design the primary objective is to achieve a simple laminar flow from top to bottom. Surface disturbance is intentionally minimised in order that dirt filtered from the water collects on the filter media as indicated at 29, which is found to actually enhance filtration towards the end of the filtration cycle Outlet 26 is also used for the inlet of the backwash water when filtration has terminated.

Figure 2a represents the improvement of filtration performance that takes place over time as contamination builds up on surfaces of thefinerfilter media, providing a substratum on these surfaces which assists filtration. At the same time the pressure drop across the filter increases due to blockage or occlusion of paths for water flow by the contamination. Water flow through the filter thus reduces over time as represented in Figure 2b. The pressure difference between the inlet 24 and the outlet 26 is detected by a filter control system and when it reaches a pre-determined threshold (typically a gauge pressure of 50,000 Pascals (0.5 Bar)) the control system puts the filter into backwash mode to clean the filter bed. Filtration performance - in terms of removal of unwanted material from the water - is at its best just before backwashing is carried out, but flow through the filter is at a minimum at that point.

During backwashing, cleaning water is supplied through the outlet 26 causing flow through the filter in the reverse of the normal direction (upwards). Flow rate of the backwash water is chosen to fluidise the finer filter media, typically by 20% of its volume. The appropriate flow rate depends on the media being used. The backwash water releases the contamination held within the filter media bed, and since the contamination is typically less dense than the filter media it is carried to and flushed out of a backwash outlet 28 at the head of the pressure vessel. All inlet/outlet connections are fitted with automatic control valves A filter 10 of this type can at best filter down to a level of 10-15 microns using conventional pressure vessels 12 and media 14-20, and this is only achieved at low filtration velocities of around 10,000 litres per square metre of filter area per hour. The resulting low inlet velocity/flow rate is required to ensure that the filter bed is not disturbed during filtration, allowing the build-up of contamination which improves filter performance. However use of media filters in this manner creates a number of major operational issues.

Because of the low rate of flow in proportion to filter area, media filters of the above described conventional type for sub 20 micron duty are large and heavy pieces of equipment which require large plant rooms with substantial foundations. Capital expenditure is thus undesirably high.

Operating at the 15-20 micron filtration level the filter media tend to retain bacteria towards the end of the operating cycle (shortly before backwashing takes place) and due to their low flow environment these bacteria are able to proliferate and colonise the filter media. The water passing through the filter, being a natural material, provides the bacteria with elements of nutriment. The consequent bacterial colonisation can cause catastrophic filter failure in a short period of time. Once established the bacteria can be difficult to remove or control effectively. The bacteria can form clumps of "jelly" which may be of higher density than the fine filter media. These clumps are consequently resistant to removal by backwashing. The process and result of the biological build up causes "chanelling or "rat holing" in the filter bed to occur, resulting in the failure of the filtration process through water bypass

Consequently traditional media filters as depicted in Figure 1 are not generally used for filtration below the 20 micron level.

An improved form of media filter is described in United Kingdom patent GB2461119 and is depicted in Figures 3 and 4. In this improved filter 30, a main inlet 32 for water to be filtered is formed in a cylindrical wall 34 of pressure vessel 36 and is aligned along a tangent to that wall (see Figure 4 in particular). Consequently during filtration a body of water 37 above the filter media continuously moves in a circular path (a vortex) about the vertical axis of the cylindrical pressure vessel, as indicated by arrows in Figure 4. The moving water serves to continuously disturb and disperse the fine media forming the top layer of the filter, preventing bacterial colonisation and so enabling the filter to be used for fine filtration at much higher flow rates than can be achieved with the Figure 1 filter.

The circular movement of the water tends to remove filter media from the periphery of the pressure vessel 36 and deposit it closer to the centre of the vessel, where water velocity is lower, to create a domed profile on the filter media. This is undesirable, potentially enabling breakthrough of water to lower media layers which might reduce filter performance. To avoid this effect, the media filter of GB2461119 has a "vortex bed stabiliser" 38, which is a distribution head arranged on the axis of the pressure vessel above the filter media, which is supplied with water to be filtered and which has discharge holes 39 oriented to project water horizontally in a direction which promotes the rotary motion of the water. Looking at Figure 4, it can be appreciated that the discharge holes lie in a circle and each project water along a respective direction 43 which is a tangent to that circle. In this way conical build-up of the filter media is reduced.

One problem encountered in use of the media filter disclosed in GB2461119 is local erosion of the pressure vessel wall in the vicinity of the main inlet 32. Water emerges from the main inlet 32 at a relatively high velocity and impinges almost immediately on a region 41 of the pressure vessel wall 34, which must deflect this flow into a circular path. In the presence of abrasive filter media the effect can be to abrade and, overthe design lifetime, to locally erode this region of the pressure vessel.

Another known arrangement of the main water inlet is depicted in Figure 5 and has an inlet tube 50 which extends roughly to the centre of the pressure vessel and is elbowed so that main water inlet 52 projects water in a direction that has a circumferential component and so creates some circular motion, as indicated by arrows in Figure 5. However in this arrangement the pattern of circular flow is not well defined and controlled, creating dead areas in which water velocity is low. These dead areas lead to premature bed fouling. Also the problem of erosion of the pressure vessel wall is not overcome because water is directed against a region 54 of the wall.

Another problem with the media filter of GB2461119 concerns assembly and disassembly. Looking again at Figure 3, the pressure vessel 36 has at its upper end a removable hatch 40 providing access to the vessel's interior. But pipework 42 leading to and carrying the vortex bed stabiliser 38 passes through and is attached to the hatch 40. To remove the hatch 40, it must therefore be raised far enough above the pressure vessel 36 to withdraw the pipework 42 and the vortex bed stabiliser 38 from the neck 42a of the vessel. This necessitates considerable headroom above the filter, and can also present problems in terms of handling, and of health and safety.

The inventor has recognised that a key factor in the operation of media filters having rotary water motion is the velocity of water entering the pressure vessel. This determines the water speed of the rotational motion in the pressure vessel, and hence the effectiveness of this flow in disturbing the surface of the media bed to prevent biological infestation. Inadequate water velocity can result in media bio-fouling. All high performance and standard media filters are specified by their manufacturers for use over a range of flow rates. That is, the throughput of water per unit time may vary considerably from one installation to another. In existing media filters, if the flow rate is low then the water velocity will be correspondingly low. The difference in water velocity between a filter operating at its minimum and maximum specified flow rates may be a factor of three or more, so the filter cannot function optimally over this entire range. This is especially true of the high performance designs

Another problem with known filters, as noted above, is that they can suffer from dead zones in the rotational flow of water above the filter media, causing locally concentrated and premature bio-fouling. This is highly deleterious to filter longevity and performance. The pattern of flow created by the inlet arrangements seen in Figures 3-5 does not prevent such infestation.

Another problem with the prior art filters of Figures 1 to 5 concerns backwashing. The simple open-ended pipes 28 used in the prior art to exhaust backwash water can admit filter media through high suction velocity, similarly to a vacuum cleaner hose. This can and often does lead to major loss of media during backwashing, thereby rendering subsequent filter cycles less effective due to the reduction in filter bed depth resulting from media loss.

According to a first aspect of the present invention there is a water filter comprising a vessel for receiving multiple layers of particulate filter media, a first inlet through which water to be filtered is introduced to the vessel, the first inlet being positioned to supply water to a body of water above the filter media in use, and oriented to cause the said body of water to rotate, an outlet for exhaust of filtered water from the vessel, the outlet being positioned such that water being filtered flows down through the vessel passing through the filter media to reach the outlet, wherein the first inlet comprises a velocity adjuster which defines a cross-sectional area for flow or water and so influences velocity of water emerging from the first inlet, the velocity adjuster being replaceable or adjustable to enable the aforesaid cross-sectional area to be varied.

The water filter according to the present invention can thus be flexibly configured to provide a desired velocity of water at a specified rate of water flow through the filter, making it possible to manufacture and supply one filter able to operate effectively and at peak performance for any of a wide range of water flow rates.

Note that an uppermost portion of the filter media may be fluidised by and entrained in the rotational flow of the said body of water. That is, the body of water may contain some of the filter media.

According to a second aspect of the present invention there is a water filter comprising a vessel for receiving multiple layers of particulate filter media, an inlet through which water to be filtered is introduced to the vessel, the inlet being positioned to be submerged in a body of water at the topmost level of media and being oriented to cause the body of water above the filter media to rotate, an outlet for exhaust of filtered water from the vessel, the outlet being positioned such that water being filtered flows down through the vessel passing through the filter media to reach the outlet, wherein the inlet has at least one opening for supply of water which is upwardly inclined to the main water flow to project water into a region above the inlet.

In this way dead areas of low water velocity in the prior art filters, especially those at or adjacent the water surface, can be avoided and biofouling reduced.

According to a third aspect of the present invention there is a water filter comprising a pressure vessel for receiving multiple layers of particulate filter media, an inlet through which water to be filtered is introduced to the vessel, the inlet being positioned to supply water to a body of water above the filter media in use, and oriented to cause the said body of water to rotate, an outlet for exhaust of filtered water from the vessel, the outlet being positioned such that water being filtered flows down through the vessel passing through the filter media to reach the outlet, wherein the pressure vessel comprises a cylindrical side wall and the first inlet comprises an inlet tube which is connectable outside the vessel to a source of water to be filtered and which enters the vessel through the cylindrical side wall.

The vessel may have an access hatch for access to the interior of the vessel. Because the inlet tube passes through the vessel wall and not the access hatch, removal of the hatch is straightforward.

According to a fourth aspect of the present invention there is a water filter comprising a pressure vessel for receiving multiple layers of particulate filter media, an inlet through which water to be filtered is introduced to the vessel, and an outlet for exhaust of filtered water from the vessel, the outlet being positioned such that that water being filtered flows down through the vessel passing through the filter media to reach the outlet, the water filter being configured to be renewed by backwashing, in which water is passed upward through the filter media, and having a backwash outlet for exhaust of water from the pressure vessel during backwashing, the backwash outlet being disposed above the level of the filter media, being upwardly facing, and having an upwardly divergent shape.

The form of the backwash outlet reduces loss of filter media during backwashing. The divergent shape of the outlet reduces flow velocity at its opening, making filter media less prone to be entrained in flow to the outlet, and the fact that the outlet faces upwards means that the relatively dense particles of the filter media are less likely to reach it.

According to a fifth aspect of the present invention there is a water filter comprising a cylindrical vessel for receiving multiple layers of particulate filter media, the vessel being provided with a first water inlet which is disposed above the level of the filter media and oriented to cause a body of water above the filter media to rotate, a second water inlet disposed on or adjacent the vessel's axis, an an outlet for exhaust of filtered water from the vessel arranged so that water being filtered flows down through the vessel to reach the outlet, passing through the filter media, wherein the second water inlet is configured to project water along radial directions.

This radially directed flow is especially effective in resisting coning of the filter media surface, serving to push the media in the central region of the vessel outward into regions where the rotational flow of the water is faster and at a constant high speed. This interception further mixes the media fines which carry a static charge to further mingle with incoming contamination and allow further time and opportunity for the interception filtration process to occur.

According to a sixth aspect of the present invention there is a water filter comprising a cylindrical vessel for receiving multiple layers of particulate filter media, a first inlet through which waterto be filtered is introduced to the vessel, the first inlet being positioned to supply water to a body of water above the filter media in use, and oriented to cause the said body of water to rotate, an outlet for exhaust of filtered water from the vessel, the outlet being positioned such that water being filtered flows down through the vessel passing through the filter media to reach the outlet, wherein the vessel has a substantially cylindrical wall having an axis and the first inlet is arranged to project water from a position which is radially offset both from the wall and from the aforesaid axis.

The offset inlet provides the required rotational flow in an efficient manner but reduces erosion of the vessel wall.

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

Figure 1 is a section in a vertical plane through a media filter belonging to the prior art;

Figure 2a is a graph of filtration performance over time for the known media filter of Figure 1;

Figure 2b is a graph of water flow through the known media filter media of Figure 1 over time;

Figure S is a section in a vertical plane through another media filter belonging to the prior art;

Figure 4 is a section in a horizontal plane through the known media filter of Figure S;

Figure 5 is a section in a horizontal plane through another known media filter;

Figure 6 is a section in a vertical plane through a media filter embodying the present invention;

Figure 7 is a section in a horizontal plane through the media filter of Figure 6;

Figures 8a, b and c are respectively views (a) from one side, (b) from one side and the front, and (c) from above of a nozzle housing and related parts of the media filter of Figure 6;

Figure 9 is an exploded view of a secondary inlet of the media filter of Figure 6; and

Figure 10 is a plan view of the secondary inlet;

Overview

The media filter 100 of Figure 6 onward is a pressure-type filter suitable for filtration of water or other liquids. While water will be referred to throughout this description it should be understood that the filter may be operated with another liquid. It has a pressure vessel 102 which is generally cylindrical but has domed upper and lower end walls 104,106. A removable access cover 108 incorporates a safety valve 110 and a pressure gauge 112. Water to be filtered enters the pressure vessel 102 via a main inlet 114 and also via a secondary inlet 116. The water passes down through multiple filter media layers 118a-d and through a nozzle plate 120 to reach an outlet 122 through which filtered water is supplied.

The media filter described herein is suited to a range of uses but can in particular be used in high performance filter applications for filtering below 10 microns particle size. The effective filtration range is as small as 0.45 micron.

The main inlet 114

There is, in use, a body of water 124 above the top layer 118a . During filtration, this body of water 124 is caused to move in a roughly circular path about the vessel's vertical axis due to the configuration of the main inlet 114. In this way the top layer of filter media is continually disturbed and fluidised during filtering. This disturbance avoids biological infestation and also enables interaction between the individual particles of the filter media and the contamination particles in the incoming water being filtered in a manner which improves filter performance. The smaller particles have a negative Zeta Potential whilst the media has the opposite charge, and unlike charges attract enabling the finer sub 10 micron contamination to be effectively removed out of the water and onto the media particles' surfaces. This is especially advantageous when using those filter media that exploit the Zeta potential of the particles of filter media.

The general arrangement of the main inlet 114 is best seen in Figures 6 and 7 and comprises an inlet tube 128 which enters the pressure vessel 102 through its cylindrical side wall 129 and which is in use connected via a suitable valve arrangement (which does not form part of the present invention and is not shown) to a source of water to be filtered. The inlet tube 128 extends horizontally and is provided, at its free end within the pressure vessel 102, with a nozzle housing 130, which performs several functions.

Figures 8a to 8c show the nozzle housing 130 on its own. It has a coupling 132 for releasably attaching the nozzle housing 130 to the inlet tube 128. In the present example this takes the form of a union connection to be received on the end of the inlet tube 128 and threadedly retained upon it, providing a passage for water to flow from the inlet tube 128 through the nozzle housing 130 to a supply opening 138. A quick twist fit coupling can alternatively be used for the connection

The nozzle housing 130 is elbowed or curved so that the supply opening 138 faces along a direction indicated by arrow 140 in Figure 7 and 8b, which is inclined to axis 142 of the coupling 132. Water is output from the nozzle housing 130 along the direction 140. This direction is chosen to promote the desired circular motion of water in the pressure vessel. The water moves in what may loosely be described as a vortex, although unlike some vortices it is not the result of a central region of low pressure.

It was noted above that in certain prior art filters flow from the main filter inlet could cause local erosion of the pressure vessel 102. This problem is ameliorated in the present embodiment by virtue of the configuration of the main inlet 114, which is neither immediately adjacent the pressure vessel wall 129, as in the prior art arrangement of Figure 4, nor centrally disposed, as in that of Figure 5. Instead the supply opening 138 is at an intermediate or offset radial position, separated from both the vessel wall 129 and the vessel's axis. In the illustrated example it is placed at 70% of the radius of the pressure vessel (that is, the separation of the radial centre of the supply opening 138 from the axis of the pressure vessel is substantially 70% of the internal radius of the vessel). The resultant water flow does not suffer from the relatively high vessel impact velocity and localised impingement upon the pressure vessel wall 129 which creates erosion in the prior art media filters. By installing the inlet at this position the already rotating water acts as a shield or buffer zone to aid against erosion from the direct water inlet

The area of the supply opening 138 is chosen to provide a desired water velocity as the water enters the pressure vessel 102. As noted above this velocity, and the resultant rotational velocity of the vortex in the pressure vessel, have a significant effect on overall filter performance. To enable this aspect of the filter's performance to be adjusted and optimised, the present embodiment comprises an interchangeable velocity adjuster 144. The water entering the pressure vessel 102 via the supply opening 138 passes through the velocity adjuster 144. The velocity adjuster 144 provides a desired cross-sectional area for through-flow. We will refer to this area as "a". The velocity of the water emerging from the velocity adjuster 144 (averaged over the area a, since it will not be entirely consistent over this area) must equal the rate of water flow divided by the area a. A smaller area a thus provides a higher water velocity. If the magnitude of the velocity of the water is v and the rate of flow through the supply opening is/then v=f/a.

Provision is made for the area a provided by the velocity adjuster 144 to be adjusted or otherwise altered to enable the flow velocity to be optimised for a given filter installation. In the present embodiment, this is achieved by virtue of the fact that the velocity adjuster 144 is replaceable, and can be interchanged with one or more other velocity adjusters providing different areas a for through-flow of water. The velocity adjuster 144 takes the form in this embodiment of a ring that seats on a shoulder formed on the open end of the nozzle housing 130. An annular retainer 146 keeps the velocity adjuster 144 in place. The annular retainer screws onto the free end of the nozzle housing and can be unscrewed to release the velocity adjuster and allow it to be replaced. The radially inner surface 148 of the velocity adjuster 144 is radiussed to reduce turbulence. The velocity adjuster 144 could take the form of a passage, in order to promote laminar flow. That is, it could have a longer extent in the direction of water flow, being formed for example as a tube.

For low flow applications, a velocity adjuster 144 with a relatively small area a will be selected. For higher flow applications, a larger area a will provide the desired vortex velocity. In this way the same filter can be adapted for high performance in a range of different applications/flow rates. If a filter is put to a new application it can be replaced - after manufacture/installation - in a very straightforward manner to optimise it for the new application, simply by replacing the velocity adjuster 144.

The velocity adjuster 144 could take other forms. It needs to provide a means of varying the cross-sectional area a but this can be achieved in many different ways. For example, this variation could be enabled by some movable oradjustable part, such as a movable gate partly occluding the flow path, or the type of adjustable nozzle in which advancing a conical part toward an opening reduces the cross-sectional area for through-flow of water.

The main inlet 114 is configured to direct at least part of the incoming flow of water along an upwardly inclined path. The inventor has observed that there are specific areas in which biofouling tends to occur in existing media filters, in particular a zone above the level of the main water inlet. . This is believed to be the result of "dead zones" in which water velocity is relatively low, or the water is static, giving better conditions for unwanted organisms to proliferate. By appropriate direction of the input water, these dead zones - and the consequent fouling - are removed In the illustrated embodiment this is achieved by provision of an inclined nozzle 150 as part of the main inlet 114. The inclined nozzle 150 takes the form of a short tube projecting from an upper portion of the nozzle housing 130 at an angle of approximately 45 degrees. Other angles may be found most effective in other embodiments. The inclined nozzle projects water along an inclined upward direction 151, toward the otherwise dead zone above the submerged main inlet 114. This water flow also has a component along the general direction of water rotation. Its effect is to ensure that the water above the level of the submerged main inlet 130 is entrained in the general vorticular flow, avoiding dead zones which might otherwise allow local fouling. It also disturbs the water surface and so prevents build of a fouling layer there.

The secondary inlet 116

In order to resist the unwanted build-up of filter media in the centre of the pressure vessel and resultant "coning" of the filter media surface, the secondary inlet 116 provides a continuous water flow in radially outward directions.

The location of the secondary inlet 116 is seen in Figures 6 and 7. It lies on the axis of the pressure vessel 102 and at or below the top surface 152 of the filter media 118. The secondary inlet directs water along directions 153 which are approximately radial (with respect to the pressure vessel axis) and are somewhat upwardly inclined. The angle of this inclination is 15 degrees, in the present embodiment.

The construction of the secondary inlet 116 is best seen in Figures 9 and 10 and comprises an inlet body 155 in the form of a shallow, rounded cup carried upon a vertical portion of a secondary inlet tube 154. An internally threaded boss 156 projects into a circular cap plate 158 which is secured in place by a bolt 160 screwed into the boss 156. A shallow platform 162 separates the boss 156 from an open end 164 of the inlet tube 154. When the secondary inlet 116 is assembled, the cap plate 158 is separated by a small distance from the upper periphery 165 of the inlet body 155, and this separation provides the nozzle opening through which water is supplied. It extends around the whole periphery of the inlet body 155, so that water is projected along all radial directions 153 (see Figure 10). This distance is determined by spacers in the form of washers 157 between the cap plate 158 and the platform 162, and can be adjusted by adding or removing spacers. The cap plate 158 has a chamfer 166 running around its undersurface. Consequently water emerges from the secondary inlet 116 along an upwardly inclined direction, so that the vectors of water velocity form a shallow, upwardly concave cone around the secondary inlet. The angle of this cone is in the region of 15 degrees in the present embodiment.

The secondary inlet 116 receives a portion of the incoming water via a connection 168 to the main inlet 128 incorporating a valve 170 and a pipe union 172.

The secondary inlet 116 of the present embodiment can be contrasted with the vortex bed stabiliser 38 of the prior art filter depicted in Figure 3 in several respects:- 1. the secondary inlet 116 projects water radially rather than along a tangent. It might be expected that this would impair the rotational flow in the pressure vessel, but this proves not to be the case. Rather, flow 153 from the secondary inlet 116 pushes filter media radially outward into the faster flow at larger radii, and so reduces coning of the filter media surface. 2. the secondary inlet 116 projects water from a level generally coincident with the top level 152 of the filter media, and is typically partly buried in the filter media. This assists its function in dispersing media build up at the centre of the vessel. 3. the secondary inlet 116 projects water along an upwardly inclined direction. Combined with its location around the top surface of the media, this further improves its effectiveness in dispersing the media into the upper reception area of the filter. 4. the secondary inlet 116 projects water along all radial directions (i.e. through a full 360 degrees about the axis of the pressure vessel), forming a flow pattern that can be visualised as a shallow cone, without the potential for gaps in the flow pattern at any circumferential location.

Another practical advantage of the embodiment of the present invention depicted in Figure 6 is that the pipework 168 leading to the secondary inlet 116 passes through the side wall 129 of the pressure vessel 102. Compare this to the Figure 3 arrangement in which the corresponding pipework passes through the hatch 40. The Figure 6 arrangement makes it possible to easily remove the access hatch 108 to gain access to the interior of the pressure vessel 102, without the problems mentioned above involved in raising the hatch sufficiently to withdraw vortex bed stabiliser 38 from the pressure vessel.

Backwash management

It was noted above that prior art media filters can be subject to loss of filter media during backwashing. The present embodiment has a backwash outlet conduit 174 which terminates in an upwardly facing backwash outlet opening 176. The form of the backwash outlet conduit 174 leading to the backwash outlet opening 176 is upwardly divergent. That is, the cross section of the conduit increases toward the outlet opening. In this way velocity of the flow of backwash water in the region where it enters the backwash outlet conduit 174 is reduced. The effect of this velocity reduction is to reduce or avoid the tendency for filer media to be drawn into the outlet conduit 174 and so lost.

In the present embodiment the upwardly divergent portion 178 of the backwash outlet conduit 174 has the form of a shallow frustum of a cone. Other divergent shapes could be used. The reduction in velocity at the backwash outlet opening 176 can be in the region of 60 to 80%. This development also makes it possible to reduce the size of valves used in the backwash exhaust path, to reduce cost.

Claims

1. A water filter comprising a vessel for receiving multiple layers of particulate filter media, a first inlet through which water to be filtered is introduced to the vessel, the first inlet being positioned to supply water to a body of water above the filter media in use, and oriented to cause the said body of water to rotate, an outlet for exhaust of filtered water from the vessel, the outlet being positioned such that water being filtered flows down through the vessel passing through the filter media to reach the outlet, wherein the first inlet comprises a velocity adjuster which defines a cross-sectional area for flow or water and so influences velocity of water emerging from the first inlet, the velocity adjuster being replaceable or adjustable to enable the aforesaid cross-sectional area to be varied.

2. A water filter as claimed in claim 1 in which the first inlet comprises an inlet tube which is connectable outside the vessel to a source of water to be filtered and which extends into the vessel, and the velocity adjuster comprises a body which is mountable at a mouth of the inlet tube within the vessel and which provides a supply opening through which water emerging from the inlet tube passes, the mounting of the velocity adjuster being releasable to enable the filter to be fitted with a velocity adjuster having a supply opening whose area is selected with reference to the specified rate of flow of water through the filter.

3. A water filter as claimed in claim 2 in which the velocity adjuster comprises a plate providing the supplying opening which is mountable across the mouth of the inlet tube by means of a screw-threaded fitting.

4. A kit of parts comprising a water filter as claimed in claim 2 or claim 3 and a set of velocity adjusters having supply openings of different areas.

5. A water filter as claimed in any preceding claim provided with a secondary nozzle arranged to project water along an upwardly inclined direction.

6. A water filter as claimed in any preceding claim in which the vessel has a substantially cylindrical wall having an axis and the first inlet is arranged to project water from a position which is radially offset both from the wall and from the aforesaid axis.

7. A water filter as claimed in claim 6 in which the aforesaid position is offset from the axis by between 20% and 80% of the vessel's radius.

8. A water filter as claimed in any preceding claim further comprising a second inlet disposed in a central zone of the vessel and configured to project water radially outwardly.

9. A water filter as claimed in any preceding claim in which the second inlet is arranged to project water from a location at or adjacent the level of the filter media's top surface.

10. A water filter as claimed in claim 8 or claim 9 in which the second inlet is configured to project water along directions which are radially outward and upwardly inclined.

11. A water filter as claimed in any of claims 8 to 10 in which the second inlet projects water along all radial directions.

12. A water filter as claimed in claim 1 in which the vessel comprises a cylindrical side wall and in which the first inlet comprises an inlet tube which is connectable outside the vessel to a source of water to be filtered and which enters the vessel through the cylindrical side wall.

12. A water filter as claimed in any preceding claim which is configured to enable backwashing, having a backwash outlet arranged in use above the level of the filter media, the backwash outlet having an upwardly facing and upwardly divergent mouth through which backwash water passes to exit the vessel.

13. A water filter as claimed in claim 12 in which the mouth of the backwash outlet is shaped as a frustum of a cone.

14. A water filter comprising a vessel for receiving multiple layers of particulate filter media, an inlet through which water to be filtered is introduced to the vessel, the inlet being positioned to be submerged in a body of water above the filter media in use and being oriented to cause the body of water above the filter media to rotate, and an outlet for exhaust of filtered water from the vessel, the outlet being positioned such that water being filtered flows down through the vessel passing through the filter media to reach the outlet, wherein the inlet has at least one opening for supply of water which is upwardly inclined to project water into a region above the inlet.

15. A water filter as claimed in claim 14 in which the inlet comprises a main supply opening which is substantially horizontally directed, and a nozzle which is upwardly inclined.

16. A water filter as claimed in claim 14 or claim 15 further comprising a second inlet disposed in a central zone of the vessel and configured to project water radially outwardly.

17. A water filter comprising a pressure vessel for receiving multiple layers of particulate filter media, an inlet through which water to be filtered is introduced to the vessel, the inlet being positioned to supply water to a body of water above the filter media in use, and oriented to cause the said body of water to rotate, an outlet for exhaust of filtered water from the vessel, the outlet being positioned such that water being filtered flows down through the vessel passing through the filter media to reach the outlet, wherein the pressure vessel comprises a cylindrical side wall and the first inlet comprises an inlet tube which is connectable outside the vessel to a source of water to be filtered and which enters the vessel through the cylindrical side wall.

18. A water filter comprising a pressure vessel for receiving multiple layers of particulate filter media, an inlet through which water to be filtered is introduced to the vessel, and an outlet for exhaust of filtered water from the vessel, the outlet being positioned such that that water being filtered flows down through the vessel passing through the filter media to reach the outlet, the water filter being configured to be renewed by backwashing, in which water is passed upward through the filter media, and having a backwash outlet for exhaust of water from the pressure vessel during backwashing, the backwash outlet being disposed above the level of the filter media, being upwardly facing, and having an upwardly divergent shape.

19. A water filter as claimed in claim 18 in which the backwash outlet is shaped as a frustum of a cone.

20. A water filter comprising a cylindrical vessel for receiving multiple layers of particulate filter media, the vessel being provided with a first water inlet which is disposed above the level of the filter media and oriented to cause a body of water above the filter media to rotate, a second water inlet disposed on or adjacent the vessel's axis, an an outlet for exhaust of filtered water from the vessel arranged so that water being filtered flows down through the vessel to reach the outlet, passing through the filter media, wherein the second water inlet is configured to project water along radial directions.

21. A water filter as claimed in claim 20 in which the second water inlet is configured to project water in upwardly inclined radial directions.

22. A water filter as claimed in claim 20 or claim 21 in which the second water inlet is disposed to project water from a level at or adjacent the level of a top surface of the filter media.

23. A water filter as claimed in any of claims 20 to 22 in which the second water inlet is configured to project water along all radial directions, so that the velocity vectors of water emerging from it form a shallow, upwardly divergent cone.

24. A water filter as claimed in any of claims 20 to 23 in which the second water inlet is at least partly buried in the filter media.

25. A water filter as claimed in any of claims 20 to 24 in which the second water inlet comprises a cup part having a circular upper periphery and a cover part, a separation between the upper periphery of the cup part and the cover part forming an opening for supply of water.

26. A water filter comprising a cylindrical vessel for receiving multiple layers of particulate filter media, a first inlet through which water to be filtered is introduced to the vessel, the first inlet being positioned to supply water to a body of water above the filter media in use, and oriented to cause the said body of water to rotate, an outlet for exhaust of filtered water from the vessel, the outlet being positioned such that water being filtered flows down through the vessel passing through the filter media to reach the outlet, wherein the vessel has a substantially cylindrical wall having an axis and the first inlet is arranged to project water from a position which is radially offset both from the wall and from the aforesaid axis.

27. A water filter as claimed in claim 26 in which the position from which the first inlet projects water is offset from the axis by between 20% and 80% of the vessel's radius.

28. A water filter as claimed in claim 26 or claim 27, further comprising a second inlet disposed in a central zone of the vessel and configured to project water radially outwardly.

29. A water filter substantially as herein described with reference to, and as illustrated in, accompanying Figures 6 onwards.