GB2330088A

GB2330088A - Moving filter with elongate flow channel

Info

Publication number: GB2330088A
Application number: GB9721475A
Authority: GB
Inventors: Sean Keeping; Dieter Binz; Albrecht Vogel
Original assignee: ABB Kent Taylor Ltd
Current assignee: ABB Instrumentation Ltd
Priority date: 1997-10-09
Filing date: 1997-10-09
Publication date: 1999-04-14
Anticipated expiration: 2017-10-09
Also published as: GB2330088B; GB9721475D0

Abstract

A filter assembly 10 arranged to reduce the effects of clogging comprises a filter element 14 in contact with a contaminated source on one side and in contact with a filter holder 12 on the other to define an elongate flow channel 16, motion of the assembly moving the filtered fluid along the flow channel towards a collection chamber 26. The elongate channel may be in the form of a spiral or helix (fig 3), and the filter holder may be disc shaped, and capable of rotation. The assembly may be used to obtain liquid samples from waste water heavily contaminated with suspended solid material, the filter disc being immersed in the waste water at a depth sufficient to cause filtration due to hydrodynamic effect. Rotation of the filter assembly then causes the filtered fluid to move along the elongate flow channel towards the centre of the disc, and into the collection chamber. Periodically fluid accumulated within the collection chamber is withdrawn through a collection tube (28, fig 2) connected to a pump. Alternatively, the filter holder may be cylindrical, with the flow channel in the form of a helical groove on the cylinder surface.

Description

FLUID FILTRATION AND COLLECTION DEVICE This invention relates to a device for collecting filtered fluid from a source which is contaminated with suspended particulate matter. The invention is particularly concerned with devices for obtaining filtered samples of liquids from contaminated sources, in particular from liquids such as waste waters which are heavily contaminated with solids.

Obtaining filtered samples of waste waters, for example industrial effluent, or from raw, partially treated, or fully treated sewage, is important in environmental management, because it is essential for such effluents to be monitored for levels of contamination, including carrying out assessments of biological oxygen demand (BOD), heavy metal contamination and contamination by organic chemicals which are deleterious to the environment. River water or ground water also need to be monitored and analysed in order to maintain an appropriate degree of control on pollution levels.

Most if not all of the liquids referred to suffer high levels of contamination with suspended solid particles and the presence of such contamination can interfere with analytical methods carried out on recovered samples. It is desirable, therefore, for fluid samples to be recovered in a clarified or filtered state.

However, past efforts to devise a system for obtaining filtered samples continuously or intermittently over a long period of time have been hampered by the fact that existing filtering and fluid recovery systems suffer the disadvantage that the surface of the filter that is in contact with the contaminated liquid becomes fouled by the suspended matter and the flow rate is reduced drastically as time passes.

Thus, for example, in the case where it is desired to take small samples of a heavily contaminated liquid from the inlet of a waste water plant, filters having a pore size of 0.2m are customarily employed to prevent bacteria from entering the analytical apparatus. These may operate satisfactorily for a short period of time, but after a longer interval, which could several days, or (where the level of contamination is relatively low) after a period of months, the level of fouling of the filters becomes excessive and replacement of the filters is needed.

Previous efforts to overcome these problems have resorted to back-flushing with purified liquid, chemical treatment with cleaning agents such as sodium hydroxide or citric acid, use of ultrafiltration techniques and use of cross-flow filtration. However none of these techniques is wholly successful and their use can introduce processing complications, for example the need to include programming devices in order to commence a cleaning cycle intermittently, and, in the case of the use of chemical treatments, the need to maintain supplies of reagents.

The present invention provides a fluid filtration device which overcomes disadvantages associated with prior art devices.

According to one aspect of the present invention, there is provided a device for collecting fluid from a source which is contaminated with suspended particulate matter, comprising an assembly which includes a filter carrier and a filter element carried by the filter carrier, one surface of the filter element being arranged in use to be in contact with the contaminated source and the other surface defining a wall of an elongated flow channel, wherein means are provided for continuously moving the assembly relative to the contaminated source, whereby in use, filtered fluid passing from the contaminated source into the flow channel is translocated to a collection point as a result of motion of the assembly relative to the filtered fluid.

As indicated, the filtered fluid that has passed from the contaminated source into the flow channel is translocated to a collection point as a result of motion of the assembly relative to filtered fluid.

This relative motion may be the result of one or more physical interactions between the assembly and the filtered fluid, including inertial and hydrodynamic effects.

Thus the motion towards the collection point of the filtered fluid that has passed from the contaminated source into the flow channel may be the net result of a number of physical effects including one or more of (i) inertia of the filtered fluid resulting in relative movement between the filter carrier and the filtered fluid, (ii) hydrodynamic forces induced in the filtered fluid as a result of relative movement between the filter carrier and the filtered fluid, (iii) centripetal forces acting on the filtered fluid as a result of relative rotational movement between the filter carrier and the filtered fluid.

Most preferably in devices according to the invention the filtered fluid passing from the contaminated source into the flow channel is translocated to a collection point as a result of rotational motion of the assembly relative to filtered fluid, in which case the elongated flow channel is advantageously in the form of a spiral or helix. Thus the elongated flow channel may be in the form of a spiral and the assembly which includes the filter carrier and the filter element carried by the filter carrier is arranged to rotate around the axis of the spiral. In this embodiment, the filter carrier may be in the form of a disc, and in use, the filtered fluid is forced by the relative movement of the groove to move to the centre of the disc.

The elongated flow channel is conveniently formed as a groove in a planar surface of the filter carrier, and the filter element is carried on the planar surface of the filter carrier. In this arrangement, a surface of the filter element and the walls of the groove can in part define the elongated flow channel.

In embodiments in which the filter carrier is in the form of a disc, the disc may be in the form of a disc having one or more spiral grooves on one surface thereof extending around the axis of the disc, and the filter element may be carried on a planar surface of the disc whereby a surface of the filter element and the wall or walls of the groove define the elongated flow channel.

In one preferred arrangement, the disc, in use, is arranged horizontally, with the surface that is provided with the spiral groove facing downwardly and the collection point is adjacent the centre of the disc. Optionally, a reservoir is provided at the collection point for receiving filtered fluid.

As indicated above, in use of devices according to the invention, a variety of physical interactions can induce the filtered liquid to flow to the collection point.

A common feature of these is that they result from the movement of the assembly which includes a filter carrier and a filter element carried by the filter carrier. Thus, for example, in the case where the assembly which includes the filter carrier and the filter element undergoes rotational movement, centrifugal force may be important in determining the rate at which the filtered liquid flows to the collection point. Thus at speeds of rotation which are too high, the flow to the collection point may be inhibited by centrifugal forces. Generally, however, the inertia of the filtered fluid may be a key feature in inducing the filtered liquid to flow to the collection point.

This may be illustrate by the embodiment in which the elongated flow channel is in the form of a single start spiral and the filter carrier rotates about the spiral axis. In this arrangement, in use, the filter carrier is rotated in the "direction of winding" of the spiral. As used herein, the expression "in the direction of winding" as used in relation to a spiral groove, refers by analogy to the rotational direction that corresponds to the direction in which a spiral spring is wound. By employing a multiple start spiral groove, more than one concentric spiral flow channel can be provided. Thus a two-start spiral with the starts diametrically oppositely arranged can provide two concentric spiral grooves.

When rotated in the direction of winding, fluid within the spiral groove tends to move in a direction contrary to the direction of winding, i.e. towards the centre of the spiral. This movement is essentially the result of inertial effects. Thus, fluid that has passed through the filter into the spiral channel will initially be at rest in relation to the body of contaminated fluid. On the other hand the assembly in which the spiral flow channel is formed will be in motion and fluid will consequently move relative to the spiral flow channel in the opposite direction to the direction of rotation of the assembly. This relative motion will persist provided that (1) fluid dynamic forces (e.g. viscosity and frictional forces) do not eliminate the movement of the fluid relative to the flow channel and (2) rotational forces (e.g. centrifugal forces) do not cause the fluid to be flung outwardly so that nett movement of fluid out of the outer part of the flow channel occurs.

Thus, it will be appreciated that at high rotational speeds, centrifugal forces may tend to force fluid in the opposite direction, i.e. away from the centre of the spiral. Accordingly, for a spiral groove (or spiral grooves) of given dimensions, there will a maximum rotational speed, below which fluid will tend to flow to the centre of the spiral and above which, it will tend to flow outwardly.

The relative movement between the assembly which includes a filter carrier and a filter element carried by the filter carrier and the contaminated fluid tends to have a self-cleaning effect on the filter element. This effect has a greater magnitude at relatively high speeds. Accordingly, in the case where the collection point for filtered fluid is at the centre of the spiral, it is necessary to select a speed at which the filtered fluid moves towards the centre (i.e. centrifugal forces must not be excessive), but not so slow that the desired anti-fouling effects are absent.

An embodiment of fluid collection device according to the invention will now be described by way of example, with particular reference to the accompanying drawings, of which: Figure 1 represents a vertical cross section through a fluid collection device in accordance with the invention.

Figure 2 is an enlarged view of part of the device of Figure 1, Figure 3 is a plan view of the filter carrier of the fluid collection device of Figure 1.

Figure 4 is a vertical cross section through the filter carrier of Figure 3, and Figure 5 is a diagrammatic view of part of the filter carrier of Figure 4 on an enlarged scale.

Referring to the drawings, fluid filtration and collection device 10 consists of a filter carrier 1 2 in the form a disc provided with a spiral groove 1 6 on its lower surface.

A filter element 1 4 is attached to the lower groove surface of the filter carrier 1 2 and is retained at its outer margin by a retaining ring 24.

The filter carrier 1 2 may be formed of any suitable material which is not adversely affected by the environment to which the device is intended to be exposed. Thus, for example, it could be constructed of any suitable metallic or plastics material. In extreme environments it may be desirable for the disc to be constructed of corrosion resistant stainless steel, but where the fluid filtration and collection device of the invention is intended to recover samples of waste water, plastics materials may be sufficiently robust. In the embodiment illustrated, the filter carrier 1 2 was constructed of an acrylic plastics material (polymethylmethacrylate). The retaining ring 24 was formed of a similar material.

The filter element 14 is formed of an acrylic copolymer having a 2 pm pore size. The membrane was adhered to the grooved surface of the filter carrier by applying chloroform to the surface of the carrier so as to partially dissolve a surface layer portion thereof. The membrane was pressed in place and its outer margin secured by fitting retaining ring 24.

The filter carrier and the filter membrane both had an overall diameter of about 10 cm.

The filter carrier 1 2 is mounted on a hollow drive shaft 1 8 which is supported by a ball-race (not shown). Also not shown is a motor which is installed off-centre and arranged to drive the drive shaft/filter carrier assembly via a belt.

A collection chamber 26 is mounted inside the drive shaft at its lower end and the region below the collection chamber is enclosed by a casing 20 which is sealed against an O-ring 22 placed on the underside of the filter membrane 1 4. Collection tube 28 is also mounted inside the drive shaft with its lower end arranged to enable fluid accumulating within collection chamber 26 to be withdrawn.

At its inner end 30 (see Fig. 3) the flow channel defined between the membrane and spiral groove 1 6 communicates with the interior of the collection chamber 26 via flow passages 32.

As shown in Figure 5, groove 1 6 has a 3mm pitch and the side walls of the groove are set at angles of 600. The "lands" between the grooves are 1 .5mm in width.

In use, the collecting device 10 was immersed in waste water and rotated at various speeds. As indicated in Figure 3, the direction of rotation is in the direction of winding of the spiral groove.

During operation, waste water passes through the filter membrane 14 into the flow passage defined between the membrane and the spiral groove 16. The flow of filtered fluid through the filter membrane 1 4 was primarily as a result of the pressure differential across the membrane was due to the head of waste water corresponding to the depth of immersion of the device. Once within the flow passage, the rotation of the filter carrier caused the filtered waste water in the spiral groove 1 6 to move along the groove towards the centre of the filter carrier.

Movement in this direction is believed to be due to the combination of forces acting on the fluid, together with the inertia of the fluid, resulting in net inward flow along the spiral groove.

For a given design and size of support structure there was a optimum speed of rotation in order to achieve an acceptable flow rate towards the centre of the disc. Thus, for example, if the speed is too high, centrifugal forces will dominate and the liquid will tend to move towards the outer edge of the disc. Conversely if the speed is too low, the movement of the spiral would not be sufficient for inertial forces to move the liquid to the centre of the disc where it is capable of entering the collection tube. For the disc illustrated in the accompanying Figures the optimum speed was found to be about 800 rpm.

The device described was tested at this speed using a membrane with a 0.2 i'm pore size and even after several weeks of testing in a waste water treatment plant, a flow rate of more than 16i'l/min was obtained.

Further, the movement of the membrane through the water at this speed of rotation prevented accumulation of debris on the underside of the filter membrane.

In other words, the filter membrane was self-cleaning.

As fluid reached the inner end 30 of the flow channel provided by spiral groove 16, fluid passes through flow passages 32 into collection chamber 26.

Periodically, fluid accumulating within collection chamber 26 is withdrawn via collection tube 28 and then through a conduit (not shown) connected to a pump. If desired, a sensor may be provided to operate the pump intermittently when sufficient volume of fluid accumulates in the collection tube.

Modifications of the device illustrated can be made within the general inventive concept of the present invention. For example the filter carrier could be cylindrical, in which case the flow channel would be in the form of a helical groove on the cylinder surface.

Claims

CLAIMS 1. A device- for collecting fluid from a source which is contaminated with suspended particulate matter, comprising an assembly which includes a filter carrier and a filter element carried by the filter carrier, one surface of the filter element being arranged in use to be in contact with the contaminated source and the other surface defining a wall of an elongated flow channel, wherein means are provided for continuously moving the assembly relative to the contaminated source, whereby in use, filtered fluid passing from the contaminated source into the flow channel is translocated to a collection point as a result of motion of the assembly relative to the filtered fluid
2. A device according to Claim 1 wherein the motion of the assembly relative to filtered fluid that has passed from the contaminated source into the flow channel is at least partly due to inertia of the filtered fluid resulting in relative movement between the filter carrier and the filtered fluid.
3. A device according to Claim 1 wherein the motion of the assembly relative to filtered fluid that has passed from the contaminated source into the flow channel is at least partly due to hydrodynamic forces induced in the filtered fluid as a result of relative movement between the filter carrier and the filtered fluid.
4. A device according to Claim 1 wherein the motion of the assembly relative to filtered fluid that has passed from the contaminated source into the flow channel is at least partly due to centrifugal forces acting on the filtered fluid as a result of relative rotational movement between the filter carrier and the filtered fluid.
5. A device according to Claim 1 wherein the motion of the assembly relative to filtered fluid that has passed from the contaminated source into the flow channel is at least partly due to centripetal forces acting on the filtered fluid as a result of relative rotational movement between the filter carrier and the filtered fluid.
6. A device according to Claim 1 wherein the motion of the assembly relative to filtered fluid that has passed from the contaminated source into the flow channel is at least partly due to one or more physical interactions between the two selected from: (i) inertia of the filtered fluid resulting in relative movement between the filter carrier and the filtered fluid, (ii) hydrodynamic forces induced in the filtered fluid as a result of relative movement between the filter carrier and the filtered fluid, (iii) centripetal forces acting on the filtered fluid as a result of relative rotational movement between the filter carrier and the filtered fluid.
7. A device according to any preceding claim wherein the filtered fluid passing from the contaminated source into the flow channel is translocated to a collection point as a result of rotational motion of the assembly relative to filtered fluid.
8. A device according to any preceding claim wherein the elongated flow channel is in the form of one or more spiral or helix.
9. A device according to any of Claims 1 to 7 wherein the elongated flow channel is in the form of a spiral.
1 0. A device according to Claim 9 wherein the elongated flow channel is in the form of a spiral and the assembly which includes the filter carrier and the filter element carried by the filter carrier is arranged to rotate around the axis of the spiral.
11. A device according to any preceding claim wherein the elongated flow channel comprises a groove formed in a planar surface of the filter carrier.
1 2. A device according to any preceding claim wherein the filter element is carried on the planar surface of the filter carrier and a surface of the filter element and the walls of the groove in part define the elongated flow channel.
1 3. A device according to any preceding claim wherein the filter carrier is in the form of a disc.
1 4. A device according to any preceding claim wherein the filter carrier is in the form of a disc having one or more spiral grooves on one surface thereof extending around the axis of the disc, the filter element is carried on a planar surface of the disc and a surface of the filter element and the wall or walls of the groove define the elongated flow channel.
15. A device according to Claim 14, wherein in use, the disc is arranged horizontally, with the surface that is provided with the spiral groove facing downwardly.

1 6. A device according to any of Claims 1 3 to 1 5 wherein the collection point is adjacent the centre of the disc.

1 7. A device according to any preceding claim wherein a reservoir is provided at the collection point for receiving a pool of filtered fluid.

1 8. A device according to any preceding claim in use, wherein the speed of motion of the assembly relative to the contaminated source inhibits fouling of the filter element by suspended particulate matter.