GB2457543A

GB2457543A - Filtration method and apparatus

Info

Publication number: GB2457543A
Application number: GB0803215A
Authority: GB
Inventors: Sanjay Bellara
Original assignee: Nuclear Decommissioning Authority
Current assignee: Nuclear Decommissioning Authority
Priority date: 2008-02-22
Filing date: 2008-02-22
Publication date: 2009-08-26
Also published as: GB0803215D0

Abstract

The invention comprises an apparatus and method for effecting the filtration of a fluid (2), the apparatus comprising a cross-flow filtration unit (3) comprising a liquid eductor (4). The liquid eductor (4) causes fluid circulation in the apparatus, and thereby drives the fluid flow through the filtration unit (3). The liquid eductor (4) preferably comprises a venturi jet device which uses pressurised liquid to entrain greater volumes of liquid through a flow system or channel. The apparatus and method of the invention are preferably used for the handling of fluids which are contaminated with significant levels of radioactivity. Most specifically, the apparatus additionally comprises an ion-exchange unit (10) and is used for the removal of significant levels of soluble b / q radioactivity.

Description

NOVEL FILTRATION METHOD AND APPARATUS

The present invention is concerned with an apparatus and method having particular application in the filtration of fluids with which there is associated particular concern in terms of safety and environmental considerations, for example fluids having a potentially high level of toxicity. More specifically, the apparatus and method finds particular application in the filtration of fluids having significant levels of radioactivity.

Various standard, well known methods are used for the filtration of industrial liquors but, in general, these rely on the use of mechanical pumps to drive the flow of liquor.

In general, such arrangements are perfectly satisfactory. However, on those occasions when the liquors are of high toxicity, or otherwise involve particular safety or environmental considerations, such arrangements are usually not desirable, since the pumps become contaminated and subsequent repair and maintenance work involving the pumps can be extremely hazardous.

Clearly, therefore, there is a need for an alternative arrangement in cases where such fluids -which may, for example, comprise highly toxic industrial waste liquors or liquors which are contaminated with significant levels of radioactivity -are being handled. Specifically, it would be preferred that, in such instances, contact between the contaminated fluids and the mechanical components of a pump should be avoided, and that the driving force for promoting movement and circulation of the liquor should be provided by alternative means.

Thus, the present invention seeks to provide an apparatus and method which eliminates the requirement for a mechanically driven fluid flow to circulate the contaminated liquor through the system. The apparatus and method of the invention allows for the filtration of fluids using alternative pumping means to drive the circulation of the contaminated fluid through the filtration means and relies on the use of a liquid eductor to drive the cross flow operation in a cross flow filtration unit.

Thus, according to a first aspect of the present invention, there is provided an apparatus for effecting the filtration of a fluid, the apparatus comprising a cross-flow filtration unit comprising a liquid eductor. The liquid eductor causes fluid circulation to occur in the apparatus, and thereby drives fluid flow through the filtration unit.

A liquid eductor preferably comprises a Venturi jet device which uses pressurised liquid to entrain greater volumes of liquid through a flow system or channel. Thus, the eductor essentially uses the liquid at high pressure to eject a high speed jet which effectively sucks other liquid along in its path, thereby driving the fluid flow.

In the present invention, the flow system comprises the interior of a cross flow filter module and the entrained liquid is obtained from the liquid in a holding container, such as a pond. The invention relies on cross flow filtration, wherein the flow is tangential to the filter surface. The filtration unit comprising the liquid eductor may be located within, or external to, the liquid medium. Preferably, however, said unit is located within the liquid and, most preferably, said filtration unit is wholly submerged below the surface of the liquid. In the event that the unit is located external to the liquid, it is necessary that a supply of liquid should be provided to the liquid eductor in order to drive the said eductor.

The apparatus according to the first aspect of the invention preferably also comprises a permeate reservoir, in which the fluid which permeates through the filtration unit is able to collect. The collected fluid may then be utilised for backflushing the filtration unit. Thus, in a preferred arrangement, the permeate reservoir is connected to the filtration unit, and is also attached to a vacuum pump and compressed air supply.

Activation of the vacuum pump provides a pressure differential across the filtration unit and causes fluid to be drawn into the permeate reservoir. Once a desired level of fluid is present in the reservoir, a predetermined volume of the fluid can be caused to flow back through the filtration unit by deactivating the vacuum pump and opening the compressed air supply for a period of time.

The apparatus of the invention is preferably used for the handling of fluids which are contaminated with significant levels of radioactivity. In such cases the apparatus typically also comprises means for the removal of said radioactive contamination. A preferred example of such means comprises an ion-exchange unit which is capable of removing soluble f3/7 activity contained in the fluid. The decontaminated fluid flowing out of such an ion-exchange unit is preferably allowed to collect in a suitable tank or reservoir, from which it may then be pumped by means of a conventional pump, since the fluid, at that stage, will comprise only trace active liquors. The apparatus of the invention is particularly suited to the handling of bulk fluids.

In a preferred arrangement, a conventional pump is used to recycle the treated liquid to the liquid eductor unit. By virtue of the eductor principle, the recycled liquid then causes further contaminated liquid to be entrained into the flow channel into the filtration unit, thereby allowing the process to continue.

The present invention also envisages a process for the treatment of contaminated fluids which involves the use of the apparatus according to the first aspect of the invention. Thus, according to a second aspect of the present invention, there is provided a method for the filtration of a fluid, said method comprising circulating the fluid through an apparatus comprising a cross-flow filtration unit comprising a liquid eciuctor.

The apparatus and method of the invention will now be exemplified, though without limitation, by reference to the drawing shown as Figure 1, which illustrates an in- pond treatment system for removal of radioactivity using an eductor driven cross-flow filtration unit.

Thus, in Figure 1 there is shown a pond I in which is contained a fluid 2 which is contaminated with soluble 1317 radioactivity. A filtration module 3, comprising a cross-flow filtration unit having a filter membrane, is placed below the surface of the fluid 2, and associated with this module 3 is a liquid eductor unit in the form of a Venturi jet device 4 including inlets 5.

The filtration module 3 is connected via pipework incorporating valve 6 to permeate reservoir 7, which itself is linked via further pipework to vacuum pump 8, compressed air supply 9, and ion-exchange unit 10. The respective linkages to these three components include further valves 11, 12 and 13. The ion-exchange unit includes an exit pipe which passes via filter 14 to collection reservoir 15, from which a further pipe connects to pump 16, whereby recirculation of fluid to the filtration module 3 is possible via pipe 17 incorporating valve 18.

In operation, the vacuum pump 8 is activated with valves 6 and 11 open and valves 12 and 13 closed in order to provide a pressure differential across the filter membrane comprised in filtration module 3, thereby causing fluid to be drawn into the prermeate reservoir 7. Once the reservoir 7 is full, valve 11 is closed and valve 12 briefly opened in order to force some liquid back through the filter membrane, thereby backflushing the filtration module 3. After a few seconds, valve 6 is closed and valve 13 is opened in order that the liquid which has collected in the penneate reservoir is forced, by means of the compressed air, through the ion-exchange unit 10, wherein the soluble (3by radioactivity contained in the fluid may be removed.

After passage through the ion-exchange unit 10, the treated fluid travels through filter 14 into collection reservoir 15. In order for the treatment process to continue, further fluid must flow through the filtration module 3 and, in view of the fact that the said module 3 comprises a cross-flow filtration unit, it is necessary that the fluid should flow tangentially across the surface of the filter membrane. Thus, the fluid contained in the collection reservoir 15 is recirculated by means of pipework 17, the flow being driven by pump 16 on opening the valve 18.

The recirculated fluid is driven at high pressure to the filtration module 3, via the Venturi jet device 4, wherein the eductor principle is effective in entraining pond water fluid, via the inlets 5, into the filter flow channel. Thus, the process of the invention may be carried out without the contaminated fluid in the pond coming into contact with any mechanical pumps, since the fluid which passes through pump 16 is the treated liquor which contains only trace levels of activity. The hazards associated with any repair and maintenance work relating to the said pump are, therefore, negligible, and the apparatus and process of the invention thereby provides significant advantages in terms of health and safety over the known procedures for the handling of such contaminated fluids, since such procedures have previously inevitably required the direct flow of the fluids through mechanical pumping devices.

The apparatus and method of the invention which is herein exemplified may be optimised in order that substantially less liquid than is produced in the permeate reservoir requires to be recirculated in order to drive the process.

Claims

CLAIMS1. An apparatus for effecting the filtration of a fluid, the apparatus comprising a cross-flow filtration unit comprising a liquid eductor, wherein said liquid eductor drives fluid flow through said filtration unit.
2. An apparatus as claimed in claim 1 wherein the liquid eductor comprises a Venturi jet device.
3. An apparatus as claimed in claim I or 2 wherein said cross-flow filtration unit is wholly submerged below the surface of the said fluid.
4. An apparatus as claimed in claim 1, 2 or 3 which also comprises a penneate reservoir.
5. An apparatus as claimed in claim 4 which additionally comprises a vacuum pump and a supply of compressed air.
6. An apparatus as claimed in any one of claims I to 5 comprising means for the removal of soluble contamination.
7. An apparatus as claimed in claim 6 wherein said soluble contamination comprises soluble I3/ radioactivity.
8. An apparatus as claimed in claim 6 or 7 wherein said means comprises an ion-exchange unit.
9. An apparatus as claimed in any previous claim which comprises a pump for recirculating decontaminated liquor.
10. A method for the filtration of a fluid, said method comprising circulating the fluid through an apparatus as claimed many preceding claim.
11. A method as claimed in claim 10 which comprises cross-flow filtration.
12. A method as claimed in claim 10 or 11 which comprises a process for the decontamination of a pond comprising soluble 13/? radioactivity.
13. An apparatus as hereinbefore described and with reference to the accompanying drawing.
14. A method substantially as hereinbefore described and with reference to the accompanying drawing.