GB2285975A

GB2285975A - Effluent treatment

Info

Publication number: GB2285975A
Application number: GB9500867A
Authority: GB
Inventors: John Christopher Gale
Original assignee: COMENCO DESIGN ENGINEERING COM
Current assignee: COMENCO DESIGN ENGINEERING COM
Priority date: 1994-01-17
Filing date: 1995-01-17
Publication date: 1995-08-02
Also published as: GB9400768D0; AU1421795A; WO1995019213A1; GB9500867D0

Abstract

Effluent is continuously fed via an inlet (3) to vessel (1) and flows upward through closely packed beads (2) of a submerged density somewhat less than that of the effluent whereby the beads (2) tend to rise but are prevented from doing so by a partition (15) which contains openings smaller than the beads (2), the volume of beads and the interstices between them acting as a habitat for micro-bacteria. Gas is injected into the bottom of the vessel (1) by means (11) and a higher volume of gas is injected in the vessel by means (26) whereby the higher volume of air serves to strip any micro-bacteria away from the beads and transports the detached micro-bacteria and other impurities through the openings in the partition (15) to the free surface (27) of the effluent. The purified effluent is withdrawn through outlet (4), the beads (2) being continuously removed via an outlet (19) and returned via duct (20) to the bottom of the vessel (1). <IMAGE>

Description

EFFLUENT TREATMENT PROCESS AND APPARATUS This invention relates to a continuous process and apparatus for the treatment of liquid effluent which may be domestic, industrial or agricultural.

It is known to use a fixed bed or volume of closely packed beads made of a plastic or other material whose submerged density is close to that of water to provide a host for organisms and to pass effluent through the packed volume of beads to expose the effluent to the biological activity of the organisms. It is further known to submerge the closely packed volume of beads in the effluent and to continually add new effluent to the beads while simultaneously withdrawing effluent from the beads in such a way as to expose the flow of effluent to the beads for a specified time.

The organisms inhabiting the beads within effluent treatment processes of this type survive either by attachment to the beads, attachment to other organisms or organic films attached to the beads or by generally inhabiting the interstices between the beads. Such inhabiting organisms are referred to hereafter as "biofilm".

Biofilm effluent treatment systems are suitable for the reduction in the quantity of suspended material passing through the closely packed beads and for a reduction in the biological and chemical oxygen demand exerted by the effluent by virtue of its solid, colloidal and soluble constituents.

One of the problems with treatment systems of this type is that periodically the volume of organisms and suspended solids present in the closely packed volume of beads will reach a level where the volume of effluent able to pass through the volume of beads will be seriously reduced to a level that renders the treatment process uneconomic. When this happens, the process has to be temporarily terminated and the volume of beads exposed to a flow of treated effluent or other supply of water in a flow direction generally opposite to the flow direction of the main treatment process. This procedure is generally known as "back-washing" and it is known to accompany "back-washing" with pressurised air released into the bottom of the volume of beads in order to disassociate them from the biofilm and other debris. When a "back-wash" flow as just described is applied to the beads, it is possible depending on their density to change them from being generally of a positive buoyancy to being generally of a negative buoyancy by virtue of the volume of air temporarily present in the effluent.

This change in buoyancy enhances the effectiveness of the "back-wash".

It is further known to use a fixed filter bed or volume of closely packed small mineral beads of a density generally several times that of water to form a host for organisms, and to pass effluent through the packed volume of beads to expose it to the biological activity of the organisms.

Generally the effluent is passed through the mineral beads in a downward direction at a rate sufficiently low for a considerable volume of air to occupy the interstices of the packed volume of beads.

None of these prior art processes allows the beads to be continually cleaned and recirculated to the filter bed so they generally suffer from a down time which renders them uneconomic while the filter is being cleaned.

It is an object of the present invention therefore to provide a process for the continuous treatment of aqueous effluent in which the material from which the filter is made is continually cleaned and the cleaned material is recirculated to the bottom of the filter bed so that the process can run on a continuous basis.

According to the invention there is provided a process for the continuous treatment of aqueous effluent comprising feeding the effluent through tank containing a filter made up from a closely packed volume of particles immersed in said effluent and of a submerged density slightly less than that of the effluent whereby, in use, the particles become coated with contaminant and/or the interstices therebetween become filled with contaminant thereby cleaning the effluent, removing treated effluent from the filter, continuously cleaning contaminant from the particles or the interstices therebetween in the filter at a location downstream of the location where the treated effluent is removed and continually removing said cleaned particles from the filter and returning them to the upstream region thereof, the particles moving through the filter in a controlled manner throughout the treatment process.

Preferably the effluent is passed generally upwardly through the filter and the particles are prevented from rising to the free surface of the effluent at a location above that where the treated effluent is removed. Pressurised gas may be supplied to the bottom region of the filter at a pressure and rate sufficient to promote the growth of biofilm on or between the particles if the process is to be used as a biological filter. However, it is envisaged within the scope of the invention that the process could be used as a non-biological filter by not feeding any gas to the bottom of the vessel.

If the gas is fed to the bottom of the vessel, it is preferably supplied in a diffuse manner to the filter.

Gas is preferably injected into the effluent to clean the particles at a rate and pressure sufficient to detach the contaminant therefrom and/or from the interstices therebetween. In the preferred embodiment, the gas to clean the particles is supplied to the filter in a diffuse manner, for instance from a sparge pipe.

Preferably the cleaned particles are removed from the filter using moving means which can be screw or some other form of conveyor. The cleaned particles can be returned directly to the filter by means of a return path and it is envisaged that gas may be added to the return path prior to their reentry into the filter to assist the re-entry process.

Preferably the contaminant removed from the particles floats to the top surface of the effluent and is continually removed therefrom.

The gas supplied to the filter can be air, an air/gas mixture, oxygen, methane or gases largely containing methane.

Preferably any cleaned particles which may become stuck to the moving means are removed therefrom and directed into the re-entry path back to the filter.

If the moving means is a screw conveyor, gas can be supplied to the interior of the screw conveyor to exit therefrom via outlets to assist in the cleaning process.

According to another aspect of the invention there is provided apparatus for the continuous treatment of aqueous effluent through a vessel from an inlet to an outlet, the vessel containing a volume of closely packed particles immersed in said effluent and of a submerged density less than that of the effluent to provide a filter for the effluent, a liquid permeable partition extending across the vessel downstream of the outlet, the particles substantially filling the volume in the vessel between the inlet and said partition and being prevented from rising to the free surface of the effluent by said partition, means for removing treated effluent from the outlet, means for continually cleaning from the effluent contaminant collected on the particles and/or in the interstices therebetween in the vessel upstream of said partition, and moving means for removing cleaned particles from the vessel and returning them to the bottom region thereof, the arrangement being such that, in use, the entire volume of particles moves through the vessel in a controlled manner and contaminant removed from the particles and/or the interstices therebetween floats to the top surface of the effluent in the vessel downstream of said partition where it collects as a sludge and means for removing said sludge from the vessel.

Preferably the inlet through which the effluent is fed into the vessel is located at a low point in said vessel and the effluent, in use, flows generally upwardly through the vessel.

Conveniently means for supplying pressurised gas to the filter to promote the growth of a biofilm on the particles or in the interstices therebetween are located in the bottom region of the vessel. However, this is not essential as it is envisaged that the apparatus of the present invention can be used as a non-biological filter by omitting the supply of gas to the bottom region of the filter.

Preferably cleaning means are provided adjacent the partition which are operable to supply gas at a rate and pressure sufficient to clean contaminant from the particles. Conveniently the cleaning means comprises a plurality of gas injection nozzles or a sparge pipe.

In the preferred embodiment, the moving means for the cleaned particles is a screw conveyor (other forms of conveyor can however be used) which extends across the width of the vessel immediately adjacent the partition, said conveyor, in use, feeding cleaned particles from the vessel adjacent the partition into a return duct connected to the bottom region of the vessel whereby said cleaned particles can be returned and reintroduced to the bottom of the filter in the vessel.

Conveniently the return duct is connected to the inlet to the vessel through which effluent is admitted thereto and means for admitting gas to said return duct can also be provided to assist the reintroduction process by reducing the buoyancy of the particles in the end region of the return duct where it joins the effluent inlet.

Means are preferably associated with the screw conveyor to remove any particles stuck thereto for admission into the return duct.

Conveniently the screw conveyor is an auger and the partition is shaped to closely surround the upper half of said auger.

Preferably the partition is rendered liquid permeable by providing a plurality of openings in it of a size which allows effluent but not the particles to flow therethrough.

Preferably rotary means in the form of a rotary brush cooperates with the screw conveyor to positively drive particles from the vessel into the return duct. The rotary brush is preferably rotated by drive means operable whereby there is a relative peripheral velocity between the circumference of the rotary brush and the apparent linear velocity of the auger spiral at the point where the brush meshes with the auger. If necessary, the interior of the screw conveyor can be connected to a gas supply, said gas exiting, in use, from the interior of the auger via a plurality of holes therein.

In the preferred embodiment, the treated effluent outlet is connected to a pipe which extends upwardly and terminates in an outlet which is adjustable in height, vertical movement of said outlet controlling the upper level of the sludge in the top of the vessel.

Preferably the vessel includes a weir in its upper region over which the sludge flows for removal from the vessel.

In one embodiment, the vessel outlet is connected to a pipe which extends upwardly and terminates in an immovable outlet, the weir including adjusting means whereby the volume of sludge flowing over the weir can be varied.

In the preferred process, the effluent is passed generally upwardly through the volume of closely packed beads of individual submerged density slightly less than the effluent, the beads being entirely submerged in the effluent, pressurised air being added continually to the underside of the beads in a diffuse manner at a rate and volume which will promote and serve the needs of biological growth on and between the beads while not causing upward velocity sufficient to detach and mobilise the biological growth. At a point towards the top of the closely packed volume of beads and above the point at which the treated effluent is removed from the process, additional volumes of gas are added in a diffuse manner, the additional volumes of rising gas creating an upward velocity sufficient to detach contaminant from the beads and/or from the interstices between the beads, the detached contaminant then floating to the surface of the effluent for removal therefrom.

Preferably the biofilm or contaminant and other debris stripped from the beads or the interstices therebetween is floated to the surface of the effluent and continuously removed therefrom together with a small quantity of the effluent via a weir situated just below the surface of said effluent.

The preferred system for the removal of biofilm or contaminant and other debris is provided with means whereby the rate of removal of the biofilm and debris in conjunction with a small quantity of effluent is determined by the height adjustment of an adjustable bell mouth which discharges the treated effluent from the process.

The cross sectional shape of the vessel is not critical. It can for instance be circular, oval, square or rectangular.

Furthermore, the walls need not be parallel along the whole length of the vessel as they can taper in one or both directions.

The gas added to the said volume of closely packed beads for the purpose of promoting and serving biological growth on and between the beads can be air, an air gas mixture, oxygen or methane or a mixture thereof.

The beads are made of a material whose submerged density is slightly less than that of the effluent, a plastic material such as polypropylene being the preferred material. Beads of any size and shape can be used. A maximum size of 12mm has been found to be particularly suitable.

In a preferred device, the effluent is continuously fed into a low point within the vessel and flows in a generally upward direction through a fixed volume of closely packed particles of a submerged density somewhat less than that of the effluent such that the entire volume of particles tends to rise to the free surface of the effluent in the said vessel but is prevented from doing so by a member across a generally horizontal cross section of the vessel, said member containing a plurality of openings of individual area somewhat smaller than the particles, the volume of particles and the interstices between the particles acting as a habitat for micro-bacteria which have a purifying effect on the effluent. Air is injected into the bottom of the said vessel to provide the oxygen requirements of the said microbacteria and a higher volume of air is injected at a higher zone in the vessel immediately below said member such that the higher volume of air serves to strip the micro-bacteria away from the particles above that point in the overall volume of particles and lifts said detached micro-bacteria and other impurities within the effluent through the openings in the said member to the free surface of the effluent within the vessel. At that point, the microbacteria and impurities accumulate as a floating mass of sludge and the effluent purified by exposure to the said micro-bacteria on and between the volume of particles is withdrawn from the vessel at a point immediately below the injection of the higher volume of air, the particles being continuously removed from a point in the vessel immediately below said member and returned to a point near the bottom of the vessel such that they float upwardly to re-join the said volume of particles.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 is a side elevation cross sectional view through a first apparatus of the invention; FIGURE 2 is an end view of the apparatus of Figure 1 partly in cross section; FIGURE 3 is a side elevation of a modified apparatus; FIGURE 4 is an end elevation, partly in section, of the modified apparatus of Figure 3; FIGURE 5 is a cross section through the upper region of the apparatus of Figures 3 and 4; and FIGURE 6 is a cross sectional end view of the part of the apparatus shown in Figure 5.

Referring now to the embodiment shown in Figures 1 and 2 of the drawings, there is shown a vessel or tank 1 of generally square cross section with a tapered base la. The tank is substantially filled with particles such as beads 2 made of polypropylene or some other material of a submerged density slightly less than the effluent in which they are submerged whereby the beads tend to float to the top surface thereof.

The beads are closely packed in the tank and substantially fill the space in the tank between the bottom thereof and a collector or partition 15 located adjacent the upper region thereof leaving a small volume 2a with no beads at the bottom of the tank 1. The closely packed beads 2 provide a filter bed through which the effluent flows in a controlled manner and is cleaned thereby.

Effluent enters the tank 1 via inlet pipe 3 near the bottom of the vessel and the effluent and beads flow upwardly through the closely packed volume of beads 2 in a controlled manner and out through an outlet pipe 4 situated at a higher point in the vessel 1. The pipe 4 is connected by a further pipe 5 to a bell mouth outlet 6 located adjacent the upper end of the tank and it is from this bell mouth 6 that the clean effluent is discharged into tank 8a and removed therefrom via outlet pipe 6a. The bell-mouth 6 incorporates means (not shown) to adjust its height relative to the floor of the tank 8a and hence the upper level 26 of the effluent 7 in the adjacent tank 8. The entrance to the pipe 4 is protected by a mesh having apertures 10 therein of a size less than that of the beads so that they cannot pass into the pipe 4.

If the system is to be used as a biological filter system, gas is pumped into the bottom of the vessel 1 in a diffuse manner from a gas diffuser 11 (see Figure 1) fed by pipe 11a (see Figure 2) in order to promote biological growth in the volume of beads 2. The gas passes upwardly through the closely packed volume of beads 2 and into the collector 15 (see Figure 2) adjacent the top of the vessel 1 comprising a partition or divider wall 16 in which a mesh of apertures 17 of a dimension less than that of the beads 2 is formed.

Moving or conveyor means are contained within the top of the collector 15 in the form of an Archimedes type screw conveyor 18 driven by motor 25 to transport the beads across the top of the vessel 1 through aperture 19 in the side wall of the vessel 1 and a return conduit 20 where they are fed to rejoin the inlet pipe 3 at the bottom of the vessel 1.

The cleaned beads 2 re-enter the inlet pipe 3 and thus join the flow of effluent to be filtered entering the vessel 1 which is then fed into the empty space 2a beneath the underside 2c of the beads 2.

The upward rate of the gas and effluent in the tank below the clean effluent outlet 4 is arranged to be such that biofilm formed on the volume of beads 2 is not disturbed.

At a point immediately above the outlet 4, further gas is introduced into the volume of beads via a sparge pipe 26 which creates a higher flow velocity in the region of the vessel 1 above the outlet 4 whereby biological material (i.e. biofilm and other contaminant) is stripped from the beads 2 and the interstices therebetween and carried by the air bubbles to the effluent top surface 27 where it collects as a sludge.

The solid material washed from the beads 2 migrates upwardly by virtue of the small bubbles of gas that will become attached to it. The solid material passes through the collector mesh 16 and flows continuously upwardly, in association with a small volume of supernatant effluent to form the sludge at the upper surface 27 where it flows over weir 28 and into tank 29. Solid material sludge and supernatant effluent is continuously removed from the tank 29 through pipe 30.

An alternative embodiment is shown in Figures 3-6 which will now be described.

Referring now to the apparatus shown in Figures 3-6, this can be seen to comprise an upright tank 1 having a tapered base la. The tank 1 is substantially filled with closely packed particles or beads 2, preferably made of a plastic material such as polypropylene, of a density slightly less than the effluent in which they are submerged whereby the beads tend to float to the top surface thereof. The closely packed beads substantially fill the space in the tank between the bottom thereof and collector or partition 16 located adjacent the upper region thereof leaving a small volume 2a with no beads at the bottom of the tank. The upper region of the tank 1 narrows into a conical section 1b to which an upper tank part lc is attached. A screw conveyor 18 is housed in this tank part lc and feeds beads 2 from the top of the tank 1 where they collect against partition 16 via return pipe 20 back to the bottom of the tank in much the same way as has been described with reference to Figures 1 and 2.

A gas diffuser 11 is located slightly above the effluent inlet 3 at the bottom of the tank. The tank 1 has a clean effluent outlet 34 in the upper section lc and this is connected by means of a pipe 35 to a bell mouth 36 located within a tank 45. The height of the bell mount 36 relative to the bottom of the tank 45 can be adjustable by means (not shown). A duct 46 leads from the tank 45 to remove treated effluent therefrom. A second tank 45a receives sludge from the upper effluent surface 27 in the tank part lc via weir 60 which can either be fixed or pivoted as illustrated.

An optional recirculation pipe 35a is shown in Figure 4 for supplying clean effluent to the bottom of the tank, in certain circumstances and if required.

As can be seen more clearly in Figures 5 and 6, the upper region lc of the tank has a partition 16 extending across it which provides a collector and is shaped to fit around the contours of the conveyor 18 as can be seen more clearly in Figure 6. This partition 16 has a plurality of perforations 16a in it of a diameter insufficient to permit beads 2 to pass through it to the upper liquid level 27 in the tank.

A gas diffuser 46 is preferably located in the upper tank section lc intermediate the clean effluent outlet 34 and the conveyor 18. Additionally, the interior of the screw conveyor 18 can be hollow and provided with outlets 38 whereby gas can be supplied from the interior of the screw conveyor 18 to remove any beads which may be stuck thereto.

Rotary means in the form of wheel 31 with bristles 32 attached around its periphery is located in the upper part lc of the tank 1 immediately adjacent the end of the screw conveyor 18 in the region where it passes through a bead outlet 19 in the tank to communicate with bead return duct 20. The bristles 32 engage with the screw conveyor 18 in such a way that they provide a positive displacement drive to feed the beads 2 into the outlet duct 20. Any beads stuck to the conveyor 18 will also be removed by the bristles 32. A wall 47 divides the upper region of the tank 1 into a separate chamber 56, the upper edge of the wall 47 being located closely adjacent the bristles 32. A perforated coverplate 48 extends around the periphery of the bristles 32 from the bead outlet 19 to the top of the wall 47 and is mounted on supports 48a. Any sludge 55 which may collect in the chamber 56 can be removed therefrom via outlet 57.

The wheel 31 has a pulley 39 attached to it which is driven by a belt 40 (see Figure 6) at a speed whereby the bristles are in constant mesh with the screw conveyor 18. The screw conveyor 18 is driven by motor 50 connected to one end thereof, the other end driving a reduction gear 51 having a pulley (not shown) connected to it, said pulley driving the belt 40 and thus the brush wheel 31 at a peripheral speed at the point where the brush engages with the screw conveyor 18 which is equal to the linear speed of the screw conveyor or the brush can be driven at a slightly higher peripheral speed to assist cleaning beads from the screw conveyor.

The apparatus shown in Figures 3-6 operates in much the same way as has already described with reference to that shown in Figures 1 and 2 in that the tank 1 is substantially filled with closely packed plastic beads 2 and the effluent to be treated is fed into the bottom of the tank at 3 and allowed to pass upwardly therethrough. Gas is pumped into the bottom of the vessel 1 through pipe 11a to the diffuser 11 if biological growth in the volume of beads 2 within the tank is to be promoted. The gas and effluent passes upwardly through the volume of closely packed beads 2 towards the screw conveyor 18, the beads themselves also moving upwardly through the tank in a controlled manner. By the time it reaches the level of outlet 34, it has been thoroughly cleaned and clean effluent therefore flows out of the tank through the outlet 34.

The upward rate of the gas and effluent in the tank below the clean effluent outlet 34 is arranged to be such that biofilm formed on the volume of beads 2 is not disturbed.

Further gas can optionally be introduced into the closely packed volume of beads via the sparge pipe 46 to create a higher velocity in the region of the vessel lc above the outlet 34 whereby biological material such as biofilm and other contaminants is stripped from the beads 2 and can float up to the top level 27 of the liquid in the tank through the perforated collector partition 16 where it collects as a sludge and can flow therefrom over weir 60 (see Figure 4). This weir can either be of the fixed type or it can be adjustable by means of a hinged flap as illustrated to vary the operational height of the weir.

Beads 2 are removed from the tank 1 by means of the screw conveyor 18 which is of the Archimedes type and transports them across the top of the tank towards the outlet aperture 19. Any dirty beads 2 which tend to stick to the screw conveyor 18 are removed therefrom by means of the brush 32 where they fall into the return duct 20 where they are fed back into the empty space 2a in the base of the tank 1 for recycling and re-use on a continuous basis. If necessary, an independent air supply can be introduced into the end region 20a of the return duct to reduce the buoyancy of the beads and encourage their reintroduction back into the tank 1.

The perforated screen on shield 48 allows clean effluent to enter the chamber 56 but stops any beads entering it.

Sludge 55 can however collect at the bottom of the chamber 56 but this can be removed via outlet 57.

The gas supplied to the inlets 11 and 46 can be air, an air gas mixture, oxygen or methane or gases largely containing methane. It is also envisaged that the apparatus of Figures 3-6 can be used without the introduction of gas to the filter bed of beads 2 either at the base or intermediate the clean effluent outlet 34 and the screw conveyor 18.

From the foregoing description it will be appreciated that the volume of air supplied to the closely packed beads is not sufficient to create a fluidised bed of beads 2.

Instead, it merely aerates the filter bed in the lower region to a degree which will promote the growth of biofilm and to agitate the beads sufficiently in the upper region of the tank to strip the biofilm therefrom and clean the beads prior to their removal from the tank by means of the screw conveyor 18.

Because the beads are continually removed from the tank and returned via the duct 20 to the base thereof on a continuous basis, they tend to move up the tank in a controlled manner whereby, in general terms, the same group of beads which start off adjacent each other at the bottom of the tank arrive at the top of the tank adjacent the same beads. It should be noted therefore that the process is not a fluidised bed process as is common in the prior art.

Claims

1. A process for the continuous treatment of aqueous effluent comprising feeding the effluent through a tank containing a filter made up from a closely packed volume of particles immersed in said effluent and of a submerged density slightly less than that of the effluent whereby, in use, the particles become coated with contaminant and/or the interstices therebetween become filled with contaminant thereby cleaning the effluent, removing treated effluent from the filter, continually cleaning contaminant from the particles or the interstices therebetween in the filter at a location downstream of the location where the treated effluent is removed and continually removing said cleaned particles from the filter and returning them to the upstream region thereof, the particles moving through the filter in s controlled manner throughout the treatment process.

2. A process as claimed in claim 1 wherein the effluent is passed generally upwardly through the filter and the particles are prevented from rising to the free surface of the effluent at a location above that where the treated effluent is removed.

3. A process as claimed in claim 1 or claim 2 wherein pressurised gas is supplied to the bottom region of the filter at a pressure and rate sufficient to promote the growth of biofilm on or between the particles.

4. A process as claimed in claim 3 wherein the gas is supplied in a diffuse manner to the filter.

5. A process as claimed in any preceding claim wherein gas is injected into the effluent at a rate and pressure sufficient to detach the contaminant therefrom and/or from the interstices therebetween to clean the particles.

6. A process as claimed in claim 5 wherein the gas to clean the particles is supplied to the filter in a diffuse manner.

7. A process as claimed in any preceding claim wherein the cleaned particles are removed from the filter using moving means.

8. A process as claimed in claim 7 wherein gas is added to the return path of the cleaned particles prior to their reentry into the filter.

9. A process as claimed in any preceding claim wherein contaminant removed from the particles and/or the interstices therebetween floats to the top surface of the effluent and is continuously removed therefrom.

10. A process as claimed in any preceding claim wherein the gas is air.

11. A process as claimed in any of claims 3 to 9 wherein the gas is oxygen.

12. A process as claimed in any of claims 3 to 9 wherein the gas is methane or a mixture of gases largely containing methane.

13. A process as claimed in any of claims 3 to 9 wherein the gas is methane or a mixture of gases largely containing methane.

14. A process as claimed in claim 7 wherein any cleaned particles stuck to the moving means are removed therefrom and guided into the return path back to the filter.

15. A process as claimed in claim 7 wherein the moving means is a screw conveyor and gas is supplied to the interior thereof and exits therefrom via outlets therein.

16. A process for the continuous treatment of effluent substantially as herein described with reference to the accompanying drawings.

17. Apparatus for the continuous treatment of aqueous effluent comprising means for feeding the effluent through a vessel from an inletKto an outlet, the vessel containing a volume of closely packed particles immersed in said effluent and of a submerged density less than that of the effluent to provide a filter for the effluent, a liquid permeable partition extending across the vessel downstream of the outlet, the particles substantially filling the volume in the vessel between the inlet and said partition and being prevented from rising to the free surface of the effluent by said partition, means for removing treated effluent from the outlet, means for continually cleaning from the effluent contaminant collected on the particles and/or in the interstices therebetween in the vessel upstream of said partition and moving means for removing cleaned particles from the vessel and returning them to the bottom region thereof, the arrangement being such that, in use, the entire volume of particles moves through the vessel in a controlled manner and contaminant removed from the particles or the interstices therebetween floats to the top surface of the effluent in the vessel downstream of said partition where it collects as a sludge and means for removing said sludge from the vessel.

18. Apparatus as claimed in claim 17 wherein the inlet through which the effluent is fed into the vessel is located at a low point in said vessel and the effluent, in use, flows generally upwardly through the vessel.

19. Apparatus as claimed in claim 17 or claim 18 wherein means for supplying pressurised gas to the filter to promote the growth of biofilm on the particles or in the interstices therebetween are located in the bottom region of the vessel.

20. Apparatus as claimed in any of claims 17-19 wherein cleaning means are provided adjacent the partition operable to supply gas at a rate and pressure sufficient to clean contaminant from the particles.

21. Apparatus as claimed in claim 20 wherein said cleaning means comprises a plurality of gas injection nozzles.

22. Apparatus as claimed in any of claims 17-21 wherein the moving means comprises a screw conveyor.

23. Apparatus as claimed in claim 22 wherein the screw conveyor extends across the width of the vessel immediately adjacent the partition and is operable, in use, to feed cleaned particles from the vessel adjacent the partition into a return duct connected to the bottom region of the vessel whereby said cleaned particles can be returned and re-introduced to the bottom of the vessel on a continuous basis.

24. Apparatus as claimed in claim 23 wherein the return duct is connected to the vessel inlet.

25. Apparatus as claimed in claim 24 including means for admitting air to said return duct in the region of the vessel inlet.

26. Apparatus as claimed in any of claims 22-25 wherein means are associated with the screw conveyor to remove any particles stuck thereto for admission into the return duct.

27. Apparatus as claimed in any of claims 23-26 wherein the screw conveyor is an auger and the partition is shaped to closely surround the upper half of said auger.

28. Apparatus as claimed in any of claims 17-27 wherein the partition contains a plurality of openings of a size which allows effluent but not the particles to flow therethrough.

29. Apparatus as claimed in any of claims 26-28 wherein rotary means comprising a rotary brush cooperates with the screw conveyor to positively drive particles from the vessel into the return duct.

30. Apparatus as claimed in claim 29 wherein the rotary brush is rotated by drive means operable to rotate said brush at a peripheral velocity at the point where the brush engages with the auger which equals the linear speed of the auger.

31. Apparatus as claimed in any of claims 22-30 wherein the interior of the screw conveyor is connected to a gas supply, said gas, in use, exiting via a plurality of holes in said screw conveyor.

32. Apparatus as claimed in any of claims 17-31 wherein the vessel outlet is connected to a pipe which extends upwardly and terminates in a bell-mouth which is adjustable in height, vertical movement of said bell-mouth controlling the upper level of the sludge formed on the top of the vessel.

33. Apparatus as claimed in any of claims 17-32 wherein the vessel includes a weir in its upper region over which the sludge flows for removal from the vessel.

34. Apparatus as claimed in claim 33 wherein the vessel outlet is connected to a pipe which extends upwardly and terminates in an immovable outlet, said weir including adjusting means whereby the volume of sludge flowing over said weir can be varied.

35. Apparatus as claimed in any preceding claim wherein the particles are plastic beads.

36. Apparatus substantially as herein described with reference to Figures 1 and 2 or Figures 3-6 of the accompanying drawings.