WO1997011767A1

WO1997011767A1 - Electro-osmotic dewatering of sludges

Info

Publication number: WO1997011767A1
Application number: PCT/EP1996/004186
Authority: WO
Inventors: Horst Christian Schulte; André VOGEL
Original assignee: Steinmuller Mining And Piping Systems (Proprietary) Limited; Seprotech (Proprietary) Limited
Priority date: 1995-09-26
Filing date: 1996-09-25
Publication date: 1997-04-03
Also published as: AU7214196A

Abstract

The invention concerns a method and apparatus for dewatering a sludge. In the method the sludge is gravitationally dewatered as a first step and electroosmotically dewatered as a second step. In accordance with the invention, in the second step the sludge is passed between a lower electrode and an upper electrode in the form of an endless, perforated belt (36) having upper and lower runs. In the first step the sludge is gravitationally dewatered by depositing it on the upper run of the endless belt (36) of the upper electrode and liquid is allowed to filter gravitationally out of the sludge through the perforations in the belt. According to another aspect of the invention, the lower electrode is also in the form of a perforated, endless belt (44) and suction is applied to the belt. The belts can be driven at different linear speeds to generate shear forces on solid particles of the sludge to promote liberation of the liquid content.

Description

"ELECTRO-OSMOTIC DEWATERING OF SLUDGES"

BACKGROUND TO THE INVENTION

THIS invention relates to electro-osmotic dewatering of sludges.

Electro-osmotic dewatering may, for instance, be used to dewater waste water sludges such as those encountered in sewage treatment plants. The electro-osmotic technique relies on the negative charge carried by the individual sludge particles and makes use of an upper anode and a lower cathode which have the effect of drawing the water content of the sludge downwards and the solid particle content upwards, thereby achieving or enhancing the separation of the solid and liquid phases of the sludge.

It has already been proposed, for instance in South African patent 91/0538, to perform electro-osmotic dewatering of a sludge by passing the sludge between two endless, perforated belts with the upper belt being an anode and the lower belt a cathode.

US patent 4,101,400 also describes an electro-osmotic dewatering method and apparatus. In this case, sludge which is to be dewatered is deposited on a lower, non-conductive screen belt which transports the sludge to a subsequent electro-osmotic dewatering stage where the sludge is compressed between the lower screen belt and an upper belt and where electro-osmotic action is created by further, upper and lower, electrically conductive belts arranged within the confines of the main compression belts. SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a method of dewatering a sludge, the method comprising gravitationally dewatering the sludge as a first step and electro-osmotically dewatering the sludge as a second step, wherein in the second step the sludge is passed between an upper electrode in the form of an endless, perforated belt and a lower electrode, and wherein in the first step the sludge is gravitationally dewatered by depositing it on the upper run of the endless belt and allowing liquid to filter out of the sludge through the perforations in the belt.

The invention extends to apparatus for use in the method according to the first aspect ofthe invention, the apparatus comprising an endless, perforated belt serving as an upper electrode, a lower electrode beneath the belt, means for depositing the sludge on the upper run of the belt so that liquid filters under gravity out of the sludge through the perforations of the belt, and means thereafter for causing the sludge to pass between the electrodes for electro-osmotic dewatering thereof to take place.

According to a second aspect of the invention there is provided a method of electro-osmotically dewatering a sludge, wherein the sludge is passed between an upper electrode and a perforated lower electrode which is located beneath the upper electrode, and wherein suction is applied to the underside of the lower electrode.

The invention also extends to apparatus for use in the method according to the second aspect of the invention, the apparatus comprising an upper electrode, a perforated lower electrode, means for causing the sludge to pass between the electrodes, and means for applying suction to the underside of the lower electrode.

According to a third aspect of the invention there is provided apparatus for electro-osmotically dewatering a sludge, the apparatus comprising an upper electrode in the form of an upper endless belt, a lower electrode, located beneath the upper electrode and in the form of a lower, perforated endless belt, means for causing the sludge to pass between the lower run of the upper belt and the upper run of the lower belt, and means for adjusting the vertical spacing of the said runs of the belts.

According to a fourth aspect of the invention there is provided a method of electro-osmotically dewatering a sludge in an apparatus which includes an upper electrode in the form of an endless belt and a lower electrode, also in the form of an endless belt, beneath the upper electrode, the method comprising the steps of driving the belts so that the lower run of the upper belt moves in the same direction as the upper run of the lower belt but at a different linear speed, and causing the sludge to pass between the said runs of the belts so that the said runs of the belts apply shear forces to solid particles of the sludge because of the difference in their speeds.

The invention extends further to an apparatus for use in the method according to the fourth aspect of the invention, the apparatus comprising an upper electrode in the form of an endless belt and a lower electrode, also in the form of an endless belt, beneath the upper electrode, means for driving the belts so that the lower run of the upper belt and the upper run of the lower belt move in the same direction but at different linear speeds, and means for passing the sludge between the said runs of the belts with the result that the said runs of the belts apply shear forces to solid particles of the sludge because of the difference in their speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a side elevation of a sludge dewatering apparatus operating in accordance with the method of the invention;

Figure 2 shows a plan view ofthe sludge dewatering apparatus; and

Figure 3 shows an enlarged side elevation of a single stage of the sludge dewatering apparatus.

DESCRIPTION OF EMBODIMENTS

The electro-osmotic sludge dewatering apparatus 10 seen in Figures 1 and 2 has three dewatering stages indicated generally by the reference numerals 12, 14 and 16 respectively. The stage 12 is seen at an enlarged scale in Figure 3.

The numeral 18 in Figures 1 and 2 indicates a preparation tank for polvmer which is to be added to the sludge prior to dewatering. The polymer is pumped from the tank 18 into a polymer storage tank 20 by means of a pump 22. A water dosing pump 24 pumps water via a proportional mixing valve 26, where the polymer is mixed in the required proportion with water, into a mixing tank 28 where the polymer is mixed with raw sludge supplied by a pump 30 from an inlet 31. The pressurisation of the tank 28 causes sludge, mixed with the required proportion of polymer, to flow from the tank along a feed line 32. At the end of the feed line 32, the sludge is distributed by a spreader chute 34 onto the upper run of a perforated, endless belt 36.

The belt 36, made of electrically conductive material, serves as the anode for the first electro-osmotic dewatering stage 12. Referring to Figure 3, the belt 36 passes around respective head and tail rollers 38 and 40 and intermediate idler rollers 42. Located beneath the belt 36 in the first stage 12 is another endless, perforated belt 44, also made of electrically conductive material. The lower belt 44 serves as the cathode in the first stage 12. The belt 44 passes around head and tail rollers 46 and 48 and intermediate idler rollers 50. Electrical current is supplied to the two belts from a DC power source (not illustrated) via suitable contacts on the belts.

The second stage 14 of the apparatus 10 has upper and lower, perforated anode and cathode belts 52 and 54 which are similar to the belts 36 and 44 of the first stage, and both belts pass around similar arrangements of head and tail rollers and intermediate idler rollers. In similar fashion, the third stage 16 has upper and lower, perforated anode and cathode belts 56 and 58 passing around head and tail rollers and intermediate rollers. Each stage is provided with its own DC electrical supply (not shown), although it is also possible for the various anode and cathode belts to be connected electrically with one another.

The belts 36, 44, 52, 54, 56 and 58 are driven in the directions indicated by the arrows by means of electric motors, indicated with the numeral 60, and suitable transmissions which drive the relevant rollers in each stage.

The spreader chute 34 spreads the sludge which is to be dewatered across the width of the upper run of the anode belt 36 of the first stage 12. The sludge is conveyed, from right to left in the drawings, by the upper run of the belt. During this movement of the sludge, it is subjected to gravitational dewatering, with a substantial proportion of the water content of the sludge filtering out of the sludge through the perforations in the belt. The water which passes through the upper run of the belt is collected in a suitable collector structure (not shown) located beneath the upper run ofthe belt, and is removed.

An advantage of performing gravitational dewatering on the upper run ofthe anode belt in the first stage is the fact that there is no need for an independent dewatering unit to perform dewatering prior to electro-osmotic dewatering. Added to this the absence of a separate dewatering unit upstream of the electro-osmotic dewatering apparatus can reduce the overall cost of the dewatering apparatus and make it more compact.

After initial gravitational dewatering as described above, the partially dewatered sludge flows on the belt over an inclined guide 62 extending over the head roller 38 of the belt 36 and is deposited onto the upper run of the cathode belt 44 of the first stage 12. The sludge is conveyed into a gap 64 between the lower run of the upper anode belt 36 and the upper run of the lower cathode belt 44. During passage through the gap, in the direction from left to right in the Figures, the sludge is subjected to electro-osmotic dewatering in accordance with known principles.

Further dewatering of the sludge is thus achieved before the sludge is discharged to the second stage 14 as indicated by the reference numeral 66. In the second stage, the sludge is subjected to further electro-osmotic dewatering as it passes through a similar gap between the upper anode and lower cathode belts 52 and 54. The sludge is discharged from the second stage to the third stage 16 as indicated by the arrow 68. In the third stage, the sludge is subjected to a further electro-osmotic dewatering treatment in the gap between the belts 56 and 58. After passage through the third stage 16, the dewatered sludge or cake is discharged to a screw conveyor 70 which removes it for further treatment or disposal.

In each electro-osmotic dewatering stage, the water which is separated from the solid phase of the sludge is able to pass through the upper run ofthe belt 44, 54, 58 for collection and removal. In each stage, the electro-osmotically induced dewatering action is enhanced by applying suction to the underside of the upper run of the belt 44, 54, 58. Referring to Figure 3, spaced apart suction heads 72 are arranged beneath, and in contact with, the upper run of the cathode belt 44. The suction heads 72 are connected to a vacuum unit 74 (Figure 2) maintained at a sub-atmospheric pressure by suitable vacuum equipment (not illustrated). The suction heads draw water out of the sludge and downwardly through the upper run of the belt 44 for disposal by the vacuum unit 74. The corresponding suction heads of the second and third stages 14 and 16 are omitted from Figures 1 and 2 in the interests of clarity of illustration, but it may be assumed that their arrangement is similar to that seen in Figure 3 for the first stage.

Although not readily apparent from the drawings, the gap 64 between the belts 36 and 44 will generally decrease in the direction in which the sludge moves, i.e. in a direction from left to right as illustrated. This taper in the gap 64 enables the upper anode belt 36 to maintain adequate contact with the sludge as the volume of the sludge decreases with progressive dewatering. In similar fashion, the gaps between the belts 52 and 54 and between the belts 56 and 58 will also taper in the direction of movement.

In accordance the invention, the gap 64, i.e. the vertical spacing between the belts 36 and 44, as well as the corresponding gaps between the upper and lower belts in the second and third stages, are adjustable. Referring by way of example to Figure 3, it will be seen that the rollers of the upper belt 36 are carried by a frame indicated generally with the numeral 76 and that the rollers of the lower belt 44 are carried by a frame 78. The frames 76 and 78 are connected to one another by vertical spacer rods 80 located towards the ends of the belts . The lower end of each rod 80 is mounted rotatably in a bearing housing 82. The upper end of each rod 80 is threaded and passes through a complemental nut in an upper housing 84. The upper extremity of the rod carries a hexagonal formation 86. A suitable spanner or other tool can be applied to the formation 86 for the purposes of rotating the rod relative to the nut. Depending on the direction of rotation, this reduces or increases the effective length ofthe rod and hence varies the vertical spacing between the frames 76 and 78, with the result that the gap 64 is varied accordingly.

It will be appreciated that by appropriately adjusting the rods 80 it is possible to vary the overall size of the gap 64 as well as the taper undergone by the gap. The facility for readily adjusting the belt spacing is considered to be an important feature of the apparatus 10, since it will enable an operator of the dewatering apparatus to vary the performance of the apparatus to suit the nature of the incoming raw sludge.

According to another important aspect of the invention, the motors driving the upper and lower belts in each stage can be independently controlled, typically by means of suitable frequency inverters. With this feature, the lower run of the upper belt can in each case be driven at a different linear speed from that at which the lower belt is driven. With the belts running at different speeds, the solid phase of the sludge which is present in the gap between the belts can be subjected to a longitudinal shearing action, the magnitude of which is dependent on the belt speed differential.

It is believed that applying a shearing action to the solid phase of the sludge will assist in liberating bound water from the solid particles. It will also assist in separating solid particles which adhere to one another, thereby facilitating the separation of water which may be bound up physically in agglomerations of solid particles.

Although the above description refers to a three-stage dewatering apparatus, it will be appreciated that the principles of the invention are equally applicable to electro-osmotic dewatering apparatuses with more or less than three stages. Although reference has been made to the "dewatering" of sludges and to "water" which is separated from the solid phase of a sludge, it should be understood that the invention is not applicable only to the separation of an aqueous liquid phase from a solid phase. The invention also finds application in the separation of liquid phases, other than water, from associated solid phases of sludges. Thus it will be understood that terms such as "dewatering" and "water" are used for convenience only.

In practice the perforated belts, such as the belts 36 and 44, are of woven steel, typically stainless steel, fibre. A woven construction gives the belt the necessary flexibility to pass around the various rollers. The interstices between the steel fibres of the woven construction form the perforations of the belt. It will accordingly be understood that the term "perforated" is used to describe a belt material which has a multitude of openings therethrough and is not limited to a belt material in which an array of regular or irregular perforations is formed through an otherwise solid membrane.

Claims

1. A method of dewatering a sludge, the method comprising gravitationally dewatering the sludge as a first step and electro¬ osmotically dewatering the sludge as a second step, characterised in that:

in the second step the sludge is passed between a lower electrode and an upper electrode in the form of an endless, perforated belt having upper and lower runs, and

in the first step the sludge is gravitationally dewatered by depositing it on the upper run of the endless belt of the upper electrode and liquid is allowed to filter gravitationally out of the sludge through the perforations in the belt.

2. A method according to claim 1 characterised in that the lower electrode is perforated and suction is applied to the lower electrode to draw liquid through the perforations in that electrode.

3. A method according to claim 1 or claim 2 wherein the upper and lower electrodes are both in the form of endless belts with upper and lower runs, characterised in that the belts are driven so that the lower run of the upper belt moves in the same direction as the upper run of the lower belt but at a different linear speed, and the sludge is caused to pass between the said runs of the belts so that the said runs of the belts apply shear forces to solid particles ofthe sludge because of the difference in their linear speeds.

4. An apparatus for electro-osmotically dewatering a sludge, the apparatus comprising an upper electrode, a lower electrode beneath the upper electrode and means for causing the sludge to pass between the electrodes for electro-osmotic dewatering thereof to take place, characterised in that the upper electrode is in the form of an endless, perforated belt having upper and lower runs and in that the apparatus includes means for depositing the sludge on the upper run of the belt so that liquid filters under gravity out of the sludge through the perforations of the belt before the sludge is passed between the electrodes.

Apparatus according to claim 4 characterised in that the lower electrode is perforated and in that the apparatus includes means which applies suction to lower electrode to draw liquid through the perforations in that electrode.

Apparatus according to claim 4 or claim 5 wherein the upper and lower electrodes are both in the form of perforated, endless belts with upper and lower runs, with the sludge passing between the lower run of the upper belt and the upper run of the lower belt, characterised by means for driving the belts so that the lower run of the upper belt and the upper run of the lower belt move in the same direction but at different linear speeds with the result that shear forces are applied to solid particles of the sludge because of the difference in the said linear speeds.

7. Apparatus according to any one of claims 4 to 6 characterised in that the apparatus includes means for adjusting the vertical spacing between the lower run of the upper belt and the upper run of the lower belt.