IE912510A1

IE912510A1 - Apparatus for controlling the solids content of sludge produced from a continuous belt filter press

Info

Publication number: IE912510A1
Application number: IE251091A
Authority: IE
Inventors: Patrick Torpey
Original assignee: Patrick Torpey
Priority date: 1991-07-17
Filing date: 1991-07-17
Publication date: 1993-01-27
Also published as: GB2257640B; GB2257640A; IE67647B1; DE4223554A1; GB9214962D0

Abstract

Sewage sludge is dewatered by gravity on an endless filter belt 11 where it passes over a first filtrate tray 13, then a variable fraction of the sludge is removed by a first scraper 15 and diverted to a hopper 16 while the rest of the sludge is carried by the belt round a pressing roller 23 before being removed by second scraper 18 to the hopper 16. The scraper 15 is adjustable, so that the overall solids content of the mixed sludges obtained from hopper 16 may be controlled. The mixture viscosity may be monitored and scraper 15 automatically controlled to maintain its constant. Scraper 15 may comprise a single bowed and flexible blade or a series of separate blades individually movable relative to the belt. Thus most of the sludge remaining on the belt is near its centre, to counteract the lateral squeezing where the belt goes round roller 23. Roller 23 may be grooved. Filtrate from tray 14 is used to backwash the belt downstream of the pressing stage, and the resulting mixture is separated by gravity.

Description

This invention relates to an apparatus and method for controlling the solids content of sludge produced from a continuous belt filter press.

For many years various types of sludge have 5 been de-watered using continuous moving filter belt apparatus .

These have taken two basic forms viz :1. Thickeners in which the sludge is de-watered by gravity alone and the thickened sludge is removed by means of a scraper at the end of the gravity zone. This produces a sludge of approximately 1%-5% solids content by weight. 2. Presses in which the sludge is first de-watered by gravity and then passed through or over one or more rollers to squeeze out more water, the resultant sludge, which depending on it's origins, would typically have a minimum solids content of approximately 11-15% solids by weight, is again removed by scraper. - 3 There are many applications, particularly in the anaerobic digestion of sludge, for which 12% solids would be too high, giving rise for example to pumping problems, and problems with mixing inside the digester.

There is therefore a need for equipment which can exceed the performance of a gravity belt thickener, while offering control over the final thickening effect. It is therefore an object of the present invention to provide an apparatus which would, inter-alia, provide for a sludge recovery preferably between 5%—11% by weight.

Accordingly, belt press equipment is known in which the entire sludge flow after de-watering by gravity can be diverted as a thickened sludge stream or alternatively, the entire sludge flow may be permitted to go forward to a pressing process for further de-watering of the sludge. It is therefore an object of the present invention to provide an apparatus for extracting liquid from a sludge which apparatus incorporates means to allow proportioning the sludge flow such that some of the sludge flow may be diverted after the gravity de-watering to a hopper and sludge flow may also be sent forward for additional de-watering by a press roller(s) prior to being fed to the hopper. Stated another way it is an object of the invention to provide an apparatus which will allow for a degree of de-watering variable at will between that associated with a typical gravity belt thickener performance and that associated with a conventional filter belt press performance.

The invention, therefore, provides an apparatus for extracting liquid from a sludge which apparatus comprises an endless belt of filter material; means for feeding a mixture of sludge and a conditioning - 4 agent from a feed source to the belt; the belt being mounted on a set of roller means which define a path of travel for the belt; the belt having an upper run which in use, is at least approximately horizontal and which has an exposed surface defining a gravity de-watering zone; a pressure de-watering zone located downstream of the gravity de-watering zone; a belt washing zone located downstream of the pressure de-watering zone; means for collecting filtrate from the gravity de-watering zone and the pressure de-watering zone; and means for collecting the washings from the belt washing zone, wherein a first scraper means is provided downstream of the gravity de-watering zone and upstream of the pressure de-watering zone for controlling the flow of solids material from the belt and means for adjusting the first scraper means relative to the belt so as to vary the final percentage value of the solids material removed from the belt.

Preferably, a second scraper means located at or downstream of the pressure de-watering zone for controlling the flow of solids material from the belt.

Preferably, means is provided for adjusting the second scraper means relative to the belt.

Preferably, the final percentage value of the solids material removed from the belt is between -11% by weight.

The invention will be understood in greater detail from the following description of preferred embodiments thereof given by way of example only and with reference to the accompanying drawings in which:IE 912510 - 5 Figure 1 is a representation of an apparatus according to the invention; Figure 2 is an enlarged view of one part of the apparatus of Figure 1 of the drawings; Figure 3 is an enlarged view of first embodiment of a roller of the apparatus of Figure 1 of the drawings; Figure 4 is an enlarged view of a second embodiment of a roller of the apparatus of Figure 1 of the drawings; and Figure 5 is an enlarged view of another part of the apparatus of Figure 1 of the drawings.

Referring now to the drawing there is shown an apparatus 10 according to the invention which comprises an endless belt 11 and rollers 21, 22, 23, 24, and 26. The belt 11 is mounted under tension on the rollers 21-26, which define a path of travel for the belt 11 in the direction of the arrow 100.

Between the rollers 21 and 22, the belt 11 has an upper run 12 which is at least approximately horizontal and which has an exposed upper surface defining a gravity de-watering zone 30. The belt 11 can be made from any suitable material but the preferred material is monofilament polyester yarn. Below the zone is a collection tray 13 which collects filtrate from the zone 30 and directs it to a tank 14. - 6 Downstream of the zone 30 is located the roller 23 known as a press roller 23 which provides a pressure de-watering zone. The press roller 23 may have a smooth surface. Located at the roller 22 is a scraper blade 15 which enables thickened cake discharge 31 to be removed from the belt 11 and collected in a hopper 16.

It will be appreciated that the scraper blade 15 is located downstream of the gravity de-watering zone 30 and upstream of the pressure de-watering zone.

The press roller 23 serves to further remove filtrate from the sludge which filtrate is collected in a tray 17. Downstream of the press roller 23 and at the roller 24 is a second scraper blade 18 which serves to remove pressed cake discharge 32 from the belt 11 which pressed cake discharge 32 is also collected in the hopper 16.

When the press roller 22 is of the type having a smooth surface there can be a tendency for the sludge to extrude axially relative to the roller 23 which can then escape from further action by the apparatus 10. De-watering of the sludge is effected by the force applied to the sludge when pressing between the belt 11 and the roller 23. The force is primarily derived from the tension of the belt 11. The amount of tension which must be applied to the belt 11 in order to transmit the necessary drive torque is of such magnitude that a sudden rise in pressure occurs as the sludge enters the nip between the belt tangent and the surface of the rollers 23. It is usually not possible to alter the tension of the belt in cases where the force is sufficient to cause axial movement of the sludge on the - 7 roller 23. In those circumstances, it would be useful to employ a roller having one or more circumferentially arranged grooves or projections thereby providing one or more passages or channels of a relatively lower pressure which would prevent axial displacement of the sludge.

In Figure 3 of the drawings, there is shown an embodiment of the roller 23 the surface 23a of which has a plurality of circumferentially disposed grooves 23b thereon.

The depth of such grooves 23b may be of the order of 10-30mm in a roller having a diameter of between 200mm and 400mm. Typically, the grooves would be approximately 20mm deep in a roller having a diameter of approximately 300mm.

In Figure 3 of the drawings the grooves 23b are essentially integral with or generated from the surface of the roller 23. An alternative means of producing grooves is shown in Figure 4 of the drawings. In Figure 4, there is shown the roller 23 and associated shaft 23b. A plurality of rings 23c may be threaded onto the roller 23 so as to provide grooves 23d. The axial length of the rings 23c together with their thickness will determine the depth and width of the grooves 23d and these dimensions may be varied as can the number of rings 23c to suit the conditions pertaining at the time of use of the apparatus 1.

It is a characteristic of the partially de-watered sludge material arriving at roller 22 that, being typically 95% water, it has little mechanical stability. As a consequence, on entering, the material - 8 tends to extrude sideways (i.e. in a direction perpendicular to the direction of belt travel), thus contaminating the filtrate.

The scraper blade 15, is accordingly 5 arranged so that it permits a proportion (between zero and 100%) of the partially de-watered sludge arriving at roller 22 to enter the pressure de-watering zone, and to by-pass the remaining partially de-watered sludge directly to the hopper 16. Preferably this first proportion is selected from the centre or middle portion of belt 11, so that, on entering the press zone, the partially de-watered sludge can suffer the inevitable sideways extrusion whilst still remaining trapped between the belt fabric and the roll surface and thereby preventing contamination of the filtrate. This proportioning effect is achieved by arranging scraper blade 15, and its support mechanism, so that the physical separation of the blade from the belt surface can be varied at different points along the width of the blade.

This is done for example by arranging the scraper blade 15 in sections, on slotted guides, so that one or more sections near the centre of the belt 11 are further removed from the belt surface than those scraper blade sections associated with the outer edges of the belt 11. In Figure 2 of the drawings, there is shown the belt 11 in relation to the scraper blade 15. In the embodiment shown in Figure 2 of the drawings, the scraper blade 15 comprises a base (not shown) on which - 9 is mounted in substantially parallel and abutting or closely spaced apart relationship a plurality of elements or sections 15a. Each section 15a has a respective slot 15b through which a suitable bolt 15c is inserted for enabling slidable engagement of the section 15a relative to the base. Each section 15a can be selectively positioned relatively near or distant from the belt 11. Thus, in Figure 2 of the drawings, the first four sections 15a and the first five sections 15a (reading from left to right) are nearer the belt 11 than the other four sections 15a therebetween. Another embodiment comprises a flexible scraper blade with a bowed or curved shape, which, by varying the degree of bowing or curvature, provides maximum separation from the belt at its mid span, and a lesser separation at outer extremes .

In order to achieve the desired overall or average thickening effect, the scraper blade 15 can be manually adjusted until the desired result is achieved.

It is also possible to arrange a control mechanism so that the blade separation can be adjusted in proportion to a characteristic of the mixed de-watered sludge. Therefore, to control the de-watered sludge solids content, a servo positioning device would be provided for the scraper blade, which would take its input signal from a viscosity monitoring transducer. Any similar means of measuring a characteristic of the de-watered sludge, which characteristic would vary in proportion to the solids content of the de-watered sludge, could similarly be used. - 10 In Figure 5 of the drawings, the point y represents the three o' clock position of the roller 22.

The positions of rollers 22 and 23 and to a lesser extent their size can have an influence on the performance of the apparatus 10. Thus, in order to capture the sludge bypassing behind the scraper 15 into the nip between the belt 11 and the roller 23, it is desirable that the centre of the roller 23 be at least directly below and preferably to the right of the point y (as viewed in Figure 5 of the drawings) relative to the roller 22 so that sludge on the belt 11 at the roller 22 will tend to remain thereon and enter the nip between the belt 11 and the roller 23. Otherwise, due to the relative fluidity of the sludge, it might tend to fall from the belt 11 thereby bypassing the roller 23 and the pressure de-watering zone. The roller 22 functions as a guide roller or drive roller and can be of relatively small diameter compared with the diameter of the roller 23. Preferably, the roller 23 has a diameter of about twice that of the roller 22.

By arranging the rollers 22, 23 as shown, with a notional line 70 tangential to the rollers 22 and 23, which line is located at an angle x of between 30° and 60°, preferably 45° relative to the horizontal belt 12, it is possible to arrange that the thickened sludge diverted from roller 22 can flow by gravity to hopper 16, where it will meet the pressed sludge removed by scraper blade 18, thereby avoiding the need for any auxiliary pumping or conveying equipment.

Although the sludge or discharge 31 scraped from the roller 22 by the scraper blade 15 has a certain fluidity, it nevertheless has a certain thixotropic - 11 characteristic to cause a build-up as it passes from the blade 15 to the hopper 16 if the pathway of travel of the discharge 31 is less than an angle of 30° relative to the horizontal belt 12. Ideally, the angle should be approximately 45° which is achieved when the notional line is approximately 45° relative to the horizontal belt 12.

The position of the roller 24 can be at any point relative to the roller 23 so as to generate a belt wrap around the roller so as to enable the roller 23 to function as a press roller. Accordingly, the roller 23 should preferably be located as far anticlockwise relative to the roller 22 (as viewed in Figure 1 of the drawings) so as to provide maximum wrap of the belt 11 around the roller 23 while not intersecting an extension of said notional line. Ideally, said notional line should be tangential to the rollers 22, 23 and 24.

Between the rollers 25 and 26 is located a belt washing zone 40. The belt washing zone 40 comprises one or more sprayer arms 41 which sprays clean water from a tank (not shown) or filtrate from the tank 14 on to the belt 11. This has the effect of removing any remaining solids from the belt 11 the upper surface of which is now facing downwards. The remaining solids and wash water is directed into a tray 42 which has a pipe 43 leading to a settling tank 44. The washed belt now continues beyond roller 26 and arrives back at the roller 21 down stream of which is deposited flocculated sludge at arrow 200. - 12 The capacity of the tank 44 need only be sufficient to hold the washings for about 10 minutes as the residual sludge in the washings which contains a flocculating agent, will be already polymerised and will, consequently, settle to the bottom of the tank 44 relatively quickly. Clean filtrate will automatically discharge by overflow from the tank 44 via a pipe 45 located near the top of the tank 44. The base of the tank 44 has a pipe 46 leading to a tank (not shown) for discharge of the sludge on to the belt 11 at 200. The flow of sludge in the pipe 46 is controlled by a pump 47. In order to ensure that only clean filtrate is discharged through the pipe 45 turbidity sensor or sludge blanket detector (not shown) are provided in the tank 44 and when appropriate the pump 47 is actuated in response to an output signal from the turbidity sensor or sludge blanket detector to pump sludge from the tank 44.

The sludge material falling into the hopper 16 is mixed by a screw conveyor 16a which may be the sole means of discharging the material, or may form a pre-charging device for a conventional positive displacement pump, and the resultant discharge is monitored for physical or chemical properties or both by conventional means to determine the equivalent percentage solids content. By means of suitable conventional monitoring equipment, predetermined values for the percentage solids may be specified and appropriate means provided to move one or both scraper devices 15, 18 in whole or in part towards or away from its associated respective roller 22, 24 to give a percentage solids content of between 5-11% by weight emanating from the screw conveyor 16a.

Claims

Claims :

1. An apparatus for extracting liquid from a sludge which apparatus comprises an endless belt of filter material; means for feeding a mixture of sludge 5 and a conditioning agent from a feed source to the belt; the belt being mounted on a set of roller means which define a path of travel for the belt; the belt having an upper run which, in use, is at least approximately horizontal and which has an exposed surface defining a 10 gravity de-watering zone; a pressure de-watering zone lo cated downstream of the gravity de-watering zone; a belt washing zone located downstream of the pressure de-water ing zone; means for collecting filtrate from the gravity de-watering zone and the pressure de-watering zone; and 15 means for collecting the washings from the belt washing zone, wherein a first scraper means is provided downstream of the gravity de-watering zone and upstream of the pressure de-watering zone for controlling the flow of solids material from the belt and means for 20 adjusting the first scraper means relative to the belt so as to vary the final percentage value of the solids material removed from the belt.

2. An apparatus as claimed in claim 1 which further comprises a second scraper means located at or 25 downstream of the pressure de-watering zone for controlling the flow of solids material from the belt.

3. An apparatus as claimed in claim 1 or claim 2 where means is provided for adjusting the position of the first scraper means or the second scraper means or 30 both relative to the belt. - 14

4. An apparatus as claimed in any of claims 1-4 wherein said first or second scraper means or both each comprises a base having mounted thereon in substantially parallel and abutting or closely spaced apart 5. Relationship a plurality of elements each of which is adapted for slidable interengagement along a respective notional line which is substantially perpendicular relative to the base so that each element may be selectively positioned relatively near or distant from 10 the belt.

5. An apparatus as claimed in any of claims 1-4 wherein the pressure de-watering zone comprises a press roller having a smooth or a non-smooth surface.

6. An apparatus as claimed in claim 5 wherein 15 the surface of the press roller has one or more circumferentially located grooves or projections thereon.

7. An apparatus as claimed in claim 6 wherein the diameter of the press roller is between 200mm and 20 400mm and the depth of each groove or projection is between 10mm and 30 mm.

8. An apparatus as claimed in any of claims 3-7 wherein the apparatus further comprises means for monitoring a characteristic of de-watered sludge 25 produced by the apparatus and for producing an output signal related to said characteristic; and means for automatically controlling the adjusting means in response to said signal. - 15

9. An apparatus as claimed in any of claims 1-8 capable of producing de-watered sludge having a solids content of between 5-11% by weight.

10. An apparatus as claimed in any of claims 5 1-9 wherein a first roller is located between the gravity de-watering zone and the pressure de-watering zone; the pressure de-watering zone comprises a second roller; said first and second rollers are located relative to each other such that a notional line 10 tangential to said first and second rollers is at an angle of between 30° and 60° relative to the upper run of the belt.

11. An apparatus as claimed in claim 10 in which said angle is about 45°. 15

12. An apparatus as claimed in claim 10 or claim 11 wherein the diameter of the second roller is approximately twice the diameter of the first roller.

13. An apparatus as claimed in any of claims 10-12 which further comprises a third roller located 15 downstream of the second roller and wherein said notional line is also tangential to the third roller.

14. An apparatus for extracting liquid from a sludge, substantially as hereinbefore described with reference to and as illustrated in the accompanying 20 drawings.