GB2152544A - Method and apparatus for washing pulp - Google Patents

Abstract

Input pulp stock (20) flows along a passageway (25) defined between a vat (18) and a perforated, filtering, cylindrical wall (16) of a rotating cylinder (10). A dilute fraction (30) of the input stock is removed from the interior of the cylinder, and a concentrated fraction in the form of a mat (38) is removed from the uprunning cylinder wall at a location above the level (23) of the stock (20) at the downstream end (29) of the passageway. The invention is characterised by the introduction of clean dilution water (86) at such downstream end (29). This water is confined to the downstream end, and is directed through the mat (38) into the interior of the cylinder whereby to enhance the washing of the mat. <IMAGE>

Description

SPECIFICATION Washing of papermaking pulp stock Background of the invention The invention relates to the washing of pulp stock during its preparation for papermaking.

In the case of pulps of wood or other fibres prepared by cooking or digesting processes, or by chemi-mechanical processes, the residual cooking chemicals must be removed. In the case of pulps prepared from the reprocessing of waste paper, often called secondary fibre pulps, it is necessary to remove the printers' ink, which otherwise would constitute a serious, undesirable contaminant in the pulp.

The process and apparatus of the present invention are well adapted to deinking, or the removal of printers' ink from pulp re-processed from waste paper. As will be apparent to those skilled in the art, the present invention is by no means limited to this application, but may be used for other pulp washing applications within the pulp and paper industry. However, for the purposes of clarity and simplicity, disclosure of the invention herein will be made using deinking of secondary fibre pulp as a specific example.

While many different types of apparatus have been used or proposed for the deink washing of secondary fibre, one common type, sometimes called a "Lancaster Washer", is in wide use in the industry. This is a relatively simple device, operating by gravity, by which is meant that neither pressure (other than normal gravity head) nor vacuum is used on the pulp stock or on the filtrate which is removed.

The present invention discloses improvements to such known apparatus, and an improved method of applying wash water to the pulp.

Brief description of the drawings The invention will be better understood by reference to the prior art and the embodiments of the invention illustrated in the drawings, in which: Figure 1 is a simplified representation, in vertical section, of a well-known prior art type of apparatus; Figure 2 is a similar representation, in vertical section, of one embodiment of the present invention; Figure 3 is a partial view, in vertical section, of another embodiment of the invention; Figure 4 is a partial view, in vertical section, of yet another embodiment of the invention.

Prior art Figure 1, representing known art, shows a cylinder 10, mounted axially on a shaft 12 by conventional support means (not shown) with its axis horizontal. At its ends, shaft 12 is supported on suitable bearings (not shown) and is arranged to be rotated in the direction shown by the arrow 14 by suitable drive means (not shown) and motor (not shown). The cylindrical wall 16 of the cylinder 10 is constituted by a relatively open filter medium, such as, for example, a woven-wire cloth. This cy- lindrical wall thus provides suitable perforations or openings to facilitate the passage of fluid through the filter medium into the interior of the cylinder.

Partially surrounding the cylinder 10 on its lower side is a vat 18 containing pulp stock 20. The apparatus is normally so arranged that somewhat more than half of the circumference of the cylinder is immersed in the pulp stock, which is a suspension of fibres in water.

Vat 18 is preferably fitted with a curved bottom part 24, generally conforming to, although radially spaced from, the cylinder, whereby to define between them a passageway 25 extending from an "upstream" end 27 (at which the input stock is added) to a "downstream" end 29. The level of the stock 20 is shown at 22 at the upstream end 27, but is somewhat higher at 23 at the downstream end 29 due to the viscous drag of the rotating cylinder.

At least one end of the cylinder is open but sealed to the end-wall of the vat by a rotary seal (not shown), which seal is the same diameter as the cylinder or nearly so, in such a manner that liquid in the interior of the cylinder can flow freely out of the cylinder through a suitable opening (not shown) in the end wall of the vat. Thus the level of filtrate inside the cylinder, shown as 30, will be low. The use of a chime ring attached to the end wall of the vat, and of a deckle strap closing the gap between the chime ring and the end of the cylinder, an arrangement well-known in the paper industry, will provide an adequate rotary seal, although other arrangements are possible. It may be desirable to equip both ends of the cylinder 10 with rotary seals such as deckle straps and chime rings. If only one end is so equipped, the other end of the cylinder 10 is imperforately closed.

A cylindrical roller 32, or "couch roll" is provided, with stub shafts (not shown) at its ends, which stub shafts are supported on suitable bearings (not shown). The couch roll 32 has a solid surface of rubber or other material and it extends axially the length of the cylinder 10. It is so mounted as to be in contact with the surface 16 of the cylinder 10, or in contact with the "mat" 38 of pulp on the surface 16. Normally the couch roll 32 is caused to rotate in the sense of the arrow 34 by friction with the cylinder 10, although it may be separately driven. A scraper or "doctor blade" 36 contacts the surface of the couch roll 32.

What has been described thus far resembles closely a gravity thickener or "decker" that is in widespread use in the paper industry. In operation, as the cylinder rotates, a relatively dilute fraction of the input stock is filtered through the wall 16 of the cylinder 10 and leaves the interior of the cylinder through the rotary seal at one end, as previously described. As filtration proceeds, fibres are left behind, adhering loosely to the surface 16 as a thickened layer or "mat" 38 of fibres. When contact is made with the couch roll 32, the mat 38 adheres preferentially to its solid surface, as depicted at 40, and, as the couch roll rotates, is removed by the doctor blade 36, as depicted at 42.The mat 38 thus constitutes a relatively concentrated fraction of the input stock, and it will be seen that the entire input stock has been removed from the system in the form of the two fractions.

Devices as described thus far are widely used and known as "deckers" for thickening paper making stock, that is, to increase the consistency, by which is meant the concentration by weight, of fibres in water, usually expressed as a percentage.

The device described thus far also bears at least a superficial resemblence to a cylinder paper machine, or a cylinder board (or cardboard) machine, as known widely in the paper industry. However, the operation is different, and the proportions of the parts are different, due to the different requirements of the processes.

The cylinders of papermaking or board making machines are operated at very much lower rotative speeds than are the cylinders of deckers and gravity pulp washers, by a ratio of perhaps ten to one.

This has considerable consequences in the operation of the devices. In cylinder paper machines, it is often a problem to maintain enough fluid motion in the vat to prevent the fibres or some of them, from settling to the bottom. In U.S. Patent No.

2,001,208 issued May 14, 1935, for example, S.V.

Minskey et al have described means to draw off continuously a flow of stock from the bottom of the vat, and return it to the vat, to maintain a more uniform vat consistency. By contrast, in deckers and washers, the high rate of rotative speed keeps the contents of the vat in continuous agitation, due to fluid friction and fluid shear, and, in fact, in the present invention, as will be further disclosed below, it is desirable to keep the turbulence confined and controlled.

In cylinder paper and paperboard machines, of course, one of the most important considerations is to produce a uniform sheet on the surface of the cylinder, without lumps or discontinuities. On the other hand, in deckers and washers, handling the maximum possible throughput with the smallest possible machine and occupying the least floor space, are of great importance, and hence the much higher rotative speed. In deckers and washers, uniformity of the sheet is of no practical importance.

In cylinder paper machines it is usual to provide a continuous overflow of stock from the vat, which overflow is recycled to maintain sufficiently high fluid velocity to prevent flocculation and settling.

This is not usually done in thickeners and washers.

H. Nisser, in U.S. Patent No. 3,330,723 issued July 11, 1967 describes as one of his embodiments (Figure 3) a cylinder papermaking machine for which he notes (column 4, line 74) that it is provided with an overflow "where excess pulp is allowed to escape, the pulp in container 4 being kept in motion thereby to prevent flocculation of the fibres". He goes on to say that "Between the screen 9 and the boundary walls of the container 4 there is sufficient space to permit the fibres to move about freely".

Nisser's screen 9 is stretched on the surface of a rotatable cylinder 40 immersed in pulp in the container 4. This need to move about freely is not a consideration in the design of deckers and washers, where only sufficient space is provided, as in the passageway 25 of Figure 1, to permit the free flow of all the stock the device can handle. In fact, by keeping the transverse dimension of the passageway 25 small, excessive turbulence in the vat of a decker or a washer can be controlled at least to some extent.

Returning now to consideration of Figure 1, it may be noted that the addition of a "repulper" converts a decker to a gravity washer, or, as it is sometimes called, a "Lancaster Washer". A repulper is simply a mixing device to mix incoming pulp suspension with dilution water, which dilution water may or may not contain some fibres. In Figure 1 an auxiliary pulp container 44, rather like a somewhat smaller vat, is provided, advantageously having a bottom 46 of semi-circular shape. Within this vat there is a shaft 48, mounted on suitable bearings (not shown) and rotated by a suitable motor and drive (not shown) in the sense indicated by the arrow 50. Along the axial length of the shaft 48 there is mounted a plurality of blades or paddles 52.

In operation, incoming thick pulp with a consistency or concentration of dry weight of fibres in water that typically might be 5%, but which can vary rather widely, enters the repulper, as shown diagrammatically by the arrow 54. Also dilution water, shown diagrammatically by the arrow 56, is added to the repulper.

Continuous flows of pulp 54 and dilution water 56 are intimately mixed by the motion of the blades 52, and flow over a weir crest 58 into the main vat 18, as a mixed flow depicted by an arrow 60. This mixed flow constitutes the input stock to the passageway 25 in the vat 18 where it is rethickened so that a relatively concentrated fraction having a consistency that might typically be once again 5% leaves the edge of the doctor blade 36 as depicted by the arrow 62. The filtrate removed leaves the inside of the cylinder 10 through the end by way of the rotary seal described previously, as a flow of so-called "whitewater" here depicted diagrammatically by the arrow 64.

It will be seen that by intimately mixing incoming pulp, which, in the case of the deinking of secondary fibre, contains an admixture of very finely divided printers' ink particles, with dilution water and then extracting some of the water, a considerable portion of the ink particles will be removed with the whitewater. It may be remarked here that it is traditional to call the filtrate whitewater by analogy with the filtrate from deckers and cylinder board machines, even though, in the case of deink washers, its actual colour may be black.

It will also be seen that the washing process is a good deal less than perfect. As already stated the foregoing apparatus is in wide use in the pulp and paper industry. It is common to use a plurality of these devices in series, that is, re-thickened stock 62 from the first stage becomes the incoming thick stock 54 for the second stage, and so on. Three stages in series are common, and more stages are known, this being necessary due to the inefficiency of a single stage. It may also be noted that when multiple stages are used it is usual to operate them counterflow to save water and conserve pulp fines in the whitewater. That is to say, the whitewater from the last stage becomes the dilution water 56 for the next-to-last stage, and so on.The dilution water 56 for the last stage should, of course, be substantially ink-free water, either fresh water or recycled water that has been passed through a clarifier to remove any ink.

Summary of the invention The object of the present invention is to provide improvements in the efficiency of a washing process of the type described above, and, more specifically, to enhance the washing of the mat.

This result is achieved by introducing dilution water into the passageway at its downstream end 29 and directing such dilution water through the mat at that location towards the interior of the cylinder. When the process is being used for the deinking of stock that has been reprocessed from waste paper, this dilution water will be substantially ink free. While this dilution water can also be substantially fibre free, it need not necessarily be so. It can contain fibres that are added to the mat and hence recovered.

It will be observed that the addition of dilution water at the downstream end of the passageway merely adds to, without interfering with, the basic operation of the washer, in that the entire input stock 60 continues to be removed from the system via the two fractions, i.e. the relatively dilute fraction 64 and the relatively concentrated fraction 62.

This is in contrast to the arrangement shown in Figure 3 of the Nisser patent referred to above where the incoming stock enters the system at an intermediate location (conduit 6) and flows in both directions around the cylinder, much of it leaving the container 4 at the overflow 7.

While Nisser adds "clear or fiber free water" (column 5, line 18) to "a second container" 5 located on the "ascending side of the cylinder mold 40", he does not direct such clean water through the mat. He maintains the liquid level 8a in the second container 5 higher than the level 8 in the main container 4. The result described by Nisser is that "the fibrous layer will be compressed and compacted and will emerge from the clear or fiber free water in container 5 in that condition on its way to the take-off roller 42" (column 5, lines 3033). The "clear or fiber free water "itself" can pass from the container 5 into the container 4" (column 5, lines 24, 25) through a communicating passageway 35 between the edge 3 of a wall 2 and the screen 9.Hence Nissen's function is (a) to compress and compact the mat, and (b) to cause the added clean water to escape from the second container 5 by flowing countercurrent in the vat, i.e. counter to the direction of rotation of the cylinder, and into the main container 4.

By contrast, in the present invention the primary function of the dilution water added at the downstream end 29 of the passageway 25 is to pass through the mat 38 to wash the same. In the preferred form of the present invention (Figure 1), a baffle is provided to inhibit countercurrent flow of the dilution water so that it passes out of the system as part of the dilute fraction 64 without entering the main body of the vat 18 to any significant extent. However, in an alternative form of the invention (Figure 3), a controlled, but minor, amount of the dilution water can be directed to a slightly upstream region between two baffles in order to expose the mat to a region of lesser dilution prior to its exposure to the region of greater dilution that takes place at the downstream end 29 of the passageay 25.

Detailed description of embodiments Figure 2 represents a first embodiment of the present invention. On the side of the vat 18 at which the cylinder 10 is uprunning, i.e. the downstream end 29 of the passageway 25, there is added a distribution plenum 66 extending axially the length of the cylinder 10 and closely facing the cylinder 10 with a distribution plate 70 containing a plurality of holes 72 uniformly spaced along its length The dimensions are so chosen that the distribution plate 70 is located radially near the mat 38 on the cylinder 10. Advantageously, the bottom side of the plenum 66 can be formed as a baffle 74 having its tip 76 extending radially near to the surface of the mat 38. Additionally, one or more further baffles 78 may be provided towards the bottom of the vat, to prevent undue turbulence and recirculatory flow of the stock in the vat.

A conduit 68 leads into the distribution plenum 66 which will be fitted with a bleed hole (not shown) at its top to allow for the release of any trapped air. In Figure 2, the main dilution water (56 in Figure 1) will now be called the first dilution water, and is here renumbered 80. The re-thickened stock is here renumbered 82, and the filtrate or whitewater is renumbered 84. Additionally, a second dilution water flow, depicted by the arrow 86, is provided.

In operation, ink-free water from the second dilution flow 86, passing through the holes 72 of the distribution plate 70, will be directed against the mat 38 to pass through it, while displacing water containing ink that forms part of the mat 38. The water containing ink that has thus been displaced will pass through to the interior of the cylinder 10, becoming part of the whitewater 84. The stock level, now shown at 21, will be slightly higher than the level 23, due to increased pressure from the incoming flow 86, although most of this pressure head will be lost in passage of the water through the holes 72.

Consideration of the relative weights of water and fibre involved at various points in the path of the fibre through the known device of Figure 1, and the device of the present invention in Figure 2, will make the advantages of the invention apparent. If the incoming thick stock 54 is at 5% consistency, this means that one pound of dry fibre is associated with 19 pounds of water. The undesirable printers' ink can be considered as very finely divided particles suspended in the water associated with the fibre. If sufficient dilution water 56 is added to bring the consistency of the mixed flow 60 down to 1%, then, at this point, one pound of fibre is associated with 99 Ibs. water.Assuming "perfect" mixing in the repulper, the ink particles which had formerly been associated with, or dispersed in, 19 Ibs. water will now be dispersed uniformly throughout 99 Ibs. of water, or a concentration of about 0.19 of that in the water forming part of the incoming thick stock.

However, the outgoing re-thickened stock 62 also contains water, and, if it is taken to be at 5% consistency, one pound of fibre is associated with 19 Ibs. water, which water, however, contains a reduced concentration of ink particles, namely 0.19 of that contained in the water of the incoming thick stock 54. It is apparent that the maximum possible ink removal efficiency of the known device of Figure 1 can at best be only 81%. In actual practice, of course, it is less that that, since the mixing in the repulper is never perfect, and since it is only a useful first approximation to consider that the ink in the whitewater is at the same concentration as the ink in the water associated with the outgoing rethickened stock 62.

It is now apparent why several devices in series are commonly used to obtain re-thickened stock from the final stage that is acceptably clean and ink-free for use in paper making.

Considering now the device of the present invention shown in Figure 2, if sufficient ink-free water 86 is added and the distribution across the axial length of the cylinder 10 is made sufficiently uniform, as provided by the holes 72, then this inkfree water will displace the ink-carrying water in the mat 38 before it emerges above the liquid surface 21. If this displacement is "perfect", then the 19 Ibs. of water associated with each pound of fibre in re-thickened stock 82 will be ink-free. Thus it is at least theoretically possible to achieve 100% ink removal efficiency in a single device.

In practice, of course, this does not happen. The efficiency of the displacement of the ink-carrying water in the mat with ink-free water is less than perfect. However the ink removal efficiency is greater with the present invention than with the devices previously known. In practice, this will often mean that two such devices in series will produce acceptably clean pulp for papermaking, in circumstances and with a feed stock 54 to the first stage that would have required three or four prior devices in series. The savings in capital cost, floor space, power requirements and water requirements are apparent.

Consistencies, weights of fibre and of water, concentrations of ink and washing efficiencies stated herein are to be understood as illustrative only, and not necessary or limiting conditions for practicing the invention.

It may be remarked here that, although it is obvious that at least for the final washing stage having the second dilution water 86 ink-free, or as inkfree as may be possible with the use of recycled clarified water, is of great importance, it is by no means necessary that the second dilution water 86 be fibre-free. In some cases, depending on the fibre balance and the water balance of the total paper mill, it may be highly desirable to use fibrecontaining, though substantially ink-free water, as the second dilution water. The second dilution water, accordingly, may be, for example, save-all filtrate or suction-box whitewater from a fourdrinier paper machine. In some cases this will lead to the recovery for re-use of valuable fibre, by incorporating it into the mat 38.

It has been noted herein that considerable turbu lence is induced in the stock contained within the vat 18 by the rotation of the cylinder 10. It is necessary to control this turbulence, so that the incoming ink-free water 86 is not mixed indiscriminately throughout the contents of the vat.

At least the greater fraction of it must be available to pass through the mat 38 in the small, downstream region between the distribution plate 70 and the immediately facing portion of the cylinder 10. To this end, the baffle 74 is provided, so that substantially all the second dilution water 86 is restricted to the area above the tip 76 of the baffle 74 on its way to the mat 38, and so that the remainder of the stock below this point does not readily pass above the tip 76. General turbulence within the vat may advantageously be suppressed by the use of one or more additional baffles, such as the baffle 78.

Moreover, reduction in turbulence is improved by displacing the longitudinal axis of the cylinder 10 to the right in the vat 18, thus making the crosssection of the passageway 25 progressively decreasing in the direction of rotation of the cylinder.

Compare the dimensions 26 and 28.

It should also be noted that in cases where the whitewater 84 is withdrawn from only one end of the cylinder, such as through a rotary seal incorporating a deckle strap and chime ring, then a type of seal or baffle arrangement must be provided at the other end of the cylinder to prevent free flow of stock around the closed end of the cylinder from the up-running side back to the down-running side.

Various arrangements are possible, including the provision of a second chime ring and deckle strap, even though the second end of the cylinder is closed.

Figure 3 represents a partial section of a second embodiment of the invention. In this embodiment, the baffle 74 is somewhat modified and contains holes 88 uniformly spaced along its axial length.

Holes 88 permit a controlled amount of the second dilution water flow 86 to enter the region 90 between the baffle 74 and the baffle 78. The liquid in the region 90 will partly be composed of stock from the vat, which passed between the baffle 78 and the mat 38 on the cylinder, but diluted to some extent by the dilution water from the holes 88. The size and spacing of the holes 88, compared to the size and spacing of the holes 72 will be so chosen that the major fraction of the incoming second dilution water 86 passes through the holes 72 and the minor fraction through the holes 88.The effect, in operation, will be to provide a transition region 90 where the mat, passing between the location of the baffle 78 and that of the baffle 74, is contacted by stock carried in water with a somewhat reduced concentration of ink, before entering the region 29 above the baffle 74, where it is contacted by ink-free water. This will contribute to good overall ink removal efficiency.

Figure 4 shows, in partial section, yet another embodiment of the invention. In this embodiment, the distribution plenum 66 is open at the top and incorporates a broad-crested weir 92, running axially the full length of the cylinder 10. The ink-free second dilution water 86 leaves the plenum 66 over the weir crest 92 as a flow 94 and falls a very short distance onto the liquid surface 21. The inkfree water then finds its way downward to the level of the tip 76 of the baffle 74. This provides a simpler and less costly construction than that shown in the other two embodiments, although not offering the same possibility of precise control of flow.

As will be apparent to those skilled in the art, various modifications may be made in the geometric configuration, the materials of construction, and the relative arrangement of parts, while remaining within the scope of the invention.

Claims (12)

1. A method of washing pulp stock for use in paper making, comprising (a) defining a passageway (25) between a vat (18) and a perforate, filtering, cylindrical wall (16) of a hollow cylinder (10), (b) introducing input pulp stock (20) containing water and fibre into an upstream end (27) of said passageway to flow along said passageway while maintaining the cylinder partially immersed in the stock, (c) rotating the cylinder about its longitudinal axis (12) with its cylindrical wall travelling downwardly at said upstream end, (d) removing from the interior of the cylinder a relatively dilute fraction (30) of the input stock that has passed through the cylindrical wall, (e) removing from the cylindrical wall at a location above the level (23) of the stock at a downstream end (29) of the passageway at which said cylindrical wall travels upwardly, a fibrous mat (38) composed of a relatively concentrated fraction of the input stock, whereby the entire input stock is removed in the form of the two said fractions, characterised by (f) introducing dilution water (86) into the passageway at its downstream end (29), and (g) directing such dilution water through the mat at said downstream end to the interior of the cylinder whereby to enhance the washing of said mat.

2. A method according to claim 1, for the deinking of pulp stock reprocessed from waste paper, wherein said dilution water is substantially ink free.

3. A method according to claim 1, wherein said dilution water contains fibres that are added to the mat and hence recovered.

4. Apparatus for washing pulp stock for use in paper-making, comprising: (a) a vat (18), (b) a hollow cylinder (10) mounted in and spaced from the vat to define therebetween a passageway (25) extending from an upstream end (27) to a downstream end (29), (c) means (44) for introducing input pulp stock (20) containing water and fibre into said upstream end, (d) the cylinder (10) having a perforate, filtering, cylindrical wall (16) for passage therethrough from the vat to the interior of the cylinder of a relatively dilute fraction (30) of the input stock and the formation on the outer surface of said wall of a relatively concentrated fraction of the input stock in the form of a fibrous mat (38), (e) means (12) for rotating the cylinder about its longitudinal axis so that its cylindrical wall travels downwardly at the upstream end and upwardly at the downstream end, (f) means for removing the dilute fraction from the interior of the cylinder, (g) means (32) for removing the mat from the cylindrical wall at a location above the level (23) of the stock in the vat at the downstream end, characterised by (h) means (66, 68) for introducing dilution water (86) into the vat at the downstream end of the passageway, and (i) means (70, 72) for directing such dilution water through the mat at the downstream end to the interior of the cylinder.

5. Apparatus according to claim 4, wherein said directing means (i) comprise a plate (70) mounted in the vat at the downstream end of the passageway closely adjacent the cylindrical wall, said plate having a plurality of holes (72) for forming streams of the dilution water to impinge directly on the mat.

6. Apparatus according to claim 4, including means (74, 78) for inhibiting counterflow of the dilution water from the downstream end towards the upstream end.

7. Apparatus according to claim 4, wherein said intro ducing means (h) comprise a distribution plenum (66) connected to receive a supply of dilution water (86) and said directing means (i) comprise a wall (70) of such plenum located closely adjacent the cylindrical wall of the cylinder at the downstream end of the passageway, said plenum wall having a plurality of distributed holes (72) for forming streams of the dilution water and directing such streams directly onto the mat.

8. Apparatus according to claim 7, wherein the plenum (66) has a lower wall that projects from the vat to a location (76) closely adjacent the cylindrical wall of the cylinder to act as a main baffle (74) to inhibit counterflow of the dilution water in the passageway.

9. Apparatus according to claim 8, including at least one further baffle (78) extending from the vat to a location closely adjacent the cylindrical wall and arranged in the passageway upstream of said main baffle (74).

10. Apparatus according to claim 9, wherein said lower wall of the plenum is perforated (88) to direct a minor fraction of the dilution water into a space in the passageway between the main baffle (74) and the adjacent said further baffle (78) whereby to expose the mat to a region (90) of lesser dilution prior to its exposure to a region (29) of greater dilution resulting from its exposure to a major fraction of the dilution water at the downstream end.

11. Apparatus according to claim 4, wherein said introducing means (h) comprise a distribution plenum (66) connected to receive a supply of dilution water (86) and said directing means (i) comprise an imperforate wall of such plenum (Figure 4) located closely adjacent the cylindrical wall of the cylinder at the downstream end of the passageway to define a narrow downwardly tapering region between such wall and the mat on said cylindrical wall, said plenum being open at its top whereby an upper edge of said wall forms a weir (92) for directing the dilution water downwardly into said region and through the mat.

12. Apparatus according to claim 4, wherein said passageway (25) has a cross-section that tapers gradually (26, 28) from the upstream end (27) to the downstream end (29).