Dewatering process

Abstract: A process for enhancing dewatering of sludge on a filter belt by the use of conical plows that elevate and mix the sludge. The angles of the plows with the horizontal may be progressively increased, and thereby increase elevation and mixing of the sludge as it moves downstream on the belt toward discharge position.

Description: The present invention relates generally to the field of separating the liquid phase from the solid phase of a sludge or slurry. This separation is hereinafter referred to as "dewatering." Generally, the sludge or slurry, hereinafter jointly referred to as "sludge," is deposited on and travels with a flight of an endless filter media with the liquid phase draining by gravity through the media and the solid phase travelling with the media to the point of discharge from the gravity dewatering part of the filtration equipment. In general, the use of plows to improve water drainage from sludge carried on an endless conveyor belt has been recognised to be advantageous. Thus, in U.S. Patent No. 4,609,467, which is commonly assigned to the assignee of the present application, plows are disclosed which extend into the longitudinal path of movement of the belt and are rotatable in order to minimise hang-up of fibrous materials that may be undesirably retained on other, conventional plows. In that patent, the rotatable plows descend into the path of the sludge and hereby distribute the sludge evenly over the face of the belt while at the same time scraping or wiping the top surface of the belt to provide exposed filter media portions. Thereafter, existing free liquid flows by the force of gravity from the sludge to and through the liquid permeable belt on which the sludge is carried. The liquid is then collected and further treated or discharged in the course of the waste management program. The solids remaining in the sludge then pass to another apparatus for further treatment, such as a press section of a belt filter press. US-A-4879033 discloses a process for dewatering sludge on a flight of a foraminous filter belt which moves longitudinally between sludge deposit and discharge positions, in which sludge is deposited on the belt flight and carried longitudinally toward a sludge discharge position past first and second rows of plows which elevate and mix the sludge as it passes them. The apparatus disclosed in that patent comprises apparatus for dewatering sludge, comprising a foraminous filter belt having a flight which is movable between sludge deposit and discharge positions, and a plurality of plows arranged in respective first and second rows extending laterally across the flight and spaced apart longitudinally of the flight, the second row (or rows) being positioned downstream of the first row (or rows), each plow having the general form of a truncated cone with a vertical axis with the plane of its base and the belt and having a side wall extending upwardly and inwardly away from the belt. Applicants also acknowledge US-A-2097529 which discloses a vacuum assisted dewatering apparatus having a variety of solids elevating devices, some stationary and some rotating about transverse horizontal axes. The present invention seeks to improve or to simplify the separation of liquid from a sludge as compared with the processes and apparatus disclosed in the above mentioned patents. The process of the present invention is characterised in that the plows in the second row or rows are shaped to elevate and mix the sludge to a greater extent than the plows in the first row or rows and the apparatus is characterised in that the base angles of the cones in the second row or rows are greater than the base angles of the cones in the first row or rows. In the process and apparatus of the present invention, it is preferred to provide third and subsequent rows of stationary plows, each row or rows elevating and mixing the sludge to a greater extent than the preceding, upstream row or rows. In the preferred embodiments of the invention there is created a series of flow elevation zones with the creation of multi-directional eddy currents to improve mixing over essentially the entire surface of the permeable filter belt that is in contact with the sludge. The solids-liquid mixture is caused to form a turbulent layer covering all or a large part of the filter belt surface which, because of the charateristics of the turbulent layer, greatly increase the rate of filtration, or drainage, of the liquid from the sludge. As a sludge initially is deposited upon the belt, an excess of free water begins to draw through the filter media carrying with it to the top surface of the media light floc and fines which tend to cover the top of the media and block or prevent maximum drainage. The first row of plows, when positioned properly, wipes the filter media and lifts and gently disturbs the light surface floc, thereby enhancing drainage and allowing the floc to build. As the sludge continues to travel toward the discharge end of the belt, additional liquid drains off and at each subsequent contact of plows of greater angle encounter increased resistance causing the sludge mass to build and at the same time releasing more liquid. Eventually the mass may become so great that upon engaging a plow of sufficiently large angle it is caused to roll over on itself laterally and longitudinally, thereby impinging and falling on oncoming sludge driving out additional liquid. Where this phonomenon of the sludge causing mixing eddies to occur and even rolling back on itself in some instances, takes place at multiple locations along the length of the filter belt in the longitudinal direction, it has been determined that the solids content of sludge carried by the belt, without pressure being exerted on that sludge and without a vaccum being drawn from beneath the belt, can be increased up to nine-fold. This can be accomplished by the expedient of using plows according to the present invention and spacing and varying those plows in a manner so as to produce optimal results. As will be apparent to those of skill in the art, such spacing and numbers of plows utilized, either in a concentrated arrangement at the discharge end or a distributed arrangement along the length of the filter media, will be dependent, inter alia , on the type of sludge being dewatered, the rate of feed of the sludge onto the filter belt, and the solids content desired in the filter cake that is discharged from the filter media. Further particular embodyments of the inventive method and apparatus are set out in dependent claims. Preferred embodiments of the invention will now be described, by way of example, in conjunction with the accompanying drawings, in which: FIG 1 is a schematic, side elevational view of a flight of an endless filter belt with plow assemblies positioned on it at preferred locations; FIG 2 is a top plan view of the belt and plow assembly of FIG 1, also in schematic form; FIG 3 is an enlarged side elevational view, partly in section, showing a preferred mounting of plow assemblies in operable position on a filter belt, and FIG 4 illustrates configurations of various forms of plows. Referring now to the drawings, and in particular to FIGS. 1 and 2 thereof, the horizontal flight 10 of an endless filter belt is shown in both views. As seen best in FIG. 1, a feed hopper 11 accumulates sludge 12 having a high liquid content. As sludges are measured in solids content, that content may typically be from one quarter to one percent (or more) solids. Flight 10 of the filter media is shown extending between rollers 15 and 16, which guide rollers rotate in a counter-clockwise direction so that the flight moves in the direction indicated by arrow 17. As guided by rollers 15 and 16, the flight 10 accumulates slurry or sludge 12 from feed hopper 11, which is deposited on the flight. The slurry then passes on the foraminous belt and, in the particular embodiment illustrated, contacts an upstream row of conical plows 20. Thereafter, the belt transports sludge to a second, downstream row of plows 21, and then further downstream to plow rows 22, 23, 24 and 25. After passing by the row of plows 25, the sludge is then discharged into a suitable receptacle as the belt flight 10 passes about discharge roller 16 in a conventional manner. The structure of an individual plow and the means by which the plow is mounted for operation are seen in a preferred mode in FIG. 3. Two plows are shown in FIG. 3, the plow assembly 30 to the left being shown in side elevation and the plow assembly 31 to the right being seen in vertical section. From FIG. 3 it will be apparent that the elements of the plow and its mounting means comprise a horizontal post 33, which can be circular or square in cross-section, extends across the width of the belt, and is suitably mounted in side walls or other conventional structure located on either side of the belt flight 10. As so mounted, post 33 extends across the belt and the sludge carried on the belt, and carries support structure for maintaining the conical plows in position to dewater the slurry or sludge carried on the belt. As seen in FIG. 3, a collar 34 is firmly fixed to the post 33. Collar 34 forms an interior, extended orifice 35. Extending within orifice 35 is a shaft 36 which terminates at one end in plow 38 and at the other end in a shaft head 39 that extends above the collar 34. There is a short length of shaft 36 between the top of the collar 34 and the bolt 39 to permit vertical movement of plow assembly 30. At its lower end, as best seen in the sectional view of the plow assembly 31, shaft 36 terminates downwardly in a nut 40 located above the conical plow 38 and a lock nut 41 that is located below the shell of the conical plow 38. In this manner, as mounted between nuts 40 and 41 at the lower end of shaft 36, the conical shell of plow 38 is firmly fixed on the shaft 36 so that the plow can only move in conjunction with shaft 36. As so mounted, since the shaft 36 has a diameter slightly less than the diameter of orifice 35 of collar 34, shaft 36 is free to rotate within collar 34. Further, shaft 36 is free to move in a generally vertical direction within the orifice 35. Motion in a downward direction by shaft 36 is limited by shaft head 39 affixed to the upper end of the shaft so that, for example, if the filter media 10 is removed for cleaning or repair, the plows 38 will move downward for a relatively short distance and will still be positioned so that on replacement of the filter media the plows will be in their proper position for dewatering sludge or slurry on the media. The plow assembly illustrated in FIG. 3, also includes an elongated sleeve that encloses the shaft 36. In operative position sleeve 37 has its axis vertically disposed and is freely rotatable independently of any movement of the shaft 36. Use of such a sleeve acts to avoid hang-up of fibrous or other materials on shaft 36, since when there is any such tendency for material to adhere to shaft 36, the presence of a light rotatable sleeve will permit rotation and movement of the material past the sleeve and, therefore, the shaft. In order to move the plows upwardly so that they can be cleaned, repaired, or replaced, we prefer to use the apparatus disclosed and claimed in U.S. Patent No. 4,602,998. Because these plows are substantially or generally conical, by the use of which term it is intended to include variants thereof, including what might be considered frusto-conical, the side walls or shells of the plows have a slope that form an angle with plane of the base of the plow. The term "base" is used whether the plow has a closed base or, as shown in the preferred embodiment, where the base of the plow is left open so that access may be had to the lock nut 41 in case it is desired to remove the plow. By providing such ease of access, it is not a difficult matter to change plows, so that a plow having one slope and forming a corresponding angle with the plane of its base may be easily replaced by a plow having a different slope and therefore having its shell form a different angle with the plane of its base. Shown herein in FIG. 4 in top plan and side elevations are several variants of cones that may be used in accordance with the present invention to elevate and mix sludge carried on a filter belt. In FIG. 4A are cones having side walls that vary between slightly greater than 0° and approaching but less than 90° angles with the horizontal. In FIG. 4B are plows where the frustum of a plow is then joined to a circular right cylinder, the cylinder being designated by numeral 43, mounted atop the frustum of circular right cone 42. Shown in FIG. 4C are two general cones 45 with offset axes 46. In FIG. 4D are shown a series of convoluted conical sections 49. Such sections are spaced closely together as shown in top plan view in FIG. 4D, and are not rotatable. While the use of any specific plows will depend on the particular sludge being dewatered, the rate of deposit of the sludge on the filter belt, and the structure of the filter media, it has been found advantageous to increase the slope and corresponding angle, and thereby provide progressively greater flow elevation and more intense multi-directional eddy mixing to the sludge between, over and around the conical plows, in the downstream direction of the media. At this time a preferred embodiment of my invention, as shown in FIGS. 1 and 2, is to employ in rows 20 and 21 plows that have a lower profile, i . e ., plows the side walls of which form a lesser angle with the planes of their bases. Then, in rows 22, 23, 24 and 25, we employ plows having a higher profile, which means that the slope of the side walls of those plows is greater than the slope of the side walls of the plows in rows 20 and 21. In this manner, the sludge will be subjected to less agitation when it has a copious supply of water which is relatively facile to liberate through the foraminous media, yet be subjected to greater flow elevation and multi-directional eddy mixing when such is necessary in order to extract liquid from the slurry when the solids content of the slurry is higher. Indeed, at the group of plows in row 25, the slope of the side walls of the plows is such as to cause the sludge to tumble back upon itself before it can pass over and between the plows of that row. With regard to the specific slopes of the plow shells, it is presently preferred that the lower profile plows have a vertical dimension of ¾ inch (18.5 mm) between the base of the plow and the upward termination of the plow just below nut 40, which plow portion might be termed its frustum. In a higher profile plow, that vertical dimension has 1½ inches (37 mm). Since the plows are 6 inches (148 mm) in diameter at their bases, in this preferred embodiment the low profile plows, rows of which are located upstream in the direction of flow of sludge on the belt, have angles of approximately 15° with the horizontal; the higher profile plows located downstream in the path of movement of the belt have side walls that make angles of approximately 30° with the bases of the plows. Because of the way the plows are mounted, all of the plows are free to float on the belt and, in preferred form, ride directly on the belt, perhaps separated by a thin layer of sludge that may adhere to the upper surface of the belt. By means of the present invention, markedly superior solids contents of sludges have been obtained. In one instance, where sludge 12 in a feed hopper 11 was deposited on a belt filter at a flow rate of approximately 425 gallons per minute (1609 dm³/min), the sludge as so deposited had from one quarter percent to one percent solids content. Sludge discharged from the belt was obtained varying between five and nine percent solids. Only gravity drainage through the permeable belt, assisted by the flow elevation and eddy mixing from rows of first low profile, then moderate profile plows of conical shape, was utilized. As a variant of the rows of plows illustrated in FIGS. 1 and 2, use has also been made of rows that are closely adjacent to each other along the longitudinal dimension of the belt. Such an instance, rather than having the plows aligned in the direction of flow of the sludge, the rows were staggered so that the gaps between adjoining plows in one row, e . g ., row 22, would be occupied by plows of the next row, e . g ., row 23, which would necessitate removal of one plow in row 23. In this manner an even more vigorous eddy mixing of the sludge carried by the flight 10 of the filter belt may be achieved, causing additional water release.

Claims: A process for dewatering sludge on a flight (10) of a foraminous filter belt which moves longitudinally between sludge deposit and discharge positions, in which sludge (12) is deposited on the belt flight (10) and carried longitudinally toward a sludge discharge position past first and second rows (20,21,22) of plows which elevate and mix the sludge as it passes them, characterised in that the plows in the second row or rows (22) are shaped to elevate and mix the sludge to a greater extent than the plows in the first row or rows (20,21). A process as claimed in claim 1, in which the sludge is moved past a third row or rows (23) of plows which mix the sludge a third time to an extent greater than the elevation and mixing thereof effected by the second row or rows. A process as claimed in claim 1 or 2, in which the second and any subsequent rows of plows elevate and mix the sludge and cause it to fall back upon itself to expose interior sludge portions. Apparatus for dewatering sludge, comprising a foraminous filter belt having a flight (10) which is movable between sludge deposit and discharge positions, and a plurality of plows arranged in respective first and second rows (20,21,22 etc.) extending laterally across the flight and spaced apart longitudinally of the flight, the second row or rows (22) being positioned downstream of the first row or rows, each plow having the general form of a truncated cone (38) with a vertical axis with the plane of its base at the belt and having a side wall extending upwardly and inwardly away from the belt, characterized in that the base angles of the cones in the second row or rows (22) are greater than the base angles of the cones in the first row or rows (20, 21). Apparatus as claimed in claim 4, in which each said cone (38) is a right circular cone. Apparatus as claimed in claim 5, in which the side wall of each cone in one row intercepts the plane of the base of said cone at an angle of about 15°. Apparatus as claimed in claim 5, in which the side wall of each said cone in the second or subsequent rows (22,23 etc.) intercepts the plane of the base of said cone at an angle of about 30°. Apparatus as claimed in claim 4, 5 or 6, wherein the plows of one row (21,22 etc.) are longitudinally aligned with gaps between the plows of the preceding upstream row (20,21 etc.). Apparatus according to any one of claims 4 to 8, comprising means for mounting each said plow with respect to said belt, including a post (33) extending horizontally across the path of said belt and a shaft (36) interconnected to said post and extending downwardly from said post, said plow body being attached to said shaft at the downwardly extending end thereof so that in operative position an outer surface of said plow, body side wall extends in a continuous, inwardly extending slope from its base at said belt to said shaft, whereby on movement of said belt toward and past said plow, the sludge carried on said belt flight will ride up on the side wall of said conical plow body, thereby elevating and disturbing the flow of sludge and mixing and agitating same to increase the flow of liquid from said sludge to and through said liquid permeable belt. Apparatus according to any one of claims 4 to 9, including a collar (34) mounted on each said post (33), said collar (34) forming therewithin an elongate orifice (35) the axis of which is substantially vertical when said plow (38) is in a sludge elevating and mixing position, a shaft (36) positioned within said orifice (35) and having a diameter slightly less than the inside diameter of said collar (34), so that said shaft may rotate within said orifice and also move axially therewithin, said plow (38) being fixed at one end of said shaft so that when said plow is in a sludge elevating and mixing position, said plow depends substantially vertically downward from said collar to said belt (10) and by the weight of said plow and shaft is maintained at or in contact with said belt. A plow assembly as claimed in claim 10, in which said plow (38) is maintained in fixed position relative to said shaft (36) at the lower end thereof. A plow assembly as claimed in claim 10 or 11, in which a shaft head (39) is provided at the top of said shaft above the orifice of said collar for limiting movement of said shaft and said plow in the direction of said belt.

Inventors: CROSBY ROBERT J, GORON JOHN, RENZA RONALD G, SODTALBERS ERICH W, CROSBY, ROBERT J., GORON, JOHN, RENZA, RONALD G., SODTALBERS, ERICH W.
Applicants: KOMLINE SANDERSON ENG CORP, KOMLINE-SANDERSON ENGINEERING CORPORATION
Application year: 1990
Main classification: B01D33
IPC codes: B01D33