Rotary press and method of operating a rotary press for dewatering sludge

Abstract

NOT VISIBLE DUE TO STATUS OF PATENT

Description

ROTARY PRESS AND METHOD OF OPERATING A ROTARY PRESS FOR DEWATERING SLUDGEBACKGROUND OF THE INVENTION

[0001] This invention relates to the treatment of sludge and waste waters.

[0002] “Sludge” is to be interpreted broadly and includes a composition comprising a mixture of a liquid and a solid material, typically of particulate or fibrous form. “Sludge” extends, by way of example only, to products produced in the paper industry, by waste water treatment, in the processing of minerals, agricultural and food products, to compositions produced by fisheries, breweries and wineries, and to products produced through chemical, oil and tar sand processes.

[0003] “Rotary press” relates to a method or apparatus for extracting liquid from a sludge.Without being restrictive “rotary press” includes a filter press with a rotatable screen e.g. of the kind described in the specifications of US7166229, US8146750 and US7946225.

[0004] A press, acting on sludge, separates liquid (filtrate) from solids in the sludge. If the solids are sufficiently dry, and compressed by the action of the press, the product which results is referred to as cake.

[0005] “TSS" refers to the total suspended solid content in a sludge.

[0006] Raw waste water normally has a very low suspended solids value, for example less than 0,15%. Frequently the sheer volume of the waste water to be processed makes effective treatment difficult particularly due to the loading of the solids in the water.

[0007] To the applicants' knowledge a rotary press cannot handle a sludge with a TSS of less than 0,5% effectively for the rate of dewatering which is required to generate a cake with a highTSS results in a significant lower sludge processing rate.

[0008] In order to obtain cake of an acceptable quality at a good production rate the sludge must be thickened before it is fed to the press. Typical approaches include the use of gravity thickeners, flotation thickening, the operation of centrifuges or gravity belt thickeners or the use of a rotary drum.

[0009] In general terms it can be said that thickening of a sludge of this kind can be difficult and conventional techniques can require a large area with meaningful capital infrastructure.

[0010] It is desirable to extract the bulk of the solids from the raw waste water fairly quickly so that the quality of the solids does not deteriorate significantly. Early deterioration can give rise to technical difficulties downstream of the thickening process. Preferentially the solids, or the bulk of the solids, should be removed quickly so that the remaining high volume liquid portion can be processed using suitable conventional techniques. The removed solids can be subjected to a suitable treatment process.

[0011] A rotary press with a typical current configuration and control philosophy cannot handle dewatering of this type of sludge directly to produce a cake, and a filtrate, each of a satisfactory quality, at an acceptable production rate.

[0012] An object of the invention is to address, at least to some extent, the aforementioned situation.SUMMARY OF THE INVENTION

[0013] The invention provides a rotary press comprising a housing, a sludge flow channel which is formed through the housing, a sludge inlet to the channel, a sludge outlet from the channel, screens which are rotatable and which form opposed walls on respective sides of the channel, a sludge feed which is operable to feed sludge into the channel through the sludge inlet, a first flow meter which provides a first signal which is dependent on the sludge flow rate to the inlet, a pressure sensor at the sludge inlet to provide a second signal which is dependent on the pressure of the sludge at the sludge inlet, a restrictor at the sludge outlet which is operable to vary, in size, a cross sectional area of the sludge outlet through which material emerging from the channel flows, a second flow meter to provide a third signal which is dependent on a filtrate flow rate from the sludge between the inlet and the outlet, and a controller, characterised in that the controller is responsive to the first signal, the second signal and the third signal to control the operation at least of the restrictor thereby to control said filtrate flow rate.

[0014] If the sludge has a low TSS then, depending on operating conditions, the sludge feed to the sludge inlet can be via a gravity feed arrangement wherein the sludge is supplied from a tank or reservoir located at a position which is elevated relative to the press. A gate or valve, operable in any suitable way, in response to a signal from the controller, is then actuable to control the sludge feed rate in a satisfactory way.

[0015] In a mechanized system a suitable pump, e.g. electrically driven, feeds sludge from a suitable source to the press. The controller can then operate, directly on the pump, to vary its speed of operation and so control the rate of sludge feed to the press.

[0016] The controller may be operable to maintain the filtrate flow rate at a constant value.Primary control of the filtrate flow rate and sludge inlet pressure may be effected by varying said cross sectional area. Generally a variation of the TSS in the incoming sludge is gradual and, in reaction thereto, the cross sectional area at the sludge outlet is varied in a commensurate incremental manner.

[0017] In addition to controlling the filtrate flow rate the controller may operate to maintain the sludge inlet pressure at a constant value.

[0018] In most situations the aforementioned control techniques are effected by incremental variations of the size of the cross sectional area of the sludge outlet. In some cases though, depending at least on the characteristics of the sludge, a variation of the size of the outlet cross sectional area may not be sufficiently effective in producing a suitable balance or value of the filtrate flow rate and of the sludge inlet pressure. The sludge inlet pressure can then be varied and the rotational speed of the screens can be adjusted to achieve a suitable operating situation i.e. one in which the production rate is acceptable with a desired rate of filtrate without forming a dry cake at the outlet. Thus the TSS of the outlet sludge is increased so that the sludge can thereafter be treated in a secondary downstream process.

[0019] A rotary press of the aforementioned kind is configured, particularly, to treat sludge with a TSS of less than 0,5%. It is known that with this type of press the sludge cannot be processed in one operation to produce a dewatered cake of an acceptable quality and a corresponding filtrate. However in this specification it is recognised that the sludge can be treated by a rotary filter press, operating in the aforementioned manner, to remove aconsiderable amount of the liquid from the sludge. The extent to which the filtrate is removed depends on a variety of operating conditions and on the nature of the sludge and could be as high as 90% or 95% of the incoming sludge volume.

[0020] The suspended solids which are in the incipient cake at the sludge outlet can then be processed or disposed of in any appropriate manner.

[0021] The controller can regulate rotation of the screens relative to the channel and thus to the sludge in the channel. Due to the action of the restrictor at the sludge outlet the flow of sludge from the channel outlet is restricted and this causes the pressure in the channel inside the housing to increase. Water in the sludge, pressurised as aforesaid, moves through mesh apertures in the screens and a significant degree of separation of the water from the solids is achieved. Entrained particulates in the water, moving through the mesh apertures, can block the mesh apertures and reduce the operating efficiency of the arrangement. To address this negative occurrence the screens are rotatable relative to the channel so that particulates which have adhered or become stuck to the screen material are wiped from the screen material. Thus the screens and their speeds of rotation play a significant role in the production of the filtrate.

[0022] The sludge is preferably treated with a polymer to promote coagulation or flocculation of particles in the sludge thereby to reduce the likelihood of small particles in the channel from passing through the screen apertures.

[0023] This invention is concerned particularly with the processing of a sludge with a low TSS.That type of sludge is not formed into a good quality cake in the channel but into what may be referred to as a “wet cake". The application of a high force or pressure to the sludge in thechannel does not necessarily produce a satisfactory result as the production of filtrate is not always enhanced. A balance is therefore called for to take account of the ratio of the sludge volume in the channel to the dewatering area of mesh apertures presented by the screens. For example if the width (thickness) of the sludge in the channel between the two opposed screens is too high dewatering is cannot significantly be increased by raising the pressure on the sludge.If the sludge thickness is such that a dry cake is formed in the channel it is likely that the channel is too narrow. In that event a blockage can occur and a production rate can suffer. Thus a channel width must be adopted which gives an effective and acceptable compromise between the efficiency of filtrate extraction and the processing rate for the sludge.

[0024] Against this background the controller acts on the restrictor to vary the cross sectional area at the outlet and, if necessary, the sludge inlet pressure is adjusted. The sludge flow rate from the sludge outlet is thereby controlled to ensure a high degree of liquid extraction from the sludge but without forming a dry cake.

[0025] The restrictor at the sludge outlet can take on different forms. The restrictor may for example include an adjustable plug mechanism or a sliding gate operable to vary the cross sectional area at the sludge outlet. In one embodiment the restrictor comprises a pinch valve which is operable to change a cross sectional area of a part of a flexible tubular member which is connected to the sludge outlet and through which the sludge exits the press. In a different approach the restrictor comprises a member e.g. in the form of a flap which is movable to vary the cross sectional area at the sludge outlet through which sludge can flow. The operation of the restrictor is such as to adjust the cross sectional area at the sludge outlet, thereby to maintain the inlet pressure to the filter at a constant rate.

[0026] The restrictor comprises an important component in the apparatus of the invention. As the TSS of the incoming sludge normally varies gradually it must be possible to control the restrictor so that the cross sectional area of the sludge outlet is changed incrementally in dependence on a small variation of the TSS in the sludge.

[0027] The invention is further described by way of example with reference to the accompanying drawings wherein:Figure 1 illustrates in a schematic form the operation of a rotary press for thickening a sludge with a low TSS according to the invention, andFigures 2 to 6 show different ways of restricting flow of a thickened sludge from the rotary press by adjusting a cross sectional area of a sludge outlet from the press .DESCRIPTION OF PREFERRED EMBODIMENT

[0028] Figure 1 of the accompanying drawings illustrates, schematically and by way of a non- limiting example only, a rotary press 10 which for practical purposes is similar to the press described in the specification of US patent number 7166229. For this reason the construction of the press 10 is not described in detail.

[0029] The press 10 includes a housing 14 through which extends an arcuate channel 16. The channel 16 has a sludge inlet 18 and a sludge outlet 20. The walls of the channel, inside the housing, are formed by two spaced apart screens 24 with mesh apertures of a suitable size.The screens are rotatable about an axis 26 by means of a drive mechanism 30.

[0030] The arrangement shown in the drawing is intended to treat a sludge 34 with a low TSS, typically a sludge wherein TSS is less than 0,5%. This value is exemplary.

[0031] The sludge is pumped by a pump 36 through a sludge flow meter 38 to a flocculator 40 and then to the inlet 18. The flow meter 38 produces a first signal 42 which is dependent on the sludge flow rate to the inlet 18. A pressure sensor 44 at the inlet monitors the pressure of the incoming sludge and produces a second signal 46 which is dependent on that pressure.

[0032] If the sludge is fed under gravity action to the flocculator a suitable valve 36A, the operation of which is regulated by a controller, as referred to hereinafter, is used to replicate the function of the pump 36 and thereby control the rate of sludge feed into the sludge inlet. The components downstream of the pump (the meter 38 and the flocculator 40) are retained.

[0033] A restrictor 48 is positioned at the outlet. The restrictor is operable by means of an actuator 50 to vary the cross sectional area of an aperture at the outlet through which the sludge exits the press. By controlling this cross sectional area the rate of flow of the sludge from the housing 14 is controlled. When the cross sectional area is reduced the sludge outflow rate is also reduced and pressure builds up on the sludge inside the channel in the housing. Filtrate52 from the sludge is expelled through the mesh apertures in the screens 24.

[0034] The filtrate flow rate is measured by a flow meter 54 which produces a third signal 56 which is dependent on the filtrate flow rate. TSS variations in the incoming sludge are gradual and it is important therefore to be able to control the effect of the restrictor in a gradual manner which is dependent on the variation in the TSS. Ideally the restrictor should be capable of continuously varying an outlet aperture area as opposed to being able to vary the outlet area ina stepped manner. In this way the outlet aperture area can be varied so that it is responsive to the TSS of the sludge and produces filtrate at a desired rate.

[0035] It can happen though that the effect of the adjustment which results from changing the outlet aperture area is not sufficient to enable a balance to be achieved between the filtrate flow rate 56 and the sludge inlet pressure 46. This occurrence is detected by the controller which, in response thereto, adjusts the sludge inlet pressure to change the filtrate production rate to match the sludge inflow. Effectively the TSS of the sludge leaving the press is controlled responsive to a variation of the TSS of the incoming sludge.

[0036] In order to change the inlet pressure the incoming sludge flow rate can be varied for if that flow rate is increased the inlet pressure is increased. It is also possible to vary the rotation speed of the screens to adjust the pressure and thereby vary the filtrate production.

[0037] For effective control, as aforesaid, the size of an area of an aperture at the outlet should be incrementally adjusted. The full extent of the outlet area must be usable and it should not be clogged i.e. no cake must be formed in the outlet. The filtrate flow rate must be measured.Preferably the sludge inflow rate should be constant although variations are permissible to achieve desired filtrate production.

[0038] Thus once the outlet aperture size has been determined it is set and thereafter is varied as the inlet pressure changes. The aperture size is decreased as the inlet pressure drops (TSS drops) and the aperture size is increased as the inlet pressure increases (TSS increases) for a given sludge flow rate.

[0039] Use is made of a controller 64 to control the rotational movement of the screens 24 via the drive mechanism 30. The signals 42, 46 and 56 are applied to the controller. The function of the controller is such as to ensure that under most operational conditions the pressure at the inlet 18 is substantially constant. A control variable in this respect is provided by the restrictor48 which is movable by means of a pressure or force applied by the actuator 50 in response to a control signal from the controller 64.

[0040] The restrictor 48, operated on by the actuator 50 in response to the controller 64, varies the cross sectional area of an opening or aperture at the outlet through which material 68 exiting the outlet 20 flows. As the size of that area is reduced the sludge flow rate through the outlet is also reduced and the pressure of the sludge in the channel 16 inside the housing is increased.This causes water to be expelled from the sludge inside the housing through the mesh apertures in the screens 24. The actuator 50 is regulated by the controller 64 to keep the pressure at the inlet 18 substantially constant. As the sludge 34 has a TSS less than 0,5% (in this example) the action of the press is inadequate to form the material 68, exiting the sludge outlet 20, into a cake of an acceptable quality. This material 68 has a liquid content which is higher than that which is achieved when a press of the kind referred to acts on a sludge with a greater TSS. A substantial degree of control of the pressure inside the channel can however be exerted through the use of the restrictor 48 which in a controlled way reduces the cross sectional area at the outlet aperture so that the material 68 is significantly dewatered.

[0041] A sludge with a known TSS is used during an initial setup stage. The TSS of the sludge represents what can be referred to as an average value of the sludge which is to be dewatered.In practice though the TSS of the sludge will vary. The relationship between the filtrate flow rate56 to the pressure 46 at the sludge inlet 18 is established through tests. An objective in this respect is to establish an operating situation in which filtrate production is effectively maximised without producing a dry cake at the outlet. The desired situation is one wherein the sludge at the outlet has an increased TSS and is referred to as a “wet cake".

[0042] In operation the inlet pressure is set to achieve the desired filtrate production rate which is matched to the incoming sludge flow rate. The important variable in this regard is the TSS of the sludge which, at the sludge inlet 18 has a low value and, at the sludge outlet 20 has a higher value.

[0043] With the equipment working in the manner described the filtrate 52 is drained from the housing 14 in a substantially conventional manner. At the outlet 20 the material 68, i.e. the wet cake, is produced. This material, which has a substantially higher TSS than the incoming sludge and thus is thicker than the sludge 34, can be processed further, for example by using a second rotary press or any other appropriate technology to produce a dry cake which is of an acceptable quality.

[0044] The high pressure which prevails inside the housing 14 can cause some particulates in the sludge to move together with the filtrate through the mesh apertures. The larger particulates are however trapped by the screens. To some extent this factor can be addressed by adding a suitable polymer 70 to the sludge as shown in Figure 1.

[0045] The polymer is drawn from a suitable source and is pumped through a flow meter 74 into the incoming sludge flow upstream of the flocculator 40 by a pump 76. A signal 78, reflecting the polymer flow rate to the flocculator, is delivered by the flow meter to the controller 64 whichis programmed to deliver a signal 80 to the polymer pump 76 to ensure a controlled flow of polymer to the incoming sludge.

[0046] The polymer 70 which is mixed with the sludge 34, exerts a flocculating function and causes particulates in the sludge to agglomerate, thereby effectively becoming larger and less likely to exit through the screen. Additionally, to unblock the mesh apertures, the screens 24 are rotated continuously by the device 30. Such rotation exerts a cleansing effect on the screens for particulates adhering to the mesh apertures are wiped from the screens. Generally this wiping action lowers the pressure in the channel 16 and, to compensate, the cross sectional area of the outlet aperture is decreased by means of a signal from the controller 64 which causes the actuator 50 to act on the restrictor 48. If the rotational speed of the screens is reduced the sludge pressure in the channel increases.

[0047] The restrictor 48 and actuator 50 can take on any appropriate form. Figures 2 to 6 illustrate possible variations.

[0048] Figure 2 shows an arrangement wherein a control arm 90 is positioned at an inner location adjacent an end of the channel 16 slightly upstream of the outlet 20. The arm is movable by means of a pressure device 94 which works in response to a control signal 96 from the controller 54. The action is such that by moving the arm 90 the free area available at the outlet20 through which the material 68 from the channel can flow is controlled and in this way a sludge restricting action is achieved, and pressure in the channel 16 can be regulated.

[0049] In the arrangement of Figures 3 to 6 a signal from the controller 64 is used to actuate the restrictor 48, generally in the manner described in connection with Figure 2.

[0050] Figure 3 illustrates a different arrangement which uses a pinch valve 100 as a restrictor.A flexible tubular member 102 is attached to the outlet 20. A compressing device 104 acts on the member 102. The device 104 in turn is controlled by means of an actuator 106 which, responsive to a control signal from the controller, causes an outlet aperture comprising a flow passage through the member 102 to open or close in a desired manner. The flow of sludge material is then regulated by varying the size of the flow passage through the tubular member102. Use of a restrictor of this type is exemplary only and is non-limiting.

[0051] Figure 4 shows the use of a plug 108, of variable size, which is positioned in the outlet20 and which acts to reduce the cross sectional area of an opening or aperture at the outlet, through which sludge exits the press by means of an actuator 110, operation of which is regulated by the controller 64. The plug is moved deeper into the outlet, or removed from the outlet, to control the cross sectional area of the outlet aperture.

[0052] Figure 5 is similar to Figure 2 and depicts a gate 116 which is movable with a sliding action by means of an actuator 118 under the control of the controller 64 to vary the cross sectional area of the outlet aperture.

[0053] Figure 6 shows an extension of the Figure 3 approach in that a single actuator 120 acts on a complex compressive device 122 which is used to adjust the cross sectional areas of flexible tubular members 102A, 102B at outlets 20A, 20B from respective channels 16A, 16B in a simultaneous manner. This technique can be extended to three or more channels, according to requirement.14

[0054] What the invention achieves in an effective and controllable manner is a dewatering of a sludge with a low TSS thereby to produce a thickened sludge which can then be treated, by using a suitable conventional process, to output a cake of acceptable quality.

Claims

15CLAIMS1. A rotary press (10) comprising a housing (14), a sludge flow channel (16) which is formed through the housing, a sludge inlet (18) to the channel, a sludge outlet (20) from the channel, screens (24) which are rotatable by means of a drive mechanism (30) and which form5 opposed walls on respective sides of the channel, a sludge feed (34) which is operable to feed sludge into the channel through the sludge inlet (18), a first flow meter (38) which provides a first signal (42) which is dependent on the sludge flow rate to the inlet (18), a pressure sensor (44) at the sludge inlet (18) to provide a second signal (46) which is dependent on the pressure of the sludge at the sludge inlet (18), a restrictor (48) at the sludge outlet (20), which is operable to10 vary, in size, a cross sectional area of an outlet aperture at the sludge outlet (20) through which material (68) emerging from the channel (16) flows, a second flow meter (54) to provide a third signal (56) which is dependent on the flow rate of filtrate (52) from the sludge between the inlet(18) and the outlet (20), and a controller (64), characterised in that the controller (64) responsive to the first signal (42), the second signal (46) and the third signal (56) controls the operation of15 at least the restrictor (48) thereby to control the flow rate of the filtrate (52).

2. The rotary press (10) of claim 1 wherein the restrictor (48) is operable to reduce the size of the cross sectional area of the outlet aperture at the outlet (20), in response to a decrease in the sludge inlet pressure (46), and to increase the size of the cross sectional area of said outlet aperture in response to an increase of the sludge inlet pressure (46).

163. The rotary press of claim 1 wherein the sludge inlet pressure (46) is varied by at least one of the following: a variation in the sludge flow rate (38) to the inlet (18); and to vary the rotational speed of the screens (24).

4. The rotary press of claim 1 wherein the restrictor (48) comprises a flexible tubular5 member (102), at the outlet (20), with a passage through which sludge flows and a compressive device (104) which acts on the tubular member and which is controlled by an actuator (106) in response to a signal from the controller (64), to vary the size of the flow passage through the tubular member (102).

5. The rotary press of claim 1 wherein the restrictor (48) comprises a gate (90) or a plug10 (108) at the outlet (20) which is movable in response to a signal from the controller (64) to vary the sludge flow rate through the outlet (20).

6. A method of operating a rotary press (10) for dewatering sludge (34), wherein the press includes a housing (14), a channel (16) in the housing, a sludge inlet (18) to the channel, and a sludge outlet (20) from the channel, wherein the method includes the step of feeding the sludge15 (34) into the sludge inlet (18), and is characterised by the steps of monitoring a flow (56) of a filtrate (52) from the sludge between the inlet (18) and the outlet (20), and responsive to at least the flow rate of the filtrate (52) and the pressure of the sludge at the inlet (18) controlling the flow rate (56) of the filtrate by varying the size of the cross sectional area of an aperture at the sludge outlet (20) through which sludge leaves the press.