GB2181661A - Discharging sludge from settling tank - Google Patents

Abstract

Settled sludge is discharged from the central sump of a settling tank through pipes 16, 19 by opening a valve 22, 26. The valve comprises a flared nozzle 25 and a tubular valve member 26, which is raised by lever 30 being engaged by a cam 35 on the rotary bridge 13 which carries the tank scrapers. A device which monitors the fluid in or emerging from the pipes moves the bridge to close the valve when thinner liquid begins to emerge from the tank. This device may be a pressure transducer 49 or a contact 36, located in pipe 18 as shown or situated to be struck by the fluid emerging from gap 41 or not, depending on its trajectory. <IMAGE>

Description

SPECIFICATION Improvements in or relating to apparatus for removing settled sludge from liquid This invention relates to separating the sludge from a volume of liquid out of which it has settled, and particularlyto sewage treatment processes and the removal of the settled sludge from any of the customary series of settlement tanks.

Aconduit, leaving such settlement tanks at low level, normally leads into a nearby empty vessel known as a desludging chamber, the outlet of the conduit within this chamber being at a height below the normal water level in the tank. The normal method of removing the settled sludgefrom thetank into the chamber has been for an operator at a predetermined time, such as the end of each working shift, to open a valve in the conduit so that the head of liquidwithinthetankdrivesthesludgefromthetank through the conduit into the chamber, and to shutthe valve again as soon as he judges that the thickness of the sludge leaving the outlet has fallen below a predetermined value.

As manpower levels have fallen and operational practices have improved within sewage treatment works, new apparatus and methods of operation have been adopted to increase automation, and also to achieve a better quality sludge with reduced water content by removing smaller quantities of sludge from the liquid at more frequent intervals. These methods have included electrically-actuated valves with timer control to effect desludging for a preset length of time at regular predetermined intervals, and also the use of pumps working in this manner in place of gravity. However, such valves and pumps are not inexpensive to install,and require regular maintenance.

The present invention aims to provide a simpler yet reliable automated system, in which the use of neither electrically-actuated flow valves nor liquid pumps is essential. According to the invention apparatusforremovingthesettledsludgefrom a body of liquid including that sludge comprises a conduit having an upward-facing outlet; means to cause the liquid to flow through the conduittowardsthe outlet; a hollow tubular closure member mounted above the outlet for vertical movement between a raised open position and a lowered closed position in which it engages with the outlet; detector means responsiveto a parameter indicative of change in consistency of the liquid leaving the outlet, and operating means responsive to those detector means responsive to those detector means to move the closure memberfrom its open to its closed position.

The detector means may respond to a change in the pattern of the discharge of the liquid from the out let.

The detector means may be mounted on the closure member.

The detector means may alternatively be located remotely from the outlet and may respond to a liquid level change which occurs within the apparatus when the consistency of liquid leaving the outlet changes. The detector means may include a liquid sensor probe.

As yetanotheralternative the detector means may be in the form of a liquid pressure sensor and may be located in the conduit at a convenient location where pressure change reflects change in consistency of the liquid leaving the outlet.

The conduit outlet may be flared and the closure member of circular section, and substantially annular, line contact may be made between the circular bottom edge ofthe closure member and the surface oftheflared outlet.

The invention also includes plant including an operating member which engages periodically with the operating means to raise the closure member to its open position. The apparatus may operate periodic allyto discharge settled sludge from a sedimentation tank, and the operating means may be a rotarycom ponent-for instance, a sludge scraper - associated with the tank. The operating means may include a lever, one end of which is attached to the closure member and the opposite end of which is depressed once per revolution of the rotary component, so raising the closure member.

The conduit may contain valve means operable to regulate the rate of flow of liquid through the conduit and thus the pattern of discharge of that liquid through the outlet when open.

The invention will now be described, by way of example, with reference to the accompanying drawings in which :- Figure 1 shows a known sedimentation tank and its associated desludging chamber in vertical section in outline only; Figure2 is a vertical section through a desludging chamber including apparatus according to the invention, with the valve member in the closed position; Figure 3 is similar to Figure 2 but shows the valve member in the open position, and Figure 4 is an outline plan view of parts of the apparatus shown in Figures 1,2 and 3.

The sedimentation tank shown in outline in Figure 1 is typical of manyto be found in sewagetreatment works. Thetank is defined by a circular side wall 1,an annular and sloping base 2 and a central recess or pit 3. The crest 4 of wall 1 serves also as a weirwhich stabilisestheworking level 5 of liquid within thetank and serves as the innerwall of an annulartrough 6 having an outer wall 7 and an outlet8 bywhich liquid which has overflowed may escape. Liquid sewage enters the tank from preiiminarystagesofthetreat- mont process (indicated diagrammatically at 9) by way of a central vertical pipe 10, and solid sludge 11 settles out by gravity into the pit 3 and onto the sloping base surface 2.Periodically a scraper 12, carried by a bridge 13 rotatable about a central pillar 14 and supported by a roller 15 running on wall 7, is rotated to dislodge sludge from the base 2 so that all the settled sludge is concentrated towards the pit 3.

A conduit 16 leads from the pit 3 into an adjacent desludging chamber 17 in which it forks into two limbs 18 and 19. Limb 18 is a vertical vent stack, the top of which lies above the working liquid level 5.

Limb 19 is the discharge limb and contains a valve 20 which could be of electrical orotherautomaticallyoperating type but which, as shown, is a manual valve operated by a handwheel 21. The open end of limb 19 constitutes an outlet22.

To remove settled sludge 11 from pit 3 the valve 20 is opened. After an initial discharge of relatively thin liquid left in conduit 16 and limbs 18 and 1 9 since the last desludging operating, the gravity head represented by the vertical distance of level 5 about outlet 22 will then drive thick settled sludge 11 from pit3 through conduit 16 and limb 19 out into chamber 17.

The operatorthen chooses when to closethe valve 20 again, so stopping the desludging operation, by referenceto two factors: firstly a thinner appearance of the liquid leaving outlet 22, and secondly an increase in the distance to which the liquid is projected clear as it emerges from the outlet 22, because the thinning ofthofluid results in less energy loss due to friction betweenthefluid andthewalls of conduit 16 and limb 19.

In apparatus according to the present invention, as shown in Figures 2 to 4, the tank is as before, as are the chamber 17, conduit 16 and stack 18. However the valve 20 is ommitedfrom limb 19 and the outlet 22 now comprises an upward-facing flared mouth 25 above which an open-ended tubular closure member 26 is supported by a rod 27 the lower end of which is pivotted to a cross-pin 28 which spans the tube. The upper end of rod 27 ispivotted at 29 to one end of a beam 30, which pivots about a fulcrum 31 supported bya pillar32 mounted on a platform 33 which isfixed to the wall 7. A roller 34 is carried on the other end of beam 30.

As Figure 2 shows, detector means in the form of a liquid sensor 36 of known type such as the Model AC1 /L level controller for conducting liquids, as sold by Hawker Electronics Ltd., Kings Norton, Birmingham, is mounted on a bracket 37 carried by member 26, the active end of the sensor being located close to but a predetermined distance from the outlet 22.

Figure 2 shows outlet 22 closed by member 26,the circular bottom edge 24 of member 26 making a circular, line seal against the inner surface of the flare 25. This is the position which the member 26 tends to take up by gravity, whenever the roller34 is free to rise and the beam 30 is therefore free to pivot anticlockwise about fulcrum 31. As will be explained, member 26 is in fact in this position from the end of one desludging operation to the beginning of the next. As soon as member 26 closes, the liquid levels 38 and 39 within member 26 and stack 18 respectively rise to about the common height at which they are shown in Figure 2.This height shown as equal to that oftheworking level 5 within the tank, but it should be appreciated that static resistance to flow in the pipework of the system may result in some inequality between levels 38,39 and 5. It should also be noted particularly that because member 26 is an open-ended hollow tube the liquid within limb 19 exercises negligible upward thrust upon the member, so allowing the mass ofthe member alone to create an adequate seal between edge 24 and flare 25.

A desludging operation starts when the bridge 13, which carries a cam 35, is rotated around the tank until it reaches the position where the cam bears on roller 34, pivotting the beam 30 about fulcrum 31 so that member 26 is lifted. The first effect that follows is that the column of liquid previously standing within member 26 discharges, the liquid gushing out through the gap 41 now presented between 24 and 25: similarly,thesurfaceofthecolumnstanding within stack 18 falls to height40, substantially level with mouth 22, and this liquid reaches the gap 41 by way of limb 19 and also gushes out.Because this liquid will be relatively thin and subject to a pressure head represented by the difference between levels 5 and 40, it will be projected from gap 41 with a substantial trajectory and so will make contact with the working end of sensor 36. The control system ofthe sensor (indicated diagrammatically at 42) is however programmed to create a time delay between the lifting of member 26 and the activation of sensor 36,this delay being long enough to cover the short initial period when thin liquid is discharging. Once that period is over, the head of liquid within the tank drivesthicksludge 11 from the pit3through conduit 16 and limb 19 so that it also discharges into chamber 17 through gap 41.Howeverdueto effects including the high viscosity and density of the sludge, and high friction between that sludge and the conduit wall, the trajectory of the sludge as it escapes through gap 41 will be less greatthan that ofthethinnerfluid pre- viously. Therefore, although the sensor 36 is now activated, the thick sludge will not reach the tip of the sensor. However as the escaping liquid thins down progressively, as it will once most of the sludge has been expelled from pit 11 and some thinner liquid from thetank is ableto enterthe conduit 16 also, the combined effects of lower density/viscosity and lower friction losses cause an increase in thetrajectory of the escaping liquid so that it again makes contactwith sensor 36.The causes the sensor control mechanism 42 to reactivate the motor 43 which drives the scraper 12 and bridge 13, so rotating the bridge 13through another small angle of are 44to a " park" position. This moves cam 35 out of contact with roller34, so that member 26 falls to close the gap 41 and make a seal again between edge 24 and flare 25.

As well as showing the closure member26 in the open position instead of the closed one, Figure 3 also shows an alternative to the arrangement of Figure 2 in thatthe sensor 36, instead of being mounted on member 26, is suspended within stack 18 from a bracket 45. Such an arrangement has the advantages of providing a more sheltered situation for the sensor, a mode of operation in which there is less relative motion between sensor and liquid when the sensor operates to end a desludging operation, and more constant contact between the sensor and liquid thus avoiding malfunction due to repeated wetting and drying of the sensor. As has already been explained with reference to Figure 2 liquid within stack 18 stands at level 39 between desludging operations and falls to around level 40 shortly after each such operation begins. Then, when thick sludge begins to movethrough conduit 16, and limb l9towardsthe outlet, the effect of the losses due to density/viscosity and friction effects within the conduit will typically be to depress the level in stack 18 a little further, say to 46. The working tip of sensor 36 should therefore be located above level 46, so that the sensor is "triggered" asthe level within stack 18 rises again from its lowest level, which it does once the consistency of the liquid escaping through outlet 22 starts to thin down. The best heig ht for the working tip of the sensor 36 relative to level 40 may or may not be sub stantially as shown in Figure 3, for itwill in practice depend on a variety of working characteristics particularto each individual installation.

Alternatives to the liquid sensor 36 shown in full lines in Figures 2 and 3 are of course possible. For example, the change in consistency of liquid leaving the outlet 22 is reflected, as has already been described, in changes in the liquid pressure within the system. Such changes could, for instance, be detected by a pressure transducer as indicated schematicallyat49, located in the region of the fork of limbs 18 and 19 and connected (as indicated at 50) to the control mechanism 42.

The rate of flow of liquid within conduit 16 and limb 19, and hence the trajectory of discharge at outlet 22 and the rate at which level 46 rises as the consistency of the liquid changes, may if desired be regulated to some extent by a variable-orifice valve 47, the setting of which can be varied by operation of a handwheel control 48.

Claims (11)

1. Apparatus for removing the settled sludge from a bodyofliquid includingthatsludge,compris- ing: a conduit having an upward-facing outlet; meanstocausetheliquidtoflowthroughthecon- duit towards the outlet; a hollowtubularclosure member mounted above the outlet for vertical movement between a raised open position and a lowered closed position in which it engages with the outlet; detector means responsive to a parameter indicative of change in consistency of the liquid leaving the outlet, and operating means responsive to the detector means to move the closure member from its open to its closed position.

2. Apparatus according to Claim 1 inwhichthe detector means responds to a change in the pattern of the discharge of the liquid from the outlet.

3. Apparatus according to Claim 2 in which the detector means is mounted on the closure member.

4. Apparatus according to Claim 1 inwhichtho detector means is located remotely from the outlet and responds to a liquid level change which occurs within the apparatus when the consistency of liquid leaving the outlet changes.

5. Apparatus according to Claim 1 in which the detector means includes a liquid sensor probe.

6. Apparatus according to Claim 1 inwhichthe detector means is a liquid-pressure sensor located in the conduit.

7. Apparatus according to Claim 1 in which the conduit outlet is flared, in which the closure member is of circular section, and in which substantially an nular, line contact is made between the circular bottom edge of the closure member and the surface oftheflared outlet.

8. A liquid treatment plant including apparatus according to Claim 2, in which the plant includes an operating memberwhich engages periodically with the operating means to raise the closure member to its open position.

9. A liquid treatment plant according to Claim 8 in which the apparatus operates periodicallyto dis chargesettledsludgefrom a sedimentation tank, and in which the operating means is a rotary componentfor instance, a sludge scraper - associated with the tank.

10. A sewage treatment plant according to Claim 9 in which the operating means includes a lever, one end of which is attached to the closure member and the opposite end of which is depressed once per revolution of the rotary component, so raising the closure member.

11. Apparatus according to Claim 1 in which the conduit contains valve means operable to regulate the rate offlow of liquid through the conduit and thus the pattern of discharge ofthat liquid through the outlet when open.