GB2157584A

GB2157584A - Removing sludge from settling tank

Info

Publication number: GB2157584A
Application number: GB08509024A
Authority: GB
Inventors: Colin Edward Brade
Original assignee: Rosewater Engineering Ltd
Current assignee: Rosewater Engineering Ltd
Priority date: 1984-04-14
Filing date: 1985-04-09
Publication date: 1985-10-30
Also published as: GB8509024D0; GB2157584B

Abstract

Settled sludge 20 is drawn from the bottom of a settling tank 10 by a pump 18, which is controlled by pressure sensing means 26, which effectively monitors the viscosity, and therefore the solids content, of sludge passing through pipe 16, and either stops the pump or diverts the sludge flow when the viscosity falls to a predetermined level. If the level in tank 10 is constant, then a single pressure sensor 26 is sufficient; otherwise two sensors monitor the pressure drop along pipe 16. Only the peak pressure is monitored if pressure varies over the pump cycle. Timers may control the restarting of the pump after it has been stopped. <IMAGE>

Description

SPECIFICATION Improved separating method and apparatus The present invention concerns improvements in or relating to a method and apparatus for separating. It is especially, but not exclusively, applicable to the removal of sludge from a sewage sedimentation tank but is also applicable, for example, to the separation of a first slurry or sludge from a second less viscous slurry or sludge. In the following specification, the term "sludge" will be used to define all sludges, slurries and solids containing liquids.

Most sewage treatment works use a sedimentation process at one stage in the purification of sewage. The process is a physical one in which settleable solids, which have been held in suspension in the sewage up to that stage, are allowed to settle in quiescent conditions whereupon they form a sludge blanket in the base of a sedimentation tank. This sludge contains about 90% water and is normally removed manually using hydrostatic head and allowed to flow by gravity down drains for subsequent treatment, for example, in an anaerobic digestion tank to which it is pumped after storage in a holding tank receiving the discharge from the drains.

Attempts have been made to mechanise this manual process, such attempts employing various devices such as air lifts, automatic valves, level sensors, flow meters and remote T.V. systems.

None has been particularly satisfactory.

In an attempt to avoid some of the disadvantages presented by the prior methods a proposal has been made to pump the sludge only for a predetermined time at intervals which are predetermined by the operator.

The success of a method of this nature depends almost entirely on the nature and flow of the sewage supplied to the sedimentation tank. In practical conditions these are never constant so that the choice of pumping duration and the intervals between pumping cycles must be a compromise.

It is an object of the present invention to obviate or mitigate these and other disadvantages.

According to the present invention there is provided a method of removing sludge from a sedimentation tank comprising pumping the sludge from the tank and while pumping is being carried out sensing the pressure drop at the pump due to changing viscosity of the sludge and discontinuing pumping when a predetermined pressure drop or average pressure drop over a period is detected.

Preferably the level of material in the tank is maintained at a substantially constant level in which case only the pressure drop at the pump is measured.

Alternatively if the level of material in the tank decreases on removal of sludge therefrom the pressure at the tank outlet is measured and subtracted from the pressure drop at the pump to provide a pressure drop reading due to reducing sludge viscosity.

Preferably where, in operation, the pressure drop fluctuates during the pumping operation only the peak pressure drop is sensed.

Preferably the peak pressure drop is detected by sensing the condition of the pump giving rise to the peak pressure drop by use, for example, of a proximity switch. Alternatively the peak pressure drop can be detected by electronic means from the pressure drop being sensed.

Preferably the method includes also an initial pumping step controlled by a timer during which the supply conduit between the pump and the sedimentation tank is purged. The method may include also recommencing the pumping operation after stoppage thereof on detection of the said predetermined pressure drop after a preselected interval of time.

Further according to the present invention there is provided apparatus for removing sludge from a sedimentation tank including an outlet pipe leading from the tank, a pump interposed in said outlet pipe, a pressure sensor at the inlet to the pump and control means operable with the pressure sensor whereby upon detecting a predetermined pressure drop in the pipe the control means cause the pump to stop.

A further pressure sensor may be provided at the tank outlet so that where the level of material in the tank is not constant by substracting the pressure read by the further sensor from the pressure read by the sensor at the pump the pressure drop due to changing viscosity can be determined.

Preferably a level sensor is provided in the tank to prevent operation of the pump if the tank level falls below a predetermined value.

Preferably means are provided for detecting the peak pressure drop sensed by the sensor if this fluctuates during operation. These means may comprise a proximity switch operable in conjunction with the pump or electronic detecting means for detecting the maximum pressure drop signalled by the pressure sensor.

Preferably first timer means are provided to actuate the pump after a predetermined delay following a prior pumping operation has elapsed.Additionally second timer means may be provided to allow pumping to continue for an introductory period so that the inlet pipe may be purged of its contents prior to the pumping of sludge from the sedimentation tank.

Preferably the discharge from the pump is connected to an anaerobic digestion tank. A pipe may be connected from the digestion tank to a circulating pump discharging into the connection between the said pump and the digestion tank.

The pump may be a reciprocating pump. In one alternative arrangement it may be an air lift pump.

According to a further aspect of the present invention there is provided a method of separating a first sludge from a second thinner sludge which lie in layers in a storage tank comprising pumping the sludge from the tank and while pumping is being carried out sensing the pressure drop at the pump and, on determining a predetermined pressure drop or rise, actuating diverting means to discharge the thick and relatively thinner sludge by different outlets.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, in which Figure 1 shows diagramatically and in elevation a sludge settlement tank and discharge pump assembly; and Figure 2 shows diagrammatically and in plan a sludge settlement tank and digester assembly.

Figure 1 shows a sludge settlement tank 10 having a base 12 which converges towards the outlet 14 from which extends a pump suction pipe 16 which has a pump console 18 interposed therein.

Settled sludge in the tank accumulates in the lower portion of the tank 10 to form a layer 20, the sludge settling from sewage fed to the tank by way of an inlet pipe 22, clarified sewage escaping over the edge of the tank.

Sludges are generally viscous in nature especially at high solid content and when stationary or flowing under laminar conditions. Viscosity is often related to solids content. If viscous sludge is pumped through a pipe the pressure drop due to the viscous nature of the sludge during laminar flow pumping conditions can be mathematically related to the sludge viscosity and, in many sludges, the solid content of the sludge. In the present apparatus a pressure transducer 26 is fixed in the suction pipe 16 close to the pump 28 included in the pump console 18.

As described above, the sedimentation tank is supplied on a continuous basis and therefore has a constant level so that it is not necessary to measure the actual pressure drop along the pipe 16, only the pressure at the pump 28.

The pressure drop down the short length of pipe used in the present apparatus is low since a large diameter pipe has to be used in order to prevent blockages. Thus a very accurate pressure transducer is used and in the present embodiment the sensitivity of the transducer is enhanced by using only the peak pressure drop during each pump cycle.

All pumps suitable for pumping sludge will tend to have a positive displacement action at low speeds and this results in large variations in sludge velocity during the pump cycle. The peak can be sensed by means of a proximity switch 24 situated at a suitable position on the pump 28 so that a specific operational condition of the pump can be sensed, or by electronic means reading the signal from the pressure sensor 26.

The pump console includes manual control means (not shown) for setting the minimum pressure level to be achieved before pumping is stopped together with a first timer means which starts the pump after a predetermined delay time has elapsed following from the previous pumping operation. A second timer means operates on receiving a signal from the first means to cause the pump to operate for a predetermined time to purge the suction pipe of the fluid between the pump 28 and the oulet 14 from the tank. After purging has taken place the pressure sensor is brought into operation as described above and when the predetermined peak pressure which indicates that sludge from the tank is beginning to thin below a desirable value, the pump is stopped until the first timer causes it to start again.

The method and apparatus described above has the advantage of reducing manpower, providing a thicker sludge for subsequent processing, eliminating blockage prone gravity pipes and increasing security of operation.

Various modifications can be made without departing from the scope of the invention. For example the pressure sensor could be located at a suitable point in the pump. The first and second timer means could readily be provided by one clock arrangement or by a computer software programme using direct digital control methods.

In a further modification the arrangement is used to separate a first sludge from a second relatively thinner sludge. It operates in a manner similar to that described above but rather than discontinuing pumping when a predetermined pressure drop is detected it operates a diverter valve so that thick sludge is pumped through one outlet and thinner sludge is pumped through another outlet by way of the diverter valve.

It will be realised that the method and apparatus of the present invention is advantageous where it is desired to produce sludges which are compacted.The lowermost layer of sludge in a tank is relatively compact as a result of the weight of material pressing down on it from above and this compact layer only can be removed by adjustment of the pressure drop to be sensed by the sensor so that pumping is stopped when the layer has been removed.

In a modification, rather than cause stoppage of the pump when a predetermined pressure drop is sensed, stoppage can be signalled by utilising the average pressure drop of a series of pressure readings taken in a predetermined time interval. In this modification and in other circumstances suitably programmed micro-processors can be utilised.

In a further modified operational method turbulent flow conditions may be experienced in the pipe leading from the tank. The method described above will still be applicable in these conditions, but with a change in the pressure to be sensed to cause pump deactuation.

In the method and apparatus described above, the level of material in the tank is maintained constant.lt may fluctuate and if so a further pressure sensor will be located at the tank outlet so that the pressure drop at the pump due to decreasing viscosity can be determined by subtracting from the pressure detected by the pump sensor the pressure detected by the further sensor.

in a batch process the further sensor may not be called for as a level sensor may be incorporated in the tank to deactuate operation of the apparatus when the tank level falls below a predetermined value.

In a still further modification the pump may be provided by an air lift arrangement in a vertical section of the discharge pipe. A pressure or level sensor in the pipe above the air lift detects the head in the downstream section of the pipe and a timer senses the recovery time to achieve the maxmum head after a discharge of sludge by the air lift. As the recovery time is proportional to the sludge thickness when the recovery time is less than a predetermined value the air lift is deactuated.

Fig. 2 shows a plan view of a settlement tank 10 and pump 18 which operates in any one of the manners described above. The discharge from the pump is fed by a pipe 30 to an anaerobic digester 32.A recirculating pipe 34 having an additional pump 36 interposed therein is connected between the digester 32 and the pipes 30 downstream of the pump 18.

This arrangement ensures without the use of a separate mixer that thick sludge being fed to the digester is well mixed with the thin sludge in the digester. It enables the reduction of the volume of the digester and gives better reaction kinetics. Additionally it tends to reduce blockages in pipe 30 as the material therein is thinned by the recirculated material which, in addition, is hot and bacteria bearing so that the breakup of fats and bacterial scouring in pipe 30 is assisted. Blockage problems can be further mitigated by causing turbulent flow in pipe 30.

Claims

1. method of removing sludge from a sedimentation tank comprising pumping the sludge from the tank and while pumping is being carried out sensing the pressure drop at the pump due to changing viscosity of the sludge and discontinuing pumping when a predetermined pressure drop or average pressure drop over a period is detected.

2. A method as claimed in claim 1, in which the level of material in the tank is maintained at a substantially constant level in which case only the pressure drop at the pump is measured.

3. A method as claimed in claim 1, in which as the level of material in the tank decreases on removal of sludge therefrom the pressure at the tank outlet is measured and subtracted from the pressure drop at the pump to provide a pressure drop reading due to reducing sludge viscosity.

4. A method as claimed in any one of claims 1 to 3, in which where, in operation, the pressure drop fluctuates during the pumping operation only the peak pressure drop is sensed.

5. A method as claimed in claim 4, in which the peak pressure drop is detected by sensing the condition of the pump giving rise to the peak pressure drop.

6. A method as claimed in claim 5, in which the peak pressure drop is measured by a proximity switch.

7. A method as claimed in claim 4, in which the peak pressure drop is detected by electronic means from the pressure drop being sensed.

8. A method as claimed in any one of the preceding claims including also an initial pumping step controlled by a timer during which the supply conduit between the pump and the sedimentation tank is purged.

9. A method as claimed in any one of the preceding claims including also recommencing the pumping operation after stoppage thereof on detection of the said predetermined pressure drop after a preselected interval of time.

10. Apparatus for removing sludge from a sedimentation tank including an outlet pipe leading from the tank, a pump interposed in said outlet pipe, a pressure sensor at the inlet to the pump and control means operable with the pressure sensor whereby upon detecting a predetermined pressure drop in the pipe due to changing viscosity of the sludge the control means cause the pump to stop.

11. Apparatus as claimed in claim 10, in which a further pressure sensor is provided at the tank outlet so that where the level of material in the tank is not constant by subtracting the pressure read by the further sensor from the pressure read by the sensor at the pump the pressure drop due to changing viscosity can be determined.

12. Apparatus as claimed in claim 10 or in claim 11, in which a level sensor is provided in the tank to prevent operation of the pump if the tank level falls below a predetermined value.

13. Apparatus as claimed in any one of claims 10 to 12, in which means are provided for detecting the peak pressure drop sensed by the sensor if this fluctuates during operation.

14. Apparatus as claimed in claim 13, in which the said means comprises a proximity switch operable in conjunction with the pump.

15. Apparatus as claimed in claim 13, in which the said means comprises electronic detecting means for detecting the maximum pressure drop signalled by the pressure sensor.

16. Apparatus as claimed in any one of claims 10 to 15, in which first timer means are provided to actuate the pump after a predetermined delay following a prior pumping operation has elapsed.

17. Apparatus as claimed in claim 16, in which second timer means are provided to allow pumping to continue for an introductory period so that the inlet pipe may be purged of its contents prior to the pumping of sludge from the sedimentation tank.

18. Apparatus as claimed in any one of claims 10 to 17, in which the discharge from the pump is connected to an anaerobic digestion tank.

19. Apparatus as claimed in claim 18, in which an additional pipe is connected from the digestion tank to a circulating pump discharging into the connection between the said pump and the digestion tank.

20. Apparatus as claimed in any one of claims 10 to 19, in which the pump is a reciprocating pump.

21. Apparatus as claimed in any one of claims 10 to 19, in which the pump is an air lift pump.

22. A method of separating a first sludge from a second thinner sludge which lie in layers in a storage tank comprising pumping the sludge from the tank and while pumping is being carried out sensing the pressure drop at the pump and, on determining a predetermined pressure drop or rise, actuating diverting means to discharge the thick and relatively thinner sludge by different outlets.

23. A method of removing sludge from a sedimentation tank substantially as hereinbefore described with reference to Fig 1 or Fig 2 of the accompanying drawings.

24. Apparatus for removing sludge from a sedimentation tank substantially as hereinbefore described with reference to the accompanying drawings.

25. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.