GB2359807A

GB2359807A - Sewage sludge dewatering plant

Info

Publication number: GB2359807A
Application number: GB9923244A
Authority: GB
Inventors: Dennis Killen
Original assignee: WESTECH PROJECTS Ltd
Current assignee: WESTECH PROJECTS Ltd
Priority date: 1999-10-02
Filing date: 1999-10-02
Publication date: 2001-09-05
Also published as: GB9923244D0

Description

1 2359807 1 Westech Projects Limited SLUDGE DEWATERING PLANT

TECHNICAL FIELD OF THE INVENTION

This invention relates to the dewatering of sludge, particularly sewage sludge.

BACKGROUND

When waste water is treated biologically in sewage plants the result is a suspension of solids in water, i.e. sludge. Generally such sludge comprises about 5% solids, but by passing it through centrifuges the water content can be reduced to bring the solids content to about 30%. Ideally the water content is reduced still further to achieve a solid content of around 95%. The conventional way of doing this is to heat the sludge under conditions which cause the water to boil off leaving the solids behind. However, the sludge has a tendency to coagulate and retain the water so that the process uses a considerable amount of energy and is very inefficient.

The present invention seeks to provide a new and inventive sludge dewatering process.

2 SUMMARY OF THE INVENTION

The present invention proposes sludge dewatering plant comprising: - a low pressure chamber having an inlet for sludge; - means for drawing water vapour from the chamber and thereby creating a reduced pressure therein; and - means for removing dewatered solids from the chamber.

The sludge is preferably dispersed within the chamber, and is preferably deposited on a surface in a thin layer. The chamber may include means for removing the dewatered solids from the said surface.

The solids are preferably removed from the chamber by a screw conveyor which is filled with the solids to maintain a pressure seal.

The water vapour is preferably drawn from the chamber through a barometric condenser. A cyclone may be interposed between the chamber and the condenser to separate fine particles of dried material from the water vapour.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings:

3 Figure 1 is a general layout of sludge dewatering plant in accordance with the invention, Figure 2 is a vertical section through one form of vacuum chamber for use therein, and Figure 3 is a similar vertical section through an alternative form of vacuum chamber.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to Fig. 1, sludge comprising about 30% solids in water is fed from existing plant through pipe 1 into a holding vessel 2. An outlet conduit 3 carries sludge from the bottom of the holding vessel to a sludge pump 4, e.g. a progressive cavity mono pump, a peristaltic pump, or any other form of pump suitable for pumping slurry. The output from the pump 4 goes via a conduit 5 to a vacuum chamber 6, and under certain operating conditions it may sometimes be an advantage to pass the sludge through a heat exchanger 7 to raise the temperature of the slurry, but this is not always necessary.

Although the term "vacuum chamber" is used for convenience it should be appreciated that the chamber is held below atmospheric pressure (i.e. not a true vacuum), typically between 40 and 200 millibars, so that water in the sludge will evaporate at a substantially reduced temperature. With this object, water vapour is drawn through a vacuum take-off 8 at the top of the 4 chamber 6, into a cyclone type dust extractor 9. Water vapour is in turn fed upwards from the top of the cyclone 9 through a pipe 10 into a condenser (not shown) located at barometric height. The condenser is of the direct contact type. Water is sprayed into the condenser causing water vapour to condense at any pressure above the vapour pressure for the temperature at which condensate leaves the condenser. Any dust removed by the cyclonic dust extractor 9 is collected at the bottom of the extractor by a catchpot 11. The dust can be carried away by a conveyor 12 for storage or disposal.

The weight of dewatered solids remaining in the vacuum chamber 6 after removal of the water vapour causes them to pass into a screw extruder 13 mounted beneath the chamber 6. The speed of the extruder is adjusted such that the solids are continuously forced through an outlet nozzle 14 thus acting as a seal to prevent air from leaking back into the chamber 6 via the extruder. Solids leaving the nozzle fall onto the conveyor 12 and are also carried away.

Fig. 2 shows one possible form of the vacuum chamber 6. The chamber comprises a cylindrical wall portion 20 closed by a top wall 21 and having a part-conical portion 22 at its lower end. Sludge feed from conduit 5 enters the chamber near a tubular shaft 23 which is mounted in bearings 24. The shaft 23 is coupled to a drive 25 and passes down through the centre of the top wall 21 to carry a scraper blade 26. The blade is supported by standoff arms 27 such that upon rotation of the shaft 23 the blade moves over the interior surface of the cylinder 20 and removes any solids which adhere thereto. By way of example, the scraper typically rotates 20 to 25 timer per minute. An inner shaft 28 passes rotatably down the centre of the shaft 23 which carries a horizontal spreader disc 29 at its lower end. The shaft 28 is coupled to a sprocket or other drive means 30 at its upper end so that the disc 29 is typically spun at 1500 to 3000 revolutions per minute.

Slurry enters the chamber 6 and falls onto the rotating disc 29 which fragments it into small particles from which water quickly evaporates under the reduced pressure. The particles are flung against the walls of the chamber where they complete the drying out process and are removed by the scraper 26, eventually passing out of the chamber via the extruder 13.

An alternative form of vacuum chamber shown in Fig. 3. The chamber again comprises a cylindrical wall 20 and a top wall 21 with a part-conical portion 22 at the bottom. However, an inner part-conical wall 36 is mounted in ring bearings 37 and rotatably driven by ring gears 38 to rotate within the conical portion 22, typically at 20 to 25 timer per minute. An adjustable scraper 39 is mounted just clear of the inside of cone 36 fixed with the chamber 6. Sludge from conduit 5 enters the chamber 6 and is sprayed into the inside of the rotating cone 36. The spraying can help to break up the sludge into droplets, but in any event, deposition of the sludge in a thin layer over the inside of the cone 36 ensures that water quickly evaporates under the reduced pressure. Again, the scraper removes the dried material which is removed by the conveyor 13.

Since the water is removed at a relatively low temperature considerably less energy is required and the speed of the dewatering process is greatly increased. Moreover, the water and dried material are both continuously 6 removed from the equipment in a way which maintains a constant low pressure within the chamber 6.

Claims

It will be appreciated that the features disclosed herein may be present

in any feasible combination. Whilst the above description lays emphasis on those areas which, in combination, are believed to be new, protection is claimed for any inventive combination of the features disclosed herein.

7 CLAIMS Sludge dewatering plant which includes: a low pressure chamber having an inlet for sludge; means for drawing water vapour from the chamber to create a reduced pressure therein; and - means for removing dewatered solids from the chamber.

2. Sludge dewatering plant according to Claim 1, which is arranged to break up and disperse the sludge upon entry into the chamber.

3. Sludge dewatering plant according to Claim 2, in which the sludge is dispersed by a spray nozzle.

4. Sludge dewatering plant according to Claim 2, in which the sludge is dispersed by a rotating spreader.

5. Sludge dewatering plant according to Claim 2, 3 or 4, which is arranged to deposit the dispersed sludge on a evaporation surface in a thin layer.

6. Sludge dewatering plant according to Claim 5, in which the chamber includes means for removing the dewatered solids from the evaporation surface.

8 7. Sludge dewatering plant according to Claim 6, In which said means for removing the dewatered solids includes a scraper blade.

8. Sludge dewatering plant according to Claim 7, in which the evaporation surface is disposed around the periphery of the chamber and the dawatered solids are removed by relative rotation between the evaporation surface and the scraper blade.

9. Sludge dewatering plant according to Claim 8, in which the evaporation surface is fixed and the scraper blade rotates.

10. Sludge dewatering plant according to Claim 8, in which the scraper blade is fixed and the evaporation surface rotates.

11. Sludge dewatering plant according to any preceding claim, in which the solids are removed from the chamber by a screw conveyor which is arranged to be continuously filled with the solids to maintain a pressure seal.

12. Sludge dewatering plant according to any preceding claim, in which the water vapour is drawn from the chamber through a barometric condenser.

13. Sludge dewatering plant according to any preceding claim, in which the a cyclone is interposed between the chamber and the condenser to separate particles of dried material from the water vapour passing therethrough.

0 so 9 14. A method of dewatering sludge in which the sludge is dispersed and deposited in a layer on an evaporation surface inside a chamber, water vapour is removed from the chamber to maintain a low pressure therein and promote the evaporation of water from the deposited material, and dewatered material is scraped off the evaporation surface and removed from the chamber.

15. Sludge dewatering plant substantially as described with reference to the drawings.

16. A method of dewatering sludge substantially as described with reference to the drawings.