GB2032794A - A method of dewatering liquid flocculant sludges and apparatus for carrying out the method - Google Patents

Abstract

The apparatus comprises a chamber (1) having an impervious bottom (3). Liquid flocculant sludge is dewatered in a settling chamber (1) with an inlet 5 and two water-pervious walls (4, 4') cutting off two regions adjacent the cylindrical wall of the chamber. Liquid drains laterally from the sludge through walls (4, 4') at a restricted rate, so that the thixotropic sludge remains too stiff to follow. Vibrator (7) may assist separation. <IMAGE>

Description

SPECIFICATION A method of dewatering liquid flocculent sludges and apparatus for carrying out the method The present invention relates to a method of dewatering liquid flocculent sludge and to apparatus for carrying out the method.

In water treatment plants, the free spilling of the liquid flocculent sludges with a predominating horizontal component of motion when the sludge water escapes causes the sludges to thicken and thus stops the free flow of the sludges and forms a dewatered layer of solid sludge.

The handling of liquid flocculent sludges during water treatment represents an important part of the costs for the overall treatment of water. With respect to the relatively low concentration of these sludges and, in the case of sludges of biological origin, with respect to a frequent biological instability it is necessark to dewaterthese sludges to a solid state enabling their easy handling and transport and/or enabling a restriction of undesired biological processes.

Currently, the most frequently used method of dewatering liquid flocculent sludges is by dewatering in sludge fields. Basically, a sludge field is formed by a flat reservoir with a water-pervious subgrade. The dewatering of the liquid flocculent sludge is effected by the seeping of the sludge water through this subgrade while the solid sludge remains in a layer on this subgrade. The main disadvantage of the sludge fields lies in a noticeable decrease of vertical water-perviousness and by a growth of the thickness of solid sludge and thus a maximum thickness of the layer of solid sludge is very limited. This method calls for considerably extensive areas of sludge fields, making the mechanical removal of solid sludge difficult which leads to functional defects during the winter season.

Other frequently used methods of dewatering liquid flocculent sludges are by the use of centrifuges or pressure band filters. Both these methods have disadvantages due to high investment and operational costs, the high investment costs being due to the complicated mechanical apparatus required and by their considerable size as a result of the mechanical properties of the sludge to be handled. The high operation costs are also due to a high consumption of energy and often due to a considerable consumption of polycoagulants necessary for a change of the mechanical properties of the sludge to enable the apparatus to handle it as well as due to high demands on experienced operators. Because of these factors, such apparatus are almost inapplicable to smaller water treatment plants.

According to one aspect of the invention there is provided a method of dewatering liquid flocculent sludges formed during water treatment by their free spilling with a predominating horizontal component of motion. when water escapes from them causing the sludges to thicken and thus stops the free flow of the sludges to form a dewatered layer of solid sludge, the improvement is to derive, due to the pressure of depositing layers of the sludge, a thixotropic flow of the dewatered lower layers while this flow is held mechanically behind the limit of free flow of liquid sludge while allowing the sludge water to flow behind this limit.

According to another aspect of the invention there is provided an apparatus for carrying out the method set forth in the preceding paragraph comprising a chamber having an impervious bottom which is surrounded at least around a part of its circumference, by water-pervious walls and an inlet for liquid flocculent sludge arranged above the impervious bottom.

Preferably, the individual strata of dewatered sludge overlaps immediately only partially the previously formed strata of dewatered sludge and leaves the remaining part with free access to the air, and/or the actuation of mechanical vibrations is interrupted.

Conveniently, the rate of water drain in the region of the mechanical adjustment of thixotropic flow of dewatered sludge is smaller than 10 m3 a day per 1 m of the horizontal width across the water flow.

Preferably to reduce space the inlet is arranged at the maximum horizontal distance from the waterpervious walls and when the bottom is of circular formation the inlet is arranged above its centre. The bottom and the walls are preferably surrounded with an impervious jacket with a roof above, while between the water-pervious walls and the jacket collectors for the sludge water are provided for emptying into the bottom.

Conveniently a vibrator is located below the inlet the vibrator being mounted on vertically arranged guides.

The lower part of the jacket is preferably provided with at least one evacuating hole.

An embodiment of the apparatus according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a diagrammatic vertical axialsection of the apparatus according to the invention, and, Figure 2 is a plan view of the apparatus shown in Figure 1.

The apparatus comprises a tank with a jacket 2 of cylindrical formation with an impervious bottom 1 and two water-pervious walls 4 and 4' extending the height of the tank. An inlet 5 is led into the upper part of tank 1 above the level of the water-pervious walls 4 and 4' and is preferably located at the maximum distance from the water-pervious walls 4 and 4'.

Under the inlet 5 is a downflow rod 6 for guiding the sludge while in the tank 1, close to the inlet 5 for the sludge, is located a guide 71 with a mechanical vibrator 7. In the bottom 3, between the walls 4 and 4' and the jacket 2 are installed collectors8 for the sludge water, these collectors being connected by piping 9 to the sump (not shown) for the raw water of a water treatment plant. Adjacent the bottom 3 of the tank 1 in the jacket 2 is formed an evacuating hole 10 and the tank 1 is closed by roof 11.

The dewatering method is based on the biological properties of liquid flocculent sludges. Namely, these sludges are formed by a suspension of sludge floccules in water and the whole behaves as Bing ham's liquid which starts to flow only on reaching the spray tension of flow. This spray tension of flow grows with the increase in the concentration of the suspension. Moreover, the liquid exhibits the property of thixotropy, i.e. a property of a colloid when the spray tension of flow as against the spray tension of free flow is decreased by pressure or vibration. Water contained in the sludge behaves as an independent Newton's liquid in a porous medium, this means that it flows through the sludge.Therefore, during the free flow of sludge, water flows out from the sludge across the face of the flowing sludge stream whereby it increases the sludge concentration until during the flow of sludge the spray tension will drop to the spray tension of flow whereby it halts the sludge flow and creates a layer of solid sludge.

From the layer of solid sludge, sludge water will continue to flow out very slowly.

By overlapping the individual layers of solid sludge across each other there is formed in succession a conical formation. With a sufficient size of the cone the pressure on the lower layers is already of such a value that it causes a thixotropic decrease of the spray tension of flow of these layers so that it flows to the sides thus forming a thixotropic spreading of the cone. This thixotropic spreading can be supported by a further decrease of the spray tension of flow of the dewatered sludge by causing vibrations. The thixotropic spreading of dewatered sludge can be mechanically stopped by the actuation of a suitable barrier to the flow, e.g. by the provision of walls which are water-pervious. The mechanical barrier of thixotropic spreading enables the creation of thick layers of dewatered sludge.Sludge water flowing out from the sludge under dewatering must incidentally flow behind the limit of the mechanical bar rierofthixotropic spreading of dewatered sludge.

Thus the barrier must be the form of the waterpervious walls 4 and 4' causing a mechanical barrier of thixotropic spreading of dewatered sludge and must be within a definite part water-pervious from the bottom as far as the upper limit of sludge filling.

With respect to this water-perviousness the mechanical barrier to the thixotropic spreading of dewatered sludge must be effected only behind the boundary where a limit of free flow of liquid sludge is reached, because otherwise the non-dewatered liquid sludge would flow through the water-pervious barrier to the thixotropic flow.

In normal liquid flocculent sludges it was experi mentally ascertained that for securing this condition it is sufficient that the rate of water in the place of the mechanical barrier to the thixotropic flow of dewatered sludge be smaller than 10 m3 a day per 1 m of the width across the water flow. Flocculent sludge is brought in through the inlet 5 while the down-flow rod 6 serves for guiding the inflowing sludge as far as the sludge level in the tank 1. With sludges having a high limit of thixotropic flow, it is impossible to use the mechanical vibrator 7 to make the sedimented sludge liquid again. This vibrator can be mounted in the guide 71 or alternatively arranged vertically in a bridgeable way.

The application of the method and apparatus of the present invention is especially suitable for the thickening of well stabilized biological sludges during a biological treatment of water and for thickening lime sludges during a chemical treatment of water. It is possible to achieve satisfactory results even in cases of other kinds of flocculent sludges, e.g. when chemically cleaning sludges with the use of hydrolyzing coagulents.

The method of dewatering liquid biological sludges and the apparatus for its performance by the invention offer many advantages. In comparison with sludge fields, they represent a substantial saving in area, the easy mechanical removal of solid sludge, and a possibility of an easy limitation of drawbacks caused during the winter season. These advantages follow from the possibility to create a thick layer of dewatered sludge and from the mechanical resistance of a firm subgrade. Compared with conventional centrifuges and pressure band filters the present invention offers substantially lower investment and operational costs; the lower investment costs resulting from the simplicity of the apparatus of the invention; the lower operational costs follow from the fact that the dewatering process does not require energy or polycoagulants and the apparatus requires the minimum of supervision by the operator.

Claims (15)

1. A method of dewatering liquid flocculent sludges formed during water treatment, by their free spilling with a predominating horizontal component of motion, when water escapes from them causing the sludges to thicken and thus stops the free flow of the sludges to form a dewatered layer of solid sludge, the improvement is to derive, due to the pressure of depositing layers of the sludge, a thixotropic flow of the dewatered lower layers while this flow is held mechanically behind the limit of free flow of liquid sludge while allowing the sludge water to flow behind this limit.

2. A method according to claim 1 in which the individual layers of dewatered sludge overlap immediately only partially the layers of dewatered sludge previously formed and thus leave the remaining part with free access to the air.

3. A method according to claim 1 in which the flocculent sludge after spilling is subjected to mechanical vibrations.

4. A method according to claim 3 in which the mechanical vibrations are interrupted.

5. A method according to claim 1 in which the rate of the water removal in the region of the mechanical barrier of the thixotropic flow of dewatered sludge is smaller than 10 m3 a day per 1 m of horizontal width across the water flow.

6. An apparatus for carrying out the method according to claim 1 comprising a chamber having an impervious bottom which is surrounded at least around a part of its circumference, by water pervious walls and an inlet for liquid flocculent sludge arranged above the impervious bottom.

7. An apparatus according to claim 6 in which the inlet is arranged at the maximum horizontal distance from the water-pervious walls.

8. An apparatus according to claim 6 or 7 in which the impervious bottom is circular, the inlet being arranged above the centre thereof.

9. An apparatus according to any of claims 6 to 8 in which the impervious bottom and the waterpervious walls are surrounded by an impervious jacketthe apparatus being covered by a roof.

10. An apparatus according to claim 9 in which collectors for sludge water emptying on the bottom are arranged between the water-impervious walls and the jacket

11. An apparatus according to any of claims 6 to 9 in which a vibrator is located below the inlet.

12. An apparatus according to claim 11 in which the vibrator is mounted on vertically arranged guides.

13. An apparatus according to claim 9 in which at least one evacuating hole is provided in the lower part of the jacket.

14. A method of dewatering liquid flocculent sludges substantially as herein described with reference to the accompanying drawings.

15. An apparatus according to claim 6 substantially as herein described with reference to the accompanying drawings.