GB2275468A - Water treatment - Google Patents

Abstract

Sludge from water-treatment and containing aluminium hydroxide is frozen to precipitate its solids content in a plate freezer (2). The sludge is introduced between plates (16) of the plate freezer and frozen into slabs (6). The slabs are partially thawed so as to release the slabs from the plates and then discharged into a separate melting tank (4) containing warm water. The tank preferably includes a grid of heated pipes (10) onto which the slabs are discharged. The slabs are melted in the tank and yield granulated sludge solids, which are then removed and disposed of. <IMAGE>

Description

WATER TREATMENT The present invention relates to an apparatus and process for freezing and melting sludge, particularly sludge from a water treatment works.

During the treatment of water to produce drinking water, it is common practice in those areas where the water has a brown peaty colour to add aluminium sulphate thereto. This results in a precipitate of aluminium hydroxide colloidal sludge of high water content, which is then removed from the clarified water. However, the disposal of the sludge in an environmentally satisfactory manner poses problems.

The dumping of sludge is generally forbidden by environmental regulations. The sludge could be dehydrated and incinerated but this is expensive and incineration is environmentally unsatisfactory. The sludge could be pressed to remove water and used as låndfill but again this is expensive.

It is known that such sludges on freezing lose their coloidal nature and change into an insoluble precipitate.

When the sludge is frozen in a confined volume, the solids content precipitates out in the form of granules which may be removed and safely disposed of.

Apparatus for freezing sludge is known. The apparatus includes a single vessel into which the sludge is filled. The vessel contains a network of pipes which, during the freezing process are connected to the evaporator of a refrigeration apparatus. Once frozen,the sludge is then melted again by attaching the pipes to the condenser of the refrigeration apparatus. As the frozen sludge melts, the solids content collects at the bottom of the vessel in the form of granules. The apparatus has a number of disadvantages. Firstly, the freezing and melting processes take a long time due to the limited rate of heat conduction through the ice. Secondly, there is a persistent problem of bursting of the vessel due to the expansion forces of the solidifying ice, which needs to be frozen within a confined volume in order to operate the process satisfactorily.

Plate-type freezers wherein ice is frozen between parallel plates are known, and it is an object of the present invention to provide an apparatus of this type which is suitable for the treatment of colloidal sludges.

The present invention provides an apparatus for freezing sludge which comprises; - a plate freezer having an inlet for aqueous sludge, the freezer comprising a series of parallel plates between which the sludge is introduced, and freezing means for freezing the sludge into one or more slabs; - defrost means provided on the plate freezer for partially thawing the frozen slab(s) such as to release the slab(s) from the plates; - means for discharging the released slab(s) from between the plates; - tank means for receiving and melting the discharged slab(s) and comprising means for providing warm water therein, the slab(s) on melting yielding water and granulated sludge solids; and - means for removing the granulated sludge solids from the tank means.

The invention also relates to a corresponding process for treating sludge.

A particular feature of the invention is the use of a separate tank means for melting the slab(s) and giving good throughput.

The plate freezer is preferably in the form of a series of substantially upright parallel plates closed around the sides and bottom but having an open top for the discharge of the frozen slab(s). Biasing means, such as spring means, hydraulic or pneumatic means, may be provided to bias the plates towards one another, such that the plates are able to move outwards slightly to accommodate the expansion of the sludge as it freezes to ice.

The spacing between the parallel plates generally represents a compromise between the time taken to freeze the slab(s) and the time involved in thawing and discharging the slab(s). Thus, a wide spacing results in a thick slab which takes a long time to freeze, but wherein discharging is required relatively infrequently for a given volume of sludge treated. Conversely a thin slab is frozen more quickly but more frequent discharge is required. In practice, it is found that a spacing of 15 to 400 mm, preferably 20 to 250 mm is useful.

Each plate preferably comprises a series of channels within its thickness (for example by forming the plates from an aluminium extrusion wherein the channels are integrally formed within the plate thickness).

The channels in the plates are then arranged to form part of the evaporator of a refrigeration apparatus whereby the plates are cooled for freezing the sludge.

The surface of the frozen slab must be thawed to unstick the slab from the plates prior to discharge from the freezer. This may be achieved by warming the plates.

Thus, the channels in the plates may be disconnected from the evaporator and connected to the hot gas outlet from the compressor of the refrigeration apparatus such as to form a condenser. The hot gas may pass through the same channels as the cold liquid refrigerant or may be provided within separate channels.

The slab(s) may now be discharged from the plate freezer using a suitable discharge means. This may be a means for rotating i.e. tilting, the plate freezer so that the slab(s) may slide out from between the plates, usually through the open top of the plate freezer. The slabs generally slide out under the effect of gravity but means for removing the slab(s) may be provided if necessary.

Generally, the plate freezer is tilted through an angle of 110 to 1700, usually 130 to 1500.

The discharged frozen slab(s) are received into a tank means where complete melting takes place. This is an advantageous feature of the present invention, since the use of a separate tank means for melting the slab(s) allows the plate freezer to be used for freezing a fresh charge of aqueous sludge while melting of the previous charge is effected. This substantially improves the throughput of the apparatus.

The slab(s) are melted in the tank means by the use of warm water. This is preferably clean water which has been heated by the condenser of the refrigeration means.

Warm water may also be sprayed onto the frozen slab(s).

In a preferred embodiment, the frozen slab(s) are first received onto a grid of heated pipes in the tank means forming part of the condenser of the refrigeration apparatus. This is advantageous since it regulates the rate of melting of the slab(s). Thus, if a significant number of slabs builds up on the hot pipes, the weight of the slabs presses the slabs down onto the hot pipes and accelerates the rate of melting.

Usually, the temperature of the warm water supply in the tank means and the temperature of the pipe grid is controlled such as to give melting of a slab in a time range of 30 to 90 minutes.

The granulated sludge solids sink to the bottom of the tank means and are removed therefrom by removal means, for example a discharge screw. The melt water is also removed from the tank means to prevent it building up.

The melt water is generally dirty and is recycled for cleaning.

An embodiment of the present invention will now be described by way of example only in conjunction with the drawings wherein.

Figure 1 is a cross-section on I-I (Figure 2) through an apparatus according to the present invention; Figure 2 is a schematic elevation of the apparatus; and Figure 3 is a representation of the refrigeration circuit.

Figure 1 shows a plate freezer 2 rotatable about an axis 8, so as to discharge frozen sludge slabs 6 into a melting tank 4 equipped with a grid of heated pipes 10 and warm water spray 12. A discharge 14 and screw (not shown) are provided for removing the granulated sludge solids.

The plate freezer 2 comprises a series of parallel plates 16 held within a frame 18 so as to leave a spacing of 20 to 25 mm between the plates for forming the sludge slabs. The sides, ends and base of the plate freezer are closed, but the top remains open for discharging the slabs.

Each plate is formed of an aluminium extrusion having a series of channels running therethrough for passage of freezing liquid and hot gas for thawing. The plates are biased within the frame by means of springs (not shown) which take up the expansion as the plates move apart slightly (for example about 2mm per plate) during freezing of the sludge.

Figure 2 is an elevation of the apparatus showing a vessel 20 for the production of warm water of desired temperature for melting the slabs.

Figure 3 shows the refrigeration circuit of the refrigeration apparatus, which typically employs R717 refrigerant (anhydrous ammonia).

Refrigerant vapour from surge tank 22 and line 60 is compressed in reciprocating compressor 24 and the hot gas is discharged through line 56 via oil removal pot 26 and passes into the pipes 10 of the hot grid in the melting tank, where it partially condenses to liquid. The warm fluid passes through further condenser pipes 32 located within the warm water vessel 20 and the cooled liquid is then returned to the surge tank 22 through an expansion valve 63.

Cold low pressure liquid refrigerant is pumped from surge tank 22 by circulating pump 28 and passes through either of two cooling circuits. The first circuit passes via pipe 52 through the plates 16 of the plate freezer for freezing the sludge slabs and returns to the surge tank 22 via pipe 54.

The second cooling circuit passes through pipe 50 to cold evaporator 34 also located within the warm water vessel 20, before returning to the surge tank 22.

During freezing the cold refrigerant is arranged to evaporate within the plates 16. During defrosting the refrigerant is arranged to evaporate within the cold evaporator 34 to provide a flow of refrigerant for defrosting purposes.

When slabs of sludge have been frozen between plates 16, the circuit is changed such that hot gas from the compressor 24 is caused to pass through the conduits in the plates 16, so as to thaw the surface of the slabs to allow discharge of the slabs. This is carried out by closing valve 40 so as to divert hot gas through line 58.

Valve 42 is closed and hot gas then passes in the reverse direction through pipe 54, through the plates 16, and returns via line 52 and line 62.

Suitable control means are provided such that the freezing of the sludge, the thawing and discharge of the frozen slabs is carried out automatically.

The invention mitigates the problems of burst pipes and poor throughput inherent in present freezing apparatus designs.

Claims (12)

1. An apparatus for freezing sludge which comprises; - a plate freezer having an inlet for aqueous sludge, the freezer comprising a series of spaced parallel plates between which the sludge is introduced, and freezing means for freezing the sludge into one or more slabs; - defrost means provided on the plate freezer for partially thawing the frozen slab(s) such as to release the slab(s) from the plates; - means for discharging the released slab(s) from between the plates; - tank means for receiving and melting the discharged slab(s) and comprising means for providing warm water therein, the slab(s) on melting yielding water and granulated sludge solids; and - means for removing the granulated sludge solids from the tank means.

2. An apparatus according to claim 1 wherein the plate freezer further comprises biasing means for biasing the plates towards one another and accommodating expansion of the sludge on freezing.

3. An apparatus according to any preceding claim wherein the spacing between adjacent plates is in the range 20 to 250 mm.

4. An apparatus according to any preceding claim wherein each plate comprises a series of channels within the thickness of the plate for containing a refrigerant fluid.

5. An apparatus according to any preceding claim wherein the tank means further comprises a grid of heated pipes onto which the slab(s) is/are discharged from the plate freezer.

6. An apparatus according to any preceding claim wherein the feezing means for freezing the sludge in the plate freezer, and the defrost means on the plate freezer are provided by a refrigeration apparatus; the refrigeration apparatus comprising a compressor for compressing refrigeration fluid to generate hot gas, a condenser for removal of heat from the hot gas to condense the gas to liquid, and an evaporator to evaporate the refrigerant liquid to form cold gas; the refrigeration apparatus further comprising a valve means for directing the cold gas to the plate freezer to provide said freezing means when the sludge is to be frozen, the valve means directing the hot gas to the plate freezer to provide said defrost means to allow the frozen slab(s) to be released from the plate freezer.

7. An apparatus according to any preceding claim, wherein the means for discharging the released slab(s) comprises means for rotating the plate freezer about a generally horizontal axis such as to discharge the released slab(s) from between the plates.

8. An apparatus according to any preceding claim, which further comprises means for spraying warm water onto slab(s) in the tank means.

9. A process for freezing sludge which comprises; - introducing aqueous sludge into a plate freezer having a series of spaced parallel plates between which the sludge is introduced, and freezing the sludge into one or more slabs; - defrosting the plate freezer such as to partially thaw the frozen slab(s) and release the slab(s) from the plates; - discharging the released slab(s) from between the plates of the plate freezer; - receiving and melting the discharged slab(s) in a tank means containing warm water, such as to yield water and granulated sludge solids; and - removing the granulated sludge solids from the tank means.

10. A process according to claim 9 wherein the tank means further comprises a grid of heated pipes onto which the slab(s) is/are discharged from the plate freezer.

11. An apparatus for freezing sludge substantially as described in conjunction with the drawings.

12. A process for freezing sludge substantially as described in conjunction with the drawings.