GB2049460A - Secondary sludge dewatering process - Google Patents

Abstract

In a process for mechanically dewatering secondary sewage sludge the sludge is passed into one end of a screw press and heated to at least 60 DEG C within the press or before entry or both. Preferably, a cylindrical substantially unagitated layer of material derived from the sludge is retained within an annular space located between the outer edge of the screw 4 and the inner surface of the porous wall 3. As shown the upper part of the press is heated by a steam jacket 9. <IMAGE>

Description

SPECIFICATION Secondary sludge dewatering process The present invention relates to a process for dewatering secondary sewage sludge such as may be derived from a typical wastewater treatment process incorporating secondary treatment steps. It is therefore not directed to the dewatering of other organic wastes such as primary sludge or wood pulp scraps. The invention more specifically relates to a process for mechanically dewatering secondary sewage sludge wherein the undewatered feed stream is passed into a cylindrical mechanical dewatering zone having a porous cylindrical sidewall and therein pressurized by a rotating helical blade.

It has long been recognized that it would be advantageous to remove water mechanically from various waste and by-product sludges such as sewage sludge. In the specific case of sewage sludge, mechanical dewatering would reduce the amount of material to be disposed or transported, or the amount of water to be evaporated during various drying steps, as in the production of solid fertilizers or soil conditioners. Many different types of dewatering apparatus have been developed, but none is believed to have gained widespread usage and acceptance. Both the difficulties encountered in mechanically dewatering sewage sludge and a process for compacting the dried slidge into fertilizer pellets are described in U.S. Patent 2,977,214.

One specific type of mechanical dewatering apparatus is a continuous filter belt which is slowly pulled through solids collection and removal areas. The device presented in U.S. Patent 2,097,529 is of this type and may be used to dewater sewage sludge.

Other sludge dewatering machines utilizing a moving filter belt are shown in U.S. Patents 4,008,158 and 4,019,431. A belt or conveyor type sewage sludge dewatering device is also shown in U.S.

Patent 3,984,329. This reference mentions the benefits obtained by breaking up the layer of solid matter which forms on the perforate conveyor belt.

These benefits include aiding the water in reaching the belt and a tendency to prevent the plugging of the openings in the belt.

U.S. Patents 3,695,173,3,938,434, and 4,041,854 present apparatus for dewatering sewage sludge in which a helical screw conveyor is rotated within a cylindrical and frustoconical dewatering chamber having perforate walls. These references all describe apparatus in which the outer edge of the screw conveyor scrapes the inner surface of perforate wall.

The inventions presented include specific coil-spring wiping blades, slot-cleaning blades or brushes attached to the outer edge of the helical blade for continuous contact with the inside surface of the perforate wall, thereby cleaning solids therefrom.

The last two patents in this group also teach an alternative embodiment in which the terminal cylindrical portion of the screw conveyor blade does not follow closely the inner surface of the perforate wall but instead has a diameter approximately one-half the diameter of the dewatered solids output opening.

Other references which utilize a rotating conveyor oraugerwithin a perforated outer barrel are U.S.

Patents 1,772,262, 3,997,441 and 1,151,186. These references illustrate the use of a precoat layer located in a space between the conveyor and the inner surface of the barrel as an aid to filtration. U.S.

Patent 1,772,262 discloses that the precoat layer or filter media may be formed from solids present in a liquid to be filtered. However, these references, and particularly U.S. Patent 1,772,262, are directed to the filtration of such materials as sugar juices, suspensions of clays, chalks, and the like rather than fibrous organic waste processed in the present invention.

These reference also do not teach the specific mechanical arrangements and temperature ranges employed herein to dewater successfully the specific feed stream treated according to the invention.

The present invention is characterised, inter alia, by its restriction to secondary sewage sludge as feed stream. Secondary sewage sludge is quite different from primary sludge in terms of consistency and character. The secondary sludge, for example, tends to behave as a fluid under pressure, unlike primary sludge which readily gives up water under pressure.

A secondary sludge is normally much thinner and more mud-like in consistency than a primary sludge.

A basic feature which characterises the subject process is the heating of the feed stream. In the process of the invention the sludge is heated to at least 60 C. and preferably to 65 C. or higher. It is believed that none of the above-cited references discloses or suggests the use of such elevated temperatures in a mechanical dewatering process and that all of them totally fail to recognize the substantial benefits to be obtained by the present invention.

According to the present invention there is provided a process for mechanically dewatering secondary sewage sludge which comprises passing a feed stream comprising secondary sewage sludge into one end of a mechanical dewatering zone containing a rotating helical blade which is concentric with the longitudinal axis of a cylindrical porous wall which encircles at least a portion of the helical blade and removing a solids stream a higher solids content than the feed stream from the other end of the dewatering zone, in which process the sewage sludge is heated to a temperature of at least 60 C.

before it is removed from the dewatering zone.

In different embodiments the feed stream may be heated to above 600C before passage into the dewatering zone or the sewage sludge located within the cylindrical porous wall of the dewatering zone may be heated above 60 C.

in one embodiment the invention provides a process for mechanically dewatering secondary sewage sludge which comprises the steps of passing a feed stream comprising secondary sewage sludge into one end of a mechanical dewatering zone bounded by a cylindrical porous wall which encircles at least a portion of a helical blade which is concentric with the longitudinal axis of the cylindrical porous wall, the outer edge of the helical blade being separated from the innermost portion of the cylindrical porous wall by a radial distance of at least 1 mm; heating the sewage sludge located within the cylindrical porous wall, or prior to feeding to the dewatering zone, to a temperature above 60 C, preferably above 65 C; pressurizing the sewage sludge located within the cylindrical porous wall to a superatmospheric pressure by rotating the helical blade and effecting the transfer of water originally present in the feed stream outwards through the cylindrical porous wall; and withdrawing a solids stream having a higher solids content than the feed stream from the dewatering zone at the other end of the cylindrical porous wall.

In another preferred embodiment the invention provides a process for mechanically dewatering secondary sewage sludge which comprises the steps of passing a feed stream comprising secondary sewage sludge into one end of a mechanical dewatering zone bounded by a cylindrical porous wall which encircles at least a portion of a helical blade which is concentric with the longitudinal axis of the cylindrical porous wall, the outer edge of the helical blade being separated from the innermost portion of the cylindrical porous wall by a radial distance of at least 0.2 cm; heating the sewage sludge located within the cylindrical porous wall to a temperature above 65 C; pressurizing the sewage sludge located within the cylindricl porous wall to a superatmospheric pressure by rotating the helical blade and effecting the transfer of water originally present in the feed stream outward through the cylindrical porous wall; and withdrawing a solids stream having a higher solids content than the feed stream from the dewatering zone at the other end of the cylindrical porous wall.

The invention is thus concerned with a dewatering process using a press wherein in a preferred embo dimentthe liquid removed from the feed stream drains through a pressure surface comprising a layer of fibrous material collected from the feed stream. If desired, a plurality of cylindrical mechanical dewatering zones are used in sequence, with the effluent of the first zone being charged to a second zone wherein additional dewatering is performed.

Of the accompanying drawings, Figure 1 is a cross-sectional view along a vertical plane of an apparatus which may be used to perform the present invention according to one embodiment, Figure 2 is an enlarged cross-sectional view of a small portion of the screw conveyor blade and porous wall shown in Figure 1; and Figure 3 is a schematic flow diagram of a second embodiment of the present invention.

Referring now to Figure 1, a feed stream of secondary sewage sludge to be dewatered enters the apparatus through an inlet throat 1 and is directed downward into one end of a dewatering zone where it makes contact with a screw conveyor having a shaft 2 and a helical blade 4. The shaft 2 extends out of the cylindrical chamber surrounding the dewatering zone through a seal and bearing 5 and is connected to a drive means (not shown) which rotates the screw conveyor. The rotation of the screw conveyor pressurizes the sludge by pushing it towards the other end of the dewatering zone and against a cylindrical porous wall 3 which encircles the screw conveyor. The outer end of the conveyor is supported by a bearing 7 at the center of a spider or cross-member 6. The spider 6 is in turn held in place by a threaded cap 8 having an opening 12 at the other end of the dewatering zone.The outer end of the arms of the spider are retained between a raised lip 13 on the inner surface of the chamber and the cap.

The sludge located within the porous wall is heated to a temperature above 6000. by steam charged to a steam jacket 9 through inlet 14. The steam jacket covers only the upper portion of the porous wall and is in contact with the porous wall.

Fibrous material derived from the entering feed stream accumulated in an annular space between the outer edge of the screw conveyor and the inner surface of the porous wall. Water is expressed radially through this built-up layer of material and through the porous wall. The water is directed into a basin 10 which surrounds the lower portions of the porous wall and is then drawn off through line 11.

Condensed steam leaves the bottom of the stream jacket through an outlet line (not shown).

The preferred construction of the cylindrical porous wall 3 is shown in detail in Figure 2. The wall is formed by parallel spiral windings of tapered wire 17 which are welded to several connecting rods 15 at the smaller outer edge of each winding. The connecting rods are in alignment with the central axis of the cylinder formed by the wall and are on the outer surface of the porous wall. The broader edge of each winding faces inwards toward the blade 4 of the screw conveyor, with each winding being separated by a uniform space 16 through which water may pass. The inner surface of the porous wall is separated from the outer edge of the helical blade by a preferably constant distance "d".

In the embodiment shown in Figure 3, a feed stream comprising secondary sewage sludge is passed into a heating zone 19 through a line 18. The feed stream is therein heated to a temperature of from 6000. to 10000. The heated sewage sludge is then passed through line 20 to a mechanical dewatering zone 21. Water is removed from the heated sludge and withdrawn in line 22, and a first solids stream having a higher solids content than the feed stream is withdrawn in line 23. The first solids stream is passed into a second mechanical dewatering zone 24 wherein additional water is removed from the sludge. The resultant second solids stream is passed through line 26 to an optional product finishing zone 27 and is then removed from the process in line 28. The water removed from the sludge in the second dewatering zone is removed in line 25 and admixed with the water from the first dewatering zone. The total water produced in the process is then carried to suitable treatment facilities in line 29. The dewatering zone 21 and 24 may be identical to that shown in Figure 1 with the steam jackets being employed to maintain the temperature of the sludge rather than increasing its temperature.

As an alternative, the steam jackets may be eliminated from the dewatering zones if the sludge remains sufficiently warm within the dewatering zones.

These drawings are presented to assure a clear understanding of the inventive concept and are not intended to limit the scope of the invention, which may also be practiced with apparatus differing from that shown.

As compared to the disclosure in U.S. Patents 3,695,173,3,938,434 and 4,041,854 the design of the dewatering zone in preferred embodiments of the present invention is distinguished by several features including the definite annular space which is preferably provided between the outer edge of the screw conveyor blade and the inner surface of the perforated outer wall. This space preferably begins at the first end of the screw conveyor, where the feed first contacts the conveyor, and continues for the entire length of the porous wall to the outlet of the apparatus. Smaller spacing between the parallel windings of the perforated outer wall also distinguishes the inventive concept.

Large amounts of secondary sewage sludge are produced daily in municipal sewage treatment plants. The production of secondary sludge will probably increase as treatment plants are improved to comply with stricter pollution standards. Secondary sludges have a less solid character than raw primary sludge and a lower fiber content. They may be fairly slimy and runny. The feed stream charged to the process will comprise a secondary sludge.

This may be a digested sludge but is preferably an undigested sludge. It may also contain a small to moderate amount of primary sludge, but the amount of primary sludge should be less than 50 wt.% and preferably less than 20 wt.%. Preferably, the sludge charged to the process contains 15-25 wt.% or more solids and 5 wt.% or more fibers on a dry basis. That is, the feed stream will preferably contain 15-25 wt.% solids before it is dewatered in the process and should contain more than 5 wt.% fibers or fibrous material on a dry basis. The feed stream may, however, contain as little as 0.4 wt.% solids or as much as 60 wt.% solids. Atypical undewatered sewage sludge will contain at least 50 wt.% water and some amount of inorganic ash.Other components of sewage sludge include various soluble salts and minerals, water-soluble hydrocarbonaceous compounds, hydrocarbons, and cellulosic fibers, as from paper products and vegetable roughage. There is no apparent upper limit on acceptable fiber contents.

It is often desirable to remove some or most of the water present in sewage sludge before it is consumed or disposed of. For instance, drying sewage sludge produces a solid material which may be formed into a very satisfactory fertilizer and soil builder. The dry form of the sludge is preferred since it is lighter for the same solids content, is less oderiferous, is easily stored in bags, and is easily applied using common types of dry fertilizer spreaders. It may also be desirable to dewater sewage sludge to convert it to a combustion-sustaining fuel, to limit water run-off, to reduce disposal problems, to reduce the weight of sludge which must be transported, to recover water for reuse, or to prepare the sludge for further processing. The inventive concept is therefore utilitarian in many different applications.

Water can normally be driven off sewage sludge by the application of heat. However, this procedure normally requires the consumption of increasingly expensive fuel and leads to its own problems, including flue gas and vapor stream discharges. It is therefore desirable to mechanically dewater sewage sludge to the maximum extent possible and feasible and utilize thermal drying only as a final drying or sterilization step.

Despite the incentive provided by the benefits to be obtained by mechanical dewatering, the various continuous belt filtration devices have apparently not evolved to the point where they produce dewatered sewage sludges containing more than about 25-30 wt.% solids. This limitation also seems to apply to the extrusion press apparatus described in the previously referred U.S. Patent 3,695,173 since it is specified as having produced sludge filtrates containing 66 and 71 percent moisture. It therefore appears that the prior art has not provided a method of mechanically dewatering sewage sludge which produces an effluent stream approaching or exceeding a 40 wt.% solids content.However, it is possible by means of the present invention to dewater secondary sewage sludge mechanically to a solids content greater than 60 wt.%, and often in excess of 75 wt.%. It is also possible to convert the undewatered secondary sludge into a useful fuel.

The present invention is preferably performed using a dewatering zone which comprises a porous cylindrical chamber which is sealed at one end except for an organic waste inlet conduit and an opening for a rotating drive shaft. The other end of the dewatering zone has an opening for the discharge of the dewatered organic waste. The terminal portions of the chamber located adjacent to the central porous section of the chamber are preferably imperforate to provide greater structural strength.

The chamber should have a length to inside diameter ratio above 2:1 and preferably from 4:1 to 20:1. The inside diameter of this chamber is preferably uniform along the length of the chamber. The cylindrical chamber of the dewatering zone may be regarded as corresponding to the barrel of an extruder. A major portion of the distance between the ends of the chamber is devoted to providing a porous outer wall through which water is expressed.

This porous wall is to be cylindrical and preferably has the same inside diamter as the rest of the chamber, with the exception that a raised lip may be present at the further end of the chamber to aid in positioning equipment located at the end of the chamber.

The porous wall is preferably fashioned from a continuous length of wedge-shaped bar which is welded to several longitudinal connecting members running along the length of the porous wall as shown in the drawing. This construction provides a continuous spiral opening having a self-cleaning shape. That is, the smallest opening between two adjacent parallel windings is at the inner surface of the porous wall, thereby providing a continuously widening space which allows any particle passing through the opening to continue outward. The outward movement of these particles is aided by the radially flowing water. If desired, the connecting members which run along the length of the porous wall may be attached to the inner surface of the windings and be located on the inner surface of the porous wall.Wedge-shaped wound screens of the desired shape are available commercially and are used as well screens and to confine particulate material within hydrocarbon conversion reactors.

Other types of porous wall construction meeting the criteria set out herein may also be used.

The distance between adjacent windings, or the equivalent structure of other screen materials, used in the porous wall is advantageously within the range of from 0.0075 to 0.013 cm. (0.003 to 0.005 inches). This distance is smaller than that specified in the previously referred to U.S. Patents 3,695,173, namely 0.006 inches, and 3,938,434, namely 0.008 inches. The process of the present invention is therefore preferably performed in an apparatus having a considerably smaller opening than called for by the prior art.

A screw conveyor having a helical blade is centrally mounted within the cylindrical chamber. The major central axis of this conveyor is preferably coextensive with the major axis of the cylindrical chamber and the porous cylindrical wall. The chamber and porous wall are therefore concentric about the screw conveyor. It is critical to the securing of optimum results in the performance of the process that the outer edge of the blade of the screw conveyor be spaced apart from the inner surface of the porous wall by a distance greater than 0.08 cm.

but less than 5 cm. Preferably, the outer edge of the screw conveyor is at least 0.2 cm. but less than 2 cm.

from the inner surface of the porous wall. It is especially preferred that a minimum distance of 0.44 cm. is provided between the outer edge of the screw conveyor and the porous wall. This distance should be substantially uniform along the distance the two elements are in juxtaposition.

The purpose of this separation between the screw conveyor and the wall is to provide a relatively unagitated layer of fibrous dewatered sludge forming a filter medium on the inner surface of the porous wall. This filter medium has an annular shape conforming to the inner surface of the porous wall and the cylinder swept by the outer edge of the screw conveyor. The term "unagitated" is intended to indicate that this filter bed is not mixed or sliced by any mechanical element extending toward the porous wall from the blade. This arrangement is contrasted with the previously referred to extrusion press apparatus in which the surface of the porous wall is "scraped" by the screw conveyor and blades or brushes are attached to the blade to clean the openings in the porous wall.

Although it is free from mechanical agitation, the annular layer of filter medium covering the inner surface of the dewatering zone will not be stagnant and undisturbed since it will be subjected to the stress and abrasion which result from the rotation of the screw conveyor. The associated shear stress will extend radially outward through the filter bed to the porous wall, thereby exerting a torque on the entire bed and causing some admixture of the filter media.

This torque may actually cause the annular layer of filter media to rotate with the screw conveyor. The speed of rotation and linear velocity of the filter bed toward the second end of the cylindrical chamber will probably at all times be less than that of organic waste solids located in the grooves of the screw conveyor. It is theorized that the filter medium may be self-cleaning because of the continuous movement occurring along both of its surfaces.

The just-described preferred dewatering zone construction is contrary to the teaching of the prior art in several areas. For instance, the prior art describes the problem of the porous wall or filter belt becoming clogged and teaches that the built-up layer of solids should be agitated or scraped from the porous wall. The preferred apparatus comprises a porous wall having smaller openings which would seem to be more easily clogged. It also provides an unagitated layer of built-up fibers which covers the entire porous wall.

The screw conveyor is rotated to move the sludge to the outlet of the dewatering zone, pressurizing the material within the dewatering zone and thereby causing water to flow radially through the layer of filter media and the porous wall. The screw conveyor may be rotated at from 10 to 150 rpm, or even more rapidly if desired. However, it is preferred to operate the dewatering zone with the screw conveyor rotating at from 20 to 60 rpm. Only a moderate superatmospheric pressure is required within the dewatering zone. A pressure of less than 35 atm. is sufficient, with the pressure preferably being less than 7.8 atm.

The screw conveyor suitably has a length to diameter ratio above 2:1 and preferably in the range of from 4:1 to about 20:1. A unitary one-piece screw conveyor is preferred. The design of the screw conveyor is subject to much variation. The pitch or helix angle of the blade need not change along the length of the screw conveyor. However constant pitch is not critical to successful performance of the process, and the pitch may be varied if so desired.

Another common variable is the compression ratio of the screw conveyor or auger. The compression ratio refers to the change in the enclosed flight volume along the length of the screw conveyor. As used herein, a 10:1 compression ratio is intended to specify that the flight volume at the terminal portion of the screw conveyor is one-tenth as great as the flight volume at the initial or feed receiving portion of the screw conveyor. The compression ratio of the screw conveyor is preferably below 15:1 and more preferably is in the range of from 1:1 to 5:1. Suitable screw conveyors, drive components and reduction gears are readily available from firms supplying these items for use in the extrusion of plastics, etc.

The improved dewatering of the present invention is a direct result of the secondary sludge being at an elevated temperature while it is within the dewatering zone(s). The average temperature of the sludge within the dewatering zone should be at least 600C.

and is preferably above 65 C. It is not necessary to heat the sludge to an average temperature above 100 C. Unless otherwise specified, sludge temperatures set out herein are intended to refer to the average temperature of the sludge as measured at the outlet of the dewatering zone. The sludge is preferably heated in an enclosed structure having only minimal vapor flow in order to reduce heat losses due to evaporation and to minimize vaporous discharges. That is, the sludge is heated in the present invention only to improve the manner in which it may be mechanically dewatered. The sludge is not heated to promote vaporization of the water.

The heat input required to heat the sludge to the desired temperature will normally be of the order of 100 BTU/lb and is considerably less than the heat required for thermal drying of the sludge.

The sludge may be heated to the desired temperature prior to its passage into the dewatering zone.

Preferably, heat is applied to the sludge while it is within the dewatering zone. The amount of heat transferred to the sludge while it is within the dewatering zone may be the total heat input required in the process. It is also possible to heat the sludge both before and during its residence within the dewatering zone, with the heat added to the sludge within the dewatering zone being adjusted to compensate for the rate at which heat is lost due to convection, evaporation and radiation.

The sludge may be heated through the use of any suitable heating means such as steam, hot oil or other heat transfer fluid, fired heaters, and microwave or electric resistance heaters. Preferably the sludge is heated by indirect heat exchange during which the heat is transferred through a solid surface.

The sludge may be heated in a heating zone which comprises a belt conveyor, screw conveyor or rotating kiln. Preferably the sludge is at least somewhat agitated to promote mixing and heat transfer.

The preferred heating zone configuration consists of an externally heated screw conveyor which also functions to transport the sludge to the inlet of the dewatering zone.

Heat is preferably transferred to the sludge within the dewatering zone through the use of a heater arranged in a manner similar to the steam jacket shown in Figure 1 of the accompanying drawing.

This fully enclosed jacket is in direct contact with the metal on the upper surface of the outside of the porous wall. The transfer of heat through the windings to other portions of the porous wall and the movement of the sludge within the dewatering zone cause the sludge to have fairly uniform temperature. The lower portion, and preferably also the sides, of the porous wall are not in contact with the heater in order to allow the expressed water to easily escape. The porous wall may also be heated by heating elements which are wrapped around the entire cylindrical porous wall. These heating elements may be either steam lines or electrical heating lines. As another alternative the screw conveyor located within the porous wall may be heated to thereby transfer heat to the sludge.

Example A mechanical dewatering zone comprising a oneinch ID porous wall having the preferred winding configuration was used in a series of comparison tests. The edge of the central screw conveyor was spaced apart from the relatively smooth inner surface of the porous wall by a distance of approximately 0.32 cm (1/8-inch). The porous wall was approximately 46 cm (18 inches) long, and the screw conveyor was rotated at about 20 rpm. The outlet end of the porous wall was not closed in any manner.

In a first test, raw primary sludge having a nominal solids content of about 20 wt.% was charged to the dewatering zone. Water was readily expelled through the porous wall and a product containing in excess of 40 wt.% solids was discharged from the outlet of the porous wall.

In a second test, a secondary municipal sewage sludge was charged to the apparatus while it was operated in the same manner as the first test. This sludge also had a nominal solids content of about 20 wt.%. During this test, very little water passed through the porous wall. The product stream discharged from the outlet of the porous wall normally had a solids content very close to that of the feed stream and did not exceed 22 wt.% solids. The product stream typically contained between 20 and 21 wt.% solids.

In a third test, a waterproof electric heater was attached to the upper outer surface of the porous wall.

The same secondary sludge as the second test was charged to the dewatering zone in the same manner as during the second test. Sufficient electrical current was passed through the heater to maintain its external surface temperature between 149 and 177 C. As a result of this heating the sludge leaving the outlet end of the porous wall had a temperature of between 60 and 650C Significant quantities of water passed through the porous wall. The product sludge discharged from the outlet end of the porous wall had a solids content which reached 50 wt.%.

It is therefore evident that the application of heat greatly increases the ability of the dewatering zone to mechanically dewater a secondary sewage sludge. The exact mechanism responsible for this surprising result is not known. It is believed that the improvement is possibly caused by the contact of the sludge with a surface maintained at a temperature sufficient to disrupt or rupture cellular structures which enclose the subsequently released water. This could resu It from the rapid vaporization of small amounts of water contained within the cellular structure rupturing the cellular structure. The vapor would be quickly condensed since the overall temperature of the sludge is relatively low.A second possible cause for the improvement provided byheating the sludge is the release of some water of hydration from the many ions and molecules present in the sludge.

It has been found that a single dewatering zone of the preferred type will normally experience some operational problems, such as clogging at the outlet, if it is attempted to reach an extremely high percentage of water rejection in a single pass. For this reason it is preferred that high solids contents, such as 60-80 wt.% solids, are achieved by repeatedly passing the sludge through a dewatering zone. A multi-pass dewatering process such as that shown in Figure 3 is therefore preferred.

A multi-pass dewatering process may be performed in a batch-type system utilizing a single dewatering zone. However, it is preferably performed using two or more separate and unattached dewatering zones in seris. For instance, the solids stream of two first-stage dewatering zones of uniform size may be passed into a single third dewatering zone which is also of the same design and is operated at the same conditions as the first two dewatering zones. These two first-stage dewatering zones would preferably produce dewatering zone solids streams having substantially the same solids content. The dewatering zone solids streams would be physically discharged from their cylindrical dewatering zones before their admixture, which preferably would be performed at or near ambient atmospheric pressure.

Claims (12)

1. A process for mechanically dewatering secondary sewage sludge which comprises passing a feed stream comprising secondary sewage sludge into one end of a mechanical dewatering zone containing a rotating helical blade which is concentric with the longitudinal axis of a cylindrical porous wall which encircles at least a portion of the helical blade, and removing a solids stream having a higher solids content than the feed stream from the other end of the dewatering zone, in which process the sewage sludge is heated to a temperature of at least 60 C.

before it is removed from the dewatering zone.

2. A process as claimed in claim 1 wherein the feed stream is heated to a temperature above 600C before it is passed into the dewatering zone.

3. A process as claimed in claim 1 wherein the sewage sludge located within the cylindrical porous wall of the dewatering zone is heated to a temperature above 600C

4. A process as claimed in any of claims 1 to 3 wherein the sewage sludge is heated to a temperature above 65 C.

5. A process for mechanically dewatering secondary sewage sludge which comprises the steps of: (a) passing a feed stream comprising secondary sewage sludge into one end of a mechanical dewatering zone bounded by a cylindrical porous wall which encircles at least a portion of a helical blade which is concentric with the longitudinal axis of the cylindrical porous wall, the outer edge of the helical blade being separated from the innermost portion of the cylindrical porous wall by a radial distance of at least 1 mm; (b) heating the sewage sludge located within the cylindrical porous wall, or prior to feeding to the dewatering zone, to a temperature above 60 C;; (c) pressurizing the sewage sludge located within the cylindrical porous wall to a superatmospheric pressure by rotating the helical blade and effecting the transfer of water originally present in the feed stream outwards through the cylindrical porous wall; and (d) withdrawing a solids stream having a higher solids content than the feed stream from the dewatering zone at the other end of the cylindrical porous wall.

6. A process as claimed in claim 5 wherein the sewage sludge is heated within the cylindrical porous wall to a temperature above 65 C.

7. A process as claimed in claim 5 wherein the feed stream is heated to above 650C prior to entry into the dewatering zone.

8. A process as claimed in any of claims 5 to 7 wherein the sewage sludge is heated to a temperature of less than 100 C,

9. A process as claimed in any of claims 5 to 8 wherein the outer edge of the helical blade is separated from the innermost portion of the cylindrical porous wall by a radial distance less than 2 cm, the helical blade is rotated at from 10 to 150 rpm and the maximum pressure applied to the sewage sludge within the dewatering zone is less than 3.5 atm.

10. A process as claimed in any of claims 1 to 9 wherein the secondary sludge is successively dewatered in a plurality of the mechanical dewatering zones operating in a similar fashion.

11. A process for mechanically dewatering secondary sewage sludge carried out substantially as hereinbefore described with reference to Figures 1 and 2 or Figures 1 to 3 of the accompanying drawings.

12. Dewatered secondary sewage obtained bya process as claimed in any of claims 1 to 11 and having a solids content greater than 60 wt.%.