GB2412655A

GB2412655A - Method and apparatus for dewatering sewage sludge

Info

Publication number: GB2412655A
Application number: GB0407032A
Authority: GB
Inventors: John Mcneil
Original assignee: Otwoempower Corp
Current assignee: Otwoempower Corp
Priority date: 2004-03-29
Filing date: 2004-03-29
Publication date: 2005-10-05
Also published as: GB0407032D0

Abstract

A method and apparatus are disclosed for dewatering sewage. The apparatus comprises a drying tube 15 connected to an air displacement device 1. The air displacement device 1 comprises fan blades 6 which rotate about an axis 8 and draw hot gas, preferably from an engine exhaust, through the drying tube 15. The hot gas passes through the tube 15 drying the sewage sludge and drawing it along the tube 15 into the air displacement device 1. The dried sewage enters the air displacement device 1 and is pulverized into fine particles by the rotating fan blades 6.

Description

24 1 2655 Method and Apparatus for Dewatering Sewage Sludge The present

invention relates to an energy efficient method of dewatering sewage sludge that can produce finely ground particles of substantially dry, pasteurised and odour free sludge.

The traditional methods that have been used for many years to dispose of sewage sludge are becoming increasingly more regulated and restricted. For example, it is no longer acceptable to dispose of sewage sludge at sea, whilst recycling sewage sludge onto agricultural land is becoming tightly controlled to reduce the risk of potential infection to the soil, animals and food crops. The disposal of potentially contaminated sewage sludge material to landfill is also subject to stringent controls.

If a market is to be retained for sewage sludge as an agricultural fertiliser, public opinion is demanding that the sludge is adequately treated before being spread onto the land, i.e. the sludge should be odour free and should not pose a risk to either the environment or human and animal health. The sewage sludge will therefore need to be substantially dry, pasteurised and odour free if the sludge is to be in an acceptable condition for use as a fertiliser or soil conditioner.

Other potential disposal routes, such as incineration and gasification, also require that the sewage sludge be in a substantially dry state to allow efficient processing of the sludge. Even the disposal of sewage sludge to landfill requires that the sludge be in a dry and pasteurised condition so as to prevent leaching and ground contamination.

Effective methods of drying and pasteurising sewage sludge are therefore essential if the sludge is to be disposed of in a safe and responsible manner that will comply with current and future legislation. Drying systems will also have to be able to deal with large volumes of wet sewage sludge; by way of example there are approximately one million dry tonnes equivalent of sewage sludge produced each year in the UK.

Unfortunately, the existing methods used to dry and pasteurise sewage sludge are both energy intensive and costly. For example, drum driers are widely used by the sewage treatment industry to dry raw sewage sludge, and in this method of drying the sludge has to be rotated in the drum drier for at least 20 minutes at temperatures up to 85 C in order to adequately dewater the sludge. The drum drying process therefore requires a substantial supply of continuous thermal energy.

To enable the sewage sludge to dry effectively in a drum drier, the sludge is usually partially dewatered by strain and belt presses before entering the drum drier.

The wet sewage sludge may also need to be pre-conditioned by mixing some previously dried sludge with the raw sludge, in order to improve the consistency of the sludge and prevent sticking inside the drum drier.

Broadly the present invention provides a method of dewatering a wet substance, for example sewage, comprising feeding the substance to a drying chamber through which is flowing a high temperature gas.

In a preferred embodiment a very hot gas is drawn along a cylindrical drying tube e.g. of metal preferably at high speed and preferably in a turbulent, cyclonic manner. As the hot gas passes along the metal drying tube it comes into contact with IS wet sewage sludge that is simultaneously being fed into the tube at a controlled rate.

This provides an improved and more energy efficient method of drying sewage sludge.

In a preferred embodiment an air displacement device, preferably a centrifugal displacement device, attached to one end of the drying tube is used to draw the hot gas through the drying tube. Rapid rotation of the centrifugal fan blades inside the air displacement device causes the stream of hot gas to pass through the tube at high speed and in a turbulent manner, so that when the hot gas contacts the raw sewage sludge the water in the sludge is immediately evaporated off. The resulting mixture of hot gas, dry steam and dried sludge is drawn out of the drying tube at high speed by the motion of the rotating fan blades and into the centrifugal air displacement device.

From a further aspect therefore the present invention provides a high temperature method of dewatering and drying raw wet sewage sludge, wherein a centrifugal air displacement device draws hot gas at high speed and in a turbulent, cyclonic manner through a cylindrical drying tube, and as the hot gas passes along the tube it contacts and dries raw sewage sludge that is simultaneously being fed into the drying tube at a controlled rate.

Preferably the hot exhaust gas from an internal combustion engine is used as the supply of hot gas to dry the sewage sludge, although the hot exhaust gas from a gas turbine would also be suitable for this purpose. The engine could either be a diesel engine, fuelled by a liquid fuel such as diesel gas oil, or more preferably a gas engine, fuelled by either natural gas or sewage gas from on-site sewage sludge digesters.

High-speed engines, which typically operate at a speed of 1500 rpm, are suitably sized to produce the appropriate amount of hot exhaust gas required for this method of drying. For example, a 1.6MW(e) engine could produce up to 24,OOO m3/hour of hot exhaust gas, which is sufficient to supply up to eight hot gas centrifugal drying devices. High-speed engines also produce a very hot exhaust gas; for example, the exhaust gas would typically be at a temperature of 450 C to 500 C.

As with any powder drying operation, there is always a risk that under certain adverse atmospheric conditions the powdered dry sludge could potentially combust or even explode whilst inside the drying system. A further advantage of using hot exhaust gas as the drying medium is that the exhaust gas will be relatively inert. For example, a typical exhaust gas from a gas engine would only contain about 6% oxygen and the remainder of the exhaust gas would mainly consist of inert nitrogen, carbon dioxide and dry steam.

Engine driven generator sets are already used at many sewage works to produce the electricity requirements of the site; however, the heat produced by these engines in the form of hot exhaust gas and hot engine coolant is frequently wasted.

Using the waste heat available from the engines to dewater sewage sludge would therefore provide an effective means of recovering valuable thermal energy that A further feature of the drying process is that the cylindrical drying tube would preferably be connected to the narrow end of a frusto-conical shaped expansion chamber, and the wide end of the conical expansion chamber would form the aperture leading into the centrifugal air displacement device.

Similar arrangements have been used to pulverise solid materials, such as minerals and rocks, and it has been suggested that the cyclonic air movement inside the cylindrical tube, combined with the pressure drop that occurs when the air and solid material passes through the conical expansion chamber, can cause solid material to fracture and shatter into small pieces.

If a similar effect occurred with the mixture of hot gas, dry steam and sewage sludge as it travelled along the drying tube and through the expansion chamber, the sludge could be broken down into smaller sized particles, and the heat within the drying system would then drive off even more water from the shattered particles of sludge.

The preferred sludge drying apparatus would therefore basically consist of a cylindrical drying tube connected to the narrow end of a frusto- conical shaped expansion chamber, and a centrifugal air displacement device connected to the wide end of the conical expansion chamber.

Rapid rotation of the centrifugal fan blades inside the air displacement device would continuously draw hot gas into the drying tube, and the hot gas would dewater sewage sludge that was being fed simultaneously into the drying tube. The resulting mixture of hot gas, dry steam and dried sludge would then be sucked along the drying tube, through the conical expansion chamber and into the air displacement device.

The centrifugal fan blades would then displace the mixture of hot gas, dry steam and dry sludge from the air displacement device into an outlet pipe that would preferably transfer the mixture to a particulate separation system, such as a cyclone 1 5 unit.

Before using the apparatus to dry the sewage sludge, hot engine exhaust gas is preferably passed through the complete drying system until the internal surfaces of all the metal components used in the system, i.e. the drying tube, the conical expansion chamber, the centrifugal air displacement device, the outlet tube and the cyclone separation unit, were brought up to a temperature of over 300 C, more preferably about 400 C to 450 C.

Pre-heating all the components in the drying system ensures that the sewage sludge would only ever come into contact with hot metal surfaces, which not only helps to dry out the sludge but also prevents the sludge from sticking to metal surfaces as the sludge passes through the process. All of the components used in the drying process, including the cyclone separation unit, would preferably be well insulated and lagged to retain heat within the system.

Because of the high thermal energy required to convert the water in raw sewage sludge into steam, a further preferred feature of the drying system is that the raw sewage sludge is pre-heated before being fed into the cylindrical drying tube.

Waste heat from the cooling system of the same engine that produced the hot exhaust gas used in the drying process could advantageously be used to pre-heat the sewage sludge to a temperature of about 75 C.

Maintaining the sewage sludge at an elevated temperature for a short period of time, i.e. for approximately 10 minutes, would also partially pasteurise the wet sludge.

For example, a temperature of 75 C can effectively destroy certain types of bacteria and pathogens, including E Coli and salmonella.

As the hot raw sewage sludge is introduced into the drying tube, the wet sludge will immediately come into contact with turbulent stream of hot gas that is passing along the drying tube at high speed. The combination of the hot gas, which will typically be at a temperature of about 450 C to 500 C, and the heat that has already built up inside the drying tube will quickly evaporate the water from the sludge. The initial drying action of the process is therefore almost instantaneous.

As the hot drying gas contacts the cooler raw sewage sludge and water is evaporated from the wet sludge, the temperature inside the drying tube may well drop from 450 C/500 C down to say 300 C/350 C. However, a temperature of 300 C inside the drying tube would still be hot enough to ensure that the steam from the evaporation process remained in a dry state.

The temperatures that prevail within the drying process, i.e. temperatures ranging from 300 C to 500 C, are also high enough to ensure that the sludge will have been thoroughly pasteurised by the time the dried sludge finally leaves the drying system. At these elevated process temperatures, any volatile organic materials present in the raw sewage sludge will also be vaporised out of the sludge, and the dry sludge will therefore be substantially odour free.

The dry steam produced by the rapid evaporation of water from the raw sewage sludge will also make the atmosphere inside the drying system even more inert.

For example, independent research has shown that the risk of dry sewage powder being able to burn or explode is significantly reduced if the atmosphere in the drying process contains no more than 6% oxygen, and as previously mentioned the exhaust gas from a gas engine would normally only contain about 6% oxygen.

However, steam is also an excellent inerting agent, and the additional dry steam produced by the drying process will make the atmosphere in the drying process even more inert, which further reduces the risk of the dry sludge being able to ignite, burn, pyrolise or explode whilst inside the drying system.

The mixture of hot gas, dry steam and dry sludge produced by the drying process will also pass through the drying system at high speed. The powdered dry sludge will therefore spend very little time inside the drying system, further reducing the risk of accidental combustion or explosion.

A further preferred feature of the drying apparatus is that there are no fan blades in the inner chamber of the air displacement device; instead the centrifugal fan blades are located in a centrifugal chamber that surrounds the inner chamber.

When the mixture of hot gas, dry steam and dried sludge passes through the conical expansion chamber, the pressure drop inside the expansion chamber may well help to break down the sludge into smaller pieces. The small pieces of sludge would pass directly into the inner chamber of the air displacement device, and the heat inside the inner chamber would dry the sludge even further.

The rapidly rotating fan blades, which would typically be rotating at about 3000 rpm, would then draw the mixture of hot gas, dry steam and dried sludge from the inner chamber of the air displacement device into the centrifugal chamber. As the dried sludge passes through the rapidly rotating fan blades, the sludge would be pulverised into very fine particles by the mechanical action of the rotating fan blades.

Because sewage sludge is a relatively soft material, there would be very little fan blade wear during the pulverising process.

The fine particles of pulverised sludge produced by the rotating fan blades will still be at an elevated temperature of at least 300 C. Any moisture remaining in the pulverised sewage sludge will therefore be completely evaporated off at this stage, and any pathogens remaining in the sludge will be destroyed.

The dry sewage sludge is preferably transferred to a cyclone particulate separation unit, and on leaving the cyclone unit the sludge should preferably consist of at least 95% dry solids, with the sludge in the form of pasteurised and odour free powdered particles. In this finely powdered state, the sludge would already be in a suitable condition for direct use as a renewable energy resource, either as a solid fuel for incineration or as a raw material for gasification.

If the dried sewage sludge were intended for agricultural purposes, the finely ground sludge particles from the cyclone unit would probably have to be pelletised to provide a product that was more suitable for being spread safely onto agricultural land.

Embodiments of the sewage sludge drying apparatus will now be described, by way of example only, with reference to the accompanying drawings in which: Figure I is a front view of a centrifugal drying device for use in the invention; Figure 2 is a cross-sectional side view of the centrifugal device of Figure 1; Figure 3 is a side view of the drying apparatus comprising a cylindrical drying tube, a frusto-conical expansion chamber and a centrifugal drying device; Figure 4 is a cross-sectional view of one means of introducing raw sewage sludge into the cylindrical drying tube; and Figure 5 is a schematic illustration of a complete system for drying sewage sludge.

With reference to Figures I and 2, a centrifugal air displacement device I comprises a spiral metal housing 2 containing a circular inner chamber 5, surrounded by a circular centrifugal outer chamber 3, and an outlet aperture 4.

During the drying process, a mixture of hot gas, dry steam and dried sludge enters the air displacement device I through an inlet aperture at the front of the inner chamber 5. A central core 7 is mounted onto a drive shaft 8 at the centre of the inner chamber 5, and the core 7 is locked in position onto the drive shaft 8 by a contoured locking nut 9.

The core 7 is shaped so as to provide a smoothly contoured surface that helps to deflect the mixture of hot gas, dry steam and dry sludge entering the inner chamber out towards the centrifugal outer chamber 3.

Drive shaft 8 is also connected to a back plate 12, and a plurality of centrifugal fan blades 6 is mounted onto the back plate 12 in a radial manner. Each centrifugal fan blade 6 is also connected to a front plate 13 so that the complete assembly of fan blades 6 and plates 12 and 13 make up the centrifugal outer chamber 3 of the air displacement device 1. The surface of core 7 and the back plate 12 can be coated with a ceramic non-stick coating to help prevent sewage sludge sticking to the inside of the inner chamber 5 as the sludge passes through the air displacement device I. The drive shaft 8 is connected to a drive wheel 11, which would be driven by a drive belt connected to a variable speed electric motor. To ensure smooth rotation, the drive shaft 8 passes through oil-cooled bearings 10, which are located on the outside of the air displacement device I to prevent overheating, before entering the inner chamber 5.

There would typically be ten, twelve or more fan blades 6 attached to plates 12 and 13, depending on the capacity of the air displacement device 1, and the blades would be equally spaced around the circumference of the centrifugal outer chamber 3.

The inlet aperture at the front of the inner chamber 5 is connected to the wide end of a hollow metal frusto-conical shaped expansion chamber 14. The narrow end of the conical expansion chamber 14 is connected to a hollow metal cylindrical tube 15, which forms the drying tube of the drying process. The cylindrical tube 15 and the conical expansion chamber 14 are manufactured from a corrosion resistant material such as stainless steel. The drying tube 15 would be available in different lengths, which allows the residence time inside the tube to be easily varied to suit the particular consistency and moisture content of the raw sewage sludge.

The operation of the sewage sludge drying apparatus will now be described with reference to Figure 1, 2, 3 and 4.

Preheated raw sewage sludge is contained in a sluice hopper 16. The sludge trickle feeds under gravity from the hopper 16 into a hollow metal tube 17 that contains a horizontal auger 18. The sewage sludge is transferred from tube 17 into the cylindrical metal drying tube 15 by the auger 18. The rate of feed of the sewage sludge is controlled by a variable speed electric motor (25, Figure 5) connected by drive belts to a drive wheel 19 at the end of the auger 18.

Although a horizontal sludge feed system is illustrated in Figure 4, a vertical in-feed system could equally be used to feed the raw sewage sludge into the drying tube 15.

The auger 18 introduces the sewage sludge into the drying tube 15 near the end of the tube 15 that is furthest away from the conical expansion chamber 14.

Rapid rotation of the centrifugal blades 6 in the air displacement device 1 draws hot engine exhaust gas, at a temperature of 450 C to 500 C, into the drying tube 15. Before any raw sludge is introduced into the drying tube 15, hot exhaust gas is passed through the complete drying system until all the internal surfaces of the system had reached a temperature of least 400 C.

Hot raw sewage sludge is then be introduced into the drying tube 15 at a controlled rate by the auger 18. The high thermal energy of the hot exhaust gas, which enters the drying tube 15 at high speed and in a turbulent manner, combined with the heat inside the drying tube 15, ensures that the water in the preheated sludge is evaporated as soon as the hot gas comes into contact with the sludge.

The resulting mixture of hot gas, dry steam and dried sludge is drawn at high speed along the drying tube 15, through the expansion chamber 14 and into the inner chamber 5 of the air displacement device 1. The rotation of the centrifugal fan blades 6 draws the mixture of hot gas, dry steam and dried sludge from the inner chamber 5 into the centrifugal outer chamber 3, and as the sludge passes through the rapidly rotating fan blades 6 it is pulverised into fine particles. The centrifugal fan blades 6 displace the mixture of hot gas, dry steam and pulverised sludge to the outlet aperture 4 of the air displacement device 1.

Figure 5 shows a typical sewage sludge drying system comprising a drying tube 15 connected to the narrow end of a frusto-conical expansion chamber 14, and the wide end of the conical expansion chamber 14 is in turn connected to a centrifugal air displacement device 1.

The exhaust 27 from a high-speed internal combustion engine (not illustrated in Figure 5) feeds hot exhaust gas 28 into a large insulated reservoir tank 26, to ensure that there is always an adequate supply of hot gas available to feed the drying system.

A pressure-regulating valve 29 situated in the exhaust from the engine will activate if the pressure in reservoir 26 deviates beyond predetermined high and low pressure limits. The exhaust gas from the engine would be by-passed temporarily into a flue stack 30, and the exhaust gas would be released to the outside atmosphere until the pressure in the reservoir tank 26 had returned to an acceptable operational level.

In accordance with normal practice, the raw sewage sludge will have been passed through a series of conventional strain and belt presses to partially dewater the sludge to a dry solids content of about 25%, before the raw sludge is supplied to the drying process.

If necessary, some previously dried sludge from the drying process could be mixed with the partially dewatered sludge to improve the consistency of the sludge and help prevent sticking inside the drying system.

The partially dewatered sewage sludge is fed through an inlet 31 into a holding tank 32, which is heated by a hot water jacket 36 using hot water introduced to an inlet 37 from a heat exchanger (not shown).

The heat exchanger would preferably use hot engine coolant, from the same engine that supplies the hot exhaust gas to reservoir 26, to heat the hot water supplied to the heating jacket 36. After circulating through the jacket 36, the hot water is returned to the heat exchanger through an outlet 38.

A paddle stirrer 34, powered by a variable speed electric motor 35, continually stirs the raw sewage sludge 33 inside the tank 32 to form a reasonably homogeneous mixture of sludge and water. The sewage sludge is heated up to a temperature of about 75 C in the tank 32 and the sludge is then held at that elevated temperature for approximately 10 minutes to partially pasteurise the sludge.

The hot sewage sludge is then gradually released at a controlled rate into the sluice hopper 16 where it is fed by gravity onto the horizontal auger (18, Figure 4) powered by a variable speed motor 25. The auger 18 then trickle feeds the sludge at a controlled rate into the drying tube 15. The length of the drying tube 15 can be varied, if necessary, to provide an appropriate residence drying time to suit the consistency and solids content of the raw sewage sludge.

Hot gas from the reservoir 26 is drawn at high speed and in a turbulent manner through the drying tube 15 by the rapid rotation of the centrifugal fan blades 6 inside the air displacement device 1. A variable speed motor 20 controls the speed of rotation of the fan blades 6. When the hot gas stream, which is at a temperature of about 450 C to 500 C, contacts the pre-heated sewage sludge, which is at a temperature of about 75 C, the water in the sewage sludge immediately evaporates from the sludge.

The resulting mixture of hot gas, dry steam and dried sludge is then sucked at high speed along the drying tube 15, through the conical expansion chamber 14, and into the inner chamber 5 of the air displacement device I. The rotating fan blades 6 draw the mixture of hot gas, dry steam and dried sludge into the centrifugal outer chamber 3, and as pieces of sludge pass through the rapidly rotating fan blades 6 they are pulverised into finely ground particles.

The centrifugal action of the fan blades 6 displaces the mixture of hot gas, dry steam and pulverised sludge into the outlet aperture 4, and a tube 21 then transfers the mixture to a cyclone particulate separator 22.

The cyclone separator 22 separates out the particles of dry sludge from the hot gas and dry steam, and the powdered sludge is collected in a suitable receptacle 23.

The waste gas 24 from the cyclone unit 22 would be passed through a catalytic oxidation reactor and a bag filter, to remove volatile organic compounds, contaminants and fine dust particles, before the waste gas was released into the atmosphere.

Before being released into the atmosphere, the hot waste gas could also be passed through a condenser, to condense out the steam, and the water collected from the condenser could be used as process water within the sewage works.

The dry sludge from the cyclone separator 22 would probably comprise at least 95% dry solids and would be in the form of a finely ground powder. The dry sludge would therefore already be in a suitable state for direct use as either a renewable solid fuel for incineration or a raw material for gasification.

After passing through the high temperature drying process, the dry sludge would also be completely pasteurised and odour free. The dry sludge would therefore be suitable for use as an agricultural fertiliser, although the powdered sludge would probably have to be converted into a pelletised form for agricultural applications.

The effectiveness, efficiency and throughput of the drying process can be influenced by a number of variable factors including, for example, the amount of water in the raw sewage sludge; the temperature of the preheated sewage sludge; the rate at which the wet sludge is fed into the drying tube; the temperature, speed and volume of the hot drying gas entering the drying tube; the length of the drying tube; the size of the conical expansion chamber; the capacity of the air displacement device; and the speed of rotation of the centrifugal fan blades.

The operating conditions of the drying process can therefore be finely adjusted in a number of different ways to cater for variations in the consistency and the moisture content of the original raw sewage sludge.

The potential throughput of a typical high temperature hot gas centrifugal drying process is illustrated with reference to the following example.

Example

Based on practical evaluations in the laboratory, it is estimated that a single hot gas centrifugal drying device could produce up to 350 tonnes a year of dry, powdered sludge from raw sewage sludge containing 25% solids, as illustrated in

Table 1.

Table 1

Drying Performance of a Single Hot Gas Centrifugal Drying Device Hot Gas Wet Sludge Dry Sludge Dry Sludge Throughput Hourly Input Hourly Output Annual Output 3000 m3/hour 219 kg/hour 44 kg/hour 350 tonnes/pa A typical 1.6MW(e) high- speed diesel or gas engine can produce up to 24000m3/hour of hot exhaust gas at a temperature of 450 C to 500 C. A single 1.6MW(e) high-speed engine could therefore produce enough hot exhaust gas to run up to eight centrifugal drying units, and in theory the complete system would be able to produce 2800 tonnes of dry sludge per annum as shown in Table 2.

Table 2

Output of Dry Sludge from a Complete Drying System Number of Centrifugal Annual Output of Dry Dry Sludge Devices Sludge From Each Device Total Annual Output 350 tonnes/pa 2800 tonnes/pa The primary purpose of the engine would be to generate electricity for on-site use, and the thermal energy used to dry the wet sewage sludge, i.e. the hot exhaust gas from the engine and the heat from the engine cooling system, would essentially be waste heat that would otherwise be lost.

The main energy requirement of the drying process would be the electricity needed to power the motors that drive the centrifugal drying devices, plus a small amount of electricity to power other electric motors used in the drying process, such as the motor used to stir the sewage sludge in the heating tank, the motor used to feed sewage sludge into the drying tubeand the motor in the cyclone particulate separation unit.

The drying system would therefore be able to cost effectively produce almost 3000 tonnes per annum of substantially dry sludge, i.e. sludge containing at least 95% dry solids, from typical raw sewage sludge, and the dry sludge would also be in a pasteurised, odour free and finely ground condition.

The drying process would be very energy efficient because all of the thermal energy used in the process would be waste heat from an engine, whilst the same engine would also generate all of the electricity used by the process. Estimates suggest the energy costs for the centrifugal drying process may well only be about 10% of the energy costs required for traditional sewage sludge drying methods. The centrifugal drying devices would also be relatively cheap; for example, eight centrifugal driers would probably cost no more than 20,000.

Because the dry, pasteurised, odour free sewage sludge from the high temperature drying process is in a finely ground powdered state, the sludge is in a suitable condition for direct use as either a cost effective renewable energy resource or a cheap but safe agricultural fertiliser.

Obviously the sewage sludge drying process is not limited to a particular size or type of engine or to a particular size or number of centrifugal drying devices. The drying process as described in the invention is therefore capable of being readily modified to suit the specific circumstances of individual sewage plants.

In addition to sewage, the drying process as described in the invention could also be used to dewater animal wastes and animal slurries, and the principles of the invention could also be applied to any powder or granular product that needed to be efficiently dewatered and dried including, for example, certain types of foodstuffs and chemicals.

Claims

Claims 1. A method of dewatering wet sewage, comprising feeding the sewage

to a drying chamber through which is flowing a high temperature gas.
2. A method as claimed in claim 1 wherein said drying chamber is a tube.
3. A method as claimed in claim 2 wherein said gas travels through said tube at high speed and in a turbulent, cyclonic manner.
4. A method as claimed in any preceding claim, wherein the sewage is preheated, preferably to about 75 C, prior to its introduction into the drying chamber.
5. A method as claimed in any preceding claim, wherein the residence time of the sewage inside the drying tube is controlled in part by the length of said drying tube and in part by the speed at which the hot gas travels along said drying tube.
6. A method as claimed in any preceding claim, wherein said hot gas is stored in a reservoir chamber connected to one end of said drying chamber.
7. A method as claimed in any preceding claim, wherein said hot gas is at a temperature of over 400 C, more preferably between about 450 C and 500 C.
8. A method as claimed in any preceding claim, wherein said hot gas is exhaust gas from an engine.
9. A method as claimed in claim 8, wherein said engine is an internal combustion engine, more preferably a diesel engine running on liquid fuel or a gas engine running on gaseous fuel.
10. A method as claimed in claim 8, wherein said hot gas is the exhaust gas from a gas turbine.
11. A method as claimed in claim 8, 9 or 10, wherein the primary purpose of the engine is to power a generator to produce on-site electricity required by a sewage plant.
12. A method as claimed in any preceding claim, wherein said hot gas is drawn into said drying chamber tube by an air displacement device.
13. A method as claimed in claim 12, wherein said air displacement device is a centrifugal air displacement device.
14. A method as claimed in claim 13 wherein said air displacement device comprises a spiral housing containing an inlet aperture leading to a circular inner chamber, a circular centrifugal chamber surrounding said inner chamber, a plurality of centrifugal fan blades located inside said centrifugal chamber, and an outlet aperture leading from said housing.
15. A method as claimed in claim 14, wherein there are ten, twelve or more centrifugal fan blades equally spaced around said centrifugal chamber.
16. A method as claimed in claim 14 or 15, wherein said centrifugal fan blades are radially mounted between a back plate, which is attached to a central drive shaft, and a front plate, so that the combination of fan blades, back plate and front plate forms said centrifugal chamber.
17. A method as claimed in claim 16, wherein a variable speed electric motor rotates said central drive shaft, which in turn rotates said fan blades located in said centrifugal chamber.
18. A method as claimed in any of claims 14 to 17, wherein rapid rotation of said fan blades by said motor sets up a centrifugal motion that draws the hot gas from the reservoir chamber into the drying chamber in a turbulent and cyclonic manner.
19. A method as claimed in any of claims 14 to 18, wherein the drying chamber or tube is connected to the narrow end of a frusto-conical expansion chamber, and the wide end of said conical expansion chamber is connected to the air inlet aperture leading to the inner chamber of the centrifugal air displacement device.
20. A method as claimed in any of claims 14 to 19, wherein the speed of rotation of the centrifugal fan blades governs the speed at which said hot gas travels along said drying chamber.
21. A method as claimed in any of claims 14 to 20, wherein the centrifugal motion of the rapidly rotating fan blades draws a mixture of hot gas, dry steam and dried sewage from said inner chamber into the centrifugal chamber of said centrifugal device, so that the dried sewage in said mixture is pulverised into finely ground particles as it passes through said rotating fan blades.
22. A method as claimed in any of claims 12 to 21, wherein the output of said air displacement device is conducted to a cyclone particle separation unit.
23. A method as claimed in any preceding claim, wherein said raw sewage is a sludge containing about 25% dry solids.
24. A method as claimed in any preceding claim, wherein the sewage is preheated
25. A method as claimed in claim 24 wherein said sewage is pre-heated in a heated holding tank.
26. A method as claimed in claim 25, wherein a hot water jacket heats said holding tank.
27. A method as claimed in claim 26 as dependent directly or indirectly upon claim 8, wherein said hot water to heat said water jacket is supplied by a heat exchanger that utilises hot engine coolant from the same engine that supplies the hot gas for the drying process to heat said water.
28. A method as claimed in any preceding claim, wherein the hot drying gas is used to preheat the internal surfaces of the components used in the drying system before the drying process commences.
29. A method as claimed in claim 28 wherein the components are heated to between about 450 C and 450 C.
30. A method as claimed in claim 28 or 29, wherein all components used in the drying system are insulated and lagged to help retain heat within the system.
31. A method of dewatering a wet substance, comprising feeding the substance to a drying chamber through which is flowing a high temperature gas.
32. Apparatus for performing the method of any preceding claim.
33. Apparatus for dewatering and drying sewage sludge comprising a source of hot gas; a source of sewage; a drying chamber having an inlet for said hot gas, an inlet for said sludge and an outlet for a mixture of said hot gas and sludge.
34. Apparatus as claimed in claim 33 wherein said drying chamber is a tube.
35. Apparatus as claimed in claim 34, wherein said hot gas source is engine exhaust.
36. Apparatus as claimed in claim 35, wherein said engine is an internal combustion engine, more preferably a diesel engine running on liquid fuel or a gas engine running on gaseous fuel.
37. Apparatus as claimed in claim 36, wherein said engine is a gas turbine.
38. Apparatus as claimed in any of claims 33 to 37 further comprising an air displacement device for drawing said hot gas into said chamber.
39. Apparatus as claimed in claim 38, wherein said air displacement device is a centrifugal air displacement device.
40. Apparatus as claimed in claim 39 wherein said air displacement device comprises a spiral housing containing an inlet aperture leading to a circular inner chamber, a circular centrifugal chamber surrounding said inner chamber, a plurality of 5centrifugal fan blades located inside said centrifugal chamber, and an outlet aperture leading from said housing.
41. Apparatus as claimed in claim 40, wherein there are ten, twelve or more centrifugal fan blades equally spaced around said centrifugal chamber.
42. Apparatus as claimed in claim 40 or 41, wherein said centrifugal fan blades are radially mounted between a back plate, which is attached to a central drive shaft, and a front plate, so that the combination of fan blades, back plate and front plate forms said centrifugal chamber.
43. Apparatus as claimed in claim 42, further comprising a variable speed electric motor for rotating said central drive shaft, which in turn rotates said fan blades located in said centrifugal chamber.
2044. Apparatus as claimed in any of claims 40 to 43 wherein a flow deflector is provided within said inner chamber to deflect flow towards said blades.
45. Apparatus as claimed in any of claims 39 to 44, wherein the drying chamber or tube is connected to the narrow end of a frusto-conical expansion chamber, and 25the wide end of said conical expansion chamber is connected to the air inlet aperture leading to the inner chamber of the centrifugal air displacement device.
46. Apparatus as claimed in any of claims 39 to 45 further comprising a cyclone particle separation unit receiving the output of said air displacement device.
47. Apparatus as claimed in any of claims 33 to 46 further comprising means to preheat the sewage.
48. Apparatus a s claimed in claim 47 wherein said sewage is pre-heated in a heated holding tank.
49. Apparatus as claimed in claim 48, wherein a hot water jacket heats said holding tank.
50. Apparatus as claimed in any of claims 33 to 49 further comprising means to preheat the internal surfaces of the components used in the drying system before the drying process commences.
51.Apparatus for dewatering and drying sewage sludge comprising a reservoir chamber for containing very hot gas, a cylindrical drying tube connected at one end to said reservoir chamber, means to supply preheated raw sewage sludge into said drying tube at a controlled rate, a frustoconical shaped metal expansion chamber connected at its narrow end to the other end of said drying tube, a centrifugal air displacement device connected to the wide end of said expansion chamber, a set of centrifugal fan blades located inside air displacement device, a variable speed electric motor to rotate said fan blades so that rotation of said fan blades will draw said hot gas from said reservoir, along said drying tube, through said expansion chamber and into said air displacement device at the required rate.
52. A system for dewatering, drying, pasteurising and deodorising sewage sludge comprising a supply of very hot gas, such as the hot exhaust gas from a high speed engine, a reservoir chamber to contain said hot gas, a supply of preheated raw sewage sludge, a drying tube, a centrifugal air displacement device to draw said hot gas into and along said drying tube at high speed and in a turbulent cyclonic manner, means to feed the raw sewage sludge into said drying tube at a controlled rate, a frustoconical shaped metal expansion chamber linking said drying tube to said centrifugal air displacement device, and an outlet pipe from said air displacement device to transfer the mixture of hot gas, dry steam and pulverised dry sludge from said air displacement device to a cyclone particle separation unit.
53. A high temperature method of dewatering, drying, pasteurising and deodorising raw sewage sludge wherein a very hot gas, which is travelling at high speed and in a turbulent, cyclonic manner along a cylindrical metal drying tube, comes into contact with pre-heated raw sewage sludge being fed simultaneously to the drying tube at a controlled rate.