The disposal of mixed collected municipal and agricultural waste presently provides a major problem. A medium sized city may produce in the region of hundreds of thousands ouf toises of waste a year. Whereas in the past it has been the practice to dump this waste in landfill sites, this is becoming less favoured, as the availability of suitable sites for landfill is decreasing. Current EU legislation has adjudicated to remove the landfill option in future for untreated mixed organic waste.

At the same time, there is concern over the rate of consumption of fossil fuels. These fuels are not replaceable and in some cases present pollution problems due to a high sulphur content. There is a corresponding interest in finding a source of low pollution supplementary fuel which does not deplete fossil fuel stocks and can compensate for C02 production.

There is a wide interest in the use of organic material as compost for horticultural purposes. In the past, materials such as coir have been used, which has a relatively low nutrient content. Peat has also been used, but there is now concern that natural peat bogs are being dangerously depleted by this use.

Accordingly, there is an interest in recycling municipal and agricultural waste to produce either a fuel material for replacement of fossil fuels or organic compost for horticultural purposes. A number of processes have been proposed in which agricultural or municipal waste is composted in a specially design apparatus so that it is treated by aerobic digestion. Examples of each process are described for example in GB1551020 and W083/02779.

The latter apparatus provides for the drying of the waste during digestion by providing an airflow over the waste. However, in practice, it is found that this apparatus is only partially successful, sometimes failing to produce a consistent quality product.

The present inventor has realised that the performance of the aerobic digester for example as used in W083/02779 depends very much on the condition of the waste being fed into the apparatus. This is not constant over time so that the reliability of the process is very variable.

The present inventor has set out to overcome the problems of the prior art. The present inventor has particularly sought to provide a process and apparatus for processing the municipal waste which has a very variable moisture level and physical composition, so that a relatively homogenous product can be obtained.

An alternative approach to the disposal of waste uses incineration, in which the waste is heated to a very high temperature to reduce its volume, leaving an ash which has to be disposed of. However, the plant required for the incineration is complex and expensive.

There are problems in the operation of the plant due to the uneven nature of the waste being collected and the high moisture content which is often encountered. There can be problems with generation of pollutants particularly NOx.

For this reason, an alternative method of thermal decomposition of waste involves a pyrolysis step, in which the waste material is heated to a temperature of about 800°C, whereby a mixture of combustible gases is produced and the solid material is reduced to a largely carbonaceous char. The char can be subsequently gasified either insitu or in a separate step, for example using steam, to generate further combustible gases. The combustible gases from the pyrolysis and gasification process can be burnt at high temperature under controlled conditions for production of heat. This heat can be used to provide power and to run the process. Generation of pollutants is effectively controlled because the degradation of the waste occurs at a temperature below that at which NOx gases are produced and the combustion of the gases can be conducted under controlled conditions which reduce the generation of such gases. However, there are problems with the control of such processes where the composition and water content of the waste being processed are not uniform from time to time. A drying step can be used, but this adds complexity.

The present inventor has realised that an aerobic process can be employed in which the heat generated digests the mixed organic waste and provides heat for allowing it to be dried, the process being controllable by taking different feeds of waste having different average moisture levels before treatment, and blending them, so that the material fed to a treatment vessel has a relatively consistent nature, thereby making the product more consistent.

Accordingly, the present invention provides in a first aspect a method of treating mixed organic waste, comprising the steps of : mixing a supply of organic waste having a first average moisture level before treatment with a supply of other solid waste, having a lower average moisture level before treatment, wherein the relative quantities by weight of the organic waste and the other waste are controlled, feeding the mixed solid waste into a treatment vessel, treating the waste by microbial activity in the treatment vessel, the mixed solid waste being agitated during treatment, the oxygen content in the gas in contact with the mixed solid waste being controlled during the treatment process so that it does not fall below 5% by volume, the waste having an average moisture level after treatment not exceeding 45% by weight, more preferably not exceeding 35% of weight and most preferably not exceeding 25%.

Subsequent drying of the product to an average moisture content of below 20% by weight can be carried out relatively easily.

Further, in a second aspect, the present provides apparatus for treating mixed organic waste, comprising: a supply for organic waste having a first average moisture level before treatment and a supply for other waste having a lower average moisture level before treatment, means for mixing the solid organic waste and the other waste, control means for controlling the relative quantities by weight of the organic waste and other waste mixed together, means for feeding the organic waste and the other waste to a treatment vessel means for agitating the solid organic waste in the treatment vessel, drying means following the treatment vessel and means for controlling the air flow through the treatment vessel, and/or the input of organic waste and other solid waste to the treatment vessel, so that the average moisture level of waste after treatment does not exceed 45% by weight, more preferably not exceeding 35% by weight and most preferably not exceeding 35% by weight, and so that the oxygen content of gas in contact with the mixed waste in the vessel does not fall below 5% by volume.

It is found that both the calorific value and the nutrient content of waste treated in this way remains high so that it is suitable as a replacement fuel or compost.

According to the first aspect of the invention, variations in the physical composition (for example calorific content) and moisture level of the organic waste (typically domestic waste, but also possibly agricultural waste) can be smoothed out, so that a product formed from treated waste from different areas or different time periods can be relatively homogeneous. For a fuel, homogeneity is required in parameters such as ash content, calorific value, moisture level and density. For an organic compost, homogeneity in parameters such as moisture level, C/N ratio, electrical conductivity density and nutrient content is required.

Preferred features of the first aspect of the invention will be described further below.

The mixed organic waste treated according to the first and second aspect of the invention is preferably solid mixed organic waste, for example mixed domestic waste, industrial waste or agricultural waste.

By"organic waste"is meant waste which has at least a proportion of organic material capable of being treated microbially. The other solid waste mixed with the mixed organic waste preferably also contains organic material. <BR> <BR> <P>By, "mixing"it is meant that at least two separate sources of waste are collected and fed into the treatment vessel in controlled relative quantities by weight. The waste from the two different sources may be mixed in a mixing device or in a shredder or they maybe mixed during agitation in the treatment vessel.

According to the invention, the mixed organic waste is treated by microbial activity.

This may be referred to as digestion. The term'digestion'is used herein to indicate the microbial breakdown of at least some of the organic matter to produce heat. This breakdown is accelerated by changes in the physical nature of the waste. Typically, the microbial activity is bacterial activity. Preferably, the microbial activity is aerobic.

The treatment process is preferably carried out using bacteria in the thermophyllic phase, which normally occurs in the temperature range 60°C-75°C, most preferably around 63°C-70°C. In this phase, very rapid digestion occurs with the production of heat. It is found that the reaction in the thermophyllic phase is much quicker than the commonly used mesophyllic phase which occurs in the range 30°C-38°C.

Accordingly, accelerated decomposition of the waste takes place. However, if the temperature rises above 75°C, there is a danger that the bacteria will be destroyed.

The reaction in the thermophyllic phase results in the natural generation of heat which breaks down the mixed organic waste to produce a material which is suitable for processing to provide a fuel or compost. The reaction will almost always provide sufficient heat to maintain itself without provision of supplementary heat. However, in practice, chemical mixing of the waste can lead to an increase in temperature which assists the commencement of the microbial activity.

Other material may be added to the treatment vessel, for example quicklime, to control pH. This is particularly suitable for the production of compost.

According to the invention, the oxygen level in the gas which is contact with the mixed solid waste being treated does not fall below 5% by volume.

As will be described further below, the treatment vessel is not normally filled completely, so there is a gas space above the mixed solid waste being treated. The oxygen content in this gas space is suitably measured. The skilled person will be aware of suitable techniques for measuring oxygen content. The moisture level may also be measured, as described below.

Preferably, the oxygen content (and, optionally moisture level) of gas removed from the treatment vessel (as will be described further below) is measured. This is a particularly convenient arrangement.

The gas in the space will typically comprise atmospheric nitrogen, oxygen, carbon dioxide and water vapour. The gas in the space can, according to the present invention, contain no methane, ammonia or hydrogen sulphide, as the microbial activity is carried out in the thermophyllic phase..

In order to maintain the oxygen level above 5% by volume, the air can be supplied to the treatment vessel. Air can be supplied continuously throughout at least part of the process or in discrete inputs of air.

In order to replace the oxygen which promotes aerobic activity and to control moisture level in the exit gas, a relatively high airflow rate is required.

The air can be supplied by some form of forced draught. For example, a fan may be provided. The fan may blow air into the treatment vessel. However, it is preferred that there is a fan to draw gas out of the vessel. Where extraction means are provided for withdrawing gas from the treatment vessel, it may be replaced by air supplied through at least one duct. Air can be supplied to the treatment vessel intermittently, but it is preferably supplied substantially continuously.

Many designs of treatment vessel are not substantially sealed so that as long as gas is removed, air will naturally flow in through openings to replace the gas removed.

As fresh air is supplied to the treatment vessel and as gas is removed from the vessel, water vapour will be removed from the mixed solid waste. This helps to control the drying effect, leading to a product having an average moisture level within the desired range.

Air supplied to the vessel may be previously dried by any suitable apparatus, to maximise the drying effect.

According to a preferred aspect of the invention, the moisture level in the gas in contact with the mixed organic waste is maintained at a level below its dew point.

This ensures that water is continuously removed from the mixed solid waste being treated into the gas space by evaporation.

Means maybe provided for monitoring the moisture level in the gas space. Any suitable means may be employed for measuring the moisture level.

The moisture level may maintained below the dew point by supplying air which has a moisture level below the dew point of the mixed solid waste being treated at the temperature of treatment. As the temperature of the treatment will be typically higher than ambient temperature, normal fresh air may be used. Alternatively, dried air, having a moisture level below the moisture level of ambient air, may be used. The main process features which maintain the oxygen level within the required range can also be used to maintain the moisture level within the required range.

The flow of air and gas through the apparatus also removes heat from the apparatus. It is found that an adequate heat balance can be achieved. That is, heat generation by the microbial activity within the concentrated mass of mixed solid waste can be balanced with heat removal by the gas flowing through the vessel so that the temperature is maintained at a desirable level.

According to the first aspect of the invention, the waste should be agitated while it is being digested. This provides further breakdown the mixed solid waste and mixing to ensure that microbes are spread throughout the material. It also exposes different parts of the mixed solid waste to the gas to ensure access of oxygen to the waste and drying of the waste by the gas. Agitation may take place by any suitable means, but it is particularly preferred that the digestion takes place in a rotating aerobic drum.

The drum may be rotated at any suitable rate, and suitably completes one revolution in a time range of 1 minute to 10 minutes, preferably 2-5 minutes, most preferably about 3 minutes. However, a higher rate of rotation may be used during loading and unloading, in order to assist these operations. Typically, the speed can be increased to one revolution per minute during loading and unloading.

As will be described further below, the drum is suitably simultaneously loaded at one end and unloaded at the other end. Loading and unloading typically take place at 4 hourly intervals and can take 30 minutes.

The drum preferably comprises a substantially parallel sided circular section cylinder.

The axis of the cylinder may be inclined to the horizontal, for example at an angle in the range 3°-10° most preferably 5°-8°, to provide gravitational flow through the drum.

Any suitable size of drum may be provided, depending upon the rate of consumption of mixed organic waste. It has been found that, for a processing rate of about 250-500 tonnes per day, a drum of diameter in the range 3.5-6m, preferably 4-6m most preferably around 5. 5m should be used. The length should be in the range from 6 to 10 times the diameter, most preferably about 8 times the diameter, suitably up to 40m.

The drum may be used of any suitable material, for example mild steel.

A rotary drum has the advantage that it is mechanically simple. There are relatively few problems of blocking and very few moving parts, which reduces the risk of breakdown.

The agitation caused by the rotation leads to attrition of the solid waste, further contributing to its breakdown. Preferably, the drum is filled to a high level with waste, being preferably initially 75% to 90% full by volume. This leads to increased attrition, rapid heat generation and also to efficient use of plant.

Average residence time of mixed solid waste in the treatment vessel is suitably in the range 18-60 hours, more preferably around 24 to 48 hours, most preferably around 36 hours The treatment vessel preferably comprises a vessel through which the mixed solid waste is moved during treatment, for example a drum as described above. The mixed solid waste suitably moves from a loading point to an unloading point within the drum. As noted above, loading and unloading suitably occur substantially simultaneously, with fresh mixed solid waste being loaded at the loading end and mixed solid treated waste being removed at the unloading end. The loading and/or unloading operation can take 10-40 minutes, preferably about 30 minutes.

One unloading operation or loading operation is preferably spaced from the following unloading or loading operation respectively by a period in the range 2-8 hours, preferably 3-5 hours, most preferably around 4 hours.

In this way, a"semi batch"process can be carried out.

During processing, it is found that the volume of the material may decrease by as much as 25%. The gas space over the material will accordingly increase.

The waste material should be discharged from the treatment vessel at a stage at which the treated waste material is sufficiently digested and sufficiently dry. This typically occurs after a period of about 48 hours. By restricting residence time to 48 hours or less, additional loss of carbon can be reduced.

In order to promote the microbial activity, some parameters of the mixed organic waste fed to the digestion step are preferably controlled.

In the first place, the mixed organic waste is preferably treated in a first process before aerobic digestion to remove particles of size in excess of 60mm, more preferably 50mm.

This process may comprise a first step in which very large objects are removed, for example by hand or by sieving and a second step in which the remaining material is treated to reduce its particle size, for example by shredding. The person skilled in the art will be able to obtain suitable shredding apparatus.

Alternatively the mixed organic waste may be subjected to an operation to reduce its particle size, for example by shredding without initially removing oversized particles.

The shredding operation is particularly significant, as it mixes the material thoroughly, spreading the microbial culture throughout the material and initiating a thermophyllic reaction, very quickly.

The second parameter which may be controlled is the average moisture content of at least some of the organic waste treated in the method. The average moisture level of this part of the waste is suitably in the range 40-75%, more preferably 55 to 65%, most preferably around 60% by weight. Waste having an average moisture level in the range 30-80%, preferably in the range 60%-70% by weight can also be used All moisture levels quoted herein are % by weight. They are average values, being averaged for quantities of at least 100kg of waste.

Moisture levels of waste may be measured by measuring the moisture level of air or gas over the waste at a fixed temperature and in equilibrium with it.

It has been found to be acceptable if the average moisture level, of mixed waste fed in to the digestion step is in the range 50-60% by weight and preferably 53-57% by weight.

If the mixed organic waste after mixing is low in organic content or moisture level, process water maybe preferably added in controlled quantities. This process water is preferably waste water from water treatment, most preferably dewatered sewage sludge.

This material has a high nitrogen content and acts as a catalyst for the microbial reaction.

According to the first aspect of the present invention, control of the moisture level is obtained by blending mixed organic waste with other waste of a lower average moisture level. It is found that mixed domestic waste typically has a moisture level in excess of 50% by weight. Agricultural waste may have a moisture level in excess of 75% by weight and sometimes 80% by weight, particularly in tropical or sub-tropical countries for crops such as bananas and pineapples. Finally, commercial waste from offices and factories is typically much drier, having a moisture level in the range 10%- 30% by weight.

The moisture level of waste fed to the digester may be manipulated by altering the mixing ratios of different types of waste. It is required that at least part of the waste fed to the digester has a moisture level in the range 40-75% by weight, preferably 55 to 65% by weight in order to promote the faster thermophyllic reaction. However, part of the waste fed to the digester may comprise a relatively dry commercial waste. The heat generated by the digestion of the moist waste is sufficient to treat the whole of the waste fed to the treatment vessel. However, during the agitation process, the commercial and domestic waste are slowly mixed together reducing the overall moisture content of the mixture, so that at the end of the processing, the moisture level does not exceed 45% by weight and preferably does not exceed 25% by weight.

Solid waste with higher moisture level may be blended with solid waste with lower moisture level in blending apparatus in a controlled manner. The relative quantities of different types of waste are controlled so that the desired average moisture level over the combined masses of mixed wastes is obtained as explained above.

The blending step also allows absorbent material such as paper and paper based material (which is particularly common in commercial waste) to be blended intimately with the moist waste (such as domestic waste). The absorbent material absorbs liquid rich in bacteria, providing a substrate for the bacteria to grow on and allowing the bacteria to be spread throughout the waste being processed. This promotes reaction and mixing, leading to an improved digestion. Further, the wetting of the paper helps it to be broken down.

In processing mixed organic waste it is particularly important to produce a product which is substantially homogeneous, at least at the scale of mm or above. The blending step helps to improve the homogeneity of the product.

However, although blending takes place, it is found that the moisture level remains concentrated in local areas of the waste, where it is sufficiently high to allow the thermophyllic reaction to commence and proceed very rapidly.

The relative quantities of different types of waste fed can be controlled using automatic weigh feeders.

By way of example, the moisture level of mixed organic waste during processing may be as follows.

Domestic waste with a high organic content and moisture level above 50% can be mixed with commercial waste having a moisture level of 20% or below in a suitable ratio to provides a blend having an average moisture level in the range 45 to 55% by weight.

During microbial treatment, a part of the moisture is absorbed by the gas and air flowing over the material being processed. The average moisture level may drop to around 30-40% by weight, preferably 25 to 30% by weight.

During emptying of the treatment vessel, the waste which still has a high residual heat level, may be dried by a forced draught as described above, so that the moisture level drops to the range 30-40% by weight, preferably 25 to 30% by weight. The treated waste may then be further dried on a drying floor as described above, so that the moisture level drops to below 25% by weight.

It has been found that the process of the first aspect of the present invention for treating domestic waste can allow between 75% and up to a maximum of 85% by weight of delivered waste to be recycled. A proportion of the organic material fed to the process is consumed by microbial activity to generate heat, which is used in the drying of the material. However, the quantity which is consumed in this way is relatively low. The remaining waste not consumed or recycled has to be rejected and disposed of by conventional means.

A further parameter which may be manipulated is the pH of the mixed organic waste.

This is suitably in the range 6.0-8. 5, preferably 6.3-7. 3, most preferably around 6.8.

Nitrogen level has an impact on microbial activity, and adjustment of pH and nitrogen content can be advantageous.

It has been further found that the density of the mixed organic waste fed to the treatment vessel digestion process is suitably not too low. Preferably, the density is not less than 450g per litre, preferably not less than 750g per litre. Again, the blending step is particularly useful here. Domestic waste can have a relatively high density. The average density can be controlled by admixing a suitable quantity of commercial waste, which has a comparatively low density.

Preliminary Treatment As described above, the mixed organic waste may be subjected to various types of treatment before the treatment process. Preferably, the previous steps include any or all of the following: 1. Picking Initial treatment to remove objects which are not combustible, such as stone, concrete, metal, old tyres etc. Objects having a size in excess of 100mm or more may also be removed The process can be carried out on a stationary surface, such as a picking floor.

Alternatively or additionally, the mixed organic waste may be loaded onto a moving surface such as a conveyor and passed through a picking station in which mechanical or manual picking of the material takes place.

2. Shredding Shredding is a highly preferred step. It is carried out to reduce the average particle size.

It can also be used to increase blending of waste from different sources. It also makes the treatment process more effective. It is found that, during the shredding process, microbial activity may commence and rapidly raise the temperature passing very quickly through the mesophyllic phase into the thermophyllic phase.

Screening The mixed organic waste may be mechanically screened to select particles with size in a given range. The given range may be from 10mm to 50mm. Material less than 10mm in size comprises dust, dirt and stones and is rejected. The mixed organic waste may be treated to at least two screening processes in succession, each removing progressively smaller fractions of particles. Material removed in the screening process as being too large may be shredded to reduce its average size. Material which is classified by the screen as being of acceptable size and, where applicable, shredded material can then be fed to the treatment vessel.

Subsequent Treatment The treated material may be subjected to a number of steps after the treatment process.

These steps may include any of the following: 1. Grading The material may be screened to remove particles in excess of a given size. For example, particles in excess of 50mm may be rejected. They may be subsequently shredded to reduce their size, returned to the aerobic digester or simply rejected.

2. Metal Separation Relatively small metal particles such as iron or aluminium may have passed through the system. They can be removed, for example by a magnetic or electromagnetic remover in a subsequent step. Metal particles removed from the system may then pass to a suitable recycling process.

3. Drying Suitably, after treatment in the treatment vessel, the material is subjected to an additional drying step. However, with the first aspect of the present invention, because the moisture level does not exceed 45% by weight, more preferably not exceeding 35% by weight and most preferably not exceeding 25% by weight, after the microbial treatment, the subsequent drying can be carried out relatively simply.

For example, in a first stage, a forced draught of air may be provided during or after the unloading phase from the treatment vessel. During this stage, the treated material will still be at high temperature (for example in the range 50-60°C) and further moisture can be removed simply by forcing air over it.

A further drying step may comprise laying the material out on a drying floor. In this step, material is laid out at a thickness of not more than 20cm over a relatively large area for a suitable period of time, during which the moisture level drops. The material may be agitated, for example by turning using mechanical or manual apparatus such as a power shovel. The material may be turned at intervals of for example of 2-4 hours preferably around 3 hours. Preferably, during this stage, the moisture level drops to below 25% by weight after which no further biological decomposition occurs. Suitably, the material is left on a drying floor for a period in the range 18-48 hours, preferably 24- 36 hours, more preferably around 24 hours.

It is also found that further drying may take place during subsequent processing, due to the mechanical input of energy.

Waste heat from other process equipment, for example from furnaces, may be used to dry the material.

4. Pelletising In order to convert the treated material to fuel, the material may be classified according to size and subsequently densified to provide pellets of suitable size which can be subsequently stored or packed for use. During this stage, further drying of the material may occur, due to heat generation caused by friction and due to further exposure to air.

Preferably, in order for pelletising to proceed well, the moisture level of the treated material is in the range 28-35% by weight.

5. Bagging Where the material is to be converted into compost, it can simply be bagged after treatment.

Further use of treated material The method and apparatus of the invention can be used to produce a product which is suitable for subsequent storage, transportation or sale.

It has been found that the method of the invention can provide a fuel, referred to as Green Coal, which has a calorific value in the order of 14, 500kJ/kg which is about half that of industrial coal. The material has an ash content of less than 20% by weight and has the additional advantage that it will contain relatively low levels of sulphur and chlorine, so reducing pollution due to acid rain from gases of combustion.

By blending different sources of waste material, fuel produced at different times or with waste from different locations can be relatively homogeneous in terms of : 1. Calorific value-suitably in the range 13,000 to 16,500 kJ/kg, 2. Density-suitably in the range 270-350g/l more preferably around 300g/l 3. Moisture level-below 30% by weight and preferably around 20% by weight.

This fuel can be used either on its own or as a supplementary fuel.

Alternatively, the material can be used as a multi purpose organic compost having a relatively high nutrient content.

The apparatus and method of the invention may alternatively form a part of a plant or system. The apparatus and method of the invention can be used to supply fuel in a plant or system. For example, the material may be fed directly to a combustion chamber for generating heat or power. The apparatus and method of the invention can be used to supply a feed to a pyrolysis process, as described below.

The treated material may be fed directly from the treatment vessel to the plant or system or it may be treated by any suitable steps such as grading, metal separation, drying, pelletising or bagging as appropriate.

In a particularly preferred embodiment, there is provided an electrical power supply system comprising apparatus according to the invention for feeding material to the combustion chamber of a boiler for providing steam for power plant.

In an alternative embodiment, there is provided a cement producing apparatus comprising a cement kiln and an apparatus according to the invention for supplying fuel to the cement kiln.

In this embodiment, complete heat transfer from the fuel to the cement producing materials is obtained as they are mixed together in the kiln. The ash from burning the material of the present invention, is absorbed into the cement material. Waste heat from the kiln can be used to dry the treated waste from the present invention or to provide power for operating the process.

The present inventor has further realised that mixed organic waste fed to a pyrolysis process can be initially treated microbially in conditions where the oxygen level of gas in contact with the waste being treated does not fall below 5% by volume.

Accordingly, in the second aspect, the present invention provides a method of treating mixed organic waste, comprising treating the waste by microbial activity under conditions in which the oxygen level in the gas in contact with the waste does not fall below 5% by volume and subsequently treating the treated waste in a pyrolysis process.

The present invention further provides in the second aspect, apparatus for treating mixed organic waste, comprising a microbial treatment vessel, means for feeding microbially treated mixed organic waste from the treatment vessel to means for pyrolysing the treated solid mixed organic waste, the aerobic microbial treatment vessel being controlled so that the oxygen content in the gas in contact with the waste during treatment does not fall below 5% by volume.

The pyrolysed material may be used as a fuel in its own right. However, in a preferred embodiment, the pyrolysed material may be fed to a gasification process in which combustible gases are produced by introduction of a gasifying agent. This will normally require the pyrolysed material to be at a high temperature and the gasification process preferably occurs directly after the pyrolysis process.

The pyrolysis process and gasification process may be carried out in separate zones, for example as described in W097/15641 and W097/15640, or in a common zone, for example as shown in GB2301659.

Suitably, the gasifying agent comprises air, steam or water vapour.

Preferred methods and apparatus for the pyrolysis and gasification processes are disclosed in GB2301659, W097/15640, W097/15641 and WO01/96501.

Suitable apparatus may also be obtained from Metso Corporation, or from the Allis Chalmers Corporation.

Preferably, the microbial treatment comprises bacteriological digestion, suitably aerobic bacterial digestion.

Suitably, it is carried out using the thermopyhilic phase, which normally occurs in the temperature range 60°C-75°C as described above in relation to the first aspect of the invention. At this temperature, substantially no methane and no ammonium compounds are generated. Suitably, the treatment takes place in a container, for example a rotating drum. Preferred aspects of the drum are as set out above for the first aspect of the invention.

Preferably, the mixed organic waste is treated in a manner which allows its moisture level to be controlled.

It is particularly preferred that the mixed organic waste is treated by a method according to the first aspect of the invention. Suitably, an apparatus according to the first aspect of the invention is used. The preferred features of the first aspect of the invention described above apply to the second aspect of the invention.

However, instead of the optional treatments of grading, metal separation, drying, pelletising and bagging described above subsequent to the microbial treatment process, the treated material is suitably fed directly to the apparatus for pyrolysis. Any suitable feed means may be used for delivering the treated waste from the microbial treatment vessel to the pyrolysis apparatus.

As the microbial treatment is typically carried out in a semi batch wise fashion, whereas the pyrolysis process typically requires a continuous feed of material, an interim storage means, for example in the form of a feed hopper may be provided. It is preferred that there is a first delivery means for receiving treated mixed organic waste from the microbial treatment process and feeding it into the interim storage means and a second feed apparatus for feeding the stored treated solid waste from the interim storage means to the pyrolysis apparatus. The second feed means is preferably operated substantially continuously. The first and second feed apparatus may comprise any suitable means, for example conveyor belts or screw feeders.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic process diagram of steps involved in the process of the first aspect of the present invention before the rotary aerobic digester.

Figure 2 is a schematic process diagram showing the steps involved in feeding the rotary aerobic digester and drying the material in the first aspect of the invention.

Figure 3 is a schematic process diagram showing further steps in the procedure.

Figure 4 is a schematic process diagram showing a process according to the second aspect of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS Figures 1,2, and 3 from a continuous process diagram which has been divided into three sections for convenience of illustration.

Figure 1 shows the steps involved in an embodiment of a process according to the first aspect of the present invention for manufacturing fuel from solid mixed organic waste.

In this embodiment, two sources of waste 101 and 102 are identified. Source 101 comprises a source of domestic waste, which comprises organic waste which typically has an average moisture in the range 45-60% by weight. Source 102 comprises a source of commercial waste which may have moisture averaging around 20% by weight.

At 103, waste from the sources 101 102 is delivered to a reception and picking floor.

On the reception and picking floor, waste supplies from the two sources 101 and 102 are kept separate. On the picking floor, waste is manually picked over to identify objects which are not suitable for further processing, for example, metal, large plastic objects, etc. The rejected objects are collected at 104 and disposed of separately, for example by tipping or by recycling if appropriate.

The dotted line in step 103 indicates separation of the solid waste from the two sources.

On the left hand side of figure 1, domestic waste which has been manually picked over is then fed at 105 onto a feed conveyor 106 using a loading hopper arrangement.

Further manual sorting of the solid waste can take place whilst the waste is on the feed conveyor 106 for example by personnel standing on both sides of the conveyor. The rejected waste can be disposed of in step 104 as described above.

The feed conveyor 106 feeds the sorted waste to a rotary screen separator apparatus 107. In a first rotary screen separator 108, particles of size less than 50mm are passed and allowed to fall onto a conveyor 109 for subsequent feeding to the rotary aerobic digester as will be described further below in figure 2. Material screened out by the rotary screen separator 108 is then screened in a further rotary screen separator 109 which passes objects of size less than 100mm.

Objects of size less than 100mm are then fed to a domestic waste shredder 110 which reduces their size to below 50mm and greater than 10mm. Thereafter, the shredded waste is passed onto to the conveyor 109.

Objects of size greater than 100mm are fed onto an"oversize conveyor 112"for further processing.

Returning to the reception floor 103, commercial waste which has been sorted is directed to a commercial waste conveyor 113. Whilst this is on the conveyor, the waste may be subject to further hand picking by personnel located adjacent to the conveyor.

Material from the over size conveyor 112 and commercial waste from the commercial waste conveyor 113 are combined and together fed into a commercial waste shredder 114 of suitable design which shreds the waste so that it has a particle size not greater than 50mm.

In Figure 2, the waste products on the conveyor 109, which will comprise unmixed domestic and commercial waste are blended together in a blender 115. Further, paper and paper based material in the commercial waste absorbs moisture in the domestic waste which is rich in bacteria, thus spreading the bacteria throughout the material and providing a substrate to promote bacterial action.

The blended waste is fed from the mixer 115 intermittently to a treatment vessel in the form of a rotary aerobic digester 116. The aerobic digester 116 comprises a cylindrical steel vessel of diameter 5. 5mm and length 33m which is mounted with its axis at an angle of approximately 7° degrees to the horizontal with waste being fed in at the loading end 117. Waste is fed into the loading 117 end at intervals of approximately 24 hours, and treated waste is simultaneously discharged at the opposite end.

A fan 148 is provided for drawing gas out of the rotary aerobic digester. The rotary aerobic digester is not sealed, so that when gas is removed by the fan 148, atmospheric air flows in to replace the gas removed, thus creating a steady current of air and gas throughout the apparatus. The fresh air introduces oxygen to replenish that consumed by the microbial activity.

A sensor 149 is provided which senses the oxygen content in the gas at the top of drum, above the level of the material being treated. Signals representing the oxygen comment of the gas from the sensor 149 are fed to a controller 151 which controls the fan, so that increased flow of air can be provided if the oxygen level in the gas in contact with the material being treated falls too low. Further, a moisture sensor 150 is provided for sensing the moisture level in the gas in contact with the material being treated. Moisture level signals are transmitted from the sensor 150 to the controller 151. The sensor 150 may also sense the temperature of the gas in the drum. Alternatively, the temperature may be sensed by a different sensor or it may be maintained at a predetermined level.

The controller is set up to determine if the difference between the moisture level in the gas in the drum differs from the dew point of the gas in the drum at the temperature determined or predetermined. If the difference between the measured moisture level and the dew point is less than a fixed amount, a control signal may be sent to the fan 148 to increase the airflow through the drum 116, so that the moisture level can be kept below the dew point and so that drying can continue.

The digester is normally rotated at a rate of approximately one revolution every 3 minutes.

During loading and unloading, the rate of rotation is increased to 1 rpm to help feed material down the digester. As a result, the waste inside the rotary aerobic digester is gently agitated. Also, as the digester is mounted at an angle to the horizontal, the waste slowly feeds downwards from the loading end 117 to the unloading end 118.

During the loading phase, as mentioned above, a mixture of domestic and commercial waste which has been pre-treated is fed from the mixer 115 into the loading end 117 of the rotary aerobic digester. By controlling the rate of feed of treated domestic waste on the conveyor 109 and treated commercial waste on the conveyor 113 and the air flow into the drum 116, the average moisture level of the solid waste fed into the loading end 117 of the rotary aerobic digester 116 can be controlled.

The temperature inside the rotary aerobic digester 116 is maintained at a temperature in the range 65-75°C, most preferably around 68-70°C. Although it is possible to provide auxiliary heating means, this is not normally required, as the moist solid waste fed into the digester undergoes a thennophyllic aerobic reaction which leads to the generation of sufficient heat to maintain the temperature at the desired level.

It can be seen that the screening and shredding apparatus can be used to ensure that the waste fed into the rotary aerobic digester does not have a particle size greater than 50mm. Further, by controlling the relative quantities of commercial and domestic waste fed into the rotary aerobic digester using the blending system, the overall moisture level of at least part of the load can be maintained at the high level leading to a high rate of reaction. Further whilst the waste is on the conveyor or picking floor, it can be tested to ensure that its pH is the correct range and suitable additives can be added to correct the pH in a manner known to the person skilled in the art if necessary. Further, the shredders can be operated to ensure that the density of the waste material does not fall below 750gm per litre.

It has been found that if the solid waste fed into the rotary aerobic digester meets the following parameters, a high rate of aerobic digestion can be obtained: 1. less than 50mm in size 2. Moisture content between 50 and 70%, ideally 60% by weight.

3. pH between 6.5 and 8.0 4. density not less than 750gm per litre.

The rotary aerobic digester is operated so that waste has a residence time of approximately 24 hours inside the digester. During an appropriate unloading phase, digested waste is unloaded at the unload end 118 where it is sieved. Digested waste with a particle size less than 50mm is collected at 119. This material is suitable for formation of fuel pellets as will be described further below. Treated waste with a particle size in excess of 50mm is collected at 120 and rejected. A grid inside the digester, over the final 1 metre length, for passing objects of size less than 50mm is provided, to reject oversize material.

In step 121, the sieved treated solid waste is treated in a metal separation stage, for separating out metals such as iron and aluminium In a manner known in the art, electromagnetic or magnetic apparatus can be used to separate various materials which can be collected at 122 for suitable recycling.

The treated waste from which metal has been received can be spread on a stockpile/ drying floor 123.

Treated waste collected at the unloading end 118 has a moisture level of not more than 35% by weight and preferably not more than 25% by weight. A certain amount of the moisture in the treated solid waste material goes into the gas over the material in the rotary aerobic digester. A certain amount of the moisture is driven off for example by a forced draught fan at the unloading stage 118 when the aerobic digester is opened the waste sieved.

The moisture level of the treated waste loaded onto the drying floor is typically in the range 20-25% by weight and the material will still be at a temperature in the range 50- 60°C from the drum. The material is fed into a layer not more than 20cm thick, where it is allowed to dry by natural evaporative drying and mechanical turning. As a result, the moisture level drops to a level of less than 25% by weight. At this stage, further bacterial decomposition of the material ceases and the product becomes stable and storable.

In Figure 3, dried treated waste from the stock pile 123 is loaded onto a feed conveyor 124 where it is fed to a classifier 125 for separating treated waste which is of too large a diameter. Particles of size in excess of 50mm are rejected and collected at 126.

Particles of size less than 50mm are collected in a hopper 127 from whence they are fed to a feed stock transfer conveyor 128 which transfers the treated waste to densifiers 129 and 130. The densifiers compress and pelletise the waste and further reduce the moisture level. Over spill from the conveyor 128 is collected at 132 and returned to the conveyor 134. Pelletised treated waste is collected on a conveyor 131 and delivered to a stockpile or bagging stage at 133.

EXAMPLE A process according to figures 1-3 was operated with a feed a comprising 75000 tons per annum of mixed municipal waste. The average moisture level of waste input to the rotary aerobic digester was 60% by weight, with some batches having higher moisture level and some batches having lower moisture level.

Treated waste collected from the rotary aerobic digester, after unloading and fan drying had a moisture level of about 30% by weight.

After 24 hours residence on the drying floor 123, the moisture level had dropped to below 15% by weight and the product was stable.

From an input of 75,000 tonnes of combined waste materials, 40,000 tonnes of GREEN COAL can be produced, having a calorific value equivalent to 15,000 tonnes of fuel oil.

The graded and pelletised product was found to have constant thermal characteristics and to be suitable for use as a supplementary fuel, going by the description of GREEN COAL. It had a minimum calorific value of 14,500 kJ/kg. It had an ash content of less than 20% by weight and very small quantities of sulphur and chlorine.

Figure 4 shows a schematic process diagram of a second embodiment of the present invention.

The embodiment of Figure 4 will employ the system as shown in Figure 1 for the pre- treatment of solid waste material and digestion in a rotary aerobic digester 116 as shown in Figure 2. However, the embodiment of Figure 4 is different to that of Figures 2 and 3 in that a different system is employed downstream of the aerobic digester. At the output end 118 of the rotary aerobic digester, the treated material is fed into a gravity hopper 118 for storage of material. The stored material is fed from the gravity hopper 134 by a screw feeder 135 into a pyrolysis chamber 136 in which the solid waste material is transported by a screw while being heated to a temperature in excess of 800°C, sufficient to cause pyrolysis of the treated waste material. As a result, a mixture of combustible gases is produced at 139. The solid material is reduced to a char, largely comprising carbon and ash.

The char may, in some embodiments, be used as a fuel. However in the process shown in Figure 4, the char is fed into a gasification chamber 138, where it is maintained at a temperature in excess of 800°C and treated with steam so that a mixture of carbon monoxide and hydrogen are generated at 140. As a result, the char is converted to a non-combustible ash 141 which is subsequently disposed of, suitably by landfill.

The combustible gases from the pyrolysis chamber 139 and the producer gas 140 from the gasification chamber 138 are burnt at high temperature in a combustion chamber 142. As a result, exhaust gas is produced. The exhaust gas has a very low content of pollutants in particular, NOX gases, because of the combustion conditions.

The combustion of the gases 139 and 140 is used to produce steam, some of which is fed at 144 into the gasification chamber and the remainder of which is fed at 143 into a steam turbine power plant 145 for the generation of electricity. A portion of the electricity from the power plant can be distributed at 146 for running the entire apparatus of the embodiment of Figure 4. Power can also be delivered at 147 to consumers, via the National grid for example.

The pyrolysis and gasification apparatus is shown schematically only. It is suitably as described for example in W097/15640 or W097/15641. Alternatively, the gasification and pyrolysis chambers may be combined, for example as shown GB2301659.