GB2208645A

GB2208645A - Processing waste

Info

Publication number: GB2208645A
Application number: GB8819374A
Authority: GB
Inventors: Peter Hood
Original assignee: FUTURE FUELS Ltd
Current assignee: FUTURE FUELS Ltd
Priority date: 1987-08-13
Filing date: 1988-08-15
Publication date: 1989-04-12
Also published as: GB8819374D0

Abstract

In a plant for processing municipal waste, the waste is passed through a reception and storage stage, a shredder, a screen, magnetic separator to an anaerobic digestion stage producing biogas, which is fed to a power generation stage, and solid material, which is fed to a filter press. The dewatered residue constitutes an ideal growing medium or fertilizer and has a high nutrient value. If waste with a high organic content is to be processed, some or all of the pro-digestion stages may be omitted.

Description

Processing waste The present invention relates to a method of and a plant for processing waste.

Waste disposal is a problem for municipal authorities.

Landfill techniques are expensive in terms of labour and transport costs and intrusive because of the smell of rotting garbage and the presence of a leachate which causes pollution when it reaches the water table.

The present invention seeks to overcome or at least reduce the above problems.

According to a first aspect of the present invention there is provided a method of processing municipal 'waste comprising sorting out at least some non biodegradable components, comminuting the remaining material and then subjecting it to a substantially anaerobic digestion process. Further sorting may occur during or after the comminuting stage.

Municipal waste is defined as a mixture of domestic waste and/or trade and commercial waste and/or civic amenity waste.

According to a second aspect of the present invention, there is provided a plant for processing municipal waste comprising a reception and sorting stage, a mechanical conditioning stage comprising means for comminuting the waste, and a substantially anaerobic digestion stage for the comminuted material.

At the sorting stage, the nature of a load is identified, e.g. by an operator, and if it comprises a substantial proportion of bio-degradable material it is passed on to the mechanical conditioning stage. The mechanical conditioning stage preferably comprises a shredding unit. After shredding, ferrous scrap may be removed. The anaerobic digestion stage produces biogas (chiefly methane) which is preferably supplied directly to a gas engine to supply electricity.

According to a third aspect of the present invention, there is provided a method of processing municipal waste comprising sorting out at least some nonbiodegradable components, comminuting the remaining material and then subjecting it to a substantially anaerobic digestion process.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawing of a municipal waste-processing plant.

Municipal waste arrives at a reception and sorting stage 10. Civic amenity waste and commercial loads, which contain substantial quantities of materials having little intrinsic energy content and which are unsuitable for processing in accordance with the present invention, are directed to a sorting zone 11.

Such materials comprise commercial equipment items, building rubble etc. Materials deposited in this zone are sorted with mobile mechanical machinery. Portions of this waste which are suitable for shredding are transported to the main loading apron for the shredder 20. The remaining portion, unsuitable for shredding, is reloaded on to transportation for landfill, indicated at 22 in the Figure.

Deliveries of waste arriving on site and which are predominantly suitable for processing in accordance with the present invention are directed into the feed hopper 12 which supplies the shredder 20. Typical moisture content at this stage is 20-40% by weight.

In order to accelerate the reaction rate and achieve a high efficiency in the anaerobic digester stage 30, it is desirable to deliver the feed in a finely divided, homogenous condition. This is achieved in the mechanical conditioning stage, comprising a shredder 20, a trummel screen 24 and a overhead magnetic separator 25. Shredder 20 is preferably a Svedala-Arbra model 18/24 shredding unit. Raw feed from the inlet hopper 12 gravitates on to the shredder feed conveyor.

This conveyor is aligned with the shredder feed opening and the raw waste is force fed into the shredder.

The shredder has a rotor which carries four hammer shafts upon which a series of hammers are pivoted. The rotating hammers pass between a static row of knives in the process of which they crush and chop the waste into small fragments. The shredder contains an outlet grating which is selected to pass particle sizes of 50mm or less. The particle size can be arranged to be adjustable. The larger particles recycle within the machine until they are sufficiently reduced in size to pass through the grating. Further details of the shredder and shredding process are given in GB patents 1248995 and 2016951.

The material leaving the shredder is then graded in trummel screen 24 with a 50mm mesh size. A small amount of oversized material, e.g. large plastic sheets, is separated out and rejected at this stage as indicated at 22. The bulk of the material will pass through the screen and is conveyed onward to overhead magnetic separator 25. At this point the ferrous materials are separated out and rejected from the process; they may be sold as scrap or alternatively rejected as indicated at 22.

At this point in the process substantially all the material remaining (except any glass present) is biodegradable, and has a moisture content of 35-60%, preferably 40%, by weight. The volatile solids content of the dry material is from 45 & to 85% and preferably averaging 60-65% by weight. The material is then passed to an anaerobic digestion or methanation stage 30 comprising one or more digester units connected in parallel. The digestion units consist of steel tanks which hold the waste at the desired reaction conditions (i.e. 500C-600C and at a slightly positive pressure) for the time necessary to complete the reactions.The nature of the process requires the units to conform to a certain height to diameter aspect ratios and the number of units for a given waste processing capacity is, therefore, affected by any height restrictions which may be imposed on the process site. Use of the maximum permissible height minimises the number of units required. On the other hand, a multiplicity of units allows the addition of a spare digester at minimal additional cost.

'Fines' from the mechanical conditioning stage are conveyed to the top of the digestor tanks where they are fed into the tanks through a sealed hopper arrangement. The tanks are maintained at a controlled temperature and the feed material gradually progresses through the tanks from top to bottom at a rate which gives it the required residence time. During its time within the tank the waste undergoes degradation reactions which result in the production of gas and a residual fibrous solid material. The gas consists mainly of methane with some carbon dioxide. This is removed from the top of the tanks and piped to the power generation stage 40. The residual solid material is removed from the base of the tanks and conveyed to a filter press 32.

Besides the temperature, the following parameters are also carefully controlled within the digestor stage 30: pressure, water content, pH, and the nature and concentration of the various microbes. To avoid imbalance in the content of the units, continuous charging every day is preferred, although the missing of a day occasionally, e.g. at weekends, is not unduly detrimental.

The viscous residue taken from the base of the digestor tanks is saturated with aqueous liquor (e.g.55% to 80% and typically 75%). This liquor is extracted from the residue by means of filter presses and returned at 34 into the digestors. The dewatered residue, which still contains 25% to 45% water and may be subjected to further drying processes, is then conveyed into an adjacent storage area 36 to await loading into lorries for disposal. The residue at 36 is no longer obnoxious, (having no leachate) is highly organic and has a high nutrient value in, for example, potassium, nitrogen, phosphate and sulphates. It constitutes an ideal growing medium and can be sold as top cover material.

One type of suitable anaerobic digestor unit suitable for use in the process of the present invention is that developed by Cardiff Laboratories for Energy and Resources Ltd; and may be as disclosed in GB patent 2155460. Alternatively there may be employed an anaerobic digestor unit of the type produced by Valorga S.A. of Vendarges.

So far processes of this type have been applied to waste of a single special type. An important feature of the present invention is the realisation that such processes can be applied to general waste providing this is suitably pre-treated.

The shredding unit of the mechanical conditioning stage 20 can readily handle 30 to 50 tonnes/hr of a feed consisting predominantly of domestic and biodegradable waste. This gives an approximate capacity of 100,000 tonnes/year through the plant. Of this 13,700 tonnes would be rejected at zone 11 and 8,000 tonnes would be rejected by trummel screen 24 and separator 25. This would leave a supply of 78,300 tonnes of shredded feed to the digestors which would produce 6 to 20 million cubic metres of biogas giving at least 2.0 MW of electrical output and at least 3.5 MW of low grade heat.

The above-described process has a number of advantages.

Its prime advantage is economic: the acceptance of municipal waste, the supply of growing medium at 36, and the supply of electricity are all sources of regular and reliable income, although the prime aim is the processing of the waste.

With the use of an anaerobic process, the presence of putrescible material is a positive advantage and improves the efficiency of the process. Energy is produced as electric power which has a virtually unlimited outlet via the national grid. The residue material is inert, odourless and can be sold as topsoil. The plant required is robust, requires little operator attention and can be left unattended for long periods. The equipment is simple and low in capital and maintenance costs, and is all concentrated at a single compact site.

The production of electricity also involves the production of low grade heat which may also find a commercial outlet, e.g. in a brewery or for drying purposes.

Various modifications may be made to the abovedescribed process. For example, the bio-gas may be stored and sold as a fuel instead of being used immediately. At the input to the plant, the glass content of the waste may be removed which improves the efficiency of the process and the quality of the growing medium at 36. Alternatively, or in addition, the paper product content of the waste may be separated at the plant input.

The invention may also be applied to the processing of solid waste, especially waste substantially comprising entirely organic matter, and the pre-digestion stages may be omitted. The comminuting stage is usually retained, however, unless the waste particle size is already smaller than 50mm. Although the application of anaerobic digestion processes to liquids is known, their application to solid waste has numerous advantages in terms of converting an unwanted bulky product into useful outputs, e.g. electricity and/or biogas, and fertilizer. The following organic waste products are particularly suitable for such processing: pineapple waste, e.g. husks, sugar products, peas, e.g.

pods, potatoes, e.g. skin, and/or fish.

Claims

1. A method of processing municipal waste comprising sorting out at least some non-biodegradable components, comminuting the remaining material and then subjecting it to a substantially anaerobic digestion process.

2. A plant for processing municipal waste comprising a reception and sorting stage, a mechanical conditioning stage comprising means for comminuting the waste, and a substantially anaerobic digestion stage for the comminuted material.

3. A plant according to claim 2, wherein the mechanical conditioning stage comprises a shredder for comminuting the waste, a screen and a magnetic separator.

4. A plant according to claim 3, wherein the shredder comprises a plurality of hammers which pass between a row of knives.

5. A plant according to claims 3 or 4 wherein the screen has a mesh size of substantially 50mm.

6. A plant according to any of claims 2 to 5, wherein the anaerobic digestion stage comprises one or more tanks are at a temperature of 500C - 600 and are at a positive pressure.

7. A plant according to any of claims 2 to 6 wherein biogas produced by the anaerobic digestion stage is supplied directly to a gas engine to produce electricity.

8. A method of processing solid waste comprising subjecting it to a substantially anaerobic digestion process.

9. A method according to claim 8, wherein the waste is substantially entirely organic waste.

10. A method according to claim 8 or 9, wherein the waste is from the food processing industry.

11. A method according to claim 10, wherein the waste is from pineapple, sugar, peas, potatoes and/or fish.

12. A method of processing municipal waste or waste with a high organic content substantially as herein described.

13. A plant for processing municipal waste or waste with a high organic content substantially as herein described with reference to the accompanying drawing.