WO2004076083A1

WO2004076083A1 - Method for treating waste

Info

Publication number: WO2004076083A1
Application number: PCT/GB2004/000745
Authority: WO
Inventors: Karl Stephen Hick; Peter Rowland Coe
Original assignee: Tass Environmental Technology Ltd
Priority date: 2003-02-25
Filing date: 2004-02-25
Publication date: 2004-09-10
Also published as: EP1596997A1; GB0304215D0

Abstract

Method for the treatment of waste, in which the waste is subjected to a high-pressure, high-temperature environment, and is then subjected to baterial digestion. The treated waste is then used or disposed of in an environmentally acceptable manner, for example as a compost.

Description

METHOD FOR TREATING WASTE

The present invention relate to a method of treating waste and more particularly to the treatment of municipal solid waste.

Municipalities such as cities, towns, and villages generate a large amount of waste which has to be collected and disposed of. A favoured method for the of disposal of such waste is to use landfill. However, the amount of waste generated is increasing by approximately 3% per annum, whilst the number of sites which are suitable for use as landfill are decreasing. Additionally, the European Union Landfill Directive limits the amount of waste which can be diverted to landfill. The Directive states that the amount of biodegradable municipal waste that can be put to landfill in the future will be limited to 75% of that landfilled in 1995 by 2010, 50% by 2013 and 35% by 2020. The Landfill Directive will also require that, in future, all waste is treated prior to disposal in a landfill site. Similar legislation may exist now or in the future in other countries. In any case, this method of disposal also has the disadvantage that poorly contained sites may lead to hazardous materials leaking into the environment. Furthermore, the areas used for landfill look unsightly and are becoming increasingly unacceptable to the public.

An alternative method of disposal to landfill is direct incineration of the waste and the production of refuse derived fuel for incineration. The disadvantages of both direct incineration and the incineration of refuse derived fuel is that if the incineration is incomplete or there is a large amount of plastics material present in the waste, this can result in the production of a number of pollutants. Furthermore, the public are strongly opposed to incineration. The incineration of waste does not help Local Authorities reach their recycling targets.

Previous proposals have been suggested as alternatives to landfill and incineration and these include treatment of the waste to separate any materials which can be recovered for recycling such as ferrous metals, aluminium, glass, plastics and textiles. However, such treatments also produce an end product consisting of a mixture of paper and putrescible material which forms a fibre-like product requiring disposal under controlled conditions as discussed above.

It has also been proposed that waste material, including municipal waste, may be treated by subjecting this to anaerobic bacterial digestion. This produces a liquid fraction, a methane rich gas fraction and a solid fraction which is more acceptable environmentally than the untreated waste. It has been proposed that the treated solid fraction may be used as a soil conditioner or as a fuel. Examples of such systems are found in GB-A- 2230004 and WO 96/23054, the disclosures of which are incorporated herein by reference. These disclosures suggest that the treatment of the solid waste is part of a system involving the treatment of liquid waste.

It is an object of the present invention to provide improvements in relation to one or more matters discussed herein or generally.

According to the invention there is provided a method for the treatment of waste, said method comprises:

a. collecting said waste material in a treatment vessel;

b. subjecting said waste to an environment comprising a pre-determined temperature and pre-determined pressure for a pre-determined time period;

c. recovering at least part of the waste material from the treatment vessel;

d. transferring said treated waste to a further treatment vessel;

e. subjecting said treated waste to bacterial digestion; and

f. recovering said treated waste for disposal in an environmentally acceptable manner.

In a preferred aspect of the invention, during the first treatment step, the waste is collected and placed in a vessel which is sealed, such as an autoclave. The waste is then subjected to an increase in temperature and pressure using the injection of steam, for example taking the temperature up to in the region of 150 to 170 degrees centigrade and 4-6 bar abs pressure. Any suitable temperature and pressure may be used as required. The temperature used is greater than that used to sterilise hospital instruments but nol high enough to melt the plastics content of the waste. The vessel is rotated and the waste is retained within the vessel for around 60 minutes although this time period may be varied as required. The steam is then removed and the pressure reduced. Once the temperature and pressure have been allowed to reduce, the waste is removed from the vessel and is further treated, for example by means of a series of screens and recovery systems such as rag separator, trommel screens, over band magnets, eddy current separators, air knives, air classifier, etc. Materials such as metals, plastics and any other materials which did not break down are sorted and removed.

More detail of a system for pressure-heat treating waste material can be found in WO 03/25101, the disclosure of which is incorporated herein by reference. This International Patent Application was published after the claimed priority date of the present application.

The remaining material, consisting of a mixture of paper and putrescible material which forms a fibre product, is separated, for example by falling through the screen. This material is then transported to a second treatment vessel. This could be achieved manually or by means of a conveyor. It is beneficial for the waste material being treated in the further treatment vessel to be at a temperature above ambient, for example around 60 degrees centigrade. The fibre material is therefore advantageously transported as promptly as possible in order that the material does not lose heat. In a preferred embodiment of the invention, a covered conveyor, sealed auger or the like may be provided to assist in retaining the temperature of the fibre material as it is transported between treatment vessels. By ensuring that the temperature of the material is not significantly reduced, the temperature of the material may be as high as possible without requiring additional energy to raise the temperature to a desired level, or the amount of energy required to raise the temperature to a desired level is minimised. Advantageously, the conveyor or auger is insulated to further assist in reducing the loss of temperature of the material. It is particularly beneficial if the conveyor, sealed auger or the like incorporates heating elements, which may enable the temperature loss to be avoided altogether. The material being conveyed to the further treatment vessel may be heated using waste heat from elsewhere in the system, for example using heat generated when a solid or gaseous fraction of the treated material is used to generate electricity. The fibre is placed into a second treatment vessel, for example filling the vessel to in the region of 75% capacity, this amount is of course variable. The second treatment vessel may be connected to a tank containing liquid, such as a bacterially active liquid, which is in turn connected to a gas collection tank. The bacterially active liquid could be liquid used in the process previously, or could be landfill leachate or similar organic liquids, any source of methanogenic culture or diluted amounts thereof. If water is initially used as a liquid, this will gradually build up its own levels of bacteria as this is exposed to the waste material. Examples of suitable bacteria are one or more of Bacteroides succinogenes, Butyrivibrio fibrisolvens, Ruminococcus flavefaciens, Clostridium thermocellum, Thermoanaerobium brockii, Clostriduim thermohydrosulphuricium, bacteroides amylpohilis, Succinivibrio dextrinosolvens, Selenomonas ruminatium, Streptococcus bovis, Veillonella alcolescens, Peptostreptococcus elsdenii, Bacteroides ruminicola, Methanobacterum ruminantium, Methanbacterium thermoautothophicum, Methanobacterium arbophilicum, Methanospirillum, Methanosarcina barkeri, Methanococcus, Methanothix, Desulphotomaculum, or Desulphovibrio. The vessel is advantageously provided with heating means such as heating bars. Again, heating may use waste heal from elsewhere in the system.

During use, bacterially active liquid can be fed into the treatment vessel to saturate the fibre and fill up the remainder of the vessel. This may be directly from the liquid tank by means of a suitable conduit or manually. The heating means within the vessel raises the temperature within the vessel to in the region of 60 degrees centigrade and the fibre is maintained at this temperature for 12 to 36 hours. This temperature encourages the activity of thermophilic bacteria within the bacterially active mixed fibre and liquid and initiates bacterial digestion of the fibre. Digestion in this temperature range assists in the elimination of parasitic and potentially pathogenic organisms. The process generally takes three days. Preheating of the fibre material helps to speed up the bacterial digestion process thereby assisting in reducing the time the material is retained within the vessel and thereby speeding up the processing of the fibre. Hence it is advantageous that as much heat as possible is retained in the fibre material during the transfer from the first treatment vessel to the second.

The bacterial digestion process may occur under anaerobic or aerobic conditions depending on particular requirements.

Once the required time period has elapsed, the liquid can drained off at which time it begins to produce gas, typically in the region of 100m³ per tonne of fibre and being approximately 65% methane. This can be collected in the gas collection tank. The fibre material is recovered from the vessel and allowed to dry. After the second treatment, the resultant digestive fibre material will be in a clean and compost-like form which may be applied to land, or potentially used for other purposes such as a fuel or incorporated into building products. In addition to being an environmentally acceptable means of disposing what was originally mixed municipal waste, the treated waste, being a source of organic matter, when added to soil may also serve to assist soil stability and reduce the effect of erosion caused by wind and water.

It is highly unexpected that a product produced from mixed waste, such as municipal waste could be subjected to a bacterial digestion process to provide a clean, environmentally acceptable compost-like product. Such systems normally require a clean organic feedstock such as farm waste (manure), food waste etc. As far as is known, no such system can accept mixed municipal waste.

This method provides an unexpected advantage resulting from the "double" treatment of mixed municipal waste namely that a fibre product which requires controlled and specialised disposal can be treated to provide a clean and compost-like material which can be applied to land. Further advantages provided by the method described herein are that the process does not require the burning or destruction of waste materials to produce gas, the "double" treatment process enables the treatment of almost anything found in a mixed municipal waste stream. This system enables treatment of waste without the need for twin bin or other separate collection systems although it can, of course, be used in addition to these systems. The "double" system runs at a sufficient temperature to ensure that pathogenic and parasitic organisms are killed and the system can result in as much as 80% and potentially more than 85% of the mixed municipal waste stream being recycled.

A further advantage of this treatment system is that, even where separate collection exists, it allows for additional re-cycling which will assist local authorities to reach their required target.

The treated material may be used to generate electricity, for example by combustion of the treated material and/or the gaseous fraction from the digestion phase. This electricity may in turn be used in the system, for example for powering the rotation of an autoclave, conveyors, augers and/or separators used in the system.

An embodiment of the invention is described by reference to the accompanying schematic diagram of the treatment system which is provided for the purposes of illustration only. The number of autoclaves and digesters used to conduct the "two stage" process will, of course, vary with requirements.

As shown in Figure 1, a loading hopper 2 is provided in association with a loading conveyor 4 to supply a quantity of waste material to an autoclave 8. The loading hopper may control the amount of waste material being supplied to the autoclave depending upon the density and nature of the waste material. When the desired amount of waste material has been added to the autoclave 8, the door 6 is closed, thereby sealing the autoclave 8. Steam is introduced into the autoclave 8 from a boiler and/or accumulator system 10. The steam raises the temperature and pressure within the autoclave, typically to a temperature of between 150 and 1 0°C, and to a pressure of between 4 and 6 bar. After treatment of the waste material for a suitable period of time, the steam may be removed from the autoclave, and the treated waste material may be unloaded through the door 6 using an unloading conveyor 12. The treated material may be separated, for example using a rotary trommel screen 14 or other separation systems. The non-fibre content of the treated and separated material may pass on a picking belt 16 from which recyclable plastics and metals may be removed.

The fibre content separated from the treated material by the rotary trommel screen or the like may pass on a conveyor or auger 18 to a digester vessel 20. The fibre in the digester vessel 20 is mixed with bacterial active liquid from a liquid storage tank 22. The fibre is maintained in the digester for a period of at least 12 hours. After this, the liquid is drained from the digester 20. After draining the liquid, gas is produced which may be collected in a gas collection tank 24. The gas, which is primarily methane, can then be used for powering a gas powered generator 26 or the like.

The disclosure in this application is intended to be by way of example only and it is to be understood that alternative materials, dimensions and means may be employed, as may be judged appropriate by a men skilled in the art, having regard to the purpose and advantage of the disclosed embodiments. Likewise, it is intended that the features specifically disclosed herein, suitably broadened in accordance with the aforegoing sentence, will, in due course, form a basis for a selection of features defining an invention having novelty and inventive step, such selection to be made in due course, in accordance with the purpose and advantages of the disclosure embodiments.

Where, in the foregoing description, certain features are disclosed in combination with each other, it is to be understood that these features are not necessarily intended to be used in combination for the purposes of the invention in general, and it is hereby stated that features disclosed herein are to be treated as independently selectable for the purpose of the invention claimed.

Claims

1. A method for the treatment of waste, comprising:

a. collecting said waste material in a treatment vessel;

b. subjecting said waste to an environment comprising a pre- determined temperature and pre-determined pressure for a predetermined time period;

c. recovering at least part of the waste material from the treatment vessel;

d. transferring said treated waste to a further treatment vessel;

e. subjecting said treated waste to bacterial digestion; and

2. A method according to Claim 1 , in which the predetermined pressures is in the range of 4 to 6 bar.

3. A method according to Claim 1 or Claim 2, in which the predetermined temperature is in the range of 150 to 170 degrees centigrade.

4. A method according to any one of the preceding claims, in which, during the first treatment step, the waste is collected and placed is a vessel which is sealed, such as an autoclave.

5. A method according to Claim 4, in which the waste is subjected to an increase in temperature and pressure using the injection of steam.

6. A method according to Claim 4 or Claim 5, in which the vessel is rotated.

7. A method according to any one of the preceding claims, in which the waste is retained at the predetermined temperature and predetermined pressure for around 60 minutes.

8. A method according to any one of the preceding claims, in which the material is conveyed to the further treatment vessel in a manner such as to reduce the reduction of the temperature of the material.

9. A method according to Claim 8, in which the material is conveyed to the further treatment vessel by a covered conveyor or sealed auger.

10. A method according to Claim 8 or Claim 9, in which the means for conveying the material to the further treatment vessel is insulated.

11. A method according to any one of Claims 8 to 10, in which the means for conveying the material to the further treatment vessel includes heater means.

12. A method according to any one of the preceding claims, in which the further treatment vessel is connected to a tank containing a bacterially active liquid, and in which the bacterially active liquid is fed into the treatment vessel to saturate the fibre.

13. A method according to Claim 12, in which the further treatment vessel is connected to a gas collection tank.

14. A method according to any one of the preceding claims, in which the material in the further treatment vessel is heated using a heating means, such as heating bars.

15. A method according to Claim 14, in which the fibre is heated to at least 60 degrees centigrade.

16. A method according to Claim 15, in which the fibre is maintained at this temperature for at least 12 hours, and preferably between 12 to 36 hours.

17. A method according to Claim 16, in which, after the required time period has elapsed, the liquid is drained off at which time it begins to produce gas.

18. A method according to any one of the preceding claims, in which the treated waste material is dried.

19. A method according to any one of the preceding claims, in which the treated waste material has compost-like form which may be applied to land.