AU701658B1 - A method of treating residual waste - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant FABER RECYCLING GMBH Invention Title: A METHOD OF TREATING RESIDUAL WASTE The following statement is a full description of this invention, including the best method of performing it known to me/us: 1A A Method of Treating Residual Waste The present invention refers to a method of treating residual waste.

Recently, regulations were imposed according to which the dumping of waste, such as domestic waste, local-authority waste, or the like, is only permitted if said waste has been treated previously. These regulations are intended to prevent the organic mass existing in the waste from being decomposed in the waste disposal body in an uncontrolled manner by microbiological processes. The pretreatment of the waste is carried out for guaranteeing that the waste disposal gases, which form in the course of microbiological decomposition processes, and heavily loaded seep water are avoided. Predominantly a thermal type of treatment has recently been suggested for carrying out the pretreatment. In the case of this treatment, the waste is virtually burned.

However, it also turned out that, under certain circumstances, a low-emission dump material that is rendered inert to a large extent can also be produced by a mechanical-biological residual-waste treatment, which is substantially less expensive than incineration.

Since, however, the two different kinds of treatments each have advantages and disadvantages, neither of said methods can be preferred without reservation. Hence, it has already been suggested in practice that, prior to the incineration, the waste should be placed in large, closed, air- and liquid-tight containers in which the material rots within several days under control of the processes taking place. In this rotting process, the easily degradable organic substances decompose and the material dries. It is therefore more suitable for the incineration taking place later on.

However, also this method requires a considerable investment 2 because it is necessary to develop containers and transport systems which are specially suitable for the rotting material and in which the waste can be taken up and treated.

It is therefore the object of the present invention to provide a method of treating residual waste, in the case of which a treatment is carried out in a simple and economypriced manner with optimum adaptation to the utilisability of a great variety of substances existing in said residual waste, said treatment representing an optimum and, where appropriate, modular combination of the advantages of known *methods without entailing the disadvantages of said known According to the present invention there is provided a method of treating residual waste comprising the following steps: 20 a) the waste is first subjected to a mechanical treatment; b) the waste is then subjected to an aerobic biological treatment; c) subsequently, the waste is separated so as to form at least one fine and at least one coarse fraction; d) the coarse fraction is thermally utilised and, subsequently, the residues are dumped; e) the fine fraction is then further treated in such a way that a 3mm part is screened out and reused as building material; f) the residual part of the fine fraction is dumped.

By means of this combination of individual method steps, it is possible to treat the waste by the technical method which is the most advantageous one in each individual case and the largest possible amount of substances, which can be utilised later on, can be recovered from the waste. An H:\paulad\Keep\speci\69973-95.doc 10/12/98 buidin maeral 2a optimum treatment is carried out on the one hand for waste disposal and on the other hand for the recovery of useful material. By a suitable combination of mechanicalbiological and thermal treatment steps, it is, on the one hand, possible to reduce the incineration input mass, since a substantial percentage of the biologically degradable organic mass can be decomposed in step b. The calorific value of the material increases because the waste is dried by the biological treatment. The mass flow to the thermal treatment can be rendered more uniform because the material is stored during the mechanical-biological treatment (steps a and b) so that the mass flow supplied for the purpose of incineration can be rendered more uniform. On the other hand, low-emission storage is achieved. The waste 15 composition permits an improved burn out and consequently also an improved quality of the

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\\MELB01 \home$\ARymer\Keep\Speci\ 69973 -95.doc 7/12/98 3 slag which is finally formed and dumped. The use of the aerobic biological waste treatment offers the great advantage that peaks in the delivery to the thermal incineration plant can be avoided in this way. When aerobic biological methods are used, the waste can stored intermediately without any problems even over a prolonged period of time.

Hence, peak loads in the delivery of waste, e.g. in touristic areas during the high season, do not have to be used as a reference load for dimensioning the thermal plant. Nor is it necessary to provide a preceding second line for outage times in the thermal treatment, since the waste can again be stored in the biological treatment stage during such times.

The investment expense for the thermal treatment plant can therefore be reduced to a substantial extent. It follows that a substantial advantage of the method claimed is to be seen in the combination of thermal incineration with a pre- S ceding aerobic biological treatment step, as indicated in S step b.

A further substantial advantage of the combination of the mechanical-biological residual waste treatment with the ft thermal residual waste treatment is the possibility of providing facilities for recovering useful material also subsequently. By means of a simple mechanical treatment of the type provided in step c, a fraction (coarse fraction) of ~high calorific value and a fine fraction can be separated from the residual waste. By means of step c, it can be achieved that 50% of the waste can be recycled for further use. In particular by means of the screening out of 3 mm particles provided in step d, particles are obtained which can be reused as building materials, especially as additives to building materials, in an optimum manner. The remaining residual substances can be dumped such that little emission is produced, and, prior to being dumped, they can be subjected to further biological or thermal treatments.

In addition to the improvement of the storability of the 4 waste, it is therefore especially the reduction of the incineration costs that matters because less mass has to be thermally treated and because the plant can have smaller dimensions consequently.

In accordance with an advantageous embodiment, the method is so conceived that the the waste is reduced in size and homogenized in step a. The aim of this treatment is to achieve a well mixed product in which individual fractions of the waste are no longer recognizable. For the fraction consisting of domestic waste and of industrial waste that is similar to domestic waste,- a treatment in e.g. a homogenization drum will suffice. Fractions with very coarse waste (e.g.

9. 9.

bulky refuse) can be reduced in size separately. Subsequent- *ly, also this fraction can be added to the fraction in the homogenization drum. A batch mixer is ideally suitable for thoroughly mixing the individual fractions. The waste mate- S rial homogenized and prepared in this way is then particularly suitable for further treatment.

For the biological treatment in the aerobic biological treatment stage b, static or dynamic methods can be used.

9 9 In the case of dynamic methods, the waste is aerated by movement. Big machines turn the waste at specific intervals and aerate the waste due to its movement through the air. In contrast to this, the waste is not moved in the case of static methods. Aeration is caused by technical means. Forcedair systems can be used, in which the air is supplied e.g.

via pressure and suction lances, or, in accordance with a very advantageous embodiment of the present invention, it is possible to use self-aerated systems, e.g. the chimneydraught method according to Spielmann and Collins. In the case of the chimney-draught method, vertical small chimneys are inserted in the waste, said chimneys communicating with the ambient air via horizontally perforated conduits at the base of the rick. Due to the biological conversion processes, an increase in temperature is achieved by means of 5 which a draught is produced in the chimneys. Due to this draught, fresh air is always drawn into the rick automatically. This method can be carried out at a very moderate price and causes the waste to rot efficiently in a very effective manner.

The best way of carrying out the separation provided in step c is screening. For this purpose, a screening drum can be used; the average screening value can preferably lie between and 120 mm. The coarse fraction screened out in this way contains a high percentage of plastic material (carbon) in the case of conventional waste charges and is therefore a fraction of high calorific value. Screening at this point, i.e. after the biological treatment, is advantageous because organic material adhering to the plastic material is removed by the preceding rotting arrangement. The calorific value of the coarse fraction is therefore clearly increased (Hu 20000 KJ/Hg), and also an improved storability as well as transportability of the waste is achieved.

o In the following, the sequence of method steps according to the present invention is explained and described in more S. detail on the basis of the process sequence chart shown in the figure.

S. S SThe materials which occur as local-authority waste, industrial waste, bulky refuse or domestic waste and which are referred to as residual waste in their entirety are transported in the usual way by suitable transport vehicles to the treatment plant where they are first mechanically treated in a first step (a of claim By means of this mechanical treatment, the waste is prepared for the subsequent treatment steps by size reduction and homogenization. Coarse waste can be reduced in size several times. The mixing and homogenization is preferably carried out in a homogenization drum.

6 When the material has been prepared and mixed in this way, a first biological treatment is carried out, said treatment taking place in an aerobic treatment process. This treatment can alternatively be carried out in a static rick-type rotting arrangement with a self-aerated system or a forced-air system or in a dynamic rotting arrangement in which forced aeration takes place. A measure which proved to be particularly advantageous is the use of the chimney-draught method in this step. In the case of this method, a self-aerating effect is produced in the system. Due to biological conversion processes an increase in temperature is achieved which produces a draught in the chimneys extending through the waste. This draught has the effect that fresh air is always drawn into the rick automatically. This method is known in the literature as chimney-draught method according to Spielmann/Collins.

oe This aerobic treatment can last for several months. It can be carried out at a moderate price, especially when a selfaerating system is used. After the aerobic decomposition of the waste, the waste is subjected to further mechanical treatment (mechanical treatment II) in the course of which it is divided at least into a coarse fraction and a fine fraction. The coarse fraction, whose average screening value preferably exceeds 60 mm, consists of carbonaceous components (plastic materials) and represents consequently a fraction of high calorific value which is adapted to be thermally utilized (incineration) in a particularly advantageous manner. The fine fraction 60 mm) is subjected to further screening so as to screen out particles 3 mm. These particles are recycled in building materials. The 3 mm components of the fine fraction can be dumped either directly or they can be subjected to further thermal or biological treatment before they are dumped. The biological further treatment takes place as an aerobic treatment. This biological treatment can again be carried out in a self-aerated rick-type rotting arrangement (chimney-draught method) or it 7 can also take place in a triangular rick. In so doing, the residual waste is re-treated in such a way that it can be dumped in a thin layer and such that little emission is produced.

Especially in view of the combination of the .biological aerobic pretreatment and the subsequent incineration of the coarse fraction having a high calorific value, the method according to the present invention has various advantages.

These advantages are, in particular, a reduction of the incineration input mass, an increase in the calorific value of the material, a greater uniformity of the mass flow due to the buffer effect of the aerobic biological waste treat- S ment plant, an improvement in the transportability of the waste, an improvement in the storability, and a reduction of the incineration costs. Advantageous possibilities of reuse are obtained for at least part of the intermediate products formed in the individual stages; building materials are, for example, such a possibility of reusing the particles 3 mm.

S It follows that the present invention combines in an ideal manner the advantages of the various waste treatment processes, without having to put up with the respective disadvantages of these processes, and it permits an economy-priced and effective, ecologically desirable waste treatment which S is adapted to the respective mass flux (mass flux-specific) Any use of the words "comprising", "comprises" or "comprise" in the specification is meant to mean "including" or "amongst other things" and is not limited to the claimed integers.

Claims (7)

1. A method of treating residual waste comprising the fol- lowing steps: a) the waste is first subjected to a mechanical treat- ment; b) the waste is then subjected to an aerobic biological treatment; c) subsequently, the waste is separated so as to form at :b least one fine and at least one coarse fraction; the coarse fraction is thermally utilized and, sub- sequently, the residues are dumped; e) the fine fraction is then further treated in such a e way that a 3mm part is screened out and reused as building material; f) the residual part of the fine fraction is dumped.

2. A method according to claim 1, o" characterized in that the waste is reduced in size and homogenized in step a). S

3. A method according to claim 1 or 2, characterized in that the treatment in step d) is effected by means of a self- aerating system.

4. A method according to any one of claims 1 to 3, characterized in that the separation provided in step c) is carried out by means of screening.

A method according to claim 4, characterized in that 9 the average screening value of the screening operation lies between approx. 60 mm and 120 mm.

6. A method according to any one of claims 1 to characterized in that, between steps e) and the residual part of the fine fraction is subjected to a thermal or biological further treatment.

7. A method according to claim 6, characterized in that the biological further treatment comprises an aerobic treatment. Dated this llth day of December 1998 FABER RECYCLING GmbH By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent Attorneys of Australia C C C