CH621715A5 - Process for recycling waste, especially household rubbish - Google Patents

Description

The present invention relates to a process for recycling rubbish, in particular household waste by a treatment allowing the industrial production of a gas whose composition and production are controlled.

The recovery and destruction of household waste poses several important problems. There is, on the one hand, nuisance and pollution and, on the other, the waste of raw materials which are destroyed by present means.

Two methods are currently used. Household waste is spread in 2.50 m layers and covered with earth or is incinerated.

35 - The first solution causes underground infiltration, the composition and path of which are not controlled, which represents a danger for groundwater and waterways.

- The second solution has the drawback of producing 40 ashes, the volume of which is around 30 to 40% of the materials burned, which can hardly be reused and therefore pose the problem of their destruction. This can only be done by burying it in the ground but, following this operation, the latter becomes unsuitable for both agriculture and construction.

In addition, all of the methane gas used both in industry and for household purposes currently comes from the exploitation of slicks of natural gas. Depending on the location of the deposits, the composition of natural gases varies.

50 Some, such as Lacq gas, contain components that must be eliminated. This operation is expensive and has a heavy influence on the sale price. Lacq gas, as used, has a calorific value of 9960 kcal / m3. Other gases, such as that of Groningen, have non-combustible components, such as carbon dioxide and nitrogen. In order not to burden the cost price, these elements are not eliminated, but their presence has an unfavorable effect on the calorific value, which is around 8400 kcal / m3.

Other gases, such as that of Hassi R'Mel in Algeria, have an ideal composition 6o which, in addition to the fact that they do not require prior treatment, gives them a significant calorific value of the order of 10700 kcal / m3 . These latter cases are nevertheless very rare.

The object of the invention is to produce a gas with a high content of 65 methane, therefore having good calorific value, obtained from a base product without value. The production of such a gas could be used in petroleum refineries where heavy fuel oil is currently used as a combustion element.

3

621,715

tibia during refining operations. It should be noted that half of the production of heavy fuel oil is currently used for this purpose. The use of gas therefore makes it possible to save on heavy fuel and to use the part saved for other purposes.

The process which the invention relates to consists in collecting and treating detritus with organic matter in order to obtain, by fermentation, a gas mixture containing methane and hydrogen.

The pre-fermentation and fermentation phases can be carried out in cells equipped so as to control the temperature as well as the anaerobic conditions. These cells also include means for supplying an additional substrate to replenish them, if necessary, by adding methanogenic bacteria.

It is interesting to mix with detritus such as household waste other organic matter and, among them, sewage sludge and faeces. According to an advantageous form of implementation of the invention, the shredding of household waste is carried out at the level of residential or industrial premises by an apparatus which, comprising a crusher associated with a tank for recovering the wastewater from said local, is equipped with control means causing the release of the waste water contained in the tank when the grinder is started, the apparatus being connected to the sewage system, so that the detritus and the faeces are conveyed by the same connection to a main collector, to a sewage treatment plant just before which are arranged settling tanks allowing to separate:

- household waste, faeces, sewage sludge and other products such as paper, cotton and others which, by their respective densities, remain at the bottom of the tanks,

- waste water and detergents sent to the purification station, the materials of the first group mentioned are then kneaded before being spread in the cells.

This solution eliminates household waste collections, as they are currently practiced, public landfills and incineration plants.

In a preferred form of implementation of this process, the different phases of pre-fermentation and fermentation of methanogenic bacteria are carried out in at least three vertically offset cells, the cell in which the first phase occurs being the highest, in order to allow the passage of products by gravity from one cell to another adjacent cell.

The advantage of this mode of implementation lies in the dynamics of sludge by simple gravity.

Therefore, it is possible to use as carrier gas the gas production obtained in the cells where the pre-fermentation and fermentation phases of methanogenic bacteria are carried out.

In fact, if the cells were on the same level, an outside and industrial carbon dioxide supply would be required to transfer the sludge from one cell to another adjacent one. However, such a practice is harmful to gas production since it causes a reduction in the percentage of methane produced.

In the present case, it is not necessary to carry out such a supply of carbon dioxide, which confers operating autonomy on the device.

The gas mixture can contain a large part of methane, and a little carbon dioxide. The separation of these two gases is carried out very simply, by dissolving carbon dioxide in a supersaturated solution of KOH potassium hydroxide. After dissolving the carbon dioxide, there remains a gas containing 99.5% methane and 0.5% other components, such as nitrogen.

This gas has an extremely interesting calorific value of the order of 9600 kcal / m3. It is of course possible to increase this calorific value to bring it to values from 12,000 to

13,000 kcal / m3 by adding hydrocarbons, such as ethane or ethylene.

Advantageously, the method consists in carrying out, during an operation, the transfer of only part of the products contained in a cell to the next cell, so as not to upset the reaction equilibrium in the different cells. This allows continuous production of gas, without having to start the reaction each time.

In a preferred form of implementation of this method, the product, obtained at the end of the treatment in the last enclosure, is recycled in the first enclosure to carry out the seeding thereof in methanogenic bacteria.

In addition, this process can consist in carrying out the oxygenation of the products in the first cell then, before transfer into the second cell, in placing oneself in anaerobic conditions by injection of carbon dioxide, and possibly in carrying out in the other cells a supply of carbon dioxide, hydrogen, while maintaining a temperature between 35 and 45 ° C, and preferably between 37.5 and 41 ° C, methane recovery being made in these latter cells.

In any case, the invention will be clearly understood with the aid of the description which follows, with reference to the appended schematic drawing representing, by way of examples, two embodiments of an installation for the implementation of this process :

fig. 1 is a very schematic sectional view of part of an installation of great importance;

fig. 2 is a sectional view of a minor processing unit along 2-2 of FIG. 3;

fig. 3 is a top view of the unit in FIG. 2.

Each treatment unit has four separate cells: A allows the preparation and oxygenation of the sludge, B ensures the pre-fermentation, C allows the acceleration of the fermentation producing carbon dioxide and the initiation of the methane fermentation and D ensures the methane enrichment of gas production.

In the embodiment shown in FIG. 1, each cell has a width of approximately 3 m, a height of 2.50 m and a length varying between 9 and 90 m, this length being a function of the size of the treatment center produced.

The cells are bounded longitudinally by reinforced concrete 6 walls 60 cm thick. The floors and ceilings separating the cells are produced by hollow metal plates 7 inside which circulates a temperature regulation fluid.

Each floor serves as a support at the longitudinal edges of the cells to pierced tubes 8, allowing the supply of components promoting the reaction, that is to say as the case may be, in air, in substrate, in support gas such than hydrogen and carbon dioxide, or hydrocarbons for the heat enrichment of gases.

Inside each cell, stirring means are provided, constituted for example by horizontal disks 9 driven in rotation from electric motors 10.

In the embodiment shown in FIG. 1, the four cells of the same unit are superimposed. The cells communicate with each other by conduits 12 on each of which is mounted at least one valve 13.

These conduits allow gravity transfer of the sludge from an upper cell to a cell placed below. When they arrive at the treatment center, the various detritus is sorted in order to separate the metals. The other products are reduced in a crusher 14, and then feed a cell A. This is filled over a good part of its height by liquids recovered in cells D, and brought back by a conduit 15 equipped with a pump 16 The conduit 15 opens,

on the one hand, at the lower part of a cell D and, on the other hand, at the upper part of a cell A. The detritus coming from the crusher 14 is added to these liquids.

5

10

15

20

25

30

35

40

45

50

55

60

65

621715

4

The homogenization of the sludge is carried out by the discs 9. The sludge becomes extremely compact without increasing its volume. From the start of the brewing operation, air is injected by the spray bars 8,

in order to carry out the oxygenation of the mixture. After a certain reaction time, part of the sludge is transferred to cell B. Before the transfer operation, the tank is put into an anaerobic condition by distribution of carbon dioxide by the ramps 8.

Between cell A and cell B, the conduit 12 has a special feature since, on this, two valves 13 are mounted between which is placed a filter 17. The two valves, preferably automated, are opened and closed simultaneously.

Under the effect of the pressure in cell A, the sludge passes into cell B, through the filter 17. To clean the filter 17, it suffices to operate when the two valves 13 are in the closed position.

Cell B provides the pre-fermentation producing carbon dioxide. To accelerate this, it is possible either to increase the temperature of the mixture by actuation of the discs 9, or to provide an additional gas through the conduits 8, such as carbon dioxide or a mixture of hydrogen and carbon dioxide.

After a reaction time in this cell, the sludge is transferred to cell C. The transfer takes place in the same way as that from cell A to cell B. In cell B, the acceleration takes place of the fermentation producing carbon dioxide and the initiation of methane fermentation. It is possible to speed up the process by carrying out, as in the case of cell B, a supply of carbon dioxide and hydrogen and by maintaining an optimal temperature of 41 ° C. for example. After a certain reaction time, part of the sludge is transferred from cell C to cell D. Cell C ensures the methane enrichment of the gas production. In this cell, the temperature is maintained between 35 and 45 ° C and advantageously between 37.5 and 41 ° C, the ideal temperature range for obtaining optimal methane production.

This enrichment is advantageously obtained by injecting the gas production from cell B into cell C, and the gas volume produced in cell C in cell D, via conduits 18 connecting the gas volumes of two adjacent cells and of conduits, not shown in the drawing connecting the gas volume of a cell with the distribution ramps 8 of the adjacent cell.

The gas volume produced in cell B contains 85% carbon dioxide and 15% methane, the gas volume produced in cell C contains 70% carbon dioxide and 30% methane, and the gas volume produced in cell D contains 60% methane and 40% carbon dioxide. The fact of injecting by the distribution ramps mounted in cells C and D the gas produced in cells B and C is very interesting, because it allows an enrichment of the sludge by contribution of a substrate and methanogenic bacteria. It is obvious that valves are provided for distributing the passage of the gas volumes directly and indirectly through the ramps 8.

The final flow will therefore be the sum of the gas flows in the three cells B, C and D. The gas produced in the three cells is used as the carrier gas.

Insofar as the gaseous product of cells B, C and D I0 does not reach a sufficient flow rate of the order of 120 to 1501 / h and per cubic meter of sludge, it is possible to inject to supplement it with gas industrial carbon dioxide. This external contribution can slightly lower the methane content. This is nevertheless unimportant since, at the outlet of cell D, the carbon dioxide 15 is dissolved in a supersaturated solution of KOH potassium hydroxide. The crystals remaining at the bottom of the tanks are collected to be introduced into cell A and to maintain a pH greater than or equal to 7 there, this decreasing during fermentation in cells B and C before going back to cell D.

20 In the case of the installation shown in fig. 2 and 3, the different cells A to D are not superimposed, but simply arranged in stairs, the main thing being that the passage of sludge can be carried out by gravity from one cell to the adjacent one located below.

25 In addition to the energy source which it makes it possible to obtain under advantageous financial conditions, this process is interesting in the sense that there is no discharge of gas or sludge, therefore no pollution of the atmosphere, streams or groundwater.

It is also possible to recycle in cell A only part, for example 90%, of the liquid obtained in cell D at the end of fermentation, the other part, that is to say the remaining 10% in the example considered, being sown with methanogenic bacteria previously isolated. This fraction enriched in 35 methanogenic bacteria is then mixed with products such as vase, brackish water, dead leaves, or the like, in order to carry out a controlled fermentation of these, in order to obtain methane, using 'an apparatus comprising several cells with specific functions, such as that described above. It is quite obvious that, if one proceeds in this way, it is advisable to make in cell A a new volume of sludge rich in organic products, corresponding to the volume of the liquid of D not recycled, in order to keep the volume constant in cell A.

45 The treatment process in the cells would remain the same insofar as the shredding of the refuse would be carried out, as indicated above, at the level of residential or industrial premises with the addition of faeces and sewage sludge.

R

2 sheets of drawings

Claims (15)

621715 1. A method of recycling litter, especially household waste, characterized in that it consists in collecting and treating the litter with organic matter in order to obtain, by fermentation, a gaseous mixture containing methane and hydrogen. 2. Method according to claim 1, characterized in that it consists in finely grinding the detritus after having separated them from the metal compounds and the glass, mixing them with organic materials, then spreading the mixture thus produced in equipped cells so as to control the temperature and the ana-robiosis during fermentation. 2 3. Method according to claim 2, characterized in that during the fermentation, the sludge formed is periodically agitated using carbon dioxide under pressure, and in that it is carried out after spreading the sludge in a cell - replacing ambient air with carbon dioxide which creates anaerobic conditions and which serves as the first substrate for microorganisms and then, during fermentation, for the injection into sludge of a complementary substrate for replenish, if necessary, by adding bacteria. 4. Method according to claim 3, characterized in that a mud enriched with bacteria from cultures is injected to activate or restart the production of a cell. 5. Method according to claim 1, characterized in that it consists in mixing faeces and sewage sludge with the detritus. 6. Method according to claim 1, characterized in that the different phases of prefermentation and fermentation of methanogenic bacteria are carried out in at least three cells offset vertically, the cell in which the first phase occurs being the highest, in order to allow the passage of products by gravity from one cell to another adjacent cell. 7. Method according to claim 6, characterized in that the carrier gas used is the gas production obtained in the cells where the pre-fermentation and fermentation phases of methanogenic bacteria are carried out. 8. Method according to one of claims 6 or 7, characterized in that it consists in carrying out, during an operation, the transfer of only part of the products contained in a cell to the next cell, in order to do not upset the reaction balance in different cells. 9. Method according to one of claims 6 to 8, characterized in that the product obtained at the end of the treatment in the last cell is recycled in the first cell to carry out the seeding thereof into methanogenic bacteria. 10. Method according to claim 6, characterized in that it consists in carrying out the oxygenation of the products in the first cell and then, before transfer into the second cell, in placing themselves under anaerobic conditions by injection of carbon dioxide, and to possibly provide in the other cells a supply of carbon dioxide and hydrogen, while maintaining a temperature between 35 and 45 ° C, and preferably between 37.5 and 41 ° C, the recovery of methane being made in these last cells. 11. Method according to one of claims 9 or 10, characterized in that it consists in recycling in the first cell only part of the product obtained in the last cell, the other part being seeded with methanogenic bacteria, beforehand isolated, before being mixed with products containing organic materials, such as vase, brackish water and dead leaves, in order to carry out their methane fermentation. 12. Device for implementing the method according to claim 1, characterized in that it comprises four cells offset vertically relative to each other, each cell communicating with at least one neighboring cell by at least one conduit on which are placed two valves between which a filter is mounted. 13. Device according to claim 12, characterized in that each cell is equipped at its bottom with distribution ramps for products promoting the reaction. 14. Device according to one of claims 12 or 13, character-ized in that each cell is equipped with at least one product stirring device, consisting of a disc driven in rotation by an electric motor, and also allowing to influence the temperature of the cell in question. 15. Device according to one of claims 12 to 14, character-io risé in that the bottom of the last cell is connected to the upper part of the first cell by a conduit on which is mounted a pump. 16. Device according to one of claims 12 to 15, characterized in that the means for transferring the gaseous production 15 from a cell to another adjacent cell are constituted by conduits connecting, on the one hand, the gas volumes of the two cells considered and, on the other hand, the gas volume of the first cell with the distribution ramps of the second.