WO1984001363A1 - A method and a system for decomposing organic waste material by bacterial action so as to generate combustible gas - Google Patents

Abstract

A plant for decomposing organic waste material (43) by bacterial action so as to generate combustible gas comprises a plurality of processing containers (11-14). Fresh waste material may be fed from a supply container (44) to any of the processing containers (11-14), and processed material may be discharged from each of these processing containers to any of the others, and finally processed material may be discharged from the system through at discharged conduit (68). The supply of fresh waste material from the supply container (44) to the processing containers (11-14), and transfer from partly processed material from one processing container to another is controlled by an electronic control device (109) based on empirical data and on values measured within the system by means of sensors (48, 49, 53, 70-77). So as to obtain an optimum gas production and/or a desired variation in the gas production with respect to time, and/or a desired degree of the composition of the waste material.

Description

A METHOD AND A SYSTEM FOR DECOMPOS I NG ORGAN IC WASTE MATER IAL BY BACTERIAL ACTION SO AS TO GENERATE COMBUST¬ I BLE GAS

The present invention relates to a method and a system for decom¬ posing organic waste material by bacterial action so as to generate combustible gas, hereinafter also referred to as "bio-gas" .

A substantial part of the waste materials produced in the modern society is of an organic character and comprises carbohydrates , proteins and/or fats . All of the substances may be decomposed under the action of anaerobe bacteria in a so-called bio-gas process in which a combustible bio-gas normally comprising methane (Cl-L) and carbon dioxide (CO-) , is generated . Generally, the bio-gas process comprises three successive steps, namely a hydrolyzing step during which organic macromolecules are decomposed by bacterial action into smaller units (cellulose is decomposed into glucoses, proteins into amino acids, etc. ) , an acid generating step du ring which the smaller organic molecules are converted by bacterial action into volatile fat acids (especially acetic acid) , carbon dioxide, and hydrogen , and a methane generating step du ring which methane generating bacteria convert the acetic acid, the carbon dioxide and the the hydrogen formed during the preceding step into methane and carbon dioxide.

While the hydrolyzing and the acid generating steps normally proceed rather quickly (within some hours) and is caused by several rather different, partly facultative anaerobe bacteria, the methane generating step is caused by special methane bacteria, which are absolutely anaerobe, propagate rather slowly, and have a rather low activity . Furthermore, the methane bacteria are much more sensitive to various factors than the other kinds of bacteria involved in the process . Thus, the activity of the methane bacteria is reduced by high acidity and high concentrations of cations, amonia, various poisonous sub¬ stances, and by sudden temperature variations . It is known to decompose organic materials and to simultaneously produce combustible bio-gas by a bio-gas generating process of the type described above. I n a conventional bio-gas system, such as a digesting tank of a sewage processing system, the sewage or waste material is continuously fed into the container or tank containing a suitable bacterial flora, and processed material is simultaneously discharged (normally through a simple overflow) from the container at a rate corresponding to the feeding rate. The container content is mixed so as to secure that fresh material just supplied is mixed with elder material containing the active bacteria . Due to the bacterial , action within the container or tank the sewage or waste material is decomposed as described above, and bio-gas generated may be with¬ drawn continuously or intermittently from the container or tank. The average residence or retention time of the waste material in the con- tainer or tank - normally referred to as "the hydraulic residence time" - may be calculated by dividing the active volume of the con¬ tainer or tank with the volume of waste material fed to the container or tank per time unit. I n the known bio-gas generating systems the hydraulic residence time is normally within the range of 15-30 days.

The known bio-gas generating systems involve several problems and disadvantages. Thus, because it is necessary to mix the material within the process container, the processed material discharged there¬ from will contain a relatively high percentage of rather fresh material, unless the hydraulic residence time or average residence or retention time within the container is made relatively long. Furthermore, the chemical and bacterial processes taking place within the processing container are rather complicated, and some of the desired processes may be impeded by the existence of certain substances . As an example, a reduction of the activity of the methane bacteria for any of the reasons indicated above may cause an increased acidity of the con¬ tainer content which further reduces the activity of the methane bacteria, and so on . At worst the methane production may stop com¬ pletely.

The present invention provides an improved method of the above type by means of which it is possible to obtain an improved decomposition of the organic waste material and a correspondingly increased produc¬ tion of bio-gas.

The method according to the invention is of the type comprising continuously or intermittently feeding the waste material into a first processing container, mixing the material therein , and continuously or intermittently discharging mixed waste material therefrom so as to obtain a desi red average residence or retention time for the waste material in the said first processing container, and the method accord¬ ing to the invention is characterized in that at least part of the waste material discharged from the first processing container is fed into and further decomposed by bacterial action in one or more additional processing containers .

The maximum production of bio-gas from an organic waste material depends i . a . on the composition of the material, the bacterial flora within the processing container, the temperatu re and other conditions in the container influencing the bacterial action , etc. If a batch of a certain organic waste material is arranged within a processing con¬ tainer under given bacterial and temperature conditions the produc¬ tion of gas may be plotted versus the residence or retention time of the material within the container. I n case the material is continuously or intermittently fed into and discharged from the processing con¬ tainer at a certain rate, while the content of the container is mixed, the distribution of the material within the container on various resi¬ dence times may be determined mathematically . Provided that the conditions within a continuously functioning processing container are equal to the conditions within that in which the batch material was processed, the total production of bio-gas in the continuously func¬ tioning container may be calculated by multiplying the graph showing the gas production as a function of residence time with the graph showing the distribution on residence times within the container. It has been found that the maximum gas production is normally obtained at a residence time of about 10 days . Therefore, in order to increase the gas production , the amount of processed material discharged from the processing system and having a total residence time of less than 10 days should be reduced. It may be shown, that if, in accordance with the present invention, the material discharged from the first processing container is fed into an additional or second processing container, the distribution on residence times in the second container is much more favourable than in the first container, whereby an in- creased total gas production and a better decomposition of the waste material being processed may be obtained . I n the method according to the invention the waste material may be processed in any number of processing containers connected in series and/or in parallel, and the material discharged from the first container and from any of the additional processing containers may be fed into a single succeeding processing container or be divided and fed into two or more succeed¬ ing containers . This means that the distribution of the material on residence times in the various additional processing containers may be varied within rather wide limits . As explained above the content of fresh or young material (i . e. material with short total residence times) in the additional processing container will be much less than in the said first processing container, and therefore much more stable conditions are obtained and the risk of poisoning of the process or of a too high acidity of the content is eliminated or considerably re- duced .

According to the invention the processed waste material may be re¬ turned from one or more of the additional processing containers to the first container and/or any preceding additional container. By such backfiow of the processed material from an additional container to the first container it is possible to obtain a favourable distribution of residence times and stable processing conditions also in the first processing container, and the possibility of backfeeding material from one of the additional processing containers to another further in¬ creases the flexibility of the process .

As explained above, the production of bio-gas by decomposition of a certain organic material under certain conditions may be determined empirically, and the distribution on residence or retention times of the material within each container may be calculated mathematically on the basis of the composition of the material fed to the container and on the hydraulic or average residence time in the respective container. These facts may be utilized for controlling the production of bio-gas and/or the degree of decomposition of the organic waste material in a desired manner. Thus, according to the invention, feeding of waste material to and discharge of the material from the various processing containers may be controlled on the basis of em¬ pirical data showing the varying rate of gas production from a batch of the respective material in a process container, as a function of the residence time of the material in that container so as to obtain a desired total gas production and/or a desired degree of decomposition of the waste material in the various process containers .

Alternatively or additionally, the feeding of waste material to and the discharge of material from the various processing containers may be controlled on the basis of measured values of physical and/or chemical conditions within the various containers such as acidity, temperatu re, concentration of poisonous substances, etc.

It has been found that use of a plu rality of processing containers connected in series in accordance with the present invention renders it possible to obtain a production of bio-gas which is increased by 50-100% compared to the gas production which may be obtained by using a known bio-gas system with a single continuously operating processing container.

Methane bacteria have a pronounced ability to provide mutations with an increased efficiency under given physical and/or chemical con¬ ditions . Therefore, it is expected that different bacteria floras will develop in the various processing containers adapted to the special physical and chemical conditions therein, whereby the efficiency of the system may be fu rther increased .

Most of the known bio-gas systems are operating within the mesophilic area, which means that the decomposition of the organic material is caused by micro-organisms with optimum temperature for growth between 20-45°C, even though it is well-known that the efficiency of the process would be considerably increased if the process was ther- mophilic, which means that the decomposition of the organic material

OMPI is caused by micro-organisms with optimum temperature for growth above 45°C. The reason is that a thermophilic process is more un¬ stable and more difficult to control than the mesophilic process, and that the thermophilic process requires a supply of heat energy.

As explained above, the method according to the invention allows for a more efficient control of the bio-gas process within the various con¬ tainers, and also makes it possible to obtain a better process stability. Therefore, it would involve no serious problems to control a thermo¬ philic process by using the method according to the invention . A thermophilic process requires that the waste material is heated in one or more of the processing containers . However, in accordance with the present invention , the costs involved in the necessary heating of the waste material may be considerably reduced by arranging each of the containers in which the waste material is heated within an outer processing container, and in the preferred embodiment the processing containers, which are substantially cylindrical, are arranged coaxially within each other. When the contents of the inner container is heated to a temperature above normal ambient temperature, the temperatures in the other containers may vary from the relatively high temperature of the inner or central container to substantially ambient temperature within the outer container. The loss of heat from the central container is then used for heating the adjacent surrounding containers so that the central container or containers may be operated thermophilic, while the surrounding outer containers may be operated partly ther- mophilic or totally mesophilic. By such arrangement the heat supply to the bio-gas system is utilized to a very high degree so that the system may be operated much more economically than known ther¬ mophilic systems .

Normally, at least the major part of the fresh organic waste material is supplied to the outer mesophilic container or containers from which the material is continuously or intermittently discharged into the inner thermophilic container or containers . Due to the high bacterial activity within the thermophilic container the hydraulic residence time within the mesophilic containers may be relatively short. Of course, it is also possible to supply the major part of the fresh waste material into the inner thermophilic containers from which the contents may be discharged into the outer mesophilic container or containers .

I n some cases it may be advantageous to heat the decomposed waste material to a sterilizing temperature, for example when the waste material is in the form of sludge or sewage, because such sterilization renders it possible to use the decomposed material as a fertilizer, and because a heating of the sewage or sludge makes it easier to obtain a dewatering thereof . Therefore, in accordance with the invention the processed waste material may be heated to a sterilizing temperature in a sterilization container which is surrounded by one or more of the processing containers . Thus, in case of a coaxial arrangement, the inner container may be a sterilization container while the adjacent inner container or containers may be thermophilic processing con¬ tainers which are heated by heat transmitted from the sterilization container.

As explained above, the bio-gas process may be retarded or stopped in a processing container due to the generation of substances, such as acids or poisonous substances, counteracting or preventing the growth of the active bacteria in the process . It has been found that such retarding or poisonous substances are normally dissolved in the water content of the waste material, while they are generated by the decomposition of the solid waste material . I n order to reestablish good processing conditions in a container in which a excessive amount of retarding or poisoning substances have been generated, it may there- fore be sufficient to replace some of the poisoned water content of the waste material with fresh water. Thus, according to the invention a watery fluid containing substances restraining the desi red bacteria action may be separated from the waste material in at least one of the process containers and discharged therefrom, while a corresponding amount of fresh water may be supplied to the waste material in the container. Thus , the processing container may include water per¬ meable filtering means dividing the container into a waste material compartment communicating with a fresh water inlet and a water discharge compartment communicating with water discharge means .

OMPI The major part of the substances restraining the desired bacterial action is acids and ammonia (NH,) . The separated water also contains great amounts of carbon dioxide in solution . In accordance with the invention the watering fluid discharged from the waste material may be passed through a porous material bearing bacteria for converting these restraining substances to useful or harmless substances. Thus, for example, the ammonia may be converted into cell mass (protein) , while the carbon dioxide may be converted into CH , and bound to the carbon atoms in the cell mass . Furthermore, the acids may be con¬ verted into CH₄ and CO-, which may be converted into protein and CH₄ during later steps .

The organic waste material being treated is normally in the form of a liquid or paste, depending on the contents of solid matter. The bacteria which are active in decomposing the waste material, reside on the particles of solid matter in the waste material . Therefore, in case a watery fluid having a relatively small content of solid matter is treated, the number of bacteria per unit of volume is small, and the process activity is correspondingly low. I n order to increase the bacterial activity one or more of the processing containers may con- tain a porous inactive material bearing bacteria for decomposing the organic waste material .

The invention will now be further described with reference to the drawings, wherein

Fig. 1 diagrammatically illustrates a first embodiment of the plant or system according to the invention,

Fig. 2 diagrammatically shows a second embodiment having concen¬ trically arranged processing compartments or containers, Fig. 3 diagrammatically illustrates a third embodiment of the plant or system according to the invention, Fig . 4 is a graph in which the amount of bio-gas produced from a batch of waste material is plotted as a function of the residence time of the material within a processing container, and

Fig . 5 shows graphs illustrating the distribution on residence times within various containers.

OMP Fig. 1 shows a bio-gas system comprising a tank 10 which is divided into fou r separate processing compartments 11 , 12, 13, and 14. The tank 10 is provided with an outer heat insulating layer 15, and each of the processing compartments is provided with a man hole closed by a removable cover or lid 16, 17, 18, and 19, respectively. The gas space in each of the processing compartments is connected to a gas measuring unit 20 by means of gas discharges lines or conduits 21 , 22, 23, and 24, respectively, and the lower portion of each of the processing compartments is connected to a feeding and discharge system by means of feeding and discharge lines or conduits 25, 26, 27, and 28, respectively. The various gas discharge lines 21 -24 are interconnected by a connecting line or conduit 29 via remotely con¬ trolled closure valves 30, 31 , 32, and 33, respectively, such as magnet valves . The connecting line 29, which includes a pressu re gauge 34, a pump 35, a closure valve 36, and a flow meter 37, is in communication with gas nozzle tubes 38 arranged at the bottom of the tank compartments 11 -14, via remotely controlled valves 39, 40, 41 , and 42.

An organic waste material 43 to be processed in the system may be fed from a supply container 44 through a macerating or grinding device 45, a remotely controlled valve 46, a line or conduit 47, and temperature and acidity sensors 48 and 49, respectively, into a dosage container 50. The material 43 is transported from the supply container 44 to the dosage container 50 by means of a pump 51 inserted in the conduit 47 and being of the type which is able to pump the material in either direction through the conduit. A transparent level control tube 52 coextends and is connected in parallel with the dosage con¬ tainer 50, and a differential pressu re sensor 53 is in communication with the upper gas space of the dosage container 50 and in pressu re transmitting communication with the lower part of the container 50 through a remotely controlled valve 54 and a chamber 55 containing a flexible pressu re transmitting membrane. A similar membrane chamber 56 and a remotely controlled valve 57 is arranged in a line or conduit 58 which connects the conduit 47 with a line or conduit 59 extending between the sensor 53 and the valve 54. Suitable amounts of sodium hydroxide (NaOH) may be supplied to the dosage container 50 th rough a supply line or conduit 60 having a remotely controlled valve 61 , and water may be supplied through a line or conduit 62 containing a remotely controlled valve 63.

As described more in detail below, waste material may be pumped from the dosage container 50 to any of the processing compartments 11 -14 by means of the pump 51 through the line or conduit 47 and the respective one of the feeding conduits 25-28, which are provided with remotely controlled closure valves 64, 65, 66, and 67, respectively. A discharge conduit 68 containing a remotely controlled valve 69 is connected to the conduit 47 downstream of the valve 46, and the discharge line 68 is passed th rough the supply container 44 in heat transmitting contact with the waste material contained therein . Each of the processing compartments 11 -14 contains electrical heating means designated 70, 71 , 72, and 73, respectively, and a temperature sensor designated 74, 75, 76, 77, and 78, respectively.

The gas measuring unit 20 comprises four gas meters 78, 79, 80, and 81 , each of which is allocated a respective one of the processing compartments 11 -14. Gas produced in these compartments may be supplied to the gas meters through the gas discharge conduits 21 -24 containing flow meters 82, 83, 84, and 85, and after having passed the flow meters the gas may flow through an outlet conduit 86 to a storage container or a gas consuming device, not shown . Each of the gas discharge conduits 21 -24 is connected to the gas space of the dosage container 50 and to the differential pressure sensor 53 by means of a connecting line or conduit 87 th rough remotely controlled closure valves 88, 89, 90, and 91 , and each of the gas discharge conduits 21 -24 is communicating with a by-pass conduit 92, 93, 94, and 95, respectively, which is normally closed by a manually operate- able closure valve 96, 97, 98, and 99, respectively . Manually operate- able closure valves 100, 101 , 102, and 103, respectively, are also arranged at the inlet and the outlet of each of the gas meters . There¬ fore, when the closure valves 100-103 are closed, while the valves 96-99 are opened, gas may flow from the processing compartments 11 -14 to the gas outlet conduit 86 via the by-pass conduits 92-95. Each of the gas discharge conduits 21 -24 is connected to a condensate drainage line 104 through flow meters 105, 106, 107, and 108.

OMPI The function of all of the remotely controlled valves, the pumps, and the electrical heating means of the system or plant described above is controlled by a micro-processor 109 or another suitable electronic control device on the basis of signals received from the various sensors of the system, and on the basis of empi rical data regarding the characteristics of the specific waste material being processed so as to obtain an optimum production of gas, or a desi red variation in gas production in respect to the time of the day, or the day of the week.

When a given amount of an organic waste material is decomposed by bio-gas producing bacteria under certain physical conditions the gas production will vary with respect to time. The gas production will normally vary from zero, when the waste material is fresh, to a maximum, when the material has been exposed to the bacteria flora for a number of days, normally 10-15 days, and then gradually de¬ crease with respect to time.

Fig . 4 shows a graph illustrating the variation of the gas production from a batch of visceral contents arranged in a processing container and mixed with a suitable culture of bio-gas producing bacteria , i n Fig . 4 the gas production (combustible gas and carbon dioxide) for the first thi rty days is plotted as the percentage of the total gas production obtainable from the batch of waste material . Fig . 4 shows that the gas production increases from zero - when the waste material is fresh - to a maximum after about 10 days , and that the production of gas on the 10th day amounts to about 4.5% of the total production from the batch of material .

When the waste material is fed continuously or intermittently at rela¬ tively small time intervals to a processing container and thoroughly mixed therein , while processed material is discharged from the con- tainer at a discharge rate corresponding to the feeding rate, the distribution on residence times of the material in the container may be calculated . I n Fig . 5 the graph 1 illustrates distribution on residence or retention times of the waste material in a single processing con¬ tainer in which the so-called hydraulic residence time is 10 days . From the graph 1 it is seen that more than 10% of the total content of the processing container will be rather fresh material with a residence time of less than 1 day, and that less than 4% of the content of the container will have a residence time of 10 days . Provided that the contents of the processing container is thoroughly mixed, the material discharged from the container will have exactly the same distribution on residence times . This means that more than 10 percent of the material discharged from the processing container is rather fresh and has been within the processing container for less than one day. The graphs designated by 2, 3, and 4 in Fig . 5 illustrates the distribution on residence times in the last processing container, when the single processing container has been replaced by two, three, and four processing containers, respectively, connected in series . It is seen that even though the average or hydraulic residence time is the same as in the single processing container, the. amount of fresh waste material in the processed material discharged from the last or down¬ stream container, is drastically reduced, while the amount of dischar¬ ged material having a residence time exceeding 10 days is increased . ^■Thus, Fig. 5 shows that the use of two or more interconnected pro- cessing containers in accordance with the present invention causes an improved composition of the waste material , and, consequently, an in¬ creased gas production .

It is understood that a value of the total gas production obtainable in a bio-gas producing system having one, two, three, or four proces- sing containers connected in series, may be obtained by multiplying the ordinate of the respective residence time distribution graph (Fig . 5) with the corresponding ordinate of the relevant gas production graph (Fig . 4) . The shape of the residence time distribution graph of a bio-gas system with two or more processing containers may be changed by feeding some of the processed material discharged from a downstream processing container back to a processing container arranged upstream thereof, and/or by feeding fresh waste material to one or more downstream containers. This means that based on em¬ pirical data regarding the waste material being processed it is possible to control the process in the bio-gas system so as to obtain a desired variation in gas production and/or a desired degree of decomposition of the waste material . The function of the bio-gas producing system illustrated in Fig. 1 will now be described more in detail .

An organic waste material 43, such as visceral contents and other kinds of offal, is supplied to and stored in the supply container 44. At certain time intervals, for example twice an hour, an amount of fresh waste material is fed from the supply container 44 into the processing compartments 11 and/or any of the other compartments 12-14. Such a feeding operation may proceed as follows : The valves 46 and 54 are opened and the pump 51 is started so as to pump waste material from the supply container 44 into the dosage container 50. ^' When the dosage container 50 has been filled to a predetermined degree, the differential pressure sensor 53 generates a signal causing the control device 109 to close the valve 46, to open the valve 64 or any other of the valves 64-67, and to reverse the pump 51 , whereby the waste material is pumped from the dosage container into the processing compartment 11 , or another of the compartments 12-14 chosen . When the dosage container 50 has been emptied the differen¬ tial pressure sensor 53 generates a signal causing the valves 54 and

64 to close and the pump 51 to stop .

Material may be transferred from one of the compartments 11 -14 to any of the other compartments . Thus, for example, material may be transferred from the compartment 11 to the compartment 12 by the following procedure: The valves 54 and 64 are opened and the pump 51 is started to pump material from the compartments 11 through the conduits 25 and 47 into the dosage container 50. When the dosage container has been filled to a predetermined degree the differential pressu re sensor 53 generates a signal causing the control device 109 to close the valve 64, to open the valve 65, and to reverse the pump 51 so that the material may be pumped from the dosage container 50, through the conduits 47 and 26 into the compartment 12. When the container 50 has been emptied the sensor 53 causes the valves 54 and

65 to close and the pump 51 to stop . When the material is transported to and from the dosage container 50 it passes the temperature and acidity sensors 48 and 49. If the acidity of the material exceeds a predetermined value, the acidity sensor may generate a signal causing

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OMPI the control device 109 to open the valve 63 and/or 61 for supplying a predetermined amount of sodium hydroxide and/or water into the material contained in the dosage container 50. When material is to be discharged from the system, for example from the compartment 14, the valves 54 and 67 are opened and the pump 51 is started to pump material from the compartment 14 through the conduits 28 and 47 into the dosage container 50. When a desired amount has been filled into the container 50 the differential pressure sensor 53 causes the control device 109 to close the valve 67, to open the valve 69, and to reverse the pump 51 . The material is then pumped from the dosage container 50, through the conduit 47, through the discharge conduit 68, and out of the system. As the discharge conduit 68 is in heat transmitting contact with the fresh material within the supply container 44 heat from the discharged processed material will be transferred to the fresh material in the container 44. When the dosage container 51 has been emptied, the sensor 53 generates a signal, whereafter the con¬ trol device 109 causes the valves 54 and 69 to close and the pump 51 to stop.

The gas produced in the processing compartments 11 -14 continuously flow through the gas discharge conduits 21 -24, through the gas meters 78-81 , and out of the system th rough the outlet conduit 86, and possible condensate may be drained from the system through the drainage line 104. The contents of the processing compartments 11 -14 are mixed at certain time intervals. This may be done by circulating gas from the upper part of the respective compartment, for example the compartment 11 , to the gas nozzle tube 38 arranged at the bottom thereof. When the waste material 43 within the compartment 11 is to be mixed the control device 109 causes the valves 30 and 41 to open and the pump 35 to start, whereby gas is pumped from the upper gas space of the compartment 11 into the gas nozzle tube 38 at the bottom of the compartment. The gas bubbles rising upwardly through the waste material 43 will then cause the waste material 43 to become mixed .

The control device 109 may control the level of the waste material 43 within any of the compartments 11 -14 at predetermined time intervals. Thus, for example, the level within the processing compartment 11 may be determined by opening the valves 57, 64, and 88, whereby the differential pressure sensor 53 will be exposed to the pressure at the upper part of the compartment 1 1 th rough the conduit 21 and to the pressure at the bottom of the container th rough the conduits 25, 47, and 59.

It is understood that the control device 109 may control the operation of the bio-gas system illustrated in Fig . 1 in accordance with a pre¬ determined program on the basis of empirical data of the waste mate- rial being processed and on the basis of measu rements made by the various sensors of the system, such as the temperature sensors 48 and 74-77, the acidity sensor 49, and the differential pressure sensor 53. The control device 109 may also control the heating means 70-73 within the processing compartment so as to maintain a desired tempe- rature within these compartments .

The function of the system described may be controlled so as . to maintain good conditions for bacterial growth within the various processing compartments, for example so as to avoid excessive pro¬ duction of acids and other poisonous substances in order to avoid instability. Fu rthermore, the function of the system may be controlled so as to obtain an optimum gas production, and/or a desired variation in gas production corresponding substantially to the consumption of the produced gas, for example for heating purposes . Thus, for example, it may be desirable to obtain a reduced production of bio- gas at weekends and/or in the night time.

Fig . 2 illustrates a modified embodiment of a plant or system accord¬ ing to the invention , and parts of the plants illustrated in Fig . 2 corresponding to similar parts in Fig . 1 are designated by similar reference numerals . I n Fig . 2 the tan k 10 is a substantially cylin- drical tan k with a vertical axis, and the processing compartments 11 -14 are defined therein by cylindrical partition walls so that the central cylindrical compartment 14 is su rrounded by the concentrically arranged annular compartments 11 , 12, and 13. While the tank 10 and the supply container 44 is preferably arranged above the ground level 111 as shown in Fig. 2, the remaining part of the plant or system may be arranged below ground level in a cellar 112.

I n Fig . 2 the processing compartments 11 -14 are preferably provided with heating means like the heating means 70-73 indicated in Fig. I . The waste material 43 arranged within the central compartment 14 is then preferably heated to a temperature within the thermofillic area, for example within the temperature range 50-60°C. The waste material within the surrounding compartment 13 is then partly heated by heat transmission from the material within the compartment 14, but also by heating means within the compartment 13, so that the temperature in the compartment 13 will be somewhat lower than in the compartment 14. Similarly, the waste material within the compartments 12 and 11 may be heated, but to an even lower temperature, so that the tem¬ perature within the compartment 11 may be above the ambient tem- perature, but within the mesophilic temperature area, for example 32-37°C. Alternatively, the waste material within the central com¬ partment 14 may be heated to a sterilizing temperature, for example to a temperatu re above 60°C, so as to obtain a sterilized product leaving the system through the discharge conduit 68. In that case, the temperature within the processing compartment 13 may be within the thermophilic area .

Fig. 3 illustrates diagrammatically a system or plant composed by two cooperating systems 113 and 114 of the same general type as that shown in Figs. 1 and 2, and parts in Fig . 3 corresponding to parts in Fig . 1 are designated by similar reference numerals provided with one or two dots .

Each of the systems 113 and 114 in Fig . 3 operates in substantially the same manner as described in connection with Figs . 1 and 2, and the operation is controlled by a common electronic control device, not shown . Each of the processing compartments I T, 12' , 13', and 14' in the system 113 is divided into a water chamber 115 and a surrounding chamber 116 for solid matter by means of a partition wail 117 forming an osmotic filter. The water chamber in each of the processing cham¬ ber is in communication with a water discharge conduit 118 via a

OMPI remotely controlled valve 119, 120, 121 , and 122, respectively, and via an intermediate chamber 123 which is filled with a porous, but inactive material of a type which may form the basis for bacterial growth . The water discharge conduit 118 is in communication with a pump 124, by means of which water may be pumped from the water chamber 115 of any of the processing chambers 11 '-14' through the porous material within the chamber 123, and the removed water may be replaced by pure water which may be added to the solid matter chamber 116 through a water supply line 125, 126, 127, and 128, respectively.

If the process within one of the compartments 11 '-14' is not at its optimum due to excessive production of poisoning substances, such as acid and NH-, part of the polluted water may be pumped from the water chamber of the respective processing chamber by means of the pump 124 and replaced by pure water supplied through any of the water supply lines 125-128. The polluted water will then pass the porous material within the chamber 123, and provided that a suitable bacterial culture is present in this porous material , the bacteria may convert the poisoning NH- into protein and the CO- into CH₄ and into carbon bound to the protein mass . The acids present in the polluted water may be converted into CH , and CO- which may be fu rther converted into protein and CH , , as mentioned above. The water thus treated may be pumped into any of the processing compartments 11 "-14" of the system 114 through a supply conduit 129 and remotely controlled valves 130, 131 , 132, and 133, respectively . The system 114 may, for example, be adapted to treat waste material with a high water content, such as a slurry or sewage water, and in order to obtain a better basis for the bacterial culture within the processing compartments 11 "-14" each of these compartments preferably contains a suitable porous, inactive material .

When the water content of the material treated in the system 114 is high and, consequently, the content of solid matter is low, the pro¬ duction of poisonous substances will also be low. Therefore, the system 114 is able to receive poisoned water from the system 113, in which waste material with a high content of solid matter is treated, and in which a relatively big amount of poisonous matter is generated .

It should be understood that the embodiment described above may be modified in any desired manner within the scope of the appended claims . Thus, for example, the processing compartments in one or both of the systems shown in Fig . 3 may be in a concentrical ar¬ rangement as that shown in Fig . 2, and the central compartment may then be a sterilization compartment in which the processed material is heated to a sterilization temperature, for example 60-90°C.

Claims

1. A method of decomposing organic waste material (43) by bacterial action so as to generate combustible gas, said method comprising continuously or intermittently feeding the waste material into a first processing container (11), mixing the material therein, and continu¬ ously or intermittently discharging mixed waste material therefrom so as to obtain a desired average residence time for the waste material in the said first processing container, c h a r a c t e r i z e d in that at least part of the waste material (43) discharged from the first processing container (11) is fed into and further decomposed by bacterial action in one or more additional processing containers (12-14).

2. A method according to claim.1, c h a r a c t e r i z e d in that processed waste material is retur- ned from one or more of the additional processing containers (12-14) to the first container (11) and/or any preceeding additional container (12, 13).

3. A method according to claim 1 or 2, c h a r a c t e r i z e d in that feeding of waste material to and discharge of material from the various processing containers are controlled on the basis of empirical data showing the varying rate of gas production from a batch of the respective material in a process container, as a function of the residence time of the material in that container, so as to obtain a desired total gas production and/or a desired degree of decomposition of the waste material in the various processing containers.

4. A method according to any of the claims 1-3, c h a r a c t e r i z e d in that the feed of material to and the discharge of material from each of the processing containers are con- trolled so as to obtain a desired distribution on residence times, on the basis of physical and/or chemical conditions measured in the processing containers.

5. A method according to any of the claims 1-4, c h a r a c t e r i z e d in that the waste material is heated in one or more of the processing containers.

6. A method according to claim 5, c h a r a c t e r i z e d in that the container or each of the con¬ tainers in which the waste material is heated is arranged within an outer processing container.

7. A method according to claim 6, c h a r a c t e r i z e d in that the processing containers are arranged coaxially within each other.

8. A method according to claims 6 or 7, c h a r a c t e r i z e d in that processed waste material is heated to a sterilizing temperature in a sterilization container (14), which is surrounded by one or more of the processing containers (11-13).

9. A method according to any of the claims 1-8, c h a r a c t e r i z e d in that a watery fluid containing sub¬ stances restricting the desired bacterial action, is separated from the waste material in at least one of the processing containers and dis¬ charged therefrom while a corresponding amount of fresh water is supplied to the waste material in the container.

10. A method according to claim 9, c h a r a c t e r i z e d in that the watery fluid discharged from the waste material is passed through a porous material (123) bearing bacteria for converting the restraining substances to useful or harm- less substances.

11. A method according to any of the claims 1-10, c h a r a c t e r i z e d in that the waste material is mixed by circulating combustible gas generated in the respective container upwardly through the material.

12. A system for decomposing organic waste material (43) by bacterial action so as to generale combustible gas, said system comprising a first processing container (11), means (25,64) for continuously or intermittently feeding waste material into the container, means (35,38) for mixing the waste material within the container, and means (25,64) for continuously or intermittently discharging waste material from the container, c h a r a c t e r i z e d in one or more additional processing con¬ tainers (12-14) communicating with the discharge means (25,64) of the first container (11).

13. A system according to claim 12, c h a r a c t e r i z e d in that the discharge means of at least one of the additional processing containers (12-14) is communicating with the first container (11) and/or another of the additional containers arranged upstream of said one additional container.

14. A system according to claim 12 or 13, c h a r a c t e r i z e d in electronic control means (109) for con¬ trolling the feed of waste material to and the discharge of waste material from the various processing containers (11-14) so as to obtain a desired distribution on residence times in the respective containers.

15. A system according to any of the claims 12-14, c h a r a c t e r i z e d in that heating means (71-73) are provided in at least one of the additional processing containers (12-14).

16. A system according to any of the claims 12-15, c h a r a c t e r i z e d in that at least one of the containers

(12-14) containing the heating means is arranged within an outer processing container (11).

17. A system according to any of the claims 12-16, c h a r a c t e r i z e d in that the processing containers (11-14, Fig. 2) are arranged coaxially within each other.

18. A system according to any of the claims 12-17, c h a r a c t e r i z e d in that at least one of the processing containers (11-14, Fig. 3) includes water permeable filtering means (117) dividing the container into a waste material compartment (116) communicating with a fresh water inlet (125-128), and a water dis¬ charge compartment (115) communicating with water discharge means (124).

19. A system according to any of the claims 12-18, c h a r a c t e r i z e d in that at least one of the processing containers contains a porous inactive material (123) bearing bacteria for decomposing the organic waste material.

20. A system according to claims 18, c h a r a c t e r i z e d in that a porous inactive material bearing bacteria is arranged within the water discharge compartment (115), the bacteria being able to convert uπdesired poisonous or process restraining substances into useful or harmless compositions.