A process and device for anaerobic fermentation

Abstract: The invention relates to a process and a device for the essentially anaerobic fermentation of organic material, for example from domestic waste or from vegetable waste. Said organic material is after a pre-treatment transported into one of the fermentation compartments of an anaerobic fermentation installation comprising several fermentation compartments in which said mass is heated and it is kept at the required temperature, preferably by injection of steam, until said organic material is essentially fermented. The fermentation temperature ranges from 30 to 65°C, and preferably from 50 to 65°C.

Description: The present invention relates to a process for the essentially anaerobic fermentation of organic material, for example originating from domestic waste or vegetable waste, as well as to a device for implementation of said process. (A) WO-A-82/02059 discloses a process for anaerobic fermentation of organic waste which is converted into slurry in a mixing tank. After crushing said slurry is pumped into a closed holding container and then pumped via a heat exchanger into fermentation vessels. Such a process is further known from, amongst others, the European patent application 0 131 319. In the described process organic materials are anaerobically fermented in a (semi-) continuous process, wherein during the feed of the device with fresh, fermentable material a part of the partially fermented material is removed from the reaction vessel, it is mixed with new organic material and it is recycled into said reaction vessel. Said organic material may originate from waste, from which heavy components, such as metals, glass etc. are removed, which organic material, if desired, may be reduced and homogenized. A disadvantage of such a (semi-) continuous process is that in one single reaction vessel all stages of the fermentation process occur simultaneously, whereas each stage has its specific and optimum conditions. It should be clear that by the use of such a (semi-) continuous process in only one single fermentation vessel in favour of the occurrence of all stages of the fermentation process in said vessel the optimum conditions may not be met simultaneously. An additional disadvantage of such a (semi-) continuous process is the requirement of a technical facility in the reaction vessel, with which material present on the bottom of the reaction vessel may be removed by means of a discharge canal, and which is permanently in contact with the material to be fermented. This gives rise to additional wear and maintenance. Furthermore, with the process described in the above-mentioned European patent application, a large portion of the (partially) fermented material is once again introduced into the fermentation vessel, which results in unnecessary circulation of not completely fermented organic material, which is disadvantageous for the economy of the process. Moreover additional wear of the fermentation vessel and its components occurs by the repeated feed of mass on top and removal of mass at the bottom of the fermentation vessel. Accordingly the object of the present invention is to provide a process and a device which effectively eliminate these disadvantages and allow batchwise, anaerobic fermentation of organic material. This goal is now achieved by the present invention. Therefore the invention provides a process of the kind described in the preamble, characterized in that said organic material is fed into a rotating drum-type container and there thoroughly mixed with a solid and/or liquid graft material containing anaerobic bacteria, after which the such obtained mass having a moisture content of 40 to 80%, by weight, is transferred into one of the fermentation compartments of an anaerobic fermentation installation comprising several rectangular fermentation compartments adjacent to each other in which said mass is heated and it is kept at the required temperature until the organic material is almost completely fermented while the biogas obtained from said fermentation is removed and whereafter the such obtained fermented material is removed entirely from said fermentation compartment. An advantage of the present process is that in one single fermentation compartment organic material is present which occurs in one single fermentation stage of the anaerobic fermentation, resulting in almost ideal process conditions for the said stage in said fermentation compartment. All stages of the fermentation process are completed in succession in this compartment. A further advantage is that the thus obtained, after fermentation to be removed material, is a homogeneously fermented mass, which improves the quality of the final product considerably. Excess circulation of organic material is also avoided with the present process of batchwise fermentation which prevents unnecessary wear of parts of the device. Another feature of the process of the invention is that prior to the anaerobic fermentation in a fermentation compartment of the fermentation device, the organic material is treated and simultaneously aereted in a rotating drum or tubular container which is preferably a so-called DANO drum (DANO is a registered trademark). The effect of said aeration is, that due to aerobic decomposition processes the temperature of the organic material to be processed increases but does not achieve yet the optimum temperature that is required for the anaerobic fermentation, to wit about 55°C. It is in particular advantageous with the present process when the organic material in said drum which is preferably a so-called DANO (registered trademark) drum is aerated prior to mixing it in the mixer and homogenizing the same. It should be noted that the aeration of the organic material also may take place in the storage. Aeration of the organic materials in the storage happens specially if the waste to be processed mainly consists of vegetable material, for example originating from market withdrawn horticultural products. Accordingly, the organic waste materials which have to be processed, are, in addition to increasing the temperature by natural oxidation, usually heated in a rotating drum (which is preferably a DANO (registered trademark) drum) by injection of steam. A further characteristic of the process according to the invention is that mixing occurs under aerobic conditions. In this variant the processable material, particularly domestic waste, is introduced in a container together with the graft material containing anaerobic bacteria, wherein said mass is thoroughly mixed. Usually, one uses for this purpose a rotating drum, preferably the so-called DANO (registered trademark) drum which is a rotating horizontally mounted tubular container. Such a tubular container is in open contact with the air. Prior to transport the thus obtained homogenized mass into a fermentation compartment of a fermentation device it is necessary to remove coarse non-fermertable components from the mass for example by means of sieves and magnets. In said tubular container, which is preferably the so-called DANO (registered trademark) drum, said organic materials are squashed and reduced in order to improve the subsequent anaerobic degradation significantly. An advantageous variant of the process of the invention is when during the aerobic treatment additional air is introduced resulting in a further rise of the temperature of the material to be fermented. Another variant of the present process is characterized in that after the transfer of the obtained thoroughly mixed mass into one of the fermentation compartments said mass is heated to 30-60°C, preferably to 50-65°C, which temperature is maintained for the anaerobic fermentation of said mass. It is noted that the required heat can be partly obtained by aeration whereas the optimum required heat preferably may be obtained by injection of steam in the rotating drum which is preferably a DANO (registered trademark) drum. By doing so it is possible to bring the temperature of the mass to be fermented prior to its transfer into the fermentation compartment at a temperature required for the anaerobic fermentation, to wit 30-65°C. According to another feature of the invention said graft material is heated prior to mixing it with said organic material to a temperature of 30-65°C, and preferably to 50-65°C. By doing so the material to be fermented keeps the desired temperature after introduction of said graft material. Furthermore, it is possible according to the present invention that said organic material optionally together with said graft material is heated to 30-65°C, and preferably to 50-65°C, and preferably by injection of steam. As anaerobic graft material preferably use is made of fermented sewage sludge, fermented solid organic material or press liquid from dehydrators for fermented solid organic material or to this end specially grown biomass. By using the fermented organic material or press liquid such graft material may be recovered from the fermentation compartment of the present anaerobic fermentation installation, so that there is no need to purchase and transport this graft material, which improves the economy of the process. For a favourable working of the process of the invention said graft material is used in such an amount that excessive acidification of the mass in the particular fermentation compartment is prevented. Acidifying bacteria may not be present in excess. The medium should be such that the methane forming bacteria can exist. Excessive acifidication inhibits or even prevents proper fermentation. An advantageous aspect of the proces of the invention is that the moisture content of the material to be fermented, introduced into a fermentation compartment, ranges from 40 to 80%, by weight. This allows processing of various organic waste materials with a relatively high dry weight content. A practical feature of the present process is the fact that the transport of graft material and material to be fermented to the particular fermentation compartment is effected with a conveying system. The advantage of such a conveying system is that it permits the transport of material to be fermented with a relatively high dry weight content. A further preferred aspect of the present process is that said transport is carried out by means of a closed conveying system, preferably a closed conveyor belt. This prevents unnecessary heat loss of for example said thoroughly mixed mass and simultaneously the spreading of foul smells. For the transport use may be made of jack screw transport and prop transport means which are used for transport of wet sand. Preferably use is made of a traditional closed chain conveyor system or more preferably a closed conveyor belt. Another feature of the present process concerns the fact that per fermentation compartment an amount of organic material to be fermented is introduced which corresponds to the average daily supply of new processable organic material. In this respect it is favourable that in one fermentation compartment only organic material is present which is in the same fermentation stage, which organic material is kept there without interruption during the essentially entire period of the fermentation. This period varies depending on the process conditions from 15 to 50 days. According to a variant of the process according to the invention the biogas formed during the fermentation is blown through the material to be fermented in a fermentation compartment. This results in a loosening up of the fermentable mass and in promoting the release of biogas. Usually, at the end of the fermentation the fermented material in the particular fermentation compartment is blown through with an inert gas or air. The advantage of this is that as much as possible biogas is obtained, because the obtained biogas is completely removed from the mass. Finally, is it possible according to the present invention that the fermented material is treated aerobically before its removal from the fermentation compartment or after its removal. By doing so the not completely degraded material is aerobically processed and stabilized, which improves the quality of the obtained fermented final product. The invention also comprises a device for carrying out the process of the invention. To this end the device is characterized in that said device is provided with at least a container for solid graft material and a container for liquid graft material, as well as a rotating drum-type container for thoroughly mixing of the organic material to be fermented and the solid and/or liquid graft material and which device further is provided with a conveying system for transport of the thoroughly mixed mass having a moisture content of 40 to 80%, by weight, from said drum-type container to one of the anaerobic rectangular fermentation compartments adjacent to each other of an anaerobic fermentation installation comprising several heatable fermentation compartments. In such a device the mixing, usually under aerobic conditions, of the material to be fermented and the graft material may be carried out. According to the invention each of the separate fermentation compartments may be provided with at one or more segmentation walls, if necessary, which depends on the size and capacity of the fermentation installation. Favourable results are obtained when in the fermentation compartment the segmentation walls are placed in a way that the relative distance between each other and the walls of the compartment is about the same. Furthermore, it is preferred when the height of the segmentation wall amounts about 5-15% less than the height of the partition wall of the fermentation compartment. Advantageously such a segmentation wall as well as the partition wall of the compartment are provided with one or more heating elements which preferably comprise hot water heating elements. According to a favourable embodiment of the present invention the device is provided with at least one container for fresh, fermentable organic material, in which it can be subjected to an aerobic treatment. This embodiment will especially be used in case of with relatively clean, virtually not-contaminated, vegetable waste. In such a case the container is a simple mixing container. According to the preferred embodiment of the device of the invention each of the fermentation compartments is provided with concrete partition walls wherein preferably a heating element is built in. Said concrete walls do have the advantage of a very good installation resulting in the maintenance of an optimum thermophilic temperature during the fermentation. Both the rear wall and the roof construction of the compartments are preferably made from an isolating sandwich panel or they are also made from concrete, in which case this is provided with isolating material at the outside surface in order to achieve an optimum isolating construction. Preferably, each of the mentioned fermentation compartments is provided with heatable floors which also contributes to bringing the mass to be fermented to a higher temperature as well as to maintain the required fermentation temperature. As mentioned before the heating is preferably achieved by means of a hot water heating system. According to another feature of the device of the invention the fermentation compartments are moisture and air tight in order to enable an optimum anaerobic fermentation. It is preferred when the device of the invention is provided with a closed conveying system between said separate containers for transport of said organic mass in any form, and/or said solid and/or said liquid graft material. Generally, a closed conveyor belt is used. Said closed conveying system enables the transport of the mass streams without unnecessary heat loss and/or the causing of a stench inconvenience to the employees and environment. The invention will be further explained by means of the accompanying drawings 1 and 2, wherein Fig. 1 is a diagrammatic representation of the preferred embodiment of the invention, and Fig. 2 provides a diagrammatic representation of an embodiment wherein the fermented material is subjected to a post-treatment. Figure 1 As mentioned before Fig. 1 represents the preferred embodiment of the device of the invention. Herein the material to be fermented is first, if required, pre-treated, in order to remove inert and/or interfering substances such as plastics and metals as much as possible. The said pre-treated organic material is generally transported into container 3 which is usually a rotating drum and preferably a so-called DANO (registered trademark) drum, wherein said material is reduced and thoroughly mixed. However, it is also possible to introduce the pre-treated material into a container 6 prior to its transportation into the container 3, wherein said organic material is subjected to aerobic degradation, which may be promoted by introducing air. To the organic material in a container 3 solid graft material from container 1 and liquid graft material from container 2 are added. However, it should be noted that said liquid graft material advantageously may be added to the organic material after leaving said container 3 and prior to its transportation into the fermentation compartiment 5a, 5b, 5c, ... by means of transportation installation 4. This option is not shown in Fig. 1. By doing so the anaerobic micro-organism content of the organic material to be fermented may easily be adjusted to the optimum level required for an optimum anaerobic fermentation in said fermentation compartment. It should be clear that the mentioned pre-treatment of the organic material to be fermented is dependent on the origin of the waste material. In many cases the waste material is suitable for direct treatment in container 3, which is preferably a DANO (registered trademark) drum. Said DANO (registered trademark) drum is a horizontally arranged rotating tubular vessel, wherein the organic material to be fermented is mixed thoroughly in the presence of air. During this treatment aerobic degradation occurs, which causes the increase of the temperature of the moisture. However the temperature in said container 3 advantageously may be increased by injection of steam prior to its transfer to said fermentation compartment. The temperature of the organic mass is usually increased to 30-65°C, and preferably to 50-65°C. Between the container 3 and the transport installation 4 an additional container may be placed (not shown in the drawing) in order to obtain additional heating, grafting and water supply. The size of a fermentation compartment, which compartment is preferably rectangular, for example 5a, depends on the average daily supply of new organic material to be fermented. Per working day one fermentation compartment will be filled, whereupon that compartment is closed off for the duration of the fermentation. The fermentation takes place during 15-50 days. The production of the next day of material to be fermented is introduced into fermentation compartment 5b etc. In this way a batchwise charging of the fermentation installation is obtained. It is noted that each of the fermentation compartments 5a, 5b, 5c, ... shown in the figure is provided with two segmentation walls 15 which preferably are provided with one or more heating elements (not shown in the figure). Said heating elements are advantageously hot water heating elements. However, the heating elements also may be the electrical elements. In case of hot water heating systems the required heat is provided by waste heat, which is developed by the gas engines, which convert the obtained biogas in electricity. Such fermentation compartments appear to ensure a good and efficient fermentation. However, it should be clear that the invention is not restricted to two segmentation walls per fermentation compartment. Dependant on the dimensions and therefore the capacity of the fermentation compartment one segmentation wall or three or more may be used, whereas in case of a small capacity no segmentation wall is used at all. During operation of the fermentation installation essentially no organic material except biogas is withdrawn. Optionally press liquid which may be formed in the fermentation compartment may be recycled into the fermentation compartment in order to keep the water content of the organic mass at the required level. The capacity of the fermentation compartments may be for example about 250 to 750m³. It will be clear that the smaller or larger fermentation compartments can be used. Usually the stacking height after charging of the material in the fermentation compartments is about 4.5 to 5.5 meters, whereas the capacity of the fermentation compartment may be accommodated by adapting the length of the compartments. However, the total capacity of the fermentation installation may be increased by increasing the number of fermentation compartments. The fermentation compartments may be constructed of many different materials. However, an acid resistant concrete construction is preferred. Each of the fermentation compartments (5a, 5b, 5c ...) preferably comprises a concrete floor, concrete partition walls and concrete segmentation walls, if any, whereas the rear wall and roof are made from isolating sandwich panels or they also may be made from concrete in which case the outer walls and roof are provided with an isolating material at the outside. During the fermentation of the organic material biogas is formed, which is then removed. After the fermentation has been completed the material which is remained in the particular fermentation compartment is optionally treated with an inert gas or air. Then the contents of the particular fermentation compartment is removed. The above mentioned treatment with an inert gas or air also can take place after removal of the fermented material from the particular fermentation compartment. Post-treatment of the fermented material Figure 2 The fermented material may be removed from the fermentation compartment by means of an excavator. The fermented material from a fermentation compartment (for instance 5b) may be transported partially through transportation means such as a conveyor belt etc. 7 to a storage container 8 and it may serve as graft material for new fermentable organic material. The fermented sewage sludge may be brought to or maintained at a temperature of about 40-65°C. This permits that preferably each batch of new, fermentable organic material has the optimum temperature during the pre-treatment stage. It is also possible to transport the fermented material from a fermentation compartment, for example 5a, via transportation means 9 to a dehydrator 10, in which solid fermented organic material and press liquid from the dehydrator are transported by means of transport means 11 to a storage container 12 and by means of transportation means 13 to a storage container 14, respectively. The solid organic material from storage container 12 may subsequently be dried and/or carried off. The press liquid from storage container 14 and the fermented solid organic material from storage container 12 may be re-used as graft material for new, fermentable material. The amount of graft material should always be sufficient to suppress the predominance of acid forming bacteria in the first stage of the fermentation process. Once sufficient fermented material is obtained in the installation, which comprises several fermentation compartments, it is possible to use 30% of this fermented material as graft material in order to properly graft the new fermentable organic mass. By the simultaneous use of fermented sewage sludge or press liquid from the dehydration of fermented organic material the percentage solid graft material may be reduced.

Claims: A process for the essentially anaerobic fermentation of organic material, which originates for example from domestic waste or vegetable waste, characterized in that said organic material is fed into a rotating drum-type container and there thoroughly mixed with a solid and/or liquid graft material containing anaerobic bacteria, after which the such obtained mass having a moisture content of 40 to 80%, by weight, is transferred into one of the fermentation compartments of an anaerobic fermentation installation comprising several rectangular fermentation compartments adjacent to each other in which said mass is heated and it is kept at the required temperature until the organic material is almost completely fermented while the biogas obtained from said fermentation is removed and whereafter the such obtained fermented material is removed entirely from said fermentation compartment. The process according to claim 1, characterized in that said organic material is aerated prior to the anaerobic fermentation. The process according to claim 2, characterized in that said organic material is aerated prior to the mixing. The process according to claim 1 or 2, characterized in that the mixing is carried out under aerobic conditions. The process according to any of the claims 2 to 4, characterized in that during said aerobic treatment additional air is supplied. The process according to any of the claims 1 to 5, characterized in that after transfer of the obtained thoroughly mixed mass into one of the fermentation compartments, said mass is heated to 30 to 65°C, which temperature is maintained for the anaerobic fermentation of said mass. The process according to claim 6, characterized in that said mass is heated to 50-65°C, which temperature is maintained. The process according to any of the claims 1 to 7, characterized in that said graft material is heated prior to mixing it with said organic material. The process according to claim 8, characterized in that said graft material is heated to a temperature of 30-65°C, and usually to 50-65°C. The process according to any of the claims 1 to 7, characterized in that said organic material together with said graft material is heated to 30-65°C, and usually to 50-65°C, generally by injection of steam. The process according to any of the claims 1 to 10, characterized by the use of fermented sewage sludge, fermented solid organic material or press liquid from dehydrators for fermented solid organic material or to this end specially grown biomass as anaerobic graft material. The process according to any of the claims 1 to 11, characterized in that the graft material is used in such an amount that excessive acidification of the mass in the particular fermentation compartment is prevented. The process according to claim 1, characterized in that said moisture content of the material to be fermented is from 50 to 70%, by weight. The process according to any of the claims 1 to 13, characterized in that the transport of graft material and material to be fermented to the particular fermentation compartment is effected with a conveying system. The process according to claim 14, characterized in that said transport is carried out by means of a closed conveying system, such as a closed conveyor belt. The process according to any of the claims 1 to 15, characterized in that per fermentation compartment an amount of organic material to be fermented is introduced which corresponds to the average daily supply of new, processable organic material. The process according to any of the claims 1 to 16, characterized in that biogas formed during the fermentation is blown through the fermentable material in a fermentation compartment. The process according to any of the claims 1 to 17, characterized in that at the end of the fermentation the fermented material in the particular fermentation compartment is blown through with an inert gas or air. The process according to any of the claims 1 to 18, characterized in that the fermented material is treated aerobically before its removal from the fermentation compartment. A device for the essentially anaerobic fermentation of organic material characterized in that said device is provided with at least a container (1) for solid graft material and a container (2) for liquid graft material, as well as a rotating drum-type container (3) for thoroughly mixing of the organic material to be fermented and the solid and/or liquid graft material and which device further is provided with a conveying system (4) for transport of the thoroughly mixed mass having a moisture content of 40 to 80%, by weight, from said drum-type container (3) to one of the anaerobic rectangular fermentation compartments adjacent to each other of an anaerobic fermentation installation (5) comprising several heatable fermentation compartments (5a, 5b, 5c, ...). The device according to claim 20, characterized in that each of the separate fermentation compartments (5a, 5b, 5c, ...) is provided with one or more segmentation walls (15). The device according to claim 21, characterized in that segmentation wall (15) and the partition walls are provided with one or more heating elements. The device according to claim 22, characterized in that said heating elements are hot water heating elements. The device according to claims 20 to 23, characterized in that it is provided with at least one container (6) for fresh, fermentable organic material, in which it can be subjected to an aerobic treatment. The device according to any of the claims 20 to 24, characterized in that each of the fermentation compartments (5a, 5b, 5c, ...) comprises a concrete floor and concrete partition walls whereas the rear wall and roof are made from isolated sandwich panels or also from concrete, wherein the outer walls and roof are provided with an isolation material. The device according to any of the claims 20 to 25, characterized in that each of the fermentation compartments (5a, 5b, 5c, ...) comprises a heatable bottom, if desired. The device according to any of the claims 20 to 26, characterized in that the fermentation compartments (5a, 5b, 5c, ...) are moisture and air tight fermentation compartments. The device according to any of the claims 20 to 27, characterized in that said device is provided with a closed conveying system between said separate containers for transport of said organic mass in any form, and/or said solid and/or said liquid graft material.

Inventors: MASSELINK PAUL HERMAN ANDREAS, MASSELINK, PAUL HERMAN ANDREAS MARIA
Applicants: DUTCH ENVIRONMENTAL MANAGEMENT, DUTCH ENVIRONMENTAL MANAGEMENT GROUP B.V.
Application year: 1991
Main classification: C12M1
IPC codes: C05F17, C12M1, C02F3, C05F11, B09B3