WO1999067191A1

WO1999067191A1 - A bioreactor for composting

Info

Publication number: WO1999067191A1
Application number: PCT/DK1999/000303
Authority: WO
Inventors: Per Eduard Robert Bjerager; Leif Berthelsen
Original assignee: Perox Bioplant Aps
Priority date: 1998-06-08
Filing date: 1999-06-08
Publication date: 1999-12-29
Also published as: DK174691B1; DK77098A; AU4256699A

Abstract

A process for composting moist organic material in a continuous, aerobic bioreactor. The organic material is placed in a tray with a grating bottom which is placed on top of a stack of trays standing in an insulated, vertical shaft (101). Individual trays are transported one step downwards through the shaft when a tray (102g) with compost is removed from below. The stack of trays in an individual shaft is aerated by supplying composting air through a pipe stub or connecting piece (106) below and by removing composting air through a pipe stub or connecting piece (104) above. By means of this process, efficient aeration of the organic material and simple transport of the organic material through the bioreactor is achieved. In addition, the bioreactor ensures efficient composting of moist organic material with a relatively high water content because the bioreactor's insulation and compact construction minimize loss of heat energy from the composting by conduction.

Description

A Bioreactor for Composting.

The invention relates to a process for composting moist organic material in an insulated, continuous, aerobic bioreactor with a vertical composting profile, counterflow aeration and trays for the organic material. The invention also relates to a process in a bioreactor for combining the composting of moist organic material with biofiltration of composting air and drying of compost. Furthermore, the invention relates to the construction of the bioreactor as well as to the use of the bioreactor and processes for treatment of moist organic material.

Moist organic material consists of dry matter and water. Composting is an aerobic biological process in which microorganisms degrade dry matter in the organic material primarily to carbon dioxide, water and heat. The main purpose of the invention is a high rate of degradation of dry matter and thereby the composting of a large amount of fresh organic material in a short time.

A prerequisite for a high rate of degradation is favourable conditions for the microorganisms. It is general knowledge that aerobic microorganisms require a moist environment and aeration that supplies oxygen and removes the composting products water, heat and carbon dioxide. These requirements are normally fulfilled to only a poor extent because it is difficult in practice to have a high water content in the organic material and at the same time ensure good aeration.

The organic material must have an airy structure to ensure good aeration. The highest rate of degradation is achieved by maximizing the water content while taking into account the fact that aeration must not be inhibited by the water. Inhibition can occur as a result of small air spaces in the organic material becoming filled with water, or by the mass of the water compressing the material and thereby distroying the small air spaces.

To avoid an increase in the water content of the organic material during composting it is necessary to remove both the free water formed by the composting and part of the original water content of the organic material, corresponding to the degradation of dry matter.

A continuous bioreactor for composting is a closed container with active aeration and with continuous supply of fresh organic material and removal of compost. A bioreactor with a vertical composting profile receives fresh organic material at the top and contains partially composted material in the middle, whilst compost is removed at the bottom. With counterflow aeration, composting air flows first through composted material and finally through fresh material.

The big advantage of a continuous composting reactor with vertical composting profile and counterflow aeration is that a stable composting process with a high rate of degradation can be achieved. This is possible because the reactor contains organic material at all stages of degradation, and because the aeration fits in with the need for removal of water and energy.

Fresh organic material contains dry matter which is easy to degrade as well as dry matter which is difficult to degrade. During composting, an exponential drop in the rate of degradation will take place after degradation of the easily degraded dry matter. As a result there is a corresponding decrease in the need for removal of water and energy. This is taken into account with counterflow aeration, since heating of the composting air takes place from the time at which it first flows through composted material and until it flows through fresh material. This heating results in an exponential increase in the capacity of the air to contain water and energy.

The heat generated in composting is removed from a composting reactor by means of conduction and convection. Conduction takes place by transmission of heat through the walls of the reactor, and convection takes place through heating of the composting air and through evaporation of water which is removed with the composting air. Convection must be maximized in order to achieve the highest possible rate of degradation (= fastest composting). The background herefor is that convection removes water, and with strong convection the water content in the organic material may be high without the water content in the organic material increasing during composting.

The use of trays for composting moist organic material is well known. US patent No. 5,441,552 and French patent publication No. 2 594434 describe trays with perforated sides and bottoms. This perforation ensures passive aeration of the organic material during composting. The uninsulated perforated sides of the trays have two major disadvantages. Firstly, a large part of the heat developed in the composting will be removed through the sides by conduction, and secondly, it is not possible to regulate the aeration of the organic material during composting. German patent publication DE 40 06 239 A1 employs an insulated storage building for after-ripening or after-composting of composted material on pallets. The storage building itself is ventilated, but there is no active aeration of individual pallets in the building. The storage building contains only composted material, and the need for aeration is therefore low.

DE 297 06411 U1 describes a "fast composter" which has the same basic construction as in DE 4006 239 A1 , but which employs perforated trays with inclined sides instead of pallets. The inclined sides result in good passive aeration of the organic material in the individual trays. It is not possible to regulate the aeration of individual trays - only the aeration of the closed, insulated space with the trays.

In principle, a tray system with vertical composting profile for composting organic material is known from PCT publication WO 94/19296 (Nigel Nattrass), trays with fresh material being placed on top of trays with composted material. The individual trays are rectangular and have a grating at the bottom as well as small openings on two opposing sides for passive aeration of the organic material. On top of the stack of trays there is a lid, and the stack of trays is placed on a special tray with a solid bottom and a pibe stub for draining off liquid.

The tray system in question (WO 94/19296) has been developed further by Nigel Nattrass with special focus on passive ventilation of the organic material by counterflow aeration, known from PCT publication WO 98/05606. In this case, the individual trays have closed sides, and the lowest tray stands on legs which are constructed to lead air up through the stack of trays when the wind blows. On top of the stack of trays there is a lid with holes for removing the air which flows through the stack. The ventilation in this system must be regarded as passive since it is dependent on the wind.

The tray systems known from WO 94/19296 and WO 98/05606 cannot be characterized as bioreactors since they are not closed containers. Moreover, these tray systems have a distinct disadvantage in that the bottom tray with compost can only be removed by first removing all the trays above. Since the trays are not insulated, both tray systems undergo major heat loss by conduction. It will therefore not be possible to avoid an increase in the water content when composting organic material with a high water content. Patent application DK 0235/97 (Per Bjerager) describes an insulated bioreactor with active counterflow aeration. In this case, the organic material is held and transported through the reactor in closed containers. The containers in the reactor are connected in series by pipes, so that the composting air first flows through containers with composted material and finally through containers with fresh organic material. A major disadvantage is that the containers are equipped with pipes and a lid.

In DK 0235/97 the piping between the containers is a major but necessary disadvantage, since pipes have to be assembled and disassembled when introducing and removing containers. This disassembly and assembly work has to be done manually since the piping between the containers has to be flexible and leak-tight.

In DK 0235/97, the lids of the containers are a major but necessary disadvantage, especially when using large containers (covering more than 1 m²). A lid is expensive to make, and it has to have a substantial thickness (height) in order to be able to cover a large container without becoming deformed (deformation leads to leaks). Consequently, the lid takes up a significant amount of space in the bioreactor. In addition, it is an significant disadvantage that the lid has to be fitted after filling the container with fresh organic material, and that the lid has to be removed before emptying the container.

The purpose of the invention is now to provide a process for composting moist organic material in a bioreactor of the type mentioned initially which can easily be supplied with fresh organic material, from which compost is easily removed, and which has minimal conduction.

This is achieved according to the invention by a process for composting moist organic material in a bioreactor of the type mentioned initially, characterized in that the organic material is placed in a tray which has a grating bottom and which is placed on top of a stack of trays standing in a vertical shaft in a bioreactor having one or more shafts, that the individual trays are transported one step downwards through the shaft when a tray with compost is removed from below, and that the stack of trays in an individual shaft is aerated by supplying composting air through a pipe stub or connecting piece in a closed space under the bottom tray and by removing composting air through a pipe stub or connecting piece in a closed space above the top tray. In this manner, efficient composting with a very high rate of degradation is achieved, since organic material with a relatively high water content can be composted as a consequence of the minimal conduction of the reactor and the active counterflow aeration. Conduction is minimal because the reactor is insulated and very compact. The reactor is very compact because there is no lid on the trays, because the trays can be stacked to a considerable height, and because the distance between shafts is minimal.

The transport of trays through the reactor is very simple, since gravity is exploited. The trays are of a simple construction relative to containers, since the trays have no lid and piping. Moreover, it is easier to use the trays than containers, since the trays do not have the disadvantages associated with lids and piping.

Composting air under particular conditions and with a particular chemical composition can be supplied through the pipe stub or connecting piece below the lowest tray in an individual shaft. The flow of air through the stack of trays is regulated by the removal of composting air through the pipe stub or connecting piece above the top tray in an individual shaft, and the suction makes it possible to treat the air removed.

In a particular embodiment of the process, the removed composting air is mixed with fresh air, and part of this air mixture, corresponding to the amount of composting air removed, is supplied to the composting reactor through the pipe stub or connecting piece below the bottom tray. This procedure ensures supply of oxygen for the composting with the fresh air.

In another embodiment of the process, composting is combined with biofiltration of composting air by mixing the removed composting air with fresh air, by recirculating this air mixture to the bioreactor through a pipe stub or connecting piece on the side of the reactor, and by withdrawing an amount of air corresponding to the fresh air supplied through the pipe stub or connecting piece below the bottom tray. Biological purification of the excess composting air in the trays below the pipe stub or connecting piece on the side of the bioreactor is achieved hereby.

In a third embodiment of the process, composting is combined with biofiltration of composting air and drying of compost by supplying drying air through the pipe stub or connecting piece below the bottom tray and by removing both purified composting air and moist drying air through a pipe stub or connecting piece positioned on the side of the reactor between the pipe stub or connecting piece below the bottom tray and the pipe stub or connecting piece for supplying composting air.

Drying of the compost in the trays below the pipe stub or connecting piece for removing both purified composting air and moist drying air takes place thereby. The composting and drying results in a very substantial reduction in the mass of the organic material. Furthermore, the drying stops the composting process so that storage-stable compost is obtained.

The invention moreover provides a bioreactor for use in the processes according to the invention. The bioreactor is characterized in that it is insulated, that it consists of one or more vertical shafts, that there is a stack of trays with grating bottoms in the individual shafts, that there above the top tray in the stack is a closed space with a pipe stub or connecting piece for removal of air, that there below the bottom tray in the individual stacks is a close space for supplying air, and that the bottom tray in the stack can be removed from below.

The individual trays can be constructed with horizontal plates around the gratings, the plates having a width of 5-30 cm and fitting tightly against the sides of the container. With this construction, the formation of air channels up the sides of the tray is avoided.

In another construction for an individual tray, the tray has conical sides with an angle af 5-40°, preferably 20°, to the vertical, so that the inside width of the tray is greatest at the top. The conical sides counteract the formation of air channels up the sides of the tray, and in addition ensure good aeration of the organic material along the sides. The conical sides also ensure that organic material can easily be emptied from the tray by simply inverting the tray.

The individual shafts in the bioreactor can be constructed with one or more retaining devices for holding the trays above the bottom tray in the shaft during removal of the bottom tray. Individual retaining devices can be in the form of a pawl which can be activated by a pressure mechanism and deactivated by a spring. The retaining devices/pawls make it possible to remove the bottom tray in the individual shafts without affecting the trays above. It is extremely simple to activate and deactivate an individual pawl by means of the pressure mechanism and spring. Individual shafts in the bioreactor - only for composting - can be constructed with a lid with a pipe stub or connecting piece on the top tray, a bottom with a pipe stub or connecting piece below the bottom tray and with gaskets or seals between the lid and top tray, between the individual trays, and between the bottom tray and the bottom. In this way, the lid, trays and bottom together form a bioreactor.

The individual shafts in the bioreactor for composting/biofiltration/drying can be constructed as a closed container with an airtight hatch at the top for introducing trays, with an airtight hatch at the bottom for removing trays, with one or more pipe stubs or connecting pieces on the side of the shaft, and with gaskets or seals between the individual trays and the sides of the shaft. It is thus possible to introduce or remove composting air through a pipe stub or connecting piece on the side of the shaft between two particular trays, and the gaskets or seals ensure that flow of air vertically around the individual trays is avoided.

In the individual shafts in the bioreactor for composting, biofiltration and drying, the gaskets or seals between the individual trays and the sides of the shaft can be inflatable elastic membranes. This construction ensures efficient sealing. When the trays are to be transported one step downwards through the shaft, air is removed from the membranes. Friction between the membranes and the trays during transport is hereby avoided.

As mentioned, the invention also relates to the use of the reactors and the processes. The invention is to be used primarily for treatment of moist organic material with a dry matter content of 20-30%.

The invention is explained in more detail in the following with reference to the drawings, in which

Fig. 1 shows a bioreactor according to the invention for composting

Fig. 2 shows a bioreactor according to the invention for composting, biofiltration and drying

Fig. 3a shows an embodiment of a tray with horizontal plates Fig. 3b shows a special embodiment of a tray with conical sides.

Figure 1 shows an embodiment of a composting bioreactor with one shaft. This bioreactor consists of an insulated shaft 101 with seven trays 102a-102g containing organic material. On top of the top tray 102a is a tightly sealing lid 103 with a pipe stub or connecting piece 104 for removing air. The bottom tray 102g is placed on a leak-tight bottom 105 with a pipe stub or connecting piece 106 for supplying air.

The trays 102a-102g, the lid 103 and the bottom 105 constitute a bioreactor (closed container) since there are airtight gaskets or seals between the units. One of the gaskets or seals is indicated on the figure by the numeral 107. The trays 102a-102g with the lid 103 and the bottom 105 are kept in place in the shaft by means of pawls 108. The individual pawls 108 are equipped with a spring 109 and a pressure mechanism 110. The pawl 108 is activated by the pressure mechanism 110, and the pawl 108 is deactivated by the spring 109 when the pressure mechanism 110 is deactivated.

The trays 102a-102g are transported one step downwards through the shaft 101 by removing the bottom tray 102g from below. This is done in practice by 1) lifting the trays 102a-102g with the lid 103 and the bottom 105 slightly, 2) deactivating the pawls 108, 3) lowering the trays 102a-102g with the lid 103 and the bottom 105 - until the tray 102f second from the bottom is just above the pawls 107, 4) activating the pawls 108, 5) lowering the bottom tray 102g and the bottom 105, 6) removing the bottom tray 102g, and 7) lifting the bottom 105 up over the pawls 108. When the trays are transported one step downwards through the shaft 101, a new tray is introduced by lifting the lid 103.

In the new tray there is fresh organic material, and as a result of the composting and the transport of the trays downwards through the shaft, the bottom tray will contain moist compost.

The composting air flows from the pipe stub or connecting piece 106 up through the stack of trays 102a-102g and out through the pipe stub or connecting piece 104. By means of the flow of the air through the trays 102a-102g, the desired heating of the air and increase in the water content of the air, as well as delivery of oxygen and uptake of carbon dioxide, takes place as a result of the composting.

Fig. 2 shows the construction of a bioreactor for composting, biofiltration and drying. This bioreactor consists of a closed, insulated shaft 201 in which ten trays 202a-202j with organic material are stacked on top of each other. Air can flow between the trays out into the air space between the trays and the inner sides of the shaft. Gaskets or seals are positioned between the individual trays 202a-202j and the sides of the shaft and prevent air from flowing vertically around the trays 202a-202j. One of the gaskets or seals is indicated on the figure by the numeral 203.

At the top of the side of the shaft 201 there is a pipe stub or connecting piece 206 for withdrawing hot composting air. Between the tray 202e and the tray 202f there is a pipe stub or connecting piece 207 on the side of the shaft 201 for supplying cold composting air. At the bottom of the side of the shaft 201 there is a pipe stub or connecting piece 209 for supplying drying air, and between the tray 202h and the tray 202i there is a pipe stub or connecting piece 208 for withdrawing both moist drying air and purified composting air.

With this construction of the shaft 201 , composting of the organic material will take place in the trays 202a, -b, -c, -d, -e. In the trays 202f, -g, -h there is moist compost which purifies cold composting air by biofiltration. Biofiltration takes place when more cold composting air is supplied through pipe stub or connecting piece 207 than is removed through pipe stub or connecting piece 206.

Drying of compost takes place in trays 202e, -j as a result of supplying drying air through pipe stub or connecting piece 209.

The trays are transported one step downwards through the shaft 201 by opening an airtight hatch 204 at the bottom of the side of the shaft 201 and withdrawing the bottom tray 202j. A new tray with organic material is then introduced through an airtight hatch 205 at the top of the side of the shaft 201.

Fig. 3a shows one construction of a tray 301a with vertical sides, a grating bottom 302a and a horizontal plate 303 around the grating bottom 302a. The plate 303 is sealed against the sides of the tray.

Fig. 3b shows a special construction of a tray 301b with a grating bottom 302b and conical sides with an angle a relative to the vertical .

Claims

1. A process for composting moist organic material in an insulated, continuous, aerobic bioreactor with a vertical composting profile, counterflow aeration and trays for the organic material, characterized in that the organic material is placed in a tray with grating bottom which is placed on top of a stack of trays standing in a vertical shaft in a bioreactor having one or more shafts, that the individual trays are transported one step downwards through the shaft when a tray with compost is removed from below, and that the stack of trays in an individual shaft is aerated by supplying composting air through a pipe stub or connecting piece in a closed space below the bottom tray and by removing composting air through a pipe stub or connecting piece in a closed space above the top tray.

2. A process according to claim 1, characterized in that the removed composting air is mixed with fresh air, and that part of this air mixture, corresponding to the amount of removed composting air, is supplied to the composting reactor through the pipe stub or connecting piece below the bottom tray.

3. A process according to claim 1 for composting moist organic material and biofiltering composting air in compost, characterized in that the removed composting air is mixed with fresh air, that this air mixture is circulated to the bioreactor through a pipe stub or connecting piece on the side of the reactor, and that an amount of air corresponding to the fresh air supplied through the pipe stub or connecting piece below the bottom tray is withdrawn.

4. A process according to claim 1 and 3 for composting, biofiltration of composting air and drying of compost, characterized in that drying air is supplied through the pipe stub or connecting piece below the bottom tray, and that both purified composting air and moist drying air are removed through a pipe stub or connecting piece placed on the side of the reactor between the pipe stub or connecting piece below the bottom tray and the pipe stub or connecting piece for supplying composting air.

5. An aerobic, continuous bioreactor for use in performing the process according to claims 1to4, characterized in that it is insulated, that it consists of one or more vertical shafts, that a stack of trays with grating bottoms is placed in the individual shafts, that there is a closed space with a pipe stub or connecting piece for removing air above the top tray in the stack, that there is a closed space for supplying air below the bottom tray in the stack, and that the bottom tray in the stack can be removed from below.

6. An aerobic bioreactor according to claim 5, characterized in that the individual trays are constructed with horizontal plates around the grating, that the plates have a width of 5-30 cm, and that the plates fit tightly against the sides of the container.

7. An aerobic bioreactor according to claim 5, characterized in that the individual trays have conocal sides with an angle of 5-40┬░, preferably 20┬░, to the vertical, so that the inside width of the tray is greatest above.

8. An aerobic bioreactor according to claim 5, characterized in that the individual shafts are constructed with one or more retaining devices which retain the trays above the bottom tray in the shaft during removal of the bottom tray.

9. An aerobic bioreactor according to claim 5 and 8, characterized in that the individual retaining devices are in the form of a pawl which can be activated by a pressure mechanism and deactivated by a spring.

10. An aerobic bioreactor according to claim 5, characterized in that the individual shafts in the bioreactor are constructed with a lid with a pipe stub or connecting piece on the top tray, a bottom with a pipe stub or connecting piece below the bottom tray, and with gaskets or seals between the lid and the top tray, between the individual trays and between the bottom tray and the bottom.

11. An aerobic bioreactor according to claim 5, characterized in that the individual shafts in the bioreactor are constructed as a closed container with an airtight hatch at the top for introducing trays, with an airtight hatch at the bottom for removing trays, with one or more pipe stubs or connecting pieces on the side of the shaft and with gaskets or seals between the individual trays and the sides of the shaft.

12. An aerobic bioreactor according to claim 8, characterized in that the gaskets or seals between the individual trays and the sides of the shaft are inflatable elastic membranes.

13. Use of a bioreactor and processes according to any of the preceding claims for the treatment of organic material, preferably moist organic material having a dry matter content of 20-30%.