WO2005085156A2

WO2005085156A2 - A composting system and a method for composting a biomass

Info

Publication number: WO2005085156A2
Application number: PCT/DK2005/000125
Authority: WO
Inventors: Per Vestergaard; Poul Wiid
Original assignee: Compsoil Danmark Aps
Priority date: 2004-03-03
Filing date: 2005-02-24
Publication date: 2005-09-15
Also published as: WO2005085156A3; EP1723090A2

Abstract

The present invention relates to a method of composting a biomass. The method comprises feeding an amount of sludge finto a mixer, feeding an amount of sand into the mixer, feeding an amount of green waste into the mixer, mixing the sludge, sand and green waste to form at least part of a biomass, and composting the biomass. It is preferred that the percentages by weight of the amounts of sludge, sand and green waste in the biomass is within predetermined ranges. It is furthermore preferred that the amounts of sludge, sand and green waste and the content of water in the sludge, sand and green waste are selected so as to obtain a biomass having a water content in the range of 40-65% by weight. The invention further relates to a composting system, which comprises a container for biomass and/or organic waste having outer sidewalls and a bottom wall, which this container has an inner perforated bottom floor being arranged above the bottom wall to form a bottom cavity between the bottom wall and the bottom floor. In a first embodiment of the system the container may further have at least one inner beam being arranged between the outer sidewalls and at a height above the inner bottom floor. The container may further have at least two tilted or angled inner sidewall parts with each of the two outer sidewall parts having at least one of said tilted sidewall parts being arranged along the inside. In a second embodiment of the system, the container further has inner perforated sidewalls arranged along at least part of the outer sidewalls to form side cavities between the outer sidewalls and the corresponding inner sidewall, with openings provided between the bottom cavity and the side cavities to thereby allow a flow of air between these cavities.

Description

A COMPOSTING SYSTEM AND A METHOD FOR COMPOSTING A BIOMASS

FIELD OF THE INVENTION

The present invention relates generally to a method of composting a biomass and in particular to a composting method wherein the biomass comprises a mixture of sludge, sand and green waste. Furthermore, the present invention relates to a composting system having a container for the composting of a biomass.

DESCRIPTION OF THE PRIOR ART

At present there are several composting technologies available for large scale composting of organic wastes. The method of maintaining aerobic conditions in the compost mass depends on the type of organic waste composted. Homogenous wastes such as sludge mixed with wood chips are commonly composted under static conditions ("static" because the compost mass is not agitated or turned during the active compost cycle of approximately 21 days). Aeration is provided to the static compost mass by blowing air through the material. Heterogeneous wastes, such as mixed municipal solid waste, generally require agitation in order to provide even aeration during composting.

A common method of composting is to arrange shredded and mixed organic waste in open piles, as in windrows. The piles may be aerated with air blowers or the piles may periodically be turned or agitated to expose the organic waste to air.

Some of the disadvantages of pile composting are overcome by in-vessel processes. Such processes enclose the compost in a reactor vessel. They can also mechanize materials handling and mixing of the organic materials to be composted. Organic waste is typically fed into an opening at one end of the reactor and compost is removed from the other end. The material may be moved through the reactor either by a complex moving floor apparatus or by a hydraulic ram. Aeration can be provided by pressurized air forced through the organic waste by air vents located throughout the moving apparatus. In- vessel reactors at least provide potential for collection of odorous emission. The compost process can be enclosed and the air routed through a filtration system. U.S. Pat. No. 5,312,754 discloses a container for composting that contains an inflatable bladder at the bottom of the container. At the start of the composting process, material is added to the container leaving enough empty space to allow inflation of the bladder. Inflation of the bladder lifts and then deflation lowers the compost to provide some mixing of the composting material. The system is portable and will work in commercial containers. Air circulation controls the temperature in the reactor and exhaust air is filtered to diminish odorous emissions. Process control includes mixing the original organic material by inflating the bladder and air circulation through the material. Air circulation is provided via a perforated second floor positioned above the bottom floor.

U.S. Pat. No. 6,281,001 also discloses a container for composting, which container has a perforated second floor positioned above the bottom floor, to thereby allow the introduction of air into the composting material. It is also disclosed that the composition of the organic material to be composted can be adjusted by amending the organic material and by mixing the amended organic material with a bulking agent and an inoculant. Here, the bulking agent amongst others may be wood chips, shredded brush or straw. The inoculant may be previously composted organic material.

It should be understood that in order to obtain an efficient composting process it is important to have and maintain a sufficient oxygen content in the biomass or material being composted. An increased oxygen content in the biomass or material to be composted may be obtained by having a high amount of air in the biomass to be composted. Furthermore, in order to control a composting process within a container, it is important to obtain an efficient aerating of the biomass during the composting process.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a method of composting a biomass, said method comprising: feeding an amount of sludge into a mixer, feeding an amount of sand into the mixer, feeding an amount of green waste into the mixer, mixing the sludge, sand and green waste to form at least part of a biomass, and composting the biomass. Here, a predetermined amount of sludge may be fed into the mixer, a predetermined amount of sand may be fed into the mixer, and a predetermined amount of green waste may be fed into the mixer.

5 By having the sludge mixed with both the green waste and the sand, an increased amount of air spaces or voids may be created in the biomass. These air spaces contain oxygen, which is required for microbial activity in the composting process, thereby increasing the efficiency of the composting process.

10 It is preferred that the percentage by weight of the amount of sludge in the biomass is within a predetermined range, the percentage by weight of the amount of sand in the biomass is within a predetermined range, and the percentage by weight of the amount of green waste in the biomass is within a predetermined range.

15 It is preferred that the amount of sludge in the biomass is in the range of 40-65% or in the range of 50-60% by weight. It is also preferred that the amount of green waste in the biomass is in the range of 10-35% or in the range of 15-30% by weight. Accordingly, it is preferred that the amount of sand in the biomass is in the range of 10-30% or in the range of 15-25% by weight.

20 For the sludge being fed into the mixer, it is preferred that the content of water is not above 85% and/or not below 70% by weight. It is also preferred that the content of water in the sand being fed into the mixer is not above 10% or not above 5% by weight. For the green waste being feed into the mixer, it is preferred that the content of water is not above

25 40%, not above 30 %, or not above 25% by weight.

Preferably, the amounts or selected amounts of sludge, sand and green waste and the content of water in the sludge, sand and green waste are selected so as to obtain a biomass having a water content in the range of 40-65% by weight or in the range of 50- 30 60% by weight.

In order to obtain a biomass resulting in an effective composting process it is preferred that a major part of the particles of the sand being fed into the mixer have no dimensions larger than 10 mm or larger than 5 mm. Preferably, a major part of the particles of the sand being fed into the mixer have no dimensions larger than 4 mm. Here, said major part may be at least 60%, 70%, 80% or 90% of the particles of the sand.

It is also preferred that no more than 50% or 40% or 30% of the particles of the green waste have a dimension being larger than 10 mm. It is further preferred that no more than 75% or 60% or 50% of the particles of the green waste have a dimension being larger than 4 mm. It is also preferred that at least 20% or 25% or 30% of the particles of the green waste have a largest dimension being equal to or smaller than 4 mm.

According to an embodiment of the invention a major part of the particles of the green waste being fed into the mixer may have no dimensions larger than 50 mm or larger than 20 mm. It is also within an embodiment of the invention that a major part of the particles of the green waste being fed into the mixer have no dimensions larger than 8 mm or larger than 4 mm. Here, said major part may be at least 50%, 60%, 70% or 80% of the particles of the green waste.

According to an embodiment of the invention, the mixer is a rotatable flow mixer having a feeding input end and a discharge output, and the retention time from the feeding input to the discharge output of the materials being fed into the mixer is in the range of 0.5-2 minutes or about 1 minute, during which passage time the sludge, sand and green waste is being mixed into the biomass mixture.

It is preferred that the mixing step is followed by a discharging step in which the biomass is discharged from the mixer to a composting system for performing said composting step.

It should be understood that different forms of sludge may be used for the biomass of the present invention. The sludge may be organic sludge, which may partly comprise inorganic material. It is preferred that the sludge to be used for the biomass may comprise sludge selected from a list containing: sludge from sewage treatment works, industrial, agricultural and fisheries treatment works and the like.

It is also within the present invention to use different forms of sands for the biomass. Here, the sand may comprise sand selected from a list containing: sand from sewage treatment works, road gutters, gravel pits, beach sand and the like. It is preferred that the green waste to be used for the biomass comprises vegetative waste, and the green waste may comprise green and/or woody vegetation remains. Preferably, the green waste comprises green and/or woody remains selected from a list containing: vegetation remains from parks and/or gardens or from public garden departments, public disposal, farms, nurseries, industrial, beach cleaning, forest and the like.

According to an embodiment of the invention, air may be introduced into the biomass comprising sludge, sand and green waste during the step of mixing.

It should be understood that according to the present invention, the composting step is an aerobic process. Here, aeration of the biomass during composting may take place by means of a self-propelled air lift system from bottom to top through the biomass, a so- called "chimney effect". Thus, the air in the air spaces of the biomass may be exchanged and renewed with the surrounding air by this chimney effect.

It is also within the present invention that the composting step comprises a hygienic treatment having a heating step during which the biomass is heated to a temperature not less than 70 °C for at least 1 hour. Here, it is preferred that during said heating step the biomass is heated to a temperature not larger than 80 °C for said at least 1 hour.

According to an embodiment of the invention the composting step may be followed by a sorting or filtering process to thereby sort out particles having a dimension larger than a predetermined max. size. Here, at least part of the sorted out particles may be feed into the mixer to thereby form part of a new, mixed biomass.

According to a second aspect of the present invention, there is provided a composting system comprising: a container for biomass and/or organic waste having outer sidewalls and a bottom wall, said container further comprising an inner perforated bottom floor being arranged above the bottom wall to form a bottom cavity between the bottom wall and the bottom floor, and at least one inner beam being arranged between the outer sidewalls and at a height above the inner bottom floor. Here, the inner beam may be arranged substantially parallel to the sidewalls and the inner bottom floor, and at a height above the inner bottom floor being in the range of 20-60% or in the range of 20-40% of the total height of the container. According to an embodiment of the composting system of the invention, the container may further comprise at least two tilted or angled inner sidewall parts with each of the two outer sidewall parts having at least one of said tilted sidewall parts being arranged along the inside. Here, each tilted sidewall part may be arranged on a sidewall at a downwards facing angle being no larger than 60 ° or 50 °, preferably about 45 °.

It is also within an embodiment of the composting system of the invention, that the container further comprises: inner perforated sidewalls arranged along at least part of the outer sidewalls to form side cavities between the outer sidewalls and the corresponding inner sidewall, wherein openings are provided between the bottom cavity and the side cavities to thereby allow a flow of air between these cavities.

According to a third aspect of the present invention, there is provided a composting system comprising: a container for biomass and/or organic waste having outer sidewalls and a bottom wall, said container further comprising an inner perforated bottom floor being arranged above the bottom wall to form a bottom cavity between the bottom wall and the bottom floor, and inner perforated sidewalls arranged along at least part of the outer sidewalls to form side cavities between the outer sidewalls and the corresponding inner sidewall, whereby openings are provided between the bottom cavity and the side cavities to thereby allow a flow of air between these cavities.

By having both a bottom cavity and side cavities and allowing a flow of air between these cavities, a more efficient aerating of the biomass can be obtained during the composting process, when compared to the known composting containers.

In order to obtain a container, which can be closed, the containers of the second and third aspects of the invention should preferably have two end walls and two sidewalls.

For the containers of the second and third aspects of the invention it is preferred that the bottom floor has a downwards U or V shape, whereby the distance from the bottom floor to the bottom wall is larger along the middle of container compared to the distance closer to the outer sidewalls.

It is further preferred that the composting system also comprises a cover to enclose the container, and openings or valves positioned beneath the cover for discharging gases.

It is also within the second an third aspects of the invention that the composting system comprises a temperature controlled aeration system, wherein the temperature controlled aeration system comprises of a heat sensing means and a blower, the heat sensing means being mounted on the side and/or end walls within the container to detect temperature changes in the biomass or waste and to activate the blower. Here, the blower may be adapted for delivering air or pressurized air to the bottom and/or side cavities, whereby the air may aerate the biomass or waste through the perforated bottom floor and the inner sidewalls. The heat sensing means may be mounted on the sidewalls of the container and stretching partly across the inside of the container.

It is preferred that the composting system further comprises heating means connected to the blower to thereby provide heated air to the blower to thereby deliver heated air or pressurized heated air to the container.

It should be understood that it is within the scope of the present invention to use a composting system according to the second or third aspects of the invention when composting the biomass mixture being mixed according to the first aspect of the invention. Here, the biomass may be placed in a biomass container being part of the composting system of the second or third aspects of the invention.

Other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram of an embodiment of a mixing and composting system according to the present invention,

Fig. 2 is a schematic diagram showing a mixer for mixing added material to form a biomass according to an embodiment of the present invention, Fig. 3 is a schematic view of a composting system including a biomass container according to an embodiment of the present invention,

Fig. 4 is a cross section view of a first embodiment of the biomass container of Fig. 3, and

Fig. 5 is a cross section view of a second embodiment of the biomass container of Fig. 3.

DETAILED DESCRIPTION OF THE INVENTION

The first aspect of the present invention relates to a composting process in which a biomass comprising a mixture of sludge, green waste and sand is composted. In Fig. 1 is shown a schematic diagram of an embodiment of a mixing and composting system according to the present invention, which may be used for the mixing and composting process of the invention. The system of Fig. 1 comprises a sludge silo 101 for storage of sludge, a sand silo 102 for storage of sand, a green waste container 103 for storage of green waste, a green waste feeder 104, a biomass mixer 105 for mixing introduced sludge, sand and green waste into a biomass, conveyer means 109, 108, 107 for feeding sludge, sand and green waste, respectively, into the mixer 105. Conveyer means 106 are provided for feeding mixed biomass being discharged from the mixer 105 into a composting container 110. For the system shown in Fig. 1 the conveyer means 106 may be rotated in order to fill a first container 110 and then to fill another container arranged at an angle to the first container. When a container 110 is filled with biomass, it may be moved to another place for the composting process as indicated in Fig. 1 by the container 112. Here the container 112 may be covered and provided with an aerating system indicated by 111 in order to deliver air to the container 112 during the composting process.

An embodiment of a biomass mixer 105 is shown in more details in Fig. 2. The biomass mixer of Fig. 2 is a rotatable flow mixer with a feeding input end 201 , an inlet 203 and an outlet 207 having a discharge output for discharging of biomass as indicated by 202. The rotatable part is mounted in a casing 204 and comprises an axle 205 on which mixing knifes 206 are positioned in order to mix the input materials into a biomass and to move the biomass from the inlet 203 to the outlet 207. The mixer is secured to a support 208 with anchoring means 209 for fastening the mixer to the ground. The rotation direction in Fig. 2 is indicated by 210. In order to get access to the interior of the mixer, such access can be obtained via a hatch as indicated by 211. In an embodiment of the invention, the casing 204 has an outer diameter of 600 mm and the a length of 2000 mm. Thus, during the mixing process the biomass is moved a length around 2000 mm from the inlet 203, which is at a first end of the casing 204, to the outlet 207, which is at the other end of the casing 204.

The mixer in Fig. 2 may have at total of 48 mixing knifes 206 and two hatches 211 , which allows renewal of the knifes 206. The mixer may be rotated by a motor having a power of 15 kW and rotated by about 400 revolutions per minute, resulting in a continuous mixing capacity of about 0,5 m³ biomass per minute.

The second aspect of the invention relates to a composting system including a biomass container. An embodiment of such a system is illustrated in Fig. 3. The system of Fig. 3 has a biomass container 301 with outer sidewalls 41 (see Fig. 4), a bottom wall 43 (see Fig. 4) and end walls 302 and 303. The container 301 further has an inner perforated bottom floor 49 (see Fig. 4) arranged above the bottom wall 43 to form a bottom cavity 44 (see Fig. 4) between the bottom wall 43 and the bottom floor 49. The container 301 further has perforated inner sidewalls 45 (see Fig. 4) arranged inside the outer sidewalls 41 to form side cavities 42 (see Fig. 4) between the outer sidewalls 41 and the inner sidewalls 45. Openings are provided between the bottom cavity 44 and the side cavities 42 to thereby allow a flow of air between these cavities. The bottom floor 49 has a downwards V shape, so that the distance from the bottom floor 49 to the bottom wall is larger along the middle of the container 301 compared to the distance closer to the outer sidewalls 41.

The container 301 has a cover 307, which may be a tarpaulin, to enclose the container 301 , and openings or valves 304, 305 positioned beneath the cover for discharging gases. For the inlet of air to the bottom cavity 44 and thereby the side cavities 42, there are openings 308, 309 in the end walls 303 and 302 near the bottom wall 43. The sidewalls 41 of the container 301 also have openings 306 for holding heat sensors or heat sensing means 46 (see Fig. 4).

The composting system further includes a temperature controlled aeration system, wherein the temperature controlled aeration system comprises the heat sensing means 46 being mounted on the side walls 41 within the container via holes 306 to detect temperature changes in the biomass or waste and to activate a blower 310. The blower 310, which may be a high-pressure blower, is adapted for delivering air or pressurized air to the bottom and/or side cavities, whereby the air may aerate the biomass or waste through the perforated bottom floor 49 and the perforated inner sidewalls 45. Heating means 311 , which may be an oil burner, is connected to the blower 310 in order to supply 5 heated air to the blower to thereby deliver heated air or pressurized heated air to the container 301.

During the composting process, the valves 305 and 308 may be open. However, if there is a desire for exchanging the heat of the air being outputted from the container, then the

10 valves 305 and 308 should stay closed and the air being outputted from valves 304 may be fed 315 to a heat exchanger 312, which again is connected to the heating means 311. The heat exchanger 312 may have an air inlet 314 and an air outlet 313. When air is delivered to the container 301 via the blower 310 and the opening 309, the opening 308 should stay closed.

15 Fig. 4 is a cross section view of a first embodiment of the biomass container of Fig. 3. In Fig. 4, the bottom wall is indicated by 43, the perforated inner bottom floor is indicated by 49, the sidewalls are indicated by 41 , the perforated inner sidewalls are indicated by 45, the bottom cavity is indicated by 44, the side cavities are indicated by 42, and the heat

20 sensing means are indicated by 46. In Fig. 4, the container is filled with biomass, where the top of the biomass is indicated by 47, showing that the openings 304 and 305 are between the top of the biomass 47 and the cover 307. Pressurized air may be conducted into the cavities 44 and 42 via the opening 309.

25 Fig. 5 is a cross section view of a second embodiment of the biomass container of Fig. 3. The same reference numerals are used in Fig. 4 and Fig. 5 when referring to similar parts of the container. When comparing the container of Fig. 4 to the container of Fig. 5 the inner perforated sidewalls 45 are omitted for the container of Fig. 5. For the container in Fig. 5 an inner beam 51 is arranged above the centre of the perforated inner bottom floor

30 49, with the beam 51 having the same length as the sidewalls 41. The beam 51 is secured to the container via a three pairs of rigid suspensions, 53a, 53b, arranged at the front part, the central part, and the rear part of the container. The inner beam 51 may further be secured to the front end wall. Furthermore, or optionally, two downwards tilted sidewall parts, 52a, 52 b, are arranged along the sidewalls 41 , with one tilted sidewall part

35 52a or 52 b arranged along each of the sidewalls 41. It should be understood that the present invention also covers embodiments of a biomass container comprising two or more inner beams 51 arranged above the perforated inner bottom floor. The present invention also covers embodiments of a biomass container comprising two or more tilted sidewall parts 52a, 52b arranged above each other along each of the sidewalls 41.

According to an embodiment of the invention, the container may have inner dimensions where the height and width are around 2.4 m and the length is around 6 m. The openings 308, 309 may have a diameter of around 400 mm, the height of the bottom cavity may be around 600 mm, and the inner cavities 42 may have a height of around 1100 mm and a thickness in the range of 40-80 mm or around 50 mm. The perforated inner bottom wall 49 and the inner side walls 45 may be made of 3 mm thick steel plates having am area of perforation giving a total opening of around 40%. When the container 301 is filled with biomass, the biomass may reach a height of 1.5 m at the sidewalls 41 and end walls 302, 303 and a height of 1.8 m in the middle of the container 301.

For the container of Fig. 5, the beam may be made of steel with a thickness of 10 mm and having a width of 150 mm and be arranged about 500 mm above the centre of the inner bottom floor 49. Each of the tilted sidewall parts 52a, 52b may be arranged at an angle of 45 ° in relation to the sidewall 41 , and sidewall parts may be made of 3 mm thick steel plates and having a width of 200 mm.

It is preferred that that the composting system and biomass container of Fig. 3 is used as part of the mixing and composting system of Fig. 1. Thus, the container 110 in Fig. 1 may correspond to the biomass container 301 of Fig. 3.

In the following is given a description of a preferred biomass mixing and composting process according to the present invention.

It is preferred that the sludge, sand and green waste is feed into the mixer so that the ratio by weight is 56% sludge, 21 % sand and 23% green waste. Here, the sludge may have a water content around 80%, the sand between 0-5% water, and the green waste around 25% water. It is preferred that for one mixing process a total biomass of around 35 tons is mixed and fed into the two biomass containers 110. It is also preferred that for one mixing process a total biomass of around 52 or 70 tons is mixed and fed into three or four biomass containers 110, respectively. It is preferred that the sand being fed into the mixer 105 has been sorted or cleaned before entering the sand silo 102, so that no sand particles have any dimension larger than 4 mm. It is also preferred that the green waste has been sorted or divided into particles having no dimension larger than 4 mm. However, the green waste may also comprise particles being larger than 4 mm. Thus, according to another embodiment the green waste may comprise 20-35% of particles being larger than 10 mm, 25-40% of particles in the range of 4-10 mm, 15-30% of particles in the range of 2-4 mm and 10-25% of particles in the range of 0-2 mm.

The green waste may be sorted or divided into the right size before entering the green waste container 103, or a slicing or dividing means may be arranged at the green waste container 103 in order to ensure that the green waste fed into the mixer 105 has the correct size. By using such small sand and green waste particles, the final output material of the composting process may be widely used.

In a preferred embodiment, the sludge is from sewage treatment plants or sewage works, the green waste is green and woody remains from parks and/or gardens, and the sand may also be from sewage treatment plants or sewage works or from gravel pits.

The mixer 105 is dimensioned so as to be able to output about 0.5 m³ of mixed biomass per minute, and the sludge, sand and green waste should be fed into mixer at a rate so as to keep up with the capacity of the mixer 105. When the biomass containers 110, 301 have been filled each with about 17-18 tons of biomass, the containers 110, 301 are transported to another area, where the composting process will take place.

When the biomass is filled in a container 301 , the container 301 may be closed by the closure 307, and the composting process starts. However, it is within a preferred composting process that the container is open during the first step of the composting process. During this first step of the composting process, no heated air is introduced into the container 301 , and the openings 308 and/or 309 are open to allow a free air intake, and the valves or openings 304 and 305 are open to allow a free air outlet. During this first step the biomass will be heated by itself due to the microbial activity taking place, and a "chimney effect" will be obtained whereby fresh air and thereby oxygen is passed from the bottom to the top of the biomass. Thus, an aeration of the biomass takes place. During this, first step the temperature of the biomass should be monitored by use of the heat sensing means 46 being arranged at several places within the biomass.

According to one embodiment of the invention, the first composting step may run for about 10-15 days, where after a second composting step is initiated. The second composting step comprises a hygienic treatment during which treatment process the biomass is heated to a temperature of at least 70 °C and not above 80 °C for at least one hour. During this heating process, the container 301 is closed by the closure 307, the opening 308 is closed and heated air is introduced via the opening 309 by use if the oil burner 311 and the blower 310. It is preferred that valves or openings 305 are closed and that the heated air being output via the valves 304 is re-circulated to the burner 311 or is heat recirculated via the heat exchanger 312. During this second step the temperature of the biomass should be monitored by use of the heat sensing means 46 to ensure that all of the biomass has obtained the desired temperature. After the hygienic treatment, a third and final composting step starts, during which step there is not introduced any heated air and both openings 308, 309 are open to allow a free air inlet, and the valves 304, 305 are also open to allow a free air outlet. This third step is allowed to take place for about 10-11 days, so that total time for the whole composting process takes about 21 days in total.

According to a second embodiment of the invention, the first composting step may run for about 19-20 days, where after the second composting step is initiated. The second composting step comprises the hygienic treatment discussed above during which treatment process the biomass is heated to a temperature of at least 70 °C and not above 80 °C for at least one hour. After the hygienic treatment, the third and final composting step starts, during which step there is not introduced any heated air and both openings 308, 309 are open to allow a free air inlet, and the valves 304, 305 are also open to allow a free air outlet. For this second embodiment, the third step is only used for cooling down the biomass and should only take place for about 1-2 days, so that total time for the whole composting process takes about 21 days in total.

When the composting process including the hygienic treatment is finished, the composted biomass may be sorted or filtered, so that larger particles may be sorted out. The obtained larger particles may be used as structural material or may be used as inoculation material and feed back into the mixer. The monitoring of the temperature of the biomass may be used in order to control start and stop of the hygienic treatment process. Here, the temperature surveillance system may include a remote sensing system including a data logging and radio transmitting system at each container and a tele-log head station with computer and data processing software for controlling the start and stop of the hygienic treatment based on preset temperature levels. The remote sensing system may secure that the temperature in each container is heated to at least 70 °C for at least one hour in order to fulfil the requirements set by the authorities.

It should be understood that when using the second embodiment of a biomass container described if Fig. 5, the inner beam 51 may take part of the pressure delivered from the biomass above the inner beam 51 , resulting in a better stream of air from the bottom to the top of the biomass during the composting step and the hygienic treatment. The tilted sidewall parts, 52a, 52 b, may also take part of the pressure from the biomass during the composting step. The tilted sidewall parts 52a, 52b may also reduce the intake of false air from the top of the biomass along the sidewalls 41 , while at the same time the sidewall parts 52a, 52b may reduce the risk of a blow-out of biomass along the sidewalls 41 during the hygienic treatment.

While the invention has been particularly shown and described with reference to particular embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention, and it is intended that such changes come within the scope of the following claims.

Claims

1. A method of composting a biomass, said method comprising:

feeding an amount of sludge into a mixer,

feeding an amount of sand into the mixer,

feeding an amount of green waste into the mixer,

mixing the sludge, sand and green waste to form at least part of a biomass, and

composting the biomass.

2. A method according to claim 1 , wherein a predetermined amount of sludge is being fed into the mixer, a predetermined amount of sand is being fed into the mixer, and a predetermined amount of green waste is being fed into the mixer.

3. A method according to claim 1 or 2, wherein the percentage by weight of the amount of sludge in the biomass is within a predetermined range, the percentage by weight of the amount of sand in the biomass is within a predetermined range, and the percentage by weight of the amount of green waste in the biomass is within a predetermined range.

4. A method according to any one of the claims 1-3, wherein the amount of sludge in the biomass is in the range of 40-65% or in the range of 50-60% by weight.

5. A method according to any one of the claims 1-4, wherein the amount of green waste in the composting mixture is in the range of 10-35% or in the range of 15-30% by weight.

6. A method according to any one of the claims 1-5, wherein the amount of sand in the biomass is in the range of 10-30% or in the range of 15-25% by weight.

7. A method according to any one of the claims 1-6, wherein the content of water in the sludge being fed into the mixer is not above 85% and/or not below 70% by weight.

8. A method according to any one of the claims 1-7, wherein the content of water in the sand being fed into the mixer is not above 10% or not above 5% by weight.

9. A method according to any one of the claims 1-8, wherein the amounts of sludge, sand and green waste and the content of water in the sludge, sand and green waste are selected so as to obtain a biomass having a water content in the range of 40-65% by weight or in the range of 50-60% by weight.

10. A method according to any one of the claims 1-9, wherein a major part of the particles of the sand being fed into the mixer have no dimensions larger than 10 mm or larger than

5 mm.

11. A method according to any one of the claims 1-10, wherein a major part of the particles of the sand being fed into the mixer have no dimensions larger than 4 mm.

12. A method according to claim 10 or 11, wherein said major part is at least 80% or 90% of the particles of the sand.

13. A method according to any one of the claims 1-12, wherein no more than 50% or 40% of the particles of the green waste have a dimension being larger than 10 mm.

14. A method according to any one of the claims 1-13, wherein no more than 75% or 60% of the particles of the green waste have a dimension being larger than 4 mm.

15. A method according to any one of the claims 1-14, wherein at least 20% or 25% of the particles of the green waste have a largest dimension being equal to or smaller than 4 mm.

16. A method according to any one of the claims 1-15, wherein a major part of the particles of the green waste being fed into the mixer have no dimensions larger than 50 mm or larger than 20 mm.

1 . A method according to any one of the claims 1-16, wherein a major part of the particles of the green waste being fed into the mixer have no dimensions larger than 8 mm or larger than 4 mm.

18. A method according to claim 16 or 17, wherein said major part is at least 70% or 80% of the particles of the green waste.

5 19. A method according to any one of the claims 1-18, wherein the mixer is a rotatable flow mixer having a feeding input end and a discharge output, and the passage time from the feeding input to the discharge output of the materials being fed into the mixer is in the range of 0.5-2 minutes or about 1 minute, during which passage time the sludge, sand and green waste is being mixed. 10

20. A method according to any one of the claims 1-19, wherein the mixing step is followed by a discharging step in which the biomass is discharged from the mixer to a composting system for performing said composting step.

15 21. A method according to any one of the claims 1-20, wherein the sludge comprises sludge selected from a list containing: sludge from sewage treatment works, industrial, agricultural and/or fisheries treatment works.

22. A method according to any one of the claims 1-21 , wherein the sand comprises sand 20 selected from a list containing: sand from sewage treatment works, road gutters, gravel pits, beach sand.

23. A method according to any one of the claims 1-22, wherein the green waste comprises vegetative waste.

25 24. A method according to any one of the claims 1-23, wherein the green waste comprises green and/or woody vegetation remains.

25. A method according to any one of the claims 1-24, wherein the green waste 30 comprises green and/or woody remains selected from a list containing: vegetation remains from parks and/or gardens or from public garden departments, public disposal, farms, nurseries, industrial, beach cleaning and forest.

26. A method according to any one of the claims 1-25, wherein air is introduced into the 35 biomass comprising sludge, sand and green waste during the step of mixing.

27. A method according to any one of the claims 1-26, wherein the composting step is an aerobic process.

28. A method according to any one of the claims 1-27, wherein the composting step comprises a hygienic treatment having a heating step during which the biomass is heated to a temperature not less than 70 °C for at least 1 hour.

29. A method according to claim 28, wherein during said heating step the biomass is heated to a temperature not larger than 80 °C for said at least 1 hour.

30. A method according to any one of the claims 1-29, wherein the composting step is followed by a sorting or filtering process to thereby sort out particles having a dimension larger than a predetermined max. size.

31. A method according to claim 30, wherein at least part of the sorted out particles are feed into the mixer to thereby form part of a new, mixed biomass.

32. A composting system comprising: a container for biomass and/or organic waste having outer sidewalls and a bottom wall, said container further comprising an inner perforated bottom floor being arranged above the bottom wall to form a bottom cavity between the bottom wall and the bottom floor, and at least one inner beam being arranged between the outer sidewalls and at a height above the inner bottom floor.

33. A composting system according to claim 32, wherein the inner beam is arranged substantially parallel to the sidewalls and the inner bottom floor, and at a height above the inner bottom floor being in the range of 20-60% of the total height of the container.

34. A composting system according to claim 32 or 33, wherein the container further comprises at least two tilted or angled inner sidewall parts with each of the two outer sidewall parts having at least one of said tilted sidewall parts being arranged along the inside.

35. A composting system according to claim 34, wherein the tilted sidewall parts are arranged on a sidewall at a downwards facing angle being no larger than 60 ° or 50 °.

36. A composting system according to any one of the claims 32-35, wherein the container further comprises: inner perforated sidewalls arranged along at least part of the outer sidewalls to form side cavities between the outer sidewalls and the corresponding inner sidewall, wherein openings are provided between the bottom cavity and the side cavities to thereby allow a flow of air between these cavities.

37. A composting system comprising: a container for biomass and/or organic waste having outer sidewalls and a bottom wall, said container further comprising an inner perforated bottom floor being arranged above the bottom wall to form a bottom cavity between the bottom wall and the bottom floor, and inner perforated sidewalls arranged along at least part of the outer sidewalls to form side cavities between the outer sidewalls and the corresponding inner sidewall, wherein openings are provided between the bottom cavity and the side cavities to thereby allow a flow of air between these cavities.

38. A composting system according to any one of the claims 32-37, wherein the container has two end walls and two sidewalls.

39. A composting system according to any one of the claims 32-38, wherein the bottom floor has a downwards U or V shape, whereby the distance from the bottom floor to the bottom wall is larger along the middle of container compared to the distance closer to the outer sidewalls.

40. A composting system according to any one of the claims 32-39, further comprising a cover to enclose the container, and openings or valves positioned beneath the cover for discharging gases.

41. A composting system according to any one of the claims 32-40, further comprising a temperature controlled aeration system, wherein the temperature controlled aeration system comprises of a heat sensing means and a blower, the heat sensing means being mounted on the side and/or end walls within the container to detect temperature changes in the biomass or waste and to activate the blower, the blower being adapted for delivering air or pressurized air to the bottom and/or side cavities, whereby the air may aerate the biomass or waste through the perforated bottom floor and/or the inner 5 sidewalls.

42. A composting system according to claim 41 , wherein the heat sensing means is mounted on the sidewalls of the container and stretching partly across the inside of the container.

10 43. A composting system according to claim 41 or 42, further comprising heating means connected to the blower to thereby provide heated air to the blower to thereby deliver heated air or pressurized heated air to the container.

15 44. A method according to any one of the claims 1-31 , wherein the composting step comprises the use of a composting system according to any one of the claims 32-43.

45. A method according to claim 44, wherein the biomass is placed in the biomass container during said composting step.