GB2517180A

GB2517180A - Improved apparatus for composting and digestion of biodegradable waste

Info

Publication number: GB2517180A
Application number: GB1314536.2A
Authority: GB
Inventors: Gopalakrishnan Jeyasundra
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-08-14
Filing date: 2013-08-14
Publication date: 2015-02-18
Also published as: EP3071342A2; WO2015022543A2; GB201314536D0; WO2015022543A3

Abstract

An organic material composting apparatus formed of a unitary system comprising an inlet 5, an outlet 17, one or more homogenizer tanks 6, one or more rotary digesters 11 and means 9 for transferring material from the homogenizer to the rotary digester wherein the homogenizer comprises a stirrer 7 to create a vortex in the fluid and an air supply at a location in the tank adjacent the vortex, the rotary digester comprises a circulator arranged to circulate heated air within the digester and to input fresh heated air to the digester (fig 2; 30). Ideally, the stirrer is a paddle stirrer and the rotary digester is a drum which rotated about the horizontal axis and is driven by a motor and chain drive. Preferably, heated air from the rotary digester is transferred to the homogenizer.

Description

IMPROVED APPARATUS FOR COMPOSTING AND

DIGESTION OF BIODEGRAI)ABLE WASTE This invention relates to apparatus for eomposting and digestion of biodegradable waste.

Biodegradable organic waste matter forms a major component of municipal solid waste. This is collected in open bins and disposed of in landfills to undergo natural decomposition. This may release a number of organic acids and foul smelling gas compounds. This may have a negative environmental impact.

Aerobic composting requires a degradable substrate, oxygen, water and warmth. Normally the material to be composted generates its own heat. There may be two stages of composting. Initially slow mesophilic composting occurs. When the temperature has built up rapid composting occurs at thermophilic conditions between 400 and 65°C. Most large scale composting, for example, carried out by local authorities, is anaerobic, methane being produced once oxygen has been exhausted.

The methane may be used to generate energy.

The main benefit of anaerobic digestion is the production of energy rather than the efficient production of usable digestate.

GB-A-246265 1 discloses apparatus for in-vessel composting and digestion of biodegradable waste which comprises a number of fermenters, a rectangular heater duct including a number of finned tube heaters arranged inside the heater duct to provide a maximum heating surface area, a heater pipe coupling the heater duct to the fermenter, a pressure supply blower for forcing air to enter the fermenter via the heater duct, a centrifugal blower for sucking air outside the fermenter through a plurality of exhaust pipes via a mechanical seal, a plurality of thenno-couples positioned at various locations inside the apparatus, at least one drive motor for driving the plurality of fermenters, a plurality of microwave guns coupled to the plurality of fermenters via the mechanical seal, and a control panel mounted on the app aratus.

According to the present invention organic material composting apparatus comprises a unitary system comprising an inlet, an outlet, one or more homogeniser tanks and one or more rotary digesters; the inlet being arranged to input fluid undigested organic material to the homogeniser; the homogeniser including a stirrer arranged to stir in the fluid and including an air supply at a location in the tank adjacent the vortex in use; means for transferring the material from the tank to a said rotary digester; the rotary digester including a circulator arranged to circulate heated air within the digester and further arranged to input fresh heated air to the digester; the outlet being arranged to discharge digested material from the drum; wherein material moves in a single direction from the inlet to the outlet.

The apparatus may be unitary in that the components are connected together to form a single apparatus so that the organic material is sealed within the system during the entire digestive process.

The stirrer is preferably a paddle stirrer.

The stirrer may be arranged to create a vortex when rotated rapidly to improve aeration of the fluid. Alternatively, the stirrer may be rotated more slowly so that a vortex is not formed.

The apparatus may further comprise control means for controlling transfer of material from the tank to the drum.

The present invention has the advantage of enabling production of good quality stable fertilizer in a continuous process using aerobic digestion in a single rotary digester, together with a homogeniser tank. Use of heaters, such as microwave guns are not required because the air is warm enough to raise the temperature of the digesting material to 4O and 65°C during digestion and above 70°C for around one hour during the subsequent pasteurization phase.

The organic material is processed in two stages. Firstly a mesophilic stage occurs where the temperature is between 200 to 45°C. This is followed by a thermophilic stage where the temperature is between 45° and 65°C or higher.

Subsequently a pasteurization phase is used to achieve a material temperature of 70°C for a period of one hour. This may be achieved by raising the inlet air temperature to above 75°C.

The time required to digest the material may be about I day for manure, 3 to 4 days for food waste, or 10 to 14 days for wood. This may be compared to a period of 30 days required for food waste in a conventional digester.

The digested material is stable in that it does not produce methane.

Anaerobic digesters do not produce stable material and when the majority of methane has been extracted during anaerobic digestion the digestate must be stored in tanks for a further six months to complete the removal of any remaining methane and sulphur.

The material may be spread onto fields but only when it is stable in the sense that no further methane is produced. Also, the material may be able to be reactivated by contact with external enzymes.

In contrast the digestate made by the apparatus of the present invention is stable in that all biological activity has ceased. Anaerobic digesters generally produce material in a liquid form and this can only be spread onto fields two or three times each year. It is restricted in use because biologically active compounds may remain.

Apparatus in accordance with the present invention may additionally comprise microwave guns or other heating means, but these are not required for the purpose of digestion. A stable material can be produced without any need for the use of microwave guns because digestion is thoroughly accomplished by use of warm air circulated at a temperature between 3S° and 65°C, together with aeration and an appropriate moisture level. Microwave guns may be provided so that they can be used as a precautionary measure to eliminate pathogens and to increase the drying efficiency.

In preferred apparatus a single rotary digester may be used. The homogeniser tank may be used to collect material for a period of, for example, about two days or longer, this being the period that it takes for the thermophilic digestion process to be completed in the rotary digester. The homogeniser tank serves to predigest the material until it is transferred to the rotary digester for full digestion and stabilisation to occur.

The apparatus may include sensors and control means for measurement and control of the digestion time, temperature and moisture and oxygen levels within the digester. Two temperature probes may be placed at opposite ends of the rotary drum.

Wireless temperature probes may be employed. These may be connected to a control unit by a slip ring provided on the drum.

In use of the apparatus temperature readings are recorded and a temperature/time profile is measured. A decrease in temperature indicates the digestion process is complete. Once digestion is complete the moisture level may be reduced due to evaporation. Consumption of oxygen and production of carbon-dioxide also decreases at the end of the digestion stage.

A filtration unit, for example a sieve, may be provided in the homogeniser tank in order to facilitate decontamination of the material before transfer to the rotary digester.

Apparatus in accordance with the present invention confers several advantages. No storage tanks are required and the size of the digester may be significantly reduced in comparison to prior art apparatus. The water content of the organic matter may be reduced. This reduces the volume of material handled. The digested product can be used immediately without restriction. Therefore it is possible to operate the apparatus in urban areas. The carbon footprint is also reduced.

Existing anaerobic systems can be converted with the storage tanks fed into new digesters.

It is an important feature of the present invention that heat produced by anaerobic digestion is used to dry the slurry in the digester to form a usable fertilizer.

The invention is further described by means of example but in any limitative sense with reference to the accompanying drawings of which; Figure 1 is a side elevation of apparatus in accordance with the invention; and Figure 2 is a flow diagram of apparatus in accordance with this invention.

Figure 1 is a side elevation of the general layout of apparatus in accordance with this invention.

A hopper (1) for collecting and storing organic material is connected by a screw feed (2) driven by a motor (3) to an inlet pipe (4) connected to an inlet (5) of a homogeniser tank (6). The tank (6) is cylindrical on a vertical axis and has a paddle stirrer (7) driven by motor (8). Rotation of the stirrer (7) creates a vortex in fluid organic matter contained within the tank (6). A valve (8) at a lower end of the tank (6) may be opened to allow organic material to be transferred through pipe (9) to inlet (10) of a rotary digester (II). Air inlet (12) and outlet (13) are located in an upper part of the tank to provide circulation of air to oxygenate material. Oxygenation is promoted by contact of the air with the vortex surface.

The rotary digester is mounted on a raised support (14) and driven by motor (15) and chain drive (16). An outlet (17) maybe opened downwardly to discharge digestive material into collector (18) having an outlet (19) through which the digestive material may be removed from the apparatus by a discharge auger.

Figure 2 is a schematic view of the apparatus. Hopper (1) is loaded using a bin lifter (2) and has a level sensor (21) to determine the substrate level within the hopper. Screw feeder (2) is driven by motor (3) to supply undigested organic material via pipe (4) to inlet (5) of homogeniser tank (6). A level sensor (22) provides a signal when the tank (6) is fully charged. An electric heater (23) may be used to adjust the temperature within the tank (6) as measured by temperature sensors (24,25). Paddle stirrer (7) is driven by motor (8) and creates a vortex within the fluid or liquid material to promote dissolution of oxygen. Air is supplied to the upper part of the tank (6) by fan (26) and is removed by optional exhaust fan (27) via carbon filter (28).

Air from the rotary digester is circulated to the homogeniser tank as described below.

Outlet valve (8) may be opened to discharge material to screw pump (29) and pipe (9) leading to inlet (10) of rotary drum (11). Rotary drum (11) is driven by motor (15) and chain drive (16) to rotate about a horizontal axis.

Air is circulated within the drum (11) by a fan (30) and electrically powered heater (31) controlled by thermostat (32). Temperature sensor (33) provides a signal indicative of the temperature of air delivered to the rotary dryer (11). Feedback airflow restrictor (34) and mixer (35) control circulation of air through the heater (31).

Fresh air (36) may be delivered to the mixer through an inlet (36) which is connected to the outlets (41) from the rotary drum to provide a closed circuit preventing loss of heated air from the system in use. A microwave generator (37) provides optional heating to the drum. Temperature sensors (3 8,39) and level sensor (40) provide control signals to a controller (not shown).

Air is circulated from the drum through outlet (41) via humidity sensor (42) and oxygen sensor (43) to fan (26) and outlet (28) to the homogeniser tank. Air passing from the outlet (28) and optional fan (27) escapes to atmosphere or may be passed through a condenser.

In use of the apparatus liquid or semi-solid waste organic material can be digested to form solid digestate suitable for immediate use as fertiliser. Withdrawal of partially digested material for aeration is not necessary.

Slurry having a water content of 80% can be pumped, but slurry having a water content of 75% or less is difficult to pump. The system of the present invention may handle slurry having 70%-75% water content. The energy required to dry a low water content slurry, for example having 75% water, is much less than is required to dry 85% water content slurry.

It is an advantage of the present invention that the homogeniser tank does not need to be full in order to operate efficiently, in contrast to conventional systems. The homogeniser tank is used as a feeder tank for the rotary digester. It is not important to the quality of the digestion process to monitor the extent to which the tank is filled.

Injection of air below the liquid surface is not required.

It is an important feature of the present invention that material being digested is moved in one direction from the homogeniser tank inlet to the digester outlet. It is not necessary to add and remove material through a single aperture. This removes the possibility of adding material to the rotary digester in mid cycle, thereby avoiding a risk of contamination of the product with incompletely digested material. The risk of human error is reduced and there is less need for skilled management of the digestion process. The efficiency of the thermophillic processing conditions in the rotary digester are optimised. The speed of digestion is also improved. The digestion time may be about I day in the homogeniser followed by 1-3 days in the rotary tank. This contrasts with standard anaerobic digestion, which requires 20-60 days in a non-aerated homogeniser drum followed by 6 months storage.

Addition of nutrients is facilitated to enhance fertiliser quality. It is difficult to add nutrients to previously ktown processes because the product never achieves biological stability and therefore the nutrients may interfere with the digestion process.

Claims

CLAIMS1. An organic material composting apparatus comprising a unitary system comprising an inlet, an outlet, one or more homogeniser tanks and one or more rotary digesters; the inlet being arranged to input fluid undigested organic material to the homogeniser; the homogeniser including a stirrer arranged to stir in the fluid and including an air supply at a location in the tank adjacent thc vortex in use; means for transferring the material from the tank to a said rotary digester; the rotary digester including a circulator arranged to circulate heated air within the digester and further arranged to input fresh heated air to the digester; the outlet being arranged to discharge digested material from the drum; wherein material moves in a single direction from the inlet to the outlet.
2. An organic material composting apparatus as claimed in claim I, wherein the apparatus is unitary so that organic material is sealed within the system during the entire composting process.
3. An organic material composting apparatus as claimed in claim 1 or 2, wherein the stirrer is a paddle stirrer.
4. An organic material composting apparatus as claimed in any preceding claim, wherein the material in the digester is processed at a first stage where the temperature is between 20°C and 45°C followed by a second stage where the temperature is greater than 45°C.
5. An organic material eomposting apparatus substantially as hereinbefore described with reference to the accompanying drawings.