AU5401501A - Improved composting system - Google Patents

Description

P/00/011 28/5/91 Regulation 3.2(2)

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Application Number: PQ8308 Lodged: 22 June 2000 r r.

Invention Title: IMPROVED COMPOSTING SYSTEM The following statement is a full description of this invention, including the best method of performing it known to me.

IMPROVED COMPOSTING SYSTEM Field of the Invention This invention relates to composting systems for treatment of organic waste in a manner accepted to be environmentally sustainable. More generally, the invention may relate to systems and methods for stabilizing organic materials, for example, in fruit and vegetable storage and preservation, or sterilization of organic materials.

Background to the Invention Each day a large amount of solid municipal domestic waste is produced.

Of all solid municipal domestic waste produced in Australia approximately consists of food and garden waste. Other components of the waste include paper plastics glass metals and other inorganics The organic fraction has a strong detrimental impact on the environment and may be hazardous.

Detriment results from the large volume of organic waste which may occupy 50-70% of landfill space. The waste is of a putrescible nature, thus making it a potential source of pathogenic organisms. A large volume of greenhouse gases, e.g. carbon dioxide and methane are released during uncontrolled decomposition. Finally, and significantly, decomposing organic matter causes odour, attracts pests and is a major contributor to groundwater pollution through dissolution and its role as a carrier or inorganic pollutants such as heavy metals. In certain cases such pollution may make groundwater unsuitable for safe use.

Therefore, one of the main challenges in any integrated waste management strategy is the appropriate and effective treatment of organic waste.

The current practice of landfilling organic waste is rapidly becoming an inappropriate waste treatment practice and will be unsustainable in the long term.

Landfilling consumes large areas of land, results in low land value and is strongly objected to by residents. Consequently, landfilling is fast disappearing in major cities as a sole waste disposal strategy. In cities with low housing densities, transport costs may become prohibitive as suitable landfill space becomes available only well away from waste generation centres.

Organic waste nevertheless has considerable potential as a resource when stabilised through composting. It is high in organic matter and contains nutrients such as nitrogen phosphorus potassium and trace elements.

Composting is the process whereby organic matter is decomposed by a range of microorganisms using oxygen. The process is appropriate for treatment of a combination of fibrous waste green organics) and putrescible waste such as food waste, sewage sludge and industrial and commercial organic residues. Composting has the advantage of reducing the waste volume by In addition, product compost has significant benefits as a soil conditioner.

A typical composting process may comprise four stages. Each stage is characterised by the activity of different generations of bacteria, fungi, protozoa and actinomycetes. During each stage the microbes use original organic compounds present in the waste as well as by-products of the metabolism of the previous generation as a nutrient and energy source. Thus the organic matter :decomposes until a stable humus is formed.

•ooo• The incubation or mesophilic phase lasts for approximately 24 hours during which the organic matter is rapidly invaded by mesophilic composting organisms including bacteria, actinomycetes and fungi. These organisms thrive at a temperature of 250 450C. The mesophilic organisms grow in this phase on the more easily assimilated substances present in the organic waste, for example: sugars, soluble protein, starch and organic acids.

The high metabolic activity of the organisms and the exothermic decomposition processes that result, in combination with the insulating properties of the composting material, causes the temperature to rise. The temperature rise strongly favours thermophilic sporogenous bacteria. The activity of these bacteria takes the process into the thermophilic phase.

During the thermophilic phase, organic matter is decomposed rapidly.

Temperatures may reach 700C in the core of the composting material. This is undesirable, since at this temperature most process participating microbes, including some thermophilics, are killed. This may considerably reduce the decomposition rate of organic material. About 450- 500C is optimum and above 550C is typically required for pathogen destruction, thus 550C is accepted as an optimum temperature compromising between these factors, at which the decomposition rate is highest. These temperatures assist in accelerating the process and sanitizing the material from pathogens, weed seeds and plant disease causing agents. This temperature, and below, allows the development of eumycetes and actinomycetes which are the main decomposers of long chain polymers, cellulose and lignin. The oxygen demand is very high in this phase and aeration is required. This phase may last for 2 3 weeks depending on aeration and substrate.

The cooling phase commences when there is insufficient exothermic organic substrate left to maintain the high temperature. Accordingly, water evaporation and heat convection cause temperature to drop. If the temperature drops below 450C mesophilic bacteria and other organisms may reinvade the fresh compost. This phase may last a few days.

A maturation or stabilisation phase is required to allow the toxicity of fresh °oooo compost to fall to enable effective utilisation by plants. The activity of fungi, protozoa and actinomycetes may be highest during this phase, while bacterial activity slowly falls. At this stage, large polymers such as lignin and cellulose are decomposed and a humidification process sets in. The activity of actinomycetes ooooo produces the compound "geosmine" which gives matured compost a fresh earthy smell. Three to four weeks may be sufficient to enable completion of this phase.

Such a composting process is to be distinguished from ensilage which is a fermentation process.

The Applicant's earlier International Patent Application No. WO/97/19901 describes a composting system comprising an area having a base for holding and treating a mass of compostable material. The mass of compostable material is covered with a removable cover, conformable to the mass which is secured to the base to form an environment, a condition of which may be controlled to optimise the composting process. An aeration means flows an oxygen containing 4 gas through the enclosed composting environment supplying oxygen which is consumed in the composting process.

The system works well but there is room for improvement in achieving efficient aeration. The aeration means may fail to achieve a uniform air flow throughout the mass of compostable material. In addition, portion of the aeration means may be incorporated in the base of the system and this may be subject to damage by vehicles transferring compostable materials to the system for composting.

Summary of the Invention It is the objective of the present invention to provide a composting system and method that may enable the respective phases of the composting process to be completed in an efficient manner, under conditions that enable the most advantageous conditions for highly efficient aeration and aerobic microbial decomposition of an organic substrate to occur. In addition, the system may be designed in such a way as to reduce the risk of damage to an aeration structure by vehicles delivering compostable material to the system.

With this objective in view, the present invention provides a composting .g..e system including; an area having a base for holding and treating compostable material; in use, a mass of compostable material located in the area; a removable cover conformable to the mass of compostable material and securable to said base for defining with said base a composting environment; o:oa means for securing the cover to the base for forming said composting environment; o aerating means for flowing an oxygen containing gas through the composting environment and including diffusion means for distributing the flow of oxygen containing gas through said mass; and a control unit for controlling a condition in the composting environment.

In particular, the weatherproof cover is preferably to be secured or appropriately sealed to prevent ingress of water or other environmental, especially climatic, impacts detrimental to efficient composting such as excessive drying out. Entry of pests and vermin is also prevented. A flexible or modular construction for the cover is preferred, particularly a cover which is readily conformable to the volume of material to be composted. The volume of the environment is dictated primarily by the volume of compostable material to be treated and flexibility in this respect is a feature of the system.

In this respect, the invention is predicated on the discovery that the composting process is dependent upon a number of variables, the control of which ensures a more efficient composting process. Further, the cover prevents escape of odour, water ingress and generation of leachate. Leachate generation is a particular problem in open air composting systems such as windrow composting and static pile forced aeration composting which are strongly dependent on weather conditions. Where rainfall is high, water may drain through compost windrows, leaching nutrients and soluble organic matter from the compost. The generated leachate may usually require treatment before °o.o.i discharge to surface or ground water and compost quality is reduced.

For a first example, the control of aeration may be important to conducting ~:the composting process. Aeration provides the oxygen necessary to sustain the aerobic organisms that promote composting. In a static pile, oxygen levels can ooooa drop to below 1% by volume and carbon dioxide levels can reach 20% by volume. Such levels may be inhibitive to composting.

."°Therefore, aeration means to aerate the mass of compostable material are to be included within the composting system. The aeration means may comprise aeration structures located in the base of the area for holding compostable material through which oxygen containing gas such as air flows to or from the mass of the compostable material. Incorporation of diffusion means with aeration means allows achievement of a distributed flow of air to better aerate the mass of compostable material. A grid structure, as used to cover trenches or ditches in roadways, may be used for this application. More suitably and advantageously used are aerobic drainage cells which allow support of the mass of compostable material which also allowing distribution of the air flow to allow better aeration of the mass of compostable material.

Australian Application No. 70742/87 (assigned to Humberto Urriola) describes aerobic drainage cells of the kind suitable for use in accordance with the present invention. Such a cellular structure may be a rigid cell structure comprising first and second substantially parallel perforate planar members maintained in a fixed spaced relationship from each other by means of a plurality of spacer members. The perforations in the planar members may comprise at least 40% of their surface area and all areas between perforations are adapted for load bearing at a loading of at least 40 kilograms per square metre (approximately 0.06 pounds per square inch). It is this load bearing feature that is most desirable to allow vehicles transporting compostable material to the composting area to safely traverse the rigid cellular structure forming the diffusion means. In use, the structure may be arranged to be covered with a relatively thin layer of compostable or composted material. This affords still greater damage resistance.

"In practice, the rigid cellular structure is located in an aeration ditch, cavity, trench, recess or other aeration structure recessed into the area for holding the compostable material. A number of such aeration structures may be used, particularly where desired aerated area is high as provision of an appropriate air Ssupply may be facilitated by reducing the required aerated area into a number of units. Generally, two units may be provided, perhaps arranged in parallel. These structures, which may be referred to as aeration structures, may be communicated with a supply of oxygen-containing gas, typically air. A substantial portion of the base may be occupied by a recess in which the rigid cellular structure is located to allow more controlled air distribution throughout the mass.

Connectors may be used to secure the rigid cellular structure into position in the recess.

The rigid cellular structure may be designed to be robust to everyday operation of the system. Grids, of metal or suitable polymer, may be arranged relative, usually above, the rigid cellular structure to protect it. A finer mesh screen, for example of polymer or wire, may be arranged between the rigid cellular structure and grid. A finer mesh assists to prevention of clogging of the apertures of the rigid cellular structure with composting material. Either the grid or screen or both could be omitted.

In a further aspect of the invention, the rigid cellular structure may be employed in a system preferably of the above kind, used for aeration or bioremediation applications other than composting.

The aeration means may provide a variable controllable proportion of recycled process air and fresh air assisting in maintenance of the compostable mass moisture at desired levels importantly preventing drying out of compost, and may provide oxygen levels within the mass of 10 18% by volume.

Control over the level of carbon dioxide in the mass of compostable material may also be desirable. In this respect, air flow may be controlled such that carbon dioxide levels are maintained below 10% by volume.

In this respect, the 02 and CO2 levels are interlinked and add up to 21%.

Thus if oxygen is 15%, carbon dioxide is 6%.

Accordingly, the composting system may advantageously include means for maintaining an appropriate moisture level. In this respect, recirculation of spent process air or oxygen through the mass of compostable material may be conducted to cause a flow of moist air which maintains moisture in the mass at desired levels and a carbon dioxide level of approximately 15% by volume.

Recirculation of air or aeration by other means may also assist in achievement of a homogeneous moisture level throughout the mass avoiding stratification or formation of dry spots which adversely affect composting. Fresh air may then be introduced by blower or other air compressing means to maintain a ratio of recycled air to fresh air, sufficient to maintain the desired oxygen carbon dioxide level. This air may replace a portion of spent process air which may be discharged through a biofilter to atmosphere. Alternatively, fresh air or oxygen may be introduced at any time to maintain a desired recycle air to fresh air ratio and/or carbon dioxide concentration. Spent process air may be discharged by suitable venting arrangements from the holding area or an overhead pipe arrangement.

This ratio may be maintained at the desired level in dependence upon monitored oxygen or carbon dioxide level but may also be controlled as a function of other composting process variables, such as the temperature of the core of the mass of compostable material. Suitable sensors may be provided for these purposes and the composting system may be under the control of a microprocessor or like device.

Where warm, moist air contained within the cover is mixed with fresh air and recirculated through a blower or like means, the temperature and moisture level of the air entering the core of the compost windrow increases preventing drying out and/or premature cooling.

The aeration structure, whatever the diffusion means employed, may be designed to allow variable flow of air through the mass of compostable material.

Thus the aeration structure may be divided into aeration units connected through suitable valving arrangements to the supply of oxygen-containing gas or air. In *this way, variable flow of gas may be achieved for each aeration unit. If such an "arrangement is employed the gas may be controlled in terms of fresh air/recycle air and OJ/CO2 ratios as described above.

The weatherproof cover may be sealed at its edge by a low cost sealing means such as sandbags, soil, a water jacket, beams rods or other means. It is important that the sealing be achieved in a manner that enables effective control over the microenvironment within the weatherproof cover. Space may be at a premium. In this respect, the system is ideally designed to exclude climatic influences over the composting process, chiefly drying influences and excess .rainfall which may base excess moisture and leachate generation or prolonged hot and dry conditions which may dry the compost to a point where microbiological activity ceases. The system is also advantageously flexible to suit variation in the mass of material to be treated and sealing means may be selected with this in view.

A clear manifestation of climatic influence is rainfall. Rainfall may markedly interfere with a composting process because the level of moisture in the mass of compostable material is an important process parameter. While the composting microbes may require a minimum moisture content of about 40% to avoid reduced activity, levels of moisture above about 60% may lead to the occurrence of anaerobic conditions which change the process from a composting process to a fermentation (or rotting) process. This will occur when the pores in the substrate fill with water to an extent that effective aeration is impaired.

Further, excess water may be a cause of leachate generation, undesirable for environmental reasons, for example, base metal pollution and uncontrolled nutrient loss to the environment. Hence, the weatherproof cover is ideally to be a material that prevents ingress of water to the composting system due to rainfall and which prevents excess moisture loss due to drying, especially of the edges, during hot weather. Ideally, the material should facilitate collection of moist spent air and dry fresh air.

In addition, temperature may be important. Composting involves a thermophilic stage and drop in ambient temperature may effect this. Thus the material may be of a kind that prevents ingress of ambient air to the mass of "compostable material and escape of odour. If necessary, provision for addition of make up fresh air may be made to minimise the effects of ambient temperature.

Typically, the system through controlled aeration and exclusion of water enables compost to be held at a sufficient temperature, defined by some °eee° standards as 55iC or higher, for a sufficient period, a few, typically three, days to kill pathogenic organisms present in the material or biosolids.

Pathogens, both plant and human, are inherent to most types of organic wastes. In order to minimise risks to public health and flora, i.e. crops, such materials must typically be processed such that substantially complete pathogen S: destruction is achieved.

The provision of air circulation through the compostable material may assist in avoiding a situation where low temperature zones are formed at the base of a pile or a periphery of a windrow where excessive heat loss to the atmosphere and lack of insulation may prohibit temperatures reaching thermophilic levels. Drying out may also occur at this periphery in open windows.

Product compost which may typically have less than ten (10) total coliforms per gram compost in comparison with ARMCANZ guidelines (see Agriculture and Resource Management Council of Australia and New Zealand Water Technology Committee, Guidelines for Sewage Systems Biosolids Management Occasional Paper WTC No 1/95 October, 1995) which specify that Class A compost may contain a maximum pathogen concentration of 100 thermotolerant coliforms per gram compost.

Typically, the compostable material may be turned once during the process (duration usually approximately 8 weeks) although turning may be conducted more frequently. However, frequent turning is undesirable as labour and equipment costs may be increased and it is not an aspect of a preferred embodiment of the present invention.

In a further aspect, the present invention provides a method for composting a mass of compostable material comprising: forming a mass of compostable material in an area having a base for holding said material; ooo°i conforming a removable cover to said mass of compostable material for defining with said base an enclosed composting environment on securement to the base; securing the cover to said base to form said composting environment; flowing an oxygen containing gas through an aeration means including diffusion means for distributing the flow of air through said mass of compostable material; and controlling a condition in the enclosed composting environment with a control unit.

The composting system and method of the present invention may provide a number of advantages. Primarily, use of a diffusion means which non evenly spreads the airflow along the base of the composting system achieves more efficient composting. Hot spots are generally avoided. Further, the system 11 allows control over odour, the "balloon" formed by the preferably flexible weather proof cover around the mass of the compostable material including biosolids preventing odour emission. Recycled air may be deodorized by the compost mass acting as a biofilter and excess air which requires venting may be led through a biofilter for substantially complete odour removal. Leachates are not generated in any significant amounts and may be contained and not released to the surrounding environment, at least in an untreated state. In addition, the "balloon" creates a homogeneous microenvironment which is controllable to the benefit of efficient composting. The control achieved over moisture level and pathogens may allow, for example, a more rapid composting rate. In addition, the system offers a benefit of low cost with the various components available in most locations at low cost. Low cost also offers the advantage of plant mobility with low capital risk when the system is moved from place to place.

The invention may be extended to other systems that require gas flow through a mass of organic material. Accordingly, in a further broad aspect of the invention there is provided a system for flowing a gas through a mass of organic material subject to microbial activity to stabilize same as defined below and *including: a holding zone for holding and treating the organic material; in use, a mass of organic material in the holding zone; gas flow means for flowing a gas through the holding zone, wherein said holding zone includes a base supporting the organic material, which base includes diffusion means for distributing the flow of gas through the mass of organic material to reduce undesirable microbial activity relative to the organic material.

The system may further include a control unit for controlling a condition in S. the holding zone such as gas pressure or gas composition. In one case, the gas composition may be controlled to promote activity of desired microbes, as in the case of composting.

The diffusion means may be a grid structure for distributing flow of gas through the holding zone. The gas may be an oxygen containing gas selected, for example, from air, oxygen or carbon dioxide or any other suitable gas for stabilizing the mass of organic material. Inert gases are not precluded.

By "stabilizing" or "stabilization" is intended the prevention of undesirable microbial activity. It may include flowing gas through the mass to reduce putrescence of the organic material under conditions favourable to activity of desired microorganisms in the case of composting. In this regard, organic materials may contain stable fractions such as cellulose based or fibrous materials and a putrescible fraction which it may be desired to make more stable by reduction of microbial decomposition rate as measured by reduction of the oxygen uptake rate or odour emissions and so on. The systems of the invention are suitable for this purpose.

Also included within the definition of "stabilizing" or "stabilization" is prevention of undesirable microbiological activity relative to the organic material.

Thus, in addition to prevention of rotting or similar processes in organic materials, vegetables, fruit and so on, it may be desirable to prevent mould growth on organic materials such as fruit, vegetables or paper which may become mouldy ;under damp conditions. This may be relevant to a drying or storage application.

The diffusion means may be a cellular structure which may incorporate a rigid cell structure including first and second substantially perforate planar members maintained in a fixed spaced relationship from each other by means of a plurality of spacer members having a load bearing capacity for supporting the organic material.

Such systems may incorporate other features, as discussed in relation to the above composting system which is a particularly preferred embodiment of the invention. However, such system has broader application in the field of stabilization of organic material. Important applications may include sterilization of organic matter, storage and preservation of organic materials such as fruit and vegetables. Other organic materials may be treated by the system.

In a fruit/vegetable storage application, by way of example, a gas including carbon dioxide may be passed through the holding zone by the diffusion means.

Such a technique would be applicable, for example, to the storage of potatoes.

Such a system may also be used in drying applications using an appropriate gaseous substance to reduce microbiological degradation of an organic material, for example paper, subject to microbial activity while achieving drying. Mould growth may be preventable in this way.

Brief Description of the Drawings The invention may be more fully understood from the following description of a non-limiting embodiment thereof made with reference to the accompanying drawings in which: Figure 1 is a transverse section through a composting system in accordance with one embodiment of the invention; Figure 2 is a plan view of the composting system in accordance with the embodiment of Figure 1; Figure 3 is a temperature profile of a region of the system constructed in one embodiment of the present invention; Figure 4 shows a transverse section of a second preferred embodiment of the composting system of the present invention; Figure 5 is a transverse section of a third preferred embodiment of the composting system of the present invention; Figure 6 is a plan view of the composting system in accordance with the embodiment of Figure Figure 7 is a detailed section view of a base of the composting system taken at section A of Figure 6; Figure 8 is a detailed section view of a base of the composting system .•.taken at section B of Figure 6; Figure 9 shows a transverse section of a composting system in accordance with a fourth preferred embodiment of the present invention; Figure 10 is detail A of Figure 9; and Figure 11 is detail B of Figure Detailed Description of the Drawings Referring now to Figure 1, there is shown a transverse section of the composting system which comprises an area or base 1 on which is piled a mass of compostable material 2. The compostable material comprises organic waste, for example from domestic sources, biosolids, though other sources of waste, such as abattoirs, may be available. It may be necessary to separate the organic waste or green waste from a mixed solid waste containing paper, glass, metals, plastics and other refuse by appropriate steps, for example a system of mechanical sorters, air classifiers, magnetic conveyor belts and the like. The material may require particle size reduction by means of shredding or other forms of comminution, for example in mills (usually hammer mills) to obtain optimum particle size from the point of view of porosity structure and specific surface area.

Heavy metal analysis for acceptability may also be conducted prior to treatment commencement. If desired, an adsorbent for heavy metals, such as bauxite processing residue, may be added in accordance with the method of applicant's Australian Patent No. 661703, the contents of which are hereby incorporated by reference. Non-organic waste may account for 10 20% of the total mass of waste. Any putrescible waste may be delivered to area 1, if desired, as an amendment to the organic green waste. Moisture content of the compostable material 2 may be controlled to prevent drainage of excess liquid a* from the material. Any excess process water may condense against the inside of 0:69 cover 7 and drain sideways for collection by discharge trench 3 or like means.

:09% This effectively distilled water may be collected in a ground collection tank or similar means preferably to be used to humidify any influent air to the process.

The area 1 comprises a solid base, made for example, from a layer of concrete, brick, compacted limestone or other possibly impermeable material which provides an all weather working surface for the mass of compostable material 2 and ideally appropriate insulation to the climatic environment. A concrete layer may be placed on a compacted limestone layer. However, as S. leachate is not generated in significant quantities it is not essential that the base be water impermeable. The base may be modular and portable and may also advantageously insulate the mass 2 from surrounding ground advantageously avoiding of low temperature regions at the bottom of the mass. Such low temperature regions do not compost as desired and pathogen levels may remain high. Indeed, temperature is relatively homogeneous allowing composting and entry to thermophilic phase by a substantial portion of the mass, resulting in general pathogen destruction. The base, together with the cover 7 define the composting environment, the environment has a volume sufficient to enable efficient composting of the mass of compostable material and, typically, the cover 7 will be in close proximity to the mass of compostable material. It is not desirable that the non-composting volume be at all significant as this may detrimentally affect composting process control.

Compostable material may be delivered to area 1 by front end loader or other suitable vehicle or means, optionally (but advantageously) following separation of non-compostable matter.

At the bottom of the mass 2 is an aeration means including air inlet or aeration trench 3 which forms one portion of the means to aerate the mass 2. Air is circulated through the trench 3 by means of a blower 4, shown in plan view in Figure 2. The trench 3 is of approximately rectangular section though the ditch is not required to be restricted to rectangular geometry. The geometry could readily be semi-circular, ovoid or any other convenient geometry. Similarly, the trench 3 might be replaced with another kind of aeration means, located anywhere ooooo beneath the surface of the mass, for example the aeration means could take the form of a ditch or perforated tubing. A number of ditches, tubes or trenches could be employed, above or below ground surface level.

At the top of the ditch 3 is a grid 5, optionally made from a metallic mesh.

The mesh is ideally constructed of a corrosion resistant material, such as stainless steel, to resist the corrosive influence of moist air from the mass 2.

The grid 5 serves two purposes. Firstly, the grid 5 prevents subsidence of compostable material into the ditch 3 which would reduce the effectiveness of aeration by blocking the air supply to the compostable material. Further the apertures of the grid 5 tend to distribute the flow of air so as to better aerate the compostable material. Trench 3 or alternative means of similar function may also serve as a collection device for condensed water allowing recovery, preferably for use in humidifying influent air to the process.

At the edge of the area 1 is a gutter 6 that defines the perimeter of the area for holding compostable material. As well as receiving rainwater and allowing diversion of water away from the composting system, the gutter 6 provides a convenient location for sealing of the weatherproof cover 7 that surrounds the mass of compostable material 2. Rainfall may run off cover 7 and may be directed to flow into the gutter or spoon drain 6 which may be permitted to run into a usual stormwater discharge system.

As observed from Figure 1, the mass of compostable material 2 forms a longitudinally extending, approximately triangular pile to which the cover 7, being of a flexible low cost material, is conformed to create a microenvironment within the cover 7 in which composting can take place.

The cover 7 is conveniently of a low cost weatherproof material, such as high density polyethylene (HDPE) or PVC, woven or in the form of a film, though other polymers may also be suitable, that is waterproof and durable to prevent entry by vermin and insects, ingress of water due to rainfall which may cause a leachate flow to ground 11 surrounding area 1. The cover 7 also excludes undesirable materials and chemicals such as weed seeds and pesticides from contaminating the compost. It is also advantageously robust to other climatic influences such as windy and dry conditions which may cause drying out of the mass and a fall in composting rate to unacceptably low levels. Ideally, the cover 7 may be of air tight material to prevent ingress of air and prevent escape of odour and moisture caused by formation of volatile organic compounds and ooo•• ammonia during the composting process. The cover 7 may be made of area go•* sufficient to adapt to varying mass and volume of compostable material and appropriate folding in the region of edge 8 to achieve the desired area may be undertaken for this purpose. Flexibility or modular construction to allow for varying volumes of compostable material may enhance this advantage. The o.

cover 7 itself could be provided with sealing means, for example, water pockets, as described below.

The edge 8 of the cover is sealed against ingress of water and escape of odour by a water jacket 9 above the edge 8 of the cover preferably forming an 17 air-tight seal for the latter purpose. The volume of water imposes sufficient force to maintain a seal, though other means for achieving a seal may of course be adopted. For example, sufficiently heavy objects such as beams, rods, sandbags or soil may be laid in the gutter 6 to achieve the same end. Alternatively, sealing may be achieved by straps extending over the cover 7 and surface area to enable securement and sealing. Simple sealing means facilitate the conformance of the composting environment to varying volumes of compostable material contributing to system flexibility.

The aeration means is completed by the air recycle pipe 10 which takes spent process air away from the mass of compostable material 2 under the influence of blower 4 by suction. The pipe 10 may be simply perforated or agricultural tubing which collects process air and recycles it to the blower 4 which, in a preferred embodiment as shown in Figure 1, has a substantial portion thereof provided along, or in proximity with the bottom edge(s), one being shown in plan view in Figure 2, of the mass of compostable material 2, and which takes circulated air away from the mass of compostable material 2 under the influence of blower 4. Alternatively, as shown in Figure 4, a substantial portion of the recycle pipe 10 may be provided at the top of the system, the flexible pipe being fixed by a portion of the cover 7 bridging section 7a and 7b thereof by suitable fastening means.

Turning now to the plan view of the composting system shown in Figure 2 it may be seen that air inlet trench 3 extends longitudinally along the base of the mass of compostable material 2 preferably substantially the whole length thereof.

The air passing through the air recycle pipe 10 is returned to the blower 4 and, upstream thereof may be located a moisture, oxygen or carbon dioxide concentration sensor 11 which monitors the concentration of one or other of the gases. In accordance with the monitored gas concentration, a valve or other means, as understood in the art of blowers, may be set either manually or automatically to proportion fresh air, by means of blower 4, into the air inlet ditch 3 to achieve the desired levels of moisture, oxygen and carbon dioxide through, control via the blower and proportioning means, an appropriate ratio of recycled air to fresh air. If a valve is used, the valve may be of solenoid or other suitable type as may be understood by those involved in engineering of pneumatic systems. The recycle/fresh air mixture passes through pipe 13 to air inlet ditch 3.

pipe 13 may be of any convenient plastic material, e.g. PVC. Fresh air addition assists in maintaining preferred composting conditions. Any air discharged to maintain the balance of fresh to recycled air may be treated, for example, by a suitable biofilter to remove odour forming compounds.

The rate of aeration of the mass of compostable material of the mass of compostable material 2 may also be controlled in accordance with the temperature sensed by temperature sensors located at desired locations within the system. The most advantageous locations for temperature sensors are at the air inlet, the core and in the surface regions of the mass of compostable material 2. Temperature at these locations generally reflects the efficiency of the composting process.

Spent process air recycling may serve an important role in reducing odour as the odour causing compounds may typically be adsorbed by the compostable material which may act as a biofilter after recycling.

The system may include a covered raw material storage area for material awaiting pretreatment and conditioning prior to being placed in the system and air from this area may be extracted from this area by blower 4 for use in aeration of the windrow or otherwise treated for odour removal. Thus the potentially odorous air from the raw material storage area may be vented to the system accordingly, further assisting in odour emission control.

The system may be conveniently placed under manual or automatic control, for example, of an electronic control unit. A computer control system is desirable for this purpose. In accordance with measured variables, such as temperature, 02 concentration, CO 2 concentration, moisture level, the blower 4 may be operated at a desired recycle air: fresh air ratio to maintain composting conditions at an optimal level from the point of view of microbiological activity, and composting may proceed with little intervention from personnel.

Composting should continue for sufficient duration to diminish the levels of phytotoxic compounds caused by intermediate metabolites and high ammonia levels in immature compost. Ideally composting should proceed at least three weeks and preferably eight weeks per tonne of waste.

The contained nature of the system allows stable composting under controlled conditions. The final product is more acceptable from the standpoint of both environmental and commercial considerations and may be implemented at relatively low cost with reduced processing time.

The present invention may be particularly advantageous where medium to large quantities of waste need to be processed in environments where odour emissions and leachates are of concern. Therefore, residential areas, such as cities, may be areas of typical application.

There will now be described the performance of a development system operated in accordance with the system and method of the present invention as described above with particular reference to the preferred embodiment. In this example, the effectiveness of the system in processing a mixture of green waste and biosolids, in particular sewage sludge (anaerobically digested primary sludge, not typically treated in composting), was evaluated.

Process Heat Distribution.

The ARMCANZ guidelines (see Agriculture and Resource Management Council of Australia and New Zealand Water Technology Committee, Guidelines ~for Sewage Systems Biosolids Management Occasional Paper WTC No 1/95 October, 1995) require composting to be carried out at thermophilic temperatures 55 0 C) for at least three continuous days to produce grade 1A compost. The ARMCANZ grade 1A composting requirements were met throughout the windrow using the method and system of the invention, including at the windrow surface.

Reference is made to a Figure 3 which shows a temperature profile at the windrow surface and various depths within the pile. It has been found in previous practice that in open windrow composting systems the surface temperatures are too low (near ambient, at 15 30 0 C) to meet these guidelines. It is thought, without wishing to be bound by any theory, that the preferably flexible cover of the system insulates the windrow from climatic conditions thereby preventing heat loss. Materials for the cover may be selected with this in view. Such a cover may also trap solar radiation which may assist in heating the windrow surface. It has been found that windrow surface temperatures may reach up to 700C during composting and this a special advantage of the present system. An air layer containing trapped air will typically be present and acids in insulation of the compost mass.

The thermophilic stage may be extended by including "thermophilic" amendments such as sawdust or wastewater skimmings to the compost. Without wishing to be bound by any theory, skimmings, chaff or materials providing more energy per mole of carbon than sawdust may be preferred as an amendment though sawdust may be used. The lipid content of skimmings is a factor in providing a higher energy output.

It will be noted that in forced aeration composting systems known to the prior art, the windrow core is often subjected to excessive evaporative cooling such that the above guidelines may not be met. In the present system, this problem may be alleviated by regulating aeration with a computer controlled system to satisfy the composting aeration requirements without excessive cooling. Further, recycling of moist and warm windrow exhaust air back into the °ooeo system may further reduce evaporative cooling.

Pathogen Levels.

was found that pathogen levels in compost made by biosolids/green waste using the present system complied with the above guidelines for compost oo...i of the highest standard, grade 1A compost. MPN <2 is only limited by the detection limit of the methodology. Therefore product compost from the system was found to be suitable for unrestricted distribution. In an open windrow system, pathogen reinfestation of sanitized material from inside the windrow may occur at or after turning by mixing of the sanitized material with the pathogen infested 0. surface material or and/or bringing the sanitized material from inside the windrow to the surface thus exposing the material to vectors like insects and vermin.

Table 1.

Pathogen Levels in the Final Biosolids Compost (82 days old) Compared to Grade 1A Biosolids.

Pathogens in ADPS Grade 1A Grade 1B compost Pathogen (MPN) (MPN) (MPN) Sample location 1 2 34 Salmonella <2 <2 <2 <1 (per 50g) <2 Thermotolerant 43 23 9 coliforms <100 <1000 <3 (per g) NB 1 surface; 2 surface; 3 1 m below surface; 4 15 cm below surface It may be seen from Table 1 the product compost complies with ARMCANZ guidelines for grade 1A compost.

Leachate Generation.

The system of the invention is designed to prevent generation of leachate during processing or composting. This is one of the benefits of the system. The ground directly beneath the windrow was tested for leachate. Ammonia nitrogen (found in higher concentrations in the windrow material) was used as a tracer to indicate leaching. The (compost) concentration of ammonia nitrogen in 16 water extracts of the windrow material was 28 ppm compared to the soil ambient level of between 1 and 2 ppm. The ammonia nitrogen levels of the soil 15 cm below the ground surface under the windrow were at ambient levels, i.e. at 1 to 2 ppm. Therefore it was found in the experiment that the transfer of ammonia nitrogen (hence leachate) from the windrow material into the ground did not occur and this lack of leachate percolation was advantageous.

Odour.

This was assessed qualitatively and only a slight smell could be detected 22 by persons located at the composting site, the smell being described as a "sweet" compost smell. None described the odour as unpleasant, as with pure untreated biosolids. The flexible plastic cover may provide a seal against emission odours which would otherwise have been much higher and this may be advantage of the system.

General Compost Quality.

This is most important to the marketability of compost especially as a soil amendment. Product complied with Standards Australia Draft Standard DR 95301. Quality parameters of produced compost in the experiment are shown in Table 2.

Table 2.

General Composting Parameters of the Biosolids/Green Waste Mixture.

Final Product Parameter Initial Material wk (8 weeks) Total organic carbon, 31.1 28.7 Total organic nitrogen, 1.67 1.95 Carbon to nitrogen ratio 18.6 14.7 Total phosphorus 0.61 0.72 Buffer capacity, CaCO 3 16.7 21.1 Ash, 40.1 50.7 Salinity (Elec conductivity in 2.37 2.46 mS) pH 7.6 6.7 Typically composting increases quality of material by concentrating nitrogen and phosphorus. The C/N ratio typically suitable for land application after composting. Composting typically increases ash content as it is expected to remove the readily putrescible organic matter which may encourage pathogen regrowth. It was found that the electrical conductivity (salinity) and pH of compost a 4 a. a.

a.

were ideal for land application.

Figures 5 to 8 show transverse section, plan and detailed section views, respectively, of a further embodiment of the system of the present invention.

These figures show a base 1 a portion of which is formed with an aeration structure 80 in which is located rigid cellular structure 70 used as a diffusion means to controllably distribute air flow throughout the mass of compostable material 2. The cellular structure 70 is formed of PVC or other suitable materials and may be obtained from Atlantis Corp Pty Ltd.

The rigid cellular structure 70, as conveniently shown in Figure 6 may occupy a recess 60 formed in the area 1 to which is communicated an air supply manifold 65, extending along one edge la of area 1 through five branch pipes 66, corresponding with five separate aeration units. Recess 60 has chamfered edges 60a, arranged at an angle of about 450 to the horizontal. Each branch pipe 66 is provided with a control means such as solenoid valve 66a, outside the perimeter of area 1, the opening of which is controlled to achieve a desired flow of air along the length of area 1 and, hence, the mass of compostable material 2.

In this way, the proportion and rate of air flow along the length of the mass of compostable material 2 may be controlled by an operation of the composting system. Such rate and proportion of the air flow may be controlled by the control unit supervising operation of the composting system.

Larger areas of aeration structure may optionally be supplied with air by dual air supply manifolds 214 as shown in Figure 9 each being connected to a common blower though this is not essential. Of course, any number of air supply manifolds could be provided subject to cost considerations.

The rigid cellular structure 70 may be connected to the base of the recess by securing means, in this case bolts 60b extending through flat holding plates that hold the structure 70 into position, allowing limited if any movement.

Figure 7 is a convenient illustration of this. This provides a secure base for holding the mass of compostable material.

More description of the rigid cellular structure 70 employed is provided in Australian Patent Application No. 70742/87, the contents of which are hereby 24 incorporated herein by reference.

The load bearing characteristics of such a cellular structure 70 are such as to facilitate passage of vehicles transferring compostable material to area 1. To prevent any substantial risk of damage by loading vehicles such as front end loaders, the upper surface 70a of cellular structure 70 is located below the upper extremity of area 1 leaving a clearance 70b thereof in which composted or compostable material may accumulate to form a protective layer. If, for example, the recess 60 is 50mm deep and the cellular structure 70 is 30mm deep, a clearance of 20mm will be left for this purpose. Further, the width of each aeration structure 80 may be selected to be less than the width of loading vehicles, for example, front end loaders. In particular, the width of the aeration structure 80 may be selected to be less than the width of a front end loader bucket to prevent collision with the cellular structure As shown in the embodiment of Figures 9 to 11, the structure 70 may be further protected by inclusion of a suitable protective grid 71 of metal or suitable polymer overlying it. A steel grid 71 may be preferred. This grid 71 may simply be appropriately sized and fitted into position as a drainage grid but fastening means may be employed for this purpose.

o Further, a fine mesh screen 72 of nylon or other suitable material may be arranged between structure 70 and grid 71 to assist in prevention of clogging of o* the apertures of the cellular structure with composting material. Either the grid 71, screen 72 or both could be omitted from the system.

~In this case, the aeration structure 70 may be formed with steps 73 in the concrete base layer. Mesh screen 72 may be connected to steps 73 by fastening means such as pins 74 which should not pass through branch pipes 66. Slipping of the screen 73 would be undesirable.

The embodiment of Figures 5 to 8 employs a different arrangement for collection of spent process air. Extending along one side of area 1 within the perimeter thereof, there is arranged a U shaped drain 105 through which air is drawn by suction of blower 14 into air recycle manifold 110. A grate 106 prevents blockage of this drain. Drain 105 replaces overhead recycle pipe 10 of Figures 1 and 2. A clearance is left between mass 102 and the drain 105 to allow collection of spent process air. A portion of air, dependent on the selected recycle ratio, is discharged through lines 114 and biofilter 200 to atmosphere. Biofilter 200 is located subsoil, taking the form of a ditch lined with a plastic liner filled with old compost material. Line 114 delivers the discharged portion of air (which may be loaded with condensate) to the bottom of biofilter 200. After passage of air through the biofilter 200, it is substantially deodorized and suitable for discharge to atmosphere.

Spoon drain 116 extends around the perimeter of holding area 1 and this drain, which may be connected to a stormwater treatment system, serves the same function as drain 6 of Figures 1 to 4. Sufficient concrete is left between air supply manifolds(s) and branch pipes 66 to ensure no damage to the latter.

Once again, flexible cover 107 is secured in similar manner to that described in relation to Figures 1 to 4.

Otherwise, operation of the embodiment of Figures 5 and 6 is in accordance with the description made with reference to Figures 1 to 4.

Further modifications are possible. In particular, the system need not be controlled identically with the description above. Furthermore, the system may comprise an area with other than approximately rectangular geometry. For larger .:.•areas of aeration structure, the area may conveniently be divided into a number ;of units of smaller area. Generally two such aeration structure units may be provided which may have same or individual air supply and/or air removal S° systems. The units may be separated by an area of concrete pad. The units may be arranged in parallel.

o The air could be treated for odour removal, for example by treatment with an adsorbent prior to recirculation if adsorbent capacity of the compostable material is insufficient.

So. Further, a full scale plant might comprise a number of, preferably parallel covered systems, each system of construction as described above, with air recirculation possibly being under the control of an integrated control system.

Claims (19)

1. A composting system including: an area having a base for holding and treating compostable material; in use, a mass of compostable material located in the area; a removable cover conformable to the mass of compostable material and securable to said base for defining with said base a composting environment; means for securing the cover to the base for forming said composing environment; aerating means for flowing an oxygen containing gas through the composting environment which includes diffusion means for distributing the flow of oxygen-containing gas through said mass; and a control unit for controlling a condition in the composting environment.

2. The system of claim 1, wherein said aeration means includes an aeration structure located in the base including diffusion means for distributing flow of oxygen-containing gas through the composting environment.

3. The system of claim 2, wherein said aeration structure is recessed in said •base, the diffusion means being a grid structure for distributing flow of oxygen- containing gas supplied to the aeration system.

4. The system of claim 2, wherein said aeration structure is recessed in said base, the diffusion means being a cellular structure. .e The system of claim 4, wherein said cellular structure is a rigid cell structure including first and second substantially perforate planar members maintained in a fixed spaced relationship from each other by means of a plurality of spacer members having a load bearing capacity to allow vehicles transporting compostable material to said area for holding and treating compostable material.

6. The system of claim 5, wherein said rigid cell structure is covered with a layer of composted or compostable material.

7. The system of claim 5, wherein said aeration structure is rectangular in plan extending substantially the length of said area for holding and treating compostable material.

8. The system of claim 7, wherein a plurality of aeration structures are located in said area.

9. The system of claim 8, wherein said aeration structures are arranged in parallel. The system of claim 8, wherein said aeration structure is connected to an oxygen-containing gas supply manifold delivering air to each aeration structure °ooo. through a branch pipe.

11. The system of claim 10, wherein said branch pipe for each aeration structure includes a control means to achieve a desired flow of oxygen-containing gas to each aeration structure. The system of claim 10 or 11, wherein the width of each aeration structure is less then the width of a vehicle delivering compostable material to said area. So 13. The system of claim 10, wherein a protective grid overlies said rigid cellular o° structure.

14. The system of claim 13, wherein a mesh screen is arranged between said rigid cellular structure and said protective grid. The system of any one of claims 1 to 14, wherein said area includes a drain, communicating with a gas recycle manifold, and through which gas is drawn by suction of a blower.

16. The system of claim 15, including a biofilter for deodorizing portion of gas drawn through said drain prior to discharge to atmosphere.

17. A system for flowing a gas through a mass of organic material subject to microbial activity to stabilize same as hereinbefore defined including: a holding zone for holding and treating the organic material; in use, a mass of organic material in the holding zone; gas flow means for flowing a gas through the holding zone wherein the holding zone includes a base supporting the organic material which includes diffusion means for distributing the flow of gas through the mass of organic material to reduce undesirable microbial activity relative to the organic material.

18. The system of claim 17, including a control unit for controlling a condition in oo i the holding zone.

19. The system of claim 17 or 18, wherein said gas flow means includes an aeration structure located in the base including diffusion means for distributing .ooo°i flow of a gas through the holding zone.

20. The system of claim 19, wherein said aeration structure is recessed in said base, the diffusion means being a grid structure for distributing flow of gas supplied to the gas flow means. o.i 4

21. The system of claim 20, wherein said aeration structure is recessed in said base, the diffusion means being a cellular structure. 29

22. The system of claim 21, wherein said cellular structure is a rigid cell structure including first and second substantially perforated planar members maintained in a fixed spaced relationship from each other by means of a plurality of spacer members.

23. A method of composting organic material including composting of said organic material in the composting system of any one of claims 1 to 16.

24. A method of stabilizing organic material including treating said organic material in the system of any one of claims 16 to 22, for reduction of undesirable microbial activity relative to the organic material. DATED this 21st day of June, 2001. HARRIE HOFSTEDE WATERMARK PATENT TRADEMARK ATTORNEYS 2 1 ST FLOOR, "ALLENDALE SQUARE TOWER" 77 ST GEORGE'S TERRACE PERTH WA 6000 S S. S Se S S. S S S C