EP0723582A1 - An improvement in the management of wastes - Google Patents

Abstract

A method of treating biodegradable organic waste material in which a suspension of the waste material in liquid at a solids content of at least 40 % by volume on a dry solids basis is put into a lined enclosure (1) which is then sealed with a fluid impervious flexible cover (3) in the absence of air to form a bioreactor cell, and heat is then applied to promote a decomposition reaction. The process is a batch one and the enclosure can be emptied after use and refilled for further use.

Description

An Improvement in the Management of Wastes

This invention relates to waste management. Waste in this context refers broadly to domestic, household, commercial and non-hazardous industrial refuse. Waste, as defined, would normally contain biodegradable material such as garden cuttings, paper, food material, wood and similar. Manures, agricultural slurries or sewage could be added to help accelerate, increase or enhance biodegradability. The natural decomposition of wastes, of the kind previously detailed, under traditional landfill disposal methods is a well appreciated phenomenon. The bacterial break down involved is also widely understood to result in the production of quantities of biogas (often referred to as landfill gas) over time. This biogas is typically composed of approximately 60% methane and 40% carbon dioxide. Both the use of the methane fraction of this gas to drive electrical generators and its flaring off at landfill sites are well known. Migration of methane to areas away from the site, and the build up of the gas on site are both well recognised potential hazards associated with landfills, as are the implications for land use subsequent to the end of tipping. Another aspect of landfill operation that has been of concern from an environmental point of view is the production of leachate, that is liquid contaminated with substances derived either directly from, or arising as a result of the decomposition of, the waste in the tip. Pollution of groundwater by such leachate is a serious problem.

Changes in working practice have altered the nature of leachate problems at waste sites. Increases in the size of sites, improvements in waste compaction ratios and the use of efficient water and air excluding covers have altered the details of decomposition. Rapidly established anaerobic conditions produce a very high Biological Oxygen Demand (BOD) which gives rise to large quantities of biogas as decomposition proceeds.

Historically, the production of biogas and the formation of leachate have been frequently treated as separate.

However, they are closely linked, particularly in a modern contained landfill site.

It is known from our earlier application W093/01456 published 21st January 1993 to provide a reservoir involving an excavated or naturally occurring cavity covered by an impervious flexible membrane and containing a biodegradable material. The present invention seeks to provide improvements in this kind of system, although not necessarily incorporating a reservoir.

According to the present invention, a significant quantity of waste is allowed to undergo decomposition to provide energy, which is accelerated by the provision of additional energy to the system. This enables evolution of biogas to be managed more effectively, and particularly at start up.

A method of treating biodegradable organic waste material of the kind foundsin domestic, household, commercial and industrial waste, said method comprising the steps of mixing the waste with a liquid to form a suspension in which the waste material represent at least 40% by volume on a dry solid basis and putting the said organic waste material suspension into an enclosure lined with a fluid impermeable liner and sealed by a cover to form a bioreactor cell, which is substantially gas tight and excludes air, applying heat from an external source so as to raise the temperature of said material to a temperature above ambient so that a microbial composition reaction can commence and continue at an enhanced rate drawing off gas formed by the biological decomposition and, on completion of decomposition, removing the decomposed material from said cell. The invention can be carried out by use of a waste management container incorporating a cavity formed by a structure, pit or excavation, lined with a fluid impermeable liner and sealed by a cover to create a bioreactor cell, in which biodegradable material is arranged to be subjected to a decomposition process to thereby provide usable energy, said container further comprising means for provision of a controlled quantity of thermal or other energy in addition to said energy provided by decomposition so as to bring the cell to a chosen temperature for commencement and then for continuation of said decomposition process.

In a practical execution of the invention a large excavated cavity, trench or pit is lined with impermeable material, filled with waste, and a fluid impermeable top layer placed above. The system can be regarded as providing an individual containment cell for the biological degradation of the waste within.

The cavity is of truncated substantially catenoid shape that is its under surface is as near to a catenoid shape as is practical in that excavation is normally carried out by forming planar cuts in the soil and these link to equate approximately to a catenoid and form a shallow dish shaped cavity. The construction of the cell dictates that the organic waste material degrades chiefly anaerobically, that is in th absence of air. This in turn means that the waste is subject first to an acetogenic first stage of breakdown resulting in the production of volatile fatty acids and high concentrations of ammoniacal nitrogen. The liquids produced in this have high Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD) .

In the methanogenic second stage the organic compounds resulting from the first stage are largely changed into biogas.

These processes take place under natural conditions within a landfill site, but they do so (a) over relatively lengthy time-frames and (b) non-uniformly throughout the waste. Both of these make management of the decomposition difficult. Under the present invention, the waste breaks down at a more uniform rate, and in a considerably reduced time.

The methane fraction of the biogas is not an unwanted consequence, as it can be collected and used for the generation of heat or electrical energy, or simply flared. The reintroduction of the thermal energy from the said flaring or generation (making use of the otherwise waste heat produced) or electrical energy generated can provide the required additional energy in accordance with the invention, as can the aerobic degradation of waste.

In a practical application of the invention, a series of such cells as described, can be employed to bring about the required result, namely the bio-degradation of waste with the potential for biogas production.

In a relatively small scale application, the additional heat energy is provided by a burner/boiler unit burning coppice fuel. Other energy sources and other heaters can also be utilised to provide the extra thermal energy to warm the bioreactor cell(s) which embody the core of the invention. The full integration of such alternative providers of the thermal energy into the management of waste, in, for example, the sorting of waste types, or the calorisation of waste sub-fractions before, during or after, digestion can form part of the present invention; as can the potential use of waste, waste derived fuels or combined heat and power systems to the same ends.

The amount of additional energy is determined by the requirement to fulfill two linked objectives:

a) raise the temperature of the digestion cell initially to one at which the bacterial cultures responsible for "" anaerobic digestion and methanogenesis will thrive, grow and multiply.

b) maintain the digestion cell subsequently in a relatively steady state, at this appropriate temperature, for these anaerobic processes to be continued.

Clearly the input of energy required to raise the temperature above ambient and maintain it, to satisfy these requirements, will^" be subject to a number of variables. Such factors include time of year, site topography and location, exact composition of the waste, local heat losses, initial ambient temperature and so on. While, knowing the ambient temperature at onset and the constituent fill of a given digestion cell, it should be possible to calculate the necessary thermal input to raise the contents to the required temperature, predictions beyond that would be difficult. Accordingly, rather than try to specify the requisite heat addition in terms of a particular value per cell, or per unit volume, it is more pertinent to define it in relation to the overall temperature levels needed.

In a preferred practical embodiment, the temperature would lie in the mesophilic range, between 32 - 370C, though it would be possible to run the system at a lower temperature (and, thus, slower degradation) or at a more elevated, thermophilic range, around 55oc (at a faster rate of decomposition). Thus, the heat energy requirement can be said to be that which is necessary to raise the temperature initially to, and subsequently maintain it at, one appropriate for the requirements of the bacteria responsible for anaerobic digestion and methanogenesis; or to raise the temperature initially to, and subsequently maintain it at, one within the required range for the growth of mesophilic bacteria responsible for anaerobic digestion and methanogenesis.

Since the required heat addition is to be defined in terms of the overall thermal environment of the waste cell, measuring and verifying this becomes a relatively simple task. In essence, it involves taking the temperature, whether by direct means, a thermometer, for example, or indirectly, by electronic sensors, or similar, within the cell contents. If the direct route is used, it may be necessary to abstract some of the liquid from the cell to allow this to be done. The preferred option is to have sensors in situ.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross section through the complete system in accordance with the invention;

Figure 2 is a schematic representation of the relationship of the heat energy generator to the digestion cells.

Figure 3 is a plan view of the waste cell at ground level according to the invention;

As has previously been stated, the invention uses a container defined by a structure, pit or excavation, and this is lined with a fluid impermeable liner. The shape of this container is important since the liner should retain its integrity and be capable of re-use. It is therefore a preferred feature of the invention that the container and defining liner are of truncated substantially catenoid shape. This minimises stresses at the boundary layer so that cracks are unlikely to appear from the mass and movement of the biodegradable material within the container.

Referring now to the drawings, a relatively large area of land is excavated to produce a pit or trench, either totally by digging out, or by cut and fill techniques. Alternatively a pre-existing natural or artificial hole of suitable dimensions may be used provided the approximate catenoid shape can be achieved. This excavation is lined with a fluid impermeable liner 1, laid on an adequately prepared surface.

This lined hole provides a void space 2, into which refuse containing biodegradable waste, with the possible addition of sewage, manure or food industry waste liquor is deposited. The waste is mixed with a liquid such as water to form a suspension having a relatively high solids ratio, that is at least 40% by volume measured on a dry solids basis.

The shape of this void space is substantially that of a truncated catenoid, i.e. it is formed from flat cuts equating to a catenoid and is in a shallow dish like configuration.

Moisture levels are maintained within the waste to enhance conditions for anaerobic digestion. The liner 1 is equipped with a leachate sump 6, which is served by a leachate pipe 7.

When the deposition of waste into void space 2 is completed the impermeable liner 3 is laid across the surface of the refuse, forming an effective gas-tight seal on the vessel which excludes air and, can be regarded as a sealed bioreactor cell. The impermeable liner normally needs to have a degree of flexibility to accommodate fluctuations in volume of the cell during the decomposition process. Alternatively some form of buffer tank in the take off circuit with appropriate values could be used to accommodate these fluctuations.

The waste within the cell, which had been prepared as a suspension of optimised humidity/water content for high solid digestion, is heated via the integral thermal inlet 8, by the addition of external energy from an external heat energy generator 9, to a temperature elevated above ambient, and at which microbial decomposition is facilitated and/or enhanced. This acts to reduce the time of degradation, and to provide a quicker, more manageable biogas supply than in a traditional landfill.

Within the void space 2 there is provision for a gas collection system 4 to gather the biogas produced by the biological decomposition of the waste contained within the cell. These gases are removed for use for heat or power generation, or disposal, via an above ground exit pipe 5. Heat energy derived from the gas (either directly or indirectly) may also or alternatively be used to warm the decomposing waste by being fed back into the system to accelerate or enhance the processes of decomposition. Liquid is circulated within the digester cell to enhance microbial action, and to maintain relative wetness of the waste. This is pumped from the leachate sump 6 to the top of the cell via pipe 7, so that on returning into the cell at the top it maintains relative wetness and provides a degree of agitation.

When the decomposition of the waste is complete in the void space 2, the system is opened and the residual digestate remaining is removed either for use or disposal. Anaerobic digestion produces a relatively large reduction in the volume of material, so disposal in a landfill after this process is less wasteful of land, and safer since the methane and leachate production potential has been significantly reduced. Alternatively, the material could be further treated for other end uses.

The now empty void space 2 is available for reuse, and the cycle previously described can be repeated many times, assuming the liners involved maintain their integrity.

Claims

1. A method of treating biodegradable organic waste material of the kind found in domestic, household, commercial and industrial waste, said method comprising the steps of mixing the waste with a liquid to form a suspension in which the waste material represent at least 40% by volume on a dry solid basis and putting the said organic waste material suspension into an enclosure lined with a fluid impermeable liner and sealed by a cover to form a bioreactor cell, which is substantially gas tight and excludes air, applying heat from an external source so as to raise the temperature of said material to a temperature above ambient so that a microbial decomposition reaction can commence and continue at an enhanced rate drawing off gas formed by the biological decomposition and, on completion of decomposition, removing the decomposed material from said cell.

2. A method according to claim 1 in which during decomposition a leachate liquid is recirculated by being abstracted from the cell and then is fed back into an upper region of the cell, so as to agitate and maintain relative wetness of the decomposing waste material.

3. A method according to claim 1 or claim 2 in which some of the gas derived from biological decomposition is formed into heat energy and fed back into the cell in order to help promote the decomposition reaction.

4. A waste management container for use in the method according to any preceding claim and incorporating a waste management container incorporating a cavity formed by an enclosure, pit or excavation, lined with a fluid impermeable liner and sealed by a cover to create a bioreactor cell, in which biodegradable material is arranged to be subjected to a decomposition process to thereby provide usable energy, said container further comprising means for provision of a controlled quantity of thermal or other energy in addition to said energy provided by decomposition so as to bring the cell to a chosen temperature for commencement and then for continuation of said decomposition process.

5. A waste management container as claimed in claim 4 which is integrally linked with an external energy source.

6. A waste management container as claimed in claim 4 or 5, which is integrally linked or associated with an external heat generator source.

7. A waste management container according to any of claims 4 to 6 in which said cavity is of substantially catenoid shape.