WO2011042846A1

WO2011042846A1 - A process for removing methane from landfill biogas streams, and relevant biofiltering unit and plant

Info

Publication number: WO2011042846A1
Application number: PCT/IB2010/054442
Authority: WO
Inventors: Pietro Paolo Cella Mazzariol; Gian Francesco Galanzino
Original assignee: Co2Balance Italia S.R.L.
Priority date: 2009-10-05
Filing date: 2010-10-01
Publication date: 2011-04-14
Also published as: IT1396293B1; EP2485826A1; ITTO20090756A1

Abstract

The present invention refers to a process, and to the relevant plant, for removing methane from landfill biogas streams and/or bioaerosols through at least one biofiltering unit (1) equipped with a double covering (3, 30) comprising, in its turn, a permeable sheet (30), equipped with a suitable supporting system (33), and a cover (3), said double covering (3, 30) allowing to achieve the optimum humidity and temperature conditions for the biological process as well as to minimise, and even to nullify, the release of the methane contained in the biogas into the surrounding environment.

Description

"A process for removing methane from landfill biogas streams, and relevant biofiltering unit and plant"

DESCRIPTION

Technical Field

The present invention refers to a process for removing methane from landfill biogas streams.

More precisely, the present invention refers to a process for removing methane from landfill biogas streams and/or bioaerosols.

The present invention also refers to a biofiltering unit for removing methane from landfill biogas streams and/or bioaerosols .

The present invention also refers to a plant for removing methane from landfill biogas streams and/or bioaerosols comprising at least one biofiltering unit.

Known Art

The biogas developed in the landfills for municipal solid j

waste (MS ) is normally tapped by a collecting network; the term "landfill biogas" means a mixture of different kinds of gases that spontaneously forms by anaerobic bacterial fermentation of organic material, mainly with generation of methane (the so-called methanisation of the organic compounds) as well as and of carbon dioxide and molecular hydrogen .

Biogas that contains methane and/or other combustible contaminants with concentrations higher than 25 % can be used for generating electric power, for instance by conveying it to a cogeneration system, or it can be burned, for instance by torch combustion.

On the contrary, biogas that contains methane and/or other combustible contaminants with concentrations lower than 25 % is usually released to the atmosphere.

However, particularly methane is a greenhouse gas over twenty times more dangerous than carbon dioxide: the emission of 1 kg of CH₄, on a time frame of 100 years, is equivalent to emit 21 kg of C0₂.

It therefore results necessary to minimise the biogas dispersion into the environment of the biogas, and particularly of the methane therein contained, thus reducing the environmental risks; specifically, it is desirable to degrade methane into carbon dioxide and water.

Technical solutions such as that disclosed in the United States Patent application no. US 2003/0024686 are not suitable for solving the above-stated problem since they are addressed to the aerobic treatment of organic material in order to generate firstly thermal energy, and fertilisers and other by-products as well; according to such document it is to be noticed that, in particular, the treatment process produces methane as a by-product of the higher hydrocarbons disintegration reactions because of the biomass microbic decomposition and that the described plant comprises a biofilter external to the actual treatment plant and only able to remove all the potential smells.

The technical problem faced by the aforesaid document concerns, besides the generation of thermal energy, the transfer and utilisation of the generated thermal energy, but it does not concern the methane removal from landfill biogas streams and/or bioaerosols.

It is opportune to highlight that composting processes, which involve the aerobic decomposition of organic material, generally release negligible amounts of methane and that, therefore, the problem of treating landfill biogases for removing methane and/or other combustible contaminants therein contained, and the problem of biofiltering the gaseous streams exiting the composting plants substantially to remove smells therefrom, are distinct technical problems. Solutions allowing the purification of the landfill biogas insufficient to be used for electric purposes or to be torch burned are known in the reference technical fields; some of such solutions provide for removing the so-called "poor" biogas through biofiltration before their final release to the atmosphere.

Biofiltration is a biological process for removing methane and the smells contained in the gaseous stream that uses a natural removal process carried out by a heterogeneous microbial population present on the surface of the filtering bed.

The microbial flora metabolises the most of the organic and inorganic compounds by means of a series of oxidative reactions that convert the inlet organic compounds into reaction products that are no longer smelling and less dangerous from the environmental point of view.

The biological oxidation reaction of methane is the following :

CH₄ + 20₂ → C0₂ + 2H₂0

whose reaction products are carbon dioxide, water, new biomass and energy produced in the form of heat.

In order to maximise the efficiency of the biological process, optimum humidity and temperature conditions have to be guaranteed as it has been proved, for instance, by the studies performed by Cuhls et al. (2002) according to which the methane degradability through biofiltration is greatly limited by lacks of oxygen or excesses of humidity as well as by the experimental test performed by Gebert et al. (2003) highlighting that the methane degradation depends on temperature .

There still exists, therefore, the need of finding a solution for the biological treatment of the "poor" biogas extracted from landfills that guarantees optimum humidity and temperature conditions and that allows to minimise, and even to nullify, the release of the methane contained in the biogas into the surrounding environment.

As said above, the operating principle of a biofilter provides for using a heterogeneous microbial population consisting of, for instance, bacteria, actinomycetes and fungi that, by colonising the filtrating matrix, are able to metabolise the substances present in the gaseous effluents passing therethrough, irrespective of whether they are natural or synthetic, inorganic or organic, aliphatic or aromatic compounds.

The biofilters efficiency in removing smells and potentially toxic substances is high; however, a worry raised in their employ concerns the potential emission from the filtering surface of the so-called bioaerosols, that is of vapours and powders containing moulds, bacteria and other organisms potentially bad, at high concentration, for the health of people working near the plants.

Among the found mycetes species, the spores of Aspergillus fumigatus result to be particularly worrying, Aspergillus fumigatus that, though being a non-pathogen mould, can however support, in case of prolonged and massive inhalation, unhealthy forms with proliferation at pulmonary level (aspergillosis) ; Aspergillus fumigatus has an optimum growth temperature of about 37 °C.

According to some analysis carried out on biofilters in the active phase, i.e. fed with hot air coming from municipal solid waste (MSW) treatment plants, the presence of fungal spores is generally lower than 1,000 Colony-Forming Units (CFU) /Nm³, comparable to those normally existing in rural environments; however, there has been recently highlighted the possibility that the emissions from biofilter exceed 10,000 CFU/Nm3, with significant percentages of Aspergillus fumigatus . Though specifying that, at present, no regulations exist in this respect, the indications at European level place the acceptable risk contents at about 1,000 CFU/Nm3 for the overall fungi and bacteria; some studies, on the contrary, deem that concentrations of Aspergillus fumigatus spores below 2,000 CFU/Nm3 are acceptable.

There still exists, therefore, also the need of finding a solution for the biological treatment of the bxoaerosols that overcomes the disadvantages above described.

Description of the Invention

The present invention intends to overcome drawbacks and limitations of the prior art solutions and to meet the still unsatisfied need for a solution for the biological treatment of the "poor" biogas extracted from landfills that allows to minimise, and even to nullify, the release of the methane contained in the biogas into the surrounding environment.

In particular the present invention intends to meet the still unsatisfied need for a solution for the biological treatment of the "poor" biogas extracted from landfills that guarantees optimum humidity and temperature conditions for the biological process and minimises, or even nullifies, the release of the methane contained in the biogas into the surrounding environment.

The present invention also intends to meet the still unsatisfied need for a solution for the biological treatment of bxoaerosols.

Such object is achieved with the biofiltering unit, and the relevant plant and process, used for removing landfill biogas and/or bxoaerosols according to the invention that, advantageously because being equipped with a double covering, allows to obtain optimum humidity and temperature conditions for the biological process and to minimise, and even to nullify, the release of the methane contained in the biogas into the surrounding environment as well as to decrease bioaerosols .

More precisely, the biofiltering unit according to the invention comprises said double covering comprising, in its turn, a permeable sheet, equipped with a suitable supporting system, and a cover.

Advantageously, thanks to the solution according to the invention, optimum humidity and temperature conditions for the biological process are achieved and the release of the methane contained in the biogas into the surrounding environment is significantly reduced and even completely removed .

Advantageously, furthermore, thanks to the solution according to the invention, a significant decrease of bioaerosols is obtained.

Advantageously, furthermore, thanks to the application diversification of the solution according to the invention, it is possible to optimise the biogas management together with a traditional system, thus allowing to treat also the volumes having a low calorific value; more precisely, the solution according to the invention allows to treat the biogas also at the first cultivation stage of the landfill, when a conventional system is not yet active, as well as at the production "tail" stage, when a conventional system is no more applicable.

Advantageously - and surprisingly - the inventors have found that their solution works not only on smells (mainly due to volatile organic substances containing more atoms of carbon and to the inorganic substances produced by degradation under merely anaerobic conditions of the organic fraction of wastes conferred on landfill) but also on methane, which is a very different specific case since it is an odourless molecule and with only one atom of carbon and, in particular, it involves a more difficult biological degradation. Advantageously - and surprisingly - the inventors have also found that the covering according to their solution, and more precisely the permeable sheet, helps to remove methane.

Synthetic Description of the Drawings

The invention will be now described with reference to the following drawings, given by way of non-limiting example, in which :

- FIG. 1 is a front perspective view of the biofiltering unit according to the invention;

- FIG. 2 is a rear perspective view of the biofiltering unit according to the invention;

- FIG. 3 is a top perspective view of the biofiltering unit according to the invention;

- FIG. 4 is a front view, with open door, of the preferred embodiment of the biofiltering unit according to the invention;

- FIG. 5 is a side view of FIG. 4;

- FIG. 6 is a section taken along line A-A of FIG. 5;

- FIG. 7 is a section taken along line B-B of FIG. 6;

FIG. 8 is a top view of the supporting system of the permeable sheet of the double covering of the biofiltering unit according to the invention;

- FIG. 9 is a section taken along line A-A of FIG. 8;

- FIG. 10 is a top plan view of FIG. 5, which shows the wetting system of the biofiltering unit according to the invention ;

- FIG. 11 is a section taken along line B-B of FIG. 10;

- FIG. 12 is a section taken along line A-A of FIG. 10, which shows the condensate and leachate collection and discharge system of the biofiltering unit; and

- FIG. 13 is an enlargement of detail D of FIG. 12, which shows the condensate storage gutter of the biofiltering unit according to the invention. Detailed Description of the Preferred Embodiment of the Invention

FIGS. 1, 2 and 3 show a biofiltering unit 1 for removing landfill biogas according to the invention.

Said biofiltering unit 1 preferably comprises a roll-off container; according to a variant of the invention, said biofiltering unit 1 may comprise a marine container.

Said biofiltering unit 1, preferably made as standard- version, may also be provided with flameproof protection.

Always with reference to FIGS. 1, 2 and 3, the roll-off container 2 of the biofiltering unit 1 is provided with ribbings 20 and it is preferably made of steel, more preferably of Fe 360 steel; preferably, said container 2 has an epoxy painting both on the outside and on the inside surfaces thereof, the outside painting being able to be personalised with logos and or drawings for instance made of metal or aluminium, adhesives, serigraphated, etc..

Preferably, said container 2 has a size of about 6.5 m x 2.5 m x 2.65 m, equal to a useful internal volume of about 22 m³. With reference to FIG. 1, the roll-off container 2 of the biofiltering unit 1 comprises an openable cover 3, an inlet 4 for the biogas to be purified, a condensate and leachate collection and discharge system 5 and at least two sliding rolls 10; preferably, said container also comprises a first stairway 11 to facilitate the access to the cover top from the front side.

With reference to FIG. 2, the roll-off container 2 of the biofiltering unit 1 further comprises a charge/discharge door 6 and a housing 7 for the electrical panel of the adjustment and control system of the biofiltering unit 1; preferably, said container also comprises a second stairway 12 to facilitate the access to the cover top from the rear side. With reference to FIG. 3, the roll-off container 2 of the biofiltering unit 1 further comprises a photovoltaic solar panel 8, preferably adjustable in height, for supplying the electrical occupancies and a chimney 9, preferably equipped with backflow cap 9a and sleeve 9b for sampling and measuring emissions; said panel 8 and said chimney 9 are accessible through said first stairway 11 and said second stairway 12 respectively .

The preferred embodiment of the biofiltering unit 1 according to the invention will be now described in detail with reference to FIGS. 4 to 13.

The roll-off container 2 of the biofiltering unit 1 is visible in FIG. 4, said container comprising an openable cover 3, an inlet 4 for the biogas to be purified, a condensate and leachate collection and discharge system 5, a charge/discharge door 6, a housing 7 for the electrical panel of the adjustment and control system of said biofiltering unit 1 (better visible in FIG. 5) , a photovoltaic solar panel 8 for supplying the electrical occupancies and a chimney 9, preferably equipped with backflow cap 9a and sleeve 9b for sampling and measuring emissions, at least two sliding rolls 10 and ribbings 20.

Said inlet 4 for the biogas to be purified is preferably made of stainless steel and it is connected to the collecting network, as shown.

Said photovoltaic solar panel 8 is preferably adjustable in height, as illustrated by the dotted line in figure, in order to maximise the exploitation of the solar energy depending on the change of the sun position.

Said cover 3 is preferably of the flag-kind, as illustrated by the dotted line in figure, and it is preferably openable through a manual oleodynamic lifting system.

Said cover 3 is accessible through the second stairway 2 provided on the rear side of said container 2, as it is well visible in FIG. 5. It is clear that any kind of stiff covering, even not fastened to the container 2 (such as, for instance, an independent roof) , known to the person skilled in the field can replace said cover 3 for the purposes of the present invention .

Said openable cover 3 together with a permeable sheet 30, which is also visible in FIG. 4, forms a double covering 3, 30 of the biofiltering unit 1 allowing to obtain optimum humidity and temperature conditions for the biological process and to minimise, and even to nullify, the release of the methane contained in the biogas into the surrounding environment and also to decrease bioaerosols ; said permeable sheet 30 is equipped with a suitable supporting system 33. In FIG. 4 it is also visible that the roll-off container 2 further comprises a settling chamber, or plenum, 13 placed in the lower part thereof; the biogas to be purified is blown into said settling chamber 13 through said inlet .

With reference to FIG. 6, the double covering 3, 30 and the ventilation bottom 40 are visible; the filtering matrix 50 is also represented in figure.

Said permeable sheet 30 preferably consists of a fabric coupled with a plastic film, said coupling being preferably carried out by means of sticking, thermal welding or other known techniques.

Said fabric of said permeable sheet 30 is preferably either a natural fibre fabric, such as hemp or cotton, or a synthetic fibre fabric, such as acrylic, polyester or polyamide fibres; more preferably said fabric of said permeable sheet 30 is polyester .

Said plastic film of said permeable sheet 30 preferably consists of a plastic, for instance polyethylene or PVC, film with very small pores, preferably having diameter < 2 micron. Some parameters of the permeable sheet 30, and the corresponding illustrative values, are reported in the following table:

Said double covering 3, 30 of the biofiltering unit 1 performs the combined action of an element preserving the humidity of said filtering matrix 50, specifically performed by said cover 3, and of an additional filter for the gas wastes, specifically performed by said permeable sheet 30; particularly, said permeable sheet 30 also allows to remove bioaereosols because of the plastic film structure, whose pores are smaller than the bioaerosols spores and that, therefore, acts as a "sieve" and, consequently, as a filter. Said double covering 3, 30 of the biofiltering unit 1 further allows an optimal protection against the atmospheric agents, such as UV rays, wind, rain and other inclement weather conditions, as well as the full segregation from the surrounding environment, which helps the installation thereof possibly also in zones next to areas of human settlement.

FIG. 7 clearly shows the pipe forming the inlet of the biogas to be treated in the biofiltering unit 1, the ventilation bottom 40 and the supporting system 33 of said permeable sheet 30.

Said ventilation bottom 40 substantially consists of a holed paving made of modular grid tiles 41, preferably made of glass-fibres reinforced polypropylene and preferably having plan unit size of 500 x 500 mm, and of frusto-conical supports 43, preferably made of polypropylene and preferably being 500 mm high; thanks to the implemented construction type, said ventilation bottom 40 is removable, this helping the cleaning and maintenance operations of the biofiltering unit, and it turns out to be suitable to assure a low speed proper passage of the forced air as well as the homogeneous distribution of said air inside said biofiltering unit.

A constructive variant of the ventilation bottom 40 can consist of steel plates.

Said biofiltering matrix 50, preferably consisting of a preconstituted mixture, comprising ligno-cellulosic oversize material coming from composting processes, compost, peats and other materials, with the addition of additives such as microbial inocula and/or organic/inorganic specific activators for optimising the biological activity inside the filtering material, is placed above said ventilation bottom 40.

An example of biofiltering matrix 50 has the following characteristics:

In order to optimise the ventilation of said biofiltering matrix, a forced air ventilation, for instance by means of a fan (not shown), is preferably provided. Said supporting system 33 of the permeable sheet 30 is illustrated in detail with reference to FIGS. 8 and 9.

Said supporting system 33 consists of a series of truss-beams

23.

Said truss-beams 23 are preferably made of stainless steel; said truss-beams 23 are provided to be removable to facilitate the recurring replacement of the biofiltering matrix 50.

The permeable sheet 30 is placed on the structure formed by the truss-beams 23 and anchored, along its perimeter, to a suitable frame 25, said frame too being preferably made of stainless steel and fastened to container 2 by means of known methods, for instance by welding.

Said support system 33 of the permeable sheet 30 has optimum lightness characteristics, combined with a good assembly easiness and with realisation and installation limited costs. A constructive variant of the supporting system 33 of the permeable sheet 30 can consist of independent frames to be applied to the frame of the truss-beams 23; in this case the anchoring frame 25 of the permeable sheet will appear to be unnecessary and, in its whole, the structure will become even lighter .

Said support system 33 also supports the wetting system 60, used for wetting the filtering matrix 50 of the biofiltering unit 1 and shown in detail in FIGS. 10 and 11.

Said biofiltering matrix 50 can be periodically wetted, for instance with water preferably coming from a nozzle system 66, preferably having a temporised opening, mounted above the surface of said biofiltering matrix 50; the wetting frequency and intensity can be settled and changed depending on the requirements of the specific application.

With reference to FIGS. 12 and 13 it is finally visible the condensate and leachate collection and discharge system 5 of the biofiltering unit 1. Some tests carried out by the Applicant proved that the permeable sheet 30, when wetted, has a considerable increase in terms of yield; therefore, said system 5 is necessary to remove the condensate excess that tends to form inside the double covering 3, 30, and more precisely between said permeable sheet 30 and said cover 3, in order to guarantee the optimum humidity and temperature conditions of the biofiltering unit 1.

Specifically, the above-mentioned increase in terms of yield, obtained thanks to the presence of the wetted permeable sheet 30, is in the order of 10-30 % with optimum humidity and temperature conditions of the biofiltering unit 1, that is with humidity of 50-65 % and temperature of 20-60°C.

With reference to FIG. 13, the condensate storage gutter 35 of the biofiltering unit 1 is visible; said gutter is connected to the collection and discharge system 5.

Preferably, the condensate and leachate discharge pipe is equipped with a threaded ring.

The activity of the biofiltering unit 1 can be monitored through the measurement of parameters such as, for instance, flowrate, concentration and temperature of the inlet biogas; concentration of the outlet biogas; temperature, relative humidity and pressure gradient between inlet and outlet of the biofiltering matrix.

Reference parameters are considered to be the following ones:

- inlet biogas temperature: 10-60°C

- biofiltering matrix temperature: 20-60°C

- biofiltering matrix relative humidity: 45-80 %

- pressure gradient between inlet and outlet of the biofiltering matrix: 5-40 mmH₂0

And furthermore: Inlet biogas properties

Minimum value Maximum value Measurement unit methane concentration 1 25 % v/v

:arbon dioxide concentration 3 40 % v/v

axygen concentration 8 19 % v/v

biofiltering unit inlet flowrate 20 200 m³/h

methane removal efficiency 20 95 %

The monitoring/adjusting system of the operative main parameters of the biofiltering unit 1 is preferably housed, according to the preferred embodiment of the invention, inside the housing 7.

Said system can be put in connection with the chimney 9, specifically with the sleeve 9b for sampling and measurement of the emissions, and also with the wetting system 60, so as to obtain an integrated and fully automated detecting/monitoring/adjusting system for controlling the management parameters. In case, the methane concentrations at the entrance into and at the exit from the system, the inlet biogas flowrate, the amounts of wetting water, the inlet biogas and the biofiltering material temperatures could be monitored.

Said system can further comprise a GSM modem, for transmitting data by sms .

Said system is supplied by the photovoltaic solar panel 8 placed on the top of the biofiltering unit 1.

The Applicant has experimentally tested the validity of the present invention; the operating characteristics of the biofiltering unit 1 according to the invention are reported in the following table: Inlet biogas Inlet methane Methane Outlet methane flowrate concentration removal concentration [m³/hj [%] efficiency [%]

[%]

50 15 84 2.40

100 15 77 3.45

150 15 70 4.50

150 25 70 7.50

280 25 51 12.25

330 25 44 14.00

The Applicant has further tested that the biofiltering unit 1 according to the invention guarantees the removal of smelling impact; the olfactometric surveys carried out according to UNI EN 13725-2004 regulation, have in fact highlighted a removal efficiency higher than 90 % of the inlet olfactometric units.

The data relevant to the performance of four different biofiltering units having been operated in parallel are reported in the following table:

The Applicant has further tested that the biofiltering unit 1 according to the invention helps to reduce the greenhouse gas (GHG) emissions since a single biofiltering unit allows to remove the methane contained in the landfill biogas in measure up to about 4,400 tons/year of C0₂ equivalent, varying depending on the biogas flowrate and on the inlet methane concentration; it has been however assessed that the average contribution of a single biofiltering unit according to the invention is equivalent to the positioning of about 1,700 trees/year on a land area of 1.7 hectars.

The possibility to achieve also the removal of bioaerosols appeared within the experiments carried out by the Applicant, since the sizes of the bioaerosols spores, and particularly the sizes of the Aspergillus fumigatus spores, are larger than the microholes of the plastic film of the permeable sheet 30 and they are therefore kept on the surface thereof. The check of the right functioning of the biofiltering unit 1 according to the invention was also performed by means of periodical measurements of the microbic colonisation status of the matrix; more precisely, both the quantitative evolution of the microbic consortium expressed as "total bacterial load" and the quantitative evolution of the microbic consortium expressed as "total mycetic load" have been determined.

As far as the microbic colonisation is concerned, from a point of view of both the bacterial load and the mycetic load, no poisoning symptoms of the biofiltering matrix due to the biogas subjected to treatment have been found; the evolution of the consistence of the microbic populations seems to indicate a selection in favour of the bacterial component, to which the methanotrophic micro-organisms belong .

In conclusion the Applicant has found that, surprisingly, the double covering 3, 30 helps to remove methane.

Such surprising result is confirmed by the microbiological analyses performed on the aqueous film that is formed on the intrados of the permeable sheet applied on the biofiltering unit, reported in the below table, from which a significant presence of active microbic populations is deduced: Sample Parameter Result

Permeable sheet Total bacterial load 500 CFU/cm² buffer Total mycetic load 1 CFU/cm²

The Applicant believes that the formation of said aqueous film, consisting of water droplets containing active microbic colonies, considerably contributes to the improvement of the invention biofiltering unit efficiency in terms of biogas purification, that is in terms of removal of methane therein contained, since inside said film an additional filtration would occur.

A plant for removing landfill biogas and/or bioaerosols according to the present invention comprises at least one biofiltering unit 1.

However, in order to meet the actual requirements for biogas treatment, it is possible to combine two or more biofiltering units 1 as above described; thanks to the modularity of said biofiltering units, the system turns out to be easily adaptable to the specific applications and the overall treatment efficiency can be improved.

Said plant will comprise, furthermore, at least a feed system of the biogas to be treated, a system of release to the atmosphere of the purified biogas and in case other ancillary apparatuses, such as mixers and fans, known to the person skilled in the field and, therefore, not described in detail herein .

In particular, depending on the application needs, said plant can be provided with a predilution device, or mixer, of the biogas to be treated with ambient air, in order to produce a mixture wherein the concentration of the methane entering said biofiltering unit 1 is approximately within the threshold of 25 % v/v; said biogas is fed to said mixer preferably through a fan, and said biogas/air mixture is then conveyed to the biofiltering unit 1, which carries out the purification thereof and from which a purified gas stream containing a reduced methane amount is discharged.

The predilution system of the inlet biogas uses the Venturi principle. The object of the dilution with ambient air, besides lowering the inlet methane concentrations, is also to be able to enrich the biogas with oxygen, in case the concentration of said element is too low.

Moreover, optionally, said plant can also comprise an external energy source, whose main function will be to supply electrical apparatuses (for instance, thermal exchangers) that will allow to maintain the inlet biogas temperature, and therefore the biofiltering material temperature too, always optimal, with the result of maintaining high performances of methane removal.

As to the process for removing methane from landfill biogas streams and/or bioaerosols according to the invention, it comprises the following steps:

providing at least one container 2 having the above- described features;

implementing a ventilation bottom 40;

introducing a biofiltering matrix 50 inside said container 2, on said ventilation bottom 40;

placing a supporting system 33 inside said container 2, above said biofiltering matrix 50;

placing a permeable sheet 30 on said supporting system 33 and a cover 3 on said permeable sheet 30, thus forming a double covering for the container 2 and therefore creating at least one biofiltering unit 1;

feeding a biogas stream to be treated into said at least one biofiltering unit 1, said stream containing a methane amount in the range 1 - 25 % v/v;

allowing the removal of methane from landfill biogas and/or bioaerosols inside said at least one biofiltering unit 1 up to a methane value in the range 0.2 - 13.2 % v/v; controlling the formation of condensate that tends to form inside said double covering 3, 30, and more precisely between said permeable sheet 30 and said cover 3, by means of a collection and discharge system 5, thus assuring the optimum humidity and temperature conditions of the biofiltering unit 1; and

allowing the purified gaseous stream to be discharged from said at least one biofiltering unit 1, said purified gaseous stream containing a methane amount in the range 0.2 - 13.2 % v/v.

Optionally, said process for purifying biogases and/or removing bioaerosols further comprises the following steps: providing a wetting system 60, supported by said supporting system 33 and equipped with a nozzle system 66, preferably having a temporised opening;

periodically spraying said biofiltering matrix 50 with water coming from said nozzle system 66.

Optionally, said process for removing landfill biogases and/or bioaerosols according to the invention further comprises the following step:

providing a detecting/monitoring/adjusting system of the main operative parameters connected to said at least one biofiltering unit 1.

providing a predilution system of the biogas to be treated with ambient air, in order to produce a mixture wherein the concentration of the methane entering said at least one biofiltering unit 1 is approximately within the threshold of 25 % v/v.

From the description above explained in detail, the advantages ensuing from the biofiltering unit, as well as from the relevant plant and process, used for removing the landfill biogases according to the present invention result clear; in particular:

the removal of methane emissions into the atmosphere that, as known, cause a greenhouse effect 21 times more dangerous than that caused by C0₂; the methane reduction achieved with the present invention guarantees a C0₂ credits gain obtainable by following international standards such as "Voluntary Carbon Standard" and "Carbon Emission Reduction" through the implementation of possible "Clean Development Mechanism - CDM" projects;

the minimisation of the biogas dispersion into the surrounding environment, with consequent reduction of environmental risks as well as of those linked to the safety of areas near the landfill;

the removal of the biogas odorous impact, higher than 90 % of the inlet olfactometric units;

the high flexibility of the system to the different landfill operating conditions, even in combination with biogas treatment conventional systems;

the mobility of the system, which can be positioned near the biogas tapping system;

the modularity of the system, which allows the sizing thereof to meet the actual treatment needs and the easy adjustment thereof to the specific application;

the installation and utilisation simplicity;

the limited operational and maintenance costs; and the continuous and reliable functioning.

A particularly advantageous aspect of the biofiltering unit, as well as of the relevant plant and process, used for removing the landfill biogas according to the present invention consists in its excellent operative flexibility; in particular : the biofiltering unit according to the invention allows to treat the biogas streams in the production initial stage, when the energy recovery systems can not be employed yet;

the biofiltering unit according to the invention makes the biogas reduction possible at the stage following the landfill closure, when the conventional systems are no longer used; and

the biofiltering unit according to the invention can be employed in the production final stage in order to optimise the biogas purification together with the energy recovery systems, thus allowing the purification of biogas streams not suitable to the energetic recover.

It is clear that the biofiltering unit, as well as the relevant plant and process, used for removing the landfill biogases according to the present invention, herein described by means of preferred embodiments given by way of non- limiting example, can be modified according to ways known to the person skilled in the technical field without, for this reason, departing from the scope of the present invention.

Claims

1. A process for removing methane from landfill biogas streams and/or bioaerosols comprising the following steps:

providing at least one container (2) equipped with a ventilation bottom (40);

introducing a biofiltering matrix (50) inside said container (2), on said ventilation bottom (40);

placing a supporting system (33) inside said container (2), above said biofiltering matrix (50);

placing a permeable sheet (30) on said supporting system (33) and a cover (3) on said permeable sheet (30), thus_' forming a double covering for the container (2) and therefore creating at least one biofiltering unit (1);

feeding a biogas stream to be treated into said at least one biofiltering unit (1), said stream containing a methane amount lower than 25 % v/v;

allowing the removal of methane from landfill biogas and/or bioaerosols inside said at least one biofiltering unit (1) up to a methane value in the range 0.2 - 13.2 % v/v;

controlling the formation of condensate that tends to form inside said double covering (3, 30), and more precisely between said permeable sheet (30) and said cover (3), by means of a collection and discharge system (5), thus assuring the optimum humidity and temperature conditions of the biofiltering unit (1); and

allowing the purified gaseous stream to be discharged from said at least one biofiltering unit (1), said purified gaseous stream containing a methane amount in the range 0.2 - 13.2 % v/v.

2. A process according to claim 1, further comprising the following steps:

providing a wetting system (60), supported by said supporting system (33) and equipped with a nozzle system (66), preferably having a temporised opening; periodically spraying said biofiltering matrix (50) with water coming from said nozzle system (66).

3. A process according to claim 1 or 2, further comprising the following step:

providing a detecting/monitoring/adjusting system of the main operative parameters connected to said at least one biofiltering unit (1), preferably connected to said wetting system (60) and to a chimney (9), placed on said biofiltering unit (1), for sampling and measuring the emissions.

4. A plant for removing methane from landfill biogas streams and/or bioaerosols comprising at least one biofiltering unit (1) through the process according to one of the claims 1 to 3.

5. A biofiltering unit (1) for removing methane from landfill biogas streams and/or bioaerosols through the process according to one of the claims 1 to 3, comprising: a container (2) equipped with an ventilation bottom (40) and containing a biofiltering matrix (50);

a supporting system (33) placed inside said container (2), above said biofiltering matrix (50);

a permeable sheet (30) set on said supporting system

(33) ;

a cover (3) set, in its turn, on said permeable sheet (30) , so as to form a double covering (3, 30) for said container (2); and

a collection and discharge system (5) to control the formation of condensate that tends to form inside said double covering (3, 30), and more precisely between said permeable sheet (30) and said cover (3), so as to guarantee the optimum humidity and temperature conditions of the biofiltering unit (1) .

6. A biofiltering unit (1) according to claim 5, further comprising a wetting system (60), supported by said supporting system (33) and equipped with a nozzle system (66), preferably having a temporised opening, for wetting said biofiltering matrix (50).

7. A biofiltering unit (1) according to claim 5 or 6, further comprising a detecting/monitoring/adjusting system of the main operative parameters connected to said biofiltering unit (1), preferably connected to said wetting system (60) and to a chimney (9), placed on said biofiltering unit (1), for sampling and measuring the emissions.

8. A biofiltering unit (1) according to claim 5, wherein said permeable sheet (30) consists of a fabric coupled to a plastic film, said coupling being preferably carried out by means of sticking or thermal welding.

9. A biofiltering unit (1) according to claim 8, wherein said fabric of said permeable sheet (30) is a fabric of natural fibre or of synthetic fibre, preferably of synthetic fibre, more preferably of polyester.

10. A biofiltering unit (1) according to claim 8, wherein said plastic film of said permeable sheet (30) consists of a plastic, preferably a polyethylene, film with very small pores having diameter < 2 micron.