Gas and liquid extraction system and method

A gas and liquid extraction system and method to efficiently capture the gas generated from the decomposition of organic matter, generally referred herein as the biomass, present in a landfill is disclosed herein. The system and method taking in account the presence of high amount of water and proposing landfilling methodology to allow the efficient capture of an as high as possible amount of gas from each and every ton of biomass contained in the landfill which is heterogeneous, anisotropic in nature and unsteady in term of fluid flow behavior.

FIELD

The present invention generally relates to gas and liquid extraction. More specifically, embodiments of the present invention are concerned with systems and methods to extract gas produced from anaerobic decomposition of organic matter and with systems and methods to extract liquid occurring from rain water, water content in organic matter and condensate.

BACKGROUND

The generation of gas produced from the anaerobic decomposition of organic matter is a natural process occurring all around the world.

In recent decades the organic matter generated as by-product of human activities has been stored in large cells usually confined with under and overlaying membranes. The overlaying membrane could be described as daily cover and final cover.

The organic matter thus trapped is slowly drying since no more rainfall precipitations can penetrate the impermeable body of organic matter. Even during the drying period, the organic matter can produce gas as a by-product of anaerobic decomposition, but to a limited extend and for a limited period of time, because such decomposition requires water to occur.

The microbiological cycle of gas production requires as high moisture and temperature as possible to activate and accelerate the microbiological decomposition of the organic matter under anaerobic conditions.

Before this discovery, it was found that the artificial introduction of water into the biomass of a landfill, even trough it includes impermeable membranes at the bottom and at the top, improves the amount and duration of gas production from organic matter. This was generally called bioreactor landfill system.

However, the presence of standstill water below the surface level of the biomass of a landfill generally prevents the efficient capture of landfill gas using vertical wells. Therefore, the efficient extraction of liquid present in the biomass and of gas generated from the decomposition of the organic matter using conventional methods is more difficult.

It has also been observed that the internal pressure profile varies in the landfill following cycles of high and low positive pressure generating a pressure wave over time which has varying amplitude and a varying frequency.

The pressure wave frequency and amplitude varies according to internal properties of the body of waste such as waste type, waste density, waste moisture content, waste porosity, waste layering, and the waste age. Because these properties of waste layers and composition are not constant within the body of the landfill it creates anisotropic and heterogeneous conditions. These inherent conditions coupled with landfill internal variation of temperature, moisture content, organic matter content and waste distance from the atmosphere cause the pressure wave amplitude and frequency, at a given point, to be unsteady over time. This means that any pressure wave amplitude and frequency is difficult to predict.

However, observations are showing that:

Pressure wave formation is essentially caused by a sequence of pressure build-up in the landfill due to organic matter decomposition followed by pressure release towards the atmosphere (the zone of lowest positive pressure) through micro and/or macropores creating pathways to ensure that the landfill gas escape towards the atmosphere, whether located upwards or sideways.

Younger waste with a combination of high temperature, high organic matter content, high moisture content under a predetermined waste density, depth and porosity will have a high pressure wave frequency (seeFIG. 1).

Older waste with a combination of low temperature, low organic matter content, low moisture content under a predetermined waste density, depth and porosity will have a low pressure wave frequency (seeFIG. 2).

Older waste may also have lower amplitude than the amplitude of younger waste because of lower pressure built-up caused by preferential path created over time within the network of micro and macro pores towards the atmosphere (seeFIG. 3).

Also, at a predetermined waste density, depth and porosity, the absence or depletion of moisture, temperature, organic content or a combination thereof will create low frequency/low amplitude pressure wave and indicate a reduction in landfill gas flow rate potential.

DETAILED DESCRIPTION

In accordance with an illustrative embodiment, there is provided a gas and liquid extraction system to be installed in a landfill to extract gas and liquid from the biomass present in the landfill; the extraction system comprising:

a perforated well so positioned in the biomass as to have a downward slope;

a pipeline connected to the perforated well and having a downward slope; the pipeline including a liquid extraction mechanism to extract liquid from the sloping pipeline;

a vacuum source connected to the pipeline to selectively apply a vacuum to the pipeline and to the attached perforated well to extract gas from the sloping pipeline.

In accordance to another aspect, there is provided a gas and liquid extraction system to be installed in a landfill to extract gas and liquid from the biomass present in the landfill; the extraction system comprising:

a plurality of perforated wells so positioned in the biomass as to have a downward slope;

a vacuum source;

a pipeline system interconnecting the plurality of perforated wells to the vacuum source via individual actuating valves; the pipeline system having a downward slope and including a liquid extraction mechanism to extract liquid from the sloping pipeline;

a sensing device provided in the vicinity of a perforated well;

a receiver-controller so configured as to control the actuation of the valves and to receive data from the sensing device;

wherein the receiver-controller determines the actuation of the valves depending on the data received from the sensing device.

In accordance to another aspect, there is provided a gas and liquid extraction method to extract gas and liquid from a biomass provided in a landfill; the extraction method comprising:

installing a perforated well on the upper part of a first biomass layer; the perforated well being so installed as to have a downward slope;

providing a second biomass layer on top of the first biomass layer;

connecting the perforated well to a vacuum source via a pipeline including a liquid extraction mechanism;

selectively applying a vacuum to the perforated well to extract gas from the biomass; and

extracting liquid from the pipeline.

In accordance to another aspect, there is provided a gas and liquid extraction method to extract gas and liquid from a biomass provided in a landfill; the extraction method comprising:

installing a first perforated well on the upper part of a first biomass layer; the first perforated well being so installed as to have a downward slope;

providing a second biomass layer adjacent to of the first biomass layer;

connecting the first perforated well to a vacuum source via a first pipeline including a liquid extraction mechanism;

selectively applying a vacuum to the first perforated well to extract gas from the biomass;

extracting liquid from the first pipeline;

when the second biomass layer reaches a predetermined size:installing a supplemental perforated well adjacent to the first perforated well;providing a third biomass layer adjacent to the second biomass layer;connecting the supplemental perforated well to the vacuum source via a supplemental pipeline;selectively applying a vacuum to the supplemental perforated well to extract gas from the biomass; andextracting liquid from the supplemental pipeline.

The term “about” is used to indicate that a value includes an inherent variation of error for the device or the method being employed to determine the value.

It is to be noted that the expression “perforated well” is to be construed herein and in the appended claims as any tubular member configured to allow gas and liquid through its surface.

It is to be noted that the expression “adjacent well” is to be construed herein and in the appended claims as a well that is spaced apart from another well, either horizontally, vertically or diagonally.

It is to be noted that the expression “adjacent well” is to be construed herein and in the appended claims as a well that has a distance from another well that could vary according to landfill characteristics.

Similarly, the term “adjacent” when used in conjunction with the terms “biomass” or “biomass layer” is to be construed herein and in the appended claims as meaning being horizontally, vertically or diagonally adjacent.

It is also to be noted that the expressions “negative pressure”, “depression” and “vacuum” are used interchangeably in the present disclosure.

Generally stated, illustrative embodiments disclose a gas and liquid extraction system to efficiently capture the gas generated from the decomposition of organic matter, generally referred herein as the biomass, present in a landfill; taking in account the presence of high amount of water and proposing landfilling methodology to allow the efficient capture of an as high as possible amount of gas from each and every ton of biomass contained in the landfill.

Horizontal Well Technology

FIG. 4of the appended drawings illustrates a landfill10provided with a gas and liquid extraction system12according to a first illustrative embodiment.

The gas and liquid extraction system12includes a gently sloping perforated well14connected to a pipeline16, a liquid extraction mechanism in the form of two inverted siphons18, a vacuum pump20connected to the pipeline16via a main valve21located downstream of the siphons18. A manifold gas valve23is provided between the two siphons18and a leachate & gas valve25is provided between the landfill10and the siphons18.

Since liquid can enter the gently sloping perforated well14and be evacuated via the siphons18as will be described hereinbelow, the biomass located above the well14is an unsaturated biomass22, while the biomass located below the well14is a saturated biomass.

As a non-limiting example, the gently sloping perforated well14can be a 10 inches (about 25 cm) generally cylindrical hollow tube provided with perforations and so installed in the biomass as to present a slope of about 2 degrees. Optionally, the perforated well14can be surrounded by porous drainable material to increase the nominal diameter of the perforated well14.

Turning now toFIG. 5, one of the inverted siphons18is illustrated. The siphon18is a J-shaped tube including a downward straight portion23and an integral curved portion24. As is generally known, the liquid level26in the straight portion23is the same as the level28of the free open end30of the siphon18. It is to be noted that the vacuum pump20is not operational.

FIG. 6illustrates the siphon18ofFIG. 5when the vacuum pump20is energized carrying both gas and liquid from the landfill. When this is the case, the liquid level26in the straight portion23is raised by the addition of liquid from the pipeline16and by the vacuum created in the pipeline16. Additional increase of the liquid level26forces the liquid from the integral curved portion to exit the siphon18by the open end30(see arrow32). The leachate is therefore discharged from the siphon by gravity.

In operation, the vacuum pump20creates a vacuum in the pipeline16to therefore draw the gas produced in the landfill through the gently sloping perforated well14and the pipeline16. This gas may then be supplied to a system that either destroys or transforms the gas into heat and/or power. These destruction and transformation systems are believed known to those skilled in the art and will therefore not be discussed in details herein.

The liquid present in the landfill10above the perforated well14, is evacuated through the perforated well14, the pipeline16and the inverted siphons18under the influence of the gravity and of the suction generated by the vacuum pump20to thereby leave an unsaturated biomass above the level of the perforated well14.

The liquid and gas getting in the pipeline16via the perforated well14are therefore separated by the combined action of the siphons18and the vacuum pump20.

A main valve21is installed on the pipeline16, usually downstream from the last siphon18. This valve21isolates the landfill gas and liquid from the vacuum pump. Valve21is opened only to create a depression on the perforated well14for gas and liquid to escape more rapidly from the landfill. Under passive conditions, valve21and23are closed leaving valve25to drain out the excess water from the landfill through the siphon. Once it is decided to extract gas from the landfill, valves21and23are fully opened and valve25slowly primes the pipeline16with the suction applied by the vacuum pump20and regulates the flow of gas in the pipeline16. The priming is usually done at the beginning of the gas extraction, when the perforated well14and the pipeline16will carry additional liquid being trapped in the surroundings of the well14. During the priming, the pipeline16may flow full of liquid for a time to empty the surrounding area of the perforated well over the whole longitudinal distance of the perforated well and for lateral distances that can vary depending on the hydraulic conductivity of the porous mass. The priming is generally done slowly and carefully since the vacuum applied by the vacuum pump to the siphon18increases the speed of the fluid in the pipeline16and may cause fluid to bypass the siphons and travel to the vacuum pump, which may damage the pump. A second siphon is usually installed to overcome the first siphon by-pass.

Of course, one skilled in the art will understand that the number of inverted siphons18could vary depending on the configuration and size of the landfill.

It will be understood that the use of a generally horizontally laid perforated well14used in illustrative embodiments is interesting since the horizontal well has a greater contact surface with the saturated biomass leading to a more efficient liquid extraction and also a greater contact with the unsaturated biomass, leading to a more efficient gas extraction.

An example of a method used to install the gas and liquid extraction system12in the landfill10will now be described.

The gently sloping perforated well14is laid out when a thickness of about 3 to about 5 meters of waste biomass material has been placed in the landfill10. The well14is then buried with waste material to allow the gas and liquid extraction system12to be started.

In other words, a perforated well is installed on the upper part of a first biomass layer and a second biomass layer is provided on top of the perforated well.

FIGS. 7 to 9are front elevational views illustrating the introduction of a perforated well14in a liquid saturated landfill50. More specifically,FIG. 7illustrates the landfill50before the introduction of the well14.FIG. 8illustrates the landfill50when the well14is introduced. The introduction of the well14triggers the drainage of the liquid out of the biomass22, i.e. the organic biomass waste material, done during the priming of the well and gradually lowers the level of the liquid saturated biomass.FIG. 9illustrates the landfill50after the well14has been introduced for an adequate time. The adequate time is at least partially determined by the natural hydraulic conductivity of the porous biomass.

It is to be noted that any liquid that flows in the biomass, for example rainfall precipitations or liquid present in the additional biomass put on top of the existing drained biomass, is drained out with the gaseous fluid and separated by the siphons as described hereinabove.

It is also to be noted that should the vacuum be stopped in a well for a sufficiently long time, it might be necessary to re-prime the well as discussed above.

It is believed that one skilled in the art will be in a position to determine the level of vacuum applied to the carrying pipeline16by the vacuum pump20. The following considerations may be taken for the determination of the vacuum level:The vacuum level of the vacuum pump should compensate for the friction head losses in the pipeline;The nominal vacuum level at the entry of the well should be higher than about five time the average pressure found in the landfill to compensate for the friction head losses in the porous biomass matrix; andThe nominal vacuum level at the entry of the well is set according to the naturally occurring pressure variations at the depth of the well.

The following features are believed interesting in the gas and liquid extraction system described hereinabove:The length of the straight portion of the siphons is determined by the suction applied to the pipeline by the vacuum pump;Since the curved portion of the siphons is filled with liquid, no outside gas is introduced in the pipeline;The oxygen level should not exceed about 1% in volume; the oxygen level is an indication of aerobic metabolic conversion of biomass or the presence of preferential flow paths from the atmosphere to the well; in both cases the presence of oxygen when detected causes a reduction in the methane fraction of the landfill gasThe pipeline should not have humps or slumps that exceed the nominal pipeline diameter to allow liquid flow by gravity;The diameter of the tube of the siphons shall be greater than or about the same than the nominal diameter of the pipeline;The well, illustrated herein as a perforated pipe, could be replaced by any element acting as a drainage conduit for the liquid and gas, connected to an external pipeline; andThe siphon also extracts any liquid condensate that may be produced as the gas flows in the pipeline.

Turning now toFIG. 10of the appended drawings, a gas extraction system100according to a second illustrative embodiment will be described. It is to be noted that since the gas extraction system100is very similar to the gas extraction system12ofFIG. 4, only the differences between these systems will be described hereinbelow, for concision purpose.

The gas extraction system100includes two vertically distanced and gently sloped perforated wells102and104respectively connected to a common vacuum pump110via pipelines106and108and via valves110,112,114,116,118and120.

This arrangement allows the landfill10to be deeper while maintaining an efficient gas extraction. Of course, more than two vertically distanced and gently sloped perforated wells could be used.

Turning now toFIG. 11of the appended drawings, a gas extraction system200according to a third illustrative embodiment will be described. It is to be noted that since the gas extraction system200is very similar to the gas extraction system100ofFIG. 10, only the differences between these systems will be described hereinbelow.

The gas extraction system200includes three horizontally distanced perforated wells202,204and206provided in the same landfill208. The wells202-206are connected to the same vacuum pump (not shown) via pipelines, siphons and valves (also not shown). This arrangement allows the landfill208to be wider while maintaining an efficient gas extraction. Of course, more than three horizontally distanced perforated wells could be used. Similarly, more than one vacuum pump could be used.

Turning now toFIG. 12of the appended drawings, a gas extraction system300according to a fourth illustrative embodiment will be described. Again, since the gas extraction system300is very similar to the gas extraction system200ofFIG. 11, only the differences between these systems will be described hereinbelow.

The gas extraction system300includes a first row of three horizontally distanced perforated wells302,304and306and a second row, vertically distanced from the first row, of three horizontally distanced perforated wells308,310and312, all provided in the same landfill314. The wells302-312are connected to the same vacuum pump (not shown) via pipelines, siphons and valves (also not shown). This arrangement allows the landfill314to be both wider and deeper while maintaining an efficient gas extraction. Of course, more than three horizontally distanced perforated wells and more than two rows of wells could be used. Similarly, more than one vacuum pump could be used.

When multiple vertically separated gently sloping horizontal wells are installed as illustrated inFIGS. 10 and 12, it has been found interesting to vertically separate the wells of a distance not exceeding about 10 meters since this facilitates the installation of the horizontal well placed in a trench as the landfill is filled with biomass.

Similarly, when multiple horizontally separated gently sloping horizontal wells are installed as illustrated inFIGS. 11 and 12, it has been found interesting to horizontally separate the wells of a distance not exceeding about 30 meters.

It is to be noted that the vacuum pump20illustrated herein could be replaced by any adequate source of vacuum.

When multiple perforated wells are used in the same landfill or in adjacent landfills, a single vacuum pump can be connected thereto via a manifold.

It is to be noted that while all the multiple perforated well illustrative embodiments have the perforated well laid in the same direction, perforated wells laid in different directions could be advantageous in some landfill configurations.

Turning now toFIG. 13of the appended drawings, a gas extraction system350according to a fifth illustrative embodiment will be described.

The system350includes a sloped perforated well352, connected to a pipeline354that directs the gas in a first direction (see arrow356) towards a vacuum pump358. Because the perforated well352is sloped, the leachate is directed in a second, opposite, direction (see arrow360) towards a sink hole362. In other words, the leachate is discharged from the horizontal well352into the sink hole362. A sump pump364is used to remove the leachate from the sink hole362. It is to be noted that the sink hole362is hermetically closed to the atmosphere.

This configuration is interesting when the perforated well is provided under the natural soil level366and that the vacuum pump358is provided above the natural soil level366.

Phase Implementation of Horizontal Wells

Turning now toFIGS. 14 to 16of the appended drawings the phase implementation of horizontal wells will be described.

As discussed hereinabove with reference toFIGS. 10 to 12, the size of landfills is generally such that more than one horizontal well are often required. However, the horizontal wells do not required to be installed at the same time and may be added as the biomass present in the landfill increases.

FIG. 14illustrates, in a front cross-sectional view, a first phase implementation of horizontal wells provided in a landfill400. Three phases402,404and406are completed in the landfill400. Phases402and404extend about 12 meters below ground while phase406extends about 12 meters above the ground. Each phase402-406includes a corresponding horizontal well408,410and412similar to the perforated well14described hereinabove. Of course, as described hereinabove, pipelines, a liquid extraction mechanism, valves and a vacuum source (all not shown inFIG. 14) are also provided to extract the produced gas in phases402-406.

The landfill400is ready to receive a third phase414provided adjacent to phases404and406.

FIG. 15illustrates, in a side cross-sectional view, a second phase implementation of horizontal wells provided in a fully filled landfill500including fourteen completed phases. Indeed, each layer502-506includes a number of side-by-side phases.

Each layer502-506is about 12 meters deep and about 400 meters long. Of course, layers can be more or less deep and more or less long than illustrated.

FIG. 16illustrates a third phase implementation of horizontal wells provided in a landfill600. Three side-by-side phases602-606are provided. These phases share3horizontal wells608-612all connected to a manifold618via pipelines, siphons and valves (not shown). The gas extracted by the vacuum pumps620is either supplied to a combustion system622or to an energy producing system624.

Again, the landfill600is ready to receive other phases besides, below and above phase606. Accordingly, the manifold618Includes supplemental inlets626.

Control of the Individual Horizontal Wells to Optimize Gas Production

As mentioned hereinabove, it has been observed that the internal pressure profile will vary in the landfill following cycles of high and low positive pressure generating a pressure wave over time which has a varying amplitude and a varying frequency

In order to optimize landfill gas production under these heterogeneous, anisotropic fluid conductivity characteristics and unsteady fluid flow conditions, commonly found in all landfills it has been found interesting to use a pressure feedback mechanism.

The feedback mechanism includes pressure measuring devices, such as piezometers, installed between the landfill level and the well depth. For example, the pressure measuring devices can be installed substantially at the same level as the perforated well. The piezometers provide a feedback on the amplitude of internal pressure variation.FIG. 17schematically illustrates a piezometer700provided between two horizontal wells702and704.

The piezometers are used to fingerprint of the pressure variation amplitude and pressure wave length at a given point in the landfill, over time.

A plurality of piezometers is used to map the different landfill gas conditions prevailing in the landfill at rest, i.e. submitted to atmospheric pressure only.

The feedback mechanism also includes well heads equipped with a depression device, for example a vacuum pump, which creates a depression in the body of waste all along the perforated wells, as described hereinabove. These vacuum pumps create a new gradient and a new direction for the landfill produced gas to escape.

The new depression conditions, caused by the vacuum pump, dampers the amplitude of the pressure wave over time and at a given point in space. When this is observed, a direct relationship can be made between landfill gas production and dampening of the pressure wave amplitude over time.FIG. 18illustrates the pressure wave amplitude over time when vacuum is applied to the horizontal wells.

As can be seen fromFIG. 18, it has been observed that the landfill submitted to the depression of the wells show negative pressure peaks measured at the pressure monitoring device

It is to be noted that the head losses between the measured point and the well on which a depression is applied can be defined as the difference between the suction head in the well and the pressure (negative or positive) at the measured point at any given time minus the pressure at rest (or under no depression.

When the suction is done over a long period of time, the landfill gas may start showing depletion that could be due to a reduction of moisture content, a reduction of temperature, a preferential flow from the atmosphere and/or a reduction of organic waste to be decomposed.

Turning now toFIG. 19of the appended drawings a landfill800provided with nine perforated horizontal wells802-818and with six piezometers820will be described. As can be seen from this figure, the piezometers820are provided between adjacent wells provided on a same level and are connected to a common receiver-controller822receiving data therefrom. Each of the wells802-818is coupled to a vacuum source (not shown) via a corresponding actuating valve (not shown) that is so connected to the controller806as to be independently actuated.

Using the horizontal gas and liquid extraction wells802-818coupled to the piezometers820, it is possible to understand the landfill behaviour even for these inherent anisotropic and heterogeneous conditions. The landfill behaviour can be mapped and can provide useful insights for the extraction of landfill gas despite the unsteadiness of its naturally occurring generation of landfill gas.

For example, the mapping can be achieved by submitting the landfill to different depression mode at each individual well802-818since each well is equipped with a separate actuating valve.

Each separate valve can be closed, partially open of totally open, hence affecting the depression and consequently the landfill gas extraction flow regime and state.

By sequencing the opening and the closing of each valve according to a predetermined pattern, it is possible to measure the effect of the depression from different well individually or combined together on each individual pressure measuring device. This enables the receiver-controller822to determine how to activate the valves in view of increasing the gas extraction from the biomass.

It is possible to open all or some valve according to different predetermined patterns of actuation of the valves, hereinafter referred to as “modes”. For example, three modes are described hereinbelow.

MODE A: All the Wells are Under Depression

This mode is illustrated inFIG. 20. All the valves of the wells802-818are fully or partly opened in negative pressure. The flow rate of the produced gas is adjusted according to CH4 & O2 concentration. The arrows inFIG. 20illustrate the produced gas flow towards the wells.

MODE B: Alternate Wells Under Vacuum

In this mode, two distinct steps are performed. In a first step, illustrated inFIG. 21, the receiver-controller822controls the valves so that wells802,806,808,812,814and818are under negative pressure.

In the second step, illustrated inFIG. 22, the receiver-controller822controls the valves so that wells804,810and816are under negative pressure.

MODE C: Staggered Wells Under Vacuum

Again, in this mode, two distinct steps are performed. In a first step, illustrated inFIG. 23, the receiver-controller822controls the valves so that wells802,806,810,814and818are under negative pressure.

In the second step, illustrated inFIG. 24, the receiver-controller822controls the valves so that wells804,808,812and816are under negative pressure.

By going through the three modes described hereinabove, it is possible to determine the efficiency of landfill gas extraction for individual wells. It is even possible to determine the pumping rate strategy due to well that might become blocked over time.

By varying the mode of depression in the landfill body it is possible to induce a movement, as shown by the arrows inFIGS. 20-24, of leachate within the organic matter of the landfill which then distributes more evenly the moisture content among organic matter. By having a more even distribution of moisture within the organic matter, more organic matter is allowed to decompose which then increase the production of landfill gas over a given period for the same quantity of organic matter.

By applying a depression on a given well, the pressure wave amplitude is dampened over time and under the depression conditions of the well as measured by the piezometer. By applying a depression to opposite wells, it is expected that the same piezometer will be showing a different dampening of the pressure wave amplitude. The wells positioned above and below the horizontal wells will also, under depression, exercise a different dampening effect on the pressure wave as measured by the piezometer.

The decision process is made simpler and more thorough, by having a proper mapping for a given point in time and over time combined with the other measuring points in time and over time of the unsteadiness of landfill gas flow rate due to the landfill anisotropy and heterogeneity.

For example, should one of the piezometer820detect an increase in landfill pressure amplitude, while suction is applied to the neighbouring well; the valves of the neighbouring horizontal wells can be open further to relieve the excess pressure.

To the opposite, should one of the piezometer820detect an increase in landfill depression amplitude (below 0), while suction is applied to the neighbouring well to a point where oxygen is introduced from the atmosphere; the valves of the neighbouring horizontal wells can be reduced further to prevent the excess depression.

Should a piezometer820detect no depletion of the pressure amplitude while depression is applied to neighboring well, it could mean that the vacuum applied from the well has no impact on the landfill at the measuring point; in this case either the well is blocked or is too far from the measuring point to influence pressure variation over time.

In the latter case, a depression created by the opposite neighboring well may cause a different reading on the same piezometer over time meaning a different flow condition from the other direction

Should a piezometer820detects a very high depression which approaches the depression level applied to neighboring well; this could be a consequence of a very low flow occurring between the neighboring well and the monitoring point (i.e. suction built-up) even though the neighboring well has an impact on the piezometer reading.

In this case, a depression created by the opposite neighboring well may cause a different reading on the same piezometer over time, which could mean a different flow condition from the other direction.

In all situations of the above examples, the operator obtains a better understanding of the landfill characteristics and is able to plan accordingly to maximize landfill gas capture.

It is to be noted that while pressure variation can be measured by the pressure measuring device700over time, other varying biological, electrical, optic, mechanical, chemical parameters of concern such as Temperature ° C.; electrical current Mvolts; respiratory rate O2/CO2; metabolic rate CH4, for example, can be measured by other measuring devices (not shown) positioned near the piezometer700. Variation over time which could create consequent wave formation for a given parameter, i.e. temperature wave; Mvolt wave; O2/CO2/CH4wave and so on can also be recorded.

It is to be noted that while only one piezometer820is illustrated between adjacent wells in the appended figures, more than one piezometer may be installed along the entire length of the wells. For example, three piezometers can be installed alongside each well.

As mentioned herein, landfill gas generation relies on the presence of organic matter decomposed under anaerobic condition with given temperature and moisture. Excess moisture under saturated conditions will impede the generation of landfill gas. Absence of moisture will also impede the production of landfill gas.

Accordingly, once the leachate water is extracted with the horizontal wells described herein, the landfill gas generation is accelerated until the moment when the moisture content of the biomass is depleted.

It has been shown that leachate water recirculation can maintain ideal moisture conditions to compensate the gradual depletion of landfill gas production.

One potential way to recirculate leachate in a system comprising a plurality of horizontal wells described herein would be to re-introduce the leachate at the upstream end of wells that are not under vacuum. For example, the free open end30of the siphon18(FIG. 5) could be connected to an irrigation line (not shown) that would carry the leachate to the upstream end of adequate wells. If a sink hole356is used (FIG. 13) the outlet of the sump pump358could bring the leachate to the upstream end of adequate wells.

Turning now toFIGS. 25-27of the appended drawings, another technique to optimize the gas production in a landfill will be described.

This technique aims at determining the proximity and position of the perforated horizontal wells.

InFIG. 25, a first row of three horizontal wells902,904and906is laid in a partially filled landfill900. Piezometers908are provided between the wells.

The operator may thus monitor the pressure wave detected by the piezometers908, and therefore understand the particulars of the landfill900regarding the gas collection efficiency to take decisions regarding the later installation of supplemental wells.

FIG. 26shows the landfill900when second and third rows of horizontal perforated wells910and914, including respective piezometers912and916, are installed. In the example ofFIG. 26, each row910and914includes four wells and the density of wells above the wells902-906is increased since the amount of gas generated is higher than the gas collected by wells902-906as shown by piezometers908reading and pressure wave variation over time.

FIG. 27illustrates an alternate configuration of the second and third rows910′ and914′. In this configuration, the wells are staggered and equidistant.

It is to be understood that the invention is not limited in its application to the details of construction and parts illustrated in the accompanying drawings and described hereinabove. The invention is capable of other embodiments and of being practiced in various ways. It is also to be understood that the phraseology or terminology used herein is for the purpose of description and not limitation. Hence, although the present invention has been described hereinabove by way of illustrative embodiments thereof, it can be modified, without departing from the spirit, scope and nature of the subject invention.