Patent Publication Number: US-2007116525-A1

Title: Landfill including highly permeable

Description:
BACKGROUND OF THE INVENTION  
      (1) Field of the Invention  
      This invention concerns landfills including permeable zones comprising one or more vertically separated highly permeable material layers and at least one gas extraction well that penetrates and withdraws landfill gases from the at least one highly permeable material layer.  
      (2) Description of the Art  
      Landfills that are filled with compostable waste materials, such as municipal solid waste materials, commonly include vertical gas extraction wells. The vertical gas extraction wells provide for the controlled removal of landfill decomposition by-product gases including methane from landfills thereby reducing landfill odor and accelerating landfill decomposition.  
      A standard landfill typically includes a plurality of vertical gas extraction wells typically separated by 100 to 200 feet or more. The vertical gas extraction wells are generally placed in a landfill cell after the cell filled with waste material by excavating a bore hole approximately 3 feet in diameter in the landfill cell, locating a casing in the excavated borehole and filling the annulus between the casing the bore hole with a porous media such as gravel.  
      Landfill gas extraction methods and structures are well known in the art. U.S. Pat. No. 4,798,801 discloses methods for producing methane from the anaerobic fermentation of waste materials such as municipal solid waste. U.S. Pat. No. 5,857,807 discloses landfill cells including a methane gas containment layer. The disclosed landfill cells include leachate collection pipes and gas extraction wells for removing methane and other gases from the cells. U.S. Pat. Nos. 6,505,681 and 6,338,386 disclose methods for improving methane gas recovery from landfills by forming fissures in the municipal solid waste placed in a landfill using carbon dioxide gas pressurization techniques.  
      Despite improvements made to landfill gas recovery methods and apparatuses, there remains a need for methods and for landfills in which landfil gases, including methane, can be recovered more efficiently from a larger landfill areas with fewer gas extraction wells.  
     SUMMARY OF THE INVENTION  
      One aspect of this invention are methods for constructing a landfill waste disposal cells comprising the steps of: placing a first layer of waste material in a landfill wherein the waste material includes an exposed surface; placing a first highly permeable material layer on top of the first layer of waste material such that the first highly permeable material layer at least partially covers the exposed surface of the first waste material layer; placing a second layer of waste material on the first highly permeable material layer; and installing a gas extraction well including a perforated casing in the landfill cell such that the casing passes through the first horizontal highly permeable material layer.  
      Another aspect of this invention includes methods for constructing a permeable zone in a landfill comprising the steps of: placing a first layer of waste material in a landfill wherein the waste material includes an exposed surface; placing a first highly permeable material layer on top of the first layer of waste material such that the first highly permeable material layer at least partially covers the exposed surface of the first waste material layer; placing a second layer of waste material on the first highly permeable material layer wherein the second layer of waste material has an exposed surface; placing a second highly permeable material layer on top of the second layer of waste material such that the second highly permeable material layer at least partially covers the exposed surface of the second waste material layer, the combination of the first highly permeable layer and second highly permeable layer defining a permeable zone; and placing a third layer of waste material on top of the second highly permeable material layer.  
      Still another aspect of this invention are one or more permeable zones located in a landfill comprising: a plurality of highly permeable layers, each highly permeable layer separated from one another wherein a waste material layer is located between at least two adjacent highly permeable layers; and a gas extraction well including a casing having a first end located in the landfill a second end located outside of the landfill and a perforated portion located between the casing first end and second end wherein the casing passes through at least one of the highly permeable material layers such that casing perforations are located in the highly permeable material layer.  
    
    
     DESCRIPTION OF THE FIGURES  
       FIG. 1  is an overhead schematic of a landfill waste disposal cell embodiment of this invention including a plurality of vertically separated horizontal highly porous material layers;  
       FIGS. 2A and 2B  are elevation views of a landfill waste disposal cell embodiment of this invention depicting the placement of a plurality of vertically separated horizontal highly porous material layers;  
       FIG. 3  is an elevation view of a landfill waste disposal cell embodiment of this invention including a plurality of horizontal highly porous material layers located at vertical intervals that are slightly offset from non-contiguous or stripped horizontal daily cover layers;  
       FIG. 4  is an elevation view of a landfill waste disposal cell embodiment of this invention including a plurality of horizontal highly porous material layers located at vertical intervals that are offset from contiguous horizontal daily cover layers;  
       FIG. 5  is a landfill waste disposal cell embodiment of this invention including a plurality of vertically separated horizontal highly porous material layers wherein several of the horizontal highly porous material layers encroach upon the landfill cell slope; and  
       FIGS. 6A and 6B  are aspects of an expemplary gas collection well useful in the landfill waste disposal cells and methods of this invention.  
    
    
     DESCRIPTION OF THE CURRENT EMBODIMENT  
      The present invention relates to landfill waste disposal cells and to methods for constructing landfill waste disposal cells having a highly permeable zone comprising one or more horizontal highly permeable material layers and further including at least one vertical gas extraction well located in the landfill cell such that the at least one vertical gas extraction well penetrates one or more of the plurality of highly permeable material layers. This invention also relates to permeable zones comprising two or more highly permeable material layers associated with at least one landfill gas extraction well and to landfills and landfill cells including one or more permeable zones.  
      The methods and landfill waste disposal cells of this invention allow for the progressive installation of vertical landfill gas collection wells during the construction of a landfill waste disposal cell. In one embodiment, the methods of this invention are accomplished by placing one or more layers of highly permeable material one on top of the other at different vertical locations in the waste disposal cell as it is being filled with municipal solid waste. The layers of highly permeable material provide a preferential flow path (or “path of least resistance”) for landfill gases and liquids to flow from the landfill into one or more gas extraction wells. The locations of the highly permeable material layers are recorded as they are installed using standard surveying practices or using GPS techniques. Once the construction of a landfill waste disposal cell is complete, the approximate center of each permeable zone comprising one or more highly permeable layers is located using, for example GPS or traditional surveying methods, and a conventional vertical gas extraction well is placed in each of the approximate centers of each permeable zone.  
      The methods and waste disposal cells of this invention allow for increased gas yield and drainage in a defined landfill cell area by providing a preferential flow path for liquids and for landfill gases. Moreover, the methods of this invention involve landfill construction techniques that are familiar to landfill contractors.  
       FIG. 1  is an overhead view of a landfill waste disposal cell  10  embodiment of this invention. Landfill waste disposal cell  10  includes a boundary  12  defining the periphery of landfill waste disposal cell  10 . Landfill waste disposal cell  10  further includes a plurality of highly permeable zones  18  located at defined or undefined intervals throughout the cell. Each highly permeable zone  18  further includes at least one landfill gas extraction well  16  preferably placed in approximately the center of each highly permeable zone  18 . However, landfill gas extraction wells  16  are still effective so long as they contact any portion of highly permeable material associated with highly permeable zone  18 .  
      As will become apparent from subsequent Figures, each highly permeable zone  18  will include at least one and preferably a plurality of highly permeable material layers  28 . Each highly permeable material layer  28  is separated from one another by a layer of material and preferably a layer of compostable waste material. For example highly permeable zone  18 ′ of  FIG. 2A  will generally consist of two or more vertically separated highly permeable material layers  28 .  
      The horizontal area of a landfill waste disposal cell  10  (X 1 ×Y 1 ) is not critical. The larger the landfill cell boundary  12 , the larger the number of highly permeable zones  18  that may be located in landfill waste disposal cell  10 . In general, landfill waste disposal cell will have an Y 1  dimension of approximately 200 to 1000 feet or more and an X 1  dimension of from about 400 to 2000 feet or more. Highly permeable zones  18  may have a variable area (X 2 ×Y 2 ) depending upon many factors including the dimensions of landfill cell boundary  12 . However, it is preferred that highly permeable zones  18  have a length X 2  of from about 10 to 200 feet or more and a width Y 2  of from 10 to 200 feet or more. In a more preferred aspect, highly permeable zones will have a length X 2  ranging from about 50 to 100 feet or more and a width Y 2  ranging from about 50 to 100 feet or more.  
       FIGS. 2A and 2B  are elevation views of a landfill waste disposal cell embodiment of this invention including a plurality of highly permeable zones  18  each having a plurality of horizontal highly porous material layers  28 . The landfill waste disposal cell  10  of  FIG. 2A  is partially completed. The landfill waste disposal cell  10  of  FIG. 2B  is a completed landfill waste disposal cell. Landfill waste disposal cell  10  may be constructed for example by placing a liner system  24  at the bottom of landfill waste disposal cell  10 . Next, landfill waste disposal cell  10  is filled with waste materials in layers  26 . Waste material layers  26  consists of a volume of waste material that may be defined by any known method. The volume may, for example, be the amount of waste material added to a particular area of the landfill waste disposal cell over a period of time such as a day, a week or any other period of time. Alternatively, waste material layer  26  may be an amount of waste material having a particular depth before or after compaction. In yet another alternative, a waste material layer  26  may be defined by the depth of waste material located between highly permeable material layers  28  and/or between highly permeable material layers  28  and the top or bottom of a landfill cell. For example, in  FIG. 2A , waste material layers  26  have a depth of from 10 to 50 feet or more depending upon their vertical placement in landfill waste disposal cell  10 . One highly permeable material layer  28  will typically be located or adjacent to each waste material layer  26 . In  FIG. 2A , highly permeable material layers  28  are located at the interface between landfill layers  26 . As will be shown in  FIGS. 3-5  below, the location of highly permeable material layers  28  in relationship to waste material layers  26  may vary.  
       FIG. 2B  is a side view of a completed landfill waste disposal cell  10 . Landfill waste disposal cell  10  of  FIG. 2B  includes two permeable zones  18 , each including a plurality of vertically separated highly permeable material layers  28 . A vertical landfill gas extraction well  16  is located in the central portion of each permeable zone  18 . A partial final cap  22  covers the waste material  20  located in landfill waste disposal cell  10 .  
      The geometry of the horizontal highly permeable material layers  28  is not critical. Highly permeable material layers  28  may be square, they may be round, they may be oval or they may be amorphous in shape. Moreover, it is not critical that the plurality of highly permeable material layer  28  are located precisely one over the other. Nor is it critical that each of the highly permeable material layers  28  in a single permeable zone  18  all has the same geometry. Instead, highly permeable material layers  28  in a single permeable zone  18  may have different geometries or they may be offset from one another depending upon many factors including the contours of the landfill cell and the composition of the waste material in the cell. However, placing highly permeable material layers  28  in the same permeable zone  18  one over the other and constructing the plurality of highly permeable material layers  28  in a single permeable zone  18  to have essentially the same dimensions and shape allows for the placement of a single vertical gas extraction well  16  in the approximate center of permeable zone  18  where it is able to most efficiently and effectively withdraw landfill gases from the section of landfill waste disposal cell  10  associated with the permeable zone  18 .  
      Generally each highly permeable material layer  28  may have an area ranging from about 100 to 10,000 square feet or more. More preferably, each highly permeable material layer  28  will have an area ranging from about 2500 to 10,000 square feet or more. This corresponds to a square having a dimension for example of from about 10 feet by 15 feet to about 100 feet by 100 feet.  
      Highly permeable material layer  28  will consist of a highly porous or permeable material. Any material reasonably permeable to gas may be used to form highly permeable material layer  28 . Examples of useful reasonably gas permeable materials include, but are not limited to sand, crushed stone, construction and demolition debris, rubber tire chips, auto fluff, plastic bottles, wood debris, geotextiles, geo-et with drain layer, glass cullet and combinations of these materials. If a compressible material such as tire chips or shreds are used, then the tire chips may preferably have a nominal size of 4 inches to 6 inches or more. Useful non-compressible materials such as stone may generally have a nominal diameter size of from 1 to 4 inches or more. Highly permeable material layers  28  are constructed or compressed and/or non-compressible materials may have a thickness ranging from about 1 inch to about 18 inches or more. With the preferred thickness ranging from about 2 to about 8 inches. If a geotextile is used as highly permeable material layer  28 , then the geotextile composite drain net thickness can range from ⅜″ to 2″ or more.  
       FIG. 3  is a side elevation view of a landfill cell embodiment of this invention including a plurality of highly permeable material layers  28  located adjacent to the cover layer  30 . Daily cover material layer  30  is generally less permeable than the permeable layer material. Daily cover material layer  30  will typically consist of soil, sand, sludge or other similar material that is used to cover the waste material added to a landfill in a single day in order to keep the materials from blowing away and in order to inhibit the emission of odors from the landfill. Daily cover material layer  30  in  FIG. 3  is non-contiguous. That is, daily cover material layer  30  does not cover the entire layer of waste material upon which it is placed. Instead, the daily cover layer  30  is non-contiguous in the area occupied by one or more highly permeable material layers  28 . When daily cover layer is non-contiguous, gases in waste material layers  26  located above and below the non-contiguous daily cover layer may be removed from the landfill via a single permeable zone  28  located in the non-contiguous portions of daily cover layer  30 . Daily cover layer  30  may be applied in a non-contiguous manner along with the highly permeable material layer  28  to form a complete layer over landfill waste material. Alternatively, daily cover layer may be uniformly applied to landfill waste material and thereafter a portion of the daily cover material layer may be removed to form a non-contiguous portion in which to install a highly permeable material layer  28 .  
      In many jurisdictions, daily cover material cannot be removed from a landfill cell. In such landfills, highly porous material layers  28  may be placed anywhere in landfill including between two daily cover layers  30 . In such landfills it is preferred that highly porous material layers  28  are placed at a distance and more preferably about equal distance between daily cover material layers  30  so that gases generated in the waste material located between two daily cover layers  30  can be efficiently removed via highly permeable material layer  28  using landfill by gas extraction well  16 .  
      In  FIG. 4 , the plurality of highly permeable material layers  28  lie essentially in between daily cover layers  30 . In addition, daily cover layers  30  are contiguous. In the embodiment shown in  FIG. 4 , each highly permeable material layer  28  is able to draw landfill gases from waste layer  26  defined as the waste material located between two daily cover layers  30  and  30 ′.  
      The landfill cells  10  of  FIGS. 2B, 3 ,  4  and  5  include a temporary intermediate grade  21 . When the cell goes to final grade, temporary grade  21  is often left behind (as indicated by the dash lines). When a highly permeable material layer protrudes through intermediate grade  21 , drainage and gas collection is improved. The same applies for temporary haul roads erected on partially completed landfill cells.  
       FIG. 5  is a side view of a landfill waste disposal cell embodiment wherein vertical drilled landfill gas extraction well  16 ′ is located on the slope of the landfill.  FIG. 5  demonstrates that a permeable zone  18  consisting of one or more highly permeable material layers  28  may be located along the slope of a landfill. Moreover,  FIG. 5  demonstrates that highly permeable material layers  28  do not all need to have the same dimensions. Instead, the shape of highly permeable material layers  28  may vary in order to avoid drawing air into the gas extraction well  16  from shallow areas of the landfill. For example, highly permeable material layers  28 ,  28 ′ and  28 ″ have different horizontal configurations depending upon their proximity to the landfill perimeter.  
      There are several factors that may be considered when locating a highly porous material layer  28  in a landfill. It is desirable, for example, to avoid drawing air into the landfill gas extraction well via highly permeable material layer  28 . Air infiltration can be minimized by insuring that the perimeter of a highly permeable material layer  28  is not near the edge or top of landfill cell  10 . In order to minimize air infiltration, the perimeter of a highly permeable material layer  28  should be located a distance of 5 feet or more and preferably 10 feet or more from the landfill cell liner. Moreover, the perimeter of highly permeable material layer  28  should be located at least 5 feet and preferably at least 15 feet or more from the landfill cell perimeter and from the landfill cell cap.  
      The distance between two highly permeable material layers  28  may also vary significantly depending upon factors such as the location of layer  28  in the landfill, landfill geometry, the type of waste in the landfill, the layer position in the landfill cell and so forth. Generally, highly permeable material layers  28  will be separated by a distance of 10 feet or more and preferably 20 feet or more. Likewise the thickness of a highly permeable material layer  28  can be varied depending upon the same factors as well as upon the selection of the highly permeable material. The highly permeable material layer thickness will generally range from 1 to 18 inches but can range up to 1″ to 5 feet or more.  
       FIGS. 6A and 6B  show aspects of a vertical landfill gas extraction well  16 . According to the present invention, the vertical drilled landfill gas extraction well  16  may be constructed as landfill lifts are being added to landfill waste disposal cell  10 . Alternatively, a plurality of highly permeable material layers  28  may be added to landfill waste disposal cell  10  to form a highly permeable zone and thereafter one or more landfill gas extraction wells can be placed into the landfill waste disposal cell  10  such that the gas extraction well passes through one or more and preferably all of the highly permeable material layers  28  in a permeable zone  18 .  
      A standard landfill gas extraction well is comprised of a bore hole  39  of approximately 3 feet in diameter. A 6 to 10 inch casing  40  including a plurality of perforations  41  is inserted into bore hole  39 . Casing  40  includes a first end associated with wellhead  42  and a second end  58  located in bore hole  39 . Second end  58  may be capped or it may remain uncapped to allow fluids to drain from casing second end  58 . The cylindrical gap between the casing and the bore hole wall is filled with a porous media such as gravel  43 . Bore hole  39  is capped with one or more essentially impervious materials. In  FIG. 6A , gravel  43  in bore hole  39  is capped with a clay layer  54  over which is located a sand layer  52  on top of which is placed a second clay layer  50 . A final cap material layer  46  is placed over bore hole and the remaining landfill materials. Casing  40  is located in bore hole  39  such that perforations  41  in casing  40  are at least associated with highly permeable material layer  28 .  
      The gas extraction well depicted in  FIGS. 6A and 6B  is merely an example of a gas extraction well that may be located in the landfills of this invention. Any embodiments of landfill gas extraction wells that are known to those of ordinary skill in the art may be employed in the present invention.  
      The plurality of vertical drilled landfill gas extraction wells  16  will generally be connected via pipe  44  to a piping manifold (not shown) which in turn is connected to a vacuum pump or to a vacuum system in order to draw landfill gases from landfill waste disposal cell  10 .  
      Casing pipe  40  may be perforated along its length, it may include perforations only in the vicinity of each of highly permeable material layers  28  through which it passes, or it may be perforated in any pattern that allows landfill gasses to be drawn into casing  40 . It is preferred that at least one vertical landfill gas extraction well  16  is located in the approximate center (as viewed from above) of each permeable zone  28 . However, it is within the scope of this invention to include two or more vertical drill landfill gas extraction wells in each permeable zone  28  or to install landfill gas extraction well away from the center of a highly permeable zone  18 .  
      The approximate center of each permeable zone  18  may be identified using GPS or other surveying techniques. In one embodiment, surveying techniques are used to identify the level above grade as well as the approximate corners of each highly permeable material layer  18  as a landfill waste disposal cell is being erected. Once construction of landfill waste disposal cell  10  is complete, the same or similar surveying techniques may be used to identify the approximate centers of each permeable zone  18  for purposes of installing landfill gas extraction well  16 .  
      The methods and landfill cells of this invention are useful for landfills that accept compostable materials that degrade anaerobically or aerobically to form noxious gases such as methane and carbon dioxide. Examples of useful compostable waste materials include, municipal solid waste such paper, wood, food waste and so forth; construction waste including an organic content such as wood fiber, paper and so forth; agricultural waste, and any other types of waste that is organic in nature.  
      The landfill cells and methods for their construction may be utilized in a variety of landfills. As mentioned above, the methods and landfill cells of this invention are useful in landfills accepting organic waste materials such as municipal solid waste. Additionally, the landfill cells of the methods and landfill cells of this invention may be employed in wet landfills and in landfills that use leachate recovery and/or in bioreactor landfills. The methods of this invention are also useful in landfills that must leave large amounts of cover in place or that have cover material that is revenue generating such as bottom ash. When cover is left in place, gas that is formed during the degradation process moves toward the outside of the landfill as directed by the confining cover layers. The placement of horizontal layers, in between these cover layers allows for more efficient collection by having a wider zone of influence and promoting draining of liquids in the confirming layers. Moreover, the methods and landfills of this invention are useful in dry landfills which are more porous and prone to air infiltration. In dry landfills, because their greater natural porosity, the spacing of gas extraction wells and the location of permeable zones  18  may be enlarged thereby reducing the number of gas extraction wells per acre. Furthermore in landfills with Evapotranspiration Covers, air infiltration is prevented by placing these porous layers deeper in the waste cells so that gas is preferentially collected in areas of the landfills where moisture and gas production is highest. The net effect is more wells per unit area of landfill cover. Finally, the methods of this invention are useful in landfills with large impervious layers of biosolids that prevent gas collection and drainage and in landfills with long or steep exterior slopes which preclude the installation of conventional gas collection wells on the slopes.  
      There are several advantages that may or may not be achieved by the methods and landfill cells of this invention. The methods and landfill cells of this invention may increase the capacity of gas extraction wells by increase in the void space in the landfill and allowing for a larger zone of gas extraction influence. The landfill embodiments of this invention may allow for drainage in a heterogeneous mass as well as a decrease in air emissions by improving extraction efficiency. Moreover, the landfill embodiments of this invention may allow for geometric preplanning of a gas extraction well field regardless of the nature of the incoming waste material. Finally, the landfill embodiments of this invention may artificially enlarge the gas extraction well surface area.  
      It is preferred that highly permeable material layers  28  are essentially horizontal. The terms “horizontal” or “essentially horizontal” means a layer is horizontal in nature. The term does not require that the highly permeable material layer be precisely horizontal. A horizontal layer may deviate from horizontal by up to 20 degrees. Moreover, if the term horizontal is not used in the specification and claims in describing a layer such as the highly permeable material layer  28 , then this definition of the term “horizontal” does not apply to the layer. In essence, all that is required of the highly permeable material layer  28  is that it be located entirely within a landfill cell, that at least one gas extraction well pass through it and that the gas extraction well are capable of extracting landfill gas from the highly permeable material layer.  
      The methods and landfill cells of this invention may be useful for assisting in the removal of perched and accumulated liquids in wet landfills. In addition to providing a route for gas to escape the landfill via gas extraction well  16 , highly permeable material layers  28  also provide a route for leachate to enter gas extraction well  16 . Any liquid entering gas extraction well  16  can exit a hole in the bottom of gas extraction well  16  or it can be removed—in landfills where there is a great amount of water that seeps into gas extraction well  16 —by a sump located at the bottom of one or more gas extraction wells.