Abstract:
A system and method for removing contaminants from water that has a relatively low oxygen content with naturally occurring chemoautotrophic bacteria, ferrous iron, manganese, sulfide, and other contaminants. The system comprises an oxygenation vessel for oxygenating the water and a filtration vessel having bio-filtration media with surfaces that are exposed to the oxygenated water. The chemoautotrophic bacteria propagate in the presence of the oxygenated water and deposit certain of the contaminants on the bio-filtration media as by-products. The contaminants are also precipitated in the presence of the oxygenated water and settle on the bio-filtration media in at least the filtration vessel. A secondary filtration stage can further remove contaminants from the water that were not sufficiently removed by the first filtration stage.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention is directed to filtration systems and, more particularly, to a system and method for removing contaminants from groundwater associated with underground natural gas wells, mining operations, and other environments.  
           [0002]    It is often necessary to remove water from underground geological formations in order to release natural gas associated with the underground formations. Oftentimes, the formations are more than 1,000 feet below the surface of the earth. A typical formation can comprise several separate layers of the liquid and gas or can comprise a single large reservoir. A bore hole is drilled into the earth and passes through the different layers of the formation until the target layer is reached. The location and depth of the bore hole is carefully controlled because of the great expense associated with drilling the bore hole. In order to prevent collapse of the bore hole after drilling, it is usually lined with a casing along its entire length. The casing also helps to control reservoir pressure and protect surface water from contamination. The casing is cemented in place and sealed at the ground surface by a wellhead.  
           [0003]    One or more pipes or tubes extend into the bore hole from the wellhead. One of the tubes is typically used to carry liquid to the surface. The internal pressure of many geological formations is often insufficient to naturally raise commercial quantities of the natural gas from the formation through the bore hole or does so at an inadequate flow rate. Oftentimes, a large volume of liquid is present in the underground formation and must be removed on a continuous basis in order to recover natural gas from the formation. An artificial lift system is used in conjunction with the tube(s) to remove the liquid from the underground formation. Currently, many different types of artificial lift systems are available to lift the liquid from the formation, the most common of which are progressive cavity pumps, beam pumps and subsurface gas lift (SSGL) systems.  
           [0004]    No matter what artificial lift system is used, water retrieved from underground coal formations often contains contaminants such as iron and inorganic and organic sulfur compounds, manganese, sodium, barium, arsenic, and other trace metals, and coal fines. Some constituents indigenous to groundwater associated with underground coal seams, such as iron and manganese, are pH dependent, while other constituents, such as sulfur, are more oxygen dependent. These constituents typically exist in soluble forms in the groundwater. When the groundwater is ejected or pumped to the surface and exposed to air, iron, sulfur and manganese are oxidized, resulting in the deposition of insoluble forms and precipitate on contact surfaces causing discoloration. The precipitates may also impart foul taste and odor to the water.  
           [0005]    Federal and state regulations dictate minimum water quality standards that must be met before water from underground formations, mines or other underground structures can be discharged into the environment. When such standards are not met, the environment may be adversely affected and gas production or other operations may be halted. Due to the excessive amounts of contaminants in the discharge water, many natural gas producers are experiencing difficulties in obtaining discharge permits from state agencies.  
           [0006]    Prior attempts to filter the contaminated water have included activated carbon filters, capacitive deionization systems, and the like. The filters can become quickly clogged and therefore must be constantly monitored, cleaned and/or replaced, leading to great expense and reduced efficacy over time. This problem is exacerbated by the relatively large flow rates that must be accommodated. By way of example, a filtration system may be required to process approximately 100 gallons of groundwater per minute over a 24-hour period of time, depending on the number of wells associated with the filtration system, the volume of groundwater to be lifted from each well, and the frequency at which the groundwater is lifted. Thus, approximately 144,000 gallons or 3,429 barrels of groundwater may pass through the filtration system every 24 hours.  
           [0007]    In addition, natural gas wells are typically located at remote locations where power from electrical grids may not be available. In such locations, the wells may be operated through wind, solar or gas powered generators. Accordingly, filtration systems at remote locations should require little or no electrical power to operate.  
           [0008]    It would therefore be desirable to provide a filtration system that is capable of removing large amounts of contaminants from groundwater under large flow rates in a relatively quick and efficient manner without substantial degradation of the filtration system. It would also be desirable to provide a filtration system that requires little or no electrical power to operate.  
         SUMMARY OF THE INVENTION  
         [0009]    According to the invention, a method is provided for removing contaminants from water that has a relatively low oxygen content with naturally occurring chemoautotrophic bacteria, and at least one of iron in the ferrous state and sulfide. The method comprises oxygenating the water and directing the oxygenated water to a filtration vessel. The filtration vessel has bio-filtration media with surfaces that are exposed to the oxygenated water. The chemoautotrophic bacteria propagate in the presence of the oxygenated water and at least one of the ferrous iron and sulfide. At least one of ferric iron and sulfate are deposited on the bio-filtration media as a by-product of the chemoautotrophic bacteria. At least one form of ferric iron and iron sulfate are precipitated in the presence of the oxygenated water. The water can then be removed from the filtration vessel and either discharged into the environment or directed to a secondary filtration stage.  
           [0010]    Further according to the invention, a system is provided for removing contaminants from water that has a relatively low oxygen content with naturally occurring chemoautotrophic bacteria, and at least one of ferrous iron and sulfide. The system comprises an oxygenation vessel for oxygenating the water and a filtration vessel having first bio-filtration media with surfaces that are exposed to the oxygenated water. With this arrangement, at least one form of ferric iron and iron sulfate can be precipitated in the presence of the oxygenated water and can be deposited on the bio-filtration media as a by-product of the chemoautotrophic bacteria to thereby remove ferric iron and/or iron sulfate from the water. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0011]    The preferred embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, wherein like designations throughout the drawings denote like elements, and wherein:  
         [0012]    [0012]FIG. 1 is a schematic block diagram of a system for removing contaminants from water according to the present invention;  
         [0013]    [0013]FIG. 2 is a schematic diagram of a system for removing contaminants from water according to the present invention and showing the system connected to a well head associated with an underground formation;  
         [0014]    [0014]FIG. 3 is a schematic diagram of a system for removing contaminants from water according to a further embodiment of the present invention;  
         [0015]    [0015]FIG. 4 is an enlarged perspective view of an aeration tower that forms part of the system of the present invention;  
         [0016]    [0016]FIG. 5 is a top plan view of a filtration component that forms part of the system of the present invention;  
         [0017]    [0017]FIG. 6 is a side elevational view of the filtration component; and  
         [0018]    [0018]FIG. 7 is an exploded perspective view of a filtration tank that forms part of the system of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Referring now to the drawings, and to FIG. 1 in particular, a system  10  for removing contaminants from groundwater according to the present invention is illustrated. The system  10  includes a first filtration stage  12 ,  12 A that is connected between a well  14  and a waste removal stage  16 . The well  14  may be associated with an underground gas producing formation, such as an underground coalbed formation where groundwater is removed through an artificial lift system to recover methane gas. A second filtration stage  18  is preferably connected to the first filtration stage  12 ,  12 A for removing particles that pass through the first filtration stage. The second filtration stage  18  is also connected to the waste removal stage  16 . As shown, clean water  20  exits from the second filtration stage for safe disposal in the environment or other uses. Alternatively, depending on the type and amount of contaminants to be removed from the groundwater, the second filtration stage may be eliminated, so that the water is directly discharged to the environment after the first filtration stage.  
         [0020]    As will be described in greater detail below, and according to a preferred embodiment of the invention, the first filtration stage  12 ,  12 A is constructed to extract relatively large amounts of iron, sulfur, manganese, coal fines and other relatively large particles that would otherwise plug up the second filtration stage  18 . Without the first filtration stage  12 ,  12 A, the second filtration stage would require constant maintenance or would be rendered partially or wholly inoperative, and therefore would not be an economically feasible approach in and of itself.  
         [0021]    The second filtration stage can be constructed to remove sodium, barium, arsenic, and other contaminants that may not be sufficiently removed by the first filtration stage. Many different types and configurations of filters and filter systems well known to those of ordinary skill in the art can be used for the second stage, such as reverse osmosis, ion exchange, and activated charcoal filters. According to a preferred embodiment of the invention, a Capacitive Deionization Technology (CDT) filter system is used for the second filtration stage  18 . In the CDT system, sheets of thin carbon aerogel material are formed into cells and placed at opposite boundaries of a flow path of the incoming water. By polarizing the cells, oppositely charged ions migrate to the oppositely charged sheets of aerogel material. A second filtration stage constructed in this manner can be controlled electronically through software, requires little power consumption to operate, and is easier to maintain than other filter systems. More details of the CDT system can be found in U.S. Pat. No. 5,425,858 issued to Joseph Farmer on Jun. 20, 1995, the disclosure of which is hereby incorporated by reference.  
         [0022]    As shown in FIG. 2, the first filtration stage  12  is connected to the well  14 , such as a coalbed methane well, through an underground pipe  32 . The well  14  includes a well head  30  and a casing  34  that extends into a reservoir  36  of an underground formation  38  from the well head. The reservoir  36  may include groundwater  40  that must be removed before gas can be produced from the formation  38 . An artificial lift system  42 , such as a down-hole pump or the like, is connected to a production tube  44 , which is in turn connected to the underground pipe  32 . The artificial lift system  42  moves water from the underground reservoir  36 , through the tube  44 , and into the pipe  32 .  
         [0023]    The first filtration stage  12  includes a chemical treatment system  50  that is connected to the underground pipe  32 , an aeration tower  52  located at a discharge end of the pipe  32 , and a filtration tank  54  connected to the aeration tower through a pipe  56 .  
         [0024]    The chemical treatment system  50  includes a storage tank  60  containing oxidation media. A metering pump  62  is connected to the storage tank  60  through tubing  64 . An injection nozzle  66  extends between the underground pipe  32  and the metering pump  62  for delivering oxidation media to the water within the pipe  32 . A measurement electrode  68  extends into the pipe  32  downstream from the injection nozzle  66  for monitoring oxidation of the water within the pipe  32 . In addition to chemical oxidation, an air injector can be installed in the pipe  32  to assist in oxidizing the water, and thus the contaminants carried by the water.  
         [0025]    Prior to treatment, groundwater  40  from the reservoir  36  can be sampled and analyzed for a variety of constituents including trace metals, non-metals and major ions. Particular attention can be focussed on the soluble levels of iron, sulfur and manganese, as well as the factors that affect their oxidation and removal from the groundwater, such as pH level, temperature, alkalinity, and the presence of catalysts. This information can then be used to establish the initial oxidant concentration and application rate for the groundwater. The set point of the chemical metering pump  62  can be determined from the desired water quality parameters. Oxidant can then be continuously metered into the groundwater through the nozzle  66  and monitored with the electrode  68 . Preferably, the electrode  68  generates a millivolt (mV) signal from the reduction-oxidation (redox) process. The signal can then be compared to the set point of the metering pump and the set point can be automatically adjusted to provide an adjusted oxidant dosage. According to a preferred embodiment of the present invention, the oxidant is potassium permanganate (KMnO 4 ). The chemical reactions for iron and manganese oxidized with potassium permanganate are as follows:  
         3Fe 2 +KMnO 4 +7H 2 O         3Fe(OH) 3 (s)+MnO 2 +K + +5H +   
         3Mn 2 +KMnO 4 +2H 2 O         5MnO 2 (s)+2K + +4H +   
         [0026]    Following the oxidation stage, the groundwater is directed to the aeration tower  52  through the underground pipe  32 . The underground pipe  32  helps to maintain the groundwater at an ideal temperature level, as will be described in greater detail below.  
         [0027]    With additional reference to FIG. 4, the aeration tower  52  includes a generally cylindrical housing  70  that can be constructed of polyvinyl chloride (PVC) or other material well known to those of ordinary skill in the art. The housing  70  includes a continuous wall  72  that forms a hollow interior  74 . Openings  76  are formed in the wall circumferentially around the housing  70  and extend between the hollow interior  74  and the outside of the housing. A catch basin  78  surrounds the housing  70  for catching groundwater that may flow through the openings  76 . The height of the catch basin is approximately equal to the height of the housing  70 , as shown in FIG. 4. Alternatively, the height of the catch basin can be much smaller, as shown in FIG. 2. An enclosure  79  (FIG. 2) constructed of fiberglass or other material, may surround the tower  52 .  
         [0028]    Bio-filtration media, shown here as a plurality of bio-media balls  80 , are located in the hollow interior  74  of the housing  70 , and preferably substantially fill the hollow interior.  
         [0029]    As shown in FIGS. 5 and 6, each of the bio-media balls  80  according to an exemplary embodiment of the invention is constructed of a plastic material and has a plurality of axially extending pins  82 ,  84 ,  86 ,  88 ,  90 , and  92  arranged on concentric, annular rings or webs  94 ,  96 ,  98 ,  100 ,  102 , and  104 , respectively. Arms  106  extend radially from a centrally located rod  108  and are formed integral with the annular webs for holding the webs together. The pins are preferably integrally formed with their respective annular webs and progressively increase in length from the outer annular web  94  to the inner annular web  104 . By way of example, the bio-media balls are about 1.5 inches in diameter and preferably include over one hundred pins. The pins provide both mechanical and biological filtration by physically blocking larger particles and by promoting the growth of bacteria over the relatively large surface area of the pins. The pins also help produce turbulence as the groundwater flows over the bio-media balls, aiding in the oxygenation process of the groundwater and the precipitation of contaminants on the surfaces of the pins. Although the bio-media balls have been shown as having a particular configuration, it will be understood by those of ordinary skill in the art that other bio-media configurations can be used. The bio-media balls are commonly used in freshwater and saltwater wet/dry systems and pond filters for breaking down toxic ammonia produced by aquatic life into less toxic nitrates and are commercially available from Lee&#39;s Aquarium &amp; Pet Products (www.leesaqpet.com). Suitable bio-media balls and other bio-media configurations are commercially available from petsolutions® (www.petsolutions.com).  
         [0030]    According to the invention, the bio-media balls are used for precipitating iron and sulfur on their surfaces, and are compatible with relatively high flow rates of the groundwater. Iron and sulfur bacteria are diverse in their taxonomy. Their primary usefulness and importance to the present invention is their ability to transform ferrous iron to ferric iron. These bacteria obtain energy by the oxidation of iron from the ferrous to the ferric state. Discharge from coal-bed methane wells is typically basic, although it may be acidic, and has carbonates, iron and sulfur forms. Ferric iron readily combines with the carbonates and sulfur to form precipitates which can be settled. The ferric form of iron is precipitated as ferric hydroxide. Water borne iron is available in many natural water systems, such as coal bed methane wells.  
         [0031]    The bacterial oxidation of ferrous iron to the ferric form is generally as follows:  
         4Fe 2+ +O 2 +4H +           4Fe 3+ +2H 2 O  
         [0032]    In the present invention, one or more bacteria of the chemoautotrophic type, including members of the bacteria genus  Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, Sulfolobus acidocaldarius, Sulfobacillus thermosulfidooxidans,  and other bacteria exhibiting similar environmental, ecological, and/or metabolical properties, may be present in the groundwater from coalbed methane gas wells, drainage from mining operations, and other environments. Oxidation of ore and iron by a consortia of bacteria generally takes place at a higher rate than with pure cultures, and is ideally suited to the system and method of the present invention since more than one type of bacteria may be present in the groundwater. The consortia may include mixtures of bacterial genera, species and types that can react synergistically to produce impacts to the microenvironment. Thus, it is believed that no single genera or group of organisms is exclusively responsible for the process.  
         [0033]    Groundwater from coalbed methane wells offer several key energy and nutritional circumstances that provide an acceptable and productive growth environment. Before the groundwater is oxygenated, the bacteria are in a substantially dormant state, and do not require nor consume iron and sulfur, and thus do not deposit the iron and sulfur byproducts of such consumption onto surfaces. Where oxidative and reductive environments change in the presence of iron and the large surface area of the bio-media balls, selected bacterial populations propagate. The relatively large surface area of the bio-media balls provide an excellent surface for the bacteria to adhere to and on which to deposit the contaminants, such as ferric hydroxide, sulfur, arsenic, and so on, from the water. Iron, sulfur and manganese may also precipitate independent of the bacteria and settle on the surfaces of the bio-media balls or at the bottom of the tower and tank. The propagation of the selected bacterial populations increases at a logarithmic rate, which in turn increases the metabolic requirements. The iron and sulfur present in the oxygenated ground water are therefore consumed at a higher rate, leading to increased filter efficiency.  
         [0034]    Although the tower  52  is primarily used for aeration of the groundwater  40 , the ferrous iron may be precipitated and deposited on the surfaces of the bio-media balls  80 .  
         [0035]    The tower  52 , by way of example, may be constructed to continuously oxygenate ground water at a flow rate of approximately one hundred gallons per minute. For such a flow rate, the tower  52  can have an inside diameter of approximately  20  inches and a height of approximately 6.5 feet. The openings  76  are about one inch in diameter and are spaced on about two inch centers within the same row. The openings of adjacent rows can be spaced on one foot centers. It will be understood by those of ordinary skill in the art that these dimensions are given by way of example only, and can greatly vary depending on numerous factors, including the size and type of the bio-media, the water flow rate and the amount of oxygen and/or contaminants already present in the water.  
         [0036]    Once the groundwater  40  exits the aeration tower  52 , it is directed, preferably under pressure from the aeration tower  52  and gravity, to the filtration tank  54 .  
         [0037]    Referring now to FIG. 7, the tank  54  has a tank housing  120  with elongate side walls  122  and  124  and end walls  126  and  128  connected at opposite ends of the side walls. A floor  130  extends between the side and end walls to form an hollow interior  132 . A lip  134  extends around the upper periphery of the tank housing  120  for supporting a lid  136 . The lid  136  encloses the hollow interior  132  and normally prevents the ingress of foreign matter into the tank housing  120  and the egress of water and contaminants from the tank housing.  
         [0038]    Flow control weirs  140 ,  142 ,  144 ,  146 , and  148  are positioned in the hollow interior  132  of the tank housing  120  and extend between the side walls  122  and  124  to divide the interior into an entrance chamber  150 , filtration chambers  152 ,  154 ,  156 , and  158 , and an exit chamber  159 . The weirs  140 ,  144  and  148  have a bottom edge  160  that is spaced from the floor  130  so that groundwater and contaminants can flow between the floor  130  and the weirs  140 ,  144  and  148 . Likewise, the weirs  142  and  146  have an upper edge  162  spaced from the lid  136  when closed so that groundwater and contaminants can flow between the lid  136  and the weirs  142  and  146 . In this manner, the groundwater must pass through the tank in a serpentine fashion from the entrance chamber  150  to the exit chamber  159 , as represented by dashed line  164 . The entrance chamber  150  and exit chamber  159  help to slow movement of the groundwater. An L-shaped flange extension  166  is attached to the end wall  128  below an exit pipe  168  to ensure that the filtered groundwater completely fills the exit pipe  168 .  
         [0039]    A filtration basket  180  is removably positioned in each of the filtration chambers  152 ,  154 ,  156 , and  158 . Each filtration basket  180  includes a housing  182  that is sized to be received in one of the filtration chambers. The housing  182  has end walls  184 ,  186  and side walls  188 ,  190  that extend between the end walls. A floor  192  extends between the side and end walls to form a hollow interior  194 . Each wall has an opening  196  with a screen  198  located in the opening. A plate  200  extends generally horizontally across each opening to support the screen  198 . Bio-media balls  80  are located in the hollow interior  194  of each basket  180  to deposit, precipitate and filter out contaminants from the ground water, as previously described with respect to the aeration tower  52 . A lid  202  is hingedly connected to the housing  182  and includes an opening  196  with a screen  198  formed in the opening. The lid  202  holds the bio-media balls  80  in the basket during use and can be opened for emptying and filling the basket.  
         [0040]    In use, the groundwater from the aeration tower  52  first enters the entrance chamber  150  to slow the water velocity prior to filtration. The water then flows upwardly through a first basket  180  in the first filtration chamber  152 , followed by flowing downwardly through a second basket  180  in the second filtration chamber  154 , flowing upwardly through a third basket  180  in the third filtration chamber  156 , then flowing downwardly through a fourth basket  180  in the fourth filtration chamber  158 , as shown by the dashed line  164 . Finally, the water flows upwardly through the exit chamber and is discharged out of the tank to either the environment as surface drainage or to the second filtration stage  18  (FIG. 1). For a flow rate of one hundred gallons per minute, by way of example, the filtration tank  54  is preferably dimensioned to hold about 2,000 gallons of liquid, and each basket  180  is dimensioned to hold approximately 25,000 bio-media balls  80  of 1.5 inch diameter. With this example, the groundwater is in the filtration tank  54  for approximately twenty minutes, which in many cases gives enough time for the contaminants in the groundwater to be deposited as bacteria by-products onto the bio-media balls and other surfaces, and allows iron and sulfur precipitates, coal fines, sediment, and other relatively large particles in the groundwater to settle to the bottom of the baskets  180 . In this manner, a substantial amount of contaminants are removed from the water.  
         [0041]    As the bio-media balls become coated with metal oxides, the coatings serve as catalysts for oxidation and removal of additional precipitate from the groundwater. According to a preferred embodiment of the invention, the bio-media balls are pre-coated with metal oxides before being placed in the baskets  180  and inserted into the tank to expedite their effectiveness. When the surface area of the bio-media balls  80  is reduced to a predetermined value due to the build-up of metal oxides and other contaminants, the baskets can be removed and the bio-media balls replaced. The spent bio-media balls can then be cleaned and then recoated with metal oxide and stored for later use. Alternatively, a substantial portion of the spent bio-media balls can be cleaned, such as seventy-five percent, with the remaining portion being used as catalysts during the next filtration cycle. The bio-media balls can be cleaned by cracking the oxide layers through movement and/or vibration, and/or exposing them to surfactants. Larger particles that have collected in the bottom of the baskets, such as iron and sulfur precipitates, coal fines, debris, and the like, can also be removed from the baskets during removal of the spent bio-media balls.  
         [0042]    Although four filtration chambers and baskets are shown, and a particular number of bio-media balls have been indicated for each basket, it is to be understood that these numbers can vary greatly, as well as the size of the filtration tank, baskets and chambers, depending on the amount of contaminants in the water, the water flow rate, the desired filtration time, and other factors.  
         [0043]    With reference now to FIG. 3, a first filtration stage  12 A according to another preferred embodiment of the invention is shown, wherein like parts in the previous embodiment are represented by like numerals. The first filtration stage  12 A is similar in construction to the first filtration stage  12  (FIG. 2), with the exception that the chemical treatment system  50  is removed. Surprisingly, it has been found that that the first filtration stage  12 A is more efficient in removing contaminants from the groundwater than the first filtration stage  12 . With the exemplary first filtration stage  12 A as described, it has been found that approximately 80% iron and 66% manganese can be removed from the groundwater, with no visible staining when the groundwater is discharged into the environment. The first filtration stage  12 A thus greatly improves the quality of the water before it is discharged into the environment or directed to the second filtration stage  18 . By removing a substantial portion of the iron precipitates and other related contaminants at the first filtration stage  12 A, the operating and maintenance costs for the second filtration stage can thus be greatly reduced.  
         [0044]    It is to be understood that the various representative dimensions and capacities for the aeration tower, filtration tank, filtration baskets, and the bio-media balls as shown and described are given by way of example only. The representative dimensions and capacities illustrate only the relative proportions of the preferred embodiment of the system. It is to be understood that the overall dimensions, including the relative proportions and capacities, can be varied without departing from the spirit and scope of the present invention.  
         [0045]    In each of the described embodiments, the groundwater can be inoculated with a particular type of bacteria or with a consortium of bacteria species, preferably before the groundwater enters the aeration tower. In this manner, bacteria beneficial to the filtration of a particular contaminant or group of contaminants, that otherwise may not be naturally present in the groundwater, can be used to remove the contaminants. The bacteria may be retrieved from other water sources or cultured in a laboratory or the like. The metering pump  62  (FIG. 2) can be used to infuse the bacteria into the groundwater.  
         [0046]    It may also be desirable to remove the bacteria after the groundwater exits the filtration tank  54  and prior to entering the second filtration stage. The bacteria can be removed by infusing an oxidizer into the filtered groundwater, which burns the cell walls of the bacteria and destroys them. Preferably, Ozone is injected into the filtered groundwater since it will naturally diffuse into the atmosphere downstream of the injection point. However, other oxidizers can be used, such as chlorine or potassium permanganate.  
         [0047]    While the invention has been taught with specific reference to the above-described embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention. Thus, the described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.