Patent Publication Number: US-2002008057-A1

Title: Organic slurry storage basin cover and method of constructing and deploying cover

Description:
CROSS REFERENCE TO RELATED APPLICATION  
     [0001] This application claims the benefit of U.S. provisional application 60/179,919 filed Feb. 3, 2000. 
    
    
     
       OBJECT OF THE INVENTION  
       [0002] The purpose of the invention is to successfully fabricate, deploy and secure a geotextile cover which will reduce malodorous emissions from the surface of an open outdoor, earthen basin, sloped sidewall, organic slurry storage structure. This invention comprises such a cover, a method of constructing such a cover and a method of deployment of such a cover, as well as the resultant lagoon.  
       DESCRIPTION OF OLD METHODS OR APPARATUS AND DISADVANTAGES  
       [0003] Malodorous emissions from open organic slurry storage structures have been a nuisance for nearby receptors throughout history. In the past few years, the advent of large scale livestock facilities and subsequent malodorous gas emissions therefrom have become a source of national controversy.  
       [0004] Many methods have been tried to control malodorous emissions on open organic slurry storage basins: impervious covers with and without gas collection systems, bioaugmentation, chemical additives, enzymes and full and partial aerobic treatment systems. Impervious covers are quite costly and gas collection systems require significant initial capital investment as well as ongoing maintenance expense. Bioaugmentation, chemical additives and enzymes have not delivered consistent odor control and usually require significant ongoing labor and maintenance costs in addition to product costs. Aerobic methods work most convincingly, but excessive cost has prevented widespread acceptance of aeration as the preferred method of odor control. Floating biomats of straw provide partial aeration, have performed well and are reasonably economical. But it is difficult to place and maintain a solid straw cover on areas greater than two hundred feet wide. They also require regular maintenance to cover increasing slurry storage surface area as sloped earthen basins fill and to fill holes that appear as straw sinks to the bottom of the basin. Crossover piping and recycle pumps and plumbing have been clogged with sunken straw, potentially compromising basin capacity and requiring additional manpower to periodically unclog problem appurtenances. Sinking straw also increases organic loading in the basin, potentially compromising design storage capacity. 
     
    
    
     DESCRIPTION OF DRAWINGS  
     [0005]FIG. 1 is a top view of an embodiment of the invention in an early stage of deployment.  
     [0006]FIG. 1A is a close-up view of a portion of the embodiment of FIG. 1.  
     [0007]FIG. 2 is a side view of the embodiment of FIG. 1 at a later stage of deployment.  
     [0008]FIG. 3 is a top view of the embodiment of FIGS. 1 and 2 at a later stage of deployment.  
     [0009]FIG. 3A is a clasp view of a portion of the embodiment of FIG. 3.  
     [0010]FIG. 4 a top view of the embodiment of FIGS.  1 - 3  at a later stage of deployment.  
     [0011]FIGS. 4A and 4C are clasp views of alternative embodiments of FIG. 4.  
     [0012]FIG. 4B is a clasp view of a portion of the embodiment of FIG. 4.  
     [0013]FIG. 5 is a side view of an alternative embodiment of the invention at a later stage of deployment.  
    
    
     DESCRIPTION OF INVENTION  
     [0014] A lagoon basin cover is fabricated on site from geotextile, a polypropylene fiber, non-woven, needle punched, fabric stabilized to resist degradation due to ultraviolet light exposure. This felt-like material has a specific gravity less than 1 and, therefore, floats like a sponge on water. It will stretch half of its original length, has a grab tensile strength of 200 lbs. or more, weighs 8 oz/yd or more, has an apparent opening size equal to a 100 U.S. Sieve, and has a permittivity of 1.5 sec or less.  
     [0015] Stock rolls of geotextile are fabricated into a single cover approximately the size of the top inside dimensions of the open storage structure.  
     [0016] Each basin to be covered is evaluated to select the most efficient side of the basin upon which to build the cover. A minimum fabrication area width is required along the length of the berm upon which the cover is fabricated to accommodate the width of the rolls of geotextile being used. If the area width constraint is met, the berm selected will be the longest berm if a rectangle, the longest side adjacent to the right angle if the basin is a right triangle, along the hypotenuse if an obtuse triangle, along either long side of an isosceles triangle, and along the longest side of a polygon. A circular or oblong basin cover will be fabricated in an area of the above described minimum width and parallel to the longest distance across the surface area of the basin. This evaluation and design process may or may not entail the use of specially developed computer software.  
     [0017] A roll of geotextile fabric is rolled out along the berm selected for fabrication and another layer of geotextile is rolled out on top of the first. The fabric is laid out in fanfold fashion. One edge of the under layer of geotextile is rolled back to overlap the edge of the next layer by approximately three inches. The seaming process described below is employed to join the two layers of geotextile fabric together. Another layer of geotextile is then rolled out on top of the top layer of seamed geotextile. The unseamed edge of the layer of geotextile under the top layer is rolled back to overlap the top layer of geotextile by approximately three inches. This process will continue until the last layer of geotextile necessary to cover the inside top surface area dimensions of the basin has been seamed to make a one-piece cover. Individual geotextile layer lengths are cut to the length necessary and/or are shifted along the length of the layer beneath it to create the desired shape coverage of surface area.  
     [0018] Seaming is best accomplished using heat to fuse overlapping seams together, but may also be accomplished with a sewing machine, though when sewed an ultraviolet light inhibiting thread must be used.  
     [0019]FIG. 1, by way of example, shows a lagoon basin  10  in top plan view that is generally rectangular in shape. A cover  14  has been constructed on site along a long berm  12 . The cover  14  is constructed of multiple interconnected layers  16 ,  18  of geotextile fabric of the type described above. A heat seaming device  20  has been used to connect adjacent edges.  
     [0020] The unseamed edge of the top layer of the cover is wrapped around floatation and tacked together using a hot air gun to lightly fuse the adjoined geotextile material together. This is done to create a leading edge of the cover that will float over scum and debris on top of the basin when the cover is pulled into place across the basin. Deployment ropes the same size as the tethering ropes are attached to the floating leading edge of the cover at up to eight points. These deployment ropes are stretched across the basin to the opposite berm. FIG. 2 shows a float tube  22  connected to the top or leading edge  24  of the cover  14 . The leading edge  24  is wrapped around the float tube  22  and secured at spot weld points  26 . A number of deployment ropes  28  are connected at one end to the float tube  22 .  
     [0021] Posts with flange plates are pressed into the middle of the top flat area of the berm approximately equidistant to each other. These posts have pulleys attached through which the deployment ropes are threaded and are brought to a central point where a single mandrel is located. FIG. 3 shows the cover  14  preparatory to deployment over the lagoon basin  10 . Eight deployment ropes  28  have been extended to the berm  30  opposite berm  12 . Posts  32  are secured to berm  30  at points directly aligned with attachment points of deployment ropes  28  to the float tube  22 . Each post  32  has a flange  34  (see FIG. 3A) that carries a pulley assembly  36 . The deployment rope is trained through the pulley assembly  36  and extends to a mandrel  40 . The cover is deployed using a single segmented mandrel  40  to which the deployment ropes  28  are attached. The mandrel is rotated to wind the deployment ropes onto the mandrel. This pulls the cover squarely across the basin as the fan-folded cover on the berm unfolds to cover the surface area.  
     [0022] The cover is normally tethered from points approximately every thirty feet around the perimeter of the cover back to secure points  42 , described below, but varying distances may be used. The tether  44  is connected to the cover without puncturing the geo-textile material and the tethering media is ultraviolet light stabilized and possesses sufficient tensile strength not to break under weather related stress. Traditional steel fence posts or ground anchors may be used as tethering points to attach the cover to the earthen berm of the basin. They are inserted into the ground a depth sufficient to withstand weather related stress at a point set back from the inside berm so as not to impair the integrity of a compacted clay liner. The cover must be positioned squarely in place to cover the entire surface area of the organic slurry storage structure FIG. 4 is a top plan view of the cover  14  deployed over the lagoon basin  10 . Tether points  42  are installed along the berms of the lagoon as indicated above. For example, the edges of the covers may be secured to fence posts as indicated in FIG. 4A, or to ground anchors as indicated in FIG. 4C. As indicated in FIG. 4B, the edge of the tarp  14  is wrapped around a ball  45  to provide a neck to tie the tether  44 .  
     [0023] As an alternate form of tethering in high wind prone geography, the margin of the cover may be placed in a trench with a vertical side wall that is dug around the perimeter of the basin above the operational high water line, typically a distance A of about two and one-half feet from the edge. The trench is back filled after the margin of the cover has been placed in the open trench. The margin of the cover is buried at a depth sufficient to withstand weather related stress. FIG. 5 shows a vertical side walled trench formed in a berm. A wide edge margin  48  of the cover lies in the trench and extends out of it. A fill material  50  fills the trench and anchors the cover.  
     [0024] Once in place, the cover suppresses the release of malodorous gases in four ways:  
     [0025] First, the cover fabric is porous and it restricts the release of gas to a volume equal to the permittivity. Second, vapor pressure under the cover causes small areas of the cover to inflate and rise from the surface of the organic slurry in a low profile manner. Under observation, the cover appears to develop sand dune-like ripples. These elevated areas create a humid, aerobic environment along the unsubmerged surface of the cover. The presence of oxygen enables endemic facultative and/or aerobic bacteria to exist and break down malodorous gases passing through the cover to the atmosphere. Of particular note is significant nitrification/denitrification stimulated in this environment. Third, the anaerobic zone on the underside of the cover provides a substrate to which anaerobic bacteria attach and break down malodorous gases passing through the cover to the atmosphere. Fourth, the thin layer of geotextile forms a barrier to mass transfer of gas, especially air-phase limited gases, based on two-film theory. This provides especially effective reduction in the transfer of short chain carbon ring volatile organic compounds through the cover to the atmosphere. Reduced disruption of the equilibrium of water-phase limited gases improves the retention time and subsequently improves reduction and release of these gases. The result is effective odor control.  
     [0026] The physical barrier of the cover plays an important metering function. In explanation, large volumes of malodorous gases are normally stripped from open structures due to agitation caused by wind, wave action or pummeling rainfall. The cover prevents stripping of gases and the permittivity (or porosity) of the fabric helps to meter malodorous gas through the biofilm environment ubiquitous in and under the cover.  
     [0027] The cover will remain in place year around once in position and will not require regular inspection or maintenance. The geotextile fabric has superior elongation properties that allow it to accommodate freeze-thaw without damage. If the storage structure is emptied periodically, the cover may be loosened and pulled back during agitation and pump-out of stored organic slurry. It is extremely important to secure the loosened cover so it is not drawn into agitation and pumping equipment. After removal has been completed, the cover should be retethered. The cover may also be supported by cables attached to posts that, when tightened, create an access point for agitation and pumping equipment under the cover. This option is preferable and will remove the need to loosen and pull back the cover during agitation and pump-out, which is labor intensive. Ultraviolet light degradation will likely be the eventual cause of cover failure. It is anticipated that the geotextile used will last at least three full calendar years, though actual experience for this duration has not yet been recorded. When the fabric has deteriorated, remaining pieces should be removed from the surface of the storage structure, piled, and left to drain and dry. The material may be incinerated, or it may be taken to a landfill. A spent cover may also be salvaged as underlayment to support site roadbeds and reduce soft spots in roadways during wet weather. A new cover should be fabricated and positioned in place.