Abstract:
A floating fine-bubble aeration system for dissolving a gas in a liquid in which the system is floated. A grid of uniformly spaced fine-bubble diffusers is used to establish large area of bubble-laden liquid, having a substantially uniform density. The uniform density prevents the formation of any upwardly directed currents within the interior portion of the area. Bubble residence time is maximized, due to the absence of the currents, thus increasing the efficiency of dissolving the gas in the liquid.

Description:
FIELD OF THE INVENTION 
     This invention relates to an apparatus for aerating liquids held in a containment structure. More particularly the invention relates to an apparatus for aerating liquids contained in large structures wherein the apparatus is maintained in an operating position by floatation. 
     BACKGROUND OF THE INVENTION 
     In the treatment of wastewater, in the conditioning of water for aquatic life, and for various industrial and environmental processes, it is necessary to dissolve oxygen or other gases in a liquid so as to promote bacterial action, provide oxygen for survival of aquatic life, chemically oxidize substances and various other reasons. In processes requiring oxygen, it is well known to compress air, (which contains approximately 21% oxygen) and inject it in bubble form beneath the surface of a liquid so as to dissolve a portion of the oxygen of the air bubbles into the liquid being treated. Factors such as size of the bubbles, bubble residence in the liquid, temperature of the air and liquid, depth of injection, etc. determine the percentage of the oxygen that is dissolved in the liquid prior to the oxygen-containing bubbles reaching the top surface of the liquid. By optimizing various factors a more efficient aerating process can be carried out so as to maximize the oxygen dissolved per unit energy input to the aeration system. The factors contributing most to the efficiency of the system are bubble size and bubble residence in the liquid. 
     When bubbles are produced from a given quantity of air, the area of gas/liquid interface is greater for small bubbles formed from that quantity of air than for larger bubbles formed from that quantity of air. 
     Bubble residence time in a liquid is primarily dependent on 1) size of the bubble, and 2) factors other than buoyancy that move a bubble in a vertical direction toward the top surface of the liquid. Regarding vertical movement due to bubble size and buoyancy, the smaller the bubble the slower the vertical movement. 
     The primary factor in bubble residence, other than buoyancy and its relation to bubble size, is upwardly directed currents in the liquid which add velocity to the bubbles and decreases the time it takes a bubble to reach the top surface of the liquid. The upwardly directed currents can be caused by various conditions, however, a prime cause found with prior art aeration devices is liquid density induced currents. Liquid density induced currents are described with reference to FIGS. 1 and 2. In FIG. 1, the body of liquid  22  has portions  20 , having bubbles distributed throughout, and portions  24  which are substantially free of bubbles. Such a condition is found, for example, where concentrated areas of bubble-producing devices such as  26  are spaced apart a relatively large distance (for example 20-40 ft.) in a wastewater treatment pond. When a condition as described exists, portions  20 , having bubbles throughout, have a lower density than the surrounding bubble-free portions  24  and upwardly directed currents, indicated by arrows  28 , are induced by density gradients. 
     Another example of the density induced currents is described with reference to FIG.  2 . In FIG. 2, reactor tank  30 , has bubble-providing devices  32  located solely along two sides of the tank. Rolling currents  34  are induced as a result of the density gradients and they increase the upward vertical velocity of bubbles  36  thus reducing the bubble residence time. 
     An additional problem found with some prior art aeration devices having bottom support members, experienced especially during installation or maintenance, is the need to drain the containment structure. Such a need can present enormous problems for many installations. The devices of FIGS. 1 and 2 are both bottom mounted. 
     The apparatus and methods of the present invention overcome those problems and other deficiencies found in prior art aerators. 
     SUMMARY OF THE INVENTION 
     The present invention includes a gas distribution network having input and output apertures for receiving a gas and conveying it to output apertures which are in communication with fine-bubble producing devices which receive the gas, form bubbles, and discharge the fine bubbles into the liquid in which the apparatus is submerged. A floatation device is used to maintain the fine-bubble producing devices at a selected depth below the top surface of the liquid absent any vertical support from structural members bearing on the containment structure holding the liquid. 
     The fine-bubble producing devices, such as membrane disc diffusers or membrane tube diffusers are spaced uniformly to form a two dimensional grid with a spacing which provides a substantially uniform density of bubbles above the grid. Liquid density induced currents are minimized and/or prevented from developing over a large portion of the grid because of the substantially uniform density of bubbles throughout the grid area. A grid size is determined which minimizes the affect of the unavoidable liquid density induced currents found near the periphery of the grid. 
     Other specific features and contributions of the invention are described in more detail with reference being made to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of a prior art aeration system for describing liquid density induced currents; 
     FIG. 2 is an elevational view of a prior art aeration system having bottom support members in use in a reactor tank; 
     FIG. 3 is a plan view of an aeration apparatus of the invention incorporating a rigid support system for components of the apparatus; 
     FIG. 4 is a plan view of the aeration apparatus of FIG. 3 shown without the support system; 
     FIG. 5 is a perspective view of an aeration apparatus of the invention shown in floating operating position in liquid of a containment structure; 
     FIG. 6 is an elevational view of an aeration device of the invention for describing liquid density induced currents found at the periphery of a grid of fine-bubble diffusers; 
     FIG. 7 is a cross-sectional view of the apparatus of the invention taken in a plane indicated at  7 — 7  of FIG. 4; 
     FIG. 8 is a vertical cross-sectional view of a fine-bubble membrane disc diffuser; 
     FIG. 9 is a perspective view of a feeder conduit of the invention for describing attachment means for fine-bubble disc diffusers; 
     FIG. 10 is a cross-sectional view of the apparatus of the invention taken in a plane indicated at  10 — 10  of FIG. 4; 
     FIG. 11 is a plan view of a second embodiment of the invention wherein cylindrically shaped membrane diffusers are utilized; 
     FIG. 12 is a cross-sectional view of the second embodiment of the invention taken in a plane indicated at  12 — 12  of FIG. 11; 
     FIG. 13 is an elevational view of apparatus for a method used to determine the efficiency of aeration systems; 
     FIG. 14 is a graph showing standard wire aeration efficiency vs delivered power density for various aeration systems; 
     FIG. 15 is a graph showing oxygen transfer efficiency vs delivered power density for various aeration systems. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 is a plan view of a portion of the preferred embodiment of the apparatus for aerating a liquid. Elongated manifold  40  has a plurality of elongated feeder conduits  42  equally spaced and rigidly attached. The manifold  40  and feeder conduits  42  are preferably fabricated of stainless steel tubing having a rectangular cross-section. However, other types of piping of differing material and differing cross sections can be used. The attachment is preferably made by welding. Referring to FIG. 7, the attachment of each feeder conduit  42  to the manifold  40  is made at a site of an output aperture  44  formed in the manifold  40  by drilling, metal punching, or the like. In order to provide the necessary volume of gas to each of the feeder conduits  42 , the manifold  40  is of a larger cross-section than the feeder conduits  42  and it is preferable to align bottoms of the conduits and manifold in the same plane as shown at  46  to facilitate supporting the conduits as discussed below. Although the aperture is shown as being circular in shape other shapes are possible. In the preferred embodiment, the manifold  40  has a nominal dimension of 4″×6″, and the feeder conduits  42  have a nominal dimension of 2″×2″. The attachment is made to the manifold on the 6″ face. Both ends of the manifold and non-attached ends of the conduits are closed. Gas is supplied to the manifold through at least one input aperture as shown at  48  of FIG. 3 which can include a threaded fitting for connecting a gas supply line. Additional input apertures can be provided if required for the volume of air being supplied. In a wastewater treatment pond installation, for example, the gas is supplied from an on-shore compressor via a flexible hose. 
     A plurality of fine-bubble producing means are attached to each feeder conduit. In FIG. 3, membrane disc diffusers are shown at  50 , substantially evenly spaced along conduits  42 . FIG. 8 is a vertical cross-section of a membrane disc diffuser  50  showing gas chamber  52 , membrane support disc  54 , perforated membrane  56 , and base  58 . Other types of fine-bubble producing means are available such as cylindrically shaped membrane diffusers referred to as membrane tube diffusers. Any fine-bubble diffuser of the membrane type can be used to carry out the invention. 
     FIGS. 9 and 10 show the preferred method for attaching disc diffusers to the conduits. In FIG. 9 a threaded member  59 , such as a bolt, is welded or otherwise attached to conduit  42 . Near the threaded member, at least one aperture is provided through the wall of the conduit as shown at  60 . Referring to FIG. 10, an “O”-ring  62  is placed between diffuser  50  and conduit  42  prior to placing the diffuser over bolt  59 . Nut  64  secures the diffuser onto the conduit. In operation, gas from conduit  42  passes through aperture  60 , through passages  66  in the diffuser base and into gas chamber  52 . The gas then passes through an aperture  68  in support plate  54  to slightly inflate perforated membrane  56 . Fine bubbles are formed when the gas passes through the perforations which are very small in cross section. Bubbles 1 mm to 10 mm in diameter are typically formed. Bubbles having a diameter less than 5 mm are preferred. In fabricating the conduits, it is preferred to provide an excess of uniformly spaced threaded members  59  and associated apertures  60  for use if an increase in aeration is required in the future. Plugs for the apertures are easily inserted to prevent the escape of the gas. 
     In order to provide increased rigidity and in some cases additional weight, a frame  70  (FIG. 3) preferably fabricated of “I”-beams is provided below the gas distribution means of the manifold  40  and feeder conduits  42 . The rigid frame provides support under the manifold and under the conduits at a point on each conduit toward its non-attached end. Any means for attaching the manifold and conduits to the frame is acceptable. As will be described below, the above described assembly, when in operation, must have a weight greater than the liquid it displaces when submerged in the liquid as it is important that the portion of the apparatus, as depicted in FIG. 3, not float. The selection of manifold, conduit, and frame material and dimensions must be coordinated to achieve that requirement. Although “I”-beams of mild steel are preferred, other materials and shapes can be selected for use in applications where the liquid is highly corrosive or other conditions prevail. FIG. 4 depicts components of the invention, without the frame support, for better clarity in viewing the manifold  40 , feeder conduits  42 , and diffusers  50 . 
     FIG. 5 shows the complete apparatus for aerating liquid as positioned in a liquid when in use. In addition to the assembly shown in FIG. 3, the apparatus includes floatation means for properly positioning the fine-bubble producing diffusers. The floatation means includes buoyant members  72  and cables  74  attached to attachment devices  76  on frame  70 . The buoyant members are of any suitable fabrication to provide the buoyancy necessary to maintain the grid of uniformly spaced diffusers  50  at a selected depth below the top surface of the liquid (indicated at  77 ) in which they are submerged. The diffusers, which substantially lie in a plane, are held parallel to the top surface of the liquid by the floatation means. Although not shown cables can be extended from the floatation mean in a generally horizontal direction to maintain positioning of the apparatus in a wastewater treatment pond or the like. FIG. 6 is an elevational view of the apparatus in working position. Attachment devices  78 , which are free to rotate about the floatation device  72  are used to attach cables  74  and can also be used for the horizontal positioning described above. 
     The primary objective of the present invention is to obtain a high oxygen transfer efficiency; that is oxygen dissolved in a liquid per energy input. Although the description below will focus on oxygen from air being dissolved in a liquid such as wastewater, the system is applicable to other gases being dissolved in other liquids. 
     A high oxygen transfer efficiency is obtained by providing fine bubbles, thereby providing more gas/liquid interface area, as discussed above, and by promoting a maximum bubble residence time in the liquid. 
     The present invention uses fine-bubble diffusers, as discussed, so as to maximize the gas/liquid interface area and to minimize the upward vertical velocity due to buoyancy. The method for increasing residence time is now discussed. The concept of liquid density induced currents in liquids was discussed in relation to FIGS. 1 and 2 wherein, in the prior art practice of locating aeration means at a plurality of locations in a wastewater treatment pond, for example, bubbles discharged from diffusers  26  are propelled to the top surface of the liquid by both buoyancy and the currents, depicted at  28 , caused by the difference in liquid densities at  20  and  24 . The present invention overcomes most of the influence due to those currents. 
     The improvement to the oxygen transfer efficiency is obtained by configuring the apparatus such that the bubbles acted on by the induced currents are a small percentage of the total bubbles discharged from the diffusers. That condition is obtained by providing a large field of liquid having substantially the same density in order that only a small portion of the bubbles, that is those at the periphery of the field, are influenced by liquid density induced currents. 
     Referring to FIG. 6, aerating apparatus  80  has uniformly spaced diffusers  50  positioned a depth d below the top surface  77  of liquid  82  which is being aerated. The diffusers are substantially uniformly spaced in a two dimensional grid as shown in FIG. 4. A 2-dimensional orthogonal coordinate system for referring to spacing in the grid is indicated at  84 . Referring again to FIG. 6, with proper spacing in the x-y directions a substantially uniform bubble density, and thus liquid density, can be achieved in a horizontal plane, such as the plane indicated as h-h, beginning at a distance of approximately 2 to 4 feet above the diffusers  50 . As depicted in FIG. 6 the discharged bubbles rise from the diffusers initially in a truncated cone shaped pattern so as to form the uniform density area above the entire grid. With a uniform density in the volume of liquid defined horizontally by the plane indicated by h-h and the top surface  77  of the liquid  82 , and defined vertically by the four peripheral edges of the grid, there are no significant liquid density gradients within that volume to cause liquid density induced currents. The only portions of the system at which the currents are generated are at boundaries of the grid as depicted by arrows at  86  in FIG.  6 . The relative upward vertical velocities of the bubbles are indicated by arrows  88  and  90 . The majority of the bubbles have a velocity and direction indicated by arrows  88 . A small proportion of the bubbles, at the periphery of the grid, have a greater velocity and direction indicated by larger arrows  90 . 
     The detrimental currents are found at the grid boundary, therefore the percentage of bubbles being influenced by the currents in relation to the total amount of bubbles discharged can be reduced by increasing the size of the grid of diffusers. The relationship of area to edges for a square, having edge “A”, for example, is A 2  to 4A and it can be seen by substituting numbers of increasing value that the area to edge relationship increases with increasing size. The same type relationship occurs with a rectangular grid configuration. A practical size for a preferred diffuser grid is about 16 feet by 32 feet as the advantages in size are surpassed by practical problems encountered when the size is too large. A minimum size for a rectangular grid is about 12 feet by 12 feet. A size wherein at least 25% of the bubbles discharged are not influenced by density induced currents improves the efficiency significantly. Larger sized grids are preferred so as to increase the percentage of bubbles not influenced. 
     FIG. 11 shows a second embodiment of the apparatus of the invention which is provided with membrane tube diffusers  92  having a cylindrical shape. Diffusers  92  are attached to a rectangular manifold  94 , preferably of tubing having a rectangular cross-section, having at least one input aperture  96  for inputting the aerating gas. The diffusers are equally spaced along both sides of the longer legs of the rectangular shaped manifold. Attachment can be made, as shown in FIG. 12 by providing apertures in opposing walls of the manifold as at  98  and passing a threaded nipple  100 , into which at least one gas supply aperture  102  has been provided, through the opposing apertures and then threading a membrane tube diffuser  92  onto each threaded end of the nipple. The nipple is of a length so as to position a base  104  of each diffuser against sides of the manifold tubing. A gasket  106  is positioned between each base and manifold. A suitable frame underneath the manifold is provided for rigidity and weight, if needed, and the assembly is suspended in a manner similar to that shown in FIG. 5 with reference to the first embodiment. 
     In the membrane disc diffuser system of FIG. 4, the membrane tube diffuser system of FIG. 11, or any other system wherein diffusers are uniformly spaced in a grid, a two dimensional array of uniform repeating spacing areas such as  107  of FIGS. 4 and 11 can be described for defining an acceptable “uniform” density of diffusers and thus “uniform” liquid density. For purposes of design, it is convenient to divide the area of the grid into a plurality of uniformly shaped and repeating areas which fill the area of the grid and provide one diffuser per uniformly spaced area. With such a method for describing the uniformity of the system, the shape of the diffusers is not of concern and the definition of “uniform” can be quantified. The ideal shape of the spacing areas is a square. However, it has been determined that an oblong spacing area having side and end measurements with a ratio of up to 4:1 only slightly lowers the efficiency of the system. For systems wherein the diffuser is not square or circular, it is preferable to generally match the shape of the diffuser with the shape of the spacing areas as is shown in FIG. 11 which depicts the cylindrically shaped diffusers in oblong spacing areas. It is preferred that a maximum dimension for a side of a spacing area be 60 inches. 
     The efficiency of oxygen transfer for differing systems can be measured using apparatus depicted in FIG. 13. A collecting hood  108  is placed above the aeration system to be evaluated such that edges of the hood extend into the liquid to form a closed cavity  110 . A suction line  112 , in communication with cavity  110 , conveys the sample offgas, from bubbles which have surfaced, to an oxygen analysis device  114  which analyses the percent oxygen in the captured gas. Knowing the percent oxygen in the gas captured in cavity  110  from the bubbles and the percent oxygen in the compressed air supplied to the manifold, the percent transferred to the liquid can be calculated. To enable comparisons of different systems the above analysis is typically carried out under process water conditions. 
     An important consideration in the operation of aeration processes in water treatment plants and the like is energy expended per quantity of oxygen dissolved in the liquid. Graphs showing the efficiency of different systems are presented in FIGS. 14 and 15. 
     Data for both of the graphs were obtained in tests conducted in clean water with diffusers located at a depth of 15 feet below the top surface of the water. In FIG. 14, the x axis denotes delivered power density expressed in hp/1000 ft 3  of air compressed; the y axis denotes pounds of oxygen dissolved per wire hp-hr (wire hp is the power input to the compressor or the like). 
     Curve A denotes the efficiency of a coarse bubble aerator wherein the efficiency is strongly decreased because of the relatively low gas/water interface area per given volume of gas and the greater upward velocity that larger bubbles have in comparison with smaller bubbles. 
     Curve B denotes the efficiency of an aerator wherein a liquid pump and a gas compressor are both used to dissolve the gas in the liquid. The additional energy required for the liquid pump, not require in processes discussed above, decreases the efficiency of that type system. 
     Curves C and D denote the efficiencies of systems located in large vertical walled tanks similar to that shown in FIG.  2 . In the system of curve C, fine-bubble diffusers, similar to those of the present invention, were installed along two long walls of the tank as shown in FIG.  2 . As a result of having areas of differing water density, liquid density induced currents such as those indicated by arrows  34  of FIG. 2 were present. The use of fine-bubble producing diffusers improve the efficiency over coarse-bubble diffusers. Curve D denotes the efficiency of an aerator system in a large vertical walled tank, such as in FIG. 2, however, the diffusers were arranged in a grid having uniform spacing so as to provide a substantially uniform density liquid throughout the tank. Such uniform density prevented liquid density induced currents from being established. The high efficiency, denoted by curve D, is attributable to 1) small bubbles having a favorable gas/liquid interface area, and 2) a long residence time due to the absence of any pronounced upward currents due to density gradients. 
     An aeration efficiency approaching that indicated by curve D is achievable with a large floating fine-bubble system of the invention as the conditions are similar within a large portion of the system. As discussed above, only a portion of the system of the present invention is influenced by liquid density induced currents at the periphery of the grid. 
     The graph of FIG. 15 expresses the efficiency of aeration systems as standard oxygen transfer efficiency express as a percent of oxygen transferred to the liquid. Conditions for curves A-D correspond to those described in relation to the graph of FIG.  14 . 
     While specific materials, dimensions, fabricating steps, etc. have been set forth for purposes of describing embodiments of the invention, various modifications can be resorted to, in light of the above teachings, without departing from the applicant&#39;s novel contributions; therefore in determining the scope of the present invention, reference shall be made to the appended claims.