Abstract:
A means for mixing drinking water stored in large storage tanks, preventing stratification of the water, detects incipient stratification of water along thermoclines, and, responsive to thermocline detection, generates large mixing bubbles toward the bottom of the tank, causing mixing of layers of water in the tank through turbulence created as the bubbles rise through the tank.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. provisional application No. 61/127,376, filed May 12, 2008, entitled WATER SUPPLY MIXING PROCESS. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods for ensuring purity of potable water supplies. More specifically, this invention relates to methods for avoiding stagnation of such water supplies in storage tanks. 
     2. Description of the Related Art 
     Stagnant water is a leading cause of the deterioration of drinking water stored in water storage tanks. When a large capacity tank is underutilized, differential thermal conditions in the tank can cause the contents to stratify in thermoclines, where warmer layers of water meet cooler layers. As is well known in the art, the accumulation and growth of algae, protozoan pathogens such as cryptosporidium and other undesirable organisms is favored at such thermoclines. 
     If, as is often the case, a tank with stratified contents is both filled and emptied from a limited portion of the tank, water supplied by the tank will be from recently filled, fresher strata, while the remaining strata in the tank may age and harbor increasing microbial populations, becoming stagnant. 
     In many public water systems, water is disinfected before it enters the storage tank to ensure that potentially dangerous microbes are killed before the water enters the distribution system. Because residual disinfectant remains in the water after treatment, disinfectant agents such as chlorine, chloramines or chlorine dioxide provide further protection from microbial reproduction after water enters the distribution system. The efficacy of such residual disinfectants diminishes with time, however. When disinfected water is allowed to stratify in storage tanks, older layers of water may lose disinfectant protection altogether, leading to the possibility that such portions of the tank become stagnant despite disinfectant treatment of water prior to transport to the tank. 
     What is needed is a method of preventing or remediating stratification of water in storage tanks. As will be understood by those in the art, stratification can be obviated by sufficient vertical mixing of water in the tank. 
     A number of means for mixing liquids are available to de-stratify stored water. A mechanical mixer, comprised of a screw or blade that is turned by a motor, is commonly employed to mix various liquids. Mechanical mixers, however, are subject to a number of shortcomings for mixing drinking water in storage tanks. 
     Mixing the strata in a typical large water storage tank with a mechanical mixer requires a large amount of energy relative to the amount of water that is actually mixed. Further, agitation of the water in the tank by mechanical mixers can disturb sediment settled in the bottom of the tank, resulting in suspended sediment degrading the aesthetics of the water for drinking. Further still, mechanical mixers are often inefficient, mixing some but not all strata in a storage tank. In addition, acquisition costs can be high for a mechanical mixer having sufficient capacity to mix all the strata in a large storage tank. Yet further, costs are high to retrofit an existing water storage tank with a mechanical mixer, retrofitting further often entailing a need to drain the tank or otherwise temporarily remove the tank from the water distribution system. What is needed are more economical and efficient means of mixing water to eliminate stratification with minimal disturbance to sediment in the tank. What is needed further is such means that can be retrofitted to a water storage tank operation economically and without a need to take the water tank off-line. 
     Mixing water in the tank more than is needed for destratification is undesirable, not only because unnecessary mixing is uneconomical but also because mixing may disturb sediments in the tank, affecting the aesthetic quality of the drinking water. Accordingly, it is further desirable that the mixer that is used to obviate stratification be engaged only when needed, i.e. only when stratification is taking place. The present invention, therefore, provides a means for detecting thermocline formation and engaging the mixer when thermoclines are found. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention detects incipient stratification of water along thermoclines. When thermocline formation is indicated, the mixer generates large mixing bubbles below the thermocline, causing mixing of layers of water in the tank through turbulence created as the bubbles rise through the tank. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing objects, as well as further objects, advantages, features and characteristics of the present invention, in addition to methods of operation, function of related elements of structure, and the combination of parts and economies of manufacture, will become apparent upon consideration of the following description and claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures, and wherein: 
         FIG. 1  is a diagram of an embodiment of the present invention in a rectangular storage tank; 
         FIG. 2   a  is a diagram of a bubble forming plate according to an embodiment of the present invention; 
         FIG. 2   b  is a diagram of an alternative embodiment of a bubble forming plate; 
         FIG. 3  is a diagram illustrating mixing of drinking water in a storage tank by turbulence caused by rising bubbles according to an embodiment of the invention such as illustrated in  FIG. 1 ; 
         FIG. 4  is a diagram illustrating a thermocline detection arrangement according to an embodiment of the present invention; and 
         FIG. 5  is a diagram illustrating a thermocline detection arrangement according to an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a perspective view of a water storage tank  18  in which is installed a mixer according to an embodiment of the invention. The tank  18  is filled with drinking water from below by standpipe  25 . 
     Located in tank  18 , a mixer  26  injects a gas safe for drinking water, such as air, to generate large mixing bubbles. As further discussed in reference to  FIG. 3  below, the mixing bubbles are large enough to move a substantial amount of water as they rise toward the water&#39;s surface. For effective and efficient mixing of drinking water, bubbles generated by the present invention should be large, from approximately one half meter to several meters in diameter. The mixing current, resulting from turbulence from displaced water as the large bubbles rise, mixes the water to obviate stratification. 
     The mixer  26  includes a forming plate  28  to form mixing bubbles from the injected gas, and a valve  29  to permit or prevent the gas from reaching the forming plate  28 . The mixer  26  also includes a distribution line  30  to supply the forming plate  28  with the gas when the corresponding valve  29  is open, and a controller  32  to open and close the valve  29 . For example, in one embodiment, the mixer  26  includes five forming plates  28 , five valves  29 , and five distribution lines  30 , and the controller  32  includes a memory (not shown) and a processor (not shown) to allow a user to input data to control when and how long each valve. 
     Each forming plate  28 , one embodiment of which is shown in  FIG. 2   a , includes an orifice  36 . When the valve  29  is opened, air flows through the distribution line  30  toward the forming plate  28 , and then exits the distribution line  30  through the orifice  36 . The forming plate  28  prevents the air from rising toward the surface of the water until the valve  29  injects more air than the forming plate  28  can hold, at which time most of the air escapes from under the forming plate  28  and forms a large mixing bubble. The large mixing bubble then rises toward the surface of the water. When the valve  29  is closed, air does not flow through the orifice  36 . 
     By outfitting with strong, permanent magnets, plates  28  may be installed in an active, filled water storage tank that is comprised of ferromagnetic or ceramic magnetic material. In such a case, distribution lines  30  are flexible and plates  28  are simply dropped into tank  18 . Referring now to  FIG. 2   b , illustrated is plate assembly  14 , comprising plate  28  operatively connected to flexible distribution line  30 . Plate  28  is further attached to permanent flat magnet  10  via support legs  12 , providing a space between plate  28  and magnet  10 , thereby elevating forming plate  28  some distance above the bottom of tank  18  to allow for the formation of large mixing bubbles on the underside of plate  28 . As will be appreciated by those of skill in the art, the required length of support legs  12  may be varied with considerable tolerance, from an inch or so to a dozen or more inches. In embodiments using magnets for this purpose, it is important that magnet  10  be sufficiently strong to retain plate assembly  14  on the bottom of tank  18  against the buoyancy of both distribution line  30  and plate assembly  14  when large bubbles are formed on the underside of plate  28 . 
       FIG. 3  illustrates the mixing caused by the large bubbles generated by a mixer such as that illustrated in  FIG. 1 . The mixing bubbles  40  generate the mixing currents indicated by the arrows  42  ( 28  arrows shown but only 5 labeled with the reference number  42  for clarity) that mix the water  50 . The strength of the mixing currents  42  depends on the speed at which each mixing bubble  40  travels through the water and the size of each bubble  40 . 
     The speed of the mixing bubble  40  depends on the density of the gas employed in the invention relative to the density of water  50 , and the bubble&#39;s shape. The greater the difference between the densities of water  50  and the gas, the faster the mixing bubbles  40  rise through water  50 . The more aerodynamic the shape of the bubble  40  becomes the faster the bubble  40  rises through water  50 . For example, in one embodiment, the bubble  40  forms an oblate spheroid—a sphere whose dimension in the vertical direction is less than the dimension in the horizontal direction. In other embodiments, the bubble  40  forms a squished sphere having the trailing surface—the surface of the bubble  40  that is the rear of the bubble  40  relative to the direction in which bubble  40  moves—that is convex when viewed from the direction that the bubble  40  moves. 
     The size of the mixing bubble  40  depends on the flow rate of the gas into water  50 . The flow rate depends on the size of the orifice  36  and the gas&#39;s injection pressure. As one increases the gas injection pressure, one increases the amount of gas injected into water  50  over a specific period of time that the valve  29  is open. And, as one increases the area of the orifice  36 , one increases the amount of gas injected into water  50  over a specific period of time that the valve  29  is open. As one increases the diameter of the forming plate  28  one increases the amount of gas the forming plate  28  can hold before the gas escapes it. For example, in one embodiment the size of the bubble  40  is approximately 0.5 meters across its largest dimension. In other embodiments, the bubble  40  is approximately 3 meters or greater across in largest dimension. 
       FIG. 4  illustrates the thermocline detection aspect of this invention. Depicted is a cylindrical tank  402  in which a thermocline  404  has developed. Embodiments of the present invention detect the presence of thermocline  404  by comparing temperature readings from sensors  405  through  408 . The presence of thermocline  404  is indicated by significant difference between temperature readings from sensors  407 ,  408  located above thermocline  404  and sensors  405 ,  406  located below thermocline  404 . While the actual value of a temperature difference indicating a thermocline will vary with tank configuration, prevailing weather conditions, etc., a temperature difference of 4 to 10 degrees C. between different levels of water in the tank may indicate the presence of thermocline  404 . Responsive to detecting such a temperature difference, controller  32  directs valves  29  to provide pressurized gas to supply line  30 , providing gas to form bubbles under plates  28  as discussed above in reference to  FIG. 1 . 
     As will be appreciated by those of skill in the art, sensors  405 - 408  may be any form of electronic sensor, such as a thermistor, capable of measuring temperatures in the range of 0 to 100 degrees C. For tanks comprised of ferromagnetic material, sensors  405 - 408  may comprise a means of magnetic adhesion, for ease of installation. For tanks without substantial thermal insulation, temperature sensors  405 - 408  may adhere to the exterior of tank  402 . 
     Alternatively, sensors  405 - 408  may detect a parameter other than temperature that indicates the formation of a thermocline and/or stagnation of water in portions of tank  402 . Such parameters may include levels of free chlorine, oxygen, nitrates, biological oxygen demand, and other parameters known to those of skill in the art, whose differential values at different levels in the tank indicate that water stratification is taking place. 
     In the depicted embodiment, four sensors  405 - 408  are illustrated. As will be appreciated by those of skill in the art, the actual number and location of sensors required for accurate thermocline detection are determined by several factors, principal among which is the geometry of tank  402 . Accurate thermocline detection may be obtained with sensors spaced farther apart vertically in tanks that are taller and narrower than in tanks that are shorter and wider. Spacing between sensors may vary from a meter or less to a dozen or more meters vertically. 
     The actual number of sensors employed may be as few as two or as many as ten or more. What is required is that a sufficient plurality of sensors be employed and placed so that there is a difference in parameter measurement between at least one of the sensors and the rest of the sensors to indicate that a thermocline is forming or has formed. 
       FIG. 5  depicts an alternative embodiment of the thermocline detection aspect of the present invention. Depicted is tank  502 , in which cable  508  is affixed to run vertically from near the bottom of tank  502  to near the top of tank  502 . In some embodiments, cable  508  may be affixed to the floor of tank  502  by means of a strong permanent magnet adhering to the floor of tank  502 . Affixed on cable  508  approximately 18 inches from the floor of tank  502  is lower sensor  505 . Floating on the surface  512  of water in tank  502  is float  510 , fashioned and disposed to traverse up and down the length of cable  508  as water level  512  rises and falls in tank  502 . Affixed to float  510 , and depending about 12 inches below it, is upper sensor  506 . In a manner similar to that described in relation to  FIG. 4  above, sensors  505  and  506  provide temperature data to controller  32 . When the difference between the temperature detected by sensor  505  and the temperature detected by sensor  506  exceeds a predetermined value, presence of a thermocline  504  is indicated, and, responsive to such indication, controller  32  directs valves  29  to open, providing gas through distribution line  30  to forming plate  28 , thereby forming large bubbles which generate mixing currents, mixing the contents of tank  502  to break up thermocline  504 . 
     Just as observed in relation to  FIG. 4  above, sensors  505  and  506  may, in alternative embodiments, detect a parameter other than temperature that indicates the formation of a thermocline and/or stagnation of water in portions of tank  502 . Such parameters may include levels of free chlorine, oxygen, nitrates, biological oxygen demand, and other parameters known to those of skill in the art, whose differential values at different levels in the tank indicate that water stratification is taking place. 
     Although the detailed descriptions above contain many specifics, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Various other embodiments and ramifications are possible within its scope, a number of which are discussed in general terms above. While the invention has been described with a certain degree of particularity, it should be recognized that elements thereof may be altered by persons skilled in the art without departing from the spirit and scope of the invention. Accordingly, the present invention is not intended to be limited to the specific forms set forth herein, but on the contrary, it is intended to cover such alternatives, modifications and equivalents as can be reasonably included within the scope of the invention.