Abstract:
A process and system mixes drinking water stored in large water tower type storage tanks, preventing stratification of the water, by generating large mixing bubbles in the tank&#39;s standpipe, causing mixing of layers of water in the tank through turbulence created as the bubbles rise through the tank. Embodiments of the present invention, adapted for use in storage tanks in which the standpipe serves as both water inlet and outlet, detect flow in the standpipe and provide mixing only when the standpipe is not serving as a water outlet.

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 
     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. 
     For economy, 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. Accordingly, it is desirable to have a means for determining when mixing is needed and for engaging the mixer only at such times. 
     It is further desirable that the mixer system be easy to install and easy to operate. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a means for mixing drinking water stored in large water tower type storage tanks, preventing stratification of the water, by generating large mixing bubbles in the tank&#39;s standpipe, causing mixing of layers of water in the tank through turbulence created as the bubbles rise through the tank. Embodiments of the present invention, adapted for use in storage tanks in which the standpipe serves as both water inlet and outlet, detect flow in the standpipe and provide mixing only when the standpipe is not serving as a water outlet. 
    
    
     
       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 water tower type storage tank; 
         FIG. 2  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 ; and 
         FIG. 3  is a flow chart for operation of an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates an embodiment of the present invention. In this embodiment for elevated oblate spheroid water tank  10 , gas supply  36  makes available a supply of pressurized gas to supply line  30 , interposed in which are valves  29  under control of controller  32 . In response to signaling from controller  32 , valves  29  open to allow the passage of pressurized gas through supply line  30 , which disperses the gas through orifice  33  into the lower portion of tank inlet standpipe  20 . Controller  32  opens valves  29  at such a rate and with such a volume of gas as to generate large mixing bubbles  40  that cause mixing of the water in tank  10  to prevent or remove thermoclines. Controller  32  may open valves  29  in response to data from sensors (not depicted) that indicate a thermocline is forming or has formed. Such sensors may be thermistors indicating temperature differences in different portions of the tank corresponding to the formation of thermoclines. Alternatively, sensors may be used to detect a parameter other than temperature that indicates the formation of a thermocline and/or stagnation of water in portions of tank  10 . 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 yet other embodiments, no such sensors are used: rather, controller  32  opens valves  29  according to a program schedule designed to provide large bubbles for sufficient mixing sufficiently frequently to reduce the incidence of thermoclines in the tank. 
     In any case, because of the high pressure of the head of water over the lower portion of standpipe  20 , bubbles  40  emitted at orifice  33  are initially small and spherical. However, as they rise through standpipe  20  to enter tank  10 , the pressure diminishes with diminishing head of water and bubbles  40  therefore become larger, assuming an oblate shape as they travel upward. By the time bubbles  40  enter tank  10 , they have become large, on the order of 0.5 to 3 or more meters in diameter along the largest dimension, providing mixing currents as indicated by arrows  42 . 
     For some tanks  10 , standpipe  20  serves as both an inlet and an outlet pipe. Preferred operation of the present invention takes place when there is no net outflow in standpipe  20 . Accordingly, for such tanks with two-way flow in the standpipe, it is preferred to add a sensor  31  for water flow in standpipe  20  so that controller  32  opens valves  29  to provide pressurized gas to tank  10  only when there is no net outflow from the tank in standpipe  20 , as illustrated in the flow chart provided in  FIG. 3  discussed below. 
       FIG. 2  is an illustration providing more detail of the mixing effects of large bubbles  40  in tank  10 . The mixing bubbles  40  generate the mixing currents indicated by the arrows  42  (12 arrows shown but only four 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  33  and the gas&#39;s injection pressure. Typical orifices  33  may vary from about ½ inch for small tanks to 1½ inches for very large tanks. Injection pressures may vary from about 40 to about 100 or more pounds per square inch for larger tanks and taller heads of water. 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  33 , one increases the amount of gas injected into water  50  over a given period of time that the valve  29  is open. The size of bubble  40  can be varied by varying the volume of gas injected into water  50  and the period of time taken to inject a given quantity or pulse of gas. Controller  32  creates a pulse of gas by causing valve  29  to open for a short period of time. Depending upon the embodiment, this pulse may last from 0.3 to 0.6 seconds. In one embodiment in which a relatively small pulse of gas is injected over a moderately long period of time, the size of bubbles  40  is approximately 0.5 meters across the largest dimension. In other embodiments in which a large quantity of highly pressurized gas is injected quickly through a larger orifice, the bubbles  40  are approximately 3 meters or greater across in largest dimension at the top of the tank. 
     Turning to  FIG. 3 , depicted is a simple flowchart for operation of embodiments of the invention when the standpipe  20  serves as both an inlet and an outlet for tank  10 . As discussed above, in some embodiments of the invention, stratification of water is indicated by the presence of thermoclines or other indicators of stratification in tank  10  as detected by sensors in the tank. In other embodiments of the invention, it is assumed that stratification develops in tank  10  over time, such stratification indicated simply by the passage of time since the water  50  in tank  10  was last mixed by the release of large mixing bubbles  42  by the invention. In any case, when a stratification condition is indicated  302 , controller  32  detects  304  whether there is outflow in standpipe  20  based upon data supplied by sensor  31 . Only if no outflow is detected  304  in standpipe  20  will controller  32  direct valves  29  to open  306 , thereby releasing gas into standpipe  20  to form large mixing bubbles  42   
     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. The invention is limited only by the following claims and their equivalents.