Abstract:
A system and method of wastewater treatment in a tank provides large mixing bubbles generated in the lower portion of the tank. In embodiments providing aerobic wastewater treatment, the system further provides oxygen to the wastewater by way of tiny aerating bubbles provided by diffusers. At least one sensor in the tank provides measurements of at least one wastewater treatment parameter such as total suspended solids, dissolved oxygen, ammonium or nitrate. An automatic controller in the system, responsive to measurements provided by the sensor, adjusts the rate of mixing provided by the large mixing bubbles. In some aerobic embodiments, the controller, responsive to measurements from the sensor, further adjusts the rate of oxygenation supplied to the wastewater by the tiny aerating bubbles.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from U.S. provisional application No. 60/681,717, filed May 16, 2005, entitled MIXER AND PROCESS CONTROLLER FOR USE IN WASTEWATER TREATMENT PROCESSES. 

   BACKGROUND 
   Water is frequently used to transport unwanted materials—waste—to a facility where the waste is removed or neutralized in the water. For example, water carries most sewage and industrial waste, such as chemicals, in the form of wastewater to a treatment facility where the water is treated and then returned to the environment for future use. The wastewater treatment process typically includes three general phases. The first phase, or primary treatment, involves mechanically separating the dense solids in the wastewater from the less dense solids and liquid in the wastewater. This is typically done in sedimentation tanks with the help of gravity. The second phase, or secondary treatment, involves the biological conversion of carbonaceous and nutrient material in the wastewater to more environmentally friendly forms. This is typically done by promoting the consumption of the carbonaceous and nutrient material by bacteria and other types of beneficial organisms already present in the wastewater or mixed into the wastewater. The third phase, or tertiary treatment, involves removing the remaining pollutant material from the wastewater. This is typically done by filtration and/or the addition of chemicals and/or UV light and/or Ozone to neutralize harmful organisms and/or remove pollutant material. 
   The second phase of the wastewater treatment process typically includes an aerobic—with oxygen—portion in which bacterial and other microorganisms are provided dissolved oxygen to promote their consumption of the carbonaceous and nutrient materials, and an anoxic—oxygen from a nitrate/nitrite source—portion in which the bacteria and other microorganisms use the oxygen in the nitrate/nitrite for their metabolic functions. The second phase may also include an anaerobic—without oxygen—portion in which bacteria and other microorganisms metabolically function without oxygen. The aerobic, anoxic and anaerobic portions are typically carried out in tanks that are divided into aerobic, anoxic and anaerobic zones. The tank may include one zone in which the aerobic portion operates and one in which the anoxic portion operates and one in which the anaerobic portion operates, or the tank may include any combination of any number of these zones. In some applications, a tank may be solely dedicated to one of the three aerobic, anoxic and anaerobic portions. 
   In the aerobic process, wastewater that includes ammonium (NH 4 ) and organic waste containing nitrogen, for example Urea ((NH 2 ) 2 CO), enters the aerobic zone. In the presence of dissolved oxygen (O 2 ), bacteria and other microorganisms convert the ammonium into nitrate (NO 3 ) via nitrite (NO 2 ). The nitrate can then be anoxically processed into nitrogen gas (N 2 ), which is harmless in the environment. A blower and diffusers supply the dissolved oxygen to the wastewater. The blower provides air to the diffusers, and the diffusers generate and release tiny bubbles so that the oxygen in the bubbles will dissolve in the wastewater. As the aerobic process progresses, the amount of ammonium in the wastewater decreases while the amount of nitrate and dissolved oxygen increases. The amount of dissolved oxygen increases because the demand for the dissolved oxygen decreases as the amount of nitrate increases. After most of the ammonium has been converted into nitrate, the wastewater is ready to be anoxically processed. 
   In the anoxic process, wastewater that includes nitrate and the organic waste containing nitrogen enters the anoxic zone. In the absence of dissolved oxygen, bacteria and other microorganisms convert the nitrate into nitrogen gas and the organic waste containing nitrogen into ammonium. As the anoxic process progresses, the amount of nitrate decreases and the amount of ammonium increases. After most of the nitrate has been converted into nitrogen gas, the wastewater is ready to be aerobically processed or treated in the tertiary treatment phase. 
   Mixing the contents in each of the aerobic and anoxic zones promotes the conversion reactions in each zone by increasing the contact of the components, such as the dissolved oxygen (aerobic zone), nitrite/nitrate (anoxic zone), wastewater, and bacteria and other microorganisms, with the other components in each zone. In the aerobic zone, the wastewater is typically mixed by the movement of the tiny bubbles through the wastewater and a mechanical mixer that includes a screw or blade that is turned by a motor. In the anoxic zone, a mechanical mixer typically only mixes the wastewater because the anoxic process requires little or no dissolved oxygen, which is provided in the aerobic zone by the tiny bubbles from the diffusers. 
   During the aerobic process, the amount of dissolved oxygen and ammonium in the wastewater, along with the total suspended solids (TSS) of the wastewater, are monitored to determine whether the amount of dissolved oxygen injected into the wastewater needs to be increased or decreased, whether or not the wastewater is ready to be processed anoxically, and whether or not the wastewater should be mixed more aggressively. Similarly, during the anoxic process, the amount of nitrate and ammonium in the wastewater, and the TSS of the wastewater are monitored to determine whether or not the wastewater is ready to be processed aerobically or treated in the tertiary phase, and whether or not the wastewater should be mixed more aggressively. With this information, one then determines whether or not to inject more tiny bubbles into the wastewater to increase the amount of dissolved oxygen or to more aggressively mix the wastewater. If the amount of dissolved oxygen in the wastewater should be increased, then the operator turns up the blowers to the diffusers. If the wastewater should be more aggressively mixed, then the operator turns up the blower and/or mechanical mixer. 
   To monitor these process parameters, one periodically retrieves a sample of the wastewater from the processing zone, analyzes the sample, and then evaluates the results. Alternatively, sensors located in the aerobic and anoxic zones can periodically sense the desired parameter and provide the information to an operator, who then analyses and evaluates the information. 
   Disadvantageously, the prior art practice of having someone monitor the aerobic and anoxic process parameters and adjust the output of the blowers and mechanical mixers is time consuming and unnecessarily expensive. Because someone analyzes and evaluates the process parameters and then adjusts the blowers and mechanical mixers accordingly, the process takes time to complete resulting in concomitant costs in time and labor. Thus, for economical reasons, in practice the number of times the aerobic and anoxic process parameters are retrieved, analyzed and evaluated is kept to a minimum. If sensors are used someone typically still has to analyze and evaluate the information the sensors provide and then accordingly adjust the blower to the diffusers and/or mechanical mixer. 
   Furthermore, the typical prior art means for mixing the wastewater in the aerobic and anoxic zones is subject to several limitations. Mixing the aerobic zone with the movement of the tiny bubbles through the wastewater requires a substantial amount of tiny bubbles to be injected into the wastewater to sufficiently mix the wastewater. Disadvantageously, the demand for dissolved oxygen in the wastewater may decrease to the point where the amount of tiny bubbles injected into the wastewater to satisfy the demand would not be enough to sufficiently mix the wastewater. When this happens the amount of tiny bubbles injected into the wastewater is typically kept high enough to sufficiently mix the wastewater. Thus, the diffusers consume more power than required to oxygenate the wastewater and can inject more dissolved oxygen into the aerobic zone than required. 
   Mixing the aerobic and anoxic zones with a mechanical mixer consumes a large amount of power relative to the amount of wastewater that it mixes, and often mixes some, but not all, of the wastewater in each zone. Thus, some of the sludge in the aerobic and anoxic zones remains on the bottom of the tank after it settles there. In the aerobic zone, the settled sludge can plug some of the diffusers. This can reduce the amount of dissolved oxygen injected into the wastewater, and thus requires one to clear the plugged diffusers. Furthermore, when sensors are used to measure wastewater parameters, settled sludge can clog the sensors, resulting in erroneous wastewater parameter measurements. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect of the invention, a system for treating wastewater includes a tank having an aerobic zone in which bacteria and other microorganisms convert pollutants in the presence of dissolved oxygen, and an anoxic zone in which bacteria and other microorganisms convert pollutants in the absence of dissolved oxygen to a more environmentally friendly form. The system also includes a blower and diffusers to inject dissolved oxygen into the aerobic zone, a mixer that generates large mixing bubbles, and a control system to monitor the aerobic and anoxic processes and adjust, accordingly, the output of the blower and mixer. For example, the control system can monitor the amount of ammonium and dissolved oxygen as the aerobic process progresses in the aerobic zone of the tank, and can monitor the amount of nitrate and ammonium as the anoxic process progresses in the anoxic zone of the tank. With the information obtained by monitoring these process parameters, the control system can then adjust the output of the blower and mixer accordingly. With the control system retrieving, analyzing and evaluating the process parameters, and then adjusting the output of the blower and mixer accordingly, someone does not have to perform these functions as the system treats wastewater. 
   The mixer generates large mixing bubbles, for example a bubble having a largest dimension of 6 inches to 10 feet, and is located in the aerobic and anoxic zones. The mixing bubbles are large enough to move wastewater as they rise to the surface and generate a mixing current in the wastewater. The mixing current mixes the wastewater, and bacteria and other microorganisms to promote the bacteria and other microorganisms&#39; conversion of the pollutants contained in the wastewater. Because the mixer requires less energy than a typical mechanical mixer, the mixer costs less to operate. With the mixer mixing the wastewater in the aerobic zone the output of the blower can be reduced to match the demand for dissolved oxygen, which may be below the output required to mix the wastewater. In addition, the mixing bubbles are large enough that the amount of oxygen that they release into the wastewater as they move through it is negligible. Thus the anoxic portion remains anoxic as the large bubbles from the mixer rise toward the surface of the wastewater. 
   In another aspect of the invention, the system for treating wastewater can include a tank having one zone that can change processes over time as desired. For example, during the first six hours of a daily cycle, the tank can be used to aerobically process the wastewater and then during the next six hours of the daily cycle the tank can be used to anoxically process the wastewater. The ability to have the same zone of the tank available to process the wastewater aerobically and anoxically allows the wastewater treatment process to easily adapt to fluctuations in the amount of wastewater that can enter the treatment facility over time. 

   
     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 schematic diagram of a typical wastewater treatment plant that includes a primary treatment process, a secondary treatment process, a tertiary treatment process, and a waste sludge treatment process; 
       FIG. 2  is a perspective view of a tank, a blower, a mixer, and a control system that are included in a system for treating wastewater, according to an embodiment of the invention; 
       FIG. 3  is a schematic diagram of the control system in  FIG. 2 , according to an embodiment of the invention; 
       FIG. 4  is a flow chart of how the control system in  FIGS. 2 and 3  monitors the aerobic and anoxic processes and accordingly adjusts the output of the blower and mixer, according to an embodiment of the invention; 
       FIG. 5  is a perspective view of a forming plate that is included in the mixer in  FIG. 2 , according to an embodiment of the invention; and 
       FIG. 6  shows an embodiment with mixing bubbles that are generated by a mixer traveling through wastewater and mixing the wastewater. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a schematic diagram of a wastewater treatment process that includes a primary treatment process, a secondary treatment process and a tertiary treatment process. The primary treatment process includes a clarification stage  10   a  to separate dense portions of the wastewater, typically heavy solids, from less dense portions of the wastewater, typically light solids and liquid. The secondary treatment process includes a biological nutrient conversion stage  12  that converts the biological nutrient material contained in the light solids and liquid into a more environmentally friendly form. For example, in one embodiment, wastewater is first clarified into heavy solids, and light solids and liquid, in the clarification stage  10   a  using conventional techniques. The heavy solids are directed to a sludge processing stage  14  that processes the heavy solids using conventional techniques. The light solids and liquid are directed to the biological nutrient conversion stage  12  where they are subject to an aerobic and anoxic conversion process as discussed in greater detail in conjunction with  FIGS. 2 and 3 . During the biological nutrient conversion stage  12 , the bacteria and other microorganisms convert the nutrient material contained in the wastewater to a form that is more environmentally friendly. From the biological nutrient conversion stage  12 , the wastewater is directed to another clarification stage  10   b  that clarifies the liquid and any remaining heavy and light solids using conventional techniques. From the clarification stage  10   b , the heavy sludge, which contains a predominance of bacteria, is partially directed to the sludge processing stage  14  that processes the heavy solids using conventional techniques and partially returned to the secondary treatment stage. The very light solids, along with liquid that does not contain excessive amounts of biologically nutrient material, is directed to the tertiary treatment process  16  where remaining pollutant material is removed from the wastewater. 
     FIG. 2  is a perspective view of a tank  18 , a blower  20 , a mixer  22 , and a control system  24  that are included in a system for treating wastewater, according to an embodiment of the invention. The tank  18  includes zones  26   a  and  26   b  in which bacteria and other microorganisms aerobically and anoxically convert pollutants in the wastewater to more environmentally friendly forms. In one embodiment, for example, the tank  18  includes two zones  26   a  and  26   b , an inlet  28  through which wastewater enters the tank  18 , an outlet  30  through which wastewater exits the tank  18  after flowing through the zones  26   a  and  26   b , and a portal  32  through which the wastewater leaves the zone  26   a  and enters the zone  26   b . The zone  26   a  includes bacteria and other microorganisms (not shown) that aerobically convert pollutants in the wastewater, and the zone  26   b  includes bacteria and other microorganisms (not shown) that anoxically convert pollutants in the wastewater. 
   In addition, an Integrated Fixed-film Activating Sludge (IFAS) system that includes media (omitted for clarity) may exist in zones  26   a  and  26   b . The media provides the bacteria and other microorganisms (not shown) a structure to hold onto and may be freely suspended in the wastewater. In other embodiments, the IFAS may include a net or web (not shown) that is anchored in the zones  26   a  and  26   b . In still other embodiments the IFAS may include both the net or web and the media. 
   The blower  20  delivers air to the diffusers  34  ( 22  shown but only four labeled with a reference number for clarity) via distribution lines  36 . The diffusers  34  generate tiny bubbles (not shown) that travel through the wastewater toward the surface of the wastewater. As the tiny bubbles ascend through the wastewater, they release oxygen into the wastewater. Once the oxygen is in the wastewater, the bacteria and other microorganisms can use it to convert ammonium into nitrate. 
   The mixer  22  injects any fluid, such as air, that is less dense than the combination of the wastewater, bacteria and other microorganisms to generate large mixing bubbles (discussed in greater detail in conjunction with  FIG. 6 ). The mixing bubbles are large enough to move a substantial amount wastewater as they rise toward the wastewater&#39;s surface, and thus generate a mixing current in the wastewater. The mixing current mixes the wastewater, bacteria and other microorganisms to promote biological activity for removal of pollutants from the wastewater. 
   The mixer  22  includes a forming plate  38  to form mixing bubbles from the injected fluid, and a valve  39  to permit or prevent the fluid from reaching the forming plate  38 . The mixer  22  also includes a distribution line  40  to supply the forming plate  38  with the fluid when the corresponding valve  39  is open. Each forming plate  38 , one embodiment of which is shown in  FIG. 5 , includes an orifice  44 . When the valve  39  is opened, air flows through the distribution line  40  toward the forming plate  38 , and then exits the distribution line  40  through the orifice  44 . The forming plate  38  prevents the air from rising toward the surface of the wastewater until the valve  39  injects more air than the forming plate  38  can hold, at which time most of the air escapes from under the forming plate  38  and forms a large mixing bubble. The large mixing bubble then rises toward the surface of the wastewater. When the valve  39  is closed, air does not flow through the orifice  44 . For additional discussion on the forming plate  38  and an embodiment of an injector see U.S. Pat. No. 6,629,773, titled IMPROVED METHOD AND APPARATUS FOR GAS INDUCED MIXING AND BLENDING OF FLUIDS AND OTHER MATERIALS, issued to Parks on 7 Oct. 2003, which is herein incorporated in its entirety. 
   Still referring to  FIG. 2 , the forming plates  38  may be arranged throughout the aerobic and anoxic zones  26   a  and  26   b  as desired to provide any desired mixing current arrangement. In one embodiment, the forming plates  38  are located a few inches above the bottom of the tank  18 . The forming plates  38  each may be located closer to the bottom of the tank  18  or further away from the bottom of the tank  18  in either or both zones. Preferred embodiments employ one or more forming plates  38  located on the bottom of tank  18  or at most a few inches above the bottom, in order to maximize the efficacy of the mixing afforded by the large bubbles. 
   As depicted in a preferred embodiment, the forming plates  38  are spatially arranged in the anoxic zone  26   b  to form a rectangle with an additional forming plate  38  located in the middle of the rectangle. As will be appreciated by those in the art, numerous other spatial arrangements of the plates  38  are possible in each zone, including circular and other arrangements, as required for a given wastewater treatment system configuration. 
   Still referring to  FIG. 2  the valves  39  may also be opened and closed in any desired sequence to provide any desired mixing current within each of the zones  26   a  and  26   b . For example, in one embodiment, four valves  39  corresponding to the four forming plates  38  in the anoxic zone  26   b  that are closest to the sidewalls of the tank  18  may first permit air to flow toward the forming plates  38 . Then, after these valves  39  have closed, the remaining valves  39  that correspond to the remaining forming plates  38  may permit air to flow toward the forming plates  38 . This sequence would cause a turbulence in the mixing currents generated by the four forming plates  38  and may promote mixing the wastewater, bacteria and other microorganisms through out the anoxic zone  26   b.    
   The control system  24  monitors the aerobic and the anoxic processes that occur in the respective zones  26   a  and  26   b  of the tank  18 . The control system  24  includes a controller  48  (discussed in greater detail in conjunction with  FIGS. 3 and 4 ) that analyses and evaluates information regarding process parameters of both the aerobic and anoxic processes as these processes progress, and accordingly adjusts the output of the blower  20  and mixer  22 . The control system  24  also includes sensors  50 ,  52 ,  54 .  56  and  58  located in respective zone  26   a  and  26   b  of the tank  18  to sense certain process parameters and convey the information to the controller  48  via conventional means (not shown). Suitable sensors may be obtained from WTW Wissenschaftlich-Technische Werkstätten GmbH, of Weilheim, Germany. 
   In one embodiment, the sensors  50 ,  52  and  54  are located in the aerobic zone  26   a , and sensors  56  and  58  are located in the anoxic zone  26   b . Sensor  50  senses the presence of dissolved oxygen in the wastewater in the aerobic zone  26   a , sensor  52  senses the presence of ammonium, and sensor  54  senses turbidity, which, as is known to those of skill in the art, correlates to total suspended solids (TSS). Sensor  56  senses the presence of nitrate in the wastewater in the anoxic zone  26   b  and sensor  58  also senses turbidity to measure TSS. For additional discussion on the control system  24  see PCT Patent Application PCT/US2004/011248, titled APPARATUS AND METHOD FOR GAS INDUCED MIXING AND AGITATING OF A FERMENTING JUICE IN A TANK DURING VINIFICATION, filed 8 Apr. 2004 which is herein incorporated in its entirety. 
     FIG. 3  is a schematic diagram of the control system  24  in  FIG. 2 , according to an embodiment of the invention. The control system  24  includes the sensors  50 - 58  and a controller  48  to analyze and evaluate the data generated by the sensors  50 - 58 , and generate instructions to adjust the outputs of the blower  20  and the mixer  22 . The controller  48  includes circuitry  62  that can store and generate data and instructions based on the data the circuitry receives from the sensors  50 - 58 , and a processor  64  to execute instructions stored or generated in the circuitry  62 . The controller  48  also includes an input  66  that one can use to enter data into the circuitry  62 . For example, in one embodiment, one can enter limits for the amount of dissolved oxygen, ammonium and nitrate that the controller  48  can compare with respective amounts determined to exist in the wastewater. One can also enter a limit for the degree of total suspended solids (TSS) in the wastewater in each of the zones  26   a  and  26   b  that the controller  48  can compare with the degree of TSS determined in each of the zones  26   a  and  26   b . One can also enter a limit for the outputs of the blower  20  and mixer  22  that the controller  48  can compare with the output that the controller  48  determines should be used based on the data from the sensors  50 - 58 . 
   In other embodiments, the control system  24  may include a set of instructions to switch the data and instructions stored and generated by the circuitry  62  from those used to monitor an aerobic or anoxic process to those used to monitor an anoxic or aerobic process, respectively. This may be desirable when the tank  18  includes one zone that processes wastewater aerobically for a period of time and then process the wastewater anoxically for another period of time. 
     FIG. 4  is a flow chart of the control system in  FIGS. 2 and 3  monitoring the aerobic and anoxic processes, and accordingly adjusting the output of the blower and mixer, according to an embodiment of the invention. In operation, the control system  24  can monitor the aerobic process while it monitors the anoxic process (as shown in  FIG. 2 ), and can accordingly and independently adjust the blower and mixer outputs in the aerobic zone  26   a  relative to the mixer output in the anoxic zone  26   b . As previously discussed, in other embodiments, the control system  24  can sequentially monitor one of the conversion processes and accordingly adjust the mixer&#39;s output or the blower and mixer&#39;s output, whichever is applicable. 
   In one embodiment, the control system  24  monitors the amount of TSS determined to be in the wastewater in the aerobic and anoxic zones  26   a  and  26   b  ( FIG. 2 ) during the aerobic and anoxic processes. When the level of TSS is determined to be less than a desired predetermined degree, the control system  24  instructs the mixer  22  to change one or more of the bubble generation parameters that the mixer  22  uses to generate mixing bubbles (discussed greater detail in conjunction with  FIG. 6 ) to increase the TSS. For example, the mixer  22  may increase the frequency of the mixing bubbles that one or more of the forming plates  38  ( FIG. 2 ) generates and releases into the wastewater. When the level of TSS is determined to be greater than a desired predetermined degree, the control system  24  instructs the mixer  22  to change one or more of the bubble generation parameters to decrease the TSS. For example, the mixer  22  may decrease the size of each mixing bubble that one or more forming plates  38  generates and releases. The one or more bubble generation parameters that the control system  24  chooses to have the mixer  22  change depends on many variables that include the difference between the determined level of TSS and the desired level, how quickly one wants to correct this difference, and the capability of the mixer  22 . 
   In one embodiment, the control system  24  monitors the amount of dissolved oxygen and ammonium determined to be in the wastewater in the aerobic zone  26   a  ( FIG. 2 ) during the aerobic process. The control system  24  then compares the determined amounts of ammonium and dissolved oxygen in the wastewater and then accordingly adjusts the output of the blower  20  ( FIG. 2 ). When the amount of ammonium is greater than a desired predetermined amount, and the amount of dissolved oxygen is less than a desired predetermined amount, the control system  24  instructs the blower  20  to change one or more of the parameters that define the airflow toward the diffusers to increase the amount of dissolved oxygen in the wastewater. For example, the blower  20  may increase the flow rate of air to the diffusers  34  ( FIG. 2 ) or the blower  20  may deliver air that has a higher concentration of oxygen to the diffusers  34 . When the amount of ammonium and dissolved oxygen is greater than respective, desired predetermined amounts, the control system  24  instructs the blower  20  to change one or more of the bubble generation parameters to decrease the amount-of dissolved oxygen. When the amount of ammonium is less than a desired predetermined amount, and the amount of dissolved oxygen is greater than a desired predetermined amount, the control system  24  instructs the blower  20  to change one or more of the parameters that define the airflow toward the diffusers to decrease the amount of dissolved oxygen in the wastewater. When the amount of ammonium and dissolved oxygen is less than respective, desired predetermined amounts, the wastewater is ready to be anoxically processed, and the control system  24  confirms that this portion of the wastewater is about to enter the anoxic zone  26   b . In other embodiments, the control system  24  switches from monitoring the aerobic process to monitoring the anoxic process while the wastewater remains in the same zone of the tank. 
   In one embodiment, the control system  24  monitors the amount of nitrate determined to be in the wastewater in the anoxic zone  26   b  during the anoxic process. When the amount of nitrate exceeds a desired predetermined amount, the control system  24  confirms that this portion of the wastewater still has a significant amount of processing time to progress through. When the amount of nitrate is less than a desired predetermined amount, the control system  24  confirms that this portion of the wastewater is ready to leave the anoxic zone  26   b . If the amount of ammonium, which the control system  24  may also monitor in the anoxic zone, exceeds a desired predetermined amount, the wastewater is ready to be aerobically processed again, and the control system confirms that the wastewater is about to enter another aerobic zone (not shown). In other embodiments, the control system  24  switches from monitoring the aerobic process to monitoring the anoxic process while the wastewater remains in the same zone of the tank. 
   In addition, in one embodiment of the control system  24  the control system monitors the time of day that it receives specific data from the sensors  50 - 58  and analyzes and evaluates the data. By keeping track of the time of day, the control system  24  can compare the data it receives and generates with data that it should receive and generate for the time of day, and can determine whether or not a malfunction in the sensors  50 - 58 , blower  20 , mixer  22  and control system  24  might exist. 
     FIG. 6  is a view of one of the zones  26   a  and  26   b  in  FIG. 2 . The mixing bubbles  68  generate the mixing currents indicated by the arrows  70  (28 arrows shown but only 5 labeled with the reference number  70  for clarity) that mix the wastewater  72 , bacteria (omitted for clarity) and other microorganisms (also omitted for clarity). The strength of the mixing currents depends on the speed at which each mixing bubble  68  travels through the wastewater and the size of each bubble  68 . 
   The speed of the mixing bubble  68  depends on the density of the fluid relative to the density of the wastewater  72 , and the bubble&#39;s shape. The greater the difference between the densities of the wastewater  72  and the fluid, the faster the mixing bubbles  68  rise through the wastewater  72 . The more aerodynamic the shape of the bubble  68  becomes, the faster the bubble  68  rises through the wastewater  72 . For example, in one embodiment, the bubble  68  forms an oblate sphere—a sphere whose dimension in the vertical direction is less than the dimension in the horizontal direction. In other embodiments, the bubble  68  forms a distorted oblate sphere having the trailing surface—the surface of the bubble  68  that is the rear of the bubble  68  relative to the direction the bubble  68  moves—that is convex when viewed from the direction that the bubble  68  moves. 
   The size of the mixing bubble  68  depends on the flow rate of the fluid into the wastewater  72 . The flow rate depends on the size of the orifice  44  and the fluid&#39;s injection pressure. As one increases the fluid-injection pressure, one increases the amount of fluid injected into the wastewater  72  over a specific period of time that the valve  39  is open. And, as one increases the area of the orifice  44 , one increases the amount of fluid injected into the wastewater  72  over a specific period of time that the valve  39  is open. As one increases the diameter of the forming plate  38  one increases the amount of fluid the forming plate  38  can hold before the fluid escapes it. For example, in one embodiment the size of the bubble  68  is approximately 6 inches across its largest dimension. In other embodiments, the bubble  68  is approximately 10 feet across it largest dimension. 
   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.