Abstract:
A method and apparatus is disclosed for maintaining fluid in suspension in a mixing tank including particles includes providing a reversible mixer, rotating the mixer in a normal direction in which particles buildup on the mixer, and, rotating the mixer in an abnormal direction to shed the particles from the mixer.

Description:
CLAIM TO PRIORITY 
       [0001]    This application claims priority to European Patent Application No. 10306296.4, which was filed Nov. 25, 2010. 
       BACKGROUND 
       [0002]    This application relates to wastewater treatment, and more particularly to elimination of fibers on a mixer impeller in wastewater treatment. Sewage treatment involves the removal of contaminants from waste water and household sewage to produce solid or semisolid waste and an effluent suitable for discharge back into the environment. Sewage is created by residential, institutional, commercial and industrial establishments and includes household waste, liquid from toilets, baths, showers, kitchens, sinks, etc. 
         [0003]    Conventional sewage treatment may involve primary, secondary and tertiary treatment steps. During primary treatment, sewage is held in a basin where heavy solids generally settle and light contaminants float to the surface. The sediment and floating materials are removed and the remaining liquid may be discharged or subject to secondary treatment. Secondary treatment generally removes dissolved and suspended biological matter and is performed by introducing micro organisms in a managed habitat. Secondary treatment may require a separation process to remove the micro organisms from the water prior to discharge or to tertiary treatment. In tertiary treatment treated water is sometimes disinfected chemically or physically prior to discharge to the environment. 
         [0004]    Many municipal plants churn the sewage constantly during treatment steps to encourage separation and to introduce oxygen to allow the micro organisms to consume the biodegradable soluble organic contaminants like sugars, fats, etc. Some systems use aerated lagoons in which an electric motor driven impeller draws air into the water to allow the micro organisms to function efficiently. 
       SUMMARY 
       [0005]    According to an exemplar method disclosed herein for maintaining fluid in suspension in a mixing tank including particles includes providing a reversible mixer, rotating the mixer in a normal direction in which particles buildup on the mixer, and, rotating the mixer in an abnormal direction to shed the particles from the mixer. 
         [0006]    According to a further exemplar disclosed herein an apparatus for maintaining fluid in suspension in a mixing tank including fibers includes a reversible mixer and a controller providing commands to the mixer to rotate in a normal direction in which fibers may buildup on the mixer, and the controller providing commands to the mixer to rotate in an abnormal direction to shed the fibers from the mixer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
           [0008]      FIG. 1  shows a motor driving a blade attached to a hub within a sewage treatment containment area. 
           [0009]      FIG. 2  shows a motor of  FIG. 1  contaminated by fibers. 
           [0010]      FIG. 3  shows the motor of  FIG. 2  in which the rotor is driven in an opposite direction to remove fibers attached to the blade and hub. 
           [0011]      FIG. 4  shows a clockwise rotation where the fibers are suspended in a media as shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Referring now to  FIG. 1 , a mixing tank  10  for a waste water treatment plant (not shown) in which a mixer  15  is fitted in the mixing tank  10 . The mixer  15  keeps fine particles  20  including fibers  50  in suspension and allows proper aeration and homogenation in the mixing tank  10 . The fibers  50  may come from textiles, hair, paper, tissues or the like. The fibers  50  may have many properties and behaviors, for instance, they may be short, long, curled or elastic. 
         [0013]    The mixer  15  includes a shaft  25 , a gear box  30 , a reversible motor  35 , a hub  40  and an impeller  45 . The mixer  15  is controlled by controller  55 . 
         [0014]    Referring now to  FIG. 2 , over time, the particles  20  including fibers  50  may become entrapped around the shaft  25 , hub  40  and the impeller  45  and may build up much in the same way in which wool thread is made. For instance, the fibers  50  may be “spun” like wool thread creating stringy snags  65  (see  FIG. 2 ) that may wind around the shaft  25 , hub  40  and the impeller  45 . If the fiber  50  is allowed to build up around the shaft  25 , hub  40  and the impeller  45  there may be unbalances and vibrations on the shaft  25 , hub  40  and the impeller  45  that increase the power required which may cause a mixer to stop and mechanical damage may occur. For instance, the gear box  30  may break. 
         [0015]    While impellers  45  may be designed to shed these fibers  50  and avoid the problems that may occur due to the entrapment of fibers  50 , changing the shape of the impeller  45  might make the impeller inappropriate for use in waste treatment. That is, a redesigned impeller (not shown) may change the absorbed power and the hydrodynamics that is presently provided by the impeller  45 . In such a situation, a redesigned impeller (not shown) may not be able to provide smooth flow if flash mixing for high shear or flocculation is required. Combining an impeller  45  that is able to shed the fiber and provide the specific functions required by the mixer  15 , including energy savings, has not yet been found. 
         [0016]    Referring now to  FIG. 3 , if fibers  50  are wrapped around the shaft  25 , hub  40  and the impeller  45  due to the normal, clockwise rotation of the impeller  45 , the controller  55  may command the shaft  25 , hub  40  and the impeller  45  to rotate in a counter-clockwise direction, that is, in an abnormal direction of rotation. 
         [0017]    The controller  55  may require abnormal rotation on a regular basis. For example, for every hour of normal, clockwise rotation, the controller  55  may provide commands to the mixer  15  that may be rotated in an abnormal counter-clockwise direction for a period of time such as fifteen minutes. The mixer  15  may also be sensor controlled. For instance, the controller  55  may have a sensor  60  therein that senses excessive drag on the shaft  25 , hub  40  and the impeller  45  by sensing an increase in voltage or current required by the motor  35 . If such increase in voltage or current is sensed, the controller may provide commands to the mixer  15  to reverse rotation to shed the particles  20  including fibers  50  and unwind any snags  65  for a period of time. Other types of sensors regarding a buildup of particles  20  including fibers  50  are contemplated herein. 
         [0018]    The reverse or abnormal rotation of the shaft  25 , hub  40  and the impeller  45  pushes the particles  20  and fibers  50 , as exhibited by arrows A away from the shaft  25 , hub  40  and the impeller  45  due to centrifugal forces. During the time period, the mixer  15  operates in the abnormal or reverse direction of rotation, the presence of particles  20  and fibers  50  are minimized and the mixer  15  can operate again in the normal direction (see  FIG. 4 ) and the controller  55  so instructs the mixer  15  to rotate in a normal direction. 
         [0019]    Removing the particles  20  and the fibers  50  from the mixer  15  by means of counter-clockwise rotation minimizes power and operation costs; minimizes vibrations and loads caused by overloaded and/or an unbalanced shaft  25 , hub  40  or the impeller  45  that may damage the mixer  15  and require a waste water treatment plant to shut down; and, minimizes potentially hazardous manual labor to clean the shaft  25 , hub  40  and the impeller  45 . Further, no extra system, such as a scraper (not shown), is added into the water and the efficiency of the mixer  15  is not impaired. 
         [0020]    The foregoing description is exemplary rather than defined by the limitations within. Various non-limiting embodiments are disclosed herein, however, one of ordinary skill in the art would recognize that various modifications and variations in light of the above teachings will fall within the scope of the appended claims. It is therefore to be understood that within the scope of the appended claims, the disclosure may be practiced other than as specifically described. For that reason the appended claims should be studied to determine true scope and content.