Abstract:
A screw press has a cylindrical screening section mounted to a frame. The screening section is able to rotate but can be prevented from rotating. A sprayer system is operable to spray water on the screening section. In a cleaning process, the screening section is rotated while water is sprayed against it. Optionally, the power required to rotate the screen may be provided by an auger drive mechanism. The screening section may have openings sized to retain flocculated solids. The screw press may be used to thicken a sludge from a waste water treatment plant.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/636,000 filed Apr. 20, 2012, which is hereby incorporated by reference. 
     
    
     FIELD 
       [0002]    This specification relates to separating solids from liquids, screw presses and to methods of thickening sludge. 
       BACKGROUND 
       [0003]    In general, a typical screw press comprises a screw, also called an auger, rotating within a fixed cylindrical body comprising screen panels. A solid-liquid mixture to be treated is fed into an inlet end of the screw press. The auger conveys the mixture through the cylindrical body while pressing the mixture against the screens. Liquid and fine solids are collected from the outside of the screens. Retained solids and residual liquid are collected from an outlet end of the screw press. The cylindrical body is dismantled when the screens need to be cleaned. 
         [0004]    In International Publication Number WO02/062564, a screw press is used to separate solids from sewage. A screw is driven by an electric motor. The direction of rotation of the screw is reversed periodically to clean the screw press. 
         [0005]    In International Publication Number WO99/52704, a screw press has a cylindrical body made up of a stack of annular plates. Some of the plates are fixed and some of the plates are movable. An eccentric shaft passing through the stack of plates rotates when the auger rotates. The rotation of the eccentric shaft causes the movable plates to oscillate relative to the fixed plates. 
       INTRODUCTION TO THE INVENTION 
       [0006]    The following introduction is intended to introduce the reader to the detailed description but not to limit or define the claimed invention. 
         [0007]    A screw press described in this specification has a cylindrical screening section mounted to a frame of the press such that the screening section is able to rotate. Restraints between the screening body and the frame may be configured to allow the screening body to rotate or to inhibit or prevent the screening body from rotating. A sprayer system is provided on the outside of the screening body. A motor may be used to cause the screening body to rotate. Optionally, the motor may also drive an auger within the screening body. 
         [0008]    A screw press described in this specification has a screening body with opening sizes appropriate to retain flocculated solids. Optionally, a sprayer system may be provided on the outside of the screening body. Optionally, an auger shaft may have an increased diameter towards an outlet end of the press. 
         [0009]    A screw press cleaning method described in this specification comprises steps of spraying water at a screening section of the screw press from outside of the screening section. Optionally, the screening section may be rotated while water is sprayed at it. 
         [0010]    The invention provides an alternative device for separating solids from a liquid or an alternative method of operating a screening device. The device and method may be used, for example, to thicken sludge from a wastewater treatment plant such as primary or activated sludge from a municipal sewage treatment plant. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]      FIG. 1  shows a schematic cross sectional view of a screw press with a screening section. 
           [0012]      FIG. 2  shows a portion of the screening section of  FIG. 1  with optional screening section mounts and restraints and optional drive mechanisms for rotating the screening body. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  shows a screw press  10  having an inlet end  6  and an outlet end  8 . The screw press  10  has a cylindrical screening body  12  and a frame  16 . The screening body  12  has a non-porous inlet section  20 , a porous screening section  22  and a non-porous outlet section  24 . The frame  16  similarly has an inlet portion  26 , a central portion  28  and an outlet portion  30 . The inlet portion  26  of the frame  16  may share components with or support the inlet section  20  of the screening body  12 . The central portion  28  of the frame  16  at least partially surrounds the screening section  22  of the screening body  12 . The outlet portion  30  of the frame  16  may share components with or support the outlet section  24  of the screening body  12 . 
         [0014]    The screw press  10  also has an auger  14  and a drive mechanism  18 . The auger  14  is located within the screening body  12  and is supported by the frame  16 . In particular, the auger  14  and screening body  12  are concentric about a central axis  32  of the screw press  10 . One end of a shaft  60  of the auger  14  is supported through a bearing (not shown) on a stationary post  34  attached to the outlet portion  30  of the frame  16 . The other end of the auger shaft  60  of the auger  14  is supported on a drive shaft  36  of the drive mechanism  18 . A blade  58  of the auger  14  is attached in a spiral around the auger shaft  60  and extends towards, or optionally touches, the inside of the screening body  12 . The drive mechanism  18  is attached to the inlet portion  26  of the frame  16  and comprises a motor  38  and a gearbox  40 . The frame  16  is supported on the ground through a frame  41 . 
         [0015]    The inlet portion  26  of the frame  16  has an inlet  42  to receive a feed mixture  44 . The central portion  28  of the frame  16  has a liquid outlet  46  to discharge a liquid fraction  48  of the feed mixture  44 . The outlet portion  30  of the frame  16  has a solids outlet  50  to discharge a solids fraction  52  of the feed mixture. The liquid fraction  48  may have some solids remaining in it but at a reduced solids concentration relative to the feed mixture  44 . The solids fraction  52  may have some liquid in it but at a higher solids concentration than the feed mixture  44 . 
         [0016]    In operation, the feed mixture  44  is pumped into the inlet  42  at an initial pressure. The drive mechanism  18  rotates the auger  14  causing the blade  58  to convey the feed mixture  44  along the screening body  12 . The auger  14  preferably also increases the pressure of the feed mixture  44 . Liquid and fine solids in the feed mixture  44  are forced through the screening body  12 . These liquids and fine solids are collected in the central portion  28  of the frame  16  and discharged through liquid outlet  46 . The remainder of the feed mixture  44  exits the screening body  12  after passing by a counter pressure cone  60 . Counter pressure cone  60  is biased towards the screening body  12  by a biasing mechanism  62  such as a set of springs or a pneumatic cylinder. The solids fraction  52  drops from the end of the screening body  12  and is discharged from the frame  16  through the solids outlet  50 . 
         [0017]    The internal volume of the screening section  22  preferably decreases towards the outlet end  8  of the screw press  10 . This helps maintain pressure in the screening section  22  even though the liquids fraction  48  is removed from the feed mixture  44 . A decreasing volume may be obtained by reducing the diameter of the screening section  22 . Alternatively, the pitch of the blades  58  may be decreased towards the outlet end  8  of the screw press. Both of these methods, however, prevent the use of a blade  58  having a uniform outer diameter and pitch, which is more easily manufactured to a tight fit with the screening section  22 . In the screw press  10  of  FIG. 1 , a decreasing internal volume is provided by increasing the diameter of at least a portion of the shaft  60  towards the outlet end  8  of the screw press  10 . 
         [0018]    The screening section  22  of the screening body  12  has openings of a size and shape adapted to provide a selected degree of separation. For example, the screening section  22  may remove fibrous materials from the digestate of an anaerobic digester treating manure or other agricultural wastes. Alternatively, the screening section  12  may have smaller openings suited to separating flocculated solids from primary or activated wastewater treatment sludge. For example, the screening section  22  may be made from wedgewire with a slot opening in the range from about 0.25 mm to about 0.75 mm. 
         [0019]    Although any screw press may benefit from having a convenient cleaning method, cleaning is required more frequently when the screening section  22  has small openings. In particular, when the screw press  10  is used to thicken sludge, dismantling the screw press  10  for cleaning is undesirable. The screw press  10  is fitted with a sprayer system  70  to allow cleaning by spraying water against the outside of the screening section  22 . When cleaning is required, water is pumped through a manifold  72  to a series of sprayer heads  74  located inside of the central portion  28  of the frame  16 . The screw press  10  of  FIG. 1  has one manifold  72 , but there may be multiple manifolds  72  spaced around the circumference of the screening body  12 . 
         [0020]    The water sprayed against the screening section  22  moistens and breaks up accumulations of solids caught in the openings of the screening section. Some of the water may also force its way through the openings of the screening section  22  in a reverse direction. The water is preferably heated. In order to assist the water in cleaning the screening section  22 , the supply of feed mixture  44  may be stopped while the auger  14  continues to rotate for a period of time before the water is sprayed. This reduces the volume or pressure, or both, of the feed mixture  44  inside the screening section  22 . 
         [0021]    Referring to  FIG. 2 , the screening section  22  may be made up of screen panels  82  having openings  84 . In the construction shown in  FIG. 2 , the screening section  22  is made up of segments  84  each having a cylindrical screen panel  82  attached to a forward flange  86  and a rearward flange  88 . The screening section is built up by attaching the rearward flange  88  of one segment  84  to the forward flange  86  of another segment  84 , optionally by way of fasteners  90 . Other methods of constructing a screening section  22  may also be used. 
         [0022]    Referring back to  FIG. 1 , a forward flange  86  at one end of the screening section  22  is held within a receiver  94  attached to the frame  16  through the inlet section  20  of the screening body  12 . Alternatively, the receiver  94  may be attached directly to the frame  16 . The receiver  94  contains a bearing, such as a brass or plastic ring or a race of ball bearings, and allows rotation of the screening section  22 . Similarly, a receiver  94  is attached to the frame  16  and holds, but allows rotation of, the other end of the screening section  22 . This second receiver  94  may hold a rearward flange  88  or a supplementary flange  92  fastened to a rearward flange  88 . In this way, at least the screening section  22  of the screening body  12  is allowed to rotate about the central axis  32 . Optionally, one or more non-porous parts of the screening body  12  may also be allowed to rotate. Optionally, intermediate bearing mechanisms  96  may be provided to support, but allow rotation of, the screening section  22 . 
         [0023]    Referring back to  FIG. 2 , each bearing mechanism  96  has a roller  98  supported through a post  100  by the frame  16 . The roller  98  spins on an axle  102  supported by the post  100 . Three or more bearing mechanisms  96  may be spaced around the circumference of the frame  16  at each longitudinal position shown in  FIG. 1  to better support and center the screening section  22 . A lever  104  supported on the end of an axle  102  is attached to an actuator  106 . When the actuator  106  is moved to the left, the lever  104  bears against a forward flange  86 . The lever  104  may apply friction to reduce the speed of rotation of the screening section  22  or to stop the screening section  22  from rotating. Moving the actuator  106  to the right lessens or removes the friction. Moving the actuator  106  even further to the right moves the lever  104  to the outside of the forward flange  86  so that the screening section  22  can be pulled out of, or inserted into, the frame  16 . 
         [0024]    An alternative mechanism for supporting or facilitating a desired rotation of the screening section  22  is shown at the top of  FIG. 2 . In this alternative, rearward flanges  88  are made in the form of ring gears. A gear shaft  110  supported by the frame  16  is fitted with gears  112  that engage the rearward flanges  88 . The gear shaft  110  may be supported by the frame  16  at intermediate positions to allow the gears  112  to help support or center the screening section  22 . Alternatively or additionally, the gear shaft  110  may be attached to a brake such that the gear shaft  110  can be used to stop or slow the rotation of the screening section  22 . Alternatively or additionally, the gear shaft  110  may be connected to the gearbox  40  of the drive mechanism  18 , or to a separate drive mechanism, so that the gear shaft  110  can be used to drive the rotation of the screening section  22 . 
         [0025]    When rotation of the screening section  22  is not restrained, the screening section  22  will tend to rotated with the auger  14  due to friction between the auger  14  and the screening section  22 . Optionally, the screening section  22  may be forced to rotate with the auger  14  by actuating a releasable connection (not shown) between the auger  14  and the screening section  22 . Alternatively, the screening section  22  may be driven by the drive mechanism  18  without applying force through the auger  14 , for example by use of the gear shaft  110  of  FIG. 2 . In all of these examples, the motor  38  is used, directly or indirectly, to rotate the screening section  22 . A separate motor may also be used to rotate the screening section  22 . In some of the examples above, the screening section  22  may be rotated when the auger  14  is not rotating or at a different speed than the auger  14 . In some other examples, such as applying some friction to the screening section  22  while the auger  14  is rotating, the screening section  22  can only be rotated while the auger  14  is rotated but the screening section  22  may rotate at a different speed than the auger  14 . 
         [0026]    The screening section  22  is preferably rotated while water is sprayed against it during the cleaning process described above. Rotating the screening section  22  allows each part of the screening section to pass under a line of sprayer heads  74 . Multiple manifolds  72  are not required. Rotating the screening section  22  also allows each part of the screening section  22  to be placed at or near the top of the screening section  22  so that gravity may assist in the cleaning. Rotating the screening section  22  also avoids having the bottom of the screening section  22  accumulate solids that fall from upper parts of the screening section  22  during cleaning. Optionally, the screening section  22  may be stopped and the auger  14  may be rotated for a period of time during or after the cleaning procedure to convey backwashed solids out of the screening section  22 . Alternatively, the auger  14  may be rotated at a faster speed than the screening section  22  during the cleaning process to convey material released during cleaning out of the screening section  22 . 
         [0027]    After the cleaning procedure, the screening section  22  is restrained from rotating, the auger  14  resumes or continues normal rotation, and the supply of feed mixture  44  is restarted. The screening section  22  is typically prevented from rotating during operation of the filter press  10  except when cleaning the screening section  22 .