Abstract:
A biofilm carrier screen is adapted for use with a moving bed bioreactor (MBBR). The screen has a screening body connected to an effluent pipe. The effluent pipe optionally has two parts separated by a union, preferably a pitless adapter. The screening body can be lifted from a bioreactor tank for maintenance after disconnecting the union. The screening body may have an aerator to provide bubbles to disperse biofilm carriers from the screening body. The screening bod may extend downwards from a mount near the top of the tank. In the case of an anaerobic MBBR, the screening body may extend downwards from a service box through a cover over the tank.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit under 35 USC 119 of, and priority to, U.S. Provisional Application No. 61/676,131 filed on Jul. 26, 2012 which is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    This disclosure relates to water treatment with bioreactors and in particular to moving bed biofilm bioreactors (MBBR). 
       BACKGROUND 
       [0003]    The following discussion is not an admission that anything discussed below is citable as prior art or common general knowledge. 
         [0004]    In biological water treatment, a growth of microorganisms, for example aerobic or anaerobic bacteria, consumes one or more components dissolved or suspended in the water. In general, the microorganism may be in the form of an attached growth, a suspended growth, or both. 
         [0005]    In some attached growth systems, a biofilm containing the microorganisms grows on a moving surface. In a moving bed biofilm reactor (MBBR), a large number of biofilm carriers, sometimes called carriers for brevity, in a tank provide supporting surfaces for biofilm. The biofilm carriers are typically made of plastic or another material having a density near the density of water. The size of individual biofilm carriers may be in the range of a few millimeters to a few centimeters. Small biofilm carries may be in the form of spheres or cylinders. Larger carriers may have shapes that provide a large protected inner surface area within an outer shell. When operating, one or more mixing devices such as an aerator or a mixing paddle distribute the biofilm carriers around the tank and keep at least some of them in suspension in the water. In some cases, there is also suspended growth in the MBBR. For example, in the integrated fixed film activated sludge (IFAS) process, return activated sludge (RAS) recirculates through an MBBR. 
       INTRODUCTION 
       [0006]    In a moving bed biofilm reactor (MBBR), water flows through a tank and contacts biofilm attached to carriers confined to the tank. A screen covers the outlet of the tank to prevent the biofilm carriers from leaving the tank with the treated water. However, the flow of water through the screen tends to collect carriers against the screen. This interferes both with operation of the screen and suspension of the carriers. The screen may also need to be cleaned or repaired at some times but the water around the screen may be murky and contain pathogens. In the case of an anaerobic MBBR, the biofilm also releases biogas. The biogas is both a greenhouse gas and a valuable product and so a cover is placed over the tank to contain the biogas. The cover and the toxicity of the biogas under the cover make it more difficult to access the screen for cleaning or other maintenance. 
         [0007]    This specification describes a screen adapted for use with a bioreactor, for example an MBBR. The screen has a screening body connected to at least part of an effluent conduit. The screening body may extend downwards from a supporting structure that is adapted for mounting near the top of a tank. In operation, the screening body extends downwards into water in the tank. Water flows through the screening body to reach an inlet of the effluent conduit open to the inside of the screening body. Screened water then travels through the effluent conduit to leave the tank. The screening body prevents particles of a selected size, for example biofilm carriers, from reaching the inlet of the effluent conduit. In the case of an anaerobic bioreactor, the supporting structure may be part of a service box allowing access to the inside of the tank through a cover. 
         [0008]    Preferably, a mount for the screening body allows the screening body to be raised or lowered relative to the tank. If the effluent conduit is rigid, it is preferably in two or more parts separated by a union such as a pitless adapter. One part of the effluent conduit, located on one side of the union, passes through the tank wall. Another part of the effluent conduit, located on the other side of the union, connects to the screening body. In the case of a pitless adapter, raising the screening body automatically disconnects the pitless adapter while lowering the screening body re-connects the pitless adapter. In the case of an anaerobic bioreactor, there may be a hatch in a tank cover or service box over the screening body. In use, the hatch allows the screening body to be lifted clear of water in the tank. 
         [0009]    Screened particles may be discouraged from collecting against the outside of the screening body by placing the screening body near or over an agitator, for example an aerator or paddle, in the tank. Preferably, the screen has an agitator connected to the screening body. For example, the screening body may have an aerator connected to the outside of the screening body near the bottom of the screening body. The aerator produces a curtain of bubbles that rise along, or parallel to, the outer surface of the screening body. Preferably, the aerator can be lifted clear of water in the tank by raising the screening body. In the case of an anaerobic MBBR, the bubbles may be produced by recirculating biogas from within a cover of the tank to the aerator. 
         [0010]    This specification also describes a moving bed biofilm reactor and water treatment method. The reactor has a tank and a screening body attached to an effluent conduit through a pitless adapter. In the case of an anaerobic MBBR, the screening body may extend downwards from a service box that passes through a cover of the tank. In the method of treating water, water flows into the tank and contacts biofilm carriers in the tank. Microorganisms, for example anaerobic methanogens, grow on the carriers. Treated water flows through the screening body to the inlet of an effluent conduit open to the inside of the screening body. Optionally, bubbles produced outside of the screening body help disperse biofilm carriers from the screening body. In another option, water flowing into the tank may contain a suspended growth of microorganisms, for example anaerobic methanogens. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic cross-sectioned side view of a bioreactor with a biofilm carrier screen. 
           [0012]      FIG. 2  is an enlarged view of a portion of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The detailed description to follow describes a biofilm carrier screen for use with a moving bed bioreactor (MBBR) used to treat wastewater. The reactor has carriers contained in a tank by the screen. The screen may be removed from the water for maintenance or replacement without emptying the water from the tank. This reduces disturbance to reactions in the reactor, which may take weeks or more to be established. In the case of an anaerobic MBBR, the screen may also be removed from the water without having to vent the headspace of the tank. This avoids contaminating the reactor or a biogas product with air. The configuration of the screen also provides a large screening area. Optionally, an attached agitator helps keep carriers away from the screen in use. By being attached, the agitator can act directly on the screen and be removed with the screen for maintenance. In another option, a pitless adapter may be used to avoid having to separately disconnect a union in an effluent conduit (which may be immersed in sludge) when removing the screen. In the example shown in the Figures, all of these features are combined in an anaerobic MBBR. However, in other embodiments, any subset of these features may be used for their individual advantages. 
         [0014]      FIGS. 1 and 2  show a bioreactor  10 , in particular an MBBR. The bioreactor  10  may operate under aerobic or anaerobic conditions. In use as an MBBR, the bioreactor  10  contains biofilm carriers. The carriers are not shown in the Figures since the individual carriers are typically too small to be shown in detail. Any custom made or commercially available type of carrier may be used. 
         [0015]    The bioreactor  10  comprises a tank  12 , a cover  14  and a service box  26 . The cover  14  is typically omitted for an aerobic MBBR. The tank  12  can be any structure that will contain the contents of the bioreactor  10 . The cover  14  engages the top of the tank  12  in a generally airtight manner to capture biogas produced in the bioreactor  10 . The service box  26  may be generally similar to service boxes sold by UTS Biogas but with modifications according to the description below. 
         [0016]    The bioreactor  10  also includes an inlet pipe  16  and an effluent conduit  17 . The inlet pipe  16  introduces wastewater  18  to the bioreactor  10  for treatment. The wastewater  18  may be industrial wastewater, municipal wastewater treatment sludge, agricultural or food processing waste, or another feed stream that can be treated by microorganisms, for example methanogens, attached to biofilm carriers within the bioreactor  10 . In some cases, the feed stream may be a high solids suspension or slurry and other feeding mechanisms may be used. The microorganisms break down one or more components of the wastewater  18  to produce sludge  19  in the tank  12 . For example, the sludge  19  may have reduced biochemical oxygen demand. 
         [0017]    The microorganisms may also produce a biogas product. The biogas is typically a mixture of methane, carbon dioxide, and other gasses. The biogas is initially contained within a headspace  20  between the surface of the sludge  19  and the cover  14 . The biogas is extracted from the headspace  20  for use, for example, to produce heat or electricity. The biogas is preferably removed from the headspace  20  through a biogas collection pipe  70  passing through a wall of the service box  26 . Alternatively, the biogas collection pipe  70  can extend the through a section of the cover  14  or an upper section of the tank  12 . 
         [0018]    Biofilm carriers, not shown, are suspended within the sludge  19  to support the microorganisms. The carriers are typically inert and do not contribute to the reactions that occur within the bioreactor  10 . The carriers typically have a high surface area per unit volume to support a large area of biofilm while being easily moved throughout the tank  12  to contact the sludge  19 . The carriers may be of any commercially available carrier type. 
         [0019]    The bioreactor  10  can include one or more mixers  24 , alternatively called agitators, to distribute and move the biofilm carriers throughout the tank  12 . A generally even distribution of the carriers assists the reactions between the microorganisms and the sludge  19 . A mixer  24  may be a mechanical mixer with paddles, blades or other suitable features. Alternatively, a mixer  24  may be an aerator, sparger or diffuser that releases gas bubbles into the sludge  19 . The gas used to make the bubbles may be biogas drawn from the headspace  20 . 
         [0020]    The bioreactor  10  also includes the service box  26 . The service box  26  may be supported on a wall or the floor of the tank  12  by a frame  28 . The frame  28  can be located inside or outside of the tank  12 . Optionally, the frame  28  distances the service box  26  from the inner wall of the tank  12 . The service box  26  preferably includes a platform  27  suitable to support a person who is working on the bioreactor  10 . At least a portion of the service box  26  extends through the cover  14 . A generally gas tight seal is formed between the exterior of the service box  26  and the cover  14  to prevent the escape of biogas. The service box  26  includes a lid  30  that, when opened, allows equipment to be lowered into the tank  12  through the service box  26 . Optionally, the service box  26  may also have a hatch  31  to provide a separate way for a person working on the bioreactor  10  to enter the service box  26 . 
         [0021]    The service box  26  also supports a carrier screen  32 . The carrier screen  32  comprises a screening body  34 . The screening body  34  is at least partially made up of a mesh, perforated or other material having openings. The openings have a shape and size that contains a selected type of biofilm carrier in the tank  12 . The screening body  34  may have an elongated prismatic shape. For example, the screening body  34  may be in the shape of a tube. The bottom of the screening body  34  is covered with a material that may be solid or have openings. The top of the screening body  34  may also be covered but it is preferably left open. In this way, the interior of the screening body  34  can be reached for maintenance, for example to clean the screening body  34  by spraying water through it from the inside out. 
         [0022]    The carrier screen  32  may also have an agitator  36  and a collection pipe  38 . The collection pipe  38  may be part of the effluent conduit  17  when the carrier screen  32  is in use. The carrier screen  32  may be suspended from the service box  26  when in use by way of a lip  71  of the carrier screen resting on a flange  72  in a curtain  73  of the service box  26 . Optionally, the lip  71  may be attached to the flange  72 , for example by a bolt, clamp or pin. Opening the lid  30  and releasing the attachment, if any, allows the carrier  32  to be lifted out of the sludge  19  for installation, maintenance or replacement. 
         [0023]    While the carrier screen  32  is in use, the screening body  34  extends from the service box  26  into the tank  12  and below the surface level of the sludge  19 . The depth that the screening body  34  extends into the sludge  19  may vary. If biogas is removed through a biogas collection pipe  70  in the service box  26 , the sludge  19  is kept below the curtain  73  of the service box  26 , for example at line A in the Figures. Preferably, the top of the carrier screen  32  is always above the surface of the sludge  19  so that it can be reached without reaching through the sludge  19  and so that carriers do not enter the interior of the screening body  34  even if it has an open top. Sludge  19  passes through the screening body  34  to the interior of the carrier screen  32  while the biofilm carriers are retained in the tank  12 . 
         [0024]    The agitator  36  may be located near the bottom of the screening body  34 . Preferably, the agitator is attached to the screening body  34 . Alternatively, the agitator  36  may be supported on a separate frame (not shown) that supports the agitator  36  at or near the bottom of the screening body  34  from the floor or wall of the tank  12 . The agitator  36  preferably has holes spaced around the periphery of the bottom of the screening body  34  and provides a curtain of bubbles that rise from the agitator  36  to the surface of the sludge  19 . For example, if the cross-sectional shape of the screening body  34  is circular, the agitator  36  may be a ring or torus shaped diffuser. The agitator  36  may be fed biogas from a pump (not shown) with its inlet in communication with the headspace  20  and its outlet in communication with a gas feed line  76 . The gas feed line  76  extends from the service box  26  to the agitator  36  inside or outside of the screening body  34 . The gas feed line  76  preferably has a union  77  that allows it to be released from the pump when the carrier screen  32  is removed from the sludge  19 . In use, bubbles travel very close to, and perhaps along, the sides of the screening body  34  to move carriers away from the screening body  34  and help avoid concentrating carriers near the screening body  34 . 
         [0025]    Sludge  19  that passes through the screening body  34  flows into the collection pipe  38 . The collection pipe  38  has an inlet in communication within the interior of the screening body  34  and an outlet in fluid communication with the remainder of the effluent conduit  17  through a union  44 , alternatively called a pipe coupler. The union  44  is preferably a snap-on type coupled that can be disengaged by lifting the carrier screen  32 . For example, the pipe coupler  44  may be a pitless adapter of the type used to insert a pump into a well shaft. 
         [0026]    To perform maintenance on the carrier screen  32 , the sludge  19  is raised to level B or a movable portion  50  of the curtain  73  is lowered into the sludge  19 . This prevent biogas from flowing from the headspace  20  to the interior of the service box  26 . Alternatively, with a biogas collection pipe  70  that does not pass through the service box  26 , the sludge  19  may be already at level B. Biogas in the service box  26 , if any, is vented, for example through lid  30 , hatch  31  of biogas collection pipe  70 . If necessary, an isolation valve  46  in the effluent conduit  17  is closed. In the example shown, a handle  47  to close the isolation valve  46  is reached through the service box  26 . Alternatively, the isolation valve  46  may be located outside of the tank  12 . Union  77  in the gas feed line  76 , if any, is separated. The carrier screen  26  may then be lifted from the sludge  19 , passing though the lid  30  if necessary. Lifting the carrier screen  26  automatically separates the union  44  if it is a pitless adapter or snap-on coupler. Otherwise, the union  44  is disconnected through the service box  26  before lifting the carrier screen  26 . The flange  72  may have a notch (not shown) to allow the collection pipe  38  and attached part of the coupler  44  to pass through the flange  72 . Optionally, the carrier screen  26  can be completely removed from the tank  12  and service box  26 . These steps are performed generally in reverse order to re-install the carrier screen  26 . 
         [0027]    When the bioreactor  10  is in use, wastewater  18  treated flows into the tank  12 . Organic matter within the wastewater  18  is broken down by microorganisms supported on the carriers and sludge  19  is produced. Breaking down the organic matter may also produce biogas. Some sludge  19  flows out of the tank  12 . The carriers are separated from the effluent sludge  19  by passing it through the carrier screen  32 . The treatment of the wastewater  18  optionally includes a step of producing bubbles near the screening body  34  to disperse carriers from it. When necessary, the carrier screen  32  can be removed from the sludge  19  as described above for maintenance. 
         [0028]    This written description uses examples to disclose the invention and also to enable any person skilled in the art to practice the invention. The scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art.