Abstract:
A media bed in a biological reactor is backwashed in a plurality of short pulses. An outlet of the reactor is preferably located above, but close to, the top of the bed. During a pulse, the outlet is closed and the bed may be fluidized or otherwise expanded to above the level of the outlet. Between pulses, the bed settles to below the level of the outlet.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a non-provisional application of U.S. Application Ser. No. 62/131,328, filed Mar. 11, 2015. U.S. Application Ser. No. 62/131,328 is incorporated by reference. 
     
    
     FIELD 
       [0002]    This specification relates to a method for cleaning a biological reactor. 
       BACKGROUND 
       [0003]    The following paragraphs are not an admission that any of the information below is common general knowledge or citable as prior art. 
         [0004]    Selenium is an essential trace element, but becomes toxic at very low concentrations. Selenium accumulates in the bodies of plants and fish that live in selenium-contaminated water and in the bodies of wildlife and people that eat those plants and fish. In people, elevated selenium concentrations may cause neurological damage and hair and nail loss. 
         [0005]    Selenium has been treated in biological reactors, for example as described in U.S. Pat. No. 6,183,644 and International Publication Number WO 2007/012181, and as used in ABMet™ reactors sold by the GE Water and Process Technologies. In such reactors, dissolved selenium is removed from contaminated water by treating the water in a reactor containing selected endemic and other selenium reducing organisms. Microbes may be isolated from the specific water or imported from other selenium contaminated water. The microbes are then screened for ability to reduce selenium under the site specific environmental conditions. The selected microbes are optimized for selenium reduction, then established in a high density biofilm within a reactor. The selenium contaminated water is passed through the reactor with optimized nutrient mix added as needed. The elemental selenium is precipitated and removed from the water. The entirety of U.S. Pat. No. 6,183,644 is incorporated herein by this reference to it. 
         [0006]    The high density biofilm may be supported on a media bed, as described in US Publication No. 2013/0270181. Activated carbon may be employed as the medium and provides a large surface area available for microbial growth. The activated carbon may be in the form of granular activated carbon (GAC) or pelletized activated carbon. Other media may be used, for example polymeric fibers, crushed stone, pumice, sand, plastic media or gravel. As selenium, and possibly other solids, accumulate in the media bed, the pressure drop across the media bed will increase. As a selected time interval or pressure drop set point, the media bed is backwashed to remove the grown biomass and solids retained in the media bed. The upflow velocity during backwashing may be about 80 ft/hr. Backwashing may be required from between once every two weeks to only a few times each year, for example once a month. Backwashing may take, for example, 30 minutes. The upflow velocity applied during flushing may result in an upward expansion of the media bed by up to 30%. The backwash liquid and entrained solids are removed through troughs located above the expected media expansion area and connected to backwash effluent line. The entirety of US Publication No. 2013/0270181 is incorporated herein by this reference to it. 
       SUMMARY 
       [0007]    The following summary is intended to introduce the reader to this specification but not define any invention. Inventions may reside in the combination of one or more of the apparatus elements or process steps described anywhere in this document. 
         [0008]    The backwashing method described above necessitates a large space to be reserved above the media bed to accommodate media expansion and settling without the media outflowing with the backwash liquid through the overflow troughs. In addition, the method described above creates stable laminar flow of backwash liquid through the media bed, which undermines backwash efficiency in removing the impurities and solids retained in the media bed. Further, solids retained below the overflow troughs are not discharged. 
         [0009]    This specification describes a method of backwashing a media bed in a biological reactor. The bed is backwashed in a plurality of short pulses. An outlet of the reactor is preferably located above, but close to, the top of the bed. During a pulse, the outlet is closed and the bed may be fluidized or otherwise expanded to above the level of the outlet. Between pulses, the bed settles to below the level of the outlet. 
         [0010]    Without intending to be limited by theory, the inventors believe that the pulses generate more turbulence to remove solids, for example elemental selenium, from the bed relative to a continuous backwash of the same total duration and flow rate. Relative to the prior art reactor described above, replacing the overflow troughs with a relatively low outlet allows more of the backwashed solids to be released, or conserves space in the bioreactor, or both. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]      FIG. 1  is a schematic cross-sectional view of a biological reactor. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  shows a treatment system  10  including a biological reactor  12 . Biological reactor  12  can be adapted to remove various impurities from wastewater influent  14 , For example, biological reactor  12  can be adapted to remove selenium. Reactor  12  may be operated as an attached film, upwards or downwards plug flow reactor. 
         [0013]    In the biological reactor  12  shown, media bed  16  provides a location on which a population of microorganisms will grow and be retained within the biological reactor  12 . Activated carbon may be employed as the medium and provides a large surface area available for microbial growth. The activated carbon may be in the form of granular activated carbon (GAO) or pelletized activated carbon. Other media might be used, for example polymeric fibers, crushed stone, pumice, sand, plastic media or gravel. 
         [0014]    The biological reactor  12  has a backwash port  18 , which may be connected to a distribution system  20 , for example one or more perforated horizontal pipes. In the preferred embodiment, the backwash port  18  and the distribution system  20  are positioned below the media bed  16 . Aggregate  22  may be installed around the distribution systems  20  below the media bed  16  to aid in flow distribution while also preventing breakthrough of media to the distribution systems  20 . This arrangement is preferred but other systems and arrangements may be suitable for distributing backwashing liquid through media bed  16 . 
         [0015]    During normal operation, wastewater influent  14  enters biological reactor  12  through upper port  24  and flows downwards through media bed  16 . Treated effluent  26  exits the reaction vessel through lower port  28 . While passing through the media bed  16 , impurities are biologically removed from the wastewater and are retained within the media bed  16 . For example, soluble forms of selenium may be reduced to elemental selenium located inside or outside of the microorganisms. As solids (including living and dead microorganisms) accumulate in the media bed  16 , the pressure drop across the media bed  16  will increase deteriorating filter efficiency. 
         [0016]    At a selected time interval or pressure drop set point, the flow of wastewater influent  14  is stopped by closing the influent valve  30 . The wastewater influent  14  in the biological reactor  12  is preferably allowed to pass through the media bed  16  and drain through lower port  28 . The wastewater influent  14  in the reactor  12  may alternatively be allowed to drain to a selected minimum level, which may be above, at or below the top of the media bed  16 . Effluent valve  38  is then closed and a backwash cycle may be initiated. 
         [0017]    At the start of the backwash cycle, a pulse of backwash liquid  32  is supplied from backwash supply  34  by opening backwash influent valve  36  and, optionally, by operating a pump if necessary. The backwash liquid  32  is supplied through the backwash port  18  to flush or backwash the media bed  16 , The backwash liquid  32  may be supplied, for example, at the rate of 5-20, preferably about 10, gpm/ft 2 . As the backwash liquid  32  travels through the media bed  16 , solids are removed from the media bed  16  and entrained in the backwash liquid  32 . 
         [0018]    The pulse may continue, for example, for less than 5 minutes, preferably 2 to 4 minutes, The turbulence created by the passage of backwash liquid  32  may expand the media bed  16  beyond its volume during normal operation. At the end of pulse, the supply of backwash liquid  32  is stopped. 
         [0019]    The media bed  16  is then allowed to at least partially settle. Preferably, the media bed  16  settles to at least below the level of an opening in the reactor  12  to a backwash effluent line  40 . At the end of the settling period, backwash liquid  32  containing entrained solids is drained from the biological reactor  12  through the backwash effluent line  40  by opening a backwash effluent valve  42 . After the water level in the reactor  12  is near or at the opening from the reactor  12  to the backwash effluent line  40 , the backwash effluent valve  42  is closed. 
         [0020]    The pulse and drain steps described above may be repeated one or more times. For example, the steps may be repeated two to five times. The reactor  12  is then returned to normal operation. 
         [0021]    The method described above provides a means of effectively cleaning the media bed by providing a dynamic and unstable backwash flow to produce shear on the media. This is in contrast to backwashing with a continuous flow for 15 minutes or more, which creates a laminar flow and generates less turbulence on the media due to a stabilized flow pattern. To allow for more complete draining of solids released during a pulse, the opening from the reactor  12  to the backwash effluent line  40  is located above, but near, the height of the media bed  16  when fully settled. For example the opening from the reactor  12  to the backwash effluent line  40  may be located less than 30 cm above or less than 15 cm above, the height of the media bed  16  when fully settled. 
         [0022]    The process described above is intended to provide an example and not to limit or define any claimed invention. Other cleaning processes may be used within the scope of an invention defined in one of the following claims.