Abstract:
The present invention relates to a vertically adjustable screened decanter system to replace prior art stationary or pivoting effluent weirs in water clarifiers and settling basins. The screened decanter has no physical weir and relies instead on maintaining a desired flow rate by controllably varying the depth of immersion of a screened box. The decanter is periodically raised into a hood that provides spray cleaning and disinfection of the screened box. The system is capable of removing up to 85% of the BOD in a wastewater stream.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of water treatment; more particularly, to settling tanks in water treatment systems wherein grit and dense solids are allowed to settle from the influent, and buoyant solids (fats, oil, grease, non-dense solids) are prevented from entering into an effluent decanter; and most particularly, to a vertically driven screen box assembly (SBX) comprising a screen for separating liquids from solids. 
     BACKGROUND OF THE INVENTION 
     In developed and developing countries, primary treatment and disinfection of waste water discharges from collection systems and waste water treatment facilities is the first step to improving water quality. As the countries continue to advance, secondary and tertiary waste water treatment processes are added to provide additional treatment of the primary effluent. 
     Primary treatment removes large solids via screening and gravitational settling to remove light and dense solids, allowing neutrally buoyant matter to pass into the secondary treatment process or receiving body of water. Primary treatment utilizing gravitational settling or clarification is recognized as removing 20-33% of the organic load as measured in Biochemical Oxygen Demand (BOD). Secondary treatment removes another 50+% of the organic load by converting the BOD to biomass (bacteria) and CO 2 . 
     Secondary treatment provides an environment of adequate temperature, volume, mixing, and oxygen or the absence of oxygen in anaerobic processes to sustain the bacterial population necessary to consume the BOD and nutrients remaining in the waste water after primary treatment. New organic matter enters the treatment facility continuously so a portion of the existing bacterial population is removed from the process to promote the growth of new bacteria. The effectiveness of primary treatment directly affects secondary process or the receiving body of water if discharged from the collection system. 
     Primary clarifiers or settling basins are recognized as being the most economical means to reduce BOD as there is little energy required and no biomass to maintain. Primary treatment has no biomass therefore no aeration energy; no process controls to monitor the biomass to determine the health of the biomass by the types and quantity of the bacteria; no need to separate and remove or waste the bacteria by moving to a side-stream digester; no need to aerate the digester; and no need to dewater and dispose of the surplus bacteria, also called secondary sludge. The lack of complexity of primary treatment is well suited for developing nations and begins an effective recovery of their surface waters and aquifers resulting in reduced health issues. 
     Prior art primary clarifiers may be circular or rectangular tanks and are volumetrically and geometrically sized to provide a horizontal fluid velocity lower than the solids settling velocity. The horizontal travel time and distance of the liquid from the inlet to the effluent weir must be greater than the settling time and distance of the suspended solids so that solids settle to the bottom of the tank prior to reaching the elevated effluent weir. These settled solids contain a majority of the BOD in raw sewage. This is an important first stage because the more solids that exit the primary clarifier (or if there is no primary clarifier), the higher the BOD entering the secondary treatment process or the effluent-receiving body of water. The higher the BOD entering the secondary treatment process, the larger the required secondary process equipment and tanks, the more biomass required, generated, and disposed of, the more processing energy that must be expended. The higher the BOD of the effluent stream entering the receiving body of water the greater the eutrophication of the water body and the more detrimental to the health, due to poor disinfection. 
     An example based on standard design parameters to achieve 33% BOD reduction is shown as follows: 
     Minimum depth=10′; Surface Overflow Rate=1,000 Gallons per day (GPD)/square foot (design) and 1,500 GPD/SF (Peak); Weir Loading @ Peak Hourly=20,000 GPD/linear foot; 
     Use Design Flow=1,000,000 GPD (1.55 CFS); Peak Hourly=2,500,000 GPD (3.87 CFS); 
     Design=1,000,000 GPD/1,000 GPD/SF=1,000 SF; Peak=2,500,000/1,500=1,667 SF 
     Typical design seeks a length about 3 times the width so, 1,667 SF=24′ wide×70′ long×10′ deep; Forward velocity=3.87 CFS/(10′×24′)=0.016 Ft. per Second (FPS). 
     An EPA study provided a summary of settling data from multiple wastewater plants. The table below is an average of pertinent findings to support the design parameters as they relate to BOD reduction: 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Organic 
                 Average 
                   
                   
               
               
                 Suspended 
                 % Primary 
                 (BOD) 
                 Settling 
                 % &gt;50 
                 % BOD 
               
               
                 Solids 
                 Sewage 
                 Content 
                 Velocity 
                 microns 
                 Reduction 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Settleable 
                 45 
                 50% 
                 0.106 FPS 
                 64% 
                 22.5%   
               
               
                 (&gt;100 microns) 
               
               
                 Supracolloidal 
                 35 
                 30% 
                   
                 68% 
                 0% 
               
               
                 (1-100 microns) 
               
               
                 Colloidal 
                 20 
                 20% 
                   
                 0% 
                 0% 
               
               
                 (0.2-1.0 microns) 
               
               
                   
               
             
          
         
       
     
     The values in the above table are averages taken from several WWTP that include storm water, combined sewer systems, and sanitary sewage. The settleable solids have a settling velocity range from 0.016 to 0.115 FPS with an average of 0.106 FPS as stated in the table. 
     The design example above results in a forward velocity of 0.016 FPS which is less than the average settling velocity of 0.106 FPS. The tank is 10′ deep so the solids will settle in 94 seconds. The forward distance travelled in 94 seconds is 1.5 Feet so the solids will settle before the liquid reaches the effluent weir. The EPA study expressed considerable difficult in establishing a consistent average for the supracolloidal and colloidal solids as they vary from site to site and range from 0.0007 to 0.002 FPS. The forward velocity is 0.016 FPS and the tank is 70 Ft long therefore the travel time=4,375 seconds therefore the depth of settling is 3′ to 8.75′. 
     The effluent weir is 2,500,000 GPD/20,000 GPD/Ft.=a minimum of 125′, the tank is 24′ wide therefore use 3-double sided weirs providing 144′ of weir length so the flow is 2,500,000 GPD/144=17,361 GPD/Ft or 0.027 CFS/Ft. at the weir. The velocity of the liquid at 3′ from the weir is 0.0057 FPS and at 8.75′ the liquid velocity is 0.002 FPS. Some portion of the supracolloidal solids will be removed as per this mathematical exercise on clarifier velocities, but very little of the colloidal solids. 
     It would be reasonable to expect the primary clarifier in this design example to reduce the BOD to the receiving stream or secondary treatment process by 33%. 
     Developed and developing nations, as well as the environment, would significantly benefit from removing more than 20-33% of the organic matter from the waste water in the primary treatment because;
         Less CO 2  would be released to the atmosphere.   Less energy consumed to convert the organic matter (BOD) to biomass (secondary sludge)   Less secondary sludge to pump, store, aerate, dewater, and send to landfill   Fewer trucks hauling secondary sludge to landfill or composting facilities   Landfills would have a longer operational life and release less methane to the atmosphere   Smaller secondary treatment system would be possible resulting in significant capital costs savings for the developed and developing countries allowing more to be done sooner   Lower operational and maintenance costs for the secondary treatment systems   Higher quality primary effluent would accelerate improvements to the receiving waters and reduce environmental health and safety issues   The higher concentration of organics in the primary sludge significantly increases the energy generation potential in anaerobic digesters. Anaerobic Digesters capture and utilize the methane gas created from the high volatile primary sludge to produce energy versus releasing most of the methane to atmosphere due to poor capture systems in landfills.   Waste water treatment plants become a renewable resource recovery facility creating more energy than they consume as the organic load to the secondary treatment process is reduced and the organic fuel for the anaerobic digesters is increased.   Anaerobic Digestion creates less bacteria and results in a Class A sludge that can be used for composting.       

     The organic removal rate of primary clarifiers can be improved from 33% to approximately 50% by the addition of coagulating chemicals. This improvement is called Chemically Enhanced Primary Treatment (CEPT) and CEPTs have demonstrated all of the above described benefits. There were no physical or operational modifications to the primary clarifier tank, influent flow baffle, sludge scrapper mechanisms, scum trough or effluent trough. The coagulant forms a floc or gel net that is larger and more dense than the individual suspended solids. As this floc settles it gathers some supracolloidal and colloidal particles thus reducing the BOD and suspended solids flowing to the secondary treatment process. 
     The Ballasted Floc Reactor (BFR) followed the CEPT in an attempt to remove more BOD and reduce capital costs. The BFR technology removes approximately 50% of the BOD, the same as CEPT, but with a smaller clarifier because the solid settling rate is much higher. 
     Developing nations would likely not be able to see the benefits of enhanced BOD reduction with the CEPT or BFR products because the chemicals and skilled operators may not be available. 
     In summary, conventional primary clarifiers, BFRs and CEPTs do not have screened effluent weirs to retain the supracolloidal and colloidal organic particles. Simple placement of a screen at existing effluent weirs will not work because a) such screens would foul in a short time frame due to the high flow velocity at the weir design liquid flow velocities; b) such screens would be stationary so there is no backwashing; and c) such screen would foul due to organic growth on the screen since the screen is in the liquid all of the time. The forward velocity from the inlet to the effluent weir is constant so there is an inertia imparted into the solids keeping them moving towards the effluent weir; there is no velocity control within the tank as the tank is always full so if 10 gallons of liquid enters the tank, 10-gallons of liquid must exit the tank at the same rate as it was added; and the sludge removal equipment in the tank is continually moving and disturbing the settled sludge creating eddies that keep neutrally buoyant constituents and colloidals in suspension moving towards the effluent weir at a high effluent weir entrance velocity. 
     A screened decanter comprising an effluent weir is disclosed in U.S. Pat. Nos. 7,972,505 and 8,398,864, the relevant disclosures of which are incorporated herein by reference. The movement of a screened decanter is an arc rotating about a pivot. The vertical movement of the screened decanter about a pivot comprises both horizontal and vertical movement in the direction of motion. Depending upon the depth of the tank, the length of the pivot arm requires that the decanter assembly occupy a relatively large footprint in the tank. 
     What is needed in the art is a screen assembly in the form of a rectangular box or cylinder that is controllably driven in the vertical direction to optimize the exposure of the screen to the wastewater to varying wastewater levels and that can be lifted from the wastewater for backflushing and sterilization in a dedicated overhead apparatus. Because the motion of the screen assembly is only vertical, the required footprint can be relatively small. 
     What is further needed is an assembly comprising a ganged plurality of such screen box assemblies for wastewater systems having high flows, limited surface area, and/or shallow active tank volumes. 
     It is a principal object of the invention to provide a high and constant effluent flow rate from a wastewater treatment facility over a wide range of influent flow rates. 
     SUMMARY OF THE INVENTION 
     Briefly described, the present invention provides a screen assembly in the form of a rectangular box or cylinder that is controllably driven in the vertical direction to optimize the exposure of the screen to the wastewater to varying wastewater levels in a wastewater clarifier and that can be lifted from the wastewater for backflushing and sterilization in a dedicated overhead apparatus. 
     A screen box (“SBX”) assembly in accordance with the present invention comprises an ultrafine screen; a screen frame of flat plate and hollow tubing that incorporates air scouring at the lowest elevation of the screen, the frame being sealed to prevent liquids and solids from bypassing the screen so all must pass through the screen; a flexible discharge hose that may have swivel joints or may extend and compress in an accordion fashion to minimize forces on the screened decanter; guiderails to define the vertical and horizontal movement of the invention; a lifting device to raise and lower the invention in the liquid at controlled descent speed and multiple rise rates; an effluent flow manifold with openings to allow liquid to flow to the screen from below the screen; a deflector plate with drain ports; an encoder to position the screen box in the tank to measure headloss and to insure the appropriate amount of screen is in contact with the wastewater; a protective maintenance hood to backwash, disinfect, and thaw the screen; controls, sensors, actuated valves, modulating valve, flow meter, and in some cases a filtrate pump if required by the existing hydraulic gradient. 
     Multiple units of the invention may be necessary to meet the needs of each application; similarly, multiple units of the invention may be used in the same tank to provide a redundant system as desired. 
     A SBX assembly defines a physical barrier providing a very low horizontal velocity to the wastewater exiting the clarifier so as to retain most of the supracolloidal and colloidal solids. The physical barrier has openings small enough to keep a majority of the supracolloidal solids within the primary clarifier. The deflector plate prevents the disturbance of the settled solids below the deflector plate and increases the travel time of liquid to discharge at the screen. 
     The fundamental difference between a prior art weir structure and a novel vertical screen structure in accordance with the present invention is that a weir structure permits only a relatively shallow layer of fluid from the top of the fluid mass in the tank to pass over the weir to exit the tank, thus creating comparatively high horizontal flow velocities which work against providing sufficient time for solids to settle below the level of the weir. A vertical screen structure, to the contrary, permits horizontal flow from the tank into the screen structure over a comparatively large surface area of screen and depth of flow, thus requiring only very low horizontal flow velocities to separate relatively large volumes of fluid from the tank fluids. 
     The vertical position of the SBX is controllably adjustable to provide a change in liquid elevation and a rest period with no forward velocities that allow the supracolloidal and colloidal solids in suspension to mix with the coagulant and settle, as there is no velocity towards the discharge. Such controls include a modulating screened effluent discharge valve, flow meter, and electronic control system that adjusts the screen surface area in contact with the liquid to maintain a screen loading rate (GPM/Sq. Ft. of Screen) based on discharge velocity, resulting in reduced screen fouling. Pressure transducers, encoders, and controls to measure headloss through the screen and to control the movement of the screened decanter are included in the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages of the invention, as well as presently preferred embodiments thereof, will become more apparent from a reading of the following description in connection with the accompanying drawings in which: 
         FIG. 1  is an elevational cross-sectional view of an SBX assembly in accordance with the present invention, showing the SBX screens being scoured by introduced air bubbles; 
         FIG. 2  is an elevational cross-sectional view like that shown in  FIG. 1 , showing the SBX screens being ⅔ clogged; 
         FIG. 3  is an elevational cross-sectional view like that shown in  FIG. 2 , showing the SBX screens being further immersed to permit continued operation of the unit with fresh screen surface; 
         FIG. 4  is an elevational cross-sectional view like that shown in  FIG. 1 , showing the SBX being supported on a lifting column having slotted exit ports; 
         FIG. 5  is an elevational cross-sectional view like that shown in  FIG. 4 , showing the exit ports being screened; 
         FIGS. 6 through 10  are elevational views of alternate configurations of exit ports in a lifting column; 
         FIG. 11  is an isometric view from above of an SBX and central lifting column, showing a lifting cable attachment; 
         FIG. 12  is an enlarged view of the lifting cable attachment shown in  FIG. 11 ; 
         FIG. 13  is an elevational view of an SBX disposed for cleaning and disinfection in first embodiment of a hood in accordance with the present invention; 
         FIG. 14  is an elevational view of an SBX disposed for cleaning and disinfection in second embodiment of a hood in accordance with the present invention; 
         FIG. 15  is an elevational cross-sectional view of a complete wastewater treatment system, showing an SBX in raised position inside a cleaning hood; 
         FIG. 16  is an elevational view of a water treatment system, showing a hydraulic or pneumatic power pack for lifting the SBX; 
         FIG. 17  is an elevational cross-sectional view like that shown in  FIG. 15 , showing an SBX in lowered position, freshly cleaned and entering into service; 
         FIG. 18  is an elevational cross-sectional view like that shown in  FIG. 17 , showing an SBX having been controllably lowered in accordance with the present invention to follow a drop in tank level to maintain a desired immersion level of the SBX; 
         FIG. 19  is an elevational cross-sectional view like that shown in  FIG. 18 , showing an SBX having been controllably lowered still farther to follow a further drop in tank influent level to maintain a desired immersion level of the SBX; 
         FIG. 20  is an elevational cross-sectional view like that shown in  FIG. 19 , showing an SBX having been controllably raised from immersion to permit backwash of the screens in the SBX; 
         FIG. 21  is an elevational cross-sectional view of a dual-tank wastewater treatment system, showing the SBX in one tank being backwashed while the SBX in the other tank continues in normal service; 
         FIG. 22  is an isometric view from above, showing an SBX single-tank wastewater treatment system similar to that shown in  FIG. 15 ; 
         FIG. 23  is an isometric view from above, showing multiple SBXs in a single tank wastewater treatment system; 
         FIG. 24  is an isometric view from above, showing a single SBX in a single-tank wastewater treatment system having a circular tank and circular SBX; 
         FIG. 25  is an isometric view from above of a larger circular wastewater treatment tank having a plurality of ganged cylindrical SBX units; 
         FIGS. 26, 26   a  are elevational and plan views of a prior art wastewater treatment system, showing the footprint required by a prior art pivoting decanter; 
         FIG. 27, 27   a  are elevational and plan views of a prior art wastewater treatment system, showing the footprint required by a retrofitted vertical lift SBX decanter system in accordance with the present invention; 
         FIG. 28  is an isometric view showing multiple racks mounted to a single discharge manifold with retractable air hose reels above in a single tank; 
         FIG. 29  is a plan view of multiple screen racks with square ends; 
         FIG. 30  is a plan view of multiple screen racks with rounded ends to create a volute shape to improve horizontal flow; 
         FIG. 31  is a plan view of multiple screen racks with triangular ends to improve horizontal flow patterns; 
         FIG. 32  is an isometric view of the spray header typically located inside a spray hood; 
         FIG. 33  is an isometric view of a spray bar having unique shaped orifices to send a horizontal fan of high pressure/low volume water to both inside faces of the screen box; 
         FIG. 34  is an isometric view of the backwash spray manifold and spray bars in the spray hood above a SBX having multiple screen racks; 
         FIG. 35  is an isometric view showing the upward movement of the SBX into the spray hood. The backwash water is activated when the top of the screen reaches the spray bar elevation and continues to backwash the SBX as it slowly rises in the spray hood and then shuts off when the bottom of the screen reaches the spray bar elevation; 
         FIG. 36  is an isometric of the multiple rack SBX inside the spray hood; 
         FIG. 37  is a cross-section view showing the spray bar and backwash manifold positioned inside the screen racks of the SBX; 
         FIG. 38  is a plan view of an LPSBX manifold; and 
         FIG. 39  is an isometric view of the LSBX manifold shown in  FIG. 38 , shown in inverted position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1-39 , there is shown an SBX system  10  in accordance with the present invention, comprising the following elements: 
     Screen Box (SBX) 
     The top  14  of the SBX  12  ( FIG. 1 ) is normally open to allow occasional screen washing via hose or automated spray system (spray ball for the symmetrical shapes or spray bar for the long rectangular boxes) and to access instruments located inside of the screen box. 
     Some applications (not shown) may require a closed and sealed top when the screen box operates completely submerged except for air vents. These air vents also serve to store screened liquid  11  to provide additional backwash volume. 
     The bottom  16  of screen box  12  is a solid plate with open areas to allow screened liquid  11  to exit the screen box and thus the tank. The solid plate  16  and closed effluent valve  18  ( FIGS. 15-23 ) requires all screened liquid inside of screen box  12  to exit via the screened sidewalls to improve screen backwashing at the end of each decant cycle. 
     The sides  20  of screen box  12  consist of screen  22  and screen framing members  24  that may be vertical (perpendicular to the liquid surface) or sloped so that the top of the screen box is wider than the bottom creating a frustum shape. This allows for more screen surface to be in contact with the influent liquid  13 , and liquid  13  enters from all sides thus decreasing the approach velocity  15  to the screen. 
     Some screen boxes may only have screened surfaces below the surface of the liquid with a solid vertical plate above the screen. The solid portion may be partially submerged to increase the volume of screened liquid inside of the screen box used for backwashing of the screen. This solid portion also will not foul due to fats, oils, and grease on the surface of the liquid. 
     Screens that are elongated and spaced closely to other screen boxes or racks may have a rounded or triangular end pieces to direct horizontal flow to between the racks with less turbulence in a more laminar flow. 
     Preferably, each screen rack is formed of fiberglass to avoid the corrosive decay to which metal racks and gaskets may be subject. Each screen is laminated to a flat sheet of FRP with an air scour header  24 ′ laminated across the base of the screen. Preferably, header  24 ′ contains low pressure air on the inside with small openings (not visible in  FIG. 1 ) in the top of header  24 ′ to provide air bubbles  26  to air scour to the screen surface. It is critical that screen box  12  be sealed along all edges to prevent the liquid  13  in the tank from entering screen box  12  by any means other than passing through screen elements  22 . Gasketing may be provided as necessary, although non-gasketed arrangements are preferable. 
     Air  26  is released at the base of the screen surface through the tubular screen frame as described above. The vertical flow of air scours the external surface of the screen. Solids that may be pressed against the exterior surface of the screen by liquid moving through the screen are disturbed and carried upward. The vertical flow of air and solids also aligns elongated fibers vertically, or perpendicular to the openings in the screens, to reduce passage of solids through the screens. 
     Preferably, an oxidant solution (e.g., aqueous sodium hypochlorite or potassium permanganate) is injected into the compressed air line. 
     The ultrafine screen currently preferred is a SS wire woven as a fabric. Screens of different materials and opening sizes may be used in certain applications. 
     Multiple SBX modules  98  with individual synchronized lifting devices ( FIG. 23 ) are likely for large flow installations and as redundant units. The features of each module include the previously described screen, screen attachment, air scour, hood, solid plate bottom, and may or may not include a closed top with air vents and other features described below. 
     Referring to  FIGS. 24 and 25 , a second embodiment  12 ′ of an SBX in accordance with the present invention may be cylindrical (circular) or conical (not shown). To provide added capacity, a plurality of SBXs may be ganged in parallel, as shown in  FIG. 25 . A cylindrical SBX is especially useful in an installation having a cylindrical tank. The structure and operation of a cylindrical SBX is similar to that of a polyhedral SBX  12 . 
     Comparison of Prior Art Clarifier Weir with a Screen Box Decanter 
     Preferably, the present screen box system incorporates coagulation and an ultrafine screen. 
     For a conventional primary clarifier weir, the horizontal velocity of fluid at the weir may be calculated as follows:
 
20,000 gallons per day/foot of weir=0.0309 cubic feet per second/foot of weir.
 
     If the liquid depth over the weir is 3 inches, the horizontal fluid velocity at the weir=0.124 FPS. 
     To the contrary, an SBX in accordance with the present invention can provide a horizontal fluid velocity of &lt;0.009 FPS. Combining the use of a coagulant, ultrafine screen, and effluent velocity approximately 13 times lower than the conventional primary clarifier can produce a BOD removal of 65% to 85+%. 
     In addition to the previously stated benefits related to organic (BOD) reduction, secondary wastewater treatment processes can see additional benefits from the invention in:
         Improved oxygen transfer efficiency to further reduce energy consumption.   Removal of fibers that cause fouling of hollow fiber and flat plate membranes so reduced air scour energy, increase the membrane life, and reduce operational issues requiring Clean-In-Place (CIP) activities.       

     Without chemical addition, an SBX system in accordance with the present invention can remove approximately 55% of the BOD. The ultrafine screen has openings smaller than the supracolloidal particles; the air scour causes an upward velocity greater than the forward velocity of the exiting liquid causing fibers to align vertically or perpendicular to the screen openings; the reduced velocities at the screen improve settling; the deflector plate increases the travel distance of the settled BOD laden solids under the screened decanter (as shown in  FIG. 18 ) and stops the vertical velocities of the rising air bubbles from disturbing and carrying the settled BOD up towards the screen. 
     Screen box  12  replaces the effluent weir  100  used in all prior art clarifiers, (see, e.g.,  FIG. 26 ). The benefits of the screen box over the conventional effluent weir or launder are: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Conventional 
                   
                   
               
               
                 Effluent Weir 
               
               
                 Or Launder 
                 Screen Box 
                 Benefit 
               
               
                   
               
             
             
               
                 Stationary 
                 Moves 
                 A vertically moving weir changes dynamics of 
               
               
                 Effluent Weir 
                 Vertically 
                 clarification by allowing the liquid level in the tank to 
               
               
                   
                   
                 change as a stationary effluent weir maintains a 
               
               
                   
                   
                 minimum liquid level in the clarifier/tank equal to the 
               
               
                   
                   
                 elevation of the weir. Water enters the clarifier and the 
               
               
                   
                   
                 liquid near the weir immediately exits at the same rate 
               
               
                   
                   
                 as water does not compress and the tank does not 
               
               
                   
                   
                 expand to store this additional water. The invention 
               
               
                   
                   
                 decants the liquid in the clarifier to a low level then 
               
               
                   
                   
                 rises out of the tank. 
               
               
                   
                   
                 Water enters the clarifier having a low level and fills to 
               
               
                   
                   
                 a higher level. During this filling process there is no 
               
               
                   
                   
                 means for the contained water to exit the clarifier as the 
               
               
                   
                   
                 (SBX) is out of the tank. Therefore there is no 
               
               
                   
                   
                 directional flow or inertia or energy instilled into 
               
               
                   
                   
                 neutrally buoyant solids and there is no scouring or 
               
               
                   
                   
                 suspension of settled solids near the bottom of the 
               
               
                   
                   
                 clarifier that would occur if the water were continually 
               
               
                   
                   
                 moving towards an effluent weir. 
               
               
                 Weir that rotates 
                 Weir that 
                 Vertical movement has no horizontal dimension. 
               
               
                 about a pivot 
                 Moves 
                 Movement about a pivot has both horizontal and 
               
               
                   
                 vertically 
                 vertical dimensions. The horizontal motion must be 
               
               
                   
                   
                 considered in the design of a new clarifier or the retrofit 
               
               
                   
                   
                 of an existing clarifier. In all cases the horizontal space 
               
               
                   
                   
                 is larger for a pivoting than a vertical moving weir. 
               
               
                   
                   
                 A fixed weir that rotates about a pivot is limited to the 
               
               
                   
                   
                 width of the tank and receives flow in one direction, 
               
               
                   
                   
                 towards the weir. If a second weir is added to the same 
               
               
                   
                   
                 pivoting decant arm in an attempt to reduce the liquid 
               
               
                   
                   
                 velocity at the weir, the weir with the shortest radius 
               
               
                   
                   
                 will always be lower in elevation than the weir traveling 
               
               
                   
                   
                 along a longer radius. The weir and decanting arm uses 
               
               
                   
                   
                 gravity flow so the potential range of motion is limited 
               
               
                   
                   
                 to 9:00 to 12:00 or 12:00 to 3:00 (At 12:00 the decanter 
               
               
                   
                   
                 is out of the water and at 3:00 there is no hydraulic 
               
               
                   
                   
                 gradient so there is no flow at the ends of this range). 
               
               
                   
                   
                 The liquid will travel to and over the weir with the 
               
               
                   
                   
                 lowest elevation in the water at a disproportionate rate 
               
               
                   
                   
                 creating uneven flow patterns through the screen 
               
               
                   
                   
                 causing regionalized fouling issues. 
               
               
                   
                   
                 A vertically moving screen (no pivot) can have more 
               
               
                   
                   
                 than one weir or one continuous weir that remains at the 
               
               
                   
                   
                 same elevation throughout the full vertical range of 
               
               
                   
                   
                 motion. The weir is screened so an increased amount 
               
               
                   
                   
                 of screen is receiving equal flow thus reducing the 
               
               
                   
                   
                 velocity at the water/screen interface. 
               
               
                 Physical Weir 
                 No physical 
                 The liquid must flow over a physical edge and free 
               
               
                   
                 weir 
                 fall. The free fall of water creates a slight pulling 
               
               
                   
                   
                 action and no frictional headloss. Both of these create 
               
               
                   
                   
                 a high weir entrance velocity. As an example, a 3′ long 
               
               
                   
                   
                 weir with 1′ depth of water over the weir has a 
               
               
                   
                   
                 discharge flow rate of 35.4 GPM or 0.079 CFS/0.083 
               
               
                   
                   
                 SF = 0.95 FPS @ weir. 
               
               
                   
                   
                 There is no weir in the screen box with the liquid level 
               
               
                   
                   
                 set by the effluent flow and selected screen loading rate 
               
               
                   
                   
                 (GPM/Sq. Ft. of screen). Using a screen loading rate 
               
               
                   
                   
                 of 4 GPM/SF and the same flow rate of 35.4 GPM the 
               
               
                   
                   
                 required screen surface area is 35.4 GPM/4 GPM SF = 
               
               
                   
                   
                 8.85 SF of screen, The screen box is positioned based 
               
               
                   
                   
                 on screen configuration to a depth placing 8.85 SF of 
               
               
                   
                   
                 screen in contact with the liquid. The velocity of the 
               
               
                   
                   
                 liquid at the screen is 4 GPM/448.8 = 0.009 FPS. 0.95 
               
               
                   
                   
                 FPS/0.009 FPS = 106.6 times lower velocity at the 
               
               
                   
                   
                 screen surface than at the weir. 
               
               
                   
                   
                 The low 0.009 FPS horizontal (created by the deflector 
               
               
                   
                   
                 plate) exit velocity through the screen, positioned near 
               
               
                   
                   
                 the liquid surface far from the settled solids, results in 
               
               
                   
                   
                 less scouring and disturbance of the settled solids and 
               
               
                   
                   
                 organic matter. 
               
               
                   
                   
                 No physical weir allows a greater liquid depth and 
               
               
                   
                   
                 360° horizontal flow of liquid moving towards the exit, 
               
               
                   
                   
                 thus significantly larger cross-sectional area of liquid at 
               
               
                   
                   
                 every flow radius. The larger the cross-sectional area 
               
               
                   
                   
                 the slower the velocity for the same volume of liquid 
               
               
                   
                   
                 exiting the system. 
               
               
                 No Deflector 
                 Deflector Plate 
                 Previously described, but in summary it creates a 
               
               
                 Plate 
                   
                 horizontal flow pattern versus a 180° flow pattern 
               
               
                   
                   
                 towards a fix effluent weir. 
               
               
                   
                   
                 Existing effluent weirs do not have horizontal deflector 
               
               
                   
                   
                 plates or baffles as all flow must exit at the liquid 
               
               
                   
                   
                 surface. There is a Stamford Baffle that was developed 
               
               
                   
                   
                 to deflect the solids away from the effluent weirs as the 
               
               
                   
                   
                 liquid rose from the sludge blanket level towards the 
               
               
                   
                   
                 fixed effluent weir. 
               
               
                   
                   
                 The Stamford baffle is a 45° plate to allow a vertical 
               
               
                   
                   
                 flow vector. The invention&#39;s flat deflector plate 
               
               
                   
                   
                 discourages all vertical flow patterns because the SBX 
               
               
                   
                   
                 lowers with the liquid at the same rate to maintain a 
               
               
                   
                   
                 fixed screen surface area thus not requiring any vertical 
               
               
                   
                   
                 flow to exit. 
               
               
                 Weir is located 
                 Screen Box is 
                 Water exiting near the center of the tank reduces short 
               
               
                 at opposite end 
                 positioned 
                 circuit caused by placing a stationary weir near a side 
               
               
                 of inlet 
                 nearer the 
                 wall. The wall reduces the cross-sectional area of the 
               
               
                   
                 center of the 
                 water moving towards the exit causing higher velocities. 
               
               
                   
                 tank 
               
               
                   
               
             
          
         
       
     
     Deflector Plate 
     A deflector plate  60  is placed below air scour  24 ′ to stop disturbance of settled solids that may be caused by vertical currents created by rising air bubbles from the air scour. Deflector plate  60  also increases the horizontal travel distance to the screen surface for any supracolloidal or colloidal solids that may be disturbed and start to move towards the tank discharge/screen. 
     The deflector plate is sized to extend several feet (some distance) past the edge of the screen box  12 . The actual size and shape of the deflector is dependent on the size and shape of the screen box and tank. The deflector plate edge nearest the tank wall may have a flexible sealing strip  62  mounted to the deflector plate if the distance to the wall and edge of the deflector plate is within 3 feet or the tank configuration requires such to stop transient rising currents. Sealing strip  62  connection to deflector plate  60  preferably is via slotted holes to allow the strip to be adjusted closer to or farther away from the wall and then tightened into final position. Sealing strip  62  should be within 1/16 inch or actually touching the tank sidewall to minimize vertical flow from below. 
     Deflector plate  60  preferably has drain ports  64  that open with low pressure to allow liquid above the deflector plate to pass through the plate when the screen box is moving upward. The drain ports may be low tension flap valves, molded polycarbonates with resilient properties, or the like. 
     Deflector plate  60  may be made of a flexible material that bends downward to allow liquid above the plate to flow easily off the edges. Such type of plate would obviate the need for the drain ports. 
     Preferably, the edges of deflector plate  60  facing the influent feed troughs  66  are raised at an angle to increase the travel distance and deflect supracolloidal and colloidal solids rising from below the deflector plate towards the influent feed troughs and away from the screen box as the screen box lowers in the liquid. 
     Screen Box Lifting Apparatus 
     A screen box lifting apparatus  28  may be pneumatic, hydraulic, winch and cable, or other mechanical apparatus to raise and lower the SBX  12  in a path perpendicular to the surface  30  the liquid  13 . The vertical (up/down) movement of the SBX allows the SBX system to be installed in relatively small clarifier tanks of circular or square geometry. 
     The currently preferred lifting apparatus  28  comprises a combined winch  32 , cable  34 , a pulley or pulleys  36 , and a winch drive  40 . The winch and cable provide an unlimited range of vertical motion, whereas the range of pneumatic, hydraulic, and mechanical actuators are limited (at this time) to about 8 feet due to lateral stresses created by the liquid movement. As development of pneumatic and hydraulic actuators proceeds, their incorporation in SBX systems may increase. An overhead pulley arrangement keeps the SBX assembly centered in the tank. 
     The lifting range of motion typically is from the bottom of the tank (likely low level is 1-5 feet) to 6 feet above the top of the tank. 
     Preferably, winch drive  40  is a vector motor, which can operate at 0-RPMs without overheating. A vector motor is desirable to ensure that the SBX descends at the same rate as the change in liquid level, which is critical to not disturbing the supracolloidal and colloidal constituents in the waste water, to promoting horizontal versus vertical currents towards the screen box, and to maintaining the liquid/screen contact area to control the screen solids loading rate. 
     As shown in  FIGS. 20-21 , at the conclusion of a decant cycle, raising of SBX  12  starts slowly to reduce an energy spike/demand to conserve energy and then quickly accelerates to increase the exit velocity of the filtrate from inside SBX  12 , in the reverse direction through the screen, creating a vigorous backwash  42  of the screen. This action is initiated and controlled by control system  44 . 
     Cable  34  is connected to a baffled lifting column  28  for small units and to a support frame  46  of larger units. A ball and socket device  48  allows screen box  12  to move laterally as needed to reduce stress on the lifting device and to provide additional scouring of the screen box via slight horizontal motion caused by air scour and discharge hose rigidity. 
     Vertical guiderails are provided on the tank to guide SBX  12  in its vertical path. Guiderails interface with support frame  46  to align the SBX with the hood. The guiderails may be placed in various positions relative to the SBX depending on the configuration of the tank. 
     An encoder (not shown) tracks the vertical position of screen box  12  in the tank. Knowing the position of the screen box in the liquid is critical to knowing headloss through the screen and thus to having the correct amount of screen surface area in contact with the liquid for a specific screen loading rate and effluent flow rate. An algorithm to the SCADA provides control feedback on current RPM to slow or increase the motor to the proper speed. 
     Baffled Lifting Column and Stub Effluent Pipe for the SBX 
     Baffled Lifting Column  28  is a slotted or perforated circular pipe that is internally or externally threaded at the base to connect to the SBX Stub Effluent Pipe  52 . Lifting column(s)  28  (the long rectangular screen racks have (3) lifting columns and not all are used for lifting and all are centered and equally spaced in the screen racks) is centered in the SBX with openings  54  to encourage flow distribution through the screen. In rectangular or square frustum SBX shapes preferably there is more open area on the Baffled Lifting Column facing the box corners so as to pull more liquid from the corner or more distant screen. The open area closest to the screen will have the lowest surface area. If the screen is an equal distance from the Baffled Lifting Column, as in a cylindrical SBX, then the open area is the same around the circumference of the circular lifting column. 
     Preferably, the open area of the Baffled Lifting Column is lowest at the bottom and increases with elevation, creating headloss at the lower portion of the lifting column to equalize travel distance and pressure, and thus to equalize flow through the screen from the lowest point to the highest point of liquid contact. 
     Various configurations of suitable openings (vertical slots  54  tapering or of variable length, horizontal slots  54   a , holes  54   b , and screening  54   c ) are shown in  FIGS. 4-10 . 
     Baffled Lifting Column  28  connects to SBX Stub Effluent Pipe  52  that connects directly to a flexible discharge hose  68  that directs the filtrate/effluent to effluent exit valve  18 . 
     Liquid Level and Effluent Flow Controls 
     Referring to  FIGS. 15 and 17-20 , for gravity discharge flow applications, the flow rate of screened wastewater exiting the tank is controlled by a modulating exit valve  18  that opens or closes incrementally to maintain a target flow rate set by the controls  44  and measured by a flow meter  70  located upstream or downstream of the modulating exit valve. 
     The elevation of the discharge end of the screened wastewater pipe  72  is fixed as are the diameter and length of pipe connecting the SBX, SBX Stub Effluent Pipe, Discharge Hose, Flow Meter, and Modulating Valve to the discharge end. The piping and discharge location and elevation are a component on the infrastructure and not subject to change. 
     The change in liquid elevation within screen box  12  and the change in elevation of the screen box in the tank from a high liquid level  74  to a low liquid level  76  affects the hydraulic pressure in the screened effluent piping. The greater the elevation difference between inlet liquid elevation and discharge liquid elevation, the greater the pressure difference and thus flow. The lower the difference, the lower the pressure and thus flow. 
     Screen box  12  starts a decant cycle at the high liquid level  74  in the tank ( FIG. 17 ). Screen box  12  lowers at the same rate as the liquid level in the tank. When the tank liquid level reaches the low level set point, the screen box then is lifted upwards. The captured screened liquid exits outwards through the screen on the screen box. The faster the rise rate, the higher the exit velocity of the screened liquid moving through the screen. The high velocity creates a more vigorous backwash resulting in a more thorough cleaning of the screen. 
     The system employs a pair of pressure transducers  78 , 80  ( FIG. 15 ) disposed within the screen box and the tank, respectively. The control system  44  uses input from the flow meter  70 , pressure transducers  78 , 80 , and tank encoder to automatically position the screen box in the liquid to provide the defined screen surface area in contact with the liquid. The controls can automatically adjust the screen/liquid contact area to any desired value when the differential volume of the tank exceeds standard allowable deviations as in an abnormal flow condition that activates an alarm followed by adjustments in the target flow and decant cycles. 
     The flow rate of screened wastewater exiting the tank can also be controlled by a pump (not shown) instead of a modulating valve  18 . A pump may be used when there is inadequate active volume (volume between high and low liquid level—depth of decant) or the discharge elevation and the liquid level in the screen box is not adequate to flow by gravity at the required rate. A variable frequency drive (VFD) provides the incremental discharge flow control. 
     Discharge Hose 
     As described above, flexible discharge hose  68  is connected to pipe  52  near the bottom of the tank for gravity discharge (the more normal situation) and higher in the tank if the filtrate is pumped. The hose connection to the SBX  12  is to the internal flow distribution, lifting column  28  and SBX stub pipe  52  of a smaller single SBX unit or to the filtrate manifold  82  if multiple SBXs are used to provide more screen surface area. Hose  68  may have swivel connections to allow the hose to twist as the screen box moves up and down in the tank or the hose may be an accordion type of hose/duct to increase in length as the screen box rises up to above the tank to the hood or contracts as the screen box decants to the low liquid level in the tank. It is currently preferred to use an accordion type hose as it provides less disturbance of the settled sludge. 
     Screen Box Hood 
     An enclosing hood  84  that may contain a heater  86 , screen spray system  88 , and/or UV disinfection apparatus  90  is placed above the tank over each screen box  12 . Lifting cable  34  passes through an opening in the center of the hood. The hood  84  is mounted to the pulley support or other structure above the tank. The hood has an open bottom and hinged or flexible sides to allow access to the screen box, heater, screen spray system, UV disinfection, control instrumentation, etc. If UV is used, then a flexible protective seal (not shown) and sidewalls (not shown) and interlocking controls to deactivate the UV prior to lowering the SBX are provided to avoid accidental exposure. 
     In addition, in operation, hood  84  blocks the sun from the screen, preventing the growth of algae that could foul the screen. 
     Instruments and Controls Specific to Screen Box Functions 
     As described above, a pressure transducer (PT)  80  in the tank provides the controls with the liquid depth in the tank. A PT  78  in the screen box provides the liquid depth in the box. An encoder provides the position of the screen box in the tank. 
     These 3-inputs provide basic information necessary to perform the following functions: 
     1. Screen Surface Area Adjustment 
     The screen surface area for each incremental elevation of screen is entered into the control system, as the screen sizes may vary. The operator sets a) a screen loading rate in GPM/SF, b) the desired Target Flow (TF) or discharge flow. These two variables then dictate the depth of the screen in the liquid to provide the correct screen surface area. The controls adjust the screen depth and thus surface area in the liquid to match the operator entered screen loading rate and effluent flow. 
     2. Lowering of the Screen Box at the Start of a Decant Cycle 
     The air scour starts when the lower level of the screen reaches the liquid level. This is done to keep the liquid from flowing into the screen box without the air scour, to reduce fouling. Air scour could be activated at the start of decent but it consumes energy for no process benefit. 
     3. Lifting and Flushing of the Screen Box at the End of a Decant Cycle 
     The lifting of the screen box was partially described above. 
     When the low liquid level is reached and it is time to raise the screen box out of the liquid, the effluent valve on the filtrate discharge piping is closed to prevent the screened wastewater/filtrate in the screen box from exiting via the discharge hose when the screen box is lifted. The screened wastewater reverses flow and exits through the screen, thus flushing the solids on the outside surface of the screen away from the screen surface. 
     With the effluent valve still closed, the screen box is lowered a set distance into the liquid in the tank to increase the volume of filtered liquid in the screen box. The entrance velocity of the liquid entering the screen box to fill the additional volume of filtered wastewater is low due to the slow descent and no discharge. This is done to prevent the solids laden lower liquid from fouling the screen. By refilling the screen box, the volume of backwash effluent is increased. 
     With the desired volume of filtered wastewater inside of the screen box, the screen is raised slowly at first for a short period of time and then quickly accelerates to increase the backwash flow velocity. As the screen box reaches a certain elevation, the vertical motion of the screen box slows and continues to slow as it reaches the hood and then stops at a set elevation or contact switch or other position detection device. 
     4. Activation of Screen Spray System, Heater, UV Disinfection 
     The controls allow the operator to set the frequency of screen spray and UV disinfection cycles as needed based on a count of decant cycles. The systems will be activated when the screen is properly positioned and a contact switch in the hood is activated. The duration of the backwash in the hood is set by the amount of screen surface area and the available flow and pressure of the site. 
     The screen spray system will be automatically activated on the next cycle if the screen headloss reaches a user-defined set point. 
     The heater is temperature-controlled and deactivated to conserve energy when the screen box is not in the hood. 
     Low Profile Screen Box 
     Referring now to  FIGS. 28-37 , a Low Profile Screen Box (LPSBX)  112  can be useful for applications of high flows, limited surface area to place a screen box, and/or shallow active volumes (the vertical distance between high and low water levels) of existing primary clarifiers. The low profile minimizes the height the SBX occupies from the bottom of the deflector plate to the top of the screen surface area. 
     Multiple screen boxes  112  or racks are ganged in parallel to provide the necessary screen surface area at a controlled screen loading rate. 
     The application requires the screen racks be placed close together with limited space between the racks ( FIGS. 29-31 ). This limited space can result in high horizontal velocities that would create uneven flow to and through the screen surface area, which uneven flow would result in fouling of high velocity areas of the screen. To create lower velocities and more uniform distribution of flow the screened surface of each rack is submerged with either a sealed top with air vents or an open top and solid vertical plates to enclose and seal the area above the screened surface. The LPSBX filtrate manifold  82  is connectible to flexible discharge hose  68 . This is done to increase the pathways and cross sectional area of flow to the center of the elongated racks which lowers the velocities to the screen, and the enclosed volume above the racks serves to increase the volume of screened liquid to backwash the screen. There are multiple screen racks  112  mounted to LPSBX filtrate manifold  82 , a deflector plate below  114 , and a modular lifting frame  116 . 
     The width of the rack  112  is determined by the open area between the rack and the filtrate manifold. The more open cross-sectional area connecting the rack to the manifold, the narrower the rack can be. 
     Referring to  FIGS. 36-39 , LPSBX filtrate manifold  82  comprises a central drain channel  83  terminating in an outlet  85  connectible to a flexible drain hose  68  ( FIG. 13 ) via fitting  52  ( FIG. 1 ) as just described. Central drain channel  83  is transected by a plurality of feeder channels  87  that drain into central drain channel  83 . In turn, the multiple screen racks  112  transect and drain into feeder channels  87  via mating ports  89  that are sealed between racks  112  and channels  87 . 
     Referring to  FIGS. 29-31 , the narrow vertical ends of the racks  112  may be rounded  12   a  or triangular  12   b  to reduce turbulence and promote laminar horizontal flow towards the center of the rack thus reducing vertical flows from top and bottom. 
     Referring now to  FIGS. 32-37 , LPSBX  112  is cleaned and sanitized in a manner similar to the cleaning of a single SBX  12  as described above. 
     A spray header assembly  300  comprises a plurality of spray elements  302  (equal to the number of SBXs) connected in parallel via piping  304  to one or more water inlets  306 . Assembly  300  is mounted in a openable hood  308  that in turn is mounted to a framework  310  for attachment to a clarifier tank (not shown) containing LPSBX  112 . Assembly  300  and LPSBX are aligned such that upon raising of the LPSBX the spray elements enter the LPSBX or SBX units, spraying the inside screen surface outward to displace solids on the exterior screen face. The raising and lowering cycle may be repeated as may be needed for proper cleaning of the screens. The cleaning effluent drains into the deflector plate  114  and through openings therein into the clarifier tank below. 
     Installation into a Prior Art System 
     Referring now to  FIGS. 26-27   a , an SBX system in accordance with the present invention may be installed in existing clarifiers  200  of conventional design or the new clarifier design. The preferred installation is the new clarifier design that comprises a single primary settling tank  202  that performs grit removal, flow equalization, primary clarification, and fine screening such as is disclosed in the above-incorporated US patents. 
     The location of the SBX  12  in a retrofit of a conventional clarifier is dependent on the size and shape of the clarifier tank, the configuration of the internal sludge and scum mechanisms, a mapping of the COD within the clarifier under different flow conditions, the settling characteristics of the solids, peak/average/minimum flows, and hydraulic profile. In some cases the existing sludge withdrawal mechanisms, scum troughs, and effluent weirs may need to be modified. 
     In the new style clarifiers, the SBX is placed in the center of the tank over the sludge hoppers, equal distance from the influent feed trough. This is done because most solids have settled in the center of the tank in the sludge hoppers as a result of feeding equal flow, equal distances from opposite sides of the tank towards the center, at equal velocities. The SBX deflector plate prevents the disturbance of the solids below the plate. There will be some slight disturbance of the light solids from the invention moving downward. These disturbed solids then must travel both vertically and horizontally around the deflector plate. This additional travel distance and time at a low exit velocity will reduce the amount of solids reaching the screen. 
     The SBX has several different configurations useful for different flow ranges, types of liquid being decanted, and new or old style clarifier. 
     The installation of the SBX into an existing clarifier requires modifications to the operation of the conventional clarifier to provide beneficial flow patterns similar to the new style clarifier. The influent flow is directed to the clarifiers that have a low liquid level. The clarifiers with a high liquid level are in the process of resting or decantation. This is accomplished by alternating the clarifier influent gates or valves from open to close through the inventions&#39; control system. Actuators may be affixed to the existing gates or valves to allow automatic operation. Individual pumps dedicated to specific tanks may also be used. 
     There is no decanting or discharge during the fill cycle because energy imparted into the flowing liquid keeps the BOD in suspension. Preferably, after filling of the tank a rest period with no discharge allows the fluid inertia and energy to dissipate, improving the settling of the supracolloidal and colloidal solids. Such a rest period can assist in achieving solids removal levels of about 85%. The exception to this operational mode is during high flow events in which the sewage is highly diluted, having a lower solids and BOD concentration, than both tanks may be operated to handle the excessive volume of liquid. Currently such events wash the settled solids out of the clarifier and aeration tanks and into the receiving body of water or into the secondary treatment process. The physical barrier of the SBX contains the solids within the clarifier tank. There may also be redundant SBX systems within each clarifier tank that can be brought into operation to assist in screening of the excessive flow. This is automatically done via the SBX control system detecting and quantifying the excessive flow and deviations to normal or experienced flow patterns. 
     The SBX moves vertically with no pivot at the base discharge so there is no horizontal movement. This makes the horizontal footprint of the invention smaller so it can fit into narrow deep tanks. 
       FIGS. 26-26   a  show that a prior art pivoting weir  100  occupies a footprint that may be fully half of a clarifier tank.  FIGS. 27-27   a  show that an SBX  12  in accordance with the present invention may occupy a footprint scarcely larger than the diameter of the SBX. In a clarifier retrofit, the pivoting weir  100  is simply removed at the pipe pivot joint  102  and replaced by connection of the collapsible SBX hose  68 . 
     From the foregoing description, it will be apparent that there has been provided an improved decanter system for a wastewater clarifier. Variations and modifications of the herein described decanter system, in accordance with the invention, will undoubtedly suggest themselves to those skilled in this art. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense.