Abstract:
An SBX assembly comprising an ultrafine screen; a three-dimensional screen frame supportive of the screen and sealed along the screen edges to prevent liquids and solids from bypassing the screen; and an air plenum attached to the frame that provides air scouring at the lowest elevation of the screen. The air plenum is attached to the lower region of the SBX. The plenum is supplied from a source of compressed air and is provided with a plurality of exit holes. Air exiting the plenum flows along the upstream surface of the SBX screening below the level of liquid in the primary settling tank, thereby scouring accumulated residues from the screening surface. Upon reaching the liquid surface, the bubbles migrate horizontally away from the SBX assembly and toward one or more nearby scum troughs in the primary settling tank.

Description:
RELATIONSHIP TO OTHER APPLICATIONS AND PATENTS 
       [0001]    The present application is a Continuation-In-Part of a pending U.S. patent application Ser. No. 14/142,197 (&#39;197), filed Dec. 27, 2013, the relevant disclosure of which is incorporated herein by reference. This application is related to co-pending U.S. patent application Ser. No. 14/464,870 (&#39;870), APPARATUS AND METHOD FOR REMOVAL OF FLOATABLES AND SCUM IN A WATER TREATMENT SYSTEM, which is incorporated herein in its entirety for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    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 or fixed discharge pipe; and most particularly, to a method and apparatus for using air scouring in a water treatment facility comprising a vertically-driven screen box assembly (SBX). Beneficially, the scouring air also creates low-velocity surface currents in the influent in the primary settling tank flowing away from the screened decanter that assist in driving floatables into one or more scum troughs for disposal. 
       BACKGROUND OF THE INVENTION 
       [0003]    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. 
         [0004]    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 . 
         [0005]    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. 
         [0006]    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. 
         [0007]    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. 
         [0008]    A screened decanter comprising an effluent weir is disclosed in U.S. Pat. No. 7,972,505 (&#39;505), PRIMARY EQUALIZATION TANK SETTLING TANK, and U.S. Pat. No. 8,398,864 (&#39;864), SCREENED DECANTER ASSEMBLY FOR A SETTLING TANK, both of which are incorporated in their entirety for all purposes. 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. 
         [0009]    The parent application, Ser. No. 14/142,197, filed Dec. 27, 2013, discloses an improved screen assembly in the form of a box, oval, 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. 
         [0010]    In continued use of screen apparatus in wastewater treatment, fouling and blockage of the screening is an important operational problem. What is needed is an apparatus and method for simply and automatically preventing fouling of the upstream surface of any screen assembly. 
         [0011]    It is a principal object of the invention to maintain suitable flow of influent through any screen assembly. 
         [0012]    It is a further object of the invention to assist in clearing floatable materials from the surface of the influent reservoir in the primary settling tank. 
       SUMMARY OF THE INVENTION 
       [0013]    Briefly described, the present invention provides an air scouring method and apparatus for simply and automatically preventing fouling of the upstream surface of a screen surface used to screen fluid being decanted from a settling tank. In one aspect of the present application, a screen box (“SBX”) assembly in the form of a rectangular box, oval, or cylinder is controllably driven in the vertical direction to optimize the exposure of the screen to the wastewater. As used herein, the term “SBX” should be taken to mean all forms of a screened box for filtering solids from liquids, including a low-profile SBX (LPSBX) as disclosed in the parent application, Ser. No. 14/142,197. In another aspect of the application, the screen surface is affixed. to or disposed upstream of a static discharge pipe as disclosed in U.S. Pat. No. 7,972,505 (&#39;505). 
         [0014]    An SBX assembly in accordance with the present invention comprises an ultrafine screen; a three-dimensional screen frame supportive of the screen and sealed along the screen edges to prevent liquids and solids from bypassing the screen; and an air plenum attached to the frame that provides air scouring at the lowest elevation of the screen. 
         [0015]    The air plenum is conveniently attached to the lower region of the SBX assembly and is mounted generally parallel to the surface of the liquid in the primary settling tank. The plenum is supplied from a source of compressed gas, preferably air, and is provided with a plurality of exit holes such that air exiting the plenum is directed generally toward the upstream surface of the SBX screening below the level of liquid in the primary settling tank. Preferably, the SBX screening is inclined outward from bottom to top such that air bubbles from the plenum are forced by gravity to maintain contact with the screening as they migrate upward along the screening surface to reach the liquid surface, thereby scouring accumulated residues from the screening surface. Upon reaching the liquid surface, the bubbles migrate horizontally away from the SBX assembly and toward one or more nearby scum troughs in the primary settling tank. The air flow from the plenum thus primarily scours the screening and then secondarily directs scum and floatables on the surface of the liquid toward the scum troughs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    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: 
           [0017]      FIG. 1  is a partial-cutway perspective view of a prior art single tank waste water treatment installation; 
           [0018]      FIG. 2  is an elevational cross-sectional view of an SBX having a first embodiment of an air scouring apparatus; 
           [0019]      FIG. 3  is an elevational cross-sectional view of an SBX having a second embodiment of an air scouring apparatus; 
           [0020]      FIG. 4  is an elevational cross-sectional view of an SBX having a third embodiment of an air scouring apparatus; 
           [0021]      FIG. 5  is a schematic elevational cross-sectional view of prior art single tank waste water treatment installation in accordance with the disclosure of the &#39;505 patent; 
           [0022]      FIG. 6  is an elevational cross-sectional view of the installation shown in  FIG. 5  comprising a first embodiment of an air scouring apparatus disposed upstream of a screen and fixed discharge pipe; 
           [0023]      FIG. 7  is an elevational cross-sectional view of the installation shown in  FIG. 5  comprising a second embodiment of an air scouring apparatus disposed upstream of a screen and fixed discharge pipe; and 
           [0024]      FIG. 8  is an elevational cross-sectional view of the installation shown in  FIG. 5  comprising a third embodiment of an air scouring apparatus disposed upstream of a screen and fixed discharge pipe. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the specification and drawings are to be regarded as illustrative rather than restrictive. It is to be further noted that the drawings are not to scale. 
         [0026]      FIGS. 1 through 4  illustrate first embodiments of the invention with respect to an SBX in a fluid treatment system.  FIGS. 5 through 8  illustrate embodiments of the invention with respect to a fixed discharge pipe in a fluid treatment system. The present invention is a system (method and apparatus) for air scouring materials from the upstream side of a screen on an SBX or a static discharge pipe in a primary waste water treatment system. 
         [0027]      FIG. 1  shows a rectangular primary settling tank as disclosed in co-pending U.S. patent application Ser. No. 14/142,099, FLOATABLES AND SCUM REMOVAL APPARATUS FOR A WASTE WATER TREATMENT SYSTEM, which is incorporated in its entirety for all purposes.  FIG. 1  primary settling tank  10  receives waste water from a waste water collection system through an influent pipe  12  controlled by a control valve (not shown). The waste water treatment system may also be used in other applications that benefit from an equalized flow into the waste water treatment processes, such as industrial batch discharges, storm water, and septic receiving at a waste water treatment plant. Waste water reaches the waste water treatment system as a result of gravity, the operation of pumps, or both. The primary setting tank has outer walls  14 . A bar rack  16  is placed in the primary settling tank  10  between the outer wall  14  and the interior  18  of the primary settling tank  10 . Bar rack  16  keeps large inorganic solids and floatables from entering the waste water treatment system. 
         [0028]    Primary settling tank  10  is sized based on the daily flow patterns for the collection system using generally known engineering practices. The size of the primary settling tank  10  is large compared to the diameter of influent pipe  12  such that the velocity of the incoming flow decreases dramatically upon entrance of the water into primary settling tank  10 . Preferably, the incoming velocity is further reduced by splitting the flow so there are two influent pipes  12  at opposite ends of tank  10  (second influent pipe  12  is not visible in cutaway  FIG. 1 ). An influent feed trough  20  receives the incoming waste water and directs the flow in the direction of arrow  22 . As the water level rises to the level of the bar screen  16 , scum and sludge (not shown) pass through the bar screen  16 , over a fixed weir  24 , and into the interior  26  of tank  10 . Large floatables (not shown), such as plastic solids, are prevented from passing through the bar screen  16 . Sludge collects by gravity in the sludge hopper  28 , and can be removed from the sludge hopper  28  through outlet  30 . A scum and floatables trough  32  is provided to collect scum and floatables that pass through bar screen  16 . A handle  34  is connected to the scum and floatables trough  32  to control the angular position of the scum and floatables trough  32 . A scum and floatables collection box (not shown) is in fluid communication with the scum and floatables trough  32 . 
         [0029]    Primary settling tank  10  is shown with a screen box assembly (SBX)  36  and baffle plate  38 . The vertical position of the SBX determines the level of liquid in the primary settling tank  10  and may be adjusted by a controlled lifting mechanism (not shown)attached to a central baffled lifting column  40  that also serves as a drain outlet for screened influent within SBX  36 . A tank overflow outlet  42  prevents water from flowing over the sides of the primary setting tank  10  if the water level rises too high. 
         [0030]    Referring now to  FIG. 2 , a first embodiment  136  of an improved SBX in accordance with the present application is shown. 
         [0031]    The top  144  of SBX  136  is preferably open to allow occasional screen washing via hose or automated spray system (spray ball for symmetrical shapes or spray bar for low-profile rectangular boxes) and to access instruments located inside of the screen box. 
         [0032]    The bottom  146  of SBX  136  comprises a solid plate with an opening  148  to allow screened liquid to exit the screen box and thus the tank. Opening  148  preferably is provided with a nipple  150 , which may include hose barbs (not shown) formed in known fashion on the outer surface thereof. 
         [0033]    The sides  152  of SBX  136  include portions of screen  154  and solid wall  156   a,    156   b.  Sides  152  may be vertical (perpendicular to the liquid surface) or preferably are sloped outward as shown so that the top of the SBX is wider than the bottom creating a frustum shape. 
         [0034]    In a currently preferred embodiment, each SBX is formed of fiberglass to avoid the corrosive decay to which metal racks and gaskets may be subject. 
         [0035]    An internal air plenum  158  is formed within SBX  136  by installing a plate  160  diagonally between side  156   b  and bottom  146 . A plurality of holes  162  are formed in side  156   b  to permit air to escape from plenum  158  and form bubbles for air scouring of the outside surfaces of screens  154  as the bubbles rise. For substantially square SBXs having screening on all four sides, similar to SBX  62  shown in  FIG. 1 , plenum  158  may comprise a continuous tube supplied from a single air source, whereas for elongated SBXs, the shown left and right plenums  158  may be independently supplied via T-connection from a single source (connections not shown). 
         [0036]    Preferably, plenum  158  is large enough in cross-sectional area that longitudinal pressure drop is negligible. Holes  162  must be large enough to create bubbles that rise rapidly and scour the screens. In a currently preferred embodiment, holes  162  are 0.067 inch in diameter and spaced at 1.5 inches on center. Preferably, air flow through plenum  158  is about 1 SCF/min and is held constant while SBX  136  is immersed in the influent in tank  10 . 
         [0037]    Preferably, a solution of sodium hypochlorite (NaClO), e.g. 15%, is added to the air flow to eliminate buildup of biofilm on screens  154 . Preferably, air temperature is above ambient such that the solution is vaporized and dispersed through plenum  158 . The NaClO solution may be dripped into the air stream or fed continuously, depending upon the size of the facility and consequent volume demand for solution. 
         [0038]    Referring to  FIG. 3 , a second embodiment  236  of an improved SBX in accordance with the present invention is shown. SBX  236  is substantially identical with first embodiment  136  except that plenums  258  are formed integrally with sides  256   b  and bottom  246 . The size and placement of holes  262  are the same as disclosed above. 
         [0039]    Referring to  FIG. 4 , a third embodiment  336  of an improved SBX in accordance with the present invention is shown. SBX  336  is substantially identical with first and second embodiments  136 , 236  except that plenums  358  are formed integrally with sides  356   b  on the outside thereof. The size and placement of holes  362  are the same as disclosed above. Note that holes  362  are formed exclusively in plenums  358  and not in sides  356   b.    
         [0040]    In a method for air scouring SBX screens in accordance with the present invention, the air scour starts as the SBX is lowered into the influent in tank  10  when the lower edge 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 descent but it consumes energy for no process benefit. Air scour continues preferably at a fixed air flow rate during submerged operation of the SBX. 
         [0041]    Beneficially, the scouring air bubbles also create low-velocity surface currents in the influent in the primary settling tank flowing away from SBX  136 ,  236 ,  336  that assist in driving floatables into one or more scum troughs  32  ( FIG. 1 ) for disposal. 
         [0042]    Referring to  FIG. 5 , a schematic elevational cross-sectional drawing of a prior art alternative primary waste water treatment system  400  is shown, substantially as disclosed as  FIG. 3  in the incorporated &#39;505 patent. The discharge mechanism is a fixed discharge pipe  402  passing through a wall  404  of primary settling tank  406  near the bottom  408  thereof. Screen  410  is mounted essentially vertically ahead of discharge pipe  402 , is sealed along its edges to the walls (not visible in  FIG. 5 ) of primary settling tank  406 , and extends below the closed end  412  of pipe  402  on either side thereof; influent to be discharged enters pipe  402  via openings  411  (see  FIG. 6 , not visible in  FIG. 5 ) along the top of pipe  402 . Screen  410  can vary in diameter or surface area and must extend above high water level  414  and may be static or mechanical. In one embodiment, screen  410  has a greater surface area as it rises up from its base. Fixed discharge pipe  402  has an actuated valve  416  controlled by programmable controller  900  which controls the rate at which screened water exits primary settling tank  406 . 
         [0043]    Referring now to  FIG. 6 , a schematic drawing of another embodiment  500  of a system for air scouring of a screen upstream of a discharge pipe is shown. In this embodiment, the waste water treatment system is substantially as shown in  FIG. 5 . A perforated plenum  558  similar to plenums  158 , 258 , 358  is disposed transversely of tank  406  near the bottom of screen  410  on the upstream side thereof and may be mounted conveniently on the closed end  412  of pipe  402 . Plenum  558  is connected to a controllable source of compressed gas (not shown). Bubbles emanating from plenum  558  rise up along the upstream side of screen  410  to scour accumulated materials therefrom. 
         [0044]    Referring to  FIG. 7 , embodiment  600  is similar to embodiment  500  except that discharge pipe  402  is not positioned on the bottom  408  of tank  406 , but rather is positioned in wall  404  above the anticipated level of the sludge blanket that accumulates on the bottom of the tank. Plenum  658  is still positioned near the bottom of screen  410 . 
         [0045]    Referring to  FIG. 8 , in embodiment  700 , discharge pipe  402  is elevated as in embodiment  600  but is provided with a bench  702  for receiving pipe  402  and the lower end of screen  410 . Preferably, pipe  402  is closed at end  412  and perforated along the upper length for receiving screened influent as in embodiments  500 , 600 . Plenum  758  is preferably mounted to closed end  412 . 
         [0046]    In any of the above embodiments, the entire top half of pipe  402  may be open to receive fluid that has passed through the screen. 
         [0047]    From the foregoing description, it will be apparent that there has been provided an improved method and apparatus for cleaning the upstream screen, surface of an SBX in a wastewater clarifier. Variations and modifications of the herein described air scour 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.