Abstract:
A recycle system for use in a waste treatment facility utilizing either partially fluidized or combined fluidized bed filtration principles comprising the use of discrete passageways to allow flow between the bottom of the clarifier and the aeration compartment, a baffle positioned between bubblers in the aeration compartment and the clarifier openings, and a conduit positioned in the clarifier to provide flow between the clarifier and the anoxic compartment that helps prevent the formation of settled sludge pockets, allows for almost complete evacuation of the solids during “no flow” conditions, and improves on other inefficient conditions inherent in treatment systems using prior art recycle designs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/669,656 entitled “Improved Clarifier Recycle System Design for use in Wastewater Treatment System”, filed Apr. 8, 2005, the disclosure of which is hereby incorporated by reference. 
     
    
     FIELD  
       [0002]     The present invention relates to waste material management, and more particularly to a recycle system for use therein, for use in purifying a waste influent material in a biological waste treatment plant utilizing aeration and anoxic compartments and upflow sludge blanket filtration techniques.  
       BACKGROUND  
       [0003]     For the purposes of biological treatment, compact biological reactors, which include anoxic (denitrification), aeration (nitrification) and sludge separation (clarifier) zones, and activated sludge recirculation system within one compact design are increasingly being used. Wastewater is treated by biological process using activated sludge, a mixture of microorganism, which requires ingredients contained in the wastewater for its growth and survival. During the process, ammonia compounds are oxidized to nitrite/nitrates (nitrification) and subsequently nitrite/nitrates are reduced to nitrogen gas (denitrification). In the separation zone, activated sludge is separated from the treated effluent by fluidized bed type filtration system and the separated activated sludge is recycled by means of airlift or mechanical pumps back to the anoxic zone.  
         [0004]     The use of fluidized bed filtration-type processes for water treatment in such systems is also well known. Such processes generally rely on a decreasing upward velocity gradient that is formed by water flowing upwardly in a separator (or clarifier) through a fluidized bed created by agglomerated solids that are flowing downwardly to the bottom of the separator. Processes using this general concept are fully shown in described in U.S. Pat. Nos. 6,620,322, 5,755,966 and 5,720,876 and U.S. application, Publication No. US 2005/0000907 A1, the contents of which are herein incorporated by reference. The technical applications of such processes involve a wide range of chemical treatment of underground and surface water for communal and industrial use, and prevailing portion of municipal, industrial and agricultural biological wastewater treatment. Processes such as these rely on the flocculation of solids and a decreasing upward velocity profile through a “sludge blanket” formed by the flocculated solids. This sludge blanket, acting much like a fluidized bed, is responsible for the filtering of the generally upwardly flowing fluid. In order to get the desired decreasing upward velocity gradient, the separating space is usually formed with upwardly broadening diffuser shape cross sections. Common shapes that are used for this purpose include a simple inverted truncated cone, a longitudinal prism, a toroidal prism (which can also be described as an inverted truncated cone with inserted central cone or cylinder), among others.  
         [0005]     The fluidized bed that is formed in the aforementioned process, generally called a sludge blanket, consists of a fluidized layer of flocculated solid waste particles. Inside the sludge blanket there is preferably formed a dynamic equilibrium: on one side the flocculation of smaller flocs leads to the creation of larger flocs and on the other side the larger flocs are disintegrated by hydrodynamic forces resulting from local turbulence thereto. The result of those two counteracting processes is a certain, generally uniform mean floc diameter and floc size distribution in a given place. In this manner, a fluidized bed like blanket may be formed having particles of generally standard size and shape.  
         [0006]     Needless to say, in order to maintain dynamic equilibrium in a system utilizing a clarifier such as the one discussed above, there must be removal of suspended solids from the sludge blanket proportionally corresponding to the solids in the liquid flowing into the clarifier section. It is this removal that generally distinguishes the type of filtering that is being utilized. More specifically, in a fully fluidized bed system, the solids are generally withdrawn from the top of the sludge blanket. In a combined fluidized bed system, the solids are generally withdrawn from the middle of the sludge blanket. And in a partially fluidized bed system, the solids are generally withdrawn from the bottom of the sludge blanket. As will be discussed in detail below, the improved clarifier disclosed and claimed herein is generally useful in the partially or combined fluidized bed type systems.  
         [0007]     In a partially fluidized bed system, the density current flows along the walls of the clarifier allowing the excess flocs to be removed at bottom of sludge blanket. Early on, this was accomplished using a simple return of separated suspended solids through the input. Later on, it was determined that increased performance could be obtained by forced withdrawal of the separated solids from below the propagated density currents. Because the concentration of flocs in density currents is higher than what is required for a full fluidization, the partially fluidized sludge blanket is particularly suitable for separation of concentrated suspensions such as may be found in various typical biological wastewater applications. An example of a clarifier using this method is disclosed and discussed in aforementioned U.S. Pat. No. 6,620,322.  
         [0008]     Also advantageous is what may be described as a “combined” fluidized bed sludge blanket. In a combined sludge blanket system the bottom part of the sludge blanket behaves similar to a fully fluidized bed while the upper part behaves more like a partially fluidized bed. The fully fluidized bottom part distributes the water and solids into the upper partially fluidized part and the excess flocs are withdrawn from the density currents at the walls of the clarifier from the side at the middle part of the sludge blanket. Due to the fact that withdrawn density currents do not flow against liquid flow in the region of high apparent velocity, the hydraulic load can be substantially higher than in a pure partially fluidized sludge blanket system. As noted above, the partially fluidized and combined sludge blankets are particularly well suited to biological wastewater treatment facilities. As is well known, biological processes generally include systems for aerobic activation, aerobic sludge stabilization, nitrification, denitrificaton, dephosphorization and selector action.  
         [0009]     In order to meet the requirements for effective activation process, the sludge blanket needs a significant concentration of activated sludge, generally obtainable only by sludge recirculation. More specifically, using an internal circulation loop, mixed liquor suspended solids (sludge) enter the clarifier from the aeration compartment at the bottom and are filtered out of the effluent by the filter media consisting of flocculated suspended solids themselves. The effectiveness of this process is critical to filtration efficiency. In fact, given that the portion of sludge that is “activated” represents only a small percentage of the total sludge in the sludge blanket, and given that the efficiency of the system depends on the effective activation of the sludge, the overall efficiency of the waste treatment system is very much dependent on the efficient recirculation and activation of the sludge at or near the bottom of the sludge blanket (in a partial fluidized bed system) or closer to the middle (in a combined fluidized bed system).  
         [0010]     In order to insure most efficient activation of the sludge, great care must be taken to ensure that the sludge is removed from the bottom of the clarifier evenly across the length of the clarifier and that no ‘pockets’ of settled sludge are formed. This is because such pockets may lead to partial plugging and an uneven withdrawal of sludge from the clarifier, and these may cause anoxic conditions within the pocket, nitrogen gas generation due to denitrification and pockets of non-activated sludge rising to the surface of the clarifier, all of which detrimentally effect the overall efficiency of the treatment system. In doing this, care must also be taken to make sure that bubbles of oxygen from the aeration diffusers located in the oxidation zone are not accidentally introduced into the clarifier. In summary, it has been found that overall treatment efficiency can be detrimentally effected by recycle systems which do not allow for complete evacuation of the solids during “no flow” conditions.  
         [0011]     Accordingly, it is desired to have a recycle system for use in a clarifier in a sludge blanket filtration system utilizing either partially fluidized or combined fluidized conditions that improves on prior designs with respect to the removal and recirculation of suspended solids, helps prevent the formation of settled sludge pockets, allows for almost complete evacuation of the solids during “no flow” conditions, and improves on other inefficient conditions inherent in treatment systems using prior art recycle designs.  
       SUMMARY  
       [0012]     What is provided is an improved clarifier recycle system for use in a wastewater treatment system that utilizes fluidized bed-type filtration and a method for using the same. More specifically, what is provided is an improved recycle system comprising a number of distinct passageways in the clarifier which allow for the flow of mixed liquor between the clarifier and the aeration compartment. Preferably the passageways are located proximate the bottom of the clarifier. Also provided in one preferred embodiment of the invention is the use of a baffle that may be placed in front of the discrete passageways to prevent oxygen/air from the aeration bubblers from entering the clarifier but which still allows for flow of solids and liquids underneath the baffle thereby not detrimentally effecting flow conditions. The use of such a baffle is not only advantageous in that it prevents oxygen/air bubbles from entering the clarifier, but that it allows the bubblers to be placed much closer to the clarifier than otherwise would be possible thereby making the oxidation chamber that much more efficient while helping to prevent undesirable anoxic sludge “pockets.” 
         [0013]     Also provided is a tube, pipe or conduit, having a number of distinct openings for the collection and recycling of sludge to the anoxic compartment. Preferably, the tube or pipe is located proximate the bottom of the clarifier (in a partial fluidized bed arrangement) or the middle of the clarifier (in a combined fluidized bed arrangement) and that the openings are located on a portion of the tube, pipe or conduit located opposite, and preferably staggered from, the passageways for mixed liquor flow between the clarifier and the aeration compartment.  
         [0014]     As will be discussed in detail below, this arrangement of the recycling components of the sludge and mixed liquor from the clarifier provides greater filtration efficiency by helping to eliminate sludge “pockets” that lead to inefficient anoxic conditions within the pocket, nitrogen gas generation due to denitrification and pockets of sludge rising to the surface of the clarifier and allows for essentially complete evacuation of the solids during “no flow” conditions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a front perspective view of an embodiment of a clarifier for use in connection with a waste treatment system in accordance with the present invention;  
         [0016]      FIG. 2  is a side elevational view of the clarifier of  FIG. 1 .  
         [0017]      FIG. 3  is a rear view of the recycle pipe in the clarifier of  FIG. 1 ;  
         [0018]      FIG. 4  is a depiction of passageways having various geometries useful in the clarifier of  FIG. 1 ;  
         [0019]      FIG. 5  is a side elevation view of a portion of a waste treatment system in accordance with the present invention;  
         [0020]      FIG. 6  is a side elevation detail view of a baffle as shown in  FIG. 5 ; and  
         [0021]      FIG. 7  is a front elevation view of the baffle of  FIG. 6 . 
     
    
     DETAILED DESCRIPTION  
       [0022]     What follows is a detailed description of an exemplary embodiment of the present invention. This description or the drawings associated therewith should not be construed as constituting a limitation of the invention. In particular, it is particularly noted that a combined sludge blanket system is not shown in the drawings but would still be considered within the scope of the present invention.  
         [0023]     A clarifier  10  for use in connection with a fluidized bed-type filtration process is provided. Mixed liquor suspended solids  12  (sludge) enters the clarifier  10  from the aeration compartment  14  at the bottom  16  of the clarifier  10  and are filtered out of the effluent  18  by the filter media  20  consisting of flocculated suspended solids themselves. Driven by the stream vertical velocity uplift force  22 , the suspended solids  20  grow by agglomeration on compact with other suspended solids or flocs of suspended solids. When the flocculated solids  20  become heavier than the vertical velocity uplift force  22 , which progressively decreases due to the clarifier gradually increasing cross-section, they descend to the bottom  16  of the clarifier  10  and are subsequently recycled to the anoxic compartment  24  of the reactor.  
         [0024]     In order to insure that the sludge  12  is removed from the bottom of the clarifier  10  evenly across the length of the clarifier  10  and that no ‘pockets’ of settled sludge are formed, a number of discrete passageways  26  alongside the bottom  16  of the clarifier  10  maybe formed, through which mixed liquor enters  12  or is allowed to exit  28  the clarifier  10 . A tube, pipe or conduit  30 , located near the bottom  16  of the clarifier  10  is also provided. The tube  30  includes openings  32  to allow for the collection of sludge  12  for recycling to the anoxic compartment  24 . A pipe  34  may be connected to the tube  30  and is preferably operatively attached to a pump (not shown) for recycling the sludge  12  to the anoxic compartment  14 .  
         [0025]     When in use, there are essentially two modes of the clarifier  10  operation; incoming sewage mode and no incoming sewage (“no flow”) mode. When sewage is added to the anoxic compartment  24  in the first mode, it is mixed with mixed liquor  12  recycled from the bottom  16  of the clarifier  10 , utilizing the equipment described above. The mixed liquor (sludge)  12  then flows by gravity to aeration compartment  14  and enters the clarifier  10  through the passageways  26 . Vertical velocity of the rising suspended solids  22  forming the sludge blanket  20  gradually slows down due to the prism or conical shape of the clarifier  10  and due to weight gain of the flocculating solids  20 . This eventually causes the solids  20  to descend via gravity to the bottom  16  of the clarifier  10 . From the bottom of the clarifier  10  the solids  20  enter the tube  30  via the openings  32  and are recycled, via the pipe  34  to the anoxic compartment  24 .  
         [0026]     When no sewage is added in the second mode, vertical velocity and the solids uplift force  22  drop to zero and all solids of the sludge blanket  20  start descending to the bottom  16  of the clarifier  10 . It is in this mode that the possibility of the settled sludge pockets formation is the most acute. The function of the passageways  26  in this mode is to allow the settling solids  20  to exit  28  the bottom  16  of the clarifier  10  into the aeration compartment  14 . Once the excess solids  20  are evacuated from the bottom  16  of the clarifier  10 , flow of mixed liquor  12  into the clarifier  10  through passageways  26  driven by activated sludge recycle may be resumed. Thus within relatively short time intervals, the direction, the velocity and the orientation of the flow of mixed liquor within the bottom of the clarifier dramatically changes or reverses itself. The result is a continuous and complete evacuation (“clean-out”) of the bottom of the clarifer, and elimination of the sludge settling and all its negative effects on the biological plant performance and operation.  
         [0027]     The openings  32  may be of any suitable shape or size and are preferably positioned on a side of the tube  30  opposite the passageways  26  in such a way that the centerlines of the passageways  26  are staggered from the centerlines of the openings  32 . Referring to  FIG. 3 , the size of the openings may preferably be determined using the following equation: 
 
 Q   RAS   =Q   DES   ×RRR  
 
A RAS =ΣA O  
 
 v   1 =0.3−1.5 m/s 
 
 v   RAS =0.3−1.5 m/s 
 
 RRR= 2−6 
 
 Where 
 
         [0028]     Q RAS  Return Activated Sludge flow  
         [0029]     Q DES  Plant design average or maximum daily flow  
         [0030]     RRR Return Activated Sludge flow recycle rate  
         [0031]     A RAS  Return Activated Sludge pipe area  
         [0032]     A O  Discreet openings area  
         [0033]     v Flow velocity  
         [0034]     The passageways  26  may be of any suitable shape or size, but are preferably shaped and sized such that even transfer and withdrawal of recycled sludge is accomplished. As shown best in  FIG. 4 , examples of specific shapes of the passageways include triangular, rectangular, square or semi-cylindrical and may be sized as is appropriate given specific flow and wastewater treatment conditions.  
         [0035]     In an exemplary embodiment, as best shown in  FIGS. 5-7 , the aeration compartment  14  is provided with bubblers  40  for bubbling air and/or oxygen into the aeration compartment  14  that are fed via a gas-line  42 . The bubblers  40  are preferably positioned on the floor  44  of the waste treatment system  46  and are positioned in a manner to insure efficient distribution of the air/oxygen throughout the aeration compartment  14  as is generally known in the art in a manner to prevent the introduction of the air/oxygen gas directly into the clarifier  10 . In an exemplary embodiment, this may be accomplished using a baffle  48  positioned between the bubblers  40  and the clarifier  10 . The use of a baffle  48  allows the bubblers  40  to be placed closer to the clarifier  10  which can improve the efficiency of the aeration compartment  14  and help prevent the formation of the aforementioned anoxic sludge pockets.  
         [0036]     Preferably the baffle  48  includes a main body member  50  and supports  52  and the main body member  50  is spaced from the floor  44  providing an opening  54  therebetween to allow for circulation and flow of solids and liquids through and around the aeration compartment  14 . In particular, opening  54  helps to prevent the formation of anoxic sludge “pockets” and aids in the circulation of sludge throughout the entire aeration compartment thereby contributing to the overall efficiency of the system  46 . Preferably the opening  54  is positioned in a manner such that gas from the bubblers  40  is prevented from entering the clarifier  10 . Additionally, depending on the geometry of the clarifier  10  and the size and the placement of the bubblers  40 , the main body member  50  may include an extension  56  on a top portion thereof to prevent gas from the bubblers  40  from entering the clarifier  10 . In an embodiment of the invention, the extension  56  may be slanted to match the angle of the clarifier  10  wall.  
         [0037]     As discussed herein the aeration compartment  14  immediately adjacent to the clarifier  10  passageways  26 , the passageways  26 , and the withdrawal pipe  34  are designed to facilitate entrained air separation and to accommodate the various flow streams as functions of the influent flow. Activated sludge flow may at times be into the clarifier  10  or out of the clarifier  10  but it will often be simultaneous in and out flow depending on the influent flow rate and the clarifier bottom to the aeration sludge densities difference. Since the flow out of the clarifier  10  is as mentioned densities difference driven with no other energy input, the sludge flocs are not physically damaged (broken) which improves their settling characteristics and results in much improved flows at the bottom part of the clarifier  10 .  
         [0038]     The specific embodiments and examples set forth above are provided for illustrative purposes only and are not intended to limit the scope of the following claims. Additional embodiments of the invention and advantages provided thereby will be apparent to one of ordinary skill in the art and are within the scope of the claims.