Abstract:
A constant velocity serpentine anoxic reactor incorporates a multiple cell vertical serpentine path, as well as a horizontal serpentine path, through the anoxic chamber. A fixed film media is mounted within each cell of the anoxic chamber to provide a structure on which the bacteria can grow to sustain the biological reaction, which convert nitrates into nitrogen gas. The fixed film media can be a cross-flow media and can optionally include a web of textile material integrated within the fixed film media to enhance bacterial growth within the fixed film media or optionally the anoxic vertical serpentine configuration could be applied to an activated sludge operation. A nitrate recycle pump recycles about 75% of the effluent from the aerobic chamber back into the anoxic chamber to provide a nitrate source for the digestion of the BOD within the influent wastewater.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to biological wastewater treatment systems that remove BOD and nitrogen contaminants from wastewater and, more particularly, to a critical-velocity vertical-serpentine anoxic reactor design that employs fixed-film media to support biological populations in contact with the wastewater while maintaining suspension of suspended bacteria and other particulates, without application of additional mechanical mixers. 
   BACKGROUND OF THE INVENTION 
   The treatment processes for municipal and industrial wastewater have evolved due to identification of harmful environmental conditions created by nitrogen rich wastewater treatment plant effluents in rivers and estuarian environments and subsequent and necessary regulatory changes required to protect the environment. Biochemical Oxygen Demand (BOD), ammonia-nitrogen, nitrite-nitrogen, nitrate-nitrogen and organic-nitrogen are now commonly restricted components of the effluents of wastewater treatment systems. Although biological wastewater treatment systems are routinely engineered to remove the BOD, ammonia-nitrogen and much of the organic-nitrogen from wastewater streams, in doing so, these wastewater treatment plants, known as nitrification systems, create a nitrate rich effluent that has been recognized as being harmful to some aquatic environments. 
   There are biological systems designed to remove the nitrate-nitrogen from nitrification system effluents and the process of biological nitrate-nitrogen removal is called de-nitrification. There are a variety of process configurations for de-nitrification and many of these may be incorporated directly within the conventional biological nitrification system. For example, in the most common approach to de-nitrification, the engineer establishes a de-nitrification reactor or reactors, also called an anoxic reactor(s) as the first reactor(s) in a series of separate reactors in which the latter reactors are operated aerobically with oxygen or air present with the intent to biologically oxidize the ammonia-nitrogen and organic-nitrogen to form nitrate-nitrogen. The first reactor is termed anoxic because no elemental oxygen or air containing oxygen is introduced for aeration or mixing in that reactor even though the anoxic reactor is also rich in organic matter because it receives the influent wastewater. 
   In the anoxic/aerobic reactor scheme described above the nitrate-nitrogen would exit the last tank to contaminate the environment and thus in processes with the anoxic/aerobic sequence, a large stream of the nitrified wastewater is pumped from the nitrification reactor back to the anoxic reactor where-in the nitrate-nitrogen is used as a source of nitrate-oxygen for facultative heterotrophic bacteria which use it processing the BOD of the incoming wastewater in the anoxic tank. 
   In the anoxic de-nitrification/aerobic nitrification process described above, a means of mixing the influent wastewater with the large recycled stream of nitrate rich water is critical to the effective contact of the mixture with the bacteria. Wherein the overall system is operating as an activated sludge system with suspended bacteria a mechanical mixing device is sufficient, albeit it requires more energy and capital investment. In some instances the series of reactors, both anoxic and aerobic, may use fixed-film biological populations that grow on surfaces fixed within the reactor vessel. Fixed-film systems do not lend them selves as easily to the application of mechanical mixers and again the additional cost for mixers and power are a factor in the economics of these anoxic/aerobic nitrification de-nitrification systems. 
   The typical anoxic/aerobic nitrification/de-nitrification system requires a 3:1 ratio of recycled nitrate rich water to influent wastewater to substantially reduce effluent nitrate concentration. At this ratio, a typical system can remove 70-75% of the available nitrates thus reducing the negative effects of nitrate-nitrogen on the receiving water systems. Increasing the recycle beyond the 3:1 ratio produces diminished returns because the effluent flow of the nitrification reactor will always contain nitrate-nitrogen at a concentration approximating that of the ammonia concentration entering the nitrification section. The nitrate removal effect is in fact created because the nitrate recycle stream, which does not contain ammonia at the end of the nitrification reactor, dilutes the influent ammonia concentration in and subsequently out of the de-nitrification reactor. The ammonia concentration of the liquid in the anoxic reactor does not vary greatly from the diluted concentration in the de-nitrification reactor but because it has already been diluted as it enters nitrification reactor, the nitrate-nitrogen concentration generated in the nitrification reactor is reduced. 
   The need to contact the nitrate rich final effluent wastewater with the bacteria in suspension or by forcing flow through the fixed media, or both, has always required the addition of mixing energy to the systems with mechanical devices, as is disclosed in U.S. Pat. No. 4,599,174, granted on Jul. 8, 1986, to Curtis S. McDowell, to maintain efficient contact between the wastewater and the biological fixed-film and/or to maintain the suspension of the facultative bacteria. 
   It would be desirable to provide a biological de-nitrification reactor that does not require additional mixers to maintain the required mixing of the wastewater and recycled nitrate stream and at the same time effectively contacts the mixture of these elements with the facultative bacteria. It would also be desirable to utilize the fluid flow and mixing energy provided by the nitrate recycle pumps to accomplish the required mixing of the influent wastewater and recycled nitrate stream and proper contact without requiring an additional input of mechanical energy or equipment. 
   SUMMARY OF THE INVENTION 
   It is an object of this invention to overcome the disadvantages of the prior art by providing a serpentine anoxic bioreactor having an anoxic chamber or series of chambers formed in an alternating up/down vertical serpentine path. 
   It is an object of this invention to provide an anoxic bioreactor that has fixed film media within each vertical path of the anoxic chamber. 
   It is a feature of this invention that a nitrate recycle pump directs a flow of liquid from the aerobic chamber into the anoxic chamber for mixture with the wastewater influent. 
   It is an advantage of this invention that the energy added to the liquid flow from the aerobic chamber by the nitrate recycle pump is the only energy needed to direct flow through the anoxic chamber. 
   It is another advantage of this invention that mixing energy is not needed in the anoxic chamber to keep suspended bacteria from settling to the bottom of the anoxic chamber. 
   It is a feature of this invention that the fixed film media within each cell of the anoxic chamber provides a structure on which bacteria can grow to perform the anoxic reactions to convert nitrates into nitrogen gases. 
   It is another feature of this invention that the effluent is pushed through the anoxic chambers by the energy added by the nitrate recycle pump with the effluent moving vertically up and down through the fixed film media. 
   It is still another advantage of this invention that the cells of the anoxic chamber are sized to maintain critical vertical velocities through the serpentine path defined by the anoxic chamber sufficient to maintain any biological solids in suspension. 
   It is a further advantage of this invention that the fixed film media can be formed as a cross-flow media that provides a non-linear flow path through the fixed film media. 
   It is a further feature of this invention that the fixed film media can be integrated with textile material within the structure of the media to enhance the ability of the bacteria to grow on the fixed film media. 
   It is still a further advantage of this invention that the energy inputted into the system by the nitrate recycle pump is operable to keep the solids in suspension within the effluent without the requirement of additional mixers within the anoxic chamber. 
   These and other objects, features and advantages are accomplished according to the instant invention by providing a constant velocity serpentine anoxic reactor that incorporates a multiple cell vertical serpentine path, as well as a horizontal serpentine path, through the anoxic chamber. A fixed film media is mounted within each cell of the anoxic chamber to provide a structure on which the bacteria can grow to sustain the biological reaction, which convert nitrates into nitrogen gas. The fixed film media can be a cross-flow media and can optionally include a web of textile material integrated within the fixed film media to enhance bacterial growth within the fixed film media or optionally the anoxic vertical serpentine configuration could be applied to an activated sludge operation. An air-lift pumping action through the fixed film towers in the aerobic chamber ensures contact of the wastewater with the media. A nitrate recycle pump recycles about 75% of the effluent back into the anoxic chamber to provide a nitrate source for the digestion of the BOD within the influent wastewater. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages of this invention will be apparent upon consideration of the following detailed disclosure of the invention, especially when taken in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a top plan view of an anoxic bioreactor incorporating the principles of the instant invention, the horizontal serpentine flow paths being indicated with arrows; 
       FIG. 2  is a schematic cross-sectional view through the anoxic bioreactor taken along lines  2 - 2  of  FIG. 1 ; 
       FIG. 3  is a schematic representation of the anoxic chamber expressed in a linear arrangement to reflect the vertical serpentine flow pattern represented by the arrows, the numbers of the individual cells corresponding to the cell numbers displayed in  FIG. 1 ; 
       FIG. 4  is a schematic cross-sectional view through the anoxic bioreactor taken along lines  4 - 4  of  FIG. 1 ; 
       FIG. 5  is a schematic representation of an individual cell of the aerobic chamber to reflect the vertical serpentine flow pattern represented by the arrows; 
       FIG. 6  is an enlarged top plan view of the aerobic chamber to reflect the horizontal serpentine flow pattern represented by the arrows; 
       FIG. 7  is an elevational view of a fixed film media tower mounted on a stand to keep the fixed film media above the floor of the reactor; 
       FIG. 8  is an elevational view of a fixed film media tower mounted on a support chimney to maintain the fixed film media above the floor of the reactor; and 
       FIG. 9  is a process flow diagram showing the incorporation of the principles of the instant invention into a conventional treatment system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring first to  FIGS. 1-6 , an anoxic bioreactor incorporating the principles of the instant invention can best be seen. The anoxic bioreactor  10  includes an anoxic chamber  20 , which is the subject of this invention, that would typically be coupled to and an aerobic reactor chamber  30 , although anoxic chamber  20  can be utilized as a stand alone system. When coupled to an aerobic chamber  30  as shown in  FIG. 1 , the influent enters the anoxic chamber  20  through an inlet  29  and immediately mixes with the treated effluent being discharged from the aerobic chamber  30 , as will be described in greater detail below, via a recycle pump  15  which directs the nitrate rich aerobic chamber effluent into the anoxic chamber  20  for treatment of the influent wastewater. The anoxic chamber  20  includes a plurality of fixed film towers  25  that provide structure for the attachment and growth of facultative bacteria for the de-nitrification of the recycled effluent. Furthermore, and specific to this invention, the only energy input into the anoxic chamber  20  of the treatment system  10  is the energy provided by the recycle pump  15 . 
   The disclosed bioreactor  10  incorporates multiple reactor cells  22  within the anoxic chamber  20  placed in flow communication in series with alternating vertical flow patterns, as is represented in  FIG. 3 . Preferably, the anoxic chamber  20  has an even number of individual reactor cells  22  so that the first cell  22 , numbered  1  in  FIGS. 1 and 3 , has a vertically downward flow path forced by the divider wall  23  that is spaced from the floor  21  of the anoxic chamber  20  to allow the flow to pass horizontally under the divider wall  23  to reach the adjacent reactor cell numbered  2  in  FIGS. 1 and 3 . The divider wall  24  between reactor cells numbered  2  and  3  is lower than the divider wall  23 , the top of the divider wall  24  being below the fluid level  29  within the anoxic chamber  20 , to allow the flow to pass upwardly through the reactor cell numbered  2  and pass horizontally over the divider wall  24  into reactor cell  3 . 
   Alternating the divider walls  23 ,  24  requires the flow path to follow a vertically serpentine path from one reactor cell  22  to the next reactor cell  22 , with alternating vertical flow directions, until the last reactor cell  12  is reached. The discharge from the last reactor cell  12  is directed over a weir  28  in the wall  18  separating the anoxic chamber  20  from the aerobic chamber  30 . The anoxic chamber  20  could be formed with an odd number of reactor cells  22  so long as the first cell is arranged to provide flow of the mixed influent and nitrate recycle through the first column  22  and the subsequent columns  22  in an alternating vertical serpentine flow path to the last reactor cell for discharge to the aerobic chamber  30 . In an anoxic chamber having an odd number of reactor cells  22 , the mixed influent and nitrate recycle could enter the bottom of the first reactor cell  22  to provide an upward flow path so that the last reactor cell  22  would have the discharge therefrom over the weir  28  in the wall  18 . The juxtaposition of the anoxic chamber  20  to the aerobic chamber and the number of columns  22  therein is not limiting in this disclosure. 
   Each reactor cell  22  may or may not be filled with a tower  25  of fixed film media depending on whether the reactor system  10  is a fixed film reactor or a suspended solids reactor, as in an activated sludge process. In a fixed film reactor system wherein each cell  22  is filled with fixed film media such that the effluent flow must pass through the fixed film tower  25  to pass into the adjacent reactor cell  22 , the fixed film media is preferably cross-flow media formed by a plurality of sheets of fixed film connected together to form interconnected diagonal paths within the media that force the effluent to move throughout the fixed film tower  25 . A straight vertical flow path formed by the fixed film media will work also. As an example, fixed film media comprising individual sheets of formed film interconnected to form a tower is disclosed in U.S. Pat. No. 6,544,628, issued on Apr. 8, 2003, to Richard J. Aull, et al and assigned to Brentwood Industries, Inc. Such fixed film media would work properly within each reactor cell to provide a structure that will facilitate the growth of the facultative bacteria as the effluent passes through the tower  25  to move to the adjacent reactor cell  22 . 
   The opening at the bottom of the divider walls  23 , through which the effluent passes into the adjacent reactor cell  22 , is sized so that the average system design flow will result in a liquid velocity of one foot/second through the opening. This velocity of one foot/second is recognized as being sufficient to scour suspended biological solids from the floor  21  of the reactor chamber  20  and is the recommended velocity for suspension of such solids in an activated sludge system. The floor  21  is depicted in the drawings as being generally planar. An alternate configuration would be for the floor of each reactor cell  22  to slope toward the opening in the divider wall  23 . In other words, the floor of the odd numbered reactor cells  22  having a downward flow path would slope downwardly toward the opening under the wall  23 , while the adjacent reactor cell  22  having an upward flow path would slope upwardly away from the opening under the divider wall  23 . The presence of a sloped floor configuration will increase the velocity of the effluent over the floor  21 . The sloped floor will also promote a uniform down flow velocity through the cross-sectional area of the reactor cell  22 , as the effluent exiting the vertical column near the inlet wall will make the 90 degree turn and flow through a smaller vertical cross-sectional area than the effluent flowing form the middle of the vertical column or the effluent flowing downward at the dividing wall  24 . 
   The plan view area of each reactor cell  22 , is critical to design of the anoxic chamber  20 . The upward velocity of the effluent moving through the even numbered reactor cells, as depicted in  FIG. 4 , is the most critical design factor in creating the multiple cell serpentine anoxic reactor  20 . The cross-sectional area of the reactor cell  22  having an upward flow path must be such that the rise velocity in these even numbered reactor cells  22  is greater than 30 ft/hr. (or 0.5 ft/min). At this velocity, solids will be kept entrained in the upward flow within the matrix of the fixed film tower  25 . Thus, the surface area of the even numbered reactor cells  22  is defined by the need to achieve the proper upward flow velocity, given the volume of effluent passing through the reactor cells  22 . The rise velocity must be sufficient to exceed the settling velocity of the majority of particles suspended in the effluent and/or those particles that slough off the fixed film media within the reactor cells  22 . In addition, the use of cross-flow fixed film media in the tower  25  will create turbulence as the effluent flows through the cross-flow media, reducing the boundary layer in the manner of a static mixer aiding in this process. 
   The disclosed process, can be defined as a critical velocity serpentine anoxic reactor, and can be incorporated into conventional treatment systems in many different ways. The process flow diagram of  FIG. 9  depicts one way in which the anoxic reactor  20  is incorporated into an activated sludge treatment system wherein full conversion of ammonia and degradable organic nitrogen to nitrate is assumed to occur through the nitrifying secondary treatment system, as will be described in greater detail below. The nitrifying secondary treatment system downstream of the anoxic chamber  20  could be an activated sludge system (shown), a trickling filter, a rotating biological disk system, a moving bed bioreactor or other effective nitrification bioreactor system. 
   The movement of the fully nitrified effluent from the secondary treatment system, i.e. the aerobic chamber  30 , as described in greater detail below, is normally transferred to the anoxic chamber  20  by centrifugal pumps  15  or other known pumping devices. This movement of the nitrate rich effluent recycled from the aerobic chamber  30  to the front of the anoxic reactor  20  requires the input of energy. This energy is sufficient in the anoxic chamber  20  to keep the solids in suspension without the requirement of supplemental mixers in the anoxic chamber  20 . Accordingly, the energy provided to the anoxic chamber  20  by the recycle pumps  15  is the only energy required to maintain solids suspension in the entire anoxic chamber  20  and to discharge denitrified effluent into the aerobic chamber  30 . 
   The volume of the anoxic chamber  20  is determined by the stoicheometry of the nitrate removal process and/or a minimum hydraulic retention time sufficient to allow the facultative bacteria to extract the nitrate from the effluent under BOD rich conditions. If sufficient BOD is not naturally present in the feed stream, a supplemental BOD source such as sugars from industrial waste generators, methanol or acetate can be used to accelerate the biological conversion of the nitrates into nitrogen gas. In an aerobic bioreactor, the rate of oxygen consumption for very food rich conditions may approach 30 to 45 mg O 2 /L-hr. If nitrate supplies the equivalent of 2.85 mgs of oxygen per ppm of nitrate-nitrogen removed, then the nitrate removal rate could be about 10.5 to 15.8 mg NO 3 -/L-hr. Accordingly, the volume of the reactor per pound of nitrate-nitrogen/day to be removed would thus range from 475 gallons to 316 gallons. The volume of the anoxic chamber  20  can thus be determined once the amount of nitrate to be removed has been established and hence the hydraulic detention time determined. However, because of the large volume of nitrate recycle, usually 300% of the influent flow, it is common to specify a minimum hydraulic retention time of one hour in the anoxic chamber based on total flow if the hydraulic retention time based on the anoxic chamber volume required to remove the indicated pounds of nitrate, does not equal at least one hour. 
   Further, consideration must be given to the residual dissolved oxygen (DO) returning with the recycle effluent stream, as this dissolved oxygen consumes additional BOD before de-nitrification can take place. The DO at the back of the secondary system, the aerobic chamber  30 , can approach 6-7 ppm. This oxygen is returned with the nitrate recycle effluent into the anoxic chamber  20 . Since the facultative bacteria will consume the DO before the bacteria will consume nitrates, the size of the anoxic chamber  20  must incorporate sufficient volume to allow the excess DO to be consumed via the biological process. The DO must be reduced to near zero to allow for anoxic conditions before conversion of the nitrate to nitrogen gas can occur. Accordingly, for every pound of oxygen returned to the front of the anoxic chamber  20 , the volume of the anoxic chamber  20  must be increased by as much as 167 gallons. 
   Once the anoxic chamber  20  discharges the denitrified effluent over the weir  28  into the aerobic chamber  30  described in the attached figures, or into another specified aerobic biological process, the conversion of ammonia into nitrates will occur. The nature of the nitrification aerobic process following is not limited to a fixed film process nor is the process described in the Figures in this disclosure a part of this invention. A known example of the aerobic chamber  30  is divided into a horizontal serpentine flow path by divider walls  33 , creating, preferably, four linear aerobic cells  32 , as is depicted in  FIG. 6 . Each aerobic cell  32  is provided with a plurality of spaced apart towers  35  of fixed film media, preferably the cross-flow type of media. An air supply is bubbled up through each tower  35  by an air supply member  37  to create an air-lift pumping action, as is represented by the arrows in  FIG. 5 . The injection of air into the towers  35  decreases the density of the effluent within the tower  35  causing the effluent to rise, pushing the effluent down between the adjacent towers  35  where the downwardly flowing effluent enters the bottom of the tower to be aerated. With effluent moving from one tower  35  to the next adjacent downstream tower  35  through the aerobic cells  32 , and with the energy inputted by the recycle pump  15 , the effluent moves from one tower  35  to another through the aerobic chamber  30  from one aerobic cell  32  to another until reaching the end of the last aerobic cell D. 
   At the end of the aerobic chamber  30 , the recycle pump moves about 75% of the now nitrate rich effluent and inserts the recycled effluent into the front of the anoxic chamber  20 , as is described above. The remaining effluent is discharged from the reactor  10  through the outlet opening  39 . The discharge rate will be substantially equal to the inflow rate of the influent inputted through the inlet opening  29 . 
   As is represented in  FIGS. 7 and 8 , the fixed film media towers can be supported on a stand  40  that is mounted on the floor  21  of the reactor  10  to keep the bottom of the fixed film tower  25 ,  35  off the floor  21 . Alternatively, the fixed film tower  25 ,  35  can be mounted in a chimney  45  and suspended therefrom. The chimney  45  is preferably supported by the adjacent divider or separator walls, while the fixed film media tower  25 ,  35  is hung from the chimney  45  so that the bottom of the tower  25 ,  35  is spaced above the floor  21  of the reactor  10 . 
   In operation, the BOD rich influent arriving through the inlet opening  29  is mixed with the recycle effluent which is rich in nitrates. After the dissolved oxygen from the recycled effluent is dissipated, the organic food source of the BOD material, primarily a carbon source, is converted by the facultative bacteria growing on the fixed film media towers  25  into ammonia, which is largely dissolved within the effluent, and nitrogen which is discharged to the atmosphere. The vertical and horizontal serpentine paths created by the anoxic chamber  20  through the towers  25  of fixed film media provide adequate time for the conversion of the nitrates into ammonia and expelled gases and new bacterial growth. 
   When the ammonia rich effluent is discharged over the weir  28  in the wall  18 , the oxygenation of the effluent within the aerobic towers  35  convert the ammonia into first nitrites and then nitrates, and water. Thus, the effluent in the aerobic chamber  30  re-nitrifies the effluent so that the recycle pumps  15  can recycle the converted nitrate rich recycle effluent back into the anoxic chamber  20 . Meanwhile, the BOD is substantially exhausted from the effluent. The remaining nitrates in the effluent discharged from the reactor  10  through the outlet opening  39  can be treated subsequently by known de-nitrification processes before being discharged to the environment. 
   It will be understood that changes in the details, materials, steps and arrangements of parts which have been described and illustrated to explain the nature of the invention will occur to and may be made by those skilled in the art upon a reading of this disclosure within the principles and scope of the invention. The foregoing description illustrates the preferred embodiment of the invention; however, concepts, as based upon the description, may be employed in other embodiments without departing from the scope of the invention.