Abstract:
An efficient system and process for removing nitrogen from wastewater while enriching seed sludge in the mainstream treatment process. Bioaugmentation of seed autotrophic organisms facilitate the nitrification reactions by enhancing the rates of reaction advantageously within a smaller volume or within a shorter activated sludge solids retention time. Likewise, bioaugmentation of seed denitrification organisms will also enhance rate of reaction within a smaller volume or shorter activated sludge solids retention time. Separate treatment of high ammonia digester reject water is an efficient method to treat nitrogen in recycle streams as well as to enrich the seed nitrifying and denitrifying cultures.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/730,035, filed on Oct. 26, 2005, the disclosure of which is incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to wastewater treatment and in particular to the treatment of digest reject water.  
       BACKGROUND OF THE INVENTION  
       [0003]     With reference to  FIG. 1 , a block diagram of a conventional wastewater treatment process is shown. The treatment includes influent wastewater  111  being settled in a primary settling basin  110 . Settled sludge  115  (i.e., primary sludge) is sent to an anaerobic sludge digester  140 , while the settled wastewater  112  is sent for secondary treatment. This secondary treatment may include biological aeration in aeration tanks  120  and a final settling process in a final settling basin  130 . A portion of the sludge  114  from the final settling basin  130  is returned to the secondary treatment in order to maintain biological compounds in the influent. Effluent water stream  118  that meets water quality standards is output from the final settling basin  130 . The remainder of the waste activated sludge  113  from the final settling basin  130  is sent to the anaerobic sludge digester  140 . After the anaerobic sludge digestion process, the removed water  116  is mixed with the incoming influent wastewater  111  and the stabilized solids (biosolids)  117  are now safe for application as fertilizers, for example.  
         [0004]     One problem that plagues conventional wastewater treatment plants is nitrogen removal to meet effluent discharge water quality standards. There are various sources of nitrogen in municipal wastewater, including human feces, industrial wastes, and other garbage. Typically, nitrogen removal at wastewater treatment plants is achieved by a series of nitrification and denitrification steps. Specifically, nitrifying bacteria convert ammonia to nitrite and subsequently to nitrate, followed by denitrification of nitrite or nitrate to nitrogen gas. The general chemical equations for these processes are:  
                         
 
         [0005]     The capability to remove nitrogen is constrained by the rate limiting aerobic nitrification reactions to convert ammonia to nitrite and/or nitrate by slow growing autotrophic organisms. The cumulative volume requirements for nitrogen removal depends on the completion of these reactions. Accordingly, there is a need and desire for a more efficient nitrogen removal process by encouraging the nitrification/denitrification processes in the mainstream reactor.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     The present invention, as illustrated in the various exemplary embodiments, includes an efficient process for removing nitrogen from wastewater while enriching seed sludge in the mainstream treatment process. Bioaugmentation of seed autotrophic organisms will facilitate the nitrification reactions by enhancing the rates of reaction within a smaller volume or within a shorter activated sludge solids retention time (“SRT”). Likewise, bioaugmentation of seed denitrification organisms will also enhance the rate of reaction within a smaller volume or shorter activated sludge solids retention time. Additionally, separate treatment of high ammonia digester reject water is an efficient method to treat nitrogen in recycle streams as well as to enrich the seed nitrifying and denitrifying cultures.  
         [0007]     In accordance with one exemplary aspect of the invention, direct mainstream bioaugmentation of sludge is performed and at least a part of the treated seed sludge is returned to a part of the mainstream reactor.  
         [0008]     In accordance with a second exemplary aspect of the invention, sidestream bioaugmentation is performed in a seed production reactor, and at least a part of the bioaugmentation sludge is returned to a mainstream reactor. In accordance with a third exemplary aspect of the invention, at least a part of the bioaugmentation sludge is returned to a mainstream reactor and a remaining portion of the bioaugmentation sludge is returned to the seed production reactor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The foregoing and other aspects of the invention will be better understood from the following detailed description of the invention, which is provided in connection with the accompanying drawings, in which:  
         [0010]      FIG. 1  is a block diagram of a conventional wastewater treatment process;  
         [0011]      FIG. 2  is a block diagram of a portion of a wastewater treatment process in accordance with a first exemplary embodiment of the invention;  
         [0012]      FIG. 3  is a block diagram of a portion of a wastewater treatment process in accordance with a second exemplary embodiment of the invention; and  
         [0013]      FIG. 4  is a block diagram of a portion of a wastewater treatment process in accordance with a third exemplary embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof and show by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that changes may be made without departing from the spirit and scope of the present invention. The progression of processing steps described is exemplary of embodiments of the invention; however, the sequence of steps is not limited to that set forth herein and may be changed as is known in the art, with the exception of steps necessarily occurring in a certain order.  
         [0015]     In accordance with the invention, and as described in more detail below with respect to  FIGS. 2-4 , exemplary wastewater treatment systems include a mainstream and a seed production reactor. It should be understood that with each exemplary system, other wastewater treatment processes (not shown) may be used in conjunction with the described processes. These other processes in no way affect the scope of the present invention.  
         [0016]     In accordance with the invention, seed sludge is generated in a seed production reactor. The seed production reactor is a nitrification and denitrification process that receives a low strength ammonia wastewater that is first nitrified and subsequently denitrified using an external carbon substrate such as methanol, ethanol, acetic acid, sugar, glycol or glycerol. These denitrifying carbon substrates will produce specialized organisms for denitrification. The influent to the seed production reactor consists of mainly ammonia and very little carbonaceous substrate. Autotrophic conditions are promoted, allowing organisms to use ammonia as an energy source and convert it to nitrate, thus producing an enriched population of autotrophic (nitrifying) seed organisms. Subsequently, anoxic conditions are promoted for denitrification using external carbon, thus producing an enriched population of denitrifying seed organisms. The effluent is then sent to a settling basin.  
         [0017]     Portions of a first exemplary waste water treatment system  200  are shown in  FIG. 2 . It should be understood that the wastewater flow  212  entering the system  200  may be similar to the flow  112  described above with reference to effluent from a primary clarifier (settling basin  110 ) in  FIG. 1 . In accordance with this first exemplary bioaugmentation process, a steady state quantity of the autotrophic seed sludge  220  is recovered and sent to the mainstream high rate bioaugmentation reactor, or mainstream reactor  201 , where nitrification is promoted. The control of ammonia flow in the effluent of the mainstream flow  215  to the seed production reactor  202  for seed generation and maintenance of steady state seed quantity is important for process stability. This control is achieved by varying the mainstream flow  215  or volume subject to seed nitrification. The steady state quantity of seed is achieved by sending the seed to only part of the mainstream bioaugmentation process (between 30-60% flow or process volume), in a manner to encourage sufficient but not excessive nitrification in the mainstream process. Sufficient ammonia is allowed to flow into the seed production reactor  202  for seed regeneration and steady state maintenance.  
         [0018]     The mainstream bioaugmentation  201  and seed  202  reactors are operated at normal seasonal water temperatures (10° C. to 27° C.) and pH (6.5-7.5). The mainstream reactor  201  is operated aggressively at a low SRT of 0.5-3 days for carbonaceous substrate removal with seed enhanced nitrification and simultaneous or staged step-feed denitrification/nitrification. The seed production reactor  202  is operated in the SRT range of 7-20 days; with an optimum range of 10-15 days.  
         [0019]     As shown in  FIG. 2 , return activated sludge  208 ,  209  of each of the mainstream  201  and the seed  202  reactors, respectively, may be used as a recycle stream  208 ,  209  to further enhance biological reactions in these reactors  201 ,  202 . In addition, the waste sludge  217  can be further treated using lime stabilization, anaerobic digestion, or other known sludge treatment techniques. The treated waste sludge  217  may then be sent for final disposal and/or management. Effluent water  225  from the seed production reactor  202  may be further processed, as desired. Other processes for treating the seed production reactor effluent water  225 , such as tertiary treatments, are beyond the scope of the present invention.  
         [0020]     Thus, unlike conventional seed treatment processes, the first exemplary system  200  provides for the seeding of a mainstream reactor process. This advantageously helps to: (1) maintain the steady state seed mass, (2) control addition of seed to a partial flow/volume in the mainstream process, and (3) and provide for denitrification of seed derived nitrate simultaneously or sequentially within the mainstream process using a step-feed process.  
         [0021]     The advantages of this first exemplary system  200  include: lower methanol requirements of between 25-50% for denitrification through reductions in overall ammonia and subsequent nitrate loads in the seed production reactor, lower denitrification volume requirements of between 25-50% through reductions in nitrate loads, and lower nitrification volume requirements of between 25-50% through reductions in ammonia loads.  
         [0022]     Turning to  FIG. 3 , a second exemplary wastewater treatment system  300  is shown. The second system  300  includes a mainstream reactor  301  having a wastewater influent  312  and a mainstream effluent  315  that is sent to a seed production reactor  302 . In accordance with the second exemplary embodiment of the invention, a steady state quantity of seed organism  307  generated by the seed production reactor  302  is sent to a sidestream process, the Sidestream Bioaugmentation and Enrichment Reactor (“SBER”)  310 . The SBER is also fed a high strength anaerobic digester reject water recycle  316 , and may receive additional inputs of carbon sources for denitrification and alkalinity, as necessary to maintain the desired pH levels. Accordingly, the sludge stabilization technique for a system in accordance with this embodiment is likely anaerobic digestion in order to produce the high strength reject water  316 . Return activate sludge  308 ,  309  is used as a recycle stream in mainstream and seed production reactors,  301 ,  302 .  
         [0023]     The SBER  310  is operated at a temperature somewhat higher than the mainstream process  301 . The temperature of the SBER  310  is approximately between 2 and 20 degrees Celsius higher than the mainstream reactor  301  and represents a volume-averaged temperature of the higher temperature incoming reject water  316  recycle and the seed sludge  307 . This temperature is high enough to improve rates of nitrification and denitrification in the SBER  310 , but low enough to allow the seed population to grow in both the SBER  310  and mainstream reactor  301 . The solids retention time (SRT) of the SBER  310  is maintained between 1 and 5 days aerobic SRT and between 1 and 5 days anoxic SRT. The pH in the SBER  310  is maintained between 6.0-8.5 with an optimum range of 6.5-7.5. The dissolved oxygen concentration can be maintained as high as 5 mg/L and as low as 0.2 mg/L, during aerobic operations. The optimum dissolved oxygen concentration will depend on the final reactions desired in the SBER If the reactions need to stop at nitrite, the optimum dissolved oxygen is lower. If the reaction proceeds to produce nitrate, the optimum dissolved oxygen concentration is higher. In accordance with an embodiment, the optimum dissolved oxygen concentration is 2 mg/L.  
         [0024]     The reject water  316  is treated; nitrified and then denitrified (using the same external carbon source as the seed production reactor) in this initial bioaugmentation step. This step also serves as a seed enrichment step, to increase the yield of seed nitrifying and denitrifying sludge. The enriched seed  320  is then sent to the mainstream reactor  301  to perform bioaugmentation. The enriched seed  320  has a high capability to perform nitrification in the mainstream reactor  301 . The mainstream  301  and seed production reactors  302  are operated at normal seasonal water temperatures (10° C. to 27° C.) and pH (6.5-7.5). The mainstream reactor  301  is operated aggressively at a low SRT of within the range of about 0.5-3 days for carbonaceous substrate removal with seed enhanced nitrification and simultaneous or staged step-feed denitrification/nitrification. The seed production reactor  302  is operated in the SRT range of 7-20 days, with an optimum range of 10-15 days. For maintenance of steady-state seed sludge, the same description in the first exemplary system  200 , applies.  
         [0025]     As discussed above with reference to  FIG. 2 , a part of the effluent of each of the mainstream  301  and the seed  302  reactors may be used as recycle streams  308 ,  309 , respectively, to further enhance biological reactions in these reactors  301 ,  302 . In addition, part of the effluent  319  from the mainstream reactor  301  may be sent to the SBER  310  for bioaugmentation processing directly and if necessary to control the formation of nitrate. It may be desirable to stop the nitrification reaction at nitrite by reducing the dissolved oxygen concentration, ammonia or nitrite inhibition, or by the addition of effluent  319 . These operations will result in the wash out of organisms responsible for promoting the second step of nitrification reaction. This partial nitrification process can limit the amount of air and external carbon necessary. Waste sludge  317  can be further stabilized and treated using known sludge treatment techniques. Effluent water  325  from the seed production reactor may be further processed, using known tertiary or other treatments, as desired.  
         [0026]     Thus, unlike conventional processes, the second exemplary system  300  also provides for the seeding of a mainstream reactor process. Thus, the second exemplary system  300  appreciates the same advantages from seeding the mainstream process as discussed above.  
         [0027]     Other advantages realized by this option include lower methanol requirements of 25-50% for denitrification through reductions in overall ammonia and subsequent nitrate loads in the seed production reactor, lower denitrification volume requirements of 25-50% through reductions in nitrate loads, lower nitrification volume requirements of 25-50% through reductions in ammonia loads, and the capability to treat high-strength reject water  316 .  
         [0028]     A third exemplary system  400  in accordance with the invention is shown in  FIG. 4 . The third system  400  includes a mainstream reactor  401  having a wastewater influent  412  and a mainstream reactor effluent  415  that is sent to a seed production reactor  402 . Return activated sludge  408 ,  409  are used as recycle streams for each both reactors,  401 ,  402 , respectively. In accordance with the invention, a steady state quantity of seed organism  407  is sent to a sidestream process, SBER  410  in the third exemplary system  400 . The SBER  410  is also fed a high strength anaerobic digester reject water recycle  416 , and other inputs may include carbonaceous substances and alkalinity. The sludge stabilization technique for a system in accordance with this exemplary embodiment is likely anaerobic digestion in order to produce the high strength reject water  416 .  
         [0029]     The SBER  410  is operated at a temperature somewhat higher than the mainstream process  401 . The temperature of the SBER  410  is approximately between 2 and 20 degrees Celsius higher than the mainstream reactor  401  and represents a volume-averaged temperature of the higher temperature incoming reject water  416  recycle and the seed sludge  407 . This temperature is high enough to improve rates of nitrification and denitrification in the SBER  410 , but low enough to allow the seed population to grow in both the SBER  410  and mainstream reactor  401 .  
         [0030]     The solids retention time (SRT) of the SBER  410  is preferably maintained between about 1 and 5 days aerobic SRT and between 1 and 5 days anoxic SRT. The pH in the SBER  410  is maintained between 6.0 and 8.5 with an optimum range of 6.5 to 7.5. The dissolved oxygen concentration can be maintained as high as 5 mg/L and as low as 0.2 mg/L during aerobic operations. The optimum dissolved oxygen concentration will depend on the final reactions desired in the SBER. If the reactions need to stop at nitrite, the optimum dissolved oxygen is lower at approximately 0.5 mg/L. If the reaction needs to proceed to nitrate, the optimum dissolved oxygen concentration is higher, at approximately 2 mg/L.  
         [0031]     The reject water  416  is treated; nitrified and then denitrified (using the same external carbon source as the seed production reactor) in this initial bioaugmentation step. This step also serves as a seed enrichment step, to increase the yield of seed nitrifying and denitrifying sludge. The enriched seed sludge  420  from the SBER  410  can be sent in part or in entirety to the mainstream reactor  401  and the remaining portion of the enriched seed is sent to the seed production reactor to maintain process stability in case of inhibition and process upsets and to perform additional bioaugmentation, if desired. Thus, a smaller stream  420  is sent to the mainstream reactor  401  to perform mainstream bioaugmentation.  
         [0032]     The mainstream  401  and seed production reactors  402  are operated at normal seasonal water temperatures (10° C. to 27° C.) and pH (6.5-7.5). The mainstream reactor  401  is operated aggressively at a low SRT with the range of about 0.5-3 days for carbonaceous substrate removal with seed enhanced nitrification and simultaneous or staged step-feed denitrification/nitrification. The seed production reactor  402  is preferably operated in the SRT range of about 7-20 days, with an optimum range of 10-15 days. For maintenance of steady-state seed sludge, the same description described above with reference to  FIG. 2  applies.  
         [0033]     As discussed above with reference to  FIG. 2 , a part of the effluent of each of the mainstream  401  and the seed  402  reactors may be used as recycle streams  408 ,  409 , respectively, to further enhance biological reactions in these reactors  401 ,  402 . In addition, the waste sludge  417  can be further treated using known sludge stabilization and treatment techniques. Effluent water  425  from the seed production reactor may be further processed, using tertiary or other known processing techniques, as desired. Another optional location for waste sludge is shown as effluent  422  from the SBER  410 .  
         [0034]     Like the first two exemplary embodiments, a steady-state seed sludge  420  is sent to the mainstream process  401  to perform nitrification and denitrification, but sufficient ammonia  421  is allowed to flow to the seed production reactor  402  for seed regeneration.  
         [0035]     The third exemplary system  400  may be easier to control than the second exemplary system  300 , since there is flexibility to send seed sludge  420 ,  421  to either process (mainstream reactor  401  or seed production reactor  402 ), thus the seed production reactor  402  can be sustained through the seed recycle  409 , and does not need to completely depend on ammonia from mainstream reactor  401  for regeneration. It should be noted that waste sludge  422  may also be produced in this SBER  410  for external treatment, such as sludge stabilization prior to land disposal.  
         [0036]     Another advantage of system  400  is the seeding of denitrifiers to the seed production reactor  402 . Since the same external carbon source is used in both the seed production reactor  402  and SBER  410 , the denitrifying populations are also enriched in the SBER  410  and available for bioaugmentation in the seed production reactor  402 . Thus the denitrification volume requirements are reduced even more than in the first two exemplary systems  200 ,  300 . In system  400 , the high strength load from the digester reject water recycle  416  is treated (nitrified and denitrified) simultaneously as the seed sludge  407  is enriched.  
         [0037]     Thus, unlike conventional processes, the third exemplary system  400  also provides for the seeding of a mainstream reactor process. Thus, the third exemplary system  400  appreciates the same advantageous from seeding the mainstream process as discussed above with reference to exemplary systems  200 ,  300 .  
         [0038]     Other advantages realized by system  400  include: lower methanol requirements of 25-50% for denitrification through reductions in overall ammonia and subsequent nitrate loads in the seed production reactor, lower denitrification volume requirements of 25-50% through reductions in nitrate loads and through seeding f denitrifiers, lower nitrification volume requirements of 25-50% through reductions in ammonia loads, and capability to treat high-strength reject water.  
         [0039]     The processes and devices described above illustrate preferred methods and typical devices of many that could be used and produced. The above description and drawings illustrate embodiments, which achieve the objects, features, and advantages of the present invention. However, it is not intended that the present invention be strictly limited to the above-described and illustrated embodiments. For example, the mainstream reactor may consist of several tanks in parallel, some of which may undergo bioaugmentation while others remain unbioaugmented. Additionally, any modifications, though presently unforeseeable, of the present invention that come within the spirit and scope of the following claims should be considered part of the present invention.