Abstract:
A waste treatment process comprised of an ammonification system to convert soluble organic nitrogen into ammonia nitrogen, followed by a physico-chemical process to remove a substantial amount of the ammonia as a recovered ammonium sulfate fertilizer or ammonium hydroxide (“aqua ammonia”), and followed by an ammonia oxidation process to oxidize the remaining ammonia from the physico-chemical process. The process reduces ammonia and carbonaceous organic matter to less than 10 mg/l and recovers ammonia in the form of either ammonium sulfate or ammonium hydroxide.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Applicants claim the priority benefits of U.S. Provisional Patent Application No. 61/277,844, filed Sep. 30, 2009. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to waste treatment systems, and in particular, to a method for removing nitrogen and recovering ammonia content from waste streams with high concentrations of nitrogen, a portion of which is organically bound nitrogen, such as in the supernatant of anaerobically digested sludge, landfill leachate or industrial wastewaters. The method is comprised of a biological process (aerobic or anaerobic), followed by a physico-chemical process and, in some cases, followed by an aerobic biological process. 
     An example of an application for the present invention process with an aerobic first step is for municipal wastewater treatment plants with anaerobic digestion. The supernatant from the anaerobic digestion of blended or secondary sludge contains high concentrations of ammonium that, as a recycle stream to the influent of a wastewater treatment plant, may account for as much as 17-20% of the nitrogen loading to the plant. This waste stream together with other nitrogen rich recycles may contribute 20-30% of total nitrogen load to the plant. These recycle streams introduce additional difficulty to treatment, within the main plant, because (1) they typically do not contain the requisite carbon to nitrogen ratio (C/N) for traditional biological nitrogen removal; and (2) they are generated by sludge handling operations that typically do not occur every day and, therefore, result in shock loads to the main plant. However, the increasingly stringent nutrient limits being applied to wastewater treatment plants require management of these recycle streams in order to consistently achieve low effluent nitrogen concentrations. 
     SUMMARY OF THE INVENTION 
     The present invention process combines several core technologies to substantially reduce ammonia to less than 10 mg/l or lower. The invention process combines both biological and physico-chemical processes together in order to achieve energy conservation and resource recovery simultaneously. The invention process is hereinafter referred to as the BioCAST process. The invention process is designed to increase ammonia recovery, to reduce electrical energy requirements, to reduce chemical consumption, to reduce space requirements when compared to biological processes alone, and to be economically feasible while achieving very low total nitrogen concentrations in the recycle stream. 
     The core technologies for the BioCAST process are (1) an ammonification system to convert soluble organic nitrogen into ammonia nitrogen, (2) a physico-chemical process to remove a substantial amount of the ammonia as a recovered ammonium sulfate fertilizer or ammonium hydroxide (“aqua ammonia”), and (3) an ammonia oxidation process to oxidize the remaining ammonia from the physico-chemical process. The combined technologies in the invention process reduce ammonia and carbonaceous organic matter to less than 10 mg/l and recovers ammonia in the form of either ammonium sulfate or ammonium hydroxide. The BioCAST process first maximizes ammonia in the wastewater and then recovers the ammonia in the form of commercial-grade ammonium sulfate or other ammonium salt. 
     The ammonification system is a biological process designed to convert soluble organic nitrogen into ammonia nitrogen. Ammonification is the release of ammonia nitrogen which occurs as amino acids and other nitrogen containing carbonaceous compounds undergo biodegradation either aerobically or anaerobically. In the aerobic process ammonification occurs as heterotrophic bacteria oxidize nitrogen containing soluble organic matter. In the anaerobic process multiple pathways including hydrolysis, fermation and acidogenesis, result in the degradation of the compounds containing organically bound nitrogen. The choice of whether to use the BioCAST process with an aerobic or an anaerobic first stage is dependent on the type of wastewater to be treated. For example, with waste streams with high BOD content, an anaerobic first stage may be the most effective. 
     In this first stage of the invention, i.e., the ammonification reactor, the purpose is to reduce the carbonaceous matter, some of which contains nitrogen which when released is converted to ammonia. The result will be an increase in ammonia nitrogen prior to entering the physico-chemical process. 
     The physico-chemical process removes ninety percent of the nitrogen as a recovered ammonium sulfate, ammonium nitrate, or ammonium phosphate that can be used as a fertilizer. Alternatively the ammonia can be recovered as an ammonium chloride, or other ammonium salt, and sold as a commodity product. One additional desirable product is aqua ammonia (Ammonium Hydroxide). The ammonia gas can be introduced directly into demineralized water to produce ammonium hydroxide, i.e., aqua ammonia. 
     The physico-chemical process is based upon a vacuum separation process, sold by CASTion Corporation, of Worcester, Mass., as the R-CAST process. Various aspects of the R-CAST process are described in U.S. Pat. Nos. 4,770,748 and 4,880,504, incorporated herein by reference. The R-CAST process performs two major functions. Its primary function is to remove approximately 90% of the ammonia from the influent centrate. A secondary function is to strip ammonia from ammonification/AOx backwash wastewater. An R-CAST unit can be operated individually, in series, or in parallel for high flow streams or to treat high concentration ammonia streams. The R-CAST unit for recovery of the ammonia from the ammonification unit normally operates in a batch mode set by the respective backwash frequencies. 
     The effluent containing ammonia from the ammonification process is directed as influent to the R-CAST unit, comprised of a vacuum assisted flash stripping tower, where the ammonia is removed from the effluent. During operation, ammonium is converted to ammonia gas by raising the pH of the wastewater to approximately 11. The ammonia gas is drawn out of the stream into a vapor stream. The vapor leaves the R-CAST in a separate vapor tube where it is condensed and introduced to a water stream containing acid. The removed ammonia gas is placed in contact with water or water containing an acid such as sulfuric, nitric, hydrochloric, phosphoric, and/or another acid. In the case of sulfuric acid, there is a reaction creating ammonium sulfate, a salable product that can be used in the fertilizer industry or other areas as raw material. Tests have shown that the purity of ammonium sulfate is high due to the minimal passage of volatile organic compounds which are primarily removed in the upstream biological process. 
     The ammonia oxidation process is a biological process based on an ammonia oxidation reactor sold under the trademark AOx by F.R. Mahony &amp; Associates, Inc., Rockland, Mass. The effluent from the R-CAST unit contains 100-200 mg/l of ammonia. Acid is introduced to lower the pH of the R-CAST effluent and convert the ammonia back to ammonium. The AOx reactor is designed to achieve ammonia oxidation to nitrite and subsequent denitrification with a supplemental electron donor. Within the AOx reactor the first step of nitrification (ammonia oxidation to nitrite) occurs as described by the following mass-based equation normalized to ammonium.
 
NH 4   + +2.457O 2 +6.716HCO 3   −             0.114C 5 H 7 O 2 N+2.509NO 2   − +1.036H 2 O+6.513H 2 CO 3 .

     Several environmental conditions conducive to partial nitrification (i.e. oxidation of ammonia to nitrite), exist within the AOx reactor. First, it has been shown that the relative ratios of C DO /C NH4-N  and C DO /C FA  are indicative of the nitrogen species (i.e. nitrite or nitrate) predominating in the effluent, where: 
     C DO —concentration of dissolved oxygen (mg/l) 
     C NH4-N —concentration of total ammonia mg (NH 3 —N/1) 
     C FA —concentration of free ammonia (mg NH 3 —N/1) 
     In order to have nitrite as the predominating species produced from the biological transformation of ammonia, a C D0 /C NH4-N  ratio of less than one (1) and a C DO /C FA  ratio of less than ten (10) is maintained within the reactor. Therefore one of the control mechanisms for partial nitrification is to limit the dissolved oxygen (DO) concentration. Controlling the concentration of dissolved oxygen (C DO ) is achieved by intermittently aerating the bioreactor. Secondly, in a high ammonia concentration waste stream with a slightly elevated pH (such as the effluent from the R-CAST unit), there is a high concentration of free ammonia which inhibits the conversion of nitrite to nitrate (i.e. stops the reaction as described above). Finally, the temperature of the effluent from the R-CAST step is approximately 40° C. or higher and will be cooled to approximately 35° C. At this temperature the growth rate of ammonia oxidizing bacteria (AOB) is approximately twice that of the nitrite oxidizing bacteria (NOB). Therefore with adequate removal of sludge (i.e. backwashing of the reactor) the NOB will be constantly removed from the system. 
     Microbiological studies of the biofilm indicate that operating the reactor with intermittent aeration coupled with the free ammonia toxicity inherent in the waste stream tend to exclude nitrite oxidizing bacteria (NOB) from the biofilm, not just suppress them. The result is stable partial nitrification (i.e. ammonia oxidation only). 
     In order to remove the nitrogen, denitrification will also be achieved within the AOx reactor. Since the intermittent aeration provides anoxic conditions within the bioreactor addition of a supplemental electron donor (carbon source likely) would result in denitrification of nitrite as described by the following simplified reaction:
 
6NO 2   − +3CH 3 OH           3N 2 +6HCO 3   − +3H 2 O

     The BioCAST process fits into an overall plant process flow and offers several advantages for the management of high strength nitrogen waste streams. The BioCAST process maximizes the amount of ammonia that may be recovered by flash distillation. The BioCAST process also achieves very low total nitrogen numbers, but with significantly reduced oxygen and chemical requirements. There are three reasons for this. Firstly, only a small fraction of the total nitrogen in the waste stream is treated biologically. Secondly, the biological portion of the process requires 25% less oxygen and 40% less supplemental carbon than conventional biological nitrogen removal. Thirdly, the BioCAST process requires approximately five (5) times less reactor area to treat a given waste stream than conventional biological treatment. 
     These together with other objects of the invention, along with various features of novelty, which characterize the invention, are pointed out with particularity in the claims annexed hereto and forming a part of the disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a typical wastewater treatment plant process schematic with the invention process. 
         FIG. 2  is a schematic of an aerobic ammonification processor. 
         FIG. 3  is a schematic of an anaerobic ammonification processor 
         FIG. 4  is a block diagram of an R-CAST unit. 
         FIG. 5  is a schematic of an AOx processor. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring to the drawings in detail wherein like elements are indicated by like numerals, there is shown a process flow for a typical wastewater treatment plant  10 , and the BioCAST process  30  installed in the wastewater treatment plant  10 . The BioCAST process has three major components, i.e., ammonification reactor  40 , R-CAST vacuum separation unit  70 , and an AOx reactor  90 . 
     Referring more particularly to  FIG. 1 , a typical wastewater treatment plant  10  will have a raw influent  11  passed to the plant  10  through an influent pipe. Solids within the raw influent will be filtered out through a screening assembly  12  and passed out for disposal. The screened raw influent  13  is then passed to a primary clarifier  14  where solids still within the screened raw influent will separate into a sludge component  15  and a liquid component  16 . The sludge component  15  is passed to a thickener  17  with the waste sludge  18  then passed to an anaerobic digester  27 . The clarifier liquid component  16  and a thickener liquid component  19  are then brought into the treatment plant&#39;s main biological process  20 . The biological process output is then passed to a secondary clarifier  21  for further settling out and separation of liquid from solids. The liquid from the secondary clarifier  21  is then passed out for a tertiary processing  22 . A portion of a sludge component  23  from the secondary clarifier  21  is recycled into the biological process  20  as return sludge while the remaining sludge is passed to a first centrifuge  24  for further separation of liquid from solids. The centrifuge solids  25 , i.e., wasted sludge, and thickened wasted sludge  18  are passed to the anaerobic digester  27  for further processing. The first centrifuge liquid component  26 , i.e., first centrate, is formed into a recycle stream and brought back into the biological process  20 . The digester  27  digests the wasted sludge  25  and outputs the digested sludge to a second centrifuge  28 . Solids from the second centrifuge  28  are separated out for disposal. The second centrifuge liquid component  29 , i.e., second centrate, is a high strength supernatant and provides the influent for the BioCAST process. The second centrate has a high ammonium concentration. Due to the high ammonium concentration in the second centrate, a typical wastewater treatment plant may see 10-20% of the nitrogen from the second centrate entering the plant. 
     Traditionally, the biological reactor  20  processes the influent stream, including the recycle stream, and oxidizes the ammonia content of the stream converting the ammonia, in a two-step process, into nitrite and then to nitrate. The nitrate is then passed through a denitrification process whereby the nitrate is converted to nitrogen gas and released into the atmosphere. The process of oxidizing ammonia to nitrate requires substantial quantities of oxygen from external air blowers. If denitrification is required as well, substantial quantities of chemicals to provide the carbon source may be required. Supplying air to the biological reactor is one of the biggest cost factors in operating a wastewater treatment plant. 
     In the present invention, the BioCAST process treats the second centrate so that the ammonia content in the second centrate is substantially removed. The BioCAST ammonification and vacuum separation operations convert ammonia to an end product of about 40% ammonium sulfate for sale as fertilizer, or to ammonium hydroxide, i.e., aqua ammonia, and second biological process further reduce nitrogen, carbonaceous matter and TSS (total suspended solids). Any remaining nitrogen, carbonaceous matter or TSS may be (1) returned to the treatment plant&#39;s main stream, (2) used as process water prior to return to the main stream, or (3) returned to the main stream effluent prior to disinfection. 
     Referring to  FIG. 2  there is shown an aerobic ammonification processor  40 .  FIG. 3  shows an anaerobic processor  40 . The difference between the two types is the aerobic air pipe  58  bringing air from an external source and diffusing the air in the filter  53 . The aenerobic processor also has means  59  for venting methane gas. The ammonification processor  40  is comprised of a biological reactor  45  and a clear well  60 . Nitrogen in the second centrate  29  providing an influent  41  to the BioCAST system is comprised of ammonia and organically bound nitrogen. The biological reactor  45  has a top  46 , bottom  47 , receiving side  48 , discharge side  49 , front side (not shown) and rear side  51 , said top, bottom, and sides defining a biological reactor interior  52 . The reactor interior  52  has a filter  53  made up of at least two layers, an upper sand layer  54  and an adjacent lower gravel layer  55 . A sump  56  is formed beneath the filer  53  adjacent the interior bottom  47 . The reactor interior  52  receives the influent  41  protruding through the reactor receiving side  48 . The influent pipe  41  opens into the reactor interior  52  just above the reactor filter  53 . The influent is an anoxic waste stream and flows through the filter layers and collects in the reactor sump  56 . In an aerobic system biochemical transformation of the organic material in the waste stream by bacteria attached to the filter  53  oxidizes the organic carbonaceous matter causing the release of organically bound nitrogen. This nitrogen, typically in the form of amines (mR2-), is immediately converted to nitrogen. The resulting effluent in the sump  56  therefore has a higher percentage of ammonia nitrogen than the influent. The resulting effluent is brought out through the reactor discharge side  49  via a discharge pipe  57  into the clear well  60 . The clear well  60  has a top  61 , bottom  62 , receiving side  63 , discharge side  64 , front side (not shown) and rear side  66 , said top, bottom and sides defining a clear well interior  67 . A first clear well pump  68  within the clear well interior  67  draws the biological reactor effluent, rich in ammonia nitrogen, into the clear well interior  67 . A second clear well pump  69  discharges effluent from the clear well interior  67  into the invention R-CAST unit influent pipe  71 . Backwash water from the ammonification reactor is sent to a solids separation tank and the supernatant is pumped and mixed with the influent. 
     The R-CAST unit  70  strips ammonia from the influent  71  utilizing a pH shift, control of partial pressures and temperature. The ammonia may be further processed for commercial products or stored  78  for later use. Alkali  72  is added to the influent  71  to convert ammonium to ammonia gas. Water  73  is introduced to the reaction chamber  74  through a spray nozzle (Flashed)  75  to yield a high degree of exposed surface for the ammonia gas to leave the aqueous phase and be transported to a venturi  76  or the vacuum pump. 
     The R-CAST reaction vessel is under vacuum at approximately −28 inches of mercury or lower so that the partial pressure of ammonia at the surface of the liquid droplets is far lower than its equilibrium pressure. A temperature above ambient is needed to raise the equilibrium pressure driving force and achieve a high rate of ammonia removal. The second centrate  29  is normally produced at 90 to 100° F., which is often adequate for efficient ammonia removal. Higher temperatures up to 140° F. improve process efficiency. 
     There are three critical elements in the R-CAST operation, namely, high flow rate recirculation pump, a baffle system to prevent entrainment, and a venturi or vacuum pump to create the vacuum and capture the ammonia from an R-CAST vapor tube. The recirculation pump circulates the distillation bottoms from the R-CAST back to the spray nozzle in the mid section of the reactor vessel. The liquid is maintained at a minimum level in the reactor vessel with an extended suction stand pipe to provide sufficient net positive suction pressure for the R-CAST recirculation pump. A baffle arrangement is staged near the top of the reaction vessel. The baffles are designed to minimize containment carry over to the vapor tube. The ammonia vapors are drawn into the suction side of the vacuum pump or venturi. Sulfuric acid water or other acids are used to create the liquid seal on the vacuum pump and reacts with the ammonia to create an ammonium sulfate by-product, aqua ammonia or other ammonia salts. Sulfuric acid can also be fed under pressure to provide the motive force for a venturi. The suction side of the venturi then draws in the ammonia vapors and a small fraction of the second centrate fluid. The ammonia concentration at this point in the system is approximately 100 ppm NH 3 —N. 
     The ammonia separation process in the R-CAST is employed to treat the ammonia rich centrate and any spent backwash wastewater from the ammonification unit and the AOx unit. While the centrate is treated in a semi-continuous process, the spent backwash wastewater is introduced to the R-CAST process on an episodic basis or bled into the feed to the BioCAST process. In either case, gaseous ammonia from the R-CAST vessel is drawn down the vapor tube to the suction side of the vacuum pump. A sulfuric acid feed system is used for the liquid seal in the vacuum pump, creating the ammonium sulfate product as previously described. 
     The sulfuric acid/ammonium sulfate solution circulates back to a product storage tank(s). The produced ammonium sulfate can be sold as a 21:24 S and N commodity to the open market. Once the desired concentration of ammonium sulfate is reached, the ammonium sulfate tank and its contents are transferred to a receiving truck for recovery. 
     Recycle streams such as the centrate from anaerobically digested sludge typically contain sufficient amounts of ammonia, phosphate and magnesium to form struvite, magnesium ammonium phosphate (MgNH 4 PO 4 .6H 2 O), a common form of struvite. The solubility of struvite decreases with pH and therefore, forms in the R-CAST during the addition of caustic. The struvite precipitate is subsequently removed in the AOx reactor and then removed from the AOx reactor during backwash. This step will result in the removal of phosphate from the centrate. This is a desirable yet unintentional consequence of the process. 
     The remaining effluent  77  from the R-CAST process is directed to the AOx reactor  90 . The ammonia concentration at this point in the BioCAST system is approximately 100 ppm NH 3 —N. The AOx reactor then removes ammonia down to less than 10 ppm NH 3 —N as well as any residual organic carbonaceous matter and suspended solids. 
     Referring to  FIG. 5  there is shown an AOx reactor  90 . The AOx process  90  is comprised of a biological reactor  95  (AOx reactor) and a clear well  110 . The remaining R-CAST effluent  77  provides an influent  91  to the AOx process. The biological (AOx) reactor  95  has a top  96 , bottom  97 , receiving side  98 , discharge side  99 , front side  100  and rear side  101 , said top, bottom, and sides defining a biological reactor interior  102 . The reactor interior  102  has a filter  103  made up of at least two layers, an upper sand layer  104  and an adjacent lower gravel layer  105 . A sump  106  is formed beneath the filer  103  adjacent the interior bottom  97 . The reactor interior  102  receives the influent  91  via a pipe protruding through the reactor receiving side  98 . The influent pipe  91  opens into the reactor interior  102  just above the reactor filter  103 . The influent flows through the filter layers and collects in the reactor sump  106 . Oxidation of the ammonia nitrogen is carried out by chemolithoautotrophic bacteria under aerobic conditions. Reduction of nitrite to nitrogen gas is mediated by heterotrophic bacteria under anoxic conditions. These two different populations of bacteria are present in the filter  103  due to the intermittent aeration of the biological reactor  95 . The effluent within the filter is simultaneously treated aerobically by an air pipe  108  causing bacteria to oxidize ammonia nitrogen to nitrite nitrogen, a process known as nitritation. The low dissolved oxygen in the reactor coupled with low concentrations of free ammonia toxicity tends to exclude nitrite oxidizing bacteria (NOB) from the biofilm, not just suppress them. The result is stable partial nitrification (i.e. ammonia oxidation only). 
     The resulting effluent in the sump  106  has a low concentration of total nitrogen. The resulting effluent is brought out through the reactor discharge side  99  via a discharge pipe  107  into the clear well ii 0 . The clear well  110  has a top  111 , bottom  112 , receiving side  113 , discharge side  114 , front side (not shown) and rear side  116 , said top, bottom and sides defining a clear well interior  117 . A first clear well pump  118  within the clear well interior  117  draws the AOx biological reactor effluent, into the clear well interior  117 . A second clear well pump  119  discharges effluent from the clear well interior  117  into the main plant biological process  20 . 
     It is understood that the above-described embodiment is merely illustrative of the application. Other embodiments may be readily devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.