Abstract:
Removal of biological nutrients from a wastewater yielding a low phosphorous (e.g., less than 0.25 mg/L) output includes providing a serial multistage bioreactor containing activated sludge having in hydraulic series an anaerobic zone and a downstream aerobic zone, with each zone having an upstream inlet and a downstream outlet. The wastewater is provided to the anaerobic zone inlet. A quantity of chemical sufficient to precipitate soluble and particulate phosphorous is added to the downstream aerobic zone in an amount sufficient to yield a low phosphorous output. Treated water is separated from the activated sludge and precipitated phosphorous and a return activated sludge separated from the treated water is recycled to the anaerobic zone.

Description:
TECHNICAL FIELD  
       [0001]     The present invention is directed toward wastewater treatment processes, and more particularly toward an activated sludge treatment process using a membrane bioreactor (“MBR”) and using biological and chemical phosphorus removal adapted to prevent phosphorus limiting conditions in the activated sludge treatment process.  
       BACKGROUND ART  
       [0002]     Biological treatment processes for the removal of biological nutrients such as biological oxygen demand (“BOD”), nitrates and phosphates are well known. A typical biological treatment process is an activated sludge process in which the wastewater is aerated and agitated with an activated sludge and then purged of a variety of microorganisms. Often this aerobic stage is combined with an anaerobic stage, i.e., a stage operated in the absence of induced oxygen, either soluble or derived from nitrites or nitrates (NO x ) and an anoxic stage, i.e., where oxygen is absent but nitrites or nitrates are present. Phosphorus removal is accomplished by the presence of phosphorus-accumulating organisms (“PAOs”) in the anaerobic stage which release phosphorus into the wastewater as part of the process of accumulating organic matter (i.e., volatile fatty acids) used for cell growth. In a downstream aerobic or anoxic zone, the organisms metabolize the accumulated organic matter and accumulate the released phosphorus into cells as part of the growth process. A number of prior art patents disclose multi-zoned bioreactors with some recycling of flows between the various zones to maintain concentrations of useful microorganisms and to improve biological nutrient removal. For example, Daigger, U.S. Pat. No. 6,517,723, the contents of which are incorporated herein by reference. Other examples include Daigger, U.S. Pat. No. 5,480,548; Hawkins, U.S. Pat. No. 5,601,719; Marsman, U.S. Pat. No. 5,342,522; Strohmeier, U.S. Pat. No. 5,798,044; Hong, U.S. Pat. No. 5,650,069; Timpany, U.S. Pat. No. 5,354,471; Wittmann, U.S. Pat. No. 4,961,854; Nicol, U.S. Pat. No. 4,787,978; and Yang, U.S. Pat. No. 5,942,108.  
         [0003]     In certain circumstances, very low concentrations of phosphates in effluents are required and in such circumstances chemical precipitation is used for phosphate or phosphorous removal. In chemical precipitation methods, soluble salts, such as ferrous/ferric chloride or aluminum sulfate, are added to the wastewater to form insoluble phosphate metal salts. The insoluble phosphate metal salts are then gravity separated or filtered from the wastewater to yield an effluent with low concentrations of total phosphate (“TP”). Low levels of TP are defined herein to be in a range of less than 0.25 mg/L.  
         [0004]     As disclosed in Husain, U.S. Pat. No. 6,406,629, a type of biological treatment known as a membrane bioreactor can be combined with phosphate precipitation techniques. Husain sets forth an example of addition of phosphate precipitating chemicals to an aerobic tank connected to a membrane filter. This combination is criticized, however, because the presence of metallic precipitates increases the rate of membrane fouling or forces the operator to operate the system at an inefficient long sludge retention time. Another disadvantage of a combined system as described is if the system includes recycle of activated sludge, which is typical in biological processes, removal of phosphorous in excess of the stoichoimetric amount of phosphorous required to support growth of the activated sludge in the biological treatment process can degrade the efficiently of the biological treatment process&#39; removal of other nutrients.  
         [0005]     Husain describes one attempted solution to the problems discussed above. Husain provides side stream processes operating in parallel to a conventional multistage activated sludge biological treatment process to remove excess phosphorous. In a first side stream process, a liquid lean in solids but containing phosphates is extracted from anaerobic mixed liquor from an anaerobic stage of the activated sludge process. Phosphates are precipitated from that mixed liquor to produce a phosphorus lean liquid which leaves the process as effluent or is returned to an anoxic or aerobic zone. In an alternate side stream process, an aerobic mixed liquor is removed to a reaction zone and treated to form a liquid rich in insoluble phosphates. The liquid rich in insoluble phosphates is treated in a hydro cyclone to separate out insoluble phosphates and create a liquid lean in insoluble phosphates. The liquid lean in insoluble phosphates is returned to the anoxic zone. While the solution proposed in Husain may provide for effective phosphorous removal, it requires additional processes which increase treatment costs along with the space required to perform the treatment process. In addition, the first side stream process may result in a phosphorous deficiency in biological process stages downstream from the anaerobic zone.  
         [0006]     The present invention is directed toward overcoming one or more of the problems discussed above.  
       SUMMARY OF THE INVENTION  
       [0007]     An aspect of the present invention is a method for removal of biological nutrients from a wastewater yielding a low phosphorous (e.g., less than 0.25 mg/L) output. A serial multistage bioreactor containing activated sludge having in hydraulic series an anaerobic zone and a downstream aerobic zone, each zone having an upstream inlet and a downstream outlet is provided. A wastewater is provided to the anaerobic zone inlet. A quantity of chemical sufficient to precipitate soluble and particulate phosphorous is added to the downstream aerobic zone in an amount sufficient to yield a low phosphorous output. Treated water is separated from the activated sludge and precipitated phosphorous and a return activated sludge separated from the treated water is recycled to the anaerobic zone.  
         [0008]     In a preferred embodiment, the separating of the treated water from the activated sludge and precipitated phosphorous is performed by filtering the treated water, activated sludge and precipitated phosphorous through an immersed membrane filter operatively associated with the downstream aerobic zone. The multistage bioreactor may further include an anoxic zone in hydraulic series intermediate the anaerobic and downstream aerobic zone. Alternatively the multistage bioreactor may include an upstream anoxic zone, and upstream aerobic zone and a downstream anoxic zone in hydraulic series between the anaerobic zone and the downstream aerobic zone. A variety of recycling options are available. For example, the return activated sludge may first be recycled to near an inlet of the anoxic zone and then recycled from near an outlet of the anoxic zone to the anaerobic zone. Alternatively, the return activated sludge may be first recycled to near an inlet of the upstream aerobic zone and then recycled from near the outlet of the upstream aerobic zone to near then inlet of the upstream anoxic zone and then recycled from near the outlet of the upstream anoxic zone to the anaerobic zone.  
         [0009]     The method of the present invention allows for obtaining low phosphorous output by a combined biological treatment process and chemical phosphorous removal. Surprisingly, recycle of return activated sludge including residuals of the phosphorous precipitating chemical does not cause sufficient precipitation of released phosphorous to inhibit the downstream biological processes. The anaerobic zone in essence functions as a phosphorous buffer to assure a supply of phosphorous for downstream biological processes during simultaneous chemical removal of phosphorous in the downstream aerobic zone. Thus, the method allows for chemical phosphorous removal within a conventional biological treatment process including an anaerobic zone without inhibiting and still promoting robust nutrient uptake in the downstream biological zones. In addition to easing the need to provide precise amounts of phosphorous removal agents in the downstream aerobic zone, the method eliminates the need for side stream phosphorous removal processes or subsequent downstream phosphorous removal and clarification, thus minimizing space requirements and attendant cost. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic representation of a first embodiment of an apparatus for treating wastewater in accordance with the present invention having an anaerobic and an aerobic treatment zone;  
         [0011]      FIG. 2  is a schematic representation of a second embodiment of an apparatus for treating wastewater in accordance with the present invention having an anaerobic, anoxic and aerobic zone;  
         [0012]      FIG. 3  is a schematic representation of a third embodiment of an apparatus for treating wastewater in accordance with the present invention having an anaerobic zone, an upstream anoxic zone, an upstream aerobic zone, a downstream anoxic zone and a downstream aerobic zone; and  
         [0013]      FIG. 4  is a schematic representation of an alternate embodiment of the apparatus of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]     A first embodiment of a membrane bioreactor apparatus  10  for the treatment of wastewater to produce a low phosphorous effluent is illustrated in  FIG. 1 . The apparatus  10  consists of a multistage bioreactor having in hydraulic series an anaerobic zone  12  and an aerobic zone  14  divided by a weir  16 . Those skilled in the art will appreciate that separate vessels connected by conduits could be employed as an alternative to the two zones separated by the weir  16 . A membrane filter  18  is operatively associated with the downstream aerobic zone  14 . In a preferred embodiment as illustrated in  FIG. 1 , the membrane filter  18  is immersed in the aerobic zone  14 . Alternatively, the membrane filter could be in a downstream zone. The aerobic zone  14  is preferably provided with an aerator  20  in the form of a diffuser attached to air supply  21  situated below the membrane filter  18  which both provides air to the aerobic zone  14  and helps purge the membrane filter  18  of adhering solids. A recycle conduit  22  is provided with an inlet in the aerobic zone  14  and an outlet near an inlet  24  to the anaerobic zone  12  for recycling return activated sludge. A mixer  25  is preferably provided to maintain the mixed liquor suspended solids in suspension. A pump  26  may be provided for promoting the return activated sludge recirculation. A chemical supply  26  is provided in communication with the aerobic zone  14  for providing a chemical suitable for precipitating phosphates in the aerobic zone  14 . Suitable chemicals for phosphate precipitation include soluble salts such as ferrous/ferric chloride or aluminum sulfate.  
         [0015]     In use, wastewater is provided through the inlet  24  to the anaerobic zone  12 . In the anaerobic zone  12 , phosphorous-accumulating organisms release phosphorous into the wastewater as they accumulate fatty acids used for cell growth. In the aerobic zone  14 , the phosphorous-accumulating organisms metabolize the accumulated organic matter and accumulate the released phosphorous into cells as part of the growth process. In this manner, phosphorous is removed from the effluent filtered through the membrane filter  18 . In order to achieve low concentrations of phosphorous (e.g., less than 0.25 mg/L), the apparatus  14  provides for addition of soluble salts to the aerobic zone for forming insoluble phosphate metal salts from any residual phosphorous. These salts are added from the supply  26  as needed to achieve the low concentrations of phosphorous effluent results. Precipitated phosphorous and activated sludge are separated from the effluent by the membrane filter  18 . Return activated sludge (which may include some insoluble phosphate metal salts and soluble phosphorous precipitating salts) is recycled through conduit  22  to the anaerobic zone  12 . A quantity of waste activated sludge, including insoluble phosphate salts, is removed from the aerobic tank through conduit  28 .  
         [0016]     Use of the anaerobic zone  12  in combination with the aerobic zone  14  ensures that adequate phosphorous will be available in the aerobic zone  14  for the digestion of BOD, COD and other nutrients from the wastewater  24 . While the phosphorous precipitating salts lower the phosphorous content in the aerobic zone  14 , the anaerobic process in zone  12  acts as a phosphorous buffer by continuing to release phosphorous into the aerobic zone  14 . The anaerobic zone  12  also functions as a buffer against insoluble salts which are recycled as part of the return activated sludge through conduit  22  to the anaerobic zone  12 .  
         [0017]      FIG. 2  is a second embodiment of a membrane bioreactor apparatus  30  producing a low phosphorous effluent. Like elements of  FIG. 2  will have the same reference numbers used above in describing  FIG. 1 . The primary difference between the apparatus  30  of  FIG. 2  and the apparatus  10  of  FIG. 1  is the inclusion of an upstream anoxic zone  32  between the anaerobic zone  12  and the downstream aerobic zone  14 . The anoxic zone  32  functions as a de-nitrification zone wherein nitrate/nitrite nitrogen in the effluent is converted to elemental nitrogen. There is substantially no dissolved oxygen present in the anoxic zone  32 . The conversion of the NO x  to elemental nitrogen occurs because the micro organisms in the anoxic zone  32  seek oxygen through the reduction of NO x  compounds to nitrogen gas. The nitrogen gas is then able to escape the liquid phase to the atmosphere. A nitrogen rich recycle conduit (“NRCY”)  34  recycles return activated sludge from the downstream aerobic zone  14  to near the inlet of the anoxic zone  32 . An anoxic recycle conduit  36  recycles de-nitrified mixed liquor from near the outlet of the anoxic zone  32  to near the inlet of the anaerobic zone  12 . Alternatively, as shown by a phantom line, a conduit  38  may be provided to recycle mixed liquor suspended solids directly from the aerobic zone  14  to near the inlet of the anaerobic zone  12 . The embodiment illustrated in  FIG. 2  performs the same phosphorous removal and phosphorous buffering functions as the embodiment in  FIG. 1 , but includes the upstream anoxic zone  32  for the promotion of denitrification.  
         [0018]      FIG. 3  is a third embodiment of a membrane bioreactor apparatus  40  for treating wastewater to produce a low phosphorous effluent. Identical reference numbers will be used for identical elements in the third embodiment  40  as used in the embodiments illustrated in  FIGS. 1 and 2 . The third embodiment  40  includes an upstream aerobic zone  42  and a downstream anoxic zone  44  between the upstream anoxic zone  32  and the downstream aerobic zone  40  of the apparatus  30  illustrated in  FIG. 2 . The upstream aerobic zone  42  and downstream anoxic zone  44  are provided for enhanced nutrient removal. The third embodiment  40  includes a conduit  46  for recycling return activated sludge from the downstream aerobic zone  14  to near the inlet of the upstream aerobic zone  44 . A NRCY conduit  48  recycles NRCY from near the outlet of the upstream anaerobic zone  42  to near then inlet of the upstream anoxic zone  32 . Finally, an ARCY conduit  50  recycles activated sludge from near the outlet of the upstream anoxic zone  32  to near then inlet of the anaerobic zone  12 .  
         [0019]      FIG. 4  illustrates a fourth embodiment of a membrane bioreactor apparatus  56  for treating wastewater to produce a low phosphorous effluent. This embodiment is similar to the third embodiment  40  illustrated in  FIG. 3  and again identical reference are used for identical elements. The principal difference between the third embodiment  40  and the fourth embodiment  56  is provision of a de-aeration zone  58  upstream of the anaerobic zone  12  and a RAS recycle conduit  60  recycling return activated sludge from the aerobic zone  14  to the de-aeration zone  58 . Alternatively, the anaerobic zone  12  could be made large and the RAS recycle conduit  60  could flow directly to the anaerobic zone  12 .  
       EXAMPLE  
       [0020]     A pilot testing program was conducted to test the nutrient removal capability of the third embodiment of the membrane bioreactor (“MBR”) and method for treating wastewater described with reference to  FIG. 3  above. The pilot testing program included a number of objectives. The overall objective of the program was to determine whether the MBR could achieve the effluent limitation goals shown in Table 1.  
                         TABLE 1                           Treatment Goals            Parameter   Treatment Goals               Biochemical Oxygen Demand (BOD), mg/L   Not applicable (NA)       Total Suspended Solids (TSS), mg/L   1       Chemical Oxygen Demand (COD), mg/L   10.0 a         Total Nitrogen (TN), mg/L   3.0-8.0       Total Phosphorus (TP), mg/L   0.1       Turbidity   0.5 NTU       (Nephelometric Turbidity Units, NTU)       Coliform (per 100 mL)   &lt;2/100 mL                   a COD limit to be achieved through the post-treatment of MBR effluent with activated carbon.             
 
         [0021]     The wastewater used for the pilot testing program consisted of municipal sewage from a community. The wastewater sources were mostly domestic in nature (i.e., few industrial inputs). Table 2 summarizes the typical characteristics of the raw wastewater source and for the effluent from the primary treatment facility that was used as the input to the membrane bioreactor pilot.  
                                           TABLE 2                           Typical Wastewater Characteristics                    Primary           Raw   Effluent       Parameter   Wastewater   (Pilot Influent)                    Biochemical Oxygen Demand   273   133       (BOD), mg/L       Total Suspended Solids (TSS), mg/L   211   82       Chemical Oxygen Demand (COD), mg/L   Not   283           Available       Total Kjeldahl Nitrogen (TKN), mg/L   28   36       Ammonia (NH 3 —N), mg/L   22   21       Total Phosphorus (TP), mg/L   6   5.5       BOD/TKN Ratio   10.2   3.8                  
 
         [0022]     With regard to phosphorus removal, the pilot testing equipment was operated in the configuration shown in  FIG. 3  of the application for 112 days. To maximize nitrogen removal, the system was operated with methanol addition (a supplemental carbon source) during the entire period. The methanol dose averaged about 52 mg/L during the testing period.  
         [0023]     Alum was not added during days 1-51 to determine the base amount of phosphorus that could be removed by biological activity alone. Alum was added during days 52-112 to maximize the amount of phosphorus removal within the reactor and to verify that the process could achieve an effluent concentration of 0.1 mg/L for total phosphorus (TP). The alum dose varied between 23 and 73 mg/L, and the average alum dose was 43 mg/L.  
         [0024]     Table 3 summarizes the results from the pilot testing program for the periods described above. Included on Table 3 are the average, maximum, and minimum values for effluent total nitrogen, total phosphorus, and chemical oxygen demand (COD), another important indicator of treatment efficacy, for the membrane bioreactor portion of the pilot. COD was measured in place of biochemical oxygen demand (BOD), which was removed to near the detection limit.  
         [0025]     To simplify the presentation, the pilot data is shown for the entire operating period and includes variations in chemical doses, recycle flow rates, etc. (not shown) that result in subsequent treatment variations. In terms of phosphorus removal, the data clearly shows that low levels of effluent phosphorus can be achieved. Overall, the pilot equipment was able to meet all of the goals listed in Table 1. Post-treatment of the MBR effluent with activated carbon was needed to meet the effluent COD requirement of 10 mg/L.  
         [0026]     Without alum addition, the effluent TP averaged 2.76 mg/L with a minimum value of 1.88 mg/L. These values are lower than conventional plants (typically about 4 mg/L) without biological or chemical phosphorus removal capabilities.  
         [0027]     With alum addition, the effluent TP averaged 0.24 mg/L with a minimum value of 0.02 mg/L. The operating period included several days during which effluent TP was less than the 0.1 mg/L treatment goal listed on Table 1. The treatment goals for BOD, TSS, COD, TN, TP, turbidity and coliform were achieved during the several days of extremely low TP (less than 0.1 mg/L).  
                                                                                             TABLE 3                           Effluent Data from the Pilot Testing Program                    Effluent   Effluent   Effluent               Total   Total   Chemical               Nitrogen   Phosphorus   Oxygen               (TN)   (TP)   Demand (COD)       Time Period       (mg/L)   (mg/L)   (mg/L)                    5-Stage Operation with Methanol Addition (without Alum Addition)            Days 1-51   Average   9.1   2.76   18.7           Maximum   24.6   3.94   38.1           Minimum   0.8   1.88   10.0            5-Stage Operation with Methanol and Alum Addition            Days 52-112   Average   3.59   0.24   14.6           Maximum   13.5   1.73   18.2           Minimum   1.0   0.02   11.6