Abstract:
In an activated sludge process for treating wastewater, waste activated sludge is directed to a digester and subjected to sequential aerobic, anoxic and anaerobic treatment. Internal recycling of sludge may be carried out. The oxidation reduction potential during the anoxic and anaerobic cycles is carefully managed so that there is a significant reduction in the quantity of sludge that must be disposed of.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    None. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates generally to the field of wastewater treatment and more particularly to an activated sludge treatment process in which waste activated sludge is treated in an improved manner. 
       BACKGROUND OF THE INVENTION 
       [0003]    Wastewater has long been treated using activated sludge processes in which the influent is biologically treated in a basin to produce a mixed liquor. Clarification or another technique is used to separate the mixed liquor into a clear liquid effluent and a solid biomass which takes the form of activated sludge. The effluent is discharged, whereas part of the activated sludge is returned to the activated sludge basin in order to maintain a sufficient bacteria concentration for effective treatment of the influent. 
         [0004]    The part of the activated sludge that is not return activated sludge is referred to as waste activated sludge. The waste activated sludge must be removed from the system and disposed of by incineration, deposit in a landfill, or in some other way such as use as fertilizer. Handling and disposal of waste activated sludge is a significant problem that makes it highly desirable to minimize the quantity of the waste activated sludge that is generated in a treatment plant. While various processes have been developed attempting to reduce the amount of waste activated sludge, they have not been wholly satisfactory. 
         [0005]    Such processes are generally either a form of biological digestion or a mechanical process in which the cell tissue is physically ruptured. Mechanical methods have included high frequency sonic cell disruption and high pressure/shear cell destruction. Both of these methods require high capital expenditures and they are both subject to high energy requirements. 
         [0006]    The most common biological process that attempts to achieve a low waste activated sludge yield involves use of a reactor known as an interchange reactor. The interchange reactor modifies the biological population spectrum from the activated sludge basin so that different organisms predominate. Treated sludge from the interchange reactor is added to the influent and introduced back into the activated sludge basin. Even though processes using interchange reactors can achieve improvement, there are significant drawbacks including the need for additional equipment, plumbing, pumps and instrumentation that call into question whether the benefits outweigh the added cost and complexity. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with the present invention, both aerobic and anaerobic digestion techniques are used in a unique process that reduces the quantity of waste activated sludge by increasing the solids destruction compared to what is achieved by either aerobic or anaerobic methods alone. The waste activated sludge may be screened, alone or together with the return activated sludge. Aeration of the waste activated sludge is carried out in a digester and may be interrupted periodically for solids settling and the decanting of clear liquid. 
         [0008]    Either in the same zone or a different zone or zones of the digester, subsequent anoxic and anaerobic conditions are cycled through in sequence, optionally followed by aeration. The anoxic treatment denitrifies nitrate that might be present due to the aerobic treatment. The anaerobic treatment results in liberation of biodegradable intracellular material and ammonia nitrogen as well as moderate acidification. Some of the aerobic cell tissue is converted to anaerobic organisms which yield a much reduced mass to further decrease the net amount of sludge. The aerobic and anaerobic treatments can be carefully managed using oxidation reduction potential measurements to create the optimum conditions for achieving minimum sludge quantities and other desired benefits. 
         [0009]    Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0010]    In the accompanying drawings, which form a part of the specification and are to be read in conjunction therewith in which like reference numerals are used to indicate like or similar parts in the various views: 
           [0011]      FIG. 1  is a schematic diagram of a method and apparatus for treating waste activated sludge in accordance with one embodiment of the present invention; 
           [0012]      FIG. 2  is a schematic diagram of a method and apparatus for treating waste activated sludge in accordance with a modified embodiment of the invention; and 
           [0013]      FIG. 3  is a schematic diagram of a method and apparatus for treating waste activated sludge in accordance with another modified embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Referring now to the drawings in more detail and initially to  FIG. 1 , numeral  10  designates an activated sludge basin which receives wastewater influent along an influent line  12 . In the basin  10 , the wastewater is treated in accordance with conventional or advanced multi-stage activated sludge treatment processes. Mixed liquor that results from the process may be delivered to a clarification means  14  (such as a sedimentation basin, flotation basin or membrane) in which the mixed liquor is separated into a liquid effluent which is discharged on an effluent line  16  and activated sludge which is discharged from the clarification means  14  on line  18 . 
         [0015]    The sludge on line  18  may be passed through a screen  20  having screen openings that may range from 0.02 inch to 0.08 inch. The screen material can be wedge wire, woven mesh, perforated plate or any other suitable material. Larger solid materials that are removed by the screen  20  are discharged as trash on line  22 . The sludge that passes through the screen  20  is separated into return activated sludge and waste activated sludge. The return activated sludge is delivered on line  24  to the influent line  12  and is thus recycled into the activated sludge basin  10  to maintain the proper concentration of bacteria needed for the activated sludge process. The waste activated sludge is delivered on line  26  to a digester  28 . It should be noted that only the waste activated sludge may be screened, although it is usually preferred that both the return sludge and the waste sludge be passed through the screen  20 . 
         [0016]    The digester  28  may be divided into two or three stages such as zones  30  and  32  which may be separated by a partition  34  or in some other manner. The waste activated sludge that is delivered to the digester  28  on line  26  is first treated aerobically in zone  30 . Conventional aerators (not shown) may operate in zone  30  to provide the aeration. Preferably, the dissolved oxygen concentration in zone  30  is maintained at a level equal to or greater than 0.5 mg/l for the majority of the time during the aeration stage of the process. The aeration in zone  30  can be interrupted periodically to allow for solids settling for thickening and concentration of the sludge, along with decanting. Clear supernatant water that is decanted in zone  30  may be delivered on line  36  to the influent line  12  and then into the activated sludge basin  10 . After an appropriate period of time (which may range from a few hours to several days) of retention time in the aerobic stage  30 , the sludge is then transferred into zone  32  of the digester  28 . This transfer of the waste activated sludge may be carried out either on a batch basis or continuously. 
         [0017]    In zone  32 , the waste activated sludge is subjected sequentially to treatment under anoxic conditions, then under anaerobic conditions, and then optionally under aerobic conditions. The principal purpose of the anoxic cycle of treatment is to denitrify any nitrate that may be transferred from zone  30  into zone  32 . During the anoxic period of treatment, the oxidation reduction potential is maintained in a range of about +50 MV to about −200 MV. The anoxic portion of treatment can be a separate zone. 
         [0018]    In the subsequent anaerobic treatment cycle, the oxidation reduction potential is allowed to drop below about −200 MV and is preferably maintained in the range of approximately −200 MV to approximately −400 MV. During the anaerobic treatment, lyses and hydrolysis of cell tissue occurs, liberating biodegradable intracellular material. These reactions also liberate ammonia nitrogen. Although moderate acidification also occurs, no significant reduced sulfur compounds are produced. Some of the aerobic cell tissue is converted to anaerobic organisms which have a much lower mass yield in order to further reduce the net quantity of sludge. In addition, the anaerobic zone promotes phosphorus release by phosphorus accumulating organisms (PAOs) and encourages subsequent luxury phosphorus uptake upon return to the aerobic zone of the digester, resulting in a supernatant low in phosphorus, which is important for biological nutrient removal (BNR) processes. After the oxidation reduction potential reaches the selected lower limit (−400 MV, for example), the sludge in zone  32  may be aerated to minimize the production of objectionable odors. 
         [0019]    A recycle line  38  is provided to recycle sludge from zone  32  back to zone  30 . The recycled sludge which passes through line  38  returns soluble biodegradable cell material and ammonia nitrogen to the aerobic zone  30  where nitrification and further degradation take place. Sludge may be removed from the digester  28  and discharged from the treatment facility from either zone  30  or  32 , such as along line  40 . 
         [0020]    Control of the aerobic/anoxic/anaerobic conditions can be either manual or automatic. The infeed, settling, decanting, intradigester sludge transfer and waste sludge discharge can be controlled either manually or automatically as well. Along with effective screening of the sludge, the amount of waste activated sludge that must be handled and disposed of can be reduced by 50% to 80% compared to conventional practice. 
         [0021]      FIG. 2  depicts an alternative embodiment of the invention in which most of the process is identical to the process of  FIG. 1 . The components of the system of  FIG. 2  are for the most part the same as in the embodiment of  FIG. 1  and are identified by the same reference numerals. 
         [0022]    The principal difference in the embodiment of  FIG. 2  is that a digester  128  is used that has a single stage rather than being partitioned into two separate zones as is the case with digester  28  in the embodiment of  FIG. 1 . During operation of the system shown in  FIG. 2 , the digester  128  is operated so that the conditions to which the waste activated sludge is subjected cycle sequentially through aerobic conditions, anoxic conditions, and anaerobic conditions. During these stages of the treatment of the waste activated sludge, the dissolved oxygen concentration is preferably about the same as in the case of the  FIG. 1  embodiment during the aerobic treatment, and the oxidation reduction potential ranges set forth for the  FIG. 1  embodiment are maintained in the treatment system of  FIG. 2  for the anoxic and anaerobic stages of the treatment. 
         [0023]    Because the embodiment of  FIG. 2  uses a single stage digester  128 , the reaction kinetics are somewhat slower than in the embodiment of  FIG. 1  so the total tank volume is larger. The sludge quantity in the embodiment of  FIG. 2  may be reduced by approximately 50%-70% compared to conventional practice. 
         [0024]      FIG. 3  depicts another embodiment of the invention in which most of the process is identical to the processes of  FIGS. 1 and 2 . The components of the system of  FIG. 3  are for the most part the same as in the embodiments of  FIGS. 1 and 2  and are identified by the same reference numerals. 
         [0025]    The principal difference in the embodiment of  FIG. 3  is that a digester  228  is used that has three separate stages, including an initial aerobic stage or zone  231 , an anoxic stage or zone  233 , and an anaerobic stage or zone  235 , each of which is separate from the others. Zones  231  and  233  are separated by a partition  234   a , and zone  233  is separated from zone  235  by another partition  234   b . A recycle line  238   a  provides for recycling of sludge from zone  235  to zone  233 . Sludge from either or both of zones  233  and  235  may be recycled to zone  231  on another recycle line  238   b.    
         [0026]    During operation of the system shown in  FIG. 3 , the digester  228  is operated so that the conditions to which the waste activated sludge is subjected cycle sequentially through aerobic conditions in zone  231 , anoxic conditions in zone  233 , and anaerobic conditions in zone  235 . During these stages of the treatment of the waste activated sludge, the dissolved oxygen concentration is preferably about the same as in the case of the  FIG. 1  embodiment during the aerobic treatment, and the oxidation reduction potential ranges set forth for the  FIG. 1  embodiment are maintained in the treatment system of  FIG. 3  for the anoxic and anaerobic stages of the treatment in zones  233  and  235 , respectively. The embodiment of  FIG. 3  has the advantage that the environment in each zone is maintained rather than being cycled through different conditions. 
         [0027]    From the foregoing it will be seen that this invention is one well adapted to attain all ends and objects hereinabove set forth together with the other advantages which are obvious and which are inherent to the structure. 
         [0028]    It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. 
         [0029]    Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative, and not in a limiting sense.