Abstract:
A wastewater treatment process providing nitrogen, phosphorus, biochemical oxygen demand (BOD) and suspended solids removal comprises the steps of:  
     passing wastewater containing ammonia nitrogen, phosphate, BOD and suspended solids, said wastewater mixed with recycled activated sludge from a subsequent step, into a first anoxic zone therein reducing nitrate nitrogen from the recycled sludge to molecular nitrogen;  
     passing effluent from the first anoxic zone to a first aerobic zone therein oxidizing at least a portion of the BOD and oxidizing at least a portion of the ammonia nitrogen to nitrate nitrogen;  
     recycling a portion of the contents at the end of the first aerobic zone back to the first anoxic zone;  
     passing the effluent of the first aerobic zone to a second anoxic zone;  
     introducing volatile fatty acid such as acetic acid into the second anoxic zone therein releasing phosphorus into a liquid phase;  
     passing effluent from the second anoxic zone including the volatile fatty acid to a second aerobic zone therein substantially absorbing phosphorus into biomass and removing and/or oxidizing ammonia nitrogen;  
     passing effluent from the second anoxic zone to a final settling zone therein separating:  
     (i) a purified wastewater having decreased nitrogen, phosphorus, BOD and suspended solids and  
     (ii) a sludge containing suspended solids, phosphate and BOD; and recycling at least a portion of the sludge (ii) to the first anoxic zone.  
     In another embodiment, at least a portion of the sludge (ii) is also recycled to the second anoxic zone. In yet another embodiment, the second anoxic zone is divided into a first section and a second section. In the first section, anoxic conditions are established and in the second section, volatile fatty acid is added after anoxic conditions have been established.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates broadly to process improvements in nutrient removal wastewater treatment processes. More specifically, this invention relates to methods for improving the efficiency of nitrogen and phosphorous removal. Even more specifically, this invention relates to methods for integrating an activated sludge process with methods for improving nutrient removal in wastewater treatment processes. In addition this invention relates to the conduct of such processes in order to improve the efficiency of removal of suspended solids and biochemical oxygen demand.  
         BACKGROUND OF THE INVENTION  
         [0002]    There are basically two processes for the biological or secondary treatment of wastewater, including a slurry type process and a process utilizing a biological fixed-film. In both processes, raw wastewater is settled in a primary settling zone before effluent is passed to a secondary or biological oxidation zone. The settling zone removes suspended solids resulting in a reduction in particulate Biochemical Oxygen Demand (BOD), nitrogen and phosphorous.  
           [0003]    One of the biological oxidation processes is characterized by the slurry type of process in which suspended solids in the mixed liquor (MLSS) are aerated and mixed in an aeration tank which can be either a complete mix reactor, or more usually, a plug flow reactor. After the aeration tank, the treated wastewater with its suspended solids passes to a final settling tank where the suspended solids are settled and a portion of the settled suspended solids is recycled back to the entrance to the aeration tank. Another portion of the settled suspended solids is removed from the system as waste activated sludge (WAS). The process can also be done on a batch basis in a sequencing batch reactor (SBR).  
           [0004]    The second type of secondary biological treatment is characterized by a biological fixed film. One type of biological fixed-film is a trickling filter which passes settled wastewater over a static rock or plastic media. The effluent from the trickling filter media containing treated wastewater and suspended solids is passed to a final settling tank where a portion of the clarified effluent is recycled back to the feed to the trickling filter. The settled suspended solids in the final settling tank are removed from the liquid system. The other type of fixed-film treatment is the rotating biological contactor (RBC) in which the fixed-film is attached to a rotating disk in a tank containing the wastewater being treated. This process usually is divided into three or four stages of rotating disks. The effluent containing treated wastewater and suspended solids is passed to a final settling tank separating the clarified effluent from the settled suspended solids. The settled suspended solids in the final settling tank are removed from the liquid system.  
           [0005]    An Activated Sludge process treats settled wastewater by the use of aeration tanks under aerobic conditions where dissolved oxygen is present at a minimum of about 2 mg/l. This aeration process converts soluble and colloidal organic matter into new biomass and carbon dioxide by oxidation. The biomass is settled in a final settling tank or clarifier and the settled biomass or active sludge is recycled back to the entrance to the aeration tank where it is mixed with the settled wastewater. The biomass is also reduced by endogenous respiration.  
           [0006]    Many variations of the activated sludge process are possible. The one of interest to this invention is the MLE Activated Sludge Process (Modified Ludzac-Ettinger Process). The original process, LE, was the conversion of the initial part of the aeration tank to an anoxic zone (no dissolved oxygen but some nitrate) where return or active sludge is first mixed with settled wastewater in a zone without air or an anoxic zone. In this modification, any oxidation of ammonia nitrogen to nitrate by the aerobic section of the tank is exposed to the anoxic zone where nitrate nitrogen is reduced to nitrogen gas by denitrification. To increase the efficiency of nitrate removal, an inside recycle from the end of the aeration tank back to the entrance to the aeration tank or anoxic zone is added. This process flow is named the MLE Process.  
           [0007]    The activated sludge process was later improved to remove not only nitrogen but also phosphorous. If an initial zone is anaerobic (no oxygen and no nitrate), phosphorous is released to the bulk liquid from the biomass. Following the anaerobic zone, an aerobic zone removes the released P from the liquid phase back into biomass. Many variations of this process have been developed including the Bardenpho and AO Processes.  
           [0008]    These variations of the activated sludge process for nutrient removal present some difficulties. For example, sludge settling is a problem in the final settling tank as well as excess foaming in the aeration tank. In addition, there is a problem of insufficient removal of nitrogen and phosphorous so that additional steps are required. This includes the addition of chemicals such as alum or ferric chloride for phosphorus removal and chemicals such as methanol for denitrification. These additions result in excess sludge in phosphorus removal and excess tankage in nitrogen removal. The operation of these variations for nutrient removal also cause control problems such as the inability to match methanol with changing nitrate concentration resulting in overdosing of the methanol.  
           [0009]    Fixed-film systems such as the trickling filter and RBC can contribute to biological nutrient removal only by oxidation of ammonia nitrogen to nitrate nitrogen. If nitrate and phosphorous are to be removed, extra tankage and/or chemicals are required. The general trend has been either to abandon the fixed-film process and construct an activated sludge nutrient removal plant, or to add an activated sludge nutrient removal system following the fixed-film process. These add excessive costs and extra operational problems for both options.  
         OBJECTS OF THE INVENTION  
         [0010]    It is thus the primary object of this invention to improve the efficiency of activated sludge nutrient removal processes.  
           [0011]    It is a further and related object of this invention to improve the activated sludge MLE process for nutrient removal.  
           [0012]    It is still a further and related object of this invention is to improve the removal of BOD, SS and turbidity.  
           [0013]    It is still a further object of this invention to provide process modifications for future and existing activated sludge plants which enable such plants to remain as activated sludge plants with nutrient removal as a integral part of the process.  
         SUMMARY OF THE INVENTION  
         [0014]    This invention broadly includes methods for increasing nutrient removal in an activated sludge process. The invention broadly resides in a wastewater treatment process which includes treating wastewater with an activated sludge process including a two part anoxic zone, a two part aerobic zone and a settling zone with the recycle of settled biomass back to a first stage anoxic zone. In particular, the process of the invention involves recycle of settled biomass back to a first stage anoxic zone, followed by a first aerobic zone, and then to a second anoxic zone with addition of volatile fatty acid such as acetic acid, and then followed by a second aerobic zone.  
           [0015]    In embodiments of the invention, a wastewater treatment process providing nitrogen, phosphorus, biochemical oxygen demand (BOD) and suspended solids removal comprises the steps of:  
           [0016]    passing wastewater containing ammonia nitrogen, phosphate, BOD and suspended solids, said wastewater mixed with recycled activated sludge from a subsequent step, into a first anoxic zone therein reducing nitrate nitrogen from the recycled sludge to molecular nitrogen;  
           [0017]    passing effluent from the first anoxic zone to a first aerobic zone therein oxidizing at least a portion of the BOD and oxidizing at least a portion of the ammonia nitrogen to nitrate nitrogen;  
           [0018]    passing the effluent of the first aerobic zone to a second anoxic zone;  
           [0019]    introducing volatile fatty acid such as acetic acid into the second anoxic zone therein releasing phosphorus into a liquid phase;  
           [0020]    passing effluent from the second anoxic zone including the volatile fatty acid to a second aerobic zone therein substantially absorbing phosphorus into biomass and removing and/or oxidizing ammonia nitrogen;  
           [0021]    passing effluent from the second anoxic zone to a final settling zone therein separating:  
           [0022]    (i) a purified wastewater having decreased nitrogen, phosphorus, BOD and suspended solids and  
           [0023]    (ii) a sludge containing suspended solids, phosphate and BOD; and recycling at least a portion of the sludge (ii) to the first anoxic zone.  
           [0024]    In one embodiment, a portion of the contents at the end of the first aerobic zone is recycled to the first anoxic zone. In another embodiment, at least a portion of the sludge (ii) is also recycled to the second anoxic zone. In yet another embodiment, the second anoxic zone is divided into a first section and a second section. In the first section, anoxic conditions are established and in the second section, volatile fatty acid is added after anoxic conditions have been established.  
           [0025]    The process can be used in existing plants or in new plants to substantially remove N and P. The process can be adapted for the unsettled affluent of fixed-film wastewater treatment processes such as rotating biological contractors (RBC) or trickling filters. The process of the invention results in substantial and significant improvement in the reduction of N and P levels in an economical manner. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    FIGS.  1 - 14  are schematic process diagrams of preferred processes incorporating the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    The invention relates broadly to wastewater treatment processes and more specifically to such processes which employ activated sludge as a nutrient removal process. The invention can be used with domestic, agricultural and/ or industrial wastewater. Certain types of industrial wastes are difficult to treat biologically because they lack certain nutrients such as nitrogen and phosphorous. In order to biologically treat such wastes, nutrients such as nitrogen and phosphorus may be added to make up for their limited concentration or complete absence. The treatment of paper wastes is an example where available N and P are added for biological activated sludge to maintain ratios of a part N per 20 parts BOD and 1 part P per 75 parts BOD.  
         [0028]    It has been found that nitrogen and /or phosphorous removal can be facilitated by a process wherein, with respect to N removal, activated sludge oxidizes ammonia nitrogen to nitrate nitrogen and a first anoxic zone reduces nitrate nitrogen to molecular nitrogen gas. With respect to P removal, volatile acid is supplied to a second anoxic zone to release P into the liquid phase, followed by an aerobic zone to incorporate the P into the biomass from the liquid phase.  
         [0029]    In a preferred embodiment of the process, as depicted in FIG. 1, raw wastewater enters a primary settling zone  5  where some solids are separated from the wastewater. Settled wastewater from the primary settling zone  5  containing suspended solids, BOD, N and P is conveyed to the first anoxic zone  10  via line  7  where the settled wastewater containing suspended solids is mixed with settled sludge from final settling zone  30  va lines  32  and  7 . The first anoxic zone effluent is passed via line  14  to a first aerobic zone  15  where the BOD is converted to suspended solids and carbon dioxide and a portion of the ammonia nitrogen is converted to nitrate nitrogen. Nitrate formed in the first and second aerobic zones is reduced to nitrogen gas in the first anoxic zone  10 . Nitrogen conversion from ammonia to nitrate is referred to as nitrification. In order for nitrification to occur by microbial oxidation, the BOD must be significantly decreased, such as to a level of 14 mg/l or less. This is because autotrophic bacteria such as the species nitrosommonas and nitrobacter are responsible for the conversion of ammonia nitrogen to nitrate nitrogen. Initially, the activity of the heterotrophic bacteria such as bacillus predominate in the biological oxidation zone  15  as these heterotrophs metabolyze BOD. This heterotrophic activity successfully limits the activity of the nitrifying autotrophs until the BOD has decreased to a sufficiently low level that heterotrophic activity is limited and autotrophic activity can dominate. The same effect, i.e., autotrophic dominance would inherently be achieved with wastewater that started with sufficiently low BOD, such as 14 mg/l or less.  
         [0030]    In one embodiment, at the end of the first aerobic zone  15 , a portion of the contents from the first aerobic zone  15  in an inner recycle  16  can be recycled back to the first anoxic zone  10  via lines  17  and  7 .  
         [0031]    The effluent from the first aerobic zone  15  is passed via line  18  to second anoxic zone  20  as is volatile acid  21  via line  19 . Bacteria in the presence of the volatile acids and under anoxic conditions, will release phosphate from the sludge to the liquid in the second anoxic zone  20 . The effluent from second anoxic zone  20  is passed via  22  to a second aerobic zone  25 . In aerobic zone  25 , bacteria rapidly take up phosphate in the liquid phase, acting to remove not only the phosphate released in the second anoxic zone  20  but also phosphate content from line  7 . Effluent from aerobic zone  25  is passed via line  27  to final settling zone  30 . The settled sludge containing suspended solids (return activated sludge  31 ) is recycled via lines  32  and  7  to the first anoxic zone  10 . Excess settled sludge (waste activated sludge  33 ) is removed from the system via conduit  34 . Purified wastewater (final effluent  35 ) having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  30  via line  36 .  
         [0032]    As used throughout, the following terms have the following meanings:  
         [0033]    By “main aerobic biological zone” is meant any of the known aerobic biological secondary wasterwater treatments such as the activated sludge process and its various modifications. Also included are the fixed film systems as RBC and trickling filter and slurry systems as stabilization ponds, lagoons and ditch oxidation processes. Such aerobic biological oxidation zones include any operation wherein the major thrust is the reduction of BOD by aerobic biological treatment.  
         [0034]    By “aerobic conditions” as in the aerobic or aeration zone are meant aeration operating conditions as may be achieved in known process equipment including aerators, mixers and the like. The addition of air or oxygen creates aerobic conditions which means containing a finite amount of dissolved oxygen (DO). Preferred aerobic conditions are those wherein the DO is greater than one mg/l.  
         [0035]    By “PENReP Process” is meant a tertiary process designed to follow secondary wastewater systems such as activated sludge, trickling filter or rotating biological contractors (RBC). The first anoxic zone and first aerobic zone represent an activated sludge process in the MLE mode and is a main biological oxidation zone or (MBOZ) which precedes the PENReP Process represented by a second anoxic zone and a second aerobic zone. The actual location of the PENReP Process at the end of the activated sludge aeration tank would depend on the activated sludge design and operating hydraulic retention time (HRT). The HRT of the PENReP Process is independent of the HRT of the activated sludge process.  
         [0036]    By “anoxic conditions” are meant conditions in which no DO is present in the bulk liquid but chemical bound oxygen as in nitrate is available for microbial metabolism. Air or oxygen is not usually added.  
         [0037]    By “anaerobic conditions” are meant conditions wherein no DO nor nitrate is present in the bulk liquid so that only anaerobic microorganisms can grow. Air or oxygen is not usually added.  
         [0038]    By “anoxic/anaerobic conditions” are meant conditions which are at least anoxic (no DO) but there may be or may not be combined oxygen present as nitrate. Air or oxygen is not usually added.  
         [0039]    The term “settling” as used herein refers broadly to any solids separation process known in the art, e.g., filtering and centrifuging.  
         [0040]    Th e term “volatile acid” as used herein mean s water soluble fatty acids that can be distilled at atmospheric pressure and includes soluble fatty acids of up to 6 carbon atoms. It also includes the water soluble carboxylates of the volatile acids.  
         [0041]    The term “methanol” as used herein means a biological oxygen consuming organic such as methyl alcohol (or methanol) which can reduce nitrate-nitrogen to gaseous nitrogen in anoxic systems.  
         [0042]    The term “SVI” as used herein is the Sludge Volume Index which represents the settleability of the activated sludge (or any biological sludge) suspended solids. The SVI represents the settling value in ml of the activated sludge in a graduated cylinder for a 30 minute duration that is divided by the suspended solids (as mg/l) in the activated sludge. The resulting number is SVI as mg/l that ranges from about 60 to about 250.  
         [0043]    The term “ECP” as used herein is Extracellular Polymer which represents the polymeric material on the exterior of the bacteria in a biological sludge that is an aid in the settling of the biological sludge.  
         [0044]    The term “COD” as used herein is Chemical Oxygen Demand which is a chemical oxidation step of wastewater with acid and dichromate to oxidize organic material at high temperature.  
         [0045]    The term “SCOD” as used herein is Soluble COD which represents the soluble portion of a wastewater as defined by filtration through a membrane filter with COD of the filtrate.  
         [0046]    The type of reactor used in any of the zones described in this invention (aerobic zone, anoxic zone, etc.) might be classified as biological slurry or fixed-film. In addition the two types can be combined as a slurry/fixed-film reactor. An example of the slurry reactor is the aeration tank as used in the activated sludge process. An example of a fixed-film reactor is a trickling filter or a rotating biological contactor (RBC). Combined or hybrid slurry/fixed-film reactors can be of various types such as a slurry system with a stationary or mobile fixed-film. An example of a stationary fixed-film system in an activated sludge aeration tank would be a RBC unit while an example of a mobile fixed-film system would be a mobile media suspended in the activated sludge aeration tank. Other examples are slurry feed to a fixed-film reactor or a settled suspended biological solids feed to a fixed film reactor.  
         [0047]    In preferred embodiments of the process of FIG. 1, the wastewater supplied to the first anoxic zone may first be passed through a primary solids separation zone wherein a portion of the BOD and suspended solids is removed. The process conditions within the several zones described in FIG. 1 are set forth in detail above.  
         [0048]    In a preferred embodiment of the process, as depicted in FIG. 2, effluent from primary settling zone  40  is passed to a first anoxic zone  45  via line  42  wherein the effluent is mixed with settled sludge (return activated sludge  68 ) from final settling zone  65  which is returned via lines  67  and  42 . The first anoxic zone  45  effluent is passed via line  46  to first aerobic zone  50  where carbon is oxidized to carbon dioxide and biomass and a portion of the ammonia nitrogen are oxidized to nitrate. The effluent from the first aerobic zone  50  is passed via line  51  to the second anoxic zone  55 , as is volatile acid  57  via line  58 .  
         [0049]    In one embodiment, at the end of the first aerobic zone  50 , a portion of the contents (inner recycle  53 ) from the first aerobic zone  50  can be recycled back to the first anoxic zone  50  via lines  52  and  42 .  
         [0050]    In second anoxic zone  55 , bacteria in the presence of the volatile acids and under anoxic conditions will release phosphate from the sludge to the liquid. The effluent from the second anoxic zone  55  is passed via line  56  to a second aerobic zone  60 . In aerobic zone  60 , bacteria rapidly take up phosphate in the liquid phase, acting to remove not only the phosphate released in the second anoxic zone  55  but also phosphate content from the line  42 . Effluent from the second aerobic zone  60  is passed via line  61  to final settling zone  65 . Settled sludge (return activated sludge  68 ) containing suspended solids is recycled both via lines  67  and  42  to the first anoxic zone  45  and also via line  69  to the second anoxic zone  55 . Excess settled sludge (waste activated sludge  66 ) is removed from the system via conduit  72 . Purified wastewater (final effluent  70 ) having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  65  via line  71 .  
         [0051]    In a preferred embodiment of the process, as depicted in FIG. 3, settled wastewater  76  containing suspended solids, BOD, N and P is conveyed through line  77  to first anoxic zone  80  wherein it is mixed with settled sludge (return activated sludge  106 ) from final settling zone  105  via line  107 . The first anoxic zone  80  effluent is passed via line  81  to first aerobic zone  85  where carbon is oxidized to carbon dioxide and biomass and a portion of the ammonia nitrogen are oxidized to nitrate.  
         [0052]    In one embodiment, at the end of the first aerobic zone  85 , a portion (inner recycle  87 ) of the contents from first aerobic zone  85  can be recycled back to the first anoxic zone  80  via lines  87  and  77 .  
         [0053]    The effluent from the first aerobic zone  85  is passed via line  86  to a first section  89  of second anoxic zone  90 . The effluent from first section  89  of the second anoxic zone  90  is passed via line  91  to a second section  95  of the second anoxic zone  90  as is volatile acid  83  via line  84 . Bacteria in the presence of the volatile acids and under anoxic conditions will release phosphate from the sludge to the liquid in the second section  95  of the second anoxic zone  90 . The effluent from the second section  95  of the second anoxic zone  90  is passed via  96  to second aerobic zone  100 . In aerobic zone  100 , bacteria rapidly take up the phosphate in the liquid phase, acting to remove not only the phosphate released in the second section of the second anoxic zone  90  but also phosphate content from line  77 . Effluent from the second aerobic zone  100  is passed via line  101  to final settling zone  105 . Settled sludge (return activated sludge  106 ) containing suspended solids is recycled via lines  107  and  77  to first anoxic zone  80 . Excess settled sludge (waste activated sludge  110 ) is removed from the system via conduit  111 . Purified wastewater (final effluent  108 ) having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  105  via line  109 .  
         [0054]    In a preferred embodiment of the process, as depicted in FIG. 4, raw wastewater  103  enters a primary settling tank  112  via line  102  where over half of the solids are separated from the wastewater containing particulate BOD, N and P as a sludge  104  via line  113 . Settled effluent from primary settling zone  112  is passed to the first anoxic zone  115  via line  114  wherein the effluent is mixed with settled sludge as return activated sludge  139  from final settling zone  135  which is returned via lines  136  and  114 . The first anoxic zone  115  effluent is passed via line  116  to first aerobic zone  120  where carbon is oxidized and biomass and a portion of the ammonia nitrogen is oxidized to nitrate The effluent from the first aerobic zone  120  is passed via line  121  to the second anoxic zone  125  as is volatile acid  132  via line  128  and methanol  129  via line  127 .  
         [0055]    In one embodiment at the end of the first aerobic zone  120 , a portion of the contents as inner recycle  123  from the first aerobic zone  120  can be recycled back to the first anoxic zone  115  via lines  122  and  114 .  
         [0056]    In second anoxic zone  125 , bacteria in the presence of the methanol and volatile acid and under anoxic conditions will reduce nitrate to gaseous nitrogen and release phosphate from the sludge to the liquid. The effluent from the second anoxic zone  125  is passed via line  126  to a second aerobic zone  130 . In aerobic zone  130 , bacteria rapidly take up phosphate in the liquid phase, acting to remove not only the phosphate released in the second anoxic zone  125  but also the phosphate content from the line  114 . Effluent from the second aerobic zone  130  is passed via line  131  to final settling zone  135 . Settled sludge as return activated sludge  139  containing suspended solids is recycled both via lines  136  and  114  to the first anoxic zone  115 . Excess settled sludge as waste activated sludge  137  is removed from the system via conduit  138 . Purified wastewater as final effluent  141  having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  135  via line  140 .  
         [0057]    In a preferred embodiment of the process, as depicted in FIG. 5, raw wastewater  144  enters a primary settling tank  145  via line  143  where over half of the solids are separated from the wastewater containing particulate BOD, N and P as a sludge  147  via line  146 . Settled effluent from primary settling zone  145  containing suspended solids, BOD, N and P is passed to the first anoxic zone  150  via line  148  wherein the effluent is mixed with settled sludge as return activated sludge  176  from final settling zone  175  via line  177 . The first anoxic zone  150  effluent is passed via line  151  to first aerobic zone  155  where BOD is converted to suspended solids as biomass and carbon dioxide and a portion of the ammonia nitrogen is oxidized to nitrate.  
         [0058]    In one embodiment at the end of the first aerobic zone  155 , a portion as inner recycle  156  of the contents from first aerobic zone  155  can be recycled back to the first anoxic zone  150  via lines  157  and  148 .  
         [0059]    The effluent from the first aerobic zone  155  is passed via line  156  to a first section  160  of second anoxic zone  164 . The effluent from first section  160  of the second anoxic zone  164  is passed via line  161  to a second section  165  of the second anoxic zone  164  as is volatile acid  162  via line  163 . Bacteria in the presence of the volatile acids and under anoxic conditions will release phosphate from the biomass to the liquid in the second section  165  of the second anoxic zone  164 . The effluent from the second section  165  of the second anoxic zone  164  is passed via line  166  to a second aerobic zone  170 . In aerobic zone  170  bacteria rapidly take up phosphate in the liquid phase, acting to remove not only the phosphate released in the second section  165  of the second anoxic zone  164  but also phosphate content from line  148 . Effluent from the second anoxic zone  164  is passed via line  171  to final settling zone  175 . Settled sludge as return activated sludge  176  containing suspended solids is recycled via lines  177  and  148  to first anoxic zone  150 . Excess settled sludge as waste activated sludge  178  is removed from the system via conduit  179 . Purified wastewater as final effluent  180 ) having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  175  via line  181 .  
         [0060]    In a preferred embodiment of the process, as depicted in FIG. 6, raw wastewater  184  enters a primary settling tank  185  via line  183  where over half of the solids are separated from the wastewater containing particulate BOD, N and P as a sludge  186  via line  187 . Settled effluent from primary settling zone  185  containing suspended solids, BOD, N and P is passed to the first anoxic zone  190  via line  187  wherein the effluent is mixed with settled sludge as return activated sludge  217  from final settling zone  215  via line  218 . The first anoxic zone  190  effluent is passed via line  192  to first aerobic zone  195  where BOD is converted to suspended solids as biomass and carbon dioxide and a portion of the ammonia nitrogen is oxidized to nitrate.  
         [0061]    In one embodiment at the end of the first aerobic zone  195 , a portion as inner recycle  193  of the contents from first aerobic zone  195  can be recycled back to the first anoxic zone  190  via lines  191  and  187 .  
         [0062]    The effluent from the first aerobic zone  195  is passed via line  196  to a first section  200  of second anoxic zone  204  as is methanol  196  via line  197  to reduce nitrate into gaseous nitrogen. The effluent from first section  200  of the second anoxic zone  204  is passed via line  201  to a second section  205  of the second anoxic zone  204  as is volatile acid  198  via line  199 . Bacteria in the presence of the volatile acids and under anoxic conditions will release phosphate from the biomass to the liquid in the second section  205  of the second anoxic zone  204 . The effluent from the second section  205  of the second anoxic zone  204  is passed via line  206  to a second aerobic zone  210 . In aerobic zone  210  bacteria rapidly take up phosphate in the liquid phase, acting to remove not only the phosphate released in the second section  205  of the second anoxic zone  204  but also phosphate content from line  187 . Effluent from the second aerobic zone  210  is passed via line  211  to final settling zone  215 . Settled sludge as return activated sludge  217  containing suspended solids is recycled via lines  218  and  187  to first anoxic zone  190 . Excess settled sludge as waste activated sludge  221  is removed from the system via conduit  222 . Purified wastewater as final effluent  220  having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  215  via line  216 .  
         [0063]    In a preferred embodiment of the process, as depicted in FIG. 7, raw wastewater  223  enters a primary settling tank  225  via line  224  where over half of the solids are separated from the wastewater containing particulate BOD, N and P as a sludge  226  via line  227 . Settled effluent from primary settling zone  225  containing suspended solids, BOD, N and P is passed to the first anoxic zone  230  via line  228  wherein the effluent is mixed with settled sludge as return activated sludge  256  from final settling zone  255  via line  257 . The first anoxic zone  230  effluent is passed via line  231  to first aerobic zone  235  where BOD is converted to suspended solids as biomass and carbon dioxide and a portion of the ammonia nitrogen is oxidized to nitrate.  
         [0064]    In one embodiment at the end of the first aerobic zone  235 , a portion as inner recycle  238  of the contents from first aerobic zone  235  can be recycled back to the first anoxic zone  230  via lines  239  and  228 .  
         [0065]    The effluent from the first aerobic zone  235  is passed via line  236  to a first section  240  of second anoxic zone  204  as is methanol  237  via line  236  to reduce nitrate into gaseous nitrogen. The effluent from first section  240  of the second anoxic zone  204  is passed via line  241  to a second section  245  of the second anoxic zone  204  as is volatile acid  242  via line  243 . Bacteria in the presence of the volatile acids and under anoxic conditions will release phosphate from the biomass to the liquid in the second section  245  of the second anoxic zone  204 . The effluent from the second section  245  of the second anoxic zone  204  is passed via line  246  to a second aerobic zone  250 . In aerobic zone  250  bacteria rapidly take up phosphate in the liquid phase, acting to remove not only the phosphate released in the second section  245  of the second anoxic zone  204  but also phosphate content from line  228 . Effluent from the second aerobic zone  250  is passed via line  251  to final settling zone  255 . Settled sludge as return activated sludge  256  containing suspended solids is recycled via lines  257  and  228  to first anoxic zone  230  and also via line  258  to the second section  240  of the second anoxic zone. Excess settled sludge as waste activated sludge  252  is removed from the system via conduit  253 . Purified wastewater as final effluent  260  having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  255  via line  261 .  
         [0066]    In a preferred embodiment of the process, as depicted in FIG. 8, raw wastewater  263  enters a primary settling tank  265  via line  264  where over half of the solids are separated from the wastewater containing particulate BOD, N and P as a sludge  266  via line  267 . Settled effluent from primary settling zone  265  containing suspended solids, BOD, N and P is passed to the first anoxic zone  270  via line  268  wherein the effluent is mixed with settled sludge as return activated sludge  296  from final settling zone  295  via line  297  and  268 . The first anoxic zone  270  effluent is passed via line  271  to first aerobic zone  275  where BOD is converted to suspended solids as biomass and carbon dioxide and a portion of the ammonia nitrogen is oxidized to nitrate. In one embodiment at the end of the first aerobic zone  275 , a portion as inner recycle  276  of the contents from first aerobic zone  275  can be recycled back to the first anoxic zone  270  via lines  277  and  268 .  
         [0067]    The effluent from the first aerobic zone  275  is passed via line  278  to a first section  280  of second anoxic zone  204  as is methanol  272  via line  273  to reduce nitrate into gaseous nitrogen. The effluent from first section  280  of the second anoxic zone  204  is passed via line  281  to a second section  285  of the second anoxic zone  204  as is volatile acid  282  via line  283 . Bacteria in the presence of the volatile acids and under anoxic conditions will release phosphate from the biomass to the liquid in the second section  285  of the second anoxic zone  204 . The effluent from the second section  285  of the second anoxic zone  204  is passed via line  286  to a second aerobic zone  290 . In aerobic zone  290  bacteria rapidly take up phosphate in the liquid phase, acting to remove not only the phosphate released in the second section  285  of the second anoxic zone  204  but also phosphate content from line  268 . Effluent from the second aerobic zone  290  is passed via line  291  to final settling zone  295 . Settled sludge as return activated sludge  296  containing suspended solids is recycled via lines  297  and  268  to first anoxic zone  270  and also via line  279  to the first section  280  of the second anoxic zone and also via line  284  to the second section  285  of the second anoxic zone. Excess settled sludge as waste activated sludge  298  is removed from the system via conduit  299 . Purified wastewater as final effluent  300  having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  295  via line  301 .  
         [0068]    In a preferred embodiment of the process, as depicted in FIG. 9, raw wastewater  303  enters a primary settling tank  305  via line  304  where over half of the solids are separated from the wastewater containing particulate BOD, N and P as a sludge  307  via line  308 . Settled effluent from primary settling zone  305  containing suspended solids, BOD, N and P is passed to the first anoxic zone  310  via line  306  wherein the effluent is mixed with settled sludge as return activated sludge  336  from final settling zone  325  via line  337 . The first anoxic zone  310  effluent is passed via line  311  to first aerobic zone  315  where BOD is converted to suspended solids as biomass and carbon dioxide and a portion of the ammonia nitrogen is oxidized to nitrate.  
         [0069]    In one embodiment at the end of the first aerobic zone  315 , a portion as inner recycle  317  of the contents from first aerobic zone  315  can be recycled back to the first anoxic zone  310  via lines  318  and  306 .  
         [0070]    The effluent from the first aerobic zone  315  is passed via line  316  to a first section  320  of second anoxic zone  204  as is volatile acid  322  via line  323 . Bacteria in the presence of the volatile acids and under anoxic conditions will release phosphate from the biomass to the liquid in the first section  320 . The effluent from first section  320  of the second anoxic zone  204  is passed via line  321  to a second section  325  of the second anoxic zone  204  as is methanol  326  via line  327  to reduce nitrate into gaseous nitrogen. The effluent from the second section  325  of the second anoxic zone  204  is passed via line  326  to a second aerobic zone  330 . In aerobic zone  330  bacteria rapidly take up phosphate in the liquid phase, acting to remove not only the phosphate released in the first section  320  of the second anoxic zone  204  but also phosphate content from line  306 . Effluent from the second aerobic zone  330  is passed via line  331  to final settling zone  335 . Settled sludge as return activated sludge  336  containing suspended solids is recycled via lines  337  and  306  to first anoxic zone  310 . Excess settled sludge as waste activated sludge  338  is removed from the system via conduit  339 . Purified wastewater as final effluent  340  having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  335  via line  341 .  
         [0071]    In a preferred embodiment of the process, as depicted in FIG. 10, raw wastewater  343  enters a primary settling tank  345  via line  344  where over half of the solids are separated from the wastewater containing particulate BOD, N and P as a sludge  347  via line  348 . Settled effluent from primary settling zone  345  containing suspended solids, BOD, N and P is passed to the first anoxic zone  350  via line  346  wherein the effluent is mixed with settled sludge as return activated sludge  376  from final settling zone  375  via line  377  and  346 . The first anoxic zone  350  effluent is passed via line  351  to first aerobic zone  355  where BOD is converted to suspended solids as biomass and carbon dioxide and a portion of the ammonia nitrogen is oxidized to nitrate.  
         [0072]    In one embodiment at the end of the first aerobic zone  355 , a portion as inner recycle  357  of the contents from first aerobic zone  355  can be recycled back to the first anoxic zone  350  via lines  358  and  346 .  
         [0073]    The effluent from the first aerobic zone  355  is passed via line  356  to a first section  360  of second anoxic zone  204  as is volatile acid  362  via line  363 . Bacteria in the presence of the volatile acids and under anoxic conditions will release phosphate from the biomass to the liquid in the first section  360  of the second anoxic zone. The effluent from first section  360  of the second anoxic zone  204  is passed via line  361  to a second section  365  of the second anoxic zone  204 . The effluent from the second section  365  of the second anoxic zone  204  is passed via line  366  to a second aerobic zone  370 . In aerobic zone  330  bacteria rapidly take up phosphate in the liquid phase, acting to remove not only the phosphate released in the first section  360  of the second anoxic zone  204  but also phosphate content from line  346 . Effluent from the second aerobic zone  370  is passed via line  371  to final settling zone  375 . Settled sludge as return activated sludge  376  containing suspended solids is recycled via lines  377  and  346  to first anoxic zone  350 . Excess settled sludge as waste activated sludge  378  is removed from the system via conduit  379 . Purified wastewater as final effluent  380  having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  375  via line  381 .  
         [0074]    In a preferred embodiment of the process, as depicted in FIG. 11, raw wastewater  383  enters a primary settling tank  385  via line  384  where over half of the solids are separated from the wastewater containing particulate BOD, N and P as a sludge  386  via line  387 . Settled effluent from primary settling zone  385  containing suspended solids, BOD, N and P is passed to the first anoxic zone  390  via line  388  wherein the effluent is mixed with settled sludge as return activated sludge  416  from final settling zone  415  via line  417 . The first anoxic zone  390  effluent is passed via line  391  to first aerobic zone  395  where BOD is converted to suspended solids as biomass and carbon dioxide and a portion of the ammonia nitrogen is oxidized to nitrate.  
         [0075]    In one embodiment at the end of the first aerobic zone  395 , a portion as inner recycle  397  of the contents from first aerobic zone  395  can be recycled back to the first anoxic zone  390  via lines  398  and  388 .  
         [0076]    The effluent from the first aerobic zone  395  is passed via line  396  to a first section  400  of second anoxic zone  204  as is volatile acid  402  via line  403 . Bacteria in the presence of the volatile acids and under anoxic conditions will release phosphate from the biomass to the liquid in the first section  400  of the second anoxic zone. The effluent from first section  400  of the second anoxic zone  204  is passed via line  401  to a second section  405  of the second anoxic zone  204  as volatile acid  407  via line  408  to further release phosphate from the biomass to the liquid in the second section  405  of the second anoxic zone. The extra volatile acid is needed in the second section  405  when the nitrate level in first section  400  is very high since the volatile acid will reduce the nitrate preferentially over the phosphate release. The effluent from the second section  405  of the second anoxic zone  204  is passed via line  406  to a second aerobic zone  410 . In aerobic zone  410  bacteria rapidly take up phosphate in the liquid phase, acting to remove not only the phosphate released in the first section  400  of the second anoxic zone  204  but also phosphate content from line  388 . Effluent from the second aerobic zone  410  is passed via line  411  to final settling zone  415 . Settled sludge as return activated sludge  416  containing suspended solids is recycled via lines  417  and  388  to first anoxic zone  390 . Excess settled sludge as waste activated sludge  418  is removed from the system via conduit  419 . Purified wastewater as final effluent  420  having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  415  via line  421 .  
         [0077]    In a preferred embodiment of the process, as depicted in FIG. 12, raw wastewater  424  enters a primary settling tank  425  via line  423  where over half of the solids are separated from the wastewater containing particulate BOD, N and P as a sludge  427  via line  428 . Settled effluent from primary settling zone  425  containing suspended solids, BOD, N and P is passed to the first anoxic zone  430  via line  426  wherein the effluent is mixed with settled sludge as return activated sludge  456  from final settling zone  455  via line  457 . The first anoxic zone  430  effluent is passed via line  431  to first aerobic zone  435  where BOD is converted to suspended solids as biomass and carbon dioxide and a portion of the ammonia nitrogen is oxidized to nitrate.  
         [0078]    In one embodiment at the end of the first aerobic zone  435 , a portion as inner recycle  436  of the contents from first aerobic zone  435  can be recycled back to the first anoxic zone  430  via lines  437  and  426 .  
         [0079]    The effluent from the first aerobic zone  435  is passed via line  436  to a first section  440  of second anoxic zone  204 . The effluent from first section  400  of the second anoxic zone  204  is passed via line  441  to a second section  445  of the second anoxic zone  204  as is methanol  442  via line  443  wherein nitrate will be reduced. The effluent from the second section  445  of the second anoxic zone  204  is passed via line  446  to a second aerobic zone  450 . Effluent from the second aerobic zone  450  is passed via line  451  to final settling zone  455 . Settled sludge as return activated sludge  456  containing suspended solids is recycled via lines  457  and  426  to first anoxic zone  430 . Excess settled sludge as waste activated sludge  458  is removed from the system via conduit  459 . Purified wastewater as final effluent  460  having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  455  via line  461 .  
         [0080]    In a preferred embodiment of the process, as depicted in FIG. 13, raw wastewater  464  enters a primary settling tank  465  via line  463  where over half of the solids are separated from the wastewater containing particulate BOD, N and P as a sludge  467  via line  468 . Settled effluent from primary settling zone  465  containing suspended solids, BOD, N and P is passed to the first anoxic zone  470  via line  466  wherein the effluent is mixed with settled sludge as return activated sludge  496  from final settling zone  495  via line  497 . The first anoxic zone  470  effluent is passed via line  471  to first aerobic zone  475  where BOD is converted to suspended solids as biomass and carbon dioxide and a portion of the ammonia nitrogen is oxidized to nitrate.  
         [0081]    In one embodiment at the end of the first aerobic zone  475 , a portion as inner recycle  477  of the contents from first aerobic zone  475  can be recycled back to the first anoxic zone  470  via lines  478  and  466 .  
         [0082]    The effluent from the first aerobic zone  475  is passed via line  476  to a first section  480  of second anoxic zone  204  as is methanol  482  via line  483  wherein nitrate will be reduced. The effluent from first section  480  of the second anoxic zone  204  is passed via line  481  to a second section  485  of the second anoxic zone  204  The effluent from the second section  485  of the second anoxic zone  204  is passed via line  486  to a second aerobic zone  490 . Effluent from the second aerobic zone  490  is passed via line  491  to final settling zone  495 . Settled sludge as return activated sludge  496  containing suspended solids is recycled via lines  497  and  466  to first anoxic zone  470 . Excess settled sludge as waste activated sludge  498  is removed from the system via conduit  499 . Purified wastewater as final effluent  500  having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  495  via line  501 .  
         [0083]    In a preferred embodiment of the process, as depicted in FIG. 14, raw wastewater  503  enters a primary settling tank  505  via line  504  where over half of the solids are separated from the wastewater containing particulate BOD, N and P as a sludge  506  via line  507 . Settled effluent from primary settling zone  505  containing suspended solids, BOD, N and P is passed to the first anoxic zone  510  via line  506  wherein the effluent is mixed with settled sludge as return activated sludge  536  from final settling zone  535  via line  537 . The first anoxic zone  510  effluent is passed via line  511  to first aerobic zone  515  where BOD is converted to suspended solids as biomass and carbon dioxide and a portion of the ammonia nitrogen is oxidized to nitrate.  
         [0084]    In one embodiment at the end of the first aerobic zone  515 , a portion as inner recycle  517  of the contents from first aerobic zone  515  can be recycled back to the first anoxic zone  510  via lines  518  and  506 .  
         [0085]    The effluent from the first aerobic zone  515  is passed via line  516  to a first section  520  of second anoxic zone  204  as is methanol  522  via line  523  wherein nitrate will be reduced. The effluent from first section  520  of the second anoxic zone  204  is passed via line  521  to a second section  525  of the second anoxic zone  204  as is methanol  527  via line  528  wherein nitrate will be further reduced. The effluent from the second section  525  of the second anoxic zone  204  is passed via line  526  to a second aerobic zone  530 . Effluent from the second aerobic zone  530  is passed via line  531  to final settling zone  535 . Settled sludge as return activated sludge  536  containing suspended solids is recycled via lines  537  and  506  to first anoxic zone  510 . Excess settled sludge as waste activated sludge  538  is removed from the system via conduit  539 . Purified wastewater as final effluent  540  having reduced N, P, BOD, SS and turbidity is passed from the final settling zone  535  via line  541 .  
       EXAMPLE  
       [0086]    An embodiment of the process of FIG. 1 according to the invention will be termed PENReP (including activated sludge MLE-type inner recycle). The process was tested in the field with primary settled wastewater from the Rockland County, NY, (Sewer District No. 1) wastewater treatment plant in Orangeburg, N.Y., USA. The test data covered the period from Sept. 15, 1999 to Jan. 11, 2000. The operating conditions for the test period are shown in TABLE 1 and the test results are shown in TABLE 2.  
                                                             TABLE 1                                       Acetic                           acid       Run No.   Flow (Gpm)   HRT (hours)   SRT (days)   (mg/l)   Qr/Q                                1   2.5   8   10   50   1       2   4   5   5   50   1       3   4   5   5   50   0.5                  
 
         [0087]    [0087]                                                             TABLE 2                           Total                           Inorganic               Absorbance           Nitrogen,   o-PO 4     SS   Soluble   (355 nm)       Process Stream   mg/l   as P, mg/l   mg/l   COD mg/l   units                                Run 1                           Settled Primary   32.64   2.98   75   113    —       Effluent       PENReP   1.15   0.05   5.2   20   0.049       Effluent       Run 2       Settled Primary   41.98   3.21   69   96   —       Effluent       PENReP   1.5   0.11   2.5   18   0.041       Run 3       Settled Primary   45.11   3.3   74   130    —       Effluent       PENReP   1.52   0.08   3   19   0.046                    
         [0088]    HRT is hydraulic retention time  
         [0089]    SRT is solids retention time  
         [0090]    Qr/Q is the total recycle of the final settled solids divided by the total flow  
         [0091]    SS is suspended solids  
         [0092]    COD is chemical oxygen demand  
         [0093]    The testing of the Activated Sludge-single sludge PENReP Process (SSPP) is shown above in Runs 1, 2 and 3. The removals are based on a settled wastewater as the feed.  
         [0094]    Run 1 with a hydraulic retention time (HRT) of 8 hours and a solids retention time (SRT) of 10 days shows excellent TIN (Total Inorganic Nitrogen) (ammonia, nitrite and nitrate nitrogen) removal of 96.48%, excellent o-PO 4  (ortho-phosphate) removal of 98.32%, excellent SS (suspended solids) removal of 93.06% and excellent SCOD (soluble chemical oxygen demand) removal of 82.3%. Run 2 reduced the HRT to 5 hours and the SRT to 5 days and still showed excellent results. The TIN was reduced by 96.43% and the o-PO 4  was reduced by 96.57%, the SS was reduced by 96.38% and the SCOD was reduced by 81.25%. Run 3 kept the same HRT and SRT as Run 2 but reduced the cycle ratio (Total recycle of the final settled solids divided by total flow) to 0.5. The results were still excellent. The TIN was reduced by 96.63%, the o-P 04  was reduced by 97.58%, the SS was reduced by 95.95% and the SCOD was reduced by 85.38%.  
         [0095]    Absorbance at 355 nm in Runs 1, 2 and 3 was measured against tap water and represents the relative absorption or the clarity of the effluent produced by the process. The effluent could be described in words such as “water white”. The effluent can also be better described with numbers. The effluent absorbance divided by the tap water absorbance was 3.88 for Run 1, 3.41 for Run 2, and 3.42 for Run 3.