Abstract:
A wastewater treatment system comprises a septic tank which flows substantially all of its liquid effluent through an aerobic filter having a filter medium to produce a nitrified filtrate of reduced biological oxygen demand (BOD) and total suspended solids (TSS). The majority of the filtrate is returned to the septic tank for denitrification followed by further recirculation through the aerobic filter. All permanent discharge of effluent from the system is in the form of filtrate from an aerobic filter. Permanent discharge of effluent directly from the septic tank is prevented.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a sewage wastewater treatment system comprising a septic tank and aerobic filter arranged in a manner which maximizes the quality of the effluent permanently discharged from the system. 
     A septic tank typically provides primary treatment for domestic wastewater where municipal treatment facilities are unavailable. In a conventionally operated septic tank, raw untreated sewage wastewater having a significant concentration of waste solids is introduced into the tank from an adjacent building. In the septic tank, solids separate from the liquid portion of the sewage. Solids having a lower density than the liquid move to the top of the liquid to form a scum layer, and solids having a higher density than the liquid sink to the bottom of the tank to form a sludge layer, resulting in a relatively clear liquid layer between the scum and the sludge. This liquid portion of the wastewater, which exits the discharge end of the tank by means of gravity, a pump, or a siphon, is the septic tank effluent. 
     The quality of the septic tank effluent primarily determines its subsequent disposition, including the size and kind of any required secondary waste treatment facilities. Such effluent quality is generally measured by the biochemical oxygen demand (BOD), total suspended solids (TSS), and total nitrogen present in the effluent. 
     Nitrogen in raw untreated wastewater is primarily organic nitrogen combined in proteinaceous material and urea. Decomposition of the organic material by bacteria present in the anaerobic environment of the septic tank changes the organic nitrogen to ammonia nitrogen. Thus, in conventionally treated septic tank effluent, nitrogen is present primarily as ammonia nitrogen. 
     Secondary treatment of septic tank effluent is typically an aerobic treatment. In addition to its reduction of BOD and TSS, the aerobic environment of secondary treatment causes bacteria to oxidize ammonia nitrogen to nitrate nitrogen, a process known as nitrification. Thus, in a conventional system, nitrogen in the secondary treatment effluent is primarily nitrate nitrogen. The secondary treatment effluent is either discharged directly from the system, or undergoes at least partial recirculation through a recirculation tank as shown in U.S. Pat. No. 5,480,561. However, the nitrogen content of the secondary treatment effluent is often unacceptable. 
     In order to reduce the nitrogen content of the secondary treatment effluent, the nitrate nitrogen must be converted to a readily removable gaseous form of nitrogen. As disclosed in U.S. Pat. No. 5,531,894, this conversion can be accomplished biologically under anaerobic conditions by denitrifying bacteria. Denitrifying bacteria are capable of converting nitrate to nitrite, followed by production of nitrogen gas (N,) which is released to the atmosphere and thus removed entirely from the effluent. To accomplish such removal, however, the denitrifying bacteria require a source of carbon for cell synthesis. Conventional nitrate-laden secondary treatment effluent does not contain a sufficient source of carbon for the denitrifying bacteria, since the aerobic process which produces the secondary treatment effluent removes carbon sources by reducing the BOD. Nor does a recirculation tank such as that shown in the aforementioned U.S. Pat. No. 5,480,561 supply sufficient carbon. However, as disclosed in U.S. Pat. No. 5,531,894, an actual septic tank which receives raw untreated sewage wastewater does supply sufficient carbon to provide significant nitrogen removal by denitrification. 
     Nonetheless, the system shown in U.S. Pat. No. 5,531,894 fails to maximize the overall quality of the effluent permanently discharged from the system. This is because the system permanently discharges its effluent directly from a septic tank compartment, which necessarily means that the effluent is a mixture of some denitrified secondarily treated effluent and some primarily treated effluent which has not yet undergone the nitrification or the reduction in BOD and TSS accomplished by secondary treatment in an aerobic filter. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is one object of the present invention to maximize the quality of septic tank effluent with respect to its BOD, TSS and total nitrogen levels by arranging a septic tank and an aerobic filter in a novel recirculating treatment assembly which provides denitrification but prevents permanent discharge from the treatment assembly of any liquid other than as a filtrate from the aerobic filter. 
     It is a separate object of the present invention, independent of the previous object, to provide a novel filtrate flow splitter capable of recirculating part of the filtrate through a tank and permanently discharging the remainder while preventing permanent discharge of liquid directly from the tank. 
     It is a further separate object, independent of the previous objects, to provide denitrification in a meander-type septic tank. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a sectional side view of an exemplary embodiment of a system in accordance with the present invention. 
     FIG. 2 is a top view of the system of FIG.  1 . 
     FIG. 3 is al top view of a second exemplary embodiment of a system in accordance with the present invention. 
     FIG. 4 is an enlarged view of an exemplary filtrate flow splitter, taken along line  4 — 4  of FIG.  2 . 
     FIG. 5 is a partially sectional view taken along line  5 — 5  of FIG.  4 . 
     FIG. 6 is a partially sectional view of an alternative embodiment of a flow splitter. 
     FIG. 7 is a partially sectional view of a further alternative embodiment of a flow splitter. 
     FIG. 8 is a partially sectional view of a still further alternative embodiment of a flow splitter. 
     FIG. 9 is a side view of a still further alternative embodiment of a flow splitter. 
     FIG. 10 is a sectional view taken along line  10 — 10  of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the exemplary embodiment of the invention shown in FIG. 1, a septic tank  10  receives untreated, raw sewage wastewater, having a significant concentration of waste solids, through an inlet  12  directly from a residence or other building  14 . The septic tank may be made of a suitable material such as fiberglass or concrete. In the septic tank, gravity operates to divide the wastewater into three layers. The solids separate from the wastewater and distribute into a lower horizontal sludge layer  16  and an upper horizontal scum layer  18  with a relatively clear central horizontal liquid layer  20  therebetween having an upper surface  21 . Meanwhile, anaerobic bacteria convert the organic nitrogen in the wastewater to ammonia nitrogen. The liquid layer  20  continually communicates between the septic tank  10  and a recirculation tank  11  through an aperture  23   a  formed in a dividing partition  23  which also has an air vent  23   b . Alternatively, the partition  23  could be eliminated so that the entire tank is a septic tank, with no recirculation tank  11 . 
     A pump assembly, indicated generally as  22 , includes an electric effluent pump  24  preferably located in a cylindrical housing  26  having an encircling horizontal row of apertures  28  exposed to the liquid layer  20 . Within the housing  26  is a group of tubular filters  30  which filter solids from the liquid entering the housing  26  through the apertures  28 , as described in more detail in U.S. Pat. No. 5,492,635 which is hereby incorporated by reference. Other types of pump assemblies, with or without accompanying housings or filters, could be employed in place of the pump  24  and housing  26 . Alternatively, gravity or siphon outlet systems, with or without housings or filters, could be employed. 
     A control panel (not shown) controls the operation of the effluent pump  24  through an electrical conduit  34  connected to a splice box  36 . The pump  24  is activated intermittently in response to a conventional float switch assembly which includes a redundant off/low level alarm float  40 , a timer override on/off float  41 , and a timer override on/alarm float  42 . When activated, the pump  24  pumps the ammonia-laden liquid  20  through a tank outlet  44  to an aerobic filter  52 . If desired, the tank outlet  44  can include one or more intervening liquid-containing tank or chambers (not shown). 
     The aerobic filter  52  is preferably an attached growth treatment system containing a filter media  54  having one or more preferably textile layers suitable for the support and growth of an ecosystem of microorganisms, including nitrifying bacteria, capable of performing substantial organic and inorganic process reductions. Aerobic filters of this type are shown in the systems described in U.S. Pat. Nos. 5,531,894 and 5,480,561, which are hereby incorporated by reference. A distribution manifold  56  receives the liquid  20  from the tank outlet  44  and distributes it through the filter media  54 . The filter media  54  is kept constantly in an aerated condition by an air intake fan (not shown) so as to support the aerobic microorganisms which degrade or oxidize the organic material present in the liquid and thereby reduce the BOD and TSS. Meanwhile, the nitrifying bacteria convert the ammonia nitrogen present in the liquid to nitrate nitrogen. The liquid passes through the filter media to the slotted underdrain  60  which collects the nitrate-laden filtrate and conducts it through a filtrate outlet conduit  62  to a filtrate inlet  72  of the septic tank  10 . 
     Depending upon the height of the upper surface  21  of the liquid layer  20  within the septic tank, a portion of the filtrate is returned to the septic tank from the filtrate inlet  72  in a manner to be described hereafter. In the septic tank, heterotrophic bacteria under anoxic conditions convert the nitrate nitrogen in the filtrate to gaseous nitrogen products (a process called denitrification) which permanently separate from the blended filtrate and raw wastewater and are released to the atmosphere. The organic matter present in the septic tank  10  is necessary to provide the denitrifying bacteria with sufficient carbon necessary for cell growth. Sufficient organic matter for this purpose is not present in the recirculation tank  11 . 
     The objective of maximizing the quality of the septic tank effluent permanently discharged from the septic tank/aerobic filter treatment assembly is accomplished by substantially preventing permanent discharge from the treatment assembly of any septic tank liquid other than in the form of filtrate from the aerobic filter  52 , while compatibly returning filtrate to the septic tank  10  for denitrification to permanently remove nitrogen. This is why substantially all of the output of the pump  24  is directed to the filter  52 , permitting substantially no permanent discharge of the liquid  20  in any bypassing relationship to the filter  52 . 
     An alternative arrangement of the components of FIGS. 1-2 is shown in FIG. 3, with identical components being labeled identically. In FIG. 3 there is no recirculation tank  11 , but rather a septic tank  10   a  which has a partition  100  only partially extending longitudinally within the tank so that, unlike FIGS. 1-2, all of the contents of the septic tank, including the sludge layer  16  and scum layer  18 , exist on both sides of the partition  100 . The partition  100  extends above the upper surface  21  of the liquid layer  20  and creates a conventional “meander” flowpath within the tank  10   a  indicated by the arrow  102  through which the liquid layer  20  flows sequentially around the opposite sides of the partition  102 . When used in the past in conventional septic tanks, such meander flowpath was effective to extend the flowpath between the inlet and outlet of the tank and thereby extend the time during which solids could separate from the liquid by gravity. In the present case, however, the extended flowpath provides the additional unique function of extending the time during which denitrification of filtrate returned to the septic tank occurs, thereby maximizing gaseous nitrogen removal. This approach of maximizing denitrification by utilizing a meander-type flowpath is useful not only in the novel process described above, but also in other denitrification processes such as that described in U.S. Pat. No. 5,531,894 mentioned above. 
     A filtrate flow splitter or proportioning valve, indicated generally as  74 , receives the filtrate from the filtrate inlet  72  and splits it into a first portion which is returned to the septic tank  10  for denitrification, and a second portion which is permanently discharged from the treatment assembly through a filtrate discharge conduit  76 . The second portion can be discharged to a drain field or to any desired further treatment facility. With further reference to FIGS. 2-5, one preferred embodiment of the flow splitter  74  comprises an open-topped cylindrical housing  78  into which the filtrate  70  from the filtrate inlet  72  can flow by gravity or, alternatively, under the influence of other means such as a pump. The filtrate  70  is returned to the liquid layer  20  in the septic tank  10  preferably through one or more filtrate one-way recirculation valves  80  at the bottom of the housing  78  in parallel relation to the filtrate discharge conduit  76 . A typical one-way recirculation valve  80  is formed of a flexible resilient material such as silicone with a slit  82  biased to a closed position by the resilient material of the valve other types of yieldably biased or otherwise normally closed one-way valves could alternatively be used for this purpose. The slit  82  is openable only when the depth-dependent filtrate pressure inside the valve exceeds the depth-dependent liquid pressure in the septic tank outside the valve plus the mechanical biasing force of the resilient valve material. Thus, the one-way valve  80  permits flow only in one direction, i.e., from the housing  78  to the septic tank  10 , and prevents flow in the opposite direction. 
     When the upper surface  21  of the liquid layer  20  within the septic tank is at a level  21   a  as exemplified in FIG. 5, and the upper surface of the filtrate  70  in the housing  78  is at a level  70   a , the pressure difference between the liquid  20  tending to close the valve  80 , and the filtrate  70  tending to open the valve, is sufficient to overcome the resilient biasing force and open the valve to cause all of the filtrate  70  to be returned to the septic tank, with none of the filtrate being permanently discharged through the filtrate discharge conduit  76 . This, of course, is only a temporary condition, because any addition of sewage through the inlet  12  from the residence  14  raises the level  21   a  thereby increasing the pressure of the liquid  20  and causing the valve either to close or at least restrict the flow of the filtrate  70  into the septic tank. This raises the level of the filtrate  70  within the housing  78  and allows at least a portion of the filtrate to be permanently discharged through the discharge conduit  76 . If the upper surface of the liquid  20  in the septic tank rises to a sufficiently high level such as  21   b , the valve  82  closes entirely and all of the filtrate  70  in the housing  78  is temporarily discharged through the conduit  76  until the level of the liquid  20  in the septic tank decreases. Between the levels  21   a  and  21   b  of the liquid  20 , the valve  82  will have a variably restricted opening depending upon the pressure differential between the filtrate  70  and the liquid  20 . However, the valve  80  could alternatively merely be of an on-off type with no variability in its opening. 
     An adjustable or replaceable restrictor plate  84 , having a weir opening  86  or other flow-restrictive opening, is optionally provided for adjusting the size of the opening and thus the flow rate of the permanently-discharged filtrate through the discharge conduit  76 . 
     During operation, the surface  21  of the liquid layer  20  within the septic tank will normally be somewhere between the lower, full-recirculation level  21   a  and the upper, full-discharge level  21   b  shown in FIG. 5, so that a first portion of the filtrate  70  is returned to the septic tank and a second portion of the filtrate is permanently discharged through the discharge conduit  76 . The level of the filtrate  70  interior of the housing  78  is dependent upon the exterior level of the liquid layer  20  within the septic tank, the interior filtrate level rising and falling in offset relation to the rise and fall of the exterior liquid level in the septic tank. As the filtrate level rises within the housing  78 , the proportion of filtrate discharged through the conduit  76  increases until it approximately reaches equilibrium with the filtrate flow rate from the inlet  72 . Conversely, as the filtrate level drops in response to a drop in septic tank liquid level, the proportion of filtrate discharged diminishes until all of the filtrate is returned to the septic tank through the recirculation valve  80 . 
     Possible alternative embodiments of the filtrate flow splitter  74  are shown in FIGS. 6-10, and are intended to be merely exemplary and not exclusive. Other alternative embodiments (not shown) could discharge the filtrate through the filtrate discharge conduit by mechanical or air-lift pump or dosing siphon, rather than by gravity, for example. 
     The embodiment  74   a  of the flow splitter shown in FIG. 6 includes the addition of a filter  88 , such as a mesh screen tube filter. Filtrate from the housing  78   a  enters the interior of the filter tubes  90  and flows outwardly through the tubular mesh screen walls and downwardly through openings  92  toward the one-way recirculation valve  80 . The filter  88  can be of any type suitable for separating solids from the filtrate returning to the septic tank. 
     In the embodiment  74   b  of FIG. 7, a flapper-type, gravity-biased, one-way recirculation valve  80   a  replaces the resiliently biased one-way recirculation valve  80  shown in FIGS. 6 and 7, but functions in a similar yieldably biased, normally-closed fashion. 
     The embodiment  74   c  of FIG. 8 receives filtrate through a filtrate inlet  72   a  and permanently discharges a portion of the filtrate through a filtrate discharge conduit  76   a  while returning another portion of the filtrate to the septic tank through a conduit  94 , level-sensitive cutoff float valve  96 , and one-way recirculation valve  80 . Port  77  is an optional port for returning filtrate to the septic tank if the float valve  96  is closed. The float valve  96  is not redundant to the one-way recirculation valve  80 . The float valve  96  is merely sensitive to fluid level, and cannot reliably prevent liquid from the septic tank from passing upwardly past the valve  96  and into the filtrate discharge conduit  76   a . In contrast, the one-way recirculation valve  80  substantially prevents any passage of septic tank liquid upwardly toward the filtrate discharge conduit. 
     The flow splitter embodiment  74   d  of FIGS. 9 and 10 receives filtrate through a filtrate inlet  72   b  and permanently discharges a portion of the filtrate through a filtrate discharge conduit  76   b  surrounded by the inlet conduit  72   b  so as to create an inlet annulus  97  between the two conduits. A resilient seal  98  separates the annulus  97  from the discharge end of the conduit  76   b . Another portion of the filtrate is returned to the septic tank through a one-way recirculation valve  80  via a conduit  100  into which the inlet conduit  72   b  empties. A slidably detachable coupling assembly  99   a ,  99   b  is optionally provided for quick disconnection of the flow splitter  74   d  from the conduits  72   b  and  76   b . A lifting handle  104  enables easy removal of the disconnected flow splitter  74   d  from the septic tank. Tube  102  provides ventilation for the lower portion of the flow splitter  74   d.    
     The exemplary alternative filtrate flow splitter embodiments  74 ,  74   a ,  74   b ,  74   c  and  74   d  are useful in any type of a tank or chamber where flow splitting is desired, such as a recirculation tank, and are not limited to use in a septic tank. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.