Abstract:
A compact high efficiency combined biological and physical unsaturated wastewater treatment filter configured for periodic backwashing is disclosed. The filter utilizes packed media and removes pollutants and pathogens from wastewater and other dirty water sources. The filter comprises a periodic backwashing means to less the chance that biofilm growth will clog the media pores. However, such backwashing still leaves sufficient biofilm attached to the media to maintain a very high level of treatment. In the preferred embodiment, the filter utilizes high frequency dosing to cause pore saturation at or near the surface during dosing and shortly thereafter in order to maximize distribution uniformity and to induce downward airflow into the media. The system provides a number of unexpected benefits; including allowing a very low distribution pressure and providing high oxygen transfer at low energy utilization into the filter media for aerobic biodegradation of pollutants.

Description:
RELATED APPLICATIONS 
       [0001]    This application is related to previously filed U.S. provisional patent applications having Ser. Nos. 61/958,222 and 61/057,645, both filed May 30, 2008, and both entitled “Pulse-Fed Backwashing Unsaturated Filter”. Both are incorporated by reference herein as if set out in full. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention is related to wastewater treatment filters, and in particular to a compact, high efficiency, combined biological and physical treatment filter for removing pollutants and pathogens from wastewater and other dirty water sources. 
         [0004]    2. Background of the Invention 
         [0005]    Decentralized wastewater treatment systems typically comprise a septic tank into which raw wastewater is introduced from one or more residences or businesses. In the septic tank or other settling means, solids are typically removed by gravity settling, and other constituents are partially removed by passive anaerobic biological treatment. When a higher level of treatment is required, the septic tank wastewater is generally directed to a biological filter or other treatment system. After treatment, the treated effluent is typically discharged into a lateral field comprising a plurality of underground parallel drainpipes or irrigation tubes that discharge the treated effluent into the underground environment through a plurality of openings on each lateral drainpipe or tube. 
         [0006]    One type of biological filter is the biological wastewater trickling filter. Conventional trickling filters apply wastewater to the top of the trickling filter from an overhead rotary distributor. The filter is generally a fixed coarse medium comprising rocks or pieces of shaped plastic. Biological life on the surfaces of the filter media provides the primary means for wastewater treatment in such filters. The media is unsaturated to provide maximum opportunity for oxygen transfer to aerobic microbes on the media. Filters of this design are cost efficient and have wide commercial acceptance. There are several factors, however, that prevent conventional trickling filters from meeting some of the stricter established parameters for effluent discharge into the environment. Because they use a fixed media bed, these system do not rely on the superior method of backwashing to remove excess biological growth and solids from the trickling filter effluent, but instead rely on gravity sloughing, followed by settling in an external secondary clarifier. The external clarifier adds significant cost and complexity to the system that otherwise would not be present. Intermittent sand filters and recirculating sand filters are types of trickling filters that use sand instead of rocks or larger media in order to achieve a higher degree of both biological and physical treatment than conventional trickling filters. Intermittent and recirculating sand filters also rely on substantial degradation of excess solids within the media pores rather than removal in an external clarifier. Because they employ a fixed bed, intermittent and recirculating sand filters are loaded at very low rates to prevent plugging, and are also not backwashed. Finally, they do not produce final treated effluent with the quality equaling that of treated effluent from fine media filters. 
         [0007]    A second type of biological filter is the biological aerated filter, which is used to remove nitrogen compounds and carbonaceous material from wastewater. These filters utilize submerged media to provide sites for attached fixed-film microbial growth. Biological aerated filters may be of upflow or downflow design, and are periodically backwashed. Biological aerated filters are operated in a saturated state and utilize pressurized bubble aeration to supply oxygen for aerobic microbes. Therefore, such filters require a substantial amount of energy to operate. 
         [0008]    A third type of filter is the rapid sand filter, which is in common use in the source water treatment industry for physical filtration and in the wastewater industry as a final wastewater polishing treatment. Unlike trickling filters, rapid sand filters are backwashed through a process that fluidizes the media bed. However, rapid sand filters are run at high hydraulic loading rates and are fully saturated by maintaining a water head or pressure over the filter surface. Rapid sand filters are principally designed and operated to provide physical filtration. Biological treatment is not an objective because biological growth substantially reduces the hydraulic capacity of rapid sand filters to perform their primary function of physical filtration. Furthermore, the continuously saturated conditions in rapid sand filters are not conducive to efficient aerobic biological treatment of wastewater. 
         [0009]    Thus, conventional solutions for aerobic biological wastewater treatment using unsaturated filter media involve fixed (that is, non-fluidizing and normally non-backwashed) media. Biological aerated filters utilize media that is periodically backwashed, but are operated in a saturated state and use bubble aeration to supply oxygen to aerobic microbes on the media. For those physical water treatment solutions that do involve backwashing, the media is saturated by pressurized water that is essentially continuously maintained above the media surface, and biological growth is viewed as a detriment to physical filtration performance rather than as a desired object of design. 
       DESCRIPTION OF RELATED ART 
       [0010]    One of the closest known prior art systems is the Amphidrome® by F. R. Mahoney. According to company statements, it “is a submerged attached growth bioreactor process, designed around a deep-bed sand filter.” It is specifically designed for the simultaneous removal of soluble organic matter, nitrogen and suspended solids within a single reactor. The Amphidrome is a sand media filter that works with a septic tank and an effluent storage tank and has a periodic backwash cycle. Similar to a conventional biological aerated filter, the Amphidrome is continuously saturated and uses a pressurized air diffuser at the bottom of the bioreactor to provide oxygen to the biofilm (see U.S. Pat. Nos. 5,989,427, 5,776,344 and others). Further, the Amphidrome system is not vertically compact and requires a relatively large amount of energy usage. There is thus a need to provide a more vertically compact filter requiring a low amount of energy. 
         [0011]    Textile biofilters are one of the most recent innovations in small wastewater systems. Textile biofilters utilize an open-structure textile mat instead of sand or gravel as a fixed biological filter media. They are pulse-fed (that is, periodically dosed with wastewater), unsaturated, and allow some of the highest wastewater loading rates of currently available fine media unsaturated wastewater treatment filters. However, textile biofilters cannot be effectively backwashed and their recommended surface area loading rates, expressed in gallons per day per square feet (gpd/sf) of filter surface area, must be maintained at moderate levels in order to prevent long-term plugging. For example, textile biofilters are typically designed for a net loading rate of 20 gpd/sf. This is an improvement over the typical net loading rate of 5 gpd/sf for fixed bed recirculating gravel filters, but is still far below the net loading rate of 100 gpd/sf exhibited by the present invention. Therefore surface area loading rate is one area in which textile biofilters and other fixed bed fine wastewater filters are still lacking. Even biofilters with large amounts of porosity tend to build up biofilm and solids, eventually causing some hydraulic short-circuiting of the media. Finally, textile filters sometimes require hose washing to clear buildup. See products and patents by Orenco Systems, Inc. for more details (U.S. Pat. Nos. 6,372,137; 5,980,748; 5,531,894; 5,480,561; 5,360,556; 5,492,635; 4,439,323; D461,870; and D445,476). There is thus a need for a biofilter that provides a high degree of treatment similar to conventional unsaturated sand and textile biofilters, but that does so without the drawbacks associated with the relatively low surface area loading rates commonly found in such systems. 
         [0012]    U.S. Pat. No. 7,309,434 to Potts discloses a pulse-fed unsaturated media filter for wastewater treatment that provides intermittent forced air above the media prior to each wastewater dose. The airflow that is induced into the media provides oxygen for nitrification of ammonia to nitrate. Interruptions in the supply of air allow anoxic conditions to develop, providing denitrifying conditions. This is useful for specific cases where enhanced nitrogen removal is required, but it does not provide any improvement in surface area loading rate capacity over conventional sand and textile biofilters, resulting in a physically large filter for a given amount of treatment capacity. Potts refers to a dosing rate “most preferably” in the range of 3 to 10 gpd/sf, beyond which the bed is subject to progressive clogging or being “overwhelmed.” There is thus a need to provide a more compact unsaturated filter capable of being backwashed, and that provides higher surface area loading rates with more efficient and natural air circulation. Neither Potts&#39; nor other known unsaturated wastewater treatment filters contain the proper geometry, tanks, pump(s), valve(s), controls, and other associated components to enable them to be effectively backwashed through fluidization. 
         [0013]    There is thus no known wastewater treatment device that provides a combination of generally unsaturated conditions suitable for aerobic biological fixed film treatment and periodic backwashing through fluidizing the filter media. 
         [0014]    Because the conventional unsaturated systems that utilize fine media are limited to much lower hydraulic loading rates, existing fixed sand or textile media filters must rely on a pressurized spray or squirt application system for distributing wastewater over the filters&#39; large surface areas. In addition, because of the spray or squirt type of application, these filters do not typically create a uniform downward flow of air into the upper part of the media to enhance oxygen availability for aerobic treatment. 
         [0015]    The present invention overcomes these and other problems that are inherent with existing biological fixed-film filter wastewater treatment systems by providing a system which is capable of high surface loading rates on fine media at very low pressure, where long-term plugging of the media is prevented by periodic backwashing, and aerobic conditions can be maintained with minimal energy throughout most of the filter to provide the maximum rate of aerobic biological treatment. 
         [0016]    It is therefore a first object of the present invention to provide a means for maintaining optimal moisture and oxygen conditions for biological wastewater treatment in a predominantly unsaturated filter with relatively fine media. 
         [0017]    It is a further object of the invention to provide a compact and energy efficient wastewater treatment filter. 
         [0018]    It is a further object of the invention to provide a compact unsaturated backwashing filter with a high surface loading rate and that provides efficient and natural air circulation. 
         [0019]    It is a further object of the invention to provide effective wastewater treatment in a compact filter that enables treatment similar to that achieved in conventional unsaturated sand media biological wastewater filters approximately twenty times larger. 
         [0020]    It is a further object of the invention to provide an air vent located below the surface of an unsaturated backwashing filter to allow ready air movement down through and out of the filter. 
         [0021]    It is a further object of the invention to provide a biological wastewater filter that maintains an optimum level of biofilm growth while simultaneously remaining free from clogging that is normally associated with extended periods of biological growth. 
         [0022]    It is a further object of the invention to provide a filter which, through the use of sufficiently fine media, provides effective physical filtration concurrent with biological treatment. 
         [0023]    It is a further object of the invention to provide a highly energy efficient means for transferring oxygen to the aerobic bacteria treating wastewater in a treatment system. 
         [0024]    It is a further object of the invention to maintain an average downward flow of air through the filter such that odorous compounds at the surface of the filter are adsorbed and oxidized in the depth of the filter. 
         [0025]    It is a further object of the invention to utilize recycle flow from the filter to an anoxic or septic tank to enhance total nitrogen removal in the combined septic tank—filter system where nitrogen removal is desired. 
       SUMMARY OF THE INVENTION 
       [0026]    The applicant discloses a compact high efficiency combined biological and physical unsaturated wastewater treatment filter configured for periodic backwashing that removes pollutants and pathogens from wastewater and other dirty water sources. In the present system, wastewater processed in a septic tank or by other primary treatment means to remove most settleable solids enters a dosing chamber through a distribution head. In the dosing chamber, the wastewater is passed through and is treated by a filter media coated by microbial biofilm. In the preferred embodiment, high frequency pulsed applications of wastewater are utilized to better distribute the wastewater evenly across the entire surface of the media. This provides a high uniformity of wastewater flow down through the pores of the media, which pushes air downward ahead of the wastewater pulse and also pulls air in through the media surface above the wastewater pulse as the pulse moves down into the media, thereby promoting air circulation through the media for optimal biofilm growth and removal of pollutants. 
         [0027]    From the dosing chamber the treated wastewater (or for purposes of clarity, “treated effluent”) is collected in an underdrain system and directed to a holding tank. From the holding tank, the treated effluent is discharged, in exemplary cases either into the environment above ground, in an irrigation system for reuse, or into a lateral field comprising a plurality of drain pipes under the ground that discharge the treated effluent into the underground environment through a plurality of holes on each drain pipe. 
         [0028]    Treated effluent in the holding tank is periodically directed back through the underdrain to fluidize and backwash the filter media in the dosing chamber, thereby removing the most hydraulically restrictive portion of the biofilm growth out of the media pores and ultimately preventing excess growth from clogging the system. Backwashing also redistributes the media to eliminate preferential flow channels that may have formed and caused hydraulic short-circuiting and therefore inadequate treatment of wastewater. Such backwashing still leaves sufficient biofilm attached to the media to maintain a very high level of treatment. 
         [0029]    The system provides a number of unexpected benefits; including enabling unexpected and surprisingly high sustained average wastewater loading rates, a very low wastewater application pressure, enhanced natural airflow into the filter media, excellent distribution of treatment throughout the filter media, retention of sufficient biofilm on the media after backwashing for continued treatment, and the capture and treatment of odors. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0030]    The foregoing aspects and many of the attendant advantages of the invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0031]      FIG. 1  is a sectional side view of a summary diagram of a preferred embodiment the claimed filter system; 
           [0032]      FIG. 2  is a sectional view of seven media particles representative of the filter media during the predominant unsaturated conditions. 
           [0033]      FIG. 3  is a detailed sectional side view diagram of the backwash mode of the preferred embodiment of the claimed filter system; 
           [0034]      FIG. 4  is a detailed sectional side view diagram of the conditions near the end of the application of a pulse of wastewater in the preferred embodiment of the claimed filter system, wherein temporary surface ponding and media saturation conditions during each brief application of wastewater to the filter are shown, along with air being forced down below the wetting front and ultimately out the a vent; 
           [0035]      FIG. 5  is a detailed sectional side view diagram of media conditions shortly after wastewater application has ceased in a preferred embodiment of the claimed filter system, wherein air is depicted being drawn into the filter above the saturated pulse, shown moving downwards; 
           [0036]      FIG. 6  is a detailed sectional side view diagram of a preferred embodiment of the claimed filter system, depicting the predominant media condition between wastewater applications wherein the media is in an unsaturated condition; 
           [0037]      FIG. 7  is a sectional side view of an alternative embodiment of the claimed filter system, wherein the filter system is shown in conjunction with a buried septic tank; 
           [0038]      FIG. 8  is a sectional side view of an additional alternative embodiment of the claimed filter system, wherein the system is configured in an above-ground fashion and wherein multiple filter vessels are shown; and 
           [0039]      FIG. 9  is a chart showing data from an electrical conductivity tracer test performed on an exemplary embodiment of the claimed filter system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    The following description is presented to enable a person of ordinary skill in the art to make and use various aspects and examples of the present invention. Any descriptions of specific materials, techniques, and applications are provided only as examples. Various modifications to the examples described herein will be readily apparent to those of ordinary skill in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the invention. Thus, the present invention is not intended to be limited to the examples described and shown, but is to be accorded the scope consistent with the appended claims. 
         [0041]    In a preferred embodiment of present invention as shown in  FIG. 1 , a wastewater tank  10  receives raw wastewater and provides passive removal of settleable solids  11  and floating solids (not shown). Wastewater from such primary treatment is then directed using a pump, gravity, or other means into filter inlet  12 . The wastewater next flows from inlet pipe  12  to a distribution head  13  that substantially evenly distributes the wastewater over a top surface area of a filter media  14 . In the preferred embodiment of the invention, the wastewater is distributed intermittently and repeatedly. Although the intermittent application of wastewater to the filter media  14  is not required for proper functioning of the filter, intermittent application improves the movement of air through the filter, thereby improving filter capacity and treatment effectiveness. The compact nature of the filter combined with a low head splash style surface distribution device allows the wastewater to be distributed at a high uniformity to the filter surface with very little pressure head. This allows an energy efficient device such as a small dosing siphon to be used for providing the intermittent application flow to the filter. The splash or spray distribution head may in an alternative embodiment of the invention be changed to a flooding style of distribution for the lowest possible pressure head requirement. 
         [0042]    The filter media  14  and the other filter components are contained by the filter vessel  15 . The filter media  14  comprises filter media particles  30  on which microbial growth occurs (herein after termed biofilm  31 ) and which assimilates and biodegrades wastewater constituents as the wastewater flows by gravity through filter media  14 . See  FIGS. 1 and 2 . Turned back to  FIG. 1 , treated effluent is collected near the bottom of filter vessel  15  in underdrain  16 , and flows through treated effluent conveyance pipe  17  through a check valve  18  into treated effluent holding tank  19 . Treated effluent pump  20  periodically pumps treated effluent into discharge pipe  21  for either discharge through a first discharge control valve  22   a  or for filter backwashing through a second control valve  22   b . Although the preferred embodiment is described as above and as shown in  FIG. 1 , first and second control valves may in an alternative embodiment be combined into a single three-way valve for reason of compactness. 
         [0043]    During backwash, treated effluent flows from discharge pipe  21  through control valve  22   b  thence via treated effluent pipe  17  to filter underdrain  16  where it is discharged into filter vessel  15  thereby fluidizing and backwashing filter media  14 . The underdrain in a preferred embodiment is configured to fluidize the media uniformly, but it could be configured to fluidize the media in a manner to encourage some vertical media circulation within the filter. Alternatively, wastewater or other liquids could be backwashed through the system either by discharge pipe  21  or another inlet in filter vessel  15 . Backwash flow containing excess biofilm and captured solids dislodged from the filter media  14  is conveyed upward to the backwash overflow trough  24  for conveyance through backwash discharge pipe  25  back to the wastewater tank  10  for additional passive clarification treatment. During backwashing the relatively high density of the filter media  14  is sufficient to retain it in filter vessel  15  in a fluidized state and prevent it from entering backwash overflow trough  24 . Treated effluent pump  20  and control valves  22   a  and  22   b  are in a preferred embodiment electrically controlled by a timed controller, not shown. Air circulation through the filter media  14  is enhanced by a vent  27  that allows air to escape from the lower portion of the filter media through vent pipe  28 . 
         [0044]    Turning briefly to  FIG. 2 , a detailed depiction of several contacting particles of the filter media during general forward flow treatment operation of the filter is shown. Upon packing together, the filter media  14  ( FIG. 1 ), which comprises sand or other relatively fine media particles, forms pores  29  for water flow, air flow, and physical filtration. The surfaces of media particles  30  provide sites for attached biofilm  31  and biological treatment. A majority of the time the filter is in use it is in an unsaturated condition, wherein air fills the pores  29  between media particles and water forms a film over the surfaces of the media particles  30  with the attached biofilm growth  31 . Air in the media pores  29  provides oxygen to the attached microbial growth  31  to enable the microbial growth to efficiently consume the waste constituents in the water. Although not directly evident in the two-dimensional depiction in  FIG. 2 , the air in the larger media pores is contiguous throughout the unsaturated filter media. During backwashing, the media is fluidized and excess microbial growth and trapped particles are scoured from the media. The filter performance is affected by the depth of the filter media, which influences the physical filtration and the contact time between the wastewater and the biofilm on the media. A deeper filter media bed will therefore provide higher treated effluent quality, although the depth of the media bed may be limited by hydraulic grade line and cost considerations. 
         [0045]    To reduce the likelihood of clogging, the present invention in a preferred embodiment has a backwash mode, as depicted in detail in  FIG. 3 . Backwashing keeps the biofilm from becoming excessively thick and reduces mass transfer limitations from the pore liquid to the microbes performing biodegradation of the pollutants. When the system is in backwash mode, the first discharge control valve  22   a  that directs treated wastewater out of the system is closed, while the second discharge control valve  22   b  is open. When in backwash mode, check valve  18  closes automatically due to a pressure differential. This allows treated effluent pump  20  to move water “upstream” through the discharge pipe  21  and treated effluent conveyance pipe  17  and back into underdrain  16  of the filter and thence the packed media filter  14 , substantially fluidizing the media in the process as shown in  FIG. 3 . Backwash overflow trough  24  comprising weir or other openings catches the mixture of backwash water and excess solids as it approaches the top of filter vessel  15 , and redirects it through backwash discharge pipe  25  to wastewater tank  10  (not shown in  FIG. 3 ) or other wastewater settling/equalization upstream of the media filter. Backflow through filter inlet  12  is restricted by an upstream orifice, check valve, or other means. 
         [0046]    Based on the above description and related figures, treated effluent holding tank  19  thus serves the dual purpose of both storing treated water for discharge or irrigation reuse and storing treated water for the next backwash event. Primary wastewater tank  10  or similar (as described above and shown in  FIG. 1 ) upstream of the filter likewise serves multiple purposes in the operational scheme, providing settling treatment for both the raw wastewater and for the solids released from the filter during backwashing. Wastewater tank  10  provides a third purpose in that the treated effluent recycled back to the tank during backwashing typically contains some nitrate, which will undergo denitrification in the anaerobic environment of the wastewater tank  10 . This is even more a benefit where nitrate is a pollutant of concern. 
         [0047]    Continuing with  FIG. 3 , also shown in detail is an alternative embodiment of the present invention, wherein an optional surface spray wash nozzle system comprising one or more spray nozzles  32  is present. This surface wash system is supplied with higher-pressure water from discharge pipe  21  during backwashing through a surface spray conveyance pipe  33 . First gate valve  23   a  and second gate valve  23   b  or other throttling means are used to control the flow rates to treated effluent conveyance pipe  17  and spray conveyance pipe  33 . The surface spray wash nozzles  32  provide spray streams  52  that impinge the media surface, thereby providing higher energy scouring of the media surface to improve media cleaning in the critical top surface area. Nozzles  32  also help mix the media vertically to improve biofilm distribution within the filter. 
         [0048]    In the preferred embodiment, wastewater is dosed into filter vessel  15  from the upstream wastewater tank  10  on a high frequency basis, typically for several seconds every few minutes. The large-scale movement of wastewater and air in the filter during and between doses of wastewater is depicted in  FIGS. 4 ,  5 , and  6 . The pulse-feed application of wastewater to the filter provides brief intermittent episodes of saturation at the filter surface, resulting in high distribution uniformity, while driving air downwards into the filter media to provide oxygen for aerobic biological treatment.  FIG. 4  depicts the condition of media saturation near the filter surface during each brief application of wastewater through the distribution head  13  above the filter media  14 . Here ponded wastewater  34  on the surface of the filter media  14  is depicted. For coarse media, actual ponding may not occur or may be very brief. A saturated wetting front  35  (the instantaneous boundary between the saturated media above and the unsaturated media below) for the upper saturated zone  36  as well as a continuously saturated lower zone  37  is also depicted. In the preferred embodiment of the invention employing fine media, lower zone  37  is kept in a continuously saturated state by the capillary action of the media pores above the treated effluent at the level of the underdrain  16 . As saturated wetting front moves downwards due to gravity and capillary action, air begins to move into vent  27  and out vent pipe  28 . In situations where greater nitrogen removal or filter residence time is desired, it may be beneficial to back the treated wastewater up into the lower portion of the media to increase the volume of the lower saturated zone  37 . 
         [0049]      FIG. 5  shows media conditions shortly after wastewater application has ceased. Note that as compared to  FIG. 4 , in  FIG. 5  gravity and capillary action has had time to cause the upper saturated zone  36  and its saturated wetting front  35  to move further downwards in the filter vessel  15 . Air trapped between the downward moving zone of saturation  36  and continuously saturated lower zone  37  near the bottom of filter media  14  is allowed to continue escaping through vent  27  and vent pipe  28 . As this occurs, new air is drawn into the upper layer (not labeled) of filter media  14  through the filter media surface to fill in the pores  29  on the media above the downward moving upper saturated zone  36 . As the wastewater continues to flow down into the filter due to gravity, it eventually spreads out vertically in the media due to capillary forces until there are only small scattered saturated pockets  38 , with the vast majority of the media pores becoming unsaturated as depicted in  FIG. 6 . During this phase, the downward wastewater velocity slows, and the thickness of the water film around media particles decreases, allowing more time and closer contact with the biofilm around the media particles for biological treatment. The phase depicted in  FIG. 6  with media water film conditions as shown in  FIG. 2  represents the predominant unsaturated condition of the filter for the relatively long durations between the brief dosing periods. 
         [0050]      FIG. 7  depicts an additional alternative embodiment of the present invention, wherein in this exemplary case is a subsurface embodiment of the filter system in conjunction with a buried septic tank. Although again this figure is diagrammatic, the scale of the individual components is more representative of the actual scale for the system in use for a residential or small commercial application. In this alternative embodiment, raw wastewater enters a standard two-chamber septic tank  53  at an inlet pipe  39 , receiving passive clarification treatment to remove settleable solids  11  prior to passing through a port in a dividing wall  40  into a second chamber of septic tank  53 . Within the second chamber is a smaller isolated chamber  41  with an orifice entry  42  to control the rate of flow into said isolated chamber. Within isolated chamber  41  is a dosing siphon  43 , which intermittently doses wastewater through filter inlet pipe  12  into the filter vessel  15 . First riser  44 , second riser  45 , third riser  46 , and fourth riser  47  provide access to the various parts of the system from ground surface  58 . The size and geometry of filter vessel  15  are such that the filter vessel could fit within fourth riser  47  if desired for compactness. In the exemplary embodiment as shown, the backwash discharge pipe  25  discharges near the beginning of the first chamber of septic tank  53  to provide the greatest opportunity for solids in the backwash water to settle. 
         [0051]      FIG. 8  depicts an above-ground alternative embodiment of the filter system demonstrating some additional features to further enhance filter performance in this and other embodiments. Here, septic tank  53  comprises a pump chamber  48  that itself comprises a pump  49  that sends wastewater through filter inlet  12  to first filter unit  54  and second filter unit  55 . An electrical controller utilizing float switches and timers, not shown, controls operation of pump  49  to dose wastewater to the filters on an intermittent basis, while maintaining maximum and minimum water levels in wastewater tank  10  as influent flows vary. Filter units  54  and  55  comprise multiple distribution heads  13 . Treated effluent leaves the filter units through their respective underdrains (shown but not labeled), through first treated effluent pipe  56  and second treated effluent pipe  57 , to treated effluent holding tank  19 . Treated effluent holding tank  19  comprises a separate backwashing pump  50  and discharge pump  51 , thereby eliminating both the need for check valves on the treated effluent pipelines and the need for a control valve on the discharge pipeline. During backwashing, first control valve  59   a  and second control valve  59   b  are actuated sequentially to backwash first filter unit  54  and second filter unit  55  separately and in sequence, although not necessarily in that order. Backwash from both filters is returned to septic tank  53  through backwash discharge pipe  25 . The system depicted in  FIG. 8  could be further modified to include yet additional filter units, and is shown with just two in this embodiment for purposes of simplicity and efficiency. 
         [0052]    In an alternative embodiment of the present invention not shown here, the filter media may be natural or man-made, depending upon desired cost and hydraulic performance considerations. Media with a high specific surface area, good backwashing characteristics, and good water holding capacity are generally preferred. Preferably, most filter components should be made of materials that resist corrosion or degradation in wet conditions, such as plastic or stainless steel. In yet other alternative examples of the present invention, multiple layers of differing media may provide benefits for high strength wastewater. For instance, a coarser, less dense media over a finer, denser media would require less frequent backwashing than a uniform single media when treating high strength wastewater. Alternative media may provide benefits in treatment effectiveness and/or economics. The use of zeolite as the media in the present system may provide additional treatment because of zeolite&#39;s high capacity for adsorbing some potential pollutants. The use of fine pumice as the media may provide enhanced performance because of the low density, surface roughness, and high moisture holding capacity of fine pumice. 
         [0053]    In yet an additional alternative embodiment of the present invention, vent  27  and vent pipe  28  (see  FIG. 1 ) are not present. In this embodiment, air may escape from the filter by forcing its way through one of the saturated zones after air pressure has built up sufficiently. Such a system would reduce the wastewater infiltration rate and the amount of oxygen transferred into the media, and therefore should only be used where the vent  27  and vent pipe  28  were not a possible option. Overall, venting improves the filter&#39;s performance. To assist with venting, in an additional alternative embodiment of the invention, a small fan to draw air through the vent can further enhance oxygen transfer and venting, which may be particularly advantageous when treating high strength wastewater. Although not a preferred embodiment of the invention, an air vent in communication with a sufficiently powerful fan may also be located below the lower capillary fringe ( 37  in  FIG. 4 ) or downstream of the underdrain  16  to overcome the air flow resistance in the capillary fringe and still provide supplemental air circulation through the filter media. 
         [0054]    As described previously, the preferred embodiment of the present system utilizes a special manifold incorporating a check valve that allows treated effluent to gravity flow from the treated effluent storage tank, but backwash flow can be pressurized through a discharge control valve in a high location and/or more accessible location than the location shown for discharge control valve  22   b.    
         [0055]    The present invention provides many advantages over traditional wastewater treatment systems. First, it provides a high level of wastewater treatment and physical filtration in a much smaller footprint than conventional unsaturated wastewater filters and without needing secondary clarification treatment. Similar to fine non-backwashing wastewater filters, it provides both physical filtration and aerated biological treatment, but unlike the conventional filters, incorporates backwashing previously only used in high rate saturated filters. The present system is capable of reducing biochemical oxygen demand, suspended solids, and turbidity from wastewater to the very low levels similar to those found in conventional “tertiary” wastewater treatment plants of approximately 10 mg/L for both biochemical oxygen demand and suspended solids. This is accomplished at surface area loading rates from 5 to 20 times as high as conventional fixed media non-backwashing wastewater filters such as biotextile filters and recirculating gravel filters. 
         [0056]    The present system is also relatively inexpensive because an internal structure is not required as with fixed media filters. Furthermore, it provides a greater specific surface area for microbial growth than many other unsaturated filters because finer media can be used without plugging. The present system also provides an extremely energy-efficient means for transferring oxygen to the aerobic microbes biodegrading the wastewater without the need for large blowers, mixers, or pressurized air diffusers. Supplying sufficient oxygen for aerobic microbes is important for a compact system because aerobic microbes provide a much faster rate of pollutant biodegradation than anaerobic microbes. 
         [0057]    A benefit to coupling the present invention to a septic tank such as in one of the above-described alternative embodiments is that the amount of treated effluent recycled back to a septic tank either as part of the backwash cycle or as a direct return can be controlled to substantially increase the total nitrogen removal of the combined system due to enhanced biological denitrification in the septic tank. 
         [0058]    Data collected from an installation of the system provides insight into influent and treated effluent water quality, as well as application drain time. A tracer test as well as other operational testing was also performed. In this test system, an 18-inch diameter filter vessel was constructed using a 40-gallon flat bottom polyethylene tank. The underdrain in this test system was assembled from ¾ inch slotted PVC pipe with 0.020-inch wide slots. Pea gravel was placed on the underdrain, above which was placed a 1-inch layer of coarse #3 sand, followed by a 10 inch layer of #20 silicon filter sand and finally 12 inches of 14×30 zeolite placed on top of the filter sand layer. Municipal wastewater was fed to the filter vessel from a 1200-gallon septic tank using a submersible pump controlled by a timer. A float switch was used to control a pump feeding raw domestic wastewater to the septic tank, thereby keeping a nearly constant average daily inflow and level in the septic tank. A 300-gallon polyethylene tank was used for the treated effluent holding tank, and a submersible treated effluent pump was set therein. The treated effluent holding tank pump was set up for timer control to provide backwash flow to the filter several times per day. An internal air vent, a small vent fan, and surface wash nozzles were added. 
         [0059]    Within a few weeks of startup the prototype filter was removing most of the carbonaceous biochemical oxygen demand (BOD) from the applied wastewater. The filter took several more weeks to begin nitrifying. This is not unusual as nitrifying bacteria are relatively slow growing. Drain time performance for each application was improved substantially by the addition of surface wash nozzles. Overall filter performance was excellent within a few months, and showed very low effluent BOD, ammonia, and turbidity at a net raw wastewater surface loading rate of 320 gallons per day (gpd) with four 2 minute backwashes per day (gross wastewater loading rate of 440 gpd from the septic tank including backwash recycle). Based on nitrate measurements, partial denitrification was also occurring. In related testing, total nitrogen removal was substantially enhanced by recycling a greater amount of backwash water into the septic tank. 
         [0060]    Average concentrations of nitrogen compounds after acclimation are shown below in Table 1, below. Raw wastewater ammonia-N concentrations during the period averaged approximately 25 mg/L. As can be seen in the table, the prototype filter provided essentially full nitrification treatment and partial denitrification. Biochemical oxygen demand in treated effluent from the filter was measured at 3.5 mg/L one month after startup and non-detect in subsequent tests versus an average biochemical oxygen demand of 106 mg/L for the raw wastewater during the test period. Substantial fecal coliform bacteria reduction (2-4 logs) was also measured for the prototype filter. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Water Quality Results 
               
             
          
           
               
                   
                 Septic Tank Effluent 
                   
                 Filter Treated Effluent 
                   
               
             
          
           
               
                   
                 Ammonia-N 
                 Nitrate-N 
                 Ammonia-N 
                 Nitrate-N 
               
               
                   
                   
               
               
                   
                 18.0 
                 0.0 
                 0.5 
                 13.6 
               
               
                   
                   
               
             
          
         
       
     
         [0061]      FIG. 9  shows additional test results from an electrical conductivity (EC) Tracer Test performed on the prototype filter. The Y-axis in this chart shows electrical conductivity and is expressed as reciprocal micro ohms per centimeter (μmhos/cm). The X-axis shows time, in minutes. A mean residence time of approximately 17 minutes is derived. This is greater than the typical residence time in trickling filters, indicating a better opportunity for a higher level of treatment. This residence time is similar to residence time for textile media filters. 
         [0062]    With respect to the above description then, it is to be realized that material disclosed in the applicant&#39;s drawings and description may be modified in certain ways while still producing the same result claimed by the applicant. Such variations are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and equations and described in the specification are intended to be encompassed by the present invention. 
         [0063]    Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact disclosure shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.