A septic field dosing system is described which utilizes a self starting true siphon to deliver a dose of septic tank effluent into a conventional septic field. The precision siphon unit not only eliminates the need for electrical services to the septic tank, thereby eliminating the need for pumps and solenoid operated valves and switches at the septic tank, but also contains the effluent handling equipment in a small unit. For example, the drawdown of the septic tank is used to entirely control the operation of the dosing siphon. Further, the dimensional configuration of the precision siphon operates within the boundaries of the draw down to deliver a proper size dose to the septic field. While a typical septic tank can hold of the order of a thousand or more gallons, the precision siphon can easily fit into a five gallon bucket.

This is a Continuation-in-Part application of Ser. No. 11/699,151, filed on Jan. 29, 2007 now abandoned, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to the treatment of wastewater and more particularly, to a septic system design with true siphon dosing and effluent filtration with backwash. 
     (2) Background of the Invention and Description of Previous Art 
     It is common knowledge among sanitary engineers that to prolong the life of septic fields it is necessary to clean or filter the effluent from the septic tank and to rapidly discharge a measured quantity (dose) of filtered effluent to flood the septic fields. The dose volume is normally about 70% of the septic field volume. This procedure extends the life of the septic field by distributing the effluent, and more importantly, the residual solids contained in the effluent, over the entire field rather than only near the entrance to the field where they will accumulate and eventually clog the first few feet of the septic field thus rendering a portion of the field's capacity to percolate effluent useless. Once this deterioration starts it will overload the remaining functioning portion of the field, which will lead to a total failure of the field in due time. The replacement of a failed septic system is very costly and messy operation. 
     Before describing the prior state of the art in this field it would be useful to keep in mind that septic tanks are buried underground to prevent freezing or for esthetic reasons. There is, in general, six inches to a foot or two of earth on top of the tank. To remove the tank covers for pumping out the tank contents or to inspect for malfunctioning components of the system, earth over the tank covers must be dug out to gain access. Access to the septic tank is not easy and is generally beyond the aptitude of most building owners. Typically a functioning septic tank should be pumped out once every two or three years. 
     The following patents were examined to ascertain the prior state of the art in this field. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Hanford, William E., 
                 U.S. Pat. No. 4,040,962 
               
               
                   
                 Ball, Harold L., 
                 U.S. Pat. No. 4,439,323 
               
               
                   
                 Gavin, Norman, 
                 U.S. Pat. No. 4,838,731 
               
               
                   
                 Daniels, Byron C., 
                 U.S. Pat. No. 5,198,113 
               
               
                   
                 Graves, Jan D., 
                 U.S. Pat. No. 5,207,896 
               
               
                   
                 Richard, James G., 
                 U.S. Pat. No. 5,290,434 
               
               
                   
                 Ball, Eric S., 
                 U.S. Pat. No. 5,492,635 
               
               
                   
                 Stuth, William L., 
                 U.S. Pat. No. 5,690,824 
               
               
                   
                 Wilkins, Charles A., 
                 U.S. Pat. No. 6,231,764 
               
               
                   
                   
               
             
          
         
       
     
     Methods and apparatus for improving the performance of septic system as described in the above mentioned patents have found limited use due to one or more of the following drawbacks.
         1. High initial investment.   2. Cannot be retrofitted in existing installations.   3. Costly maintenance and operation.   4. Complexity and bulk.   5. High elevation differential requirements.   6. Not true siphons.   7. No dosing provision.   8. Insufficient or no filtration.   9. No provision to indicate the need for backwash or filter replacement.   10. Need for external power (electric pumps).   11. Need for frequent refill of chemicals.   12. Not intended for Septic Systems.       

     It is important to recognize the enormity of the job a residential septic system must perform over its intended life time which is typically between 20 or more years. A typical single family consumes approximately 1000 gallons of water every day. Over a 25 year period the septic system processes over 9 million gallons or 76 million pounds of effluent. The quality of domestic waste dumped into the septic system varies greatly with the lifestyle of each family, particularly if a food disposal unit is utilized to grind kitchen waste and send it to the septic tank. The amount of sludge and flotsam removed by frequent tank pump outs will also vary over a wide range. 
     The suspended solids are of most concern because they clog up the septic fields. Daniels, '113 utilizes open cell polymeric foam to filter the effluent from the septic tank. No information is provided regarding the particle size of the solids, which will pass through the filter medium. However it is easy to guess from the general description that the particle size will be fairly small. Although the filter will remove most of the suspended solids from the effluent, frequent replacement of the filter element is necessary. This requires shoveling away the earth to expose the cover, removing the cover and replacing the filter, replacing the cover and the earth, not a welcome task with the ground frozen solid in winter. The reference also requires an electric pump to move the filtered effluent from the dosing chamber to the septic septic fields, thereby requiring the provision of electric service at the tank. 
     Ball, '635 describes a series of multiple size filters the smallest of which has an opening of ⅛ th  of an inch. Here too an electric pump is required, and has no backwash system. A ⅛ th  of an inch opening in the filter will pass 3,175-micron particles. Graves, '896 describes a multistage filtration process, which aims to filter particles as small as 1000 microns. However this process is dependent upon chlorination, aerobic agitation, and optional de-chlorination, thus requiring electrical power and chemicals. When the filters clog, the unit must be removed and cleaned. As with the previous reference this requires exposing and opening the tank to clean or replace the filter, again a substantial undertaking. No dosing mechanism is provided so, with the exception of the filter, the septic system has the site limitations of a simple gravity fed system. 
     Filtration systems are generally categorized by the particle size, which will pass through them. Particle size is generally measured in microns. A Micron is one millionth of a meter or 40 millionth of an inch. For reference the high quality drinking water filters block particles larger than 5 microns from passing through them. Some coarse drinking water filters would pass 30 micron particles. With respect to filtration in a septic system, to pass solids of over 3,000 microns is tantamount to no filtration at all. A great majority of suspended particles in a septic tank are much smaller; therefore much finer filter media are necessary to clean the effluent significantly. It becomes clear why Ball, '635 cites that the filter requires cleaning only as often as the container (the septic tank) requires pumping to remove accumulated sludge. 
     It is difficult to establish the suspended particle size distribution of the effluent, because each family's life style is different. Assuming a linear distribution of particle size the following table will illustrate the importance of filtration medium. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Quantity of suspended solids in 76 million lbs (38,000 Tons) of 
               
               
                 effluent processed over a lifetime of 25 years. 
               
             
          
           
               
                   
                 Percent (by Weight) 
                   
                 Quantity 
               
               
                   
                   
               
             
          
           
               
                   
                 1.0% 
                 (10,000 ppm.) 
                 380 Tons 
               
               
                   
                 0.1% 
                  (1,000 ppm.) 
                  38 Tons 
               
               
                   
                 0.001%  
                   (100 ppm.) 
                  3.8 Tons 
               
               
                   
                 0.0001%   
                    (10 ppm.) 
                 760 lbs. 
               
               
                   
                   
               
             
          
         
       
     
     Even at the lower concentrations there is sufficient quantity and volume of suspended solids, which if not removed by filtration would plug up any septic field. Clearly, the importance of good effluent filtration cannot be overemphasized. 
     By comparison this invention filters particles as small as 100 to 200 microns by using a super fine filter. A 100 mesh screen (10,000 holes per square inch) will filter 180 micron or larger particles and a 150 mesh screen (22,500 holes per square inch) which will filter 100 micron or larger particles, and the effluent will be almost as clean with respect to suspended particles as the domestic water supply, a great benefit for the life of the septic field. 
     The present invention shows the use of a precision and true siphon to dose and filter the liquid extracted from the central clear zone of a septic tank. The unit is contained in a housing mounted and ported on the discharge side of a septic tank. Liquid flows into the housing from the bottom thereof, passes through a fine mesh basket strainer or filter, and rises in the housing until enough is collected to start the siphon. Then the siphon is initiated through the lifting of a float and the proper dose is delivered to a distribution box. After delivery of the dose volume the siphon is broken, and a predetermined short time thereafter a remote timer triggers a solenoid valve, which sends pressurized domestic water to backwash the filter for a predetermined time after which the system becomes ready for the next dosing cycle. 
     Referring now to  FIG. 11 , there is shown a typical septic system  120  comprising a septic tank  122 , a drainpipe  124 , and a distribution box  126 . The inflow  127  to the septic system  120  is delivered through a pipe  121  emanating from a building or house (not shown) and received at the inlet of the septic tank  122 . A septic field towards which the outflow  128  from the distribution box  126  is directed is not shown. 
     The total elevation difference  130  is defined as the difference in elevation between the bottom of the inlet pipe  121  and the bottom of the lowest level in the distribution box  126 . The total elevation difference  130  can be further broken down to the sum of the septic tank drop  132 , the pitch drop  134 , and the distribution box drop  136 . 
     The selection of an effluent delivery system i.e. a gravity siphon, or a pump system depends on the total elevation difference  130 . In most health jurisdictions the minimum required difference  132  between the inlet and outlet of the septic tank is about three inches, but in some cases it can be as much as six inches or more. The pitch drop  134  depends upon the distance  135  between the septic tank and the distribution box. Most health departments require that a pitch or gradient of 1 in 100 or about ⅛ of an inch per foot of drainpipe length be maintained. The distribution box drop  136  is normally about one inch. The pitch drop  134  dictates the choice of an effluent disposal system as follows:
         a. If the pitch drop  134  is insufficient to maintain the required pitch or if the distribution box is at a higher elevation than the liquid level in the septic tank, then it becomes necessary to install a pumping system.   b. If the pitch drop  134  is just enough to maintain a pitch of 1 in 100, then a simple gravity system is the only choice.   c. If the pitch drop  134  is large enough to meet the incremental elevation differential requirements, then a classical Bell Siphon (not shown) or the so-called siphon systems (some of which are included in the list of patents cited i.e. Ball, '323 and Richard, '434) on the market can be used.       

     Referring to  FIG. 12 , these systems  140  require a dosing chamber  148  downstream of the septic tank  142 , which holds the entire dose volume. Depending upon the dose volume, which governs the dimensions of the dosing chamber  148 , the incremental siphon drop  147  can be anywhere from 6 to 18 inches on top of the pitch and distribution box drop  149 . The pitch drop here is measured from the input to the drainpipe  144  near the bottom of dosing chamber  148 . The dose volume is denoted in the figure by  146 . The septic tank drop  143  is measured between the bottoms of the entry and exit pipes of the tank  142 , and plays no role in the performance of the above mentioned so called siphon systems. 
     Neither, Ball, '323 nor Richard, '434 are true siphons, because the effluent is always under a positive hydrostatic head, and there is no vacuum anywhere in the drainpipe. A true siphon is defined as a continuous tube (siphon tube) that allows liquid to drain, without requiring pumping assistance, from a reservoir at a higher elevation to a point at a lower elevation, where the tube passes through an intermediate point that is higher than the reservoir. The up flow from the reservoir is driven by the pressure difference created by the vacuum formed by the siphon process at the highest point of the siphon tube. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an economical and reliable true siphon operated dosing system to prolong the life of septic fields of residential dwellings or commercial buildings. 
     It is yet another object of this invention to provide an economical and reliable true siphon operated dosing system that can be deployed in cases where the pitch drop is insufficient even for a simple gravity system. 
     It is still another object of this invention to provide an economical and reliable true siphon operated dosing system that utilizes the septic tank drop to provide additional hydrostatic head to increase the flow rate of the siphon. 
     It is another object of this invention to provide an economical and reliable true siphon operated dosing system that utilizes the full difference in elevation to drive the siphon flow at a high velocity. 
     It is yet another object of this invention to provide an economical and reliable true siphon operated dosing system that utilizes a float-operated valve to initially block the flow of screened effluent into the drainpipe. 
     It is still another object of this invention to provide an economical and reliable true siphon operated dosing system that passes the effluent through a fine mesh screen filter prior to its entry into the working section of the siphon. 
     It is still another object of this invention to provide an economical and reliable true siphon operated dosing system that backwashes the fine mesh filter screen after each dosing cycle. 
     These objects are accomplished by a precision siphoning unit containing two cylindrical control floats arranged respectively above and below, and concentric with a stationary cylindrical member having large central flow passages surrounded by a plurality of smaller flow passages. An elastic sealing surface on the underside of the upper float provides a seal across inner and outer valve seats on top of the cylindrical stationary member thereby blocking flow through the large flow passages. An elastic sealing surface on top of the lower float seals the plurality of smaller flow passages protruding out of the bottom of the stationary member. The floats and the stationary member are housed in a cylindrical barrel having a top cover, an open bottom, and a side opening. The stationary member is sealed to the inside of the barrel. A fine mesh screen basket filter of a diameter, slightly larger than that of the barrel is supported in a wire mesh basket, which in turn is fastened to the bottom of the barrel. A central pipe passes through the barrel, the floats, and the stationary member. A rotatable sprinkler arm is attached to the bottom of the central pipe with a rotatable seal. The sprinkler arm fits inside the basket filter. The barrel assembly with the basket filter and sprinkler are contained in a larger diameter cylindrical outer housing with a top cover. A water pipe passes through the top covers of the barrel and the outer housing, and is connected, through a solenoid valve, to a pressurized domestic water supply in the house or building, which is served by the septic system. The water pipe enters the central pipe concentrically and terminates therein. The outer cylindrical housing is mounted on the discharge side of the septic tank service the house or building. 
     Effluent from the clear zone of the septic tank passes through an inlet pipe into the bottom of the outer cylindrical housing, where it first passes through the basket filter. As the liquid level rises in the septic tank due to incoming waste, the now filtered effluent is blocked from passing through to the discharge port of the precision siphon unit by cooperation of the floats and the stationary member. When the liquid rises above the end of the inner water pipe, the air pressure therein begins to rise. The liquid continues to rise in the housing and in the central pipe, building up head, until it passes through spillover ports at the top of the annulus between inner and outer central pipes. The spilled over liquid falls into the compartment surrounding the upper float. The float then becomes buoyant, rises, and releases a sudden surge of flow through the central flow passages. The flow passes through the exit port of the barrel taking along with it most of the air in the upper float compartment, and the drainpipe. 
     Just prior to the upper float becoming buoyant a pressure switch, located in the building and connected to the water pipe, senses the increase in air pressure in the water pipe. At a preset pressure, the pressure switch triggers a timer which, after a time delay, initiates the opening of a solenoid valve in the building which sends a flow of high pressure domestic water through the water pipe for a short time period (about one minute). This sudden rush of high-pressure water pushes the remaining air out of the siphon unit, the drainpipe, and the system, now primed, initiates the siphon flow. The flow of liquid continues at a gradually diminishing rate as the liquid level in the tank drops. When the liquid level between the barrel and the outer housing falls below vent openings in the barrel, which are located below the level of the floats, air enters the barrel, the floats drop, and the siphon is broken. 
     After a time delay to assure that the siphon flow has ceased, the timer in the building or house again opens the solenoid valve for about five minutes to send a second flow of pressurized domestic water through the central pipe causing the sprinkler in the siphon unit to back flush the fine mesh basket screen filter, thereby driving the accumulated particulate matter on the filter screen back into the septic tank. 
     It is yet another object of this invention to provide a method for retrofitting the precision siphon unit of this invention into an existing conventional gravity septic system without removing any of the components of the original system. 
     This object is accomplished by lowering the exit port of the existing septic tank by creating a new exit port and plugging the old port, placing a new smaller diameter flexible drainpipe into the existing drainpipe, and fitting a new discharge pan into the existing distribution box. 
     It is another object of this invention to provide an economical and reliable true siphon operated dosing system to prolong the life of septic fields of residential dwellings or commercial buildings wherein backwashing of said fine mesh filter is accomplished without a water supply connection of the dosing unit to the building being serviced thereby eliminating the need for a control box in the building. 
     This object is accomplished by backwashing the fine mesh filter with the volume of liquid trapped in the dosing unit after the siphon is broken. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross section of a modern two-compartment septic tank with the precision siphon unit of the present invention attached on the outside and downstream of the septic tank. 
         FIG. 2  shows a cross section of the precision siphon-dosing unit taught by this invention. 
         FIG. 3  is an isometric view of the core floats and spray assembly of the precision siphon unit taught by this invention. 
         FIG. 4  is a top view of the lower float of the float and spray assembly of this invention. 
         FIG. 5   a  is a top view of the stationary central unit of the float and spray assembly of the invention. 
         FIG. 5   b  is a top view of the upper float of the float and spray assembly of the invention. 
         FIG. 6  is a block diagram showing the layout of the interface between the building or house and the precision siphon-dosing system (installed on the outside of the septic tank) of this invention. 
         FIGS. 7   a  and  7   b  are a cross sectional views showing the configuration of a distribution box arrangement including a discharge pan and discharge elbow which cooperates with the precision siphon-dosing unit taught by this invention. 
         FIG. 7   c  is a top view of the discharge pan illustrated in  FIGS. 7   a  and  7   b  showing an arrangement of drain holes in the pan and the locations of webs which physically connect the discharge elbow to the pan. 
         FIGS. 8   a  through  8   e  are cross sections of the float assembly region of the precision siphon unit of this invention showing the position of the floats and the location of effluent within the unit as the effluent level rises within the unit and falls during siphon flow. 
         FIG. 9  is a diagram of the configuration of a control box for the precision siphon dosing system taught by this invention 
         FIG. 10  is a diagram showing a retrofit conversion of an existing conventional septic tank system to the system using the precision siphon dosing system taught by this invention. 
         FIG. 11  is a diagram showing the configuration of a conventional septic tank waste disposal system. 
         FIG. 12  is diagram showing a conventional septic system utilizing prior art dosing technology. 
         FIG. 13  is a cross sectional diagram showing the installation of the precision siphon unit taught by this invention inside a conventional septic tank. 
         FIG. 14  shows a vertical cross section of the precision siphon-dosing unit taught by a second embodiment of this invention. 
         FIG. 15   a  is an isometric view of the chamber partition of the second embodiment of this invention as seen from above. 
         FIG. 15   b  is an isometric view of the chamber partition of the second embodiment of this invention as seen from below. 
         FIG. 16  is an isometric drawing showing details of the retainer stem of the second embodiment of this invention. 
         FIG. 17  is a horizontal cross-section of the second embodiment of this invention denoted by the line A-A in  FIG. 14  as viewed from above, showing the upper flow passages  225  which extend from the outer chamber  214  through openings in the inner tube  229  into a space over the metering orifice  228 . 
         FIG. 18   a  through  18   f  are vertical cross sections of the of the second embodiment of this invention illustrating the operation thereof by showing the position of the floats and the location of effluent within the unit as the effluent level rises within the unit and falls during siphon flow. 
         FIG. 19  is a view of a portion of the vertical cross section of the precision siphon-dosing unit taught by a second embodiment of this invention illustrating the clearance volume. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a first embodiment of this invention the construction and functioning of an economical and reliable true siphon operated dosing system to prolong the life of septic fields of residential homes or commercial buildings is described. Referring first to  FIG. 1 , there is shown a cross section of a modern two-compartment septic tank  10 . The tank is generally made of cast concrete but may be constructed of a tough polymer material. The tank  10  is provided with removable covers  11  and  11   a , which provide access to compartments  12  and  13  respectively. In the figures boxed arrows indicate the flow of effluent             Septic waste effluent  15   a  emanating from a building or a house (not shown) enters the first compartment  12  of the tank  10  through an inlet port  16 . Over time, a settling process takes place as well as fermentation caused by the action of anaerobic bacteria. Solids in the effluent settle to the bottom to form a sludge layer  17 . A scum layer  18  of buoyant solids forms on top of a relatively clear liquid  15 . Clear liquid from the first compartment flows through an opening in a baffle  19  into the second compartment where the settling process continues but far less scum and sludge are produced.
     The precision siphon apparatus  20  of the invention is enclosed in an outer housing  21 . Effluent  15  from the clear center zone of the septic tank  10  enters the housing  21  through an inlet conduit  22  at the bottom and rises therein as the flow accumulates in the septic tank. The bottom of the outer housing  21  is conical, and the inlet conduit  22  is pitched down towards the septic tank  10  to facilitate the return of debris to the septic tank during backwash of the siphon unit. The draw down  71  in the septic tank  10  and precision siphon  20  combination, that is, the difference between the highest  23  and lowest  24  liquid levels is determined by the configuration of the precision siphon apparatus  20 . The draw down  71  will be presented later when the functioning of the precision siphon is described. An air vent  26  from the top of outer housing  21  into the top portion of the septic tank above the highest liquid level is provided. A water pipe  27  is connected to the domestic water supply and a control box in the building or house (not shown). Note that the level  23  represents the highest level of septic effluent in the combined septic tank  10  and precision siphon  20  and is determined by a feature of the precision siphon unit, which will be discussed later. 
     Referring now to  FIG. 2  a cross section of the precision siphon-dosing unit  20  as taught by this invention is shown. The isometric view of the core float and spray assembly  28  of the unit shown in  FIG. 3  may be of help in visualizing the construction. The float and spray assembly  28  consists of a lower float  29 , an upper float  30 , and a stationary central unit  31  in between, the three components operating within a cylindrical barrel  34 . The two floats have a smaller diameter than the inside diameter of the barrel so that the may travel freely along the axis of the barrel, guided by a central pipe  27   a . The barrel  34  is formed of PVC, high density polyethylene, or other inert polymer material, which will not corrode or deform in the septic effluent environment. The barrel  34  has an inner diameter of about 4 inches. The upper float  30 , the larger of the two floats, has a diameter approximately 1/16 th  of an inch smaller than the inner diameter of the barrel  34 . This clearance also allows liquid to flow around the float towards the bottom. The float  30  preferably consists of an injection molded outer shell made of the same material as the barrel  34 . The cavity is filled with a closed cell foam and is weighted to meet the performance requirements for the float  30 . An elastic gasket disk  35  is bonded onto the lower surface of the float  30 . The lower float  29  is also preferably made of the same material as the barrel and the upper float. Both upper and lower floats are designed so that their weight is about half the weight of the volume of displaced liquid respectively. It is the intent that when fully submerged in the liquid, each float has a net upward buoyancy approximately equal to its weight so that it can exert sufficient force to close the respective flow passages without leakage. 
     The elastic gasket disk  35  is formed of a soft durable rubber or a synthetic elastic polymer. The thickness of the elastic gasket disk is approximately one sixteenth of an inch. The upper float  30  has an additional design requirement that it be of sufficient weight to block the passage of effluent thru the main flow passages  47  by exerting sufficient down force on the seating surfaces  45  and  46  when the liquid is near its highest level  23 . 
     The weight of the upper float  30  including the gasket disk  35  bonded thereto is preferably between about 10 and 20 percent greater than the maximum upward hydrostatic force exerted on the bottom of the float between the inner and outer sealing seats  45  and  46  in the closed position when the liquid level in the septic tank is at its highest. The volume of the upper float  30  preferably should be such that its net buoyancy when totally immersed in liquid is equal to or more than the combined weight of the upper float  30  and the gasket disk  35 . 
     Referring to  FIG. 5   b , there is shown a top view of the upper float  30 . In operation the upper float becomes buoyant only when liquid enters the upper chamber  64  from above. The liquid passes down along the sides of the upper float  30 . In order to assure free flow of liquid to the base of the float, channels  48  are formed along the sides of the float. 
     The lower float  29  has an outer segment  36  and a concentric inner segment  37  ( FIG. 2 , not shown in  FIG. 3 ) connected to the outer segment by hollow structural webs (not shown). Large flow passages  38  between the inner segment  37  and outer segment  36  assure rapid deployment of the dose once the siphon cycle begins. Vertical sleeves  52   b  pass through the body of the lower float allow the passage of retaining bolts  52 . The retaining bolts  52  serve as alignment guides as well as supports when the lower float is not buoyant. An elastic gasket  40 , made of the same material as elastic gasket  35 , is bonded to the top surface of the lower float  29 . The gasket  40  has punched openings corresponding to the flow passages  38  and retaining bolt sleeves  52   b . The hollow cavity inside each float segment is filled with closed cell foam. The combined weight of the float  29  and the elastic gasket  40  should be about half the weight of displaced volume of liquid. The elastic gasket material forms a watertight seal when the lower float rises and presses against drain tubes  42 , which pass through and extend beneath the bottoms of the inner and outer sections of the stationary member  31  as will be explained later. When the lower float is not buoyant it rests upon the wide heads  52   a  of the retaining bolts  52  which are threaded into the lower part of the central unit  31  through the lower float. 
     A top view of the lower float  29  is shown in  FIG. 4 . The places of contact  43  where the drain tubes  42  press into the elastic gasket  40  are shown in phantom in the figure. The structural webs which connect the outer  36  and inner  37  segments lie under the regions  39  of the gasket  40 . 
     The stationary member unit  31  is a solid, cast or molded body which houses the seating surfaces for the two floats. The stationary member  31  is sealed into the barrel  34  and onto the central pipe  27   a  to make it water tight all around its outer and inner perimeters. The sealing seats for the lower float  29  consist of the machined sharp edged bottoms of the drain tubes  42 , which project beneath the bottom of the stationary member  31  and have already been mentioned supra. The seating surfaces  45  and  46  for the upper float are machined and sharp edged to seal off the main flow passages  47  which pass through the stationary member  31  and the subjacent lower float  29 , thereby preventing the flow of filtered effluent around the outer portion of the upper float  30  and inner portion around the central pipe  27 , to prevent it from becoming buoyant and prematurely releasing the main surge which will initiate the siphon.  FIG. 5   a  is a top view of the stationary member  31  showing the configuration of the seals  45  and  46  and the drain tubes  42 . The structural webs  32  between the flow passages  47  connect the inner and outer portions of the stationary member  31 . The retaining bolts  52  which pass through the sleeves  52   b  in the lower float are threaded into the underside of the stationary member  31 . The threaded holes  52   c  which receive these bolts are shown in phantom in  FIG. 5   a.    
     Returning to  FIGS. 2 and 3 , the remaining features of the precision siphon are described. A fine mesh screen filter  53   a  is supported on a rigid wire basket  53   b . The basket  53   b  is fastened to a lip  25   a  on the base of the barrel  34  and is also supported by a second lip  25   b  extending from the housing  21 . A rotatable sprinkler tube  50  is attached to the bottom of the central pipe  27   a  with a rotatable seal  51 . The sprinkler tube  50  fits within the basket  53   b . A plurality of small openings  54  are arranged along the bottom and one side of the sprinkler tube  50  so that when pressurized water is forced through the openings the sprinkler tube  50  rotates slowly, dislodging and flushing collected particulates from over the entire surface of the filter  53   a  in a backwashing action. 
     The outer housing  21  has a tightly fitting top cover  21   a  through which the vent pipe  26  and the water pipe  27  enter the unit. The bottom portion  21   b  of the housing  21 . is funnel shaped towards the inlet conduit  22 . 
     The exit port of the precision siphon-dosing unit consists of a drainpipe  60  emanating at approximately at the same level as the upper float  30  at rest on its seats  45  and  46 . A tube  63  is routed from an opening  62  at the top of the drainpipe  60  to the chamber  64  in the top of the barrel  34 , which is fitted with an airtight cover  65 . 
     A plurality of spillover ports  66  is located in the top of the annulus between the central pipe  27   a  and the extension  27   b . of the water pipe  27 . The spillover ports allow effluent to flow into the upper chamber  64 . The elevation of the spillover ports  66  in the dome  65   a  determines the highest level  23  of liquid in the entire system. The lowest liquid level  24  is defined in the vicinity of openings  70  at the base of the barrel  34 . The distance  71  now becomes the draw down of the entire system. 
     To start the operation of the siphon, on the cue of a pressure sensor, a short burst of high pressure domestic water is delivered into the unit via the water pipe  27 . The water comes out of the spillover ports  66  after the upper float  30  has either lifted or is about to be lifted, the water fills the upper chamber  64  completely, and pushes any remaining air through the siphon tube  63  into the drainpipe  60  and eventually out of the system. The top of central pipe  27   a  protrudes into a dome  65   a  at the top of the cover  65  and is fastened to the inner pipe  27   b  which is the extension of the water pipe  27  into the unit. 
     The net flow through area of the fine mesh screen filter  53   a  is preferably at least about ten times the flow area of the drainpipe  60 . This reduces the impact velocity of the suspended particles against the fine mesh screen thus making it easier to dislodge them by the subsequent backwash action. The net flow through area of a 100 mesh screen is about 45% of the face area and a 150 mesh screen is about 35% open. 
     The draw down  71  and the diameter of drainpipe  60  determine the internal configuration of the precision siphon unit. If the inner diameter  73  of the drainpipe  60  is fixed at 2 inches nominal, the minimum height of the barrel  34  above the bottom of drainpipe  60  or from the top of the seats  45  and  46  should be the sum of the inner diameter  73  of the drainpipe and the top clearance  74  which must be the height of upper float  30  including the thickness of the soft elastic disk  35 . This height plus the height of spillover ports  66  in the dome  65   a  determines the maximum hydrostatic pressure that the upper float  30  must withstand in order to keep the liquid from discharging into the drainpipe  60 . After the liquid level in the annulus between the central pipe  27   a  and concentric inner pipe  27   b  pipes has reached the spillover ports  66  the upper chamber  64  of the barrel begins to fill. For the upper float  30  to become buoyant it is necessary that liquid should accumulate in the space around the lower half of the float and on the outside of seating surfaces  45  and  46 . The upper float  30  should be immersed in the liquid to a depth, which is near the depth to which the float will sink while freely floating in the liquid. To achieve this accumulation of liquid, a drainpipe uplift  75  is provided. The cross sectional area of the flow passages  38  and  47  must be equal to or greater than the internal flow area of the drain pipe  60  to assure efficient operation of the siphon. 
     Referring now to  FIG. 6  there is shown a block diagram of a complete septic system taught by this invention including a building  92 , from which the septic flow emanates and enters the septic tank  10  and precision siphon  20  assembly (FIG.  1 , 2 ). The drainpipe  60  delivers a dose from the siphon unit to a discharge pan  83  in the distribution box  80  ( FIG. 7   a ), which in turn passes it into the septic field  95 . The pipe  27  is connected to a domestic water supply in the building  92  through a solenoid switch. The dashed line  96  indicates the grade of the land. The pipe  27  must be buried or otherwise insulated to protect from freezing and damage. It is also laid from the building to the dosing unit at a downward pitch to allow for drainage into the dosing unit  20  when the water is turned off. 
     A control box  94  located within the building  92 , illustrated in detail in  FIG. 9 , communicates with the precision siphon unit  20  through the water pipe  27 . When the pipe is empty and at atmospheric pressure, the system senses the dormancy of the siphon cycle. When sensor switch  97  senses a predetermined increase in air pressure in the pipe due to rising effluent in the dosing unit, it indicates that the liquid level in the annulus between inner and outer pipes  27   a  and  27   b  in the siphon unit  20  is at or near the top. At this time the sensor switch  97  triggers the timer  98  which, in turn, energizes the solenoid valve  99 . The control box  94  receives electrical power to operate its components from the buildings electrical supply E. The solenoid valve  99  operates to deliver timed flows of pressurized water from the buildings domestic water supply W through the water pipe  27  and into the dosing unit, first to initiate the siphon flow and later to backwash the filter  53   a . The timer  98  is set to allow a fixed time to elapse to assure that the siphon cycle is completed before starting the backwash. The timer  98  then actuates the solenoid valve  99  for a predetermined period, typically about five minutes, causing a flow of domestic water to operate the backwash sprinkler  50  for a predetermined time after which operation ceases and the siphon unit  20  is returned to its starting configuration, ready to accumulate the next dose. Using a sensing system as described here, eliminates the need to supply electric service to the siphon unit  20 , which would have been costly and hazardous. 
     The distribution box  80 , which receives the dosed output of the precision dosing unit  20  through the discharge pipe  60 , also must be modified in order to cooperate with the dosing unit during a siphon flow. 
       FIG. 7   a  shows installation of discharge pan  83  in a standard distribution box  81 . The box  80  is fitted with a cover  82 . The discharge pan  83  is connected to the drainpipe elbow  87  by connecting webs to secure the spacing between the two. The webs  83   a  are illustrated in the top view and cross section of the pan assembly shown in  FIGS. 7   b  and  7   c  respectively. There are small openings  85  on the lower side and bottom of the drain pan  83 , which allow the tray to drain after the siphon ceases. During siphon flow, drainage through the small openings  85  is insignificant and the tray is quickly filled and the primary discharge therefrom is overflow. However, it is essential to drain the tray  83 , particularly in colder climates, to avoid freezing. The small openings  85  serve this purpose. The overflow is discharged from the distribution box  80  though the exit opening  90  at the bottom of the box and into the septic field (not shown). 
     During a siphon operation in the precision siphon unit  20  the filtered effluent discharge therefrom is delivered into the distribution box  80  though the drainpipe  60  entering at the input port  86 . of the distribution box  80 . A discharge elbow  87  is fitted on the end of the pipe  60  to deliver the effluent vertically into the tray  83 . The discharge elbow  87  must extend into the tray  83  so that when the tray is filled, the discharge end of the elbow  87  must be submerged at least one fourth of an inch in the liquid to form an air lock. In order to provide unrestricted flow area, the end of discharge elbow  87  must be above the bottom of the tray  83  one fourth of the inside diameter at the end of the elbow. This geometry is fixed by the connecting structural webs  83   a  between the tray  83  and the elbow  87 . 
     Referring now to  FIG. 7   b , there is shown a cross section of the discharge pan at the end of the discharge elbow  87 , illustrating the fastening of the elbow to the pan with structural webs  83   a .  FIG. 7   c  is a top view of a horizontal cross section of the discharge pan taken at the level b-b′. where four webs and four drainage holes are shown. While additional openings and webs may be added, it is found that the arrangement of openings and webs shown in the figures is sufficient. 
     The detailed step-by-step operation of the precision siphon of this embodiment will now be described and is illustrated in  FIGS. 8   a  through  8   e  which show the status of the precision siphon unit at several liquid levels  68 . The left hand sides of these figures show the corresponding liquid level  68   a  in a portion of the septic tank  10  during each step. 
     The starting point of the siphon cycle is chosen here to be the point at which the level of septic effluent  15  has reached the openings  70  in the bottom of the barrel  34 . This point is reached on the initial filling of the septic tank  10  and also thereafter when the siphon is broken at the end of each dose delivery. 
     In  FIG. 8   a  clear effluent  15  from the septic tank  10  has risen in the unit  20  through the input pipe  22 , (see also  FIG. 2 ), passed through the fine mesh screen filter  53   a , where any residual particles are trapped and reached the level of the openings  70 . The liquid level continues to rise in the housing  21  and into the bottom of a cylindrical barrel  34  where it reaches the bottom of a lower float  29 . In the absence of liquid, the lower float  29  rests upon wide heads of the orientation retaining bolts  52 . Referring to  FIG. 8   b , as the liquid continues to rise, the lower float  29  lifts from the heads of the support bolts  52  and rises to meet the bottom of the drain tubes  42 . The elastic layer  40  seals off the drain tubes  42  preventing flow into the region of the barrel  34  surrounding the upper float  30 . In  FIG. 8   c , the effluent level has risen further in the region between the barrel  34  and the surrounding housing  21  as well as in the water pipe  27   a  but the sealed drain tubes  42  have kept the effluent from entering the upper region of the barrel above the stationary member  31 , thereby keeping the upper float  30  from becoming buoyant. 
     When the liquid rises above the bottom of the water pipe extension  27   b  the pressure in the air filled water pipe  27  begins to increase because the solenoid valve in the building is closed. This pressure increase is sensed by the pressure switch  97  in the control box  94 . The pressure switch is set to trigger the timer  98  to start when the pressure reaches a pre-set value which can be determined either by experiment or by calculation from the overall volume of the water pipe. This value is the pressure reached in the pipe  27  approximately when the effluent level is near the level  23  of the spillover ports  66 . 
     When the level of the effluent finally reaches spillover ports  66  as shown in  FIG. 8   c , the liquid begins to overflow  78  into the upper chamber  64 . The elevation  75  in the attached drain pipe  60  prevents the outflow of effluent from the barrel  34  to the drainpipe  60  and allows sufficient accumulation to cause the upper float  30  to become buoyant. Once this occurs the seal between the seating surfaces  45  and  46  and the elastic polymer  35  under the upper float  30  is broken and a sudden rush of effluent passing though the main passages  38  and  47  is released slamming the upper float  30  against the barrel top cover  65  as shown in  FIG. 8   d.    
     At about the same time, the timer, which has been started by the pressure switch, completes a preset time delay and triggers the solenoid valve  99  to open releasing a burst of water through the water pipe  27  driving any residual air from the upper regions of the chamber  64  and the tube  63  causing the onset of siphon flow indicated by the boxed arrow in  FIG. 8   d . The burst of water is maintained only for approximately a minute or less after which the timer triggers the solenoid valve  99  to close. 
     Once the siphon begins and the drain pipe  60  is filled, the pan  83  in the distribution box fills and provides an air tight seal to sustain the siphon until the entire dose is delivered, and the level of effluent in the precision siphon unit  20  drops down to the level of the vent holes  70  in the barrel  34 , allowing air through the vent pipe  26  on top of the septic tank  10  to enter the flow and break the siphon. 
     The space at the top of septic tank  10  is connected to the atmosphere via the vents in the plumbing system of the building. This keeps the pressure on top of the liquid layer in the tank always at atmospheric level. If this were not so, then immediately after the start of the siphon, a vacuum would start to develop at the top of liquid in the septic tank  10 , and the siphon will cease to operate. 
       FIG. 8   e  shows the liquid levels  68  and  68   a  in the siphon unit and in the septic tank respectively near the end of the siphon flow. As the liquid level outside of the barrel  34  continues to drop, the openings  70  are eventually exposed, and air rushes into the lower portion of the barrel. The barrel begins to drain, and the siphon is broken. Now the upper float  30  drops down, and seals off the larger passages  38  and  47 . The lower float  29  also drops down, and opens the heretofore sealed drain tubes  42  which now permit total drainage of the barrel  34 , returning the liquid level in the barrel to the lower limit line  24 . Air now fills the space in the barrel  34  and the water pipe  27  all the way up to the solenoid valve  99  in the control box. 
     After a short delay the timer causes the solenoid valve  99  to open once again at a time when the siphon cycle has reliably been completed. Pressurized domestic water again flows through the water pipe  27  down the central pipe  27   a  of the siphon unit  20  and out the openings  54  of the sprinkler arm  50 , which initiates the post siphon backwash of the filter  53   a  within the precision unit. The backwashing is sustained for between 5 and 10 minutes after which the timer closes the solenoid valve  99 . The backwashing time is preset in the timer  98  and is determined mainly by the amount of debris collected during the dosing period which depends on the particular application. Typically the back wash period is between about 5 and 10 minutes. Once the backwashing is complete the water now drains out of the pipe  27  and the cycle is complete. 
     While in the foregoing embodiment the precision dosing unit  20  was mounted externally on the septic tank  10 , it may also be mounted in the septic tank on the wall having the exit port.  FIG. 13  illustrates a suitable installation wherein the unit  20  is supported by its inlet pipe  141  which is now perforated at the top to accommodate the incoming liquid and the returning backwash debris. Alternately the unit may be fastened onto the inner wall of the septic tank  10  (not shown). The vent pipe  26  is now already in the tank. The water pipe  27  and the drain pipe  60  are fed in through side openings in the tank. The compartment cover  11   a  provides ready access to the unit. An advantage of this internal mounting is added protection of the unit. 
     The key component of the present invention, with regard to dosing, is the upper float  30  which, when resting on the sealing surfaces  45 ,  46 , blocks the flow of liquid into the drain pipe as well as into the upper region of the barrel surrounding the upper float while the liquid level elsewhere in the siphon unit and in the septic tank rises to a higher level, thereby building up the dose volume and hydrostatic head beneath the float. The maximum head achieved when the liquid level has risen well above the upper float, is not sufficient to force the float off the seals. However, as liquid begins to come out of the spillover ports  66 , and accumulates around the lower half of the upper float, it becomes buoyant, and breaks the seal at  45  and  46 . The sudden rush of liquid from below slams the upper float up against the cover. The resulting surge of liquid, supplied by the large volume in the septic tank, quickly forces most of the air out of the drainpipe. Any remaining air in the system is quickly expelled by the entrance of high-pressure water from pipe  27  via the sprinkler jets  54  and the spillover ports  66 , and the siphon starts. 
     The lower float  29  serves only to block the flow of liquid into the upper float region through the drain tubes  42 . The drain tubes  42  are needed to drain the liquid remaining in the upper chamber of the barrel after the upper float has dropped back and re-sealed the main flow passages  47  at the end of the siphon cycle. 
     While the sprinkler backwash assembly plays no role in the dose accumulation and delivery, it is nevertheless a necessary item, which greatly extends the functionality of the fine mesh filter  53   a ; so much so that the filter needs no service even when the septic tank is pumped out and cleaned. 
     Referring now to  FIG. 10  there is shown a block diagram, which illustrates how the precision siphon can be retrofitted into an existing conventional gravity fed septic system. The existing septic tank  100  can be either a single or double compartment unit. The conventional exit port of the septic tank is typically too high for use with the precision siphon system and must therefore be plugged  101 . A new opening  102  is made below the original and the precision siphon unit  103  described supra is mounted onto the septic tank connecting the inlet pipe  104  to the new opening. The siphon units vent tube  105  is also fitted into a second new opening in the top air region of the septic tank  100 . Because the drain pipe required for the precision siphon unit is significantly smaller in diameter (about 2 inches) than the conventional drain pipe (nominally 4 or 5 inches), the new flexible drain pipe  106  is easily inserted within the original drain pipe  107 . Depending on the original layout, this may have to be done prior to mounting the new unit  103 . The new drainpipe is mechanically protected by the original pipe  107  and may be made of a flexible material or of a polymer such as PVC. 
     The existing distribution box  108  may also be re-used and outfitted with a pan  109 . The end of the drainpipe  106  is fitted with a discharge elbow  110  which is fastened to the pan, having the same relationship to the pan  109  as the corresponding items  87  and  83  in the distribution box  80  supra. The output  111  of the distribution box  108  is left connected to the septic field as is. The input  112  to the septic tank  100  is left undisturbed. Finally a control box and water pipe connection  114  must be made connecting the retrofitted unit to the buildings water and electric supply. This retrofit clearly requires a very minimum (5-10 cubic feet) of excavation and labor making it highly cost effective. 
     The precision siphon dosing system can be deployed even in those cases where some pitch drop is available, but is insufficient for a simple gravity system, and would normally require the installation of a pump system. If the pitchdrop is enough to maintain a slope of 1 in 200 or even 1 in 300 or 400. The precision siphon dosing can be used as explained below. 
     If the effluent is cleaned by filtration, as it is in this embodiment, then the customary  1  in 100 pitch is excessive, and there is no justification for it. For comparison the pitch in natural streams or other channels is generally in the range of 1 in 1000, and it still makes the water rapidly flow forward in the downhill direction. Reducing the pitch in half or 1 in 200 or less will still generate sufficient open channel flow velocity to empty the drainpipe quickly after the siphon is broken. It is necessary to empty the drainpipe quickly to prevent freezing of effluent in colder climates. 
     The precision siphon described by present invention provides the following advantages:
         1. It eliminates the need for hazardous and costly electrical service to the septic tanks, thereby eliminating the need for pumps, and other electrical devices in the septic tank.   2. It also eliminates the need for costly, bulky, and separate dosing chambers by effectively using the internal volume of the septic tank, by incorporating into the dose volume the presently unutilized volume represented by the septic tank drop of three inches or more. To make up the entire dose volume it only needs about four inches of the volume below the exit port of the conventional septic tanks. The above-mentioned volume is never available to delay the need for pumping out the sludge from the septic tank, because if the sludge has accumulated to a level to block the passage of liquid through the partition baffle  19  then the system is not functioning and the tank needs to be pumped out anyway.   3. The precision siphon uses the full force of the head provided by the difference in elevation between the bottom of the entrance pipe to the septic tank and the bottom of the distribution box.   4. The precision siphon unit  20  is small enough to fit in a five-gallon bucket.       

     In a second embodiment of this invention the construction and functioning of an economical and reliable true siphon operated dosing system to prolong the life of septic fields of residential homes or commercial buildings wherein the backwashing of the fine mesh filter is accomplished entirely within the unit without the use of a water pipe or any other support from the building which is serviced by the septic system. The siphon dosing unit is housed entirely within the septic tank and backwashes its filter by return flow of the septic effluent trapped within the unit after the siphon breaks. 
     Referring to  FIG. 14 , there is shown a cross section of the siphon dosing unit  200  housed in a conventional septic tank  10  having a cover  11   a . The outlet pipe  232  of the unit  200  passes through the septic tank wall and to a distribution box configured in the same manner as that of the first embodiment described supra and shown in  FIGS. 7   a  and  7   b . The unit  200  is supported within the tank either by a post structure as shown in  FIG. 13  or by the exit pipe  232  itself. 
     A chamber partition  209  supports both upper  221  and lower  205  floats. The chamber partition is sandwiched between the upper  215  and lower  235  portions of the barrel housing of the dosing unit, providing an airtight connection, and is shown in greater detail in  FIGS. 15   a  and  15   b . The chamber partition  209  controls flow between upper and lower chambers by operation of the floats  205  and  221 . 
     A siphon tube  204 , supported on the bottom flange  234 , passes vertically through and is sealed, to make the connection airtight, onto the top cover  226  of the unit  200 , and extends several inches above the highest liquid level  23  in the septic tank  10 , thereby assuring a continuous exposure of the opening of this tube to atmosphere. A tee  270  is included on top of siphon tube to prevent debris from entering the tube. A hole  203  on the side and near the bottom of the siphon tube  204  determines the lowest liquid level  24  in the septic tank  10 . As in the first embodiment, the siphon dosing operates between these two levels. The difference between these two levels, as in the first embodiment is referred to as the drawdown  71  of the dosing unit. 
     A fine mesh filter screen  201  is sandwiched across the bottom input collar  237  of the dosing unit  200  between the inner collar  236  and the locking collar  238 . The locking collar  238  may be a snap-ring/O-ring combination permitting easy removal and replacement of the filter  201 . 
     Referring now also to  FIG. 15   a , the chamber partition  209  is provided with multiple openings  217  which connect the outer chamber  214  (the annular space between the outer cylindrical barrel  215  and the inner cylindrical barrel  216 ) of the dosing unit with the lower chamber  202 , to enable liquid to freely flow between the upper and lower chambers. In the same pattern, opening  204   a  provides a passthrough for the siphon tube  204 , while the groove  216   a  receives the bottom of the inner cylindrical barrel which is sealed thereto, separating the inner  218  and outer  214  chambers. 
     The outer drain passages  211  (i.e., outside of upper float seals  223 ) pass through the three structural webs  239  to connect to the vertical inner drain passages  210 , which drain into the space between the lower seals  207  and  208 . The lower float  205  is suspended from the center of the chamber partition  203  by a retainer stem  212  which also contains a drain passage  230  to empty the central tube  229  at the end of the siphon cycle. The features of the lower float can be best seen in  FIG. 16  where the drain  230  in the retainer stem  212  is illustrated showing a vertical passage  230   a  which connects to a horizontal passage  230   b . On assembly the retainer stem  212  is glued into the opening in the center of the lower float  205 . When assembled, the horizontal passage  230   b  in the retainer stem is just above the top of the sealing gasket  206  as shown in  FIG. 14 . In operation, when the lower float  205  rises, the gasket  206  engages the inner and outer circular seats  207  and,  208 , thereby sealing off the openings from drain passages  210  and  211 , as well as the passage  230  in the retainer stem  212 , thereby preventing liquid from flowing into the upper chamber  218  as well as into the central tube  229 . 
     The circular seats  207  and  208  engaged by the lower float  205  as well as the circular seats  222  and  223  engaged by the upper float gasket  222  are machined on the bottom and top surfaces respectively of the chamber partition  209  which is important in order to obtain a tight seal.  FIG. 15   b , illustrating the underside of the chamber partition  209 , shows the lower float sealing surfaces  207  and  208  and the openings  210  of the vertical drain passages. 
     The top cover of the siphon dosing unit  200  contains the features for filling the upper chamber  218  and starting the siphon flow when the septic fluid level reaches its highest level  23 .  FIG. 17  illustrates a section of the top cover  226  perpendicular to the line A-A′ in  FIG. 14 . Referring now to  FIG. 14  with reference to  FIG. 17  there are three passages  225  in the top cover  226  through which septic effluent flows from the outer chamber  214  into the inner tube  229  when the highest level  23  in the septic tank has been reached. The effluent passes through metering orifice  228  located within the top of the inner tube  229 . When tube  229  is filled liquid overflows through openings  231  into the inner chamber  218 , eventually causing the upper float assembly  221 ,  220  to become buoyant and starting the siphon flow. This process is similar to that described in the first embodiment and will be detailed for the present embodiment later. The elevated bend  235  in the discharge tube  232  has the same function as in the first embodiment and is designed to permit just the right amount of liquid to accumulate to make the upper float assembly  221 , 220  buoyant enough to lift, and to start the rapid onset of the siphon flow. The volume of the upper float assembly  221 , 220  is such that its maximum buoyancy when totally immersed in liquid is about twice its weight. For the upper float assembly  221 , 220  to become buoyant it is necessary that liquid should accumulate in the space around the lower half of the float and on the outside of seating surfaces  222  and  223 . The upper float assembly  221 , 220  should be immersed in the liquid to a depth, which is near the depth to which the float will sink while freely floating in the liquid. To achieve this accumulation of liquid, a discharge tube uplift  235  is provided. 
     Referring now to  FIG. 18   a , the operational cycle of the second embodiment of this invention will be described. In the figure the septic liquid level is at its lowest  24 . The lower float is suspended from the retainer stem  212 . As more liquid enters the septic tank  10 , the level in the dosing unit  200  rises with liquid flowing through the fine mesh screen filter  201  and into the lower chamber  202 . In  FIG. 18   b  the level has risen sufficiently to cause the lower float assembly to become buoyant, rising to seal the passages  210  to the upper chamber  218  as well as the passage  230  at the base of the inner tube  229 . 
     In  FIG. 18   c  the liquid continues to rise unobstructed into the siphon tube  204  through siphon port  203 , and into the outer chamber  214  through openings  217  in the chamber partition  209 . In  FIG. 18   c , when the liquid level has reached the maximum level  23  and begins to flow through the passages  225  in the top cover  226  and down through the opening  228   a  in the metering orifice  228 , thereby beginning to fill  252  the inner tube  229  from the top. 
     When the liquid level in the inner tube  229  reaches the overflow passages  231  the liquid begins to spill over  254  into the inner chamber  218  as shown in  FIG. 18   d . In the figure, enough liquid has flowed into the inner chamber  218  to bring the level in the discharge tube  232  to near the top of the bend  235 , and the upper float  221  is about to break free. 
     Further rise of the liquid level in the inner chamber  218 , but before overflow at the bend  235  occurs, provides enough incremental buoyancy (by design) to lift the upper float  221 , when said overflow occurs and raise it to the bottom of top cover  226 . The upper float assembly  221  pushes the air in the inner chamber  218  into the discharge tube  232 . There is ample clearance between the float  221  and inner wall of the inner barrel  216  and the outer wall of the inner tube  229  to allow the escape of air. 
     In a very short period (a few seconds) the inner chamber  218  is filled with liquid and the discharge pipe  232  starts filling up. Quickly thereafter all the air in the system is pushed out through the discharge pipe  232  and the discharge pan  83  in the distribution box  80  (see  FIG. 7   a ), and the siphon operation begins. 
     As the siphon proceeds, the liquid level in the septic tank  10 , siphon tube  204 , and the outer chamber of the dosing unit  200 , begins to drop. Referring now to  FIG. 18   e , when the liquid level in the septic tank  10  and in the siphon tube  204  drops to near its lowest level  24 , the siphon port  203  becomes exposed to air. Air  260  begins to bubble in and through the annular passages  217  and  219  and starts entering simultaneously into the inner chamber  218  and outer chamber  214 , thus breaking the siphon, and causing the trapped liquid to begin to drain downwards. Liquid from the outer and inner chambers flows  262  back into the septic tank  10  through the fine mesh filter  201 . This flow backwashes and cleans the filter. 
     After a few seconds the upper float assembly  221 ,  220  starts to drop. Liquid trapped in the outer chamber  214  only starts to drop once air bubbles enter the device, and flow through passages  217 . Trapped liquid displaced by air in chamber  218  flows down through passages  219 . 
     Before the inner chamber  218  is fully emptied, the upper float assembly  221 ,  220  falls back onto the seals  222  and  223 , thereby blocking further flow of liquid from the inner chamber  218  into the lower chamber  202 , and leaving residual liquid in a clearance volume  266 , as shown in  FIG. 18   f . The clearance volume  266  is illustrated in  FIG. 19  and represents the maximum amount of liquid that can remain in the upper chamber after the upper float assembly has dropped. The residual liquid  267  ( FIG. 18   f ) in the clearance volume  266  must be removed so that upper float assembly  221 ,  220  does not rise prematurely during the next siphon cycle. 
     Liquid continues to flow back into the septic tank to seek equilibrium. Eventually the lower float  205  drops to its suspended position, releasing the seals against the drain passages  210 / 211  and thereby allowing the residual liquid in the clearance volume  266 , as well as liquid in the inner tube  229 , to drain back into the lower chamber, through passages  210 / 211  and through discharge tube  232 , respectively. At the same time the inner tube drains through the passage  230  in the stem retainer  212  completing the filter backwash and returning the liquid level in the dosing unit to the initial condition shown in  FIG. 18   a . When all the liquid above the lowest liquid level  24  has drained back the siphon cycle is completed, and the unit is ready for the next cycle. 
     While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit, principles, and scope of the invention.