Abstract:
A system for limiting the size of particulate matter entering a septic tank pump contains a strainer that has a straining screen through which the pump draws liquid from the septic tank. A nozzle receives some of the effluent being pumped by the pump and is aimed toward the screen for directing effluent toward the screen to dislodge particulate matter from an area of the screen while the pump draws liquid through the screen. The screen and nozzle are arranged for relative movement so that the area of the screen being acted on by the effluent from the nozzle changes as the pump operates. Various embodiments are disclosed.

Description:
REFERENCE TO RELATED APPLICATION AND PRIORITY CLAIM  
       [0001]    This application expressly claims the benefit of earlier filing date and right of priority from the following patent application: U.S. Provisional Application Ser. No. 60/239,537 filed on Oct. 10, 2000 in the names of Charles A. Wilkins and James O. Stoneburner. The entirety of that earlier-filed, co-pending patent application is hereby expressly incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to treatment of septic tank effluent before it is discharged to a drain field, and more particularly it relates to systems and apparatus for straining the effluent.  
         BACKGROUND OF THE INVENTION  
         [0003]    Certain wastes introduced into a septic tank tend to separate into distinct layers: a bottom sludge layer, a top scum layer, and a noticeably distinct intermediate liquid layer that is to a large extent free of scum and sludge. As waste accumulates, liquid is periodically pumped out of the tank as effluent that is discharged to a drain field. An electric-operated pump is typically used for pumping, and it is desirable to communicate the suction inlet of the pump to the intermediate layer because that layer is generally freer of solids and particulate matter than are the scum and sludge layers.  
           [0004]    To avoid the entry of undesired particulate matter into the pump, a stationary screen may be used to screen the liquid before it enters the pump. The particle size that can be effectively strained is determined by the screen mesh. Generally speaking, the finer the mesh, the smaller the particle that can be screened; however, the finer the mesh, the greater the tendency for the screen to clog. Actual screen mesh size may be a compromise based on conflicting factors. Because a screen is typically disposed within a tank or within a vault that is disposed within a tank, it may be inconvenient to regularly clean. Because a homeowner may not maintain a septic tank at the intervals recommended by a manufacturer, a manufacturer may decide that the mesh of a strainer screen may be deliberately sized more coarsely to guard against clogging due to lack of homeowner maintenance. This means that larger particulate material may enter the pump, and while this may be undesirable as far as the pump is concerned, it may be deemed preferable to the risk of screen clogging.  
           [0005]    Commonly assigned U.S. Pat. No. 6,231,764 of Charles A. Wilkins discloses a pump arrangement, including a self-cleaning rotary strainer that is effective in straining liquid that is pumped out of a septic tank as effluent to a drain field.  
           [0006]    The exemplary pump arrangement disclosed in that patent comprises an electric-motor-operated effluent pump disposed generally upright, and coaxially within, an upright main tube that extends downward from a top wall of a septic tank to a location at or near a bottom wall of the tank. The tube passes through the top scum layer, through the intermediate liquid layer, and into the bottom sludge layer. Sensors, or switches, that sense the level of waste in the tank control operation of the pump. When the level rises beyond an upper limit, the pump operates to pump fluid out of the tank until the level drops to a lower limit at which the pump shuts off.  
           [0007]    A strainer strains liquid entering the main upright tube from the intermediate layer, and includes a cylindrical filter screen disposed coaxial with the tube to constrain the effluent to flow radially through the filter screen and into the interior of the strainer. An electric pump unit is disposed coaxially within the tube and comprises an inlet port which is disposed downstream of the strainer along the direction of effluent flow through the system. The effluent provides some cooling of the motor as it flows along the motor exterior toward the inlet port where it enters the pump unit. An outlet pipe that is communicated to an outlet port at which pumped effluent exits the pump unit conveys pumped effluent out of the septic tank.  
           [0008]    A nozzle is communicated to the pump unit outlet port and arranged to emit some of the pumped effluent toward the cylindrical filter screen opposite the effluent flow through the filter screen so as to cause the filter screen to be acted upon by both radial and circumferential flow components. A journal mounts one of the screen and the nozzle for rotation relative to the other such that effluent emitted from the nozzle is effective both to turn the screen and nozzle relative to each other and to dislodge debris from the filter screen.  
           [0009]    In some embodiments, the nozzle is disposed within the interior of the strainer and arranged to emit effluent in a radially outward direction, and the flow of effluent through the filter screen is radially inward toward the interior of the strainer. In others, the nozzle is disposed in the exterior of the strainer and arranged to direct effluent radially inward toward the interior of the strainer, and the flow of effluent through the filter screen is radially outward toward the exterior of the strainer.  
           [0010]    It is believed that the self-cleaning action enables the screen to have a finer mesh, yet avoid clogging, when the septic system is used in compliance with manufacturer recommendations. In a septic system where strainer clogging may be a limiting factor, the system disclosed in the Wilkins patent can offer the potential for extending the length of time between maintenance intervals.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention relates to further improvements in self-cleaning septic tank strainers. Various embodiments of improvements are disclosed, and each possesses certain unique features within generic aspects of the present invention. The strainers use materials that are suited to provide long service life, but when needed, service may be conveniently accomplished. The inventive strainers are adapted for retrofitting existing septic systems, as well as for installation as original equipment in new septic systems.  
           [0012]    Moreover, various embodiments make the inventive strainer suitable for use in various types of septic systems. Certain embodiments are suited for use with electric-motor-operated effluent pumps of the type described at length above with reference to the Wilkins patent. Certain embodiments are suited for use with other types of septic tank pumps that comprise electric-operated pumps housed within pump vaults that are disposed within septic tanks. Certain embodiments are intended for integrated assembly with a pump to form a pump/strainer unit that can be installed within a septic tank. Certain embodiments can provide for the strainer to be remotely located from the pump. Strainers that embody principles of the invention can be used in single and multiple septic tank systems.  
           [0013]    A general aspect of the invention relates to a system for straining liquid pumped as effluent from a septic tank to a drain field. The apparatus comprises a strainer for straining liquid that is being pumped out of a septic tank by a pump before the liquid enters the pump. The strainer comprises a straining screen through which the pump draws the liquid. Solid material that is sucked against a face of the screen by pump suction is dislodged from the screen by returning some of the pumped effluent to a nozzle that is aimed toward the screen. The effluent emitted from the nozzle can act on a limited area of the screen while liquid is drawn through the remaining screen area. The screen and nozzle move relative to one another so that the area of the screen being acted on by the effluent from the nozzle is continually changing. This relative motion eventually enables the full extent of the screen to be cleaned, with the cleaning being repeated as long as the pump continues to operate.  
           [0014]    The various embodiment of the invention provide for different types of relative motion and different screen and nozzle geometries. The nozzle may be stationary while the screen moves, or alternatively the screen may be stationary while the nozzle moves. The nozzle may execute either rotary motion or oscillatory motion. The screen may have a circular annular shape that extends 360° about an axis. The screen may be mounted on a wall in covering relation to a hole in the wall, and as such may be either flat or curved.  
           [0015]    Because a septic tank that comprises a self-cleaning strainer embodying principles of the present invention allows the screen to have a finer mesh, yet avoid clogging, any particles that pass through the screen will have a smaller size. The pump is therefore not taxed by larger size particles, there is less likelihood that the nozzle or nozzles that clean the screen will clog. Because the inventive strainer prevents larger particles from being pumped out of the tank with the effluent, it may also be helpful in extending the useful life of a drain field.  
           [0016]    The foregoing features, advantages, and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims, which are accompanied by drawings. The drawings disclose a presently preferred embodiment of the invention according to the best mode contemplated at this time for carrying out the invention. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0017]    [0017]FIG. 1 shows an elevation view, partly broken away, of a first embodiment in accordance with principles of the present invention.  
         [0018]    [0018]FIG. 2 shows an elevation view of a second embodiment.  
         [0019]    [0019]FIG. 3 is a vertical cross section view in the direction of arrows  3 - 3  in FIG. 2.  
         [0020]    [0020]FIG. 4 is a view similar to FIG. 3, showing a modification.  
         [0021]    [0021]FIG. 5 is a vertical cross section view through another embodiment.  
         [0022]    [0022]FIG. 6 is a vertical cross section view through another embodiment similar to FIG. 5.  
         [0023]    [0023]FIG. 7 is a vertical cross section view through still another embodiment.  
         [0024]    [0024]FIG. 8 is a vertical cross section view through still another embodiment similar to FIG. 7.  
         [0025]    [0025]FIG. 8A is a vertical elevation view, in cross section, through an exemplary septic tank containing a pump-strainer unit according to principles of the invention.  
         [0026]    [0026]FIG. 9 is a vertical cross section view through still another embodiment.  
         [0027]    [0027]FIG. 10 is a vertical cross section view through still another embodiment.  
         [0028]    [0028]FIG. 11 is a vertical cross section view through still another embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    [0029]FIG. 1 shows a self-cleaning septic tank strainer  302 . The strainer may be associated with a pump in any of several different ways. One way is to dispose strainer  302  in underlying relation to the bottom horizontal wall  300  of a pump vault  292  so that both pump vault  292  and strainer form a unit that is supported within a septic tank  290  in any suitable manner. Preferably the unit is supported such that strainer  302  is disposed in the intermediate zone of the septic tank. Pump vault  292  contains a pump, shown schematically at  291 , that when running, is effective to draw liquid through strainer  302 .  
         [0030]    Strainer  302  comprises a circular perforated screen  304  supported uprightly on the outer margin of an imperforate horizontal circular bottom wall  306 . The screen is retained in a circular shape by uprights, or posts,  308 . The upper ends of posts  308  join to an imperforate rim  310  that is stepped to provide an outwardly directed horizontal flange  312  that is supported on a circular ledge  314  forming the margin of a circular hole  316  in wall  300 . Flange  312  is captured on ledge  314  by an overlying wall  318  which has a flanged circular hole  320  at its center and is formed as part of bottom wall  300  as shown. When the pump is drawing liquid through the strainer, the liquid flows in the direction of the arrows  321 , passing through screen  304  and thence upward through hole  320  to the pump inlet. The pump pumps the liquid as effluent through a discharge pipe  323  ultimately leading to a drain field.  
         [0031]    Adjacent hole  320 , a supply pipe  322 , that is teed into discharge pipe  323  through which the pump pumps effluent, extends through wall  318  to terminate in one or more nozzles  324  facing the interior side of the circular screen  304 . Supply pipe  322  conveys some of the pumped effluent to the nozzle(s). The nozzle(s) are arranged at a suitable angle not only to clean the screen but also to impart a horizontal force that is effective to rotate strainer  302  about a vertical centerline CL that is shared by the strainer, the screen, flange  312 , and hole  320 . In this way the screen rotates past the nozzle(s) where it will be continuously cleaned by the nozzle(s) to dislodge accumulated debris from the exterior side of the screen. Because a limited circumferential extent of the screen is being cleaned at any given time, liquid can be drawn through the remainder of the screen to be pumped as effluent. Joints of the upper and lower edges of the circular screen are tight to the bottom wall and the rim so that debris will not be sucked through the joints. Flange  312  has close running clearance to the crevice within which it is captured between walls  300  and  312 .  
         [0032]    Alternatively, wall  300  can be a separate wall that is disposed against the bottom wall of a pump vault with hole  320  positioned in registration with a suction inlet of the pump extending through the vault bottom wall.  
         [0033]    Another way to associate strainer  302  with a pump is like the way described in commonly assigned U.S. Pat. No. 6,231,764 where wall  300  is disposed within the interior of a cylindrical vault tube. Suitable modification is made to the strainer so that it will fit to the axial flow type pump shown in that patent.  
         [0034]    [0034]FIGS. 2 and 3 illustrate a second embodiment of strainer  330  that comprises a circular imperforate cup  332  having an open upper rim  334  that is fit tight to wall  300  to prevent debris intrusion between them. The side wall of the cup comprises a circular hole  336  that is covered by a perforated screen  338  having a tight fit of the screen edge to the margin of the hole to prevent debris intrusion between them. Wall  300  forms the bottom of the pump vault  292  containing pump  291  that draws liquid through the screen to pump out the septic tank. A tap  340  at the pump outlet returns some of the pumped effluent through supply pipe  322  for cleaning the screen.  
         [0035]    A rotary spray arm  342  is mounted within the interior of cup  332 , and contains one or more nozzles  344  aimed at the interior side of screen  338 . The liquid pressure delivered to spray arm  342  is effective to cause it to rotate about a horizontal axis that intersects the center of the screen as sprays are also being emitted from its nozzles. Because the nozzles spray only a limited area of the screen at any given instant of time to dislodge debris from the exterior of the screen, the remainder of the screen remains open so that the pump can draw liquid through it. It is believed beneficial to place a wall  346  over the screen in relatively close outwardly spaced relation to limit the influence of the nozzle spray on the unfiltered waste.  
         [0036]    The second embodiment can be modified to an embodiment (not shown by a drawing) wherein such a rotary spray arm is mounted above the level of wall  300  to emit spray toward an existing screen in the side wall of a tube mounted within the vault within which the pump is disposed.  
         [0037]    [0037]FIG. 4 shows an embodiment that has a strainer like the one of FIGS. 2 and 3. It differs in that it allows the pump to be remotely located with the suction inlet of the pump being coupled to the strainer interior by a suction tube  350 .  
         [0038]    [0038]FIG. 5 shows another embodiment where a pump/strainer unit  500  is disposed within a vault  502  having a closed bottom wall  504  and a side wall  506 . Unit  500  is supported in any suitable manner within vault  502 , such as being supported upright on bottom wall  504 . Vault  502  is itself supported in any suitable manner within the interior of a septic tank  505 . Side wall  506  extends vertically upward from bottom wall  504  to bound the vault interior. Side wall  506  is closed except for being open at the top and having a series of holes  508  spaced in succession around the side wall circumference shortly below the open top of the vault. Liquid in septic tank  505  that is above the level of holes  508  spills into the interior of vault  502  where pump/strainer unit  500  is disposed. In use, the interior of vault  502  will typically be completely filled with liquid so that pump/strainer unit  500  is completely immersed in liquid.  
         [0039]    Unit  500  comprises an electric pump like one shown in commonly assigned U.S. Pat. No. 6,231,764. A pump/motor sleeve  34  is disposed upright coaxial with and within the interior of a support tube  36  of larger diameter. Both sleeve  34  and tube  36  can be commercial PVC pipe. Tube  36  is closed at the bottom, either by a sealed fit to bottom wall  504  of the vault (as shown), or by a closure (not shown) that is fit to the lower end of the tube in a fully sealed manner. Sleeve  34  and tube  36  are associated by any suitable construction that keeps the lower end of the sleeve open.  
         [0040]    Pump/strainer unit  500  comprises a strainer  10  having a walled enclosure  512  that is fit to, and closes, the upper end of support tube  36 . Enclosure  512  has an interior that serves to communicate support tube  36  to the interior of vault  502 . Enclosure  512  comprises a vertical side wall  514  containing a hole  516  covered by a mesh screen  518 .  
         [0041]    An electric pump unit  48  comprising an electric motor  48   m  and a pump  48   p  driven by the motor is disposed coaxially within sleeve  34 . When unit  48  operates, it draws liquid in vault  502  through strainer  10 . The flow path through unit  500  is shown by the arrows. Liquid is strained by screen  518  as it enters the interior of enclosure  512  at hole  516 . It passes through the interior to enter support tube  36 , thence downwardly through the annular space between sleeve  34  and tube  36 , and thence around the lower edge of sleeve  34  where it enters the sleeve. The liquid then flows upward through the sleeve to enter the pump, which will pump the liquid out as effluent through a discharge conduit, or pipe,  49  extending from the pump discharge outlet. As the liquid passes along the exterior of motor  48   m , heat from the motor can transfer to the liquid whereby the liquid provides motor cooling. As the unit is being operated, liquid under pressure is being delivered to a spray nozzle  28  through a supply pipe  50  to clean screen  518 . Because the pumped effluent has been strained by strainer  10 , it may be used to supply spray nozzle  28  by teeing pipe  50  into conduit  49 . The upper end of sleeve  34  is closed to the top horizontal wall  520  of enclosure  512 , and both conduit  49  and pipe  50  pass through, and are sealed to, holes in that wall. Thus, within the liquid in vault  502 , unit  500  is totally enclosed except for the opening to the interior of enclosure  512  through screen  518 .  
         [0042]    The liquid drawn from the interior of enclosure  512  is continually replenished by flow through screen  518 . Screen  518  strains the liquid that enters enclosure  512  so that particulate and other material larger than a certain size is prevented from entering the enclosure. In this way the liquid is strained before it ever reaches the pump.  
         [0043]    Spray nozzle  28  comprises a spray mechanism like that of U.S. Pat. No. 5,058,806, hereby incorporated by reference. That mechanism is effective to create a spray that is directed outwardly against the inside of screen  518  to dislodge adhering debris from the outside of the screen. The spray washes only a limited area of screen  518  at any given time, but it moves across the screen to eventually wash the entire screen area. The spray motion may be rotary or oscillatory. The screen may be flat or curved. Because a limited zone of the screen is being cleaned at any given time, liquid can be drawn through the remainder of the screen by the pump.  
         [0044]    In a specific embodiment, enclosure  512  comprises an upright cylindrical tube that is closed at both top and bottom. Tube  36  is also closed at both ends. A short horizontal tube  513  extends between confronting portions of the side walls of the two vertical tubes just below the closed upper ends of the two tubes. Tube  513  serves to communicate the interior of the tube forming enclosure  512  to the interior of tube  36 .  
         [0045]    [0045]FIG. 6 shows an embodiment that is like that of FIG. 5 except that vault  502  is not used, and pump/strainer unit  500  is instead disposed directly within septic tank  505 . Like elements in both FIGS. 5 and 6 are identified by the same reference numerals, and so a detailed description of unit  500  will not be repeated in connection with FIG. 6. When unit  500  is placed directly within septic tank  505  without vault  502 , it is preferable to locate screen  518  in the intermediate zone of liquid in the tank. When vault  502  is used, the vault aids in preventing larger material suspended in liquid from entering the vault.  
         [0046]    [0046]FIG. 7 shows another embodiment of pump/strainer unit  700  that is disposed within a vault  502  like the one of FIG. 5. Vault  502  is in turn disposed within a septic tank  505 . Unit  700  comprises a pump like the one described in connection with FIG. 5, but does not use a tube  36  because the associated self-cleaning strainer  10 A communicates to the bottom of sleeve  34 . Sleeve  34  is still disposed upright and is closed at the top so that liquid in the septic tank cannot enter that end.  
         [0047]    Strainer  10 A comprises a walled enclosure  512 A having an interior for conveying strained liquid into the lower end of sleeve  34 . At its upper end, the strainer comprises a mesh screen  712  that is cleaned by a spray nozzle  28  disposed interior of the screen. In a specific embodiment, enclosure  512 A comprises an upright cylindrical tube that is closed at the bottom. A short horizontal tube  513 A extends between confronting portions of the side wall of the tube forming enclosure  512 A and the side wall of sleeve  34 .  
         [0048]    Nozzle  28  may be like the one previously described, with either rotary or oscillatory motion of the spray across the screen. The screen may be flat or curved, and it may have a full or partial circumferential extent. FIG. 7 shows a specific example where screen  712  is circular with its upper and lower edges secured to the margins of circular walls  714 ,  716 . Holes are present at the centers of walls  714 ,  716 . The body of spray nozzle  28  passes through the hole in wall  714 , and a cap  720  that supports nozzle  28  closes the hole. The hole in the center of wall  716  is fit to the upper end of the tube forming enclosure  512 A. The spray emitted from nozzle  28  traverses screen  712  with circumferential motion.  
         [0049]    [0049]FIG. 8 shows an embodiment that is like that of FIG. 7 except that vault  502  is not used, and pump/strainer unit  700  is instead disposed directly within septic tank  505 . Like elements in both FIGS. 7 and 8 are identified by the same reference numerals, and so a detailed description of unit  700  will not be repeated in connection with FIG. 8. When unit  700  is placed directly within septic tank  505  without vault  502 , it is preferable to locate screen  712  in the intermediate zone of liquid in the tank.  
         [0050]    [0050]FIG. 8A shows an example of a septic system that comprises an in-ground septic tank  505 . Tank  505  encloses a rectangular volume into which liquid waste, such as household sewage, is introduced through an inlet pipe  101 . A pump/strainer unit  700  is disposed within tank  505  for drawing liquid from the tank and pumping it out as effluent through a discharge pipe  49  to a drain field. The top of tank  505  is closed by a tank lid, or cover,  104  that contains two access risers  105 ,  106  extending upward to above ground level. The risers are cylindrical in shape and are closed at the top by removable caps  107 ,  108 . When removed from the risers, caps  107 ,  108  allow access to inlet pipe  101  and to pump/strainer unit  700 .  
         [0051]    When a new septic tank is being installed, the tank can be set in an excavation with or without pump/strainer unit  700  installed. Before tank lid  104  is placed on the tank and the tank covered with fill, unit  700  can be easily placed into the tank because the top is fully open. After the unit has been placed, a pipe leading from the unit to the drain field can be connected to discharge pipe  49  in any appropriate manner. FIG. 8A shows such a pipe passing through the sidewall of riser  106 , above lid  104 , but below cap  108 . The Figure also shows a float switch tree  111  as part of the unit. The tree may be supported upright from a stand that forms the base of the unit. The switches of the tree control the operation of the electric operated pump to pump out effluent when the level rises to a certain level in the tank and to then shut off the pump once the tank has been pumped out to below a certain level. The switches are at elevations intended to keep strainer  10 A in the intermediate zone of liquid in the tank. FIG. 8 further shows electric connections  113 ,  115  to both the tree and the pump motor.  
         [0052]    In an existing septic tank installation that has a pump, but lacks the self-cleaning strainer of the present invention, the tank is below ground, and so the only access for retrofitting the existing installation with a self-cleaning strainer  10 A is via riser  106 . The maximum lateral dimension of unit  700  allows it to pass through conventional risers that may have diameters as small as twenty inches. Hence, the inventive unit can retrofit, and be serviced when needed, via the existing riser.  
         [0053]    [0053]FIG. 9 shows another embodiment where a strainer  910  is associated with a pump vault  905  containing a pump  906  to form a unit  900 . Pump  906  may be a centrifugal type, electric operated pump. Although a septic tank is not shown, unit  900  is disposed in any suitable manner within the tank, such as simply resting on the bottom wall of the tank. Vault  905  comprises a walled enclosure that is closed except for having an entrance opening  907  and an exit opening  908  in a top horizontal wall of the vault. Strainer  910  is disposed in covering relation to entrance opening  907 . Exit opening  908  provides for discharge pipe  911  to pass upward from the discharge outlet of pump  906  to transport pumped effluent out of the tank. A supply pipe  914  is teed into pipe  911  for returning some of the pumped effluent to a spray nozzle  28  that is disposed within the interior of strainer  910 . Strainer  910  comprises a walled enclosure containing a screen  912 . The screen may be flat or curved, and it may have a full or partial circumferential extent. Nozzle  28  may be like the one previously described, with either rotary or oscillatory motion of the spray across the screen. The walled enclosure of strainer  910  fits onto the top wall of vault  905  over entrance opening  907 . The bottom of the walled enclosure is open to expose the enclosure interior to the interior of vault  905 .  
         [0054]    [0054]FIG. 9 shows a specific example where screen  912  is circular with its upper and lower edges secured to circular edges of the enclosure side wall. Supply pipe  914  passes through the otherwise closed top wall of the enclosure and spray nozzle is attached to and supported from the end of the supply pipe. The spray emitted from nozzle  28  traverses screen  912  with circumferential motion.  
         [0055]    [0055]FIG. 10 shows another embodiment of pump/strainer unit  1000  that is disposed within a vault  1002  that in turn is disposed within a septic tank  1005 . Unit  1000  comprises a pump  906  and discharge pipe  911  like those described in connection with FIG. 9. A supply pipe  914  is teed into pipe  911  for returning some of the pumped effluent to a spray nozzle  28  that is disposed within the interior of a strainer  1010  that is associated with the pump and vault to form unit  1000 . Strainer  1010  is disposed on the exterior of vault  1002  beneath the bottom horizontal wall of the vault in covering relation to entrance opening  1007  in the bottom wall of the vault. Supply pipe  914  passes through the interior of the vault and entrance opening  1007  to spray nozzle  28 . Strainer  1010  comprises a screen  1012  that may be flat or curved, and that may have a full or partial circumferential extent. Nozzle  28  may be like the one previously described, with either rotary or oscillatory motion of the spray across the screen.  
         [0056]    [0056]FIG. 10 shows a specific example where screen  1012  is circular with its upper edge configured in any suitable way to close against the bottom wall of the vault. The lower edge of the screen is fit to the margin of a circular wall of the strainer enclosure that closes the bottom of the strainer. The embodiment of FIG. 10 is intended to be suspended in a septic tank to place the strainer in the intermediate zone of liquid in the tank.  
         [0057]    [0057]FIG. 11 shows an embodiment similar to FIG. 10 except that the screen mounts directly in the side wall of vault  1002  at a level above the bottom wall of the vault. This allows the vault to be mounted on the bottom wall of the septic tank. The screen covers a hole in the vault wall.  
         [0058]    In embodiments where a pump/strainer unit is disposed directly in a septic tank, without a pump vault, a float switch tree, like tree  111  in FIG. 8A, may be associated with the pump/strainer unit to operate the pump motor so that the intermediate zone of liquid in the tank is maintained at the level of the mesh strainer screen. In embodiments using a pump vault that has openings  508  near the top of the pump vault, opening that are above the self-cleaning strainer, a switch tree may be external to the pump vault, to keep the intermediate zone in the tank at the same level as openings  508  so that liquid that enters the pump vault from the tank through openings  508  will come from the intermediate zone of liquid in the tank.  
         [0059]    In any of the embodiments shown using a spray nozzle  28 , a different type of nozzle may be substituted, with the screen shape possibly being modified to accommodate the different nozzle. For example, the rotary spray arm shown in FIGS.  2 - 4  may be substituted, in which case, the screen may not extend around the full circumferential extent of the strainer. In any embodiment the spray must have sufficient strength to dislodge debris from the screen, but at the same time, the spray should not be so strong as to excessively disturb the contents of the tank, especially when the outer face of the screen does not face a wall of a pump vault like it does in FIGS. 5 and 7. Hence, in installations like those of FIGS. 6, 8,  9 ,  10 , and  11 , it may be desirable to place a wall of suitable size and shape a short distance from the outer face of the screen, in the same way that wall  346  is associated with the screen in FIG. 3. While the invention may be practiced in various embodiments, such as those specifically illustrated, the inventive principles also contemplate uses where a self-cleaning strainer is installed in a septic system that is already in use, i.e. retrofitting of an existing septic system. Certain embodiments that have been illustrated and described are especially suited for existing septic tank pumps and vaults.  
         [0060]    While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles of the invention are applicable to other embodiments within the scope of the following claims.