Abstract:
A screen cleaning apparatus for use, for example, in connection with a discharge conduit of a sewage pumping station, may include a housing, a screen, a rake and a float. The housing may operate to contain a fluid (and/or define a container for the other components) and include a fluid inlet and a fluid outlet. The screen may be positioned in the housing and define one or more apertures. The rake may also be positioned in the housing and have one or more protrusions, with each protrusion extending into a respective aperture in the screen. The float may apply a force to the rake so that the rake moves relative to the screen and in dependence upon variations of a volume of fluid received in the housing. Accordingly, the rake may rise and fall will the fluid level to remove debris from the screen.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. national phase of PCT/GB2008/000664 filed Feb. 27, 2008, which claims priority of Great Britain Patent Application 0703823.5 filed Feb. 27, 2007 and Great Britain Patent Application 0704557.8 filed Mar. 8, 2007. 
     
    
     BACKGROUND TO THE INVENTION 
       [0002]    The present invention relates to an automatic screen-cleaning apparatus. In particular, the present invention relates to an automatic screen cleaning apparatus for use as part of a sewage pumping station or the like. 
         [0003]    Sewage pumping stations are conventionally underground structures that foul or surface water sewage is discharged into There are various types of sewage stations, but in smaller systems the sewage station is typically a wet well i.e. where the pumping station houses one or more submersible pumps which pump the sewage onwards to its next destination. In a larger station there may be a separate dry well, adjacent to the wet well, which houses the pumps. Alternatively, on some pumping stations the pumps may be housed above ground near the wet well. 
         [0004]    In storm conditions or other extreme circumstances which would be sufficient to flood the pumping station with sewage in volumes which exceed the intended capacity of the sewage pumping station, many pumping stations have consent to discharge sewage in a controlled manner into a nearby river or stream. Such consent is issued by the Environmental Agency in an effort to avoid a breach of raw sewage at ground level where the pumping station is situated. In the event of such an extreme situation arising, sewage is discharged from the pumping station via an outfall&#39; into a nearby river or stream. 
         [0005]    The most common causes of flooding of pumping stations include heavy rainfall, electrical power failure, for example caused by power-cuts, mechanical breakdown of pumps, and downstream blockage of the sewage transit pipe system. 
         [0006]    Most pumping stations are required to filter the sewage to prevent larger particles of sewage solids or Rag being discharged. Rag is the generic name for any non-organic material that is present in the sewage liquid and includes materials such as toilet tissue, old dish cloths and other fabric materials, and other items that may be found floating on the top of the sewage surface. 
         [0007]    If the screen becomes blocked, less filtered sewage is discharged and thus, if more sewage is entering the pumping station that is able to be discharged through the blocked or partially blocked screen, the sewage level at the pumping station rises and eventually the sewage will breach the ground surface, rather than being safely discharged into a nearby river or stream. As some pumping stations are located in residential areas, this is both unhygienic and undesirable. 
         [0008]    One prior art solution that attempts to address this problem involves a high pressure jet arrangement located so as to spray the residue or rag off the screen in a cleaning action, thereby keeping the screen in an unblocked state. Such systems may be automated when used in combination with a sonar device, but have expensive installation and operating costs. For most pumping stations the cost of automated screen cleaning is not economical, and so the screen is cleaned manually using a rake or a mobile pressure washer when an operator visits the site. The manual cleaning is normally triggered after a storm event, but this operation is often missed and the screen remains at least partially blocked. 
         [0009]    There is therefore a need for a low cost way to facilitate efficient automated screen cleaning. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention seeks to address the problems of the prior art. 
         [0011]    Accordingly, a first aspect of the present invention provides an automatic screen cleaning apparatus comprising:
       a. a housing for containing a liquid, the housing provided with a liquid inlet, a liquid outlet and a screen member;   b. the screen member defining at least one aperture therethrough;   c. a rake member received within the housing and having one or more protrusions, the or each protrusion extending into a respective aperture defined by the screen member;   d. a resilient member arranged so as to apply a first force to a first surface of the rake member; and   e. a float member adapted to apply a second opposing force to the rake member.
 
such that the rake member is moveable relative to the screen member in dependence upon variations in the volume of liquid received within the housing.
       
 
         [0017]    Thus, when sewage enters the housing, the rake member rises with the rising surface level of the sewage due to the presence of the float member. This causes the protrusions of the rake member to rise upwards within the respective apertures. 
         [0018]    The screen member may comprise a plurality of elongate spacers each pair of elongate spacers defining a gap therebetween. Alternatively, the screen member may comprise a substantially planar member defining a plurality of apertures therethrough. 
         [0019]    The dimensions of the apertures in the screen member determine the size of rag that is prevented from passing into the housing. 
         [0020]    The float member may be located above or below the rake member and may comprise any suitable buoyant member and may comprise and suitable material known to the skilled person including, but not restricted to, for example, moulded plastic air-filled floats, polyurethane foam filled buoyancy devices and the like. 
         [0021]    Preferably, a greater force is applied to the second surface of the rake member than to the first surface of the rake member. 
         [0022]    As the liquid level rises, the buoyancy forces exert an upwards pressure on and through the float member to the rake member to overcome the resilient member, thus allowing the rake member to rise with the sewage surface level against the force applied by the resilient member. 
         [0023]    The resilient member may comprise any resilient arrangement known to the skilled person and suitable for the purpose. For example, the resilient member may comprise a spring, or a resiliently compressible member or a resilient plunger arrangement or a resiliently bendable rod arrangement or the like. 
         [0024]    The protrusions of the rake member may comprise any suitable protrusions known to the skilled person, including but not limited to prongs, teeth, bars and rods. For example, where the screen member comprises a substantially planar member defining a plurality of apertures therethrough, the protrusions may comprise rods with a cross-sectional shape corresponding to the shape of the apertures and dimensioned to fit closely within the apertures. 
         [0025]    When the sewage surface level drops, the rake member lowers by a corresponding amount as the float member will continue to allow movement of the rake member with the sewage surface level. However, by application of force to a first surface of the rake member by the resilient member, the rake member is able to dislodge any adhered rag from the screen in a scraping or pushing action and move downwards whilst remaining at the sewage surface level. The dislodged rag is then either returned to the surface of the sewage or is dislodged so as to fall outside of the screen and returns to float in the wet well. At the normal operating range (start/stop levels) the pumps may draw the rag in under suction and pass it forward. Otherwise, the rag will be removed by tanker during the scheduled well clean. 
         [0026]    Thus, the rake member position is determined by the balance of forces acting at the first and second opposing surfaces of the rake member. 
         [0027]    The movement of the rake member relative to the screen member results in relative movement of the or each protrusion relative to the respective aperture into which it extends. 
         [0028]    The or each protrusion may be dimensioned to correspond substantially with the width of the respective elongate gap through which it extends. In this way, the protrusions can effectively dislodge even small pieces of rag which have adhered to the screen. 
         [0029]    An automatic screen cleaning apparatus in accordance with the present invention may comprise a plurality of rake members. 
         [0030]    In one embodiment, the rake member may be integral with the float member. For example, the float member may define the rake member protrusions. One way in which this may be realised is by casting or moulding the rake protrusions integrally with the float member as a single integral unit. 
         [0031]    A second aspect of the present invention provides an automatic screen cleaning apparatus comprising:
       a. a housing for containing a liquid, the housing provided with a liquid inlet, a liquid outlet and a screen member;   b. the screen member defining at least one aperture therethrough;   c. An orifice plate defining an orifice in fluid communication with the liquid outlet of the housing;   d. A float member received within the housing, the float member comprising a rake member having one or more protrusions, the or each protrusion aligned with a respective aperture defined by the screen member,
 
wherein the float member is moveable relative to the screen member in dependence upon variations in the volume of liquid received within the housing such that the float member obstructs the orifice of the orifice member to varying amounts in dependence upon the position of the float member within the housing.
       
 
         [0036]    Preferably, the orifice of the orifice plate comprises an orifice of increasing diameter in a direction extending away from the screen member. For example, the orifice may have a “Y” shaped aperture. However, it will be appreciated that any other suitable shape may be used, such as, but not limited to, a “V”-shaped orifice or a “U” shaped orifice or the like. 
         [0037]    The total area of the apertures defined by the screen member is preferably greater than the area of outlet  50  to allow for partial blocking of the apertures without affecting fluid flow through the housing. 
         [0038]    The total area of the orifice is preferably substantially the same as the area of the liquid outlet. If the area of the orifice is less than that of the liquid outlet, fluid flow through the housing may be detrimentally restricted. 
         [0039]    A further aspect of the present invention provides a method of automatically cleaning a screen, the method comprising the steps of:
       a. providing an automatic screen cleaning apparatus in accordance with any preceding Claim; and   b. passing liquid through the inlet in the housing so as to vary the volume of liquid received within the housing.       
 
         [0042]    A further aspect of the present invention provides a sewage pumping station comprising an automatic screen cleaning apparatus in accordance with a first aspect of the present invention. 
         [0043]    A further aspect of the present invention provides a rake member for automatic cleaning of a screen of a pumping station, the rake member comprising one or more protrusions, the or each protrusion adapted for location within an aperture defined by the screen, a resilient member arranged to apply a first force to the rake member and a float member located so as to apply a second opposing force to the rake member. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]    Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: 
           [0045]      FIG. 1  is a side view of a pumping station comprising an embodiment of an automatic screen cleaning apparatus in accordance with a first aspect of the present invention; 
           [0046]      FIG. 2  is a cross-sectional view of a first embodiment of the screen arrangement of  FIG. 1 ; 
           [0047]      FIG. 3  is a cross-sectional view of the screen arrangement of  FIG. 2  with an increased volume of sewage received within the housing of the automatic screen cleaning apparatus relative to  FIG. 2 ; 
           [0048]      FIG. 4  is a cross-sectional view of the screen arrangement of  FIG. 2  with the rake member shown at its highest point; 
           [0049]      FIG. 5  is a cross-sectional view of the screen arrangement of  FIG. 2  with a decreased volume of sewage received within the housing of the automatic screen cleaning apparatus relative to  FIG. 4 , and showing rag dislodged from the screen; 
           [0050]      FIG. 6  is a cross-sectional view of the screen arrangement of  FIG. 2  and overflow with the rake member at a low position; 
           [0051]      FIG. 7  is a cross-sectional view of the screen arrangement of  FIG. 2  and overflow with the rake member at a high position; 
           [0052]      FIG. 8  is a cross-sectional view of a second embodiment of an automatic screen cleaning apparatus in accordance with a first aspect of the present invention with the rake member shown at its highest position; 
           [0053]      FIG. 9  is a cross-sectional view of the embodiment of  FIG. 8  with the rake member at its lowest position and; 
           [0054]      FIG. 10  is a plan view of an embodiment of a single unit integral screen member/rake member arrangement; 
           [0055]      FIG. 11  is a side view of the single unit of  FIG. 10 ; 
           [0056]      FIG. 12  is a plan view of further embodiment of a single unit integral screen member/rake member arrangement; 
           [0057]      FIG. 13  is a side view of the single unit of  FIG. 12 . 
           [0058]      FIG. 14  is a cross-sectional view of a first embodiment of an automatic screen cleaning device in accordance with a second aspect of the present invention with the float in the lowest position; 
           [0059]      FIG. 15  is a plan view of the screen of the screen cleaning device of  FIG. 14 ; 
           [0060]      FIG. 16  is an enlarged view of a portion of  FIG. 14  showing the detail of the screen cleaning rods and their interaction with the screen; 
           [0061]      FIG. 17  is a cross-sectional view of the “Y” notch orifice plate; 
           [0062]      FIG. 18  is a cross-sectional view of the embodiment of  FIG. 14  in a semi-open position with the float partially raised; 
           [0063]      FIG. 19  is a cross-sectional view of the embodiment of  FIG. 14  in a fully open position with the float in its highest position; and 
           [0064]      FIG. 20  is a cross-sectional view of the embodiment of  FIG. 14  with the screen blocked and the float/rake descending to clean the screen. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0065]    An embodiment of the present invention will now be described with reference to all of  FIGS. 1 to 20 , in which common features are indicated using common figure references. 
         [0066]      FIG. 1  shows a pumping station  10  comprising a housing  20  containing a pump  25 , the housing  20  having a sewage inlet  30 , through which sewage is received from sewage inlet pipe  35 , and a sewage outlet  40  through which sewage is pumped for onward travel. The housing is also provided with an outfall outlet  50  through which sewage may leave the housing  20  when the sewage level within the housing  20  exceeds overflow level  55 . 
         [0067]    Sewage leaving housing  20  through outfall outlet  50  passes through outfall pipe  60  and finally exits outfall pipe  60  at a nearby river or stream  70 . 
         [0068]    A screen cleaning apparatus  100  is located within with housing  20  (shown in more detail in  FIGS. 2 to 7 ) substantially adjacent the inlet  65  to outfall pipe  60 . Screen cleaning apparatus  100  comprises a screen member  110  and a rake member  120 . Rake member  120  comprises a body  122  with prongs  124  extending therefrom. A float member  130  is located adjacent rake member  120  and resilient members  126  in the form of springs are provided in engagement with float member  130  and the upper interior surface of screen cleaning apparatus  100 . Screen member  110  has a number of elongate spacers  112  defining gaps  114  therebetween, with each prong of rake member  120  extending through a respective elongate gap  112  of screen member  110  to clear any potentially adhered rag. 
         [0069]    In use, sewage  140  containing rag  150  is received into housing  20  of pumping station  10  through inlet  30  of sewage inlet pipe  35 . Pump  25  then pumps the sewage  140  and rag  150  through sewage outlet  40  onward to its next destination. During the process, some rag may get drawn into the suction of the pump and passed forward under pressure to the next pumping station. However, the volume of sewage  140  received within housing  20  may vary depending on several influencing factors including, but not limited to, the flow of sewage  140  passing through sewage inlet pipe  35  and/or the efficiency/malfunctioning of pump  25 . When the level of sewage  140  received within housing  20  rises above overflow level  55 , sewage passes through screen cleaning apparatus  100  into outfall pipe  60  through outlet  50 . However, as sewage  140  passes through screen member  110 , rag  150  floating within sewage  140  may adhere or become lodged against spacers  112  of screen member  110 . Sufficient adhesion/lodging of rag  150  in this manner may lead to full or partial blocking of gaps  114  in screen member  110 , thereby preventing passage of sufficient volumes of sewage  140  through screen member  110  to prevent overflow of sewage from housing  20  leading to flooding of sewage  140  at ground level. Thus, it is important the screen member  110  is kept clear of obstructions at all times. 
         [0070]    As the sewage surface level rises, rake member  120  moves rises with the sewage surface level due to the application of force exerted on rake member  120  by float member  130 . 
         [0071]    Once the sewage surface level drops, any pieces of rag which have become lodged within the gaps  114  defined by spacers  112  of screen member  110  are dislodged by the movement of prongs  124  downwards between gaps  114 . The dislodged rag  150  is dislodged back into the volume of sewage contained within the housing  20  of pumping station  10 . In this way, the gaps  114  between spacers  112  of screen member  110  are automatically kept free of obstruction, thereby avoiding the blockage of screen member  110  and the overflow of sewage from pumping station  10  at ground level. The size of the gaps in the screen limits the dimensions of the rag passing through to overflow. 
         [0072]      FIGS. 8 and 9  show a further embodiment of an automatic screen cleaning apparatus in accordance with a first aspect of the present invention. 
         [0073]    Screen cleaning apparatus  100 A of  FIGS. 8 and 9  closely resembles apparatus  100  of  FIGS. 1 to 7 . However, screen member  110 A comprises a substantially planar member  114 A defining a plurality of apertures  112 A therethrough with 6 mm diameters. 
         [0074]    Rake member  120 A comprises a plurality of rods  124 A dimensioned so as to fit through respective apertures  112 A in screen member  110 A. 
         [0075]    Support rods  200  are provided to keep the rods  124 A of rake member  120 A aligned with respective apertures  112 A in screen member  110 A. 
         [0076]    When the rake member  120 A is in a raised position (see  FIG. 8 ), rods  124 A clear respective apertures  112 A in screen member  110 A thereby allowing fluid to flow through apertures  112 A into screen cleaning apparatus  100 A and out through outfall pipe  60 . 
         [0077]    However, when rake member  120 A is in a lowered position, rods  124 A of rake member  120 A extend through respective apertures  112 A, thereby blocking the passage of fluid into apparatus  100 A. 
         [0078]    Due to the dimensions of the apertures  112 A, there is a limitation on the size of rag that is able to pass through apertures  112 A into apparatus  100 A when rake member  120 A is in a raised position. Typically, the apertures  112 A will be dimensioned to prevent rag sized greater than 6 mm from passing through into apparatus  100 A. However, other dimensions may be applied to the apertures  120 A as required. 
         [0079]    Blockage of apertures  112 A in screen member  110 A is prevented as any lodged rag is dislodged by the movement of rods  124 A through respective apertures  112 A. 
         [0080]    It will be appreciated that the dimensions and shape of apertures  112 A may be varied as desired in order to control the dimensions of rag which are prevented from passing through apertures  112 A into apparatus  100 A. 
         [0081]    It will be appreciated that the screen member and float member may be provided integrally, as shown in  FIGS. 10 and 11 . Integral screen/float member  200  comprises a body  210  defining protrusions  220  extending from the body  210 . Body  210  of  FIGS. 10 and 11  is equivalent to float member  130  of  FIGS. 2 to 5 , and protrusions  220  are equivalent to prongs  124  of rake member  120  of  FIGS. 2 to 5 . 
         [0082]    A second embodiment of such a single unit screen member/float member arrangement is shown in  FIGS. 12 and 13  in which integral screen/float member  200  comprises a body  210  defining protrusions  220  extending from the body  210 . 
         [0083]    Body  210  of  FIGS. 12 and 13  is equivalent to float member  130  of  FIGS. 8 and 9  and protrusions  220  of  FIGS. 12 and 13  are equivalent to prongs  124 A of rake member  110 A of  FIGS. 8 and 9 . 
         [0084]    An integral screen member/float member arrangement may comprise a one piece moulding, or the body (i.e. float member portion) may have protrusions cast into it. Alternatively, any other suitable means of manufacture known to the skilled person may be employed. For example, the body and protrusions may be manufactured separately and engaged with one another prior to installation within a screen cleaning apparatus in accordance with the present invention. 
         [0085]    A further embodiment of an automatic screen cleaning device in accordance with a second aspect of the present invention is shown in  FIGS. 14 to 20  in which screen cleaning device  100   b  is located within housing  20 . A “Y” notch orifice plate  80  located at the output of screen cleaning device  100   b  such that it is substantially adjacent the inlet end of outfall outlet  50 . Screen cleaning device  100   b  further comprises a screen member  110   b  defining a plurality of spaced apertures  112   b,  a float  130   b  located above the screen member  110   b  with rods  124   b  extending from float  130   b  towards screen  110   b  and aligned with apertures  112   b.    
         [0086]      FIG. 15  shows a plan view of screen  110   b.  Screen  110   b  comprises a substantially planar member  114   b  defining a plurality of apertures  112   b.  Apertures  112   b  preferably have a diameter of substantially 6 mm. It will be appreciated however that the cross-sectional shape and dimensions of apertures  112   b  may be varied depending on the dimensions of rods  124   b  and/or composition of rag  150  in order control the dimensions of rag that will be able to pass through screen  110   b.    
         [0087]    Effluent  140  flows through apertures  112   b,  which are partially obstructed by shank  121  of the rod  124   b,  across the top of screen  110   b,  through the narrow portion  83  of “Y” notch orifice plate  80 , and through the outfall outlet  50 . 
         [0088]      FIG. 14  shows a cross-sectional view of the screen cleaning device  100   b  in the closed position. (See  FIG. 16  for enlarged view of a portion of  FIG. 14 ) The closed position is defined as the float  130   b  resting on seat  101  and occurs when the level of effluent  140  is lower than the upper surface  102  of seat  101 .  FIG. 14  shows the level of effluent  140  being above the level of the screen  110   b  and below the level of upper surface  102  of seat  101 . 
         [0089]      FIG. 17  shows an end view of “Y” notch orifice plate  80 . The total area of the apertures  112   b  is significantly greater than the area of outlet  50  in order to permit sufficient fluid flow through apparatus  100   b  even when there is partial blockage of apertures  112   b.  For example, the total area of the “Y” notch orifice  81  may be around one quarter of the sum of the areas of apertures  112   b.  It will be appreciated that other orifice shapes may be used such as but not limited to a “V” or “Y” shape. A feature of the orifice is that it comprises a portion  82  which defines a region of increasing aperture in a direction extending away from narrowed portion  83  as shown in  FIGS. 17 . It will further be appreciated that the dimensions of orifice  81  relative to the dimensions of the apertures  112   b  may be varied. 
         [0090]    The total area of “Y” notch orifice  81  is substantially the same as the area of outlet  50 . For example, if the area of “Y” notch orifice  81  were substantially smaller than that of outlet  50 , the fluid flow out of apparatus  100   b  would be restricted. There would be no added value to making the area of the “Y” notch orifice substantially greater than that of outlet  50  as the area of outlet  50  determines the rate of fluid flow through apparatus  100   b.    
         [0091]    The purpose of the Y notch orifice  81  or any other shaped orifice is to regulate the flow going out through outfall pipe  60  so that when the flow into the housing  100   b  exceeds that of the flow out through outfall pipe  60 , the float is induced to rise, thereby allowing more flow into housing  100   b  through screen  110   b.  The greater the flow into the housing  100   b,  the further the float is lifted until the maximum flow through “Y” notch orifice  81  is achieved (within the constraints of the dimensions of the outflow pipe  60 ). Furthermore, if the apertures  112   b  in screen  110   b  become blocked then the “Y” notch orifice  81  allows the housing  100   b  to empty of liquid through outfall pipe  60 , thus resulting in the lowering of the float  130   b  until rods  124   b  extend through apertures  112   b  in screen  110   b,  thereby cleaning any obstructing rag from the screen  110   b.  This cycle will then repeat itself as liquid flows into housing  100   b  again lifting the float  130   b.  This results in the screen being kept clean of rag. 
         [0092]    When float  13   b  is in its lowest position with respect to seat  101  (see  FIG. 14 ), if the flow rate of effluent  140  through narrow portion  83  of “Y” orifice  81  is less than the flow rate of effluent  140  through screen  110   b  the level of effluent  140  in housing  20  and therefore in screen cleaning device  100   b  rises and float  130   b  lifts off seat  101  pulling rods  124   b  through apertures  112   b  in screen  110   b.  As each rod head  122  passes through respective aperture  112   b  the aperture will be substantially closed, preventing flow of effluent  140  through aperture  112   b.  However, because rods  124   b  comprise two different lengths, only fifty percent of apertures  112   b  will be obstructed by rod heads  122  at the same time. The apertures  112   b  and “Y” orifice  81  are dimensioned such that when the float is at a height such that fifty percent of the apertures  112   b  are substantially blocked by rod heads  122  and fifty percent of apertures  112   b  are partially blocked by rod stems  121  the flow of effluent  140  through screen  110   b  required to raise float  130   b  is permitted. It will be appreciated that a greater variety of lengths of rods  124   b  could be used to further reduce the restriction of flow of effluent  140  through apertures  112   b  due to obstruction of apertures  112   b  by rod heads  122 . 
         [0093]    Rods  124   b  may be discreet components coupled to float  130   b.  Alternatively, the plurality of rods  124   b  may be mounted on a body to form a rake member which is, in turn, mounted on the float  130   b.  Alternatively rods  124   b  may be formed integrally with float  130   b.    
         [0094]    As the level of effluent  140  rises, float  130   b  rises and increased area of “Y” notch orifice  81  is exposed allowing a greater flow of effluent  140  through “Y” notch orifice  81  and through outfall outlet  50 . 
         [0095]    In  FIG. 18  the level of effluent  140  has risen to lift float  130   b  away from seat  101 . The shorter rods  124   b  have been completely removed from their respective apertures  112   b  which are now substantially free from obstruction. The flow rate of effluent  140  through screen  110   b  is therefore increased. If a nett inflow of effluent  140  to screen cleaning device  100   b  continues the level of effluent  140  and float  130   b  will continue to rise until the fully open position shown in  FIG. 9  is achieved, i.e. the float  130   b  has reached its highest position within housing  20 . 
         [0096]      FIG. 19  shows the screen cleaning device in the fully open position. In this position apertures  112   b  and “Y” orifice  81  are substantially free of obstruction. 
         [0097]    The dimensions of “Y” orifice  81  are such that the flow rate through “Y” notch orifice  81  is sufficient to meet the overflow requirements of pumping station  10 . 
         [0098]      FIG. 20  shows the screen cleaning device  100   b  in the fully open position with the screen  110   b  blocked by rag  150 . This may be caused by the flow of effluent  140  through screen  110   b  causing pieces of rag  150  to become lodged in apertures  112   b.    
         [0099]    If apertures  112   b  become partially or completely blocked the flow of effluent  140  through screen  110   b  will be restricted. If, as a result of complete or partial blockage of apertures  112   b,  the flow of effluent  140  into the screen cleaning device  100   b  becomes less than the flow of effluent  140  out of the screen cleaning device  100   b,  via “Y” orifice  81 , the level of effluent  140  in the screen cleaning device  100   b  will fall, as will float  130   b.  As the float  130   b  moves closer to the screen  110   b  the aligned rods  124   b  to pass through the apertures  112   b  in screen  110   b  dislodging any obstruction therein. The weight of the float is sufficient to dislodge any obstruction. As rods  124   b  extend below screen  110   b  rag  150  trapped against the underside of screen  110   b  is pushed away form the surface of screen  110   b.  The dislodged rag will be swept away from the screen by the turbulence of the effluent and return to float in the wet well. At the normal operating range the pumps may draw the rag in under suction and pass it forward. Otherwise, the rag will be removed by tanker during the scheduled well clean. 
         [0100]    As a result of removing rag  150  from screen  110   b  the flow rate of effluent  140  through screen  110   b  will increase. If the flow rate of effluent  140  through screen  110   b  again exceeds the flow rate of effluent  140  through “Y” orifice  81 , now partially obstructed by float  130   b,  the level of effluent  140  inside screen cleaning device  100   b  will again rise as will float  130   b,  again removing rods  124   b  from apertures  112   b.  The transitioning between open and closed positions will repeat as necessary as long as an overflow situation exists. 
         [0101]    Screen  110   b,  rods  124   b  and “Y” notch orifice plate  80  comprise materials such as but not limited to stainless steel, which will suffer substantially no deterioration when placed in contact with effluent for prolonged periods of time. 
         [0102]    Although aspects of the invention have been described with reference to the embodiments shown in the accompanying drawings, it is to be understood that the invention is not limited to the precise embodiments shown and that various changes and modifications may be effected without further inventive skill and effort. For example, it will be appreciated that an automatic screen cleaning member in accordance with the present invention also has application in other environments, for example, in brewing or any other filtration systems in which it is desired to keep screens free from blockages.