Abstract:
A subsurface drip wastewater disposal system and method are disclosed that eliminate the need for headworks, reduce the risk of damage to the system, and permit optimal pump sizing by simultaneously dosing, flushing the pipe network and flushing the filter. A tank and pump package provide effluent under pressure to the field piping network comprising a supply manifold, a return manifold, and a plurality of emitter lines. A discharge filter that is capable of simultaneous filtering and flushing is provided on the discharge line, and a first flow restrictor is provided on the filter flush return line. Effluent is discharged at a rate and pressure that permits simultaneous pipe flushing and dosing, with the return manifold returning to the tank through a field return line having a second flow restrictor. The size of the first and second flow restrictors is selected to provide the desired flow splits.

Description:
BACKGROUND 
     Subsurface drip disposal (“SDD”) systems are systems for disposing of wastewater such as septic tank effluent and the like. Subsurface drip disposal provides a shallow, slow rate, pressure-dosed system used for land application of pretreated wastewater. In general, SDD systems are characterized by: (1) uniform distribution of effluent, (2) dosing and resting cycles, and (3) very shallow placement of trenches. SDD systems typically use small diameter piping with subsurface drip emitters. The effluent must be adequately filtered before distribution through the underground emitter system, and filters and the piping network must be routinely flushed or otherwise cleared of trapped particles. 
     Well-designed SDD systems distribute effluent uniformly at a relatively low application rate over an absorption field, also called the drip field. Waste fluids are applied at a controlled rate in the plant root zone, which tends to minimize percolation of the effluent. Hydraulic loading rates may vary, for example between 0.1 and 1.6 gallons per day per square foot. 
     Conventional SDD systems include valving and control systems referred to as headworks that direct and control the flow of the effluent. In a conventional SDD system the headworks include two or three solenoid valves that control the timing and sequence of fluid flows. A conventional SDD system may include a tank wherein the effluent is accumulated for dispersal, a pump for removing effluent from the tank, a supply manifold and return manifold, and a number of emitter lines that extend between the supply and return manifolds and are disposed in the drip field. The emitter lines include a number of small emitters distributed along their length through which the effluent is dispersed in the drip field. 
     In a typical conventional SDD system the pump is periodically engaged, and a first solenoid valve is opened, to send flow to the drip field for dispersal through the drip emitters. A valve on the return manifold is typically closed such that the pumped fluid flows uniformly away from the tank, and is dispersed through the emitters. This is typically referred to as “dosing” cycle, and may occur, for example, twelve times a day, for periods of 5-10 minutes. It will be appreciated that this frequency and duration for the dosing cycle is by way of example, and the actual timing selected will depend on the particular application. 
     In order to avoid accumulation of matter in the emitter lines and manifolds, cooperatively referred to herein as the “field piping network”, a conventional SDD system will periodically engage a field piping network flushing cycle wherein relatively high-velocity effluent is pumped through the field piping network to clean out the pipes, with the valve for the return manifold open such that the effluent is partially returned to the tank. The minimum required fluid velocity for the flushing operation is often specified by local and/or state regulations. The field piping network flushing cycle may be engaged, for example, every 12-48 hours, and typically pressurizes the entire system such that effluent is also dispersed to the drip field, although the amount of such dosing is typically difficult to determine and/or unknown. 
     A conventional SDD system also includes a filter that prevents or reduces the amount of solid matter that is pumped from the SDD tank to the emitter lines, in order to prevent clogging of the emitters. Conventional SDD systems periodically engage a filter flush cycle wherein a third valve is opened to allow flow to go through a filter flushing port and return to the tank. In the filter flushing cycle fluids at a relatively high velocity are provided to remove matter from the filter. Typically the pump pressurizes the entire system, resulting in effluent also being dispersed through the emitters, although again the amount of fluid discharged may be difficult to determine or predict. In an exemplary septic tank application, a filter flush cycle may be engaged once for every 5-20 dose cycles. 
     SDD systems, particularly in cold weather climates, are designed and installed to drain back from the field piping (e.g., the emitters and plenums) into the dose tank after each dose, so that effluent does not freeze in the lines, potentially damaging the system. However, the headwords active valving systems used in most conventional SDD systems tend to interfere with proper drainage from the field piping, which can result in damage to the field piping network Moreover, the valving systems add significant costs and complexity to the drip disposal system. In addition, conventional SDD systems require three different pumping operations. There is one pressure and flow requirement for dosing the field, a different pressure and flow requirement for flushing the field piping network, and a third flow and pressure requirement for flushing the discharge filter. The differences in these three operating conditions make it difficult to select a suitable pump, and requires operation of the selected pump at non-optimal conditions at least some of the time. 
     To avoid the disadvantages associated with conventional SDD systems having headworks, a system without full headworks has been proposed in  Design  &amp;  Performance of Drip Dispersal Systems in Freezing Environments  (published online at http://www.geoflow.com/research_w.html), by S. Wallace. However, the systems described therein include a solenoid valve for drain back of the effluent, and a throttle valve on the return head. Moreover, the disclosed system does not appear to include a filter, or means for flushing a filter. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     A subsurface drip effluent disposal system is disclosed having a wastewater tank, such as a septic effluent tank, and a subsurface field piping network for dispersing flow from the tank. A pump is provided in the tank that pumps fluids through a discharge line to a discharge filter that is capable of simultaneous flushing and filtering. A spin filter is a suitable type of discharge filter. A portion of the flow received by the discharge filter is used to flush the filter and returns to the tank, and another portion of the flow is discharged to the field piping network. The field piping network includes a supply manifold that is fluidly connected to one end of a plurality of emitter lines having spaced apart emitters for discharging a portion of the flow. The opposite end of the emitter lines are connected to a return manifold that returns a portion of the flow to the tank. Flow restrictors are provided, preferably on the filter flush return line, and on the field return line, such that the flow split between the filter, the filter flush return, the drip dose, and the field return can be predetermined. The present system simultaneously flushes the discharge filter, doses the field, and flushes the field piping network, and does not require the use of active headworks, such as a solenoid operated valve systems. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of an exemplary subsurface drip effluent disposal system in accordance with the present invention, shown in isolation for clarity; 
         FIG. 2  is a partially side view of the wastewater tank shown in  FIG. 1 , shown installed with the tops of the risers at approximately ground level; 
         FIG. 3A  is a three-quarter front perspective view of the piping in the first riser of the wastewater tank shown in  FIG. 1 ; 
         FIG. 3B  is a three-quarter rear perspective view of the piping in the first riser of the wastewater tank shown in  FIG. 1 ; and 
         FIG. 4  is an exploded perspective view of an exemplary flow restrictor for the drip effluent disposal system shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     A perspective view of an exemplary subsurface drip effluent disposal system  100  in accordance with the present invention is shown in  FIG. 1 . The disposal system  100  includes a wastewater tank  102 , for example a septic system effluent tank, which are well-known in the art. For example, the wastewater tank  102  may be an injection-molded fiberglass-reinforced polyester septic tank. The exemplary wastewater tank  102  shown in  FIG. 1  includes oppositely-disposed first and second risers  104  and  106 , respectively. Of course, other suitable tank construction may alternatively be used. A tank fluid inlet  108  provides a fluid conduit for supplying fluids to the wastewater tank  102 . In a current embodiment of the drip effluent disposal system  100 , for example, the inlet  108  is formed from four inch ABS piping. The inlet  108  may be fluidly coupled to a pretreatment tank or system (not shown), for example to substantially remove solids and/or harmful organisms, from the wastewater. 
     A supply manifold  110 , extends from the wastewater tank  102 , terminating in an air/vacuum relief valve  112 . A plurality of generally U-shaped emitter lines  115  are provided having a first end  114  fluidly connected to the supply manifold  110 , and a second end  116  fluidly connected to a return manifold  120 . The emitter lines  115  include a number of spaced emitters (not shown), as are well-known in the art. For example, pressure-compensated emitters are adapted to produce a relatively constant outflow over a range of fluid pressures. Non-pressure-compensated emitters may alternatively be used. The inflow manifold  120  terminates at one end with a second air/vacuum relief valve  112 , and at the other end returns to the tank  102  through a field return line  154 . A portion of the effluent pumped through the emitter lines  115  is expelled through the emitters, thereby dosing the drip field. It will be appreciated by the artisan that in a typical alternative configuration the supply manifold  110  and return manifold  120  may be spaced a distance apart, with the emitter lines  115  extending therebetween. 
     In regions where freezing is a consideration, the emitter lines  115  are generally installed to slope towards the supply manifold  110  and/or the return manifold  120  such that when the system  100  is not pressurized, fluid in the emitter lines  115  will be gravity-driven towards one or both of the manifolds  110 ,  120 . In these regions it is desirable that the emitter lines  115  not have any sag that would trap fluids therein. The supply manifold  110  and return manifold  120  are installed to slope towards the wastewater tank  102 , such that when the system is not pressurized, fluid in the manifolds  110 ,  120  will flow under gravity towards the wastewater tank  102 . In regions where freezing is not a consideration, the grading of the field piping is not a primary consideration. 
     Refer now to  FIG. 2 , which shows an installed, partially cut-away side view of the wastewater tank  102 . In the disclosed embodiment, a pump  131  is disposed in a pump package  130 , which may include other related components such as waste filtering and/or treatment components  132  such as products marketed by Orenco Systems, Inc. under the trademark Biotube®. A pump discharge line  133  receives effluent from the pump  131 , which is thereby delivered under pressure to the supply manifold  110  ( FIG. 1 ). A manual shutoff valve  134  may be provided on the discharge line  133 . A conventional junction box  125  is conveniently disposed in the second riser  106 , for providing electrical power to the system  100 . 
     Refer now also to  FIG. 3A  and  FIG. 3B  which show the piping in the first riser  104  (the first riser  104  is shown in phantom, for clarity), from different perspectives to better show the piping.  FIG. 3A  shows a generally three-quarter front perspective view and  FIG. 3B  shows a generally three-quarter rear perspective view. The discharge line  133  is fluidly connected to a discharge filter  136  that is capable of simultaneously filtering effluent and flushing the filter  136 , for example a spin filter. The discharge filter  136  removes solids from the effluent prior to its discharge into the supply manifold  110 . The discharge filter  136  has one end attached to a filter flush return line  138  that returns effluent with filtered solids captured by the discharge filter  136  to the wastewater tank  102 . A first flow restrictor  140  is disposed between the discharge filter  136 , and the discharge filter flush return line  138 . Suitable flow restrictors are known in the art. An exemplary flow restrictor  140  having a flow balance orifice  146  is shown in  FIG. 4 . In this embodiment, the first flow restrictor  140  includes a fitting  142  that supports a disk-shaped blocking member  144  with a calibrated aperture  146  therethrough, and a second fitting  143  that is attachable to the first fitting  142 , as shown. An o-ring  145  provides a seal about the flow balance orifice  146 . The function of the first flow restrictor  140  is discussed in more detail below. 
     An optional flow meter  150  and pressure gauge  152  are also provided on the discharge line  133 , upstream of the supply manifold  110 . 
     Referring again to  FIG. 1 , the effluent flow through the supply manifold  110  is distributed to the emitter lines  115 , where a portion of the flow is dosed to the drip field. A portion of the effluent flow enters the return manifold  120 , and is recirculated back to the wastewater tank  102 . The return manifold  120  fluidly connects to a field return line  154  that extends into the wastewater tank  102 . Referring now again to  FIG. 3A  and  FIG. 3B , the field return line  154  includes a second flow restrictor  148  (discussed below) located prior to the return flow being discharged into the wastewater tank  102 . The second flow restrictor  148  is similar in structure to the first flow restrictor  140  shown in  FIG. 4 . An optional pressure gauge  152  is also provided on the field return line  154 . 
     The operation of the present subsurface drip effluent disposal system  100  will now be described. Generally at prescheduled intervals the pump  131  is activated and pumps fluids from the wastewater tank  102  through the discharge line  133 . As the flow encounters the discharge filter  136 , a portion of the flow flushes the discharge filter  136  and returns to the wastewater tank  102  through the filter flush return line  138 , and a portion of the flow is discharged to the supply manifold  110 . The supply manifold flow is then distributed to the emitter lines  115 , wherein a portion of the effluent is dosed to the drip field, and a portion is returned to the return manifold  120 , and thereby to the wastewater tank  102 , with flow velocities throughout the field piping network that are sufficient to flush the pipes. For a given effluent disposal system  100 , the flow splits between the filter flush portion, the field dose portion, and the piping flush portion are determined by the size of the apertures in the first and second flow restrictors  140 ,  148 . 
     Frequently, subsurface drip effluent disposal systems must comply with local regulations, manufacturer recommendations and/or practical limitations regarding the amount of dosing that can be applied to a given drip field over a given period of time, piping network flushing flow velocity requirements, and filter flushing requirements. 
     With the present system, the dosing operation, piping network flushing and filter flushing can occur simultaneously. In particular, as disclosed herein the design of the SDD system can be accomplished using the following steps: 
     1. Specify the required wastewater discharge rate, for example in gal/day. 
     2. Identify the drip field soil type, and the allowable loading rate, for example in gal/ft^2/day. 
     3. Determine the emitter flow rate, the number of emitters required and the emitter lateral and in-line spacing requirements. 
     4. Calculate the required dimensions of the drip field. 
     5. Determine the piping flushing and filter flushing minimum flow requirements. 
     6. Determine the size of the supply manifold, emitter lines, and return manifold required to achieve the desired minimum flushing velocities. 
     7. Determine the optimal field piping network pressure, and pressure range for emitter lines. 
     8. Calculate the flow restrictor sizes required to achieve the desired flow splits to produce the desired minimum flow velocity in the field piping network, and to achieve the desired dosing and filter flushing flow rates. 
     It will be appreciated that the above method permits the calculation of the flow rate and head or piping pressures, which allows selection of the appropriate pump size. In order to optimize the system, a standard pump size may then be selected and the flow restrictor sizes re-optimized for the selected pump. Typically, the filter flushing flow rate may be increased to improve filter flushing without adversely impacting the pressure in the field piping network. 
     The SDD system  100  disclosed above provides significant advantages over the prior art. The active headworks with electronically-controlled valves is eliminated. Flow restrictors are provided that may be fixed aperture or pre-set upon installation. Although fixed aperture flow restrictors are shown and currently preferred, it is contemplated that the flow restrictors  140 ,  148  may alternatively be field-adjustable, such that the user may adjust the system, for example to optimize the flow rates during installation. The elimination of the headworks reduces cost and complexity, and increases the reliability of the system by promoting complete drain back to the tank between dosing cycles. 
     It will be appreciated that the present system and method provides for a uniform pumping cycle that simultaneously doses the drip field, flushes the discharge filter, and flushes the piping network. A properly designed system will achieve the requisite fluid velocities and flows required. This configuration reduces the number of times that the pump must be activated, and permits the designer to select an optimal pump size. In prior art systems wherein the flushing and dosing operations are separately conducted, the required flow rates are dramatically different, resulting in the pump operating outside of its optimal range for many cycles. The present system and method provides for a single dosing/flushing operation, and therefore one pump operating condition, allowing selection of a pump that will operate at or near its best efficiency point. 
     While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.