Abstract:
A water-tight fiberglass septic tank is strong enough to withstand live loads. The fiberglass septic tank is preferably designed with integral ribs to for structural strength. In one preferred embodiment, the tank is made in two unequal halves. The lower half comprises the major portion of the tank and includes a bottom, two endwalls and two sidewalls. The upper half comprises a cover. This arrangement allows the horizontal seam to be placed above the internal waterline, thereby protecting the seam from exposure to sewage. At least one of the endwalls includes a raised top portion that allows a pipe to be located near the top of the lower portion without passing through the seam between the upper and lower portions of the tank. The endwalls are also curved to allow for angled inlet and outlet pipe connections. The ribs are preferably internal and hollow. The lower portion of the tank is also preferably flat bottomed. A provision has been made for a self-locating baffle if required. In another preferred embodiment, a cylindrical male molded tank is created on a mandrel. The tank circumference is stepped, which both provides for built-in stepped ribs and eases removal of the tank from the mandrel. In preferred embodiments of the tank, a baffle is located at the position of a stepped rib so that the stepped rib supports the baffle.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to septic tanks generally, and more particularly to a water-tight septic tank. 
     2. Discussion of the Background 
     Tanks for the first phase treatment of home waste have traditionally been referred to as septic tanks. Septic tanks slow down and temporarily hold a waste flow from a home so that a natural cleansing action involving anaerobic bacteria can occur. As sewage enters the tank, solids separate from fluids and fall to the bottom. Anaerobic bacteria break the solids down to reduce the volume. However, the volume of the solids is never reduced to zero; therefore, a residue remains. It is this residue that must be removed from the septic tank. 
     An example of a typical septic tank system  100  is shown in FIG.  1 . The system  100  includes a tank  110  with an inlet  120  and an outlet  130 . The tank  100  also includes an access riser  140 . As sewage enters the tank  110 , it separates into one of three distinct layers in the tank. The top layer  170  is referred to as scum. This layer includes components of the sewage such as cooking oils and fats, cigarette filters, and other materials that float. Most of the scum does not decompose in the septic tank  110 . The middle layer  160  is referred to as the clear layer. The scum layer  170  actually floats on the middle layer  160  such that approximately three quarters of the volume of the scum layer  170  is below the liquid level  161  of the middle layer  160  and approximately one quarter of the volume of the scum layer  170  is above the liquid level  161 . The middle layer  160  comprises mostly liquids with suspended solids. Bacteria digest many of the suspended solids in this area. The bottom layer  150  is referred to as sludge. This layer contains most of the sewage solids and is generally oxygen free. Anaerobic bacteria reduce the volume of solids in this layer greatly, but not entirely. 
     In operation, raw sewage enters the inlet  120 . The inlet  120  is typically shaped as a “T” or “Y” to prevent clogging of the inlet  120  by the scum layer  170 . The raw sewage separates into the three layers  150 ,  160 ,  170  as discussed above, where the bacteria begin breaking the sewage down. As sewage enters the tank  110  through the inlet  120 , a corresponding volume of sewage from the clear layer  160  exits the tank  110  through the outlet  130 , which is also “T” or “Y” shaped for the same reason the inlet is shaped that way. As long as the fluid in the clear area  160  has been present in the tank for a sufficient amount of time for the bacteria to do their work, the fluid exiting the tank through the outlet  130  is relatively clear. The exiting fluid is typically released into the surrounding soil, where further biodegradation continues. 
     However, if the fluid in the clear layer  160  has not been in the tank  110  for a sufficient amount of time, solids are released with the fluid through the outlet  130  and into the surrounding soil, causing the surrounding soil to become clogged or blocked. At this point the system  100  has failed. One main cause of septic system  100  failure is the entry of ground water into the tank  110 , which causes the premature release of fluid in the layer  160  from the tank  110 . Therefore, it is important that the tank  110  be watertight. 
     The use of baffles has become popular in recent years. A baffle  190  is shown in phantom in FIG.  1 . The baffle  190  is essentially a vertical wall that separates the tank  100  into two portions, usually such that the first portion (the portion nearest the inlet  120 ) is at least ⅔ of the volume of the tank. The baffle  190  usually includes a passage located in the clear layer  160 . The idea is to allow clear fluids only past the baffle so that further biodegradation can occur before the fluids are expelled through the outlet  130 . Because only fluids pass through the baffle  190 , the size of the sludge and scum layers  150 ,  170  is theoretically greatly reduced relative to the clear layer  160  in the portion of the tank  110  downstream of the baffle  190 . There is ongoing dispute within the industry as to the effectiveness of baffles. 
     Prior art tanks are often made of concrete or rotocast thermoplastic. The problem with concrete tanks is that historically they have been fabricated poorly. Also, concrete leaks unless it is professionally designed and manufactured and then carefully installed by a trained and experienced field installer. When those precautions are taken, the cost of the tank is increased and becomes non-competitive in many instances. A leaking septic tank interrupts the treatment process as described above. On the other hand, thermoplastic tanks are often watertight, but are not strong. Over time, they collapse under loads when being driven over by trucks or under other hydrostatic loads. Another weakness is that such rotocast thermoplastic tanks will often collapse when pumped dry during the periodic sludge removal process. 
     Another popular method for handling home sewage is to pipe the sewage from the home to a central treatment plant. This plant is usually built by a municipality. The waste that is pumped to the plant is not treated before it leaves the home and is therefore thicker because it includes solids. Transporting waste in this manner requires large pipes made of concrete. Such pipes are usually six inches or larger in diameter. In addition, getting the waste to the plant requires either expensive pumping stations or installing the pipes with the correct pitch for a gravity flow system, which can add significantly to the expense in installing the pipes at the requisite depths. 
     An additional problem is that the market is changing. Some municipalities are no longer accepting new subscribers. Developers are often now required to install their own treatment systems when they build development sites. 
     SUMMARY OF THE INVENTION 
     The invention solves the aforementioned problems to a great extent by providing a fiberglass septic tank that is watertight and strong enough to withstand live loads such as those resulting from vehicles being driven over the tanks, hydrostatic loads, and ground forces exerted on the tank when the tank is pumped dry. In preferred embodiments, the fiberglass septic tank is designed with integral ribs for structural strength. 
     In one preferred embodiment, the tank is made in two unequal halves. The lower half comprises the major portion of the tank and includes a bottom, two endwalls and two sidewalls. The upper half comprises a cover. This arrangement allows the horizontal seam to be placed above the internal waterline, thereby protecting the seam from exposure to sewage. Because of the size and light weight of the cover, it can be installed manually at the site without the use of power equipment, resulting in significant savings. At least one of the endwalls includes a raised top portion that allows the fluid inlet to be located near the top of the lower portion without passing through the seam between the upper and lower portions of the tank. The endwalls are also curved to allow for angled inlet and outlet pipe connections. The ribs are preferably internal, integral and hollow and the tank is preferably shaped such that multiple tanks may be stacked during shipment to reduce shipping costs. The internal ribs also provide support for an optional baffle. The lower portion of the tank is also preferably flat bottomed, which both eases installation (because the tank will stand upright on a bed of pea gravel) and increases the volume of the sludge storage area. The tank is preferably sized at approximately 500 to 3,000 gallons for residential use. In some preferred embodiments, the bottom internal ribs are filled with foam. The foam acts as a spacer or dunnage when tanks are stacked during shipping. The foam also fills the space under the ribs, thus eliminating the need for backfill in that area. 
     In another preferred embodiment, a cylindrical male molded tank is created on a mandrel. The tank circumference is stepped, which both provides for built-in stepped ribs and eases removal of the tank from the mandrel. In preferred embodiments of the tank, a baffle is located at the position of a stepped rib so that the stepped rib supports the baffle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is a cross sectional view of a prior art septic tank. 
     FIG. 2 is a perspective view of a septic tank according to a preferred embodiment of the invention. 
     FIGS. 3 a,b,c  are is a side views of a sidewall rib, a filled bottom rib and a closed top rib, respectively, of the tank of FIG.  2 . 
     FIGS. 4 a  and  4   b  are a cross sectional views of alternative forms of joints between the upper and lower portions of the tank of FIG.  2 . 
     FIG. 5 is a side view of the septic tank of FIG. 2 installed in the ground. 
     FIG. 6 is a top view of the septic tank of FIG.  2 . 
     FIG. 7 is a side view of a septic tank according to a second preferred embodiment of the invention. 
     FIG. 8 is a side view of the tank of FIG.  7 . 
     FIG. 9 is an end view of the tank of FIG.  7 . 
     FIG. 10 is a side view of differently-sized tanks according to the preferred embodiments of the present invention. 
     FIG. 11 a  is a perspective view of a portion of a septic tank having a relocatable baffle according to a preferred embodiment of the present invention. 
     FIG. 11 b  is an end view of a portion of a septic tank having a relocatable baffle according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, a perspective view of a septic tank  200  is shown in FIG.  2 . The tank  200  is unequally divided into a lower portion  1  and an upper portion  2 . The lower portion  1  includes a number of integral, spaced apart ribs  4 . The ribs  4  are sized and spaced to provide structural support for the tank  200  so that it can absorb hydrostatic loads as well as loads exerted by vehicles such as pickup trucks driving over the tank  200 . The tank  200  is also capable of withstanding ground forces exerted on the tank  200  when it is installed and later pumped dry. 
     In preferred embodiments, the ribs  4  are hollow and internal. As shown in FIG. 3 a , the ribs  4  are preferably trapezoidally shaped. Exemplary dimensions for the rib  4 , when used with a tank  200  having a 1500 gallon capacity and one quarter inch thick fiberglass construction, are a base width of four inches, a top width of two inches, and a height of one and three quarters inches. As will be apparent to those of ordinary skill in the art, the dimensions of the walls and ribs, as well as the number of ribs, may be varied to meet the desired tank capacities as well as site-specific conditions (soil conditions, burial depths, expected traffic over the tank, government regulations, etc.). 
     Referring now back to FIG. 2, the lower portion  1  is preferably shaped such that it is stackable, especially when provided with internal hollow ribs  4 . The tanks  200  may be stacked, for example,  10  tanks high with as many as  4  stacks on a flatbed truck. This is a great improvement over cement tanks, which are not stackable and therefore could only be shipped four to a truck. Another advantage associated with using internal ribs  4  is that one rib  4  provides support for an optional baffle  7  installed in the tank  200 . The baffle  7  may be attached to the desired rib  4  by an adhesive  9 . In an alternate embodiment shown in FIGS. 11 a  and  b , the baffle  7  may be held in place by bolt-on (which may be installed in the field) or molded guide clips  72 . Normally, the baffle  7  retains solids but is not required to be watertight. However, if a watertight baffle  7  is required, then an adhesive may be used between the baffle  7  and the internal rib  4  to create the required watertight seal. 
     Referring now back to FIG. 2, the upper portion  2  of the tank  200  includes two access risers  13 . The access risers  13  allow access to the tank  200  when periodic sludge removal becomes necessary. Two access risers  13  are provided because the tank is divided by the baffle  7 . If no baffle  7  is included, then only a single access riser  13  is necessary. 
     The lower portion  1  is provided with a flange  6 . A corresponding flange  5  is provided on the upper portion  2 . The flanges  5 ,  6  provide a large surface area for the formation of a chemical bond between the upper portion  2  and the lower portion  1  as shown in FIG. 4 a . An adhesive or reinforced resin  10  is placed on one or both of the flanges  5 ,  6 , which are then placed together (using pressure if necessary for the particular adhesive  10 ) to form the joint shown in FIG. 4 a . The mating surfaces of the flanges  5 , 6  must be properly cleaned and prepared (e.g., abraded) to ensure that a watertight bond is formed. 
     An alternative joint is illustrated in FIG. 4 b . In this configuration, the adhesive  36  is applied to the outside surfaces of the upper and lower portions  2 , 1 , again to keep the adhesive isolated from the sewage in the tank  200  as much as possible. 
     Referring now back to FIG. 2, it can be seen that the upper portion  2  includes ribs  11 . Unlike the ribs  4  in the lower section  1 , the ribs  11  are external ribs. In preferred embodiments, the ribs  11  are hollow and trapezoidally shaped with dimensions similar to or the same as the ribs  4 . The ribs  11  may be closed off as shown in FIG. 3 b.    
     FIG. 5 is a side view illustrating an installed tank  200 . The tank  200  resides in a pit  15  formed in the surrounding soil  14 . The pit  15  is typically backfilled with a self-compacting material such as pea gravel. The backfill is important because it must both support the tank and endure loads that are created when, for example, a pickup truck is driven over the tank  200  (as opposed to dumping excessive load on the tank  200  itself). 
     In practice, a level bed of pea gravel is formed in the pit  15  before the tank  200  is installed. If the pea gravel is well graded, clay and other materials will not flow into the backfill and create voids in the native soil which can later cause the soil to settle or collapse around the tank  200 . Next, the tank  200  is set on the level bed of pea gravel. Because the bottom of the tank  200  is substantially flat, the tank  200  is level and stable. This feature of the tank  200  means that the tank  200  will not need to be shimmed to hold it straight and level while backfill is applied along the sides of the tank  200 . The voids created on the bottom of the tank by the internal ribs  4  may be filled with foam  414  as shown in FIG. 3 c , thereby eliminating concerns regarding backfill in these voids. 
     When the tank  200  is positioned correctly and the sides are backfilled, the tank inlet  16  can be connected to the sewage line  27  from the house and the tank outlet  18  can be connected to the discharge line  28 , which may lead to a discharge field in the soil or to another processing facility as discussed further below. 
     The tank  200  includes a baffle  7 , which preferably divides the tank  200  into a first compartment  39  that encompasses approximately ⅔ of the total volume of the tank  200  and a second compartment  23  that encompasses approximately ⅓ of the total volume of the tank  200 . A pipe  8  in the shape of an inverted “U” is installed in the baffle to prevent buoyancy-neutral solids (e.g. condoms) from clogging the passage in the baffle  7 . Also provided in the tank  200  is a filter  21 , which screens any solids that may enter the smaller compartment  23  from exiting the tank into the drain field through the outlet  18 . The filter  21  preferably comprises a wire mesh. If site conditions do not permit a gravity flow system, a pump (not shown in FIG. 5) may be connected between the filter  21  and the outlet  18 . 
     FIG. 5 illustrates several access points to the tank  200 . An access pipe  17  is provided from the inlet “T”  16   a  to the surface. The access pipe  17  preferably protrudes above the surface, although many installers and homeowners choose to keep the pipe  17  beneath the surface. The pipe  17  provides access to the inlet “T”  16   a  in the event of a clog. A pump out pipe  25  is also provided. The pump out pipe  25  may be mated to the tank  200  with a rubber boot  26 , or alternatively may be glassed in to the tank  200 . The pump out pipe  25  is used for the periodic sludge removal process. A riser  19  is also provided. The riser  19  may be any size, but is commonly 18, 24 or 30 inches in diameter. The riser  19  provides access to the filter  21  as well as to a pump if one is provided. The riser  19  may also be used to remove sludge from the smaller compartment  23  should that ever become necessary. Although the riser  19  is shown as buried in FIG. 5, it may also be at ground level as shown in FIG.  1 . 
     The tank  200  is shown in FIG. 5 with a layer of scum  30  and a layer of sludge  31  floating on a clear layer  32 , with approximately three quarters of the volume of the scum layer  30  below the liquid level  33  of the clear layer  32  and approximately one quarter of the volume of the scum layer  30  below the liquid level  33 . The liquid level  33  is also shown near the top of the tank  200 , but the liquid level  33  as well as the top of the scum layer  30  are both preferably below the top of the lower portion  1 . This keeps sewage away from the joint between the lower portion  1  and the upper portion  2 . Another advantage of the present invention is provided by the raised portions  500  of the tank endwalls  202 . The raised endwall portions  500  allow the inlet  16  and/or outlet  18  to pass through the endwalls  202  at a height near the top of the endwalls  202  without intersecting any seam between the lower portion  1  and upper portion  2 . 
     Referring now to FIG. 6, a top view of the tank  200 , it can be seen that the end walls  500  are curved. This curvature allows both the inlet  16  and the outlet  18  (shown in phantom in FIG. 6) to be connected to the tank  200  at different, non-zero angles with respect to the main axis Z while intersecting the end wall  500  nearly perpendicularly, thereby simplifying the pipe/wall joint. 
     A second embodiment of a tank  700  is illustrated in FIG.  7 . The tank  700  is generally cylindrical, but each half (relative to the center vertical axis V) of tank  700  includes a number of stepped ribs  710 . What is meant by “stepped ribs” is that the stepped ribs  710  are vertical surfaces that connect sections of the tank  700  having different circumferences. The stepped ribs  710  provide structural support to the tank much in the same way as internal or external ribs would provide support. However, unlike external or internal ribs, the stepped ribs can be molded into the tank  700  directly using an appropriately-shaped male mold. In other words, the tank  700  can be constructed by placing a male mold corresponding to one half of a tank on a mandrel, applying the resin and fiberglass mat or chop over the mold, allowing the resin to cure, removing the cured tank half from the mold and joining it at seam  780  with another half tank prepared in the same manner and possibly using the same mold. 
     A baffle  707  is also included in the tank  700 . The baffle  707  is installed in a tank half before the halves are joined to form the tank  700 . The baffle  707  is located at a step rib  710 , which provides support for the baffle  707 . An adhesive or reinforced resin  790  preferably forms a watertight seal between the baffle  707  and the step rib  710 . 
     As discussed above, the location of the baffle  707  preferably divides the tank  700  into two unequal portions  723 ,  739  in a 2:1 ratio. The baffle  707  also includes a “U” pipe  708  with the ends of the “U” turned downward to provide for fluid communication between the tank halves and to prevent the pipe  708  from becoming clogged by buoyant-neutral debris. The baffle  707  may extend a portion of the diameter of the tank  700  (as shown in FIG. 7) or may extend across the entire diameter. 
     Leveling blocks  703  are provided at each reduced-diameter end  799  of the tank  700  to provide support when the tank  700  is at rest on a horizontal surface H. The leveling blocks  703  extend downward a distance D from the tank ends  799  to the horizontal surface H. 
     Access risers  713  are also provided at each end  799  of the tank  700 . As can be seen with reference to FIG. 8, which illustrates a tank  700  installed in a pit  702  in the ground  701 , access riser  713   a  provides access to the inlet “T”  715  and access pipe  717  on one tank end  799  and access riser  713   b  provides access to the outlet filter  721  on the other tank end  799 . Risers  781  provide access to the access risers  713   a,b  from just below ground  740  level. Another access pipe  725  and riser  713   c  are provided for periodic removal of accumulated sludge  731 . 
     An end view of the tank  700  is shown in FIG.  9 . The width W of the leveling block  703  is preferably 1-2 feet, the only requirement being that the width W be sufficient to stabilize and support the tank end  799 . FIG. 9 also illustrates that the inlet  716  is located above the outlet  718 , as was the case for the tank  200  discussed above. 
     FIG. 10 illustrates several different sizes of tanks  900   a-d , each with different numbers of stepped ribs. One 500 gallon tank  900   a  has no stepped ribs. The preferred baffle location B is indicated for each tank  900   a-d.    
     Although the embodiment of the tank  900  discussed above has a single, vertical seam, those of skill in the art will recognize that it is also possible to build a tank having a horizontal seam. 
     Both of the septic tanks described above are water-tight, thereby preventing groundwater from prematurely forcing unclarified waste out of the septic tank. Because the fluids that exit the tanks according to the present invention are clarified, the outlet pipes may be of smaller diameter, such as 1-3″ plastic pipes, than previously possible. The flexibility of such plastic pipes allows installation of the pipes just below the frostline and following the terrain. The cost benefits associated with such an installation are substantial as compared to a traditional six inch or larger concrete pipe installation. 
     Another advantage of the water-tight tanks according to the present invention is that they may be used with alternative treatment plans that are becoming increasingly common in light of municipal regulations and/or the refusal of many municipal sewage treatment plants to accept new customers. One example of an alternative treatment plan for which tanks according to the present invention are especially useful is a system in which 750-3000 gallon septic tanks according to the present invention are connected to receive waste from 1-3 dwelling units for first phase treatment. Clarified waste leaves these smaller tanks through small diameter pipes and feeds into sand filters or a larger tank sized to accept clarified waste from several 750-3000 gallon septic tanks, which in turn feeds into sand filters. The entire waste treatment process occurs in these systems without any connection to a municipal waste treatment facility. small diameter pipes and feeds into sand filters or a larger tank sized to accept clarified waste from several 750-3000 gallon septic tanks, which in turn feeds into sand filters. The entire waste treatment process occurs in these systems without any connection to a municipal waste treatment facility. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.