Abstract:
A low profile transportable holding tank for accepting waste from portable toilets. The holding tank is carried on a flatbed vehicle and has a flat deck for cargo, thus effectively increasing the cargo-carrying capacity of the vehicle compared with that available if the vehicle were carrying a conventional holding tank. Internal reinforcement enables the holding tank to withstand a partial vacuum imposed for pumping waste into the tank.

Description:
FIELD OF THE DISCLOSURE 
     This disclosure pertains to a transportable sewage container, and in particular to a sewage container intended for servicing portable toilets. 
     BACKGROUND 
     Portable toilets are a feature of construction sites and certain other temporary workplaces, and also of transient public events such as festivals and some sporting occasions. To service these toilets, it is necessary not only to transport them to and from sites where they are used; it is also necessary to transfer accumulated waste into a holding tank, which is transported to a treatment plant for proper disposal. Usually, the same vehicle (usually a flatbed truck) is used to transport both the toilets and the holding tank. Waste is transferred from the toilets to the holding tank by means of a pump that evacuates the holding tank to a vacuum of at least about 12″ of mercury. If the portable toilets are at a particular site for an extended time, they may need periodic servicing. Otherwise, they would be serviced at a central location, which may or may not be the location of the treatment plant. In any event, it is often convenient to be able to transport both the waste and the toilets themselves simultaneously on the same vehicle. 
     The ideal shape for a vessel needing to withstand a high pressure differential is a perfect sphere. A convenient way of describing a general configuration of a three-dimensional vessel is its aspect ratio, which may be defined as the ratio of the size of an object measured in two orthogonal dimensions. Clearly, a spherical vessel has an aspect ratio of 1, since it measures the same distance across in all dimensions. A cubic vessel also has an aspect ratio of 1. While not as strong as a sphere, it is stronger than a comparably shaped vessel with an aspect ratio greater than 1. As a rule of thumb, it may be stated that for any two vessels with comparable shapes and volumes but different aspect ratios, the one having a higher aspect ratio will be weaker with respect to pressure differential, all other factors being equal. A given object may have different aspect ratios depending on the directions of measurement. 
     It has hitherto been a feature of the aforementioned sewage holding tanks that they are generally cylindrical in shape to provide maximum strength, and do not have an aspect ratio greatly exceeding 1. They can be considered the best practical approximation to a sphere. A customary holding tank occupies a significant area of the truck bed, which obviously limits how many portable toilets can be accommodated. It is possible to envisage a low-profile holding tank occupying substantially the entire area of the truck bed, the tank itself having a flat cargo-bearing surface similar in area to that of the truck bed. 
     Conventional holding tanks typically have a low aspect ratio, and occupy a smaller area than the envisaged low profile tanks, but are not configured to support additional cargo. 
     Sometimes dedicated tanker trucks are used for carrying waste, having no additional cargo area for portable toilets. The evacuable, waste-containing portions of these tanker trucks are generally cylindrical. While their dimensions vary, the greatest aspect ratio (in this case evacuable cylinder length divided by diameter) is typically less than 3. 
     Typically, a flatbed truck carrying a conventional holding tank might have additional space for two portable toilets, while the envisaged low profile holding tank would provide space for at least four. In many situations this would be a distinct economic advantage, since it could cut to a half or a third the number of necessary trips, representing a considerable saving in time and labor. 
     There is therefore a need for a low profile holding tank configured to provide increased cargo carrying space. Such a configuration must necessarily have an aspect ratio significantly greater than 1, and it would normally be expected to be too weak to withstand the required evacuation. Therefore, there is further a need for a low profile holding tank capable of withstanding such evacuation. 
     SUMMARY OF THE INVENTION 
     The holding tank of the invention has a length and width conforming generally to the area of cargo space of a vehicle such as a flat bed or similar truck, and a height much less than the length or width. Consequently, the holding tank has an aspect ratio which may be as high as 24. The holding tank has a floor, walls, and a flat top or deck and is made from mild steel sheet stock. 
     An internal reinforcement is provided which allows relatively light sheet stock to be used for fabricating the holding tank. The reinforcement includes reinforcement strips inside the floor, support pillars attached to the reinforcement strips, and support strip attached atop the support pillars for absorbing pressure from the deck when the holding tank is evacuated. 
     Since the flat deck approximates the area of the truck bed, the low-profile holding tank essentially provides the same space for carrying cargo; the holding tank itself therefore uses up no cargo space, and additional space is therefore available for other payload. In particular, the freed up space can be used for transporting more portable toilets per trip than would otherwise be possible. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially cut-away perspective view of a low profile holding tank for a transportable sewage system. 
     FIG. 2 is a top plan view of the holding tank of FIG.  1 . 
     FIG. 3 is a side elevation of the holding tank of FIG.  1 . 
     FIG. 4 is an end elevation of the holding tank of FIG.  1 . 
     FIG. 5 is a side elevation of a support assembly of the holding tank. 
     FIG. 6 is a schematic of the transportable sewage system connected to a portable toilet. 
     FIG. 7 is a side view of the transportable sewage system aboard a vehicle. 
     FIG. 8 is a rear view of the transportable sewage system aboard the vehicle. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, a low profile transportable sewage system  100  is used for receiving waste from portable toilets  170 . The system  100  has a holding tank  110 , shown in FIGS. 1-4, sized to conform generally with a cargo area of a vehicle  180  such as a truck with a flat truckbed  182 . The holding tank  110  can be partially evacuated to receive the waste for transportation to a treatment plant, and has a surface capable of accommodating the transport portable toilets  170  for transportation when necessary. 
     The holding tank  110  has a floor  112 , a deck  114  and walls  116  attached to the floor  112  and the deck  114 . Both the floor  112  and the deck  114  are generally flat. As will be described later, it is intended to draw waste into the holding tank  110  by establishing a partial vacuum therein. 
     The holding tank  110  is typically formed from steel sheet stock, preferably 0.25″ thick mild steel. It is fabricated by any suitable combination of cutting and shaping or welding the sheet stock. For example, the floor  112  and at least some of the walls  116  may originate from a single blank of sheet stock. Alternatively, the walls  116  could originate from a single strip of stock that is shaped to conform with the intended perimeter of the holding tank  110 ; it would then be welded to the floor  112  and the deck  114  finally welded atop the walls  116 . In any event, any line along which a wall  116  meets the floor  112 , the deck  114  or a neighboring wall  116  must be made fluid-tight by welding where appropriate. 
     Inside the holding tank  110  are spaced apart longitudinal support assemblies  130  configured as shown in FIG.  5 . Each support assembly  130  has a reinforcing strip  132  welded to the floor  112 , a series of support pillars  134  welded to the reinforcing strip  132 , and a support strip  136  atop the support pillars  134  and welded thereto. The support assemblies  130  are provided to preclude failure that would otherwise occur under a desired pressure differential of  12 ″ of mercury. The low profile system  100  of the invention necessitates that the holding tank  110  should have a high aspect ratio. As explained earlier, such a vessel is less able to withstand evacuation than one that more closely approximates a sphere. Without internal support, the floor  112 , deck  114  and wall  116  of the holding tank  110  would have to be prohibitively massive. 
     In the fabrication of the holding tank  110 , the support assemblies  130  obviously have to be put in place before the deck  114  is welded to the walls  116 . 
     A transverse drainage channel  140  is provided at one end of the floor  112 . The drainage channel  140  slopes down towards an effluent discharge port  142  below the level of the floor  112 . The discharge port  142  is typically equipped with a gate valve. 
     The holding tank  110  is configured so that there is generally a small clearance between the deck  114  and each of the support assemblies  130 , although contact between the deck  114  and the support assembly  130  is not precluded. As the holding tank  110  is evacuated, external air pressure tends to deform the deck  114  inward so that it increasingly contacts the support assemblies  130 . In this regard, the flexibility of mild steel renders it particularly suitable for the holding tank  110 , although some stainless steels may be used. Other materials such as hard steel are less suitable. Given the small clearance between the deck  114  and the support assemblies  130 , the degree of deformation is also small and the deck  114  remains essentially flat, thus enabling the holding tank  110  to carry cargo in a stable manner. 
     Preferably, the deck  114  is made of diamond plate steel to facilitate any intentional movement of objects thereon. A transverse headboard  160  is mounted atop the deck  114  by a suitable fastening means. The headboard  160  could made of lumber and be securely bolted to an L-shaped attachment member (not shown) welded to the deck  114 . A transverse retaining member  162  is attached to the headboard as best seen in FIG.  1 . The retaining member  162  is typically a length of standard “four-by-four” lumber, which actually has a 3.75″ square cross-section. The retaining member  162  is located on the headboard  160  so that a gap  164  of about 4″ remains between the retaining member  162  and the deck  114 . In an alternative construction, the headboard  160  is a steel frame and the retaining member  162  is a length of angle iron attached thereto so as to project rearward and provide the required 4″ gap. 
     The headboard  160  defines a line dividing the deck  114  into a front deck  118  and a rear deck  120 , the rear deck  120  corresponding to the end of the holding tank  110  with the drainage channel  140 , and having an area at least twice that of the front deck  118 . The retaining member  162  faces rearward. The headboard  160  is used to secure one or more portable toilets  170  carried atop the rear deck  120  for transportation to or from a use site. Projecting beyond the bottom of many portable toilets  170  is a skid base  172  made up of “four by fours”. When a portable toilet  170  abuts the headboard, the skid base  172  is received into the gap  164 . The headboard  160  has fastening means such as a strap  174  that passes through anchors  176 ; the strap is passed around the portable toilet  170  and secured with a buckle. The strap  174  may for example be a ratchet strap and is selected so that it is long enough to pass around multiple portable toilets  170 , typically up to four. The interlocking of the skid base  172  with the retaining member  162  provides stability when the vehicle  180  is in motion; wind pressure tends to tilt the portable toilet  170  rearward, upwardly urging the skid base  172  towards the retaining member  162 , which limits further tilting. Portable toilets lacking the projecting skid-base  172  as described have an equivalent interlocking feature, for example a recess that can receive the retaining member  162 . 
     Clearly, the retaining member  162  must be securely attached to the headboard  160  to withstand the effects of wind pressure. With the wooden headboard  160  and retaining member  162 , the attachment means is for example heavy bolts. With the metal frame construction, the headboard  160  and retaining member  162  may be welded or bolted together. 
     In addition to the discharge port  142 , the holding tank  110  has an evacuation port  144  with a connection  150  to a standard pumping system  148  that includes a vacuum pump, typically driven by an electric-start internal combustion engine, although other types of drive may be used. The pumping system  148  is carried atop the front deck  118  along with other equipment such as an aluminum 250 gallon water tank, a wash down pump, and a water hose and hose reel. Although the holding tank  110  is removable from the vehicle  180 , it generally remains on the truckbed  182  for an extended time, and the connection  150  may therefore be hard-plumbed. 
     The holding tank  110  has at least one inlet port  146  for admitting waste from the portable toilet  170 . In fact, there are usually two inlet ports  146 , one on either side of the deck  114 , either of which can be used as convenient. The inlet ports  146  have valves (not shown) for admitting air to equalize internal and external pressures when pumping is complete. Typically, a 2″ diameter flexible vacuum hose  152  is used to connect the inlet port  146  and the portable toilet  170 , using aluminum cam lock quick disconnect fixtures. The holding tank  110  has an indent  122  on each side of the front deck  118  to provide space for stowing the vacuum hose  152  and drainage hoses when not in use. FIG. 6 is a schematic of the low profile transportable sewage system  100  as coupled to the portable toilet  170 . 
     At least one waste-level indicator is provided on the holding tank  110 . Typically, 2″ diameter “fish-eye” sight-glasses  124  are located in a forward-facing wall of the driver&#39;s-side indent  122 . These are hemispheres of 0.25″-thick polycarbonate, threaded to engage threaded boiler flanges welded into the wall  116 , there being a vacuum-tight seal between each sight-glass  124  and the corresponding boiler flange. Usually, three such sight glasses are disposed in a vertical alignment to allow viewing of a wide range of waste levels. 
     Also, the holding tank  110  has attachment points such as hooks or handles  178  for lifting-equipment required when moving the holding tank  110  on or off the vehicle  180 . 
     The holding tank  110  is typically 10′, 11′ or 12′ long and about 7.5′ wide. There are normally three support assemblies, spaced 22.5″ apart between centers. The reinforcing strips  132  and the support strips  136  are mild steel strips 3″ wide and 0.25″ thick. The support pillars  134  are made from tubular mild steel having an outside diameter of 1.5″ and a wall thickness of 0.25″. Within each support assembly  130 , the support pillars  134  are spaced at 16″ intervals. Of the indents  122 , one has an indented area of 28″×10″ while the other has an area of 28″×6″. Relative to the headboard  160 , the front deck  118  extends about 2.75′ forward while the rear deck  120  extends about 7.75′ rearward. The drainage channel  140  is 6″ wide and extends 8″ below the floor  112  at its lower end. The depth of the holding tank  110  is usually 1′. Some holding tanks may be as long as 16′ and have a 1′ or an 8″ depth. 
     Given the above measurements of the holding tank  110 , various values of the aspect ratio can be calculated. For example, with the 11′-long holding tank  110  one may divide the depth (1′) into the width at the indents (6′), into the more general width of 7.5′, or into the length of 11′. The holding tank  110  therefore has aspect ratios of 6, 7.5 and 11. Even the least of these values represents a significant departure from the “ideal” aspect ratio of 1. A holding tank  110  that is 16′ long and 8″ deep has aspect ratios as high as 24. 
     To summarize, the holding tank  110  is configured to be evacuated to a partial vacuum and to withstand that vacuum. The required pressure differential (about 12″ of mercury) would most likely bring about the collapse of the holding tank  110  with the aforementioned aspect ratios, were it not for the internal support assemblies  130 . However, as the holding tank  110  is evacuated, the support assemblies  130  increasingly absorb the pressure differential. 
     The pumping system  148  includes primary and secondary vacuum traps which preclude the entry of fluids from the holding tank  110  into the pump, and also isolate the pump from the holding tank  110  once the latter has been filled to some preselected level. 
     The low profile transportable sewage system  100  may be used as follows for transporting portable toilets  170 . It is assumed that the system  100  is installed on the vehicle  180  as shown in FIGS. 7 and 8 with enough rear overhang of the holding tank  110  for the drainage channel  140  to clear the truckbed  182 . The vehicle  180  is moved into proximity with the portable toilet  170  which can then be loaded atop the deck  114 . Any convenient method of loading can be used, although it is conventional in transporting portable toilets to use a vehicle lift-gate (not shown). Once the portable toilet  170  is at deck level, it is slid forward along the diamond-plate deck  114  until the skid base  172  is fully received into the gap  164 , the portable toilet  170  then abutting the headboard  160 . If a second toilet  170  needs to be transported on the same trip, it is loaded in the same way and moved until it abuts the headboard  160  adjacent the first toilet  170 , again with the skid base  172  received into the gap  164 . Although the exact dimensions of “standard” portable toilets may vary, they are generally sized to have a “footprint” approximating to a square with a side in the range 3′ to 3.5′. In practice, this means two, but no more than two, portable toilets  170  may be accommodated within the width of a typical vehicle. It follows, therefore, that the same limitation applies to the holding tank  110  of this invention. Additional toilets (to a typical total load of four, or six if the holding tank is 16′ long) can be loaded and slid along the deck  114  into positions behind the first and second toilets  170 ; of course, these additional toilets do not abut the headboard  160 . When the required toilets  170  are in position, at least one strap  174  is passed around all of them and secured with a suitable fastening means such as a strap  174  and in particular a ratchet strap. The foregoing procedure is reversed for unloading the toilets. 
     Regarding the actual filling and draining of the low profile holding tank  110 , this is equivalent to filling and draining conventional holding tanks. Given that the holding tank  110  and pumping system  148  are present and functional on the vehicle, the procedure may be summarized as follows: actuating the pumping system  148  to evacuate the holding tank  110  so that the support assembly  130  increasingly absorbs pressure from the deck  114 ; using the vacuum hose  152 , fluidly connecting the holding tank  110  and the portable toilet  170  such that waste from the portable toilet  170  is transferred to the holding tank  110 ; disconnecting the holding tank  110  and the portable toilet  170  after the waste has been transferred and uncoupling the vacuum hose  152 ; shutting off the pumping system  148 ; transporting the holding tank  110  to a treatment plant; and draining the holding tank  110  through the discharge port  142 . 
     Depending on the particular treatment plant, the discharge port  142  may be positioned directly above a receiving area and the holding tank  110  drained by gravity, without any physical connection, or the holding tank  110  may be drained through a hose connecting the discharge port  142  with the treatment plant. 
     While the toilets  170  are being transported, slipstream wind exerts a pressure on forward-facing portions of the first and second toilets  170 , tending to tilt them rearward. Any such tilting urges the skid base  172  upward against the retaining member  162 , which limits further tilting. In fact, increasing wind pressure increases pressure between the skid base  172  and the retaining member  162 , improving the frictional contact between them and precluding unwanted movement. 
     The high aspect ratio of the low profile holding tank  110  allows it to be configured to occupy generally the entire cargo space of the truck bed  182 . The deck  114  being flat, this in effect provides a raised but undiminished cargo-carrying area for the vehicle  180 . Some of this surface is used for equipment such as pumping machinery necessary for the holding tank  110  to function, but such equipment is equally necessary for conventional holding tanks and cannot be accommodated thereon. Overall, the low profile holding tank  110  allows a user to make much better use of cargo space than is possible with conventional holding tanks. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope and spirit of the appended claims.