Gravity Distribution: Do You Need a Drop Box or Distribution Box?

Gravity Distribution: Do You Need a Drop Box or Distribution Box?

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As I travel across the country and discuss gravity distribution, one of the biggest variations I see is how the effluent is delivered to the trenches. Flow to gravity distribution systems is directly related to use at the source: effluent from the septic tank flows to the soil treatment area whenever sewage enters the septic tank.

Gravity distribution of septic tank effluent has been the most common design over the history of onsite treatment in the U.S. Less expensive to install and maintain than systems in which effluent is pumped, gravity distribution systems take advantage of natural elevation differences. Effluent flows down from its sources to the septic tank, then on to the soil treatment system in either serial or parallel distribution. The goal with both systems is the even distribution of effluent facilitated by a biomat, as shown below. Until the biomat reaches the end of a trench or trenches truly uniform distribution is not achieved.

Serial Distribution

With serial distribution, septic tank effluent flows into the first trench until effluent has ponded and the trench reaches capacity. Then, the effluent flows into the second trench until it, too, reaches capacity, then into the third. The first trench should be at capacity before effluent is delivered to the second trench. The system is composed of a watertight pipe from the septic tank and then most commonly to the first drop box. A drop box is a distribution device used for the serial gravity application of sewage tank effluent to a soil dispersal system. An outlet, near the bottom of the drop box, connects to the distribution pipe of the trench. Another outlet near the top of the drop box connects to a watertight pipe leading to the drop box of the lower trench.

Aside from the order in which effluent reaches them, the trenches function independently, each receiving effluent at the rate it is accepted in that trench. If one is draining more slowly than the others, perhaps because it is in less permeable soil, it will accept less effluent. If one tends to drain quickly, perhaps because it receives more sunlight on the surface and more water is lost through evaporation in the warmer months, it will receive more effluent. Since the trenches are not directly connected, there is no hydraulic head from trench to trench — effluent does not move more quickly into or through the second or third trenches because they are downhill from the first one. 

 Application and advantages: 

  1. Encourages biomat formation and unsaturated flow in sandy or coarse textured soils 
  2. Sloping sites (no maximum slope limits) 
  3. When variable trench lengths are needed to accommodate structures and trees, etc. 
  4. Flexibility to construct and connect new trenches as needed 
  5. Quick inspection at the box and ends of the trenches 
  6.  No standing effluent in solid pipe between septic tank and drop boxes

Parallel Distribution

Parallel distribution is the distribution of septic tank effluent by gravity flow, loading all sections of the soil treatment and dispersal system equally at the same time. The parallel distribution system directs effluent flow into all trenches in the soil treatment unit simultaneously. Trenches are constructed to be of equal length and depth and to be suited for the same type of soil, so treatment occurs at the same rate in each trench.

The parallel distribution system can have effluent in the solid pipes between the trenches and the distribution box. A distribution box is a device commonly used to achieve parallel distribution which is designed to distribute septic tank effluent concurrently and equally by gravity to multiple segments of a soil treatment and dispersal system. 

Even when there is no backflow problem, there may be significant hydraulic head between the top of the system (the distribution box) and the trenches. Therefore, the allowable slope is not very steep (typically less than 5%). All trenches must be the same length, and each must be able to treat the same amount of effluent as the others throughout its operational life. All the outlets of the distribution box must be at the same elevation when installed and after the system has been backfilled.

Many devices have been developed to overcome the problems associated with distribution boxes settling. Some manufacturers have developed distribution boxes that can be leveled after they are installed. A leveling device can be inserted in the end of the 4-inch outlet pipes, which makes the outlet inverts somewhat level. These levelers need to be adjusted periodically. The distribution box outlet adjuster is a device that can be placed inside the distribution box into each pipe leaving the distribution box. These adjusters can be turned or dialed so that the inverts of the small openings are at the same elevations or at different elevations to provide trench resting. The advantage of this approach is that it allows readjustments to be made when one side of the distribution box settles or when freeze-thaw activity or shrink-swell in the soil causes the distribution box to become slightly out of level.

Application and advantages:

  1. Level sites (<5% slope)
  2. Sites with limited separation to the limiting condition where the loss of elevation in the drop boxes may impact the required separation
  3. When equal length trenches are feasible
  4. In soils which have more clay and silt (versus sand) to help distribute the wastewater the length of the trench

About the author
Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the president-elect of the National Onsite Wastewater Recycling Association and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to


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