Gravity Distribution

Over the course of onsite treatment in the U.S., gravity distribution of septic tank effluent has been the most popular design. Gravity distribution systems make use of elevation differences in order to distribute waste water more cheaply than systems that pump waste. From its origins, effluent descends to the septic tank before proceeding in either a serial or parallel distribution to the soil treatment system. The objective of both systems is to distribute effluent equally, which is made possible by a biomat, as seen below. A genuinely uniform dispersion cannot be accomplished until the biomat reaches the end of a trench or trenches.

Serial Distribution

When using serial distribution, septic tank effluent runs into the first trench and continues to fill it up until it ponds. The effluent then runs into the second trench until it fills up as well before entering the third. Before effluent is transferred to the second trench, the first trench needs to be filled to capacity. The system consists of a watertight pipe that runs from the septic tank to the first drop box, which is usually how it is set up. A drop box is a distribution tool used to apply sewage tank effluent in series, gravity, to a soil dispersal system. The distribution pipe of the trench is connected to an outlet that is located close to the bottom of the drop box. A watertight pipe leading to the drop box of the lower trench is connected to another exit near the top of the drop box.

The trenches operate independently, receiving wastewater at the pace that is accepted in each trench, regardless of the sequence in which effluent enters them. Less effluent will be absorbed if one drains more slowly than the others, maybe because it is in less permeable soil. One will receive more effluent if it has a tendency to drain quickly, possibly because it receives more sunshine on the surface and more water evaporates throughout the summer. There is no hydraulic head from trench to trench since the trenches are not connected directly.Because they are downhill from the first trench, the effluent does not flow through or into the second or third trenches any faster.

Benefits

Promotes the growth of biomats and unsaturated flow in soils with a sandy or coarse texture.

Sloping terrain (no maximum slope limits)

When the requirement for varying trench lengths is necessary to accommodate buildings, trees, etc.

Flexibility to build and link new trenches as necessary

Observing the box and the ends of the ditches quickly

Between the septic tank and the trash cans, there must be no standing waste in solid pipes.

Parallel Distribution

Parallel distribution is the process of distributing septic tank effluent by gravity flow, equally loading each section of the soil treatment and dispersal system at once. All of the trenches in the soil treatment unit get effluent flow concurrently thanks to the parallel distribution system. Because trenches are built to be the same width, depth, and kind of soil, treatment happens in each trench at the same rate.Between the trenches and the distribution box, the parallel distribution system is capable of having effluent in the solid pipes. An instrument frequently used to accomplish parallel distribution is a distribution box, which is made to transfer septic tank effluent concurrently and evenly via gravity to various parts of a soil treatment and dispersal system.

There may be a sizable hydraulic head between the top of the system (the distribution box) and the trenches even when there is no backflow issue. As a result, the permitted slope is often not very steep (less than 5%). Throughout their operating lives, each trench must be able to treat the same volume of effluent as the others and must all have the same length. The distribution box's outlets must all be at the same height both during installation and after the system has been backfilled.

The issues caused by distribution boxes settling have been solved by a variety of technologies. Distribution boxes that can be leveled after installation have been produced by a few manufacturers. The 4-inch outlet pipes' ends can be fitted with a leveling mechanism, which substantially levels the outlet inverts. Periodically, these levelers need to be adjusted. A device known as the distribution box outlet adjuster can be inserted into each pipe leaving the distribution box. To allow trench resting, these adjusters may be rotated or dialed to position the small apertures' inverts at either the same height or a different elevation.This method has the advantage of allowing readjustments when the distribution box becomes slightly out of level due to soil shrinkage or expansion or when one side of the distribution box settles, freeze-thaw activity, or other environmental factors.

Benefits

Level Sites (<5% slope) Sites with restricted separation to the limiting condition where the needed separation may be impacted by the drop boxes' loss of elevation When it is possible to create equal-length trenches To assist spread the wastewater over the length of the trench in soils with more clay and silt than sand