Our flush has now caused effluent to flow from the tank to the leaching area through the tank's 4" diameter outlet pipe. Often a specific part of the leaching area will be fed first, and only after that part is full will the next part be fed. This distribution is performed with a distribution box (D-box), a simple concrete (or plastic, rarely) box that has a "knock-out" hole in each side that allows a pipe to be inserted. One, or sometimes two, of the holes is higher than the others and can only receive effluent when the level in the box rises to a point about 1" above the lower holes. Thus, when the first section of the leaching area is full, the liquid "high-overflows" to the next. With a four-hole box with a single high-hole as shown below, the pipe to the left would most commonly be the feed pipe and must originate at a higher elevation than the high-overflow pipe shown on the right.
There are a variety of leaching types or structures that may be utilized for the leaching area, depending on the ability of the soil to drain water, the size of the area available within the setback constraints, the depth to a "restrictive layer", etc..
Commonly found is the traditional "leaching field", more properly called leaching trenches. Their profile has become more shallow, as well as wider, over the years. Each trench usually has its own distribution box, or "D-box", from which is fed both the trench itself and subsequent trenches. If the high hole in the box feeds the next trench downhill the distribution is termed "serial"; if a low hole in the box feeds the next trench on a more level piece of land the distribution is called "equal", as the idea is to feed all trenches equally, at the same time.
The trench itself consists of clean crushed stone and perforated pipe that is covered by an approved filter fabric. A minimum of 6" of stone is needed under the pipe. The holes in the perforated pipe are lined up at "4 o'clock and 8 o'clock".
Below: A Leaching Field or Trench Under Construction