Septic Components - Distribution
Gravity Distribution: Gravity distribution of sewage in septic systems has been the most common design for a long time. Gravity distribution systems take advantage of a natural phenomenon: water flows downhill. Wastewater flows down from its sources to the septic tank, then to the leach lines or absorption area. 
Gravity distribution systems have a series of trenches. The trenches are sometimes long (max. 100') and are filled with rock and topped with a fairly shallow layer of topsoil. (See "Septic Component: Leach Lines"). Each trench is connected to the septic tank (usually through a distribution box) so that wastewater leaving the tank flows into the trench - the most typical arrangement of this flow is called "serial distribution".
Serial Distribution: Serial distribution is a type of gravity distribution where wastewater from the septic tank flows into the first trench until the trench is full. Then the water flows into the second trench until it, too, is full, then into the third and so on. The first trench will tend to be full all the time. When the water level in that trench drops, it will receive wastewater immediately. But aside from the order in which water reaches them, the trenches function independently, each receiving wastewater at the rate it is treated in that trench. If one is draining more slowly than the others, perhaps because it's located in less permeable soil, it will receive less wastewater. If one tends to drain quickly, perhaps because it receives more sunlight and more water is lost 
through evaporation, it will receive more wastewater. Since the trenches are not directly connected, there is no hydraulic head from trench to trench - water does not move more quickly into or through the second or third trenches because they are downhill from the first one.
Serial distribution allows for flexibility. If one of the trenches fails, another trench can be dug and connected to the septic tank without any alterations to the existing trenches or their distribution lines. Serial distribution also allows for quick inspections - check only the ends of the trenches (See "Septic Components: Inspection Ports").
When the effluent leaves the septic tank, it is sent to the distribution box (D-box). The D-box usually has a single inlet (from the tank) 
and outlets leading to an individual leach line in the absorption area and another leading to the next D-box.
Other Methods of Distribution - Pipe Fitting (Crossover Unit)
