Trenches are long, narrow systems that allow the soil to treat the effluent. The method of distribution is usually gravity (See
"Septic Components: Distribution").
The materials used in trenches have changed over the years. Typically, systems have been constructed out of 4" pipe and rock. A number of plastic products have become available recently to replace the rock and gravel-less trenches are becoming more common. (See
Septic Components: Leach Lines").
It is important that the bottom of the leach line trench is a minimum of 5 feet from groundwater to prevent contamination.
It is also important that the bottom of the trench is level, allowing for an even flow of effluent.