Replace Drain Field

The Septic Tank Owners Manual, Drain Field

(Chapter 5) Components of Your Septic System: Drain Field

The Drain Field

There are many design variations of absorption component. Regardless of the design, all serve the same primary function. They disperse the partially treated septic sewage from the septic tank into the sub-surface of the ground. The dimensions provided below are examples of a typical septic system. In many cases site conditions and state and local codes determine the dimensions. An overwhelming majority of household septic systems utilize a septic drain field or absorption trenches.

The Septic Drain Field is a large rectangular hole dug into the ground, partially filled with washed gravel to a depth of approximately 12 inches. At the inlet end of the field is a cylindrical chamber called a distribution box. The distribution box may be made of concrete, fiberglass, plastic or steel. The distribution box is usually a cylinder, square or rectangle of approximately 24 inches in diameter and 18 inches to 24 inches deep with three or more holes in its sides. One of the holes contains the effluent pipe from the septic tank or settling chamber. Connected to the other holes are perforated pipes called laterals. On top of the 12" of washed gravel a series of pipes are laid out parallel to each other and spaced approximately 3 to 6 feet apart. The laterals are interconnected with each other on each end utilizing 90-degree pipe elbows. The laterals are then connected into a tee near the center of the distribution field at the end opposite of the distribution box. Connected to the tee is a vertical pipe that runs upward out of the ground and is called the vent pipe. On top of the perforated pipe, approximately 12 inches or more of washed gravel is placed in the trench. On top of this washed gravel a filter fabric is laid. The filter fabric allows rainwater to infiltrate the surface but does not allow dirt to settle into the field. On top of the filter fabric, topsoil is used to fill the trench up to the original grade

An absorption trench is very similar to an Septic Drain Field with the exception that undisturbed soil is retained in the area between the laterals. The trenches have more sidewall surface area than does a rectangular pit. The advantage of the trench as compared to the field is that the trench allows the effluent to be absorbed through the increased area of the sidewalls of the trench. A very common absorption component of the past was the seepage pit or dry well.

This design was very popular in sandy soil that has very good drainage capability. This design is constructed by digging a circular pit of approximately 8 - 10 feet in diameter and approximately 6 - 8 feet deep. A 4 - 6 foot diameter cylinder is constructed from the bottom of the pit utilizing concrete or cinder blocks or bricks. The block or bricks are laid on their side with the openings running horizontally. The cylinder is built to a height of 4 - 6 feet and capped with a concrete or steel cover. The gap between the constructed cylinder and the dug pit is filled with washed stone to the top of the cylinder. The remainder of the pit is filled with topsoil to the original grade. A very popular absorption component design is the mound system.

This is an above grade distribution component in areas such as slowly permeable soils, shallow permeable soils over a limiting layer, or permeable soils with high water tables. The mound system is similar to an absorption field except it is constructed above grade. The mound system site is carefully selected for ideal slope. Once the location is selected, the topsoil is plowed utilizing a proven technique. On top of the plowed topsoil, a layer of medium sand is spread to a specific depth. On top of the sand, a layer of washed stone is laid in place. As in the distribution field, perforated pipe is buried within the layer of washed stone. The washed stone is then covered with a filter fabric. The filter fabric is covered with subsoil and then with topsoil. The outside of the mound is shaped by excavation to have a prescribed slope to efficiently handle rainwater runoff.

Drain Field, Drywell / Pit, Mound System or Cesspool.

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The Septic Tank Owners Manual, Drain Field

(Chapter 5) Components of Your Septic System: Drain Field

The Drain Field