WHERE DOES THE WASTEWATER GO AFTER IT LEAVES THE TANK?
Most commonly, wastewater goes to a drainfield, (also called leachfield or disposal field). It can also go to a mound (seep. 89), a seepage bed (seep. 24), or a seepage pit (seep. 25). The general and more technical term for all these methods of handling the wastewater from the tank is the soil absorption system. We will begin here with the most common type, the drainfield or leachfield.
WHAT THE DRAINFIELD DOES
Once sewage undergoes primary treatment in the septic tank, the clarified effluent flows to the drainfield, where it is discharged into the soil for final treatment and disposal.
Note: A typical drainfield consists of several relatively narrow and shallow gravel-filled trenches with a perforated pipe near the top of the gravel to distribute the wastewater throughout each trench. In most cases, drainfields will perform almost indefinitely if the system is designed, used, and maintained properly.
CONSTRUCTION OF DRAINFIELD
The typical drainfield trench is a level excavation, rectangular in cross section, with a level bottom. Trenches are typically 1 to 3 feet wide, 2 to 3 feet deep, and usually no more than 100 feet long. The trench is partially filled with a bed of clean gravel (%- to 2Vi-inch diameter) to within 1 or 2 feet of ground surface. A single line of perforated pipe, 3 to 4 inches in diameter, is installed level (no slope) on top of the gravel and covered with an additional 2 or 3 inches of gravel.
the main job of the drainfield is to purify and disperse the effluent flowing out of the septic tank.
Cross section — typical drainfield. For an EPA diagram showing the pathway of air to the soil in a drainfield, seep. 159 in the Appendix.
The gravel is covered with a semipermeable geotextile fabric and the remaining 1 to 2 feet of the trench is backfilled with soil up to the ground surface. The fabric barrier keeps the backfill soil out of the gravel and prevents the fine soil particles from clogging the pores of the gravel. (Before the advent of geotextile fabric, a layer of straw or hay or untreated building paper was used for the soil-gravel barrier.)
A NOTE ABOUT DRAINFIELD DESIGNS
The size, design, and location of a drainfield for a given home depend on a variety of factors, such as local soil characteristics, the amount of wastewater flow, ground slope, and depth to groundwater or bedrock. The objective is to distribute the effluent into an area with an adequate depth of suitably permeable, unsaturated soil. The drainfield also should be located as far as possible from drinking-water wells, streams, lakes, steep hillsides, road-cuts, property lines, etc.
Specific design criteria are provided by local building or sewer codes and often vary from one locality to the next. Local health departments often require a permit before allowing any work on a drainfield and frequently can offer helpful advice. A comprehensive presentation of the many and various local design rules is beyond the scope of this book. Rather, our intent is to provide a basic understanding of what the drainfield is and how it works.