One of Those Things Everyone Should Know III

A reprint from Penn State Extension, Faith Kibuye, Extension Program Specialist, Water Resources: https://extension.psu.edu/septic-system-basics
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The drain field is also known as a leach field or absorption area. This is where the partially treated wastewater or septic effluent is disposed of for further treatment. It comprises of a network of perforated pipes laid on aggregate materials and a suitable soil layer. Once laid, the pipes are covered with additional aggregate material, geotextile material, and backfill soil. These pipes receive septic effluent through the distribution box and uniformly disperse to the underlying aggregate layer allowing it percolate through the suitable soil layers.

How drain fields provide additional wastewater treatment
Once wastewater is in the drain field, it trickles through the underlying aggregate and the suitable soil layer. As the wastewater moves downward, the soil within the drain field acts as a filter that traps particles and organic matter in septic effluent. Additionally, the effluent comes in contact with naturally occurring bacteria in soil that further break down pollutants in wastewater. Some of the minerals in the soil can also react with the chemicals in wastewater to help break them down. These processes help filter the wastewater naturally before eventually reaching and mixing with nearby groundwater or surface water bodies. 

Types of drain fields
There are different types of drain fields, and the type permitted for construction in a home varies based on site-specific factors. Some factors considered include lot size and shape, soil type and characteristics such as its ability to absorb water, topography and slope, underlying geology, depth to the groundwater table, existing infrastructure (e.g., location of private wells), among other environmental factors and local regulations. These factors are evaluated to determine the most suitable type of drain field to effectively treat septic effluent while minimizing contamination of nearby groundwater and surface water resources. For example, based on Pennsylvania code, the top of a limiting zone, i.e., groundwater table or bedrock, should be a minimum of 4 feet below the bottom of the gravel aggregate layer of the drain field. This separation distance is beneficial as it ensures that there is sufficient suitable soil to filter septic effluent before mixing with groundwater. The types of drain fields include:

Standard trenches: This type of drain field consists of a network of perforated pipes laid in narrow (12 to 72 inches width) parallel trenches following land surface contours (See Figure 1).

Seepage bed: These rectangular excavations contain several perforated pipes to distribute septic effluent. The depth of aggregate materials in the seepage bed should be 12 to 36 inches.

Elevated sand mound: A sand mound is also commonly called a raised bed or a mound system. This type of drain field is a bed built above ground on top of a man-made sand plateau. Effluent is pumped and dispersed into the mound of sand. Sand mounds are used in areas where the soil is unsuitable or the water table is too high to have a conventional trench or seepage bed. The major difference between elevated sand mounds and conventional in-ground-absorption systems is the addition of sand to the top of the absorption area in order to increase the depth to the limiting zone to the required minimum of 4 feet.

Subsurface sand filter beds and trenches: According to the Pennsylvania Code, subsurface sand filters cannot be utilized on soils where the limiting zone is less than 6 feet below the soil surface. System design should meet the requirements of the standard trenches or seepage beds.  As the name suggests, they consist of a sand filter bed layered in an excavated area with a maximum depth of 5 feet. Pipes evenly dispersing effluent are laid on top of the sand bed and are covered with soil backfill.

Alternative drain fields: Newer designs for drain fields exist. Some include drip irrigation, individual residential spray irrigation systems (IRSIS), evaporation systems, and constructed wetlands, among many others.

Caring for your drain field
The drain field is the most expensive and difficult septic system component to correct or replace. Regular care and maintenance can help extend the lifespan and the entire septic system.

Here are some things you can do:
(I had to abbreviate the rest due to space considerations)- Have your system professionally inspected; do regular maintenance on it (have it pumped every 3-5 years); divert excess water away from it; and keep heavy equipment, traffic, and trees away from it.
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The Bradford County Conservation District is committed to helping people manage resources wisely.  You can visit the Bradford County Conservation District at 200 Lake Rd in Wysox across from the Wysox Fire Hall. Contact us at (570) 485-3144 or visit our web page at www.bccdpa.com