Septic System Trench

In the typical absorption field, each length of leach line or leaching chamber is set in its own individual trench. This results in an absorption field consisting of a series of parallel trenches that are about 18" to 30" deep. Because the aerobic bacteria that decompose waste thrive mostly in this upper area of the soil, this type of system provides the most thorough treatment of wastewater. It also helps protect ground water from possible contamination by affording the greatest possible separation distance between trench bottom and the water table.

Occasionally, however, space limitations may require different approaches. If soil and ground water conditions permit, seepage pits are an option. Another alternative is a stone bed where all the leach line is laid in a single, wide trench with stone aggregate.

All absorption areas, regardless of type, are subject to the same aging processes. In every case, a biomat forms on the trench walls. Initially this biomat aids wastewater treatment by straining out pathogenic bacteria.

As more bacteria and waste add to the biomat, the trench walls become more impervious to the passage of wastewater. Siltation and soil compaction can further slow the absorption rate. Eventually, wastewater might either back up into house drainage plumbing or seep out onto the ground surface.
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