Understanding Your Septic System

How Your System Works:

Home septic systems consist of two principle components: a septic tank where solid waste is stored and an absorption area where wastewater is treated.

Bathroom, kitchen and laundry waste drains through a pipe (house sewer) into your septic tank where it separates out into three layers:

1. Solids settle to the bottom and, through the action of anaerobic bacteria, decompose to form a sludge.
2. Insoluble greases and oils, which are lighter than water, form a floating surface layer of scum.
3. The wastewater that remains after solids and scum have separated out forms a middle layer.

The anaerobic decomposition that occurs in the septic tank is very incomplete. Septic tanks must be routinely pumped (usually every 2 to 3 years) to remove accumulation of bottom sludge and surface scum. Fortunately, however, home septic systems are designed to treat the separated wastewater much more thoroughly. Beyond the septic tank, your septic system consists of a delivery means for distributing wastewater beneath the ground surface where it will undergo further decomposition through the action of aerobic bacteria present in the soil.

Wastewater leaves your septic tank through an outlet baffle. These baffles help ensure that wastewater flowing to the absorption area is relatively free of scum and solids which could seriously shorten the working life of your system by clogging leach lines and blocking trench walls.

From the septic tank, wastewater flows to the distribution box. This box distributes wastewater through header pipes to perforated leach lines set in absorption trenches of stone aggregate. New, “gravel-less” systems offer an alternative to trenches filled with stone. These systems replace conventional perforated leach lines either with leaching chambers or with pipe wrapped in geotextile.

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 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. But 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 the house drainage plumbing or seep out onto the ground surface.

With constant usage, this aging process is inevitable. Usually, however, moderate maintenance can prevent premature failure.