How the Biomat Affects Drainfields

To best understand how a biomat affects your drainfield, it is helpful to use a liner for a swimming pool as an analogy. Liners for swimming pools are typically made of vinyl and form an impermeable barrier along the bottom and sides of the swimming pool; water is prevented from absorbing into the ground on all surfaces below ground level.

Typical septic tank and gravity fed drainfield. Bio-mat is growing at bottom edge of gravel bed in drainfield

Biomats start growing at the bottom of septic system drainfields between the gravel and the surrounding soil. A failed septic system contains a biomat that has grown not only on the bottom of the drainfield, but up the sidewalls, as well, sealing the septic tank liquid effluent inside the drainfield. The only way for the liquid to leave the drainfield is to pond on the surface and evaporate, an extraordinarily slow, odorous and visually repulsive process! As you can see, the biomat is performing the same function as a swimming pool liner, trapping the liquid and preventing it from being absorbed into the surrounding soil.

Bio-mat growth moving up sides of gravel bed in drainfield

Biomats actually seal better than swimming pool liners. When a swimming pool liner is punctured, water exits through the puncture; a human intervention is required to repair the opening. When the biomat is punctured, however, it can seal itself. Incoming septic tank organic matter or anaerobic bacterial waste will eventually fill in any void or crack in the biomat and return it to an impermeable state.

The analogy is complete; the biomat functions very much like a swimming pool liner.

Bio-mat growth seals gravel bed in drainfield.

In appearance, however, the two couldn’t be further apart. Whereas the swimming pool liner conjures up relaxing images, the biomat is a massive, active, thick and slimy living organism that puts a death grip on a drainfield, sewage water on the lawn and an odor in the air.


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