Biomat Layer in Septic Drainfields

Build-up of biomat over the life span of a septic system.

What is biomat in the septic drainfields (a.k.a. leachfields)?

Biomat is composed of anaerobic micro-organisms (organisms that don’t require high levels of oxygen).  These organisms anchor to the soil and gravel surrounding the drainfields.

“The biomat forms first along the trench bottom near the perforations [in the drainfield piping which delivers the effluent along the trench] where effluent is discharged, and then up along the trench walls.”

How does biomat affect a septic system?

Biomat coated soil and gravel is less permeable than fresh soil and gravel.  As shown in the photo above, the biomat increases over time and the surrounding soil and gravel are able to absorb less and less water.  Eventually the water flow rate becomes higher than the soil absorption rate because of the biomat layer. This build-up causes the effluent to “break out” to the surface in pools of water in various places along or at the end of the drainfields.  Or in some cases the effluent water pushes back into the house drains.

This can process can take anywhere from 10 to 60 years depending upon: 1) water usage – the more water used the quicker the biomat builds up;  2) soil type – the denser the soil the quicker the biomat builds up; and 3) septic maintenance – poorly timed clean-outs can cause solids to near the surface of the tank and pass into the pipes.  The average duration of fields is 20 years as pictured above.

What should be done about biomat clogs?

There are products on the market that promise to restore biomat clogged drainfields, but these products are unlikely to produce lasting results and some even run the risk of contaminating the environment with toxic or caustic chemicals.

Once drainfields are completely clogged with biomat the only definitive remedy is to replace the fields with new ones.  This can be done by any licensed septic installer in your area.  To schedule an appointment with Precision Excavating Inc. to replace septic fields click here.