Category: Pipe and Gravel System

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.

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.



Pipe and Gravel vs. Chamber (Infiltrator) Systems

The pipe and gravel field system has been around much longer than the plastic chamber systems and have been proven to work more efficiently.

Chambers can, theoretically, store a higher capacity of water.  The chambers themselves can hold 3 or 4 times more water than the equivalent length of 4” pipe.  However, the trench size for both products is the same – the pipe or chamber is laid in a 2’ wide trench and then covered with gravel.  The trench for the chamber is completed with 1/3 the amount of gravel that the pipe system receives OR the pipe system has 3 times the amount of gravel of the chamber system.  The gravel is the “filter” for the septic water that leaves the tank.  More gravel equates to cleaner water being both absorbed into the soil and evaporated/transpired into the air.

Septic systems work best when they disperse the water over the largest area possible which is the fundamental operating principal of the pipe and gravel system.  Chamber systems can store more water in the chambers, but this does not increase the absorption rate of the soil and, therefore, is no added benefit in most septic applications.  Fields are not meant to store water, but to disperse water over the largest area available.

The best use for chambers is in small situations with excellent soil percolation rates.  In clay or heavy soils, the water tends to sit in the chamber longer and this retention causes the growth of scum which can cause the system to fail sooner than a pipe and gravel system.