Septic Tank

What is a septic tank?

Modern Septic Tank with access riser

A septic tank is a buried, watertight container usually made of concrete. It holds the wastewater long enough for bacteria digestion and settling to occur. In a properly working tank, the contents of  the tank will separate into three distinct layers.

1.  Sludge on the bottom of the tank
2.  A liquid effluent layer in the center part of the tank
3.  A scum layer, made of oils and fats at the very top of the tank (Yum!)

To keep the tank from getting agitated (or stirred up),  the inlet flow needs to be baffled.  This is usually accomplished by a concrete baffle (as part of the tank) or a PVC tee fitting on the inlet pipe.

The outlet pipe is on the other end of the tank.  The outlet pipe is equipped with a tee fitting that extends down into the liquid (middle) portion of the tank.  This allows only the liquid effluent to leave the tank and enter the absorption field.

Effluent filters are often installed on the outlet side of the tank to help keep any residual suspended “Stuff” from leaving the tank.
A “dead tank” or blackwater tank describes an unhealthy septic tank where there are NOT distinct layers inside the tank.  This can be caused by medications, softener discharge or hydraulic overloading of your septic system (too much water).

If you have a “Dead tank”, your effluent filter will clog up immediately, making you aware of the problem
before all those suspended solids make there way to your absorption field and do damage!

Effluent filters are inexpensive and are always a good idea.

Concrete Inlet Baffle

Concrete Inlet Baffle

Existing “septic tanks” are often inadequate and made out of bricks, metal or even wood.  Many of these old septic tanks are grossly undersized and structurally unsound (dangerous).

Below is an example of  crock style septic tanks.   These tanks are set in series.   These are only 24″ in diameter.  I found them at an old two bedroom farm house.


Crock Septic Tank(s). Note baffles on inlets and outlets

Close-up of Crock septic tank construction

Close-up of Crock septic tank construction

See related… 
How to find your septic tank
and Septic Care pages

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