How A Septic System Works

The septic system is a natural method of treatment and disposal of household wastes for homeowners who live in an area where there is no municipal sewage disposal system. It is estimated that there are about 30 million residences in the U.S. that fall into this category. If your home has a septic system it important for you to know something about how it works. This often-overlooked part of your home can be very expensive to repair or replace. A little knowledge has the potential to save homeowners thousands of dollars.

A Brief Overview
Although individual designs vary, a typical gravity fed septic system is composed of three parts:

1. Septic tank -- 2. Distribution box -- 3. Drain field or leachfield

Wastewater and solids exit the home through toilets and drains and end up in the septic tank. The function of the septic tank is to separate solid material from the wastewater. The solids are broken down by naturally occurring bacteria that inhabit the tank.

Septic tanks may contain one or two chambers. Scum is the material, which floats on the top of the water in the tank where aerobic (air breathing) bacteria digest it. Sludge is undigested solid matter, which settles to the bottom of the tank. Here anaerobic (without air) bacteria digest some of the matter, the undigested part must be removed periodically by a licensed septic system pumping service every 2 - 5 years depending on use. In a septic tank that is not pumped on a regular basis, undigested solids accumulating in the tank may make their way to the drainage bed where they will clog soil passages.

Effluent is the clarified partially treated liquid portion of household waste, which makes its way from the tank to the distribution box where it is then distributed to the various lateral lines in the drain field.

A distribution box (D Box) can be made of concrete or plastic.

The drain field, also known as the leachfield, absorption bed, disposal field or seepage field performs the task of accepting and processing wastewater, or effluent, from the septic tank before it returns to natural aquifers below the earth's surface.

A typical drainfield consists of several gravel-filled trenches with perforated drainpipe buried just below the surface of the gravel running the length of the trenches. The effluent is distributed through a distribution box to the various lateral drainpipes exiting through the perforations into the surrounding gravel bed. Secondary processing begins in these beds as the effluent fills the trench then seeps through the biomat and into the surrounding soil.

The biomat is a naturally occurring tar-like substance that forms on the bottoms and sides of the drainfield trenches. It is made up of living anaerobic (without oxygen) organisms, which feed on organic matter in the wastewater. As the biomat matures it grows thicker slowing down the flow of wastewater to the surrounding soil. This is part of the natural process, which clarifies the effluent. As the wastewater passes through the biomat pathogenic organisms and viruses are removed. On the outside of the trench, beyond the biomat where the soil is not saturated, are living colonies of aerobic (with oxygen) bacteria. These aerobic bacteria colonies feed on the biomat and keep it from becoming so thick that wastewater will no longer pass through.

When soil floods these aerobic bacteria colonies will die off and no longer keep the biomat in check. The biomat will grow too thick and drainage will stop.