Septic Tank Overview

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Septic Tank Overview

What are septic tanks?

Septic system

Your septic tank is a key component of your water treatment and recycling system. It serves as a settling basin where solids accumulate and gradually get broken down by bacterial action.

Some of the organic waste is actually liquefied by this "natural bacterial decomposition"; however the rest of the waste accumulates in the bottom as a layer of sludge. Additionally, a small percentage of this waste (mostly fats and oils) may float to the top of the tank to form a layer of semi-solid scum.

How Do Septic Tanks Work?

The population living in metropolitan areas (who have never had the pleasure of maintaining a septic system, or who have never experienced waddling in their leach field, or even had the experience of pumping out their systems) simply flush their toilets and "away go troubles down the drain". However, most of these people pay a quarterly sewer bill, usually between $75 to $180, for this service.

Those of us living in more rural areas have been forced to learn about the maintenance and working of the sewerage treatment facility attached to our home, "the septic system". Usually a septic tank is connected to a drainage field or seepage pit of some kind. If properly maintained, a well-designed system can last almost indefinitely. However, if it is neglected for too long a time, it can back up and clog the drainage field. This neglect can result in an expensive excavation and even a replacement of the drainpipes that could cost thousands of dollars.

Design of the Septic Tank

Septic tank cross section

Although designs vary, most septic tanks consist of a watertight, below ground tank that has one or two manhole covers (buried a few inches below the ground) to provide access for pumping, cleaning and inspection. Effluent from the house flows into the tank through an inlet pipe near the top on one side. It flows out through a discharge or overflow pipe at the other side. The pipe may end in a large tee-fitting or into a baffle (wall) preventing the effluent or scum from flowing straight across from one pipe to the other.

The incoming effluent will be diverted downward with a minimum of splashing, allowing the solids to sink to the bottom.

Outgoing effluent is drawn from the several feet below the top layer of the floating waste (grease, oil, scum) so that only liquid waste or solids that have been liquefied by the BACTERIAL ACTION going on at the bottom of the septic tank are discharged out into the drainage field. In the drainage field, further decomposition of the soluble organics will occur releasing the basic building blocks back to the environment.

Many new tanks installed today have two compartments built in to the unit side-by-side. Having the tank divided into discrete units is done for several reasons. One is to minimize the potential for solids carryover to the drainage field. The majority of the solids are allowed to accumulate and digest in the first section of the tank. The potential for solids to accumulate in the bottom of the second compartment and to be carried over to the leach field is significantly reduced.

In some newer aerated systems, which accelerate the digestion of solids and breakdown of organics (BOD) from the sewage through introduction of air to keep the first chamber aerobic, the second compartment can be used as a settling chamber and also to oxidize nitrogenous components of the sewage once the organics are removed.