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Septic System Mechanics

The septic tank was patented in London around 1900. Webster's dictionary defines the septic tank as "a tank in which waste matter is decomposed through bacterial action." The modern septic tank is a watertight box usually made of precast concrete, concrete blocks, or reinforced fiberglass. When household waste enters the septic tank several things happen:

  1. Organic solid material floats to the surface and forms a layer of what is commonly called "scum.” Bacteria in the septic tank biologically convert this material to liquid.

  2. Inorganic or inert solid materials and the by-products of bacterial digestion sink to the bottom of the tank and form a layer commonly known as "sludge.”

  3. Only clear water should exist between the scum and sludge layers. It is this clear water-and only this clear water-that should overflow into your drainfield.


A drainfield is typically a rectangular underground area comprised of porous piping laying in a bed of stone covered by soil. The discharge pipe from the septic tank feeds into a header pipe with multiple porous pipes branching out perpendicular to the header. (Think of tines on a fork.) Drainfield site, size and construction are all important. Suitable soil for a drainfield easily absorbs water, i.e. clay is poor whereas sand is best. The soil bed of the drainfield should not be compacted, constructed with a bulldozer. The size of the drainfield should be proportional to number of bedrooms, number of facilities, and anticipated water flows.


Avoid solid material overflowing into your drainfield. It is this solids overflow that clogs soil pores and causes systems to fail. Two main factors cause solid material to build up enough to overflow:

  1. lack of sludge removal - the failure to have tanks vacuumed cleaned regularly and

  2. bacterial deficiency.


Bacteria MUST BE PRESENT in the septic tank to break down and digest the organic solids. Normal household waste provides enough bacteria to digest the solid UNLESS any harm is done to the bacteria. Bacteria are very sensitive to environmental changes. Check the labels of products you normally use in the home. Products carrying harsh warnings such as "HARMFUL OR FATAL IF SWALLOWED" will harm bacteria. The following commonly-used home care products - even when used according to directions - can reduce the bacteria population required for proper septic tank operations:

  • detergents

  • bleaches

  • cleaning compounds

  • disinfectants

  • acids

  • sink & tub cleaners

  • toilet cleaners

  • polishes

  • caustic drain openers


Bacteria must be present to digest the scum. If not digested, the scum will accumulate until it presses down so scum flows through the baffle into the drainfield, clogging the drainfield.


The sludge in the septic tank - inorganic and inert material - is not biodegradable and will not decompose. If not removed, the sludge will accumulate until it eventually overflows, clogging your drainfield.

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