The design of a septic system requires careful and intelligent planning.
The character of the soil is the first important factor that must be considered. The soil that is most favorable for the disposal of the purified liquid waste of the septic system is of sandy nature. Sand or gravel absorbs moisture and permits rapid leaching of the liquid waste into the subsoil.
Another favorite condition is having the contour or slope of the soil at such distance from the purification unit of the septic tank that a gravity flow in the discharge line is possible.
The soil must have an accepted percolation or absorption rate, without interference from ground water or impervious layers below the level of the absorption system.
In general, two conditions must be met:
1. The percolation or absorption time
2. The maximum elevation of the groundwater table should be at least four feet below the bottom of the trench. Unless these two conditions are satisfied, the site is unsuitable for a sub surface sewage disposal system.
The septic tank is a device used to expedite the decomposition of the elements contained in raw sewage wastes. Raw sewage consists of water and settleable solid materials, such as grit, grease, fats, vegetable and animal materials in a suspended state and non-settleable materials of a vegetable and mineral nature in solution. It has a milky color and may have an extremely offensive odor.
The settleable solids, usually referred to as organic materials, can be liquified in a septic tank in a short time. Approximately a 24-hour period of retention offers satisfactory results. The solid organic materials, being more dense than water, tend to settle to the bottom of the tank and are technically referred to as sludge. The lighter organic materials, being less dense than water, rise to the surface of the water and usually are referred to as scum. The suspended materials constitute only a small part of the raw sewage by actual weight.
The non-organic materials in solution present a more difficult problem. They cannot be liquified in the septic tank and must undergo further treatment by processes other than liquification. The liquid content of raw sewage does not present a problem other than that of discharge into the subsoil or natural drainage terminal.
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