Absorption

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Absorption, or more generally "sorption," is the process by which one material (the sorbent) takes up and retains another (the sorbate) to form a homogenous concentration at equilibrium.

The general term is "sorption," which is defined as adhesion of gas molecules, dissolved substances, or liquids to the surface of solids with which they are in contact. In soils, three types of mechanisms, often working together, constitute sorption. They can be grouped into physical sorption, chemiosorption, and penetration into the solid mineral phase. Physical sorption (also known as adsorption) involves the attachment of the sorbent and sorbate through weak atomic and molecular forces. Chemiosorption involves chemical bonds similar to holding atoms in a molecule. Electrostatic forces operate to bond minerals via ion exchange, such as the replacement of sodium, magnesium, potassium, and aluminum cations (+) as exchangeable bases with acid (-) soils. While cation (positive ion) exchange is the dominant exchange process occurring in soils, some soils have the ability to retain anions (negative ions) such as nitrates, chlorine and, to a larger extent, oxides of sulfur.

In on-site wastewater treatment, the soil absorption field is the land area where the wastewater from the septic tank is spread into the soil. One of the most common types of soil absorption field has porous plastic pipe extending away from the distribution box in a series of two or more parallel trenches, usually 1.5–2 ft (30.5–61 cm) wide. In conventional, below-ground systems, the trenches are 1.5–2 ft deep. Some absorption fields must be placed at a shallower depth than this to compensate for some limiting soil condition, such as a hardpan or high water table. In some cases they may even be placed partially or entirely in fill material that has been brought to the lot from elsewhere.

The porous pipe that carries wastewater from the distribution box into the absorption field is surrounded by gravel that fills the trench to within a foot or so of the ground surface. The gravel is covered by fabric material or building paper to prevent plugging. Another type of drainfield consists of pipes that extend away from the distribution box, not in trenches but in a single, gravel-filled bed that has several such porous pipes in it. As with trenches, the gravel in a bed is covered by fabric or other porous material.

Usually the wastewater flows gradually downward into the gravel-filled trenches or bed. In some instances, such as when the septic tank is lower than the drainfield, the wastewater must be pumped into the drainfield. Whether gravity flow or pumping is used, wastewater must be evenly distributed throughout the drainfield. It is important to ensure that the drainfield is installed with care to keep the porous pipe level, or at a very gradual downward slope away from the distribution box or pump chamber, according to specifications stipulated by public health officials. Soil beneath the gravel-filled trenches or bed must be permeable so that wastewater and air can move through it and come in contact with each other. Good aeration is necessary to ensure that the proper chemical and microbiological processes will be occurring in the soil to cleanse the percolating wastewater of contaminants. A well-aerated soil also ensures slow travel and good contact between wastewater and soil.

According to the 1990 U.S. Census, there are about 24.7 million households in the United States that use septic tank systems or cesspools (holes or pits for receiving sewage) for wastewater treatment. This figure represents roughly 24% of the total households included in the census.

According to a review of local health department information by the National Small Flows Clearinghouse, 94% of participating health departments allow or permit the use of septic tank and soil absorption systems. Those that do not allow septic systems have sewer lines available to all residents. The total volume of waste disposed of through septic systems is more than one trillion gallons (3.8 trillion l) per year, according to a study conducted by the U.S. Environmental Protection Agency's Office of Technology Assessment, and virtually all of that waste is discharged directly to the subsurface, which affects groundwater quality.

Elliott, L. F., and F. J. Stevenson, Soils for the Management of Wastes and Waste Waters. Madison, WI: Soil Science Society of America, 1977.

Fact Sheet SL-59, a series of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. February 1993.