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The septic tank is a watertight structure that is the main collection point for human waste by-products. It is at this tank that the solid waste is separated from the liquid waste, and the biological digestion of the waste matter takes place. A septic tank provides anaerobic treatment - it does a good job of settling out the solids, but not such a good job of removing nutrients and breaking down organic matter. Septic tanks are sized according to the amount of liquid waste they must process - this is based on the number of bedrooms.
= All septic tanks have an opening for the waste to enter the tank and another one for the waste to exit the tank. The entrance is called the inlet. Inside the tank there will be a PVC, "T"-shaped fitting, consisting of a short section of horizontal piping leading into a slightly longer, vertical section of piping that is open on both the top and the bottom.
= All septic tanks have an opening for the waste to exit the tank. The exit is called the outlet. Inside the tank, there will be a PVC "T"-shaped fitting, consisting of a short section of horizontal piping leading into a slightly longer vertical section of piping that is open on both the top and the bottom. The top of the vertical section must extend above the level of the scum layer, and the bottom of the vertical section must extend below the bottom level of the scum layer. The outlet tee is usually several inches below the level of the inlet tee.
= Excessive discharge of solids to the drain field can cause it to plug and lose efficiency in treatment and dispersal of the normal liquid flow. If the problem persists, the drain field may need to be replaced. Septic tank effluent filters provide a relatively inexpensive means of preventing solids discharge. In new septic installation, effluent filters are required at the outlet of the septic tank, in the outlet sanitary "T", collecting solids that may be discharged from the tank. Solid accumulation in the filter will cause poor performance of the septic tank, but creates a problem that is far easier and less expensive to clean and maintain than solids accumulation in the drain field.
= This is buoyant waste made up of greases and soaps. When a septic tank is opened, this is usually the first thing that is seen floating on top. If periodic maintenance is not performed (i.e. pumping the tank), this waste can build up to the point of going above the top of the inlet and outlet tees and clogging the inlet into the tank, as well as possibly clogging the soils in the absorption area.
= The liquid effluent is made up of the remaining liquids and semi-buoyant waste particles after the sludge and scum waste have separated. A normally operating septic tank maintains a constant effluent level at the height of the bottom of the outlet tee opening. Consequently, when new waste enters the tank, the liquid effluent level rises and the effluent is forced out of the tank through the outlet into the distribution box and into the absorption area for dispersment and continued treatment. Septic tank effluent is usually cloudy and contains suspended solids and pathogens, including disease-causing bacteria and viruses. This condition requires more bacterial action for treatment than can occur in the tank alone.
= The sludge layer consists of the heavier waste solids that separate and settle to the bottom. The sludge layer is where the decomposition process continues by means of bacteriological interaction. These bacteria live and grow without the presence of air in what is called an anaerobic treatment. Although decomposition is a continual process, the breakdown is not complete, which can eventually result in waste residue build-up if not pumped out on a regular basis . This residue can build up to the bottom of the inlet or outlet tee and block flow into and/or out of the tank.
The septic tank removes solids by holding wastewater in the tank, which allows the solids to settle and scum to rise to the top. To accomplish this, wastewater should be held in the tank for at least 24 hours. Up to 50 percent of the solids retained in the tank decompose. The remaining solids accumulate in the tank. Biological and chemical additives are not needed to aid or accelerate decomposition. As the septic system is used, sludge continues to accumulate in the bottom of the septic tank. Properly designed tanks have enough space for up to three years safe accumulation of sludge. When the sludge level increases beyond this point, sewage has less time to settle properly before leaving the tank. As the sludge level increases, more solids escape into the absorption area. If sludge accumulates too long, no settling occurs before the sewage escapes directly to the soil absorption area. To prevent this, the tank must be pumped periodically. The material pumped out of the tank is known as "septage."
Capacity of septic tank.
Flow of wastewater (related to size of household).
Volume of solids in wastewater (more solids if garbage disposal is used).
The table below gives the estimated pumping frequencies according to septic tank capacity and household size. The frequencies were calculated to provide a minimum of 24 hours of wastewater retention assuming 50 percent digestion of the retained solids.
Estimate Septic Tank Pumping Frequencies in Years (For Year-Round Residence)
Note: More frequent pumping needed if garbage disposal is used.
It is important to note that the soil absorption field will not fail immediately when a full tank is not pumped. However, the septic tank is no longer protecting the soil absorption field from solids. Continued neglect will result in failure and the soil absorption field may need to be replaced. In some cases, replacement of the absorption area may not be possible due to site limitations.