3.1. Selection of the septic tank. Assuming that there is sufficient area to accommodate a leaching system, and that construction of the system will be approved by local authority, the next step will be selection of a septic tank. Some knowledge of the functions of a septic tank will help in making the proper selection.

3.2. Functions of septic tanks. Untreated liquid household wastes (sewage) will quickly clog all but the most porous gravel formations. The tank conditions sewage so that it may be more readily percolated into the subsurface formation. Therefore, the most important function of a septic tank is to provide protection for the absorptive ability of the subsoil. Three functions take place within the tank to provide this protection.

3.3. Removal of solids. Clogging of soil with tank effluent varies directly with the amount of suspended solids in the liquid. As sewage from a building sewer enters a septic tank, its rate of flow is reduced so that larger solids sink to the bottom or rise to the surface. These solids are retained in the tank and the clarified effluent is discharged.

3.4. Biological treatment. Solids and liquids in the tank are subjected to decomposition by bacterial and natural processes. Bacteria present are of a variety called anaerobic which thrive in the absence of free oxygen. This decomposition or treatment of sewage under anaerobic conditions is termed “septic”, hence the name, septic tank. Sewage which has been subjected to such treatment causes less clogging than untreated sewage due to a reduction of suspended solids.

3.5. Sludge and scum storage. Sludge is an accumulation of solids at the bottom of the tank, while scum is a partially submerged mat of floating solids that may form at the surface of the liquid in the tank. Sludge, and scum to a lesser degree, will be digested and reduced into a smaller volume. However, no matter how efficient the process is, an accumulation of inert solid material will remain in the tank. Space must therefore be provided in the tank to store this material during the interval between cleanings; otherwise, sludge and scum will eventually be scoured from the tank and clog the disposal field.

If properly designed, constructed, maintained and operated, septic tanks are very effective in accomplishing their purpose.

The relative position of a septic tank in a typical subsurface disposal system is illustrated in Figure 3.

The heavier sewage solids settle to the bottom of the tank, forming a blanket of sludge. The lighter solids, including fats and greases, rise to the surface and form a layer of scum. A considerable portion of the sludge and scum are liquefied through decomposition or digestion. During this process, gas is liberated from the sludge, carrying a portion of the solids to the surface, where they accumulate with the scum. Ordinarily, they undergo further digestion in the scum layer, and a portion settles again to the sludge blanket. Furthermore, there are relatively wider fluctuations of flow in small tanks than in the large units. This effect has been recognized in TABLE 4. which shows the recommended minimum liquid capacities of septic tanks.

3.6. Location. Septic tanks should be located where they cannot cause contamination of any well, spring, or other source of water supply. Underground contamination may travel in any direction and for considerable distance, unless filtered effectively. Underground pollution usually moves in the same general direction as the normal movement of the groundwater in the locality. Groundwater usually moves in the direction of the slope or gradient of the water table (i.e., from the area of high water table to area of lower water table). In general, the water table follows the general contour of the ground surface. For this reason, septic tanks should be located downhill from wells or springs. Sewage from disposal systems occasionally contaminates wells having high surface elevations, since the elevations of disposal systems are almost always higher than the level of water in wells located nearby; therefore pollution from a disposal system on a lower surface elevation may still travel downward to the water-bearing stratum. It is necessary, therefore, to rely upon horizontal as well as vertical distances for protection.

The septic tank or effluent line shall be located 100 feet or more from any source of private water supply.

The septic tank shall be located 10 feet or more from any building. The tank should not be located in poorly drained areas, nor in any areas subject to flooding. In general, the tank should be located where the largest possible area will be available for the disposal field. Consideration should also be given to the location from the standpoint of cleaning and maintenance. Septic tanks shall be so located that septic tank cleaning equipment trucks can be driven within reasonable vertical and horizontal distance of the tank for the purpose of tank cleaning. Where public sewers may be installed at a future date, provision should be made in the household plumbing system for connection to such sewer.

The location of all septic tanks shall meet the following minimum horizontal distance requirements as listed in TABLE 3.

Contrary to popular belief, septic tanks do not accomplish a high degree of bacteria removal. Although the sewage undergoes treatment in passing through the tank, this does not remove all dangerous bacteria, therefore, septic tank effluent cannot be considered safe. The liquid that is discharged from a tank also may cause an odor nuisance unless properly treated. As previously explained, its primary purpose is to condition the sewage so that it will cause less clogging of the disposal field.

3.8. Approved Pipe Material. Material used in the construction of the sewage disposal system shall meet the following requirements:

Any pipe under driveways, roadways, parking areas, any area where traffic will pass over, or instances where less than six (6) inches of cover is used, shall be constructed of Schedule 40 PVC, cast iron, or use a steel sleeve.