filtration also remove bacteria. This combination of factors results in the eventual
purification of the wastewater effluent.
b. Location. Septic tanks must 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 distances unless filtered
effectively. Underground pollution usually moves in the same general direction as the
normal movement of the ground water in the locality. For this reason, septic tanks
should be located downhill from wells or springs. Wastewater from disposal systems
occasionally contaminates wells having higher surface elevations; therefore, it is also
necessary to rely on horizontal as well as vertical distances for protection. Tanks
should never be closer than 50 feet to any source of water supply; greater distances are
preferred where possible. The septic tank should not be located within 5 feet of any
building. It should not be located in swampy areas or in areas subjected 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.
c. Capacity. Capacity is one of the most important considerations in septic tank
design. Liberal tank capacity is not only important from a functional standpoint, but is
also good economy. The required tank capacity is based upon the number of bedrooms
for individual residences (see Table 1-5) and upon the daily wastewater flow for
institutions (see Figure 1-13).
Figure 1-13. Septic tank capacities for wastewater flows up to 14,500 gallons per day.
d. Design. Septic tanks may be of many designs. They may be either
rectangular or cylindrical. They must be watertight and should be constructed of
materials such as concrete, coated metal or vitrified clay, which are not subject to