MIL-HDBK-1138
System Operation Principles. Wastewater flows out
3.1.2
of the building through a pipe into the septic tank. In the
tank, bacteria attack and digest organic matter by anaerobic
digestion. The wastewater itself provides the bacteria for this
process. The anaerobic digestion process changes the waste into
gas, biosolids (residual organic and inorganic material), and
treated effluent. The gas escapes into the air, the treated
effluent is discharged to the leaching system, and the residual
solids remain in the tank. The solids should be pumped out of
the tank periodically. The treated effluent is discharged into
the soil through the perforated or open-jointed pipes in the
drain field. Soil bacteria destroy remaining organic material in
the effluent.
Septic Tank System O&M. The primary O&M
3.2
requirement for the septic tank system is periodic removal of
settleable solids. In addition, take preventative care of the
system by monitoring waste disposed to the system and ensuring
that trees or shrubs are not planted over any of the system
components. It is not necessary to add yeast or bacteria to the
system as a maintenance procedure. As long as human and kitchen
wastes are being discharged to the system, there will be
sufficient bacteria in the tank for treatment.
Inspecting the Septic Tank. Check the septic tank
3.2.1
every 3 to 5 years to determine if solids need to be removed.
Check the tank once a year if garbage disposals discharge to the
tank.
Caution: Exercise extreme care when inspecting the
septic tank. Never inspect a septic tank alone or enter a tank.
Toxic gases are produced by the natural treatment processes in
septic tanks and these gases can kill in minutes. Details on
confined space entry requirements are found in the Sacramento
Series Operation of Wastewater Treatment Plants, Volume 2,
Chapter 14.
Measuring Solids and Scum Inside the Tank. The
3.2.1.1
following information comes from Pipeline: Maintaining Your
Septic System--A Guide for Homeowners, National Small Flows
Clearinghouse, 1995.
There are two frequently used methods for measuring the
solids and scum layers inside your tank. The contractor may use
a hollow clear plastic tube that is pushed through the different
layers to the bottom of the tank. When brought back up, the tube
retains a sample showing a cross-section of the inside of the
tank. The layers can also be measured using a long stick. To
measure the scum layer using a stick, a 3-inch (8 cm) piece of
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