(From Michigan State University Extension Water Quality Bulletin, WQ-39 (formerly E-1521), February 1993)
A septic
system consists of two basic parts: a septic tank and a soil absorption
field or drainfield. Wastes flow from the house into the septic tank where
most solids are separated to the bottom and are partially decomposed by
bacteria to form sludge. Some solids float and form a scum mat on top of
the water.
The liquid effluent from the septic tank, carrying disease-causing organisms and liquid waste products, is discharged into the soil absorption field.
In the absorption field, the water is further purified by filtration and decomposition by microorganisms in the soil. The semi-purified wastewater then percolates to the groundwater system.
The typical septic tank is a large, buried, rectangular, or cylindrical container made of concrete, fiberglass, or polyethylene. Wastewater from the toilet, bath, kitchen and laundry flow into the tank. Heavy solids settle to the bottom where bacterial action partially decomposes them to digested sludge and gases. Most of the lighter solids, such as fats and grease, rise to the top and form a scum layer.
Septic tanks may have one or two compartments. Two compartment tanks do a better job of settling solids and may be required for new systems. Tees and baffles are provided at the tank's inlet and outlet pipes. The inlet tee slows the incoming wastes and reduces disturbance of the settled sludge. The outlet tee keeps the solids or scum in the tank. All tanks should have accessible covers for checking the condition of the baffles and for pumping both compartments. If risers extend from the tank to or above the ground surface, they should be secure to prevent accidental entry into the tank.
Solids that are not decomposed remain in the septic tank. If not removed by periodic pumping, solids will accumulate until they eventually overflow into the drainfield, leading to costly reparis or replacement. Remember that retention time, or the time available for solids to settle out of wastewater, decreases as the sludge layer increases in your septic tank.
Septic tanks must be pumped once every three to five year, depending on the tank size, number of persons in the household, and whether or not occupants are minimizing release of unnecessary solids into the wastewater. Harmful materials include:
fats
solvents
oils
disinfectants
paints
chemicals
pesticides
poisons
coffee grounds
paper towels
disposable diapers, sanitary
napkins and tampons
The septic tank requires pumping if:
the top of the sludge
deposit is within 12 inches of the outlet baffle;
the bottom of the floating
scum mat is within three inches of the bottom of the outlet baffle;
the top of the floating
scum mat is within one inch of the top of the outlet baffle or;
the floating scum mat is
more than 10-12 inches thick.
The drainfield receives septic tank effluent. It has a network of perforated pipes laid in gravel-filled trenches, or beds in the soil. Wastewater trickles out of the pipes, through the gravel layer, and into the soil. The size and type of drainfield depends upon estimated daily wastewater flow and soil conditions.
The soil below the drainfield provides the final treatment and disposal of the septic tank effluent. After the effluent has passed into the soil, most of it percolates downward and outward, eventually entering the groundwater. A small percentage is taken up by plants through their roots, or evaporates from the soil.
The soil filters effluent as it passes through the pore spaces. Physical and biological processes treat the effluent before it reaches groundwater, or a restrictive layers, such as hardpan, bedrock or clay soils. These processes work best where the soil is somewhat dry, permeable, and contains plenty of oxygen for several feet below the drainfield.
