The typical septic tank is a large buried rectangular or cylindrical container made of concrete, fiberglass, or polyethylene. A septic tank's purpose is to separate solids
from the wastewater, store and partially decompose as much solid material as possible, while allowing the liquid (or effluent) to go to the drainfield.
Wastewater from your toilet, bath, kitchen, and laundry flows into the tank and remains there for up to 24 hours (known as the retention time) before it passes to the
drainfield. This helps prevent clogging of the drainfield, which can lead to failure and costly repairs.
Give It Time to Sink and Float
The retention time is necessary to allow the solids to properly separate from the liquids—heavy solids settle to
the bottom as sludge and the lighter particles rise to the top, forming a scum layer. Although bacterial action
partially decomposes some of the solids, up to 50 percent remain in the tank.
Until the mid-1970's, septic tanks had one compartment; however, current regulations require two chambers, which do
a better job of settling solids. In residential systems, the tank size is determined by the number of bedrooms and should
be enough to handle approximately three years worth of sludge and scum. In commercial establishments, the tank size is determined by the amount of daily flow.
As wastewater flows into the tank, a tee (or baffle) at the tank's inlet pipe slows the incoming wastes and reduces
the disturbance of the settled sludge. The outlet tee keeps the solids or scum in the tank. In tanks installed since
1995, an effluent filter is attached to the outlet baffle (going to the drainfield) to keep solids in the tank instead
of entering the drainfield. Effluent filters are an excellent addition to an older tank, and can be installed by a pumper or other septic system professional.
As the volume of sludge and scum builds up, there is less space and time for the solids to separate before the
wastewater leaves the tank, which causes the system to be less effective. With not enough time for solids to settle,
they can pass into the drainfield with the wastewater (or clog the effluent filter, if there is one). This causes the
drainfield to gradually plug and eventually fail, causing sewage to back up into the house or effluent to surface
outside. Also, the closer the thickening scum and sludge layers come to the outlet tees, the greater the risk that they can plug the tank inlet or pass into the drainfield.
Consequently, it is important that solids be removed by periodic pumping, so they do not overflow into the
drainfield. Most septic tanks need to be pumped every 3 to 5 years, depending on the tank size and the amount and type of solids entering the tank.
All tanks should have accessible covers for checking the condition of the tees and for pumping both compartments. To
eliminate the time and nuisance of digging down to the access covers, risers can be installed. The riser(s) should
be secure to prevent accidental entry into the tank, and should also be watertight to prevent groundwater from entering the riser cavity, which may cause the tank to flood.