The septic tank effluent is allowed to flow to the drainfield by gravity or is dosed by pump or siphon. The effluent enters the soil and is treated as it percolates to the groundwater.
The soil acts as biological filter to remove nearly all harmful substances including disease-causing bacteria and viruses, toxic organics and other undesirable wastewater constituents remaining in the septic tank effluent.
In addition to acting as a sedimentation chamber and providing storage for the sludge and scum, the septic tank also digests or breaks down the waste solids. Micro-organisms that thrive without oxygen feed on the solids to reduce the volume of sludge and scum.
In the process, carbon dioxide, hydrogen sulfide and other gases are produced which must be vented from the tank through the plumbing vent on the roof.
Only about 40% of the sludge and scum volume can be reduced in this manner, so the tank must be pumped regularly to remove the accumulated solids. If not done, the tank will fill with sludge and the solids will be washed out into the drainfield where they will quickly clog the soil.
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