septic tank separates and processes wastes. From the waste water that flows into the tank, heavy solids settle to the bottom, forming a layer of sludge. Greases, oils, and lighter solids rise to the top, creating a layer of scum. The area between these two layers is filled with liquid effluent that can flow through the outlet pipe to the drainfield system. Inside the tank, anaerobic and facultative micro-organisms feed on the solids in the sludge and scum, breaking down their volume. This process creates gases--carbon dioxide, hydrogen sulfide, and others--that exit through the vent stack at the roof.
Because only a portion of the sludge and scum are broken down (about 40 percent), a septic tank must be pumped periodically. Otherwise, accumulated solids fill the tank and are forced out into the drainfield, where they hinder the soil's ability to percolate.
A tank is usually sized according to the number of bedrooms in a house and will typically store solids for three to 12 years. The tank should be watertight--built so that groundwater cannnot leak into the tank and septage cannot leak out. If there is water intrusion coming into the tank from groundwater, it will raise the dissolved oxygen level in the tank, which will inhibit the biological treatment and lead to early failure of the drainfield.
