These solids either settle to the bottom of the tank, known as sludge, or float to the surface and become a scum layer. The clear liquid or effluent is then passed on to additional treatment processes or is distributed to the soil absorption area, such as a sand mound. Removing the solids from the wastewater in the septic tank protects the soil absorption area from clogging and failure. Up to 50 percent of the solids retained in the tank decompose; the remainder accumulates in the tank. Biological and chemical additives are not needed to aid or accelerate decomposition.

Properly sized septic tanks are large enough to safely store up to three years of sludge and scum. If you go beyond three years and the tank fills with sludge and scum, the clear wastewater is retained in the tank for less time because there is less space in the tank for the liquid to remain there. As this occurs, the solids removal process becomes less effective and more solids escape into the soil absorption area. If too much sludge accumulates, the solids will flow to the soil absorption field causing system failure. System failure can lead to wastewater coming to the surface of your yard causing hazardous wet areas or it could cause wastewater to back up into your home.  To prevent this, the tank must be pumped periodically. The material pumped is known as septage.

The frequency of pumping depends on several factors: capacity of septic tank, volume of wastewater (related to size of household), and amount of solids in the wastewater (for example, use of garbage disposals produce more solids).

Homeowners should get in the habit of having the septic tank pumped on a routine basis. If you are able to have your septic tank pumped on a schedule, such as every 2 years, it may be possible to further enhance the effectiveness of your entire onlot wastewater disposal system. Research at Penn State has shown that your soil absorption system will benefit from periodic resting (a period during which no wastewater is added to the system). To get the greatest benefit from pumping your septic tank, it is recommended that you have your septic tank pumped every two years on the day before you, and your family, leave for your summer vacation. This means the whole system, especially the soil absorption area, will have the opportunity to dry out and any partially decomposed organic waste that may have moved into the soil absorption area will quickly decompose in the absence of water.

A 900 gallon septic tank is the minimum size that may be used for a home with three bedrooms or less. If six people reside in a three-bedroom house, the tank should be pumped every 1.3 years. If the same system serves a family of two, the tank would need pumping every 5.2 years. Systems installed before the current rules and regulations were implemented may need to be pumped more often, perhaps every year or less.