A septic tank is a single or dual chamber tank that receives the raw wastewater from the home. Until recently, septic tanks were most often single chamber tanks. Septic tanks are designed to facilitate the removal of particles heavier than water by encouraging these heavy particles to settle to the tank floor, thus creating the sludge layer. Septic tanks are also designed to retain particles lighter than water by encouraging these lighter particles to float to the surface and be retained in the tank creating a scum layer. In addition, during the approximate two to three days wastewater resides in the septic tank, the biodegradable organics in the septic tank are expected to decompose, in the absence of oxygen, into less complex organic compounds.

This decomposition process is slow and largely ineffective because septic tanks are as cold as the soil around them and the anaerobic bacteria need higher temperatures to effectively decompose the organic material in the wastewater, thus reducing the biological oxygen demand (BOD) of the wastewater. Finally the anaerobically treated wastewater leaves the septic tank and is piped to additional treatment units or distributed to the soil absorption area. Retaining the heavy (settle-able) and lighter (float-able) solids slowly fills the septic tank with solids from the bottom up and top down. When the septic tank no longer has room to store these captured particles, the particles begin to escape from the tank with the exiting wastewater and will begin to clog the soil absorption area.

In septic tanks that have been fitted with an exit filter, this filter will capture and reduce the flow of solids to the absorption area. The filter will help protect the absorption area, but it will increase the volume of solids captured and stored in the septic tank. Thus it is important that every septic tank be pumped periodically to remove these captured, partially decomposed organic particles. The two-chamber tank provides enhanced removal of solids by holding the wastewater in each of the two tank chambers. A small percentage 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.

As the wastewater disposal system is used, sludge and scum continue to accumulate in the septic tank. Properly sized septic tanks (see image below) are large enough to safely store up to about three years of sludge and scum. As the tank volume filled with sludge and scum increases, wastewater is retained in the tank for less time and the solids removal process becomes less effective. If too much sludge accumulates, large amounts of the waste-water’s solids will flow to the soil absorption field causing system failure (especially from older tanks that do not have exit filters. To prevent this, the tank must be pumped periodically. The material pumped is known as septage.