Wastewater Management - Septic Tanks

University Curriculum Development for Decentralized Wastewater Management
Septic Tanks
Seabloom, Bounds, Loudon


"Although relatively inconspicuous, and much maligned, the septic tank is the essential
component of the small scale decentralized wastewater management option utilized by
approximately 25% of the US population. The wastewaters generated in the residence or from
small commercial and institutional activities are collected and transported by the plumbing
system drain pipes directly from the building into a unit called the septic tank. The septic tank is
a single or multi-chambered watertight vault which provides the first and very important
pretreatment in the typical small scale onsite wastewater treatment system. As the wastewater
enters the tank the velocity of flow is reduced providing relatively quiescent conditions, which
allows portions of the suspended solids to settle to the bottom, permits grease and other
floatables to rise to the surface and be retained, and provides storage space for the very complex
physical, chemical, and biological processes to occur. The septic tank is probably the single
most important treatment unit in the small scale decentralized wastewater management system
concept, and accomplishes approximately 50% of the ultimate treatment within the tank.
Without this treatment, the discharge of residential wastewater to the soil-absorption system
would most certainly lead to premature or excessive clogging of the drainfield. A typical
household septic tank system is shown in Figure 1a.
 Figure 1a. A typical household septic tank soil absorption system.
 University of Wisconsin (1975) 
Septic Tanks
Seabloom, Bounds, Loudon,
Page 2
M. Mouras of France is generally credited with developing the modern septic tank and in 1881
obtained a patent on a device he named the “Mouras Automatic Scavenger”. An excerpt from
Mouras’ overly optimistic patent description follows:
Mouras Automatic Scavenger
“A mysterious contrivance consisting of a vault hermetically closed by a hydraulic seal.
By a mysterious operation, and one which reveals an entirely novel principle, it rapidly
transforms all the excrementitious matters it receives into a homogeneous fluid, only
slightly turbid, and holding all the solid matter in suspension in the form of scarcely
visible filaments. The vault is self emptying and continuous in its workings.” (Dunbar,
The last sentence in the Mouras patent, “the vault is self emptying and continuous in its
workings,” was very misleading and may have led to many later failures. However, it probably
was not used as it is today. It may have been a storage receptacle which was emptied
periodically into a local sewer.
In 1895 Donald Cameron, of Great Britain, more correctly described the septic actions and
processes within the vault and named it the septic tank.(Crites and Tchobanoglous, 1997)
Subsequently, a variety of tank configurations have developed although the fundamental concept
remains the same, basically to provide a place for physical, chemical, and biological pretreatment of the wastewater.
The use of septic tanks for primary treatment of household wastewater first started in the
United States in the late 1880s, but surprisingly it would take another 60 years or so for
subsurface dispersal of the effluent to become common practice. (Kreissl, 2003) "