The septic tank diagram is from the US EPA website's A Home Owner's Guide to Septic Systems. This is a very helpful 15 page brochure, which gives good detail on maintenance and system failure.
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The septic tank is buried and holds wastewater long enough to allow solids to settle out (forming sludge), and oil and grease to float to the surface (as scum). It also allows partial decomposition of the solid materials.
The depth of the tank and the T-shaped outlet pictured above prevent the sludge and scum from leaving the tank and traveling into the drainfield area. After a period of time which depends upon the size of the system, the size of the household, and the household water use and waste disposal practices, the scum and sludge layers thicken and the remaining liquid layer is reduced in size. Eventually, these layers have to be removed by pumping.
Liquid effluent which flows through the T-shaped outlet proceeds into a deep layer of unsaturated soil where soil and microorganisms growing in the soil remove pollutants before they can enter the groundwater or surface water. Untreated or under-treated wastewater can contaminate nearby wells, groundwater, and other drinking water sources with dangerous nutrients and disease-causing bacteria. Untreated or under-treated wastewater is a significant source of surface water pollution.
New Ohio Health Department rules will require 2000 gallons in two tanks for new systems in 4 bedroom homes. Existing systems in older homes may fall below this requirement, but they are grandfathered under the rules.