Agricultural and Biosystems Engineering
Most rural homes use a septic, on-site wastewater treatment system for household waste disposal. In a properly operating septic system, wastes are flushed into a septic tank where heavy solids are trapped and sink to the bottom. Lighter solids float on top of the water and form a scum layer. Anaerobic (without oxygen) bacteria break down wastes inside the septic tank. Water soluble compounds are carried out into a drain field trench where they are absorbed into the soil. Once in the soil absorption field, aerobic (with oxygen) bacteria continue to break down the effluent.
Derickson, Russell, "Septic System Maintenance" (2002). SDSU Extension Extra Archives. 18.