A septic tank is an enclosed watertight container consisting of one or two compartments. It collects wastewater and provides primary treatment by allowing solids to separate from the water. At the same time solids are settling to the bottom of the tank

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A septic tank is an enclosed watertight container
consisting of one or two compartments. It
collects wastewater and provides primary
treatment by allowing solids to separate from the
water. At the same time solids are settling to
the bottom of the tank (sludge), fats, oils and
greases rise to the top (scum). Water should
remain in the tank for at least 24 to 36 hours to
provide time for solids to settle. Solids are
partially decomposed in the tank. Scum and
sludge are retained in the tank and must be
removed periodically through maintenance. Much of
the organic matter, solids, nutrients, and
pathogens remain in the effluent as it leaves the
septic tank.