How do septic systems work?  
Household wastewater leaves the house through the line and goes into a tank, which is usually made of concrete, fiberglass or polyethylene.  Solids will settle as sludge, and oil and grease will float to the top as scum.  The wastewater then moves out to the drain field, where it percolates into the soil. Soil filters out bacteria, viruses and nutrients.  Periodic pumping ensures that sludge and scum do not get into the drain field.