In a typical septic system, waste leaves a house through a pipe then enters a septic tank, where solids settle in a layer at the bottom and oils and grease float to the top. The liquid then flows into a drainfield and eventually the soil, where microbes digest most contaminants before they can reach the ground water. Septic systems should be located in suitable soils. Avoid steep slopes, high water tables, and small lot sizes. Septic systems should be inspected every 2–3 years by a licensed contractor and pumped as recommended (generally every 3–5 years depending on the size of the system and number of people living in the home).
Wastewater and sewage from septic systems can contaminate drinking water sources during flood events or system breakdowns. According to the US Environmental Protection Agency, approximately 10–25% of septic systems fail at some point. If these failures release untreated wastewater and sewage into source water, risks to human health and the environment increase. When septic systems are properly designed, constructed, and maintained, most of these risks are minimized if not eliminated. Properly maintained septic systems also help to preserve property values and avoid costly repairs.