This link has a good illustration of how the typical conventional septic system works.
So, how does a septic system work? Septic Systems
A pipe collects all the home’s wastewater and transfers it to an underground, watertight septic tank. Here, solids (known as “sludge”) settle to the bottom, and floatable materials (known as “scum”) float to the top; both are contained by the tank and are periodically pumped out by a professional.
The middle layer contains liquid wastewater (known as “effluent”) that exits the tank into a buried drainfield in the yard, where the wastewater disperses into the soil. The soil filters out contaminants and beneficial bacteria break down any organic materials.
Specifically, this is how a typical septic system works:
- All water runs out of your house from one main drainage pipe into a septic tank.
- The septic tank is a buried, water-tight container usually made of concrete, fiberglass, or polyethylene. Its job is to hold the wastewater long enough to allow solids to settle down to the bottom forming sludge, while the oil and grease floats to the top as scum.
Compartments and a T-shaped outlet prevent the sludge and scum from leaving the tank and traveling into the drainfield area.
- The liquid wastewater (effluent) then exits the tank into the drainfield.
- The drain field is a shallow, covered, excavation made in unsaturated soil. Pretreated wastewater is discharged through piping onto porous surfaces that allow wastewater to filter though the soil. The soil accepts, treats, and disperses… wastewater as it percolates through the soil, ultimately discharging to groundwater.
If the drainfield is overloaded with too much liquid, it will flood, causing sewage to flow to the ground surface or create backups in toilets and sinks.
- Finally, the wastewater percolates into the soil, naturally removing harmful coliform bacteria, viruses and nutrients. Coliform bacteria is a group of bacteria predominantly inhabiting the intestines of humans or other warm-blooded animals. It is an indicator of human fecal contamination.
EPA – Septic System Guidelines Septic Systems (Onsite/Decentralized Systems)
Regarding the inherent risk of any septic system on the same property as drinking water, here is a study from 2010 discussing life cycle of common flu viruses and other contaminants found in most septic systems. Over time migration of waste effluent through soil particles eventually attenuates risk of affecting on site drinking water. This highlights the need to maintain proper safe separation distances between septic and wells for conventional septic systems.
Onsite Sewage Treatment and Disposal Systems: Viruses1– Mary Lusk, Gurpal S. Toor, and Thomas Obreza2
Massachusetts Title 5 – Septic System Regulations and Information on Innovative Technologies approved for use in the state of Massachusetts.