How a Septic Tank and Septic System Works

How a Septic System Works How a Septic System Works

The septic system is a sewage treatment and disposal system. A basic system consists of a septic tank and drainage area. All flows from the house are directed by way of a main sewer line to the septic tank. 40% of household sewage is from the toilet, 30% is from bathing, 15% is from laundry and 10% is from the kitchen.

What is a Septic Tank?

The septic tank is a watertight chamber constructed of concrete or poly material. An average size is approximately 1000 gallons to 1500 gallons in capacity. Most septic tanks have one or two compartments. Two compartment tanks, or two single compartment tanks in series, provide better settling of the solids.

Each septic tank has an inspection port over each baffle as well as a manhole access port. The manhole lid needs to be accessed for the tank to be pumped. These can be found at or below the ground surface. Typically you will find 4” diameter plastic lids at the ground surface that are the inspection ports over either of the baffles on the tank and not where the tank is to be pumped through.

The baffles of the tank are one of the most important components in the septic tank. The inlet baffle forces the wastewater from the sewer line down into the tank instead of across the surface of the tank and into the outlet pipe leading to the absorption area. The outlet baffle prevents the scum layer from moving into the soil absorption area. In a properly functioning septic tank the solids and sludge settle to the bottom and accumulate, scum (lightweight materials including paper, fats and greases) rises to the surface and the effluent (liquid) in the tank existing between those layers overflows to the absorption area.

Septic Systems Diagram #1 Septic Systems Diagram #2

The absorption area uses the ability of the stone and soil to filter and treat the remaining effluent. Examples of absorption areas are seepage beds, trenches, sand mounds or older cesspools / seepage pits. A cesspool is a block walled dirt bottom pit. Cesspools are no longer an installation choice but there are many properties that still have functioning cesspools. Odors and gasses from the septic system, that are always present, are vented through pipes on the house roof.

Septic Systems Diagram #1

For further information:
- On Lot Sewage System Owner Manual
- A Homeowner's Guide to Septic Systems - by EPA

Septic System Information