It holds the wastewater long enough to allow solids to settle to the bottom of the tank forming what is called “sludge” and oil and grease to float to the top of the tank to form a semi-solid layer called “crust” or “scum”. Between these two layers is a clear zone of liquid. Found in all three of these layers are billions of bacteria that live naturally in the tank and perform the first phase of treatment to break down solid matter. They digest the solid materials. In the process, gases are produced, which are vented from the septic tank through the plumbing vent on your rooftop. Compartments and a T-shaped outlet in the septic tank prevent the sludge and crust from leaving the tank and traveling into the drain field area.

The primary treatment of waste starts in the septic tank, where bacteria digest organic materials in the wastewater. The wastewater exits the septic tank and flows into the drain field or leach field where a secondary treatment occurs. A drain field generally consists of a network of perforated pipes laid in a gravel-lined trench. A drain field typically has two or more trenches.

Here, bacteria complete the digestion and purification process as the wastewater slowly leaches or infiltrates into the soil. Microbes in the soil digest or remove most contaminants from wastewater before it eventually reaches groundwater. If the drain field is overloaded with too much liquid, it will flood, causing sewage to flow to the ground surface or create backups in plumbing fixtures preventing the secondary treatment of all wastewater. A reserve drain field, required by many states, is an area on your property suitable for a new drain field system if your current drain field fails. Treat this area with the same care as your septic system. Soils that are porous or sandy are best for an optimally functioning drain field.