How your typical septic tank drainfield system works

Your septic system consists of a buried tank and drainfield or soil absorption system. The tank is usually concrete or fiberglass plastic and can be a single or double (multi) compartment tank. The drainfield is made up of perforated pipe and gravel or chambers that channel the liquid discharge (effluent) from the septic tank – out over a large area of the soil. The septic tank is baffled to prevent excess solids and trash from leaving the tank and entering the drainfield.

Upon using the plumbing facilities in your home wastewater enters the septic tank through a gravity flow connecting pipe. The heavier solids settle to the bottom as sludge and lighter solids float to the surface as scum. Anaerobic bacteria start to slowly decompose the wastewater. Anaerobic bacteria are bacteria that require no oxygen and are therefore slow at treating the wastewater. Because of this the septic tank only provides about 35% to 40% treatment before the effluent gravity flows to the drainfield. The drainfield is designed to treat the remainder of the wastewater in the effluent and then dispose of the treated effluent in the surrounding soil or environment.

Treating wastewater effluent in the drainfield: As the pretreated effluent enters the drainfield anaerobic bacteria form on the bottom and sides of the drainfield or soil interface and form what is commonly called biomat. It is this biomat or anaerobic bacteria that provides the rest of the effluent treatment as it passes through the anaerobic bacteria and soil interface before it safely returns to the groundwater or environment.

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