What is a septic tank?
A septic tank is an underground, watertight tank in which domestic wastewater is pre-treated. It is used for homes that are not connected to the public sewer, for example in rural areas.
How does a septic tank work?
- Settling: the wastewater flows into the tank and comes to rest. Heavy particles sink to the bottom and form sludge. Fats and light particles float up and form a scum layer.
- Digestion: in the sludge, bacteria break down part of the organic material without oxygen.
- Compartments: the tank is usually divided into several compartments. The water flows from one to the next, so that fewer and fewer solids are carried along.
- Outflow: the pre-treated water flows to a soil infiltration system, a further treatment stage or surface water, depending on what is permitted.
A septic tank only partially treats the water: the solids are largely retained, but dissolved pollutants and pathogens remain in the water to a large extent.
Application
Rules in the Netherlands: in the Netherlands, a septic tank is a form of IBA (Individuele Behandeling van Afvalwater, individual wastewater treatment). The simplest variant, IBA class I, is a septic tank with a capacity of at least 6 m³, divided into three compartments. Whether that is sufficient, or whether a heavier IBA class with further treatment is required, depends on where you discharge and on the rules of the municipality and the water authority. So always check beforehand; within a certain distance of the sewer, connecting to it is usually compulsory. Other countries have their own rules; check with your local authority or environmental regulator.
Maintenance:
- Have it emptied: sludge and scum must be pumped out regularly by an approved company. If you do not, sludge washes through to the outlet and an infiltration field becomes clogged.
- Inspect: check the sludge and scum levels and the inlet and outlet for blockages.
- Ventilation: a septic tank must be vented, usually via the soil stack of the house; a blocked vent causes odours.
Common mistakes:
- Flushing fats, frying oil, wipes and chemicals down the sink or toilet: these disrupt the bacterial process or block the tank.
- Connecting rainwater to the septic tank: it then becomes overloaded and sludge washes through. Drain rainwater separately.
- Simply leaving an old cesspit or septic tank in the ground: a tank that is no longer used must be emptied and demolished or filled, so that it cannot collapse.
Related Terms
- IBA (individual wastewater treatment)
- Cesspit
- Wastewater
- Sewer
- Infiltration
- Infiltration in your own garden
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