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Septic Tanks: How they work and tank sizing

Posted on July 4, 2012 by

How does a septic tank work?

The purpose of a septic tank is to separate solids and fats, oils and greases (FOG) from the wastewater, emitting only a clarified liquid. As can be seen in the image below, solids settle to the bottom of tank creating the sludge layer while FOG rises to the top creating the scum layer. This leaves a clarified liquid in the centre which is discharged into the percolation area.  A septic tank should also have tees at the inlet and outlet, the purpose of the inlet tee is to prevent the influent flow from disturbing the scum layer, dissolving it back into the clarified zone. The purpose of the outlet tee is to prevent the scum layer being discharged into the percolation area. The contents of the septic tank is anaerobic, meaning there is no oxygen present. This results in the production of gases such as methane, hence, ventilation of the tank is required to release these gases. A septic tank should be designed to allow wastewater stay in the tank long enough to allow solids and FOGs separate from the clarified liquid. If this dwell time is insufficient, un-clarified effluent will be emitted from the septic tank which can cause the percolation area to break down.

Sizing a septic tank

EPA CoP 2009 states that a septic tank should have a minimum volume of 2600 litres, this figure increases with occupancy, add 150 litres per person. However, to reduce de-sludging frequency, the tank can be larger.  If a septic tank is not de-sludged the scum and sludge layers will build-up reducing the volume of the clarified zone. This will reduce the dwell time in the tank, resulting in un-clarified effluent leaving the tank causing the percolation area to breakdown. De-sludging frequency depends on usage and tank size, it is recommended that the tank should be de-sludged once the sludge consumes one third of the tank.


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