How A Septic Tank Works

Septic tanks are split into two chambers. The chambers are divided by a baffle wall. Solids are retained in the first chamber, and liquids then pass through the baffle into the second chamber and on to the percolation area.

Septic Tank Internal

The T-pieces shown in the drawing are essential for a properly working tank. At the inlet side a T-piece allows new wastewater into the tank without disrupting the crust which has formed in the top layer of the tank. A crust is a sign of a healthy septic tank and the crust, combined with the T-piece help keep smells down. T-pieces should be fitted with a blanking cap to prevent smells travelling through to your vent pipe. On the outlet side a T-piece blocks floating solids passing through to your percolation area. This is essential to maintain a long life for your percolation area.

Septic Tanks are designed to take all the wastewater from inside your house, including sinks, showers, washing machines and dishwashers. All modern septic tanks are sized and designed to deal with this flow, and as long as detergents, cleaners, etc. are used in normal quantities they won’t cause any issues.

The effluent from a septic tank flows to a distribution box which splits the flow from the septic tank into percolation pipes (You can read more about distribution boxes here). The effluent will then flow down through the stone underneath the percolation pipes and through the soil underneath the stone. As the effluent passes through the stone and soil it is naturally treated before reaching any groundwater. Septic tank effluent will eventually reach the groundwater, so it is important to have a properly constructed percolation area to ensure no pollution occurs. The EPA (Environmental Protection Agency) produces guidelines about how to construct a percolation area, which your architect or engineer can advise you about.