A Septic Tank is a multi-chamber storage tank allowing liquid and solid waste to separate: The liquid is allowed to flow out of the tank and be disposed of separately. Firstly the sewage enters a settlement chamber, allowing solid waste (sludge) to sink and the liquid to rise to the surface. The surface liquid makes contact with oxygen and the organic matter starts to breakdown biologically. This liquid still contains sewage but in small enough particles to be carried through the discharge outlet and into the ground (soakaway).
Septic Tanks only partially treat sewage, and the treated is often low quality and smelly. Many areas of the UK and Ireland prohibit the installation of Septic Tanks and recommend a Sewage Treatment Plant instead.
Septic tanks separate solid and liquid waste by allowing the solids to settle. The liquids should then soak away through a land drain. For this reason, septic tanks are only suitable if the soil at your property allows liquid to drain away easily.
It's worth noting that many old systems discharge unsightly, foul-smelling liquid into watercourses. This shouldn't be happening and cases like this should be reported to the Environment Agency.
If you're discharging to a watercourse, or you're dealing with large volumes of waste (if you're running a pub or hotel for example), you may need permission from the Environment Agency before installing the tank.
Waste water leaves your house via the plumbing system and enters the septic tank. Here, gravity separates solids and liquids. The heavy solids (known as sludge) build up at the bottom of the tank, while lighter greases and oils, or scum, float to the top of the liquid layer. Naturally present micro-organisms begin to break down the contaminants in the liquid, partially treating the waste. This process generates natural gases, which are released through plumbing vents.
The liquid in the tank, called effluent, flows into a soakaway system. Ideally, this will be a series of pipes, set underground in a bed of crushed rock, that allow the effluent to seep slowly into the ground. If you don't have enough land for this, the soakaway could be as simple as a large hole filled with crushed rock. The rock ads as a filter, while natural bacteria helps break down the waste even further.