Septic Tank


Septic tank is the term given to a holding tank which is used as an off mains sewage system.  The septic tank is installed where is it not possible to for a property to connect to a main sewer system. The Environment Agency and section H of the UK Building Regulations are the main governmental departments in the UK that regulate and oversee  all installations and discharges from a septic tank.


Let me start by saying that a septic tank is not a cesspool.  A cesspool is a sealed holding tank that allows liquid in, and then when it is full it needs to be emptied.  Cesspools are often installed when there is no possibility of installing septic tank, due to the risk of sewage pollution.


A septic tank is a holding tank with a soakaway or drain field attached to it.  Once the liquid in the septic tank reaches a certain level, the liquid is then discharged via gravity into the soakaway.  The soakaway then allows the liquid to drain away naturally into the soil or another type of medium.  The main function of the septic tank is to trap and contain any solids, or solid particles that enter it.  This is normally achieved by a baffle, or a series of chambers built into the tank. The reason the solid particles are separated from the sewage effluent is so they don’t enter the soakaway and block it up.


Septic tanks have been growing in popularity in the UK for the last 150 years, and for the last 500 years septic tanks have been constructed out of stone or brick.  Today, septic tanks can be constructed from a variety of materials such as concrete, fibre glass or plastic and come in many shapes and sizes.  The two main shapes for a septic tank are the onion shape and the cylindrical or low profile shape as it is known.  The onion shape septic tank is made from fibre glass and range from 7-12 feet in height.  These onion septic tanks also need a concrete base and blanket around them, to stop them from popping out of the ground.  The low profile septic tanks are around 4 feet in height, and do not need concreting in the ground.  Instead they are installed with a granular backfill such as gravel or shingle.


The size of the septic tank depends on the number of bedrooms a property has, for example the Environment Agency and UK Building Regulations state, that for each bedroom you must allow for up to 2 people. So for example, a three bedroom property would need a septic tank that would cater for up to 6 people. Each person in a property, produces about 180 L of liquid per day. The smallest septic tank you can buy in the UK is 2800 L, which will cater up to a three bedroom property. The next size septic tank is 3800 L, this would do 10 people or five bedrooms. The next size septic tank is 4600 L, this will cater for up 18 people or nine bedrooms.  Generally, a septic tank needs emptying once a year. The reason the septic tank is emptied is because the solids at the bottom of the septic tank get higher and higher overtime. If the solids are allowed to get too high then there is a danger that they will discharge into the soakaway drain field and block it up.  In turn this could result in the new installation of a septic tank.