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Last edited 15 Dec 2020
Septic tanks are generally installed at properties that are not connected to the public sewer network. They are typically fabricated from concrete, fiberglass or plastic and installed below ground where they collect and then to naturally treat sewage (a mixture of waste water and excrement) so that it can be drained safely to the surrounding environment.
They generally consist of two chambers. Sewage enters the first chamber via a drain. The solids within the sewage slowly settle to the bottom, and grease and oil remains at the top. These layers are slowly broken down by bacteria. This leaves a layer of clarified water in the middle that is then drained to a second chamber and then out to sub-soils in a septic drain field (also referred to as seepage field or leach field) which disperses the discharge to ensure it does not cause pollution. Drain fields should remain free from trees or hard landscaping and should not be used by traffic.
Material that remains in the tank will need to be removed occasionally by a professional disposal company (this is referred to as de-sludging) to prevent a build up causing overflowing into the drain field.
The most common problem associated with septic tanks is blockages. Blockages may be apparent by poor drainage in the property itself, or by smells, or backing up at manholes and so on. Items such as food waste, nappies, sanitary towels, tampons, cotton buds, wet wipes and so on may cause blockages to form or may hinder bacterial action and so should not be flushed or drained into septic tanks. Excessive use of bleach, disinfectants or washing liquids may also disrupt the balance of bacteria in a septic tank.
Septic tanks must not cause pollution. Historically, septic tanks were permitted to discharge to a watercourse rather than a septic drain field, however, on 1 January 2015 The Environmental Permitting (England and Wales) (Amendment) Regulations 2014 came into force, creating General Binding Rules (GBRs) requiring that by 1 January 2020 (or sooner if a property is sold) septic tanks may only discharge to a sewer, or to a septic drain field. If this is not possible, the septic tank should be replaced by a small treatment plant that uses mechanical processes to treat sewage so that it does not pollute the watercourse.
Installation of a septic tank requires planning permission and building regulations approval, and may require a permit depending on the volume discharged. For more information see: https://www.gov.uk/permits-you-need-for-septic-tanks
 Septic Tank Servicing
The servicing of your septic tank is of paramount importance as they are only able to handle a certain amount of sewage. It is recommended that you have your septic tank serviced and emptied every 3-5 years for a standard residential sewage system. However, this may vary based on your actual usage and the number of people living in your home. Larger households will require more regular servicing due to increased water usage, however a single occupancy household may only require their tank to be emptied every 10 years.
Maintaining the septic tank is key to ensuring it is not overloaded which requires a measured approach to how much water is being used within the property. Excessive water use from washing machines, dishwashers, baths and showers are all advised against. And, as mentioned, any blockage inducing materials or substances such as oil and grease should be kept away from each of these areas.
Septic tanks must be pumped occasionally to ensure that they operate efficiently and reliably. A septic tank which is not working as it should can pose a serious biohazard to your property with sewage potentially becoming blocked in your drains or the sewage bubbling up to the ground outside your property.
There will be a number of signs to indicate that the septic tank connected to the property needs servicing and emptying. Toilets and washing machines will begin to work inefficiently, becoming slow and unable to operate as they should. If a toilet cannot successfully complete a flush cycle, it is advised to check the septic tank for problems.
Waste which is flushed from your home to the septic tank will separate upon reaching the tank. Liquids will travel to the drainage pit, but any heavier substances will sink to the base of the tank and remain there as sludge. Eventually, the sludge will need emptying from the tank as it will reach a point where it will begin to create a blockage. This will become evident when the toilets and drains within the property show signs of slowing down.
There will be a more visible and therefore more serious sign that a septic tank must be emptied, and this is when there is visible overflow surrounding the surface of the tank. The odour caused by this spillage will be evident, at which point it will require professional intervention. In order to prevent any serious damage to the property and the septic tank it is advisable to call in a professional off-mains drainage specialist. In this instance they will undertake an assessment to determine what repairs are needed and whether a new tank is required.
Septic tank maintenance requires a preventative approach in order to be successful and keep your system working efficiently. It is important that property owners do not wait until there is a problem with their tank and instead, implement a schedule in which the tank can be serviced and emptied every few years- dependant on water usage and the size of the household.
 Related articles on Designing Buildings Wiki
- Air admittance valve.
- Approved Document H.
- Clarified water.
- Difference between drains and sewers.
- Drainage stack.
- Grease management.
- Private sewer.
- Public sewer.
- Reed bed.
- Safe working in drains and sewers.
- Sanitary pipework.
- Section 102 existing sewer adoption.
- Section 104 new sewer adoption.
- Soil vent pipe.
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