Septic Tanks – An alternative to mains drainage

Guest article from Duncan - www.propertyhive.org

Under the Water Resources Act 1991, it remains a criminal offence to discharge effluent that is not of a sufficient quality into a watercourse without the consent of the Environment Agency. This can result in a three month prison sentence or a fine of up to £100,000. Therefore it is important to ensure your installations are up to date.

Source: http://www.mtmdrains.co.uk/

Consideration of drainage is an important part of any development and in the vast majority of cases UK developments will enjoy the benefit of being able to connect to ‘main drainage’.  In rural or more isolated areas, where mains drainage is not generally available, alternative methods of controlling and disposing of drainage, particularly foul waste, need to be considered.  Septic tanks are one such option:

The first recorded septic tank was installed in 1896 by Donald Cameron who was the city Surveyor for Exeter. This septic tank served the entire town and was 20 x 6m to a depth 2.1m. Since then there has been some progress in the development of septic tanks, but the principles are basically the same.

Source: http://www.septicrepairny.com/

Septic tanks are covered in UK Building Regulations under Approved Document H, and it is an offence not to build new structures to the standards as set out in the Building Regulations under the Building Act 1990. However as long as the British Standards are followed or bettered this should not be a problem. See BS6297:2007 for further guidance. Building Regulations do not apply retrospectively and so septic tanks that pre-date the 1985 Building Regulations are not affected. This is despite the fact that many septic tanks are no longer fit for purpose due to the increase in water usage rates over the last fifty years. However under the Water Resources Act 1991, it remains a criminal offence to discharge effluent that is not of a sufficient quality into a watercourse without the consent of the Environment Agency. This can result in a three month prison sentence or a fine of up to £100,000. Therefore it is important to ensure your installations are up to date.

Treatment  - All foul drainage should enter a tank including water from W.C’s, showers, sinks, baths and domestic appliances such as washing machines etc. Washing machines and dishwashers are often discharged into rainwater pipes as it is easier, but this can have a significant impact on the local environment and should always be avoided.

A Septic Tank is a self contained installation that processes and treats raw sewage. It is a tank which stores the waste allowing enough time for the organic matter to decompose through natural processes. The sewage becomes liquefied, with a thick oily scum forming on the top and sludge that settles on the bottom. What is left is a central layer of clear liquid, which in older tanks is often emptied straight to a watercourse (which is illegal and harmful to the environment), but in new installations it is treated further before been discharged. The sludge at the bottom is made up of everything that is too heavy to remain in suspension and the oily layer that sits atop is the matter that can decompose naturally.

The secondary treatment usually comes in the form of a soakaway, where the water is drained into a hole formed underground, using plastic formwork that can be described as looking like milk crates, where the water will be soaked up slowly by the ground. Soakaways come with their own set of legal requirements.

The other type of secondary treatment comes in the form of a bio-filter. These have sweeper arms that drip the effluent over clinker beds. Please note that a septic tank is not the same thing as a cess pit, which is a container that stores sewage until it can be emptied and treated elsewhere. Older tanks are often square in shape with brick walls, although modern installations are often pre-fabricated uPVC units that can be square or circular. The septic tank is made of either two separate tanks or one tank split into two

Capacity - A septic tank should be 180 litres in size for every person it serves, with an additional 2000 litres of capacity regardless of the number of people using it.

Litres = 180P + 2000

Where P = the amount of persons served by the tank.

The above calculation should treat children as adults and allows for emptying on an annual basis.

This is so the sewage can be stored long enough that the decomposition of the waste can occur. Anything smaller than this will mean untreated sewage is discharged and solids may cause blockages in the pipework, or if installed the bio-filter. If untreated sewage is allowed to drain into the local environment, there is likely to be an increase in disease, rat infestation and damage to the local environment.

End Product - One of the end products of a septic tank is sludge. This is raw sewage that remains in the tank, and then emptied periodically. The other is water that consists of nitrates and tiny fragments of sewage.

General Problems - An excess of detergent will inhibit separation of the three layers. An excess of disinfectants will kill of the bacteria involved in the treatment process. Day to day domestic use should not cause any problems in a well designed and maintained system. Water softeners however produce salts which in excess can kill of the bacteria required to make the system work.

De-sludging - How often to de-sludge depends largely upon the occupancy of the household and size of tank, but for an average household of 4 people it would be reasonable to assume an annual de-sludging would be sufficient.

Mains Drainage - If there is a nearby sewer you have the right to connect to it. Just contact the local sewerage company and they will give you further details on how to arrange for a connection.

Construction Requirements - Reference should be made to BS6297:2007, which contains recommended materials for use. Rain and groundwater must be prevented from entering the Septic tank, as clean water should not be contaminated. Therefore the tank should be covered, which also helps with the safety issues of having an open tank. Access must be maintained with an access hatch of at least 600mm square. This access should allow for inspection of the inlets and outlets, and should be large enough to allow rodding. Tanks should be situated;
1. Not within 15 metres of the dwelling in question.
2. Not within 10 metres of a watercourse.
3. Not on land that it is regularly flooded, or land that has a high water table.
4. So that a tanker can access for emptying (e.g within 30 metres of vehicle access).
5. Downwind of nearby dwellings if at all possible.

For more information on a variety of property and construction related articles please visit www.propertyhive.org

Please feel free to share this article and other articles on this site with friends, family and colleagues who you think would be interested

Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog.