Septic Tanks in France - Fosse Septiques
Why do we need a Septic Tank in France?
Individual French properties, sometimes even whole hamlets, can not be connected with the mains drainage network, generally because they are too distant or it is too costly or too difficult to connect them or create a new sewage treatment plant for only a few houses. In this case, in order not to dump untreated waste water into the natural environment, these houses must be equipped with an individual system for sewage treatment (called “l'assainnisement autonome" or “l'assainissement non collectif”), better known by the general public by the name of “septic tank” or fosse septique in France.
Originally septic tanks just had the sewage coming from the toilets of the house and waste water from sinks, showers etc. would either go into a soak away in the ground or sometimes (read that as often in rural France) would simply run out into a ditch. Today the standard for a septic tank system is a 'fosse tous eaux' that is to say that the septic tank takes all of the waste water from the house, including baths, kitchen sinks, washing machines etc. This is to prevent too many chemicals going directly into the French water courses.
The general rule of thumb is 1000 litres of septic tank capacity per bedroom in your property, so a 4 bedroom house in France would require a septic tank with a 4000 litre capacity. The smallest and most readily available septic tanks size in France is 3000 litres.
Septic Tank Alternatives in France micro-stations d'épuration
Today under European rules we can install micro-stations d'épuration which are mini sewage treatment plants designed to cope with individual houses in France. These package sewage treatment plants can be considered to be the 'Gold Standard' of home sewage treatment and we discuss some of the benefits and sewage treatment plants available in France in this article about micro-stations d'épuration
The Septic Tank Sewage Treatment Process or How does a Septic Tank installation in France work?
There are principally two stages to a septic tank installation in France. There is the Pre-treatment stage and the Treatment stage. Contrary to popular opinion the septic tank is not the treatment stage but simply the pre-treatment stage.
Septic Tank Pre-treatment Stage
The pre-treatment makes it possible to prepare the effluent to undergo the treatment stage and to protect the treatment stage from getting clogged up with solids in the waste water.
The pre-treatment stage has 3 main functions:
Buffer for changing flow rates of the waste water and sewage coming from your house. For example you're having some people round and there are lots of people flushing toilets, the septic tank slows down the flow of the waste water and allows decantation to occur.
Collection of solid matter by decantation and floating matter like oils, greases by flotation. The matter which decants or settles out to the bottom of the tank forms a sludge layer while the floating oils etc. form a scum layer.
Biological action within your septic tank in France will cause liquefaction by anaerobic digestion (merde eating which thrives in the absence of oxygen) of the sludge and scum layers within your septic tank. Solids will have to be pumped out of your septic tank every 2-4 years.
The bacteriological activity within your septic tank produces gases like H2S (hydrogen sulphide - smells of rotten eggs) and CH4 (methane - this is what human farts are made of and smells like....) which can be toxic with certain levels of concentration. This is why it is necessary to have a ventilation pipe fitted to your septic tank installation in France. Normally this pipe will run up your gable end as high as possible or appear through the roof in the form of a mushroom capped pipe. The end of this pipe should obviously be as far as possible from openings such as windows, doors and Velux etc.
If your septic tank is more than 10 m away from your house in France, or if the risk of a build-up of solidifying greases and oils in the drain pipe to the septic tank is a high risk then a grease trap will have to be fitted to prevent the grease from entering into the drain pipe. This grease trap will require emptying every 6 to 12 months in general.
In order to further refine the pre-treatment process of your septic tank installation in France, it is possible to add a volcanic rock filter. Sounds like witch craft but in fact it's usually just a basket of volcanic rock (a bit like you find on a gas BBQ) that all the outflow from the septic tank has to pass through.
Technical septic tank details in French are available with this link fosse toutes eaux
Septic Tank Treatment Stage
The treatment stage makes it possible to purify the pre-treated water and to then disperse it into the environment.
How does the septic tank treatment stage work?
Waste water arriving at the treatment stage of the septic tank system will be purified thanks to the action of the micro-organisms which develop within the filter medium or ground. The organic matter contained in the waste water (carbon, nitrogenizes, phosphorus) are the nutrients for the micro-organisms in the ground and they then will consume these materials in order to live and develop. One the waste water has past through the treatment stage of the septic tank installation it will either simply soak away into the ground or flow out into a drainage ditch, stream etc. (correct approval should be sout to outpout into water courses and ditches etc.)
The types of filter and methods for the Final Treatment stage?
In order to carry out the treatment, the waste water will be delivered evenly over a certain surface of ground, where the waste water will undergo the final treatment stage. The form of the final treatment stage will vary according to various factors like the constitution of the land, its permeability, the slope, the proximity of a freshwater spring or the level of the water table etc.
In the case of a purification and evacuation of the waste water by soaking away into the ground the most common method if final treatment from a septic tank system is:
Shallow drainage ditches which have perforated pipes laid within a filing of gravel, covered with a geotextile and finally covered in a thin layer of soil and probably a grassed finish. The rule of thumb for the length of trenches is 20m linier per bedroom in your house. So for a 3000 litre tank 60 linier meters of drainage trench are required and are usually laid out in three or five parallel trenches. The gravel is where the micro-organisms live that digest all of the organic matter before the waste water finally soaks away into the ground. You'll not be able to grow trees or shrubs on this land and it will have to be more than 3 meters from your land boundry.
In the case of a the waste water from your septic tank system being purified before rejection into a ditch or stream due to the impermeability of the ground the following technique is commonly used:
Vertical sand filters are sited in a large hole not dissimilar in size to a medium sized swimming pool. The hole is lined with a one piece plastic liner to make it water tight and the outflow pipes are laid out in a thin layer of gravel covering the bottom of the hole. These pipes allow treated waste water to run out of the sand filter once it has filtered down through the special washed filtration sand which fills up the bulk of the hole.
Above the filtration sand is another layer of gravel, this holds the input top level of perforated pipes which evenly delivers the waste water from you septic tank to the top of the sand filter. The top of the gravel is covered in a geotextile and can then be covered in a thin layer of soil and probably grassed to finish. Just as in the previous method, you can not have this installed too close to the land boundary, trees or shrubs and absolutely no driving over it in anything heavier than a sit on mower.
For more information on vertical sand filters we have some more reading available in French here: sand filter or filtre a sable
Don't want or can't have a septic tank in France?
If you have limited land available and can not install the large filter system which is normally associated with a septic tank in France, or you don't want to have the nasty smells associated with a septic tank, or even if you are environmentally conscientious and want to fit a system that is less polluting than a normal septic tank in France? The solution could be a package sewage treatment plant in France known as micro-stations d'épuration. There are some EU approved (EN 12566-3 2005) compact sewage treatment plants available on the market which have undergone the CE certification process and can be installed into a small area with less impact and less smells than a septic tank as well as much cleaner outputs than septic tanks....... then read our article about micro-stations d'épuration
To find an installer of Septic Tanks in Brittany (fosse Septique) or WPL package sewage treatment plants and Klargester systems...