Fosse Septique (Septic Tank) Drainage Systems

In this section of the website, you will find detailed information regarding Septic Tanks (Fosse Septiques) for private properties in France.

The French Technology is based on a massive amount of research and experience - anyone requiring a UK septic tank would benefit from following the same rules.

I. Design & Installation

Before the new regulations came into force in France several years ago, the water from toilets was directed into a concrete tank called a Fosse Septique. The water from showers, baths and sinks could be discharged into any local ditch or waterway - and often was!

Fosse Toutes Eaux

Now all the water has to go into one tank - hence the name - Fosse all types of water - roughly!

The systems that are now required include the concrete tank and a filter bed that varies in type depending on ground conditions. About 25% of the water purification takes place in the tank and the remaining 75% takes place in the filter bed.

II. History & Background

Historically the French seperated the grey water (Eau Us�e) from showers, sinks, washing machine etc and the sewage from the WC (Eau Vanne).

The "eau us�e" was taken anywhere convenient such as into the ditch alongside the road or even into a local stream.

The "eau vanne" was either:

The invention of the septic tank is attributed to a French man named Jean-Louis Mouras who built the first septic tank in Vesoul in 1860 and the first use of a septic tank in France dates back to 1881.

The fosse septiques (septic tank) were often very small but the fosse d'etanche can be 4 cubic metres or more. Fosse d'�tanches were often tampered with to reduce emptying costs - by simply knocking a hole in the bottom - but this only worked in areas where the limestone rock or gravel prevailed and the water could drain down and pollute the same acquifer that was being used for drinking water. It also often backfired because in non-draining soils the tanks will often fill up with rainwater and require even more frequent emptying. Many people wonder why their tanks need to be emptied more in the winter and cannot understand why they "use more water in the winter than in summer".

Once in the tank, the waste starts to divide itself - the lightest parts of the waste (fat, oil, grease) start to float to the surface where they form a crust called Suspended Solids. These will be digested by aerobic bacteria and transformed into carbon dioxide and water. (Aerobic bacteria : a micro-organism that requires oxygen to live).

The heavier solids will drop and settle at the bottom of the tank. This is called the sludge. They will be digested by anaerobic bacteria. (Anaerobic: opposite of aerobic) and the liquid will circulate in between the two masses.

Every few years a specialist firm's tanker will need to be commissioned to pump the sludge out from the tank and cart it way to the local sewage works.


The septic tank does not really treat waste. Its main purpose is to store the waste for a period during which the waste will undergo its first transformation called the pre-treatment. This "pre-treatment" only removes about 25% of the pollutants and the remaining 75% contains germs and pathogens (such as hepatitis, polio, cholera, para typhoid etc.) which pose real threats of life threatening disease.

The remaining 75% is purifed by the filter bed that is now required for every installation - this is the "treatment" that just does not exist in most existing installations. It has been estimated that about 3 out of 4 French rural properties do not comply.

European law has forced the French government to act - and not before time!

III. Septic Tanks & Main Drainage Connections

The new regulations on the installation of fosse toutes eaux are tough and now in place. On the 3rd of January 1992, all towns and villages in France saw their responsibilities increased and the Maire is now responsible for every aspect of wastewater treatment, be it collective or individual and is directly responsible for the control and regular maintenance of your septic tank.

The new laws impose an obligation on each commune to prepare a map - called "Carte d'aptitude des sols" that designates the area that should have systems "collectif" or "non-collectif". In the areas where non-collectif systems (fosse toutes eaux) are designated each commune has to have a pollution monitoring system in place by the end of 2005 that will control the disposal of all sewage.

Because most of the Maires are not specialists in the field of wastewater treatment, they have in many cases appointed a syndicate or a professional body to enforce the law. They are often (but not always) known as S.P.A.N.C. (Service Public d’Assainissement Non Collectif) and will service a group of communes from a local office.

The responsible regional body is "Les Directions Regionale et Departmentales des Affairs Sanitaires et Sociales (DRASS) of your region." Each department in each region will also have a "Les Directions Departmentale et Departmentales des Affairs Sanitaires et Sociales (DDASS) of your department" These offices will monitor and administer the implementation of the new laws by the mairies of each commune.
The law is all about pollution control and does not impose any construction obligation on communes that have designated "collectif" systems. So if a commune does not have the money for a system "collectif" then do not buy a house in the designated area because you will probably not get consent for a "non-collectif" fosse toutes eaux.

If your property is in a designated "collectif" area, where the main drains have been installed you will eventually be obliged to stop using your existing fosse septique and connect to the mains drainage in the road.

Many of you will have noticed that your water bill contains a line called “Redevance Service Public d’Assainissement Non Collectif”. This is a charge applied by your water company to help finance the implementation of the law. The cost does vary depending on where you live.

The main purpose of the control is to ensure that all the systems which are not, or which cannot be, linked to the main sewage system are in good condition, function properly and are capable of dealing with the amount of wastewater to be treated.

A visit to your property will be arranged by SPANC and during the visit the technician will verify the :

You will then receive a report that will indicate whether or not your system is working correctly and that will explain to you how to proceed with any required changes.

IV. Requirement For Documentation

All recent installations should have been documented and the records placed with the Mairie.

If you are purchasing an old house in the country ask to see the legal certificate of approved drainage. It is unlikely that there is one and it will be the responsibility of the new owner to get it. The notary will need it before you can resell the property in the future.

A visit to the Mairie is probably all that will be necessary to decide whether or not a new fosse toutes eaux or mains drainage connection is required.

V. Application For A New Fosse Toutes Eaux

This is known as the Demande d'installation d'un dispoitif d'assainissement non collectif.

The application requires:

VI. Costs of Fosse Toutes Eaux

The costs of Fosse Toutes Eaux installations depends on the number of main rooms - because this directly affects the size of the fosse elements.

The cost of concrete tanks can vary between 400� and 800� depending on the size and the builders discount from the merchants.

The cost of polymer tanks will probably lie in the same range even though the published price lists of many major suppliers such as Frans Bonhomme indicate much higher prices. The polymer tanks do need sand backfill around them and need more careful installation.

The pipework and fittings between the house and the filter bed will also cost several hundred euros. Additional drainage runs can add a day to the programme and increase the cost by up to 1000�.

The cost of the filter bed materials bed will be approximately 30� per square metre.

A straightforward installation for a 5 main room residence will take about 4 days - this could increase to 7 for a large complex installation. A team of builders with their plant and equipment will cost about 750� per day.

Hence total costs are likely to be as in the following table. The cost of carting the excavation arisings off site can increase the cost by 500� or more depending on the filter size and the haulage distance to the local tip (dechetterie).

Indicative Costs:

The costs exclude pipe connections, pumping, vent stacks, travelling and accommodation costs.

Number of main rooms (No of bedrooms + 2)

Fosse toutes eaux costs in euros

less than or equal to 5 5000
6 6000
7 7000
Each extra room on top 1000

The conclusion to be drawn is - do not overestimate the number of rooms and never build a fosse now for a future dwelling.

Costs can be quite a bit more for several reasons including:

VII. Mains Drainage

If mains drainage has been or is about to be installed then the house-owner will have been asked where they would prefer the connection point to be located. When this request is not answered the communes drainage contractor will make the decision and mark the connection point on a plan.This will be available for reference in the future and the connection location will be required before your installation contractor will be able to provide a fixed quotation.

The mains drainage connection needs to be planned before the installation of a pool because it is surprising how often the best locations for both result in a clash. Hence the pool and main drainage connections need to be just as carefully planned as a pool and fosse toutes eaux installations.

The mains drainage connections will cause quite severe damage to an established garden (As fosses also do) and so it is preferable to install pool and these connections at the same time both to reduce cost and some of the disruption.

Mains drainage connection costs:

These can vary more than fosse toutes eaux costs because the number of manholes and length / depth of drain runs will vary significantly from project to project. However in broad terms most mains drainage connections should be a bit less than the cost of a fosse installation.

VII. Maintenance of Fosse Systems

The law requires that the fosse toutes eaux is emptied at 4 year intervals or less. It is essential that the property owner retains the invoice (facture) from the licensed fosse evacuation contractor as the Mairie will ask to see this during any inspection in the future.

When a fosse has been emptied it should be refilled with fresh water and treated with a packet of EPARCYL (can be bought in most supermarkets). This also applies to old septic tanks after they have been emptied. A fosse that has stopped working for any reason can also be kickstarted by emptying, refilling and treating with EPARCYL.


Problem Potential Causes
Smells in the house

No water seal in toilets and traps

Evaporation of water seals

Lack of high level ventilation

Smells outside the house

Fosse vent pipes <100 mm

Fosse vent pipe blocked or not carried above the roof

Lack of extractor on top of vent

Lack of primary vent on the WC pipe

Fosse vent pipe not installed properly

Manholes on fosse out of position

Corrosion of the steel reinforcement in the fosse

Lack of fosse ventilation

Blockages in internal drains


Insufficient falls on pipework

Pipe diameters too small

Fosse full

Fosse needs emptying

Fosse to small for loading

Foreign materials in the entry manhole to the filter bed

Fosse needs emptying

Fosse is too small

Rainwater getting into the fosse network

Prefilter in fosse needs to be changed

Blockage of the final manhole in the filter

Sand filter is sealed up

IX. Fosse Toutes Eaux Technical Details

The installation will always consist of two main systems:

Occasionally a grease trap (bac degraisseur of 500 litres) may be required but this should not be necessary in non commercial premises and should be avoided if possible because it is just something else to maintain or go wrong. If it is not possible to keep the distance between the kitchen and the fosse down to 10 metres or less a grease trap is advisable and will probably be a technical requirement of the consent documentation.

When an existing fosse system is being replaced it must be cut out of the pipework system entirely. It can be sealed up and left full of water - any odour from it should reduce to nothing within a few days. Alternatively, depending on its location, it may be preferable to have it pumped dry and filled with sand.

Fosse Toutes Eaux Volumes:

The volume of the fosse toutes eaux can be determined from the following table:

Number of main rooms (No of bedrooms + 2 Fosse toutes eaux volume in cubic metres
less than or equal to 5 3
6 4
7 5
Each extra room + 1

Tank sizes:

These are shown in the following table:

Number of main rooms 4 5 6 7
Fosse toutes eaux capacity (m3) 3 3 4 5
Depth of tank 1.35m 1.35m 1.72m 1.85m
Length of tank 2.52m 2.52m 2.52m 2.52m
Width of tank 1.19m 1.19m 1.19m 1.40m
Weight 1600kg 1600kg 1950kg 2050kg


Two vents are always required and these are the most important part of the installation if odours are to be avoided. Both vents must be taken above the property roof eaves levels.

Primary vent - 100 mm and taken from the downpipe from the WC straight up through the roof.

Secondary vent - 100mm and taken from the top of the fosse outlet pipe, back to the property and then up an outside wall - can be taken through or outside the roof tiling.

Filter Beds:

The size and type of filter bed is a function of the number of main rooms, permeability of the soil, the maximum ground water levels and the slope of the land on which it is to be built.

The size can be determined from the following table:

Soil type Filter type Length of trenches or minimum surface area of sand filter
5 main rooms or less Each extra room
Permeable sand or silt with a minimum thickness of 1 metre

Land drain trenches

500 mm wide x 500 mm deep filled with 10/40 mm gravel surrounding slotted pipes

45m 15m
A thick layer of soil on fissured permeable rock

Vertical sand filter without drainage

Slotted pipes in 200mm gravel over a 700 mm deep sand bed covered with a minimum 200 mm topsoil

25 m2 5m2
Impermeable soil such as clay or when there is a risk of contaminating an underground water resource

Vertical sand filter with drainage.

As filter without drainage but with a system of slotted water collection pipes in a 200 mm layer of gravel below the sand filter. The water is then piped or pumped to a suitable ditch.

25 m2 5 m2
Land prone to flooding or where the ground water is very near the ground surface.

An above ground sand filter with sloping sides

As vertical sand filter without drainage but above ground

Base area - 60 m2

Top area - 25 m2

Base area - 20 m2

Top area - 5 m2