Fosse Toutes Eaux

The Design and Installation of Fosse Septiques or Septic Tank Drainage systems for private houses and homes

Fosse Septiques or Septic Tanks

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.

Bluepools and Fosse Toutes Eaux

Bluepools can undertake the installation of Fosse Toutes Eaux when concurrent installation of fosse and pool is required

This section of the website describes everything that you need to know about Fosse installations.

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

History and Background

Historically the French seperated the grey water (eau usee ) from showers, sinks, washing machine etc and the sewage from the WC (eau vanne).

The eau usee 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'etanches 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) starts 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 "pretreatment" 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!

Septic Tanks and 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) de your region." Each department in each region will also have a "Les Directions Departmentale et Departmentales des Affairs Sanitaires et Sociales (DDASS) de 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.

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. Bluepools can undertake the application on behalf of the client where required

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:

The application form completed and signed - including number of main rooms, slope of the land, the ground water level and the type and size of filter.
A location plan (1:25,000 or 1:10,000)
A layout plan showing existing and future buildings, the proposed location of the fosse installation, any wells or streams and the slope and direction of slope of the land.

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 euros 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 euros.

The cost of the filter bed materials bed will be approximately 30 euros 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 euros 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 euros 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






Each extra room on top


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:

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.

Maintenance of Fosse Systems

The law requires that the fosse toutes eaux in 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.


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
  • Grease
  • 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

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
Each extra room
+ 1


Tank sizes

These are shown in the following table:

Number of main rooms





Fosse toutes eaux capacity (m3)





Depth of tank





Length of tank





Width of tank











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



A thick layer of soil on fissured permeable rock

Vertical sand filter without drainage

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

25 m2


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


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