Print Posted By Lost in France on 30 May 2006 in French Property - Renovating French Property

A Guide to Septic tanks - Fosse Septiques

Regulations applying to individual wastewater systems - Primarily septic tanks Mini Wastewater Treatment plant and fat traps in France

The 1992 wastewater law
Application of the law by the Mayors
What is a septic tank?
How do they work?
Drawing of a septic tank
The control
The visit
Regular maintenance of your system
New and replacement installation
Bio-Solv products and principals

The 1992 Waste-water law

On the 3rd of January 1992 all towns and villages in France saw their responsibilities increased.
The Maire is now responsible for every aspect of wastewater treatment, be it collective or individual.
Your Maire is directly responsible for the control and regular maintenance of your septic tank .
Because all 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 generally known as S.P.A.N.C. (Service Public de Assainissement Non Collectif).

Application of the 1992 wastewater law

The body selected by your Maire has until December 2005 to put in place a plan of action.
This plan will identify what is the best way to treat the wastewater based on population level, cost and other factors. It will also identify every single septic tank in order to control them.
This action plan must also provide a sound financial structure for the body chosen to remain profitable whilst enforcing the law.

Application of the 1992 wastewater law

Many of you have noticed that your water bill contains a line called 'Redevance Service Public de Assainissement Non Collectif', basically 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.
Many of you have also been invited to attend a public information meeting organised by the body appointed to implement the new rules.
In almost every region the action plan is already in place and the control of the septic tanks have started.

What is a septic tank

  • A septic tank is a large tank, where all the wastewater from your home is stored, there it receives very basic treatment.
  • The initial concept was to create an environment that was both aerobic and anaerobic in order to deal with the various matters present in the tank.
  • 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.
  • The official use of a septic tank in France dates back to 1881.

How do they work?

Once in the tank the waste starts to divide itself.
The lightest of the waste (fat, oil, grease) starts to float to the surface where they form a crust, this is called Suspended Solids. They will be digested by aerobic bacteria and transformed into carbon dioxide and water. (Aerobic: a life form 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's. (Anaerobic: opposite of aerobic).
The liquid will circulate in-between the two masses.
  • A 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. This is called the pre-treatment.
  • It is fair to say that at the moment 70% or more of the pollution contained in the waste coming in the tank goes out again.
  • This pollution contains germs and pathogens which pose real threats of contamination and diseases and is therefore very dangerous to human life.
  • This is why the law is now coming into force. Now, waste coming out of the septic tank into the ground must be filtered. This is called the treatment.
  • There are many types of filtration systems available, but the choice is completely dependant on the soil composition.

Diagrammatic of a septic tank

This type of septic tank (fosse toutes eaux in France) is the most commonly used, it receive the wastewater from the bathroom, the toilets and the kitchen.
Other systems have a separate fat trap which only receives the wastewater from the kitchen.

Diagram of a sceptic tank

Drawing of a septic tank with 2 compartments. The septic tank receives all the wastewater from the house, bathroom, toilets and kitchen. The waste undergoes a digestion process. The bacteria's present in the tank feed on the waste transforming it into gas, carbon dioxide and water this is called the pre-treatment. The second stage is to treat the waste coming out of the septic tank which contains a large number of germs and pathogens. This is achieved by using a filtration system. This system will be chosen after a soil test as been done. This is called the treatment.
Please note that it is a serious offence to release untreated wastewater directly into the soil or into a water stream. The risk of diseases and contamination is very high.

The control

The main purpose of the control is to ensure that all the systems which are not or 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.
So what will happen?
The body responsible for the control will contact you to setup a meeting at a time convenient to both party's.

The visit

During the visit an engineer will:
  • Verify the access to the tank
  • The condition of the tank, is it leaking? is it cracked?
  • The location of the tank
  • The ventilation of the tank
  • The volume of the tank
  • The sludge level
  • The drain going to the filtration system
  • The nature of the soil
  • The proximity of water source, above and underground
  • The filtration system

Depending on his findings, the engineer will give you a report, which will indicate whether or not your system is working correctly.
If your system does not function correctly or if it needs some modification the engineer will explain to you how to proceed with the required changes.

The causes and risks of a mal- functioning septic tank

  • Modern life does not facilitate your septic tank's life. In many cases it is too small to deal with the volume of wastewater it receives.
  • Toilets, showers, baths, washing machines and dishwashers have considerably increased the volume of wastewater.
  • In France a family of 3 produce on average 2.6m3 of wastewater per week.
  • This is compounded by the over use of detergents, disinfectants, antiseptics and antibiotics which slow or completely stop your septic tank.

Regular maintenance of your system

Do not dispose of any toxic products in any of the pipes and drains linked to your septic tank. For details regarding the products concerned contact [email protected]
You must ensure that the bacterial activity of your septic tank, fat trap and filtration is replenished regularly by using a top quality product such as Bio-Solv septic tank activator.
Every 4 months do a visual check of your system.
When the sludge content of your septic tank is more than 2/3 of the total volume pump out a third to half of the sludge. Never empty your tank completely.
A septic tank without any sludge in it will not work and will start to smell. 20 to 30 % of the sludge must remain in the tank at all time. It can take up to a year for an emptied tank to get back to normal.

For new build houses or septic tank replacement

For a new installation or a replacement you will need to go to your Mairie and ask for an application form. (Demande de Installation de un Dispositif de Assainissement Non Collectif).
Provide all the information requested on the form (Please contact use if you need any help with this form).
The application addresses two points:
The pre treatment of the waste (the septic tank or MWWTP)
(the wastewater arriving from the house into the septic tank)
The treatment of the waste (the filtration)
(the wastewater coming out of the septic tank)

Information required
On the drawing, mark the area and the location of the land reserved for the wastewater system (septic tank + filtration).
The topography of the land, is the land flat or on a slop?
The nature of the soil between 50 and 100cm deep. Does the ground absorb water slowly, quickly or not at all?
Is the Land at risk from flooding?
Is there any drinkable water source on the land be it Public or Private?
Number of people living in the house.
Does your commune have an existing wastewater system or methods in place?
The type of system you would like to use and its size.
A soil test to ensure that the filtration system chosen is compatible with the type of soil on your property.

The size of your septic tank

  • Water consumption in France is on average 175 litres per person per day
  • You need to add to that 50% in order to take into account the space occupied by the waste already present in the septic tank
  • The water height in the septic tank must be 1 metre
  • The width must be 1.20 metres
  • The length of the septic tank is equal to: capacity in M3/Water Height x Width
  • The retention period must be of 2 to 3 days

Therefore the minimum septic tank size for a property occupied by 3,4,5 or 6 people is:

Diagram of a sceptic tank

A property with 3 occupants must be equipped with a septic tank with a minimum capacity of 2.36 M3 or 2360 Litres
It is always advisable to choose a septic tank larger then required as this will give the following advantages:
  • Reduce the number of pump out required
  • Reduce the impact of accidental release of toxic products in the septic tank.
  • A longer retention time will seriously reduce the risque of blockages of the drain going from your septic tank to the soak away and the soak away.

Please note that the curent regulation governed by document (DTU 64-1) requires the installation of a septic tank with a minimum capacity of 3.00 M3 or 3000 Litres

Wastewater system layout


  • 1. Fat Trap (optional) unless your Septic Tank is more than 10m away from the house.
  • 2. Septic Tank
  • 3. Filter
  • 4. Rainwater (Must never come in contact with your wastewater system).
  • 5 .Drains ventilation.
  • 6. Septic tank ventilation.
  • 7. 7m distance between house and Septic Tank. If the distance between the house and 8. the Septic Tank is over 10m you must install a Fat Trap.
  • 9. 35m minimum distance between filter and water source.
  • 10. Neighbouring fence minimum 3m.
  • 11. Large Trees and Shrubs must be at least 3m away to ensure that the roots system does not damage the septic tank and the filter.
  • 12. Access & Observation points for Fat Trap, Septic Tank and Filter.

If you are planning to upgrade your existing system or install a new one:
You should consider using a mini WWTP.
There are many benefits in using a mini WWTP instead of a conventional septic tank.

© Jean-Claude Bardot
To receive further technical data or general information absolutely free contact BIO-SOLV
E-mail: [email protected]
Web site:


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