What is a Fosse Septique?

What is a Fosse Septique (fosse toutes eaux) and how does it work

Most British households discharge the waste from their kitchen, WC and bathroom into a mains drainage system and consequently know very little about the types of systems used when this is not possible. The alternatives are either a storage tank, commonly known in the UK as a Cesspit, or a treatment plant. As a cesspit only retains the waste for eventual removal by an authorised waste removal company, it is usually only considered when a treatment plant is not feasible. A treatment installation plant is made up of several parts, each contributing an essential role. The initial stage is usually the septic tank (fosse septique), unless a grease trap is necessary and has to be placed between the house and the septic tank. Its main role is to separate and store the waste solids and start the bacterial digestion process. The main treatment therefore takes place in the filter bed where the majority of toxins, chemicals and other suspended waste matter is removed. The following links show examples of each component and its role in the treatment process.

Grease Trap – Bac Degraisseur

Cess Pit/Pool – Fosse d’Étanche

Septic Tank – Fosse Septique

Waste Water Treatment Plant – Station d’Epuration Individuelle

Filter Bed – Traitement


The new law of 1992 and the fosse toutes eaux

In many areas of France, especially in the countryside where mains drainage is not available, sewage has to be treated on site using a Fosse Septique (Septic Tank), or similar treatment plant. In 1992 a new law was passed handing responsibility for overseeing and regulating old and new waste water installations to the local communes, i.e. the Marie. The new law stipulates that all household waste liquids have to be processed in the correct manner, by means of a Fosse Toutes Eaux (a septic tank that accepts all waste waters) and filtration system. In the past, many households, especially in rural areas, have allowed water from the kitchen and bathrooms to bypass the tank altogether, allowing the water to drain into the surrounding soil or nearby ditch. Now all waste has to be fully treated before it is released into the environment.

Diagram showing the various stages of a French domestic waste water treatment plant


Stage 1 (Collection): Collection of all household waste water and matter from the kitchen, bathroom and WC. If the fosse septique is sited more than 10 metres from the kitchen, the bathroom, and particularly the kitchen waste, must first pass though a Grease Trap.

Stage 2 (Pre-treatment): All waste must then flow directly into the septic tank where separation of the solid matter takes place and treatment begins. About 30% of the waste (the sludge lying on the bottom of the tank, scum floating on the top and the liquid effluent) is consumed by bacteria. The bacterial action for both effluent and sludge is known as anaerobic as the environment is devoid of oxygen. The bacterial action affecting the floating scum layer is aerobic as it relies on oxygen. When the effluent leaves the tank it first passes through the ‘prefilter’ which captures much of the remaining suspended matter.

Stage 3 (Treatment): The effluent now passes through a filter bed, or filtered land drain, where further treatment takes place, removing many more waste and toxic particles.

Stage 4 (Evacuation): Finally the treated liquid is either allowed to soak away into the soil or siphoned off into a nearby ditch, or similar.

SPANC, DDASS and the requirements of a technical survey of your existing Septic Tank

Many communes across France have created their own professional body to enforce the new law. This is commonly known as Service Public d’Assainissement Non Collectif (SPANC). There is also a department and region office that monitors the implementation of the new law by each commune and are known as Les Directions Departmentale et Departmentales des Affairs Sanitaires et Sociales (DDASS), of each department and Les Directions Regionale et Departmentales des Affairs Sanitaires et Sociales (DRASS), of each region, respectfully. SPANC will arrange a professional technician to visit your property to assess your current drainage system particularly to verify the following points:

  • Exact location of your current Fosse and Grease Trap
  • Volume
  • Condition
  • Access points
  • Ventilation
  • Sludge and scum levels
  • Condition of the pipe work
  • Filtration system
  • Nature of the surrounding soil
  • Lay of the land
  • Proximity to any water sources, below or above ground

The property owner will receive a copy of the resulting technician’s report which should outline any problems with the system, and any work necessary to correct it.If you are installing a new system, the appropriate permission has to be sought. The application form (Demande d’Installation d’un Dispositif d’Assainissement Non Collectif) can be collected from your local Marie. If you are employing the services of a company to install your new treatment plant you may find that they will complete all the necessary paperwork on your behalf.

Important points to consider when installing a new French waste water treatment plant or refurbishing your existing system

  • Regardless of your requirements, the minimum tank size allowed is now 3 cubic metres (3000 litres).
  • If your finances allow, choose a tank size that exceeds your requirements as it will reduce the frequency of pump-outs, dilute any accidental input of unwanted chemicals (i.e. high dosage of bleach) and allow a greater retention period, thereby giving the waste more time to separate and the bacteria better chance to consume the waste material.
  • If the fosse is to be sited more than 10 metres from the kitchen, a grease trap needs to be installed as close to the property as possible.
  • Leave good access to all tanks and manholes (grease trap, fosse septic, observation holes for the filter bed etc.)
  • Primary and secondary ventilation is necessary to avoid unwanted odours and to help the flow of waste matter. The primary ventilation should be taken from a WC and either vented directly up through the roof or fitted with a one-way valve to allow air in and not out. Secondary ventilation should be taken from the top of the fosse, or the outlet pipe, and vented up an outside wall to a height greater that the highest opening window, normally above the eaves of the property. Both vents should be in 100mm pipe work. See diagram above.
  • The fosse should not be sited within 3 metres of a neighbour’s boundary
  • There should be at least 3 metres between the fosse/filter bed and the nearest trees or large shrubs
  • Rain water should not enter the fosse at all, but should be directed to alternative drainage systems/soak-away.
  • There should be at least 35 metres between the filter bed and any water source, above or below ground.
  • In the future, every 4 years, your Marie is obliged to make sure your fosse is checked and functioning in the correct manner. Within the intervening period, you are required to have your fosse emptied by a licensed waste collector; make sure you retain your receipt as proof.

Caring for your Fosse Septique

  • Try to moderate your use of detergents, disinfectants and other cleaning agents as an overload will kill the bacteria in the fosse. Read the labels and look for products that are recommended for use with a septic tank.
  • Be aware that antibiotics may stop a fosse from working as they too will kill the essential bacteria in your tank. If this happens, you may have to empty the tank and start again.
  • Don’t use the WC as a general waste bin for sanitary and other bathroom products as they will not degrade and eventually block the system.
  • Check the sludge and scum layers on a regular basis, perhaps every 3 or 4 months.
  • If the level of the sludge is more than 60% full, it will need emptying and restarting
  • If you suspect your tank has stopped working, i.e. if you have been away from your property for a long period, reactivate using a bacterial activator, which can be purchased from most supermarkets.
  • Remove and clean the prefilter on a reguler basis as it is likely to clog if ignored.

Grease Trap – Bac Degraisseur
Cess Pit/Pool – Fosse d’Étanche
Septic Tank – Fosse Septique
Waste Water Treatment Plant – Station d’Epuration
Filter Bed – Traitement

About Gorseinonboy

Hi I'm Vernon Goddard, retired and currently living in Lincoln having spent time abroad in France. My wife Carol and I are enjoying life away from having to make a living; instead we're making plenty of new friends in Lincoln. We have new plans for our time together and new adventures to achieve. Hell Lloyds TSB are still paying for it all and you taxpayers in the UK. So thanks ~ we really appreciate your regular contributions to our spending money. And why not stop and contribute on this site. If you have a view about anything I write you're welcome to post a comment or get in touch with me. When I'm not blogging, I write a little, garden a little less, drink and eat some, exercise when pushed, talk for Wales and think about the grandchildren. Well that's me. Gorseinonboy.
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