In rural areas of France where houses are spaced so far apart that a centralised sewerage system (or tout a l’egout) would be too expensive to install, people install their own, private sewage treatment plants. These are called septic tanks or fosse septiques or the full term in French is l'assainissement non collectif
Traditionally a septic tank or fosse septique in France is simply a big concrete or plastic tank that is buried in the garden. Why do you think that the French are accredited with inventing perfume?!
The tank should hold a minimum of 3000 litres of waste water in a toutes eaux system and as a rule of thumb for the sizing of a French fosse septique you should allow 1000 litres for each bedroom at your property in France.
Wastewater flows into the tank at one end and leaves the tank at the other in what is basically only really a pre-filtered state and still containing up to 75% of the bacteria levels of raw effluent. Hence why there are new regulations for fosse septiques in France being enforced.
A standard tank looks something like this in cross-section:
In this picture, you can see three layers. Anything that floats rises to the top and forms a layer known as the scum layer. Anything heavier than water sinks to form the sludge layer. In the middle is a liquid effluent layer. This body of water contains bacteria and chemicals like nitrogen and phosphorous that can act as fertilizers, but it is largely free of solids.
In a toutes eaux (all water) system, wastewater comes into the septic tank from the sewer pipes in the house, as shown below, although on more modern systems where the septic tank system or fosse septic is more than 10 metres from the house; the sink, shower and bath wastes will pass through a grease trap first. All parts of the fosse must be a minimum of three metres from a boundary, trees or bushes and be on near level ground. To see the technical details in French for a fosse toutes eaux see here
A septic tank or fosse septique naturally produces waste gases (caused by bacteria breaking down the organic material in the wastewater), and these gases can smell being methane based. Outlets inside the house should therefore have loops of pipe called P-traps or sometimes bottle traps that hold water in the lower loop and block the gases from flowing back into the house. The gases flow up a vent pipe instead – on older fosses this will be a thin pipe coming through the roof or maybe even something that looks like a grey mushroom poking up through the lawn. A modern system is required to have a 100mm pipe that vents at the ridge height of the property, so it often pays to be cautious when siting new velux windows!
As new water enters the tank, it displaces the water that's already there. This water flows out of the septic tank and into a soakaway or tranche d’épandage. Traditionally this is made of perforated pipes buried in trenches filled with gravel and you hope for the best that it would soak away. The following diagram shows an overhead view of a house, septic tank, distribution box and soakaway, though in a French fosse sceptique there is normally a return at the end of the drains back to another inspection hatch or règard : (To see the technical details in French for a tranche d’epandage see here
A typical drain field pipe is 10 centimetres in diameter and is buried in a trench that is about 1.5 m deep 0.6 m wide. The gravel fills the bottom 60cm of the trench and dirt covers the gravel, like this:
The water from the fosse septique or septic tank is slowly absorbed and filtered by the ground in the soak-away. The size of this epandage is determined by how well the ground absorbs water, which is now determined by having a soil test done – or etude de sol. Even if you hadn’t planned on replacing your fosse septique you will probably soon have a visit from the SPANC The Service Public d’Assainissement Non Collectif to give you a fosse inspection, whether you like it or not. In places where the ground is hard clay, rocky and absorbs water very slowly or too quickly, the etude de sol will make recommendations as follows:
Lit d’épandage; This is for ground of medium permeability and involves digging a huge hole, filling it with gravel, laying down a specified pattern and length of drainage pipes, covering this with geotextile fabric, to stop the soil filtering down into the gravel drainage bed and then covering the whole thing over with top soil. This then becomes the greenest part of your lawn!
LIT FILTRANT VERTICAL NON DRAINE ; Where the ground is insufficiently permeable or conversely is too permeable (calcaire) you will have to install a lit filtrant. To see the technical details in French for a lit filtrant see here
FILTRE A SABLE VERTICAL DRAINE ; Where your soil is unsuitable for a natural drainage the sand filter outlet or filtre a sable is used for dealing with the treated waste water coming out of your septic tank. To see the technical details in French for a sand filter or filtre a sable see here
Tetre d’infiltration; These exceptional measures are only taken where your soil is really unsuitable for a traditional type drainage bed and has the water table or a watercourse, stream or river close by. This is the raised sand bed filter and looks like you have a nuclear bunker under your lawn! This is also, unfortunately, often the most expensive "traditional" type of fosse septique or septic tank as it usually requires an electric pumping station to lift the effluent water to the top of your sand bed. To see the technical details in French for a raised sand bed filter or tertre d’infiltration see here
Fosse septiques Compacte or compact septic tanks; in an exciting new development there are now approved compact fosse septiques available for installation in France. This range of compact sewage treatment plants are specifically designed for the self-build market, offering simple installation, high quality and odour free treatment, low maintenance and minimal ongoing costs. With purpose sized plants, the range caters for installation from 1 to 55 people and can be used for single or multiple properties. Typically these compact fosse septiques have been used on new builds, barn conversions and renovations or to simply replace existing septic tanks which no longer comply with the new fosse septique regulations in France. These compact fosse septiques are particularly suitable where a traditional septic tank installation would involve ruining a mature and attractive domestic garden, or where the property in France simply doesn’t have enough land to accommodate a normal fosse sceptic. These systems are known in France as Micro Station d'Epuration and in the UK as Package sewerage treatment plants. There are currently two makes of Package Sewerage treatment plants that conform and have been approved to EN12566-3, one is Klargester and the other is WPL. The WPL units offer longer desludge periods than the Klargester Biodisc units, whereas the Klargester package sewerage treatment plants make a litle less noise in opperation. Both can be bought in France from a company registered in France.
The entire range is CE marked and certified to EN12566-3 2005, the new European standard covering wastewater treatment plants, making it fully compliant with UK Building Regulations, normal Environment and SEPA specifications and French DTU regulations . With over 40,000 installed worldwide, this compact fosse septique is fast becoming the preferred treatment solution worldwide.