Traditionally a septic tank or fosse septique in France is simply a big concrete or plastic tank that is buried in the garden. 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. This is why there are new regulations to check on the conformity of existing tanks to the regulations.
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, 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. This is because after 10 metres grease can cool and solidify, blocking the pipes. All parts of the fosse must be a minimum of three metres from a boundary, trees or bushes and be on near level ground.
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 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.
Inspections of tanks and their drainage have started to be done by SPANC The Service Public d’Assainissement Non Collectif, whether you the owner likes 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 for a filter bed.
Fosse Septiques Compacte or Compact Septic Tanks are now approved for installation in France. They are particularly suitable where a traditional septic tank installation would involve ruining a mature and attractive domestic garden, or where the property simply doesn’t have enough land to accommodate a normal fosse sceptic.