What is a Septic Tank (Fosse Septique or Fosse Toutes Eaux) and how does it work

The septic tank is a partial treatment system in that it will process about 30% of the waste product. All the household waste from the WC, bathroom and kitchen enter the tank where it separates allowing lighter particles to float to the surface creating a ‘scum’ layer, heavier particles to sink to the floor of the tank creating a ‘sludge’ layer, and inbetween, the 'effluent', a liquid with the remaining suspended particles.

New Fosse Septiques (or fosse toutes eaux) usually consists of at least two consecutive chambers allowing; a controlled flow of liquid through the tank, a better chance for the waste particles to separate, anaerobic (not reliant on oxygen) bacteria chance to digest some of the suspended waste, and a small portion of the sludge, before the ‘clarified’ liquid continues its journey to the filter bed via the prefilter on the outlet of the tank. The recommended flow rate for the effluent passing through the septic tank should be between 2 - 3 days, any faster than this hinders the treatment process.

Guide to required Septic Tank sizes
(the following information is intended as a guide)

All tanks must have a water depth of at least 1 metre and a width of 1.2 metres. Therefore, with a water retention period of 2 - 3 days, as mentioned above, and an average water consumption of 262.5 litres/day per person (includes 50% extra to compensate for high sludge levels), a property with 3 occupants would require a septic tank with a minimum capacity of 2362.5 litres.

3 (occupants) x 262.2 (water consumption) x 3 (water retention period) = 2362.5 litres (or 2.3625 cubic metres)

Therefore a 3000 litre tank will be required as the minimum size laid down by current goverment regulation is 3 cubic metres.

Occupants	Water consumption per person (litres)	Water retention period	Minimum septic tank size (litres)
3	262.5	3	2362.5
4	262.5	3	3150
5	262.5	3	3937.5
6	262.5	3	4725

Fosse Septique maintenance

You are now obliged by law to have your Fosse Toutes Eaux emptied at least every 4 years, by a licenced waste removal company (don't forget to keep your receipt safe as proof); you will be reminded of this as your local Marie is also obliged to routinely inspect it during the same period. When a Fosse Septique has been emptied it is necessary to refill it immediately with fresh water and treat it with a bacterial reactivater i.e. Eparcyl.

Safety note: if you are replacing an old system with a new one, make sure you remove all pipe connections to and from it, have it pumped dry and then fill it with sand or any other fine aggregate.