subject: The key to autonomy: the septic tank [print this page] The key to autonomy: the septic tank
When the term is mentioned, one has to make a difference between "fosse septique eaux vannes" and "fosse septique toutes eaux".
The "fosse septique eaux vannes" collects waters from the toilets.
The "fosse septique toutes eaux" collects all polluted waste waters, including the "eaux vannes"
The septic tank is an airtight container made of concrete, polyethylene, or fibreglass which is buried in the ground. Older septic tanks can be made out of steel or wood, but risks are respectively rust or rot.
Its size can vary according to the size of the house itself, the number of people living in it, and the amount of water used. However, in France, the size of a septic tank is defined by the number of main rooms in the house, that is to say 3 cubic metres for up to 5 main rooms, then 1 additional meter for any other room.
A septic tank can be partitioned into two distinct volumes (linked by a slit of little pipes) to ease the transportation of clear waters after the retention of solid content as wells as their anaerobic digestion. A grease trap can also be installed, in order to collect the oil and fat solids from cooking water for example.
The efficiency of a septic tank relies on 3 steps:
- The arrival of waters in the tank
- Their stay in the tank
- Their release from the tank
There are 7 rules of use for a septic tank:
- To choose a dry and impermeable ground on a flat ground or with a gentle slope
- To make sure the tank's volume is sufficient for liquid retention but also for mud and foam stocking
- To have a retention time long enough for the waste waters' solid content to settle
- To make sure the liquid is stable
- To ensure there is no obstruction so that the airing is carried out properly
- To have an easy access to the tank for control and maintenance
- To schedule a tank cleanup by a specialised company every 2 years, every 4 years as a minimum
The decomposition process is very slow to get under way. It may be a good idea to add mud collected in another functioning tank to yours. The decomposed organic elements will accelerate the digestion of waste waters entering the septic tank.
The following elements should not be put in the tank:
- Rainwater
- Chemical products (pesticides, antifreeze, petrol, oil, glaze, paint)
Chemical products such as mere detergents slow down considerably the decomposition process.
A septic tank should be cleaned up every 2 or 3 years. The "fosse septique toutes eaux" is usually cleaned up every 4 years. The time between two emptying can depend on different considerations, like the house's size, the amount of water used, the average temperature etc.
One of the problems is that a septic tank can keep on working even if it is completely full: waste waters will simply go through the tank instead of staying there. The emptying should be performed when the mud (between the bottom and the liquid) reaches 2/3 of the tank's overall height. However, a septic tank should not be completely cleaned, for it would need mud and its indispensable bacteria to be able to work again.
When a septic tank has to be installed, one has to withdraw a file and to hand it out to the town hall.
Usually, several indications have to be put in writing: the surface and the location of the plot of land, its slope, the possible existence of an underground water sheet, the ground's composition on 1 meter deep and its permeability, the liability to flooding, the supply of drinking water (local or private), the number of rooms in the house and the number of people living in it.
It is the owner's responsibility to make sure the tank is working properly. A septic tank installation cost is around 1000 if done by yourself, about the double if installed by a builder.
However, it has the great advantage of treating beforehand and efficiently waste waters before the following official treatment process.
A septic tank is a real alternative when the linking with public sewage is impossible or too expensive.
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