What is a Filter Bed (Traitement) and how does it work

This stage of the treatment has been rather neglected in the past but is perhaps the most important part of the treatment process. The filter bed is effectively an underground soil treatment system receiving partially treated effluent from the fosse. The liquid then passes through a layer of sand, removing pathogens, nutrients and other organic material before either soaking into the surrounding subsoil or drained away into an appropriate ditch.

Type and size of filter bed required

The type and size of filter bed required will depend on your soil type, size of fosse (which is governed by the number of people or main rooms in a property), lay of the land and the maximum water table height. The following table will give you a rough idea of what is expected with certain soil conditions

Type of soil	Filter type suitable	Length of pipe work required or surface area for 5 main rooms + for each extra room
Permeable soil or silt, minimum thickness 1 metre	Land drain trench	45m + 15m
Thick layer of soil on fissured permeable rock	Vertical sand filter without drainage	25m² + 5m²
Impermeable soil i.e. clay or risk of contaminating an underground water source	Vertical sand filter with drainage	25m² + 5m²
Land prone to flooding or where the water table is close to the surface	Above ground sand filter with sloping sides and without drainage	Base area = 60m² + 20m² Top area = 25m² + 5m²

Please note that it is absolutely forbidden to discharge untreated waste liquids directly into the soil, ditch or stream.

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Diagram showing the basic layout of a land drain trench

The land drain system should consist of the following elements:

• 20cm of top soil
• Layer of Geotextile
• Drainage pipes bedded in and on a bed (50cm x 50cm) of 10/40 aggregate (non calcaire - limestone)
• The drainage pipes should be placed in parallel lines, a maximum length of 30 metres per run and at least 1.5 metres apart

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Diagram showing the basic layout of a vertical sand filter without drainage

The filter bed should consist of the following elements:

• 20cm of top soil
• Layer of geotextile
• Drainage pipes bedded in the upper half of a 20cm layer of 10/40 aggregate (non calcaire - limestone)
• 70cm layer of sand
• Another layer of geotextile

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Diagram showing the basic layout of a vertical sand filter with drainage

The filter bed should consist of the following elements:

• 20cm of top soil
• Layer of geotextile
• Drainage pipes bedded in the upper half of a 20cm layer of 10/40 aggregate (non calcaire - limestone)
• 70cm layer of sand
• Another layer of geotextile
• Drainage pipes bedded in the lower half of a 20cm layer of 10/40 aggregate (non calcaire - limestone)
• All surrounded by a non permeable film

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Above ground sand filter without drainage

This filter system is identical to the 'Vertical sand filter without drainage', shown above, but sitting on the surface of the ground instead of below it. The sides of the top soil covering the filter bed should be sloped at an angle. Therefore a filter bed with an affective area of 25m² should have a base area of 60m², thus giving a typical angle of 38.7° from the ground.

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Pipe gradient for good drainage

Generally, in all the examples shown above, the drainage pipes should be installed at a very shallow gradient, approximately 1:200, to allow seepage along the whole length of the pipe run, thus avoiding a sump effect in the lowest lying inspection chamber (Regard).