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The “Etude de Sol” in France, calculating dimensions for a soakaway in France
(tranches d’épandage).
You may be encouraged to have a costly “étude de sol” as the first step when
replacing a fosse sceptic in France, however, if you wish to benefit from the
reduced ecological impact, lack of odeurs, and convenience of a packaged
domestic wastewater treatment plant in France the “etude de sol” becomes a
little superfluous and with a little patience you can calculate the permeability
of the soil yourself. A DIY etude de sol! This should save you between 4-500
euros too, which can go towards your domestic sewage treatment plant . . . . .
Domestic wastewater treatment plants in France should ideally have a drainage
field or soakaway to disperse the water produced by the WWTP as this is what the
relevant authorities prefer, and it saves your local council being as meticulous
at keeping the roadside drains clear.
The infiltration zone around the sub surface irrigation pipe becomes
biologically active and enables additional treatment of the effluent too. The
unsaturated zone beneath this provides attenuation and a pathway for oxygen
diffusion for further wastewater treatment. A saturated zone provides further
dispersion and dilution of the treated effluent.
The siting and design of the drainage field depends on many factors, the most
important of which is that there is sufficient suitable land with subsoil
capable of absorbing the daily effluent discharge on a long term basis.
In the absence of any recent French regulations for septic tank or wastewater
treatment plant installation we have based the following document on BS 6297:
2007 which is a new code of practice for the design and installation of drainage
fields for use in wastewater treatment. It provides recommendations and guidance
to aid preliminary planning, detailed site investigation to identify suitable
drainage field locations and the assessment of site characteristics. It provides
systems designs and layouts including percolation testing and determination of
trench area.
Site assessments must be made before equipment is purchased or installed as not
all land is suitable for drainage field use. A percolation test establishes the
length and area of infiltration trench required to disperse the effluent. Soil
porosity can vary across a site and the percolation test should be carried out
at the intended location of the proposed soakaway. It should not be carried out
in extreme weather conditions such as drought, frost or heavy rain.
In order to assess the infiltration zone of your soakaway for your wastewater
treatment plant in France a percolation test is required and a deeper trial hole
should be used to identify the soil and conditions beneath the drainage field,
including the position of the seasonally highest water table as ideally you need
a minimum of 1.2 m of unsaturated soil above this position.
Etude de sol, the Soakaway Percolation Test and calculations in France
How to Calculate Soakaway trench area and trench lengths
The Vp is used to determine the total floor area of the drainage trenches and
therefore the total length of irrigation drain. For domestic premises, the floor
area of the drainage field required may be calculated as follows.
A = p x Vp x 0.25 for septic tanks
Or
A = p x Vp x 0.20 for package wastewater treatment plants
(i.e. 20 % less, because the effluent has received additional treatment)
The calculated area A should be converted to an amount of linear trench based on the width of the trench which is usually between 0.3 m to 0.9 m. The layout of the trench network will depend upon the soil porosity and the availability of land but the legs of the trenches should be connected so as to form complete loops.
Where the calculated result indicates the need for a long drainage trench length (200 metres) serious consideration should be given to the use of a packaged wastewater treatment system which has been CE certified to EN12566-3 to produce a better quality of effluent. This effluent may be fed into a water course or open culvert. All equipment (for less than <50 pop equivalent) should meet the requirements of their relevant standard, i.e. EN 12566 part 1 for septic tanks, or EN 12566 part 3 for package treatment plants.
Effluent processed through a wastewater treatment plant in France contains far fewer pollutants and fine solids than that from a septic tank in France. Therefore the irrigation system is better protected and less likely to block and can be smaller. The standard recognizes this fact and differentiates. As an example: Irrigation systems for domestic applications
In all cases treated effluent
SOAKAWAY SYSTEM DESIGN in France
Your recommended certified Installer is the ideal person to design the soakaway or drainage field for your specific site. BS You can work to some basic guidelines for your French wastewater soakaway. e.g. not closer than 7m to a building. Drainage fields for septic tanks should ideally be a minimum of 10 m away from a water course or ditch, although soakaways for WWTPs can be closer. 3m away form site boundaries, away from trees and plants with extensive root systems, and from existing supply services, access roads and other drainage fields. The detailed preliminary assessment should have identified other limiting criteria such as wells, rivers containing pisciculture etc.
In brief, after the septic tank or treatment unit, the effluent pipe should be connected to an inspection &/or distribution chamber which leads to the drainage field. This should be designed as a closed circuit with facilities for inspection and maintenance. The layout should ensure even distribution throughout the absorption field, avoiding steep gradients on sloping sites. An inspection chamber at the furthest point from entry on each leg or loop is advisable.
The drainage / sub-surface irrigation system should be very carefully constructed using 110 mm downward facing perforated pipes laid in trenches with a uniform gradient not steeper than 1:200. The trenches should be between 300mm and 900 mm wide and minimum 1m wide strips of undisturbed ground should be maintained between parallel trenches. The pipes should be laid on a 200- 300 mm layer of clean gravel granular fill material graded either 16-32mm or 20-50mm. The trenches should be filled with the same material to a level 50mm above the pipe and covered with geotextile material to prevent the entry of silt. The remainder of the trench can be filled with normal soil. Pipes should be laid at a minimum depth of 200mm below the surface. Corrugated pipes designed specifically for land drainage should not be used.
We also supply a packaged irrigation system (Ezy Drain) that replaces the traditional construction methods. Based on an innovative design that has been used overseas for more than 20 years the system combines pipe work and an integrated filter. The integrated filter utilizes modern recycled material and provides a high performance, well voided, freely draining irrigation medium. Ezy Drain is supplied in three meter length and replaces the stone / gravel and perforated pipe work traditionally used for drainage field soakaways therefore is much quicker and simpler to install, especially for DIY applications. For further information on Ezy Drain see here, or for a quotation for the Ezy Drain solution click here. We hope you found our page on DIY Etude de sol in France useful, you can always give us your feedback here.