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Beyond the mains: understanding fosse septique systems
Types of private wastewater systems, their maintenance and planning considerations
Wastewater management may not be the most glamorous topic, but it's important to understand the basics - especially if you own a property in France that doesn't connect to a communal drainage system. Around 5 million homes in France use a fosse septique or other private drainage system for the disposal of sewage and wastewater from toilets, and 'grey' water from sinks and bathrooms etc; this article explains how they work, the importance of a properly functioning system for day-to-day living, and why they are relevant to planning applications, as well as the different options.
What is a fosse septique?
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A fosse septique is a self-contained wastewater treatment system that processes household sewage through natural biological processes. In the past, these systems only treated toilet waste. Nowadays modern systems treat all household water including from toilets, sinks, showers, and other plumbing fixtures. That’s why you may see them referred to as fosse toutes eaux – literally “all waters tanks.” In the USA and UK, similar systems are known as septic tanks.
Typically, a fosse septique is an underground, watertight container made of concrete, fibreglass, or plastic. The tank can be thought of as a settling system, the solids sink to the bottom whilst lighter materials like grease and oils float to the top, creating a scum layer. In the middle, a liquid layer containing partially treated wastewater, known as effluent, remains. Anaerobic bacteria in the tank break down the solids, reducing their volume. The effluent then flows out of the tank and into a drain or filter field, or champ d'épandage, where soil and microorganisms further treat it.
Issues and considerations
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Several issues can arise with a fosse septique system. One common problem is the buildup of sludge and scum. If you don't empty the tank regularly, the layers accumulate and reduce its capacity, which can cause blockages and backups into the house. A lack of maintenance can lead to unpleasant odours, slow drains, and even sewage overflows.
Another issue is the failure of the drain field. If the soil doesn’t allow proper drainage or the field becomes saturated, it fails to absorb and treat the effluent effectively. This can result in surface water contamination, foul odours, and potential health hazards. Factors such as heavy rainfall, soil compaction, or tree roots can contribute to drain field failure. Additionally, the type of soil plays a crucial role.
Planning applications and fosse septiques
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If you need to make a planning application for a new construction, or a renovation or extension to an existing building and the property is not connected to the communal mains drainage system the planning authorities will want to ensure that the property and any existing private drainage system has adequate volume, complies with current regulations, and does not pose a risk to public health or the environment. The required volume of a system is calculated based upon the number of living spaces within the home so if there is an application to increase that number the capacity of the tank would need to match the increase.
If a property doesn't include a compliant fosse septique system or if the existing system can't support the planned development, the authorities may reject the planning application or approve it with conditions. You may need to conduct a soil test (etude de sol) to assess whether the site is suitable for a simple system or whether a more complex microstation type system is required.. In order to validate a planning application the authorities require details of the proposed system, including its size, design, and location.
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What type of system do you need?
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There are various options when deciding on the appropriate wastewater treatment system:
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Fosse Septique: ideal for large plots with permeable soil, offering low maintenance but requiring significant space.
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Sand Filter Bed: for less permeable soil, it provides a more compact solution, though with higher installation and potential sand replacement costs.
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Compact Filter System: ideal for smaller plots due to its minimal land use. These systems generally have higher upfront costs and more frequent replacement needs.
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Micro-Station d’Épuration: a compact, high-treatment plant suitable for limited space and strict discharge standards, but it's more expensive and requires electricity and regular servicing.
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Reed Bed System: offers an eco-friendly, natural treatment for larger plots, though they are more complex to design.
Your choice should always align with soil permeability, plot size, local regulations, and your budget.
Installing a fosse septique or equivalent wastewater system requires knowledge and expertise to get it right and avoid future problems. In France, people refer to the artisan or professional who installs these systems as a terrassier or an entreprise de terrassement.
The role of SPANC
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SPANC (Service Public d'Assainissement Non Collectif) is the public non-collective sanitation service responsible for inspecting and regulating fosse septique systems. They conduct periodic checks (every 4 to 10 years, depending on the commune) to ensure compliance and may issue reports detailing any issues or necessary repairs. A negative report from SPANC may impact planning applications or property sales. As part of the planning application process, if a new fosse is required the design must be approved by SPANC before the application can be validated so it's important to understand if the scheme requires a fosse update as the approval of new systems can impact the timeline of a planning approval.
Contact us
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French Plans has over 20 years’ experience helping people to apply for building permits or planning consents in France. We are fully qualified English speaking architects and planning consultants.
We also work with building professionals of all kinds, including a network of artisans such as installers of wastewater systems in France.
If you are looking to realise your own project in France, whether it is a renovation, a new build, an extension or another project, residential or commercial – please get in touch.
+33 (0) 6 75 05 36 86