Saint Bonnet le Froid

Saint Bonnet le Froid : a 900 m², vertical flow reed bed (French system) plus a 600 m² second stage unsaturated vertical flow red bed for the treatment of  domestic effluent from a village with variable population.

Additional Info

In operation since: 2011

Type of wastewater treated: Domestic

Hydraulic load: 125 m3/d

Organic load (PE): 750 PE

Organic design load (kg BOD/day): 50

Location : Saint Bonnet le Froid, Haute Loire, France

Client : Town of Saint Bonnet le Froid

Stage 1 type: Vertical flow reed bed system (French system)

Stage 1 surface area (m²): 900

Stage 2 type: Vertical flow, unsaturated

Stage 2 surface area (m²): 600 m²

Cost: 473,000.00 €

Designed by SINBIO



The project was to create a reed bed filter (2 stages) to replace an old Imhoff tank. Currently 420 residents are living there but there are an important number of non permanent visitors. The treatment has been designed for a maximal capacity of 835 people.

Majors design constraints were:

  • Population fluctuation and specific activities (restaurants and laundry),
  • High altitude of the project (1000 m) with a long period of snow cover,
  • Strong slope of the available land.


Two stage reed bed filters, fed by gravity flow siphons. There is no primary treatment; the raw, only coarse screened sewage is fed directly on the first stage beds with storage of the filtered dried and mineralized solids on the first sage beds for at least 10 years.


This solution allows for a high quality of treatment during summer and winter seasons. Maintenance requirements are minimal compared to intensive solutions.


Additional Info

  • Hydraulic load: m³/day
  • Organic load (PE): PE
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