Phyto-purification or lagooning: operation, advantages, disadvantages, costs

The phyto-purification is the generic name for all the ecosystems capable of clearing a site soiled by organic, mineral or to a certain extent chemical pollution. More commonly intended for industrial uses or for equipping small and medium-sized communities, phyto-purifiers can constitute ecological solutions accessible to independent private sanitation.

In this dossier, we will deal more specifically with the treatment of domestic wastewater by lagooning, practical application on the model of natural wetlands.

Legality of non-collective sanitation by lagooning

This equipment does not benefit fromno approval health authorities. Before even giving the groundwork for the first time, or submitting a building permit, the user must absolutely ensure that two conditions are met:

  • Check that no network sewerage does not pass near, because the law makes obligation owners to connect their buildings to these collectors within a maximum period of 2 years after their commissioning.
  • Get derogatory authorization prior to installation with the Departmental Service of Water Police (SDPE). A serious study and recourse to the Public Non-Collective Sanitation Service (SPANC) of the municipality, can provide decisive help to support this kind of case.

Principle of lagoon sanitation

The combined action, microorganisms, suspended plants and aquatic plants, transforms the pollution carried by wastewater into a complete food chain, extending from bacteria to fish. The lagooning site is a set of artificial ponds (lagoons) intended to improve the concentration and fixation of these nutrients in order to create a confined environment conducive to the growth of microscopic flora and fauna. The circuit and the flow time are specifically designed to optimize each step of the treatment. At the end of the loop, the water must be sufficiently purified to be infiltrated if the permeability of the ground is sufficient. Evacuation to a pond, a stream or a ditch may constitute a last resort, subject to study and authorization. The reuse for domestic or industrial use, follows the same procedure.

Physiognomy of a lagoon industry

After pre-treatment, the stream of water to be treated crosses at slow speed (between 1 week and 1 month), a network of at least 3 lagoons connected hydraulically by pipes or by an overflow edge. The basins have different characteristics depending on their level, the type of waste and the flow of the tributaries. They can be built different materials, simply dug in the clay soil, coated a layer ofcompacted clay or a geomembrane, but they are still waterproof. The basins are organized in hierarchical sector according to their function. Rectangular lagoons offer better performance.


First wastewater treatment, the screen mechanically retains heavy, bulky or hardly biodegradable elements (sand, stones, bottles, metal objects, wipes, etc.). It is essential to prevent clogging of pipes or clogging of settling tanks. It is also a life insurance policy for any pumps located downstream.

Maintenance: vClean the receptacle 3 to 4 times / year.


The coarse de-oiling of the effluents is carried out using a grease trap of suitable dimensions. Rarely used in collective installations, it becomes essential for private installations in order to reduce the land area of ​​the sector. It is mandatory for installations equipped with all-water tanks located more than 10 meters from buildings or dedicated to catering professions.

Interview : skim the surface layer 2 to 3 times / year.

First lagoon: settling

The lagoon of settling occupies more than half of the lagoon space. It is a deep buffer basin, in which heavy solids are transformed:

  • In gas (Carbon dioxide, Methane, Hydrogen sulfide….), Before dispersing in ambient air and / or diluting in water.
  • In minerals, treated over the course of the following basins.

These elements are the residues of digestion of deep sludge by microorganisms anaerobic, naturally present in the environment. The efficiency of installations can be improved by densifying useful species to the detriment of those whose abundance creates dysfunctions or generates unpleasant odors. These crops are very dependent on local climatic conditions and the composition of the waste to be treated. On leaving the first basin, the water has lost up to 75% of its impurities.

Interview : pump the sludge every 7 to 10 years.

Alternatives to settling ponds

Some lagoon systems do without a settling lagoon, by entrusting, in the same basin, all the degradation processes to microorganisms, aerobic and algae at the surface, and anaerobic at depth. These devices require very malls land, times of long treatment and are energy consumers for oxygenation by mixing.

In individual installations, the surface economy often encourages the replacement of the settling tank by a all water pit, providing the same service. The lagoon system is then reserved, only for final biological and mineral purification.

Second lagoon: biological and mineral treatments

The second lagoon is shallower and of lesser surface. Its role is to complete the organic cleaning and to rid the water of most of its loads, pathological microbial (coliforms, faecal streptococci, etc.) and minerals (nitrates, phosphorus, etc.). Part of the oxygenation is provided by a flora of microscopic algae (phytoplankton) feeding on minerals and decomposition gases, hence the predominantly green color of the basin. In addition, sunshine and wind promote oxygenation by photosynthesis and brewing. These algae and bacteria in turn serve as meals on the upper echelon, the zooplankton. The deep sludge serves as a fixative for the plants and mineralizes slowly.

Interview : limited to the maintenance of the banks and the control of bacterial loads.

Third lagoon: The finishes

At this stage, the water is clear. Zooplankton and plants take the last pollutant samples, supporting the life of a host of living filters (amphibians, crustaceans, insects, etc.). The borders of this landscaped lagoon are planted of plants macrophytes (rushes, bulrushes, papyrus, etc.) whose roots in turn participate in the oxygenation of the water.

Residual water can be infiltrated, channeled towards new inhabited amenity ponds (ornamental fish) or enhanced (watering, breeding, fish farming, etc.).

Interview : Weed cutting and water quality monitoring. Ducks and other birds are responsible for thinning plantations and regulating aquatic fauna.

Advantages of lagooning systems

  • Ecological ecosystem, respectful of the environment
  • Landscape integration into the site
  • Higher decontamination efficiency than most conventional systems
  • Moderate investment
  • Low operating costs, low maintenance
  • Great durability

Disadvantages of lagooning systems

  • Requires large areas of land.
  • Administrative exemption.
  • Operation depending on the seriousness of the preliminary study.
  • Variable yield depending on the season.
  • Fast, but regular maintenance.
  • May attract rodents and mosquitoes.

Construction costs

The investment cost depends on its treatment capacity, the conformation of the land and the requirement for landscaping.

Count, on average:

  • Between 3,500 and € 5,000 tax included for the sanitation of a family house with 5 rooms.
  • Between 500 and € 1,000 Tax incl. for the preliminary study.
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