Lagunage

The lagunage is a natural technique of purification of water based on the dice Eutrophisation. It takes as a starting point the natural systems by purification and filtration by watery Micro-organismes, algae and plants. Sometimes, one also makes stream water through the roots of plantations of trees, possibly of the willows treated in coppices cut in short rotation (TCR).

Description

That consists in establishing a slow flow by gravity of waste water in several retaining tanks (3 to 5) not very deep by eliminating the risk from infiltration in subterranean water.

This technique shows the characteristic to require an important surface, between 15 and 20 m ², for this reason one allot the extensive property to him. The residence time must be high (minimum 30 days, even more). Muds concentrating on the bottom and intervening in the biology of the system should be evacuated only after some 5 to 10 years.

The lagoons are generally built in the form of ground basins, equipped with slope having a slope of a third. If a protection against the infiltration in subterranean water is necessary, the funds and the embankments must be clogged. They are generally rectangular.

One distinguishes the nonventilated lagunage which is a technique Anaérobie and the ventilated lagunage which belong to the techniques Aérobie S. the depth of the basins varies 0,5m with 1,5m without artificial ventilation (lagunage natural) and of 2,5m with 3m with an artificial ventilation.

The nonventilated lagunage is used to decrease the content of rough or mechanically clarified waste water substances organic (primary decantation). Without ventilation nor artificial mixings oxygen has three sources:

dissolution of oxygen in air in water by contact on the surface;
production of oxygen by the photosynthetic algae;
contribution by mixture with the water saturated with oxygen.

The ventilated lagunage is used for the biological purification of the not elutriated and nondissolved organic substances contained in waste water pre-purified mechanically in sedimentation ponds or another process. Moreover, artificial ventilation allows:

to improve absorption, the distribution and the use of oxygen;
a more homogeneous distribution of the polluting matters and micro-organisms in the lagoon;
more independence with respect to the noncontrollable natural factors (photosynthetic temperature, wind, contributions,…) by having the possibility of controlling the oxygen contribution and the output of purification.

The most important condition, for the purification of waste water in the lagoons, is that the contents of these last are not at any time in putrefaction. A preprocessing of water consists of: Cleaning, Sifting, Desanding and De-oiling.

The organizations taking part in the processes of purification, in particular the bacteria and the algae, are classified according to their activity in heterotrophic organizations and autotrophic organizations.

The first basin is the lagoon with Microphyte S where the microscopic bacteria and algae are found. The mineralization of the soluble organic matter in suspension is ensured by the aerobic bacteria, they transform it into water, Carbonic gas, Nitrate S and Phosphate S. These simple compounds will be comparable by the algae which thanks to the light it sun will carry out the Photosynthèse to ensure their metabolism and to release from essential oxygen for the aerobic life of the bacteria in the lagoon. The organic matter forming a deposit at the bottom of the basins (decantation) is degraded by the anaerobic bacteria according to the process of anaerobic digestion producing the mineralization of muds and the gaseous emissions (nitrogenized) fixed for some by the algae. Water remains approximately 50 days in the basin with microphytes.

The lagoons with macrophytes generally constitute the second and the third basin (if there are three of them). It lives there in addition to the macroscopic algae and the watery plants able to absorb inorganic substances in particular the mineral shapes of nitrogen and phosphorus, ammonium, the nitrate, the O-phosphate and to emphasize them. They allow the reduction of manures thus. The watery plants also fix rock salt for their growth, it develops micro-organism then which nourish plants they-even. Appearance of Zooplancton (daphnid, cyclops,…) allows to improve filtration of water. It is thus established food chains between the bacteria, the Phytoplancton, the zooplancton and the plants. Water remains approximately 40 days in this type of lagoon.

The lagunage is placed on the level D as regards impact of the rejections on the receiving medium, in agreement with the following minimal quantities:

total suspended matter (MY): 120 mg/l;
chemical request oxygenates some (DCO): 120 mg/l;
biological request oxygenates some in 5 days (DBO5): 40mg/l.

One notes also a great effectiveness on the reduction of the concentrations out of phenols, hydrocarbons, detergents and manure in water at exit of lagunage. The long exposure of water to the rays ultra-violet of the sun makes it possible to reduce considerably the number of agent Pathogène which can be bacteria, viruses and parasites.

A power of 20 W/m ³ allows that the whole of muds remains in suspension and is transported with the effluent. If a recycling of muds coming from the secondary decantation is envisaged, the installation is then an installation with activated sludge with great durations of ventilation.

Most widespread is the mixed lagunage. A power from 3 to 4 W/m ³ is enough then to make circulate and feed out of oxygen the totality of water. But in this case, mud settles at the bottom and acts on the nutritive matters brought back above it, like a fixed mud, with aerobic biological action, similar to biological film of the bacterial bed.

Other advantages of this technique are the low costs of investment and operation, good landscape integration and the aquicultural and agricultural valorization of the produced biomass.

Certain disadvantages appear with the proofing of the ground, the risk of odor, great specific pollution, great chemical pollution, the seasonal variation of the quality of water at exit and to take care not to reach the state of putrefaction.

The natural lagunage as proceeded of purification of water is advised for stations having a capacity from 250 to 1500 éq-hab (but can meet from 100 to 2000 éq-hab). The ventilated lagunage is advised from 400 to 2000 éq-hab (but can see itself from 200 to 3000 éq-hab).

History

The lagunage is used since centuries, but its optimization and the scientific measurement of its performances dates only from the beginning of the 20th century, with for example the studies of the professor Albert Calmette in Lille (photo opposite).

See too

Environment
Pollution the water
Phytoremédiation

External bonds

definition on Ekopédia
Site on the management of water

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