L'utilisation des lits de séchage traditionnels est de plus en plus abandonnée par les petites collectivités, du fait de leurs grandes contraintes d'exploitation. L'article débute par une présentation du parc de station par taille et types de traitement. Il se poursuit par une rapide analyse des coûts d'utilisation des lits de séchage traditionnels et d'épandage en agriculture. Viennent ensuite une bibliographie puis une description dans un site expérimental en France constitué de 3 bassins de 20 m2 chacun dont un n'est pas planté. Les lits plantés ont reçu pendant 6 mois 15 kg de matière sèche par m2. | The use of traditional drying beds is more and more abandoned by small communities, due to their high requirements. The article starts with a presentation of the station population by size and treatment type. It is followed by a quick analysis of the costs of operation of traditional drying beds and spreading in agriculture. A bibliography and a description of an experimental site in France made of 3 20 m2 basins, one of which is not planted, are then given. The planted beds have received 15 kg of dry matter per m2 during 6 months.

Description of the wastewater treatment system: drainage basins filled with gravel, where reeds are grown. Presentation of installations monitored by the Cemagref. At Gensac la Pallue, a supply of raw effluent on a first floor receiving an organic load of 30 to 35 g of BOD5/m2 leads to a 70% drop in organic matter and a 85% drop in suspended matter. | Description de la filière de traitement des eaux usées à retenir : bassins drainés, garnis de graviers, plantés de phragmites, alimentés en alternance et par bachées. Présentation des installations suivies par le CEMAGREF. A Gensac la Pallue un apport d'effluent brut sur un premier étage recevant une charge organique de 30 à 35 g de DBO5/m2 conduit à un abattement de 70 % de la matière organique et 85 % des MES.

The French coasts exhibit two kiruJs of eutrophicati.on; the prolif erati.on of green macroa/gae (Ulva sp.) affects the very shallow Mediterranean /agoons as we/1 as a lot of widely open beaches in Brittany, whereas phytoplanktonic mass blooms are to be f ound in front of the large estuaries and in some Atlantic -embayments. Bath these types of eutrophicati.on seem to have increased during the fast twenty years, probably in relation with the increase of the nutrients /oadings from agriculture and sewage. However, the severity of eutrophicati.on is not on/y related to the absolute amount of /oadings. The local residence time of the waters, deduced from hydrodynamics, is a crucial parameter; numerica/ mode/s have highlighted the raie of tidal residua/ circulation. Reduction of eutrophicati.on requires a precise knowledge of the in-situ limiting nutrient. If green macroalgae are c/early nitrogen limited, phytoplankton may be either phosphorus or nitrogen limited. Ecological numerical mode/s may help to quanti/y the effects of various strategies of lowering the loadings of nutrients.