The Fight against Eutrophication in the Inlet of “Odense Fjord” by Reaping of Sea Lettuce (Ulva Lactuca)

Odense, which is the third largest city in Denmark, is situated at the head of the inlet of “Odense Fjord” - a shallow inlet area of 54 sq km. Including the discharge of industrial wastewater the total wastewater load corresponds to approx. 400,000 p.e. which is treated in a mechanical/biological treatment plant. There is today a considerable plant production in the inlet, especially of macrophytes, for instance Ulvalactuca, and at the moment the Municipality of Odense is extending its wastewater treatment to include the removal of nutrient salts, nitrogen and phosphorus. Calculations of the total discharge to the inlet show that 80 per cent of the nitrogen and 20 per cent of the phosphorus load are derived from the catchment area which has an intensive agricultural production. In an attempt to reduce this discharge, tests have been made over a number of years with reaping of Ulvalactuca in the inlet, and calculations of the effects of the reaping have been carried out. Experience shows that there is a need both for an increased knowledge of how to optimize the growth of Ulvalactuca and for development of a second generation of reaping machines.