JD-Contractor A/S restores the deepest lake in Denmark

JD-Contractor A/S has a contract for restoring Furesøen over the next 10 years. The project can already be described as a success.

We are working with some of the best biologists in this field and we are therefore capable of delivering and carrying out complete oxidation systems to lakes as well as inlets.

The lake Furesøen is the deepest lake in Denmark. At the deepest parts the depth is very close to 40 metres. Around the year 1900, Furesøen was one of the clearest lakes in Europe with a magnificent variation of seaweed, algae and moss. During the century Furesøen became more and more polluted as a result of the increasing population in the region lack of sewerage, which resulted in a lot of untreated wastewater being sent out into the lake. In the sixties it was usual to see clouds of dishwashing detergent sail along the River Mølleåen. In the late sixties a building of sewage treatment works started and sewage water, from population centres in the region, was redirected into the Øresund. This has improved the conditions in the lake substantially but still today the lake suffers from the malpractice in the past. There are among other things large sediments of phosphor on the seabed. To speed up the recovering it was decided to start a treatment with pure oxygen to the deepest parts in the lake. In the winter of 2002 there was put out a competitive tender for the building and running of a system of diffusers, which from a tank on the shore area would be able to supply oxygen to the lake.

JD-Contractor A/S who specialise in the laying of pipes on the seabed made an offer and in heavy competition with a series of contractors we won the tender. We won because we have experienced staff and the equipment needed to carry out work at these depths and also because we together with our subcontractors created a team of the best specialists in the country for this kind of job, which also contains a great deal of innovation.

In March 2003 the work started with the construction of a landing stage for the fishermen who through the next years are going to remove a lot of unwanted fish from the lake. Soon after the work was initiated on the tank yard and the pipes led down to the distribution well, from where the three pipes are taken out to the three deepest positions, the “Kalvene”.

From the distribution well the three pipes are laid out into the diffuser area. In order to protect the pipes, they were buried 1 meter down into the seabed.

The diffuser system consists of three manifolds from where 5.2 km of rubber hose with a minute perforations are distributed on the seabed. The oxygen is released through the holes in the hoses. The bubbles are so fine that the oxygen is already absorbed in the water after a few metres.

The manifolds are placed the centre of the deepest holes in the lake from where they will oxygenate the water and contribute to an increasing decomposition of the unwanted deposits. In this way a quick clarification of the water and a flowering of the favourable species of seaweeds in the lake are gained. It will start a favourable development of the lake and bring it back to the conditions of a hundred years ago.

In the deepest holes remains a survived population of crustaceans from the time when Furesøen was isolated as a lake during the ending of the ice age. They are dependent on a low temperature and are not able to leave the holes in the summer when the oxygen content in the water decreases. To them the cleaning of the water will give a huge improvement in their living conditions and secure their existence in the centuries to come.

The oxygen started to flow into the water in the middle of May 2003. The oxygen treatment in the lake will continue up to 10 years but with a reduced need for oxygen every year.