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New life for lakes

Sludge no longer needs to rob lakes of life. A new and inexpensive method swiftly shows the difference between toxic and non-toxic sludge.
Two municipal departments in the Danish city of Aarhus - Nature & Environment and Water & Wastewater - have been working with COWI to develop a method of determining the degree of mineral oil contamination in sludge and other highly organic types of earth.

A 30-year accumulation of sludge is being removed from the bed of three small lakes and a stream in the Marselisborg Forest near Aarhus. In some places the sludge is more than a metre thick and it has been a long time since even the smallest fish has been spotted in these lakes.

Things are about to change

But things are about to change. When the treatment operation ends in the summer of 2006, it’s not just the animal and plant life that will get another chance.

Local inhabitants, who frequent this large recreation area, will enjoy new benefits too.

The results have now revealed that the majority of the sludge is not as contaminated as was first feared. It contains leaves, branches and other completely harmless natural materials.

So rather than depositing it at great expense at special waste disposal sites, the sludge from three lakes will be used to expand Aarhus harbour.

Huge potential

This will save close to 95% on costs, which for this project amounts to almost DKK 3 million.

But perhaps the best thing is that the new method has huge potential for other uses, such as determining the actual level of oil contamination in a range of different materials, and it can be used anywhere in the country.

"Used on a nationwide basis this method could save millions", conclude the two environmental experts behind the method, Helle Ravn-Sørensen of COWI in Aarhus and Kim Gulvad Svendsen, Head of Department of Nature & Environment at the municipality of Aarhus. Helle Ravn-Sørensen is project manager of the lake treatment project.

By Eva Isager
Published: 24.04.2006

The method

To date laboratories have had difficulty separating leaves and other natural organic matter from mineral oil, when analysing sludge samples to determine the contamination rate.

The oil emanates from traffic and it seeps into the lakes via the stream when it rains. Both oil and decomposing leaves contain their own hydrocarbon compounds. When the total volume of hydrocarbons began to exceed Aarhus Municipality’s limits for slightly contaminated soil, the only solution seemed to be expensive disposal of the sludge.

Helle Ravn-Sørensen and Kim Gulvad Svendsen have worked very hard to find a way to prove their theory and they finally combined two or three well-known methods. This combination enabled them to separate the different types of hydrocarbons and provide an accurate breakdown of the contaminants.

LAST UPDATED: 16.09.2016