Text: Dorte Larsen, Communications Specialist, Aalborg University
Photo: L.Studio

The rePair research project in Ormstrup Lake, Viborg Municipality, aims to develop a gentle method for dredging and cleaning lakes. Ormstrup Lake, with its high phosphorus concentration and poor water quality, is an ideal candidate for developing new restoration methods, as it resembles many other Danish lakes.

The project has developed new technologies to remove nutrient-rich sediment, separating it into two fractions:

1. Purified lake water
2. Phosphorus and carbon, which can be used for fertilizer and soil improvement

The need for a standard solution
Denmark needs standard solutions for lake restoration, as almost all the country’s 1,000 lakes are in poor ecological condition. Only five lakes are in good ecological condition and meet the EU’s Water Framework Directive. The rest suffer from oxygen depletion, algae blooms, and fish deaths.

The problems in the lakes are due to various factors, such as phosphorus discharge from agriculture, treatment plants, stormwater overflows, unsewered properties, aquaculture, and industry being the biggest challenges.

The rePair project in Ormstrup aims to develop a gentle standard solution for cleaning all types of lakes in Denmark. The most used method has been to add chemicals to the lake to bind phosphorus in the sediment, but a long-term solution is to remove the sediment so that phosphorus can be reused.

Robotic vacuum cleaner
For lake restoration in Ormstrup, a robot has been developed to gently suck up sediment from the lake bottom. Previous cleaning projects used a rather harsh method, removing sediment with a large excavator, which not only removed the sediment but also damaged the lake bottom and vegetation, leaving the lake’s ecosystem in poor condition. Therefore, a standard solution for all lakes is being developed, one that can be used at various depths, and controlled by sonar. The robotic vacuum cleaner can handle depths between 1 and 6 meters. In Ormstrup Lake, work is being done down to a depth of 6 meters.

Researchers estimate that six tons of phosphorus can be extracted from Ormstrup Lake, which can be used as agricultural fertilizer. The sediment from the dredging can also be reused instead of just being deposited, as has been customary. The sediment can be used in cement and brick production, among other things.

How is sediment dewatered?
The sediment must be dewatered as much as possible, which Associate Professor Morten Lykkegaard Christensen from Aalborg University’s Department of Chemistry and Bioscience is involved in. He explains:

Today, synthetic polymers are added to make the water easier to remove and to retain particles when filtered. AAU researchers have also investigated the use of natural polymers, which show results as good as synthetic ones. It is also explored whether suitable polymers can be produced from wastewater and used for this purpose.

Testing the purity of lake water before returning it
After dewatering and handling the sediment, the purity of the remaining water is tested.

Researchers conduct ecotoxicological tests on the water remaining after phosphorus has been removed. Fortunately, the results of these tests show that it is safe to return the water from the sediment to the lake.

The remaining sediment is used as a solid fertilizer product. In some lakes, the sediment will be contaminated with heavy metals and organic pollutants. The project is developing technologies to treat and clean contaminated sediment so that phosphorus can also be reused here.

Morten Lykkegaard Christensen states:

"In the rePair project, we have gained new knowledge and developed new technologies that can be used as the future standard method for cleaning lakes. We need to move away from patch solutions where chemicals are added to the lake to bind phosphorus to the lake's bottom.

We have proven that we can gently dredge the sediment and remove the water by adding natural polymers. By sucking up the sediment and separating the phosphorus, we can utilize the sediment for useful purposes, just as the valuable phosphorus can be reused as a fertilizer product. We are ready to work on restoring the rest of the polluted lakes in Denmark."

The state of Danish lakes
In Denmark, nearly 1,000 lakes are polluted, and only a handful are in good condition, even though Denmark has committed to the EU’s Water Framework Directive from 2000 to restore the water environment in lakes, rivers, and streams by 2027.

The Ministry of Environment has previously launched a work program for the water area plans 2021-2027, which aims to ensure cleaner water in Denmark’s coastal waters, lakes, rivers, and groundwater following the EU’s Water Framework Directive. The water area plans include 986 lakes, of which only five are in good condition both ecologically and chemically.

Project details about rePair

• The rePair research project runs from January 2021 to March 2025.
• Funding: The Poul Due Jensen Foundation, a sub-project in the Lake Stewardship Project.
• Participants: Aalborg University (AAU), Aarhus University (AU), Technical University of Denmark (DTU), Danish Engineering Service (DIS), and University of Southern Denmark (SDU)
• Participants from AAU: Associate Professor Morten Lykkegaard Christensen and Associate Professor Jens Muff, Aalborg University Esbjerg
• Project leader is Ole Wolf, Poul Due Jensen Foundation.

Contact:

• Morten Lykkegaard Christensen. Associate Professor, Department of Chemistry and Bioscience, Aalborg University. Tel. 99 40 84 64 / 21 62 72 16, mlc@bio.aau.dk
• Press contact for Aalborg University: Tor Bagger. Journalist, Aalborg University. Tel. 30 17 31 65, toba@adm.aau.dk