August 28, 2018

Binding phosphorus into sediment

Binding phosphorus into sediment – Stockholm university

In the project, three pilot examples with heat activated limestone application to phosphorus-rich bottoms will be implemented in coastal bays in Sweden and Finland. The pilots has started with a development phase, risk assessments and selecting of the pilot sites, after which the measure will be implemented. After application, the effects of the measure on phosphorus concentrations as well as on sediments and biota will be assessed. If the pilot tests are successful, the results will be compiled into guidelines for marl application as a method for binding phosphorus to bottom sediments in the Baltic Sea region.

The aim of the project is to reduce phosphorus loading in the Baltic Sea, by using a sorbent, based on marl/limestone residue, that can bind phosphorus into the bottom sediment. The project started with a development phase, in which the treatment method and the dosage of the sorbent was determined. The laboratory tests showed that heat-treated marl with a particle size of under 2 mm was suitable. This material is called activated limestone and has an uptake of phosphate which is approximately 500 times higher than the untreated marl. Based on the results, a dosage of 100 grams of activated limestone per square meter was estimated to be enough to capture 1 gram of phosphorus per square meter.

Activated limestone is spread to the sea bottom to bind phosphorus into the sediment. Picture: Eva Björkman / Stockholm University

The next step was to produce the activated limestone. A rough estimation gave that 30 tons of activated limestone was needed for the three pilots. The production of the activated limestone was done at Cementa in Slite, Gotland, and included design and optimization of the furnace together with the production of the sorbent. The marl was donated by Nordkalk, Storugns, Gotland. Read more about the treatment of marl and about our visit to Cementa AB, where the marl is treated.

The last step is to implement the measure in three different pilot sites. In one, 12 tons of activated limestone has been spread by helicopter, in one of the other sites a boat will be tested. The handling of the necessary permissions for the spreading have taken much longer than planned, which must be considered in the future.

Results from this pilot were presented in the SEABASED Final Webinar on January 26th, 2021. See the presentation behind this link.

For more information, please contact

Sven Blomqvist (sven.blomqvist(at)
Eva Björkman (eva.bjorkman(at)

Stockholm University
Department of Ecology, Environment and Plant sciences