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Contaminated sediments and the circular economy

2023-01-16 Tamara Parkin
The beneficial use of contaminated sediments for agricultural purposes and how this supports a circular economy model for the dredging industry.

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The beneficial use of sediments has been a topic of importance in the dredging sector, particularly as the industry lowers its environmental footprint and adopts strategies that reflect the UN sustainable development goals. Past studies on the beneficial use of sediments have demonstrated their use for habitat development, beach nourishment, erosion control, and as construction materials for civil engineering projects such as port development. 

However, regulations on sediment quality standards or disposal often categorise contaminated sediments as waste. Sediments can contain high concentrations of micro- and macronutrients, which is particularly beneficial when the nutrient it contains has limited reserves. The lack of flexible regulations to facilitate the beneficial use of contaminated sediments has prevented the use of globally available resources at a time of material shortages.

The manner of reuse of sediments is determined by the characterisation of the material, available treatment technologies and the local demand of such material. For example, highly organic sediment has restricted use in construction due to the possibility that the organic matter could negatively impact the quality of the final product.

Reusing sediments, regardless of contamination status, would extend the life cycle of sediments, reduce waste, and create further value for the material. This would then produce a more sustainable model of production and consumption in the dredging and construction sectors, leading to a circular economy. 

Reclaiming lost nutrients 

Dredged sediment is an ideal resource for nutrient rich materials, with phosphorus being a prime example. Phosphorus is an important component of fertiliser for plant growth and is a primary component in fertilisers. Though phosphorus is traditionally sourced from mines, these are few and far between, making supplies of the mined resources vulnerable to geopolitical factors. Furthermore, analysts predict that there will be phosphorus depletion by the end of the 21st century, unless the rate of extraction decreases. 

Given tightening supplies, phosphorus-rich dredged sediment could be a way for the agriculture industry to source much needed nutrients with low environmental impact. In addition to acting as a source of phosphorus, organic dredged sediments can also act as a soil amendment. Furthermore, it can replace the use of peat for propagation of horticultural plantlets in the plant nursery sector - which is being phased out across the EU as the extraction of this material is associated with ecosystem loss and climate change emissions. 

Substituting dredged materials for peat would reduce the need for disposal methods for dredged sediments such as open ocean discharge and landfilling - and lessen the burden of sourcing raw materials. However, using sediments as a source of nutrients creates several challenges, including managing the impact of pollutants in sediments, legislation compliance, societal acceptance, and gaps in knowledge of application. 

Treating cadmium 

It is worth noting that managing the risk from pollutants in dredged sediment relies on proper processes, including appropriate assessment and treatment. Past studies have shown the need to manage the concentration of specific metals in dredged sediment. In particular, Cadmium has been a reoccurring problem when using dredged material for growing crops for human consumption. Crops like lettuce have absorbed high amounts of the element, resulting in concentration values that exceed maximum regulated thresholds. High doses of Cadmium is toxic to humans, posing a health risk when absorbed into edible crops. 

Some national markets have set legislation to limit the amount of cadmium per phosphorus in fertilisers. A study conducted in Sweden in 2021 investigated the use of dredged sediments as a plant-growing substrate to grow lettuce and determined that this could potentially impact human health via edible vegetable crops. Dredged sediments came from Malmfjärden Bay, a shallow, semi-enclosed bay located in Kalmar, southeast of Sweden. The cadmium levels in the substrate used exceeded the maximum metal concentration allowed for soil by the Swedish Environmental Protection Agency (SEPA). Cadmium levels tested in the grown lettuce also exceeded the maximum permissible concentrations.

The study did note, however, that sediment pre-treatment could potentially reduce cadmium uptake. The method of treatment for future use of cadmium-rich sediment was phytoremediation, which is a process to absorb the heavy metals in the soil by using plants and trees to remediate the soil. An alternative method would be the addition of biochar (organic matter that has been carbonised under high temperatures) to the substrates as this will absorb the heavy metals.  

However, the Swedish researchers warned that the treatment of sediment is dependent on the site configuration, whether the contaminant is chemical or physical, treatment goals and local governance. 

Fueling the bioeconomy 

The use of treatments, such as those mentioned in the Swedish study, hold the potential to help with global food shortages and improve the quality of agricultural soil - underpinned by sediment from the dredging sector. A further reciprocity would be in the form of biofuel created from plants fertilised by this dredged sediment, which would create a closed loop economy. 

The need for additional research into the use of dredged sediment is abundantly clear, particularly as the world works to embrace more sustainable practices and mitigate the impact of climate change. Readers keen to know more would do well to read the  2019  CEDA position paper about ‘Assessing the benefits of using contaminated sediments’, which includes information on use of sediment for agricultural practices, habitat restoration, construction and a lot more.

CEDA members working on projects involving the use of dredged sediment who would like to see their work featured in a future edition of CEDA Industry News should contact: or

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