Oskarshamn is a Swedish port city with a rich industrial history. In the past, a copper production site and battery factory discharged their contaminated waste water into the sea, resulting in the whole harbour basin becoming contaminated with heavy metals, PCB and dioxins. As shipping continues to sail into and out of the port, the polluted sediments were regularly dispersed over the Baltic Sea. Envisan has been entrusted with the remediation work to help clean up the port of Oskarshamn sediments, in what is currently the biggest environmental project ever executed in Sweden.
"This is the biggest Envisan project during the last five years."
In March 2016, Envisan started with the construction of the sediment dewatering and water treatment plant in the port of Oskarshamn. The plant was designed entirely in-house. Project Manager Sofie Herman was involved in the project from the start: “There was absolutely nothing here, just the port and a large asphalt site that we could use. We brought all components with us and set up the plant on site, from filter presses and thickeners to all pipes, scaffolding and steel structures.” The purpose of the whole operation is to decontaminate the seabed in the harbour basin, a surface of 500,000 square metres. For Envisan, this is the biggest project during the last five years.
From sediment to cake
The Petrus Plancius has been converted into an environmental dredging vessel; the draghead of the dredger was equipped with valves to remove the layers of contaminated sediments. For the areas with a lot of debris on the seabed, including stones, the trailing suction hopper dredger Pinta was deployed, which has a higher loading capacity and a bigger pump to pump the sediments into the treatment plant.
Sofie Herman: “The Pinta pumps the dredged sediments into the plant, where initially the sediments are screened so as to separate out all coarser material. Subsequently, the sediments are transferred to thickeners which, through a settling process, makes the first separation between water and sediments. The water is pumped to the water treatment plant, and the sediments onto a silo for conditioning and further dewatering. In the silo, we add a lime suspension to accelerate the dewatering process. The final step in the dewatering process is executed in filter presses, where the remaining water is physically pressed out of the sediments, leaving behind a hard cake. These cakes are then discharged to a licensed landfill.” The filtrate water from the filter presses is decontaminated and partly re-used as process water for the plant, and the remainder partly re-used in the plant as process water, partly discharged into the sea after a quality inspection.
The project started in 2016 and will be completed in 2018. This long programme has everything to do with the winter in Sweden. Kristien Veys: “In November, it already starts freezing. When the dredger supplies the last sediments, these must still be processed onshore. Afterwards, we make everything ready for winter: shielding, insulating, making it waterproof and a technical inspection.” In 2017, we dredged until mid-November. The plant will be re-started in April 2018.
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