Sediments in Valdemarsvik Port, Sweden were heavily contaminated with chromium, and to a lesser extent mercury, following the earlier discharge of untreated wastewater from the former Lundbergs Läder tannery. In operation between 1873 and 1960, Lundbergs Läder was one of the largest tanneries in Sweden. Chromium had been used to soften the leather, which lead to the site producing up to 250 kg of chromium emissions a year.
At the site the polluted sediment from the fjord was dredged using an excavator and transported to shore using barges. After treatment, the sludge was deposited in a landfill on site.
During the site activities two separate flows of water were identified and had to be treated – a flow from the barges and from the landfill.
After transport, the sludge had settled in the barge, leaving a layer of water on top, which had to be pumped off. This flow contained large amounts of suspended solids (up to 10 g/l) but also small amounts of heavy metals. Additionally, some leaching occurred from the treated sediments. This leachate was collected in a ditch and mixed with rainwater that fell onto the landfill. The landfill water had a high pH (up to pH 13) and contained suspended solids.
Given the difference in quality and quantity of the two water flows, they were treated in two separate lines, which eventually converge into a large settling basin. The treatment of the barge water mainly focused on the removal of suspended solids. It consisted of a coagulation followed by several neutralisation processes and a flocculation with a poly-electrolyte. This coagulated the suspended solids into flocks that then settled in a lamella separator, resulting in an overflow of clear water at the top of the separator. The sludge that settled in the separator was mixed with the sludge from the barges and treated accordingly.
The treatment of the landfill water concerned the removal of suspended solids and the reduction of chromium.
The landfill water was pumped from the ditch into an open, 20ft buffer. Following neutralisation phases to reduce the pH level, poly-electrolyte was dosed to conglomerate the coagulated particles into flocks. The flocks settled in the settlement basin, where the water was also mixed with the treated barge water.
From the settlement basin, the water mixture was pumped to a sand filter for the removal of any remaining suspended solids and discharged.
The entire installation, including the large lamella separator, was containerised. This made the installation easy to transport and mobilise, which is perfect for short-term projects. It also helps protect the installation from the weather.