As issues of land and water pollution become more serious, innovative developers are looking for new ways of turning contaminated land into available land. In Sweden the Port of Gothenburg is using innovative techniques to reuse polluted sediment and clean displaced water in order to build a new freight terminal using clay dredged from the seabed.
Back in the days before environmental awareness became the norm, it was not uncommon for sea and river beds to become contaminated by a variety of harmful substances. In Gothenburg, the clay at the bed of the Göta Älv River contains a range of pollutants including TBT (tributyltin), an environmental toxin that can disrupt hormone levels. Until 2008, TBT was widely used in antifouling paints on the hulls of vessels.
When the Port of Gothenburg set about planning a new freight terminal, it decided to build it using clay spoils from Göta Älv. However, due to the presence of contaminants in the mud, including TBT (tributyltin), significant volumes of residual water need to be pH-neutralised and purified before being released back into the sea.
The Port of Gothenburg is strategically located on the west coast of Sweden. Approximately 70% of Sweden’s total population and industrial capability is located within a 500-km radius of the port, which currently manages close to 30 per cent of Swedish foreign trade and operates more than 130 direct services to destinations in Europe, Asia, the Middle East, Africa and North America.
“The material we’re working with is alkaline and the high pH level creates a greater risk of the TBT being freed and released into the environment,” explains Kristina Bernstén, environmental project manager at the Port of Gothenburg.
When it comes to pH, a level of seven is neutral. Anything below six is acidic, while anything above eight is alkaline. Both extremes are potentially harmful to people, animals and the environment. Due to the stricter environmental regulations in place these days, wastewater can only be discharged into outlet channels if it is within a narrow pH range around the neutral point.
In the case of the Port of Gothenburg, the displaced water from the Göta Älv clay spoils is being pH neutralised using a technology from Linde known as SOLVOCARB, which uses carbon dioxide (CO2) gas to neutralise the alkaline waters. When dissolved in water, carbon dioxide forms carbonic acid and reduces the pH value to the appropriate level.
“By maintaining the pH of the displaced water at a neutral level and by purifying the water through subsequent sedimentation and filters, we ensure that the TBT remains safely bound up, thereby making it inaccessible to the aquatic organisms in the water,” she continues, adding that this is one of the main reasons why the displaced water has to undergo several purification steps before being released back into the sea.
Carbon dioxide can be used for pH control across a variety of industries from food to textile, pulp and paper, chemical and petroleum. In addition to wastewater treatment, it can be used to pH control process water and for increasing the alkalinity and remineralisation of drinking water.
Whereas most pH-neutralisation solutions use mineral acids to reduce alkalinity, Kristina Bernstén says CO2 is a safer, more sustainable option.
“Using CO2 for pH adjustment comes with two major advantages: Firstly we can avoid potentially reducing the pH in the water so much that it becomes too acidic. Secondly, it is far safer from a working environment perspective, since handling acids can always be a risky business,” she explains.
The new freight terminal at the Port of Gothenburg, which is being built on an area measuring some 220,000 square metres, is scheduled to be operational in 2024-2025.
Text: Isabelle Kliger