Efficient treatment of pollutants from bottom sediment

To meet the needs of a modern infrastructure in a growing city, Borås Energi and Miljö are constructing a new combined heat and power plant, as well as a wastewater treatment plant, at Sobacken just south of Borås. District heating pipelines are being drawn from Sobacken to the existing Ryaverket, built in the 1930s and centrally located in the city. Pipelines are also being extended to the Gässlösa wastewater treatment plant in Borås.

On the route between Sobacken and Ryaverket lies the river Viskan, through which the pipelines need to pass. The work of laying pipelines through Viskan was carried out in the second half of 2017. Analysis and engineering consulting firm WSP is responsible for environmental control during construction.

The challenge

It was not possible to push the transfer pipeline under Viskan – it just got stuck. Instead, the decision was made to bury the pipeline at the bottom of the river. The problem is that along the sides of the river, where the water flow is slightly lower, there are organic sediments contaminated with substances like dioxins and metals. However, in the middle of the watercourse, the bottom consists of sandy sediment that is not contaminated.

The solution

WSP explored various solutions to clean contaminants during the excavation work on the bottom of Viskan. They concluded that dioxin is a compound that is quite difficult to remove. It is possible to use activated carbon, for example, but it significantly restricts the flow and takes a long time – resulting in higher costs.

WSP found only one method that could effectively meet the purification requirements – Swehydro's environmentally friendly passive chitosan flocculation with biodegradable flocculating agents.

– We conducted a test that showed the filtration worked well, so we decided to use passive flocculation, says Titti Johansson, environmental consultant, and project manager at WSP.

To prevent spreading contaminants from the organic sediment to the free water or sandy sediment, two areas were delimited on each side of the river using sheet piles.

After the excavation work, the water from the sheet-piled area was pumped to a sedimentation container, then to a flocculation facility from Swedish Hydro Solutions, and finally to a second sedimentation container.

– We had a relatively tight workspace, so the equipment setup was crucial. Swedish Hydro Solutions participated in the preparatory meetings to plan the work, and they handled the entire facility. It has worked great.

Once all the work causing turbidity was completed, the sheet piles remained in place for some time to allow any remaining suspended substances to sediment again.

The result

In total, 99,392 m3 were treated from Viskan.

– We are satisfied with the purification levels; we brought down the levels of both dioxins and metals below the limits, in line with the usual free water in Viskan, says Titti Johansson.

The verdict

– I would recommend Swedish Hydro Solutions because their chitosan solution really works. It is also easy to set up. Moreover, there is a lot of knowledge within the company, so we have been able to bounce ideas and discuss with them. They have been a great support in the project.

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