In the extensive rock excavation project Johannelund – a part of the E4 Stockholm Bypass connecting northern and southern Stockholm under Lake Mälaren – fine-grained injection cement was used to keep intruding groundwater away from the tunnels. However, this type of fine-grained cement results in a high content of suspended material in the drainage water and also contains chromium, for which the requirement levels were recently lowered by 80 percent.

Challenges in Water Treatment

Conventional treatment techniques can face challenges in handling turbid water with suspended material from fine-grained injection cement. Additionally, being able to effectively remove chromium poses another challenge. Therefore, the Swedish Transport Administration began exploring treatment techniques beyond the traditional ones.

– We needed a technology to separate the fine particles and reduce the chromium levels. That's when we became aware of chitosan flocculation technology, says Linn Elmlund, formerly an environmental specialist at the Swedish Transport Administration, who, through the Västlänken project, got in touch with Swedish Hydro Solutions.

Chitosan's Role in Reduction

Chitosan is a highly effective flocculating agent and causes fine-grained injection cement to coalesce into flocks. At the same time, it reduces the amount of total chromium in the water since the majority of chromium is particle-bound and attaches to the flocks. However, the more toxic hexavalent chromium is not affected as it is highly water-soluble. Recognizing hexavalent chromium as a growing problem, Swedish Hydro Solutions had already started developing and testing, on a smaller scale, an additional treatment step called RedOx3. It is a liquid reducing agent that efficiently reduces hexavalent chromium to trivalent chromium.

– Trivalent chromium is less toxic and, above all, less water-soluble. Therefore, we can subsequently precipitate the chromium with additional treatment chemistry and then flocculate the particles with chitosan, says Andreas Pettersson, project manager at Swedish Hydro Solutions.

After an initial test in the Swedish Hydro Solutions laboratory, the Swedish Transport Administration decided to adopt the technology.

Handling High Water Flows through Collaboration

Since the contractor Implenia already had a water treatment facility in place, it evolved into a collaborative project. Implenia, the Swedish Transport Administration, and Swedish Hydro Solutions worked together to address water treatment. They connected Swedish Hydro Solutions' equipment to the project's existing water treatment facility, consisting of a container for pH adjustment, a container with dosing of RedOx3, and two sedimentation containers.

– Combining our systems gave us the best result while being able to manage treatment on the limited space available, says Linn Elmlund.

The collaboration among the three entities soon proved to be a strength for the project. The water flow was higher than initially estimated. In six months, they treated more than 200,000 cubic meters of water – an average flow of 7,700 m³ per week or 46 m³ per hour. This is equivalent to about three tanker trucks per hour, around the clock, seven days a week, totaling 13,000 tanker truck trips – one way.

Thanks to the collaboration, they could handle the high flow, which could temporarily reach over 60 m3 per hour. By installing backup pumps, activated during exceptionally high flows, they could redirect water to Implenia's sedimentation containers, integrating them into Swedish Hydro Solutions' flocculation process.

– It has been easy to connect our different treatment systems. When we disconnected Swedish Hydro Solutions' facility, it has been straightforward to reconnect, says Therese Rönnkvist-Mickelson, Country Head Sustainability Sweden at Implenia.

Despite the water flow being very high during the most intense periods, with elevated chromium levels, they managed to halve the total chromium concentration, and hexavalent chromium decreased by a remarkable 82 percent.

– The guideline for total chromium is set at 10 micrograms per liter. In the latest analyses, the result was around 1–3 micrograms per liter. That is very good, says Linn Elmlund, a sentiment echoed by Therese Rönnkvist-Mickelson:

– The treatment technology is fantastic! I am very impressed with the treatment of hexavalent chromium and total chromium. These are precisely the types of solutions we want to work with, where we can treat process water without adding hazardous chemicals.

The treatment results have pleased many, especially Stockholm Water and Waste.

– Stockholm Water and Waste were concerned about chromium levels. But now they are calm and very satisfied with the chromium treatment.

Swedish Hydro Solutions is also pleased.

– These are the largest water flows we have ever handled through a single treatment facility, but we have still lowered the concentrations very well, says Andreas Pettersson.

– We at Implenia are very satisfied with the project, especially with the results. Swedish Hydro Solutions has been very helpful with the questions and concerns we have had to make the system work optimally.