Challenge
In the summer of 2011, HMVT was asked to come up with a practical solution to the purification of 600m³ of contaminated water in an old cellar in Zwolle. At the site of this project a soil remediation and demolition of former business premises was being carried out at the time. The location was severely contaminated with chromium-3, chromium-6 and nickel. Due to the wet summer, the old cellar that was yet to be demolished was full of contaminated water, with concentrations as a high as 400,000 µg/l. A mobile water purification installation based on ion exchange, supplied by a third party, was delivering insufficient results.

Solution
Our solution involved the on-site precipitation of the heavy metals and then pumping off the purified water into the sewer system. The contaminated sludge that ended up at the bottom would then be transported along with the excavated contaminated soil and concrete rubble for processing.

Before carrying out this work, we first collected water from the site and tested it at our lab to see whether we could realise a sufficient return and to determine the blend of chemicals that would be necessary. This was then translated into a practical application.

Results
The first laboratory tests revealed that the water contained primarily chromium-6. The concentrations were not too bad: 25-36.000 µg/l. A three-phase treatment, in which firstly the pH was corrected and then the chromium was precipitated with iron (III) chloride and polymer, produced the best lab results. This allowed us to bring the chromium-6 levels down to below 100 µg/l. We also saw a fine sedimentation and clear discolouration of the water.

We then converted the lab results to full-scale doses necessary for the project. In the field, the water in the old cellar was treated in much the same way as in the lab. As the total amount of water turned out not to be 600m³ but had in the meantime increased to 1,500m³, the water was treated in a number of rounds. And in the field, too, after some time we managed to get the same fine results regarding precipitation: a reduction to less than 100µg/l chromium-6.

In addition to the on-site treatment of the water in the cellar, we also provided at this site the pumps that pumped away the clean water, as well as a minor purification unit to hold back any sludge flakes (sand filtration).