In the early 20th century, local factories in Flanders produced felt with a process that entailed using mercury and over the decades the soil and the groundwater at most of these factory sites became heavily polluted.
During the remediation of one of these former felt factories in Lokeren, Belgium, the soil had to be excavated to a depth of 4 m, which meant that the groundwater level had to be lowered. As very high levels of mercury - up to 5000 µg/l - were found in the groundwater, it had to be treated to an acceptable level of 2 µg/l before being discharged in a nearby sewer.
After an extensive testing period in the laboratory with specialised equipment, the best treatment method was identified to achieve the required standard and crucially, it was also economically feasible.
First a commercial complexant was dosed into the area and this was followed by microfiltration. The complexant converts the dissolved mercury into a very fine solid suspension that can then be filtered out by microfiltration.
However, the fine solids quickly start blocking the membrane, reducing the capacity of the installation. Hence, to ensure sufficient flow, the membranes are regularly backwashed.
To reduce noise to the neighbouring houses and to increase the mobility of the installation for future use, the microfiltration filters and accompanying equipment were installed in a special, insulated container. This had the added benefit of keeping the filter free of frost during the winter months.