Heavy metal, soft rock
Over the last five years, the Belgian based DEME (Dredging, Environmental and Marine Engineering) Group has been focusing on the need for more successful removal of heavy metals from soil and groundwater, especially where a variety of contaminants are contained. This effort has resulted in two new technologies, which, although entirely different, are complimentary in application. The first is a mineralogical approach that uses metasorb, Soils¹ new granular adsorbent, while the second is an electrochemical treatment using bipolar electrolysis.
The coarser grained metasorb-Water can be applied as an end-of-pipe treatment of process waters or pumping and treatment of polluted groundwater. It is typically used in percolation filters, often set up in series for larger scale products. Metasorb-Water has an average metal adsorption capacity of up to 15%, and can reduce most heavy metals to below 100 microgrammes per litre.
Bipolar electrolysis works by passing an electric current through water as it is pumped through a cell containing two poles. Pollutants are then oxidised or otherwise transformed into chemicals capable of extraction by decantation and filtration.
As a result of successful laboratory studies, Soils were asked by Union Minière, one of the world¹s largest non-ferrous production groups, to carry out a pilot project near their zinc plant in Belgium, where leaching of various metals has caused widespread, low-level contamination of groundwater.
In the first stage of the pilot, Soils designed, engineered and constructed a mobile treatment plant, with a capacity of 20m3/hour, housed within a single standard freight container.
'This project presented a double challenge,' says Stany Pensaert, Soils NV's
project manager in electrolysis systems R&D. 'A plant of this kind had never
been biult before, and the biploar electrolysis process had never been
tested beyond laboratory scale.'
The pilot also provided data for economic analysis. Results confirmed the
laboratory findings: all metals could be removed to below legislative
standards and operational costs were as low as E0.4-0.5/m3.
'The process can be even more cost-effective when treating higher levels of
pollution,' says Pensaert. 'Because high metal concentrations often create
high water conductivity, correspondingly less electricity is requires to
power the operation.'