Bugs feed on zinc contamination

Scientists from the United Kingdom-based research laboratory of Shell have joined forces with Dutch experts to solve the problem of contamination from the site of one of the world’s most efficient zinc plants.

Their solution is to use bacteria. The Budelco plant in Holland, which produces more than 200,000 tonnes of special high-garde zinc annually, stands on a site which was once used to produce zinc by an old process.

The soil and groundwater under the site have been contaminated by zinc, cadmium and sulphate, and a study has shown that the contamination would eventually spread outside the site.

To prevent this spread, a geohydrological containment system was installed. From 12 strategically situated wells, the system pumps the contaminated groundwater to the surface, so preventing it moving outside the site. But the need to remove the heavy metals and sulphate, once the water is brought to the surface, posed a problem.

Scientists considered three options before turning to a family of bacteria, namely sulphate-reducing bacteria (or SRBs), to do the job. The Shell team was asked to make a detailed study of the bugs to establish whether or not they could clean up the contamination.

“The natural habitat of the SRBs could be mimicked in an oxygen-free aqueous environment — an anaerobic bioreactor,” a Shell spokesman said. “We needed to know whether these bacteria, fed with the appropriate organic compound, would reduce the sulphate in the Budelco groundwater to sulphide and precipitate the heavy metals so that they could be recovered. “Our laboratory studies revealed that the SRBs could perform the required task and Budelco built a 9-cubic-metre demonstration unit at its site. The success of this project led to the installation of a larger integrated pilot plant.”

A full-scale water purification plant incorporating the SRB process is now operational at the Budelco site, treating 250 cubic metres of contaminated groundwater per hour.

When it enters the plant, this water contains about 400 milligrams per litre of sulphate, 50 mg per litre of zinc and 1 mg per litre of cadmium. After treatment, each litre of water contains fewer than 200 mg sulphate, 0.3 mg zinc and 0.01 mg cadmium.

Further research is trying to discover ways to reduce processing costs. A possibility is to find a less expensive alternative to the ethanol that is used as the feedstock for the bugs.