Chlorocarbons and chlorohydrocarbons were long considered harmless to the environment, and were therefore used in considerable quantity as cleaning or degreasing agents in many branches of industry. As a result they are now a frequent cause of contamination in soil and ground water. Together with the Institute for Sanitary Engineering and the Institute for Microbiology at the University of Stuttgart, the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Stuttgart has developed a biological process for purifying the ground water. The cleaning-up is done by special microorganisms: they are capable of splitting the chlorine atoms from the compounds and converting them to harmless substances.

"The biggest problem was to find suitable organisms", explains Dr. Dieter Bryniok, project manager at the IGB. "Many processes fail because the bacteria are poisened by the epoxides which are formed when they break down the chloroethenes. Our organisms possess a special enzyme to prevent this." The next step was to find the right substrate for them. This serves as a source of carbon and energy for the microorganisms. The Stuttgart researchers use ethene (ethylene), whose structure is very similar to that of a chlorinated ethene and is degraded in the same way.

The IGB's purification process for ground water contaminated with tetrachloroethene and trichloroethene takes place in two stages. The contaminated water is first passed through a fixed-bed reactor, where the bacteria split off one or two chlorine atoms from the chlorocarbons to produce harmless chloride; this takes places under anaerobic conditions – without the use of oxygen. The next step, the aerobic stage, takes place in a percolation reactor which is supplied with ethene. Here, the microorganisms completely degrade the dichloroethene which was formed in the first stage, together with any vinyl chloride (monochloroethene) present in the ground water. With their bioremediation process the IGB researchers avoid the main difficulty with conventional methods of cleaning up soil and ground water: these leave large quantities of contaminated activated charcoal that must be disposed of somehow. In the long run, it should be possible to clean up numerous ground water aquifers using this environment-friendly process.