Careless handling of chemicals repeatedly led to the contamination of soils in the past. The consequences were often only seen in excavation work for construction. Every former site of industrial or commercial activities has finally to be considered as being contaminated.

Comparably serious, however, are the effects when seeping storm water washes chemical substances, e.g. harmful halogenated hydrocarbons, down to groundwater bodies. This has caused a serious threat to drinking water supply in many places. In order to avoid the worst the contaminated groundwater had to be pumped off on a large scale and had to be purified by costly procedures consuming huge amounts of energy and resources.

Traditional methods

Traditionally there are several methods available for groundwater purification, e.g. the adsorption of pollutants to activated carbon and the stripping of substances from water into the air – followed by an adsorption to activated carbon. These treatments have the big shortcoming that they do not destroy the polluting substance, but only transfer it from one medium to another. Other energy and resource-intensive steps are therefore necessary to completely eliminate the pollutants.

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Smart bacteria

For 25 years already, science has known bacteria that are able to completely decompose synthetic – even halogenated hydrocarbons – into harmless compounds like water, carbon dioxide, and table salt. These bacteria are already being used in the biological steps of industrial wastewater treatment plants. “When we started our work for biological groundwater purification at the beginning of the 90ies we entered into a completely new field. In addition, the concentration of contamination in groundwater is 10,000 times lower than in the effluents of an industrial plant,” explains Dr. Markus Thüer, the head of the group Environmental Technology at Ciba Spezialitätenchemie, and he continues: “The question arose whether a biological degradation in concentrations of a few micrograms per litre – in the ppb-range – is happening at all.”

Extensive lab and pilot tests showed that specialised micro-organisms from landfills and industrial wastewater treatment plants are indeed able to completely decompose halogenated hydrocarbons often found in groundwater even in the trace range under aerobe conditions. And this in lingering periods of less than twenty minutes! Researchers work with genuine groundwater and test different reactor types already operating on large technological scale under field-like conditions. Before the groundwater reached the bio-reactors, metals like iron and manganese were oxidised with air oxygen and subsequently eliminated by filtration. “Although the biological activity set in after a few weeks only, the results were overwhelming as to degradation and turnover rates,” is Markus Thüer happy to say. “Even the 1,2,4-trichlorobenzol could not be traced in the treated water.”

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Practical tests passed

To verify the findings the method was tested in real life in groundwater near a former pharmaceutical production plant in Lübeck, Germany. 1,2-dichloroethane used for the extraction of pancreatin from calf pancreases had for decades seeped through leaking pipes into the ground and subsequently into the groundwater, where the concentration of the substance amounted to up to 60 mg/l.

To eliminate this problem, a procedure with two sand filters followed by activated carbon filters was chosen. The amount of activated carbon used was very high due to the poor adsorption properties of 1,2-dichloroethane.

Project leader Dr. Gerhard Stucki from Ciba Umwelttechnik describes the new method: “We inoculate the carbon filters with a specific culture of bacteria. To increase biological decomposing properties, we complete the procedure with a superposed rotation trickle body. In the new procedure the main load of 1,2-dichloroethane is decomposed by bacteria on the surface of the rotation trickle body, while the sand and carbon filters only retain a police function. “In the last seven years of the cleanup we could do without changing the carbon, even though the mean concentration of contamination in the groundwater sometimes rose from 4 mg/l to up to 20 mg/l,” is Stucki glad to say and continues: “The degradation rate of the procedure is above 99.5 percent; the effluent contained less than 10 µg/l of pollutants. During the past ten years of uninterrupted cleansing more than 2,000 kg of pollutants were biologically converted and mineralised.”

The application of this new method also helped to save considerable amounts of resources and thus of funds. The treatment costs for one cubic metre of contaminated groundwater dropped from about € 1.53 to € 0.25. Thus, € 1.017 million were saved during the cleanup period thanks to the biological solution.

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Other applications

Meanwhile this ecologically efficient method has been patented in Europe and the USA and has received the 1997 environmental prize of the European Fair for Environmental Technology (Europäische Messe für Umwelttechnik, M.U.T).

This success naturally motivated the researchers to use this procedure in other contaminated groundwater bodies and also to extend it to contaminated soils. An underground lake that had been polluted by a metal processing plant with 2 to 15 mg/l of tetrachloroethylene (PER) was thus cleansed successfully. Markus Thüer: “In this multi-step process we used micro-organisms in different redox potentials and could thus purify 1.5 cubic metres of groundwater per hour. A sand filter served as denitrification reactor, the reductive dechlorination took place in a methanogeneous solid bed reactor and a biologically active, aerobe carbon filter mineralised the metabolites completely.” Decontamination of soils by bacteria is still in the pilot phase.

Gerhard Stucki explains the status of research as yet: “In co-operation with the Institute of Plant Biology/Microbiology at the University of Zurich, we are presently preparing a procedure to clean up soils contaminated by chlorophenol for practical application. Chlorophenols were used for a long time in agriculture and served as intermediate products in the chemical industry. What we do is combining an alkaline soil extraction with a subsequent biological mineralisation of the pollutants in the solid bed reactor.” More work has certainly to be done until the procedure can be applied in practice, but the scientists are optimistic about the results. There will definitely be no lack of applications in the future: in Switzerland and Austria alone there are thousands of sites suspected to be contaminated that have to be cleaned up sooner or later.
(Source: aqua press Int. 06/2000)

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Information & contact:
Ciba Spezialitätenchemie
Schweizerhalle AG
Dr. Markus Thüer
Mailbox 1130
CH–4133 Pratteln 1
Tel. +41 61 636 92 51
Fax +41 61 636 93 29