Degradation of CHC pollutants in groundwater

On the premises of a former manufacturer of textile machinery, contamination of the soil with volatile halogenated hydrocarbons (VHHC) occurred in the course of use. These pollutants are mainly contained in solvents and were introduced on the site in the area of a chip centrifuge and a solvent storage.

Currently, a groundwater remediation plant is operated on the site, which pumps out polluted water, cleans it and discharges it again. Although such a pump-and-treat process cleans the water very well, it usually has a very long runtime and the achievement of the remediation goal is not guaranteed. This makes long-running pump-and-treat measures very expensive, as there are ongoing operating costs for electricity, filter substrate and expert monitoring.

Biological in-situ remediation is an alternative method for the remediation of CHC contamination. In this remediation method, the natural biodegradation of the CHC pollutants in the soil is stimulated and supported by the targeted addition of various additives such as sugars or alcohols. If there is no or insufficient natural biodegradation potential, cultures of suitable microorganisms can also be introduced into the soil through a so-called inoculum in order to initiate the biodegradation of the CHC pollutants.

At the site, biological degradation products of the CHC pollutants were detected in the downstream of the contamination during groundwater investigations. This suggests that natural biodegradation of the pollutants by microorganisms in the groundwater has already taken place to a certain extent. In order to prove this and to examine the extent to which biological in-situ remediation is possible at the site, MuP Umwelttechnik GmbH conducted laboratory tests to examine the microbiological degradation potential. For this purpose, so-called microcosm studies were carried out in the company's own laboratories. For this purpose, several test series were set up with site water in the laboratory and regularly sampled. The various test series included untreated preparations mixed with nutrients and co-substrate as well as preparations into which a suitable microorganism culture for complete anaerobic degradation of CHC (inoculum) was introduced.

The result of the microcosm study is that in-situ biological remediation is generally possible at the site, provided that the milieu conditions required for optimal biodegradation can be established in the groundwater.

For the implementation of the bioremediation measure at the site, two options for implementation are derived from the results of the laboratory tests carried out:

1. An anaerobic/aerobic process through stimulation of the existing, native microorganisms
2. A strictly anaerobic procedure by adding a suitable microorganism culture (inoculum).

To enable the degradation of chlorinated compounds by microorganisms, anaerobic, i.e. oxygen-free, conditions must first be established for both options. For this purpose, easily usable organic substrates such as sugars or alcohols can be introduced into the soil. In order to continue the degradation of the CHC by the native microorganisms afterwards, the damaged area must be "aerobised" again. This means that oxygen must be added again. Alternatively, the addition of a special microorganism culture (inoculum) can bring about complete degradation (dechlorination) of the CHC pollutants under continuously anaerobic conditions.

The advantage of biological in-situ remediation is that, compared to other methods, the process removes the CHC pollutant load in the subsoil relatively quickly and sustainably.

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