Miljøprojekt Nr. 1195 2007 – In situ biologisk nedbrydning som oprensningsmetode ved Høfde 42

In situ biologisk nedbrydning som oprensningsmetode ved Høfde 42

Summary and conclusions

The substances at the Høfde 42 (Breakwater 42) site that pose the greatest risk to the surroundings are all biodegradable and suited to biological in situ cleanup. One of the components, sulfotep, has not been tested experimentally, and it is therefore recommended that an experimental degradation study is carried out to ensure that sulfotep also is sufficiently degradable.

In accordance with the survey based on literature, the chemical analyses, which have been carried out as part of the investigations performed, show that degradation either is ongoing or has taken place with respect to parathion, methyl parathion, malathion, diethyl thio phosphoric acid and diethyl dithio phosphoric acid.

The pH at the site is very low in some places which potentially can inhibit biological degradation. It is recommended that laboratory experiments are carried out to determine the influence of pH on the possibilities for biological in situ cleanup. This should include degradation tests at various pH values, bacterial counts and the determination of genes related to parathion hydrolase.

Contamination levels may at some places be so high that this will inhibit the degradation. It is also recommended that investigations are carried out to clarify this potential problem.

The nutrients N, P and S are probably present in sufficient amounts such that they will not obstruct an in situ biological degradation process.

Stoichiometrically, it will require 12.9 x 106 mol oxygen or 412 tons of oxygen to metabolize the 250 tons of contaminants. Supplying 412 tons of oxygen dissolved in water will require the circulation of 10,000 m³ oxygen saturated water every day for 10 years, or 250 m³ of air every day for 10 years. Therefore it is recommended to supply oxygen in the form of air, either through bioventing or air sparging or a combination of the two.

Under simplified assumptions a project sketch has been drawn up encompassing injection of air below the groundwater table and above the dividing clay layer through approx. 1,000 rammed ventilation borings. The estimated cost for the establishment of such an installation is approx. DKK 4 million, while the running costs for the use and maintenance of the installation for 10 years are estimated to approx. DKK 16 million, primarily made up of energy costs. In the project sketch it is assumed that the surplus air does not have to be cleaned. This might prove necessary and should be investigated as part of a pilot study.

As the basis for the design of a full scale installation, a pilot study is suggested. This would include the determination of the degradability of sulfotep and the influence on the degradability of all substances of too high concentrations and/or too low pH. The design parameters for the full scale plant should also be determined as part of the pilot study. The pilot study should be carried out on a small area within the contaminated site. In the pilot study, various versions of the filter part of the air injection installation should be investigated together with various running methods for the installation, e.g. with respect to injection rate pressure to obtain the optimal ratio of influence with the minimal vaporisation of oxygen and contaminants to the atmosphere.