REFERENCES

Context

In a car garage, a buried hydrocarbon tank impacted the soil (schiste silt) and the groundwater (GWL:5,5 m-bgl), contaminated with mineral oils (C16-22) and BTEX. The consultancy planned the realization of a venting coupled with a bioremediation for the treatment of the volatiles in the unsaturated zone. For the treatment of the groundwater, the hydraulic permeability being too low to pump efficiently the water and treat it, so it was decided to treat the groundwater in situ by injections.

Injectis intervened to advise on the products to be injected to find the best possible treatment with the best cost/benefit/duration ratio. The 4 options suggested were a sorption on activated carbon combined with anaerobic biological oxidation, an in situ chemical oxidation (ISCO), an anaerobic biological oxidation (ABO), a combination of the 2 techniques (ISCO+ABO).

Reaction

This combination ISCO+ABO has been chosen as the most suitable for the project. This choice allows a fast and efficient action on the pollutants, while strongly reducing the oxidant dose necessary for remediation by combining the further treatment by biological degradation in anaerobic conditions. Although rapid, ISCO, realizing alone, required the injection of a large quantity of oxidant, imposing two interventions and high costs. ABO alone did not ensure a sufficiently fast treatment of BTEX, the most mobile and harmful contaminants, implying a longer treatment.

Reagent

Sodium persulphate (Na2S2O8) is a strong oxidant (redox potential > sodium or potassium permanganate or sodium percabonate). It is effective to treat hydrocarbon pollution such as fuel oil and diesel (in light to medium fractions). The oxidation reaction of the contaminant releases sulphate. In the case of low permeability aquifers and where there is few o no oxygen (or where the bacteria themselves have already consumed all the oxygen) so that the bacteria can not continue to degrade the pollutants aerobically, the presence of sulphate is very important because it will be used instead of oxygen as an electron acceptor and will allow the micro-organisms present to continue to degrade the pollutants biologically. Moreover, it has the advantage of being very soluble (more than O2) and easily available to bacterias.

In addition to this first chemical degradation pathway (ISCO), the biological degradation pathway described is also used, in this case we speak of anaerobic biological oxidation.