Biological soil treatments

In soils, pollutants are distributed according to their density, solubility in water, volatility and capacity to adsorb onto the solid matrix of the porous medium, between the unsaturated zone, which corresponds to the soil layer above the piezometric level, and the saturated zone, which constitutes the aquifer.

Compounds found on polluted sites are of both organic and inorganic origin. The main organic pollutants are hydrocarbons (fuels) and halogenated products (chlorinated solvents, polychlorinated biphenyls, polychlorinated phenols).

Inorganic pollutants are compounds based on heavy metals such as lead, mercury, zinc and cadmium...

The diversity and adaptability of micro-organisms (bacteria, fungi, yeasts) mean that they are naturally present in soils, attached to the solid medium or suspended in the residual water of the saturated zone or groundwater. Their variable numbers can be estimated at between 10 ⁵ and 10 ⁹ micro-organisms per gram of soil. Microorganisms are capable, after adapting their metabolism, of degrading a wide variety of natural or xenobiotic products.

Biodegradation of a product may be partial, meaning that micro-organisms have transformed it, often through oxidation mechanisms, or it may be total. In the latter case, we no longer speak of biodegradation, but of mineralization, since the products of microbial action are essentially carbon dioxide and water.

A single micro-organism does not possess all the enzymes needed to degrade a multitude of products, as is the case with petroleum product pollution. This is why, more often than not, degradation is achieved by a community of several species whose actions are complementary.

As soils are rarely sterile, the microflora in place when it encounters pollution will, depending on the chemical and physico-chemical conditions of its environment, seek to metabolize the pollutants. All these phenomena are referred to as natural attenuation. Biological treatments use and stimulate the ability of micro-organisms to use various types of organic and mineral compounds for their growth, energy requirements and life support.

An initial distinction will be made between biological processes that have already been validated and those still under development. The former are divided into two sub-groups: in-situ treatments and ex-situ treatments, which involve excavating polluted soil before decontaminating it on or off site.