New soil remediation methods: progress and prospects

For decades soil remediation meant one thing in practice: dig out the contaminated soil, take it away and replace it with clean material. Effective, but expensive, disruptive and — paradoxically — a burden on the environment through transport and the need to deal with the waste somewhere else. The direction of the industry is therefore clear: clean up where the contamination arose.

Biological methods — slow, but the gentlest

Bioremediation accelerates and optimises the microbial breakdown that occurs in the ground anyway. Microorganisms decompose hydrocarbons; the contractor's job is to give them the right conditions — aeration, moisture, temperature and nutrients.

The related technique of phytoremediation uses higher plants, through phytoextraction, phytostabilisation, phytodegradation, phytovolatilisation and phytofiltration — all based on the ability of plants to take up contaminants through their root system.

The advantage is minimal interference with the structure of the ground and a low unit cost. The limitation is time: the process runs over seasons, not weeks, so it rarely suits a project with a tight schedule.

In situ oxidation and soil washing

Where time matters, chemical methods that work without excavation come into play. Chemical and photochemical oxidation break petroleum substances down into simpler, less harmful compounds. Soil washing flushes contaminants out with a solution that is then recovered and treated.

Effectiveness depends above all on the permeability of the ground. In sands the reagent reaches everywhere; in clays and silts it can bypass whole zones, leaving untreated pockets behind. That is why the results of ground and water condition testing determine the choice of method as much as the type of contamination does.

Thermal desorption and physical methods

Physical methods include vapour extraction, aeration, sorting, barriers and electro-remediation. Thermal techniques — evaporation, incineration and vitrification — form a separate group, applied mostly ex situ after the soil has been removed.

Thermal desorption is highly effective against volatile and semi-volatile substances, but it is energy-intensive and needs a support base. In practice it is used where concentrations are high and the site must be returned to use quickly.

The outlook: combining methods rather than seeking one perfect answer

Experience shows that "which method is best" is the wrong question. These techniques combine and complement one another depending on the type and extent of contamination — preliminary oxidation, for instance, can bring concentrations down to a level at which bioremediation starts to work efficiently.

Biological methods bring the greatest benefit to the environment because they introduce no new substances into the ground. Wherever they can be used without risk to the schedule, they are the first choice. The rest is a matter of thorough site investigation before any decision is made — see our soil remediation service.