Contaminated soil remediation
1. Soil washing
The soil washing process separates the colloidal fraction of contaminated soil from the inert fraction: all surface soil is, in fact, made up of a fine fraction (silt or clay) and a coarser fraction (sand or gravel), as well as organic matter, water and gas. Pollutants tend to bind (chemically and/or physically) to the colloidal fraction (silt, clay, and organic material) which in turn tends to physically adhere to sand and gravel.
When soil contains a predominant inert fraction, the contaminated colloidal fraction can be separated from the coarse, larger grade fraction by the use of water.
This separation removes the hazardous pollutants from the soil and concentrates them into a reduced volume. Washing separates the fine fraction from contaminated soil (or from waste), and from sand and gravel; at the end of the treatment, most of the pollutants are concentrated in the fine fraction of the soil, which can be further treated using authorised processes or disposed of, as required.
The washed inert fractions are characterised separately and are managed as:
- a product, if they satisfy the Transfer Test under Annex 3 of Law DM 186/2006,
- as waste, if they do not satisfy the above-mentioned Transfer Test.
In the latter case, the matrix treated as waste is sent to appropriate plants for recovery or disposal.
The treatment process
The simplified diagram of the process is shown at:
- 1. The first step of the treatment process is excavation and storage in an equipped area close to the treatment system.
- 2. Subsequently the soil is screened to separate the larger debris.
- 3. The larger debris is fed into the washing system, where it is mixed with a washing solution and shaken; as mentioned earlier, the washing solution is composed of water, with the addition where necessary of detergents, surfactants, and acid buffers to improve the efficacy of removal of the contaminants.
- 4. At the exit of the washing system, sand and gravel is separated from the washing silt (water with contaminated silt and clay) by screening.
- 5. At the end of the process, the contaminant content of the inert fraction is checked and it is then used for backfilling on site (or put through a new washing cycle).
- 6. Washing silt, which contains pollutants, is treated so as to again remove the silt and clay, which contain most of the pollutants, from the washing water. The silt and clay, in specific cases, can be further treated in order to render them recoverable, or they are sent for disposal. However, the washing water can be re-used for numerous treatment cycles: it is only purified at the end of the process (if necessary on site), and discharged into the sewage system.
2. Soil vapour extraction and air sparging
Soil Vapour Extraction (SVE) is a treatment for physically extracting contaminants from the soil. In particular, this technology is aimed at extracting contaminants in the form of vapour and is used for the removal of volatile and semi-volatile compounds from the saturated area. Operationally, this system requires the digging of extraction wells for the removal of vapours kept under light depression using a blower, and often linked to injection wells to increase the air flow and the rate of removal of the contaminants.
Before release into the air, the gases extracted must be pre-treated by adsorption through activated carbon rather than through catalytic oxidation or condensation of the extracted solvents.
Often air injection notably increases the rate of biological removal of biodegradable contaminants.
Where contaminants are present even under the subsurface, SVE is used in conjunction with Air Sparging which consists of blowing air into the subsurface to strip the volatile and semi-volatile contaminants from the liquid phase and transfer them to the unsaturated zone, where they can be finally removed through the SVE extraction wells. The following diagram shows the operation of a typical SVE/Air Sparging system.
Ecomed has experience using both the systems described above in various remediation projects, some of which have been completed while others are still underway, thereby gaining specialised knowledge in the sector and placing the company in the forefront of on-site treatment processes.
In particular, the experience gained has allowed the company to work in areas which normally would not favour technology: areas characterised by a predominance of fine materials (such as silty sand and sandy silt) along with superficial groundwater and salt water, and semi-volatile compounds.
