Thousands of sites have been contaminated by previous industrial use, often associated with traditional processess which are no longer used. These sites may present a hazard to the general environment, but there is a growing need to reclaim and redevelop. There is increasing pressure to reuse land which is affected by contamination rather than develop greenfield sites such as parks or woodland.
Remediation is considered as the management of the contaminant at a site so as to prevent, minimize or mitigate damage to human health, property or the environment. It is a broader term than clean up in that remediation options can include physical actions such as removal, destruction and containment. Development and use of remediation technologies have progressed and a large number of clean-up alternatives have evolved and improved over the past decade.
The in-situ and ex-situ section of this digest outlines the major clean-up technologies which have become well-established in the commercial soil remediation sector. Diagrams accompany each technology description to provide a more vivid picture. References are also included in the library, should the user require more specific information and soil remediation cases.
Classes of soil remediation
Two distinct classes of soil remediation can be defined:
In-situ
Ex-situ (with on-site and off-site interventions)
In-situ cleanups - meaning that no excavation of the contaminated soil occurs - are often preferred because they are generally less expensive. However, they generally take a longer time to effect treatment to the desired limits and there is less certainty about the uniformity of treatment because of the inherent variability in soil and aquifer characteristics and difficulty in monitoring progress.
On the other hand, excavating a contaminated area (ex-situ approach) and treating the material on the same site (ex-situ, on-site) or transporting it to a remote site for cleaning (ex-situ, off-site) can often be more complicated and expensive. Nevertheless, ex-situ remediation has the added bonus of taking the bulk of contaminants away before they can spread further. It also allows homogenization of the contaminated soil before treatment and ensures monitoring so that soils are cleaned to the desired limits within a relatively short time.
Some technologies can have both in-situ and ex-situ applications. While the principle of the technique remains the same, the technological set-up differs. In this digest, four (4) technologies are presented which can be applied as in-situ, as well as, ex-situ. The illustration for each technology indicates the different technological installation.
Categories of remediation technologies
In general, remediation technologies can be grouped into categories based on their treatment mechanism: biological, physical, chemical, electrical, and thermal. These are further subdivided into in situ and ex situ processes (as indicated above). However, in this digest, physical, chemical and electrical mechanisms have been abridged into one group, called physico-chemical, because these mechanisms normally occur together and overlap in the treatment process. “Thermal” has been listed separately because the driving force for the decontamination is heat.
The various techniques usually work well when applied to a specific type of soil pollution, though no readily available treatments were implemented that could clean all types of pollutants. Due to the complex nature of many polluted soils and the fact that pollution, in many situations, is due to the presence of a “cocktail” of different types of contaminants, it is frequently necessary to apply several remediation techniques (treatment train) to reduce the concentrations of pollutants to acceptable levels.
Biological Treatments
Biological treatment is a process whereby contaminants in soil, sediments, sludge or groundwater are transformed or degraded into innocuous substances such as carbon dioxide, water, fatty acids and biomass, through the action of microbial metabolism.
Biological processes are typically implemented at low cost. Contaminants can be destroyed and often little to no residual treatment is required. However, the process requires more time and it is difficult, in general, to determine whether contaminants have been completely destroyed. Additionally, microbes may often be sensitive to toxins or highly concentrated contaminants in the soil.
Physico-Chemical Treatments
Physico-chemical treatments use the physical and/or chemical and/or electrical properties of the contaminants or of the contaminated medium to destroy (i.e., chemically convert), separate, or contain the contamination. In the physical processes the phase transfer of pollutants is induced. In the chemical processes the chemical structure (and then the behaviour) of the pollutants is changed by means of chemical reactions to produce less toxic or better separable compounds from the solid matrix.
These treatments are typically cost effective and can be completed in short time periods (in comparison with biological treatment). Equipment is readily available and is generally not engineering or energy-intensive. Certain in-situ physico-chemical treatment technologies are sensitive to certain soil parameters. For example, the presence of clay or humic materials in soil causes variations in horizontal and vertical hydraulic parameters, which, in turn, cause variations in physicao-chemical process performance.
Thermal Treatments
Thermal treatments, where the driving force of the process is heat, generally offer quick cleanup times but are typically the most costly treatment group. This difference, however, is less in ex-situ applications than in in-situ applications. Cost is driven by energy and equipment costs and is both capital and Operation & Maintenance (O & M) intensive.
Thermal processes use heat to increase the volatility, to burn, decompose, destroy or melt the contaminants. Cleaning soil with thermal methods may take only a few months or several years. The time it takes depends on three major factors that vary from site to site: type and amounts of chemicals present; size and depth of the polluted area; type of soil and conditions present.