Classes of remediation

Two distinct remediation classes can be defined: in-situ and ex-situ, the latter also distinguished in on-site and off-site interventions. In-situ cleanups 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.

Categories of remediation technologies

In general, remediation technologies can be grouped into categories based on their treatment mechanism: physical, chemical, biological and thermal. These are further subdivided into in situ and ex situ processes (as indicated above). However, in this survey, physical and chemical mechanisms have been abridged into one group, called physical-chemistry, because these two 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 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 groundwater bodies 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 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.

Physical/chemical treatment uses the physical and/or chemical 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 behavior) 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 physical/chemical treatment technologies are sensitive to the presence of clay or humic materials, causing variations in horizontal and vertical hydraulic parameters, which, in turn, cause variations in physical/chemical process performance.

Thermal treatments 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 and sediments 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 or sediments and conditions present.

Thermal destructive technologies comprehend Incineration, Pyrolysis, High-Pressure Oxidation and Vitrification, introducing highly effective processes to destroy contaminants on sediments by heating a wide variety of elements and compounds for several hundreds or thousands of degrees above ambient temperature. They are used to destroy completely PCBs, PAHs, chlorinated dioxins and furans, petroleum hydrocarbons, and pesticides. Some technologies, such as vitrification, may immobilize metals in a glassy matrix. Volatile metals, particularly mercury, must to be removed by equipments for emission control.

Thermal desorption technologies comprehend some techniques such as X*TRAX System, Desorption and Vaporization Extraction, High-Temperature Thermal System, Low-Temperature Thermal System, Low-Temperature Thermal Aeration and Anaerobic Thermal Processor, introducing highly effective processes to destroy contaminants on sediments by heating a wide variety of elements and compounds for several hundreds or thousands of degrees above ambient temperature. Soils and sediments may be heated to temperatures ranging from 90 to 540°C and contaminants are condensed and collected as liquid, captured on activated carbon, and/or destroyed in an afterburner. These techniques offer several advantages over thermal destructive processes, including reduced energy requirements, less potential for formation of toxic emissions, and smaller volumes of gaseous emissions. Disadvantages include the need for a follow-on destruction process for the volatilized organic compounds and reduced effectiveness for less volatile organic compounds.