Direct Thermal Treatment of Contaminated Soil - Literature review Example

Running Head: Direct thermal treatment of contaminated soil Name Lecturer Date Direct thermal treatment of contaminated soil Out of the many substances that are released to the soil in a given environment, some can contaminate the soil making it unusable or usable but with poor productivity. These soils can be treated to make them recover their original status or become productive in certain ways. Soil treatment therefore refers to the various ways of undoing any fumigation that may be present in the soil and making it usable again (ART Engineering, 2013, De Koning et al, 2008). Among these methods of treatment is the thermal treatment of the contaminated soil which consists of the direct and indirect thermal treatment of soil (Midwest Soil Remediation, 2013). This paper embarks on explaining on the various thermal treatment methods as well as their effectiveness. It will also tackle the benefits that are drawn from the treating of soil, as well as cite an example of where such have been employed. Special attention however will be given to the direct treatment of contaminated soil and its appropriateness in places where it is utilized. As mentioned above, there are various methods of treating contaminated soil. Among these methods are: the physical or chemical treatment of the soil; thermal treatment and biological treatment. The Physical treatment method is quite diverse; Dermont et al (2008) observe that it involves the five different ways of administering remedy to a soil. They include the extraction of soil vapour which involves aerating the soil by digging it, and using a vacuum sucker to remove the harmful particles as noted in the Information Bulletin (2011). Solidification is another physical mode of treatment that involves the solidification of the harmful material, which after being consolidated is treated (Information Bulletin, 2011, Dermont et al, 2008). Chemical oxidation also lies in this category where, by the chemical processes of reduction and oxidation a substance is rendered harmless or less harmful. Such involves the stabilizing, immobilizing or making inert the substances that are in the soil (Tsitonaki et al, 2010). There is also soil flushing where a liquid solution is passed to the contaminated soil and then removed once it absorbs the particles that contaminate the soil (ART Engineering, 2013). Finally, electro-kinetic separation is another mode of physical treatment that is used and that involves the spreading of current through the soil to separate it with any heavy metals lodged in it. It is noted that physical treatments of contaminated soils are effective though they tend to be expensive as a result of the tools along with the machines and the chemicals used (Information Bulletin, 2011). Biological treatment on the other hand is unique in its own way. It involves the use of organisms or vegetation that in turn alters the harmful substances present in the soil by transforming them into compounds that are harmless. Biological treatment involves two ways, namely, bioventing and phytoremediation (Forster Wheeler Environmental Corporation & Batelle Corporation, 1998). Bioventing is the release of gas under the soil surface of an area that is contaminated. The gas helps in breaking down any harmful chemicals that may be resident in the soil. Phytoremediation is the use of plants in extracting or decimating the spread of contamination. Some of the plants include the use of compost in place of plants, the use of microbes or even fungi (Forster Wheeler Environmental Corporation, 1998). There are two ways of treatment as noted in the ART Engineering (2013), namely; the indirect thermal desorption (IDTD) and direct thermal desorption (DTD). Indirect thermal desorption (IDTD) is a method that involves the recovering and re-using. Some of the materials that are recovered through this method include hydrocarbons acquired from petroleum products, from soils that are contaminated and sludge. The indirect thermal desorption method is used to treat soils that have concentrations of carbon exceeding 3 percent (ART Engineering, 2013). The direct thermal treatment (DTD) technology which is of concern in this paper is used to destroy any contaminating agents in the soil, sludge among other materials (Remediation Australasia, 2013). This method is basically used in removing contaminating agents which have very little recovery or are totally irrecoverable and can persistently continue thriving in the environment as observed in the Remediation Australasia (2013). The direct thermal treatment of soil is used where the level of carbon does not exceed 3 percent and is in line with the best practices of controlling emissions in industries. Ordinarily, the temperatures range from 750o – 900oF in accordance with the material from which a cylinder is made. The total time of heating is usually between three and fifteen minutes (United States Environmental Protection Agency, 2006). Direct thermal desorption is a method that has persisted for some time. It has been utilized for soils contaminated with petroleum substances as well as those that contain RCRA hazardous materials (Orasche et al, 2011). Around every six to ten contractors have between fifteen and twenty portable systems that are commercially available. The direct thermal desorption systems use an afterburner (secondary chambers of combustion) or catalytic oxidizers that destroy, by heating, the organics that are volatilized (United States Environmental Protection Agency 2006). Some of the systems are fitted with scrubbers and quenchers immediately after the oxidisers. The scrubber or the quench allows for treatment of soils that contain organics with chlorine, especially pesticides and solvents. The cylinder for desorbing that is utilized for large systems that are transportable is basically 4-10 feet in its diameter and the lengths of heating range from 20-50 feet (United States Environmental Protection Agency, 2006). In general, According to Forster Wheeler Environmental Corporation & Batelle Corporation (1998), the main difference between the direct and indirect thermal systems is determined by the methods in which heat is applied to the material that is contaminated and the way gas treatment system treats the emitted gases. The source heat can be directly applied by radiation emanating from a burning flame or by convection coming from emanating from burning gases (Orasche et al, 2011). As a result, there comes the invocation of the direct-fired thermal desorption system and the Indirect-fired thermal desorption system. Another difference is noted in the broad characterization of the continuous feed as well as the batch-feed varieties (Forster Wheeler Environmental Corporation & Batelle Corporation, 1998). The systems that have continuous feeds are regarded as ex-situ processes and the contaminated substance or material is usually retrieved from the original placement and handled to some level, then it is channelled to a treatment chamber where direct-fired or indirect-fired equipment may be used. The direct-fired thermal desorption differs from the indirect-fired thermal desorption in that the former uses a rotary drier and the latter uses a conveyor that is fitted with a screw on top of the rotary drier (United States Environmental Protection Agency, 2006). According to Bates et al (2008), direct thermal treatment systems have three different processes, namely; the first, second and third-generation – direct- contact thermal desorption processes. The first-generation process uses basic elements of rotary driers, a baghouse made of fabric filter along with an afterburner respectively Bates et al (2008). These elements are good for boiling points which do not exceed 500o – 600o F especially for contaminants which are nonchlorinated. The baghouse makes the system incapable of handling organics that have high boiling points as Riser-Roberts, (2010) observes. This is as a result of the compounds which have a high molecular weight that condenses together with increasing the dropping of pressure across the various bags (Bates et al, 2008). The second generation direct-contact thermal desorption systems came about to handle nonchlorinated contaminants with higher boiling points exceeding 600oF as Bates et al, (2008) note. The systems use rotary driers, afterburners, gas coolers, as well as a baghouse as the basic elements in the process respectively (Bates et al, 2008). Organics with high boiling points can be treated because the materials are heated to higher temperature by the dryer. As a result, the off-gas temperatures which are higher are produced while the baghouse remains intact (Reddy & Cameselle, 2009). The baghouse is positioned at the end in the treatment sequence to enable the removal of particulates inherent in the off-gas. Meanwhile, the temperatures of the gas stream are maintained at a range of 450o – 500o F (United States Environmental Protection Agency, 2006). Moreover, the organics that are vaporized become annihilated in the afterburner making the condensation of high-molecular-weight organics inside the baghouse impossible (Reddy & Cameselle, 2009). Normally, thermal desorption systems of the second-generation are capable of heating the residue of the treated material to a temperature of around 500o – 1200oF (Bates et al, 2008). Materials or soils with higher contamination of heavier oils are therefore to be treated by these systems since they remain in the category of nonchlorinated compounds. The reason to this is because the capacity to control the emissions of hydrochloric acid from the burning of chlorinated compounds is not in place in such systems as maintained by Reddy & Cameselle, (2009). The third generation of direct-contact thermal desorption systems aims at treating contaminants which on top of having high boiling points are chlorinated (Bates et al, 2008). The materials are combusted a temperature ranging from 500oF to 1200oF within the rotary dryer. The off-gas on the other hand is oxidized within the afterburner at a temperature ranging between 1400oF to 1800oF. In other cases, the temperature may be as high as 2000oF (Reddy & Camaselle, 2009). The off-gas is subsequently cooled and thereafter it is passed through the baghouse which acts as a system in the second generation. However, in the final stage of the treatment, there is an acid gas neutralizer that is introduced with the purpose of controlling the emission of hydrochloric acid (HCL) to the environment around or the atmosphere (Kulkarni et al, 2008). A scrubber, according to Kulkarni et al (2008) that is wet and which uses water enriched with caustic soda is used to control the fumes of the acid gas from spreading from the system. Since the scrubber can be made plastic hat is reinforced with fibreglass, there a low permissibility of operating temperature, there is an upstream emanating from the quench stage from the baghouse that is used to cool gas before it enters the scrubber as Herberlein & Murphy (2008) maintain. An additional wet gas scrubber assists to a certain level in controlling any other components that may be contained in the water according to Reddy & Cameselle (2009). The wet scrubber system also helps in the achieving of a particulate collection. The particulate consolidates into sludge within the system of wastewater treatment requiring its removal and management before discharging it (Herberlein & Murphy, 2008). The above discussion delves into the nature of the direct thermal desorption (DTD) which is by itself is important in various ways. By the various types of direct soil treatment methods, there is an opportunity for the removal of contaminants, be they of high boiling points or lower boiling points thus there is a possibility of adaptability as Gan et al (2009) note. Compared to the indirect method, the direct thermal desorption (DTD) is much less expensive compared to the indirect thermal desorption considering the intricacies involved in the latter which may involve an extra cost (Taube et al, 2008). In Australia, the direct thermal desorption (DTD) was utilized on the Rhodes Peninsula in Sydney. The soil was contaminated with dioxin and was treated using rotary plants. The off-gas emitted was put under control by the use of a system that is comprised of cyclones or dust removers, thermal oxidizers for combusting any organic compounds, evaporative cooler for quenching the emitted off-gas; a baghouse for removing particles; a scrubber for removing acid gas (Hydrochloric acid or HCL), a draft inductile draft fan and a stack. The evaporative cooler rapidly quenches the off-gas by the introduction of a watery mist which in turn reduces the reformation of dioxin. The evaporative cooler was vital step in the prevention of reformation of dioxin during the thermal processes (Remediaton Australasia, 2013). The soil where the site is built has a large proportion of moisture of an approximate percentage of 25 percent and fine grains of lime that were used in filing the site. There is an average concentration of dioxins and furans which have a toxic equivalence (TEQ) of 50 ug/kg. There is a need of Soil Treatment Temperature (STT) of about 525oC and a residence time of around 10min that meets the standard of treatment of 1miligram per kilogram toxic equivalence of dioxins together with furans. High moisture content as well as the high STT has led to a relatively minimal soil feed rate approximately rated at around 18 t/h. The thermal oxidizer ran at 930oF and 960oF and the concentration of dioxin standard in the off-gas was around 0.1ng TEQ/Nm3. The plant is effective and according the concepts of best practice regarding STT and oxidizer operating temperature (OOT) to reduce the production of off-gas could produce an approximate 40 percent of greenhouse gas. There has been no further debate on how the two can be balanced but everything in terms of the plant is set (Remediation Australasia, 2013). To conclude on this discussion, it is important to acknowledge that there are various ways of treating contaminated soil. The direct thermal treatment is just one way which has several other modifications which make it more adaptable to given contaminants in the soil. Applying the method can be of importance because it can proof more suited to various soil conditions creating the possibility of modifying it. Though it may be expensive too, the treatment can be specific in comparison to bioremediation and the chemical or physical treatment. It is therefore more logical to advice for more use of this treatment method in the decontamination of affected materials. References ART Engineering (2013). Thermal desorption plants. retrieved on July 16, 2013. Bates, M., Bruno, P., Caputi, M., Caselli, M., de Gennaro, G., & Tutino, M. (2008). Analysis of polycyclic aromatic hydrocarbons (PAHs) in airborne particles by direct sample introduction thermal desorption GC/MS. Atmospheric Environment, 42(24), 6144-6151. De Koning, S., Kaal, E., Janssen, H. G., van Platerink, C., & Brinkman, U. A. T. (2008). Characterization of olive oil volatiles by multi-step direct thermal desorption–comprehensive gas chromatography–time-of-flight mass spectrometry using a programmed temperature vaporizing injector. Journal of Chromatography A, 1186(1), 228-235. Dermont, G., Bergeron, M., Mercier, G., & Richer-Laflèche, M. (2008). Metal-contaminated soils: remediation practices and treatment technologies. Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management, 12(3), 188-209. Forster Wheeler Environmental Corporation & Batelle Corporation (1998). Overview of thermal desorption technology. Naval facilities engineering service centre. June 1998. Gan, S., Lau, E. V., & Ng, H. K. (2009). Remediation of soils contaminated with polycyclic aromatic hydrocarbons (PAHs). Journal of Hazardous Materials, 172(2), 532-549. Heberlein, J., & Murphy, A. B. (2008). Thermal plasma waste treatment.Journal of Physics D: Applied Physics, 41(5), 053001. Information bulletin (2011). Thermal treatment technologies. retrieved on July 16, 2013. Kulkarni, P. S., Crespo, J. G., & Afonso, C. A. (2008). Dioxins sources and current remediation technologies—a review. Environment international, 34(1), 139-153. Midwest Soil Remediation (2013). Direct Fired Thermal Desorption Systems. retrieved on July 16, 2013. Orasche, J., Schnelle-Kreis, J., Abbaszade, G., & Zimmerman, R (2011). In-Situ derivatization thermal desorption GC-TOFMS for direct analysis of particle bound non-polar and polar organic species. Atmospheric chemistry and physics discussions.11, 1255-15295. Reddy, K. R., & Cameselle, C. (2009). Electrochemical remediation technologies for polluted soils, sediments and groundwater. Wiley. com. Remediation Australasia (2013). Thermal treatment technologies. retrieved on July 16, 2013. Riser-Roberts, E. (2010). Remediation of petroleum contaminated soils: biological, physical, and chemical processes. CRC Press. Taube, F., Pommer, L., Larsson, T., Shchukarev, A., & Nordin, A. (2008). Soil remediation–mercury speciation in soil and vapour phase during thermal treatment. Water, Air, and Soil Pollution, 193(1-4), 155-163. Tsitonaki, A., Petri, B., Crimi, M., Mosbæk, H., Siegrist, R. L., & Bjerg, P. L. (2010). In situ chemical oxidation of contaminated soil and groundwater using persulfate: a review. Critical Reviews in Environmental Science and Technology,40(1), 55-91. United States Environmental Protection Agency (2006). Engineering Forum Issue Paper. In situ treatment technologies for contaminated soil. July 2006. retrieved on July 16, 2013. Read More