Control of gaseous pollutants

Gas and vapor control technologies applicable to SVE operations

Control of gas and vapor pollutants can be carried out in several ways. Elimination of the source of pollution should be the first consideration. Unfortunately, there are relatively few economic alternatives to soil vapor extraction processes (SVE) when only the soil above the groundwater table (vadose zone) requires treatment. In some instances the contamination extends through the vadose zone to the groundwater, and a treatment technology used to treat the groundwater may be combined to treat the vadose zone soil. Additional such technologies are described under alternatives to SVE (click here or use Alternatives button on sidebar). More typically, SVE is a favorable economic remedy, and toxic vapors will require treatment to prevent their release to the atmosphere.

Commonly applied technologies for treating gases and vapor include the following: absorption, adsorption, condensation, thermal oxidation (catalytic and non-catalytic) and biofiltration. Each of these treatment techniques are typically economic over a certain range of concentrations, and are better suited to treat some pollutants than others. Because pollutant concentrations from SVE operations often decrease over the period of treatment from a moderate (about 1000 ppm) to low (about 10 ppm or less) level, condensation and absorption have rarely been used for SVE.[1] Those two processes also do not destroy the pollutant, but they can be combined with processes that do destroy the compounds. Those two technologies will not be discussed further here.

Adsorption is an interfacial phenomenon, molecules adhere at the surface of the solid adsorbent. A large surface area is required to collect the pollutant molecules resulting in a larger physical size and container. Adsorption from dilute gas streams is complicated by the occurrence of high humidity and the presence of other compounds whose removal is not desired, but which also occupy adsorption sites. Competition for adsorption sites reduces the capacity of the adsorbent for the target molecules. At high relative humidity, a phenomenon known as "capillary condensation" results in further loss of adsorption capacity further increasing the need for adsorbent and the size of the unit. Although hydrophobic adsorbents are now available, used in industrial applications, and perform better under high humidity conditions than granular activated carbon GAC, they are too expensive for use with non-regenerable systems. Adsorption systems used for SVE operation have traditionally been non-regenerable systems because of the low concentrations. Since pollutants are only trapped, and not destroyed by adsorption, disposal of the spent adsorbent must be taken into consideration, since it may need to be treated as a hazardous waste.

Thermal oxidation is a term that includes both catalytic and non-catalytic oxidation units. The term "thermal oxidizer" as used by the DTSC refers to "non-catalytic" units that are typically "recuperative" to recover heat energy. Both types of units heat the gases containing the pollutant molecules to a temperature sufficiently high to quickly react the pollutant to less harmful end products in the unit, e.g., carbon dioxide and hydrochloric acid in the case of chlorinated solvents. Thermal oxidation is commonly applied in those situations for which it is important to destroy the pollutant molecules, e.g., toxic compounds such as chlorinated solvents. The energy required to heat the gases is costly so that non-catalytic thermal oxidizers that are efficient at recovering heat, or catalytic units that operate at lower temperature, are commonly used for SVE. A small burner operated with petroleum gas is commonly used to heat the units. Control of acidic gases produced when treating chlorinated solvent vapors, e.g., hydrochloric acid, is usually accomplished with a scrubber.

Figure 1. A thermal oxidation unit at an industrial site treating SVE gases. It is likely that the unit is not treating chlorinated solvents since a scrubber for removing acid gases that would be formed is not present.

Biofilters are packed bed reactors through which contaminated air is either blown or drawn, as indicated in Figures 2 and 3. Microbial communities grow as biofilms on the packing surface. Biofilms are composed of a community of microbial cells (principally bacteria), extracellular polysaccharides (materials exuded by the microorganisms), and bound water. A liquid film must exist around the bacteria because they extract all of their required nutrients from the liquid phase. The contaminants must first dissolve into the liquid phase to be available for microbial metabolism. Compounds that are amenable to biological treatment include petroleum hydrocarbons, non-halogenated solvents, sulfides (e.g. H₂S), ammonia and to a lesser degree, a few halogenated solvents. Biofilters have not been widely used for chlorinated solvent vapors, though some demonstration projects have been carried out. These have not provided sufficiently consistent and robust results for the technology to be considered reliable for use on chlorinated solvent vapors at this time.

Figure 2a. A biofilter installation treating a 300 cfm flow from styrene storage tanks. The figure is provided to provide a sense of the size of a small unit. Photo provided courtesy of PPC Biofilter, Long View, Texas, copyright 2002.

Figure 2b. Biofilters operating in parallel at a remediation site.

Advantages of biofiltration systems over other air pollution control alternatives include low capital and operating costs, low energy requirements, and the absence of residuals and by-products requiring further treatment or disposal. (The liquid streams, i.e., "leachate" from biofilters can normally be discharged to a sanitary sewer.) Disadvantages of biofiltration are that the microbial process can be upset, resulting in periods of lower performance so that there is lower reliability. Commonly, when used with toxic compounds, a back-up system is utilized, e.g., GAC). The biofilter is used to reduce the amount of GAC that is consumed, by degrading competing molecules and the primary pollutant. A summary of existing VOC control technologies, process residuals and by-products, energy costs, and process limitations is shown in Table 1.

Table 1. Comparison of Vapor Phase Pollutant Control Technologies

treatment technology	residuals/byproducts	energy costs	Comments
absorption (not commonly used for SVE)	waste water/ chemical sludges	moderate	limited to soluble compounds (e.g. H₂S, acetone, methanol)
adsorption	typically non-regenerable for SVE applications/spent adsorben such as activated carbon	moderate to high	limited to low to moderate concentration and molecular weights typically above approximately 45
condensation (not commonly used for SVE)	compound not destroyed, however, potential for product recovery	high	low range of compounds at high concentrations
thermal oxidizer	CO₂, HCl, products of incomplete combustion, e.g., CO, low NO_x,	high	stable performance with sufficient time, temperature, and turbulence
catalytic oxidizer	CO₂, HCl, products of incomplete combustion, e.g., CO	moderate to high	H₂S, HCl, or particulate matter can damage catalyst
biofiltration (not commonly used for chlorinated solvents)	leachate disposal to sanitary sewer/compost packing media (if used) changed every 2-5 years	low	low to moderate concentration of biodegradable emissions, moderate to large foot print
biotrickling filter	synthetic media, low flow rate cell waste liquid stream	low to moderate	low to high concentration biodegradable emissions, moderate to large foot print, works better with soluble compounds

Rapid oxidation can also occur with a variety of other processes not directly involving heat, e.g. production of non-thermal plasmas by electric discharge or photo-catalysis. These are "emerging" technologies that have only recently begun to be evaluated for possible application to SVE systems. For more detailed information about various air pollution control technologies, you may download the following file: VOCCntrl.pdf.

[1] Condensation of pollutants is a physical mechanism generally applicable to relatively concentrated streams (greater than about 1000 ppm). It is typically employed when product recovery is desired. This is because the costs of cooling are generally high and it becomes very expensive to cool dilute air streams sufficiently for efficient condensation of many vapors. It is rarely applied directly to SVE streams unless a concentrating unit is also present. The condensed vapors would also require further treatment.

Absorption involves dissolution into the liquid phase, i.e. absorption, can be applied to both relatively dilute and to concentrated streams, but for dilute concentrations a large contact area and sufficient time to allow for diffusion into the absorbent are required. Successful absorption requires that the pollutant be soluble in the absorbent. Several chlorinated solvents are poorly soluble in water and use of other solvents in non-industrial settings is rare. The absorbed vapors in the solution would require further treatment.