Contaminant removal from groundwater

Air strippers are ex situ devices used to physically transfer volatile organic contaminants (VOCs) from groundwater, surface water, or wastewater to air. Contaminants are not destroyed by air stripping, but once they are transferred to the airstream, they may be destroyed by oxidation or incineration, or removed using activated carbon absorption. 

Phase-transfer oxidation is a technology for destruction of organic contaminants. It was developed to treat contaminated liquid streams using adsorption for contaminant removal and advanced oxidation processes for spent adsorbent regeneration. It was used in testing to treat the contaminated effluent from groundwater extraction technologies. 

Natural attenuation by itself, however, often is not sufficient to achieve a desired extent or rate of contaminant removal from an aquifer. In these instances, one remedial option may be to enhance the natural rate of biodegradation of pollutant chemicals in the aquifer. This strategy, called in situ bioremediation, is considered to be one of the most attractive remedial techniques from a cost perspective, because many of the high costs associated with pumping and treating groundwater or excavating contaminated aquifer material are avoided. Furthermore, the potential exposure of cleanup workers to pollutant chemicals is reduced if many of the contaminants are mineralized while still in the aquifer. 

The metabolic product tert-butyl alcohol (TBA), which is often present with MTBE, is also considered a groundwater contaminant. Due to its physical properties, it is much more difficult to remove from groundwater than MTBE through the physical processes mentioned [20]. Air sparging and sorption unto GAC cannot be considered as viable options for TBA removal from groundwater. 

Carbon adsorption. A treatment system where contaminants are removed from groundwater or surface water when the water is forced through tanks containing activated carbon, a specially treated material that attracts the contaminants. 

When we refer to water purification, it makes little sense to discuss the subject without first identifying the contaminants that we wish to remove from water. Also, the source of the water is of importance. Our discussion at this point focuses on drinking water. Groundwater sources are of a particular concern, because there are many communities throughout the U.S. that rely on this form. The following are some of the major contaminants that are of concern in water purification applications, as applied to drinking water sources, derived from groundwater. 

As indicated earlier, heavy contamination can be buried, sealed or removed. Burying of the material should be well below the root growth zone, and this is normally taken as 3.0 m below the final ground-surface level. Sealing for heavy contamination to prevent vertical or lateral leaching through groundwater flow can be with compacted clay or proprietary plastic membranes. Removal from site of the contaminants is normally only contemplated in a landscaped scheme where the material, even at depth, could be a hazard to public health directly or phytotoxic to plant life. 

In an air sparging system, the primary mechanism for contaminant removal is by the transfer of contaminants from the dissolved to the vapor phase. The extent to which this transfer can take place during air sparging depends on the Henry s law constant, which is an indication of the extent to which each will partition between the dissolved state and the vapor state under equilibrium conditions. A contaminant with a greater Henry s law constant is more readily stripped from groundwater by air sparging than one with a lower Henry s law constant. 

Other than aerial application over swamps for mosquito abatement, disulfoton is not known to be used over water. Potential sources of release into surface water include discharge of waste water from disulfoton manufacturing, formulation, and packaging facilities (HSDB 1994). Leaching and runoff from treated fields, pesticide disposal pits, or hazardous waste sites may contaminate both groundwater and surface water with disulfoton. Entry into water can also occur from accidental spills. Small amounts of volatilized disulfoton may be removed from the atmosphere as a result of wet deposition and may enter surface water (Racke 1992). 

The Stripperator was used as part of a pump-and-treat system installed at Camp Lejeune in North Carolina. This system, which was used to remove VOCs, had an average cost of 95,000 per pound of contaminant removed. According to the U.S. Navy, 175,000 was spent on the system to remove 3 lb of contaminants, and 325,000 was spent to remove an additional 0.5 lb. The Navy claims that the high cost of treatment at the site resulted from inefficiencies in groundwater extraction methods and was not caused by the use of the Stripperator (D22770N, p. ES-1, ES-3, 3-18-3-28). 

Reclaim cannot remove metals or other ionic compounds from groundwater nor catalyze chemical reactions. Also, the success of Reclaim is in relative proportion to the permeability of the geologic components comprising the contaminated site, the hydraulic gradient, and the concentrations and vapor pressures of the contaminants. As permeability, contaminant concentrations, vapor pressure, and hydraulic gradients decrease, so does the rate of recovery. In addition, Reclaim requires vendor-supplied, on-site service and support on a periodic basis. 

Geokinetics International, Inc., has developed other applications for this technology as weU. It can be set up as an electrokinetic ring fence to recover ionic contamination from groundwater as it flows past the electrodes. It may also be used as a soil heating element in conjunction with soil vapor or groundwater extraction to remove organics from soil. 

MYCELX is a proprietary chemical that can be applied to a variety of substrates to remove organic and inorganic compounds from contaminated surface water, groundwater, and wastewater. The substrates are selected based ou the application and marketed as individual techuologies. The prices aud brief descriptious of several products are provided in Table 1 (D17717X D220633). 

The vendor states that tetrachloroethane (PCE), trichloroethene (TCE), and other volatile compounds are difficult to remove from saturated soils because they are relatively insoluble. The vendor states that the technology is especially applicable to sites contaminated with dense non-aqueous-phase liquids (DNAPLs). Using the ISSZT technology creates an unsaturated zone from which these contaminants can be readily air stripped. Other contaminants such as polychlorinated biphenyls (PCBs) or metals can be isolated from groundwater and contained within barriers preventing the spread of contamination. 

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