In Situ Subsurface Volatilization And Ventilation Combined Saturated Zone Sparging And In Situ Vadose Zone Vapor Stripping

The contaminated soil and groundwater in the saturated zone can be remediated for VOCs removal through sparging. The technology involves the use of combined saturated zone sparging and in situ vadose zone vapor stripping [34]. It is also called subsurface volatilization and ventilation [22-25], in situ sparging, in situ air stripping, in situ aeration, and aeration curtain.

There are two broad approaches to the process, which involves sparging volatile organics compounds (VOCs) from the saturated zone using compressed air:

1. Throughout the contaminated zone. Individual sparging wells are placed with a combination of saturated zone sparging and in situ vadose zone vapor stripping throughout the contaminated zone to remove VOCs and SVOCs from across a wide area. Wells are screened over a narrow interval located at the bottom of an aquifer or below the deepest contamination within the aquifer. Compressed air is forced from the well screen and flows radically outward and upward through the contaminated soils. As the air bubbles move upward through the contaminated groundwater and soils,

VOCs and SVOCs dissolved in the groundwater and absorbed to the soil particles' surface are volatilized and swept to the unsaturated zone with the air bubbles. The extracted air is then collected by vacuum through the screened vacuum extraction well, and further purified by air purification means (such as dryer, activated carbon, or equivalent) before its release to the ambient air. Biodegradation may occur within the remediation system, thus reducing the need for off-gas treatment. 2. Combination of saturated zone sparging and in situ vadose zone vapor stripping to form aeration curtains oriented at right angles to the flow of the groundwater plume. Aeration curtains can be created in trenches backfilled with porous media. The trenches have a horizontal slotted pipe (air injection well, or air distribution pipe) near the bottom of the trench to supply compressed air. As the groundwater flows through the trench, the rising air bubbles strip the VOCs and SVOCs to the top of the trench, reaching the unsaturated zone with the air bubbles. The extracted air is then collected by vacuum through the screened vapor recovery pipe (or vacuum extraction well) and further purified by air purification means (such as dryer, activated carbon, or equivalent) before its release to the ambient air. Biodegradation may occur within the remediation system, thus reducing the need for off-gas treatment.

A well-established subsurface volatilization and ventilation system (SVVS) is presented below as a case study. The SVVS (Fig. 4) was developed by Billings and Associates, Inc. (BAI), Albuquerque, NM, United States, and operated by several other firms under a licencing agreement, uses a network of injection and extraction wells (collectively, a reactor nest) to treat subsurface

Figure 4 Subsurface volatilization and ventilation system (SVVS). (Courtesy of USEPA.)

VOCs and SVOCs contamination through in situ biodegradation using compressed air below the water table combined with soil vacuum extraction in the vadose zone (above the water table). Each system is custom designed to meet site-specific conditions. A series of compressed air injection wells and vacuum extraction wells is installed at a site. One or more vacuum pumps create negative pressure to extract contaminant vapors, while an air compressor simultaneously creates positive pressure, sparging air through the subsurface treatment area. This placement allows the groundwater to be used as a diffusion device. Control is maintained at a vapor control unit that houses pumps, control valves, gages, and other process control hardware.

The number and spacing of the wells depends on the modeling results of applying a design parameter matrix, as well as the physical, chemical, and biological characteristics of the site. The exact depth of the injection wells and screened intervals are additional design considerations.

To enhance vaporization, solar panels are occasionally used to heat the injected compressed air. Additional valves for limiting or increasing air flow and pressure are placed on individual reactor nest lines (radials) or, at some sites, on individual well points. Depending on groundwater depths and fluctuations, horizontal vacuum screens, "stubbed" screens, or multiple-depth completions can be applied. The system is dynamic: positive and negative air flow can be shifted to different locations at the site to place the most remediation stress on the areas requiring it. Negative pressure is maintained at a suitable level to prevent the escape of vapors.

Because it provides oxygen to the subsurface, the SVVS, or equivalent, can enhance in situ bioremediation at a site. Thus, it can decrease site remediation time significantly. These processes are normally monitored by measuring dissolved oxygen levels in the aquifer, recording carbon dioxide levels in transmission lines and at the emission point, and periodically sampling microbial populations. If air quality permits require, VOC emissions can be treated by a biological treatment process unit that uses indigenous microbes from the site.

The SVVS, or equivalent, is applicable to sites with leaks or spills of gasoline, diesel fuels, and other hydrocarbons, including halogenated compounds. The system is very effective on methyl tertiary-butyl ether (MTBE), benzene, toluene, ethylbenzene, and xylene (BTEX) decontamination. It can also contain contaminant plumes through its unique vacuum and air injection techniques.

The technology should be effective in treating soils contaminated with virtually any material that has some volatility or is biodegradable. The technology can be applied to contaminated soil, sludges, free-phase hydrocarbon product, and groundwater. By changing the injected gases to induce anaerobic conditions and by properly supporting the microbial population, the SVVS can remove nitrates from groundwater. The aerobic SVVS or equivalent raises the redox potential of groundwater to precipitate and remove heavy metals.

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