Common Treatment Technologies for Fuels in Ground Water, Surface Water, and Leachate
It may be necessary to know other subsurface information to remediate fuels in ground water. Treatability testing to characterize contaminant biodegradability and nutrient content may be needed for any biodegradation technology. A subsurface geologic characterization would be particularly important to characterize the migration of NAPLs. Recovery tests are usually necessary to design a product/ground water pumping scheme that will ensure that the nonaqueous fuel layer can be recovered and that contaminated ground water can be treated effectively at the design flow. Ground water models are also often needed to predict flow characteristics, changes in contaminant mixes and concentrations, capture zones, and times to reach cleanup levels.
Technologies most commonly used to treat fuels in ground water include air stripping, carbon adsorption, dual phase extraction, fluid/vapor extraction, and bioslurping. These are all ex situ treatment technologies requiring ground water extraction.
Air stripping involves the mass transfer of volatile contaminants from water to air. For ground water remediation, this process is typically conducted in a packed tower or an aeration tank. The generic packed tower air stripper includes a spray nozzle at the top of the tower to distribute contaminated water over the packing in the column, a fan to force air countercurrent to the water flow, and a sump at the bottom of the tower to collect decontaminated water. Auxiliary equipment that can be added to the basic air stripper includes automated control systems with sump level switches and safety features such as differential pressure monitors, high sump level switches and explosion proof components, and discharge air treatment systems such as activated carbon units, catalytic oxidizers, or thermal oxidizers. Packed tower air strippers are installed either as permanent installations on concrete pads, on a skid, or on a trailer.
Liquid phase carbon adsorption is a full-scale technology in which ground water is pumped through a series of vessels containing activated carbon to which dissolved contaminants are adsorbed. When the concentration of contaminants in the effluent from the bed exceeds a certain level, the carbon can be regenerated in place; removed and regenerated at an off-site facility; or removed and taken off-site for disposal. Adsorption by activated carbon has a long history of use in treating municipal, industrial, and hazardous wastes.
For dual phase extraction, undissolved liquid-phase organics are removed from subsurface formations, either by active methods (e.g., pumping) or a passive collection system. This process is used primarily in cases where a fuel hydrocarbon lens is floating on the water table. The free product is generally drawn up to the surface by a pumping system. Following recovery, it can be disposed of, re-used directly in an operation not requiring high-purity materials, or purified prior to re-use. Systems may be designed to recover only product, mixed product and water, or separate streams of product and water (i.e., dual pump or dual well systems). Dual phase extraction is a full-scale technology.
In fluid/vapor extraction, a high vacuum system is applied to simultaneously remove liquid and gas from low permeability or heterogeneous formations. The vacuum extraction well includes a screened section in the zone of contaminated soils and ground water. It removes contaminants from above and below the water table. The system lowers the water table around the well, exposing more of the formation. Contaminants in the newly exposed vadose zone are then accessible to vapor extraction, which can remove contaminants more efficiently than pump-and-treat.
Bioslurping is the adaptation and application of vacuum-enhanced dewatering technologies to remediate hydrocarbon-contaminated sites. Bioslurping combines vacuum-assisted free-product recovery with bioventing to simultaneously recover free product and bioremediate vadose zone soils. Bioslurping can improve free-product recovery efficiency without extracting large quantities of ground water. Bioventing of vadose zone soils is achieved by drawing air through the soil via the recovery well. The system is designed to minimize environmental discharge of ground water and soil gas. When free-product removal activities are completed, the bioslurping system is easily converted to a conventional bioventing system to complete the remediation.
Air sparging is an in situ technology in which air is injected through a contaminated aquifer. Injected air traverses horizontally and vertically in channels through the soil column, creating an underground stripper that removes contaminants by volatilization. This injected air helps to flush (bubble) the contaminants up into the unsaturated zone where a vapor extraction system is usually implemented in conjunction with air sparging to remove the generated vapor phase contamination. This technology is designed to operate at high flow rates to maintain increased contact between ground water and soil and strip more ground water by sparging.