Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 150

In situ techniques do not require excavation of the contaminated soils, so it may be less expensive, create less dust, and cause less release of contaminants than ex situ techniques. Also, it is possible to treat a large volume of soil at once. In situ techniques, however, may be slower than ex situ techniques, may be difficult to manage, and are most effective at sites with permeable soil (i.e., sandy or uncompacted soil).

The goal of aerobic in situ biodegradation is to supply oxygen and nutrients to the microorganisms in the soil. Aerobic in situ techniques can vary in the way they supply oxygen to the organisms that degrade the contaminants. Two such methods are (i) bioventing and (ii) injection of hydrogen peroxide. Oxygen can be provided by pumping air into the soil above the water table (bioventing) or by delivering the oxygen in liquid form as hydrogen peroxide. In situ biodegradation may not work well in clays or in highly layered subsurface environments because oxygen cannot be evenly distributed throughout the treatment area. In situ remediation often requires years to reach cleanup goals, depending mainly on how biodegradable specific contaminants are. Less time may be required with easily degraded contaminants.

Bioventing systems deliver air from the atmosphere into the soil above the water table through injection wells placed in the ground where the contamination exists. The number, location, and depth of the wells depend on many geological factors and engineering considerations. An air blower may be used to push or pull air into the soil through the injection wells. Air flows through the soil, and the oxygen in it is used by the microorganisms. Nutrients may be pumped into the soil through the injection wells. Nitrogen and phosphorous may be added to increase the growth rate of the microorganisms.

Injection of hydrogen peroxide is a process that delivers oxygen to stimulate the activity of naturally occurring microorganisms by circulating hydrogen peroxide through contaminated soils to speed up the biodegradation of organic contaminants. Since it involves putting a chemical (hydrogen peroxide) into the ground (which may eventually seep into the groundwater), this process is used only at sites where the groundwater is already contaminated. A system of pipes or a sprinkler system is typically used to deliver hydrogen peroxide to shallow contaminated soils. Injection wells are used for deeper contaminated soils.

The in situ biodegradation of groundwater speeds up the natural biodegradation processes that take place in the water-soaked underground region that lies below the water table. For sites at which both the soil and groundwater are contaminated, this single technology is effective at treating both.

Generally, an in situ groundwater biodegradation system consists of an extraction well to remove groundwater from the ground, an above-ground water treatment system where nutrients and an oxygen source may be added to the contaminated groundwater, and injection wells to return the “conditioned” groundwater to the subsurface where the microorganisms degrade the contaminants. A limitation of this technology is that differences in underground soil layers and differences in the density of each of the layers may cause reinjected conditioned groundwater to follow certain preferred flow paths and, consequently, the conditioned water may not reach some areas of contamination.

Another frequently used method of in situ groundwater treatment is air sparging (also known as in situ air stripping and in situ volatilization), which involves pumping air into the groundwater to help flush out contaminants and is often used in conjunction with a technology called soil vapor extraction.

The air sparging method is used for the treatment of saturated soils and groundwater contaminated with volatile organic compounds (VOCs). Typically, the process involves the use of multiple air injection points and multiple soil vapor extraction points that can be installed in contaminated soils to extract vapor phase contaminants above the water table. Contamination must be at least 3 ft deep beneath the ground surface in order for the system to be effective. A blower is attached to wells, usually through a manifold, below the water table, creating pressure. The pressurized air forms small bubbles that travel through the contamination in and above water column. The bubbles of air volatilize contaminants and carry them to the unsaturated soils above. Vacuum points are installed in the unsaturated soils above the saturated zone and facilitate the extraction of the vapors through a soil vapor extraction system. In order for the vacuum to avoid pulling the air from the surface, the ground has to be covered with a tarp or other method of sealing out surface air to prevent vapors from breaking through to the surface above. Surface air intrusion into the system reduces efficiency and can reduce the accuracy of system metrics.

While the air sparging system typically treats the off-gases (referred to as contaminated vapors and extracted air), the process can also be employed to treat arsenic-contaminated groundwater treated by air sparging and what the treatment does is to remove arsenic at a certain percentage using a solution of iron and arsenic.

One of the advantages of in situ biodegradation is that it can be effectively applied to treat wastes in place. The process usually entails introduction of nutrients, microorganism, and air to the soil/waste through a series of injection wells or infiltration trenches. The term bioventing has also been applied to this technology, although the term could just as easily be applied to composting or to soil heaping.

If the soil does not have sufficient moisture content, water may also have to be added. In situ biodegradation is often applied in conjunction with groundwater pump and treat systems and soil flushing activities.

There is also the concept of gene manipulation as a means degrading polynuclear aromatic hydrocarbon derivatives. The concept offers promise for many sites (such as town gas sites where wastes containing polynuclear aromatic hydrocarbon derivatives are evident). However, the degradation products from such interactions may require cleanup. But it is quite possible that the degradation products are easier to clean than the original polynuclear aromatic hydrocarbon derivatives. There is also the concept of using biodegradation on such wastes where the waste has been reduced to residual saturation by flushing technologies. A final flushing to remove the biodegraded material will be necessary.

See also: Biodegradation, Biodegradation Processes, Biodegradation – Slurry Phase, Biodegradation – Solid Phase.