resulting products from biofragmentation can be assimilated into microbial cells; this is the assimilation stage. Some of the products from fragmentation are easily transported within the cell by membrane carriers. However, other products of the biofragmentation stage still have to undergo biotransformation reactions to yield products that can then be transported inside the cell. Once inside the cell, the products enter catabolic pathways that either lead to the production of adenosine triphosphate (ATP) or elements of the structure of the cell.
The biodegradation process is, in general, an important process for the removal of chemical compounds (especially organic chemicals) from the environment. The versatility and activity of microbial enzymes as catalysts mean that biodegradation is much more significant than purely chemical reactions such as hydrolyses and redox reactions. Enzymatically catalyzed transformation also occurs in higher organisms, but this process is quantitatively less important than the contribution from microorganisms. Some of the most important microorganism-mediated chemical reactions in aquatic and soil environments are those involving nitrogen compounds and the cycle of such compounds throughout the Earth system. Among the biochemical transformations in the nitrogen cycle are (i) nitrogen fixation, whereby molecular nitrogen is fixed as organic nitrogen, (ii) nitrification, the process of oxidizing ammonia to nitrate, (iii) nitrate reduction in which nitrogen in nitrate ions is reduced to nitrogen in a lower oxidation state, and (iv) denitrification, the reduction of nitrate and nitrite to ammonia.
Physical-chemical and biological treatment processes are employed as for wastewater treatment. In addition, chemicals are introduced for precipitation of nutrients, followed by coagulation and filtration for removing solids remaining after biological treatment. In some cases, granular activated carbon or membrane filtration or a combination of membrane-assisted solvent extraction is used for additional purification of the groundwater streams and waste streams. This higher level of treatment is advisable because of the damage that any visual traces of chemical waste can do to the appearance of the waters. In addition, the treatment may combat the potential eutrophic effect that the nutrients phosphorus and nitrogen can have on a water system.
For the most part, anthropogenic compounds resist biodegradation much more strongly than do naturally occurring compounds. This is generally due to the absence of enzymes that can cause an initial attack on the compound. A number of physical and chemical characteristics of a compound are involved in its amenability to biodegradation. Such characteristics include hydrophobicity, solubility, volatility, and affinity for lipids. Some organic structural groups impart particular resistance to biodegradation. These structural groups include branched carbon chains, ether linkages, meta-substituted benzene rings, chlorine, amines, methoxy groups, sulfonates, and nitro groups.
Usually the products of biodegradation are molecular forms that tend to occur in nature and that are in greater thermodynamic equilibrium with their surroundings than the starting materials. Detoxification refers to the biological conversion of a toxic chemical to a less toxic chemical species.
The biodegradability of a compound is influenced by its physical characteristics, such as solubility in water and vapor pressure, and by its chemical properties, including molecular mass, molecular structure, and presence of various kinds of functional groups, some of which provide a “biochemical handle” for the initiation of biodegradation. With the appropriate organisms and under the right conditions, even substances such as phenol that are considered to be biocidal to most microorganisms can undergo biodegradation.
See also: Biodegradation In Situ, Biodegradation Processes, Biodegradation – Slurry Phase, Biodegradation – Solid Phase.
Biodegradation – In Situ
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