User:StolteKate/Bioremediation

Introduction
Bioremediation is a process used to treat contaminated media, including water, soil and subsurface material, by altering environmental conditions to stimulate growth of microorganisms and degrade the target pollutants. Cases where bioremediation is commonly seen is oil spills, underground pipe leaks, and crime scene cleanups. These toxic compounds are metabolized by enzymes present in microorganisms. Most bioremediation processes involve oxidation-reduction reactions where either an electron acceptor (commonly oxygen) is added to stimulate oxidation of a reduced pollutant (e.g. hydrocarbons) or an electron donor (commonly an organic substrate) is added to reduce oxidized pollutants (nitrate, perchlorate, oxidized metals, chlorinated solvents, explosives and propellants). Bioremediation is used to reduce the impact of byproducts created from anthropogenic activities, such as industrialization and agricultural processes. In many cases, bioremediation is less expensive and more sustainable than other remediation alternatives. Other remediation techniques include, thermal desorption, vitrification, air stripping, and soil washing. Biological treatment, bioremediation, is a similar approach used to treat wastes including wastewater, industrial waste and solid waste. The end goal of bioremediation is to remove or reduce harmful compounds to improve soil and water quality.

Contaminants can be removed or reduced with varying bioremediation techniques that are in-situ or ex-situ. Bioremediation techniques are classified based on the treatment locality. In-situ techniques treats polluted sites in a non-destructive manner and cost-effective. Whereas, ex-situ techniques commonly require the contaminated site to be excavated which increases costs. In both these approaches, additional nutrients, vitamins, minerals, and pH buffers may be added to optimize conditions for the microorganisms. In some cases, specialized microbial cultures are added (biostimulation) to further enhance biodegradation. Some examples of bioremediation related technologies are phytoremediation, bioventing, bioattenuation, biosparging, composting (biopiles and windrows), and landfarming.

Bioventing (formally Aerobic)
Bioventing is a process that increases the oxygen or air flow into the unsaturated zone of the soil which increases the rate of natural in situ degradation of the targeted hydrocarbon contaminant. Bioventing, an aerobic bioremediation, is the most common form of oxidative bioremediation process where oxygen is provided as the electron acceptor for oxidation of petroleum, polyaromatic hydrocarbons (PAHs), phenols, and other reduced pollutants. Oxygen is generally the preferred electron acceptor because of the higher energy yield and because oxygen is required for some enzyme systems to initiate the degradation process. Microorganisms can degrade a wide variety of hydrocarbons, including components of gasoline, kerosene, diesel, and jet fuel. Under ideal aerobic conditions, the biodegradation rates of the low- to moderate-weight aliphatic, alicyclic, and aromatic compounds can be very high. As the molecular weight of the compound increases, so does the resistance to biodegradation. This results in higher contaminated volatile compounds, due to their high molecular weight, are more difficult to remove from the environment.

Biostimulation (formally Additives)
Bioremediation can be carried out by bacteria that is naturally present in the environment or adding nutrients, this process is called biostimulation.

Bacteria, also known as microbia, are naturally occurring in the environment and are used to degrade hydrocarbons. Many biological processes are sensitive to pH and function most efficiently in near neutral conditions. Low pH can interfere with pH homeostasis or increase the solubility of toxic metals. Microorganisms can expend cellular energy to maintain homeostasis or cytoplasmic conditions may change in response to external changes in pH. Anaerobes have adapted to low pH conditions through alterations in carbon and electron flow, cellular morphology, membrane structure, and protein synthesis.

In the event of biostimulation, adding nutrients that are limited to make the environment more suitable for bioremediation, nutrients such as nitrogen, phosphorus, oxygen, and carbon may be added to the system to improve effectiveness of the treatment. Nutrients are required for the biodegradation of oil pollution and can be used to reduce the negative output on the environment. Specific to marine oil spills, nitrogen and phosphorus have been key nutrients in biodegradation.

Bioattenuation
During bioattenuation, biodegradation occurs naturally with the addition of nutrients or bacteria. The indigenous microbes present will determine the metabolic activity and act as a natural attenuation. While there is no anthropogenic involvement in bioattenuation, the contaminated site must still be monitored.

Biosparging
Biosparging is the process of groundwater remediation as oxygen, and possible nutrients, is injected. When oxygen is injected, indigenous bacteria are stimulated to increase rate of degradation. However, biosparging focuses on saturated contaminated zones, specifically related to ground water remediation.

Biopiles
Biopiles, similar to bioventing, are used to reduce petroleum pollutants by introducing aerobic hydrocarbons to contaminated soils. However, the soil is excavated and piled with an aeration system. This aeration system enhances microbial activity by introducing oxygen under positive pressure or removes oxygen under negative pressure.

Windrows
Windrow systems are similar to compost techniques where soil is periodically turned in order to enhance aeration. This periodic turning also allows contaminants present in the soil to be uniformly distributed which accelerates the process of bioremediation.

Landfarming
Landfarming, or land treatment, is a method commonly used for sludge spills. This method disperses contaminated soil and aerates the soil by cyclically rotating. This process is an above land application and contaminated soils are required to be shallow in order for microbial activity to be stimulated. However, if the contamination is deeper than 5 feet, then the soil is required to be excavated to above ground.

Heavy metals
Heavy metals become present in the environment due to anthropogenic activities or natural factors. Anthropogenic activities include industrial emissions, electronic waste, and ore mining. Natural factors include mineral weathering, soil erosion and forest fires. Heavy metals including cadmium, chromium, lead and uranium are unlike organic compounds and cannot be biodegraded. However, bioremediation processes can potentially be used to reduce the mobility of these material in the subsurface, reducing the potential for human and environmental exposure. Heavy metals from these factors are predominantly present in water sources due to runoff where it is uptake by marine fauna and flora.

Limitations of bioremediation
Even though bioventing is an inexpensive method to bioremediate contaminated sites, this process is extensive and can take a few years to decontaminate a site.

In agricultural industries, the use of pesticides is a top factor in direct soil contamination and runoff water contamination. The limitation or remediation of pesticides is the low bioavailability. Altering the pH and temperature of the contaminated soil is a resolution to increase bioavailability which, in turn, increased degradation of harmful compounds.

Acrylonitrile is commonly produced in industrial setting but adversely contaminates soils. Microorganisms containing nitrile hydratases (NHase) degraded harmful acrylonitrile compounds into non-polluting substances.

Since the experience with harmful contaminants are limited, laboratory practices are required to evaluate effectiveness, treatment designs, and estimate treatment times. Bioremediation processes may take several months to several years depending on the size of the contaminated area.