Bioremediation is a method used to clean up polluted areas by using natural or added microorganisms. The term "in situ" means "on site" and describes a process that happens at the location of the pollution without moving the polluted materials. Bioremediation helps reduce harmful substances such as hydrocarbons, chlorinated compounds, nitrates, toxic metals, and other pollutants through chemical reactions. Microorganisms used in bioremediation can be introduced to a site or grown there by adding fertilizers and nutrients. Common areas treated with bioremediation include groundwater, aquifers, and polluted soil. Bioremediation has also improved aquatic environments affected by oil spills, such as the Deepwater Horizon oil spill in 2010 and the Exxon Valdez oil spill in 1989. There are two types of bioremediation, determined by where the process takes place. Ex situ bioremediation happens at a location away from the polluted site and requires moving the contaminated material. In situ bioremediation occurs at the polluted site and can be further divided based on the type of metabolism involved (aerobic or anaerobic) and the level of human involvement.

History

In 1972, the Sun Oil pipeline spill in Ambler, Pennsylvania led to the first commercial use of in situ bioremediation to clean hydrocarbons from polluted areas. In 1974, Richard Raymond filed a patent titled "Reclamation of Hydrocarbon Contaminated Ground Waters," which helped make in situ bioremediation available for commercial use.

Classifications of In situ Bioremediation

Accelerated in situ bioremediation is a method where specific microbes are encouraged to grow by adding nutrients or electron donors to a polluted area. In aerobic metabolism, oxygen is the nutrient added to the soil. Anaerobic in situ bioremediation often requires various electron donors or acceptors, such as benzoate and lactate. In addition to nutrients, microbes can be directly introduced to the site during accelerated in situ bioremediation. Adding extra microbes to a site is called bioaugmentation. This is used when the right microbes to break down pollution are not naturally present or are not in enough numbers to be effective. Accelerated in situ bioremediation is used when the desired microbes are not naturally present in sufficient numbers to clean the pollution. It is also used when the necessary nutrients are not available or not in high enough amounts to support microbial growth.

The Raymond Process is a type of accelerated in situ bioremediation developed by Richard Raymond. It involves adding nutrients and electron acceptors to a polluted site. This process is mainly used to treat polluted groundwater. In the Raymond Process, a loop system is created. Contaminated groundwater from downstream is pumped to the surface, mixed with nutrients and an electron donor, such as oxygen, and then returned to the groundwater upstream. This introduces nutrients and electron donors to the site, helping specific microbes grow.

In polluted areas where the desired microbial metabolism is aerobic, adding oxygen can increase the population of targeted microbes. Oxygen can be injected into the ground through injection wells or injection galleries. The presence of oxygen often determines how quickly and effectively an in situ bioremediation process works.

Ozone can also be used to add oxygen to a polluted site. Although ozone is a strong chemical that might harm some microbes, it dissolves easily in water, making it efficient for spreading oxygen. Within 20 minutes after injection, half of the ozone breaks down into oxygen. Ozone is often introduced to soil in either dissolved or gaseous form.

In accelerated anaerobic in situ bioremediation, electron donors and acceptors are added to a polluted site to increase the population of anaerobic microbes.

Monitored Natural Attenuation is a type of in situ bioremediation that happens with little or no human help. It relies on the natural microbes in the area to reduce pollution over time. During this process, the site is monitored to track the progress of the cleanup. Monitored Natural Attenuation is used in areas where the pollution source is no longer active, often after other cleanup methods have been used.

Uses of In Situ Bioremediation

Hydrocarbons are sometimes called polycyclic aromatic hydrocarbons (PAHs), which have ring-shaped structures. These pollutants form when materials like fossil fuels are burned. They are found in both land and water environments, which can harm the environment, biodiversity, and food safety. Hydrocarbons can cause changes in DNA and increase cancer risk, which is dangerous for humans and animals.

Aerobic and anaerobic methods can break down these pollutants. One effective anaerobic method is Biochar. Biochar is made from plant materials, microbes, and agricultural waste through a process called pyrolysis. It has a porous and absorbent structure that helps microbes break down pollutants in soil. Biochar is rich in carbon, which traps PAH pollutants. Higher pyrolysis temperatures create more surface area in Biochar, improving its ability to absorb pollutants. To enhance Biochar’s effectiveness, acids and alkalis are often used as activating agents. Biochar works in both aerobic and anaerobic conditions but is most effective under anaerobic conditions because it supports microbial growth. Byproducts from Biochar can improve soil quality. Biochar is used to create biogas, a renewable fuel made from waste materials like methane and carbon dioxide. This happens in digesters where Biochar’s structure helps microbes break down waste. Digestates, which remain after the process, can be used as nutrient-rich fertilizer but may also contain harmful heavy metals.

Hydrocarbons are divided into two types: low molecular weight PAHs (LMW-PAHs) with fewer than four rings and high molecular weight PAHs (HMW-PAHs) with more than four rings. The more rings a PAH has, the less soluble it becomes, making it harder to break down.

Soil naturally contains microbes that use hydrocarbons for energy and growth. Up to 20% of soil microbes can break down hydrocarbons. These microbes can reduce hydrocarbon pollution through natural or accelerated processes. Most hydrocarbon breakdown happens through aerobic metabolism. The final products of this process are carbon dioxide and water. Hydrocarbons vary in how easily they break down based on their structure. Long chains of aliphatic carbon are easiest to break down, while short, branched, or quaternary aliphatic hydrocarbons are harder to break down. Saturated alkenes are easier to break down than unsaturated ones. Many microbes in soil can break down aromatic hydrocarbons, and some can do so without oxygen. The ability to break down polynuclear aromatic hydrocarbons depends on the number of rings in the molecule. Compounds with two or three rings break down easily, but those with four or more rings are harder to remove. Microbes in soil break down smaller polynuclear aromatic hydrocarbons through aerobic processes. Larger molecules require cometabolism, a process where microbes break down other substances while also breaking down the target compound. The fungus Phanerochaete can break down some polynuclear aromatic hydrocarbons under anaerobic conditions using a special enzyme called peroxidase.

Microbes use several methods to break down chlorinated aliphatic compounds. These include anaerobic reduction, oxidation of the compound, and cometabolism under aerobic conditions. Few microbes in the environment can break down these compounds. Shorter chlorinated compounds with few chlorine atoms are most easily broken down by soil microbes. Cometabolism is the most common method for breaking down chlorinated aliphatic compounds.

Chlorinated aromatic hydrocarbons are hard to break down, and many microbes cannot degrade them. These compounds are often broken down through reductive dechlorination under anaerobic conditions. Polychlorinated biphenyls (PCBs) are mainly broken down through cometabolism. Some fungi can also break down PCBs. Studies show that adding biphenyl to a site increases PCB breakdown because enzymes that break down biphenyl also help break down PCBs.

Benefits

When bioremediation happens at the polluted site, there is a lower chance of spreading pollution to other areas compared to when polluted material is moved to different locations for treatment. This method can also cost less and clean up contamination more effectively than moving the polluted material to another place for treatment.