IN SITU BIOREMEDIATION (ISB)
In situ bioremediation(ISB) is the usage of MO to degrade the occurring contaminants/pollutants on the land of pollution (in situ) in order to produce non-dangerous final compounds. ISB is applied for the degradation of contaminants/pollutants in saturated soils and underground waters, and in unsaturated areas, also.
The technique has been developed to be less costly, more efficient than standard pumping and treatment techniques used to degrade aquifers and soils polluted with chlorinated solvents, petroleum hydrocarbons, explosives, nitrates, and toxic metals. ISB has advantages such as: complete destruction of contaminants/pollutants, lower health risks for land workers, lower expenditure for installation and operation. ISB can be classified according to metabolism or depending on the degree of human intervention.
The two types of metabolism are aerobic metabolism and anaerobic metabolism. The type of metabolism for a particular ISB system will be set as target according to the type of contaminants/pollutants to be destroyed. Some contaminants/pollutants are degraded by aerobic treatment (such as petroleum) others by anaerobic treatment (such as carbon tetrachloride), while other contaminants/pollutants can be biodegradable both by aerobic and anaerobic routes (such as trichloroethene). Since, the soil treatment doesn’t require excavation and transport; the treatment expenditure is significantly reduced, thus posing a gr eat advantage. However, this remediation requires longer periods, and the uniformity of treatment is less reliable given the variability of soil and aquifer characteristics. In addition, it is more difficult to control the effectiveness of the technique.
Bioremediation techniques are destructive techniques aimed at stimulating the multiplication of MO by using contaminants/pollutants as sources of food and energy.
Causing favorable conditions for the development of MO usually involves providing certain combinations of oxygen, nutrients, and humidity, as well as controlling the temperature and pH.
Sometimes, to improve the technique, MO adapted to degrade certain contaminants/pollutants is added. The usage of biological remediation methods is usually done with low expenditure. Contaminants/pollutants are destroyed and rarely additional residue treatment is required (Majoneet al., 2015).
Some disadvantages arise with specific contaminants/pollutants. Such as, biodegradation of polycyclic aromatic hydrocarbon (PAH) leads to ground degradation to PAH with large, recalcitrant, and potentially carcinogenic masses. Polyhalogenated compounds are highly biodegradable, and some of them are transformed by biodegradation into secondary and more toxic products (such as, conversion of trichloroethene to vinyl chloride). If adequate control techniques will not be used, these by-products may be mobilized by groundwater.
ISB requires a thorough characterization of soil, aquifer, and contaminants/ pollutants. Sometimes it may be necessary to extract and treat the ground- water, and then the low polluted groundwater may be recirculated through the treated area to provide the necessary humidity (Nernecek et al., 2018).
ACCELERATED "IN-SITU" BIOREMEDIATION
Accelerated “in-situ” bioremediation stimulates the growth of a bacterial consortium through by adding substrate or nutrients to the aquifer. Despite the usual target bacteria being indigenous to the land, a good alternative is represented by external enriched cultures from other lands. These cultures should be especially effective in degrading a certain contaminant (bioanr- plifrcation). Accelerated “in-situ” bioremediation is used for specific areas, where an increase in the rate of biotransformation of the contaminant is desired, the speed being limited by the lack of nutrients, the lack of donors or electron acceptors. The type of modification required depends on the target metabolism and the target contaminant.
In-situ aerobic bioremediation may require only an addition of oxygen (electron acceptor), whereas the anaerobic can require both the addition of electron donors (such as lactate, benzoate) and electron acceptors (ex. nitrate, sulfate).
Chlorinated solvents often require the addition of carbon substrate to stimulate reductase degradation. The goal pursued in accelerated “in-situ” bioremediation technique is to increase the amount of biomass inside the polluted aquifer and thereby to achieve effective biodegradation of the dissolved and absorbed contaminant.
Although it usually is a faster solution compared to others, accelerated “in-situ” bioremediation does normally require a bigger investment in equipment, labor work and materials (Tong et al., 2015).
ADVANTAGES OF ACCELERATED IN SITU BIOREMEDIATION (ISB)
Contaminants can be fully turned into harmless substances such as water, ethane, and carbon dioxide. The volumetric treatment in-situ bioremediatiorr provides is able to treat both saponified and dissolved contaminants. The length of ISB treatment of underwater soil pollution can often be faster than if pumping-treatment technologies are used. ISB tends to expenditure less, whereas other remediation options are often more expensive due to the technologies used. Since bioremediation treatment follows the aquifer movement, otherwise inaccessible areas are reached; therefore the area of treatment is greater than when using different remediation technologies (Abel and Akkanen, 2019).
RESTRICTIONS OF ACCELERATED IN-SITU BIOREMEDIATION
Some contaminants could be only partially turned into harmless compounds, depending on the specificity of the land. Intermediate products which formed in the biotransformation technique can potentially have a higher mobility or toxicity than the parent compound.
Moreover, certain contaminants are recalcitrant to biodegradation, meaning they are not biodegradable. If it is inappropriately applied, the addition of nutrients, electron acceptors, and/or donors could lead to intense microbial growth, which in turn could lead to clogging of the injection wells.
Since nutrient transport is limited in aquifers with low permeability, it is more difficult to apply accelerated in-situ bioremediation. The activity of indigenous MO can be inhibited by toxic concentrations of organic compounds and heavy metals. Populations of acclimated MO, which are usually required for ISB, cannot be developed for recalcitrant compounds or for recent waste MO (Carrara et al., 2011).