Bioremediation management strategies and application
Bioremediation uses living organisms that interact directly or indirectly with the environment to neutralize or remove contaminants (Vidali 2001). Bioremediation can be applied both in situ (i.e., field conditions) and ex situ (i.e., mesocosm/controlled conditions). Many small-scale examples exist of both approaches using plants, fungi, and bacteria as bio-remediators of organic contaminants, although results have been varied (Vidali 2001, Haims et al. 2011). However, applications of important bio-remediators at a large scale has been reported by Boopathy 2000, Adriaens et al. 2006, Gonzalez et al. 2019. The classification of bioremediation processes is given in Figure 2.
In situ bioremediation
In situ bioremediatiou means that you allow bioremediation to take place while leaving the soil or water in its original location. In situ bioremediation can be designed with or without plant species. Plant species have been used because they take up vast amounts of water that assists in controlling contaminated water, such as a groundwater contaminant plume, in the soil. Since in situ bioremediation, mostly applications, is based on aerobic biodegradation, anaerobic-based processes have also become of interest. That can be essential to improve the aerobic degradation of organic compounds (Azubuike et al. 2016). Some of the in situ bioremediation methods have been addressed below.
Natural bioremediation has been happening for millions of years. Naturally, existing bacteria and fungi or plants themselves degrade or detoxify substances hazardous to human health and the
Figure 2. Classification of bioremediation processes. Source: (Oyetibo et al. 2017).
Figure 3. Natural attenuation and bioremediation are widely accepted environment clean-up procedures. Source: (Shukla
et al. 2014).
environment. Biodegradation of dead animals or decay of vegetation is a kind of natural remediation. Bioremediation is a namral part of the carbon, nitrogen, and sulfur cycles. Chemical energy present in waste matters is used by microbes to grow while they transform organic carbon and hydrogen to water and C02 (Figure 3).
Managed or engineered bioremediation
A type of remediation that enhances the growth and degradative activity of microorganisms by using engineered systems that supply nutrients (e.g., nitrogen and phosphorus), electron acceptors, and other growth-stimulating materials (Fiedler 2000). An example of managed bioremediation is land fanning, which refers to the regulated organic compounds degradation that are distributed onto the soil surface and then tilled. Since around 1980, prepared bed practices are used for bioremediation. In this procedure, contaminated soil is excavated and deposited with suitable fertilizers into a shallow layer over an impenneable base. Conditions are managed to achieve degradation of the contaminants of concern. Essential techniques of engineered bioremediation are described below with the results of some of the case studies.
This technique has gained popularity among other in situ bioremediation techniques, especially in restoring sites polluted with light spilled petroleum products (Hohener and Ponsin 2014). This technique includes controlled stimulation of airflow by carrying oxygen to the unsaturated zone to increase bioremediation, by enhancing movements of indigenous microbes. In bioventing, amendments are made by supplementing nutrients and moisture to improve bioremediation, with the final goal being to produce a microbial transformation of pollutants to a safe state (Philp and Atlas 2005).
A study by Hong and Xingang (2011) showed the effect of air injection rate on volatilization, biodegradation, and biotransformatiou of toluene-contaminated location by bioventing. It was seen that at two separate air injection rates (81.504 and 407.52 m3/d), no significant difference in contaminant elimination was seen (200 days). Though, at the earlier stage of the study (day 100), it was observed that high ah injection rates resulted in enhanced toluene removal by volatilization compared to low air injection rates. In other words, a high airflow rate does not bring about an increase in the biodegradation rate, nor makes pollutant biotransformatiou more efficient. That is due to the initial saturation of ah hi the subsurface for oxygen requirement during biodegradation. However, a low ah injection rate resulted in a significant improvement in biodegradation. It, therefore, shows that in bioventing, the air injection rate is among the essential parameters for pollutant dispersal, redistribution, and surface loss.
The bioremediation method, particularly in treating the vadose zone polluted with chlorinated compounds, is generally recalcitrant under aerobic conditions. In tins process, in pure oxygen, a mixture of nitrogen together with low concentrations of C02 and H2 can also be injected to bring about loss of chlorinated vapor, with H2 acting as the electron donor (Mihopoulos et al. 2000, Shah et al. 2001). In soil with low-permeability, injection of pure oxygen might lead to higher oxygen concentration compared to air injection. Moreover, ozonation might be beneficial for the incomplete oxidation of recalcitrant compounds to accelerate biodegradation (Philp and Atlas 2005). Unlike bioventing that relies on improving the microbial biodegradation process at the vadose-zone by controlled air injection, soil vapor extraction (SYE) maximizes volatile organic compound (VOCs) volatilization via vapor extraction (Magalhaes et al. 2009).
Soil vapor extraction (SYE) may be regarded as a physical method of remediation due to its mechanism of pollutant removal. During on-site field trials, obtaining similar results achieved during lab investigations is not always achievable due to additional environmental factors and various characteristics of the unsaturated zone to which air is injected. As a result, bioventing treatment tune may be extended. High airflow rate begins to transfer volatile organic compounds (VOCs) to the soil vapor phase, which needs off-gas treatment of the resulting gases before releasing into the atmosphere (Burgess et al. 2001).