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Ex-situ bioremediatioii

Ex-situ bioremediation techniques entail the removal of the contaminated object to be treated elsewhere (Pliilp and Atlas 2005, Tomei and Daugulis 2013). For instance, in the case of material polluted by kerosene, phenols, cresols, polycyclic aromatic hydrocarbons and semi-volatile organic compounds, these techniques involve excavating pollutants from contaminated zones and finally transporting them to another site for treatment. Ex-sita bioremediation techniques are usually considered based on the cost of treatment, depth of pollution, type of pollutant, degree of contamination, geographic location, and geology of the polluted site (EPA 2006). Performance criteria also determine the selection of ex-situ bioremediation techniques (Pliilp and Atlas 2005, Tomei and Daugulis 2013). Some of the ex-situ bioremediatioii methods have been addressed below.

Biopile-mediated bioremediation

A biopile-mediated method is the most effective method in remediating pollutants such as BTEX, phenols, PAHs, and explosives such as TNT and RDX. This is a frill-scale technology which involves above-ground piling of excavated polluted soil on a treatment area, followed by nutrient supplement, and sometimes forced aeration is used to enhance bioremediation by basically increasing microbial activities (Chemlal et al. 2013, Dias et al. 2015). The components of the biopile technique are aeration system, irrigation, nutrient and leachate collection systems, and a treatment bed. The use of this ex situ technique is increasingly being considered due to its useful features, including cost- effectiveness, which enables effective biodegradation on the condition that nutrients, temperanire, oxygen, moisture, and pH are well controlled (Whelan et al. 2015). The utilization of biopile by polluted sites can help limit the volatilization of low molecular weight (LMW) contaminants. It can also be used efficiently to remediate extremely polluted enviromnents such as the frigid regions (Gomez and Sartaj 2014. Whelan et al. 2015).

A study was conducted by Gomez and Sartaj (2014) to investigate the effects of the different applications of microbial consortia rates, i.e., 3 and 6 ml/in3, with compost at the rate of 5 and 10% on total petroleum hydrocarbon (TPH) reduction in field-scale biopiles at low-temperanire conditions. The result showed that at the end of 94 days, 90.7% TPH reduction in the bioaugmented and biostimulated setups was achieved over the control setup. The high TPH removal was attributed to the synergistic interaction between bioaugmentation and biostimulation, thus demonstrating the flexibility of biopiles for bioremediation. Biopile-mediated bioremediation setup can readily be scaled up to a pilot system to get similar performance obtained during lab experiments (Chemlal et al. 2013).

Essential to the efficiency of biopiliug is the sieving and aeration of contaminated soil before processing (Delille et al. 2008). Bulking agents such as straw, sawdust, bark, or wood chips, and other organic materials have been added to enhance the remediation process in a biopile construct (Rodriguez-Rodriguez et al. 2010). Although biopile-mediated bioremediation systems conserve space compared to other ex situ bioremediation methods, including land farming, robust engineering, cost of maintenance and operations, lack of power supply particularly at remote places, which would facilitate uniform dispersion of ah' in contaminated gathered soil through air pump, are some of the limitations of this system. More so, extreme heating of ah' can lead to drying of soil undergoing bioremediation, which results in inhibition of microbial activities and increase in volatilization rather than biodegradation (Delille et al. 2008. Rodriguez-Rodriguez et al. 2010).

Windrows

Windrow system is just like composting, another form of ex-situ solid-phase bioremediation. Windrows rely on the periodic turning of piled polluted soil to enhance aeration, which increases degradation activities of indigenous and transient hydrocarbon clastic bacteria present in contaminated soil (McCutcheon et al. 2004). The regular turning of polluted soil, together with adding water, brings the uniform distribution of pollutants, nutrients, and microbial degradative enzymatic activities, thus speeding up bioassimilation, biotransformation, and mineralization rate to achieve efficient bioremediation (Bair et al. 2002).

Windrow-mediated treatment, when compared with biopile-mediated tr eatment, showed a greater hydrocarbon removal. However, the high efficiency of the windrow towards hydrocarbon removal is a result of the soil type, which is stated to be more friable (Coulon et al. 2010). Nevertheless, due to the requirement of periodic turning in windrow treatment, especially in the case of soil polluted with toxic volatiles, it is not considered as the better option to adopt in bioremediation. The use of windrow-mediated treatment has been associated with CH4 (greenhouse gas) release due to the creation of anaerobic conditions within piled polluted soil (Hobson et al. 2005).

 
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