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Ex situ bioremediation

This bioremediation method involves transfer or excavation of pollutants from the site of contamination and treated safely away from the site of origin. Ex situ bioremediation has no complex strategies for selecting the most suitable technique for treatment of polluted soils because the soils are transported from contaminated sites to new sites for treatment. However, safe transport and outspread of soil contaminants are taken care of following the nature and stability of pollutants, environmental health and biogeochemical conditions of the treatment sites (Pliilp and Atlas 2005, Fnitos et al. 2010, Azubuike et al. 2016). Excavation and treatment costs, nature of contaminated soils, active transport system required to transport the contaminated soils, environmental policies and their social impact determine the preference of active treatment methods for ex situ bioremediation (Azubuike et al. 2016). Ex situ bioremediation is often considered as a traditional method of treatment of polluted soils amending conventional sewage and solid waste treatment methods. Depending on the substrate used for bioremediation, ex situ method is categorized into two systems known as solid phase system and slimy phase system. The former involves treatment of contaminated soil through land fanning, developing piles of organic wastes (biopiles) and periodical turning of the biopiles, and the latter includes treatment of contaminated soils in the form of slimy' or liquid suspension in bioreactors. Techniques used in ex situ bioremediation are discussed below.

4.2.1 Landfarming

Landfanning is performed by applying excavated polluted soils over the land chosen as a treatment site and allowing degradation of contaminants by microorganisms growing on it. It is a common practice for treating petroleum contaminated soils—the volatile fraction evaporates as gases and the remaining fractions are biodegraded through microbial activity (Al-Awadhi et al. 1996,

Cerqueira et al. 2014, Khan et al. 2014, Nikolopoulou and Kalogerakis 2016). This technique is also considered as advantageous over other methods because its requirements are cost-effective and Mfilled through minimal energy consumption, ensuring low risk of contaminant outspread, complying with government rules and regulations, following environmental policies and assuring benefits to the society and geographical location (Maila and Cloete 2004, Besaltatpour et al. 2011, Nikolopoulou and Kalogerakis 2016, Azibuke et al. 2016. Alfke et al. 1999). Landfarming depends on physicochemical properties of soil, pollutant characteristics and climatic conditions (Morgan and Watkinson 1989, Zhu et al. 2004, Khan et al. 2004, Paudyn et al. 2008, Besaltatpour et al. 2011). This technique performs well with addition of autochthonous microorganisms, nutrient amendment, proper supply of water through irrigation and oxygen through tillage (Philp and Atlas 2005, Nikolopoulou and Kalogerakis 2016, Silva-Castro et al. 2015). These activities provide essential conditions such as soil moisture, growth factors and aeration to enhance contaminant degrading ability of microorganisms under favorable climatic conditions. In extreme climatic conditions like in desert areas, amendment of nutrients and water may lead to soil compaction (Philp and Atlas 2005, Hejazi and Husain 2004). Laudfarmiug has few limitations: (i) requirement of huge land areas (ii) management of highly volatile organic compounds hi highly cold and highly hot and humid climate (iii) sustaining microorganisms in unfavorable environmental conditions (Philp and Atlas 2005, Khan et al. 2004, Azibuke et al. 2016, Maila and Colete 2004).

4.2.2 Biopiling

This technique involves piling or heaping of excavated soils in a defined area for treatment with microorganisms. Biopiling is often defined as a modified landfarming bioremediation limiting irrigation and tillage of treated soils, promoting growth of aerobic and anaerobic microorganism in the heap of contaminated soils (Mani and Kumar 2013). Microbial degradation in biopiling is enhanced with nutrient, moisture and oxygen amendment in a well-organized treatment bed. Contaminant leaching from treatment bed is prevented by applying an impermeable layer or lining that impedes or limits the flow of leachate into ground water. Biopiliug treatment beds are also equipped with leachate drainage and collection unit that collects seeping leachates for further treatment. Sometimes bulking agents like straw, bark, wood chips and similar biomass may be added to the biopliles to improve bioavailability of soil contaminants and enhance microbial remediation (Azibuke et al. 2016, Rodriguez-Rodriguez et al. 2010). This method is effortlessly utilized in treating low molecular weight (LMW) organic pollutants. Biopiling is less costly as it requires low capital input and easily controllable parameters ensuring effective degradation of soil contaminants in favorable as well as extreme climatic conditions. It depends on soil physicochemical characteristics, nutrient concentration and constituent characteristics of soil contaminants (Whelan et al. 2015, Gomez and Sartaj 2014). In biopiling, aeration is induced by ah' pumps, thereby continuous heating and drying of biopiles affect the microbial flora and their degrading capabilities (Sanscartier et al. 2009). Other limitations include (i) lack of uniform contaminant mixing with soils due to poor aeration, and (ii) low efficiency for high molecular weight organic pollutants, heavy metals inhibiting microbial growth and rapid volatilization of untreated lower fractions of organic pollutants (USEPA 1995).

4.2.3 Windrow

Windrow is a method of turning piles of contaminated soils at regular intervals to provide aeration for enhanced degradation of soil contaminants. It ensures proper mixing of contaminated soils with nutrients, and uniform distribution of oxygen and water for better results. Windrow has a very good option in removal of greenhouse gases like methane produced during anaerobic conditions of degradation, and periodic turning of soils which causes release or oozing of methane gas from the anaerobic zone of biopiles (Andersen et al. 2010). Nutrient, oxygen and water amendments in biopiling can be immediately followed by windrow treatment; this will lead to better integration of the amendment components and improve microbial degradation activities (Shi et al. 1999). Windrow is effectively utilized in treating crude oil contaminated soils with the help of specialized machinery

  • (Al-Daher et al. 1998, Baiba et al. 1998). This technique depends on soil physicochemical properties and degree of bioavailability and biodegradability of soil contaminants (Coulon et al. 2010, Azibuke et al. 2016, Semple et al. 2001). Drawbacks of windrow treatment include (i) rapid removal of toxic untreated volatile gases challenging the environmental health, and (ii) requirement of ample of amount of time and labor (Azibuke et al. 2016).
  • 4.2.4 Bioreactor

Treatment of polluted soils in bioreactors is carried out in liquid phase system known as slurry bioreactors. Contaminated soils are fed into bioreactors in the form of slurry with optimized bioprocess parameters such as pH, temperature, aeration, microbial population, and nutrient concentration (Azibuke et al. 2016). Slimy bioreactors are supplied with nutrients for growth of microorganisms and surfactants for increasing bioavailability of soil contaminants. These are easily manipulated and controlled, allowing enhanced and faster degradation of soil contaminants in an optimized operation mode and design of the bioreactor system (Robles-Gonzalez et al. 2008, Mueller et al. 1991). This technique is very effective in treating polluted soils with high concentration of toxic and recalcitrant contaminants (Robles-Gonzalez et al. 2008, Mueller et al. 1991, Christodoulatos et al. 1998). Slimy bioreactors are artificially instigated with microbes and their growth factors following bioaugmentation, i.e., introduction of autochthonous microorganisms and biostimulation, i.e., input of nutrients for better growth and performance of microorganisms (Robles-Gonzalez et al. 2008). Advanced smdies on slimy bioreactor system and bioprocessing showed that DNA (molecular) fingerprinting may be used for identifying changes in the pattern of microbial communities corresponding to varied experimental conditions. This may serve as useful information in optimizing the bioreactor parameters for better performance of the microorganisms (Rizzo et al. 2010). These bioreactors depend on bioavailability and biodegradability of soil contaminants, microorganisms and bioprocess parameters such as pH, temperature, nutrient concentration and exchange of gases. However, slimy bioreactors are limited to small scale operations where space and manpower constraints prevail (Robles-Gonzalez et al. 2008). Another drawback of slurry bioreactor is that the contaminated soils require pre-treatment like crushing and sieving before incorporation into the bioreactor unit (Woo and Park 1999). Moreover, cost of the bioreactor system narrows down its applicability and most of the times it is inconvenient to conduct slimy bioreactor treatment in absence of proper optimization of bioreactor operation unit, operation design and bioprocess parameters (Pliilp and Atlas 2005). Different in situ and ex situ methods are given in Figure 1.

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