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Bioreactor-mediated bioremediation

Bioreactors, which can be applied in bioremediation strategies, are tanks in which pollutants are converted, by living organisms, into a specific product(s) following a series of biological reactions. In bioreactors-mediated bioremediation, various operating modes are used like batch, fed-batch, sequencing batch, continuous, and multistage, etc. (Castaldi and Ford 1992, Wang and Vipulanandam 2001, Adriaens et al. 2006). The selection of operating mode often depends on financial and capital expenditure. Conditions in a bioreactor support the natural process of cells by mimicking and maintaining then natural environment to provide optimum growing conditions. Polluted samples are fed into a bioreactor either as dry solid or slimy to treat contaminated soil. It has many benefits as compared to other ex-situ bioremediation procedures. In bioremediation, bioreactor provision of controlling all bioprocess parameters (t'.e., pH, agitation and aeration rates, temperature, substrate loading, and inoculum concentrations) makes it more meaningful than the others (Azubuike et al. 2016).

The provision to control and manipulate process parameters in a bioreactor helps in speeding up biological reactions to reduce bioremediation time effectively. Most importantly, in controlled bioaugmeutation practice, where cultured microbes and nutrients are added into the subsurface for biodegrading specific contaminants, increased pollutant bioavailability, and mass transfer (contact between pollutants and microbes), can conclusively be established in a bioreactor, thus making bioreactor-based bioremediation more efficient (Wang and Vipulanandam 2001, Adriaens et al. 2006). It can be applied to efficiently treat soil or water polluted with volatile organic compounds (VOCs), including ethylbenzene, xylenes (BTEX), benzene, and toluene.

Castaldi and Ford (1992) treated petroleum waste sludges containing 680 mg kg'1 of 2-3 rings PAHs and 38 mg kg"1 of 4-6 rings PAHs in a continuous flow multistage А-SB that operated at relatively short residence times with the minimal loss of volatile constituents. The higher molecular weight PAHs, including the four-ring derivatives of pyrene, benzo[a]pyrene. and chrysene, were removed with efficiencies higher than 90%. In another study, Wang and Vipulanandam (2001) observed the effect of naphthalene concentration on the А-SB remediation potential. Naphthalene was generally found to rapidly desorb from the spiked soil by undergoing rapid and extensive biodegradation. 500 mg kg"1 of naphthalene was reduced to 20 mg kg'1 after 65 h days of treatment, whereas 5000 mg kg'1 of the same pollutants were reduced to 40 mg kg'1 after 100 h.

Laiidfarming

Landfarming is a full-scale bioremediation technology, which generally combines liners and additional methods to control leaching of pollutants, which needs excavation and placement of polluted soils, sediments, and sludges. Contaminated and polluted media is applied to lined beds and periodically turned over to enhance aeration, which improves bioremediation efficiency. Landfarming often needs controlled and optimized soil conditions to increase the contaminant degradation rate. Controlled conditions are typically maintained by irrigation or spraying (for moisture content), tilling the soil (for aeration), liming (for pH), and nutrient supplement (nitrogen, phosphorus, and potassium). In most cases, it is regarded as ex situ bioremediation, while, in some cases, it is viewed as an in-situ bioremediation technique (Kao et al. 2008).

In agriculture, especially in conventional practice, polluted soils are usually excavated and tilled, but the site of treatment determines the type of bioremediation. When excavated contaminated soil is treated on-site, it can be regarded as in-sitw, otherwise, it is ex-situ as it has more in common with other ex-situ bioremediation techniques (EPA 2000). It has been stated that when a pollutant lies 1 m below ground surface, bioremediation continued without excavation contaminated media, while pollutant lying 1.7 m below needs transportation on the ground surface for practical bioremediation (Nikolopoulou et al. 2013). Generally, excavated polluted soils are carefiilly applied to fixed layer support above the ground surface to allow aerobic biodegradation of pollutants by microorganisms (Pliilp and Atlas 2005, Silva-Castro et al. 2015).

Moreover, landfarming is not fit for soil polluted with toxic volatiles treatment because of its design and pollutant removal mechanism (volatilization), particularly in hot (tropical) climate regions. These limitations make landfarming-based bioremediation tune consuming and less efficient compared to other ex-situ bioremediation techniques. One of the notable advantages of ex-situ bioremediation techniques is that they do not require an extensive preliminary assessment of polluted sites before remediation; this makes the initial stage short, less complicated, and less expensive.

 
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