Bioremediation: A Sustainable Approach for Environmental Cleanup 5

been volatilized pass into the unsaturated zone, where they are vacuum-extracted by a soil vapor

extraction system (Hardisty and Ozdemiroglu 2005, Neilson and Allard 2008).

1.4.2 Ex-situ Bioremediation

Ex-situ bioremediation procedures treat pollutants elsewhere than where they were initially

discovered. Ex-situ bioremediation techniques are further divided into other categories, such as

landfarming, composting, biopiling, bioreactors and biofilters, based on the type of pollutant,

depth and degree of pollution, treatment cost and geographical and geological characteristics of

the contaminated location (Atlas and Philp 2005). Some of the ex-situ bioremediation strategies as

shown in Figure 1.1 are discussed below:

1.4.2.1 Biopiling

A biopile is one of the several bioremediation strategies for treating hydrocarbon-contaminated soil

that involves piling the dirt on top of an air distribution system and aerating it. Landfarming and

composting processes are combined in the biopiling approach and are mostly used in the treatment

of areas polluted with petroleum hydrocarbons. This whole setup includes a treatment bed and

nutrients, an aeration system, an underground irrigation system and a leachate collecting system

(Azubuike et al. 2016). For its cost-effectiveness and the ability to manage temperature, pH, and

nutrient conditions, this approach is increasingly being employed for bioremediation (Whelan et al.

2015).

1.4.2.2 Landfarming

Landfarming is a technique that includes dumping polluted material onto the soil surface and tilling

it to mix and aerate it (Harmsen et al. 2007, Maciel et al. 2009). Fuels, non-halogenated volatile

organic carbons, pesticides and herbicides are among the contaminants that this technology is meant

to handle. The idea is to encourage indigenous biodegradative bacteria and make it easier for them

to degrade pollutants aerobically. Landfarming appears to be limited to the treatment of 10–35 cm

of surface soil (Kumar et al. 2018). This technology is commonly used to clean up polluted areas

with aliphatic and polycyclic aromatic hydrocarbons, as well as PCBs (Silva-Castro et al. 2012).

1.4.2.3 Bioreactors

In this process, the pollutant is degraded in a reactor or container under controlled conditions. The

bioreactor technique is used to remediate soil or water that has been polluted by volatile organic

pollutants such as BTEX (benzene, toluene, ethylbenzene and xylene). A slurry bioreactor is a

vessel and apparatus which is used to increase the bioremediation rate of soil-bound and water-

soluble pollutants as a contaminated soil and biomass slurry (Kumar et al. 2011). Batch, continuous,

sequential batch biofilm, membrane, fluidized bed, biofilm and airlift bioreactors are among the

many types of bioreactors available globally. Before being placed in a bioreactor, the contaminated

soil must be pretreated or the contamination can be eliminated from the soil in different ways

(EPA 2000).

1.4.2.4 Biofilters

Biofilters are most commonly used to remove gaseous contaminants. The removal of gaseous

contaminants is accomplished using columns packed with microorganisms by converting unwanted

elements, such as CO2, H2O and cell mass, into harmless objects (Boopathy 2000). Biofilters use

a combination of basic techniques such as assimilation, adsorption, desorption and debasement of

gaseous phase contaminants. Microorganisms form a biofilm that adheres to the surface of a solid-

packed medium. The filter bed medium is often composed of natural materials with a wide surface

area in the zone and some supplement supplies. Biofilters are commonly used in water supply

systems to manage moisture and add nutrients (Gopinath et al. 2018).