# Bioaugmentation Treatment of a PAH-Polluted Soil in a Slurry Bioreactor
###### Abstract
In biostimulation, the soil conditions are adjusted with nutrients mainly containing nitrogen and phosphorous; other amendments to control pH or conductivity; and/or a (bio)surfactant in order to increase the bioavailability of the hydrophobic contaminants, thereby increasing bioremediation efficiency [13]. Moreover, soil aeration can increase the microbial count, thereby enhancing the metabolic activities of naturally occurring microbial organisms and accelerating pollutant degradation [14]. The efficiency of bioremediation can be predicted by monitoring the microbial communities [15]. In this regard, bacteria are the prokaryotic microorganisms actively tangled in the degradation of organic pollutants in contaminated sites, and numerous bacterial species can degrade PAHs [12; 16]. Contaminated environments, such as soil or sediments, are rich in microorganisms that can metabolize PAHs; thus, a variety of bacteria capable of degrading compounds such as two-ring or three-ring PAHs have been reported [17; 18; 19]. Nevertheless, less is known about the capacity of bacteria to metabolize PAHs with five or more rings, such as benzo[a]pyrene and benz[a]anthracene, [20; 21; 22]. Nucleic acid-based techniques are feasible approximations [23; 24; 25] to carry out microbial monitoring, and molecular techniques have demonstrated their capacity to assess the microbial diversity of several types of environmental samples. Among the latter, fingerprinting methods such denaturing gradient gel electrophoresis (DGGE) allows for the direct visualization of microbial diversity and the rapid comparison of environmental samples [23; 26]. The degree and rate of PAH bioremediation depend on many factors, including environmental conditions, PAH properties, and microbial capacity [27]. In this regard, ex situ methods, such as bioreactors, permit better control of temperature and amendments to enhance degradation [28]. In particular, slurry bioreactors (SBs) have proven to be suitable for the bioremediation of contaminated soil under controlled environmental conditions [23]. SBs are engineered devices that generally comprise the following four parts: an area for polluted soil handling and conditioning after excavation, the bioreactor battery itself, an area for the handling and disposal of treated soil, and supplementary equipment for the treatment of process by-streams [29]. The main feature of an SB is that the soil inside the reactor is kept in aqueous suspension by mixing, so that the biological activity of the microbial population occurs in saturated conditions and an almost homogeneous suspension [30]. The treatment of soils and sediments in SBs is another option for the bioremediation of soils polluted under controlled conditions [23; 31; 32]. SBs are also very useful in pilot-scale tests to rapidly evaluate the feasibility of other bioremediation techniques, such as landfarming and biopsies [33; 34]. Actually, under slurry conditions, the rate of pollutant depletion depends mainly on the degradation activity of the microorganisms, and the results obtained largely echo the true biological depruation potential of the polluted soil [35]. Furthermore, in contrast to in situ bioremediation techniques, SB technology implies monitoring several environmental parameters, which can lead to enhanced treatment of the polluted soils. In this regard, SBs can involve the addition of nutrients, inoculum (in the case of using bioaugmentation), a variety of possible electron acceptors, or an increased pollutant availability by means of surfactants addition or biosurfactant production, thereby facilitating the degradation of recalcitrant contaminants or severely weathered mixtures [36; 37]. Bioslurry treatments for PAHs at different scales have previously been described [38; 39]. Based on our previous experience using a biopile approach, here, we designed a pilot-scale SB to test the capacity of a bioslurry approach to clean highly PAH-polluted soil (see [27] for details).