# Bioaugmentation Treatment of a PAH-Polluted Soil in a Slurry Bioreactor
## 4 Conclusions  
This work has revealed the usefulness of bioaugmentation for degrading PAHs in slurry bioreactor experiments, as well as the complexity of the microbial relationships during the treatments. A notable reduction of PAH (above 80%) content was achieved in very short periods (less than two weeks), irrespective of the use of the autochthonous microbial populations (biostimulation) or the addition of previously isolated specialized microbes (bioaugmentation). The control of the environmental conditions in the bioreactors facilitated the bioavailability of the pollutants (linked to the production of bacterial biosurfactants), and subsequently allowed notable yields of PAH depletion. In this sense, in a previous real-scale study, around 40% of the PAHs were degraded in biopsies in two months (mostly two- and three-ring PAHS were depleted), whereas in this work, the yields were duplicated in a shorter period (15 days), and the degradation notably affected heavy PAHs and other organics, as shown by GC-MS qualitative and quantitative analyses. Therefore, although bioslurry approaches are less cost-effective than landfarming or biopsies, they should be considered when designing real-scale treatments of heavy PAHs or other recalcitrant organics. Given that the scale-up needed to treat high volumes of soil might not be affordable, the treatment of only heavily polluted fine fractions (such as those tested in this study) may be a good option, in combination with soil washing or other technologies. In turn, bioaugmentation with autochthonous bacteria was revealed in this work as a promising tool. In particular, _P. stutzeri_ could be helpful to accelerate the process in a real-scale treatment involving several larger bioreactors. A detailed analysis of the microbial interactions in these processes will provide complementary information to facilitate the rational selection of the most suitable bacteria for bioaugmentation purposes.  
Figure 6: Denaturing gradient gel electrophoresis (DGGE) analysis of the bioslurry reactors. (**A**) biostimulation treatment at times 0, 3, 7, and 15 days; (**B**) _Rhodococcus_ biostimulation-bioaugmentation experiment; (**C**) the same from _Pseudomonas_ biostimulation–bioaugmentation experiment; R and P bands correspond to _R. erythropolis_ and _P. stutzeri_ alone, respectively (arrow).  
**Author Contributions:** Conceptualization, C.S., J.L.R.G and A.I.P.; experimentation and methodology, I.L. and D.B.; resources, J.L.R.G and A.I.P.; writing--original draft preparation, R.F., J.L.R.G. and A.I.P.; writing--review and editing, R.F., J.L.R.G. and A.I.P. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported by the project CTM2016-75894-P (funded by AEI/FEDER, UE). We would like also to thank the Environmental Testing Unit of the Scientific and Technical Resources of the University of Oviedo for their support. [PERSON] obtained a grant from the \"Formacion del Profesorado Universitario\" program, financed by the \"Ministerio de Educacion, Cultura y Deporte de Espana\".  
**Conflicts of Interest:** The authors declare no conflict of interest.