## 4 Advanced Bioreactors for Hydrocarbon-Containing Wastewater Treatment
### Aerobic bioreactors
The simultaneous process of hydrocarbon biodegradation and production of mostly extracellular neutral lipids revealed the possibility of bioconversion of toxic industrial wastes into valuable compounds. Fluidized bed bioreactors (FBBs) (Figure 1), in which the biomass is fixed on bed particles fluidized by liquid up-flow, were successfully applied to aerobic biological treatment of industrial and domestic wastewaters [38; 58]. The use of smaller particles than those in MBR and MBBR systems results in much thicker biofilms and therefore FBBs are proven to be able to withstand large loads and operate at lower HRT values compared to typical bioreactor. Large biofilm-liquid interfacial area, high immobilized biomass concentration and improved mass transfer are the major advantages of this type reactors [62]. A biotereatment of petroleum refinery wastewater was investigated in the three-phase (gas-liquid-solid) FBB, in which polypropylene particles were inoculated with the refinery activated sludge sample and fluidized by an upward flow of air [38]. Clogging and channeling of the bed were eliminated through the intensive motion of the particles. Approximately a 90% COD reduction (from 34,250 to 3100 mg/L), was achieved after two weeks of steady-state biomass loading. More recently, a batch FBB inoculated with the pelleted mycelium of the white rot fungus _Trantes versicolor_ ATCC 42530 was used to study the degradation of pharmaceutical micropollutants present in urban wastewaters [58]. Complete removal of 7 out of 10 initially detected pharmaceuticals, including antibiotics and non-steroid anti-inflammatory compounds, was achieved in a non-sterile treatment. It should be noted that successful FBB treatments of hydrocarbon-containing water are often hindered by limitations of mass transfer between three phases: aqueous, hydrophobic (hydrocarbons) and solid (biocatalyst). Thus, greater requirements apply to the carrier that must have an optimal particle size and density to be easily fluidized at the operational flow rate, be non-toxic and absorbing hydrocarbons and microbial cells and be resistant to chemical and mechanical damages. We previously tested [59] three different materials, namely hydrophobized sawdust, granulated macroporous cryogel made of poly(vinyl alcohol) and sponge-like poly(acrylamide) cryogel for the immobilization of  
Figure 1: Different type fluidized-bed bioreactors (FBB): (**a**) two-phase (liquid–solid); (**b**) three-phase (gas–liquid–solid); (**c**) three-phase (gas–liquid–solid) with particle circulation. _Rhodococcus ruler_ IEGM 615 and _Rhodococcus opacus_ IEGM 249 association (Figure 2). It was found that hydrophboized sawdust had a largest surface area available for bacterial adhesion, and, as a result, demonstrated highest level of bacterial cell immobilization. This biocatalyst was used to treat synthetic petroleum wastewater in FBBs continuously operating at different flow rates (0.6 or 1.2 mL/min) for 3 weeks. The FBB with a faster flow rate was most successful, providing 70-100% removal of \(n\)-alkanes (C\({}_{10}\)-C\({}_{19}\)) and 46-70% removal of 2-3-ring PAHs from contaminated water after two weeks. More recently, we evaluated a feasibility of the FBB process using sawdust co-immobilized _Rhodococcus_ cultures for the purification of highly concentrated (COD of 10.3 g/L; TPH of 3.74 g/L) oilfield wastewater [41], resulting in 70% biodegradation efficiencies for alkanes and PAHs within two weeks. Furthermore, the 75-96% removal of heavy metals (Al, Cr, Cu, Fe, Hg, Zn, Mn) resulted from combined effects of physical sorption by sawdust and biosorption by the _Rhodococcus_ biomass. While a treatment of highly saline oilfield wastewater was less efficient in the FBB with _Rhodococcus_ co-cultures immobilized in poly(vinyl alcohol) granules [60], such gel-encapsulated cells were more resistant to salinity and hydrocarbons and could be easily recovered from the bioreactor for further reuse.