## 4 Combined Applications of NCs and BCs for Elimination of Hydrocarbon Pollutants  
It is known today that chemical-only or biocatalytic-only approaches are rarely used for deep wastewater treatment containing hydrocarbon pollutants. The mechanisms of action of NCs and BCs for individual application are described in detail in many articles and mainly depend on the chemical composition of hydrocarbon pollutants and catalysts, as well as the conditions for the treatment of contaminants [4; 9; 15; 20; 25; 48; 67]. The possibility of their combination at the secondary and tertiary stages of wastewater treatment is most often considered [10; 94]. The development and implementation of NCs and NComs (Table 1) in combination with highly efficient BCs (Table 2) is the basis for successfully solving urgent problems related to the processes of water and soil purification from hydrocarbon pollutants. Two approaches to the integration of chemical and biocatalytic stages of wastewater treatment are known: (a) sequential physical-chemical and biocatalytic treatment conducted in different combinations and (b) the simultaneous carrying out of physical-chemical treatments and biodegradation in one reactor. To choose a hybrid approach to the removal of pollutants, one of the key parameters is the initial chemical content of the cleaning object and the initial concentration of toxic components. At high concentrations of phenol in effluents, it is recommended to carry out preliminary chemical treatment in combination with further biocatalytic stages [95]. Petroleum refinery wastewater is a complex mixture of hydrocarbons, sulfides, ammonia, oils, suspended and dissolved solids and heavy metals. Depending on the type of wastewater (desalinated, acidic, spent alkaline and oily wastewaters), various sequential combined physical-chemical-biocatalytic purification methods are proposed at the secondary treatment stage (ozonation with treatment in a biofilm reactor with a movable layer or photocatalysis with treatment in a biofilm reactor with a compacted layer, etc.) [94,96]. For the purification of oilfield-produced water containing a mixture of suspended substances (including PAHs: pyrene, phenanthrene, anthracene and naphthalene) and dissolved organic (a mixture of phenol, benzene, xylenes, toluene and ethylbenzene) and non-organic (cations of barium, calcium, magnesium, sodium, potassium and heavy metals and anions such as chloride) compounds, biocatalytic treatment is usually carried out at the secondary stage of purification. Aerobic biocatalytic treatment is followed by a tertiary deep purification stage, which includes one or more stages: electrodialysis, macroporous polymer extraction (MPPE), microporous membrane treatment, electrolysis, ion exchange and AOPs [10]. In general, with the subsequent combination of several approaches to purification, the biocatalytic stages are now successfully combined with membrane technology, advanced oxidation processes (AOPs), electrochemical methods and other modern purification technologies. Among the advantages of hybrid sequential or one-time implemented processes, their environmental friendliness and the possibility of saving resources are indicated. Thus, for the complete mineralization of organic compounds, photocatalysis usually requires too much energetic costs and is often accompanied by the formation of dangerous by-products. The combination of photocatalysis and biodegradation joins the advantages of both technologies; is characterized by simple functioning, low energy consumption and high cleaning efficiency; and represents an actual direction of the current trend of research [97,98]. Membrane bioreactors are increasingly being offered as the technological design of hybrid processes associated with the deep purification of real wastewater [68,99]. Among the main competitive advantages of membrane technologies, it is possible to note the production of highly purified wastewater and the possible separation of growing biomass in a membrane bioreactor.