## 1 Introduction  
In the 21 st century, industries related to the extraction, transportation, storage, processing and use of hydrocarbon raw materials continue to actively develop. Various wastewaters of oil-producing, oil-refining, chemical, transporting and other fields of industry form multi-tonnage effluents containing residual concentrations of hydrocarbons [1; 2; 3; 4]. The volume of oilfield-produced water (OPW) alone is on average at least 250 million barrels per day worldwide [2; 5; 6]. Crude oil emulsions and hydrocarbon-contaminated wastewaters and soils are considered sources of potential threat to the environment and living objects [7]. Polyaromatic hydrocarbons (PAHs) and benzene, toluene, ethylbenzene, and xylenes (BTEX) usually predominate among dissolved organic compounds in hydrocarbon-containing wastes [8]. Hydrocarbon-containing wastewaters may contain various toxic organic compounds: aromatic substances, MTBE (methyl tert-butyl ethers), naphthalene acids, methanol, ketones, ethers, brominated organic compounds, estrogens, PCBs (polychlorinated biphenyls), phthalates, linear alkyl benzene sulfonates, furans and others [1; 4; 9; 10]. However, BTEX are rarely considered due to their high volatility and rapid decomposition in water as a source of serious ecotoxicological effects. It is believed that phenols and PAHs (naphthalene, acenaphthylene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, chrysene and others) pose the greatest danger to the environment and human health. Alkylphenols, especially with long (C7-C9-carbon) chain, and naphthenic acids can have a significant negative effect on the endocrine system, causing estrogenic effects in vertebrates [6; 11]. Today, the effective use of chemical, physical-chemical and biological approaches to the purification of hydrocarbon-containing liquid effluents is known (Figure 1). Physical methods using gravity separators, hydrocyclones and reactors with flocculants are applied at the first stage for the primary processing of oily wastewaters and separation of suspended particles [12]. Then, deeper secondary and tertiary stages of purification based on a combination of various physical-chemical (freezing/thawing, membrane treatment, adsorption, oxidation, etc.) and biological methods are carried out [13]. Aerobic, anaerobic and hybrid bioreactors are actively used for the process of elimination of hydrocarbon pollutants [1]. Anaerobic biocatalytic conversion allows a part of the energy potential of hydrocarbon-containing wastewaters to be used in the form of accumulating biogas. At the same time, toxic hydrocarbons can inhibit the activity of methanogens and microbial processes with their participation [14]. At high concentrations of hydrocarbons in the utilization of waste, prior to their introduction into the bioreactor, it is advisable to carry out preliminary physical-chemical treatment, which ensures the rapid destruction of molecules that are difficult to biotransformation [1]. The combinatory treatments involving physical and/or biological and chemical treatments not only provide the effective transformation of pollutants but also allow the conservation of resources and the energy contained in them [4]. Figure 1: Different catalysts and processes based on their use in the treatments of oil-containing wastewaters. As a rule, the purpose of catalytic treatment of polluted media is to reduce the level of their contamination to the limits permissible for their discharge. Due to the complex chemical composition, high concentrations and stability of polluting components, it is difficult for existing purification systems of hydrocarbon-containing wastes and wastewaters to ensure that the results obtained meet the strict requirements imposed on them. In this regard, the search for new effective approaches to the elimination of hydrocarbon pollutants remains relevant.