## 1 Introduction  
PAHs, which are formed by direct connection, bending connection, or aggregation of multiple benzene rings [1; 2; 3], mainly occur during incomplete combustion [4] of fossil fuel, such as coal, oil, and gas, waste incineration sites, or the formation of crude oil [5; 6], and are a class of compounds with genotoxicity, carcinogenicity, mutagenicity, and teratogenicity [7; 8]. Studies have shown that PAHs and their derivatives could pose a threat to almost all organisms. PAHs have long-term health effects even at low levels, such as cataracts, kidney and liver damage, and jaundice, because they are persistent in the environment. They are highly lipophilic and readily adsorb onto particles in both water and aerosols, indicating that exposure to PAHs can occur through inhalation of atmospheric particles, ingestion of contaminated particles (e.g., soil), or dermal contact with contaminated material (e.g., soil, sediment, and water) [9; 10; 11; 12; 13; 14; 15]. In the early 1980s, the United States Environmental ProtectionAgency (EPA) listed 16 PAHs as environmental priority monitoring pollutants [16; 17]. The European Commission and China also identified PAHs as priority hazardous substances causing environmental and human health concerns [18]. Because of rigid aromatic structure, high partition coefficient, and high resonance energy, this type of persistent organic compounds easily absorbs on organic substances above the soil or sediment particle. Moreover, PAHs exist in crude oil and incomplete combustion products of oil, so oil sludge contains a large number of PAHs. The low fluidity, high emulsification, and complex components (e.g., residual oils, benzene series, aromatic hydrocarbons, polychlorinated biphenyl, asphaltenes, colloids, bacteria, and toxic heavy metal elements) of oil sludge also increase the difficulty of PAHs degradation. Therefore, PAHs in oily sludge has become one of the difficulties and hotspots of its harmless. Although PAHs in oily sludge can be degraded or eliminated by volatilization, ultraviolet photolysis, or chemical oxidation [19; 20], biodegradation technology [21; 22] has the incomparable superiority compared to the traditional method with cost-effectiveness and non-invasion advantages. Natural terrestrial plants in nature, tall fescue, willow, reed, and sudangrass [23; 24; 25; 26; 27] can promote the decomposition of PAHs through the rhiz-degradation effect. In addition, various forms of bacterium that can survive in oily sludge (_Pseudomonas_, _Bacillus_, and _Acinetobacter_) and fungus (_White rot fungi_) [28; 29; 30; 31] can be isolated and cultured and then reused for the degradation of PAHs in oily sludge, which can not only quickly adapt to the complex degradation environment but also contribute important roles in the aspect of PAHs' biodegradation. Furthermore, many studies have noticed that the bacterial consortium promoted better organics degradation effects from individual bacteria [32]. The possible reason is that, in the process of bacterial consortium degradation, since the intermediate biotransformation product of one bacteria can be used as the basis for the catabolism and growth of others, a better degradation efficiency of mixed PAHs could be achieved by bacterial consortium [33]. Based on strong synergistic metabolism, the response to complex environment, and degradability to multiple pollutants [34] advantages, bacterial consortium technology has become the first option for PAHs and heterocyclic derivatives degradation method in recent years. The research focused on the PAHs mixture of oily sludge, the bacteria in oily sludge were used to construct the bacterial consortium and reused for the degradation of PAHs mixture in oily sludge. In this study, 10 bacteria isolated from oily sludge were used to catabolize bicyclic NAP (naphthalene), tricyclic PHE (phenanthrene) and tetracyclic PYR (pyrene). The single variable method was used to screen and construct, optimized and improve the P&A bacterial consortium activity.