## 1 Introduction  
With the strengthening of public crisis awareness, governments have adopted stricter environmental protection policies and increased environmental supervision and punishment. The amount of petrochemical excess sludge is increasing year by year with the increase in demand for petrochemicals, and the potential environmental risks are increasing due to the particularity of the industry. The rational disposal of petrochemical excess sludge is of great significance to public health and environmental protection. The excess sludge has the characteristics of high water content, large quantity, and complex pollution components. Therefore, reasonable disposal methods of the remaining sludge should be dependent on its pollutant composition analysis and hazard research. At present, various approaches for excess sludge pollutants treatment, such as harmless treatment, sludge utilization, pyrolysis [1; 2], and incineration [3; 4; 5], have been widely utilized. Harmlesstreatment involves bioleaching of heavy metals from sewage sludge [6] and biochemical treatment of sewage sludge [7], etc. Sludge utilization involves preparation of biochar [8] and adsorbent [9], recovery of phosphorus from excess sludge [10], methane production by anaerobic digestion of excess sludge [11; 12], and additive of the cement [13], etc. For example, [PERSON] et al. [14] adopted the biodegradable chelating agent N,N-bis(carboxymethyl) glutamic acid to remove harmful substances, such as cadmium, nickel, and copper, in sludge. [PERSON] et al. [12] utilized excess sludge to produce methane via anaerobic digestion after hydrothermal pretreatment of sludge at 180 \({}^{\circ}\)C for 76 min. [PERSON] et al. [15] employed conical spouted bed reactor to transform excess sludge into bio-oil at the temperature ranging from 450 to 600 \({}^{\circ}\)C. [PERSON] et al. [3] used a fluidized bed to investigate residual sludge incineration pollutant emission reduction process. Although a large variety of reports on the disposal of excess sludge, the environmental risks arising from the disposal of excess sludge need to be evaluated. For example, with respect to the excess sludge for land using, the potential for soil improvement and utilization [16] and the impact on plants [17; 18] should be analyzed. Meanwhile, the migration of heavy metals of excess sludge in the process of soil utilization should be studied [19]. With respect to thermal treatment of the excess sludge, it is necessary to evaluate the production of environmentally hazardous substances such as Polychlorinated dibenzodioxin and Polychlorinated dibenzofuran (PCDD/Fs) [20], hydrocarbons (PAHs) [21], and particulate matter [22] during sludge pyrolysis or incineration. Additionally, the migration [23; 24] or removal [25] of heavy metals from the excess sludge should be studied. Furthermore, some review studies are listed to summarize the existing technologies for disposal of sludges. For example, [PERSON] et al. [26] summarized the treatment of sludge in supercritical water avoiding the pre-drying procedure and secondary pollution, and analyzed supercritical water gasification, supercritical water partial oxidation and supercritical water oxidation technologies of sludge. Recent studies showed that through vermicomposting, industrial sludge could be transformed into matured organic fertilizer or vermicompost in a shorter period. To this end, [PERSON] et al. [27] analyzed the utilization of vermicomposting process to manage industrial sludge in order to assess the feasibility of the technology. In order to improve the energy yield, [PERSON] et al. [28] evaluated the anaerobic fermentation process of sludge which was capable of obtaining bioenergy by biogas production under the functions of microbes. For the purpose of obtaining a final product of sludge which can be used as fertilizers in farm soils, [PERSON] et al. [29] discussed removal technologies of toxic metals from sewage sludge through chemical, physical, and biological treatments.