Analysis of the current status of oil-containing sludge treatment at home and abroad

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Analysis of the Current Status of Treatment for Oil-Contaminated Sludge at Home and Abroad Abstract: A large amount of waste oil-contaminated sludge is generated during oil extraction and processing. If such sludge is discharged into the environment without treatment, it will pose a serious threat to environmental safety and human health. Therefore, various countermeasures are being actively taken at home and abroad in an effort to mitigate the hazards caused by oily sludge. It reviews the current status of treatment technologies for oily sludge at home and abroad, and outlines the future development directions of such technologies. 1 Hazards and sources of oily sludge Oily sludge is a by-product of oil production; it is one of the main sources of pollution in oil production and represents a significant challenge to the environmental quality of oil fields and their surrounding areas. Oil-containing sludge contains large amounts of organic matter as well as abundant nutrients such as nitrogen, phosphorus, and sulfur. If such sludge is discharged into water bodies without any stabilization treatment, the organic matter and ammonia nitrogen in it will consume a large amount of oxygen in the water, leading to deteriorated water quality and severely affecting the survival of aquatic organisms. Moreover, these nutrients can cause eutrophication in water bodies, resulting in red tides and green tides in coastal areas. Furthermore, oily sludge with different compositions poses varying levels of harm to the environment and humans. 1.1 Hazards of oily sludge: The composition of oily sludge from oil fields is extremely complex; it constitutes a highly stable suspension emulsion system that contains large amounts of aged crude oil, waxes, asphaltenes, colloids, solid suspended particles, bacteria, salts, acidic gases, corrosion products, etc. It also includes a large quantity of water treatment agents such as coagulants, corrosion inhibitors, scale inhibitors, and bactericides that are added during the production process. Due to its large size, once released it not only occupies a large amount of arable land but also pollutes the surrounding soil, water bodies, and air. Our country has now placed greater emphasis on the discharge of oily sludge. It is now explicitly stipulated that the reckless discharge of untreated oily sludge will result in a fine of 1,000 yuan per m3. While this limits the emission of some pollutants, it still cannot solve the problem at its root. Therefore, the treatment of oily sludge is the primary issue that oil fields need to address. Furthermore, sludge contains a large amount of pathogenic bacteria, parasites (eggs), heavy metals, radioactive isotopes, and other difficult-to-degrade toxic and harmful substances. This poses a serious threat to human health and is the cause of many fatal diseases; it can be considered an invisible killer. As such, oilfield sludge has been classified as hazardous solid waste. 1.2 Sources of oily sludge 1.2.1 Oily sludge generated during crude oil extraction The oily sludge produced during crude oil extraction mainly comes from the surface treatment systems. It also includes oily sludge generated during the treatment of oil production wastewater, as well as flocs formed by coagulants used in wastewater purification, corrosion products and deposits from equipment and pipelines, and bacteria (their dead bodies), all of which contribute to the composition of oily sludge. 1.2.2 Oil-containing sludge is generated during the oil field gathering and transportation process. This oil-containing sludge can also originate primarily from oil tanks, sedimentation tanks, wastewater tanks, and the sludge in oil separation tanks at transfer stations and combined processing plants; as well as from the oil sand and sludge removed from oil treatment facilities in refineries and light hydrocarbon processing plants, as well as from crude oil and oil-containing sludge that results from drilling operations, pipeline leaks. When storing oil in storage tanks, small amounts of mechanical impurities, sand particles, soil, heavy metal salts, as well as heavy oil components such as paraffins and asphaltenes settle at the bottom of the tank, forming bottom sludge. In addition, the discharge from the primary sedimentation tank, secondary sedimentation tank, and well washing water recovery tank can also produce oily sludge. 1.2.3 Oil-containing sludge generated in refinery wastewater treatment plants The oil-containing sludge from refinery wastewater treatment plants mainly comes from the sediment at the bottom of oil separation tanks, the scum in flotation tanks, and the sediment at the bottom of crude oil storage tanks; it is commonly referred to as \"the three types of sludge.\" These oil-containing sludges have varying compositions, with an oil content typically ranging from 10% to 50%, along with a certain amount of solids. 2 Current status of oil-containing sludge treatment at home and abroad. The common methods for treating oil-containing sludge worldwide include incineration, biological treatment, thermal washing, solvent extraction, chemical demulsification, solid-liquid separation, coking, modification of oil-containing sludge properties, and comprehensive utilization of oil-containing sludge. However, many of these methods have certain problems and are not suitable for practical applications. Methods such as incineration are energy-intensive and prone to causing secondary pollution, and oil resources are not recycled ； The biological treatment method takes 41 days to biodegrade 97% of the petroleum hydrocarbons, and the oil resources are not recovered either ； The problems with solvent extraction are its long process, complex procedures, and high treatment costs; it is only suitable for oily sludge containing large amounts of hard-to-degrade organic compounds ； For oil-containing sludge with severe emulsification, chemical demulsification requires the addition of a demulsifier and heating ； The solid-liquid separation method has a low oil recovery rate for oily sludge with high oil content and severe contamination. Therefore, these methods have not been effectively popularized and applied domestically. Therefore, when exploring treatment technologies for oily sludge, the key is to find a technology suitable for the country’s specific conditions, rather than blindly pursuing advanced and expensive treatment methods. Under this principle, we must first familiarize ourselves with some advanced foreign technologies, then identify those aspects of these technologies that can be applied in our country, and combine them with local traditional methods in order to develop an efficient and practical treatment approach. 2.1 Current status of treatment technologies abroad Abroad, a long journey has been taken in searching for suitable treatment technologies for oily sludge, with various treatment methods having been tested. Through extraction and separation methods, crude oil and organic substances that can be directly utilized can be obtained, while the remaining slurry is converted back into usable resources, thereby eliminating pollution and serving as one of the effective measures for environmental protection in oil fields. However, this method has issues such as a long process, complex techniques, and high processing costs, and it still needs further improvement. Pyrodesorption is a modified high-temperature treatment method for sludge. Even if the oily sludge is heated under anaerobic conditions to a temperature between the boiling point of water and the pyrolysis temperature of hydrocarbons, the light hydrocarbons and water are then recovered in a flash tower through evaporation and condensation ； The heavy hydrocarbons and inorganic substances are removed from the separation tower in the form of slurry; after solid-liquid separation, the heavy hydrocarbons are recovered. The high-temperature treatment process does not pollute the air because it recovers hydrocarbon vapors through condensation. The tempering-mechanical separation treatment technology is already quite mature abroad, and in terms of the chemical tempering of sludge, a range of new, highly efficient solvent extraction technologies as well as polymer flocculants have been developed. This process utilizes demulsifiers to alter the structure of oil-containing sludge particles and disrupt the stability of colloids, thereby improving the dewatering efficiency of the sludge. Subsequently, a three-phase centrifuge is used to separate the oil, water, and sludge phases from the treated sludge, allowing for a crude oil recovery rate of over 90%. The hot water washing method is the preferred approach used by the U.S. Environmental Protection Agency to treat oily sludge. Petrochemical companies in France and Germany mostly use incineration, with the ash being used for road construction or buried in designated ash landfills. The heat generated by combustion is used for heating and power generation. 2.2 Current status of domestic treatment technologies Although there are already successful examples of many advanced foreign technologies, the various constraints in our country prevent these efficient technologies from being widely applied here. Our country can only draw on its essence to innovate sludge treatment technologies suitable for our national conditions. Below is a brief analysis of the application of some of these technologies in our country and the problems that have arisen. 2.2.1 Current status of modification and mechanical separation technologies for oily sludge in China In the oily sludge modification systems used in domestic refineries, the flocculants commonly employed include high-molecular-weight inorganic flocculants such as polyaluminum chloride and polyferric sulfate, as well as high-molecular-weight organic flocculants such as highly polymerized non-ionic, cationic, and anionic polyacrylamides and other long-chain macromolecules. Most facilities use cationic organic flocculants, or a combination of organic flocculants and lime, before mechanical dewatering in order to improve the efficiency of flocculation and dewatering, replacing the previously used single inorganic flocculants whose treatment effects varied. In China, mechanical dewatering processes are commonly used for the pre-treatment of refinery sludge, with belt filter presses and centrifuges being the main methods; the moisture content of the sludge after dewatering is 75% to 80%. Based on various experimental results, it has been determined that among the different processes available for treating oily sludge, the process of sludge conditioning followed by mechanical dewatering is the most maturely developed. 2.2.2 Current status of oil-containing sludge incineration technology in China: The vast majority of refineries in China are equipped with sludge incineration facilities, and the waste most commonly treated by incineration is the oil-containing sludge from sewage treatment plants; for example, the Jingmen Petrochemical Plant and Changling Petrochemical Plant in Hubei use counter-current rotary incinerators ； The fluidized bed incinerator used by Yanshan Petrochemical. After incineration, various harmful substances in oily sludge are almost completely removed, achieving good results. However, in our country, sludge incineration still requires large amounts of diesel or used oil; it generates a lot of heat that is not recovered, resulting in high costs and substantial investment. Moreover, the incineration process often causes severe air pollution, as well as a large amount of dust, which leads to a low actual utilization rate of the incineration facilities. 2.2.3 Current status of biological treatment technologies for oily sludge in China Since the 1980s, biological treatment methods have received increasing attention. Both domestically and abroad, many researchers are actively studying efficient microorganisms for degrading organic substances, and many new high-efficiency strains have been developed, thereby expanding the range of applications possible. Methods such as soil cultivation, composting, and sludge bioreactor methods all belong to biological treatment technologies. The greatest advantage of the in-situ treatment method for oily sludge is that it converts petroleum hydrocarbons into harmless soil components through natural processes, resulting in low operating costs. However, the soil cultivation method has a slow purification process, is not suitable for areas with long winters, and causes the accumulation in farmland of hydrocarbons that are difficult for organisms to degrade (mainly high-molecular-weight waxes and asphaltenes). The composting method is only applicable to treating oily sludge with a high hydrocarbon content. Generally speaking, the treatment process is time-consuming; it has poor effectiveness in treating naphthenes, aromatics, and heterocycles, struggles with sludge containing high levels of oil, and fails to allow for the reuse of resources – these are the problems associated with current biological treatment technologies. 3. Ideas regarding the electrochemical treatment of oily sludge: In the current sludge treatment processes used in China’s oil fields, there are essentially no efficient comprehensive treatment systems. As a result, a large amount of sludge that has not been properly treated lacks suitable disposal methods, posing a significant burden on oil field operations. Therefore, researching new efficient processing methods and equipment has become an urgent task. Treating with electrochemical methods essentially involves first washing the sludge using certain activators. By thorough mixing and stirring with the oily sludge, the organic matter in the sludge is completely dissolved in the washing solution. The washing solution is then subjected to electrochemical treatment. By electrolyzing a metal anode, it is dissolved in the liquid to be treated in the form of metal ions; under the influence of the electric field force, these ions combine with OH- ions in water to form polymeric hydroxide flocs. After these flocs adsorb and agglomerate the pollutants in the wastewater, they are pushed to the surface of the liquid by H+ ions (released from the cathode), or they settle and can then be filtered to remove the pollutants. This method is expected to be developed into an efficient treatment process for oily sludge. 4 Conclusions Oil-containing sludge mainly refers to the sludge and oil sands generated during the production processes in the petroleum exploration and development industry as well as the petrochemical industry. It is characterized by large production volumes, high oil content, a high proportion of heavy oil components, few comprehensive utilization methods, and high treatment difficulties. The presence of oily sludge has an adverse effect on the environmental quality in its surrounding area, and it represents a significant challenge in solid waste treatment at present. From the perspective of the current technical status of comprehensive utilization, there is a lack of advanced treatment and utilization technologies with high added value. From a long-term perspective, recycling used oil and making comprehensive use of sludge are effective ways to achieve harmlessness and resource utilization. Local areas should adapt measures to their specific conditions in order to find the best approaches for the comprehensive utilization and resolution of sludge pollution issues. At the same time, it is necessary to appropriately draw on some advanced foreign treatment and management technologies; this will not only fill the gaps in China’s capabilities regarding the treatment of oily sludge but also help to **reduce related costs, making a significant contribution to environmental protection and human health.