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This post was last edited by I am kind on 2016-6-13 08:33. I. Sources and characteristics of petrochemical wastewater 1. Characteristics of petrochemical wastewater 2. Classification and sources of petrochemical wastewater 1. Oil-containing wastewater: This is the largest category of wastewater generated in oil refining processes; it contains crude oil, refined oils, lubricants, as well as small amounts of organic solvents and catalysts. Oil in water exists in wastewater primarily in the forms of floating oil, dispersed oil, emulsified oil, and dissolved oil. Oil-containing wastewater mainly comes from condensed water in the plant, oil and gas condensate water, water used for washing oil products by vacuum extraction, and water used for cleaning equipment. 2 Sulfur-containing wastewater mainly comes from the wastewater generated by devices such as tower top oil-water separators, rich gas water washing units, liquid hydrocarbons water washing units, liquid hydrocarbons storage tank water removal systems, and blended gasoline water washing units in secondary processing units like catalytic cracking, catalytic pyrolysis, delayed coking, and hydrocracking in refineries. The discharge volume is not large, but the concentration of pollutants is relatively high. In addition to large amounts of hydrogen sulfide, ammonia, and nitrogen, wastewater also contains phenols, **, and oily pollutants; it has a strong foul odor and is corrosive to equipment. When the pH value is low, sulfides tend to decompose, releasing hydrogen sulfide gas that pollutes the environment. This wastewater should not be discharged directly into a centralized treatment facility; instead, it needs to undergo stripping pretreatment. 3 Alkaline-containing wastewater: This type of wastewater originates from the washing water resulting from the alkaline treatment of diesel, aviation kerosene, and gasoline in units such as atmospheric and vacuum distillation systems as well as catalytic cracking units, as well as the washing water resulting from the alkaline treatment of liquid hydrocarbons. The wastewater contains free caustic soda, petroleum products, as well as small amounts of phenol and sulfur, etc. 4 Saline wastewater. It mainly comes from the drainage from the crude oil electrodesalination and dewatering tanks, as well as the drainage from the production of naphthenates. This wastewater has a high salt and oil content, as well as other impurities; it is severely emulsified and difficult to treat. Phenol-containing wastewater mainly originates from atmospheric and vacuum distillation, catalytic cracking, delayed coking, electrorefining, and polymerization units. Among them, except for the wastewater discharged from the oil-water separator at the top of the fractionation tower in the catalytic cracking unit, which contains high levels of phenols and accounts for more than half of the total phenol content in the wastewater discharged by the refinery, the wastewater discharged from the other units has lower phenol concentrations but larger volumes. If left untreated, this wastewater is highly hazardous, can cause widespread pollution, and has serious effects on humans, crops, and natural water bodies. 6 Production wastewater mainly originates from circulating water, cooling water discharge, boiler water discharge, spray cooling water for oil tanks, and unpolluted surface water, etc. This type of wastewater is rarely polluted; generally, its COD value is less than 60 mg/L, meeting the requirements of ** or local discharge standards. II. Treatment process for petrochemical wastewater: Refinery wastewater contains petroleum-based substances such as floating oil and emulsified oil, as well as high concentrations of organic pollutants like chemical oxygen demand (COD) and phenols; therefore, it is difficult to subject it to direct biochemical treatment. It is necessary to undergo oil separation and flotation treatments to remove most of the floating oil, suspended solids, and some organic pollutants; only then can biochemical treatment be carried out to remove ammonia nitrogen, sulfur, phosphorus, as well as BOD and COD, so that the effluent meets discharge standards. III. Treatment methods for petrochemical wastewater. The wastewater treatment technologies used in refineries are classified into primary treatment, secondary treatment, and tertiary treatment based on the level of purification achieved. The methods used for primary treatment include screens, grit removal, pH adjustment, demulsification, oil separation, air flotation, and coagulation. Secondary treatment methods mainly involve biological treatment, such as activated sludge, biochemical aeration tanks, biofilm processes, biological filters, contact oxidation, and oxidation pond systems. Tertiary treatment methods include adsorption, chemical oxygen demand treatment, membrane processes, etc. Generally, wastewater from oil refineries can meet discharge standards after secondary treatment; however, few enterprises in China employ tertiary treatment. Based on the treatment principles, all treatment methods can be classified into three categories: physical treatment, chemical treatment, and biochemical treatment. 1 Physical treatment 1 Oil separator – gravity method. The oil separator is a common treatment device in the wastewater treatment process for the petrochemical industry. Suspended solids are removed based on the fact that they have different relative densities compared to water in boiling water. This method can only remove larger water or oil droplets; as a primary treatment, it is low-cost but moderately efficient. The most commonly used oil separation tanks in China are horizontal flow oil separation tanks and inclined plate oil separation tanks. 2. Flotation method: Its principle involves the mutual adhesion between bubbles in water and fine suspended particles, resulting in the formation of a suspension that rises to the water surface as foam or scum, which can then be removed. It is commonly used for separating fine particles whose density is close to or less than that of water; as a preliminary treatment in mixing processes, it can effectively remove floating oil, emulsified oil, and suspended solids from refinery wastewater. 3 Filtration: This method is suitable for removing tiny particles from suspensions with relatively low concentrations. In wastewater treatment, filtration is often used as a pretreatment method for adsorption, ion exchange, membrane analysis, etc., and also serves as a tertiary treatment following biological treatment. Filtration using porous materials (1) Screens for removing coarser suspended particles. Typical equipment includes grills, screens, and hair removal machines. (2) Porous filter media excluding particles with very fine particle sizes. (3) Devices that use special semipermeable membranes as filtration media, such as reverse osmosis, ultrafiltration, nanofiltration, and electrodialysis. Filtration of granular materials utilizes the pores between the filter particles to allow water to pass through while retaining suspended solids. It is commonly used to ensure that the turbidity of treated water meets water usage requirements. 4 Stripping and stripping-off: By introducing a carrier gas into the wastewater, full contact between the two phases is achieved; the gases dissolved in the wastewater as well as volatile solutes are transferred from the liquid phase to the gas phase through mass transfer, thereby removing the pollutants. The two main pollutants in petrochemical wastewater that require stripping and air stripping treatments are H2S and ammonia; they originate primarily from organic nitrogen and organic sulfur compounds that are destroyed during desulfurization, denitrification, and hydrogenation processes. Phenol can also be removed by this method, but the efficiency is lower than that of sulfur and nitrogen. 2 Chemical treatment 1 Chemical coagulation: This method is used to remove fine suspended particles and colloidal particles that are difficult to eliminate through natural sedimentation in wastewater; air flotation and filtration are generally employed simultaneously. To further improve the efficiency of air flotation treatment, a certain flocculant is added to the wastewater in the recirculation pressurized dissolved-air flotation process. This causes the colloidal suspended particles or emulsified pollutants that are difficult to settle in water to become unstable; under the effect of mutual collisions, they aggregate, coalesce, or link together to form larger particles or flocs. As a result, the pollutants can be more easily removed by settling or floating to the surface. 2 Electrolysis method: A direct current of a certain voltage is applied to the electrolytic cell, and the wastewater is passed through it; this causes the anions of the electrolytes in the wastewater to move toward the anode, where they lose electrons and get oxidized ； Cations move toward the cathode, where they gain electrons and are reduced. This reaction is used to convert the contaminating substances into water-insoluble precipitates, or to produce gases that escape from the water, thereby purifying the wastewater. 3 Neutralization method: The pH value of biological treatment systems should be maintained between 6.5 and 8.5 to ensure optimal activity of microorganisms; therefore, petrochemical wastewater with acid or alkaline concentrations exceeding certain limits needs to be neutralized prior to biological treatment. Methods of neutralization include neutralization of acid-base wastewater, chemical neutralization of acidic wastewater, and filtration neutralization of acidic wastewater, etc. Among them, the neutralization of acidic and alkaline wastewater against each other is a simple and economical treatment method; it can handle both acidic and alkaline wastewater simultaneously. However, this method requires that both types of wastewater be present during the production process, as well as sufficient capacity for adjustment. The chemical neutralization method is suitable for various types of acidic wastewater and has strong adaptability to fluctuations in water quality and quantity. Lime is the most commonly used, as it is widely available and inexpensive. 3 Biological treatment 1 Activated sludge process The purpose of the activated sludge process is to remove both dissolved and undissolved organic substances from wastewater, and to convert these substances into flocculent microbial suspensions that are easy to precipitate, so that they can be separated using solid-liquid separation techniques. The activated sludge process includes the plug-flow activated sludge process, the completely mixed activated sludge process, the delayed aeration method, and the oxidation ditch system. Basic process of the activated sludge method: 2 Anoxic-aerobic biological treatment process. The anoxic-aerobic biological treatment process is a wastewater treatment method that combines an anoxic stage with an aerobic stage. In addition to removing organic pollutants from wastewater, it can also eliminate ammonia and nitrogen, which is why it is widely used. Anoxic-aerobic biological treatment process: 3IMBR-A/O process. The process flow of the IMBR-A/O process is as follows: The raw wastewater first passes through a grid to remove large particulate suspended solids and then settles; it is subsequently pumped into the raw water tank, and from there it goes to the inclined plate sedimentation tank before reaching the pre-denitrification section A (anaerobic tank). It then overflows into section O of the aerobic reactor (the aerobic tank), and under the suction of the effluent pump, membrane-filtered water is obtained; the aerobic tank is continuously aerated. 4. The biological film process (biological aerated filter) operates primarily through the oxidative decomposition by microorganisms attached to the biological film on the packing material within the reactor, the adsorption and retention by the packing material and biological film, the sequential predation along the food chain in the direction of water flow, as well as denitrification occurring in the microenvironment within the biological film and in its anaerobic zones. It has the advantages of strong resistance to shock loads, good sludge sedimentation performance, ability to treat low-concentration wastewater, and ease of maintenance and operation. An aerated biological filter can remove carbon-containing organic matter and retain solid suspended particles. Based on the way in which the biofilm comes into contact with wastewater, they can be divided into packed and submerged types. 5 Hydrolysis-acidification - aerobic biological treatment process: Oil refining wastewater is a type of high-concentration organic wastewater with poor biodegradability; moreover, the complex nature of the oil refining process often results in unstable water quality. As a pretreatment process for refinery wastewater, hydrolysis-acidification can significantly improve the biodegradability of the wastewater, providing a solid foundation for subsequent aerobic treatment processes.