With the acceleration of oilfield development, oilfield wastewater is increasing day by day, which seriously pollutes the ecological environment. The quality of oilfield sewage is complex and oily. Demulsifier, salt, phenol, sulfur and other substances that pollute the environment. Oilfield wastewater generally has the following characteristics: high oil content (1000 mg/l); High salinity (20000-50000 mg/L); /L)； The PH value is alkaline (7.5-8.5); The wastewater contains bacteria (sulfate reducing bacteria SRB5- 10μm). ?

Oilfield sewage mainly includes crude oil water (also known as oilfield produced water), drilling sewage and other types of oily sewage in the station. According to the factors of oilfield production and environment, there are many ways to treat oilfield sewage. When the oilfield needs water injection, the oilfield sewage will be re-injected into the formation after treatment. At this time, many indexes such as suspended solids and oil in water should be strictly controlled to prevent them from causing damage to the formation. Most oil production units are concentrated in arid areas, and water resources are seriously lacking. How to turn waste water produced in the process of petroleum production into treasure is of great practical significance. ?

In the oilfield developed by water injection, most of the water from the reservoir is injected into the ground with crude oil through oil wells. This water must be removed before crude oil is transported abroad and exported. The discharged sewage contains crude oil, so it is called oilfield produced water. With the extension of oilfield production life, the water content of produced water is rising, and some blocks have reached more than 90%. These oily sewage have become the main source of water injection in oil fields. With the continuous development of low permeability oil fields and off-balance-sheet oil reservoirs, the requirements for the quality of injected water in oil fields are more stringent. The purpose of oilfield sewage treatment is to remove oil, suspended solids, additives and other unfavorable components that hinder water injection and easily cause corrosion and scaling of water injection system. The technologies used include gravity separation, coarse granulation, flotation, filtration, membrane separation and biological method. The water quality components of each oilfield or block are complex and vary greatly, and the water quality requirements of reinjection water after treatment are also different, so the treatment process should be selected. The development of new equipment, new chemicals, new technologies and new technologies has become a new trend of oilfield sewage treatment?

2? Present situation of oilfield sewage treatment technology?

Oil field water treatment processes are generally "oil separation-filtration" and "oil separation-air flotation (or cyclone oil removal)-filtration", commonly known as? "Old Three Sets", its process is mainly to remove oil and suspended matter from wastewater. For a long time, this process has been widely used in the treatment of produced water in various oil fields, and the effect is good. The treated water quality can generally meet the requirements of reinjection. ?

2. 1? Technical classification?

According to the different requirements of oilfield sewage treatment degree and water quality, sewage treatment technology is usually divided into primary treatment, secondary treatment and tertiary treatment. See table 1 for objects removed or processed at all levels. Generally speaking, the primary treatment belongs to pretreatment, and the secondary treatment can remove about 90% of degradable organic matter and 90~95% of solid suspended matter. However, it is difficult to completely remove heavy metal poisons, high-carbon compounds of biodegradable organic compounds and nitrogen and phosphorus during biochemical treatment, and three-stage treatment is still needed. Treatment technologies at all levels mainly include gravity separation, coarse granulation, flotation, filtration, membrane separation and biological method. ?

The primary and secondary treatment mainly adopts the methods of filtration, precipitation and flotation to remove suspended solids from sewage. Remove minerals and most solid suspended matter, oil, etc. In the waste water. The main methods are gravity separation, centrifugal separation, filtration, coarse granulation, neutralization and biological treatment. These technologies are very mature at home and abroad. ?

Liquid-liquid cyclone separation technology is a new industrial water treatment technology developed in 1980s. Centrifugal separation is to make the container filled with wastewater rotate at high speed to form a centrifugal force field. Because the quality of particles and sewage is different, the centrifugal force is also different. Under the action of large centrifugal force, the big ball is thrown outside, while the small ball stays inside and is discharged through different outlets to achieve the purpose of separating pollutants. Firstly, it has been widely used in overseas offshore oil fields. Compared with other degreasing equipment, such as various grease traps, hydrocyclone has high removal efficiency for emulsified oil with small oil droplet size, small floor space and no wearing parts. Hydrocyclone has the advantages of small volume, light weight, good separation performance, safe and reliable operation, and so on. Tertiary treatment is an advanced oilfield sewage treatment method. The main methods are as follows: First, chemical method is mainly used to treat some colloids and soluble substances in wastewater that cannot be removed by physical or biological methods alone, especially emulsified oil in oily wastewater. Include condensation and precipitation, chemical conversion and neutralization; Adsorption can be divided into three types: surface adsorption, ion exchange adsorption and exclusive adsorption. Second, physical and chemical treatment methods usually include air flotation and adsorption. Biological methods are divided into aerobic biological treatment and anaerobic biological treatment. Air flotation is to inject air into water in the form of tiny bubbles, so that the tiny bubbles attach to the oil particles suspended in water, and float up because its density is less than that of water, forming a scum layer to separate from water. Flotation agents are often added to improve the flotation effect. On the one hand, flotation agent has demulsification and foaming function, on the other hand, it has adsorption and bridging function, which can make colloidal particles aggregate and float with bubbles. Adsorption method mainly uses solid adsorbent to remove various pollutants in wastewater. According to the different adsorption force of solid surface, adsorption can be divided into three types: surface adsorption, ion exchange adsorption and exclusive adsorption. The adsorption method used in oilfield sewage treatment mainly uses lipophilic materials to adsorb oil in water. The commonly used adsorption material is activated carbon. Because of its limited adsorption capacity, high cost, difficult regeneration and limited use, it is generally only used for advanced treatment of oily wastewater. Therefore, in recent years, research has been carried out to find new oil absorbents; Third, the biological method is to use the biochemical action of microorganisms to decompose complex organic substances into simple substances and convert toxic substances into non-toxic substances, thus purifying wastewater. ?

Membrane separation technology is considered as "2 1 century water treatment technology", which mainly includes microfiltration, ultrafiltration, nanofiltration and reverse osmosis. These membrane separation products all use the interception ability of special porous materials to remove impurities with a certain particle size in water by physical interception. In particular, ultrafiltration has made some progress in the related research of oil removal, and gradually moved from laboratory to practical application stage. ?

2.2? General technology of oilfield sewage treatment?

The composition of oilfield wastewater is complex, and the oil content and the existing forms of oil in the water are also different. In most cases, it is often mixed with other wastewater, and the single method is often ineffective. At the same time, because all kinds of mechanical methods have their limitations, in practical application, it is usually two or three methods combined to make the effluent quality meet the discharge standard. In addition, the different production methods, environmental protection requirements and the use of treated water in each oilfield make the oilfield sewage treatment process quite different. In these processes, the common primary treatment is gravity separation, flotation and centrifugal separation, which mainly removes oil slick and lipophilic solids; Secondary treatment includes filtration, coarse granulation, chemical treatment, etc. , mainly demulsifying and removing dispersed oil; Advanced treatment includes ultrafiltration, activated carbon adsorption and biochemical treatment. , mainly to remove dissolved oil. The most common oilfield sewage treatment process is shown in figure 1.

2.3? Membrane bioreactor process?

Membrane bioreactor is a new type of water treatment technology, which consists of membrane separation unit and biological treatment unit. The membrane module replaces the secondary sedimentation tank to maintain the high activated sludge concentration in the bioreactor, reduce the occupation of sewage treatment facilities, and reduce the sludge volume by maintaining a low sludge load. Compared with traditional biochemical water treatment technology, MBR has the following main characteristics: high treatment efficiency and good effluent quality; Compact equipment and small floor space; Easy to realize automatic control and simple to operate and manage. Since 1980s, this technology has been paid more and more attention and become one of the research hotspots. At present, membrane bioreactor has been used in more than ten countries, such as the United States, Germany, France and Egypt, with a scale of 6m? 3/d to 13000m? 3/d is not equal. ?

In China, membrane bioreactor is a high-tech wastewater resource, and its development and research are getting deeper and deeper. Although there are few practical applications of membrane bioreactor in China at present, with the increasingly scarce water resources, with the development of membrane technology, the development of new membrane materials and the gradual decline of membrane material costs, membrane bioreactor will have a better application prospect. ?

3? What is the development trend of oilfield sewage treatment technology?

With the aggravation of global water shortage and the deepening of people's understanding of environmental pollution, the reuse of oilfield sewage after treatment has been paid more and more attention. Recent research has the following trends:

(1) Research and development of new water treatment chemicals. Coagulant is an important agent for treating oilfield produced water and drilling sewage. It is the direction of water treatment agent developers to develop a new coagulant with strong coagulation ability, fast demulsification, fast settling speed, small floc volume and equivalent effect under alkaline and neutral conditions. In recent years, the development and application of polyaluminum, iron, silicon and other coagulants with wide sources of raw materials have become hot spots, and the varieties of inorganic polymer coagulants have gradually formed a series; In the organic aspect, the screening of compound formula of organic coagulant and the grafting of polymer are the key points of research. ?

(2) Research and popularization of membrane separation technology. The application of membrane separation technology in oilfield wastewater treatment is still in the industrial test stage, and the main reasons for its difficulty in large-scale industrial application are membrane cost and membrane pollution. Therefore, the future research focuses are: developing new material films with high quality and low price; Methods to reduce membrane pollution; Optimization of cleaning methods and development of cleaning agents. ?

(3) Develop more advanced composite reactors to improve the treatment efficiency and reduce the occupied area. MBR is an effective combination of membrane separation technology and biological treatment technology. Its origin is that membrane separation technology is used to replace the secondary sedimentation tank in activated sludge process for solid-liquid separation. This process not only effectively achieves the purpose of mud-water separation, but also has incomparable advantages over the traditional tertiary sewage treatment process. As a combination of membrane separation technology and biological treatment technology, membrane bioreactor technology has concentrated the advantages of the two technologies and has been applied in some industrial wastewater treatment, but there is no report on its application in oilfield wastewater treatment at present. But as far as its own characteristics are concerned, the application trend of membrane bioreactor in oilfield sewage treatment is irreversible. Therefore, in the long run, the application scope of membrane bioreactor in water treatment will be wider and wider. Today, with the increasingly strict water environment standards, MBR has shown its great development potential and will become a strong competitor to replace the traditional sewage treatment technology in the new century.