How to treat oily wastewater correctly

According to the three existing states of oil in water - suspended state, emulsified state and dissolved state, and the characteristics of wastewater in different states, this paper expounds the main treatment methods of different kinds of oily wastewater, including physical method, chemical method and biological method, and summarizes the characteristics and application conditions of various methods.

1 source, harm and form of oily wastewater
1.1 source of oily wastewater
Oily wastewater is a kind of wastewater with large amount, wide range and serious harm. Its main sources are oily wastewater produced by oil refineries in the petroleum industry; Cooling lubricant and emulsified oil wastewater produced in machinery manufacturing industry; Textile industry, food processing industry and catering industry will also discharge a large amount of oily wastewater.

1.2 hazards of oily wastewater
Oil substances form a thin film on the surface of the water body, which prevents the oxygen in the air from dissolving in the water, resulting in the decrease of dissolved oxygen in the water and the death of plankton in the water body due to hypoxia; At the same time, oil film also hinders the photosynthesis of aquatic plants and affects the self purification ability of water body. Fish, shrimp and shellfish live in oily wastewater for a long time, resulting in taste deterioration and inedibility. Toxic and harmful substances may harm human health through the enrichment of fish and shellfish and through the food chain

1.3 existing forms of oily wastewater
According to the source of oily wastewater and the existence form of oil in water, it can be divided into four categories: floating oil, dispersed oil, emulsified oil and dissolved oil:
(1) Oil slick: the particle size of oil droplets is generally greater than 100 μ M floats on the water surface in continuous phase to form oil film or oil layer.
(2) Dispersed oil: the particle size of oil droplets is 10 ~ 100 μ m. Suspended in water with tiny oil droplets, unstable.
(3) Emulsified oil: the particle size of oil droplets is very small, most of which are 0.1 ~ 2 μ m. It is difficult to achieve oil-water separation.
(4) Dissolved oil: the diameter of oil drop is smaller than that of emulsified oil. It is a kind of particle dispersed oil dissolved chemically.

2 common treatment methods of different forms of oil

2.1 treatment method of oil slick

(1) Physical oil separation.
Common equipment is oil separator, including horizontal flow oil separator, inclined plate oil separator and corrugated inclined plate oil separator. The floating oil on the water surface of the oil separator can be discharged through the oil collecting pipe or skimmed out by special machinery such as slag skimmer, while the small oil separator can be skimmed manually. Removable particle size greater than 60 μ M and most of the solid particles in the wastewater. The method has the advantages of simple equipment, stable operation, strong adaptability and convenient installation, management and operation. However, the removal effect of oil droplets and solid substances with small particle size is poor.

(2) Filtration method.
The mechanism of interception, inertial collision, screening, surface adhesion and coalescence of granular media filter bed is used to remove oil in water, which is generally used for secondary treatment or advanced treatment. Common granular media filter materials include quartz sand, anthracite, glass fiber, walnut shell, polymer and so on. The filtration method has the advantages of simple equipment, convenient operation and low investment cost. However, with the increase of operation time, the pressure drop increases gradually, and backwashing needs to be carried out frequently to ensure normal operation. This method can also be used for the treatment of emulsified oil.

2.2 removal of dispersed oil
Air floatation is usually used to remove dispersed oil. In this method, a large number of bubbles are released from the oil-water suspension, and the tiny oil droplets dispersed in the water are adhered to the bubbles by the action of surface tension, so as to increase the buoyancy of the bubbles and float upward, so as to achieve the purpose of oil-water separation. Feng pengbang and others used the flotation column of new air flotation equipment to treat oily sewage, and the removal rate was about 90%. This method has low energy consumption and low cost. However, it covers a large area, uses a large amount of reagent and generates scum.

2.3 treatment method of emulsified oil
(1) Air floatation
Air floatation technology is a water treatment technology widely used in the treatment of oily wastewater at home and abroad. Its principle is to introduce air or other gases into the water to produce fine bubbles, so that some fine suspended oil beads and solid particles in the water adhere to the bubbles, float to the water surface together with the bubbles to form scum, and the floating speed can be increased by nearly a thousand times, so as to complete the separation of solid and liquid. The oil-water separation efficiency of this method is very high. According to the different ways of generating bubbles, it can be divided into pressurized dissolved air flotation, impeller flotation and aeration flotation. In order to improve the flotation effect, inorganic or organic polymer flocculants can be added to the wastewater, that is, flocculation flotation method.

This method has been widely used in the treatment of oilfield wastewater, petrochemical wastewater and food oil production wastewater. At present, the research on air flotation at home and abroad mostly focuses on the innovation and improvement of air flotation device and the optimal combination of air flotation process. For example, the structure of flotation cell has been changed from square to round, which reduces the dead angle; Overflow weir plate is used to remove scum and mud scraping machinery. In addition, some new devices are also studied.

(2) Chemical method
A method of purifying wastewater by adding chemicals to convert pollutant components in wastewater into harmless substances. Coagulation method is mainly used for oily wastewater, that is, flocculant is added to oily wastewater. After hydrolysis in water, positively charged micelles and negatively charged emulsified oil produce electric neutralization, oil particles gather and the particle size becomes larger. At the same time, flocs are generated to adsorb small oil droplets, and then oil-water separation is realized by sedimentation or air flotation. Common flocculants include inorganic flocculants such as polyaluminium chloride (PAC), ferric chloride, aluminum sulfate and ferrous sulfate, and organic polymer flocculants such as acrylamide and polyacrylamide (PAM). This method is suitable for emulsified oil droplets and other fine suspended solids that cannot be separated by gravity sedimentation.

(3) Physical degreasing

Emulsified oil in water can be separated by high-speed centrifuge (rotating speed higher than 12000 R · min-1). The oil content of the effluent can be reduced to 20 ~ 30 mg / L. The application of this method is limited because of its high energy consumption.

(4) Membrane separation
Membrane separation technology is a new high-efficiency and precision separation technology successfully developed in the 20th century. It uses the screening mechanism to filter and separate according to the characteristics of solution and the size of molecules. Water has strong polarity. Oil is a simple hydrocarbon and a non-polar hydrophobic substance. They are often mixed with surfactants and other chemicals to become a difficult oil-water system. The typical emulsified oil and dissolved oil have small oil droplets and complex surface properties, while the inorganic membrane is blocked by the membrane based on the size of oil droplets due to its physical and chemical properties, such as hydrophilicity and charging. Based on the molecular interaction between membrane and solute, dissolved oil is blocked by membrane, so as to realize the separation and purification of oil-water system.

Membrane process for treating wastewater containing emulsified oil can generally realize oil-water separation without demulsification process, and there is no oily sludge in the process of membrane process for oil-water separation, and the concentrated liquid can be incinerated. The permeate flow and water quality are relatively stable and do not change with the fluctuation of oil concentration in the influent. It is especially suitable for the treatment of high concentration emulsified oil wastewater [2]. Membrane separation technology has the advantages of simple operation, good separation effect and recoverable oil. However, the membrane used has serious pollution, is not easy to clean, and the operation cost is high. It is necessary to further develop membrane materials and membrane pollution control technology with excellent performance to reduce the cost. The development trend is that various membrane treatment methods are combined with each other or with other methods, such as ultrafiltration and microfiltration, membrane separation and electrochemical method, so as to achieve the best treatment effect.

2.4 treatment method of dissolved oil
(1) Biological method. A / O anaerobic aerobic two-stage process is commonly used in oily wastewater treatment.
Firstly, the wastewater enters the anaerobic section. Under the condition of no molecular oxygen, through the action of anaerobic microorganisms (including facultative microorganisms), hydrolysis and acidification, the refractory organics in the wastewater are transformed into easily degradable organics, and the long-chain organics are transformed into short-chain fatty acids, alcohols, aldehydes and other simple organics, so as to improve the biodegradability of the wastewater. The wastewater can remove part of COD under the action of anaerobic bacteria. At the same time, under the action of hydrogen producing and methanogenic bacteria, part of organic matter is decomposed and transformed into hydrogen, methane, carbon dioxide, etc.

Secondly, the wastewater enters the aerobic section. Under the condition of sufficient oxygen supply, the fatty acids, alcohols, aldehydes and short chain hydrocarbons in the wastewater are oxidized by aerobic microorganisms into inorganic substances such as carbon dioxide and water, so as to reduce the COD and oil content in the wastewater. In order to improve the biomass in the reactor, some elastic fillers can be added into the reaction tank, so that there are both evenly distributed biofilm and a large amount of suspended sludge in the tank, which increases the biomass in the reaction tank, greatly strengthens the treatment capacity and enhances the impact load resistance of a / O.

(2) Adsorption method
Adsorption method uses lipophilic materials to adsorb dissolved oil and other dissolved organic matter in wastewater. The most commonly used oil absorbing material is activated carbon, which can adsorb dispersed oil, emulsified oil and dissolved oil in wastewater. This method has strong adsorption capacity and wide application range, but the cost is high and the regeneration of adsorbent is difficult.
There are many oily wastewater treatment methods in the oily wastewater treatment process system, but a single treatment method has great limitations. Therefore, a set of oil removal process system should be developed by integrating physical, chemical and biological treatment methods to improve the oil removal effect and make the effluent quality meet the discharge standard. Oily wastewater is treated continuously by oil separation air flotation anaerobic aerobic biofilter. The results show that the treatment process has the advantages of good treatment effect, stable effluent quality (up to the first-class standard in GB8978-1996) and convenient operation and management. It is one of the effective methods for treating oily wastewater.

3.1 oil separation
The wastewater contains a large amount of suspended oil. Therefore, in this section, the oil separator is used to remove most of the suspended oil, and the oil is removed regularly by mechanical means, so as to reduce the load impact on the subsequent section and create conditions for the later biochemical part. At the same time, the oil collected in this section can be sold, so as to compensate part of the operation cost and reduce the operation cost.

3.2 air flotation
Most of the suspended oil is removed through the oil separator, but the sewage also contains a large amount of dispersed oil and emulsified oil. Therefore, an integrated dosing air flotation tank is used in this section to further treat the sewage,

First, the dispersed oil can be floated to the liquid level through the tiny bubbles generated by the air flotation tank and removed by the scraper,

Second, the disturbance of small bubbles generated by the air flotation tank can make the oil sludge in the wastewater constantly rub against each other, separate the sludge from the oil adsorbed on its surface, float the oil to the liquid level through small bubbles, sink the solid particles to the bottom of the tank and discharge them regularly through the sludge pump,

Third, the flocculant polyaluminium chloride and coagulant aid polyacrylamide can be added to the sewage to promote the adhesion of small oil droplet particles to the floc and form a floating body with a specific gravity less than water to float on the water surface, so as to separate oil and water The scum of the air flotation device is scraped into the sludge drying tank, and the sludge cake is transported out after drying.

3.3 anaerobic treatment
The sewage in this stage contains a small amount of oil, chemical raw materials and domestic sewage. The purpose of collecting domestic sewage and oily sewage in this section is to provide an appropriate ratio of COD ∶ n ∶ P for anaerobic bacteria, which can make anaerobic bacteria grow better and play its role of high COD removal rate As the main structure of the project is located on the riverside in winter, the temperature of the mixed steel plant can be considered to be the lowest. Therefore, the mixed steel plant can be used as the main structure of the project. At the same time, it can play a role in maintaining the temperature of the lawn in winter

3.4 aerobic treatment
The COD value of comprehensive sewage in this stage is relatively low, so aerobic biological filter is used to further remove COD in sewage, and the main equipment is upward flow biological aerated filter (BAF) The particle size of the filter material is small, which can cut and block bubbles, increase the gas-liquid contact area and improve the oxygen utilization rate; High volume load and small volume required by the reactor; High biomass, fast biofilm renewal and good impact load resistance; It integrates biodegradation and solid-liquid separation, and then there is no need to set up a secondary sedimentation tank, which saves the floor area and infrastructure investment The blower fills the BAF unit with oxygen to provide dissolved oxygen required for microbial growth; Regularly start the backwashing pump and switch the fan to backwash each BAF tank; The backwash water of Ba unit and the filtrate of sludge drying tank flow to the oil separation regulating tank for reprocessing

4 Conclusion
At present, there are many treatment methods for oily wastewater, but if only one method is used, it has great limitations. With the improvement of people's environmental requirements, the development trend of oily wastewater treatment methods mainly focuses on:

(1) Develop new, efficient and stable treatment methods and systems on the basis of existing processes and technologies.
(2) Adopt the process of combining multiple methods to give full play to the advantages of each method and avoid its limitations.
(3) Further explore the oil removal mechanism to provide a theoretical basis for realizing the process with high efficiency and low cost.
(4) The most fundamental and effective way to reduce the amount of oily wastewater is to reduce pollution from the source and pay attention to cleaner production.
(5) Water is a valuable resource for human beings, but the available water is decreasing at an extremely fast speed. A large amount of water is polluted every day. Therefore, we should try our best to make the treated oily wastewater meet the reuse standard.