1 Source and hazard of oily sewage

1.1 Source analysis of oily sewage

The source of oily sewage in China is very extensive. Oily sewage will be produced in the process of steel refining, industrial production, oil exploitation, pesticide and food processing and production, and these oil pollutants mainly exist in four forms, namely, dissolved oil, dispersed oil, floating oil and emulsified oil.

(1) Petrochemical industry. In China's petrochemical industry, almost any stage from the initial exploitation to the final transportation and consumption will produce oily sewage. With the rapid development of China's science and technology, China's tertiary oil recovery technology has also been widely used, which improves the oil displacement effect, but also makes the composition of sewage more complex.

(2) Chemical and pharmaceutical engineering. Its main source is oily sewage of high concentration process. During the manufacturing process, a large amount of water and lubricating oil will be used in the stages of raw material reaction, product separation and raw material pretreatment, so a large amount of oily sewage will be generated in the later stage.

(3) Metal smelting industry. In the process of smelting metal, we need to cool, clean and lubricate the materials and equipment in contact with oil products, and the lubricating oil may also directly contact with them, thus forming oily sewage.

(4) Food processing and production. In the process of food processing and production in China, oily sewage is produced in the stages of equipment cleaning and machine lubrication.

1.2 Hazard analysis of oily sewage

(1) Oily sewage pollutes drinking water sources. If our daily drinking water source is polluted by oily sewage, not only people and animals will be infected with diseases, but also food poisoning may be caused, which is very harmful; In addition, oily sewage water also contains a certain amount of carcinogens, so it may increase the incidence of cancer in areas polluted by oily sewage;

(2) Oily sewage is discharged into rivers and lakes. The density of oily sewage is smaller than that of normal purified water, so once the oily sewage is discharged into rivers and lakes, it will adhere to the water surface, the air and water gas cannot be exchanged normally, the oxygen content in the water continues to decline, then the aquatic plants cannot grow normally, and the quality of the water body is seriously affected, The utilization value of water resources is greatly reduced;

(3) Oily sewage enters the soil. If the oily sewage is used as irrigation water to irrigate the soil, then the oil stains will be deposited on the surface of the crops. The soil cannot effectively exchange with the outside air, and the metabolism speed of the soil will slow down, which will affect the normal growth of the crops, and even lead to the death of the crops. If the crops containing oil stains are eaten by humans, it will also bring harm to our health.

2 Process flow of oily sewage treatment

Generally, in the process of treating sewage containing oil, the process flow is to separate oil from water for the first time, and then further separate oil from water by coagulation or flotation. At this time, we should add PAM and PAC quantitatively to ensure the full occurrence of flocculation reaction and coagulation reaction, This process can avoid oil blocking the treatment device, and the oil removal performance of each device can also be fully exerted. When the oily sewage enters the high-efficiency combined flotation, a large amount of SS and oil have been removed. At this time, we should first measure the water quality. If the water quality still does not meet the standard, we should use activated carbon filter tank or quartz sand filter tank to filter it and ensure that it meets the quality standard before discharge. The main purpose of our first oil-water separation is to reduce the degree of emulsification of oily sewage. If it is oily sewage with high freezing point and large particle size, heat preservation and heating equipment should be provided during treatment. If it is oily sewage with a small specific gravity difference between oil and water, filtering devices should be used. When selecting materials for treatment devices, we should fully consider the temperature parameter. When the highly efficient combined air flotation scum is discharged to the sludge storage tank, it is pumped by the pneumatic diaphragm pump to the chamber filter press for filtering and dewatering, and finally transported outside for treatment.

3 Key technical methods for oily sewage treatment

3.1 Coagulation method. This method is mainly aimed at the separation of tiny suspended oil particles and colloidal oil particles in oily sewage. First, we should add a certain amount of chemicals to the oily sewage to enable it to have sufficient chemical reaction, and then it will gradually condense into flocs or a relatively stable mixture; After that, we will add coagulant to the sewage, so that the colloidal oil particles in the original sewage are no longer negatively charged, but are electrically neutral, and the flocculent polymer or stable mixture will slowly sink. In the actual treatment process, we often use coagulants such as ferric chloride, basic aluminum oxide, aluminum sulfate and ferrous sulfate, and the accelerated clarifier is usually used as a structure.

3.2 Filtration method. The so-called filtration method is to intercept the particles in the oily sewage under the action of the filter membrane, so as to separate the oil-water and achieve the ideal purification effect. Generally, the filtration method should be the next level treatment method of coagulation method and upward flotation method. After forming polymer or stable mixture, the gelatinous oil stains in sewage can be removed by filtration method. With this treatment method, the oil content of the oily sewage finally treated does not exceed 10mg/l, and the pressure filter and ordinary fast filter are usually used as structures. The management process of adopting the filtration method is difficult, and the hot water backwashing or air reverse aeration operation should be carried out, otherwise the problem of filter material blockage is likely to occur.

3.3 Air flotation method. This method is mainly used to remove emulsified oil and small oil particles from oily sewage. The oil content of the oily sewage treated by this method does not exceed 30mg/l. Its working principle is: first pour a certain amount of air into the oily sewage, so that a large number of bubbles will appear in the sewage, and the bubbles will also float up. At this time, a In an uneven system composed of water and oil, bubbles will combine with oil with more similar density and gradually move upward, which will achieve the effect of oil-water separation. According to the different ways of generating bubbles, we can divide the upward floating method into the following categories:

(1) Dissolved air flotation method. This method realizes oil-water separation by separating bubbles from saturated oily sewage, adding oily sewage and air into the air tank and gradually pressurizing them to ensure that the air has been well dissolved in the sewage. The dissolution time is about 4 minutes, and then the sewage is sent to the flotation tank. When the air suddenly depressurizes, many small bubbles will appear, Bubbles and oil particles float together. The biggest advantage of this method is that sewage and air can be fully integrated;

(2) Air distribution air float method. The working principle of this method is to cut the air dissolved in the water into pieces. The impeller air flotation, pump suction pipe suction flotation, diffuser aeration flotation, jet air flotation and other equipment are commonly used. This method is easy to operate and manage, and its energy consumption is reduced. However, the degree of bubble fragmentation cannot be accurately controlled, and the effect of flotation may be affected;

(3) Electric float. This method is also called electrolytic condensation gas floatation method. Its working principle is to install a positive and negative electrode in the oily sewage, so that under the action of DC, electrolysis will occur and bubbles will also be generated at the cathode. Oil particles will also gradually combine with the bubbles and float upward, finally realizing oil-water separation of oily sewage.