Oil separation tank is a treatment structure that uses the difference of specific gravity between oil and water to separate and remove suspended oil with large particles in sewage. The petroleum industry and petrochemical industry discharge wastewater containing a large amount of oil products in the production process; Coking and gasification industries that discharge wastewater from coal with high concentration of tar; Wool spinning industry and meat industry discharge oily wastewater. If these oily wastewater is discharged into water, it will cause pollution, and irrigating farmland will block soil pores and be harmful to crop growth. If the oil in wastewater is recycled, it can not only avoid environmental pollution, but also obtain considerable economic benefits.
Supplement:
Generally, the specific gravity of oil in wastewater is smaller than that of water, and there are three states: ① Suspended state: oil particles are larger and there is a grease trap.
The diameter of oil droplets is above 0. 1 mm, which floats on the water surface and is easily separated from water. In the petroleum industry, this kind of oil accounts for about 60 ~ 80% of the oil content of wastewater. (2) Emulsified state: the dispersed particle size of oil is small, and the oil droplet diameter is below 0. 1 mm, which is in emulsified state and is not easy to float and separate from water. This kind of oil accounts for about 10 ~ 15% of the oil content in wastewater. ③ Dissolved state: the solubility of oil in water is very small, and the oil dissolved in water accounts for 0.2 ~ 0.5% of the oil content of wastewater. The grease trap is mainly used to separate and remove suspended oil from wastewater, while emulsified oil needs to be removed by flotation or coagulation and sedimentation.
Principle:
The basic principle of wastewater treatment in oil separation tank and sedimentation tank is the same, and they all use different proportions of suspended solids and water in wastewater to achieve the purpose of separation. The structure of grease trap mostly adopts advection. Oily wastewater enters the rectangular oil separation tank through the water distribution tank and flows slowly along the horizontal direction. During the flowing process, the oil product floats to the water surface, is pushed into the oil collecting pipe by the oil collecting pipe or the oil scraper installed on the tank surface, and flows into the dehydration tank. Heavy oil and other impurities precipitated in the oil separation tank gather in the sludge hopper at the bottom of the tank and enter the sludge pipe through the sludge discharge pipe. After oil separation, the wastewater overflows to the drainage tank for subsequent treatment to remove emulsified oil and other pollutants.
Most grease traps are made of reinforced concrete or masonry. On the rectangular plane, it is divided into 2 ~ 4 grids along the direction of water flow, and the width of each grid is generally less than 6 meters, so as to evenly distribute water. The effective water depth is less than 2 meters, and the length of the oil separation tank is generally more than 4 times the width of each tank. Oil slick and sludge at the bottom of the oil separation tank are scraped off by chain scraper and sludge scraper respectively. Generally, each grid is equipped with a set of oil scraper and mud scraper and a mud bucket. If a baffle is added in the middle of each compartment, and an oil scraper and sludge bucket are installed on both sides of the baffle, it is called a two-stage oil separation tank (see figure [two-stage oil separation tank]), which can improve the oil removal efficiency, but the equipment increases and the energy consumption increases. If several inclined plates are added to the oil separation tank, the oil removal efficiency can also be improved, but the construction investment is high. In cold areas, in order to prevent oil from freezing in winter, steam pipes can be installed at the bottom of oil collecting pipes for heating. Generally, the grease trap should be covered with a steam pipe under the cover plate, which can keep warm, prevent the grease trap from catching fire and oil products from volatilizing, and prevent dust and sand from entering.
Oil separator is a natural floating separation device. Commonly used oil separators are: horizontal flow oil separator (API oil separator), parallel plate oil separator (PPI oil separator) and inclined plate oil separator (CPI oil separator). The oil content of oil separator is generally less than 50 mg/L.