Grain and oil wastewater treatment combined with air flotation pretreatment A/O activated sludge method

This paper mainly studies the refining wastewater and press wastewater treatment technology of grain and oil plant. The wastewater discharge is 800t/d, and the principle of clear and turbidity diverting and segmentation treatment is used, aiming at the characteristics of high oil content, high COD and many impurities in the refining wastewater of grain and oil plant. Membrane technology is often used in the treatment of oily wastewater, but the membrane technology has a higher effect on the treatment of dissolved oil, and the pollution phenomenon of concentrated membrane is common. Therefore, the development of oily wastewater treatment technology with stable operation and resource material recovery is of great significance to improve water resource protection and reduce environmental pollution.

In this design, air flotation equipment is used for pretreatment, combined with biochemical technology, and the treated wastewater is discharged up to standard. The water temperature of the refining wastewater is 45℃, and the water temperature of the pressing wastewater is 58℃. The medium temperature cooling tower is used to reduce the pressing water temperature and facilitate the growth of microorganisms. After cooling, the wastewater enters the A/O activated sludge biochemical reaction tank, and the effluent reaches the first level discharge standard of GB8978-1996 "Comprehensive Sewage Discharge Standard".

1. Water quality characteristics of wastewater

Grain and oil wastewater is a kind of organic wastewater with high oil and COD content. In a grain and oil factory in Hebei Province, the wastewater is mainly divided into refining wastewater and pressing wastewater due to the characteristics of the production process. Among them, the oil in the refining wastewater is usually mainly distributed in four states in the water, and different forms of oil have different physical and chemical properties:

(1) Oil slick: the particle size of the oil slick is generally greater than 100μm, the dispersed particles in water are larger, and the oil can float faster after stable standing, generally floating on the water surface with continuous phase oil film;

(2) Dispersed oil: dispersed oil particle size of 10~100μm, suspended in water with tiny oil beads, unstable nature, after standing for a period of time to form an oil slick;

(3) Emulsified oil: the particle size of the emulsified oil is less than 10μm, generally 0.1~2μm. The surfactant was added to the wastewater to make the oil beads form a stable emulsion.

(4) Dissolved oil: the particle size of dissolved oil is generally less than a few microns, generally dispersed in the water body in a molecular state or chemical way, forming a stable homogeneous system, which can be removed by biological method.

It can be seen from the wastewater quality that the oil content in the refining wastewater is high, the COD concentration is large, and it is difficult to reach the standard only through biochemical treatment, and other processes must be combined. The horizontal flow gravity separation method was used to separate the oil float and dispersed oil. At the same time, some of the emulsified oil was raised to the surface by adding sulfuric acid demulsification. The effluent water quality after pretreatment is good, which can reduce the load of oil and COD removal in the subsequent process, thus reducing the operating cost and investment of the whole process system. After the wastewater is biochemical, the discharge water quality is as follows

2. Treatment process and its effect
The wastewater treatment process is shown in Figure 1.

According to the characteristics of water quality and quantity of wastewater discharged from the plant and the requirements of effluent index, the refining wastewater of the grain and oil plant is pretreated with air flotation equipment and combined biochemical technology by using the principle of diverting and segmentation treatment. The water temperature of the pressing wastewater is 58℃, and the medium temperature cooling tower is used to reduce the pressing water temperature, and the wastewater enters the biochemical reaction tank after cooling, and the wastewater is discharged after treatment.

2.1 Pretreatment process

This design adopts the pretreatment technology of oil trap, primary air float and secondary air float formation. Among them, air flotation can effectively remove the emulsified oil in the wastewater and remove COD caused by oil substances. Under certain conditions, a large amount of air is dissolved in water to form dissolved air water. As a working medium, a large number of fine bubbles are produced by releasing sudden decompression and rapid release. The scum and oil slick are removed by a special scraping device to achieve the purpose of solid-liquid separation.

2.2 Cooling tower process

In the cooling tower, water is generally used as a circulating coolant, and cold and heat exchange is carried out after water and air flow contact and steam is generated, and steam volatilization takes away heat to achieve evaporative heat dissipation, heat transfer and radiation heat transfer principle to disperse waste heat to reduce water temperature. In this project, the water temperature of the refining wastewater is 45℃, and the water temperature of the pressing wastewater is 58℃. After pretreatment, the water temperature of the two wastewater enters the comprehensive regulating tank is 40~45℃, so the medium temperature cooling tower is selected, and the temperature difference is 10℃, which is reduced to about 33℃.

2.3 A/O activated sludge process

The A/O process is also an anoxic/aerobic process, which was developed in the early 1980s. After the wastewater passes through the regulating tank, it first enters the anoxic tank, and ammoniating bacteria are used to convert organic nitrogen in the wastewater into NH4-N, which enters the aerobic tank together with NH4-N in the original wastewater. In the aerobic tank, the activated sludge method is used to oxidize and decompose carbon-containing organic matter, and under appropriate conditions, nitrosifying bacteria and nitrobacteria are used to nitrate NH4-N in the wastewater to produce NOX-N. In order to achieve the purpose of nitrogen removal of wastewater, the nitrification mixture in the aerobic pool is returned to the anoxic pool through internal circulation, and the organic carbon in the original wastewater is used as the electron donor for denitrification, and NOx--N is reduced to nitrogen.

Compared with the traditional biological nitrogen removal process, the A/O system can use the organic matter in the original wastewater as the carbon source for denitrification to achieve the purpose of simultaneously reducing COD and removing nitrogen. When the anoxic tank is located before the aerobic tank, when the alkalinity of the water is insufficient, the alkalinity can be compensated for partly in the nitrification process due to the increase of denitrification.

The wastewater from the refining workshop has complex organic composition, high content and high COD concentration, and the insoluble suspended substances and some macromolecular colloid are removed from the wastewater by grease trap and secondary air flotation pretreatment. There are still a lot of soluble organic matter, ammonia nitrogen, phosphate and other pollutants in the wastewater. After pretreatment, the wastewater enters the A/O sludge biochemical stage, and the organic pollutants in the wastewater can be degraded by anaerobic and aerobic bacteria.

3. Operation effect

The project has been completed and accepted in December 2020 and put into operation. After the operation parameters of each equipment and facility are confirmed, the wastewater treatment system shall be debugging and running to ensure the designed water quantity, the system shall run automatically, and partial data of the operation process shall be obtained, as shown in the following table.

As can be seen from the data in Table 2, the removal rate of COD of pollutants in wastewater by this treatment process reaches 92.5% in the pretreatment stage, 94.2% in the A/O stage, 98.7% in the SS removal rate, 83.3% in the ammonia nitrogen removal rate, 99.7% in the animal and plant oil removal rate, and 98.7% in the A/O stage. The phosphate removal rate reaches 43.1%, the effluent pH value is between 6 and 9, and the effluent water quality meets the first-level discharge standard and meets the design requirements.

4. Summary

Since the operation of the project, the operation has been stable, COD, SS, ammonia nitrogen, phosphate and other effluent water quality indicators can meet the "Comprehensive sewage discharge standard" (GB8978-1996) level 1 discharge standard. This design uses A two-stage air-float combination A/O activated sludge biochemistry, with the following characteristics:

(1) The principle of clear diverting and segmented treatment can treat refining wastewater and pressing wastewater well, improve treatment efficiency and reduce treatment costs;

(2) The use of air flotation device, combined with oil trap to form a pretreatment section to treat high-concentration oil-containing wastewater, can achieve a good oil removal effect, COD, SS indicators are also significantly reduced;

(3) Pressurized dissolved gas gas flotation method, under certain conditions, a large amount of air is dissolved in water to form dissolved gas water, as the working medium, through the release of sudden decompression, rapid release, produce a large number of fine bubbles, by the special scraping device to remove scum, oil slick, to achieve the purpose of solid-liquid separation. It has the features of compact structure, small footprint, automatic blowdown valve at the bottom can remove sediment sludge, and module design can greatly shorten installation time, which is suitable for engineering promotion.

The process design is reasonable and practical operation is good, and it has high promotion and application value in the treatment of refining wastewater in grain and oil processing industry. Air flotation pretreatment combined with A/O activated sludge biochemical process can efficiently remove oil and degrade high concentration organic pollutants, which is the main feature of this process.