In recent years, with the growth of the national economy, the catering industry has developed rapidly, and the amount of wastewater discharged has become larger and larger. Since the wastewater contains a high concentration of animal and vegetable oils and a large amount of solid suspended matter, it has become an important source of water pollution. The treatment of oily wastewater in the catering industry can not only protect the ecological environment, but also reduce the burden on urban sewage treatment plants, with obvious environmental and economic benefits. The pollution content in the oily wastewater of the catering industry is very high, and the main pollutants are animal and vegetable oils, chemical oxygen demand and suspended matter. At present, in the catering industry, the use of detergents makes the oil emulsified and floats in the water, thereby greatly increasing the value of chemical oxygen demand in the water, generally reaching several thousand mg/L. Secondly, due to the small specific gravity of oil and detergent, they are bonded together with the suspended matter in the wastewater, making it difficult to settle. It can be seen that animal and vegetable oils are key pollutants. Reducing the oil content in the oily wastewater of the catering industry has a direct impact on reducing chemical oxygen demand and suspended matter. The use of detergents causes the oil in the oily wastewater of the catering industry to be emulsified. It is difficult for general oil-water treatment equipment to separate the oil and water, and the emulsified oil must be demulsified. At the same time, since animal and vegetable oils are white pastes at room temperature, they can easily clog pipes during oil-water separation, which also makes the wastewater treatment difficult.

1 Materials and methods

1.1 Materials

1.1.1 Wastewater for testing

For oily wastewater from the catering industry, the same restaurant has different dates.

The components of the wastewater collected may not be the same, and cannot represent the situation under different conditions. There are also many interfering factors. Therefore, in addition to the wastewater taken from different catering industries, simulated wastewater was also prepared. The test water samples are: 1#, 2#, and 3# water samples were taken from different hotels. 4# water sample was prepared in the laboratory according to the properties of wastewater from the catering industry.

1.1.2 Demulsifier

In view of the nature of oily wastewater from the catering industry, we first discuss how to demulsify the emulsified oil in the wastewater. At present, the demulsifiers and flocculants commonly used in the treatment of machinery, metallurgy, petroleum and other industries in China are: aluminum sulfate, polyaluminum chloride, polyferric sulfate, alum, sodium ferrous sulfate humate, polyacrylamide, etc. In the experiment, we used them in combination to compare the treatment effects. The results are shown in Table 2. The experimental results show that the mixed use of polyferric sulfate, sodium humate and polyacrylamide has obvious effects, and the oil residue and water layer are separated rapidly. The main principle is as follows: cationic polyferric sulfate is used to reduce the surface activity of anionic detergents, destroy the hydration membrane of emulsion oil droplets and its double electric layer structure, so that the oil is precipitated, and then the flocculant is used to condense the oil droplets and separate them from the water. At the same time, polyferric sulfate is acidic, which converts the fatty acid soap in the emulsion into water-insoluble fatty acids, reduces the surfactant in the water, and increases the demulsification effect. Then sodium humate and polyacrylamide are used to assist in flocculation, and the demulsified oil is further condensed, and the oil and water are separated by gravity.

1.2.1 Effect of pH value on demulsification

In order to find out the effect of acidification on demulsification and determine the optimal pH value, the effect of pH on wastewater deoiling was studied under the premise that the dosage of other reagents remained unchanged. The test conditions were: polyferric sulfate was 0.40 mg/L, pH was 7.0, and polyacrylamide was 80 mg/L. Table 3 shows the results of sample 4# under different pH conditions, in which the oil content was determined by non-dispersive infrared spectrophotometry.

It can be seen that within the pH range studied, the deoiling effect is very good, and the oil can be removed to below 20 mg/L. At the same time, within the pH range, there is a faster stratification speed. It was found during the experiment that the pH change at the end will cause the change in the form of the polyferric sulfate hydrolysis product. Under different pH conditions, the percentage of polyferric sulfate hydrolysis products is different, resulting in different deoiling effects. For this reason, certain conditions (polyferric sulfate is 0.40 mg/L, pH is 1.5, and polyacrylamide is 80 mg/L) were controlled to study the different termination pH conditions. The results are shown in Table 4. Table 4 shows that the treatment effect is better when the pH is in the range of 6 to 9, and this range just meets the discharge requirements, so the termination pH can be set to 7.0.

1.2.2 Effect of the dosage of polyferric sulfate

During the experiment, the conditions were controlled as follows: pH start = 1.5, pH end = 7.0, and polyacrylamine was 80 mg/L. The demulsification of different amounts of polyferric sulfate was studied. The test results are shown in Table 5.

It can be seen from Table 5 that the optimal dosage of polyferric sulfate is between 0.30 and 0.60 mg/L. When the dosage is lower than or higher than this range, the oil removal effect is reduced and the stratification time is prolonged.

1.2.3 Effect of polyacrylamide dosage on demulsification

Under the experimental conditions: pH start = 1.5, pH end = 7.0, polyferric sulfate 0.40 mg/L, the effect of polyacrylamide dosage on demulsification was investigated. The results are shown in Table 6.

The results show that the amount of polyacrylamide added has a great influence on the sedimentation and stratification. With the increase of the amount of polyacrylamide, the sedimentation and stratification speed increases. When the amount of polyacrylamide is greater than 40mg/L, the stratification can be completed in about 3 minutes. When the amount of polyacrylamide is greater than 80mg/L, the sedimentation and stratification can be completed in only 2 minutes. At the same time, polyacrylamide has no obvious effect on oil removal within the studied range. During the test, it was seen that the addition of polyacrylamide can make the flocs coarse and dense. This is mainly due to the strong adsorption and bridging effect of the polyacrylamide coagulant, which improves the floc structure and thus improves the purification effect.

1.2.4 Effect of Sodium Humate Dosage

Sodium humate is a selective flocculant with strong adsorption capacity. It has many active groups in its structure. Considering the deep oil removal and purification, it is necessary to find out the effect of sodium humate dosage on oil removal. During the test, the final pH was controlled to be 7.0, the polyferric sulfate was 0, the initial pH was 1.5, and the polyacrylamide was 50 mg/L. The experimental results are shown in Table 7.

As shown in Table 7, sodium humate has a significant effect on removing oil from wastewater. As the amount of sodium humate increases, when the amount of sodium humate is greater than 0.15 mg/L, the oil can be removed to below 20.0 mg/L. However, the use of sodium humate alone has obvious disadvantages, mainly loose slag, slow stratification speed, and the pH value of the effluent is difficult to control. In summary, the results obtained from the conditional experiment are summarized.

After comprehensive analysis, the optimal conditions for oily wastewater separation were determined to be: 0.4 mg/L polyferric sulfate, initial pH 3.0, final pH 7.0, 80 mg/L polyacrylamide, and 0.20 mg/L sodium humate. Under these conditions, the oil content in the purified water was 4.5 mg/L, the chemical oxygen demand was 61.5 mg/L, and the sedimentation time was 2 min. Finally, the 1#, 2#, and 3# water samples were used for the experiment.

From the results, we can see that under the optimal conditions, the effluent quality is clear and transparent, the demulsification efficiency reaches 95%, the residual amount of clear liquid is lower than the national standard, and the chemical oxygen demand is greatly reduced.

1.3 Animal oil anti-coagulation experiment

Heat the animal oil until it is completely melted, then cool it and conduct a solidification temperature test to find out the solidification temperature of the animal oil.

The solidification temperature of animal oil is 26°C. Based on this temperature, a heating device can be added to the oil extraction pipe mouth. When the temperature is low, the pipe mouth temperature can be kept above 26°C to ensure smooth oil suction.

2 Conclusion

Experimental studies have shown that for oily wastewater from the catering industry, taking 0.4 mg/L of polyferric sulfate, adjusting the initial pH to 3.0 and the final pH to 7.0,

Polyacrylamide is 80mg/L, and sodium humate is 0.36mg/L, which can be used as technical control indicators in wastewater treatment. Controlling the temperature above 26℃ can solve the problem of oil pumping pipe blockage.