Magnetic flocculation technology is the introduction of magnetic loading materials in the conventional flocculation precipitation separation process, which effectively combines the flocs produced by flocculation with the loading materials, enhances the flocculation effect, increases the specific gravity of flocs, accelerates the sedimentation rate of flocs, and quickly purifies the water body. The effluent is clear and transparent. The sludge in the clarification tank is first sent to the drum magnetic powder recycling machine for recycling of magnetic powder, and at the same time, the sludge is discharged to the sludge dewatering system. Magnetic flocculation technology is widely used in deep treatment of large, medium, and small sewage plants.

1. The advantages of magnetic flocculation technology

The addition of magnetic powder and flocculant to the magnetic flocculation mixing system can effectively remove bacteria, viruses, and various small particles. Therefore, the SS removal rate is higher than traditional processes, and the effluent quality and stability are high. At the same time, the hydraulic load is high, with an upward flow rate of 15-30m/h and a short residence time. Its high-speed sedimentation performance is excellent, and compared with traditional processes, it has many advantages such as fast speed, high efficiency, small footprint, and low investment.

2. Magnetic flocculation precipitation for deep phosphorus removal of domestic wastewater

The conventional chemical phosphorus removal process faces problems such as loose flocs, slow sedimentation, large footprint, high chemical dosage, large sludge volume, and high operating costs, which have hindered the development of conventional chemical phosphorus removal processes.

Magnetic flocculation precipitation technology, as a physical and chemical phosphorus removal method, overcomes the shortcomings of traditional chemical phosphorus removal. It has the characteristics of dense flocs, fast settling speed, less chemical addition, small footprint, and low operating costs, and is widely used in domestic sewage treatment. Traditional chemical phosphorus removal does not set up sludge reflux due to the unsuitable settling of flocs. Magnetic flocculation sedimentation technology adds a sludge reflux system in the system, and after the sludge reflux, it can still settle at high speed in the sedimentation tank. The main function of sludge reflux is to increase flocculation efficiency and reduce the dosage of flocculants.

Experimental studies have shown that the optimal order for the application of magnetic flocculation technology for deep phosphorus removal in urban wastewater is PAC → magnetic powder → PAM. By comparing the results of two sets of experiments, the use of magnetic flocculation technology has a significant effect on wastewater containing a large amount of phosphate, with a removal rate of 97% and a effluent concentration of 0.15mg/L; However, magnetic flocculation technology has a relatively poor removal effect on organic phosphorus. For wastewater containing a large amount of organic phosphorus, the removal rate of total phosphorus is 75%, and the effluent concentration is 1.43mg/L. Through orthogonal experiments on two sets of experiments, it was found that the impact on the phosphorus removal experiment of magnetic flocculation technology is as follows: PAC concentration>magnetic particle concentration>PAM concentration>precipitation time.

Compared to traditional technologies, magnetic flocculation technology not only has good results in the removal of suspended solids and COD, but also reduces time by 33%, allowing for the use of smaller sedimentation tanks and other buildings to reduce costs. Research has confirmed that the particle size of magnets ranges from 75 to 105 μ The effect of m is the best. In turbulent vortices, due to the addition of magnets, the force on the floc particles changes, leading to continuous changes in direction. The probability of collision between flocs and magnets in water increases, making it easier to form magnetic flocs and improving the removal rate. If the magnetic particle size is too large or too small, it will cause the centrifugal inertia force to be too small or too large, leading to the instability or destruction of the floc, so the removal effect is not ideal. However, during the flocculation stage, the large vortices generated by the larger diameter magnetic powder can synergistically promote the formation of flocs, resulting in an increase in removal efficiency. When magnetic induction is strengthened, the water treatment effect is improved. The experimental study reduced the settling time from 10 minutes without using magnetic flocculation technology to 2 minutes by adding magnetic powder, and the amount of PAC consumed decreased from 17.20mg/L of the control to 9.98mg/L, greatly reducing the settling time and dosage.

3. Application of Magnetic Flocculation Technology in Mine Tailings Water Treatment

During the mining process, a large number of fine-grained particles are generated due to crushing processes such as ore dressing. These fine-grained particles in the tailings water of the mine have the characteristics of small particle size, good hydrophobicity, and high negative potential, making it easy to create stable suspended solids. Conventional flocculation and sedimentation processes are difficult to handle sedimentation. In recent years, magnetic flocculation technology has been widely used as a new tailings water treatment technology in various mines. Compared with traditional flocculation and sedimentation processes, mine tailings water has the advantages of low energy consumption, good efficiency, and low cost.

The magnetic seed flocculation technology was used to treat the tailings water of a certain hematite mine. The research results showed that adding magnetic powder can produce more dense flocculation and significantly increase the particle size of the flocculant. The optimal synergistic effect is that the dosage of anionic polyacrylamide and natural magnetic seeds is 2mg/L and 2mg/L, respectively. There is a long-range interaction force between the magnetic seeds and tailings particles, and anionic polyacrylamide has a chemical adsorption bridging effect on the surface of the magnetic seeds and hematite particles.

The relevant research results of magnetic flocculation process for treating mine water show that the optimal coagulant in magnetic flocculation process is PAC, with an optimal dosage of 60mg/L. The type of coagulant PAM has a small impact on the treatment effect. Considering the cost, anionic PAM is selected as the optimal coagulant, with an optimal dosage of 4mg/L. The addition of magnetic seeds has a relatively small impact on the improvement of effluent turbidity and SS removal rate, but it can significantly shorten the sedimentation time. The floc sedimentation time is less than 15 seconds, and the effluent turbidity removal rate can reach over 95%. The stirring intensity of the three-stage reaction tank should decrease to 300, 200, and 100r/min.

4. Application of Magnetic Flocculation Technology in Coking Wastewater Treatment

The industrial wastewater generated from high-temperature coal coking, gas purification, coking product recovery, and refined operations is called coking wastewater. The chemical composition of coking wastewater mainly consists of ammonium salts of inorganic compounds, phenols, monocyclic and polycyclic aromatic compounds in organic compounds, and heterocyclic compounds containing nitrogen, sulfur, and oxygen. Currently, the use of biochemical methods to treat coking wastewater is a choice for many countries. Due to its difficult biodegradability, it is difficult to use biochemical methods to treat wastewater to meet standards, and therefore, for the deep treatment of coking wastewater, mixed methods are often used.

At present, relevant departments generally use coagulation and adsorption methods for the advanced treatment of coking wastewater, while magnetic flocculation technology, as an emerging water treatment technology, has attracted more and more people to conduct research due to its unique advantages. The magnetic powder added to the wastewater diffuses in the water, relying on its magnetic force to obtain a large specific surface area and generate a magnetic field to attract charged particles. At the same time, the suspended magnetic particle particles in wastewater increase the number of suspended particles, thereby increasing the collision opportunity of colloidal particles, making it easier to form flocs, and the removal rate also increases with the enhancement of flocculation effect.

Experimental results have shown that in the initial stage of adding magnetic powder, the various indicators of coking wastewater increase with the increase of dosage. When the dosage reaches 400mg/L, the removal effect is the best, with a CODcr removal rate of 62.5%, a turbidity removal rate of 92.2%, and an ammonia nitrogen removal rate of 22.3%. Compared to the removal rate without the addition of magnetic powder, the removal rate increased by 1.5%, 2.4%, and 1.6%, respectively. However, with the addition of magnetic powder, the removal rate decreased. The experimental observation shows that the combination of magnetic powder and floc forms a composite magnetic floc with smaller volume, tighter binding, and faster settling rate. Analysis shows that during the process of increasing magnetic powder, more magnetic particles can be combined with flocs, resulting in an increase in removal rate. However, as the amount of magnetic powder added exceeds the optimal value, not only does it no longer form magnetic flocs, but the magnetic powder also interferes with the adsorption of pollutants by the flocculant, resulting in a decrease in removal efficiency.

5. The application of magnetic flocculation technology in the upgrading and renovation of sewage treatment plants

Due to the lack of per capita water resources and the pollution of urban water resources, the efficient treatment of sewage has received increasing attention. At present, the treatment process for domestic sewage generally includes primary physical treatment and secondary biochemical treatment, but the effect is not ideal. Magnetic flocculation technology, as a type of advanced sewage treatment technology, has become a new approach for upgrading and renovating sewage plants due to its technological advantages. Many scholars have studied the application of magnetic flocculation technology in the upgrading and renovation of sewage treatment plants in order to meet the requirements of the first level A standard in the "Pollutant Discharge Standards for Urban Sewage Treatment Plants" (GB18918-2002) for the discharge indicators of effluent.

Within a certain range, increasing the dosage of PAC and magnetic powder can improve the removal rate of SS, but with the increase of PAM, the removal rate of SS shows a trend of first increasing and then decreasing. Increasing the dosage of magnetic powder and PAC can improve the turbidity removal rate of wastewater; With the increase of PAM dosage, the turbidity removal rate shows a downward trend, and the stirring speed also seriously affects the removal effect. Both too fast and too slow results in poor results. At the same time, it is proposed that the magnetic powder shows a trend of first increasing and then decreasing; Mixing too fast or too slow results in poor removal efficiency. The earlier the magnet is added, the more favorable it is for the removal of SS. The order of drug addition is magnetic powder, PAC, followed by stirring for 4 minutes before adding PAM.

The total construction scale of Zhangpu Sewage Treatment Plant is 6.0 × 104m3/d. In 2018, it underwent a first-class A standard upgrade and transformation, using magnetic flocculation sedimentation technology. When designing, a dedicated mixing form is used, with downward mixing advancing rapidly and upward pushing rising slowly. Through this optimization design, both rapid mixing and slow flocculation of PAC can be achieved in one tank. Adopting a 4-tank design, the coagulation reaction tank consists of 2 tanks with a total residence time of 5 minutes. The loading reaction tank has a residence time of 2.5 minutes, and the flocculation reaction tank has a residence time of 2.5 minutes. In actual operation, the surface load of the sedimentation tank is 16m3/(m2 · h), and in later operation, the effluent of the sedimentation tank is stable and meets the first-class A standard.

The total construction scale of the sewage treatment plant in Jimo District, Qingdao is 20.0×104m3/d. Due to long-term operation and pressure, the effluent indicators of the sewage plant have not been able to meet the discharge requirements, so the standard improvement has been carried out. However, the land use of the plant area is limited, with only 1500m2 of available land in the southeast corner of the plant area, which cannot meet the requirements of conventional deep treatment land. Finally, Jimo City's sewage treatment can only be improved through small-scale, pilot, and scheme demonstration. Finally, the addition of a magnetic flocculation sedimentation tank is chosen as the renovation process. The flocculation sedimentation tank only covers an area of 1350m2, with a designed hydraulic retention time of 20 minutes and a surface load of less than 20m3/(m2·h). After the renovation is completed, the overall operating cost is expected to be less than 0.09 yuan/m ². 3. The water outflow effect is excellent.

6. Conclusion

Magnetic flocculation technology has been greatly applied in deep phosphorus removal of domestic sewage, treatment of mine tailings water, deep treatment of coking wastewater, and upgrading of sewage treatment plants. The application of magnetic flocculation technology in these fields can reduce the amount of medication, shorten settling time, and improve removal quality, leading to progress in wastewater treatment. This reflects the great prospects of magnetic flocculation technology in the field of wastewater treatment and provides new ideas for the development of flocculation technology.