Performance Verification, Parameter Optimization and Techno-Economic Evaluation

Abstract

Objective This study aims to analyze the advantages and application prospects of magnetic coagulation compared with conventional coagulation in the field of industrial wastewater treatment. Methods The experiment was carried out in the industrial wastewater treatment plant of Yancheng National High-Tech Development Zone. Wastewater treated by grating, mixing reaction, primary sedimentation, hydrolytic acidification, anaerobic-anoxic-oxic (AAO) process and secondary sedimentation was used as the target influent, and further treated by magnetic coagulation for deep pollutant removal. On the premise of ensuring normal production of the plant, the test was conducted in four phases: stopping magnetic powder dosing, deactivating magnetic recovery circulation, and halting magnetic sludge return sequentially. By monitoring the influent and effluent indices of the magnetic mixing clarifier under different operating conditions throughout the test period, the changes of pollutant concentrations and chemical consumption before and after each operation adjustment were compared, and the influence of magnetic powder on coagulation performance was discussed. Results Compared with conventional coagulation, magnetic coagulation can ensure the effluent meets the Class 1A standard of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002), with an additional consumption of 12.5 kg/d magnetic powder and 0.78 kW·h/kt electricity. The removal rates of total phosphorus (TP) and suspended solids (SS) increased by 7.36% and 6% respectively, while the consumption of polyferric sulfate (PFS) and anionic polyacrylamide (APAM) decreased by 16.67% and 27.27% respectively. The recovery rate of magnetic powder reached 95.21%. Conclusion Magnetic coagulation has higher phosphorus removal efficiency, lower chemical consumption and faster reaction rate than conventional coagulation in the advanced treatment of low-concentration industrial wastewater, which is an effective approach to achieve energy saving, consumption reduction, quality improvement and efficiency enhancement in water treatment.

1. Introduction

Industrial wastewater treatment has long been a key challenge in the water treatment field. Coagulation-sedimentation process is one of the most widely used conventional wastewater treatment technologies due to its convenient operation, high decontamination efficiency and mature technology. In recent years, driven by technological upgrading, stricter discharge standards and internal pressure of quality and efficiency improvement in the water industry, modified coagulation-sedimentation processes have developed rapidly.

However, conventional coagulation processes still have limitations such as high chemical consumption, large sludge production, unstable effluent quality, narrow applicable concentration range, large space requirement and complex operation. Magnetic enhanced coagulation-sedimentation technology (referred to as “magnetic coagulation”) has been widely used in domestic and industrial wastewater treatment due to its high efficiency, short reaction cycle and small footprint.

The magnetic coagulation process can be divided into four stages according to its mechanism:

1. Dosing stage: Magnetic powder particles are added to increase the system density and improve the effective collision probability of particles.
2. Flocculation stage: High-density magnetic flocs are formed with magnetic powder as the core, usually forming aggregates with magnetic powder clusters as the core.
3. Settling stage: Magnetic powder increases the gravity of flocs and accelerates sedimentation.
4. Separation stage: Magnetic powder accelerates floc sedimentation under the action of magnetic field and realizes magnetic-sludge separation.

Through sludge recirculation, re-sedimentation and magnetic powder recycling, the magnetic powder continuously maintains high particle density in the reaction system, realizing rapid nucleation, adsorption and flocculation of particles, thereby accelerating sedimentation, achieving rapid removal of target pollutants in limited space and reducing chemical consumption.

Existing studies have confirmed the obvious advantages of magnetic coagulation over conventional coagulation, but there are few studies on its application in advanced treatment of low-concentration industrial wastewater. To fill this gap, this study was conducted based on the industrial wastewater treatment plant of Yancheng National High-Tech Development Zone, investigating the actual performance of magnetic coagulation in treating low-concentration industrial wastewater, comparing the energy consumption changes before and after magnetic powder application, and analyzing the performance and limitations of the process, so as to provide experience and reference for its future market promotion.

2. Process Flow and Experimental Methods

2.1 Project Background and Process Flow

The industrial wastewater of Yancheng National High-Tech Development Zone comes from 69 enterprises covering feed, food, packaging, waste heat, machinery, microelectronics, new energy, automobile, intelligent equipment and electrical appliances. After pretreatment, the wastewater is discharged into the sewage pipeline and flows to the industrial wastewater treatment plant. The magnetic mixing clarifier is set as the advanced treatment unit after the AAO bioreactor and before chlorination disinfection.

The structure and process flow of the magnetic mixing clarifier are shown in Figure 1. The main equipment includes 4 reaction tanks, 4 agitators, 1 inclined tube sedimentation tank, 1 sludge scraper, 1 sludge storage tank, 1 sludge return pump, 1 magnetic powder recovery pump, 1 high-speed shearing machine and 1 magnetic separator.

The reaction principle is as follows: PFS is dosed at the front end of the reaction tank, mixed rapidly with the influent through the first and second reaction tanks. In the third reaction tank, particles are mixed with magnetic powder to accelerate floc formation. In the fourth tank, flocs are adsorbed and bridged with APAM to form settleable flocs, which then enter the inclined tube sedimentation tank. Finally, sludge-water separation is realized through the zigzag baffle filter.

Part of the settled sludge at the bottom of the tank is returned to the third reaction tank through the sludge return pump. The other part is pumped to the high-speed shearing machine through the magnetic powder recovery pump. Under the action of the magnetic separator, magnetic powder is recovered and re-injected into the third reaction tank for recycling. The remaining sludge with very low magnetic powder content is discharged to the sludge storage tank and pumped to the sludge thickening tank.

The effluent of the magnetic mixing clarifier is designed to meet the Class 1A discharge standard. After AAO treatment, COD and TN already meet the discharge standard, while TP and SS do not. Therefore, this experiment mainly focuses on the actual removal effect of the magnetic mixing clarifier on TP and SS in low-concentration wastewater after AAO, as well as its impact on COD and TN.

2.2 Experimental Design

The experiment was divided into 4 cycles: the first cycle lasted 14 days (normal operation), and the remaining three cycles each lasted 7 days:

• Cycle 1: Normal operation with full magnetic coagulation system (coagulation mixing + sludge return + magnetic recovery circulation + magnetic powder dosing)
• Cycle 2: Stop magnetic powder dosing, retain sludge return and magnetic recovery circulation
• Cycle 3: Stop magnetic recovery device, retain sludge return and coagulation mixing
• Cycle 4: Stop sludge return system, only retain basic coagulation mixing

Samples were collected from the inlet and outlet of the magnetic mixing clarifier at 7:00, 15:00 and 23:00 every day. COD was tested by dichromate method, TP by ammonium molybdate spectrophotometry, TN by alkaline potassium persulfate digestion UV spectrophotometry, and SS by gravimetric method.

The magnetic powder recovery rate (R) was calculated by detecting the magnetic powder content in the sludge return pipeline and the sludge discharge pipeline, as shown in Equation (1):

Where:

• = magnetic powder recovery rate, %
• = mass concentration of magnetic powder in the sludge return pipeline, g/L
• = mass concentration of magnetic powder in the sludge discharge pipeline, g/L

3. Results and Discussion

3.1 TP Concentration Variation

The daily average TP concentration of influent and effluent under four operating conditions was monitored. The influent TP concentration ranged from 1.26 to 1.77 mg/L, and the effluent TP ranged from 0.17 to 0.45 mg/L. The average influent TP of the four cycles was 1.45, 1.67, 1.58 and 1.58 mg/L respectively, and the average effluent TP was 0.26, 0.30, 0.24 and 0.26 mg/L respectively.

When magnetic powder dosing was stopped in Cycle 2, the effluent TP increased sharply, and PFS dosage had to be increased to ensure discharge compliance, which is the combined effect of stopping magnetic powder and rising influent TP concentration. Stopping magnetic recovery and stopping sludge return both led to the decline of TP removal efficiency, indicating that both magnetic powder and sludge return can strengthen the coagulation process. Overall, magnetic coagulation increased TP removal rate by 7.36% compared with conventional coagulation.

3.2 TP Removal Efficiency per Unit PFS Consumption

The average TP removal per kg of PFS consumption in four cycles was 19.25, 17.93, 17.63 and 14.85 g/kg respectively. Comparing Cycle 1 and Cycle 2, magnetic coagulation with 12.5 kg/d magnetic powder consumption achieved 1.32 g/kg higher TP removal effect than conventional coagulation, indicating that magnetic powder promotes TP removal by PFS.

The TP removal efficiency in Cycle 3 was slightly lower than that in Cycle 2, indicating that magnetic circulation is also conducive to TP removal. The TP removal effect per unit mass of PFS in Cycle 4 was significantly lower than other cycles, confirming that stopping sludge return is unfavorable for phosphorus removal by coagulants.

3.3 SS Concentration Variation

The average influent SS concentration of the four cycles was 9.36, 12.29, 12.43 and 10.14 mg/L respectively, and the average effluent SS was 3.64, 5.57, 5.57 and 6.86 mg/L respectively. The SS removal rates were 61.11%, 54.68%, 55.19% and 32.35% respectively.

The SS removal rate with magnetic powder dosing (Cycle 1) was 1.12 times that without magnetic powder (Cycle 2). The difference between Cycle 2 and Cycle 3 was small, while the SS removal rate in Cycle 3 was 1.71 times that in Cycle 4, indicating that sludge return is an important condition to accelerate SS sedimentation and removal.

Monthly monitoring data from February to July showed that the average monthly SS removal rate was 93.33% before magnetic powder application and 93.91% after application, with a 6% increase in removal efficiency. The change was relatively small because the plant dynamically adjusted coagulant dosage according to actual operation to ensure effluent compliance while reducing cost.

3.4 TN Concentration Variation

The average influent TN concentration of the four cycles was 5.72, 6.20, 5.82 and 6.95 mg/L respectively, and the average effluent TN was 5.57, 6.04, 5.67 and 6.65 mg/L respectively. The TN concentration difference between influent and effluent was basically the same in the first three cycles, indicating that magnetic powder addition has no significant effect on TN removal.

The TN removal amount in Cycle 4 was twice that in Cycle 3, suggesting that stopping sludge return may be beneficial to TN removal, possibly because sludge return causes re-release of dissolved nitrogen through mechanical agitation. The specific mechanism needs further experimental verification.

3.5 COD Variation and Magnetic Powder Recovery Rate

The average influent COD of the four cycles was 19.07, 20.43, 18.14 and 19.00 mg/L respectively, and the average effluent COD was 15.64, 14.57, 13.57 and 14.43 mg/L respectively. The COD removal rates were 17.99%, 28.68%, 25.19% and 24.05% respectively.

The COD removal rate in Cycle 2 was 1.59 times that in Cycle 1, indicating that magnetic powder addition is not conducive to COD removal. Laboratory verification tests showed that adding magnetic powder increased the COD of water samples from 19.0 mg/L to 26.5 mg/L, as magnetic powder introduces a small amount of soluble reducing substances into water, but it still ensures effluent COD meets the discharge standard.

Magnetic powder recovery rate detection was carried out in Cycle 1, with samples collected at three time points. The average daily recovery rate of magnetic powder reached 95.21%, as shown in Table 1.

Table 1 Magnetic Powder Concentration and Recovery Rate at Inlet and Outlet of Magnetic Separator

3.6 PFS Daily Consumption Variation

The average daily PFS consumption showed an upward trend across the four cycles, with consumption rates of 0.065, 0.078, 0.077 and 0.093 kg/d respectively. PFS consumption in Cycle 1 was 16.67% lower than that in Cycle 2, confirming that magnetic powder addition helps reduce PFS consumption and save chemical cost.

PFS dosage in Cycle 4 was significantly higher than that in Cycle 3, indicating that sludge return also helps save PFS consumption, which is consistent with the analysis that sludge return improves PFS utilization rate.

3.7 APAM Monthly Consumption and Energy Cost Comparison

Comparison of APAM consumption three months before and after magnetic powder application showed that the average monthly consumption was 0.55 kg/kt before dosing and 0.40 kg/kt after dosing, meaning magnetic powder addition reduced APAM consumption by 27.27%.

The electricity consumption comparison of the magnetic mixing clarifier is shown in Table 2. Based on the electricity price of 0.67 yuan/(kW·h), the average electricity cost under magnetic coagulation condition was 46.50 yuan/kt, 4.3% higher than 44.60 yuan/kt of conventional coagulation.

Table 2 Electricity Consumption Comparison Before and After Magnetic Powder Dosing (February–July)

3.8 Overall Operating Cost Comparison

The overall operating cost comparison between conventional coagulation and magnetic coagulation is shown in Table 3. Under the current actual operation scale (only 1/8 of the design capacity), magnetic coagulation can still save 8,474 yuan per year compared with conventional coagulation. With the increase of actual treatment water volume approaching the design scale, the cost saving effect will be more significant, with an estimated annual cost saving of 67,792 yuan.

Table 3 Overall Operating Cost Comparison Between Conventional and Magnetic Coagulation

3.9 Mechanism of Magnetic Coagulation Advantages

The above results confirm that magnetic powder enhances TP and SS removal in low-concentration influent systems. Magnetic powder accelerates particle collision, floc formation and sedimentation, thus improving TP and SS removal efficiency. In addition, the increased cost of magnetic powder, electricity and maintenance is lower than the reduced cost of PFS and APAM, without affecting the compliance of effluent TN and COD, which proves that magnetic coagulation is superior to conventional coagulation.

4. Engineering Application Insights

4.1 Applicable Scenarios

Magnetic coagulation process is particularly suitable for the following scenarios:

• Upgrading and reconstruction of industrial and municipal wastewater treatment plants requiring stable Class 1A or higher effluent standards, especially for TP and SS deep polishing
• Low-temperature and low-turbidity wastewater treatment, where conventional coagulation has poor floc formation and slow sedimentation
• Wastewater treatment projects with limited site area, as magnetic coagulation has 1/3–1/2 of the footprint of conventional sedimentation processes
• Emergency water quality improvement projects, with short construction period and fast commissioning speed
• Advanced treatment of AAO bioreactor effluent, avoiding the impact of phosphorus removal agents on the activated sludge system

4.2 Key Design Considerations

• Magnetic recovery system matching: The magnetic powder recovery system should be designed according to the actual treatment scale and sludge production. Excessive recovery capacity will cause unnecessary energy waste, while insufficient capacity will lead to high magnetic powder loss and increased operating cost.
• Dosing point optimization: PFS, magnetic powder and APAM should be dosed in stages with corresponding stirring intensity. Rapid stirring is required for coagulant dosing, medium stirring for magnetic powder mixing, and slow stirring for flocculant reaction to avoid breaking flocs.
• Sludge return ratio setting: Appropriate sludge return ratio can maintain high particle concentration in the reaction system, improve coagulation efficiency and reduce chemical consumption. The return ratio is usually recommended to be 2%–5% of the influent flow, and can be adjusted according to actual effluent quality.
• Sludge discharge control: Excessive sludge discharge will increase magnetic powder loss, while insufficient discharge will lead to sludge accumulation and deterioration of effluent quality. Regular detection of magnetic powder content in discharged sludge is recommended to optimize sludge discharge volume.

4.3 Operation and Maintenance Best Practices

• Establish a dynamic adjustment mechanism for chemical dosage based on influent TP and SS concentrations, avoid excessive dosing, and maximize the cost advantage of magnetic coagulation.
• Regularly monitor magnetic powder loss rate, check the operation status of magnetic separator and high-speed shearing machine, and clean and maintain them regularly to ensure stable recovery efficiency.
• Pay attention to the change of effluent COD. When the influent COD is close to the discharge limit, appropriately reduce magnetic powder dosage or optimize the recovery process to avoid COD increase caused by magnetic powder dissolution.
• Formulate a regular backwashing and cleaning plan for the inclined tube sedimentation zone to prevent sludge accumulation and magnetic powder deposition on the inclined tube wall, which affects sedimentation efficiency.

5. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that magnetic coagulation is a highly efficient advanced treatment technology with outstanding advantages in deep phosphorus removal, SS removal and land saving, which is an ideal solution for wastewater treatment plant upgrading and capacity expansion under strict discharge standards.

Our modular magnetic coagulation system adopts integrated design of reaction, sedimentation and magnetic recovery, with factory prefabrication and standardized configuration, which can greatly shorten on-site construction period. Equipped with our intelligent dosing control platform, the system realizes real-time dynamic adjustment of PFS, APAM and magnetic powder dosage based on influent load and effluent quality monitoring, further reducing comprehensive chemical consumption by 10–15% compared with conventional fixed-dosage operation. We also provide full-cycle technical services including process scheme design, equipment supply, commissioning and operation guidance, helping wastewater treatment plants achieve stable compliance while minimizing total operating cost and maximizing the value of quality and efficiency improvement.

6. Conclusion and Prospect

6.1 Conclusion

1. Magnetic powder can promote coagulation reaction. Even under the limitations of low influent concentration and actual treatment scale far below the design capacity, magnetic coagulation still shows better cost performance than conventional coagulation. The advantage will be more significant with the increase of treatment scale and pollutant concentration.
2. Compared with conventional coagulation, magnetic coagulation increases TP removal rate by 7.36% and SS removal rate by 6%, reduces PFS consumption by 16.67% and APAM consumption by 27.27%, with a magnetic powder recovery rate of 95.21%. The total operating cost is lower than conventional coagulation, achieving cost reduction and efficiency improvement.
3. Magnetic powder has a slight negative impact on COD in low-concentration industrial wastewater systems, which is opposite to the conclusion in high-concentration wastewater treatment. It has no significant impact on TN removal, and does not affect the overall effluent compliance.
4. The combined process of AAO + magnetic coagulation for advanced treatment can give full play to the synergistic advantages of both processes, and avoid the impact of phosphorus removal agents on the activated sludge system, providing reliable reference for the combination of magnetic coagulation and activated sludge process for low-concentration industrial wastewater treatment.

6.2 Prospect

1. The water treatment industry is in a period of technological upgrading. With the deepening of research and market popularization, magnetic coagulation may become one of the core processes in the future water treatment industry, applied to various fields of wastewater and raw water treatment.
2. The current magnetic powder recovery system still has problems such as high investment, high energy consumption, difficult dispersion of recovered magnetic powder and structural defects of the device itself, which need further in-depth research and optimization.
3. In future application, equipment specifications should be matched with the actual scale of the plant. High-purity, large specific surface area iron powder or ferroferric oxide magnetic powder is an effective way to reduce cost and improve efficiency for low-concentration industrial wastewater treatment. High-efficiency magnetic powder with low COD interference will be another direction for magnetic coagulation improvement.
4. It is recommended to extend the test period in future studies to weaken the impact of influent pollutant concentration fluctuation, and analyze energy consumption based on the amount of magnetic powder and coagulant consumed per unit mass of pollutant removal, for more scientific and accurate evaluation.

FAQ

Q1: What is the core difference between magnetic coagulation and conventional coagulation?
The core difference is that magnetic coagulation adds micron-sized magnetic powder as the crystal nucleus of flocs during the coagulation process. The high-density magnetic powder greatly increases the density of flocs, making the sedimentation rate 5–10 times faster than conventional coagulation flocs. At the same time, magnetic powder increases the collision probability of particles in the system, improving coagulation efficiency, reducing chemical consumption and greatly reducing the footprint of the sedimentation unit. The magnetic powder can be recovered and recycled through the magnetic separation system, with low loss rate.

Q2: What factors affect the recovery rate of magnetic powder?
The main factors affecting magnetic powder recovery rate include: first, the performance of the magnetic separator, including magnetic field strength, structure design and treatment capacity; second, the effect of the high-speed shearing machine, which needs to fully break the flocs to release the magnetic powder wrapped inside; third, the properties of magnetic powder itself, high-purity and strong-magnetism powder has higher recovery efficiency; fourth, sludge discharge control, excessive sludge discharge will take away more unrecovered magnetic powder and increase loss.

Q3: Does magnetic coagulation increase the COD value of effluent?
For high-concentration wastewater, the COD removal effect of magnetic coagulation far exceeds the trace COD introduced by magnetic powder, so it shows net COD removal. For low-concentration industrial wastewater with very low influent COD, the trace soluble reducing substances in magnetic powder will cause a slight increase of effluent COD, but the increase is small and will not affect the discharge compliance. By using high-purity magnetic powder and optimizing the recovery process, this effect can be further reduced.

Q4: Is magnetic coagulation more expensive than conventional coagulation?
From the perspective of comprehensive operating cost, magnetic coagulation has lower total cost than conventional coagulation under normal design load. Although it increases magnetic powder consumption, partial electricity cost and maintenance cost, it significantly reduces the dosage of coagulant and flocculant, and saves land cost and civil construction investment. The larger the treatment scale, the more obvious the cost advantage. Only when the actual operation scale is far lower than the design value, the cost advantage will be weakened.

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