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Article

A Study on the Effects and Response Mechanisms of Different Composite Magnetic Materials in Enhancing Municipal Wastewater Biological Treatment

by
Shengshu Ai
,
Rui Hao
,
Yongtai Gao
,
Wenhua Tong
,
Shangjing Zeng
,
Hong Qu
and
Dejun Bian
*
Key Laboratory of Urban Sewage Treatment of Jilin Province, Changchun Institute of Technology, Changchun 130012, China
*
Author to whom correspondence should be addressed.
Water 2026, 18(9), 1009; https://doi.org/10.3390/w18091009
Submission received: 18 March 2026 / Revised: 17 April 2026 / Accepted: 21 April 2026 / Published: 23 April 2026
(This article belongs to the Section Wastewater Treatment and Reuse)
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Abstract

This study systematically compared the performance of different composite magnetic materials in enhancing activated sludge treatment of municipal wastewater. A sequencing batch reactor (SBR) process was used as the model system. Four different composite magnetic materials were examined: Fe3O4 composite activated carbon, Fe3O4 composite diatomite, Fe3O4 composite composite kaolin, and Fe3O4 composite composite fly ash. Their performance in enhancing activated sludge treatment of municipal wastewater was evaluated in terms of pollutant removal, sludge physicochemical properties, and molecular biology analysis. Furthermore, the two best-performing composite magnetic materials were further investigated at different dosages to examine their pollutant removal performance and mechanisms. Furthermore, the two best-performing composite magnetic materials were further investigated at different dosages to examine their pollutant removal performance and mechanisms. The results showed that, compared with the control system (without material addition), the addition of four composite magnetic materials improved the average removal efficiencies as follows: for the Fe3O4 composite activated carbon, Fe3O4 composite diatomite, Fe3O4 composite kaolin, and Fe3O4 composite composite fly ash systems, COD removal increased by 2.55%, 2.77%, 1.67%, and 4.10%, respectively; TN removal increased by 1.71%, 3.33%, 4.82%, and 0.82%, respectively; and TP removal increased by 4.96%, 7.02%, 6.01%, and 5.76%, respectively. In the Fe3O4 composite diatomite system, the highest average removal efficiencies of COD, TN, and NH4+-N were achieved at a dosage of 50 mg/L, whereas the highest average TP removal efficiency was achieved at a dosage of 200 mg/L. In the Fe3O4 composite kaolin system, the highest average removal efficiencies of COD, TP, and NH4+-N were achieved at a dosage of 50 mg/L, while the highest average TN removal efficiency was achieved at a dosage of 200 mg/L. High-throughput sequencing indicated that the highest activity of denitrifying genera was observed in the Fe3O4 composite diatomite system at a dosage of 50 mg/L and in the Fe3O4 composite kaolin at a dosage of 200 mg/L, respectively. The addition of composite magnetic materials enhances the efficiency of municipal wastewater biological treatment. These findings provide theoretical and technical guidance for the selection of magnetic composite materials in municipal wastewater treatment.

1. Introduction

The activated sludge process is widely used in wastewater treatment because of its advantages such as high treatment efficiency, simple operation, and low investment costs [1,2]. However, with continuous economic development, the quality of influent water at wastewater treatment plants has become increasingly complex, and the standards for effluent water quality have become increasingly stringent, posing significant challenges to the traditional activated sludge process [3,4]. In practical engineering applications, this method has also revealed issues such as poor sludge settling performance, loose structure, and susceptibility to sludge bulking [5]. Therefore, improving the activated sludge process and enhancing wastewater treatment efficiency have become urgent priorities [6,7].
Magnetic materials are widely used in wastewater treatment because of their unique advantages, including excellent adsorption capacity, high specific surface area, and ease of separation. Their removal mechanisms primarily involve adsorption, catalytic degradation, flocculation, and the regulation of microbial processes [8]. For example, adding an appropriate amount of Fe3O4 magnetic powder to an activated sludge system can effectively improve the structure of sludge flocs, enhance sludge settling performance, increase sludge concentration, and suppress sludge bulking by leveraging its magnetization and biomagnetic effects [9]. Studies on the long-term effects of magnetic nanomaterials on the performance of activated sludge treatment systems have shown that their addition can enhance the activity of nitrate reductase and nitrite reductase, increase the abundance of nitrifying bacteria, and ultimately improve the system’s nitrogen removal performance [10].
Over the past three decades, magnetic particle technology has achieved significant advancements in the field of wastewater treatment [11,12]. Evolving from initial basic adsorbents to multifunctional integrated systems, single-component magnetic materials primarily provide magnetic responsiveness for separation or adsorption; however, they suffer from limitations such as uniform surface properties, restricted functionality, susceptibility to agglomeration, and vulnerability to corrosion. Consequently, researchers have developed composite magnetic materials by combining magnetic components with other functional elements, such as porous carriers, catalysts, and bio-affinity materials. These composites maintain the advantage of magnetic separation while integrating and enhancing multiple functions, including adsorption, catalysis, and microbial immobilization, demonstrating substantial potential in areas such as heavy metal adsorption, organic pollutant degradation, microplastic removal, and enhanced biological nitrogen removal [13,14]. Chen et al. utilized magnetic polystyrene composite microparticles to enhance biological denitrification. Compared with the traditional activated sludge method, the removal rates of ammonia nitrogen, total nitrogen (TN), and the system’s simultaneous nitrification and denitrification rates were improved by 7.08%, 24.27%, and 19.53%, respectively [15]. Wang et al. developed a core-shell structured amino-functionalized magnetic nanomaterial, which exhibits strong adsorption capacity for heavy metal ions such as Pb2+ in water [16]. Moreover, magnetic composite materials can serve as efficient microbial carriers and are widely applied in biological treatment processes such as moving bed biofilm reactors, anaerobic digestion, and nitrogen and phosphorus removal [17,18]. Compared with other carrier materials, the primary advantage of magnetic composites is their effective microbial adhesion capability and unique magnetically driven recoverability [19]. They address the limitations of traditional carriers, which are difficult to separate, recycle, and offer limited functionality, while also featuring straightforward preparation methods and low cost [20].
Although numerous studies have demonstrated that composite magnetic materials can optimize the performance of activated sludge systems, most of these studies focused on a single material or only a few materials; to date, no research has systematically compared multiple composite magnetic materials under the same conditions. This study systematically compared the effects of four types of composite magnetic materials, namely, Fe3O4 composite activated carbon, Fe3O4 composite diatomite, Fe3O4 composite kaolinite, and Fe3O4 composite fly ash, on the performance of municipal wastewater treatment using the activated sludge process under identical experimental conditions. Two materials with superior performance were selected, and their impacts on pollutant removal efficiency at different dosages (50, 200, and 400 mg/L) were investigated. Furthermore, the response mechanisms of denitrifying bacterial activity were elucidated through high-throughput sequencing analysis. The research results provide new experimental evidence and theoretical support for the rational selection of composite magnetic material types and dosages based on target pollutants in municipal wastewater.

2. Materials and Methods

2.1. Preparation of Composite Magnetic Materials

Composite magnetic materials were prepared via coprecipitation [21]. Using Fe3O4 composite diatomite as an example, the specific preparation process was as follows. We weighed 3.332 g of FeCl3·6H2O and 1.904 g of FeSO4·7H2O and dissolved them in 100 mL of deionized water. We added 1.5 g of modified diatomite to the mixture, which was ultrasonicated for 10 min to ensure uniform distribution in the solution. Under a water bath at 70 °C, the mixture was stirred continuously at a speed of 200 rpm while slowly adding 20 mL of ammonia solution. After reacting for 15 min, we let it stand for 30 min. The resulting product was washed repeatedly with deionized water and ethanol until the supernatant was neutral and then filtered. The product was dried in a forced-air drying oven (Hangzhou Aipu Instruments & Equipment Co., Ltd. Hangzhou, Zhejiang, China) at 55 °C to obtain Fe3O4 composite diatomite. The preparation methods for the three other composite magnetic materials were the same as above. Details regarding the purity, suppliers, and particle sizes of all reagents used in this study are provided in Supplementary Material Table S1.

2.2. Experimental Apparatus and Operating Conditions

This experiment was conducted in two stages: the first stage involved the optimization of different composite magnetic materials, with five identical sequencing batch reactor (SBR) units set up, as shown in Figure 1. Reactor 1 served as the control group without any magnetic materials added, Reactor 2 received 800 mg/L Fe3O4 composite activated carbon, Reactor 3 was supplemented with 800 mg/L Fe3O4 composite diatomite, Group 4 was treated with 800 mg/L Fe3O4 composite kaolinite, and Group 5 received 800 mg/L Fe3O4 composite fly ash. Each reactor was supplemented with 800 mg of composite magnetic material every 5 d. The reactor functioned in two cycles daily, each lasting 12 h, comprising 0.25 h for influent, 5 h for aeration, 1 h for settling, 0.25 h for effluent discharge, and 5.5 h of idle time. The aeration rate was 600 mL/min, and the sludge retention time (SRT) was 45 d.
The second stage investigated the effects of different dosages of optimized materials on the performance of the enhanced activated sludge process. Seven identical SBRs were established, with SBR1 serving as the control group without any composite magnetic materials. SBR2–4 received 50, 200, and 400 mg/L Fe3O4 composite diatomite, respectively. SBR5–7 were dosed with 50, 200, and 400 mg/L Fe3O4 composite kaolinite, respectively. The material supplement amounts for each reactor every 5 d were 75, 300, and 600 mg. The reactors operated for three cycles daily, each cycle lasting 8 h, with the following phases: influent flow for 0.25 h, aeration for 3.5 h, sedimentation for 1 h, effluent discharge for 0.25 h, and idle time for 3 h. The aeration rate was 600 mL/min, and the SRT was 30 d. The first phase lasted 45 d, and the second phase lasted 60 days; data collection for all reactors commenced only after they had undergone long-term operation and reached stable operating conditions. Details regarding the specific experimental conditions are provided in Supplementary Material Table S2.

2.3. Experimental Water Quality and Inoculum Sludge

The experimental feedwater used simulated domestic wastewater, and the detailed formulation is provided in Supplementary Material Table S3. Tap water was used as the source water. The water was allowed to stand for 24 h before use to remove residual chlorine. The carbon, phosphorus, and nitrogen sources in the feedwater were provided by sodium acetate (CH3COONa·3H2O), potassium dihydrogen phosphate (KH2PO4), ammonium chloride (NH4Cl), beef extract, and peptone, whereas trace elements were provided by FeSO4·7H2O, CuSO4·5H2O, and ZnCl2 [22]. Sodium bicarbonate (NaHCO3) was used to adjust the pH of the feedwater. The specific feedwater quality parameters are shown in Table 1.
All reactors in this study were inoculated with sludge from the same batch and source. The experimental inoculum sludge was obtained from the final stage of the aerobic tank at a wastewater treatment plant in Changchun City. The initial sludge was filtered through a sieve to remove impurities, aerated continuously for 24 h to restore activity, and subjected to equal-volume inoculation. The initial mixed liquor suspended solids (MLSS) concentration in each reactor was adjusted to approximately 4000 mg/L, ensuring uniform initial sludge quality and biomass across all reactors.

2.4. Analytical Methods

2.4.1. Material Characterization Methods

The crystal structure and phase composition of the four composite magnetic materials were characterized using a high-resolution X-ray diffractometer (XRD, SmartLab, Rigaku, Akishima-shi, Tokyo, Japan) operated in the phase analysis mode. Measurements were performed with Cu Kα radiation (λ = 0.15406 nm) at a tube voltage of 40 kV and a tube current of 40 mA under continuous scanning conditions. The scanning range was set from 2θ = 10° to 80° with a scanning rate of 6°/min and a step size of 0.02°. Qualitative analysis of the crystal structure was carried out using MDI Jade 6.0 software with reference to standard PDF cards [23].
The surface microstructure of the materials was characterized by field emission scanning electron microscopy (FE-SEM, Sigma 300, Zeiss, Oberkochen, Germany). Before testing, the samples were uniformly dispersed on conductive adhesive and subjected to gold coating by ion sputtering for 60 s to enhance surface conductivity and prevent charge accumulation during the testing process. The test acceleration voltage was set to 5 kV, and the working distance was adjusted to 8.5 mm. Morphology was captured at different magnifications based on the sample’s morphological features [24].

2.4.2. Water Quality Analysis Methods

During the experimental period, influent and effluent samples were collected every 3 d. Chemical oxygen demand (COD) was determined using the dichromate method (HJ 828-2017) [25]. TN was measured by the alkaline potassium persulfate digestion-UV spectrophotometric method (HJ 636-2012) [25]. Ammonia nitrogen (NH4+-N) was analyzed via Nessler’s reagent spectrophotometric method (HJ 535-2009) [25]. Total phosphorus (TP) was quantified using the ammonium molybdate spectrophotometric method (GB 11893-89) [25]. MLSS concentration was measured by the gravimetric method (GB 11901-89) [25]. Temperature, dissolved oxygen (DO), and pH were monitored continuously using a portable DO meter (Multi 340i, WTW, Munich, Germany).

2.4.3. Microbial Community Structure Analysis

Activated sludge samples were collected from the inoculum and during stable operation of the reactors. Immediately after collection, samples were placed in sterile centrifuge tubes and stored in a −80 °C ultra-low temperature freezer. The samples were then shipped to Paisennuo Biotechnology Co., Ltd. (Shanghai, China) for DNA extraction and sequencing analysis. Total DNA was extracted using the Omega Soil DNA Kit (D5635-02) (Omega Bio-tek, Inc., Norcross, GA, USA). The extracted DNA was subjected to 0.8% agarose gel electrophoresis to assess molecular size, followed by quantification using a Nanodrop spectrophotometer. Using the extracted DNA as a template, PCR amplification was performed with universal primers targeting the V3–V4 region of the bacterial 16S rRNA gene: 338F (5′-barcode+ ACTCCTACGGGAGGCAGCA-3′) and 806R (5′-GGACTACHVGGGTWTCTAAT-3′). The amplification products were recovered via 2% agarose gel electrophoresis and subjected to paired-end 250 bp sequencing on the Illumina NovaSeq 6000 platform. Raw sequences were processed using QIIME2 (version 2022.11) for quality control, merging, and chimera removal. Sequences were then clustered at 100% similarity to generate amplicon sequence variants (ASVs) and abundance tables [26].

3. Results and Discussion

3.1. Selection of Composite Magnetic Materials

3.1.1. Characterization of Different Composite Magnetic Materials

The four composite magnetic materials were characterized by XRD. As shown in Figure 2, the four composite magnetic materials exhibited distinct diffraction peaks at 2θ values of 30.1°, 35.5°, 43.1°, 53.5°, 57.0°, and 62.6°, which corresponded to the Fe3O4 standard card (PDF#89-0691) [27] for the (220), (311), (400), (422), (511), and (440) crystal planes, respectively. The diffraction patterns of all four materials closely matched the standard card for Fe3O4. Thus, the composite magnetic materials synthesized using activated carbon, diatomite, kaolinite, and fly ash as carriers did not alter the original crystal structure of Fe3O4 [28].
Figure 3 shows the SEM images of the four composite magnetic materials. Fe3O4 nanoparticles were uniformly dispersed on the surface of the composite materials. This uniform distribution enhanced the magnetic properties of the composite materials [29]. As shown in Figure 3a, the activated carbon carrier exhibited a loose and porous block-like morphology, with a rough surface and irregularly distributed channels. Fe3O4 nanoparticles predominantly adhered to the pore walls and outer surface, showing relatively uniform particle sizes. In Figure 3b, the surface of diatomite exhibited a well-ordered pore structure with a rod-like morphology. Fe3O4 was loaded on the surface of diatomite and within the pores, with a relatively dense distribution. Figure 3c clearly shows that the kaolinite surface possessed numerous agglomerates, resulting in a rough surface morphology and increased pore structures in the composite material, which helped improve its adsorption efficiency [30]. Figure 3d shows that the fly ash carrier mostly consisted of spherical or spheroidal particles of varying sizes with relatively smooth surfaces. Fe3O4 nanoparticles were attached to the surface of the fly ash in discrete forms or as thin layers, with some particles occupying the gaps between the carrier particles. The overall loading was uniform.

3.1.2. Removal Efficiency of Pollutants by Different Composite Magnetic Materials

To directly compare the enhancement effects of the four composite magnetic material systems on the wastewater treatment system relative to the control system (without material addition), we summarized the average effluent concentrations and removal performance of COD, TP, TN, and NH4+-N for each system. This table can be found in Supplementary Material Table S4. The effluent quality of this study adhered to the Class 1A standard specified in the “Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant” (GB 18918-2002) [31], with the core pollutant limits as follows: COD ≤ 50 mg/L, TP ≤ 0.5 mg/L, TN ≤ 15 mg/L, and NH4+-N ≤ 5 mg/L.
Figure 4a shows the COD removal efficiency of different composite magnetic materials. The average COD concentration in the effluent of the system without added materials was 37.21 mg/L. By contrast, the average COD concentrations in the effluent from the systems with Fe3O4@AC, Fe3O4@DE, Fe3O4@K, and Fe3O4@FA were 28.65, 27.87, 31.66, and 23.29 mg/L, respectively, all of which were lower than that of the system without added materials. The addition of composite magnetic materials enhanced the adsorption capacity of bacterial flocs, helping reduce the loss of free bacteria and improving the biological activity within the system [32]. Additionally, composite magnetic materials had a large specific surface area, facilitating sufficient contact between organic matter and sludge particles, enhancing the degradation capacity of organic matter, and consequently improving the system’s COD removal efficiency [33]. Different composite magnetic materials exhibited significant differences in COD removal efficiency, with removal rates decreasing in the order of Fe3O4@FA > Fe3O4@DE > Fe3O4@AC > Fe3O4@K. This may be attributed to differences in the physicochemical properties of the composite magnetic materials, such as pore structure and specific surface area [34].
Figure 4b shows the removal efficiency of TP by different composite magnetic materials. The average TP concentration in the effluent of the system without added materials was 0.53 mg/L, and the TP concentration in the effluent increased significantly after 33 days of operation, with a sharp increase on the 39th day of operation. By contrast, the average TP concentrations in the effluent of the systems treated with Fe3O4@AC, Fe3O4@DE, Fe3O4@K, and Fe3O4@FA were 0.28, 0.19, 0.24, and 0.25 mg/L, respectively, and the TP concentrations began to rise slowly after 40 days of operation. Compared with the system without added materials, the composite magnetic material systems significantly improved TP removal efficiency, with removal rates decreasing in the order of Fe3O4@DE > Fe3O4@K > Fe3O4@FA > Fe3O4@AC. Among these, the Fe3O4@DE system demonstrated a significant advantage in TP removal, with an average TP removal rate that was 7.02% higher than that of the system without added materials. This difference was attributed to Fe3O4@DE’s abundant pore structure and large specific surface area, which facilitated the efficient adsorption of phosphates in water [35].
Figure 4c shows the removal efficiency of TN by using different composite magnetic materials. The average TN concentration in the effluent of the system without added materials was 13.85 mg/L. The average TN concentrations in the effluent of the systems with added Fe3O4@AC, Fe3O4@DE, Fe3O4@K, and Fe3O4@FA were 13.17, 12.50, 11.96, and 13.42 mg/L, respectively. The average TN removal efficiency of the composite magnetic material addition systems, from highest to lowest, was Fe3O4@K > Fe3O4@DE > Fe3O4@FA > Fe3O4@AC, all of which were superior to the system without material addition. The addition of composite magnetic materials enhanced the compactness of the sludge structure in the system, which facilitated the attachment of nitrifying and denitrifying bacteria with long generation times to activated sludge particles, thereby reducing their loss during the drainage process [36]. Among these, Fe3O4@K exhibited the highest TN removal efficiency, improving the system performance by 4.82% compared with the system without added materials. This may be attributed to the rough surface and porous structure of Fe3O4@K, which could effectively retain and enrich a large number of denitrifying bacteria, thereby enhancing the denitrification rate and TN removal efficiency of the system [15].
Figure 4d shows the removal efficiency of NH4+-N by different composite magnetic materials. The average effluent NH4+-N concentration in the system without added materials was 0.33 mg/L, whereas the systems with Fe3O4@AC, Fe3O4@DE, Fe3O4@K, and Fe3O4@FA exhibited average effluent NH4+-N concentrations of 0.41, 0.32, 0.38, and 0.37 mg/L, respectively. Under a dosage of 800 mg/L, the addition of composite magnetic materials only slightly influenced the NH4+-N concentration in the effluent. The average ammonia nitrogen removal rates for the system without added materials and those with Fe3O4@DE, Fe3O4@K, and Fe3O4@FA all exceeded 98%. The ammonia nitrogen effluent concentrations throughout the entire operation period were far below the Class 1A standards specified in the “Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant” (GB 18918-2002) [31], which is 5 mg/L.

3.1.3. Analysis of the Settling Performance of Activated Sludge Using Different Composite Magnetic Materials

The sludge volume index (SVI) is a key indicator for evaluating the settling performance of activated sludge. A low SVI indicates superior sludge settling performance and enhanced solid–liquid separation efficacy. As shown in Figure 5, after the reactor operated for 21 days, the SVIs of the systems treated with Fe3O4@AC, Fe3O4@DE, and Fe3O4@K were significantly lower than those of the untreated system, indicating that the addition of the three composite magnetic materials significantly improved sludge settling performance. This phenomenon was attributed to the small particle size and large specific surface area of the added composite magnetic materials, which enhanced their binding with bacterial flocs and increased the specific gravity of sludge flocs [32]. Under the influence of a magnetic field, the composite magnetic materials could rapidly aggregate and carry the adsorbed sludge particles to settle together, thereby accelerating the settling rate of activated sludge [37]. During the entire operation period, the SVI of the Fe3O4@FA system was the highest. This may be due to the high density of fly ash, which accumulated at the bottom of the reactor during aeration and failed to fully mix with the sludge, resulting in a loose sludge structure. Therefore, the Fe3O4@FA system exhibited poorer sludge settling performance compared with the three other composite magnetic materials.
In summary, among the four composite magnetic materials, Fe3O4@DE and Fe3O4@K demonstrated the best nitrogen and phosphorus removal efficiency in the system while improving the settling performance of activated sludge within the system. Therefore, Fe3O4@DE and Fe3O4@K were selected as the preferred materials to investigate the effect of their dosage on the performance of the enhanced activated sludge process for municipal wastewater treatment and its underlying mechanism.

3.2. Effect of Dosage on the Performance of the Composite Magnetic Material-Enhanced Activated Sludge Process

3.2.1. Analysis of the Effect of Composite Magnetic Material Dosage on Pollutant Removal Efficiency

Figure 6a shows the removal efficiency of COD with different dosages of composite magnetic materials. The average COD concentration in the effluent of the system without added materials was 39.43 mg/L. At Fe3O4@DE dosages of 50, 200, and 400 mg/L, the corresponding average COD concentrations in the effluent were 31.21, 33.96, and 35.16 mg/L, respectively. At Fe3O4@K dosages of 50, 200, and 400 mg/L, the average COD concentrations in the effluent were 30.11, 34.91, and 33.41 mg/L, respectively. These results indicate that the COD removal efficiency of the Fe3O4@DE system decreased with increasing dosage, with the optimal removal efficiency achieved at 50 mg/L, representing a 2.46% increase in the average COD removal rate compared with the system without added material. At 400 mg/L, the removal efficiency of COD was the lowest. High dosages can lead to particle agglomeration, resulting in a sharp reduction in the effective specific surface area, which weakens adsorption capacity and leads to poor COD removal performance [38]. The COD removal efficiency of the Fe3O4@K system exhibited an initial decrease followed by an increase with increasing dosage. Similarly, the optimal COD removal efficiency was achieved at 50 mg/L, with an average COD removal rate that was 2.86% higher than that of the system without added material.
Figure 6b shows the removal efficiency of TP with different dosages of composite magnetic materials. The average TP concentration in the effluent of the system without added materials was 0.35 mg/L. At Fe3O4@DE dosages of 50, 200, and 400 mg/L, the average TP concentrations in the effluent were 0.34, 0.34, and 0.36 mg/L, respectively. At Fe3O4@K dosages of 50, 200, and 400 mg/L, the average TP concentrations in the effluent were 0.32, 0.35, and 0.33 mg/L, respectively. These experiments indicate that the addition of the two composite magnetic materials within the aforementioned ranges had no significant effect on TP removal efficiency. However, as shown in Figure 4b, Fe3O4@DE and Fe3O4@K exhibited significantly enhanced TP removal efficiency when the addition amounts reached 800 mg/L. At low dosage levels, the number of active sites available from the composite magnetic materials may be insufficient to facilitate an effective phosphorus fixation reaction. Li et al. explicitly noted that the high phosphorus removal efficiency results from the high-affinity adsorption sites formed after Zn(II) incorporation into the lattice structure, and the saturation of these sites requires a sufficient material dosage to match the phosphate concentration in solution [39]. Therefore, the TP removal efficiency of both systems was closely related to the dosage of composite magnetic materials, and a certain threshold must be reached to enhance TP removal performance.
Figure 6c shows the removal efficiency of TN with different dosages of composite magnetic materials. The average TN concentration in the effluent of the system without material addition was 13.58 mg/L, whereas the average TN concentrations in the effluent of the systems with Fe3O4@DE at 50, 200, and 400 mg/L were 9.47, 10.89, and 11.90 mg/L, respectively. For Fe3O4@K at 50, 200, and 400 mg/L, the average TN concentrations in the effluent were 11.17, 10.02, and 11.24 mg/L, respectively. These results indicate that the TN removal efficiency of the Fe3O4@DE system showed a gradually decreasing removal efficiency with increasing dosage, with the optimal removal efficiency at 50 mg/L, and the average removal rate was 11.97% higher than that of the untreated system. In the study by Pan et al. on the impact of magnetic nanoparticles on the formation of aerobic granular sludge and microbial communities, they found that 50 mg/L Fe3O4 nanoparticles can significantly shorten the granulation time and enhance pollutant removal efficiency. However, the effect was closely related to concentration, and high concentrations might lead to increased mass transfer resistance [40]. The TN removal rate of the Fe3O4@K system initially increased and then decreased with increasing dosage, reaching the optimal value at 200 mg/L, with a removal rate increase of 10.14%. An excessively high dosage may induce the agglomeration of the material in the aqueous phase. Such agglomeration can significantly reduce the effective specific surface area of the material, thereby directly impairing its adsorption capacity for TN intermediates or the enzymes involved in the reactions, ultimately resulting in reduced TN removal efficiency [41].
Figure 6d shows the removal efficiency of NH4+-N as a function of the dosage of different composite magnetic materials. The average concentration of NH4+-N in the effluent of the system without added materials was 1.15 mg/L. At Fe3O4@DE dosages of 50, 200, and 400 mg/L, the average concentrations of NH4+-N in the effluent were 0.47, 0.83, and 0.73 mg/L, respectively. At Fe3O4@K dosages of 50, 200, and 400 mg/L, the average NH4+-N concentrations in the effluent were 0.73, 0.87, and 0.92 mg/L, respectively. The study found that the removal efficiency of the Fe3O4@DE system for NH4+-N initially decreased and then increased with increasing dosage, with the optimal removal efficiency at 50 mg/L. The removal efficiency of the Fe3O4@K system for NH4+-N decreased with increasing dosage, peaking at 50 mg/L. At high dosage levels, composite magnetic materials may lead to limited DO mass transfer. Insufficient DO supply directly inhibits the activity of key enzymes such as ammonia monooxygenase (amo), preventing the effective oxidation of ammonia to nitrite and resulting in the accumulation of NH4+-N in the system [42].
In summary, the Fe3O4@DE system achieved the optimal removal efficiency for pollutants at 50 mg/L. The Fe3O4@K system demonstrated the optimal removal efficiency for organic matter and TP at 50 mg/L, with the optimal removal efficiency for TN occurring at 200 mg/L. Compared with dosages of 50 and 200 mg/L, both systems exhibited poorer nitrogen and phosphorus removal performance at 400 mg/L, indicating that excessive dosages may inhibit the system’s performance in removing nitrogen and phosphorus.

3.2.2. Effect of Magnetic Composite Material Dosage on Activated Sludge Characteristics

Figure 7a shows the effect of different composite material dosages on the extracellular polymeric substance (EPS) content in the system. The EPS content in the system without added materials was 71.29 mg/g MLSS. At dosages of 50, 200, and 400 mg/L for the Fe3O4@DE system, the EPS contents were 58.20, 59.95, and 70.43 mg/g MLSS, respectively, showing a trend of an initial slight decrease followed by an increase (approaching the control group). Thus, Fe3O4@DE exhibited concentration-dependent regulatory effects on microbial EPS secretion (short-term effects at low concentrations and promotion of metabolic homeostasis recovery at high concentrations) [43]. At Fe3O4@K dosages of 50, 200, and 400 mg/L, the EPS contents were 97.46, 76.41, and 74.93 mg/g MLSS, respectively. As the dosage increased, the total EPS content decreased while the PN/PS ratio increased, indicating that increased dosage enhanced sludge hydrophobicity and optimized settling performance (the PN ratio in EPS increased, and the sludge structure became increasingly compact), providing a sludge-level explanation for the inhibitory effect on pollutant removal at high dosages (Figure 6) (dense sludge restricts mass transfer efficiency) [44].
Figure 7b shows the effect of different composite material dosages on the DHA content in the system. The dehydrogenase activity (DHA) content in the system without added materials was 11.35 mg TF/(gMLSS·h). At Fe3O4@DE dosages of 50, 200, and 400 mg/L, the DHA contents were 26.26, 33.71, and 35.67 mg TF/(gMLSS·h), respectively. As the dosage increased, the DHA content gradually increased, peaking at 400 mg/L, indicating that elevated concentrations of Fe3O4@DE enhanced metabolic activity (DHA was positively correlated with substrate metabolic capacity). This result was consistent with the partial recovery of NH4+-N removal at high dosages (0.73 mg/L vs. 0.83 mg/L) shown in Figure 6d, where high metabolic activity facilitates pollutant degradation [45]. At Fe3O4@K dosages of 50, 200, and 400 mg/L, the DHA contents were 51.80, 36.59, and 27.43 mg TF/(gMLSS·h), respectively, showing a continuous decrease with increasing dosage. However, it remained 16.08 mg TF/(gMLSS·h) higher than the DHA content in the system without Fe3O4@K addition. Therefore, Fe3O4@K exerted a crucial activating effect on microbial metabolism, exhibiting extremely strong metabolism at low dosages but reduced activity at high dosages due to sludge densification (Figure 6a). Nevertheless, it remained superior to the control group, demonstrating its sustained positive regulation of the microbial community [46].
In summary, Fe3O4@DE enhanced pollutant removal by improving metabolic activity, whereas Fe3O4@K improved sludge structure and settling performance by optimizing sludge structure. The regulatory mechanisms of these two materials on microbial metabolism and sludge properties differed at different dosage levels, collectively influencing the system’s nitrogen removal and phosphorus removal efficiency (Figure 6), thereby providing a theoretical basis for optimizing the dosage of composite magnetic materials.

3.2.3. Analysis of Microbial Community Structure Characteristics

During the stable operation period, the alpha diversity of the microbial community was evaluated under different dosages of Fe3O4@DE and Fe3O4@K. As shown in Table 2, all samples had a Good’s coverage value > 0.995, indicating sufficient sequencing depth and reliable results. For Fe3O4@DE, microbial abundance and diversity first increased and then decreased with increasing dosage. The Chao1, Shannon, and Simpson indices peaked at 200 mg/L and dropped at 400 mg/L, suggesting the highest species number, optimal microbial diversity/abundance, and maximum dominant microorganism proportion at 200 mg/L. For Fe3O4@K, microbial richness and diversity decreased gradually with rising dosage. Chao1 declined from 1590.98 to 1436.87, Shannon fell from 7.10 to 6.49, and Simpson decreased from 0.97 to 0.95. The Chao1, Observed_species, Shannon, and Simpson indices all peaked at 50 mg/L and declined at 400 mg/L, indicating the highest number of observed species and community richness at 50 mg/L. In summary, dosage significantly affected microbial community richness and diversity. Excessively high dosages (e.g., 400 mg/L) generally reduced diversity indices, implying potential harm to microbial community survival and development.
Community analysis was performed using a Venn diagram to investigate the effect of dosage on species composition. As shown in Figure 8, the composite magnetic material-added system and the non-added system shared 183 species, which were essential in maintaining the basic functions of activated sludge. Compared with the composite magnetic material-added system, the non-added system had the fewest unique species, indicating that the addition of composite magnetic materials helped enhance species diversity in the system. In the Fe3O4@DE system, the number of unique species in the system gradually decreased with increasing dosage, peaking at 50 mg/L. Excessive dosage may inhibit species diversity in the system. In the Fe3O4@DE system, the number of unique species in the system initially decreased and then increased with increasing dosage. Thus, the species diversity within the system varied significantly under different dosages.
Figure 9a shows the relative abundance of the top 10 dominant bacterial genera at the door level at different dosages of Fe3O4@DE and Fe3O4@K. The main dominant bacterial phyla in the system were Proteobacteria, Patescibacteria, Bacteroidota, Actinobacteriota, and Chloroflexi. Notably, Proteobacteria is the dominant microbial phylum in wastewater treatment systems, ensuring the removal of nitrogen and phosphorus [47], and its proportion is minimal in the system without added materials. In the Fe3O4@DE system, the proportion of Proteobacteria increased gradually with increasing dosage, peaking at 400 mg/L. In the Fe3O4@K system, the proportion of Proteobacteria initially decreased and then increased, with the highest proportion also observed at an addition dose of 400 mg/L. Patescibacteria is a recently discovered bacterial group. Studies have shown that it often coexists with denitrifying bacteria and possesses denitrification functions [48]. In this study, the Patescibacteria phylum reached the highest proportion in the Fe3O4@DE and Fe3O4@K systems at dosages of 50 and 200 mg/L, respectively, and the corresponding systems exhibited optimal total nitrogen removal efficiency. We speculated that this phylum may enhance denitrification capacity via denitrification in this system.
To further elucidate the changes in dominant functional bacteria and microbial communities at different dosages, we analyzed the 10 genera with the highest relative abundance. Figure 9b illustrates that Saccharimonadales, a genus with relatively high relative abundance in all systems, belongs to the Patescibacteria phylum and is a typical denitrifying bacterial genus [49]. In the Fe3O4@DE system, Saccharimonadales had the highest proportion at 50 mg/L, with a relative abundance of 38.78%, which decreased to 5.2% at 400 mg/L. In the Fe3O4@K system, the relative abundance of Saccharimonadales was highest at 200 mg/L, reaching 35.79%, and then decreased to 11.88% at 400 mg/L. In the system without added materials, the relative abundance of Saccharimonadales was 20.23%, which was higher than that in the two systems at 400 mg/L, indicating that excessive addition may inhibit the proliferation of denitrifying bacteria. Rhodoferax was also one of the genera with high relative abundance in all systems. Studies have shown that Rhodoferax is an efficient phosphorus-removing bacterium [50]. In the Fe3O4@DE system, the relative abundance of Rhodoferax peaked at 15.52% at an addition dose of 200 mg/L, corresponding to the optimal TP removal efficiency. In summary, significant differences were found in the denitrifying and phosphorus-removing bacterial communities within the composite magnetic material systems at varying addition doses. The excessive addition of composite magnetic materials may inhibit the proliferation of denitrifying bacteria within the system. The optimal abundance of denitrifying bacteria for Fe3O4@DE and Fe3O4@K was observed at dosages of 50 and 200 mg/L, respectively.
Functional gene abundance of microbial communities in different samples under varying dosages was predicted using PICRSt2, with results shown in Figure 9. Figure 10a shows the functional genes associated with the nitrification process in each reactor, including ammonia monooxygenase genes (amoA/B/C) and hydroxylamine oxidoreductase genes (hao). The hao gene is a key gene in the oxidation of NH2OH to nitrite (NO2−-N) [51]. In the Fe3O4@DE system, the abundance of the hao gene gradually decreased with increasing dosage, peaking at 50 mg/L, consistent with the optimal NH4+-N removal efficiency at this dosage. Thus, this dosage effectively promoted the nitrification process in the system. In the Fe3O4@K system, the total abundance of nitrification-related functional genes (amoA/B/C and hao) initially increased and then decreased with increasing dosage, reaching a peak at 200 mg/L (1.99 times that at 50 mg/L). This result suggested that an excessively low Fe3O4@K dosage may inhibit the nitrification process in the system.
The functional genes associated with the denitrification process in various systems are shown in Figure 10b, including nitrate reductase (narG, narH, narI, napA, napB, napC, and napD), nitrite reductase (nirK and nirS), nitric oxide reductase (norB and norC), and nitrous oxide reductase (nosZ). Nitrate reductase catalyzes the reduction of NO3− to NO2−, where the NAP gene belongs to the heterotrophic nitrate reductase gene family and is one of the key enzymes in the denitrification process [52]. As the dosage of Fe3O4@DE increased, the total gene abundance of NAP in the system showed a gradual decreasing trend, with the highest total gene abundance at 50 mg/L, which was 1.84 times that at 400 mg/L. For the Fe3O4@K system, the total abundance of nitrate reductase genes and nitrite reductase genes in the system showed a gradual increase as the dosage increased, indicating that the dosage of composite magnetic materials significantly affected the denitrification performance of the system.

3.3. Techno-Economic Analysis and Engineering Implications

Based on the experimental results presented above, the evaluation of this technology will be systematically considered from three key dimensions: compliance reliability, operational costs, and techno-economic feasibility. During the preliminary screening phase (dosage: 800 mg/L), the absolute reductions in effluent concentrations for indicators such as COD and TN in systems supplemented with composite magnetic materials appeared limited (<10 mg/L), but the effluent TP concentration in the unamended system was close to or even exceeded the discharge limit (e.g., TP ≤ 0.5 mg/L). By contrast, systems with material addition maintained effluent TP levels strictly below the standard, which is crucial for wastewater treatment plants. Through dosage optimization, this study found that Fe3O4@DE at a low dosage (50 mg/L) achieved enhanced nitrogen removal (TN reduction of 4.11 mg/L compared with the unamended system). Thus, superior treatment performance could be attained while simultaneously reducing material dosing costs, resulting in notable economic benefits. Furthermore, the significant improvement in sludge settleability (sludge volume index, SVI)—specifically, the addition of Fe3O4@K optimized and stabilized SVI from >120 mL/g to 40–60 mL/g—remarkably mitigated the risk of sludge bulking, a common challenge in northern regions. Consequently, this technology demonstrated clear engineering application value for the enhancement and modernization of existing wastewater treatment plants.

4. Conclusions

This study systematically compared the effects of four composite magnetic materials (Fe3O4@FA, Fe3O4@DE, Fe3O4@K, and Fe3O4@AC) on the performance of municipal wastewater treatment and sludge settleability in an activated sludge system. It further examined the impacts of the selected optimal materials at different dosages (0–400 mg/L) on pollutant removal, EPSs, DHA, and the microbial community. The main findings are as follows:
(1)
The addition of four composite magnetic materials effectively enhanced the system’s removal efficiency for COD, TN, and TP. Among them, the Fe3O4 composite diatomite and Fe3O4 composite kaolinite systems exhibited superior performance, with the average concentrations of COD, TN, and TP in the effluent from these two systems being reduced by 9.34, 5.55, and 1.35 mg/L and 1.89, 0.34, and 0.29 mg/L, respectively, compared with the system without added materials. After 21 days of operation, the SVI of both systems was significantly lower than that of the untreated system.
(2)
Different dosages significantly alter the pollutant removal efficiency and activated sludge performance of the system. The Fe3O4@DE system exhibited optimal pollutant removal efficiency at 50 mg/L; EPS and DHA increased with increasing dosage. The Fe3O4@K system achieved the highest removal rates for COD, TP, and NH4+-N at 50 mg/L and the best TN removal efficiency at 200 mg/L. The EPS and DHA concentrations decreased with increasing dosage. At 400 mg/L, both systems exhibited inhibited denitrification and phosphorus removal performance.
(3)
High-throughput sequencing results indicate that different dosages of Fe3O4 affect the richness and diversity of the microbial community in the system. The abundance of Patescibacteria decreased continuously with increasing dosage in the Fe3O4@DE system, whereas it showed an initial increase followed by a decrease in the Fe3O4@K system. The relative abundance of denitrifying bacteria peaked at dosages of 50 (Fe3O4@DE) and 200 mg/L (Fe3O4@K), accounting for 38.78% and 35.79%, respectively, corresponding to the optimal denitrification performance of the systems.
In summary, this study clarified the significant advantages of Fe3O4@DE and Fe3O4@K in enhancing pollutant removal and improving sludge settleability and identified the optimal dosage parameters of the two materials for targeting different pollutants. From the perspectives of pollutant removal efficiency, physicochemical properties of sludge, and microbial community response, the research revealed the intrinsic mechanisms by which magnetic composite materials enhanced biological nitrogen and phosphorus removal. The findings provide theoretical and technical references for the enhancement of municipal wastewater treatment plants and the selection of magnetic composite materials.

5. Future Perspectives

This study provides valuable insights into the enhancement of municipal wastewater biological treatment using composite magnetic materials. However, certain limitations remain. Future research could be further explored in the following aspects:
(1)
The recovery rate of used composite magnetic materials from residual sludge should be systematically investigated, efficient regeneration methods should be developed, and the long-term impact of their cyclic use on treatment performance should be clarified.
(2)
The material costs, energy consumption of magnetic separation, and accumulating effects of materials in sludge should be comprehensively evaluated to accurately determine their potential impact on subsequent sludge resource recovery (e.g., anaerobic digestion and land application).

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w18091009/s1, Table S1: Chemicals Used and Their Specifications; Table S2: Formulation of synthetic domestic wastewater (added per liter of tap water); Table S3: Summary of experimental conditions; Table S4: Pollutant Removal Performance under Different Conditions (Without and With Four Composite Magnetic Materials).

Author Contributions

S.A.: Conceptualization, Methodology, Software, Investigation, Formal Analysis, Writing—Original; R.H.: Data Curation, Writing—Original; Y.G.: Visualization, Review and Editing; W.T.: Resources, Supervision; S.Z.: Visualization, Investigation; H.Q.: Software, Validation; D.B. (Corresponding Author): Conceptualization, Funding Acquisition, Resources, Supervision, Writing—Review and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by the Joint Funds of the National Natural Science Foundation of China (U24A20186), Jilin science and technology development plan project (YDZJ202401342ZYTS), Jilin Provincial Department of Housing and Urban-Rural Development (2023-K-01).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Abbreviations

Abbreviation/SymbolDefinition
SBRSequencing Batch Reactor
CODChemical Oxygen Demand
TNTotal Nitrogen
TPTotal Phosphorus
NH4+-NAmmonia Nitrogen
SVISludge Volume Index
EPSExtracellular Polymeric Substances
DHADehydrogenase Activity
DODissolved Oxygen
MLSSMixed Liquor Suspended Solids
XRDX-ray Diffractometer
SEMField Emission Scanning Electron Microscope
DNADeoxyribonucleic Acid
PCRPolymerase Chain Reaction
16S rRNA16S ribosomal Ribonucleic Acid
Fe3O4@ACFe3O4 composite activated carbon
Fe3O4@DEFe3O4 composite diatomite
Fe3O4@KFe3O4 composite kaolinite
Fe3O4@FAFe3O4 composite fly ash
ASV/OTUAmplicon Sequence Variant / Operational Taxonomic Unit
amoA/B/CAmmonia monooxygenase gene
haoHydroxylamine oxidoreductase gene
narG, narH, narINitrate reductase gene
napA, napB, napC, napDNitrate reductase gene
nirK, nirSNitrite reductase gene

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Water 18 01009 g001
Figure 1. (a) Schematic of the SBR system. (b) Physical photo of the SBR system.
Figure 1. (a) Schematic of the SBR system. (b) Physical photo of the SBR system.
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Figure 2. XRD patterns of composite magnetic materials.
Figure 2. XRD patterns of composite magnetic materials.
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Figure 3. SEM images of composite magnetic materials: (a) Fe3O4@AC; (b) Fe3O4@DE; (c) Fe3O4@K; (d) Fe3O4@FA.
Figure 3. SEM images of composite magnetic materials: (a) Fe3O4@AC; (b) Fe3O4@DE; (c) Fe3O4@K; (d) Fe3O4@FA.
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Figure 4. (a) Removal efficiency of COD by different composite magnetic materials; (b) removal efficiency of TP by different composite magnetic materials; (c) removal efficiency of TN by different composite magnetic materials; (d) removal efficiency of NH4+-N by different composite magnetic materials.
Figure 4. (a) Removal efficiency of COD by different composite magnetic materials; (b) removal efficiency of TP by different composite magnetic materials; (c) removal efficiency of TN by different composite magnetic materials; (d) removal efficiency of NH4+-N by different composite magnetic materials.
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Figure 5. Sludge Volume Index of Different Composite Magnetic Material Systems.
Figure 5. Sludge Volume Index of Different Composite Magnetic Material Systems.
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Figure 6. (a) Removal efficiency of COD with different dosages of composite magnetic materials; (b) removal efficiency of TP with different dosages of composite magnetic materials; (c) removal efficiency of TN with different dosages of composite magnetic materials; (d) removal efficiency of NH4+-N with different dosages of composite magnetic materials.
Figure 6. (a) Removal efficiency of COD with different dosages of composite magnetic materials; (b) removal efficiency of TP with different dosages of composite magnetic materials; (c) removal efficiency of TN with different dosages of composite magnetic materials; (d) removal efficiency of NH4+-N with different dosages of composite magnetic materials.
Water 18 01009 g006
Water 18 01009 g007
Figure 7. (a) Effect of different dosages of composite magnetic materials on EPS; (b) effect of different dosages of composite magnetic materials on DHA.
Figure 7. (a) Effect of different dosages of composite magnetic materials on EPS; (b) effect of different dosages of composite magnetic materials on DHA.
Water 18 01009 g007
Water 18 01009 g008
Figure 8. ASV/OTU Venn diagram of microorganisms in the system under different dosages.
Figure 8. ASV/OTU Venn diagram of microorganisms in the system under different dosages.
Water 18 01009 g008
Water 18 01009 g009
Figure 9. Relative abundance of microorganisms in the system at different dosages (a) relative abundance at the phylum level and (b) relative abundance at the genus level.
Figure 9. Relative abundance of microorganisms in the system at different dosages (a) relative abundance at the phylum level and (b) relative abundance at the genus level.
Water 18 01009 g009
Water 18 01009 g010
Figure 10. Functional gene abundance related to denitrification pathways in different systems at various dosages: (a) genes related to nitrification process; (b) genes related to denitrification process.
Figure 10. Functional gene abundance related to denitrification pathways in different systems at various dosages: (a) genes related to nitrification process; (b) genes related to denitrification process.
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Table 1. Inlet water quality during operation.
Table 1. Inlet water quality during operation.
Inlet Water Quality ParametersCOD
(mg/L)		TN
(mg/L)		NH4+-N
(mg/L)		TP
(mg/L)
Range395.8–295.742.7–30.7139.54–25.444.11–5.6
Average335.2936.4630.114.85
Table 2. Microbial alpha diversity in the system under different dosages.
Table 2. Microbial alpha diversity in the system under different dosages.
SampleChao1Goods CoverageObserved SpeciesShannonSimpson
SBR11200.20.9971140.15.790880.926414
SBR21793.480.9951706.56.457790.945834
SBR31813.270.9951704.76.966080.967745
SBR41228.120.9961152.26.098840.944992
SBR51590.980.99715467.097730.972083
SBR61486.890.9961393.96.315670.953717
SBR71436.870.9961351.96.493310.953341
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Ai, S.; Hao, R.; Gao, Y.; Tong, W.; Zeng, S.; Qu, H.; Bian, D. A Study on the Effects and Response Mechanisms of Different Composite Magnetic Materials in Enhancing Municipal Wastewater Biological Treatment. Water 2026, 18, 1009. https://doi.org/10.3390/w18091009

AMA Style

Ai S, Hao R, Gao Y, Tong W, Zeng S, Qu H, Bian D. A Study on the Effects and Response Mechanisms of Different Composite Magnetic Materials in Enhancing Municipal Wastewater Biological Treatment. Water. 2026; 18(9):1009. https://doi.org/10.3390/w18091009

Chicago/Turabian Style

Ai, Shengshu, Rui Hao, Yongtai Gao, Wenhua Tong, Shangjing Zeng, Hong Qu, and Dejun Bian. 2026. "A Study on the Effects and Response Mechanisms of Different Composite Magnetic Materials in Enhancing Municipal Wastewater Biological Treatment" Water 18, no. 9: 1009. https://doi.org/10.3390/w18091009

APA Style

Ai, S., Hao, R., Gao, Y., Tong, W., Zeng, S., Qu, H., & Bian, D. (2026). A Study on the Effects and Response Mechanisms of Different Composite Magnetic Materials in Enhancing Municipal Wastewater Biological Treatment. Water, 18(9), 1009. https://doi.org/10.3390/w18091009

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