Magnetic bio-flocculation for cost-effective fast organic matter pre-concentration for sewage with enhanced capture and settling of sludge

Abstract

Sewage pre-concentration has been promising for carbon-neutral anaerobic wastewater treatment. Magnetic bio-flocculation, which combines the advantages of adsorption/bio-oxidation processes and magnetic separation for cost-effective, fast, and eco-friendly chemical-additive-free pre-concentration, is proposed in this study. The results show that 85.5–89.5% chemical oxygen demand (COD) removal within 30 min can be achieved. The maximum recovery rate of organic matter (OM) can be increased from 79.6% to 88.2% by replacing aeration with stirring coupled with a magnetic field (MF). This can be attributed to the increased production of loosely bound extracellular polymeric substances (LB-EPS) by 23.7–62.9 mg L⁻¹ and decreased production of the tightly bound EPS (TB-EPS) by 3.8–5.7 mg per g VSS in the stirring set, where the magnetic force alleviated the microbial surface negative charge, strengthened the attachment of magnetic seeds (MS) to sludge through MF-induced extra fragmented LB-EPS and compensated for the negative impact of LB-EPS by intensifying microbial OM capture. The decreased median diameter by 0.93–4.13 μm and the improved fractal dimension ranging from 2.15 to 2.18 further support the above effects. The combination of MS and the MF improved the sludge settleability, which is supported by the maximized hindered settling velocity accelerated by 1.6–7.2 m h⁻¹. The V₀/n values calculated from the Vesilind function, which mean the mass of sludge passing through a unit section per unit time, reached a maximum value of 52.71 after coupling the MS and MF. As such, magnetic bio-flocculation with stirring is found to be promising for achieving optimized practical sewage pre-concentration.

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Water impact

There is growing concern about the emerging concept of carbon neutrality through redesigning carbon flow in raw sewage, which either optimizes the efficiency of the carbon recovery or minimizes the consumption of the methods. This study proposed a technology that combines bio-flocculation and magnetic separation to cope with the shortcomings of both the addition of chemical additives and the poor efficiency of bio-based carbon pre-concentration technologies. Magnetic bio-flocculation with stirring was demonstrated to achieve effective carbon pre-collection which is similar to membrane-based technologies with much less reagents and energy consumption by accelerating the hindered settling velocity of sludge aggregates. A fast, sustainable and eco-friendly method was obtained.

1. Introduction

Carbon neutrality, or energy neutrality, is an emerging concept for achieving sustainable environmental protection.¹ The conventional sewage treatment process that uses activated sludge has been widely deemed insufficient to reach the goal of carbon neutrality due to high energy consumption and carbon oxidation caused by aeration.² Anaerobic digestion (AD) is a potential technology for achieving carbon neutrality through the use of energy stored in sewage organic matter (OM).^3,4 This advantage of AD is diminished in the traditional application scenario due to the low concentration of OM in raw sewage or sewage sludge after intensive mineralisation in the previous stage.^5,6 As such, organic-conservative and energy-saving technologies are in demand to improve the viability of AD for wastewater treatment that can achieve carbon neutrality.

Sewage pre-concentration is one such emerging technology that aims to retain the most raw sewage OM in an energy-saving and cost-effective way, based upon the fact that 60–80% of OM in raw sewage is particulate and/or colloidal and readily separable.^7,8 Energy stored in sewage OM can be effectively recovered by subsequent coupling with AD⁹ to develop a low-carbon solution suitable for water reuse (e.g. crop, landscape, or apartment complex irrigation) through sewage pre-concentration.^1,3,10,11 In addition, the pre-concentrated effluent can also be combined with anammox to improve the loss of bacterial activity due to high OM content and achieve more economical nutrient removal.¹²

Bio-flocculation is an environmentally friendly separation technology that is effective in redirecting carbon in sewage without chemical addition. Microbial flocs in this process are able to trap and aggregate slowly biodegradable colloidal and/or particulate OM in the mixed liquor by surface adsorption and physical flocculation through extracellular polymeric substances (EPS) produced by microorganisms.^8,13 This process is widely applied for the A-stage of the adsorption–bio-oxidation (AB) process as a form of high-rate activated sludge system (HRAS).^14,15 The A-stage (adsorption) features a very high food-to-microorganism ratio of influent having the potential to maximize the recovery of OM from sewage for direct AD. The sludge retention time (SRT), hydraulic retention time (HRT), and dissolved oxygen (DO) are related to the intrinsic biological activity. These key parameters are crucial for colloidal and particulate OM capture, but have little if any impact on the removal of soluble organics.^16,17 Therefore, it is pivotal for carbon redirection to control and maximize the separation of colloidal and particulate chemical oxygen demand (COD) along with minimized mineralization and hydrolysis of such slowly biodegradable COD.¹ The COD removal rates and recovery efficiencies in the A-stage increased with a decrease of HRT due to the enhanced flocculation and sedimentation.¹⁸ A low SRT may promote sludge to adsorb higher fractions of the particulate and colloidal substrates compared to a higher SRT.¹⁹ However, the recovery efficiency, bio-flocculation sufficiency, and bio-sorption response are yet to be optimized for low-strength wastewaters such as sewage.²⁰ As such, magnetic separation technology, which can substantially raise the settling velocity of the sludge through magnetic-field (MF)-induced magnetic forces, is incorporated as an efficient alternative for separation improvement.

Traditional magnetic separation is carried out through chemical-induced rapid agglomeration of particles and/or colloids and magnetic-seed (MS)-induced fast separation by magnetic forces.²¹ As such, the SRT and HRT can be dramatically reduced compared to those in the traditional A-stage, which improves sludge/water separation efficiency.^22–25 However, the demand for chemical coagulants and/or flocculants can increase the cost and carbon emission of the process and even exhibit negative impacts on subsequent microbial AD processes.^26,27 Until now, few (if any) of the reported bio-flocculation-based sewage pre-concentration processes have considered the effectiveness of incorporating magnetic separation. As such, modified magnetic separation where activated sludge replaces chemicals is incorporated in this study to comprise a fast and cost-effective biologically-based OM pre-concentration process, namely magnetic bio-flocculation. The schematic blueprint is shown in Fig. 1. OM in raw sewage is mixed with activated sludge and MS to form a concentrate settled and collected using a MF. The particle-and/or-colloid-free effluent can be reused for agricultural irrigation. MS trapped in activated sludge during bio-flocculation can be cost-effectively removed and instantly recycled using a magnetic cyclone separator,²⁸ leaving almost no manufactured additives. The biologically-based concentrate will be transferred to an AD unit for cogeneration and subsequently produce a qualified digestate without chemical-induced toxicity.²²

Fig. 1 Schematic diagram of the (a) aeration system and (b) stirring system.

The aim of this study is to explore the preliminary performance of lab-scale magnetic bio-flocculation in sewage OM pre-concentration which meets the key objectives stated above. The roles of the key parameters in the pre-concentration process, including the MS, MF, and mixing conditions, were investigated and detailed. The mixing and separation units were optimized. Moreover, stirring, as an alternative mixing method, was compared with aeration to explore the potential of reducing the OM mineralisation and achieving optimized carbon conservation. The settling dynamics was modelled to quantify and compare the settling characteristics of sludge under different operational conditions. The fundamental mechanisms determining the improvement of mixture separability were also tested and analyzed, including aspects of particle characteristics and EPS properties, which have great impacts on microbial aggregates and floc formation.²⁹

2. Methods and materials

2.1 Sewage and sludge samples

Raw sewage and activated sludge samples were collected from a wastewater treatment plant in the Beijing suburbs, P. R. China, without industrial wastewater inflow. Raw sewage was collected from pipes behind fine screens. The average values of COD, ammonia, total nitrogen and total phosphorus are around 450 mg L⁻¹, 45 mg L⁻¹, 58 mg L⁻¹ and 5.5 mg L⁻¹, respectively. The activated sludge used for magnetic bio-flocculation was collected from the mixed liquor of the aeration tank. The concentrations of COD and mixed liquor suspended solids (MLSS) in activated sludge were 8237.3–8898.2 mg L⁻¹ and 7.6–8.8 g L⁻¹, respectively. Permanent magnets (N45, China) are composed of Nd–B–Fe, providing a maximum MF intensity of 250 mT.

2.2 Experimental operations

Jar-test-based batch testing was used to simulate real application scenarios as depicted in Fig. 1. Tests were replicated 20 times in order to ensure the reliability of experimental results. During tests, MS (Fe₃O₄, Andy Metal Materials Co., Ltd.) with different average diameters, namely 150 mesh, 325 mesh, 500 mesh, and 1000 mesh, were added to a 1 : 1 mixture of raw sewage and activated sludge. Based upon preliminary experiments, 1 g L⁻¹ MS was selected to achieve enough interaction between MS and sludge and subsequent floc settling. Meanwhile, 30 min mixing and another 30 min settling were applied for the observation of the whole separation performance according to the common A-stage operation in the AB process. The stirrer (ZR4-4, ZHONGRUN, China) for stirring scenarios was operated at a 300 rpm mixing speed, and the aerator (SOBO, China) for aeration scenarios was operated at an air flow rate of 1 L min⁻¹. The pH of the system was in the range of 7.7–7.8. The sludge settling characteristics with and without a MF were compared at the end of settling. The COD values of the raw sewage and original sludge, as well as the supernatant and concentrate after settling, were all measured and recorded for mass balance calculations.

2.3 Analytical procedures

2.3.1 COD removal and COD mass balance. COD was measured according to the Chinese National Environmental Protection Agency standard method.³⁰ The COD removal rate was calculated using the COD mass balance with eqn (1):where R(%), C_i, and C_e represent the removal rate and the concentration in the influent and supernatant (mg L⁻¹), respectively.

The COD mass balance was calculated to evaluate the effectiveness of the concentration. In general, OM entering system is finally existed in forms of supernatant and concentrates, and lost due to mineralization. The concentration efficiency was quantitatively evaluated from the volume concentration factor (X) and concentration factor (X_c) according to eqn (2)³¹ and (3):

X = V_i/V_c (2)

X_c = C_c/C_i (3)

where X is the volume concentration factor, V_i is the cumulative volume of the influent (mL), V_c is the cumulative volume of the concentrate (mL), X_c is the concentration factor, C_c is the final COD concentration of the concentrate (mg L⁻¹), and C_i is the COD concentration of the influent (mg L⁻¹).

The cost effectiveness was also roughly evaluated from the effective concentration factor (η) with eqn (4):

η = X_c/X (4)

where η is the effective concentration factor, X_c is the concentration factor, and X is the volume concentration factor.

2.3.2 Physicochemical characteristics of the sludge. The EPS in the mixture and concentrate, including the loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), were extracted by modified thermal extraction.³² The methods for testing the concentration of EPS followed those from a previous study.³³ Particle size distributions (PSDs) were measured using a Malvern Mastersizer 3000 (Malvern, UK) to verify the median diameter (D₅₀) of the mixture. The fractal dimensions (FDs), which represent the compactness of particle aggregates and the separability of the mixture, were also calculated from the raw light scattering data using the Malvern Mastersizer 3000 to characterize the concentrate mixture.²⁸

2.3.3 Numerical modelling of sludge settling. Activated sludge settling characteristics are often measured by batch settling tests. The thirty-minute settled sludge volume (SV₃₀), as an index to characterize the settling performance of sludge flocs,³⁴ can be calculated by eqn (5):where V₀ and V₃₀ are the sludge blanket volumes (mL) at the beginning and end of the 30 min settling, respectively. The model of the Vesilind function was applied to quantitatively evaluate the effect of MS and MF on the sludge separation efficiency, where the settling velocities were calculated from the sludge blanket height data at different times, and the concentrations of the sludge were set by dilution of the sludge from the external recirculation flow with raw sewage.³⁵ Specifically, the hindered settling velocities were calculated from the steepest slope of consecutive data points which are in front of the sludge blanket height curve. As such, a settling curve could be obtained by plotting the hindered settling velocities versus the corresponding initial sludge concentrations. The Vesilind function can be fitted through data points on the curve and the parameter values can be quantitatively compared to illustrate the detailed mechanism.

In general, most models used to describe the settling behaviour of activated sludge are empirical.³⁶ The Vesilind exponential function, as a typical model, is most often used to describe the hindered settling and compression settling, given by eqn (6):³⁷

V(X) = V₀e^−nX (6)

where V(X) is the settling velocity (m h⁻¹), X is the sludge concentration (mg L⁻¹), and V₀ and n are positive parameters to be calibrated.

3. Results and discussion

3.1 COD removal and concentration performance

Generally, the particulate and colloidal OM in the A-stage of the AB process can be adsorbed on the surface of microbes before they are subjected to microbial conversion, e.g. hydrolysis.^38,39 The adsorption, flocculation and sedimentation of sludge can be enhanced due to the shortened HRT and SRT.^18,19 As such, the improvement of COD capture mainly relies on the enlarged grasping of particulate and colloidal OM by sludge and the shortened HRT of the separation process. As depicted in Fig. 2(a), the particulate species comprised 84.4% of the total COD in raw sewage which is favourable for the capture of OM. As shown in Fig. 2(b), the average COD removal rate varied from 85.5% to 89.5%, which is higher than the average value of 55% for the A-stage AB process with an SRT of 1.31 d.⁴⁰ This suggests the relatively stable COD harvesting ability of the sludge applied in this study despite the different operational conditions. This stable ability can be attributed to the robust surface adsorption of colloidal and/or particulate OM by microbes, which is rarely affected by cell viability.⁴¹ The minimal COD concentrations of effluents in the aeration and stirring groups are 42.7 mg L⁻¹ with 1000 mesh MS and 50.6 mg L⁻¹ with 500 mesh MS and MF, respectively. Both values match the most stringent Chinese standards (less than 60 mg L⁻¹) for irrigation water⁴² and hence effluents after OM pre-concentration are able to fit for reclamation, especially in the arid or semi-arid northern China area.

Fig. 2 (a) COD fraction of raw sewage; (b) COD removal and concentration under different operational conditions.

The evaluation of the operation efficiency of different scenarios in terms of the volume concentration factor (X), the concentration factor (X_c), and the effective concentration factor (η) is displayed in Table 1. The optimized values of X were 3.3 and 3.5 in the stirring and aeration groups with 325 mesh MS and MF, while the maximum values of X_c appeared to be 36.6 and 36.7 in those with 1000 mesh MS and MF. Both of the above values are almost double the values in the respective control groups. Moreover, the coupling of MF notably improved the X and X_c values in the groups with only MS used. This indicates that the magnetic force plays a positive role in separation efficiency enhancement compared to gravitational force in common bio-flocculation.⁴³ Meanwhile, the combination of the MS and MF increased the η values from 9.3 in the control group to 14.1 in the stirring groups and from 9.8 to 12.3 in the aeration groups, indicating the simultaneous improvement in OM recovery and separation. It is also worth mentioning that the MF in the stirring groups achieved a greater X value enhancement than that of X_c compared to the groups only using MS, which gave rise to the relatively smaller η values in the groups with a MF. This implies an even more compact concentrate volume induced by MF and a subsequent effective disposal strategy with lower transportation cost. The above results also demonstrate the possibility of utilizing the direct interaction between MS and microbes without the addition of chemicals to improve its robustness,⁴⁴ a cost-effective method for fast OM redirection and separation, whose mechanism shall be explored and discussed in later sections.

Table 1 Volume concentration factors (X), concentration factors (X_c) and effective concentration factors (η) in different working scenarios

	Stirring group			Aeration group
	X	X c	η	X	X c	η
None	1.7	16.0	9.3	1.9	18.3	9.8
150 mesh	2.0	27.6	14.1	2.5	26.3	10.5
150 mesh + MF	3.2	34.5	10.7	3.1	35.7	11.4
325 mesh	2.1	27.5	13.2	2.4	29.1	12.3
325 mesh + MF	3.3	37.6	11.5	3.5	34.7	9.9
500 mesh	2.1	28.8	13.7	2.5	27.7	11.3
500 mesh + MF	2.9	34.3	11.9	3.3	32.9	10.1
1000 mesh	2.1	29.3	14.0	2.5	28.6	11.4
1000 mesh + MF	2.9	36.6	12.5	3.2	36.7	11.5

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The organic mass balance was calculated to quantitatively evaluate the pre-concentration efficiency, as shown in Fig. 3. The OM/COD entering the pre-concentration system can be divided into three groups: the species in the concentrate, the species in the effluent, and the species lost due to microbial mineralisation.²⁰ It is notable that the recovered COD in the stirring group was 0.03–14.07% higher than that in the aeration group. This can be attributed to the lower concentration of DO in the stirring group than that in the aeration group, where the soluble OM oxidation by heterotrophic bacteria can be alleviated.⁸ Meanwhile, the uptake velocity of substrates for aerobic microorganisms also surpasses that for anoxic microorganisms, which metabolically mitigates the COD loss of raw sewage.^45,46 As such, stirring can minimize oxidation and maximize the recovery of OM during redirection compared to aeration, corresponding to a previous membrane-based study.⁴⁷ Therefore, an enhancement in OM recovery can be achieved by the mixing method optimization.

Fig. 3 COD mass balance at the end under different operational conditions (S: stirring group; A: aeration group).

Fig. 3 also shows that the COD recovered in the concentrate increased with a decreased MS size. In the aeration group with a MF, the COD recovery rates with 500 and 1000 mesh were 4.0% and 8.9% higher, respectively, than those with 150 and 325 mesh. While the rates in the stirring group with a MF increased by 6.4%, 9.5%, and 13.8% with decreased MS sizes, indicating the beneficial effect of small-sized MS on OM capture. Furthermore, MF facilitates the recovery of OM regardless of the mixing method. This can be attributed to the carbon conservation originating from the MF-induced inhibition of the metabolic activity of microorganisms and the improvement of the separability of sludge flocs on account of alterations in EPS, particle size, fractal dimension, etc.^48,49 The optimized OM recovery was 88.2% in the stirring group with 1000 mesh MS and MF with a minimized mineralization of 10.9%. This originates from the transfer of mineralized OM into the concentrate. All recoveries in the different stirring groups are higher than the 25% recovery in a conventional activated sludge system⁹ and the 60–70% obtained in common bio-flocculation and combined coagulation microfiltration systems.¹⁰ Magnetic bio-flocculation integrated with stirring is a promising way to achieve effective pre-concentration.

3.2 Impact of MS and MF on the excretion of EPS

EPS determine the adhesion and cohesion performance between particulate, colloidal and/or soluble OM and the biomass in activated sludge through complex interactions such as London forces, electrostatic interactions and hydrogen bonding.⁵⁰ As such, the detailed influence of the MS and MF on EPS production was measured and is shown in Fig. 4. LB-EPS in the outer layer in the colloidal state diffuses in the liquid phase⁵¹ and TB-EPS attaches on the cytoderm of microorganisms.⁵² The roles of these two kinds of EPS in OM capture turn out to be different, where LB-EPS improve the weak interaction between cells and control the sludge attachment, flocculation and adhesion,⁵³ and TB-EPS contain abundant hydrophilic compounds which interact with molecules of water and affect the inner floc structure.^54,55

Fig. 4 The contents of (a) LB-EPS and (b) TB-EPS produced in different scenarios (AM: aeration mixture, AC: aeration concentrate, SM: stirring mixture, SC: stirring concentrate).

As shown in Fig. 4a, the LB-EPS increased in the aeration groups while decreasing in the stirring groups. The improved LB-EPS production in the aeration groups is probably due to the higher concentration of DO under aeration conditions. However, a higher value of DO can also facilitate elevated microbial activity, which leads to more organics consumption.⁵⁶ Meanwhile, an increase in LB-EPS should have enhanced the porosity of sludge flocs with a low density, and hence should have deteriorated the settleability of sludge.³² However, the sludge settlement was improved with MS as shown by the results in Fig. 6. This can be ascribed to the fact that the strengthened adsorptive and electrostatic bridging interactions between LB-EPS and cations outside the MS surface could help to form flocs with elevated density.⁵⁷ The enhanced content of LB-EPS would also support cell interaction and adhesion.³² Moreover, the added MS could function as a nucleus for bacterial attachment and decrease the surface negative charge of microbes.^56,58,59 It is interesting to note that the LB-EPS in the aeration groups was reduced when a MF was applied whereas it increased in the stirring groups. This demonstrates the impact of the MF on the excretion and transformation of LB-EPS where a magnetic field could promote the disintegration of EPS and conserve fragmented LB-EPS under stirring conditions without consumption due to elevated microbial activity under aeration conditions.⁶⁰ In general, the interaction between MS and EPS coupled with the improvement in separability from the MF compensates for the deterioration of sludge settling and flocculation performance that comes from the increase in LB-EPS.

According to the fluctuation in Fig. 4b, the TB-EPS in the aeration groups decreased with the reduction of the particle size of MS and did not change much with the addition of a MF after the use of MS. However, the alteration of the particle size of MS had few impacts on the TB-EPS in the stirring groups, while the MF explicitly decreased the TB-EPS after the use of MS. As mentioned above, the MF could disintegrate the EPS components, especially TB-EPS, and hence could transfer fragmented TB-EPS within the inner sludge matrix to the outer surface in the form of stretched LB-EPS or to the bulk liquid in the form of solubilized OM. As such, the change of the collision between the sludge flocs and OM in raw sewage could be improved. The microbial capture efficiency of OM could be subsequently increased along with intensifying the magnetic force on the whole sludge matrix induced by MS.⁶¹

Fig. 4a and b show that the MS and MF reduced the production of total EPS and TB-EPS, which was consistent with a previous study,²⁸ but the trend for LB-EPS was the opposite. As such, it can be inferred that the MF transformed TB-EPS into LB-EPS in the concentrate. Comprehensively, LB-EPS plays a role in transferring organic pollutants through bio-sorption, while TB-EPS has the ability to preserve OM outside the cell wall of bacteria without being released. Moreover, LB-EPS plays a positive role in sludge aggregation, and the influence of TB-EPS on sludge aggregation is governed by the separation distance between sludge cells.⁶² Though the bound water within LB-EPS blocks the binding sites on the sludge flocs⁶³ and weakens the adhesion between cells, deteriorating the bio-flocculation and settling performance of sludge flocs,³² the combination of MS and MF could lead to a huge reduction of the water content of EPS.⁴⁹ With the decreased water content, more OM would be readily accessible to the absorption sites in the LB-EPS, and the bio-flocculation and OM capture could be improved.⁶⁴ Therefore, the application of MS and MF helps to improve the OM recovery through their positive impacts on the EPS concentration and composition.

3.3 Median diameter (D₅₀) and fractal dimension (FD)

In the activated sludge process, the sludge forms dense, firm, and large enough flocs to ensure successful sludge/water separation through gravity.⁶⁵ Aeration could give rise to larger and looser flocs by air infiltration and air-induced floc disruption⁶⁶ and flocs with a porous structure would have a higher shear sensitivity with poor sludge/water separability. As shown in Fig. 5, the D₅₀ values of the aeration groups increased with MS by 0.80–1.99 μm, but were not sensitive to the particle size of the MS. The MS could not only attract each other due to their magnetic properties but also capture bacteria, leading to the larger mean size of activated sludge flocs.^67,68 Moreover, flocs in the stirring groups indeed achieved higher D₅₀ and FD values than those in the aeration groups, indicating larger and more compact microbial aggregates in the stirring groups. The addition of MS increased the D₅₀ values of the stirring groups by 0.93–2.04 μm, and the size growth increased as the particle size of MS decreased. This indicates that the regular force given by the stirring impeller was conducive to the formation of a homogeneous mixture system, and to improving mixing performance and flocculation. It would help to form denser and more compact flocs and accordingly improve the OM capture and separation efficiency.

Fig. 5 Median diameter (D₅₀) and fractal dimension (FD) of the concentrates (the arrows point to the corresponding y-coordinates).

Meanwhile, the D₅₀ values of flocs with a MF decreased by 0.93–4.13 μm compared to those under the conditions with only MS in the two groups and their FD values ranged from 2.15 to 2.18. This reveals that the MF decreased the particle diameters and increased the FD compared to those of the non-MF-settled flocs in the two groups. With exposure to a MF, sludge flocs combined with magnetic particles are subjected to a greater downward force, and the fragile flocs split from the weakest point into smaller aggregates, thus forming flocs with a smaller, more compact and stronger structure than the parent flocs.⁶⁹ Combining the results in Fig. 2 and 6, the settling efficiency improvement with the addition of a MF did not deteriorate the quality of the effluent. The MF promotes the formation of compact aggregates with improved FD values beneficial for settling and avoids sludge bulking, which occurs when aggregates do not compact and form a loose, low-density floc.⁷⁰

Fig. 6 (a) Settling velocity as a function of the solids concentration. The data points represent the measured settling velocities and the curves the calculated settling velocities after calibration of the Vesilind function. (b) The parameter V₀/n of the Vesilind function for different conditions with stirring.

3.4 Improvement in separation efficiency

The SV values of each scenario are shown in Table 2. The SV after 30 min settling was reduced by 12.8% in the stirring group with 325 mesh MS compared to that of the control and was further reduced to 30.9% with a coupled MF. The decreased values of SV range between 23.8% and 31.0% for the stirring groups and between 21.3% and 29.3% for the aeration groups. This indicates that the application of MS and MF achieved dense concentrates with reduced volume and allowed the system with a shortened HRT and lower settling space. It subsequently improves the efficiency and cost-effectiveness of the treatment process.⁷¹

Table 2 Settling volume (SV₃₀) under different operational conditions after 30 min of settling

SV30	Stirring group	Aeration group
None	61.8%	59.1%
150 mesh	52.5%	43.8%
150 mesh + MF	31.4%	37.8%
325 mesh	49.0%	45.1%
325 mesh + MF	30.9%	29.8%
500 mesh	48.0%	44.4%
500 mesh + MF	38.0%	36.5%
1000 mesh	47.9%	42.8%
1000 mesh + MF	36.0%	37.0%

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The original purpose of the incorporation of MS and MF was to improve the efficiency of solid–liquid separation⁷² and subsequently achieve concentrates with enhanced loading rates and qualified effluents. As mentioned above, a greater OM recovery occurred with stirring instead of aeration, and hence the kinetics mechanism of MF's effects on sludge separability in the stirring group was investigated and the results (p < 0.05) are displayed in Fig. 6.

As shown in Fig. 6(a), the addition of MS and MF can explicitly increase the maximum hindered settlement velocity of sludge by 1.6–7.2 m h⁻¹. Such an effect was strengthened with the decrease of MS size, where the enhancement with 1000 mesh MS and MF was almost four times the value of the enhancement with 150 mesh MS and MF. Quantitatively, well-settling sludges have high V₀ values and low n values obtained from regression of the Vesilind hindered settling function. As such, the ratio V₀/n was proposed as a measure of sludge settling properties which means the mass of sludge passing through a unit section per unit time. A V₀/n with high values (30–40) indicates a good settling sludge and low values (10–15), a poor settling sludge.⁷³ As shown in Fig. 6(b), after MS were combined with sludge, the V₀/n values ranged between 32.3 and 33.7 for different particle sizes, which increased significantly compared with 15.5 of the control group, and these values for MS with different particle sizes did not change much. Further settling improvement occurred when a MF was incorporated, with a maximum V₀/n value of 52.71 in the scenario with 1000 mesh MS and MF. The kinetic simulation results suggested that combining MS and MF could improve the sludge settleability, especially small sized MS. As such, MS and MF can optimize the recovery of OM in sewage without worsening the effluent quality.

4. Conclusions

This study investigated the feasibility of magnetic bioflocculation for sewage carbon concentration. The addition of MS and MF could promote the recovery of organic matter in sewage and accelerate the sludge–water separation. The optimized organic matter recovery rate was 88.2%. The increase of LB-EPS and the enhanced D₅₀ and FD values indicated the better performance of sludge flocs in concentration and separation. The maximized hindered settling velocity was accelerated with the combination of MS and MF, especially with MS of a small size. The calculated V₀/n values from the Vesilind hindered settling function were also improved by MS and MF. In general, magnetic bio-flocculation with stirring in sewage OM pre-concentration is a feasible way to promote OM recovery and solid–liquid separation after further optimisation. Future studies will be carried out on a larger scale to explore and solve problems in practical applications of magnetic particles and magnetic fields.

Nomenclature

AbbreviationFull definition

AB	Adsorption bio-oxidation
AD	Anaerobic digestion
COD	Chemical oxygen demand
D 50	Median diameter
DO	Dissolved oxygen
EPS	Extracellular polymeric substances
FD	Fractal dimension
HRAS	High-rate activated sludge treatment
HRT	Hydraulic retention time
LB-EPS	Loosely bound extracellular polymeric substances
MF	Magnetic field
MLSS	Mixed liquor suspended solids
MS	Magnetic seeds
OM	Organic matter
PSD	Particle size distribution
SRT	Sludge retention time
SV	Settling volume
TB-EPS	Tightly bound extracellular polymeric substances
VSS	Volatile suspended solids

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors would like to thank the financial support from the National Natural Science Foundation of China (Grant No. 51608298), the special fund of the State Key Joint Laboratory of Environmental Simulation and Pollution Control (Grant No. 19K06ESPCT), and the Youth Teachers' Research Ability Improvement Plan for Minzu University of China (Grant No. 2020QNPY91). And we would like to thank Editage (http://www.editage.cn) for English language editing.

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Footnote

† These authors contributed equally to this work.

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