Enhancement of nitrogen removal iron-based with an electric-magnetic field in an Upflow microaerobic sludge reactor (UMSR)

doi:10.21203/rs.3.rs-1753793/v1

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Enhancement of nitrogen removal iron-based with an electric-magnetic field in an Upflow microaerobic sludge reactor (UMSR)

https://doi.org/10.21203/rs.3.rs-1753793/v1

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Traditional denitrification often produces high operating costs and excessive sludge disposal expenses due to conventional carbon sources. And Fe⁰ reactivity enhancement methods are complex and hazardous. Researchers are interested in prolonging the release of Fe²⁺, Fe³⁺, and H⁺ from Fe⁰ corrosion, accelerating the reduction of Fe³⁺ to Fe²⁺, and reactors' hydraulic retention time (HRT). Novel coupling of electric-magnetic field (MF) with Fe⁰ and C-Fe⁰ powder in an upflow microaerobic sludge reactor (UMSR) proved excellent in removing nitrogen from wastewater without organic carbon resources. In the current study, the reactor was operated for 78 ± 2 days, divided into five stages (without Fe⁰, with Fe⁰, coupling with MF, without coupling with MF, and coupling with MF again), at a hydraulic retention time (HRT) of 2.5 hours, with an influent loading of ammonium (NH₄⁺-N) 50 ± 2 mg/L, at 25°C–27°C, and less than 1.0 mg/L dissolved oxygen (DO). After coupling the electric (MF) 48mT with Fe⁰, the removal efficiencies were enhanced by 76% for (NO₃⁻-N) with an effluent concentration of 8.7 mg/L, 72% for (NH₄⁺-N) with an effluent concentration of 13.6 mg/L, and 76% for total nitrogen removal (TN), respectively. The sludge was rich in ammonia-oxidizing bacteria, heterotrophic nitrifying bacteria, and denitrifying bacteria. After coupling (MF) with Fe⁰, the denitrification bacteria community underwent the most substantial change.

Magnetic field

Nitrogen removal

Nitrate removal

Nitrite removal

microbial community richness

In anoxic conditions, heterotrophic or/and autotrophic denitrification bacteria utilize organic carbon sources as electron donors to convert nitrite and nitrate into free nitrogen gas. A lack of organic material often slows denitrification; thus, a carbon source is added to the process to convert nitrate into nitrogen gas. However, external carbon sources in wastewater treatment plants (WWTPs) have increased the breadth of safety problems, high risk, and high operating management cost of heterotrophic denitrification associated with selecting optimal carbon sources. Instead of employing organic carbon, zero-valent iron (Fe⁰) is used as the electron donor. Zero-valent iron (Fe⁰) is a widely available, low-cost, non-toxic, easy-to-handle material (Fu, Dionysiou et al. 2014). Many contaminants, including halogenated organics, nitrate, dyes, and phenol, have been successfully removed from groundwater and wastewater using Fe⁰ (Jiang, Li et al. 2008, Siddiqui, Momin et al. 2013, Fu, Dionysiou et al. 2014). However, Fe⁰ is used alone as an electron donor to remove nitrate from the environment by autotrophic and heterotrophic denitrification (Aslan and Türkman 2005, Hosseini, Ataie-Ashtiani et al. 2011, Zhao, Zhang et al. 2012). As a result of the Fe⁰ corrosion process, H₂, Fe²⁺, and Fe³⁺ are released, and hydrogen can be used as the final electron donor for heterotrophic denitrification (Zhang 2002). Fe⁰ might oxidize organic pollutants by donating 2e⁻ to O_2, releasing hydrogen peroxide H₂O₂; another 2e⁻ arising from Fe⁰ will alter H₂O₂ to H₂O, as in Eq. (1–2) (González-Dávila, Santana-Casiano et al. 2006).

Fe⁰ + O₂ + 2H⁺→ Fe²⁺ + H₂O₂ (1)

Fe⁰ + H₂O₂ + 2H⁺→ Fe²⁺ + 2H₂O (2)

Furthermore, Fe⁰ corrosion results, and oxidizing of Fe²⁺releasing H⁺, as shown in Eq. (3–6), e.g., FeOOH, Fe₃O₄, Fe₂O₃ (Noubactep, Schöner et al. 2009).

2Fe⁰ + 2H₂O + O₂ → Fe²⁺ + 4OH⁻ (3)

6Fe²⁺ + 6H₂O + O₂ → 2Fe₃O₄↓ +12H⁺ (4)

4Fe²⁺ + 6H₂O + O₂ → 4FeOOH↓+8H⁺ (5)

4Fe²⁺ + 4H₂O + O₂ → 2Fe₂O₃↓ + 8H⁺ (6)

as a resulting, different forms of iron oxides would be produced during the Fe⁰ corrosion, such as lepidocrocite (g-FeOOH), goethite (a-FeOOH), hematite (a-Fe₂O₃), maghemite (g-Fe₂O₃), magnetite (Fe₃O₄), carbonate minerals (e.g., siderite (FeCO₃) (Yin, Zhang et al. 2011, Hassan and Rahman 2016, Xu, Zhang et al. 2016, Zhao, Zhang et al. 2018, Lee, Yap et al. 2019).

However, when Fe⁰ is used to treat wastewater, oxidants such as dissolved O₂, H₂O, and NO₃⁻ consume the bulk of the Fe⁰ (Noubactep, Schöner et al. 2009); decreasing NO₃⁻ concentration by Fe⁰ can represent the following Eq. (7) (Park, Yeon et al. 2008):

4Fe⁰ + NO₃⁻ + 7H₂O → 4Fe²⁺ + NH₄⁺ +10OH⁻ (7)

The nitrite formation occurs by nitrate reduction to nitrite by iron surface (Liu and Wang 2019), as shown in Eq. (8). it is an initial chemical step to reduce nitrate when electrons transfer from the Fe⁰ surface to NH₄⁺, as shown in Eq. (7), or reduced to nitrite by Fe²⁺, as shown in Eq. (8), or an electron donor in anoxic, or by some bacteria as shown in Eq. (9).

Fe⁰ + 2NO₃⁻ + 4H⁺ → 4Fe²⁺ + NO₂⁻ +2H₂O (8)

2Fe²⁺ + NO₃⁻ → NO₂⁻+ 2Fe³⁺ (9)

The nitrate-dependent Fe²⁺ oxidized (NDFO) reaction has recently been found to oxidize NO₃⁻ to NH₄⁺ by Fe²⁺ in an abiotic environment (Carlson, Clark et al. 2013)or chemically oxidized to N₂, as shown in in Eq. (10,11) (S⊘ rensen 1987), or by bacteria in presence of hydrogen ion [32].

10Fe²⁺_(aq) + 2NO₃⁻ +24H₂O →N_2(g) + 10Fe(OH)_3(s) + 10H⁺_(aq) (10)

10Fe²⁺+2NO₃⁻ +12H⁺→N₂ + 10 Fe³⁺+6H₂O (11)

Consequently, NH₄⁺ oxidation with Fe³⁺ reduction (Feammox) was defined as oxidizing NH₄⁺ to produce N₂, NO₂⁻ or NO₃⁻ through the reduction of Fe³⁺ and reducing to Fe²⁺, as shown in Eq. (12).

NH₄⁺ + 2H₂O + 6Fe³⁺ → NO₂⁻ + 6Fe²⁺ + 8H⁺ (12)

Through the Feammox process, Fe³⁺ is utilized as an electron acceptor to oxidize NH₄⁺. At the same time, Fe²⁺ is employed as an electron donor to decrease NO₃⁻. Because Fe³⁺ has fewer electrons and is more stable than Fe²⁺, coupling of the NDFO and Feammox reactions was studied (Xu, Sun et al. 2016), with Fe cycling used as a catalyst to decrease the need for Fe ions to prevent sludge mineralization.

However, the conversion between nitrite and Fe²⁺ and decreasing pH will produce N₂O (Park, Yeon et al. 2008), then N₂O can act as an electron acceptor by electrons donated by NO₃⁻ to produce N₂O and N₂ following Eq. (13–15).

NO₃⁻ + 2e⁻+2H⁺→NO₂⁻ + 6H₂O (13)

NO₂⁻ + 2e⁻+2H⁺→0.5NO₂ + 1.5H₂O (14)

0.5NO₂ + 2e⁻+H⁺→0.5N₂ + 0.5H₂O (15)

When Fe⁰ is used, the aging of Fe⁰ and its limited reactivity are critical issues for Fe⁰-based technology (Xu, Sun et al. 2016). Therefore, finding practical ways of significantly increasing Fe0 reactivity is crucial. Researchers employed various technologies to enhance Fe⁰ reactivity, such as ultrasonic, acid washing, H₂-reducing pretreatment (Lai and Lo 2008), electrochemical shorthand (Chen, Jin et al. 2012), Fe⁰-based bimetals (Lim, Feng et al. 2007), and nanosized Fe⁰-nFe⁰ (Huang, Liu et al. 2013). These technologies are always complicated, costly, and hazardous to the environment (Jiang, Qiao et al. 2015). As a result, other technologies for increasing the reactivity of aging Fe⁰, such as pre-magnetization or applying a weak permanent magnetic field (WMF) with Fe⁰, were used to eliminate high concentrations of p-nitrophenol (PNP), SO₂ removal (Jiang, Li et al. 2008, Siddiqui, Momin et al. 2013), accelerate chloroacetamide removal from drinking water, enhance phenol degradation, partial nitrification, CH₄ production, triethyl phosphate degradation, and antibiotic degradation (Wang, Sun et al. 2015, Wang, Liu et al. 2017, Chen, Wang et al. 2019, Huang, Yang et al. 2019, Pan, Zhang et al. 2019).

Additionally, several studies have found that the effects of permanent magnets or electromagnets can change water pH, oxidation-reduction potential (ORP) (Yin, Zhang et al. 2011, Hassan and Rahman 2016), increase electron density, and promote electron transfer for redox reactions (Yap, Lee et al. 2021). The magnetic field mainly affects the material's properties, structure, photocatalysis, electrodynamics, synthesized reaction, isomerization reaction, nuclide enhancement reaction, increasing electron density in water, and metabolic reaction (Hassan and Rahman 2016). Coupling magnetic field (MF) with Fe⁰ is chemical-free (Xu, Zhang et al. 2016); it contributes to the release of Fe²⁺ from the Fe corrosion process (Li, Zhou et al. 2017, Ren, Li et al. 2018). Which can significantly improve the pollutants' targeting, reduce Fe⁰ doses, extend the operating pH range (Sun and Guan 2019, Wang, Zhao et al. 2020), give energy to the donor electron (Salehani, Esmaeilzadeh et al. 2010), and influence the anions' movement simultaneously with paramagnetic Fe²⁺ to keep electroneutrality (Sun and Guan 2019). The theoretically essential point of coupling electric-magnetic (MF) with Fe⁰ in this study is to promote the increase of Fe²⁺, accelerating the Fe³⁺ reduction to Fe²⁺ because the reduction rate of Fe³⁺ is much slower than the oxidation rate of Fe²⁺ because the concentration of Fe²⁺ would decrease rapidly during pollutants' depredation (Chu, Su et al. 2021).

However, many researchers are working to achieve a low dose of Fe²⁺ with effective pollutant degradation and removal from wastewater (Taherdanak, Zilouei et al. 2016). Fe²⁺ will promote decreasing pH and increase the reaction rate of non-target substrates (H⁺ and O₂) with Fe⁰ and would undoubtedly increase the Fe⁰ corrosion rate concomitantly with pollutants (Guan, Sun et al. 2015). Therefore, most bacteria species use Fe²⁺ as the electron donor for metabolism (Straub, Schönhuber et al. 2004), promoting microbial abundance diversity and increasing bacteria's metabolism by promoting Fe²⁺ (Li, Feng et al. 2017). Anaerobic denitrifying Fe²⁺ oxidation bacteria grow by using Fe²⁺ as a source of energy and electron donor in marine environments or freshwater with a narrow range of Ph range to 7 (Kappler, Schink et al. 2005, Hedrich, Schlömann et al. 2011). An essential point in this study is to enhance nitrogen removal efficiency in the upflow microaerobic sludge reactor (UMSR) by coupling the electric-magnetic field (MF) with Fe⁰. By coupling the electric-magnetic field (MF) with Fe⁰, there is a possibility of continuously releasing H₂ and Fe²⁺ from the Fe⁰ corrosion process.

Additionally, coupling the electric-magnetic field (MF) with Fe⁰ may prevent Fe³⁺ precipitation on the bacterial cell surface by chemically reducing Fe³⁺ to Fe²⁺. By constantly releasing H₂ and Fe²⁺ in the process, this will give activity to some heterotrophic nitrifying bacteria and denitrifying bacteria that depend on Fe²⁺ or H₂ for metabolism in the absence of the organic carbon source. This research aimed to study a novel upflow microaerobic sludge reactor (UMSR) operated for 78 ± 2 days continuously, under five operating stages with and without coupling electric-magnetic field (MF) with Fe⁰. At the same time, the NH₄⁺-N, NO₃⁻-N, NO₂⁻-N, TN removal, and microbial community structure were evaluated in this research.

As shown in Fig. 1., the UMSR has an inner volume of 4.9 L, and the HRT was 2.5 h; the influent was injected from the bottom of the reactor. A reflux aeration tank controlled dissolved oxygen (DO) in the reactor was held at less than 1.0 mg/L by a reflux aeration tank. An extra heating system maintained the temperature at 25–27°C by a supplementary heating system. The electric-magnetic field of 40 mT was adjusted by 6V-50Hz alternate current (AC) and put deep into the sludge. The UMSR continuously operated for (78 ± 2) days, divided into five operating stages of (15 ± 1) days for each stage (without Fe⁰, with Fe⁰, coupling with MF, without coupling with MF, and coupling with MF again) until the Fe⁰ corrosion was half completed.

Synthetic nutrients and sludge

The nutrients in this synthetic wastewater were comprised of (mg/L): NH₄Cl (190), KH₂PO (25.2), KHCO₃ (124.8), CaCl₂ (300), MgSO₄ (200). The inoculated sludge was taken from the Taiyuan wastewater treatment plant, with the mixed liquor suspended solids (MLSS) and the mixed liquor volatile suspended solids (MLVSS) being 2.42 mg/L and 1.936 mg/L, respectively. The ratio of Fe⁰ powder and Fe-C powder was 3:1.

2.3. Reactor monitoring and analytical methods

Before experimenting with new conditions, the reactor was operated at its optimized condition for more than 360 days at 25–27°C. An external heating wire and a temperature controller are used to keep the inside temperature of the reactor (XH-W2140, China) at the regulated level. The pH and temperature were measured using pH and temperature meters (Hanna Instruments CAL Check™ HI5221). The concentrations of NH₄⁺-N, NO₃⁻-N, NO₂⁻-N, and TN in effluent were measured daily using standard methods (Association 1915).

Microbial community analysis

The sludge samples were taken from three operation periods (before adding Fe⁰, after adding Fe⁰, and after coupling MF with Fe⁰) labeled S1, s2, and s3. They were sent to Shanghai Lingen Biological Technology Co., Ltd to analyze the microbial diversity. After completing the genomic DNA extraction, genes were amplified using the specific primer with the barcode, 16S V4-V5: 515F–907R, 18S V9: 1380F–1510R, ITS1: ITS1F–ITS2R. The library quality was assessed on the Qubit@ 2.0 Fluorometer (Thermo Scientific) and the Agilent Bioanalyzer 2100 system. The library was sequenced on an Illumina MiSeq platform, and 250bp/300bp paired-end reads were generated.

Parameters with operating conditions and reactor performance

Table 1. This table summarizes the performance of the UMSR and coupling electric-magnetic field (MF) system concerning the NH₄⁺-N, NO₃⁻-N, NO₂⁻-N, and TN removal for each stage of operation. For these data, the process continued for 78 ± 2 days until the Fe⁰ was corroded under an anaerobic condition with electric-magnetic field effects.

Table 1

Parameters and operating conditions
Operating stages at (HRT) 2.5 h.	Effluent NH₄⁺-N (mg/L)	Effluent NO₂⁻-N (mg/L)	Effluent NO₃⁻-N (mg/L)	Nitrification rate (mg/L-d)	Effluent PH	NO₃⁻-N removal (%)	NH₄⁺-N removal (%)	TN removal(%)
Stage (1) Stage (2) Stage (3) Stage (4) Stage (5)	26.2	0.1	9.9	1.2	7.8	58	47	58
	21.9	2.3	9.2	1.3	7.1	64	56	59
	13.6	0.5	8.7	2.3	5.7	76	72	75
	12.4	0.7	11.2	2.5	6.8	55	75	68
	12.9	0.1	8.7	2.4	7.5	76	74	76

NO₃⁻-N removal efficiency

As shown in Fig. (2), the concentration of NO₃⁻-N in the effluent dropped quickly for the first few days after Fe⁰ was added to the reactor. After a few days, the concentration of NO₃⁻-N increased again. After two periods of coupling MF with Fe⁰ (first coupling with MF and second coupling with MF), the NO₃⁻-N concentration dropped to a stable level. The NO₃⁻-N removal efficiencies were 58% before adding Fe⁰, 64% after adding Fe⁰, 76% at first coupling with MF, 55% without coupling MF, and 76% at second coupling with MF, respectively. The highest and most stable removal was observed during MF coupled with Fe⁰. Coupling Fe⁰ with electric-magnetic provides stability for nitrate removal by producing H₂ and Fe²⁺. It is one of the factors that donate to denitrification by Fe⁰ corrosion. It can also happen that hydrogen can be used as an electron donor because of the corrosion of Fe⁰ in a magnetic field (Liu and Lowry 2006, Němeček, Lhotský et al. 2014). The magnetic field affects electrode kinetics by the accelerated effect on the cathodic reaction of the single Fe⁰ corrosion process. It will affect the Fe³⁺ Reduction to Fe²⁺ in this Fe⁰ recycling. It is possible for nitrate-dependent Fe²⁺ oxidized (NDFO) to occur by producing more Fe²⁺ because the reductive efficiency of nitrate by Fe⁰ increased by the increase in dissolved ferrous ion concentration (Fe^2 + _aq). It may also release cathodic H₂ by reducing water-derived protons (Weathers, Parkin et al. 1997), as shown in Eq. (16,17).

Fe⁰_(s) + H₂O → Fe (OH)_2(aq) + H_2(g) (16)

Fe⁰_(s) + 2H⁺ → Fe²⁺_(aq) + H_2(g) (17)

As a favored electron donor, hydrogen gas produced by Fe⁰ corrosion can be an advantage for several metabolic groups of denitrifying bacteria. The Fe²⁺ oxidation rate could influence various abiotic factors, including a concentration in anionic species; for instance, concentration in anionic species such as Cl⁻ in water will reduce the Fe²⁺ oxidation rate and laded to a higher accumulation of Fe²⁺ (Adeleye, Keller et al. 2013). The oxidation of Fe²⁺ increases the hydrogen ion H⁺ concentration in the water, which reflects on the pH, as shown in Eq. (7–9). The pH is an important parameter that strongly affects the nitrate reduction by Fe⁰ because the nitrate requires protons participation, which affects Fe⁰ corrosion rate and Fe corrosion product, and Fe²⁺ occurs at lower pH (Weathers, Parkin et al. 1997). It was decreased at the first stage of adding Fe⁰ to the reactor. The primary way to reduce NO₃⁻ by Fe⁰, the Fe⁰ act as an electron donor to reduce NO₃⁻ by Fe⁰ surface. The nitrate reduction depends on electron transfer efficiency from the Fe⁰ surface Fe⁰→Fe²⁺ + 2e⁻.

The existence of Fe⁰ and Fe²⁺ may involve other reactions. They are sources of electrons and protons.

3Fe⁰ + 4H₂O→Fe₃O₄ + 8H⁺ + 8e⁻ (18)

3Fe²⁺ + 4H₂O→Fe₃O₄ + 8H⁺ + 2e⁻ (19)

Hematite (Fe₂O₃) and magnetite (Fe₃O₄) were discovered in these probable reactions, as shown in Eq. (20, 21). would play a part in the pH decreasing due to Fe⁰ corrosion serving as an electron donor from the Fe⁰ surface.

Fe⁰ − 2e→ Fe²⁺

2Fe⁰ + 3H₂O − 6e⁻ →Fe₂O₃ + 6H⁺ (20)

3Fe⁰ + 4H₂O − 8e⁻ → Fe₃O₄ + 6H⁺ (21)

Moreover, the nitrate reduction with H⁺ will be consumed at neutral pH.

NO₃⁻ + 10H⁺ + 8e⁻→NH₄⁺ + 3H₂O (22)

When magnetic coupling in all phases, the lowest pH was at the first operating conditions when MF coupling with Fe⁰. The pH value decreased to 5.2 at the first stage of coupling MF with Fe⁰. The decreasing pH was low and the most stable at all stages of coupling MF with Fe⁰, as shown in Figs. (2–4).

NH₄⁺-N removal efficiency

The effluent NH₄⁺-N concentration increased significantly from 21 to 35 mg/L in the first four days after Fe⁰ was added to the reactor, followed by a decrease in NO₃⁻-N concentration from 11 to 6 mg/L. Because the reaction between Fe⁰ and NO₃⁻ converts NO₃⁻ to NH₄⁺, as shown in Eq. (7), increases the NH₄⁺ concentration. The nitrogen removal efficiency could not increase during the first few days after Fe⁰ was applied to the reactor. It was noticed that the highest NH₄⁺-N removal occurred during stage (3) when MF was first coupling with Fe⁰. the coupling of NH₄⁺ oxidation to NO₂⁻ with Fe³⁺ reduction is possible, as shown in Eq. (23). After a few days, the NH₄⁺-N concentration became lower day by day and stable, as shown in Fig. (3); the NH₄⁺-N removal efficiencies were 47% before adding Fe⁰, 56% after adding Fe⁰, 72% at the first coupled MF with Fe⁰, 74% without coupling with MF, and 75% at the second coupled MF with Fe⁰, respectively.

NH₄⁺ + 2H₂O + 6Fe³⁺ → NO₂⁻ + 6Fe²⁺ + 8H⁺ (23)

thus, the NO₂⁻ which produced from NH₄⁺ oxidation with Fe³⁺, It will be involved in the ANAMMOX reaction of ammonia, as in Eq. (24).

NH₄⁺ + NO₂⁻ → N₂ + 2H₂O (24)

NO₂⁻-N removal efficiency

The NO₂⁻-N concentration increased to 2.3 mg/L during the stage (2) of adding Fe⁰ to the reactor due to nitrite formation caused by nitrate reduction, as shown in Eq. (8) or by NH₄⁺ oxidation with Fe³⁺ (Feammox) process, as shown Eq. (12), whereas the NO₂⁻-N concentration before adding Fe⁰ was 0.1mg/L, 0.5 mg/L at first coupled MF with Fe⁰, 0.7 mg/L without coupled MF, as shown in Fig. (4). Thus, due to the ANAMMOX reaction that makes an increases NO₂⁻ concentration, as in Eq. (25)

2NH₄⁺ + 3O₂ → 2NO₂⁻ + 4H⁺+ 2H₂O (25)

while the Fe³⁺ serves as an electron acceptor, Fe³⁺ oxidized NH₄⁺ into NO₂⁻ and NO₃⁻, and the effluent's NO₂⁻-N concentration was accumulated. On the other hand, by accumulated NO₂⁻, the ANAMMOX reactions with NH₄⁺ and NO₂⁻ play a part in chemical oxidation, as shown in Eq. (26)

NH₄⁺ + 1.32NO₂⁻ +0.13H⁺→1.02N₂ + 0.26NO₃⁻ + 2.03H₂O (26)

As well, the electron donated by NO₃⁻ or which released by Fe⁰ surface with a high concentration of H⁺, the NO₂⁻ will act as an electron donator to produce N₂O, and then N₂O will serve as an electron acceptor to produce N_2, as shown in Eq. (13–15), which gives removal stability and a decrease in NO₂⁻ accumulation. Moreover, other research has shown that the adsorption of Fe²⁺ on iron oxides (e.g., Fe₃O₄, FeOH, -FeO(OH)) is crucial in decreasing (NO₃⁻, NO₂⁻, NH₄⁺). Fe²⁺ ions ionize H⁺, which may be adsorbed on the surface and reduced to active NO* by hydrogen produced by Fe corrosion after reducing NO₃⁻ to NO₂⁻. This approach was not discussed in this work regarding electric-magnetic field effects.

Structure and function of the microbial community in the reactor

As shown in Fig. (5), Microbial Community Analysis detected that relative abundance changed at the phylum level due to the changes in conditions (The relative domain abundance > 1%). Proteobacteria were the most dominant phylum in S1 with 56%, 43% in S2, and 33% in S3, followed by Acidobacteriota in S1 with 11%, 4%, and 5% in S2, S3 respectively. Chloroflexi were by 2% in S1, 21% in S2, 28% in S3, Bacteroidota were by 6% in S1, 8% in s2, 6% in s3, Gemmatimonadota by 6% in S1, 3% in s2, 4% in s3, Planctomycetota were by 4% in S1, 3% in s2 and s3, Nitrospirota was by 5% in S1, Firmicutes were by 4% in s2, 5% in s3, while it was absent in sample S1. Armatimonadota were by 2% in s2 and s3, and Actinobacteriota were by 6% in s2 and s3. At the same time, it was absent in sample S1, and Patescibacteria was by 1% in S1, 2% in s2, and 3% in s3, respectively.

A notable increase and changes in variety and a decrease in certain bacteria were detected in all samples, linked to the three operating conditions. After adding Fe⁰ and coupling it with the magnetic field, there were changes in the richness, abundance, and inhibition of certain bacteria at the genus level, as shown in Fig. (6). (The relative domain abundance > 1%). Related to Proteobacteria, before changing the conditions in the sample, S1 was rich with Rhodanobacter by 16%, which is capable under acidic, and anaerobic conditions of producing N₂ and N₂O by using nitrate, nitrite, and nitrous oxide as electron acceptors (Van Den Heuvel, Van Der Biezen et al. 2010). Denitratisoma by 14% is anaerobic oxidative reduce Nitrate to N₂O and N₂ and may lad nitrite accumulation with some N₂ fixation strains (Fahrbach, Kuever et al. 2006). SWB02 by 4%, Ellin6067 by 2% which oxidize nitrites to nitrates (NOB) (Fumasoli, Morgenroth et al. 2015). Thermomonas by 4%, sulfur oxidizers, and carbon fixing convert CO₂ to carbonate, where nitrogen fixation is related (Castelán-Sánchez, Meza-Rodríguez et al. 2020). Limnobacter by 3% is heterotrophic sulfur oxidation and may adapt with anaerobic methane oxidizers bacteria (AOM) (Chen, Feng et al. 2016). Thiobacillus, by 1%, obtains energy from oxidation and reduction of ferrous and sulfide. Moreover, it reduces nitrate, nitrite, and other nitrogen compounds to dinitrogen(Kelly and Wood 2000). Blastocatellaceae_uncultured was by 4%, using nitrate as an electron acceptor to reduce nitrate to nitrite in anaerobic conditions(Huber, Geppert et al. 2017). DS-100_norank by 3%, 11–24_norank by 2%, and Subgroup_10 by 2%. These are potentially involved in nitrate reduction to nitrite, related to Acidobacteriota. The Acidobacteriota with the genus Blastocatellaceae_uncultured decreased by 1% in s2, 2% in s3, and Subgroup_10 by1% in s2 and s3, after Fe⁰ was added to the reactor, respectively.

While richness in diversity and diminishing were observed in all samples s2 and s3 for some bacteria which are related to Proteobacteria, with genus Stenotrophomonas which can use nitrate as a terminal electron acceptor for growth with oxygen absence (Crossman, Gould et al. 2008) it was by 4% in s2, Brevundimonas which have potential of N₂ fixation it was by 2% in s2, Diaphorobacter under anaerobic conditions reduces nitrate and nitrite to N₂ (Qiu, Zuo et al. 2015) was by 1% in s2, Qipengyuania by 1% in s2 it has some strains that reported can participate in biogenic nitrogen cycling (Gao, Wu et al. 2019), Pseudoxanthomonas was by 2% in s2 and s3 it is anaerobic methane oxidation bacteria (AOM) it can utilize methane as electron donor to reduce NO₂⁻, NO₃⁻, Fe³⁺, and SO₄²⁻, it typically require additional electron donors such as hydrogen (Fu, Bai et al. 2019), Pseudomonas by 6% in s2, 4% in s3 it has been reported that hydrocarbon-degrading bacteria and may provide a carbon source and energy to other bacteria (Wang, Li et al. 2020), Acinetobacter by 4% in s2, 2% in s3 which use Fe²⁺ as electron donor be capable of denitrifying (Kiskira, Papirio et al. 2017), Hydrogenophaga by 4% in s3 it is capable nitrate denitrification and using the oxidation of H₂ as an energy source and CO₂ as a carbon source (Xu, Huang et al. 2021), Limnobacter by 2% in s2, 3% in s3, SC-I-84_norank by 1% in s2 and s3, Steroidobacteraceae_uncultured was by 1% in s2, 2% in s3, Thermomonas by 1% in s2 and s3, and Xanthobacteraceae_uncultured by 1% in s3, respectively as well as observed that in the level of the Chloroflexi, it was not the most dominant bacteria before added Fe⁰. At the same time, it became the most predominant bacteria after added Fe⁰ and Fe⁰ coupled with MF with genus SBR1031_norank by 13% in s2, 18% in s3, which is desulfurization and denitrification bacteria, Anaerolineaceae_uncultured by 1% in s2, 2% in s3 it is heterotrophs chemolithotrophs denitrifying bacteria use Fe²⁺ as electron doner, OLB14_norank by1% in s2 and s3, and KD4-96_norank by 1% in both samples s2, and s3, respectively. While proportions of Bacteroidota, with genus Microscillaceae_uncultured by 1% in S1, 8% in s2, 6% in s3, NS9 marine group_norank by 1% in s2, and SJA-28_norank by 3% in both samples s2 and s3 respectively, except that there was a slight change in abundance after adding iron, as well as the richness inhibition after coupled magnetic field with Fe⁰ such as Gemmatimonadaceae_uncultured by 3% in s2, 4% in s3 which is related to Gemmatimonadota. Nitrospira (NOB) was 5% in S1; a divergent decrease in samples s2 and s3 by 0.0089% and 0.07%, related to Nitrospirota, respectively. While Firmicutes were absent in sample S1, it was observed after adding Fe⁰ and after Fe⁰ coupled with MF with genus Lysinibacillus by1% in both samples s2 and s3. The Planctomycetota shared by Schlesneria by 2% in, S1, which utilizes an organic nitrogen compound as nitrate and ammonium as nitrogen sources, Saccharimonadales_norank by 1% in s2 and 2% in s3, and Patescibacteria was by 1% in S1, 2% in s2, and 3% in s3 respectively. Armatimonadota_norank by1% in S1 and 2% in samples s2 and s3 related to Armatimonadota. Meiothermus by 1% in S1, 2% in s2, and 1% in s3 is chemoorganoheterotroph and uses nitrate as the terminal electron acceptor related to Deinococcota. Actinobacteriota was absent in S1, while it has grown up in s2 and s3, with genus 67 − 14_norank by 2% in both samples.

As a result, Fe⁰ corrosion under an electric-magnetic field may provide Fe²⁺, Fe³⁺ and be used as an electron donor to oxidize NO₃⁻ to NH₄⁺. By producing H₂ and decreasing pH and using it as an electron donor donated by NO₃⁻ to produce N₂O and N₂, The NH₄⁺-N, NO₃⁻-N, and TN removal efficiencies in the UMSR are enhanced under MF with Fe⁰. Through coupling electric-magnetic with Fe⁰, the energy would be served to electron donors from the Fe⁰ surface and increase electron density. The denitrification direction of electron transfer from Fe⁰ to nitrate would be directly converted into N₂; on the other hand, NO₃⁻ converted into NH₄⁺ and NO₂⁻ before the coupling stage. After the iron is combined with electric-magnetic in the reactor, it gives high richness to some anammox and denitrification bacteria which depend on ferric iron as the source of bacterial metabolism or hydrogen ions as an energy source in the absence of organic carbon in anaerobic conditions. The nitrogen fixation bacteria found after Fe⁰ was added may provide energy to the anammox bacteria. After coupled MF, some bacteria had an apparent richness, and the functions have not been determined by researchers yet. There needs to be more research to determine how the temperature effects and coupling electric-magnetic with Fe⁰ in the UMSR reactor.

Author contribution Abdulatti Algonin; sample collection, data analysis, and original draft preparation. BoweiZhao; writing review, and data analysis. Xiuping Yue; supervisor, funding, and support. writing review; Ying Cui. Fei Xie; writing review.

Funding This research was supported by the Ministry of Environmental Protection of the People's Republic of China (Major Science and Technology Program, No. 2019YFC0408601 and 2019YFC0408602), the National Natural Science Foundation of China (NSFC, No. 52070139), the Foundation Research Program of Shanxi Province, China (No. 202103021224099), the Natural Science Foundation of Shanxi Province (Youth), China (No. 20210302124479).

Data availability Not applicable

Ethical approval Not applicable

Consent to participate All authors have given consent to their contribution.

Consent for publication All authors have agreed with the content, and all have given explicit consent for publication.

Competing interests The authors declare no competing interests.

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Enhancement of nitrogen removal iron-based with an electric-magnetic field in an Upflow microaerobic sludge reactor (UMSR)

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Abstract

Figures

Introduction

Methods And Materials

Synthetic nutrients and sludge

Microbial community analysis

Results And Dissuasions

Parameters with operating conditions and reactor performance

NO₃⁻-N removal efficiency

NH₄⁺-N removal efficiency

NO₂⁻-N removal efficiency

Structure and function of the microbial community in the reactor

Conclusions

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References

Status:

Version 1

Enhancement of nitrogen removal iron-based with an electric-magnetic field in an Upflow microaerobic sludge reactor (UMSR)

Status:

Version 1

Abstract

Figures

Introduction

Methods And Materials

Synthetic nutrients and sludge

Microbial community analysis

Results And Dissuasions

Parameters with operating conditions and reactor performance

NO3−-N removal efficiency

NH4+-N removal efficiency

NO2−-N removal efficiency

Structure and function of the microbial community in the reactor

Conclusions

Declarations

References

Status:

Version 1

NO₃⁻-N removal efficiency

NH₄⁺-N removal efficiency

NO₂⁻-N removal efficiency