Elsevier

Chemosphere

Volume 285, December 2021, 131322
Chemosphere

The effects of Fe(III) and Fe(II) on anammox process and the Fe–N metabolism

https://doi.org/10.1016/j.chemosphere.2021.131322Get rights and content

Highlights

  • The effects of series Fe(II)/Fe(III) concentrations on anammox were compared.

  • 5 mg/L-Fe(II) was the optimal concentration for promoting the anammox activity.

  • The reactor was destroyed dramatically by continuous addition of 30 mg/L-Fe(III).

  • Fe(II)-dependent nitrate reduction was induced in reactors under Fe(II) stress.

  • OM27_clade and norank_f__Burkholderiaceae might be candidates for Fe–N metabolism.

Abstract

This study aims to compare the effects of different Fe stress on anammox (anaerobic ammonium oxidation) process, therefore seven identical reactors were operated under different Fe(II)/Fe(III) concentrations. After 38 days of operation, the anammox activity was highest (10.49 ± 0.41 mg-TN/(g-VSS·h)) under conditions of 5 mg/L-Fe(II), while under 30 mg/L-Fe(III) displayed severe inhibition. The results showed that continuous addition of 30 mg/L-Fe(III) would damage the composition of EPS (extracellular polymeric substances) and make anammox bacteria more sensitive to environmental stress. While high Fe(II) concentrations could result in precipitates encasing granular sludge, affecting substrate utilization. Moreover, the results of ΔNO3--N/ΔNH4+-N indicated that Fe(II)-dependent nitrate reduction was induced in reactors added with Fe(II). OM27_clade and norank_f__Burkholderiaceae might be candidates for this process according to the correlation of genera and functional genes (based on the PICRUSt 2 functional prediction). Overall, this research is expected to provide new ideas to the effects of Fe(II)/Fe(III) on anammox and to the practical application of coupled system based on anammox in wastewater treatment.

Introduction

Anaerobic ammonium oxidation (anammox) is a promising nitrogen removal processing for treating high ammonia wastewater. Compared to the conventional nitrification denitrification process, it is more cost-effective and environmentally friendly as it can reduce oxygen demand and the emission of CO2 and N2O with no organic carbon consumption and lower sludge yields (Op Den Camp et al., 2006; Kuenen, 2008). However, anammox bacteria (AnAOB) are quite sensitive to the surrounding environment and have a long generation cycle of 11–20 days (Jetten et al., 2009). Therefore, the recovery of anammox activity is time-consumed once inhibited by interfering substances contained within wastewater (such as heavy metal, antibiotics, etc. (Pulicharla et al., 2015; Chen et al., 2016). Consequently, increasing the activity of anammox and accelerating the recovery of reactors remain important challenges.

Iron can promote the synthesis of heme c and Fe–S proteins in AnAOB which are essential for their metabolism (Ferousi et al., 2017). Besides, AnAOB contain a large number of dense iron particles (van Niftrik et al., 2008) which may relate to iron respiration. Therefore, the addition of Fe might be a potential method to enhance the activity of AnAOB. To date, previous studies showed that the addition of Fe could stimulate anammox bacteria. For example, the start-up period reduced by 20 days (Bi et al., 2014) and the specific anammox growth rate enhanced by 45.8% (Liu and Ni, 2015) with the addition of 5 mg/L-Fe(II). The activity of AnAOB increased five times with the addition of 3.68 mg/L-Fe(III) (Chen et al., 2014). Moreover, the promotion of zero-valent iron (Ren et al., 2015, 2016; Zhang et al., 2017), Fe3O4 (Gao et al., 2014), and Fe electrode (Zhang et al., 2012; Xie et al., 2020) on anammox have also reported, while reports on the inhibition of anammox by Fe are relatively few. Therefore, concentrations of 5 mg/L (relatively optimal), 15 mg/L and 30 mg/L (relatively high) were chosen to investigate the facilitation and suppression of Fe(II)/Fe(III) on anammox. More importantly, the majority of continuous experiments have been conducted in a single reactor, in which the Fe concentration was increased sequentially in a stepwise manner, rather than compared directly in several parallel (independent) reactors. A sequential experimental scheme makes it difficult to determine the actual effective concentration of Fe as it can easily precipitate and be absorbed into the anammox sludge. Therefore, comparative experiments are required to help understand and reduce the cumulative effects of Fe.

On the other hand, Fe is a potential energy source that can be used as either the electron donor or acceptor with its different valence states. The redox reactions of Fe could be carried out by microbes or abiotically. Moreover, Fe plays a significant role in nitrogen removal and nitrogen cycling (Melton et al., 2014), such as anaerobic ammonium oxidation coupled with ferric reduction (Feammox), Nitrate-dependent Fe(II) oxidation (NDFO), and Fe(II)-dependent dissimilatory nitrate reduction to ammonium (Fe(II)-dependent DNRA). Feammox coupled with anammox contributes to nitrogen removal in wetlands and paddy soils (Yang et al., 2012; Ding et al., 2014, 2020). Meanwhile, Fe(II)-dependent nitrate reduction (NDFO and DNRA) could reduce nitrate with Fe(II) as the electron donor, providing a new pathway for total nitrogen removal (Han et al., 2020; Li et al., 2020a). Generally, there is a coupling of anammox, nitrification, DNRA, Feammox, and NDFO in majority anammox reactors (Shu et al., 2016). However, these Fe–N metabolism pathways are often neglected, even though they are likely to be present within reactors due to the continuous addition of Fe. Thus, it is necessary to take these reactions into consideration when researching the effects of Fe on anammox. Furthermore, how to establish a coupled system to achieve high treatment efficiency also requires continued study and could potentially be beneficial to the engineering application of anammox.

Consequently, the main objectives of this study were to (1) compare the facilitation and suppression of different Fe(II)/Fe(III) concentrations (5, 15 and 30 mg/L) on anammox process; (2) explore the reactions which might be induced into systems under different Fe stress. Therefore, seven identical reactors were operated and nitrogen removal performance, EPS, and community structure of reactors were investigated. Meanwhile, the correlation of genera and genes (results of PICRUSt 2 functional prediction) were also assessed to determine the potential functional bacteria of reactions related to iron metabolism.

Section snippets

Experimental set-up and operation strategy

Seven identical UASB, made of polymethyl methacrylate and a working volume of 0.42 L (diameter of 3.0 cm and height of 60.0 cm), were utilized for experiments. A thermostatic water-box was equipped to maintain a fixed temperature of 35 ± 1 °C. The schematic diagram and reactor designations are shown in Fig. 1. Anammox granular sludge, refrigerated (4 °C) for over one year, was inoculated into the reactors with an initial biomass concentration of 8.0–10.0 g-VSS/L. NH4+-N and NO2-N were in form

Effects of Fe(III) and Fe(II) on AnAOB activity

The variation of nitrogen removal performance of the seven reactors after 38 days of operation is showed in Fig. 2(a ~ c). During the phase 1, the influent NH4+-N and NO2-N concentrations were 28.54 ± 2.00 mg/L and 37.68 ± 1.73 mg/L, respectively. NLR was in the range of 0.61–0.72 kg-N/(m3·d). The maximum NH4+-N concentration in effluents of the seven reactors was at 23.0–25.0 mg/L (NO2-N was 21.0–25.8 mg/L) on day 1 and stabilized at 0 mg/L after 3–4 days. At the end of this period, the

Effects of Fe(III) and Fe(II) on anammox

Our results showed that the seven reactors displayed similar performance during the initial period (Fig. 1), which might be due to the high biomass concentrations (8.0–10.0 g-VSS/L) and low substrate concentrations. The difference emerged only after increasing the NLR. The activity of anammox could be enhanced slightly by the continuous addition of relatively low concentrations of Fe(III) (<15 mg/L), but would be severely inhibited by high concentration (30 mg/L). The nitrogen removal of

Conclusion

In this work, the effects of three Fe(III) and Fe(II) concentrations (5, 15, and 30 mg/L) on anammox were investigated. It was found that 5 mg/L-Fe(II) was optimal for anammox with the SAA of 10.49 ± 0.41 mg-TN/(g-VSS·h), while the continuous addition of 30 mg/L-Fe(III) would inhibit AnAOB severely. The results showed that long-term addition of high Fe(III) concentration damaged the composition of EPS, while that of high Fe(II) concentration affected the mass-transfer of anammox granular.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Beijing Outstanding Young Scientist Project (grant numbers: C19H100010) and National Natural Science Foundation of China (grant numbers: 51908029).

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