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