Open AccessArticle

Significance of Influent C/N Ratios in Mainstream Anammox Process: Nitrogen Removal and Microbial Dynamics

Yandong Yang

Shichong Liu

Lei Liu

Yanan Long

Chao Wang

and

Changqing Liu

School of Environmental and Municipal Engineering, Qingdao University of Technology, Qingdao 266520, China

Author to whom correspondence should be addressed.

Water 2025, 17(4), 562; https://doi.org/10.3390/w17040562

Submission received: 8 January 2025 / Revised: 12 February 2025 / Accepted: 13 February 2025 / Published: 15 February 2025

(This article belongs to the Special Issue ANAMMOX Based Technology for Nitrogen Removal from Wastewater)

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"> Figure 1
The TN removal performance (a), effluent nitrogen concentrations (b), and ΔNO2−/ΔNH4+ and ΔNO3−/ΔNH4+ ratios (c) of the mainstream anammox reactor at an influent C/N of 0.5. "> Figure 2
The TN removal performance (a), effluent nitrogen concentrations (b), and ΔNO2−/ΔNH4+ and ΔNO3−/ΔNH4+ ratios (c) of the mainstream anammox reactor at an influent C/N of 1.0. "> Figure 3
The TN removal performance (a), effluent nitrogen concentrations (b), and ΔNO2−/ΔNH4+ and ΔNO3−/ΔNH4+ ratios (c) of the mainstream anammox reactor at an influent C/N of 2.0. "> Figure 4
Variations in organic and nitrogen concentrations in the typical cycles of mainstream anammox reactors with influent C/N ratios of 0.5 (a), 1.0 (b), and 2.0 (c). "> Figure 5
Core genera of the mainstream anammox reactors with various influent C/N ratios. ">

Versions Notes

Abstract

Achieving simultaneous anammox and denitrification is a feasible approach for enhancing nitrogen removal in mainstream anammox processes. Nevertheless, the optimal C/N range and microbial dynamics driving this process are still not fully understood. In this study, three mainstream anammox reactors were operated with varying influent C/N ratios. The results demonstrated a remarkable nitrogen removal of 92.6% achieved by combining partial denitrification and anammox with the C/N ratio set at 1.0. However, the nitrogen removal efficiency decreased when the C/N ratio was either 0.5 or 2.0, causing the accumulation of nitrate and ammonium in the effluent, respectively. These results suggest a narrow optimal range of the influent C/N for mainstream anammox processes. Additionally, a transition in the predominant denitrifier population from Denitratisoma to Thauera was noted when the C/N ratio increased. The denitrifying phenotype of Thauera was significantly influenced by the C/N ratio. Thauera can effectively collaborate with anammox bacteria only at a suitable C/N ratio, where it partially reduces the nitrate generated in the anammox reaction. With a high influent C/N, Thauera primarily performed nitrite reduction, notably inhibiting anammox activity. The results of this study are valuable for the optimal design of the mainstream anammox process.

Keywords:

simultaneous anammox and denitrification; nitrogen removal; mainstream wastewater; C/N ratio; Thauera

Graphical Abstract

1. Introduction

The mainstream anammox process, which removes nitrogen from mainstream municipal wastewater using the anammox pathway, shows great potential for achieving sustainable wastewater treatment [1,2]. In this process, ammonium is coverted to N₂ in an anaerobic environment with nitrite as the electron acceptor. Mainstream wastewaters generally contain only trace amounts of nitrite. Therefore, various types of partial nitritation reactors have been developed as a pretreatment method to supply nitrite substrate for anammox processes [3,4,5,6]. This has led to the formation of the two-stage partial nitritation/anammox process [7,8].

While a favorable nitrite/ammonium ratio for anammox can be achieved, it is important to note that the effluent of a partial nitritation reactor may still contain some undesirable substances, such as excessive nitrate and residual dissolved oxygen (DO). Specifically, the entrance of oxygen into the anammox reactor can stimulate the proliferation of nitrite-oxidizing bacteria (NOB) and hinder the anammox activity [9,10,11,12,13]. As a result, nitrate levels increase in the final effluent, resulting in low nitrogen removal efficiencies of 30–60% in the mainstream anammox reactor [9,10,11,12].

Recently, the simultaneous process of anammox and denitrification has emerged as an effective method to decrease nitrate production and improve nitrogen removal in mainstream anammox reactors [9,14,15,16]. This can be achieved by introducing the appropriate amount of biodegradable organic matter into the mainstream anammox reactor. In some cases, nitrogen removal efficiencies have been enhanced to levels exceeding 90%, resulting in effluent with an extremely low total nitrogen (TN) concentration [14]. Additionally, it has been documented that denitrification can work in conjunction with the anammox process [9,14]. The partial reduction of nitrate to nitrite can supply extra substrate to anammox bacteria, thus enhancing their activity. This symbiotic relationship ultimately contributes to the overall long-term efficiency and stability of mainstream anammox processes. Despite these advantages, there are still some issues that need to be addressed. Firstly, the optimal range of the ratio of chemical oxygen demand (COD) to TN, also known as the C/N ratio, in the influent for mainstream anammox processes has yet to be determined [16]. A low influent C/N is not effective for controlling nitrate production. On the other hand, a high influent C/N ratio can trigger the overgrowth of heterotrophic bacteria and a decline in anammox activity and population [17]. Secondly, further research is needed to explore the microbial dynamics involved in the effect of the influent C/N level. This investigation has the potential to offer new insights into how to optimize the mainstream anammox process.

In the present study, three sequencing batch reactors (SBRs) were employed to perform mainstream anammox process with various influent C/N ratios. This research focused on examining how these varying ratios affected both the nitrogen removal pathway and the microbial population. The results led to a discussion about the optimal influent C/N range and operating strategies for the mainstream anammox process.

2. Materials and Methods

2.1. Reactor Setup

Three 3L SBRs were operated to investigate the impact of different influent C/N ratios on the mainstream anammox process. Except for the influent organic concentration, all operational patterns and parameters were consistent across the three SBRs, referred to as R1, R2, and R3. The SBRs were operated at four cycles per day. Each cycle lasted for a total of 6 h, consisting of 20 min feeding, 270 min anoxic stirring, 40 min settling, 4 min effluent withdrawn, and 26 min idling. Although the reactors were not aerated, DO levels were maintained at a range of 0.03–0.08 mg/L, likely due to the influx of oxygen from the influent and air at the top of the reactor. The pH levels in the reactor were monitored but not actively regulated, fluctuating between 7.2 and 8.4. The volume exchange ratio of the SBRs was 75%. As a result, the hydraulic retention time (HRT) was 8 h. The temperature in the reactors was carefully regulated at 25 ± 3 °C by a heater. During the operation of the reactors, there was no discharge of sludge. The estimated sludge retention time (SRT) exceeded 30 d.

2.2. Wastewater and Inoculum

Synthetic wastewater that mimics mainstream wastewater after partial nitritation treatment was used as the feed for the mainstream anammox reactors [14]. Acetate was selected as the organic carbon source due to its prevalence as a rapidly biodegradable form of COD commonly found in actual municipal wastewater. The synthetic wastewater was prepared using tap water and contained the following components: acetate (at various dosages for the three SBRs), NH₄Cl (20 mg N/L), NaNO₂ (25 mg N/L), NaHCO₃ (1200 mg/L), KH₂PO₄ (30 mg/L), CaCl₂·2H₂O (150 mg/L), and FeSO₄ (6.25 mg/L), as well as trace elements solutions I and II (1 mL/L). The composition of trace elements I and II was consistent with that described in the literature [18]. The key characteristics of the synthetic wastewater are outlined in Table 1. The influent C/N ratios of R1, R2, and R3 were 0.5, 1.0, and 2.0, respectively.

The inoculum was collected from a 10 L laboratory granular anammox reactor treating ammonium-rich wastewater. The initial biomass concentrations of the three SBRs were approximately 2000 mg SS/L each. The biomass concentrations in R1, R2, and R3 were relatively stable and reached levels of 1920 mg SS/L, 2270 mg SS/L, and 2460 mg SS/L, respectively, by the end of the experiment.

2.3. Analytical Methods

The concentrations of COD, ammonium, nitrite, nitrate, total nitrogen (TN), mixed liquor suspended solids, and mixed liquor volatile suspended solids were detected in accordance with the standard methods [19]. The temperature, pH, and DO levels in the reactors were monitored online by a pH/Oxi 340i analyzer (WTW Company, Munich, Germany). Sludge samples were collected for analysis of the microbial communities using high-throughput sequencing, following the established methods [20].

3. Results and Discussion

3.1. Nitrogen Removal Performance of the Mainstream Anammox Reactors at Various C/N Ratios

The nitrogen removal performance of R1, with an influent C/N of 0.5, exhibited high instability. Although efficient nitrogen removal was realized in the first 20 days with TN removal efficiencies averaging 85.8% (Figure 1a), there was a gradual accumulation of nitrate concentration in the effluent during subsequent operations (Figure 1b). The ratio of ΔNO₃⁻-N/ΔNH₄⁺-N steadily increased to a high level of 1.34 (Figure 1c), suggesting a significant increase in nitrite oxidation activity [21]. In addition, the concentrations of effluent ammonium and nitrite significantly rose between day 30 and day 70 (Figure 1b), indicating a decrease in the in situ anammox activity during this period. Due to the build-up of effluent ammonium, nitrite, and nitrate in the effluent, the TN removal efficiency of R1 finally decreased to just 30.3% (Figure 1).

A robust and efficient nitrogen removal was achieved in R2 when the influent C/N was 1.0, as shown in Figure 2a. While effluent levels of ammonium and nitrite initially accumulated during the start-up period (day 1–12), their concentrations gradually decreased and remained consistently low in subsequent operations (Figure 2b). During the stable operational period (day 31–70), effluent TN concentrations reached a minimum of 3.5 ± 1.0 mg/L, with TN removal efficiencies reaching as high as 92.6 ± 2.3%. In addition, a remarkably low ΔNO₃⁻-N/ΔNH₄⁺-N ratio of 0.04 ± 0.07 was achieved (Figure 2c), suggesting that nitrate reduction was occurring, possibly through the denitrification pathway [21].

A gradual decline in nitrogen removal efficiency was observed in R3, where the influent C/N was 2.0, dropping from over 90% to approximately 60% (Figure 3a). There were differences in the removal performance of ammonia and nitrite. Throughout the experiment, nitrite was effectively removed, leaving a low residual concentration of 0.43 ± 0.33 mg/L. On the other hand, the performance of ammonium removal showed a consistent decline. The residual ammonium concentration reached 19.0 mg/L by the end of the operation (Figure 3b), which is very close to the concentration in the influent (20.5 ± 1.6 mg/L). This minimal removal of ammonium suggests a near-total decline in the in situ anammox activity. It can be inferred that nitrogen removal in R3 was mainly achieved through the denitrification pathway rather than the anammox pathway.

This study demonstrated that nitrogen removal in the mainstream anammox process was significantly improved when the influent C/N was 1.0. However, nitrogen removal efficiency decreased when the influent C/N was either 0.5 or 2.0, causing the accumulation of residual nitrate and ammonium, respectively. These findings emphasize the significance of maintaining an optimal influent C/N level for maximizing nitrogen removal in anammox reactors. According to the literature, the anammox process can achieve enhanced nitrogen removal with an influent C/N of 0.9 [9], 1.1 [14], and 1.8 [18]. After reviewing these findings, it is recommended that the ideal influent C/N ratio for the mainstream anammox process falls within the range of 0.9–1.8.

3.2. Nitrogen Transformation in the Typical Cycles

In the typical R1 cycle, ammonium and nitrite were steadily removed as the reaction advanced, with a notable accumulation of nitrate observed (Figure 4a). The ratios of ΔNO₂⁻-N/ΔNH₄⁺-N and ΔNO₃⁻-N/ΔNH₄⁺-N were found to be 2.07 and 0.85, respectively. Both values were notably higher than the theoretical values (1.32 and 0.26, respectively) [21], suggesting that nitratation (NO₂⁻→NO₃⁻) was occurring in the reactor.

In the typical R2 cycle, limited nitrate was produced during the removal of ammonium and nitrite (Figure 4b). The low ratio of ΔNO₃⁻-N/ΔNH₄⁺-N at 0.05 suggests nitrate reduction was occurring, likely through the pathway of denitrification. In addition, the ΔNO₂⁻-N/ΔNH₄⁺-N ratio (1.17) was found to be lower than the expected theoretical value (1.32) [21]. The gap in nitrite substrate needed for the anammox reaction could potentially be filled by the partial denitrification of nitrate (NO₃⁻→NO₂⁻). Overall, with a proper influent C/N of 1.0, the integration of partial denitrification and anammox significantly enhanced the nitrogen removal efficiency of the mainstream anammox reactor, achieving an impressive level of 92.6%.

In R3, despite the influent containing 25 mg/L of nitrite, the initial nitrite concentration was only 1.42 mg/L (Figure 4c). This suggests that the nitrite was rapidly consumed through denitrification with acetate as the electron acceptor during the feeding period. During the following anoxic period, the in situ anammox activity was significantly impaired due to the shortage of nitrite substrate, resulting in a minimal removal of ammonium (1.0 mg/L).

Overall, the cycle analysis uncovered different nitrogen transformation pathways depending on the influent C/N ratios. When the influent C/N was low at 0.5, there was a noticeable increase in nitrite oxidation activity, resulting in effluent nitrate accumulation. With a moderate influent C/N of 1.0, a successful integration of partial denitrification with anammox led to an efficient nitrogen removal (92.6%). However, at a high influent C/N of 2.0, intense denitrification activity exhausted the nitrite substrate, ultimately suppressing anammox activity entirely.

3.3. Microbial Dynamics at Various Influent C/N Ratios

A consistent decline in the anammox population and an enrichment of NOB were observed in R1, which had the lowest influent C/N ratio (Figure 5). Candidatus Brocadia, the predominant anammox bacteria, experienced a decrease in abundance from 5.17% to 2.20%. This decline is in accordance with the reduction in the in situ anammox activity (Figure 1). In contrast, the population of NOB, primarily consisting of the genus Nitrospira, grew in the reactor, with their relative abundances rising significantly from 0.09% to 0.84%. The increased presence of NOB resulted in elevated nitrate production and decreased anammox activity, primarily as a result of competition for nitrite substrate. These factors were the main contributors to the deterioration of nitrogen removal. Nitrosomonas (AOB) were also detected but in low and decreasing abundances (0.34–0.18%). Furthermore, Denitratisoma, identified as a potential denitrifier, emerged as the predominant genus in the initial seeding sludge and continued to maintain its leading presence in R1, with relative abundances ranging from 11.4% to 12.47%. Despite being highly abundant, the denitrification activity of Denitratisoma was limited by organic constraints, as evidenced by the high nitrate production in the reactor (Figure 1).

In R2, the abundance of anammox bacteria decreased from 5.17% to 1.89% in the initial 25 days, but then recovered to 3.16% during the subsequent operation (Figure 5). This trend was in line with the changes in the ammonium and nitrite removal performance (Figure 2). It is worth noting that there was an observed transition in the denitrifier population. The abundance of Denitratisoma steadily decreased from 12.47% to 6.72%. On the other hand, Thauera, another type of denitrifier, was enriched in the reactor with its abundance increasing from 0.08% to 5.56%. It is hypothesized that a symbiotic relationship exists between Thauera and Candidatus Brocadia, given their enrichment in the reactor and the successful achievement of highly efficient nitrogen removal. The abundances of AOB (Nitrosomonas) and NOB (Nitrospira) were relatively low, at 0.22% and 0.18%, respectively, on day 60.

In R3, there was a notable decrease in the population of anammox bacteria (Candidatus Brocadia) from 5.17% to 0.63% (Figure 5), which correlated with the minimal in situ anammox activity. On the other hand, Thauera showed a significant enrichment with a relative abundance of 10.10% on day 45, exceeding that of Denitratisoma (4.21%). Although Thauera is known for its role in partial denitrification [22,23], it also possesses the capacity for complete denitrification [24,25,26]. Due to the high abundance of Thauera and sufficient organics, nitrite was rapidly depleted during the feeding phase (Figure 3). This led to a notable suppression of the anammox activity and growth.

High-throughput sequencing revealed the microbial mechanisms responsible for the variations in nitrogen removal efficiency seen at different C/N ratios. Only at a suitable C/N ratio of 1.0 can enhanced nitrogen removal be achieved through the proper cooperation of anammox bacteria and denitrifying bacteria (Figure 1). At a low C/N ratio of 0.5, the Nitrospira-like NOB were found to proliferate in the mainstream anammox reactor. Previous studies have shown that Nitrospira-like NOB have a strong affinity for oxygen [9,27], allowing them to effectively utilize the limited DO in the reactor. The high activity of the NOB resulted in elevated nitrate levels in the effluent (Figure 1) and a decline in the population of anammox bacteria (Figure 5), possibly due to competition for nitrite substrate. Therefore, the efficiency of nitrogen removal decreased significantly. At a high C/N ratio of 2.0, denitrifying bacteria underwent excessive growth (Figure 5). The nitrite present in the influent was quickly utilized through denitrification, resulting in a shortage of nitrite substrate for anammox bacteria (Figure 4). As a result, both the population and in situ activity of anammox bacteria experienced a notable decrease (Figure 5). The reactor primarily performed denitrification, with a minimal removal of ammonium occurring (Figure 1).

Unlike the stable composition of anammox bacteria and nitrifiers, an evolution in the population of denitrifiers occurred as the influent C/N ratio increased. Two primary genera of denitrifying bacteria were identified, namely Denitratisoma and Thauera. When the influent C/N rose from 0.5 to 2.0, the abundance of Denitratisoma decreased from 11.40% to 4.21%, while the abundance of Thauera increased from 1.38% to 10.10% (Figure 5). The results demonstrated the competitive advantages of Thauera over Denitratisoma at relatively high influent C/N levels when acetate was the sole organic carbon. Moreover, this study has uncovered that the denitrifying behavior of Thauera is significantly affected by the influent C/N ratio in the mainstream anammox reactor, where nitrite is the primary source of nitrogen oxides in the influent. It was found that Thauera is able to collaborate effectively with anammox bacteria only with a suitable C/N ratio (1.0). In doing so, it is capable of partially reducing the nitrate generated in the anammox reaction and recycling nitrite substrate back to anammox bacteria. Consequently, there was a simultaneous increase in the populations of Thauera and Candidatus Brocadia (Figure 5). At a high influent C/N ratio, Thauera primarily performed nitrite reduction, which notably inhibited anammox activity and resulted in a decline in the abundance of anammox bacteria (Figure 5).

3.4. Implication of This Work

This study has successfully demonstrated the effectiveness of enhancing nitrogen removal by combining the process of anammox and denitrification at a suitable influent C/N ratio. A remarkable nitrogen removal (92.6%) was realized with a low residual TN (3.5 mg/L), meeting stringent discharge standards (Figure 2). Furthermore, the enrichment of anammox bacteria highlights the operational resilience of the process (Figure 5). Overall, simultaneous anammox and denitrification proves to be a practical and efficient approach for removing nitrogen from municipal wastewater.

In this study, it was found that a narrow range of the optimal influent C/N ratio is crucial for the efficiency of mainstream anammox reactors. This underscores the importance of carefully managing the inflow of biodegradable organic carbon. After undergoing partial nitritation treatment, the influent of the mainstream anammox reactor typically contains minimal levels of biodegradable organic carbon. In practical applications, one effective strategy for introducing biodegradable organic carbon is to divert a portion of raw municipal wastewater to the mainstream anammox reactor [9,14]. However, a significant amount of ammonium would also be introduced into the mainstream anammox reactor. It may be challenging to simultaneously maintain appropriate levels of both the C/N ratio and the NO₂⁻/NH₄⁺ ratio. An alternative approach involves utilizing the fermentation liquid from primary sludge as a sustainable source of biodegradable organic carbon [28,29]. Previous studies have shown that operating a fermenter with a short SRT of 2 d can lead to the maximum production of soluble COD without causing an excessive release of ammonia into the system [28]. Therefore, this approach may be more suitable for regulating the biodegradable organic carbon input to the mainstream anammox reactor while minimally impacting the NO₂⁻/NH₄⁺ ratio. Online monitoring devices are essential for real-time monitoring of the influent flow rate, TN concentration of the mainstream anammox reactor, and COD concentration of the sludge fermentation liquid. These devices help in determining the influent nitrogen load of the mainstream anammox reactor and the influent carbon load of the sludge fermentation liquid. By adjusting the influent flow rate of the sludge fermentation liquid, the optimal C/N ratio for the mainstream anammox reactor can be achieved.

This study successfully demonstrated the potential for a simultaneous anammox and denitrification process in an SBR using synthetic municipal wastewater at a moderate temperature of 25 ± 3 °C. To implement this process in real wastewater treatment plants, further considerations must be made regarding the type of wastewater, reactor configuration, and temperature. The previous literature has indicated that this process can also be effective for treating actual municipal wastewater [18]. However, it is important to consider that the optimal influent C/N ratio may need to be higher for actual wastewater due to the presence of hard-to-biodegrade organics. Through this research, a C/N ratio of 0.9–1.8 was identified as optimal for achieving simultaneous anammox and denitrification using synthetic wastewater. It is crucial to customize this ratio based on the ratio of biochemical oxygen demand (BOD) to COD when treating real municipal wastewater. Regarding the reactor configuration, to the best of the authors’ knowledge, utilizing simultaneous anammox and denitrification processes in plug-flow reactors is also a viable option. However, further verification is necessary in future research. Some special reactor designs may be necessary, particularly for retaining the slow-growth anammox bacteria. Due to the challenges associated with cultivating granular sludge in plug-flow reactors, incorporating biocarriers for the attached growth of anammox bacteria is a beneficial alternative. The moving bed biofilm reactor and integrated fixed-film activated sludge reactor are both excellent options for this configuration [30]. Finally, the process’s feasibility should be further verified with seasonal temperature variations. Due to the sensitivity of anammox bacteria to temperature changes, there is a possibility of a decrease in both the rate and efficiency of nitrogen removal at lower temperatures [31]. Additionally, it is important to note that the ideal influent C/N ratio may vary as temperatures decrease. In colder conditions, a narrower range of ideal C/N ratios is anticipated.

4. Conclusions

This study explored the nitrogen removal and microbial dynamics of mainstream anammox reactors with varying C/N ratios. The key findings are as follows:

(1): The optimal influent C/N for the mainstream anammox process falls within a narrow range of 0.9–1.8. Operating the mainstream anammox reactors with influent C/N ratios lower or higher than this range led to a decrease in nitrogen removal efficiency, as nitrate and ammonium accumulated in the effluent, respectively.
(2): A remarkable nitrogen removal efficiency of 92.6% can be achieved when the influent C/N is 1.0. The enhancement of nitrogen removal is due to the effective combination of partial denitrification and anammox, catalyzed by the bacteria Thauera and Candidatus Brocadia, respectively.
(3): Variations in the influent C/N can potentially impact the composition of the denitrifier in mainstream anammox reactors. Specifically, a transition in the dominant denitrifier from Denitratisoma to Thauera was observed as the influent C/N increased. Additionally, the denitrifying phenotype of Thauera was greatly impacted by the influent C/N.

Author Contributions

Conceptualization, Y.Y. and S.L.; methodology, S.L., L.L. and C.W.; validation, S.L.; formal analysis, L.L.; investigation, S.L., Y.L. and L.L.; data curation, Y.Y., S.L. and Y.L.; writing—original draft preparation, Y.Y.; writing—review and editing, S.L. and C.L.; project administration, C.L.; funding acquisition, Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Shandong Province, China (Grant No. ZR2023ME202).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The TN removal performance (a), effluent nitrogen concentrations (b), and ΔNO₂⁻/ΔNH₄⁺ and ΔNO₃⁻/ΔNH₄⁺ ratios (c) of the mainstream anammox reactor at an influent C/N of 0.5.

Figure 2. The TN removal performance (a), effluent nitrogen concentrations (b), and ΔNO₂⁻/ΔNH₄⁺ and ΔNO₃⁻/ΔNH₄⁺ ratios (c) of the mainstream anammox reactor at an influent C/N of 1.0.

Figure 3. The TN removal performance (a), effluent nitrogen concentrations (b), and ΔNO₂⁻/ΔNH₄⁺ and ΔNO₃⁻/ΔNH₄⁺ ratios (c) of the mainstream anammox reactor at an influent C/N of 2.0.

Figure 4. Variations in organic and nitrogen concentrations in the typical cycles of mainstream anammox reactors with influent C/N ratios of 0.5 (a), 1.0 (b), and 2.0 (c).

Figure 5. Core genera of the mainstream anammox reactors with various influent C/N ratios.

Table 1. The key characteristics of the synthetic municipal wastewater.

COD (mg/L)	TN (mg/L)	NH₄⁺-N (mg/L)	NO₂⁻-N (mg/L)	NO₃⁻-N (mg/L)	PO₄³⁻-P (mg/L)	pH
R1: 25.6 ± 3.4, R2: 53.4 ± 5.2 R3: 108.1 ± 6.0	48.3 ± 2.5	20.6 ± 1.7	25.7 ± 2.0	1.7 ± 0.8	7.1 ± 0.6	7.2–8.0

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Yang, Y.; Liu, S.; Liu, L.; Long, Y.; Wang, C.; Liu, C. Significance of Influent C/N Ratios in Mainstream Anammox Process: Nitrogen Removal and Microbial Dynamics. Water 2025, 17, 562. https://doi.org/10.3390/w17040562

AMA Style

Yang Y, Liu S, Liu L, Long Y, Wang C, Liu C. Significance of Influent C/N Ratios in Mainstream Anammox Process: Nitrogen Removal and Microbial Dynamics. Water. 2025; 17(4):562. https://doi.org/10.3390/w17040562

Chicago/Turabian Style

Yang, Yandong, Shichong Liu, Lei Liu, Yanan Long, Chao Wang, and Changqing Liu. 2025. "Significance of Influent C/N Ratios in Mainstream Anammox Process: Nitrogen Removal and Microbial Dynamics" Water 17, no. 4: 562. https://doi.org/10.3390/w17040562

APA Style

Yang, Y., Liu, S., Liu, L., Long, Y., Wang, C., & Liu, C. (2025). Significance of Influent C/N Ratios in Mainstream Anammox Process: Nitrogen Removal and Microbial Dynamics. Water, 17(4), 562. https://doi.org/10.3390/w17040562

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