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Article

Unraveling Nitrogen Removal and Microbial Response of Integrated Sulfur-Driven Partial Denitrification and Anammox Process in Saline Wastewater Treatment

by
Xiangchen Li
*,
Jie Sun
,
Zonglun Cao
,
Junxi Lai
,
Haodi Feng
and
Minwen Guo
Environmental Protection Research Institute, SINOPEC Beijing Research Institute of Chemical Industry, Beijing 100013, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(15), 2284; https://doi.org/10.3390/w17152284
Submission received: 2 July 2025 / Revised: 20 July 2025 / Accepted: 29 July 2025 / Published: 31 July 2025
(This article belongs to the Special Issue Biological Nitrogen Removal in the Multi-Omics Era: Coupling Anammox with Microbial-Mineral Synergy)
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Abstract

Increasing the discharge of saline wastewater from an industrial field poses a challenge for applicable Anammox-based technologies. This study established the integrated partial sulfur-driven denitrification and Anammox (SPDA) system to explore the effects of different salinity levels on nitrogen conversion features. The results of batch tests suggested that sulfur-driven denitrification exhibited progressive suppression of nitrate reduction (97.7% → 12.3% efficiency at 0% → 4% salinity) and significant nitrite accumulation (56.4% accumulation rate at 2% salinity). Anammox showed higher salinity tolerance but still experienced drastic TN removal decline (97.6% → 17.3% at 0% → 4% salinity). Long-term operation demonstrated that the SPDA process could be rapidly established at 0% salinity and stabilize with TN removal efficiencies of 98.1% (1% salinity), 72.8% (2% salinity), and 70.2% (4% salinity). The robustness of the system was attributed to the appropriate strategy of gradual salinity elevation, the promoted secretion of protein-dominated EPS, the salinity-responsive enrichment of Sulfurimonas (replacing Thiobacillus and Ferritrophicum) as sulfur-oxidizing bacteria (SOB), and the sustained retention and activity of Brocadia as AnAOB. The findings in this study deepen the understanding of the inhibitory effects of salinity on the SPDA system, providing a feasible solution for saline wastewater treatment with low cost and high efficiency.

1. Introduction

The conventional anaerobic–anoxic–oxic (AAO) process faces challenges in wastewater treatment, including a high carbon source demand and elevated operational costs. In China, the low C/N ratio of wastewater has become a critical limiting factor for efficient biological treatment [1]. Current solutions often rely on external carbon sources to enhance heterotrophic denitrification. However, this approach not only increases operational costs but also leads to higher greenhouse gas emissions due to the degradation of organics, which contradicts the strategic goal of carbon neutrality [2,3]. Therefore, developing high-efficiency, low-consumption, and sustainable wastewater treatment technologies is of great significance.
The anaerobic ammonium oxidation (Anammox) process offers a novel solution due to its unique metabolic pathway. In this process, Anammox bacteria (AnAOB) directly convert ammonium (NH4+-N) into nitrogen gas (N2) using nitrite (NO2−-N) as an electron acceptor (Equation (1)), eliminating the demand of aeration and external organic carbon [4,5,6]. Since its discovery, Anammox technology has attracted widespread attention for its remarkable energy-saving potential [7,8]. However, practical application is limited by the slow growth rate of AnAOB, their high sensitivity to environmental factors such as salinity, and the lack of stable NO2−-N substrates supply.
NH4+ + 1.32NO2− + 0.066HCO3− + 0.13H+ → 1.02N2 + 0.26NO3− + 0.066CH2O0.5N0.15 + 2.03H2O
NO2−-N, as a key substrate for Anammox, can be supplied via partial denitrification (NO3−-N → NO2−-N) [9]. Sulfur-driven partial denitrification (SPD) refers to the promising process in which sulfur-oxidizing bacteria (SOB) use reduced sulfur compounds (e.g., elemental sulfur, S2−, and S2O32−) as electron donors to reduce NO3−-N to NO2−-N (Equation (2)). Compared to organics-driven partial denitrification, SPD saves 100% of external organic carbon demand, reduces 55% of sludge production, decreases 80% of operational costs, and lowers 95–100% of greenhouse gas emissions. Moreover, the biomass yield (0.15 g-biomass/g-N) and growth rate (0.04/h) of SOB are close to those of AnAOB (biomass yield: 0.1 g-biomass/g-N; growth rate: 0.015/h), facilitating their coexistence and preventing competitive inhibition of AnAOB with excessive denitrifying bacteria [10,11,12,13]. Zhang et al. [14] reported that sulfate produced from sulfur-driven denitrification can enhance the nitrogen metabolism activity of AnAOB, thereby strengthening autotrophic nitrogen removal efficiency. Thus, the integration of SPD with Anammox (SPDA) presents significant technical advantages and application potential in treating wastewater containing NH4+-N and NO3−-N.
3NO3− + S0 + H2O → 3NO2− + SO42− + 2H+
Facing the challenge of limited freshwater availability, more coastal cities are using seawater to flush toilets, introducing additional salinity to municipal sewage as a result. Meanwhile, the global sodium chloride traded was approximately 314 million ton in 2018 with a constant growing rate expected in the upcoming years [15]. The amount of industrial wastewater with high salinity has also increased rapidly in the past decades due to the rising demands of food, industrial raw materials, and pharmaceutical and personal-care products [16,17,18]. Saline wastewater accounts for approximately 20–40% of total wastewater discharge [19]. Unlike domestic wastewater, saline wastewater is typically characterized by high salinity (0.1–3.2%), poor biodegradability (BOD/COD < 0.3), and complex nitrogenous pollutants [20,21,22]. It has been reported that acclimated activated sludge can maintain nitrogen removal performance with salinity below 2.0%, but nitrification and denitrification are significantly inhibited when salinity exceeds 2.5–3.0% [23]. Li et al. [24] found that when the salinity increased to 3.0%, the nitrate removal efficiency of the sulfur-driven denitrification system decreased by 16.8%. High salinity induces enzyme denaturation and cellular damage, ultimately impairing nitrogen removal performance [25]. Additionally, elevated Na+ levels in high-salinity wastewater increase intracellular osmotic pressure, leading to cell membrane rupture and even lysis [26,27]. Although SPDA technology has achieved 98.0% total nitrogen (TN) removal in low-salinity wastewater with a nitrogen removal rate of 4.1 kgN/(m3·d) [28], its nitrogen removal performance in high-salinity wastewater treatment remains underexplored. Meanwhile, the knowledge gap remains to be filled regarding the start-up, domestication, and operational strategy for the SPDA system to achieve stable and satisfactory performance in saline wastewater treatment.
This study aims to (a) investigate nitrogen transformation characteristics under increasing salinity in sulfur-driven denitrification and Anammox systems, respectively, through two series of batch experiments, (b) evaluate the nitrogen removal efficiency and stability of the SPDA process under high-salinity conditions during long-term operation, (c) systematically analyze the dynamic changes in extracellular polymeric substances (EPS) of SPDA system, and (d) reveal microbial community succession and specialized functional bacteria under elevated salinity through high-throughput sequencing technology. These findings provide theoretical and technical support for the engineering application of SPDA in high-salinity industrial wastewater treatment.

2. Materials and Methods

2.1. Experimental Setup and Operating Phases

Batch tests were carried out in 500 mL glass bottles in order to explore the effect of salinities on sulfur-driven denitrification and Anammox processes, respectively. The initial substrate in batch tests on sulfur-driven denitrification was 30 mg/L NO3−-N, while in batch tests on Anammox, they were 20 mg/L NH4+-N and 30 mg/L NO2−-N. In all batch tests, pH was maintained at 7.3 ± 0.2 by titration with NaOH and HCl solutions twice a day.
Long-term experiments were carried out in an upflow anaerobic sludge bed (UASB) with a volume of 1.5 L in order to evaluate the nitrogen removal performance of the SPDA system under saline stress. The influent contained 20 mg/L NH4+-N and 30 mg/L NO3−-N. The operational temperature and hydraulic residence time were maintained at 30.0 ± 2.0 °C and 4 h, respectively.
The salinity baseline from the nitrogen-containing compound, alkalinity, and trace elements in feed wastewater was less than 200 mg/L, referred to as 0% salinity in this study. Wastewater with 1%, 2%, and 4% salinity was prepared by supplementing 10 g/L, 20 g/L, and 40 g/L NaCl, respectively. For batch tests, multiple bottles were operated to compare their performance under different salinity levels, which were fixed throughout the operation. Each condition was conducted with three identical bottles in parallel. For long-term experiments, two USAB, the test unit and the control unit, were operated in parallel with identical initial conditions. The salinity in the test unit was varied by feeding wastewater with different salinity in different operating phases, as specified in Table 1. On the first day of phase 2, phase 3, and phase 4, the salinity of feeding wastewater for the test unit was changed to 1%, 2%, and 4%, respectively. The salinity in the control unit was fixed at 0% to reflect performance change in the SPDA system over time. The performance differences between the test unit and the control unit should be attributed to differences in saline stress.

2.2. Sludge and Wastewater Composition

Two types of activated sludge were used for inoculation: One was collected from the return sludge of a municipal wastewater treatment plant in Beijing with an anaerobic–anoxic–oxic (AAO) process. Influent wastewater of the plant features 40–60 mg/L NH4+-N and 150–280 mg/L COD. The sludge features an MLSS of 8000 mg/L. The other was obtained from an Anammox pilot-scale reactor with a nitrogen removal rate of 0.5 kgN/(m3·d). The sludge features a MLSS of 5000 mg/L. These two types of inoculum sludge were inoculated into the UASB reactor at a volumetric ratio of 3:1.
The wastewater used in the batch tests and the long-term experiments was prepared with tap water. Besides the nitrogen-containing pollutants and NaCl specified before, it also included 5 mg/L MgSO4, 20 mg/L CaCl2, 2.2 mg/L KH2PO4, and 50 mg/L NaHCO3.

2.3. Sampling and Analytical Methods

Liquor samples were regularly collected from the influent and effluent of the UASB and were immediately filtered using 10 μm filter paper. The NH4+-N, NO2−N, and NO3−-N concentrations were determined using ion chromatography (Agilent, Santa Clara, CA, USA). Methods for the extraction of extracellular polymeric substances (EPSs) and the characterization of EPS compositions were referenced from previous studies [29].

2.4. DNA Extraction and Microbial Community Analysis

The sludge samples were collected from the inoculum sludge and UASB on the 18th and 82nd days. These samples were then subjected to DNA extraction using a FastDNA SPIN Kit for Soil (MP Biomedicals, Solon, OH, USA) and sequencing analysis using a MiSeq platform (Majorbio Bio-Pharm Technology, Shanghai, China) [30]. Universal primers 338F and 806R targeting the V3-V4 regions of the 16S rRNA genes were employed. Analysis of the microbial community structure was performed using the Majorbio Cloud Platform (Majorbio Bio-Pharm Technology, Shanghai, China).

2.5. Data Analysis and Statistical Processing

Experiments for each condition in the batch tests were conducted in triplicate. Boxes and whiskers were plotted using Origin 2022 with default settings. The median empty square represents the mean value of the data, the box represents the quartile (25–75%), and the whiskers represent the 95% confidence band. Significant differences were derived by a one-way ANOVA test using Origin 2022 (* 0.01 < p < 0.05, ** 0.001 < p < 0.01, *** p < 0.001). Two UASB units for long-term experiments were operated in parallel as described in Section 2.1 with no replicates.

3. Results and Discussion

3.1. Performance of Nitrogen Conversions Under Different Salinities in Batch Tests

3.1.1. Saline Stress on Sulfur-Driven Denitrification

A series of batch tests was performed in order to explore the nitrogen conversion of the sulfur-driven denitrification process under different salinities. As shown in Figure 1, when the salinity was 0%, the nitrate removal efficiency reached 97.7% within the 240 min reaction. The effluent NO3−-N concentration was as low as 0.7 mg/L, without detectable nitrite accumulation. When the salinity rose to 1%, the nitrate removal efficiency declined to 74.0–78.2%, while the nitrite accumulation was detectable at 4.1 mg/L on 240 min. As the salinity further rose to 2%, the phenomenon of nitrite accumulation is more pronounced, with a NO2−-N concentration of 7.5 mg/L on 240 min. However, the nitrogen conversion was inhibited, manifested by a decrease in nitrate reduction efficiency to 42.2–47.7%. This situation became even worse when the salinity further rose to 4%, with the nitrate removal efficiency decreasing to 12.3–17.1%.
This indicates that the sulfur-driven denitrification process was sensitive to salinity. Compared to the nitrate reduction process, the nitrite reduction process was more vulnerable under saline stress. As shown in Figure 1d, the nitrite accumulation rate increased gradually from 0.3% to 56.4% with salinity rising from 0% to 2%, whereas it declined to 0.0–2.5% with 4% salinity. This is possibly due to the activity of the nitrite reduction enzyme (NIR) being suppressed more at lower salinity levels (1–2%) than that of the nitrate reduction enzyme (NAR), while when salinity further reached 4%, the activities of both enzymes were severely suppressed, resulting in poor nitrate reduction efficiency.

3.1.2. Saline Stress on Anammox

As for the Anammox process, the TN removal performance was also evaluated through batch tests. Figure 2a shows that under the salinities of 0% and 1%, the effluent TN concentration was in the range of 0.2–4.5 mg/L, with TN removal efficiencies in between 90.8 and 97.6%. But when the salinity continued to increase to 2%, the Anammox process was significantly suppressed, manifested by an increased TN concentration to 22.1–26.1 mg/L and decreased TN removal efficiency to 48.8–55.4%. As salinity reached 4%, the TN removal efficiency was as low as 17.3–24.0%, indicating that the Anammox process was severely inhibited.
In comparing Figure 2b to Figure 1e, the TN removal efficiency of Anammox is significantly higher than that of the sulfur-driven denitrification process under 1% and 2% salinities. Despite smaller advantages under 4% salinity, Anammox always performs better than the sulfur-driven denitrification process, indicating its higher robustness under saline stress. Anammox process plays an important role in the nitrogen cycle of marine ecosystems, with over 50% of TN removal contributed by AnAOB [31,32]. It has been reported that AnAOB has a remarkable saline tolerance, resulting in promising potential for saline wastewater treatment [33]. Lin, Pratt, Li, and Ye [15] found that 80–90% TN removal efficiency could be achieved under salinities of 0.2–1.2%. Windey et al. [34] found that 84% TN removal efficiency could be achieved under salinity of 3.0% after adaption. Consistent with conclusions from the previous studies above, the results from this study prove the potential of Anammox-based technology for high-salinity wastewater treatment. Furthermore, the sulfur-driven denitrification process could provide accessible NO2−-N for AnAOB under saline stress, implying the advantages of integrating Anammox and sulfur-driven denitrification.

3.2. Long-Term Operational Performance of SPDA System

The SPDA performance under different salinity conditions is shown in Figure 3. In phase 1 (0% salinity), the SPDA integrated system could be rapidly launched, suggested by the steep increase in TN removal efficiency to 93.0% within 14 days. In phase 2 (1% salinity), a temporal fluctuation in TN removal performance was observed in the initial days when salinity increased to 1%, with TN removal efficiency declining to 78.4%. As the SPDA system adapted to the saline environment, TN removal efficiency was recovered and maintained at 92.5–98.1%. This indicates the outstanding TN removal performance of the SPDA system under low-salinity conditions. When salinities continued increasing to 2% and 4% in phase 3 and phase 4, respectively, more severe inhibition and longer adaption periods were observed in this SPDA system. Effluent TN concentration reached 25.3 mg/L during the adaption period, and stabilized at 9.3–15.5 mg/L, which was higher than that in phase 2. The average TN removal efficiencies in phase 3 and phase 4 were 72.8% and 70.2%, respectively, which were relatively close, indicating that saline stress has a similar impact on the SPDA in the 2–4% range.
Combined with the results of batch inhibitory tests, it could be inferred that SPDA achieved satisfactory TN removal efficiency under the condition of salinity < 4% after long-term domestication (Figure 3 and Figure S1). This is mainly due to the following: (a) The Anammox process has good adaptability to a high-salinity environment. (b) In a high-salinity situation, sulfur-driven denitrification naturally leads to nitrite accumulation, providing the key substrate for AnAOB. (c) Despite both the sulfur-driven denitrification and the Anammox systems performing poorly under shock saline stress, revealed by batch tests, with the appropriate strategy of gradual salinity elevation, both processes can adapt to high-salinity environments and achieve comparable performance to that in low-salinity environments.
As shown in Figure 3c, the average contribution of the Anammox pathway to TN removal gradually increased from 80.4% to 91.8% as salinity increased from 0% to 4%. This suggests that the system with two integrated nitrogen removal pathways can adapt to a wider range of salinity conditions. Considering the trend of frequent fluctuation in industrial wastewater salinity, such flexibility of the SPDA technology can be applied to challenging situations where traditional methods cannot achieve stable treating results.

3.3. Changes in EPS Characteristics During Long-Term Operation

EPS secreted by active microorganisms plays an important role in sustaining cell structure, resisting adverse environments, and promoting biofilm formation [35]. It mainly contains proteins (PN), polysaccharides (PS), humus, and a small amount of DNA fragments. PN and PS are the main components of EPS, making critical contributions in reflecting microbial status and facilitating functional consortia metabolism in high-salinity environments.
As shown in Figure 4a, during long-term operation, total EPS concentration gradually increased from 66.6 mg/g·VSS in phase 1 to 113.5 mg/g·VSS in phase 4, in which the PN concentration increased from 53.0 mg/g·VSS to 95.6 mg/g·VSS, and the PS concentration increased from 13.5 mg/g·VSS to 17.9 mg/g·VSS. Faster increase in PN compared to PS in EPS indicated that the promotion of the PN synthetic enzyme activity was the primary adaption made by microorganisms with the increase in salinity. Previous studies have established that PN in EPS exhibits a stronger correlation with the surface properties of zoogloea, particularly hydrophobicity and surface charge, compared to PS in EPS [36,37]. And PS in EPS could promote cell adhesion and assemble polymeric matrices to enhance the stability of microbial aggregates [38]. In this study, as indicated by the PN/PS ratios increasing from 4.0 to 5.4, as shown in Figure 4b, PN in EPS played a dominant role over PS under salt stress. A higher PN/PS ratio is correlated with the increased hydrophobicity, which alleviated the disturbance of regular cell metabolism from increased osmotic pressure around cells. This indicates that PN out-weighed PS as the decisive EPS component for alleviating saline inhibition.

3.4. Evolution of Microbial Community

16S rRNA Illumina sequencing was performed to reveal the evolution dynamics of the microbial community during the long-term operational period (inoculate sludge, phase 1, and phase 4). The effective numbers of sequencing after noise reduction in these three samples were 49,383, 48,656, and 57,292, respectively. As shown in Figure 5, the Chao index and Shannon index both decreased along with the establishment of the SPDA system at 0% salinity. This indicates that both microbial richness and diversity decreased, mainly due to the selection and enrichment of functional microbes for sulfur-driven denitrification and Anammox processes. When the salinity was elevated to 4%, the microbial richness and diversity went back up. Together with the results of TN removal performance in consideration, the complex microbial community formed a reliable microbial network, resulting in sustainable metabolisms under a high-salinity environment.
At the phylum level (Figure 6a), Proteobacteria, Bacteroidota, Chloroflexi, Patescibacteria, and Actinobacteriota were predominant in the UASB. The relative abundance of phylum Proteobacteria increased from 52.9% to 65.4% along with the SPDA start-up, whereas it decreased to 24.3% in the 4% salinity situation. In contrast, the relative abundance of phyla Chloroflexi, Patescibacteria, and Actinobacteriota gradually increased with the elevation of salinity from 0% to 4%.
Further analysis at the genus level revealed that microorganisms related to nitrification, denitrification, organic matter degradation, phosphorus removal, and other functions were commonly found in the inoculated sludge. With the establishment of the SPDA system, the relative abundance of genera Thiobacillus and Ferritrophicum rapidly increased to 26.7% and 19.2%, respectively, becoming the dominant genera in this system. Genera Thiobacillus and Ferritrophicum have been considered two typical SOB involved in sulfur-driven denitrification [39,40]. However, when salinity increased to 4%, the relative abundance of both Thiobacillus and Ferritrophicum decreased to undetectable, while Sulfurimonas increased from 0% to 1.2%. Genus Sulfurimonas was also regarded as the SOB, indicating that Sulfurimonas is the primary functional microorganism under saline stress. As for the Anammox process, genus Brocadia was the sole AnAOB no matter the salinity in this study. The relative abundance of Brocadia varied between 0.1% and 0.7%. Despite low richness, genus Brocadia made a major contribution in the nitrogen removal process. In addition, genus Sulfurimonas might also contribute a small portion of nitrogen removal by converting NO3−-N directly to N2.
Venn analysis illustrated that only 11.1% of the OTUs were found as shared OTUs between the microbial communities at 0% and 4% salinities. This suggests that salinity could cause the deviation in microbial community structure in the SPDA system. According to the result of the significant difference test shown in Figure 7, in contrast with the Thiobacillus and Ferritrophicum discussed above, genera Saccharimonadales, SBR1031, Saprospiraceae, Acinomarinales, Truepera, and HOC36 were enriched as salinity increased. These several genera have been reported for their ability to degrade complex organic compounds [41,42,43]. Considering nonexternal organic carbon was added in the operation, the organic compounds might be derived from cell lysis and EPS under saline stress.

4. Conclusions

In this study, batch tests and lab-scale long-term tests were conducted to investigate the nitrogen conversion performance of the SPDA under different salinity conditions. The main conclusions were drawn as follows:
(1)
Batch tests suggested that nitrate reduction in sulfur-driven denitrification could be suppressed under 1–4% salinity to different levels, with nitrite accumulation occurring under 1–2% salinity.
(2)
Batch tests suggested that TN removal efficiency of Anammox decreased from 97.6% to 17.3% with salinity increasing from 0% to 4%.
(3)
Long-term tests revealed that the SPDA system could be rapidly established under low-salinity conditions. With the appropriate strategy of gradual salinity elevation, SPDA could adapt to a high-salinity environment and achieve comparable performance to that in a low-salinity environment.
(4)
Adapted secretion of proteins in EPS, responsive enrichment of genus Sulfurimonas, and unaffected genus Brocadia retention enhanced the robustness of the SPDA system under a high-salinity environment.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/w17152284/s1. Figure S1. TN removal performance of the control SPDA system under 0% salinity: (a) nitrogen concentration, (b) TN removal efficiency, and (c) Anammox contribution to TN removal.

Author Contributions

Conceptualization, X.L. and J.S.; methodology, X.L.; software, M.G.; validation, X.L., Z.C. and J.S.; formal analysis, J.L.; investigation, X.L.; resources, X.L.; data curation, H.F.; writing—original draft preparation, X.L.; writing—review and editing, X.L.; visualization, H.F.; supervision, J.S.; project administration, J.S.; funding acquisition, J.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by SINOPEC, grant number 321018.

Data Availability Statement

Data will be made available on request from the corresponding author.

Conflicts of Interest

Xiangchen Li, Jie Sun, Zonglun Cao, Junxi Lai, Haodi Feng and Minwen Guo were employed by SINOPEC (Beijing) Research Institute of Chemical Industry Co., Ltd. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest, and this study received funding from SINOPEC. The funder was not involved in the study design; collection, analysis, and interpretation of data; the writing of this article; or the decision to submit it for publication.

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Figure 1. Nitrogen conversion characteristics of sulfur-driven denitrification under different salinities: (a) nitrate concentration, (b) nitrite concentration, (c) nitrate removal efficiency, (d) nitrite accumulation rate, and (e) TN removal efficiency. *** represents p < 0.001.
Figure 1. Nitrogen conversion characteristics of sulfur-driven denitrification under different salinities: (a) nitrate concentration, (b) nitrite concentration, (c) nitrate removal efficiency, (d) nitrite accumulation rate, and (e) TN removal efficiency. *** represents p < 0.001.
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Figure 2. Nitrogen conversion characteristics of Anammox under different salinities: (a) TN concentration and (b) TN removal efficiency.
Figure 2. Nitrogen conversion characteristics of Anammox under different salinities: (a) TN concentration and (b) TN removal efficiency.
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Figure 3. TN removal performance of SPDA system under different salinities: (a) nitrogen concentration, (b) TN removal efficiency, and (c) Anammox contribution to TN removal.
Figure 3. TN removal performance of SPDA system under different salinities: (a) nitrogen concentration, (b) TN removal efficiency, and (c) Anammox contribution to TN removal.
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Figure 4. EPS characteristics in SPDA system during long-term operation: (a) EPS compositions and contents and (b) the ratio of protein to polysaccharides in EPS.
Figure 4. EPS characteristics in SPDA system during long-term operation: (a) EPS compositions and contents and (b) the ratio of protein to polysaccharides in EPS.
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Figure 5. Microbial variations in SPDA system along the start-up and operation: (a) microbial richness and (b) microbial diversity.
Figure 5. Microbial variations in SPDA system along the start-up and operation: (a) microbial richness and (b) microbial diversity.
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Figure 6. Microbial community structure at the phylum (a) and the genus (b) levels.
Figure 6. Microbial community structure at the phylum (a) and the genus (b) levels.
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Figure 7. Fisher’ exact test bar plot on genus level of samples at 0% and 4% salinity. *** represents p < 0.001.
Figure 7. Fisher’ exact test bar plot on genus level of samples at 0% and 4% salinity. *** represents p < 0.001.
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Table 1. Operating phases of the SPDA system.
Table 1. Operating phases of the SPDA system.
PhasesDaysSalinity
Phase 11–210% (<0.2 g/L)
Phase 222–401% (10 g/L)
Phase 341–612% (20 g/L)
Phase 462–854% (40 g/L)
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Li, X.; Sun, J.; Cao, Z.; Lai, J.; Feng, H.; Guo, M. Unraveling Nitrogen Removal and Microbial Response of Integrated Sulfur-Driven Partial Denitrification and Anammox Process in Saline Wastewater Treatment. Water 2025, 17, 2284. https://doi.org/10.3390/w17152284

AMA Style

Li X, Sun J, Cao Z, Lai J, Feng H, Guo M. Unraveling Nitrogen Removal and Microbial Response of Integrated Sulfur-Driven Partial Denitrification and Anammox Process in Saline Wastewater Treatment. Water. 2025; 17(15):2284. https://doi.org/10.3390/w17152284

Chicago/Turabian Style

Li, Xiangchen, Jie Sun, Zonglun Cao, Junxi Lai, Haodi Feng, and Minwen Guo. 2025. "Unraveling Nitrogen Removal and Microbial Response of Integrated Sulfur-Driven Partial Denitrification and Anammox Process in Saline Wastewater Treatment" Water 17, no. 15: 2284. https://doi.org/10.3390/w17152284

APA Style

Li, X., Sun, J., Cao, Z., Lai, J., Feng, H., & Guo, M. (2025). Unraveling Nitrogen Removal and Microbial Response of Integrated Sulfur-Driven Partial Denitrification and Anammox Process in Saline Wastewater Treatment. Water, 17(15), 2284. https://doi.org/10.3390/w17152284

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