Deciphering the concurrence of comammox, partial denitrification and anammox in a single low-oxygen mainstream nitrogen removal reactor

Role of comammox bacteria in granular bioreactor for nitrogen removal via partial nitritation/anammox

In this study, two bioprocess models were first constructed with the newly-discovered comammox process described as one-step and two-step nitrification and evaluated against relevant experimental data. The validated models were then applied to reveal the potential effect of comammox bacteria on the granular bioreactor particularly suitable for undertaking partial nitritation/anammox (PN/A) under different operating conditions of bulk dissolved oxygen (DO) and influent NH4+. The results showed although comammox bacteria-based PN/A could achieve > 80.0 % total nitrogen (TN) removal over a relatively wider range of bulk DO and influent NH4+ (i.e., 0.25-0.40 g-O2/m3 and 470-870 g-N/m3, respectively) without significant nitrous oxide (N2O) production (< 0.1 %), the bulk DO should be finely controlled based on the influent NH4+ to avoid the undesired full nitrification by comammox bacteria. Comparatively, conventional ammonium-oxidizing bacteria (AOB)-based PN/A not only required higher bulk DO to achieve > 80.0 % TN removal but also suffered from 1.7 %∼2.8 % N2O production.

A critical review of comammox and synergistic nitrogen removal coupling anammox: Mechanisms and regulatory strategies

Nitrification is highly crucial for both anammox systems and the global nitrogen cycle. The discovery of complete ammonia oxidation (comammox) challenges the inherent concept of nitrification as a two-step process. Its wide distribution, adaptability to low substrate environments, low sludge production, and low greenhouse gas emissions may make it a promising new nitrogen removal treatment process. Meanwhile, anammox technology is considered the most suitable process for future wastewater treatment. The diverse metabolic capabilities and similar ecological niches of comammox bacteria and anammox bacteria are expected to achieve synergistic nitrogen removal within a single system. However, previous studies have overlooked the existence of comammox, and it is necessary to re-evaluate the conclusions drawn. This paper outlined the ecophysiological characteristics of comammox bacteria and summarized the environmental factors affecting their growth. Furthermore, it focused on the enrichment, regulatory strategies, and nitrogen removal mechanisms of comammox and anammox, with a comparative analysis of hydroxylamine, a particular intermediate product. Overall, this is the first critical overview of the conclusions drawn from the last few years of research on comammox-anammox, highlighting possible next steps for research.

Future directions in microbial nitrogen cycling in wastewater treatment

Discoveries in the past decade of novel reactions, processes, and micro-organisms have altered our understanding of microbial nitrogen cycling in wastewater treatment systems. These advancements pave the way for a transition toward more sustainable and energy-efficient wastewater treatment systems that also minimize greenhouse gas emissions. This review highlights these innovative directions in microbial nitrogen cycling within the context of wastewater treatment. Processes such as comammox, Feammox, electro-anammox, and nitrous oxide mitigation offer innovative approaches for sustainable, energy-efficient nitrogen removal. However, while these emerging processes show promise, advancing from laboratory research to practical applications, particularly in decentralized systems, remains a critical next step toward a sustainable and efficient wastewater management.

Comammox Nitrospira cooperate with anammox bacteria in a partial nitritation-anammox membrane bioreactor treating low-strength ammonium wastewater at high loadings

Research has revealed that comammox Nitrospira and anammox bacteria engage in dynamic interactions in partial nitritation-anammox reactors, where they compete for ammonium and nitrite or comammox Nitrospria supply nitrite to anammox bacteria. However, two gaps in the literature are present: the know-how to manipulate the interactions to foster a stable and symbiotic relationship and the assessment of how effective this partnership is for treating low-strength ammonium wastewater at high hydraulic loads. In this study, we employed a membrane bioreactor designed to treat synthetic ammonium wastewater at a concentration of 60 mg N/L, reaching a peak loading of 0.36 g N/L/day by gradually reducing the hydraulic retention time to 4 hr. Throughout the experiment, the reactor achieved an approximately 80 % nitrogen removal rate through strategically adjusting intermittent aeration at every stage. Notably, the genera Ca. Kuenena, Nitrosomonas, and Nitrospira collectively constituted approximately 40 % of the microbial community. Under superior intermittent aeration conditions, the expression of comammox amoA was consistently higher than that of Nitrospira nxrB and AOB amoA in the biofilm, despite the higher abundance of Nitrosomonas than comammox Nitrospira, implying that the biofilm environment is favorable for fostering cooperation between comammox and anammox bacteria. We then assessed the in situ activity of comammox Nitrospira in the reactor by selectively suppressing Nitrosomonas using 1-octyne, thereby confirming that comammox Nitrospira played the primary role in facilitating the nitritation (33.1 % of input ammonium) rather than complete nitrification (7.3 % of input ammonium). Kinetic analysis revealed a specific ammonia-oxidizing rate 5.3 times higher than the nitrite-oxidizing rate in the genus Nitrospira, underscoring their critical role in supplying nitrite. These findings provide novel insights into the cooperative interplay between comammox Nitrospira and anammox bacteria, potentially reshaping the management of nitrogen cycling in engineered environments, and aiding the development of microbial ecology-driven wastewater treatment technologies.

Effects of carbon to nitrogen ratio on nitrogen removal in a single-stage microaerobic system: A model-based evaluation

Single-stage microaerobic systems have been proven to be effective for concurrent removal of ammonium and organic carbon from sewage. While mechanistic models derived from activated sludge models (ASMs) have simulated nutrients removal under microaerobic conditions, classic ASMs exhibit limitations in capturing the intricate effects of carbon to nitrogen (C/N) ratio on nitrogen removal performance. To address this issue, a mechanistic model modified from the classic ASMs was proposed to capture the combined inhibitory effects of carbon and ammonium on microaerobic systems. This modified model was established based on experimental data from a single-stage microaerobic reactor encompassing simultaneous nitrification-denitrification and anammox processes. The inhibition coefficient of C/N ratio was integrated into the process rate equations, and its effectiveness was validated through model performance evaluation. Compared to the classic models, the modified one achieved superior predictions for nitrite and nitrate nitrogen concentrations. Simulations revealed that under optimized conditions with a C/N of 4.57 and a dissolved oxygen (DO) of 0.41 mg/L, the system could achieve up to 95.5% of total nitrogen (TN) removal efficiency. Based on the simulation of substrate uptake/production rate, increasing the nitrogen loading rate (NLR) rather than organic loading rate (OLR) was crucial for efficient nitrogen removal. The proposed modified model served as a valuable tool for designing and optimizing similar biological wastewater treatment systems.

Emerging biotechnological applications of anaerobic ammonium oxidation

Anaerobic ammonium oxidation (anammox) is an energy-efficient method for nitrogen removal that opens the possibility for energy-neutral wastewater treatment. Research on anammox over the past decade has primarily focused on its implementation in domestic wastewater treatment. However, emerging studies are now expanding its use to novel biotechnological applications and wastewater treatment processes. This review highlights recent advances in the anammox field that aim to overcome conventional bottlenecks, and explores novel and niche-specific applications of the anammox process. Despite the promising results and potential of these advances, challenges persist for their real-world implementation. This underscores the need for a transition from laboratory achievements to practical, scalable solutions for wastewater treatment which mark the next crucial phase in the evolution of anammox research.

Competitive enrichment of comammox Nitrospira in floccular sludge

The discovery of complete ammonium oxidation (comammox) has subverted the traditional perception of two-step nitrification, which plays a key role in achieving biological nitrogen removal from wastewater. Floccular sludge-based treatment technologies are being applied at the majority of wastewater treatment plants in service where detection of various abundances and activities of comammox bacteria have been reported. However, limited efforts have been made to enrich and subsequently characterize comammox bacteria in floccular sludge. To this end, a lab-scale sequencing batch reactor (SBR) in the step-feeding mode was applied in this work to enrich comammox bacteria through controlling appropriate operational conditions (dissolved oxygen of 0.5 ± 0.1 g-O2/m3, influent ammonium of 40 g-N/m3 and uncontrolled longer sludge retention time). After 215-d operation, comammox bacteria gradually gained competitive advantages over counterparts in the SBR with a stable nitrification efficiency of 92.2 ± 2.2 %: the relative abundance of Nitrospira reached 42.9 ± 1.3 %, which was 13 times higher than that of Nitrosomonas, and the amoA gene level of comammox bacteria increased to 7.7 ± 2.1 × 106 copies/g-biomass, nearly 50 times higher than that of conventional ammonium-oxidizing bacteria. The enrichment of comammox bacteria, especially Clade A Candidatus Nitrospira nitrosa, in the floccular sludge led to (i) apparent affinity constants for ammonium and oxygen of 3.296 ± 0.989 g-N/m3 and 0.110 ± 0.004 g-O2/m3, respectively, and (ii) significantly low N2O and NO production, with emission factors being 0.136 ± 0.026 % and 0.023 ± 0.013 %, respectively.

Rapid anaerobic culture and reaction kinetic study of anammox bacteria on microfluidic chip

Anammox bacteria are being increasingly investigated as part of an emerging nitrogen removal technology. However, due to the difficulty in culturing, current understanding of their behavior is limited. In this study, anaerobic microfluidic chips were used to study anammox bacteria, showing great advantages over reactors. On-chip fluorescence in situ hybridization (FISH) showed the relative abundance of free form anammox bacteria increased by 56.1 % after one week's culture, an increase that is three times higher than that of bioreactor (17.1 %). For granular form cultures, the nitrogen removal load reached 2.34 ∼ 2.51 kg-N/(m3·d), which was also substantially higher than the bioreactor (∼1.22 kg-N/(m3·d)). Furthermore, studying the kinetics of nitrite inhibition of granular sludge with different particle sizes (100-900 μm) showed that the maximum ammonia load and the nitrite semi-saturation coefficient noticeably decreased for smaller particle sizes. These results illustrate the usefulness of the microfluidic method for in-depth understanding anammox process and its implementation.

Efficient nitrogen removal from municipal wastewater by an autotrophic-heterotrophic coupled anammox system: The up-regulation of key functional genes

The metabolic pathways based on key functional genes were innovatively revealed in the autotrophic-heterotrophic coupled anammox system for real municipal wastewater treatment. The nitrogen removal performance of the system was stabilized at 88.40 ± 3.39 % during the treatment of real municipal wastewater. The relative abundances of the nitrification functional genes ammonia oxidase (amoA/B/C), hydroxylamine oxidoreductase (hao), and nitrite oxidoreductases (nxrA/B) were increased by 1.2-2.4 times, and these three nitrification functional genes were mostly contributed by Nitrospira that dominated the efficient nitrification of the system. The relative abundance of anammox bacteria Candidatus Brocadia augmented from 0.35 % to 0.75 %, accompanied with the increased expression of hydrazine synthase (hzs) and hydrazine dehydrogenase (hdh), resulting in the major role of anammox (81.24 %) for nitrogen removal. The expression enhancement of the functional genes nitrite reductase (narG/H, napA/B) that promoted partial denitrification (PD) of the system weakened the adverse effects of the sharp decline in the population of PD microbe Thauera (from 5.7 % to 2.2 %). The metabolic module analysis indicated that the carbon metabolism pathways of the system mainly included CO2 fixation and organic carbon metabolism, and the stable enrichment of autotrophic bacteria ensured stable CO2 fixation. Furthermore, the enhanced expression of the glucokinases (glk, GCK, HK, ppgk) and the abundant pyruvate kinase (PK) achieved stable hydrolysis ability of organic carbon metabolism function of the system. This study offers research basics to practical application of the mainstream anammox process.

Enhanced denitrification of the AO-MBBR system used for expressway service area sewage treatment: A new perspective on decentralized wastewater treatment

Decentralized wastewater treatment warrants considerable development in numerous countries and regions. Owing to the unique characteristics of high ammonia nitrogen concentrations and low carbon/nitrogen ratio, nitrogen removal is a key challenge in treating expressway service area sewage. In this study, an anoxic/oxic-moving bed biofilm reactor (A/O-MBBR) and a traditional A/O bioreactor were continuously operated for 115 days and their outcomes were compared to investigate the enhancement effect of carriers on the total nitrogen removal (TN) for expressway service area sewage. Results revealed that A/O-MBBR required lower dissolved oxygen, exhibited higher tolerance toward harsh conditions, and demonstrated better shock load resistance than traditional A/O bioreactor. The TN removal load of A/O-MBBR reached 181.5 g‧N/(m3‧d), which was 15.24% higher than that of the A/O bioreactor. Furthermore, under load shock resistance, the TN removal load of A/O-MBBR still reached 327.0 g‧N/(m3‧d), with a TN removal efficiency of above 80%. Moreover, kinetics demonstrated that the denitrification rate of the A/O-MBBR was 121.9% higher than that of the A/O bioreactor, with the anoxic tank biofilm contributing 60.9% of the total denitrification rate. Community analysis results revealed that the genera OLB8, uncultured_f_Saprospiraceae and OLB12 were the dominant in biofilm loaded on carriers, and OLB8 was the key for enhanced denitrification. FAPROTAX and PICRUSt2 analyses confirmed that more bacteria associated with nitrogen metabolism were enriched by the A/O-MBBR carriers through full denitrification metabolic pathway and dissimilatory nitrate reduction pathway. This study offers a perspective into the development of cost-effective and high-efficiency treatment solutions for expressway service area sewage.

Metabolic insights into the interaction between nitrogen removal and 4-chlorophenol reduction of anammox consortia

The response characteristic and performance stabilization of anammox process under the stress of the potential organic pollutants support the application of ammonia-nitrogen wastewater treatment. In the present study, nitrogen removal performance was significantly suppressed with the addition of 4-chlorophenol. The activity of anammox process was inhibited by 14.23% (0.1 mg/L), 20.54% (1 mg/L) and 78.15% (10 mg/L), respectively. Metagenomic analysis revealed a significant decrease in the abundance of KEGG pathways associated with carbohydrate and amino acid metabolism with increasing 4-chlorophenol concentration. Metabolic pathway profiles suggest that putrescine is down-regulated at high 4-chlorophenol stress due to inhibition of nitrogen metabolism processes, while it is up-regulated to reduce oxidative damage. In addition, the presence of 4-chlorophenol induced an enhancement of EPS and bacterial debris decomposition, and a partial conversion of 4-chlorophenol to p-nitrophenol. This study unravels the mechanism of effect on anammox consortia in response to 4-CP, which could provide supplementary to facilitate its full-scale application.

Global variations and controlling factors of anammox rates

Soil anammox is an environmentally friendly way to eliminate reactive nitrogen (N) without generating nitrous oxide. Nevertheless, the current earth system models have not incorporated the anammox due to the lack of parameters in anammox rates on a global scale, limiting the accurate projection for N cycling. A global synthesis with 1212 observations from 89 peer-reviewed papers showed that the average anammox rate was 1.60 ± 0.17 nmol N g-1  h-1 in terrestrial ecosystems, with significant variations across different ecosystems. Wetlands exhibited the highest rate (2.17 ± 0.31 nmol N g-1  h-1 ), followed by croplands at 1.02 ± 0.09 nmol N g-1  h-1 . The lowest anammox rates were observed in forests and grasslands. The anammox rates were positively correlated with the mean annual temperature, mean annual precipitation, soil moisture, organic carbon (C), total N, as well as nitrite and ammonium concentrations, but negatively with the soil C:N ratio. Structural equation models revealed that the geographical variations in anammox rates were primarily influenced by the N contents (such as nitrite and ammonium) and abundance of anammox bacteria, which collectively accounted for 42% of the observed variance. Furthermore, the abundance of anammox bacteria was well simulated by the mean annual precipitation, soil moisture, and ammonium concentrations, and 51% variance of the anammox bacteria was accounted for. The key controlling factors for soil anammox rates differed from ecosystem type, for example, organic C, total N, and ammonium contents in croplands, versus soil C:N ratio and nitrite concentrations in wetlands. The controlling factors in soil anammox rate identified by this study are useful to construct an accurate anammox module for N cycling in earth system models.

Elucidating prioritized factor for mainstream partial nitritation between C/N ratio and dissolved oxygen: Response surface methodology and microbial community shifts

Recently, C/N ratio is suggested as a promising control factor with dissolved oxygen (DO) achieving mainstream partial nitritation (PN); however, their combined effects on mainstream PN are still limited. This study evaluated the mainstream PN with respect to the combined factors, and investigated the prioritized factor affecting the community of aerobic functional microbes competing with NOB. Response surface methodology was performed to assess the combined effects of C/N ratio and DO on the activity of functional microbes. Aerobic heterotrophic bacteria (AHB) played the greatest role in oxygen competition among functional microbes, which resulted in relative inhibition of nitrite-oxidizing bacteria (NOB). The combination of high C/N ratio and low DO had a positive role in the relative inhibition of NOB. In bioreactor operation, the PN was successfully achieved at ≥ 1.5 of C/N ratio for 0.5-2.0 mg/L DO conditions. Interestingly, aerobic functional microbes outcompeting NOB were shifted with C/N ratio rather than DO, suggesting C/N ratio is more prioritized factor achieving mainstream PN. These findings will provide insights into how combined aerobic conditions contribute to achieve mainstream PN.

Co-Occurrence and Cooperation between Comammox and Anammox Bacteria in a Full-Scale Attached Growth Municipal Wastewater Treatment Process

Cooperation between comammox and anammox bacteria for nitrogen removal has been recently reported in laboratory-scale systems, including synthetic community constructs; however, there are no reports of full-scale municipal wastewater treatment systems with such cooperation. Here, we report intrinsic and extant kinetics as well as genome-resolved community characterization of a full-scale integrated fixed film activated sludge (IFAS) system where comammox and anammox bacteria co-occur and appear to drive nitrogen loss. Intrinsic batch kinetic assays indicated that majority of the aerobic ammonia oxidation was driven by comammox bacteria (1.75 ± 0.08 mg-N/g TS-h) in the attached growth phase, with minimal contribution by ammonia-oxidizing bacteria. Interestingly, a portion of total inorganic nitrogen (∼8%) was consistently lost during these aerobic assays. Aerobic nitrite oxidation assays eliminated the possibility of denitrification as a cause of nitrogen loss, while anaerobic ammonia oxidation assays resulted in rates consistent with anammox stoichiometry. Full-scale experiments at different dissolved oxygen (DO = 2 - 6 mg/L) setpoints indicated persistent nitrogen loss that was partly sensitive to DO concentrations. Genome-resolved metagenomics confirmed the high abundance (relative abundance 6.53 ± 0.34%) of two Brocadia-like anammox populations, while comammox bacteria within the Ca. Nitrospira nitrosa cluster were lower in abundance (0.37 ± 0.03%) and Nitrosomonas-like ammonia oxidizers were even lower (0.12 ± 0.02%). Collectively, our study reports for the first time the co-occurrence and cooperation of comammox and anammox bacteria in a full-scale municipal wastewater treatment system.

Use of iron-loaded biochar to alleviate anammox performance inhibition under PFOA stress conditions: Integrated analysis of sludge characteristics and metagenomics

The negative effects of perfluorooctanoic acid (PFOA) on biological nitrogen removal performance in wastewater treatment plants, are receiving increasing attention due to the widespread reporting of this issue. In this study, pomelo peel iron-loaded biochar (Fe-PBC) was added to an anammox bioreactor to alleviate the negative effects of PFOA. Results showed that the addition of Fe-PBC increased the ammonia and nitrite removal efficiencies from 77.7 ± 9.6 % and 79.5 ± 5.6 % to 94.45 ± 5.1 % and 95.9 ± 5.0 %, respectively. In addition, Fe-PBC promoted the removal of PFOA from wastewater, increasing the PFOA removal efficiency from 5.2 % to 29.2 ± 4.3 % from 100 to 200 days. The introduction of iron-loaded biochar into the anammox bioreactor increased the CO ratio by 13.64 % by 150 days. In addition, a CO fitting peak was detected in the Fe-PBC, indicating that the Fe-PBC was loaded with microorganisms. Microbial community analysis showed a decrease in the relative abundances of Proteobacteria and Nitrospirae from 31 % and 3.4 % to 16.8 % and 0.9 %, respectively, while the relative abundance of Planctomycetes increased from 26.8 % to 44.1 %. Metagenomic analysis found that the functional genes hzsB and hdh increased from 98,666 ± 11,400 and 3190 ± 460 to 119,333 ± 15,534 and 138,650 ± 11,233 copy numbers/MLSS. The increase in anammox biomass may be attributed to the presence of iron, an essential element for the synthesis of key anammox enzyme. Furthermore, iron was also associated with the enhanced extracellular electron transfer in the anammox system induced by Fe-PBC.

Tailored Bimetallic Ni-Sn Catalyst for Electrochemical Ammonia Oxidation to Dinitrogen with High Selectivity

Direct electrochemical ammonia oxidation reaction (eAOR) is an efficient and sustainable strategy to process wastewater containing ammonia, and it endures overoxidation and severely competitive oxygen evolution reaction (OER). Herein, we synthesized a Ni(OH)₂/SnO₂ composite catalyst by a multistep strategy and applied it to the eAOR process. Ni(OH)₂/SnO₂ exhibited a N₂-N Faradaic efficiency (FE_N2-N) of 84.2%, with a N₂ partial current density (j_N2-N) of 2.7 mA cm^-2 at 1.55 V vs reversible hydrogen electrode (RHE) in 0.5 M K₂SO₄ with 10 mM NH₃-N (pH 11). The oxophilic Sn promoted NH₃ absorption on Ni sites while suppressing the OER. As the active species, NiOOH accelerated the dimerization of intermediates (*NH₂ or *NH) to form N₂.

Effect of free nitrous acid on nitritation process: Microbial community, inhibitory kinetics, and functional biomarker

This work comprehensively deciphered the effect of free nitrous acid (FNA) on the microbial community, inhibitory kinetics, and nitrifiers in nitritation process. Nitritation was first successfully achieved through selective inhibition of free ammonia (FA) on nitrite oxidizers (NOB). Then, batch tests clearly showed that FNA significantly inhibits the ammonia oxidation rate (r_su) and the growth rate (μ) of ammonia oxidizers (AOB), which was well described by the Hellinga model (K_I = 0.222 mg·L^-1). The structural equation model indicated that FNA was significantly and negatively associated with r_su, μ, Nitrosomonas, Commamons, Nitrospira, and Nitrotoga and positively correlated with Paracoccus. Furthermore, Nitrosomonas significantly drove the ammonia utilization and growth of AOB and was identified as the most important functional biomarker indicating the nitritation in response to FNA levels using random forest model. This study provides helpful information on the kinetics of the mechanism underlying the FNA inhibition on nitrification.

Insights into deterioration and reactivation of a mainstream anammox biofilm reactor response to C/N ratio

In-depth knowledge of the deterioration and reactivation of the anaerobic ammonium oxidation (anammox) induced by carbon-to-nitrogen (C/N) is still lacking. Herein, the anammox performance was investigated in an anaerobic sequence biofilm batch reactor fed with low-strength partial nitration effluent in the range of C/N ratio from 0.5 to 3. The anammox was hardly deteriorated at C/N lower than 1.5, while became worsen if C/N was above 2.0. The specific anammox activity (SAA) experiments showed an 85% decrease of SAA at C/N of 3.0 compared with the maximum value (C/N:0). However, anammox capacity was rapidly recovered once influent C/N was adjusted back to zero. Moreover, C/N also highly affected the composition, structure and function of extracellular polymeric substance of the anammox biofilm. High-throughput sequencing revealed a close correlation between C/N change and microbial structure shift. Finally, the potential inhibition and restoration mechanism of the C/N-dependent anammox were proposed based on metagenomic analysis. This research provides some insights into the reinstatement of a mainstream anammox biofilm process after it is interrupted by high C/N influent.