Co-existence of anammox and denitrification for simultaneous nitrogen and carbon removal--Strategies and issues

Strategies for efficient enrichment of anaerobic ammonia oxidizing bacteria in activated sludge

Anaerobic ammonia oxidation (Anammox) is an economical and sustainable wastewater nitrogen removal technology, and its application in the mainstream process is the inevitable trend of the development of Anammox. However, how to effectively enriching Anammox bacteria from the activated sludge remains challenging and restricts its extensive applications. In this study, the rapid and efficient enrichment of Anammox bacteria was achieved by raising the reflux ratio and nitrogen loading rate (NLR) using conventional activated sludge as the inoculant. In the screening phase (days 1-90), the reflux ratio was increased to discharge partial floc sludge, resulting in the relative abundance of Candidatus Brocadiaceae increased from 0.04% to 22.54%, which effectively reduced the matrix and spatial competition between other microorganisms and Anammox bacteria. On day 90, the stoichiometric ratio of the Anammox process closely approached the theoretical value of 1:1.32:0.26, indicating that the Anammox reaction was the primary nitrogen removal process in the system. In the enrichment phase (days 91-238), the NLR increased from 0.43 to 1.20 kgN/(m3·d) and removal efficiency was 71.89%, resulting in the relative abundance of Candidatus Brocadiaceae increased to 61.27% on day 180. The reactor operated steadily from days 444 to 498, maintaining the nitrogen removal rate (NRR) of 3.00 kgN/(m3·d) and achieving successful sludge granulation with the particle size of 392.4 µm. In short, this study provided a simple and efficient approach for enriching Anammox bacteria from the activated sludge, supporting to start an Anammox process efficiently.

Anammox-based technologies: A review of recent advances, mechanism, and bottlenecks

The removal of nitrogen via the ANAMMOX process is a promising green wastewater treatment technology, with numerous benefits. The incessant studies on the ANAMMOX process over the years due to its long start-up and high operational cost has positively influenced its technological advancement, even though at a rather slow pace. At the moment, relatively new ANAMMOX technologies are being developed with the goal of treating low carbon wastewater at low temperatures, tackling nitrite and nitrate accumulation and methane utilization from digestates while also recovering resources (phosphorus) in a sustainable manner. This review compares and contrasts the handful of ANAMMOX -based processes developed thus far with plausible solutions for addressing their respective bottlenecks hindering full-scale implementation. Ultimately, future prospects for advancing understanding of mechanisms and engineering application of ANAMMOX process are posited. As a whole, technological advances in process design and patents have greatly contributed to better understanding of the ANAMMOX process, which has greatly aided in the optimization and industrialization of the ANAMMOX process. This review is intended to provide researchers with an overview of the present state of research and technological development of the ANAMMOX process, thus serving as a guide for realizing energy autarkic future practical applications.

Evaluating the role of high N2O affinity complete denitrifiers and non-denitrifying N2O reducing bacteria in reducing N2O emissions in river

Freshwater rivers are hotspots of N2O greenhouse gas emissions. Dissolved organic carbon (DOC) is the dominant electron donor for microbial N2O reduction, which can reduce N2O emission through enriching high N2O affinity denitrifiers or enriching non-denitrifying N2O-reducing bacteria (N2ORB), but the primary regulatory pathway remains unclear. Here, field study indicated that high DOC concentration in rivers enhanced denitrification rate but reduced N2O flux by improving nosZ gene abundance. Then, four N2O-fed membrane aeration biofilm reactors inoculated with river sediments from river channel, estuary, adjacent lake, and a mixture were continuously performed for 360 days, including low, high, and mixed DOC stages. During enrichment stages, the (nirS+nirK)/nosZ ratio showed no significant difference, but the community structure of denitrifiers and N2ORB changed significantly (p < 0.05). In addition, N2ORB strains isolated from different enrichment stages positioned in different branches of the phylogenetic tree. N2ORB strains isolated during high DOC stage showed significant higher maximum N2O-reducing capability (Vmax: 0.6 ± 0.4 ×10-4 pmol h-1 cell-1) and N2O affinity (a0: 7.8 ± 7.7 ×10-12 L cell-1 h-1) than strains isolated during low (Vmax: 0.1 ± 0.1 ×10-4 pmol h-1 cell-1, a0: 0.7 ± 0.4 ×10-12 L cell-1 h-1) and mixed DOC stages (Vmax: 0.1 ± 0.1 ×10-4 pmol h-1 cell-1, a0: 0.9 ± 0.9 ×10-12 L cell-1 h-1) (p < 0.05). Hence, under high DOC concentration conditions, the primary factor in reducing N2O emissions in rivers is the enrichment of complete denitrifiers with high N2O affinity, rather than non-denitrifying N2ORB.

One-stage anammox and thiocyanate-driven autotrophic denitrification for simultaneous removal of thiocyanate and nitrogen: Pathway and mechanism

The coupled process of anammox and reduced-sulfur driven autotrophic denitrification can simultaneously remove nitrogen and sulfur from wastewater, while minimizing energy consumption and sludge production. However, the research on the coupled process for removing naturally toxic thiocyanate (SCN-) is limited. This work successfully established and operated a one-stage coupled system by co-cultivating mature anammox and SCN--driven autotrophic denitrification sludge in a single reactor. In this one-stage coupled system, the average total nitrogen removal efficiency was 89.68±3.33 %, surpassing that of solo anammox (81.80±2.10 %) and SCN--driven autotrophic denitrification (85.20±1.54 %). Moreover, the average removal efficiency of SCN- reached 99.50±3.64 %, exceeding that of solo SCN--driven autotrophic denitrification (98.80±0.65 %). The results of the 15N stable isotope tracer labeling experiment revealed the respective reaction rates of anammox and denitrification as 106.38±10.37 μmol/L/h and 69.07±8.07 μmol/L/h. By analyzing metagenomic sequencing data, Thiobacillus_denitrificans was identified as the primary contributor to SCN- degradation in this coupled system. Furthermore, based on the comprehensive analysis of nitrogen and sulfur metabolic pathways, as well as the genes associated with SCN- degradation, it can be inferred that the cyanate (CNO) pathway was responsible for SCN- degradation. This work provided a deeper insight into coupling anammox with SCN--driven autotrophic denitrification in a one-stage coupled system, thereby contributing to the development of an effective approach for wastewater treatment involving both SCN- and nitrogen.

Isotope analysis of nitrogen removal pathways and N2O production potential in the SDAD-anammox system under different N/S ratios

This study explored the impact of varying nitrate to sulfide (N/S) ratios on nitrogen removal efficiency (NRE) in the sulfide-driven autotrophic denitrification and anammox (SDAD-anammox) system. Optimal nitrogen removal was observed at N/S ratios between 1.5 and 2.0. Isotope tracing results showed that the contribution of anammox to nitrogen removal was enhanced with increasing N/S ratios, reaching up to 37 % at the N/S ratio of 2.5. Additionally, complex nitrogen pathways were identified, including dissimilatory nitrate reduction to ammonium (DNRA). Furthermore, isotope tracing was innovatively applied to investigate N2O emissions, demonstrating that higher N/S ratios significantly reduced N2O emissions, with the lowest emissions at N/S ratio of 2.5. Gene expression analysis indicated that nitrogen and sulfide transformation genes decreased with increasing N/S ratios, while anammox-related genes first increased and then decreased, reflecting the system's microbial dynamics. These findings offer insights into nitrogen transformation pathways and N2O production mechanisms in the SDAD-anammox process.

Culturing partial-denitrification (PD) granules in continuous flow reactor with waste sludge as inoculum: performance, granular sludge characteristics and microbial community

Partial denitrification granular sludge (PDGS) can provide long-term stable nitrite for anaerobic ammonia oxidation (anammox). The cultivation of ordinary activated sludge from wastewater treatment plants into PDGS can further promote the application of PD in practical engineering. In this study, the feasibility of fast start-up of PDGS was explored by inoculating waste sludge in up-flow anaerobic sludge blanket (UASB) reactor with synergistic control of nitrogen load rate (NLR, 0.05-0.65 kg N/m3/d) and electron donor starvation (EDS) (240-168 mg L-1), and system performance, particle characteristics and microbial structure were studied. The results showed that PD-UASB started successfully within 48 days, the average nitrite accumulation rate (NTR) and nitrate removal ratio (NRR) reached 79.6% and 82.5% after successful initiation, accompanied by high abundance of PD bacteria (Thauera, Pseudomonas, unclassflied commamonadaceae and Limnobacter) (25.3%). The increase of PD activity, and the difference between nitrate reductase (NAR) and nitrite reductase (NIR) contributed to nitrite production. Besides, the sludge shifted from flocculated (≤0.5 mm, 95.37%) to granulated state (0.5-2 mm, 64.74%), which could be due to the increase of extracellular polymers (EPS) (especially T-EPS) and metabolism of specific microorganisms (Bacteroidota and Chloroflexi, 19.92%). Good sludge granulation promoted the settleability of PD (the SVI5 was 47.248 mL/ g. ss after successful start-up). In summary, good PD sludge granulation process could be achieved in a short time by synergistically controlling NLR and EDS.

Potential electron acceptors for ammonium oxidation in wastewater treatment system under anoxic condition: A review

Anaerobic ammonium oxidation has been considered as an environmental-friendly and energy-efficient biological nitrogen removal (BNR) technology. Recently, new reaction pathway for ammonium oxidation under anaerobic condition had been discovered. In addition to nitrite, iron trivalent, sulfate, manganese and electrons from electrode might be potential electron acceptors for ammonium oxidation, which can be coupled to traditional BNR process for wastewater treatment. In this paper, the pathway and mechanism for ammonium oxidation with various electron acceptors under anaerobic condition is studied comprehensively, and the research progress of potentially functional microbes is summarized. The potential application of various electron acceptors for ammonium oxidation in wastewater is addressed, and the N2O emission during nitrogen removal is also discussed, which was important greenhouse gas for global climate change. The problems remained unclear for ammonium oxidation by multi-electron acceptors and potential interactions are also discussed in this review.

Enhanced nitrogen removal by an isolated aerobic denitrifying strain in a vertical-flow constructed wetland

The addition of bacterial agents is an effective method for improving nitrogen removal from wetlands. Herein, an aerobic denitrifier, RC-15, was added to a vertical-flow constructed wetland (CW), and the presence of functional genes and microbial communities was investigated at different CW depths. For the RC-15-treated CW, the removal of NO3- and TN during the process was significantly greater than in the control. Quantitative PCR revealed that nirS is a dominant denitrifying gene for treating WWTP tailwater. Moreover, the presence of the RC-15 strain significantly enhanced the abundance of the napA gene and nirK gene in the CWs. The napA gene was concentrated in the upper layer of the CWs, and the nirK gene was concentrated in the middle and bottom layers. Compared to the control, the addition of the bacterial agent Trial resulted in a more diverse denitrification pathway, a greater abundance of 16Sr RNA, and a greater number of denitrifying strains. According to the microbial community analysis, Proteobacteria and Chloroflexi dominated denitrification in the CWs. Greater abundances of Thauera, Aeromonas and Ardenticatenales were found at the genus level, indicating that these genera have potential applications in future nitrogen removal projects.

Fe(II)-driven spatiotemporal assembly of heterotrophic and anammox bacteria enhances simultaneous nitrogen and phosphorus removal for low-strength municipal wastewater

The mainstream anaerobic ammonium oxidation (anammox) faces considerable challenges with low-strength municipal wastewater. A Fe(Ⅱ)-amended partial denitrification coupled anammox (PD/A) process was conducted and achieved a long-term and efficient nitrogen and phosphorus removal, yielding effluent total nitrogen and phosphorus concentrations of 1.97 ± 1.03 mg/L and 0.23 ± 0.13 mg/L, respectively, which could well meet more stringent effluent discharge standard of some wastewater treatment plants in specific geographical locations, e.g., estuaries. Fe(Ⅱ)-driven vivianite formation provided key nucleuses for the optimization of the spatial distribution of heterotrophic and anammox bacteria with enhanced extracellular polymeric substances as key driving forces. Metagenomics analysis further revealed the increase of key genes, enhancing anammox bacteria homeostasis, which also bolstered the resistance to environmental perturbations. This study provided a comprehensive sight into the function of Fe(Ⅱ) in mainstream PD/A process, and explored a promising alternative for synergetic nitrogen and phosphorus removal for low-strength municipal wastewater treatment.

Stable continuous flow CANDAN process transitioning from anammox UASB reactor by facilitating indigenous nitrite-producing denitrification community

The emerging nitrogen removal process known as CANDAN (Complete Ammonium and Nitrate removal via Denitratation-Anammox over Nitrite) has been developed in Sequencing Batch Reactors (SBRs). Yet, starting up and maintaining stability in continuous-flow reactors remain challenging. This study explores the feasibility of transitioning the CANDAN process from an anammox-dominated process by introducing appropriate external organics to facilitate indigenous nitrite-producing denitrification community in an Upflow Anaerobic Sludge Blanket (UASB) reactor. 150-day operation results indicate that under feeding rates of domestic wastewater at 0.54 L/h and nitrate-containing wastewater at 1.08 L/h, excellent N removal was achieved, with effluent TN below 10.0 mg N/L. Adding external sodium acetate at a COD/NO3--N = 2.0 triggered denitratation, ex-situ denitrification activity tests showed increased nitrite production rates, maintaining the nitrate-to-nitrite transformation ratio (NTR) above 90 %. Consequently, anammox activity was consistently maintained, dominating Total Nitrogen (TN) removal with a contribution as high as 78.3 ± 8.0 %. Anammox functional bacteria, Brocadia and Kuenenia were identified and showed no decrease throughout the operation, indicating the robustness of the anammox process. Notably, the troublesome of sludge flotation, did not occur, also contributing to sustained outstanding performance. In conclusion, this study advances our understanding of the synergistic interplay between anammox and denitrifying bacteria in the Anammox-UASB system, offering technical insights for establishing a stable continuous-flow CANDAN process for simultaneous ammonium and nitrate removal.

Relationship between denitrification and anammox rates and N2 production with substrate consumption and pH in a riparian zone

Denitrification and anaerobic ammonium oxidation (anammox) are the key processes to quantitatively remove nitrate (NO3-) and balance the nitrogen (N) budget of the ecosystem. In this paper, a slurry-based 15N tracer approach was used to study the correlation and quantitative relation of substrate consumption and pH with rates of denitrification and anammox in a riparian zone. The results showed that the fastest rates of 0.93 µg N h-1 and 0.32 µg N h-1 for denitrification (Denitrif-N2) and anammox (Denitrif-N2), respectively. N2 produced by denitrification occupied 74.04% and produced by anammox occupied 25.96% of the total N2, proving denitrification is the dominant process to remove NO3-. The substrate content (NO3-, NH4+ and TOC) and pH varied during incubation and were significantly correlated with Dentrif-N2 and Anammox-N2. Nitrate and TOC as the substrates of denitrification demonstrated a significant correlation with Anammox-N2, which was associated with the products of denitrification involved in the anammox process. This proved a coupling of denitrification and anammox. A quantitative relationship was observed between Dentrif-N2 and Anammox-N2 in the range of 2.75-2.90 when TOC, NH4+ and NO3- consumption per unit mass or pH changed per unit. Nitrogen mass balance analysis showed that 1 mg N substrate (NO3-+NH4+) consumption in the denitrification and anammox can produce 1.05 mg N2 with a good linear relationship (r2 = 0.9334). This could be related to other processes that produced extra N2 in denitrification and anammox system.

Mechanistic insight into the aggregation ability of anammox microorganisms: Roles of polarity, composition and molecular structure of extracellular polymeric substances

The chemical characteristics of extracellular polymeric substances (EPS) of anammox bacteria (AnAOB) play a crucial role in the rapid enrichment of AnAOB and the stable operation of wastewater anammox processes. To clarify the influential mechanisms of sludge EPS on AnAOB aggregation, multiple parameters, including the polarity distribution, composition, and molecular structure of EPS, were selected, and their quantitative relationship with AnAOB aggregation was analyzed. Compared to typical anaerobic sludge (anaerobic floc and granular sludge), the anammox sludge EPS exhibited higher levels of tryptophan-like substances (44.82-56.52 % vs. 2.57-39.81 %), polysaccharides (40.02-53.49 mg/g VSS vs. 30.22-41.69 mg/g VSS), and protein structural units including α-helices (20.70-23.98 % vs. 16.48-19.32 %), β-sheets (37.43-42.98 % vs. 25.78-36.72 %), and protonated nitrogen (Npr) (0.065-0.122 vs. 0.017-0.061). In contrast, it had lower contents of β-turns (20.95-27.39 % vs. 28.17-39.04 %). These biopolymers were found to originate from different genera of AnAOB. Specifically, the α-helix-rich proteins were mainly derived from Candidatus Kuenenia, whereas the extracellular proteins related to tryptophan and Npr were closely associated with Candidatus Brocadia. Critically, these EPS components could drive anammox aggregation through interactions. Substantial amounts of tryptophan-like substances facilitated the formation of β-sheet structures and the exposure of internal hydrophobic clusters, which benefited the anammox aggregation. Meanwhile, extracellular proteins with high Npr content played a pivotal role in the formation of mixed protein-polysaccharide gel networks with the electronegative regions of polysaccharides, which could be regarded as the key component in the maintenance of anammox sludge stability. These findings provide a comprehensive understanding of the multifaceted roles of EPS in driving anammox aggregation and offer valuable insights into the development of EPS regulation strategies aimed at optimizing the anammox process.

Rhizosphere-Associated Anammox Bacterial Diversity and Abundance of Nitrogen Cycle-Related Functional Genes of Emergent Macrophytes in Eutrophic Wetlands

Rhizospheric microbial community of emergent macrophytes plays an important role in nitrogen removal, especially in the eutrophic wetlands. The objective of this study was to identify the differences in anammox bacterial community composition among different emergent macrophytes and investigate revealed the the main factors affecting on the composition, diversity, and abundance of anammox bacterial community. Results showed that the composition, diversity, and abundance of the anammox community were significantly different between the vegetated sediments of three emergent macrophytes and unvegetated sediment. The composition of the anammox bacterial community was different in the vegetated sediments of different emergent macrophytes. Also, the abundance of nitrogen cycle-related functional genes in the vegetated sediments was found to be higher than that in the unvegetated sediment. Canonical correspondence analysis (CCA) and structural equation models analysis (SEM) showed that salinity and pH were the main environmental factors influencing the composition and diversity of the anammox bacterial community and NO2--N indirectly affected anammox bacterial community diversity by affecting TOC. nirK-type denitrifying bacteria abundance had significant effects on the bacterial community composition, diversity, and abundance of anammox bacteria. The community composition of anammox bacteria varies with emergent macrophyte species. The rhizosphere of emergent macrophytes provides a favorable environment and promotes the growth of nitrogen cycling-related microorganisms that likely accelerate nitrogen removal in eutrophic wetlands.

Carbon-Restricted Anoxic Zone as an Overlooked Anammox Hotspot in Municipal Wastewater Treatment Plants

The anoxic zone serves as the core functional unit in municipal wastewater treatment plants (MWWTPs). Unfortunately, in most cases, the downstream range of the anoxic zone is severely lacking in available organic carbon and thus contributes little to the removal of nutrients. This undesirable range is termed the "carbon-restricted anoxic zone", representing an insurmountable drawback for traditional MWWTPs. This study uncovers a previously overlooked role for the carbon-restricted anoxic zone: a hotspot for anaerobic ammonium oxidation (anammox). In a continuous-flow pilot-scale plant treating municipal wastewater (55 m3/d), virgin biocarriers were introduced into the carbon-restricted anoxic zone (downstream 25% of the anoxic zone with BOD5 of 5.9 ± 2.3 mg/L). During the 517-day monitoring, anammox bacteria highly self-enriched within the biofilms, with absolute and relative abundance reaching up to (9.4 ± 0.1) × 109 copies/g-VSS and 6.17% (Candidatus Brocadia), respectively. 15N isotopic tracing confirmed that anammox overwhelmingly dominated nitrogen metabolism, responsible for 92.5% of nitrogen removal. Following this upgrade, the contribution ratio of the carbon-restricted anoxic zone to total nitrogen removal increased from 9.2 ± 4.1% to 19.2 ± 4.2% (P < 0.001), while its N2O emission flux decreased by 84.5% (P < 0.001). These findings challenge stereotypes about the carbon-restricted anoxic zone and highlight the multiple environmental implications of this newfound anammox hotspot.

Effects of hydraulic retention time and influent nitrate concentration on solid-phase denitrification system using wheat husk as carbon source

Solid-phase denitrification shows promise for removing nitrate (NO₃^--N) from water. Biological denitrification uses external carbon sources to remove nitrogen from wastewater, among which agriculture waste is considered the most promising source due to its economic and efficiency advantages. Hydraulic retention time (HRT) and influent nitrate concentration (INC) are the main factors influencing biological denitrification. This study explored the effects of HRT and INC on solid-phase denitrification using wheat husk (WH) as a carbon source. A solid-phase denitrification system with WH carbon source was constructed to explore denitrification performance with differing HRT and INC. The optimal HRT and INC of the wheat husk-denitrification reactor (WH-DR) were 32 h and 50 mg/L, respectively. Under these conditions, NO₃^--N and total nitrogen removal rates were 97.37 ± 2.68% and 94.08 ± 4.01%, respectively. High-throughput sequencing revealed that the dominant phyla in the WH-DR operation were Proteobacteria, Bacteroidetes, and Campilobacterota. Among the dominant genera, Diaphorobacter (0.85%), Ideonella (0.38%), Thiobacillus (4.22%), and Sulfurifustis (0.60%) have denitrification functions; Spirochaeta (0.47%) is mainly involved in the degradation of WH; and Acidovorax (0.37%) and Azospira (0.86%) can both denitrify and degrade WH. This study determined the optimal HRT and INC for WH-DR and provides a reference for the development and application of WH as a novel, slow-release carbon source in treating aquaculture wastewater.

Enhanced nutrient removal in agro-industrial wastes-amended hybrid floating treatment wetlands treating real sewage: Laboratory microcosms to field-scale studies

The use of natural agro-industrial materials as suspended fillers (SFs) in floating treatment wetlands (FTWs) to enhance nutrient removal performance has recently been gaining significant attention. However, the knowledge concerning the nutrient removal performance enhancement by different SFs (alone and in mixtures) and the major removal pathways is so far inadequate. The current research, for the first time, carried out a critical analysis using five different natural agro-industrial materials (biochar, zeolite, alum sludge, woodchip, flexible solid packing) as SFs in various FTWs of 20 L microcosm tanks, 450 L outdoor mesocosms, and a field-scale urban pond treating real wastewater over 180 d. The findings demonstrated that the incorporation of SFs in FTWs enhanced the removal performance of total nitrogen (TN) by 20-57% and total phosphorus (TP) by 23-63%. SFs further enhanced macrophyte growth and biomass production, leading to considerable increases in nutrient standing stocks. Although all the hybrid FTWs showed acceptable treatment performances, FTWs set up with mixtures of all five SFs significantly enhanced biofilm formation and enriched the abundances of the microbial community related to nitrification and denitrification processes, supporting the detected excellent N retention. N mass balance assessment demonstrated that nitrification-denitrification was the major N removal pathway in reinforced FTWs, and the high removal efficiency of TP was attributable to the incorporation of SFs into the FTWs. Nutrient removal efficiencies ranked in the following order among the various trials: microcosm scale (TN: 99.3% and TP: 98.4%) > mesocosm scale (TN: 84.0% and TP: 95.0%) > field scale (TN: -15.0-73.7% and TP: -31.5-77.1%). These findings demonstrate that hybrid FTWs could be easily scaled up for the removal of pollutants from eutrophic freshwater systems over the medium term in an environmentally-friendly way in regions with similar environmental conditions. Moreover, it demonstrates hybrid FTW as a novel way of disposing of significant quantities of wastes, showing a win-win means with a huge potential for large-scale application.

From hybrid process to pure biofilm anammox process: Suspended sludge biomass management contributing to high-level anammox enrichment in biofilms

The application of mainstream anammox is highly desirable for municipal wastewater treatment. However, enrichment of anammox bacteria (AnAOB) is challenging, particularly given the vicious competition from denitrifying bacteria (DB). Here, suspended sludge biomass management, a novel operational strategy for hybrid process (suspended sludge/biofilm), was investigated for 570 days based on a modified anaerobic-anoxic-oxic system treating municipal wastewater. By successively decreasing the suspended sludge concentration, the traditional hybrid process was successfully upgraded to a pure biofilm anammox process. During this process, both the nitrogen removal efficiency (NRE) and rate (NRR) were significantly improved (P < 0.001), from 62.1 ± 4.5% to 79.2 ± 3.9% and from 48.7 ± 9.7 to 62.3 ± 9.0 g N/(m³·d), respectively. Mainstream anammox was improved in the following: Candidatus Brocadia was enriched from 0.70% to 5.99% in anoxic biofilms [from (9.94 ± 0.99) × 10⁸ to (1.16 ± 0.01) × 10¹⁰ copies/g VSS, P < 0.001]; the in situ anammox reaction rate increased from 8.8 ± 1.9 to 45.5 ± 3.2 g N/(m³·d) (P < 0.001); the anammox contribution to nitrogen removal rose from 9.2 ± 2.8% to 67.1 ± 8.3% (P < 0.001). Core bacterial microbiome analysis, functional gene quantification, and a series of ex situ batch experiments demonstrated that the stepwise decreases in suspended sludge concentration effectively mitigated the vicious competition of DB against AnAOB, enabling high-level AnAOB enrichment. This study presents a straightforward and effective strategy for enriching AnAOB in municipal wastewater, shedding fresh light on the application and upgradation of mainstream anammox.

Multi-parameter control-based operation strategy for mainstream deammonification in an integrated anaerobic biofilm reactor-step feed MBR

The mainstream deammonification of municipal wastewater has been recognized as one of the greatest challenges in wastewater engineering. The conventional activated sludge process has disadvantages of high energy input and sludge production. To tackle this situation, an innovative A-B process, where an anaerobic biofilm reactor (AnBR) was functioned as the A stage for energy recovery, and a step-feed membrane bioreactor (MBR) was functioned as the B stage for mainstream deammonification, was constructed for carbon-neutral wastewater treatment. For addressing the challenge associated with selective retention of ammonia-oxidizing bacteria (AOB) over nitrite oxidizing bacteria (NOB), a multi-parameter control-based operation strategy was developed with synergistic control of influent COD redistribution, dissolved oxygen (DO) concentration and sludge retention time (SRT) in the innovative AnBR - step-feed MBR system. Results showed that more than 85% of wastewater COD could be removed with the direct production of methane gas in the AnBR. A relatively stable partial nitritation, which is a prerequisite of anammox, was achieved with the successful suppression of NOB, leading to 98% of ammonium-N and 73% of total nitrogen removed. Anammox bacteria could well survive and enrich in the integrated system, and the contribution of anammox to the total nitrogen removal was more than 70% at optimal conditions. Reactions network involved in the nitrogen transformation in the integrated system was further constructed through the mass balance and microbial community structure analyses. Consequently, this study demonstrated a practically feasible process configuration with high operation and control flexibility towards stable mainstream deammonification of municipal wastewater.

Performance of anammox enchanced by pulsed electric fields under added organic carbon sources using integrated network and metagenomics analyses

This paper aims to investigate the function of a pulsed electric field (PEF) in the anaerobic ammonia oxidation (anammox) process after adding certain chemical oxygen demand (COD) through integrated network and metagenomics analyses. The findings showed that the presence of COD was detrimental to anammox, but PEF could significantly reduce the adverse effect. The total nitrogen removal in the reactor for applying PEF was 16.99% higher on average than the reactor for only dosing COD. Additionally, PEF upgraded the abundance of anammox bacteria subordinate to the phylum Planctomycetes by 9.64%. The analysis of molecular ecological networks promulgated that PEF resulted in an increase in network scale and topology complexity, thereby boosting the potential collaboration of the communities. Metagenomics analyses demonstrated that PEF dramatically promoted anammox central metabolism in the presence of COD, specifically enhancing pivotal N functional genes (hzs, hdh, amo, hao, nas, nor and nos).

Treatment of high-phosphorus load wastewater by column packed with non-burning compound filler/gravel/ceramsite: evaluation of performance and microorganism community

Cost-effective and environmental-friendly substrates are essential for the constructed wetlands (CWs). In this study, the column test was used to explore the differences in pollutant purification performance, microbial community structure and abundance between non-burning compound filler and conventional CWs substrates (i.e. gravel and ceramsite) at low temperature (0-15℃). It was found that the maximum phosphorus removal efficiency of compound filler (99%) was better than gravel (18%) and ceramsite (21%). Besides, the proportion of aerobic heterotrophic bacteria capable of ammonium oxidation, nitrification and denitrification (i.e. Pseudomonas, Acinetobacter, and Acetoanaerobium) was enhanced by compound filler, which has an excellent potential for nitrogen removal in the subsequent purification process. These results demonstrated that the self-made non-burning compound filler was a potential substrate for CWs, which was of great significance for the resource utilization of solid wastes such as polyaluminum chloride residue.

Energy-neutral municipal wastewater treatment based on partial denitrification-anammox driven by side-stream sulphide

"Low-carbon" has become an important evaluation index of modernisation construction. In the area of wastewater treatment has also caused considerable concern. Anaerobic ammonium oxidation (anammox) is a novel autotrophic nitrogen removal process that provides an opportunity for low-carbon remodelling of municipal wastewater treatment plants (MWTPs). The stable supply of nitrite is of great significance for the application of anammox. As a process with stable nitrite supply, partial denitrification (PD) is of great significance in the coupling nitrogen removal with anammox in municipal wastewater. Furthermore, innovation of the low-carbon nitrogen removal process can enable the recovery of abundant bioenergy resource from MWTPs. The low-carbon nitrogen removal via PD-anammox process and the bioenergy recovery for municipal wastewater in the previous studies has been summarised. On this basis, a novel energy-neutralisation municipal wastewater treatment process based on partial denitrification-anammox driven by sulphide produced in the side-stream has been proposed. The long-term retention of mainstream anammox and improvement of energy recovery efficiency under the requirement of ensuring nitrogen removal require additional detailed investigation.

Responses of anammox and sulfur/pyrite autotrophic denitrification in one-stage system to high nitrogen load: Performance, metabolic and bacterial community

To remove residual nitrate from anammox process and achieve efficient nitrogen removal, a two-stage system (TAS) with the two individual reactors and a one-stage system (OAS) with the spatial functional areas in one reactor were established via anammox coupling sulfur autotrophic denitrification. The total nitrogen removal efficiency (TNRE) of OAS system (97.85 ± 1.92%) was higher than that of TAS system (93.63 ± 1.87%) under the influent NH₄⁺-N and NO₂^--N of 227 and 300 mg/L. Meanwhile, the responses of microbial metabolism to high nitrogen load were investigated in term of microbial metabolites, electron transfer and metabolic activity. Microbial metabolites characteristics demonstrated that the OAS system secreted more EPS with lower protein (PN)/polysaccharide (PS) ratio than that in the TAS system, which was beneficial to protect bacteria from high nitrogen load. Electrochemical analysis suggested that the secretion of electron conductive substance (such as PN, PS) and redox active substances (such as flavin mononucleotide, the binding of flavins and cytochrome c on the outer membrane) were increased in the OAS system, which promoted the electron transfer efficiency. Moreover, the electron transport system activity (ETSA) values and ATP contents in OAS system were higher than that in the TAS system, which indicated that metabolic activity was improved in OAS system under the stimulation of high nitrogen load. Additionally, the bacterial community analysis indicated that the functional bacteria of Candidatus_Kuenenia and Armatimonadetes_gp5 had higher abundance in the OAS system than that in the TAS system, which was beneficial to realize the stable nitrogen removal performance. Overall, the responses mechanism of the OAS system was established to explain the resistant to high nitrogen load.

Resilience of anammox application from sidestream to mainstream: A combined system coupling denitrification, partial nitritation and partial denitrification with anammox

Anaerobic ammonium oxidation (anammox) is a potential process to achieve the neutralization of energy and carbon. Due to the low temperature and variation of municipal sewage, the application of mainstream anammox is hard to be implemented. For spreading mainstream anammox in practice, several key issues and bottlenecks including the start-up, stable NO₂^--N supply, maintenance and dominance of AnAOB with high activity, prevention of NO₃^--N buildup, reduction of sludge loss, adaption to the seasonal temperature and alleviation of COD impacts on AnAOB are discussed and summarized in this review in order to improve its startup, stable operation and resilience of mainstream anammox. Hence a combined biological nitrogen removal (CBNR) system based on conventional denitrification, shortcut nitrification-denitrification, Partial Nitritation and partial Denitrification combined Anammox (PANDA) process through the management of organic matter and nitrate is proposed correspondingly aiming at adaptation to the variations of seasonal temperature and pollutants in influent.

Nitrogen removal of thermal hydrolysis-anaerobic digestion liquid: A review

Thermal hydrolysis (TH) pretreatment, as an anaerobic digestion (AD) pretreatment, has not only been verified in the laboratory but also frequently employed in actual engineering. However, the properties of anaerobic digestion liquid (ADL), such as high organic matter concentration, high ammonia nitrogen (NH₄⁺-N) concentration, and low carbon-nitrogen ratio (C/N), have posed some difficulties in the follow-up treatment. To address the above issues, the autotrophic nitrogen removal (ANR) process is developed to treat ADL. Due to the NH₄⁺-N, organic materials, toxic and harmful substances in the ADL that might directly impact the activity of functional bacteria, the ADL should be treated before being fed into the ANR process. This paper provided a focused review of the thermal hydrolysis-anaerobic digestion process (TH-ADP) mechanism and the ANR mechanism, summarized the existing difficulties in the treatment of thermal hydrolysis-anaerobic digestion liquid (TH-ADL), assessed the research status thoroughly, and offered the potential solutions to the problems.

Fast start-up and stable operation of mainstream anammox without inoculation in an A2/O process treating low COD/N real municipal wastewater

It is of great significance to start up the anammox process in the most commonly used anaerobic-anoxic-oxic (A²/O) process in treating mainstream municipal wastewater. Recently, partial-denitrification/anammox (PD/A) has attracted increasing interest as a new avenue in mainstream. This study investigated the in situ start-up of PD/A process in a traditional A²/O process. The PD/A system was rapidly started up within 60 days by adding virgin carriers into the anoxic zone and then run stably for the next 90 days. The in situ anammox activity reached 1.0 ± 0.1 mg NH₄⁺-N/L/h contributing 37.9 ± 6.2% of total nitrogen removal. As a result, the nitrogen removal efficiency of the system increased by 16.9%. The anammox bacteria (AnAOB) on the anoxic biofilms were enriched with a doubling time of 14.53d, and the relative abundance reached 2.49% on day 150. Phylogenetic analysis showed the dominant AnAOB was related to Ca. Brocadia sp. 40, which was the only detected anammox genus in the anoxic biofilm from start-up to stable operation. Batch tests and qPCR results revealed that compared with the floc sludge, the anoxic biofilms exhibited NO₂^- accumulation driven by PD and performed a better coordination between denitrifiers and AnAOB. Overall, this study provides great confidence for the in situ fast start-up of mainstream anammox using conventional activated sludge.

Partial S(0)-driven autotrophic denitrification process facilitated the quick natural enrichment of anammox bacteria at room temperature

Anaerobic ammonium oxidation (anammox) is well-known to be an environmental and promising biotechnology. However, the natural enrichment of anammox bacteria is still a challenging topic. In this study, partial S(0)-driven autotrophic denitrification (PSAD) was developed to stably supply nitrite, and natural enrichment of anammox bacteria was rapidly realized in a single sequencing moving bed biofilm reactor at room temperature. With the initiation of PSAD, anammox bacteria spontaneously emerged within 12 days, and PSAD-anammox coupling system was realized successfully. And then, the influent concentration of ammonium continuously increased to the same concentration as the nitrate, and the mean total nitrogen removal efficiency reached 92.77 %, which was mainly contributed by anammox. Moreover, the coupling of PSAD and anammox reduced the risk of sulfate emissions. cDNA high throughput sequencing revealed that the relative abundance of Candidatus Brocadia and Thiobacillus reached 39.03 % and 13.48 % at the 88th day. Oligotyping analysis illustrated that GATTTAAT and GTCCCA were the dominant Ca. Brocadia and Thiobacillus oligotypes in PSAD-anammox coupling system, respectively. DNA-based stable isotope probing further deciphered that Thiobacillus was the actual performer of PSAD and supported the nitrite for anammox bacteria in PSAD-anammox coupling system. Overall, this work provided a new strategy to naturally enrich anammox bacteria.

The organics-mediated microbial dynamics and mixotrophic metabolisms in anammox consortia under micro-aerobic conditions

The engineering applications of mainstream anaerobic ammonium oxidation (anammox) have raised increasing attention due to its energy-efficient, however, the organics-mediated microbial dynamics and mixotrophic metabolisms in anammox consortia under micro-aerobic conditions are still elusive. Here, the response of the anammox process to sodium acetate and glucose at a C/N ratio ranging from 0 to 0.5 was investigated under micro-aerobic conditions, respectively. Results showed that the additional glucose could promote the nitrogen removal efficiency (NRE) and nitrogen removal rate (NRR) of anammox processes at a low C/N ratio (0.3), representing 84.00% and 0.53 N kg·m^-3·d^-1. The introduced organics could regulate the diversity of the microbial community and simplify the microbial relationship in anammox consortia. Anammox could not benefit from the introduced sodium acetate, while glucose could effectively enhance the anammox activity and microbial interactions in anammox consortia. Glucose might also stimulate the mixotrophic mechanism of Ca. Kuenenia, further promotes the proliferation of anammox sludge under micro-aerobic conditions. This study reveals that glucose could positively mediate microbial interactions and mixotrophic metabolism in anammox consortia under micro-aerobic conditions, which raises a new horizon for the proliferation of anammox sludge for mainstream engineering applications.

A mechanistic review on aerobic denitrification for nitrogen removal in water treatment

The traditional biological nitrogen removal technology consists of two steps: nitrification by autotrophs in aerobic circumstances and denitrification by heterotrophs in anaerobic situations; however, this technology requires a huge area and stringent environmental conditions. Researchers reached the conclusion that the denitrification process could also be carried out in aerobic circumstances with the discovery of aerobic denitrification. The aerobic denitrification process is carried out by aerobic denitrifying bacteria (ADB), most of which are heterotrophic bacteria that can metabolize various forms of nitrogen compounds under aerobic conditions and directly convert ammonia nitrogen to N₂ for discharge from the system. Despite the fact that there is no universal agreement on the mechanism of aerobic denitrification, this article reviewed four current explanations for the denitrification mechanism of ADB, including the microenvironment theory, theory of enzyme, electron transport bottlenecks theory, and omics study, and summarized the parameters affecting the denitrification efficiency of ADB in terms of carbon source, temperature, dissolved oxygen (DO), and pH. It also discussed the current status of the application of aerobic denitrification in practical processes. Following the review, the difficulties of present aerobic denitrification technology are outlined and future research options are highlighted. This review may help to improve the design of current wastewater treatment facilities by utilizing ADB for effective nitrogen removal and provide the engineers with relevant references.

A review on upgrading of the anammox-based nitrogen removal processes: Performance, stability, and control strategies

The anaerobic ammonia oxidation (anammox) process is a promising biological nitrogen removal technology. However, owing to the sensitivity and slow cell growth of anammox bacteria, long startup time and initially low nitrogen removal rate (NRR) are still limiting factors of practical applications of anammox process. Moreover, nitrogen removal efficiency (NRE) is often lower than 88 %. This review summarizes the most common methods for improving NRR by increasing microorganism concentration, and modifying reactor configuration. Recent integrated anammox-based systems were evaluated, including hydroxyapatite (HAP)-enhanced one-stage partial nitritation/anammox (PNA) process for a high NRR of over 2 kg N/m3/d at 25 °C, partial denitrification/anammox (PDA) process, and simultaneous partial nitrification, anammox, and denitrification process for a high NRE of up to 100 %. After discussing the challenges for the application of these systems critically, a combined system of anaerobic digestion, HAP-enhanced one-stage PNA and PDA is proposed in order to achieve a high NRR, high NRE, and phosphorus removal simultaneously.

Coupling sulfur-based denitrification with anammox for effective and stable nitrogen removal: A review

Anoxic ammonium oxidation (anammox) is an energy-efficient nitrogen removal process for wastewater treatment. However, the unstable nitrite supply and residual nitrate in the anammox process have limited its wide application. Recent studies have proven coupling of sulfur-based denitrification with anammox (SDA) can achieve an effective nitrogen removal, owing to stable provision of substrate nitrite from the sulfur-based denitrification, thus making its process control more efficient in comparison with that of partial nitrification and anammox process. Meanwhile, the anammox-produced nitrate can be eliminated through sulfur-based denitrification, thereby enhancing SDA's overall nitrogen removal efficiency. Nonetheless, this process is governed by a complex microbial system that involves both complicated sulfur and nitrogen metabolisms as well as multiple interactions among sulfur-oxidising bacteria and anammox bacteria. A comprehensive understanding of the principles of the SDA process is the key to facilitating the development and application of this novel process. Hence, this review is conducted to systematically summarise various findings on the SDA process, including its associated biochemistry, biokinetic reactions, reactor performance, and application. The dominant functional bacteria and microbial interactions in the SDA process are further discussed. Finally, the advantages, challenges, and future research perspectives of SDA are outlined. Overall, this work gives an in-depth insight into the coupling mechanism of SDA and its potential application in biological nitrogen removal.

Exploiting Biofilm Characteristics to Enhance Biological Nutrient Removal in Wastewater Treatment Plants

Biological treatments are integral processes in wastewater treatment plants (WWTPs). They can be carried out using sludge or biofilm processes. Although the sludge process is effective for biological wastewater systems, it has some drawbacks that make it undesirable. Hence, biofilm processes have gained popularity, since they address the drawbacks of sludge treatments, such as the high rates of sludge production. Although biofilms have been reported to be essential for wastewater, few studies have reviewed the different ways in which the biofilm properties can be explored, especially for the benefit of wastewater treatment. Thus, this review explores the properties of biofilms that can be exploited to enhance biological wastewater systems. In this review, it is revealed that various biofilm properties, such as the extracellular polymeric substances (EPS), quorum sensing (Qs), and acylated homoserine lactones (AHLs), can be enhanced as a sustainable and cost-effective strategy to enhance the biofilm. Moreover, the exploitation of other biofilm properties such as the SOS, which is only reported in the medical field, with no literature reporting it in the context of wastewater treatment, is also recommended to improve the biofilm technology for wastewater treatment processes. Additionally, this review further elaborates on ways that these properties can be exploited to advance biofilm wastewater treatment systems. A special emphasis is placed on exploiting these properties in simultaneous nitrification and denitrification and biological phosphorus removal processes, which have been reported to be the most sensitive processes in biological wastewater treatment.

Nitrate Addition Increases the Activity of Microbial Nitrogen Removal in Freshwater Sediment

Denitrification and anammox occur widely in aquatic ecosystems serving vital roles in nitrogen pollution removal. However, small waterbodies are sensitive to external influences; stormwater runoff carrying nutrients and oxygen, flows into waterbodies resulting in a disruption of geochemical and microbial processes. Nonetheless, little is known about how these short-term external inputs affect the microbial processes of nitrogen removal in small waterbodies. To investigate the effects of NO₃⁻, NH₄⁺, dissolved oxygen (DO) and organic C on microbial nitrogen removal in pond sediments, regulation experiments have been conducted using slurry incubation experiments and ¹⁵N tracer techniques in this study. It was demonstrated the addition of NO₃⁻ (50 to 800 μmol L⁻¹) significantly promoted denitrification rates, as expected by Michaelis-Menten kinetics. Ponds with higher NO₃⁻ concentrations in the overlying water responded more greatly to NO₃⁻ additions. Moreover, N₂O production was also promoted by such an addition of NO₃⁻. Denitrification was significantly inhibited by the elevation of DO concentration from 0 to 2 mg L⁻¹, after which no significant increase in inhibition was observed. Denitrification rates increased when organic C was introduced. Due to the abundant NH₄⁺ in pond sediments, the addition demonstrated little influence on nitrogen removal. Moreover, anammox rates showed no significant changes to any amendment.

The intrinsic relevance of nitrogen removal pathway to varying nitrate loading rate in a polycaprolactone-supported denitrification system

Up to date, the intrinsic association of nitrate loading rate (NLR) with treatment performance of solid-phase denitrification (SPD) systems is still ambiguous. To address this issue, three continuous up-flow bioreactors were configured. They were packed with polycaprolactone (PCL) under a filling ratio of 30%, 60% or 90% and were operated under a varying NLR of 0.34 ± 0.01-3.99 ± 0.12 gN/(L·d). Results showed that the denitrification efficiency was high (RE > 96%) and stable except the case with the highest NLR, which was mainly attributed to the lack of available carbon sources. At the phylum or genus level, most of the detected dominant bacterial taxa were either associated with organics degradation or nitrogen metabolism. The difference in bacterial community structure among the three stages was mainly caused by NLR rather than the filling ratio. Moreover, as the NLR got higher, the Bray-Curtis distance between samples from the same stage became shorter. By the results of gene or enzyme prediction performed in PICRUSt2, the main nitrogen metabolism pathways in these reactors were denitrification, dissimilatory nitrate reduction to ammonium (DNRA), assimilatory nitrate reduction to ammonium (ANRA) and nitrogen fixation. Moreover, aerobic and anaerobic nitrate dissimilation coexisted in the systems with the latter playing a dominant role. Finally, denitrification and DNRA occurred under both high and low NLR conditions while nitrogen fixation and ANRA preferred to occur under low NLR environments. These findings might help guide practical applications.

Full-scale transition from denitrification to partial denitrification-anammox (PdNA) in deep-bed filters: Operational strategies for and benefits of PdNA implementation

This study shows for the first time more than 2 years of operation of a mainstream anammox application at full-scale under temperate climate. This implementation of partial denitrification-anammox (PdNA) in deep bed filters at the HRSD York River treatment plant was demonstrated to achieve the benefits of shortcut nitrogen removal without nitrite oxidizing bacteria (NOB) out-selection. The transition from denitrification to PdNA filters required bleeding ammonium to the filters using an optimized ammonium versus NOx (AvN) control in the upstream aeration tanks and maintaining a nitrate residual in the filter effluent through feedforward/feedback control. The latter actions led to savings of 85% in methanol, 100% in alkalinity, and 35% in capacity enhancement. Up to 6 mg NH₄ ⁺ -N/L with an average of 2.2 ± 0.98 mg NH₄ ⁺ -N/L was removed through the anammox pathway, which accounted for about 15% of the overall plant nitrogen removal. Anammox enrichment was confirmed by activity testing and molecular analysis. The large excess of AnAOB capacity present in the filters (5-10 times more than normal operation) resulted in stable and reliable operation through winter conditions and showed potential for further intensification. PRACTITIONER POINTS: For the first time, long-term mainstream anammox was established full-scale through PdNA implementation in deep-bed filters. PdNA implementation required upstream aeration control optimization to provide a blend of ammonium and nitrate to the filters. Efficient anammox enrichment and retention resulted in reliable PdNA performance under different seasonal and influent conditions. PdNA implementation resulted in significant methanol and alkalinity savings and upstream capacity enhancement as ammonia removal depended less on aerobic nitrification. In the event of NOB out-selection and presence of nitrite, carbon savings in PdNA polishing filters can be enhanced via partial nitritation-anammox.

Applications of Sponge Iron and Effects of Organic Carbon Source on Sulfate-Reducing Ammonium Oxidation Process

The typical characteristics of wastewater produced from seafood, chemical, textile, and paper industries are that it contains ammonia, sulfate, and a certain amount of chemical oxygen demand (COD). The sulfate-reducing ammonium oxidation process is a biochemical reaction that allows both ammonia and sulfate removal, but its low growth rate and harsh reaction conditions limit its practical application. Due to the adsorption properties of the iron sponge and its robust structure, it provides a suitable living environment for microorganisms. To reduce the negative impact on the environment, we employed 4.8 kg of sponge iron in a 2.0 dm3 anaerobic sequencing batch reactor (ASBR). We investigated the effects of the type and concentration of carbon sources on the performance of the sulfate-reducing ammonium oxidation (SRAO) process. The results demonstrated that during a start-up period of 90 days, the average ammonium removal efficiency and the sulfate conversion efficiency of the reactor containing the sponge iron were 4.42% and 8.37% higher than those of the reactor without the sponge iron. The addition of the sponge iron shortens the start-up time of this greenhouse gas-free denitrification process and reduces future costs in practical applications. The removal of total nitrogen (TN) significantly increased after adding organic carbon sources and then declined sharply, while the most considerable reduction of ammonium removal efficiency from 98.4% to 30.5% was observed with adding phenol. The performance of the group employing glucose as the carbon source was recovered on the 28th day, with the average ammonium removal efficiency increasing from 49.03% to 83.5%. The results of this simulation study will help the rapid start-up of SRAO in the water treatment industry and can precisely guide the application of the SRAO process for wastewater containing different organic carbon sources.

Cross-effect of wetland substrates properties on anammox process in three single-substrate anammox constructed wetlands for treating high nitrogen sewage with low C/N

Constructing a stable and efficient anammox-driven constructed wetlands (CWs) system for efficiently treating high-nitrogen wastewater with low C/N remains a challenge, due to slow growth rate and high sensitivity of anammox bacteria to changing environmental conditions. Notably, sensitive anammox bacteria is still affected by the physicochemical properties of wetland substrates and their effects are still unknown. Therefore, three single-substrate (gravel, zeolite, and oyster shell) CWs were constructed with the goal of enhancing total nitrogen (TN) removal by anammox-driven/dominant process and determining the effect of substrate on anammox process. The gravel, zeolite and oyster shell systems achieved desired TN removal rates of 20.50, 14.25 and 22.15 g·(m²·d)^-1 when influent TN load was 32.57 g·(m²·d)^-1 without carbon source and costly aeration, respectively. Oyster shell system exhibited the highest removal ability and better capacity for resistance to influent nitrogen load, followed by gravel and zeolite systems (p < 0.05). Integrated analyses indicated anammox-driven/dominant process was the foremost reason accounted for the enhanced nitrogen treatment performance in all systems. The abundance of anammox gene was higher than the total abundance of denitrifying genes in the three CWs when influent TN load reached 14.85 g·(m²·d)^-1. Path analysis further demonstrated anammox process was the foremost nitrogen removal pathway. [anammox] had a highest positive direct contribution (97.3%) on TN transformation rate in gravel system; [anammox/(napA+narG+nirK+nirS+nosZ)] showed highest positive direct contribution (92.4% and 97.4%) on that in zeolite and oyster shell systems, respectively. Substrate configurations significantly affected nitrogen transformation pathway and microbial communities, particularly those of anammox bacteria. Anammox genera of Candidatus Brocadia (primary anammox genera) and Candidatus Kuenenia exhibited different evolutions among the three CWs. Machine learning of Least absolute shrinkage and selection operator (LASSO) analyses showed pH, Ca, Mg, EC, and K were the key physicochemical properties of wetland substrates affecting anammox gene and anammox genera. In conclusion, Oyster shell was the optimal substrate for anammox bacteria growth.

A Solution for Sustainable Utilization of Aquaculture Waste: A Comprehensive Review of Biofloc Technology and Aquamimicry

Biofloc technology (BFT) is gaining traction as a strategic aquaculture tool for boosting feed conversions, biosecurity, and wastewater recycling. The significant aspect of BFT is aquaculture with highest stocking density and minimal water exchange. It not only improves the water quality of a system by removing inorganic nitrogen from wastewater but also serves as a suitable feed supplement and probiotic source for cultured species. This technology is commonly used for shrimp and tilapia culture and can be used for both semi-intensive and intensive culture systems. Biofloc, when combined with formulated diets, forms a balanced food chain that improves growth performance. Nutrients in this system are continuously recycled and reused and form an efficient alternative system in aquaculture. In addition to the reduction in water exchange, it is also considered as a bio-security measure, since it prevents entry of disease from outside sources. Aquamimicry is an innovative concept that simulates natural estuarine conditions by developing copepods that act as supplementary nutrition especially for shrimp culture. The review highlights the process, significance, and development of BFT, its microbial interactions, nutritional value, transition from biofloc to copefloc, and concept of aquamimicry to sustainably improve aquaculture production.

Mainstream partial denitrification-anammox (PD/A) for municipal sewage treatment from moderate to low temperature: Reactor performance and bacterial structure

Anammox is sensitive to temperature, which can limit its practical application in wastewater treatment. In this study, a step-feed anoxic-oxic (A/O) process coupled with PD/A was operated steadily from 26.8 °C to 13.1 °C for wastewater treatment for 200 days. The effluent total inorganic nitrogen (TIN) and phosphorus concentrations were 10.2 mg/L and 0.29 mg/L at C/N ratio of 4.6 and 15.0 °C even with increasing nitrogen loading rate (NLR). The anammox activity was 5.60 mg NH₄⁺-N/gMLSS/d even at 14 °C, moreover, anammox abundance on the biocarriers increased with decreasing temperature. It was observed that the effect of partial denitrification (PD) was enhanced under low temperature, thus the contribution of anammox for nitrogen removal was improved. The pathway of anammox for nitrogen removal accounted for 48% and the effect of effluent did not deteriorate under low temperature. This study states that PD/A has advantages under low temperature operation, which is suitable for treatment of wastewater with low C/N ratio.

Nitrogen removal characteristics and potential application of the heterotrophic nitrifying-aerobic denitrifying bacteria Pseudomonas mendocina S16 and Enterobacter cloacae DS'5 isolated from aquaculture wastewater ponds

Two biosafety strains, identified as Pseudomonas mendocina S16 and Enterobacter cloacae DS'5, were isolated from freshwater aquaculture ponds and showed significant heterotrophic nitrification-aerobic denitrification abilities. Within 48 h, the inorganic nitrogen removal efficiencies in the two strains were 66.59 %-97.97 % (S16) and 72.27 %-96.44 % (DS'5). The optimal conditions for organic nitrogen removal of the two strains were temperature 20-35 °C and carbon/nitrogen (C/N) ratio 10-20 while using sodium citrate as the carbon source. Sequence amplification demonstrated the presence of the denitrification genes in both the two strains, and quantitative real-time PCR results showed that the coupled expression of nap + nar would improve the nitrate removal rate in S16. The nitrogen removal efficiencies of the two strains in immobilization culture systems were 79.80 %-98.58 % (S16) and 60.80 %-98.40 % (DS'5). This study indicated the great potential application of the two strains in aquaculture tail water treatment.

A novel simultaneous partial nitritation, denitratation and anammox (SPNDA) process in sequencing batch reactor for advanced nitrogen removal from ammonium and nitrate wastewater

This study presented a novel simultaneous partial nitritation (PN), denitratation and anammox (SPNDA) process for treating ammonium and nitrate wastewater. Results indicated that SPNDA could achieve a great total nitrogen (TN) removal of 97.6 ± 0.5%, leading to effluent TN concentration of only 3.4 mg/L. Mass balance indicated that nitrogen removal rates via anammox, simultaneous nitrification and denitrification were 96.7% and 3.3%, respectively. Extended aerobic duration (12 h) and low dissolved oxygen (DO) concentration (0.15 mg/L) could improve ammonia-oxidizing bacteria (AOB) activity and maintain PN stability. The stable suppression of nitrite-oxidizing bacteria activity was attributed to the low DO (0.15 mg/L) and high free ammonia (3.63 mg/L) in SPND. Besides, the nitrogen conversion mechanisms for SPNDA were revealed based on a typical operational cycle. Microbial analysis showed that AOB (Nitrosomonas) and partial denitrifying bacteria (Thauera and Denitratisoma) coexisted with anammox bacteria (Candidatus Brocadia and Candidatus Anammoxoglobus) in the mixotrophic bio-community.

Synergistic partial denitrification, anammox and in-situ fermentation (SPDAF) process for treating domestic and nitrate wastewater: Response of nitrogen removal performance to decreasing temperature

A synergistic partial denitrification, anaerobic ammonium oxidation (Anammox), and in-situ fermentation (SPDAF) system was established to solve problems of wastewater treatment plants (WWTPs) in combined treatment of domestic sewage, and nitrate wastewater discharged from industrial areas. The SPDAF system was started up at decreasing temperatures (26.8-18.9 ℃), and remained robust at abrupt temperature drop and drastic temperature fluctuations (20.7-14.1 ℃). The influent and effluent total inorganic nitrogen (TIN) were 97.0 ± 3.7 mg/L and 10.3 ± 4.0 mg/L, respectively. In-situ fermentation supplemented electron donors for NO₃^--N reduction. A high TIN removal efficiency, of 89.5 ± 3.9% was obtained. Specifically, Anammox contributed 90.9 ± 5.2% to TIN removal. Furthermore, the abundances of hydrolysis and acidogenesis bacteria were 14.02% and 29.47% in the low and high zones, respectively, which promoted fermentation and the use of complex organics. This study provided novel insights for actual operation of WWTPs.

Responses of simultaneous anammox and denitrification (SAD) process to nitrogen loading variation: Start-up, performance, sludge morphology and microbial community dynamics

The effects of loading variation on the efficiency, EPS, sludge morphology and microbial population of simultaneous anammox and denitrification (SAD) were thoroughly investigated with the low-abundance SAD sludge. Results indicated that the first stage lasted the longest (33d), and the average removal rate of TN can be maintained above 95%. The specific anammox activity (SAA), specific denitrification activity and PN/PS continued to increase, but the excessive loading caused the effluent to deteriorate rapidly, and SAA and PN/PS also decreased slightly, but it could be recovered quickly. The contribution rate of anammox and denitrification to N removal reached 87.6% and 12.4% eventually, respectively. The abundance of AnAOB was 10.68%-18.01%, 9.01%-15.54%, 5.74%-12.88% in the upper, middle and lower layers, respectively. Candidatus Kuenenia was always the dominant AnAOB, especially after high loading inhibition. The abundance of denitrifying bacteria (mainly Bacillus, Comamonas and Denitratisoma) gradually became the highest.

Submerged plants alleviated the impacts of increased ammonium pollution on anammox bacteria and nirS denitrifiers in the rhizosphere

Excess nitrogen input into water bodies can cause eutrophication and affect the community structure and abundance of the nitrogen-transforming microorganisms; thus, it is essential to remove nitrogen from eutrophic water bodies. Aquatic plants can facilitate the growth of rhizosphere microorganisms. This study investigated the impact of ammonium pollution on the anammox and denitrifying bacteria in the rhizosphere of a cultivated submerged macrophyte, Potamogeton crispus (P. crispus) by adding three different concentrations of slow-release urea (0, 400, 600 mg per kg sediment) to the sediment to simulate different levels of nitrogen pollution in the lake. Results showed that the ammonium concentrations in the interstitial water under three pollution treatments were significantly different, but the nitrate concentration remained stable. The abundance of anammox 16S rRNA and nitrite reductase (nirS) gene in rhizosphere sediments exhibited no significant differences under the three pollution conditions. The increase in the nitrogen pollution levels did not significantly affect the growth of anammox bacteria and nirS denitrifying bacteria (denitrifiers). The change trend of the abundance ratio of (anammox 16S rRNA)/nirS in different nitrogen treatment groups on the same sampling date was very close, indicating that this ratio was not affected by ammonium pollution levels when P. crispus existed. The redundancy analysis showed that there was a positive correlation between the abundance of anammox 16S rRNA and nirS gene and that the abundance of these bacteria was significantly affected by the mole ratio of NH₄⁺/NO₃^-. This study reveals that submerged plants weaken the environmental changes caused by ammonia pollution in the rhizosphere, thereby avoiding strong fluctuation of anammox bacteria and nirS denitrifiers.

Treatment of wastewater with high carbon-to-nitrogen ratio using a waterfall aeration biofilm reactor combined with sequencing batch reactor: Microbial community structure and metabolism analysis

A low-cost and high-efficiency waterfall aeration biofilm reactor (WABR) combined with a sequencing batch reactor (SBR) was established to treat wastewater with a C/N ratio of 50. Three WABR-SBR systems with different fillers were used. In the stable operation phase, the removal efficiency of chemical oxygen demand was R1 (approximately 99%), R2 (97-99%), and R3 (96-99%); the effluent concentration of NH₄⁺-N was 0.5 mg/L without nitrite or nitrate accumulation. High-throughput 16S rRNA sequencing revealed that the dominant phyla in the microbial community structure were Proteobacteria, Bacteroidetes, and Planctomycetes. Quantitative PCR was used to quantify the nitrification and denitrification gene expressions (Nitrobacter, nirS, and nirK) to evaluate the simultaneous nitrification and denitrification processes. Both anammox and denitrifying bacteria were abundant. Metagenomic annotation of genes that revealed the metabolic pathways of carbohydrates, amino acids, and the two dominant enzymes (GH and GT) provide valuable information for microbial ecology analysis.

Organic carbon bioavailability: Is it a good driver to choose the best biological nitrogen removal process?

Organic carbon can affect the biological nitrogen removal process since the Anammox, heterotrophic and denitrifying bacteria have different affinities and feedback in relation to carbon/nitrogen ratio. Therefore, we reviewed the wastewater carbon concentration, its biodegradability and bioavailability to choose the appropriate nitrogen removal process between conventional (nitrification-denitrification) and Anammox-based process (i.e. integrated with the partial nitritation, nitritation, simultaneous partial nitrification and denitrification or partial-denitrification). This review will cover: (i) strategies to choose the best nitrogen removal route according to the wastewater characteristics in relation to the organic matter bioavailability and biodegradability; (ii) strategies to efficiently remove nitrogen and the remaining carbon from effluent in anammox-based process and its operating cost; (iii) an economic analysis to determine the operational costs of two-units Anammox-based process when compared with the commonly applied one-unit Anammox system (partial-nitritation-Anammox). On this review, a list of alternatives are summarized and explained for different nitrogen and biodegradable organic carbon concentrations, which are the main factors to determine the best treatment process, based on operational and economic terms. In summary, it depends on the wastewater carbon biodegradability, which implies in the wastewater treatment cost. Thus, to apply the conventional nitrification/denitrification process a COD_b/N ratio higher than 3.5 is required to achieve full nitrogen removal efficiency. For an economic point of view, according to the analysis the minimum COD_b/gN for successful nitrogen removal by nitrification/denitrification is 5.8 g. If ratios lower than 3.5 are applied, for successfully higher nitrogen removal rates and the economic feasibility of the treatment, Anammox-based routes can be applied to the wastewater treatment plant.

Recent advances in partial denitrification-anaerobic ammonium oxidation process for mainstream municipal wastewater treatment

To solve the bottleneck of the unstable accumulation of nitrite in the partial nitrification (PN)-anammox (AMX) in municipal wastewater treatment, a novel process called partial denitrification (PD)-AMX has been developed. PD-AMX, which is known for cost-efficiency and environmental friendliness, has currently exhibited a promising potential for the removal of biological nitrogen from municipal wastewater and has attracted much research interest regarding its process mechanisms, as well as its practical applications. Here, we review the recent advances in the PD process and its coupling to the anammox process, including the development, basic principles, main characteristics, and critical process parameters of the stable operation of the PD-AMX process. We also explore the microbial community and its characteristics in the system and summarize the knowledge of the dominant bacteria to clarify the key factors affecting PD-AMX. Then, we introduce the engineering feasibility and economic feasibility as well as the potential challenges of the process. The induction and implementation of partial denitrification and maintenance of mainstream anammox are critical issues to be urgently solved. Meanwhile, the implementation of a full mainstream anammox application remains burdensome, while the mechanism of partial denitrification coupled to anammox needs to be further studied. Additionally, stable operation performance and process control¹ methods need to be optimized or developed for the PD-AMX system for better engineering practice. This review can help to accelerate the research and application of the PD-AMX process for municipal wastewater treatment.

Fatigue of anammox consortia under long-term 1,4-dioxane exposure and recovery potential: N-kinetics and microbial dynamics

Long-term exposure of anammox process to 1,4-dioxane was investigated using periodic anammox baffled reactor (PABR) under different 1,4-dioxane concentrations. The results generally indicated that PABR (composed of 4 compartments) has robust resistance to 10 mg-dioxane/L. The 1^st compartment acted as a shield to protect subsequent compartments from 1,4-dioxane toxicity through secretion of high extracellular polymeric substance (EPS) of 152.9 mg/gVSS at 10 mg-dioxane/L. However, increasing 1,4-dioxane to 50 mg/L significantly inhibited anammox bacteria; e.g., ~ 93% of total nitrogen removal was lost within 14 days. The inhibition of anammox process at this dosage was most likely due to bacterial cell lysis, resulting in the decrease of EPS secretion and specific anammox activity (SAA) to 105.9 mg/gVSS and 0.04 mg N/gVSS/h, respectively, in the 1^st compartment. However, anammox bacteria were successfully self-recovered within 41 days after the cease of 1,4-dioxane exposure. The identification of microbial compositions further emphasized the negative impacts of 1,4-dioxane on abundance of C. Brocadia among samples. Furthermore, the development of genus Planococcus in the 1^st compartment, where removal of 1,4-dioxane was consistently observed, highlights its potential role as anoxic 1,4-dioxane degrader. Overall, long-term exposure to 1,4-dioxane should be controlled not exceeding 10 mg/L for a successful application.

Advances in Biological Nitrogen Removal of Landfill Leachate

With the development of economy and the improvement of people’s living standard, landfill leachate has been increasing year by year with the increase in municipal solid waste output. How to treat landfill leachate with high efficiency and low consumption has become a major problem, because of its high ammonia nitrogen and organic matter content, low carbon to nitrogen ratio and difficult degradation. In order to provide reference for future engineering application of landfill leachate treatment, this paper mainly reviews the biological treatment methods of landfill leachate, which focuses on the comparison of nitrogen removal processes combined with microorganisms, the biological nitrogen removal methods combined with ecology and the technology of direct application of microorganisms. In addition, the mechanism of biological nitrogen removal of landfill leachate and the factors affecting the microbial activity during the nitrogen removal process are also described. It is concluded that the treatment processes combined with microorganisms have higher nitrogen removal efficiency compared with the direct application of microorganisms. For example, the nitrogen removal efficiency of the combined process based on anaerobic ammonium oxidation (ANAMMOX) technology can reach more than 99%. Therefore, the treatment processes combined with microorganisms in the future engineering application of nitrogen removal in landfill leachate should be paid more attention to, and the efficiency of nitrogen removal should be improved from the aspects of microorganisms by considering factors affecting its activity.