Effective nitrogen removal in a granule-based partial-denitrification/anammox reactor treating low C/N sewage

Rapid start-up of mainstream partial denitrification /anammox and enhanced nitrogen removal through inoculation of precultured biofilm for treating low-strength municipal sewage

This study investigated the rapid start-up of mainstream partial denitrification coupled with anammox (PD/A) and nitrogen removal performance by inoculating precultured PD/A biofilm. The results showed mainstream PD/A in the anaerobic-anoxic-aerobic (A2O) process was rapidly established within 30 days. Nitrogen removal efficiency (NRE) improved by 23.8 % contrasted to the traditional A2O process. The mass balance showed that anammox contribution to total nitrogen (TN) removal were maintained at 37.9 %∼55.7 %, and reducing hydraulic retention time (HRT) strengthened simultaneously denitrification and anammox activity. The microbial community showed that the dominant bacteria such as denitrifying bacteria (DNBs) and glycogen accumulating organisms (GAOs) both in biofilm and flocculent sludge (floc), integrating with anammox bacteria (AnAOB) in biofilm might lead to enhanced nitrogen removal. Overall, this study offered a fast start-up strategy of mainstream PD/A with enhanced nitrogen removal, which are valuable for upgradation and renovation of existed municipal wastewater treatment plants (WWTPs).

Strategy to mitigate substrate inhibition in wastewater treatment systems

Global urbanization requires more stable and sustainable wastewater treatment to reduce the burden on the water environment. To address the problem of substrate inhibition of microorganisms during wastewater treatment, which leads to unstable wastewater discharge, this study proposes an approach to enhance the tolerance of bacterial community by artificially setting up a non-lethal high substrate environment. And the feasibility of this approach was explored by taking the inhibition of anammox process by nitrite as an example. It was shown that the non-lethal high substrate environment could enhance the nitrite tolerance of anammox bacterial community, as the specific anammox activity increasing up to 24.71 times at high nitrite concentrations. Moreover, the system composed of anammox bacterial community with high nitrite tolerance also showed greater resistance (two-fold) in response to nitrite shock. The antifragility of the system was enhanced without affecting the operation of the main reactor, and the non-lethal high nitrite environment changed the dominant anammox genera to Candidatus Jettenia. This approach to enhance tolerance of bacterial community in a non-lethal high substrate environment not only allows the anammox system to operate stably, but also promises to be a potential strategy for achieving stable biological wastewater treatment processes to comply with standards.

Metagenomic unveils the promotion of mainstream PD-anammox process at lower nZVI concentration and inhibition at higher dosage

The partial-denitrification-anammox (PdNA) process exhibits great potential in enabling the simultaneous removal of NO3--N and NH4+-N. This study delved into the impact of exogenous nano zero-valent iron (nZVI) on the PdNA process. Adding 10 mg L-1 of nZVI increased nitrogen removal efficiency up to 83.12 % and maintained higher relative abundances of certain beneficial bacteria. The maximum relative abundance of Candidatus Brocadia (1.6 %), Candidatus Kuenenia (1.5 %), Ignavibacterium (1.3 %), and Azospira (1.2 %) was observed at 10 mg L-1 of nZVI. However, the greatest relative abundance of Thauera (1.3 %) was recorded under 50 mg L-1. Moreover, applying nZVI selectively enhanced the abundance of NO3--N reductase genes. So, keeping the nZVI concentration at 10 mg L-1 or below is advisable to ensure a stable PdNA process in mainstream conditions. Considering nitrogen removal efficiency, using nZVI in the PD-anammox process could be more cost-effective in enhancing its adoption in industrial and mainstream settings.

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.

Efficient nitrogen removal by multi-stage A/O mud membrane composite process with segmented influent: Performance and microbial community structure

In this paper, a multi-stage A/O mud membrane composite process with segmented influent was constructed for the first time and compared with the traditional activated sludge process and the multi-stage A/O pure membrane process with segmented influent. The nitrogen removal efficiency of the process under different influencing factors was studied. Under the optimum conditions, the highest removal rate of ammonia nitrogen can reach 99%, and the average removal rate of total nitrogen was 80%. The removal rate of COD in effluent reached 93%. The relative abundance of Proteobacteria was the highest in the multi-stage A/O mud membrane composite reactor with segmented influent. The community diversity and richness of activated sludge and biofilm in aerobic pool were the highest. Dechloromonas, Flavobacterium and Rhodobacter were dominant bacteria, and they were aerobic denitrifying bacteria that significantly contributed to the removal rate of ammonia nitrogen.

Distinct microbial characteristics of the robust single-stage coupling system during the conversion from anammox-denitritation to anammox-denitratation patterns

Simultaneous anammox-denitrification is effectively operated in two types, i.e., the anammox-denitritation (SAD pattern) and the anammox-denitratation (PDA pattern). The nitrate derived from inevitable nitrite oxidization likely determines the practical operational pattern of the coupling system, while little information is available regarding the microbial characteristics during the pattern conversion. Here, the single-stage bioreactor coupling anammox with denitrification was operated under conditions with a changed ratio of influent nitrite and nitrate. Results showed that the bioreactor exhibited a robust performance during the conversion from SAD to PDA patterns, corresponding with the total nitrogen removal efficiency ranging from 89.5% to 92.4%. Distinct community structures were observed in two patterns, while functional bacteria including the genera Denitratisoma, Thauera, Candidatus Brocadia, and Ca. Jettenia steadily co-existed. Meanwhile, the high transcription of hydrazine synthase genes demonstrated a stable anammox process, while the up-regulated transcription of nitrite and nitrous oxide reductase genes indicated that the complete denitrification process was enhanced for total nitrogen removal during the PDA pattern. Ecologically, stochastic processes dominantly governed the community assembly in two patterns. The PDA pattern improved the interconnectivity of communities, especially for the cooperative behaviors between dominant denitrifying bacteria and low-abundant species. These findings deepen our understanding of the microbial mechanism underlying the different patterns of the coupling system and potentially expand its engineering application.

Advanced nitrogen removal from municipal wastewater through partial nitrification-denitrification coupled with anammox in step-feed continuous system

Combined partial nitrification-denitrification/anammox (PN-PD/A) processes have attracted great attention from researchers in recent years to achieve high nitrogen removal from low carbon /nitrogen (C/N) municipal wastewater. In this context, a step-feed anoxic/oxic (A/O) process was conducted in this study through the combination of the partial nitrification-anammox (PN/A) and partial denitrification-anammox (PD/A) to remove N from municipal wastewater with low C/N. The enhancement of the PN-PD/A process resulted in N removal efficiency of 85.6% at C/N of 2.8. The contributions of the anammox reached 36.4 and 8.8% in the anoxic and oxic chambers, respectively. The biocarriers added to the anoxic and oxic chambers increased the relative abundance of the anammox bacteria in biofilms from 0.61% to 1.51% and 1.02%, respectively. This study demonstrated that employing the step-feed A/O process can create optimal conditions for the anammox bacteria growth, thereby ensuring advanced N removal from low C/N municipal wastewater.

Stimulating extracellular polymeric substances production in integrated fixed-film activated sludge reactor for advanced nitrogen removal from mature landfill leachate via one-stage double anammox

Introducing carbon sources to achieve nitrogen removal from mature landfill leachate not only increases the costs and carbon emissions but also inhibits the activity of autotrophic bacteria. Thus, this study constructed a double anammox system that combines partial nitrification-anammox (PNA) and endogenous partial denitrification-anammox (EPDA) within an integrated fixed-film activated sludge (IFAS) reactor. In this system, PNA primarily contributes to nitrogen removal pathways, achieving a nitrite accumulation rate of 98.23%. The production of extracellular polymer substances (EPS) in the IFAS reactor is stimulated by introducing co-fermentation liquid. Through the utilization of EPS, the system effectively achieves EPDA with the nitrite transformation rate of 97.20%. Under the intermittent aeration operation strategy, EPDA combined with PNA and anammox in the oxic and anoxic stages enhanced the nitrogen removal efficiency of the system to 99.70 ± 0.12%. The functional genus Candidatus kuenenia became enriched in biofilm sludge, while Thauera and Nitrosomonas predominated in floc sludge.

A comprehensive literature mining and analysis of nitrous oxide emissions from different innovative mainstream anammox-based biological nitrogen removal processes

The biological nitrogen removal (BNR) process in wastewater treatment plants generates a substantial volume of nitrous oxide (N2O), which possesses a potent greenhouse gas effect. A limited number of studies have systematically investigated the N2O emissions of anammox-based systems with different BNR processes under mainstream conditions. Based on extensive big data statistical analysis, it had been revealed that simultaneous nitritation, anammox and denitrification (SNAD), partial nitritation anammox (PNA) and partial denitrification anammox (PDA), exhibit significantly lower N2O emission factors when compared to traditional BNR processes. The median values for N2O emission factors were determined to be 1.01 %, 1.15 % and 1.43 % for SNAD, PNA and PDA, respectively. Based on nitrogen removal data and N2O emission factors, the N2O emissions from PNA, SNAD and PDA processes were calculated to be 0.016 g·d-1, 0.037 g·d-1 and 0.008 g·d-1, respectively. Furthermore, the machine learning models (SVM and ANN) exhibited excellent predictive performance for N2O emissions in the BNR processes. However, after removing environmental factors, the R2 value of the SVM model sharply decreased. The SHAP feature analysis demonstrated the significant impact of environmental factors on the accuracy of predictive performance in machine learning models. Spearman correlation analysis was employed to investigate the relationship between N2O emissions and operational factors as well as microbial communities. The results demonstrated a negative correlation between HRT, temperature and C/N with N2O emissions. Moreover, strong associations were observed between Nitrosomonas, Nitrospira, Denitratisoma, Thauera species and N2O emissions. The contribution of N2O production via AOB pathways played a key role that was quantitatively calculated to be 93 %, 80 % and 48 % in the PNA, SNAD and PDA processes, respectively. These findings highlight the potential of these innovative BNR processes in mitigating N2O emissions.

Granulation of partial denitrification sludge: Advances in mechanism understanding, technologies development and perspectives

The high-rate and stably efficient nitrite generation is vital and still challenges the wide application of partial denitrification (PD) and anammox technology. Increasing attention has been drawn to the granulation of PD biomass. However, the knowledge of PD granular sludge is still limited in terms of granules characterization and mechanisms of biomass aggregation for high nitrite accumulation. This work reviewed the performance and granulation of PD biomass for high nitrite accumulation via nitrate reduction, including the system start-up, influential factors, granular characteristics, hypothetical mechanism, challenges and perspectives in future application. The physiochemical characterization and key influential factors were summarized in view of nitrite production, morphology analysis, extracellular polymer substance structure, as well as microbial mechanisms. The PD granules exhibit potential advantages of a high biomass density, good settleability, high hydraulic loading rates, and strong shock resistance. A novel granular sludge-based PD combined with anammox process was proposed to enhance the capability of nitrogen removal. In the future, PD granules utilizing different electron donors is a promising way to broaden the application of anammox technology in both municipal and industrial wastewater treatment.

Nitrite accumulation in activated sludge through cyclic anaerobic exposure with acetate

Achieving nitrite accumulation still remains challenging for efficient short-cut biological nitrogen removal in municipal wastewater treatment. To tackle the problem of insufficient carbon in incoming wastewater for biological nutrient removal, a return activated sludge (RAS) fermentation method has been proposed and demonstrated to enable producing supplemental volatile fatty acids (VFAs) and enhance biological phosphorus removal via sludge cycling between mainstream and a sidestream anaerobic reactor. However, the impacts of long anaerobic exposure with acetate on nitrifying bacteria, known as the aerobic chemoautotrophic microorganisms, remains unexplored. In this study, the activated sludge underwent a cyclic anaerobic treatment with the addition of acetate (Ac), the effects on nitrification rate, abundance and microdiversity of nitrifying communities were comprehensively assessed. Firstly, batch activity tests proved the direct addition of high acetate (above 1000 mg/L) could cause inhibition on the nitrification rate, moreover, the inhibitory effect was stronger on nitrite-oxidizing bacteria (NOB) activity than that of ammonia-oxidizing bacteria (AOB). Then, a sequencing batch reactor (SBR) was applied to test the nitrogen conversion performance for low-strength ammonium wastewater. Nitrite accumulation could be achieved via the cyclic anaerobic exposure with 1000-5000 mg Ac/L. The maximum effluent concentration of nitrite was 40.8 ± 3.5 mg N/L with nitrite accumulation ratio (NAR) of 67.6 ± 3.5%. The decrease in NOB activity (72.7%) was greater than AOB of 42.4%, promoting nitrite accumulation via nitritation process. Furthermore, the cyclic anaerobic exposure with acetate can largely reshape the nitrifying communities. As the dominant AOB and NOB, the abundance of Nitrosomonas and Nitrospira were both decreased with species-level microdiversity in the nitrifying communities. However, the heterotrophic microorganism, Thauera, were found to be highly enriched (from 0 to 17.3%), which may act as the potential nitrite producer as proved by the increased nitrate reduction gene abundance. This study can provide new insights into achieving mainstream nitrite accumulation by involving sidestream RAS fermentation towards efficient wastewater treatment management.

Multi-omics analysis revealed the selective enrichment of partial denitrifying bacteria for the stable coupling of partial-denitrification and anammox process under the influence of low strength magnetic field

The microbial consortium involving anaerobic ammonium oxidation (anammox) and partial denitrification (PD), known as PD-anammox, is an emerging energy-efficient and lower carbon nitrogen removal process from wastewater. However, maintaining a stable PD process by locking nitrate reduction until nitrite was challenging. This study established the first stable connection of anammox with constant nitrite generation by PD bacteria under a low-strength (1.3 mT) magnetic field (MF). When the nitrogen loading rate was 1.81 kg-N/m3/d, the nitrogen removal efficiency of the control reactor (R1) was 75%, lower than that of the experimental reactor (R2), which was 85%. The expression of Thauera and Zoogloea, potential PD bacteria was substantially lower in R1 (5.75% and 1.21%, respectively) than in R2 (10.25 and 6.61%, respectively), according to a meta-transcriptomic analysis. At the same time, the mRNA expression of anammox genera Candidatus Brocadia and Candidatus Kuenenia was 33.53% and 3.83% in R1 and 22.86% and 1.87% in R2. Moreover, carbon and nitrogen metabolism pathways were more abundant under the influence of low-strength MF. The selective enrichment of PD bacteria can be attributed to the increased expression of carbon metabolic pathways like the citrate cycle, glycolysis/gluconeogenesis, and pyruvate metabolism. Interestingly, the control reactor was dominated by a hydroxylamine-dependent anammox process while a low-strength MF-enhanced nitric-oxide-dependent anammox process. For successful anammox-centered nitrogen removal from wastewater, this study demonstrated that low-strength MF is a convenient and applicable technique to lock the nitrate reduction until nitrite.

A predictive model for determining the nitrite concentration in the effluent of an anammox reactor using ensemble regression tree algorithm

Anaerobic ammonium oxidation (anammox) is a cost-effective biological nitrogen removal method for treating wastewater. Nitrite has strong negative effect on microbial activity of anammox bacteria, while the conventional equitment available for determining nitrite on-line is challenging due to high price. By knowing the concentration of nitrite in the effluent, its concentration in the reactor can be controlled accordingly. To investigate this, an ensemble regression tree algorithm was used to establish the predictive model proposed in the current work. Moreover, the Bayesian algorithm was adopted to systematically optimize various parameters of machine learning algorithms. The predicted concentrations of nitrite were in good agreement with the observed values, and the coefficient of determination (R2) and root mean squared error (RMSE) values reached 0.91 and 4.81, respectively. Furthermore, the model established by the ensemble regression tree algorithm was compared with models established by commonly used machine learning algorithms. Finally, the established models were applied to another anammox reactor, and the predicted results of ensemble regression tree model were found to be in good agreement with the experimental values with R2 and RMSE values of 0.84 and 6.34, respectively.

An Advanced Synergy of Partial Denitrification-Anammox for Optimizing Nitrogen Removal from Wastewater: A Review

Anammox is a widely adopted process for energy-efficient removal of nitrogen from wastewater, but challenges with NOB suppression and NO₃^- accumulation have led to a deeper investigation of this process. To address these issues, the synergy of partial denitrification and anammox (PD-anammox) has emerged as a promising solution for sustainable nitrogen removal in wastewater. This paper presents a comprehensive review of recent developments in the PD-anammox system, including stable performance outcomes, operational parameters, and mathematical models. The review categorizes start-up and recovery strategies for PD-anammox and examines its contributions to sustainable development goals, such as reducing N₂O emissions and saving energy. Furthermore, it suggests future trends and perspectives for improving the efficiency and integration of PD-anammox into full-scale wastewater treatment system. Overall, this review provides valuable insights into optimizing PD-anammox in wastewater treatment, highlighting the potential of simultaneous processes and the importance of improving efficiency and integration into full-scale systems.

Integration of EBPR with mainstream anammox process to treat real municipal wastewater: Process performance and microbiology

The mainstream application of anaerobic ammonium oxidation (anammox) for sustainable N removal remains a challenge. Similarly, with recent additional stringent regulations for P discharges, it is imperative to integrate N with P removal. This research studied integrated fixed film activated sludge (IFAS) technology to simultaneously remove N and P in real municipal wastewater by combining biofilm anammox with flocculent activated sludge for enhanced biological P removal (EBPR). This technology was assessed in a sequencing batch reactor (SBR) operated as a conventional A2O (anaerobic-anoxic-oxic) process with a hydraulic retention time of 8.8 h. After a steady state operation was reached, robust reactor performance was obtained with average TIN and P removal efficiencies of 91.3 ± 4.1% and 98.4 ± 2.4%, respectively. The average TIN removal rate recorded over the last 100 d of reactor operation was 118 mg/L·d, which is a reasonable number for mainstream applications. The activity of denitrifying polyphosphate accumulating organisms (DPAOs) accounted for nearly 15.9% of P-uptake during the anoxic phase. DPAOs and canonical denitrifiers removed approximately 5.9 mg TIN/L in the anoxic phase. Batch activity assays, which showed that nearly 44.5% of TIN were removed by the biofilms during the aerobic phase. The functional gene expression data also confirmed anammox activities. The IFAS configuration of the SBR allowed operation at a low solid retention time (SRT) of 5-d without washing out biofilm ammonium-oxidizing and anammox bacteria. The low SRT, combined with low dissolved oxygen and intermittent aeration, provided a selective pressure to washout nitrite-oxidizing bacteria and glycogen-accumulating organisms, as relative abundances of.

Sustainable nitrogen removal in anammox-mediated systems: Microbial metabolic pathways, operational conditions and mathematical modelling

Anammox-mediated systems have attracted considerable attention as alternative cost-effective technologies for sustainable nitrogen (N) removal from wastewater. This review comprehensively highlights the importance of understanding microbial metabolism in anammox-mediated systems under crucial operation parameters, indicating the potentially wide applications for the sustainable treatment of N-containing wastewater. The partial nitrification-anammox (PN-A), simultaneous PN-A and denitrification (SNAD) processes have demonstrated sustainable N removal from sidestream wastewater. The partial denitrification-anammox (PD-A) and denitrifying anaerobic methane oxidation-anammox (DAMO-A) processes have advanced sustainable N removal efficiency in mainstream wastewater treatment. Moreover, N2O production/emission hotspots are extensively discussed in anammox-based processes and are related to the dominant ammonia-oxidizing bacteria (AOB) and denitrifying heterotrophs. In contrast, N2O is not produced in the metabolism pathways of AnAOB and DAMO-archaea; Moreover, the actual contribution of N2O production by dissimilatory nitrate reduction to ammonium (DNRA) and DAMO-bacteria in their species remains uncertain. Thus, PD-A and DAMO-A processes would achieve reduction in greenhouse gas production, as well as energy consumption for the reliability of N removal efficiencies. In addition to reaction mechanisms, this review covers the mathematical models for simultaneous anammox, partial nitrification and/or denitrification (i.e., PN-A, PD-A, and SNAD). Promising NO3- reduction technologies by endogenous PD, sulfur-driven autotrophic denitrification, and DNRA by anammox are also discussed. In summary, this review provides a better understanding of sustainable N removal in anammox-mediated systems, thereby encouraging future investigation and exploration of the sustainable N bio-treatment from wastewater.

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.

Coupling effects of nitrate reduction and sulfur oxidation in a subtropical marine mangrove ecosystem with Spartina alterniflora invasion

The mangrove ecosystem has a high nitrate reduction capacity, which significantly alleviates severe nitrogen pollution. However, current research on nitrate reduction mechanisms in the mangrove ecosystem is limited. Furthermore, Spartina alterniflora invasion has disrupted the balance of the mangrove ecosystem and the effect of S. alterniflora on nitrate reduction has not yet been fully elucidated. Nitrate reduction was comprehensively investigated in a subtropical mangrove ecosystem in this study, which has been invaded by S. alterniflora for 40 years. Results showed that S. alterniflora significantly increased the relative and absolute abundance of nitrate reduction genes, especially nirS (nitrite reductase), in the mangrove ecosystem. Dissimilatory nitrate reduction to ammonium was the main pathway of nitrate reduction in the mangrove ecosystem. Nitrate reduction was mainly performed by Desulfobacterales and occurred in the shallow layers (0-10 cm) of mangrove sediments. A strong positive correlation was found between nitrate reduction and sulfur oxidation (especially sulfide oxidation), and the sulfide content was significantly positively correlated with the relative abundance of nitrate reduction genes. Moreover, 207 metagenomic assembled genomes (MAGs) were constructed, including 50 MAGs with high numbers (≥ 10) of nitrate reduction genes. This finding indicates that the dominant microbes had strong nitrate reduction potential in mangrove sediments. Our findings highlight the impact of S. alterniflora invasion on nitrate reduction in a subtropical marine mangrove ecosystem. This study provides new insights into our understanding of nitrogen pollution control and contributes to the exploration of new nitrogen-degrading microbes in mangrove ecosystems.

Modified integrated fixed-film activated sludge process: Advanced nitrogen removal for low-C/N domestic wastewater

Actual low-C/N domestic wastewater was treated using the high-concentration powder carrier bio-fluidized bed (HPB) process comparing diatomite and Fe-C as the carriers. The total nitrogen removal efficiencies were increased from 50.08% to 65.40% and 78.58%, respectively. The diatomite HPB process increased the relative abundance of autotrophic N-cycle bacteria to more than twofold and the sludge size. Therefore, the contributions for nitrogen removal by anammox and simultaneous nitrification-denitrification were increased. The Fe-C HPB process improved the nitrogen removal efficiency mainly by increasing the biodegradability and activities of electron transfer system and key enzymes. The key device (hydrocyclone separator) of the HPB process significantly improved the recovery efficiency of the carriers. It also improved the capacity of microbial aggregations for adsorbing pollutants. Furthermore, it reduced the relative abundance of filamentous bacteria. This study demonstrated the feasibility and mechanism of the HPB process for improving the nitrogen removal efficiency for low-C/N wastewater.

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.

Development of a post-treatment system using a downflow hanging sponge reactor - an upflow anaerobic reactor for natural rubber processing wastewater treatment

This study aimed to evaluate the nitrogen removal of a post-treatment system for natural rubber processing wastewater (NRPW) under low chemical oxygen demand to total nitrogen (COD/TN) ratios without any supplemental external carbon source. The system including a downflow hanging sponge (DHS) reactor and an upflow anaerobic reactor (UAR) was operated in two phases. In phase 1 (day 0-102), under a nitrogen loading rate (NLR) of 0.23 ± 0.06 kgN m^-3 d^-1 and COD/TN ratio of 0.63 ± 0.47, the DHS-UAR system removed 82.5 ± 11.8% and 83.9 ± 7.6% of TN and ammonium concentrations, respectively. In phase 2 (day 103-229), higher COD/TN ratio of 1.96 ± 0.28 was applied to remove increasing NLRs. At the highest NLR of 0.51 kgN m^-3 d^-1, the system achieved TN and ammonium removal efficiencies of 93.2% and 93.7%, respectively. Nitrogen profiles and the 16S rRNA high-throughput sequencing data suggested that ammonium, a major nitrogen compound in NRPW, was utilized by nitrifying and ammonium assimilation bacteria in DHS, then removed by heterotrophic denitrifying and anammox bacteria in the UAR. The predominance of Acinetobacter detected in both reactors suggested its essential role for the nitrogen conversion.

Genome-centric metagenomics insights into functional divergence and horizontal gene transfer of denitrifying bacteria in anammox consortia

Denitrifying bacteria with high abundances in anammox communities play crucial roles in achieving stable anammox-based systems. Despite the relative constant composition of denitrifying bacteria, their functional diversity remains to be explored in anammox communities. Herein, a total of 77 high-quality metagenome-assembled genomes (MAGs) of denitrifying bacteria were recovered from the anammox community in a full-scale swine wastewater treatment plant. Among these microbes, a total of 26 MAGs were affiliated with the seven dominant denitrifying genera that have total abundances higher than 1%. A meta-analysis of these species suggested that external organics reduced the abundances of genus Ignavibacterium and species MAG.305 of UTPRO2 in anammox communities. Comparative genome analysis revealed functional divergence across different denitrifying bacteria, largely owing to their distinct capabilities for carbohydrate (including endogenous and exogenous) utilization and vitamin (e.g., pantothenate and thiamine) biosynthesis. Serval microbes in this system contained fewer genes encoding biotin, pantothenate and methionine biosynthesis compared with their related species from other habitats. In addition, the genes encoding energy production and conversion (73 genes) and inorganic ion transport (53 genes) putatively transferred from other species to denitrifying bacteria, while these denitrifying bacteria (especially genera UTPRO2 and SCN-69-89) likely donated the genes encoding nutrients (e.g., inorganic ion and amino acid) transporter (64 genes) for other members to utilize new metabolites. Collectively, these findings highlighted the functional divergence of these denitrifying bacteria and speculated that the genetic interactions within anammox communities through horizontal gene transfer may be one of the reasons for their functional divergence.

Nitrogen removal capacity and carbon demand requirements of partial denitrification/anammox MBBR and IFAS processes

A pilot study was conducted to investigate the carbon demand requirements and nitrogen removal capabilities of two mainstream partial denitrification/anammox (PdNA) processes: a two-zone, moving bed biofilm reactor (MBBR) process and an integrated fixed-film activated sludge (IFAS) process. The first MBBR zone conducted PdNA, while the second operated as an anammox zone. Operation of the IFAS process was conducted in two phases. The first phase of the operation involved minor external carbon addition, while the second phase of the operation involved controlled external carbon addition. The MBBR process produced an average effluent TIN concentration and chemical oxygen demand (COD)/TIN ratio of 2.81 ± 1.21 mg/L and 2.42 ± 0.77 g/g. The average effluent TIN concentrations and COD/TIN ratios for the IFAS process were 4.07 ± 1.66 mg/L and 1.08 ± 0.38 g/g during phase 1 and 3.30 ± 0.96 mg/L and 2.18 ± 0.99 g/g during phase 2. Despite having relatively low and unstable partial denitrification (PdN) efficiencies, both mainstream PdNA processes exhibited low effluent TIN concentrations and carbon requirements compared to nitrification/denitrification. Successful operation of the PdNA IFAS process indicates that mainstream PdNA can be implemented with minimal capital costs in a water resource recovery facility's second anoxic zone. PRACTITIONER POINTS: Low effluent TIN concentrations can be maintained in mainstream PdNA MBBR and IFAS processes with low external carbon demand. MBBR and IFAS PdNA processes do not require consistent or high PdN efficiencies to maintain low effluent TIN concentrations. IFAS and MBBR PdNA processes exhibit similar TIN and NH3 removal efficiencies. PdNA can be implemented in a second anoxic zone, using IFAS technology for anammox retention, with minimal capital costs.

Synergistic simultaneous endogenous partial denitrification/anammox (EPDA) and denitrifying dephosphatation for advanced nitrogen and phosphorus removal in a complete biofilm system

To achieve simultaneous biological nitrogen and phosphorus removal from municipal wastewater, the endogenous partial denitrification/anammox (EPDA) was combined with denitrifying dephosphatation in a complete biofilm reactor. Advanced nitrogen and phosphorus removal were achieved with effluent total nitrogen (TN) and PO₄^3--P concentrations of 7.77 ± 0.33 mg/L and 0.35 ± 0.10 mg/L, respectively. Anammox took a major role in the system, accounting for 76 ± 7% of nitrogen removal. 16S rRNA high-throughput sequencing results showed that the anammox bacteria co-existed with the denitrifying glycogen accumulating organisms (DGAOs) and the denitrifying phosphorus accumulating organisms (DPAOs). Anammox bacteria were mainly distributed in the inner layer, while DGAOs and DPAOs existed in the outer layer of EPDA biofilms. Furthermore, based on the EPDA biofilm system, a promising advanced nitrogen and phosphorus removal process was suggested to achieve lower requirements for energy and reagent consumption.

Towards a more labor-saving way in microbial ammonium oxidation: A review on complete ammonia oxidization (comammox)

In the Anthropocene, nitrogen pollution is becoming an increasing challenge for both mankind and the Earth system. Microbial nitrogen cycling begins with aerobic nitrification, which is also the key rate-limiting step. For over a century, it has been accepted that nitrification occurs sequentially involving ammonia oxidation, which produces nitrite followed by nitrite oxidation, generating nitrate. This perception was changed by the discovery of comammox Nitrospira bacteria and their metabolic pathway. In addition, this also provided us with new knowledge concerning the complex nitrogen cycle network. In the comammox process, ammonia can be completely oxidized to nitrate in one cell via the subsequent activity of the enzyme complexes, ammonia monooxygenase, hydroxylamine dehydrogenase, and nitrite oxidodreductase. Over the past five years, research on comammox made great progress. However, there still exist a lot of questions, including how much does comammox contribute to nitrification? How large is the diversity and are there new strains to be discovered? Do comammox bacteria produce the greenhouse gas N₂O, and how or to which extent may they contribute to global climate change? The above four aspects are of great significance on the farmland nitrogen management, aquatic environment restoration, and mitigation of global climate change. As large number of comammox bacteria and pathways have been detected in various terrestrial and aquatic ecosystems, indicating that the comammox process may exert an important role in the global nitrogen cycle.

An Eco-Friendly Conversion of Aquaculture Suspended Solid Wastes Into High-Quality Fish Food by Improving Poly-β-Hydroxybutyrate Production

The aquaculture industry is vital in providing a valuable protein food source for humans, but generates a huge amount of solid and dissolved wastes that pose great risks to the environment and aquaculture sustainability. Suspended solids (in short SS), one of the aquaculture wastes, are very difficult to be treated due to their high organic contents. The bioconversion from wastewater, food effluents, and activated sludge into poly-β-hydroxybutyrate (PHB) is a sustainable alternative to generate an additional income and could be highly attractive to the agricultural and environmental management firms. However, little is known about its potential application in aquaculture wastes. In the present study, we first determined that 7.2% of SS was PHB. Then, the production of PHB was increased two-fold by the optimal fermentation conditions of wheat bran and microbial cocktails at a C/N ratio of 12. Also, the PHB-enriched SS showed a higher total ammonia nitrogen removal rate. Importantly, we further demonstrated that the PHB-enriched SS as a feed could promote fish growth and up-regulate the expression of the immune-related genes. Our study developed an eco-friendly and simple approach to transforming problematic SS wastes into PHB-enriched high-quality food for omnivorous fish, which will increase the usage efficiency of SS and provide a cheaper diet for aquatic animals.

Deciphering waste bound nitrogen by employing psychrophillic Aporrectodea caliginosa and priming of coprolites by associated heterotrophic nitrifiers under high altitude Himalayas

Himalayan ecosystem is characterized by its fragile climate with rich repositories of biodiversity. Waste collection and disposal are becoming increasingly difficult due to topographical variations. Aporrectodea caligenosa, a versatile psychrophillic soil dweller, is a useful biocatalyst with potent bio-augmented capability for waste treatment at low temperatures. Microcosm experiments were conducted to elucidate the comprehensive nature of biogenic nitrogen transformation to NH₄⁺ and NO₃⁻ produced by coupling of earthworm-microbes. Higher biogenic recovery of NH₄⁺-N from coprolites of garden soil (47.73 ± 1.16%) and Himalayan goat manure (86.32 ± 0.92%) with an increment of 14.12 and 47.21% respectively over their respective control (without earthworms) with a linear decline beyond 4th week of incubation was reported. NO₃^–-N recovery progressively sustained in garden soil and goat manure coprolites during entire incubation with highest 81.81 ± 0.45 and 87.20 ± 1.08 µg-N g⁻¹dry weight recorded in 6th and 5th week of incubation respectively and peak increments as 38.58 and 53.71% relative to respective control (without earthworms). Declined NH₄⁺–N in coprolites at low temperature (15.0 ± 2.0 °C) evidenced increased nitrification rates by taking over the process by abundant nitrifying microbes. Steady de-nitrification with progressive incubation on an average was 16.95 ± 0.46 ng-N g⁻¹ per week and 21.08 ± 0.87 ng-N g⁻¹ per week compared to 14.03 ± 0.58 ng-N g⁻¹ per week and 4.50 ± 0.31 ng-N g⁻¹ per week in respective control treatments. Simultaneous heterotrophic nitrification and aerobic denitrification (SHNAD) was found to be a prominent bioprocess at low temperature that resulted in high and stable total nitrogen and nitrate accumulation from garden soil and goat manure with relative recovery efficiency of 11.12%, 14.97% and 14.20%; 19.34%. A. caligenosa shows promising prospects for mass applicability in biogenic N removal from manure of Himalayan goat.

Enhanced treatment of low-temperature and low carbon/nitrogen ratio wastewater by corncob-based fixed bed bioreactor coupled sequencing batch reactor

In this study, a combined corncob-based fixed bed bioreactor and sequencing batch reactor system (CCF-SBR) was developed to treat low-temperature (3-12 °C) and low carbon/nitrogen ratio (C/N = 2) wastewater with a single SBR as the control. Results showed similarly low COD concentration of CCF-SBR (20.4 ± 3.7 mg·L^-1) and control SBR (24.9 ± 6.7 mg·L^-1) effluent. However, the total nitrogen (TN) removal rate of CCF-SBR was significantly higher than that of control SBR (29.6 ± 2.7% vs 8.6 ± 2.3%). According to the nitrification and denitrification activities and the analysis of microbial community, CCF mainly played the role of denitrification based on fermentation genera and denitrifying genera, and SBR mainly implemented nitrification with Nitrospira and Acinetobacter. This study explores a promising way for agricultural waste resource utilization and wastewater treatment under low-temperature and low C/N ratio.

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.

Multidisciplinary characterization of nitrogen-removal granular sludge: A review of advances and technologies

Nitrogen-removal granular sludge (NRGS) is a promising technology in wastewater treatment, with advantages of efficient nitrogen removal, less footprint, lower sludge production and energy consumption, and is a way for wastewater treatment plants to achieve carbon-neutrality. Aerobic granular sludge (AGS) and anammox granular sludge (AnGS) are two typical NRGS technologies that have attracted extensive attention. Mounting evidence has shown strong associations between NRGS properties and the status of NRGS systems; however, a holistic view is still missing. The aim of this article is to provide an overview of NRGS with an emphasis on characterization. Specifically, the integrated nitrogen transformation pathways inside NRGS and the performance of NRGS treating various wastewaters are discussed. NRGS properties are categorized as physical-, chemical-, biological- and systematical ones, presenting current advances and corresponding characterization technologies. Finally, the future prospects for furthering the mechanistic understanding and engineering application of NRGS are proposed. Overall, the technological advancements in characterization have greatly contributed to understanding NRGS properties, which are potential factors for optimizing the performance and evaluating the working status of NRGS. This review will provide guidance in characterizing NRGS properties and boost the introduction of novel characterization technologies.

Designing Multi-Stage 2 A/O-MBR Processes for a Higher Removal Rate of Pollution in Wastewater

Multi-stage A/O-MBR processes were designed to improve wastewater treatment efficiency; three different designs were carried out and compared in this study. The 2(A/O)-MBR process, i.e., with two sets of anoxic/oxic tanks in series, showed better effluent quality than A/O-MBR and 3(A/O)-MBR processes. The removal rates of COD, NH4+-N, TP and TN were 95.29%, 89.47%, 83.55% and 78.58%, respectively, complying satisfactorily with China’s urban sewage treatment plant pollutant discharge standards. In terms of membrane fouling, the 3(A/O)-MBR process demonstrated the lowest fouling propensity. The microbial community structure in each bioreaction tank was analyzed, the results from which matched with the process efficiency and fouling behavior.

Achieving Partial Nitrification-Anammox Process Dependent on Microalgal-Bacterial Consortia in a Photosequencing Batch Reactor

Partial nitrification coupled with anammox (PN/A) process is an energy-efficient approach for nitrogen removal from low C/N wastewater. In this study, PN/A was achieved with optimal oxygen supply from a green microalga, Chlorella sorokiniana. The PN process was first initiated within 35 days, and the following algae-intensified PN then reached the steady state within the next 32 days. The dissolved oxygen (DO) concentration was gradually maintained at 0.6 mg L-1 via adjusting the photoperiod to 6-h light/18-h dark cycles, when the accumulation ratio of NO2 --N and the removal ratio of NH4 +-N were both more than 90%. The nitrogen removal capability of anammox was acclimated via elevating the individual effluent NH4 +-N and NO2 --N levels from 100 to 200, to 300 mg L-1. After acclimation, the removal rates of NH4 +-N and total nitrogen (TN) reached more than 70 and 80%, respectively, and almost all the NO2 --N was removed. Then, the algae-intensified PN/A, algammox biofilm system, was successfully started up. When the NH4 +-N level increased from 100 to 300 mg L-1, the TN removal varied between 78 and 82%. In the photosequencing bioreactor, C. sorokiniana, ammonia-oxidizing bacteria (AOB), and anammox coexisted with an illumination of 200 μmol m-2 s-1 and a 6-h light/18-h dark cycles. The DO levels ranged between 0.4 and 0.5 mg L-1. In addition, the microbial community analysis by Illumina MiSeq sequencing showed that the dominant functional bacteria in the algae-intensified PN/A reactors included Nitrosomonas (AOB) and Candidatus Brocadia (anammox), while Nitrospira and Nitrobacter (nitrite oxidizing bacteria), together with Denitratisoma (denitrifier) were largely inhibited. Further studies are required to optimize the microalgal-bacterial consortia system to achieve superior nitrogen removal rates under controllable conditions.

Effect of addition sites on bioaugmentation of tea polyphenols-NZVI/PE composite packing: Nitrogen removal efficiency and service life

Although efficient improvement of the nitrogen removal from wastewater by adding iron was achieved in wastewater process, the influence mechanism of addition sites is unclear. The study was based on the A/O-MBR treating simulated domestic wastewater, and tea polyphenol-nano zero-valent iron/polyethylene packing (TP-NZVI/PE) was added into the anoxic tank, aerobic tank and membrane effluent end of the process, respectively. The effect of the different addition sites on the nitrogen removal performance of A/O-MBR was investigated. Combine with the corrosion rate of NZVI on the packing surface to optimize TP-NZVI/PE addition site. The enhancement mechanism of TP-NZVI/PE under different addition site was explored through the calculation of the materials balance (carbon, nitrogen, phosphorus). The results showed that the pollutant removal of A/O-MBR was significantly increased with the TP-NZVI/PE added. In particular, the TP-NZVI/PE was added into the aerobic tank, and the pollutant removal rate was increased 31.71% (TN) and 53.00% (total phosphorus), respectively. Meanwhile, the service life of TP-NZVI/PE in the aerobic tank was 66 days. The anti-oxidation and dispersion of NZVI was improved with the encapsulation of tea polyphenols and support of packing, and it also played a certain slow-release effect, so that the service life of NZVI was further prolonged in aerobic condition. Combined with the material balance analysis, the result showed that the environmental structure made diversity in the aerobic tank by added the TP-NZVI/PE, and the simultaneous nitrification and denitrification process was achieved. The dependence of the denitrification process on the carbon source was greatly reduced. Besides, it promoted the adsorption and chemical precipitation process of the system for phosphor pollutant and achieved the denitrifying phosphorus removal performance.

Current state of the art biotechnological strategies for conversion of watermelon wastes residues to biopolymers production: A review

Poly-3-hydroxyalkanoates (PHA) are biodegradable and compostable polyesters. This review is aimed to provide a unique approach that can help think tanks to frame strategies aiming for clean technology by utilizing cutting edge biotechnological advances to convert fruit and vegetable waste to biopolymer. A PHA manufacturing method based on watermelon waste residue that does not require extensive pretreatment provides a more environmentally friendly and sustainable approach that utilizes an agricultural waste stream. Incorporating fruit processing industry by-products and water, and other resource conservation methods would not only make the manufacturing of microbial bio-plastics like PHA more eco-friendly, but will also help our sector transition to a bioeconomy with circular product streams. The final and most critical element of this review is an in-depth examination of the several hazards inherent in PHA manufacturing.

Advanced nitrogen removal from landfill leachate via a two-stage combined process of partial nitrification-Anammox (PNA) and partial denitrification-Anammox (PDA)

In this study, a two-stage combined process of partial nitrification-Anammox (PNA) and partial denitrification-Anammox (PDA) was established achieving advanced nitrogen removal from landfill leachate. The PNA sludge used to treat reject water adapted to the leachate in 37 days, resulting in fast start-up of the PNA process with a nitrogen removal rate (NRR) of 0.22 kgN/(m³·d). Partial denitrification (PD) was induced using sodium acetate and proceeded in a stepwise manner using sludge fermentation liquid (SFL), achieving a NO₃^--N to NO₂^--N transformation ratio (NTR) of 52.1 ± 1.1% within 16 days. PDA was established via the addition of mature Anammox biofilms. The nitrogen removal efficiency (NRE) of this system was 97.6 ± 1.5%, of which PNA and PDA contributed 74.8 ± 4.0% and 18.7 ± 4.1%, respectively. Nitrosomonas (2.6% in PNA), Thauera (16.0% in PDA) and Candidatus Brocadia (23.0% in PNA, 1.4% in PDA) were dominant in the two-stage system. This study provides valuable and novel insights, supporting the practical application of PNA-PDA processes in landfill sites.

Applicability of two-stage anoxic/oxic shortcut nitrogen removal via partial nitrification and partial denitrification for municipal wastewater by adding sludge fermentation products continuously

Partial nitrification and partial denitrification combined with anammox is a promising process for sewage treatment. In this study, real municipal wastewater was treated in a continuous two-stage anoxic/oxic (A/O) reactor. External mixed sludge fermentation products were added in the anoxic zone, simultaneously achieving partial nitrification and partial denitrification and achieving a high and relatively stable accumulation of nitrite. The maximum accumulation rates of NO₂^--N in A1.2 and A2.1-A2.4 zones of the reactor reached 70% and 61%-37%, respectively, which improved denitrification efficiency and created conditions that supported the coupling of subsequent anammox. The influent nitrogen load of the system was 0.078 kg/(m³•d), and the mean influent and effluent total nitrogen were 51 and 12 mg/L, respectively. The mean total nitrogen removal rate reached 76%. Further analysis revealed that Hyphomicrobium (incomplete denitrifiers) and Nitrosomonas (ammonia oxidizing bacteria) were enriched, which may have facilitated the high nitrite accumulation. Moreover, the batch test showed that adding sludge fermentation during denitrification significantly suppressed nitrite reduction, resulting in the nitrite accumulation.

Enhancing the treatment performance of partial denitrification/Anammox process at high nitrogen load: Effects of immobilized strain HFQ8C/Non the sludge characteristics

A novel strategy based on quorum sense (QS) was proposed to improve the treatment performance of the partial denitrification/Anammox (PD/A) process at high loads by adding immobilized Pseudomonas sp. HFQ8_C/N, which could release high concentrations of N-butyryl-DL-homoserine lactone (C4-HSL), N-octanoyl-DL- homoserine lactone (C8-HSL) and N-decanoyl-DL-homoserine lactone (C10-HSL). The results showed that adding immobilized HFQ8_C/N improved the sludge activity and settleability, contributing to higher nitrogen removal efficiency at the high nitrogen loading rate (NLR). It was proved that C4-HSL promoted the abundances of Thauera and Candidatus Kuenenia at NLR 1.68-2.52 kg N/(m³·d), while C10-HSL promoted the abundance of Candidatus Brocadia. Besides, C8-HSL and C10-HSL played different regulation roles in the production of protein (PN) in tightly bound extracellular polymeric substances (TB-EPS) at different loads, improving the sludge settleability. This study provided a new way to improve the treatment performance of high-load PD/A processes.

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.

Achieving advanced nitrogen removal in a novel partial denitrification/anammox-nitrifying (PDA-N) biofilter process treating low C/N ratio municipal wastewater

For achieving mainstream anammox, a novel partial denitrification/anammox-nitrifying (PDA-N) biofilter process to treat municipal wastewater was developed. This process achieved a total inorganic nitrogen (TIN) removal efficiency of 81%, with an average effluent TIN of 7.31 mg·L^-1, when the ratio of influent chemical oxygen demand (COD) to TIN was 3.2. Approximately 97% of the TIN was removed by anammox in the PDA biofilter. Nitrite was provided by partial denitrification for anammox. Partial denitrification was driven by Thaurea in the middle and lower regions of the PDA biofilter, while anammox was mainly driven by Candidatus Brocadia in the middle and upper regions. When treating real municipal wastewater, the TIN was efficiently removed in the PDA-N biofilter, with the effluent TIN of 5.96 mg·L^-1. Anammox played a primary role, achieving approximately 98% of the TIN removal. Compared to the traditional nitrification/denitrification process, this process can economize organic carbon demand and oxygen consumption.

Pilot-scale evaluation of partial denitrification/anammox on nitrogen removal from low COD/N real sewage based on a modified process

The nitrogen removal performance of a pilot-scale biosystem was significantly improved via partial denitrification/anammox (PD/A) in real sewage with low COD/N ratio. The modified pilot plant was designed as an anaerobic/anoxic/oxic (AAO) reactor combined with a biological aerated filter. The inoculation of biocarriers into anaerobic and anoxic zones enhanced anammox and the nitrogen removal performance. Despite a COD/N ratio of 3.1, effluent total inorganic nitrogen decreased from 17.1 to 9.8 mg N/L. The anoxic unit developed as the PD/A hotspot, which was associated with the enrichment of Ca. Brocadia (2.00%) and partial denitrification functional groups (OLB14, 13.50%; Thauera, 5.45%) in the anoxic-carrier biofilms and contributed 34.1% towards total nitrogen removal. Besides improving the PD/A process, enhanced denitrifying dephosphatation was simultaneously realized, suggesting that the integration of PD/A into this modified system is a promising approach to enhance nutrient removal of low COD/N wastewater.

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.

Comparisons of nitrite accumulation, microbial behavior and nitrification kinetic in continuous stirred tank (ST) and plug flow (PF) moving bed biofilm reactors

Two types of continuous stirred tank moving bed biofilm reactors (ST-MBBR) and plug flow MBBR (PF-MBBR) were compared for nitrification. PF-MBBR showed strong shock resistance to temperature, and ammonium oxidation ratio (AOR) was 9.63% higher than that in the ST-MBBR, although the average biomass and biofilm thickness of ST-MBBR were 7.32-18.59%, 9.44-14.06% higher than those in the PF-MBBR. Meanwhile, a lower nitrite accumulation ratio (NAR) was observed (54.88%) in the PF-MBBR than the ST-MBBR (78.92%) due to different operation modes, and the divergence was demonstrated by the microbial quantitative analysis. Nitrification kinetics revealed that the temperature coefficient (θ) in the ST-MBBR (1.068) was much higher than that in the PF-MBBR (1.006-1.015), proving the contrasting nitrification performances caused by temperature shock. According to the Monod equation, the half-saturation coefficient (K_N) in the ST-MBBR was 0.19 mg/L while it varied around 0.12-0.24 mg/L in the PF-MBBR, revealing various NH₄⁺ affinity owing to different biofilm thickness and microbial composition. Finally, MBBR optimization related to operation mode, temperature, and free ammonium (FA) inhibition for nitrite accumulation was discussed.

Rapid cultivation of anammox bacteria by forming free cells in a membrane bioreactor

The low growth rate of anammox bacteria is one of the main obstacles to its broad use in removing biological nitrogen in sewage treatment. Free cells show the characteristics of relatively rapid growth. To increase the growth rate, this study cultivated free-living anammox bacteria using a membrane bioreactor. The particle size distributions showed that more than 95.48% of biomass was less than 50 μm in size, and the growth rate of free-living anammox bacteria was shortened to 5.68 days after cultivation. Candidatus Brocadia was the most important anammox genus, with a relative abundance of 8.83%. The key functional genes, including hzs, hdh, and hao, were identified and expressed using metagenomic and metaproteomic analyses. After the rapid cultivation of the free-living anammox bacteria, a potential method shortening the start-up time was proposed to rapidly form anammox biofilm by attaching to carriers. PRACTITIONER POINTS: The free-living anammox bacteria are cultivated in a membrane bioreactor. More than 95.48% of biomass size is less than 50 μm. The doubling time of these free anammox bacteria was 5.68 days. The key functional genes of anammox bacteria were identified and expressed. A novel method is proposed to rapidly form anammox biofilm.

Effectively stimulating partial denitrification to utilize dissolved slowly-biodegradable organic matter by introducing in-situ biosorption and hydrolytic acidification

In this study, partial denitrification (PD, nitrate → nitrite) using dissolved slowly-biodegradable organic matter (DSBOM) was effectively established by introducing biosorption and hydrolytic acidification (HA) as a pretreatment for carbon capture and conversion. After 119 days of optimized operation, an efficient nitrate to nitrite transformation of 80% was achieved, with an influent nitrate level of 40 mg/L and DSBOM level of 183.8 mg/L. There was a significant shift from exogenous PD to endogenous PD, with energy supplied by HA products of captured DSBOM, i.e., acetate, saccharide and intracellular poly-hydroxyalkanoates (PHAs), jointly facilitating nitrite production. This was well explained by that genera Dechloromonas (26.7%), possibly responsible for carbon HA and nitrite production, were enriched; while abundant enzymes for glycolysis, acetate fermentation and PHAs storage, and 2.6 times more nitrate reductases than nitrite reductases were identified. These results highlight a novel carbon capture reuse and PD-based anammox strategy to cost-effectively treat nitrogen.

Insights into two stable mainstream deammonification process and different microbial community dynamics at ambient temperature

How to achieve stable mainstream deammonification is still a huge challenge. In this work, satisfactory nitrogen removal were achieved in a deammonification granular sludge reactor (R1, 0.42 ± 0.03 kg N / (m³·d)) and a mixed flocculent with granular sludge reactor (R2, 0.39 ± 0.04 kg N / (m³·d)) at ambient temperature (21-28 ℃) . The good adaptability of anammox bacteria (Candidatus Jettenia) to ambient temperature ensured its efficient activity (0.84-1.54 mg N/(g VSS·h)). The overexpression ammonia monooxygenase gene abundances in ammonia oxidizing bacteria (Nitrosomonas) was also predicted. The inhibition of hydrazine and the competition of denitrifying bacteria (Denitratisoma) to nitrite nitrogen, leading to a low Nitrospira relative abundances (0.2%-2.1%) . It was also found that R1 was more resistant to the unfavorable condition. For R2, higher Denitratisoma abundances (9.2%-18.5%) and predicted metabolic pathway abundances related to carbon metabolism were observed.

Achieving simultaneous nitritation, anammox and denitrification (SNAD) in an integrated fixed-biofilm activated sludge (IFAS) reactor: Quickly culturing self-generated anammox bacteria

In this study, by inoculating nitritation suspended sludge, simultaneous nitritation, anammox and denitrification (SNAD) was established quickly in an integrated fixed-biofilm activated sludge (IFAS) reactor to treat high-ammonia municipal wastewater. Results showed that, deep-level total nitrogen and chemical oxygen demand removal efficiencies (92.8% and 78.8%, respectively) were achieved, and their effluent concentrations were 13.2 and 39.3 mg/L, respectively. Excess generation of nitrate was once occurred under continuous aerobic condition, but it could be solved by suppressing nitrite oxidizing bacteria activity stably via switching to intermittent aeration mode (alternate 7 min of aerobic and 21 min of anoxic) and rising influent ammonium concentration temporarily (lasted 31 days). High-throughput sequencing analysis revealed that, Candidatus_Brocadia, as dominant anammox bacteria, was self-generated in flocs (2.93%) but mainly biofilm (7.67%), whereas uncultured_f_Nitrosomonadaceae as ammonia oxidizing bacteria was mainly found in flocs (2.4%). This work not only demonstrated that anammox bacteria could be self-generated and retained in the SNAD-IFAS system, but also suggested a promising application of the SNAD-IFAS in wastewater treatment plants.

Semi-starvation fluctuation driving rapid partial denitrification granular sludge cultivation in situ by microorganism exudate metabolites feedbacks

In this study, semi-starvation fluctuation driving PD granules cultivation in situ by microorganism exudate metabolites feedbacks was firstly investigated. The PD granules of high nitrite production were cultivated with an excellent mean nitrate-to-nitrite transformation rate (NTR) of 56.39% in just 30 days. The granules size was improved from the initial size of 0.09 ± 0.01 mm in diameter to a size above 2 mm when the extracellular polymeric substance (EPS) content increased from 80.21 ± 10.20 mg/g MLVSS to 777.00 ± 22.13 mg/g MLVSS. Acyl-homoserine lactone signals (AHLs) ultimately increased ten-fold more than the initially through 30 days of cultivation. Meanwhile, Thauera had been identified as the main function bacteria of PD, which enriched from 0.47% to 10.67%. Results demonstrated that AHLs, EPS, PD bacteria and the PD granules cultivation were closely associated. Semi-starvation fluctuation produced oligotrophic stress on bacterial community, a part of bacteria would be eliminated on starvation for oligotrophic stress and AHLs of bacteria regarded as distress signals resulted in the rapid formation of PD granules. A mechanism for PD granular cultivation with semi-starvation fluctuation was proposed from the aspect of oligotrophic stress. A better strategy for rapid PD granules cultivation was obtained and it could be useful for the mainstream granule-based PD combined with the anammox process application in the future.

Highly enriched anammox within anoxic biofilms by reducing suspended sludge biomass in a real-sewage A2/O process

This study proposes a novel strategy of stably enriching anammox in mainstream, based on the competitive difference to NO₂^- between anoxic biofilms and suspended sludge. A modified anaerobic-anoxic-oxic (A²/O) process run for 500 days with actual municipal wastewater. Microbial analysis revealed that anoxic-carrier biofilms had a significantly higher abundance of anammox (qPCR: 0.74% - 4.34%) than suspended sludge (P< 0.001). Batch tests showed that anammox within anoxic-carrier biofilms contributed to significant nitrogen removal, coupled with partial-denitrification (NO₃^- → NO₂^-). The anammox genus, Ca. Brocadia, was highly enriched when suspended sludge was accidentally lost. Further batch tests found that reducing suspended biomass helped anammox enrichment in anoxic-carrier biofilms, because the suspended sludge had strong NO₂^- competition (NO₂^- → N₂) with anammox (increased nirK). Metagenomic sequencing revealed that Ca. Brocadia dominates in the anoxic-carrier biofilms, and is the most important narG contributor to NO₃^- → NO₂^-, which could have promoted the competition of NO₂^- with heterotrophic bacteria. For this A²/O process, the low effluent total nitrogen (8.9 mg ± 1.0 mg N/L) was attributed to partial-denitrification coupling with anammox, demonstrating that this process is applicable to the general influent N-concentration range (30 mg - 50 mg NH₄⁺-N/L) of municipal wastewater treatment plants (WWTPs). Based on the special competitive preference of anammox for NO₂^-, this study provides a promising and practical alternative for enriching anammox bacteria in municipal WWTPs.