Selective genes expression and metabolites transformation drive a robust nitrite accumulation during nitrate reduction under alternating feast-famine condition

Nitrite production via denitrification has been regarded as a key approach for survival of anaerobic ammonium oxidation (anammox) bacteria. Despite the important carbon substrate, little is known about the role of differential genes expression and extracellular metabolite regulation among diverse microbial communities. In this study, a novel alternating feast-famine strategy was proposed and demonstrated to efficiently accumulate nitrite in a low-nitrogen loading rate (NLR) (0.2∼0.8 kg N/m3/d) denitrification system. Highly selective expression of denitrifying genes was revealed as key regulators. Interestingly, in absence of carbon source (ACS) condition, the expression of narG and narI/V genes responsible for reduction of nitrate to nitrite jumped to 2.5 and 5.1 times higher than that in presence of carbon source (PCS) condition with carbon to nitrate ratio of 3.0. This fortunately facilitated a rapid nitrite accumulation once acetate was added, despite a significantly down-regulated narG and narI/narV and up-regulated nirS/nirK. This strategy selected Thauera as the most dominant denitrifier (50.2 %) with the highest contribution to narG and narI/narV genes, responsible for the high nitrite accumulation. Additionally, extracellular xylose, pyruvate, and glucose jointly promoted carbon-central metabolic pathway of key denitrifiers in ACS stage, playing an important role in the process of self-growth and selective enrichment of functional bacteria. The relatively rapid establishment and robust performance obtained in this study shows an engineering-feasible and economically-favorable solution for the regulation of partial denitrification in practical application.

Mechanisms of endogenous and exogenous partial denitrification in response to different carbon/nitrogen ratios: Transcript levels, nitrous oxide production, electron transport

Nitrite as an important substrate for Anammox can be provided by partial denitrification (PD). In this study, endogenous partial denitrification (EdPD) and exogenous partial denitrification (ExPD) sludge were domesticated and their nitrite transformation rate reached 74.4% and 83.4%, respectively. The impact of four carbon/nitrogen (C/N) ratios (1.5, 3.0, 5.0 and 6.0) on nitrous oxide (N2O) emission and denitrification functional genes expression in both PD systems were investigated. Results showed that elevated C/N ratios enhanced most denitrification genes expression, but in EdPD, high nitrite levels suppressed nosZ genes expression (from 9.4% to 1.4%), leading to increased N2O emission (0 to 3.4%). EdPD also exhibited lower electron transfer system activity, resulting in slower nitrogen oxide conversion efficiency and more stable nitrite accumulation compared to ExPD. These findings offer insights for optimizing PD systems under varying water quality conditions.

Achieving advanced nitrogen removal from mature landfill leachate in continuous flow system involving partial nitrification-anammox and denitrification

Considering the challenges associated with nitrogen removal from mature landfill leachate, a novel combined continuous-flow process integrating denitrification and partial nitrification-Anammox (PN/A) was developed using an internal circulation (IC) system and a biological aerated filter (BAF) biofilm reactor (IBBR). In this study, IBBR successfully operated for 343 days, and when influent NH4+-N concentration of mature landfill leachate reached 1258.1 mg/L, an impressive total nitrogen removal efficiency (TNRE) of 93.3 % was achieved, along with a nitrogen removal rate (NRR) of 1.13 kg N/(m3·d). The analysis of the microbial community revealed that Candidatus Kuenenia, the dominant genus responsible for anammox, accounted for 1.7 % (day 265). Additionally, Nitrosomonas, Thauera and Truepera were identified as key contributors to the efficient removal of nitrogen from mature landfill. As a novel nitrogen removal strategy, the practical application of the IBBR system offers novel perspectives on addressing mature landfill leachate.

Enhancement of nitrate reduction in microbial fuel cells by acclimating biocathode potential: Performance, microbial community, and mechanism

The enhancement of nitrate reduction in microbial fuel cells (MFCs) by acclimating biocathode potential was studied. An MFC system was started up, and measured by cyclic voltammetry to determine a suitable potential region for acclimating biocathode. The experimental results revealed that potential acclimation could efficiently improve denitrification performance by relieving the phenomenon of nitrite accumulation, and optimum performance was obtained at -0.4 V with a total nitrogen removal efficiency of 87.4 %. Subsequently, the characteristics of electron transfer behaviors were measured, suggesting that a positive correlation between nitrate reduction and the contribution of direct electron transfer emerged. Furthermore, a denitrification mechanism was proposed. The results indicated that potential acclimation was conducive to enhancing denitrifying enzyme activity and that the electron transport system activity could be increased by 5.8 times. This study provides insight into the electron transfer characteristics and denitrification mechanisms in MFCs for nitrate reduction at specific acclimatization potentials.

Oxygen affinity and light intensity induced robust phosphorus removal and fragile ammonia removal in a non-aerated bacteria-algae system

Non-aerated bacteria-algae system gaining O2 through photosynthesis presents an alternative for costly mechanical aeration. This study investigated oxygen supply and performance of nutrients removal at low and high light intensity (LL and HL). The results showed that P removal was high and robust (LL 97 ± 1.8 %, HL 95 % ± 2.9 %), while NH4+-N removal fluctuated dramatically (LL 66 ± 14.7 %, HL 84 ± 8.6 %). Oxygen generated at illumination of 200 μmol m-2 s-1, 6 h was sufficient to sustain aerobic phase for 2.25 g/L MLSS. However, O2 produced by algae was preferentially captured in the order of heterotrophic bacteria (HB), ammonia oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB). Oxygen affinity coupled with light intensity led to NOB suppression with stable nitrite accumulation ratio of 57 %. Free nitrous acid (FNA) and light stimulated the abundance of denitrifying polyphosphate accumulating organism (DPAO) of Flavobacterium, but with declined P-accumulating metabolism (PAM) of P release, P/C, K/P and Mg/P ratios. Flavobacterium and cyanobacteria Leptolyngbya, along with biologically induced CaP in extracellular polymeric substances was the key to robust P removal. AOB of Ellin6067 and DPAO of Flavobacteria offer a promising scenario for partial nitrification-denitrifying phosphorus removal.

The negative effects of the excessive nitrite accumulation raised by anaerobic bioaugmentation on bioremediation of PAH-contaminated soil

Nitrite accumulation in anaerobic bioaugmentation and its side effects on remediation efficiency of polycyclic aromatic hydrocarbon (PAH)-contaminated soil were investigated in this study. Four gradient doses of PAH-degrading inoculum (10^4, 10^5, 10^6 and 10^7 cells/g soil) were separately supplied to the actual PAH-contaminated soil combining with nitrate as the biostimulant. Although bioaugmented with higher dose of inoculum could effectively improve the biodegradation efficiencies in the initial stage than sole nitrate addition but also accelerated the accumulation of nitrite in soil. The inhibition effects of nitrite swiftly occurred following the rapid accumulation of nitrite in each experiment group, restraining the PAH-degrading functionality by inhibiting the growth of total biomass and denitrifying functions in soil. This study revealed the side effects of nitrite accumulation raised by bioaugmentation on soil microorganisms, contributing to further improving the biodegrading efficiencies in the actual site restoration.

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.

Integrating thermodynamics and mathematical modelling to investigate the stoichiometry and kinetics of sulphide oxidation-nitrate reduction with a special focus on partial autotrophic denitrification

In the present study, the stoichiometry of the Sulphur Oxidizing-Nitrate Reducing (SO-NR) process, with a focus on Partial Autotrophic Denitrification (PAD), has been evaluated through a thermodynamic-based study whereas a model-based approach has been adopted to assess process kinetics. Experimental data on process performance and biomass yields were available from a previous work achieving efficient PAD, where a biomass yield of 0.113 gVSS/gS was estimated. First, the free Gibbs energy dissipation method has been implemented, in order to provide a theoretical framework exploring the boundaries for sulphur oxidizing biomass yields. Second, a screening of available mathematical models describing SO-NR process was conducted and five published models were selected, in order to assess the most suitable model structure for describing the observed PAD kinetics. To the best of our knowledge, none of reported biomass yields are estimated in systems operating PAD as the main process and, analogously, none of the proposed models have been applied to case studies aiming at partial denitrification only. The work showed that the very low biomass yield of 0.117 ± 0.007 gVSS/gS, observed in a PAD system in our previous work, suggests that the conditions applied to achieve partial denitrification resulted in a high energy-dissipating metabolism compared to complete denitrification applications. Models' analysis revealed that nitrite accumulation can be described by a classical Monod kinetics if different μmax are adopted for each intermediate reaction, with Theil Inequality Coefficient values lower than 0.21 for both NO3- and NO2-. Nonetheless, adopting Haldane-type kinetics for nitrite uptake inferred higher identifiability to the model structure, resulting in confidence intervals below ±10% for all the parametric estimations. The thermodynamic and modelling outcomes support the experimental results obtained in the reference study and the critical comparison of model suitability to represent PAD process is believed pivotal to pave the way to its real-scale implementation.

Multi-chambers of pilot-scale reactor enhanced partial nitritation performance

Nowadays, applying anammox to treat high nitrogenous side-stream wastewater has taken a step forward. However, the partial nitritation process is sensitive to the ammonium concentration and the nitrogen loading rate, which significantly influences the nitrogen removal performance. This study investigated the performance of a novel nitritation pilot-scale reactor which was divided into four chambers. The nitrite accumulation efficiency reached more than 90 % in the rural wastewater treatment process. As the reactor was divided into four chambers, the comprehensive statistical results showed that the concentration of free ammonium in the front chambers had been effectively improved. The proportion of free ammonium concentration (>0.1 mg NH₃·L^-1), which could inhibit the activity of nitrite oxidizing bacteria, in first chamber (PN1) was 2 times higher than in the last chamber (PN4). Meanwhile, Nitrosomonas, responsible for ammonium oxidation to nitrite, was highly enriched in the first two chambers even though the dissolved oxygen was maintained at 1.5 ± 0.3 mg·L^-1. Compare to conventional reactor, the resistance of the novel reactor to volumetric shock loading has been enhanced. Even though the ammonium loading rate fluctuated greatly, the effluent was still stable and could meet the demand following the anammox process. This study demonstrated that the reactor with multi-chambers could effectively improve the nitrite accumulation efficiency in the partial nitritation process and thus provide a new perspective on the partial nitritation process in a single reactor and further promote the anammox performance in the wastewater treatment process.

Response of nitrite accumulation to elevated C/NO- 3-N ratio during partial denitrification process: Insights of extracellular polymeric substance, microbial community and metabolic function

This study investigated the response of nitrite accumulation to elevated COD/NO₃^--N ratio (C/N) during partial denitrification (PD). Results indicated nitrite was gradually accumulated and remained stable (C/N = 1.5 ∼ 3.0), while that rapidly declined after reaching the peak (C/N = 4.0 ∼ 5.0). The polysaccharide (PS) and protein (PN) content of tightly-bound extracellular polymeric substances (TB-EPS) reached the maximum at C/N of 2.5 ∼ 3.0, which might be stimulated by high level of nitrite. Illumina MiSeq sequencing showed Thauera and OLB8 were dominated denitrifying genera at C/N of 1.5 ∼ 3.0, while Thauera was further enriched with fading OLB8 at C/N of 4.0 ∼ 5.0. Meanwhile, the highly-enriched Thauera might enhance the activity of nitrite reductase (nirK) promoting further nitrite reduction. Redundancy analysis (RDA) showed positive correlations between nitrite production and PN content of TB-EPS, denitrifying bacteria (Thauera and OLB8) and nitrate reductases (narG/H/I) in low C/N. Finally, their synergistic effects for driving nitrite accumulation were comprehensively elucidated.

Pilot-scale study of step-feed anaerobic coupled four-stage micro-oxygen gradient aeration process for treating digested swine wastewater with low carbon/nitrogen ratios

This study developed an innovative step-feed anaerobic coupled four-stage micro-oxygen gradient aeration process to treat digested swine wastewater. An anaerobic zone was used for prepositive denitrification; four micro-oxygen reactors (zones O1-O4) were used for simultaneous partial nitrification and denitrification through low-dissolved oxygen gradient control, step-feed, and swine wastewater-digested swine wastewater distribution. The nitrogen-removal efficiency was satisfactory (93 ± 3 %; effluent total nitrogen, 53 ± 19 mg/L). Mass balance coupled with quantitative polymerase chain reaction analysis revealed that simultaneous partial nitrification and denitrification was achieved in four micro-oxygen zones. Zones O1 were the major denitrification zones for nitrogen removal; nitrification was primary happened in zones O2 and O3. Correlation analysis confirmed that low-dissolved oxygen gradient control was the key to achieving efficient nitrogen removal. This study provides a low oxygen energy consumption method to treat digested swine wastewater with a low carbon/nitrogen ratio (<3).

Evaluation of sludge anaerobic fermentation driving partial denitrification capability: In view of kinetics and metagenomic mechanisms

Partial denitrification (PD) provides a promising approach of efficient and stable nitrite (NO₂^--N) generation for annamox. In this study, the feasibility of short-term sludge anaerobic fermentation driving PD was evaluated. It was found that a higher NO₂^--N accumulation in nitrate (NO₃^--N) reduction was obtained with the 5-days fermented sludge compared to 8 and 15-days fermentation. Moreover, compared to acetate as carbon source, sludge fermentation products (SFPs) induced the higher NO₂^--N production with nitrate-to-nitrite transformation ratio (NTR) nearly 100 %. Denitrification activity of fermented sludge were obviously improved with SFPs as electron donor. Metagenomic analysis revealed that Thauera was the dominant bacteria, which was assumed to be the key contributor to PD performance by harboring the highest narGHI and napAB but much lower nirS and nirK. Under the conditions of SFPs and fermented sludge, Thauera was speculated to have higher resistance than other denitrifiers attributed to versatile carbon metabolic capabilities utilizing SFPs with the significantly improved genes for metabolism of complex organic carbon via glycolysis after anaerobic fermentation. A novel integration of sludge fermentation driving PD and anammox for mainstream wastewater treatment and sidestream polishing was proposed to offer a promising application with reduced commercial carbon source consumption and waste sludge reduction.

Regulation of denitrification performance and microbial topology by lights: Insight into wavelength effects towards microbiota

The low efficiency of conventional complete denitrification, as well as the unstable nitrite supply for partial-denitrification coupled anammox (PD/A) restrict the efficient removal of nitrogen from industrial wastewaters. Herein, we proposed an optical strategy to bidirectionally regulate denitrification by introducing lights at different wavelengths, and the underlying mechanisms were elucidated accordingly. It turned out that yellow light at wavelength of 590 nm accelerated denitrification by 35.4%, while blue light delayed denitrification with stable nitrite accumulation above 86.9% and high nitrate removal (99.8%). Microbial physiology and viability further supported the positive effects of yellow light on microbial activity. Additionally, despite the sluggish denitrification aroused by blue light, negligible cellular damage was observed. Antioxidant capability divergence, microbial community shifting and metabolic flux redirection contributed to the wavelength-dependent effects. Halomonas and Pseudomonas were identified as high-credit taxonomic biomarkers of yellow and blue light. As revealed by metabolomics, pantothenate and CoA biosynthesis, glutamate metabolism and alkaloid biosynthesis presented high impact values. Co-analysis of metabolomics and metagenomics based on microbial topology further distinguished pivotal metabolic pathways and genes. Oxidative phosphorylation contributed to the divergent denitrification performance through electron transfer chains, whereas glutamate and glutathione metabolism contributed to oxidative stress alleviation and mediated the metabolic flux between peroxisome and nitrogen metabolism. This study shed a light on the application of optical strategy to regulate denitrification performance and achieve either complete denitrification or PD/A.

Superior nitrogen removal and efficient sludge reduction via partial nitrification-anammox driven by addition of sludge fermentation products for real sewage treatment

Efficient retention and enrichment of anammox bacteria (AnAOB) are essential for the application of municipal wastewater anammox. Herein, an innovative process for highly enriching AnAOB within suspended carrier was developed in a single-stage anaerobic/oxic/anoxic reactor with 5.5 % carrier filling ratio for real sewage. Addition of sludge fermentation products promoted stable maintenance of partial nitrification (nitrite accumulation rate > 90.0 %) and achieved efficient external sludge reduction (27.6-37.9 %). Continuous nitrite supply and carrier addition promoted AnAOB enrichment (2.4 × 1011 gene copies/g dry sludge). Candidatus Brocadia was the predominant bacteria in carriers (18.6 %). The average effluents of total inorganic nitrogen (TIN) and NH4+-N were 1.9 and 0.8 mg/L with removal rates of 97.0 % and 98.7 %. In the anoxic stage, TIN removal rate reached 71.5 %, and the proportion of anammox to nitrogen removal accounted for 82.7 %. This study broadens the application of mainstream sewage anammox and the resource utilization of waste activated sludge.

Novel synergistic metabolic processes for phenanthrene biodegradation by a nitrate-reducing phenanthrene-degrading culture and an anammox culture

The synergistic metabolism by anammox cultures and nitrate-reducers for anaerobic PAH biodegradation is largely unknown, including whether anammox culture and which kind of anammox bacterium can perform nitrogen metabolism in the anaerobic PAH biodegradation processes, the inhibitory effect of PAH on anammox activity and nitrite on PAH-degrading nitrate-reducers activity, and the synergistic metabolic processes. Herein, an anammox culture that can eliminate nitrite accumulation and decrease inorganic carbon emission during anaerobic phenanthrene (a model of PAH in this study) biodegradation, the synergistic mechanism for phenanthrene biodegradation by a nitrate-reducer and such anammox culture, and the inhibition effect of phenanthrene on such anammox culture and nitrite on a phenanthrene-degrading nitrate-reducer were newly discussed. The results showed that nitrite largely accumulated during anaerobic phenanthrene biodegradation (nitrate accumulation is a common phenomenon for the biodegradation of refractory matter, including PAHs, by nitrate-reducers) by a nitrate-reducer, PheN2, which mineralizes phenanthrene to inorganic carbon, and nitrite was verified as an inhibiting factor for further biodegradation. Anaerobic phenanthrene biodegradation rates and nitrite concentrations (0-7 mM) appeared to have a negative linear correlation. The anammox culture that mainly contained Candidatus Kuenenia was newly found to efficiently reduce nitrite accumulation and inorganic carbon emissions and significantly promote biodegradation efficiency by ∼1.94-fold. Our results showed that phenanthrene absorbed in and on anammox cells had a more direct relationship with the inhibitory effect on anammox activity than phenanthrene in the environment, and 15.2 mg/gVSS phenanthrene absorbed in and on the cells (4 mM concentration in the culture) showed nearly complete inhibition of anammox culture in this study. In addition, few (less than 2% abundance) anammox bacteria were found to be enough for the removal of nitrite produced from anaerobic phenanthrene biodegradation. In an ideal world, co-pollutants of ammonia, nitrate, phenanthrene, and nitrite could be converted to nitrogen gas and biomass by the synergistic metabolism of anammox cultures and nitrate reducers. Our study reveals a new synergistic process that may exist in our environments for PAH elimination by an anammox culture and a nitrate-reducer, which provides a new strategy for the bioremediation of PAH-polluted anoxic zones.

Rapid enrichment of anammox bacteria and transformation to partial denitrification/anammox with nitrification/denitrification sludge

The stable nitrite (NO₂^--N) generation and rapid startup of anammox-based process are the main bottlenecks hindering its application in mainstream municipal wastewater treatment. In this study, a Partial-Denitrification (PD) system reducing nitrate (NO₃^--N) to NO₂^--N was rapidly developed within 40 days, using the nitrification/denitrification sludge from wastewater treatment plant. The NO₃^--N to NO₂^--N transformation ratios achieved 80.6 %. Significantly, a fast self-enrichment of anammox bacteria in this system was subsequently obtained, resulting in the successful transformation to an efficient PD/Anammox (PD/A) process after 79-day operation. The total nitrogen removal efficiency increased from 12.4 % to 90.0 % with influent ammonia and nitrate of 45.9 mg N/L and 62.2 mg N/L, corresponding to the anammox activity significantly increasing to 6.0 mgNH₄⁺-N/g VSS/h without seeding anammox sludge. Abundance of anammox increased from 6.7 × 10⁸ to 2.0 × 10¹¹ copies/g dry sludge. High-throughput sequencing results showed that Candidatus Brocadia was the only known anammox genus and accounted for 1.08 % during the PD/A stage. Functional bacteria for PD, assumed to be the Thauera, was enriched from 1.99 % to 60.06 % but decreased to 32.49 % during the improvement of anammox activity. It demonstrated that the PD system with stable NO₂^--N accumulation enabled a rapid self-enrichment of anammox bacteria and sufficient nitrogen removal with ordinary nitrification/denitrification sludge. This provides new insights into the scaling application of anammox by integrating PD with shortened startup periods and improved TN removal efficiency.

Accelerating denitrification and mitigating nitrite accumulation by multiple electron transfer pathways between Shewanella oneidensis MR-1 and denitrifying microbial community

The bio-denitrification was usually retarded by the unbalance of electron generation and consumption. In this study, mixing S. oneidensis MR-1 with denitrifying microbial community increased the nitrogen removal rate by 74.74 % via the interspecies electron transfer (IET), and reduced the accumulated nitrite from 9.90 ± 0.81 to 0.02 ± 0.03 mg/L. Enhanced denitrification still appeared but relatively decreased, when S. oneidensis MR-1 was separated by a dialysis bag (MW < 3000), indicating mediated interspecies electron transfer (MIET) counted in IET. The results of electron transfer activity and sludge conductivity suggested DIET and MIET jointly transfer electrons from MR-1 to electroactive denitrifying bacteria (EDB), improving denitrifying reductase activities. Electron distribution among denitrifying reductases was found to be associated with the IET rate. Microbial insights showed the total abundance of EDB was increased, and denitrifying genes were correspondingly enriched. Pseudomonas was found to cooperate with exoelectrogens in a complicated microbial community.

Bioaugmentation for low C/N ratio wastewater treatment by combining endogenous partial denitrification (EPD) and denitrifying phosphorous removal (DPR) in the continuous A2/O - MBBR system

Endogenous partial denitrification (EPD) and denitrifying phosphorous removal (DPR) were combined in a novel A²/O - MBBR (Anaerobic Anoxic Oxic - Moving Bed Biofilm Reactor) system for low carbon/nitrogen (C/N) ratio wastewater treatment. The DPR performance was compared and the nutrient metabolism was elucidated based on the optimization of hydraulic retention time (HRT, 4-12 h) and nitrate recycling (R, 200%-600%). In the continuous-flow, the nitrate (NO₃^-) denitrification accompanied by nitrite (NO₂^-, via EPD) accumulation with the nitrate-to-nitrite transformation ratio (NTR) of 35.87%-43.31% in the anoxic zones. At HRT of 12 h with R of 500%, batch test initially revealed the DPR mechanism using both NO₃^- and NO₂^- as electron acceptor, where denitrifying phosphorus accumulation organisms (DPAOs) and denitrifying glycogen accumulation organisms (DGAOs) were the main contributors for EPD with incomplete denitrification (NO₃^- → NO₂^-). Furthermore, stoichiometry-based functional bacteria analysis displayed that higher bioactivity of DPAOs (NO₂^-→N₂, 57.30%; NO₃^-→N₂, 35.85%) over DGAOs (NO₃^-→N₂, 6.85%) facilitated the anoxic NO₃^- reduction. Microbial community analysis suggested that Cluster I of Defluviicoccus-GAO group (∼4%) was responsible for stable NO₂^- accumulation performance via EPD, while increased Accumulibacter-PAO group (by ∼15%) contributed to the advanced nutrient removal. Based on the achievement of NO₂^- accumulation, the application feasibility of integrated EPD - DPR - Anammox for deep-level nutrient removal was discussed.

Successful sulphide-driven partial denitrification: Efficiency, stability and resilience in SRT-controlled conditions

Partial denitrification is emerging as a valuable solution for NO₂^- supply in Anammox systems. When reduced sulphur compounds are used as electron donors, S-driven Partial Autotrophic Denitrification (PAD) can also be achieved, allowing for an integrated autotrophic nitrogen (N) and sulphur (S) removal from liquid and gaseous streams. The aim of the present work was to maximise NO₃^- reduction to NO₂^- coupled with complete HS^- oxidation, by the selective control of influent S/N ratio and sludge retention time (SRT). A 2.5-L chemostat was operated for 115 days and three operational phases were tested at decreasing SRT of 40, 23 and 13 h, testing S/N ratios in the range of 0.5-1 gS/gN. Successful sulphide-driven PAD was achieved and lead to average NO₂^- conversion efficiencies as high as77±17% at all the conditions tested, with the highest value of 99% at the lowest S/N of 0.58 gS/gN and SRT of 23 h. Respirometric tests showed that NO₃^- uptake rate was stable at 90±10 mgN/gVSS/h, when NO₃^- was present as sole electron acceptor or at NO₂^- levels as high as 120 mgN/l; on the contrary, NO₂^- uptake rates were very sensitive to the applied conditions. Metabarcoding analyses revealed that the microbial community was highly enriched in Sulphur Oxidizing Bacteria (SOB>80%) and stable S-limiting conditions appeared to favour Thiobacillus over Sulfurimonas genus. A preliminary assessment of N₂O potential emission was also performed. To the best of our knowledge, this is the first work evaluating the synergic effect of SRT and influent S/N ratio on nitrite accumulation in highly SOB-enriched systems and the NO₂^- conversion efficiencies achieved are among the highest reported in literature.

Enrichment of a denitratating microbial community through kinetic limitation

Denitratation, or the intentionally engineered accumulation of nitrite (NO₂^-) from selective reduction of nitrate (NO₃^-), can be combined with downstream anammox to reduce chemical and energy use associated with conventional nitrification and denitrification. This study aimed to enrich a denitratating microbial community capable of significant NO₂^- accumulation by applying added kinetic limitation to an already stoichiometrically-limited, glycerol-driven denitratation process. Operation at solids residence time, SRT=3.0 d, resulted in optimal denitratation performance and a microbial community dominated by NO₃^--respirers, noted by one order of magnitude lower total copy numbers of nirS and nirK gene transcripts compared to longer SRTs. Selective NO₃^- reduction to NO₂^- was achieved at all SRTs although longer SRTs (less kinetic limitation) supported microbial communities more capable of full denitrification as described by a lower NO₂^- accumulation ratio (NAR=42±5%) and higher steady-state nitrous oxide (1.5 mg/L N₂O-N) accumulation. Shorter SRTs (more kinetic limitation) led to higher observed yields (Y=0.63 mg-COD/mg-COD) with more electrons dedicated for cell synthesis (f_s=0.56±0.10), which potentially contributed to the accumulation of NO₃^-. Enrichment of a denitratating-dominant microbial community by optimizing kinetic limitation operating parameters could support significant NO₂^- accumulation and reduce chemical and energy use for biological nitrogen removal when combined with downstream anammox.

Denitrification performance and mechanism of denitrification biofilm reactor based on carbon-nitrate counter-diffusional

This study researched denitrification performance and mechanism of denitrification biofilm reactor with different HRTs and carbon sources dosages. Experimental group (EG) had better nitrate and COD removal performance than control group (CG) with different HRTs or carbon doses, and the maximum nitrate-to-nitrite transformation ratio (NTR) of them reached 7.91 ± 1.60% and 17.50 ± 1.92%, respectively. Because organic carbon sources were added to the carrier's interior in EG, forming high local concentrations in biofilms and counter-diffusional with nitrate. By contrast, carbon sources and nitrate were provided from the aqueous phase in CG. Thus, the EG system has more active regions of the biofilm than CG. In addition, EG had higher proportions of microorganisms and enzymes related to denitrification and carbon metabolism. The most dominant phylum, genus, and species were Proteobacteria, Thaurea, and Thauera_sp._27, respectively. The transcript of acetyl-CoA synthetase (K01895) and denitrification (M00529) was mainly originated from unclassified_g__Pseudomonas and unclassified_g__Thauera, respectively.

Mitigating nitrite accumulation during S0-based autotrophic denitrification: Balancing nitrate-nitrite reduction rate with thiosulfate as external electron donor

This study was carried out to determine the effect of influent nitrate loading on nitrite accumulation during elemental-sulfur based denitrification process, and proposed to enhance the nitrogen removal efficiency by mitigating nitrite accumulation with thiosulfate as external electron donor. Along with increasing the nitrate influent loading (from 0.09 kg N/m³/d to 1.73 kg N/m³/d) by shortening the empty bed contact time (EBCT) (from 5 h to 0.25 h), the nitrate removal loading increased from 0.08 to 0.83 kg N/m³/d. Meanwhile, the raise of the nitrate influent loading obviously aggravated the nitrite accumulation. Herein, nitrite began to accumulate since the nitrate influent loading was over 0.86 kg N/m³/d, and a maximum nitrite accumulation of 2.39 mg/L was observed under the 0.25 h of EBCT and 15 mg/L of nitrate influent concentration condition. Thiosulfate was used as the external electron donor to accelerate the nitrite reduction rate in order to mitigate the nitrite accumulation. As a result, the nitrite accumulation significantly decreased from 2.39 mg/L to 0.17 mg/L with the thiosulfate dosage of 13.36 mg/L. However, the nitrite accumulation bounced with the on-going increase of the thiosulfate dosage, indicating that the nitrate reduction rate and nitrite reduction rate were accelerated alternatively. After dosing thiosulfate, the relative abundances of sulfurimonas and ferritrophicum grew up significantly.

Efficient nitrite accumulation and elemental sulfur recovery in partial sulfide autotrophic denitrification system: Insights of seeding sludge, S/N ratio and flocculation strategy

Partial sulfide autotrophic denitrification (PSAD) has been proposed as a promising process to achieve elemental sulfur recovery and nitrite accumulation, which is required for anaerobic ammonium oxidation reaction. This study investigated the effect of seeding sludge on the start-up performance of PSAD process, with different sludge taken from the oxidation zone (S-o) of wastewater treatment plants, partial denitrification reactor (S-_PD), and anoxic/oxic reactor (S-_A/O). The results showed that the PSAD process could be achieved rapidly in three systems on day 22, 29 and 26, respectively. In particular, the S-_O system completed the start-up in the shortest time of 22 d, with NO₃^--N and S^2- removal efficiency of 85.3% and 99.3%, respectively. Selected the S-_O system to operate long term, the nitrite (NO₂^--N) and biological elemental sulfur (S⁰) accumulation efficiencies were systematically investigated under different S/N ratios (in a range of 0.71-1.2). The maximum NO₂^--N and S⁰ accumulation efficiencies were 85.2% and 73.5%, respectively, at the S/N ratio of 1.1. In addition, the separation and recovery of S⁰ in effluent was achieved by employing polyaluminum chloride (PAC) as a flocculant. Using 2D Gaussian function as quadratic model for the maximizing of S⁰ flocculant efficiency (SFR), an optimal condition of PAC dosage 7.92 mL/L and pH 5.14 was obtained, and the SFR reached 94.1%, under such conditions. The findings offered useful information to facilitate the application of the PSAD process.

N2O reduction during denitrifying phosphorus removal with propionate as carbon source

Denitrifying phosphorus removal was realized in sequencing batch reactors using different carbon sources (acetate, propionate, and a mixture of acetate/propionate). Nutrient removal and nitrous oxide (N₂O) production were investigated, and the factors affecting N₂O production were explored. Nitrogen removal was 40.6% lower when propionate was used as the carbon source instead of acetate, while phosphorus removal was not significantly different. N₂O production was greatly reduced when propionate was used as the carbon source instead of acetate. The emission factor in the propionate system was only 0.43%, while those in the acetate and mixed-carbon source system were 16.3% and 1.9%, respectively. Compared to the propionate system, ordinary heterotrophic organisms (i.e., glycogen-accumulating organisms) were enriched in the acetate system, explaining the higher N₂O production in the acetate system. The lower nitrite accumulation in the propionate system compared to the acetate system was the dominant factor leading to the lower N₂O production.

Hydroxylamine metabolism in mainstream denitrifying ammonium oxidation (DEAMOX) process: Achieving fast start-up and robust operation with bio-augmentation assistance under ambient temperature

Nitrogen removal from mainstream wastewater via DEnitrifying AMmonium OXidation (DEAMOX) is often challenged by undulated actual temperature and high loading rate. Here, we discovered NH₂OH addition (HA) and bio-augmentation (BA) tactics on start-up and operation performance of DEAMOXs (R1 and R2) under ambient temperature (11.3-31.7 °C). Over 340-day operation suggested that R2 received 10 mg/L HA and 1:25 BA ratio (v/v, anammox/partial denitrification sludge) achieved desirable nitrogen removal efficiency (NRE) of 97.22% after 145-day, while R1 under higher BA ratio of 1:12.5 without HA obtained lower NRE (90.86%) after 184-day. Batch tests revealed that nitrate-nitrite transformation ratio reached 98.64% at low COD/NO₃^--N of 2.6 with HA. Significantly, compared with R2, R1 recovered quickly with satisfactory effluent total nitrogen of 4.21 mg/L despite nitrogen loading rate greater than 0.15 kg N/m³/d and temperature decreased to 14.6 °C. The abundant narG represented high nitrate reduction potential, hzsA and hdh were extensively detected as the symbolisation of anammox metabolism. Thauera, Denitratisoma and unclassified f Comamonadaceae dominated nitrite accumulation. Ca. Brocadia as the dominant anammox bacteria, and its population maintained stable against low temperature and load shocks by NH₂OH intensification. Overall, this study offers an opportunity for the wide-applications of DEAMOX treating mainstream wastewater.

Non-submerged attached growth process for domestic wastewater treatment: Influence of media types and internal recirculation ratios

This study is aimed to comprehend the treatment of non-submerged attached growth systems using bio-sponge, bio-cord, and bio-cloth media. Three reactors were set up with internal recirculation ratio of 1 (IR = 1) and similar media surface area. Bio-sponge and bio-cloth reactors showed removal of COD (79 vs. 76%) and NH₄⁺-N (78 vs. 73%). While bio-cord treatment was deteriorated due to time-dependent process. Multiple linear regression revealed that alkalinity governed the formation degree of the anaerobic zone in bio-sponges, partially affecting nitrification. Increasing IR from 1 to 3 caused sloughing of the attached biomass and was positively correlated with effluent nitrite nitrogen concentration, indicating the sensitivity of nitrification to spatial distribution effects. In addition, bio-sponge system obtained superior performance at IR of 2 while bio-cloth one might be also an effective media for wastewater treatment if having good durability.

Perturbation of clopyralid on bio-denitrification and nitrite accumulation: Long-term performance and biological mechanism

The contaminant of herbicide clopyralid (3,6-dichloro-2- pyridine-carboxylic acid, CLP) poses a potential threat to the ecological system. However, there is a general lack of research devoted to the perturbation of CLP to the bio-denitrification process, and its biological response mechanism remains unclear. Herein, long-term exposure to CLP was systematically investigated to explore its influences on denitrification performance and dynamic microbial responses. Results showed that low-concentration of CLP (<15 mg/L) caused severe nitrite accumulation initially, while higher concentrations (35-60 mg/L) of CLP had no further effect after long-term acclimation. The mechanistic study demonstrated that CLP reduced nitrite reductase (NIR) activity and inhibited metabolic activity (carbon metabolism and nitrogen metabolism) by causing oxidative stress and membrane damage, resulting in nitrite accumulation. However, after more than 80 days of acclimation, almost no nitrite accumulation was found at 60 mg/L CLP. It was proposed that the secretion of extracellular polymeric substances (EPS) increased from 75.03 mg/g VSS at 15 mg/L CLP to 109.97 mg/g VSS at 60 mg/L CLP, which strengthened the protection of microbial cells and improved NIR activity and metabolic activities. Additionally, the biodiversity and richness of the microbial community experienced a U-shaped process. The relative abundance of denitrification- and carbon metabolism-associated microorganisms decreased initially and then recovered with the enrichment of microorganisms related to the secretion of EPS and N-acyl-homoserine lactones (AHLs). These microorganisms protected microbe from toxic substances and regulated their interactions among inter- and intra-species. This study revealed the biological response mechanism of denitrification after successive exposure to CLP and provided proper guidance for analyzing and treating herbicide-containing wastewater.

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.

Aerobic granulation of nitrifying activated sludge enhanced removal of 17α-ethinylestradiol

The positive correlation between the nitrification activity of activated sludge and 17α-ethinylestradiol (EE2) removal has been widely reported. However, up to now the effect of the granulation of nitrifying activated sludge (NAS) on EE2 removal has not been determined. In this study, nitrifying granular sludge (NGS) exhibited more effective EE2 removal efficiency with 3.705 μgEE2∙(gMLSS∙h)^-1 in a sequential batch reactor (SBR). Through the artificial neural network (ANN) model and Spearman correlation analysis, nitrite accumulation was demonstrated to be the key factor affecting EE2 removal. Notably, under the same aeration condition (0.15 L/min), nitrite accumulation was more easily achieved in NGS because of its dense structure. Full-length 16S rRNA gene sequencing suggested that EE2 could strongly influence the microbial communities of NAS and NGS. NGS exhibited an increase in community diversity and richness, but NAS exhibited a decrease. In addition, the relative abundance of Nitrosomonas (ammonia-oxidizing bacteria, AOB) decreased considerably in both NAS and NGS, whereas the expression of amoA and nirK genes in Nitrosomonas was upregulated. It was suggested that Nitrosomonas was forced to regulate its gene expression to resist the negative effects of EE2. Denitrifying bacteria, such as Comamonas, were enriched in both NAS and NGS, and there were more species of heterotrophs that can degrade micropollutants in NGS with exposure to EE2. The transformation pathways of EE2 were uniform in NAS and NGS. Ammonia monooxygenase (AMO) in AOB directly biotransformed EE2 while reactive species produced by AOB chemically transformed EE2. Heterotrophs degraded EE2 and its transformation products (TPs) generated by AOB. According to TPs and microbial structure, NGS exhibited better performance than NAS regarding the collaborative removal of EE2 by AOB and heterotrophs. These results provide important information for the development and application of NGS to treat wastewater containing estrogen and high-strength ammonium.

Metagenomics reveals functional species and microbial mechanisms of an enriched thiosulfate-driven denitratation consortia

In this study, thiosulfate-driven denitratation (TDD) system was successfully established under optimal S/N molar ratio of 1.00, with nitrite accumulation efficiency (NAE) of 82.24 ± 17.09%. This work highlighted that thiosulfate significantly preferred the reduction of nitrate than nitrite. However, after the depletion of thiosulfate, the in-situ formed intermediate product element sulfur (S⁰) served as the main electron donor, and significantly favored the reduction of nitrite than nitrate, which constrained nitrite accumulation and nitrate removal. In addition, metagenomic sequencing revealed that the functional denitratation species might be Thiobacillus_sp._65-29, but the occurrence of Nir-annotated species would decrease nitrite accumulation. Under S/N ratio of 1.00, the decreased abundant Nir-annotated species (e.g., Thiobacillus_sp.), as well as the down-regulated quorum sensing interactions between Nar- and Nir-annotated species were key microbial metabolisms of high NAE in the TDD system. Overall, this work provides new sight into the metagenome-base functional species and metabolic potential of thiosulfate-driven denitratation.

Application of humin-immobilized biocathode in a continuous-flow bioelectrochemical system for nitrate removal at low temperature

Solid-phase humic substances (humin) can work as an additional electron donor to support the low temperature denitrification but the reducing capacity of its non-reduced form is limited. In this study, a continuous-flow denitrifying BES with a humin-immobilized biocathode (H-BioC) was established. Humin was expected to function as a redox mediator and be persistently reduced on the cathode to provide reducing power to a denitrifying biofilm. Results showed that the H-BioC maintained a stable denitrification capacity with low nitrite accumulation for more than 100 days at 5 °C, and the specific microbial denitrification rate and electron transfer rate were 3.97-fold and 1.75-fold higher than those of the unaltered cathode. The results of repeated cycles of humin reduction and oxidation experiments further suggested that the redox activity of humin was stable. Acidovorax was the most dominant genus in both H-BioC biofilm and unaltered cathodic biofilm, while Rhodocyclaceae (unclassified_f_) was more enriched in H-BioC biofilm. Further Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analyses indicated that biofilm formation, electron transfer, and nitrate reduction functions were more abundant in H-BioC, suggesting a possible enhancement mechanism by humin. The results of this study raise the possibility that immobilization of solid-phase humin may be a useful strategy for electrostimulated heterotrophic denitrification in groundwater where the indigenous bacteria have poor electroactivity.

A predictive model for N2O production in anammox-granular sludge reactors: Combined effects of nitrite/ammonium ratio and organic matter concentration

Once the use of anammox reactors has been increasing on a global scale, it is important to understand the mechanisms of N₂O emissions and how to minimise the emissions by optimising the operating conditions. In this study, the influence of chemical oxygen demand (COD) (from 0 mgO₂ L^-1 to 100 mgO₂ L^-1) and nitrite/ammonium ratio from 0.79 to 2.21 (maintaining ammonium at 100 mgN L^-1 and varying nitrite from 79 mgN L^-1 to 221 mgN L^-1) in the N₂O emissions from anammox-granular sludge reactor was investigated in two steps. Step 1 consisted of batch tests, using central composite design, and Step 2, long-term operation of a 6.5 L continuous up-flow reactor. The results showed that the N₂O emissions were minimized by controlling, in the influent, the NO₂^--N/NH₄⁺-N ratio from 1.1 to 1.3 and maintaining the COD concentration below 100 mgO₂ L^-1. TN removal efficiencies were higher than 70% in all conditions tested".

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.

Realization of nitrite accumulation in a sulfide-driven autotrophic denitrification process: Simultaneous nitrate and sulfur removal

The study was based on the removal of nitrate and sulfide, and aimed to nitrite accumulation. The process of autotrophic denitrification driven by sulfide as an electron donor was investigated in a sequencing batch reactor. The research showed that autotrophic denitrification successfully started on day 22, and the removal rates of NO₃^--N and S^2--S were 95.8% and 100%, respectively, when the S/N molar ratio was 1.45. When the S/N ratio was reduced to 0.94, the phenomenon of NO₂^--N accumulation was observed. NO₂^--N continuously accumulated, and the maximum accumulation rate was 55.3% when the S/N ratio was 0.8. In the batch test, the study showed that NO₂^--N accumulation was optimal when the S/N ratio was 0.8, and the NO₂^--N concentration increased with increasing NO₃^--N concentration at the same S/N ratio. Microbial communities also changed based on the high-throughput analysis, and Proteobacteria (59.5%-84%) was the main phylum. Arenimonas (11.4%-28.2%) and uncultured_f_ Chromatiaceae (5.7%-27.5%) were the dominant bacteria, which complete denitrification and desulfurization throughout the operating system. Therefore, this study provided a theoretical basis for the simultaneous removal of NO₃^--N and S^2--S, as well as the accumulation of nitrite, and provided material support for anaerobic ammonia oxidation technology.

Biochar amendment mitigated N2O emissions from paddy field during the wheat growing season

Biochar may variably impact nitrogen (N) transformation and N-cycle-related microbial activities. Yet the mechanism of biochar amendment on nitrous oxide (N₂O) emissions from agricultural ecosystems remains unclear. Based on a 6-year long-term biochar amendment experiment, we applied a dual isotope (¹⁵N-¹⁸O) labeling technique with tracing transcriptional genes to differentiate the contribution of nitrifier nitrification (NN), nitrifier denitrification (ND), nitrification-coupled denitrification (NCD) and heterotrophic denitrification (HD) pathway to N₂O production. Then the field experiment provided quantitative data on dynamic N₂O emissions, soil mineral N and key functional marker gene abundances during the wheat growing season. By using ¹⁵N-¹⁸O isotope, biochar decreased N₂O emission derived from ND (by 45-94%), HD (by 35-46%) and NCD (by 30-64%) compared to the values under N application. Biochar increased the relative contribution of NN to total N₂O production as evidenced by the increase in ammonia-oxidizing bacteria, but did not influence the cumulative NN-derived N₂O. The field experiment found that the majority of the N₂O emissions peaked following fertilization, in parallel with soil NH₄⁺ and nitrite dynamics. Soil N₂O emissions during the wheat growing stage were effectively decreased (by 38-48%) by biochar amendment. Based on the correlation analyses and random forest analysis in both microcosm and field experiments, the decrease in nitrite concentration (by 62-65%) and increase in N₂O consumption were mainly responsible for net N₂O mitigation, as evidenced by the decrease in the ratios of nitrite reductase genes/transcripts (nirS, nirK and fungal nirK) and N₂O reductase gene/transcripts (nosZI and nosZII). Based on the extrapolation from microcosm to field, biochar significantly mitigated N₂O emissions by weakening the ND processes, since NCD and HD contributed little during the N₂O emission "peaks" following urea fertilization. Therefore, emphasis should be put on the ND process and nitrite accumulation during N₂O emission peaks and extrapolated to all agroecosystems.

A novel strategy for enhancing the partial denitrification to treat domestic wastewater by feeding sludge fermentation liquid

Partial-denitrification (PD; NO₃^-→NO₂^-) has recently been proposed to be a feasible choice of NO₂^--N supply for anammox bacteria. In this study, an aerobic/anoxic process for treating domestic wastewater was operated for 176 days to evaluate the feasibility of using sludge fermentation liquid for partial denitrification of the wastewater. Results show that, with the ratio of C/N (COD/ NO₃^--N) increased at anoxic stage, the average NO₂^--N concentration in the effluent and nitrate-to-nitrite transformation ratio (NTR) at anoxic stage showed relative growth. High-throughput sequencing analysis demonstrated that the enhancement of PD can be explained by the increases of Thauera, Paracoccus and Enterobacteriaceae. Moreover, Candidatus_Brocadia (0.13%) was detected as the predominant anammox bacteria. Ex-situ isotopic tracing technique analysis assessed that the ratio of anammox role (ra%) was 7.29%. This study has a great potential for being coupled with the anammox bacteria for advanced nitrogen removal.

Achieving efficient nitrite accumulation in glycerol-driven partial denitrification system: Insights of influencing factors, shift of microbial community and metabolic function

Partial denitrification (PD), which could provide sufficient nitrite for subsequent anaerobic ammonium oxidation, is a novel strategy for mainstream nitrogen removal. In this study, the performance of using glycerol as electron donor for nitrite accumulation in PD process was evaluated. Results showed that a C/N of 4.5 was effective for nitrite production (average nitrite accumulation rate: 34.32 mg N h^-1 gMLVSS^-1; average nitrate-to-nitrite transformation ratio (NTR): 91.1%) with pH ranging from 6.0 to 9.0. Also, a stable nitrite accumulation was achieved in long-term operation with the average NTR of 80.1%. Mechanism investigation found that the denitrifying bacteria Saccharibacteria (77.9%) was enriched in glycerol-driven reactors. Moreover, the enzymatic activity as well as the encoding genes (i.e. narG, narH and napA) involved in nitrate reduction were much higher than that for nitrite reduction (i.e. nirK), and this disparity was responsible for the efficient nitrite accumulation in glycerol-driven PD system.

Performance and microbial structure of partial denitrification in response to salt stress: Achieving stable nitrite accumulation with municipal wastewater

The effects of inorganic salts on partial denitrification (PD) was investigated in a sequencing batch reactor for simultaneously treating saline nitrate sewage and municipal wastewater. After 230-day operation, a high nitrate-to-nitrite transformation ratio (NTR) of ~ 80% was achieved with the salinity of 1.25 wt% and the initial chemical oxygen demand to nitrate ratio of 3.7. Microbial community analysis revealed that, Thauera was remained predominant in PD system but with a relative abundance decreasing from 53.02% (0.0 wt%) to 42.36% (1.25 wt%). Moreover, as a suitable ratio of nitrite to ammonia (~1.6) in effluent was obtained, it would be a promising method to treat saline nitrate sewage by combing PD with anammox.

Enhancing the nitrogen removal of anammox by treating municipal wastewater with sludge fermentation products in a continuous flow reactor

In this study, a novel process was developed to treat real sewage with a low chemical oxygen demand/total nitrogen ratio (COD/TN = 3.2) and to obtain enhanced nitrogen removal through Anaerobic ammonia oxidation (anammox). Anaerobic/aerobic/anoxic/aerobic (AOAO) reactor processes were amended with a fixed anammox biofilm in the anoxic zone. During an operational period of 212 days, an average effluent TN of 13.7 mg/L with a removal efficiency of 72.0% was obtained with an influent of 47.0 mg/L ammonium. Mass balance analysis suggested that the anammox resulted in removal of 33.6% of the TN. Besides, by adding sludge fermentation products, nitrite accumulation occurred via nitration while in the aerobic zone for the anammox process. This study demonstrated an alternative way to apply a sewage anammox process via excess sludge fermentation products triggering nitrite production in a continuous flow reactor.

Exploring the linkage between free nitrous acid accumulation and nitrous oxide emissions in a novel static/oxic/anoxic process

In this work, four batch tests were conducted to comprehensively explore the effects of free nitrous acid (FNA) accumulation on nitrous oxide (N₂O) emissions in a novel energy-saving and N₂O-reducing static/oxic/anoxic (SOA) process. With the accumulation of FNA, the N₂O emission factor increased from 1.51% to 4.32%, and the N₂O emission ratio contributed by ammonia-oxidizing bacteria (AOB) increased from 74.0% to 78.6%, accordingly. Mechanism studies show that produced FNA and weakened aerobic metabolism bring synergy to competition between reductases. Aeration conditions and FNA cytotoxicity exert a greater impact on nitrite-oxidizing bacteria than on AOB, thus enhancing the potential for nitrite accumulation. Considering the removal of nitrogen and phosphorus and the reduction of N₂O emissions in the SOA process, it is feasible to keep the average dissolved oxygen above 2.0 mg/L under the premise of nitrite accumulation.

Achieving partial denitrification-anammox in biofilter for advanced wastewater treatment

Recently, partial denitrification (PDN) - anaerobic ammonium oxidation (anammox) process has been widely studied in activated sludge for nitrate wastewater treatment. However, achieving PDN-Anammox in biofilter for domestic wastewater treatment was never reported. In this study, two lab-scale PDN biofilter and Anammox biofilter were built up to treat simulated domestic wastewater. The results showed that stable nitrogen removal performance was kept with averaged effluent nitrogen of 10.2 mg/L. Stable nitrite accumulation performance was achieved with low abundance of nitrite reductase gene, while influent composition influenced nitrogen transformation pathway in PDN biofilter. When treating domestic wastewater, nitrification and partial denitrification led to the higher nitrite accumulation ratio of 75.4%. The percentage contribution of anammox biofilter was 74.6% for nitrogen removal, and Candidatus Brocadia was dominant genus. After long-term operation, limited substrate concentration caused interspecific competition among various anammox bacteria, leading to an increasing proportion of Candidatus Brocadia fulgida. PDN-Anammox biofilter is a feasible process to advanced wastewater treatment, which could save aeration consumption and carbon source addition, and reduce sludge production.

In situ enrichment of anammox bacteria in anoxic biofilms are possible due to the stable and long-term accumulation of nitrite during denitrification

In situ enrichment of anammox bacteria in anoxic biofilms has been observed, but the specific conditions for anammox competition with denitrification for nitrite are not yet fully understood. Therefore, an anoxic sequencing batch biofilm reactor (SBBR) was used to investigate nitrite production during denitrification. In each SBBR cycle, with nearly 80% of nitrate reduced in 3 h, over 9.7 mg/L nitrite was gradually accumulated and maintained for a long time, despite temperatures gradually decreasing from 32 to 5 ℃. The long-term existence of nitrite was due to the low biofilm nitrite reduction rate (1.2 mgN gVSS^-1 h^-1), which was about 10-fold less than the nitrate reduction rate. Accordingly, nitrite reduction via denitrifiers was continuously suppressed, which was favorable for nitrite reduction through the anammox pathway. Indeed, anammox bacteria were successfully enriched here (Candidatus_Brocadia, 0.1%). This study confirms the potential of anoxic biofilm in enriching anammox bacteria and provides insight into understanding.

Coupling of sulfur(thiosulfate)-driven denitratation and anammox process to treat nitrate and ammonium contained wastewater

This study investigated the feasibility of a new biological nitrogen removal process that integrates sulfur-driven autotrophic denitratation (NO₃^-→NO₂^-) and anaerobic ammonium oxidation (Anammox) for simultaneous removal of nitrate and ammonium from industrial wastewater. The proposed sulfur(thiosulfate)-driven denitratation and Anammox process was developed in two phases: First, the thiosulfate-driven denitratation was established in the UASB inoculated with activated sludge and fed with ammonium, nitrate and thiosulfate for 52 days until the nitrite level in the effluent reached 32.1 mg N/L. Second, enriched Anammox biomass was introduced to the UASB to develop the integrated thiosulfate-driven denitratation and Anammox (TDDA) bioprocess (53-212 d). Results showed that nitrate and ammonium could be efficiently removed from synthetic wastewater by the integrated TDDA system at a total nitrogen (TN) removal efficiency of 82.5 ± 1.8% with an influent NH₄⁺-N of 101.2 ± 2.2 mgN/L, NO₃^--N of 101.1 ± 1.5 mgN/L and thiosulfate of 202.5 ± 3.2 mg S/L. It was estimated that Anammox and autotrophic denitritation (NO₂^-→N₂) contributed to about 90% and 10% of the TN removal respectively at stable operation. The established TDDA system was further supported by high-throughput sequencing analysis that sulfur-oxidizing bacteria (e.g., Thiobacillus and Sulfurimonas) coexisted with Anammox bacteria (e.g., Ca. Kuenenia and Ca. Anammoxoglobus) in this syntrophic biocenosis. Additionally, batch experiments were conducted to reveal the kinetic rates and to reconcile the stoichiometry of the electron donor/acceptor couples of the TDDA process. The results unraveled the mechanisms in the new bioprocess: i) sulfite and elemental sulfur (S⁰) were initially generated from branched thiosulfate; ii) oxidation of sulfite and elemental sulfur coupled with fast and slow denitratation; iii) nitrite produced from denitratation together with ammonium were effectively converted to dinitrogen gas via Anammox.

New insights on the sludge fermentation liquid driven denitrification: Evaluation of the system performance and effluent organic matter (EfOM)

Sludge fermentation liquid (FL) is a promising and sustainable carbon source for biological denitrification. In this study, four types of FLs were prepared using different chemical treatments to enhance denitrification. The profile of effluent organic matter (EfOM) were evaluated. Results indicate that the FL_NaOH (NaOH treatment at pH 10) contained the maximum amount of soluble organics, but led to the worst nitrate removal performance. Nitrate removal efficiency with FL_control was much higher than that of FL_CaO2, FL_Ca(OH)2, and FL_NaOH. On the other hand, it was found some organics in FL was hard to be utilized by denitrifiers. In particular, EfOM with FL_NaOH presented the highest building blocks, high molecular weight (HMW) protein and HMW polysaccharide. It also contained much higher C-(C/H) group. This study provided a new insight on the FL driven denitrification. The necessarity of pursuing the alkali pretreatment to produce carbon source were discussed and evaluated.

Partial denitrification providing nitrite: Opportunities of extending application for anammox

Anaerobic ammonium oxidation (anammox) has been extensively investigated for cost-efficient nitrogen removal from wastewater. However, the major issues of nitrate (NO₃^--N) residue and instability in the current combination of nitritation and anammox process necessitates being addressed efficiently. The recently proposed partial-denitrification (PD), terminating NO₃^--N reduction to nitrite (NO₂^--N), has been regarded as a promising alternative of NO₂^--N supplying for anammox bacteria. Given the engineering practices, the steadily high NO₂^--N production, alleviating organic inhibition, and reducing greenhouse gas of PD process offers a viable and efficient approach for anammox implementation. Moreover, it allows for the extending applications of anammox process due to the NO₃^--N removal availability. Here we comprehensively review the important new outcomes and discuss the emerging applications of PD-based anammox including the process development, mechanism understanding, and future trends. Significant greater stability and enhanced nitrogen removal efficiency have been demonstrated in the novel integrations of PD and anammox process, indicating a broad perspective in dealing with the mainstream municipal sewage, ammonia-rich streams, and industrial NO₃^--N contained wastewater. Furthermore, researches are still needed for the predictable and controllable strategies, along with the detailed microbiological information in future study. Overall, the achievement of PD process provides unique opportunity catalyzing the engineering applications of energy-efficient and environmental-friendly wastewater treatment via anammox technology.

New direction in biological nitrogen removal from industrial nitrate wastewater via anammox

Anaerobic ammonium oxidation (anammox) is an important scientific discovery in the field of wastewater treatment. This process is a sustainable option in nitrogen removal due to its energy-efficient and cost-effective advantage. Great effort has been made recently to remove ammonium from industrial and municipal wastewater via the anammox process with a preceding partial nitrification (PN) converting part of NH₄⁺ to NO₂^-. Anammox process is seldom involved in the nitrate removal. Nitrate (NO₃^-), one of the main nitrogen compounds produced from various industries, is typically converted to nitrogen gas via denitrification process where a large amount of carbon source is consumed. Within this context, we reviewed the current technologies for high-strength nitrate wastewater treatment. It is found that nitrite accumulation often occurs during nitrate reduction, and its accumulating level would be increased at certain conditions (i.e., low C/N ratio and high pH). Hence, this provides a great opportunity to employ the anammox process to further convert nitrite in a more sustainable way. In this review, we highlight a new approach for industrial nitrate wastewater treatment via partial denitrification coupled with anammox process (PD-A). We also discuss the conditions to achieve successful PD-A process, economic and environmental benefits, and potential challenges as well as the future perspectives in practical application.

Free nitrous acid-based nitrifying sludge treatment in a two-sludge system obtains high polyhydroxyalkanoates accumulation and satisfied biological nutrients removal

A novel strategy to achieve substantial polyhydroxyalkanoates (PHA) accumulation in waste activated sludge (WAS) was developed, which was conducted in a two-sludge system consisted of an anaerobic/anoxic/oxic reactor (AAO-SBR) and a nitrifying reactor (N-SBR), where the nitrifying-sludge was treated by free nitrous acid (FNA). Initially, 0.98 ± 0.09 and 1.46 ± 0.10 mmol-c/g VSS of PHA were respectively determined in the control-SBR and AAO-SBR. When 1/16 of nitrifying sludge was daily treated with 1.49 mg N/L FNA for 24 h, ∼46.5% of nitrite was accumulated in the N-SBR, ∼2.43 ± 0.12 mmol-c/g VSS of PHA was accumulated in WAS in AAO-SBR without deteriorating nutrient removal. However, nutrient removal of control-SBR was completely collapsed after implementing the same FNA treatment. Further investigations revealed that the activity and abundance of nitrite oxidizing bacteria (NOB) was decreased significantly after FNA treatment. Finally, sludge with high PHA level to generate more methane was confirmed.

Evaluating the potential for sustaining mainstream anammox by endogenous partial denitrification and phosphorus removal for energy-efficient wastewater treatment

This study demonstrated a novel process configuration for sustaining mainstream anammox by integrating the anammox and endogenous partial denitrification-and-phosphorus removal (EPDPR) in two-stage sequencing batch reactors (SBRs). In the EPDPR-SBR, high nitrate-to-nitrite transformation (68.2%) and P removal (99.3%) were achieved by adjusting the anaerobic/anoxic/aerobic durations and influent nitrate concentration, providing a suitable NO₂^--N/NH₄⁺-N (∼1.37) for subsequent anammox reaction. In the Anammox-SBR, ∼95% of TN was removed without external carbon and oxygen demands. Satisfactory effluent quality (∼6 mgTN/L and 0.2 mgP/L) achieved in the integrated EPDPR/anammox opens a new window towards the energy-efficient wastewater treatment. Microbial analysis further revealed that Dechloromonas (1.6-9.6%) and Candidatus Competibacter (6.4-5.8%) respectively conducted P removal and NO₂^--N production (79.2%) from NO₃^--N denitrification in the EPDPR-SBR, whereas Candidatus Kuenenia (7.0-29.7%) dominated NO₂^--N and NH₄⁺-N removal (91.3% and 99.5%) in the Anammox-SBR, with 10 genera identified as denitrifying bacteria (0.6-8.1%) further reduced 18.9% of the produced NO₃^--N.

Achieving partial denitrification using carbon sources in domestic wastewater with waste-activated sludge as inoculum

Partial denitrification (PD, nitrate → nitrite) using carbon sources in domestic wastewater with waste-activated sludge as inoculum was firstly achieved in this study. Through controlling influent pH at about 9.0 and anoxic reaction time of 1 h in the start-up, the nitrite (NO₂^--N) production reached as high as 25.2 mg/L, with influent nitrate (NO₃^--N) of about 30 mg/L and chemical oxygen demand (COD) to NO₃^--N ratio of 5.9. Furthermore, PD performance remained stable without pH control during subsequent operations. Efficient NO₂^--N production was closely related to the consumed amount of readily biodegradable COD (Ss) fraction, with optimal Ss/NO₃^--N ratio of about 3.5. Thauera (19.1%), norank_f__Xanthomonadaceae (5.2%), and Thiobacillus (5.0%) were enriched during the 208-day operation, which may be responsible for high NO₂^--N production. These findings provided a novel strategy for promoting mainstream PD/Anammox application, without additional nitrite-accumulating denitrifying sludge and external carbon sources.

Long-term effect of pH on denitrification: High pH benefits achieving partial-denitrification

Partial-denitrification (nitrate to nitrite) can supply nitrite for anammox which can reduce organic matter consumption in wastewater treatment plants (WWTPs). In order to achieve stable partial-denitrification, the effect of pH on denitrification were investigated for 420 days in three reactors with influent pH of 5.0, 7.0 and 9.0. The results indicate that the nitrite accumulation rate (NAR) increased with pH, with average effluent NARs being 21%, 38% and 57% in the above reactors, respectively. The sludge cultivated at a high pH of 9.0 was resistant to pH shock, with a high NAR being maintained at 83% when it was exposed to a low pH of 5.0. Metagenomic analysis showed that the higher NAR at pH 9.0 was correlated with an enrichment of Thauera, which harbored more nitrate reductase (8098 hits) than nitrite reductase (2950 hits). Based on these findings, a novel process was proposed for achieving partial-denitrification/anammox in mainstream WWTPs.