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• Partial denitrification was conveniently realized in activated sludge in 6 days. • PD and bio-carriers were conducive to the enrichment of anammox biomass. • Optimum TN in the effluent was 4.7 mg/L with NRE of 90.5%. • Ca. Brocadia was mainly enriched in bio-carriers. • Thauera was mainly enriched in floc sludge. Anammox is widely considered as the most cost-effective and sustainable process for nitrogen removal. However, how to achieve the enrichment of anammox biomass remains a challenge for its large-scale application, especially in mainstream wastewater treatment. In this study, the feasibility of enrichment of anammox biomass was explored through the realization of partial denitrification and the addition of bio-carriers. By using ordinary activated sludge, a sequencing batch reactor (SBR) followed by an up-flow anaerobic sludge bed (UASB) was operated at 25 ± 2°C for 214 days. The long-term operation was divided into five phases, in which SBR and UASB were started-up in Phases I and II, respectively. By eliminating oxygen and adjusting the inflow ratios in Phases III-V, advanced nitrogen removal was achieved with the effluent total nitrogen being 4.7 mg/L and the nitrogen removal efficiency being 90.5% in Phase V. Both in-situ and ex-situ activity tests demonstrated the occurrence of partial denitrification and anammox. Moreover, 16S rRNA high-throughput sequencing analysis revealed that Candidatus Brocadia was enriched from below the detection limit to in biofilms (0.4% in SBR, 2.2% in UASB) and the floc sludge (0.2% in SBR, 1.3% in UASB), while Thauera was mainly detected in the floc sludge (8.1% in SBR, 8.8% in UASB), which might play a key role in partial denitrification. Overall, this study provides a novel strategy to enrich anammox biomass driven by rapid achievement of partial denitrification through the addition of bio-carriers, which will improve large-scale application of anammox processes in mainstream wastewater treatment. [Display omitted] [ABSTRACT FROM AUTHOR]

For low carbon-to-nitrogen ratio municipal wastewater, full utilization of carbon sources is positive in reducing carbon emissions and carbon source addition. This study evaluated and proposed appropriate anaerobic duration control to improve nitrogen and phosphorus removal performance in Denitrifying Phosphorus Removal and Partial Denitrification coupled with Anammox (DPR-PDA) system. Excessive anaerobic duration resulted in wasted carbon source, anaerobic stage should be stopped after carbon source had been utilized. Hydraulic Retention Time-Anaerobic (HRT-An) of R1 was shortened from 5h to 0.56h, while R2 was maintained at 5h. The removal efficiency of nitrate (NO 3 −) were 91.7% in R1 and 71.5% in R2. Peak of NO 2 − accumulation (Peak-NO 2 --N) during denitrification in R1 was 4.5 times than in R2. Candidatus_Brocadia as anammox bacteria was self-enriched, the abundance in R1 was 4.67 times than in R2. This provided a practical guide for optimization of wastewater treatment processes and application of anammox technology. [Display omitted] • The effect of HRT-An on N and P removal was examined in two DPR-PDA systems. • Re-NO 3 --N of R1 (HRT-An: 5. → 0.56h) and R2 (HRT-An: 5h) were 91.7% and 71.5%. • Peak-NO 2 --N during denitrification process in R1 was 4.5 times higher than in R2. • Candidatus_Brocadia was self-enriched, abundance in R1 was 4.67 times than in R2. [ABSTRACT FROM AUTHOR]

The partial denitrification/anammox (PD/A) process is receiving increasing attention due to its cost-effectiveness advantages. However, effective strategies to alleviate organic matter inhibition and promote anammox activity have been proven to be a big challenge. This study investigated the effects of three types of iron (nano zero-valent iron (nZVI), Fe(II), and Fe(III)) on the PD/A process. It is worth noting that nZVI of 5–50 mg/L and Fe(III) of 5–120 mg/L promoted both PD and anammox activity. Long-term intermittent addition of nZVI (50 mg/L) resulted in a nitrogen removal efficiency of 98.2% in the mixotrophic PD/A system driven by iron and organic matter. The contribution of anammox for nitrogen removal reached as high as 93.8%. The organic carbon demand decreased due to the external electron donor provided by nZVI for PD. Multiple Fe–N metabolic pathways, primarily involving ammonia oxidation by Fe(III) and nitrate reduction by nZVI, play a crucial role in facilitating nitrogen transformation. Conversely, the direct addition of 30–120 mg/L Fe (II) resulted in a significant decrease in pH to below 5.0 and severe inhibition of PD and anammox activity. Following prolonged operation in the presence of nZVI, it was demonstrated that there is an enhancing effect on robust nitrite production for anammox. This was accompanied by a remarkable up-regulation of genes encoding nitrate reductase and iron-transporting proteins dominated by Thauera. Overall, this study has provided an efficient approach for advanced nitrogen removal through organic- and iron-driven anammox processes. [Display omitted] • Multiple pathways of Fe–N metabolism jointly promote nitrogen removal of PD/A. • PD and anammox activities were promoted by nZVI and Fe(III) below 50 mg/L. • Long-term enhancement of PD and anammox activity by nZVI with 98.2 % TN removal. • Fe(Ⅱ) inhibited PD and anammox activity due to a significant decrease in pH. • Iron transporting proteins genes dominated by Thauera were up-regulated. [ABSTRACT FROM AUTHOR]

[Display omitted] • Role of iron in collaboration of anammox and denitrification was revealed. • Iron as electron donor and transporter promoted NH 4 +-N removal. • PD/A with iron exhibited resilience in shocking effect of COD, pH and salinity. • Thauera hold versatile metabolism pathway for NO 2 −-N production utilizing Fe(II). • Candidatus Brocadia participated in iron metabolism with Fe(II) as electron donor. Autotrophic nitrogen removal via anaerobic ammonium oxidation (anammox) technology currently faces significant challenges in treating low-strength wastewater due to inefficient retention of bacteria and unstable activity under fluctuating conditions. This study demonstrates enhanced resilience and stability of anammox coupled with partial denitrification (PD) in granular sludge, assisted by ferric iron (Fe(III)) and nano zero-valent iron (nZVI). The systems treated wastewater with low concentrations of ammonia (NH 4 +-N of 50 ∼ 80 mg/L) and nitrate (NO 3 −-N of 60 ∼ 100 mg/L). Fe(III) and nZVI were intermittently added to two PD coupled with anammox (PD/A) systems, which were subjected to increasing nitrogen loading rates (NLRs) and decreasing temperature. Remarkably, both Fe(III)- and nZVI-assisted systems achieved satisfactory total nitrogen (TN) removal efficiencies of 98.1 % and 96.6 %, respectively, even as the temperature dropped from 28.5℃ to 13.4℃ and the NLR increased from 0.22 to 0.72 kgN/m3/d. Significantly, these two systems exhibited pronounced resilience under the shocking effect of excessive organic load, extremely low pH, and high salinity. The presence of iron, acting as an accelerated electron transporter, enhanced anammox activity and microbial NH 4 +-N oxidation. Additionally, iron reduced the requirement for an organic carbon source, further stimulating the competitiveness of anammox for nitrite (NO 2 −-N) against denitrification. Interestingly, metagenomic analysis revealed that the functional bacteria Thauera, responsible for PD, employed versatile metabolic pathways for NO 3 −-N reduction to NO 2 −-N using Fe(II) as an electron donor. Overall, this study provides new insights into microbial interactions and versatile metabolism in iron-assisted anammox systems, contributing to more energy-efficient wastewater treatment. [ABSTRACT FROM AUTHOR]

[Display omitted] • The largest NSFD system (32 m3) ran 150 days; achieved 96.7 % NRE and 62.2 % SRE. • PH and ORP are key factors affecting the performance of NSFD system. • Microfaunae are enriched and deeply involved in sludge reduction. • SRT causes community differences by altering biofilm adsorption. • Niche synergy is the primary driver of excellent performance of NSFD systems. The sludge fermentation-coupled denitrification process, utilized for sludge reduction and nitrogen removal from wastewater, is frequently hindered by its hydrolysis step's efficacy. This study addresses this limitation by extending the sludge retention time (SRT) to 120 days. As a result, the nitrate removal efficiency (NRE) of the nitrification-sludge fermentation coupled denitrification (NSFD) pilot system increased from 67.1 ± 0.2 % to 96.7 ± 0.1 %, and the sludge reduction efficiency (SRE) rose from 40.2 ± 0.5 % to 62.2 ± 0.9 %. Longer SRT enhanced predation and energy dissipation, reducing intact cells from 99.2 % to 78.0 % and decreasing particle size from 135.2 ± 4.6 μm and 19.4 ± 2.1 μm to 64.5 ± 3.5 μm and 15.5 ± 1.6 μm, respectively. It also created different niches by altering the biofilm's adsorption capacity, with interactions between these niches driving improved performance. In conclusion, extending SRT optimized the microbial structure and enhanced the performance of the NSFD system. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ultra-high nitrogen removal efficiency (NRE) of 95.5% at C/N of 2.7 was achieved. • Lower NAR (range 40 %∼70 %) instead resulted in better anammox contribution and NRE. • High-efficiency EPDA with less NO X - loss dominated in ammonium removal than PN/A. • Enriching NOB and GAOs in PN/A process instead favored co-metabolism stability. • The nitrate introduction effectively attenuated the competition of ED for nitrite. Higher nitrite accumulation, which is challenging to achieve reliably, is always sought to obtain better nitrogen removal performance in traditional partial nitritation-anammox (PN/A) process. This study developed a modified PN/A process by introducing nitrite-oxidizing bacteria and endogenous metabolism. Advanced nitrogen removal performance of 95.5 % was achieved at a low C/N ratio of 2.7 under nitrite accumulation ratio (NAR) fluctuations. Higher nitrate accumulation at lower NAR (70 ∼ 40 %) resulted in superior anammox contribution (60 ∼ 75 %) and nitrogen removal performance (93 ∼ 98 %). This was attributed to the higher nitrogen removal efficiency of the post-anoxic endogenous partial denitrification coupling anammox process, although the PN/A process occurring first possessed a faster anammox rate of 2.0 mg NH 4 +-N /(g VSS⋅h). The introduction of nitrate allowed more nitrite flow to anammox, promoting a high enrichment of anammox bacteria (Ca. Brocadia, 0.3 % to 2.8 %). This study provides new insights into the practical application of the PN/A process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Iron Sulfide primarily enhances autotrophic partial dentrification performance. • Various sulfur valence states transitions promotes the electron buffers. • Iron sulphide facilitates the establishment of carbon–nitrogen-sulfur cycle. • Carbon-Nitrogen-Sulfur cycle drives nitrogen removal via electron transfer. This study successfully established Iron Sulfide-Mediated mixotrophic Partial Denitrification/Anammox system, achieving nitrogen and phosphorus removal efficiency of 97.26% and 78.12%, respectively, with COD/NO 3 –-N of 1.00. Isotopic experiments and X-ray Photoelectron Spectroscopy analysis confirmed that iron sulfide enhanced autotrophic Partial Denitrification performance. Meanwhile, various sulfur valence states functioned as electron buffers, reinforcing nitrogen and sulfur cycles. Microbial community analysis indicated reduced heterotrophic denitrifiers (OLB8 , OLB13) under lower COD/NO 3 –-N, creating more niche space for autotrophic bacteria and other heterotrophic denitrifiers. The prediction of functional genes illustrated that iron Sulfide upregulated genes related to carbon metabolism, denitrification, anammox and sulfur oxidation–reduction, facilitating the establishment of carbon–nitrogen-sulfur cycle. Furthermore, this cycle primarily produced electrons via nicotinamide adenine dinucleotide and sulfur oxidation–reduction processes, subsequently utilized within the electron transfer chain. In summary, the Partial Denitrification/Anammox system under the influence of iron sulfide achieved effient nitrogen removal by expediting electron transfer through the carbon–nitrogen-sulfur cycle. [ABSTRACT FROM AUTHOR]

[Display omitted] • Quick start-up of Anammox process was achieved within 21 days. • The SNAD operates consistently and efficiently with an average DO of 0.051 mg/L. • The accumulation and dominance of AnAOB occur in the inner layers of biofilms. • The inhibition of NOB was effectively achieved in a continuous hypoxic environment. • The intermittent aeration operation mode was investigated. In this study, a SNAD-SBBR process was implemented to achieve ammonia removal and carbon reduction of mature landfill leachate under extremely low dissolved oxygen conditions (0.051 mg/L) for a continuous operation of 266 days. The process demonstrated excellent removal performance, with ammonia nitrogen removal efficiency reaching 100 %, total nitrogen removal efficiency reaching 87.56 %, and an average removal rate of 0.180 kg/(m3·d). The recalcitrant organic compound removal efficiency reached 34.96 %. Nitrogen mass balance analysis revealed that the Anammox process contributed to approximately 98.1 % of the nitrogen removal. Candidatus Kuenenia achieved a relative abundance of 1.49 % in the inner layer of the carrier. In the SNAD-SBBR system, the extremely low DO environment created by the highly efficient partial nitrification stage enabled the coexistence of AnAOB, denitrifying bacteria, and Nitrosomonas , synergistically achieving ammonia removal and carbon reduction. Overall, the SNAD-SBBR process exhibits low-cost and high-efficiency characteristics, holding tremendous potential for landfill leachate treatment. [ABSTRACT FROM AUTHOR]

• Novel interspecies cooperation in photogenerated electron uptake was firstly proposed. • Anammox bacteria became the main photogenerated electron sink in microbial community. • Photogenerated electrons constructed the reductive glycine pathway in anammox bacteria. • The rGlyP-WLP carbon fixation directed photogenerated electron to nitrogen metabolism. • Multi-pathway nitrogen removal achieved 100 % ammonia removal and 94.25 % TNRE. Photocatalytic materials-microbial biohybrid systems pave the way for solar-driven wastewater nitrogen removal. In this study, interspecies cooperation in photogenerated electron transfer and efficient nitrogen removal mechanism in the g-C 3 N 4 -anammox consortia biohybrid system were first deciphered. The results indicated that the essential extracellular electron carriers (cytochrome c and flavin) for anammox genomes were provided by associated bacteria (BACT3 and CHLO2). This cooperation, regulated by the ArcAB system and electron transfer flavoprotein, made anammox bacteria the primary photogenerated electron sink. Furthermore, an efficient photogenerated electron harness was used to construct a reductive glycine pathway (rGlyP) in anammox bacteria inventively, which coexisted with the Wood–Ljungdahl pathway (WLP), constituting a dual-pathway carbon fixation model, rGlyP-WLP. Carbon fixation products efficiently contributed to the tricarboxylic acid cycle, while inhibiting electron diversion in anabolism. Photogenerated electrons were targeted channeled into nitrogen metabolism-available electron carriers, enhancing anammox and dissimilatory nitrate reduction to ammonium (DNRA) processes. Moreover, ammonia assimilation by the glycine cleavage system in rGlyP established an alternative ammonia removal route. Ultimately, multi-pathway nitrogen removal involving anammox, DNRA, and rGlyP achieved 100 % ammonia removal and 94.25 % total nitrogen removal efficiency. This study has expanded understanding of anammox metabolic diversity, enhancing its potential application in carbon-neutral wastewater treatment. [Display omitted] [ABSTRACT FROM AUTHOR]

• Rapid and high efficiency of nitrite accumulation was achieved via a novel feast-famine strategy. • Genes expression for nitrate reductase jumped to much higher levels without carbon. • Extracellular metabolites facilitated functional bacterial survival at famine stage. • Selective genes expression and microbial cross-feeding were key reason for rapid PD. 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. [Display omitted] [ABSTRACT FROM AUTHOR]

• Organic carbon addition enhanced denitrification rate and S0 accumulation. • The lowest N 2 O emissions was achieved at Ac−-C/N ratio of 1.3. • Organic carbon influenced nitrate reduction genes, increasing nap A and decreasing nar G. • Isotope fractionation (18ε/15ε) indicated autotrophic or heterotrophic denitrification pathway. • 18ε and 15ε were related to kinetic parameter changes and nitrate reduction genes. Additional organics are generally supplemented in the sulfide-driven autotrophic denitrification system to accelerate the denitrification rate and reduce sulfate production. In this study, different concentrations of sodium acetate (NaAc) were added to the sulfide-driven autotrophic denitrification reactor, and the S0 accumulation increased from 7.8% to 100% over a 120-day operation period. Batch experiments revealed a threefold increase in total nitrogen (TN) removal rate at an Ac−-C/N ratio of 2.8 compared to a ratio of 0.5. Addition of organic carbon accelerated denitrification rate and nitrite consumption, which shortened the emission time of N 2 O, but increased the N 2 O production rate. The lowest N 2 O emissions were achieved at the Ac−-C/N ratio of 1.3. Stable isotope fractionation is a powerful tool for evaluating different reaction pathways, with the 18ε/15ε values in nitrate reduction ranging from 0.5 to 1.0. This study further confirmed that isotope fractionation can reveal denitrifying nutrient types, with the 18ε (isotopic enrichment factor of oxygen)/15ε (isotopic enrichment factor of nitrogen) value approaching 1.0 for autotrophic denitrification and 0.5 for heterotrophic denitrification. Additionally, the 18ε/15ε values can indicate changes in nitrate reductase. There is a positive correlation between the 18ε/15ε values and the abundance of the functional gene nap A, and a negative correlation with the abundance of the gene nar G. Moreover, 18ε and 15ε were associated with changes in kinetic parameters during nitrate reduction. In summary, the combination of functional gene analysis and isotope fractionation effectively revealed the complexities of mixotrophic denitrification systems, providing insights for optimizing denitrification processes. [Display omitted] [ABSTRACT FROM AUTHOR]

· The S0PDA process implementing granular sludge was successfully established. · The S0PDA process exhibited rapid startup and robust operation. · NRR and NRE reached up to 1.75 ± 0.07 kg-N/(m3·d) and 88.5 ± 2.0 %, respectively. · Anammox contributed 82.3 ± 5.6 % of N removal under an ultra-short HRT of 2.0 h. · The synergy of SOB on S0 particles and AnAOB in granular sludge promoted the S0PDA. This study proposed a novel strategy that integrates S0 particles (diameter: 2–3 mm) and granular sludge to establish S0-driven partial denitrification coupled with anammox (S0PDA) process for autotrophic nitrogen removal from NH 4 +- and NO 3 −-containing wastewaters. This process was evaluated using an up-flow anoxic sludge bed bioreactor, operating continuously for 240 days. The influent concentrations of NH 4 + and NO 3 − were 29.9 ± 2.7 and 50.2 ± 2.7 mg-N/L, respectively. Throughout the operation, the hydraulic retention time was shortened from 4.0 h to 2.0 h, while the effluent concentrations of NH 4 + and NO 3 − were maintained at a desirable level of 1.45–1.51 mg-N/L and 4.46–6.52 mg-N/L, respectively. Despite an autotrophic process, the nitrogen removal efficiency and rate reached up to 88.5 ± 2.0 % and 1.75 ± 0.07 kg-N/(m3·d), respectively, indicating the remarkable robustness of the S0PDA process. Autotrophic anammox and sulfur-oxidizing bacteria (Candidatus Brocadia and Thiobacillus) were the predominant bacterial genera involved in the S0PDA process. Candidatus Brocadia was primarily enriched in the granular sludge, with a relative abundance of 6.70 %. Thiobacillus occupied a unique niche on the S0 particles, with a relative abundance as high as 57.6 %, of which Thiobacillus thioparus with partial denitrification function (reducing NO 3 − to NO 2 − without further reduction to N 2) accounted for 78.0 %. These findings challenge the stereotype of low efficiency in autotrophic nitrogen removal from wastewater, shedding fresh light on the applications of autotrophic processes. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • EdPD has more stable NTR (74.5% to 61.0%) with rising C/N ratios (from 3 to 6). • ETSA of ExPD is about 3–5 times higher than that of EdPD. • N 2 O emissions by EdPD (from 0 to 3.4%) was positively related to C/N ratio. • Nitrite accumulation suppresses nosZ gene expression in EdPD. 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 (N 2 O) 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 N 2 O 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel IBBR system was developed to treat landfill leachate and operated 343 days. • PN/A process was quickly started through inoculated activated sludge within 86 days. • The thicker biofilm enables the enrichment of denitrifying bacteria in BAF. • High TNRE of 93.3 % and NRR of 1.13 kg N/(m3·d) was achieved. • A high abundance of AnAOB (Ca. Kuenenia) was enriched, accounting for 1.7%. 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 N H 4 + -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. [ABSTRACT FROM AUTHOR]

• SBR achieved up to 93 % above partial nitrification of municipal wastewater. • SBR effluent and mature landfill leachate in UASB had an 89.4 % anammox contribution. • UASB achieved simultaneous anammox and partial denitrification with an HRT of 3.8 h. • The PN-SAPD process achieved an Eff.TIN of 3.4 mg/L and a removal efficiency of 95 %. • Ca. Brocadia had the highest abundance and was the only AnAOB for nitrogen removal. Anaerobic ammonia oxidation (anammox) of municipal wastewater is a research focus, especially the combined treatment with mature landfill leachate is a current research hotspot. In this study, municipal wastewater was treated by partial nitrification via sequencing batch reactor (SBR), and its effluent and mature landfill leachate were then mixed into an up-flow anaerobic sludge blanket (UASB) for simultaneous anammox and partial denitrification reaction. Through partial nitrification, a high nitrite accumulation rate (93.0 ± 3.8 %) was achieved by low dissolved oxygen (0.5–1.6 mg/L) and controlled aerobic time (3.5 h) in SBR. The UASB system was responsible for 78.8 ± 2.1 % nitrogen removal of the entire system with a hydraulic reaction time (HRT) of 3.8 h, accompanied by the anammox contribution up to 89.4 ± 6.0 %. The overall partial nitrification-simultaneous anammox and partial denitrification (PN-SAPD) system was controlled at a total COD/TIN of 2.8 ± 0.3 and a total HRT of only 10.2 h, achieving the nitrogen removal efficiency and effluent TIN were 95.2 ± 2.2 % and 3.4 ± 1.5 mg/L, respectively. The qPCR results showed functional genes (hzsA(B), hdh) associated with anaerobic ammonia-oxidizing bacteria (AnAOB), whose high gene copy abundance and transcription expression ensured the removal of major nitrogen from municipal wastewater and mature landfill leachate. 16S amplicon sequencing showed that the Ca. Brocadia (9.72–12.6 %) was further enrichment after sodium acetate was added, and the transcription expression of Thauera (0.5–7.0 %) caused nitrate to nitrite. The high abundance of related enzymes (hao, hzs, hdh, narGHI) involved in anammox and partial denitrification processes were found in the macrogenomic sequencing, and only Ca. Brocadia was involved in multi-pathway nitrogen metabolism in AnAOB. Based on the efficient nitrogen removal by AnAOB and denitrifying bacteria, this modified PN-SAPD process provides a new option for the co-treatment of mature landfill leachate in municipal wastewater treatment plants. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Established NH 2 -MIL-101 (Fe)-based photo-Fenton reaction in anammox reactor. • AnAOB maintained normal nitrogen removal activity at 0.1‰ H 2 O 2. • The TNRE of the photo Fenton-Anammox process was up to 91.9 ± 1.5 %. • Photo-Fenton reaction improved the removal of TOC by 3.3-fold in anammox process. • Electron transfer realized a promotion of the photo-Fenton reaction and AnAOB. In this study, H 2 O 2 (0.1 ‰) and NH 2 -MIL-101(Fe)-driven (150 mg/L) photo-Fenton-coupled anammox were proposed to simultaneously improve the removal efficiency of nitrogen and humic acid. Long-term experiments showed that the total nitrogen removal efficiency was increased by the photo-Fenton reaction to 91.9 ± 1.5 % by altering the bioavailability of refractory organics. Correspondingly, the total organic carbon removal efficiency was significantly increased. Microbial community analyses indicated that Candidatus_Brocadia maintained high activity during photo-Fenton reaction and was the most abundant genus in the reactor. Dissimilatory nitrate reduction to ammonium process and denitrification process were enhanced, resulting in reduced NO 3 –-N production. The establishment of electron transfer between microorganisms and NH 2 -MIL-101 (Fe) improved the charge separation efficiency of the quantum dots and increased the intracellular adenosine triphosphate content of anammox bacteria. These results indicated that photo-Fenton-anammox process promoted the removal of nitrogen and refractory organics in one reactor which had good economic value and application prospects. [ABSTRACT FROM AUTHOR]

• PN was achieved with an NAR of 90.4 ± 5.4 % under normal DO conditions (1-2 mg/L). • Average TIN and PO 4 3−-P in effluent were 1.8 and 0.3 mg/L for low-carbon sewage. • DPR process using nitrite as the main electron acceptor saved energy notably. • Total nitrogen and phosphorus removal contributions by nDNPR were 23.6 % and 44.8 %. • The enrichment and balance of AOB, DGAOs and DPAOs ensured the effluent effect. Achieving economic and efficient removal of nutrients in mainstream wastewater treatment plants (WWTPs) continues to be a challenging research topic. In this study, a continuous-flow anaerobic/aerobic/anoxic system with sludge double recirculation (AOA-SDR), which integrated partial nitrification (PN), endogenous denitrification (ED) and nitrite-type denitrifying phosphorus removal (nDNPR), was constructed to treat real carbon-limited municipal wastewater. The average effluent concentrations of total inorganic nitrogen (TIN) and PO 4 3−-P during the stable operation period were 1.8 and 0.3 mg/L, respectively. PN was achieved with an average nitrite accumulation ratio of 90.4 % by combined strategies. Adequate storage of polyhydroxyalkanoates and glycogen in the anaerobic zone promoted the subsequent nitrogen removal capacity. In the anoxic zone, nitrite served as the main electron acceptor for the denitrifying phosphorus removal process. Mass balance analysis revealed that nDNPR contributed to 23.6 % of TIN removal and 44.7 % of PO 4 3−-P removal. The enrichment of Nitrosomonas (0.45 %) and Ellin 6067 (1.31 %), along with the washout of Nitrospira (0.15 %) provided the bacterial basis for the successful implementation of PN. Other dominant endogenous heterotrophic bacteria, such as Dechlormonas (10.81 %) and Candidatus Accumulibacter (2.96 %), ensured simultaneous nitrogen and phosphorus removal performance. The successful validation of integrating PN, ED and nDNPR for advanced nutrient removal in the AOA-SDR process provides a transformative technology for WWTPs. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • High active NH 2 -MIL-101(Fe)@EPS photocatalysts were facile prepared. • EPS-embedding strategy accelerated the separation of electron-hole pairs. • Photocatalysis promoted the nitrate byproduct reduction in the anammox process. • EPS accelerated the electron derived from photocatalyst transferring to bacteria. • Increased abundance of flavin and complex III accelerated electron transfer. Anaerobic ammonium oxidation (anammox) is an efficient nitrogen removal process; however, nitrate byproducts hampered its development. In this study, extracellular polymeric substances (EPS) were embedded into NH 2 -MIL-101(Fe), creating NH 2 -MIL-101(Fe)@EPS to reduce nitrate. Results revealed that chemical nitrate reduction efficiency of NH 2 -MIL-101(Fe)@EPS surpassed that of NH 2 -MIL-101(Fe) by 17.3 %. After adding 0.5 g/L NH 2 -MIL-101(Fe)@EPS within the anammox process, nitrate removal efficiency reached 63.9 %, consequently elevating the total nitrogen removal efficiency to 92.4 %. 16S rRNA sequencing results elucidated the predominant role of Candidatus Brocadia within NH 2 -MIL-101(Fe)@EPS-anammox system. Concurrently, sufficient photogenerated electrons were transferred to microorganisms, promoting the growth of Desnitratisoma and OLB17. Additionally, photogenerated electrons activated flavin and Complex III, thereby up-regulating crucial genes involved in intra/extracellular electron transfer. Subsequently, denitrification and dissimilatory nitrate reduction to ammonium were activated to reduce nitrate. In summary, this study achieved a notable rate of photocatalytic nitrate reduction within anammox process through the NH 2 -MIL-101(Fe)@EPS photocatalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • High NO 2 – accumulation was achieved and NTR reached 87.0 % within 18 days. • Sludge size increased from 131.5 μm to 400.9 μm in 49 days to achieve granulation. • PD bacteria can quickly adapt to the treatment of electroplating tail wastewater. • Thauera was significantly enriched with abundance increasing from 12.5% to 76.4%. The nitrite (NO 2 –) accumulation in partial denitrification (PD) offers the possibility of widespread application of anammox process. In this study, the rapid establishment of PD granular system was achieved by increasing nitrogen loading rates (NLR) from 0.9 to 4.8 kg N/(m3·d), with the nitrate-to-nitrite transforming ratio (NTR) increasing rapidly to 87.0 % within 18 days. Growth evidence indicated that the functional genus Thauera was significantly enriched (12.5 %→76.4 %), with nitrate (NO 3 –) reduction rates (S NO3) improving by 5.4 times from 13.0 to 70.7 mg N/(g VSS·h). Importantly, the rapid aggregation of PD biomass as granules ensured robustness and resistance of PD feeding with the electroplating tail wastewater (NO 3 –-N of 103.0 ± 5.0 mg/L), obtaining stable NTR above 91.5 %. This study demonstrated the achievability of the fast development of PD granules and the adaptability and robustness of treating nitrate-containing industrial wastewater, which provided a promising method for efficient nitrogen transformation in industrial applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Rising NO 2 –-N stress accelerated the growth of NPG with stable NO 2 –-N production. • EPS was stimulated with increasing size and offered supplementary electron donors. • Highly enriched functional bacteria Thauera was evenly distributed around NPG. • Enhanced electron transfer for NO 3 –-N reductase played a key role in efficient NPG. Nitrite-producing denitrification granular sludge (NPG) is emerging as a novel approach to employing anammox for cost-efficient nitrogen removal from high-strength nitrate-containing wastewater. However, the underlying impact of increasing stress of nitrite (NO 2 –-N) accumulation on microbial metabolism and cooperation of denitrifying bacteria and heterotrophs in NPG was still not clear. This study first demonstrated a high-rate NPG system with elevating nitrate (NO 3 –-N) from 52.4 mg/L to 613.2 mg/L. During this operation, a rapid increase in granular size from 322.8 µm to 1124.0 µm was observed with a relatively stable nitrate-to-nitrite transformation ratio (NTR) as high as 83.0 %, and effluent NO 2 –-N of increasing from 16.5 mg/L to 331.9 mg/L. Such an increasing NO 2 –-N stress improved the capability of the TCA cycle with rising intermediates of amino acid metabolism. This further stimulated the production of exocellular organic matter that replenished as electron donors and promoted the growth of granular size. The homogeneously spatial distribution of key bacteria (Thauera) surrounding the NPG with a calcium-dominated inorganic core inside was revealed to play a key role in enhanced substrate transfer capability. Genes nar and nap encoding NO 3 –-N reductase dominated by key species belonging to Thauera showed much higher abundance than that encoding NO 2 –-N reductase, which was the key reason for relatively stable NO 2 –-N accumulation. It illustrated the excellent persistence of NPG to high NO 2 –-N stress with special structure of functional bacteria inside granules, providing a great potential for the treatment of high-strength nitrate-containing wastewater. [ABSTRACT FROM AUTHOR]

• Mixotrophic system showed stable nitrogen removal performance of 94 % under organics stress. • SAD in floc sludge was the primary NO 2 −-N supply pathway for anammox. • Autotrophic subsystem dominated nitrogen removal with anammox contributing to 65.1 %. • Significantly strengthened SSA and SAA were observed with increasing OLR. • SOB and anammox bacteria expressed actively on transcriptional level under high OLR. Sulfur-based autotrophic denitrification (SAD) coupled with anammox is a promising process for autotrophic nitrogen removal in view of the stable nitrite accumulation during SAD. In this study, a mixotrophic nitrogen removal system integrating SAD, anammox and heterotrophic denitrification was established in a single-stage reactor. The long-term nitrogen removal performance was investigated under the intervention of organic carbon sources in real municipal wastewater. With the shortening of hydraulic retention time, the nitrogen removal rate of the mixotrophic system dominated by the autotrophic subsystem reached 0.46 Kg N/m³/d at an organic loading rate of 0.57 Kg COD/m³/d, with COD and total nitrogen removal efficiencies of 82.5 % and 94 %, respectively, realizing an ideal combination of autotrophic and heterotrophic systems. The 15NO 3 −-N isotope labeling experiments indicated that thiosulfate-driven autotrophic denitrification was the main pathway for nitrite supply accounting for 80.6 %, while anammox exhibited strong competitiveness for nitrite under the dual electron supply of sulfur and organic carbon sources and contributed to 65.1 % of nitrogen removal. Sludge granulation created differential functional distributions in different forms of sludge, with SAD showing faster reaction rate as well as higher nitrite accumulation rate in floc sludge, while anammox was more active in granular sludge. Real-time quantitative PCR, RT-PCR and high-throughput sequencing results revealed a dynamically changing community composition at the gene and transcription levels. The decrease in heterotrophic denitrification bacteria abundance indicated the effectiveness of the operational strategy for introduction of thiosulfate and maintaining the dominance of SAD in denitrification process in suppressing the excessive growth of heterotrophic bacteria in the mixotrophic system. The high transcriptional expression of sulfur-oxidizing bacteria (SOB) (Thiobacillus and Sulfurimonas) and anammox bacteria (Candaditus_Brocadia and Candidatus_Kuenenia) played a crucial role in the stable nitrogen removal. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • A pilot-scale AOA system treated sewage under HRT of 5 ∼ h with 63.2 ∼ g/m3·d of NRR. • Short aeration (＜1.5 h) saved the loss of Gly for high-rate denitrification. • More energy supply benefited for GAOs' enrichment under long SRT. • Dominant Dechloromonas helped achieve simultaneous nitrogen and phosphorus removal. • HRT optimization offers AOA processes an energy/cost-saving potential. Applications of post-denitrification processes are subjected to low reaction rates caused by a lack of carbon resources. To offer a solution for reaction rate promotion, this research found a pilot-scale anaerobic/aerobic/anoxic bioreactor treating 55–120 m3/d low-strength municipal wastewater for 273 days. A short hydraulic retention time (HRT, 5–6 h) and a high nitrogen removal rate (63.2 ± 9.3 g-N/m3·d) were achieved using HRT optimization. The effluent total nitrogen concentration was maintained at 5.8 ± 1.4 mg/L while operating at a high nitrogen loading rate of 86.2 ± 12.8 g-N/m3·d. The short aeration (1.25–1.5 h) minimized the Glycogen loss. The endogenous denitrification rate increased to above 1.0 mg/(g-VSS·h). The functional genus Ca. Competibacter enriched to 2.3 %, guaranteeing the efficient post-denitrification process. Dechloromonas rose to 1.1 %, aiding in the synchronous phosphorus removal. These findings offered fresh insights into AOA processes to achieve energy/cost-saving wastewater treatment. [ABSTRACT FROM AUTHOR]

[Display omitted] • PD/A granules achieved high-rate and enhanced N removal with continuous feeding. • Dynamic migration of anammox bacteria to outer edge of granules was found in CSTR. • Stratified structure benefited for cross-feeding between anammox and denitrifiers. • Versatile metabolism of anammox bacteria for reducing NO 3 −-N was assumed in PD/A process. • Anammox bacteria could be efficiently enriched at cost of decreased PD bacteria. Anammox coupling with denitrification has attracted increasing attention in low-strength wastewater treatment. However, its wide application in continuous-flow process is seriously restricted by insufficient anammox capacity due to the failed biomass retention. The functional capacities of communities inhabiting the fluctuating habitat has been little understood. This study developed a novel granular sludge-based partial denitrification coupling with anammox (PD/A) process with the feeding strategy shifting from sequencing to continuous flow in a Continuous Stirred Tank Reactor (CSTR), treating low-strength wastewater containing ammonia (NH 4 +-N) of 46.1 mg/L and nitrate (NO 3 −-N) of 60.7 mg/L. Significantly enhanced specific anammox activity was demonstrated, attributed to the robust nitrite (NO 2 −-N) generation with NO 3 −-N to NO 2 −-N transformation ratio (NTR) averaged 84.1 %. Remarkable total nitrogen (TN) removal of 95.4 % was thus achieved with anammox pathway contribution as high as 88.3 %. Interestingly, a dynamic and stratified microbial structure was obtained in CSTR, in which anammox migrated to outer layer together with Thauera capable of partial denitrification (PD) in big granules, resulting in more efficient cross-feeding and strengthening the cooperation between anammox and heterotrophic PD bacteria in CSTR. Metagenomic-based metabolic inferences provide first insight into the versatile microbial metabolic potential of anammox bacteria for reducing NO 3 −-N with acetate as electron donor in PD/A process. They were successfully enriched and retained with the relative abundance of 8.4 %, being attributed to the enhanced mass transfer efficiency. On the contrary, dominant denitrifier Thauera decreased sharply to 4.2 %. This study provides a new alternative of anammox coupling with heterotrophics granular sludge for efficient low-strength wastewater treatment under continuous feeding, and open up new perspectives for selecting distinctive communities that stand out by expansive metabolic versatility and excellent microbial collaboration. [ABSTRACT FROM AUTHOR]

[Display omitted] • Research progress on N 2 O emissions in novel processes was first fully summarized. • The advantages and challenges of novel processes in N 2 O emissions were analyzed. • Novel Anammox-based processes can effectively mitigate N 2 O emissions. • The potential of ED to mitigate N 2 O emissions was discussed. • The role of bioaugmentation technology in reducing N 2 O emissions was discussed. The proliferation of novel wastewater treatment processes has marked recent years, becoming particularly pertinent in light of the strive for carbon neutrality. One area of growing attention within this context is nitrous oxide (N 2 O) production and emission. This review provides a comprehensive overview of recent research progress on N 2 O emissions associated with novel wastewater treatment processes, including Anammox, Partial Nitrification, Partial Denitrification, Comammox, Denitrifying Phosphorus Removal, Sulfur-driven Autotrophic Denitrification and n-DAMO. The advantages and challenges of these processes are thoroughly examined, and various mitigation strategies are proposed. An interesting angle that delve into is the potential of endogenous denitrification to act as an N 2 O sink. Furthermore, the review discusses the potential applications and rationale for novel Anammox-based processes to reduce N 2 O emissions. The aim is to inform future technology research in this area. Overall, this review aims to shed light on these emerging technologies while encouraging further research and development. [ABSTRACT FROM AUTHOR]