Synergy of partial-denitrification and anammox in continuously fed upflow sludge blanket reactor for simultaneous nitrate and ammonia removal at room temperature

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

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

Rapid start-up and operational characteristics of partial denitrification coupled with anammox driven by innovative strategies

The Partial Denitrification-Anammox (PD/A) process established a low-consumption, efficient and sustainable pathway for complete nitrogen removal, which is of great interest to the industry. Rapid initiation and stable operation of the PD/A systems were the main issues limiting its engineering application in wastewater nitrogen removal. A PD/A system was initiated in a continuous stirred-tank reactors (CSTRs) in the presence of low concentration of organic matter, and the effects of organic matter types and COD/NO3--N ratios on the performance of the PD/A system, and microbial community characteristics were explored. The results showed that low concentrations of organic matter could promote the rapid initiation of the Anammox process and then the strategy of gradually replacing NO2--N with NO3--N could successfully initiate the PD/A system at 70 days. The type of organic matter had a significant effect on the initiation of the Anammox and the establishment of the PD/A system. Compared to glucose, sodium acetate was more favorable for rapid start-up and the synergy among microorganisms, and organic matter was lower, with an optimal COD/NO3--N ratio of 3.0. Microorganisms differed in their sensitivity to environmental factors. The relative abundance of Planctomycetota and Proteobacteria in R2 was 51 %, with the presence of three typical anammox bacteria, Candidatus_Brocadia, Candidatus_Kuenenia, and Candidatus_Jettenia in the system. This study provides a new strategy for the rapid initiation and stable operation of the PD/A process.

Development of partial denitrification process in upflow-anaerobic sludge blanket and effect of electric field on partial denitrification performance

Partial denitrification (PD) is an alternative to providing NO2- for the anaerobic ammonium oxidation (anammox) process. In this study, three upflow anaerobic sludge blankets (UASB) were used to investigate the effect of an external electric field on PD performance. The results indicated that the maximum nitrite transformation ratio (NTR) reached 76.3 %, with an average NTR of 54.1 %, in the presence of external electric field, whereas the average NTR of the control was only 49.8 %. The fitted maximum specific nitrate reduction rates of PD1, PD2, and PD3 were 83.7, 90.5, and 92.3 mg N g-1VSS h-1, respectively, according to the Haldane model analysis. Microbial community analysis demonstrated that the abundance of Thauera, Comamonas, and Accumulibacter increased with electric assistance. In summary, UASB reactor with electrodes set in the upper region was most feasible for the stable PD process, providing an alternative for developing a coupled PD-anammox process.

Simultaneous nitrogen and phosphorus removal through an integrated partial-denitrification/anammox process in a single UAFB system

With the aim of obtaining enhanced nitrogen removal and phosphate recovery in mainstream sewage, we examined an integrated partial-denitrification/anaerobic ammonia oxidation (PD/A) process over a period of 189 days to accomplish this goal. An up-flow anaerobic fixed-bed reactor (UAFB) used in the integrated PD/A process was started up with anammox sludge inoculated and the influent composition controlled. Results showed that the system achieved a phosphorus removal efficiency of 82% when the influent concentration reached 12.0 mg/L. Batch tests demonstrated that stable and efficient removal of chemical oxygen demand (COD), nitrogen, and phosphorus was achieved at a COD/NO3--N ratio of 3.5. Scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis indicated that hydroxyapatite was the main crystal in the biofilm. Furthermore, substrate variation along the axial length of UAFB indicated that partial denitrification and anammox primarily took place near the reactor's bottom. According to a microbiological examination, 0.4% of the PD/A process's microorganisms were anaerobic ammonia oxidizing bacteria (AnAOB). Ca. Brocadia, Ca. Kuenenia, and Ca. Jettenia served as the principal AnAOB generals in the system. Thauera, Candidatus Accumulibacter, Pseudomonas, and Acinetobacter, which together accounted for 27% of the denitrifying and phosphorus-accumulating bacteria, were helpful in advanced nutrient removal. Therefore, the combined PD/A process can be a different option in the future for sewage treatment to achieve contemporaneous nutrient removal.

Performance optimization and nitrogen removal mechanism of up-flow partial denitrification/anammox process

This study aimed to remediate the problems of sludge floating and uneven mass transfer in up-flow partial denitrification/anammox (PDA) reactors and dissect the nitrogen removal mechanism. Two up-flow PDA reactors were operated, whereby in R1 combined biological carriers were added, while in R2 mechanical stirring was applied, the reactors were inoculated with PD sludge and anammox sludge. Results showed the TN removal rates at the end of the operation were 89% (R1) and 92% (R2). The addition of both strategies suppressed the occurrence of sludge upwelling and deterioration of settling performance, even when the granule diameter of the granular zone in R1 and R2 reached 1.921 and 2.006 mm, respectively. 16SrRNA sequencing revealed R1 had a higher abundance of anammox bacteria (AAOB, 14.53%-R1, 9.06%-R2, respectively), and R2 had a higher quantity of denitrifying bacteria (61.92%-R1, 67.11%-R2, respectively). And the nitrogen removal was contributed by anammox and denitrification in combination, with contributions of 82.17%, 17.83% (R1), and 85.07%, 14.93% (R2), respectively. In summary, both strategies prevented sludge flotation and uneven nitrogen mass transfer. However, mechanical agitation had a more substantial positive effect on the performance of PDA than the addition of biocarriers because it achieved a more adequate mass transfer.

Dual-edged effects and mechanisms of hydroxylamine in partial denitrification-anaerobic ammonium oxidation system

The combination of partial denitrification (PD) and anaerobic ammonium oxidation (anammox) is a novel and promising nitrogen removal process. Regulating the synergistic reaction between denitrifiers and anammox bacteria (AnAOB) is the key to achieving stable and efficient PD-anammox performance. In this study, 10 mg/L of hydroxylamine (NH2OH) was considered to efficiently promote the bacterial activity, microbial energy flow, and the synergy of functional microflora. As a result, the nitrogen removal rate (NRR) significantly increased from 0.05 to 0.30 g N/L/d in parallel with an increase in the nitrogen loading rate (NLR) from 0.10 to 0.40 g N/L/d. However, the dual-edged effect of NH2OH was also confirmed. The long-term presence of NH2OH caused overgrowth of complete-denitrifying bacteria and decreased the NRR to 0.11 g N/L/d. Additionally, NH2OH enhanced nitrous oxide (N2O) emissions via chemical pathways as well as enhanced denitrification Fortunately, the inhibition caused by NH2OH was reversible by stopping the dosing, the reactor restored to stable operation with an NRR of 0.27 g N/L/d. Analysis of metabolic intensity and pathways revealed the effecting process and mechanism of NH2OH on the PD-anammox system. This study verified the dual-edged effects and mechanisms of NH2OH, therefore proving a theoretical basis and technical reference for the application of PD-anammox.

The long-term effect of glutaraldehyde on the bacterial community in anaerobic ammonium oxidation reactor

A 160-day incubation was performed with two anammox reactors (GA and CK) to investigate the effect of glutaraldehyde. The results indicated that anammox bacteria were very sensitive when glutaraldehyde in GA reactor increased to 40 mg/L, the nitrogen removal efficiency sharply decreased to 11%, only one-quarter of CK. Glutaraldehyde changed spatial distribution of exopolysaccharides, caused anammox bacteria (Brocadia CK_gra75) to disassociate from granules (24.70% of the reads in CK but only 14.09% in GA granules). Metagenome analysis indicated glutaraldehyde led to the denitrifier community succession from strains without nir (nitrite reductase) and nor (nitric oxide reductases) genes to those with them, and the rapid growth of denitrifiers with NodT (an outer membrane factor)-related efflux pumps replacing those with another TolC -related ones. Meanwhile, Brocadia CK_gra75 lacks the NodT proteins. This study provides important insight into community adaptation and potential resistance mechanism in an active anammox community after exposure to disinfectant.

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

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

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

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

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

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

Highly efficient transformation of slowly-biodegradable organic matter into endogenous polymers during hydrolytic fermentation for achieving effective nitrite production by endogenous partial denitrification

The utilization of slowly-biodegradable organic matter (SBOM) to provide nitrite efficiently for anaerobic ammonia oxidation (anammox) process is an essential topic. High nitrite concentration without inhibition of exogenous organic matter is optimal condition for anammox process. In this study, hydrolytic fermentation (HF) of SBOM was applied to drive an endogenous partial denitrification (EPD) process (nitrate to nitrite) during an anaerobic-anoxic operation in a starch-fed system. With a limited production of exogenous organic matter (22.3 ± 4.9 mg COD/L), 79.0% of SBOM was transformed into poly-hydroxyalkanoates (PHA) through a pathway of simultaneous HF-absorption and endogenous polymer synthesis, corresponding to a hydrolytic fermentation ratio of 86.0%. A high nitrate to nitrite transformation ratio of 85.4% was achieved under an influent carbon to nitrogen ratio of 4.8. Denitrifying glycogen-accumulating organisms (DGAOs) was enriched from 0.6% to 10.9%, with an increase from 0.7 to 1.0 of nitrate reductase genes to nitrite reductase genes ratio. Subsequently, nitrate reduction rate was 5.6-fold higher than the nitrate reduction rate. A prominent migration of exogenous complete denitrification to EPD was accomplished. Furthermore, the starch-fed system exhibited performance with significant adaptability and stability in the presence of different SBOMs (dissolved protein and primary sludge). Therefore, the HF-EPD system achieved efficient nitrite production through EPD with the addition of various SBOMs, providing a potential alternative to anammox systems for the treatment of SBOM-rich wastewater.

Strategy of nitrate removal in anaerobic ammonia oxidation-dependent processes

The anaerobic ammonium oxidation (anammox), a microbial process that is considered as a low-cost and high efficient wastewater treatment, has received extensive attention with an attractive application prospect. The anammox process reduces nitrite (NO₂^-) to nitrogen gas (N₂) with ammonium (NH₄⁺) as the electron donor. However, some nitrate (NO₃^-) equivalent to 11% of total nitrogen (TN) is generated in this process, which limits the development of anammox. To overcome this problem, many efforts have been made in this regard, mainly combining with other biological treatment methods (denitrification, denitrifying anaerobic methane oxidation, etc.), introducing the substance into anammox process, etc. Herein, we summarized a detailed review of previous researches on the removal of NO₃^- in the anammox-dependent processes. It is hoped that this review could serve as valuable guidance in future research and practical applications of anammox.

Insights into integrated glycerol-driven partial denitrification-anaerobic ammonium oxidation system using bioinformatic analysis: The dominance of Bacillus spp. and the potential of nitrite producing via assimilatory nitrate reduction

Partial denitrification-anaerobic ammonium oxidation (PD/A) was considered a novel technology for biological nitrogen removal. In this study, a glycerol-driven PD/A granular sludge reactor was constructed, and its nitrogen removal efficiency and microbial mechanisms were investigated systematically. After optimization, the PD/A reactor achieved 92.3 % of the nitrogen removal (~90 % by anammox) with the influent COD/NO₃^--N ratio of 2.6, and approximate 1.36 mol NO₃^--N was required for removing 1 mol NH₄⁺-N. Granular sludge with layered structure (anaerobic ammonium oxidizing bacteria (AnAOB) was wrapped by the heterotrophic bacteria) was successfully developed, which resulted in the sludge floating. Bacillus was firstly found to be the dominant genus in PD/A system with an abundance of 46.1 %, whereas the AnAOB only accounted for 0.2-2.8 %. Metatranscriptomic analysis showed that the metabolic characteristics obviously changed during the operation, and the differential expressing genes mainly belonged to ABC transport and quorum sensing pathway. Further analysis about the expressing patterns of nitrogen metabolism related genes indicated that the anammox related genes (mainly from Candidatus Brocadia and Candidatus Jettenia) exhibited a much higher expressing level than other genes. Interestingly, the assimilatory nitrate reduction process in Bacillus showed great NO₂^--N producing potential, so it was considered to be an essential pathway participating in PD/A process. This study provided a comprehensive insight into the glycerol-driven PD/A system.

Extracellular polymeric substances trigger microbial immigration from partial denitrification (PD) to anammox biofilms in a long-term operated PD/anammox process in low-strength wastewater

The immigration of microbial communities in a synergistic partial denitrification/anammox (SPDA) system was investigated in a moving bed biofilm reactor (MBBR) inoculated with partial denitrification (PD) and anaerobic ammonium oxidation (anammox) biofilms. The SPDA system was operated at 25 ± 1 °C over 260 days. The total nitrogen (TN) of the effluent was only 3.71 ± 0.92 mg·L^-1 in the stable phase with a TN removal efficiency of 95.23%. The anammox process was the dominant nitrogen removal pathway with an average contribution of 74.31% to TN removal. The results of the in situ activity and key enzymatic activity revealed that the nitrate-reducing bacteria tended to immigrate to anammox biofilms. Correspondingly, the abundance of the genus Thauera, the second most dominant bacteria in anammox biofilms, quickly increased from 0.78 to 10.69% on day 50 and eventually to 16.45% on day 221 according to the Illumina MiSeq sequencing data. The microbial immigration might be caused by different extracellular polymeric substance (EPS)-mediated mechanisms in PD and anammox biofilms. For fast-growing denitrifiers, PD biofilms tend to increase the ability of mass transfer by excreting more polysaccharides to form loosely-bound EPS at the expense of the ability to harbor the nitrate-reducing bacteria. However, for the slow-growing anaerobic ammonium oxidizing bacteria (AnAOB), the anammox biofilms tend to increase the retention of AnAOB by excreting more proteins to form enhanced tightly-bound EPS at the expense of the mass transfer ability, thereby causing the detached nitrate-reducing bacteria to immigrate into anammox biofilms.

Integrating conventional nitrogen removal with anammox in wastewater treatment systems: Microbial metabolism, sustainability and challenges

The various forms of nitrogen (N), including ammonium (NH₄⁺), nitrite (NO₂^-), and nitrate (NO₃^-), present in wastewaters can create critical biotic stress and can lead to hazardous phenomena that cause imbalances in biological diversity. Thus, biological nitrogen removal (BNR) from wastewaters is considered to be imperatively urgent. Therefore, anammox-based systems, i.e. partial nitrification and anaerobic ammonium oxidation (PN/anammox) and partial denitrification and anammox (PD/anammox) have been universally acknowledged to consider as alternatives, promising and cost-effective technologies for sustainable N removal from wastewaters compared to nitrification-denitrification processes. This review comprehensively presents and discusses the latest advances in BNR technologies, including traditional nitrification-denitrification and anammox-based systems. To a deep understanding of a better-controlled combining anammox with traditional processes, the microbial community diversity and metabolism, as well as, biomass morphological characteristics were clearly reviewed in the anammox-based systems. Explaining simultaneous microbial competition and control of crucial operation parameters in single-stage anammox-based processes in terms of optimization and economic benefits makes this contribution a different vision from available review papers. The most important sustainability indicators, including global warming potential (GWP), carbon footprint (CF) and energy behaviours were explored to evaluate the sustainability of BNR processes in wastewater treatment. Additionally, the challenges and solutions for BNR processes are extensively discussed. In summary, this review helps facilitate a critical understanding of N removal technologies. It is confirmed that sustainability and saving energy would be achieved by anammox-based systems, thereby could be encouraged future outcomes for a sustainable N removal economy.

Effects of hydroxylamine on treatment of anaerobic digestate of pig manure in partial nitrification-anaerobic ammonium oxidation

Partial nitrification-anaerobic ammonium oxidation (PN-anammox) was started up within 40 days by bioaugmentation and aeration control, and its performance in the treatment of anaerobic digestate of pig manure (ADPM) was evaluated. Inhibitors in ADPM decreased the nitrogen removal rate (NRR) by 0.24 g N/L/d. The effect and mechanism of hydroxylamine (NH₂OH) alleviation of PN-anammox inhibition during ADPM treatment were investigated. As an intermediate product of anammox and ammonia-oxidizing bacteria, NH₂OH strengthened energy metabolism, improved the activity and abundance of functional bacteria, and eliminated miscellaneous bacteria, increasing the average NRR by 31%. However, the average nitrous oxide emission was increased by 10.1% via hydroxylamine oxidation. The results showed that synergy and competition among nitrogen-transforming microorganisms were crucial for NRR and that NH₂OH played an essential role in maintaining efficient operation. This study lays a foundation for restoring PN-anammox for treating livestock wastewater.

Enhanced nitrogen removal from municipal wastewater via a novel combined process driven by partial nitrification/anammox (PN/A) and partial denitrification/anammox (PD/A) with an ultra-low hydraulic retention time (HRT)

Anaerobic ammonia oxidation (Anammox) is a highly productive research area in municipal wastewater treatment. A novel combined process driven by partial nitrification/anammox (PN/A) and partial denitrification/anammox (PD/A) was established in this paper using a sequencing batch reactor (SBR) and two up-flow sludge beds (USBs). Municipal wastewater after carbon removal pretreatment in SBR entered PN/A-USB. PN/A process was initiated and enhanced by optimizing the intermittent aeration mode under low dissolved oxygen (DO). After enhancing and stabilizing the PD/A process, PN/A effluent entered the PD/A-USB along with raw municipal wastewater at a ratio of 4:1 and the combined system was established. Through this, this study achieved a nitrogen removal efficiency (NRE) of 84.9 % from municipal wastewater at an ultra-low total hydraulic residence time (HRT) of 3.9 h. Candidatus Brocadia (1.8 % in PN/A, 1.0 % in PD/A) was the only functional anammox bacterium in the combined process.

Insight into the microbial interactions of Anammox and heterotrophic bacteria in different granular sludge systems: effect of size distribution and available organic carbon source

Microbial interactions between Anammox and heterotrophic bacteria in different granule distributions in an Anammox system (AMX) and partial denitrification coupled with Anammox system (PDA) were analyzed in this paper. Candidatus Brocadia was the main Anammox microorganism in granules of 1.0 > d > 0.5 mm with the highest abundance of 21.5 % in AMX, significantly higher than the maximum proportion of 2.3 % in PDA sludge > 2.0 mm. However, the total nitrogen (TN) removal of 77.9 % in AMX was lower than PDA (94.0 %) because of the excessive NO3--N generated by nitrite-oxidizing bacteria (NOB). Anammox activity could be stimulated by heterotrophs via simple organic carbon, which decreased with the increasing size of sludge in AMX but increased in PDA. This highlighted that regulation of the distribution of sludge size and organic carbon source had an essential effect on efficient nitrogen removal of Anammox technology.

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

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

Integrated moving bed biofilm reactor with partial denitrification-anammox for promoted nitrogen removal: Layered biofilm structure formation and symbiotic functional microbes

Partial denitrification/anaerobic ammonia oxidation (anammox) (PD/A) is currently an advanced nitrogen removal process. This study developed a PD/A system in a moving bed biofilm reactor. Results showed that the nitrogen removal efficiency reached 76.60% with a COD/NO₃-N of 2.0, and the contribution of anammox was 88.01%. Further analysis showed that the biocarriers could form layered pH and dissolved oxygen structures to promote the aggregation of different functional bacteria at various depths, thus stabilizing the coupled process. Microbial structure analysis showed that the abundance of Saccharimonadales, responsible for denitrification, increased from 0% to 36.27% between day 0 and day 120, while the abundance of Candidatus Jettenia, responsible for anammox, decreased from 10.41% to 2.20%. The synergistic effect of Saccharimonadales and Candidatus Jettenia enabled stable and efficient removal of nitrogen. This study proposed a novel configuration of the PD/A process and provided a theoretical basis for its promotion and application.

Treatment of nitrate containing wastewater by adsorption process using polypyrrole-modified plastic-carbon: Characteristic and mechanism

Polypyrrole-modified plastic-carbon (PET-PPy) composite was prepared by using high porosity plastic-carbon materials and a special doping mechanism of polypyrrole to remove nitrate from water to achieve waste recycling. As a result, PET-PPy-500 showed remarkable nitrate adsorption in both acidic and alkaline wastewater. The pseudo-second-order kinetic and Langmuir isotherm models were fit for the nitrate adsorption by PET-PPy-500, and the maximum adsorption capacity predicted by the Langmuir model was 10.04 mg NO₃-N/g (45.18 mg NO₃^-/g) at 30 °C. The ion exchange and electrostatic attraction were the main mechanisms of removing NO₃^- by PET-PPy-500, which was demonstrated by the interface characterization and theoretical calculation. The doped ions (Cl^-) and/or other anions produced by charge transfer interaction were the main exchange ions in the process of NO₃^- adsorption. The main binding sites in the electrostatic adsorption process were nitrogen-containing functional groups, which can be confirmed by the results of XPS and density functional theory (DFT). Furthermore, DFT results also showed that the adsorption of nitrate by PET-PPy was a spontaneous exothermic process, and the adsorption energy at the nitrogen site was the lowest. The findings of this study provide a feasible strategy for the advanced treatment of nitrate containing wastewater.

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

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

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.

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.

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.

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

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

In-situ fermentation coupling with partial-denitrification/anammox process for enhanced nitrogen removal in an integrated three-stage anoxic/oxic (A/O) biofilm reactor treating low COD/N real wastewater

Mainstream partial-denitrification with anammox (PD-anammox) process faced the challenge of complex organics involved in real sewage. Herein, PD-anammox coupled with in-situ fermentation was successfully achieved in a full biofilm system formed by three-stage anoxic/oxic reactor to treat real wastewater with low COD/N of 3.6. The total nitrogen (TN) removal efficiency was enhanced to 78.4% ± 3.6% with average TN and ammonium concentrations in effluent of 10.6 and 0.5 mg N/L, respectively. Batch tests confirmed that partial-denitrification was the major nitrite provider for anammox in the anoxic biofilm, while in-situ fermentation could decompose the complex organics to readily-biodegradable organics for full- or partial-denitrification. Additionally, a significant anammox bacteria (Candidatus Brocadia) population was detected in the second (3.53%) and third (4.46%) anoxic zones, while denitrifiers and fermentative bacteria were mainly enriched in the first anoxic zone. This study presents a feasible approach for PD-anammox process in practical application under mainstream condition.

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

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

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

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

Nitrate removal by anammox biomass with intracellular carbon source as electron donors via DNRA pathway

In this work, a novel nitrate (NO₃^-) reduction pathway by anaerobic ammonium oxidation (anammox) biomass was firstly discovered with the intracellular carbon sources as the only electron donors. And the possible reaction mechanism was deduced to be intracellular dissimilatory nitrate reduction to ammonium (DNRA) pathway according to the experimental results. In batch experiments, without any external electron donors, NO₃^--N (about 50 mg/L) was reduced to N₂ within 48 h, and a small amount of NO₂^--N was detected (the maximum of 2 mg/L) with the anammox biomass concentration of 4400 mg/L. Acetylene (4.46 mmol/L) addition resulted in obvious NH₄⁺ accumulation during NO₃^- degradation by anammox biomass, since acetylene mainly interfered in hydrazine (N₂H₄) generation from NH₄⁺ and NO. Without HCO₃^- addition, the NO₃^--N degradation rate was slower than that with HCO₃^- addition. Simultaneously, glycogen contents inside anammox biomass decreased to 133.22 ± 1.21 mg/g VSS and 129.79 ± 1.21 mg/g VSS with and without HCO₃^-, respectively, from 142.20 ± 0.61 mg/g VSS. In the long-term experiment, anammox biomass stably degraded NO₃^--N without external electron donors addition, and the maximum removal efficiency of NO₃^--N reached 55.4%. The above results indicated the anammox bacteria utilized the DNRA pathway to reduce NO₃^- to NO₂^- and further NH₄⁺, then normal anammox metabolism would continue to convert the produced NO₂^- and NH₄⁺ to N₂. The intracellular stored carbon sources (e.g., glycogen) were supposed to be electron donors for NO₃^- degradation. This capability would enhance the viability and living space of anammox bacteria in different natural ecosystems, and make it plausible that complete nitrogen removal could be implemented only by the anammox process.

Effect of organic concentration on biological activity and nitrogen removal performance in an anammox biofilm system

The effects of different concentrations of organic matter on the biological activity and nitrogen removal performance of the anaerobic ammonium oxidation (anammox) system was studied. The results showed that under the conditions of low influent total organic carbon (TOC ≤ 100 mg/L), the activity rate of anammox bacteria was basically unaffected, the anammox bacteria and denitrifying bacteria formed a good synergistic effect, and the maximum total nitrogen (TN) removal efficiency reached 95.77%. However, when the influent TOC concentration was up to 200 mg/L, the activity of anammox bacteria was seriously inhibited. At this time, denitrification becomes the main pathway of nitrogen removal, the effluent ammonia nitrogen content increases, and the TN removal efficiency decreases to 64.17%. High-throughput sequencing analysis showed that with the increase in organic matter concentration, the relative abundance of Proteobacteria and Planctomycetes changed significantly. In particular, the relative abundance proportion of Proteobacteria increased from 21.06% to 25.57%, the Planctomycetes dropped from 10.01% to 3.03% and the Candidatus Brocadia genus had the largest decrease. In conclusion, the concentration range of organic matter for collaborative denitrification was proposed in this study, which provided theoretical reference for the practical application of anammox biofilm process.

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.

Rapid initiation and stable maintenance of municipal wastewater nitritation during the continuous flow anaerobic/oxic process with an ultra-low sludge retention time

Rapid achievement of nitritation of mainstream municipal wastewater in a continuous-flow process is attractive since it favors the involvement of the anammox process and reduces the operational costs. In this study, a feasible and economical strategy is proposed to rapidly achieve the nitritation of municipal wastewater. By aggressively discharging excess sludge during the seasonal warming period (temperature increasing from 18°C to 22°C), nitritation was established in 15 days with a nitrite accumulation ratio of 85.09% in a continuous-flow anaerobic/oxic (An/O) reactor. Meanwhile, qPCR results revealed that amoA abundance increased from (1.78±0.10) × 10⁸ copies/(g VSS) to (1.05±0.11) × 10¹⁰ copies/(g VSS) while the abundance of nitrite-oxidizing bacteria decreased from (1.1±0.02) × 10¹⁰ copies/(g VSS) to (5.01±0.02) × 10⁸ copies/(g VSS). The temperature gradually stabilized at 26°C during the following operational period and stable nitritation was maintained with a nitrite accumulation ratio above 90%, which was mainly attributed to a short sludge retention time (SRT) of 4.3 days and a low dissolved oxygen of 0.86 ± 0.5 mg/L. Falling temperature negatively impacted the stability of nitritation, but nitritation could be restarted by aggressively discharging excess sludge during another temperature increase period. Overall, this study provides a feasible strategy to start-up nitritation that has great potential applications for municipal wastewater treatment.

Development of a novel denitrifying phosphorus removal and partial denitrification anammox (DPR + PDA) process for advanced nitrogen and phosphorus removal from domestic and nitrate wastewaters

A novel energy-efficient DPR + PDA (denitrifying phosphorus removal and partial denitrification anammox) process for enhanced nitrogen and phosphorus removal was developed in the combined ABR-CSTR reactor. After 220 days operation, excellent total inorganic nitrogen (TIN) and phosphorus removal (97.57% and 95.66%, respectively) were obtained under external C/NO₃^--N of 0.7, with the effluent TIN and PO₄^3--P concentrations of 3.51 mg/L and 0.28 mg/L, respectively. At the steady period, DPR contributed major TN removal (58.65%), while PDA mediated an increasingly considerable impact and finally achieved 37.07%, in which anammox accounted for a significant percentage. Batch tests demonstrated that efficient PD with nitrate-to-nitrite transformation ratio of 97.67% supplying stable nitrite for anammox, and phosphorus was mainly removed using nitrate as electron acceptor via DPR with the ideal phosphorus release/uptake rate (7.73/22.17 mgP/gVSS/h). Accumulibacter (6.24%) dominated high phosphorus removal performance, while Thauera (8.26%) and Candidatus Brocadia (2.57%) represented the superior nitrogen removal performance.

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

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

Impacts of organics on the microbial ecology of wastewater anammox processes: Recent advances and meta-analysis

Anaerobic ammonium oxidation (anammox) represents a promising technology for wastewater nitrogen removal. Organics management is critical to achieving efficient and stable performance of anammox or integrated processes, e.g., denitratation-anammox. The aim of this systematic review is to synthesize the state-of-the-art knowledge on the multifaceted impacts of organics on wastewater anammox community structure and function. Both exogenous and endogenous organics are discussed with respect to their effects on the biofilm/granule structure and function, as well as the interactions between anammox bacteria (AnAOB) and a broad range of coexisting functional groups. A global core community consisting of 19 taxa is identified and a co-occurrence network is constructed by meta-analysis on the 16S rDNA sequences of 149 wastewater anammox samples. Correlations between core taxa, keystone taxa, and environmental factors, including COD, nitrogen loading rate (NLR) and C/N ratio are obtained. This review provides a holistic understanding of the microbial responses to different origins and types of organics in wastewater anammox reactors, which will facilitate the design and operation of more efficient anammox-based wastewater nitrogen removal process.

Simultaneous enhanced biological phosphorus removal and semi-nitritation (EBPR-SN) followed by anammox process treating municipal wastewater at seasonal temperatures: From summer to winter

This work investigated the feasibility of a novel simultaneous enhanced biological phosphorus removal and semi-nitritation (EBPR-SN) plus anammox process treating real municipal wastewater from summer to winter (28.1- 15.3 °C). Two lab-scale sequential reactors were used in this study, namely EBPR-SN and Anammox sequencing batch reactors (SBRs). Long-term operation suggested that ammonium oxidizing bacteria abundance decreased from 1.67% to 0.89% whereas nitrite oxidizing bacteria decreased to nearly undetected in the EBPR-SN SBR, maintaining the stable nitritation (nitrite accumulation ratio: 98.3 ± 1.0%). Lowering airflow rate was effective to retain nitritation with temperature decrease. Reliable nutrient removal was still maintained in winter (16.4 ± 0.7 °C), i.e. the removal efficiencies for nitrogen and phosphorus were 80.0 ± 3.5% and 95.4 ± 5.2%, respectively, with short aerobic HRT (6.4 h) and low dissolved oxygen (0.2-1.5 mg/L). The percentage of anammox contribution to nitrogen-removal increased with temperature decrease, although Candidatus Brocadia abundance decreased. Additionally, the protection of extracellular polymeric substances was important to the successful performance.

Deammonification process in municipal wastewater treatment: Challenges and perspectives

The deammonification process has been proved to be an efficient nitrogen removal process in treating high NH₄⁺-N concentration wastewater (sidestream deammonification). It is very hopeful to bring WWTP close to energy autarky. However, the feasibility of applying mainstream deammonification to sewage treatment need to be further explored. Therefore, this review attempts to give an overview of challenges in applying mainstream deammonification and to discuss the impacts of unfavorable conditions on main functional species. In addition, some novel control strategies to maintain the dominant position of desired species were summarized. Efficient solution to the conflict between AnAOB (Anaerobic ammonium-oxidizing bacteria) biomass retention and NOB (Nitrite oxidizing bacteria) wash out was also reviewed. Ultimately, we suggested further studies including effective improved process that achieve combination of autotrophy and organotrophy species based on the metabolic diversity of AnAOB.

Feasibility of partial denitrification and anammox for removing nitrate and ammonia simultaneously in situ through synergetic interactions

In this study, the single-stage partial denitrification-anammox (PD-A) process was started-up in 22 days in a lab-scale up-flow sludge blanket (UASB) reactor to treat wastewater containing NH₄⁺-N and NO₃^--N simultaneously. The TN removal rate reached 97.08% with a low effluent TN of 10 mg/L. High-throughput sequencing results revealed the dominant bacterial strains were related to the genus of Thauera and Candidatus Kuenenia. The PD-A system was started-up based on the optimized PD process via inoculated exogenous anammox sludge attributing to the improvement of bacterial adaptation and co-existence by EPS. The PD process was realized in 18 days with the abundance of PD functional bacterium Thauera through fluctuated C/NO-3-N conditions. Moreover, the detrimental effects of starvation on anammox was weaker than that on PD bacteria. The PD-A process was expected to open a new possible perspective in designing NO₃^--N and NH₄⁺-N wastewater treatment plants.

Development of novel denitrifying nitrite accumulation and phosphorus removal (DNAPR) process for offering an alternative pretreatment to achieve mainstream Anammox

For achieving mainstream anaerobic ammonium oxidation (Anammox), there is a need to achieve organic carbon and phosphorus removal meanwhile supplying nitrite (NO₂^--N). Based on this demand, a novel anaerobic/anoxic/aerobic operated denitrifying nitrite accumulation and phosphorus removal (DNAPR) process was proposed for treating synthetic municipal and nitrate (NO₃^--N) wastewaters simultaneously (volume ratio of 5:1). By adjusting influent composition, discharging anaerobic-end supernatant, shortening anoxic duration, and adding a short aerobic stage, DNAPR process achieved promising and stable nitrate-to-nitrite transformation (78.35%) and phosphorus removal (98.34%) performance. Moreover, effluent with chemical oxygen demand of 16.63 mg/L, nitrite of 54.16 mg/L, orthophosphate of 0.37 mg/L, and nitrite to ammonia ratio of 1.3 were finally obtained after 141-day operation. Microbiological analysis showed that Thauera (34.9%) and unclassified_f_Rhodobacteraceae (6.79%) were both responsible for DNAPR. Therefore, DNAPR, serving as promising alternative pretreatment, might possess significance for achieving mainstream Anammox.

Effect of fulvic acid on bioreactor performance and on microbial populations within the anammox process

The long-term effect of fulvic acid (FA) on bioreactor performance and on microbial populations within the anammox process were firstly investigated in this study. The average nitrogen removal rate showed an upward trend when the influent TOC concentration of FA was 25.2-65.1 mg/L. However, when FA was increased to 80.3 mg/L, the reactor performance was slightly inhibited. In addition, judging from the particle size and settling properties, FA can promote anammox sludge granulation. After 53 days of exposure to FA, the genus Anaerolineaincreased in number, while Denitratisoma decreased. Candidatus Jettenia and Candidatus Kuenenia survived and enriched in the changed environment, potentially due to the interaction between anammox bacteria and some heterotrophic bacteria, which could protect anammox bacteria from adverse environments. These results indicate that FA can change the bacterial community and trigger different microbial interaction mechanisms within the anammox reactor.

Enhancement of nitrite production via addition of hydroxylamine to partial denitrification (PD) biomass: Functional genes dynamics and enzymatic activities

This study investigated the activity of partial denitrification (PD) biomass/key enzymes, functional gene expressions in response to 0 ~ 50 mg/L hydroxylamine (NH₂OH) addition. Results indicated that NH₂OH contributed to nitrite (NO₂^--N) production, facilitating the maximum increase of nitrate (NO₃^--N) to NO₂^--N transformation ratio to 80.47 ± 2.82%, leading to 2.56-fold NO₂^--N higher than those of control. The observed transient inhibitory effect on NO₃^--N reduction process was attributed by high-level NH₂OH (35 ~ 50 mg/L). Enzymatic assays revealed the enhanced activity of both NO₃^--N and NO₂^--N reductase while the former showed obvious superiority which led to high NO₂^--N accumulation. These results were further confirmed by the corresponding functional genes (narG, napA, nirS and nirK). Besides, negative influence of NH₂OH addition was limited to PD aggregates, due to the increasing secretion of extracellular polymeric substances (EPS) as well as proteins/polysaccharides ratios in tightly-bound structure of EPS.

Enhancement of pollutants removal from saline wastewater through simultaneous anammox and denitrification (SAD) process with glycine betaine addition

Enhanced pollutants removal from saline wastewater was investigated in simultaneous anammox and denitrification (SAD) process with glycine betaine (GB) addition. Long-term operation indicated the optimal GB dose was around 0.4 mM, which enhanced both anammox and denitrifying activity by 30% and 45%, respectively. The total nitrogen and organic removal rates were 0.38 ± 0.2 kgN/m³/d and 0.34 ± 0.3 kgCOD/m³/d, respectively, which increased by 34.5% and 20.5%. Independent of GB dose, denitrifying activity was promoted, but anammox activity was drastically deteriorated after excessive GB addition. The optimal GB dose predicated by both Gaussian and Modified-Boltzmann models were 0.42-0.45 mM. Besides, the bacterial activity recovery after excessive GB addition could be analyzed by the Modified-Boltzmann model. With 1.5 mM GB, granular floatation occurred since numerous gas bubbles were inside the granules. In general, exogenous GB addition can mitigate salinity inhibition and promote pollutants removal from saline wastewater.

Advances and challenges of mainstream nitrogen removal from municipal wastewater with anammox-based processes

Anaerobic ammonium oxidation (anammox) is a novel process of deammonification that exhibits superior ecological and economic potential compared to that of traditional heterotrophic processes. Although this process has been successfully implemented in treating high-strength nitrogen-contaminated wastewater, it still faces many challenges in treating mainstream municipal wastewater. This review aims to provide an overview of the status and challenges of mainstream anammox-based processes. The different configurations and crucial factors are discussed in this review. Finally, the future needs for feasible application are stated. PRACTITIONER POINTS: Factors restricting mainstream application of anammox-based processes are reviewed. Control strategies for selecting and maintaining anammox bacteria are discussed. Recent advances in nitrite production via partial nitrification or denitrification are summarized. Future needs for the feasible application of anammox-based nitrogen removal technology for mainstream municipal wastewater treatment are outlined.

Facilitating sludge granulation and favoring glycogen accumulating organisms by increased salinity in an anaerobic/micro-aerobic simultaneous partial nitrification, denitrification and phosphorus removal (SPNDPR) process

This study used salinity (0.5 wt%, 0.75 wt%) to accelerate the formation of ammonia oxidizing bacteria (AOB)-enriched aerobic granular sludge in a lab-scale anaerobic/micro-aerobic simultaneous partial nitrification, denitrification and phosphorus removal (SPNDPR) reactor. Results confirmed that the average granule diameter increased from 298.7 to 425.4 µm after 45 days of salinity stress even with low dissolved oxygen. Extracellular polymeric substances increased from 149.5 to 387.7 mg/g VSS after salinity (0.75 wt%) treatment, in turn accelerating granulation. Partial nitrification was maintained under the salinity condition due to the relative high activity and abundance of AOB, and the observed nitrite accumulation ratio averaged 98.9%. Salinity favored glycogen-accumulating organisms over polyphosphate-accumulating organisms (PAOs)/denitrifying-PAOs, with the abundance of Candidatus_Competibacter increasing from 4.86% to 15.34% and the simultaneous partial nitrification-denitrification efficiency increasing from 74.4% to 91.1%, promoting N-removal potential. The P-removal performance was good under 0.5 wt% salinity but was inhibited under 0.75 wt% salinity.

Development of a denitrification system using primary sludge as solid carbon source - Potential to couple with anammox process

Heterotrophic denitrification is a robust and reliable process for nitrogen removal from wastewater. However, wastewater often faces the issue of lacking carbon source. In this study, the feasibility of using primary sludge, a by-product of wastewater treatment plants, to support denitrification of high-strength nitrite wastewater was investigated. Results suggest the desired performance can be achieved with the influent nitrite concentration of 400 to 1200 mg N/L, and the optimal primary sludge dosage for the complete nitrite removal was 3.6 g VSS/g N. Ammonium removal was also detected along with nitrite removal. Microbial analysis reveals various types of denitrifying bacteria and a large number of macromolecular organics degrading bacteria existed in the microbial community. Notably, anammox bacteria, Candidatus Brocadia, was also identified with an abundance of 0.1%. The slow kinetics of carbon source release from primary sludge was likely the reason for the existence of anammox process. This study developed a promising nitrogen removal process using an alternative carbon source for denitrification, and it shows great potential to couple denitrification with anammox to reduce ammonium residue.

Performance of the anammox process treating low-strength municipal wastewater under low temperatures: Effect of undulating seasonal temperature variation

In the anammox process treating low-strength municipal wastewater, the effect of common seasonal temperature variation (15.1 °C-22.2 °C) on performance was studied. In autumn and winter, the nitrogen removal rate (NRR) decrement of 0.038kgN/(m³·d) (17.9 °C → 15.1 °C) was nearly threefold higher than 0.014kgN/(m³·d) (22.2 °C → 17.9 °C), which showed that lower temperature laid more negative impact on nitrogen removal. ¹⁵N isotope tracing tests confirmed that the contribution of denitrification to nitrogen removal was far less than anammox, and anammox contributed more at 15.1 °C (91.7%) than 21.9 °C (78.9%). Anammox bacteria could adapt to lower temperature after short-term acclimatization, especially the dominant genus Ca. Brocadia increased from 1.8% to 2.5% and its abundance was significantly correlated with nitrogen consumption (p < 0.05). Above findings suggest that the adaptability of Ca. Brocadia could provide the possibility to maintain nitrogen removal performance at lower temperature. In spring, the improved maximum anammox activity from 2.85 to 3.23mgNH₄⁺-N/(gVSS·h) indicated the recovered removal capacity.

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.

Nutrient removal and microbial community in a two-stage process: Simultaneous enhanced biological phosphorus removal and semi-nitritation (EBPR-SN) followed by anammox

This study developed a two-stage process, including simultaneous enhanced biological phosphorus-removal and semi-nitritation (EBPR-SN) sequencing batch reactor (SBR), followed by Anammox SBR, to achieve advanced nitrogen (N) and phosphorus (P) removal from real sewage with low carbon/nitrogen (2.82). The long-term operation suggested that removal efficiencies for TIN (86.2 ± 3.5%) and P (95.0 ± 5.5%) were stably obtained, with nitrite accumulation ratio of 98.7% in EBPR-SN SBR. Mechanism analysis indicated contribution of anammox to N-removal being 57.3%-73.7% and superior P-removal due to the majority of removed organics (~74.5%) being stored by polyphosphate-accumulating organisms (PAOs). In EBPR-SN SBR, high-throughput sequencing showed ammonium-oxidizing bacteria was 0.03% while nitrite-oxidizing bacteria was not detected, and PAOs accounted for 30.07%. In Anammox SBR, Candidatus Brocadia (9.75%) was the only anammox bacteria. Remarkably, short aerobic hydraulic retention time (4.29 h) with low DO (0.3-1.2 mg/L) during the whole process provided desirable energy-saving.