Overcoming Nitrite Oxidizing Bacteria Adaptation through Alternating Sludge Treatment with Free Nitrous Acid and Free Ammonia

Achieving rapid granulation and long-term stability of partial nitritation /anammox process by uniquely configured airlift inner-circulation partition bioreactor

To maintain the long-term stability and efficiency of the partial nitritation/anammox (PN/A) process, a novel partition bioreactor featuring a uniquely sieve plate was developed to improve the airlift inner-circulation. The bioreactor achieved startup within 38 days, effectively handling influent containing 150 mg-N/L ammonium nitrogen and 50 mg-N/L nitrite. By reducing hydraulic retention time, nitrogen loading rate was escalated to 0.60 kg-N/m3/d, maintaining over 80 % nitrogen removal. Additionally, fluctuations in nitrite-oxidizing bacteria (NOB) were automatically controlled through dissolved oxygen (DO) partitioning. Moreover, the average granules size expanded from 85 μm to 338 μm by day 127, coinciding with robust anammox activity reaching 1.02 ± 0.05 g-N/g-VSS/d by day 179. The results demonstrate that the bioreactor effectively enhanced the enrichment of functional bacteria, enabled spatial distribution of DO, promoted NOB self-regulation and sludge granulation. This approach provides an efficient solution for rapid granulation while maintaining stable performance in the PN/A process.

Initial free ammonia is critical for robust acidic partial nitrification in sequencing batch reactor

The full implementation of partial nitrification (PN) is still limited by low influent NH4+-N concentrations. The stability and boundary conditions for nitrite-oxidizing bacteria (NOB) inhibition in the sequencing batch reactor (SBR) mode for low-strength wastewater with dynamic environments remain ambiguous. This study attempted to identify the critical factors and extended boundary conditions for the stable PN in SBR. In this study, based on the long-term operation (580 days) of a conventionally configured SBR, we investigated the robustness of the more complex combined inhibition by dynamic free ammonia (FA) and free nitrous acid conversion on different NOB in SBR. The experimental results revealed that the initial FA concentration was critical for establishing PN under low-alkalinity influent conditions. The NOB genus Nitrospira, which grows in periodically acidic environments, was extremely sensitive to FA inhibition (KI_FA=0.35 mg NH3-N/L) and significantly reduced the FA concentration required for NOB inhibition. When the pH was lowered to 5.4, the acid-intolerant NOB genus Ca. Nitrotoga was effectively inhibited, which was essential for maintaining PN. Further, the influent concentration boundaries and corresponding discharge ratios for achieving PN in the SBR mode were predicted by measuring kinetic parameters and model development, and the results suggest that the initial FA concentration in the reactor is critical for achieving acidic PN. The results of this study can provide guidance for achieving robust acidic PN in conventionally configured SBR for low strength wastewater.

A Novel Perspective on the Instability of Mainstream Partial Nitrification: The Niche Differentiation of Nitrifying Guilds

Short-cut biological nitrogen removal (sBNR) favors the paradigm shift toward energy-positive and carbon-neutral wastewater treatment processes. Partial nitrification (PN) is a key approach to provide nitrite for anammox or denitritation during sBNR, and its stability is the precondition for achieving robust nitrogen removal performance. However, maintaining a stable mainstream PN process has been a long-standing challenge. This review analyzes the mainstream PN process from a microbial ecology perspective, focusing on the niche differentiation among nitrifiers. First, we propose that mainstream PN systems are ecologically unstable, and the failure of the mainstream PN process due to the reactivation of nitrite-oxidizing bacteria (NOB) can be regarded as a behavior to restore system stabilization. Thus, maintaining mainstream PN systems primarily relies on enhancing the niche differentiation between ammonia-oxidizing bacteria (AOB) and NOB. We then summarize the realized niches of indigenous nitrifiers within nitrification systems and discuss their ecophysiological characteristics (e.g., cell structure and substrate affinity) that define their specific ecological niches. By comparing the niche breadths of AOB and NOB on various niche axes, we further discuss their niche differentiation and identify the different responses of AOB (resistance) and NOB (resilience) to exogenous perturbations. Finally, we propose outlook for achieving a stable mainstream PN process through an ecological lens. This review provides ecological insights into the instability of the mainstream PN process, which is intended to guide the derivation of optimized strategies from a single-factor approach to integrated solutions.

Intelligent FA/FNA alternating strategy for nitrite-oxidizing bacteria inhibition: Data-driven prediction and process control

Alternating treatment with free ammonia (FA) and free nitrous acid (FNA) is an effective strategy to inhibit nitrite-oxidizing bacteria (NOB) in partial nitrification (PN) process. However, the current alternating treatment relies on manual assessment of nitrite accumulation rate (NAR), which poses challenges for automation. This study proposed an intelligent control system for automatic real-time switching between FA/FNA inhibition strategy and real-time control of inhibition concentration to realize stable NOB inhibition. By comparing the prediction performance of three models with different complexities for both classification and regression methods, support vector machine (SVM) was used to determine whether to alternate the strategies based on whether NAR was below 96 % and multilayer perceptron (MLP) was used for real-time control of FNA concentration by predicting the nitrite concentration. The high prediction accuracy of these two sub-models provides a solid foundation for the automatic control model of FA/FNA. Both models were trained and tested using an experimental dataset with manual alternating FA/FNA strategy over 180 days of a continuous flow PN reactor. After optimizing algorithms, the SVM had a 91.67 % classification accuracy, while the MLP showed an R2 of 0.96 and an RMSE of 53.16. During the real-time control of the intelligent control system, the SVM showed a classification accuracy of 97.5 % compared to actual measurements, and the R2 between the controlled FNA and the actual FNA is 0.83, with an RMSE of 0.04. The real-time operation demonstrates that the intelligent control system can promptly realize FA/FNA alternating and accurately control FNA concentration, maintaining NAR above 95.55 % while ammonium removal efficiency above 52.99 %. Compared to the current alternating treatment, the intelligent control strategy simplifies the manual operations and enables the automation of the FA/FNA alternating inhibition strategy, contributing to the stable and efficient operation of the PN process and promoting the application of PN/A process.

Insights into the start-up of acidic nitritation using conventional activated sludge: Process dynamics, nitrifiers succession, and pilot-scale demonstration

Acidic nitritation driven by acid-tolerant ammonia-oxidizing bacteria (AOB) has gained wide attention due to its potential in sustainable wastewater and sludge treatment. However, limited knowledge of initiating acidic nitration using conventional activated sludge hindered the wider studies and application of this technology at lab- and field-scale. This study evaluates three strategies for initiating acidic nitritation: a constant low hydraulic retention time (HRT); an extended initial HRT followed by manual HRT reduction; and pH-controlled HRT. All strategies successfully started acidic nitritation using seed sludge from a local wastewater treatment plant (WWTP) containing undetectable acid-tolerant AOB. Among the three strategies, pH-controlled HRT was the most efficient, with a smoother (minimal fluctuations) and faster (around 30 days) start-up process than the other two strategies. This was attributed to an initial redundancy in ammonia oxidation capacity (i.e. making the proton generation rate caused by ammonium oxidation exceed the alkalinity supply rate by influent), allowing AOB to overcome the activity valley during the transition from neutral to acid pH Level. Using pH as a real-time proxy of AOB activity also leveraged the unique low buffer capacity at acidic pH. Based on these findings, a pilot-scale acidic nitritation reactor treating diluted sidestream wastewater was initiated for the first time using the pH-controlled strategy. The pilot reactor immediately achieved nitrite accumulation and reached the target hydraulic loading rate quicker than the lab reactor, indicating higher influent nitrogen concentration may facilitate NOB suppression and a higher growth rate of acid-tolerant AOB. Based on those results, the versatile start-up strategies using both mainstream or sidestream wastewater were further discussed. Overall, this work greatly expands potential applications of acidic nitritation and paves the way for future field-scale applications.

Stable partial nitrification was achieved for nitrogen removal from municipal wastewater by gel immobilization: A pilot-scale study

As an energy and carbon saving process for nitrogen removal from wastewater, the partial nitrification and denitrification process (PN/D) has been extensively researched. However, achieving stable PN in municipal wastewater has always been challenging. In this study, a gel immobilized PN/D nitrogen removal process (GI-PN/D) was established. A 94 days pilot-scale experiment was conducted using real municipal wastewater with an ammonia concentration of 43.5 ± 5.3 mg N/L at a temperature range of 11.3-28.7℃. The nitrogen removal performance and associated pathways, shifts in the microbial community as well as sludge yield were investigated. The results were as follows: the effluent TN and COD were 0.6 ± 0.4 mg/L and 31.1 ± 3.8 mg/L respectively, and the NAR exceeding 95%. GI-PN/D achieved deep nitrogen removal of municipal wastewater through stable PN without taking any other measures. The primary pathways for nitrogen removal were identified as denitrification, simultaneous nitrification-denitrification, and aerobic denitrification. High-throughput sequencing analysis revealed that the immobilized fillers facilitated the autonomous enrichment of functional bacteria in each reactor, effectively promoting the dominance and stability of the microbial communities. In addition, GI-PN/D had the characteristic of low sludge yield, with an average sludge yield of 0.029 kg SS/kg COD. This study provides an effective technical for nitrogen removal from municipal wastewater through PN.

Achieving stable partial nitrification by exploiting lag phase of NOB recovery for selective washout

Stable inhibition of nitrite-oxidizing bacteria (NOB) is a significant challenge in achieving partial nitrification (PN) and partial nitrification-anaerobic ammonia oxidation (PNA). Growing evidence suggested that NOB can develop resistance to suppression over time, leading to the re-enrichment of NOB within reactors. To address these issues, this study aimed to achieve stable PN by regulating SRT to selectively washout NOB during the lag phase of activity recovery following FA/FNA exposure. The effects of this new strategy on nitrifying bacteria, sludge characteristics, and microbial interspecies relationships were investigated over a 150-day trial. Under the new strategy operation, the effluent ammonia, nitrite, and nitrate concentrations were 4.72 mg/L, 51.81 mg/L, and 3.30 mg/L, respectively, at a dissolved oxygen (DO) concentration of 0.18 mg/L, with a nitrite accumulation rate (NAR) remaining above 95%. This was attributed to the increasing relative abundance of ammonia-oxidizing bacteria (AOB) (Nitrosomonas) up to 25.86% and the decreasing relative abundance of NOB (Nitrospira, Nitrobacter and Candidatus Nitrotoga) to below the detection limit (1%) during the stabilization period. Analysis of the PN sludge characteristics revealed a tendency for sludge to form loosely structured aggregates, facilitating the potential rapid start-up of the PNA biofilm system or granular sludge. These findings suggest that the new strategy is a straightforward and effective method for achieving PN.

Unraveling rapid start-up and stable maintenance of partial nitrification in domestic wastewater under high dissolved oxygen

Partial nitrification (PN), is a promising nitrogen removal technology in wastewater treatment. Contrary to the dogma that low dissolved oxygen (DO) is more conducive to achieving PN, this study successfully established PN within 7 days under high DO conditions (> 6 mg/L). Ultra-stable PN was maintained over 143 days with an average nitrite accumulation ratio of 98 % treating real domestic wastewater. Kinetics indicated that the maximum activity difference increased to 40 folds between ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacterium (NOB), resulting in AOB prospering while NOB declined. High DO operation reshaped the nitrifier community with AOB genera relative abundance increased substantially (0.1 %-1.2 %), while the predominant NOB Nitrospira was below the detection limit. Batch test confirmed the reproducibility of this strategy to achieve PN using ordinary activated sludge. This study provides an update on developing a feasible approach for the rapid realization and stable maintenance of mainstream PN.

Formation and Fate of Reactive Nitrogen during Biological Nitrogen Removal from Water: Important Yet Often Ignored Chemical Aspects of the Nitrogen Cycle

Hydroxylamine, nitrous acid, and nitric oxide are obligate intermediates or side metabolites in different nitrogen-converting microorganisms. These compounds are unstable and susceptible to the formation of highly reactive nitrogen species, including nitrogen dioxide, dinitrogen trioxide, nitroxyl, and peroxynitrite. Due to the high reactivity and cytotoxicity, the buildup of reactive nitrogen can affect the interplay of microorganisms/microbial processes, stimulate the reactions with organic compounds like organic micropollutants (OMP) and act as the precursors of nitrous oxide (N2O). However, there is little understanding of the occurrence and significance of reactive nitrogen during biological nitrogen conversions in engineered water systems. In this review, we evaluate the formation and fate of reactive nitrogen produced by microorganisms involved in biological nitrogen removal (BNR) processes, i.e., nitritation/nitrification, denitratation/denitrification, anammox, and the combined processes. While the formation of reactive nitrogen intermediates is entirely controlled by microbial activities, the consumption can be either biological or purely chemical. Changes in environmental conditions, such as redox transition, pH, and substrate availability, can imbalance the production and consumption of these reactive intermediates, thus leading to the transient accumulation of species. Based on previous experimental evidence, environmental relevance of reactive nitrogen in BNR systems, particularly related to abiotic N2O production and OMP transformation, is demonstrated.

Saturated dissolved oxygen-driven high-rate and ultrastable partial nitrification in municipal wastewater

Achieving stable and high-rate partial nitrification (PN) remains a worldwide technical conundrum in low-strength mainstream conditions. This study successfully achieved ultrarapid mainstream PN within 8 days under a saturated dissolved oxygen (DO) supply strategy, reaching a record-breaking PN rate of over 1.0 kg N m-3 d-1 treating municipal wastewater. Stable PN was maintained for over 200 days with an ultrahigh nitrite accumulation ratio of 98.5 ± 0.9 %, resilient to seasonal fluctuations in temperature (16.0-25.6 °C) and load (NH4+-N, 40-80 mg N/L). Kinetics revealed a remarkable 159.1-fold increase in the maximum activity ratio of ammonia-oxidizing bacteria (AOB) to nitrite-oxidizing bacteria (NOB). The faster response of AOB to saturated DO stimulated its highest activity difference with NOB, contributing to the AOB (Nitrosomonas oligotropha) boom and the elimination of NOB groups (-99.9 %). Our results highlight the importance of promoting AOB rather than solely focusing on NOB suppression for initiating and stabilizing high-rate mainstream PN.

Coarse bubble mixing in anoxic zone greatly stimulates nitrous oxide emissions from biological nitrogen removal process

The biological nitrogen removal process in wastewater treatment inevitably produces nitrous oxide (N2O), a potent greenhouse gas. Coarse bubble mixing is widely employed in wastewater treatment processes to mix anoxic tanks; however, its impacts on N2O emissions are rarely reported. This study investigates the effects of coarse bubble mixing on N2O emissions in a pilot-scale mainstream nitrite shunt reactor over a 50-day steady-state period. Online and offline N2O monitoring campaigns show that coarse bubble mixing in the anoxic zones significantly elevates N2O emissions, yielding an extremely high emission factor of 15.5 ± 3.5 %. Intensive sampling and isotopic analyses suggest that the elevated emissions are primarily due to the inhibition of the N2O denitrification process by oxygen in the anoxic phase introduced by coarse bubbling. Substituting coarse bubble mixing with submersible pump mixing resulted in a substantial reduction of N2O emissions, decreasing the emission factor by an order of magnitude to 1.2 ± 0.8 %. The findings reveal that a previously overlooked factor, coarse bubble mixing, can significantly stimulate N2O emissions. The use of coarse bubble mixing in anoxic tanks of biological nitrogen removal warrants caution.

Achieving nitrite shunt using in-situ free ammonia enriched by natural zeolite: Pilot-scale mainstream anammox with flexible nitritation strategy

The mainstream partial nitritation/anammox (PN/A) process represents a significant innovation in decarbonizing municipal wastewater treatment. However, its implementation is considerably hampered by the challenge of stable nitrite supply. In this study, a pilot-scale PN/A system receiving real sewage (20 m3) was operated at room temperature for nearly one year. Remarkable PN performance with relatively high nitrite accumulation ratio of 75.04 ± 10.05 % was obtained via in-situ free ammonia (FA) strategy. The ammonium concentration enriched in the zeolite increased significantly by 548.8 times compared to that in the aqueous phase by ion exchange. This substantial increase robustly inhibited nitrite-oxidizing bacteria (NOB), resulting in high relative abundance ratio of ammonia-oxidizing bacteria (AOB) to NOB of 37.93 ± 12.61 in the zeolite biofilm, compared to 10.22 ± 1.67 in suspended floc sludge. The significant differences in FA concentrations between zeolite biofilm and suspended floc sludge resulted in distinct spatial distribution disparities of AOB and NOB, which were central to achieving stable nitrite accumulation without complex multiple selective pressures. Consequently, compliant effluent with total nitrogen of 10.91 ± 4.23 mg N/L was achieved at 10.4-31.1 °C without external carbon source addition. The biocarriers in the anammox process played a key role in enhancing functional genes and electron flow, supporting anammox-dominated nitrogen removal. This study presents a flexible and adaptable strategy for mainstream nitrite shunting, highlighting its potential for large-scale implementation of mainstream anammox treatment.

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

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

Rapid start-up and stable operation of pilot scale denitrification-partial nitritation/anammox process for treating electroplating tail wastewater

This study explored a novel economical and efficient process for treating actual low-ammonia nitrogen electroplating tail wastewater. A pilot scale system of denitrification-partial nitrification/anaerobic ammonium oxidation (DN-PN/A) was constructed and operated for 190 days. The partial nitrification (PN) reactor, filled with zeolite, increased free ammonia concentration beyond the nitrite oxidizing bacteria threshold and successfully supplied NO2--N, with nitrite accumulation rate exceeding 90 %. Over 109 days, the total nitrogen removal rate achieved was 80.2 ± 7.41 %, and the chemical oxygen demand removal rate reached 79.68 ± 9.53 %. The dominant functional bacteria were Nitrosomonas (5.45 %) and Candidatus Anammoxoglobus (28.84 %) in PN reactor and anaerobic ammonium oxidation (Anammox) reactor. This process, characterized by rapid start-up, strong shock resistance, and low cost, alleviates the pressure of ammonium pollution control, promotes the sustainable development of the electroplating industry and has the potential for application in the treatment of other industrial wastewater.

Dynamic pH regulation drives Nitrosomonas for high-rate stable acidic partial nitritation

How to intensify the ammonia oxidation rate (AOR) is still a bottleneck impeding the technology development for the innovative acidic partial nitritation because the eosinophilic ammonia-oxidizing bacteria (AOB), such as Nitrosoglobus or Nitrosospira, were inhibited by the high-level free nitrous acid (FNA) accumulation in acidic environments. In this study, an innovative approach of dynamic acidic pH regulation control strategy was proposed to realize high-rate acidic partial nitritation driven by common AOB genus Nitrosomonas. The acidic partial nitrification process was carried out in a laboratory-scale sequencing batch moving bed biofilm reactor (SBMBBR) for long-term (700 days) to track the effect of dynamic acidic pH on nitrifying bacterial activity. The results indicated that the influent NH4+-N concentration was about 100 mg/L, the nitrite accumulation ratio was exceeding 90%, and the maximum AOR can reach 14.5 ± 2.6 mg N L-1h-1. Although the half-saturation inhibition constant of NOB (KI_FNA(AOB)) reached 0.37 ± 0.10 mg HNO2N/L and showed extreme adaptability in FNA, the inactivation effect of FNA (6.1 mg HNO2N/L) for NOB was much greater than that of AOB, with inactivation rates of 0.61 ± 0.08 h-1 and 0.06 ± 0.01 h-1, respectively. The effluent pH was gradually reduced to 4.5 by ammonia oxidation process and the periodic FNA concentration reached 6.5 mg HNO2N/L to inactivate nitrite-oxidizing bacteria (NOB) without negatively affecting Nitrosomonas during long-term operation. This result provides new insights for the future implementation of high-rate stabilized acidic partial nitritation by Nitrosomonas.

Achieving Long-Term Stability of Partial Nitrification and Autotrophic Denitrification in an MABR via Sulfide Dosing

While partial nitrification (PN) has the potential to reduce energy for aeration, it has proven to be unstable when treating low-strength wastewater. This study introduces an innovative combined strategy incorporating a low rate of oxygen supply, pH control, and sulfide addition to selectively inhibit nitrite-oxidizing bacteria (NOB). This strategy led to a stable PN in a laboratory-scale membrane aerated biofilm reactor (MABR). Over a period of 260 days, the nitrite accumulation ratio exceeded 60% when treating synthetic sewage containing 50 mg NH4+-N/L. Through in situ activity testing and high-throughput sequencing, the combined strategy led to low levels of nitrite-oxidation activity (<5.5 mg N/m2 h), Nitrospira species (relative abundance <1%), and transcription of nitrite-oxidation genes (undetectable). The addition of sulfide led to simultaneous PN and autotrophic denitrification in the single-stage MABR, resulting in over 60% total inorganic nitrogen removal. Sulfur-based autotrophic denitrification consumed nitrite and inhibited NOB conversion of nitrite to nitrate. The combined strategy has potential to be applied in large-scale sewage treatment and deserves further exploration.

Superior mainstream partial nitritation in an acidic membrane-aerated biofilm reactor

Shortcut nitrogen removal holds significant economic appeal for mainstream wastewater treatment. Nevertheless, it is too difficult to achieve the stable suppression of nitrite-oxidizing bacteria (NOB), and simultaneously maintain the activity of ammonia-oxidizing bacteria (AOB). This study proposes to overcome this challenge by employing the novel acid-tolerant AOB, namely "Candidatus Nitrosoglobus", in a membrane-aerated biofilm reactor (MABR). Superior partial nitritation was demonstrated in low-strength wastewater from two aspects. First, the long-term operation (256 days) under the acidic pH range of 5.0 to 5.2 showed the successful NOB washout by the in situ free nitrous acid (FNA) of approximately 1 mg N/L. This was evidenced by the stable nitrite accumulation ratio (NAR) close to 100 % and the disappearance of NOB shown by 16S rRNA gene amplicon sequencing and fluorescence in situ hybridization. Second, oxygen was sufficiently supplied in the MABR, leading to an unprecedentedly high ammonia oxidation rate (AOR) at 2.4 ± 0.1 kg N/(m3 d) at a short hydraulic retention time (HRT) of a mere 30 min. Due to the counter diffusion of substrates, the present acidic MABR displayed a significantly higher apparent oxygen affinity (0.36 ± 0.03 mg O2/L), a marginally lower apparent ammonia affinity (14.9 ± 1.9 mg N/L), and a heightened sensitivity to FNA and pH variations, compared with counterparts determined by flocculant acid-tolerant AOB. Beyond supporting the potential application of shortcut nitrogen removal in mainstream wastewater, this study also offers the attractive prospect of intensifying wastewater treatment by markedly reducing the HRT of the aerobic unit.

Electrochemical-coupled sulfur-driven autotrophic denitrification for nitrogen removal from raw landfill leachate: Evaluation of performance and mechanisms

The cost-effective and environment-friendly sulfur-driven autotrophic denitrification (SdAD) process has drawn significant attention for advanced nitrogen removal from low carbon-to-nitrogen (C/N) ratio wastewater in recent years. However, achieving efficient nitrogen removal and maintaining system stability of SdAD process in treating low C/N landfill leachate treatment have been a major challenge. In this study, a novel electrochemical-coupled sulfur-driven autotrophic denitrification (ESdAD) system was developed and compared with SdAD system through a long-term continuous study. Superior nitrogen removal performance (removal efficiency of 89.1 ± 2.5 %) was achieved in ESdAD system compared to SdAD process when treating raw landfill leachate (influent total nitrogen (TN) concentration of 241.7 ± 36.3 mg-N/L), and the effluent TN concentration of ESdAD bioreactor was as low as 24.8 ± 5.1 mg-N/L, which meets the discharge standard of China (< 40 mg N/L). Moreover, less sulfate production rate (1.3 ± 0.2 mg SO42--S/mgNOx--N vs 1.7 ± 0.2 mg SO42--S/mgNOx--N) and excellent pH modulation (pH of 6.9 ± 0.2 vs 5.8 ± 0.4) were also achieved in the ESdAD system compared to SdAD system. The improvement of ESdAD system performance was contributed to coexistence and interaction of heterotrophic bacteria (e.g., Rhodanobacter, Thermomonas, etc.), sulfur autotrophic bacteria (e.g., Thiobacillus, Sulfurimonas, Ignavibacterium etc.) and hydrogen autotrophic bacteria (e.g., Thauera, Comamonas, etc.) under current stimulation. In addition, microbial nitrogen metabolic activity, including functional enzyme (e.g., Nar and Nir) activities and electron transfer capacity of extracellular polymeric substances (EPS) and cytochrome c (Cyt-C), were also enhanced during current stimulation, which facilitated the nitrogen removal and maintained system stability. These findings suggested that ESdAD is an effective and eco-friendly process for advanced nitrogen removal for low C/N wastewater.

Competitive bio-augmentation overcoming unusual direct inhibitor inefficacy in mainstream nitrite-oxidizing bacteria suppression: Unveiling the underpinnings in microbial and nitrogen metabolism aspects

The long-stabilized mainstream partial nitritation/Anammox (PN/A) process continues to encounter significant challenges from nitrite-oxidizing bacteria (NOB). Therefore, this study aimed to determine an efficient, rapid, and easily implementable strategy for inhibiting NOB. A laboratory-scale reactor was operated continuously for 325 days, experiencing NOB outbreak in mainstream and recovery with simulated sidestream support. The results show that direct inhibitory strategies including intermittent aeration and approximately 35 mg/L free ammonia had unusual weak inhibitory effects on NOB activity. Subsequently, the exogenous Anammox from sidestream employed as a competitive bio-augmentation approach rapidly inhibited NOB dynamics. Evidence suggests that the damaged hydroxyapatite granules under low pH conditions might have contributed to NOB dominance by diminishing Anammox bacteria activity, thereby creating a substrate-rich environment favoring NOB survival. In contrast, the introduction of exogenous Candidatus Kuenenia facilitated the nitrogen removal efficiency from 32.5 % to over 80 %. This coincided with a decrease in the relative abundance of Nitrospira from 16.5 % to 2.7 % and NOB activity from 0.34 to 0.07 g N/(g mixed liquor volatile suspended solid)/d. Metagenomic analysis reveals a decrease in the functional potential of most nitrite transport proteins, coupled with a significant increase in eukaryotic-like serine/threonine-protein kinase involved in cellular regulation, during the Anammox activity recovery. This study's findings reveal the feasibility of the bio-augmentation based on substrate competition, wherein sidestream processes support the mainstream PN/A integration, offering significant potential for practical applications.

Contrasting response of comammox Nitrospira, ammonia oxidising bacteria, and archaea to soil pH and nitrogen inputs

This study aimed to investigate the effect of soil pH change, and nitrogen amendment on ammonia oxidiser abundance and comammox Nitrospira community composition. The experimental design used soil mesocosms placed in a temperature-controlled incubator for 90 days. A Templeton silt loam was used as its physiochemical properties are typical of the region's dairy farms. The results showed that comammox Nitrospira clade B preferred the natural (pH 6.1-6.2) soil pH with no applied nitrogen. Furthermore, synthetic urine (N700) decreased the abundance of comammox Nitrospira clade B. This may have been because the large amounts of available ammonia in the N700 treatments inhibited the growth of comammox Nitrospira. These results suggest that while comammox Nitrospira clade B are present in New Zealand dairy farm soils, but their role in nitrification in the very high nitrogen environment under a urine patch in grazed pastures may be limited. Further research is needed to confirm this. In contrast to comammox, the AOB community (dominated by Nitrosospira) responded positively to the application of synthetic urine. The response was greatest in the high pH soil (7.1), followed by the natural and then the low pH (4.9) soils. This may be due to the difference in ammonia availability. At high pH, the ammonia/ammonium equilibrium favours ammonia production. Calculated ammonia availability in the N700 treatments accurately predicted the AOB amoA gene abundance. Interestingly, the AOA community abundance (which was predominantly made up of Thaumarchaeota group I.1b clade E) seemed to prefer the natural and high pH soils over the low pH. This may be due to the specific lineage of AOA present. AOA did not respond to the application of nitrogen.

Feasibility of low-intensity ultrasound treatment with hydroxylamine to accelerate the initiation of partial nitrification and allow operation under intermittent aeration

Partial nitrification is a key aspect of efficient nitrogen removal, although practically it suffers from long start-up cycles and unstable long-term operational performance. To address these drawbacks, this study investigated the effect of low intensity ultrasound treatment combined with hydroxylamine (NH2OH) on the performance of partial nitrification. Results show that compared with the control group, low-intensity ultrasound treatment (0.10 W/mL, 15 min) combined with NH2OH (5 mg/L) reduced the time required for partial nitrification initiation by 6 days, increasing the nitrite accumulation rate (NAR) and ammonia nitrogen removal rate (NRR) by 20.4% and 6.7%, respectively, achieving 96.48% NRR. Mechanistic analysis showed that NH2OH enhanced ammonia oxidation, inhibited nitrite-oxidizing bacteria (NOB) activity and shortened the time required for partial nitrification initiation. Furthermore, ultrasonication combined with NH2OH dosing stimulated EPS (extracellular polymeric substances) secretion, increased carbonyl, hydroxyl and amine functional group abundances and enhanced mass transfer. In addition, 16S rRNA gene sequencing results showed that ultrasonication-sensitive Nitrospira disappeared from the ultrasound + NH2OH system, while Nitrosomonas gradually became the dominant group. Collectively, the results of this study provide valuable insight into the enhancement of partial nitrification start-up during the process of wastewater nitrogen removal.

Ferric and sulfate coupled ammonium oxidation enhanced nitrogen removal in two-stage partial nitrification - Anammox/denitrification process for food waste liquid digestate treatment

Liquid digestate of food waste is an ammonium-, ferric- and sulfate-laden leachate produced during digestate dewatering, where the carbon source is insufficient for nitrogen removal. A two-stage partial nitrification-anammox/denitrification process was established for nitrogen removal of liquid digestate without pre-treatment (>300 d), through which nitrogen (95 %), biodegradable organics (100 %), sulfate (78 %) and iron (100 %) were efficiently removed. Additional ammonium conversion (20 %N) might be coupled with ferric and sulfate reduction, while produced nitrite could be further converted to di-nitrogen gas through anammox (75 %) and denitrification (25 %). Notably, since increasingly contribution of hydroxylamine producing nitrous oxide, and up-regulated expression of electron transfer and cytochrome c protein, the enhanced ammonium oxidation was probably conducted through extracellular polymeric substances-mediated electron transfer between sulfate/ferric-reducers and aerobic ammonium oxidizers. Thus, the established partial nitrification-anammox/denitrification process might be a cost-efficient nitrogen removal technology for liquid digestate, benefitting to domestic waste recycling and carbon neutralization.

Effective recovery of the nitritation process through hydrogen peroxide

This study successfully achieved stable nitritation by adding hydrogen peroxide (H2O2) to the nitrification sludge whose nitritation stability had been destroyed. The batch experiment demonstrated that, the activity of ammonia-oxidizing bacteria (AOB) was restored more rapidly than that of nitrite oxidizing bacteria (NOB) after the addition of H2O2, thereby selectively promoting AOB enrichment and NOB washout. When the H2O2 concentration was 6.25 mg/L, the NOB activity was significantly reduced and the nitrite accumulation rate (NAR) was more than 95% after 18 cycles of nitrifying sludge restoration. As a result, H2O2 treatment enabled a nitrifying reactor to recover stable nitritation performance via H2O2 treatment, with the NAR and ammonia removal efficiency (ARE) both exceeding 90%. High-throughput sequencing analysis revealed that H2O2 treatment was successful in restoring nitritation, as the relative abundance of Nitrosomonas in the nitrifying reactor increased from 6.43% to 41.97%, and that of Nitrolancea decreased from 17.34% to 2.37%. Recovering nitritation by H2O2 inhibition is a low operational cost, high efficiency, and non-secondary pollution nitritation performance stabilization method. By leveraging the varying inhibition degrees of H2O2 on AOB and NOB, stable nitrification can be efficiently restored at a low cost and without causing secondary pollution.

Research progress of anaerobic ammonium oxidation (Anammox) process based on integrated fixed-film activated sludge (IFAS)

The integrated fixed-film activated sludge (IFAS) process is considered one of the cutting-edge solutions to the traditional wastewater treatment challenges, allowing suspended sludge and attached biofilm to grow in the same system. In addition, the coupling of IFAS with anaerobic ammonium oxidation (Anammox) can further improve the efficiency of biological denitrification. This paper summarises the research progress of IFAS coupled with the anammox process, including partial nitrification anammox, simultaneous partial nitrification anammox and denitrification, and partial denitrification anammox technologies, and describes the factors that limit the development of related processes. The effects of dissolved oxygen, influent carbon source, sludge retention time, temperature, microbial community, and nitrite-oxidising bacteria inhibition methods on the anammox of IFAS are presented. At the same time, this paper gives an outlook on future research focus and engineering practice direction of the process.

Startup of a large height-diameter ratio bioreactor by alternate feeding: performance of partial nitrification and enrichment of ammonia-oxidizing bacteria (AOB)

In order to solve the complicated control of dissolved oxygen (DO) for partial nitrification in bioreactors treating high NH 4 + - N wastewater, a large height-diameter ratio anammox pre-reactor system was developed. And in this reactor, NO 2 - - N accumulation rate can reach 85.76% by alternate feeding with high NH 4 + - N wastewater (150 mg NH 4 + - N / L ) and low NH 4 + - N wastewater (50 mg c) with low DO (0.19 mg/L-0.62 mg/L). Based on 16S rRNA identification technology, it was found that Nitrosomonas had a significant effect on NH 4 + - N oxidization in this study. And when the reactor treated higher concentration wastewater (250 mg NH 4 + - N / L ), the growth rate of Nitrosomonas was higher than that of Nitrospira (nitrite-oxidizing bacteria, NOB), which was conducive to improving the NO 2 - - N accumulation rate and realizing partial nitrification stably. It was also found that the material exchange frequency of the microbial flora during alternate feeding with different NH 4 + - N concentration wastewaters was higher than that during feeding with higher NH 4 + - N concentration wastewater (250 mg/L) by Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolism pathways analysis. This study can provide valuable insights and lay the foundation for building anammox pre-reactors.

Recurrent multi-stressor floc treatments with sulphide and free ammonia enabled mainstream partial nitritation/anammox

Selective suppression of nitrite-oxidising bacteria (NOB) over aerobic and anoxic ammonium-oxidising bacteria (AerAOB and AnAOB) remains a major challenge for mainstream partial nitritation/anammox implementation, a resource-efficient nitrogen removal pathway. A unique multi-stressor floc treatment was therefore designed and validated for the first time under lab-scale conditions while staying true to full-scale design principles. Two hybrid (suspended + biofilm growth) reactors were operated continuously at 20.2 ± 0.6 °C. Recurrent multi-stressor floc treatments were applied, consisting of a sulphide-spiked deoxygenated starvation followed by a free ammonia shock. A good microbial activity balance with high AnAOB (71 ± 21 mg N L-1 d-1) and low NOB (4 ± 17 % of AerAOB) activity was achieved by combining multiple operational strategies: recurrent multi-stressor floc treatments, hybrid sludge (flocs & biofilm), short floc age control, intermittent aeration, and residual ammonium control. The multi-stressor treatment was shown to be the most important control tool and should be continuously applied to maintain this balance. Excessive NOB growth on the biofilm was avoided despite only treating the flocs to safeguard the AnAOB activity on the biofilm. Additionally, no signs of NOB adaptation were observed over 142 days. Elevated effluent ammonium concentrations (25 ± 6 mg N L-1) limited the TN removal efficiency to 39 ± 9 %, complicating a future full-scale implementation. Operating at higher sludge concentrations or reducing the volumetric loading rate could overcome this issue. The obtained results ease the implementation of mainstream PN/A by providing and additional control tool to steer the microbial activity with the multi-stressor treatment, thus advancing the concept of energy neutrality in sewage treatment plants.

Potential stimulation of nitrifying bacteria activities and genera by landfill leachate

With the increasing complexity of influent composition in wastewater treatment plants, the potential stimulating effects of refractory organic matter in wastewater on growth characteristics and genera conversion of nitrifying bacteria (ammonium-oxidizing bacteria [AOB] and nitrite-oxidizing bacteria [NOB]) need to be further investigated. In this study, domestic wastewater was co-treated with landfill leachate in the lab-scale reactor, and the competition and co-existence of NOB genera Nitrotoga and Nitrospira were observed. The results demonstrated that the addition of landfill leachate could induce the growth of Nitrotoga, whereas Nitrotoga populations remain less competitive in domestic wastewater operation. In addition, the refractory organic matter in the landfill leachate also would have a potential stimulating effect on the maximum specific growth rate of AOB genus Nitrosomonas (μmax, aob). The μmax, aob of Nitrosomonas in the control group was estimated to be 0.49 d-1 by fitting the ASM model, and the μmax, aob reached 0.66-0.71 d-1 after injection of refractory organic matter in the landfill leachate, while the maximum specific growth rate of NOB (μmax, nob) was always in the range of 1.05-1.13 d-1. These findings have positive significance for the understanding of potential stimulation on nitrification processes and the stable operation of innovative wastewater treatment process.

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

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

Regulation mechanism of hydrazine and hydroxylamine in nitrogen removal processes: A Comprehensive review

As the new fashioned nitrogen removal process, short-cut nitrification and denitrification (SHARON) process, anaerobic ammonium oxidation (anammox) process, completely autotrophic nitrogen removal over nitrite (CANON) process, partial nitrification and anammox (PN/A) process and partial denitrification and anammox (PD/A) process entered into the public eye due to its advantages of high nitrogen removal efficiency (NRE) and low energy consumption. However, the above process also be limited by long-term start-up time, unstable operation, complicated process regulation and so on. As intermediates or by-metabolites of functional microorganisms in above processes, hydroxylamine (NH2OH) and hydrazine (N2H4) improved NRE of the above processes by promoting functional enzyme activity, accelerating electron transport efficiency and regulating distribution of microbial communities. Therefore, this review discussed effects of NH2OH and N2H4 on stability and NRE of above processes, analyzed regulatory mechanism from functional enzyme activity, electron transport efficiency and microbial community distribution. Finally, the challenges and limitations for nitric oxide (NO) and nitrous oxide (N2O) produced from regulation of NH2OH and N2H4 are discussed. In additional, perspectives on future trends in technology development are proposed.

Nitrous oxide emissions in novel wastewater treatment processes: A comprehensive review

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 (N2O) production and emission. This review provides a comprehensive overview of recent research progress on N2O 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 N2O sink. Furthermore, the review discusses the potential applications and rationale for novel Anammox-based processes to reduce N2O 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.

How to Provide Nitrite Robustly for Anaerobic Ammonium Oxidation in Mainstream Nitrogen Removal

Innovation in decarbonizing wastewater treatment is urgent in response to global climate change. The practical implementation of anaerobic ammonium oxidation (anammox) treating domestic wastewater is the key to reconciling carbon-neutral management of wastewater treatment with sustainable development. Nitrite availability is the prerequisite of the anammox reaction, but how to achieve robust nitrite supply and accumulation for mainstream systems remains elusive. This work presents a state-of-the-art review on the recent advances in nitrite supply for mainstream anammox, paying special attention to available pathways (forward-going (from ammonium to nitrite) and backward-going (from nitrate to nitrite)), key controlling strategies, and physiological and ecological characteristics of functional microorganisms involved in nitrite supply. First, we comprehensively assessed the mainstream nitrite-oxidizing bacteria control methods, outlining that these technologies are transitioning to technologies possessing multiple selective pressures (such as intermittent aeration and membrane-aerated biological reactor), integrating side stream treatment (such as free ammonia/free nitrous acid suppression in recirculated sludge treatment), and maintaining high activity of ammonia-oxidizing bacteria and anammox bacteria for competing oxygen and nitrite with nitrite-oxidizing bacteria. We then highlight emerging strategies of nitrite supply, including the nitrite production driven by novel ammonia-oxidizing microbes (ammonia-oxidizing archaea and complete ammonia oxidation bacteria) and nitrate reduction pathways (partial denitrification and nitrate-dependent anaerobic methane oxidation). The resources requirement of different mainstream nitrite supply pathways is analyzed, and a hybrid nitrite supply pathway by combining partial nitrification and nitrate reduction is encouraged. Moreover, data-driven modeling of a mainstream nitrite supply process as well as proactive microbiome management is proposed in the hope of achieving mainstream nitrite supply in practical application. Finally, the existing challenges and further perspectives are highlighted, i.e., investigation of nitrite-supplying bacteria, the scaling-up of hybrid nitrite supply technologies from laboratory to practical implementation under real conditions, and the data-driven management for the stable performance of mainstream nitrite supply. The fundamental insights in this review aim to inspire and advance our understanding about how to provide nitrite robustly for mainstream anammox and shed light on important obstacles warranting further settlement.

The microbiome of two strategies for ammonia removal with the sequencing batch moving bed biofilm reactor treating cheese production wastewater

IMPORTANCE

Cheese production facilities must abide by sewage discharge bylaws that prevent overloading municipal water resource recovery facilities, eutrophication, and toxicity to aquatic life. Compact treatment systems can permit on-site treatment of cheese production wastewater; however, competition between heterotrophs and nitrifiers impedes the implementation of the sequencing batch moving bed biofilm reactor (SB-MBBR) for nitrification from high-carbon wastewaters. This study demonstrates that a single SB-MBBR is not feasible for nitrification when operated with anerobic and aerobic cycling for carbon and phosphorous removal from cheese production wastewater, as nitrification does not occur in a single reactor. Thus, two reactors in series are recommended to achieve nitrification from cheese production wastewater in SB-MBBRs. These findings can be applied to pilot and full-scale SB-MBBR operations. By demonstrating the potential to implement partial nitrification in the SB-MBBR system, this study presents the possibility of implementing partial nitrification in the SB-MBBR, resulting in the potential for more sustainable treatment of nitrogen from cheese production wastewater.

The stable operation of nitritation process with the continuous granular sludge-type reactor and microbial community analysis

The nitritation step is the fundament for the biological nitrogen removal regardless of the traditional nitrification and denitrification process, the nitrite shunt process or the anammox process. Thus, exploring the effective nitritation performance is an important aspect of biological nitrogen removal. This study explored the upper limit of nitritation rate by increasing hydraulic residence time with the well-mixed and continuous granular sludge-type reactor characterized with low complexity and easy operation. The results showed that with the nitrogen loading rate of 1.0 kg/m3/d, the nitrite production rate could reach up to 0.65 kg/m3/d with the nitrite production efficiency of 63.49%, which is remarkable compared to that in the previously similar research. The microbial analysis indicated that ammonia-oxidizing bacteria was successfully enriched (13.27%) and genus Nitrosomonas was the dominant bacteria type. Besides, the activity of ammonium oxidizing bacteria in the continuous flow reactor was higher than that of other reactor types. The growth of vorticella on the sludge was also found in the reactor. The test of specific sludge activity and the microbe analysis both indicated that the nitrite-oxidizing bacteria was well inhibited during the whole experiment, which indicated the strategy of simply adjusting the dissolved oxygen is effective for running of nitritation process. The phosphorus removal performance was also achieved with a removal efficiency of 23.53%. The functional composition of the microbial community in the samples was predicted and finally transformation mechanism of nitrogen in sludge was drawn. In sum, this study indicated the superior performance of the granule sludge-type nitritation process and give a reference for the application of biological nitrogen removal technology.

Dynamic Simulation of Nitrifying Microbial Communities for Establishing Acidic Partial Nitritation in Suspended Activated Sludge

Acidic partial nitritation (PN) is a promising technology to achieve low-cost and energy-efficient shortcut nitrogen removal from wastewater. However, a comprehensive understanding of the acidic PN under dynamic changes of pH in a sequencing batch reactor (SBR) is still lacking. In this study, we successfully established acidic PN (NO2- accumulation ratio >80%) under dynamic pH variation from 7.0 to 4.5 in a lab-scale SBR. By accumulating in situ free nitrous acid (FNA) generation based on the dynamic pH change, acidic PN maintained stability even at a low NH4+ concentration of 100 mg N L-1. The microbial community analysis revealed that two ammonium-oxidizing bacteria (AOB) genera, Nitrosospira and Nitrosomonas, successfully coexisted and cooperated during acidic PN. None of the species of nitrite-oxidizing bacteria (NOB) showed adaptation to intermittent inhibition of in situ FNA even under high DO conditions (>4.0 mg O2 L-1). Furthermore, we innovatively incorporated the classic nitrification model with the growth and decay of different nitrifying bacterial species and their inhibition by pH, FNA, and free ammonia (FA) to predict the nitrifying microbial communities shifting for establishing acidic PN. The extended model was calibrated by using short-term batch experiments and was validated by using long-term dynamic data of the nitrifying microbial community during SBR operation. The validated model was further used to identify feasible influent conditions for the SBR PN process, including influent HCO3- concentration, NH4+ concentration and molar ratio (HCO3/NH4+). Outcomes from this study support the optimal design of acidic PN-based short-cut nitrogen removal processes for future application.

Comparison of typical nitrite oxidizing bacteria suppression strategies and the effect on nitrous oxide emissions in a biofilm reactor

In mainstream partial nitritation/anammox (PN/A), suppression of nitrite oxidizing bacteria (NOB) and mitigation of N2O emissions are two essential operational goals. The N2O emissions linked to three typical NOB suppression strategies were tested in a covered rotating biological contactor (RBC) biofilm system at 21 °C: (i) low dissolved oxygen (DO) concentrations, and treatments with (ii) free ammonia (FA), and (iii) free nitrous acids (FNA). Low emerged DO levels effectively minimized NOB activity and decreased N2O emissions, but NOB adaptation appeared after 200 days of operation. Further NOB suppression was successfully achieved by periodic (3 h per week) treatments with FA (29.3 ± 2.6 mg NH3-N L-1) or FNA (3.1 ± 0.3 mg HNO2-N L-1). FA treatment, however, promoted N2O emissions, while FNA did not affect these. Hence, biofilm PN/A should be operated at relatively low DO levels with periodic FNA treatment to maximize nitrogen removal efficiency while avoiding high greenhouse gas emissions.

Nitrite soaking pretreatment induced initial denitrifying nitrite accumulation

Partial denitrification (PD) could be another method for obtaining nitrite. However, PD startup takes a long time limiting its investigation and application. This study proposed nitrite soaking as a pretreatment method for starting PD. Results showed that denitrifying nitrite accumulation (4.20 mg/L) emerged after previously soaking by 10 mg/L nitrite for 9 h. When the duration was 6 h, comparing different soaked nitrite concentrations, the highest denitrifying nitrite accumulation amount (4.92 mg/L) was obtained in the 20 mg/L group. Nevertheless, high pH of 9 and frequent feeding could further advantage denitrifying nitrite accumulation. Pretreatment as a disturbance would impel the microbial community to change from complete denitrification towards PD.

Insights on biological phosphorus removal with partial nitrification in single sludge system via sidestream free ammonia and free nitrous acid dosing

The sidestream sludge treatment by free ammonium (FA)/free nitrous acid (FNA) dosing was frequently demonstrated to maintain the nitrite pathway for the partial nitrification (PN) process. Nevertheless, the inhibitory effect of FA and FNA would severely influence polyphosphate accumulating organisms (PAOs), destroying the microbe-based phosphorus (P) removal. Therefore, a strategic evaluation was proposed to successfully achieve biological P removal with a partial nitrification process in a single sludge system by sidestream FA and FNA dosing. Through the long-term operation of 500 days, excellent phosphorus, ammonium and total nitrogen removal performance were achieved at 97.5 ± 2.6 %, 99.1 ± 1.0 % and 75.5 ± 0.4 %, respectively. Stable partial nitrification with a nitrite accumulation ratio (NAR) of 94.1 ± 3.4 was attained. The batch tests also reported the robust aerobic phosphorus uptake based on FA and FNA adapted sludge after exposure of FA and FNA, respectively, suggesting the FA and FNA treatment strategy could potentially offer the opportunity for the selection of PAOs, which synchronously have the tolerance to FA and FNA. Microbial community analysis suggested that Accumulibacter, Tetrasphaera, and Comamonadaceae collectively contributed to the phosphorus removal in this system. Summarily, the proposed work presents a novel and feasible strategy to integrate enhanced biological phosphorus removal (EBPR) and short-cut nitrogen cycling and bring the combined mainstream phosphorus removal and partial nitrification process closer to practical application.

Nitrite Oxidation in Wastewater Treatment: Microbial Adaptation and Suppression Challenges

Microbial nitrite oxidation is the primary pathway that generates nitrate in wastewater treatment systems and can be performed by a variety of microbes: namely, nitrite-oxidizing bacteria (NOB). Since NOB were first isolated 130 years ago, the understanding of the phylogenetical and physiological diversities of NOB has been gradually deepened. In recent endeavors of advanced biological nitrogen removal, NOB have been more considered as a troublesome disruptor, and strategies on NOB suppression often fail in practice after long-term operation due to the growth of specific NOB that are able to adapt to even harsh conditions. In line with a review of the history of currently known NOB genera, a phylogenetic tree is constructed to exhibit a wide range of NOB in different phyla. In addition, the growth behavior and metabolic performance of different NOB strains are summarized. These specific features of various NOB (e.g., high oxygen affinity of Nitrospira, tolerance to chemical inhibitors of Nitrobacter and Candidatus Nitrotoga, and preference to high temperature of Nitrolancea) highlight the differentiation of the NOB ecological niche in biological nitrogen processes and potentially support their adaptation to different suppression strategies (e.g., low dissolved oxygen, chemical treatment, and high temperature). This review implicates the acquired physiological characteristics of NOB to their emergence from a genomic and ecological perspective and emphasizes the importance of understanding physiological characterization and genomic information in future wastewater treatment studies.

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

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

Start-up and maintenance of indigenous microalgae-bacteria consortium treating toilet wastewater through partial nitrification and nitrite-type denitrification

Microalgae-bacteria consortium (MBC) provides an alternative to sustainable treatment of human toilet wastewater (TWW) and resource recovery. This study compared the conventional activated sludge system and wastewater indigenous MBC system (IMBC) for nitrogen removal in TWW through the coupled partial nitrification (PN) and nitrite-type denitrification process. PN was firstly established by alternating FA and FNA. Subsequently, the successful PN maintenance with the nitrite accumulation rate ranging between 90.1-95.3% was achieved using two strategies: light irradiation with the appropriate specific light energy density at 0.0188-0.0598 kJ/mg VSS and the timely nitrite-type denitrification with the algae-secreted organics as the carbon source, eventually resulting in the nitrite accumulation rate ranging between 90.1-95.3%. In the IMBC-PN system, bacterial metabolism contributed to 91.5% of nitrogen removal and the rest was through microalgal assimilation. This study offers a sustainable hybrid IMBC-PN process for high NH₄⁺-N strength wastewater treatment (e.g., TWW), which theoretically saves 23.5% aeration and 34.2% carbon source as well as reduces 17.0% sludge production.

One-year stable pilot-scale operation demonstrates high flexibility of mainstream anammox application

Mainstream nitrogen removal via anammox is widely recognized as a promising wastewater treatment process. However, its application is challenging at large scale due to unstable suppression of nitrite-oxidizing bacteria (NOB). In this study, a pilot-scale mainstream anammox process was implemented in an Integrated Fixed-film Activated Sludge (IFAS) configuration. Stable operation with robust NOB suppression was maintained for over one year. This was achieved through integration of three key control strategies: i) low dissolved oxygen (DO = 0.4 ± 0.2 mg O₂/L), ii) regular free nitrous acid (FNA)-based sludge treatment, and iii) residual ammonium concentration control (NH₄ ⁺ with a setpoint of ∼8 mg N/L). Activity tests and FISH demonstrated that NOB barely survived in sludge flocs and were inhibited in biofilms. Despite receiving organic-deficient wastewater from a pilot-scale High-Rate Activated Sludge (HRAS) system as the feed, the system maintained a stable effluent total nitrogen concentration mostly below 10 mg N/L, which was attributed to the successful retention of anammox bacteria. This study successfully demonstrated large-scale long-term mainstream anammox application and generated new practical knowledge for NOB control and anammox retention.

Novel Use of a Ferric Salt to Enhance Mainstream Nitrogen Removal from Anaerobically Pretreated Wastewater

This study aims to demonstrate a new technology roadmap to support the ongoing paradigm shift in wastewater management from pollutant removal to resource recovery. This is achieved by developing a novel use of an iron salt (i.e., FeCl₃) in an integrated anaerobic wastewater treatment and mainstream anammox process. FeCl₃ was chosen to be dosed in a proposed sidestream unit rather than in a primary settler or a mainstream reactor. This causes acidification of returned activated sludge and enables stable suppression of nitrite-oxidizing bacterial activity and excess sludge reduction. A laboratory-scale system, which comprised an anaerobic baffled reactor, a continuous-flow anoxic-aerobic (A/O) reactor, and a secondary settler, was designed to treat real domestic wastewater, with the performance of the system comprehensively monitored under a steady-state condition. The experimental assessments showed that the system had good effluent quality, with total nitrogen and phosphorus concentrations of 12.6 ± 1.3 mg N/L and 0.34 ± 0.05 mg P/L, respectively. It efficiently retained phosphorus in excess sludge (0.18 ± 0.03 g P/g dry sludge), suggesting its potential for further recovery. About half of influent organic carbon was recovered in the form of bioenergy (i.e., methane). This together with low energy consumption revealed that the system could produce a net energy of about 0.11 kWh/m³-wastewater, assessed by an energy balance analysis.

In a quest for high-efficiency mainstream partial nitritation-anammox (PN/A) implementation: One-stage or two-stage?

Partial nitritation-anammox (PN/A) process is known as an energy-efficient technology for wastewater nitrogen removal, which possesses a great potential to bring wastewater treatment plants close to energy neutrality with reduced carbon footprint. To achieve this goal, various PN/A processes implemented in a single reactor configuration (one-stage system) or two separately dedicated reactors configurations (two-stage system) were explored over the past decades. Nevertheless, large-scale implementation of these PN/A processes for low-strength municipal wastewater treatment has a long way to go owing to the low efficiency and effectiveness in nitrogen removal. In this work, we provided a comprehensive analysis of one-stage and two-stage PN/A processes with a focus on evaluating their engineering application potential towards mainstream implementation. The difficulty for nitrite-oxidizing bacteria (NOB) out-selection was revealed as the critical operational challenge to achieve the desired effluent quality. Additionally, the operational strategies of low oxygen commonly adopted in one-stage systems for NOB suppression and facilitating anammox bacteria growth results in a low nitrogen removal rate (NRR). Introducing denitrification into anammox system was found to be necessary to improve the nitrogen removal efficiency (NRE) by reducing the produced nitrate with in-situ utilizing the organics from wastewater itself. However, this may lead to part of organics oxidized with additional oxygen consumed in one-stage system, further compromising the NRR. By applying a relatively high dissolved oxygen in PN reactor with residual ammonium control, and followed by a granules-based anammox reactor feeding with a small portion of raw municipal wastewater, it appeared that two-stage system could achieve a good effluent quality as well as a high NRR. In contrast to the widely studied one-stage system, this work provided a unique perspective that more effort should be devoted to developing a two-stage PN/A process to evaluate its application potential of high efficiency and economic benefits towards mainstream implementation.

Composite carrier enhanced bacterial adhesion and nitrogen removal in partial nitrification/anammox process

The rapid start-up and stable operation of one-stage (Partial nitrification/anammox) PN/A process for low-ammonium wastewater are difficult to be achieved, and many carriers are designed to solve this problem. Here, a composite carrier was developed, in which sepiolite and non-woven fabrics were assembled in polypropylene spherical shells. At the start-up phase, P_A reactor using the composite carriers reached a higher nitrogen removal rate of 134.50 ± 19.60 mg·N·L^-1d^-1, in contrast to that of 48.85 ± 19.64 mg·N·L^-1d^-1 in the P_B reactor without sepiolite carriers. When the final influent ammonium concentration of PN/A process is 100 mg/L, the total nitrogen removal efficiency can reach 72 ± 0.03 %. High biomass immobilization ability of composite carrier was evidenced by the greater adsorption trend between sludge and sepiolite than that between sludge and non-woven fabrics, where hydrophobic interaction and Van der Waals interaction played a major role. Extracellular protein (PN) content and the ratio of PN and extracellular polysaccharide of samples in P_A were significantly higher than those in P_B, verifying higher biofilm formation ability on the composite carrier. The composite carrier also increased the abundance of dominant bacteria in PN/A process, especially AOB, the relative abundance of which reached 46.11 %. And it increased the abundance of essential functional genes for nitrogen conversion as their perfect acid neutralizing effects. This study is of great significance in improving the start-up speed and stable operation of this process.

Achieving nitrogen removal with low material and energy consumption through partial nitrification coupled with short-cut sulfur autotrophic denitrification in a single-stage SBR

An innovative partial nitrification and short-cut sulfur autotrophic denitrification (PN-SSAD, NH₄⁺-N → NO₂^--N → N₂) coupled system in a single-stage SBR was proposed to treat low C/N wastewater with low material and energy consumption. Nearly 50 % alkalinity consumption and 40 % sulfate production were reduced in S⁰-SSAD compared with S⁰-SAD, whereas the autotrophic denitrification rate was increased by 65 %. In S⁰-PN-SSAD, the TN removal efficiency reached almost 99 % without additional organic carbon. Furthermore, pyrite (FeS₂) rather than S⁰ served as the electron donor to optimize the PN-SSAD process. The practical sulfate production in S⁰-PN-SSAD and FeS₂-PN-SSAD were about 38 % and 52 % lower than complete nitrification and sulfur autotrophic denitrification (CN-SAD), respectively. Thiobacillus was the major autotrophic denitrification bacteria in S⁰-PN-SSAD (34.47 %) and FeS₂-PN-SSAD (14.88 %). Nitrosomonas and Thiobacillus played a synergistic effect in the coupled system. FeS₂-PN-SSAD is expected as an alternative technology for nitrification and heterotrophic denitrification (HD) in treating low C/N wastewater.

The mitigation effect of free ammonia and free nitrous acid on nitrous oxide production from the full-nitrification and partial-nitritation systems

The potentials of using endogenous free ammonia (FA) and free nitrous acid (FNA) as nitrous oxide (N₂O) mitigators were investigated in treatment of both mainstream and sidestream wastewater. Although the N₂O emission factor of a sidestream partial-nitritation (PN) reactor (averaged 1.70 % ± 0.39 %, n = 30) was about 2.4 times higher than a mainstream full-nitrification (FN) reactor (averaged 0.72 % ± 0.24 %, n = 30) (P < 0.01), one-hour exposure of PN sludge to 1.5 mg HNO₂-N/L FNA could virtually abolish N₂O emission. As for FN sludge, both 45 mg NH₃-N/L FA and 0.015 mg HNO₂-N/L FNA successfully mitigated N₂O production at varying dissolved oxygen (DO) levels (50 % vs 61 %), while 1.5 mg HNO₂-N/L FNA not only reduced more N₂O (92 %) but also altered the N₂O dependency on DO. Both FNA and FA sludge treatment were effective N₂O mitigation strategies with FNA toward the end of carbon neutrality and FA being more economically appealing (2 % cost saving).

Achieving robust nitritation in a modified continuous-flow reactor: From micro-granule cultivation to nitrite-oxidizing bacteria elimination

In this study, a modified continuous-flow nitrifying reactor was successfully operated for rapid cultivation of micro-granules and achieving robust nitritation. Results showed that sludge granulation with mean size of ca. 100 µm was achieved within three weeks by gradually increasing settling velocity-based selection pressure from 0.48 to 0.9 m/hr. Though Nitrospira like nitrite-oxidizing bacteria (NOB) were enriched in the micro-granules with a ratio between ammonia-oxidizing bacteria (AOB) and NOB of 5.7%/6.5% on day 21, fast nitritation was achieved within one-week by gradually increasing of influent ammonium concentration (from 50 to 200 mg/L). Maintaining ammonium in-excess was the key for repressing NOB in the micro-granules. Interestingly, when the influent ammonium concentration switched back to 50 mg/L still with the residual ammonium of 15-25 mg/L, the nitrite accumulation efficiency increased from 90% to 98%. Experimental results suggested that the NOB repression was intensified by both oxygen and nitrite unavailability in the inner layers of micro-granules. Unexpectedly, continuous operation with ammonium in excess resulted in overproduction of extracellular polysaccharides and overgrowth of some bacteria (e.g., Nitrosomonas, Arenimonas, and Flavobacterium), which deteriorated the micro-granule stability and drove the micro-granules aggregation into larger ones with irregular morphology. However, efficient nitritation was stably maintained with extremely high ammonium oxidation potential (> 50 mg/g VSS/hr) and nearly complete washout of NOB was obtained. This suggested that smooth and spherical granule was not a prerequisite for achieving NOB wash-out and maintaining effective nitritation in the granular reactor. Overall, the micro-granules exhibited a great practical potential for high-rate nitritation.

Achieving robust mainstream nitritation by implementing light irradiation: long-term performance and microbial dynamics

The effective inhibition of nitrite-oxidizing bacteria (NOB) is widely acknowledged to be a critical issue for mainstream short-cut biological nitrogen removal. This study demonstrated a stable mainstream nitritation by implementing light irradiation. A sequencing batch reactor with ultraviolet-A (UVA) irradiation was operated for 250 days, and a high nitrite accumulation ratio was achieved and stabilized at about 90 %. UVA irradiation also positively impacts denitrification activity, with total nitrogen removal up to 63 %. Microbial community analysis confirmed that the UVA effectively and stably decreased the abundance of Nitrospira (the only detected NOB) from 6.0 % to 0.1 %, while it showed no effect on Nitrosomonas. The enriched genus Rhodocyclaceae was the major contributor to the increase in denitrification activity in the light-induced nitritation system. The proposed UVA irradiation strategy has the potential to be integrated with an anoxic/aerobic (A/O) or integrated fixed-film activated sludge (IFAS) process for achieving mainstream short-cut biological nitrogen removal.

Biological nitrogen removal from low carbon wastewater

Nitrogen has traditionally been removed from wastewater by nitrification and denitrification processes, in which organic carbon has been used as an electron donor during denitrification. However, some wastewaters contain low concentrations of organic carbon, which may require external organic carbon supply, increasing treatment costs. As a result, processes such as partial nitrification/anammox (anaerobic ammonium oxidation) (PN/A), autotrophic denitrification, nitritation-denitritation and bioelectrochemical processes have been studied as possible alternatives, and are thus evaluated in this study based on process kinetics, applicability at large-scale and process configuration. Oxygen demand for nitritation-denitritation and PN/A is 25% and 60% lower than for nitrification/denitrification, respectively. In addition, PN/A process does not require organic carbon supply, while its supply for nitritation-denitritation is 40% less than for nitrification/denitrification. Both PN/A and nitritation-denitritation produce less sludge compared to nitrification/denitrification, which saves on sludge handling costs. Similarly, autotrophic denitrification generates less sludge compared to heterotrophic denitrification and could save on sludge handling costs. However, autotrophic denitrification driven by metallic ions, elemental sulfur (S) and its compounds could generate harmful chemicals. On the other hand, hydrogenotrophic denitrification can remove nitrogen completely without generation of harmful chemicals, but requires specialized equipment for generation and handling of hydrogen gas (H2), which complicates process configuration. Bioelectrochemical processes are limited by low kinetics and complicated process configuration. In sum, anammox-mediated processes represent the best alternative to nitrification/denitrification for nitrogen removal in low- and high-strength wastewaters.