Nitrite oxidizing bacteria (NOB) contained in influent deteriorate mainstream NOB suppression by sidestream inactivation

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.

Significant in situ sludge yield reduction in an acidic activated sludge system

Minimizing sludge generation in activated sludge systems is critical to reducing the operational cost of wastewater treatment plants (WWTPs), particularly for small plants where bioenergy is not recovered. This study introduces a novel acidic activated sludge technology for in situ sludge yield reduction, leveraging acid-tolerant ammonia-oxidizing bacteria (Candidatus Nitrosoglobus). The observed sludge yield (Yobs) was calculated based on the cumulative sludge generation and COD removal during 400 d long-term operation. The acidic process achieved a low Yobs of 0.106 ± 0.004 gMLSS/gCOD at pH 4.6 to 4.8 and in situ free nitrous acid (FNA) of 1 to 3 mg/L, reducing sludge production by 58 % compared to the conventional neutral-pH system (Yobs of 0.250 ± 0.003 gMLSS/gCOD). The acidic system also maintained effective sludge settling and organic matter removal over long-term operation. Mechanism studies revealed that the acidic sludge displayed higher endogenous respiration, sludge hydrolysis rates, and higher soluble microbial products and loosely-bounded extracellular polymer substances, compared to the neutral sludge. It also selectively enriched several hydrolytic genera (e.g., Chryseobacterium, Acidovorax, and Ottowia). Those results indicate that the acidic pH and in situ FNA enhanced sludge disintegration, hydrolysis, and cryptic growth. Besides, a lower intracellular ATP content was observed for acidic sludge than neutral sludge, suggesting potential decoupling of catabolism and anabolism in the acidic sludge. These findings collectively demonstrate that the acidic activated sludge technology could significantly reduce sludge yield, contributing to more cost- and space-effective wastewater management.

Acidophilic partial nitrification (pH<6) facilitates ultra-efficient short-flow nitrogen transformation: Experimental validation and genomic insights

Partial nitrification (PN) represents an energy-efficient bioprocess; however, it often confronts challenges such as unstable nitrite accumulation, nitrite oxidizing bacteria shocks, and slow reaction rate. This study established an acidophilic PN with self-sustained pH as low as 5.36. Over 120-day monitoring, nitrite accumulation rate (NAR) was stabilized at more than 97.9 %, and an ultra-high ammonia oxidation rate of 0.81 kg/m3·d was achieved. Notably, least NAR of 77.8 % persisted even under accidental nitrite oxidizing bacteria invasion, aeration delay, alkalinity fluctuations, and cooling shocks. During processing, despite detrimental effects on bacterial diversity, there was a discernible increase in acid-tolerant bacteria responsible for nitrification. Candidatus Nitrosoglobus, gradually dominated in nitrifying guild (2.15 %), with the substantially reduction or disappearance of typical nitrifying microorganisms. Acidobacteriota, a key player in nitrogen cycling of soil, significantly increased from 0.45 % to 9.98 %, and its associated nitrogen metabolism genes showed a substantial 215 % rise. AmoB emerged as the most critical functional gene influencing acidophilic PN, exhibiting significantly higher unit gene expression than other nitrification genes. Blockade tricarboxylic acid cycle, DNA damage, and presence of free nitrous acid exert substantial effects on nitrite-oxidizing bacteria (NOB), serving as internal driving forces for acidophilic PN. This highlights the reliable potential for shortening nitrogen transformation process.

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.

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.

A critical review of improving mainstream anammox systems: Based on macroscopic process regulation and microscopic enhancement mechanisms

Full-scale anaerobic ammonium oxidation (anammox) engineering applications are vastly limited by the sensitivity of anammox bacteria to the complex mainstream ambience factors. Therefore, it is of great necessity to comprehensively summarize and overcome performance-related challenges in mainstream anammox process at the macro/micro level, including the macroscopic process variable regulation and microscopic biological metabolic enhancement. This article systematically reviewed the recent important advances in the enrichment and retention of anammox bacteria and main factors affecting metabolic regulation under mainstream conditions, and proposed key strategies for the related performance optimization. The characteristics and behavior mechanism of anammox consortia in response to mainstream environment were then discussed in details, and we revealed that the synergistic nitrogen metabolism of multi-functional bacterial genera based on anammox microbiome was conducive to mainstream anammox nitrogen removal processes. Finally, the critical outcomes of anammox extracellular electron transfer (EET) at the micro level were well presented, carbon-based conductive materials or exogenous electron shuttles can stimulate and mediate anammox EET in mainstream environments to optimize system performance from a micro perspective. Overall, this review advances the extensive implementation of mainstream anammox practice in future as well as shedding new light on the related EET and microbial mechanisms.

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.

Insights on pretreatment technologies for partial nitrification/anammox processes: A critical review and future perspectives

For almost 20 years, partial nitrification-anammox (PN/A) has been the subject of intensive study and development. Pretreatment of wastewater for PN/A is crucial because the inhibitory substances in the influent may reduce the performance of PN/A. In this review, the current PN/A pretreatment technologies are comprehensively summarized. The selection of pretreatment technology for PN/A depending on the source of the wastewater and its main characteristics (high-strength wastewater or municipal wastewater, organic matters, suspended solids). Comparison of pretreatment technologies through multiple perspectives including wastewater characteristics, the objectives of the wastewater treatment (treating requirement, energy and resource recovery demand), reactor configuration of PN/A. Based on the discussion, two integrated processes, HRAS + one-stage PN/A and advanced AD + two-stage PN/A, are recommended as the preferred processes for treating municipal wastewater and wastewater with a high-strength ammonium, respectively. This review aims to provide guidance for future research and development of PN/A.

Impact of electrochemically generated iron on the performance of an anaerobic wastewater treatment process

Anaerobic treatment of domestic wastewater has the advantages of lower biomass yield, lower energy demand and higher energy recover over the conventional aerobic treatment process. However, the anaerobic process has the inherent issues of excessive phosphate and sulfide in effluent and superfluous H₂S and CO₂ in biogas. An electrochemical method allowing for in-situ generation of Fe²⁺ in the anode and hydroxide ion (OH^-) and H₂ in the cathode was proposed to overcome the challenges simultaneously. The effect of electrochemically generated iron (e‑iron) on the performance of anaerobic wastewater treatment process was explored with four different dosages in this work. The results showed that compared to control, the experimental system displayed an increase of 13.4-28.4 % in COD removal efficiency, 12.0-21.3 % in CH₄ production rate, 79.8-98.5 % in dissolved sulfide reduction, 26.0-96.0 % in phosphate removal efficiency, depending on the e‑iron dosage between 40 and 200 mg Fe/L. Dosing of the e‑iron significantly upgraded the quality of produced biogas, showing a much lower CO₂ and H₂S contents in biogas in experimental reactor than that in control reactor. The results thus demonstrated that e‑iron can significantly improve the performance of anaerobic wastewater treatment process, bringing multiple benefits with the increase of its dosage regarding effluent and biogas quality.

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.

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.

Resilience of anammox application from sidestream to mainstream: A combined system coupling denitrification, partial nitritation and partial denitrification with anammox

Anaerobic ammonium oxidation (anammox) is a potential process to achieve the neutralization of energy and carbon. Due to the low temperature and variation of municipal sewage, the application of mainstream anammox is hard to be implemented. For spreading mainstream anammox in practice, several key issues and bottlenecks including the start-up, stable NO₂^--N supply, maintenance and dominance of AnAOB with high activity, prevention of NO₃^--N buildup, reduction of sludge loss, adaption to the seasonal temperature and alleviation of COD impacts on AnAOB are discussed and summarized in this review in order to improve its startup, stable operation and resilience of mainstream anammox. Hence a combined biological nitrogen removal (CBNR) system based on conventional denitrification, shortcut nitrification-denitrification, Partial Nitritation and partial Denitrification combined Anammox (PANDA) process through the management of organic matter and nitrate is proposed correspondingly aiming at adaptation to the variations of seasonal temperature and pollutants in influent.

The coexistence and competition of canonical and comammox nitrite oxidizing bacteria in a nitrifying activated sludge system - Experimental observations and simulation studies

The second step of nitrification can be mediated by nitrite oxidizing bacteria (NOB), i.e. Nitrospira and Nitrobacter, with different characteristics in terms of the r/K theory. In this study, an activated sludge model was developed to account for competition between two groups of canonical NOB and comammox bacteria. Heterotrophic denitrification on soluble microbial products was also incorporated into the model. Four 5-week washout trials were carried out at dissolved oxygen-limited conditions for different temperatures (12 °C vs. 20 °C) and main substrates (NH₄⁺-N vs. NO₂^--N). Due to the aggressive reduction of solids retention time (from 4 to 1 d), the biomass concentrations were continuously decreased and stabilized after two weeks at a level below 400 mg/L. The collected experimental data (N species, biomass concentrations, and microbiological analyses) were used for model calibration and validation. In addition to the standard predictions (N species and biomass), the newly developed model also accurately predicted two microbiological indicators, including the relative abundance of comammox bacteria as well as nitrifiers to heterotrophs ratio. Sankey diagrams revealed that the relative contributions of specific microbial groups to N conversion pathways were significantly shifted during the trial. The contribution of comammox did not exceed 5 % in the experiments with both NH₄⁺-N and NO₂^--N substrates. This study contributes to a better understanding of the novel autotrophic N removal processes (e.g. deammonification) with nitrite as a central intermediate product.

Combined partial denitrification-anammox with urea hydrolysis (U-PD-Anammox) process: A novel economical low-carbon method for nitrate-containing wastewater treatment

For the sake of exploring a new economical and low-carbon alternative for real nitrate-containing wastewater treatment, a new combined partial denitrification-anammox with urea hydrolysis (U-PD-Anammox) process was developed. The nitrogen removal performance of this process was investigated through long-term operation in a sequencing batch reactor (SBR) and two submerged anaerobic biological filters (SABF). Results showed that the average NO₃^--N to NO₂^-N transformation ratio improved to 82.6% with organic carbon source to NO₃^-N ratio of 1.8, and urea hydrolysis provided sufficient NH₄⁺-N and inorganic carbon to anammox process for nitrogen removal. The influent NH₄⁺-N/NO₂^--N ratio for subsequent anammox reactor could be adjacent to the optimal ratio of 1.32 during the whole operation. The combined process showed efficient nitrogen removal performance with 85% NO₃^--N removal, 93.8% total nitrogen removal and total nitrogen loading rate as 1.1 ± 0.5 kg N/(m³·d). High-throughput sequencing analysis results revealed that Genera Thauera, Hyphomicrobium and Candidatus Brocadia were the dominant species responsible for partial denitrification, urea hydrolysis and anammox, respectively. The proposed process was more economically and environmental-friendly than the traditional denitrification process with 51.7% operational cost reduction, 99.7% N₂O and 60% CO₂ emission decrement, facilitating the sustainable development of the nitrate-containing wastewater treatment industry in the future.

Novel insights into aerobic duration control-based partial nitritation in source-separated blackwater treatment: Growth type, inoculation source, and comammox threat

Aerobic duration control (ADC), whereby aeration is terminated before nitrite is extremely oxidized during the nitrification process, is an effective strategy to achieve partial nitritation (PN) for blackwater. This study evaluated the effects of microbial growth type, influent ammonia-oxidizing organisms (AOO), and comammox bacteria from seeding sludge to ADC-based PN. The long-term operation of lab-scale reactors and model simulations were implemented to select the best growth type. The biofilm formed on the inner wall of the activated sludge reactor decreased the nitrite accumulation ratio (NAR) from 99.2% to 77.2%. Meanwhile, the NAR of the pure-biofilm reactor decreased from 95.9% to 47.8%. The deteriorated PN of the biofilm-related reactors was due to the extended solid retention time and increased substrate saturation constants of AOOs compared with those of nitrite-oxidizing organisms (NOO). Periodic biofilm carrier regeneration and biofilm thickness control can recover PN performance but are difficult to implement. In contrast, the optimized activated sludge reactor exhibited high (NAR >94%) and stable (>3 months) PN performance when treating real blackwater. Nitrifiers were found in blackwater, and chemically enhanced high-rate activated sludge pretreatment removed more NOOs than AOOs (41.8% vs. 24.3%) and increased the influent AOO/NOO ratio. Interestingly, the influent AOOs supported fast PN start-up in the moving-bed biofilm reactor without the initial inoculation of activated sludge. Moreover, model simulations verified that high and stable PN could also be realized in an activated sludge reactor by the continuous inoculation of influent AOOs, which is a novel PN start-up strategy. Metagenomic analyses showed that the comammox bacteria from the seeding sludge eventually disappeared owing to their intrinsic specific growth rates and free ammonia inhibition. The findings of this study will provide insightful guidelines for PN application in decentralized and semi-centralized wastewater treatment systems.

Biomass retention and microbial segregation to offset the impacts of seasonal temperatures for a pilot-scale integrated fixed-film activated sludge partial nitritation-anammox (IFAS-PN/A) treating anaerobically pretreated municipal wastewater

Partial nitritation-anammox (PN/A) is a promising deammonification process to develop energy-neutral wastewater treatment plants. However, the mainstream application of PN/A still faces the challenges of low nitrogen concentration and low temperatures, and has not been studied under a realistic condition of large-scale reactor (kiloliter level), real municipal wastewater (MWW) and seasonal temperatures. In this research, a pilot-scale one-stage PN/A, with integrated fixed-film activated sludge (IFAS) configuration, was operated to treat the real MWW pretreated by anaerobic membrane bioreactor. The removal efficiency of total nitrogen (TN) was 79.4%, 75.7% and 65.9% at 25, 20 and 15°C, corresponding to the effluent TN of 7.3, 9.7 and 12.0 mg/L, respectively. The suppression of ammonium-oxidizing bacteria (AOB) and anammox bacteria (AnAOB) occurred at lower temperatures, and the significant decrease in AOB treatment capacity was the reason for the poorer nitrogen removal at 15°C. Biomass retention and microbial segregation were successfully achieved. Specifically, Candidatus_Brocadia and Candidatus_Kuenenia were main AnAOB genera and mainly enriched on carriers, Nitrosomonas and uncultured f_Chitinophagaceae were main AOB genera and mainly distributed in suspended sludge and retained by sedimentation tank. Moreover, nitrite-oxidizing bacteria (NOB) were sufficiently suppressed by intermittent aeration and low dissolved oxygen, the presence of heterotrophic bacteria upgraded the PN/A to a simultaneous partial nitritation, anammox, denitrification, and COD oxidation (SNADCO) system, which improved the overall removal of TN and COD. The results of this investigation clearly evidence the strong feasibility of PN/A as a mainstream nitrogen removal process in temperate climates.

Stable nitrogen removal in the novel continuous flow anammox system under deteriorated partial nitrification: Significance and superiority of the anaerobic-oxic-anoxic-oxic operation mode

The collapse of mainstream anammox system caused by deterioration of partial nitrification (PN) is easy to occur and it is vital to quickly restore the stable nitrogen elimination performance. Herein, a novel continuous push-flow anaerobic-oxic-anoxic-oxic (AOAO) process treating sewage was used to restore the nitrogen elimination performance rapidly under deteriorated PN. The increased abundances of Nitrospira and Candidatus Nitrotoga was responsible for the deterioration of PN. Effluent total inorganic nitrogen of 8.7 mg N/L and a stable nitrogen removal rate of 0.083 kg N/m³·d were obtained with the aerobic hydraulic retention time (HRT) of 3.75 h even PN deteriorated. Endogenous partial denitrification coupled anammox in the anoxic zone was essential to maintain stable nitrogen removal under the deterioration of PN and the anammox contribution increased from 17.2 % to 23.6 %. The AOAO system shows robustness on nitrogen removal even PN deteriorated under the decrease of HRT from 16 to 12 h.

Achieving robust mainstream nitrite shunt at pilot-scale with integrated sidestream sludge treatment and step-feed

As a promising energy- and carbon efficient process for nitrogen removal from wastewater, mainstream nitrite shunt has been extensively researched. However, beyond the laboratory it is challenging to maintain stable performance by suppressing nitrite-oxidising bacteria (NOB). In this study, a pilot-scale reactor system receiving real sewage was operated in two stages for >850 days to evaluate two novel NOB suppression strategies for achieving nitrite shunt: i) sidestream sludge treatment based on alternating free nitrous acid (FNA) and free ammonia (FA) and ii) sidestream FNA/FA sludge treatment integrated with in-situ NOB suppression via step-feed. The results showed that, with sidestream sludge treatment alone, NOB developed resistance relatively quickly to the treatment, leading to unstable nitrite shunt. In contrast, robust nitrite shunt was achieved and stably maintained for more than a year when sidestream sludge treatment was integrated with a step-feed strategy. Kinetic analyses suggested that sludge treatment and step-feed worked in synergy, leading to stable NOB suppression. The integrated strategy demonstrated in this study removes a key barrier to the implementation of stable mainstream nitrite shunt.

Determining Factors for Nitrite Accumulation in an Acidic Nitrifying System: Influent Ammonium Concentration, Operational pH, and Ammonia-Oxidizing Community

Acidic nitrification is attracting wide attention because it can enable robust suppression of nitrite-oxidizing bacteria (NOB) in wastewater treatment. This study reports a comprehensive assessment of the novel acidic nitrification process to identify the key factors that govern stable nitrite accumulation. A laboratory-scale moving-bed biofilm reactor receiving low-alkalinity wastewater was continuously operated under acidic conditions (pH < 6) for around two years, including nine stages varying influent and operational conditions. The results revealed that nitrite accumulation was related to three factors, i.e., influent ammonium concentration, operating pH, and ammonia-oxidizing microbial community. These three factors impact nitrite accumulation by altering the in situ concentration of free nitrous acid (FNA), which is a potent inhibitor of NOB. The critical FNA concentration is approximately one part per million (ppm, ∼1 mg HNO₂-N/L), above which nitrite accumulation is stably maintained in an acidic nitrifying system. The findings of this study suggest that stable nitrite accumulation via acidic ammonia oxidation can be maintained under a range of influent and operational conditions, as long as a ppm-level of FNA is established. Taking low-strength mainstream wastewater (40-50 mg NH₄⁺-N/L) with limited alkalinity as an example, stable nitrite accumulation was experimentally demonstrated at a pH of 4.35, under which an in situ FNA of 2.3 ± 0.6 mg HNO₂-N/L was attained. Under these conditions, Candidatus Nitrosoglobus became the only ammonia oxidizer detectable by 16S rRNA gene sequencing. The results of this study deepen our understanding of acidic nitrifying systems, informing further development of novel wastewater treatment technologies.

Wastewater Primary Treatment Using Forward Osmosis Introduces Inhibition to Achieve Stable Mainstream Partial Nitrification

Achieving stable long-term mainstream nitrite oxidizing bacteria (NOB) suppression is the bottleneck for the novel partial nitrification (PN) process toward energy- and carbon-efficient wastewater treatment. However, long-term PN stability remains a challenge due to NOB adaptation. This study proposed and demonstrated a novel strategy for achieving NOB suppression by the primary treatment of mainstream wastewater with a forward osmosis (FO) membrane process, which facilitated two external NOB inhibition factors (salinity and free nitrous acid, FNA). To evaluate the proposed strategy, a lab-scale sequencing batch reactor was operated for 200 days. A stable PN operation was achieved with a nitrite accumulation ratio of 97.7 ± 2.8%. NOB were suppressed under the combined inhibition effect of NaCl (7.9 ± 0.2 g/L, as introduced by the FO direct filtration) and FNA (0.11 ± 0.02 mg of HNO₂-N/L, formed as a result of the increased NH₄⁺-N concentration after the FO process). The two inhibition factors worked in synergy to achieve a more stable PN operation. The microbial analysis showed that the elevated salinity and accumulation of FNA reshaped the microbial community and selectively eliminated NOB. Finally, an economic and feasibility analysis was conducted, which suggests that the integration of an FO unit into PN/A is a feasible and economically viable wastewater treatment process.

A versatile control strategy based on organic carbon flow analysis for effective treatment of incineration leachate using an anammox-based process

Anammox-based process provides an alternative for the sustainable treatment of incineration leachate that has high-load ammonium and high residual heat, but the high concentrations of organics in such leachates brought challenges for the process control. For the first time, a two-stage partial nitrification (PN)-anammox process coupled with a pre-enhanced anaerobic digestion (AD) was established to achieve efficient nitrogen removal from incineration leachate. Satisfactory nitrogen and chemical oxygen demand (COD) removal efficiencies were achieved-with the average values of 90% and 78%, respectively-despite fluctuating influent properties [1100-2000 mg-total nitrogen (TN)/L and 3800-15800 mg-COD/L]. A versatile control strategy was developed to create an optimum autotrophic environment for nitrifier and anammox bacteria: i) enhanced AD set before the PN-anammox process captured nearly 50% of the influent COD; ii) in the PN unit, ammonia-oxidizing bacteria were well adapted to COD concentrations of 1420-2400 mg/L, and dissolved oxygen (0.2-0.4 mg/L) controlling combined with a high free nitrous acid concentration (>0.08 mg/L) ensured a nitrite accumulation rate of >95%; and iii) in the anammox unit, a suitable influent NO₂^--N/NH₄⁺-N ratio (the average value of 1.27) was achieved by mixing AD effluent with PN effluent (1:1.78, v/v), contributing to a high TN removal of 78 ± 2.4%. Nevertheless, 980-1560 mg/L of COD remained in the influent of the anammox unit; biorefractory humic acids in this (245.6 ± 3 mg/L) might be the main component that caused the observed 66 ± 2% decrease in anammox activity. The proliferation of denitrifying bacteria and sulfate-reducing bacteria induced by the organic compounds may have led to the observed decline in the abundance of the anammox bacterium Candidatus Kuenenia. The proposed strategy guaranteed the robust operation of the PN-anammox process and provides a promising approach for the sustainable treatment of incineration leachate.

Mainstream short-cut N removal modelling: current status and perspectives

This work gives an overview of the state-of-the-art in modelling of short-cut processes for nitrogen removal in mainstream wastewater treatment and presents future perspectives for directing research efforts in line with the needs of practice. The modelling status for deammonification (i.e., anammox-based) and nitrite-shunt processes is presented with its challenges and limitations. The importance of mathematical models for considering N₂O emissions in the design and operation of short-cut nitrogen removal processes is considered as well. Modelling goals and potential benefits are presented and the needs for new and more advanced approaches are identified. Overall, this contribution presents how existing and future mathematical models can accelerate successful full-scale mainstream short-cut nitrogen removal applications.

Culturing partial denitrification biofilm in side stream incubator with ordinary activated sludge as inoculum: One step closer to mainstream Anammox upgrade

Recently, adding carriers into anoxic zone is proposed for mainstream Anammox upgrade, which relied on the denitrifiers responsible for partial denitrification (PD) to generate essential nitrite for Anammox bacteria. Still, their low abundance in the naturally formed biofilm leads to insufficient nitrite supply. This study investigated the sequential culturing of PD biofilm. By inoculating ordinary activated sludge, the PD process was quickly established within 54-day. During that, decreasing carbon to nitrogen ratio and anoxic duration in order might be effective strategies. Adding carriers shifted the microbial community, especially the proliferation of Flavobacterium. When solely using the mature PD biofilm, high nitrate to nitrite transformation ratio (>70%) was obtained. Meanwhile, both nitrate-reducing and nitrite-generating processes slowed down and lasted ∼90 min. In addition, abundant Simplicispira candidate for PD was detected in biofilm. This study also suggests that regularly harvesting PD-related functional bacteria from a side-stream incubator promotes mainstream Anammox upgrade.

Nitrogen removal by a Hydroxyapatite-enhanced Micro-granule type One-stage partial Nitritation/anammox process following anaerobic membrane bioreactor treating municipal wastewater

Nitrogen removal from wastewater by the partial nitritation/anammox (PNA) technology is promising from both economic and environmental perspectives. However, this technology has not been popularized in the mainstream because of low biomass retention and the growth of the nitrite oxidizing bacteria. In this study, a one-stage PNA process with hydroxyapatite (HAP)-enhanced granules was used to treat effluent from a mainstream anaerobic membrane bioreactor. The HAP-enhanced reactor allowed an enriched high biomass of 6.9 ± 0.2 g/L at a low hydraulic retention time of 2 h. A nitrogen removal efficiency of 80 ± 6.0 %, a nitrogen removal rate of 0.36 ± 0.05 kg/m³/d and a COD removal efficiency of 54 ± 15 % were achieved stably, leading to a low total nitrogen concentration of 8.5 ± 2.7 mg/L and a low COD concentration of 19.7 ± 5.9 mg/L in the effluent. Anammox bacteria of Candidatus Kuenenia stuttgartiensis and ammonium oxidizing bacteria of Nitrosomonas were found to be the two most predominant bacteria.

A successful start-up of an anaerobic membrane bioreactor (AnMBR) coupled mainstream partial nitritation-anammox (PN/A) system: A pilot-scale study on in-situ NOB elimination, AnAOB growth kinetics, and mainstream treatment performance

In this pilot-scale study, an innovative mainstream treatment process that couples the anaerobic membrane reactor (AnMBR) with a one-stage PN/A system was proposed for advancing the concept of carbon neutrality in the municipal wastewater treatment plant. This work demonstrates the start-up procedure of a pilot-scale one-stage PN/A system for mainstream treatment. The 255-day start-up of the one-stage PN/A system involved the cultivation of ammonium-oxidizing bacteria (AOB) from the activated sludge, suppression of nitrite-oxidizing bacteria (NOB), investigation of in-situ growth kinetics of anammox bacteria (AnAOB), and the 50-day operation of the pilot-scale AnMBR-PN/A process for natural mainstream treatment. It is verified in the pilot-scale system for the first time that the in-situ free ammonia (FA) and free nitrous acid (FNA) exposure could effectively eliminate the Nitrospira (the NOB genus) while retaining the Nitosonomas (the AOB genus) community in the suspended sludge. NOB community rebounding was not detected even at the mainstream conditions with low nitrogen concentrations (Influent ammonium concentration=38±6 mg-NH₄⁺-N/L) by intermittent aeration to control the system dissolved oxygen (DO) below 0.5 mg/L. The results of the mainstream treatment showed that the average effluent total nitrogen (TN) in the coupled process was generally lower than 10 mg-N/L, which meets the discharge limits of most prefectures in Japan. The investigated results of the in-situ anammox bacteria (AnAOB) growth kinetics suggested that the promoted start-up strategy of taking advantage of the warm months with higher mainstream temperature to achieve the rapid in-situ growth of the AnAOB is applicable in the investigated regions. From the perspective of the removal performance of the TN and organic substance, the AnMBR-PN/A process has great potential as the layouts of the carbon-neutral mainstream wastewater treatment plants.

Oxygen control and stressor treatments for complete and long-term suppression of nitrite-oxidizing bacteria in biofilm-based partial nitritation/anammox

Mainstream nitrogen removal by partial nitritation/anammox (PN/A) can realize energy and cost savings for sewage treatment. Selective suppression of nitrite oxidizing bacteria (NOB) remains a key bottleneck for PN/A implementation. A rotating biological contactor was studied with an overhead cover and controlled air/N₂ inflow to regulate oxygen availability at 20 °C. Biofilm exposure to dissolved oxygen concentrations < 0.51 ± 0.04 mg O₂ L^-1 when submerged in the water and < 1.41 ± 0.31 mg O₂ L^-1 when emerged in the headspace (estimated), resulted in complete and long-term NOB suppression with a low relative nitrate production ratio of 10 ± 4%. Additionally, weekly biofilm stressor treatments with free ammonia (FA) (29 ± 1 mg NH₃-N L^-1 for 3 h) could improve the NOB suppression while free nitrous acid treatments had insufficient effect. This study demonstrated the potential of managing NOB suppression in biofilm-based systems by oxygen control and recurrent FA exposure, opening opportunities for resource efficient nitrogen removal.

Insights into complete nitrate removal in one-stage nitritation-anammox by coupling heterotrophic denitrification

Nitritation-anammox has been considered to be the most promising process for nitrogen (N) removal from wastewater. However, the anammox reaction still produces an amount of nitrate, which cannot be removed further. This study hypothesizes that heterotrophic denitrification can be an appealing option to remove the residual nitrate in the one-stage nitritation-anammox process. Through monitoring N-removal performance and microbial community succession of a laboratory microaerobic reactor, the effect of four different levels of oxygen supply on nitrate removal was investigated. The reactor was continuously fed with real manure-free piggery wastewater containing ~240 mg NH₄⁺-N/L and chemical oxygen demand (COD)/total nitrogen (TN) ratio of less than 1 for 180 days. With a high influent loading rate of 0.7 kg N/(m³·d), efficient total nitrogen removal (>80 %) was achieved during stable operation of dissolved oxygen (DO) concentrations between 0.3 and 0.6 mg O₂/L, indicating N-removal via the nitritation-anammox pathway in the low-carbon wastewater treatment. At the same time, the effluent nitrate reduced with decreased oxygen supply and completely depleted at DO of 0.3 ± 0.1 mg O₂/L. In addition to oxygen, preventing ammonia nitrogen from falling to very low levels (<10 mg/L) could be also useful for the complete nitrate removal and stable nitritation-anammox. 16S rRNA gene-based analyses confirmed a complex microbial community including nitrifiers, denitrifiers and anammox bacteria in the biomass of the reactor. Collectively, this study provides new insights into high-level N-removal of a nitritation-anammox process by complete nitrate depletion.

Achieving stable mainstream nitrogen and phosphorus removal assisted by hydroxylamine addition in a continuous partial nitritation/anammox process from real sewage

Hydroxylamine (NH₂OH) as the putative intermediate for anammox ensures the robustness of partial nitritation/anammox (PN/A) process; however, the feasible for NH₂OH addition to improve the stability of PN/A process under low-strength ammonia (NH₄⁺-N) condition need to be further investigated. In this study, the restoration and steady operation of mainstream PN/A process were investigated to treat real sewage with in situ NH₂OH added in a continuous alternating anoxic/aerobic with integrated fixed-film activated sludge (A³-IFAS) reactor. Results showed that the deteriorated PN/A process caused by nitrate (NO₃^--N) built-up was rapidly restored with a distinct decrease of the NO₃^--N_produced/NH₄⁺-N_consumed ratio from 28.7% to <10.0% within 20 days, after 5 mg N/L of NH₂OH was added daily into the aerobic zone of A³-IFAS reactor. After 230 days of operation, the average total nitrogen (TN) and phosphate (PO₄^3--P) removal efficiencies of 80.8% and 91.5%, respectively were stably achieved, with average effluent sCOD, NH₄⁺-N, TN and PO₄^3--P concentrations reaching 23.1, 2.3, 7.7 and 0.4 mg/L, respectively. Microbial community characterization revealed Candidatus Brocadia (3.60% and 2.92%) and Ignavibacteriae (1.56% and 2.66%) as the dominant anammox bacteria and denitrifying bacteria, respectively, jointly attached in the biofilm_1 and biofilm_2, while Candidatus Microthrix (5.17%) dominant in floc sludge was main responsible for phosphorus removal. This study confirmed that NH₂OH addition is an effective strategy for nitrite-oxidizing bacteria suppression, contributing to the in situ restoration of PN/A process and high stable mainstream nitrogen and phosphorus removal in a continuous PN/A process from real sewage.

Stable nitritation of mature landfill leachate via in-situ selective inhibition by free nitrous acid

In-situ free nitrous acid (FNA) and free ammonia (FA) treatments are more feasible than side-stream methods to achieve nitritation. To assess the optimum conditions and long-term performance of in-situ inhibition by FNA, batch tests and a sequencing batch reactor (SBR) treating mature landfill leachate were conducted and established. As a result, the selective inhibition characteristic by FNA was more conspicuous than FA, and FNA (0.175 mg N/L, 6 h) treatment are more biocidal to nitrite oxidizing bacteria (NOB). Moreover, ammonia oxidizing bacteria (AOB) were more sensitive to the FA environment but its activity recovered preferentially compared to NOB. The SBR achieved a sustained nitrite accumulation rate above 90% for 200 days, with a significant decrease of NOB activity and microbial abundance according to qPCR and 16S rRNA gene sequencing results. In-situ selective inhibition by FNA (0.175 mg N/L, 6 h) has been proved to be effective to maintain stable nitritation.

Light Irradiation Enables Rapid Start-Up of Nitritation through Suppressing nxrB Gene Expression and Stimulating Ammonia-Oxidizing Bacteria

Nitritation facilitates the application of anaerobic ammonium oxidation (Anammox)-based processes for cost-efficient nitrogen removal from wastewater. This study proposed light irradiation as a novel strategy to rapidly start up nitritation by stimulating both the activities and growth of ammonia-oxidizing bacteria (AOB) while suppressing that of nitrite-oxidizing bacteria (NOB). Batch assays and kinetic model jointly suggested that AOB activity presented an initial increase followed by a decline while NOB decreased continuously throughout the light energy densities applied. Under optimal light energy densities (0.03-0.08 kJ/mg VSS), the highest nitrite accumulation ratio of 70.0% was achieved in sequencing batch reactors with both mainstream online and sidestream offline light treatments when treating real or synthetic municipal wastewater. Light irradiation induced different responses of AOB and NOB, leading to microbial structure optimization. Specifically, the expression of nxrB was downregulated, while the expression of amoA was upregulated under appropriate light irradiation. Moreover, although Nitrosomonas as typical AOB disappeared, the family Nitrosomonadaceae was doubled with enrichment of Ellin6067 and another four Nitrosomonadaceae genera that were only identified in light-treated reactors, thus ensuring AOB predominance and stable nitritation. These findings offer a new approach to rapidly establishing nitritation using light irradiation in municipal wastewater, especially for nitritation/microalgae system.

Centralized iron-dosing into returned sludge brings multifaceted benefits to wastewater management

Iron salts (i.e. FeCl₃) are the most used chemicals in the urban wastewater system. Iron is commonly dosed into sewage or the mainstream system, which provides multiple benefits such as enhanced phosphorus removal and improved sludge settleability/dewaterability. This study reported and demonstrated a new approach that dosed FeCl₃ into returned sludge in order to bring two more benefits to wastewater management: short-cut nitrogen removal via the nitrite pathway and less biomass production. This approach is achieved based on our findings that with similar amount of FeCl₃, centralized iron dosing into a sidestream sludge unit generated iron concentration two orders of magnitude higher than the common mainstream dosing (e.g. 10-40 mg Fe/L-wastewater), leading to sludge acidification (pH = 2.1) with Fe (III) hydrolysis. Together with accumulated nitrite in the supernatant of the sludge, ppm-level of free nitrous acid was generated and thus enabled sludge disintegration, cell lysis, and selective elimination of nitrite-oxidizing bacteria (NOB). Long-term effects on nitrifying bacteria and overall reactor performance were evaluated using two laboratory reactor experiments for over one year. The experimental reactor showed stable nitrite accumulation with an average NO₂^-/(NO₂^- + NO₃^-) ratio above 80% and ∼30% observed biomass yield reduction compared to those in control reactors. In addition, the centralized sludge dosing strategy still provided benefits such as improved settleability and dewaterability of sludge and enhanced phosphorus removal.

Partial Nitrification in a Sequencing Moving Bed Biofilm Reactor (SMBBR) with Zeolite as Biomass Carrier: Effect of Sulfide Pulses and Organic Matter Presence

This work aimed to achieve partial nitrification (PN) in a Sequencing Moving Bed Biofilm Reactor SMBBR with zeolite as a biomass carrier by using sulfide pulses in the presence of organic matter as an inhibitor. Two conditions were evaluated: sulfide (HS−) = 5 mg S/L and vvm (air volume per liquid volume per minute, L of air L−1 of liquid min−1) = 0.1 (condition 1); and a HS− = 10 mg S/L and a vvm = 0.5 (condition 2). The simultaneous effect of organic matter and sulfide was evaluated at a Chemical Oxygen Demand (COD) = 350 mg/L and HS− = 5 mg S/L, with a vvm = 0.5. As a result, using the sulfide pulse improved the nitrite accumulation in both systems. However, Total Ammonia Nitrogen (TAN) oxidation in both processes decreased by up to 60%. The simultaneous presence of COD and sulfide significantly reduced the TAN and nitrite oxidation, with a COD removal yield of 80% and sulfide oxidation close to 20%. Thus, the use of a sulfide pulse enabled PN in a SMBBR with zeolite. Organic matter, together with the sulfide pulse, almost completely inhibited the nitrification process despite using zeolite.

Understanding the effect of free ammonia on microbial nitrification mechanisms in suspended activated sludge bioreactors

During nitrification, the varieties of microbial structures, metabolic pathways and functional profiles in four parallel laboratory-scale sequencing batch reactors (SBRs) with 0.5, 5, 10 and 15 mg/L of free ammonia (FA) concentrations were analyzed by high-throughput sequencing of the 16S rRNA gene. The SBRs were named S_0.5, S₅, S₁₀ and S₁₅, respectively. Ammonia removal via the nitrate pathway was achieved in S_0.5 and S₅ throughout the whole experimental period, while ammonia removal via the nitrite pathway was established in S₁₀ and S₁₅ after 89 and 146 day, respectively. The key finding of this study is that both the microbial diversity and richness were significantly affected (p < 0.05) by the FA concentration at different taxonomic levels. The most dominant taxa of S₅, S₁₀ and S₁₅ were same, and mainly included Thauera while S_0.5 was mainly composed of Zoogloea. Linear discriminant analysis (LDA) effect size (LEfSe) analysis was used to identify unique biomarkers in SBR activated sludge (AS) sample. The functional genera and enzyme in the four SBRs are similar but different in abundance and they are responsible for the removal of organics and nitrogen. Moreover, metabolic pathways are similar by PICRUSt analysis. The relative proportions of pathway-specific genes involved in some metabolic pathways differed to some extent. The ammonia oxidation rate was positively linked to Nitrosomonas and amo (both Spearman correlation coefficients (ρ) = 0.777) while the nitrite oxidation rate was positively linked to Nitrospira (ρ = 0.777) by co-occurrence network analysis. This work deciphered the response of microbial characteristics to different FA constraints in AS process and could provide helpful information for revealing the biological mechanism of FA inhibition on nitrogen removal.

Application of Anammox-Based Processes in Urban WWTPs: Are We on the Right Track?

The application of partial nitritation and anammox processes (PN/A) to remove nitrogen can improve the energy efficiency of wastewater treatment plants (WWTPs) as well as diminish their operational costs. However, there are still several limitations that are preventing the widespread application of PN/A processes in urban WWTPs such as: (a) the loss of performance stability of the PN/A units operated at the sludge line, when the sludge is thermally pretreated to increase biogas production; (b) the proliferation of nitrite-oxidizing bacteria (NOB) in the mainstream; and (c) the maintenance of a suitable effluent quality in the mainstream. In this work, different operational strategies to overcome these limitations were modelled and analyzed. In WWTPs whose sludge is thermically hydrolyzed, the implementation of an anerobic treatment before the PN/A unit is the best alternative, from an economic point of view, to maintain the stable performance of this unit. In order to apply the PN/A process in the mainstream, the growth of ammonia-oxidizing bacteria (AOB) should be promoted in the sludge line by supplying extra sludge to the anaerobic digesters. The AOB generated would be applied to the water line to partially oxidize ammonia, and the anammox process would then be carried out. Excess nitrate generated by anammox bacteria and/or NOB can be removed by recycling a fraction of the WWTP effluent to the biological reactor to promote its denitrification.

Unravelling adaptation of nitrite-oxidizing bacteria in mainstream PN/A process: Mechanisms and counter-strategies

Stable suppression of nitrite-oxidizing bacteria (NOB) is still a major challenge for the implementation of partial nitritation and anammox (PN/A) in mainstream treatment. Despite numerous suppression strategies demonstrated, it is increasingly recognized that NOB could develop resistance to these strategies, threatening the long-term stability of the mainstream PN/A process. This study aims to understand adaption mechanisms and develop counter-strategies to overcome the adaptation. To this end, three previously-demonstrated suppression strategies, including NOB inactivation via side stream sludge treatment with free ammonia (FA), the use of low dissolved oxygen (DO), and the use of anammox to scavenge nitrite, were stepwise applied, over a period of 800 days, to a laboratory-scale reactor treating effluent from a high-rate activated sludge (HRAS) plant. FA sludge treatment alone sustained nitrite accumulation for about two months, after which NOB adaptation occurred causing PN to fail. The FA adaptation was induced by a shift in the NOB community from Nitrospira to Ca. Nitrotoga. The latter was found to have higher resistance to FA and also a higher maximum specific growth rate. Low DO at 0.2-0.4 mg O₂ L^-1 was then applied, in conjunction with FA treatment, which successfully eliminated Ca. Nitrotoga and re-established PN. However, new and unidentified NOB with a higher apparent oxygen affinity emerged in three months, again leading to PN failure. Lastly, as the third strategy for NOB suppression, anammox was introduced as an in-situ nitrite-scavenger. The combo-strategy delivered reliable NOB suppression with no further adaptation in the remaining experimental period (eight months). The resulted one-stage PN/A reactor achieved a nitrogen removal efficiency of 84.2 ± 5.37%. A control reactor, operated in parallel under the same conditions but without FA treatment, only achieved 10.4 ± 4.6% nitrogen removal, with anammox completely outcompeted by NOB in the last phase.

A continuous plug-flow anaerobic/aerobic/anoxic/aerobic (AOAO) process treating low COD/TIN domestic sewage: Realization of partial nitrification and extremely advanced nitrogen removal

The realization of stable partial nitrification and advanced nitrogen removal are not acquired effectively in conventional pre-denitrification biological nitrogen removal processes treating domestic sewage. Herein, a novel anaerobic/aerobic/anoxic/aerobic (AOAO) continuous plug-flow reactor, characterized with double sludge reflux and a bypass of anaerobic mixed liquor conveyed to anoxic zone, was first constructed to realize stable partial nitrification in treating domestic sewage. The alternating anoxic/aerobic conditions and longer anoxic sludge retention time might be responsible for the partial nitrification. Nitrite accumulation ratio reached 89.3 ± 3.3% with the maximum activity ratio of AOB to NOB increasing from 0.72 to 8.17. A content total inorganic nitrogen (TIN) removal efficiency (93.7 ± 2.2%) and effluent TIN concentration (2.9 ± 0.9 mg N/L) were obtained after 238 days' operation. Specifically, nitrogen balance of the typical cycle showed that about 30.1% of TIN was removed through simultaneous partial nitrification and denitrification (SND) in aerobic zone and 48.2% by endogenous denitrification in anoxic zone. The AOAO process is an economic treatment for domestic sewage with aerobic hydraulic retention time (HRT) of 4 h.

A review of partial nitrification in biological nitrogen removal processes: from development to application

To further reduce the energy consumption in the wastewater biological nitrogen removal process, partial nitrification and its integrated processes have attracted increasing attentions owing to their economy and efficiency. Shortening the steps of ammonia oxidation to nitrate saves a large amount of aeration, and the accumulated nitrite could be reduced by denitritation or anammox, which requires less electron donors compared with denitrification. Therefore, the strategies through mainstream suppression and sidestream inhibition for the achievement of partial nitrification in recent years are reviewed. Specifically, the enrichment strategies of functional microorganisms are obtained on the basis of their growth and metabolic characteristics under different selective pressures. Furthermore, the promising developments, current application bottlenecks and possible future trends of some biological nitrogen removal processes integrating partial nitrification are discussed. The obtained knowledge would provide a new idea for the fast realization of economic, efficient and long-term stable partial nitrification and biological nitrogen removal process.

Robust Nitritation Sustained by Acid-Tolerant Ammonia-Oxidizing Bacteria

Oxidation of ammonium to nitrite rather than nitrate, i.e., nitritation, is critical for autotrophic nitrogen removal. This study demonstrates a robust nitritation process in treating low-strength wastewater, obtained from a mixture of real mainstream sewage with sidestream anaerobic digestion liquor. This is achieved through cultivating acid-tolerant ammonia-oxidizing bacteria (AOB) in a laboratory nitrifying bioreactor at pH 4.5-5.0. It was shown that nitrite accumulation with a high NO₂^-/(NO₂^- + NO₃^-) ratio of 95 ± 5% was stably maintained for more than 300 days, and the obtained volumetric NH₄⁺ removal rate (i.e., 188 ± 14 mg N L^-1 d^-1) was practically useful. 16S rRNA gene sequencing analyses indicated the dominance of new AOB, "Candidatus Nitrosoglobus," in the nitrifying guild (i.e., 1.90 ± 0.08% in the total community), with the disappearance of typical activated sludge nitrifying microorganisms, including Nitrosomonas, Nitrospira, and Nitrobacter. This is the first identification of Ca. Nitrosoglobus as key ammonia oxidizers in a wastewater treatment system. It was found that Ca. Nitrosoglobus can tolerate low pH (<5.0), and free nitrous acid (FNA) at levels that inhibit AOB and nitrite-oxidizing bacteria (NOB) commonly found in wastewater treatment processes. The in situ inhibition of NOB leads to accumulation of nitrite (NO₂^-), which along with protons (H⁺) also produced in ammonium oxidation generates and sustains FNA at 3.0 ± 1.4 mg HNO₂-N L^-1. As such, robust PN was achieved under acidic conditions, with a complete absence of NOB. Compared to previous nitritation systems, this acidic nitritation process is featured by a higher nitric oxide (NO) but a lower nitrous oxide (N₂O) emission level, with the emission factors estimated at 1.57 ± 0.08 and 0.57 ± 0.03%, respectively, of influent ammonium nitrogen load.

Nitrous oxide emission during denitrifying phosphorus removal process: A review on the mechanisms and influencing factors

Excessive emissions of nitrogen (N) and phosphorus (P) pollutants are leading to increased eutrophication of water bodies. Biological N and P removal processes have become a research priority in the field of sewage treatment with the aim of improving sewage discharge standards in countries worldwide. Denitrifying P removal processes are more efficient for solving problems related to carbon source competition, sludge age conflict, and high aeration energy consumption compared to traditional biological N and P removal processes, but they are easy to produce nitrous oxide (N₂O) in the process of sewage treatment. N₂O is a greenhouse gas with a global warming potential approximately 190-270 times that of CO₂ and 4-21 times that of CH₄, which was produced and released into the environmental in denitrifying P removal systems under conditions of a low C/N ratio, high dissolved oxygen, and low activity of denitrifying phosphorus accumulating organisms (DPAOs). This paper reviews the emission characteristics and influencing factors of N₂O during denitrifying P removal processes and proposes appropriate strategies for controlling the emission of N₂O. This work serves as a basis for the development of new sewage treatment processes and the reduction of greenhouse gas emissions in future wastewater treatment plants.

An integrated mainstream and sidestream strategy for overcoming nitrite oxidizing bacteria adaptation in a continuous plug-flow nutrient removal process

An integrated mainstream aeration and sidestream sludge treatment was demonstrated to be effective in overcoming the adaptationof nitrite oxidizing bacteria (NOB) in an anoxic/oxic process. Results showed that by employing the alternating free nitrous acid and free ammonia (FNA/FA) sidestream sludge treatment alone, nitritation was established but varied, which was addressed by integrating alternating aeration with step feeding (ALASF) in reactor. Two critical considerations contributed to stable effluent nitrite accumulation (>83.8 %)and nitrogen removal (>83.0 %): 1) aerobic sludge rather than return sludge should be taken for FNA/FA treatment to avoid anoxic starvation which facilitated NOB recovery; 2) ALASF ensured timely denitritation and created constant anoxic disturbance for NOB inhibition. Nitrospira and Nitrobacter after 540-day operation were 0.38 % of seed sludge.A20 % reduction of operating cost was obtained in this nitritation process. This study moved nitritation one step closer to application in continuous plug-flow process from municipal wastewater.

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.

Critical Factors Facilitating Candidatus Nitrotoga To Be Prevalent Nitrite-Oxidizing Bacteria in Activated Sludge

Nitrite oxidation is the primary pathway that generates nitrate in engineered systems. However, little is known about the role of a novel nitrite-oxidizing bacteria (NOB) genus Candidatus Nitrotoga in activated sludge systems. To elucidate key factors that impact NOB community composition, laboratory-scale sequencing batch reactors (SBRs) were designed and operated under the same conditions as real wastewater treatment plants to achieve considerable nitrogen removal and similar community; then, different conditions including temperature (T), dissolved oxygen (DO), free nitrous acid (FNA), and free ammonia (FA) were applied. The 16S rRNA gene-based PCR and sequence analysis illustrated that Ca. Nitrotoga were abundant even at ambient temperature, thus further challenging the previous conception of them being solely cold-adapted. Ca. Nitrotoga are less competitive than Nitrospira during oxygen deficiency, indicating its lower affinity to dissolved oxygen. Ca. Nitrotoga are the dominant nitrite oxidizers under regular exposure to FNA and FA due to their relatively higher resistance than other NOB toward these two effective biocides. Therefore, this study demonstrates that Ca. Nitrotoga can play an important role in biological nitrogen removal and also highlights the need for multiple strategies for NOB suppression for the next-generation, shortcut nitrogen removal.

Autotrophic nitrogen removal characteristics of PN-anammox process enhanced by sulfur autotrophic denitrification under mainstream conditions

Stabilization of nitrification process and reduction of NO₃^--N concentration in effluent are the keys to realize mainstream application of partial nitrification-anaerobic ammonia oxidation (PN-anammox) process. The sulfur-based autotrophic denitrification (SADN) process was coupled with the PN-anammox in a single reactor to enhance and stabilize the nitrogen removal performance, and the feasibility and reaction characteristics of the coupling system under mainstream conditions were investigated. The results showed that the NO₃^- of PN-anammox effluent dropped from 22 to 24 mg/L to 5 mg/L after the SADN process coupled, and the total nitrogen removal efficiency and total nitrogen removal rate reached 83.5% and 0.15 kg/(m³·d), respectively. This coupling system doesn't need to over-strengthen PN control. Batch experiments showed that sulfur autotrophic oxidizing bacteria used O₂ to oxidize S^2- in the coupling system, which competed with SADN to remove NO₃^-. Moreover, Nitrosomonas, Candidatus Brocadia and Thiobacillus were the main genera for nitrogen and sulfur conversion.

Mitigating nitrous oxide emissions at a full-scale wastewater treatment plant

Mitigation of nitrous oxide (N₂O) emissions is of primary importance to meet the targets of reducing carbon footprints of wastewater treatment plants (WWTPs). Despite of a large amount of N₂O mitigation studies conducted in laboratories, full-scale implementation of N₂O mitigation is scarce, mainly due to uncertainties of mitigation effectiveness, validation of N₂O mathematical model, risks to nutrient removal performance and additional costs. This study aims to address the uncertainties by investigating the quantification, development and implementation of N₂O mitigation strategies at a full-scale sequencing batch reactor (SBR). To achieve this, N₂O emission dynamics, nutrient removal performance and operation of the SBR were monitored to quantify N₂O emissions, and identify the N₂O generation mechanisms. N₂O mitigation strategies centered on reducing dissolved oxygen (DO) levels were consequently proposed and evaluated using a multi-pathway N₂O production mathematical model before implementation. The implemented mitigation strategy resulted in a 35% reduction in N₂O emissions (from the emission factor of 0.89 ± 0.05 to 0.58 ± 0.06%), which was equivalent to annual reduction of 2.35 tonne of N₂O from the studied WWTP. This could be mainly attributed to reductions in N₂O generated via the NH₂OH oxidation pathway due to the lowering of DO level. As the first reported mitigation strategy permanently implemented at a full scale WWTP, it showcased that the mitigation of N₂O emissions at full-scale is feasible and that widely accepted N₂O mitigation strategies developed in laboratory studies are also likely effective in full-scale plants. Furthermore, the close agreement between the validated and predicted N₂O emission factors (0.58% vs 0.55%, respectively), showed that the N₂O mathematical model is a useful tool to evaluate N₂O mitigation strategies at full-scale. Importantly this work demonstrated that N₂O mitigation does not necessarily require additional operational cost to meet reduction targets. In contrast, the N₂O mitigation applied here reduced energy requirements for aeration by 20%. Equally important, long-term monitoring identified that N₂O mitigation did not affect the nutrient removal performance of the plant. Finally, with the knowledge acquired in this study, a standard approach for mitigating N₂O emissions from full-scale treatment plants was proposed.

Activated sludge and other aerobic suspended culture processes

This paper provides an overview of activated sludge related to suspended growth processes for the year 2019. The review encompasses process modeling of activated sludge, microbiology of activated sludge, process kinetics and mechanism, nitrogen and phosphorus control, design, and operation in the activated sludge field. The fate and effect of xenobiotics in activated sludge, including trace organic contaminant and heavy metal xenobiotics, which had influence on the growth of suspended sludge, are covered in this review. Compared to past reviews, many topics show increase in activity in 2019. These include, biokinetics process of aerobic granular sludge formation, pyrolysis kinetic mechanism of granular sludge. These topics are referred to formation and disintegration of granular sludge. Other sections include activated sludge settling model, toxicity resistant microbial community, nitritation-anammox processes for nitrogen removal, and respirometry used in the operation of real wastewater treatment plant are especially highlighted in this review. PRACTITIONER POINTS: Biokinetics process of aerobic granular sludge formation Toxicity resistant microbial community in activated sludge Nitritation-anammox processes for nitrogen removal in activated sludge.

Adaptation of nitrifying community in activated sludge to free ammonia inhibition and inactivation

Sludge treatment using free ammonia (FA) is an innovative approach that was recently reported effective achieving stable mainstream nitrogen removal via the nitrite pathway. This study aims to investigate the adaptation of nitrifying community and the response of nitrification performance to high-level of FA exposure under real wastewater conditions. Two parallel lab-scale sequencing batch reactors were operated and fed with real municipal wastewater, with one receiving sludge treatment by FA and another as a control. While the FA approach rapidly achieved partial nitrification with a nitrite accumulation ratio (NAR) of approximately 60%, the partial nitrification eventually failed due to nitrite-oxidizing bacteria (NOB) adaptation to FA inactivation. NOB activity in the inoculum was suppressed by 82% after exposure to FA at ~220 mg NH₃-N/L. However, towards the end of the experiments, significantly higher NOB activities were observed after exposure to the same level of FA. Distinct behaviours of NOB observed in batch tests during the study supported the reactor operational data and strongly suggested the adaptation of NOB under the FA stress. Furthermore, microbial community analysis revealed the underlying mechanism of the observed adaptation: the dominant NOB changed from Nitrospira to Candidatus Nitrotoga. It is for the first time shown that Ca. Nitrotoga are highly resistant to FA inhibition and inactivation in comparison to Nitrospira and Nitrobacter. In addition, while the Nitrosomonas genus was always the dominant ammonia-oxidizing bacteria (AOB) throughout the study, different shift in a species level was observed.

The differential proliferation of AOB and NOB during natural nitrifier cultivation and acclimation with raw sewage as seed sludge

Nitrifier immigration from sewers to wastewater treatment systems is attracting increasing attention for understanding nitrifier community assembly mechanisms, and improving process modeling and operation. In this study, nitrifiers in raw sewage were cultivated and acclimated in a sequencing batch reactor (SBR) for 90 days to investigate the characteristics of the influent nitrifiers after immigration. During the experiment, specific nitrite utilization rate (SNUR) exceeded specific ammonia utilization rate (SAUR) when floc size reached 224 ± 46 μm, and nitrogen loss occurred at the same time. The ratio of nitrite oxidizing bacteria (NOB) to ammonia oxidizing bacteria (AOB) increased from 0.84 to 2.14 after cultivation. The Illumina MiSeq sequencing showed that the dominant AOB was Nitrosomonas sp. Nm84 and unidentified species, and the three most abundant Nitrospira were Nitrospira defluvii, Nitrospira calida, and unidentified Nitrospira spp. in both raw sewage and cultivated activated sludge. The shared reads of raw sewage and activated sludge were 48.76% for AOB and 89.35% for Nitrospira. These indicated that nitrifiers, especially NOB, immigrated from influent can survive and propagate in wastewater systems, which may be a significant hinder to suppress NOB in the application of advanced nitrogen remove process based on partial nitrification in the mainstream.

Recovering partial nitritation in a PN/A system during mainstream wastewater treatment by reviving AOB activity after thoroughly inhibiting AOB and NOB with free nitrous acid

Starting up or recovering partial nitritation is a major challenge for achieving or maintaining stable partial nitritation/anammox (PN/A) during mainstream wastewater treatment. This study presents a novel strategy for recovering the nitrite pathway by selectively reviving ammonium oxidizing bacteria (AOB) after thoroughly inhibiting AOB and nitrite oxidizing bacteria (NOB) using free nitrous acid (FNA). A sequencing batch reactor was operated for PN/A to treat real domestic wastewater for 423 days, during which twice FNA treatment was temporarily implemented. Results showed that with a single 0.45 mg/L FNA treatment on flocculent sludge, the NO₃^--N concentration during the aerobic period showed an uptrend again and the partial nitritation performance was deteriorated. In contrast, 1.35 mg/L FNA treatment induced the inhibition of both AOB and NOB leading to regressive ammonium oxidation, but a subsequently higher DO (1.5 mg/L) and longer aeration duration recovered partial nitritation. For the relative abundances of the acquired biomass related to nitrogen conversion, Nitrosomonas, Nitrospira and Nitrolancea increased to 9.65%, 10.27% and 4.35%, respectively, at the beginning of the 1.35 mg/L FNA treatment, and Nitrospira and Nitrolancea decreased to 2.80% and 0.03% whereas Nitrosomonas declined to 8.71% after 76 days. Ca. Brocadia showed less resilience after the 1.35 mg/L FNA treatment, with the relative abundance decreasing from 13.38% to 0.62% due to insufficient nitrite. Molecular ecological network analysis indicates that among anammox taxa, Ca. Kuenenia and Ca. Brocadia formed important links with other N cycle processes. Moreover, the proposed strategy shows operational flexibility because it can be easily used to control NOB in mainstream PN/A applications offered by flocculent sludge systems.

Improving wastewater management using free nitrous acid (FNA)

Free nitrous acid (FNA), the protonated form of nitrite, has historically been an unwanted substance in wastewater systems due to its inhibition on a wide range of microorganisms. However, in recent years, advanced understanding of FNA inhibitory and biocidal effects on microorganisms has led to the development of a series of FNA-based applications that improve wastewater management practices. FNA has been used in sewer systems to control sewer corrosion and odor; in wastewater treatment to achieve carbon and energy efficient nitrogen removal; in sludge management to improve the sludge reduction and energy recovery; in membrane systems to address membrane fouling; and in wastewater algae systems to facilitate algae harvesting. This paper aims to comprehensively and critically review the current status of FNA-based applications in improving wastewater management. The underlying mechanisms of FNA inhibitory and biocidal effects are also reviewed and discussed. Knowledge gaps and current limitations of the FNA-based applications are identified; and perspectives on the development of FNA-based applications are discussed. We conclude that the FNA-based technologies have great potential for enhancing the performance of wastewater systems; however, further development and demonstration at larger scales are still required for their wider applications.