Intermittent Aeration Suppresses Nitrite-Oxidizing Bacteria in Membrane-Aerated Biofilms: A Model-Based Explanation

Tolerant and highly-permeable membrane aerated biofilm reactor enabled by selective armored membrane

Membrane aerated biofilm reactor (MABR) is a promising technology for dramatically reducing aeration energy consumption in wastewater treatment. However, the crucial membranes, including microporous hydrophobic membranes and dense membranes, are intolerant to fouling and possess high oxygen transfer resistance respectively, hindering their application potential. Herein, we developed a tolerant and highly-permeable membrane aerated biofilm reactor (THMABR) with a selective armor layer on the membrane to support the biofilm. The selective permeability of the selective armor layer enabled oxygen transfer efficiently and prevented interference by water, surfactant and microbial extracellular polymers. Besides, the composite of the 5 μm selective armor layer and microporous support significantly shortened the distance for solution-diffusion, reducing the transmembrane energy barrier of oxygen molecules. The THMABR's excellent and stable oxygen permeability solved the oxygen substrate concentration's limitation on oxidation rate, enabling functional bacteria to possess a higher oxidation potential and more abundant ecological niche. Based on the novel design, oxygen selective armor membrane (OSAM) performed notably higher oxygen transfer rates (9.61 gO2·m-2d-1) compared to the fouled microporous hydrophobic membrane (3.31 gO2·m-2d-1) and the dense membrane (4.04 gO2·m-2d-1). Besides, the OSAM exhibited more stable fouling resistance to water infiltration and pollutant intrusion compared to the microporous hydrophobic membrane after surfactant pretreatment. Municipal wastewater treatment tests further confirmed that the novel membrane support-selective armored layer-biofilm structure of THMABR can high-efficiently remove nitrogen. The structural characteristics, mechanisms of fouling resistance and oxygen transfer, as well as wastewater treatment performance of the THMABR and OSAM are discussed in detail. This work introduces a new design concept to overcome the bottleneck of traditional MABRs involving the disunity of tolerance and permeability, being expected to support the low-carbon and stable operation of wastewater biological treatment.

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.

Multi-omics reveals mechanism of hydroxylamine-enhanced ultimate nitrogen removal in pilot-scale anaerobic/aerobic/anoxic system

Hydroxylamine (HA) dosing is an effective strategy for promoting partial nitrification (PN); however, its impact on endogenous denitrification remains underexplored. In this study, long-term continuous HA dosing (1.4 mg/L) was introduced for over 110 days in a pilot-scale anaerobic/aerobic/anoxic (AOA) system treating municipal wastewater (66.7-75 m3/d). The HA dosing significantly increased the nitrite accumulation ratio to 67.6 ± 5.0 % (p<0.001) and reduced the effluent total inorganic nitrogen concentration from 6.2 ± 2.0 to 2.4 ± 1.1 mg/L (p<0.001), achieving a nitrogen removal efficiency of 87.4 ± 4.5 % (p<0.001) at a hydraulic retention time of 8 h. During the HA dosing, aerobic nitrogen removal contribution increased from 2.4 ± 3.4 % to 25.8 ± 8.1 % (p<0.001), and the anoxic nitrogen removal rate improved from 1.63 ± 0.11 to 2.35 ± 0.13 mg N/(L·h) (p<0.001). Enhanced nitrogen removal was not only achieved through the rapid establishment of PN but also driven by the long-term impact of HA dosing on microbial community dynamics. Multi-omics analyses revealed that HA disrupted the polyphosphate (poly-P) cycle, evidenced by enhanced transcription of ppx (poly-P degradation) and suppressed ppk (poly-P synthesis), thereby reducing energy availability for phosphate-accumulating organisms (PAOs) and shifting the carbon source competition toward glycogen-accumulating organisms (GAOs), with Ca. Competibacter abundance increased from 0.16 % to 1.13 % (p < 0.001). The economic analysis demonstrated that HA reduced sludge production by 11.2 % and saved operating costs by 31.4-42.8 % compared to conventional carbon sources. These findings highlight the potential of HA dosing to achieve sustainable and highly efficient wastewater treatment.

Maximizing the efficiency of single-stage partial nitrification/Anammox granule processes and balancing microbial competition using insights of a numerical model study

Granulation is an efficient approach for the rapid growth of anaerobic ammonia oxidation (Anammox) bacteria (X ANA $$ {X}_{ANA} $$ ) to limit the growth of nitrite-oxidizing bacteria (X NOB $$ {X}_{NOB} $$ ). However, the high sensitivity of Anammox bacteria to operational conditions and the competition with other microorganisms lead to a critical challenge in maintaining sufficientX ANA $$ {X}_{ANA} $$ population. In this study, a one-dimensional steady-state model was developed and calibrated to investigate the kinetic constants ofX ANA $$ {X}_{ANA} $$ growth and mass transport in individual granules, including the liquid film. According to the model calibration results, the range of the maximum specific growth rate constant ofX ANA $$ {X}_{ANA} $$ (μ ANA $$ {\mu}_{ANA} $$ ) was 0.033 to 0.10 d-1. In addition the other kinetic constants ofX ANA $$ {X}_{ANA} $$ were 0.003 d-1 for decay rate constant (b ANA $$ {b}_{ANA} $$ ), 0.10 mg-O2/L for oxygen half-saturation constant (K O 2 ANA $$ {K}_{O_2}^{ANA} $$ ), 0.07 mg-N/L for ammonia half-saturation constant (K NH 4 ANA $$ {K}_{NH_4}^{ANA} $$ ), and 0.05 mg-N/L for nitrite half-saturation constant (K NO 2 ANA $$ {K}_{NO_2}^{ANA} $$ ). The model simulation results showed that the dissolved oxygen of about 0.10 mg-O2/L was found to be optimal to maintain highX ANA $$ {X}_{ANA} $$ population. In addition, minimal COD concentration is required to control heterotrophs (X H $$ {X}_H $$ ) and improve ammonia oxidation by ammonia-oxidizing bacteria (X AOB $$ {X}_{AOB} $$ ). It was also emphasized that moderate mixing conditions (L f $$ {L}_f $$ ≅ $$ \cong $$ 100 μm) are preferable to decrease the diffusion of oxygen to the deep layers of the granules, controlling the competition betweenX ANA $$ {X}_{ANA} $$ andX NOB $$ {X}_{NOB} $$ . A single-factor relative sensitivity analysis (RSA) on microbial kinetics revealed thatμ ANA $$ {\mu}_{ANA} $$ is the governing factor in the efficient operation of the single-stage PN/A processes. In addition, it was found that nitrite concentration is a rate-limiting parameter on the success of the process due to the competition betweenX ANA $$ {X}_{ANA} $$ andX NOB $$ {X}_{NOB} $$ . These findings can be used to enhance our understanding on the importance of microbial competition and mass transport in the single-stage PN/A process. PRACTITIONER POINTS: A one-dimensional steady-state model was developed and calibrated for simulating the single-stage partial nitrification/Anammox (PN/A) granule process. Moderate liquid films (L f $$ {L}_f $$ ≅ $$ \cong $$ 100 μm) are preferable for better performance of Anammox growth in single-stage PN/A processes. Moderate dissolved oxygen (DO≅ $$ \cong $$ 0.10 mg-O2/L) is highly recommended for efficient growth of Anammox bacteria in single-stage PN/A granulation. Minimal COD (COD≅ $$ \cong $$ 0) is preferable for successful operation of the single-stage PN/A granule process. Nitrite concentration is a rate-limiting parameter on the competition between Anammox and nitrite-oxidizing bacteria in the single-stage PN/A processes.

Biofilm formation and microbial interactions in moving bed-biofilm reactors treating wastewater containing pharmaceuticals and personal care products: A review

The risk of pharmaceuticals and personal care products (PPCPs) has been paid more attention after the outbreak of COVID-19, threatening the ecology and human health resulted from the massive use of drugs and disinfectants. Wastewater treatment plants are considered the final stop to restrict PPCPs from wide spreading into the environment, but the performance of conventional treatment is limited due to their concentrations and characteristics. Previous studies have shown the unreplaceable capability of moving bed-biofilm reactor (MBBR) as a cost-effective method with layered microbial structure for treating wastewater even with toxic compounds. The biofilm community and microbial interactions are essential for the MBBR process in completely degrading or converting types of PPCPs to secondary metabolites, which still need further investigation. This review starts with discussing the initiation of MBBR formation and its influencing parameters according to the research on MBBRs in the recent years. Then the efficiency of MBBRs and the response of biofilm after exposure to PPCPs are further addressed, followed by the bottlenecks proposed in this field. Some critical approaches are also recommended for mitigating the deficiencies of MBBRs based on the recently published publications to reduce the environmental risk of PPCPs. Finally, this review provides fundamental information on PPCPs removal by MBBRs with the main focus on microbial interactions, promoting the MBBRs to practical application in the real world of wastewater treatment.

Exploring Aeration Strategies for Enhanced Simultaneous Nitrification and Denitrification in Membrane Aerated Bioreactors: A Computational Approach

In this study we employ computational methods to investigate the influence of aeration strategies on simultaneous nitrification-denitrification processes. Specifically, we explore the impact of periodic and intermittent aeration on denitrification rates, which typically lag behind nitrification rates under identical environmental conditions. A two-dimensional deterministic multi-scale model is employed to elucidate the fundamental processes governing the behavior of membrane aerated biofilm reactors (MABRs). We aim to identify key factors that promote denitrification under varying aeration strategies. Our findings indicate that the concentration of oxygen during the off phase and the duration of the off interval play crucial roles in controlling denitrification. Complete discontinuation of oxygen is not advisable, as it inhibits the formation of anaerobic heterotrophic bacteria, thereby impeding denitrification. Extending the length of the off interval, however, enhances denitrification. Furthermore, we demonstrate that the initial inoculation of the substratum (membrane in this study) influences substrate degradation under periodic aeration, with implications for both nitrification and denitrification. Comparison between continuous and periodic/intermittent aeration scenarios reveals that the latter can extend the operational cycle of MABRs. This extension is attributed to relatively low biofilm growth rates associated with non-continuous aeration strategies. Consequently, our study provides a comprehensive understanding of the intricate interplay between aeration strategies and simultaneous nitrification-denitrification in MABRs. The insights presented herein can contribute significantly to the optimization of MABR performance in wastewater treatment applications.

Genetic programming expressions for effluent quality prediction: Towards AI-driven monitoring and management of wastewater treatment plants

Continuous effluent quality prediction in wastewater treatment processes is crucial to proactively reduce the risks to the environment and human health. However, wastewater treatment is an extremely complex process controlled by several uncertain, interdependent, and sometimes poorly characterized physico-chemical-biological process parameters. In addition, there are substantial spatiotemporal variations, uncertainties, and high non-linear interactions among the water quality parameters and process variables involved in the treatment process. Such complexities hinder efficient monitoring, operation, and management of wastewater treatment plants under normal and abnormal conditions. Typical mathematical and statistical tools most often fail to capture such complex interrelationships, and therefore data-driven techniques offer an attractive solution to effectively quantify the performance of wastewater treatment plants. Although several previous studies focused on applying regression-based data-driven models (e.g., artificial neural network) to predict some wastewater treatment effluent parameters, most of these studies employed a limited number of input variables to predict only one or two parameters characterizing the effluent quality (e.g., chemical oxygen demand (COD) and/or suspended solids (SS)). Harnessing the power of Artificial Intelligence (AI), the current study proposes multi-gene genetic programming (MGGP)-based models, using a dataset obtained from an operational wastewater treatment plant, deploying membrane aerated biofilm reactor, to predict the filtrated COD, ammonia (NH4), and SS concentrations along with the carbon-to-nitrogen ratio (C/N) within the effluent. Input features included a set of process variables characterizing the influent quality (e.g., filtered COD, NH4, and SS concentrations), water physics and chemistry parameters (e.g., temperature and pH), and operation conditions (e.g., applied air pressure). The developed MGGP-based models accurately reproduced the observations of the four output variables with correlation coefficient values that ranged between 0.98 and 0.99 during training and between 0.96 and 0.99 during testing, reflecting the power of the developed models in predicting the quality of the effluent from the treatment system. Interpretability analyses were subsequently deployed to confirm the intuitive understanding of input-output interrelations and to identify the governing parameters of the treatment process. The developed MGGP-based models can facilitate the AI-driven monitoring and management of wastewater treatment plants through devising optimal rapid operation and control schemes and assisting the plants' operators in maintaining proper performance of the plants under various normal and disruptive operational conditions.

A vertical-flow constructed wetland-microalgal membrane photobioreactor integrated system for treating high-pollution-load marine aquaculture wastewater: A lab-scale study

Individual biological water treatment techniques often prove ineffective in removing accumulated high concentrations of nitrogen and phosphorus in the late stages of biofloc aquaculture. To address this issue, we integrated a previously developed autotrophic denitrification and nitrification integrated constructed wetland (ADNI-CW) with a microalgal membrane photobioreactor (MPBR). Under high nitrogen and phosphorus pollution loads in the influent, the standalone ADNI-CW system achieved removal rates of only 24.17 % ± 2.82 % for total nitrogen (TN) and 25.30 % ± 2.59 % for total phosphorus (TP). The optimal conditions for TN and TP degradation and microalgal biomass production in the Chlorella MPBR, determined using response surface methodology, were an inoculum OD680 of 0.394, light intensity of 161.583 μmol/m2/s, and photoperiod of 16.302 h light:7.698 h dark. Under the optimal operating conditions, the integrated ADNI-CW-MPBR system achieved remarkable TN and TP removal rates of 92.63 % ± 2.8 % and 77.46 % ± 8.41 %, respectively, and a substantial microalgal biomass yield of 54.58 ± 6.8 mg/L/day. This accomplishment signifies the successful achievement of efficient nitrogen and phosphorus removal from high-pollution-load marine aquaculture wastewater along with the acquisition of valuable microalgal biomass. A preliminary investigation of the microbial community composition and algal-bacterial interactions in different operational stages of the MPBR system revealed that unclassified_d__Bacteria, Chlorophyta, and Planctomycetes were predominant phyla. The collaborative relationships between bacteria and Chlorella surpassed competition, ensuring highly efficient nitrogen and phosphorus removal in the MPBR system. This study laid the foundation for the green and sustainable development of the aquaculture industry.

Low alkalinity, free ammonia, and free nitrous acid cooperatively stabilize partial nitrification under excessive aeration condition

In this study, a lab-scale sequencing batch reactor (SBR) was established to assess the feasibility of maintaining partial nitrification in treating high-ammonium wastewater under the condition of excessive aeration (1 L/min, 1289 min). Results showed that the average ammonia (NH4+-N) removal efficiency (ARE) was 93.4 ± 2.7% and the average nitrite accumulation ratio (NAR) was 90.7 ± 2.8% during 15-50 d. In a typical cycle, free ammonia (FA) and free nitrous acid (FNA) alternately inhibited the activity of nitrite oxidizing bacteria (NOB), while low alkalinity limited further nitrification at even excessive aeration. Metagenomic analysis revealed that the relative abundance of Nitrosomonas, as the ammonia oxidizing bacteria (AOB), was up to 1.61%, while NOB were not detected. Taken together, this study indicated partial nitrification was almost not affected by excessive aeration, demonstrating its robustness depending on the cooperative strategy of the low alkalinity limitation and the inhibition of FA and FNA.

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.

Challenges of suppressing nitrite-oxidizing bacteria in membrane aerated biofilm reactors by low dissolved oxygen control

Membrane aerated biofilm reactor (MABR) and shortcut nitrogen removal are two types of solutions to reduce energy consumption in wastewater treatment, with the former improving the aeration efficiency and the latter reducing the oxygen demand. However, integrating these two solutions, i.e., achieving shortcut nitrogen removal in MABR, is challenging due to the difficulty in suppressing nitrite-oxidizing bacteria (NOB). In this study, four MABRs were established to demonstrate the feasibility of initiating, maintaining, and restoring NOB suppression using low dissolved oxygen (DO) control, in the presence and absence of anammox bacteria, respectively. Long-term results revealed that the strict low DO (< 0.1 mg/L) in MABR could initiate and maintain stable NOB suppression for more than five months with nitrite accumulation ratio above 90 %, but it was unable to re-suppress NOB once they prevailed. Moreover, the presence of anammox bacteria increased the threshold of DO level to maintain NOB suppression in MABRs, but it was still incapable to restore the deteriorated NOB suppression in conjunction with low DO control. Mathematical modelling confirmed the experimental results and further explored the differences of NOB suppression in conventional biofilms and MABR biofilms. Simulation results showed that it is more challenging to maintain stable NOB suppression in MABRs compared to conventional biofilms, regardless of biofilm thickness or influent nitrogen concentration. Kinetic mechanisms for NOB suppression in different types of biofilms were proposed, suggesting that it is difficult to wash out NOB developed in the innermost layer of MABR biofilms because of the high oxygen level and low sludge wasting rate. In summary, this study systematically demonstrated the challenges of NOB suppression in MABRs through both experiments and mathematical modelling. These findings provide valuable insights into the applications of MABRs and call for more studies in developing effective strategies to achieve stable shortcut nitrogen removal in this energy-efficient configuration.

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.

Municipal wastewater contains antibiotic treatment using O2 transfer membrane based biofilm reactor: Interaction between regular pollutants metabolism and sulfamethoxazole degradation

The antibiotic sulfamethoxazole (SMX) is frequently detected in wastewater treatment plant effluents and has attracted significant attention owing to its significant potential environmental effects. We present a novel O₂ transfer membrane based biofilm reactor (O₂TM-BR) to treat municipal wastewater to eliminate containing SMX. Furthermore, conducting metagenomics analyses, the interactions in biodegradation process between SMX and regular pollutants (NH₄⁺-N and COD) were studied. Results suggest that O₂TM-BR yields evident advantages in SMX degradation. Increasing SMX concentrations did not affect the efficiency of the system, and the effluent concentration remained consistent at approximately 17.0 μg/L. The interaction experiment showed that heterotrophic bacteria tend to consume easily degradable COD for metabolism, resulting in a delay (>36 h) in complete SMX degradation, which is 3-times longer than without COD. It is worth noting that the taxonomic and functional structure and composition in nitrogen metabolism were significantly shifted upon the SMX. NH₄⁺-N removal remained unaffected by SMX in O₂TM-BR, and the expression of K10944 and K10535 has no significant difference under the stress of SMX (P > 0.02). However, the K00376 and K02567 required in the nitrate reductase is inhibited by SMX (P < 0.01), which hinders the reduction of NO₃^--N and hence the accumulation of TN. This study provides a new method for SMX treatment and reveals the interaction between SMX and conventional pollutants in O₂TM-BR as well as the microbial community function and assembly mechanism.

Free ammonia-free nitrous acid based partial nitrification in sequencing batch membrane aerated biofilm reactor

Membrane aerated biofilm reactor (MABR) has attracted a lot of attention as an energy-efficient integrated nitrogen removing technology in recent years. However, it is lacking of understanding to realize stable partial nitrification in MABR because of its unique oxygen transfer mode and biofilm structure. In this study, free ammonia (FA) and free nitrous acid (FNA) based control strategies for partial nitrification with low NH₄⁺-N concentration were proposed in a MABR of sequencing batch mode. The MABR was operated for over 500 days under different influent NH₄⁺-N concentrations. With the influent NH₄⁺-N of around 200 mg-N/L, partial nitrification could be established with relatively low concentration of FA (0.4-2.2 mg-N/L) which suppressed nitrite oxidizing bacteria (NOB) on the biofilm. With lower influent NH₄⁺-N concentration of around 100 mg-N/L, the FA concentration was lower and strengthened suppression strategies based on FNA were needed. With the final pH of operating cycles below 5.0, the FNA produced in the sequencing batch MABR could stabilize partial nitrification by eliminating NOB on the biofilm. Since the activity of ammonia oxidizing bacteria (AOB) was lower without the blow-off of dissolved carbon dioxide in the bubbleless MABR, longer hydraulic retention time was required to reach a low pH for high concentration of FNA to suppress NOB. After exposures to FNA, the relative abundance of Nitrospira was decreased by 94.6%, while the abundance of Nitrosospira increased greatly which became another dominant AOB genus in addition to Nitrosomonas.

Aerobic biodegradation of quinoline under denitrifying conditions in membrane-aerated biofilm reactor

Aerobic denitrification is being investigated as a novel biological nitrogen removal process, yet the knowledge on aerobic denitrification is limited to pure culture isolations and its occurrence in bioreactors remains unclear. This study investigated the feasibility and capacity of applying aerobic denitrification in membrane aerated biofilm reactor (MABR) for biological treatment of quinoline-laden wastewater. Stable and efficient removals of quinoline (91.5 ± 5.2%) and nitrate (NO₃^-) (86.5 ± 9.3%) were obtained under different operational conditions. Enhanced formation and function of extracellular polymeric substances (EPS) were observed at increasing quinoline loadings. MABR biofilm was highly enriched with aerobic quinoline-degrading bacteria, with a predominance of Rhodococcus (26.9 ± 3.7%) and secondary abundance of Pseudomonas (1.7 ± 1.2%) and Comamonas (0.94 ± 0.9%). Metagenomic analysis indicated that Rhodococcus contributed significantly to both aromatic degradation (24.5 ± 21.3%) and NO₃^- reduction (4.5 ± 3.9%), indicating its key role in aerobic denitrifying quinoline biodegradation. At increasing quinoline loadings, abundances of aerobic quinoline degradation gene oxoO and denitrifying genes of napA, nirS and nirK increased; there was a significant positive correlation of oxoO with nirS and nirK (p < 0.05). Aerobic quinoline degradation was likely initiated by hydroxylation, encoded by oxoO, followed by stepwise oxidations through 5,6-dihydroxy-1H-2-oxoquinoline or 8-hydroxycoumarin pathway. The results advance our understanding of quinoline degradation during biological nitrogen removal, and highlight the potential implementation of aerobic denitrification driven quinoline biodegradation in MABR for simultaneous removal of nitrogen and recalcitrant organic carbon from coking, coal gasification and pharmaceutical wastewaters.

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.

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.

Heterotrophic nitrification-aerobic denitrification performance of a novel strain, Pseudomonas sp. B-1, isolated from membrane aerated biofilm reactor

A heterotrophic nitrification-aerobic denitrification (HN-AD) strain isolated from membrane aerated biofilm reactor (MABR) was identified as Pseudomonas sp. B-1, which could effectively utilize multiple nitrogen sources and preferentially consume NH₄-N. The maximum degradation efficiencies of NO₃-N, NO₂-N and NH₄-N were 98.04%, 94.84% and 95.74%, respectively. The optimal incubation time, shaking speed, carbon source, pH, temperature and C/N ratio were 60 h, 180 rpm, sodium succinate, 8, 30 °C and 25, respectively. The strain preferred salinity of 1.5% and resisted heavy metals in the order of Mn²⁺ > Co²⁺ > Zn²⁺ > Cu²⁺. It can be preliminarily speculated from the results of enzyme assay that the strain removed nitrogen via full nitrification-denitrification pathway. The addition of strain into the conventional MABR significantly intensified the HN-AD performance of the reactor. The relative abundance of the functional bacteria including Flavobacterium, Pseudomonas, Paracoccus, Azoarcus and Thauera was obviously increased after the bioaugmentation. Besides, the expression of the HN-AD related genes in the biofilm was also strengthened. Thus, strain B-1 had great application potential in nitrogen removal process.

Increasing nitrogen and organic matter removal from swine manure digestate by including pre-denitrification and recirculation in single-stage partial nitritation/anammox

A novel two-stage process comprising pre-denitrification and single-stage partial nitritation/anammox was developed to treat swine manure digestate with a constant nitrogen loading rate of 1.0 gN/L/d. As the influent NH₄⁺-N concentration increased from 500 to 1500 mg/L, a nitrogen removal efficiency of 88 %-96 % and 5-day biochemical oxygen demand removal efficiency of 93 %-97 % were achieved. Owing to the high influent chemical oxygen demand (COD)/nitrates and nitrites (NO_X) ratio of 8.2-9.2 and high COD utilization of denitrifying bacteria (DB), the NO₂^--N and NO₃^--N removal efficiencies in the denitrification reactor reached 96 %-99 % and 97 %-99 %, respectively. The contribution of anammox bacteria to nitrogen removal was 70.9 %-84.3 %, whereas that of DB was 11.7 %-18.3 %. The contributions of DB and ordinary heterotrophic organisms to COD removal were 19.5 %-49.3 % and 17.9 %-39 %, respectively. This study will help guide the anammox process in swine wastewater treatment.

Chemical oxygen demand and nitrogen removal from real membrane-manufacturing wastewater by a pilot-scale internal circulation reactor integrated with partial nitritation-anammox

A pilot-scale system integrating internal circulation and partial nitritation-anammox successfully treated real high-strength membrane-manufacturing wastewater in this study. With this pilot-scale system, a high chemical oxygen demand (COD) removal efficiency of 85 % and a nitrogen removal of 90 % are achieved at an organic loading rate of 6.0 kg COD/m3/d. The nitrogenous organic matters in the internal circulation zone are degraded into ammonia nitrogen. In the partial nitrification zone, nitrite accumulation is achieved, providing a suitable NH4+-N/NO2--N ratio for anammox reaction. Partial nitritation is achieved by maintaining an operational temperature at 30-35 °C, free ammonia concentration at 5-7 mg/L and dissolved oxygen at 0.4-0.7 mg/L with a reflux ratio of 150 %. The COD to nitrogen ratio in the internal circulation effluent is maintained below 3.0 to inhibit nitrite oxidizing bacteria. This study demonstrates that a pilot-scale system can efficiently remove organic matters and nitrogen from wastewater of membrane-manufacturing industry.

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.

Model study on real-time aeration based on nitrite for effective operation of single-stage anammox

Anaerobic ammonia oxidation (Anammox) is an innovative technology for cost-efficient nitrogen removal without intensive aeration. However, effective control of the competition between nitrite oxidizing bacteria (X_NOB) and Anammox bacteria (X_ANA) for nitrite is a key challenge for broad applications of single-stage Anammox processes in real wastewater treatment. Therefore, a real-time aeration scheme was proposed to determine dissolved oxygen (DO) based on nitrite concentration for effective control of X_NOB growth while maintaining the X_ANA activity in a single-stage Anammox process. In this study, a non-steady state mathematical model was developed and calibrated using previously reported lab-scale Anammox results to investigate the efficiency of the proposed real-time aeration scheme in enhancing the Anammox process. Based on the calibrated model simulation results, DO of about 0.10 mg-O₂/L was found to be ideal for maintaining effective nitrite creation by ammonia oxidizing bacteria (X_AOB) while slowing down the growth of X_NOB. If DO is too low (e.g., 0.01 mg-O₂/L or lower), the overall rate of the ammonia removal is limited due to slow growth of X_AOB. On the other hand, high DO (e.g., 1.0 mg-O₂/L or higher) inhibits the growth of X_ANA, resulting in dominancy of X_AOB and X_NOB. According to the simulation results, nitrite concentration was found to be a rate-limiting parameter on effective nitrogen removal in single-stage Anammox processes. We also found that nitrite concentration can be used as a real-time switch for aeration in a single-stage Anammox process. A schematic aeration method based on real-time nitrite concentration was proposed and examined to control the competition between X_ANA and X_NOB. In the model simulation, the X_ANA activity was successfully maintained because the schematic aeration prevented an outgrowth of X_NOB, allowing energy-efficient nitrogen removal using single-stage Anammox processes.

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.

Nitrate respiration occurs throughout the depth of mucoid and non-mucoid Pseudomonas aeruginosa submerged agar colony biofilms including the oxic zone

Pseudomonas aeruginosa is an opportunistic pathogen and well characterized biofilm former. P. aeruginosa forms strong oxygen gradients inside biofilms due to rapid oxygen respiration in the top layers and the poor solubility of oxygen coupled with diffusion limited transport. Transcriptomic evidence from in vitro and ex vivo sampling suggests that denitrification is occurring in biofilms in ostensibly oxic environments. It is hypothesized that in the presence of nitrate there is stratification with aerobic respiration occurring in the outer oxic layer and denitrification in the lower anoxic zone. We used submerged agar colony biofilms grown from mucoid (FRD1) and non-mucoid (PAO1) strains to simultaneously measure depth microprofiles of oxygen and nitrous oxide in the same colony with microelectrodes. Oxygen respiration occurred at the top of the colony as expected but denitrification occurred throughout the entire depth, even in the oxic region. Local denitrification rates were highly variable suggesting heterogenous metabolic activity within the colony. We also assessed the short-term influence of tobramycin on aerobic respiration within a PAO1 colony. Although there was an immediate reduction in respiration it was never completely arrested over a 2 h period. On tobramycin removal the oxygen gradient steadily reestablished, demonstrating immediate recovery of metabolic activity.

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.

Coupling of membrane-based bubbleless micro-aeration for 2,4-dinitrophenol degradation in a hydrolysis acidification reactor

Micro-aeration hydrolysis acidification (HA) is an effective method to enhance the removal of toxic and refractory organic matter, but the difficulty in stable dosing control of trace oxygen limits its wide application. Membrane-based bubbleless aeration has been proved as an ideal aeration method because of its higher oxygen transfer rate, more uniform mass transfer, and lower cost than HA. However, the available information on its application in HA is limited. In this study, membrane-based bubbleless micro-aeration coupled with hydrolysis acidification (MBL-MHA) was exploited to investigate the performance of 2,4-dinitrophenol (2,4-DNP) degradation via comparing it with bubble micro-aeration HA (MHA) and anaerobic HA. The results indicated that the performances in MBL-MHA and MHA were higher than those in HA during the experiment. 2,4-DNP degradation rates under redox microenvironments caused by counter-diffusion in MBL-MHA (84.43∼97.28%) were higher than those caused by co-diffusion in MHA (82.41∼94.71%) under micro-aeration of 0.5-5.0 mL air/min. The 2,4-DNP degradation pathways in MBL-MHA were nitroreduction, deamination, aromatic ring cleavage, and fermentation, while those in MHA were hydroxylation, aromatic ring cleavage, and fermentation. Reduction/oxidation-related, interspecific electron transfer-related species, and fermentative species in MBL-MHA were more abundant than that in MHA. Ultimately, more reducing/oxidizing forces formed by more redox proteins/enzymes from these rich species could enhance 2,4-DNP degradation in MBL-MHA.

NOB suppression strategies in a mainstream membrane aerated biofilm reactor under exceptionally low lumen pressure

Integrating the aeration-efficient membrane aerated biofilm reactor (MABR) with anaerobic ammonium oxidation (anammox) could yield further reduction in energy in wastewater treatment facilities. However, nitrite oxidizing bacteria (NOB) suppression remained challenging due to the absence of intrinsic inhibition factors in mainstream conditions. This study investigated selective NOB suppression strategies in MABR under <5 kPa lumen pressure. Three MABRs were seeded from different seeding sludge, and operated under various ammonium loading rates, aeration pressure, and temporary inhibitory shock conditions. The three reactors were operated for 170-456 days depending on studied parameters. The results showed that higher ammonium loading could create a substrate-oxygen imbalance and quickly contain emergent NOB activity when aeration pressure was not excessive. In addition, lowering of aeration pressure reversed nitrite oxidizing activities without affecting ammonium oxidizing bacteria (AOB). Cultivating partial nitritation biofilm under zero positive aeration pressure slowed down the growth of NOB yet resulted in self-induced anammox activities. With the aid of temporary free ammonia (FA)/free nitrous acid (FNA) treatment, full-nitrifying biofilm could be transformed to stable partial nitritation biofilm. More than 84% nitrite accumulation ratio (NAR) was sustained during stable operation in each reactor together with an ammonium removal rate of more than 100 mg-N/L/d. Microbial analysis revealed that Nitrosomonas was the main AOB taxon in the three reactors while K-strategist Nitrospira showed presence despite low nitrite oxidizing activities. Under zero positive pressure, proliferation of Nitrospira was much slower while Candidatus Brocadia was self-induced. Furthermore, Nitrospira showed downturn after temporary inhibition treatment.

Assessing membrane aerated biofilm reactor configurations in mainstream anammox applications

Partial nitritation anammox (PNA) membrane aerated biofilm reactors (MABRs) have the potential to be employed in mainstream wastewater treatment and can drastically decrease the energy and carbon requirements for nitrogen removal. Previous PNA MABR studies have looked at 1-stage systems, but no study has holistically compared the performance of different MABR configurations. In this study, a PNA MABR was mechanistically modelled to determine the impact of the reactor configuration (1-stage, hybrid, or 2-stage system) on the location of the preferred niche for anammox bacteria and the overall nitrogen removal performance. Results from this study show that the 2-stage configuration, which used an MABR with a thin biofilm for nitritation and a moving bed biofilm reactor for anammox, had a 20% larger nitrogen removal rate than the 1-stage or hybrid configurations. This suggests that an MABR should focus on maximizing nitrite production with anammox implemented in a second-stage biofilm reactor to achieve the most cost-effective nitrogen removal. However, the optimal configuration will likely be facility specific, as each facility differs in operating costs, construction costs, footprint, and effluent limits. Additional experimentation is required to confirm these results, but this work narrows the number of viable configurations that need to be tested. The results of this study will inform researchers and engineers how to best implement PNA MABRs in mainstream nitrogen removal at larger scales.

Unraveling encapsulated growth of Nitrosomonas europaea in alginate: An experimental and modeling study

Encapsulation is a promising technology to retain and protect autotrophs for biological nitrogen removal. One-dimensional biofilm models have been used to describe encapsulated systems; they do not, however, incorporate chemical sorption to the encapsulant nor do they adequately describe cell growth and distribution within the encapsulant. In this research we developed a new model to describe encapsulated growth and activity of Nitrosomonas europaea, incorporating ammonium sorption to the alginate encapsulant. Batch and continuous flow reactors were used to verify the simulation results. Quantitative PCR and cross-section fluorescence in situ hybridization were used to analyze the growth and spatial distribution of the encapsulated cells within alginate. Preferential growth of Nitrosomonas near the surface of the encapsulant was predicted by the model and confirmed by experiments. The modeling and experimental results also suggested that smaller encapsulants with a larger surface area to volume ratio would improve ammonia oxidation. Excessive aeration caused the breakage of the encapsulant, resulting in unpredicted microbial release and washout. Overall, our modeling approach is flexible and can be used to engineer and optimize encapsulated systems for enhanced biological nitrogen removal. Similar modeling approaches can be used to incorporate sorption of additional species within an encapsulant, additional nitrogen-converting microorganisms, and the use of other encapsulation materials.

Recent progress using membrane aerated biofilm reactors for wastewater treatment

The membrane biofilm reactor (MBfR), which is based on the counter diffusion of the electron donors and acceptors into the biofilm, represents a novel technology for wastewater treatment. When process air or oxygen is supplied, the MBfR is known as the membrane aerated biofilm reactor (MABR), which has high oxygen transfer rate and efficiency, promoting microbial growth and activity within the biofilm. Over the past few decades, laboratory-scale studies have helped researchers and practitioners understand the relevance of influencing factors and biological transformations in MABRs. In recent years, pilot- to full-scale installations are increasing along with process modeling. The resulting accumulated knowledge has greatly improved understanding of the counter-diffusional biological process, with new challenges and opportunities arising. Therefore, it is crucial to provide new insights by conducting this review. This paper reviews wastewater treatment advancements using MABR technology, including design and operational considerations, microbial community ecology, and process modeling. Treatment performance of pilot- to full-scale MABRs for process intensification in existing facilities is assessed. This paper also reviews other emerging applications of MABRs, including sulfur recovery, industrial wastewater, and xenobiotics bioremediation, space-based wastewater treatment, and autotrophic nitrogen removal. In conclusion, commercial applications demonstrate that MABR technology is beneficial for pollutants (COD, N, P, xenobiotics) removal, resource recovery (e.g., sulfur), and N₂O mitigation. Further research is needed to increase packing density while retaining efficient external mass transfer, understand the microbial interactions occurring, address existing assumptions to improve process modeling and control, and optimize the operational conditions with site-specific considerations.

Bacterial-algae biofilm enhance MABR adapting a wider COD/N ratios wastewater: Performance and mechanism

Although membrane aerated biofilm reactor (MABR) is promising in nitrogen removal due to its counter-diffusion biofilms structure, it still cannot adapt a wider COD/N ratios wastewater. In this condition, expanding the MABR applicability range in different COD/N ratio wastewater is necessary. In this study, a bacterial-algae biofilm, instead of bacteria biofilm, supporting membrane aerated biofilm reactor (MABAR) was constructed, and the performance was compared to MABR. Results showed that the total nitrogen (TN) removal efficiency was promoted significantly in MABAR regardless of the COD/N ratio. Compared to MABR, effluent TN concentration in COD/N ratio of 2, 5, and 8 declined by 14.34 mg/L, 0.50 mg/L, and 12.10 mg/L, respectively. Nitrification inhibition test suggested that algae assimilation made an obvious contribution (at least 18.18 mg/L) to the NH₄⁺-N removal in MABAR. Besides, redundancy analysis (RDA) indicates that MABAR has a negative correlation with Nitrospirae but is positively correlated with NH₄⁺-N removal load. These results are consistent with the kinetics result that algae assimilation, instead of nitrification-denitrification, is responsible for the nitrogen removal in MABAR. Therefore, the change of nitrogen removal route further gave MABAR excellent adaptability and impact resistance to address wastewater with different COD/N ratios, which is conducive to its wide application.

Development of Strategies for AOB and NOB Competition Supported by Mathematical Modeling in Terms of Successful Deammonification Implementation for Energy-Efficient WWTPs

Novel technologies such as partial nitritation (PN) and partial denitritation (PDN) could be combined with the anammox-based process in order to alleviate energy input. The former combination, also noted as deammonification, has been intensively studied in a frame of lab and full-scale wastewater treatment in order to optimize operational costs and process efficiency. For the deammonification process, key functional microbes include ammonia-oxidizing bacteria (AOB) and anaerobic ammonia oxidation bacteria (AnAOB), which coexisting and interact with heterotrophs and nitrite oxidizing bacteria (NOB). The aim of the presented review was to summarize current knowledge about deammonification process principles, related to microbial interactions responsible for the process maintenance under varying operational conditions. Particular attention was paid to the factors influencing the targeted selection of AOB/AnAOB over the NOB and application of the mathematical modeling as a powerful tool enabling accelerated process optimization and characterization. Another reviewed aspect was the potential energetic and resources savings connected with deammonification application in relation to the technologies based on the conventional nitrification/denitrification processes.

Free ammonia resistance of nitrite-oxidizing bacteria developed in aerobic granular sludge cultivated in continuous upflow airlift reactors performing partial nitritation

Free ammonia (FA) inhibition has been taken advantage as a strategy to suppress the growth of nitrite-oxidizing bacteria (NOB) in aerobic granules stabilized in a continuous upflow airlift reactor to achieve partial nitritation. However, after nearly 18 months of continuous exposure of aerobic granules to FA in the reactor, the FA inhibition of NOB was proven ineffective, and the partial nitritation gradually shifted to partial nitrification even though the long-term granule structural stability remained excellent under the continuous-flow mode. The extent of NOB resistance to FA inhibition was quantified based on the kinetic response of NOB to various FA concentrations in the form of an uncompetitive inhibition coefficient. It was confirmed that the NOB immobilized in larger granules under longer term exposure to FA tend to become more resistant to FA. Thereby, it was concluded that NOB can develop strong resistance to FA after continuous exposure, and thus, FA inhibition is not a reliable strategy to achieve partial nitritation in mainstream wastewater treatment. PRACTITIONER POINTS: Nitrifying aerobic granules can remain structurally stable in continuous-flow bioreactors. NOB developed free ammonia resistance after 6-month continuous exposure. Larger aerobic granules tended to develop stronger free ammonia resistance. Free ammonia inhibition is not a reliable strategy for mainstream anammox.

Bifurcation Analysis of an Impulsive System Describing Partial Nitritation and Anammox in a Hybrid Reactor

Low-energy nitrogen removal under mainstream conditions is a technology that has received significant attention in recent years as the water industry drives toward long-term sustainability goals. Simultaneous partial nitritation-Anammox (PN/A) is one process that can provide substantial energy reduction and lower sludge yields. Mathematical modeling of the PN/A process offers engineers insights into the operating conditions necessary to maximize its potential. Laureni et al. (Laureni et al. Water Res. 2019, 14) have recently published a simplified mechanistic model of the process operated as a sequencing batch reactor that investigated the effect of three key operating parameters on performance (Anammox biofilm activity, dissolved oxygen concentration and fraction of solids wasted). The analysis of the model was limited, however, to simulation with relatively few discrete parameter sets. Here, we demonstrate through the use of bifurcation theory applied to an impulsive dynamical system that the parameter space can be partitioned into regions in which the system converges to different fixed points that represent different outcomes: either the washout of nitrite-oxidizing bacteria or their survival. Mapping process performance data onto these spaces allows engineers to target suitable operating regimes for specific objectives. Here, for example, we note that the nitrogen removal efficiency is maximized close to the curve that separates the regions in parameter space where nitrite-oxidizing bacteria washout from the region in which they survive. Further, control of solids washout and Anammox biofilm activity can also reduce oxygen requirements while maintaining an appropriate hydraulic retention time. The approach taken is significant given the possibility for using such a methodology for models of increasing complexity. This will enable engineers to probe the entire parameter space of systems of higher dimension and realism in a consistent manner.

Optimization of the Aeration Strategies in a Deammonification Sequencing Batch Reactor for Efficient Nitrogen Removal and Mitigation of N2O Production

In deammonification systems, nitrite-oxidizing bacteria (NOB) suppression and nitrous oxide (N₂O) mitigation are two important operational objectives. To carry out this multivariable analysis of response, a comprehensive model for the N cycle was developed and evaluated against experimental data from a laboratory-scale deammonification granular sludge sequencing batch reactor. Different aeration strategies were tested, and the manipulated variables comprised the dissolved oxygen (DO) set point in the aerated phase, aeration on/off frequency (F), and the ratio (R) between the non-aerated and aerated phase durations. Experimental results showed that a high ammonium utilization rate (AUR) in relation to the low nitrate production rate (NPR) (NPR/AUR = 0.07-0.08) and limited N₂O emissions (E_N2O < 2%) could be achieved at the DO set point = 0.7 mg O₂/L, R ratio = 2, and F frequency = 6-7 h^-1. Under specific operational conditions (biomass concentration, NH₄⁺-N loading rate, and temperature), simulation results confirmed the feasible aeration strategies for the trade-offs between the NOB suppression and N₂O emission. The intermittent aeration regimes led to frequent shifts in the predominating N₂O production pathways, that is, hydroxylamine (NH₂OH) oxidation (aerated phase) versus autotrophic denitrification (non-aerated phase). The inclusion of the extracellular polymeric substance mechanism in the model explained the observed activity of heterotrophs, especially Anaerolineae, and granule formation.

Enhanced removal of cephalexin and sulfadiazine in nitrifying membrane-aerated biofilm reactors

Nitrification process has been reported to be capable of degrading various pharmaceuticals due to the cometabolism of ammonia-oxidizing bacteria (AOB). The membrane aerated biofilm reactor (MABR) is an emerging configuration in wastewater treatment with advantages of high nitrification rate and low energy consumption. However, there are very few studies investigating the degradation of antibiotics at environmentally relevant levels in nitrifying MABR systems. In this study, the removal of two widely used antibiotics, cephalexin (CFX) and sulfadiazine (SDZ), was evaluated in two independent MABRs with nitrifying biofilms. The impacts of CFX and SDZ exposure on the nitrification performance and microbial community structure within biofilms were also investigated. The results showed that nitrifying biofilms were very efficient in removing CFX (94.6%) and SDZ (75.4%) with an initial concentration of 100 μg/L when hydraulic retention time (HRT) was 4 h in the reactors. When HRT decreased from 4 h to 3 h, the removal rates of CFX and SDZ increased significantly from 23.4 ± 1.0 μg/(L·h) and 18.7 ± 1.1 μg/(L·h), respectively, to 27.7 ± 1.3 μg/(L·h) (p＜0.01) and 20.8 ± 2.4 μg/(L·h) (p＜0.05), while the removal efficiencies decreased to 86.0% and 61.5%, respectively. Despite the exposure to CFX and SDZ, the nitrification performance was not affected, and microbial community structure within biofilms also remained relatively stable. This study shows that nitrifying MABR process is a promising option for the efficient removal of antibiotics from domestic wastewater.

Nighttime aeration mode enhanced the microalgae-bacteria symbiosis (ABS) system stability and pollutants removal efficiencies

Utilizing external aeration to enhance the performance of microalgae-bacteria symbiosis (ABS) system has been extensively studied. However, inappropriate aeration damaged ABS system stability. A nighttime aeration mode (NA-ABS) in different aeration intensities (20, 50, 100 mL/min) was adopted to compare to continuous aeration microalgae-bacteria symbiosis (CA-ABS) mode and no-aerated mode on pollutants removal efficiencies and system stability. Results showed that NA-ABS system performed better on total organic carbon (TOC), NH₄⁺-N, total nitrogen (TN) and PO₄^3- removal than CA-ABS system, especially under the aeration intensity of 20 mL/min (NAI20), with the removal efficiencies of 96.59%, 99.18%, 90.30% and 89.16%, respectively. These results were because NA-ABS system prevented CO₂ stripping and provided more dissolved inorganic carbon (DIC) for the microalgae growth. Furthermore, less CO₂ stripping released the competition between microalgae and autotrophic bacteria for the DIC, leading to a more stable ABS system during long-term operation. This paper suggested that NA-ABS system would provide some new insights into ABS system and be helpful for further study.

Organic carbon promotes algae proliferation in membrane-aeration based bacteria-algae symbiosis system (MA-BA)

In the bacteria-algae (BA) system, the amount of oxygen produced by the algae is always insufficient for the organic carbon degradation, resulting in less inorganic carbon (IC) production. Meanwhile, the conventional extra aeration method always causes CO₂ stripping and IC loss. Both two reasons limited the algae boosting. Membrane aeration (MA) has the excellent capability of organic carbon thorough degradation and gas blown-off control. In this study, MA-BA was employed to investigate the effect of organic carbon on the algae growth. Results showed that COD had a positive correlation with Chlorophyll-a (Chl-a) and algae proliferation in MA-BA system according to the redundancy analysis (RDA). The biggest Chl-a concentration (20.95 mg/cm²) occurred in R₄ (COD = 400 mg/L). Stimulated algal population changed nutrient removal pathway from bacterial action to algae action. Meantime, Soared algae accumulation would selectively excite the abundance of bacteria that supported the algae growth, such as Acinetobacter, which exhibited a growing trend as the increase of influent COD, especially in the inner biofilm. This paper provided new insight into the effect of organic carbon on the algae in a novel MA-BA system, which will be helpful for future research.

Common principles and best practices for engineering microbiomes

Despite broad scientific interest in harnessing the power of Earth's microbiomes, knowledge gaps hinder their efficient use for addressing urgent societal and environmental challenges. We argue that structuring research and technology developments around a design-build-test-learn (DBTL) cycle will advance microbiome engineering and spur new discoveries of the basic scientific principles governing microbiome function. In this Review, we present key elements of an iterative DBTL cycle for microbiome engineering, focusing on generalizable approaches, including top-down and bottom-up design processes, synthetic and self-assembled construction methods, and emerging tools to analyse microbiome function. These approaches can be used to harness microbiomes for broad applications related to medicine, agriculture, energy and the environment. We also discuss key challenges and opportunities of each approach and synthesize them into best practice guidelines for engineering microbiomes. We anticipate that adoption of a DBTL framework will rapidly advance microbiome-based biotechnologies aimed at improving human and animal health, agriculture and enabling the bioeconomy.

Membrane Aerated Biofilm Reactors - How longitudinal gradients influence nitrogen removal - A conceptual study

Membrane-aerated biofilm reactors are becoming more important for nitrogen removal in the wastewater sector. One-dimensional (1D) models are widely used to study the performance of such systems; however, 1D models are not able to simulate the longitudinal gradients that exist in the reactor. Although there is experimental evidence that points to the existence of longitudinal gradients simple modeling approaches that consider these gradients are not yet developed. This study proposes a novel multi-compartment model that simulates the longitudinal substrate and oxygen gradients. It assesses the effects of temperature, biofilm thickness, number of compartments, and flow configuration (liquid and gas phase) on the modeling results. Additionally, it compares the capabilities of a traditional 1D model with those of the novel multi-compartment model. Our results show that a classical 1D model predicts a lower total dissolved nitrogen concentration (TDN) in the effluent in contrast to the predictions of the multi-compartment model. In the worst-case scenario, the TDN predicted by the traditional 1D model was three times lower than the prediction of the multi-compartment model. The results delivered by the models differ also in the axial gradients. The traditional 1D model, for example, predicted an oxygen concentration at the membrane surface of 0.4 mg-O₂/l while the multi-compartment model predicted a concentration of 2.9 mg-O₂/l. Finally, the results of this study show that the longitudinal oxygen gradient has an important effect on both, biomass distribution and effluent TDN, whereas the longitudinal substrate exclusively affected the effluent TDN.

Hydrophobic Gas Transfer Membranes for Wastewater Treatment and Resource Recovery

Gaseous compounds, such as CH₄, H₂, and O₂, are commonly produced or consumed during wastewater treatment. Traditionally, these gases need to be removed or delivered using gas sparging or liquid heating, which can be energy intensive with low efficiency. Hydrophobic membranes are being increasingly investigated in wastewater treatment and resource recovery. This is because these semipermeable barriers repel water and create a three-phase interface that enhances mass transfer and chemical conversions. This Critical Review provides a first comprehensive analysis of different hydrophobic membranes and processes, and identifies the challenges and potential for future system development. The discussions and analyses were grouped based on mechanisms and applications, including membrane gas extraction, membrane gas delivery, and hybrid processes. Major challenges, such as membrane fouling, wetting, and limited selectivity and functionality, are identified, and potential solutions articulated. New opportunities, such as electrochemical coating, integrated membrane electrodes, and membrane functionalization, are also discussed to provide insights for further development of more efficient and low-cost membranes and processes.

Initial nitrite concentration promote nitrite-oxidizing bacteria activity recovery from transient anoxia: Experimental and modeling investigations

Transient anoxia due to the periodic anoxic/aerobic operation is beneficial for the nitrite-oxidizing bacteria (NOB) suppression. A continuous reactor of modified University of Cape Town process treating municipal wastewater was equipped with alternating anoxic/aerobic zones to maintain nitritation. Higher nitrite accumulation ratio in the oxic zones was achieved through transient anoxia and shorter aerobic actual hydraulic retention time (15 min), but it steeply deteriorated from above 95.0% to 21.0% after elevated temperature (25 °C). Batch experiments indicated that the existence of initial nitrite at the starting of aerobic phase promoted the recovery of NOB activity from transient anoxia and inhibited the activity of ammonium-oxidizing bacteria. Furthermore, a supplemental modeling further confirmed that the specific growth rates of NOB (μ_NOB) decreased at the anoxic phase and the recovery extent of μ_NOB after anoxic exposure have a positive correlation with the initial concentrations of nitrite, leading to the failure of maintaining nitritation.

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

Sidestream sludge treatment approaches have been developed in recent years to achieve mainstream nitrite shunt or partial nitritation, where NOB are selectively inactivated by biocidal factors such as free nitrous acid (FNA) or free ammonium (FA) in a sidestream reactor. The existence of NOB in raw wastewater has been increasingly realized and could pose critical challenge to stable NOB suppressions in those systems. This study, for the first time, evaluated the impact of influent NOB on the NOB suppressions in a mainstream nitrite shunt system achieved through sidestream sludge treatment. An over 500-day sequential batch reactor operation with six experimental phases rigorously demonstrated the negative effects of influent NOB on mainstream NOB control. Continuously seeding of NOB contained in influent stimulated NOB community shifts, leading to different extents of ineffective NOB suppression. The role of primary wastewater treatment in NOB removal from raw wastewater was also investigated. Results suggest primary settling and High Rate Activated Sludge system could remove a large part of NOB contained in raw wastewater. Primary treatment for raw wastewater is necessary for ensuring stable mainstream NOB suppressions.

Full-scale comparison of N2O emissions from SBR N/DN operation versus one-stage deammonification MBBR treating reject water - and optimization with pH set-point

To be able to fulfill the Paris agreement regarding anthropogenic greenhouse gases, all potential emissions must be mitigated. Wastewater treatment plants should aim to eliminate emissions of the most potent greenhouse gas, nitrous oxide (N₂O). In this study, these emissions were measured at a full-scale reject water treatment tank during two different operation modes: nitrification/denitrification (N/DN) operating as a sequencing batch reactor (SBR), and deammonification (nitritation/anammox) as a moving bed biofilm reactor (MBBR). The treatment process emitted significantly less nitrous oxide in deammonification mode 0.14-0.7%, compared to 10% of total nitrogen in N/DN mode. The decrease can be linked to the changed feeding strategy, the lower concentrations of nitrite, a lower load of ammonia oxidized, a shorter aeration time, the absence of non-optimized ethanol dosage or periodic lack of oxygen as well as the introduction of biofilm. Further, evaluation was done how the operational pH set point influenced the emissions in deammonification mode. Lower concentrations of nitrous oxide were measured in water phase at higher pH (7.5-7.6) than at lower pH (6.6-7.1). This is believed to be mainly because of the lower aeration ratio and increased complete denitrification at the higher pH set point.

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

Stable suppression of nitrite oxidizing bacteria (NOB) is one of the major bottlenecks for achieving mainstream nitrite shunt or partial nitritation/anammox (PN/A). It is increasingly experienced that NOB could develop resistance to suppressions over an extended time, leading to failure of nitrite shunt or PN/A. This study reports and demonstrates the first effective strategy to overcome NOB adaptation through alternating sludge treatment with free nitrous acid (FNA) and free ammonia (FA). During over 650 days of reactor operation, NOB adaptation to both FNA and FA was observed, but the adaptation was successfully overcome by deploying the alternate treatment strategy. Microbial community analysis showed Nitrospira and Nitrobacter, the key NOB populations in the reactor, have the ability to adapt to FNA and FA, respectively, but do not adapt to the alternation. Stable nitrite shunt with nitrite accumulation ratio over 95% and excellent nitrogen removal were maintained for the last 10 months with only one alternation applied. N₂O emission increased initially as the attainment of nitrite shunt but exhibited a declining trend during the study. By using on-site-produced nitrite and ammonium, the proposed strategy is feasible and sustainable. This study brings the mainstream nitrite shunt and PN/A one step closer to wide applications.

Factors affecting performance and functional stratification of membrane-aerated biofilms with a counter-diffusion configuration

Membrane-aerated biofilms (MABs) developed with a novel counter-diffusion configuration in oxygen and substrate supply were examined for the effect of biofilm thickness on the functional activity and microbial community structure of the biofilm with the simultaneous degradation of acetonitrile, and nitrification and denitrification. Results demonstrated that different biofilm thicknesses under different surface loading rates (SLRs) caused substantially varied profiles of the microbial activities with distinct functions in the biofilm. Both thick and thin MABs achieved high-rate performance in terms of acetonitrile removal (>99%), but the performance differed in the removal efficiencies of total nitrogen (TN), which was 1.3 times higher in the thick MAB (85%) than in the thin MAB (36.3%). The specific ammonia-oxidizing rate (SAOR) and the specific acetonitrile-degrading rate (SADR) exhibited similar declining and ascending trends in both the thin and thick MABs, respectively. In contrast, the specific denitrifying rate (SDNR) was relatively uniform at a concentration near the detection limit in the thin MAB but exhibited a hump-shaped variation with the highest rate occurring in an intermediate region in the thick MAB. Microbial community analysis revealed a dramatic shift in the dominant bacteria of the community composition with low diversity across the biofilm. This study suggests that the biofilm thickness developed under SLRs, which controls the mass transfer of oxygen and substrates into biofilms, is an important factor affecting the structural and functional stratification of bacterial populations in a single MAB treating organonitrile wastewater.

Biofilm thickness is a key factor affecting structural and functional stratification of community in counter-diffusion membrane-aerated biofilms (MABs) with the simultaneous degradation of acetonitrile, and nitrification and denitrification.

Effects of free nitrous acid treatment conditions on the nitrite pathway performance in mainstream wastewater treatment

Inline sludge treatment using free nitrous acid (FNA) was recently shown to be effective in establishing the nitrite pathway in a biological nitrogen removal system. However, the effects of FNA treatment conditions on the nitrite pathway performance remained to be investigated. In this study, three different FNA treatment frequencies (daily sludge treatment ratios of 0.22, 0.31 and 0.38, respectively), two FNA concentrations (1.35 mgN/L and 4.23 mgN/L, respectively) and two influent feeding regimes (one- and two-step feeding) were investigated in four laboratory-scale sequencing batch reactors. The nitrite accumulation ratio was positively correlated to the FNA treatment frequency. However, when a high treatment frequency was used e.g., daily sludge treatment ratio of 0.38, a significant reduction in ammonia oxidizing bacteria (AOB) activity occurred, leading to poor ammonium oxidation. AOB were able to acclimatise to FNA concentrations up to of 4.23 mgN/L, whereas nitrite oxidizing bacteria (NOB) were limited by an FNA concentration of 1.35 mgN/L over the duration of the study (up to 120 days). This difference in sensitivity to FNA could be used to further enhance nitrite accumulation, with 90% accumulation achieved at an FNA concentration of 4.23 mgN/L and a daily sludge treatment ratio of 0.31 in this study. However, this high level of nitrite accumulation led to increased N₂O emission, with emission factors of up to 3.9% observed. The N₂O emission was mitigated (reduced to 1.3%) by applying two-step feeding resulting in a nitrite accumulation ratio of 45.1%. Economic analysis showed that choosing the optimal FNA treatment conditions depends on a combination of the wastewater characteristics, the nitrogen discharge standards, and the operational costs. This study provides important information for the optimisation and practical application of FNA-based sludge treatment technology for achieving the mainstream stable nitrite pathway.