Microbial dynamics of biofilm and suspended flocs in anammox membrane bioreactor: The effect of non-woven fabric membrane

Study on the performance of a new type of combined packing biofilm reactor treating wastewater

The present work investigates the performance of a biofilm reactor filled with a new type of combined packing used to treat wastewater and explores a new technology approach for the application of coral sand and waste non-woven fabric. The combined packing was made of coral sand and waste non-woven fabric, which was used as a biofilm carrier to treat sewage. The experimental results showed that the removal efficiencies of COD, NH4+-N and TN in the biofilm reactor containing the combined packing were 92.9%, 72.9% and 63.2%, respectively. The maximum removal efficiencies of COD, NH4+-N and TN in the biofilm reactor containing single packing were 89.0%, 63.4% and 55.2%, respectively. The properties of the combined packing were characterized by Fourier Transform Infrared (FTIR), specific surface area, SEM and dehydrogenase activity. Infrared analysis showed that there were hydroxyl, carboxyl and carbonyl groups on the surface of coral sand and non-woven fabric which were beneficial for biofilm growth and wastewater treatment. The large pores in the interior of coral sand and non-woven fabric could provide a comfortable environment for microbes to grow and reproduce. The dehydrogenase activity of the biofilm on the surface of coral sand in the third biofilm reactor was 49.91 μgTF·g-1·h-1, which was significantly higher than that of the other two biofilm reactors. The new type of combined packing is suitable for biofilm carriers with low cost, which can be applied to actual sewage treatment projects. This study provides a reference for the practical application of the technique.

Degradation performance and potential protection mechanism of the anammox consortia in response to capecitabine

The potential risks of anti-cancer drugs such as capecitabine have attracted considerable attention due to their continuous release. Understanding the response of removal performance and protective mechanism to the presence of emerging contaminants is crucial for the application of anammox techniques in wastewater treatment. Capecitabine affected the nitrogen removal performance slightly in the activity experiment. Due to bio-adsorption and biodegradation, up to 64-70% of the capecitabine can be removed effectively. However, 10 mg/L of capecitabine significantly decreased the removal efficiency of capecitabine and total nitrogen at repeated load of capecitabine. Metabolomic analysis revealed the metabolites 5'-deoxy-5-fluorocytidine and alpha-fluoro-beta-alanine, while metagenomic analysis confirmed the biodegradation pathway and underlying gene distribution. The potentially protective mechanisms of the system against capecitabine were the increased heterotrophic bacteria and secretion of sialic acid. Blast analysis confirmed the presence of potential genes involved in the complete biosynthesis pathway of sialic acid in anammox bacteria, some of which are also found in Nitrosomonas, Thauera, and Candidatus Promineofilum.

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

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

Characterization of Enrichment Cultures of Anammox, Nitrifying and Denitrifying Bacteria Obtained from a Cold, Heavily Nitrogen-Polluted Aquifer

Anammox bacteria related to Candidatus Scalindua were recently discovered in a cold (7.5 °C) aquifer near sludge repositories containing solid wastes of uranium and processed polymetallic concentrate. Groundwater has a very high level of nitrate and ammonia pollution (up to 10 and 0.5 g/L, respectively) and a very low content of organic carbon (2.5 mg/L). To assess the potential for bioremediation of polluted groundwater in situ, enrichment cultures of anammox, nitrifying, and denitrifying bacteria were obtained and analyzed. Fed-batch enrichment of anammox bacteria was not successful. Stable removal of ammonium and nitrite (up to 100%) was achieved in a continuous-flow reactor packed with a nonwoven fabric at 15 °C, and enrichment in anammox bacteria was confirmed by FISH and qPCR assays. The relatively low total N removal efficiency (up to 55%) was due to nonstoichiometric nitrate buildup. This phenomenon can be explained by a shift in the metabolism of anammox bacteria towards the production of more nitrates and less N₂ at low temperatures compared to the canonical stoichiometry. In addition, the too high an estimate of specific anammox activity suggests that N cycle microbial groups other than anammox bacteria may have contributed significantly to N removal. Stable nitrite production was observed in the denitrifying enrichment culture, while no "conventional" nitrifiers were found in the corresponding enrichment cultures. Xanthomonadaceae was a common taxon for all microbial communities, indicating its exclusive role in this ecosystem. This study opens up new knowledge about the metabolic capabilities of N cycle bacteria and potential approaches for sustainable bioremediation of heavily N-polluted cold ecosystems.

Initiating an anaerobic ammonium oxidation reactor by inoculation with starved anaerobic ammonium oxidation sludge and modified carriers

Prolonged starved anammox sludge (SAS) obtained during initial rejuvenation was inoculated into a reactor together with activated sludge (AS), anaerobic granular sludge (AGS) and modified carriers consisting of honeycomb carrier with high biological interception and activated carbon carrier with high adsorption performance. SAS accounted for 5% of the inoculated sludge. The anammox process was started and operated at around 25℃. After 160 days, the nitrogen loading rate and nitrogen removal rate reached 1.12 kgN·m^-3·d^-1 and 0.97 kgN·m^-3·d^-1, respectively. Obvious red anammox biofilms were observed on the modified carriers. Microbial community analysis showed that the relative abundance of anammox bacteria increased from < 0.1% to 22.96%. Candidatus Jettenia and Candidatus Brocadia were the dominating anammox species. This work demonstrates the potential to reuse SAS to improve the start-up efficiency of anammox reactors, which makes good economic sense.

Activation of N-acyl-homoserine lactone-mediated quorum sensing system improves long-term preservation of anammox microorganisms by vacuum lyophilization

The long-term preservation of anaerobic ammonium oxidation (anammox) microorganisms via vacuum lyophilization process would help commercialize the technique. In this study, vacuum lyophilization was evaluated for the cost-effective long-term preservation of such microorganisms. Skim milk was found to be the most effective cryoprotectant for maintaining the physiological properties (heme c, EPS, and the PN/PS ratio) of anammox microorganisms. Conversely, the vacuum lyophilization technique was shown to cause serious damage to the quorum sensing (QS) system of anammox, so that anammox activity was not adequately recovered afterwards. To overcome this limitation, activation of the AHL-mediated QS system were applied to the vacuum lyophilization process. Endogenous (i.e., fresh anammox sludge of 10%) and exogenous (i.e., C6-HSL of 60 mg/L) QS autoinducers significantly increased anammox activity to 88.2 ± 12.2 and 130.0 ± 12.2 mgTN/gVSS/d, respectively, after 56 d of reactivation. In addition, nitrogen removal potentials were estimated to be 123.5 and 87.5 gTN/m³/d, respectively. The effect of the exogenous QS autoinducer on anammox reactivation was reconfirmed through the comparison experiment. The results of this study will be greatly significant to this field since they improve the feasibility of the once-underestimated vacuum lyophilization technique.

Challenges, solutions and prospects of mainstream anammox-based process for municipal wastewater treatment

Anaerobic ammonia oxidation (anammox) process has a promising application prospect for the mainstream deammonification of municipal wastewater due to its high efficiency and low energy consumption. In this paper, challenges and solutions of mainstream anammox-based process are summarized by analyzing the literature of recent ten years. Slow growth rate of anammox bacteria is a main challenge for mainstream anammox-based process, and enhancement of bacteria retention has been recognized to be necessary. Compared with directly increasing sludge retention time (SRT) with membrane bioreactors or sequencing batch reactors, culturing anammox bacteria in the form of biofilm or granule sludge is more promising for its feasibility of eliminating nitrite oxidizing bacteria (NOB). Besides, adding external electron donors or conductive materials and enriching the concentration of ammonia with absorption materials have also been proved helpful to improve the activity of anammox bacteria. Other challenges include the elimination of NOB and achieving ideal ratio of NH₄⁺ and NO₂^-. To solve these problems and achieve stable partial nitrification, composite control strategies based on low SRT and limited aeration are needed based on the special characteristics of ammonia oxidizing bacteria (AOB) and NOB. When treating actual wastewater, interference of low temperature and components in the influent is another problem. Relatively high activity of anammox bacteria has been realized after artificial acclimation at low temperature and the mechanism was also preliminary explored. Different pre-treatment sections have been designed to reduce the concentration of COD and S^2- from the influent. As for the nitrate produced by the anammox reaction, coupling processes are useful to reduce the concentration of nitrate in the effluent. In brief, suitable reactor and coupling process should be selected according to the temperature, influent quality and discharge targets of different regions. The future prospects of the mainstream anammox-based process are also put forward.

Treatment of wastewater with high carbon-to-nitrogen ratio using a waterfall aeration biofilm reactor combined with sequencing batch reactor: Microbial community structure and metabolism analysis

A low-cost and high-efficiency waterfall aeration biofilm reactor (WABR) combined with a sequencing batch reactor (SBR) was established to treat wastewater with a C/N ratio of 50. Three WABR-SBR systems with different fillers were used. In the stable operation phase, the removal efficiency of chemical oxygen demand was R1 (approximately 99%), R2 (97-99%), and R3 (96-99%); the effluent concentration of NH₄⁺-N was 0.5 mg/L without nitrite or nitrate accumulation. High-throughput 16S rRNA sequencing revealed that the dominant phyla in the microbial community structure were Proteobacteria, Bacteroidetes, and Planctomycetes. Quantitative PCR was used to quantify the nitrification and denitrification gene expressions (Nitrobacter, nirS, and nirK) to evaluate the simultaneous nitrification and denitrification processes. Both anammox and denitrifying bacteria were abundant. Metagenomic annotation of genes that revealed the metabolic pathways of carbohydrates, amino acids, and the two dominant enzymes (GH and GT) provide valuable information for microbial ecology analysis.

Advanced nitrogen removal from anaerobically pre-treated potato wastewater via partial nitritation-anammox in a continuous fed SBR

Partial nitritation-anammox was carried out successfully in a continuous fed Sequencing Batch Reactor (cf-SBR), composed of 3 compartments operated in continuous mode. The reactor was operated with floccular biomass (flocs) and biofilm to remove nitrogen from the anaerobic effluent from the potato industry at different nitrogen loading rates (0.16 g TN L^-1 d^-1 - 0.8 g TN L^-1 d^-1). At the maximum nitrogen loading rate (NLR) evaluated the nitrogen removal and ammonia oxidation achieved were 62% and 74% respectively. During the evaluation of the NLR, it was observed an improvement of the characteristics of the sludge, improving the Sludge Volumetric Index (SVI) from 228 to 63 mL g^-1 MLSS. Moreover, molecular analysis (qPCR) confirmed the presence of anammox bacteria on the flocs and in the biofilm from the cf-SBR. The results showed the capability of the reactor to carry out the partial nitritation-anammox in the same reactor at pilot scale. The cf-SBR was presented as a suitable and feasible technology for advanced nitrogen removal under partial nitritation and anammox conditions.

Microbial nitrogen removal in synthetic aquaculture wastewater by fixed-bed baffled reactors packed with different biofilm carrier materials

Fixed-bed baffled reactors packed with carbon fiber (CFBR), polyurethane, or non-woven fabrics were developed to support microbial nitrification-denitrification reactions for nitrogen removal from synthetic aquaculture wastewater. The CFBR showed the best performance, with a short hydraulic retention time and low C/N ratio. Microbial communities in the reactor's biofilms and deposited sludge were analyzed using high-throughput sequencing and quantitative polymerase chain reactions. The biofilms efficiently enriched the nitrifying and denitrifying bacteria in the CFBR. Moreover, bacteria capable of denitrification under aerobic conditions were detected in the aerobic chamber biofilm, showing positive correlations with the main nitrifiers and denitrifiers, which provides potential synergistic interactions for simultaneous nitrification-denitrification in the aerobic chamber. A network analysis revealed that the CFBR had more complex cooperative interactions than others. This study provides insights into the influence of different carrier materials on biofilm formation, proving that the CFBR has potential applications in aquaculture wastewater treatment.

Achieving single-stage partial nitritation and anammox (PN/A) using a submerged dynamic membrane sequencing batch reactor (DM-SBR)

Single-stage partial nitration and anammox (PN/A) process was achieved in a sequencing batch reactor (SBR) using a submerged dynamic membrane (DM) in this study. The reactor was stably operated for 200 days, and the nitrogen removal efficiency (NRE) was sustained at 70.3 ± 7.2% at a nitrogen loading rate (NLR) ranging from 0.1 to 0.3 kgNm^-3  day^-1 with a hydraulic retention time (HRT) of 24 hr. When the NLR was 0.2 kgN m^-3 day^-1 , the NRE achieved was high as 80% with a low concentration of dissolved oxygen (DO) of 0.13 mg/L. In addition, the specific activity of anammox bacteria and ammonia-oxidizing bacteria (AOB) reached was 2.72 and 16.80 gN gVSS^-1 day^-1 , respectively. The DM intercepted the biomass due to the lamellar, intact, dense biofilm self-generated on the surface of the supporting material, which had an effluent turbidity of 10 NTU. The enriched anammox functional bacteria were Candidatus Jettenia (11.06%) and the AOB-like functional bacteria consisted primarily of Nitrosomonas, with a relative abundance of 2.76%, which ensured the PN/A process proceeding. This study provides a novel reactor configuration of the single-stage PN/A process in the view of practical applications. PRACTITIONER POINTS: Single-stage partial nitration and anammox (PN/A) process was achieved using a submerged dynamic membrane (DM) in this study. The reactor was stably operated for 200 days, and the nitrogen removal efficiency was sustained at 70.3 ± 7.2%. The feasibility of the PN/A system with DM is evaluated. The main objective is to provide a control strategy of the DM-SBRs for practical applications.

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

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

Membrane processes

This literature review provides a review for publications in 2018 and 2019 and includes information membrane processes findings for municipal and industrial applications. This review is a subsection of the annual Water Environment Federation literature review for Treatment Systems section. The following topics are covered in this literature review: industrial wastewater and membrane. Bioreactor (MBR) configuration, membrane fouling, design, reuse, nutrient removal, operation, anaerobic membrane systems, microconstituents removal, membrane technology advances, and modeling. Other sub-sections of the Treatment Systems section that might relate to this literature review include the following: Biological Fixed-Film Systems, Activated Sludge, and Other Aerobic Suspended Culture Processes, Anaerobic Processes, and Water Reclamation and Reuse. This publication might also have related information on membrane processes: Industrial Wastes, Hazardous Wastes, and Fate and Effects of Pollutants.

Insight into the influence of microbial aggregate types on nitrogen removal performance and microbial community in the anammox process - A review and meta-analysis

The anaerobic ammonium oxidation (anammox) process has been paid close attention in the wastewater treatment field because of its energy-saving advantages. Different microbial aggregates have been used in the anammox process, and there is an urgent need to evaluate the comparative efficiencies of the widely used types of microbial aggregates with respect to their nitrogen removal performance as well as microbial community. To address this, 1724 published papers concentrating on three types of microbial aggregates, namely granules, biofilm, and flocs were compiled. A quantitative meta-analysis was carried out to compare the standard error of nitrogen removal efficiencies among these three microbial aggregates. The data sources of this meta-analysis comprised articles on granules (42%), followed by those on biofilm (33%) and flocs (25%). The granular sludge appeared to be competent in achieving the highest average nitrogen removal efficiencies of 81.1%, followed by biofilm (80.8%). Flocs provided comparatively poor removal of nitrogen pollutants with the lowest removal efficiency of 74.1%. Biofilm had the highest abundance of functional microbial communities with 43.4% on Candidatus Kuenenia and 11.2% on Candidatus Brocadia, which were detected in the anammox system as common genera. This meta-analysis suggested that the microbial aggregate types of granules and biofilm had a relatively low heterogeneity and high total nitrogen removal efficiencies for the anammox process and were the recommended microbial aggregates for anammox bacteria cultivation and operation of the anammox process.

A novel control strategy for the partial nitrification and anammox process (PN/A) of immobilized particles: Using salinity as a factor

The impact of the addition of salinity on partial nitrification and anammox (PN/A) was investigated in this study. The sludge was immobilized by polyethylene glycol (PEG)-modified polyvinyl alcohol (PVA)-sodium alginate (SA) immobilization technology, and the effective diffusion coefficient (D_e) of the immobilized particles was measured to be 0.313 × 10^-9 m²·s^-1, indicating that the system has excellent mass transfer performance. An experiment was carried out by adding NaCl to create a salinity gradient. It was found that the initiation of partial nitrification was achieved at a concentration of 10 g·L^-1 NaCl and the nitrite accumulation rate (NAR) reached 81.03%, which could provide sufficient NO₂^--N for subsequent anammox. Additionally, an anammox reactor operating at the same salinity maintained a stable state after acclimation, and the removal rates of NH₄⁺-N and NO₂^--N reached 80%. The dominant population in the anammox system was Planctomycetes. Salinity is a feasible factor for controlling the PN/A process.

Extractive membrane bioreactor (EMBR): Recent advances and applications

Combining bioreactor and membrane, known as a membrane bioreactor (MBR), has been considered as an attractive strategy to solve the limitations of conventional activated sludge process, such as biological instability, poor sludge quality, and low concentration of mixed liquor suspended solid. Unlike the other MBRs, extractive membrane bioreactor (EMBR) focuses on enhancing the efficiency of wastewater treatment through toxic compounds extraction by using a selective membrane. Even though EMBR has been successfully demonstrated in wastewater and waste gas treatment by several studies, it still faces some obstacles such as biofilm formation and low selectivity of the membrane towards a specific component. Appropriate biofilm formation control strategies and membrane with high selectivity are needed to solve those problems. This paper reviews EMBR including its potential applications in wastewater treatment, denitrification process, and waste gas treatment. In addition, challenges and outlook of EMBR are discussed.

Nitrogen removal from food waste digestate using partial nitritation-anammox process: Effect of different aeration strategies on performance and microbial community dynamics

The feasibility of employing anammox and partial nitritation-anammox (PN/A) processes for nitrogen removal from food waste (FW) digestate was investigated in this study. The effects of different aeration strategies on the microbial community were also investigated. To achieve this, after anammox enrichment (Phase 1), the reactor was fed with digestate supplemented with nitrite (Phase 2), and subsequently different aeration strategies were evaluated to establish PN/A. Aeration strategies with high anoxic periods (30 and 45 min) in relation to aerobic periods (15 min) coupled with low air flow rates (0.026 L  min^-1. L_reator^-1) were found to be better for establishing PN/A, as coefficients of produced nitrate/removed ammonium were closer to those reported previously (0.17 and 0.21). Aeration conditions considerably altered the microbial community. Candidatus Brocadia was replaced by Candidatus Jettenia, after the first aeration strategies. These results support the feasibility of FW digestate treatment using anammox and PN/A processes and provide a better understanding of the effect of aeration on microbial dynamics in PN/A reactors.

Highly heterogeneous interior structure of biofilm wastewater for enhanced pollutant removals

Biofilm processes are widely used in wastewater treatment. The biofilm has highly heterogeneous interior structure, which can significantly affect the transport processes and the biological reactions over the biofilm. This study for the first time detailed the complicated velocity and concentration fields of substrate in a real biofilm structure. With a real biofilm interior being profiled and meshed to numerical solutions, the flow-through mode has significant distortion of inflow velocity fields and concentration distributions, which lead to enhanced biological reactions at regimes nearby major pores. Conversely, the crossflow mode depends weakly on the biofilm interior structure. The uniform biofilm model fails to describe the real biofilm processes. Future research needs based on real biofilm structures were discussed.

Keystone taxa regulate microbial assemblage patterns and functional traits of different microbial aggregates in simultaneous anammox and denitrification (SAD) systems

Understanding the impacts of the ecological patterns and niche-based selection on microbial community assembly and nitrogen-cycling network is crucial for achieving energy-neutral wastewater treatment. However, little is known about the niche differentiation and microbial nitrogen-cycling traits of keystone taxa in flocs and granules in anammox-based systems. Herein, the aspects of community assemblage patterns, metabolic functions and nitrogen transformation pathways were explored. The findings discovered that the treatment performance and bacterial community assembly were regulated by core taxa and flocs and granules communities harbored core taxa based on their functional traits. Both niche differentiation and environmental filtering have profound influences on functional bacteria. Furthermore, a combined analysis showed that nitrogen removal in flocs and granules was regulated by different nitrogen transformation pathways. These results suggest that core taxa are the key drivers for the microbial nitrogen-cycling network and improve the understanding of cross-feeding and metabolic pathways between anammox and nitrogen-cycling-related microorganisms.

NOB suppression in partial nitritation-anammox (PNA) process by discharging aged flocs: Performance and microbial community dynamics

The partial nitritation-anammox (PNA) process is the most promising technique to treat municipal sewage; however, nitrite oxidizing bacteria (NOB) are a hindrance to achieve PNA. This study investigated the effects of selectively discharging flocs (<200 μm) to washout NOB in a sequencing batch reactor (SBR) over 200 d. The experiment was divided into three phases with different floc sludge retention times (SRTs; 30, 20 and 30 d). When the SRT of the flocs was reduced from 30 to 20 d to washout NOB, a significant reduction of ammonia oxidizing bacteria (AOB) and anaerobic ammonium-oxidizing (anammox) bacteria in the flocs was found. This indicates that a low floc SRT (20 d) leads to the loss of AOB and anammox bacteria in the flocs (<200 μm) and destroys PNA. Activity tests and qPCR analysis revealed the variations of functional bacteria in the granules and flocs, indicating that the enrichment of AOB, NOB, anammox bacteria in the granules is caused by the long-term discharging of flocs. High-throughput sequencing analysis revealed that the microbial shift of Tetrasphaera was significant in the flocs and may be connected to the enrichment of anammox bacteria and the stability of the PNA requires further research. All the obtained NOB sequences were affiliated with the genera Nitrospira and could further influence the PNA system. Overall, this study provides an in-depth understanding of the impact of discharging flocs to washout NOB and promotes the application of combing granules/floc PNA in sewage treatment.

Nitric oxide removal from flue gas with ammonium using AnammoxDeNOx process and its application in municipal sewage treatment

A novel AnammoxDeNOx process was designed to simultaneously remove NOx in flue gas and ammonium wastewater, with the aim of exploring the possibility of using NO as a long-term and stable electron acceptor for anammox bacteria. The performance of the AnammoxDeNOx process indicated a NOx removal efficiency from simulated flue gas (including CO₂, SO₂, O₂ and NO₂) of 87-96% using simulated ammonium wastewater. With municipal wastewater, the removal efficiencies for NOx were 70-90%, total nitrogen 40-70%, and COD 80-90% (NO concentration: 100-500 ppm). The anammox genus underwent considerable changes from the dominant Candidatus Kuenenia in the stage of domestication to the predominant Candidatus Brocadia, which then became the dominant species in the simulated flue gas and actual municipal wastewater stages.