Start-up and long-term performance of anammox moving bed biofilm reactor seeded with granular biomass

Rapid start-up strategy and microbial population evolution of anaerobic ammonia oxidation biofilm process for low-strength wastewater treatment

The implementation of the anaerobic ammonium oxidation (anammox) process in treating low-strength wastewater is limited by the difficulty in enriching anammox bacteria (AnAOB). Here, the first enrichment of AnAOB at a high nitrogen (N) loading rate (NLR) as a strategy was proposed to achieve the rapid start-up of the anammox biofilm process treating low-strength wastewater. The long-term stability of the anammox biofilm process after start-up operating at a low NLR of 0.2-0.4 kg N/(m3⋅d) was evaluated. Results showed that the N removal efficiency was up to 75 % under a low NLR of 0.2 kg N/(m3⋅d) condition. Low-strength organic matter promoted the metabolic coupling between partial denitrifying bacteria (PDB) and AnAOB. The genus Candidatus Brocadia as AnAOB (18 %-27 %) can coexist with Limnobacter (PDB, 9 %-12 %) for efficient N removal. This study offers a rapid start-up strategy of anammox biofilm process in treating low-strength wastewater.

Iron-modified carriers accelerate biofilm formation and resist anammox bacteria loss in biofilm reactors for partial denitrification-anammox

The slow formation of anammox biofilms presents a bottleneck for resolving anammox bacterial loss and achieving stable performance in biofilm-based partial denitrification-anammox (PD-A) processes. This study utilized iron-modified (K1/Fe3O4 NPs) carriers, which were prepared and used for the first time in PD-A processes. Parallel moving bed biofilm reactors (MBBRs) indicated that iron-modified carriers facilitated the formation of biofilms at a faster rate than K1 carriers, consequently improving the nitrogen removal performance of the process by over 40 %. 16S rDNA analysis showed that anammox bacteria were approximately four times more abundant in the iron-modified carrier biofilm than in the K1 carrier biofilm. XPS and zeta potential analysis suggested that the improved microbial affinity of the iron-modified carrier surface caused this. As a result, the iron-modified carriers facilitated the formation of anammox biofilms and enhanced PD-A performance.

Enhancing start-up strategies for anammox granular sludge systems: A review

The anaerobic ammonium oxidation (anammox) process has been developed as one of the optimal alternatives to the conventional biological nitrogen removal process because of its high nitrogen removal capacity and low energy consumption. However, the slow growth rate of anammox bacteria and its high sensitivity to environmental changes have resulted in fewer anammox sludge sources for process start-up and a lengthy start-up period. Given that anammox microorganisms tend to aggregate, granular-anammox sludge is a frequent byproduct of the anammox process. In this study, we review state-of-the-art strategies for promoting the formation of anammox granules and the start-up of the anammox process based on the literature of the past decade. These strategies are categorized as the transformation of alternative sludge, the addition of accelerators, the introduction of functional carriers, and the implementation of other physical methods. In addition, the formation mechanism of anammox granules, the operational performance of various strategies, and their promotion mechanisms are introduced. Finally, prospects are presented to indicate the gaps in contemporary research and the potential future research directions. This review functions as a summary guideline and theoretical reference for the cultivation of granular-anammox sludge, the start-up of the anammox process, and its practical application.

Rapid start-up and long-term stable operation of the anammox reactor based on biofilm process: Status, challenges, and perspectives

Anammox-biofilm processes have great potential for wastewater nitrogen removal, as it overcomes the slow growth and easy loss of AnAOB (anaerobic ammonium oxidation bacteria). Biofilm carrier is the core part of the Anammox-biofilm reactor and plays a key role in the start-up and long-term operation of the process. Therefore, the research on the biofilm carrier of Anammox-based process was summarized and discussed in terms of configurations and types. In the Anammox-biofilm process, fixed bed biofilm reactor is a relatively mature biofilm carrier configuration and has advantages in terms of nitrogen removal and long-term operational stability, while moving bed biofilm reactor has advantages in terms of start-up time. Although the long-term operational stability of fluidized bed biofilm reactor is good, its nitrogen removal performance needs to be improved. Among the different biofilm carrier categories, the inorganic biofilm carrier has an advantage in start-up time, due to the enhancement of the growth and metabolic of AnAOB by inorganic materials (such as carbon and iron). Anammox-based reactors using organic biofilm carriers, especially suspension carriers, are well-established and more stable in long-term operation. Composite biofilm carriers combine the advantages of several materials, but their complex preparation procedures lead to high costs. In addition, possible research directions for accelerating the start-up and keeping the long-term stable operation of Anammox reactor by biofilm process were highlighted. It is hoped to provide a possible pathway for the rapid start-up of Anammox-based process, and references for the optimization and promotion of process.

A strategy for fast anammox biofilm formation under mainstream conditions

One of the bottlenecks to applying anaerobic ammonium oxidation (Anammox) is the long start-up time, especially under mainstream conditions. This study proposed a strategy for fast anammox biofilm formation under mainstream conditions. By first cultivating an aerobic heterotrophic biofilm, and then transferring to anoxic conditions, a pre-cultivated heterotrophic biofilm can be formed in 12 days. The pre-cultivated heterotrophic biofilm then functions as a "glue" to accelerate anammox bacteria adhesion and biofilm formation. Secondary settled effluent with externally added 15-30 mg-N·L^-1 ammonium and nitrite was applied as reactor influent. With a single inoculation of suspended growth anammox-laden biomass and no bioaugmentation, an anammox-enriched biofilm formed after 5 months of operation under uncontrolled temperature of 15-20 °C. Both the nitrogen removal rate and specific anammox activity exponentially increased over the course of study, corresponding to an estimated anammox doubling time of 10.8 days. The biofilm thickness on primed carriers was 2-3 times higher than on the non-primed carriers over the first 5 months of operation, and the hszA gene copy number in primed biofilms revealed was consistently 1 to 2 times higher than the non-primed carrier biofilm, indicating that biofil m carrier priming via selection for a pre-cultivated heterotrophic biofilm base can effectively improve the anammox enrichment rate at early stages of reactor operation. Time, rather than the type of biofilm (primed versus non-primed), had a stronger influence on microbial community structure over the full 230 days of reactor operation. Candidatus Brocadia was the only detected anammox bacteria genus. Overall, pre-cultivation of heterotrophs on biofilm carriers provides a simple route to accelerate anammox-enriched biofilm formation under mainstream conditions.

Rapid Start-Up Characteristics of Anammox under Different Inoculation Conditions

The long multiplication time and extremely demanding enrichment environment requirements of Anammox bacteria (AAOB) have led to difficult reactor start-ups and hindered its practical dissemination. Few feasibility studies have been reported on the recovery of AAOB activity initiation after inlet substrate disconnection caused by an unfavorable condition, and few factors, such as indicators of the recovery process, have been explored. Therefore, in this experiment, two modified expanded granular sludge bed reactors (EGSB) were inoculated with 1.5 L anaerobic granular sludge (AGS) + 1 L Anammox sludge (AMS) (R1) and 2.5 L anaerobic granular sludge (AGS) (R2), respectively. After a long-term (140 days) starvation shock at a high temperature (38 °C), the bacteria population activity recovery experiments were conducted. After 160 days, both reactors were successfully started up, and the total nitrogen removal rates exceeded 87%. Due to the experimental period, the total nitrogen removal rate of R2 was slightly higher than that of R1 in the final stage. However, it is undeniable that R2 had a relatively long activity delay during startup, while R1 had no significant activity delay during startup. The sludge obtained from R1 had a higher specific anammox activity (SAA). Analysis of the extracellular polymer substances (EPS) results showed that the extracellular polymer content in R1 was higher than that in R2 throughout the recovery process, indicating that R1 had higher sludge stability and denitrification performance. Scanning electron microscopy (SEM) analysis showed that more extracellular filamentous bacteria could be seen in the R1 reactor with better morphology of Anammox bacteria. In contrast, the R2 reactor had fewer extracellular hyphae and micropores as a percentage and higher filamentous bacteria content. The results of microbial 16SrDNA analysis showed that R1 used AAOB as inoculum to initiate Anammox, and the reactor was enriched with Anammox bacteria earlier and in much greater abundance than R2. The experimental results indicated that inoculating mixed anaerobic granular sludge and Anammox sludge to initiate an anammox reactor was more effective.

Improving the biomass retention and system stability of the anammox EGSB reactor by adding a calcium silicate hydrate functional material

Improving the biomass retention and the sludge system stability to promote the full-scale application of anammox process is the focus of current related research. In this study, a calcium silicate hydrate functional material with calcium-releasing ability and weak alkalinity was used for an enhanced anammox process. In the long-term operation, an increase in the nitrogen removal rate (NRR) from 2.75 to 13.38 gN/L/d was achieved after 50 days of operation, with the abundance of Candidatus Kuenenia increased from 40.1 % to 47.0 %. The anammox activity was strengthened from 0.089 to 0.55 gN/gVSS/d over 50 days, with a growth rate being fitted at 0.0310 d^-1. The resilience of the EGSB anammox system after inhibitions was investigated by substrate shock and low pH shock in long-term operation and batch test. Besides that, the phosphorus removal efficiency of the reactor reached up to 90 % under the positive effect of functional material. The functional material was shown to continuously provide calcium in the long-term for the reaction of hydroxyapatite (HAP) formation, which further improved the granular properties of the sludge and the biomass retention ability of the reactor. The promotion effect of functional material on the sludge granulation and anammox microbes retaining efficiency was the key for a high-resilience anammox EGSB reactor.

N2O production and emission pathways in anammox biofilter for treating wastewater with low nitrogen concentrations

Although anaerobic ammonia oxidation (anammox) is a cost-effective nitrogen removal process, nitrous oxide (N₂O) production will greatly reduce the advantages of this process. It is important to identify the N₂O emission pathways and then reduce the N₂O production in anammox system. To date, very limited research has been done to investigate the N₂O production and N₂O emission pathways in anammox biofilter. In this study, N₂O production were investigated under different filtration rates in anammox biofilter for treating wastewater with low nitrogen concentrations, and N₂O emission pathways were analyzed with batch tests using N₂O microsensor and stable isotope mass spectrometry. The results showed N₂O production increased with the increase of filtration rates in anammox biofilter, where the N₂O emission factor increased from 0.012 % at 1.0 m/h to 0.496 % at 3.0 m/h. And the optimal operation condition was at filtration rate of 1.5 m/h, where NH₄⁺-N and NO₂^--N removal efficiencies reached 99 % and N₂O concentration was the lowest. qPCR showed that anammox bacteria, nitrifying bacteria and denitrifying bacteria were all present in anammox biofilter, with anammox bacteria in the highest abundance. And nitrifying bacteria and denitrifying bacteria provided the possibility of N₂O production. The batch tests and stable isotope mass spectrometry analysis indicated that nitrifier denitrification, hydroxylamine oxidation and endogenous heterotrophic denitrification were N₂O production pathways in aerobic zone and anoxic zone of anammox biofilter, respectively. In addition, batch tests under different conditions showed no oxygen environment could reduce N₂O production. Therefore, the production of N₂O in anammox system is a problem that cannot be ignored and should be paid more attention to.

Nitrogen removal performance and rapid start-up of anammox process in an electrolytic sequencing batch reactor (ESBR)

In this study, the electrolytic sequencing batch reactor (ESBR) with different current densities was constructed to investigate the nitrogen removal performance and rapid start-up of anaerobic ammonia oxidation (anammox) process. The changes of total nitrogen removal rate (TNRR), specific anammox activity (SAA) and nitrogen concentration under different current densities were analyzed, and then the effect of the optimal current density on the start-up of anammox in ESBR was explored. The results showed that ammonium nitrogen removal efficiency (92.7%), nitrite nitrogen removal efficiency (15.5%) and total nitrogen removal efficiency (28.1%) were obtained with the TNRR and SAA were 0.0118 g N L^-1 d^-1 and 0.0050 g N (g Vss d)^-1, respectively under the optimal conditions (i.e., current density = 0.10 mA cm^-2, temperature = 36 °C and pH = 7.6). In addition, the stoichiometric ratio indicated that anammox was initiated successfully for 91 days in ESBR with the current density of 0.10 mA cm^-2, which was shortened by 10 days compared with the conventional SBR without current density. These results suggest that an array of rapid start-up processes of anammox can be developed through applying current density to stimulate the activity of anammox bacteria (AnAOB).

Long-Term Performance of Nitrogen Removal and Microbial Analysis in an Anammox MBBR Reactor with Internal Circulation to Provide Low Concentration DO

The anammox process is considered as a revolutionary new denitrification technology. In this study, the anammox process was started in a single-stage moving bed biofilm reactor (MBBR) and the mechanism of excess removal of ammonia nitrogen was studied. At stage I (day 0-51), anammox bacteria (AnAOB) was enriched by feeding synthetic sewage without adding organic carbon. The removal rate of ammonia nitrogen was maintained at about 54% and the removal rate of total inorganic nitrogen was maintained at about 62%. At stage II (day 52-91), internal circulation was added into the MBBR. After adding internal circulation, the ammonium removal efficiency reached about 96% (at day 56) and the total nitrogen removal efficiency reached about 86%. At day 90, the biofilm sample was drowned out for high-throughput sequencing. The results showed that the relative abundance of AnAOB was 23.23%. The dominant anammox genus was Candidatus Brocadia. The relative abundance of Nitrosomonas (ammonia oxidizing bacteria, AOB) was 0.63%. The excess ammonia nitrogen was removed by AOB and AnAOB through the partial nitrification and anammox (PNA) process.

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.

A review of biomass immobilization in anammox and partial nitrification/anammox systems: Advances, issues, and future perspectives

Two biomass immobilization techniques; entrapment and carrier-based, attract increasing attention in anammox and partial nitrification/anammox (PN/A) systems. This paper provides a comprehensive review of the advances, outstanding issues, and future research directions in this field. The application of both entrapment and carrier-based biofilm immobilization for reactor start up, improving the nitrogen removal performance, and protecting autotrophic bacteria from environmental fluctuations in anammox and partial nitrification/anammox systems are summarized and discussed. The key characteristics of carriers for biomass immobilization are biocompatibility for supporting microbial growth, permeability for effective mass transfer, and physical/chemical stability for long-term use. Carriers without these characteristics must be improved and re-evaluated for their feasibility in applications. Lab-scale, pilot, and full-scale studies are needed to overcome the potential obstacles of preliminary studies, and to investigate the long-term performance of biomass immobilization techniques, especially using real wastewater as influent, which may introduce more complexity and threaten the carrier's immobilization. In addition, calculating the 'nitrogen removal rate normalized by the packing ratio of carriers (NRR-C)' in the immobilization system is strongly suggested to obtain a direct comparison of immobilization performance/limitations from different studies. This review will improve understanding of the major challenges of immobilization technology in anammox and PN/A systems and provide insights into the next-stage of research and full-scale applications.

Media selection for anammox-based polishing filters: Balancing anammox enrichment and retention with filtration function

Retrofitting conventional denitrification filters into partial denitrification-anammox (PdNA)- or anammox (AnAOB)-based filters will reduce the needs for external carbon addition. The success of AnAOB-based filters depends on anammox growth and retention within such filters. Studies have overlooked the importance of media selection and its impact on AnAOB capacity, head loss progression dynamics, and shear conditions applied onto the AnAOB biofilm. The objective of this study was to evaluate viable media types (10 types) that can enhance AnAOB rates for efficient nitrogen removal in filters. Given the higher backwash requirement and lower AnAOB capacity of the conventionally used sand, expanded clay (3-5 mm) was recommended for AnAOB-based filters in this study. Owing to its surface characteristics, expanded clay had higher AnAOB activity (304- vs. 104-g NH₄ ⁺ -N/m² /day) and higher AnAOB retention (43% more) than sand. Increasing the iron content of expanded clay to 37% resulted in an increase in zeta potential, which led to 56% more anammox capacity compared to expanded clay with 7% iron content. This work provides insight into the importance of media types in the growth and retention of AnAOB in filters, and this knowledge could be used as basis in the development of PdNA filters. PRACTITIONER POINTS: Expanded clay showed the lowest head loss buildup and most likely will result in longer runtime for full-scale PdNA applications The highest AnAOB rates were achieved in expanded clay types and sand compared with smaller media typically used in biofiltration Expanded clay resulted in better AnAOB retention under shear, whereas sand could not withstand shear and required more frequent backwashing Expanded clay iron coating enhanced AnAOB enrichment and retention, most likely due to increased surface roughness and/or positive charge.

Carrier surface modification for enhanced attachment and growth of anammox biofilm

This study investigates and compares the ammonia removal kinetics, attachment, biofilm development and anammox bacteria enrichment on various surface modified carriers throughout the 163 days of start-up of an MBBR system: virgin, dextran-functionalized carriers, silica-functionalized and pre-seeded denitrifying carriers. Silica-functionalized carriers along with pre-seeded denitrifying carriers induced significant higher kinetics, faster biofilm growth and greater anammox bacteria enrichment during the 64 days of operation compared to non-modified virgin and dextran-functionalized carriers. The elevated anammox bacteria counts along with the elevated kinetics of all carriers measured at day 106 indicated that the completed biofilm growth and biofilm maturation are achieved prior to or at day 106 of start-up. The NH₄⁺-N removal rate for virgin, dextran-functionalized, silica-functionalized and pre-seeded denitrifying carriers were achieved 0.684 ± 0.019, 0.608 ± 0.016, 0.634 ± 0.017 and 0.665 ± 0.018 g NH₄⁺-N/m²/d, respectively, at day 106. The results demonstrate that the silica-functionalized and pre-seeded denitrifying carriers offer advantages during the early stage of start-up while the dextran-functionalized carriers did not reduce the start-up period for anammox biofilm.

Nitrate modulation of Bacillus sp. biofilm components: a proposed model for sustainable bioremediation

The presence of different pollutants in wastewater hinder microbial growth, compromise enzymatic activity or compete for electrons required for bioremediation pathway. Therefore, there is a need to use a single microorganism that is capable of tolerating different toxic compounds and can perform simultaneous bioremediation. In the present study, nitrate reducing bacteria capable of decolorizing azo dye was identified as Bacillus subtillis sp. DN using protein profiling, morphological and biochemical tests X-ray diffraction pattern, Raman spectroscopy and cyclic voltammetry confirm that the bacterium under study possesses membrane-bound nitrate reductase and that is capable of direct electron transfer. The addition of nitrate concentrations (0-50 mM) resulted in increased biofilm formation with variable exopolysaccharides, protein, and eDNA. Fourier Transform Infrared spectrum revealed the presence of a biopolymer at high nitrate concentrations. Effective capacitance and conductivity of the cells grown in different nitrate concentrations suggest changes in the relative position of polar groups, their relative orientation and permeability of cell membrane as detected by dielectric spectroscopy. The increase in biofilm shifted the removal of the azo dye from biodegradation to bioadsorption. Our results indicate that nitrate modulates biofilm components. Bacillus sp. DN granular biofilm can be used for simultaneous nitrate and azo dye removal from wastewater.

Fast start-up and enhancement of partial nitritation and anammox process for treating synthetic wastewater in a sequencing bath biofilm reactor: Strategy and function of nitric oxide

In this study, the partial nitritation and anammox (PN-A) process was initiated within 30 days in a sequencing batch biofilm reactor (SBBR) by employing pre non-aeration and post non-aeration with fixed aeration rates. The average ammonia removal efficiency (ARE), total nitrogen removal efficiency (TNRE) of 98.5 ± 1.5% and 89.5 ± 1.6% were achieved. By doubling aeration rate and agitation rate and adopting pre non-aeration, the TNRR was promoted from 0.135 ± 0.013 kg N·m^-3·d^-1 to 0.285 ± 0.015 kg N·m^-3·d^-1, obtaining an average ARE and TNRE of 97.5 ± 1.5% and 85.5 ± 2.6%. Nitric oxide might induce anaerobic ammonia oxidation bacteria (AnAOB) during the start-up stage, and could be an indicator for synergetic state between ammonia oxidation bacteria (AOB) and AnAOB. Lower nitrous oxide emission factor of 0.51% was obtained. The abundance of AOB, AnAOB and nitrite oxidation bacteria (NOB) accounted for 1.6%, 19.3% and 0.3%, respectively.

Critical Analysis of Biomass Retention Strategies in Mainstream and Sidestream ANAMMOX-Mediated Nitrogen Removal Systems

ANAMMOX (anaerobic ammonium oxidation) represents an energy-efficient process for biological nitrogen removal, particularly from wastewater streams with low chemical oxygen demand (COD) to nitrogen (C/N) ratios. Its widespread application, however, is still hampered by a lack of access to biomass-enriched with ANAMMOX bacteria (AMX), slow growth rates of AMX, and their sensitivity to inhibition. Although the coupling of ANAMMOX processes with partial nitrification is already widespread, especially for sidestream treatment, maintaining a functional population density of AMX remains a challenge in these systems. Therefore, strategies that maximize retention of AMX-rich biomass are essential to promote process stability. This paper reviews existing methods of biomass retention in ANAMMOX-mediated systems, focusing on (i) granulation; (ii) biofilm formation on carrier materials; (iii) gel entrapment; and (iv) membrane technology in mainstream and sidestream systems. In addition, the microbial ecology of different ANAMMOX-mediated systems is reviewed.

Effects of Fe3+ on microbial communities shifts, functional genes expression and nitrogen transformation during the start-up of Anammox process

In this study, the effect of Fe³⁺ on the start-up of Anammox process was investigated. Four EGSB reactors were operated with the addition of 0 (R1), 0.04 (R2), 0.08 (R3) and 0.14 (R4) mmol/L Fe³⁺, respectively. The results showed that Fe³⁺ remarkably improved the nitrogen loading rate (NLR) and operation efficiency of the reactor. After 180 days, the influent NH₄⁺-N concentration in the four reactors was 201.4, 301.8, 343.2, 380.2 mg N/L, and the NLR was 589.3, 877.6, 993.0, 1105.8 mg N/(L·d), respectively. And the nitrogen removal rate (NRR) in R2, R3 and R4 was respectively 1.54, 1.73 and 1.94 times of that in R1. High throughput sequencing revealed that Fe³⁺ could promote the enrichment of Anammox bacteria Candidatus Brocadia. Moreover, the analysis by qPCR indicated that the abundance of Anammox 16S rRNA gene and the functional gene hzsB increased, which showed a positive correlation with the concentration of Fe³⁺.

Anammox attachment and biofilm development on surface-modified carriers with planktonic- and biofilm-based inoculation

This study investigates the kinetics, attachment, biofilm development and anammox bacteria enrichment of a novel detached anammox biofilm inoculation method on non-modified virgin MBBR carriers and pre-seeded denitrifying carriers. The study compares these results to the more common use of attached anammox carriers for anammox MBBR inoculation. The anammox bacteria specific attachment-growth rates for virgin carriers inoculated with detached anammox biofilm mass were 38.1% greater for the first 25 days, leading to approximately 30% less time required to achieve complete biofilm coverage than those measured in attached biofilm carrier inoculated systems during the attachment and early biofilm growth stages. The biofilm thickness increase rate was also 52.3% higher for virgin carriers with detached biofilm inoculum. Further, inoculation using pre-seeded denitrifying carriers compared to virgin carriers demonstrated a 13.8% preferential increase in anammox bacteria specific attachment-growth rate and a corresponding 47.2% higher NH₄⁺-N removal rate at the time of biofilm maturation.

Characteristics of rapid-biofiltering anammox reactor (RBAR) for low nitrogen wastewater treatment

This research provides an important approach for rapid treatment of low nitrogen wastewater through anaerobic ammonium oxidation (anammox), which was realized in a rapid-biofiltering anammox reactor (RBAR). The operation mode of continuous upward flow and gradually shortened hydraulic retention time (HRT) accumulated anammox bacteria effectively in RBAR, where carmine anammox granular sludge and thick biofilm were co-existed, leading the biomass concentration and the specific anammox activity to reach 21.61 gSS/L and 0.82 gN/gVSS·d in the main functional zone. Moreover, the relative abundance of anammox bacteria in the whole reactor was more than 50%, and the relative abundance of Candidatus Brocadia in the biofilm of 20-47 cm zone reached 71.10%. Results showed that the removal rate and effluent concentration of total nitrogen remained stable at 86.24% and 14.20 mg/L (below 15 mg/L) averagely, under HRT of 32 min when the the nitrogen loading rate was 4.86 kgN/m³·d.

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.

Efficient transition from partial nitritation to partial nitritation/Anammox in a membrane bioreactor with activated sludge as the sole seed source

A lab-scale membrane bioreactor (MBR) was employed to carry out the partial nitritation/Anammox (PN/A) process from conventional activated sludge. Seed sludge was cultivated under microaerobic conditions for 10 days before seeding into the MBR. The bacterial community was analyzed on the basis of cloning and sequencing of 16S rRNA gene. Relative slow ammonia oxidation rates (3.2-13.0 mgN/L/d) were established in the microaerobic cultivation period. In the continuous MBR operation, the nitritation was achieved in the first 16 days and the reactor produced a balanced ratio between ammonia and nitrite which favored the proliferation of Anammox bacteria. Efficient transition from PN to PN/A was achieved in two months which was supported by appearance of reddish spots on the reactor inner wall and the concurrent consumption of ammonium and nitrite. The PN/A performed a robust and high-rate nitrogen removal capability and achieved a peak nitrogen removal of 1.81 kg N/m³/d. 16S rRNA gene-based analysis indicated that "Nitrosomonas sp." and "Candidatus Jettenia sp." accounted for ammonia oxidation and nitrogen depletion, respectively. Denitratisoma facilitated denitrification in the reactor. The present study suggested that a pre-cultivation of seed sludge under microaerobic conditions assists fast realization of PN and further convoyed efficient transition from PN to PN/A. Knowledge gleaned from this study is of significance to initiation, operation, and control of MBR-PN/As.

Characterizing the biofilm stoichiometry and kinetics on the media in situ based on pulse-flow respirometer coupling with a new breathing reactor

Biofilm based systems and the hybrid between activated sludge and biofilms have been popularly applied for wastewater treatment. Unlike the suspended biomass, the biofilm concentration and kinetics on the media cannot be easily measured. In this study, a novel and easy-to-use approach has been developed based on pulse-flow respirometer to characterize the biofilm stoichiometry and kinetics in situ. With the new designed breathing reactor, the mutual interference between the magnetic stirring and biofilm media that happened in the conventional breathing reactor was solved. Moreover, Microsoft Excel based programs had been developed to fit the oxygen uptake rate curves with dynamic nonlinear regression. With this new approach, the yield coefficient, maximum oxidation capacity, and half-saturation constant of substrate for the heterotrophic biofilms in a fix bed reactor were determined to be 0.46 g-VSS/g-COD, 67.0 mg-COD/(h·L-media), and 4.4 mg-COD/L, respectively. Those parameters for biofilm ammonia oxidizers from a moving bed biofilm reactor were determined to be 0.17 g-VSS/g-N, 18.6 mg-N/(h·L-media), and 1.2 mg-N/L, respectively, and they were 0.11 g-VSS/g-N, 20.9 mg-N/(h·L-media), and 0.98 mg-N/L for nitrite oxidizers in the same biofilms. This study also found that the maximum specific substrate utilization rate for detached biofilms increased by 3.2 times, indicating that maintaining biofilm integrity was very important in the kinetic tests. Using this approach, the biofilm kinetics on the media can be regularly measured for treatment optimization.

Anammox bacteria enrichment and denitrification in moving bed biofilm reactors packed with different buoyant carriers: Performances and mechanisms

Anaerobic ammonium oxidation (anammox) is recognized as the most cost-effective process for nitrogen removal from wastewater. In this study, effects of polyethylene plastics, nonwoven fabric, granular activated carbon (GAC) and polyurethane sponge as buoyant carriers were evaluated in lab-scale moving bed biofilm reactors (MBBRs). The overall performance of MBBRs with four types of carriers from priority to inferiority was noticed as, GAC, nonwoven fabrics, polyurethane sponge and polyethylene plastics under the same packing ratio of 20 v% and an average carrier size of 4 × 4 × 4 mm. The hydrophobic surface of GAC could selectively adsorb hydrophobic protein and favor anammox bacteria attachment, which contributed to achieving a total nitrogen removal rate of 0.40 kg-N/(m³·d) in 60 days. In conclusion, our results provide compelling evidence for achieving effective anammox process in an MBBR with GAC carriers and would benefit towards accomplishing a stable partial nitritation-anammox process in the future.

Start-up of a full-scale partial nitritation-anammox MBBR without inoculum at Klagshamn WWTP

Partial nitritation and anaerobic ammonium oxidation (PNA) is a useful process for the treatment of nitrogen-rich centrate from the dewatering of anaerobically digested sludge. A one-stage PNA moving bed biofilm reactor (MBBR) was started up without inoculum at Klagshamn wastewater treatment plant, southern Sweden. The reactor was designed to treat up to 200 kgN d^-1, and heated dilution water was used during start-up. The nitrogen removal was >80% after 111 days of operation, and the nitrogen removal rate reached 1.8 gN m^-2 d¹ at 35 °C. The start-up period of the reactor was comparable to that of inoculated full-scale systems. The operating conditions of the system were found to be important, and online control of the free ammonia concentration played a crucial role. Ex situ batch activity tests were performed to evaluate process performance.

Achieving stable two-stage mainstream partial-nitrification/anammox (PN/A) operation via intermittent aeration

The mainstream anammox process has attracted extensive attention recently. Compared to single-stage partial-nitrification/anammox (PN/A) system, two-stage PN/A process was more advantageous for achieving mainstream anammox. However, complex control strategy in partial-nitrification reactor (N-SBR) might not be feasible in practical application. The aim of this study was to provide an easy operation strategy to achieve two-stage PN/A process. Firstly, intermittent aeration was investigated to achieve 100% conversion of ammonium to nitrite in N-SBR. The effluent nitrite concentrations increased from 19.96 to 38.62 mg/L when intermittent aeration ratio (IAC) varied from 30 min/30 min-30 min/15 min. During 125 d's operation of N-SBR, stable partial nitrification performance was obtained through intermittent aeration, without coupling with low dissolve oxygen or short sludge retention time. Then, raw municipal wastewater was directly mixed with N-SBR effluent to provide suitable feed to anaerobic sequencing batch reactor (A-SBR).When the mixture ratio between the raw wastewater and the N-SBR effluent was 2.5, the effluent ammonium and total inorganic nitrogen (TIN) was only 0.97 and 2.52 mg N/L, respectively. Additionally, carbon-based pollutants was also removed in the proposed system without any pretreatment, which made the process easier to operate in practice.

Enhanced selective enrichment of partial nitritation and anammox bacteria in a novel two-stage continuous flow system using flat-type poly (vinylalcohol) cryogel films

To improve stability of nitrogen removal in partial nitritation (PN)-anammox process, flat-type cryogel films using poly (vinylalcohol) named as FT-CPVAF were applied in continuous reactors. Stable PN operation was maintained with short acclimation of 8 days and ammonium oxidation rate of 1.68 ± 0.12 kg N m^-3 d^-1 comparatively higher than previous studies. The nitrogen removal, initially inhibited by an oxygen shock, was immediately reactivated with short lag-period by immobilization of anammox bacteria in FT-CPVAF. A novel two-stage PN-anammox process was operated in a continuous flow using FT-CPVAF for treatment of ammonium-rich synthetic wastewater (influent 315 mg NH₄⁺-N L^-1) showing 89.6 ± 0.76% of nitrogen removal at short hydraulic retention time (7.7 h). The use of FT-CPVAF enhanced selective enrichment of AOB and anammox bacter ia confirmed by high-throughput sequencing of i.e., relative abundances of Nitrosomonas europaea C-31 (37.14% in PN reactor) and 'Candidatus Jettenia caeni' (34.36% in anammox reactor).

Recent development of advanced biotechnology for wastewater treatment

The importance of highly efficient wastewater treatment is evident from aggravated water crises. With the development of green technology, wastewater treatment is required in an eco-friendly manner. Biotechnology is a promising solution to address this problem, including treatment and monitoring processes. The main directions and differences in biotreatment process are related to the surrounding environmental conditions, biological processes, and the type of microorganisms. It is significant to find suitable biotreatment methods to meet the specific requirements for practical situations. In this review, we first provide a comprehensive overview of optimized biotreatment processes for treating wastewater during different conditions. Both the advantages and disadvantages of these biotechnologies are discussed at length, along with their application scope. Then, we elaborated on recent developments of advanced biosensors (i.e. optical, electrochemical, and other biosensors) for monitoring processes. Finally, we discuss the limitations and perspectives of biological methods and biosensors applied in wastewater treatment. Overall, this review aims to project a rapid developmental path showing a broad vision of recent biotechnologies, applications, challenges, and opportunities for scholars in biotechnological fields for "green" wastewater treatment.

Metagenomic insights into functional traits variation and coupling effects on the anammox community during reactor start-up

Anammox technology is an energy-efficient wastewater treatment process and anammox community structure has gained extensive attention. However, the dynamics of community functional traits are still elusive. Here, we combined the long-term reactor operation and metagenomic, multiple bioinformatic and network analyses to reveal the succession of anammox community and function traits during reactor start-up. We found the cooperation of denitrifiers that affiliated to the phylum Proteobacteria could reduce nitrite to dinitrogen gas. These organisms and genes had higher abundance after the inhibition phase, which could contribute to nitrite consuming and reactor performance recovery. Importantly, the Terrimonas and Anaerolinea organisms had ability of extracellular polymers secretion or aggregate formation. They had the highest abundance at the end of the lag phase, which could benefit for promoting the nitrogen removal rate (NRR). Meanwhile, Terrimonas and Anaerolinea bacteria could cooperate with methanogenic and nitrite-denitrifying methanotrophic organisms based on H₂ and CH₄, respectively. Since these organisms also had higher abundance after the inhibition phase, their cooperation could prevent anammox bacteria from nitrite inhibiting when the influent nitrite concentration was higher. The analysis of community and function shift is expected to emphasize the importance of functional bacteria in anammox process and provides a potential control strategy for nitrogen-containing wastewater treatment process.

Start-up and operational performance of Anammox process in an anaerobic baffled biofilm reactor (ABBR) at a moderate temperature

A lab-scale anaerobic baffled biofilm reactor (ABBR) was used as a novel reactor to start up Anammox process at a moderate temperature around 20 °C and an innovative filling module was adopted as support material. Quick start-up of Anammox process from the aerobic activated sludge was achieved after 47 days operation. The max nitrogen loading rate and nitrogen removing rate attained 1.00 kg N m^-3 d^-1 and 0.90 kg N m^-3 d^-1 after 161 days operation. Scanning electron microscope photographs showed that the structure as well as the states of the micro-aggregates (micro-aggregates sticking on a non-woven fiber, entangling non-woven fibers and enwrapped by non-woven fibers) enhanced biomass retention for Anammox bacteria. Microbial community analysis showed that Anammox bacteria were effectively enriched with Candidatus Brocadia, Candidatus Jettenia and Candidatus Kuenenia being the main Anammox species in the mature biofilms. This contributed to the excellent Anammox operation performance at the moderate temperature.

Start-up evaluations and biocarriers transfer from a trickling filter to a moving bed bioreactor for synthetic mariculture wastewater treatment

Mariculture wastewater treatment by nitrification requires a long start-up time due to high salinity stress. This study aimed to verify the faster start-up of a trickling filter (TF) compared to a moving bed bioreactor (MBBR) treating synthetic mariculture wastewater, and to investigate the feasibility of transferring mature biocarriers from the TF to a new MBBR (TF-MBBR). The nitrogen removal performance, biofilm physicochemical properties and microbial communities were investigated. The results obtained showed that, the TF started up 41 days faster than the MBBR, despite the richer microbial diversity in the latter. Lower biofilm roughness and protein content as well as higher adhesive force and polysaccharide content in the TF were obtained compared to the MBBR. Adhesive force was found to be negatively correlated with roughness (r = -0.630, p = 0.069). Transmittance assigned to amide II (1538 cm^-1) and amid III (1243 cm^-1) through Fourier transform infrared spectroscopy (FTIR) determination was only obtained in the TF, which was likely related to the faster start-up. Nitrosomonas and Nitrospira were detected as the predominant nitrifiers in both reactors. In addition, the new MBBR, incubated with the mature biocarriers transferred from the TF, had a satisfactory nitrification performance with no lag time. Interestingly, the transfer action increased the microbial diversity and made the biofilm physicochemical characteristics shift toward those of the MBBR. Taken together, the study confirmed that MBBR nitrification start-up can be accelerated via TF and biocarrier transfer.

Quick start-up and stable operation of a one-stage deammonification reactor with a low quantity of AOB and ANAMMOX biomass

In this study, a quick start-up of one-stage deammonification in an immobilized aerobic ammonium oxidizing bacteria (AOB) and anoxic ammonium oxidizing (ANAMMOX) bacteria up-flow reactor (IAAR) was successfully achieved. With the aid of gel layers, AOB and ANAMMOX bacteria had excellent spatial distribution, theoretically meeting dissolved oxygen requirements for the simultaneous processes of aerobic and anaerobic ammonium oxidizing. The results indicated that an IAAR containing 0.4 g-VSS L^-1 immobilized biomass achieved a nitrogen removal rate (NRR) of 0.53 kg-N m^-3 d^-1 after only 10 days of operation and subsequently reached a maximum nitrogen removal rate (NRR_max) of 3.73 kg-N m^-3 d^-1. The micro-profiles of DO and pH were measured using microelectrodes to help understand the stratification of the microbial processes inside the gel layers. The distribution of AOB and ANAMMOX bacteria within the gel layers was verified using fluorescence in situ hybridization (FISH) analysis. The community distribution in the FISH three-dimensional images closely corresponded to the micro-profiles of DO concentration and pH, enabling rapid adaptation and stable operation of the reactor seeded with a quite low quantity of biomass.

Anammox granule as new inoculum for start-up of anaerobic sulfide oxidation (ASO) process and its reverse start-up

The feasibility of implementing anaerobic ammonium oxidation (anammox) granules to start up high-loading anaerobic sulfide oxidation (ASO) in an upflow anaerobic sludge bed (UASB) reactor was investigated. An innovation method of the reverse start-up of anammox was also validated. Firstly, the reactor was operated to treat sulfide-rich wastewaters into which nitrite was introduced as an electron acceptor. An high-rate performance with sulfide and nitrate removal rates of 105.5 ± 0.11 kg S m^-3 d^-1 and 28.45 ± 3.40 kg N m^-3 d^-1, respectively, was accomplished. Sulfurovum were enriched with the increase of the substrate load and then conquered Candidatus Kuenenia to be the predominant bacteria. Excitation-emission matrix (EEM) spectroscopy showed that the intensities of fluorescence decreased and protein-like substrates were the main components associated with the process of start-up. FT-IR analysis found that the main functional groups indicator were O-H groups. Secondly, the reverse start-up of anammox (achieving 90% TN removal) was achieved immediately when the substrate changed. 16S rRNA analysis indicated the successfully enrichment of anammox bacteria (Candidatus Kuenenia). These results suggest that anammox granules can act as inoculum of high-loading ASO process and the reverse start-up provides a new perspective for the fast initiation of anammox process.

Evaluation and optimization of anammox baffled reactor (AnBR) by artificial neural network modeling and economic analysis

Anammox baffled reactor (AnBR) had a moderate start-up period of 53 days. Interestingly, tangled relationships between key parameters affecting anammox performance were observed, i.e., polynomial function for nitrogen loading rate (NLR) with extracellular polymeric substances (EPS), linear relationships between EPS with granules diameter, granules diameter with settling velocity, and settling velocity with biomass concentration. The correlation coefficients (R2) were 0.97, 0.84, 0.86, and 0.88, respectively. Furthermore, a multi-layered feed forward artificial neural network (ANN) was utilized for simulating and predicting the performance of AnBR. An ANN structure of two hidden layers with four neurons at 1st layer and eight neurons at 2nd layer achieved the best goodness of fit with the minimum mean squared error (MSE) and maximum R² of 0.002 and 0.99, respectively. Additionally, economic assessment stated that using AnBR at NLR of 4.04 ± 0.10 kg-N/m³/day achieved the maximum net present value of $48100.9.