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

Substrate utilization and secondary metabolite biosynthesis in the phylum Planctomycetota

The phylum Planctomycetota is changing our understanding of bacterial metabolism, driving critical biogeochemical processes through the transformation of complex polymeric substrates into valuable bioactive compounds. Sophisticated methods for cultivation, genome sequencing and genetic strain engineering developed in the last two decades have stimulated detailed studies on cell propagation, metabolic capabilities and potential applications of phylum members beyond the mere isolation and characterization of novel taxa. This review synthesizes recent advances in understanding the Planctomycetota physiology with a focus on the degradation of phototroph-derived polysaccharides, anaerobic ammonium oxidation (anammox) and biosynthesis of secondary metabolites. New data especially collected over the last 5 years justifies more intensive research of the yet uncharacterized pathways of substrate uptake and utilization, as well as genome mining-assisted bioprospection to exploit the phylum's chemical repertoire. KEY POINTS: • Planctomycetes can degrade high-molecular-weight sugars produced by algae • Anaerobic ammonium oxidation (anammox) is used in technical applications • The first secondary metabolites were discovered in the last 5 years.

Efficacy of nitrate and biochar@birnessite composite microspheres for simultaneous suppression of As(III) mobilization and greenhouse gas emissions in flooded paddy soils

Elevated As(III) pollution and greenhouse gas (GHG) emissions are two primary environmental concerns associated with flooded paddy soils. Herein, a novel biochar@birnessite composite microsphere was engineered using a biochar, birnessite and sodium alginate formulation. The microspheres were applied along with nitrate to examine their efficacy in suppressing As(III) mobilization and GHG emissions in an As-contaminated flooded paddy soil. After a 10-day incubation period, the combined nitrate + microsphere treatment achieved desirable remediation effects versus a nitrate-alone treatment, with mobile As(III) (initially 0.1 mM in flooded layer) completely immobilized and N2O, CH4 and CO2 emissions declining by 89 %, 73 % and 31 %, respectively. As(III) immobilization was ascribed to oxidation/adsorption/coprecipitation by FeOx/MnOx regenerated from successive cycles of Feammox/Mnammox and nitrate-reduction coupled with Fe(II) oxidation (NRFO)/nitrate-reduction coupled with Mn(II) oxidation (NRMO). Moreover, NRFO/NRMO-derived full denitrification displayed high thermodynamic feasibility, leading to full denitrification with the generation of N2 rather than N2O. The co-occurrence of anaerobic oxidation of methane (AOM) driven by biochar-shuttling and coupled reduction of nitrate/FeOx/MnOx fostered anaerobic oxidation of CH4 to CO2. A portion of the resulting CO2 was incorporated into poorly-soluble carbonate minerals leading to lower CO2 emission and soil carbon sequestration. Metagenomic sequencing revealed that the nitrate + microsphere treatment enriched the abundances of key microorganisms linked to As/Fe/Mn oxidation and GHG mitigation (e.g., Geobacter, Streptomyces, Cupriavidus and Chloroflexus). Our findings document the efficacy of nitrate + biochar@birnessite microsphere treatment as an effective remediation strategy to simultaneously mitigate As(III) pollution and GHG emissions in flooded paddy soils.

Roles of waste iron scraps in anammox system treating sulfide-containing wastewater: Alleviating sulfide inhibition, promoting novel anammox bacteria enrichment, and enhancing nitrogen removal capacity

In this study, waste iron scraps (WIS) were exerted to alleviate sulfide inhibition on anammox bacteria and promote anammox nitrogen removal from sulfide-containing wastewater.Short-term batch experiments showed that WIS-addition led to the anammox bacteria activity increasing by 124.8 % at an initial sulfide concentration of 40 mgS/L. During the long-term experiments, the nitrogen removal rate (NRR) reached to 8.76 kg/(m3·d) in the WIS-added reactor, while the maximum NRR was only 3.77 ± 0.31 kg/(m3·d) in the non-WIS reactor. In contrast to anammox bacteria development in the non-WIS reactor, the relative abundance of Candidatus Kuenenia (1.4-3.7 %) declined significantly in the WIS-added reactor, but novel potential anammox bacteria Brocadiaceae_unclassified (60.1 %-78.6 %) were highly enriched. Overall, the experimental evidence suggested that WIS-addition not only mitigated the sulfide inhibition on anammox bacteria, but also promoted novel anammox bacteria proliferation. The findings of this work provide a promising solution for wide engineering applications of anammox treating sulfide-containing wastewater.

Temperature effects on nitrogen removal and N2O emissions in anammox reactors

Mainstream anammox faces challenges in adapting to non-optimal temperatures and managing greenhouse gas emissions. This study investigates nitrogen removal and N2O emissions in attached-growth anammox reactors subjected to rapid temperature shifts (15-55 °C). Temperature reductions to 15-25 °C had minimal impact on the anammox bacterial populations, with nitrogen removal rates of 0.37±0.11 gN/(L⋅d) and 0.88±0.10 gN/(L⋅d) at 15 °C and 25 °C, respectively. In contrast, increasing temperatures to 45-55 °C significantly diminished both anammox biomass and bioactivity. The reactor at 35 °C exhibited the lowest N2O emissions (< 1.0 mgN/(L⋅d)), while emissions rose to approximately 5.0 mgN/(L⋅d) at 15 °C and 3.4 mgN/(L⋅d) at 55 °C (during 295-395 d), primarily due to denitrification performed by coexisting ammonia-oxidizing bacteria and denitrifying microbes. This study provides insights into temperature adaptability and N2O emission risks, supporting mainstream anammox applications.

A chemically enhanced primary treatment and anammox-based process for sustainable municipal wastewater treatment: The advantage and application prospects

Low-carbon nitrogen removal, bioenergy production, and phosphorus recovery are key goals for sustainable municipal wastewater treatment. Traditional activated sludge processes face an energy demand conflict. Anaerobic ammonium oxidation (Anammox) offers a solution to this issue, with the A-B process providing a sustainable approach. Stable and cost-effective nitrite supply for mainstream anammox has gained attention, while the interactions between A-B stage processes are also crucial. This paper reviews the benefits and challenges of mainstream anammox, bioenergy, and phosphorus recovery. A combined process of chemically enhanced primary treatment, partial denitrification and anammox is identified as effective for sustainable treatment. Additionally, the stable nitrite supply from the sidestream partial nitrification provides a 54% nitrogen removal contribution to the mainstream anammox. Anaerobic digestion with sulfate reduction is proposed as an efficient method for simultaneous bioenergy and phosphorus recovery from iron-enhanced primary sludge. The recycling of iron and sulfate reduces excess sludge and cuts costs. A novel wastewater treatment scheme, supported by a mass balance analysis, is presented; the proposed process is capable of recovering >50% of the carbon and phosphorus, while reduced 40% dosing of Fe and S chemicals, reducing the cost of chemical dosing and treatment of the digestate while meeting the high-quality effluent. The paper also explores the potential for transitioning from conventional activated sludge processes and suggests directions for future research.

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.

Revealing microbial compatibility of partial nitritation/Anammox biofilm from sidestream to mainstream applications: Origins, dynamics, and interrelationships

Biofilms offer a solution to the challenge of low biomass retention faced in mainstream partial nitritation/Anammox (PN/A) applications. In this study, a one-stage PN/A reactor derived from initial granular sludge was successfully transformed into a biofilm system using shedding carriers. Environmental stressors, such as ammonium nitrogen concentration and organic matter, significantly affected the competitive dynamics and dominant species composition between Ca. Kuenenia and Ca. Brocadia. Under approximately 500 mg/L NH4+-N, Ca. Brocadia emerged as the dominant anammox bacteria species, but was subsequently replaced by Ca. Kuenenia in the presence of approximately 54 mg COD/L CH3COONa. Moreover, Chloroflexi species on the original biofilm exhibited an associated relationship with the growth of Ca. Kuenenia in new biofilm. The biofilm assembly and microbial community migration uniquely reveal the microbial niche dynamics. This study provides valuable insights for PN/A biofilm applications facing diverse challenges of environmental stresses in the transition from sidestream to mainstream.

Toward a circular nitrogen bioeconomy: integrating nitrogen bioconcentration, separations, and high-value products for nitrogen recovery

Recovering nitrogen (N) from wastewater is a potential avenue to reduce reliance on energy-intensive synthetic nitrogen fixation via Haber-Bosch and subsequent treatment of N-laden wastewaters through nitrification-denitrification. However, many technical and economic factors hinder widespread application of N recovery, particularly low N concentrations in municipal wastewater, paucity of high-efficiency separations technologies compatible with biological treatment, and suitable products and markets for recovered N. In this perspective, we contextualize the challenges of N recovery today, propose integrated biological and physicochemical technologies to improve selective and tunable N recovery, and propose an expanded product portfolio for recovered N products beyond fertilizers. We highlight cyanophycin, an N-rich biopolymer produced by a diverse range of bacteria, as a potential target for N bioconcentration and downstream recovery from municipal wastewater. This perspective emphasizes the equal importance of integrated biological systems, physicochemical separations, and market assessment in advancing nitrogen recovery from wastewater.

Effective organic matter removal via bio-adsorption prior to anammox process and utilization of carbon-rich sludge

Excessive organic matter in the anaerobic ammonia oxidation (Anammox) leads to the growth of a large number of heterotrophic bacteria, which disrupts the anaerobic ammonia oxidation. The adsorption-anaerobic ammonia oxidation process can effectively reduce excessive organic matter, capturing it instead of consuming it, which is a sustainable development technology. In this study, utilizing the excellent adsorption performance of aerobic granular sludge (AGS), an adsorption-regeneration process was employed to remove organic matter at the front end of the Anammox process through bio-adsorption in an artificial simulated domestic sewage environment, and it was successfully used for denitrification. Stirring rate is a key factor affecting sludge granulation. As a parallel experiment of sludge granulation, two Sequencing Batch Reactors (SBRs) (R1 and R2) were operated simultaneously at different stirring rates. After 153 days, the particle size of the two reactors was analyzed, revealing that the proportion of particles larger than 200 μm was over 50%, and granular sludge was successfully formed in both reactors. Long-term operational results indicate that at a temperature of 16.5 ± 1 °C, varying initial pH levels (6.5, 6.7, 7.2, and 8.5) significantly affect the removal efficiency of chemical oxygen demand (COD). COD is rapidly adsorbed and removed within a short period. Among the tested initial pH values, a pH of 6.7 yielded the best total chemical oxygen demand (tCOD) removal efficiency, achieving up to 95%. Additionally, the study examined the effects of different carbon sources on denitrification, revealing that under carbon-rich conditions, the denitrification rate was highest, reaching 1.44 mg N/(g VSS·h). Compared to endogenous denitrification, the denitrification rate increased by 40%, and the nitrate (NO₃⁻-N) removal efficiency reached 100%.

Comprehensive performance of a new-type hybrid membrane bioreactor applied to mainstream anammox process

The new-type submerged granular sludge membrane bioreactor (S-GSMBR) was constructed by installing a membrane module inside an upflow anaerobic sludge blanket. S-GSMBR achieved the fast start-up (47 d) and long-term stable operation (133 d) of mainstream Anammox process as well as the effective control of membrane fouling. The maximum nitrogen removal rate and efficiency were 0.52 kg N m-3 d-1 and 88.32%. The flow field analysis indicated that the turbulent effects were found around the membrane module and benefited for alleviating the membrane fouling at the special axial flow velocity at the central section of the membrane module (4.89 × 10-7 m s-1 to 1.30 × 10-6 m s-1). On day 133, S-GSMBR achieved sludge granulation with 68.63% of particles larger than 200 μm. Meanwhile, Candidatus Brocadia and Candidatus Kuenenia were typical Anammox bacteria in mature Anammox granular sludge. The relative abundance of Candidatus Brocadia and Candidatus Kuenenia was 21.22% and 0.36%, indicating that Candidatus Brocadia occupied the main niche under mainstream Anammox conditions. The feasibility of application of S-GSMBR in mainstream Anammox process was confirmed and moreover S-GSMBR was proven to be a robust mainstream Anammox reactor. This study lays the important theoretical foundation and technical support for the engineering application of the mainstream Anammox process.

Full-scale simultaneous partial nitrification, anammox, and denitrification for the efficient treatment of carbon and nitrogen in low-C/N wastewater

A full-scale simultaneous partial nitrification, anaerobic ammonia oxidation (anammox), and denitrification (SNAD) reactor was initiated to address the problem of high energy consumption for the treatment of low C/N wastewater. The SNAD system achieved a nitrogen removal rate of 0.9 kg/(m3·d) at an influent NH₄+-N concentration of 500 mg/L after 450 days of stable operation. Partial nitrification was achieved by maintaining free ammonia levels at 0.8 ± 0.3 mg/L and dissolved oxygen concentrations between 0.3 mg/L and 1.2 mg/L, which resulted in synergistic nitrogen removal, with anammox contributing 61 % and denitrification contributing 39 %. Microbiological analyses indicated that the dominant microorganisms were Candidatus Brocadia, Thauera, Denitratisoma, and Nitrosomonas. In conclusion, study provides a solid foundation for the broader implementation of the SNAD process in wastewater treatment systems.

Advanced N removal from low C/N sewage via a plug-flow anaerobic/oxic/anoxic (AOA) process: Intensification through partial nitrification, endogenous denitrification, partial denitrification, and anammox (PNEnD/A)

Achieving low-cost advanced nitrogen (N) removal from municipal wastewater treatment plants (WWTPs) remains a challenge. A plug-flow anaerobic/oxic/anoxic (AOA) system with a mixtures bypass (MBP) integrating partial nitrification (PN), endogenous carbon denitrification (EnD), partial denitrification (PD), and anaerobic ammonium oxidation (Anammox), was constructed to treat actual sewage with a low C/N ratio. The effluent concentrations and removal efficiency of total inorganic nitrogen (TIN) during stable operation were 2.9 ± 0.9 mg/L and 93.1 ± 2.0 %, respectively. EnD was enhanced by the MBP through the efficient utilization of polyhydroxyalkanoates generated in the anaerobic zone. PD was promoted by the addition of carries and sodium acetate to the anoxic tank and the subsequent implantation of the Anammox biofilm successfully coupled PD/A. Stable PN was obtained with a satisfactory nitrite accumulation ratio of 92.6 %, facilitated by carriers and the introduction of hydroxylamine in the oxic zone. Mass balance analysis revealed that EnD and Anammox contributed 40.8 % and 48.2 % of TIN removal, respectively. The enrichment and synergistic effects of ammonia-oxidizing bacteria, denitrifying bacteria, glycogen-accumulating organisms, and anaerobic ammonia-oxidizing bacteria formed a diverses bacterial basis for the establishment of PN, EnD, PD, and Anammox (PNEnD/A) in the AOA system. The successful integration of PNEnD/A into the AOA process provides an innovative approach for low-cost advanced N removal in WWTPs.

Pure water and resource recovery from municipal wastewater using high-rate activated sludge, reverse osmosis, and mainstream anammox: A pilot scale study

In response to the escalating global water scarcity and the high energy consumption associated with traditional wastewater treatment plants, there is a growing demand for transformative wastewater treatment processes that promise greater efficiency and sustainability. This study presents an innovative approach for municipal wastewater treatment that integrates high-rate activated sludge with membrane bio-reactor (HRAS-MBR), reverse osmosis (RO) and partial nitrification-anammox (PN/A). With an influent of 8.4 m³/d, the HRAS-MBR demonstrated a removal efficiency of approximately 85 % for chemical oxygen demand (COD), with over 70 % of it being recovered for energy production. The RO system achieved a recovery rate of 75 % for the influent, producing pure water with an electrical conductivity of 50 μS/cm. Concurrently, it concentrated ammonia, thereby enhancing the effectiveness of the PN/A process for nitrogen removal in the mainstream, resulting in a removal efficiency exceeding 85 %. Notably, the HRAS-MBR achieved significant phosphorus removal without chemical additives, attributed to the presence of influent calcium and magnesium ions. Overall, this integrated system reduced the net energy consumption for reclaimed water production by about 26 % compared to conventional methods. Additionally, the new process produced a revenue of 0.75 CNY/m³, demonstrating considerable economic and environmental benefits. This pilot-scale study offers a viable alternative for wastewater treatment and water reuse in water-scarce regions, contributing to sustainable water resource management.

Nitrate formation in anammox process: Mechanisms and operating conditions

Anaerobic ammonium oxidation (anammox) is an energy-efficient technology for wastewater nitrogen removal. However, the byproduct nitrate has hindered development and application of anammox process. Meanwhile, the knowledge of nitrate formation during anammox process is insufficient, which prohibits high nitrogen removal. This review firstly summaries and discusses valuable findings on nitrate formation, including molecular mechanism of nitrate production, microbial pathway of nitrate reduction and its net formation. Specially, influences of operating conditions on mechanisms and patterns of nitrate formation are analyzed. Then, based on nitrate formation mechanism, current strategies of nitrate removal from anammox process are reevaluated. Finally, the key knowledge gaps and further process development are presented. Overall, this review sheds light on the understanding of nitrate formation of anammox process, which would further facilitate and optimize the process design and operation for high performance nitrogen removal.

Sulfide promotes nitrogen removal in anammox low-strength ammonium wastewater treatment system

Anammox has been widely used for denitrification from different wastewaters due to its low energy and carbon sources consumption. Nevertheless, the presence of nitrate in the effluent has been found to impede the enhancement of total nitrogen removal efficiency (TNRE). In this study, anammox was employed in conjunction with sulfur autotrophic denitrification (SAD) in order to enhance TNRE. During a long-term test in an UASB reactor with 30 mg L-1 ammonium influent concentration, it was observed that sulfide facilitated both nitrate removal efficiency (NRE) and TNRE. Specifically, compared with 0 mg L-1 sulfide addition, the NRE and TNRE were enhanced from 92.55% and 74.56% to 94.10% and 89.51%, respectively with 10 mg L-1 sulfide. However, with sulfide concentration increased to 20 mg L-1, 81.13% of TNRE was only found. Notably, the anammox performance was observed to enhancement when the sulfide level was reduced to 0 mg L-1 again, the result indicated that the inhibitory effect of 20 mg L-1 sulfides can be reversed. Further exploration revealed that sulfide not only suppressed the activity of nitrifying bacteria but also acted as electron donor, aiding the conversion of NO3--N to N2 through SAD process, this made the nitrate in effluence as low as 2.31 mg L-1. The results of microbiological analysis demonstrated that the population of microorganisms associated with SAD increased in response to the addition of sulfide. The findings suggested a potential equilibrium and collaboration between SAD and anammox to further increase TNRE in low-strength ammonium wastewater treatment.

Microbial coadaptation drives the dynamic stability of microecology in mainstream and sidestream anammox systems under exposure of progesterone

Microbial cooperation determines the efficacy of wastewater biological treatment, and the adaptability of microorganisms to environmental stresses varies. Recently, extensive use of hormones results in their inevitable discharge into aquatic environment. Therefore, mainstream and sidestream anammox reactors were constructed in this study to evaluate their removal performance of progesterone and nitrogen simultaneously, the adaptability of anammox consortia to progesterone stress and the corresponding regulation mechanism. Both anammox processes had the resilience to progesterone stress, with the average nitrogen removal efficiency exceeding 90 %. At the same time, progesterone removal efficiency also exceeded 70 %. In contrast, microbial community in the mainstream reactors was more susceptible to progesterone interference. The adaptation of anammox consortia mainly depended on microbial cooperation and molecular regulation. Initially, bacteria secreted more extracellular polymeric substances to detain progesterone. Biodegradation also contributed to mitigating the side effect of progesterone, which was demonstrated by the proliferation of potential degrading bacteria such as Bacillus salacetis, Bacillus wiedmannii and Rhodococcus erythropolis. In addition, the enhancement of microbial interaction intensity drove their cooperation to enhance adaptability and maintain stable performance. Combined with metagenomic and metatranscriptomic analyses, such microbial adaptability was enhanced through molecular regulations, including the energy redistribution for amino acid synthesis and alteration of key metabolic pathways. Related functional gene expressions and microbial interactions were, in turn, regulated by quorum sensing. This work verifies the feasibility of anammox process in hormone-containing wastewater treatment and provides a holistic understanding of molecular mechanism of microbial interaction and coadaptation to stress.

Fimbriimonadales performed dissimilatory nitrate reduction to ammonium (DNRA) in an anammox reactor

Bacteria belonging to the order Fimbriimonadales are frequently detected in anammox reactors. However, the principal functions of these bacteria and their potential contribution to nitrogen removal remain unclear. In this study, we aimed to systematically validate the roles of Fimbriimonadales in an anammox reactor fed with synthetic wastewater. High-throughput 16S rRNA gene sequencing analysis revealed that heterotrophic denitrifying bacteria (HDB) were the most abundant bacterial group at the initial stage of reactor operation and the abundance of Fimbriimonadales members gradually increased to reach 38.8 % (day 196). At the end of reactor operation, Fimbriimonadales decreased to 0.9 % with an increase in anammox bacteria. Correlation analysis demonstrated nitrate competition between Fimbriimonadales and HDB during reactor operation. Based on the phylogenetic analysis, the Fimbriimonadales sequences acquired from the reactor were clustered into three distinct groups, which included the sequences obtained from other anammox reactors. Metagenome-assembled genome analysis of Fimbriimonadales allowed the identification of the genes narGHI and nrfAH, responsible for dissimilatory nitrate reduction to ammonium (DNRA), and nrt and nasA, responsible for nitrate and nitrite transport. In a simulation based on mass balance equations and quantified bacterial groups, the total nitrogen concentrations in the effluent were best predicted when Fimbriimonadales was assumed to perform DNRA (R2 = 0.70 and RMSE = 18.9). Moreover, mass balance analysis demonstrated the potential contribution of DNRA in enriching anammox bacteria and promoting nitrogen removal. These results prove that Fimbriimonadales compete with HDB for nitrate utilization through DNRA in the anammox reactor under non-exogenous carbon supply conditions. Overall, our findings suggest that the DNRA pathway in Fimbriimonadales could enhance anammox enrichment and nitrogen removal by providing substrates (nitrite and/or ammonium) for anammox bacteria.

Gel beads prepared by polyvinyl alcohol cross-linking with phytic acid and Fe as novel microbial carriers for simultaneous partial nitrification, anammox and denitrification

Enhancing the stability of biomass and ensuring a stable activity of anaerobic ammonia oxidizing bacteria are crucial for successful operation of the simultaneous partial nitrification, Anammox, and denitrification (SNAD) process. In this study, gel beads of polyvinyl alcohol/phytic acid (PVA/PA) and polyvinyl alcohol/phytic acid/Fe (PVA/PA/Fe) were prepared as innovative bio-carriers. Theoretical simulations and analyses revealed that these carriers are predominantly connected via hydrogen and borate bonds, with PVA/PA/Fe also featuring metal coordination bonds. The total nitrogen removal efficiency of reactors with PVA/PA/Fe and PVA/PA increased by 13.5 % and 9.0 %, respectively, compared to reactor without carriers. The iron-enriched PVA/PA/Fe carriers significantly improve SNAD by promoting Anammox, Feammox, and nitrate-dependent Fe2+ oxidation reactions, leading to faster nitrogen conversion and higher nitrogen removal rate than reactor without carriers and with PVA/PA. Using of PVA/PA and PVA/PA/Fe gel beads as bio-carriers offers benefits to the SNAD process, including cost-effective and low carbon requirement.

Response and self-regulation of PD/A granular sludge to oxytetracycline stress

This paper investigated the operational characteristics and self-regulation mechanism of the partial denitrification/anammox (PD/A) granular system under the stress of oxytetracycline (OTC), an emerging pollutant that accumulates in municipal wastewater treatment plants through various pathways, posing significant challenges for its future promotion in engineering applications. The results indicated that OTC concentrations below 100 mg/L intensified its short-term inhibition on the PD/A granular sludge system, decreasing functional bacterial activity, while between 150 and 300 mg/L, PD's NO3--N to NO2--N conversion ability diminished, and Anammox activity was significantly suppressed. Under long-term high OTC stress (20-30 mg/L), nitrogen removal suffered, and batch tests revealed significant inhibition of PD's NO3--N to NO2--N conversion, dropping from 73.77 % to 50.17 %. Anammox bacteria activity sharply declined from 1.81 to 0.39 mg N/gVSS/h under OTC stress. Extracellular polymeric substances (EPS) content rose from 185.39 to 210.86 mg/gVSS, indicating PD/A sludge's self-protection mechanism. However, EPS content fell due to cell lysis at high OTC (30 mg/L). The decreasing relative abundance of Candidatus_Brocadia (2.32 % to 0.93 %) and Thaure (12.63 % to 7.82 %) was a key factor in the gradual deterioration of denitrification performance. This study was expected to provide guidance for the PD/A process to cope with the interference of antibiotics and other emerging pollutants (short-term shock and long-term stress).

Enhanced wastewater treatment with an AnF-AAO system for improved internal carbon source utilization

The main challenge in removing nutrients from municipal wastewater in China is the lack of available carbon sources. While hydrolysis acidification tanks can improve wastewater biodegradability by effectively utilizing internal carbon sources, high sludge concentrations are difficult to control in traditional tank variants. In this study, an innovative anaerobic filter (AnF) hydrolysis acidification reactor composed of a continuously stirred tank reactor (CSTR) and cloth media filter was designed to regulate and maintain high sludge concentrations in the hydrolysis acidifier. The reactor was used as a pretreatment unit for the anaerobic/anoxic/oxic (AAO) units and combined into an AnF-AAO system to explore the effectiveness of internal carbon source utilization in wastewater. The results indicate that as the sludge concentration in the hydrolysis acidifier increased, the hydrolysis and acidification processes became more efficient. The optimal sludge concentration was 40 g/L, which significantly increased the production of soluble chemical oxygen demand and volatile fatty acids. Above this concentration, the efficiency decreased. Compared to traditional AAO processes, the AnF-AAO system achieved superior total nitrogen and phosphorus removal with shorter hydraulic retention times and reduced sludge production by a significant amount of 35%. Due to its capacity for enhancing internal carbon source utilization, the AnF-AAO system constitutes a promising approach for sustainable urban wastewater treatment.

Emerging biotechnological applications of anaerobic ammonium oxidation

Anaerobic ammonium oxidation (anammox) is an energy-efficient method for nitrogen removal that opens the possibility for energy-neutral wastewater treatment. Research on anammox over the past decade has primarily focused on its implementation in domestic wastewater treatment. However, emerging studies are now expanding its use to novel biotechnological applications and wastewater treatment processes. This review highlights recent advances in the anammox field that aim to overcome conventional bottlenecks, and explores novel and niche-specific applications of the anammox process. Despite the promising results and potential of these advances, challenges persist for their real-world implementation. This underscores the need for a transition from laboratory achievements to practical, scalable solutions for wastewater treatment which mark the next crucial phase in the evolution of anammox research.

Nitrogen metabolism network in the biotreatment combination of coking wastewater: Take the OHO process as a case

As steel production increases, large volumes of highly toxic and nitrogen-rich coking wastewater (CWW) are produced, prompting the development of a novel oxic-hydrolytic-oxic (OHO) biological treatment combination designed for highly efficient removal of nitrogen-contained contaminants. However, previous studies have not comprehensively explored the CWW biotreatment from the perspective of nitrogen metabolism functional genes and pathways. Based on the investigation of taking the full-scale OHO biotreatment combination as a case, it was found that the O1 and O2 bioreactors remove nitrogen through the ammonia assimilation accounting for 33.87% of the total nitrogen (TN) removal rate, while the H bioreactor removes nitrogen through the simultaneous nitrification-denitrification accounting for 61.11% of the TN removal rate. The major ammonia assimilation taxa include Thauera, Immundisolibacter and Thiobacillus; the major nitrifying taxa include Nitrospira and Nitrosomonas; and the major denitrifying taxa include Thiobacillus, Lautropia and Mesorhizobium. Additionally, the H bioreactor exhibits the potential to be optimized for simultaneous nitrification-denitrification coupled with anaerobic ammonium oxidation (Anammox). These understandings will guide the optimization of engineering design and operational practices, contributing to more effective and sustainable wastewater treatment strategies.

Nitrogen removal performance and the biocenosis with microalgae consortium Nitrosifying and anammox bacteria in an upflow reactor

This study introduced an innovative pathway utilizing an algal anaerobic ammonium oxidation (ALGAMMOX) system to treat ammonium wastewater. Lake bottom sludge and anammox sludge were used to cultivate functional microorganisms and microalgae for nitrogen removal in an upflow reactor made of transparent materials. The results showed that the ALGAMMOX system achieved 87.40 % nitrogen removal when the influent NH4 +-N concentration was 100 mg-N/L. Further analysis showed that anammox bacteria Candidatus Brocadia (8.87 %) and nitrosobacteria Nitrosomonas (3.74 %) were crucial contributors, playing essential roles in nitrogen removal. The 16S rRNA gene showed that the anammox bacteria in the sludge transitioned from Candidatus Kuenenia to Candidatus Brocadia. The 18S rRNA gene revealed that Chlamydomonas, Bacillariaceae and Pinnularia were the dominant microalgae in the system at a relative abundance of 7.99 %, 3.64 % and 3.14 %, respectively. This novel approach provides a theoretical foundation for ammonium wastewater treatment.

Mechanisms of anammox granular sludge reactor effluent as biostimulant: Shaping microenvironment for anammox metabolism

Effluent from anammox granular sludge (AnGS) bioreactor contains microbes and microbial products. This study explored mechanisms of utilizing AnGS-effluent as biostimulant for anammox process enhancement. Compared with no AnGS-effluent supplemented control reactor, 5.0 and 1.3 times higher ammonium nitrogen and total inorganic nitrogen removal rates, respectively were obtained with continuous AnGS-effluent supplementation after 98 days' operation. Anammox bacteria from Candidatus Brocadia accounted for 0.1 % (DNA level) and 1.3 %-1.5 % (RNA level) in control reactor, and 2.9 % (DNA level) and 54.5 %-55.4 % (RNA level) in the AnGS-effluent-fed reactor. Influent microbial immigration evaluation showed that bacterial immigration via AnGS-effluent supplementation was not the main contributor to active anammox community development. Amino acids biosynthesis, B-vitamins and coenzymes metabolism related pathways were facilitated by AnGS-effluent supplementation. AnGS-effluent supplementation aided anammox metabolic activity by shaping microenvironment and microbial interactions. This study provides insights into enhancing anammox bacterial metabolism with AnGS-effluent microbial products as biostimulant.

Future directions in microbial nitrogen cycling in wastewater treatment

Discoveries in the past decade of novel reactions, processes, and micro-organisms have altered our understanding of microbial nitrogen cycling in wastewater treatment systems. These advancements pave the way for a transition toward more sustainable and energy-efficient wastewater treatment systems that also minimize greenhouse gas emissions. This review highlights these innovative directions in microbial nitrogen cycling within the context of wastewater treatment. Processes such as comammox, Feammox, electro-anammox, and nitrous oxide mitigation offer innovative approaches for sustainable, energy-efficient nitrogen removal. However, while these emerging processes show promise, advancing from laboratory research to practical applications, particularly in decentralized systems, remains a critical next step toward a sustainable and efficient wastewater management.

Recovery strategies and mechanisms of anammox reaction following inhibition by environmental factors: A review

Anaerobic ammonium oxidation (anammox) is a promising biological method for treating nitrogen-rich, low-carbon wastewater. However, the application of anammox technology in actual engineering is easily limited by environmental factors. Considerable progress has been investigated in recent years in anammox restoration strategies, significantly addressing the challenge of poor reaction performance following inhibition. This review systematically outlines the strategies employed to recover anammox performance following inhibition by conventional environmental factors and emerging pollutants. Additionally, comprehensive summaries of strategies aimed at promoting anammox activity and enhancing nitrogen removal performance provide valuable insights into the current research landscape in this field. The review contributes to a comprehensive understanding of restoration strategies of anammox-based technologies.

Chemomixoautotrophy and stress adaptation of anammox bacteria: A review

Anaerobic ammonium oxidizing (anammox) bacteria, which were first discovered nearly three decades ago, are crucial for treating ammonium-containing wastewater. Studies have reported on the biochemical nitrogen conversion process and the physiological, phylogenic, and ecological features of anammox bacteria. For a long time, anammox bacteria were assumed to have a lithoautotrophic lifestyle. However, recent studies have suggested the functional versatility of anammox bacteria. Genome-based analysis and experiments with enrichment cultures have demonstrated the association of the metabolic activities of anammox bacteria with different stress conditions, revealing the importance of utilizing specific organic substances, including organoautotrophy, for growth and adaptation to stress conditions. Our understanding regarding the utilization and metabolism of organic substances and their associations with anammox reactions in anammox bacteria is growing but still incomplete. In this review, we summarize the effect of the utilization of organic substances by anammox bacteria under environmental stress conditions, emphasizing their potential organoautotrophic activity and metabolic flexibility. Although most anammox bacteria may utilize specific organic substances, Ca. Brocadia exhibited the highest level of mixoautotrophic activity. The environmental factors that substantially affect the organoautotrophic activities of anammox bacteria were also examined. This review provides a new perspective on the organoautotrophic capacity of anammox bacteria.

Iron particle-integrated anammox granules in baffled reactor: Enhanced settling property and nitrogen removal performance

This study evaluates iron particle-integrated anammox granules (IP-IAGs) to enhance wastewater treatment efficiency. The IP-IAGs resulted in notable improvements in settleability and nitrogen removal. The settling velocity of IP-IAGs increased by 17.91 % to 2.92 ± 0.20 cm/s, and the total nitrogen removal efficiency in batch mode improved by 6.82 %. These changes indicate enhanced biological activity for effective treatment. In continuous operation, the IP-IAGs reactor showed no accumulation of nitrite until 40 d, reaching a peak nitrogen removal rate (NRR) of 1.54 kg-N/m3·d and a nitrogen removal efficiency of 82.61 %. Furthermore, a partial nitritation-anammox reactor that treated anaerobic digestion effluent achieved a NRR of 1.41 ± 0.09 kg-N/m3·d, proving the applicability of IP-IAGs in real wastewater conditions. These results underscore the potential of IP-IAGs to enhance the efficiency and stability of anammox-based processes, marking a significant advancement in environmental engineering for wastewater treatment.

Rapid start-up and operational characteristics of partial denitrification coupled with anammox driven by innovative strategies

The Partial Denitrification-Anammox (PD/A) process established a low-consumption, efficient and sustainable pathway for complete nitrogen removal, which is of great interest to the industry. Rapid initiation and stable operation of the PD/A systems were the main issues limiting its engineering application in wastewater nitrogen removal. A PD/A system was initiated in a continuous stirred-tank reactors (CSTRs) in the presence of low concentration of organic matter, and the effects of organic matter types and COD/NO3--N ratios on the performance of the PD/A system, and microbial community characteristics were explored. The results showed that low concentrations of organic matter could promote the rapid initiation of the Anammox process and then the strategy of gradually replacing NO2--N with NO3--N could successfully initiate the PD/A system at 70 days. The type of organic matter had a significant effect on the initiation of the Anammox and the establishment of the PD/A system. Compared to glucose, sodium acetate was more favorable for rapid start-up and the synergy among microorganisms, and organic matter was lower, with an optimal COD/NO3--N ratio of 3.0. Microorganisms differed in their sensitivity to environmental factors. The relative abundance of Planctomycetota and Proteobacteria in R2 was 51 %, with the presence of three typical anammox bacteria, Candidatus_Brocadia, Candidatus_Kuenenia, and Candidatus_Jettenia in the system. This study provides a new strategy for the rapid initiation and stable operation of the PD/A process.

Iron Recycle-Driven Organic Capture and Sidestream Anaerobic Membrane Bioreactor for Revolutionizing Bioenergy Generation in Municipal Wastewater Treatment

The practicality of intensifying organic matter capture for bioenergy recovery to achieve energy-neutral municipal wastewater treatment is hindered by the lack of sustainable methods. This study developed innovative processes integrating iron recycle-driven organic capture with a sidestream anaerobic membrane bioreactor (AnMBR). Iron-assisted chemically enhanced primary treatment achieved elemental redirection with 75.2% of chemical oxygen demand (COD), 20.2% of nitrogen, and 97.4% of phosphorus captured into the sidestream process as iron-enhanced primary sludge (Fe-PS). A stable and efficient biomethanation of Fe-PS was obtained in AnMBR with a high methane yield of 224 mL/g COD. Consequently, 64.1% of the COD in Fe-PS and 48.2% of the COD in municipal wastewater were converted into bioenergy. The acidification of anaerobically digested sludge at pH = 2 achieved a high iron release efficiency of 96.1% and a sludge reduction of 29.3% in total suspended solids. Ultimately, 87.4% of iron was recycled for coagulant reuse, resulting in a theoretical 70% reduction in chemical costs. The novel system evaluation exhibited a 75.2% improvement in bioenergy recovery and an 83.3% enhancement in net energy compared to the conventional system (primary sedimentation and anaerobic digestion). This self-reliant and novel process can be applied in municipal wastewater treatment to advance energy neutrality at a lower cost.

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

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

Enhancing anammox process with granular activated carbon: A study on Microbial Extracellular Secretions (MESs)

Granular activated carbon (GAC), a porous carbon-based material, provides increased attachment space for functional microorganisms and enhances nitrogen removal by facilitating extracellular electron transfer in the anammox process. This study investigates the effects of GAC on the biosynthesis of microbial extracellular secretions (MESs) and explores the roles of these secretions in anammox activities. Four lab-scale reactors were operated: two downstream UASB reactors (D1 and D2) receiving effluents from the upstream UASB reactors (U1: no-GAC, U2: yes-GAC). Our results indicate that MESs were enhanced with the addition of GAC. The effluent from U2 exhibited a 59.62 % higher amino acid content than that from U1. These secretions contributed to an increase in the nitrogen loading rate (NLR) in the downstream reactors. Specifically, NLR in D1 increased from 130.5 to 142.7 g N/m3/day, and in D2, it escalated from 137.5 to 202.8 g N/m3/day, likely through acting as cross-feeding substrates or vital nutrients. D2 also showed increased anammox bacterial activity, enriched Ca. Brocadia population and hao gene abundance. Furthermore, this study revealed that D2 sludge has significantly higher extracellular polymeric substances (EPS) (48.71 mg/g VSS) and a larger average granule size (1.201 ± 0.119 mm) compared to D1 sludge. Overall, GAC-stimulated MESs may have contributed to the enhanced performance of the anammox process.

Temperature-resilient superior performances by coupling partial nitritation/anammox and iron-based denitrification with granular formation

Partial nitritation-anammox (PN/A), an energy-neutral process, is widely employed in the treatment of nitrogen-rich wastewater. However, the intrinsic nitrate accumulation limits the total nitrogen (TN) removal, and the practical application of PN/A continues to face a significant challenge at low temperatures (<15 °C). Here, an integrated partial nitritation-anammox and iron-based denitrification (PNAID) system was developed to address the concern. Two up-flow bioreactors were set up and operated for 400 days, with one as the control group and the other as the experiment group with the addition of Fe0. In comparison to the control group, the experiment group with the Fe0 supplement showed better nitrogen removal during the entire course of the experiment at different temperature levels. Specifically, the TN removal efficiency of the control group decreased from 82.9 % to 53.9 % when the temperature decreased from 30 to 12 °C, while in stark contrast, the experiment group consistently achieved 80 % of TN removal in the same condition. Apart from the enhanced nitrogen removal, the experiment group also exhibited better phosphorus removal (10.6 % versus 74.1 %) and organics removal (49.5 % versus 65.1 %). The enhanced and resilient nutrient removal performance of the proposed integrated process under low temperatures appeared to be attributed to the compact structure of granules and the increased microbial metabolism with Fe0 supplement, elucidated by a comprehensive analysis including microbial-specific activity, apparent activation energy, characteristics of granular sludge, and metagenomic sequencing. These results clearly confirmed that Fe0 supplement not only improved nitrogen removal of PN/A process, but also conferred a certain degree of robustness to the system in the face of temperature fluctuations.

Tertiary treatment of municipal wastewater in an IBFR dominated by PD/A with unique niche

To explore the feasibility of biofilter reactor to treat municipal secondary effluent deeply without extra carbon source, this paper proposed an integrated biofilter reactor (IBFR) coupling partial denitrification (PD) with anammox (A) to treat the secondary effluent and raw sewage with the flow ratio of 3:1 together. The results show that the effluent concentration of TN and COD in IBFR could be reduced to 10 mg/L and 15 mg/L, respectively, under hydraulic retention time of 1.5 h and nitrogen loading rate of 0.55 kg/(m3·d). The highest specific anammox activity (19.2 mg N/(g TVS·d)) and the maximum extracellular polymeric substance (EPS) content (107.21 mg/g TVS) occurred at the 25-50 cm section of IBFR, where Thauera, Candidatus Anammoximicrobium and Candidatus Brocadia were the dominant denitrifiers and anammox bacteria. Furthermore, the cyclic self-stratification occurred along the reactor height, where the utilization, decomposition, transformation and cross-feeding of EPS enhanced the performance stability of nitrogen and carbon removal, strengthened the niche structure and promoted the synergistic symbiosis. In conclusion, IBFR coupling PD and A demonstrated the possibility to treat secondary effluent without additional carbon sources, which is expected as an alternative approach for tertiary treatment of municipal wastewater.

Self-cultivating anammox granules for enhancing wastewater nitrogen removal in nitrification-denitrification flocculent sludge system

The feasibility of self-cultivating anammox granules for enhancing wastewater nitrogen removal was investigated in a nitrification-denitrification flocculent sludge system. Desirable nitrogen removal efficiency of 84 ± 4 % was obtained for the influent carbon to nitrogen ratio of 1-1.3 (NH4+-N: 150-200 mg N/L) via alternate anaerobic/oxic/anoxic mode. Meanwhile, some red granular sludge was formed in the system. The abundance and activity of anaerobic ammonia oxidation bacteria (AnAOB) increased from 'not detected' in seed sludge to 0.57 % and 29.4 ± 0.7 mg N/(g mixed liquor volatile suspended solids·h) in granules, respectively, suggesting successful cultivation of anammox granules. Furthermore, some denitrifying bacteria with capability of partial denitrification were enriched, such as Candidatus Competibacter (2.45 %) and Thauera (5.75 %), which could cooperate with AnAOB, facilitating AnAOB enrichment. Anammox was dominant in nitrogen removal with the contribution to nitrogen removed above 68.8 ± 0.3 %. The strategy of self-cultivating anammox granules could promote the application of anammox.

The enrichment characterisation of Nitrospira under high DO conditions

Nitrite-oxidizing bacteria (NOB) are crucial to nitrification and nitrogen elimination in wastewater treatment. Mass reports exist on the links between NOB and other microorganisms, for instance, ammonia-oxidizing bacteria (AOB). However, a few studies exist on the enrichment characterisation of NOB under high dissolved oxygen (DO) conditions. In this study, NOB was designed to be enriched individually under high DO conditions in a continuous aeration sequencing batch reactor (SBR), and the kinetic characterisation of NOB was evaluated. The analysis revealed that the average NO2--N removal rate was steady above 98%, with DO and NO2--N being 3-5 mg L-1 and 50-450 mg L-1, respectively. The NO2--N removal efficiency of the system was significantly enhanced and better than in other studies. The high-throughput sequencing suggested that Parcubacteria_ genera_incertae_sedis was the first dominant genus (21.99%), which often appeared in the NOB biological community with Nitrospira. However, the dominant genus NOB was Nitrospira rather than Nitrobacter (8.49%). This result suggested that Nitrospira was capable of higher NO2--N removal. But lower relative abundance indicated that excessive NO2--N had an adverse effect on the enrichment and activity of Nitrospira. In addition, the nitrite half-saturation constant (KNO2) and the oxygen half-saturation constant (KO) were 1.71 ± 0.19 mg L-1 and 0.95 ± 0.10 mg L-1, respectively. These results showed that the enriched Nitrospira bacteria had different characteristics at the strain level, which can be used as a theoretical basis for wastewater treatment plant design and optimisation.

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

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

The coupling of anammox with microalgae-bacteria symbiosis: Nitrogen removal performance and microbial community

The partial nitrification-anammox process for ammonia nitrogen wastewater treatment requires mechanical aeration to provide oxygen, which is not conducive to energy saving. The microalgae-bacteria symbiotic system (MaBS) has the advantages of low carbon and energy saving in wastewater biological nitrogen removal. Therefore, this study combined the MaBS with an anammox process to provide oxygen, through the photosynthesis of microalgae instead of mechanical aeration. We investigated the nitrogen removal efficiency and long-term operation of a co-culture system comprising microalgae, nitrifying bacteria (NB), denitrifying bacteria (DnB), and anaerobic ammonium-oxidation bacteria (AnAOB) in a sequencing batch reactor without mechanical aeration. The experiment was divided into three steps: firstly, cultivating NB; then, adding three kinds of microalgae which were Chlorella sp., Anabaena sp., and Navicula sp. to the bioreactor to construct a microalgae-bacteria symbiotic system; finally, adding anammox sludge to construct the anammox and microalgae-bacteria symbiosis (Anammox-MaBS) system. The results demonstrated that nitrification, denitrification, and anammox processes were coupled successfully, and the maximum TN removal efficiency of the stable Anammox-MaBS system was 99.51 % when the concentration of the influent NH4+-N was 100 mg/L. The addition of microalgae in ammonia wastewater promoted the enrichment of DnB and AnAOB, which were Denitratisoma, Haliangium, unclassified_Rhodocyclaceae, and Candidatus_Brocadia. Furthermore, the unique biofilm structure could effectively alleviate the photoinhibition of light-sensitive bacteria, which may be the reason for the long-term adaptation of Candidatus_Brocadia to light conditions. This research can provide a low-cost solution to bacterial photoinhibition in the coexistence system of microalgae and bacteria without mechanical aeration, offering theoretical support for low-carbon and energy-efficient treatment of wastewater.

Rapid anaerobic culture and reaction kinetic study of anammox bacteria on microfluidic chip

Anammox bacteria are being increasingly investigated as part of an emerging nitrogen removal technology. However, due to the difficulty in culturing, current understanding of their behavior is limited. In this study, anaerobic microfluidic chips were used to study anammox bacteria, showing great advantages over reactors. On-chip fluorescence in situ hybridization (FISH) showed the relative abundance of free form anammox bacteria increased by 56.1 % after one week's culture, an increase that is three times higher than that of bioreactor (17.1 %). For granular form cultures, the nitrogen removal load reached 2.34 ∼ 2.51 kg-N/(m3·d), which was also substantially higher than the bioreactor (∼1.22 kg-N/(m3·d)). Furthermore, studying the kinetics of nitrite inhibition of granular sludge with different particle sizes (100-900 μm) showed that the maximum ammonia load and the nitrite semi-saturation coefficient noticeably decreased for smaller particle sizes. These results illustrate the usefulness of the microfluidic method for in-depth understanding anammox process and its implementation.

Dynamic pulse approach to enhancing mainstream Anammox process stability: Integrating sidestream support and tackling nitrite-oxidizing bacteria challenges

Nitrite-oxidizing bacteria (NOB) seriously threaten the partial nitritation and Anammox (PN/A) process, hindering its mainstream application. Herein, a one-stage PN/A reactor was continuously operated for 245 days under nitrogen loading rate lifted from 0.4 g N/L/d to 0.6 g N/L/d and 0.8 g N/L/d with the nitrogen removal efficiency of 71 %, 64 %, and 41 %, respectively. Furthermore, the NOB species over time was identified as Nitrospira_sp._OLB3, exhibiting an increase of the relative abundance from 0.9 % to 4.3 %. The hydroxyapatite (HAP) granules gradually lost their microbiological function of Anammox bacteria then aged, leading to NOB dominance. Therefore, one "pulse therapy" was introduced and combined with "continuous enhancement" of Anammox sludge supported by sidestream to competitively limit the NOB dynamics. The treatment's effect persisted for around two months. The strategy that returning at least 50 % of the impaired HAP granular sludge to the sidestream for recultivation could fulfill the bottlenecks of mainstream PN/A.

Achieving ultra-high nitrogen and phosphorus removal from real municipal wastewater in a novel continuous-flow anaerobic/aerobic/anoxic process via partial nitrification, endogenous denitrification and nitrite-type denitrifying phosphorus removal

Achieving economic and efficient removal of nutrients in mainstream wastewater treatment plants (WWTPs) continues to be a challenging research topic. In this study, a continuous-flow anaerobic/aerobic/anoxic system with sludge double recirculation (AOA-SDR), which integrated partial nitrification (PN), endogenous denitrification (ED) and nitrite-type denitrifying phosphorus removal (nDNPR), was constructed to treat real carbon-limited municipal wastewater. The average effluent concentrations of total inorganic nitrogen (TIN) and PO43--P during the stable operation period were 1.8 and 0.3 mg/L, respectively. PN was achieved with an average nitrite accumulation ratio of 90.4 % by combined strategies. Adequate storage of polyhydroxyalkanoates and glycogen in the anaerobic zone promoted the subsequent nitrogen removal capacity. In the anoxic zone, nitrite served as the main electron acceptor for the denitrifying phosphorus removal process. Mass balance analysis revealed that nDNPR contributed to 23.6 % of TIN removal and 44.7 % of PO43--P removal. The enrichment of Nitrosomonas (0.45 %) and Ellin 6067 (1.31 %), along with the washout of Nitrospira (0.15 %) provided the bacterial basis for the successful implementation of PN. Other dominant endogenous heterotrophic bacteria, such as Dechlormonas (10.81 %) and Candidatus Accumulibacter (2.96 %), ensured simultaneous nitrogen and phosphorus removal performance. The successful validation of integrating PN, ED and nDNPR for advanced nutrient removal in the AOA-SDR process provides a transformative technology for WWTPs.

Impacts of crude glycerol on anaerobic ammonium oxidation (Anammox) process in wastewater treatment

This work investigated the impact of a waste-derived carbon source, crude glycerol (CG), on Anammox. Batch bioassays were conducted to identify inhibitory component(s) in CG, and the relationship between Anammox activity and the concentration of CG, pure glycerol, and methanol were assessed. The results showed that the half-maximal inhibitory concentration of CG and methanol are 434.5 ± 51.8 and 143.0 ± 19.6 mg chemical oxygen demand (COD) L-1, respectively, while pure glycerol at 0-2283 mg COD L-1 had no significant adverse effect on Anammox. The results suggested methanol is the major inhibitor in CG via a non-competitive inhibition mechanism. COD/total inorganic nitrogen ratio of > 1.3 was observed to cause a significant Anammox inhibition (>20 %), especially at low substrate level. These results are valuable for evaluating the feasibility of using CG for nitrogen removal in water resource recovery facilities, promoting sustainable development.

Metagenomics reveals the microbial community and functional metabolism variation in the partial nitritation-anammox process: From collapse to recovery

Mainstream partial nitritation-anammox (PNA) process easily suffers from performance instability and even reactor collapse in application. Thus, it is of great significance to unveil the characteristic of performance recovery, understand the intrinsic mechanism and then propose operational strategy. In this study, we combined long-term reactor operation, batch tests, and metagenomics to reveal the succession of microbial community and functional metabolism variation from system collapse to recovery. Proper aeration control (0.10-0.25 mg O2/L) was critical for performance recovery. It was also found that Candidatus Brocadia became the dominant flora and its abundance increased from 3.5% to 11.0%. Significant enhancements in carbon metabolism and phospholipid biosynthesis were observed during system recovery, and the genes abundance related to signal transduction was dramatically increased. The up-regulation of sdh and suc genes showed the processes of succinate dehydrogenation and succinyl-CoA synthesis might stimulate the production of amino acids and the synthesis of proteins, thereby possibly improving the activity and abundance of AnAOB, which was conducive to the performance recovery. Moreover, the increase in abundance of hzs and hdh genes suggested the enhancement of the anammox process. Changes in the abundance of key genes involved in nitrogen metabolism indicated that nitrogen removal pathway was more diverse after system recovery. The achievement of performance recovery was driven by anammox, nitrification and denitrification coupled with dissimilatory nitrate reduction to ammonium. These results provide deeper insights into the recovery mechanism of PNA system and also provide a potential regulation strategy for the stable operation of the mainstream PNA process.

Microbial activity along the depth of biofilm in the simultaneous partial nitrification, anammox and denitrification (SNAD) system

Simultaneous partial nitrification, anammox and denitrification (SNAD) is a sustainable and cost-effective technology for nitrogen removal from low-strength wastewater. However, knowledge of the biofilm microenvironment of the SNAD system is currently unsatisfactory. The purpose of this study was to evaluate organic carbon effects on the microenvironment and microbial growth in the SNAD biofilm system. Microelectrodes were used to investigate microbial activity in-depth within biofilms. ORP distribution of the SNAD system was positively related to anammox activity(R2 = 0.9), and had some influence on microbial community structure. The synergistic effect of anammox bacteria and denitrifiers could be achieved when the abundance ratio of anammox bacteria to denitrifying bacteria is greater than 1.2.

Efficient nitrogen removal and substrate usage in integrated fixed-film activated sludge-anammox system under seasonal temperature variation

To elucidate how integrated fixed-film activated sludge (IFAS) system favors nitrogen removal performance under seasonal temperature variations, two push-flow reactors were operated with and without carriers under the same operating conditions. The results show that the IFAS system had significant advantages in shock response and low temperature adaptation, with a nitrogen removal rate of 0.37-0.53 kg-N(m3·d)-1 at the temperature of 8-12 °C. Anammox bacteria on carriers were almost unaffected by temperature variation, and its nitrogen removal contribution rate stabilized at 55 % in the IFAS system. The Haldane model reveals that the specific anammox activity in the IFAS system was 28 % to 49 % higher than that in the control system at 13 °C. Candidatus_Jettenia, with the highest abundance of 45 %, was the dominant species in the IFAS system and preferred to attach to the carriers. This study provides a feasible scheme for the application of anammox process.

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.

Biofilm reactors for the treatment of used water in space:potential, challenges, and future perspectives

Water is not only essential to sustain life on Earth, but also is a crucial resource for long-duration deep space exploration and habitation. Current systems in space rely on the resupply of water from Earth, however, as missions get longer and move farther away from Earth, resupply will no longer be a sustainable option. Thus, the development of regenerative reclamation water systems through which useable water can be recovered from "waste streams" (i.e., used waters) is sorely needed to further close the loop in space life support systems. This review presents the origin and characteristics of different used waters generated in space and discusses the intrinsic challenges of developing suitable technologies to treat such streams given the unique constrains of space exploration and habitation (e.g., different gravity conditions, size and weight limitations, compatibility with other systems, etc.). In this review, we discuss the potential use of biological systems, particularly biofilms, as possible alternatives or additions to current technologies for water reclamation and waste treatment in space. The fundamentals of biofilm reactors, their advantages and disadvantages, as well as different reactor configurations and their potential for use and challenges to be incorporated in self-sustaining and regenerative life support systems in long-duration space missions are also discussed. Furthermore, we discuss the possibility to recover value-added products (e.g., biomass, nutrients, water) from used waters and the opportunity to recycle and reuse such products as resources in other life support subsystems (e.g., habitation, waste, air, etc.).

Metagenomics insight into the long-term effect of ferrous ions on the mainstream anammox system

Anaerobic ammonium oxidation (anammox) bacteria have a high requirement for iron for their growth and metabolism. However, it remains unclear whether iron supplementation can sustain the stability of mainstream anammox systems at varying temperatures. Here, we investigated the long-term effects of Fe2+ on the mainstream anammox systems. Our findings revealed that the nitrogen removal efficiency (NRE) of the anammox system supplemented with 5 mg/L Fe2+ decreased from 76.5 ± 0.76% at 35 °C to 39.0 ± 9.9% at 25 °C. Notably, higher dosages of Fe2+ (15 mg/L and 30 mg/L) inhibited the anammox system, resulting in NREs of 15.9 ± 8.1% and 2.5 ± 1.1% at 25 °C, respectively. The results of microbial communities and function profiles suggested that the high Fe2+ dosage seriously affected the iron assimilation and utilization in the mainstream anammox system. This was evident from the decreased abundance of genes associated with Fe(II) transport and uptake, which in turn hindered the biosynthesis of intracellular iron-cofactors, resulting in decrease in the absolute abundance of Candidatus Brocadia, a key anammox bacterium, as well as a decline in NRE. Furthermore, our results showed that the anammox process was more susceptible to iron supplementation at 25 °C compared to 35 °C, which may be due to the oxidative stress reactions induced by combined lowered temperature and a high Fe2+ dosage. Overall, these findings offer a deeper understanding of the effect of iron in mainstream anammox systems, which can contribute to improved stability maintenance and effectiveness of anammox processes.

Anaerobic Wastewater Treatment and Potable Reuse: Energy and Life Cycle Considerations

Anaerobic secondary treatment has the potential to facilitate energy-positive operations at wastewater treatment plants, but post-treatment of the anaerobic effluent is needed to recover dissolved methane and nutrients and remove sulfide. In this study, a life cycle assessment was conducted to compare hypothetical full-scale wastewater treatment trains and direct potable reuse trains that combine the staged anaerobic fluidized membrane bioreactor (SAF-MBR) with appropriate post-treatment. We found that anaerobic wastewater treatment trains typically consumed less energy than conventional aerobic treatment, but overall global warming potentials were not significantly different. Generally, recovery of dissolved methane for energy production resulted in lower life cycle impacts than microbial transformation of methane, and microbial oxidation of sulfide resulted in lower environmental impacts than chemical precipitation. Use of reverse osmosis to produce potable water was also found to be a sustainable method for nutrient removal because direct potable reuse trains with the SAF-MBR consumed less energy and had lower life cycle impacts than activated sludge. Moving forward, dissolved methane recovery, reduced chemical usage, and investments that enable direct potable reuse have been flagged as key research areas for further investigation of anaerobic secondary treatment options.

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

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