Conductive carrier promotes synchronous biofilm formation and granulation of anammox bacteria

Adsorption and nitrogen removal of NH4+-N based on Mn (II)/α-MnO2 cycle in bio-electrochemical system

This paper developed a single-chamber α-MnO2-coupled microbial electrolysis cell (α-MnO2-MEC) system to enhance the oxidation denitrification rate of ammonia nitrogen (NH4+-N) in order to overcome the electrode repulsion problem between NH4+ and the anode. The α-MnO2 material with an equilibrium adsorption capacity of 10.6 mg·g-1 for NH4+-N was developed. The removal rate of total nitrogen in the α-MnO2-MEC reactor is 95.8 %, and NH4+ oxidation efficiency is 100 % in 20 h, which is 78.7 % and 47.8 % higher than in the α-MnO2 reactor and the MEC reactor, respectively. The Mn(II)/α-MnO2 cycle was realized in α-MnO2-MEC reactor, avoiding the loss of the Mn(II). The 16S rRNA gene sequencing identified key microbial genera involved in the ammonia removal are Candidatus_Brocadia, SC-I-84, and Thauera. This study demonstrates that combining α-MnO2 with bioelectrochemistry provides a novel strategy for ammonia nitrogen wastewater treatment, offering a new insight for optimizing electrochemical-microbial coupled nitrogen removal.

Feδ+ diaspora titanium dioxide and graphene: A study of conductive powder materials and coating applications

Developing new conductive primers to ensure electrostatic spraying is crucial in response to the call for lightweight production of new energy vehicles. We report a stabilized material, Fe-T/G, of Fe-doped TiO2 composite graphene synthesized by a simple hydrothermal and electrostatic self-assembly method. The resistivity decreases from 0.9165 Ω·cm to 0.0943 Ω·cm for T/G. XPS and EPR confirm that Fe3+ is doped into the TiO2 lattice to generate OVs, and Feδ+ is introduced to activate the surrounding Fe-Ti active sites to break the obstruction of the Ti-O bond and build the Ti-O-C and Feδ+-O-C bonds between TiO2 and graphene to connect the two tightly. Multiple electron transfer channels promote Feδ+ impurity energy levels in the TiO2 valence and conduction bands to build electron leaping pathways, enhancing the electron transport ability. Based on this, Fe-T/G can be used as a conductive coating material to develop various insulator surfaces. With Fe-T/G powder material and PVDF, a conductive primer coating can be made and overlaid on a plastic plate to simulate actual applications. The resistance can still be maintained below 25 Ω·cm. Through contact angle experiments, it has been confirmed that the superhydrophobic surface has a water contact angle of 142.8°, with self-cleaning potential.

Iron-reduction driven extracellular electron transfer widely promotes microbial reductive dechlorination metabolism

Biological iron reduction and reductive dehalogenation occur in similar ecological environments, however, how Fe(III)/Fe(II) redox cycles impact the microbial dehalogenation processes remains controversial. In this study, the favorable microbial reductive dechlorination activity has been widely observed in iron-rich river sediments by national sampling, with the dechlorination efficiency showing a positive correlation with the concentration of Fe(III). Microcosm experiments demonstrated that the addition of nano-hematite resulted in a maximum increase of 2.16 times in the dechlorination rate constant (k) for 2,4,6-trichlorophenol, achieved via synergistic interactions with Fe(III) reduction. Multi-tools, including transcriptomic analyses, revealed that the addition of nano-hematite enhanced the process of Fe(III) reduction by upregulating genes associated with extracellular electron transfer (e.g., CYC, pliM) and conductive biofilm formation (e.g., livH, secY, wza). This synergistic Fe(III) reduction further facilitated intracellular carbon metabolism, energy production, and reductive dechlorination, as confirmed by the upregulated functional genes identified through transcriptomics and RT-qPCR. The discovery of the novel phenomenon involving synergistic Fe(III) reduction and dehalogenation broadens our understanding of the biochemical cycling of organohalides (e.g., chlorinated phenols) in iron-rich environment, and provides a feasible strategy for improving biodehalogenation through the regulation of carbon and electron flow at sites contaminated with organohalides.

Enhancing anammox granular sludge for mainstream anammox process by adding iron-loaded diatomite: Performance and intrinsic mechanism

Iron-loaded diatomite (Fe-DE) was developed as the innovative material to enhance anammox granular sludge (AnGS) and mainstream anammox performance. By adding Fe-DE with the Fe:DE ratio of 1:20 and the dosage of 3 g/L, the start-up period of mainstream anammox process was shortened from 29 d to 17 d and its nitrogen removal rate was increased from 0.234 kg N/(m3·d) to 0.437 kg N/(m3·d). AnGS generated more hydrophobic functional groups and redox substances, forming the robust particle structure and improving the electron transfer of anammox reaction. In addition, the key genes PleC, PleD and TrpE/G, related to quorum sensing, increased from 502.69, 91.18 and 18.25 CPM to 532.84, 103.66 and 19.96 CPM. The key genes hzs and hdh related to anammox process also increased by 30.76% and 24.26%. As a result, formation of AnGS was promoted and the enrichment level of Candidatus Brocadia was improved. This study provides a novel insight into the development of innovative material for enhancing mainstream anammox process.

Deciphering the key role of biofilm and mechanisms in high-strength nitrogen removal within the anammox coupled partial S0-driven autotrophic denitrification system

Anammox coupled partial S0-driven autotrophic denitrification (PS0AD) technology represents an innovative approach for removing nitrogen from wastewater. The research highlighted the crucial role of biofilm on sulfur particles in the nitrogen removal process. Further analysis revealed that sulfur-oxidizing bacteria (SOB) are primarily distributed in the inner layer of the biofilm, while anammox bacteria (AnAOB) are relatively evenly distributed in inner and outer layers, with Thiobacillus and Candidatus Brocadia being the dominant species, respectively. Except for anammox and PS0AD processes, 15N isotope labeling tests determined that sulfur reshaped nitrogen metabolism pathways, providing solid evidence for the occurrence of sulfammox process. SOB and AnAOB collaborate in nitrogen and sulfur conversion, with SOB-drived PS0AD processes reducing nitrate to nitrite for AnAOB to remove ammonia. Conversely, the nitrate produced from anammox process can be reused by SOB. Metagenomic analyses verified that SOB drove the PS0AD process through encoding soxBYZ gene, while AnAOB might play an important role in simultaneously driving the anammox and sulfammox processes. These findings underscore the importance of biofilm and clarify the nitrogen-sulfur cycle mechanisms within the coupled system.

Screening, identification, and application of anaerobic ammonia oxidizing bacteria in activated sludge systems: A comprehensive review

Anaerobic ammonium oxidation (Anammox) has garnered significant attention due to its ability to eliminate the need for aeration and supplementary carbon sources in biological nitrogen removal process, relying on the capacity of anaerobic ammonium oxidizing bacteria (AnAOB) to directly convert ammonium and nitrite nitrogen into nitrogen gas. This review consolidates the latest advancements in AnAOB research, outlining the mechanisms and enzymatic processes of Anammox, and summarizing the molecular biological techniques used for studying AnAOB, such as 16s rRNA sequencing, qPCR, and metagenomic sequencing. Additionally, it also overviews the currently identified AnAOB species and their distinct metabolic traits, while consolidating strategies to improve their performance. It further delineates coupled processes that utilize Anammox technology, offering practical insights for process selection. Eventually, the review concludes by suggesting future research directions and highlighting critical areas for further investigation. This review serves as a theoretical reference for the enrichment and cultivation of AnAOB, environmental impact management, and the selection of effective treatment processes.

Multi-faceted effects and mechanisms of granular activated carbon to enhance anaerobic ammonium oxidation (anammox) for nitrogen removal from wastewater

Nitrogen removal via anammox is efficient but challenged by their slow growth. Adding granular activated carbon (GAC) increased the total nitrogen removal rate to 66.99 g-N/m3/day, compared to 50.00 g-N/m3/day in non-GAC reactor. Both reactors dominated by Candidatus Brocadia (non-GAC: 36.25 %, GAC: 35.5 %) but GAC improved specific anammox activity. Functional metabolic profiling from metagenomic analysis unveiled that GAC enhanced pathways associated with electron shuttle production, potentially promoting intra/extracellular electron transfer. In nitrogen metabolism, GAC is indicated to facilitate anammox N2H4 synthesis process, and inhibit nitrification and full denitrification processes, functioned by Nitrosomonas and Castellaniella which are more abundant in the non-GAC reactor. GAC also enhanced dissimilatory nitrate reduction to ammonium and partial denitrification processes, providing anammox with NH4+/NO, which was conducted by Anaerolineae members (29.7 % in GAC-reactor and 7.8 % in non-GAC reactor sludge). This research illuminated the intricate microbial nitrogen cycling networks affected by GAC in anammox systems.

A comprehensive review of antibiotics stress on anammox systems: Mechanisms, applications, and challenges

Anaerobic ammonia oxidation (anammox), an energy-efficient technology for treating ammonium-rich wastewater, faces the challenge of antibiotic stress in sewage. This paper systematically evaluated the impact of antibiotics on anammox by considering both inhibitory effects and recovery duration. This review focused on cellular responses, including extracellular polymeric substances (EPS), quorum sensing (QS), and enzymes. Then, the physiological properties of cells and the interactions between nitrogen and carbon metabolism under antibiotic stress were discussed, particularly within the anammoxosome. The microbial community evolution and the development and transfer of antibiotic resistance genes (ARGs) were further analyzed to reveal the resistance mechanisms of anammox. To address the limitations imposed by antibiotics, the development of bio-augmentation and combined processes based on molecular biology techniques, such as bio-electrochemical systems (BES), has been suggested. This review offered new insights into the mechanisms of antibiotic inhibition during the anammox process and aimed to advance their engineering applications.

Comparation on the responses and resilience of single-Anammox system and synergistic partial-denitrification/anammox system to long-term nutrient starvation: Performance and metagenomic insights

Starvation disturbance was a common problem in biological sewage treatment processes. However, understanding about the responses and resilience of different active anammox biomass in autotrophic and heterotrophic systems to long-term nutrient starvation remains limited. This study compared responses and potential recovery mechanisms of autotrophic single-Anammox and heterotrophic synergistic partial-denitrification/anammox (PD/anammox) systems to prolonged starvation (31-40 days). After starvation, total inorganic nitrogen (TIN) removal efficiency of single-Anammox and synergistic PD/anammox systems decreased to 62.16 % and 78.52 %, respectively, of their original level. After the nutrient resupply, the performance of both systems gradually recovered to a similar-to-pre-starvation level at the rate of 1.26 %/day and 1.89 %/day, respectively. Compared with single-Anammox system, complex synergistic relationship of microorganisms and effective quorum sensing (QS) regulation strategies might mitigate the negative influences were caused by starvation and ensure the performance quickly return of synergistic PD/anammox system. This study would contribute to promote the application of Anammox technology.

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.

Role of membrane fouling layer in microbial fuel cell-membrane bioreactor (MFC-MBR) for controlling sulfamethoxazole and corresponding resistance genes

Integrated MFC-MBR systems effectively remove antibiotics and control the release of antibiotic resistance genes (ARGs). However, the fouling layers on membranes can potentially act as reservoirs for ARGs. This study aims to elucidate the roles of membrane fouling layers and levels in influencing sulfamethoxazole (SMX) removal and ARGs control within an MFC-MBR system. Our findings demonstrate that low-intensity bioelectricity (400-500 mV) mitigates membrane fouling rates. The membrane fouling layer significantly contributes (39%-47%) to SMX removal compared to the cathode/anode zones. Higher extracellular polymeric substance (EPS) content and a lower protein/polysaccharide (PN/PS) ratio favor SMX removal by the membrane fouling layer. Across different levels of membrane fouling, the PN/PS ratio rather than EPS concentration plays a crucial role in SMX removal efficiency. The MFC-MBR with low fouling achieved superior SMX removal (69.1%) compared to medium (54.3%) and high fouling conditions (46.8%). The presence of ARGs in the membrane fouling layer increases with fouling formation, with intrinsic ARGs prevailing. Dense membrane fouling layers effectively retain ARGs, thereby reducing the risk of extracellular ARGs (eARGs) diffusion in effluents. These results provide insights into controlling ARGs in MFC-MBR systems and underscore the significant role of membrane fouling layers in antibiotics and ARGs removal.

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.

Deciphering Fe@C amendment on long-term anaerobic digestion of sulfate and propionate rich wastewater: Driving microbial community succession and propionate metabolism

This study evaluated the reflection of long-term anaerobic system exposed to sulfate and propionate. Fe@C was found to efficiently mitigate anaerobic sulfate inhibition and enhance propionate degradation. With influent propionate of 12000mgCOD/L and COD/SO42- ratio of 3.0, methane productivity and sulfate removal were only 0.06 ± 0.02L/gCOD and 63 %, respectively. Fe@C helped recover methane productivity to 0.23 ± 0.03L/gCOD, and remove sulfate completely. After alleviating sulfate stress, less organic substrate was utilized to form extracellular polymeric substances for self-protection, which enhanced mass transfer in anaerobic sludge. Microbial community succession, especially for alteration of key sulfate-reducing bacteria and propionate-oxidizing bacteria, was driven by Fe@C, thus enhancing sulfate reduction and propionate degradation. Acetotrophic Methanothrix and hydrogenotrophic unclassified_f_Methanoregulaceae were enriched to promote methanogenesis. Regarding propionate metabolism, inhibited methylmalonyl-CoA degradation was a limiting step under sulfate stress, and was mitigated by Fe@C. Overall, this study provides perspective on Fe@C's future application on sulfate and propionate rich wastewater treatment.

Identification of extracellular polymeric substances layer barrier in chloroquine phosphate-disturbed anammox consortia and mechanism dissection on cytotoxic behavior by computational chemistry

The over-dosing use of chloroquine phosphate (CQ) poses severe threats to human beings and ecosystem due to the high persistence and biotoxicity. The discharge of CQ into wastewater would affect the biomass activity and process stability during the biological processes, e.g., anammox. However, the response mechanism of anammox consortia to CQ remain unknown. In this study, the accurate role of extracellular polymeric substances barrier in attenuating the negative effects of CQ, and the mechanism on cytotoxic behavior were dissected by molecular spectroscopy and computational chemistry. Low concentrations (≤6.0 mg/L) of CQ hardly affected the nitrogen removal performance due to the adaptive evolution of EPS barrier and anammox bacteria. Compact protein of EPS barrier can bind more CQ (0.24 mg) by hydrogen bond and van der Waals force, among which O-H and amide II region respond CQ binding preferentially. Importantly, EPS contributes to the microbiota reshape with selectively enriching Candidatus_Kuenenia for self-protection. Furthermore, the macroscopical cytotoxic behavior was dissected at a molecular level by CQ fate/distribution and computational chemistry, suggesting that the toxicity was ascribed to attack of CQ on functional proteins of anammox bacteria with atom N17 (f-=0.1209) and C2 (f+=0.1034) as the most active electrophilic and nucleophilic sites. This work would shed the light on the fate and risk of non-antibiotics in anammox process.

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.

Management of biofilm by an innovative layer-structured membrane for membrane biofilm reactor (MBfR) to efficient methane oxidation coupled to denitrification (AME-D)

Aerobic methane oxidation coupled with denitrification (AME-D) has garnered significant attention as a promising technology for nitrogen removal from water. Effective biofilm management on the membrane surface is essential to enhance the efficiency of nitrate removal in AME-D systems. In this study, we introduce a novel and scalable layer-structured membrane (LSM) developed using a meticulously designed polyurethane sponge. The application of the LSM in advanced biofilm management for AME-D resulted in a substantial enhancement of denitrification performance. Our experimental results demonstrated remarkable improvements in nitrate-removal flux (92.8 mmol-N m-2 d-1) and methane-oxidation rate (325.6 mmol m-2 d-1) when using an LSM in a membrane biofilm reactor (L-MBfR) compared with a conventional membrane reactor (C-MBfR). The l-MBfR exhibited 12.4-, 6.8- and 3.4-fold increases in nitrate-removal rate, biomass-retention capacity, and methane-oxidation rate, respectively, relative to the control C-MBfR. Notably, the l-MBfR demonstrated a 3.5-fold higher abundance of denitrifying bacteria, including Xanthomonadaceae, Rhodocyclaceae, and Methylophilaceae. In addition, the denitrification-related enzyme activity was twice as high in the l-MBfR than in the C-MBfR. These findings underscore the LSM's ability to create anoxic/anaerobic microenvironments conducive to biofilm formation and denitrification. Furthermore, the LSM exhibited a unique advantage in shaping microbial community structures and facilitating cross-feeding interactions between denitrifying bacteria and aerobic methanotrophs. The results of this study hold great promise for advancing the application of MBfRs in achieving efficient and reliable nitrate removal through the AME-D pathway, facilitated by effective biofilm management.

Electrically-assisted anaerobic digestion under ammonia stress: Facilitating propionate oxidation and activating methanogenesis via direct interspecies electron transfer

Electrical assistance is an effective strategy for promoting anaerobic digestion (AD) under ammonia stress. However, the underlying mechanism of electrical assistance affecting AD is insufficiently understood. Here, electrical assistance to AD under 5 g N/L ammonia stress was provided, by employing a 0.6 V voltage to the carbon electrodes. The results demonstrated remarkable enhancements in methane production (104.6 %) and the maximal methane production rate (207.7 %). The critical segment facilitated by electro-stimulation was the microbial metabolism of propionate-to-methane, rather than ammonia removal. Proteins in extracellular polymer substances were enriched, boosting microbial resilience to ammonia intrusion. Concurrently, the promoted humic/fulvic-substances amplified the microbial electron transfer capacity. Metagenomics analysis identified the upsurge of propionate oxidation at the anode (by e.g. unclassified_c__Bacteroidia), and the stimulations of acetoclastic and direct interspecies electron transfer-dependent CO2-reducing methanogenesis at the cathode (by e.g. Methanothrix). This study provides novel insights into the effect of electrical assistance on ammonia-stressed AD.

Interactions between biofilms and PFASs in aquatic ecosystems: Literature exploration

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) have been detected in most aquatic environments worldwide and are referred to as "forever chemicals" because of their extreme chemical and thermal stability. Biofilms, as basic aquatic bioresources, can colonize various substratum surfaces. Biofilms in the aquatic environment have to interact with the ubiquitous PFASs and have significant implications for both their behavior and destiny, which are still poorly understood. Here, we have a preliminary literature exploration of the interaction between PFASs and biofilms in the various aquatic environments and expect to provide some thoughts on further study. In this review, the biosorption properties of biofilms on PFASs and possible mechanisms are presented. The complex impact of PFASs on biofilm systems was further discussed in terms of the composition and electrical charges of extracellular polymeric substances, intracellular microbial communities, and overall contaminant purification functions. Correspondingly, the effects of biofilms on the redistribution of PFASs in the aqueous environment were analyzed. Finally, we propose that biofilm after adsorption of PFASs is a unique ecological niche that not only reflects the contamination level of PFASs in the aquatic environment but also offers a possible "microbial pool" for PFASs biodegradation. We outline existing knowledge gaps and potential future efforts for investigating how PFASs interact with biofilms in aquatic ecosystems.

Novel insights into carbon nanomaterials enhancing anammox for nitrogen removal: Effects and mechanisms

Carbon nanomaterials (CNMs) possess the properties including large specific surface area, high porosity, and stable chemical structures, presenting significant application advantages in wastewater treatment. Indeed, CNMs are considered to be added to anammox systems to strengthen anammox function, especially to resolve the challenge of anammox technology, i.e., the slow growth rate of anammox bacteria, as well as its high environmental sensitivity. This paper systematically reviews the promotion effects and mechanisms of CNMs on the nitrogen removal performance of anammox system. Among the zero-, one-, and two-dimensional CNMs, two-dimensional CNMs have best promoting effect on the nitrogen removal performance of anammox system due to its excellent conductivity and abundant functional groups. Then, the promotion effects of CNMs on anammox process are summarized from the perspective of anammox activity and bacteria abundance. Furthermore, CNMs not only enhance the anammox process, but also stimulate the coupling of denitrification pathways with anammox, as well as the improvement of system operational stability (alleviating the inhibitions of low temperature and pH fluctuation), thus contributing to the promoted nitrogen removal performance. Essentially, CNMs are capable of facilitating microbial immobilization and electron transfer, which favor to improve the efficiency and stability of anammox process. Finally, this review highlights the gap in knowledge and future work, aiming to provide a deeper understanding of how CNMs can strengthen the anammox system and provide a novel perspective for the engineering of the anammox process.

Effective enrichment of anaerobic ammonia oxidation sludge with feast-starvation strategy: activity, sedimentation, growth kinetics, and microbial community

To address the issue of floating and loss of floc sludge caused by gas production in anaerobic ammonia oxidation (anammox) reactors, this study proposes a limited nitrite supply strategy to regulate gas production during the settling and enhance sludge retention. Results indicate that the effluent suspended solids in the anammox reactor can be reduced to as low as 0.11 g/L under specific feast-starvation conditions. Even under long-term intermittent nitrite-starvation stress, the maximum growth rate of Candidatus_Kuenenia can still reach 0.085d-1, with its abundance increasing from 0.47% to 8.83% within 69 days. Although the combined effects of starvation and sedimentation would lead to a temporary decrease in anammox activity, this reversible inhibition can be fully restored through substrate intervention. The limited nitrite supply strategy promotes the sedimentation of anammox sludge without significantly affecting its growth rate, and effective sludge retention is crucial for enriching anammox sludge during initial cultivation.

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.