Achieving superior nitrogen removal performance in low-strength ammonium wastewater treatment by cultivating concentrated, highly dispersive, and easily settleable granule sludge in a one-stage partial nitritation/anammox-HAP reactor

Nondestructive and three-dimensional visualization by identifying elements using synchrotron radiation microscale X-ray CT reveals microbial and cavity distributions in anaerobic granular sludge

We developed a nondestructive three-dimensional microbial visualization method utilizing synchrotron radiation X-ray microscale computed tomography to better understand the relationship between microorganisms and their surrounding habitats. The method was tested and optimized using a mixture of axenic Escherichia coli and Comamonas testosteroni. The osmium-thiocarbohydrazide-osmium method was used to stain all the microbial cells, and gold in situ hybridization was used to detect specific phylogenetic microbial groups. The stained samples were embedded in epoxy resin for microtomographic analysis. Differences in X-ray absorbances were calculated by subtracting the pre-L3-edge images from the post-L3-edge images to visualize the osmium and gold signals. Although we successfully detected cells stained with osmium, those labeled with gold were not detected, probably because of the insufficient density of gold atoms in the microbial cells. We then applied the developed technique to anaerobic granules and visualized the distribution of microbial cells and extracellular polymeric substances. Empty spaces were highlighted to determine the cavity distribution in granules. Numerous independent cavities of different sizes were identified in the granules. The developed method can be applied to various environmental samples for deeper insights into microbial life in their habitats.

IMPORTANCE

Microorganisms inhabit diverse environments and often form biofilms. One factor that affects their community structure is the surrounding physical environment. The arrangement of residential space within the formed biofilm plays a crucial role in the supply and transportation of substances, as well as the discharge of metabolites. Conventional approaches, such as scanning electron microscopy and confocal laser scanning microscopy combined with fluorescence in situ hybridization, have limitations as they provide information primarily from the biofilm surface and cross-sections. In this study, we developed a method for detecting microorganisms in biofilms using synchrotron radiation X-ray microscale computer tomography. The developed method allows nondestructive three-dimensional observation of biofilms at a single-cell resolution (voxel size of approximately 200 nm), facilitating an understanding of the relationship between microorganisms and their physical habitats.

Anaerobic membrane bioreactors for municipal wastewater: Progress in resource and energy recovery improvement approaches

Anaerobic membrane bioreactor (AnMBR) offer promise in municipal wastewater treatment, with potential benefits including high-quality effluent, energy recovery, sludge reduction, and mitigating greenhouse gas emissions. However, AnMBR face hurdles like membrane fouling, low energy recovery, etc. In light of net-zero carbon target and circular economy strategy, this work sought to evaluate novel AnMBR configurations, focusing on performance, fouling mitigation, net-energy generation, and nutrients-enhancing integrated configurations, such as forward osmosis (FO), membrane distillation (MD), bioelectrochemical systems (BES), membrane photobioreactor (MPBR), and partial nitrification-anammox (PN/A). In addition, we highlight the essential role of AnMBR in advancing the circular economy and propose ideas for the water-energy-climate nexus. While AnMBR has made significant progress, challenges, such as fouling and cost-effectiveness persist. Overall, the use of novel configurations and energy recovery strategies can further improve the sustainability and efficiency of AnMBR systems, making them a promising technology for future sustainable municipal wastewater treatment.

Deciphering the role of micro/nano-hydroxyapatite in aerobic granular sludge system: Effects on treatment performance and enhancement mechanism

Hydroxyapatite (HAP), a mineral nucleus identified within aerobic granular sludge (AGS), plays a vital role in enhancing the AGS systems. However, the microscopic mechanism underlying their roles remains largely unexplored. Herein, a systematic investigation was carried out to elucidate the impact and enhanced mechanisms associated with HAP of different sizes, i.e. micro-HAP (mHAP) and nano-HAP (nHAP), on the aerobic granulation, nutrient removal and microbial diversity of AGS. Results showed that the presence of nHAP and mHAP significantly shortened the granulation process to 15 and 20 days, respectively. This might be ascribed to the fact that the large specific surface area of nHAP aggregates was conducive to microbial adhesion, biomass accumulation and sludge granulation. Compared with mHAP, the granules with nHAP showed better settlement performance, mechanical strength and larger diameter. The X-ray diffraction (XRD) and Raman spectrometer analysis confirmed the presence of HAP within the granules, which was found to stimulate the secretion of extracellular polymeric substance, improve the compactness of granule structure and suppress the growth of filamentous bacteria, thereby contributing to a stable AGS system. The presence of HAP, especially nHAP, effectively enriched the functional microorganisms, such as nitrifying and denitrifying bacteria (e.g. Candidatus_Competibacter) and phosphorus accumulating organisms (e.g. Flavobacterium), leading to the improved nutrient removal efficiencies (COD > 96%, TN > 76%, and TP > 74%). Further analysis revealed the up-regulation of functional enzymes (e.g. nitrite oxidoreductase and polyphosphate kinase) involved in nutrient metabolism, underlying the inherent mechanisms for the excellent nutrient removal. This study deepens the understanding of granulation mechanisms from the perspective of mineral cores, and proposes an economically feasible strategy for rapid initiation and stabilization of AGS reactors.

A temperature-resilient anammox process for efficient treatment of rare earth element tailings wastewater via synergistic nitrite supply of partial nitritation and partial denitrification

Rare earth elements result in substantial tailings wastewater with high ammonium and nitrate during extraction. In this study, a temperature-resilient Anammox process was employed for efficient treatment of rare earth element tailings wastewater through implementing synergistic nitrite supply by partial nitritation (PN) and partial denitrification (PD). Enhancing temperature resilience of Anammox process relies on dynamic management of DO and COD inputs to shift the dominant nitrite supplier from PN to PD, stable PD (NAR ≥ 90 %) can boost nitrogen removal by Anammox to 97.8 %. The nitrogen removal rate and nitrogen removal efficiency at 10.6 °C could maintain at 0.12 kgN/m3·d-1 and 92.5 %, respectively. Microbial analysis reveals that Nitrosomonas, Thauera, and Candidatus_Kuenenia are the predominant genera responsible for nitrite supply and nitrogen removal, localized within the gas channels of granules, flocs, and micro-granules, respectively. Keeping the influent C/NO3--N ratio below 1.7 is ideal to prevent overgrowth of Thauera and maintain system stability.

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.

Concurrent removal of carbon and nutrients in a one-stage dual internal circulation airlift A2O bioreactor from milk processing industrial wastewater: Process optimization, sludge characteristics and operating cost evaluation

In this work, a one-stage dual internal circulation airlift anaerobic/anoxic/aerobic (DCAL-A2O) bioreactor was continuously operated for concurrent removal of nutrients and organics from milk processing wastewater (MPW). Special configuration of the airlift A2O bioreactor created possibility of the formation of desired anaerobic, anoxic and aerobic zones in a single unit. The process functionality of the bioreactor was examined under three influential operating variables i.e. hydraulic retention time (HRT; 7-15 h), air flow rate (AFR; 1-3 L/min) and aerobic volume ratio (AVR; 0.324-0.464). The optimum region was identified at HRT of 13h, AFR of 2L/min and AVR of 0.437, leading to TCOD, TN and TP removal efficiency of 94.5 %, 59.6 %, and 62.2 %, respectively, and effluent turbidity of 8 NTU. The impact of feed biodegradability on the process performance of the bioreactor treating the MPW, soft drink wastewater (SDW) and soybean oil plant wastewater (SOW) was also assessed. From the results, the feed characteristics affected significantly the nutrients removal. Moreover, the feeding location played an effective role in the nutrient removal while treating the MPW at optimum operating conditions. In this study, the change in residual organic matters as soluble microbial products (SMP) was monitored at various operating conditions. In addition, the impact of SMP extracted from sludge, extracellular polymeric substances (EPS) comprising of loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) was analyzed on sludge characteristics as bio-flocculation and settleability properties. According to the obtained data, the increase in operating variables led to the reduction in contents of effluent SMP, sludge SMP, LB-EPS, turbidity, and SVI, thereby, the enhancement in the sludge characteristics. Meanwhile, analysis of microbial communities verified the presence of various functional bacterial species. The cost operating evaluation confirmed the cost effectiveness of the airlift A2O bioreactor in reduction of energy consumption for the MPW treatment.

Kinetic and elemental characterization of HAP-based high-rate partial nitritation/anammox system orienting stability and inorganic elemental requirements

Anammox-based processes are attractive for biological nitrogen removal, and the combination of anammox and hydroxyapatite (HAP) is promising for the simultaneous removal of nitrogen and phosphorus from wastewater. However, the kinetics of one-stage partial nitritation/anammox (PNA) in which ammonia-oxidizing bacteria (AOB) and anammox bacteria (AnAOB) exist in a reactor are poorly understood. Moreover, inorganic elements are required to promote microbial cell synthesis and growth; therefore, monitoring of elements to prevent the limitation and inhibition of the process is critical. The minimum amounts of inorganic elements required for a one-stage PNA process and the elemental flow remain unknown. Therefore, in this study, kinetics, stoichiometry, and element flow in the long-term, high-rate, continuous, one-stage HAP-PNA process with microaerobic granular sludge at 25 °C were determined using process modeling, parameter estimation, and mass balance. The biomass elemental composition was determined to be CH2.2O0.89N0.18S0.0091, and the biomass yield (Yobs) was calculated to be 0.0805 g/g NH4+-N. Therefore, a stoichiometric reaction equation for the one-stage HAP-PNA system was also proposed. The maximum specific growth rate (μm) of AnAOB and AOB were 0.0360 and 0.0982 d-1 with doubling times of 19 and 7.1 d, respectively. Finally, the elemental requirements for stable and high-rate performance were determined using element flow analysis. These findings are essential for developing the anammox-based process in a stable and resource-efficient manner and determining engineering applicability.

Mechanism of gel immobilization driving efficient anammox in mainstream partial nitritation/anammox process: Structural characterization and multi-perspective microbial analysis

Here, the mechanism of encapsulated anammox bacteria (AnAOB) driving efficient nitrogen removal in the mainstream partial nitritation/anammox process is revealed. The results show that a high nitrogen removal rate (1.21±0.02 kgN·(m3·d)-1) was achieved due to the abundant micropore structure inside the anammox immobilized filler, ensuring good connectivity, and a stable aggregation capacity, reducing dependence on extracellular polymeric substances. AnAOB were uniformly distributed throughout all regions of the immobilized filler, and their abundance was higher than that of the control anammox granular sludge (AnGS). Conversely, cracks appeared on the surface of the AnGS, and hollows formed inside. The metagenome analysis revealed that the immobilized filler supported the coexistence of multiple AnAOB, and the appropriate niche enhanced coordination between the AnAOB and dominant companion microorganisms. In contrast, AnGS exhibited stronger NH4+-N and NO2--N loops, potentially reducing the total nitrogen removal efficiency. This study promotes the mainstream application of anammox.

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.

Formation of autotrophic nitrogen removal granular sludge driven by the dual-partition airlift internal circulation: Insights from performance assessment, community succession, and metabolic mechanism

Granular sludge has been recognized as an effective method for the application and industrialization of the anammox-based process due to its good biomass retention capacity and environmental tolerance. In this study, a one-stage autotrophic nitrogen removal (ANR) dual-partition system with airlift internal circulation was implemented for 320 days. A high nitrogen removal efficiency of 84.6% was obtained, while the nitrogen removal rate reached 1.28 g-N/L/d. ANR granular sludge dominated by Nitrosomonas and Candidatus Brocadia was successfully cultivated. Results showed that activity and abundance of functional flora first increased with granulation process, but eventually declined slightly when particle size exceeded the optimal range. Total anammox activity was observed to be significantly correlated with protein content (R2 = 0.9623) and nitrogen removal performance (R2 = 0.8796). Correlation network revealed that AnAOB had complex interactions with other bacteria, both synergy for nitrogen removal and competition for substrate. Changes in abundances of genes encoding the Carbohydrate Metabolism, Energy Metabolism, and Membrane Transport suggested energy production and material transfer were possibly blocked with further sludge granulation. Formation of ANR granular sludge promoted the interactions and metabolism of functional microorganisms, and the complex nitrogen metabolic pathways improved the performance stability. These results validated the feasibility of granule formation in the airlift dual-partition system and revealed the response of the ANR system to sludge granulation.

Iron cycle-enhanced anaerobic ammonium oxidation in microaerobic granular sludge

Granule-based partial nitritation and anaerobic ammonium oxidation (PN/A) is an energy-efficient approach for treating ammonia wastewater. When treating low-strength ammonia wastewater, the stable synergy between PN and anammox is however difficult to establish due to unstable dissolved oxygen control. Here, we proposed, the PN/A granular sludge formed by a micro-oxygen-driven iron redox cycle with continuous aeration (0.42 ± 0.10 mg-O2/L) as a novel strategy to achieve stable and efficient nitrogen (N) removal. 240-day bioreactor operation showed that the iron-involved reactor had 37 % higher N removal efficiency than the iron-free reactor. Due to the formation of the microaerobic granular sludge (MGS), the bio(chemistry)-driven iron cycle could be formed with the support of anaerobic ammonium oxidation coupled to Fe3+ reduction. Both ammonia-oxidizing bacteria and generated Fe2+ could scavenge the oxygen as a defensive shield for oxygen-sensitive anammox bacteria in the MGS. Moreover, the iron minerals derived from iron oxidation and Fe-P precipitates were also deposited on the MGS surface and/or embedded in the internal channels, thus reducing the size of the channels that could limit oxygen mass transfer inside the MGS. The spatiotemporal assembly of diverse functional microorganisms in the MGS for the realization of stable PN/A could be achieved with the support of the iron redox cycle. In contrast, the iron-free MGS could not optimize oxygen mass transfer, which led to an unstable and inefficient PN/A. This work provides an alternative iron-related autotrophic N removal for low-strength ammonia wastewater.

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

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

Nitrogen removal efficiency and mechanisms of an improved anaerobic-anoxic-oxic system for decentralized sewage treatment

The unstable operation and poor effluent quality often associated with decentralized sewage treatment systems due to fluctuating water flows have garnered significant attention. In this study, a novel integrated process combining anoxic denitrification and simultaneous nitrification and denitrification was developed to address these challenges. The improved anaerobic-anoxic-aerobic system achieved average effluent concentrations of 20.83 mg/L and 4.63 mg/L for chemical oxygen demand and NH4+-N, with average removal rates of 91 % and 68 %, respectively. Moreover, the aerobic zone demonstrated an impressive efficiency of 40.8 % for simultaneous nitrification and denitrification. The key bacteria groups driving the system's performance were heterotrophic and aerobic nitrifying bacteria, which dominated the microbial populations. Overall, the system optimizes the traditional anaerobic-anoxic-aerobic process, providing an effective solution for fluctuating wastewater flows. It establishes a successful coexistence model for multiple microbial populations, highlighting its applicability for superior nitrogen removal performance, and reference for optimizing rural sewage treatment. TAKE HOME MESSAGE SENTENCE: The improved anaerobic-anoxic-aerobic system for fluctuating wastewater treatment has superior nitrogen removal performance depending on multiple microbial populations.

Rapid formation of denitrification granules for nitrite accumulation by increasing nitrogen loading rates and resistance to industrial wastewater

The nitrite (NO2-) accumulation in partial denitrification (PD) offers the possibility of widespread application of anammox process. In this study, the rapid establishment of PD granular system was achieved by increasing nitrogen loading rates (NLR) from 0.9 to 4.8 kg N/(m3·d), with the nitrate-to-nitrite transforming ratio (NTR) increasing rapidly to 87.0 % within 18 days. Growth evidence indicated that the functional genus Thauera was significantly enriched (12.5 %→76.4 %), with nitrate (NO3-) reduction rates (SNO3) improving by 5.4 times from 13.0 to 70.7 mg N/(g VSS·h). Importantly, the rapid aggregation of PD biomass as granules ensured robustness and resistance of PD feeding with the electroplating tail wastewater (NO3--N of 103.0 ± 5.0 mg/L), obtaining stable NTR above 91.5 %. This study demonstrated the achievability of the fast development of PD granules and the adaptability and robustness of treating nitrate-containing industrial wastewater, which provided a promising method for efficient nitrogen transformation in industrial applications.

Spatial difference in nitrogen removal pathways and microbial functional diversity in an EGSB reactor during the start-up of PD/Anammox

The start-up of a relatively high nitrogen load PD/Anammox in an EGSB reactor was achieved through strategies of bioaugmentation, mass transfer enhancement, and COD/NO3--N control, with NRR of 5.2 g N/L/d. Longitudinal heterogeneity in EGSB reactor induced divergent nitrogen conversion pathways and enriched different functional microorganisms between stratified sludge. Along the elevation of the reactor, the proportion of removed nitrogen through anammox increased continuously from bottom, middle and up, which were 65.0 %, 79.8 %, and 84.1 %, respectively, consistent with the trend of ex-situ activities calculated with Gompertz model. The bottom zone played a role in mixed nitrogen conversion to provide NO2--N accumulation and nitrogen removal, with higher abundance of Thauera, Denitratisoma and Ignavibacterium. The middle part was enriched Candidatus_Kuenenia (12.51 %), and up inhibited completed denitrification, together forming the anammox dominant zone. The proposed functional zones in the EGSB reactor provided approaches for the optimisation of high-load PD/Anammox systems.

Effect and microbial mechanism of pharmaceutical and personal care product exposure on partial nitrification process and nitrous oxide emission

This study focused on the short- and long-term exposure of pharmaceutical and personal care products (PPCPs) to the partial nitrification process and nitrous oxide emission. The corresponding microbial mechanisms were also explored. The results revealed a concentration-dose effect on the partial nitrification process. Moreover, the PPCP concentration of ≥2 μg/L featured inhibitory effects on the process. The solo effect of PPCP on the partial nitrification process was analyzed through microcosmic experiments, and the results revealed significant variations in PN. A dose-effect relationship existed between the PPCP concentration and N2O emission intensity. After exposure to PPCPs, the N2O emission released during the partial nitrification process was significantly reduced. Different PPCPs featured various effects in mitigating N2O emissions. Low PPCP concentrations led to a reduction in the richness and diversity of microbes, but their community structure remained significantly unchanged. High PPCP concentrations (≥5 μg/L) resulted in increased species richness and diversity, but their microbial community composition was significantly affected. The function prediction and nitrogen metabolic pathway analysis indicated that PPCP exposure led to the inhibition of the ammonia oxidation process. However, all genes encoding denitrification enzymes were upregulated. The microorganisms in the microbial community featured modular structural properties and wide synergistic relationships between genera. This study provides valuable insights into the effect of PPCP exposure on the particle nitrification process and corresponding changes in the microbial community.

Extracellular polymeric substances and microbial community shift during the start-up of a single-stage partial nitritation/anammox process

A sequencing batch reactor (SBR) was operated to investigate variations of extracellular polymeric substances (EPS) and microbial community during the start-up of the single-stage partial nitritation/anammox (SPN/A) process at intermittent aeration mode. The SPN/A system was successfully started on day 34, and the nitrogen removal efficiency and total nitrogen loading rate were 82.29% and 0.31 kg N/(m3 ·day), respectively. Furthermore, the relationship between the protein secondary structures and microbial aggregation was strongly related. The α-helix/ (β-sheet + random coil) ratios increased obviously from 0.20 ± 0.03 to 0.23 ± 0.01, with the sludge aggregation mean size increased from 56 to 107 μm during the start-up of SPN/A. During the start-up of SPN/A, Candidatus Kuenenia was the primary anammox bacteria, whereas Nitrospira was the main functional bacteria of nitrite-oxidizing bacteria. Correlation between the microbial community and EPS components was performed. The EPS and microbial community played important roles in keeping stable nitrogen removal and the formation of sludge granules. PRACTITIONER POINTS: Intermittent aeration strategy promoted SPN/A system start-up. EPS composition and protein secondary structure were related with the sludge disintegration and aggregation. Microbial community shift existed and promoted the stability of sludge and reactor performance during SPN/A start-up.

Anammox-Mediated Hydroxyapatite Granules: Physicochemical Properties, 3D Hierarchy, and Biofilm Thickness

Biomineralization inspired the development of simultaneous biological transformations and chemical precipitation for simultaneous nitrogen removal and phosphorus recovery from wastewater, which could compensate for the incapacity of phosphorus management in the new biological route of anaerobic ammonium oxidation (anammox). In this study, we strengthened anammox-mediated biomineralization by long-term feeding of concentrated N, P, and Ca substrates, and a self-assembled matrix of anammox bacteria and hydroxyapatite (HAP) was fabricated in a granular shape, defined as HAP-anammox granules. HAP was identified as the dominant mineral using elemental analysis, X-ray diffraction, and Raman spectroscopy. The intensive precipitation of HAP resulted in a higher inorganic fraction and substantially improved settleability of anammox biomass, which facilitated HAP precipitation by acting as nucleation and metabolically elevated pH. By using X-ray microcomputed tomography, we visually represented the hybrid texture of interwoven HAP pellets and biomass, the core-shell layered architecture of different-sized HAP-anammox granules, and their homogeneously regulated thickness of the outer biofilm (from 118 to 635 μm). This unique architecture endows HAP-anammox granules with outstanding settleability, active biofilm, and tightly bonded biofilm with the carrier, which may explain the excellent performance of these HAP-anammox granules under various challenging operational conditions in previous studies.

Improved Properties and Enhancement Strategies of Hydroxyapatite-Based Functional Granular Sludge for a High-Rate Partial Nitritation/Anammox System

Retaining sufficient anammox bacteria (AnAOB) while keeping the anammox-based process stable is the focus of the study of anammox technology, especially in a one-stage partial nitritation/anammox (PNA) process. The use of hydroxyapatite (HAP) granules in an anammox-based process is innovative for its potential to improve the nitrogen removal rate and achieve simultaneous removal of phosphorus. In this study, the HAP-based granular sludge was employed using enhancement strategies for an excellent nitrogen removal performance in a one-stage PNA process. Compared to those of other granular sludge PNA systems, a remarkable sludge volume index of 7.8 mL/g and an extremely high mixed liquor volatile suspended solids of 15 g/L were achieved under a low hydraulic retention time of 2 h. Consequently, an unprecedented nitrogen removal rate as high as 4.8 kg N/m3/d at 25 °C was obtained under a nitrogen loading rate of 6 kg N/m3/d. After a long-term operation of 870 days, the enhancement strategies underlying the superior performance of the granular sludge were identified. These findings clearly demonstrate that the enhancement strategies are crucial for the superior operating performance of the PNA process, and they can promote the application of the anammox-based process.

Towards advanced simultaneous nitrogen removal and phosphorus recovery from digestion effluent based on anammox-hydroxyapatite (HAP) process: Focusing on a solution perspective

In this paper, the state-of-the-art information on the anammox-HAP process is summarized. The mechanism of this process is systematically expounded, the enhancement of anammox retention by HAP precipitation and the upgrade of phosphorus recovery by anammox process are clarified. However, this process still faces several challenges, especially how to deal with the ∼ 11% nitrogen residues and to purify the recovered HAP. For the first time, an anaerobic fermentation (AF) combined with partial denitrification (PD) and anammox-HAP (AF-PD-Anammox-HAP) process is proposed to overcome the challenges. By AF of the organic impurities of the anammox-HAP granular sludge, organic acid is produced to be used as carbon source for PD to remove the nitrogen residues. Simultaneously, pH of the solution drops, which promotes the dissolution of some inorganic purities such as CaCO₃. In this way, not only the inorganic impurities are removed, but the inorganic carbon is supplied for anammox bacteria.

Double-edged sword effects of dissimilatory nitrate reduction to ammonium (DNRA) bacteria on anammox bacteria performance in an MBR reactor

Dissimilatory nitrate reduction to ammonium (DNRA) bacteria imposing double-edged sword effects on anammox bacteria were investigated in an anammox-membrane bioreactor (MBR) experiencing an induced crash-recovery event. During the experiment, the anammox-MBR was loaded with NH₄⁺-N:NO₂^--N ratios (RatioNH₄⁺-N: NO₂^--N) of 1.20-1.60. Initially, the anammox-MBR removed over 95% of 100 mg/L NH₄⁺-N and 132 mg/L NO₂^--N (RatioNH₄⁺-N: NO₂^--N = 0.76, the well accepted stoichiometric RatioNH₄⁺-N: NO₂^--N for anammox) in the influent (Stage 0). Then, we induced a system crash-recovery event via nitrite shock loadings to better understand responses from different guilds of bacteria in anammox-MBR, loaded with 1.60 RatioNH₄⁺-N: NO₂^--N with 100 mg/L NO₂^--N in the influent (Stage 1). Interestingly, the nitrogen removal by anammox bacteria was maintained for about 20 days before starting to decrease significantly. In Stage 2, we further increased influent nitrite concentration to 120 mg/L (1.33 RatioNH₄⁺-N: NO₂^--N) to simulate a high nitrite toxicity scenario for a short period of time. As expected, nitrogen removal efficiency dropped to only 16.8%. After the induced system crash, anammox-MBR performance recovered steadily to 93.2% nitrogen removal with a 1.25 RatioNH₄⁺-N:NO₂^--N and a low nitrite influent concentration of 80 mg/L NO₂^--N. Metagenomics analysis revealed that a probable causality of the decreasing nitrogen removal efficiency in Stage 1 was the overgrowth of DNRA-capable bacteria. The results showed that the members within the Ignavibacteriales order (21.7%) out competed anammox bacteria (17.0%) in the anammox-MBR with elevated nitrite concentrations in the effluent. High NO₂^--N loading (120 mg N/L) further caused the predominant Candidatus Kuenenia spp. were replaced by Candidatus Brocadia spp. Therefore, it was evident that DNRA bacteria posed negative effects on anammox with 1.60 RatioNH₄⁺-N: NO₂^--N. Also, when 120 mg/L NO₂^--N fed to anammox-MBR (RatioNH₄⁺-N: NO₂^--N = 1.33), canonical denitrification became the primary nitrogen sink with both DNRA and anammox activities decreased. They probably fed on lysed microbial cells of anammox and DNRA. In Stage 3, a low RatioNH₄⁺-N: NO₂^--N (1.25) with 80 mg/L NO₂^--N was used to rescue the system, which effectively promoted DNRA-capable bacteria growth. Although anammox bacteria's abundance was only 7.7% during this stage, they could be responsible for about 90% of the total nitrogen removal during this stage.

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

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

Resilience of anammox application from sidestream to mainstream: A combined system coupling denitrification, partial nitritation and partial denitrification with anammox

Anaerobic ammonium oxidation (anammox) is a potential process to achieve the neutralization of energy and carbon. Due to the low temperature and variation of municipal sewage, the application of mainstream anammox is hard to be implemented. For spreading mainstream anammox in practice, several key issues and bottlenecks including the start-up, stable NO₂^--N supply, maintenance and dominance of AnAOB with high activity, prevention of NO₃^--N buildup, reduction of sludge loss, adaption to the seasonal temperature and alleviation of COD impacts on AnAOB are discussed and summarized in this review in order to improve its startup, stable operation and resilience of mainstream anammox. Hence a combined biological nitrogen removal (CBNR) system based on conventional denitrification, shortcut nitrification-denitrification, Partial Nitritation and partial Denitrification combined Anammox (PANDA) process through the management of organic matter and nitrate is proposed correspondingly aiming at adaptation to the variations of seasonal temperature and pollutants in influent.

The phosphorus harvest from low-temperature mainstream wastewater through iron phosphate crystallization in a pilot-scale partial nitritation/anammox reactor

The phosphorus harvest along nitrogen removal in the partial nitritation/anammox (PNA) reactor is promising for saving space and simplifying the management of mainstream wastewater treatment facilities. In this study, the phosphorus recovery from the low-temperature mainstream wastewater was explored through iron phosphate crystallization in a pilot-scale PNA reactor. With the COD-alleviated municipal wastewater as the influent, the ammonium concentration of about 50 mg/L and the phosphorus concentration ranged from 5.4 to 7.1 mg/L, under the temperature of 15 °C and the addition of external ferrous iron of 14 mg/L, the achieved nitrogen removal efficiency and the phosphorus removal efficiency were 37.6 % and 62.7 %, respectively. The good settleability of sludge indicated that the formed iron phosphate was well combined with the biomass. The quantitative analysis confirmed that the main iron phosphate in dry sludge was graftonite, and qualitative analysis confirmed that the equivalent of P₂O₅ content in the sludge was 5.8 %, which was suitable as fertilizer on agricultural land to realize the direct recycle of discharged phosphorus. In all, this study proposed a pioneering scheme to realize the nitrogen removal and phosphorus cycle in human society and given a meaningful reference for further research and application.

Start-up and optimization of a one-stage partial nitrification-anammox (PN-A) process treating low ammonium concentration wastewater: experimental results and modeling investigation

Although the partial nitrification-anammox process (PN-A) has achieved great success in nitrogen removal for the high ammonium concentration wastewater, its application is still limited in low ammonium concentration wastewater treatment due to its instability and low nitrogen removal efficiency. In this study, a sequencing batch reactor (SBR) with continuous aeration was employed to enrich ammonia oxidation bacteria (AOB) and suppress nitrite oxidation bacteria (NOB) first; then, the SBR was operated intermittently aerated SBR (IASBR), to which the anammox granular sludge (AMX) was added to achieve complete autotrophic nitrogen removal under low influent ammonium concentration of 100 mg/L. A mathematical model was used to optimize the IASBR aeration strategy to achieve sub-optimal nitrogen removal. The experimental results showed that high nitrite accumulation efficiency (above 80%) in the SBR and a fast start-up within 100 days and a stable TN (total nitrogen) removal efficiency of 70% were achieved in the IASBR. Meanwhile, the simulation results indicated that keeping aeration duration at 4 h, k_La (oxygen transfer coefficient) at 50 day^-1, or aeration duration at 2.5 h, k_La at 80 day^-1 could obtain a higher total nitrogen removal efficiency (TNR) (TNR > 80%), and the TN removal could also be improved by increasing hydraulic retention time (HRT) under the optimal oxygen supply rate.

A comparison of nitrogen removal systems through cost-coupled life cycle assessment and energy efficiency analysis

The global water crisis reflects the necessity of exploring the best approaches for the water supply. Therefore, for the first time, the current study compares nitrogen removal systems (NRSs) from life cycle assessment (LCA), economic, kinetic, thermodynamic, and synergistic perspectives. The assessed systems were sequential batch reactor (SBR), oxic/anoxic (OA), and oxic/anaerobic/oxic (OAO) bioreactors. Among all, the SBR configuration showed the best efficiency (98.74 %) for nitrogen removal. The environmental impacts notably presented by marine + freshwater ecotoxicity (53.76 %), and climate change categories (16.39 %), significantly because of metal emissions. Non-renewable sources supplied 95 % of total energy demand. The operation of NRSs showed the most impact on human health (63.67 %) through CH₄ and CO₂ emissions. The total costs significantly belonged to the construction (<86.37 %) > amortization> operation. The influent COD illustrated the most role in environmental burdens (16.44 %) based on the sensitivity analysis. The removal reaction was endothermic, physical, non-spontaneous, and followed a pseudo-second-order kinetic model (R² > 0.98). The chemical exergy provided the major portion of the total calculated exergy (83 %). The exergetic efficiency of the system was 69 %, which was predominantly supplied by biogas (∼50.75 %). Accordingly, this study can present a stepwise guideline for further related investigations.

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

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

Anammox-based granulation cycle for sustainable granular sludge biotechnology from mechanisms to strategies: A critical review

Anaerobic ammonium oxidation (anammox) granular sludge is a promising biotechnological process for treating low-carbon nitrogenous wastewater, and is featured with low energy consumption and footprint. Previous theoretical and experimental research on anammox granular sludge processes mainly focused on granulation (flocs → granules), but pay little attention to the granulation cycle including granulation and regeneration. This work reviewed the previous studies from the perspective of anammox granules lifecycle and proposed various sustainable formation mechanisms of anammox granules. By reviewing the anaerobic, aerobic, and anammox granulation mechanisms, we summarize the mechanisms of thermodynamic theory, heterogeneous growth, extracellular polymeric substance (EPS)-based adhesion, quorum sensing (QS)-based regulation, biomineralization-based growth, and stratification of microorganisms to understand anammox granulation. In the regeneration process, the formation of precursors for re-granulation is explained by the mechanisms of physical crushing, quorum quenching and dispersion cue sensing. Based on the granulation cycle mechanism, the rebuilding of the normal regeneration process is considered essential to avoid granule floatation and the wash-out of granules. This comprehensive review indicates that future research on anammox granulation cycle should focus on the effects of filamentous bacteria in denitrification-anammox granulation cycle, the role of QS/ quorum quenching (QQ)-based autoinducers, development of diversified mechanisms to understand the cycle and the cycle mechanisms of stored granules.

Long-term performance, microbial evolution and spatial microstructural characteristics of anammox granules in an upflow blanket filter (UBF) treating high-strength nitrogen wastewater

Granule formation, microstructure and microbial spatial distribution are crucial to granule stability and nitrogen removal. Here, an upflow blanket filter (UBF) reactor with porous fixed cylinder carriers was fabricated and operated for 234 days to investigate overall performance and the formation mechanism of anammox granules. Results showed that the UBF performed the highest nitrogen removal efficiency of 93.19 ± 3.39% under nitrogen loading rate of 3.6 kg-N/m³/d and HRT of 2 h. The tryptophan-like proteins as the key component in EPS were vital for granules formation. Further 16 s rRNA analysis indicated that SBR1031 with a relative abundance of 40.5% played an important role in cell aggregation. Thus, anammox granules were developed successfully with a two-layered spatial structure where outer-layer was ammonia oxidizing bacteria and inner-core was anaerobic ammonia oxidizing bacteria. Together, introduction of porous fixed cylinder carriers is a valid method to avoid biomass loss and floatation.

Material mass balance and elemental flow analysis in a submerged anaerobic membrane bioreactor for municipal wastewater treatment towards low-carbon operation and resource recovery

The anaerobic membrane bioreactor (AnMBR) has gained huge attention as a municipal wastewater (MWW) treatment process that combined high organics removal, a low sludge yield and bioenergy recovery. In this study, a 20 L AnMBR was set up and operated steadily for 70 days in temperate conditions with an HRT of 6 h and a flux of 12 LMH for the treatment of real MWW, focusing on the behavior of the major elements (C, N, P and S) from an elemental balance perspective. The results showed that the AnMBR achieved more than 85 % COD removal, a low sludge yield (0.081 gVSS/gCOD_removed) and high methane production (0.31 L-CH₄/gCOD_removed) close to the theoretical value. The elemental flow analysis revealed that the AnMBR converted 77 % of the influent COD to methane (57 % gaseous and 20 % dissolved) and 6 % of the COD for sludge production. In addition, the AnMBR converted 34 % of the total carbon to energy-generated carbon, and only 3 % was in the form of CO₂ in the biogas for further upgradation, which was in line with the concept of carbon neutrality. Since little nitrogen or phosphorus were removed, the permeate was nutrient-rich and further treatment to recover the nutrients would be required. This study illustrates the superior performance of the AnMBR for MWW treatment with a microscopic view of elemental behavior and provides a reference for implementing the mainstream AnMBR process in carbon-neutral wastewater treatment plants.

Enhancing nitrogen removal through directly integrating anammox into mainstream wastewater treatment: Advantageous, issues and future study

Anaerobic ammonium oxidation (anammox) has great potential to be applied to the process of nitrogen removal from mainstream wastewater. However, directly applying complete anammox to the mainstream is typically hindered by low temperatures, a low ammonia concentration, and high organic matter concentrations. Directly integrating anammox into mainstream treatment by enhancing the in-situ enrichment of anammox bacteria in wastewater treatment plants (WWTPs) could effectively improve the nitrogen removal efficiency and reduce the treatment cost. A certain anammox bacteria abundance in full-scale WWTPs provides the feasibility of directly integrating anammox into mainstream treatment and realizing partial mainstream anammox. The technical development status of partial anammox and the mechanisms of achieving partial mainstream anammox by aeration and organic control are summarized. This review provides an enhanced understanding of this novel technical route of partial mainstream anammox treatment for improving the quality, performance, and prospects for this technology to be used in upgrading WWTPs.

Rapidly recovering and maintaining simultaneous partial nitrification, denitrification and anammox process through hydroxylamine addition to advance nitrogen removal from domestic sewage

The collapse of simultaneous partial nitrification, denitrification and anammox (SPNDA) system, caused by the destruction of partial nitrification (PN), is the most likely phenomenon to occur. Therefore, recovering the process quickly and maintaining efficient nitrogen removal is a valuable topic for research. In the anaerobic/aerobic/anoxic operation mode, SPNDA process was used to treat domestic sewage in a sequencing batch biofilm reactor. After the deterioration of PN effect, with the addition of hydroxylamine, the activity of ammonia-oxidizing bacteria in the nitrobacteria increased (61.0-91.3 %), whereas the accumulation of nitrite quickly recovered to 90.4 % within 5 days. Meanwhile, the nitrogen removal efficiency improved (61.8-95.6 %) and the effluent TN was 2.1 mg/L. Furthermore, Candidatus Brocadia was enriched (0.50-1.82 %) in the system. The results indicated that the addition of hydroxylamine was an effective strategy to recover and economically maintain the SPNDA process for advanced nitrogen removal from domestic sewage.

A newly isolated microalga Chlamydomonas sp. YC to efficiently remove ammonium nitrogen of rare earth elements wastewater

The aim of this study was to establish a practical approach to remove ammonium nitrogen of rare earth elements (REEs) wastewater by an indigenous photoautotrophic microalga. Firstly, a new microalgal strain was successfully isolated from REEs wastewater and identified as Chlamydomonas sp. (named Chlamydomonas sp. YC). The obtained results showed that microalga could completely remove the NH₄⁺-N of 10% REEs wastewater after 10 days of cultivation; however, the highest NH₄⁺-N removal rate was attained by microalga to treat undiluted REEs wastewater. Then, three cultivation modes including batch, semi-continuous and continuous cultivation methods were developed to evaluate the ability of NH₄⁺-N removal rate by this microalga to treat diluted (10%) and undiluted REEs wastewater. It was found that, Chlamydomonas sp. YC exhibited superior performance towards NH₄⁺-N removal rates (32.75-61.05 mg/(L·d)) by semi-continuous and continuous processes for the treatments of 10% and undiluted REEs wastewater in comparison to the results (19.50-30.38 mg/(L·d) by batch process. Interestingly, under the same treatment conditions, among the three cultivation modes, microalga exhibited the highest removal rates of NH₄⁺-N in undiluted REEs wastewater by semi-continuous (61.05 mg/(L·d)) and continuous (57.10 mg/(L·d) processes. In term of the biochemical analysis, microalgal biomass obtained from the wastewater treatment had 35.40-44.40% carbohydrate and 4.97-6.03% lipid, which could be potential ingredients for sustainable biofuels production. And the highest carbohydrate and lipid productivities attained by Chlamydomonas sp. YC in the continuous mode were 226.36 mg/(L·d) and 32.98 mg/(L·d), respectively. Taken together, the established processes mediated with Chlamydomonas sp. YC via continuous cultivation was the great promising approaches to efficiently remove NH₄⁺-N of REEs wastewater and produce valuable biomass for sustainable and renewable biofuels in a simultaneous manner.

Advanced Research on Polymer Floating Carrier Application in Activated Sludge Reactors

This research estimates the efficiency of domestic wastewater treatment in the removal of organic pollutants and nitrogen compounds with a two-stage treatment sequence (an activated sludge reactor in the first stage, and a trickling filter in the second stage), and with the application of floating carriers in the activated sludge reactor. The materials “Polyvom”, “Polystyrene” and “Bioballs” were adopted as floating carriers with previously determined filling ratios in the reactor volume of 10%, 20% and 20%, respectively. After the first stage of the study, it was found that the most effective treatment was achieved using the “Polyvom” material. Therefore, only this floating carrier was considered in the second and third stages of the study. Within the stages of the research, lab-scale benches operated under different operation modes of the treatment sequence. At the end of the study, it was possible to achieve the following levels of purification: BOD₅ (2.1 mg/L), NH₄ (0.4 mg/L), NO₂ (1.0 mg/L), and NO₃ (25 mg/L). The mean values of the concentrations of BOD, NH₄, and NO₃ met the requirements, but the concentration of NO₂ exceeded the requirements (1.0 mg/L vs. 0.08 mg/L). These results were achieved under a hydraulic retention time in the activated sludge reactor of 8 h, and the MLSS for the free-floating and immobilized activated sludge was 0.2 and 0.9 g/L, respectively.

Biofilm growth characterization and treatment performance in a single stage partial nitritation/anammox process with a biofilm carrier

Biofilm carriers can avoid microorganism washout while maintaining a high amount of biomass, but are also associated with a long biofilm formation period and biofilm aging. A single stage partial nitritation/anammox process (single stage PN/A) reactor was setup to study the biofilm growth characterization and treatment performance under an NLR of 0.53 to 0.90 gN/L/d over one year. Biofilm growth was divided into three stages: the formation stage, maturation stage and aging stage. The initial biofilm was observed at day 84. A nitrogen removal efficiency of 83.4% was achieved at an NLR of 0.90 gN/L/d during the mature biofilm stage. Starvation, nitrogen gas accumulation and hydroxyapatite formation resulted in biofilm aging. After mechanical stirring treatment, biofilm reactivation was achieved by biofilm re-formation within one month. There is clear potential for phosphorus recovery, as indicated by the 5.24% - 6.29% phosphorus content in the biofilm (similar to the 5%-7% phosphorus content in enhanced biological phosphate removal sludge). The AnAOB genera abundance in the biofilm maintained at a high level of 18.25%-32.31%, while the abundance of AnAOB increased from the initial 4.10% to 13.78% after mechanical stirring treatment in the suspended sludge ensured biofilm reactivation. The results of this study clearly show that mechanical stirring treatment can be used to achieve the biofilm reactivation as the biofilm fills with the hollow cylindrical carrier. This study has potential as a useful reference for the realization of the wide application of the biofilm single stage PN/A process in the future.

Treating the filtrate of mainstream anaerobic membrane bioreactor with the pilot-scale sludge-type one-stage partial nitritation/anammox process operated from 25 to 15 °C

At ambient temperature condition, the one-stage partial nitritation/anammox (PNA) process has been successfully adopted to treat the filtrate from the mainstream anaerobic membrane bioreactor (AnMBR). However, there is no investigation of the performance of this process at low-temperature condition. In this study, the nitrogen removal performance of a pilot-scale PNA reactor at the temperature of 15 °C for treating the filtrate of a mainstream AnMBR was investigated. The nitrogen removal rate of 0.09 kg/m³/d and the nitrogen removal efficiency of 37.6% were achieved. The anammox reaction was the rate-limiting step of the nitrogen removal. Nitrogen removal was attributed in part to denitrification activity. The microbial community analysis confirmed that the main functional bacteria comprised of genus Nitrosomonas and genus Kuenenia. In sum, this research demonstrated the applicability of PNA process for mainstream AnMBR filtrate treatment to some extent and enriched the related knowledge.

Multidisciplinary characterization of nitrogen-removal granular sludge: A review of advances and technologies

Nitrogen-removal granular sludge (NRGS) is a promising technology in wastewater treatment, with advantages of efficient nitrogen removal, less footprint, lower sludge production and energy consumption, and is a way for wastewater treatment plants to achieve carbon-neutrality. Aerobic granular sludge (AGS) and anammox granular sludge (AnGS) are two typical NRGS technologies that have attracted extensive attention. Mounting evidence has shown strong associations between NRGS properties and the status of NRGS systems; however, a holistic view is still missing. The aim of this article is to provide an overview of NRGS with an emphasis on characterization. Specifically, the integrated nitrogen transformation pathways inside NRGS and the performance of NRGS treating various wastewaters are discussed. NRGS properties are categorized as physical-, chemical-, biological- and systematical ones, presenting current advances and corresponding characterization technologies. Finally, the future prospects for furthering the mechanistic understanding and engineering application of NRGS are proposed. Overall, the technological advancements in characterization have greatly contributed to understanding NRGS properties, which are potential factors for optimizing the performance and evaluating the working status of NRGS. This review will provide guidance in characterizing NRGS properties and boost the introduction of novel characterization technologies.

Seasonal temperatures impact on the mass flows in the innovative integrated process of anaerobic membrane bioreactor and one-stage partial nitritation-anammox for the treatment of municipal wastewater

A pilot-scale anaerobic membrane bioreactor (AnMBR) integrated with a one-stage partial nitritation-anammox (PN/A) reactor was operated for the treatment of municipal wastewater (MWW) at seasonal temperatures of 15-25 °C. The removal efficiencies of COD and total nitrogen (TN) were always > 90% and > 75% respectively. The methanogenesis and PN/A were identified as the primary removal pathways of COD and TN, respectively, and were suppressed at low temperatures. With the temperature dropped from 25 °C to 20 °C to 15 °C, the methane-accounted COD decreased from 63.1% to 59.6% to 48.4%, and the PN/A-accounted TN decreased from 58.1% to 51.7% to 45.3%. The AnMBR and PN/A mutually complement each other in this combined process, as the AnMBR removed 8.5%-16.1% of TN by sludge entrainment and the PN/A reactor removed 2.6%-3.4% of COD by denitrification and aerobic oxidation. These results highlighted the strong feasibility of applying the AnMBR-PN/A process to the treatment of MWW in temperate climate.

The startup of the partial nitritation/anammox-hydroxyapatite process based on reconciling biomass and mineral to form the novel granule sludge

The synchronous nitrogen elimination and phosphorus (P) recovery can be realized by the novel one-stage partial nitritation/anammox (PN/A)-hydroxyapatite (HAP) crystallization (PN/A-HAP) process, which seems promising in actual application. This research firstly conducted the startup of the PN/A-HAP process based on reconciling biomass and mineral to cultivate the novel sludge with the strategy of alternating enhancement of biomass accumulation and mineral formation. Within three months, the nitrogen removal rate of 1.1 kg/m³/d and the P removal efficiency of 54.2% were achieved. The biomass reached to 3.7 g/L and the average particle size of sludge granules was about 260 μm. The microbial analysis indicated that in sludge the ammonium-oxidizing bacteria (AOB) mainly belonged to the genus Nitrosomonas, and the anammox bacteria mainly the genus Kuenenia. The main mineral in sludge was identified as HAP. This startup strategy is guidable for the application of one-stage PN/A-HAP process in actual wastewater treatment.

Nitrogen removal by a Hydroxyapatite-enhanced Micro-granule type One-stage partial Nitritation/anammox process following anaerobic membrane bioreactor treating municipal wastewater

Nitrogen removal from wastewater by the partial nitritation/anammox (PNA) technology is promising from both economic and environmental perspectives. However, this technology has not been popularized in the mainstream because of low biomass retention and the growth of the nitrite oxidizing bacteria. In this study, a one-stage PNA process with hydroxyapatite (HAP)-enhanced granules was used to treat effluent from a mainstream anaerobic membrane bioreactor. The HAP-enhanced reactor allowed an enriched high biomass of 6.9 ± 0.2 g/L at a low hydraulic retention time of 2 h. A nitrogen removal efficiency of 80 ± 6.0 %, a nitrogen removal rate of 0.36 ± 0.05 kg/m³/d and a COD removal efficiency of 54 ± 15 % were achieved stably, leading to a low total nitrogen concentration of 8.5 ± 2.7 mg/L and a low COD concentration of 19.7 ± 5.9 mg/L in the effluent. Anammox bacteria of Candidatus Kuenenia stuttgartiensis and ammonium oxidizing bacteria of Nitrosomonas were found to be the two most predominant bacteria.

Insights into the synergy between functional microbes and dissolved oxygen partition in the single-stage partial nitritation-anammox granules system

The rapid start-up and stable operation of the single-stage partial nitritation-anammox (PNA) process remains a challenge in practical applications. An integrated investigation of nitrogen removal performance, sludge characteristics, activity and abundance, and microbial dynamics was implemented for 360 days via an airlift internal circulation reactor. During long-term operation, the reactor realized a stable dissolved oxygen (DO) partition and cultivated granular sludge. The nitrogen removal rate increased from 0.15 kg-N/m³/d to 1.24 kg-N/m³/d, and a high nitrogen removal efficiency of 82.6% was obtained. A stable DO partition further accelerated the bioreaction rates and enhanced the activity of functional microbes. The activities of ammonia oxidation and anammox reached 1.21 g-N/g-VSS/d and 1.43 g-N/g-VSS/d, respectively. Sludge granulation efficiently enriched the abundances of Candidatus Brocadia (7.4%) and Nitrosomonas (5.2%). These results demonstrated that efficient DO partition and stable culture of granular sludge could enhance the synergy of functional microbes for autotrophic nitrogen removal.

The first pilot-scale demonstration of the partial nitritation/anammox-hydroxyapatite process to treat the effluent of the anaerobic membrane bioreactor fed with the actual municipal wastewater

Recently, it was reported that for synthetic low-strength wastewater, the excellent nitrogen removal rate (NRR) accompanied with phosphorus removal could be achieved through the partial nitritation/anammox (PNA)-hydroxyapatite (HAP) process. Thus, this research further investigated the performance of the pilot-scale PNA-HAP process treating the effluent of an anaerobic membrane bioreactor (AnMBR) fed with the actual municipal wastewater. The results showed that with the hydraulic retention time of 4.0 h, the influent ammonium concentration ranging from 36.0 to 41.0 mg/L, and the BOD₅ ranging from 6.3 to 12.7 mg/L, the average NRR and the nitrogen removal efficiency was 0.13 kg/m³/d and 63.38%, respectively. The specific activity test of sludge confirmed that the PNA process was the main nitrogen metabolism pathway. The effluent nitrate and the BOD₅ were almost zero, indicating the existence of denitrification activity in reactor. Given that the oxygenation condition, the heterotrophic organic matter oxidization activity also occurred in reactor. The sludge analysis confirmed the phosphate formation in sludge. Thus, in the reactor, four kinds of biological activities and chemical crystallization occurred harmoniously in sludge. From the mixed liquid volatile suspended solid of 2.4 g/L and the low distribution range of granule size, it was obvious that the sludge had a high dispersity. Based on the well settling ability of sludge during the operation, it was inferred that there was a close bond between biomass and HAP in sludge, which was helpful to enhance the settleability of sludge granule. Besides, the phosphorus-containing sludge was suitable as the fertilizer. In all, this study demonstrated that the PNA-HAP process is an ideal alternative treating the effluent of the AnMBR process in the municipal wastewater treatment.

Distinct granulation pathways of anammox granular sludge under biofilm enhancement

The simultaneous partial nitrification and anammox (PN/A) granular sludge process in a plug-flow reactor has been difficult to achieve. This study provides a novel way to enhance granulation using biofilm detachment. In a plug-flow reactor, a fixed carrier was added to the activated sludge, and a PN/A biofilm gradually formed during the operation. Mature biofilm detachment appeared and caused the emergence of micro-granule. Then the fixed carriers were removed from the reactor, but the nitrogen removal rate (NRR) of the reactor was barely affected. This result suggests granular sludge is a feasible replacement for biofilm. Moreover, the particle size of the granule increased from 212 to 425 μm, and the NRR was 1.63 kg N/(m³·d), with a maximum nitrogen removal efficiency of 86.5%. Overall, this study implies that it is feasible to maintain granular sludge in a plug-flow PN/A reactor, and biofilm detachment significantly favors the granulation process.

Chemical oxygen demand and nitrogen transformation in a large pilot-scale plant with a combined submerged anaerobic membrane bioreactor and one-stage partial nitritation-anammox for treating mainstream wastewater at 25 °C

A novel municipal wastewater treatment process towards energy neutrality and reduced carbon emissions was established by combining a submerged anaerobic membrane bioreactor (SAnMBR) with a one-stage partial nitritation-anammox (PN/A), and was demonstrated at pilot-scale at 25 °C. The overall COD and BOD₅ removal efficiencies were 95.1% and 96.4%, respectively, with 20.3 mg L^-1 COD and 5.2 mg L^-1 BOD₅ remaining in the final effluent. The total nitrogen (TN) removal efficiency was 81.7%, resulting 7.3 mg L^-1 TN was discharged from the system. The biogas yield was 0.222 NL g^-1 COD _removed with a methane content range of 78-81%. Approximately 90% of influent COD was removed in the SAnMBR, and 70% of influent nitrogen was removed in the PN/A. The denitrification which occurred in the PN/A enhanced overall COD and nitrogen removal. The successful operation of this pilot-scale plant indicates the SAnMBR-PN/A process is suitable for treating real municipal wastewater.

High nitrogen removal performance of anaerobically treated fish processing wastewater by one-stage partial nitritation and anammox process with hydroxyapatite (HAP)-based syntrophic granules and granule structure

The one-stage partial nitritation and anammox process with the hydroxyapatite (HAP)-based syntrophic granules was studied for the ammonium nitrogen removal from the effluents of a self-agitated anaerobic baffled reactor treating the fish processing wastewater. When the ammonium in the influent was 1140 mg N·L^-1, a high nitrogen removal rate and nitrogen removal efficiency of 1.51 ± 0.10 kg N·m^-3·d^-1 and 88.2% were obtained, respectively. Anammox bacteria of Candidatus Kuenenia stuttgartiensis and ammonium oxidizing bacteria of Nitrosomonas were the two most predominant bacteria, while nitrite oxidizing bacteria activity was low and could be neglected during the treatment. The inorganic element properties of the sludge were analyzed by several methods to confirm the existence of HAP granules. Optical microscopic observation and scanning electron microscopy analysis revealed the structure of the granular sludge.This study supports the feasibility and potential of this process for high-efficiency nitrogen removal from fish processing wastewater.