Anaerobic ammonium oxidation (anammox) under realistic seasonal temperature variations: Characteristics of biogranules and process performance

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.

Insights into the carbon and nitrogen metabolism pathways in mixed-autotrophy/heterotrophy anammox consortia in response to temperature reduction

While the multi-coupled anammox system boasts a substantial research foundation, the specific characteristics of its synergistic metabolic response to decreased temperatures, particularly within the range of 13-15 °C, remained elusive. In this study, we delve into the intricate carbon and nitrogen metabolism pathways of mixed-autotrophy/heterotrophy anammox consortia under conditions of temperature reduction. Our macrogenomic analyses reveal a compelling phenomenon: the stimulation of functional genes responsible for complete denitrification, suggesting an enhancement of this process during temperature reduction. This adaptation likely contributes to maintaining system performance amidst environmental challenges. Further metabolic functional recombination analyses highlight a dramatic shift in microbial community composition, with denitrifying MAGs (metagenome-assembled genomes) experiencing a substantial increase in abundance (up to 200 times) compared to autotrophic MAGs. This proliferation underscores the strong stimulatory effect of temperature reduction on denitrifying species. Notably, autotrophic MAGs play a pivotal role in supporting the glycolytic processes of denitrifying MAGs, underscoring the intricate interdependencies within the consortia. Moreover, metabolic variations in amino acid composition among core MAGs emerge as a crucial adaptation mechanism. These differences facilitate the preservation of enzyme activity and enhance the consortia's resilience to low temperatures. Together, these findings offer a comprehensive understanding of the microbial synergistic metabolism within mixed-autotrophy/heterotrophy anammox consortia under temperature reduction, shedding light on their metabolic flexibility and resilience in dynamic environments.

Potential electron acceptors for ammonium oxidation in wastewater treatment system under anoxic condition: A review

Anaerobic ammonium oxidation has been considered as an environmental-friendly and energy-efficient biological nitrogen removal (BNR) technology. Recently, new reaction pathway for ammonium oxidation under anaerobic condition had been discovered. In addition to nitrite, iron trivalent, sulfate, manganese and electrons from electrode might be potential electron acceptors for ammonium oxidation, which can be coupled to traditional BNR process for wastewater treatment. In this paper, the pathway and mechanism for ammonium oxidation with various electron acceptors under anaerobic condition is studied comprehensively, and the research progress of potentially functional microbes is summarized. The potential application of various electron acceptors for ammonium oxidation in wastewater is addressed, and the N2O emission during nitrogen removal is also discussed, which was important greenhouse gas for global climate change. The problems remained unclear for ammonium oxidation by multi-electron acceptors and potential interactions are also discussed in this review.

Robustness of the anammox process at low temperatures and low dissolved oxygen for low C/N municipal wastewater treatment

Low water temperatures and ammonium concentrations pose challenges for anammox applications in the treatment of low C/N municipal wastewater. In this study, a 10 L-water bath sequencing batch reactor combing biofilm and suspended sludge was designed for low C/N municipal wastewater treatment. The nitrogen removal performance via partial nitrification anammox-(endogenous) denitrification anammox process was investigated with anaerobic-aerobic-anoxic mode at low temperatures and dissolved oxygen (DO). The results showed that with the decrease of temperature from 30 to 15℃, the influent and effluent nitrogen concentrations and nitrogen removal efficiencies were 73.7 ± 6.5 mg/L, 7.8 ± 2.8 mg/L, and 89.4 %, respectively, with aerobic hydraulic retention time of only 6 h and DO concentration of 0.2-0.5 mg/L. Among that, the stable anammox process compensated for the inhibitory effects of the low temperatures on the nitrification and denitrification processes. Notably, from 30 to 15℃, the anammox activity and relative abundance of the dominant Brocadia genus were increased from 39.7 to 45.5 mgN/gVSS/d and 7.3 to 12.0 %, respectively; the single gene expression level of the biofilm increased 9.0 times. The anammox bacteria showed a good adaptation to temperatures reduction. However, nitrogen removal by anammox was not improved by increasing DO (≥ 4 mg/L) at 8-4℃. Overall, the results of this study demonstrate the feasibility of the mainstream anammox process at low temperatures.

Metagenomic insights into the symbiotic relationship in anammox consortia at reduced temperature

Anammox as a promising biological nitrogen removal technology has attracted much attention. However, cold temperature would limit its wide application and little is known about the microbial interactions between anammox bacteria (AnAOB) and heterotrophic bacteria at cold temperature. Here, we observed reduced temperature (25-15 °C) promoted the secretion of EPS and thus stimulated bigger size of granular sludge in a laboratory-scale anammox reactor. We further combined co-occurrence network analysis and genome-centered metagenomics to explore the potential interactions between AnAOB and heterotrophic bacteria. Network analysis suggested 22 out of 25 positively related species were reported as definite heterotrophic bacteria in subnetwork of AnAOB. Genome-centered metagenomics analysis yielded 23 metagenomic assembly genomes (MAGs), and we found that Acidobacteriota-affiliated bacteria could biosynthesize most polysaccharides (PS) precursors and contain the most glycosyltransferases and transporters to facilitate exopolysaccharides biosynthesis, together with partial PS precursors produced by AnAOB. AMX1 as the only anammox genome could synthesize most amino acids and cross feed with some heterotrophs to affect the extracellular protein function. Additionally, Bacteroidota, Planctomycetota, Chloroflexota, and Proteobacteria could contribute folate and molybdopterin cofactor for AMX1 to benefit their activity and growth. Superphylum Patescibacteria could survive by cross-feeding with AnAOB and heterotrophic organisms about organic compounds (Glyceraldehyde-3P and lactate). These cross-feedings maintained the stability of anammox reactor performance and emphasize the importance of heterotrophs in anammox system at reduced temperature.

Physiology of anammox adaptation to low temperatures and promising biomarkers: A review

The adaptation of bacteria involved in the anaerobic ammonium oxidation (anammox) to low temperatures in the mainstream of WWTP will unlock substantial treatment savings. However, their adaptation mechanisms have begun to be revealed only very recently. This study reviewed the state-of-the-art knowledge on these mechanisms from -omics studies, crucially including metaproteomics and metabolomics. Anammox bacteria adapt to low temperatures by synthesizing both chaperones of RNA and proteins and chemical chaperones. Furthermore, they preserve energy for the core metabolism by reducing biosynthesis in general. Thus, in this study, a number of biomarkers are proposed to help practitioners assess the extent of anammox bacteria adaptation and predict the decomposition of biofilms/granules or slower growth. The promising biomarkers also include unique ladderane lipids. Further proteomic and metabolomic studies are necessary for a more detailed understanding of anammox low-temperature adaptation, thus easing the transition to more cost-effective and sustainable wastewater treatment.

Mainstream partial denitrification-anammox (PD/A) for municipal sewage treatment from moderate to low temperature: Reactor performance and bacterial structure

Anammox is sensitive to temperature, which can limit its practical application in wastewater treatment. In this study, a step-feed anoxic-oxic (A/O) process coupled with PD/A was operated steadily from 26.8 °C to 13.1 °C for wastewater treatment for 200 days. The effluent total inorganic nitrogen (TIN) and phosphorus concentrations were 10.2 mg/L and 0.29 mg/L at C/N ratio of 4.6 and 15.0 °C even with increasing nitrogen loading rate (NLR). The anammox activity was 5.60 mg NH₄⁺-N/gMLSS/d even at 14 °C, moreover, anammox abundance on the biocarriers increased with decreasing temperature. It was observed that the effect of partial denitrification (PD) was enhanced under low temperature, thus the contribution of anammox for nitrogen removal was improved. The pathway of anammox for nitrogen removal accounted for 48% and the effect of effluent did not deteriorate under low temperature. This study states that PD/A has advantages under low temperature operation, which is suitable for treatment of wastewater with low C/N ratio.

Achieving high-rate partial nitritation with aerobic granular sludge at low temperatures

Partial nitritation is necessary for the implementation of the mainstream anammox (anaerobic ammonium oxidation) process in wastewater treatment plants. However, the difficulty in outcompeting nitrite-oxidizing bacteria (NOB) at mainstream conditions hinders the performance of partial nitritation. The present work aimed to develop a high-rate partial nitritation process for low-ammonium wastewater treatment at low temperatures by seeding aerobic granules. Experimental results suggested that both stratified structure of nitrifiers developed in the granules and sufficient residual ammonium concentration (18-35 mg N L^-1) in the bulk liquid contributed to efficient NOB repression. With the hydraulic retention time progressively shortened from 1.0 to 0.17 h, the influent nitrogen loading rate of the partial nitritation process reached 6.8 ± 0.4 kg N m^-3 d^-1 even at 10-15 °C. The high concentration (7.5 gVSS L^-1) and activity (0.48 g N g^-1 VSS d^-1 at 11 °C) of granular sludge made the reactor possess an overcapacity evaluated by the ratio between the actual ammonium oxidation rate of the granules and their maximum potential. The overcapacity helped the reactor to face the adverse effect of decreasing temperatures. Overall, this work indicated the great potential of applying aerobic granules to achieve high-rate partial nitritation at mainstream conditions. Moreover, anammox bacteria with a relative abundance of 2.8% was also identified in the partial nitritation granules at the end of this study, suggesting that the granules provided a habitable niche for anammox bacteria growth. Note that these results cannot fully relate to the treatment of real domestic/municipal wastewater, they are a source of important information increasing the knowledge about low temperature partial nitrification.

The short-term and long-term effects of Fe(II) on the performance of anammox granules

Fe(II) is one of the commonly used additives in wastewater treatment and proved to be beneficial for promoting microbial activity. In this study, the effects of Fe(II) on the specific anammox activity (SAA) and reactor performance were proved to be concentration-dependent. In the short-term experiment, low concentration of Fe(II) (5-80 mg/L) significantly enhanced the SAA, while high concentration of Fe(II) (120-300 mg/L) inhibited the SAA. It was confirmed that anammox can be domesticated after long-term exposure to low Fe(II) concentration, and the SAA could be further enhanced by higher Fe(II) concentration in the following phases. In addition, as an important factor for anammox granulation and maintaining the SAA, the extracellular polymeric substance (EPS) was also affected by Fe(II) addition. In spite of the effects on SAA and EPS, Fe(II) was proved to be the key factor that enhances the N₂ O emission via abiotic pathway in the anammox reactor. PRACTITIONER POINTS: Low Fe(II) concentrations enhanced SAA, while high concentrations inhibited SAA. Long-term acclimatization by Fe(II) improved the tolerance of anammox to Fe(II). Fe(II) affects the amount and constituent of EPS and the performance of anammox granules. Accumulation of Fe(II) in the AAFEB reactor promoted the N₂ O emission.

Holistic insights into extracellular polymeric substance (EPS) in anammosx bacterial matrix and the potential sustainable biopolymer recovery: A review

Anaerobic ammonia oxidation (anammox) process has been proven to be a favorable and innovative process, for treatment of nitrogen-rich wastewater due to decreased oxygen and carbon requirements at very high nitrogen loading rates. Anammox process is mainly operated through biofilm or granular sludge structures, as for such slow-growing microorganisms, elevated settling velocity of granules allows for adequate biomass retention and lowered potential risk of washouts. Stability of granular sludge biomass is extremely critical, yet the formation mechanism is poorly understood. There are number of important functions linked to Extracellular Polymeric Substance (EPS) in anammox bacterial matrix, such as; structural stability, aggregation promotion, maintenance of physical structure in the granules, water preserving and protective cell barrier. There is an increasing demand to introduce accurate methods for proper EPS extraction and characterization, to expand the perception of anammox granule stability and potential resource recovery. Analyzing EPS with a focus on various (mechanical and physical) properties can lead to biopolymer production from granular sludge. Biopolymers such as EPS are attractive alternatives substituting the conventional chemical polymers furthermore their recovery from the waste sludge and the potential applications in industrial sectors, leads to a radical enhancement of both environmental and economical sustainability, accelerating the circular economy advancements. Here, this study aims to overview the newest understanding on the structure of anammox sludge EPS, obtained recently and to assess the potential challenges and prospects to identify the knowledge gaps towards constructing an inclusive anammox EPS recovery and characterization procedure.

Insight: High intensity and activity carrier granular sludge cultured using polyvinyl alcohol/chitosan and polyvinyl alcohol/chitosan/iron gel beads

The newly developed carrier granular sludge (CGS) with polyvinyl alcohol/chitosan (PVA/CS) and PVA/CS/Fe gel beads assistance showed higher intensity and anammox activity than the natural granular sludge (NGS). Through comprehensive investigation, it was found: (1) the gel beads provided a stable framework of cells entangle with extracellular polymeric substances (EPS) to enhance the sludge intensity. In this framework, β-polysaccharides are distributed at the edge of CGS as a protection layer, α-polysaccharides and proteins are spread in the whole cross-section as backbones, and Fe²⁺/Fe³⁺ in CGS-PVA/CS/Fe act as bridges to link with the negatively charged groups on bacterial surfaces and proteins. (2) The porous gel beads satisfied a relatively unimpeded mass transfer. Thus, the sludge activity, microbe's metabolism, membrane transportation and environmental adaption in CGS were apparently improved. The results improved the understanding about the advantages of the CGS and indicated their possible application in full-scale anammox processes.

Influence of seasonal temperature change on autotrophic nitrogen removal for mature landfill leachate treatment with high-ammonia by partial nitrification-Anammox process

In this study, a denitrification (DN)-partial nitritation (PN)-anaerobic ammonia oxidation (Anammox) system for the efficient nitrogen removal of mature landfill leachate was built with a zone-partitioning self-reflux biological reactor as the core device, and the effects of changes in seasonal temperature on the nitrogen removal in non-temperature-control environment were explored. The results showed that as the seasonal temperature decreased from 34°C to 11.3°C, the total nitrogen removal rate of the DN-PN-Anammox system gradually decreased from the peak value of 1.42 kg/(m³•day) to 0.49 kg/(m³•day). At low temperatures (<20°C), when the nitrogen load (NLR) of the system is not appropriate, the fluctuation of high NH₄⁺-N concentration in the landfill leachate greatly influenced the stability of the nitrogen removal. At temperatures of 11°C-15°C, the NLR of the system is controlled below 0.5 kg/(m³•day), which can achieve stable nitrogen removal and the nitrogen removal efficiency can reach above 96%. The abundance of Candidatus Brocadia gradually increased with the decrease of temperature. Nitrosomonas, Candidatus Brocadia and Candidatus Kuenenia as the main functional microorganisms in the low temperature.

Enhanced production of methane in anaerobic water treatment as mediated by the immobilized fungi

Anaerobic digestion of organic waste and wastewater represents an attractive sustainable bio-technology to produce methane as an alternative to fossil energy. In response to improvement of methane production via enhancing methanogenesis, current strategies of the addition of external biological/non-biological materials have to confront either the loss of materials, high cost and/or possible destruction of the microbial community. Here, we report the first case of using immobilized fungi Aspergillus sydowii 8L-9-F02 to optimize the microbial community, achieving remarkable improvement of the methane production in both batch test (1.5 times) and continuous flow operation (1.13-1.31 times). The crucial role of fungi is associated with the stimulation of enrichment of Methanosaeta and Methanobacterium for methanogenesis from 28.2 to 67.4% as well as the improved activity of enzyme F420. Moreover, fungi also increase the content of extracellular polymeric substances, facilitating the formation of bio-aggregates. This work provides a new pathway to enhance methanogenesis during anaerobic digestion of wastewater by using fungi as bio-enhancer.

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

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

Metabolic acclimation of anammox consortia to decreased temperature

Widespread application of anammox process has been primarily limited to the high sensitivity of anammox consortia to fluctuations of temperature. However, the metabolic acclimation of anammox consortia to decreased temperature remains unclear, which is the core of developing potential strategies for improving their low-temperature resistance. Here, we operated anammox reactors at 25 °C and 35 °C to explore the acclimation mechanism of anammox consortia in terms of metabolic responses and cross-feedings. Accordingly, we found that the adaptation of anammox consortia to ambient temperature (25 °C) was significantly linked to energy conservation strategy, resulting in decreased extracellular polymeric substance secretion, accumulation of ATP and amino acids. The expression patterns of cold shock proteins and core enzymes caused the apparent metabolic advantage of Candidatus Brocadia fulgida for acclimation to ambient temperature compared to other anammox species. Importantly, strengthened cross-feedings of amino acids, nitrite and glycine betaine benefited adaptation of anammox consortia to ambient temperature. Our work not only uncovers the temperature-adaptive mechanisms of anammox consortia, but also emphasizes the important role of metabolic cross-feeding in the temperature adaptation of microbial community.

Performance of the anammox process treating low-strength municipal wastewater under low temperatures: Effect of undulating seasonal temperature variation

In the anammox process treating low-strength municipal wastewater, the effect of common seasonal temperature variation (15.1 °C-22.2 °C) on performance was studied. In autumn and winter, the nitrogen removal rate (NRR) decrement of 0.038kgN/(m³·d) (17.9 °C → 15.1 °C) was nearly threefold higher than 0.014kgN/(m³·d) (22.2 °C → 17.9 °C), which showed that lower temperature laid more negative impact on nitrogen removal. ¹⁵N isotope tracing tests confirmed that the contribution of denitrification to nitrogen removal was far less than anammox, and anammox contributed more at 15.1 °C (91.7%) than 21.9 °C (78.9%). Anammox bacteria could adapt to lower temperature after short-term acclimatization, especially the dominant genus Ca. Brocadia increased from 1.8% to 2.5% and its abundance was significantly correlated with nitrogen consumption (p < 0.05). Above findings suggest that the adaptability of Ca. Brocadia could provide the possibility to maintain nitrogen removal performance at lower temperature. In spring, the improved maximum anammox activity from 2.85 to 3.23mgNH₄⁺-N/(gVSS·h) indicated the recovered removal capacity.

Technologies for biological removal and recovery of nitrogen from wastewater

Water contamination is a growing environmental issue. Several harmful effects on human health and the environment are attributed to nitrogen contamination of water sources. Consequently, many countries have strict regulations on nitrogen compound concentrations in wastewater effluents. Wastewater treatment is carried out using energy- and cost-intensive biological processes, which convert nitrogen compounds into innocuous dinitrogen gas. On the other hand, nitrogen is also an essential nutrient. Artificial fertilizers are produced by fixing dinitrogen gas from the atmosphere, in an energy-intensive chemical process. Ideally, we should be able to spend less energy and chemicals to remove nitrogen from wastewater and instead recover a fraction of it for use in fertilizers and similar applications. In this review, we present an overview of various technologies of biological nitrogen removal including nitrification, denitrification, anaerobic ammonium oxidation (anammox), as well as bioelectrochemical systems and microalgal growth for nitrogen recovery. We highlighted the nitrogen removal efficiency of these systems at different temperatures and operating conditions. The advantages, practical challenges, and potential for nitrogen recovery of different treatment methods are discussed.

A review on mainstream deammonification of municipal wastewater: Novel dual step process

The conventional biological nitrogen removal process is receiving increasing pressure partially due to its energy-negative operation. To address this challenge, various mainstream deammonification processes have been explored for energy-neutral municipal wastewater treatment, whereas these processes appear challenging to be sustainably and stably achieved in conventional process configurations. Therefore, this review aimed to provide a comprehensive analysis of the state-of-the-art of mainstream deammonification, while highlighting the major technical challenges. It appeared that recently developed novel dual step process, i.e. A-B processes, could provide a feasible engineering option for mainstream deammonification, where A-stage is designed for COD capture with the aim to enhance energy recovery, and B-stage is tailored for nutrient removal/recovery. This indeed may lead to a promising integrated mainstream deammonification process towards energy-efficient and environmentally sustainable nitrogen removal. Meanwhile, this review also offered an opinion on future municipal wastewater treatment, aiming for concurrent water reclamation and energy recovery.

Integrated effects of temperature and COD/N on an up-flow anaerobic filter-biological aerated filter: Performance, biofilm characteristics and microbial community

The integrated effects of temperature and COD/N ratio on performance, biofilm characteristics and microbial community in up-flow anaerobic filter-biological aerated filters (UAF-BAFs) were investigated. Results indicated that the UAF-BAF system could achieve excellent COD, NH₄⁺-N and TN removal, in which effluent quality well met the Class 1A standard. Biofilm physicochemical characteristics showed that the biomass, biofilm thickness and extracellular polymeric substance (EPS) content in the UAF-BAFs reduced with the decrease in COD/N ratio, but were enhanced under low temperature. The biofilm structure characterized by CLSM in the UAF-BAFs significantly shifted, which was closely correlated with operational conditions. Sequencing analysis revealed that Proteobacteria, Epsilonbacteraeota, Bacteroidetes and Firmicutes were dominant in the UAFs and the abundance of ammonium oxidizing bacteria (AOB) was responsible for nitrification performance in the BAFs. Functions analysis indicated that amino acid metabolism, carbohydrate metabolism, energy metabolism and lipid metabolism were clearly regulated by parameters changes.

The performance of an anaerobic ammonium oxidation upflow anaerobic sludge blanket reactor during natural periodic temperature variations

An anaerobic ammonium oxidation-upflow anaerobic sludge blanket (anammox-UASB) reactor was operated without temperature control during the four seasons and was therefore subjected to natural periodic temperature variations between 9 and 28 ℃. The anammox reactor had a high nitrogen removal ability at intermediate and low temperatures. The total nitrogen (TN) concentration of the influent increased from 200 to 1200 mg/L, the nitrogen removal efficiency was maintained at 90%, and the nitrogen removal rate (NRR) increased to 9.15 ± 0.35 kg N/m³/d. The enrichment of anammox bacteria in the UASB granular sludge reached 53.8%, and the dominant bacteria changed from Candidatus Brocadia to Candidatus Kuenenia after several seasons of cultivation. Dynamics analysis revealed that the maximum reaction rate of the anammox-UASB sludge was 62.5 kg N/m³/d, reflecting the high potential nitrogen removal ability of the reactor.

Insight into the response of anammox granule rheological intensity and size evolution to decreasing temperature and influent substrate concentration

Anammox granules are advantageous because of their relatively higher nitrogen removal rate (NRR) and biomass retention capacity in ammonia-containing wastewater treatment. However, little attention has been paid to granule rheological intensity and size evolution, especially under low temperature and substrate concentration conditions. In this study, the size evolution and variations in rheological properties associated with biochemical characteristics of anammox granules were investigated at decreasing temperatures (35 → 13 °C) and influent substrate concentrations (300 → 50 mg NH₄⁺-N L^-1). Both the specific anammox activities (SAA) and yield stress (τ_c) (or storage modulus (G')), which reflected granules' intensity, decreased with decreasing temperature and influent substrate concentration. An exponential correlation was found between SAA and τ_c (or G'). Granule size strongly decreased at low temperature (13 °C) and influent substrate concentration (50 mg NH₄⁺-N L^-1), despite slight variations in τ_c (or G'). A threshold τ_c (or G') that is closely related to the hydrodynamic shear force in the reactor may exist for the anammox granules. Once the τ_c (or G') of the anammox granules was lower than this threshold value (τ_c^∗ = 10.13-15.63 kPa), granules that could not endure hydrodynamic shear forces would disintegrate and their size would decrease substantially. Candidatus Kuenenia was the dominant genus in the expanded granular sludge bed reactor, reaching a minimum abundance of 14.6% at 16 °C because of the low-temperature shock, but increasing in abundance to 57.0% at 13 °C, indicating it has a competitive advantage at low temperatures. This contributed to achieving a high reactor nitrogen loading rate (>1.0 kg N m^-3 d^-1) even at 13 °C or with 50 mg NH₄⁺-N L^-1 influent. Overall, the results of this study will facilitate management of anammox bioreactors that run at various temperatures and influent substrate concentrations by clarifying the correlation between rheological intensity and anammox granule sludge activity and identifying the τ_c (or G') threshold.

Relationship of heme c, nitrogen loading capacity and temperature in anammox reactor

The characteristic carmine red color due to heme proteins is always observed in enriched anammox biomass. Heme c is a very important co-factor participating the main metabolic reactions with catalytic and electron-transfer potential in the anammox bacteria, and is possible for use as an indicator to evaluate anammox performance. Knowledge of the relationship between the heme c concentration and the anammox reactor performance is, however, very limited available information is constrained at an operation temperature of 35 °C. In this study, we report the heme c concentration change along with nitrogen removal rate (NRR) in three anammox expanded granular sludge bed reactors operated at different temperatures (15, 25, 35 °C). The response of specific anammox activity (SAA) to temperature was revealed for biomass originating from three reactors. The results indicate a strong relationship between heme c concentration and NRR at different culture temperatures. The possibility of evaluating the anammox performance by combining heme c quantification and the temperature is revealed.

Effects of temperature on anammox performance and community structure

A lab-scale anammox up-flow anaerobic sludge blanket (UASB) reactor was run to investigate the influence of temperature on anammox performance and community structure. The anammox system had a higher substrate tolerance at 13 °C than at 18 °C. The adverse effects caused by the use of a lower temperature (8 °C) could be restored. The nitrogen removal rate (NRR) decreased with decreasing in situ specific anammox activity (SAA). Interestingly, the ex situ SAA acclimated at 23 °C, when exposed to ex situ temperatures of 33 and 28 °C, was higher than for those acclimated at 33 and 28 °C. No shift was observed in the optimum temperature for ex situ SAA in the whole lowering process of anammox UASB. More extracellular polymeric substances were produced in response to cooler conditions (18 °C and 13 °C). Ca. Kuenenia became much more abundant (55.18% of the microbial community) and had a competitive advantage over other anammox bacteria (AnAOB) at 13 °C.

Mass transfer characteristics, rheological behavior and fractal dimension of anammox granules: The roles of upflow velocity and temperature

In this study, the mass transfer, rheological behavior and fractal dimension of anaerobic ammonium oxidation (anammox) granules in upflow anaerobic sludge blanket reactors at various temperatures (8.5-34.5°C) and upflow velocities (0.06, 0.18mh^-1) were investigated. The results demonstrated that a lower temperature increased the external mass transfer coefficient and apparent viscosity and impaired the performance of anammox granules. The external mass transfer coefficient was decreased, but efficient nitrogen removal of up to 96% was achieved under high upflow velocity, which also decreased the apparent viscosity. Furthermore, a fractal dimension of up to 2.93 achieved at low temperature was higher than the previously reported values for mesophilic anammox granules. A higher upflow velocity was associated with the lower fractal dimension. Because of the disturbance in granule flaking, the effectiveness factor was less suitable than the external mass transfer coefficient for characterization of mass transfer resistance.

Influence of temperature and pH on the anammox process: A review and meta-analysis

The anammox (anaerobic ammonium oxidation) process was considered a very efficient and economic wastewater treatment technology immediately after its discovery in 1995, thus research in this field was intensified. The anammox process is characterised by a high temperature optimum and is very sensitive to both temperature and pH fluctuations. The process can also be inhibited by many factors, including by its substrates, i.e. nitrite and ammonium (or its unionised forms: free ammonia and free nitrous acid). This paper presents a comprehensive study of the most important and recent findings on the influence of two parameters that are crucial in wastewater treatment, i.e. temperature and pH. Because both parameters may influence the anammox process simultaneously, a meta-analysis was conducted of the data from the literature. Although meta-analysis is commonly used in medical research, mathematical analysis of the literature data has become an interesting and important step in the environmental sciences. This paper presents information on the influence of both temperature and pH on process efficiency and microbial composition. Additionally, the responses of different operating systems on both temperature and pH changes are described. Moreover, the role of both adaptation to changed conditions and of pH control as well as indicated areas of process operation are discussed.

Nitrogen removal performance and operation strategy of anammox process under temperature shock

Sequencing batch reactors were used to study anaerobic ammonium oxidation (anammox) process under temperature shock. Both long-term (15-35 °C) and short-term (10-50 °C) temperature effects on nitrogen removal performance were performed. In reactor operation test, the results indicated that ammonium removal rate decreased from 0.35 kg/(m³ day) gradually to 0.059 kg/(m³ day) when temperature dropped from 35 to 15 °C. Although bacteria morphology was not modified, sludge settling velocity decreased with decreasing temperature. In batch test, apparent activation energy (E_a) increased with decreasing temperature, which suggested the activity decrease of anaerobic ammonium oxidizing bacteria (AAOB). Low temperature inhibited AAOB and weakened nitrogen removal performance. The cardinal temperature model with inflection was first used to describe temperature effect on anammox process. Simulated results revealed that anammox reaction could occur at 10.52-50.15 °C with maximum specific anammox activity of 0.50 kg/(kg day) at 36.72 °C. The cold acclimatization of AAOB could be achieved and glycine betaine could slightly improve nitrogen removal performance at low temperature.

Effects of thiocyanate on granule-based anammox process and implications for regulation

The feasibility of using anaerobic ammonium oxidation (anammox) process to treat industrial wastewater containing thiocyanate (SCN^-) was examined in this study. Anammox activity decreased with increasing thiocyanate concentration and pre-exposure time in batch tests. A typical noncompetitive model was used to fit the data for thiocyanate inhibition, and the 50% inhibition concentration (IC₅₀) of thiocyanate on anammox was 620.4mgL^-1 at 200mgL^-1 total nitrogen level. The influent thiocyanate concentration of test reactor (R₁) in phase II gradually increased from 10 to 120mgL^-1, and the average nitrogen removal efficiency (NRE) of R₁ was maintained at 83.0±7.82%. This robustness was attributed to the self-adaptation ability of anammox biomass through long-term acclimatization. The NRE was decreased to 57.1% under 130mgL^-1 thiocyanate within two days. However, the NRE of control reactor (R₀) in absence of thiocyanate was 91.23±4.11% in this phase. Under thiocyanate stress, the specific anammox activity, settling velocity and heme c content of the granules significantly decreased, and the extracellular polymeric substances content slightly increased. The short- and long-term performance inhibition could be reversed in the presence of 10mgL^-1 Fe(III).

Successful start-up of the anammox process: Influence of the seeding strategy on performance and granule properties

Successful start-up of the anaerobic ammonium oxidation (anammox) process in up-flow sludge blanket reactor was achieved by seeding denitrifying (R0) or mixed denitrifying-anammox granular sludge at a 3:1 volume ratio (R1). The results demonstrated that R1 was successfully started-up on day 40 and had a nitrogen removal rate (NRR) of 0.55kgNm(-3)d(-1). By contrast, it took 98days to start up R0 (0.54kgNm(-3)d(-1)). R0 and R1 achieved maximum NRRs of 5.70 and 12.02kgNm(-3)d(-1), respectively. The biogranules from R1 generally possessed greater specific anammox activity (SAA), higher extracellular polymeric substance (EPS) and heme c content, larger granules and greater settling velocity. Quantitative PCR (qPCR) and high-throughput sequencing techniques were used to monitor the evolution of the bacterial community. These results demonstrated the feasibility of seeding mixed denitrifying-anammox granular sludge to start-up an anammox reactor.

Effects of substrate shock on extracellular polymeric substance (EPS) excretion and characteristics of attached biofilm anammox granules

The production mechanism of an extracellular polymeric substance (EPS) and its effects on the performance of anammox granules were evaluated.

Biological nitrogen removal from sewage via anammox: Recent advances

Biological nitrogen removal from sewage via anammox is a promising and feasible technology to make sewage treatment energy-neutral or energy-positive. Good retention of anammox bacteria is the premise of achieving sewage treatment via anammox. Therefore the anammox metabolism and its factors were critically reviewed so as to form biofilm/granules for retaining anammox bacteria. A stable supply of nitrite for anammox bacteria is a real bottleneck for applying anammox in sewage treatment. Nitritation and partial-denitrification are two promising methods of offering nitrite. As such, the strategies for achieving nitritation in sewage treatment were summarized by reviewing the factors affecting nitrite oxidation bacteria growth. Meanwhile, the methods of achieving partial-denitrification have been developed through understanding the microorganisms related with nitrite accumulation and their factors. Furthermore, two cases of applying anammox in the mainstream sewage treatment plants were documented.