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

Negative role of filamentous bulking and its elimination in anammox process

In this study, the filamentous bulking (FB) with moderate and excessive levels were demonstrated to induce anammox failure by inhibiting nitrogen (N) removal and biomass retention. The low external mass transfer resulted from high liquid-surface friction and low turbulence of filamentous surface was considered the "trigger" of anammox failure, which decreased flux of nitrogen flow toward granular surface and directly limited N-removal loading, which meanwhile exposed granules with N-scarcity environment and indirectly inhibited N-removal bio-activity. Low bio-activity performed poor extracellular polymeric substances secretion further destroyed bio-aggregation with low suface hydrophobicity, which acted as "accelerator" for granule disintegration and biomass washout, ultimatly leading to anammox failure. Fortunately, incresing hydraulic shear stress could eradicate FB's negative effects without inhibiting FB itself, which promoted re-granulation and N-remval restore by enhancing external mass transfer more than hydraulic detachment. Enhancing mechanical stirring with FB level was necessary to maintain stable operation of granular anammox system.

Performance optimization and nitrogen removal mechanism of up-flow partial denitrification/anammox process

This study aimed to remediate the problems of sludge floating and uneven mass transfer in up-flow partial denitrification/anammox (PDA) reactors and dissect the nitrogen removal mechanism. Two up-flow PDA reactors were operated, whereby in R1 combined biological carriers were added, while in R2 mechanical stirring was applied, the reactors were inoculated with PD sludge and anammox sludge. Results showed the TN removal rates at the end of the operation were 89% (R1) and 92% (R2). The addition of both strategies suppressed the occurrence of sludge upwelling and deterioration of settling performance, even when the granule diameter of the granular zone in R1 and R2 reached 1.921 and 2.006 mm, respectively. 16SrRNA sequencing revealed R1 had a higher abundance of anammox bacteria (AAOB, 14.53%-R1, 9.06%-R2, respectively), and R2 had a higher quantity of denitrifying bacteria (61.92%-R1, 67.11%-R2, respectively). And the nitrogen removal was contributed by anammox and denitrification in combination, with contributions of 82.17%, 17.83% (R1), and 85.07%, 14.93% (R2), respectively. In summary, both strategies prevented sludge flotation and uneven nitrogen mass transfer. However, mechanical agitation had a more substantial positive effect on the performance of PDA than the addition of biocarriers because it achieved a more adequate mass transfer.

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

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

Enhancement of biomethane recovery from batch anaerobic digestion by exogenously adding an N-acyl homoserine lactone cocktail

Biomethane recovered through anaerobic digestion (AD) is a renewable, sustainable, and cost-effective alternative energy source that has the potential to help address rising energy demands. Efficient bioconversion during AD depends on the symbiotic relationship between hydrolytic bacteria and methanogenic archaea. Interactions between microorganisms occur in every biological system via a phenomenon known as quorum sensing (QS), in which signaling molecules are simultaneously transmitted and detected as a mode of cell-to-cell communication. However, there's still a lack of understanding on how QS works in the AD system, where diverse bacteria and archaea interact in a complex manner. In this study, different concentrations (0.5 and 5 μM) of signaling molecules in the form of an N-acyl homoserine lactone cocktail (C6-, C8-, C10-, and 3-oxo-C6-HSL) were prepared and introduced into anaerobic batch reactors to clearly assess how QS affects AD systems. It was observed that the methane yield increased with the addition of AHLs: a 5 μM AHL cocktail improved the methane yield (341.9 mL/g-COD) compared to the control without AHLs addition (285.9 mL/g-COD). Meanwhile, evidence of improved microbial growth and cell aggregation was noticed in AHLs-supplemented systems. Our findings also show that exogenously adding AHLs alters the microbial community structure by increasing the overall bacterial and archaeal population counts while favoring the growth of the methanogenic archaea group, which is essential in biomethane synthesis.

Anammox biofilter with denitrification sludge as seed in treating low nitrogen strength wastewater

Deficient seed sludge, low substrate concentrations are recognized as the major barriers for the application of anaerobic ammonia oxidation (Anammox) to treat mainstream wastewater. In this work, anammox biofilter (A-BF) was started up by inoculating denitrification sludge at low nitrogen strength at 25 °C. The total nitrogen removal efficiency (TNRE) and nitrogen removal rate (NRR) reached 74.8 ± 3.4% and 0.81 kg-N m^-3 d^-1 under nitrogen loading rate (NLR) of 1.20 kg-N m^-3 d^-1 with 7.00 mg-NH₄⁺-N L^-1 and 10.00 mg-NO₂^--N L^-1 as influent. 1.00-2.00 mg-DO L^-1 negatively impacted effluent, but the total nitrogen of effluent (TN_eff) was 10.65 ± 2.76 mg L^-1, in limit of the standard of Class 1A for municipal WWTP discharge (GB18918-2002). The abundance of Planctomycetes increased from 0.6% to 1.4-2.6%, in which, Candidatus_Brocadia was the dominant genera. The results establish the application feasibility of A-BFs as advanced nitrogen removal technique in treating mainstream wastewater.

Fast formation of anammox granules using a nitrification-denitrification sludge and transformation of microbial community

A lengthy start-up period has been one of the key obstacles limiting the application of the anammox process. In this investigation, a nitrification-denitrification sludge was used to start-up the anammox EGSB process. The transformation process from nitrification-denitrification sludge to anammox granule sludge was explored through the aspects of nitrogen removal performance, granule properties, microbial community structure, and evolution route. A successful start-up of the anammox process was achieved after 94 days of reactor operation. The highest nitrogen removal rate (NRR) obtained was 7.25±0.16 gN/L/d at a nitrogen loading rate (NLR) of 8.0 gN/L/d, and the corresponding nitrogen removal efficiency was a high 90.61±1.99%. The results of the microbial analysis revealed significant changes in anammox bacteria, nitrifying bacteria, and denitrifying bacteria in the sludge. Notably, the anammox bacteria abundance increased from 2.5% to 29.0% during the operation, and Candidatus Kuenenia and Candidatus Brocadia were the dominant genera. Distinct-different successions on Candidatus Brocadia and Candidatus Kuenenia were also observed over the long-term period. In addition, the settling performance, anammox activity and biomass retention capacity of the granules were significantly enhanced during this process, and the corresponding granule evolution route was also proposed. The results in this study indicate the feasibility of using available seed sludge source for the fast-transformation of anammox granules, it is beneficial to the large-scale application of anammox process and the utilization of excess sludge.

A mainstream anammox fixed-film membrane bioreactor with novel sandwich-structured carriers for fast start-up, effective sludge retention and membrane fouling mitigation

Novel sandwich-structured carriers were developed for fast immobilizing anammox sludge, with which a fixed-film membrane bioreactor was further established for treating municipal wastewater. Results showed that fast start-up of the fixed-film reactor with anammox bacteria could be achieved without lag phase, indicated by the respective nitrogen removal efficiency and rate of 70.58 ± 0.66% and 0.12 g N/(L·d). Meanwhile, low membrane fouling 0.0017 bar/hour was also observed. The activity of anammox sludge fixed in the novel carriers gradually stabilized at the level of 6.59 mg N/(g VSS·h), while Candidatus Kuenenia as the dominant anammox bacteria were enriched from the initial abundance of 15.16% to 39.12% after a long-term operation. Consequently, it was demonstrated that the sandwich-structured carriers developed in this study could offer a promising alternative for fast immobilization and start-up of mainstream anammox process.

Granular activated carbon promoting re-granulation of anammox-hydroxyapatite granules for stable nitrogen removal at low phosphate concentration

Anaerobic ammonium oxidation (anammox) coupled with hydroxyapatite (HAP) crystallization not only achieves simultaneous nitrogen removal and phosphorus recovery, but also cultivates excellent anammox granules. However, a floatation and wash-out of anammox-HAP granules was occurred at low phosphate concentrations. In this study, a reactor inoculated with mature anammox-HAP granules and fed with low phosphate (5 mg P/L) was added with granular activated carbon (GAC) to maintain sludge granulation and nitrogen-removing stability. At influent total nitrogen >800 mg/L and nitrogen loading rate ~ 9.8 kg/m³/d, a satisfactory nitrogen removal of around 88% was maintained during 140 days of operation. Insufficient phosphate supplement resulted in a sludge bulking, with suspended solid and sludge density decreased whereas sludge water content and expansion ratio increased due to HAP loss. Nevertheless, the sludge re-granulation was found at the later stage as the proportion of granules in 2.8- 3.35 mm went up to 37.4% after large granules disintegrated into small pieces at the initial stage. The settling velocity was finally ranging from 129.8 to 182.2 m/h. In addition, Candidatus Brocadia was increased from 2.1% to 20.1% and dominated in the microbial community. These findings suggest GAC was able to promote re-granulation of anammox-HAP granules at low phosphate concentration, which avoids sludge flotation and widens their application as an inoculum.

Dimension effect of anammox granule: Potential vs performance

Anammox granule is the key support of anammox sludge bed reactor. In this study, the anammox granules from a steady-state reactor were divided into 6 groups to investigate their dimension effects. The results of batch cultivation showed that the anammox granules with VMD (volume surface mean diameter) of 2.17 mm had the maximum SAA (specific anammox activity) of 399.6 ± 37.6 mg-N/(g-VSS·d). The bacterial community analysis demonstrated that Candidatus Kuenenia was the main detectable AnAOB genus in the anammox granules. Q-PCR together with flow cytometry indicated that the total number of viable AnAOB cells ascended with the increasing anammox granular size, suggesting the enhancement of nitrogen removal potential. On the contrary, the mass transfer efficiency descended with the increasing granular size, indicating the restriction of nitrogen removal performance. The maximum SAA was ascribed to the optimal match between nitrogen removal potential and mass transfer efficiency. The results of this study are helpful to comprehend the nitrogen removal capacity of anammox granules and to promote the optimization of anammox process.

Architecture of HAP-anammox granules contributed to high capacity and robustness of nitrogen removal under 7°C

The anaerobic ammonium oxidation (anammox) process is an autotrophic nitrogen removal process with great potential as a cost-effective and highly efficient technology in the wastewater treatment field. The main challenges yet to be overcome in this new frontier technology are operating at lower temperatures and achieving a high and stable nitrogen removal efficiency. In this study, an up-flow expanded bed reactor with hydroxyapatite (HAP)-anammox granules was operated for more than 200 days at 7°C. The nitrogen loading rate (NLR) was improved from 1.0 g-N/L/d to 3.6 g-N/L/d, together with a high-level nitrogen removal efficiency of 84-92%, which is the highest to date at extremely low temperatures in a continuous experiment. Candidatus Kuenenia was revealed to be the only dominant anammox genus, with a relative abundance of 35.3-37.5%. The optimal operational temperature was around 35°C and the apparent activation energy (E_a) was calculated as 78.37 kJ/mol. The three-layers architecture and architectural evolution of HAP-anammox granules into HAP-cores and peeling biofilms with outstanding settling performance were characterized. Under 7°C, the high capacity of nitrogen removal with robust removal efficiency using HAP-anammox granules was achieved.

Characteristics of anammox granular sludge using color differentiation, and nitrogen removal performance of its immobilized fillers based on microbial succession

The characteristics of anammox granular sludge (AnGS) based on color differentiation, and the regulation mechanism of immobilized fillers in the system were investigated. The results showed that biomass content, EPS and activity of red AnGS (R1) were higher than those of brown AnGS (R2). Moreover, R1 showed nitrification, while R2 showed denitrification. Filamentous bacteria constituted the granule skeleton of R1, while R2 mainly constituted inorganic nucleation and granulation. Additionally, immobilization improved the contribution rate of Anammox, and involved different regulatory mechanisms. High-throughput sequencing analysis showed that R1 encapsulation biomass eliminated miscellaneous bacteria and established specific flora, while mixed encapsulated biomass of R1 and R2 re-formed a functional bacterial network, which strengthened interspecies cooperation. The R2 encapsulated biomass and AnAOB copy numbers were inferior and the interspecific cooperation was weak, resulting in an unsatisfactory nitrogen removal performance. These results can strengthen the understanding and optimization of AnGS and its immobilization system.

Generalized temperature dependence model for anammox process kinetics

Temperature is a key operational factor influencing the anammox process kinetics. In particular, at temperatures below 15 °C, the specific anammox activity (SAA) considerably decreases. This study aimed to describe the temperature dependence of the anammox process kinetics in the temperature range from 10 to 55 °C, including the specific characteristics of "cold anammox". The commonly used Arrhenius and extended and modified Ratkowsky equations were examined. The Ratkowsky equations yielded a strong correlation (coefficient of determination, R² = 0.93-0.96) between the measured and predicted data over the analyzed temperature range (10-55 °C). However, these equations could not correctly reflect the anammox temperature dependence at temperatures below 15 °C (R² = 0.36-0.48). Therefore, a new generalized temperature model was proposed. The generalized temperature equation (GTE) considered the division of the analyzed temperature range into three temperature ranges: 10-15 °C, 15-35 °C and 35-55 °C. The ranges correspond to "cold anammox", "(low) mesophilic anammox" and "thermophilic anammox". The applied approach yielded a strong correlation between the measured and predicted SAA (R² = 0.97) over the temperature range from 10 to 55 °C and over the low-temperature range from 10 to 15 °C (R² = 0.99). Overall, the GTE could enhance the predictions of the temperature dependence of the anammox process kinetics. The GTE can help examine anammox-based bioaugmentation systems operating at both high temperatures (sidestream reactors) and low temperatures (mainstream reactors).

Deciphering correlation between permeability and size of anammox granule: "pores as medium"

Anammox granular sludge bed technology has been widely applied for its attractive advantages. Efficient mass transfer is an important factor for the anammox granules to play their role. In this study, steady-state anammox granules were used to investigate the correlation between the permeability and granule size with the granule pore as pivot. The results of size distribution showed that the anammox granules could be divided into 6 groups: 200-500 µm (I), 500-1000 µm (II), 1000-1500 µm (III), 1500-2000 µm (IV), 2000-3000 µm (V) and ≥3000 µm (VI). The results of settling experiment demonstrated that the permeability of anammox granules was negatively correlated with the granule size. The fluid collection efficiency declined from 39.4% to 9.3% for granule group I to III, and further to 0 for granule group IV to VI (granule size was larger than 1.5 mm). The observation of micro-CT revealed that the pore structure of anammox granules varied significantly with the increase of granule size, forming a denser surface layer and sparser interior. The chemical analysis and microscopic observation indicated that the pore plugging of surface layer by cell proliferation and EPS secretion was the main cause for the permeability deterioration. The findings of this study will help to understand the mass transfer of anammox granules and promote the development of anammox processes.

Anti-migraine effect of wine-processed Radix scutellariae: Pharmacodynamic verification in nitroglycerin-induced rats and correlation study between compounds dissolution and the fractal dimension

ETHNOPHARMACOLOGICAL RELEVANCE

Wine-processed Radix scutellariae (RS) is the processed product of RS, which is the dried root of Scutellaria baicalensis Georgi. It is recorded in Chinese traditional formula that wine-processed RS has the effect of anti-migraine, while the effect has not been confirmed and the possible mechanism remains unclear.

AIM OF THE STUDY

To verify the anti-migraine effect of wine-processed RS in nitroglycerin (NTG)-induced rats and explore the correlation between compounds dissolution and the pore structure based on fractal theory.

MATERIALS AND METHODS

In the validation of pharmacodynamics, the effects of wine-processed RS on migraines were firstly evaluated by observing the number of head-scratching of rats, then investigated by determining the levels of nitric oxide (NO), calcitonin gene-related peptide (CGRP) and the expression of c-Fos in the brain of NTG-induced rat models using ELISA and immunohistochemical assessments. In the correlation study, the stir-frying time of RS was set to 5 min, 10 min and 15 min. The scanning electron microscope (SEM) and mercury intrusion method were used to explore the pore structure and main parameters of the pore structure including pore size distribution, pore volume, porosity, surface area and fractal dimension. The compounds dissolution of total flavonoids and five major components containing baicalein, baicalin, scutellarin, wogonin and wogonoside was determined by UV-Vis spectrophotometry and HPLC separately.

RESULTS

The animal experiments had shown that wine-processed RS could significantly reduce the head-scratching times of NTG-induced rat models (p < 0.01) and markedly decrease the levels of NO (p < 0.01), CGRP (p < 0.05) and the expression of c-Fos (p < 0.01) compared with model group. The data indicated that wine-processing would affect the dissolution of compounds by changing the pore structure of RS. The order of positive correlation between pore structure parameters and compounds' dissolution was total surface area > fractal dimension (r > 0) and the order of negative correlation was average pore size > total porosity > total volume (r < 0). Compared with the other sample groups (p < 0.05), the wine-processed RS stir-fried for 10 min had a pore structure which was more favorable for compounds dissolution.

CONCLUSIONS

Wine-processing could strengthen the anti-migraine effect of RS by changing the pore structure of RS, which is linked to the dissolution of compounds. The RS stir-fried for 10 min may be more effective in treating migraine.

Anammox Granules Acclimatized to Mainstream Conditions Can Achieve a Volumetric Nitrogen Removal Rate Comparable to Sidestream Systems

The implementation of mainstream anammox has gained increasing attention. In this study, the feasibility of using sidestream anammox granules to start up mainstream reactors was investigated by comparing two switching strategies. A maximum nitrogen removal potential of 3.6 ± 0.2 kg N m^-3 d^-1 was obtained for the reactor after direct switching to mainstream conditions (70 mg TN L^-1, 15 °C). Nevertheless, the reactor preacclimatized to 25 °C (Ma) exhibited a higher nitrogen removal potential of 7.0 ± 0.3 kg N m^-3 d^-1 at 15 °C, which is the highest volumetric nitrogen removal rate of mainstream anammox reactors to date. Candidatus Kuenenia stuttgartiensis was identified as the dominant anammox bacterium, and its relative abundance in two reactors remained stable throughout the whole operation (200 days). Moreover, with the aid of acclimatization, the activation energy was reduced and the specific growth rate became higher. These results indicated that the physiological evolution of the dominant anammox bacterium instead of interspecies selection was the main reason for the high potential during the switch to mainstream conditions. Therefore, using sidestream anammox granules as seed sludge to start up mainstream reactors was demonstrated to be feasible, and a switching strategy of acclimatization at 25 °C was recommended.

Effects of exogenous N-acyl-homoserine lactones on nutrient removal, sludge properties and microbial community structures during activated sludge process

This study investigated the effects of exogenous N-acyl-homoserine lactone (AHL) signal molecules, N-hexanoyl-_l-homoserine lactone (C6-HSL) and N-octanoyl-_l-homoserine lactone (C8-HSL), on treatment performance, sludge properties and microbial community structures in activated sludge systems. Results showed that the nitrification and denitrification efficiencies were enhanced with the addition of signal molecules. The particle size, irregularity, and internal mass transfer resistance of activated sludge flocs (ASFs) increased, primarily because dosing AHLs led to a content increase and chemical composition variation of extracellular polymeric substances (EPS) in sludge. Microbial analysis indicated an increase in both the bacterial richness and diversity of the systems. The relative abundances of the key functional groups, including bacteria related to C and N removal and EPS production, varied correspondingly. This study presents an insight into the comprehensive understanding of the effects of AHL-based quorum sensing on activated sludge treatment process.

A novel strategy for anammox consortia preservation: Transformation into anoxic sulfide oxidation consortia

The preservation of anaerobic ammonia oxidation (anammox) consortia is crucial for the rapid start-up and the process stability of the anammox based bioreactor. This work proposed and evaluated the feasibility of an anammox consortia preservation strategy, in which the anammox sludge was transformed into intermediate anoxic sulfide oxidation (ASO) functional microorganisms. Initially, the ASO process was successfully started up by inoculating anammox sludge and the overall sulfide and nitrate removal rates stabilized at 57.5 ± 0.22 and 10.0 ± 0.18 kg m^-3 day^-1, respectively. Then, the bioreactor function was reversely transformed into anammox, whose nitrogen removal rate reached 1.68 kg m^-3 day^-1. Granule characteristics analysis revealed that both biomass and extracellular polymeric substance content returned to their original states after the reverse start-up. Although the population of Candidatus_Kuenenia was greatly declined during ASO process, its richness was successfully recovered after the reverse start-up of the anammox process. The inferred metagenomes analysis demonstrated that the shifts in functional microorganisms were related to variation in the main metabolic pathways. The specific activities of anammox and ASO both are regarded as key indicators for the successful start-up of bioreactor. This work revealed a novel technique for the preservation of anammox consortia and might be a potential strategy for overcoming the drawback of long start-up time.

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.

Effects of vibration on anammox-enriched biofilm in a high-loaded upflow reactor

An upflow biofilm reactor was operated for 211 days to investigate the effects of vibration on anammox treatment performance. With vibration, the highest nitrogen removal rates (20 kg-N·m^-3·d^-1) were obtained on day 180. Since the vibration could directly applied on the biofilm, it could release the dinitrogen gas accumulated in the biofilm timely and reduce the internal mass transfer resistance sharply. The specific anammox activity increased by more than 3 times with a higher vibration intensity. Meanwhile, the unique random motion caused by mechanical vibration promotes the production of extracellular proteins. Moreover, the VSS reached 20.97 g·L^-1 which was 1.6 times higher than the control reactor. Such enrichment method resulted in a hard and thick anammox biofilm with a special granular morphology, and the nitrite tolerance concentration could reach 500 mg-N·L^-1. Operated with an adequate vibration intensity could maintain the biofilm thickness and conducive to improve the stability of the reactor. In addition, this technique also allowed the microorganisms inside the biofilm and those on the surface to reach the same culture conditions. Base on the batch experiments, intermittent vibration caused a decrease in energy consumption from about 7.757 (kW·h)·(kg-N)^-1 in group 0-Lv7(60-60) to 0.912 (kW·h)·(kg-N)^-1 in group 0-Lv7(5-60). Compared to the internal recycle without vibration, the energy consumption fell by a slice over 65%. Furthermore, the high-throughput sequencing results showed that the relative abundance of Candidatus Kuenenia in reactor 1 increased from 13.2% to 43.9%.

Biogas Production and Fundamental Mass Transfer Mechanism in Anaerobic Granular Sludge

Anaerobic granules are responsible for organic degradation and biogas production in a reactor. The biogas production is entirely dependent on a mass transfer mechanism, but so far, the fundamental understanding remains poor due to the covered surface of the reactor. The study aimed at investigating the fundamental mass transfer characteristics of single anaerobic granules of different sizes using microscopic imaging and analytical monitoring under single and different organic loadings. The experiment was conducted in a micro reactor and mass transfer was calculated using modified Fick’s law. Scanning electron microscopy was applied to observe biogas production zones in the granule, and a lab-scale microscope equipped with a camera revealed the biogas bubble detachment process in the micro reactor for the first time. In this experiment, the granule size was 1.32, 1.47, and 1.75 mm, but 1.75 mm granules were chosen for further investigation due to their large size. The results revealed that biogas production rates for 1.75 mm granules at initial Chemical Oxygen Demand (COD) 586, 1700, and 6700 mg/L were 0.0108, 0.0236, and 0.1007 m3/kg COD, respectively; whereas the mass transfer rates were calculated as 1.83 × 10−12, 5.30 × 10−12, and 2.08 × 10−11 mg/s. It was concluded that higher organic loading and large granules enhance the mass transfer inside the reactor. Thus, large granules should be preferred in the granule-based reactor to enhance biogas production.

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

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

Synergy of partial-denitrification and anammox in continuously fed upflow sludge blanket reactor for simultaneous nitrate and ammonia removal at room temperature

In this study, the synergy of high nitrite (NO₂^--N) accumulated partial-denitrification (PD) and anammox in a continuously fed upflow anaerobic sludge blanket (UASB) reactor was verified for simultaneous nitrate (NO₃^--N) and ammonia (NH₄⁺-N) removal. A 225-days operation demonstrated that the relatively low total nitrogen (TN) concentration of 6.56 mg/L in effluent could be achieved with influent NH₄⁺-N and NO₃^--N both of 30 mg/L, resulting in a high TN removal of 89.1% at 17.5 °C. Batch tests revealed that the NO₃^--N-to-NO₂^--N transformation ratio (NTR) of PD stabilized at 90% during the whole operation, which played a crucial role in the desirable performance. However, the PD and anammox activity was negatively impacted by the limited mass transfer with serious sludge flotation. Significantly, hydrodynamic mixing optimization by adjusting liquid recirculation ratio effectively enhanced the nitrogen removal. Moreover, protein composition and tightly-bound structure of EPS played an important role in the sludge stability.

A novel anammox process combined with vibration technology

This study investigated a fixed bed anammox bioreactor that uses vibration techniques to treat synthetic inorganic wastewater. Continuous experiments indicated that the activity elevation period could be shorten to one third, when the nitrogen removal rate (NRR) reached 1 kg·N/m³·d with vibration. R2 achieved the maximum NRR of 3.3 kg·N/m³·d under the resonance state, which was 1.8 times higher than the control reactor. Analysis of vibration intensity suggested that anammox activity would be great improved with the increasing vibration. These results indicated that vibration played a key role in system performance. Furthermore, high-throughput sequencing showed that the reactor with the vibration had a higher proportion of anammox bacteria, which increased 7 times than the biofilm formation phase. Meanwhile, the proportion of Proteobacteria and Chloroflexi decreased by 37.1% and 7.78%, respectively. These results suggest that vibration could increase the anammox treatment performance and provide a better condition for the anammox bacteria.