Ammonia Conversion Rate Research Articles

Ni-Doped SFM Double-Perovskite Electrocatalyst for High-Performance Symmetrical Direct-Ammonia-Fed Solid Oxide Fuel Cells.

Ammonia has emerged as a promising fuel for solid oxide fuel cells (SOFCs) owing to its high energy density, high hydrogen content, and carbon-free nature. Herein, the electrocatalytic potential of a novel Ni-doped SFM double-perovskite (Sr1.9Fe0.4Ni0.1Mo0.5O6-δ) is studied, for the first time, as an alternative anode material for symmetrical direct-ammonia SOFCs. Scanning and transmission electron microscopy characterization has revealed the exsolution of Ni-Fe nanoparticles (NPs) from the parent Sr2Fe1.5Mo0.5O6 under anode conditions, and X-ray diffraction has identified the FeNi3 phase after exposure to ammonia at 800 °C. The active-exsolved NPs contribute to achieving a maximal ammonia conversion rate of 97.9% within the cell's operating temperatures (550-800 °C). Utilizing 3D-printed symmetrical cells with SFNM-GDC electrodes, the study demonstrates comparable polarization resistances and peak power densities of 430 and 416 mW cm-2 for H2 and NH3 fuels, respectively, with long-term stability and a negligible voltage loss of 0.48% per 100 h during ammonia-fed extended galvanostatic operation. Finally, the ammonia consumption mechanism is elucidated as a multistep process involving ammonia decomposition, followed by hydrogen oxidation. This study provides a promising avenue for improving the performance and stability of ammonia-based SOFCs for potential applications in clean energy conversion technologies.

Scalable Ammonia Synthesis in Fermentors Using Quantum Dot-Azotobacter vinelandii Hybrids

AbstractThis study introduces a scalable synthesis of ammonia through photochemical reactions, wherein nitrogen-fixing bacterial cells, Azotobacter vinelandii (A. vinelandii), form hybrids with colloidal quantum dots (QDs). Irradiation of the QD-A. vinelandii hybrids with visible light is found to significantly enhance ammonia production efficiency. The inherently low ammonia conversion rate of wild-type A. vinelandii is substantially increased upon incorporation of QDs. This increase is attributed to the electron transfer from QDs within the bacterial cells to intracellular bio-components. Transferring this chemistry to a large-scale reaction presents a tremendous challenge, as it requires precise control over the growth conditions. We explore the scalability of the QD-A. vinelandii hybrids by conducting the photochemical reaction in a 5-L fermentor under various parameters, such as dissolved oxygen, nutrient supply, and pH. Interestingly, ammonia was produced in media depleted of carbon sources. Consequently, a two-step fermentation process was designed, enabling effective ammonia production. Our findings demonstrate that the QD-A. vinelandii hybrid system in a bioreactor setup achieves an ammonia turnover frequency of 11.96 s−1, marking a more than sixfold increase in efficiency over that of nitrogenase enzymes alone. This advancement highlights the potential of integrating biological and nanotechnological elements for scalable ammonia production processes.

T-RFLP biomolecular indicator for partial nitritation under saline conditions and machine learning application

This study introduces the smart development of a cost-effective biomolecular indicator using terminal restriction fragment length polymorphism (T-RFLP) based on the 16S rRNA gene database obtained from operating the partial nitritation (PN) process under saline conditions. As a result of next-generation sequencing (NGS) on the 16S rRNA gene, ammonia-oxidizing bacteria (AOB) of Nitrosomonas sp. OTU0 (N. OTU0) favored higher salinity of 20–35 g-NaCl/L, while Nitrosomonas sp. OTU8 (N. OTU8) and Nitrosomonas sp. OTU11 (N. OTU11) were predominant under 20 g-NaCl/L. The T-RFLP system with the restriction enzymes of TaiI and FnuDII, which was thoroughly simulated based on the NGS data of the 16S rRNA gene, effectively differentiates N. OTU0 against N. OTU8 and N. OTU11. In addition, together with AOB’s signals, the signals of nitrite-oxidizing and heterotrophic bacteria were obtained as biomolecular indicators of successful PN under saline conditions. These findings suggest that T-RFLP can serve as an economically viable monitoring approach for the sensitive PN process under saline conditions to achieve more sustainable nitrogen removal. For further application, the designated T-RFLP signals of this study were applied to the regression of ammonia conversion rate through machine learning modeling. The regression showed R2 values of 0.9446 and 0.9458 for the training and test sets, respectively. The contribution of T-RFLP for the regression was quantified by a shapley additive explanation (SHAP).

Experimental analysis of ammonia as input fuel for molten carbonate fuel cell

Hydrogen fuel cells can generate clean and efficient energy, and ammonia as a hydrogen carrier contains more hydrogen in its molecule than hydrogen itself. Utilizing green ammonia as fuel for fuel cells can reduce dependency on fossil fuels. However, there has been less study on the electrochemical performance of ammonia molten carbonate fuel cells (MCFCs). Hence, this paper sets up a single MCFC experimental bench to conduct research on direct ammonia molten carbonate fuel cells. The electrochemical performance of a direct ammonia MCFC is investigated in terms of key parameters such as temperature and the volume flow ratio of NH3/H2 entering the anode. Additionally, Scanning Electron Microscopy (SEM) is employed to study the electrodes of the MCFC. Experimental findings demonstrate that the direct ammonia MCFC performs optimally at 680 °C with a current density of 0.059 A/cm2 and an ammonia conversion rate up to 91.6%. When the mixture gas of NH3 and H2 is used as the cell fuel, the current density (0.057 A/cm2) and the conversion rate of NH3 (84.9%) are higher when the volume flow ratio of NH3/H2 is 9, indicating that the increase of H2 proportion in the mixed gas will affect the conversion of NH3. SEM image analysis of the MCFC electrodes post-experiments reveals that ammonia does not destruct the loose and porous structure of MCFC electrodes, affirming that NH3 can be directly used as the MCFC fuel.

Thermal management of Ammonia-fed Solid oxide fuel cells using a novel alternate flow interconnector

Ammonia is a promising fuel for solid oxide fuel cells (SOFCs) as it is carbon-free and easy to store at high volumetric energy densities. However, the endothermic decomposition of ammonia in an SOFC stack significantly reduces the temperature at the entrance of the fuel flow, leading to temperature shock and nitriding. Designing a suitable interconnector for the stack to obtain a flatter temperature distribution is an efficient strategy to overcome this problem. This article proposes a novel interconnector design using an alternate flow concept for both fuel and air. A three-dimensional SOFC model was used for the thermal investigation of the proposed design under different operating conditions, including temperature, current, and fuel utilization. The results indicated that the alternate flow design exhibited a 40% lower temperature difference and up to 53 K higher average temperature than a typical cross-flow design. In addition to a flatter temperature profile, the proposed design exhibited a higher ammonia conversion rate than the conventional design, demonstrating its promise for use in robust and efficient ammonia-fed SOFC stacks.

Photocatalytic nitrogen fixation under an ambient atmosphere using a porous coordination polymer with bridging dinitrogen anions

The design of highly electron-active and stable heterogeneous catalysts for the ambient nitrogen reduction reaction is challenging due to the inertness of the N2 molecule. Here, we report the synthesis of a zinc-based coordination polymer that features bridging dinitrogen anionic ligands, {[Zn(L)(N2)0.5(TCNQ-TCNQ)0.5]·(TCNQ)0.5}n (L is tetra(isoquinolin-6-yl)tetrathiafulvalene and TCNQ is tetracyanoquinodimethane), and show that it is an efficient photocatalyst for nitrogen fixation under an ambient atmosphere. It exhibits an ammonia conversion rate of 140 μmol g-1 h-1 and functions well also with unpurified air as the feeding gas. Experimental and theoretical studies show that the active [Zn2+-(N≡N)--Zn2+] sites can promote the formation of NH3 and the detachment of the NH3 formed creates unsaturated [Zn2+···Zn+] intermediates, which in turn can be refilled by external N2 sequestration and fast intermolecular electron migration. The [Zn2+···Zn+] intermediates stabilized by the sandwiched cage-like donor-acceptor-donor framework can sustain continuous catalytic cycles. This work presents an example of a molecular active site embedded within a coordination polymer for nitrogen fixation under mild conditions.

Bimetallic NixCo10-x/CeO2 as highly active catalysts to enhance mid-temperature ammonia decomposition: Kinetics and synergies

Catalytic ammonia decomposition is an attractive method to generate hydrogen at mid-temperatures (<700 °C) but must incorporate precious metals (Pd, Ru, etc.) to ensure high reactivity. Developing Ni-based catalysts to decompose ammonia can enhance its prospect for hydrogen generation. However, the catalytic activity of Ni is hardly satisfactory at mid-temperatures. In this work, we show the bimetallic NixCo10-x/CeO2 towards mid-temperature NH3 decomposition, with the metal loading of Ni and Co tuned. Kinetics study demonstrates that the NH3 decomposition reaction follows the Temkin Pyzhev mechanism and the synergy between Ni and Co can decrease the reaction orders regarding NH3 and increase the reaction orders regarding H2. Mechanistic results indicate that the recombinative N desorption limits the reaction rate. The synergy between Ni and Co can simultaneously decrease the energy barriers of the recombinative N desorption and mitigate the H2 poisoning effect. Therefore, Ni7.5Co2.5/CeO2 displays both high ammonia conversion (96.96%) and hydrogen formation rate (1947.9 mmol/(gcat.h)) at 650 °C. We hope the mechanism in this work can be used to guide the design of inexpensive catalysts to decompose ammonia at mid-temperatures.

Performance Analysis of a Solar Heating Ammonia Decomposition Membrane Reactor under Co-Current Sweep.

Ammonia is an excellent medium for solar thermal chemical energy storage and can also use excess heat to produce hydrogen without carbon emission. To deepen the study of ammonia decomposition in these two fields, finite-time thermodynamics is used to model a solar-heating, co-current sweeping ammonia decomposition membrane reactor. According to the needs of energy storage systems and solar hydrogen production, five performance indicators are put forward, including the heat absorption rate (HAR), ammonia conversion rate (ACR), hydrogen production rate (HPR), entropy generation rate (EGR) and energy conversion rate (ECR). The effects of the light intensity, ammonia flow rate, nitrogen flow rate and palladium membrane radius on system performances are further analyzed. The results show that the influences of the palladium membrane radius and nitrogen flow rate on reactor performances are very slight. When the light intensity is increased from 500 W/m2 to 800 W/m2, the ACR, EGR, HAR and HPR increase obviously, but the ECR decreases by 14.2%. When the ammonia flow rate is increased by 100%, the ECR, EGR and HPR increase by more than 70%, the HAR increases by 15.6% and the ACR decreases by 12.9%. At the same time, the ammonia flow rate needs to be adjusted with the light intensity. The results can provide some guiding significance for the engineering application of ammonia solar energy storage systems and solar hydrogen production.

The photocatalytic generation of ammonia contained reusable water from antibiotics wastewater by BiOBr nanostructures with oxygen vacancies

An alternative approach for the photocatalytic generation of ammonia was proposed using N-containing antibiotics as nitrogen sources and BiOBr as photocatalyst. • A novel photocatalytic generation route of ammonia was proposed. • BiOBr nanostructures with oxygen vacancies via a solvent controlled method was used as photocatalysts. • N-containing antibiotics was used as nitrogen sources. • Ammonia can be generated while photocatalytic degradation of tetracycline. • The nitrogen transformation and ammonia generation process were explored. N-containing antibiotics wastes with high N content can cause serious environmental pollution. In order to solve the above problem, we prepared the bismuth oxybromide (BiOBr) nanostructures with oxygen vacancies and used as an efficient photocatalysts for the photocatalytic degradation of tetracycline (TC) wastewater to generate ammonia contained reusable water. Results indicated that the BiOBr nanostructures with oxygen vacancies prepared in ethanol solvent showed much enhanced photocatalytic activities, which can degrade TC completely in 1 h and produce 36.38 mg·L −1 ·g −1 of ammonia eventually. The ammonia conversion rate can reach to 48.53% at the end of the reaction. The obtained ammonia contained water without TC can be reused in agriculture field. The possible mechanism of N element transformation was proposed. This study provides an alternative approach for the photocatalytic generation of ammonia using N-containing antibiotics as nitrogen sources and opens a new perspective for the utilizing and reusing of environmental pollutants.

Highly selective Pd composite membrane on porous metal support for high-purity hydrogen production through effective ammonia decomposition

In this study, an alumina sol assisted pretreatment method for porous metal supports was investigated for a highly selective Pd composite membrane for ammonia decomposition. Alumina sol made of boehmite was applied to an yttria-stabilized zirconia (YSZ) filled porous Inconel support. Additionally, a vacuum-assisted, two-step electroless plating of Pd drastically increased the hydrogen selectivity. The hydrogen permeation flux and selectivity (H2/N2) measured at 723 K and a transmembrane pressure difference of 100 kPa were 3.40 × 10−1 mol m−2 s−1 and 8,050, respectively. A highly selective Pd composite membrane was applied to a membrane reactor combined with a Ru/Al2O3 catalyst to efficiently produce high-purity hydrogen by ammonia decomposition. The ammonia decomposition test showed that the membrane reactor was able to achieve a high ammonia conversion (99.6%) and a high hydrogen purity (99.99%) with a hydrogen production rate of 0.25 Nm3 h−1 at 745 K and a gauge pressure of 500 kPa. The Pd composite membrane reactor has the advantage of being able to selectively removing hydrogen, increasing the ammonia conversion rate and high-purity hydrogen production via a one-step reaction combined with purification.

Inoculation of Neurospora sp. for improving ammonia production during thermophilic composting of organic sludge

Recent attempts have been made to develop a thermophilic composting process for organic sludge to not only produce organic fertilizers and soil conditioners, but to also utilize the generated ammonia gas to produce high value-added algae. The hydrolysis of organic nitrogen in sludge is a bottleneck in ammonia conversion, and its improvement is a major challenge. The present study aimed to elucidate the effects of inoculated Neurospora sp. on organic matter decomposition and ammonia conversion during thermophilic composting of two organic sludge types: anaerobic digestion sludge and shrimp pond sludge. A laboratory-scale sludge composting experiment was conducted with a 6-day pretreatment period at 30 °C with Neurospora sp., followed by a 10-day thermophilic composting period at 50 °C by inoculating the bacterial community. The final organic matter decomposition was significantly higher in the sludge pretreated with Neurospora sp. than in the untreated sludge. Correspondingly, the amount of non-dissolved nitrogen was also markedly reduced by pretreatment, and the ammonia conversion rate was notably improved. Five enzymes exhibiting high activity only during the pretreatment period were identified, while no or low activity was observed during the subsequent thermophilic composting period, suggesting the involvement of these enzymes in the degradation of hardly degradable fractions, such as bacterial cells. The bacterial community analysis and its function prediction suggested the contribution of Bacillaceae in the degradation of easily degradable organic matter, but the entire bacterial community was highly incapable in degrading the hardly degradable fraction. To conclude, this study is the first to demonstrate that Neurospora sp. decomposes those organic nitrogen fractions that require a long time to be decomposed by the bacterial community during thermophilic composting.

Triclosan weakens the nitrification process of activated sludge and increases the risk of the spread of antibiotic resistance genes

The usage of triclosan (TCS) may rise rapidly due to the COVID-19 pandemic. TCS usually sinks in the activated sludge. However, the effects of TCS in activated sludge remain largely unknown. The changes in nitrogen cycles and the abundances of antibiotic resistance genes (ARGs) caused by TCS were investigated in this study. The addition of 1000 μg/L TCS significantly inhibited nitrification since the ammonia conversion rate and the abundance of nitrification functional genes decreased by 12.14%. The other nitrogen cycle genes involved in nitrogen fixation and denitrification were also suppressed. The microbial community shifted towards tolerance and degradation of phenols. The addition of 100 μg/L TCS remarkably increased the total abundance of ARGs and mobile genetic elements by 33.1%, and notably, the tetracycline and multidrug resistance genes increased by 54.75% and 103.42%, respectively. The co-occurrence network revealed that Flavobacterium might have played a key role in the spread of ARGs. The abundance of this genus increased 92-fold under the addition of 1000 μg/L TCS, indicating that Flavobacterium is potent in the tolerance and degradation of TCS. This work would help to better understand the effects of TCS in activated sludge and provide comprehensive insight into TCS management during the pandemic era.

Performance of a novel single-tubular ammonia-based reactor driven by concentrated solar power

Because of high concentrating ratios of Concentrated Solar Power Technologies (CSP), and maturity of industrial ammonia synthesis process, the high-temperature ammonia thermochemical energy storage based on CSP technology has a promising potential. As a prototype of a tubular ammonia reactor under un-uniform heat flux originating from a sunlight spot or line focusing, it is assumed that catalyst particles packed in it is isotropic, and the heat transfer and mass diffusion in the axial direction of the tube were ignored since the length-diameter aspect ratio of the tube is high. Consequently, employing the local thermal non-equilibrium method, a 3-dimensional model for an ammonia decomposition reaction in porous media was developed with semi-perimeter un-uniform heat flux. For further research, the diameter of catalyst particles, the porosity of packed particles and inflow modes were recognized as important factors. The results show that the particular axial or radial distributions of the porosity significantly affect the reaction characteristics. Especially, the distribution of radial linear reduction of porosity increases the ammonia conversion rate and reduced maximum wall temperature. At the same time, the smaller catalyst particles and the inflow mode of one-side in and one-side out all contribute to improving the performance of the tubular reactor. In general, the following measures can effectively improve the reactor performance: choose a smaller catalyst particle diameter, the porosity of catalyst particles packed decreases linearly along the radial direction from the center of the tube, and the inflow mode in the tube is one-side in and one-side out.

Partial Nitrification by controlling Environmental Conditions in a Swim-bed Reactor

The effects of ammonia concentration, hydraulic detention time and dissolved oxygen on the ammonia conversion rate and nitrite accumulation efficiency were studied in the swim-bed biofilm reactor with the constant temperature and pH value. The temperature was controlled by heating rod at 35 ± 1 °C, the pH value was automatically regulated at 7.5 by adding 2 mol/L NaOH, influent ammonia concentration increased from 200 mg N/L, the dissolved oxygen concentration was below 2.5 mg/L, a stable nitrite accumulation were achieved in a swim-bed reactor and nitrite oxidizing bacteria was washed out.

Comparison of activated carbon and zeolites’ filtering efficiency in freshwater

Water quality plays an important role in ensuring the healthy growth of aquatic living. Fish in ponds release nitrogen, phosphorus, ammonia, nitrates and organic waste. The presence of these chemicals in water leads to an increment in the pH level of the pond water, which could affect the growth of the fish. Therefore, it is vital to have an effective filter system for pond water to remove the unwanted waste. At current literature, the filtering effectiveness of activated carbon and zeolite in fresh water pond systems are remained inexplicit. In this work, a lab-scale filtering system was set up to study the ammonia removal efficiency of activated carbon and zeolite. The efficiency of these two types of filtering media was first studied separately and then combined as a hybrid filter with a weight ratio of 1:1. The effectiveness of the single filter media and hybrid media filter were evaluated based on the measurement of the concentration of ammonia, nitrite and nitrate, and the pH level. It was found that the hybrid filter – a combination of activated carbon and zeolite – tended to have higher efficiency in maintaining good water quality compared to the single media system; the ammonia level was reduced from 4 mg/L to 1 mg/L in 2.5 days. The greater the amount of the hybrid filter media presents in the system, the better the conversion rate of ammonia to nitrate. A combination of activated carbon and zeolite (150 g each) was found to be able to maintain suitable water quality (1 mg/L ammonia, 0 mg/L nitrite, <50 mg/L nitrate, 6.8–8.0 pH) for fish.

Conversion of lanthanum and cerium recovered from hazardous waste polishing powders to hazardous ammonia decomposition catalysts.

The polishing process in high-tech industries produces a large amount of waste polishing slurry, which is harmful to the environment, and the solid powders in the slurry contain a lot of La and Ce, which have been widely used as catalyst materials. The aim of this study was to convert this hazardous material into hazardous material decomposition catalyst. A novel and simple approach was successfully developed for the recovery of La and Ce from hazardous waste polishing powders to synthesize a composite metal oxide catalyst for decomposition of harmful ammonia. Here, La and Ce were leached from waste polishing powder by using nitric acid, and the Ce, La, and total REE recovery rates were approximately 100%, 83.3%, and 96.4%, respectively. The elemental concentrations of leached acidic solution was analyzed, after which stoichiometry was performed to replenish elements with insufficient mole numbers. Finally, the catalyst material was prepared using the glycine-nitrate combustion process. The catalyst prepared from the recovered polishing powders achieved a 100% ammonia conversion rate at the relatively low temperature of 250 °C. The proposed environmentally friendly method does not require complex purification and separation procedures and can be used for the synthesis of catalysts for decomposing other harmful pollutants.

Ultrasound-Enhanced Catalytic Ozonation Oxidation of Ammonia in Aqueous Solution.

Excessive ammonia is a common pollutant in the wastewater, which can cause eutrophication, poison aquatic life, reduce water quality and even threaten human health. Ammonia in aqueous solution was converted using various systems, i.e., ozonation (O3), ultrasound (US), catalyst (SrO-Al2O3), ultrasonic ozonation (US/O3), ultrasound-enhanced SrO-Al2O3 (SrO-Al2O3/US), SrO-Al2O3 ozonation (SrO-Al2O3/O3) and ultrasound-enhanced SrO-Al2O3 ozonation (SrO-Al2O3/US/O3) under the same experimental conditions. The results indicated that the combined SrO-Al2O3/US/O3 process achieved the highest NH4+ conversion rate due to the synergistic effect between US, SrO-Al2O3 and O3. Additionally, the effect of different operational parameters on ammonia oxidation in SrO-Al2O3/O3 and SrO-Al2O3/US/O3 systems was evaluated. It was found that the ammonia conversion increased with the increase of pH value in both systems. The NH3(aq) is oxidized by both O3 and ·OH at high pH, whereas the NH4+ oxidation is only carried out through ·OH at low pH. Compared with the SrO-Al2O3/O3 system, the ammonia conversion was significantly increased, the reaction time was shortened, and the consumption of catalyst dosage and ozone were reduced in the SrO-Al2O3/US/O3 system. Moreover, reasonable control of ultrasonic power and duty cycle can further improve the ammonia conversion rate. Under the optimal conditions, the ammonia conversion and gaseous nitrogen yield reached 83.2% and 51.8%, respectively. The presence of tert-butanol, CO32−, HCO3−, and SO42− inhibited the ammonia oxidation in the SrO-Al2O3/US/O3 system. During ammonia conversion, SrO-Al2O3 catalyst not only has a certain adsorption effect on NH4+ but accelerates the O3 decomposition to ·OH.

Inhibitory Effects of Phosphate and Recovery on a Nitrification System

The effect of phosphate concentration on nitrification was studied by using a stabilization nitrosation system, which was started up in a continuous flow reactor by inoculating sludge from a municipal wastewater treatment plant. The results showed that the nitrification system was started successfully after operating for 14 days. The conversion rate of ammonia nitrogen reached 92.2%, the nitrite accumulation rate was 73.66%, and the nitrite generation rate was 14.42 g·(m3·d)-1. There was no effect of phosphate concentration on the nitrosation system between 10 and 30 mg·L-1; and the conversion rate of ammonia nitrogen was decreased with the continuous increase in phosphate concentration. When the concentration of phosphate was 80 mg·L-1, with an ammonia conversion rate 13.6%, accumulation rate of nitrite of 18.19%, and nitrite generation rate of 0.54 g·(m3·d)-1, the reaction was severely inhibited. After reducing the influent phosphate concentration to 0, with the ammonia nitrogen conversion rate at more than 80%, nitrite accumulation rate improved to 86.96%, and the nitrite generation rate being 15.63 g·(m3·d)-1, the system recovered after operating for 14 days.

Simultaneous Fe(III) reduction and ammonia oxidation process in Anammox sludge

In recent years, there have been a number of reports on the phenomenon in which ferric iron (Fe(III)) is reduced to ferrous iron [Fe(II)] in anaerobic environments, accompanied by simultaneous oxidation of ammonia to NO2−, NO3−, or N2. However, studies on the relevant reaction characteristics and mechanisms are rare. Recently, in research on the effect of Fe(III) on the activity of Anammox sludge, excess ammonia oxidization has also been found. Hence, in the present study, Fe(III) was used to serve as the electron acceptor instead of NO2−, and the feasibility and characteristics of Anammox coupled to Fe(III) reduction (termed Feammox) were investigated. After 160days of cultivation, the conversion rate of ammonia in the reactor was above 80%, accompanied by the production of a large amount of NO3− and a small amount of NO2−. The total nitrogen removal rate was up to 71.8%. Furthermore, quantities of Fe(II) were detected in the sludge fluorescence in situ hybridization (FISH) and denaturated gradient gel electrophoresis (DGGE) analyses further revealed that in the sludge, some Anammox bacteria were retained, and some microbes were enriched during the acclimatization process. We thus deduced that in Anammox sludge, Fe(III) reduction takes place together with ammonia oxidation to NO2− and NO3− along with the Anammox process.

Enrichment and characterization of acid-tolerant nitrifying sludge

Nitrification is an acidifying process that requires the addition of external alkalinity because of the alkaliphilic nature of the most ammonia-oxidizing bacteria. In this study, aerobic activated sludge was used as inoculum in an internal loop air-lift reactor, which resulted in successful enrichment of acid-tolerant nitrifying (ACIN) sludge at pH 6.0 by sequential addition of tea orchard soil suspension. The results showed that ACIN sludge had a remarkable acid tolerant capability with a volumetric ammonia conversion rate of 1.13 kg N m−3 day−1. ACIN sludge showed a higher maximum specific ammonia conversion rate (0.29 g N g−1 VSS day−1) than neutrophilic nitrifying sludge (0.14 g N g−1 VSS day−1) at pH 6.0 and had good resistance against pH fluctuations, with a maximum specific ammonia conversion rate (0.584 g N g−1 VSS day−1) at pH 7.5. Microbial community analysis indicated that the higher abundance of acid tolerant Nitrosospira and ammonia-oxidizing archaea laid a solid foundation for the remarkable acid-tolerant capability of ACIN sludge.