Feasibility of low-intensity ultrasound treatment with hydroxylamine to accelerate the initiation of partial nitrification and allow operation under intermittent aeration

Partial nitrification is a key aspect of efficient nitrogen removal, although practically it suffers from long start-up cycles and unstable long-term operational performance. To address these drawbacks, this study investigated the effect of low intensity ultrasound treatment combined with hydroxylamine (NH2OH) on the performance of partial nitrification. Results show that compared with the control group, low-intensity ultrasound treatment (0.10 W/mL, 15 min) combined with NH2OH (5 mg/L) reduced the time required for partial nitrification initiation by 6 days, increasing the nitrite accumulation rate (NAR) and ammonia nitrogen removal rate (NRR) by 20.4% and 6.7%, respectively, achieving 96.48% NRR. Mechanistic analysis showed that NH2OH enhanced ammonia oxidation, inhibited nitrite-oxidizing bacteria (NOB) activity and shortened the time required for partial nitrification initiation. Furthermore, ultrasonication combined with NH2OH dosing stimulated EPS (extracellular polymeric substances) secretion, increased carbonyl, hydroxyl and amine functional group abundances and enhanced mass transfer. In addition, 16S rRNA gene sequencing results showed that ultrasonication-sensitive Nitrospira disappeared from the ultrasound + NH2OH system, while Nitrosomonas gradually became the dominant group. Collectively, the results of this study provide valuable insight into the enhancement of partial nitrification start-up during the process of wastewater nitrogen removal.

Favipiravir biotransformation by a side-stream partial nitritation sludge: Transformation mechanisms, pathways and toxicity evaluation

Information on biotransformation of antivirals in the side-stream partial nitritation (PN) process was limited. In this study, a side-stream PN sludge was adopted to investigate favipiravir biotransformation under controlled ammonium and pH levels. Results showed that free nitrous acid (FNA) was an important factor that inhibited ammonia oxidation and the cometabolic biodegradation of favipiravir induced by ammonia oxidizing bacteria (AOB). The removal efficiency of favipiravir reached 12.6% and 35.0% within 6 days at the average FNA concentrations of 0.07 and 0.02 mg-N L-1, respectively. AOB-induced cometabolism was the sole contributing mechanism to favipiravir removal, excluding AOB-induced metabolism and heterotrophic bacteria-induced biodegradation. The growth of Escherichia coli was inhibited by favipiravir, while the AOB-induced cometabolism facilitated the alleviation of the antimicrobial activities with the formed transformation products. The biotransformation pathways were proposed based on the roughly identified structures of transformation products, which mainly involved hydroxylation, nitration, dehydrogenation and covalent bond breaking under enzymatic conditions. The findings would provide insights on enriching AOB abundance and enhancing AOB-induced cometabolism under FNA stress when targeting higher removal of antivirals during the side-stream wastewater treatment processes.

Study of bacterial population dynamics in seed culture developed for ammonia reduction from synthetic wastewater

The waterbodies have been polluted by various natural and anthropogenic activities. The aquatic waste includes ammonia as one of the most toxic pollutants. Several biological treatment systems involving anoxic and semi anoxic bacteria have been proposed for reducing nitrogen loads from wastewater and increasing the efficiency and cost effectiveness. These bacteria play a vital role in the processes involved in the nitrogen cycle in nature. However, the enrichment, sustainability and identification of bacterial communities for wastewater treatment is an important aspect. Most of the chemolithotrophs are unculturable hence their identification and measurement of abundance remains a challenging task. In this study the different bacteria involved in total nitrogen removal from the wastewater are enriched for 700 days under anoxic condition. The synthetic wastewater containing 0.382 g/L of ammonium chloride was used. Molecular identification of the bacteria involved in various steps of the nitrogen cycle was carried out based on amplification of functional genes and 16S rRNA gene Polymerase chain reaction followed by DNA sequencing. Change in the abundance of chemolithotrophs was studied using qPCR. The mutual growth of various nitrifiers along with anaerobic bacteria were identified by molecular characterisation of DNA at various time intervals with the different genes involved in the nitrogen cycle. Nitrosomonas species like Nitrosomonas europaea were identified throughout the batch scale studies possessing the genes associated with ammonia oxidizing bacteria and nitrite oxidizing bacteria which act as a complete ammonia oxidizer. The uncultured species of Nitrospira and anammox bacteria were also observed which predicts the coexistence of the anammox and comammox bacteria in a batch scale study. The coexistence of the semi anoxic and anoxic bacteria helped in the growth of these bacteria for a longer duration of time. The nitrite produced by the comammox during nitrification can be utilized by anammox as an electron carrier. The other species of denitrifiers like Pseudomonas denitrificans and Aminobacter aminovorans were also observed. It is concluded that the enrichment of semi anoxic and anoxic bacteria was faster with the increase in growth of the bacteria involved in nitrification, comammox, anammox and partial denitrification process. The bacterial growth is enhanced and the efficiency is increased which can be further used in the development of small pilot scale bioreactor for total nitrogen removal.

Understanding the cometabolic degradation of sulfadiazine by an enriched ammonia oxidizing bacteria culture from both extracellular and intracellular perspectives

Antibiotics are widely used drugs in the world and pose serious threats to ecosystems and human health. Although it has been reported that ammonia oxidizing bacteria (AOB) can cometabolize antibiotics, little has been reported on how AOB would respond to the exposure of antibiotics on extracellular and enzymatic levels, as well as the impact of antibiotics on the bioactivity of AOB. Therefore, in this study, a typical antibiotic, sulfadiazine (SDZ), was selected, and a series short-term batch tests using enriched AOB sludge were conducted to investigate the intracellular and extracellular responses of AOB along the cometabolic degradation process of SDZ. The results showed the cometabolic degradation of AOB made the main contribution to SDZ removal. When the enriched AOB sludge was exposed to SDZ, ammonium oxidation rate, ammonia monooxygenase activity, adenosine triphosphate concentration and dehydrogenases activity were negatively affected. The amoA gene abundance increased 1.5 folds within 24 h, which may enhance the uptake and utilization of substrates and maintain stable metabolic activity. In the tests with and without ammonium, the concentration of total EPS increased from 264.9 to 231.1 mg/gVSS to 607.7 and 538.2 mg/gVSS, respectively, under the exposure to SDZ, which was mainly contributed by the increase of proteins in tightly bound extracellular polymeric substances (EPS) and polysacharides in tightly bound EPS and soluble microbial products. The proportion of tryptophan-like protein and humic acid-like organics in EPS also increased. Moreover, SDZ stress stimulated the secretion of three quorum sensing signal molecules, C4-HSL (from 140.3 to 164.9 ng/L), 3OC6-HSL (from 17.8 to 42.4 ng/L) and C8-HSL (from 35.8 to 95.9 ng/L) in the enriched AOB sludge. Among them, C8-HSL may be a key signal molecule that promoted the secretion of EPS. The findings of this study could shed more light on the cometabolic degradation of antibiotics by AOB.

Unravelling ciprofloxacin removal in a nitrifying moving bed biofilm reactor: Biodegradation mechanisms and pathways

Although moving bed biofilm reactors (MBBRs) have shown excellent antibiotic removal potentials, the information on underlying mechanisms is yet limited. This work assessed the removal of ciprofloxacin in an enriched nitrifying MBBR by clarifying the contribution of adsorption and microbial-induced biodegradation. Results demonstrated the considerable biomass adsorption (55%) in first 30 min. Limiting nitrite oxidizing bacteria growth or inhibiting nitrification would lead to lower adsorption capacities. The highest ciprofloxacin biodegradation rate constant was 0.082 L g SS^-1 h^-1 in the presence of ammonium, owing to ammonia oxidizing bacteria (AOB)-induced cometabolism, while heterotrophs played an insignificant role (∼9%) in ciprofloxacin biodegradation. The developed model also suggested the importance of AOB-induced cometabolism and metabolism over heterotrophs-induced biodegradation by analyzing the respective biodegradation coefficients. Cometabolic biodegradation pathways of ciprofloxacin mainly involved the piperazine ring cleavage, probably alleviating antimicrobial activities. It implies the feasibility of nitrifying biofilm systems towards efficient antibiotic removal from wastewater.

Novel database and cut-off value for bacterial amoA gene revealed a spatial variability pattern of the ammonia-oxidizing bacteria community from river to sea

Ammonia-oxidizing bacteria (AOB) catalyze the first step of nitrification, oxidizing ammonia to nitrite, and are characterized by amoA gene encoding ammonia monooxygenase. To analyze the AOB community effectively, an integral taxonomy database containing 14,058 amoA sequences and the optimal cut-off value at 95 % for OTU clustering were determined. This method was evaluated to be efficient by the analysis of environmental samples from the river, estuary, and sea. Using this method, a significant spatial variance of the AOB community was found. The diversity of AOB was highest in the estuary and lowest in the ocean. Nitrosomonas were the predominant AOB in the sediments of the freshwater river and estuary. Nearly all the AOB-amoA sequences belonged to uncultured bacterium in the sediments of deep sea. In general, an integral AOB taxonomic database and a suitable cut-off value were constructed for the comprehensive exploration of the diversity of AOB from river to sea.

Rapid start-up of autotrophic shortcut nitrification system in SBR and microbial community analysis

Shortcut nitrification is crucial for application of autotrophic nitrogen removal which is beneficial for treating carbon-limited wastewater. In this experiment, rapid start-up of autotrophic shortcut nitrification system was studied in a small sequencing batch reactor (SBR) built in laboratory with intermittent aeration operation mode. The influent was artificially simulated inorganic domestic wastewater (the ammonium nitrogen concentration was 35.19-57.54 mg/L), the pH value was 7.6-7.8, the hydraulic loading was 1L, the operating temperature was 24.3-28.3 °C, and the dissolved oxygen (DO) was 2-4 mg/L and 0.5-0.9 mg/L at the stage of complete nitrification sludge domestication and shortcut nitrification sludge domestication. High-throughput sequencing technology was used to analyse the composition and changes of microbial populations in sludge. The experimental results showed that on the 24th day of the experiment, shortcut nitrification was started successfully, the accumulation rate of nitrite was 81.63% and the removal efficiency of ammonium nitrogen was 99.25%; the richness of the main denitrifying bacteria phylum Proteobacteria increased from 30.21% to 42.85%; the richness of Nitrosomonas (ammonia oxidizing bacteria, AOB) increased from 0.37% to 22.43%, and at the species level, AOB was the salt-tolerant bacteria Nitrosomonas. europaea; the richness of Nitrospira (nitrite oxidizing bacteria, NOB) decreased from 2.59% to 0.47%.

Model study on real-time aeration based on nitrite for effective operation of single-stage anammox

Anaerobic ammonia oxidation (Anammox) is an innovative technology for cost-efficient nitrogen removal without intensive aeration. However, effective control of the competition between nitrite oxidizing bacteria (X_NOB) and Anammox bacteria (X_ANA) for nitrite is a key challenge for broad applications of single-stage Anammox processes in real wastewater treatment. Therefore, a real-time aeration scheme was proposed to determine dissolved oxygen (DO) based on nitrite concentration for effective control of X_NOB growth while maintaining the X_ANA activity in a single-stage Anammox process. In this study, a non-steady state mathematical model was developed and calibrated using previously reported lab-scale Anammox results to investigate the efficiency of the proposed real-time aeration scheme in enhancing the Anammox process. Based on the calibrated model simulation results, DO of about 0.10 mg-O₂/L was found to be ideal for maintaining effective nitrite creation by ammonia oxidizing bacteria (X_AOB) while slowing down the growth of X_NOB. If DO is too low (e.g., 0.01 mg-O₂/L or lower), the overall rate of the ammonia removal is limited due to slow growth of X_AOB. On the other hand, high DO (e.g., 1.0 mg-O₂/L or higher) inhibits the growth of X_ANA, resulting in dominancy of X_AOB and X_NOB. According to the simulation results, nitrite concentration was found to be a rate-limiting parameter on effective nitrogen removal in single-stage Anammox processes. We also found that nitrite concentration can be used as a real-time switch for aeration in a single-stage Anammox process. A schematic aeration method based on real-time nitrite concentration was proposed and examined to control the competition between X_ANA and X_NOB. In the model simulation, the X_ANA activity was successfully maintained because the schematic aeration prevented an outgrowth of X_NOB, allowing energy-efficient nitrogen removal using single-stage Anammox processes.

Ammonia-oxidizing bacterial communities are affected by nitrogen fertilization and grass species in native C4 grassland soils

Background

Fertilizer addition can contribute to nitrogen (N) losses from soil by affecting microbial populations responsible for nitrification. However, the effects of N fertilization on ammonia oxidizing bacteria under C₄ perennial grasses in nutrient-poor grasslands are not well studied.

Methods

In this study, a field experiment was used to assess the effects of N fertilization rate (0, 67, and 202 kg N ha⁻¹) and grass species (switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii)) on ammonia-oxidizing bacterial (AOB) communities in C₄ grassland soils using quantitative PCR, quantitative reverse transcription-PCR, and high-throughput amplicon sequencing of amoA genes.

Results

Nitrosospira were dominant AOB in the C₄ grassland soil throughout the growing season. N fertilization rate had a stronger influence on AOB community composition than C₄ grass species. Elevated N fertilizer application increased the abundance, activity, and alpha-diversity of AOB communities as well as nitrification potential, nitrous oxide (N₂O) emission and soil acidity. The abundance and species richness of AOB were higher under switchgrass compared to big bluestem. Soil pH, nitrate, nitrification potential, and N₂O emission were significantly related to the variability in AOB community structures (p < 0.05).

Envisioning role of ammonia oxidizing bacteria in bioenergy production and its challenges: a review

Ammonia oxidizing bacteria (AOB) play a key role in the biological oxidation of ammonia to nitrite and mark their significance in the biogeochemical nitrogen cycle. There has been significant development in harnessing the ammonia oxidizing potential of AOB in the past few decades. However, very little is known about the potential applications of AOB in the bioenergy sector. As alternate sources of energy represent a thrust area for environmental sustainability, the role of AOB in bioenergy production becomes a significant area of exploration. This review highlights the role of AOB in bioenergy production and emphasizes the understanding of the genetic make-up and key cellular biochemical reactions occurring in AOB, thereby leading to the exploration of its various functional aspects. Recent outcomes in novel ammonia/nitrite oxidation steps occurring in a model AOB - Nitrosomonas europaea propel us to explore several areas of environmental implementation. Here we present the significant role of AOB in microbial fuel cells (MFC) where Nitrosomonas sp. play both anodic and cathodic functions in the generation of bioelectricity. This review also presents the potential role of AOB in curbing fuel demand by producing alternative liquid fuel such as methanol and biodiesel. Herein, the multiple roles of AOB in bioenergy production namely: bioelectricity generation, bio-methanol, and biodiesel production have been presented.

Stoichiometric and kinetic characterization of an acid-tolerant ammonia oxidizer 'Candidatus Nitrosoglobus'

Recently, acidic (i.e. pH<5) nitrification in activated-sludge is attracting attention because it enables stable nitritation (NH₄⁺ → NO₂^-), and enhances sludge reduction and stabilization. However, the key acid-tolerant ammonia oxidizers involved are poorly understood. In this study, we performed stoichiometric and kinetic characterization of a new acid-tolerant ammonia-oxidizing bacterium (AOB) belonging to gamma-proteobacterium, Candidatus Nitrosoglobus. Ca. Nitrosoglobus was cultivated in activated-sludge in a laboratory membrane bioreactor over 200 days, with a relative abundance of 55.1 ± 0.5% (indicated by 16S rRNA gene amplicon sequencing) at the time of the characterization experiments. Among all known nitrifiers, Ca. Nitrosoglobus bears the highest resistance to nitrite, low pH, and free nitrous acid (FNA). These traits define Ca. Nitrosoglobus as an adversity-strategist that tends to prosper in acidic activated-sludge, where the low pH (< 5.0) and high levels of FNA (at parts per million levels) sustained and inhibited all other nitrifiers. In contrast, in the conventional pH-neutral activated-sludge process, Ca. Nitrosoglobus is less competitive with canonical AOB (e.g. Nitrosomonas) due to the relatively slow specific growth rate and low affinities to both oxygen and total ammonia. These results advance our understanding of acid-tolerant ammonia oxidizers, and support further development of the acidic activated-sludge process in which Ca. Nitrosoglobus can play a critical role.

Abundance, community structure and diversity of nitrifying bacterial enrichments from low and high saline brackishwater environments

The study reports diversity in nitrifying microbial enrichments from low (0.5-5‰) and high (18-35‰) saline ecosystems. Microbial community profiling of AOB and NOB enrichments were analysed by sequencing 16S rRNA and were processed using Mothur pipeline. The α-diversity indices showed the richness of nitrifying bacterial consortia from the high saline environment and were clustering based on the source of the sample. AOB and NOB enrichments from both the environments showed diverse lineages of phyla distributed in both groups with 38 and 34 phyla from low saline and 53 and 40 phyla in high saline sources respectively. At class level α and ϒ-Proteobacteria were found to be more dominant in both the enrichments. AOB and NOBs in enrichments from low saline environments were dominated by Nitrosomonadaceae, Gallionellaceae (Nitrotoga spp.) and Ectothiorhodospiraeceae and Nitrospira respectively. Though Chromatiaceae were present in both AOB and NOB enrichments Nitrosoglobus and Nitrosococcus dominated the AOB while NOB was dominated by uncultured genera, while Rhizobiales were found in both the enrichments. AOB and NOBs in enrichments from high saline environments were dominated by Nitrospira-like AOBs, Nitrosomonas and Nitrosococcus genera; while AOA group included Nitrosopumilus and Nitrososphaeraea genera comprising and Nitrospirae respectively. The majority of the genera obtained in both the salinities were found to be either uncultured or unclassified groups. Results of the study suggest that the AOB and NOB consortia have unique and diverse microbes in each of the enrichments, capable of functioning in aquaculture systems practiced at different salinities (0-60ppt).

Multiple strategies for maintaining stable partial nitritation of low-strength ammonia wastewater

Stable production of nitrite is an essential technical challenge for mainstream anaerobic ammonia oxidation (Anammox). Due to difficulties in the stable inhibition of nitrite oxidizing bacteria (NOB) and maintenance of long-term partial nitritation (PN), integrated multiple, rather than a single, controlling strategies were preferred especially in a continuous-flow treatment system. A mathematically model was developed to evaluate effects of integrated multiple-strategies on ammonia oxidizing bacteria (AOB) and NOB. Through experimental study and model simulation, intermittent aeration and low SRT (3.5 d) resulted in unstable nitrite accumulation. Integrated multiple-strategies of intermittent aeration, low SRT (3.5 d) and bioaugmentation achieved nitrite accumulation rate of 81% and NO₂^--N/NH₄⁺-N ratio in effluent of 1.29, which was preferable for further anammox process. Meanwhile, the richness and diversity of microbial community increased due to the bioaugmentation. The AOB/NOB ratio increased from 13.8 to 34.1 which facilitated nitrite accumulation. In combination with bioaugmentation, the observed growth rates of AOB and NOB increased from -0.0835 and -0.0282 to 0.0434 and 0.0127 d^-1, respectively, which promoted AOB outcompeting NOB in the mixed liquid.

Abundance and activity of ammonia oxidizing archaea and bacteria in bulk water and biofilm in water supply systems practicing chlorination and chloramination: Full and laboratory scale investigations

The abundance and nitrification activity of ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) in bulk water and biofilm in chloraminated and chlorinated water supply systems were investigated. The abundance of AOB varied between cold and warm periods while that was the case for AOA only in biofilm. Lower ammonia concentrations favored the abundance of AOA over AOB. AOA and AOB were found more in distal zones of the distribution system (DS). Higher numbers of AOA and AOB were observed in DS associated with chloramination compared to those associated with chlorination. Significant positive correlations between ammonia-N in bulk water and AOA indicate a possibility of involvement of AOA in nitrification in DS. A separate laboratory-based experiment simulating DS condition was conducted to understand the effects of chlorine and chloramine dosages and temperature on AOA and AOB. AOA were inhibited less than AOB in the presence of lower concentrations of chlorine and chloramine (1.5 and 2.0 mg/L chlorine; 0.05-0.1 and 0.3-0.4 mg/L chloramine) while both of them were not detected at higher dosages (2.5 mg/L chlorine and 1.5-1.6 mg/L chloramine). At a low temperature (10-12 °C), chloramine and chlorine provided similar inhibition trends in which AOB were inhibited more than AOA. At a high temperature (25 °C), chloramine was less inhibitory to AOA and AOB than chlorine.

Effects of heavy rare earth element (yttrium) on partial-nitritation process, bacterial activity and structure of responsible microbial communities

Yttrium (Y(III)) is mined commercially for industrial purposes due to its excellent physical properties. However, the effects of Y(III) in mining-wastewater on the performance of partial-nitritation process and ammonia-oxidizing bacteria (AOB) have not been explored. To elucidate Y(III) effects on biological mechanisms, kinetics was conducted to establish a correlation between Y(III) dosage and specific-oxygen-uptake-rate (SOUR). The mechanism(s) demonstrated by bacterial population to resist against toxic effects from Y(III) dose was also investigated using scanning electron microscopy-(SEM), energy-dispersive X-ray spectroscopy-(EDS), confocal laser scanning microscopy-(CLSM)，Fourier transform infrared-(FTIR) spectroscopy, and 2-dimensional correlation infrared-(2DCOS-IR) approach. The study revealed a strong correlation between ammonium oxidation rate (AOR) and Y(III) dosage. AOR promotion was more pronounced when Y(III) concentration was ≤20 mg/L (maximum AOR of 12.39 mgN/L/h, at 5 mg/L), whereas inhibition when Y(III) in influent was >20 mg/L (minimum AOR of 7.34 mgN/L/h, at 500 mg/L). Aiba model demonstrated high-performance (R² = 0.962) when Y(III) concentration ranged 0-20 mg/L, whereas linear model fitted well (R² of 0.984) to experimental data when Y(III) dose ranged 20-500 mg/L. The maximum change in SOUR (Vmax), half-rate constant (Km), and inhibition constant (Ki) reached 1.04 d^-1, 20.12 mg/L, and 4.87 mg/L, respectively, an indication that dosage of Y(III) could affect the partial-nitritation process. SEM-EDS showed that the content of extracellular polymeric substances (EPS) increased along with increasing Y(III) dosage. When 20 mg/L of Y(III) was dosed, the fraction of Y(III) within the surface elemental composition of the sludge increased gradually whereas that of calcium decreased. To further comprehend the EPS production, CLSM results further revealed β-polysaccharide as the dominant component in the EPS. FTIR/2DCOD-IR showed that the chelation of polyguluronic sections within β-polysaccharide, together with hydrazine might be the main pathways of cell resistance, but β- glucan, may have caused the hormesis.

Novel shortcut biological nitrogen removal method using an algae-bacterial consortium in a photo-sequencing batch reactor: Process optimization and kinetic modelling

This study evaluated a novel shortcut nitrogen removal method using a mixed consortium of microalgae, enriched ammonia oxidizing bacteria (AOB) and methanol utilizing denitrifier (MUD) in a photo-sequencing batch reactor (PSBR) for treating ammonium rich wastewater (ARWW). Alternating light and dark periods were followed to obtain complete biological nitrogen removal (BNR) without any external aeration and with the addition of methanol as the sole carbon source, respectively. The results showed that influent NH₄⁺ was oxidized to NO₂^- by AOB during the light periods at a rate of 8.09 mg NH₄⁺-N L^-1h^-1. Subsequently, NO₂^- was completely reduced during the dark period due to the action of MUD in presence of methanol. The high activities of ammonia monooxygenase (AMO) and nitrite reductase (NIR) enzymes revealed the strong role of AOB and MUD for achieving shortcut nitrogen removal from the wastewater. The reduced activities of nitrate reductase (NR) and nitrite oxidoreductase (NOR) at a high concentration of DO, NH₄⁺ and NO₂^-in the system further confirmed the nitrogen removal pathway involved in the process. The biomass produced from these experiments showed good settling properties with a maximum sedimentation rate of 0.7-1.8 m h^-1, a maximum sludge volume index (SVI) of 193 ml g^-1- 256 ml g^-1and floc size of 0.2-1.2 mm. In order to describe the growth and interaction among the algae, AOB and MUD for nitrogen removal in the system, the experimental results were fitted to four metabolic models, which revealed best fit of the experimental data due to the models based on algae-AOB and algae-AOB-MUD activities than with the other two models.

The effect of pH on N2O production in intermittently-fed nitritation reactors

The effect of pH on nitrous oxide (N₂O) production rates was quantified in an intermittently-fed lab-scale sequencing batch reactor performing high-rate nitritation. N₂O and other nitrogen (N) species (e.g. ammonium (NH₄⁺), nitrite, hydroxylamine and nitric oxide) were monitored to identify in-cycle dynamics and determine N conversion rates at controlled pH set-points (6.5, 7, 7.5, 8 and 8.5). Operational conditions and microbial compositions remained similar during long-term reactor-scale pH campaigns. The specific ammonium removal rates and nitrite accumulation rates varied little with varying pH levels (p > 0.05). The specific net N₂O production rates and net N₂O yield of NH₄⁺ removed (ΔN₂O/ΔNH₄⁺) increased up to seven-fold from pH 6.5 to 8, and decreased slightly with further pH increase to 8.5 (p < 0.05). Best-fit model simulations predicted nitrifier denitrification as the dominant N₂O production pathway (≥87% of total net N₂O production) at all examined pH. Our study highlights the effect of pH on biologically mediated N₂O emissions in nitrogen removal systems and its importance in the design of N₂O mitigation strategies.

Bioaugmented methanol production using ammonia oxidizing bacteria in a continuous flow process

Organic compounds such as methanol are widely used for enhancing denitrification at wastewater treatment plants (WWTPs) to meet effluent water quality permits. On the other hand, methane, which is the main feedstock for industrial methanol production, is also generated during anaerobic digestion in WWTPs, but is often flared to mitigate its greenhouse impacts. The overarching goal herein was to develop a novel continuous process for methanol production from methane using nitrifying activated sludge. The maximum AOB specific methanol production rate using hydroxylamine as electron donor was 1.61 ± 0.15 and 1.27 ± 0.15 mg-COD-CH3OH/(mg-COD-AOB*d), for hydraulic retention times (HRTs) of 7.5 h and 2 h, respectively. The corresponding production rate using ammonia as electron donor was 0.31 ± 0.08 mg-COD-CH3OH/(mg-COD-AOB*d) at a HRT of 2 h. These results show that nitrifier-mediated methanol production in a continuous-flow system can enhance the efficiency of WWTPs through internal production of biomethanol for denitrification, while simultaneously minimizing wasteful biogas flaring.

Roles of ammonia-oxidizing bacteria in improving metabolism and cometabolism of trace organic chemicals in biological wastewater treatment processes: A review

While there has been a significant recent improvement in the removal of pollutants in natural and engineered systems, trace organic chemicals (TrOCs) are posing a major threat to aquatic environments and human health. There is a critical need for developing potential strategies that aim at enhancing metabolism and/or cometabolism of these compounds. Recently, knowledge regarding biodegradation of TrOCs by ammonia-oxidizing bacteria (AOB) has been widely developed. This review aims to delineate an up-to-date version of the ecophysiology of AOB and outline current knowledge related to biodegradation efficiencies of the frequently reported TrOCs by AOB. The paper also provides an insight into biodegradation pathways by AOB and transformation products of these compounds and makes recommendations for future research of AOB. In brief, nitrifying WWTFs (wastewater treatment facilities) were superior in degrading most TrOCs than non-nitrifying WWTFs due to cometabolic biodegradation by the AOB. To fully understand and/or enhance the cometabolic biodegradation of TrOCs by AOB, recent molecular research has focused on numerous crucial factors including availability of the compounds to AOB, presence of growth substrate (NH₄-N), redox potentials, microorganism diversity (AOB and heterotrophs), physicochemical properties and operational parameters of the WWTFs, molecular structure of target TrOCs and membrane-based technologies, may all significantly impact the cometabolic biodegradation of TrOCs. Still, further exploration is required to elucidate the mechanisms involved in biodegradation of TrOCs by AOB and the toxicity levels of formed products.

Comparative assessment of endocrine disrupting compounds removal in heterotrophic and enriched nitrifying biomass

Despite the number of studies that have investigated the fate of endocrine disrupting compounds (EDCs), to date results are still contradictory and more research is required to evaluate the contribution of the microbial communities present in different engineered treatment systems. Thus, autotrophic and heterotrophic types of biomass were here compared in terms of efficiency in the removal of estrone (E1), 17β-estradiol (E2), estriol (E3), 17α-ethynilestradiol (EE2) and bisphenol A (BPA). Experiments were performed with enriched nitrifying activated sludge (NAS) and enriched ammonia oxidizing bacteria (AOB) sludge cultivated at lab-scale, as well as with conventional activated sludge (CAS) from a full-scale wastewater treatment plant. Both enriched NAS and AOB demonstrated a negligible degrading capacity. In both cases, the studied EDCs exhibited low removals (<14%) and showed no correlation with the increasing nitrification rates contradicting some of the hypothesis present in literature. Contrariwise, the biodegradation capabilities of the heterotrophic fraction of CAS were highlighted. E2 and E3 were removed by up to 100% and 78%, respectively. E1 was found to be the main transformation product of E2 (almost quantitative oxidation) and it was also highly eliminated. Finally, EE2 and BPA were more persistent biologically with removals ranging from 10% to 39%. For these two compounds similar removals were obtained during experiments with heat-inactivated biomass suggesting that sorption could be a relevant route of elimination.

The effect of temperature shifts on N2O and NO emissions from a partial nitritation reactor treating reject wastewater

Temperature has a known effect on ammonia oxidizing bacteria (AOB) activities, reducing its ammonia oxidizing rate (AOR) when temperature is lowered. However, little is known concerning its effect on N₂O and NO emissions which are produced during ammonia oxidation having a greenhouse effect. To study this, an AOB enriched partial nitrification sequencing batch reactor (PN-SBR) was operated within a two step-wise feed under 5 different temperatures (30-25-20-15-10 °C). A decrease on the specific AOR (sAOR) was detected when decreasing the temperature. N₂O emissions were also affected by the temperature but only the ones produced during the first aeration of the cycle, when AOBs shifted from a period of low activity to a period of high activity. N₂O emission factors (%) detected during the second aerobic phase were similar among all temperatures tested and lower than the emissions detected during the first aerated phase. The average N₂O emission factor was in the range of 0.15-0.70% N₂O-N/NH₄⁺-N oxidized in the first aeration phase and 0.14-0.15% N₂O-N/NH₄⁺-N-oxidized in the second aeration phase at 10 to 30 °C, respectively. On the other hand, NO emissions were very similar under all temperatures resulting in 0.03-0.06% of NH₄⁺-N oxidized.

Startup strategy for nitrogen removal via nitrite in a BAF system

A biological aerated filter (BAF) pilot plant consisting of two reactors (aerobic and anoxic one) was used to determine a strategy to remove nitrogen via nitrite. RNA/DNA analysis was performed to assess microbial activity and support chemical results. In less than 13 days the pilot plant was able to remove COD and suspended solids. Nitrogen removal via nitrite pathway could not be observed until day 130 when the empty bed contact time (EBCT) was set at 0.71 h. Nitrite was detected in the aerated BAF effluent but never nitrate. qPCR of amoA gene from RNA and DNA extracts of the aerobic biofilm confirmed that ammonia oxidizing bacteria (AOB) were present from the beginning of the operation but not active. AOB activity increased with time, reaching stability from operational day 124. The combination of both, low EBCT together with high OLR, has been demonstrated to be a feasible strategy to startup a BAF to achieve nitrogen removal via nitrite.

Ammonia oxidizers and nitrite-oxidizing bacteria respond differently to long-term manure application in four paddy soils of south of China

Nitrification plays an important role in the soil nitrogen (N) cycle, and fertilizer application may influence soil nitrifiers' abundance and composition. However, the effect of long-term manure application in paddy soils on nitrifying populations is poorly understood. We chose four long-term manure experimental fields in the south of China to study how the abundance and community structure of nitrifiers would change in response to long-term manure application using quantitative PCR and Miseq sequencing analyses. Our results showed that manure application significantly increased ammonia oxidizing archaea (AOA) abundance at the ChangSha (CS) and NanChang (NC) sites, while the abundance of ammonia oxidizing bacteria (AOB) represented 4.8- and 12.8- fold increases at the JiaXing (JX) and YingTan (YT) sites, respectively. Miseq sequencing of 16S rRNA genes indicated that manure application altered the community structure of nitrifying populations, especially at the NC and YT sites. The application of manure significantly changed AOA and nitrite oxidizing bacteria (NOB) community structures but not those of AOB, suggesting that AOA and NOB may be more sensitive to manures. Variation partitioning analysis (VPA) and redundancy analysis (RDA) indicated that soil pH, TN, NO₃^--N and water content were the main factors in shaping nitrifying communities. These findings suggest that nitrifiers respond diversely to manure application, and soil physiochemical properties play an important role in determining nitrifiers' abundance and communities with long-term manure addition.

Unraveling the potential of a combined nitritation-anammox biomass towards the biodegradation of pharmaceutically active compounds

In the past few years, anaerobic ammonium oxidation-based processes have attracted a lot of attention for their implementation at the mainstream line of wastewater treatment plants, due to the possibility of leading to energy autarky if combined with anaerobic digestion. However, little is known about the potential degradation of micropollutants by the microbial groups responsible of these processes and the few results available are inconclusive. This study aimed to assess the degradation capability of biomass withdrawn from a combined nitritation/anaerobic ammonium oxidation (combined N/A) pilot plant towards five pharmaceutically active compounds (ibuprofen, sulfamethoxazole, metoprolol, venlafaxine and carbamazepine). Batch experiments were performed under different conditions by selectively activating or inhibiting different microbial groups: i) regular combined N/A operation, ii) aerobic (optimal for nitrifying bacteria), iii) aerobic with allylthiourea (an inhibitor of ammonia monooxygenase, enzyme of ammonia oxidizing bacteria), iv) anoxic (optimal for anaerobic ammonium oxidizing bacteria), v) aerobic with acetate (optimal for heterotrophic bacteria) and vi) anoxic with acetate (optimal for heterotrophic denitrifying bacteria). Ibuprofen was the most biodegradable compound being significantly degraded (49-100%) under any condition except heterotrophic denitrification. Sulfamethoxazole, exhibited the highest removal (70%) under optimal conditions for nitrifying bacteria but in the rest of the experiments anoxic conditions were found to be slightly more favorable (up to 58%). For metoprolol the highest performance was obtained under anoxic conditions favoring anammox bacteria (62%). Finally, carbamazepine and venlafaxine were hardly removed (≤10% in the majority of cases). Taken together, these results suggest the specificity of different microbial groups that in combination with alternating operational parameters can lead to enhanced removal of some micropollutants.

Model-based assessment of estrogen removal by nitrifying activated sludge

Complete removal of estrogens such as estrone (E1), estradiol (E2), estriol (E3) and ethinylestradiol (EE2) in wastewater treatment is essential since their release and accumulation in natural water bodies are giving rise to environment and health issues. To improve our understanding towards the estrogen bioremediation process, a mathematical model was proposed for describing estrogen removal by nitrifying activated sludge. Four pathways were involved in the developed model: i) biosorption by activated sludge flocs; ii) cometabolic biodegradation linked to ammonia oxidizing bacteria (AOB) growth; iii) non-growth biodegradation by AOB; and iv) biodegradation by heterotrophic bacteria (HB). The degradation kinetics was implemented into activated sludge model (ASM) framework with consideration of interactions between substrate update and microorganism growth as well as endogenous respiration. The model was calibrated and validated by fitting model predictions against two sets of batch experimental data under different conditions. The model could satisfactorily capture all the dynamics of nitrogen, organic matters (COD), and estrogens. Modeling results suggest that for E1, E2 and EE2, AOB-linked biodegradation is dominant over biodegradation by HB at all investigated COD dosing levels. However, for E3, the increase of COD dosage triggers a shift of dominant pathway from AOB biodegradation to HB biodegradation. Adsorption becomes the main contributor to estrogen removal at high biomass concentrations.

Nitrosomonas europaea adaptation to anoxic-oxic cycling: Insights from transcription analysis, proteomics and metabolic network modeling

In suboxic or anoxic environments, nitrous oxide (N₂O) can be produced by ammonia oxidizing bacteria (AOB) as a potent greenhouse gas. Although N₂O producing inventory and pathways have been well-characterized using archetypal AOB, there is little known about their adaptive responses to oxic-anoxic cycling, which is a prevalent condition in soil, sediment, and wastewater treatment bioreactors. In this study, cellular responses of Nitrosomonas europaea 19718 to sustained anoxic-oxic cycling in a chemostat bioreactor were evaluated at transcriptomic, proteomic, and fluxomic levels. During a single oxic-anoxic transition, the accumulations of major intermediates were found at the beginning of anoxia (nitric oxide, NO) and post anoxia (hydroxylamine, NH₂OH, and N₂O). Anoxic-oxic cycling over thirteen days led to significantly reduced accumulations of NH₂OH, NO and N₂O. Distinct from short-term responses, which were mostly regulated at the mRNA level, adapted cells seemed to sustain energy generation under repeated anoxia by partially sacrificing the NO detoxification capacities, and such adaptation was mainly regulated at the protein level. The proteomic data also suggested the potential contributions of the newly discovered cytochrome P460-mediated NH₂OH oxidation pathway to N₂O productions. Flux balance analysis was performed based on a metabolic network model consisting of 49 biochemical reactions involved in nitrogen respiration, and changes in metabolic fluxes after the anoxic-oxic cycling were found to be better correlated with intracellular protein concentrations rather than mRNA levels. Previous studies focusing on single anoxic-oxic transition might have overlooked the adaptive responses of nitrifiers to anoxic-oxic cycling, and thus overestimated NO and N₂O emission levels from natural and engineered nitrification systems.

Chronic toxicity of graphene and graphene oxide in sequencing batch bioreactors: A comparative investigation

The present study investigates the chronic toxicity of graphene (G) and graphene oxide (GO) in activated sludge. Sequencing batch bioreactors were fed with influents containing 0, 1 and 5mgL^-1 of GO or G (12h cycles) for ten days. Reduction in performance of the bioreactors in relation to chemical oxygen demand, ammonia and phosphate removals was observed after three days in the bioreactors fed with 5mgL^-1 of nanomaterials. After about eight days, these reactors reached a steady state nutrient removal, which corresponded to recovery of certain groups of ammonia oxidizing bacteria and phosphate accumulating bacteria despite the increasing accumulation of nanomaterials in the sludge. These results suggested that biological treatment can be affected transiently by initial exposure to the nanomaterials, but certain groups of microorganisms, less sensitive to these nanomaterials, can potentially strive in the presence of these nanomaterials. Results of 16S rRNA gene deep sequencing showed that G and GO affected differently the microbial communities in the activated sludge. Between the two nanomaterials investigated, GO presented the highest impact in nutrient removal, gene abundance and changes in microbial population structures.

Response of ammonia oxidizing archaea and bacteria to decabromodiphenyl ether and copper contamination in river sediments

Ammonia oxidation plays a fundamental role in river nitrogen cycling ecosystems, which is normally governed by both ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB). Co-contamination of typical emerging pollutant Polybrominated diphenyl ethers (PBDEs) and heavy metal on AOA and AOB communities in river sediments remains unknown. In this study, multiple analytical tools, including high-throughput pyrosequencing and real-time quantitative PCR (qPCR), were used to reveal the ammonia monooxygenase (AMO) activity, subunit alpha (amoA) gene abundance, and community structures of AOA and AOB in river sediments. It was found that the inhibition of AMO activities was increased with the increase of decabromodiphenyl ether (BDE 209, 1-100 mg kg^-1) and copper (Cu, 50-500 mg kg^-1) concentrations. Moreover, the synergic effects of BDE 209 and Cu resulted in a higher AMO activity reduction than the individual pollutant BDE 209. The AOA amoA copy number declined by 75.9% and 83.2% and AOB amoA gene abundance declined 82.8% and 90.0% at 20 and 100 mg kg^-1 BDE 209 with a 100 mg kg^-1 Cu co-contamination, respectively. The pyrosequencing results showed that both AOB and AOA community structures were altered, with a higher change of AOB than that of AOA. The results demonstrated that the AOB microbial community may be better adapted to BDE 209 and Cu pollution, while AOA might possess a greater capacity for stress resistance. Our study provides a better understanding of the ecotoxicological effects of heavy metal and micropollutant combined exposure on AOA and AOB in river sediments.

Removal of pharmaceuticals and personal care products by ammonia oxidizing bacteria acclimated in a membrane bioreactor: Contributions of cometabolism and endogenous respiration

We carried out batch experiments using biomass from a membrane bioreactor (MBR) to study the influence of ammonia oxidizing bacteria (AOB) on the removal of 45 pharmaceuticals and personal care products (PPCPs). Kinetic parameters such as biodegradation constants and adsorption coefficients with and without AOB inhibition were estimated. No significant differences in adsorption tendency were found, but the biodegradability of most compounds was enhanced when ammonia was completely oxidized, indicating that AOB present in MBR played a critical role in eliminating the PPCPs. Moreover, target PPCPs were degraded in 2 stages, first by cometabolic degradation related to AOB growth, and then by endogenous respiration by microorganisms in the absence of other growth substrate. The compounds were classified into 3 groups according to removal performance and cometabolic degradation. Our approach provides new insight into the removal of PPCPs via cometabolism and endogenous respiration under AOB enrichment cultures developed in MBR.

Unraveling the active microbial populations involved in nitrogen utilization in a vertical subsurface flow constructed wetland treating urban wastewater

The dynamics of the active microbial populations involved in nitrogen transformation in a vertical subsurface flow constructed wetland (VF) treating urban wastewater was assessed. The wetland (1.5m²) operated under average loads of 130gCODm^-2d^-1 and 17gTNm^-2d^-1 in Period I, and 80gCODm^-2d^-1 and 19gTNm^-2d^-1 in Period II. The hydraulic loading rate (HLR) was 375mmd^-1 and C/N ratio was 2 in both periods. Samples for microbial characterization were collected from the filter medium (top and bottom layers) of the wetland, water influent and effluent at the end of Periods I (Jun-Oct) and II (Nov-Jan). The combination of qPCR and high-throughput sequencing (NGS, MiSeq) assessment at DNA and RNA level of 16S rRNA genes and nitrogen-based functional genes (amoA and nosZ-clade I) revealed that nitrification was associated both with ammonia-oxidizing bacteria (AOB) (Nitrosospira) and ammonia-oxidizing archaea (AOA) (Nitrososphaeraceae), and nitrite-oxidizing bacteria (NOB) such as Nitrobacter. Considering the active abundance (based in amoA transcripts), the AOA population revealed to be more stable than AOB in both periods and depths of the wetland, being less affected by the organic loading rate (OLR). Although denitrifying bacteria (nosZ copies and transcripts) were actively detected in all depths, the denitrification process was low (removal of 2gTNm^-2d^-1 for both periods) concomitant with NO_x-N accumulation in the effluent. Overall, AOA, AOB and denitrifying bacteria (nosZ) were observed to be more active in bottom than in top layer at lower OLR (Period II). A proper design of OLR and HLR seems to be crucial to control the activity of microbial biofilms in VF wetlands on the basis of oxygen, organic-carbon and NO_x-N forms, to improve their capacity for total nitrogen removal.

Biotransformation of acyclovir by an enriched nitrifying culture

This work evaluates the biodegradation of the antiviral drug acyclovir by an enriched nitrifying culture during ammonia oxidation and without the addition of ammonium. The study on kinetics was accompanied with the structural elucidation of biotransformation products through batch biodegradation experiments at two different initial levels of acyclovir (15 mg L^-1 and 15 μg L^-1). The pseudo first order kinetic studies of acyclovir in the presence of ammonium indicated the higher degradation rates under higher ammonia oxidation rates than those constant degradation rates in the absence of ammonium. The positive correlation was found between acyclovir degradation rate and ammonia oxidation rate, confirming the cometabolism of acyclovir by the enriched nitrifying culture in the presence of ammonium. Formation of the product carboxy-acyclovir (P239) indicated the main biotransformation pathway was aerobic oxidation of the terminal hydroxyl group, which was independent on the metabolic type (i.e. cometabolism or metabolism). This enzyme-linked reaction might be catalyzed by monooxygenase from ammonia oxidizing bacteria or heterotrophs. The formation of carboxy-acyclovir was demonstrated to be irrelevant to the acyclovir concentrations applied, indicating the revealed biotransformation pathway might be the dominant removal pathway of acyclovir in wastewater treatment.

Development of partial nitrification as a first step of nitrite shunt process in a Sequential Batch Reactor (SBR) using Ammonium Oxidizing Bacteria (AOB) controlled by mixing regime

Shortcut biological nitrogen removal is a non-conventional way of removing nitrogen from wastewater using two processes either nitrite shunt or deammonification. In the nitrite shunt process, the ammonia oxidation step stops at the nitrite stage, which is known as partial nitrification, then nitrite is directly reduced to nitrogen gas. Effective partial nitrification could be achieved by accumulating Ammonia Oxidizing Bacteria (AOB) and inhibiting Nitrite Oxidizing Bacteria (NOB). In this research, a novel control strategy has been developed to control the DO using the variable mixing regime in a suspended growth system using a Sequential Batch Reactor (SBR) in order to achieve a stable ammonia removal efficiency (ARE) and nitrite accumulation rate (NAR) at a high nitrogen loading rate (NLR). The new controlled SBR system has been successfully running at NLR up to 1.2kg/(m³.day) and achieved an ARE of 98.6±2.8% and NAR of 93.0±0.7%.

Biotransformation of pharmaceuticals by ammonia oxidizing bacteria in wastewater treatment processes

Pharmaceutical residues could potentially pose detrimental effects on aquatic ecosystems and human health, with wastewater treatment being one of the major pathways for pharmaceuticals to enter into the environment. Enhanced removal of pharmaceuticals by ammonia oxidizing bacteria (AOB) has been widely observed in wastewater treatment processes. This article reviews the current knowledge on the biotransformation of pharmaceuticals by AOB. The relationship between the pharmaceuticals removal and nitrification process was revealed. The important role of AOB-induced cometabolism on the biotransformation of pharmaceuticals as well as their transformation products and pathways was elucidated. Kinetics and mathematical models describing the biotransformation of pharmaceuticals by AOB were also reviewed. The results highlighted the high degradation capabilities of AOB toward some refractory pharmaceuticals, with their degradations being clearly related to the nitrification rate and their transformation products being identified, which may exhibit similar or higher ecotoxicological impacts compared to the parent compound.

Response of performance and ammonia oxidizing bacteria community to high salinity stress in membrane bioreactor with elevated ammonia loading

Effect of elevated ammonia loading rate (ALR) and increasing salinity on the operation of membrane bioreactor (MBR) and the response of microbial community were investigated. Results showed that MBR started up with 1% NaCl stress achieved amazing nitrification performance at high salinity up to 4% when treating wastewater containing 1000mg/L NH(+)4-N. Further increasing salinity to 7% led to failure of MBR unrecoverably. Steep decline of sludge activity contributed to the extremely worse performance. High-throughput sequencing analysis showed that both ALR and salinity had selective effects on the microbial community structure. In genus level, Methyloversatilis and Maribacter were enriched during the operation. Survival of salt-resistant microbes contributed to the rising of richness and diversity at 2% and 4% NaCl stress. Analysis of amoA-gene-based cloning revealed Nitrosomonas marina are chiefly responsible for catalyzing ammonia oxidation in high ALR at high salinity stress.

Enhanced sulfamethoxazole degradation through ammonia oxidizing bacteria co-metabolism and fate of transformation products

The occurrence of the widely-used antibiotic sulfamethoxazole (SFX) in wastewaters and surface waters has been reported in a large number of studies. However, the results obtained up-to-date have pointed out disparities in its removal. This manuscript explores the enhanced biodegradation potential of an enriched culture of Ammonia Oxidizing Bacteria (AOB) towards SFX. Several sets of batch tests were conducted to establish a link between SFX degradation and specific ammonia oxidation rate. The occurrence, degradation and generation of SFX and some of its transformation products (4-Nitro SFX, Desamino-SFX and N(4)-Acetyl-SFX) was also monitored. A clear link between the degradation of SFX and the nitrification rate was found, resulting in an increased SFX removal at higher specific ammonia oxidation rates. Moreover, experiments conducted under the presence of allylthiourea (ATU) did not present any removal of SFX, suggesting a connection between the AMO enzyme and SFX degradation. Long term experiments (up to 10 weeks) were also conducted adding two different concentrations (10 and 100 μg/L) of SFX in the influent of a partial nitrification sequencing batch reactor, resulting in up to 98% removal. Finally, the formation of transformation products during SFX degradation represented up to 32%, being 4-Nitro-SFX the most abundant.

A two pathway model for N2O emissions by ammonium oxidizing bacteria supported by the NO/N2O variation

In this work, a new model for nitritation combining two N2O emission pathways was confronted with both NO and N2O measurements during nitrification. The model was calibrated with batch experiments and validated with long-term data collected in a sequencing batch reactor (SBR). A good prediction of the evolution of N2O emissions for a varying level of nitrite was demonstrated. The NO/N2O ratio was shown to vary during nitritation depending on the nitrite level. None of the models based on a single pathway could describe this variation of the NO/N2O ratio. In contrast, the 2 pathway model was capable of describing the trends observed for the NO/N2O ratio and gave better predictions of N2O emission factors. The model confirmed that the decrease of the NO/N2O ratio can be explained by an increase of the ND pathway to the detriment of the NN pathway. The ND pathway was systematically the predominant pathway during nitritation. The combined effect of nitrite (or free nitrous acid) and dissolved oxygen (DO) on the contribution of each pathway was in agreement with practical observations and the literature.

N2O production by ammonia oxidizing bacteria in an enriched nitrifying sludge linearly depends on inorganic carbon concentration

The effect of inorganic carbon (IC) on nitrous oxide (N2O) production by ammonia oxidizing bacteria (AOB) was investigated over a concentration range of 0-12 mmol C/L, encompassing typical IC levels in a wastewater treatment reactors. The AOB culture was enriched along with nitrite-oxidizing bacteria (NOB) in a sequencing batch reactor (SBR) to perform complete nitrification. Batch experiments were conducted with continuous carbon dioxide (CO2) stripping or at controlled IC concentrations. The results revealed a linear relationship between N2O production rate (N2OR) and IC concentration (R(2) = 0.97) within the IC range studied, suggesting a substantial effect of IC on N2O production by AOB. Similar results were also obtained with an AOB culture treating anaerobic sludge digestion liquor. The fundamental mechanism responsible for this dependency is unclear; however, in agreement with previous studies, it was observed that the ammonia oxidation rate (AOR) was also influenced by the IC concentration, which could be well described by the Monod kinetics. These resulted in an exponential relationship between N2OR and AOR, as previously observed in experiments where AOR was altered by varying dissolved oxygen and ammonia concentrations. It is therefore possible that IC indirectly affected N2OR by causing a change in AOR. The observation in this study indicates that alkalinity (mostly contributed by IC) could be a significant factor influencing N2O production and should be taken into consideration in estimating and mitigating N2O emissions in wastewater treatment systems.

The combined effect of dissolved oxygen and nitrite on N2O production by ammonia oxidizing bacteria in an enriched nitrifying sludge

Both nitrite [Formula: see text] and dissolved oxygen (DO) play important roles in nitrous oxide (N2O) production by ammonia oxidizing bacteria (AOB). However, few studies focused on the combined effect of them on N2O production by AOB as well as the corresponding mechanisms. In this study, N2O production by an enriched nitrifying sludge, consisting of both AOB and nitrite-oxidizing bacteria (NOB), was investigated under various [Formula: see text] and DO concentrations. At each investigated DO level, both the biomass specific N2O production rate and the N2O emission factor (the ratio between N2O nitrogen emitted and the ammonium nitrogen converted) increased as [Formula: see text] concentration increased from 3 mg N/L to 50 mg N/L. However, at each investigated [Formula: see text] level, the maximum biomass specific N2O production rate occurred at DO of 0.85 mg O2/L, while the N2O emission factor decreased as DO increased from 0.35 to 3.5 mg O2/L. The analysis of the process data using a mathematical N2O model incorporating both the AOB denitrification and hydroxylamine (NH2OH) oxidation pathways indicated that the contribution of AOB denitrification pathway increased as [Formula: see text] concentration increased, but decreased as DO concentration increased, accompanied by a corresponding change in the contribution of NH2OH oxidation pathway to N2O production. The AOB denitrification pathway was predominant in most cases, with the NH2OH oxidation pathway making a comparable contribution only at high DO level (e.g. 3.5 mg O2/L).

Ammonia-oxidizing microbial communities in reactors with efficient nitrification at low-dissolved oxygen

Ammonia-oxidizing microbial communities involved in ammonia oxidation under low dissolved oxygen (DO) conditions (<0.3 mg/L) were investigated using chemostat reactors. One lab-scale reactor (NS_LowDO) was seeded with sludge from a full-scale wastewater treatment plant (WWTP) not adapted to low-DO nitrification, while a second reactor (JP_LowDO) was seeded with sludge from a full-scale WWTP already achieving low-DO nitrifiaction. The experimental evidence from quantitative PCR, rDNA tag pyrosequencing, and fluorescence in situ hybridization (FISH) suggested that ammonia-oxidizing bacteria (AOB) in the Nitrosomonas genus were responsible for low-DO nitrification in the NS_LowDO reactor, whereas in the JP_LowDO reactor nitrification was not associated with any known ammonia-oxidizing prokaryote. Neither reactor had a significant population of ammonia-oxidizing archaea (AOA) or anaerobic ammonium oxidation (anammox) organisms. Organisms isolated from JP_LowDO were capable of autotrophic and heterotrophic ammonia utilization, albeit without stoichiometric accumulation of nitrite or nitrate. Based on the experimental evidence we propose that Pseudomonas, Xanthomonadaceae, Rhodococcus, and Sphingomonas are involved in nitrification under low-DO conditions.

Effect of plant species on nitrogen recovery in aquaponics

Nitrogen transformations in aquaponics with different edible plant species, i.e., tomato (Lycopersicon esculentum) and pak choi (Brassica campestris L. subsp. chinensis) were systematically examined and compared. Results showed that nitrogen utilization efficiencies (NUE) of tomato- and pak choi-based aquaponic systems were 41.3% and 34.4%, respectively. The abundance of nitrifying bacteria in tomato-based aquaponics was 4.2-folds higher than that in pak choi-based aquaponics, primarily due to its higher root surface area. In addition, tomato-based aquaponics had better water quality than that of pak choi-based aquaponics. About 1.5-1.9% of nitrogen input were emitted to atmosphere as nitrous oxide (N2O) in tomato- and pak choi-based aquaponic systems, respectively, suggesting that aquaponics is a potential anthropogenic source of N2O emission. Overall, this is the first intensive study that examined the role plant species played in aquaponics, which could provide new strategy in designing and operating an aquaponic system.

Differential sensitivity of nitrifying bacteria to silver nanoparticles in activated sludge

Nitrification is known as one of the most sensitive processes affected when activated sludge is exposed to antimicrobial silver nanoparticles (AgNPs). The impact of AgNPs and their released silver ions (Ag(+) ) on the abundance, activity, and diversity of different nitrifying bacteria in wastewater treatment plants (WWTPs), however, is poorly understood. The present study investigated the impacts of 2 sizes of AgNPs (5 nm and 35 nm) and Ag(+) ions on the nitrifier community in activated sludge, including both ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). Ammonia-oxidizing bacteria were more sensitive to AgNPs than the NOB; a 5-d and 7-d exposure of activated sludge to 35 nm AgNPs (40 ppm) significantly reduced AOB abundance to 24% and 19%, respectively. This finding was confirmed further by a decrease in activated sludge ammonia oxidation activity measured by (14) C-labeled bicarbonate uptake. In contrast, neither AgNPs (up to 40 ppm) nor Ag(+) (1 ppm) affected the abundance of NOB. Both 5 nm and 35 nm AgNPs decreased the diversity of AOB, as indicated by denaturing gradient gel electrophoresis with ammonia monooxygenase gene (amoA) primers, although some unknown Nitrosomonas species were relatively resistant to AgNPs. The generally greater resistance of NOB than AOB to AgNPs suggests that the accumulation of bacteriostatic nitrite in WWTPs is unlikely to be exacerbated due to the accidental or incidental release of AgNPs.

Communities of sediment ammonia-oxidizing bacteria along a coastal pollution gradient in the East China Sea

Anthropogenic nitrogen (N) discharges has caused eutrophication in coastal zones. Ammonia-oxidizing bacteria (AOB) convert ammonia to nitrite and play important roles in N transformation. Here, we used pyrosequencing based on the amoA gene to investigate the response of the sediment AOB community to an N pollution gradient in the East China Sea. The results showed that AOB assemblages were primarily affiliated with Nitrosospira-like lineages, and only 0.4% of those belonged to Nitrosomonas-like lineage. The Nitrosospira-like lineage was separated into four clusters that were most similar to the sediment AOB communities detected in adjacent marine regions. Additionally, one clade was out grouped from the AOB lineages, which shared the high similarities with pmoA gene. The AOB community structures substantially changed along the pollution gradient, which were primarily shaped by NH4(+)-N, NO3(-)-N, SO4(2)(-)-S, TP and Eh. These results demonstrated that coastal pollution could dramatically influence AOB communities, which, in turn, may change ecosystem function.

Clarifying the regulation of NO/N2O production in Nitrosomonas europaea during anoxic-oxic transition via flux balance analysis of a metabolic network model

The metabolic mechanism regulating the production of nitric and nitrous oxide (NO, N2O) in ammonia oxidizing bacteria (AOB) was characterized by flux balance analysis (FBA) of a stoichiometric metabolic network (SMN) model. The SMN model was created using 51 reactions and 44 metabolites of the energy metabolism in Nitrosomonas europaea, a widely studied AOB. FBA of model simulations provided estimates for reaction rates and yield ratios of intermediate metabolites, substrates, and products. These estimates matched well, deviating on average by 15% from values for 17 M yield ratios reported for non-limiting oxygen and ammonium concentrations. A sensitivity analysis indicated that the reactions catalysed by cytochromes aa3 and P460 principally regulate the pathways of NO and N2O production (hydroxylamine oxidoreductase mediated and nitrifier denitrification). FBA of simulated N. europaea exposure to oxic-anoxic-oxic transition indicated that NO and N2O production primarily resulted from an intracellular imbalance between the production and consumption of electron equivalents during NH3 oxidation, and that NO and N2O are emitted when the sum of their production rates is greater than half the rate of NO oxidation by cytochrome P460.

Co-metabolic oxidation of pharmaceutical compounds by a nitrifying bacterial enrichment

The biotransformation of five selected pharmaceuticals ibuprofen (IBP), ketoprofen (KTP), carbamazepine (CBZ), dexamethasone (DXM) and iopromide (IOP) by a stable nitrifying enrichment culture was investigated at concentrations ranging between 25 μg/L and 2mg/L. Complete biotransformation was observed only for IBP and KTP, although, an inverse correlation between transformation rate and concentration was found. The transformation pattern observed is consistent with ammonia monooxygenase (AMO) activity. The metabolic succession of the compounds according to the biotransformation rates was: IBP>KTP>DXM>CBZ>IOP. A linear correlation between the calculated diffusive flux of the model compounds across a bilayer membrane and their biotransformation rates was found. Our results support the concept that augmentation with nitrifying activity can enhance the removal of trace organic pollutants during effluent treatment. Furthermore, ammonia-oxidizing activity appears as a good indicator for estimation of potential of biodegradability of pharmaceuticals, especially at low concentrations.

Control of nitratation in an oxygen-limited autotrophic nitrification/denitrification rotating biological contactor through disc immersion level variation

With oxygen supply playing a crucial role in an oxygen-limited autotrophic nitrification/denitrification (OLAND) rotating biological contactor (RBC), its controlling factors were investigated in this study. Disc rotation speeds (1.8 and 3.6rpm) showed no influence on the process performance of a lab-scale RBC, although abiotic experiments showed a significant effect on the oxygenation capacity. Estimations of the biological oxygen uptake rate revealed that 85-89% of the oxygen was absorbed by the microorganisms during the air exposure of the discs. Indeed, increasing the disc immersion (50 to 75-80%) could significantly suppress undesired nitratation, on the short and long term. The presented results demonstrated that nitratation could be controlled by the immersion level and revealed that oxygen control in an OLAND RBC should be predominantly based on the atmospheric exposure percentage of the discs.

Partial bioaugmentation to remove 3-chloroaniline slows bacterial species turnover rate in bioreactors

Bioaugmentation is a potentially powerful tool to direct community structure and metabolic capacities in bioreactors. Yet the outcome of bioaugmentation studies is usually unpredictable and effects on microbial community dynamics are poorly understood. We asked the question whether bioaugmentation could prevent a diversity shift induced by a model toxin, 3-chloroaniline (3-CA), regardless of whether 3-CA was degraded. Four replicate membrane bioreactors (MBRs) operating in parallel were amended with Pseudomonas putida UWC3 (pWDL7::rfp), a strain that carries the upper pathway genes necessary for partial degradation of 3-CA on its plasmid. Two MBRs served as controls and two MBRs were exposed to 3-CA for 71 days. Despite the selective pressure imposed by 3-CA, there was little or no 3-CA removal and neither the 16S rRNA gene of the augmented strain UWC3 nor the plasmid pWDL7::rfp proliferated in any of the reactors. Yet both host strain and plasmid were maintained at reduced levels (~10(4) host strain cells ml(-1)) in all reactors compared to the initial inoculum (~10(7) cells ml(-1); 1% of active cells). Additionally, the microbial community dynamics were evaluated for each MBR via terminal restriction fragment length polymorphism (T-RFLP) analysis (n = 15 per reactor) that targeted a portion of the 16S rRNA gene. Analysis comprised of a suite of multivariate statistics coupled with a theoretical microbial ecological approach, 'Island Biogeography', using a bacterial species time relationship (STR), within each MBR. Control MBRs had a wider range in w values than the treatment MBRs, which is attributed to the lack of a toxin selecting for biota that can withstand its toxic nature. Bioaugmentation alone strongly slowed the bacterial species turnover rate (as revealed by very low w scaling components), compared to non-bioaugmented reactors from a previous study, but did not protect the microbial community from a diversity shift caused by the toxin. Nonmetric multidimensional scaling (NMDS) analysis revealed that treatment MBRs diverged away from the control MBRs after the first 11 days, whereas control MBRs remained clustered. Individual reactors were analyzed by multi-response permutation procedures (MRPP) and a significant difference was found between each control MBR and the treatment MBRs. The study suggests that newly introduced strains can gain a foothold in established microbial communities even at low cell concentrations (about 1% of introduced concentration within the first week) regardless of selective pressure, whereas community dynamics are more affected by the presence of a selector toxin.

Effects of nitrogen application rate and a nitrification inhibitor dicyandiamide on ammonia oxidizers and N2O emissions in a grazed pasture soil

Ammonia oxidizers, including ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) are important drivers of a key step of the nitrogen cycle - nitrification, which affects the production of the potent greenhouse gas, nitrous oxide (N2O). A field experiment was conducted to determine the effect of nitrogen application rates and the nitrification inhibitor dicyandiamide (DCD) on the abundance of AOB and AOA and on N2O emissions in a grazed pasture soil. Nitrogen (N) was applied at four different rates, with urea applied at 50 and 100 kg N ha(-1) and animal urine at 300 and 600 kg N ha(-1). DCD was applied to some of the N treatments at 10 kg ha(-1). The results showed that the AOB amoA gene copy numbers were greater than those of AOA. The highest ratio of the AOB to AOA amoA gene copy numbers was 106.6 which occurred in the urine-N 600 treatment. The AOB amoA gene copy numbers increased with increasing nitrogen application rates. DCD had a significant impact in reducing the AOB amoA gene copy numbers especially in the high nitrogen application rates. N2O emissions increased with the N application rates. DCD had the most significant effect in reducing the daily and total N2O emissions in the highest nitrogen application rate. The greatest reduction of total N2O emissions by DCD was 69% in the urine-N 600 treatment. The reduction in the N2O emission factor by DCD ranged from 58% to 83%. The N2O flux and NO3(-)-N concentrations were significantly correlated to the growth of AOB, rather than AOA. This study confirms the importance of AOB in nitrification and the effect of DCD in inhibiting AOB growth and in decreasing N2O emissions in grazed pasture soils under field conditions.

Nitritation and denitrifying phosphorus removal via nitrite pathway from domestic wastewater in a continuous MUCT process

Nitritation and denitrifying P removal under mode of nitritation and nitrification was investigated in continuous MUCT process treating domestic wastewater. Nitritation was established through short hydraulic retention time to 6 h and low dissolved oxygen concentration of 0.3-0.5 mg/L. Nitritation was stabilized for 95 days with average nitrite accumulation ratio over 90%. Ammonia and total nitrogen removal under nitritation reached 99% and 83%, respectively, much better than complete nitrification. Real-time quantitative PCR assays presented that cell numbers and percentages of ammonia oxidizing bacteria (AOB) population had a clear correlation with nitrite accumulation ratios. The highest percentage of AOB was 13% of total bacterial population. P removal was mainly completed by denitrifying P removal of about 90% occurring in anoxic zone. The P removal efficiency under nitritation was 30% higher than that under complete nitrification. Denitrifying P removal under nitritation was highly beneficial to the treatment of wastewater with limiting carbon source.

Inhibition of phenol on the rates of ammonia oxidation by Nitrosomonas europaea grown under batch, continuous fed, and biofilm conditions

Ammonia oxidation by Nitrosomonas europaea, an ammonia oxidizing bacterium prevalent in wastewater treatment, is inhibited in the presence of phenol, due to interaction of the phenol with the ammonia monooxygenase enzyme. Suspended cells of N. europaea were cultured in batch reactors and continuous flow reactors at dilution rates of 0.01-0.2 d(-1). The rate of ammonia oxidation in the continuous cultures correlated to the dilution rate in the reactor. The batch and continuous cultures were exposed to 20 μM phenol and ammonia oxidation activity was measured by specific oxygen uptake rates (SOURs). Inhibition of NH3 oxidation by 20 μM phenol ranged from a 77% reduction of SOUR observed with suspended cells harvested during exponential growth, to 26% in biofilms. The extent of inhibition was correlated with ammonia oxidation rates in both suspended and biofilm cells, with greater percent inhibition observed with higher initial rates of NH3 oxidation. In biofilm grown cells, an increase in activity and phenol inhibition were both observed upon dispersing the biofilm cells into fresh, liquid medium. Under higher oxygen tension, an increase in the NO2(-) production of the biofilms was observed and biofilms were more susceptible to phenol inhibition. Dissolved oxygen microsensor measurements showed oxygen limited conditions existed in the biofilms. The ammonia oxidation rate was much lower in biofilms, which were less inhibited during phenol exposure. The results clearly indicate in both suspended and attached cells of N. europaea that a higher extent of phenol inhibition is positively correlated with a higher rate of NH3 oxidation (enzyme turnover).