Mechanisms of Persistence of the Ammonia-Oxidizing Bacteria Nitrosomonas to the Biocide Free Nitrous Acid

Integration of steel slag and zeolite enhances simultaneous nitrification and autotrophic denitrification in ultra-low carbon/nitrogen ratio wastewater: Remodeling microbiota and iron metabolism

Constructed wetlands (CWs) are widely used for nitrogen pollution control in rural aquatic environments, yet their nitrogen removal efficiency often remains suboptimal. This study firstly examines how zeolite robustly stimulates Fe-utilization of steelmaking waste (i.e., steel slag) to improve nitrification and autotrophic denitrification of low carbon-to-nitrogen (C/N) ratio wastewater (C/N ≈ 1). Steel slag, by providing alkalinity for nitrification, also serves as an electron donor for denitrification due to its low-valent iron content. As a result, the total nitrogen (TN) removal efficiency was increased by 153.5% compared to the control group. Zeolite reshaped the microbial consortia, enriching iron autotrophic denitrifying bacteria and aerobic denitrifying bacteria. More importantly, zeolite facilitated microbial iron utilization by enhancing transmembrane iron transport and intracellular iron oxidation to boost nitrification and autotrophic denitrification without additional aeration, external carbon sources, or pH regulation. Our work advances understanding the development of low carbon technologies for wastewater nitrogen removal.

Insights into in-situ free nitrous acid induced extracellular polymeric substances changes and membrane fouling mitigation in a nitritation membrane bioreactor

This study investigated the effectiveness of free nitrous acid (FNA) on mitigating membrane fouling, with the associated mechanisms, in two nitritation membrane bioreactors (MBRs) operated with Nitrosomonas-enriched culture. Results showed that FNA stress, regulated by pH and nitrite concentration, maintained a low-level fouling as opposed to the control MBR where trans-membrane pressure (TMP) exceeded 30 kPa. Compared to the control MBR, production of biofilm in the FNA stressed MBR was reduced by 68.1% in terms of mass and 78.2% in terms of thickness. Suspended biomass and biofilm extracellular polymeric substances (EPS) characterized by liquid chromatography (LC-OCD-OND) indicated FNA stress reduced the amount of low molecular weight neutrals and hydrophobic dissolved organic carbon. These components would have had high fouling potential. Excitation emission matrix (EEM) fluorescence contours indicated that exposure to FNA stimulated the production of tyrosine-like proteins but reduced those of SMP like and humic acid-like substances. This could have affected the adhesion between bacteria and membrane and so contributed to the reduced biofilm and fouling. X-ray photoelectron spectroscopy (XPS) analysis revealed marked differences in intensities of the main functionalities in the EPS for both sludge and biofilm, due to the oxidative effect of FNA, e.g. FNA stress resulted in more aliphatic C-OH, amines and amides while the control had more C=O, amino acids and amino sugars. This study showed that in-situ generated FNA could be employed to mitigate membrane fouling effectively via its biocidal and oxidative effect.

A Novel Perspective on the Instability of Mainstream Partial Nitrification: The Niche Differentiation of Nitrifying Guilds

Short-cut biological nitrogen removal (sBNR) favors the paradigm shift toward energy-positive and carbon-neutral wastewater treatment processes. Partial nitrification (PN) is a key approach to provide nitrite for anammox or denitritation during sBNR, and its stability is the precondition for achieving robust nitrogen removal performance. However, maintaining a stable mainstream PN process has been a long-standing challenge. This review analyzes the mainstream PN process from a microbial ecology perspective, focusing on the niche differentiation among nitrifiers. First, we propose that mainstream PN systems are ecologically unstable, and the failure of the mainstream PN process due to the reactivation of nitrite-oxidizing bacteria (NOB) can be regarded as a behavior to restore system stabilization. Thus, maintaining mainstream PN systems primarily relies on enhancing the niche differentiation between ammonia-oxidizing bacteria (AOB) and NOB. We then summarize the realized niches of indigenous nitrifiers within nitrification systems and discuss their ecophysiological characteristics (e.g., cell structure and substrate affinity) that define their specific ecological niches. By comparing the niche breadths of AOB and NOB on various niche axes, we further discuss their niche differentiation and identify the different responses of AOB (resistance) and NOB (resilience) to exogenous perturbations. Finally, we propose outlook for achieving a stable mainstream PN process through an ecological lens. This review provides ecological insights into the instability of the mainstream PN process, which is intended to guide the derivation of optimized strategies from a single-factor approach to integrated solutions.

A chemically enhanced primary treatment and anammox-based process for sustainable municipal wastewater treatment: The advantage and application prospects

Low-carbon nitrogen removal, bioenergy production, and phosphorus recovery are key goals for sustainable municipal wastewater treatment. Traditional activated sludge processes face an energy demand conflict. Anaerobic ammonium oxidation (Anammox) offers a solution to this issue, with the A-B process providing a sustainable approach. Stable and cost-effective nitrite supply for mainstream anammox has gained attention, while the interactions between A-B stage processes are also crucial. This paper reviews the benefits and challenges of mainstream anammox, bioenergy, and phosphorus recovery. A combined process of chemically enhanced primary treatment, partial denitrification and anammox is identified as effective for sustainable treatment. Additionally, the stable nitrite supply from the sidestream partial nitrification provides a 54% nitrogen removal contribution to the mainstream anammox. Anaerobic digestion with sulfate reduction is proposed as an efficient method for simultaneous bioenergy and phosphorus recovery from iron-enhanced primary sludge. The recycling of iron and sulfate reduces excess sludge and cuts costs. A novel wastewater treatment scheme, supported by a mass balance analysis, is presented; the proposed process is capable of recovering >50% of the carbon and phosphorus, while reduced 40% dosing of Fe and S chemicals, reducing the cost of chemical dosing and treatment of the digestate while meeting the high-quality effluent. The paper also explores the potential for transitioning from conventional activated sludge processes and suggests directions for future research.

Achieving stable partial nitrification by exploiting lag phase of NOB recovery for selective washout

Stable inhibition of nitrite-oxidizing bacteria (NOB) is a significant challenge in achieving partial nitrification (PN) and partial nitrification-anaerobic ammonia oxidation (PNA). Growing evidence suggested that NOB can develop resistance to suppression over time, leading to the re-enrichment of NOB within reactors. To address these issues, this study aimed to achieve stable PN by regulating SRT to selectively washout NOB during the lag phase of activity recovery following FA/FNA exposure. The effects of this new strategy on nitrifying bacteria, sludge characteristics, and microbial interspecies relationships were investigated over a 150-day trial. Under the new strategy operation, the effluent ammonia, nitrite, and nitrate concentrations were 4.72 mg/L, 51.81 mg/L, and 3.30 mg/L, respectively, at a dissolved oxygen (DO) concentration of 0.18 mg/L, with a nitrite accumulation rate (NAR) remaining above 95%. This was attributed to the increasing relative abundance of ammonia-oxidizing bacteria (AOB) (Nitrosomonas) up to 25.86% and the decreasing relative abundance of NOB (Nitrospira, Nitrobacter and Candidatus Nitrotoga) to below the detection limit (1%) during the stabilization period. Analysis of the PN sludge characteristics revealed a tendency for sludge to form loosely structured aggregates, facilitating the potential rapid start-up of the PNA biofilm system or granular sludge. These findings suggest that the new strategy is a straightforward and effective method for achieving PN.

Biotic and abiotic removal of acetaminophen during sidestream partial nitritation processes: Underlying mechanisms and transformation pathways

Pharmaceutical residues in sidestream wastewater pose the hazardous threats to ecosystem and human health. In this work, the biotic and abiotic degradation of acetaminophen were investigated during the sidestream partial nitritation process. Results demonstrated that the abiotic removal efficiency of acetaminophen was positively correlated with nitrite concentration, whereas the biotransformation of acetaminophen was mainly dependent on metabolic types and free nitrous acid (FNA) concentrations. 91.6 % of acetaminophen, acting as the sole carbon and/or energy source to support the growth of ammonia-oxidizing bacteria (AOB) and heterotrophs, was removed by adsorption (6.2 %) and biotransformation (consisting of 49.4 % AOB-induced metabolism and 36.0 % heterotrophs-induced metabolism) when lacking nitrite and FNA. Increasing FNA from 0.03 mg N L-1 to 0.15 mg L-1 led to decrease in acetaminophen removal (from 78.8 % to 60.1 %) and ammonia oxidation, ascribed to the inhibitory effect of FNA on AOB activity. Nitro substitution occurred under AOB-induced cometabolism, while hydroxylation was conducted by heterotrophs. N-deacetylation, ring cleavage, hydroxylation, nitro-reduction, and deamination at lower FNA levels (0.03 mg N L-1) contributed to the formation of small molecular products, supporting the feasibility of sidestream partial nitritation in the effective elimination of acetaminophen. This work provides strategies for optimizing anti-inflammatory drugs removal via the regulation of FNA in the sidestream wastewater treatment process.

Effect of Polyethylene Glycol and Humic Acid Coating on NPK Release From Controlled-Release Fertilizer

Plants require essential macronutrients such as nitrogen (N), phosphorus (P), and potassium (K), but their availability in soil is often inefficient due to evaporation, leaching, and binding. Controlled-release fertilizers (CRFs) provide a solution by regulating nutrient release over time. This study evaluates the effects of two coating materials, polyethylene glycol (PEG) and humic acid, on the release patterns of N, P, and K during an 18-week incubation using Inceptisol from Bogor, Indonesia. Various CRF treatments were tested, including uncoated (A1, A2), PEG-coated (B1, B2), and humic acid-coated (C1, C2) formulations. Results showed that CRF with PEG (B2) demonstrated slower N release, with ammonium levels decreasing from 32.22% in week 1 to 9.36% by week 18. Nitrate release increased steadily from 26.37% to 37.36% between weeks 3 and 18. In contrast, CRF with humic acid (C2) showed slower nitrate release, reaching 36.26% by the end of incubation. P release patterns were similar across treatments, while K release was lowest in the humic acid-coated treatment (C2) at 24.48%. These findings underline the potential of coating materials like PEG and humic acid to optimize nutrient release, enhancing agricultural efficiency.

Metagenomic insights into the influence of pH on antibiotic removal and antibiotic resistance during nitritation: Regulations on functional genus and genes

The changes in pH and the resulting presence of free nitrous acid (FNA) or free ammonia (FA) often inhibit antibiotic biodegradation during nitritation. However, the specific mechanisms through which pH, FNA and FA influence antibiotic removal and the fate of antibiotic resistance genes (ARGs) are not yet fully understood. In this study, the effects of pH, FNA, and FA on the removal of cefalexin and amoxicillin during nitritation were investigated. The results revealed that the decreased antibiotic removal under both acidic condition (pH 4.5) and alkaline condition (pH 9.5) was due to the inhibition of the expression of amoA in ammonia-oxidizing bacteria and functional genes (hydrolase-encoding genes, transferase-encoding genes, lyase-encoding genes, and oxidoreductase-encoding genes) in heterotrophs. Furthermore, acidity was the primary inhibitor of antibiotic removal at pH 4.5, followed by FNA. Antibiotic removal was primarily inhibited by alkalinity at pH 9.5, followed by FA. The proliferation of ARGs mediated by mobile genetic element was promoted under both acidic and alkaline conditions, attributed to the promotion of FNA and FA, respectively. Overall, this study highlights the inhibitory effects of acidity and alkalinity on antibiotic removal during nitritation.

Copper supported Dowex50WX8 resin utilized for the elimination of ammonia and its sustainable application for the degradation of dyes in wastewater

To obtain high efficient elimination of ammonia (NH4+) from wastewater, Cu(II), Ni(II), and Co(II)) were loaded on Dowex-50WX8 resin (D-H) and studied their removal efficiency towards NH4+ from aqueous solutions. The adsorption capacity of Cu(II)-loaded on D-H (D-Cu2+) towards NH4+ (qe = 95.58 mg/g) was the highest one compared with that of D-Ni2+ (qe = 57.29 mg/g) and D-Co2+ (qe = 43.43 mg/g). Detailed studies focused on the removal of NH4+ utilizing D-Cu2+ were accomplished under various experimental conditions. The pseudo-second-order kinetic model fitted well the adsorption data of NH4+ on D-Cu2+. The non-linear Langmuir model was the best model for the adsorption process, producing a maximum equilibrium adsorption capacity (qmax = 280.9 mg/g) at pH = 8.4, and 303 K in less than 20 min. The adsorption of NH4+ onto D-Cu2+ was an exothermic and spontaneous process. In a sustainable step, the resulting D-Cu(II)-ammine composite from the NH4+ adsorption process displayed excellent catalytic activity for the degradation of aniline blue (AB) and methyl violet 2B (MV 2B) dyes utilizing H2O2 as an eco-friendly oxidant.

Acidophilic partial nitrification (pH<6) facilitates ultra-efficient short-flow nitrogen transformation: Experimental validation and genomic insights

Partial nitrification (PN) represents an energy-efficient bioprocess; however, it often confronts challenges such as unstable nitrite accumulation, nitrite oxidizing bacteria shocks, and slow reaction rate. This study established an acidophilic PN with self-sustained pH as low as 5.36. Over 120-day monitoring, nitrite accumulation rate (NAR) was stabilized at more than 97.9 %, and an ultra-high ammonia oxidation rate of 0.81 kg/m3·d was achieved. Notably, least NAR of 77.8 % persisted even under accidental nitrite oxidizing bacteria invasion, aeration delay, alkalinity fluctuations, and cooling shocks. During processing, despite detrimental effects on bacterial diversity, there was a discernible increase in acid-tolerant bacteria responsible for nitrification. Candidatus Nitrosoglobus, gradually dominated in nitrifying guild (2.15 %), with the substantially reduction or disappearance of typical nitrifying microorganisms. Acidobacteriota, a key player in nitrogen cycling of soil, significantly increased from 0.45 % to 9.98 %, and its associated nitrogen metabolism genes showed a substantial 215 % rise. AmoB emerged as the most critical functional gene influencing acidophilic PN, exhibiting significantly higher unit gene expression than other nitrification genes. Blockade tricarboxylic acid cycle, DNA damage, and presence of free nitrous acid exert substantial effects on nitrite-oxidizing bacteria (NOB), serving as internal driving forces for acidophilic PN. This highlights the reliable potential for shortening nitrogen transformation process.

Review on ultrasonic technology enhanced biological treatment of wastewater

As a clean, sustainable and efficient technology of wastewater treatment, ultrasonic irradiation has gained special attention in wastewater treatment. It has been widely studied for degrading pollutants and enhancing biological treatment processes for wastewater treatment. This review focuses on the mechanism and updated information of ultrasonic technology to enhance biological treatment of wastewater. The mechanism involved in improving biological treatment by ultrasonic includes: 1) degradation of refractory substances and release carbon from sludges, 2) promotion of mass transfer and change of cell permeability, 3) facilitation of enzyme-catalyzed reactions and 4) influence of cell growth. Based on the above discussion, the effects of ultrasound on the enhancement of wastewater biological treatment processes can be categorized into indirect and direct ways. The indirect effect of ultrasonic waves in enhancing biological treatment is mainly achieved through the use of high-intensity ultrasonic waves. These waves can be used as a pretreatment to improve biodegradability of the wastewater. Moreover, the ultrasonic-treated sludge or its supernatant can serve as a carbon source for the treatment system. Low-intensity ultrasound is often employed to directly enhance the biological treatment of wastewater. The propose of this process is to improve activated sludge, domesticate polyphosphate-accumulating organisms, ammonia-oxidizing bacteria, and anammox bacteria, and achieve speedy start-up of partial nitrification and anammox. It has shown remarkable effects on maintaining stable operation, tolerating adverse conditions (i.e., low temperature, low C/N, etc.), resisting shock load (i.e., organic load, toxic load, etc.), and collapse recovery. These results indicate a promising future for biological wastewater treatment. Furthermore, virous ultrasonic reactor designs were presented, and their potential for engineering application was discussed.

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.

Potential stimulation of nitrifying bacteria activities and genera by landfill leachate

With the increasing complexity of influent composition in wastewater treatment plants, the potential stimulating effects of refractory organic matter in wastewater on growth characteristics and genera conversion of nitrifying bacteria (ammonium-oxidizing bacteria [AOB] and nitrite-oxidizing bacteria [NOB]) need to be further investigated. In this study, domestic wastewater was co-treated with landfill leachate in the lab-scale reactor, and the competition and co-existence of NOB genera Nitrotoga and Nitrospira were observed. The results demonstrated that the addition of landfill leachate could induce the growth of Nitrotoga, whereas Nitrotoga populations remain less competitive in domestic wastewater operation. In addition, the refractory organic matter in the landfill leachate also would have a potential stimulating effect on the maximum specific growth rate of AOB genus Nitrosomonas (μmax, aob). The μmax, aob of Nitrosomonas in the control group was estimated to be 0.49 d-1 by fitting the ASM model, and the μmax, aob reached 0.66-0.71 d-1 after injection of refractory organic matter in the landfill leachate, while the maximum specific growth rate of NOB (μmax, nob) was always in the range of 1.05-1.13 d-1. These findings have positive significance for the understanding of potential stimulation on nitrification processes and the stable operation of innovative wastewater treatment process.

Optimizing sludge retention time for sustainable photo-enhanced biological phosphorus removal systems: Insights into nutrient fate, microbial community, and bacterial phototolerance

Photo-enhanced Biological Phosphorus Removal (PEBPR) systems, promising wastewater treatment technology, offer efficient phosphorus removal without external oxygen. However, comprehending the impact of sludge retention time (SRT) on the system is crucial for successful implementation. This study investigated the SRT effect on nutrient fate, microbial community, and bacterial phototolerance in PEBPR systems. PEBPR systems exhibited good bacterial phototolerance at SRT of 10, 15, and 20 d, with optimal phosphorus-accumulation metabolism observed at SRT of 10 and 15d. However, at SRT of 5d, increased light sensitivity and glycogen-accumulating organisms (GAOs) growth resulted in poor P removal (71.9%). Accumulibacter-IIC were the dominant P accumulating organisms (PAOs) at SRT of 10, 15, and 20 d. Accumulibacter-I, IIC and IIF were the major PAOs at SRT of 5 d. The decrease in SRT promoted the microalgal population diversity, and Dictyosphaerium and Chlorella were the major microalgal species in this study. Flow cytometry results revealed high light intensity triggered intracellular Fe2+ efflux, limiting translation activity and metabolism. Moreover, PAOs had lower phototolerance than GAOs due to Poly-P bound intracellular Mg2+ affecting enzyme activity. This study provides an in-depth understanding of PEBPR systems operation strategy toward environmentally sustainable wastewater treatment.

Optimizing ciprofloxacin removal through regulations of trophic modes and FNA levels in a moving bed biofilm reactor performing sidestream partial nitritation

The performance of partial nitritation (PN)-moving bed biofilm reactor (MBBR) in removal of antibiotics in the sidestream wastewater has not been investigated so far. In this work, the removal of ciprofloxacin was assessed under varying free nitrous acid (FNA) levels and different trophic modes. For the first time, a positive correlation was observed between ciprofloxacin removal and FNA levels, either in the autotrophic PN-MBBR or in the mixotrophic PN-MBBR, mainly ascribed to the FNA-stimulating effect on heterotrophic bacteria (HB)-induced biodegradation. The maximum ciprofloxacin removal efficiency (∼98 %) and removal rate constant (0.021 L g-1 SS h-1) were obtained in the mixotrophic PN-MBBR at an average FNA level of 0.056 mg-N L-1, which were 5.8 and 51.2 times higher than the corresponding values in the autotrophic PN-MBBR at 0 mg FNA-N L-1. Increasing FNA from 0.006 to 0.056 mg-N L-1 would inhibit ammonia oxidizing bacteria (AOB)-induced cometabolism and metabolism from 10.2 % and 6.9 % to 6.2 % and 6.4 %, respectively, while HB-induced cometabolism and metabolism increased from 31.2 % and 22.7 % to 41.9 % and 34.5 %, respectively. HB-induced cometabolism became the predominant biodegradation pathway (75.9 %-85.8 %) in the mixotrophic mode. Less antimicrobial biotransformation products without the piperazine or fluorine were newly identified to propose potential degradation pathways, corresponding to microbial-induced metabolic types and FNA levels. This work shed light on enhancing antibiotic removal via regulating both FNA accumulation and organic carbon addition in the PN-MBBR process treating sidestream wastewater.

The response of nitrifying activated sludge to chlorophenols: Insights from metabolism and redox homeostasis

The specialized wastewater treatment plants for the chemical industry are rapidly developed in China and many other countries. But there is a common bottleneck in that the toxic pollutants in chemical wastewater often cause shock impacts on biological nitrogen removal systems, which directly affects the stability and cost of operation. As the research on nitrification inhibition characteristics is not sufficient till now, there is a great lack of theoretical guidance on the control of the inhibition. This study investigated the response of nitrifying activated sludge to chlorophenols (CPs) inhibition in terms of metabolism disorder and oxidative stress. At the initial stage of reaction (i.e., 1 h), reactive oxygen species (ROS)-induced membrane damage which might account for declining nitrification performance. Simultaneously excessive extracellular polymeric substances (EPS) were secreted to alleviate oxidative stress injury and protected microorganisms to some extent. In particular tyrosine-like substances in LB-EPS with a Fmax increase of 242.30% were confirmed to efficiently resist phenols inhibition. Thus, as the inhibition proceeded, metabolism disorder replaced oxidative stress as the main cause of nitrification inhibition. The affected metabolic processes include weakened enzyme catalysis, restricted electron transport and lessened energy generation. At 4 h, nitrifying production of sludge amended with 5 mg/L chlorophenols was 89.27 ± 9.51%-98.15 ± 9.60% lower than blank, the inhibition could be attributed to comprehensively affected metabolism. The structural equation modeling indicated that phenols restricted nitrification enzymes and bacterial electron transport efficiency which was critical to nitrification performance. Moreover, the lessened energy generation weakens enzyme activity to further suppress nitrification. These findings enriched our knowledge of nitrifiers' responses to CPs inhibition and provided the basis for addressing nitrification inhibition.

Effect of free nitrous acid on extracellular polymeric substances production and membrane fouling in a nitritation membrane bioreactor

The membrane bioreactor (MBR) with nitritation based nitrogen removal processes has attracted growing interest in recent years, although membrane fouling in the nitritation MBR is a challenging issue. In this study, the inhibitory effect of free nitrous acid (FNA) on microbial extracellular polymeric substances (EPS) production and membrane fouling in a nitritation MBR was investigated. Results showed that EPS played a critical role in the biofouling process, and EPS production was affected by FNA concentration. As FNA concentration increased from 5.10 × 10-3 mg N/L to 1.34 × 10-2 mg N/L, protein (PN) and polysaccharide (PS) contents increased from 8.20 to 60.28 mg/g VSS and 4.74-30.46 mg/g VSS, respectively. However, when FNA concentration was 1.48 × 10-2 mg N/L, PN and PS reduced by 20.0% and 10.9%, respectively, indicating that the higher FNA concentration could reduce EPS production. The EPS reduction could be attributed to reduction in the loosely bound (LB) and tightly bound (TB) EPS but not the soluble microbial products (SMP). It was further revealed that higher FNA concentrations up to 1.48 × 10-2 mg N/L consequently mitigate trans-membrane pressure (TMP) rate in terms of dTMP/dt by 25.5% in the nitritation MBR. High throughput sequencing analysis revealed that the increase in FNA led to enrichment of Nitrosomonas but reduction in heterotrophic bacteria. This study showed that the appropriate FNA concentration affected EPS production and hence membrane fouling, leading to the possibility of membrane fouling mitigation by in-situ generated FNA in the nitritation MBR.

Decoupling the simultaneous effects of NO2-, pH and free nitrous acid on N2O and NO production from enriched nitrifying activated sludge

In the pursuit of energy and carbon neutrality, nitrogen removal technologies have been developed featuring nitrite (NO2-) accumulation. However, high NO2- accumulations are often associated with stimulated greenhouse gas (i.e., nitrous oxide, N2O) emissions. Furthermore, the coexistence of free nitrous acid (FNA) formed by NO2- and proton (pH) makes the consequence of NO2- accumulation on N2O emissions complicated. The concurrent three factors, NO2-, pH and FNA may play different roles on N2O and nitric oxide (NO) emissions simultaneously, which has not been systematically studied. This study aims to decouple the effects of NO2- (0-200 mg N/L), pH (6.5-8) and FNA (0-0.15 mg N/L) on the N2O and NO production rates and the production pathways by ammonia oxidizing bacteria (AOB), with the use of a series of precisely executed batch tests and isotope site-preference analysis. Results suggested the dominant factors affecting the N2O production rate were NO2- and FNA concentrations, while pH alone played a relatively insignificant role. The most influential factor shifted from NO2- to FNA as FNA concentrations increased from 0 to 0.15 mg N/L. At concentrations below 0.0045 mg HNO2-N/L, nitrite rather than FNA played a significant role stimulating N2O production at elevated nitrite concentrations. The inhibition effect of FNA emerged with further increase of FNA between 0.0045-0.015 mg HNO2-N/L, weakening the promoting effect of increased nitrite. While at concentrations above 0.015 mg HNO2-N/L, FNA inhibited N2O production especially from nitrifier denitrification pathway with the level of inhibition linearly correlated with the FNA concentration. pH and the nitrite concentration regulated the production pathways, with elevated pH promoting the nitrifier nitrification pathway, while elevated NO2- concentrations promoting the nitrifier denitrification pathway. In contrast to N2O, NO emission was less susceptible to FNA at concentrations up to 0.015 mg N/L but was stimulated by increasing NO2- concentrations. This study, for the first time, distinguished the effects of pH, NO2- and FNA on N2O and NO production, thereby providing support to the design and operation of novel nitrogen removal systems with NO2- accumulation.

Limited dissolved oxygen facilitated nitrogen removal at biocathode during the hydrogenotrophic denitrification process using bioelectrochemical system

Effects of limited dissolved oxygen (DO) on hydrogenotrophic denitrification at biocathode was investigated using bioelectrochemical system. It was found that total nitrogen removal increased by 5.9%, as DO reached about 0.24 mg/L with the cathodic chamber unplugged (group R_Exposure). With the presence of limited DO, not only the nitrogen metabolic pathway was influenced, but the composition of microbial communities of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria were enriched accordingly. After metagenomic analysis, enriched genes in R_Exposure were found to be associated with nearly each of nitrogen removal steps as denitrification, nitrification, DNRA, nitrate assimilation and even nitrogen fixation. Moreover, genes encoding both Complexes III and IV constituted the electron transfer chain were significantly enriched, indicating that more electrons would be orientated to the reduction of NO₂^--N, NO-N and oxygen. Therefore, enhanced nitrogen removal could be attained through the co-respiration of nitrate and oxygen by means of NH₄⁺-N oxidation.

Bioaugmentation reconstructed nitrogen metabolism in full-scale simultaneous partial nitrification-denitrification, anammox and sulfur-dependent nitrite/nitrate reduction (SPAS)

To enhance nitrogen removal of fermentation pharmaceutical wastewater with high nitrogen load, a full-scale process based on simultaneous partial nitrification-denitrification/ anammox/ sulfur autotrophic denitrification (SPAS) was established via inoculating with bioaugmentation consortia in a modified two-stage AO. More than 93 % TN and 98 % NH₄⁺-N removal were obtained at a rate of 0.8 kg-N/ m³/d in the first A/O stage, in which short-cut SND was involved with 96.05 % E_SND when bioaugmented with SND, while S⁰-SAD could coordinate with anammox to exert further deep denitrification in the second A/O stage. KEGG analysis demonstrated that the SPAS process was synergism of HD, PN/PDN, SND, SAD and anammox metabolism, bioaugmentation could significantly up-regulate genes related to microbial metabolism (TCA cycle, Carbon metabolism, ABC transporters) and environmental adaptation (Two-component system, Quorum sensing) based on the FAPROTAX and Picrust2 functional prediction. This study provided a new perspective in engineering applications.

A review on upgrading of the anammox-based nitrogen removal processes: Performance, stability, and control strategies

The anaerobic ammonia oxidation (anammox) process is a promising biological nitrogen removal technology. However, owing to the sensitivity and slow cell growth of anammox bacteria, long startup time and initially low nitrogen removal rate (NRR) are still limiting factors of practical applications of anammox process. Moreover, nitrogen removal efficiency (NRE) is often lower than 88 %. This review summarizes the most common methods for improving NRR by increasing microorganism concentration, and modifying reactor configuration. Recent integrated anammox-based systems were evaluated, including hydroxyapatite (HAP)-enhanced one-stage partial nitritation/anammox (PNA) process for a high NRR of over 2 kg N/m3/d at 25 °C, partial denitrification/anammox (PDA) process, and simultaneous partial nitrification, anammox, and denitrification process for a high NRE of up to 100 %. After discussing the challenges for the application of these systems critically, a combined system of anaerobic digestion, HAP-enhanced one-stage PNA and PDA is proposed in order to achieve a high NRR, high NRE, and phosphorus removal simultaneously.

Changes in the Species and Functional Composition of Activated Sludge Communities Revealed Mechanisms of Partial Nitrification Established by Ultrasonication

To achieve energy-efficient shortcut nitrogen removal of wastewater in the future, selective elimination of nitrite-oxidizing bacteria (NOB) while enriching ammonia-oxidizing microorganisms is a crucial step. However, the underlying mechanisms of partial nitrification are still not well understood, especially the newly discovered ultrasound-based partial nitrification. To elucidate this issue, in this study two bioreactors were set up, with one established partial nitrification by ultrasonication while the other didn't. During the operation of both reactors, the taxonomic and functional composition of the microbial community were investigated through metagenomics analysis. The result showed that during ultrasonic partial nitrification, ammonia-oxidizing archaea (AOA), Nitrososphaerales, was enriched more than ammonia-oxidizing bacteria (AOB), Nitrosomonas. The enrichment of microorganisms in the community increased the abundance of genes involved in microbial energy generation from lipid and carbohydrates. On the other hand, the abundance of NOB, Nitrospira and Nitrolancea, and Comammox Nitrospira decreased. Selective inhibition of NOB was highly correlated with genes involved in signal transduction enzymes, such as encoding histidine kinase and serine/threonine kinase. These findings provided deep insight into partial nitrification and contributed to the development of shortcut nitrification in wastewater treatment plants.

Successful mainstream nitritation through NOB inactivation

The development of new wastewater treatment processes can assist in reducing the impact of wastewater treatment on the environment. The recently developed partial nitritation anammox (PNA) process, for example, consumes less energy for aeration and reduces nitrate in the effluent without requiring additional organic carbon. However, achieving stable nitritation (ammonium oxidation; NH₄⁺ → NO₂^-) at mainstream conditions (T = 10-25 °C, C:N > 10, influent ammonium < 50 mgNH₄-N/L and effluent < 1 mgNH₄-N/L) remains challenging. This study explores the potential and mechanism of nitrite-oxidizing bacteria (NOB) suppression in a bottom-fed sequencing batch reactor (SBR). Two bench-scale (11 L) reactors and a pilot-scale reactor (8 m³) were operated for over a year and were fed with organic substrate depleted municipal wastewater. Initially, nitratation (nitrite oxidation; NO₂^- → NO₃^-) occurred occasionally until an anaerobic phase was integrated into the operating cycle. The introduction of the anaerobic phase effectively suppressed the regrowth of NOB while nitritation was stable over 300 days, down to 8 °C and at ammonium influent concentrations < 25 mgNH₄-N/L. Batch experiments and process data revealed that parameters typically affecting NOB growth (e.g., dissolved oxygen, alkalinity, trace elements, lag-phase after anoxia, free nitrous acid (FNA), free ammonia (FA), pH, sulfide, or solids retention time (SRT)) could not fully explain the suppression of nitratation. Experiments in which fresh nitrifying microbial biomass was added to the nitritation system indicated that NOB inactivation explained NOB suppression better than NOB washout at high SRT. This study concludes that bottom-fed SBRs with anaerobic phases allow for stable nitritation over a broad range of operational parameters. Coupling this type of SBR to an anammox reactor can enable efficient mainstream anammox-based wastewater treatment.

Achieving Partial Nitritation by Treating Sludge With Free Nitrous Acid: The Potential Role of Quorum Sensing

Partial nitritation is increasingly regarded as a promising biological nitrogen removal process owing to lower energy consumption and better nitrogen removal performance compared to the traditional nitrification process, especially for the treatment of low carbon wastewater. Regulating microbial community structure and function in sewage treatment systems, which are mainly determined by quorum sensing (QS), by free nitrous acid (FNA) to establish a partial nitritation process is an efficient and stable method. Plenty of research papers reported that QS systems ubiquitously existed in ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB), and various novel nitrogen removal processes based on partial nitritation were successfully established using FNA. Although the probability that partial nitritation process might be achieved by the regulation of FNA on microbial community structure and function through the QS system was widely recognized and discussed, the potential role of QS in partial nitritation achievement by FNA and the regulation mechanism of FNA on QS system have not been reviewed. This article systematically reviewed the potential role of QS in the establishment of partial nitritation using FNA to regulate activated sludge flora based on the summary and analysis of the published literature for the first time, and future research directions were also proposed.

Metagenomics insights into the selective inhibition of NOB and comammox by phenacetin: Transcriptional activity, nitrogen metabolism and mechanistic understanding

Phenacetin (PNCT), a common antipyretic and analgesic drug, is often used to treat fever and headache. However, the effect of PNCT on nitrifiers in wastewater treatment processes remains unclear. The practicability of attaining partial nitrification (PN) through inhibitor-PNCT was investigated in this study. The optimal treatment conditions of soaking once for 18 h with 2.50 × 10^-3 g PNCT/(g MLSS) were applied to the PN stability experiment. The results showed that ammonia oxidation activity recovered quickly after 3 cycles of operation, while nitrite oxidation activity was suppressed steadily. In addition, average ammonium removal efficiency and nitrite accumulation ratio during 138 cycles could reach 94.94% and 85.38%, respectively. Complimentary DNA high-throughput sequencing and oligotyping analysis showed that the activity of Nitrosomonas would gradually surpass Nitrospira after PNCT treatment only once. The decrease of Nitrospira activity was accompanied by the simplification of oligotypes after PNCT treatment, while Nitrosomonas could adapt to PNCT stress by reducing the differences between oligotypes. Metagenomics revealed that the decrease in the number of NXR in the nitrogen metabolism pathways was the key reason for achieving PN. The potential mechanisms might be that the dominant nitrite-oxidizing bacteria and complete ammonia oxidizers were bio-killed by PNCT.

Reductive potential from cathode electrode as an option for the achievement of short-cut nitrification in bioelectrochemical systems

Nitrogen removal based on short-cut nitrification (SCN) have attract more attentions, in which stable nitrite accumulation is prerequisite. In this study, different reductive potential was applied to inhibit nitrite oxidizing bacteria for achievement of SCN in aerobic cathode chamber of bioelectrochemical systems with dissolved oxygen concentration of 3.5 mg/L. The results demonstrated that the applied potential facilitated nitrite accumulation with high ammonia oxidation rates. The maximum nitrate accumulation rate of 87.61% was obtained at -800 mV. The abundance of Nitrosomonas and Thauera increased while Nitrospira abundance declined with more negative reductive potentials. The activity of nitric oxide reductase was also evidently inhibited. The above-mentioned three genera were the keystone taxa in co-occurrence network with high degree and closeness centrality. Interestingly, total nitrogen (TN) removal was enhanced simultaneously in the absence of external organic carbon. Reductive potential would be a promising approach for achieving SCN and simultaneously TN removal.

Antibiotic resistance genes attenuation in anaerobic microorganisms during iron uptake from zero valent iron: An iron-dependent form of homeostasis and roles as regulators

Zero valent iron (ZVI) has been previously documented to attenuate the propagation of antibiotic resistance genes (ARGs) in microbes, while how ZVI affects the evolution of ARGs remains unclear. Herein, we investigated the influences of ZVI on ARGs dissemination in anaerobic bioreactor treating oxytetracycline (tet) containing wastewater, by deciphering the roles of iron homeostasis and regulatory effects. A net reduction of tet gene targets ranging from 0.75 to 1.88 and 0.67 to 2.08 log unit in intracellular and extracellular DNA was achieved at the optimal dosage of 5 g/L ZVI, whereas 20 g/L ZVI made no effects on ARGs reduction. The reduced ARGs abundance by ZVI was directly related to the inhibited horizontal transfer of ARGs and decreased proliferation of resistant strains (mainly Paludibacter and WCHB1-32). The potential mechanisms included the increased antioxidant capacity, the depressed efflux pump system and the weakened energy driving force by Fur regulon in microbes (especially for Cloacibacterium and Dechloromonas). The negligible influence of 20 g/L ZVI on ARGs reduction was ascribed to the iron-catalyzed oxidative damage and reduced physiological activity. This study firstly illustrated the potential relationships among activation of iron uptake regulator leading to protection against oxidative stress, alternation of physiological metabolisms and reduction of ARGs dissemination. This work extents our understanding about the priority of ZVI in mitigating ARGs proliferation and sheds light on its potential application in wastewater treatment plants.

An integrated strategy to enhance performance of anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) is an essential process in wastewater treatment plants as it can reduce the amount of waste activated sludge (WAS) for disposal, and also enables the recovery of bioenergy (i.e. methane). Here, a new pretreatment method to enhance anaerobic digestion was achieved by treating thickened WAS (TWAS) with ferric (as FeCl₃) and nitrite simultaneously for 24-hour at room temperature. Biochemical methane potential tests showed markedly improved degradability in the pretreated TWAS, with a relative increase in hydrolysis rate by 30%. A comparative experiment with the operation of two continuous-flow anaerobic digesters further demonstrated the improvement in biogas quantity and quality, digestate disposal, and phosphorus recovery in the experimental digester. The dosed FeCl₃ (i.e. ~6 mM) decreased the pH of TWAS to ~5, which led to the formation of free nitrous acid (FNA, HNO₂) at parts per million levels (i.e. ~6 mg N/L), after dosing nitrite at 250 mg NO₂^--N/L. This FNA treatment caused a 26% increase in methane yield and volatile solids destruction, 55% reduction in the viscosity of sludge in digester, and 24% less polymer required in further digestate dewatering. In addition, the dosed Fe(III) was reduced to Fe(II) which precipitated sulfide and phosphorus, leading to decreased hydrogen sulfide concentration in biogas, and increased percentage of vivianite in the total crystalline iron species in digested sludge. Our study experimentally demonstrated that combined dosing of FeCl₃ and nitrite is a useful pretreatment strategy for improving anaerobic digestion of WAS.

New Insights into the Chemical Reactivity of Dry-Cured Fermented Sausages: Focus on Nitrosation, Nitrosylation and Oxidation

Nitrite and nitrate are added to cured meat for their bacteriological, technological and sensorial properties. However, they are suspected to be involved in the formation of nitroso compounds (NOCs), such as potentially mutagenic nitrosamines, nitrosylheme and nitrosothiols. Controlling the sanitary and sensorial qualities of cured meat products by reducing these additives requires elucidating the mechanisms involved in the formation of NOCs. To this end, we studied the dose-response relationship of added sodium nitrite and/or sodium nitrate (0/0, 80/80, 0/200, and 120/120 ppm) on the formation of NOCs in dry cured fermented sausages. The results showed a basal heme iron nitrosylation in the absence of NaNO2/NaNO3 due to starter cultures. This reaction was promoted by the addition of NaNO2/NaNO3 in the other conditions. Reducing the dose to 80/80 ppm still limits lipid oxidation without the formation of non-volatile nitrosamines. Conversely, the addition of NO2/NO3 slightly increases protein oxidation through higher carbonyl content. The use of 80/80 ppm could be a means of reducing these additives in dry-cured fermented meat products.

A review of partial nitrification in biological nitrogen removal processes: from development to application

To further reduce the energy consumption in the wastewater biological nitrogen removal process, partial nitrification and its integrated processes have attracted increasing attentions owing to their economy and efficiency. Shortening the steps of ammonia oxidation to nitrate saves a large amount of aeration, and the accumulated nitrite could be reduced by denitritation or anammox, which requires less electron donors compared with denitrification. Therefore, the strategies through mainstream suppression and sidestream inhibition for the achievement of partial nitrification in recent years are reviewed. Specifically, the enrichment strategies of functional microorganisms are obtained on the basis of their growth and metabolic characteristics under different selective pressures. Furthermore, the promising developments, current application bottlenecks and possible future trends of some biological nitrogen removal processes integrating partial nitrification are discussed. The obtained knowledge would provide a new idea for the fast realization of economic, efficient and long-term stable partial nitrification and biological nitrogen removal process.

Development of Strategies for AOB and NOB Competition Supported by Mathematical Modeling in Terms of Successful Deammonification Implementation for Energy-Efficient WWTPs

Novel technologies such as partial nitritation (PN) and partial denitritation (PDN) could be combined with the anammox-based process in order to alleviate energy input. The former combination, also noted as deammonification, has been intensively studied in a frame of lab and full-scale wastewater treatment in order to optimize operational costs and process efficiency. For the deammonification process, key functional microbes include ammonia-oxidizing bacteria (AOB) and anaerobic ammonia oxidation bacteria (AnAOB), which coexisting and interact with heterotrophs and nitrite oxidizing bacteria (NOB). The aim of the presented review was to summarize current knowledge about deammonification process principles, related to microbial interactions responsible for the process maintenance under varying operational conditions. Particular attention was paid to the factors influencing the targeted selection of AOB/AnAOB over the NOB and application of the mathematical modeling as a powerful tool enabling accelerated process optimization and characterization. Another reviewed aspect was the potential energetic and resources savings connected with deammonification application in relation to the technologies based on the conventional nitrification/denitrification processes.

An integrated mainstream and sidestream strategy for overcoming nitrite oxidizing bacteria adaptation in a continuous plug-flow nutrient removal process

An integrated mainstream aeration and sidestream sludge treatment was demonstrated to be effective in overcoming the adaptationof nitrite oxidizing bacteria (NOB) in an anoxic/oxic process. Results showed that by employing the alternating free nitrous acid and free ammonia (FNA/FA) sidestream sludge treatment alone, nitritation was established but varied, which was addressed by integrating alternating aeration with step feeding (ALASF) in reactor. Two critical considerations contributed to stable effluent nitrite accumulation (>83.8 %)and nitrogen removal (>83.0 %): 1) aerobic sludge rather than return sludge should be taken for FNA/FA treatment to avoid anoxic starvation which facilitated NOB recovery; 2) ALASF ensured timely denitritation and created constant anoxic disturbance for NOB inhibition. Nitrospira and Nitrobacter after 540-day operation were 0.38 % of seed sludge.A20 % reduction of operating cost was obtained in this nitritation process. This study moved nitritation one step closer to application in continuous plug-flow process from municipal wastewater.

Low-dose Ultraviolet-A irradiation selectively eliminates nitrite oxidizing bacteria for mainstream nitritation

Nitritation is currently known as a bottleneck for mainstream nitrite shunt or partial nitritation/anammox (PN/A). Here we propose a new approach to selectively eliminate nitrite oxidizing bacteria (NOB) for mainstream nitritation by low-dose ultraviolet-A (UVA) irradiation. The results showed that mainstream nitritation was rapidly achieved within 10 days with UVA irradiation at the dose of 0.87 μE L^-1 s^-1, and nitrite accumulation ratio (NO₂^--N/(NO₂^--N + NO₃^--N) ×100%) stabilized over 80%. Microbial community analysis revealed that two typical NOB populations (Nitrospira and Ca. Nitrotoga) detected in the control reactor were suppressed efficiently in UVA irradiation reactor, whereas the Nitrosomonas genus of ammonium oxidizing bacteria (AOB) remained at similar level. Intracellular reactive oxygen species (ROS) analysis indicated that NOB-dominant sludge tends to generate more intracellular ROS compared with AOB-dominant sludge in the presence of UVA, leading to the inactivation and elimination of NOB. Additionally, amounts of microalgae found in UVA irradiation reactor could help to suppress NOB by generating ROS during photosynthesis. Briefly, the UVA irradiation approach proposed in this study was shown to be promising in NOB suppression for reliable mainstream nitritation.

New progress of ammonia recovery during ammonia nitrogen removal from various wastewaters

The recovery of ammonia-nitrogen during wastewater treatment and water purification is increasingly critical in energy and economic development. The concentration of ammonia-nitrogen in wastewater is different depending on the type of wastewater, making it challenging to select ammonia-nitrogen recovery technology. Meanwhile, the conventional nitrogen removal method wastes ammonia-nitrogen resources. Based on the circular economy, this review comprehensively introduces the characteristics of several main ammonia-nitrogen source wastewater plants and their respective challenges in treatment, including municipal wastewater, industrial wastewater, livestock and poultry wastewater and landfill leachate. Furthermore, we introduce the main methods currently adopted in the ammonia-nitrogen removal process of wastewater from physical (air stripping, ion exchange and adsorption, membrane and capacitive deionization), chemical (chlorination, struvite precipitation, electrochemical oxidation and photocatalysis) and biological (classical and typical activated sludge, novel methods based on activated sludge, microalgae and photosynthetic bacteria) classification based on the ammonia recovery concept. We discuss the applicable methods of recovering ammonia nitrogen in several main wastewater plants. Finally, we prospect the research direction of ammonia removal and recovery in wastewater based on sustainable development.

The fate and impact of TCC in nitrifying cultures

Triclocarban (TCC) is a highly effective antibacterial agent, which is widely used in a variety of applications and present at significant levels (e.g., 760 μg/L) in wastewater worldwide. However, the interaction between TCC and nitrifiers, important microbial cultures in wastewater treatment plants, has not been documented. This work therefore aimed to evaluate the fate of TCC in a nitrifying culture and its impact on nitrifiers in four long-term nitrifiers-rich reactors, which received synthetic wastewater containing 0, 0.1, 1, or 5 mg/L TCC. Experimental results showed that 36.7%-50.7% of wastewater TCC was removed by nitrifying cultures in stable operation. Mass balance analysis revealed that the removal of TCC was mainly achieved through adsorption rather than biodegradation. Adsorption kinetic analysis indicated that inhomogeneous multilayer adsorption was responsible for the removal while fourier transform infrared spectroscopy indicated that several functional groups such as hydroxyl, amide and polysaccharide seemed to be the main adsorption sites. The adsorbed TCC significantly deteriorated settleability and performance of nitrifying cultures. With an increase of influent TCC from 0 to 5 mg/L, reactor volatile suspended solids and effluent nitrate decreased from 1200 ± 90 mg/L and 300.81 ± 7.52 mg/L to 880 ± 80 and 7.35 ± 4.62 mg/L while effluent ammonium and nitrite increased from 0.41 ± 0.03 and 0.45 ± 0.23 mg/L to104.65 ± 3.46 and 182.06 ± 7.54 mg/L, respectively. TCC increased the extracellular polymeric substances of nitrifying cultures, inhibited the specific activities of nitrifiers, and altered the abundance of nitrifiers especially Nitrospira sp.. In particular, TCC at environmentally relevant concentration (i.e., 0.1 mg/L) significantly inhibited NOB activity and reduced NOB population.

Intensified simultaneous nitrification and denitrification performance in integrated packed bed bioreactors using PHBV with different dosing methods

To explore an effective approach of simultaneous nitrification and denitrification in wastewater with low C/N ratios, integrated packed bed bioreactors based on poly(3-hydroxybutyrate-hydroxyvalerate) (PHBV) with different dosing methods were designed. The removal efficiency of NH₄⁺-N in bioreactor with aeration was 88.62%, and higher NO₃^--N removal efficiency was observed in bioreactor filled with grainy PHBV (95.21%) than bioreactor filled with strip PHBV (93.34%). Microbial study indicated that microbes harboring amoA and nirS genes preferred to attach on the surface of ceramsite, and significant differences in microbial community compositions at phylum and genus levels were observed. To summarize, it is feasible to utilize grainy PHBV for simultaneous and efficient removal of NH₄⁺-N and NO₃^--N from wastewater with low C/N ratios.

Prompt nitrogen removal by controlling the oxygen concentration in sediment microbial fuel cell systems: the electrons allocation and its microbial mechanism

It has been proved that the nitrogen can be removed from the sediment in a sediment microbial fuel cell system (SMFCs), but the competition between nitrate and oxygen for electrons would be a key factor that would affect the removal efficiency, and its mechanism is not clear. Based on organic sediment fuel, an SMFC was constructed, and the influence of dissolved oxygen (DO) on nitrogen transformation and cathodic microbial communities was investigated. The results showed that the best total nitrogen removal efficiency of 60.55% was achieved at DO level of 3 mg/L. High DO concentration affected the removal efficiency through the electrons' competition with nitrate, while low DO concentration suppressed the nitrification. Comamonas, Diaphorobacter and Brevundimonas were the three dominant genera responsible for denitrification at DO concentration of 3 mg/L in this study. The establishment of SMFCs for nitrogen removal by regulating DO level would offer a promising method for sediment treatment.

Low-level free nitrous acid efficiently inhibits the conjugative transfer of antibiotic resistance by altering intracellular ions and disabling transfer apparatus

Recently, the dissemination of antibiotic resistance genes (ARGs) via plasmid-mediated conjugation has been reported to be facilitated by a series of contaminants. This has highlighted potential challenges to the effective control of this principal mode of horizontal transfer. In the present study, we found that low levels (<0.02 mgN/L) of free nitrous acid (FNA) remarkably inhibited (over 90%) the conjugative transfer of plasmid RP4, a model broad-host-range plasmid, between Escherichia coli. The antimicrobial role of FNA at the applied dosages was firstly ruled out, since no dramatic reductions in viabilities of donor or recipient were observed. Instead, FNA appeared to reduce the available intracellular free Mg²⁺, which was confirmed to be triggered by the liberation of intracellular Fe²⁺. These alterations in intracellular Mg²⁺ and Fe²⁺ concentrations were found to significantly limit the available energy for conjugative transfer through suppression of glycolysis by decreasing the activities of glycogen phosphorylase and glyceraldehyde-3-phosphate dehydrogenase and also by diverting the glycolytic flux into the pentose phosphate pathway via activation of glucose-6-phosphate dehydrogenase towards the generation of NADPH rather than ATP. Moreover, RP4-encoding genes responsible for DNA transfer and replication (traI, traJ and trfAp), coupling (traG) and mating pair formation (traF and trbBp) were all significantly down-regulated after FNA treatment, indicating that the transfer apparatus required for plasmid processing and delivery was deactivated. By validating the inhibitory effects of FNA on conjugation in real wastewater, this study highlights a promising method for controlling the dissemination of ARGs in systems such as wastewater treatment plants.

Impact of free nitrous acid shock and dissolved oxygen limitation on nitritation maintenance and nitrous oxide emission in a membrane bioreactor

This study investigated the initiation and maintenance of nitritation in a membrane bioreactor (MBR) with long solids retention time (SRT) of 43.8 days. Nitritation was initiated within 65 days in the MBR via dissolved oxygen (DO) limitation (<0.5 mg/L). However, nitrite oxidizing bacteria (NOB) (Nitrospira and Nitrobacter) acclimated to the low DO environment and proliferated from day 81, leading to nitrate accumulation. Thereafter, the combined strategy of DO limitation and in-situ generated free nitrous acid (FNA) shock successfully restored and maintained stable nitritation for >70 days. Quantitative polymerase chain reaction (qPCR) results showed that cell abundances of Nitrospira and Nitrobacter decreased by between 50.0 to 68.9% and 60.6 to 96.4%, respectively following the FNA shocks. The maximum ammonium loading rate achieved was 1.81 kg N/(m³ day) with ammonium removal ratio and nitrite accumulation ratio of over 0.97 and 0.96, respectively. Average emission rate of N₂O from the MBR was 2.1 ± 0.72% of ammonium removed. FNA shock on day 195 reduced the N₂O emission by 13.6%. The strategy developed in this study verified that spiked FNA shock together with DO limitation can be used for maintaining nitritation in MBRs with long SRTs. This method can potentially allow for maintaining nitritation at relatively low capital and operating expenditure when treating high concentration ammonium wastewater.

Stable partial nitrification of domestic sewage achieved through activated sludge on exposure to nitrite

Partial nitrification is crucial for application of autotrophic nitrogen removal which is beneficial for treating carbon-limited wastewater. This study presents an alternative strategy for achieving partial nitrification of domestic wastewater treatment, by treating activated sludge of a nitrifying reactor on long-term nitrite exposure. Initially, the nitrifying reactor obtained complete nitrification. After the sludge of the reactor was treated with nitrite at concentration of 5-30 mg·L^-1 without feeding for 32 days, the nitrite accumulation ratio (NAR) above 90% was achieved in 30 days, and then the NAR stabled at 97.9% under low temperature of 16.6 °C. Further analysis showed that the activity decay rate of ammonium-oxidizing bacteria (AOB) (0.020 d^-1) was lower than that of nitrite-oxidizing bacteria (NOB) (0.035 d^-1) under nitrite stress. Meanwhile, the NOB were effectively suppressed while AOB were dominant. These observations supported the feasibility of achieving stable partial nitrification by treating sludge on long-term exposure to nitrite.

A strategy of high-efficient nitrogen removal by an ammonia-oxidizing bacterium consortium

An ammonia-oxidizing bacterium consortium showed approximately 100% removal of NH₄⁺-N with an initial concentration of 262.28 ± 8.21 mg·L^-1 within 10 days, and only 16.54 ± 0.52% of NH₄⁺-N was converted to NO₂^--N in this study. The consortium removed ammonium by heterotrophic nitrification and aerobic denitrification (HNAD) without N₂O emission. The activity of AOB was not affected by low concentrations of FA or FNA, but completely inhibited by 0.04 mg HNO₂·L^-1. In a bioaugmentation treatment of eutrophic wastewater using the consortium, the removal efficiency of NH₄⁺-N reached 90.85 ± 0.8% and 77.88 ± 1.86% at initial concentrations of 1.80 ± 0.04 mg·L^-1 and 40.31 ± 0.57 mg·L^-1, respectively, and the dissolved oxygen level had a significant impact on the consortium activity. No significant changes in the bacterial community structure were observed after the consortium addition, and local functional bacteria were enriched by aeration and contributed to ammonium nitrogen removal with AOB.

Overcoming Nitrite Oxidizing Bacteria Adaptation through Alternating Sludge Treatment with Free Nitrous Acid and Free Ammonia

Stable suppression of nitrite oxidizing bacteria (NOB) is one of the major bottlenecks for achieving mainstream nitrite shunt or partial nitritation/anammox (PN/A). It is increasingly experienced that NOB could develop resistance to suppressions over an extended time, leading to failure of nitrite shunt or PN/A. This study reports and demonstrates the first effective strategy to overcome NOB adaptation through alternating sludge treatment with free nitrous acid (FNA) and free ammonia (FA). During over 650 days of reactor operation, NOB adaptation to both FNA and FA was observed, but the adaptation was successfully overcome by deploying the alternate treatment strategy. Microbial community analysis showed Nitrospira and Nitrobacter, the key NOB populations in the reactor, have the ability to adapt to FNA and FA, respectively, but do not adapt to the alternation. Stable nitrite shunt with nitrite accumulation ratio over 95% and excellent nitrogen removal were maintained for the last 10 months with only one alternation applied. N₂O emission increased initially as the attainment of nitrite shunt but exhibited a declining trend during the study. By using on-site-produced nitrite and ammonium, the proposed strategy is feasible and sustainable. This study brings the mainstream nitrite shunt and PN/A one step closer to wide applications.

A critical review of one-stage anammox processes for treating industrial wastewater: Optimization strategies based on key functional microorganisms

The one-stage nitritation/anammox (anaerobic ammonium oxidation) process is an energy-saving technology, which has been successfully developed and widely applied to treat industrial wastewaters. For the one-stage nitritation/anammox process, key functional microbes generally include anaerobic ammonia oxidation bacteria (AnAOB), ammonia-oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB), and heterotrophic bacteria (HB). Cooperation and competition among the key functional microbes are critical to the stability and performance of anammox process. Based upon key functional microorganisms, this review summarizes and discusses the optimized strategies that promote the operation of one-stage nitritation/anammox process. In particular, the review focuses on strategies related to: (1) the retention of anammox biomass through granular sludge or biofilm, (2) the balanced relationship between AOB and AnAOB, (3) the NOB suppression and (4) the HB management by controlling the influent organic matter. In addition, the review proposes further research to address the existing challenges.