Achieving simultaneous nitrogen and antibiotic removal in one-stage partial nitritation-Anammox (PN/A) process

Enhancing degradation of antibiotic-combined pollutants by a hybrid system containing advanced oxidation and microbial treatment, a review

Antibiotics often co-exist with other pollutants, posing a significant threat to ecosystems. This review first examines the applications and limitations of microbial treatments for various types of antibiotic-combined pollutants. Then, it explores the mechanisms and application of hybrid systems that integrate advanced oxidation with microbial treatment, categorized into two-stage and intimately hybrid systems. Finally, the review highlights key knowledge gaps in hybrid systems and provides new insight into the removal of combined pollutants.

Occurrence, distribution, and ecological risks of antibiotics and antibiotic resistance genes in the surface waters of Gaoyou Lake, China

This work examined the occurrence characteristics and ecological risks of 31 antibiotics across five classes and seven ARGs in the surface waters of Gaoyou Lake. A total of 27 antibiotics, spanning four classes, were detected in the surface waters of Gaoyou Lake, with an overall concentration ranging from 57.5 to 114 ng/L and an average of 78.8 ng/L. Sulfonamide antibiotics exhibited the highest average concentration at 32.7 ng/L. Spatial analysis revealed that antibiotic concentration levels in the western region of the lake were higher than those in other areas. Similarly, ARGs were most abundant in this area, with sulfonamide ARGs demonstrating a notably higher mean abundance than other ARGs. Correlation analysis revealed strong positive associations between sul1 and several antibiotics, including sulfadimidine, sulfamethoxazole, ciprofloxacin, lincomycin, clindamycin, erythromycin, and intl1 (P < 0.05), with intra-group correlations among sulfonamide ARGs exceeding those between different ARG groups. Ecological risk assessment indicated that erythromycin and sulfamethoxazole presented medium risks, whereas roxithromycin, azithromycin, and lincomycin were associated with low risks to aquatic organisms. The ecological risk proportions across monitoring sites were primarily low (10.6%) and moderate (16.7%), with no high-risk areas identified and 72.7% presenting no risk. The cumulative ecological risk quotient (RQcum) suggested a medium-risk level at all surveyed sites.

Achieving high nitrogen and antibiotics removal efficiency by nZVI-C in partial nitritation/anammox system with a single-stage membrane-aerated biofilm reactor

This study innovated constructed an activated carbon-loaded nano-zero-valent iron (nZVI-C) enhanced membrane aerated biofilm reactor (MABR) coupled partial nitritation/anammox (PN/A) system for optimizing nitrogen and antibiotics removal. Results showed that nitrogen and antibiotic removal efficiencies of 88.45 ± 0.14% and 89.90 ± 3.07% were obtained by nZVI-C, respectively. nZVI-C hastened Nitrosomonas enrichment (relative abundance raised from 2.85% to 12.28%) by increasing tryptophan content in EPS. Furthermore, nZVI-C proliferated amo gene by 3.92 times and directly generated electrons, stimulating Ammonia monooxygenase (AMO) co-metabolism activity. Concurrently, via antibiotic resistance genes (ARGs) horizontal transfer, Nitrosomonas synergized with Arenimonas and Comamonadaceae for efficient antibiotic removal. Moreover, nZVI-C mitigated antibiotics inhibition of electron transfer by proliferating genes for PN and anammox electron production (hao, hdh) and utilization (amo, hzs, nir). That facilitated electron transfer and synergistic substrate conversion between ammonia oxidizing bacteria (AOB) and anaerobic ammonia oxidizing bacteria (AnAOB). Finally, the high nitrogen removal efficiency of the MABR-PN/A system was achieved.

Recovery strategies and mechanisms of anammox reaction following inhibition by environmental factors: A review

Anaerobic ammonium oxidation (anammox) is a promising biological method for treating nitrogen-rich, low-carbon wastewater. However, the application of anammox technology in actual engineering is easily limited by environmental factors. Considerable progress has been investigated in recent years in anammox restoration strategies, significantly addressing the challenge of poor reaction performance following inhibition. This review systematically outlines the strategies employed to recover anammox performance following inhibition by conventional environmental factors and emerging pollutants. Additionally, comprehensive summaries of strategies aimed at promoting anammox activity and enhancing nitrogen removal performance provide valuable insights into the current research landscape in this field. The review contributes to a comprehensive understanding of restoration strategies of anammox-based technologies.

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

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

Response mechanism of a highly efficient partial nitritation-anammox (PN/A) process under antibiotic stress: Extracellular polymers, microbial community, and functional genes

The Partial nitritation-Anammox (PN/A) process can be restricted when treating high ammonia nitrogen wastewater containing antibiotics. This study aims to explore the response mechanism of the PN/A process under antibiotic stress. Results showed the PN/A process achieved a nitrogen removal rate higher than 1.01 ± 0.03 kg N/m3/d under long-term sulfamethazine stress. The increase of extracellular polymers from 22.52 to 43.96 mg/g VSS was conducive to resisting antibiotic inhibitory. The increase of Denitratisoma and SM1A02 abundance as well as functional genes nirS and nirK indicated denitrifiers should play an important role in the stability of the PN/A system under sulfamethazine stress. In addition, antibiotic-resistant genes (ARGs) sul1 and intI1 significantly increased by 8.78 and 5.12 times of the initial values to maintain the resistance of PN/A process to sulfamethazine stress. This study uncovers the response mechanism of the PN/A process under antibiotic stress, offering a scientific basis and guidance for further application in the future.

Source apportionment and predictable driving factors contribute to antibiotics profiles in Changshou Lake of the Three Gorges Reservoir area, China

Lakes, crucial antibiotic reservoirs, lack thorough exploration of quantitative relationships between antibiotics and influencing factors. Here, we conducted a comprehensive year-long investigation in Changshou Lake within the Three Gorges Reservoir area, China. The concentrations of 21 antibiotics spanned 35.6-200 ng/L, 50.3-348 ng/L and 0.57-57.9 ng/g in surface water, overlying water and sediment, respectively. Compared with abundant water period, surface water and overlying water displayed significantly high antibiotic concentrations in flat and low water periods, while sediment remained unchanged. Moreover, tetracyclines, fluoroquinolones and erythromycin posed notable risks to algae. Six primary sources were identified using positive matrix factorization model, with aquaculture contributing 21.2%, 22.7% and 25.4% in surface water, overlying water and sediment, respectively. The crucial predictors were screened through machine learning, redundancy analysis and Mantel test. Our findings emphasized the pivotal roles of water quality parameters, including water temperature (WT), pH, dissolved oxygen, electrical conductivity, inorganic anions (NO3⁻, Cl⁻ and F⁻) and metal cations (Ca, Mg, Fe, K and Cr), with WT influencing greatest. Total nitrogen (TN), cation exchange capacity, K, Al and Cd significantly impacted sediment antibiotics, with TN having the most pronounced effect. This study can promise valuable insights for environmental planning and policies addressing antibiotic pollution.

Selective removal of oxytetracycline by molecularly imprinted magnetic biochar

Molecularly imprinted magnetic biochar (MBC@MIPs) was synthesized through molecular imprinting precipitation polymerization. This material demonstrated a selective adsorption capacity of oxytetracycline (OTC) from water samples. Upon characterization of MBC@MIPs, results revealed the formation of a memory cavity shell layer on the magnetic biochar's surface, exhibiting a distinctive recognition effect alongside commendable magnetic and thermal stability. Analysis of the adsorption kinetics indicated that the OTC adsorption process aligned well with the pseudo-second-order rate equation, with chemisorption acting as the predominant mechanism for antibiotic adsorption onto MBC@MIPs. The data could be well described by the Langmuir isotherm model. At 299 K, MBC@MIPs showed a maximum binding capacity of 67.89 mg·g-1, surpassing that of MBC (38.84 mg·g-1) by 1.77 times. MBC@MIPs exhibited the highest selectivity towards OTC, with an imprinting factor (IF) of 5.64. Even amidst interference from antibiotics, MBC@MIPs maintained a significant adsorption capacity for OTC (6.10 mg·g-1), with IF of 6.70.

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.

Detection and removal technologies for ammonium and antibiotics in agricultural wastewater: Recent advances and prospective

With the extensive development of industrial livestock and poultry production, a considerable part of agricultural wastewater containing tremendous ammonium and antibiotics have been indiscriminately released into the aquatic systems, causing serious harms to ecosystem and human health. In this review, ammonium detection technologies, including spectroscopy and fluorescence methods, and sensors were systematically summarized. Antibiotics analysis methodologies were critically reviewed, including chromatographic methods coupled with mass spectrometry, electrochemical sensors, fluorescence sensors, and biosensors. Current progress in remediation methods for ammonium removal were discussed and analyzed, including chemical precipitation, breakpoint chlorination, air stripping, reverse osmosis, adsorption, advanced oxidation processes (AOPs), and biological methods. Antibiotics removal approaches were comprehensively reviewed, including physical, AOPs, and biological processes. Furthermore, the simultaneous removal strategies for ammonium and antibiotics were reviewed and discussed, including physical adsorption processes, AOPs, biological processes. Finally, research gaps and the future perspectives were discussed. Through conducting comprehensive review, future research priorities include: (1) to improve the stabilities and adaptabilities of detection and analysis techniques for ammonium and antibiotics, (2) to develop innovative, efficient, and low cost approaches for simultaneous removal of ammonium and antibiotics, and (3) to explore the underlying mechanisms that governs the simultaneous removal of ammonium and antibiotics. This review could facilitate the evolution of innovative and efficient technologies for ammonium and antibiotics treatment in agricultural wastewater.

Predicting and improving the microbial removal of organic micropollutants during wastewater treatment: A review

Organic micropollutants (OMPs) consist of widely used chemicals such as pharmaceuticals and pesticides that can persist in surface and groundwaters at low concentrations (ng/L to μg/L) for a long time. The presence of OMPs in water can disrupt aquatic ecosystems and threaten the quality of drinking water sources. Wastewater treatment plants (WWTPs) rely on microorganisms to remove major nutrients from water, but their effectiveness at removing OMPs varies. Low removal efficiency might be the result of low concentrations, inherent stable chemical structures of OMPs, or suboptimal conditions in WWTPs. In this review, we discuss these factors, with special emphasis on the ongoing adaptation of microorganisms to degrade OMPs. Finally, recommendations are drawn to improve the prediction of OMP removal in WWTPs and to optimize the design of new microbial treatment strategies. OMP removal seems to be concentration-, compound-, and process-dependent, which poses a great complexity to develop accurate prediction models and effective microbial processes targeting all OMPs.

A critical review on microbial ecology in the novel biological nitrogen removal process: Dynamic balance of complex functional microbes for nitrogen removal

The novel biological nitrogen removal process has been extensively studied for its high nitrogen removal efficiency, energy efficiency, and greenness. A successful novel biological nitrogen removal process has a stable microecological equilibrium and benign interactions between the various functional bacteria. However, changes in the external environment can easily disrupt the dynamic balance of the microecology and affect the activity of functional bacteria in the novel biological nitrogen removal process. Therefore, this review focuses on the microecology in existing the novel biological nitrogen removal process, including the growth characteristics of functional microorganisms and their interactions, together with the effects of different influencing factors on the evolution of microbial communities. This provides ideas for achieving a stable dynamic balance of the microecology in a novel biological nitrogen removal process. Furthermore, to investigate deeply the mechanisms of microbial interactions in novel biological nitrogen removal process, this review also focuses on the influence of quorum sensing (QS) systems on nitrogen removal microbes, regulated by which bacteria secrete acyl homoserine lactones (AHLs) as signaling molecules to regulate microbial ecology in the novel biological nitrogen removal process. However, the mechanisms of action of AHLs on the regulation of functional bacteria have not been fully determined and the composition of QS system circuits requires further investigation. Meanwhile, it is necessary to further apply molecular analysis techniques and the theory of systems ecology in the future to enhance the exploration of microbial species and ecological niches, providing a deeper scientific basis for the development of a novel biological nitrogen removal process.

Effects of antimicrobials in anammox mediated systems: critical review

Anammox-mediated systems are thought to be cost-effective and efficient technologies for removing nitrogen from wastewater by converting nitrite and ammonium into dinitrogen gas. However, there are inhibitory substances that reduce the effectiveness and efficiency of these processes, preventing their widespread application. Antimicrobial agents are among these substances that have been observed to inhibit anammox-mediated processes. Therefore, this review provides a comprehensive overview of the effects of various antimicrobials on the anammox-based systems with emphasis on the effects in different reactor configurations, sludge types and microbial population of anammox-based systems. In addition, this review also discusses the mechanisms by which nitrifying bacteria are inhibited by the antimicrobials. Gaps in knowledge based on this review as well as future research needs have also been suggested. This review gives a better knowledge of antimicrobial effects on anammox-based systems and provides some guidance on the type of system to use to treat antimicrobial-containing wastewater using anammox-based processes.

Spatial and temporal effects of fish feed on antibiotic resistance in coastal aquaculture farms

For the first time, both spatial and temporal effects of fish feed on changes in abundance of antibiotic resistance genes (ARGs) were investigated in South Korea via quantifying ARGs and analyzing physicochemical parameters in the influent (IN) and effluent before (BF) and 30 min after (AF) the fish feeding time of sixteen flow-through fish farms. The absolute abundance of ARGs in AF samples was 5 times higher than in BF and 12 times higher than in IN samples. Values of physicochemical parameters such as ammonia, total nitrogen, suspended solids and turbidity in the effluent significantly increased by 21.6, 4.2, 2.6 and 1.65 times, respectively, after fish feeding. Spatially, the fish farms on Jeju Island exhibited higher relative abundance (3.02 × 10^-4 - 6.1 × 10^-2) of ARGs compared to the farms in nearby Jeollanam-do (3.4 × 10^-5 - 8.3 × 10^-3). Seasonally, samples in summer and autumn showed a higher abundance of ARGs than in winter and spring. To assess risk to the food chain as well as public health, further studies are warranted to explore the pathogenic potential of these ARGs.

Free Nitrous Acid Inhibits Atenolol Removal during the Sidestream Partial Nitritation Process through Regulating Microbial-Induced Metabolic Types

Limited studies have attempted to evaluate pharmaceutical removal during the sidestream partial nitritation (PN) process. In this work, atenolol biodegradation by PN cultures was investigated by maintaining ammonium and pH at different levels. For the first time, free nitrous acid (FNA), other than ammonium, pH, and free ammonia, was demonstrated to inhibit atenolol removal, with biodegradation efficiencies of ∼98, ∼67, and ∼28% within 6 days at average FNA levels of 0, 0.03, and 0.19 mg-N L^-1, respectively. Ammonia-oxidizing bacteria (AOB)-induced metabolism was predominant despite varying FNA concentrations. In the absence of ammonium/FNA, atenolol was mostly biodegraded via AOB-induced metabolism (65%) and heterotroph-induced metabolism (33%). AOB-induced metabolism was largely inhibited (down to 29%) at 0.03 mg-N L^-1 FNA, while ∼27 and ∼11% were degraded via heterotroph-induced metabolism and AOB-induced cometabolism, respectively. Higher FNA (0.19 mg-N L^-1) substantially reduced atenolol biodegradation via heterotroph-induced metabolism (4%), AOB-induced metabolism (16%), and AOB-induced cometabolism (8%). Newly identified products and pathways were related to metabolic types and FNA levels: (i) deamination and decarbonylation (AOB-induced cometabolism, 0.03 mg-N L^-1 FNA); (ii) deamination from atenolol acid (heterotrophic biodegradation); and (iii) nitro-substitution (reaction with nitrite). This suggests limiting FNA to realize simultaneous nitrogen and pharmaceutical removal during the sidestream process.

Advanced nitrogen removal from real municipal wastewater by multiple coupling nitritation, denitritation and endogenous denitritation with anammox in a single suspended sludge bioreactor

Achieving advanced nitrogen removal based on anammox for treating mainstream municipal wastewater in a single suspended sludge bioreactor is a challenging research topic. In this study, multiple coupling nitritation, denitritation and endogenous denitritation with anammox (PNA-(E)PDA) was simultaneously achieved in a 10 L step-feed bioreactor, which enhanced stable nitrogen removal. After 223 days of operation, the total nitrogen concentrations of the influent and effluent were 70.7 ± 6.1 and 4.3 ± 1.8 mg/L, respectively, when treating municipal wastewater even at C/N ratio of 2.24 with only 5 h of aerobic time (DO: 0.5-0.8 mg/L). After the evolution of nitritation/anammox to PNA-(E)PDA, the contribution of anammox to nitrogen removal increased to 78.6% and the anammox activity increased from 4.3 ± 0.2 to 15.2 ± 0.7 mg NH₄⁺-N/gVSS/d. qPCR results showed that the abundance of anammox bacteria increased from 4.1 × 10⁹ to 4.5 × 10¹⁰ copies/ (g VSS). High-throughput sequencing further revealed that the relative abundance of Candidatus Brocadia, the dominant anammox genus, increased from 0.09 to 0.46%. Based on the strong competitiveness of anammox on nitrite, this novel PNA-(E)PDA process provides a potential strategy for enriching anammox bacteria in municipal wastewater treatment plants.

Fates of quaternary ammonium compound resistance genes and the corresponding resistant strain in partial nitrification/anammox system under pressure of hexadecyl trimethyl ammonium chloride

Hexadecyl trimethyl ammonium chloride (ATMAC-C16) is a kind of quaternary ammonium compound (QACs) which is extensively consumed as disinfectants, antimicrobials and surfactants. Here, the partial nitrification/anammox (PN/A) system was exposed to different levels of ATMAC-C16 (0-10 mg/L) and the main objective was to reveal the long-term microbiological responses of PN/A system to ATMAC-C16, importantly, explore the tolerance of PN/A to ATMAC-C16 and the key resistant strain. Nitrogen removal efficiency was influenced by environmental and extreme levels of ATMAC-C16 through mainly affecting the anammox (hzsB) gene. Two types of anammox, Candidatus Jettenia and Candidatus Kuenenia, were enriched under the pressure of ATMAC-C16, which allowed PN/A system to maintain good nitrogen removal performance. ATMAC-C16 might cause the hormesis of entire microbial population in PN/A system, leading to the enhancement of cell viability. ATMAC-C16 decreased the relative abundances of most antibiotics resistance genes (ARGs) but significantly enriched QACs resistance genes (QRGs). The tolerance of PN/A system to ATMAC-C16 might be strengthened by inducing the efflux pumps encoding genes (qacH-01/02). Microbial hosts dynamic and co-selection mechanism among ARGs and QRGs resulted in the opposite trends of qacEdeltal-01/02 and qacH-01/02. Pseudoxanthomonas mexicana was identified as the ATMAC-C16 resistant strain, and its resistance to 10 mg/L ATMAC-C16 might not only obtain by capturing the qacH gene, but also benefit from its own efflux pump system. Therefore, from the perspective of the transmission of resistance genes, especially for QRGs, the spread risk of QRGs and ATMAC-C16 resistant strain in PN/A technique should be taken seriously.

Source-specific risk apportionment and critical risk source identification of antibiotic resistance in Fenhe River basin, China

A comprehensive understanding of the sources and distribution of antibiotic resistance risk is essential for controlling antibiotic pollution and resistance. Based on surface water samples collected from the Fenhe River basin in the flood season, using the positive matrix factorization (PMF) model, the risk quotient (RQ) method and the multiple attribute decision making (MADM) method, the resistance risk and source-specific resistance risk of antibiotics were analyzed in this study. The results showed that sulfonamides (SAs) were the dominant antibiotics with a mean concentration of 118.30 ng/L, whereas tetracyclines (TCs) and macrolides (MLs) had the highest detection frequencies (100%). The significant resistance risk rate of antibiotics in the entire river basin was 48%, but no high risk occurred. The significant resistance risk rate of quinolones (QNs) was the highest (100%), followed by that of MLs and TCs. Owing to human activities, the most serious resistance risk occurred in the midstream of the river basin. The resistance risk was the lowest upstream. The antibiotics were mainly contributed by six sources. Pharmaceutical wastewater was the main source, accounting for 30%, followed by livestock discharge (22%). The resistance risk from the six sources showed clear differences, but none of the sources caused a high risk of antibiotic resistance. Pharmaceutical wastewater poses the greatest risk of antibiotic resistance in the Fenhe River basin and is widely distributed. The second greatest source was livestock discharge, which was mainly concentrated in the upstream and midstream areas. The critical sources upstream, midstream, and downstream were all pharmaceutical wastewater, whereas the sequences of other sources were different because different areas were affected by different human activities. The proposed method might provide an important reference for the identification the key source of antibiotics and management of antibiotic pollution, as well as help for the management of antibiotics in Fenhe and Shanxi Province.

Application of electric fields to mitigate inhibition on anammox consortia under long-term tetracycline stress

The feasibility of applying electric fields to mitigate inhibition of tetracycline (TC) on anammox process and improve system stability was evaluated in this study. Three electric field intensities of 1, 3 and a variable intensity of 1-6 V (VEF) were used to optimize electric field intensity under gradually increasing addition of TC (0.5, 2 and 10 mg L^-1). Results showed that the application of electric fields (3 V and VEF) could improve TC tolerance and keep relatively high-efficiency nitrogen removal performance, especially at TC ≥ 2 mg L^-1. Furthermore, applying electric fields contributed to mitigate irreversible inhibition and improve the stability of community structure. Underlying mechanism analysis indicated that the main mechanism of applying electric fields to mitigate inhibition relies on sludge structure strengthening. This study explored a novel strategy to reduce the inhibition of antibiotics on microbial denitrification and broaden the application of anammox in industrial water treatment.

Insights into complete nitrate removal in one-stage nitritation-anammox by coupling heterotrophic denitrification

Nitritation-anammox has been considered to be the most promising process for nitrogen (N) removal from wastewater. However, the anammox reaction still produces an amount of nitrate, which cannot be removed further. This study hypothesizes that heterotrophic denitrification can be an appealing option to remove the residual nitrate in the one-stage nitritation-anammox process. Through monitoring N-removal performance and microbial community succession of a laboratory microaerobic reactor, the effect of four different levels of oxygen supply on nitrate removal was investigated. The reactor was continuously fed with real manure-free piggery wastewater containing ~240 mg NH₄⁺-N/L and chemical oxygen demand (COD)/total nitrogen (TN) ratio of less than 1 for 180 days. With a high influent loading rate of 0.7 kg N/(m³·d), efficient total nitrogen removal (>80 %) was achieved during stable operation of dissolved oxygen (DO) concentrations between 0.3 and 0.6 mg O₂/L, indicating N-removal via the nitritation-anammox pathway in the low-carbon wastewater treatment. At the same time, the effluent nitrate reduced with decreased oxygen supply and completely depleted at DO of 0.3 ± 0.1 mg O₂/L. In addition to oxygen, preventing ammonia nitrogen from falling to very low levels (<10 mg/L) could be also useful for the complete nitrate removal and stable nitritation-anammox. 16S rRNA gene-based analyses confirmed a complex microbial community including nitrifiers, denitrifiers and anammox bacteria in the biomass of the reactor. Collectively, this study provides new insights into high-level N-removal of a nitritation-anammox process by complete nitrate depletion.

Response of antioxidant defense to oxidative stress induced by H2O2 and NO in anammox bacteria

Exposure to the stressful environment results in excessive generation of reactive oxygen species (ROS) or reactive nitrogen species (RNS) in anaerobes, which causes deterioration of microbial activities in biological wastewater treatment systems. Although the genes involved in oxidative stress defense have been primarily identified in the genome of Candidatus Kuenenia stuttgartiensis (a typical anammox species), their function is still not verified. Therefore, the expression of putative antioxidation genes kat, sor, and sod in anammox bacteria was studied by in situ transcription and function validated by heterologous expression under the typical ROS (H₂O₂) and RNS (NO) stress. After H₂O₂ and NO additions, the genes involved in the anammox central metabolism (nirS, hzsB, and hdh) were immediately down expressed consistent with the decreased anammox activity. However, the expression of putative antioxidation gene kat did not rise when exposed to H₂O₂; whereas, its encoding protein KAT enhanced the antioxidant actively of anammox bacteria by H₂O₂ decomposition like the oxidoreductase enzyme catalase. The sod and sor gene were upregulated with NO treatment, and SOD and SOR can combine with NO and decrease its concentration efficiently. These confirmed the important role of kat, sod, and sor as ROS/RNS scavengers in anammox bacteria, with which anammox bacteria protect themselves when they are exposed to the stressful environment. These verified functional enzymes provide directions for the future regulation of anammox systems, which helps to mitigate the inhibitory effect of the stressful environment on anammox bacteria.

Improving stability of mainstream Anammox in an innovative two-stage process for advanced nitrogen removal from mature landfill leachate

This study presents an innovative mainstream Anammox based on multiple NO₂^--N supplement pathways to treat actual mature landfill leachate over 180 days. Desirable effluent quality of 11.8 mg/L total nitrogen (TN) and nitrogen removal efficiency of 98.8% were achieved despite fluctuation conditions of 1.5-fold influent substrates and 8.0-fold dissolved oxygen overload. Nitrogen mass balance confirmed Anammox was the dominant nitrogen removal pathway, contributing up to 87.9%. Functional genes of ammonia monooxygenase (amoA), hydrazine synthase (hzsB), and ratio of nitrate/nitrite reductase were highly detected. Anammox genera, Candidatus_Kuenenia (4.1%) and Candidatus_Brocadia (5.3%) were dominant in two functional systems, respectively, due to the different affinity of nitrite, oxygen, and organic carbon. As an economical and sustainable technology, the innovative process enabled a 95.1% decrease in organic carbon demand, a 61.5% reduction in aeration energy consumption, and 77.6% lower biomass production compared with traditional nitrification-denitrification process.

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.

How anammox responds to the emerging contaminants: Status and mechanisms

Numerous researches have been carried out to study the effects of emerging contaminants in wastewater, such as antibiotics, nanomaterials, heavy metals, and microplastics, on the anammox process. However, they are fragmented and difficult to provide a comprehensive understanding of their effects on reactor performance and the metabolic mechanisms in anammox bacteria. Therefore, this paper overviews the effects on anammox processes by the introduced emerging contaminants in the past years to fulfill such knowledge gaps that affect our perception of the inhibitory mechanisms and limit the optimization of the anammox process. In detail, their effects on anammox processes from the aspects of reactor performance, microbial community, antibiotic resistance genes (ARGs), and functional genes related to anammox and nitrogen transformation in anammox consortia are summarized. Furthermore, the metabolic mechanisms causing the cell death of anammox bacteria, such as induction of reactive oxygen species, limitation of substrates diffusion, and membrane binding are proposed. By offering this review, the remaining research gaps are identified, and the potential metabolic mechanisms in anammox consortia are highlighted.

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.

Highly enhanced biodegradation of pharmaceutical and personal care products in a novel tidal flow constructed wetland with baffle and plants

Research on decentralized wastewaters deserves special focus due to the potential abundance of emerging organic pollutants including pharmaceutical and personal care products (PPCPs), which might pose serious threats to the local water bodies and even to human health. Constructed wetland (CW) is a common decentralized wastewater treatment technology, with a certain ability to eliminate PPCPs. Nonetheless, PPCPs removal in common CWs is frequently challenging, besides, the removal mechanism remains elusive. Based on our previous study, tidal flow constructed wetlands (TFCWs) is effective in nitrogen removal. Here, 3 TFCWs with different modifications (baffle, plants, both baffle and plants) were constructed to treat raw domestic sewage and specifically to evaluate the removal efficiencies and mechanism of PPCPs. 24 PPCPs including 7 antibiotics, 8 steroid hormones and 9 biocides were detected in the level of 1.10 ± 0.29 ng/L-799 ± 10.6 ng/L in the influents. Consequently, we found that modification with both baffle and plants significantly influenced the removal of PPCPs. Moreover, the highest removal rates of biocides (97.1 ± 0.29%), steroid hormones (99.8 ± 0.02%), and antibiotics (90.2 ± 1.60%) were achieved via both baffles and plants in TFCWs. Based on the mass balance analysis, microbial degradation dominated the removal of PPCPs with a percentage higher than 85.7%, followed by substrate adsorption (5.22 × 10^-2-14.3%) and plant uptake (1.66 × 10^-3-0.44%). Further, 16S rDNA sequencing analysis revealed that the presence of baffle and plants improve the removal efficiency of PPCPs by means of enhancing microbial diversity and changing dominant microorganisms. Moreover, Thaumarchaeota was potentially the key microorganism in the phylum level for PPCPs elimination by TFCWs through LEfSe (linear discriminant analysis (LDA) effect size) analysis. These findings provide new insights into the removal of PPCPs in CWs.

Enhanced removal of cephalexin and sulfadiazine in nitrifying membrane-aerated biofilm reactors

Nitrification process has been reported to be capable of degrading various pharmaceuticals due to the cometabolism of ammonia-oxidizing bacteria (AOB). The membrane aerated biofilm reactor (MABR) is an emerging configuration in wastewater treatment with advantages of high nitrification rate and low energy consumption. However, there are very few studies investigating the degradation of antibiotics at environmentally relevant levels in nitrifying MABR systems. In this study, the removal of two widely used antibiotics, cephalexin (CFX) and sulfadiazine (SDZ), was evaluated in two independent MABRs with nitrifying biofilms. The impacts of CFX and SDZ exposure on the nitrification performance and microbial community structure within biofilms were also investigated. The results showed that nitrifying biofilms were very efficient in removing CFX (94.6%) and SDZ (75.4%) with an initial concentration of 100 μg/L when hydraulic retention time (HRT) was 4 h in the reactors. When HRT decreased from 4 h to 3 h, the removal rates of CFX and SDZ increased significantly from 23.4 ± 1.0 μg/(L·h) and 18.7 ± 1.1 μg/(L·h), respectively, to 27.7 ± 1.3 μg/(L·h) (p＜0.01) and 20.8 ± 2.4 μg/(L·h) (p＜0.05), while the removal efficiencies decreased to 86.0% and 61.5%, respectively. Despite the exposure to CFX and SDZ, the nitrification performance was not affected, and microbial community structure within biofilms also remained relatively stable. This study shows that nitrifying MABR process is a promising option for the efficient removal of antibiotics from domestic wastewater.