Biological Nitrogen Removal System Research Articles

Microbial community and gene dynamics response to high concentrations of gadolinium and sulfamethoxazole in biological nitrogen removal system

The impact of high antibiotic and heavy metal pollution levels on biological nitrogen removal in wastewater treatment plants (WWTPs) remains poorly understood, posing a global concern regarding the issue spread of antibiotic resistance induced by these contaminants. Herein, we investigated the effects of gadolinium (Gd) and sulfamethoxazole (SMX), commonly found in medical wastewater, on biological nitrogen removal systems and microbial characteristics, and the fate of antibiotic resistance genes (ARGs), metal resistance genes (MRGs), and mobile genetic elements (MGEs). Our findings indicated that high SMX and Gd(III) concentrations adversely affected nitrification and denitrification, with Gd(III) exerting a strong inhibitory effect on microbial activity. Metagenomic analysis revealed that high SMX and Gd(III) concentrations could reduce microbial diversity, with Thauera and Pseudomonas emerging as dominant genera across all samples. While the relative abundance of most ARGs decreased under single Gd(III) stress, MRGs increased, and nitrification functional genes were inhibited. Conversely, combined SMX and Gd(III) pollution increased the relative abundance of intl1. Correlation analysis revealed that most genera could host ARGs and MRGs, indicating co-selection and competition between these resistance genes. However, most denitrifying functional genes exhibited a positive correlation with MRGs. Overall, our study provides novel insights into the impact of high concentrations of antibiotics and heavy metal pollution in WWTPs, and laying the groundwork for the spread and proliferation of resistance genes under combined SMX and Gd pollution.

Effect of initial ammonium concentration on a one-stage partial nitrification/anammox biofilm system: Nitrogen removal performance and the microbial community

One-stage partial nitrification coupled with anammox (PN/A) technology effectively reduces the energy consumption of a biological nitrogen removal system. Inhibiting nitrite-oxidizing bacteria (NOB) is essential for this technology to maintain efficient nitrogen removal performance. Initial ammonium concentration (IAC) affects the degree of inhibited NOB. In this study, the effect of the IAC on a PN/A biofilm was investigated in a moving bed biofilm reactor. The results showed that nitrogen removal efficiency decreased from 82.49% ± 1.90% to 64.57% ± 3.96% after the IAC was reduced from 60 to 20 mg N/L, while the nitrate production ratio increased from 13.87% ± 0.90% to 26.50% ± 3.76%. NOB activity increased to 1,133.86 mg N/m2/day after the IAC decreased, approximately 4-fold, indicating that the IAC plays an important inhibitory role in NOB. The rate-limiting step in the mature biofilm of the PN/A system is the nitritation process and is not shifted by the IAC. The analysis of the microbial community structure in the biofilm indicates that the IAC was the dominant factor in changes in community structure. Ca. Brocadia and Ca. Jettenia were the main anammox bacteria, and Nitrosomonas and Nitrospira were the main AOB and NOB, respectively. IAC did not affect the difference in growth between Ca. Brocadia and Ca. Jettenia. Thus, modulating the IAC promoted the PN/A process with efficient nitrogen removal performance at medium to low ammonium concentrations.

Dissolved oxygen benefits N-decanoyl-homoserine lactone regulated biological nitrogen removal system to resist acute ZnO nanoparticle exposure

The beneficial effects of N-decanoyl-homoserine lactone (C10-HSL), one of the typical N-acyl-homoserine lactones on biological nitrogen removal (BNR) system to resist the acute exposure of zinc oxide nanoparticles (ZnO NPs) has attracted extensive attentions. Nevertheless, the potential impact of dissolved oxygen (DO) concentration on the regulatory capacity of C10-HSL in the BNR system has yet to be investigated. This study conducted a systematic investigation of the impact of DO concentration on the C10-HSL-regulated BNR system against short-term ZnO NP exposure. Based on the findings, sufficient DO played a crucial role to improve the BNR system's resistance capacity to ZnO NPs. Under the micro-aerobic condition (0.5 mg/L DO), the BNR system was more sensitive to ZnO NPs. The ZnO NPs induced increased intracellular reactive oxygen species (ROS) accumulation, reduced antioxidant enzyme activities, and decreased specific ammonia oxidation rates in the BNR system. Furthermore, the exogenous C10-HSL had a positive effect on the BNR system's resistance to ZnO NP-induced stress, primarily by decreasing ZnO NPs-induced ROS generation and improving ammonia monooxygenase activities, especially under low DO concentrations. The findings contributed to the theoretical foundation for regulation strategy development of wastewater treatment plants under NP shock threat.

New insights into exogenous N-acyl-homoserine lactone manipulation in biological nitrogen removal system against ZnO nanoparticle shock

The effects and mechanisms of three N-acyl-homoserine lactones (AHLs) (C4-HSL, C6-HSL, and C10-HSL) on responses of biological nitrogen removal (BNR) systems to zinc oxide nanoparticle (NP) shock were investigated. All three AHLs improved the NP-impaired ammonia oxidation rates by up to 50.0 % but inhibited the denitrification process via regulating nitrogen metabolism-related enzyme activities. C4-HSL accelerated the catalase activity by 13.2 %, while C6-HSL and C10-HSL promoted the superoxide dismutase activity by 26.6 % and 18.4 %, respectively, to reduce reactive oxygen species levels. Besides, the enhancements of tryptophan protein and humic acid levels in tightly-bound extracellular polymeric substance by AHLs were vital for NP toxicity attenuation. The metabonomic analysis demonstrated that all three AHLs up-regulated the levels of lipid- and antioxidation-related metabolites to advance the system’s resistance to NP shock. The “dual character” of AHLs emphasized the concernment of legitimately employing AHLs to alleviate NP stress for BNR systems.

In-situ enrichment and application of heterotrophic nitrification-aerobic denitrification bacteria in membrane aerated biofilm reactor

In order to reduce the resource and energy consumption of traditional biological nitrogen removal (BNR) process, heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria was in situ enriched in membrane aerated biofilm reactor (MABR) by inoculating conventional activated sludge. Contaminants removal performance, EPS composition and microbial community were explored. The results indicated that the average removal efficiency of COD and TN under optimal condition reached 84.13% and 91.54%, respectively, which demonstrated that the reactor possessed excellent contaminants removal capacity. EPS analysis suggested that abundant protein especially tryptophan protein-like substance played a vital role in maintaining the microbial stability of biofilms. Multiple HN-AD genera, mainly Paracoccus, were detected with the highest relative abundance of 54.70%, which confirmed the successful enrichment of the HN-AD bacteria. Conventional nitrifiers and denitrifiers also thrived in biofilm, which demonstrated the synergistic nitrogen removal of multiple microorganisms. This study provided important insights into application of HN-AD bacteria and synergistic nitrogen removal in BNR system.

The nitrogen removal performance and microbial community on mixotrophic denitrification process

Sulfur autotrophic denitrifiers and heterotrophic denitrifiers widely exist in aquatic ecosystem, however, the response of sulfide to the microbial community structure in mixotrophic denitrification ecosystem is unknown yet. In this study, the denitrification performance and microbial community were explored by changing the molar ratio of influent C/N/S. From the level of genus, the joint action of Thauera, Pacacoccus, Fusibacter Pseudoxanthomonas, Thiobacillus, Sulfurovum and Sulfurimonas brought about the efficient denitrification performance in the mixotrophic system. Thauera increased from from 0.97% to more than 13%, and the relative abundances of Thiobacillus and Sulfurimonas were about 4.14% and 3.89% separately after adding S2−. The results of this study showed that the denitrification performance could be indeed intensified in the mixotrophic system, among which provided a theoretical basis for establishing an efficient biological nitrogen removal system.

Start-up and synergistic nitrogen removal of partial nitrification and anoxic/aerobic denitrification in membrane aerated biofilm reactor

To reduce energy consumption and improve operational stability of traditional biological nitrogen removal (BNR) system, partial nitrification and anoxic/aerobic denitrification were synergistically implemented in membrane aerated biofilm reactor (MABR) by regulating DO and pH. The results indicated that the optimal DO, pH and C/N ratio were 1–2 mg/L, 9.0 and 4–7, respectively. The corresponding average organic removal rate (ORR), total nitrogen removal rate (TNRR) and nitrite accumulation rate (NAR) reached 324 gCOD・m−3・d−1, 48 gN・m−3・d−1 and 77.70%, respectively. Extracellular polymeric substance (EPS) content in biofilm was more abundant than that in inoculated sludge. Multiple aerobic denitrifiers were detected in the biofilm with the relative abundance of 11.19%–22.71%. AQUASIM simulation implied that the distribution and proportion of substrates and bacteria were significantly affected by DO and pH regulation. Overall, this study provided some important insights in the start-up and operation of synergistic nitrogen removal process in BNR system.

Pluripotency of endogenous AHL-mediated quorum sensing in adaptation and recovery of biological nitrogen removal system under ZnO nanoparticle long-term exposure

The impacts of quorum sensing (QS) on nanoparticle (NP)-stressed biological nitrogen removal (BNR) system have seldom been addressed yet. In this study, the contributions of endogenous N-acyl-homoserine lactone (AHL)-based QS regulation to the BNR system's adaptation to the zinc oxide (ZnO) NP stress and its recovery potential were systematically investigated. Although 1 mg/L ZnO NPs exerted little impact on the BNR system, chronic exposure to 10 mg/L ones depressed the system's BNR performance which irreversibly impaired the nitrification process even when the system entered the recovery period with no NP added anymore. Meanwhile, ZnO NPs exhibited hormesis effects on the production of AHLs and extracellular polymeric substance (EPS), and activities of superoxide dismutase and catalase. During the ZnO NP exposure period, C4-HSL, C6-HSL, and C10-HSL were discovered to be positively associated with nitrogen removal efficiency, tightly-bound EPS production, and antioxidase activities. Besides, the shifts of Nitrospira, Dechloromonas, Aeromonas, Acinetobacter, Delftia, and Bosea were expected to determine the AHL's dynamic distribution. During the system's recovery stage, Dechloromonas replaced Candidatus_Competibacter as the dominant denitrification-related genus. Dechloromonas abundance elevated with the increased contents of C4-HSL in the aqueous and EPS phases and C10-HSL in EPS and sludge phases, and were expected to promote the activities of BNR-related and antioxidant enzymes, and the EPS production to assist in the recovery of the impaired system's BNR performance. The QS-related BNR genera exhibited higher resilience to ZnO NPs than quorum quenching-related ones, indicating their critical role in nitrogen removal in the restored system. This work provided an insight into the potential pluripotency of AHL-based QS regulation on the ZnO NP-stressed BNR system's adaptation and recovery.

Evaluation of the effect of peracetic acid solution on the performance of a continuous-flow biological nitrogen removal (BNR) system

• PAA addition directly to the anaerobic reactor did not affect BNR system performance. • PAA-bearing wastewater at 200 mg/L residual PAA fed to the anaerobic reactor resulted in BNR system upset. • The BNR system was more susceptible to residual PAA without the anaerobic reactor. • Fast, complete BNR system recovery within one HRT feeding with PAA-free wastewater. • Betaproteobacteriales, Sphingobacteriales and Rhodobacterales were affected by PAA. Peracetic acid (PAA) has been extensively used in poultry processing plants as an effective disinfectant alternative to chlorine-based products. However, PAA-bearing poultry processing wastewater, as part of the regular disinfection process and/or in case of accidental spills, can lead to severe upsets of biological wastewater treatment processes. The objective of this study was to assess the short- and long-term effect of PAA solution (PAA and hydrogen peroxide, H 2 O 2 ) on the performance of a continuous-flow biological nitrogen removal (BNR) system, fed with poultry processing wastewater. The system consisted of three reactors in series: anaerobic (R1), anoxic (R2), and aerobic (R3). Continuous, direct addition of PAA solution up to 200/28.6 mg/L PAA/H 2 O 2 (feed-based concentration) to the anaerobic reactor (R1) did not affect the performance of the BNR system, which achieved over 90 and 82% COD and N removal, respectively. However, both COD and N removal were negatively affected by continuous feeding with residual PAA-bearing wastewater at 80 and 200 mg/L. PAA was self-decomposed or by reaction with other wastewater components, such as proteins and transition metals, rather than being biodegraded. Fast recovery of COD and N removal was achieved within three retention times feeding with PAA/H 2 O 2 -free wastewater. The BNR system was more susceptible to residual PAA without the R1, i.e., two-reactor system. Complete recovery from the impact of residual PAA was achieved, suggesting the effect of PAA on system performance was fast reversed and not lasting. The impact on the BNR system was a combination of enzyme inhibition and cell lysis leading to biomass loss. The PAA impact in R1 (anaerobic) on Synergistales and Bacteroidales was reversed, but not on Lactobacillales and Pseudomonadales. Campylobacterales and Sphingomonadales were enriched when PAA was fed to the BNR system. Betaproteobacteriales, Sphingobacteriales and Rhodobacterales in R2 (anoxic) and R3 (aerobic) reactors were negatively affected by PAA, but the impact was fast reversed when PAA addition was ceased.

Study on the effectiveness of an independent biological phosphorus removal system based on immobilized biological fillers nitrogen removal system in municipal wastewater

This study focused on the problem that simultaneous nitrogen and phosphorus removal cannot be achieved in current urban sewage treatment systems. To address this, a pilot system was developed to first use immobilized biologically active fillers to remove nitrogen, and then use activated sludge to independently remove phosphorus. The goal was to maximize the effectiveness of biological phosphorus removal. This study mainly aimed at analyzing the post-independent phosphorus removal characteristics and mechanisms; the composition of the phosphorus removing bacteria; and the contribution rate of the system. The effluent PO43--P of the system remained stable at 0.15–0.2 mgL−1, with the phosphorus removal anaerobic tank maintaining a "micro-release" state. When the hydraulic retention time (HRT) of the aerobic phosphorus removal tank was reduced to 1 h, the contribution rate of the aerobic phosphorus removal bacteria decreased from 28.42% to 16.68%; the contribution rate of the denitrifying phosphorus removal bacteria increased from 72.58% to 83.32%; and the abundance of the dominant bacteria Dechloromonas and Thaurea significantly increased. The phosphorus removal system provides a new approach for deep phosphorus removal in main municipal wastewater systems.

Achieving combined biological short-cut nitrogen and phosphorus removal in a one sludge system with side-stream sludge treatment

Biological nitrogen (N) removal via the short-cut pathway (NH4+-N→NO2−-N→N2) is economically attractive in wastewater treatment plants (WWTPs). However, biological phosphorus (P) removal processes remain a bottleneck in these systems due to the strong inhibitory effect of nitrite or its protonated form (HNO2, free nitrous acid - FNA) on polyphosphate accumulating organisms (PAOs). In this study, a novel combined nitrogen and phosphorus removal strategy was verified and achieved in a biological short-cut nitrogen removal system via side-stream sludge treatment with FNA, and the mechanisms impacting this process were investigated. The side-stream FNA treatment process applied here led to a significant reduction in the real sludge retention time (SRT) in the mainstream (approximately 2.7 days) based on the biocidal effect of FNA to the majority of the organisms. This work also found that around 40% of the P uptake activity was still maintained at a much higher FNA level of 38 μg N/L with potential PAOs, which highly broadened the current knowledge of PAOs community. An economic analysis revealed advantages of the proposed as compared to conventional biological nitrogen and phosphorus removal (13% savings in total cost), biological short-cut nitrogen removal (via FNA treatment) with chemical phosphorus precipitation (21% savings) and conventional biological nitrogen removal with chemical precipitation (27% savings). Overall, this study presents a novel and viable retrofit strategy in integrating biological short-cut nitrogen removal with EBPR for next generation WWTPs.

Light as a Novel Inhibitor of Nitrite-Oxidizing Bacteria (NOB) for the Mainstream Partial Nitrification of Wastewater Treatment

Conventional biological nutrient removal processes in municipal wastewater treatment plants are energy-consuming, with oxygen supply accounting for 45–75% of the energy expenditure. Many recent studies examined the implications of the anammox process in sidestream wastewater treatment to reduce energy consumption, however, the process did not successfully remove nitrogen in mainstream wastewater treatment with relatively low ammonia concentrations. In this study, blue light was applied as an inhibitor of nitrite-oxidizing bacteria (NOB) in a photo sequencing batch reactor (PSBR) containing raw wastewater. This simulated a biological nitrogen removal system for the investigation of its application potential in nitrite accumulation and nitrogen removal. It was found that blue light illumination effectively inhibited NOB rather than ammonia-oxidizing bacteria due to their different sensitivity to light, resulting in partial nitrification. It was also observed that the NOB inhibition rates were affected by other operational parameters like mixed liquor suspended solids (MLSS) concentration and sludge retention time (SRT). According to the obtained results, it was concluded that the process efficiency of partial nitrification and anammox (PN/A) could be significantly enhanced by blue light illumination with appropriate MLSS concentration and SRT conditions.

Insights into chronic zinc oxide nanoparticle stress responses of biological nitrogen removal system with nitrous oxide emission and its recovery potential

The nitrogen transformation performances and greenhouse gas nitrous oxide (N2O) emissions in a sequencing batch reactor under chronic exposure to zinc oxide nanoparticles (ZnO NPs) were quantified and the system's self-recovery potentials were assessed. ZnO NPs posed a dose-dependent depression effect on the removal efficiencies of ammonia nitrogen (NH4+-N) and total nitrogen (TN), and the N2O emissions. The suppressed N2O emissions had a positive relationship with the activity ratios of nitrite/NO reductases and N2O reductase, and were expected to be caused by the inhibited heterotrophic denitrification process. The inhibition of glucose metabolism key enzymes and electron transport chain activities would be responsible for the heterotrophic denitrification performances deterioration. Furthermore, the removal efficiencies of NH4+-N and TN were recovered to control levels through the nitrite-shunt. However, the N2O emission increased significantly above the control during the recovery period mainly due to the irreversibility of the depressed nitrite oxidation activities.

Achieving rapid thiosulfate-driven denitrification (TDD) in a granular sludge system

Sulfur-oxidizing bacteria (SOB) can drive a high level of autotrophic denitrification (AD) activity with thiosulfate (S2O32−) as the electron donor. However, the slow growth of SOB results in a low biomass concentration in the AD reactor and unsatisfactory biological nitrogen removal (BNR). In this study, our goal was to establish a high-rate thiosulfate-driven denitrification (TDD) system via sludge granulation. Granular sludge was successfully cultivated by increasing the nitrogen loading rate stepwise in thiosulfate-oxidizing/nitrate-reducing conditions in an upflow anaerobic blanket reactor. In the mature-granular-sludge reactor, a nitrate removal rate of 280 mg N/L/h was achieved with a nitrate removal efficiency of 97.7%±1.0% at a hydraulic retention time of only 15 minutes, with no nitrite detected in the effluent. Extracellular polymeric substance (EPS) analysis indicated that the proteins in loosely bound and tightly bound EPS were responsible for maintaining the compact structure of the TDD granular sludge. The dynamics of the microbial-community shift were identified by 16S rRNA high-throughput pyrosequencing analysis. The Sulfurimonas genus was found to be enriched at 74.1% of total community and may play the most critical role in the high-rate BNR. The batch assay results reveal that no nitrite accumulation occurred during nitrate reduction because the nitrate reduction rate (75.90±0.67 mg N/g MLVSS/h) was almost equal to the nitrite reduction rate (66.06±1.28 mg N/g MLVSS/h) in the thiosulfate-driven granular sludge reactor. The results of this study provide support for the establishment of a high-rate BNR system that maintains its stability with a low sludge yield.

Nitrous oxide (N2O) emissions from a pilot-scale oxidation ditch under different COD/N ratios, aeration rates and two shock-load conditions

Nitrous oxide (N2O) generated from wastewater treatment plants (WWTPs) has drawn attention due to its high emission load and significant greenhouse effect. In the present study, N2O emissions from a pilot-scale Carrousel oxidation ditch under various chemical oxygen demand (COD) to nitrogen ratio (COD/N) and aeration rates were systematically investigated. The highest N2O emission factor was 0.142 ± 0.013%, at COD/N of 5 and aeration rate of 1.8 m3 h−1, which was much lower than the majority of previous studies. The results could be attributed to the high internal recycle ratio of the oxidation ditch process which lightened the burden of influent load to the system. The profiles of N2O emissions and dissolved N2O concentration along the channels showed a distinct spatial variation that N2O emissions primarily occurred in the aeration zones due to the air stripping effect. However, both the aeration and anoxic zones contributed to N2O generation due to autotrophic nitrification (AN), which was considered to be the main N2O generation process. In addition, two simulated shock-load conditions, ammonia overload shock and aeration failure shock, were carried out to explore the response of the biological nitrogen removal (BNR) system. The results indicated that both shock-loads lead to excessive N2O emissions, especially at higher aeration rates, which could be explained by the improved N2O generation by AN process during the shock-load period. This study offered new insights into the role of operational parameters to N2O emission and the alternative approach for N2O mitigation during both the steady-state operation and shock-load conditions in the oxidation ditch process.

Simulating a combined lysis-cryptic and biological nitrogen removal system treating domestic wastewater at low C/N ratios using artificial neural network

In this study, a combined alkaline (ALK) and ultrasonication (ULS) sludge lysis-cryptic pretreatment and anoxic/oxic (AO) system (AO + ALK/ULS) was developed to enhance biological nitrogen removal (BNR) in domestic wastewater with a low carbon/nitrogen (C/N) ratio. A real-time control strategy for the AO + ALK/ULS system was designed to optimize the sludge lysate return ratio (RSLR) under variable sludge concentrations and variations in the influent C/N (⩽ 5). A multi-layered backpropagation artificial neural network (BPANN) model with network topology of 1 input layer, 3 hidden layers, and 1 output layer, using the Levenberg–Marquardt algorithm, was developed and validated. Experimental and predicted data showed significant concurrence, verified with a high regression coefficient (R2 = 0.9513) and accuracy of the BPANN. The BPANN model effectively captured the complex nonlinear relationships between the related input variables and effluent output in the combined lysis-cryptic + BNR system. The model could be used to support the real-time dynamic response and process optimization control to treat low C/N domestic wastewater.

Establishment of Biological Nitrogen Removal Process for Drinking Water Treatment in Malaysia

Eutrophication arises from human activities has been recognized globally as an environmental issue. Human activities have greatly increased the input of phosphorus and nitrogen into the water bodies which place a heavy pressure on clean water resources. There are limitations in the conventional water treatment processes, where phosphorus and nitrogen could not be removed efficiently. For nitrogen removal, biological method emerges as a sustainable and economically feasible alternative in recent years. Though the biological nitrogen removal (BNR) method is widely recognized in developed countries with temperate climate, knowledge and expertise on the application of this treatment process is relatively limited in developing countries, so as Malaysia in tropical region. Thus, this research aims to provide a preliminary view on the establishment of the BNR process for drinking water treatment in the local context. A lab scale sequencing batch reactor (SBR) seeded with fish pond sludge and activated sludge in the volume ratio of 1:1 was operated for the BNR process. After four weeks of sludge cultivation, the characteristic of simultaneous nitrification-denitrification was observed. The removal of total nitrogen (TN) was found exceeding 60% and little nitrate-nitrogen (NO3̄-N) concentration was observed at the end of the process. Besides, the reduction in orthophosphate (PO43--P) concentration in a BNR system has indicated the process was able to perform nitrogen and phosphorus removal simultaneously. These findings suggest that the establishment of BNR is feasible in Malaysia.

[Effect of Alkaline Sludge Fermentation Products on the Nitrification Process and Performance].

The aim of the present study was to investigate the effect of alkaline sludge fermentation products as a carbon source on the nitrification process and performance. During the operation of a biological nitrogen removal (BNR) system with sludge fermentation mixture as the carbon source, the activities of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were inhibited at the beginning. After 16 days, the activity of AOB began to recover rapidly, but the activity of NOB was still inhibited. The specific nitrate production rate (SNaPR, N/VSS) decreased from 0.1791 g·(g·d)-1 to 0.0078 g·(g·d)-1. At the same time, the nitrite accumulation rate increased from 8.12% to 91.42% and remained stable. The sludge fermentation mixture was separated into sludge fermentation liquid and sludge fermentation sediment. The changes in nitrification activity by adding different types of fermentation products were investigated. The results showed that the activity of NOB decreased in the experimental group fed with the sludge fermentation mixture and the fermentation liquid. The SNaPR decreased from an initial 0.1793 g·(g·d)-1 to 0.1510 g·(g·d)-1 and 0.1617 g·(g·d)-1, respectively. In the experimental group fed with fermentation sediment, the activity of NOB increased. SNaPR rose from 0.1793 g·(g·d)-1 to 0.1864 g·(g·d)-1. Therefore, the activity of the NOB can be inhibited when the sludge fermentation mixture and the fermentation liquid are used as a carbon source in the nitrification process. In addition, the short-range nitrification process can be realized, which is beneficial to accelerating the reaction speed and saving investment in this type of carbon source.

Responses of nitrogen transformation processes and N2O emissions in biological nitrogen removal system to short-term ZnO nanoparticle stress

Although the adverse effects of ZnO nanoparticles (ZnO NPs) on biological nitrogen removal (BNR) processes have widely been reported, the impacts of ZnO NPs on the whole nitrogen transformation processes, especially the production of nitrous oxide (N2O), a typical greenhouse gas in a BNR system have rarely been systematically studied yet. In this study, the performances of both the nitrification and denitrification processes were investigated and the N2O emission was simultaneously monitored in a sequencing batch reactor (SBR) when exposed to 1, 25 or 50 mg/L ZnO NPs for one cycle. The dose-dependent ZnO NP depression effects were generally observed on denitrification processes, total nitrogen (TN) removal efficiency and N2O emissions but not nitrification process. Meanwhile, the N2O emission was positively correlated with NO2−-N accumulation in the oxic stage. Further investigation showed that the expressions of nitrate (NO3−) reduction associated narG gene were down-regulated with the increase of NP stress, and the transcript ratios of NO2−/NO reduction gene to N2O reduction one (nirK/nosZ and norB/nosZ) decreased. The released Zn2+ from ZnO NPs took an important role in the inhibition of denitrification processes. ZnO NPs addition also induced the dose-dependent variations in the superoxide dismutase (SOD) and catalase (CAT) activities, which probably contributed to the suppression of the excess reactive oxygen species (ROS) generations to mitigate nanotoxicity. The excessive secretion of protein (PN) in tightly bound EPS (TB-EPS) when ZnO NPs were no <25 mg/L further supported the system's potential self-regulation mechanism for nanotoxicity resistance. CapsuleThe effects of ZnO NPs on the whole nitrogen transformation processes in a biological nitrogen removal sequencing batch reactor, including the N2O emissions were investigated. The system's potential self-regulation mechanism for nanotoxicity resistance was addressed.

Co-treatment of shale-gas produced water and municipal wastewater: Removal of nitrogen in a moving-bed biofilm reactor

Co-treatment of hydraulic fracturing produced water (PW) with municipal wastewater is a cost-effective shale gas PW management option. However, little attention has been paid to the impact of PW on the reliability of bioprocess in municipal wastewater treatment plants. In this study, a mature chemical pretreatment method was used to remove a large proportion of organic matters in real PW. Then a laboratory-scale moving bed biofilm reactor (MBBR) was established for the co-treatment of pretreated PW and synthetic domestic wastewater. Results showed that CODCr removal rate reached above 90% and acute toxicity was lowered by 20% in the pretreatment of PW using consecutive Fenton–NaClO oxidation. After 68 days of slowly increasing the mixing ratio, complete nitrification–denitrification was established with a maximum mixing ratio of 50%. The mean total nitrogen (TN) removal rate reached above 60% when the MBBR operated under maximum mixing ratio for the following 38 days. Shotgun metagenomic analysis revealed high taxonomic structure similarity between suspended and attached growth biomass in the MBBR when the mixing ratio was maintained at 50%. The functional diversity of nitrite-oxidizing bacteria may be present in the saline biological nitrogen removal system and may strengthen nitrogen and other typical element cycles.