Under the influence of many factors, such as climate change, anthropogenic eutrophication, and the development of aquaculture, the area and frequency of algal blooms have showed an increasing trend worldwide, which has become a challenging issue at present. However, the coupled relationship between nitrous oxide (N2O) and algal blooms and the underlying mechanisms remain unclear. To address this issue, 15N isotope cultures and quantitative polymerase chain reaction (qPCR) experiments were conducted in Zhanjiang Bay during algal and non-algal bloom periods. The results showed that denitrification and nitrification-denitrification were the two processes responsible for the in-situ production of N2O during algal and non-algal bloom periods. Stable isotope rate cultivation experiments indicated that denitrification and nitrification-denitrification were promoted in the water during the algal bloom period. The in-situ production of N2O during the algal bloom period was three-fold that during the non-algal bloom period. This may be because fresh particulate organic matter (POM) from the organisms responsible for the algal bloom provides the necessary anaerobic and hypoxic environment for denitrification and nitrification-denitrification in the degradation environment. Additionally, a positive linear correlation between N2O concentrations and ammonia-oxidizing bacteria (AOB) and denitrifying bacteria (nirK and nirS) also supported the significant denitrification and nitrification-denitrification occurring in the water during the algal bloom period. However, the algal bloom changed the main process for the in-situ production of N2O, wherein it shifted from denitrification during the non-algal bloom period to nitrification-denitrification during the algal bloom period. The results of our study will improve our understanding of the processes responsible for the in-situ production of N2O during the algal bloom period, and can help formulate effective policies to mitigate N2O emissions in the bay.

A comprehensive hydrogeological report was conducted to determine the origin, occurrence and processes affecting nitrogen in a Mediterranean coastal aquifer-lagoon system. Water levels, hydrochemical and isotopic data was gathered during a 4-year period in the La Pletera salt marsh area (NE Spain). They were collected from the alluvial aquifer, two natural lagoons and four other permanent lagoons excavated during a restoration process (in 2002 and 2016), two watercourses (the Ter River and the Ter Vell artificial channel), 21 wells (considering six of them for groundwater sampling) and the Mediterranean Sea. Potentiometric surveys were carried out seasonally, however twelve-monthly campaigns (from November 2014 to October 2015), and nine seasonal campaigns (from January 2016 to January 2018) were conducted for hydrochemical and environmental isotopes analyses. The evolution of the water table was analysed for each well, and potentiometric maps were plotted to determine the relationship between the aquifer and the lagoons, sea, watercourses, and groundwater flow. Hydrochemical data included physicochemical data measured in situ (temperature, pH, Eh, dissolved oxygen, and electrical conductivity), major and minor ions (HCO3-, CO32-, Cl-, SO42-, F-, Br-, Ca2+, Mg2+, Na+, and K+), and nutrients (NO2-, NO3-, NH4+, Total Nitrogen (TN), PO43-, and Total Phosphorus (TP)). Environmental isotopes included stable water isotopes (δ18O and δD), nitrate (δ15NNO3 and δ18ONO3) and sulphate isotopes (δ34SSO4 and δ18OSO4). Water isotopes were analysed for all campaigns, however, nitrate and sulphate isotopes water samples were only analysed in some particular surveys (November and December 2014; January, April, June, July and August 2015). Additionally, two more surveys for sulphate isotopes were conducted in April and October of 2016. The data generated through this research may be used as a starting point to analyse the evolution of these recently restored lagoons, and their future responses to global change. In addition, this dataset may be used to model the hydrological and hydrochemical behaviour of the aquifer.

Floodplains remove nitrate from rivers through denitrification and thus improve water quality. The Danube River Basin (DRB) has been affected by elevated nitrate concentrations and a massive loss of intact floodplains and the ecosystem services they provide. Restoration measures intend to secure and improve these valuable ecosystem services, including nitrate removal. Our study provides the first large-scale estimate of the function of large active floodplains in the DRB to remove riverine nitrate and assesses the contribution of reconnection measures. We applied a nutrient emission model in 6 river systems and coupled it with denitrification and flooding models which we adapted to floodplains. The floodplains have the capacity to eliminate about 33,200 t nitrate-N annually, which corresponds to 6.5 % of the total nitrogen emissions in the DRB. More nitrate is removed in-stream at regular flow conditions than in floodplain soils during floods. However, increasing frequently inundated floodplain areas reveals greater potential for improvement than increasing the channel network. In total, we estimate that 14.5 % more nitrate can be removed in reconnected floodplains. The largest share of nitrogen emissions is retained in the Yantra and Tisza floodplains, where reconnections are expected to have the greatest impact on water quality. In absolute numbers, the floodplains of the lower Danube convert the greatest quantities of nitrate, driven by the high input loads. These estimates are subject to uncertainties due to the heterogeneity of the available input data. Still, our results are within the range of similar studies. Reconnections of large floodplains in the DRB can, thus, make a distinct contribution to improving water quality. A better representation of the spatial configuration of water quality functions and the effect of floodplain reconnections may support the strategic planning of such to achieve multiple benefits and environmental targets.

The removal of nitrogen compounds from a pretreated explosives wastewater in vertical flow constructed wetland planted with Vetiveria zizanioides (0.24 m2 × 0.70 m), filled with light expanded clay aggregates (Leca®NR 10/20), was studied. Experiments under constant hydraulic load, 50 ± 4 L m-2 d-1 and 83 ± 5 L m-2 d-1 without and with flooding level (25%), respectively, were made at different ammonium (3-48 mg NH4+-N L-1), nitrate (56-160 mg NO3--N L-1) and nitrite (0.3-1.1 mg NO2--N L-1) concentrations. Results indicate that without flooding level (unsaturated) the removal efficiencies obtained were 30 ± 9, 7 ± 1 and 96 ± 2%, respectively to NH4+-N, NO3--N and NO2--N. When using flooding level and an external carbon source (C/N ratio from 1.3 ± 0.19 to 2.5 ± 0.20), the organic matter (COD) removal efficiencies were above 90%, 75% for NH4+-N and 55% to NO3--N. Increasing the C/N ratio from 2.9 ± 0.21 to 4 ± 0.22 did not contributed to upgrade the efficiencies of COD, NH4+-N and NO3--N removal. The denitrification process was occurred in aerobic conditions and nitrite production have ben occurred, probably due to the presence of aerobic conditions that inhibited partially denitrification.

The denitrification process plays an important role in improving water quality and is a source/sink of nitrous oxide to the atmosphere. The second important rate-limiting step of the denitrification process is catalyzed by two enzymes with different structures and unrelated evolutionary relationships, namely, the Cu-type nitrite reductase encoded by the nirK gene and the cytochrome cd1-type nitrite reductase encoded by the nirS gene. Although some relevant reviews have been published on denitrifiers, most of these reviews do not include statistical analysis, and do not compare the nirS and nirK communities in-depth. However, a systematic study of the nirS-type and nirK-type denitrifying communities and their response to environmental factors in different ecosystems is needed. In this review, a scientometric approach combined with case studies was used to study the nirS-type and nirK-type denitrifiers. The scientometric approach demonstrated that Pseudomonas, Paracoccus, and Thauera are the most frequently mentioned nirS-type denitrifiers, while Pseudomonas and Bradyrhizobium are the top two most frequently mentioned nirK-type denitrifiers. Among various environmental factors, the concentrations of nitrite, nitrate and carbon sources were widely reported factors that can influence the abundance and structure of nirS-type and nirK-type denitrifying communities. Case studies indicated that Bradyrhizobium was the major genus detected by high-throughput sequencing in both nirS and nirK-type denitrifiers in soil systems. nirS-type denitrifiers are more sensitive to the soil type, soil moisture, pH, and rhizosphere effect than nirK. To clarify the relationships between denitrifying communities and environmental factors, the DNA stable isotope probe combined with metagenomic sequencing is needed for new denitrifier detections.

This is the first study that identifies nitrogen loss and drier liquor parameters in a full scale sludge drying facility. Obtained data enabled proposing novel treatment based on deammonification process that allows not only nitrogen removal from drier liquor but also from gravity thickeners supernatant, the stream that is considered too cold for deammonification. The novel treatment approach is compared with nitrification/denitrification in activated sludge process in terms of greenhouse gas emissions. The nitrogen loss during drying was calculated based on two independent methods using full scale data from a long-term measurement campaign. According to results, 9.8-11.2% of nitrogen from dewatered sludge, or 4.9-5.5 g N/kg of TS, was vaporized during drying and then was captured through the purification of process air in drier liquor. Overall, over 40 tN/a from drier liquor and 77 tN/a from gravity thickeners supernatant is recirculated to the activated sludge process and results in the additional emission of 670 426 kg CO2e/a when nitrification/denitrification is applied. That can be lowered by 398 858 kgCO2e/a when the novel approach is considered.

Anammox process has been widely applied in the wastewater nitrogen removal for its high rate and low cost. However, few researches reported the process potential in treating low-strength nitrogen wastewater. In this study, a sequencing batch reactor (SBR) was taken to explore the feasibility of low-strength nitrogen wastewater treatment by anammox process in theory and practice. The practical operation indicated that the effluent with satisfactory quality (1.90 ± 0.70 mg-TN·L-1) could be achieved, when the SBR was fed with low-strength nitrogen influent (6.20 ± 0.45 mg-NH4+-N·L-1 and 7.96 ± 0.59 mg-NO2--N·L-1). The hydraulic retention time (HRT), nitrogen removal efficiency, nitrogen removal rate (NRR) and hydraulic loading rate of SBR were 5.42 h, 86.5%, 0.054 kg-N·m-3·d-1 and 4.43 m3·m-3·d-1 during the 79-day operation, respectively. The theoretical analysis revealed the potential of anammox SBR. When SBR is stably operated, the maximum NRR would be 0.062 kg-N·m-3·d-1 if the effluent nitrogen was required to be as low as 3 mg·L-1. The NRR value is feasible for engineering. However, considering the lower specific substrates utilization rate in practice, the maximum stable NRR was calibrated and found inefficient afterwards. In order to improve the potential of anammox process, the reactors without back mixing and with periodic bioaugmentation should be taken in priority for the engineering applications. In particular, the bioaugmentation frequency and single addition amount were calculated as 7 d and 0.3 g-VSS·L-1, respectively. The results may provide guidance for the development of high-efficient and stable nitrogen removal process under low-strength condition.

To determine the reasons for the variation in the vertical distribution of nitrogen in sediment interstitial waters between different stratified reservoirs, the characteristics of overlying water-interstitial water in Xiangxi Bay, Yangtze River mainstream, and Xiaowan Reservoir were monitored. The vertical distribution of nitrogen in sediment interstitial waters in these different stratified waters were then analyzed, and the reasons for the variation in this distribution were assessed. The results showed:① the ρ(TN) in the sediment interstitial waters of the Yangtze River mainstream and Xiangxi Bay gradually increased with depth, while that of Xiaowan Reservoir reached its maximum at 12 cm and the bottom layer presented a \"C\" distribution. The ρ(NH4+) in the sediment interstitial waters of the Yangtze River mainstream and Xiangxi Bay exhibited an increasing trend with depth, while that of Xiaowan Reservoir was slightly higher in the bottom layer than in the surface layer, although the change with depth was not significant. Overall, the ρ(NH4+) in the sediment interstitial waters of the Yangtze River mainstream and Xiangxi Bay was higher than that of Xiaowan Reservoir, and the concentration ranges were as follows:0.512-8.289 mg·L-1, 0.968-9.307 mg·L-1, and 0.950-1.450 mg·L-1. The vertical distribution of the ρ(NO3-) in the sediment interstitial waters of all three waterbodies were opposite to that of ρ(NH4+). Moreover, the ρ(NO3-) in the sediment interstitial waters of Xiangxi Bay and the Yangtze River mainstream was higher than that of Xiaowan Reservoir. The concentration ranges were as follows:0.143-0.674 mg·L-1, 0.107-0.647 mg·L-1, and 0.050-0.051 mg·L-1. ② There were also significant differences in the vertical distribution of physical and chemical indices in the three water bodies. There was no significant change in the vertical distribution of the water temperature in the Yangtze River mainstream and the N2 value was <5×10-5 s-2; hence, the water was well mixed, and the vertical range of the dissolved oxygen content was 6.180-6.318 mg·L-1. The water temperature in the upper and middle reaches of Xiangxi Bay decreased vertically, while the water temperature in the lower reach presented a ladder-like distribution and the N2 values were all>5×10-5 s-2; thus, the water was in a stable stratified state and the dissolved oxygen content presented a \"C\" distribution. There was obvious stratification at the depths of 5-15 m and 54-70 m in Xiaowan Reservoir. The dissolved oxygen content decreased significantly at higher water temperature gradients, and there was no significant change along the water depth below 80 m. ③ The main reasons for the variation in the vertical distribution of nitrogen in the sediment interstitial waters of the three waterbodies were the differences in the overlying water hydrodynamics, dissolved oxygen distribution, and sediment environment. The ρ(NH4+) and ρ(NO3-) were higher in Xiangxi Bay, which may have increased the denitrification rate and subsequently have helped to remove nitrogen and reduce the nitrogen load in these waters.

The study aimed to evaluate the influence of the duration times of anaerobic phases on the bacterial biocenosis characterisation while denitrifying dephosphatation in the Integrated Fixed-Film Activated Sludge - Moving-Bed Sequencing Batch Biofilm Reactor (IFAS-MBSBBR). The experiment was conducted in a laboratory model. The study consisted of four series, which differed in terms of the ratio of the anaerobic phases. duration concerning the overall reaction time in the cycle. The anaerobic phases covered from 18 to 30% of the whole cycle duration. During the reactor performance that took 9 months, the influent and effluent were monitored by analysis of COD, TKN, NH4-N, NO2-N, NO3-N, TP, PO4-P, pH, alkalinity and the phosphorus uptake batch tests. Characterisation of the activated sludge and the biofilm biocenosis was based on fluorescent in situ hybridisation (identification of PAO and GAO) and the denaturing gradient gel electrophoresis patterns.
The organic compounds removal was high (more than 95.7%) independently of cycle configuration. The best efficiency for nitrogen (91.1%) and phosphorus (98.8%) removal was achieved for the 30% share of the anaerobic phases in the reaction time. Denitrifying PAO (DPAO) covered more than 90% of PAO in the biofilm and usually around 70% of PAO in the activated sludge. A substantial part of the polyphosphate accumulating organisms (PAO) community were Actinobacteria. The denitrifying dephosphatation activity was performed mainly by Accumulibacter phosphatis.
High nutrient removal efficiencies may be obtained in IFAS-MBSBBR using the denitrifying dephosphatation process. It was found that the length of anaerobic phases influenced denitrification and the biological phosphorus removal. The extension of the anaerobic phases duration time in the reaction time caused an increase in the percentage share of denitrifying PAO (DPAO) in PAO. The biocenosis of the biofilm and the activated sludge reveal different species patterns and domination of the EBPR community.

Woodchip bioreactors are viable low-cost nitrate (NO3-) removal applications for treating agricultural and aquaculture discharges. The active microbial biofilms growing on woodchips are conducting nitrogen (N) removal, reducing NO3- while oxidizing the carbon (C) from woodchips. However, bioreactor age, and changes in the operating conditions or in the microbial community might affect the NO3- removal as well as potentially promote nitrous oxide (N2O) production through either incomplete denitrification or dissimilatory NO3- reduction to ammonium (DNRA). Here, we combined stable isotope approach, amplicon sequencing, and captured metagenomics for studying the potential NO3- removal rates, and the abundance and community composition of microbes involved in N transformation processes in the three different full-scale woodchip bioreactors treating recirculating aquaculture system (RAS) effluents. We confirmed denitrification producing di‑nitrogen gas (N2) to be the primary NO3- removal pathway, but found that 6% of NO3- could be released as N2O under high NO3- concentrations and low amounts of bioavailable C, whereas DNRA rates tend to increase with the C amount. The abundance of denitrifiers was equally high between the studied bioreactors, yet the potential NO3- removal rates were linked to the denitrifying community diversity. The same core proteobacterial groups were driving the denitrification, while Bacteroidetes dominated the DNRA carrying microbes in all the three bioreactors studied. Altogether, our results suggest that woodchip bioreactors have a high genetic potential for NO3- removal through a highly abundant and diverse denitrifying community, but that the rates and dynamics between the NO3- removal pathways depend on the other factors (e.g., bioreactor design, operating conditions, and the amount of bioavailable C in relation to the incoming NO3- concentrations).