Total Inorganic Nitrogen Concentration Research Articles

Achieving ultra-high nitrogen and phosphorus removal from real municipal wastewater in a novel continuous-flow anaerobic/aerobic/anoxic process via partial nitrification, endogenous denitrification and nitrite-type denitrifying phosphorus removal

Achieving economic and efficient removal of nutrients in mainstream wastewater treatment plants (WWTPs) continues to be a challenging research topic. In this study, a continuous-flow anaerobic/aerobic/anoxic system with sludge double recirculation (AOA-SDR), which integrated partial nitrification (PN), endogenous denitrification (ED) and nitrite-type denitrifying phosphorus removal (nDNPR), was constructed to treat real carbon-limited municipal wastewater. The average effluent concentrations of total inorganic nitrogen (TIN) and PO43−-P during the stable operation period were 1.8 and 0.3 mg/L, respectively. PN was achieved with an average nitrite accumulation ratio of 90.4 % by combined strategies. Adequate storage of polyhydroxyalkanoates and glycogen in the anaerobic zone promoted the subsequent nitrogen removal capacity. In the anoxic zone, nitrite served as the main electron acceptor for the denitrifying phosphorus removal process. Mass balance analysis revealed that nDNPR contributed to 23.6 % of TIN removal and 44.7 % of PO43−-P removal. The enrichment of Nitrosomonas (0.45 %) and Ellin 6067 (1.31 %), along with the washout of Nitrospira (0.15 %) provided the bacterial basis for the successful implementation of PN. Other dominant endogenous heterotrophic bacteria, such as Dechlormonas (10.81 %) and Candidatus Accumulibacter (2.96 %), ensured simultaneous nitrogen and phosphorus removal performance. The successful validation of integrating PN, ED and nDNPR for advanced nutrient removal in the AOA-SDR process provides a transformative technology for WWTPs.

Performance and mechanism of pilot-scale carbon fibers enhanced ecological floating beds for urban tail water treatment in optimized ecological floating beds water surface coverage

A pilot-scale carbon fibers enhanced ecological floating beds (CF-EFBs) was constructed. Compared to EFBs without carbon fibers enhancement, CF-EFBs have the better removal of total inorganic nitrogen (TIN), total phosphorus (TP), and chemical oxygen demand (COD), the removal efficiencies were 3.19, 3.49, and 2.74 times higher than EFBs. Throughout the pilot test (under three different coverage rates), the concentrations of COD, TIN and TP of effluent were 18.11 ± 4.52 mgL−1, 1.95 ± 0.92 mgL−1 and 0.13 ± 0.08 mgL−1. Meanwhile, the average removal of TIN, TP and COD from tailwater was 0.96 gm−2d−1, 0.07 gm−2d−1 and 2.37 gm−2d−1 respectively. When the coverage was 30 %, the CF-EFBs had better nitrogen removal effectiveness (TIN purification ability of 1.49 gm−2d−1). The enrichment of denitrifying bacteria, such as Aridibacter, Nitrospira, Povalibacter, and Phaeodactylibacter increased denitrification efficiency. These results verified the feasibility of CF-EFBs in tailwater treatment at pilot-scale, which was of great significance for the practical application of CF-EFBs.

Wet deposition of sulfur and nitrogen in the Jiuzhaigou World Heritage Site, China: Spatial variations, 2010–2022 trends, and implications for karst ecosystem conservation

Excessive sulfur and nitrogen deposition is a threat to natural ecosystems. This study investigated the spatio-temporal changes of wet sulfur and nitrogen deposition in Jiuzhaigou to understand the responses of wet deposition chemistry to local and national air pollutant emission changes and to assess relevant ecological risks. The results show that local emission sources (e.g., tourism, post-earthquake reconstruction, and vegetation damages) largely elevated the concentrations of SO42−, NO3−, NH4+, and Ca2+ at the non-background sites during 2021–2022; However, the gaps of annual sulfur and nitrogen wet deposition fluxes among the background, semi-background, and non-background sites were reduced by annual precipitation. The pH was also higher at the non-background sites due to stronger acid neutralization capacities from crustal dust there, and acid precipitation (pH < 5.6) was not detected at all the sites. During the 2010 to 2022 period, the wet deposition of SO42− and TIN (i.e. total inorganic nitrogen) and the equivalent ratio of SO42−: NO3− generally presented a declining trend at the background and semi-background sites, as a result of regional and national emission reductions. The TIN reduction from 2015 to 2016 to 2021–2022 at the background site was more from wet NH4+ deposition and the site's NO3− concentrations and fluxes were relatively stable. Compared to the critical loads (CLs) of sulfur and nitrogen for acidification and eutrophication, the annual wet and wet+dry deposition fluxes of sulfur and nitrogen were lower in almost the entire watershed, except for the CLmin of nitrogen for acidification in considerable areas. However, all the wet deposition samples were calcite-unsaturated and had annual TIN concentrations higher than the national environmental standard of total nitrogen for surface water. More field investigations are needed to better assess the ecological risks.

Ultra-high anammox-based nitrogen removal contribution and robust bacterial abundance in a pure-biofilm anoxic/oxic system for real municipal wastewater treatment

For municipal wastewater treatment via partial-denitrification/anammox (PD/A) in the anoxic zone, the primary challenge is effective enrichment of anaerobic ammonium oxidation (anammox) bacteria with a stable autotrophic nitrogen removal performance. This long-term study investigated a pure-biofilm anoxic/oxic reactor using real municipal wastewater with an ammonia concentration of 49.1 ± 3.6 mg N/L, achieving a total inorganic nitrogen concentration of 11.2 ± 3.5 mg N/L in the effluent, resulting in a removal efficiency of 77.4 ± 7.2%. Driven by partial denitrification, the anammox contribution to total nitrogen removal represented an ultra-high level over 55%, and was considerable in the first anoxic zone. Candidatus Brocadia, the only anammox genus detected by 16S rRNA gene sequencing, reached robust abundances of 5.46, 6.49 and 5.79% in the three sequentially connected anoxic zones. Metagenomic sequencing further identified that the main species was Ca. Brocadia sapporoensis, which is inhibited by nitrite but not by acetate, supporting its high-level enrichment under acetate conditions in the anoxic zone. Batch tests indicate that heterotrophic denitrification activity decreased while anammox activity increased along the anoxic zones, and that excessive heterotrophic denitrifying activity was not conducive to increasing the anammox contribution. This study achieved robust anammox bacterial abundance using a pure-biofilm system, shedding fresh light on the application of mainstream anammox in municipal wastewater treatment.

Effects of superficial gas velocity on the performance of an air-lift internal circulation partial nitrification-anammox granular sludge reactor

The airlift internal circulation reactor for partial nitrification-anammox (PNA-ALR) has the advantages of a small footprint, high mass transfer efficiency, and the ease of formation of granular sludge, thus making it an effective biological treatment for ammonia-containing wastewater. Although superficial gas velocity (SGV) is an essential parameter for PNA-ALR, it is unclear how the magnitude of SGV impacts nitrogen removal performance. In this study, the nitrogen removal efficiencies of five PNA-ALRs with different SGV were measured during feeding with synthetic municipal wastewater. At an optimal SGV of 2.35 cm s−1, the PNA-ALR consistently maintained the total inorganic nitrogen (TIN) removal efficiency at 76.31% and the effluent TIN concentration was less than 10 mg L−1. By increasing or decreasing the SGV, the nitrogen removal efficiency decreased to a range between 30% and 50%. At lower SGV, the dead space in the PNA-ALR was increased by 21.15%, and the feast/famine ratio of sludge increased to greater than 0.5, which caused a disruption in the structure, and a large loss of, granular sludge. Computational fluid dynamics (CFD) simulations showed operation at a higher SGV, resulting in excessive shear stress of 3.25 N m−2 being generated from bubble rupture in the degassing section. Fluorescent staining determined a decrease of 26.5% in viable bacteria. These results have improved our understanding of the effects of SGV on a PNA-ALR during mainstream wastewater treatment.

Nitrogen removal from high-saline municipal wastewater via anammox-based process driven by both nitritation and denitratation

Nitrogen removal from saline municipal wastewater has recently attracted attention; thus, developing energy-efficient technology such as anaerobic ammonium oxidation (anammox) for advanced nitrogen removal of hypersaline wastewater is significant. In this study, the long-term salt adaptation and evolution mechanisms of an anammox-based process driven by both nitritation (NH4+→NO2—, PN) and denitratation (NO3—→NO2—, PD) were investigated under salinities ranging from 1.0% to 3.0% (10–30 g NaCl/L). At 1.0% salinity, PN evolved into the major nitrite production pathway with thorough suppression of nitrite oxidation. At 1.5% salinity, nitrite reduction was inhibited along with suspended sludge loss due to poor sludge settleability. Under 2.0% salinity with only-biofilm, the highest nitrogen removal efficiency (81.2%) was observed with influent and effluent total inorganic nitrogen concentrations of 44.5 and 8.2 mg N/L, respectively. Notably, results showed that anammox bacteria occupied up to 1.07% of the microbial community (qPCR: 1.14×1011 copies/(g dry sludge)) and has strong adaptability to salinity. With salinity increases, anammox contribution kept over 85% and Ca. Kuenenia (0.57%→0.22%) was more tolerant than Ca. Brocadia (0.46%→0.02%). Metagenomic sequencing revealed that the multiple nitrite substrates production by Nitrosomonas (amoA-enriched), Denitratisoma and Denitromonas (narG-enriched) guaranteed the high anammox contribution. This study provides a multifaceted understanding of the anammox-based process, which can enable improved application in real saline wastewater treatment.

Achieving deep autotrophic nitrogen removal from low strength ammonia nitrogen wastewater in aeration sponge iron biofilter: Simultaneous nitrification, Feammox, NDFO and Anammox

Partial nitrification and anammox process is considered to be a good choice for nitrogen removal. Nevertheless, it is difficult to achieve in low ammonia wastewater because of the need to control further oxidation of nitrite. Nitrification combined with Feammox (anaerobic ammonia oxidation coupled to Fe (III) reduction) and NDFO (nitrate/nitrite-dependent Fe (II) oxidation) can achieve autotrophic removal of low ammonia. And sponge iron can avoid the disadvantage of continuously adding iron sources in this process. Therefore, a sponge iron biofilter (SIBF) treating low ammonia wastewater under aeration was explored in this study. The optimal operating conditions for SIBF were hydraulic retention time (HRT) of 9 h and gas–water ratio (R) of 9:1, at which the total inorganic nitrogen removal efficiency was up to 77.2 %, and effluent ammonia and total inorganic nitrogen concentrations were less than 5 mg/L and 15 mg/L, respectively. This met the 1A discharge standard of municipal wastewater in China. Stable ratio isotope results indicated that Feammox, NDFO and Anammox resulted in high nitrogen loss in SIBF. And different nitrogen transformation contribution tests indicated NDFO had the dominant contribution, which accounted for 55.7 %. Besides, the production of N2O was minimum under HRT of 9 h and R of 9:1, in which the N2O emission factor was 0.7 %. Stable isotope tracing analysis showed that 92.8 % of N2O was derived from ammonia and the dominant production pathway was nitrifier denitrification. The results of microbial community showed that the nitrification bacteria-Nitrosomonas, Nitrotoga, Nitrospira, Feammox bacteria-Dechloromonas, NDFO bacteria-Thiobacillus and anammox bacteria-Brocadia played major roles in SIBF. Therefore, SIBF shows superior performance and application potential in autotrophic nitrogen removal from low-ammonia nitrogen wastewater under aeration.

How elevated nitrogen load affects bacterial community structure and nitrogen cycling services in coastal water.

Nutrient pollution in the coastal environment has been accelerated by progressively intensifying aquaculture activities. Excessive nutrients can lead to coastal eutrophication with serious economic and ecological consequences. In this study, we studied coastal planktonic microbial community over a year to understand the aquaculture impact on coastal water quality and function. We observed increased total inorganic nitrogen concentrations in active fish farms to favor the diverse Alpha- and Gammaproteobacteria. Bacterial community alpha diversity in fish farms was positively correlated with total inorganic nitrogen, and active fish farming co-influenced the bacterial structural composition and regional beta diversity. By analyzing the nitrogen cycle-related functional compositions and pathways using PICRUSt2 prediction on inferred genomes, we identified the contribution of over 600 bacterial species to four major pathways. Enhanced nitrogen load in active fish farms was positively correlated with elevated dissimilatory nitrate reduction and denitrification pathway abundances. Fallowed fish farms were characterized by a predicted high abundance of nirA and narB genes contributing to assimilatory nitrate reduction pathway due to the prevalence of Cyanobacteria. Overall, these results suggested active operation and short hiatus in coastal aquaculture practices could rapidly impact planktonic bacterial communities and further influence nitrogen cycling and associated processes. These findings will improve the understanding of the responses and interactions between microbiome and aquaculture activities. In a world of increasing aquaculture demands, this work has important implications for sustainable water resource management and development.

Start-up of the combined anaerobic ammonium oxidation and solid phase denitrification process and microbial characterization analysis

To improve the nitrogen removal performance of anaerobic ammonia oxidation (anammox)-based processes, a modified anaerobic baffled reactor (ABR) equipped with a packed bed of poly-3-hydroxybutyrate-cohyroxyvelate (PHBV) was employed to start up the combined anammox and solid phase denitrification (SPD) process for the treatment of nitrogen-rich wastewater. Results showed that a stable removal of total inorganic nitrogen (TIN) of 98.5 % was achieved with a nitrogen loading rate of 0.46 kg·(m3·d)−1, decreasing the effluent TIN concentration to <2 mg·L−1. Throughout the operation period of 180 days, sludge granulation in three compartments of the ABR was promoted by a stepwise decrease in the hydraulic retention time. The anammox reaction and endogenous denitrification mainly occurred in the first two compartments with inorganic feeding and contributed >86 % of TIN removal. 16S rRNA gene amplicon sequencing showed that anammox communities, dominated by genera Candidatus Kuenenia and Candidatus Brocadiaceae, were enriched in granular sludges with symbiotic heterotrophic bacteria. Co-cultures of hydrolytic bacteria (Burkholderiales, Kapabacteriales, and Clostridium-sensu-stricto-7) and denitrifying bacteria (Comamonadaceae, Rhodocyclaceae, and Denitratisoma) were detected within the biofilm on the surface of PHBV particles. PHBV hydrolysate input effectively drove heterotrophic denitrification in the successive compartment and increased the complexity of microbial co-occurrence networks. These findings provide insight into the synergistic mechanisms between anammox and SPD and present a feasible combined process application for advanced biological nitrogen removal.

Mainstream anammox driven by micro-oxygen nitrification and partial denitrification using step-feed for advanced nitrogen removal from municipal wastewater

This study developed a novel integrated nitrification, partial denitrification (PD), and anammox (INPDA) process in micro-aerobic sequencing batch reactor using step-feed strategy treating municipal wastewater. The results showed that the INPDA could achieve a 94.5 ± 1.1% total inorganic nitrogen (TIN) removal efficiency with effluent TIN concentration as low as 2.3 mg/L. The micro-aerobic with low dissolved oxygen concentration of 0.36 ± 0.01 mg/L allowed complete ammonium oxidation to nitrate. The applied step-feed by optimizing influent distribution ratio to 5:3 provided sufficient organics for nitrate reduction to nitrite. According to mass balance analysis, anammox was the dominant nitrogen removal pathway (87.9%). Microbial analysis indicated that aerobic nitrifiers Nitrosomonas (1.2%) and Nitrospira (0.9%) ensured full nitrification, while PD bacteria Thauera (27.1%) conducted labor metabolism coordination with anammox bacteria Candidatus Brocadia (1.3%) for nitrogen removal. These findings demonstrate that the INPDA has a good prospect for energy-efficient municipal wastewater treatment.

Effect of sewerage development on water quality and invertebrate assemblages in a Japanese river over the long term

River water degradation arising from wastewater effluents is a major factor contributing to the deterioration of river ecosystems; yet direct evidence of the beneficial influence of long-term sewerage development on such ecosystems is scarce at a basin scale. The present study, over 12 years, examined changes in sewerage connection rate, water quality, and invertebrate assemblages at 10 study sites in the Shigenobu River, southwestern Japan, and investigated the relationships between them. Sewerage connection rates increased in the three main municipalities in the basin during the study period (range: 8.6%–31.4%). At the lowland sites, the concentration of total inorganic nitrogen (TIN) decreased by 38.9% on average and the richness metrics of invertebrate assemblages significantly increased at several sites. Furthermore, the richness metrics negatively related with TIN concentration, implying that invertebrate diversity recovery was the result of water quality improvement. Our findings strongly suggest that sewerage system improvement is an effective measure for restoring river ecosystems.

Advanced nitrogen removal from municipal sewage via partial nitrification-anammox process under two typical operation modes and seasonal ambient temperatures

A novel two-stage partial nitrification-anammox (PN-A) process was developed, achieving nitrogen removal from low carbon/nitrogen ratio municipal sewage under two typical operational modes and seasonal ambient temperatures. When complete nitritation-anammox was performed at temperatures greater than 19.4 °C, the effluent concentration of total inorganic nitrogen (TIN) was 4.1 mg/L, corresponding to a nitrogen removal efficiency (NRE) of 94.3 %. In contrast, when partial nitritation-anammox was performed at temperatures below 19.4 °C, the effluent TIN was 12.3 mg/L, corresponding to a NRE of 83.6 %. The relative abundance of Nitrosomonas and Nitrosomonadaceae increased from 0.02 % to 0.28 %, while Ca. Brocadia decreased from 1.85 % to 1.30 %, with the contribution of anammox to nitrogen removal being highest under low temperatures (19.4℃ to 13.8℃), at 59.0 %. This novel two-stage PN-A process provides a new approach for the stable operation of wastewater treatment plants (WWTPs) under low ambient temperatures.

Variations of leachate CO2 and N2O concentrations on typical cultivated and natural hillslopes in southeastern hilly area of China

Greenhouse gases dissolved in the soil water and transported through subsurface flow were poorly understood in comparison to their emissions at the soil-atmosphere interface. In this study, leachate CO2 and N2O concentrations were monitored on a tea garden (TG) hillslope (cultivated) and a bamboo forest (BF) hillslope (naturally vegetated) from September 2019 to February 2022 in the south-eastern hilly area of China. Leachate CO2 and N2O concentrations ranged from 1.07 to 9.83 mg C L−1 and from 0.32 to 11.74 μg N L−1, respectively, on the TG hillslope, while they ranged from 0.64 to 25.95 mg C L−1 and from 0.23 to 5.68 μg N L−1, respectively, on the BF hillslope. On both hillslopes, leachate CO2 concentrations were the greatest in summer, while they were the lowest in winter. On the TG hillslope, the highest and the lowest leachate N2O concentrations were observed in spring and summer, respectively, while on the BF hillslope, they were observed in autumn and winter, respectively. On both hillslopes, soil temperature, precipitation during the previous 15 days, and matrix potential were positively related to the leachate CO2 concentrations but not related to the leachate N2O concentrations. Soil water content was positively related to both leachate CO2 and N2O concentrations, while such relationships with groundwater table depth were negative. Leachate total inorganic nitrogen and soil total inorganic nitrogen concentrations were positively related to the leachate N2O concentration on the TG hillslope, while such relationships were not observed on the BF hillslope. Spatial variations of leachate CO2 and N2O concentrations were mainly influenced by land covers, terrain attributes (slope, elevation), soil particle size distribution, bulk density and soil organic matter content. The findings of this study supplemented the knowledge of soil water dissolved CO2 and N2O in the cultivated and naturally vegetated land covers. Further researches are needed to quantify the CO2 and N2O losses through subsurface flow and their contributions to the total soil CO2 and N2O losses.

Achieving partial nitrification, enhanced biological phosphorus removal and in-situ fermentation (PNPRF) in continuous-flow system and mechanism analysis at transcriptional level

Simultaneous partial nitrification, biological phosphorous removal and sludge fermentation in continuous–flow system faced many challenges in wastewater treatment plants (WWTPs). In this study, a novel process was developed to achieve partial nitrification, enhanced biological phosphorus removal and in–situ fermentation (PNPRF) in continuous–flow system dominated by Tetrasphaera treating low carbon/nitrogen ratio (COD/N) real domestic wastewater. After 80 days of operation, no external carbon was added into the influent. Nitrite accumulation ratio (NAR) reached 99.4% and removal efficiency of PO43−–P was 100%. The effluent concentration of total inorganic nitrogen (TIN) was lower than 2 mg/L. Compared with traditional biological nitrogen and phosphorus removal processes, the daily sludge discharge in continuous–flow PNPRF decreased by 61.9%. DNA concentration of the sludge supernatant after anaerobic zone was 2.26-fold of that before anaerobic zone, and percentages of intact cells decreased by 6.9% and necrotic cells increased by 8.3%, indicating that cell lysis caused the reduction of sludge discharge. The negative effect of prolonged anaerobic phase on transcriptional response of nxrB gene was more significant than that of amoA and hao gene, which resulted in stable partial nitrification. The activities of ammonium–oxidizing bacteria (AOB) were higher than that of nitrite–oxidizing bacteria (NOB), and reverse transcriptional PCR (RT–PCR) showed that RNA expression of AOB was higher than that of NOB. Achieving partial nitrification, enhanced biological phosphorus removal and in–situ fermentation in continuous-flow system dominated by Tetrasphaera solved the problems of carbon source deficiency and large amount of sludge discharge in treating real domestic wastewater process.

Hong Kong's subtropical scleractinian coral communities: Baseline, environmental drivers and management implications

We surveyed 41 sites to provide an updated baseline of Hong Kong coral communities. Five community types were identified, among them the most common one inhabited oceanic waters and dominated by both massive and upward-plating corals. The 41 sites had 2.1–⁠79% coral cover; among them 21 in the eastern waters had >40% coral cover. Corals in several sites showed signs of external bioerosion or bleaching-induced damage. Sites in the southern waters had low coral cover. Both coral cover and generic richness correlated negatively with several water quality parameters including total inorganic nitrogen concentration and turbidity, indicating the development of Hong Kong's coral communities is constrained by water quality parameters. Management actions are proposed to reduce bioerosion, and to monitor sites affected by bleaching.

Atmospheric deposition of sulfur and nitrogen in the West China rain zone: Fluxes, concentrations, ecological risks, and source apportionment

The West China Rain Zone (WCRZ; 25,000 km2) is a broad mountainous ecotone between the urbanized Sichuan Basin (SCB) and the rural Qinghai-Tibetan Plateau (QTP). The WCRZ is rich in biodiversity and may be influenced by the air pollutants transported from the SCB towards the QTP. As air pollution can cause adverse effects on ecosystem health, this study investigated the fluxes, ecological risks, and sources of sulfur and nitrogen deposition by using a source-oriented community multiscale air quality (CMAQ) model for the entire WCRZ and its constituent national nature reserves in 2015. Results indicate that for sulfur (75%) and nitrogen (70%), the majority of the WCRZ area had annual total deposition fluxes higher than their critical loads. In addition, the annual volume-weighted mean (VWM) total inorganic nitrogen (TIN) wet deposition concentrations in almost the entire WCRZ exceeded the standard for total nitrogen of surface water in protected areas (0.2 mg N L−1). This exceedance is detrimental for the headwater watersheds that have precipitation as their sole water source. In each of the national nature reserves (NNRs), the average annual total deposition fluxes ranged between ~12–38 (sulfur) and ~ 15–38 kg S(N) ha−1 a−1 (nitrogen). About 40–100% (sulfur) and 5–85% (nitrogen) of the areas have annual total fluxes higher than the corresponding critical loads. Furthermore, the average VWM TIN concentration of wet deposition in each WCRZ NNR is higher than 0.2 mg N L−1. Therefore, sulfur and nitrogen deposition likely pose severe risks to some important natural ecosystems in the WCRZ. Based on the source apportionment results, to reduce sulfur and nitrogen deposition in the WCRZ, both SCB and non-SCB emissions should be controlled. In addition, the agricultural NH3 emissions and the industrial and power plant SO2 and NOx emissions should be reduced by larger extents than that of other sectors. This study provides a basis for future atmospheric and ecological observations in the WCRZ.

Comparison of hybrid membrane aerated biofilm reactor (MABR)/suspended growth and conventional biological nutrient removal processes.

Mathematical modelling was used to investigate the possibility to use membrane aerated biofilm reactors (MABRs) in a largely anoxic suspended growth bioreactor to produce the nitrate-nitrogen required for heterotrophic denitrification and the growth of denitrifying phosphorus accumulating organisms (DPAOs). The results indicate that such a process can be used to achieve a variety of process objectives. The capture of influent biodegradable organic matter while also achieving significant total inorganic nitrogen (TIN) removal can be achieved with or without use of primary treatment by operation at a relatively short suspended growth solids residence time (SRT). Low effluent TIN concentrations can also be achieved, irrespective of the influent wastewater chemical oxygen demand (COD)/total nitrogen (TN) ratio, with somewhat larger suspended growth SRT. Biological phosphorus and nitrogen removal can also be effectively achieved. Further experimental work is needed to confirm these modelling results.

A continuous plug-flow anaerobic/aerobic/anoxic/aerobic (AOAO) process treating low COD/TIN domestic sewage: Realization of partial nitrification and extremely advanced nitrogen removal

The realization of stable partial nitrification and advanced nitrogen removal are not acquired effectively in conventional pre-denitrification biological nitrogen removal processes treating domestic sewage. Herein, a novel anaerobic/aerobic/anoxic/aerobic (AOAO) continuous plug-flow reactor, characterized with double sludge reflux and a bypass of anaerobic mixed liquor conveyed to anoxic zone, was first constructed to realize stable partial nitrification in treating domestic sewage. The alternating anoxic/aerobic conditions and longer anoxic sludge retention time might be responsible for the partial nitrification. Nitrite accumulation ratio reached 89.3 ± 3.3% with the maximum activity ratio of AOB to NOB increasing from 0.72 to 8.17. A content total inorganic nitrogen (TIN) removal efficiency (93.7 ± 2.2%) and effluent TIN concentration (2.9 ± 0.9 mg N/L) were obtained after 238 days' operation. Specifically, nitrogen balance of the typical cycle showed that about 30.1% of TIN was removed through simultaneous partial nitrification and denitrification (SND) in aerobic zone and 48.2% by endogenous denitrification in anoxic zone. The AOAO process is an economic treatment for domestic sewage with aerobic hydraulic retention time (HRT) of 4 h.

Assessing critical load exceedances and ecosystem impacts of anthropogenic nitrogen and sulphur deposition at unmanaged forested catchments in Europe

Anthropogenic emissions of nitrogen (N) and sulphur (S) compounds and their long-range transport have caused widespread negative impacts on different ecosystems. Critical loads (CLs) are deposition thresholds used to describe the sensitivity of ecosystems to atmospheric deposition. The CL methodology has been a key science-based tool for assessing the environmental consequences of air pollution. We computed CLs for eutrophication and acidification using a European long-term dataset of intensively studied forested ecosystem sites (n = 17) in northern and central Europe. The sites belong to the ICP IM and eLTER networks. The link between the site-specific calculations and time-series of CL exceedances and measured site data was evaluated using long-term measurements (1990–2017) for bulk deposition, throughfall and runoff water chemistry. Novel techniques for presenting exceedances of CLs and their temporal development were also developed. Concentrations and fluxes of sulphate, total inorganic nitrogen (TIN) and acidity in deposition substantially decreased at the sites. Decreases in S deposition resulted in statistically significant decreased concentrations and fluxes of sulphate in runoff and decreasing trends of TIN in runoff were more common than increasing trends. The temporal developments of the exceedance of the CLs indicated the more effective reductions of S deposition compared to N at the sites. There was a relation between calculated exceedance of the CLs and measured runoff water concentrations and fluxes, and most sites with higher CL exceedances showed larger decreases in both TIN and H+ concentrations and fluxes. Sites with higher cumulative exceedance of eutrophication CLs (averaged over 3 and 30 years) generally showed higher TIN concentrations in runoff. The results provided evidence on the link between CL exceedances and empirical impacts, increasing confidence in the methodology used for the European-scale CL calculations. The results also confirm that emission abatement actions are having their intended effects on CL exceedances and ecosystem impacts.

Highly concentrated atmospheric inorganic nitrogen deposition in an urban, coastal region in the US

Much of our understanding of atmospheric nitrogen deposition (AD) is based on important monitoring networks, such as the National Atmospheric Deposition Program (NADP) in the United States (US). However, it is likely that rural-based monitoring data are unrepresentative of reactive nitrogen concentrations found in urban areas. In this study, event-based precipitation was collected over one-year in Providence, RI - a mid-sized city situated at the head of Narragansett Bay, the largest estuary in the northeastern US Significant concentration enhancements in inorganic nitrogen (i.e., ammonium and nitrate) were found for wet deposition in the urban area compared to nearby NADP rural and non-urban coastal monitoring sites. Moreover, the annual ratio of ammonium to nitrate was highest in urban precipitation (>1.0). From 164 precipitation events, the mean volume-weighted ammonium and nitrate concentrations were 32.9 μmol l−1 and 28.4 μmol l−1, respectively, with total inorganic nitrogen concentrations exceeding 100 μmol l−1 in at least 30 events. AD data for Narragansett Bay is limited and outdated: our results suggest inorganic nitrogen is at least 2.5 times greater than currently estimated in policy decision-making related to improving water quality and ecosystem services. This underestimation is due to an increase (by as much as 6 times) in ammonium wet deposition since 1990, likely due to an increase in urban sources of precursor ammonia.