Nitrogen Loading Research Articles

Start-up of Anammox-HAP in IC reactors: Revelation of sludge characteristics and microbial community structure.

The scarcity of seed sludge poses a significant barrier to the advancement of anaerobic ammonia oxidation (anammox) process. In this investigation, two alternative sludge (anaerobic granular sludge (AGS) and activated flocculent sludge (AFS)) were employed to start up the anammox process in internal circulation (IC) reactors with the hydroxyapatite (HAP) strategy. Both reactors achieved rapid start-up on days 83 and 53, respectively. Subsequently, a nitrogen removal rate (NRR) of 1.34 gN/L/d was attained at a nitrogen loading rate (NLR) of 1.39 gN/L/d on days 107 and 81 correspondingly. The analysis of granular properties revealed that the anammox granular sludge (AMXGS) transformed from AGS exhibited superior granular size distribution and settling performance. Furthermore, the assessment of microbial community structure demonstrated that inoculating AFS was capable of enriching anammox bacteria (AnAOB) in a shorter time. Last but most importantly, this study provides a comprehensive analysis of the distinct granulation routes of AGS and AFS. AGS predominantly underwent a "broken-adsorption-granulation" process, whereas AFS exhibited not only a typical "adsorption-granulation" process but also a "biofilm growth-granulation" cycle process. The findings of this study offer a novel approach for quickly initiating anammox process when inoculating alternative sludge.

Redistribution of dissolved inorganic nitrogen loading and transport in global rivers via surface water regulation.

Surface water (SW) regulation, including reservoir regulation and surface water use, alters the soil-river hydrological processes and then influences the dissolved inorganic nitrogen (DIN) transport from rivers to oceans. However, global response of the DIN transfer to such human activity has not been well investigated. Therefore, in this study, we have taken advantage of a recently-developed land surface model to show the effects of SW regulation on DIN loading and transport in global major rivers. Three simulations were conducted at the global scale over the period of 1991-2000. Results show that, acceleration on riverine N loading caused by the SW regulation was more noticeable. With the reservoir regulation only, removal rates of DIN in most rivers were reduced by 5%, up to 20%, while the increase of removal rate in some reaches occurred when the DIN was transferred back to soil through SW withdrawal and use. Influenced by the SW regulation, DIN flow has decreased obviously by 100-1000tNyr-1. Mean DIN flow was most significantly reduced in the Yellow River basin. Yangtze River, Mississippi River, and Nelson River suffered from the strong effect of such water regulation. The DIN exported to oceans was reduced as well. Mean reduction of 40-50% occurred for those exported to the Pacific Ocean, the Atlantic Ocean and the Indian Ocean, while slight effect for the estuaries into the Arctic Ocean. The impact on the DIN exported to the Atlantic Ocean was lasting decreasing from 50% to 40%, while the trend of the impact on that exported to the Pacific Ocean was opposite. There was no evident temporal change in the impacts on the DIN transferred to the Arctic Ocean and the Indian Ocean. The findings of this study could provide new insights for the researches on nitrogen change in rivers under the impacts of human water use.

Modified SWAT Model for Agricultural Watershed in Karst Area of Southwest China

The Soil and Water Assessment Tool (SWAT) model is extensively used globally for hydrological and water quality assessments but encounters challenges in karst regions due to their complex surface and groundwater hydrological environments. This study aims to refine the delineation of hydrological response units within the SWAT model by combining geomorphological classification and to enhance the model with an epikarst zone hydrological process module, exploring the accuracy improvement of SWAT model simulations in karst regions of Southwest China. Compared with the simulation results of the original SWAT model, we simulated runoff and nutrient concentrations in the Mudong watershed from January 2017 to December 2021 using the improved SWAT model. The simulation results indicated that the modified SWAT model responded more rapidly to precipitation events, particularly in bare karst landform, aligning more closely with the actual hydrological processes in Southwest China’s karst regions. In terms of the predictive accuracy for monthly loads of total nitrogen (TN) and total phosphorus (TP), the coefficient of determination (R2) value of the modified model increased by 10.3% and 9.7%, respectively, and the Nash–Sutcliffe efficiency coefficient (NSE) increased by 11.3% and 9.9%, respectively. The modified SWAT model improves prediction accuracy in karst areas and holds significant practical value for guiding non-point source pollution control in agricultural watersheds.

Microbial Nitrogen Removal in South San Francisco Bay: Does It Play a Role in Eutrophication Resistance?

The ecosystem response to anthropogenic nitrogen (N) loading in estuarine systems is determined by hydrodynamics, biogeochemical transformation rates, and other system-specific characteristics. Historically, San Francisco (SF) Bay has been an outlier from other estuaries with an unusual resistance to eutrophication, despite having extremely high rates of nitrogen loading. Recent increases in phytoplankton biomass and an unprecedented harmful algal bloom, however, have increased the urgency to understand rates and drivers of nitrogen removal in the system. To assess benthic N cycling rates, we conducted seasonal measurements across nine sites in South and Lower South SF Bay, the two sub-embayments with the highest rates of area-normalized N loading, to determine the rates and potential drivers of denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Denitrification rates averaged 60.6 ± 8.1 µmol m−2 h−1 and were primarily coupled to nitrification. Denitrification rates were positively correlated with DNRA rates and % clay. DNRA rates ranged from 0 to 20 µmol m−2 h−1 and on an annual basis averaged ~ 10% of total benthic nitrate reduction, with a negative correlation to % clay content in the surface sediment. The measured denitrification rates account for the removal of, on average, 14% of N loaded annually to South SF Bay, leaving a sizeable portion for alternate fates (e.g., recycling, export, or burial) and potential for substantial temporal and spatial variability (1–79%). This identifies the relative importance of sediment denitrification in ecosystems characterized by high nutrients and low productivity.

Subsurface hydrological controls on the short-term effects of hurricanes on nitrate–nitrogen runoff loading: a case study of Hurricane Ida using the Energy Exascale Earth System Model (E3SM) Land Model (v2.1)

Abstract. When the nutrient level in the soil surpasses vegetation demand, nutrient losses due to surface runoff and subsurface leaching are the major reasons for the deterioration of water quality. The lower Mississippi River basin (LMRB) is one of the sub-basins that deliver the highest nitrogen loads to the Gulf of Mexico. Potential changes in episodic events induced by hurricanes may exacerbate water quality issue in the future. However, uncertainties in modeling the hydrologic response to hurricanes may limit the modeling of nutrient losses during such events. Using a machine learning approach, we calibrated the land component of the Energy Exascale Earth System Model (E3SM), or ELM, version 2.1, based on the water table depth (WTD) of a calibrated 3D subsurface hydrology model. While the overall performance of the calibrated ELM is satisfactory, some discrepancies in WTD remain in slope areas with low precipitation due to the missing lateral flow process in ELM. Simulations including biogeochemistry performed using ELM with and without model calibration showed important influences of soil hydrology, precipitation intensity, and runoff parameterization on the magnitude of nitrogen runoff loss and the leaching pathway. Despite such sensitivities, both ELM simulations produced reduced WTD and increased runoff and accelerated nitrate–nitrogen runoff loading during Hurricane Ida in August 2021, consistent with the observations. With observations suggesting more pronounced effects of Hurricane Ida on nitrogen runoff than the simulations, we identified factors for model improvement to provide a useful tool for studying hurricane-induced nutrient losses in the LMRB region.

High-rate nitrogen loading accelerates organic phosphorus loss through enzymatic and non-enzymatic processes in a semi-arid grassland

Semi-arid grasslands suffer from increased nitrogen (N) loading, which affects phosphorus (P) cycling. Phosphorus is essential but limited for all living organisms. Organic P (Po) is crucial for P supply in grasslands response to N loading, but its transformation process remains unclear. To elucidate P cycling, we evaluated P fractions, phosphatase activities, and their coding genes (pho genes phoD, phoX, and phoC) at both DNA and RNA levels in response to a nitrogen loading gradient. The results showed that the total P and Po contents significantly decreased with increasing N loading rates. In contrast, plant and litter biomass P exhibited an opposite trend but were insufficient to offset soil P loss. Unexpectedly, neither the phosphatase activities nor microbial biomass increased with increasing N loading rates, although the DNA abundance of pho genes responded positively to N enrichment. A partial correlation network indicated that N loading rates decreased Po levels through phosphatase activities, which were significantly associated with soil pH and the RNA abundance of pho genes. Moreover, non-enzymatic mineralization may be enhanced in Po cycling in semi-arid grasslands undergoing high N loading, especially following the initial stage of enzymatic mineralization. Although the activity of phosphatases did not continue to rise with increased N loading, the strategy for addressing N-induced P limitation through overconsumption of Po via non-enzymatic processes would accelerate the degradation of the ecosystem in the future.

Immobilization of ammonia-oxidizing bacteria using mycelial pellets: Preparation, characteristics, and application for nitritation.

Ammonia-oxidizing bacteria (AOB) sourced from an aerobic granular sludge (AGS) process were rapidly enriched by progressively increasing ammonia nitrogen (NH4+-N) loads, achieving a Nitrosomonas abundance of 20.7% and a nitrite accumulation rate exceeding 80%. Mycelial pellets formed by Cladosporium, isolated from the same AGS system, provided a porous surface structure for the immobilization of the enriched AOB, creating mycelial pellet/AOB composites. Robust microbial colonization and aggregation in mycelial pellet porous matrix were facilitated by a higher level of extracellular polymeric substances (EPS) compared to conventional AGS. Static tests showed a maximum NH4+-N oxidation rate of 17.7mg/(gMLVSS·h), higher than free AOB (8.5mg/(gMLVSS·h)). In multi-recycling tests, the composites maintained 96.6% NH4+-N oxidation, demonstrating superior repeatability and stability. The results highlight advantages of mycelial pellets as biocompatible carriers in immobilizing AOB sourced from the same system, offering insights into improved nitritation performance and durability, making them promising for practical wastewater treatment.

Lacustrine groundwater discharge as an important hidden source of nutrients to a large eutrophic lake: Implications for eutrophication management.

Lake eutrophication driven by excessive nutrient inputs has become a global issue, but the potential impact of lacustrine groundwater discharge (LGD) as a nutrient source on lake eutrophication remains largely unknown. This study assessed the contribution of LGD-derived nutrient loads and revealed their potential impact on lake eutrophication in Taihu Lake, a typical large shallow and eutrophic lake in China, based on the segmented radon mass balance model and nutrient data. The total LGD flux was estimated to be 6.59×109m3 a-1, representing 57.8% of the annual flux from inflowing rivers. LGD was a significant hidden nutrient source, contributing total nitrogen (TN) and total phosphorus (TP) loads to the entire lake comparable to those of the inflowing rivers. Dissolved inorganic forms dominated these LGD-derived nutrient loads. Spatially, the majority of TN (59.9%) and TP (62.4%) loads derived from LGD originated from sub-area III (southwest), which differed from the dominant area of riverine inputs, sub-area II (northwest). In addition, the significant enrichment of nitrogen observed in LGD suggests its potential to mitigate nitrogen limitation in the lake. The increasing nitrogen limitation in Taihu Lake and the prevalence of nitrogen limitation in eutrophic lakes worldwide indicate that nitrogen is a key nutrient in managing LGD-derived nutrient loads. This study highlights the importance of integrating LGD-derived nutrient loads into nutrient reduction strategies to reverse eutrophication in large eutrophic lakes, especially those with nitrogen limitation.

Nitrogen removal from urea effluent in an anammox sequencing batch biofilm reactor with valorised dishwashing scrubber as biocarrier

BackgroundThis study aimed for efficient nitrogen removal from urea wastewater in an anammox SBBR with valorised dishwashing scrubber as biocarrier. MethodsThe study (225 d) comprised three discrete phases: Phase I [1 – 26 d; Inf. NH4+-N (Simulated ww)], Phase II [27 – 131 d; Inf. NH4+-N (Real ww) + Inf. NH4+-N (Simulated ww)] and Phase III [132 – 225 d; Inf. NH4+-N (Real ww)] in which the reactor was supplemented with varied ratios (0 – 7) of influent NH4+-N reflecting varying combinations of real and simulated wastewater. Significant findingsThe reactor exhibited average total nitrogen removal efficiency of approximately 83 %, 81 %, and 80 % in the subsequent phases I, II, and III, respectively at varied nitrogen loading rates (100 – 232 g N m−3 d−1). The 16S rRNA gene sequencing revealed significant enrichment of Candidatus Kuenenia (from 41.69 % to 48.11 %) on the 225th day which was primarily attributed to significant decline in Nitrospira from 23.9 % to 1.24 % (due to nitrite oxidizing bacteria inhibition on account of high free ammonia concentrations in the reactor) and Nitrosomonas from 4.32 % to 0.01 %. Additionally, genera that were not initially present (on the 1st day) including Stenotrophobacter (17.34 %), Sphingobium (8.41 %), Rhodococcus (5.88%) and Ohtaekwangia (5.84 %) were observed to emerge by the end of the experiment (225 d).

Integrating river transport processes and seasonal dynamics to assess watershed nitrogen export risk.

Excessive nitrogen exported to water bodies affects the balance of ecosystem and poses a threat to human health. Although the concept of water purification service helps quantify nitrogen export, the impact of river transport remains unclear. This study focused on nitrogen as a pollutant by utilizing the Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) model to assess nitrogen export in the Dongting Lake Basin, taking into account both the processes of sub-basin nitrogen export and river transport. Additionally, the monthly variations within the year were further explored, together with the identification of the priority area of returning cropland to forest land. The results showed that in 2021, the total nitrogen load outside the Dongting Lake was 11.34×108 kg·year-1, with the sub-basins retaining a total of 9.02×108 kg·year-1, accounting for 79.57% of the total nitrogen load. Notably, the total nitrogen retention by the river made up 16.87% of the total nitrogen retention, up to 1.83×108 kg·year-1. In view of monthly variation, water purification service based on the entire process was higher in winter and late autumn, and lower in summer and early autumn. The priority areas for ecological project were mainly distributed around the Dongting Lake, achieving an improvement of 58.15% in water purification service with approximately 7.02% of the total returnable cropland. This study proposed a new approach of water purification service assessment through integrating river transport processes and seasonal dynamics, aiming to assess watershed nitrogen export risk more accurately.

Dual intermittent aerations enhance nitrogen removal via anammox in anoxic/oxic biofilm process for carbon limited wastewater treatment.

Efficient nitrogen removal after organic capture is challenging through conventional nitrification-denitrification process. Two biofilm-based anoxic/oxic reactors, with a single intermittent zone (R1) or dual intermittent zones (R2), were compared in treating carbon-limited wastewater. Intermittent aeration integrated partial nitrification-anammox (PNA), partial denitrification-anammox (PDA), and denitrification, with anammox-related pathways contributing over 75% nitrogen removal in both reactors. As nitrogen loading rate increased from 0.14 to 0.19 kg-N m-3 day-1, nitrogen removal efficiency in R1 dropped from 74.3% to 46.0%, while R2 maintained 76.6% removal at low HRT of 6h. The dual intermittent aeration strategy improved nitrogen removal capacity by enhancing PNA in the first intermittent zone and reducing effluent fluctuation in the second. Anammox bacteria (Candidatus Brocadia, relative abundance: 0.95-2.48%) were enriched across all zones, supporting efficient PNA and PDA. These findings suggested that dual intermittent aeration enhanced anammox in pre-anoxic processes for carbon limited wastewater treatment.

Seasonal variations in water quality and phytoplankton–bacteria interactions mediated through dissolved organic matter in New Jersey coastal waters

New Jersey coastal areas are experiencing eutrophication due to human-induced nutrient overloading. Algal blooms occur frequently in New Jersey coastal waters, and excessive blooms shift water quality. However, phytoplankton–bacteria interactions mediated through dissolved organic matter (DOM) have not been extensively studied in New Jersey coastal waters, especially near overburdened communities. We targeted a traditionally underrepresented township area, Keyport Harbor, as a model site to investigate seasonal variabilities of phytoplankton biomass, DOM, and bacteria biomass. Chlorophyll-a concentrations were significantly higher in spring–summer (bloom) than in fall–winter (nonbloom). Nitrate + nitrite and ammonium were negatively correlated with chlorophyll-a, and the water was nitrogen-limited during bloom time while phosphorus-limited during nonbloom time, implying that regulating nitrogen loading was key to controlling algal blooms, especially during bloom seasons. Phytoplankton–bacteria interactions were assessed by monitoring dissolved organic carbon (DOC) and bacterial abundance between bloom and nonbloom time from field and incubation studies. A significantly higher DOC, but not dissolved organic nitrogen, occurred in the bloom than nonbloom period, suggesting that phytoplankton contributed to the production of more carbon-rich than nitrogen-rich compounds. DOC fueled threefold bacterial growth in the bloom period, exceeding the temperature effect and indicating strong phytoplankton–DOM–bacteria connections. Microbial remineralization incubations showed rapid phytoplankton–DOC drawdown, and more ambient DOC drawdown and bacterial growth in the bloom than nonbloom time, further supporting the important role of phytoplankton–DOC in shaping bacteria. With water quality monitoring via chemical and biological indicators, the study aimed to understand carbon cycling better, assess anthropogenic impacts on coastal environments, and help facilitate coastal management.

Geography, Trajectories, and Controls of Coastal Water Quality: More Rapid Improvement in the Shallow Zone of the Chesapeake Bay.

Many coastal ecosystems have suffered from cultural eutrophication and dead zones. In the Chesapeake Bay, water quality degradation is manifested in low dissolved oxygen, poor water clarity, and decreased submerged aquatic vegetation acreage. This research combines long-term monitoring data, science-based assessment methods, and novel data analysis approaches (i.e., machine learning) toward understanding the geography, trajectories, and controls of water quality in the Chesapeake Bay, which provides an example for the assessment and management of complex coastal ecosystems. Results showed that the attainment of water quality standards has improved in both the deep zone and the shallow zone since 1985, but the shallow zone has improved more rapidly. In addition, the attainment trajectory has been affected by mainly external drivers (i.e., nutrient reductions) and, to a lesser extent, internal drivers (i.e., water temperature and stratification). Reductions in nutrient loads would improve attainment, whereas warming and stratification would decrease attainment. Furthermore, scenario analyses demonstrated the importance of managing both nitrogen and phosphorus loads. Overall, the deep zone and the shallow zone showed different trajectories and controls, emphasizing the importance of geographical targeting with management actions.

Microalgae bioprospecting for the food industry: insights into the autotrophic biomass production and macromolecular accumulation of four microalgal species.

In this study, four microalgal strains were evaluated for their biomass production capacity and macromolecule biosynthesis. These include three strains from the phylum Chlorophyta: Monoraphidium sp. LB2PC 0120, Stichococcus sp. LB2PC 0117, and Tetraselmis sp. LB2PC 0320, and one strain from the phylum Haptophyta: Isochrysis sp. LB2PC 0220. The experiments were conducted under typical laboratory-scale setups. Additionally, phylogenetic analysis based on the 18-28S rRNA internal transcribed spacer (ITS) was performed to validate the taxonomic identity of the strains. Each strain was exposed to four different cultivation conditions based on two levels of illumination intensity [25-(LI) and 50-(HI) µmol m- 2 s- 1] and nitrogen loading [100-(LΝ) and 300-(HΝ) mg NaNO3 L- 1] in a full factorial design. All the microalgae achieved maximum biomass production under HI-HN conditions, which amounted to 1495, 919, 844, and 708mg/L for Monoraphidium sp. LB2PC 0120, Stichococcus sp. LB2PC 0117, Tetraselmis sp. LB2PC 0320 and Isochrysis sp. LB2PC 0220, respectively, after 16 days of cultivation. Among them, Stichococcus sp. LB2PC 0117 had the highest protein content (49.9% wt.) under LI-HN conditions and Monoraphidium sp. LB2PC 0120 had the highest lipid content (44.3% wt.) under HI-LN conditions. Both Monoraphidium sp. LB2PC 0120 and Tetraselmis sp. LB2PC 0320 accumulated the highest carbohydrate content (~ 37% wt.) under LI-LN and HI-LN conditions, respectively. Based on biomass and macromolecule production, Monoraphidium sp. LB2PC 0120 was identified as the most promising candidate for upscaling studies, expecting its highly manipulatable compositional profile to support multiple applications in the food industry, rendering this microalga a valuable resource.

Influence of sludge biochar at different carbonization temperatures on anammox process

ABSTRACT Adding biochar can expedite the establishment of the anaerobic ammonia oxidation (anammox) process and improve the nitrogen removal efficiency of the anammox reactor. However, the optimization research of biochar derived from dewatered sludge on anammox is relatively limited. In this study, four sequencing batch reactors (SBRs) were compared for the enrichment of anammox bacteria using synthetic wastewater with sludge biochar carbonized at temperatures of 300°C, 550°C, and 800°C, and without biochar (CK). The start-up and the nitrogen removal performance of anammox process were evaluated, as well as the effect of organic carbon on nitrogen removal. The results showed that the addition of sludge biochar at different pyrolysis temperatures all can accelerate the start-up of the anammox process, improve the nitrogen removal efficiency, and reduce the total nitrogen (TN) in the effluent. Although the reactor with biochar carbonized at 800°C showed the fastest increase in the nitrogen loading, the best TNRE occurred in the reactor with biochar carbonized at 300°C, which was 8.0% higher than those of the control (CK, p < 0.05). The predominant genus of anammox in SBRs differed between the sludge biochar reactor and the control reactor (without biochar), which were Candidatus Brocadia and Candidatus Jettenia, respectively. Additionally, the total abundances of anammox bacteria and denitrifiers increased with the addition of sludge biochar.

Isotopic evidence for preferential transport of fertilizer nitrogen into the northern Gulf of Mexico during high water discharge

Anthropogenic nitrogen inputs from the Mississippi-Atchafalaya River Basin have caused substantial environmental challenges in the northern Gulf of Mexico, such as coastal eutrophication, harmful algal blooms, and seasonal hypoxia. Addressing these issues requires a better understanding of the complex sources of nitrogen, which include fertilizers, groundwater, manure, and sewage. In this study, we analyzed the nitrogen isotopic composition of dissolved nitrate and particulate nitrogen from the Wax Lake Delta, a major distributary of the Mississippi-Atchafalaya River Basin that flows into the Gulf of Mexico. Our findings revealed that during the wet season, δ15N values of both nitrate and particulate nitrogen were consistently 2-3‰ lower compared to the dry season. This suggests that fertilizer-derived nitrogen, which has lower δ15N, is predominantly exported to the Gulf of Mexico during periods of high water discharge. These findings imply that adjusting fertilizer application timing could help reduce nitrogen loading and mitigate its environmental impact on the Gulf of Mexico.

Trends and drivers of hypoxic thickness and volume in the northern Gulf of Mexico: 1985-2018.

Hypoxia is a major environmental issue plaguing the commercially and ecologically important coastal waters of the northern Gulf of Mexico. Several modeling studies have explored this phenomenon, but primarily focus on the areal extent of the mid-summer hypoxic zone. Research into the variability and drivers of hypoxic volume and thickness is also important in evaluating the seasonal progression of hypoxia and its impact on coastal resources. In this study, we compile data from multiple monitoring programs and develop a geospatial model capable of estimating hypoxic thickness and volume across the summer season. We adopt a space-time geostatistical framework and introduce a rank-based inverse normal transformation to simulate more realistic distributions of hypoxic layer thickness. Our findings indicate that, on average, there is a seasonal lag in peak hypoxic volume and thickness compared to hypoxic area. We assess long-term trends in different hypoxia metrics (area, thickness, and volume), and while most metrics did not exhibit significant trends, mid-summer hypoxic thickness is found to have increased at a rate of 5.9 cm/year (p<0.05) over the past three decades. In addition, spring nitrogen load is found to be the major driver of all hypoxia metrics, when considered along with other riverine inputs and meteorological factors in multiple regression models. Hypoxic volume, which was also often influenced by east-west wind velocities, was found to be more predictable than hypoxic thickness.

Nitrogen flow in food production and consumption system of Inner Mongolia from 2000 to 2020

Decoupled crop and livestock production caused serious disconnection of nitrogen utilization in agro-pastoral ecotone of northern China. In this study, we employed a material flow analysis approach to investigate reactive nitrogen (Nr) flow in the food production and consumption system in Inner Mongolia from 2000 to 2020. Results indicated that agricultural production system is the primary source of Nr input for the food consumption system. The environmental nitrogen load in Inner Mongolia has been increasing progressively, from 266 Gg in 2000 to 882 Gg in 2020, primarily due to the rapid increase in livestock farming and significant losses of nitrogen in feces and urine. Our results raise the attention to promote the close integration of farmland production and livestock system in the decoupled crop and livestock production system.

Insights into the influence of organic and salinity on the two-stage partial nitritation/anammox process in treating food waste digestate

ABSTRACT Food waste digestate (FWD), which contains significant levels of ammonium, organic matter, and salinity, can interfere with treatment performance of the anammox process. In this study, a two-stage partial nitritation/anammox (PN/A) process was established to investigate nitrogen removal and microbial response in treating FWD at a nitrogen loading rate (NLR) of 0.27 ± 0.02 gN/L/d. High concentrations of free ammonia (58 mg/L) and free nitrous acid (0.3 mg/L) facilitated the initiation of the partial nitritation (PN) process, achieving an average NO2 −/NH4 + ratio of 1.28 ± 0.08. For the anammox process, a nitrogen removal rate (NRR) of 0.72 ± 0.13 gN/L/d was achieved. Free ammonia (NH3) stripping, Anammox pathway, and denitrification pathway contributed 4.1 ± 0.3%, 5.1 ± 0.2%, and 84.0 ± 1.5% of the total nitrogen removal, respectively. Nitrosomonas, a salt-tolerant ammonia-oxidizing bacteria (AOB), was enriched to 1.0%, while Nitrospira, a nitrite-oxidizing bacteria (NOB), was effectively suppressed to 0.003%. The salt-tolerant anammox genera unclassified_f__Brocadiaceae (13.9%) and Candidatus_Kuenenia (4.8%) dominated the nitrogen removal pathway. The high enrichment of unclassified_f__Brocadiaceae ensured stable operation of the anammox process at 0.62 ± 0.11% salinity, even with a high initial FA inhibition concentration of 40 mg/L. Additionally, norank_f_A4b (1.34%) and norank_f_norank_o_SBR1031 (52.1%) facilitated the hydrolysis of refractory organic matter. Denitrifying bacteria, including Hyphomicrobium, Truepera, and unclassified_c__Alphaproteobacteria, played significant roles in nitrate removal, with a CODconsumed/NO3 − removed ratio of 2.7 ± 0.2. This study highlights the application of a two-stage PN/A process for rapid startup and effective nitrogen removal from FWD.

The major role of riverine outflows in shaping the current and future habitats of Harmful Algal Blooms: the case of the North Sea

Abstract This study investigates the extension of the potential habitat of Harmful Algal Blooms (HABs) in the North Sea using observational data and model experiments under current and future climate scenarios. We assess the combined effects of temperature, salinity, and nutrient availability, particularly the nitrogen-to-phosphorus ratio, on HABs in the region. Climate change projections indicate a decrease in salinity concomitant with an increase in surface temperature, potentially leading to an offshore extension of HAB habitat. Reducing nitrogen and phosphorus loads in rivers differentially affects dissolved inorganic nitrogen (DIN) and phosphorus (DIP) levels, with DIN being more sensitive to load reduction, thereby constraining HAB habitat extension. We underscore the importance of considering both physical and biogeochemical factors in assessing HAB habitat dynamics and the potential impacts of climate change and nutrient reduction measures on HAB expansion in the North Sea. These findings have significant implications for environmental policy and management.