Treated wastewater and weak removal mechanisms enhance nitrate pollution in metropolitan rivers

Raising wastewater collection and discharge standards will reduce greenhouse gas emissions from metropolitan rivers

Urban rivers are increasingly recognized as significant sources for greenhouse gas (GHG) emissions. Few studies, however, quantify emissions of all three GHG from rehabilitating urban rivers that receive treated wastewater. This study analyzed carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) concentrations and diffusive fluxes from the Suzhou Creek in Shanghai that were investigated across the four seasons in 2021. Our results show that Suzhou Creek behaves as a source of atmospheric GHG emissions. The mean concentrations of CO2 CH4 and N2O, in the main and tributaries were 80.62 ± 37.81 versus 82.07 ± 50.77 μmol L-1, 0.38 ± 0.31 versus 0.73 ± 0.87 μmol L-1, and 37.33 ± 17.70 versus 51.26 ± 35.84 nmol L-1, respectively. The corresponding fluxes of CO2, CH4, and N2O were 3.04 ± 2.36 versus 2.78 ± 2.25 mmol m-2 h-1, 17.82 ± 18.91 versus 35.35 ± 49.51 μmol m-2 h-1, and 1.44 ± 1.25 versus 2.2 ± 2.95 μmol m-2 h-1, respectively. GHG emissions from Suzhou Creek are lower than global urban rivers. N2O generation in the nitrate-rich mainstem may primarily be attributed to denitrification and nitrification, and ammonium-rich tributaries may mainly associate with nitrification and coupling nitrification-denitrification. Tributaries are more suitable for CH4 generation. CO2 in the basin comes mainly from heterotrophic respiration of organic matter, and the high nutrient load and Chlorophyll a concentration in tributaries support photosynthesis. Although wastewater treatment plants and sewage treatment stations provide direct inputs of GHG and nutrient substrates, respectively, their input load (including GHG and nutrient substrates) is lower than that of other urban rivers. The study highlights that with the improvement of sewage collection capacity and treatment discharge standards in large cities, the input load and water pollution situation in urbanized areas will be greatly improved, thus reducing GHG emissions from urban rivers.

Impact of diverse irrigation water sources on olive oil quality and its physicochemical, fatty acids, antioxidant, and antibacterial properties

This study investigates the impact of irrigation water sources on the quality of olive oil from the Chemlal olive variety in the Hadjadj region, northeast of Mostaganem, Algeria, a coastal area known for its semi-arid climate and intensive olive cultivation. Olive trees (n = 50 per irrigation group) were irrigated with treated wastewater, spring water, and normal water, and the resulting oils were assessed for physicochemical properties, fatty acid composition, and bioactive compound profiles. Treated wastewater demonstrated distinct water quality characteristics, including elevated temperature (15.00 °C), chemical oxygen demand (COD: 58.38 mg/L), biochemical oxygen demand (BOD5: 29.00 mg/L), ammonium (15.60 mg/L), nitrite (2.55 mg/L), suspended solids (14.00 mg/L), pH (7.40), and conductivity (2.80 µS/cm), reflecting residual organic material and ionic content post-treatment. Heavy metal concentrations in all water sources were within permissible limits for irrigation and drinking purposes, affirming their safety for agricultural use. Olive oil from treated wastewater-irrigated trees exhibited superior quality parameters, including low acidity (1.99%), low peroxide value (6.8 meq O2/kg), enhanced oxidative stability, higher fat content (96.5%), and favorable saponification values. Fatty acid analysis revealed a higher oleic acid content (62.6 mg/kg), known for cardiovascular health benefits. Bioactive compound analysis indicated significantly elevated levels of α-tocopherol (180.25 mg/kg), squalene (7500.8 mg/kg), carotenoids (25.1 mg/kg), and polyphenols (604.76 mg GAE/kg), contributing to increased antioxidant capacity (63.50% DPPH inhibition, a measure of free radical scavenging) and lower lipid peroxidation (0.25 TBARS, an index of oxidative degradation), indicative of superior oxidative stability. Spring water-irrigated oils showed higher acidity, peroxide values, and linoleic acid concentrations, alongside notable antibacterial efficacy against Escherichia. coli, Pseudomonas. aeruginosa, and Staphylococcus. aureus. Oils from normal water irrigation were characterized by higher linolenic acid levels, providing a more balanced fatty acid profile. These findings underscore treated wastewater's potential to enhance olive oil's nutritional and functional qualities, particularly its antioxidant activity and stability, while highlighting the role of spring water in enhancing antibacterial properties despite slightly reduced antioxidant stability. These findings are relevant to water-scarce Mediterranean and arid regions, informing sustainable irrigation strategies in line with global climate-resilient agriculture policies.

Metagenomics reveals the potential transmission risk of resistomes from urban park environment to human

Urban parks play a significant role in urban ecosystems and are strongly associated with human health. Nevertheless, the biological contamination of urban parks - opportunistic pathogens and antibiotic resistance genes (ARGs) - has been poorly reported. Here, metagenomic and 16 S rRNA sequencing methods were used to study the distribution and assembly of opportunistic pathogens and ARGs in soil and water from nine parks in Lanzhou city, and further compared them with local human gut microbiomes to investigate the potential transmission risk. Our results revealed that the most important type of drug resistance in urban parks was multidrug resistance, with various resistance mechanisms. Approximately half of ARGs were shared between human gut and park environment, and it was noteworthy that cross-species transmission might exist among some high-risk ARGs, such as mepA and mdtE, with a significant enrichment in human gut. Metagenomic binning uncovered several bacterial genomes carrying adjacent ARGs, MGEs, and virulence genes, indicating a possibility that these genes may jointly transfer among different environments, particularly from park environment to human. Our results provided a reference point for the management of environmental pollutants in urban parks.

Sources and transformations of riverine nitrogen across a coastal-plain river network of eastern China: New insights from multiple stable isotopes

Riverine nitrogen pollution is ubiquitous and attracts considerable global attention. Nitrate is commonly the dominant total nitrogen (TN) constituent in surface and ground waters; thus, stable isotopes of nitrate (δ15N/δ18O-NO3-) are widely used to differentiate nitrate sources. However, δ15N/δ18O-NO3- approach fails to present a holistic perspective of nitrogen pollution for many coastal-plain river networks because diverse nitrogen species contribute to high TN loads. In this study, multiple isotopes, namely, δ15N/δ18O-NO3-, δ18O-H2O, δ15N-NH4+, δ15N-PN, and δ15Nbulk/δ18O/SP-N2O in the Wen-Rui Tang River, a typical coastal-plain river network of Eastern China, were investigated to identify transformation processes and sources of nitrogen. Then, a stable isotope analysis in R (SIAR) model-TN source apportionment method was developed to quantify the contributions of different nitrogen sources to riverine TN loads. Results showed that nitrogen pollution in the river network was serious with TN concentrations ranging from 1.71 to 8.09 mg/L (mean ± SD: 3.77 ± 1.39 mg/L). Ammonium, nitrate, and suspended particulate nitrogen were the most prominent nitrogen components during the study period, constituting 45.4 %, 28.9 %, and 19.9 % of TN, respectively. Multiple hydrochemical and isotopic analysis identified nitrification as the dominant N cycling process. Biological assimilation and denitrification were minor N cycling processes, whereas ammonia volatilization was deemed negligible. Isotopic evidence and SIAR modeling revealed municipal sewage was the dominant contributor to nitrogen pollution. Based on quantitative estimates from the SIAR model, nitrogen source contributions to the Wen-Rui Tang River watershed followed: municipal sewage (40.6 %) ≈ soil nitrogen (39.5 %) > nitrogen fertilizer (9.7 %) > atmospheric deposition (2.8 %) during wet season; and municipal sewage (59.1 %) > soil nitrogen (30.4 %) > nitrogen fertilizer (4.1 %) > atmospheric deposition (1.0 %) during dry season. This study provides a deeper understanding of nitrogen dynamics in eutrophic coastal-plain river networks, which informs strategies for efficient control and remediation of riverine nitrogen pollution.

Unveiling Microbial Nitrogen Metabolism in Rivers using a Machine Learning Approach

Microbial nitrogen metabolism is a complicated and key process in mediating environmental pollution and greenhouse gas emissions in rivers. However, the interactive drivers of microbial nitrogen metabolism in rivers have not been identified. Here, we analyze the microbial nitrogen metabolism patterns in 105 rivers in China driven by 26 environmental and socioeconomic factors using an interpretable causal machine learning (ICML) framework. ICML better recognizes the complex relationships between factors and microbial nitrogen metabolism than traditional linear regression models. Furthermore, tipping points and concentration windows were proposed to precisely regulate microbial nitrogen metabolism. For example, concentrations of dissolved organic carbon (DOC) below tipping points of 6.2 and 4.2 mg/L easily reduce bacterial denitrification and nitrification, respectively. The concentration windows for NO3--N (15.9-18.0 mg/L) and DOC (9.1-10.8 mg/L) enabled the highest abundance of denitrifying bacteria on a national scale. The integration of ICML models and field data clarifies the important drivers of microbial nitrogen metabolism, supporting the precise regulation of nitrogen pollution and river ecological management.

Identification of nitrate accumulation mechanism of surface water in a mining-rural-urban agglomeration area based on multiple isotopic evidence

The accumulation of nitrate (NO3-) in surface waters resulting from mining activities and rapid urbanization has raised widespread concerns. Therefore, it is crucial to develop a nitrate transformation information system to elucidate the nitrogen cycle and ensure sustainable water quality management. In this study, we focused on the main river and subsidence area of the Huaibei mining region to monitor the temporal and spatial variations in the NO3- content. Multiple isotopes (δD, δ18O-H2O, δ15N-NO3-, δ18O-NO3-, and δ15N-NH4+) along with water chemistry indicators were employed to identify the key mechanisms responsible for nitrate accumulation (e.g., nitrification and denitrification). The NO3- concentrations in surface water ranged from 0.28 to 7.50 mg/L, with NO3- being the predominant form of nitrogen pollution. Moreover, the average NO3- levels were higher during the dry season than during the wet season. Nitrification was identified as the primary process driving NO3- accumulation in rivers and subsidence areas, which was further supported by the linear relationship between δ15N-NO3- and δ15N-NH4+. The redox conditions and the relationship between δ15N-NO3- and δ18O-NO3-, and lower isotope enrichment factor of denitrification indicated that denitrification was weakened. Phytoplankton preferentially utilized available NH4+ sources while inhibiting NO3- assimilation because of their abundance. These findings provide direct evidence regarding the mechanism underlying nitrate accumulation in mining areas, while aiding in formulating improved measures for effectively managing water environments to prevent further deterioration.

Simultaneous denitrification and organics removal by denitrifying bacteria inoculum in a multistage biofilm process for treating desulfuration and denitration wastewater

This study aimed to treat real wastewater from the desulfuration and denitration process in a petrochemical plant with high-strength nitrogen (TN≈200 mg/L, > 90% nitrate), sulfate (2.7%) and extremely low-strength organics (CODCr < 30 mg/L). Heterotrophic denitrification of multistage anoxic and oxic biofilm (MAOB) process in three tanks using facultative denitrifying bacteria inoculum was developed to simultaneously achieve desirable effluent nitrogen and organics at different hydraulic retention time (HRT) and carbon to nitrogen (C/N) mass ratios. The optimum condition was recommended as a C/N ratio of 1.5 and a HRT of A (24 h)/O (12-24 h) to achieve > 90% of nitrogen and organics removal as well as no significant variation of sulfate. The denitrifying biofilm in various tanks was dominant by Hyphomicrobium (8.9%-25.7%), Methylophaga (18.6%-25.8%) and Azoarcus (3.3%-19.6%), etc., containing > 20% aerobic denitrifiers. This explained that oxic zone in MAOB process also exhibited simultaneous nitrogen and organics removal.