Trace metals, anions and polybromodiphenyl ethers in settled indoor dust and their association

Contaminants in settled indoor dust are potentially health hazardous to human. Thus, identification and quantification of toxic chemicals in settled indoor dust is of great concern. In this study, the levels of major anions ( $ \mathrm{C}{{\mathrm{l}}^{-}},\mathrm{N}{{\mathrm{O}}_2}^{-},\mathrm{B}{{\mathrm{r}}^{-}},\mathrm{N}{{\mathrm{O}}_3}^{-},\mathrm{P}{{\mathrm{O}}_4}^{3-}\,\mathrm{and}\,\mathrm{S}{{\mathrm{O}}_4}^{2- } $ ), trace metals (Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, As and Pb) and polybromodiphenyl ethers (PBDEs) in settled office and home dust were determined and correlations between the contaminants investigated. Depending on the available materials in both microenvironments, the most possible sources were identified. The results showed that the settled office dusts (n?=?6 pooled samples from 85 offices) were more contaminated than home dusts (n?=?8 homes). For anions, $ \mathrm{S}{{\mathrm{O}}_4}^{2- } $ and Cl^– accounted for 87 and 97 % of the total office and home dust contaminants, respectively. For trace metals, Fe, Cu, Zn and Mn, accounted for 98 % of the contaminants in both office and home dust samples. Fe exhibited the highest percentage of 76.7 and 87.3 % in office and home dust samples, respectively. For PBDEs, the mean concentrations detected in office and home dust ranged between 5.8–86.3 and 1.5–20.6 ng?g^?1, respectively. The log-transformed correlation between the total concentrations of trace metals and major anions detected in offices and homes was positive for offices and negative for homes with a statistically significant values (r?=?0.73, p?<?0.01; r?= ?0.22, p?<?0.01, respectively). The daily exposure rates determined for the most hazardous such as As, Cd, Pb and PBDEs congeners, relative to the individual concentrations reported in the literature in settled indoor dust, were found very lower. Therefore, maybe it is possible to expect less potential health risk. Investigation of formation of coordination compounds between trace metals and PBDEs congeners is possible; however, this requires further study.     

Insights into real cotton-textile dyeing wastewater treatment using solar advanced oxidation processes

Different advanced oxidation processes (AOPs) were applied to the treatment of a real cotton-textile dyeing wastewater as a pre-oxidation step to enhance the biodegradability of the recalcitrant compounds, which can be further oxidized using a biological process. Tests were conducted on a lab-scale prototype using artificial solar radiation and at pilot scale with compound parabolic collectors using natural solar radiation. The cotton-textile dyeing wastewater presents a lilac color, with a maximum absorbance peak at 641 nm, alkaline pH (pH?=?8.2), moderate organic content (DOC?=?152 mg C L^?1, COD?=?684 mg O₂ L^?1) and low-moderate biodegradability (40 % after 28 days in Zahn–Wellens test). All the tested processes contributed to an effective decolorization and mineralization, but the most efficient process was the solar-photo-Fenton with an optimum catalyst concentration of 60 mg Fe²⁺ L^?1, leading to 98.5 % decolorization and 85.5 % mineralization after less than 0.1 and 5.8 kJ_UV L^?1, respectively. In order to achieve a final wastewater with a COD below 250 mg O₂ L^?1 (discharge limit into water bodies imposed by the Portuguese Legislation-Portaria no. 423/97 of 25 June 1997), considering the combination of a solar-photo-Fenton reaction with a biological process, the phototreatment energy required is 0.5 kJ_UV L^?1, consuming 7.5 mM hydrogen peroxide, resulting in 58.4 % of mineralization $ \left({t}_{30\mathrm{W}}=3.2\ \min; \overline{T}=30.7\ {}^{\circ}\mathrm{C};\overline{\mathrm{pH}}=2.80;{\overline{\mathrm{UV}}}_{G,n}={13\ \mathrm{W}\ \mathrm{m}}^{-2}\right). $      

Removal mechanisms and kinetics of trace tetracycline by two types of activated sludge treating freshwater sewage and saline sewage

Understanding the removal mechanisms and kinetics of trace tetracycline by activated sludge is critical to both evaluation of tetracycline elimination in sewage treatment plants and risk assessment/management of tetracycline released to soil environment due to the application of biosolids as fertilizer. Adsorption is found to be the primary removal mechanism while biodegradation, volatilization, and hydrolysis can be ignored in this study. Adsorption kinetics was well described by pseudo-second-order model. Faster adsorption rate (k ₂?=?2.04?×?10^?2?g?min^?1?μg^?1) and greater adsorption capacity (q _e?=?38.8 μg?g^?1) were found in activated sludge treating freshwater sewage. Different adsorption rate and adsorption capacity resulted from chemical properties of sewage matrix rather than activated sludge surface characteristics. The decrease of tetracycline adsorption in saline sewage was mainly due to Mg²⁺ which significantly reduced adsorption distribution coefficient (K _d) from 12,990?±?260 to 4,690?±?180 L?kg^?1. Species-specific adsorption distribution coefficients followed the order of $ K_{\mathrm{d}}^{{ + 00}} \gg K_{\mathrm{d}}^{{ + - 0}} > K_{\mathrm{d}}^{{ + - - }} $ . Contribution of zwitterionic tetracycline to the overall adsorption was >90 % in the actual pH range in aeration tank. Adsorption of tetracycline in a wide range of temperature (10 to 35 °C) followed the Freundlich adsorption isotherm well.     

The influence of hydroxyl volatile organic compounds on the oxidation of aqueous sulfur dioxide by oxygen

Although the effect of volatile organic compounds (VOCs) on the oxidation of dissolved sulfur dioxide by oxygen has been the subject of many investigations, this is the first study which examines the effect of a large number of precisely 16 hydroxy compounds. The kinetics both in the absence and the presence of VOCs was defined by rate laws (A and B): A $$ \hbox{-} \mathrm{d}\left[\mathrm{S}\left(\mathrm{IV}\right)\right]/\mathrm{dt}={R}_o={k}_o\left[\mathrm{S}\left(\mathrm{IV}\right)\right] $$ B $$ \hbox{-} \mathrm{d}\left[\mathrm{S}\left(\mathrm{IV}\right)\right]/\mathrm{dt}={R}_i={k}_i\left[\mathrm{S}\left(\mathrm{IV}\right)\right] $$ where R _o and k _o are the initial rate and first-order rate constant, respectively, in the absence of VOCs, R _i , and k _i are the initial rate and the first-order rate constant, respectively, in the presence of VOCs, and [S(IV)] is the concentration of dissolved sulfur dioxide, sulfur(IV). The nature of the dependence of k _i on the concentration of inhibitor, [Inh], was defined by Eq. (C). C $$ {k}_i={k}_0/\left(1+B\left[\mathrm{Inh}\right]\right) $$ where B is an empirical inhibition parameter. The values of B have been determined from the plots of 1/k _i versus [Inh]. Among aliphatic and aromatic hydroxy compounds studied, t-butyl alcohol and pinacol were without any inhibition effect due to the absence of secondary or tertiary hydrogen. The values of inhibition parameter, B, were related to k _inh , the rate constant for the reaction of SO₄ ^? radical with the inhibitor, by Eq. (D). D $$ B=\left(9\pm 2\right)\times 1{0}^{-4}\times {k}_{inh} $$ Equation (D) may be used to calculate the values of either of B or k _inh provided that the other is known. The extent of inhibition depends on the value of the composite term, B[Inh]. However, in accordance with Eq. (C), the extent of inhibition would be sizeable and measurable when B[Inh]?>?0.1 and oxidation of S(IV) would be almost completely stopped when B[Inh]?≥?10. B[Inh] value can be used as a guide whether the reaction step: SO₄ ^??+?organics? \( \overset{k_{inh}}{\to } \) ?SO₄ ^2??+?non-chain products: should be included in the multiphase models or not.     

A method for the assessment of long-term changes in carbon stock by construction of a hydropower reservoir

Sustainability of hydropower reservoirs has been questioned since the detection of their greenhouse gas (GHG) emissions which are mainly composed of carbon dioxide and methane. A method to assess the impact on the carbon cycle caused by the transition from a natural river system into a reservoir is presented and discussed. The method evaluates the long term changes in carbon stock instead of the current approach of monitoring and integrating continuous short term fluxes. A case study was conducted in a subtropical reservoir in Brazil, showing that the carbon content within the reservoir exceeds that of the previous landuse. The average carbon sequestration over 43 years since damming was 895 mg C m\(^{-2}\mathrm{{day}}^{-1}\) and found to be mainly due to storage of carbon in sediments. These results demonstrate that reservoirs have two opposite effects on the balance of GHGs. By storing organic C in sediments, reservoirs are an important carbon sink. On the other hand, reservoirs increase the flux of methane into the atmosphere. If the sediments of reservoirs could be used for long term C storage, reservoirs might have a positive effect on the balance of GHGs.     

Transport and degradation of propylene glycol in the vadose zone: model development and sensitivity analysis

Transport and degradation of de-icing chemical (containing propylene glycol, PG) in the vadose zone were studied with a lysimeter experiment and a model, in which transient water flow, kinetic degradation of PG and soil chemistry were combined. The lysimeter experiment indicated that aerobic as well as anaerobic degradation occurs in the vadose zone. Therefore, the model included both types of degradation, which was made possible by assuming advection-controlled (mobile) and diffusion-controlled (immobile) zones. In the mobile zone, oxygen can be transported by diffusion in the gas phase. The immobile zone is always water-saturated, and oxygen only diffuses slowly in the water phase. Therefore, the model is designed in a way that the redox potential can decrease when PG is degraded, and thus, anaerobic degradation can occur. In our model, manganese oxide (MnO₂, which is present in the soil) and NO \(_{3}^{-}\) (applied to enhance biodegradation) can be used as electron acceptors for anaerobic degradation. The application of NO \(_{3}^{-}\) does not result in a lower leaching of PG nor in a slower depletion of MnO₂. The thickness of the snowcover influences the leached fraction of PG, as with a high infiltration rate, transport is fast, there is less time for degradation and thus more PG will leach. The model showed that, in this soil, the effect of the water flow dominates over the effect of the degradation parameters on the leaching at a 1-m depth.     

Increases of dissolved organic carbon in temperate and boreal lakes in Quebec, Canada

Purpose  

Increases in dissolved organic carbon (DOC) concentrations have been reported in surface waters worldwide in the last 10 to 20 years. The causes behind these increases have been attributed to many factors, including climate change and decreasing depositions of atmospheric sulphate ( \textSO₄^{2 -} {\text{SO}}_4^{{{2} - }} ). Trends in DOC concentrations and their potential causal factors were examined in a network of 30 lakes lying in undisturbed temperate and boreal catchments in the province of Quebec, Canada.     

Biogenic nitric oxide emissions from cropland soils

Emissions of nitric oxide (NO) were determined during late spring and summer 1995 and the spring of 1996 from four agricultural soils on which four different crops were grown. These agricultural soils were located at four different sites throughout North Carolina. Emission rates were calculated using a dynamic flow-through chamber system coupled to a mobile laboratory for in-situ analysis. Average NO fluxes during late spring 1995 were: 50.9±47.7 ng N m⁻² s⁻¹ from soil planted with corn in the lower coastal plain. Average NO fluxes during summer 1995 were: 6.4±4.6 and 20.2±19.0 ng N m⁻² s⁻¹, respectively, from soils planted with corn and soybean in the coastal region; 4.2±1.7 ng N m⁻² s⁻¹ from soils planted with tobacco in the piedmont region; and 8.5±4.9 ng N m⁻² s⁻¹ from soils planted with corn in the upper piedmont region. Average NO fluxes for spring 1996 were: 66.7±60.7 ng N m⁻² s⁻¹ from soils planted with wheat in the lower coastal plain; 9.5±2.9 ng N m⁻² s⁻¹ from soils planted with wheat in the coastal plain; 2.7±3.4 ng N m⁻² s⁻¹ from soils planted with wheat in the piedmont region; and 56.1±53.7 ng N m⁻² s⁻¹ from soils planted with corn in the upper piedmont region. An apparent increase in NO flux with soil temperature was present at all of the locations. The composite data from all the research sites revealed a general positive trend of increasing NO flux with soil water content. In general, increases in total extractable nitrogen (TEN) appeared to be related to increased NO emissions within each site, however a consistent trend was not evident across all sites.     

Plot-scale spatiotemporal variations of CO2 concentration and flux across water–air interfaces at aquaculture shrimp ponds in a subtropical estuary

Human activities have increased anthropogenic CO₂ emissions, which are believed to play important roles in global warming. The spatiotemporal variations of CO₂ concentration and flux at fine spatial scales in aquaculture ponds remain unclear, particularly in China, the country with the largest aquaculture. In this study, the plot-scale spatiotemporal variations of water CO₂ concentration and flux, both within and among ponds, were researched in shrimp ponds in Shanyutan Wetland, Min River Estuary, Southeast China. The average water CO₂ concentration and flux across the water–air interface in the shrimp ponds over the shrimp farming period varied from 22.79?±?0.54 to 186.66?±?8.71 μmol L^?1 and from ??0.50?±?0.04 to 2.87?±?0.78 mol m^?2 day^?1, respectively. There was no remarkable difference in CO₂ concentration and flux within the ponds, but significantly spatiotemporal differences in CO₂ flux were observed between shrimp ponds. Chlorophyll a, pH, salinity, air temperature, and morphometry were the important factors driving the spatiotemporal patterns of CO₂ flux in the shrimp ponds. Our findings highlighted the importance and spatiotemporal variations of CO₂ flux in the important coastal ecosystems.

     

Stable sulphur and nitrogen isotopes of the moss Haplocladium microphyllum at urban,rural and forested sites

Elemental (S and N) and isotopic (δ³⁴S and δ¹⁵N) contents in the moss Haplocladium microphyllum at urban, rural and forested sites in acid rain area of South China have been analyzed for comparisons to show whether they are different and can be effectively used to identify S and N sources of atmospheric deposition. Average moss S content at rural sites (0.29 ± 0.06%) was found to be in between those at urban (0.35 ± 0.05%) and forested (0.25 ± 0.04%) sites, which are significantly different. Average N contents of urban (2.60 ± 0.56%) and rural mosses (2.84 ± 0.77%) are not significantly different, while both are significantly different to that of forested mosses at most areas, indicating that the atmosphere over rural sites has been polluted by N as seriously as that over urban sites. Nitrogen supply, relative to S supply, was in excess of the requirement for protein synthesis, especially at rural and forested sites. Moss stable isotope signatures have been proven to be effective tools for deciphering atmospheric S and N sources at these sites. Through moss δ³⁴S signatures, we found that atmospheric S at urban and forested sites was mainly from local coal combustion and domestic biomass burning, respectively, whereas northerly air masses contributed more S to forested sites. The relatively negative moss δ¹⁵N values (?7.5 ± 3.0, ?3.4 ± 2.1 and ?0.8 ± 2.1‰) demonstrated that the main form in the N deposition was NH_x in these sites. More negative δ¹⁵N signatures in urban mosses (?7.5 ± 3.0‰) indicated the contribution of NH₃ released from untreated city sewage and wastes, while relatively less negative δ¹⁵N for rural and forested mosses (3.4 ± 2.1 and ?0.8 ± 2.1‰) was largely derived from agricultural NH₃.     

Historical record of nutrients inputs into the Xin’an Reservoir and its potential environmental implication

The Xin’an Reservoir is an important water supply source and water conservation area for the Qiantang River. However, after the occurrence of the two algae blooms in 1998 and 1999, the safety of water quality has been put into question. In order to study the historical deposition of nutrients, sediment cores were collected in different regions from the Xin’an Reservoir. The stable isotopes δ¹³C and δ¹⁵N, nutrients, total organic carbon (TOC), and inorganic carbon (IC) in the sediment cores were determined. Radiometric methods (²¹⁰Pb and ¹³⁷Cs) were used to obtain sediment chronologies. Spatially, it was found that the average total nitrogen (TN) content in the upper 5 cm of sediments increased from 0.21% in the riverine zone, to 0.33%, and then to 0.57% in the lacustrine zone. The average TP content in the upper 5 cm increased from 0.67 g kg^?1 in the riverine zone, to 1.03 g kg^?1 in the estuary region, and then to 1.65 g kg^?1 in the lacustrine zone. In addition, TOC levels showed a distinct increase from 1.42% in the bottom to 5.97% in the surface of the lacustrine zone. These results demonstrated that although primary productivity and the input of nutrients constantly increased in recent years, algae blooms rarely occurred in the Xin’an Reservoir, due to “depth effect” and an aquatic environment protection-oriented fishery policy. However, high TOC flux and high bio-available phosphorus and nitrogen in surface sediment demonstrated that the reservoir is still confronted with the potential risk of algae blooms.     

Integrated mangrove-shrimp cultivation: Potential for blue carbon sequestration

Globally, shrimp farming has had devastating effects on mangrove forests. However, mangroves are the most carbon-rich forests, with blue carbon (i.e., carbon in coastal and marine ecosystems) emissions seriously augmented due to devastating effects on mangrove forests. Nevertheless, integrated mangrove-shrimp cultivation has emerged as a part of the potential solution to blue carbon emissions. Integrated mangrove-shrimp farming is also known as organic aquaculture if deforested mangrove area does not exceed 50% of the total farm area. Mangrove destruction is not permitted in organic aquaculture and the former mangrove area in parts of the shrimp farm shall be reforested to at least 50% during a period of maximum 5 years according to Naturland organic aquaculture standards. This article reviews integrated mangrove-shrimp cultivation that can help to sequester blue carbon through mangrove restoration, which can be an option for climate change mitigation. However, the adoption of integrated mangrove-shrimp cultivation could face several challenges that need to be addressed in order to realize substantial benefits from blue carbon sequestration.     

Mercury contamination in alligators (Melanosuchus niger) from Mamirauá Reservoir (Brazilian Amazon) and human health risk assessment

Mercury (Hg) concentrations in muscles of wild alligators (Melanosuchus niger) from the Mamirauá Reservoir (a reference area in the Brazilian Amazon) and the human health risks associated with its consumption were assessed. The mean Hg concentration in alligator muscles was 0.407?±?0.114 μg/g (N?=?61). Close to 5 % of the muscle samples showed Hg levels above the World Health Organization guideline for fish consumption (0.5 μg/g). A positive and significant relationship was observed between Hg concentrations in muscle and the age of the specimens. The dose-response approach suggests that close to 27.4 years is required for half of the exposed specimens to attain 0.5 μg/g. The hazard quotient (HQ) is a risk indicator which defines the ratio of exposure level and a toxicological reference dose. HQ resulted above the unity for all the specimens when the ingestion rate for riverine communities (200 g of muscle per day) is considered, indicating the existence of hazard. When the ingestion rate for market consumers (28.57 g/day) is considered, the risks are much lower (mean HQ?=?0.55), suggesting that such group is not at risk. The establishment of local and regional ingestion rates for riverine populations and market consumers is extremely recommended.     

Primary product distribution from the Cl-atom initiated atmospheric degradation of furan: Environmental implications

Furan is an aromatic hydrocarbon present in both urban and rural atmospheres, which is emitted mainly by biomass burning and the combustion of fossil fuel. Reaction of furan and Cl atoms may be important in areas where chlorine atom concentrations are potentially high such as marine and coastal regions or continental atmospheres where industrial activity emits molecular chlorine or photo-labile Cl-containing compounds. To assess the importance of this reaction and to investigate whether any unique chlorine-containing product is formed the products of the reaction of Cl atoms with furan have been determined under atmospheric conditions. For the study two different sampling/detection methods have been used: (1) Solid-Phase MicroExtraction, with subsequent analysis by thermal desorption, and gas chromatography with mass spectrometry or flame ionization detection (SPME/GC-MS/FID), and 2-“in situ” with long path fourier transform infrared spectroscopy (FTIR). The yields of primary reaction products in the absence of NO were: 5-chloro-2(5H)-furanone (64.5±10.7)%, E-butenedial (11±3)%, 5-hydroxy-2(5H)-furanone (⩽2.4%) and Z-butenedial (1.6±0.5)%. Other products generated by secondary reactions were 2(3H)-furanone (2.8±1.9)%, HCl (21.1±3%) and CO. Maleic anhydride was detected with a yield of about 2%, however, this yield may be a combination of both primary and secondary reactions. All errors are ±2σ. The observed products confirm that addition of Cl atoms to the double bond of furan is the dominant reaction pathway.     

Comparison of models used for national agricultural ammonia emission inventories in Europe: Litter-based manure systems

Six N-flow models, used to calculate national ammonia (NH₃) emissions from agriculture in different European countries, were compared using standard data sets. Scenarios for litter-based systems were run separately for beef cattle and for broilers, with three different levels of model standardisation: (a) standardized inputs to all models (FF scenario); (b) standard N excretion, but national values for emission factors (EFs) (FN scenario); (c) national values for N excretion and EFs (NN scenario). Results of the FF scenario for beef cattle produced very similar estimates of total losses of total ammoniacal-N (TAN) (±6% of the mean total), but large differences in NH₃ emissions (±24% of the mean). These differences arose from the different approaches to TAN immobilization in litter, other N losses and mineralization in the models. As a result of those differences estimates of TAN available at spreading differed by a factor of almost 3. Results of the FF scenario for broilers produced a range of estimates of total changes in TAN (±9% of the mean total), and larger differences in the estimate of NH₃ emissions (±17% of the mean). The different approaches among the models to TAN immobilization, other N losses and mineralization, produced estimates of TAN available at spreading which differed by a factor of almost 1.7. The differences in estimates of NH₃ emissions decreased as estimates of immobilization and other N losses increased. Since immobilization and denitrification depend also on the C:N ratio in manure, there would be advantages to include C flows in mass-flow models. This would also provide an integrated model for the estimation of emissions of methane, non-methane VOCs and carbon dioxide. Estimation of these would also enable an estimate of mass loss, calculation of the N and TAN concentrations in litter-based manures and further validation of model outputs.     

Gaseous and particulate water-soluble organic and inorganic nitrogen in rural air in southern Scotland

Simultaneous daily measurements of water-soluble organic nitrogen (WSON), ammonium and nitrate were made between July and November 2008 at a rural location in south-east Scotland, using a ‘Cofer’ nebulizing sampler for the gas phase and collection on an open-face PTFE membrane for the particle phase. Average concentrations of NH₃ were 82 ± 17 nmol N m^?3 (error is s.d. of triplicate samples), while oxidised N concentrations in the gas phase (from trapping NO₂ and HNO₃) were smaller, at 2.6 ± 2.2 nmol N m^?3, and gas-phase WSON concentrations were 18 ± 11 nmol N m^?3. The estimated collection efficiency of the nebulizing samplers for the gas phase was 88 (±8) % for NH₃, 37 (±16) % for NO₂ and 57 (±7) % for WSON; reported average concentrations have not been corrected for sampling efficiency. Concentrations in the particle phase were smaller, except for nitrate, at 21 ± 9, 10 ± 6 and 8 ± 9 nmol N m^?3, respectively. The absence of correlation in either phase between WSON and either (NH₃ + NH₄⁺) or NO₃^? concentrations suggests atmospheric WSON has diverse sources. During wet days, concentrations of gas and particle-phase inorganic N were lower than on dry days, whereas the converse was true for WSON. These data represent the first reports of simultaneous measurements of gas and particle phase water-soluble nitrogen compounds in rural air on a daily basis, and show that WSON occurs in both phases, contributing 20–25% of the total water-soluble nitrogen in air, in good agreement with earlier data on the contribution of WSON to total dissolved N in rainfall in the UK.     

Effect of influent C/N ratio on N2O emissions from anaerobic/anoxic/oxic biological nitrogen removal processes

The problem of producing strong greenhouse gas of nitrous oxide (N₂O) from biological nitrogen removal (BNR) process in wastewater treatment plants (WWTP) has elicited great concern from various sectors. In this study, three laboratory-scale wastewater treatment systems, with influent C/N ratios of 3.4, 5.4, and 7.5, were set up to study the effect of influent C/N ratio on N₂O generation in anaerobic/anoxic/oxic (A²O) process. Results showed, with the increased influent C/N ratio, N₂O generation from both nitrification and denitrification process was decreased, and the N₂O-N conversion ratio of the process was obviously reduced from 2.23 to 0.05%. Nitrification rate in oxic section was reduced, while denitrification rate in anaerobic and anoxic section was elevated and the removal efficiency of COD, NH₄ ⁺-N, TN, and TP was enhanced in different extent. As the C/N ratio increased from 3.4 to 7.5, activities of three key denitrifying enzymes of nitrate reductase, nitrite reductase, and nitrous oxide reductase were increased. Moreover, microorganism analysis indicated that the relative abundance of ammonium-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were positively correlated with N₂O generation, which was reduced from (8.42 ± 3.65) to (3.61 ± 1.66)% and (10.38 ± 4.12) to (4.67 ± 1.62)%, respectively. NosZ gene copy numbers of the A²O system were increased from (1.19 ± 0.49) × 10⁷ to (2.84 ± 0.54) × 10⁸ copies/g MLSS with the influent C/N ratio elevated from 3.4 to 7.5. Hence, appropriate influent C/N condition of A²O process could optimize the microbial community structure that simultaneously improve treatment efficiency and decrease the N₂O generation.

     

Spatial and temporal variations of river nitrogen exports from major basins in China

Provincial-level data for population, livestock, land use, economic growth, development of sewage systems, and wastewater treatment rates were used to construct a river nitrogen (N) export model in this paper. Despite uncertainties, our results indicated that river N export to coastal waters increased from 531 to 1,244 kg N km^?2 year^?1 in the Changjiang River basin, 107 to 223 kg N km^?2 year^?1 in the Huanghe River basin, and 412 to 1,219 kg N km^?2 year^?1 in the Zhujiang River basin from 1980 to 2010 as a result of rapid population and economic growth. Significant temporal changes in water N sources showed that as the percentage of runoff from croplands increased, contributions of natural system runoff and rural human and livestock excreta decreased in the three basins from 1980 to 2010. Moreover, the nonpoint source N decreased from 72 to 58 % in the Changjiang River basin, 80 to 67 % in the Huanghe River basin, and 69 to 51 % in the Zhujiang River basin, while the contributions of point sources increased greatly during the same period. Estimated results indicated that the N concentrations in the Changjiang, Huanghe, and Zhujiang rivers during 1980–2004 were higher than those in the St. Lawrence River in Canada and lower than those in the Thames, Donau, Rhine, Seine, and Han rivers during the same period. River N export will reduce by 58, 54, and 57 % for the Changjiang River, Huanghe River, and Zhujiang River in the control scenario in 2050 compared with the basic scenario.