Residual Nitrate Concentrations Research Articles

Spatial and temporal variations in the micronutrient Fe across the Peruvian shelf from 1984 to 2017

High dissolved iron (dFe) concentrations of the order of 10–100 nmol L−1 are a feature of waters influenced by sedimentary inputs in oxygen minimum zones (OMZ). However, the temporal development of dFe concentrations is poorly defined due to a general reliance on snapshot cross-shelf sections to study marine trace metal dynamics. Multiple cruise campaigns since the 1980s have investigated Fe dynamics over the Peruvian shelf, particularly between 9° S and 17°S where the shelf is broad, extremely productive and known to feature benthic dFe effluxes which are amongst the highest measured globally. This extensive long-term dataset uniquely allows us to study the interannual variability in dFe concentrations and their response to El Niño–Southern Oscillation (ENSO) events. By combining data from 11 cruises during the period 1984–2017 we are able to evaluate dFe dynamics on interannual timescales in a major OMZ. The region where average dFe concentrations are sensitive to variations in ENSO is confined to a subsurface layer at depths between 50 and 150 m, particularly in the narrow coastal region within 50 km of the coastline. Subsurface dFe concentrations were generally low during El Niño events (0.7–15.4 nmol L−1) and relatively high with a wider range of variability during the cold ENSO phase (1.1–52.1 nmol L−1). Inverse relationships between wind speed and surface/subsurface dFe were evident. In the subsurface layer, this may be attributable to enhanced dFe offshore transport along isopycnals when upwelling-favorable winds relax in accordance with previously outlined theories. Surface layer (<40 m) dFe variability was likely associated with a dilution and/or oxidation effect depending on the strength of wind driven water column mixing. Upwelling brings macronutrient-rich water into the euphotic zone, but its intensity had a limited impact on upper layer dFe concentrations possibly due to the influence of an onshore geostrophic flow. Interannual variability in surface chlorophyll-a (Chl-a) was found to correlate with dFe concentration in the offshore zone of northern Peru. This is consistent with bioassay experiments and climatological residual nitrate concentrations which both indicate proximal Fe limitation of phytoplankton growth over and beyond the northern Peruvian shelf. Overall, our work highlights the importance of physical factors driving short-term variations in Fe availability in one of the world’s most economically important fishery regions and suggests that, despite pronounced spatial and temporal variability in dFe concentrations, the ENSO phase has an impact on dFe availability.

Groundwater denitrification using a continuous flow mode hybrid system combining a hydrogenotrophic biofilter and an electrooxidation cell

An attached-growth continuous flow hydrogenotrophic denitrification system was investigated for groundwater treatment. Two bench-scale packed-bed reactors were used in series, without external pH adjustment or carbon source addition, while inorganic carbonate salts already contained in the groundwater were the sole carbon source used by the denitrifying bacteria. The hydrogen was produced by water electrolysis using renewable energy sources thus minimizing resource-draining factors of the treatment process. The biofilter was subjected to a combination of three groundwater retention times (13.5, 27 and 54 min, corresponding to 20, 10 and 5 mL min−1 inlet water flow rates) and two hydrogen flow values (10 and 20 mL min−1) to evaluate its efficiency under different operating parameters. In all cases, significant nitrate percentage removals were achieved, ranged between 64.1% and 100%. The treatment process appears to slow down with lower retention times and H2 flow rate values, although residual nitrate concentrations were always in the range of 0–5.1 mg L−1, values below the maximum permitted limit of 11.3 mg L−1. In cases where nitrite accumulation was detected, a continuous flow electrochemical oxidation process with three different current density values (5.0, 7.5 and 10.0 mA cm−2) was examined as a post-treatment step aiming to completely remove the toxic nitrite anions. Finally, an advanced mathematical model of the attached growth hydrogenotrophic denitrification process was developed to predict concentrations of all the substrates examined in the bio-filter (nitrate, nitrite, inorganic carbon and hydrogen).

Nitrogen leaching and grey water footprint affected by nitrogen fertilization rate in maize production: a case study of Southwest China.

Effective nitrogen (N) management measures are required to control environmental problems caused by N fertilizer use in intensive maize production systems. Soil N losses associated with high precipitation and over-fertilization in maize production can cause substantial environmental problems, whereas there is a lack of quantitative data and effective study countermeasures. A 2-year field study was conducted in the subtropical maize production system in Southwest China to quantify N leaching under varying N application rates of 0, 90, 180, 270 and 360 kg N ha-1 yr-1 . The results indicated that N leaching accounted for 16-38% of N fertilizer input. For farmer practice treatment (360 kg N ha-1 yr-1 ), N leaching loss was high at 110 kg N ha-1 yr-1 and accounted for 31% of the N applied. As an indicator of the ambient water quality pollution, the grey water footprint across all treatments ranged from 376 to 1092 m3 Mg-1 , with an average of 695 m3 Mg-1 . Reducing N rate to agronomically optimized treatment (180 kg N ha-1 yr-1 ) significantly decreased N leaching by 77%, and maintained high grain yield of 8.1Mg ha-1 . The grey water footprint was reduced by 52-63% with N rates from 270 or 360 kg N ha-1 yr-1 to 180 kg N ha-1 yr-1 . Nitrogen surplus (applied N rate minus N uptake by maize) resulted in higher soil residual nitrate concentration and consequently high N leaching. High precipitation and low soil pH were the main ecological factors leading to high N leaching. © 2021 Society of Chemical Industry.

A novel method for the synchronous absorption of SO2 and NO from marine diesel engines

A novel method based on a Na2S2O8/urea composite system was used to both synchronously absorb SO2 and NO from marine diesel engine emissions and reduce the nitrate residues in cleaning wastewater. In the Na2S2O8/urea solution, a temperature of over 60 °C and a Na2S2O8 concentration of >0.05 mol/L were found to fully absorb SO2 by overcoming the SO2 release and effectively absorb NO. Urea successfully reduced the residual nitrate concentration and improves NO absorption at a concentration below 2 mol/L. The NO and SO2 removal efficiencies reached 97.8 ± 1.5% and 100%, respectively, at 70 °C in 0.1 mol/L Na2S2O8 and 1.5 mol/L urea solutions. The residual nitrate concentration was 8.12 ± 0.25 mg/L, far below the regulatory limit of 60 mg/L. Acidic conditions (pH ≤ 5.5) could be conducive to absorbing NO and reducing the residual nitrate concentration. Based on the experimental results, this novel method is highly promising for reducing ship emissions.

Feed distribution based on sensing ammonium concentration after sub-feeding to achieve target effluent nitrogen concentration in sequencing batch reactors

Conventional sequencing batch reactors (SBRs) have been developed based on nitrogen removal efficiency rather than the effluent nitrogen concentration as a design factor. The purpose of this study was to achieve the target effluent total nitrogen (TN) concentration regardless of the influent nitrogen concentration at 10 m3 pilot SBR. The desired nitrogen removal efficiency was achieved by distributing the influent one to four times in one cycle, but effluent TN concentration fluctuated as the influent nitrogen concentration varied. To achieve the target effluent TN concentration, the residual ammonia concentration after sub-feeding was adjusted to the target nitrogen concentration based on real-time ammonia sensing. When the residual ammonia concentration after sub-feeding was set at 10 or 5 mg NH4+-N L−1, the effluent TN concentration was 9.3 ± 0.6 or 4.6 ± 0.3 mg L−1, respectively, regardless of the influent TN concentration. Furthermore, complete nitrification was achieved under low temperature, from 7.2 to 13.1 °C, by controlling the aeration phase end point based on the real-time ammonia sensing. However, when the residual nitrate concentration was adjusted to a low level, the total phosphorus (TP) removal efficiency was high owing to an increase of phosphate release activity by polyphosphate accumulating organisms.

Bioremediation of pit toilet sewage

The paper examines the efficacy of anaerobic, aerobic and denitrification reactions in reducing organic carbon (C), ammonium and nitrate concentrations in pit toilet sewage. The anaerobic character of pit toilet sewage causes nitrogen (N) to prevail as ammonium rather than as nitrate ions. Anaerobic decomposition of organic carbon is initially resorted to reduce competition for ammonium oxidation during subsequent aerobic treatment of sewage. A mixture of air-dried cattle manure, sand and gravel is used as a biobarrier medium for nitrate reduction. Cattle manure serves as an affordable organic carbon source; sand particles act as a medium for attached bacterial growth, while gravel improves the permeability of the barrier. Batch tests showed that anaerobic reactions reduce chemical oxygen demand (COD) concentration in pit toilet sewage by 85%. Comparatively, aerobic reactions reduce ammonium concentration in sewage by 77% through assimilation, nitrification and adsorption; 6–10 h of contact between the biobarrier mix and nitrate leads to acceptable levels of denitrification (residual nitrate concentration &lt; 45 mg/l). A modified twin-pit toilet that facilitates anaerobic decomposition of sewage in the first pit and aerobic treatment and denitrification of sewage in the second pit is constructed at Mulbagal town, Karnataka, India.

Efficiently Treated Sewage Sludge Supplemented with Nitrogen and Potassium Is a Good Fertilizer for Cereals

AbstractSewage sludge contains phosphorus (P), nitrogen (N), potassium (K) and organic matter in both soluble and particulate forms. However, agricultural use of sludge is limited because of possible problems due to hygiene, heavy metals, inner quality variation and non‐balanced ratios of nutrients. We tested sustainable fertilization of cereals with (i) urea‐spiked granulated sludge, (ii) composted anaerobic sludge digestate, (iii) dried (dewatered), anaerobic thermophilic digestate sludge mixture and (iv) dried (dewatered) sludge. The sludge products were applied to barley and wheat with variable combinations of N and K supplements in Nordic climate. Sludge products supplemented with N and K and mineral NPK fertilizers containing soluble 70 kgN ha−1 achieved barley dry yields above 3500 kg ha−1. The grain N content was 22·3 g kg−1 in barley fertilized with granulated sludge fortified with NK (soluble N 83 kg ha−1), while grain N content was 20 g kg−1 when fertilized with 90 kgN ha−1 NPK. The wheat dry yields were 2890 and 2850 kg ha−1 fertilized with granulated sludge fortified with NK when soluble N was 49 kg ha−1 and NPK with 110 kgN ha−1 of these grains containing N 29·2 and 29·0 g kg−1. The results mean that correctly treated and formulated sludge products can be used. The risks caused by enteric microorganisms or heavy metals could be managed. The residual nitrate concentrations in soil were low. Sludge products could be developed by optimizing nutrient ratios and their structure. Sludge producers should provide farmers with detailed physicochemical data about each sludge portion. Copyright © 2016 John Wiley &amp; Sons, Ltd.

The central California Current transition zone: A broad region exhibiting evidence for iron limitation

The transition zone (TZ) of the central California Current upwelling system (cCCS) is the boundary between the cold, saline, coastally upwelled water and the warm, less saline, oligotrophic waters of the offshore California Current (CC). The TZ is a broad region that regularly exhibits chlorophyll concentrations of 1–2μgL−1 throughout the spring, summer, and fall seasons. Surface transect and vertical profile data from three cruises (May 2010, June 1999, and August 2011) between 34 and 42°N show residual nitrate concentrations (5–15μM) and low Fe concentrations (most<0.2nmolkg−1) in the TZ. We therefore suggest that much of the TZ of the cCCS is an Fe-limited, high nutrient, lower than expected chlorophyll (HNLC) region. The main source of Fe to the cCCS is from upwelling through the benthic boundary layer (BBL) over the continental shelf sediments. Iron and NO3- in coastally upwelled water are transported via offshore moving filaments into the TZ. However, since some coastal upwelling regions with narrow continental shelves do not have much Fe to begin with, and since Fe is drawn down more rapidly relative to NO3- due to biological assimilation and scavenging, these filaments transport low concentrations of Fe relative to NO3- into the TZ. Weak wind curl-induced upwelling and vertical mixing in the TZ also deliver Fe and NO3- to the surface but at lower concentrations (and lower Fe:NO3-) than from strong coastal upwelling. Mesoscale cyclonic eddies in the TZ are important to consider with respect to offshore surface nutrient delivery because there is a marked shoaling of isopycnals and the nutricline within these eddies allowing higher nutrient concentrations to be closer to the surface. Since wind curl-induced upwelling and/or vertical mixing occurs seaward of the continental shelf, there is not enough Fe delivered to the surface to accompany the NO3-. By using Fe:NO3- ratios and calculated specific growth rates for diatoms, we demonstrate that the TZ of the cCCS shows evidence for Fe limitation of diatom blooms. The TZ also appears to progress further into Fe limitation as the upwelling season progresses from spring into late summer. This study provides some of the first field data to suggest that Fe is a critical bottom up control on the ecosystem in the TZ of the cCCS.

Effects of high hydrostatic pressure and varying concentrations of sodium nitrite from traditional and vegetable-based sources on the growth of Listeria monocytogenes on ready-to-eat (RTE) sliced ham

The objective of this study was to determine the effect the source of added nitrite and high hydrostatic pressure (HHP) had on the growth of Listeria monocytogenes on ready-to-eat (RTE) sliced ham. Use of 600MPa HHP for 3min resulted in an immediate 3.9–4.3log CFU/g reduction in L. monocytogenes numbers, while use of 400MPa HHP (3min) provided less than 1log CFU/g reduction. With the 600MPa HHP treatment, sliced ham with a conventional concentration of sodium nitrite (200ppm) was not different in L. monocytogenes growth from use with 50 or 100ppm of sodium nitrite in pre-converted celery powder. Instrumental color values as well as residual nitrite and residual nitrate concentrations for cured (sodium nitrite and nitrite from celery powder) and uncured ham formulations are discussed.

Influence of mass transfer resistance on overall nitrate removal rate in upflow sludge bed reactors

A kinetic model with intrinsic reaction kinetics and a simplified model with apparent reaction kinetics for denitrification in upflow sludge bed (USB) reactors were proposed. USB-reactor performance data with and without sludge wasting were also obtained for model verification. An independent batch study showed that the apparent kinetic constants k′ did not differ from the intrinsic k but the apparent K s ′ was significantly larger than the intrinsic K s suggesting that the intra-granule mass transfer resistance can be modeled by changes in K s. Calculations of the overall effectiveness factor, Thiele modulus, and Biot number combined with parametric sensitivity analysis showed that the influence of internal mass transfer resistance on the overall nitrate removal rate in USB reactors is more significant than the external mass transfer resistance. The simulated residual nitrate concentrations using the simplified model were in good agreement with the experimental data; the simulated results using the simplified model were also close to those using the kinetic model. Accordingly, the simplified model adequately described the overall nitrate removal rate and can be used for process design.

Innovative process using Fe0/CO2 for the removal of nitrate from groundwater

In this study, the Fe0/CO2 process was investigated for removing nitrate from aqueous solution, in terms of process efficiency, process operation mode, and post-treatment of the end product ammonium. The results show that nitrate at 30 mg/L could be removed from solution within 30 min under the conditions of 2 g/L Fe0 and 200 mL/min CO2 flow rate. Additionally, nitrite was not detected in treated solution, whereas ammonium is the predominant nitrogen-containing species. The normalized residual nitrate concentration decreased with increasing nitrate concentration (2.18–24.19 mg N/L). Nitrate removal was inhibited significantly in the presence of humic acid. Comparing operation modes, NO3− reduction efficiency with increasing number of batch operations in Mode 2 (treated solution was emptied and refilled with freshly prepared solution for the next batch treatment, containing the same level of nitrate as the previous batch) is better than that with Mode 1 (treated solution was retained in the reactor and spiked with concentrated nitrate solution to raise nitrate concentration to a level close to the one in the previous batch). However, to guarantee satisfactory nitrate removal in batch operation mode, zero-valent iron supplementation needs to be taken into consideration. For example, the nitrate removal efficiency without Fe0 supplementation is decreasing in the third batch, compared with those with supplements of 0.25 and 1 g/L. According to a preliminary study, the undesired end-product ammonium can be removed from solution by about 95% within 22.5 h with an air flow rate of 500 mL/min and a solution pH around 12; the ammonium concentration decreased from 6.4 to 0.3 mg 12 N/L. Stripping time can be further shortened by increasing air flow rate and using an efficient air diffuser.

Estimation of denitrification potential in a karst aquifer using the 15N and 18O isotopes of NO 3 −

A confined aquifer in the Malm Karst of the Franconian Alb, South Germany was investigated in order to understand the role of the vadose zone in denitrifiaction processes. The concentrations of chemical tracers Sr2+ and Cl− and concentrations of stable isotope 18O were measured in spring water and precipitation during storm events. Based on these measurements a conceptual model for runoff was constructed. The results indicate that pre-event water, already stored in the system at the beginning of the event, flows downslope on vertical and lateral preferential flow paths. Chemical tracers used in a mixing model for hydrograph separation have shown that the pre-event water contribution is up to 30%. Applying this information to a conceptual runoff generation model, the values of δ15N and δ18O in nitrate could be calculated. Field observations showed the occurence of significant microbial denitrification processes above the soil/bedrock interface before nitrate percolates through to the deeper horizon of the vadose zone. The source of nitrate could be determined and denitrification processes were calculated. Assuming that the nitrate reduction follows a Rayleigh process one could approximate a nitrate input concentration of about 170 mg/l and a residual nitrate concentration of only about 15%. The results of the chemical and isotopic tracers postulate fertilizers as nitrate source with some influence of atmospheric nitrate. The combined application of hydrograph separation and determination of isotope values in δ15N and δ18O of nitrate lead to an improved understanding of microbial processes (nitrification, denitrification) in dynamic systems.

Acetate injection into anaerobic settled sludge for biological P-removal in an intermittently aerated reactor

Injecting acetate into the sludge layer during the settling and decanting periods was adopted to enhance phosphorus release inside the sludge layer during those periods and phosphorus uptake during the subsequent aeration period in a KIST Intermittently Decanted Extended Aeration (KIDEA) process. The relationship among nitrification, denitrification and phosphorus removal was investigated in detail and analyzed with a qualitative floc model. Dependencies of nitrification on the maximum DO level during the aerobic phase and phosphorus release on residual nitrate concentration during the settling phase were significant. High degree of nitrification resulted that phosphorus release inside the sludge layer was significantly interfered with nitrate due to the limitation of available acetate and the carbon sources from influent. Such limitation was related to the primary utilization of organic substance for denitrification in the outer layer of the floc and the retarded mass transfer into the inner layer of the floc. Nevertheless, effects of acetate injection on both denitrification and phosphorus release during the settling phase were significant. Denitrification rate after acetate injection was two times as high as that before acetate injection, and phosphorus release reached about 14 mg PO4(3-)-P/g MLVSS/hr during the decanting phase after the termination of denitrification inside the sludge layer. Extremely low level of maximum DO (around 0.5 mg/L) during the aerobic phase may inhibited nitrification, considerably, and thus nearly no nitrate was present. However, the absence of nitrate increased when the phosphorus release rate was reached up to 33 mg PO4(3-)-P/g MLVSS/hr during the settling and decanting phase, and nearly all phosphorus was taken up during subsequent aerobic phase. Since the sludge layer could function as a blocking layer, phosphorus concentrations in the supernatant was not influenced by the released phosphorus inside the sludge layer during the settling and decanting period. Phosphorus removal was directly (for uptake) and indirectly (for release) dependent on the median and maximum DO concentration during the aerobic phase, and those optimal values may exist within the range from 0.2 to 0.6 mg/L and 0.4 to 1.2 mg/L, respectively.

Annual carbon fluxes in the upper Greenland Sea based on measurements and a box-model approach

Measurements of nitrate and the carbonate system parameters performed mainly from 1993 to 1997 have been used to estimate the evolution of the concentration fields over the year in the surface and underlying waters of the central Greenland Sea. This, together with synoptic surface wind data from the NCEP/NCAR reanalysis project, is used to evaluate the vertical mixing, the biological production and decay, as well as the air-sea exchange of CO2 in the region. In the winter season, the vertical mixing dominates the change of the nitrate concentration in the surface water. The mixing factor estimated for this season is used to compute the addition of chemical constituents to the surface water from below. The residual nitrate concentration change, after the mixing contribution has been subtracted, is attributed to biological production or decay. The computations are performed with a 1-day resolution and initially the advective contribution is neglected, as the horizontal gradients in the central Greenland Sea gyre are small. Following this approach, the air-sea flux of CO2 is directed into the sea all year around, with an annual uptake of 53 ± 4 g C m±2 yr±1 for the years 93 to 97. The carbon flux as driven by biology shows a strong primary production peak around Julian day 140 followed by a decrease which turns into decay of organic matter at about day 200. Summarizing the biological activity in the surface water over the year gives a new production of 34 g C m±2 r±1. The vertical flux of dissolved inorganic carbon into the surface water from below amounts to 11 g C m±2 yr±1. The build up of carbon in the surface water, 30 g C m±2 yr±1, is explained by that the temperature of the outflowing water is approximately 2°C colder than the inflowing water, giving a higher dissolved inorganic carbon concentration as a result of the increase in the solubility of carbon dioxide with lower temperatures. The uncertainties in the above stated numbers are ±20%.

Residue management for irrigated maize grain and silage production

Certain farming systems limit the opportunity to leave crop residue on the soil surface which could conserve soil and water. The objectives of this study were to determine the effects of residue management, tillage, and irrigation regime on maize ( Zea Mays L.) grain and silage yields and selected soil properties. The soil type was a Hiwassee clay loam (Rhodic Kanhapludults) and is representative of soils in the southern Piedmont region (USA). Two tillage treatments (no-tillage (NT) and conventional chisel plow/disk (CT)), in factorial combination with three sprinkler irrigation levels (none, full, and limited), were evaluated in 1988 and 1989. Each tillage/irrigation treatment was split to simulate maize stover removal (silage system) or maize stover left in place (grain system). Grain and silage yields for the 2 year period averaged 13% and 17% greater, respectively, for NT compared with CT. Residue removal decreased silage yield in 1989 from 18.9 to 17.5 Mg ha −1. Total and irrigation water use efficiencies were slightly greater in NT plots when residue remained on the soil surface. In general, both sorptivity and cumulative infiltration were greater for CT compared with NT. These same parameters were at least 200% greater in the non-trafficked interrow compared with the trafficked interrow position. Residual nitrate concentrations in the soil profile were less under NT compared with CT, reflecting the higher maize yields because of improved water availability under NT. While only small differences in yields and water use efficiencies were affected by residue management, the differences likely will increase with time under these management systems.

Spatial patterns concentrations in upland Wales in relation to catchment forest cover and forest age

Data on nitrate nitrogen were collected weekly during 1984 from 136 sites on streams in upland Wales. Mean nitrate concentrations in summer (<0.·2 to 1·5 mg litre −1) were significantly lower ( P<0·001) than in winter (<0·2 to 1·26 mg litre t−1), particularly at sites with mature conifers (<30 years old). Mean concentrations increased significantly with the average age of conifers on each catchment ( P<0·001), and with increasing areal cover by trees over 30 years old ( P<0·001). Nitrate concentrations increased significantly with stream total hardness ( P<0·001), possibly reflecting nitrogen mineralisation in soils of higher base status. Concentrations also increased with stream chloride ( P<0·001), which is predominantly atmospherically derived, implying that increased nitrate occurred where general atmospheric inputs of solutes were increased. After accounting for variation in hardness, residual nitrate concentrations still increased with the average age of the conifers ( P<0·001), and with catchment cover by mature trees ( P<0·001). We infer that some additional nitrate under older conifers is thus independent of catchment sources associated with increasing hardness. Two possibilities are increased inputs and decreased retention of nitrogen within the ecosystem of maturing conifer forest. Residual nitrate after accounting for variations in chloride also increased significantly with conifer age ( P<0·01) and cover ( P<0·01), a pattern implying that some sources of nitrate may also be independent of increased sea-salt deposition. We allude to the possibility that additional nitrogen deposition adds to nitrogen throughputs from maturing forests, and we discuss the potential ecological role of additional nitrogen in runoff.