Disappearance Of Nitrate Research Articles

Seasonal phytoplankton development and new production in the central Greenland Sea

Abstract The seasonal development of the phytoplankton in the central Greenland Sea, based on sampling at different time intervals in the period 1993–1995, is consistent with patterns observed in adjacent similar regions. Phytoplankton growth started in early May and developed gradually through May reaching a peak in early June. The spring phytoplankton was mainly composed of diatoms and the prymnesiophyte Phaeocystis pouchetii. Spring primary production rates, as determined by the radioactive carbon method, averaged 752 mg C m-2 d-1 during May and June. The summer situation was characterised by strong stratification and an upper mixed layer of about 30 metres that became completely depleted of nutrients and contained low phytoplankton biomass by August. New production in the growing season from May to August, estimated from the total production and published f-ratios was 52 g C m-2, while new production estimated from the disappearance of nitrate in the same period, along a transect across the central Gr...

Comparison between chemical and isotopic measurements of biological nitrate utilization: further evidence of low new-production levels in the equatorial Pacific

New-production (nitrate uptake) rates in the equatorial Pacific were estimated by parallel measurements of nitrate disappearance from sea water using a colorimetric method and of 15N-labelled nitrate (15NO3 −) incorporation into particulate organic nitrogen (PON) collected on GF/F filters (net nitrate uptake, conventional 15N-tracer method) and Anopore (0.2 μm) membranes. Regression analyses of 74 sample pairs gathered during 12 and 24 h productivity experiments revealed a significant positive relationship between decreasing nitrate level and 15NO3 − accumulation into PON retained on GF/F filters, but the slopes of Model I and Model II regression lines were 1.18 and 1.29, respectively, suggesting that 15 to 22% of 15NO3 − removed from the dissolved fraction were lost to another N-pool. Two possible avenues for the missing 15NO3 − have been examined: uptake by submicron particles passed through the GF/F filters, and loss as dissolved organic nitrogen (DON). Nitrate uptake by small cells not recovered on GF/F filters, could be safely eliminated as a cause of loss, since 15NO3 − uptake rates obtained from 15N entering PON collected on GF/F filters agreed well with those obtained from 15N entering PON collected on Anopore membranes (32 sample pairs). Inspection of the DON pool of 0.2 μm filtrates for excess-15N enrichment (20 samples) revealed that in nitrate-rich waters (equatorial upwelling between 1°N and 10°S), loss of 15NO3 − as DO15N accounted for 2) were probably erroneous; (2) the use of GF/F filters does not result in an underestimation of new production, although it was found to underestimate PON concentrations by up to 60%; (3) in the equatorial upwelling area (1°N to 10°S), which has high ambient nitrate levels (>2000 nmol l−1) but only slight changes in concentration (0 to 80 nmol l−1 d−1), new production is more accurately estimated by the isotopic method than by the chemical method; (4) in subtropical oligotrophic waters (from 11°S southward) with low ambient nitrate levels (0 to 100 nmol l−1), both procedures are appropriate as long as nitrate removal per incubation period is >3 nmol l−1 (lower rates are only detectable with the isotopic method); (5) the traditional 15N-tracer technique does not substantially underestimate net new-production in the equatorial Pacific, and failure to account for the loss of 15NO3 − as DON, i.e. to estimate gross nitrate uptake (gross uptake = net uptake + 15N loss) tends to underestimate new production on an average by only 10%. Overall, the apparent low level of new production in the nitrate-rich area of the central equatorial Pacific seems to be a fact, and may be ascribable to other nutrient (macro and micro) deficiencies and/or to intense in situ recycling of ammonium and nitrate (regenerated production) rather than to inaccurate nitrate uptake rates measured with the classical 15N-tracer technique.

SPRING, POND AND RIVER WATER CHEMISTRIES DERIVED FROM A TEA PLANTATION IN CENTRAL SHIZUOKA PREFECTURE

To investigate the influence of chemical fertilizers on groundwatear and surfacewater neighboring tea plantations in central Shizuoka prefecture, a time series of chemical compositions and nitrogen isotopic ratios within local wells, springs, rivers and ponds were studied.The pH of spring and pond water in some areas maintained basic acidity with neither any noticeable changes in nitrogen isotopic ratios nor any disappearance of nitrate within the ponds and low concentrations of chlorophyll and phosphorus.Therefore, the activities of photosynthesis in the ponds of this area were very low due to low concentrations of phosphorus. However, the pH of spring water in the other areas maintained basic neutrality to alkali with high concentrations of C2+a and SO2-4 and low concentrations of NO-3 because of soil and fertilizer improvement.Concentrations of Al, Co, Ni, Zn and Mn ions in the groundwater and surface water increased with the decrease of pH values.Then, soil and fertilizer improvemenit si important.

Denitrification and a numerical modelling approach for shallow waters.

A numerical model (`DiffDeni') has been developed to describe the disappearance of nitrate from the water column of 10–200 cm deep waters. The disappearance is caused by bacterial denitrification in the sediments. The model employs the molecular diffusion constant, an acceleration factor describing eddy diffusion, and three bacterial growth constants, viz. the inoculum size, the maximum growth rate and the half saturation constant for the hyperbolic process. The values of these system-constants were varied over a wide range. The curves obtained were compared with the curves for well-defined situations, viz. in which diffusion takes place without any or with a complete, immediate reaction. These cases have analytical solutions, and were simulated closely by the model `DiffDeni', though this model is based on different assumptions. It is shown that, when the bacterial growth rate is above a critical value, a negative exponential curve describes the nitrate disappearance well. On the other hand, a more complicated negative exponential equation can be used to describe the first phase of this denitrification in which bacterial activity is low and nitrate behaves as a conservative compound. The change-over period from phase 1 (no reaction) to phase 2 (complete, immediate reaction) which may vary between <1 and 50 days cannot be described analytically (mathematically correctly). The influence of temperature on denitrification is assessed and it is shown that both bacterial activity and diffusion may influence the denitrification rate.

Catalytic hydrogenation of aqueous nitrate solutions in fixed-bed reactors

Liquid-phase hydrogenation using a solid Pd–Cu bimetallic catalyst offers a promising technique for the removal of nitrates from contaminated drinking water. In this study, catalytic nitrate reduction was investigated in isothermal fixed-bed reactors at T = 298 K and atmospheric pressure. Experiments carried out in a bubble-column fixed-bed reactor in the presence of distilled water as a reaction medium, demonstrate that nitrates can be efficiently removed from the liquid-phase, and that the maximum contaminant level for ammonium ions in drinking water is not exceeded. The measured nitrate conversions are considerably influenced by the variation of volumetric flow rate of either the gas- or liquid-phase. The order of magnitude analysis of apparent rate constant and mass transfer coefficients confirms that the observed reaction rate is governed by the mass transfer of hydrogen from the gas- into the bulk liquid-phase. Due to shorter mean residence times, lower nitrate conversions are measured in a trickle-bed reactor. At the given reaction conditions, catalyst particles were directly exposed to the gas-phase in this reactor system, which drastically enhanced ammonia production. When drinking water is used as a reaction medium instead of distilled water, the nitrate disappearance rate as well as reaction selectivity decrease appreciably, which is attributed to the presence of dissolved ionic species. Additionally, traces of nitrites were detected in the reactor effluent.

Nitrate Disappearance during Batch Settling in Sequencing Batch Reactor Activated Sludge Process

Nitrate Disappearance during Batch Settling in Sequencing Batch Reactor Activated Sludge Process

Biodegradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) under nitrate-reducing conditions

Hexahydro-1,3,5-trinitro-1,3,5-triazine is a widely used military explosive that is more commonly known as Royal Demolition Explosive (RDX). Because of concerns with the potential toxicity of RDX, increasingly stringent regulations are anticipated for wastewater discharges from munitions manufacturing facilities. At the Holston Army Ammunition Plant (HAAP), where RDX is currently produced in the U.S., the treatment sequence includes an anoxic filter prior to aerobic filters and activated sludge reactors. The intent of the anoxic filter is to remove the high levels of nitrate that are often present in munitions wastewater, as well as RDX and other nitrated energetic compounds. However, RDX removal across the filter is typically not adequate to meet the anticipated regulatory target, and RDX tends to persist under aerobic conditions. The objective of this study was to evaluate the reason for poor RDX removal in the anoxic filter. Microcosms were set-up with HAAP wastewater and biofilm scraped from the anoxic filter. Nitrate-reducing conditions were readily established, with organics present in the wastewater as well as acetate serving as electron donors. However, as long as nitrate was present, no decrease in RDX (15 μM) occurred. As soon as nitrate was depleted and some primary substrate was still available, RDX was rapidly biotransformed to metabolites that included mononitroso, dinitroso, and trinitroso derivatives. The disappearance of nitrate, followed by biotransformation of RDX, coincided with a decrease in redox potential to below −200 mV. These results were confirmed with an ethanol-grown nitrate-reducing enrichment culture that was inoculated with biofilm from the HAAP anoxic filter. The presence of RDX had no apparent effect on nitrate reduction, whereas RDX biotransformation was completely inhibited until all of the nitrate was consumed. Nitrate removal was achieved by repeated additions of ethanol. Thus, in order to ensure efficient biotransformation of RDX, nitrate removal must be complete, which requires an electron donor supply that exceeds the acceptor capacity of the nitrate.

River-floodplain interactions: nutrient concentrations in the Lower Paraná River

Changes in water chemistry along the lower 500 km stretch of the Parana River were assessed. The water composition of the river and of a representative floodplain marsh were compared. Short-term changes in nutrient concentrations were monitored in marsh enclosures filled with river water in order to simulate the nutrient dynamics under floodplain inundation. Experiments were performed by resuspending river suspended matter (SM) in synthetic river water acidified to floodplain pH values and short term changes in soluble reactive phosphorus (SRP) and calcium recorded. Marsh water exhibited strong depletions of oxygen and nitrate. Dissolved free CO 2 was an order of magnitude higher and pH lower than in river waters. In the marsh, the net heterotrophic metabolism below the water surface seems to be the most plausible explanation for such features. The lack of a significant downstream increase in nitrate concentration in the river, in spite of large cultural inputs, was consistent with the low nitrate concentrations observed in marsh waters and the fast nitrate disappearance in enclosure experiments, suggesting large losses by denitrification. A decrease in SM along the Lower Parana River would indicate high sediment retention within the floodplain. Calcium, bicarbonate, and soluble reactive phosphorus (SRP) concentrations were higher in the marsh than in the river. High marsh SRP concentrations are likely to originate from the weathering of river SM upon sedimentation, in response to the reducing and acidic marsh environment. The observed release of calcium and SRP upon acidification of SM is consistent with the higher contents observed in the marsh. Downstream increases in calcium, bicarbonate, and SRP along the river course suggest the effect of a large water exchange between the river and its floodplain. The downstream increase in the P content of SM was correlated to SRP concentration. Mean inorganic nitrogen/SRP ratio, by weight, decreased from 8 in the upmost sampling site, to 3.6-3.8 close to the river mouth, being 0.6-0.8 in the floodplain marsh. The TOC and POC correlations to hydrometric level and the fact that both parameters attained their maxima at the receding stage after the flood peak, suggest the floodplain origin of the river organic loading. Conductivity, sodium, potassium, chloride, and sulphate increased from Santa Fe to Rosario, without further changes downstream, suggesting an important contribution from the Salado River, a tributary of high salinity that merges with the Parana River downstream from Santa Fe.

Hardness and Salt Effects on Catalytic Hydrogenation of Aqueous Nitrate Solutions

Liquid-phase hydrogenation using a solid Pd–Cu bimetallic catalyst provides a potential technique for the removal of nitrates from waters. In this study, hardness and salt effects on both the nitrate disappearance rate and the reaction selectivity were quantitatively evaluated. Reduction experiments were performed for a wide range of nitrate conversions in an isothermal semibatch slurry reactor. It was discovered in a series of runs using various nitrate salts as a source of nitrate ions that the apparent surface reaction rate constant increases in the order K+<Na+<Ca2+<Mg2+<Al3+and changes proportionally with the ionization potential of a cation present in the aqueous solution. Permanent hardness of drinking water exhibits no inhibitive impact either on the extent of nitrate removal or on reaction selectivity. On the other hand, the nitrate disappearance rate as well as nitrogen production yield decrease appreciably in the presence of hydrogencarbonates, which is attributed to identical structures of nitrate and hydrogencarbonate ions resulting in competitive adsorption of these species to Pd–Cu active sites. A detailed analysis of experimental data confirmed that efficiency of the Pd–Cu bimetallic catalyst in catalytic nitrate reduction is influenced by the migration of produced hydroxide ions from the Helmholtz layer.

Biodegradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) under nitrate-reducing conditions

Hexahydro-1,3,5-trinitro-1,3,5-triazine is a widely used military explosive that is more commonly known as Royal Demolition Explosive (RDX). Because of concerns with the potential toxicity of RDX, increasingly stringent regulations are anticipated for wastewater discharges from munitions manufacturing facilities. At the Holston Army Ammunition Plant (HAAP), where RDX is currently produced in the U.S., the treatment sequence includes an anoxic filter prior to aerobic filters and activated sludge reactors. The intent of the anoxic filter is to remove the high levels of nitrate that are often present in munitions wastewater, as well as RDX and other nitrated energetic compounds. However, RDX removal across the filter is typically not adequate to meet the anticipated regulatory target, and RDX tends to persist under aerobic conditions. The objective of this study was to evaluate the reason for poor RDX removal in the anoxic filter. Microcosms were set-up with HAAP wastewater and biofilm scraped from the anoxic filter. Nitrate-reducing conditions were readily established, with organics present in the wastewater as well as acetate serving as electron donors. However, as long as nitrate was present, no decrease in RDX (15 μM) occurred. As soon as nitrate was depleted and some primary substrate was still available, RDX was rapidly biotransformed to metabolites that included mononitroso, dinitroso, and trinitroso derivatives. The disappearance of nitrate, followed by biotransformation of RDX, coincided with a decrease in redox potential to below −200 mV. These results were confirmed with an ethanol-grown nitrate-reducing enrichment culture that was inoculated with biofilm from the HAAP anoxic filter. The presence of RDX had no apparent effect on nitrate reduction, whereas RDX biotransformation was completely inhibited until all of the nitrate was consumed. Nitrate removal was achieved by repeated additions of ethanol. Thus, in order to ensure efficient biotransformation of RDX, nitrate removal must be complete, which requires an electron donor supply that exceeds the acceptor capacity of the nitrate.

The geochemical effects of benzene, toluene, and xylene (BTX) biodegradation

The geochemical effects of microbially mediated degradation of aromatic hydrocarbons were observed as changes in solution composition of an artificial groundwater in packed-sand laboratory columns. Benzene, toluene, and xylene, both individually and in a combined fashion, were used as substrates in biodegradation experiments conducted under oxygenated and anoxic conditions in columns filled with quartz, calcite, or Fe 3+-coated quartz sand. Typically, column effluent had increased concentrations of dissolved inorganic C, decreased pH, and decreased concentrations of NO 3 and dissolved O 2 relative to column influent. Efficiency of CO 2 generation was similar for the three different substrates, ranging from 22.5 to 26.6% organic C converted to CO 2. When all three substrates were combined, the percentage of CO 2 produced fell within the range observed in the single substrate experiments. Nitrate disappearance was more varied as a function of substrate identity, with greatest amounts lost when toluene was the substrate. Calcite dissolved as a result of CO 2 generated during the biodegradation reactions, and empirically calculated dissolution rates varied between 1.9 and 4.0 x 10 −9 mmol cm −2 s −1. The calcite dissolution rate was slower than the biodegradation rate, as evidenced by excess generation of CO 2 relative to Ca 2+ production. The decrease in pH was less in experiments with calcite present than in those with quartz sand present due to buffering by calcite dissolution. Dissolution of Fe oxyhydroxides was not observed under any experimental conditions.

Hyperproductivity of the Ross Sea (Antarctica) polynya during austral spring

Although satellite data of surface layer pigments have suggested that the daily productivity in the Ross Sea is among the largest found in any marine system, no modern oceanographic cruise has entered the Ross Sea polynya to quantitatively assess the austral spring productivity over time scales of days to weeks. We conducted a cruise to the Ross Sea polynya in November–December, 1994 to measure the contribution of phytoplankton during the austral spring to the annual productivity of the region and found markedly enhanced levels of phytoplankton biomass. Chlorophyll concentrations were greater than 3 µg l−1 in mid‐November, a time when the polynya was covered by a thin (ca. 20–30 cm) layer of ice. Particulate matter concentrations increased through time, and by early December chlorophyll and particulate carbon concentrations exceeded 10 µg l−1 and 53 µmol l−1, respectively. Primary productivity also increased through time: the mean productivity in early December equaled 3.53 g C m−2 d−1, and maximum measured rates exceeded 6 g C m−2 d−1. Productivity based on nitrate disappearance averaged 1.52 g C m−2 d−1 (with a maximum rate of 2.49 g C m−2 d−1), suggesting that the bloom's new production was also substantial. The Ross Sea polynya is the most southerly location in the Antarctic where phytoplankton growth is initiated this early and which supports such high standing stocks by early December. Inclusion of this production in a carbon budget for the region suggests that this area supports an annual production of 200 g C m−2, the largest of any region in the Southern Ocean, and confirms its hyperproductive nature.

Seasonal dynamics of denitrification activity in two water meadows

Seasonal variation in denitrification activity was measured in twoflooded water meadows, one on peaty and one on sandy soil, over a three-yearperiod. Measurements were taken during flooded and drained periods, usingthe acetylene-blockage technique, and the rates were compared to massbalance estimates of nitrate removal in the percolating water.Denitrification activity was higher in sandy soil than in peaty soil. Higherwater infiltration rate and thereby higher nitrate load was considered to bethe cause of the higher denitrification in the sandy soil. Floodingsignificantly increased denitrification, and the rates were higher in autumnand winter than in spring. This was considered to be a result of highernitrogen concentration in inflowing stream water during winter. Annualdenitrification was estimated to 430–460 kg N ha-1yr-1 in the sandy soil meadow, and 220 kg N ha-1yr-1 in the peaty soil meadow. In the sandy soil there was alarge discrepancy between nitrate removal rates and denitrification rates,which can be explained by nitrification of ammonium released from the soil.In the peaty soil nitrate disappearance and denitrification correspondedfairly well.

Kinetics of the catalytic liquid-phase hydrogenation of aqueous nitrate solutions

Liquid-phase reduction using a solid Pd/Cu bimetallic catalyst provides a potential technique for the removal of nitrates from waters. Kinetic measurements were performed for a wide range of reactant concentrations and reaction conditions in an isothermal semi-batch slurry reactor operating at atmospheric pressure. The effects of catalyst loading and initial nitrate concentration on the reaction rate were also investigated. The proposed intrinsic rate expression for nitrate disappearance is based on the conventional Langmuir-Hinshelwood kinetic approach, considering both equilibrium nitrate as well as dissociative hydrogen adsorption processes to different types of active sites, and assuming an irreversible bimolecular surface reaction between adsorbed reactant species to be the rate-controlling step. The apparent activation energy for catalytic liquid-phase nitrate reduction and the heat of nitrate adsorption, in the temperature range 280.5–293 K, were found to be 47 and 22 kJ/mol, respectively. It is confirmed that the process of catalytic liquid-phase hydrogenation of aqueous nitrate solutions undergoes a redox mechanism.

Quantitative assessment of Soil nitrate disappearance and N 2O evolution during denitrification : Nitrate disappearance during denitrification

Nitrate dynamics in the soil environment often confound investigations into the denitrification process and contribute to the extreme variability witnessed. NO 3 − dynamics and evolution of N 2O were investigated in a soil chemically-treated to control the amount and form of carbon, nitrogen and the microbial population. This was achieved by removing soil organic matter with hydrogen peroxide, leaving the soil inorganic skeleton. Treatments were: 1. chemically-treated soil (low in C and N) amended with 500 mg NO 3 −N kg −1 and 600 mg glucose-C kg −1 ( Control); 2. chemically treated soil amended with 500 mg NO 3 −N kg −1 and 600 mg glucose-C kg −1 and a culture of Pseudomonas denitrificans, equivalent to 10 6 denitrifiers g −1 of soil ( Dentreat) and 3. field soil amended with 500 mg NO 3 −N kg −1 and 600 mg glucose-C kg −1 and a culture of P. denitrificans ( Soil). Flasks were purged of O 2, shaken and anaerobically incubated with 10% C 2H 2 (v/v) for 96 h. The N 2O and CO 2 production and NO 3 − and NO 2 − concentrations were monitored. Approximately 10, 150 and 146 mg NO 3 −N kg −1 disappeared from the soil solution in 2 h from the Control, Dentreat and Soil samples respectively. At the same time there was no concurrent increase in soil NO 2 − concentration or N 2O evolution, whilst CO 2 concentrations bore no stoichiometric relationship with N disappearance until the end of the incubation (96 h). The production of N 2O did not reflect the microorganisms' capacity to remove NO 3 − from the soil environment. An approximate 80% balance of NO 3 − disappearance and N 2O evolution was seen for the Dentreat and Soil samples at 96 h respectively. The ability to remove large quantities of NO 3 − quickly under non-limiting conditions and then to utilise this at a later date, may benefit some soil microorganisms. Such an excessive disappearance has implications for the use of in-situ NO 3 − concentrations as a predictor of soil denitrification potential in the field. A proportion of soil N would be temporarily immobilised within the microbial-biomass and could later be denitrified, leading to an underestimate of potential gaseous N loss.

Denitrification kinetics in an activated sludge system

For all water bodies especially for sea, lakes, and ground waters it is very important that nutrient pollution is the least possible (minimal). From municipal and industrial waste waters where considerable amount of nutrients is expected it is possible to remove nutrients with biological treatment. The first step is usually aerobic activated sludge treatment and the second step is combined anaerobic-anoxic-aerobic step in different combinations. A very effective process for nitrogen removal is anoxic activated sludge system. It is very important that in the first aerobic step complete nitrification is achieved. In our laboratory denitrification kinetics was studied. The objective of this study was determination of denitrification velocity and attendance of different parallel phenomena such as nitrite, disappearance of nitrate and nitrite, sulphide formation, etc. Denitrification process was studied in two systems, in closed system (such as in closed reactors for denitrification) and in open systems (such as in open denitrification tanks). We found out that denitrification rate in closed systems was from about 8 to 52 mg/l.h and was dependent on activated sludge type, sludge concentration, and temperature of the system. In open system (such as in an anoxic aeration tank) the rate of denitrification was lower than in closed system; it was from 4 to 15 mg/l.h dependent on activated sludge type, sludge concentration, and temperature of the system. Experiments were carried out in the range of temperature from 20° to 30°C.

An ORP screening protocol for biological phosphorus removal in sequencing batch reactors

This research discusses two strategies for adding acetate to sequencing batch reactors operating as biological removal (Bio-P) systems. The control (fixed-time) reactor adds the acetate at a set time of 1 h 25 min, which is an assumed time for complete denitrification. The experimental (real-time) reactor adds the acetate when a computer detects the disappearance of nitrates, as indicated by a distinctive "breakpoint" or "kink" in the oxidation-reduction potential versus time profile. This control strategy is therefore based upon a known time for complete denitrification. The time-of-occurrence of the nitrate breakpoint is utilized in the development of a screening protocol for interpreting the behaviour (in terms of nitrate reactions) for reactors operating in biological phosphorus removal mode. The protocol involves categorizing the timing of the nitrate breakpoint into two groupings. A "failure" category corresponds to acetate being added prior to the breakpoint, because, in these cases, the acetate is used partially for denitrification and partially for Bio-P carbon storage. A "success" category corresponds to breakpoints occurring prior to the addition of acetate. In such cases, acetate is used solely for carbon storage by Bio-P organisms. Key words: oxidation-reduction potential, biological phosphorus removal, sequencing batch reactor, real-time computer control.

Denitrification kinetics in an activated sludge system

For all water bodies especially for sea, lakes, and ground waters it is very important that nutrient pollution is the least possible (minimal). From municipal and industrial waste waters where considerable amount of nutrients is expected it is possible to remove nutrients with biological treatment. The first step is usually aerobic activated sludge treatment and the second step is combined anaerobic-anoxic-aerobic step in different combinations. A very effective process for nitrogen removal is anoxic activated sludge system. It is very important that in the first aerobic step complete nitrification is achieved.In our laboratory denitrification kinetics was studied. The objective of this study was determination of denitrification velocity and attendance of different parallel phenomena such as nitrite, disappearance of nitrate and nitrite, sulphide formation, etc.Denitrification process was studied in two systems, in closed system (such as in closed reactors for denitrification) and in open systems (such as in open denitrification tanks). We found out that denitrification rate in closed systems was from about 8 to 52 mg/l.h and was dependent on activated sludge type, sludge concentration, and temperature of the system. In open system (such as in an anoxic aeration tank) the rate of denitrification was lower than in closed system; it was from 4 to 15 mg/l.h dependent on activated sludge type, sludge concentration, and temperature of the system. Experiments were carried out in the range of temperature from 20° to 30°C.

A physical estimate of new production in the equatorial Pacific along 150°W

New production was estimated for the equatorial Pacific by quantifying the supply of nitrate into the euphotic zone. The turbulent flux of nitrate, estimated by assuming gradient transport and by direct measurements of dissipation and nitrate concentration along a cross‐equatorial transect, was maximum (1–3 mmol m−3 d−1) at 0° and 1°S. Poleward of the equatorial region, there was little vertical diffusion of nitrate (&lt; 10−3 mmol m−2 d−1). An estimate was made of the two‐dimensional advective balance of nitrate between upwelling and the meridional divergence in the equatorial region. The net supply of nitrate into the euphotic zone (vertical supply minus meridional loss) was 4.3 mmol m−2 d−1 between 1°N and 1°S; between 5°N and 5°S, it was 1.9 mmol m−2 d−1. The total supply of nitrate was larger than the concurrently measured incorporation of nitrate into particles but was comparable to the integrated local rate of nitrate disappearance. The discrepancy between new production estimated from physical supply or from nitrate disappearance and that estimated from nitrate incorporation into particles suggests that uptake by small cells (&lt; 1 µm), dissolved organic N, or both play an important role in the equatorial region.

Influence of dissolved oxygen and oxidation–reduction potential on the denitrification rate of activated sludge

The influence of low concentrations of dissolved oxygen (DO) and the oxidation-reduction potential (ORP) on the denitrification activity of activated sludge has been studied in batchwise experiments. The ORP was maintained at different levels by automatic titration with air and the denitrification activity was determined by following the disappearance of nitrate. Oxygen was found to have a negative effect on denitrification even at lower concentrations than can be measured with conventional oxygen probes (&amp;lt;0.1 mg/L). The ORP was found to be a useful indicator of the DO concentration at this low level and the denitrification rate was found to decrease linearly with increasing ORP. However, the effect of the ORP on denitrification differed between sludges from different treatment plants. A linear relationship was also found between the ORP and the DO concentration in the region of measurable DO concentrations. Extrapolation of this straight line into the region where DO was under the detection limit indicated that oxygen exerts an inhibitory effect on denitrification at such low concentrations as a few μg/L.