High Dissolved Oxygen Concentration Research Articles

Optimization of Culture Conditions for Enhanced Lysine Production Using Engineered Escherichia coli

In this study, culture conditions, including dissolved oxygen (DO) content, presence of osmoprotectants, residual glucose concentration, and ammonium sulfate-feeding strategies, were investigated for decreasing the inhibition effects of acetic acid, ammonium, and osmotic stress on L-lysine fermentation by Escherichia coli. The results revealed that higher DO content and lower residual glucose concentration could decrease acetic acid accumulation, betaine supplementation could enhance osmotic stress tolerance, and variable speed ammonium sulfate-feeding strategy could decrease ammonium inhibition. Thus, with 25 % DO content, 0-5.0 g/L of residual glucose concentration, and 1.5 g/L of betaine supplementation, 134.9 g/L of L-lysine was obtained after 72 h of culture, with L-lysine yield and productivity of 45.4 % and 1.9 g/(L · h), respectively.

Morphological and metabolic shifts of Yarrowia lipolytica induced by alteration of the dissolved oxygen concentration in the growth environment

Yarrowia lipolytica, an ascomycete with biotechnological potential, is able to form either yeast cells or hyphae and pseudohyphae in response to environmental conditions. This study shows that the morphology of Y. lipolytica, cultivated in batch cultures on hydrophilic (glucose and glycerol) and hydrophobic (olive oil) media, was not affected by the nature of the carbon source, nor by the nature or the concentration of the nitrogen source. By contrast, dissolved oxygen concentration (DOC) should be considered as the major factor affecting yeast morphology. Specifically, when growth occurred at low or zero DOC the mycelial and/or pseudomycelial forms predominated over the yeast form independently of the carbon and nitrogen sources used. Experimental data obtained from a continuous culture of Y. lipolytica on glycerol, being used as carbon and energy source, demonstrated that the mycelium-to-yeast form transition occurs when DOC increases from 0.1 to 1.5 mg l(-1). DOC also affected the yeast physiology, as the activity of enzymes implicated in lipid biosynthesis (i.e. ATP-citrate lyase, malic enzyme) was upregulated at high DOC whereas the activity of enzymes implicated in glycerol assimilation (such as glycerol dehydrogenase and kinase) remained fundamentally unaffected in the cell-free extract.

Nitritating-anammox biomass tolerant to high dissolved oxygen concentration and C/N ratio in treatment of yeast factory wastewater

Maintaining stability of low concentration (<1 g L−1) floccular biomass in the nitritation-anaerobic ammonium oxidation (anammox) process in the sequencing batch reactor (SBR) system for the treatment of high COD (>15,000 mg O2 L−1) to N (1680 mg N L−1) ratio real wastewater streams coming from the food industry is challenging. The anammox process was suitable for the treatment of yeast factory wastewater containing relatively high and abruptly increased organic C/N ratio and dissolved oxygen (DO) concentrations. Maximum specific total inorganic nitrogen (TIN) loading and removal rates applied were 600 and 280 mg N g−1 VSS d−1, respectively. Average TIN removal efficiency over the operation period of 270 days was 70%. Prior to simultaneous reduction of high organics (total organic carbon>600 mg L−1) and N concentrations>400 mg L−1, hydraulic retention time of 15 h and DO concentrations of 3.18 (±1.73) mg O2 L−1 were applied. Surprisingly, higher DO concentrations did not inhibit the anammox process efficiency demonstrating a wider application of cultivated anammox biomass. The SBR was fed rapidly over 5% of the cycle time at 50% volumetric exchange ratio. It maintained high free ammonia concentration, suppressing growth of nitrite-oxidizing bacteria. Partial least squares and response surface modelling revealed two periods of SBR operation and the SBR performances change at different periods with different total nitrogen (TN) loadings. Anammox activity tests showed yeast factory-specific organic N compound-betaine and inorganic N simultaneous biodegradation. Among other microorganisms determined by pyrosequencing, anammox microorganism (uncultured Planctomycetales bacterium clone P4) was determined by polymerase chain reaction also after applying high TN loading rates.

Response of dissolved oxygen and related marine ecological parameters to a tropical cyclone in the South China Sea

It is well known that tropical cyclones can cause upwelling, decrease of sea surface temperature, increase of chlorophyll-a (Chl-a) concentration and enhancement of primary production. But little is known about the response of dissolved oxygen (DO) concentration to a typhoon in the open ocean. This paper investigates the impact of a typhoon on DO concentration and related ecological parameters using in situ and remote sensing data. The in situ data were collected 1week after the passage of the super-typhoon Nanmadol in the northern South China Sea in 2011. An increase in DO concentration, accompanied by a decrease in water temperature and an increase in salinity and Chl-a concentration, was measured at sampling stations close to the typhoon track. At these stations, maximum DO concentration was found at a depth of around 5m and maximum Chl-a concentration at depths between 50 and 75m. The layer of high DO concentration extends from the surface to a depth of 35m and the concentrations stay almost constant down to this depth. Due to the passage of the typhoon, also a large sea level anomaly (21.6cm) and a high value of Ekman pumping velocity (4.0×10−4ms−1) are observed, indicating upwelling phenomenon. At the same time, also intrusion of Kuroshio waters in the form of a loop current into the South China Sea (SCS) was observed. We attribute the increase of DO concentration after the passage of the typhoon to three effects: (1) entrainment of oxygen from the air into the upper water layer and strong vertical mixing of the water body due to the typhoon winds, (2) upwelling of cold nutrient-rich water which stimulates photosynthesis of phytoplankton and thus the generation of oxygen, which also increases the DO concentration due to cold water since the solubility of oxygen increase with decreasing water temperature, and, possibly, (3) transport of DO enriched waters from the Western Pacific to the SCS via the intrusion of Kuroshio waters.

Bathymetric and Geochemical Analysis of Lake Al-Saad, Abha, Kingdom of Saudi Arabia Using Geoinformatics Technology

This study investigates the potential for remote sensing of lake water bathymetry and geochemi- cal by 1) examining the empirical based technique for retrieving depth information from passive optical image worldview-2 satellite data, 2) performing atmospheric correction, 3) assessing the accuracy of spectrally based depth retrieval under field condition via field measurement, 4) pro- ducing bathometry and geochemistry mapping by examining spectral variations for identifying pairs of wavelengths that produce strong linear correlation coefficient between the band ratio. The results indicate that optical remote sensing of bathymetry and geochemical investigation is not only feasible but more accurate under conditions of typical lake water, supporting field survey. The Pearson correlation matrix (R) between the examined water samples/depth and the TOA re- flectance values of the worldview-2 (WV-2) satellite data have been investigated and found good correlation. The models developed using the combination of different band pairs also show high accuracy. Cartographical maps were generated depending on the linear correlation coefficient between the measured parameters and the TOA reflectance values of the worldview-2 data. The investigation shows that dissolved oxygen (DO) of the lake water is slight lower than the permis- sible limit of Saudi standards for lake water. The shallow water has high DO concentration, whereas the deeper shows significantly lower down. Electrical conductivity measurements serve as a useful indicator of the degree of mineralization in the water sample. All the samples which have EC exceed limit. The spatial distribution of EC and TDS inferred that the EC and TDS concen- tration is the highest at the eastern part of the lake whereas concentration drops down towards the southern side. This study confirms that remote sensing incorporated with GIS and GPS could * Corresponding author.

Development of crustacean plankton in a shallow, polyhumic reservoir in the first 20 years after impoundment (northeast Poland)

Multiannual changes of structure and biomass of crustacean zooplankton were studied in the shallow, lowland, polyhumic Siemianówka Reservoir on the upper Narew River in northeast Poland. High ammonium and soluble reactive phosphorus ion availability for phytoplankton, low water transparency, and high dissolved oxygen concentration resulted in humoeutrophication and caused an intensive development of summer cyanobacteria. We compiled and analyzed long-term (1993–2011) crustacean zooplankton community data and chlorophyll a concentrations. Zooplankton biomass and structure in the summer were related to the intensity of the cyanoprokaryota blooms. As chlorophyll a concentration increased, crustacean zooplankton biomass decreased and smaller crustacean species dominated the zooplankton structure. Large species of crustaceans (Daphnia longispina, D. magna, and Eudiaptomus graciloides) disappeared from the reservoir after filamentous cyanobacteria dominated the phytoplankton. Long-term studies suggest that phytoplankton with strong dominance of filamentous cyanobacteria shape the crustacean community but are probably an insignificant food source for the crustacean zooplankton in the Siemianówka Reservoir.

유동가속부식에 영향을 미치는 수화학 인자 : pH, 용존산소, 하이드라진

Flow accelerated corrosion(FAC) of the carbon steel piping in pressurized water reactors(PWRs) has been major issue in nuclear industry. Severe accident at Surry Unit 2 in 1986 initiated the worldwide interest in this area. Major parameters influencing FAC are material composition, microstructure, water chemistry, and hydrodynamics. Qualitative behaviors of FAC have been well understood but quantitative data about FAC have not been published for proprietary reason. In order to minimize the FAC in PWRs, the optimal method is to control water chemistry factors. Chemistry factors influencing FAC such as pH, corrosion potential, and hydrazine contents were reviewed in this paper. FAC rate decreased with pH up to 10 because magnetite solubility decreased with pH. Corrosion potential is generally controlled dissolved oxygen (DO) and hydrazine in secondary water. DO increased corrosion potential. FAC rate decreased with DO by stabilizing magnetite at low DO concentration or by formation of hematite at high DO concentration. Even though hydrazine is generally used to remove DO, hydrazine itself thermally decomposed to ammonia, nitrogen, and hydrogen raising pH. Hydrazine could react with iron and increased FAC rate. Effect of hydrazine on FAC is rather complex and should be careful in FAC analysis. FAC could be managed by adequate combination of pH, corrosion potential, and hydrazine.

&lt;b&gt;Structure of planktonic ciliates community (Protist, Ciliophora) from an urban lake of southern Brazil&lt;/b&gt; - doi: 10.4025/actascibiolsci.v35i4.18579

This study examined temporal and spatial (vertical) variations of composition, richness and abundance of the planktonic ciliates community in an urban and eutrophic lake, as well as identified the limnological factors involved in determining the patterns observed. To this end, samples of ciliates and measures of limnological variables were taken at different depths and hydrological periods. 35 ciliate species were identified, among which 14 species occurred in all periods and strata, being Prostomatida the most specious, followed by Gymnostomatida, Oligotrichida, Peritrichida, and Scuticociliatida. The patterns found for composition, abundance and species richness evidenced a vertical and temporal variation of these attributes. However, in general the composition and species richness have varied more along the vertical gradient than between periods, whereas for the abundance, only temporal differences could be detected. Results observed herein indicated that benthic species seem to be mainly influenced by the amount of organic matter, phosphorus and ammonia, while planktonic ones, abundant at the surface, were mainly associated with higher dissolved oxygen concentrations. In this way, seasonal and vertical variations of limnological characteristics were decisive for the patterns of occurrence and abundance of ciliate species.

A simple water quality model as a tool for the evaluation of alternative river basin management plans

&lt;p&gt;A key component in the implementation of Water Framework Directive is the development of a river basin management plan for each river basin district. Water quality models are important tools to test the effectiveness of alternative management plans on the water quality of the respective water bodies. The main objective of the present study was to develop and demonstrate the use of a rather simple water quality model as a tool for the evaluation of alternative water management scenarios for the river basin district of Evrotas. Furthermore an extension of the water quality model based on Monte Carlo simulation to provide for uncertainty identification is also exhibited. The model is based on the basic principles of the Streeter-Phelps model. A hierarchical approach was developed in order to delineate a complex hydrographic network into a series of water bodies being connected by convective terms. The pollution loads which were used as input data were related both to point sources and non-point sources. Based on the results a substantial removal of organic carbon load originating from the industrial activities is needed in order to achieve high dissolved oxygen concentrations throughout the entire hydrosystem even at the dry period.&lt;/p&gt;

Effect of process parameters and operational mode on nitrous oxide emissions from a nitritation reactor treating reject wastewater

Nitrous oxide (N2O) and methane emissions were monitored in a continuous granular airlift nitritation reactor from ammonium-rich wastewater (reject wastewater). N2O emissions were found to be dependent on dissolved oxygen (DO) concentration in the range of 1–4.5 mg O2/L, increasing within this range when reducing the DO values. At higher DO concentrations, N2O emissions remained constant at 2.2% of the N oxidized to nitrite, suggesting two different mechanisms behind N2O production, one dependent and one independent of DO concentration. Changes on ammonium, nitrite, free ammonia and free nitrous acid concentrations did not have an effect on N2O emissions within the concentration range tested. When operating the reactor in a sequencing batch mode under high DO concentration (>5 mg O2/L), N2O emissions increased one order of magnitude reaching values of 19.3 ± 7.5% of the N oxidized. Moreover, CH4 emissions detected were due to the stripping of the soluble CH4 that remained dissolved in the reject wastewater after anaerobic digestion. Finally, an economical and carbon footprint assessment of a theoretical scaled up of the pilot plant was conducted.

Effect of salinity on N2O production during shortcut biological nitrogen removal from landfill leachate

Three identical SBR adapted to different salinity were applied to investigate the characteristics of the treatment performance and N2O production [Formula: see text] during shortcut biological nitrogen removal from landfill leachate under various operating parameters. Increase of salinity might deteriorate the activity of the microorganisms leading to the increase of [Formula: see text] , however, the system could be gradually adapted to the inhibition and alleviate the detrimental effect to some extent. The system acclimated to high salinity provided better performance under high salinity shock and a lower possibility of [Formula: see text] , while a sudden decrease in salinity can cause a temporary increase in [Formula: see text] . High salinity strengthened the influence of high ammonia nitrogen concentration and low DO concentration on [Formula: see text] while the strengthening effect was unconspicuous at high DO concentration. The anoxic phase did not produce a significant amount of N2O even at the lowest C/N ratio of 0.5 and was less susceptible to salinity. Characterization of the biomass composition using fluorescence in situ hybridization analysis confirmed that the relative proportion of Nitrosomonas europaea was increased with the increase of the salinity, which may be an important factor for the strengthening effect of salinity on [Formula: see text] .

Development of a sixth-generation down-flow hanging sponge (DHS) reactor using rigid sponge media for post-treatment of UASB treating municipal sewage

A sixth-generation down-flow hanging sponge reactor (DHS-G6), using rigid sponge media, was developed as a novel aerobic post-treatment unit for upflow anaerobic sludge blanket (UASB) treating municipal sewage. The rigid sponge media were manufactured by copolymerizing polyurethane with epoxy resin. The UASB and DHS system had a hydraulic retention time (HRT) of 10.6 h (8.6 h for UASB and 2 h for DHS) when operated at 10-28 °C. The system gave reasonable organic and nitrogen removal efficiencies. The final effluent had a total biochemical oxygen demand of only 12 mg/L and a total Kjeldahl nitrogen content of 6 mg/L. The DHS reactor gave particularly good nitrification performance, which was attributed to the new rigid sponge media. The sponge media helped to provide a sufficient HRT, and retained a high biomass concentration, extending the solids retention time. The DHS reactor maintained a high dissolved oxygen concentration under natural ventilation.

동시 질산화-탈질(SND) 반응을 적용한 MBR 반응조에서 질소 및 인 제거 특성

Abstract : Simultaneous nitrification and denitrification (SND) occurs concurrently in the same reactor under micro dissolved oxygen (DO) conditions. Anaerobic zone was applied for phosphorus release prior to an aerated membrane bio-reactor (MBR), and anoxic zone was installed by placing a baffle in the MBR for enhancing denitrification even in high DO concentration in the MBR. Phosphorus removal was tested by alum coagulation in the anaerobic reactor preceding to MBR. DO concentration were 2.0, 1.5, 1.0, 0.75 mg/L in the MBR at different operating stages for finding optimum DO concentration in MBR for nitrogen removal by SND. pH was maintained at 7.0~8.0 without addition of alkaline solution even with alum addition due to high alkalinity in the raw sewage. Both TCODcr and NH 4+ -N removal efficiency were over 90% at all DO concentration. TN removal efficiencies were 50, 51, 54, 66% at DO concentration of 2.0, 1.5, 1.0, 0.75 mg/L, respectively. At DO concentration of 0.75 mg/L with addition of alum, TN removal efficiency decreased to 54%. TP removal efficiency increased from 29% to 95% by adding alum to anaerobic reactor. The period of chemical backwashing of the membrane module increased from 15~20 days to 40~50 days after addition of alum.

Limited denitrification in glacial deposit aquifers having thick unsaturated zones (Long Island, USA)

The goal of this study was to demonstrate how the extent of denitrification, which is indirectly related to dissolved organ carbon and directly related to oxygen concentrations, can also be linked to unsaturated-zone thickness, a mappable aquifer property. Groundwater from public supply and monitoring wells in Northport on Long Island, New York state (USA), were analyzed for denitrification reaction progress using dissolved N2/Ar concentrations by membrane inlet mass spectrometry. This technique allows for discernment of small amounts of excess N2, attributable to denitrification. Results show an average 15 % of total nitrogen in the system was denitrified, significantly lower than model predictions of 35 % denitrification. The minimal denitrification is due to low dissolved organic carbon (29.3–41.1 μmol L−1) and high dissolved oxygen concentrations (58–100 % oxygen saturation) in glacial sediments with minimal solid-phase electron donors to drive denitrification. A mechanism is proposed that combines two known processes for aquifer re-aeration in unconsolidated sands with thick (>10 m) unsaturated zones. First, advective flux provides 50 % freshening of pore space oxygen in the upper 2 m due to barometric pressure changes. Then, oxygen diffusion across the water-table boundary occurs due to high volumetric air content in the unsaturated-zone catchment area.

Modeling of Dissolved Oxygen Applying Stepwise Regression and a Template-Based Fuzzy Logic System

This paper develops a template-based fuzzy inference system (TFIS) capable of simulating the dissolved oxygen (DO) concentration of an example stream by using daily data (2009–2012). Stepwise regression (SR) analysis and Mallows’ Cp statistic were employed to select the best set of independent parameters for the input vector of the TFIS and the regressive models. The most effective inputs were determined for temperature and specific conductivity (P-value=0.000), whereas flow rate (P-value=0.002) and pH (P-value=0.004) were found to be less effective parameters. The TFIS and SR models undersimulated the magnitude of high DO concentrations (>12.5 mg/L), whereas the low and medium DO concentrations were oversimulated. However, they were closer to the observed data. A comparison of the prediction accuracy of the TFIS and SR methods indicated that the TFIS approach was more accurate in simulating DO concentrations, except for low concentrations of DO (<10 mg/L). The TFIS method was found to be superior to the conventional SR model.

Analysis of factors affecting the performance of partial nitrification in a sequencing batch reactor

The objective of this study was to analyze the factors affecting the performance of partial nitrification in a sequencing batch reactor. During a 140-day long-term operation, influent pH value, dissolved oxygen (DO), and chemical oxygen demand/nitrogen (COD/N) ratio were selected as operating factors to evaluate the maintenance and recovery of nitrite accumulation. Results showed that high DO concentration (2-4 mg/L) could damage nitrite accumulation immediately. However, nitrite accumulation ratio (NAR) could be increased from 1.68 ± 1.51 to 35.46 ± 7.86% when increasing the pH values from 7.5 to 8.3 due to the increased free ammonia concentration. Afterwards, stable partial nitrification and high NAR could be recovered when the reactor operated under low DO concentration (0.5-1.0 mg/L). However, it required a long time to recover the partial nitrification of the reactor when the influent COD/N ratios were altered. Fluorescence in situ hybridization analysis implied that ammonium oxidizing bacteria were completely recovered to the dominant nitrifying bacteria in the system. Meanwhile, sludge volumetric index of the reactor gradually decreased from 115.6 to 56.6 mL/g, while the mean diameter of sludge improved from 74.57 to 428.8 μm by using the strategy of reducing settling time. The obtained results could provide useful information between the operational conditions and the performance of partial nitrification when treating nitrogen-rich industrial wastewater.

The effects of backfilling and sealing the waste rock on water quality at the Kimheden open-pit mine, northern Sweden

Evaluating the water quality at reclaimed mines affected by acid mine drainage is an essential step in assessing and improving the performance of mitigation techniques. At the Kimheden copper mine in northern Sweden, reclamation involved the progressive backfilling of waste rock into the two small open pits and, in 1996, the application of a till dry cover that included a sealing layer. The data from both the long-term water quality monitoring by the mining company and the repeated sampling of the surface water and groundwater in 2009 and 2010 were used to assess the success of the reclamation in mitigating the acid mine drainage production from the mine waste.A substantial decrease in the concentrations of copper, zinc and sulphate ions by 95%, 81% and 81%, respectively followed by a rapid stabilisation of element concentrations was observed at the outlet of the receiving stream since early reclamation times. This trend initially suggested successful results for the reclamation, though another explanation for the diminution of contaminant release through depletion of the limited sulphidic source could not be neglected. However, in spite of the decrease, post-reclamation metal concentrations in the stream are still not satisfactory for the discharge of the mine drainage into the natural environment, indicating that the mitigation measures were insufficient. Seepage from one of the pits in 2009 had dissolved copper, aluminium and zinc concentrations of 1.6mg/L, 4.4mg/L and 0.45mg/L, respectively and a pH of 3.0. Relatively high dissolved oxygen concentrations in the groundwater of the backfill in 2009 and 2010 (>2mg/L) suggest that the mixed outcome of the mitigation actions is due to on-going oxidation in the backfilled waste rock despite the dry cover. Moreover, stream discharge and dissolved sulphate and magnesium used as natural tracers in the drainage showed that water management in the form of ditches is not appropriate. In particular, due to poor sealing of the ditches, whilst a measurable part of the contaminated drainage in the collecting ditch is leaking to groundwater and dispersing in the surrounding natural areas, the water discharged to a treatment pond at the outlet of the stream is mostly uncontaminated background water.

Spatiotemporal Predictive Models for Juvenile Southern Flounder in Texas Estuaries

AbstractSouthern Flounder Paralichthys lethostigma supports a multimillion dollar commercial and recreational fishery in the Gulf of Mexico. Despite its economic importance, the Southern Flounder population has been declining for decades. To improve the management of this fishery, both population trends and changes in environmental conditions need to be considered. Using two different statistical modeling techniques, boosted regression tree (BRT) and artificial neural network (ANN), a 29‐year fisheries‐independent record of juvenile Southern Flounder abundance in Texas was examined to illustrate how environmental factors influence the temporal and spatial distribution of juvenile Southern Flounder. Boosted regression trees show the presence of juvenile Southern Flounder is closely associated with relatively low temperatures, low salinity levels, and high dissolved oxygen concentrations. Both ANN and BRT models resulted in high predictive performance with slight spatial differences in predicted distribution. Both models suggested high probability of occurrence in Galveston Bay and East Matagorda Bay. The ANN accurately predicted higher probability of occurrence in Sabine Lake compared with the BRT model. Our results will provide tools for fisheries managers to enhance management and sustainability of the Southern Flounder population. Moreover, these results also identify a predictive framework for proactive approaches to ecosystem management by providing more data to identify essential habitat features and understanding relationships between abiotic and biotic factors within those habitats.Received October 16, 2012; accepted May 22, 2013

Cyanobacteria in wetlands of the industrialized Sambalpur District of India

BackgroundCyanobacteria are common components of phytoplankton communities in most freshwater ecosystems. Proliferations of cyanobacteria are often caused by high nutrient loading, and as such can serve as indicators of declining water quality. Massive industrialization in developing countries, like India, has polluted fresh water bodies, including wetlands. Many industries directly discard their effluents to nearby water sources without treatment. In the Sambalpur District of India effluents reach the reservoir of the worlds largest earthen dam i.e Hirakud Dam. This study examines cyanobacteria communities in the wetlands of Sambalpur District, Odisha, India, including areas subjected to industrial pollution.Result & DiscussionThe genera Anabaena, Oscillatoria, Chroococcus, Phormidium were dominant genera of polluted wetlands of Sambalpur districts. A positive correlation was found between total cyanobacterial species and dissolved oxygen levels, but cyanobacterial diversity was inversely related to BOD, COD, TSS, and TDS. High dissolved oxygen content was also associated with regions of lower cyanobacteria biomass.ConclusionCyanobacterial abundance was positively correlated to content of oxidisable organic matter, but negatively correlated to species diversity. Lower dissolved oxygen was correlated to decreased diversity and increased dominance by Anabaena, Oscillatoria, Chroococcus, Phormidium species, observed in regions characterized by deteriorated water quality.

The Confounding Effect of Nitrite on N2O Production by an Enriched Ammonia-Oxidizing Culture

The effect of nitrite (NO2(-)) on the nitrous oxide (N2O) production rate of an enriched ammonia-oxidizing bacteria (AOB) culture was characterized over a concentration range of 0-1000 mg N/L. The AOB culture was enriched in a nitritation system fed with synthetic anaerobic digester liquor. The N2O production rate was highest at NO2(-) concentrations of less than 50 mg N/L. At dissolved oxygen (DO) concentration of 0.55 mg O2/L, further increases in NO2(-) concentration from 50 to 500 mg N/L resulted in a gradual decrease in N2O production rate, which maintained at its lowest level of 0.20 mg N2O-N/h/g VSS in the NO2(-) concentration range of 500-1000 mg N/L. The observed NO2(-)-induced decrease in N2O production was even more apparent at increased DO concentration. At DO concentrations of 1.30 and 2.30 mg O2/L, the lowest N2O production rate (0.25 mg N2O-N/h/g VSS) was attained at a lower NO2(-) concentration of 200-250 mg N/L. These observations suggest that N2O production by the culture is diminished by both high NO2(-) and high DO concentrations. Collectively, the findings show that exceedingly high NO2(-) concentrations in nitritation systems could lead to decreased N2O production. Further studies are required to determine the extent to which the same response to NO2(-) is observed across different AOB cultures.