Effect Of Dissolved Oxygen Concentration Research Articles

Determining SCC resistance of stainless steel claddings in high- temperature water by constant load crack growth tests and slow strain rate tests

Stress corrosion cracking (SCC) behaviors of 309 L and 308 L weld overlays are characterized in oxygen-free and oxygen-bearing high-temperature water using slow strain rate tests (SSRTs) and crack growth rate (CGR) tests with compact tension specimens. There is no apparent SCC indication on 308 L, while local SCC is observed on 309 L after SSRT in oxygen-free water. The SCC sensitivities of 308 L and 309 L increase with the increase of dissolved oxygen (DO) content, while 308 L shows relatively higher SCC resistance than 309 L in water with 200 ppb and 8000 ppb DO. SSRT and CGR test results show a similar effect of DO concentration on SCC of 309 L and 308 L. The fracture surface of 309 L exhibits extensive intergranular SCC, while 308 L shows only local interdendritic SCC after CGR tests. The crack tip blunting decreased the CGR in the low alloy steel. The highly island-shaped δ-ferrites and moderate strain hardening in 308 L contribute to its SCC resistance over 309 L.

Effect of regulating dissolved oxygen concentration in pulp with aerated gas on pyrite flotation

Moderate oxidation is beneficial to the flotation of sulfide ores, but the exact dissolved oxygen (DO) concentration suitable for flotation is still unclear. In this work, a self–made vacuum aeration flotation equipment was designed and assembled to accurately control the DO concentration in pulp by adjusting the relative proportion of O2/N2 during aeration, and the effect of DO concentration on the floatability and surface properties of pyrite as well as the interaction between pyrite and xanthate was studied. Microflotation experiments, contact angle, time–of–flight secondary ion mass spectrometry, X–ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, electrochemical and localized electrochemical impedance spectroscopy measurements were conducted in this investigation. The results showed that the flotation recovery of pyrite could be remarkably affected by DO concentration in pulp; it increased first and then decreased with the increase of DO concentration; an optimal DO concentration (DO = 3.3 mg/L) was helpful for pyrite flotation recovery; both the formation of dixanthogen and the avoidance of excessive iron oxide/hydroxide formed on pyrite surface were the key to improve the pyrite recovery. This research is expected to be applied in optimizing the flotation separation by DO-pH combined detection and regulation, strengthening the green flotation of sulfide ore with low collectors, and the design and manufacture of new closed and efficient sulfide ore flotation equipment.

Effect of dissolved oxygen concentration on performance and mechanism of simultaneous nitrification and denitrification in integrated fixed-film activated sludge sequencing batch reactors

Integrated fixed-film activated sludge (IFAS) is a superior system for achieving simultaneous nitrification and denitrification (SND), however, the impact of dissolved oxygen (DO) has not been fully elucidated. Therefore, this study investigated the effect of DO concentration on performance and mechanism of SND in IFAS system. Results showed that IFAS outperformed control systems and achieved optimal SND performance at a DO concentration of 0.5 mg/L, with an SND efficiency of 88.51% and total nitrogen removal efficiency of 82.78%. Typical cycles analysis demonstrated limited-DO promoted SND performance. Further analysis implied biofilms exhibited high biomass and denitrification activity with decreasing DO. Microbial community analysis revealed low DO concentrations were responsible for abundant functional groups and genes associated with SND and promoted unconventional nitrogen removal pathways. Moreover, co-occurrence network analysis elucidated microbial interactions, responses to DO, and keystone genera. This study helps understanding the roles of DO for enhanced SND in IFAS.

New insights into arsenate removal during siderite oxidation by dissolved oxygen

Nowadays, arsenic (As) pollution in aquatic environments severely threatens the health of human beings. Although it has been known that siderite is capable of As adsorption and dissolved oxygen (DO) enhances the adsorption, effects of DO concentrations on As(V) adsorption onto siderite remain elusive. In this study, As(V) removal was investigated by synthesized siderite from aqueous solutions with different DO concentrations. Arsenic(V) adsorption kinetics were conformed to the pseudo-second-order model. As(V) adsorption onto siderite was enhanced in the presence of dissolved oxygen, but the excess DO concentration did not increase As(V) adsorption since Fe(III) oxides were coated onto the pristine siderite surface, preventing the mineral from further oxidation. With the increase in DO concentration, the rate of Fe(II) oxidation decreased, which was the kinetic-limited step during As(V) removal by siderite with the presence of DO. The theoretically generated Fe(III) was stoichiometrically proportional to the consumed oxygen. Microscopic characteristics by means of XRD, SEM, TEM, FTIR and XPS indicated that the adsorption was dominated by the chemical process via the As(V) complexation with siderite and co-precipitation with produced Fe(III) oxides. This study reveals the mechanisms of As(V) adsorption during siderite oxidation under different DO concentrations and emphasizes the importance of siderite oxidation in As(V) fate in aqueous systems.

Effect of dissolved oxygen on nitrogen and phosphorus fluxes from lake sediments and their thresholds based on incubation using a simple and stable dissolved oxygen control method

AbstractLaboratory incubation of intact sediments is a popular and useful method for obtaining nutrient fluxes from lake sediments to the overlying water. However, one of the main problems facing this method is that the lake sediment core itself consumes oxygen from the overlying water due to the decomposition of organic matter. In this study, addition of headspace with controlled oxygen concentration and circulation of overlying water to the traditional intact sediment incubation method allows for incubation with a stable dissolved oxygen (DO) concentration in the overlying water. In 24 columns incubated for 179–269 h, the mean DO concentration in the overlying water ranged from 0.35 to 9.95 mgO2 L−1. In most cases, stability was high, with the standard deviation of the fluctuation in DO concentration less than 0.1 mgO2 L−1. The DO concentration of the overlying water could be adjusted by changing the partial pressure of oxygen in the headspace. This method supported a detailed description of the relationship between DO concentration and nutrient fluxes from sediments. The soluble reactive phosphorus fluxes and first‐order reaction constants of NO2− + NO3− fluxes increased rapidly when the DO concentration decreased below the thresholds of 1.5 and 3.0 mgO2 L−1, respectively, in Lake Biwa. The thresholds and precise relationships between DO concentration and nutrient fluxes provide useful information for predicting the effect of DO concentration decreases in the lake bottom on nutrient release from sediments. Our results indicate that this incubation method is a powerful tool for clarifying those relationships.

Fermentative Production of Fructo-Oligosaccharides Using Aureobasidium pullulans: Effect of Dissolved Oxygen Concentration and Fermentation Mode.

Fructo-oligosaccharides (FOS) are prebiotics with numerous health benefits. So far, the dissolved oxygen (DO) concentration control strategy for fermentative production of FOS is still unknown. In order to improve FOS production, the effects of DO concentration and fermentation mode on FOS using Aureobasidium pullulans were investigated in this study. The greatest FOS production (123.2 ± 6.2 g/L), with a yield of 61.6% ± 3.0% (g FOS/g sucrose), was obtained in batch culture under high DO concentration. Furthermore, repeated-batch culture revealed that enzyme production and FOS production were not closely associated with cell growth. By keeping the DO concentration above 5% in the repeated-batch culture, a maximum FOS concentration of 548.3 ± 37.4 g/L and yield of 68.6% ± 2.6% (g FOS/g sucrose) were obtained, which were 3.45% and 11.4% times higher than those obtained in the batch culture without DO control, respectively. Additionally, the ratios of 1-fructofuranosyl nystose (GF4) and 1,1,1,1-kestohexose (GF5) were 33.8% and 23.2%, respectively, in the product of repeated-batch culture, but these compounds were not detected in batch culture. Thus, it can be concluded that the DO concentration affects not only the yield of FOS but also the composition of FOS with different degrees of polymerization, which is the key factor in the fermentative production of FOS with a high polymerization degree.

Effects of C/N ratio and dissolved oxygen on aerobic denitrification process: A mathematical modeling study

COD to ammonium nitrogen (C/N) ratio and dissolved oxygen (DO) concentration are the most important factors affecting aerobic denitrification process, however, the effects of those on the mix-cultured aerobic denitrification process are still ambiguous. A mathematical model based on the framework of activated sludge model No. 3 (ASM3) was proposed for simulating nitrogen removal in an aerobic denitrification SBR process via anoxic/aerobic denitrification. AQUASIM 2.1G was employed for parameter estimation, sensitivity analysis and model calibration, as well as model validation. Ultimately, the impacts of the C/N ratio and the DO concentration on the aerobic denitrification process were revealed by the validated model. The model proposed well described nitrogen removal in an aerobic denitrification SBR process. The total nitrogen (TN) removal efficiency of the process increased with the increasing of C/N ratio and the decreasing of DO concentration. C/N ratio impacted the synthesis of cell internal storage products (XSTO), and the effects of DO concentration on the process resulted from the competition with substrate between heterotrophs and aerobic denitrifiers. High C/N ratio was preferred, however, the DO concentration should be maintained at a relatively lower level under the premise of ensuring the aerobic condition.

The effect of dissolved oxygen concentration on long-term stability of partial nitrification process

Dissolved oxygen (DO) concentration is regarded as one of the crucial factors to influence partial nitrification process. However, achieving and keeping stable partial nitrification under different DO concentrations were widely reported. The mechanism of DO concentration influencing partial nitrification is still unclear. Therefore, in this study two same sequencing batch reactors (SBRs) cultivated same seeding sludge were built up with real-time control strategy. Different DO concentrations were controlled in SBRs to explore the effect of DO concentration on the long-term stability of partial nitrification process at room temperature. It was discovered that ammonium oxidation rate (AOR) was inhibited when DO concentration decreased from 2.5 to 0.5 mg/L. The abundance of Nitrospira increased from 1011.5 to 1013.7 copies/g DNA, and its relative percentage increased from 0.056% to 3.2% during 190 operational cycles, causing partial nitrification gradually turning into complete nitrification process. However, when DO was 2.5 mg/L the abundance of Nitrospira was stable and AOB was always kept at 1010.7 copies/g DNA. High AOR was maintained, and stable partial nitrification process was kept. Ammonia oxidizing bacteria (AOB) activity was significantly higher than nitrite oxidizing bacteria (NOB) activity at DO of 2.5 mg/L, which was crucial to maintain excellent nitrite accumulation performance.

Influence of dissolved oxygen on the bioleaching efficiency under oxygen enriched atmosphere

The use of oxygen enriched air is a common practice in high-temperature bioleaching tests (>70°C) to overcome oxygen solubility limitation and reduced the energy costs of the process. Air is usually preferred in medium and low-temperature operations mainly for technical and economic constraints. Nevertheless, under high-sulfide loading conditions - high-grade metal sulfide concentrates and high solids concentration - the microbial and chemical demand for oxygen is significantly increased during the bioleaching process. If not satisfied, this high oxygen demand might limit the oxidation efficiency. Therefore it requires the injection of large amounts of air. Sparging with oxygen enriched gas instead of air may offer an interesting alternative process option to improve gas transfer in the bioleaching reactor and to provide an adequate oxygen supply in order to satisfy the oxygen demand. It might be useful to develop innovative alternative to the classical stirred tank reactor (STR) technology. However, the use of such conditions can lead to much higher dissolved oxygen (DO) concentrations than those encountered with air. Very few papers have been devoted to the study of the optimal range of DO concentrations for bioleaching processes. Most of them reported an inhibitory effect of DO concentrations above 5ppm. The purpose of this study was to investigate the influence of DO on the bioleaching efficiency under oxygen-enriched atmosphere in 21L stirred tank reactor at 42°C. Bioleaching experiments were performed in continuous mode with sulfide-rich tailings wastes composed mainly of pyrite (51%) and quartz using the “BRGM-KCC” bacterial consortia. The solid load was close to 20% (w/w). Using various oxygen supply conditions (partial pressure, gas rate), the DO concentration in the reactor varied between 4 and 17ppm. For a DO ranging from 4 to 13ppm, a good bacterial oxidizing activity was observed and the sulfide dissolution efficiency increased with the DO concentration. It is assumed that this improvement of the bioleaching efficiency was linked to an increase of the oxygen transfer rate from the gas phase to the liquid phase rather than a direct effect of the DO level. When the DO concentration reached 17ppm a significant decrease of the microbial activity and consequently of the oxygen consumption was noticed. These results show that there is a critical value above which the DO concentration is detrimental to the activity of the bioleach microorganisms present in the “BRGM-KCC” consortia but this value is much higher than the one usually mentioned in the literature.

The effect of dissolved oxygen on fatigue behavior of Alloy 690 steam generator tubes in borated and lithiated high temperature water

Corrosion fatigue behavior of Alloy 690 tubes used for actual steam generator (SG) was investigated in borated and lithiated high temperature water containing <5ppb (by weight) and 5500ppb dissolved oxygen (DO) using boat-shaped specimens. It was found that the fatigue life at 5500ppb DO was longer than at <5ppb DO. The surface cracks and fracture characteristics of the specimens after fatigue tests were carefully examined. The effects of DO concentration on fatigue crack initiation and propagation of Alloy 690 tubes in high temperature water are discussed.

A novel control strategy for enhancing biological N-removal in a granular sequencing batch reactor: A model-based study

Biological nitrogen removal in aerobic granular sequencing batch reactors is sensitively affected by process conditions (e.g. dissolved oxygen (DO) concentration, nitrogen loading rate (NLR), influent C/N ratio, among others). The variation of one of these process conditions affects the others, because often they are tightly linked. These interrelationships are a drawback for the experimental assessment of the target domain of process conditions required to enhance N-removal. Here, we have developed a model to determine the guidelines to design an automatic control strategy with the final aim of enhancing biological N-removal in a granular sequencing batch reactor. The model was first calibrated with experimental data from a granular sequencing batch reactor treating swine wastewater. Specific simulations were designed to elucidate the effect of DO concentration (0.5–8mgO2L−1), granule size (0.5–3.5mm), influent C/N ratio (4–10gO2g−1N) and NLR (0.41–0.82gNL−1d−1) on the nitrification–denitrification efficiency. Simulation results showed that, in general, high N-removal efficiencies (from 70% to 85%) could be obtained only setting the appropriate DO concentration. That appropriate DO concentration could be easily found based on effluent ammonium concentration. Those results were used to propose a control strategy to enhance N-removal efficiencies. The control strategy was based on a closed DO loop with variable DO set-point. The DO set-point was established at a constant value for the whole cycle (i.e. once per cycle), based on the on-line measurement of ammonium concentration at the end of the previous cycle.

A Comparative Research on Sludge Properties under Different Wastewater Quality Conditions

Objective: to explore the wastewater quality conditions which made sludge bulk easily, and the sludge properties when sludge bulking happened. Methods: wastewater was treated by SBR process, and sludge volume index (SVI) in different wastewater quality conditions was determined by controlling water temperature, dissolved oxygen (DO) and pH value. Conclusion: under low temperature conditions, effect of DO concentration on SVI was great; and the lower DO concentration, the more serious sludge bulking. But pH value had less effect on SVI, and only mild filamentous sludge bulking happened on activated sludge at low pH value (pH=6.0-6.5). Moreover, SVI tended to be steady at late period of operation, and the average was 203 mL/g.

Effects of dissolved oxygen on the biooxidation process of refractory gold ores

While multiple theories exist regarding the effect of dissolved oxygen (DO) on the biooxidation of minerals, few studies have been performed the cellular or molecular scale (e.g., genetics) and the mechanism remains unclear. In this paper, the effects of DO concentration on the biooxidation process of refractory sulfide gold ores by Acidithiobacillus ferrooxidans were investigated in the experimental stirred tank bioreactors (STRs). The results indicated that higher biooxidation and cell growth rates were correlated with higher DO concentration. The biooxidation process was restricted at 1.2 ppm DO due to oxygen limitation. Furthermore, the effects of DO on cellular and molecular scale were studied for the first time. The results demonstrated that the oxygen uptake rate (OUR), the Fe(2+) oxidation activity and the rus gene expression of A. ferrooxidans all increased with the DO concentration, which might be responsible for the increase of the biooxidation rates with the DO concentration. This study provides insight into the potential impact of molecular-level mechanisms of DO in the biooxidation process of minerals.

Effects of dissolved oxygen and shear stress on the synthesis and molecular weight of welan gum produced from Alcaligenes sp. CGMCC2428

The effects of dissolved oxygen (DO) and shear stress on the synthesis and molecular weight (MW) of welan gum were systematically investigated in the batch fermentation of Alcaligenes sp. CGMCC2428 in a 7.5L bioreactor with working volume of 4.5L. The results showed that a high shear stress (1000rpm) enhanced cell growth, but decreased MW and welan gum concentration. The welan gum concentration reached a maximum of 22.8±0.61g/L at 800rpm, and the highest MW was (9.01±0.12)×105Da at 600rpm. Furthermore, a moderate DO concentration (20%) enhanced MW and welan gum concentration; the maximum values were (9.20±0.11)×105Da and 24.5±0.53g/L, respectively. To further investigate the effects of DO concentration on the synthesis and MW of welan gum, this study also compared the metabolic flux distribution of Alcaligenes sp. CGMCC2428 at various DO concentrations. The results demonstrated that glucose-6-phosphate and acetyl-coenzyme A nodes were flexible. The flux of glucose-1-phosphate to welan gum was enhanced at DO concentration of 20%. Increased flux might also be a reason for the increased MW and welan gum concentration observed.

The microorganism community of pentachlorophenol (PCP)-degrading coupled granules

Coupled granules are self-immobilized aggregates of microorganisms under micro aerobic conditions, which have the dissolved oxygen (DO) level of 0.6 mg.L(-1). The effects of DO concentration on pentachlorophenol (PCP) reduction and its microbial community were investigated in a coupled anaerobic and aerobic reactor. Both the diversity and the dynamicity of the Eubacteria and Archaea community, which were responsible for PCP degradation, were evaluated by means of amplification by polymerase chain reaction (PCR) and separation using the denaturing gradient gel electrophoresis (DGGE) technique. The results demonstrated a major shift in the Eubacteria and Archaea community as the mixed aerobic and anaerobic seeding sludge (1:1 by volume) developed into coupled granules and finally acclimated with PCP throughout the experiment period within 60 days. The numbers of the Eubacteria population decreased from 20, 16 to 11; Shannon diversity index decreased from 2.75, 2.53 to 2.10. In contrast, the number of the Archaea population increased from 12, 14 to 18; and Shannon diversity index increased from 1.87, 1.88 to 2.43. Phylogenetic analysis based on 16SrDNA genes showed the dominance Sphingomonas, Desulfobulbus, Proteobacteria, Actinobacterium, Methanogenic and some uncultured bacteria in the PCP-degrading coupled granules. Microorganism community construction of coupled granules was also deduced.

Factors affecting submerged oviposition in a water strider: level of dissolved oxygen and male presence

A large variation in dissolved oxygen (DO) concentration of water is expected to limit activities of semiaquatic insects, including submerged oviposition. Female water striders Aquarius paludum insularis (Heteroptera: Gerridae) perform submerged oviposition either in tandem pairs (with males on their backs) or singly. We allowed A. paludum females to oviposit in high- or low-DO water to examine the effects of DO concentration and the method of submersion (tandem pair or single female) on submerged oviposition. The duration of submersion and the number and depth of eggs oviposited were all greater in the high-DO water, but the females performed more bouts of submerged oviposition in the low-DO water. The duration of submersion was longer for tandem pairs than for single females in both the high- and low-DO water. In the high-DO water, the number and depth of eggs oviposited were greater for tandem pairs than for single females. The DO concentration did not affect the frequency of submerged oviposition by tandem pairs or by single females. We suggest that submerged oviposition with an accompanying male is beneficial for both sexes, because submersion is longer than for single females.

The effects of dissolved oxygen concentration and stocking density on growth and non-specific immunity factors in Chinese shrimp, Fenneropenaeus chinensis

The effects of dissolved oxygen (DO) concentration and stocking density on growth performance, such as growth, survival, ecdysis, oxygen consumption, ingestion and food conversion efficiency (FCE), and non-specific immunity factors, such as phenoloxidase (PO), superoxide dismutase (SOD), peroxidase (POD), antibacterial activity (Ua), lysozyme (Ul) were investigated in the Chinese shrimp Fenneropenaeus chinensis. Shrimp growth (gain in length and weight), survival rate, ecdysis, ingestion and FCE were not significantly different among DO concentration treatments. The activities of SOD, POD, Ua, Ul and hemolysin were significantly affected by DO concentration. Mean oxygen consumption was significantly higher under high DO concentration (HDO) compared with medium (MDO) and low DO concentrations (LDO). Shrimp growth (gain in length and weight), survival rate, FCE and the activity of PO enzyme decreased, whereas ingestion and the activity of POD enzyme increased with increasing stocking density. The activity of hemolysin was significantly higher in high stocking density (HSD) compared with medium (MSD) and low stocking density (LSD). There was an interactive effect of DO concentration and stocking density on shrimp weight gain, ecdysis, O 2 consumption, ingestion, the activities of Ua and hemolysin. The results revealed that DO concentration was one of the key factors which affecting shrimp through influencing activities of non-specific immunity factors (SOD, POD, Ua, Ul and hemolysin), while stocking density affected the growth performance of shrimp mainly by influencing ingestion or activities of enzymes (PO, POD and hemolysin), and the interactive effects of DO concentration and stocking density ( P < 0.05) played a crucial role in the production of shrimp farming.

Controlling Biofilm Growth and Lactic Acid Production by Rhizopus oryzae in a Rotating Fibrous Bed Bioreactor: Effects of Dissolved Oxygen, Rotational Speed, and Urea Concentration

Oxygen was found to be the critical factor affecting lactic acid production by Rhizopus oryzae in a rotating fibrous bed bioreactor (RFBB), which was used to immobilize fungal mycelia and control fungal morphology during fermentation. A lack of oxygen resulted in low lactic acid production because the pathway becomes shunted to ethanol production, and it eventually leads to a loss in fungal activity and cell death. However, simply maintaining a high level of dissolved oxygen (DO) in the RFBB did not significantly improve lactic acid production, whereas increasing oxygen transfer rate led to a slight increase in lactic acid productivity. Ethanol production occurred even though a high oxygen transfer rate in the fermentation medium was maintained, indicating an anoxic condition occurring in the biofilm due to diffusion limitation. The effective diffusivities of oxygen and glucose in the biofilm were determined experimentally and then used in a diffusion model to estimate the critical biofilm thickness, which was found to be much smaller than the actual thickness of the biofilm attached on the fibrous matrix, suggesting the occurrence of oxygen starvation in the overgrown biofilm. Controlling the biofilm thickness to prevent oxygen diffusion limitation could be achieved by shaving off the fungal hyphae at the outer region or limiting cell growth by reducing the nitrogen concentration in the medium. Shaving occurs when the shear stress is higher than the hyphae tensile strength, and the shaving rate is proportional to the specific energy dissipation rate, which in turn is affected by the rotational speed of the fibrous matrix. However, increasing the rotational speed did not improve lactic acid production because the shear stress generated from the fluid motion was not high enough to shave off fungal hyphae. The growth and metabolic pathway of R. oryzae immobilized on the fibrous matrix also can be controlled by manipulating the medium composition, particularly the nitrogen source. It was found that biomass production was limited and lactic acid production increased in the absence of urea in the production phase. However, a lack of nitrogen source could reduce cell activity and product synthesis in long-term cultivation. Therefore, periodical addition of urea during the production phase is recommended to prevent biofilm overgrowth, delay sporulation, and maintain high cell viability and fermentation productivity.

Response of the blue shrimp Litopenaeus stylirostris to temperature decrease and hypoxia in relation to molt stage

The effect of different levels of dissolved oxygen (DO) and a decreased temperature combined with severe hypoxia were studied in the blue shrimp Litopenaeus stylirostris in relation to molt stage. In the first experiment, shrimps were submitted to DO concentrations of 5 to 1 mg l −1. Osmoregulatory capacity (OC) was measured in shrimps at molt stage C after 6, 24, or 48 h of exposure. No mortality was recorded but a significant negative effect of DO concentration on hypo-OC was observed for DO concentrations below 3 mg l −1. Osmoregulation improved after 48 h exposure, compared to 6 and 24 h exposure. In a second experiment, temperature was decreased from 28 to 22 °C in 24 h, before a severe hypoxia of 0.5±0.1 mg O 2 l −1 was applied. Mortality and OC were studied in relation to molt stages. Half of the shrimps died after respectively 100 min at 0.5 mg O 2 l −1 at 28 °C and 153 min at 0.4 mg O 2 l −1 at 22°C. Mortality was significantly higher in stages D 2 and B compared to stages C and D 0. Control shrimps in stage D 2 had a lower hypo-OC than shrimps in stages C and D 0 both at 28 and 22 °C. Temperature decrease had no effect on hypo-OC. Hypoxia reduced hypo-OC for all stages, whatever the temperature, but the effect was greater in stage C at 27 °C (68% decrease) compared to 22 °C (49%) and stage D 2 at both temperatures (respectively, 43% and 58% at 27 and 22 °C). Combination of temperature and hypoxia had a significant effect on hypo-OC. Low temperature reduced the effect of hypoxia presumably by slowing down the metabolism. Results are discussed in relation to pond observation.

Effect of High Oxygen Concentrations on Nitrification and Performance of High-Purity Oxygen A/O Biofilm Process

A respirometric method has shown that the dissolved oxygen (DO) concentration has an effect on the growth rate of nitrifying micro-organisms. Oxygen uptake rates were normal at DO concentrations of 5 to 15 mg/L, but at 15 mg/L and greater, oxygen uptake was reduced, and these DO levels may be toxic to the nitrifying micro-organisms. The anoxic/aerobic (A/O)-high purity oxygen submerged biofilm process was tested using synthetic wastewater influent. Four different internal recycle ratios (R), that is, R = 0.5, 1.0, 1.5, and 2.0, were tested at 0.64 kg COD/m3-day and 0.14 kg NH4+-N/m3-day for the aerobic reactor. When R was increased, the removal efficiency of COD and total Kjeldhal nitrogen (TKN) also increased to greater than 90%. The efficiency of total nitrogen (TN) removal, however, was not proportional to the R. DO concentrations in the aerobic reactor were set at 5 and 10 mg/L to evaluate the effect of DO concentrations on the nitrification reaction. A COD removal efficiency of greaer than 90% was always achieved regardless of the DO concentration when the process aerobic reactor was operated in the range of 1.12 kg COD/m3-day and 0.21 kg NH4+-N/m3-day. The nitrification rate, however, changed significantly according to the DO concentration.