Dissolved Oxygen Concentration Profiles Research Articles

Phosphate concentration and association as revealed by sequential extraction and microprobe analysis: The case of sediments from two Argentinean reservoirs

This article reports the general characteristics of the sediments of two Argentinean reservoirs, which are used for water supply. The chemical composition, granulometry, and specific surface area are presented together with a study of total phosphate concentration and phosphate association by combining sequential extraction and microprobe analysis. In general, the sediments of both reservoirs have similar characteristics. Sequential extraction reveals that the main P fractions in the studied sediments are Ca‐bound phosphate in river mouths and Fe‐bound phosphate in the rest of the reservoir stations. Microprobe analysis appears to be an important complementary technique to sequential extraction. Combined with chemical fractionation and specific surface area measurements, these analyses indicate that Ca‐bound phosphate is mainly distributed within grains or particles highly concentrated in Ca and P, whereas Fe‐bound phosphate is rather homogeneously distributed in the sediments at the surface of fine particles. Microprobe analyses also revealed an important coating of phyllosilicate surfaces with Fe (hydr)oxides, which explains the good correlation found between Fe‐bound phosphate, clay fraction, and specific surface area. The role of sediments as a potential source of phosphate is discussed in terms of P association in the solid phase and dissolved oxygen concentration profiles in the water column.

Simulation of turbulence and dissolved oxygen concentration profiles over biofilm using k–ɛ turbulence model

The objective of this study is to investigate the flow structure over biofilm experimentally and theoretically. Velocity and turbulence profiles over biofilm measured by a laser Doppler velocimetry were compared to simulated profiles using the k–ɛ turbulent model. Also, dissolved oxygen concentration profiles over biofilm are measured using a micro DO sensor. The k–ɛ turbulence model was proved to be a useful tool for the understanding of mass transfer inside and outside biofilm. Dissolved oxygen concentration profile inside and outside biofilm showed the existence of turbulent diffusion inside biofilm.

THE EFFECT OF DEVELOPING DIFFUSIVE BOUNDARY LAYER ON SEDIMENTARY OXYGEN DEMAND

A model of sedimentary oxygen demand (SOD) for a sediment bed for finite length is presented. The model describes the effect of developing a diffusive boundary layer on oxygen transfer just above the sediment/water interface and on microbial oxygen uptake inside the sediment. Variations of biomass profile inside the sediment, dissolved oxygen concentration profile near the sediment/water interface, and SOD in accordance with the developingd iffusiveb oundaryl ayer are investigatedn umerically.

THE EFFECT OF DISSOLVED OXYGEN DEPLETION DUE TO FINE SEDIMENT ON THE CLAM HABITAT IN THE TIDAL FLAT OF BANJO RIVER IN OITA

A field survey has been performed both in the estuary of the Banjo river and in the Saeki bay in order to investigate the effect of water quality on the clam habitat in tidal flat. The result of water quality measurements shows that dissolved oxygen depletes due to fine sediment in the bottom of the Banjo river. Dissolved oxygen profiles near the sediment/water interface on the tidal flat are simulated based on the experimental results of the oxygen consumption rate by the sediment. The dissolved oxygen concentration profile varies considerably near the sediment/water interface and rapidly becomes zero just below the interface regardless of the shear velocity, because all DO that reach the sediment surface are utilized instantly. There is not enough oxygen both at the sediment surface and inside the sediment. A tidal flat that has biologically active sediment accumulating is therefore an inappropriate place for the clam habitat.

Comparison of the efficiency of large-scale ceramic and membrane aeration systems with the dynamic off-gas method

The aeration systems of two full-scale activated sludge basins were compared over 2.5 years under the same operating conditions using dynamic off-gas testing. Only the material of the diffuser was different, membrane vs. ceramic tube diffusers. The experimental design took the complexity and dynamics of the system into consideration. The investigation has shown that, although the membrane diffusers have higher initial standard oxygen transfer efficiency (SOTE) and standard aeration efficiency (SAE), these decreased over time, while the SAE of the ceramic diffusers started lower, but increased slightly over the whole period. Measurement of air distribution in the basins along with dissolved oxygen concentration profiles have provided important information on improving process control and reducing energy costs. The results show that dynamic off-gas testing can effectively be used for monitoring the aeration system and to check design assumptions under operating conditions. The information can be used to improve the design of new aeration systems or in retro-fitting existing basins.

Transversal profiles of dissolved oxygen in thin-layer open outdoor algal cultures

Differential mass balance of dissolved oxygen (DO) in algal culture layer flowing down an inclined open cultivation surface was set up. Using simplified aussumptions, a differential equation was derived for the course of DO concentration profile across algal culture layer. In the development of the equation were considered: hydraulics of culture flow on cultivation surface, rate of O 2 evolution by algae, molecular diffusivity and turbulent diffusivity of oxygen inside of layer. The equation was integrated numerically for apporopriate boundary conditions. Computations performed for 6 mm thick layer of the alga Chlorella kessleri, flowing at velocity 50 cms -1 on inclined cultivation surface (inclination 1.7 %) in 28 m long and 4 m wide outdoor culture unit revealed that main restistance against DO mass transfer from algal culture into atmosphere was found in boundary layer of culture near algal culture - atmosphere interface. Thickness 8 of the boundary layer was only several percent of thickness of the whole culture layer and was proportional to the square root of: culture temperature and mean DO concentration in the bulk of algal culture. Computations showed that turbulence in algal culture on cultivation surface was sufficient to create almost uniform DO transversal (across algal culture layer) profile outside boundary layer δ.

Axial inhomogeneities in steady-state dissolved oxygen in airlift bioreactors: predictive models

Models were developed for prediction and interpretation of the observed steady-state axial dissolved oxygen concentration profiles in tall airlift bioreactors. The observed concentration profiles were non-linear because of a combination of hydrodynamic and mass transport factors. The profiles were influenced mainly by the liquid-phase axial dispersion coefficient, the volumetric overall gas–liquid mass transfer coefficient, the gas velocity, the induced liquid circulation velocity. The model-predicted concentration profiles agreed within ±2% with the measured data in a tall (working aspect ratio ∼15) airlift vessel operated under aeration regimens that are typically used during wastewater treatment. Axial inhomogeneities in dissolved oxygen increased with increasing aeration rate. This phenomenon may influence activated sludge processes in airlift and deep-shaft reactors. The maximum attainable concentration of dissolved oxygen at the bottom of a typically aerated airlift reactor, ≥3.5 m deep, always remained at <80% of air saturation value even when oxygen was not being consumed. Also, at steady state and without a net transfer of oxygen, the gas-phase mole fraction of oxygen varied by >10% axially up the reactor.

Gas–liquid mass transfer in an airlift reactor — analytical solution and experimental confirmation

Gas–liquid mass transfer behavior in an external loop airlift reactor is investigated. The riser and the downcomer of 6 m height have a diameter of 0.15 and 0.08 m, respectively. The classical ‘gassing out’ dynamic method is used to obtain the evolution with time of dissolved oxygen concentration profiles. An analytical description of the concentration profiles with time following the switch in the inlet gas concentration is presented and allows the evaluation of the gas–liquid mass transfer coefficient for the different gas velocities used. The comparison between the values of k L a determined from this simplified model and those obtained in the same reactor from a more complete and sophisticated model shows a good agreement.

Reassessment of the estimation of dissolved oxygen concentration profile and KLa in solid-state fermentation

Oxygen mass transfer in aerobic microbial growth systems is often a limiting factor for optimal growth and productivity. Oxygen mass transfer has been widely studied in submerged fermentations but has attracted as yet little attention for solid state fermentations. The parallel to submerged fermentation has led to the incorrect interpretation and use of the overall oxygen mass transfer coefficient ( K L a) to assess the ability of a particular fermentation system to supply the oxygen to microorganisms. The use of K L a, as traditionally defined, should be used with caution in solid substrate fermentation systems because there is no convection on the liquid side of the medium, and oxygen is consumed in the biofilm. Hence, K L a must be redefined for solid state fermentation. In this paper, the use of oxygen mass transfer coefficients in solid state fermentations is clarified. Published literature data were analysed with a simple pseudo-steady-state model and used to discuss the influence of the biofilm thickness, the dissolved oxygen diffusion coefficient, the convective gas mass transfer coefficient, and the gas flow rate on the oxygen mass transfer coefficient in solid state fermentations.

Qxygen transfer in gravity flow sewers

Oxygen diffused to water in gravity sewer pipes was studied in a 21 m long, 0.15 m diameter model sewer. At first, the sodium sulfide was added into the clean water to deoxygenate, then the pump was started to recirculate the water and the deoxygenated water was reaerated. The dissolved oxygen microelectrode was installed to measure the dissolved oxygen concentrations varied with flow velocity, time and depth. The dissolved oxygen concentration profiles were constructed and observed. The partial differential equation diffusion model that considered Fick's law including the molecular diffusion term and eddy diffusion term were derived. The analytic solution of the partial differential equation was used to determine the diffusivities by the method of nonlinear regression. The diffusivity values for the oxygen transfer was found to be a function of molecular diffusion, eddy diffusion and flow velocity.

Steady-state axial profiles of dissolved oxygen in tall bubble column bioreactors

A model is developed for prediction and interpretation of the observed steady-state axial dissolved oxygen concentration profiles in tall bubble columns. The observed concentration profiles are non-linear, unlike what would be expected if the hydrostatic pressure alone influenced the profiles. The non-linear profiles result from the axial mixing of liquid in the column. Several other factors influence the profiles, including the overall gas holdup, the volumetric overall gas–liquid mass transfer coefficient, and the static height of liquid in the column. The effect of mixing can be adequately accounted for using an axial dispersion coefficient. Because the axial dispersion coefficient is sensitive to the diameter of the column and to gas flow rate, the overall behavior of the profile is affected by the aspect ratio of the column and the superficial gas velocity in it. The mass transfer coefficient and the axial dispersion coefficient have mutually opposing effects on the shape of the profile. Because both those variables increase with increasing gas flow rate, the shape of the profile is affected less than would be the case if only mixing influenced the profile. The non-linearity of concentration profiles increases with increasing overall height of the column especially when the height exceeds about 2 m in a 0.24 m diameter column. The model-predicted axial concentration profiles agree closely – within ±3% – with the measured data. Using the measured profile, the model allows for calculation of the liquid-phase axial dispersion coefficients. This method does not require the use of tracers. Being a steady-state method, the operation of the bioreactor does not need to the interrupted in any way for the determination of the axial dispersion coefficient or the state of mixing. Consequently, the proposed method is particularly suited to characterizing the axial dispersion coefficient in an operating bioreactor without disturbing the operation. If the axial dispersion coefficient is known, the model allows for quantifying the spatial inhomogeneities in oxygen concentration in a bioreactor vessel.

A transient mathematical model for maximum respiration activity and oxygen diffusion coefficient estimation in non-steady-state biofilms

A transient mathematical model was established in order to evaluate oxygen diffusivity in non-steady-state biofilms. A submerged fixed bed biofilm system with efficient medium recirculation was investigated for p-toluenesulphonic acid degradation by Comamonas testosteroni T-2 in a multi-species biofilm. Static mixer elements (Sulzer Chemtech Ltd, Switzerland) were used as a support matrix for biofilm formation. Biofilm respiration was estimated using the dynamic gassing-out oxygen uptake method. Based on the dissolved oxygen concentration profiles, the oxygen diffusion coefficient and the maximum respiration activity were calculated. The values of the dissolved oxygen diffusion coefficient varied with biolfilm development and values reported here (2×10−10–1.2×10−9 m2 s−1) are in good agreement with literature data. Calculated oxygen consumption rates fit well with values obtained in respirometry tests with washed out biofilms. The knowledge of diffusivity changes in biofilms is particularly important for removal capacity estimation and appropriate reactor design. © 1998 Society of Chemical Industry

Dynamics of dissolved oxygen (DO) in ponded water of a paddy field

The transfer of heat and dissolved oxygen (DO) through water is important to understand the phenomenon of ponded water in a paddy soil. The heat from solar radiation is absorbed at the soil surface and transferred into the ponded water by convection. This study clarified the dynamics of DO, as well as the role of convection in water in DO transfer in the ponded water of a paddy field. DO concentration in the ponded water of a paddy field was measured in situ in the daytime and during the night. The results were confirmed in lab-scale model experiments. The DO concentration and temperature profiles in the ponded water of a lab-scale paddy field model were investigated under convective and non-convective conditions using solar radiation and infrared radiation, respectively. Under the ponded condition, solar radiation was absorbed at the soil surface whereas infrared radiation was absorbed at the water surface and thereby convective and non-convective conditions were generated, respectively. The diurnal variation in DO concentration was closely related to the intensity of solar radiation. Oxygen generation by micro-algae and its subsequent circulation by convection resulted in uniform DO concentration profiles, with super-saturated values in the ponded water in the daytime. Eventually oxygen was released to the atmosphere by deaeration until DO in water was depleted to the saturated level. During the night the oxygen moved from the atmosphere into the water surface by reaeration which depends on the oxygen deficit related to saturation. The oxygen deficit is caused by the respiration of microorganisms. The oxygen, that moved from the atmosphere to the water surface, was transferred to the soil surface by convection in the water layer. Thus convection plays an important role in the DO transfer in the ponded water of a paddy field. The DO dynamics is correlated with biological processes in the ponded paddy soil.

Study of toluene degradation kinetics in a flat plate vapor phase bioreactor using oxygen microsensors

The paper describes the toluene degradation process in a flat plate vapor phase bioreactor (VPBR) by a Pseudomonas putida 54G biofilm. Oxygen microelectrodes were used to measure oxygen concentration profiles through the gas, liquid, and biofilm phases. The linear shape of the dissolved oxygen concentration profile in the outer 87% of the biofilm thickness suggested an absence of reaction in this layer. Oxygen consumption in the remaining basal 13 % (0.3 mm) followed zero order kinetics with a rate constant of 102.2 g m−3 h−1, for toluene gas concentration of 1.5 g m−3. The increase in respiratory activity near the substratum was confirmed by microscopic study of cryogenic biofilm sections, and the lack of activity in the surface film was interpreted as a consequence of injury exerted by the toxic substrate. The accumulation of dead cells on the top of the biofilm contributed a resistance to the transport of substrates to deeper layers of the biofilm suggesting a protective role of the outer layer against the harmful effect of the toxic. These results highlight a new conceptual biofilm model in which both microbial growth and inactivation are controlled by substrate transport, leading to a structure that itself controls substrate availability.

Simulation of growth of a filamentous fungus in 3 dimensions

The tridimensional growth of a filamentous fungus was simulated, based on a model for the evolution of the microscopic morphology of Trichoderma reesei. When supplemented with a spatial representation of growth, the model correctly simulates the evolution from a single spore to a pellet. Diffusion of oxygen is included in the model. The simulated tridimensional structures have a fractal nature; and the fractal dimension, determined by a box-counting method, increases during growth. The fractal dimension only depends on the mass of the pellet and is not affected by model parameters such as tip extension rate and branching frequency. Realistic pictures are obtained and the radius of the pellet increases at a constant rate. The influence of model parameters (tip extension rate, branching frequency, minimum porosity) on dissolved oxygen concentration profiles, biomass concentration profiles, rate at which the pellet diameter increases, and the evolution of the fractal dimension was determined. The dissolved oxygen profiles were found to be very different from the profiles, obtained by assuming a homogenous biomass distribution within the pellet. Finally, the formation of pellets from spore aggregates is calculated and the size of the spore aggregate is found to only influence the time needed before the appearance of a pellet and not its morphology. (c) 1997 John Wiley & Sons, Inc.

Study of toluene degradation kinetics in a flat plate vapor phase bioreactor using oxygen microsensors

The paper describes the toluene degradation process in a flat plate vapor phase bioreactor (VPBR) by a Pseudomonas putida 54G biofilm. Oxygen microelectrodes were used to measure oxygen concentration profiles through the gas, liquid, and biofilm phases. The linear shape of the dissolved oxygen concentration profile in the outer 87% of the biofilm thickness suggested an absence of reaction in this layer. Oxygen consumption in the remaining basal 13 % (0.3 mm) followed zero order kinetics with a rate constant of 102.2 g m −3 h −1 , for toluene gas concentration of 1.5 g m −3 . The increase in respiratory activity near the substratum was confirmed by microscopic study of cryogenic biofilm sections, and the lack of activity in the surface film was interpreted as a consequence of injury exerted by the toxic substrate. The accumulation of dead cells on the top of the biofilm contributed a resistance to the transport of substrates to deeper layers of the biofilm suggesting a protective role of the outer layer against the harmful effect of the toxic. These results highlight a new conceptual biofilm model in which both microbial growth and inactivation are controlled by substrate transport, leading to a structure that itself controls substrate availability.

Prediction of dissolved oxygen concentration along sanitary sewers

The oxygen balance in wastewater collection systems is important in respect to the degree of biological oxidation that occurs within the stream and in respect to the control of septicity and its effects. In this paper, a simple mathematical model is presented, in order to predict dissolved oxygen concentration profiles along sanitary sewers. The mathematical model was developed based on an analytical solution of the simple differential equation of dissolved oxygen balance in sewers, and includes an empirical expression for prediction of dissolved oxygen transfer to the slime layer on the pipe walls. Because the factors controlling dissolved oxygen balance in sewers are so complex, it would be unrealistic to expect, that with this rather simple model, dissolved oxygen concentrations can be accurately predicted. Nevertheless, it is reasonable to suppose that the predictions may be adequate for some design and operation purposes.

Development of a respirometric assay to measure the transient load response of activated sludge to individual organic chemicals

This study described a new respirometric technique to monitor the transient response of activated sludge systems to individual synthetic organic compounds (SCOs). The assay used a fed-batch reactor system which contained mixed liquor from the parent reactor and received a biogenic feed and the target SOC at the same F/M ratio as was applied to the parent reactor. The fed-batch reactor was oxygenated using a hollow-fiber reaeration module to meet the steady-state oxygen demand. The reaeration module was characterized by means of KL a studies. The assay monitored the dissolved oxygen concentration profile after application of a shock load achieved by switching to a fed-batch feed with the target SOC at an elevated concentration. Mathematical descriptions of the transient profiles were derived and the experimentally observed dissolved oxygen profiles were subsequently compared to profiles predicted using extant kinetic parameters for SOC removal measured using an earlier developed assay. The use of the assay were complicated by the difficulty in maintaining a reaeration capacity at constant and low capacity. As a result, the adequacy of the measured extant kinetic parameters as predictors of the transient response could not be confirmed.

Prediction of dissolved oxygen concentration along sanitary sewers

The oxygen balance in wastewater collection systems is important in respect to the degree of biological oxidation that occurs within the stream and in respect to the control of septicity and its effects. In this paper, a simple mathematical model is presented, in order to predict dissolved oxygen concentration profiles along sanitary sewers.The mathematical model was developed based on an analytical solution of the simple differential equation of dissolved oxygen balance in sewers, and includes an empirical expression for prediction of dissolved oxygen transfer to the slime layer on the pipe walls.Because the factors controlling dissolved oxygen balance in sewers are so complex, it would be unrealistic to expect, that with this rather simple model, dissolved oxygen concentrations can be accurately predicted. Nevertheless, it is reasonable to suppose that the predictions may be adequate for some design and operation purposes.

Development of a respirometric assay to measure the transient load response of activated sludge to individual organic chemicals

This study described a new respirometric technique to monitor the transient response of activated sludge systems to individual synthetic organic compounds (SOCs). The assay used a fed-batch reactor system which contained mixed liquor from the parent reactor and received a biogenic feed and the target SOC at the same F/M ratio as was applied to the parent reactor. The fed-batch reactor was oxygenated using a hollow-fiber reaeration module to meet the steady-state oxygen demand. The reaeration module was characterized by means of KLa studies. The assay monitored the dissolved oxygen concentration profile after application of a shock load achieved by switching to a fed-batch feed with the target SOC at an elevated concentration. Mathematical descriptions of the transient profiles were derived and the experimentally observed dissolved oxygen profiles were subsequently compared to profiles predicted using extant kinetic parameters for SOC removal measured using an earlier developed assay. The use of the assay were complicated by the difficulty in maintaining a reaeration capacity at constant and low capacity. As a result, the adequacy of the measured extant kinetic parameters as predictors of the transient response could not be confirmed.