Constant Dissolved Oxygen Concentration Research Articles

Fine-Tuning the Aeration Control for Energy-Efficient Operation in a Small Sewage Treatment Plant by Applying Biokinetic Modeling

Wastewater treatment is an energy-intensive process for treating liquid-phase pollutants in urban settlements. The aerobic processes of the biological treatment involve a significant air demand. An optimal control strategy could be used to minimize the amount of excess air entering the system due to safety factors applied in the design procedures. A plant-wide mechanistic modeling approach including an activated sludge model and one-dimensional settler model was proposed as an effective tool for predicting the actual air demand and for selecting the optimal aeration strategy. In this study, a sewage treatment plant receiving strong influent flow was investigated. At the sludge ages of 14–18 days, the plant was capable of achieving a 90% organic matter reduction and 85% nutrient reduction. By applying a constant dissolved oxygen concentration of 1.5 mg/L, the air demand decreased by 25%, which could be further increased by 10% if the cascade ammonium control approach was applied at peak periods. The dependence of the aeration energy demand on the temperature and dissolved oxygen was formulated, meaning the operators could select the optimal setpoint and minimize the energy consumption while the effluent quality requirements were met.

Frequency and power dependence of ultrasonic degassing

We investigated the time variation of ultrasonic degassing for air-saturated water and degassed water with a sample volume of 100 mL at frequencies of 22, 43, 129, 209, 305, 400, 514, 1018, and 1960 kHz and ultrasonic power of 15 W. Ultrasonic degassing was evaluated by dissolved oxygen concentration. Ultrasonic degassing was also investigated at a frequency of 1018 kHz and ultrasonic powers of 5, 10, 15, and 20 W. The dissolved oxygen concentration varied with the ultrasonic irradiation time and became constant after prolonged ultrasonic irradiation. The constant dissolved oxygen concentration value depended on the frequency and ultrasonic power but not the initial dissolved oxygen concentration. The degassing rate at 101.3 kPa was higher in the frequency range of 200 kHz to 1 MHz. The frequency dependence of the degassing rate was almost the same as that of the sonochemical efficiency obtained by the potassium iodide (KI) method. Ultrasonic degassing in the frequency range of 22–1960 kHz was also investigated under reduced pressure of 5 kPa. Degassing was accelerated when ultrasonic irradiation was applied under reduced pressure. However, under a reduced pressure of 5 kPa, the lower the frequencies, the higher is the degassing rate. The sonochemical reaction rate was examined by the KI method for varying dissolved air concentrations before ultrasonic irradiation. Cavitation did not occur when the initial dissolved oxygen concentration was less than 2 mg·L−1. Therefore, the lower limit of ultrasonic degassing under 101.3 kPa equals 2 mg·L−1 dissolved oxygen concentration. A model equation for the time variation of dissolved oxygen concentration due to ultrasonic irradiation was developed, and the degassing mechanism was discussed.

Oxygen transfer as a tool for fine-tuning recombinant protein production by Pichia pastoris under glyceraldehyde-3-phosphate dehydrogenase promoter.

Effects of oxygen transfer on recombinant protein production by Pichia pastoris under glyceraldehyde-3-phosphate dehydrogenase promoter were investigated. Recombinant glucose isomerase was chosen as the model protein. Two groups of oxygen transfer strategies were applied, one of which was based on constant oxygen transfer rate where aeration rate was Q O/V=3 and 10 vvm, and agitation rate was N=900min(-1); while the other one was based on constant dissolved oxygen concentrations, C DO=5, 10, 15, 20 and 40% in the fermentation broth, by using predetermined exponential glucose feeding with μ o=0.15h(-1). The highest cell concentration was obtained as 44gL(-1) at t=9h of the glucose fed-batch phase at C DO=20% operation while the highest volumetric and specific enzyme activities were obtained as 4440UL(-1) and 126Ug(-1) cell, respectively at C DO=15% operation. Investigation of specific enzyme activities revealed that keeping C DO at 15% was more advantageous with an expense of relatively higher by-product formation and lower specific cell growth rate. For this strategy, the highest oxygen transfer coefficient and oxygen uptake rate were K L a=0.045s(-1) and OUR=8.91mmolm(-3)s(-1), respectively.

Hypoxia affects performance traits and body composition of juvenile hybrid striped bass (Morone chrysops × M. saxatilis)

Performance traits and body composition of juvenile hybrid striped bass (Morone chrysops × M. saxatilis) in response to hypoxia were evaluated in replicate tanks maintained at constant dissolved oxygen concentrations that averaged 23.0 ± 2.3%, 39.7 ± 3.0% and 105.5 ± 9.5% dissolved oxygen saturation. Fish were fed a commercially formulated feed daily to apparent satiation. Total feed intake and fish growth and yield increased linearly in response to increased dissolved oxygen concentration. Nutrient utilization was reduced significantly only at the greatest level of hypoxia. With the exception of whole body protein content, whole body compositional indices and nutrient retention efficiencies were linearly related to dissolved oxygen concentration. Results demonstrate that as hypoxia becomes more severe, juvenile hybrid striped bass feed intake is reduced, which affects growth and nutrient retention.

Determination of the reaction rate coefficient of sulphide mine tailings deposited under water

The efficiency of a water cover to limit dissolved oxygen (DO) availability to underlying acid-generating mine tailings can be assessed by calculating the DO flux at the tailings–water interface. Fick's equations, which are generally used to calculate this flux, require knowing the effective DO diffusion coefficient (Dw) and the reaction (consumption) rate coefficient (Kr) of the tailings, or the DO concentration profile. Whereas Dw can be accurately estimated, few studies have measured the parameter Kr for submerged sulphide tailings. The objective of this study was to determine Kr for underwater sulphide tailings in a laboratory experiment. Samples of sulphide mine tailings (an approximately 6 cm layer) were placed in a cell under a water cover (approximately 2 cm) maintained at constant DO concentration. Two tailings were studied: TA1 with high sulphide content (83% pyrite) and TA2 with low sulphide content (2.8% pyrite). DO concentration was measured with a microelectrode at various depths above and below the tailings–water interface at 1 mm intervals. Results indicate that steady-state condition was rapidly attained. As expected, a diffusive boundary layer (DBL) was observed in all cases. An iterative back-calculation process using the numerical code POLLUTEv6 and taking the DBL into account provided the Kr values used to match calculated and experimental concentration profiles. Kr obtained for tailings TA1 and TA2 was about 80 d−1 and 6.5 d−1, respectively. For comparison purposes, Kr obtained from cell tests on tailings TA1 was lower than Kr calculated from the sulphate production rate obtained from shake-flask tests. Steady-state DO flux at the water–tailings interface was then calculated with POLLUTEv6 using tailings characteristics Dw and Kr. For the tested conditions, DO flux ranged from 608 to 758 mg O2/m2/d for tailings TA1 and from 177 to 221 mg O2/m2/d for tailings TA2. The impact of placing a protective layer of inert material over the tailings was also investigated for tailings TA1 (with high sulphide content). A protective layer of only 5 cm reduced the DO flux into the tailings at about 5 mg/m2/d, compared to 608 mg O2/m2/d without a protective layer, or an approximately 99% reduction in flux.

Optimal aeration control in a nitrifying activated sludge process

An important tool to minimise energy consumption in activated sludge processes is to control the aeration system. Aeration is a costly process and the dissolved oxygen level will determine the efficiency of the operation as well as the treatment results. What aeration control should achieve is closely linked to how the effluent criteria are defined. This paper explores how the aeration process should be controlled to meet the effluent discharge limits in an energy efficient manner in countries where the effluent nitrogen criterion is defined as average values over long time frames, such as months or years. Simulations have been performed using a simplified Benchmark Simulation Model No. 1 to investigate the effect of different levels of suppressing the variations of the effluent ammonium concentration. Optimisation is performed where the manipulated variable for aeration (the oxygen transfer coefficient, KLa) is minimised with the constraint that the average daily flow-proportional ammonium concentration in the effluent should reach a desired level. The optimisation results are compared with constant dissolved oxygen concentrations and supervisory ammonium control with different controller settings. The results demonstrate and explain how and why energy consumption can be optimised by tolerating the ammonium concentration to vary around a given average value. In these simulations, the optimal oxygen peak-to-peak amplitude range between 0.7 and 1.8mg/l depending on the influent variation and ammonium level in the effluent. These variations can be achieved with a slow ammonium feedback controller. The air flow requirements can be reduced by 1–4% compared to constant dissolved oxygen set-points. Optimal control of aeration requires up to 14% less energy than needed for fast feedback control of effluent ammonium.

Phenol biodegradation by the thermoacidophilic archaeon Sulfolobus solfataricus 98/2 in a fed-batch bioreactor

Toxic at low concentrations, phenol is one of the most common organic pollutants in air and water. In this work, phenol biodegradation was studied in extreme conditions (80°C, pH = 3.2) in a 2.7 l bioreactor with the thermoacidophilic archaeon Sulfolobus solfataricus 98/2. The strain was first acclimatized to phenol on a mixture of glucose (2000 mg l(-1)) and phenol (94 mg l(-1)) at a constant dissolved oxygen concentration of 1.5 mg l(-1). After a short lag-phase, only glucose was consumed. Phenol degradation then began while glucose was still present in the reactor. When glucose was exhausted, phenol was used for respiration and then for biomass build-up. After several batch runs (phenol < 365 mg l(-1)), specific growth rate (μ(X)) was 0.034 ± 0.001 h(-1), specific phenol degradation rate (q(P)) was 57.5 ± 2 mg g(-1) h(-1), biomass yield (Y(X/P)) was 52.2 ± 1.1 g mol(-1), and oxygen yield factor (Y(X/O2)) was 9.2 ± 0.2 g mol(-1). A carbon recovery close to 100% suggested that phenol was exclusively transformed into biomass (35%) and CO(2) (65%). Molar phenol oxidation constant (Y(O2/P)) was calculated from stoichiometry of phenol oxidation and introducing experimental biomass and CO(2) conversion yields on phenol, leading to values varying between 4.78 and 5.22 mol mol(-1). Respiratory quotient was about 0.84 mol mol(-1), very close to theoretical value (0.87 mol mol(-1)). Carbon dioxide production, oxygen demand and redox potential, monitored on-line, were good indicators of growth, substrate consumption and exhaustion, and can therefore be usefully employed for industrial phenol bioremediation in extreme environments.

Simultaneous Nitrification, Denitrification, and Phosphorus Removal in Single‐Tank Low‐Dissolved‐Oxygen Systems Under Cyclic Aeration

Simultaneous nitrification and denitrification (SND or SNdN) may occur at low dissolved oxygen concentrations. In this study, bench-scale (approximately 6 L) bioreactors treating a continuous feed of synthetic wastewater were used to evaluate the effects of solids retention time and low dissolved oxygen concentration, under cyclic aeration, on the removal of organics, nitrogen, and phosphorus. The cyclic aeration was carried out with repeated cycles of 1 hour at a higher dissolved oxygen concentration (HDO) and 30 minutes at a lower (or zero) dissolved oxygen concentration (LDO). Compared with aeration at constant dissolved oxygen concentrations, the cyclic aeration, when operated with proper combinations of HDO and LDO, produced better-settling sludge and more complete nitrogen and phosphorus removal. For nitrogen removal, the advantage resulted from the more readily available nitrate and nitrite (generated by nitrification during the HDO period) for denitrification (during the LDO period). For phosphorus removal, the advantage of cyclic aeration came from the development of a higher population of polyphosphate-accumulating organisms, as indicated by the higher phosphorus contents in the sludge solids of the cyclically aerated systems. Nitrite shunt was also observed to occur in the LDO systems. Higher ratios of nitrite to nitrate were found in the systems of lower HDO (and, to less dependency, higher LDO), suggesting that the nitrite shunt took place mainly because of the disrupted nitrification at lower HDO. The study results indicated that the HDO used should be kept reasonably high (approximately 0.8 mg/L) or the HDO period prolonged, to promote adequate nitrification, and the LDO kept low (< or =0.2 mg/L), to achieve more complete denitrification and higher phosphorus removal. The above findings in the laboratory systems find strong support from the results obtained in full-scale plant implementation. Two plant case studies using the cyclic low-dissolved-oxygen aeration for creating and maintaining SND are also presented.

Effects of temperature on tertiary nitrification in moving-bed biofilm reactors

The effect of wastewater temperature on the rate of nitrification was studied in two pure-oxygen moving-bed biofilm reactors, fed on secondary effluent from a municipal wastewater treatment plant. The first Reactor (R1) was operated under ammonia-limiting conditions, while the second Reactor (R2) was operated under oxygen-limiting conditions. Quite surprisingly, the former showed a negligible influence of thermal changes on nitrification rates, while the latter showed a much higher dependence. In this paper, a temperature coefficient “ θ” has been defined as the actual “intrinsic” biological temperature coefficient, similar to the corresponding coefficient that is usually adopted for the design of activated-sludge processes. In addition, an “apparent” coefficient θ a has been quantified independently, which was calculated according to the actual values of nitrification rates at different temperatures. The actual biological temperature coefficient “ θ”, ranged between 1.086 and 1.109 (average value 1.098) under ammonia-limiting conditions, while under oxygen-limiting conditions was in the range 1.023–1.081 (average value 1.058). The apparent value θ a was near to unity (i.e. no temperature effect) under ammonia-limiting conditions, while only under oxygen-limiting conditions and at constant dissolved oxygen concentration “ θ a” coincided with “ θ”. An explanation was given that, under oxygen-limiting conditions, the specific biomass activity (i.e. the ratio of nitrification rate to biomass concentration) was strongly influenced by the combined effects of oxygen penetration through the biofilm and effluent temperature.

Feedforward-feedback control of dissolved oxygen concentration in a predenitrification system

As the largest single energy-consuming component in most biological wastewater treatment systems, aeration control is of great interest from the point of view of saving energy and improving wastewater treatment plant efficiency. In this paper, three different strategies, including conventional constant dissolved oxygen (DO) set-point control, cascade DO set-point control, and feedforward-feedback DO set-point control were evaluated using the denitrification layout of the IWA simulation benchmark. Simulation studies showed that the feedforward-feedback DO set-point control strategy was better than the other control strategies at meeting the effluent standards and reducing operational costs. The control strategy works primarily by feedforward control based on an ammonium sensor located at the head of the aerobic process. It has an important advantage over effluent measurements in that there is no (or only a very short) time delay for information; feedforward control was combined with slow feedback control to compensate for model approximations. The feedforward-feedback DO control was implemented in a lab-scale wastewater treatment plant for a period of 60 days. Compared to operation with constant DO concentration, the required airflow could be reduced by up to 8-15% by employing the feedforward-feedback DO-control strategy, and the effluent ammonia concentration could be reduced by up to 15-25%. This control strategy can be expected to be accepted by the operating personnel in wastewater treatment plants.

Intelligent control aeration and external carbon addition for improving nitrogen removal

Fuzzy logic can in several ways be applied to improve the control of the activated sludge system. In this paper, fuzzy logic based control strategies for external carbon flow in the anoxic zone and dissolved oxygen (DO) concentration in the aerobic zone are presented and evaluated by the simulation of the predenitrification layout of the COST 624 benchmark implemented by Matlab/Simulink. The aeration fuzzy controller was designed as high-level controller that determines the DO set points in the aerated basin; conventional PI controller was used to maintain the DO concentration at the set point levels. The ammonia concentration in the influent and effluent were used as input variables for the fuzzy controller. Compared to the operation with constant DO concentrations, the required airflow could be reduced up to 7%, the effluent ammonia concentration could be reduced up to 18% by using fuzzy logic based control strategy. At the same time, ammonia peaks in the effluent could be reduced significantly. The external carbon flow fuzzy controller which includes two inputs variable, the error ( E, nitrate in the second basin minus the preset-point) and change in error ( E C), and one output variable, which adjusts the external carbon flow, compared to the normal PI operation, the external carbon flow could be reduced up to 24.2%. Experiments show that the fuzzy controllers can be easily implemented in modern control and supervision systems, to improve the effluent quality and reduce operating cost.

Dissolved Oxygen-Stat Titration Respirometry: Principle of Operation and Validation

A method is presented to assess oxygen uptake rates which operates at constant dissolved oxygen concentration (DO). This method involves DO-stat titrations by controlled addition of a diluted H\d2O\d2 solution to a batch volume of activated sludge. The titrant flow rate is proportional to the biomass respiration rate, while the mass of H2O2 added is proportional to the oxidized organics. Classical batch respirometries (with and without gas flow, i.e., “open” and “closed”) were compared to the DO-stat technique using first NaSO\d2 as reducing chemical and then respiring aerobic biomass. An excellent correlation was found among estimates of oxygen consumption rate and short term yield coefficient by the three techniques. Inhibition by H\d2O\d2 was found to be negligible for biomass concentration as volatile suspended solids of ⩾ 1.5–2 g/L.

Biofilm Formation on Magnetic Polystyrene Particles

In this study, magnetic polystyrene particles were prepared by using a solvent evaporation technique. Particles 500–595 μm in size were used as a biofilm carrier in a fluidized bed for the biodegradation of an organic carbon source in wastewater. Activated sludge seeds were used as a mixed culture of microorganisms to obtain biofilms on the magnetic polystyrene particles. Fluidized bed experiments were performed at constant dissolved oxygen concentration, pH and temperature. The magnetic particles allowed operation of the column in a magnetic field at high liquid flow rates; hence, external diffusion limitations on biofilm surfaces were lowered. Biofilm formation on the suspended particles was quite successful and allowed effective removal of the substrate.

Determination of nitrification kinetics by the ANITA-DOstat biosensor

The ANITA biosensor, which measures ammonium oxidation rate by alkaline titration at constant pH, has been recently coupled to open respirometry making it possible to evaluate NH4+ oxidation kinetics by two different and completely independent techniques. This paper describes the use of a new instrument, also derived from the titration biosensor ANITA and named ANITA-DOstat, which incorporates dissolved oxygen (DO) control by H2O2 addition. The peroxide is very rapidly decomposed to H2O2 and O2 by bacterial catalase and supplies oxygen to the mixed liquor. The novelty lies not in the use of H2O2 addition as a source of oxygen for bacterial respiration, but in the measurement of kinetics by DOstat titrimetric respirometry which allows for evaluation of bacterial bio-oxidation parameters directly from the H2O2 flow rate at constant DO concentration. Preliminary results are presented on nitrification kinetics determination, for both ammonium oxidisers (AO) and nitrite oxidisers (NO), carried out by alkaline titration and by the new H2O2 DOstat titrimetric respirometry method.

Pullulan fermentation in a reciprocating plate bioreactor

Reciprocating plate bioreactors are particularly well suited for conducting fermentations which give rise to highly viscous broth. To evaluate their performance for polysaccharide fermentations, a series of pullulan fermentations were performed with a particular emphasis placed on the influence of aeration on both the quantity and quality of the product. Two experiments were conducted at constant aeration rates and two others with constant dissolved oxygen concentrations. For the latter two experiments, the dissolved oxygen concentration was controlled by manipulating either the aeration flow rate or the reciprocating frequency of the perforated plates.

On‐line gas analysis in animal cell cultivation: I. Control of dissolved oxygen and pH

To monitor gas reaction rates in animal cell culture at constant dissolved oxygen concentration (DO) and constant pH it was necessary to develop improved control methods. Decoupling of both controllrs was obtained by manipulation of molar fractions of oxygen and carbon dioxide in the gas phase. Two pairs of DO and pH controllers were designed and tested both in simulation and exprimental runs. The first controller pair was developed for headspace aeration only, whereas the second controller pair was designed for bubble aeration using a microsparger and flushing the headspace with helium. pH was controlled by a conventional discrete PID controller in its velocity form. For DO control two linear state space feedback controllers with parameter adaptation were established. In these controllers the oxygen uptake rate (OUR) was considered as a disturbance and was not included in the mathematical model. The feedback gain adaptation was based on the difference between the actual molar fraction of oxygen at time step n and the initial molar fraction. This difference is related to OUR and was used to increase or decrease the state feedback controller gain (k and k(1), respectively) in a slow manner. With these controllers it was possible to get an excellent online estimate of OUR. In the case of bubble aeration a simple gas phase mass balance was sufficient, whereas during the headspace aeration a liquid phase balance was required. It has been shown that determination of OUR using gas balance requires a significantly better controller performance compared to just keeping DO and pH within reasonable limits. (c) 1995 John Wiley & Sons, Inc.

Factors Affecting Vitamin E Production Using Plant Cell Culture of Carthamus Tinctorius.

Factors affecting α-tocopherol production using safflower (Carthamus tinctorius) cells were studied from the engineering aspects for plant cell culture. The specific production rate was evaluated as a function of rate-limiting factors. The conditioning factor released from cells, dissolved oxygen (DO) and medium components were considered as the limiting factors responsible for the specific production rate. Increasing the conditioning factor enhanced α-tocopherol production. Increasing DO concentration up to 9 mg-O2/l also caused a linear increase in specific production rate. The specific production rate in batch cultivation decreased, however, at a constant DO concentration with a sufficient amount of the conditioning factor. In repeated batch cultivation, the specific production rate was recovered by the addition of fresh medium. These results showed that an essential component in the medium was exhausted at an early stage of the batch cultivation and that the medium-feeding operation with sufficient CF and DO concentrations was required to maintain high productivity. By repeated batch cultivation, the content of α-tocopherol in the cell was increased to 9 mg/100 g-dry cell, which was considerably higher than that in natural safflower seeds.

Influence of dissolved oxygen concentration and shear rate on the production of pullulan byAureobasidium pullulans

Experiments were carried out withA. pullulans (ATCC 9348) at constant dissolved oxygen concentration (DO=100 and 50% related to air saturation at 1 bar) and at constant stirrer speeds (n=500 and 150 [min−1]). The highest pullulan yield was achieved at decreased constant DO in connection with decreased shear rate. Biomass production was not affected.

ストライプトバスの成長に及ぼす溶存酸素の影響

Experiments were carried out to examine the effect of nearly constant dissolved oxygen con-centrations, both below and above the air saturation level, on the growth of young striped bass (body weight 2.9-15.0g).Fish were reared for 8 days in 5 different tanks. Each of these tanks had a volume of 16 liters. These tanks were continuously supplied with sea water having different dissolved oxygen con-centrations. Oxygen concentrations were controlled by bubbling nitrogen or oxygen gas through the inflowing water. During the experiments water temperature fluctuated between 20.9 and 27.2°C and the chlorinity varied from 12.16 to 18.38%. Fish were fed three times a day with commercially available, crumbled pellet for young ayu-fish.When the dissolved oxygen concentration was less than about 53% in air saturation, the growth rate and the food conversion efficiency increased remarkably with the increase of dissolved oxygen. The increase of feeding rate was found to be gradual upto 60% in air saturation. Nearly the same levels in the growth rate and food conversion efficiency were observed, when the dissolved oxygen concentrations ranged from 53 to 250% in air saturation. In the case of feeding rate nearly same level was observed, when the dissolved oxygen concentration ranged from 60 to 250% in air saturation. No clear effect of oxygen-supersaturation was observed.

Constant dissolved oxygen concentrations in cephalosporin C fermentation: Applicability of different controllers and effect on fermentation parameters

The behaviour and applicability of several controllers for maintaining a constant dissolved oxygen concentration (DO) during the cephalosporin C production with Cephalosporium acremonium in a laboratory fermentor is described. The process controllers were realized on a MC 68000 based process computer using the real-time language PEARL. The discrete signum integral controller showed the best control action. In addition some derived fermentation data were calculated on-line by the process computer.