Stirred-tank System Research Articles

Computational Analysis of Viscoplastic Nanofluid Blending by a Newly Modified Anchorage Impeller within a Stirred Container

Several industrial fields require mixing and mechanical agitation processes. This operation is mainly used to enhance heat and mass transfer inside stirred tank systems and improve the degree of homogeneity to obtain a high-quality final product. The main goal of this research paper is to analyze the thermal and hydrodynamic behavior of non-Newtonian nanofluid (Bingham–Papanastasiou–Al2O3) inside a symmetrically stirred tank. A 3D numerical study has been conducted for a stationary laminar flow inside a symmetric cylindrical vessel under influencing parameters, including the inertia parameter (Re=1, 20, 100) and the volume fraction of nanoparticles (Ø=0.02, 0.06, 0.1) with different geometric configurations, has been introduced into the stirring system. According to the findings, with high inertia (Re=100), the heat transfer inside the stirred tank is enhanced. Furthermore, increasing the nanoparticle fraction volume had a significant impact on the acceleration of heat transfer along the stirred vessel. It has been also found that the geometric configuration of an anchor with added arm blade (Case 2) is more efficient compared with the rest of the anchor agitator.

Application of a simple unstructured kinetic and cost of goods models to support T-cell therapy manufacture.

Manufacturing of cell therapy products requires sufficient understanding of the cell culture variables and associated mechanisms for adequate control and risk analysis. The aim of this study was to apply an unstructured ordinary differential equation-based model for prediction of T-cell bioprocess outcomes as a function of process input parameters. A series of models were developed to represent the growth of T-cells as a function of time, culture volumes, cell densities, and glucose concentration using data from the Ambr®15 stirred bioreactor system. The models were sufficiently representative of the process to predict the glucose and volume provision required to maintain cell growth rate and quantitatively defined the relationship between glucose concentration, cell growth rate, and glucose utilization rate. The models demonstrated that although glucose is a limiting factor in batch supplied medium, a delivery rate of glucose at significantly less than the maximal specific consumption rate (0.05 mg 1× 106 cell h-1 ) will adequately sustain cell growth due to a lower glucose Monod constant determining glucose consumption rate relative to the glucose Monod constant determining cell growth rate. The resultant volume and exchange requirements were used as inputs to an operational BioSolve cost model to suggest a cost-effective T-cell manufacturing process with minimum cost of goods per million cells produced and optimal volumetric productivity in a manufacturing settings. These findings highlight the potential of a simple unstructured model of T-cell growth in a stirred tank system to provide a framework for control and optimization of bioprocesses for manufacture.

An ode model for optimal operation of T-cell culture in suspension system

Background & Aim Manufacturing requires sufficient understanding of the variables and associated mechanisms for adequate control and risk analysis. In the case of cell therapies, relatively complex relationships between inputs/parameter controls and process outcomes demands suitable models to define/optimise for manufacturing operation. Our aim is to take a Quality by Design approach to process control of cell therapy manufacture that addresses autologous cell therapies variability in patient-specific source material through first principles models. Methods, Results & Conclusion Our Ordinary Differential Equation (ODE) model delivers the required process operation based on the premise of T cells subpopulation specific secretome and paracrine/autocrine sensitivity. The mechanistic T cell model allows prediction of variability in process outcomes with respect to input variability and thereby will identify process operation with acceptable risk and opportunities for process optimization. Ambr15 platform was used to validate the ODE model with respect to input variability including; cell seeding density, feeding ration, frequency and glucose concentration in bulk medium. Our results demonstrated a simple unstructured model of T-cell growth in a stirred tank system which provides framework for operation with respect to timings and volume of medium delivery to maintain an uninhibited growth. Such a model can also indicate how glucose delivery can be controlled to sustain cell growth while selectively affect the growth of specific sub-population of T cells.

A new application area of electrochemical limiting diffusion current technique: measurement of mixing time and effect of some parameters

In this study, the electrochemical limiting diffusion current technique, which was employed first time for the mixing time measurement in the stirred tank system by the authors, was applied to determine the effect of some parameters on the global and local mixing times in the range of 3500 ≤ Re ≤ 10,000. The technique well sensed that increasing Reynolds number (Re), blade angle and blade number decreased the mixing time. The most effective parameter was observed to be Re, and the blade number and blade angle, in order. In addition, the mixing time was also investigated for solutions added in various volume fractions having different viscosity and density by using local mixing time measurement. It was determined that the mixing time decreased with the Reynolds number, the ratio of the initial solution volume to the total solution volume and the ratio of kinematic viscosity of the solution in the vessel initially to that of the added one viscosity. The ratio of initial solution volume to total solution volume was found to be a more effective parameter than the others in terms of mixing time.

The transient expression of CHIKV VLP in large stirred tank bioreactors.

Transient gene expression (TGE) bioprocesses have been difficult to scale up in large stirred tank bioreactors with volumes of more than 1.5L. Low production levels are often observed, but the causes have not been investigated (Gutierrez-Granados et al. in Crit Rev Biotechnol 38:918-940, 2018). Chikungunya Virus-like particle (VLP), expressed by DNA-PEI transient transfection, is a representative case study for these difficulties. Clinical materials were produced in shake flasks, but the process suffered when transferred to large stirred tank bioreactors. The resulting process was not operationally friendly nor cost effective. In this study, a systematic approach was used to investigate the root causes of the poor scale up performance. The transfection conditions were first screened in ambr® 15 high throughput mini bioreactors then examined in 3L stirred-tank systems. The studies found that production level was negatively correlated with inoculum cell growth status (P < 0.05). The pH range, DNA and PEI levels, order of the reagent addition, and gas-sparging systems were also studied and found to affect process performance. Further hydromechanical characterizations (Re, energy dissipation rates, and P/V, etc.) of shake flasks, ambr® 15, and 3-L stirred tank systems were performed. Overall, the study discovered that the shear stress (caused by a microsparger) and PEI toxicity together were the root causes of scale-up failure. Once the microsparger was replaced by a macrosparger, the scale-up was successful.

Gas Evolution Rates in Supersaturated Water-in-Oil Emulsions at Elevated Pressures

In the petroleum industry, emulsions are encountered in nearly every stage of oil production, transportation, and operation. An understanding of the mass transfer rates during gas evolution from supersaturated solutions is critical for enabling better design and operation of gas–liquid separators. The objective of this work was to elucidate the influence of surfactant and water droplet sizes present in water-in-oil emulsions on the rate of gas evolution (volumetric mass transfer coefficient) from supersaturated systems. The volumetric mass transfer coefficient during gas evolution at elevated pressure (3.45 ± 0.00689 MPa) was determined using a batch stirred tank system. The volume of the liquid phase (0.0005 m3), the initial saturation pressure (3.45 ± 0.00689 MPa), the liquid-phase temperature (298.15 ± 0.5 K), and the mixing speed during gas evolution (250 rpm) were kept constant during the experiments. Ultrahigh purity methane was used as the gas phase. Pure model oil (Tech 80) and water-in-oil emulsions (30 wt % water) were used as the liquid phases. Water-in-oil emulsions with two different droplet sizes with average droplet sizes of 6.6 ± 2.6 and 21.7 ± 7 μm were used to investigate the influence of the droplet size on gas evolution rates. We hypothesize that the presence of water droplets in the continuous oil phase increases the diffusional path length, which would result in a decrease in the volumetric mass transfer coefficient. At the investigated emulsion concentrations, our results showed that the presence of the surfactant (Span 80 at 0.1 wt %) did not affect the volumetric mass transfer coefficient of methane leaving the model oil. However, the volumetric mass transfer coefficient decreased with a decrease in the initial droplet size. The decrease in the volumetric mass transfer coefficient results in an increased time required for gas evolution. Based on our data set, one can infer that a decrease in the initial droplet size might increase the time required for the solution gas to exit the liquid in a gas–liquid separator. In addition, our results showed that gas evolution from supersaturated water-in-oil emulsions led to the destabilization of the water-in-oil emulsions. We hypothesize that this increase in the droplet size is due to the coalescence of the emulsion droplets during gas evolution.

Photoautotrophic cultivation of Chlorococcum humicola in stirred tank and airlift photobioreactors under different light settings and light supplying strategies for biomass and carotenoid production

AbstractBACKGROUNDLight quality influences biomass and carotenoid production in microalgae, yet no information is available for Chlorococcum humicola. This work performed a series of batch cultivations of Chlorococcum humicola to assess biomass and carotenoid production in stirred tank and airlift photobioreactors under different light qualities and supplying strategies.RESULTSWhite light was preferred for growth while blue light was suitable for carotenoid production during stationary phase. Batch cultivation in stirred tank photobioreactors subjected to two‐staged lighting yielded higher carotenoid productivity than single‐staged lighting, with the highest productivity obtained when providing white light at 5000 lx during growth phase and combination of white (100 000 lx) and blue (5000 lx) lights during stationary phase. Subsequent cultivation in internal‐loop airlift photobioreactors under optimal light setting based on two‐staged study produced higher biomass (855 ± 26 mg L−1) and carotenoid (8.11 ± 0.20 mg L−1 or 9.23 ± 0.23 mg g−1) than the cultivation in stirred tank systems with the same volume and operating condition. High‐performance liquid chromatography (HPLC) analysis of carotenoid from airlift photobioreactors revealed that lutein, violaxanthin and neoxanthin were major carotenoids.CONCLUSIONSThis work reported for the first time the effect of light quality and supplying strategy on growth and carotenoid production in Chlorococcum humicola. Two‐staged lighting with white light following by combination of white light at high intensity and blue light offered new strategy to improve carotenoid productivity. Cultivation in airlift photobioreactor yielded higher biomass than the cultivation in stirred tank photobioreactors and offered an alternative for future photobioreactor selection. © 2019 Society of Chemical Industry

Mechanism of molasses–water mixing behavior in bioethanol fermenter. Experiments and CFD modeling

Molasses is a first-generation bioethanol feedstock that is a byproduct of sugar production. Molasses is a viscous fluid; consequently, when it serves as a substrate, it needs to be dissolved in water to obtain the appropriate conditions for the yeast. Therefore, the mixing process of molasses and water is an essential factor that significantly affects the efficiency of the fermentation process. This work aims to study the molasses–water mixing behavior in a cylindrical cone-shaped stirred-tank system reviewed by the effect of the molasses–water rheological characteristics. The molasses–water mixing behavior was investigated through a simple visualization method by using a camera and a 650 nm red laser as an observation plane of molasses–water interface height changes. Based on the results of the rheological characterization, it is known that molasses are a non-Newtonian fluid that exhibits shear thinning behavior. A rheological model molasses–water mixture has been developed. The behavior of molasses–water mixture comprises three zone mechanisms. The Eulerian multiphase model is the closest to the experimental results, and the combination of the Eulerian multiphase model and Large Eddy Simulation can be applied to predict the mixing behavior in an industrial scale stirred-tank.

Absorption and desorption mass transfer rates as a function of pressure and mixing in a simple hydrocarbon system

Determining the time required for degassing within a gas–liquid separator is usually accomplished by assuming thermodynamic equilibrium between the two phases and calculating the time required for entrained gas bubbles to rise out of the liquid. However, if the gas–liquid separation is being performed at elevated pressures, the inlet multiphase fluid may not be at thermodynamic equilibrium, thus the chance for solution gas to evolve out of the liquid must also be accounted for. There is, however, a paucity of mass transfer data in hydrocarbon systems available at elevated pressures. For the current experimental study, the rates of absorption and desorption of methane in a n-dodecane solution were measured in a batch stirred tank system at pressures ranging from 3.45 to 10.34 MPa and mixing speeds ranging from 100 to 250 rpm. All absorption and desorption experiments were performed using the same physical and hydrodynamic conditions. Additionally, care was taken in ensuring that the gas–liquid interface during mass transfer remained flat, allowing for the explicit calculation of the liquid-side mass transfer coefficient. Within the experimental variability, the rates of absorption and desorption were found to be the same in most cases. The mixing speed was found to be the main variable affecting the rate of mass transfer, while the saturation pressure was found to have little effect. The process was well described by the theoretically derived solid surface eddy cell model presented by Lamont and Scott, resulting in an average absolute error of 12.3% between the predicted and measured mass transfer coefficients.

Biogas production by two-stage thermophilic anaerobic co-digestion of food waste and paper waste: Effect of paper waste ratio

Abstract The effects of paper waste (PW) ratio on biogas production by two-stage thermophilic anaerobic co-digestion (coAD) of food waste (FW) and PW, two main components of organic fraction of municipal solid waste, were evaluated in lab-scale continuously stirred tank systems with semi-continuous feeding. Four PW ratios (0, 20, 40, and 50%) were studied in the test. The experimental results showed the two-stage thermophilic coAD could operate stably after an adjustment period at all PW ratios. Although the increase of PW ratio decreased methane content, production and yield, reaction rates of four AD steps, it could increase the methane production of substrate when the FW was fixed. The pH, total volatile fatty acids, total alkalinity, and total ammonium of the sludge in the second stage of AD, also decreased with the increase of PW ratio in the test. The change of the substrate's organic composition, increase of carbohydrate and decrease of protein and lipid, were the main reason for the decrease of methane content, production and yield and changes of the second stage sludge characteristics. An apposite PW ratio (PW-20%) could enhance the degradation of matters (TS, VS and TCOD).

Chemical modification of protein A chromatography ligands with polyethylene glycol. I: Effects on IgG adsorption equilibrium, kinetics, and transport

Chemical modification of Protein A (ProA) chromatography ligands with polyethylene glycol (PEGylation) has been proposed as a strategy to increase the process selectivity and resin robustness by providing the ligand with a steric repulsion barrier against non-specific binding. This article comprises a comprehensive study of IgG adsorption and transport in Repligen CaptivA PriMAB resin with PEGylated ProA ligands that are modified using 5.2 and 21.5 kDa PEG chains. We studied the impact of the molecular weight of the PEG as well as the extent of PEGylation for the 5.2 kDa PEG modification. In all cases, PEGylation of ProA ligands decreases the resin average pore size, particle porosity, and static binding capacity for IgG proportional to the volume of conjugated PEG in the resin. Resin batch uptake experiments conducted in bulk via a stirred-tank system and with individual resin particles under confocal laser scanning microscopy suggests that PEGylation introduces heterogeneity into IgG binding kinetics: a fraction of the IgG binding sites are transformed from typical fast association kinetic behavior to slow kinetic behavior. pH gradient elution experiments of an IgG molecule on the modified resins show an increase in IgG elution pH for all modified resins, implying a decrease in IgG-ProA binding affinity on modification. Despite losses in static binding capacity for all resins with PEGylated ligands, the loss of dynamic binding capacity at 10% breakthrough (DBC10%) ranged more broadly from almost 0–47% depending on the PEG molecular weight and the extent of PEGylation. Minimal losses in DBC10% were observed with a low extent of PEGylation with a smaller molecular weight PEG, while higher losses were observed at higher extents of PEGylation and with higher molecular weight PEG due to decreased static binding capacity and increased mass transfer resistance. This work provides insight into the practical implications for resin performance if PEGylation is considered as a strategy for selectivity enhancement in affinity chromatography with macromolecular ligands.

Development of an advanced imaging technique for dynamic emulsion stability

Oil-water emulsions consist of droplets that are approximately spherical in shape and with a distribution in size. An image processing technique that can be used for studying emulsion droplet stability behaviour in multiphase flow has been developed. In this work, the developed technique is used to study a model system, comprising Nexbase (with and without SPAN 80 surfactants) oil and saline water emulsion in a stirred tank system. A state of the art stirred tank experimental set up was constructed that measured dynamic droplet size evolution with varying impeller speed and torque. The equilibrium droplet size measurement and torque data were used to validate with theoretical inertial (Hinze model) and viscous subrange models. Besides, this method was successfully used to obtain droplet size relaxation coefficients with varying impeller speeds.

Hepatocyte spheroids as a competent in vitro system for drug biotransformation studies: nevirapine as a bioactivation case study.

The development of metabolically competent in vitro models is of utmost importance for predicting adverse drug reactions, thereby preventing attrition-related economical and clinical burdens. Using the antiretroviral drug nevirapine (NVP) as a model, this work aimed to validate rat hepatocyte 3D spheroid cultures as competent in vitro systems to assess drug metabolism and bioactivation. Hepatocyte spheroids were cultured for 12days in a stirred tank system (3D cultures) and exposed to equimolar dosages of NVP and its two major Phase I metabolites, 12-OH-NVP and 2-OH-NVP. Phase I NVP metabolites were detected in the 3D cultures during the whole culture time in the same relative proportions reported in in vivo studies. Moreover, the modulation of SULT1A1 activity by NVP and 2-OH-NVP was observed for the first time, pointing their synergistic effect as a key factor in the formation of the toxic metabolite (12-sulfoxy-NVP). Covalent adducts formed by reactive NVP metabolites with N-acetyl-L-cysteine and bovine serum albumin were also detected by high-resolution mass spectrometry, providing new evidence on the relative role of the reactive NVP metabolites, 12-sulfoxy-NVP, and NVP quinone methide, in toxicity versus excretion pathways. In conclusion, these results demonstrate the validity of the 3D culture system to evaluate drug bioactivation, enabling the identification of potential biomarkers of bioactivation/toxicity, and providing new evidence to the mechanisms underlying NVP-induced toxic events. This model, integrated with the analytical strategies described herein, is of anticipated usefulness to the pharmaceutical industry, as an upstream methodology for flagging drug safety alerts in early stages of drug development.

Bioprocessing of human mesenchymal stem/stromal cells for therapeutic use: Current technologies and challenges

The long-term outlook for regenerative medicine predicts an increased need for scalable cell expansion technologies that utilize non-animal derived materials and are compatible with the limited number of downstream processing steps required for cell-based therapies. As more stem cell therapeutics progress through clinical testing, current in vitro culture methods using planar vessels are proving cumbersome to scale. Therefore, alternative processes are under investigation. Many human mesenchymal stem/stromal cell (hMSC) bioreactor-based manufacturing processes, in particular, are complicated by the requirement to separate cells from microcarriers with high cell yield and viability whilst maintaining target phenotypic and functional characteristics. Here we review currently available technologies and ongoing development for the expansion of cellular therapeutics, with focus on allogeneic hMSCs and microcarrier-based processes. Upstream challenges include the interplay between the cell culture substrate and media formulation, sourcing of high quality animal-free reagents, and considerations for the use of microcarriers in stirred-tank systems. Complications in downstream processes include harvest approaches for separation of cells from microcarriers and volume reduction.

Multimode Process Fault Detection Using Local Neighborhood Similarity Analysis

Traditional data driven fault detection methods assume unimodal distribution of process data so that they often perform not well in chemical process with multiple operating modes. In order to monitor the multimode chemical process effectively, this paper presents a novel fault detection method based on local neighborhood similarity analysis (LNSA). In the proposed method, prior process knowledge is not required and only the multimode normal operation data are used to construct a reference dataset. For online monitoring of process state, LNSA applies moving window technique to obtain a current snapshot data window. Then neighborhood searching technique is used to acquire the corresponding local neighborhood data window from the reference dataset. Similarity analysis between snapshot and neighborhood data windows is performed, which includes the calculation of principal component analysis (PCA) similarity factor and distance similarity factor. The PCA similarity factor is to capture the change of data direction while the distance similarity factor is used for monitoring the shift of data center position. Based on these similarity factors, two monitoring statistics are built for multimode process fault detection. Finally a simulated continuous stirred tank system is used to demonstrate the effectiveness of the proposed method. The simulation results show that LNSA can detect multimode process changes effectively and performs better than traditional fault detection methods.

CFD modeling of gas entrainment in stirred tank systems

In the present work, CFD modeling was used to study the phenomenon of gas entrainment in stirred tank systems. Two types of impellers (DT, PBTD) were simulated. VOF method was used as surface tracking technique along with LES model to study interfacial behavior at the onset of gas entrainment. Simulations were performed to study cause of entrainment and underlying interfacial mechanism at the location of entrainment. CFD simulations clearly showed differences in onset and non onset conditions in terms of the magnitudes of interfacial turbulence. As per the predictions, phenomenon of surface aeration in stirred tank systems was characterized by exchange of momentum across the interface from water side to air side. Magnitudes of instantaneous axial velocities on air side, strain rates on air side and vorticities on air side exhibited a threshold at the onset of entrainment and reduced substantially after the onset.

Fouling of a Heated Rod in a Stirred Tank System

A batch stirred tank device has been developed for measuring fouling from oil samples. The unit consists of a baffled tank equipped with a centrally mounted long blade stirrer, and an electrically heated rod located at 40% of the radius of the tank. Heat transfer from the rod was first characterized. The velocity field was measured, from which the approach velocity to the probe was determined, which allowed the wall shear on the heating probe to be calculated from a literature equation. Fouling of a heavy oil fraction was studied in 1- to 2-day experiments with bulk oil temperatures typically at 320°C, initial probe surface temperatures to 536°C, and stirrer speeds of 100–900 rpm. Micrometer-sized iron oxide particles were added to the oil, such that fouling was due to a combination of particle deposition and coke formation. Deposition rates were measured thermally from the change in heat transfer coefficient when fouling was relatively heavy, and by thickness and mass accumulation when fouling was light. Effects of oil type, film temperature, stirrer rotation speed (or probe wall shear stress), and concentration of suspended particles on deposition rate and deposit composition are presented.

Intensification of viscous fluid mixing in eccentric stirred tank systems

The mixing process of highly viscous liquids in a small stirred tank for High Throughput Experimentation (HTE) apparatus was investigated using a visualization approach, i.e., the Planar Laser-Induced Fluorescence (PLIF) technique. Segregated regions were detected despite a long period of agitation, indicating that the mixing homogeneity can be hardly attained for the mixing of highly viscous liquid. Based on the image analysis of PLIF results, we defined a desegregating time to quantify the mixing process of viscous liquids and to compare the mixing performances under different operating conditions. Eccentric agitation proved to be an efficient way to intensify the mixing of viscous liquid for the fact of significantly reduced desegregating time under the same stirring rate.

Simultaneous adsorption and biodegradation of synthetic melanoidin

Molasses spent wash contains melanoidin, a dark brown recalcitrant compound. It is not easily biodegraded and causes a number of problems such as reduction in photosynthetic activities and dissolved oxygen when discharged to aqueous environment. Being an antioxidant, melanoidin removal through purely biodegradation has been inadequate. Consequently, in the current study, simultaneous adsorption and biodegradation (SAB) was employed in a stirred tank system to remove melanoidin from synthetic wastewater. Mixed microbial consortium was immobilized onto 200 g of activated carbon and used to degrade 3.5 L of melanoidin solutions with varying chemical oxygen demand (COD) concentrations. The effects of the initial COD level, pH and temperature on COD removal were then studied. Ultimately, the SAB performance was compared to that of batch adsorption or biodegradation carried out independently. After 48 h of operation, the SAB process yielded the best COD removal efficiency of 75% as compared to 49.3 and 51.9% for adsorption and biodegradation, respectively, for the initial COD value of 10800 mg/ L at a temperature of 296 K and pH 6.97. This therefore showed that the SAB process can successfully be applied to enhance the removal of melanoidin from wastewater. Key words: Adsorption, biodegradation, melanoidin, SAB, wastewater.

The effect of CO2 availability on the growth, iron oxidation and CO2‐fixation rates of pure cultures of Leptospirillum ferriphilum and Acidithiobacillus ferrooxidans

Understanding how bioleaching systems respond to the availability of CO(2) is essential to developing operating conditions that select for optimum microbial performance. Therefore, the effect of inlet gas and associated dissolved CO(2) concentration on the growth, iron oxidation and CO(2) -fixation rates of pure cultures of Acidithiobacillus ferrooxidans and Leptospirillum ferriphilum was investigated in a batch stirred tank system. The minimum inlet CO(2) concentrations required to promote the growth of At. ferrooxidans and L. ferriphilum were 25 and 70 ppm, respectively, and corresponded to dissolved CO(2) concentrations of 0.71 and 1.57 µM (at 30°C and 37°C, respectively). An actively growing culture of L. ferriphilum was able to maintain growth at inlet CO(2) concentrations less than 30 ppm (0.31-0.45 µM in solution). The highest total new cell production and maximum specific growth rates from the stationary phase inocula were observed with CO(2) inlet concentrations less than that of air. In contrast, the amount of CO(2) fixed per new cell produced increased with increasing inlet CO(2) concentrations above 100 ppm. Where inlet gas CO(2) concentrations were increased above that of air the additional CO(2) was consumed by the organisms but did not lead to increased cell production or significantly increase performance in terms of iron oxidation. It is proposed that At. ferrooxidans has two CO(2) uptake mechanisms, a high affinity system operating at low available CO(2) concentrations, which is subject to substrate inhibition and a low affinity system operating at higher available CO(2) concentrations. L. ferriphilum has a single uptake system characterised by a moderate CO(2) affinity. At. ferrooxidans performed better than L. ferriphilum at lower CO(2) availabilities, and was less affected by CO(2) starvation. Finally, the results demonstrate the limitations of using CO(2) uptake or ferrous iron oxidation data as indirect measures of cell growth and performance across varying physiological conditions.