Carbon Dioxide Transfer Rate Research Articles

Has Extracorporeal Gas Exchange Performance Reached Its Peak?

Extracorporeal gas exchange therapies evolved considerably within the first three-four decades of their appearance, and have since reached a mature stage, where minor alterations and discrete fine-tuning might offer some incremental improvement. A different approach is introduced here, making use of modern, purely diffusive membrane materials, and taking advantage of the elevated concentration gradient ensuing from gas pressure buildup in the gas chamber of the oxygenator. An assortment of silicone membrane gas exchangers were tested in vitro as per a modified protocol in pursuance of assessing their gas exchange efficiency under both regular and high-pressure aeration conditions. The findings point to a stark performance gain when pressurization of the gas compartment is involved; a 40% rise above atmospheric pressure elevates oxygen transfer rate (OTR) by nearly 30%. Carbon dioxide transfer rate (CTR) does not benefit as much from this principle, yet it retains a competitive edge when higher gas flow/blood flow ratios are employed. Moreover, implementation of purely diffusive membranes warrants a bubble-free circulation. Further optimization of the introduced method ought to pave the way for in vivo animal trials, which in turn may potentially unveil new realms of gas exchange performance for therapies associated with extracorporeal circulation.

Blood Gas Parameters and Acid–Base Balance during Extracorporeal Lung Support with Oxygenators: Semi-Empirical Evaluation

Membrane artificial lungs (oxygenators) are used in cardiopulmonary surgery as well as, in some cases, in severe lung disease to support the natural lung by means of ECMO (extracorporeal membrane oxygenation). The oxygen and carbon dioxide transfer rates of any oxygenator are usually assessed by considering several blood gas parameters, such as oxygen saturation, hemoglobin concentration, partial pressure of oxygen and carbon dioxide, bicarbonate concentration, and pH. Here, we report a set of semi-empirical equations that calculate such parameters directly from their partial pressures and assess the acid–base balance during ECMO. The implementation of this equation set permits the evaluation of any oxygenator, existing or prototypes in development, as well as the development of clinical decision-making tools for predicting the blood gas state and acid–base balance during surgical interventions and ECMO. The predicted results are then compared with experimental data obtained from in vitro gas exchange investigations with a commercial oxygenator using fresh porcine blood. The high correlation, R2>0.95, between the predicted and the experimental data suggests a possibility of using such empirical equations in the simulation of gas transfer in a cardiopulmonary system with an oxygenator for any venous inlet blood gas data and also for estimating the acid–base balance during such therapy.

Toward standardized solid medium cultivations: Online microbial monitoring based on respiration activity.

Cultivating microorganisms on solid agar media is a fundamental technique in microbiology and other related disciplines. For the evaluation, most often, a subjective visual examination is performed. Crucial information, such as metabolic activity, is not assessed. Thus, time-resolved monitoring of the respiration activity in agar cultivations is presented to provide additional insightful data on the metabolism. A modified version of the Respiration Activity MOnitoring System (RAMOS) was used to determine area-specific oxygen and carbon dioxide transfer rates and the resulting respiratory quotients of agar cultivations. Therewith, information on growth, substrate consumption, and product formation was obtained. The validity of the presented method was tested for different prokaryotic and eukaryotic organisms on agar, such as Escherichia coli BL21, Pseudomonas putida KT2440, Streptomyces coelicolor A3(2), Saccharomyces cerevisiae WT, Pichia pastoris WT, and Trichoderma reesei RUT-C30. Furthermore, it is showcased that several potential applications, including the determination of colony forming units, antibiotic diffusion tests, quality control for spore production or for pre-cultures and media optimization, can be quantitatively evaluated by interpretation of the respiration activity.

Experimental kS estimation: A comparison of methods for Corynebacterium glutamicum from lab to microfluidic scale.

Knowledge about the specific affinity of whole cells towarda substrate, commonly referred to as kS , is a crucial parameter for characterizing growth within bioreactors. State-of-the-art methodologies measure either uptake or consumption rates at different initial substrate concentrations. Alternatively, cell dry weight or respiratory data like online oxygen and carbon dioxide transfer rates can be used to estimate kS . In this work, a recently developed substrate-limited microfluidic single-cell cultivation (sl-MSCC)method is applied for the estimation of kS values under defined environmental conditions. This method is benchmarked with two alternative microtiter plate methods, namely high-frequency biomass measurement (HFB)and substrate-limited respiratory activity monitoring (sl-RA).As a model system, the substrate affinity kS of Corynebacterium glutamicum ATCC 13032 regarding glucose was investigated assuming a Monod-type growth response. A kS of <70.7 mg/L(with 95% probability) with HFB, 8.55 ± 1.38 mg/Lwith sl-RA, and 2.66 ± 0.99 mg/L with sl-MSCC was obtained. Whereas HFB and sl-RA are suitable for a fast initial kS estimation, sl-MSCC allows an affinity estimation by determining tD at concentrations less or equal to the kS value. Thus, sl-MSCC lays the foundation for strain-specific kS estimations under defined environmental conditions with additional insights into cell-to-cell heterogeneity.

Impact of different trace elements on metabolic routes during heterotrophic growth of C. ljungdahlii investigated through online measurement of the carbon dioxide transfer rate.

Synthesis gas fermentation using acetogenic clostridia is a rapidly increasing research area. It offers the possibility to produce platform chemicals from sustainable C1 carbon sources. The Wood-Ljungdahl pathway (WLP), which allows acetogens to grow autotrophically, is also active during heterotrophic growth. It acts as an electron sink and allows for the utilization of a wide variety of soluble substrates and increases ATP yields during heterotrophic growth. While glycolysis leads to CO2 evolution, WLP activity results in CO2 fixation. Thus, a reduction of net CO2 emissions during growth with sugars is an indicator of WLP activity. To study the effect of trace elements and ventilation rates on the interaction between glycolysis and the WLP, the model acetogen Clostridium ljungdahlii was cultivated in YTF medium, a complex medium generally employed for heterotrophic growth, with fructose as growth substrate. The recently reported anaRAMOS device was used for online measurement of metabolic activity, in form of CO2 evolution. The addition of multiple trace elements (iron, cobalt, manganese, zinc, nickel, copper, selenium, and tungsten) was tested, to study the interaction between glycolysis and the Wood ljungdahl pathway. While the addition of iron(II) increased growth rates and ethanol production, added nickel(II) increased WLP activity and acetate formation, reducing net CO2 production by 28%. Also, higher CO2 availability through reduced volumetric gas flow resulted in 25% reduction of CO2 evolution. These online metabolic data demonstrate that the anaRAMOS is a valuable tool in the investigation of metabolic responses i.e. to determine nutrient requirements that results in reduced CO2 production. Thereby the media composition can be optimized depending on the specific goal.

Non-invasive and time-resolved measurement of the respiration activity of Chinese hamster ovary cells enables prediction of key culture parameters in shake flasks.

Shake flasks are frequently used for mammalian cell suspension cultures. For process development and routine culture monitoring, information on culture behavior is needed early on. Here, cell-specific oxygen uptake rates (qO2 ) of two CHO cell lines were determined from shake flask experiments by simultaneous measurement of oxygen transfer rates (OTR) and viable cell concentrations (VCC). For cell line one, qO2 decreased from 2.38·10-10 to 1.02·10-10 mmol cell-1 h-1 during batch growth. For cell line two, qO2 was constant (1.90·10-10 mmol h-1 ). Determined qO2 values were used to calculate the VCC from OTR data. Cumulated oxygen consumption and glucose consumption were correlated for both cell lines and enabled calculation of glucose concentrations from OTR data. IgG producing cell line one had an oxygen demand of ∼15 mmoloxygen gglucose -1 , cell line two consumed ∼5 mmoloxygen gglucose -1 . The established correlations for determination of VCC and glucose were successfully transferred to subsequent cultivations for both cell lines. Combined measurement of the OTR and the carbon dioxide transfer rate enabled quantitative determination of the lactate concentration (production and consumption) without sampling. Taken together, non-invasive measurement of the respiration activity enabled time-resolved determination of key culture parameters for increased process understanding in shake flasks.

Online monitoring of gas transfer rates during CO and CO/H2 gas fermentation in quasi-continuously ventilated shake flasks.

Syngas fermentation is a potential player for future emission reduction. The first demonstration and commercial plants have been successfully established. However, due to its novelty, development of syngas fermentation processes is still in its infancy, and the need to systematically unravel and understand further phenomena, such as substrate toxicity as well as gas transfer and uptake rates, still persists. This study describes a new online monitoring device based on the respiration activity monitoring system for cultivation of syngas fermenting microorganisms with gaseous substrates. The new device is designed to online monitor the carbon dioxide transfer rate (CO2 TR) and the gross gas transfer rate during cultivation. Online measured data are used for the calculation of the carbon monoxide transfer rate (COTR) and hydrogen transfer rate (H2 TR). In cultivation on pure CO and CO + H2 , CO was continuously limiting, whereas hydrogen, when present, was sufficiently available. The maximum COTR measured was approximately 5 mmol/L/h for pure CO cultivation, and approximately 6 mmol/L/h for cultivation with additional H2 in the gas supply. Additionally, calculation of the ratio of evolved carbon dioxide to consumed monoxide, similar to the respiratory quotient for aerobic fermentation, allows the prediction of whether acetate or ethanol is predominantly produced. Clostridium ljungdahlii, a model acetogen for syngas fermentation, was cultivated using only CO, and CO in combination with H2 . Online monitoring of the mentioned parameters revealed a metabolic shift in fermentation with sole CO, depending on COTR. The device presented herein allows fast process development, because crucial parameters for scale-up can be measured online in small-scale gas fermentation.

Characterization of simultaneous heat, oxygen, and carbon dioxide transfer across a nonporous polydimethylsiloxane (PDMS) hollow fiber membrane

The provision of atmospheric oxygen, removal of carbon dioxide and heat is essential to sustain crew health during human spaceflight missions. Microgravity and reduced gravity environments (such as a planetary surface) limit the use of bubble-forming technologies, since surface tension and inertial or viscous forces may dominate buoyancy, creating an adverse effect in the system. Gas-to-liquid contacting membranes would allow for non-bubbling mass transfer through solution-diffusion, with the potential for simultaneous heat transfer, as done in membrane distillation operations. Previously published work, with similar applications, have focused on characterizing microporous membranes. In this study, a commercially available, nonporous polydimethylsiloxane (PDMS) hollow fiber membrane separated carbon dioxide from an ambient gas stream (296 K) while deoxygenating a cooling-water feed (274 K). Control experiments were conducted with gas and water at ambient temperatures (296 K and 292 K, respectively). Results show that increasing water feed rate (0.5 to 1.5 LPM) reduced the percentage of oxygen transferred from the water to the gas stream (approximately 25% less). Also, a reduction in water temperature significantly reduced percent oxygen transferred (approximately 35% reduction). System heat transfer and carbon dioxide transfer rates were positively correlated to gas flow rates, showing gas-phase dependence. Coefficients derived from the experimental results were used to form semi-empirical mass transport models. Results of this study developed two findings, that a nonporous PDMS membrane could be used for simultaneous heat and mass transfer which could also be described from first-order principles.

Online monitoring applying the anaerobic respiratory monitoring system reveals iron(II) limitation in YTF medium for Clostridiumljungdahlii.

Online monitoring of microbial cultures is an effective approach for studying both aerobic and anaerobic microorganisms. Especially in small‐scale cultivations, several parallel online monitored experiments can generate a detailed understanding of the cultivation, compared to a situation where a few data points are generated from time course sampling and offline analysis. However, the availability of small‐scale online monitoring devices for acetogenic organisms is limited. In this study, the previously reported anaerobic Respiration Activity MOnitoring System (anaRAMOS) device was adapted for online monitoring of Clostridium ljungdahlii (C. ljungdahlii) cultures with fructose as the carbon source. The anaRAMOS was applied to identify conversion of different carbon sources present in commonly used YTF medium. An iron(II) deficiency was discovered in this medium for C. ljungdahlii. Addition of iron(II) to the YTF medium reduced the cultivation time and increased biomass yield of C. ljungdahlii cultures by 50% and 40%, respectively. The measurement of the carbon dioxide transfer rate was used to calculated the iron(II) contained in complex components. By demonstrating the application of the anaRAMOS device for medium optimization, it is proven that the described online monitoring device has potential for use in process development.

The metabolic switch can be activated in a recombinant strain of Streptomyces lividans by a low oxygen transfer rate in shake flasks

BackgroundIn Streptomyces, understanding the switch from primary to secondary metabolism is important for maximizing the production of secondary metabolites such as antibiotics, as well as for optimizing recombinant glycoprotein production. Differences in Streptomyces lividans bacterial aggregation as well as recombinant glycoprotein production and O-mannosylation have been reported due to modifications in the shake flask design. We hypothetized that such differences are related to the metabolic switch that occurs under oxygen-limiting conditions in the cultures.ResultsShake flask design was found to affect undecylprodigiosin (RED, a marker of secondary metabolism) production; the RED yield was 12 and 385 times greater in conventional normal Erlenmeyer flasks (NF) than in baffled flasks (BF) and coiled flasks (CF), respectively. In addition, oxygen transfer rates (OTR) and carbon dioxide transfer rates were almost 15 times greater in cultures in CF and BF as compared with those in NF. Based on these data, we obtained respiration quotients (RQ) consistent with aerobic metabolism for CF and BF, but an RQ suggestive of anaerobic metabolism for NF.ConclusionAlthough the metabolic switch is usually related to limitations in phosphate and nitrogen in Streptomyces sp., our results reveal that it can also be activated by low OTR, dramatically affecting recombinant glycoprotein production and O-mannosylation and increasing RED synthesis in the process.

Culture medium optimization for osmotolerant yeasts by use of a parallel fermenter system and rapid microbiological testing

In the present study, a culture medium for qualitative detection of osmotolerant yeasts, named OM, was developed. For the development, culture media with different concentrations of glucose, fructose, potassium chloride and glycerin were analyzed in a Biolumix™ test incubator. Selectivity for osmotolerant yeasts was guaranteed by a water activity (aw)-value of 0.91. The best results regarding fast growth of Zygosaccharomyces rouxii (WH 1002) were achieved in a culture medium consisting of 45% glucose, 5% fructose and 0.5% yeast extract and in a medium with 30% glucose, 10% glycerin, 5% potassium chloride and 0.5% yeast extract. Substances to stimulate yeast fermentation rates were analyzed in a RAMOS® parallel fermenter system, enabling online measurement of the carbon dioxide transfer rate (CTR) in shaking flasks. Significant increases of the CTR was achieved by adding especially 0.1–0.2% ammonium salts ((NH4)2HPO4, (NH4)2SO4 or NH4NO3), 0.5% meat peptone and 1% malt extract. Detection times and the CTR of 23 food-borne yeast strains of the genera Zygosaccharomyces, Torulaspora, Schizosaccharomyces, Candida and Wickerhamomyces were analyzed in OM bouillon in comparison to the selective culture media YEG50, MYG50 and DG18 in the parallel fermenter system. The OM culture medium enabled the detection of 102CFU/g within a time period of 2–3days, depending on the analyzed yeast species. Compared with YEG50 and MYG50 the detection times could be reduced. As an example, W. anomalus (WH 1021) was detected after 124h in YEG50, 95.5h in MYG50 and 55h in OM bouillon. Compared to YEG50 the maximum CO2 transfer rates for Z. rouxii (WH 1001), T. delbrueckii (DSM 70526), S. pombe (DSM 70576) and W. anomalus (WH 1016) increased by a factor ≥2.6. Furthermore, enrichment cultures of inoculated high-sugar products in OM culture medium were analyzed in the Biolumix™ system. The results proved that detection times of 3days for Z. rouxii and T. delbrueckii can be realized by using OM in combination with the automated test system even if low initial counts (101CFU/g) are present in the products. In conclusion, the presented data suggest that the OM culture medium is appropriate for the enrichment of osmotolerant yeasts from high-sugar food products.

A Review on the Advancement of Online Monitoring System for Cellulosic Ethanol Production

Bioethanol is mainly produced by sugar fermentation process. Due to global demand on energy for transportation and environmental concern, biofuels as renewable energy in replacing petrol, the non-renewable energy source, has come into picture. Utilization of lignocellulosic biomass such as woody biomass (trees), herbaceous biomass (grasses) and waste cellulosic materials (solid waste) could be used in replacing starch (such as corn and potato) as source of sugar in producing bioethanol. Recently, study on cellulosic ethanol was focussing on fermentation process using ethanologenic strain such as engineered Escherichia coli and Saccharomyces cerevisiae. Invasive method in the study during fermentation may lead to uncertain or unwanted screening strategies or metabolic pathways. This paper reviews about the online monitoring system used by researchers in order to study the growth kinetics of ethanologenic strain. Online monitoring system for the Oxygen Transfer Rate (OTR) and Carbon dioxide Transfer Rate (CTR) is found to be the important method to study kinetic model of ethanologenic strain, thus increasing metabolic yields with optimum design condition.

Impact of two ionic liquids, 1-ethyl-3-methylimidazolium acetate and 1-ethyl-3-methylimidazolium methylphosphonate, on Saccharomyces cerevisiae: metabolic, physiologic, and morphological investigations.

BackgroundIonic liquids (ILs) are considered as suitable candidates for lignocellulosic biomass pretreatment prior enzymatic saccharification and, obviously, for second-generation bioethanol production. However, several reports showed toxic or inhibitory effects of residual ILs on microorganisms, plants, and animal cells which could affect a subsequent enzymatic saccharification and fermentation process.ResultsIn this context, the impact of two hydrophilic imidazolium-based ILs already used in lignocellulosic biomass pretreatment was investigated: 1-ethyl-3-methylimidazolium acetate [Emim][OAc] and 1-ethyl-3-methylimidazolium methylphosphonate [Emim][MeO(H)PO2]. Their effects were assessed on the model yeast for ethanolic fermentation, Saccharomyces cerevisiae, grown in a culture medium containing glucose as carbon source and various IL concentrations. Classical fermentation parameters were followed: growth, glucose consumption and ethanol production, and two original factors: the respiratory status with the oxygen transfer rate (OTR) and carbon dioxide transfer rate (CTR) of yeasts which were monitored online by respiratory activity monitoring systems (RAMOS). In addition, yeast morphology was characterized by environmental scanning electron microscope (ESEM).The addition of ILs to the growth medium inhibited the OTR and switched the metabolism from respiration (conversion of glucose into biomass) to fermentation (conversion of glucose to ethanol). This behavior could be observed at low IL concentrations (≤5% IL) while above there is no significant growth or ethanol production. The presence of IL in the growth medium also induced changes of yeast morphology, which exhibited wrinkled, softened, and holed shapes. Both ILs showed the same effects, but [Emim][MeO(H)PO2] was more biocompatible than [Emim][OAc] and could be better tolerated by S. cerevisiae.ConclusionsThese two imidazolium-derived ILs were appropriate candidates for useful pretreatment of lignocellulosic biomass in the context of second-generation bioethanol production. This fundamental study provides additional information about the toxic effects of ILs. Indeed, the investigations highlighted the better tolerance by S. cerevisiae of [Emim][MeO(H)PO2] than [Emim][OAc].

Definition of culture conditions for Arxula adeninivorans, a rational basis for studying heterologous gene expression in this dimorphic yeast

The yeast Arxula adeninivorans is considered to be a promising producer of recombinant proteins. However, growth characteristics are poorly investigated and no industrial process has been established yet. Though of vital interest for strain screening and production processes, rationally defined culture conditions remain to be developed. A cultivation system was evolved based on targeted sampling and mathematical analysis of rationally designed small-scale cultivations in shake flasks. The oxygen and carbon dioxide transfer rates were analyzed as conclusive online parameters. Oxygen limitation extended cultivation and led to ethanol formation in cultures supplied with glucose. Cultures were inhibited at pH-values below 2.8. The phosphorus demand was determined as 1.55 g phosphorus per 100 g cell dry weight. Synthetic SYN6 medium with 20 g glucose l(-1) was optimized for cultivation in shake flasks by buffering at pH 6.4 with 140 mmol MES l(-1). Optimized SYN6 medium and operating conditions provided non-limited cultivations without by-product formation. A maximal specific growth rate of 0.32 h(-1) and short fermentations of 15 h were achieved. A pH optimum curve was derived from the oxygen transfer rates of differently buffered cultures, showing maximal growth between pH 2.8 and 6.5. Furthermore, it was shown that the applied medium and cultivation conditions were also suitable for non-limiting growth and product formation of a genetically modified A. adeninivorans strain expressing a heterologous phytase.

Improvement of carbon dioxide removal through artificial light intensity and temperature by constructed green microalgae consortium in a vertical bubble column photobioreactor

Aims: This study was aimed to demonstrate that environmental conditions, such as light intensity, photoperiodism and temperature, played a determining role in improving CO2 removal and cellular propagation. Photosynthesis by microalgae in a direct CO2 to biomass conversion in engineered systems such as photobioreactors, has been frequently used for CO2 removal. The goal of this study was to obtain high CO2 removal by green microalgae strains cultivated in a vertical bubble column photobioreactor. Methodology and results: Constructed consortium containing Chlorella sp., Scenedesmus obliquus, and Ankistrodesmus sp. were cultured in temperatures (°C) of 25, 30, 25; light intensities (lux) of 2500, 4000 and photoperiodisms (light/dark; hour) of 24/0, 16/8, and 12/12. The experiment demonstrated that microalgae was capable of tolerating up to 7% CO2 concentrations under variation of light intensities, photoperiodisms, and temperature conditions. Synergetic of three microalgae capable of utilizing CO2 and transformed it to become biomass. The result also showed that growth was best at light intensity of 4000 lux for 16 h a day and temperature of 30 °C. The maximum growth rate (µ) of 0.38 per day was obtained from culture injected with 5% CO2 concentration. Conclusion, significance and impact study: The CO2 removal efficiency (%) was 49.02, whereas CO2 utilization efficiency (%), carbon dioxide transfer rate (CTR; gCO2/L.h) and carbon dioxide fixation rate (qCO2/h) were 15.15, 101.29 and 42.02, respectively. Biofixation of CO2 by the constructed consortium has recently gained renewed interest as a promising strategy for CO2 mitigation.

A sensitive monitoring system for mammalian cell cultivation processes: a PAT approach

Biopharmaceuticals such as antibodies are produced in cultivated mammalian cells, which must be monitored to comply with good manufacturing practice. We, therefore, developed a fully automated system comprising a specific exhaust gas analyzer, inline analytics and a corresponding algorithm to precisely determine the oxygen uptake rate, carbon dioxide evolution rate, carbon dioxide transfer rate, transfer quotient and respiratory quotient without interrupting the ongoing cultivation, in order to assess its reproducibility. The system was verified using chemical simulation experiments and was able to measure the respiratory activity of hybridoma cells and DG44 cells (derived from Chinese hamster ovary cells) with satisfactory results at a minimum viable cell density of ~2.0×10(5) cells ml(-1). The system was suitable for both batch and fed-batch cultivations in bubble-aerated and membrane-aerated reactors, with and without the control of pH and dissolved oxygen.

Pumpless arterio-venous extracorporeal lung assist compared with veno-venous extracorporeal membrane oxygenation during experimental lung injury

Extracorporeal lung support is effective to prevent hypoxaemia and excessive hypercapnia with respiratory acidosis in acute respiratory distress syndrome. Miniaturized veno-venous extracorporeal membrane oxygenation (mECMO) and arterio-venous pumpless extracorporeal lung assist (pECLA) were compared for respiratory and haemodynamic response and extracorporeal gas exchange and device characteristics. After induction of acute lung injury by repeated lung lavage, 16 anaesthetized and mechanically ventilated pigs were randomized to mECMO (Medos Hilite/Deltastream) or pECLA (iLA Novalung) for 24 h. Improved gas exchange allowed reduced ventilation and plateau pressure in both groups. An arterio-venous shunt flow of up to 30% of cardiac output resulted in a left cardiac work of 6.8 (2.0) kg m for pECLA compared with 5.0 (1.4) kg m for mECMO after 24 h (P<0.05). Both devices provided adequate oxygen delivery to organs. The oxygen transfer of pECLA was lower than mECMO due to inflow of arterial oxygenated blood [16 (5) compared with 64 (28) ml min(-1) after 24 h, P<0.05]. Unexpectedly, the carbon dioxide transfer rate was also lower [58 (28) compared with 111 (42) ml min(-1) after 24 h, P<0.05], probably caused by a Haldane effect preventing higher transfer rates in combination with lower extracorporeal blood flow. Both devices have the potential to unload the lungs from gas transfer sufficiently to facilitate lung-protective ventilation. Although technically less complex, oxygen uptake and carbon dioxide removal are limited in pECLA, and cardiac work was increased. mECMO overcomes these limitations and might provide better cardiopulmonary protection.

Carbon dioxide stripping in aquaculture – Part II: Development of gas transfer models

The basic mass transfer equation for gases such as oxygen and carbon dioxide can be derived from integration of the driving force equation. Because of the physical characteristics of the gas transfer processes, slightly different models are used for aerators tested under the non steady-state procedures, than for packed columns, or weirs. It is suggested that the standard condition for carbon dioxide should be 20°C, 1atm, CCO2=20 mg/kg, and XCO2=0.000285. The selection of the standard condition for carbon dioxide based on a fixed mole fraction ensures that standardized carbon dioxide transfer rates will be comparable even though the value of CCO2* in the atmosphere is increasing with time. The computation of mass transfer for carbon dioxide is complicated by the impact of water depth and gas phase enrichment on the saturation concentration within the unit, although the importance of either factor depends strongly on the specific type of aerator. For some types of aerators, the most accurate gas phase model remains to be determined for carbon dioxide. The assumption that carbon dioxide can be treated as a non-reactive gas in packed columns may apply for cold acidic waters but not for warm alkaline waters.

Modeling urea biodegradation in activated sludge using combined respirometric–titrimetric measurements

Urea biodegradation kinetics determination has been performed in the literature using a two-step nitrification model that was calibrated using on-line respirometric measurements. However, the model neglected the initial hydrolysis step that converts urea to carbon dioxide and ammonia nitrogen, and assumed constant carbon dioxide transfer rate (CTR), though it is inherently a non-linear process which has an impact on the titrimetric modeling. Hence, in this paper, it is aimed to propose a complete two-step nitrification model for urea biodegradation paying attention to urea degradation pathway along with due consideration given for non-linear CTR process occurring in activated sludge system. Experiments were performed in a simple batch reactor equipped with respirometric and titrimetric set-up. Three different initial urea concentrations were added to the reactor for investigating the process kinetics. Proposed model was successfully calibrated with respirometric, titrimetric and combined respirometric–titrimetric measurements; and the estimated parameters were compared for model evaluation. Furthermore, the proposed model was validated with off-line ammonium, nitrite and nitrate measurements. The study revealed that urea was hydrolyzed at a faster rate in liquid phase. The maximum growth rates of the Nitrosomonas species and the Nitrobacter species were found to be 0.065–0.1 d–1 and 0.006–0.008 d–1 respectively.

Determination of CO2 sensitivity of micro-organisms in shaken bioreactors. II. Novel online monitoring method

In the present study, a new online monitoring method for the determination of the CO₂ sensitivity of micro-organisms, based on the values of the respiration factors [OTR (oxygen transfer rate) and CTR (carbon dioxide transfer rate)], obtained by using the RAMOS (respiratory activity monitoring system) device considering a variety of aeration rates in the measuring flask, is investigated. Based on the data of the OTR, obtained by RAMOS under a variety of specific aeration rates, the proposed new method was developed as an online monitoring method for CO₂ sensitivity of micro-organisms in shaken bioreactors. A maximum accumulated CO₂ concentration of 12% was derived in applied methods, provided that the cultivation system is carried out under optimal conditions. Additionally, to predict these conditions, an unsteady-state gas transfer model in shaken bioreactors would be very advantageous. The data of OTR obtained using the RAMOS device were analysed and recalculated by a programme considering the calibration factor (Cf). The major advantage of the new method is the possibility to determine the metabolic activity, regardless of manual sampling.