In this investigation, two separate methods for determining oxygen transfer rates were applied to the oxidation ditches of an activated sludge plant. Steady state oxygen uptake rate testing and an oxygen mass balance technique were used to propose an in-process procedure for monitoring aeration efficiency using available resources. Although some overall averages offered promise, the testing results revealed that the mass balance analysis yielded results that do not accurately represent the oxygen transfer capabilities within the individual reactors due to shared variables that control the oxygen transfer rate. The steady state method provided more favorable results. Overall averages of daily oxygen transfer rates determined using the steady state method displayed a ratio of oxygen transfer rate between the reactors that corresponds to the expected ratio of 5/6 derived from the linear feet of aerator rotor present in each reactor.

This work concerns the efficient treatment of wastewaters from pulp and paper mills by means of aerobic biological processes. For treatment processes there are many aspects of efficiency and the present study investigates both energy efficiency and purification efficiency during disturbances. Special focus is put on wood extractives, such as resin acids and fatty acids, since they can cause negative effects in fish and other organisms in the receiving waters. They can furthermore be toxic to microorganisms in a biological treatment plant. They also affect oxygen transfer, which is important for energy efficient aeration of aerobic biological treatment processes. This thesis includes five papers/studies and presents a strategy for efficient treatment of forest industrial wastewaters. The results should help creating resilient wastewater treatment strategies with efficient use of energy. One new strategy proposed here includes separation of extractives before the wastewater is treated biologically, and the use of the extra amount of sludge as an energy source, shifting the energy balance from negative to positive.

A microbubble dispersion (MBD) was used to supply oxygen for an aerobic fermentation of Baker's yeast. The 1-liter microbubble dispersion generator supplied bubbles for 20-liter and 50-liter working volume fermentations in a 72-liter pilot scale fermenter. The microbubbles were stabilized by the surfactants naturally present in the culturing broth medium. The growth patterns of yeast Saccharomyces cerevisiae, cultured at agitation speeds of 150 rpm and 500 rpm, were compared for oxygen supplied by ordinary air sparging and by MBD sparging. Both air sparged and MBD systems were supplied air at equivalent volumetric flow rates. The volumetric oxygen transfer coefficients (KLa) were estimated by the yield coefficient method. The KLa values increased from 142.5 to 458.3 h-1 and from 136.1 to 473.3 h-1 for 20- and 50- liter runs, respectively, as the agitation speed was increased from 150 to 500 rpm in the ordinary air sparged fermentations. The oxygen transfer coefficients in the MBD sparged fermentations were found to be independent of the fermenter agitation speed at approximately 480 h-1 for 20-liter runs and 340 h-1 for 50-liter runs. The growth rates for MBD at 150 rpm were essentially equivalent with air sparged fermentations at 500 rpm. The total power consumption per unit volume of broth for the 150 rpm, MBD fermentation was 68% lower than the 500 rpm, air sparged run for the 20-liter fermentations and was 55% lower for the 50-liter fermentations. / Master of Science

Animal cells are often cultured in stirred tank reactors. Having no cell wall, these animal cells are very sensitive to the fluid mechanical stresses that result from agitation by the impeller and from the rising and bursting of bubbles, which are generated within the culture medium in the stirred tank to supply oxygen by mass transfer to the cells. If excessive, these fluid mechanical stresses can result in damage/death of animal cells. Stress due to the rising and bursting of bubbles can be avoided by using a gas-permeable membrane, in the form of a long coiled tube (with air passing through it) within the stirred tank, instead of air-bubbles to oxygenate the culture medium. Fluid mechanical stress due to impeller agitation can be controlled using appropriate impeller rotational speeds. The aim of this study was to lay the foundations for future work in which a correlation would be developed between cell damage/death and the fluid mechanical stresses that result from impeller agitation and bubbling. Such a correlation could be used to design stirred-tank reactors at any scale and to determine appropriate operating conditions that minimise cell damage/death due to fluid mechanical stresses. Firstly, a validated CFD model of a baffled tank stirred with a Rushton turbine was developed to allow fluid mechanical stresses due to impeller agitation to be estimated. In these simulations, special attention was paid to the turbulence energy dissipation rate, which has been closely linked to cell damage/death in the literature. Different turbulence models, including the k-ε, SST, SSG-RSM and the SAS-SST models, were investigated. All the turbulence models tested predicted the mean axial and tangential velocities reasonably well, but under-predicted the decay of mean radial velocity away from the impeller. The k-ε model predicted poorly the generation and dissipation of turbulence in the vicinity of the impeller. This contrasts with the SST model, which properly predicted the appearance of maxima in the turbulence kinetic energy and turbulence energy dissipation rate just off the impeller blades. Curvature correction improved the SST model by allowing a more accurate prediction of the magnitude and location of these maxima. However, neither the k-ε nor the SST models were able to properly capture the chaotic and three-dimensional nature of the trailing vortices that form downstream of the blades of the impeller. In this sense, the SAS-SST model produced more physical predictions. However,this model has some drawbacks for modelling stirred tanks, such as the large number of modelled revolutions required to obtain good statistical averaging for calculating turbulence quantities. Taking into consideration both accuracy and solution time, the SSG-RSM model was the least satisfactory model tested for predicting turbulent flow in a baffled stirred tank with a Rushton turbine. In the second part of the work, experiments to determine suitable oxygen transfer rates for culturing cells were carried out in a stirred tank oxygenated using either a sparger to bubble air through the culture medium or a gas-permeable membrane. Results showed that the oxygen transfer rates for both methods of oxygenation were always above the minimum oxygen requirements for culturing animal cells commonly produced in industry, although the oxygen transfer rate for air-bubbling was at-least 10 times higher compared with using a gas-permeable membrane. These results pave the way for future experiments, in which animal cells would be cultured in the stirred tank using bubbling and (separately) a gas-permeable membrane for oxygenation so that the effect of rising and bursting bubbles on cell damage/death rates can be quantified. The effect of impeller agitation on cell damage/death would be quantified by using the gas permeable membrane for oxygenation (to remove the detrimental effects of bubbling), and changing the impeller speed to observe the effect of agitation intensity. In the third and final part of this work, the turbulent flow in the stirred tank used in the oxygenation experiments was simulated using CFD. The SST turbulence model with curvature correction was used in these simulations, since it was found to be the most accurate model for predicting turbulence energy dissipation rate in a stirred tank. The predicted local maximum turbulence energy dissipation rate of 8.9x10¹ m2/s3 at a rotational speed of 900 rpm was found to be substantially less than the value of 1.98x10⁵ m2/s3 quoted in the literature as a critical value above which cell damage/death becomes significant. However, the critical value for the turbulence energy dissipation rate quoted in the literature was determined in a single-pass flow device, whereas animal cells in a stirred tank experience frequent exposure to high turbulence energy dissipation rates (in the vicinity of the impeller) due to circulation within the stirred tank and long culture times. Future cell-culturing experiments carried out in the stirred tank of this work would aim to determine a more appropriate critical value for the turbulence energy dissipation rate in a stirred tank, above which cell damage/death becomes a problem.

Optimising illumination is essential for optimising the growth of phototrophic cells and their production of desired metabolites and/or biomass. This requires appropriate modulation of light and other key inputs and continuous online monitoring of their metabolic activities. Powerful non-invasive systems for cultivating heterotrophic organisms include shake flasks in online monitoring units, but they are rarely used for phototrophs because they lack the appropriate illumination design and necessary illuminatory power. This study presents the design and characterisation of a photosynthetic shake flask unit, illuminated from below by warm white light-emitting diodes with variable light intensities up to 2300 μmol m-2 s-1. The photosynthetic unit was successfully used, in combination with online monitoring of oxygen production, to cultivate Arthrospira platensis. In phototrophic growth under continuous light and a 16 h light/8 h dark cycle (light intensity: 180 μmol m-2 s-1), the oxygen transfer rate and biomass-related oxygen production were - 1.5 mmol L-1 h-1 and 0.18 mmol O2 gx-1 h-1, respectively. The maximum specific growth rate was 0.058 h-1, during the exponential growth phase, after which the biomass concentration reached 0.75 g L-1.

Plants produce a variety of secondary metabolites, e.g. to defend themselves against herbivores or to attract pollinating insects. Plant cell biotechnology offers excellent opportunities in order to use such secondary plant metabolites to produce goods with consistent quality and quantity throughout the year, and therefore to act independently from biotic and abiotic environmental factors. This article presents results of an extensive study of plant cell in vitro cultivation in a modern shake flask system with non-invasive online respiration activity monitoring unit. Comprehensive screening experiments confirm the successful transfer of a model culture (sunflower suspension) into the shake flask monitoring device and the suitability of this respiration activity monitoring unit as qualified tool for screening of plant in vitro cultures (sunflower and sage suspension). The authors demonstrate deviations between online and offline data due to varying water evaporation from different culture flask types. The influence of evaporation on growth-specific parameters thereby rises with increasing cultivation time. Furthermore, possibilities to minimize the impact of evaporation, either by adjusting the inlet air moisture or by measuring the evaporation in combination with an appropriate correction of the measured growth values, are shown.

Plants produce a variety of secondary metabolites, e.g. to defend themselves against herbivores or to attract pollinating insects. Plant cell biotechnology offers excellent opportunities in order to use such secondary plant metabolites to produce goods with consistent quality and quantity throughout the year, and therefore to act independently from biotic and abiotic environmental factors. This article presents results of an extensive study of plant cell in vitro cultivation in a modern shake flask system with non-invasive online respiration activity monitoring unit. Comprehensive screening experiments confirm the successful transfer of a model culture (sunflower suspension) into the shake flask monitoring device and the suitability of this respiration activity monitoring unit as qualified tool for screening of plant in vitro cultures (sunflower and sage suspension). The authors demonstrate deviations between online and offline data due to varying water evaporation from different culture flask types. The influence of evaporation on growth-specific parameters thereby rises with increasing cultivation time. Furthermore, possibilities to minimize the impact of evaporation, either by adjusting the inlet air moisture or by measuring the evaporation in combination with an appropriate correction of the measured growth values, are shown.

Optimising illumination is essential for optimising the growth of phototrophic cells and their production of desired metabolites and/or biomass. This requires appropriate modulation of light and other key inputs and continuous online monitoring of their metabolic activities. Powerful non-invasive systems for cultivating heterotrophic organisms include shake flasks in online monitoring units, but they are rarely used for phototrophs because they lack the appropriate illumination design and necessary illuminatory power. This study presents the design and characterisation of a photosynthetic shake flask unit, illuminated from below by warm white light-emitting diodes with variable light intensities up to 2300 μmol m-2 s-1. The photosynthetic unit was successfully used, in combination with online monitoring of oxygen production, to cultivate Arthrospira platensis. In phototrophic growth under continuous light and a 16 h light/8 h dark cycle (light intensity: 180 μmol m-2 s-1), the oxygen transfer rate and biomass-related oxygen production were - 1.5 mmol L-1 h-1 and 0.18 mmol O2 gx-1 h-1, respectively. The maximum specific growth rate was 0.058 h-1, during the exponential growth phase, after which the biomass concentration reached 0.75 g L-1.