Browsing by Author "Harrison, Susan TL"
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- ItemRestrictedCharacterisation of the Complex Microbial Community Associated with the ASTER™ Thiocyanate Biodegradation System(Elsevier, 2015-05-15) Huddy, Robert J; Van Zyl, A Wynand; Van Hille, Robert P; Harrison, Susan TLThe ASTER™ process is used to bioremediate cyanide- (CN-) and thiocyanate- (SCN-) 13 containing waste water. This aerobic process is able to reduce the CN- and SCN14 concentrations to below 1 mg/L efficiently in a continuous system, facilitating reuse of 15 process water or safe discharge. Such remediation systems, which completely eliminate risk 16 associated with the pollutants, are essential for sustainable mineral processing and the long 17 term minimisation of environmental burden through both pollutant destruction and exploiting 18 opportunities for nutrient recycle. Process robustness of these bioremediation options can be 19 enhanced by good understanding of the microbial community involved in the process. To 20 date, the microbial consortia associated with the ASTER™ bioprocess have been poorly 21 characterised using isolation approaches only. As a result, the relative abundance and 22 diversity of the community has been significantly under-represented. In this study, both planktonic and biofilm-associated biomass have been observed. 23 Microscopy has revealed the 24 diversity of these communities, including bacteria, motile eukaryotes, filamentous fungi and 25 algae, with the biofilm densely packed with microorganisms. The results of the molecular 26 characterisation study reported here, using a clone library approach, demonstrate that the 27 microbial community associated with the ASTER™ bioprocess system is far more complex 28 than previously suggested, with over 30 bacterial species identified thus far. On-going 29 investigations focus on identification of key microbial community members associated with 30 SCN- biodegradation and other critical metabolic functions, as well as the expected dynamic 31 response of this complex microbial community to shifts in the operating window of the 32 process.
- ItemOpen AccessEnhanced bioethanol fermentation from mixed xylose and glucose using free and immobilized cultures: mathematical model and experimental observation(2019) Ghods, Nosaibeh Nosrati; Tai, Siew L; Harrison, Susan TL; Isafiade, Adeniyi JBioethanol plays a significant role in the world of liquid biofuel. However, majority of bioethanol is produced from edible food crops such as corn and sugarcane that causes an increase in demand for vacant lands for food production and, subsequently, increase in the cost of food manufacturing. Therefore, alternative raw materials for bioethanol production are sought after, such as sugarcane bagasse which is a waste material from the sugar industry. South Africa, a net sugar exporter, has a large potential to produce bioethanol from sugarcane bagasse. This research focuses on the study of the production of bioethanol from glucose and xylose which are the two most abundant sugars in hydrolysed sugarcane bagasse. To date, no suitable wild type organisms can concomitantly ferment both glucose and xylose to ethanol efficiently. Options to address the co-fermentation of glucose and xylose include genetic modification of the selected microorganism to include both pathways - limitation in the understanding of the metabolic pathways regulations - or utilization of two microorganisms in co-culture or sequential culture e.g. Zymomonas mobilis and Pichia stipitis for efficient fermentation of glucose and xylose respectively. In this study, the dual micro-organism route is explored. There are numerous problems associated with co-culturing. Xylose, a non-preferred carbon source is only converted if the glucose concentration is adequately low due to catabolite repression. In order to increase xylose conversion, a low glucose concentration is required. Therefore, two stage sequential fermentation either in one or two reactors was tested. A high inoculum of suspended or immobilized Z. mobilis was inoculated in the first stage to convert the glucose rapidly. Varying reactor configuration, including the continuous fluidized bed, continuous stirred tank reactor (CSTR) and stirred batch reactor were considered. The products and residual substrate from this fermentation was then directed to a second stage, using either a CSTR or stirred batch configuration, with a high inoculum of P. stipitis in suspension culture for conversion of xylose. When immobilized, Z. mobilis was entrapped in calcium alginate beads. On the issue of ethanol tolerance, P. stipitis is generally more easily inhibited by ethanol (threshold ethanol concentration of 35 g L-1) compared to other ethanol producing strains such as Z. mobilis (threshold ethanol concentration of 127 g L-1) and Saccharomyces cerevisiae (threshold ethanol concentration of 118.2 g L-1). In order to overcome this, a continuous bioprocess was investigated to keep ethanol concentrations in Stage II below 35 g L-1 to prevent inhibition of metabolic reactions in P. stipitis. Further, ethanol fermentation by Z. mobilis requires obligate anaerobic conditions while xylose conversion by P. stipitis is optimum under microaerobic conditions. Therefore, oxygen was sparged into the second P. stipitis stage only. The following components were carried out in this project to improve the kinetic model and to find accurate kinetic data in the selected process of the two stage sequential fermentation. Firstly, where kinetic parameters were not available in literature, the kinetic parameter relationships of glucose and xylose utilization between different constructs of the same species were examined, for example, a wild type and engineered strain. This approach was used for glucose conversion using wild type Z. mobilis, owing to the ill-fit of available kinetic parameters with experimental results. In this study, the correction factors on estimated kinetic parameters from linear and non-linear regression when a xylose fermentation route was inserted recombinantly (S. cerevisiae RWB 217) into the native culture (S. cerevisiae CEN.PK 113-7D) were determined. From kinetic parameters of an engineered strain with the xylose-fermenting pathway (Z. mobilis ZM4 (pZB5)) and the correction factors, kinetic parameters of the wild-type Z. mobilis ZM4 were determined. Predicted rates of Z. mobilis ZM4 were then validated with experimental data generated in this study. Then, the optimum initial biomass concentration required to provide a faster volumetric rate of sugar utilisation and ethanol production, as well as the optimum oxygenation level for xylose conversion using P. stipitis achieved through appropriate aeration were investigated through experimental observation and using a MATLAB mathematical model developed through combination of the Andrews and Levenspiel's models, with oxygen, substrate, cell and product terms. Experiments were carried out to validate the kinetic model and data under anaerobic and microaerobic growth conditions in a batch process. The results showed that both increasing the initial biomass concentration (3 g L-1) and operating under optimum oxygenation levels (0.1 vvm) benefitted the ethanol production and yield by P. stipitis from xylose. It was also concluded that the addition of the oxygen effective factors in the developed model allowed for optimization of aeration in the fermentation system. Next, the custom kinetic model for fermentation process of bioethanol production was developed in Aspen Custom Modeller (ACM) and embedded in Aspen Plus. The model includes equations of vapour-liquid equilibrium (VLE), mass balance, and energy balance (e.g. molecular weight, thermodynamic phase equilibria, kinetic equation). The obtained results showed better agreement between industrial data and kinetic model (1% differences) than a stoichiometric model (9% differences). The simulation showed that ACM integrated into Aspen Plus allowed for complex biological processes to be accurately predicted for biomass growth, ethanol production and sugar consumption. Finding suitable microorganisms and process conditions for efficient glucose and xylose conversion is still currently a challenge and requires optimization. Therefore, this research focusses on improving the conversion of glucose and xylose to bioethanol, with specific emphasis on the fermentation systems used to maximize biomass efficiency, and ethanol yields and productivities. Manipulation of process conditions ranging from operation conditions (e.g. batch, fed-batch, continuous), process parameters (aeration, temperature, pH), immobilization technique and type of microorganism initially using kinetic models and thereafter validating with experimental data, therefore, offers a quick and strong foundation in improving bioethanol yields and productivities.
- ItemOpen AccessInvestigating process stresses on Saccharomyces cerevisiae using isothermal microcalorimetry(2017) Myers, Matthew; Harrison, Susan TL; Tai, Siew; Huddy, Robert; Fagan-Endres, MarijkeMaximising performance of microbial processes, including yeast-based processes, in an industrial setting requires understanding of the impact of process stresses. These may be the result of process configuration, dilution, temperature changes, hydrodynamic conditions or process perturbations. Methods to determine the microbial metabolic response to such stresses have long been sought, but are typically limited, often requiring the use of a suite of methods to assess the physiological status and state. The recent technical advances in microcalorimetry suggest potential for the use of isothermal microcalorimetry (IMC) to determine yeast viability and vitality and is investigated here. IMC is a laboratory method whereby the real-time heat produced by a chemical, biological or physical process is measured in the micro to nano watt range. It is proposed that this heat production may be correlated to the physiological state of the microbial catalyst and can be used to measure the impact of different stresses. In this study, the potential of IMC as a method for exploring process stress is investigated using Saccharomyces cerevisiae and its application in the beer brewing industry as a case study. Here, it is well known that yeast viability and vitality have commercial significance. IMC is sufficiently sensitive to detect the heat given off by 1000 yeast cells. However, IMC cannot distinguish between different heat flows within a system i.e. it is non-specific. The literature demonstrates how IMC has been used in the study of numerous microbiological fields, including the growth and metabolism of yeast. Previous studies have successfully derived the specific growth rate and cell numbers of a growing yeast population from analysing power and heat curves. The specific growth activity and specific growth retardation of yeast and how these parameters relate to bactericidal and bacteriostatic effects has also been examined by a number of authors. The key objectives of this study were to determine the viability and vitality of Saccharomyces cerevisiae using IMC and to assess the impact of stresses on yeast viability and vitality. This was achieved by measuring the thermal power produced by a growing yeast suspension as a function of its overall growth and metabolism. Two industrially relevant stresses were examined: cold shock and ethanol shock. The effect of these stresses has yet to be studied using microcalorimetry. The growth of Saccharomyces cerevisiae under ethanol stress was used as an inhibition study to isolate its effects on the growth thermogram. Following the generation of thermograms under control and stress conditions using IMC, a method for their quantitative analysis was developed. Curves were fitted to the heat data using an exponential growth equation and the time for the heat flow curve to peak was determined. From the exponential curve, the specific growth rate of the yeast was determined with a high degree of repeatability. The coefficient of the exponential term in the growth equation gave highly reproducible and distinguishable results relating to the viability and vitality of the initial yeast population. The time of peak heat flow was also affected by the initial viability and vitality of the yeast and was used to estimate the initial active cell population size.
- ItemRestrictedSpatial variations in leaching of a low-grade, low-porosity chalcopyrite ore identified using X-ray μCT(Elsevier, 2017-05-01) Fagan-Endres, Marijke A; Cilliers, Johannes J; Sederman, Andrew J; Harrison, Susan TLThis study presents an investigation, using 3D X-ray micro computed tomography (μCT), into the effect of sulfide mineral position within an ore particle on leaching efficiency. Three sections of an unsaturated mini-leaching column that had been packed with agglomerated low-grade, low-porosity chalcopyrite ore and leached with an acidified ferric iron solution were imaged at different stages of a 102 day experiment. Image analysis was used to quantify changes in the mineral content and the influence on this of the mineral distance from the ore particle surface, local voidage and radial position within the column. The main factor affecting the mineral recovery was identified to be proximity of the mineral to the ore particle surface, with recovery decreasing with increasing distance from the ore surface. A maximum leaching penetration was observed to exist at 2 mm from the surface, beyond which no recovery was achieved. Higher recoveries at the column wall indicated that preferential flow in this higher voidage had an additional, albeit smaller, impact on leaching efficiency.