226 research outputs found
Review of Symmetry: The Ordering Principle
No full textSymmetry: The Ordering Principle by architect David Wade is more discursive, and less mathematical. While there is a nod to classifications of symmetries it soon becomes clear that the author is more interested in patterns than in symmetry per se, and many of the objects under discussion, such as strange attractors and vortex streets, are not truly symmetrical. The somewhat ineffable concept of ’Li’ is introduced (though not defined, to my satisfaction at least) – the subject of another book by this author in the same series
Modelling of turbulent jets and wall layers: extensions of Lighthill's acoustic analogy with application to computational aeroacoustics
No full textTwo extensions to Lighthill’s aeroacoustic analogy are presented. First, equivalent sources due to initial conditions are derived that supplement those due to boundary conditions, as given by Ffowcs Williams & Hawkings. The resulting exact inhomogeneous wave equation is then reformulated with pressure rather than density as the wave variable, and the right-hand side is rearranged using the energy equation with no additional assumptions. Applications to computational aeroacoustics are discussed, and illustrated with examples based on 2D and 3D simulations
"The day of the great writer is gone for ever": Author surrogacy in Martin Amis’s Money and J.M. Coetzee’s Summertime.
No full textThis study focuses on the use of author surrogacy in the novels Money: A Suicide Note by Martin Amis and Summertime: Scenes from Provincial Life by J.M. Coetzee. It addresses the connection between their use of author surrogacy and their comments on what scholars classify as the postmodern cultural condition. Both authors have written themselves into their novels with a different purpose but both used strikingly similar themes to incorporate this purpose, although the stress on these themes varies. Authorial power, the distinction between the real and the imagined, and the fading line between high- and lowbrow culture are examples of the topics discussed in this study with regards to author surrogacy and the postmodern cultural condition. This study concludes that, through their use of author surrogacy, J.M. Coetzee mainly aims to critique, while Martin Amis satirises postmodern culture.
Keywords: Amis, author surrogacy, authorial power, Coetzee, fact-fiction distinction, high- and lowbrow culture, postmodern cultural condition
Biofouling of spiral wound membrane systems
No full textBiofouling of spiral wound membrane systems High quality drinking water can be produced with membrane filtration processes like reverse osmosis (RO) and nanofiltration (NF). Because the global demand for fresh clean water is increasing, these membrane technologies will increase in importance in the coming decades. One of the most serious problems in RO/NF applications is biofouling - excessive growth of biomass - affecting the performance of the RO/NF systems due to e.g. (i) increase in pressure drop across membrane elements (feed-concentrate channel), (ii) decrease in membrane permeability, (iii) increase in salt passage. These phenomena result in the need to increase the feed pressure to maintain constant production and to clean the membrane elements chemically. In practice, the first phenomenon is most dominant. The objective of this study was to relate biomass accumulation in spiral wound RO and NF membrane elements with membrane performance and hydrodynamics and to determine parameters influencing biofouling. The focus of this research was on the development of biomass in the feed-concentrate (feed-spacer) channel and its effect on pressure drop and flow distribution. These detailed studies can be used to develop an integral strategy to control biofouling in spiral wound membrane systems. Problem analysis Studies to diagnose biofouling in 15 full-scale RO and NF membrane installations with varying feed water types showed that (i) highest biomass concentrations were found at the installation feed side, (ii) the biomass related parameter adenosine-tri-phosphate was suitable for biofouling diagnosis in membrane element autopsies, (iii) measurements of biological parameters in the water were not appropriate in quantifying biofouling, and (iv) there is a need for a representative monitor and sensitive accurate pressure data to enable a reliable evaluation of the development of biofouling (Chapter 2). Based on the practical observations it was decided to develop a set of tools to study biofouling at controlled conditions. Method development A monitor was developed (Chapter 3) in combination with testing of a sensitive differential pressure drop transmitter (Chapter 4). This small monitor named Membrane Fouling Simulator (MFS) uses the same membranes and spacers as present in commercial membrane elements, has similar hydrodynamics and is equipped with a sight window. The MFS is an effective scaled-down version of a full-scale system and allows to study the biofouling process occurring in the first 0.20 m of RO/NF elements. Magnetic Resonance Imaging (MRI) provided in-situ, non-invasive, and spatially-resolved measurements of biofouling and its impact on hydrodynamics and mass transport in spiral wound membrane elements as well as in the MFS (Chapter 5). A three-dimensional computational model was developed to simulate biofouling in membrane elements, with feed spacer geometry as used in practice (Chapter 6). The model combines fluid dynamics, solute transport and biofouling. The methods described in the first part of the thesis have been used to increase the understanding of fundamental aspects of biofouling. Basic studies The development of biomass and related increase in pressure drop was not influenced by the permeate production in the elements (Chapter 7). Irrespective whether a flux was applied or not, the feed-concentrate channel pressure drop and biofilm amount increased in RO and NF membranes in monitor, test-rig, pilot and full-scale installation. Mass transport calculations supported that permeate production plays a minor role in the development of biofouling. Since fouling occurred irrespective of permeate production, the critical flux concept stating that “below a critical flux no fouling occurs” is not applicable to control RO/NF biofouling in extensively pretreated water. In essence, biofouling is a feed spacer channel problem (Chapter 8). This observation is based on (i) practical data and supported by (ii) in-situ visual observations of fouling accumulation using the MFS sight window, (iii) in-situ non-destructive observations of fouling accumulation and velocity distribution profiles using MRI, and (iv) differences in pressure drop and biomass development in monitors with and without feed spacer. MRI studies showed that already a restricted biofilm accumulation on the feed channel spacer influenced the velocity distribution profile strongly, leading to a strong decrease of the effective surface area in the membrane module and probably increasing the salt concentration in the dead-zones of the element leading to increased salt passage. Three-dimensional numerical simulations of biofilm formation and fluid flow were executed and compared with MRI and MFS studies (Chapter 9). The simulations showed similar (i) pressure drop development and (ii) patterns in flow distribution and channelling as observed in MRI and MFS studies. Feed spacers showed to have an essential role in biofouling, and are considered a prime target for improving the membrane elements. Based on the gained insights several potential methodologies to minimize the impact of biofouling have been studied and described in the last chapters of the thesis. Control studies The effect of substrate concentration, linear flow velocity, substrate load and flow direction on pressure drop development and biofilm accumulation was investigated in MFSs (Chapter 10). The pressure drop increase was related to the amount of accumulated biomass and linear flow velocity. Biomass accumulation was related to the substrate load. A flow direction change in the pressure vessels instantaneously reduced the pressure drop, accentuating that hydrodynamics, spacers and pressure vessel configuration offer possibilities to restrict the pressure drop increase caused by accumulated biomass. The impact of flow regime on pressure drop, biomass accumulation and morphology was studied (Chapter 11). In RO and NF membrane elements, at linear flow velocities as applied in practice voluminous and filamentous biofilm structures developed in the feed spacer channel, causing a significant increase in feed channel pressure drop. The amount of accumulated biomass was independent of the applied shear, depending on the substrate load. A high shear force resulted in more compact and less filamentous biofilm structure compared to a low shear force, causing a lower pressure drop increase. A biofilm grown at low shear was easier to remove during water flushing compared to a biofilm grown at high shear. Flow regimes manipulated biofilm morphology affecting membrane performance, enabling new approaches to control biofouling. Phosphate limitation as a method to control biofouling was investigated at a full-scale RO installation, characterized by low phosphate and substrate concentrations in the feed water and low biomass amounts in lead membrane modules. MFS studies showed that phosphate limitation restricted the pressure drop increase and biomass accumulation, even in the presence of high substrate concentrations (Chapter 12). Outlook Most past and present methods to control biofouling have not been very successful. Based on insights obtained by the studies described in this thesis, an overview is given of several potential complementary approaches to solve biofouling (Chapter 13). An integrated approach for biofouling control is proposed, based on three corner stones: (i) equipment design and operation, (ii) biomass growth conditions, and (iii) cleaning agents. Although in this stage chemical cleaning and biofouling inhibitor dosing seem inevitable to control biofouling, it is expected that in future – also because of sustainability and costs reasons - membrane systems will be operated without or with minimal chemical cleaning and dosing.BiotechnologyApplied Science
PHA Production in Aerobic Mixed Microbial Cultures
No full textPolyhydroxyalkanoate (PHA) is a common intracellular energy and carbon storage material in bacteria, which is considered as a bioplastic due to its plastic like properties. PHAs are versatile materials which are biodegradable and made from renewable resources. Commercial production of PHAs is currently based on pure culture processes employing either natural PHA producers or genetically modified bacteria. Pure culture processes use generally pure sterile substrates and axenic reactors, leading to high production costs and thus relatively expensive products. An alternative approach for the production of PHAs is the use of mixed culture biotechnology, using non-sterile waste streams as a substrate and open reactors. The use of cheaper substrates, less energy (no sterilization of substrate or reactors) and cheaper equipment could reduce the production costs compared to pure culture processes. However, the mixed culture PHA production process requires optimization for higher cellular PHA contents to be competitive with pure culture processes. The research described in this thesis aimed at improving the cellular PHA contents that can be achieved in open mixed cultures. A two-step process consisting of (i) a culture enrichment and growth step and (ii) a PHA production step was used. For the enrichment of a mixed culture with PHA producing bacteria a selective pressure in the form of alternating periods of short presence of the carbon substrate (feast phase) and long absence of the carbon substrate (famine phase) under fully aerobic conditions was employed. PHA storing bacteria generally outcompete other bacteria in such a feast-famine system due to their very high substrate uptake rate (which is not limited by the growth rate) and due to the ability to grow in a more balanced way throughout feast and famine phase. A sequencing batch reactor (SBR) was used to establish the feast-famine regime. The cultures enriched in the first step under different operational conditions were tested for their ability to produce PHA in the second step, the PHA production step. For this purpose the cultures were supplied with an excess of carbon source (fed-batch reactor) while withholding a suitable nitrogen source in order to avoid growth and direct as much carbon as possible into PHA storage. To simplify the system for the optimization studies a mineral medium with acetate as the sole carbon substrate was used in all experiments rather than real wastewater. Acetate yielded pure polyhydroxybutyrate (PHB) as the storage polymer. In order to compare different operational conditions, specific reaction rates and observed yields had to be calculated for the key compounds acetate, biomass, PHB, carbon dioxide, oxygen and the nitrogen source ammonia from measurements performed during a stable SBR cycle or fed-batch experiment. Both SBR and fed-batch reactor were highly dynamic systems with changing reaction rates and liquid volumes, making the evaluation of experimental data a complex task. A very detailed data analysis was carried out for each SBR cycle measurement and fed-batch experiment. The data analysis included for example the correction of measurements for sampling effects and liquid volume changes, the computation of oxygen consumption and carbon dioxide evolution, and the calculation of the best estimates for all reaction rates and total conversions at each time point with the help of a metabolic model (Chapter 2). The metabolic model was used in order to be able to describe the dynamics of the system and in order to ensure that material balances would close. The metabolic model described the measurements generally very well. The reaction rates computed with the metabolic model showed clearer trends than those calculated without the help of the model. Different operational conditions were tested for the biomass enrichment step (SBR). The first two process parameters investigated were low sludge residence times (SRTs) of 4 d, 1 d and 0.5 d and the impact of different degrees of nitrogen versus carbon limitation (Chapter 3). Low SRTs are required for a high biomass productivity in the first step. The impact of nitrogen limitation was investigated, because many waste streams that are suitable substrates for mixed culture PHA production are nutrient limited. Enrichment of a PHA storing community was successful at 4 d and 1 d SRT, but less successful at 0.5 d SRT. Nitrogen limitation in the SBR generally led to competition for nitrogen and consequently to a selective pressure for high growth rates. Carbon limitation in the SBR led to a PHB storage strategy (high acetate uptake rate) and usually to higher PHB contents (about 70 wt%) in subsequent fed-batch experiments compared to cultures enriched under nitrogen limitation. Carbon limitation in the SBR allowed PHB storing bacteria to benefit more from their ability to store PHB by being able to grow throughout the famine phase. Carbon limitation and SRTs higher than 0.5 d were identified as favourable conditions for the biomass enrichment step in the SBR. Nutrient limited wastewaters may require supplementation with nutrients for this step. Another parameter that was investigated was the reactor temperature (Chapter 4). The reactor temperature will influence the reaction rates, but also the selective pressure in the SBR. The influence on the reaction rates can be investigated by applying short-term temperature changes (i.e. one SBR cycle) while the combined effect on reaction rates and selective pressure can be studied in long-term temperature change experiments. In short-term temperature change experiments the reactor temperature of a stable SBR operated at 20°C was changed for one cycle to 15, 25, 30 or 35°C. It was found that reaction rate changes in the famine phase could be described over the whole temperature range with the Arrhenius equation with one temperature coefficient. For the feast phase different temperature coefficients were estimated for acetate uptake, PHB production and growth. These were only valid for temperatures 5°C higher or lower than the steady state temperature. After long-term changes to either 15 or 30°C the reactor performance changed considerably: At lower temperatures the feast phase was long and a growth strategy prevailed. This culture had a very low PHB storage capacity (about 35 wt%). At 30°C the feast phase was short and a PHB storage strategy dominated. This culture was able to store 84 wt% PHB. Higher SBR temperatures appear to be a good strategy to support the enrichment of PHB storing bacteria. In Chapter 5 we report the most successful operating strategy applied during this thesis. A SBR culture was enriched that was able to store 89 wt% PHB within only 7.6 h in a fed-batch experiment. This culture had been enriched with a longer cycle length of 12 h as compared to our previous studies (4 h cycle length), at 1 d SRT, 30°C and carbon limitation. Another key to the high PHB content was the long operating time under these conditions of over a year. The maximum PHB storage capacity of this culture had improved with time. The long cycle length combined with a low SRT was found to favour growth of bacteria that can store a high amount of PHB at a high rate, since this is needed in order to continue to grow throughout the much longer famine phase. After the operating conditions in the SBR had been optimized, also the PHA production step in the fed-batch reactor was investigated. The temperature in fed-batch experiments did not influence the maximum PHB storage capacity, but only the reaction rates (Chapter 4). Fed-batch experiments were typically conducted using fed-batch systems without nitrogen source in the feed. With the aim of using waste streams as a substrate for PHA production, nutrient limitation or starvation may not always be feasible. We therefore investigated the influence of nitrogen starvation, nitrogen limitation and nitrogen excess on the maximum PHB content obtained in fed-batch experiments (Chapter 6). Under nitrogen starvation conditions the biomass reached a maximum PHB content of 89 wt%, under nitrogen limitation 77 wt% and under nitrogen excess 69 wt%. In the latter two experiments PHB contents decreased after these maxima were reached, because growth led to a dilution of the PHB pool. Nutrient starvation seems thus to be the best strategy for maximal PHB production in the fed-batch step. Chapter 7 summarizes and integrates the findings from all individual studies. In this chapter also some remaining issues are discussed and recommendations for future research are provided. With the aim of using real waste streams in the future and producing other PHAs apart from PHB, the next steps would be the use of more diverse carbon source mixtures and eventually a scale-up of the system. In conclusion, mixed culture PHB production has been successfully optimized in this thesis. A mixed culture was established with the capacity to produce PHB levels as high as in pure culture production processes, and at very high PHB production rates. Cultivation conditions have been identified that lead to a selection of a stable mixed microbial culture with a superior PHA production capacity. Compared to previous work with mixed cultures, a more than four times higher cellular PHB content was obtained. Herewith a highly competitive process has been established that may contribute to the development of a more sustainable and renewable biopolymer production in a future bio-based economy.BiotechnologyApplied Science
Microbial ecology of phototrophic biofilms
No full textBiofilms are layered structures of microbial cells and an extracellular matrix of polymeric substances, associated with surfaces and interfaces. Biofilms trap nutrients for growth of the enclosed microbial community and help prevent detachment of cells from surfaces in flowing systems. Phototrophic biofilms can best be defined as surface attached microbial communities mainly driven by light as the energy source with a photosynthesizing component clearly present. Eukaryotic algae and cyanobacteria generate energy and reduce carbon dioxide, providing organic substrates and oxygen. The photosynthetic activity fuels processes and conversions in the total biofilm community, including the heterotrophic fraction. This thesis starts with a brief introduction in the ecology of phototrophic biofilms and discusses their actual and potential applications in wastewater treatment, bioremediation, fish-feed production, biohydrogen production, and soil improvement and their role in biofouling. The next chapter describes the diversity of phototrophic bacteria in hot spring microbial mats found on the east coast of Greenland. In this study we utilized a polyphasic approach using a combination of isolation techniques, microscopic observation of morphological features, and cultivation-independent molecular methods. We observed a relationship between the cyanobacterial community composition and the in situ temperatures of different microbial mat parts. Chapter 4 focuses on the successional changes in community composition of freshwater phototrophic biofilms growing under different light intensities. Our results suggest that surface colonization by heterotrophic pioneers facilitates the development of phototrophic biofilms. In Chapter 5 we compared the community composition of phototrophic biofilms cultivated in three microcosm systems operated under identical conditions but placed in different laboratories. Denaturing Gradient Gel Electrophoresis (DGGE) analysis of both 16S and 18S rRNA gene fragments showed that the communities developed differently in terms of species richness and community composition. Chapter 6 demonstrates that nifD gene sequences, coding for a nitrogenase subunit, can be used to detect and identify diazotrophic cyanobacteria in natural communities. PCR products generated using primers homologous to conserved regions in the cyanobacterial nifD genes were subjected to DGGE and clone library analysis in order to determine the genetic diversity of diazotrophic cyanobacteria in environmental samples. In the last chapter we describe the development of PCR primers targeting conserved regions within the cyanobacterial hupS gene family. This gene is involved in the hydrogen metabolism of diazotrophic microorganisms. We analyzed hupS diversity and transcription in cultivated phototrophic biofilms by the direct retrieval and analysis of mRNA that was reverse transcribed, amplified with hupS specific primers, and cloned. Overall, the community composition and species richness of phototrophic biofilms was shown to be highly variable. Cultivation-independent molecular methods proved very useful to study diversity and function in phototrophic biofilms.Applied Science
Treatment of source separated urine and its effects on wastewater systems
No full textAbstract not availableApplied Science
Polyhydroxyalkanoates production by bacterial enrichments
No full textPolyhydroxyalkanoates (PHAs) is a natural bacterial storage compound, which can be used as carbon and electron source. Their remarkable similarities in physical properties to conventional plastics, such as polypropylene, attract great commercial interest. This thesis focuses on PHAs production by bacterial enrichments. As compared to the current pure culture biotechnology, mixed culture biotechnology is much less dependent on the well defined substrate and sterile process. These properties of mixed culture biotechnology can greatly reduce the price of the final products. The major aims of this thesis are to optimize the process to select bacterial enrichments with superior PHAs-producing capacity and analyze the microbial community compositions in these enrichments. In my PhD studies, the main findings are: (1) both the maximal bacterial PHAs content and biomass specific PHAs productivity have been significantly improved. (2) the new selective pressure to enrich bacterial species with superior PHA producing capacity has been found. (3) several novel bacterial speices have been isolated and characterized. Based on these findings, converting argo-industrial waste to valuable chemical compounds can be achieved. Currently, a demo-scale production reactor is being constructed.BiotechnologyApplied Science
On the use of selective environments in microalgal cultivation
No full textThis thesis deals with selective environments in microalgal cultivation. As explained in Chapter 1 microalgae have changed the course of life on Earth dramatically by performing oxygenic photosynthesis. In oxygenic photosynthesis electrons from water are used to reduce carbon dioxide to carbohydrates or lipids using solar energy. As a waste product oxygen is produced. The production of carbohydrates and particularly lipids by microalgae attracts currently considerable scientific interest as microalgal lipids can be converted to yield biodiesel. As the CO2 emitted upon combustion of this biodiesel has recently been withdrawn from the atmosphere by the microalgae, no net increase in atmospheric CO2 level takes place. Microalgae offer advantages over other lipid production platforms as microalgae are able to reach high intracellular lipid contents and need little freshwater and arable land. Chapter 2 describes the advantages and limitations of the application of a selective environment to obtain a certain functionality in a system. A selective environment aims to give a competitive advantage to a microorganism displaying the desired functionality. By rewarding microalgae for displaying a certain functionality it becomes in the interest of microalgae themselves to display this characteristic. The best reward in nature is an increased chance of survival. A selective environment therefore tries to couple the desired characteristic to an increased chance of survival. Microalgal cultivation based on selective environment fundamentally differs from cultivation of pure cultures. Maintaining the desired culture is the goal of the latter, whereas a microalgal cultivation process based on selective environments aims to maintain a functionality in a system. The species, or multiple species, displaying the desired functionality are expected to differ at different geographical places, with changing climate conditions and over time. Under any condition the species that thrives under these specific conditions by displaying the desired characteristic is enriched. Interesting microalgal functionalities from an industrial point of view include a high carbohydrate and lipid productivity. Both of these storage compounds are produced by microalgae to endure dynamic growth conditions. By limiting the presence of the essential microalgal nutrient nitrogen (in the form of NH4 + or NO3 –) to the dark phase solely an environment is created in which production of storage compounds in the light period is an advantageous strategy. Production of these compounds in the light period will allow microalgae to metabolise the available nitrogen in the dark period by supplying carbon skeletons and energy in the dark. Chapter 3 shows that such an environment enriches carbohydrate producing microalgae from a natural inoculum. Chapter 4 shows that both the moment of nitrogen addition as the amount of nitrogen dosed per microalgae had significant influence on the metabolic behaviour of marine microalgal cultures enriched using the procedure described in Chapter 3. Carbohydrate and lipid productivity proved maximal if ammonium was supplied at the start of the dark period rather than the light period, irrespective of the amount of nitrogen dosed per microalgae. Increasing the amount of nitrogen dosed per microalgae, by increasing the volume exchange ratio from 33 to 50 percent per cycle, induced a decrease in storage compound production if ammonium was supplied in the light period whereas the storage compound productivity was comparable when ammonium was supplied in the dark period. Chapter 5 shows that the enriched microalgal community was highly dependent on an environmental parameters as the presence of silicate. If silicate was present at non-limiting concentrations the enriched culture was dominated by diatoms, whereas green algae were dominant if silicate was absent. Both cultures showed however the same functionality of producing large amounts of carbohydrates in the light period to be able to consume the supplied nitrogen source in the dark period. These results, together with the data obtained under marine conditions, showed that carbohydrate production can be achieved under various conditions, as long as a carbon fixation in the light period is uncoupled from nitrogen uptake in the dark. Diatoms have interesting characteristics for large-scale microalgal cultivation. These include a relative easy solid-liquid separation after cultivation, increased resistance to predators and the possibility to synthesize lipids under silicate limitation. Supplying NH4 + in a pulse, either at the start of the light or the dark period under non-limiting silicate levels, enriched a culture fully dominated by the diatom Nitzschia palea from a natural inoculum, as described in Chapter 6. The metabolic behaviour of the enriched culture was highly influenced by the moment of nitrogen addition. Biomass was the main photosynthetic product in the light period if nitrogen was dosed at the start of the light period, whereas carbohydrates were the main photosynthetic product if nitrogen was dosed at the start of the dark period. Subjecting the enriched cultures to prolonged periods of nitrogen or silicate limitation induced different metabolic responses. Cell numbers increased four times and carbohydrates were the main storage compounds under nitrogen limitation, while cell division abruptly ceased and lipids were the preferred storage compound under silicate limitation. In all experiments carried out in this thesis carbohydrates were the preferred microalgal storage compounds. Uncoupling of carbon fixation in the light from nitrogen uptake in the dark enriched under variable conditions (freshwater, marine, under high silicate concentrations) carbohydrate producing microalgae from a natural inoculum. Intracellular carbohydrate levels typically increased from10 to 50 % of organic dry weight in the light period. Although no liquid and gas flows leaving and entering the systems were sterilised and despite regular cleaning of the systems the enriched cultures were highly stable in time. This shows that if carbohydrate productivity is aimed for a proper selective environment has been identified and tested. A better understanding of the ecological role of lipids and carbohydrates in microalgae will help creating selective environment for lipid production. Besides drawing general conclusions, Chapter 7 elaborates more on possible strategies to enrich lipid producing microalgae. The strategy advocated in this thesis, rewarding a microalga for displaying a functionality by coupling it to an increased chance of survival by imposing a selective environment, will prove a valuable tool if the ecological role of lipids is better understood.BiotechnologyApplied Science
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